Sea level pressure July 2011

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

Comments

[Lord Soth]

With this mornings update 2011 now has the largest number of top 10 daily extent drops ever for IJIS. Ranking #3, #7 and #9 overall.

I know this current weather pattern can't last forever, but we already close to the seasonal peak tempertures (since the summer seasonal lag is only three weeks in the high arctic).

By the time the high breaks down, clouds in the arctic will trap the heat, as we move towards the time of net solar heat loss.

[Tor Bejnar]

Looking at http://gfspl.rootnode.net/klimat/arcticice.png (green 2009 line easier to follow than other graph's yellow 2009), it appears that the 2009 slope of area loss/time was steeper than 2007's during most of July. 2009, however, started out with more ice area and never caught up (and then area decreases slowed down near the end of July 2009). This steeper slope suggests the possibility that, as intense as 2007 ice loss was, Arctic ice can melt faster.

I don't know if anybody has objectively compared/contrasted this year's ice conditions (other than the timing of breaking up) with previous years, but subjective comments suggest the ice is relatively rotten.

Maybe this has to be done region by region. The http://www.arctic.noaa.gov/gallery_np.html "North Pole" cameras show today's ponding far advanced compared to mid-July 2009 and 2007. My untrained eye sees less snow in July 2011 than in July 2007 or '09. If this is true, does less snow make ponding easier? (The variables are endless! The specialist knows how to figure out what's relevant.)

[crandles]

#3 #7 and #9 ?
I got #4, #8 and #9
-201875 2007 1
-191094 2006 2
-168437 2009 3
-168125 2011 4
-162031 2007 5
-161719 2005 6
-154218 2004 7
-149844 2011 8
-149375 2011 9
-148750 2009 10
-145469 2009 11

Perhaps more important than single days is to look at periods of at least a week. The loss in last week is -845,782 more than any other week in any year except 2007 which managed -960,625 in a week and even -870,781 in 6 days.

[Artful Dodger]

The 2010 low SIE is all the more remarkable for the relative cloudiness during the month of peak loss of extent. However clouds were not the only factor at play in 2010. A long line of multiyear sea ice was advected from North of the Cdn Archipelago into the Beaufort and Chukchi seas where it acted as a barrier to warm surface water entering the fragment pack ice further to the North.

This M.Y. ice did melt by the end of Summer, but the season ended with warm water losing the race into the interior of the Ice Pack to the dropping daily insolation values of Fall.

This sea ice barrier is so pronounced that at one point in late Summer 2010, the SST gradient in the Beaufort Sea was +18 C over a distance of less than 200 km.

Summer 2010 then saw extended melt, ending in a double-dip and a near tie with 2008 for SIE. Into Fall and Winter, Hudson and Baffin Bays experienced a record late freeze up, a consequence of latent heat stored in the sea surface layer. Anybody have a Polar bear mortality update for Hudson Bay?

I take two points from this comparison of 2010 to 2011. First, the heat-trapping effects of Cloud are very close their negative albedo effects. Second, 2011 has no barrier of multiyear sea ice to block the influx of warm water into the pack ice in the Central Basin...

With maximum insolation continuing, I look for continued record decreases in SIE in July 2011.

[Neven]

Excellent summary, Lodger. I would only add that 2010's extent was artificially high because the ice pack was so spread out at the end of the melting season.

First, the heat-trapping effects of Cloud are very close their negative albedo effects.

This is a very interesting remark, as was the one by Lord Soth I believe that when circumstances switch all the energy that is being built up now will be kept there by clouds. Thanks to both of you.

Second, 2011 has no barrier of multiyear sea ice to block the influx of warm water into the pack ice in the Central Basin...

I agree. Some MY ice has been moved that way again, but definitely not as much as last year.

[Artful Dodger]

Since we're on a little IJIS break, here is a thorough treatment of the effect of Cloud on the Energy budget of the Arctic Sea Ice/Ocean/Atmosphere system:

Sedlar et.al (2010) A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing

From the Abstract:

Shortwave cloud forcing ranged between -50 W m-2 and zero and varied significantly with surface albedo, solar zenith angle and cloud liquid water. Longwave cloud forcing was larger and generally ranged between 65 and 85 W m-2, except when the cloud fraction was tenuous or contained little liquid water; thus the net effect of the clouds was to warm the surface.

"Clouds warmed the surface by 25–55 W m-2 during the 1st regime; this increased to 55–75 W m-2 during the 3rd regime" - pg 8.

[Kevin McKinney]

Many thanks for that link, Lodger--it's a very cool study (pun unintended but (obviously!) tolerated.)

Not an easy slog for the amateur, but a great snapshot of observational science "as she is done." And some definite food for thought for denizens of this noble blog.