Remarkable Change in Arctic Atmospheric Circulation: Have We Passed a Tipping Point?

Anybody who is paying the slightest bit of attention knows that Arctic summer sea ice is melting at a record pace because of warming of the atmosphere and the ocean. It is also accelerating because of feedback loops -- there's less ice to reflect solar radiation back into space and more dark water to absorb it. That means further warming and even more melting of the ice.

Now scientists have found another mechanism that is speeding the melting of the arctic: atmospheric circulation. A recent paper published in Geophysical Research Letters, suggests that since 2001 a significant change has taken place in atmospheric circulation patterns during the Arctic freezing season. Whereas previously the Arctic winds travelled East-West, they are now cycling North-South and allowing warm air further north than ever before. The change helps to account for unusually high arctic surface air temperatures and increased sea-ice melt during the following summer, and points to further evidence that a tipping point has been crossed irrevocably.

Here's how the authors report their findings in their study Recent radical shifts of atmospheric circulations and rapid changes in Arctic climate system:

The Arctic climate system change has conspicuously switched onto a fast track since the beginning of the 21st century. In particular, an extreme sea-ice coverage loss occurred in summer 2007 [e.g., Comiso et al., 2008]. Although these changes have been largely attributed to the greenhouse-gas-emissions-induced radiative forcing, the atmospheric circulation is the route by which global-warming-forcing exerts dynamic effects by driving sea-ice motions and exports, ocean currents and heat transport. The atmospheric circulation also determines formation and distribution of cloudiness as well as critically modulating surface radiative heat budgets. Accordingly, substantial Arctic climate system changes have been tightly associated, under conditions of global warming forcing, with the positively-polarized trend of the atmospheric circulation leading pattern, the Arctic/North Atlantic Oscillation (AO/NAO) [e.g., Rigor et al., 2002; Zhang et al., 2003].

However, the AO/NAO has gone to neutral in the latest decade, clearly shifting away from the fast track of the changes, and unexpected from previous studies and global-warming-forced climate simulations [e.g., Thompson and Wallace, 1998; Osborn, 2004]. The driving role of the AO/NAO trend in underlying cryospheric, hydrospheric, and terrestrial subsystem changes has been substantially weakened [e.g., Maslanik et al., 2007]. Therefore, the rapid climate change signature in atmospheric circulations and its connection to other subsystem changes remain unclear.

What this means is that the AO/NAO pattern -- the atmospheric circulation that has until recently governed the Arctic climate systm -- is no longer driving atmospheric circulation in the Arctic. The researchers use a complex statistical model to show that the AO/NAO has been replaced by a different atmospheric circulation pattern that they call the Arctic Rapid change Pattern (ARP). In their study, they examined overlapping 5 year windows beginning with the winter of 1986-87 and continuing up through 2007. They found that a tri-polar pattern has given way to a di-polar pattern:

We propose that this new atmospheric circulation leading pattern – the Arctic Rapid change Pattern (ARP) – represents an unprecedented climate change signature, and plays a decisive role in driving recent rapid Arctic climate change.

Here's how Der Spiegel explained it:

Winter in the Arctic has long been determined by what researchers refer to as a “tri-polar” pattern. The interaction among the Icelandic Low, the Azores High and the subtropical high in the Pacific led to primarily east-west winds, a pattern which effectively blocked warmer air from moving northward into the Arctic region. [emphasis--JR]

But since the beginning of the decade, the patterns have changed. Now, a “dipolar” (bipolar) pattern has developed in which a high pressure system over Canada and a low pressure system over Siberia have the say. The result has been that Arctic (sic) winds now blow north-south, meaning that warmer air from the south has no problem making its way into the Arctic region. [emphasis--JR] “It’s like a short-circuit,” says Rüdiger Gerdes, a scientist at the Alfred Webener Institute for Polar and Marine Research and one of the five authors of the study.

The phenomenon occurs during the Arctic freezing season, but it has an effect on the following summer's sea-ice melt. According to the researchers, the Surface Air Temperature "anomaly" or difference reached 12.0°C in the 2005/06 winter relative to the 1958-97 average. This heightened temperature anomaly during the Arctic freezing season has continued according to the National Snow and Ice Data Center.

Air temperatures over the Arctic Ocean stayed warm through November, partly because of continued ocean-to-atmosphere heat transfer. However, some of the warmest anomalies were located well north of the open water areas seen in September. This regional pattern of warming points to the strong role of atmospheric circulation, pumping warm air into the region from the south.
[emphasis--JR]

The differences between sea-ice melt in 2006 when the ARP was in negative phase and the record-setting year of 2007 when it was in positive phase can be seen in the images below:

Image: NSIDC

Image: NSIDC

The decline of the AO/NAO and the rise or the ARP help to explain the increased melt rate of Arctic sea ice, but just how significant is the increase? According to James Overland from the Pacific Marine Environmental Laboratory in Seattle:

Even if the Arctic circulation were to return to normal and would switch to the “dipolar” pattern just once in a decade, the situation would look grim, he said. “Each time we would see a loss of so much ice that it would be impossible to return to the initial state.” [emphasis--JR]

In other words, we have probably reached a tipping point from which there is now no turning back.

Gerdes and his co-authors fear that the changes in the Arctic could mean that a “new era of global-warming-forced climate change” has begun. [emphasis--JR] The volume of greenhouse gas emissions like CO2 and methane into the Earth’s atmosphere could have resulted in a permanent change in the global climate system.

If such is indeed the case, this will simply be one more instance in which Jim Hansen’s predictions have proven correct.

“In the case of Arctic Sea ice, we have already reached the point of no return,” [emphasis--JR] says the prominent American climate researcher James Hansen, director of the Goddard Institute for Space Studies at NASA.

Related Posts:

Final Wilkins Ice-Sheet Breakup Loom

Changes Taking Place in Arctic Sea-Ice Growth and Melt Cycles

From North to South, the Whole Damn World is Melting

When Ice Shelves Collapse: A Brief Tutorial

UPDATE: Bridge to Wilkins Ice Shelf Faces Imminent Collapse

Crossposted at Climaticide Chronicles

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