As the persistent Arctic cyclone - or PAC-2013 - of the past couple of weeks winds down, I want to discuss what I've found on the subject in a couple of research papers. But first want to refer to two excellent blog posts from last week doing just that, on the Robertscribbler blog and FishOutofWater's blog on Daily Kos.
This animation of DMI SLP maps shows the birth and demise of PAC-2013:
Perhaps I should say near-demise, but the weather forecast says it's basically over for this cyclone. Of course, other cyclones come into play in other parts of the Arctic, but it's not the PAC-2013 anymore. After approximately 25 days of wandering around, weakening and re-strengthening to a pretty powerful cyclone at times, churning ice, keeping the Arctic cold, we've witnessed another remarkable cyclone in less than 1 year's time.
There are a couple of questions that have been on my mind ever since the cyclone started to show itself persistent:
- Is PAC-2013 unprecedented?
- Is it somehow caused by the progressive loss of Arctic sea ice?
- If so, will we see these PACs more often?
- What will the effect on the sea ice be, short-term and long-term?
To answer some of these questions I did a search on Google Scholar with summer arctic cyclone as search words. Because the Arctic is changing so fast, I was mostly interested in recent research, but I remembered there was one paper from 2008 that I read last year during GAC-2012, that also has interesting things to say about Arctic summer cyclones. So, I'll start off with that one and quote the most interesting parts.
The Summer Cyclone Maximum over the Central Arctic Ocean
Mark C. Serreze and Andrew P. Barrett, 2008, Journal of Climate
From the abstract:
A fascinating feature of the northern high-latitude circulation is a prominent summer maximum in cyclone activity over the Arctic Ocean, centered near the North Pole in the long-term mean. This pattern is associated with the influx of lows generated over the Eurasian continent and cyclogenesis over the Arctic Ocean itself. Its seasonal onset is linked to the following: an eastward shift in the Urals trough, migration of the 500-hPa vortex core to near the pole, and development of a separate region of high-latitude baroclinicity. The latter two features are consistent with differential atmospheric heating between the Arctic Ocean and snow-free land. Variability in the strength of the cyclone pattern can be broadly linked to the phase of the summer northern annular mode [also known as Arctic Oscillation, N.].
Figure 2 from the paper shows the annual cycle of number of cyclone centers within a specified region of the central Arctic Ocean in the 1958–2005 period:
There's now plenty of snow-free land available in late spring and early summer (see these long-term bar graphs for May and June), and this almost certainly played a role in last year's GAC and this year's PAC. The thing I learned from this paper was the part about how cyclones get born in eastern Eurasia, then move eastwards and northwards and either make it to the Arctic or not, where they either move over to the central Arctic or feed an existing cyclone there. That's something I'm on the look-out for nowadays when checking weather forecast maps.
Here's what they say about that further in the paper:
At least some of the systems generated over eastern Eurasia migrate into the central Arctic Ocean and contribute to the summer cyclone maximum, while those formed over northern Alaska instead tend to track eastward or southeastward into the Canadian Arctic Archipelago (Serreze et al. 2001).
We show that while the summer pattern is, in part, associated with the influx of lows generated along northeastern Eurasia, where the Arctic frontal zone is especially well expressed, the broader picture involves an eastward shift in the Urals trough and migration of the 500-hPa vortex core to near the pole, associated with the influx of systems generated along a wide swath of the Eurasian continent, augmented by cyclogenesis within the Arctic Ocean itself.
Serreze et al. also describe their case study of June 1989:
At 500 hPa, a closed low persisted over the Arctic Ocean for the entire month, meandering about the region. On a daily basis, it was typically highly asymmetric, and the surface lows developed and deepened ahead of the troughs.
Another paper by Tanaka et al. (2012, PDF) also has two examples of persistent cyclones:
The cyclone in Case 1 was strongest at 12Z on 19 July 2006 and persisted for 28-days staying in the middle of Arctic Ocean.
The cyclone in Case 3 was strongest at 06Z on 22 June 2008 and persisted for 20 days. These cyclones move rather randomly in direction for sufficiently longer period, indicating the characteristics of the arctic cyclone, which is distinctly different from the movement of the mid-latitude cyclones or tropical cyclones.
This answers question 1 above: PAC-2013 isn't unprecedented.
But the conclusion of Serreze and Barrett (2008) echoes questions 2 and 3:
While there have been no trends in the strength or persistence of the summer cyclone pattern over the period of 1958–2005, it is natural to speculate on its future behavior. Climate models are in near-universal agreement that Arctic warming in response to greenhouse gas loading will be especially strong. Results from the present study suggest that, at least in part, the summer cyclone pattern owes its existence to differential atmospheric heating between the Arctic Ocean and snowfree land. If patterns of differential heating change substantially, such as through earlier springtime loss of snow cover over land, or through changes in the presently strong summer net surface heat flux over the Arctic Ocean as the sea ice cover disappears, this may invoke changes in the summer circulation.
It takes a while for changes to become apparent statistically, but we might well have entered the period of changes some time back.
The following paper discusses the effects of cyclonic activity:
Dramatic interannual changes of perennial Arctic sea ice linked to abnormal summer storm activity
James A. Screen, Ian Simmonds and Kevin Keay, 2011, Journal of Geophysical Research
From the introduction:
The perennial (September) Arctic sea ice cover exhibits large interannual variability, with changes of over a million square kilometers from one year to the next. Here we explore the role of changes in Arctic cyclone activity, and related factors, in driving these pronounced year‐to‐year changes in perennial sea ice cover. Strong relationships are revealed between the September sea ice changes and the number of cyclones in the preceding late spring and early summer. In particular, fewer cyclones over the central Arctic Ocean during the months of May, June, and July appear to favor a low sea ice area at the end of the melt season. Years with large losses of sea ice are characterized by abnormal cyclone distributions and tracks: they lack the normal maximum in cyclone activity over the central Arctic Ocean, and cyclones that track from Eurasia into the central Arctic are largely absent.
Figure 3 shows the difference in total cyclones for ice loss years and ice gain years (in relation to the preceding melting season) during May, June and July:
I can confirm what the bolded text says, as it's something I learned quite quickly in the preceding melting seasons that were covered here on the Arctic Sea Ice Blog. But like I said last year: "I feel as if all I've learned in the past two years has already turned obsolete". This quote, BTW, was used by The Guardian columnist George Monbiot, where he actually called me a climate scientist.:-P
I'm not sure if the relation still holds true today (see question 4 above), as the ice is thinner than it has been probably since the Holocene Climatic Optimum. As we saw last year, weather conditions that weren't conducive to ice cover decrease didn't really matter all that much for the rate of decrease, and well before GAC-2012 struck, the trend lines on extent and area graphs were already in record territory, or headed there. This could be due to the good start to the melting season 2012 had, but the overall thinness of the sea ice definitely played a role as well.
Screen et al. say as much in the conclusion:
Our results also suggest that a strengthening of the central Arctic cyclone maximum during MJJ helps preserve the ice cover and leads to anomalously high September sea ice. However, the relationship does not appear to be entirely linear with a clearer association between low cyclone activity and reduced sea ice than between high cyclone activity and increased sea ice.
There is no doubt that dominating high pressure systems guarantee big ice losses, as they keep skies clear (leading to high insolation) and kick the Beaufort Gyre into action, leading to ice pack compaction and increased ice transport towards Fram Strait. It could very well be that the question whether increased cyclonic activity during late spring and early summer leads to a higher minimum or not, depends on the overall thickness and state of the ice pack. Does it slow the car down or does it make it go in reverse?
Another interesting tidbit looking forward from the conclusion:
We found no significant trends in late spring or summer Arctic cyclone frequency over the period 1979–2009, and neither did Simmonds et al.  over a similar period but with different data sets. Serreze and Barrett  also found no significant trend in summer cyclone occurrence over the central Arctic during the longer period 1958–2005. Therefore, it seems unlikely that multidecadal changes in cyclone activity are a primary cause of the observed decline in perennial sea ice. However, this does not imply that future changes in Arctic storm activity will be peripheral to determining when, if as projected, a seasonally ice‐free Arctic Ocean is realized.
Again, we touch upon question 4: What will the effect of (increased) cyclone activity on the sea ice be, short-term and long-term?
For that we turn to a recent paper that looks into the effects of cyclones:
Cyclone impact on sea ice in the central Arctic Ocean
A. Kriegsmann and B. Brümmer, 2013, The Cryosphere Discuss.
From the abstract:
In general, cyclones reduce the ice concentration on the order of a few percent increasing towards the cyclone centre. This is confirmed by independent AMSR-E satellite data. The reduction increases with cyclone intensity and is most pronounced in summer and on the Siberian side of the Arctic Ocean. For the Arctic ice cover the impact of cyclones has climatologic consequences. In winter, the cyclone-induced openings refreeze so that the ice mass is increased. In summer, the openings remain open and the ice melt is accelerated via the positive albedo feedback. Strong summer storms on the Siberian side of the Arctic Ocean may have been important reasons for the recent ice extent minima in 2007 and 2012.
Figure 1 shows the study area and spatial and seasonal distribution of the cyclone detections (dots) for the period 2006–2008 (click for a larger version):
This is a very interesting, though slightly technical, research paper. 7987 cyclones in the 2006-2008 period are analyzed with the help of a model, and categorized according to intensity, season and region. Here are the interesting tidbits:
The dynamic impact of cyclones has substantial impacts on the sea ice cover. The strong, inhomogeneous wind field deforms the ice cover and causes cracks, leads and polynias. The thermodynamic effect of these openings is different in winter and summer. In winter, the openings are the places with the largest heat fluxes from the ocean to the atmosphere. The openings refreeze so that the heat fluxes decrease with time and salt is released from the newly formed ice to the ocean. In summer, refreezing does not occur, so that almost all downwelling shortwave radiation is absorbed. This leads to a local warming of the uppermost ocean layer and promotes further ice melt.
On average, each cyclone has 5.05 detections, i.e. a lifetime of about 30 h [in the 2006-2008 period, N.].
On average, the wind is weaker in summer than in autumn and winter. However, the differences are not large: the maximum is 6.2ms−1 in summer and 6.8ms−1 in winter.
The general impact of cyclones on sea ice concentration consists of a 24 h-reduction within the radius of the cyclone in all seasons (Fig. 10). The reduction is most pronounced in spring and summer. In the core of the cyclone the ice concentration is reduced within 24 h by 0.92% in winter, 1.67% in spring, 2.53% in summer and 1.17% in autumn.
The seasonal difference in the duration of a cyclone’s impact can have extensive climatological consequences. A lot of studies were done on the change of cyclone activities. Some of them found an increase of the number of cyclones in the Arctic (e.g. Sepp and Jaagus, 2011). Zhang et al. (2004) found, that the number of summer cyclones increases. If the number of cyclones in the summer increases, than the melting of sea ice can be accelerated.
And from the conclusion:
On the short time scale of a passing cyclone, freezing and melting play a minor role in the change of ice concentration. Thus, the cyclone-induced reduction of ice concentration is almost solely due to ice sheet deformation. This means that there is no loss of ice mass but that the ice is ridged. Thus, the cyclone causes more thick ice. The following processes, freezing and melting, have different longer-term or even climatologic consequences in winter and summer. In winter, the heat flux between ocean and atmosphere over the cyclone-induced open water areas is increased for a few days. This heats and moistens the shallow Arctic boundary layer. At the same time the freezing of the open water areas leads to the formation of new sea ice, so that a further important impact of the wintertime cyclones is an increase of the Arctic ice mass. In summer, the cyclone-induced reduction of sea ice concentration is largest and the open water areas remain open. This summertime impact is expected to be especially large in areas with thinner ice as, e.g. on the Siberian side and in the marginal ice areas of the Arctic Ocean. Strong summer storms in those areas can lead to increased exceptional reduction of the sea ice concentration. The strong summer cyclone between 4 and 8 August 2012 is believed to be one reason for the following record ice extent minimum in September 2012 (e.g. Beitsch, 2012).
One thing we know for sure and that's that there's a lot of first-year ice in the Arctic right now. Let's return to our 4 questions at the top of the blog post, with summarized answers:
1. Is PAC-2013 unprecedented?
No, but its effect on the sea ice might be. Previous PACs had substantially more ice below them to withstand the pulling forces of the cyclone.
2. Is it somehow caused by the progressive loss of Arctic sea ice?
It seems that decreased Arctic sea ice area and volume, combined with a radical decrease in spring and summer snow cover on the Northern Hemisphere, play a role in cyclogenesis (love that word) and cyclonic behaviour.
3. If so, will we see these PACs more often?
Could be, it's too early to tell. We had GAC-2012 last year, and PAC-2013 this year. We'll see what happens next. Or to paraphrase the historic quote by Dr Jennifer Francis: How could we not?
4. What will the effect on the sea ice be, short-term and long-term?
We still have to find out what exactly the short-term effect of PAC-2013 will be. Either the clouds and cold that the cyclone brought in, offset the churning and dispersing of the ice pack, and equalling last year's record smashing melting season is virtually impossible. Or the cyclone pre-conditioned the ice pack by causing many holes to show up from Svalbard to the New Siberian Islands, and heavy ice losses will become visible as soon as the weather switches (or even if the weather doesn't switch, like we saw last year in July).
For the long-term: If this cyclonic activity early in the melting season helps conserve the ice, even though it's thinner, AND cyclonic activity increases because of Arctic sea ice and NH snow cover loss, then we have a serious negative feedback that could postpone the occurrence of a sea ice-free Arctic.
If it this isn't the case, we have a slightly better set of conditions compared to anti-cyclonic dominance (high pressure systems with clear skies and Beaufort Gyre), but it doesn't really matter in the longer term as it doesn't help the ice pack recover.
This year might give us some clues. And so we learn...