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 …
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
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.
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?
timetochooseagain says:
May 27, 2012 at 6:56 pm
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.
kadaka (KD Knoebel) says:
May 27, 2012 at 7:22 pm
From the dictionary, emphasis mine …
w.
timetochooseagain says:
May 27, 2012 at 6:56 pm
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:
w.
Willis Eschenbach says: @ur momisugly 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
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?
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..
timetochooseagain says:
May 28, 2012 at 4:29 pm
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.
http://www.windows2universe.org/earth/atacama_desert.html
Ugh.. links to “Walker circulation” and “This decending air is very dry” – make of it what you will…
Myrrh says:
May 28, 2012 at 6:25 pm
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.
timetochooseagain says:
May 28, 2012 at 4:29 pm
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.
… 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]”
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.”
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.
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.
Willis
re Precipitation vs latitude, and Precipitation – Evaporation vs latitude
See Figure 17 in
1-1-2011 Review Article The Twentieth Century Reanalysis Project
http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1253&context=usdeptcommercepub