By Bob Henson
As Shakespeare noted about true love, the course of Arctic sea ice never does run smooth. Even though weather conditions in June and July weren’t especially favorable for melting, the ice vanished at a striking pace. Then came a midsummer tempest—and now 2012 threatens to break 2007’s records for the lowest extent of Arctic sea ice ever observed.
I knew something was afoot when I turned to the indispensible Arctic Sea Ice Blog a few days ago and saw the headline “Cyclone warning!”
Computer models were indicating that a vast, powerful area of low
pressure would develop over the central Arctic Ocean and stay in place
for days. The storm didn’t last quite as long as forecast, but it was
indeed a humdinger, one that could stimulate research for years to come.
 |
| This mosaic of NASA/MODIS
satellite images from August 5 shows a massive low-pressure center
spinning across the central Arctic Ocean on August 5, 2012. Canada and
Alaska are located to the left, with Europe and Russia to the right. (Image courtesy NASA Earth Observatory.) |
WHERE TO PLACE AN OUTLIER
Just how unusual was this cyclone? It’s surprisingly hard to get a
precise answer, since Arctic Ocean weather is so difficult to observe.
For decades, surface pressure was measured only on a few Arctic islands,
at a few land-based stations ringing the ocean, and with an array of
floating buoys. Satellite cloud photos also helped estimate surface
pressures, but routine, high-quality satellite monitoring did not begin
until 1979.
Nevertheless, it’s clear this storm was “truly remarkable,” says Ian
Simmonds (University of Melbourne). He and Mark Drinkwater (European
Space Agency) produced a paper in 2007 for the World Meteorological
Organization entitled “A ferocious and extreme Arctic storm in a time of
decreasing sea ice” (PDF).
Their study analyzed a low that spun across the Arctic in August 2006,
with a central surface pressure that dipped to 984 hectopascals—on par
with a major U.S. winter storm.
This year’s Great Arctic Cyclone leaves that “ferocious” storm in the
dust, with a central pressure that bottomed out at 963 hPa. No other
summer storms—and only a tiny number of winter storms—have been that
intense across the Arctic since 1979, according to Simmonds, who led a
study of Arctic cyclones published in 2008 in the Journal of Climate.
As Simmonds points out, central pressure is just one way to assess a
storm’s strength. Stu Ostro (The Weather Channel) dug into the data to
see whether any other Arctic storms had been so intense at middle
altitudes: 500 hPa,
several miles above the surface. Looking at the interval from July 15 to
August 15 for the period 1948–2012, Ostro found only one system
anywhere in the Northern Hemisphere with a 500-hPa intensity comparable
to the one observed earlier this month. (In this case, intensity is
measured by the height of the 500-hPa surface: the lower it is, the
stronger the system.)
It was that upper-level storm moving north from Siberia last week
that triggered the surface cyclone, says Steven Cavallo. Now at the
University of Oklahoma, Cavallo analyzed tropopause polar cyclones
with graduate advisor Gregory Hakim during his studies at the
University of Washington. Cavallo thinks the strength of the Great
Arctic Cyclone is likely a product of the strong contrast between
unusually warm waters in the East Siberian and Laptev seas and the edge
of the remaining sea ice, around 75–80°N. Normally, he says, “we see
more extreme cyclones in the autumn or spring, because there’s a greater
temperature difference between the colder atmosphere and warm ocean
waters.”
A SUMMER OUTSIDE THE ENVELOPE
If the Great Arctic Cyclone of 2012 was a truly unusual midsummer
event, what makes it virtually unprecedented was the state of the ice
beneath it. The storm’s high winds and waves pummeled a mammoth zone of
ice that was already thin and fragmented.
 |
| These maps show how much the average surface pressures for June–July 2007 (top) and 2012 (bottom) departed from average across the Arctic Ocean. In 2007, when sea ice extent dropped to record-low values, high pressure dominated the Arctic basin, centered in the Beaufort Sea and Greenland. This year, the zone of high pressure has been less extensive, but the amount of melt may exceed the 2007 record. (Images for 2007 and 2012 courtesy NOAA Earth System Research Laboratory. |
NCAR’s Jennifer Kay has studied the critical role of summer weather
in shaping how much Arctic sea ice will melt in a given year. The most
ice-destructive pattern, dubbed the Arctic Dipole, features relatively
high pressure extending from the Beaufort Sea to Greenland. This helps
keep skies clear, allowing for round-the-clock sunshine. It also tends
to push ice from the western Arctic toward the Fram Strait and into the
meltyard of the North Atlantic.
“This year’s pattern is not the canonical one for ice loss,” notes
Kay. As evident in the maps at left, there’s been a tendency toward
lower-than-usual pressure north of Alaska, which runs counter to the
classic Arctic Dipole. However, consistent with the dipole, Greenland
has seen higher-than-usual pressure (as well as headline-inducing melt
extent atop its ice sheet and total melting already at record values).
And both ocean and air temperatures across much of the Arctic have been
consistently warm. For example, at Alert, Canada—the northernmost
settlement on Earth, where average midsummer highs are around 42°F
(6°C)—temperatures soared above 50°F (10°C) on at least 14 days in July and August.
Exactly how the Great Arctic Cyclone influenced this year’s
already-depleted sea ice is still an open question. “There are many
reasons to believe that a big storm could have a large effect on the sea
ice,” observes James Screen (University of Melbourne). Such a storm
might pull warmer air into the high Arctic; its waves and winds could
break up large chunks of thin ice into smaller, easier-to-melt pieces;
and the resulting ocean currents could push ice together, reducing the
total extent of sea ice (though not fostering melt per se).
As the cyclone unfolded, many ice watchers on the Arctic Sea Ice Blog
viewed it as a potential game changer for this summer’s ice loss. In
fact, major drops did occur in most ice indexes over the first two weeks
of August, including those calculated by the National Snow and Ice Data
Center (see map). Near the cyclone’s track, a huge chunk of ice roughly
the size of Norway split off from the central Arctic. Now marooned
north of Siberia, that vast iceberg could melt completely by early
autumn.
But in the Arctic, as elsewhere, correlation doesn’t necessarily
imply causation, as experts hasten to point out. “Short-term
accelerations of sea ice melt aren’t unprecedented, and they can occur
in response to many complex factors,” says Screen.
In its August 14 update,
the National Snow and Ice Data Center stopped short of attributing the
rapid early-August ice loss in the East Siberian Sea to the cyclone,
noting that “it may be simply a coincidence of timing, given that the
low concentration ice in the region was already poised to rapidly melt
out.”
WHAT NEXT?
We’re still far from September, when the Arctic’s sea ice normally
hits its annual minimum. But signals are increasingly pointing toward
melt that’s unprecedented in Arctic records. In a set of outlooks
updated each month by a panel of experts, 12 of the 23 forecasts for sea ice extent now call for 2012 to match or exceed 2007’s record low value.
Aside from extent, there are several other ways
to gauge ice loss. Most of them are on pace to approach or set records
over the next few weeks, and their nuances are worth more discussion
down the line. For now, suffice it to say that one index—sea ice area,
as calculated by Cryosphere Today—has just dipped below 3 million square kilometers. That’s already close to the lowest value observed at any point in any year in the satellite era, with further melting yet to come.
 |
| The extent of sea ice in the Arctic Ocean broke ahead of the record-setting pace of 2007 during the first two weeks of August 2012. Click on the image for a larger view. (Illustration courtesy National Snow and Ice Data Center.) |
