Arctic Sea Ice May Reach Second-Lowest Extent on Record This Month

A burst of late-season loss over the last several weeks has put the Arctic Ocean’s ice cover within reach of the lowest extent observed in any year except 2012. The extent values tracked by the National Snow and Ice Data Center, or NSIDC (see Figure 1), show that 2015 has caught up to several other recent years in the amount of ice depleted, and is poised to surpass 2011 and 2007 if the rapid loss continues. The milestone is a timely one, given this week’s historic Alaskan visit by President Barack Obama.

NSIDC reviewed the near-term outlook for sea ice in an update posted on Wednesday: “There is still a possibility that 2015 extent will be lower than 4.3 million square kilometers, the third lowest sea ice extent, surpassing the 2011 sea ice extent minimum, and a small chance of surpassing 2007, resulting in the second-lowest daily minimum. This assumes that we continue to have sea ice loss rates at least as fast as those of 2010. This was indeed the case for the final ten days of August 2015.”

As explained by NSIDC with a swiss-cheese analogy, sea ice extent refers to the amount of ocean covered by at least 15 percent ice concentration (the dimensions of the slice of cheese), whereas sea ice area is the literal amount of ocean covered by ice, not counting the holes. Arctic ice normally reaches its maximum extent in March and its minimum in September. The ice extent drops in spring and summer largely as a result of melting (from below and above), though it can also be influenced by compaction (which pushes broken-up areas of ice together, reducing the total ice extent). Another factor is a pattern of atmospheric pressure called the Arctic Dipole, which favors Asia-to-Europe cross-polar flow that can push ice out the Fram Strait into the North Atlantic, hastening ice depletion.



Figure 1. Sea ice extent across the Arctic Ocean for the period August-November, 1979-2015. This year’s extent was just over 4.6 million square kilometers on September 2, and dropping rapidly, with only the 2012 curve falling much lower. Nine of the 10 lowest extent values have occurred within the last decade. Image credit: NSIDC.


There’s still plenty of darkness and cold air to foster ice-cover regrowth across most of the Arctic each winter, but the summer minima have plummeted in recent decades (see Figure 1). With just weeks left before net ice expansion resumes, it’s all but impossible for 2015 to catch up to 2012. That year saw the lowest extent since satellite records began in 1979 (3.41 million square km, or about 50% below the typical minimum observed in the 1980s and 1990s). However, a minimum in the vicinity of 4.2 to 4.3 million sq km seems within reach. That would be well below the average value of 4.8 million sq km predicted by an array of 38 participants in the latest monthly Sea Ice Outlook produced by the Sea Ice Prediction Network. By that time this forecast was issued (August 20), Arctic ice extent had come off near record-low values for late spring, recovered somewhat by early summer—thanks in part to a cold June across the Arctic, with relatively little melting--and dipped again in August, with fairly steady losses through the month. Then came a surprisingly strong cyclone that developed across the Beaufort Sea last week. High winds and seas from that storm helped weaken a large arm of multi-year ice extending from the central Arctic into the Beaufort Sea. The storm also brought punishing waves and erosion to the Alaskan coast near Barrow (see Figure 2).


Figure 2. High surf batters the coast near downtown Barrow, Alaska, on August 27, 2015. Image source: Barrow Sea Ice Webcam, tweeted by Brian Brettschneider


Figure 3. This intense surface cyclone disrupted a large chunk of Arctic sea ice that extended from the Central Arctic into the Beaufort Sea. Analysis for 0000 GMT on August 27, 2015, shows sea level pressure (in green); potential temperature at the tropopause, a marker of upper-level energy that can help foster surface cyclones (in black); and the extent of Arctic sea ice (grey shading, with concentration fraction shown by the bar at right). A corresponding animation shows the sequence of events beginning on August 16 and segues into a model prediction from August 2 to September 9. Image credit: Steven Cavallo, University of Oklahoma.


What’s ahead this month and beyond?
“It is still pretty stormy over the Arctic,” said Steven Cavallo, a University of Oklahoma meteorologist who specializes in polar weather. Cavallo has researched tropopause polar vortices (TPVs) and their relationship to surface cyclones. “There are a lot of TPVs around, meaning the potential for surface cyclone formation is high, so I think the forecast sensitivity is very high right now and there could still be some significant ice loss.” Recent model runs have flip-flopped in predicting additional strong cyclones over the Arctic over the next few days. Cavallo hypothesizes this could be related to the difficult-to-predict effects of tropical cyclones recurving into the polar jet stream. At the Arctic Sea Ice Blog and Forum, there’s been a lot of conversation along these lines. “People on the forum are speculating on and off about a 'train' of cyclones, either Atlantic or Pacific, injecting heat and moisture into the Arctic,” said blog/forum founder Neven Acropolis in an email. Two good places to follow the dialogue are “The 2015 melting season” thread and Acropolis’s own excellent posts.


Figure 4. Sea ice concentration for September 1, 2015, as calculated by the National Snow and Ice Data Center. Areas shaded in lighter blue denote reduced concentration (area) within the overall bounds of ice extent. Image credit: NSIDC.


The ice that’s managing to persist across the Arctic this summer doesn’t look especially healthy. Polar climate specialist Jennifer Francis (Rutgers University) calls out the warning signs conveyed in the most recent ice concentration image from NSIDC (see Figure 4, at right). “Much of the ice that's left is either slushy, severely broken up, or covered in melt ponds,” Francis noted. Depletion is especially large on the Pacific side of the Arctic, she added, which recent work suggests may favor a severe winter in parts of eastern North America. Much research in the last few years by Francis and others has worked to draw connections between Arctic sea ice loss, high-latitude warming, and midlatitude winter weather. A new entry in this mini-discipline is a paper published last week in Nature Geoscience that links two modes of warm Arctic weather to subsequent winter cold downstream across East Asia and North America.

The power of this year’s still-strengthening El Niño event may be enough to swamp whatever influence the decline of Arctic sea ice might have on the upcoming winter across North America. Sea-surface temperatures (SSTs) are cooling over the western tropical Pacific in tandem with the building El Niño warmth over the eastern tropical Pacific. A number of studies (nicely summarized by Daniel Swain at California Weather Blog) suggest that the western-Pacific cooling will help lead to more storminess over the Gulf of Alaska, which in turn could finally erode the persistently warm SSTs and the “ridiculously resilient ridge” of high pressure that have prevailed in that area for most of the last two years. If so, a pathway will be carved for the classic El Niño signature of very mild winter temperatures across most of Canada and the northern United States, in line with the latest seasonal forecasts from NOAA. If, instead, we see a third consecutive winter of unusual cold across the U.S. Midwest and Northeast, it’ll be a strong sign that another player is onstage. Judah Cohen (Atmospheric and Environmental Research) bases his North American winter forecasts in part on the apparent relationship between low Arctic sea ice extent and cold Northern Hemisphere winters. “I really do think that this could be a very interesting winter and could be very informative on the interplay of tropical vs. Arctic forcing,” said Cohen in an email. “Can the Arctic, as a forcing agent of mid-latitude weather, finally step out out of the shadow of the tropics or not?”



Figure 5. Both the Arctic and Antarctic rack up more than 12 million square klilometers of sea ice extent each winter, but the summer ice depletion is greater in the Antarctic, where the ice sits at lower latitudes. Image credit: The Cryosphere Today/University of Illinois at Urbana-Champaign.

Why it matters
Although a coating of ice does return to most of the Arctic Ocean each winter, the persistence of ice through the summer is a vital part of the region’s ecosystem. Polar bears, ringed seals, and other wildlife use the ice as a platform for hunting prey and raising their young. Many indigenous residents of the lands circling the Arctic have also relied on the presence of year-round ice for centuries. The picture is far different at the other end of the world: instead of being surrounded by ocean, the South Pole lies at the heart of the landmass of Antarctica. Sea ice in this hemisphere develops on the fringes of Antarctica, which puts it at a lower latitude than most Arctic sea ice. As a result, nearly all of the ice that forms each winter around Antarctica melts back each summer. The average wintertime extent of ice around Antarctica has actually grown slightly in recent years, for reasons not fully understood. This is often falsely presented as “balancing” the loss of Arctic sea ice, but the Arctic loss is far more substantial than the Antarctic gain, and much more important to regional climate, ecology, and economy. Ice-free navigation is now once again possible in the Arctic along the coast of Canada (the southern route of the fabled Northwest Passage), and has been open for over a month along the coast of Russia (the Northeast Passage or Northern Sea Route.) Mariners have been attempting to sail these passages since 1497; the first time they were open for ice-free navigation without an icebreaker was in 2005 for the Northeast Passage and 2007 for the Northwest Passage. The continuing erosion of summer ice cover in the Arctic has stoked interest in expanding industrial activity across the region, including oil and gas development--an ironic turn of events, given the role of fossil-fuel-produced greenhouse gases in the worldwide warming of recent decades.

Tropical Atlantic: Tenacious Fred hangs on
Tropical Storm Fred has been in “never say die” mode, hanging on as a minimal tropical storm on Thursday morning as it drifted across the eastern Atlantic. New thunderstorms blossomed on the east side of Fred’s circulation center on Wednesday night into Thursday, despite stout wind shear of more than 35 mph. By midday Thursday, only a much smaller patch of convection was located just north of Fred’s exposed center. The shear is expected to increase, and the National Hurricane Center expects Fred to become a remnant low by Friday. NHC is mentioning the possibility, though, that Fred could spring back to life in five days, when the storm will encounter lower wind shear and anomalously warm waters of 27.5°C (82°F) over 500 miles southwest of the Azores Islands.

New tropical wave moving off the coast of Africa has potential to develop
A strong tropical wave with plenty of spin and heavy thunderstorm activity will move off the coast of Africa by Thursday night, and has the potential to become a tropical depression early next week as the storm moves west at 15 - 20 mph. The 00Z Thursday (8 pm EDT Wednesday) runs of two of our top three models for predicting tropical cyclone genesis, the GFS and European models, predicted that this new wave would become a tropical depression in the waters southwest of the Cape Verde islands by Monday. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave the wave 2-day and 5-day odds of development of 10% and 40%, respectively. The tropical Atlantic is relatively moist, has the highest sea surface temperatures of the year, and is expected to have low to moderate wind shear, conditions which favor development. The wave should take about 7 - 8 days to make it to the Lesser Antilles Islands.


Figure 6.Typhoon Kilo and Hurricanes Ignacio and Jimena, all captured in this infrared image from the GOES-West satellite at 1330 GMT (9:30 am) Thursday, September 3, 2015. Image credit: NOAA/NHC and Michael Lowry, The Weather Channel.

Pacific continues to bristle with tropical cyclones
The Northeast Pacific has a new named system, Tropical Storm Kevin. As of 11 am EDT Thursday, Kevin’s top sustained winds had increased to 50 mph. Kevin is expected to live out the rest of its life below hurricane strength over open water before increasing amounts of shear and mid-level dry air take their toll. Some moisture associated with Kevin will be working its way into Colorado and New Mexico through Saturday ahead of a large upper-level trough approaching the region, enhancing shower and thunderstorm activity there. In the Central Pacific, powerful Hurricane Jimena is very slowly weakening but remains a high-end Category 2 as it embarks on a broad cyclonic loop well northeast of Hawaii over the next few days. We’ll have to keep an eye on Jimena in the long range, as the recent runs of the GFS and European models bring Jimena back toward Hawaii from the northeast late next week, still as a tropical storm. Such a scenario might be dismissed out of hand in any other year, but with SSTs so warm in the Central and Northeast Pacific, Jimena could conceivably remain over waters at or above the threshold of 26°C (79°F) over most or all of such a trek. Meanwhile, Hurricane Ignacio, now a Category 1 storm north of Hawaii, is also weakening but remains impressively well-structured, with extensive spiral banding. Ignacio is on track to plow into the Gulf of Alaska as a powerful extratropical storm by early next week. En route, Ignacio will pass over unusually warm waters along the way, with sea-surface temperatures up to 3-4°C (5.4-7.2°F) above average (though eventually too cool to support Ignacio as a tropical cyclone).

Ultra-resilient Typhoon Kilo is now in its 14th day as a tropical cyclone, including a long spell as a major hurricane. Kilo is predicted to steadily reintensify over the next 3-4 days, again reaching Category 4 strength by Tuesday as it moves on a westward loop that will likely take it several hundred miles north of Wake Island. In the long range, Kilo may pose a threat to Japan.

We’ll be back with our next update on Friday.

Bob Henson and Jeff Masters