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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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.
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.
“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.
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.]
NASA has also found connection between tropical storms and the equatorial elctrojet.
http://www.vukcevic.talktalk.net/LFC20.htm
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’.
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.
“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?
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.
Stephen Wilde says:
May 25, 2012 at 2:40 pm (Edit)
Most precipitation is over the land? I greatly doubt that, but let me take a look …
w.
Timelapse of October 2011 to March 2012 for Electo-L images:
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.
“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.
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.
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
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)?
Earlier I had said:
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.
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.
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.
“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.”
“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.
“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.
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.
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.
Stephen Wilde:
At May 25, 2012 at 11:59 pm you say;
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