Are the near-Antarctic easterly winds weakening in response to enhancement of the Southern Annular Mode?

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Abstract:

Previous studies have highlighted the sensitivity of the Southern Ocean circulation to the strengthening, poleward-shifting westerlies, associated with the increasingly positive southern annular mode (SAM). The impacts of the SAM have been hypothesized to weaken momentum input to the ocean from the easterly winds around the Antarctic margins. Using ERA-Interim data, the authors show that the circumpolar-averaged easterly wind stress has not weakened over the past 3–4 decades, and, if anything, has slightly strengthened by around 7%. However, there has been a substantial increase in the seasonality of the easterlies, with a weakening of the easterly winds during austral summer and a strengthening during winter. A similar trend in the seasonality of the easterlies is found in three other reanalysis products that compare favorably with Antarctic meteorological observations. The authors associate the strengthening of the easterly winds during winter with an increase in the pressure gradient between the coast and the pole. Although the trend in the overall easterly wind strength is small, the change in the seasonal cycle may be expected to reduce the shoreward Ekman transport of summer surface waters and also to admit more warm Circumpolar Deep Water to the continental shelf in summer. Changes in the seasonal cycle of the near-coastal winds may also project onto seasonal formation and export of sea ice, fluctuations in the strengths of the Weddell and Ross Gyres, and seasonal export of Antarctic Bottom Water from the continental shelf.

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Last updated on 04/05/2019