Wouter Lefebvre

Influence of the Southern Annular Mode on the sea ice–ocean system

[1] The global sea ice - ocean model ORCA2-LIM, driven by the NCEP/NCAR ( National Centers for Environmental Prediction-National Center for Atmospheric Research) reanalysis daily 2-m air temperatures and 10-m winds and by monthly climatologies for precipitation, cloud cover, and relative humidity, is used to investigate the impact of the Southern Annular Mode (SAM) on the Antarctic sea ice-ocean system. Our results suggest that the response of the circumpolar Southern Ocean consists of an annular and a nonannular component. For the sea ice cover, the non-annular component seems to be the most important. The annular component strongly affects the overall patterns of the upper ocean circulation. When the SAM is in its positive phase, a northward surface Ekman drift, a downwelling at about 45 degreesS, and an upwelling in the vicinity of the Antarctic continent are simulated. The non-annular component has a significant impact at the regional scale, especially in the Weddell, Ross, Amundsen, and Bellingshausen Seas. In those regions, the pressure pattern associated with the SAM induces meridional winds which advect warmer air in the Weddell Sea and around the Antarctic Peninsula and colder air in the Amundsen and Ross Seas. This implies a dipole response of sea ice to the SAM, with on average a decrease in ice area in the Weddell Sea and around the Antarctic Peninsula and an increase in the Ross and Amundsen Seas during years with a high SAM index. The long-term trend in the observed sea ice area does not appear to be related to the trend in the SAM index.

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 degrees W and 60 degrees 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 degrees W-130 degrees 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 degrees C to +5 degrees C) and precipitation rates (to >2.5 mm/d). The patterns in late 2005 are consistent with the occurrence of a weak La Nina 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 degrees W and 125 degrees 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 regime.

An analysis of the atmospheric processes driving the large-scale winter sea ice variability in the Southern Ocean

The response of the sea ice in the Southern Ocean to the variability of the atmosphere on the period 1979-2004 is investigated using both model and observational data. On the one hand, our results show that in line with previous investigations, the classical modes of atmospheric variability do not explain a large part of the winter sea ice extent variability integrated over the entire Southern Ocean. On the other hand, the regression between the ice extent and the atmospheric pressure displays a pattern with low pressure areas over the South Atlantic, over the Indian Ocean, and on the southwestern part of the Pacific Ocean and a high pressure area over the Antarctic region extending toward both the southeastern Pacific and the region south of Australia. This pattern, which does not correlate significantly with any of the known modes of atmospheric variability, induces changes in the sea ice extent by influencing the air temperature, the ice production, and both the meridional and the zonal ice velocity. However, there is no clear link between the different centers of action of this pattern. Furthermore, the correlation of the sea ice extent between the different sectors is generally low. As a conclusion, we must consider that the sea ice extent in the Southern Ocean over the last 30 years does not behave as a single entity: Its variability is the result of the combination of regional sea ice changes. Each sector should thus be examined separately.