Sea ice extent and concentration [in 'State of the Climate in 2006']

Sea ice extent anomalies varied widely during the year, from record-low January and March extent (March was –18% relative to the 1979–2006 mean) to record highs of +4% for September and October, based upon analysis of monthly mean SSM/I-derived sea ice concentration data produced by the NSIDC Sea Ice Index project (Fig. 5.22; see Fetterer and Knowles 2004). However, by December 2006, sea ice extent was below average again. Strong regional variability is also apparent in ice extent and concentration anomalies. For the austral sea ice minimum period (February–March), the record low is primarily a result of greater-than-average March retreat in the Weddell Sea. This is likely a result of the intense warmth (up to 5°C above average) in the eastern Weddell in March, and a circulation pattern that favored southward and eastward wind-driven drift of the pack. In the autumn growth season, positive anomalies in the Amundsen Sea and western Indian Ocean sectors are balanced by negative anomalies in the South Atlantic (Weddell Sea) and Ross Sea/WPO sectors. The winter ice cover in August is characterized by major negative anomalies (indicative of winddriven ice compaction) in the Bellingshausen– Amundsen Seas plus WPO sectors and a lesser negative anomaly skirting the Greenwich meridian. These are counterbalanced by equally large positive anomalies in the Weddell Sea, Ross Sea, and Indian Ocean sectors. This pattern persisted through September– October. In fact, the Indian Ocean sector experienced large positive extent and concentration anomalies from April through November. Locations of extent anomalies, and the overall record extent during the winter maximum, suggest an influence from the earlier positive SAM mode and strong positive wavenumber-3 anomaly, and the presence of lower-than-average temperatures near the winter ice edge (see Massom et al. 2006; Raphael 2004).