Search

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.

[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.

[1] We have adapted the Coupled Large-scale Ice-Ocean (CLIO) model in order to include atmospheric pressure forcing in addition to the classical atmospheric forcing ( heat and freshwater fluxes and wind stress). We have compared the modeled bottom pressure to in situ bottom pressure measurements and also to another ocean model. We have computed the induced effects on Earth orientation parameters ( polar motion and nutations) and also on Earth time-variable gravity field. We show that the response of the ocean to atmospheric pressure forcing has geodetic consequences well above the precision level and that the response of CLIO seems reasonable with respect to the few data sets we can compare with. We also show that the results obtained from the model improve the modeling of polar motion.

[1] Simulated sea ice thickness in the ORCA2-LIM coupled sea ice - ocean model is compared with thicknesses from the Antarctic Sea Ice Processes and Climate (ASPeCt) database. We find a qualitative agreement of the large-scale patterns of ice thickness distribution. Regional averages for the various sectors of the Southern Ocean yield a very good correspondence between observations and model data. Exceptions are the eastern Bellingshausen and northwestern Weddell Seas. A poor representation of the Antarctic Peninsula in the atmospheric forcing data and the related overestimation of westerly winds in this region lead to a spurious accumulation of sea ice on the western side of the peninsula and to an underestimation of sea ice coverage on the eastern side. Since the spatial scale of observations is not comparable to the size of a model grid cell, there is little agreement between individual observations and the corresponding model ice thicknesses. A model analysis of the seasonal and interannual variability indicates that the ASPeCt data underestimate the climatological ice thickness in the central and southern Weddell Sea and the eastern Ross Sea by up to 1 m. Because of a winter bias in the observations an overestimation of similar magnitude is expected in the Bellingshausen Sea. Ice thickness data in most of the Indo-Pacific sector appear to be representative for the long-term climatology. A model estimate of the bias is used to compute a revised distribution of climatological sea ice thickness.