Extreme Anomalous Atmospheric Circulation in the West Antarctic Peninsula

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Extreme Anomalous Atmospheric Circulation in the West Antarctic Peninsula Region in Austral Spring and Summer 2001/02, and Its Profound Impact on Sea Ice and Biota* R OBERT A. M ASSOM , a S HARON E. S TAMMERJOHN , b R AYMOND C. S MITH , c M ICHAEL J. P OOK , d R ICHARD A. I ANNUZZI , b N EIL A DAMS , e D OUGLAS G. M ARTINSON , b M ARIA V ERNET , f W ILLIAM R. F RASER , g L ANGDON B. Q UETIN , h R OBIN M. R OSS , h Y UKO M ASSOM , i AND H. R OY K ROUSE j a Department of the Environment and Heritage, Australian Antarctic Division, and Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Hobart, Tasmania, Australia b Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York c ICESS, and Department of Geography, University of California, Santa Barbara, Santa Barbara, California d CSIRO Division of Marine and Atmospheric Research, Hobart, Tasmania, Australia e Australian Bureau of Meteorology, and Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Hobart, Tasmania, Australia f Integrative Oceanographic Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California g Polar Oceans Research Group, Sheridan, Montana h Marine Science Institute, University of California, Santa Barbara, Santa Barbara, California i National Oceans Office, Hobart, Tasmania, Australia j Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada (Manuscript received 12 April 2005, in final form 3 October 2005) ABSTRACT Exceptional sea ice conditions occurred in the West Antarctic Peninsula (WAP) region from September 2001 to February 2002, resulting from a strongly positive atmospheric pressure anomaly in the South
Atlantic coupled with strong negative anomalies in the BellingshausenAmundsen and southwest Weddell
Seas. This created a strong and persistent north-northwesterly flow of mild and moist air across the WAP.
In situ, satellite, and NCEPNCAR Reanalysis (NNR) data are used to examine the profound and complex
impact on regional sea ice, oceanography, and biota. Extensive sea ice melt, leading to an ocean mixed layer
freshening and widespread ice surface flooding, snowice formation, and phytoplankton growth, coincided
with extreme ice deformation and dynamic thickening. Sea ice dynamics were crucial to the development
of an unusually early and rapid (short) retreat season (negative ice extent anomaly). Strong winds with a
dominant northerly component created an unusually compact marginal ice zone and a major increase in ice
thickness by deformation and over-rafting. This led to the atypical persistence of highly compact coastal ice
through summer. Ecological effects were both positive and negative, the latter including an impact on the
growth rate of larval Antarctic krill and the largest recorded between-season breeding population decrease
and lowest reproductive success in a 30-yr Ad

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