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Friday, July 12, 2013

Changing Hadley and Walker circulations due to a warming Arctic

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

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

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 ?

Artful Dodger

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.

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