Natural Variability in the Widths of the Tropics

Guest Post by Willis Eschenbach

In my previous post, “Does This Analysis Make My Tropics Look Big?“I discussed a paper called “Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone”, by Robert Allen et al, hereinafter A2012. They use several metrics to measure the width of the tropics—the location of the jet stream (JET), the mean meridional circulation (MMC), the minimum precipitation (PMIN), the cloud cover minimum (CMIN), and the precipitation minus evaporation (P-E) balance. Since writing that post, I’ve looked at what the Argo dataset says about the P-E balance, which is the precipitation minus the evaporation. Figure 1 shows the global Argo results regarding the salt concentration (salinity) in the surface of the ocean, which is a proxy for precipitation minus evaporation.

Figure 1. Global average salinity, as revealed by the Argo floats, in practical salinity units (PSU).

We can use the salinity of the ocean in the tropical and temperate regions as a very good proxy for the balance of precipitation and evaporation. In the deep meteorological tropics, just north of the equator, is the Intertropical Convergence Zone, or the ITCZ. In the ITCZ, the precipitation predominates. As a result, in that area there is more fresh-water rain than evaporation, and that makes the ocean less salty (blue).

On the other hand, at about 30° north and south of the Equator are the descending dry air branches of the Hadley cells. These create the global desert belts. Figure 2 shows a cross-section through the atmosphere illustrating the circulation of the great Hadley cells:

Figure 2. The descending branch of the Hadley cells is dry because the water has been rained out in the deep tropics. In the Northern Hemisphere this dry descending air creates the arid belts of the Sonoran desert in Northern Mexico / Southwest US as well as the Sahara, Middle Eastern, and Gobi regions. In the Southern Hemisphere, it encompasses the Atacama (South America), Kalahari (Africa), and Australian deserts.

In these arid regions, the evaporation is much greater than the precipitation and as a result the ocean is saltier in these areas. And once again this is reflected in the salinity, specifically in the areas of high salinity (red) around thirty degrees north and south of the Equator, as shown in Figure 1.

Now let me refresh people’s memory regarding the claims of the A2012 paper. Figure 3 shows their Northern Hemisphere results discussed in my previous paper:

FIGURE 3. ORIGINAL CAPTION: Figure 2 | Observed and modelled 1979–1999 Northern Hemisphere tropical expansion based on five metrics. a, Annual mean poleward displacement of each metric, as well as the combined ALL metric. … CMIP3 models are grouped into nine that included time-varying black carbon and ozone (red); three that included time-varying ozone only (green); and six that included neither time-varying black carbon nor ozone (blue). Boxes show the mean response within each group (centre line) and its 2s uncertainty. Observations are in black. In the case of one observational data set, trend uncertainty (whiskers) is estimated as the 95% confidence level according to a standard t-test.

I was interested in the P-E record (precipitation minus evaporation). The P-E results in the A2012 paper (Figure 3 above) show a net change of 0.75 degrees of latitude in twenty years in the latitude of the Northern Hemisphere maximum salinity, or about 0.36 degrees per decade. Southern Hemisphere P-E results (not shown) are about half that size, at 0.17 degrees per decade.

So I took a look year by year in the various oceanic basins to examine the natural variability in the salinity, which is our best proxy for P-E. Here are the results for the Pacific Ocean (120°E to 100°W longitude) from the Argo data. Figure 4 shows the variability, along with the decadal observational changes reported in A2012.

Figure 4. Year by year changes in the latitudinal salinity in the Pacific. Circles show the annual Southern Hemisphere peak salinities at about 30°S, the Equatorial lows in salinity, and the annual peaks in the Northern Hemisphere salinity at about 30°N. Distance between the two vertical black lines in the upper right illustrates the amount of the decadal Northern Hemisphere tropical expansion claimed in A2012 (0.38°/decade). Two vertical black lines (so close they appear as one line) in the upper left illustrates the amount of the decadal Southern Hemisphere tropical expansion claimed in A2012 (0.16°/decade). There is insufficient data in the years 2002-2003 to plot the Southern Hemisphere peaks.

There are a few things of interest here. First, in the Pacific the location of the minimum salinity at the ITCZ just north of the Equator is relatively stable. The location of the Pacific ITCZ doesn’t drift north or south too much. The Southern Hemisphere peak is more variable in latitude, and the Northern Hemisphere is more variable yet. All of them move around much, much more than the amount of the estimated decadal year trend. Bear in mind that according to A2012 this tropical expansion is driven by black carbon and ozone … note also the relative sizes of the expansion claimed in A2012, shown by the vertical black lines. [As an aside, I was surprised by the difference in the widths of the northern and southern Tropics, with the southern Tropics being twice as wide as the northern Tropics. It suggests that the outer edges of the tropics, the areas of peak salinity, are controlled by physical rather than meteorological considerations ... but I digress.]

Figure 5 shows the corresponding data for the Indian Ocean (20°E to 120°E longitude). The Indian Ocean doesn’t go far enough north to experience a minimum, so the Southern Hemisphere peak and the low at the ITCZ are shown.

Figure 5. Year by year changes in the latitudinal salinity in the Indian Ocean. Circles show the annual Southern Hemisphere peak salinities at about 30°S, and the Equatorial lows in salinity. Two vertical black lines (so close they appear as one line) in the upper left illustrates the amount of the decadal Southern Hemisphere tropical expansion claimed in A2012 (0.16°/decade).

In the Indian Ocean, the situation is reversed. Unlike in the Pacific, the peak salinity is stable, but the location of the low salinity at the ITCZ is greatly variable. In addition, the ITCZ appears to have moved generally southwards over the decade.

Finally, the Atlantic Ocean takes the tropical variability prize, as shown in Figure 6.

Figure 6. Year by year changes in the latitudinal salinity in the Atlantic. Circles show the annual Southern Hemisphere peak salinities at about 30°S, the Equatorial lows in salinity, and the peaks in the Northern Hemisphere salinity at about 30°N. Distance between the two vertical black lines in the upper right illustrates the amount of the decadal Northern Hemisphere tropical expansion claimed in A2012 (0.38°/decade). Two vertical black lines (so close they appear as one line) in the upper left illustrates the amount of the decadal Southern Hemisphere tropical expansion claimed in A2012 (0.16°/decade).

As mentioned above, the Atlantic results are all over the map, with all areas varying greatly in both salinity and distance from the Equator.

The obvious conclusion that I draw from all of this is that a trend can be significant without being meaningful. The trends in tropical expansion shown in the A2012 paper are tiny compared to the natural variations in the system. The width of the meteorological tropics varies up to eight degrees in a single year. In such a system, a few tenths of a degree of expansion per decade, even if it turns out to be both accurate and statistically significant, is trivially small.

Finally, I wanted to investigate the relationship between temperature rise and precipitation. So I took a look, for each individual Argo float, at the differences in temperature and salinity (again as a proxy for rainfall minus evaporation) in successive cycles of each float. I then plotted the ratio of the changes globally. Figure 7 shows that result:

Figure 7. Change in rainfall with temperature, as indicated by the proxy of the change in salinity with temperature. Blue areas are where the rainfall increases as the temperature increases, and red areas are where the rainfall goes down as temperatures rise.

This was an interesting result, as it shows a more complex and nuanced pattern than the usual mantra of “a warmer world is a wetter world”. It is also interesting in that there is only a small relationship, albeit statistically significant, between salinity and temperature. For each degree of temperature rise, the salinity goes up by only 0.04 PSU, a tiny amount (although the p-value is 2e-16).

Finally, here’s the strange part. Averaged over the entire globe, since salinity goes up with temperature, globally the Argo data says precipitation goes down fractionally with increasing temperature. In the tropics, the relationship is as expected, rainfall increasing with temperature. But globally, it goes the other way, rainfall decreases with increasing temperature … and there is only a minuscule effect. I didn’t expect that at all. [UPDATE—see below for why I didn't expect it]

That’s the beauty of climate science being settled … there are so many surprising results.

w.

[UPDATE] Global estimates of the water cycle are on the general order of this one:

 

Thanks to commenters in the thread below, I see now that my expectation of the direction of change in the global oceanic P-E with increasing temperature was mistaken. The basic equation for the ocean mass balance says that precipitation plus additions from the rivers (including ice melt and groundwater extraction) minus evaporation from the ocean gives mass balance change.

Now, the ice melt and groundwater extraction don’t vary much year to year. So if the ocean mass is roughly constant, we can take the basic equation as being evaporation from the ocean equals rain into the ocean plus net rain over land … what goes up must come down.

My mistake was in thinking that the actual value of the oceanic P-E overall was positive. It is not. From the data given in the table above, we can see that P-E is about – 35,000 cubic kilometres per year.

And that means that if we increase the speed of the hydrological cycle by say 10%, and we assume that all of the proportions remain the same, the value of P-E becomes more negative, not more positive as I had assumed.

In other words, I should have expected that if the temperature increased the value of P-E should go down, not up as I thought. In that regard, it appears that my finding, that oceanic P-E decreases with increasing temperature, is consonant with expectations.

Always more to learn …

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93 thoughts on “Natural Variability in the Widths of the Tropics

  1. Climate science, in contrast with politically driven cargo-cult ‘science,
    actually has surprises because it is about reality,
    It is not an ideology, which is for keeping reality
    from threatening the preconceived dogma.

  2. The large natural variations in the system show that the system response to changes in the amount of energy available at the ocean surface is very sensitive.

    That system response being invariably negative the outturn is a system that is very INSENSITIVE to forcing influences.

    Anything that speeds up or slows down the energy flow through the system is immediately countered by an equal and opposite system response.

    In so far as GHGs might slow down the rate of energy loss to space the system just speeds it up again for a zero net effect on system energy content.

    The ‘price’ of the negative system response is a shift in the air circulation pattern involving a change in the sizes positions and / or intensities of the permanent climate zones.

    Underlying the whole thing is atmospheric pressure at the surface fixing the energy cost of a given amount of evaporation but that is another story.

  3. “But globally, it goes the other way, rainfall decreases with increasing temperature”

    That article relies on salinity changes over the oceans but most precipitation is over land so relying on ocean surface rainfall is inadequate for diagnostic purposes.

    We can see from the chart that precipitaion increases within the ITCZ and of course it must then decrease on either side of the ITCZ where there are descending subtropical high pressure cells which widen as the ITCZ intensifies.

    Most of the world’s surface beneath those high pressure cells is ocean so we get a skewed impression of the global effect unless we also include all the land masses AND the mid latitude depression tracks to the poleward of those dry subtropical high pressure cells.

    Globally, one will get increased precipitation under the ITCZ, over the land masses and under the mid latitude jets but decreased precipitation over the oceanic areas beneath the subtropical high pressure cells.

    Anything that widens those subtropical high pressure cells allows more solar energy into the oceans for a warming troposphere as the oceans then shed that energy faster to the air.

    The subtropical high pressure cells can be widened by variations in the rate of energy release from the oceans below or by an active sun causing more positive AO and AAO thereby drawing the air circulation pattern poleward.

    More GHGs from human or natural sorces would have the same effect but the human portion would be lost in the natural variations caused by sun and oceans.

    In the short term the solar effect is disguised by internal system variability but it becomes apparent on multidecadal and centennial timescales such as MWP to LIA to date.

  4. BTW, you might want to correct the typo in line 5-6 where you have precipitation – evaluation instead of evaporation. Luckily it’s correct shortly thereafter which stopped me from scratching my head.

    [Thanks, fixed. -w.]

  5. Unsurprisingly, there is a reasinably strong correlation between salinity (“which is a proxy for precipitation minus evaporation“) and CO2 absorption by / emission from the oceans. ie, where there is evaporation there is also CO2 emission. In the map (Fig.1), the red areas are generally the areas of CO2 absorption and the blue areas are generally the areas of CO2 emission. See http://www.ldeo.columbia.edu/res/pi/CO2/carbondioxide/pages/air_sea_flux_2000.html
    Some of these areas are perhaps not where you might expect them to be, ie. they aren’t just ‘hot’ and ‘cold’.

  6. Great post, thank you!
    1. It would be educational to the uninitiated to show where the radiation goes out in the second figure.
    2. What happened to Farrell or was it Ferrell?
    3. It would be nice to see posts more like this one in stead of the useful idiot and malefactor bashing ones, Even though I gleefully participate in the bashing it often feels like a comic diversion and a waste of time.

  7. “Figure 2. The descending branch of the Hadley cells is dry because the water has been rained out in the deep tropics. In the Northern Hemisphere this dry descending air ——-“

    ==========

    What I am pondering is: “Where does all that falling rain that has dried out the Hadley Cell come from in the first place?” – Well, it looks to me as it evaporated in the areas of the Equator and the “deep tropics” in the first place. Therefore the fresh or less salty water can only be congregating near the surface. Yes? – No?

  8. I read into thiis more thunderstorms in the tropics, more heat transported to the Troposphere to be radiated into space, ie, W.E. has confirmation of his thermostat hypothesis.

  9. Stephen Wilde says:
    May 25, 2012 at 2:40 pm (Edit)

    “But globally, it goes the other way, rainfall decreases with increasing temperature”

    That article relies on salinity changes over the oceans but most precipitation is over land so relying on ocean surface rainfall is inadequate for diagnostic purposes.

    Most precipitation is over the land? I greatly doubt that, but let me take a look …

    w.

  10. The unique geographic constraints of the North Atlantic make it a mostly tropical ocean that impinges well into the mid latitudes. As I child I went diving on Cape Cod and was chasing horseshoe crabs around at that high latitude.

  11. “Averaged over the entire globe, since salinity goes up with temperature, globally the Argo data says precipitation goes down fractionally with increasing temperature.”

    Wouldn’t it be a more accurate assessment to say that globally there is no change in precipitation minus evaporation with temperature? A statistically insignificant relationship is one that could occur purely by chance and not be real, and moreover, you seem to be suggesting that the relationship you are looking at is between precipitation and temperature, rather than precipitation minus evaporation and temperature. I am fairly certain that claims of greater precipitation with temperature are also for greater evaporation. What you have found is that this effect basically perfectly balances over the oceans. It probably would even if one also even if one also considered the land areas. I think if you could look at just precipitation there would be a globally averaged significant positive relationship with temperature, and also with evaporation alone. The effects probably just cancel on average.

  12. Stephen Wilde says:
    May 25, 2012 at 2:40 pm

    We can see from the chart that precipitaion increases within the ITCZ and of course it must then decrease on either side of the ITCZ where there are descending subtropical high pressure cells which widen as the ITCZ intensifies.

    I disagree. Increased precipitation within the ITCZ could, and likely does, result from increased evaporation in the subtropical highs. Increased ITCZ precipitation results from increased water vapour. There is no reason this should decrease precipitation in the subtropical highs.

    The GHG warming should decrease evaporation from the oceans, because it decreases the ocean/atmosphere temperature difference. Which is why the ‘warmer world is a wetter world’ mantra is likely wrong. Although a warmer world may change the ocean/land precipitation ratio.

    What would drive increased ocean evaporation is either a cooler atmosphere or increased solar insolation.

  13. I was a little late for your last party, so here’s the cross post:

    I downloaded the U-Wind (i.e. east-west) data from the 20th Century Reanalysis Project v2. I then interpolated the latitudes which separate the easterly Trade Winds from the Westerlies. I consider this the border of the “meteorological tropics” in the Horse Latitudes. In the Northern Hemisphere for the years 1979-1999 I found a poleward 0.18 +/- 0.31 decade rate. This looks compatible with the MMC figure above which is also based on circulation.

    For the period 1911-2010 the decade rate is poleward 0.007 +/- 0.025 degrees.

    For the period 1951-2010 the decade rate is *equatorward* 0.020 +/- 0.049 degrees.

    It all looks insignificant.

    Some plots and source code can be found here:

    https://sites.google.com/site/climateadj/tropical-expansion

  14. AJ-What happens if you take the difference between the NH series and the SH series (ie the width of the whole tropics thusly defined)?

  15. Earlier I had said:

    Stephen Wilde says:
    May 25, 2012 at 2:40 pm

    “But globally, it goes the other way, rainfall decreases with increasing temperature”
    That article relies on salinity changes over the oceans but most precipitation is over land so relying on ocean surface rainfall is inadequate for diagnostic purposes.

    Most precipitation is over the land? I greatly doubt that, but let me take a look …

    w.

    Well, it took me a while, but I took a look. I found the average rainfall over the ocean and land by latitude here.

    I digitized the rainfall data in that graphic. Then I used my land mask to find the percentages of ocean and land at each latitude. I used those, along with the cosine of the latitude, to calculate the weighted averages of the total rainfall on oceans and land.

    The answer is … drum roll please … globally, about 78% of the rain falls in the ocean, and about 22% on land. Since the world is about 72% ocean and 28% land, this means that rainfall is slightly heavier over the ocean. This is as I would expect, given the torrential rains over the tropical oceans.

    As a check, I calculated the total global rainfall. It worked out to 960 mm/year, which is in good agreement with global estimates.

    w.

  16. timetochooseagain says: May 25, 2012 at 8:48 pm

    My interpretation of your question is:
    “Does the north and south jiggle of the “tropics” hide the hemispheric expansion of the tropics?”

    Good question. I don’t have a definitive answer. Right now I’m just enjoying Willis’s analysis.

  17. timetochooseagain says: May 25, 2012 at 8:48 pm

    I should have posed:
    “Does the north and south jiggle of the “tropics” hide the trend in the width of the tropics?

    I can do the programming and come up with a result, but my initial analysis indicates that the results are sensitive to the start and end time points. I prefer to examine the NH only as it has the better sampling coverage.

  18. “globally, about 78% of the rain falls in the ocean, and about 22% on land. ”

    Thank you for that additional analysis Willis.But they do say this i your data source:

    “We conclude that the GPCP data is simply too short to provide a reliable estimate of global precipitation trends over land. Trend analyses of the oceans are difficult to interpret”

    and:

    “Radar does however have a number of disadvantages. The conversion of the signal backscatter into rain-rates is not exact; surface effects and melting precipitation lead to anomalous signals, and low-level precipitation may be missed due to the upward-refraction of the radar beam through the atmosphere. Other issues include attenuation, beam blockage, beam-filling, and beam overshoot ”

    I was thinking in terms of rainfall from orographic uplift as oceanic air rises on contact with landmasses especially mountains but from your figures that may be more than offset by the dryness of continental interiors.

    However there is a residual point that the variations in salinity on the oceans do not reveal variations in precipitation over land and so are not a good enough diagnostic indicator as to whether global precipitation does indeed decrease with higher temperatures.

    I agree with timetochooseagain who said:

    “I think if you could look at just precipitation there would be a globally averaged significant positive relationship with temperature, and also with evaporation.”

  19. “Increased precipitation within the ITCZ could, and likely does, result from increased evaporation in the subtropical highs. Increased ITCZ precipitation results from increased water vapour. There is no reason this should decrease precipitation in the subtropical highs.”

    More uplift means more descent and more descent would increase the width of the subtropical highs with a decrease in rainfall in regions which experience higher surface pressure as a consequence.

    “The GHG warming should decrease evaporation from the oceans, because it decreases the ocean/atmosphere temperature difference”

    The ocean surfaces are mostly above the temperature of the overlying air so warmer ocean surfaces will increase that differential.

  20. “For the period 1911-2010 the decade rate is poleward 0.007 +/- 0.025 degrees”

    That includes two warming periods and one slight mid century cooling period and the recent cooling so I would expect a very low net figure and so it is..

    “For the period 1951-2010 the decade rate is *equatorward* 0.020 +/- 0.049 degrees”

    That suggests that the recent ten years plus the mid century cooling have now just about offset the late 20th century warming but we have yet to see the full thermal consequences.For the present the previous warming has merely ceased.

    “In the Northern Hemisphere for the years 1979-1999 I found a poleward 0.18 +/- 0.31 decade rate”

    That is the late 20th century warming period on its own unameliorated by the earlier and later cooling spells and emphasised in the northern hemisphere due to the absence of oceanic modulation so as I would expect that period in that region to give the highest poleward shift and so it does.

    As to whether it is significant then I would say yes, absolutely, because it reflects the fine detail of changes in the speed of energy flow from oceans through the air and thence to space.The system is very sensitive to small changes but in a negative fashion producing a very insensitive system overall.

    The fact that the error band is larger than the observed changes is simply due to our inadequate measuring techniques plus large internal system variability on short timescales. Notwithstanding those factors we can still discern a link between past tropospheric temperature trends and the observed positioning.

    Poleward drifting is a response to more energy entering the system and equatorward drifting a response to more energy leaving the system.

    Meanwhile total system energy content remains pretty much constant for a given level of solar input to top of atmsphere and a given atmospheric pressure on the ocean surfaces. Not that I intend Willis to fire off with ‘pressurehead’ assertions.

  21. There are a few things of interest here. First, in the Pacific the location of the minimum salinity at the ITCZ just north of the Equator is relatively stable. The location of the Pacific ITCZ doesn’t drift north or south too much. The Southern Hemisphere peak is more variable in latitude, and the Northern Hemisphere is more variable yet.

    Presumably ENSO contributes to variability in these subequatorial bands.

  22. Stephen, after I wrote the post above, I realized I probably mis-understood what you were getting at.

    The ocean surfaces are mostly above the temperature of the overlying air so warmer ocean surfaces will increase that differential.

    They are, but I was referring specifically to the effect of GHG warming. GHG warming should warm the oceans by impeding heat loss because of the reduced temperature difference. If the ocean surface is warming faster than the overlying atmosphere, the mechanism can not be GHGs.

    GHG warming should slow the hydrological cycle in the ITCZ, and by your argument decrease the size of the ITCZ. If I understand your argument correctly.

    The ‘warmer world is a wetter world’ results from warmer air holding more water vapor and more being transported poleward to mid to high latitudes, but where does the extra ocean energy to feed increased evaporation come from? A warmer atmosphere can’t warm the ocean faster than itself warms. Thus can not increase the temperature difference, and the faster the atmosphere warms, the smaller the air/ocean surface temperature difference becomes.

    Figure 7 above shows increased precip with increased SSTs in the tropics, but generally the reverse elsewhere. That looks to me like an increased solar insolation effect in the tropics. While a weaker solar insolation effect away from the tropics is being masked by something else, perhaps a warmer atmosphere warming the ocean surface.

  23. Stephen Wilde:

    At May 25, 2012 at 11:59 pm you say;

    The fact that the error band is larger than the observed changes is simply due to our inadequate measuring techniques plus large internal system variability on short timescales.

    Ummm, NO!

    The fact that the error band is larger than the observed changes always means the data tells us nothing about the changes except that the changes are unlikely to be greater than the error bands. Simply, the data does not indicate a change exists and, therefore, it is not possible to ascribe a cause to why the changes are not observed.

    If there were another data set which demonstrated the existence of the change then – and only then – it would be not reasonable to suggest a cause of why the changes are not observed (e.g. with a view to determining the validity of the suggested cause). You have not provided any empirical data that demonstrates a change exists.

    To summarise, in this case
    • we do not know if there has been a change,
    • we do not know the magnitude of the change if it exists,
    and
    • we do not know the sign of the change if it exists,
    but
    • we do know the probable limits which constrain the magnitude of the possible change if it exists.

    Richard

  24. timetochooseagain says: May 25, 2012 at 8:48 pm

    I ran the numbers adding the NH and abs(SH) together. Here are the decade trends:

    1911-2010 equatorward 0.050 +/- 0.038
    1951-2010 poleward 0.078 +/- 0.073

  25. Richard, I think my non science background caused me to misuse the concept of error bands.

    Nonetheless there are observed changes in the positioning which appear to fit changes in historical climate shifts as per my post at May 25, 2012 at 11:59 pm

    Philip,

    Yes it is proposed that GHGs raise the temperature of the ocean skin thereby reducing the temperature differential from ocean bulk to ocean skin which is supposed to reduce the energy flow from ocean to air through the skin thereby making the bulk oceans warmer and ultimately heating up the air as well.

    However I have elsewhere spent a lot of effort explaining why that proposition doesn’t work due to the net cooling effect of the evaporative process.

    Try this:

    http://climaterealists.com/attachments/ftp/TheSettingAndMaintainingOfEarth.pdf

  26. AJ,

    In the period 1911 to 2010 you have a warming spell followed by cooling, then warming then a short period of cooling again so the net overall would be very small.

    Could you run figures for the following periods ? :

    1911 to 1940

    1940 to 1970

    1970 to 2000

    2000 to 2010.

  27. AJ-Thank you. It looks like the uncertainty is larger. Hm, it might help with understanding why the trend is sensitive to endpoints if one took running averages. Eleven year centered averages should smooth over all the inter-annual noise. Then you could probably see when the trends change signs. Stephen Wilde seems to be hypothesizing that the changes should roughly coincide with the known shifts from warming to cooling trends and vice versa in the temperature record.

  28. I’ll try to run some more numbers in the next couple of days. I believe the uncertainty is larger when I consider the uncertainty in the source dataset, which is also interpolated.

  29. “Stephen Wilde seems to be hypothesizing that the changes should roughly coincide with the known shifts from warming to cooling trends and vice versa in the temperature record.”

    Correct. If not then either I would have to do some serious rethinking or, possibly, current data is inadequate for purpose.

    So:

    1911 to 1940 should show a poleward drift.

    !940 to 1970 should show an equatorward drift.

    1970 to 2000 should show a poleward drift

    2000 to 2010 might be too short a period to show an equatorward drift but it should be at least neutral.

    Go to it AJ.

  30. Well, I found some free time. Here are the requested decadal trends for the NH:

    1911 to 1940 -0.30 +/- 0.14
    1940 to 1970 0.11 +/- 0.15
    1970 to 2000 -0.04 +/- 0.15
    1999 to 2009 0.07 +/- 0.42

    2010 was an outlier which resulted in a negative trend, so I swapped it for 1999.

  31. Thanks AJ.

    You confirm that 1911 to 1940 and 1970 to 2000 are both negative and both were warming periods so I assume that negative means poleward. Is that right ?

  32. Stephen Wilde says:
    May 25, 2012 at 11:24 pm

    … However there is a residual point that the variations in salinity on the oceans do not reveal variations in precipitation over land and so are not a good enough diagnostic indicator as to whether global precipitation does indeed decrease with higher temperatures.

    I agree with timetochooseagain who said:

    “I think if you could look at just precipitation there would be a globally averaged significant positive relationship with temperature, and also with evaporation.”

    Many thanks, Stephen, and also timetochooseagain. The oceans may not be a good enough diagnostic indicator, and it may well be that timetochooseagain is right that over the planet there is a “positive relationship with temperature”.

    My point is simple—for 80% of the rainfall, falling on 70% of the planet, the trend is slightly negative, not positive at all. That may be offset over the land, we don’t know.

    But for it to just get back to zero, the P-E balance over the land would have to be a) in the reverse sense and b) four times as large as it is over the ocean …

    I don’t know if that’s the case, but it would strike me as odd if it were to be true.

    All the best,

    w.

  33. “P-E balance over the land would have to be a) in the reverse sense and b) four times as large as it is over the ocean …”

    Well, since there is very little evaporation over land and lots of orographic rainfall over high ground I don’t think that is hard to believe at all :)

  34. Stephen Wilde says:
    May 26, 2012 at 12:44 pm

    “P-E balance over the land would have to be a) in the reverse sense and b) four times as large as it is over the ocean …”

    Well, since there is very little evaporation over land and lots of orographic rainfall over high ground I don’t think that is hard to believe at all :)

    Thanks as always for your thoughts, Stephen. However, most of the precipitation is not falling on the high mountains, but in the wet tropics, where there is lots and lots of evaporation. Take a look at the figure I showed above. When it is area adjusted, it shows that over half of the rainfall on land (by volume) is in the tropics, between ± 25° N/S.

    It strikes me that we should be able to compare the average rainfall over land (which is about 200 mm) with the total flow of all of the rivers to get a first-order estimate of the P-E balance over the land … too many ideas, too little time, but it’s the weekend, I’ll see what that looks like.

    w.

  35. Willis Eschenbach-“My point is simple—for 80% of the rainfall, falling on 70% of the planet, the trend is slightly negative, not positive at all.”

    I don’t think you understood my point. The salinity is not a proxy for the rainfall, but the Precipitation minus the Evaporation. What I am saying is that globally, over the oceans, you have found that the precipitation minus evaporation response to temperature is about zero. I am saying that this is not the same thing as there being no positive relationship between precipitation and temperature change over the oceans. I am saying that precipitation and evaporation probably globally increase by the same amounts with an increase in temperature, thus the P-E relationship with temperature is indeed about zero, but it is not necessarily true, as you seem to imply, that there is no relationship with precipitation and evaporation separately. One needs data that separates the precipitation and evaporation effects to test if this is true. Salinity combines both effects.

  36. Also, Steven, I’m talking not about the direct P-E balance, but the CHANGE in the P-E balance with temperature. Over the ocean the net precipitation minus evaporation goes down slightly with temperature.

    So it would have to the other way around on the land, we’d have to get more precip and less evaporation as temperatures rise … that’s what I was saying was doubtful.

    Regards,

    w.

  37. timetochooseagain says:
    May 26, 2012 at 2:00 pm

    Willis Eschenbach-

    ”My point is simple—for 80% of the rainfall, falling on 70% of the planet, the trend is slightly negative, not positive at all.”

    I don’t think you understood my point. The salinity is not a proxy for the rainfall, but the Precipitation minus the Evaporation. What I am saying is that globally, over the oceans, you have found that the precipitation minus evaporation response to temperature is about zero. I am saying that this is not the same thing as there being no positive relationship between precipitation and temperature change over the oceans. I am saying that precipitation and evaporation probably globally increase by the same amounts with an increase in temperature, thus the P-E relationship with temperature is indeed about zero, but it is not necessarily true, as you seem to imply, that there is no relationship with precipitation and evaporation separately. One needs data that separates the precipitation and evaporation effects to test if this is true. Salinity combines both effects.

    Sorry for my lack of clarity, timetochooseagain. I am of course referring to P-E. However, your point is well taken.

    What you say is not exactly true however. You say that I “have found that the precipitation minus evaporation response to temperature is about zero.” In fact, I have found a small but statistically very significant trend in salinity (as a proxy for P-E) with respect to temperature. As the earth warms the salinity of the oceans increases, and as the earth cools the salinity of the oceans decreases. The trend is small, to be sure, but not zero.

    Of more interest to me is the geographical distribution of the changes. In the tropics, precipitation outpaces evaporation as the earth warms, but in the extra-tropics, the relationship is reversed.

    w.

  38. I can’t let this topic slide by without mentioning Joe D’Aleo posted a neat article on the Weatherbell site about recent harsh winters in South America, including a snippet about a blast of Antarctic air that ducked behind the Andes Mountains and roared north right through the Amazon and past the northern tip of Peru, to a point past the equator. Joe D’Aleo wrote, ” Temperature even fell in Roraima, where the state capital Boa Vista recorded 20C (normal lows are 25C) and the wind were blowing from the South. Boa Vista is located at 2º North of latitude, so the influence of the Antarctic cold blast crossed the Equator line and reached towns in the Northern Hemisphere.”

    How wide are the tropics, you ask? Not very wide, that day.

  39. “SW… no, in the NH negative numbers are equatorward.”

    Are you sure ?

    So what are the global numbers ?

    Do they support warmer/poleward and cooler/equatorward or not ?

    Even in the NH we saw more poleward / zonal jets during the late 20th century and during the MWP as compared to the LIA so I’m puzzled that your numbers seem to show equatorward during warmer periods in the NH.

    And previously you said this:

    “In the Northern Hemisphere for the years 1979-1999 I found a poleward 0.18 decade rate”

    Yet you now say that for the years 1970 to 1999 it was equatorward -0.04.

    I suppose that it could be that cooling continued until around 1979 and converted the sign but that would still leave us with the odd proposition that the mid century cooling period gave a poleward shift in the NH according to you but we can all see that the mid latitude jets have been more equatorward recently than during the late 20th century. Also, recent equatorward jets are more akin to what we saw during the mid century cooling period.

    It may be that the fineness of the system response makes it essential to determine precisely when the inflection points in the temperature record actually occurred and then work from them rather than on broad decadal periods which do not reflect the actual observations as regards temperature trends..

    I think we are onto something here because I have been long decrying the absence of good enough data about latitudinal climate zone shifts to make adequate comparisons with changes in tropospheric temperature trends.

    At least you are making a start even if we are having problems refining it.

    Observations clearly support a significant latitudinal shift from MWP to LIA and LIA to date with poleward for the MWP and equatorward for the LIA which is opposite to what you are now saying.

  40. “on the land, we’d have to get more precip and less evaporation as temperatures rise ”

    Warmer oceanic winds carrying more water vapour would do just that on reaching land. The amounts of rainfall dumped on mountain barriers are huge.More clouds would reduce evaporation from the land too but evaporation from dry continental interiors is minimal anyway.

  41. Willis Eschenbach says: “In fact, I have found a small but statistically very significant trend in salinity (as a proxy for P-E) with respect to temperature.”

    I am sorry, I thought I had read you saying it was statistically insignificant in the post, I guess I misread that sentence, since ‘albeit statistically signficant’ is a bit of a strange phrase, IMAO.

    I regard it as an interesting question to ask what the relationship between precipitation itself with temperature changes is. Do you think RSS’s SSM/I data could be used to examine that question?

    http://www.remss.com/ssmi/ssmi_browse.html

  42. Willis said:

    “In the tropics, precipitation outpaces evaporation as the earth warms, but in the extra-tropics, the relationship is reversed.”

    That must be right because the water vapour fuelling the tropical uplift flows in from the extratropical regions either side of the ITCZ and I think if the extratropics is measured correctly (land plus the higher latitudes) there will be a balance. After all, that is the essence of your Thermostat Hypothesis which is fine as far as it goes and there are indications that global humidity remains approximtely stable whether the system is cooling or warming so there can’t be much of a warming induced drying process overall over time.

    One could propose that tropical rainfall increases, subtropical rainfall decreases and higher latitude rainfall stays much the same. That would also fit your Fig 7 would it not ? There are hints of blue around the poleward perimeters and that could be understated by current data.

    My main suggestion is that your hypothesis needs to be extended globally and needs to incorporate latitudinal climate zone shifting especially as regards the size of the subtropical high pressure cells which would be highly sensitive to increased equatorial uplift and would have a knock on effect on higher latitudes.

    Another feature you need to add is top down solar effects on the extent of the polar air masses but that is another story.

  43. Stephen, in the link you provide above, the first reference to evaporation is,

    It cannot be determined by the energy content of the air because under an open sky warm air
    above cool water just increases evaporation for a net cooling effect which cancels out the
    extra warmth in the air.

    You have got this the wrong way round. Increased air temps decrease ocean evaporation, as you have previously agree with me in this thread.

    The problem here may be that for people who live in temperate zones, warm daytime temperatures result from cloudless days with low humidity. And obviously lower air humidity increases evaporation. But this is a humidity effect not a temperature effect.

  44. I should amend that last paragraph.

    The problem here may be that for people who live in temperate zones, warm daytime temperatures in summer result from cloudless days with low humidity. And obviously lower air humidity increases evaporation. But this is a humidity effect not an air temperature effect. And solar insolation is much higher on cloudless days warming the ocean relative to the air which will also increase evaporation. Again not an air temperature effect.

  45. So how much natural variability is there in the degrees between +23 1/2 Lat and -23 1/2 Lat anyway, I always thought it was 47 degrees right on the button.

  46. “”””” Philip Bradley says:

    May 26, 2012 at 3:28 pm

    Stephen, in the link you provide above, the first reference to evaporation is,

    It cannot be determined by the energy content of the air because under an open sky warm air
    above cool water just increases evaporation for a net cooling effect which cancels out the
    extra warmth in the air.

    You have got this the wrong way round. Increased air temps decrease ocean evaporation, as you have previously agree with me in this thread. “””””

    Well I believe evaporation depends on the Temperature of the liquid (ocean); not the Temperature of the air. The water molecules in the ocean have no idea what the atmospheric Temperature is. Now the atmospheric Temperature will affect the rate of precipitation; but it has no effect on the rate of evaporation.

  47. SW, Yep, I’m sure that negative numbers are equatorward. It’s a simple linear regression with the latitude being dependent on the year. The negative coefficient indicates that the tropical border is moving south. The reason the 1979-1999 coefficient was positive and the 1970-1999 coefficient was negative was that the values in the 1970’s were relatively high. You can see them in this plot:

    Given the uncertainties, your theory might still be correct. Alternatively, maybe the relationship is opposite of what was expected. Think of El Ninos. I believe they are characterized by a slow down in ocean upwelling. Given the ocean/atmospheric coupling, perhaps there is a slow down in the meridional winds?

  48. Careful with that one:

    Paradox alert:
    The temperature-precipitation relationship FLIPS UPSIDE DOWN annually towards the poles. (Thus precipitation and temperature variables will separate cleanly in factor analysis.)

    For example, in my location cold in winter means clear & dry while warm in winter means heavy non-stop rain. In contrast, cold in summer means overcast or raining and warm in summer means painfully clear skies for uncomfortable weeks on end. The sign reversal in the bivariate statistical relationship here occurs at ~+2 degrees C.

    Temperature change implications for local hydrology vary with time & place and hydrology is a function of absolutes, not anomalies. Willis is wise to put this front & center on readers’ radars.

  49. But globally, it goes the other way, rainfall decreases with increasing temperature … and there is only a minuscule effect. I didn’t expect that at all.
    ——————
    The overall picture is hard to understand, but one rule-of-thumb that’s has been put forward is that a warmer globe means that the wet gets wetter and the dry gets dryer. That seems to match your(Willis’ ) conclusion.

  50. Stephen Wilde (May 25, 2012 at 2:40 pm) wrote:
    “In the short term the solar effect is disguised by internal system variability but it becomes apparent on multidecadal and centennial timescales such as MWP to LIA to date.”

    Technical advances bring better vision. Annual & semi-annual solar effects can no longer hide behind ENSO:

    Old indoctrination is now outdated and must be promptly discarded by all sensible parties.

    In the climate discussion, everyone is conditioned to believe there can be no abrupt leaps in understanding. This is wrong.

  51. The problem with latitudinal summaries considered in isolation is their ignorance of surface boundary layer basin loops. So many interesting layers to explore…

  52. The water molecules in the ocean have no idea what the atmospheric Temperature is.

    Nor does the water molecule in the air know what the ocean temperature is.

    By evaporation I mean the net of these 2 exchanges.

  53. [Willis, sincere apologies for the off-topic post, but I have some T-shirts I need to unload]:

    For anyone who would like a Gleick “Fakegate” T-shirt, I have some left. Send an email to my throwaway account: themistocles2010-2020*AT*yahoo.com, with a mailing address. Include your size. I also have some Heartland “Don’t Tread On Me” T-shirts.

    No charge for WUWT readers [if you like, you can always donate a few dollars to support Anthony's "Best Science" site]. For those who have already ordered, I’ll be sending them out on Tuesday because of the holiday weekend.

    No charge! What are you waiting for??☺☺☺

  54. Stephen Wilde says:
    May 26, 2012 at 3:00 pm

    “on the land, we’d have to get more precip and less evaporation as temperatures rise ”

    Warmer oceanic winds carrying more water vapour would do just that on reaching land. The amounts of rainfall dumped on mountain barriers are huge.More clouds would reduce evaporation from the land too but evaporation from dry continental interiors is minimal anyway.

    Stephen, we seem to be talking about somewhat different things. Let me see if I can explain myself a bit more clearly. It has to do with the hydrological cycle. There is an export of water from the ocean to the land which is balanced by the runoff from the world’s rivers.

    Global estimates of the water cycle are on the general order of this one:

    Thanks to you and others, I see now that my expectation of the direction of change in the global oceanic P-E with increasing temperature was mistaken. The basic equation for the ocean mass balance says that precipitation plus additions from the rivers (including ice melt and groundwater extraction) minus evaporation from the ocean gives mass balance change.

    Now, the ice melt and groundwater extraction don’t vary much year to year. So if the ocean mass is roughly constant, we can take the basic equation as being evaporation from the ocean equals rain into the ocean plus net rain over land … what goes up must come down.

    My mistake was in thinking that the actual value of the oceanic P-E overall was positive. It is not. From the data given in the table above, we can see that P-E is about – 35,000 cubic kilometres per year.

    And that means that if we increase the speed of the hydrological cycle by say 10%, and we assume that all of the proportions remain the same, the value of P-E becomes more negative, not more positive as I had assumed.

    In other words, I should have expected that if the temperature increased the value of P-E should go down, not up as I thought. In that regard, it appears that my finding, that P-E decreases with increasing temperature, is consonant with expectations.

    I’ll change the head post to reflect that.

    w.

  55. On an unrelated note, I just realized that as a Floridian I have to object to figure two appearing to color the State slightly brown as part of a “desert belt”. We appear to be the only state marked as within the region where deserts are “supposed” to be. In fact, most of the deserts in the Americas (indeed much of the world!) appear to have little to do with the Hadley Cells at all and are in fact just “rain shadow” deserts, caused by geography, not circulation. The only place in the US that falls within the latitudes called the “desert belt” is not a desert at all, but is characterized mostly by by Cfa, Humid Subtropical Climate, specifically the kind with no distinguished dry season, and no part of Florida is classified as any kind of B Group climate, ie arid zones.

  56. Paul Vaughan,

    You have some of the most amazing charts I’ve ever seen! I can’t even comprehend that last one.

  57. “”””” Philip Bradley says:

    May 26, 2012 at 4:40 pm

    The water molecules in the ocean have no idea what the atmospheric Temperature is.

    Nor does the water molecule in the air know what the ocean temperature is.

    By evaporation I mean the net of these 2 exchanges. “””””

    About which we can say nothing, without knowing anything about the rates of removal of the reaction products (water molecules) from the reaction interface. (water surface). We do know that a NETevaporation somewhere must be balanced by a NETprecipitation, somewhere else; or else we would end up with the oceans over our heads.

  58. Willis, Could there be an underling trend towards more salinity at the surface? I can imagine that a warmer earth creates a stronger convection which results in a greater upwelling of the briney deep?

  59. AJ said:

    “Given the ocean/atmospheric coupling, perhaps there is a slow down in the meridional winds?”

    That covers a possibility that I have been careful not to exclude by using the terms zonal / poleward and meridional / equatorward.

    That being a possible scenario whereby the degree of zonality or meridionality might change but the latitudinal positioning of the climate zones ON AVERAGE stays pretty much the same.

    Thus, instead of a bodily latitudinal shift of the air circulation pattern there would be a change in the amount of north / south mixing which would fit my hypothesis just as well.

    One could still get a perception of latitudinal jetstream shifting because the stronger high pressure blocking cells would more frequently shift the tracks away from their ‘normal’ routes.

    However there are many reports that the jets actually did move poleward during the late 20th century warming period such as one that suggested a 1.5 mile per annum shift poleward during that period and don’t models anticipate poleward shifting just from more GHGs warming the Earth ?

    There is therefore still the possibility that the uncertainties in your numbers are giving a result contrary to actual observations.

  60. timetochooseagain said:

    “In fact, most of the deserts in the Americas (indeed much of the world!) appear to have little to do with the Hadley Cells at all and are in fact just “rain shadow” deserts, caused by geography, not circulation.”

    I don’t think one can deny that the descending air either side of the ITCZ results in dry conditions.As regards geography I think what happens is that the precise position of the resulting desertification is influenced by the landmass distribution especially by rainshadow effects as you say.

  61. AJ said:

    “The negative coefficient indicates that the tropical border is moving south”

    By ‘tropical border’ do you mean the border between tropics and subtropics ?

    It occurs to me that an expansion of the subtropical high pressure cells could push both poleward and equatorward so that the border between tropics and subtropics could push slightly equatorward during a warming spell AND the border between subtropics and the higher latitudes could push poleward at the same time as part of the expansion process.

    Apart from that an interesting feature of your numbers is that the warming spells are both of the same sign as are the cooling spells which my previous post told you to expect.

    Also I said that it was likely that the 2000 to 2010 period might be too short for a clear signal and you confirm that by showing that the sign changes depending on whether one takes 1999 or 2000 as the start point.

    .

  62. timetochooseagain said:

    “In fact, most of the deserts in the Americas (indeed much of the world!) appear to have little to do with the Hadley Cells at all and are in fact just “rain shadow” deserts, caused by geography, not circulation.”

    There are rainshadow deserts poleward of the Hadley cells, but that doesn’t invalidate that the HCs cause deserts. Here in the western half of Australia we have plenty of desert and no significant rain shadow. Ditto the Kalahari.

  63. Willis
    Thanks for stimulating evidence.
    Re: “Now, the ice melt and groundwater extraction don’t vary much year to year.”
    However, the trend is changing. From what I have read, irrigation is rapidly increasing the groundwater extraction. Some evaluations suggest that the increase in groundwater extraction accounts for about 25% of the rate of sea level rise.

    We estimate that since the 1960s groundwater abstraction has more than doubled (from 312 ± 37 to 734 ± 84 km3 a-1) resulting in an increase in groundwater depletion of from 126 ± 32 to 283 ± 40 km3 a-1. . . .the contribution of groundwater depletion to sea level rise to be 0.8 (±0.1) mm a-1, which is 25 (±3) % of the current rate of sea level rise of 3.1 mm a-1… and the same order of magnitude as the contribution from glaciers and ice caps.

    Wada, Y., L. P.H. van Beek, C. M. van Kempen, J. W.T.M. Reckman, S. Vasak, and M.F.P. Bierkens (2010), Global depletion of groundwater resources, GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L20402, doi:10.1029/2010GL044571, 2010

    Our results show that the contribution of groundwater depletion to sea-level increased from 0.035 (±0.009) mm yr−1 in 1900 to 0.57 (±0.09) mm yr−1 in 2000, and is projected to increase to 0.82 (±0.13) mm yr−1 by the year 2050. We estimate the net contribution of terrestrial sources to be negative of order −0.15 (±0.09) mm yr−1 over 1970–1990 as a result of dam impoundment. However, we estimate this to become positive of order +0.25 (±0.09) mm yr−1 over 1990–2000 due to increased groundwater depletion and decreased dam building. We project the net terrestrial contribution to increase to +0.87 (±0.14) mm yr−1 by 2050. As a result, the cumulative contribution will become positive by 2015, offsetting dam impoundment (maximum −31 ± 3.1 mm in 2010), and resulting in a total rise of +31 (±11) mm by 2050.

    Yoshihide Wada et al. GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L09402, 6 PP., 2012 doi:10.1029/2012GL051230 Past and future contribution of global groundwater depletion to sea-level rise

  64. David L. Hagen says:
    May 27, 2012 at 5:07 am

    …. Re: “Now, the ice melt and groundwater extraction don’t vary much year to year.”
    However, the trend is changing. From what I have read, irrigation is rapidly increasing the groundwater extraction. Some evaluations suggest that the increase in groundwater extraction accounts for about 25% of the rate of sea level rise…..
    _________________________________
    David, thanks for the links but that is only ground water extraction. We also have dammed rivers and created farm ponds. In my area farm ponds are routinely used for irrigating crops.
    Here is an article about irrigation with a bar graph from 1987 for the USA: http://ga.water.usgs.gov/edu/ircropbar.html

    Throughout the world, irrigation (water for agriculture, or growing crops) is probably the most important use of water (except for drinking and washing a smelly dog, perhaps). Almost 60 percent of all the world’s freshwater withdrawals go towards irrigation uses.Large-scale farming could not provide food for the world’s large populations without the irrigation of crop fields by water gotten from rivers, lakes, reservoirs, and wells. Without irrigation, crops could never be grown in the deserts of California, Israel, or my tomato patch….

    …. the water used for irrigation, only about one-half is reusable. The rest is lost by evaporation into the air, evapotranspiration from plants, or is lost in transit, by a leaking pipe, for example….

    For 2005, total irrigation withdrawals were about 128,000 million gallons per day (Mgal/d), or 144,000 thousand acre-feet per year….. Surface water accounted for 58 percent of the total irrigation withdrawals. About 61.1 million acres were irrigated in 2005.

    About 26.6 million acres were irrigated with surface (flood) systems, 4.05 million acres with microirrigation systems, and 30.5 million acres with sprinkler systems. The national average application rate was 2.35 acre-feet per acre…. http://ga.water.usgs.gov/edu/wuir.html

    (I never knew some one in the government could actually have a sense of humor)

    That is a heck of a lot of water tossed into the air by sprinkler systems or spread over the ground (much more efficient system) by microirrigation.

    This man made modification of the environment and the micro climate is the reason IPCC has left water out of the discussion except as a positive feed back driven by CO2. Remember the companies funding the CRU also sell petroleum products that are turned into fertilizer.

    It is amusing that althoug some chemical fertilizers are made from petroleum (Department of Horticultural Science, Minnesota University) the CAGW crowd is trying to make that fact “Disappear” too. The top google post is Once Again, Fertilizer is Not “Petroleum Based” – Depleted Cranium the googl search link

  65. @Smokey (May 26, 2012 at 7:43 pm)

    It goes several layers deeper than what I’ve so far had time to report. Once locked onto the right track, obstacles start falling like dominoes. Exhilarating…

  66. Philip Bradley says: “There are rainshadow deserts poleward of the Hadley cells, but that doesn’t invalidate that the HCs cause deserts.”

    No, but one point that you missed is that I was trying to say that there are not only deserts outside of the “desert belts” (the majority of the Earth’s major non polar deserts are rain shadow deserts, although the largest of those major one’s isn’t (but is still largely outside the “desert belt” curiously) but there are areas quite wet within the “desert belts”. Again, I come back to Florida. It’s proof that even if the Hadley Cells can cause deserts at those latitudes, they don’t necessarily.

    Stephen Wilde says: “I don’t think one can deny that the descending air either side of the ITCZ results in dry conditions.As regards geography I think what happens is that the precise position of the resulting desertification is influenced by the landmass distribution especially by rainshadow effects as you say.”

    Not quite what I am saying, Since the US Western desert has nothing to do with the desert belt effect. The Mexican desert might have something to do with it, though (combined with rain shadows?)

    Again, if the Hadley Cells must make things dry at these latitudes, Florida would be a desert. It’s not. Hadley Cells can make things dry, but evidently they don’t have to do so.

  67. timetochooseagain, you are indeed correct that the Hadley cells don’t necessarily make deserts where they descend. Local conditions, such as being surrounded by water in the case of Florida, can moisten the dry descending air. However, for many parts of the world the deserts are indeed a consequence of the Hadley cells, and are not the result of a rain shadow.


    Source

    All the best,

    w.

  68. Gail Combs says:
    May 27, 2012 at 6:07 am

    David L. Hagen says:
    May 27, 2012 at 5:07 am

    …. Re: “Now, the ice melt and groundwater extraction don’t vary much year to year.”
    However, the trend is changing. From what I have read, irrigation is rapidly increasing the groundwater extraction. Some evaluations suggest that the increase in groundwater extraction accounts for about 25% of the rate of sea level rise…..

    _________________________________
    David, thanks for the links but that is only ground water extraction. We also have dammed rivers and created farm ponds. In my area farm ponds are routinely used for irrigating crops.
    Here is an article about irrigation with a bar graph from 1987 for the USA: http://ga.water.usgs.gov/edu/ircropbar.html

    Let me give you a sense of the relative size of the quantities involved. Remember that the river flow into the oceans globally is about 36,000 cubic kilometers per year (from above).

    The FAO estimates total groundwater use for irrigation around the globe as follows:

    Total consumptive groundwater use for irrigation is estimated as 545 km3 yr−1, or 43% of the total consumptive irrigation water use of 1277 km3 yr−1.

    Note that 545 cubic kilometres per year (km3 yr-1) is only about 1.5% of the river discharge of 36,000 km3 yr-1. And as a result, as I said, year to year variations in that will be even smaller.

    Also, you need to remember that the groundwater is constantly being recharged by rainfall, so that what matters for our purposes is the net groundwater depletion (extraction – recharge).

    According to the figures given by David above the net groundwater depletion is small. Global net groundwater depletion, from his cite, is 383 km3 yr-1, or only about 1% of the river discharge volume.

    Which is why I ignored groundwater use, whether for irrigation or not, in my “first-order” analysis of the oceanic mass balance above. It is important for some things, but not worth considering in the larger global mass balance picture.

    My best to you both,

    w.

  69. Willis Eschenbach-It looks like the main Northern Hemisphere non-polar deserts that are not rain shadow deserts are the Sahara-Arabian Desert, both of which are larger than the desert belt would suggest, and thus that cannot be the only reason for them being deserts. The Middle Eastern/Eurasian deserts are either too far North or probably associated with the Himalayas. I notice a distinct lack of aridity in the belt in China south of the Gobi Desert, a rain shadow desert. Southern Africa and Australia appear to have belt deserts, albeit again too large, extending beyond the belt’s range. The desert in South America that overlaps with the belt is a rain shadow desert due to the Andes. I stand by the fact that most deserts are not desert belt deserts. BTW Somalia is in the Deep tropics: atmospheric circulation being most important would suggest it should not be as arid as it is. Why is it that arid? Is there a nearby mountain range?

  70. Andrew, you misunderstand the illustration. The black lines do not represent the exact location, nor the width, of the descending dry air from the Hadley cells. The world is not that simple, the cells have different locations over the various oceans and continents, as does the ITCZ.

    For a better grasp of what is happening, look again at Figure 1. You can see from that there are a number of areas where the downwelling dry air is not exactly at 30°N/S, but is either north or south of that. For that matter, look at Figures 4, 5, and 6. The southern dry zone in the Pacific, for example, is at about 20°S, right where the Atacama desert is located … so your claim that the “desert in South America that overlaps with the belt is a rain shadow desert” is not borne out by the facts.

    Forget about the 30° N/S, that is an ESTIMATE OF THE GENERAL POSITION of the descending branch of the Hadley cells, not holy writ. Do you see in Figure 1 where the dry air (high salinity) hits the coast of Mexico? Go stand on the shore there, with nary a rain shadow in sight, and tell me what you find … yep, desert. Study Figure 1, it shows the actual reality, not the ± thirty degrees that seems to have you confused. See how the red area encompasses all of the southern part of Australia … care to tell us what is there? Yep … desert.

    So you can “stand by” your claim about the Hadley cells not creating the great desert belts all you want, but unfortunately, the facts tell another story. Certainly there are rain shadow deserts on the planet … and just as certainly, there are deserts caused by the dry descending air from the Hadley cells.

    Finally, look at the oceans themselves. There’s not a rain shadow anywhere, but despite that, large areas of the ocean get very little rain … until your “rain shadow” theory can explain that, I hold that it is caused by the descending dry air from the downwelling branch of the great Hadley cells.

    w.

  71. Re Sahara – in this Beeb programme it was said that the area would again be green in 15,000 years time – all to do with the way the Earth moves through space and around the Sun, part of the 41,000 year cycle which swings between 22 and 24 and half degrees: http://www.bbc.co.uk/programmes/b00xztbr

    When the Sahara was green the Earth’s tilt was near its maximum and “together with small cyclical changes in the direction of the tilt and the shape of our orbit was the result that the Sun shone more intensely over the northern hemisphere powering a monsoon in the Sahara.” This failed around 5,000 years ago and within a hundred years or so became desert.

    Didn’t make a note of it, but iirc they said at that time, in 15,000 years, the desert will be further north, into southern Europe.

  72. So you can “stand by” your claim about the Hadley cells not creating the great desert belts all you want, but unfortunately, the facts tell another story. Certainly there are rain shadow deserts on the planet … and just as certainly, there are deserts caused by the dry descending air from the Hadley cells.

    There are a good descriptions here:
    short version: http://ag.arizona.edu/~lmilich/dry.html
    long version: http://www.kean.edu/~csmart/Observing/10.%20Earths%20climate%20system.pdf
    texas version: http://www.dallasnews.com/news/local-news/20110810-hadley-cell-that-keeps-sahara-dry-also-responsible-for-dallas-fort-worth_s-desert-like-heat.ece

  73. Willis Eschenbach says: “Andrew, you misunderstand the illustration. The black lines do not represent the exact location, nor the width, of the descending dry air from the Hadley cells. The world is not that simple, the cells have different locations over the various oceans and continents, as does the ITCZ.”

    Fair enough.

    “The southern dry zone in the Pacific, for example, is at about 20°S, right where the Atacama desert is located … so your claim that the “desert in South America that overlaps with the belt is a rain shadow desert” is not borne out by the facts.”

    Huh? Where do you get the idea that Atacama is not in a rain shadow? The Andes go along the entire western edge of South America.

    “So you can “stand by” your claim about the Hadley cells not creating the great desert belts all you want, but unfortunately, the facts tell another story. Certainly there are rain shadow deserts on the planet … and just as certainly, there are deserts caused by the dry descending air from the Hadley cells. ”

    You have egregiously misread me. I said “I stand by the fact that most deserts are not desert belt deserts.”

    Among the ten largest deserts in the world, two are polar deserts, the Great Basin, Patagonian, and Gobi are rain shadow deserts. That’s half already. Syrian, Sahara, and Arabian deserts seem to be really one continuous desert, really. The Saharan is partly influenced by the Atlus mountains, but is mostly belt. Then there is the Kalahari and great Victorian are apparently desert belt deserts. That’s a tie, but desert belt deserts are clearly not a majority of just those ten. Chihuahuan is in a rain shadow, but may be in the belt. “Great Sandy” is in the belt. Karakum is in the Caucasus and Hindukush rain shadow. Sonoran is another desert that looks like it’s probably in the belt, but it also has rain shadow effects. Kyzyl Kum is also in the Caucasus/Hindukush rain shadow. Taklamakan is totally surrounded by mountains. Thar is in the Himalayan, Aravalli, Kirthar, and Sulaiman ranges rain shadows. Gibson, belt. Simpson, belt. Atacama is in the rain shadow of the Andes but also in the belt. Mojave, rain shadow in the belt. Dasht-e Kavir I’m not sure. Dasht-e Lut is a rain shadow desert, Alborz and Zagros.

    The final count? 10 out of 25 are polar or unambiguously rain shadow deserts. Eight are unambiguously belt including the Sahara. The remaining deserts have rain shadow effects, except possibly Dasht-e Kavir. If you are going to argue that the majority of deserts are desert belt, I think that’s a real stretch.

  74. Willis, from your update:

    In that regard, it appears that my finding, that oceanic P-E decreases with increasing temperature, is consonant with expectations.

    As opposed to vowel with expectations?

  75. timetochooseagain says:
    May 27, 2012 at 6:56 pm

    “The southern dry zone in the Pacific, for example, is at about 20°S, right where the Atacama desert is located … so your claim that the “desert in South America that overlaps with the belt is a rain shadow desert” is not borne out by the facts.”

    Huh? Where do you get the idea that Atacama is not in a rain shadow? The Andes go along the entire western edge of South America.

    Well … as is often the case when I express an opinion, it’s ’cause I’ve been there, and sometimes even done that. I get the idea because I’ve actually been to the Atacama and travelled along that amazingly arid coast and looked at which way the wind blows. The desert marches right down to the ocean there, and the winds come off of the ocean … what is there to shadow the rain?

    Here, for example, is a photo from Google Earth of “barchan” dunes along the coast in Tanaca, Peru, at about 16°S.

    Barchan dunes are formed by the wind. They are the curved “bow-shaped” dunes you see in the center of the photo above.

    Now, an oddity about barchan dunes is that they indicate the prevailing winds. If a barchan dune were a bow, it would shoot an arrow directly into the prevailing wind.

    So as you can see from the photo, in Tanaca, the prevailing wind is off the ocean, and as the photo makes clear, there is nothing to create any “rain shadow” … and yet despite that it is one of the driest places on the planet.

    All the best,

    w.

  76. kadaka (KD Knoebel) says:
    May 27, 2012 at 7:22 pm

    Willis, from your update:

    In that regard, it appears that my finding, that oceanic P-E decreases with increasing temperature, is consonant with expectations.

    As opposed to vowel with expectations?

    From the dictionary, emphasis mine …

    con·so·nant   [kon-suh-nuhnt]
    noun
    1. …
    2. …
    adjective
    3. in agreement; agreeable; in accord; consistent (usually followed by to or with ): behavior consonant with his character.

    w.

  77. timetochooseagain says:
    May 27, 2012 at 6:56 pm

    … The final count? 10 out of 25 are polar or unambiguously rain shadow deserts. Eight are unambiguously belt including the Sahara. The remaining deserts have rain shadow effects, except possibly Dasht-e Kavir. If you are going to argue that the majority of deserts are desert belt, I think that’s a real stretch.

    Thanks, timetochooseagain, If I had made that argument about the majority of the earth’s deserts, you’d be right … but do a word search, and you’ll find that the only person using the word “majority” in this thread is … well … you.

    What I said was:

    Certainly there are rain shadow deserts on the planet … and just as certainly, there are deserts caused by the dry descending air from the Hadley cells.

    w.

  78. Willis Eschenbach says: @ May 27, 2012 at 11:23 am
    >>>>>>>>>>>
    Willis it is not the effect on sea level rise I was trying to get at but the fact we are tossing a lot of water into the air where it evaporates, increasing the humidity. We are also changing the ground cover too. This will have an effect on the micro environment. (As long as there are no goats – GRIN) I cringe to use this link but here goes: More crops for Africa as trees reclaim the desert

  79. Willis Eschenbach says: “If I had made that argument about the majority of the earth’s deserts, you’d be right”
    Except I wasn’t disputing what you were saying, you were disputing a mischaracterization of what I was saying. You said

    ““So you can “stand by” your claim about the Hadley cells not creating the great desert belts all you want”

    But the claim I was standing by was not that at all! What I said was:

    ““I stand by the fact that most deserts are not desert belt deserts.””

    As I have just demonstrated, those deserts which are exclusively due to deserts belts represent a minority among the Earth’s major deserts. Not to show that you were wrong, but that I was not. You had said that I was wrong and trying to stand by a wrong claim. Except that the claim I was allegedly standing by…wasn’t what I was standing by at all.

    You claim that Atacama sees no rain shadow effects based on your experience there. Unfortunately for me, all I have to go on is researching on the internet. Several sites I went to said that Atacama’s exceptional dryness is because of both the circulation and the rain shadow of the Andes.

    Is there a high resolution vector map of prevailing winds that could make this clearer?

  80. http://www.biosphere3.com/HowDesertsForm/index.php

    Says here that Atacama was formed by Hadley cells – that very few deserts are rain shadow:

    “With the exception of the frozen deserts in Greenland and Antarctica, most of the world’s deserts are in two belts that lie within 25 degrees of the equator.

    One reason is that the high atmospheric pressure in this region can cause dry, cold air from the upper altitudes to compress and come down to earth.

    See Hadley Cells example on right, [Courtesy of NOAA]

    This dry air is so clear that it is easily heated by the sun, causing high ground temperatures with very low humidity. The Sahara (the world’s largest desert), the Atacama on the coast of Chile, and the Kalahari in Africa have been formed in this way.
    Another type of desert has also developed in this region around the equator. A “rain shadow” desert often forms when there are two mountain ranges, one on the east and one on the west of a land expanse, which block moist ocean air from reaching the land. Instead, almost all the precipitation falls on the opposite side of each mountain range, leaving the region between the mountains dry.

    Very few deserts are formed solely by a rain shadow effect, because they are also influenced by the high atmospheric pressure. However, the North American deserts are usually called rain shadow deserts. Other famous ones are the Gobi in China (blocked by the Himalayas), and the eastern and central deserts in Australia.”

    This appears to be saying that Atacama not rain shadow, but, it is in between two mountain ranges so also rain shadow:

    http://www.nationmaster.com/encyclopedia/Atacama

    “Driest Desert
    The Atacama Desert is the driest desert on Earth (with the possible exception of the McMurdo Dry Valleys in Antarctica) and is virtually sterile because it is blocked from moisture on both sides by the Andes mountains and by coastal mountains. The average rainfall in Antofagasta — a region in Chile which is part of the Atacama — is just 1 mm per year, and there was a period of time where no rain fell there for 40 years. It is so arid, in fact, that mountains that reach as high as 6885 metres (22590 feet) are completely free of glaciers and, in the southern part from 25°S to 27°S, have possibly been glacier-free throughout the Quaternary – though permafrost extends down to an altitude of 4400 metres and is continuous above 5600 metres. Some weather stations in the Atacama have never received rain, evidence suggests that places may not have had significant rainfall for about 400 years. Earth (IPA: , often referred to as the Earth, Terra, the World or Planet Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. … Categories: Antarctica geography stubs | Geography of Antarctica | Ross Dependency | Valleys … Antofagasta is Chiles second administrative region from north to south. … A glacier is a large, long-lasting river of ice that is formed on land and moves in response to gravity and undergoes internal deformation. … For other uses, see Quaternary (disambiguation). … In geology, permafrost or permafrost soil is a thermal condition where ground material stays at or below 0°C for two or more years. …

    Some locations in the Atacama do receive marine fog, providing sufficient moisture for hypolithic algae, lichens and even some cacti. But in the region that is in the “fog shadow” of the high coastal crest-line – the crest-line of the coastal range averages 3000 m for about 100 km south of Antofagasta – the soil has been compared to that of Mars. // A rain shadow (or more accurately, precipitation shadow) is a dry region on the surface of the Earth that is leeward or behind a mountain with respect to the prevailing wind direction. … ”

    http://www.windows2universe.org/earth/atacama_desert.html

    You’re both right..

  81. timetochooseagain says:
    May 28, 2012 at 4:29 pm

    Willis Eschenbach says:

    “If I had made that argument about the majority of the earth’s deserts, you’d be right”

    Except I wasn’t disputing what you were saying, you were disputing a mischaracterization of what I was saying. You said

    ““So you can “stand by” your claim about the Hadley cells not creating the great desert belts all you want”

    But the claim I was standing by was not that at all!

    I don’t get it. You said the Atacama was a rain shadow desert. It’s not. Other than that, it seems that now you say that you agree with me, or at least that you are not disputing what I said … if so, what on earth are you on about? What is your point?

    I don’t care whether or not most deserts are not from the Hadley cells. It’s immaterial to the discussion … which is why I assumed you were in fact disputing my claim that the Hadley cells cause the great desert belts.

    If you agree with that, then it appears you have nothing to add to the actual topic under discussion. What does your listing of polar deserts have to do with variability in the widths of the tropics, or the deserts created by the Hadley cells, or anything this post is about? What am I missing?

    Thanks,

    w.

  82. Myrrh says:
    May 28, 2012 at 6:25 pm

    … This appears to be saying that Atacama not rain shadow, but, it is in between two mountain ranges so also rain shadow: …

    Thanks, Myrrh, but that’s exactly why I showed the blooming’ picture, to avoid exactly this kind of erroneous claim. The Atacama is on the coast. It is not between two mountain ranges, look at the picture above. Then you can go to Google Earth and look up the town of Tanaca, Peru, and use your own eyes instead of depending on somebody else’s bad ideas. The Atacama is between the ocean and the Andes, not between mountain ranges.

    w.

  83. timetochooseagain says:
    May 28, 2012 at 4:29 pm

    You claim that Atacama sees no rain shadow effects based on your experience there. Unfortunately for me, all I have to go on is researching on the internet. Several sites I went to said that Atacama’s exceptional dryness is because of both the circulation and the rain shadow of the Andes.

    Is there a high resolution vector map of prevailing winds that could make this clearer?

    What is it about the picture above you don’t understand? Do you see the barchan dunes there? Do you understand how they work, that the tips of the horns point in the direction of the prevailing wind?

    That’s why I showed the picture, to avoid relying on anything but verifiable physical fact. You can search all you want. The barchan dunes don’t lie. Here’s another photo for you:

    See how if they were bows they’d shoot an arrow at the sea? That shows where the wind is coming from … here’s the USGS explanation:

    Source

    So you can run around all you want trying to find people mistakenly claiming that the winds are blowing from the land to the ocean at the Atacama and that it’s in the rain shadow … that’s why I depend upon things that can’t be mistaken, like the shape of dunes, rather than what other folks might think.

    w.

  84. … This appears to be saying that Atacama not rain shadow, but, it is in between two mountain ranges so also rain shadow: …

    Thanks, Myrrh, but that’s exactly why I showed the blooming’ picture, to avoid exactly this kind of erroneous claim. The Atacama is on the coast. It is not between two mountain ranges, look at the picture above. Then you can go to Google Earth and look up the town of Tanaca, Peru, and use your own eyes instead of depending on somebody else’s bad ideas. The Atacama is between the ocean and the Andes, not between mountain ranges.

    Well, the mountain range is named, you wouldn’t give a name to a mountain range that didn’t exist. I think what you’re showing is a picture of only some of the Atacama, around 600 kilometres in length – there’s a lot of it at 7000 ft which doesn’t have an ocean lapping at that height…

    This is a complex one because of its geography, so yes basically the Hadley cell but other features add to that – perhaps that’s why it is the world’s driest, because of these.

    From the wiki page on Atacama – “Geographically, the aridity can be explained by the following reasons:

    The desert is located on the leeward side of the Chilean Coast Range, so little moisture from the Pacific Ocean can reach the desert.
    The Andes are so high that they block convective clouds, which may bring precipitation, formed above the Amazon Basin from entering the desert from the east.
    An inversion layer is created by the cold Humboldt current and the South Pacific High.
    In July 2011, an extreme Antarctic cold front broke through the rain shadow, bringing 80 cm (31.5 in) of snow to the plateau, stranding residents across the region, particularly in Bolivia, where many drivers became stuck in snow drifts and emergency crews became overtaxed with a large number of rescue calls.[12] The total rainfall for the winter of 2011 was sufficient for wildflowers to bloom in the Atacama.[13]“

  85. Sorry, forgot to close italics after your quote second paragraph. And that should be 600 miles in length not kilometres; 1,000 kilometres.

    And the Atacama is in Chile not Peru, so you may well be showing pictures of a different desert..

    Haven’t been to South America, was on my to go list but never got around to it, lots to see in the Atacama:

    http://www.explore-atacama.com/eng/

    And on that front page there’s something about unusual weather in the last couple of decades:

    “In the past 2 decades, this phenomenon was repeated in 1983, 1987, 1991 and finally, with the historical precipitation of July 12th, 1997, where the fallen water registered a record of 96mm in just 15 hours, something absolutely unusual for the Atacama Desert; the arid enviroment is transformed in a unique spectacle, with surprising colors. It starts in July and August with a green cover, reaching a colorful rainbow in Sepetemer, where flowers, insects and other animals will cover great extensions of the Atacama area…”

    That shows if nothing else, that seeds can remain viable for a long time… :)

    Anyway, this is a good a description as it gets: http://www.beachcomberpete.com/south_america/Atacama_Desert/atacama_desert.php

    “Located along South Americas Pacific coast lies the worlds driest desert, the Atacama Desert, bordered to the east by the mighty Andes Mountains and to the west the world’s largest ocean, the Pacific. Hemmed in by the Andes Mountains and Chilean Coast Range, the Atacama Desert sits on a high Plateau averaging 8,000 feet. The Atacama Desert extends from just south of the Peruvian and Chilean border 600 miles south into Chile, never exceeding 100 miles in width.

    Said to be in the neighborhood of 20 million years old, the Atacama Desert is the second driest place on earth, only drier is the McMurdo Dog Valley in Antarctica. Void of moisture, the Atacama Desert often referred to as moonlike, is 50 times drier than Death Valley located in California’s Mojave Desert in the United States. The Atacama terrain is mainly composed of extinct lava flows, sand and salt basins or the Spanish name “salars”. The region surrounding the Atacama Desert is so dry that to the east the Andes Mountains reach heights upward of 22,000 feet with no snow or glaciers on the slopes.
    It is believed that areas of the Atacama Desert have never seen rain, with the weather station in Calama never recording any moisture, it is said that the city of Antofagasta located on the southern end of the Atacama Desert receives only 1mm of rain a year. Other areas of the Atacama Desert receive a fog, flowing from the Pacific Ocean to the west, known as Camache by the locals, the fog gives life to cactus, lichens and algae. Every few years, the weather phenomenon El Nino and its warming effect on the Pacific Ocean will change weather patterns worldwide and will send rain to regions of the Atacama Desert. Temperatures in the Atacama Desert will range on average from a cold of 32 deg F. to a high of 80 deg F.
    Lack of rain, moisture and animal life has not kept humans from settling in the Atacama Desert. Oasis of human life can be found in the driest reaches of the Atacama Desert. Calama, a city of 143,000 people located in the east central region of the Atacama Desert, grew around an oasis. Today Calama, at an elevation of 7900 feet, is the gateway to Chile’s high central desert and the many geological and archaeological wonders of the Atacama Desert. Several places to visit leaving Calama are: Chuguicamata, with the world’s largest open pit mines.”

  86. Willis-You are right that the point I was making was not material to the discussion at hand. I got off track with a claim, you assumed I made a different claim that would have been relevant, and I had to step up to defend my claim that was irrelevant as being different from the claim I was misunderstood to be making.

    With regard to Atacama, you are arguing that the Andes are irrelevant because the prevailing winds blow in the opposite direction necessary…But one wonders where all the rain on the opposite side of the mountains is coming from, especially since if the air coming from the Pacific is already dry, and there is no wind bringing rain from the Atlantic, there should be an even drier desert on the other side of the Andes than Atacama. Except that there isn’t. What you have shown is that there is eastward wind coming off the Pacific. But the direction of the wind on the opposite side of the Andes is also important. Clearly at that latitude, east of the Andes there is plenty of rain for it not to be a desert, and since the wind coming off the Pacific is, as you say, dry air, there have to be winds coming from the Atlantic to make that side of the Andes wet. But that rain stops abruptly at the Andes. So the winds happen to reverse direction at the mountains so that even if the mountains weren’t there, no moisture from the Atlantic would ever reach where Atacama is? This is why I wanted a map, not pictures.

    But it is irrelevant to the larger point of the thread. I am not claiming that Atacama is entirely a desert due to rain shadow effects, although you are claiming there is zero rainshadow effect there. Whether there is or is not however doesn’t change the fact that there are unambiguous deserts we can agree are in the belt, and that is the reason for their dry conditions. I really don’t care whether you are right that Atacama should count as unambiguous or not.

    I apologize for going off the rails.

  87. Many thanks, Andrew. In answer to your question, yes, because the Andes are so tall, the winds often blow in opposite directions on the two sides of the Andes. Here’s a wind chart for January and for July:


    Note that on the Pacific side the winds are off the ocean moving northeast, and on the other side, the winds are off the ocean and moving southwest.

    All the best, thanks for raising an interesting question.

    w.

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