West Antarctic Peninsula sea ice in 2005: Extreme ice compaction and ice edge retreat due to strong anomaly with respect to climate

In September–October 2005, the juxtaposition of low‐ and high‐pressure anomalies at 130°W and 60°W, respectively, created strong and persistent northerly airflow across the West Antarctic Peninsula (WAP). This had a major impact on regional sea ice conditions, with extreme ice compaction in the Bellingshausen and East Amundsen seas (60°W−130°W) but divergence in the West Amundsen and East Ross seas. This resulted in the former in a highly compact marginal ice zone and ice cover, mean modeled ice thicknesses of >5 m, and an earlier‐than‐average maximum extent (mid‐August). While rapid ice retreat in late winter‐spring created a major negative ice extent anomaly, compact ice persisted in the subsequent summer. Other effects were anomalies in air temperature (of +1°C to +5°C) and precipitation rates (to >2.5 mm/d). The patterns in late 2005 are consistent with the occurrence of a weak La Niña and a near‐neutral Southern Annular Mode, with a quasi‐stationary zonal wave three pattern dominating hemispheric atmospheric circulation. Once a compact ice edge was created, it took only one additional week of strong winds to “solidify” the pack in place. Conditions in 2005 are analyzed in the context of 1979−2005 and compared with the springs of 1993, 1997, 1999, 2001, and 2004. A statistically significant increase of the northerly 10‐m wind component between 110°W and 125°W occurred in the Septembers of 1979−2005. No clear trends occur in other spring months. This work underlines the key importance of ice dynamics in recent changes in the WAP sea ice régime.