Changing Hadley and Walker circulations due to a warming Arctic

Comments from Neven's excellent Arctic Sea Ice News blog:

Werther,

re: NH cells This is, in my opinion, the largest effect of climate change. The weakening of the polar jet is indicating a gradual merge of the ferrell and polar cells. It is this global restructuring of global weather patterns that will produce the most immediate impacts on society and humanity's food production capabilities. This will occur due to increases in droughts in current food producing regions and floods in current population centers.

What we are also seeing in this regime of climate change is a gradual strengthening of the hadley cell as tropical evaporation increases. This is increasing desertification. The northward expansion of the 30'N desert belt will also significantly impact grain and livestock production.

Studies of these effects and predictions of their impacts are in the peer-reviewed record produced over the last 20 years. The fact that we have done nothing to act on the impacts that have been observed indicates that only a "breakthrough" event will produce real motivation for change. The longer that we wait, the more painful climate change will be and the more difficult it will be to fix it. In fact, we are approaching a point where it is becoming very likely that modernity will not survive the next 65 years.

With the arctic ice pack being the "canary in the coal mine" and the most effective messenger of these developing threats, it is important that we more fully understand and and are able to explain what is happening in the arctic today.

That is why I monitor these blogs, and sincerely appreciate the work that is being done here.

http://onlinelibrary.wiley.com/doi/10.1029/2010JD015197/abstract

"poleward shift of the subtropical dry zones (up to 2° decade−1 in June-July-August (JJA) in the Northern Hemisphere "

Posted by: Jai Mitchell | July 10, 2013 at 17:19

Werther 

In fact, what has been going on since autumn last year, is a restructuring of the three NH cells. The gradient between them is fading. The behavior of the Polar Jet Stream is directly related to that.

The geopotential difference is lower, meaning that the height of the atmospheric pressure zones, especially over the Arctic and Boreal zones have gone up.

Thanks Werther, but I'm not sure I understand this logic.
Why would the height of the atmospheric pressure zones, especially over the Arctic, go up if the gradient between the NH cells fades ?

What I understand is that in 2012 the gradient between the NH cells was low, (and the polar vortex was weak, and that is why we had a warm Arctic.

So why would 2013 show the opposite ?

Posted by: Rob Dekker | July 11, 2013 at 08:45

Rob Dekker wrote: July 11, 2013 at 08:45

Why would the height of the atmospheric pressure zones, especially over the Arctic, go up if the gradient between the NH cells fades?

Hi Rob,

I think the increased height of the troposphere is a direct consequence of more water vapour in the atmosphere due to a warmer earth, rather than the gradient between circulation cells. Wikipedia says:

As a rule, the "cells" of Earth's atmosphere shift polewards in warmer climates (e.g. interglacials compared to glacials)

So it seems the Hadley and Ferrel cells themselves are crowding the Polar cell, raising the height of the atmosphere in the Arctic. Or more exactly pushing the Polar cell further to the North.

However, as the jet stream weakens, one would also expect the loss of this natural barrier between cells to raise the average height of the polar cells.

Have you seen the polar jet stream this week? It's running from 70N to 83N in the CAA, and 75N to 85N over the Laptev sea right now (00z 11 Jul 2013).

Highly unusual, and very far North.

Cheers,
Lodger

Posted by: Artful Dodger | July 11, 2013 at 10:22

http://neven1.typepad.com/blog/2013/07/so-how-slow-was-this-start.html

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Recent trends of the tropical hydrological cycle inferred from Global Precipitation Climatology Project and International Satellite Cloud Climatology Project data

Journal of Geophysical Research: Atmospheres, Vol. 16, No. D9 (16 May 2011); doi:10.1029/2010JD015197

Recent trends of the tropical hydrological cycle inferred from Global Precipitation Climatology Project and International Satellite Cloud Climatology Project data

Y P. Zhou, Kuan-Man Xu, Y. C. Sud and A. K. Betts

Abstract

[1]Scores of modeling studies have shown that increasing greenhouse gases in the atmosphere impact the global hydrologic cycle; however, disagreements on regional scales are large, and thus the simulated trends of such impacts, even for regions as large as the tropics, remain uncertain. The present investigation attempts to examine such trends in the observations using satellite data products comprising Global Precipitation Climatology Project precipitation and International Satellite Cloud Climatology Project cloud and radiation. Specifically, evolving trends of the tropical hydrological cycle over the last 20–30 years were identified and analyzed. The results show (1) intensification of tropical precipitation in the rising regions of the Walker and Hadley circulations and weakening over the sinking regions of the associated overturning circulation; (2) poleward shift of the subtropical dry zones (up to 2° decade⁻¹ in June-July-August (JJA) in the Northern Hemisphere and 0.3–0.7° decade⁻¹ in June-July-August and September-October-November in the Southern Hemisphere) consistent with an overall broadening of the Hadley circulation; and (3) significant poleward migration (0.9–1.7° decade⁻¹) of cloud boundaries of Hadley cell and plausible narrowing of the high cloudiness in the Intertropical Convergence Zone region in some seasons. These results support findings of some of the previous studies that showed strengthening of the tropical hydrological cycle and expansion of the Hadley cell that are potentially related to the recent global warming trends.

http://onlinelibrary.wiley.com/doi/10.1029/2010JD015197/abstract

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