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 …
Careful with that one:
http://wattsupwiththat.files.wordpress.com/2012/05/change-in-rainfall-with-temperature.jpg
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.
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.
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:
http://i49.tinypic.com/2jg5tvr.png
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.
The problem with latitudinal summaries considered in isolation is their ignorance of surface boundary layer basin loops. So many interesting layers to explore…
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.
[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??☺☺☺
Stephen Wilde says:
May 26, 2012 at 3:00 pm
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.
The notion that the planet has purposeful homeostatic machanisms is dangerously close to reverse Gaia.
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.
Paul Vaughan,
You have some of the most amazing charts I’ve ever seen! I can’t even comprehend that last one.
“”””” 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.
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?
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.
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.
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.
.
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.
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.
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
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
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
@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…
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.
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.
Gail Combs says:
May 27, 2012 at 6:07 am
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:
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.
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?
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.
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.