Thursday 30 July 2015

The warming Arctic sea ice - 07/29/2015

Arctic Sea Ice Now Below 2014 in All Major Measures — Warm Storm Settles In

Polynya CAA
29 July, 2015
When looking at Arctic sea ice melt, there are trends and there are bounces. The great 1979 to 2015 melt we call a trend. The 2013 and 2014 rebound from all-time record lows during 2012? That we call a bounce. But it’s starting to look more and more like the bounce is ending and the long-term melt trend is starting to, inexorably, reassert.
Overall, it was a decent rebound. By September, minimum seasonal ice popped up by about 3,500 cubic kilometers in the PIOMAS volume record, by about 1.4 million square kilometers in the Cryosphere Today area measure, and by 1.5 million square kilometers in the NSIDC extent measure. A decent rebound, but still about 11,000 cubic kilometers lower in volume than 1979 (more than a 55 percent loss), about 1.9 million square kilometers lower than 1979 in area (more than a 36 percent loss), and about 1.9 million square kilometers lower than 1979 in extent (about a 30 percent loss).

PIOMAS Volume Trend
(Sea ice volume rate of decline as measured by PIOMAS.)

Sadly, a bump of this kind does not a trend make. Looking at the overall volume loss line (above), we can clearly see that the 2013 and 2014 rebound after 2012’s record low was plainly within the melt progression’s boundaries. Moreover, out of the last 8 years, 2014 is the only year above base-line rate of loss at 3,200 cubic kilometers per decade. A rate of loss that, if it continues would bring us within striking distance of a dreaded ‘blue ocean’ type event for the Arctic by the early 2020s.
Since this trend is polar amplification driven — an underlying aspect of phase 1 climate change forced by human greenhouse gas emissions — the only major driver with the potential to challenge Arctic melt is a large outflow of fresh water from Greenland. Such an outflow would temporarily reduce ocean ventilation of heat through the sea surface in the fresh water outflow region. The result being that surface temperatures would, for a short time, cool in the outflow zone. This would have an effect of regenerating sea ice in a larger counter-melt-trend feedback. It’s likely that melt outflows from Greenland would need to be significant enough to have profound impacts on the Arctic environment as a whole. To hit anywhere near these levels, we likely need to see in the range of at least a half centimeter of sea level rise from Greenland melt alone each year. And we are, as yet, nowhere near that rate of loss (although we might get there in a decade or two or three).

So though the recent 2012 Greenland melt high mark was likely enough to push AMO negative, to further weaken AMOC, to develop a cool pool in the ocean south and east of Greenland, to back a super hot Gulf Stream up to the US East Coast during the winter of 2014-2015, and to set off a slew of nasty weather impacts for the North Atlantic from 2012 through 2015, it was nowhere near enough to upset the overall long-term, human heat-driven Arctic melt trend. If such an event were to occur, what we would likely see is a signature not only of a North Atlantic cool pool but also of more ice in Baffin Bay, more ice in the North Atlantic itself and more ice on the Arctic side near Greenland. A signal that we do not fully see at this time.

It is thus more likely that we will see a re-assertion of the overall Arctic sea ice decline trend. And there are a growing number of indicators that some of this re-assertion is starting to come about during the summer of 2015.
All Major Monitors Now Below 2014

For the Summer of 2015, melt has been consistently strong — especially for July. During most of the month, strong high pressure systems dominated. This situation led to compaction, storm formation at the sea ice edge, and a degree of sea ice export. It amplified solar insolation at a time when the sun was near its seasonally highest angle — enhancing surface melt and melt ponding.
As of yesterday, the major extent monitors — JAXA and NSIDC — as well as Cryosphere Today’s area monitor were all below or well below the 2014 line. The ongoing and rapid July melt drove JAXA below the 2014 line late last week while NSIDC hit below 2014 just yesterday. As a result, NSIDC sits at 7.2 million square kilometers extent or 7th lowest on record (a decline of 2 places since last week) and JAXA shows a 6.79 million square kilometer extent or 5th lowest on record in the measure (also a decline of 2 place).

Cryosphere Today’s area measure, meanwhile, continued to drop — showing increasing divergence from the 2014 line and hitting a 4th lowest area on record for the 27th (a one place dip from last week).

(Sea ice area dips to 4.67 million square kilometers or the fourth lowest on record in the July 27th Cryosphere Today measure. Note the 2015 sea ice area trend line is indicated in yellow, the 2014 sea ice area trend line in red. Image source: Cryosphere Today.)

Overall, these drop-offs are consistent with a returning to the long-term melt trend in 2015. But it does not yet place 2015 in striking distance of new all-time record end melt season lows set during 2012. And we’d be quite surprised if it did. Larger ocean and atmospheric teleconnections tend to drive increasing heat in the Arctic ocean waters and airs and to increase sea ice transport to compound ice weakness during El Nino year +1 and El Nino year +2. The most recent record lows both occurred 2 years after El Nino (2007 and 2012). Given the large ocean and atmospheric drivers related to this trend, we may look to next year or, more possibly, 2017 as potential new record low years.
Weather Change on the Way

All that said, it doesn’t mean that 2015 cannot surprise us or (2005, an El Nino year, was also a record low year), at least, serve up some interesting features. Notably, there’s a change in the weather on the way.
Throughout July, we saw what was, perhaps, the worst possible atmospheric regime for sea ice melt during that month. Atmospheric heat was relatively high, clear skies dominated allowing for enhanced surface melt through direct solar heating, and the persistent high pressure systems helped to drive compaction and export. Though the action of gyres moving ice out of the Fram Strait was relatively moderate, overall melt conditions were very strong.
In particular, a synergy between the high pressure driven pole-ward pull of sea ice away from the Siberian side of the Arctic and a significant influx of warm water northward from the Pacific Ocean and through the Bering and Chukchi Seas had a marked impact. You can see the amazing melt progress led by these two influences in the excellent animation provided by The Great White Con below:

Now, however, the high pressure is sliding increasingly to the Siberian side of the Arctic. Meanwhile, a persistent storm is beginning to take hold over the Beaufort Sea and Central Arctic. Overall, it’s an increased storminess for the Arctic. One that is now driving 25-35 mph winds through the shattered ice sections of the Beaufort Sea.
There’s some argument that storms are friendly to sea ice. And, perhaps, this is more true during the June time-frame when storms can reduce insolation and melt ponding. When they can spread the sea ice out to increase overall albedo. But in the current melt regime, sea ice is, overall, far more fragile. There is more latent heat in the Arctic Ocean that is available for storm systems to tap in order to melt ice. And it is this condition that is most at play as we enter late July and early August.
(A storm is predicted to persist over the Beaufort for at least the next five days. Image source: Earth Nullschool.)

Over the next five days, the current storm is predicted to persist over the Beaufort. It will rumble along, sending its 20-35 mph winds out over the fractured multi-season ice and large stretches of open water. It will linger, gobbling up little storms rushing north over Alaska and the Bering. And it will lash the ice there with increasing wave action, breaking the surface cool water cap and pumping warm water up toward the ice from below.
In addition, this Beaufort low will form a kind of dipole with a high pressure system that will tend to remain on the Kara Sea side of the Arctic Ocean. The net effect of the dual circulation of the high over the Kara and the low over the Beaufort will be to lift the thick ice away from its base of support along the Northern Canadian Archipelago. The result is likely to be a continued widening of a large polynya already developing there.
Polynya CAA
(Winds cycling between a high pressure over the Kara and a storm over the Beaufort may further widen a large polynya north of the Canadian Archipelago over the next few days. Image source: LANCE MODIS.)

Meanwhile, milder compaction and sea ice retreat is likely to continue on the Siberian side with ice recession particularly likely in the Kara, Laptev, and East Siberian Sea regions.
Overall, these factors should continue to drive melt enough to keep the monitors at or below the 2014 line with particular risk of increased divergence in the area measure over the coming week due to storm activity in the Beaufort. There is an outside, though not entirely negligible, risk that Beaufort storm activity will greatly impact the already very fragile ice along the Chukchi Sea boundary toward the Siberia side. Such an impact would result in still greater area and extent impacts. But more likely is an enhanced winnowing of the remaining multi-year ice together with a widening of the large polynya north of the Canadian Archipelago.
Hat tip to Humorta
(Please support publicly funded, non special interest based science like the fantastic work done by NSIDC, NASA, PIOMAS and JAXA which has made this report possible)

No comments:

Post a Comment

Note: only a member of this blog may post a comment.