(Edited on June 18 due to the availability of new information. I also want to promote two excellent blog posts related to this subject. One, by Neven, over at the Arctic Sea Ice Blog, takes a closer look at the science of Perstistent Arctic Cyclones, the other, posted on the Daily Kos by FishOutOfWater, links PAC 2013 to the collapse of the polar vortex this past winter.)
Yesterday, two commenters — Sourabh in this blog post and T.O.O. in this blog post — raised some very salient questions about 2013’s Persistent Arctic Cyclone (PAC). These commenters wanted to know how critical to melting is PAC 2013, is this the first time we’re witnessing thinning of the central ice due to a long-period Persistent Arctic Cyclone, and by ‘what conditions could the central ice be expelled from the Arctic Basin?’
Long Duration Summer Cyclones Rare, But Not Unheard of
First, let’s take a look at the current PAC 2013, its forecast duration, and how it compares to other storms. For context, it is important to note that most cyclones in the Arctic basin last for 40 hours or less. By comparison, PAC 2013 began on about May 26th and has remained in the Arctic for about 16 days. Forecasts now show the storm persisting until at least June 21rst. If the storm lasts this long, it will have remained in the Arctic for 26 days.
Another comparison can be seen in the Great Arctic Cyclone of 2012 (GAC). This storm was the 13th most powerful storm ever to impact the Arctic in the modern record. It lasted for about two weeks and reached a minimum central pressure of about 966 mb. The current PAC 2013, while lasting longer (and projected to last much longer) than GAC 2012, reached a lowest central pressure of around 975 mb while averaging in a range of 985 – 995 mb.
It is worth noting that Arctic cyclones are a year-round phenomena. And that more numerous, though somewhat weaker storms, have been noted to appear from May to July. That said, the strongest, longest duration storms usually occur during winter and can last for three weeks to a month or more. During summer, Arctic cyclones are weaker, pack less of a punch, and usually don’t last as long as winter storms. What makes PAC 2013 and GAC 2012 exceptional is the fact that they were both strong, long duration storms occurring during summertime and that they occurred under conditions of record thin Arctic sea ice.
There is some research to show that the strength of summer Arctic cyclones has been increasing since the late 1970s. These researchers show that increasing levels of moisture and higher temperatures around the Arctic during summer time have added fuel to the formation of new storms. Weather records do show the strength of the most powerful summer storms generally increasing with time.
Overall, PAC 2013, though somewhat weaker than GAC 2012 at peak strength, is projected to remain in the Arctic for a very long time. And with lowest pressures rivaling that of a moderate-strength tropical cyclone, it should continue to have substantial impacts — both to Arctic weather and to sea ice.
New Event: Storms that Melt Sea Ice
The Great Arctic Cyclone of 2012 was also unique because it was the first storm to have a major impact on Arctic sea ice. Though researchers have tended to disagree over how pivotal the storm was in reducing ice to the record low values achieved during 2012, it is generally accepted that the storm melted at least 250,000 square kilometers of sea ice during early August.
The storm achieved this feat by mixing the surface ice with warmer waters lying just beneath. Wave action and cyclonic pumping of warmer waters from the depths provided a powerful force for thinning and melting the surface ice. Though no direct research on sea ice volume losses due to GAC 2012 has been published, CICE images before and after the event speak to a major thinning as a result of the 2012 Cyclone.
(Images From: US Navy)
Note the large areas of ice thinned into naught by the storm as it plowed through the East Siberian, Chukchi, and Beaufort Seas. A region of central ice was also noticeably thinned during the storm.
We can, therefore, say with some confidence that it is the ice thinning forces of the storm which caused the loss of 250,000 square kilometers of sea ice attributed to its impacts. But we can also say that a visible and, as yet, undetermined volume of ice was also lost and that this loss substantially contributed to 2012’s record low status.
A similar situation is now present during PAC 2013. Substantial thinning is now visible in all the sea ice monitors, especially on the Russian side of the North Pole. But this event is different from GAC 2012 in that is occurring during June, a period of time in which the ice is thicker and more resilient. A period of time when air and water temperatures are relatively cooler. As a result, no where near as much in the way of sea ice area losses can yet be attributed to PAC 2013. I say ‘yet’ because this storm appears determined to stick around for the long haul. So we may see major area losses arise as a result of its action.
In any case, it is worth looking at before and after thickness maps to determine the level of damage caused by PAC 2013.
(Images From: US Navy)
As is plainly visible from the above set of images, PAC 2013 has dramatically hollowed out the central sea ice. With at least another ten days of duration expected, we are still just in the preliminary phase of impacts. These should ramp higher as the days continue to progress. (Note, the last image was added on June 18th, two days prior to a possible cessation of the storm).
Both PAC 2013 and GAC 2012 are new events for the reason that they result in melt and weakening of sea ice. This is unprecedented because past storms did not generate measurable losses in summer (You can look at some of this research here and here). To the contrary, it was thought that the cloudier, cooler storms were generally protective. And this was true in a cooler climate. Now, strong storms have a potential to result in losses. And this new feature is an environmental condition brought about by human-caused climate change.
Is This the First Time We’re Witnessing A Summer Cyclone Thin the Central Ice in June?
Now that we have a little background on summer cyclones and how climate change has enabled them to both significantly thin and melt ice, we can confidently answer the question: is this the first time we’re witnessing a summer cyclone thin the central ice in June?
The short answer to this question is: yes.
In the satellite record, there is no precedent for a June storm melting and weakening the ice in the past. Though June storms have impacted and fragmented the ice before PAC 2013, this storm is the first powerful, long-duration event to have such a large, measurable melt effect in early summer. As noted above, past storms were thought to be defensive, resulting in a more resilient ice pack and less melt, overall, come end of summer.
In part, such widespread damage is due to the fact that the area currently influenced by the storm is so large — covering all of the Central Arctic. The other reason is the fact that the ice in this key region is supposed to be the most resilient to late summer losses. Instead, in early summer, we see damage and erosion.
Were the storm to end now, it would leave the central ice thinner and weaker to the assaults of late summer. But the storm hasn’t ended. It continues to churn and thin the ice even as temperatures rise.
It is possible that, if this storm lasts long enough, remains strong enough, and pulls in enough warm air, it could produce a large region of open water at the very center of the ice pack even as it shoves a large portion of the thickest ice toward the Fram Strait. Such an event would not only be unprecedented. It could be catastrophic.
Under What Conditions Could the Central Ice Be Expelled From the Fram Strait? Short Answer: Persistent, Warm Storm
So now we’ll address the nightmare scenario for this particular event. This expose is by no way a prediction. It is just an illustration of what the worst case, in this event, could look like. It is also, by no means, the only way we could lose all or most of the central ice. The ice, for example, could melt out under a sustained assault from the sun. The central ice could take a hit from a swift, powerful storm, then melt as warm air and sunlight moves in behind it. We won’t examine these and other cases. Instead, we’ll take a look at the worst case in the event of a long-lasting Persistent Arctic Cyclone that warms and churns throughout a good portion of summer.
The event could look something like this:
The Persistent Arctic Cyclone that emerged in late May continues on through June and into July. As the Arctic warms, more above freezing temperatures get wrapped into the storm. Eventually, much of the region it covers warms to a range of 0-6 degrees Celsius. Rain becomes a primary form of precipitation in the storm.
The added moisture, warmer cloud cover, and above freezing precipitation create a constant surface stress to the ice. Underneath, the constant churning pushes water temperatures above freezing due to an ongoing mixing of the cold surface layer with deeper, warmer waters. The combined result is an ice melting and thinning machine. By the end of June, a growing region of open water (concentration 20% and less) has emerged.
The open water is a breeding ground for powerful waves and a magnet for sunlight streaming down through periodic breaks in the clouds. This region of warmer water thrashes and bores through the ice as July advances, creating a pheonomena never before seen in the Arctic — a large, central region of open water surrounded by thinning ice. The result is ice edge melt occurring at the same time as central ice melt. From the cored out portion, an arm of open and or nearly open water begins to sweep around the Arctic, clearing away ice in its path. The arm extends to weakest areas of sea ice. A most likely candidate for this arm’s development is the Laptev Sea as there ice there has been weakest since start of melt.
These three factors would be devastating enough. But a fourth factor provides the coupe de grace: Fram Strait export.
The constant counter-clockwise motion of our warm storm has been shoving at the remaining thick ice anchored on Greenland and the Canadian Arctic Archipelago since early June. Increasingly, large volumes of thick ice are flushed out the Fram Strait. By end of June, as much as 10% of the thick ice has been exported. But this is just the beginning.
During late June and early-to-mid July, warm air invasions from the south have melted and thinned the Canadian Arctic Archipelago ice. Now thinning and fractures from this warming have advanced into the thick ice, weakening its anchor. During July, there is less resistance to the storm’s counter-clockwise motion so more and more thick ice ends up meeting its end through the Fram Straight.
By early August, the storm has lasted for an excessive period — nearly seventy days. But it still churns on, fed by an endless procession of storms and injections of warm, moist air from the south. In a final explosion of weather never before seen in the Arctic, much of the remaining thick ice is ejected, melted, or churned beneath a storm-tossed Arctic Ocean. By early September, the storm finally disperses, but little or no ice remains.
The above ‘Warm Storm’ scenario is pure speculation. We have no reason to believe the current PAC 2013 will last so long or will have such powerful effects. More likely, a still damaging but more moderate erosion of central sea ice combined with an enhanced Fram Straight export will occur. Should the storm last until the end of June, these damaging impacts will be more than enough to weaken the ice.
That said, should the storm last longer, then we will have an altogether unwelcome opportunity to test this ‘Warm Storm’ theory.
So we come at last, to answering the first question of our commenters:
How critical to melting is PAC 2013?
And the answer to this question will depend on the duration of the storm, its relative strength over time, how much warm air is injected into it over the course of its life-span, and how much warm water it is able to dredge up from beneath it. At the very least it has already played a major part in early season melt. Should it last for a long duration, the story of 2013 melt may well become wrapped up into that of this particularly anomalous storm.