As I keep repeating how important the distribution of high-pressure areas and low-pressure areas is during a melting season, I figured it might be a good thing to compare monthly averages for July in the period 2005-2010. Especially now that we have had a big high-pressure area dominating the Arctic Basin for over a week, significantly affecting extent decrease.
That's mainly because of two things. First, high-pressure areas cause the skies to clear of clouds, and that means a lot of sunshine (insolation), especially around this time when the Arctic circle receives many, many hours of sunshine. Second, in a high-pressure area winds are blowing in a clockwise fashion. For the Arctic this means sea ice gets transported from Canada and Alaska towards Siberia, where it gets compacted (reducing the total extent of sea ice). If this high-pressure area moves over the Canadian Archipelago and gets complemented by a low-pressure area between the Kara and Laptev Seas (also known as the Dipole Anomaly) a lot of ice gets transported through Fram Strait towards warmer water in the south.
As always, I have used the Daily Mean Composite page, compiled by the Physical Science Division of NOAA's Earth System Research Laboratory. Here's the mean SLP distribution for the first five days of July: We can clearly see that high-pressure area dominating much of the Arctic. Here's a comparison of previous years for the entire month of July (click for a larger version):
It's a bit too early to tell, but for now July 2011 resembles July 2009 most. 2005 and 2007 show a high-pressure area over the central Arctic as well. Last but not least, we can see the huge low-pressure that brought 2010's extent decrease to a virtual standstill.
Here are the numbers for the July average daily extent decrease (IJIS) in square km that clearly show the correlation with SLP:
- 2005: -83,709
- 2006: -70,025
- 2007: -98,608
- 2008: -81,068
- 2009: -92,127
- 2010: -62,601