Arctic Dipole: A Positive Feedback.

In a previously discussed paper, Zhang et al open their closing discussion with the following line:

"The atmospheric circulation change identified here
could be a mixed response to rising greenhouse-gas-emissions
forcing or unexpected natural climate variability, or both."

More recently the meteorologist Jeff Masters has stated that the Arctic Dipole may be due to reduced sea-ice here. That's an article I strongly recommend.

Overland and Wang 2010 state:

"Most authors including the present ones considered that the
persistent AO and AD patterns were mainly representative of
natural variability of the chaotic climate system in the northern
latitudes. However, due to recent sea ice loss at the end
of summer there is a direct feedback to shifts in the broader
atmospheric circulation in late autumn and winter. Heat being
stored in the upper Arctic Ocean due to reduction in the area of
Arctic summer sea ice is given back to the atmosphere in the
following autumn, which has a direct impact on the temperature
of the troposphere and thus on geopotential height and thickness
fields."

First a note of clarification: Overland and Wang use the convention that the "AD pattern has a dipole field with the positive phase associated with a negative SLP anomaly on the North American side of the Arctic." Yet Wu et al state: "When the dipole anomaly remains in its positive phase, that is, negative SLP anomalies appear between the Kara Sea and the Laptev Sea with concurrent positive SLP over from the Canadian Archipelago extending southeastward to Greenland." So the two papers are at odds using opposing definitions of the AD index. This is despite Overland and Wang citing Wu in their paper! In my earlier posts I have used Wu's convention, which agrees with the convention in all the other papers I have read, and I am minded to stick with it. However anyone reading Overland and Wang should be aware of this conflict so as to avoid confusion.

So that issue clarified...

Figure 4 of Overland & Wang 2010.

The above figure is from NCEP NCAR reanalysis, it covers the atmosphere over East Siberian/Chucki/Beaufort to the pole, at around 60deg is the Bering Strait, the period covered is October to December 2002-2008. This is an anomaly plot, as is the following graphic. So where the plot is white that means no (or least) change from the climatological baseline period, any areas of colour represent a departure from that baseline.

The most prominent warming with the highest impact is away from the pole, this is where the ocean's freeze up is delayed by the heat accumulated in anomalous areas of open water due to the rapid loss of sea-ice over that period. It may seem odd that there is also notable warming over the lattitudes covered by ice during the summer. However Overland and Wang find that the positive phase (following Wu) of the AD has happened more frequently in recent years and is associated with meridional flow over the Bering Strait into the Arctic. Furthermore I suspect that the Beaufort High will play a role in distributing heat poleward.

This lower tropospheric warming causes a marked disturbance in the vertical profile of the atmosphere. As one proceeds upwards from the surface pressure drops. So if you take two pressures, 400 and 500mBar for example, there is a height difference between them, this is the geopotential thickness, the thickness of the atmospheric column between two layers of pressure. Take the red area in the graphic below, this is where the height of pressure layers around 550 to 500 hectoPascals is over 35 metres above the average height for the baseline period, 1968 to 1996.

Figure 7 of Overland & Wang 2010.

The lower tropospheric warming produces the above disturbance in the atmospheric column. Because the atmosphere above the pole is usually highly stratified in autumn and winter the lower tropospheric warming causes the marked disturbance to the profile around the Pole, right side of figure, causing the geopotential heights to be raised. Over the open ocean itself, left side of figure, there is a low level drop in geopotential height with increases (green band) higher in the atmosphere. The region in white is the area of minimum disturbance, however the black isobar passing through the middle of that region shows that above the line there is a small increase in geopotential height, below is a decrease - note the scale below the plot, anything less than +/- 5 is white.

Overland & Wang find that these changes in the atmospheric profile cause a reduction in the zonal wind flow, of which the Arctic Oscillation (AO) is the typical mode, this reduction is 40% of the maximum climatological zonal flow. As I understand it; it is this reduction in zonal flow that gives rise to the AD having attained a dominant role in the period since 2002, as noted by Zhang et al and discussed here previously. The most marked reduction is not actually in the area around the pole but over the Beaufort Sea, where sea-ice reductions and persistence of open water in the Autumn have been most marked.

So not only does the AD (in its positive phase) act as a signficant driver to ice loss, (as discussed in an earlier post), it is in turn created by loss of sea-ice and the transfer of heat thus gained by substantial areas of open water to the atmosphere. This is why opinions are changing and the AD is starting to be seen not as 'weather' but as an active and ongoing positive feedback on loss of sea-ice.

The really observant will have noted that in the quote from Overland and Wang above there was reference to impacts on geopotential height thickness fields. I've not addressed that impact in this post but these probably play a role in the recent cold winter events. As I've gone on enough for one post, that will have to wait.


Overland & Wang, 2010, "Large-scale atmospheric circulation changes are associated with the recent loss of Arctic sea ice."
http://www.webpages.uidaho.edu/envs501/downloads/Overland%20%26%20Wang%202010.pdf

Xiangdong Zhang et al, 2008, "Recent radical shifts of atmospheric circulations and rapid changes in Arctic climate system."