Reflections on the Arctic sea ice minimum: Part I

curryja

12 years ago

by Judith Curry

How should we interpret the record low minimum sea ice extent?

Summary of Observations

Here is the basic story from the observations:

On Aug 26, the Arctic Ocean seasonal minimum sea extent surpassed the previous satellite-era record minimum set in 2007 (4.17 M sq km). The current sea ice extent is below 4 M sq km, and apparently a minimum has not yet been reached, although a minimum around 3.5 M sq km seems likely.

The Arctic sea ice extent undergoes a pronounced annual cycle, with maximum extent of about 14 M sq km in March. In the 1980’s and 199o’s, sea ice minimum extent (in September) averaged about 6.5-7.5 M sq km.

The 2007 minimum ice extent was also associated with a new record in ice volume (which includes ice thickness). Minimum ice volume records were also set in 2010, 2011, and now 2012, with the current ice volume tracking slightly below the 2010/2011 values.

Causes/attribution

For background on this issues, see my previous posts:

There are three main theories for the recent decline in sea ice extent and volume:

I. Greenhouse induced ‘spiral of death’: Mark Serreze and Peter Wadhams are the primary proponents of a rapid spiral of death. The main idea is that melting triggers more melting through the ice albedo feedback mechanism. The evidence for this theory seems based on ‘trendology’ of observed sea ice extent and volume. Spiral of death proponents project faster melting than climate model projections. More moderate versions of this theory rely primarily on climate model projections of sea ice and acknowledge a significant amount of noise from interannual variability.

II. Natural variability only: Joe Bastardi is one of the chief proponents of this theory, see his recent article. This is basically an argument whereby the PDO and AMO set up ocean circulation patterns that are melting the sea ice.

III. Climate shifts hypothesis: JC is a proponent of this (see my previous post) new evolving theory. This hypothesis is based on the network synchronization ideas of Tsonis et al. and bifurcation analysis (e.g. Livina and Lenton). In the particular instance of the past 5 years of low summer sea ice, it seems that the shift in sea ice characteristics was induced by a combination of local factors and hemispheric indices.

Livina and Lenton have a new paper that lays out some of these ideas: A recent bifurcation in Arctic sea ice cover. From the abstract:

Here we show that a new low ice cover state has appeared from 2007 onwards, which is distinct from the normal state of seasonal sea ice variation, suggesting a bifurcation has occurred from one attractor to two. There was no robust early warning signal of critical slowing down prior to this bifurcation, consistent with it representing the appearance of a new ice cover state rather than the loss of stability of the existing state. The new low ice cover state has been sampled predominantly in summer-autumn and seasonal forcing combined with internal climate variability are likely responsible for triggering recent transitions between the two ice cover states. However, all early warning indicators show destabilization of the summer-autumn sea-ice since 2007. This suggests the new low ice cover state may be a transient feature and further abrupt changes in summer-autumn Arctic sea-ice cover could lie ahead; either reversion to the normal state or a yet larger ice loss.

Mike Wallace combines the regime shift idea with the spiral of death theory:

I view the question of whether the minimum sea ice extent sets a new record this year as secondary. The important news is that in five summers the sea ice extent over the Arctic Ocean has not recovered significantly from its precipitous decline in 2007. This is one of the clearest examples of what appears to be a regime shift in the recent historical record. I think we still need to be open to the possibility that natural variability has played a role in the recent warming of the Arctic and that the summer ice could come back, but with each year that goes by without a return to the pre-2007 summertime Arctic climatology it seems a bit more likely that the remarkable change that we have witnessed will prove to be irreversible on a human time scale.

Attribution bottom line

So . . . what is the bottom line on the attribution of the recent sea ice melt?

From a new paper by Stroeve et al.: CMIP5 suggests 60% of 1979-2011 rate of decline is externally forced.

Ron Kwok has attempted an observationally based attribution for multi year ice in the Beaufort Sea. An interesting regional study, which it would be nice to see extended.

JC’s attribution assessment: likely (>66% likelihood) 50-50 split between natural variability and anthropogenic forcing, with +/-20% range.

Analysis of climate dynamics and sea ice physical processes

For some background of the relevant climate dynamics, see my previous post:

Pondering the Arctic Ocean. Part I: Climate dynamics. My related publications (slightly outdated) can be found here.

Walt Meier provides a basic explanation at WUWT of the interplay between temperature, winds, and ice processes.

ARCUS provides a discussion in July on the set up of the weather patterns and ice conditions for the 2012 season.

The following factors impact the sea ice fate during the melt season:

Thickness and compactness of sea ice at the beginning of the melt season: ice that starts out thinner is more easily melted away. Further, first year ice has different optical and thermodynamic characteristics than multi-year ice.
Transport of ice through the Fram Strait (between Greenland and Europe), which depends on a combination of atmospheric and ocean circulation patterns
Weather patterns that act to either break up or consolidate the ice
Radiative forcing (which is dominated by the cloud patterns)
Melting from below by warm ocean currents.
Melting from above by warm atmospheric temperatures.
Geographic distribution of the sea ice, which depends on a combination of all of the above

And all this is complicated by the fact that the minimum sea ice extent in an individual season doesn’t simply reflect that season’s weather processes, but also reflects the decadal history of sea ice characteristics, sea ice export and atmospheric and oceanic circulation patterns. And the sea ice extent itself influences the atmospheric and oceanic circulation patterns. Hence, the sea ice characteristics tend to be out of equilibrium with the thermal forcing in a particular season.

Here’s the basic story as I see it (refer to this diagram). During the late 1980s and early 1990s, the circulation patterns favored the motion of older, thicker sea ice out of the Arctic. This set the stage for the general decline in Arctic sea ice extent starting in the 1990’s. In 2001/2002, a hemispheric shift in the teleconnection indices occurred (Tsonis et al.), which accelerated the downward trend. A local regime shift occurred in the Arctic during 2007, triggered by summertime weather patterns conspired to warm and melt the sea ice. The loss of multi-year ice during 2007 has resulted in all the minima since then being well below normal, with a high amplitude seasonal cycle. After 2007, there was another step loss in ice volume in 2010.

In 2012, the basic pattern of this new regime was given a ‘kick’ by a large cyclonic storm in early August. This storm acted to break up the already fragile ice, making it easier to melt. Further, the storm almost certainly induced substantial extra mixing in the Arctic Ocean, possibly bringing up some of the warmer water from the halocline layer (I haven’t seen anything written on this, let me know if you’ve spotted anything). The Arctic Ocean surface waters are warmer than normal for this time of the year, which is slowing down the autumnal freeze up.

So, what is the contribution of AGW to all this? Its hard to separate it out. The polar regions are extra sensitive to CO2 forcing and water vapor feedback, owing the low amounts of water vapor. However, any radiative forcing from greenhouse gases is swamped by interrannual variability in cloud radiative forcing. In the bigger picture sense, greenhouse forcing is involved in complex nonlinear ways with the climate regime shifts. So there is undoubtedly a contribution from CO2 forcing, but it is difficult to find any particular signal in this year’s record minimum, other than the contribution of greenhouse warming to a longer term trend. In the overall scheme of what is going on with the sea ice, I think 2007 was the most significant event, followed by 2010. The big event in 2012 was the cyclonic storm, and the impact on ocean mixing may turn out to be more significant than the sea ice minimum.

Part II: I hope to have Part II posted tomorrow, which will address prospects for an ice free arctic, possible mechanisms for a recovery of the sea ice, and whether it ‘matters’ to the global and regional climates.

Share this: