We are entering the final phase of the melting season. Extent decrease rates are getting lower and lower and soon we will start seeing the first days of extent growth, followed soon after by minimum extent. I figured it might be interesting to have a look at what happened in other years, specifically the years of the 'new Arctic era': 2007, 2008 and 2009. Perhaps we'll see some parallels that give us an idea of when and how the 2010 melting season will end.
I have downloaded a lot of images from different sources and combined them to make animations of the period August 25th-September 24th. The final date is based on when 2007 reached minimum extent.
The first End Zone instalment compared air temperatures, the second one was about ice displacement, and now we'll be looking at atmospheric pressure or sea level pressure. Atmospheric pressures determine where low-pressure areas (cyclones) and high-pressure areas (anti-cyclones) are situated. Along the edges of such systems winds are either blowing in an anti-clockwise fashion (cyclones) or a clockwise fashion (anti-cyclones). The magnitude of the pressure determines how strong these winds are.
The way these pressure areas are distributed over the Arctic are a big factor in the annual extent decrease. Especially now, at the end of the melting season. The Beaufort Gyre, the clockwise movement of the ice pack from the Pacific side of the Arctic towards the Atlantic side, is dependent on a high over the Beaufort Sea. Combined with a low over the Siberian coast, preferably between the Kara and Laptev Seas, the Transpolar Drift Stream is activated and transports a lot of ice through Fram Strait. The stable positioning of these two poles opposite each other is known as the Arctic Dipole Anomaly.
This is what you get when that happens:
The sea ice gets compacted towards the coasts of Greenland, Ellesmere Island and the rest of the Canadian Archipelago, and it gets transported towards Fram Strait where it melts out in warmer southern waters. A double whammy, as the American expression goes.
When looking at the distribution of atmospheric pressure systems in 2007, 2008 and 2009, we see a confirmation of what happened with regards to ice displacement:
Like with air temperatures I've copied these images from the Daily Mean Composite page, compiled by the Physical Science Division of NOAA's Earth System Research Laboratory. Again the colour bars on the right displaying atmospheric pressure in millibar are continually switching, but it's not as disturbing as with air temperatures. The blue to purple blots are low-pressure areas, the green to red one are high-pressure areas.
As we can see in 2007 the Arctic Dipole Anomaly - with its high over the Beaufort Sea and lows over the Siberian coast - persists all the way through the end of the melting season, resulting in a relatively late minimum extent. As we saw with yesterday's ice displacement animation this has kept the PIPS arrows relatively big and pointing in the right direction (towards the Canadian Archipelago and Fram Strait).
In the same period during 2008 and 2009 we see how low-pressure areas engulf the Arctic around the 10th of September, bringing extent decrease to a screeching halt and effectively ending the melting season. Not enough winds, not enough ice displacement to resist the onset of freezing temperatures.
As a bonus I've made a short animation of what happened in 2007 after minimum extent was reached. It's very clear how the high over the Beaufort Sea gets pushed aside by a big low:
The nice thing about SLP (sea level pressure) is that we can have a look at weather models that forecast 10 days in advance. To my surprise the ECMWF operational forecast model was showing the setting up of a very strong high (see the animation in yesterdays's SIE update), meaning the melt season could be extended for a week or two. However, the forecast has changed again today:
There is still a high forecasted over the Beaufort Sea, but instead of strengthening the Arctic Dipole Anomaly, it looks like it will be hanging by a thread. If low-pressure areas take over on that side of the Arctic the melting season will end somewhere in the first 10 days of September, with PIPS ice displacement arrows scattered all over the place.
Whether 2010 will follow the End Zone patterns of 2007 or 2008 and 2009 remains to be seen, but I'll keep you up-to-date in my SIE updates.
First-Class work Neven. This kind of post-mortem analysis makes it easy to explore new theories. Now I can't wait to compare an animation of 2010's SLP maps.
Thanks again for your tireless effort, Neven. You are appreciated!
Posted by: Artful Dodger | August 30, 2010 at 13:11