Why would the mid to upper troposphere become drier with increasing temperatures?

Question

There was a recent question that asked about the tropical tropospheric 'hotspot', and someone brought up the Paltridge 2009 paper that purported to show a decrease in specific humidity above 850 hPa:
http://www.gerkynet.com/meteo/paltrigde08.pdf

I'm just wondering what kind of mechanism would allow for increasing T while decreasing q in the longterm. I can understand the argument for a negative trend in RH, but specific humidity?

I should also mention that the NCEP reanalysis is the only dataset that shows decreasing q both globally and in the tropics (IIRC). Both satellites and other reanalyses show differently
http://www.gfy.ku.dk/~kaas/Bornoecourse/Material/soden.pdf
http://geotest.tamu.edu/userfiles/229/Dessler_et_al_2008b.pdf
http://journals.ametsoc.org/doi/abs/10.1175/2009JCLI3052.1

gcnp58 · Accepted Answer

I know, I'm citing RealClimate.org, but I figure none of the skeptics will care what I say anyway so it doesn't matter this comes from RC.  

Paltridge et al. gets a passing mention here:

http://www.realclimate.org/?comments_popup=2996

scan down to comment 359.  The bottom line is that none of the other reanalysis results show the decrease in specific humidity, nor do the available satellite measurements.  

In general, in cases where an observation cannot be explained in terms of a rational physical mechanism and is at odds with all other available data on the same phenomenon, it is a very good idea to look for problem(s) in the measurements.  That is Science 101.  As pointed out, there are many problems in the radiosonde humidity measurements, which Paltridge glosses over.  I think Paltridge failed that part of the quiz and the reanalysis result is not correct.

Dana1981 · Answer

I can't think of a reason.  And we all know that deniers don't do physical explanations.  As usual all they take from this study is that something is wrong with AGW therefore the study must be correct.

Useful link provided by gcnp with Gavin Schmidt's take - basically Paltridge seems to be practicing some questionable science.  I starred the question because I was hoping somebody could come up with a plausible explanation, but it's not looking good.

As gcnp said, if a result doesn't have a good physical explanation and contradicts other studies' results, the odds are pretty good that it's wrong.  You don't assume it's right just because the results are convenient for your pre-conceived notions...unless you're in denial, of course.

Incipient_planck · Answer

So Gcn... I don't care what real climate has to say.  what I do see is projections based on speculation.  Furthermore when so-called deniers call out East Anglia for allegedly fudging temperatures your side had a hissy fit.  As a meteorologist and one who has used Skew-T data  for longer than you have been a scientist (if you ever have),   I find it ironic that you would or anyone would imply that this method of data collection is seriously flawed.   Stop wanting things to be your way.  It's not scientific and you all hate the fact that scientists are calling you out.

Anonymous · Answer

No one knows.

Why is it colder in the upper troposphere over the tropics than it is over the arctic? Your answer may lie there.

Why would the mid to upper troposphere become drier with increasing temperatures?

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