In comments on my earlier post "Arctic Dipole: A Positive Feedback" Kevin O'Neill raised an intriguing point regarding stratospheric cooling and tropospheric warming in the post 2007 period during the summer (June - August).He also pointing out that there was a mixed picture previously, see here. Kevin's graphic is here.
This has had me puzzling over it for the last few days, so yet again the posts I was going to do on the recent cold winters will be delayed. However I think I now have a reasonable explanation for the observations Kevin raised.
Note that this post concerns what is going on in the region from the Beaufort Sea to the Laptev Sea up to the pole.
Firstly here's the atmospheric profile from 1000mb (surface) to 10mb (high in the stratosphere).
Air temperature anomaly profile.
The troposphere is from the surface to about 300mb and is a region where temperature decreases with height (negative lapse rate), whereas the stratosphere, above 300mb, has a positive lapse rate; temperature increases with height. From the above graphic it can be seen that there is a warming mass above the surface and into the troposphere between the sea-ice edge and the pole. Counterpoised with this is a stratospheric cooling.
What is causing the tropospheric warming? Obviously during the summer post 2007 there's a significantly higher amount of open water. Relative humidity is down, but specific humidity is up, which suggests temperatures have risen faster than humidity (note humidity only goes up to 300mb).
Specific humidity up to 300mb.
Note that the ice edge is around 76degN in the post 2007 period, which seems to support my notion that the increase in humidity is due to the reduction in sea-ice. I'd suggest (in a handwaving sense without calculations) that the tropospheric warming is likely to be due to a combination of increased infra-red and latent heat release from condensing water vapour.
But why is the stratosphere cooling? My best guess is as follows.
Here's the geopotential height anomaly for the period after 2007.
Geopotential height anomaly.
This pattern fits with the temperature anomaly, in that the area of maximum anomaly is around 82 to 86 degN. In terms of pressure the anomaly is at its greatest around the stratopause. This pattern suggests a bulge in the troposphere that is displacing the stratosphere.
Suggested process causing stratospheric cooling.
In the above figure the dark blue line is the pre-2007 profile of temperature change with height, after 2007 is the light blue line. Temperature is on the horizontal axis, height on the vertical. Following the dark blue line upwards temperature drops with height (the troposhere), then at the tropopause temperature increases with height (stratosphere). Here I'm assuming the lapse rates for both troposphere and stratosphere stay largely unchanged.
With the warming driven bulge in the troposphere the profile moves up from the dark to the light blue profile. So at level A we see that the move from the original profile (dark blue) to the new profile (light blue) entails a warming. However at the same time above the tropopause in the stratosphere at level B the move from dark to light blue entails a cooling.
The same process also applies in the spring, March to May. In the autumn, September to October, the atmosphere has rapidly cooled, restricting the waming caused by late season open water and ice formation to the very lowest level of the troposphere, i.e. warming and geopotential height anomalies show much less of an impact throughout the column and a negligible stratospheric cooling.
So I think that in general terms the tropspheric warming driven by the loss of sea-ice is causing a bulging of the troposphere, this drives the tropopause higher, which is forcing the decrease in temperature anomaly in the stratosphere. I stress that this is a regional process and doesn't account for the global trends in stratospheric cooling, which is due to CO2 and to a lesser extent CFCs, e.g. here.
But that's just the best guess of someone who's not a professional scientist, much less an atmospheric physicist.