CONVERGENCE AND DISPOSAL OF ENERGY AND MOISTURE ON THE ANTARCTIC POLAR-CAP FROM ECMWF REANALYSES AND FORECASTS

Diagnostics of energy and moisture transport and disposal over the Ant arctic polar cap (70 degrees S to the pole) and ice sheet are extracte d from the European Centre for Medium Range Weather Forecasts (ECMWF) reanalysis archive over the 1979-93 period. Transport across 70 degree s S is obtained from the 6-hourly analyses of wind, temperature, moist ure, and geopotential, whereas top-of-the-atmosphere energy balance an d surface energy and water fluxes are evaluated from 6- and 12-h forec asts. A frill decomposition of transport is made and tabulated in term s of seasons, dynamic components (mean meridional, stationary eddy, tr ansient eddy), and type of energy (sensible, latent, geopotential). Fo r instance, in terms of type of energy, about 50% of the total converg ed to the polar cap is geopotential, which is almost entirely advected by the mean meridional circulation. Even though atmospheric moisture is very low, latent heat transport accounts for almost 20% of the tota l energy import, mostly by the transient eddies. In terms of dynamic c omponents, transient eddies alone import about 50% of the total energy in the form of sensible and latent heat. Some components actually exp ort energy from the polar cap, and the variety of signatures exhibited by the transport decomposition may prove useful to check the dynamics of climate models in the very high southern latitudes. According to t he analyses, the total annual mean energy input to the polar cap south of 70 degrees S by the atmospheric circulation is 50.8 W m(-2) of hor izontal surface. The short-term forecasts suggest that the oceanic imp ort is much smaller, of the order of model and analysis uncertainties. The interannual variability of atmospheric energy convergence is unre asonably large, and it is partly, yet not quite convincingly, correlat ed with the El Nino-Southern Oscillation. No convincing correlation is found either between moisture convergence from analyses or surface wa ter budget from forecasts and the El Nino-Southern Oscillation. This r esult contradicts a previous study using the ECMWF operational analyse s, which are more prone to spurious variability than the reanalyses an d associated forecasts used here. The interannual variability of moist ure convergence is large but reasonable, about 25% of the annual mean. It might be useful as a control against which to check the dynamics o f the hydrological cycle of climate models in the high southern latitu des.