With Arctic sea ice reaching record losses of 55% in area and 80% in volume since 1979, and with a growing risk that the Arctic will experience ice-free conditions during summer before 2020, much attention has recently turned to the sea ice.
Of primary concern is how ice melts during summer. This is a complex issue because the ice is now so fragile that a number of forces are now able to act upon it. In this blog, I’ll do my best to summarize the primary conditions that result in ice loss during summer time given the its currently fragile state.
In summary, there are four powerful forces now able to impact the ice. They are insolation (heating by the sun) during warm, clear periods, warm, wet winds, during influxes of warmer air from the south, warm fog when warm air gets trapped in the Arctic and forms a snow and ice eating fog, and summer storm.
These three conditions are examined in more detail for you below:
Conditions required: Usually still (less than 15 knot winds), warm air (5+ degrees Celsius), clear skies.
Under these conditions insolation is a powerful force to melt the ice because they enable the sun’s energy to be directly absorbed by large expanses of ice and the underlying ocean. Melt ponds, which are darker in color so they absorb more heat (reducing ice albedo), multiply over the ice. These ponds act as a lens, focusing the sun’s energy downward into and through the ice. This both heats the ice and the water underneath.
If large stretches of ice heat up under these conditions, the amount of energy transferred into the ice and the waters below is enormous. An example of insolation is the currently ongoing melt in the Northwest Passage. A description of this event is available here.
Chris Reynolds has also provided some very detailed explanations of why insolation could have a serious impact on the current sea ice as most of the ice is less than 2 meters thick, rendering it very vulnerable to this kind of melt. His explanations and analysis are certainly worth reading.
2.Warm, Wet Winds
Conditions: Warm (2+ degrees Celsius), Windy (15 Knots +), Cloudy, Moist (Humidity 85% +), may include rain and blowing fog.
These conditions usually happen when you get a punch of warm air from the south via an eroded jet stream. A bulge in the jet stream expands northward, carrying with it a pulse of warmer air. As the air interacts with the cold Arctic, clouds develop. Warm air carries more moisture, so this humidity can also form clouds, fogs, and even rain.
Moisture carries a huge amount of energy. This warmer, moister air can eat away at the ice where it contacts the air. Instances of ‘snow eating fog’ are a common event in northern locations. The warm, wet wind operates under the same principle. Warm, moisture-laden air eats through the ice.
This kind of event happened recently when Scandinavia warmed up last week (description here, and here) then some of the warmer air rushed into the Arctic to feed our storm and warm the Central Arctic. Directly in the line of fire was the thin ice just off Svalbard. As the warmer, wet, air rushed up and over the ice, it melted much of it. You can see the new gap formed in the ice off Svalbard in the image below.
(Image source: Lance-Modis)
Svalbard is in the bottom-center portion of the image. A region northeast of Svalbard was mostly covered with thin ice until recently when warm, wet winds melted much of it. Also note the thin and broken state of the ice directly to the north and west of where the wind blew in.
3. Warm Fog
Conditions: Warm (2+ degrees Celsius), still air (little or no wind), high humidity (near 100%), fog.
A warm fog is a powerful force to melt ice because it carries more energy than drier air, can persist for extended periods, and can feed off the ice as it melts through sublimation and via contact with a warmer ocean surface. Under such conditions, periods of warm fog persist for a day or more, eating away at the ice. Since the Arctic has been very dynamic recently, the conditions for warm fog formation have been mostly visible at the ice edge or in areas of recently ruptured central ice. Should such conditions arise over a broad region, I’ll dedicate a post to it in order to provide you with further explanation.
4. Summer Storm
Conditions: Low pressure (995mb or less). Windy (large wind fields of 25+ knots). Air that is not conducive for large-scale freezing during storm conditions (-5 degrees Celsius +).
Storms are tricky. One reason is that Arctic storms are cold-core. Because of this, they tend to pull a bit of colder air along with them even as warmer air tends to rush in behind and around them. The colder, cloudy conditions at center have led many to discount their melting effects.
That said, storms can invoke a range of powerful natural forces to the melt the thin layer of ice floating on the sea surface. The reason is that wind from the storm has the effect of churning and turning the ocean surface. This mixing breaks up the Cold Halocline layer, a layer of cold water that protects the sea ice. This breaking of the Cold Halocline layer reduces the ice’s resilience to further melting even as it thins the ice during the storm event.
In addition, the cyclonic action of the storm acts as a pump, sucking up water from warmer layers beneath the now fragmented Cold Halocline. All this pumping and churning can cause an overall thinning and melting of ice beneath the storms, even though air temperatures may remain just below the -2 degree Celsius freezing point of sea water. In such events, the powerful action of storm energy on the ocean surface and depths can trigger a dramatic thinning and melt of even relatively thick ice.
Arctic summer storms also tend to draw warmer air in behind them. If temperatures are consistently above freezing, this influx of warm air during and after storm events can further hasten ice melt.
Should storm temperatures rise above freezing, rain events within the storm also have a powerful effect on surface ice, covering the ice with a warmer layer of water that further eats and erodes it.
It is through these mechanisms that the current Persistent Arctic Storm of 2013 (PAS 2013), has eroded and thinned the central ice. You can view this action in the US Navy CICE/HYCOM model run below:
(Image source: US Navy)
Note the jostling and thinning of the central ice in the series of images below. These impacts are a direct result of the Persistent Arctic Storm that has remained in the Central Arctic for two weeks running. In total, many regions lost as much as a meter of sea ice thickness as a result.
Warming Air, Water, Thinning Ice
Overall, there are three factors that amplify the effects of these four melt processes. They involve thinner ice, warmer ocean temperatures (both surface layer and deeper waters), and warmer air. To greater degrees, as the ice thins and as Arctic air and ocean temperatures rise, insolation, warm, wet winds, warm fog, and summer storms will further melt and weaken the ice.