The 2007 International Polar Year (IPY) in the Antarctic was distinguished by strong regional and seasonal ice-atmosphere-ocean anomalies associated with an overall weakening of the prevailing westerly circulation. Here we assess the ice-atmosphere-ocean conditions leading up to and during two IPY field campaigns that took place in early spring (September-October): the U.S.-led Sea Ice Mass Balance Antarctica (SIMBA, 68-72°S, 90-95°W) and the Australian-led Sea Ice Physics and Ecosystems eXperiment (SIPEX, 63-67°S, 115-130°E). Our regional analysis is presented within the context of circumpolar and inter-annual variability relevant to other IPY Antarctic studies. Using satellite-derived and numerically analyzed surface and atmospheric variables, we examine relationships between (i) winds and sea-ice concentration and drift, (ii) sea-surface temperature and ice-edge anomalies, and (iii) precipitation and snow accumulation. Though Antarctic-averaged sea-ice extent in September 2007 was the second highest observed for 1979-2007, the SIMBA and SIPEX studies sampled the two regions showing the largest negative sea-ice anomalies in the Southern Ocean. Maps of sea-surface temperature (SST) and sea-ice concentration (SIC) anomalies revealed distinct regional patterns, showing warm SST/low SIC in the greater SIMBA (including all of the Bellingshausen and Amundsen seas) and western SIPEX regions, versus cool SST/high SIC in the Weddell, Ross and eastern SIPEX regions. In the SIMBA and western SIPEX regions, warm northerly winds in May (overlying the warm SSTs) brought anomalously high precipitation to those regions, but due to the regional delays in sea-ice advance (by up to 2 months), most fell on open ocean, which in turn contributed to negative and near-zero September snow depth anomalies in those two regions, respectively. During autumn (March to May), warm SSTs offshore of those regions extended from mid-to-high latitudes, resulting from meridional advection of heat associated with a wave-3 atmospheric circulation pattern. In the SIMBA and SIPEX regions, the late sea-ice advance followed unusually long ice-free summer periods, which suggests that solar ocean warming was relatively high in those regions. The warm ocean conditions may help to explain why the ice edge remained well south of its mean position despite instances during winter of cold southerly winds and northward sea-ice drift. Finally, with respect to the 1978-2008 record, year 2007 was a continuation of decreasing sea ice in the SIMBA region (e.g., decreased annual sea-ice extent and ice season duration), whereas in the SIPEX region, year 2007 was a negative departure from an otherwise slightly positive trend in annual sea-ice extent and ice season duration. |
