Seasonal Change of the Atmospheric Heat Budget over the Southern Ocean from ECMWF and ERBE Data

Abstract

The seasonal variation of the zonally averaged atmospheric energy budget between 60° and 70°S was estimated. This region is predominantly within the seasonal sea ice zone of the Southern Ocean, including some parts of the Antarctic continent. In the Southern Ocean, seasonal sea ice extent exhibits large amplitudes and affects the surface heat exchange considerably. Seasonal variation of the energy budget and its relationship to the surface condition should be clarified as the basic variation. In spite of its importance, the data to estimate energy budgets are extremely sparse in this sea ice zone. Hence, the global objective analyses with forecasting models are mainly used as data for the present study. The surface energy flux is obtained as a remainder term in the energy budget of the total atmosphere, with the energy divergence and changes to the energy content of the atmospheric column derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) objective analyses, and with the radiation budget at the top of the atmosphere estimated from the Earth Radiation Budget Experiment (ERBE) data. The derived values of the surface flux are not seasonally symmetric. This differs from other latitude bands (e.g., 50°?60°S) where the variation is symmetric and is driven by shortwave radiation change. The monthly mean surface energy flux shows an immediate increase to the maximum of 116 W m?2 in May (heating of the atmosphere), and then gradually decreases to the minimum of ?108 W m?2 in December (cooling of the atmosphere). It is suggested that the asymmetry at 60°?70°S is due to the effect of seasonal sea ice extent changes on the surface energy exchanges. A comparison of the derived values of the surface flux with the latent heat required for the change in sea ice extent provides some support for this suggestion. The rate of sea ice expansion shows a peak in May when the surface energy flux becomes maximum. The turbulent heat component of the surface energy flux is compared with other estimates of turbulent heat exchange over the Southern Ocean. Suppression of the turbulent heat exchange in late winter derived in the present study, in comparison with those values over open water area, suggests the effect of extended sea ice cover.