Internal oscillations

Based on a statistical analysis of 40 years of annual sea ice concentration (SIC) and winter SLP data, Mysak and Venegas [1998] postulate an approximately 10-year

SLP over northern North Atlantic and Greenland Sea

Greenland Sea ice cover

Beaufort-Laptev Sea ice cover

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climate cycle in the Arctic and sub-Arctic that involves the NAO/AO. They provide a conceptual feedback loop to explain the relevant processes (Figure 11.20). The diagrammatic approach has already been described in Chapter 5. Starting at the top of the loop, large positive anomalies of SIC are created in the Greenland Sea by a relatively small horizontal transport of atmospheric sensible heat, associated with a negative NAO pattern. Over the Barents Sea, the formation of a large positive SIC anomaly may be more related to weaker advection of warm Atlantic water when the NAO is negative. The SIC anomalies are then advected clockwise into the Labrador Sea by the mean ocean circulation. The southern part of the Greenland Sea then becomes relatively ice free. The open water promotes strong atmospheric heating during winter, causing the Icelandic Low to deepen, helping to change the polarity of the NAO toward its positive phase. Once the NAO becomes positive, the wind anomalies create positive SIC anomalies in the Beaufort Sea, which are advected out of the Arctic by the Beaufort Gyre and transpolar drift streams. The Greenland Sea develops extensive ice cover, cutting off the heat flux to the atmosphere during winter, weakening the Icelandic Low, and contributing to a change in the polarity of the NAO back to its negative phase. The cycle then repeats itself.

While aspects of this conceptual feedback loop are clearly speculative (and stressing that such loops are only as strong as their weakest link), it makes the important point (also suggested from the Proshutinsky and Johnson study) that our search for the causes of Arctic climate variability cannot ignore processes internal to the Arctic itself. Indeed, the basic idea proposed by Mysak and Venegas [1998] of an internal oscillator operating on decadal time scales finds support in the modeling study of Dukhovsky et al. (2004).

They suggest that Arctic variability is regulated by heat and freshwater exchange between the Arctic Ocean and GIN seas. During the negative phase of the NAO/AO,

SLP over northern North Atlantic and Greenland Sea

Greenland Sea ice cover

Beaufort-Laptev Sea ice cover

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Figure 11.20 Proposed feedback loop for a decadal Arctic climate cycle. A + indicates a positive interaction (an increase in the first quantity leads to an increase in the second). A—indicates a negative interaction (an increase in the first quantity leads to a decrease in the second quantity) (from Mysak and Venegas, 1998, by permission of AGU).

Figure 11.20 Proposed feedback loop for a decadal Arctic climate cycle. A + indicates a positive interaction (an increase in the first quantity leads to an increase in the second). A—indicates a negative interaction (an increase in the first quantity leads to a decrease in the second quantity) (from Mysak and Venegas, 1998, by permission of AGU).

the atmospheric heat flux to the Arctic Ocean is small, and the freshwater flux to the GIN sea is less than average. With a small heat flux, the atmosphere over the Arctic Ocean cools, leading to a more anticyclonic circulation and a stronger Beaufort Gyre, increasing the convergence of surface water and ice. More freshwater is retained in the Beaufort Gyre. Less freshwater outflow to the GIN sea promotes deep oceanic convection in the central Greenland Sea in winter, associated with intense vertical heat fluxes to the atmosphere. This causes positive anomalies in SAT and intensification of the Icelandic Low (a transition to the positive phase on the NAO/AO). As a result of these processes, there is a growth of SAT and SLP gradients between the two basins. The gradients build to a point where they cannot be maintained and the basins then interact. An intensified horizontal heat flux to the Arctic Ocean weakens the atmospheric anticyclone and Beaufort Gyre. The accumulated freshwater in the Beaufort Gyre spills into the GIN seas, which become fresher, suppressing deep convection. With a fresher surface, sea ice forms more easily. Both processes reduce the vertical heat flux to the atmosphere (sea ice insulating the ocean from the atmosphere), acting to weaken the Icelandic Low (a transition back to the negative phase on the NAO/AO). After several years, interaction between the basins fades. Gradients then rebuild and the cycle repeats itself.

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