Friday, September 8, 2023

Arctic Sea Ice Loss Revisited

An interesting study was published last week on the topic of Arctic sea ice loss, led by Igor Polyakov of UAF.  Here's a write-up from the Geophysical Institute:

The thrust of the paper is that a change in the Arctic atmospheric circulation has contributed to the lack of sea ice loss since 2007, when the seasonal minimum reached its first dramatic record low.  Since then, Arctic sea ice has been relatively stable, and the new work suggests this is because the new wind patterns have been transporting more fresh water into the Pacific side of the basin.  Fresh water is less dense than salt water and resides immediately under the sea ice, and it serves to insulate the ice from warmer ocean water that would otherwise tend to melt the ice from below.

The following figure from the paper shows the rather surprising hiatus in sea ice loss (based on September ice extent), and the inset map shows the change in upper ocean "available potential energy", which signifies the quantity of fresh water at the surface.  The change to more fresh water and more stable conditions is largely found in the Beaufort and northern Chukchi Seas.

The paper presents a variety of oceanographic analyses to support the hypothesis about redistribution of fresh water into the Pacific sector, and the basic idea seems plausible to me.  However, to throw in my own two cents, I also think it's likely that a portion of the fresh water volume increase has arisen because of greater precipitation and greater river inflows in the past 15 years.

Let's illustrate these two additional factors.  First, it's well-known that the Arctic hydrological cycle is intensifying in response to rapid warming, and the 2021 NOAA Arctic Report Card highlighted the increase in Arctic river discharge, particularly for Eurasian rivers (providing the bulk of Arctic Ocean inflows):

Here's the link:

The total discharge of all rivers to the Arctic Ocean is estimated to be approximately 5000 km3/year, which is equivalent to about 35 cm of fresh water depth over the entire Arctic Ocean area - but much of this water will reside close to the continents or be transported into the Pacific sector and the Beaufort Gyre, where salinity is low:

It seems plain that an increase in discharge will tend to fortify the fresh water layer in the Pacific/Canada sector of the ocean, especially with the more favorable wind circulation of recent years.  The percentage change in river flows evidently isn't large, but the recent effect may have been magnified by the change in transport patterns.  Here's a diagram of ocean and ice transport around the basin, from Polyakov et al:

The other factor may be increased precipitation.  Ground-truth precipitation measurements are very scarce and uncertain, of course, so here's the ERA5 reanalysis estimate of annual precipitation change between the two 15-year periods that are compared in Polyakov et al.

An increase of 5-15% is widespread across the Asian side of the Arctic Ocean, according to the model.  How much does this amount to?  The annual mean precipitation is very low, only about 250-300 mm (10-12 inches) across much of the area where the increase has occurred:

But in the northern Barents Sea, where annual precipitation is higher and the percentage increase has been large, the absolute increase amounts to 50-100 mm/year.

Interestingly, the total Arctic Ocean fresh water input appears to be roughly evenly split between the average ~35cm depth of river discharge and the average precipitation of about 20-50 cm.  (Some of the snow will evaporate, of course, but some of it melts in summer and feeds the surface fresh water layer.)  With both fresh water sources producing higher volume in recent years, it seems this has probably contributed to the build-up of fresh water and thereby to the longevity of Arctic sea ice.

The question remaining in my mind is how long can sea ice melt be postponed by these mechanisms in the face of rapid atmospheric (and presumably ocean subsurface) warming?  Polyakov et al suggest that the atmospheric circulation regime of the last 15 years is likely to reverse back to the opposite phase very soon, which would be unfavorable for sea ice and perhaps bring about another rapid decline.  It does seem unlikely that sea ice will hold on much longer, unless something very unexpected happens.  Courtesy of Zack Labe's website, Arctic temperatures are on a steep uptrend, including since 2007:

But time will tell, and I imagine that a few more surprises lie ahead.

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