The Atmospheric Hydrologic Cycle over the Southern Ocean and Antarctica from Operational Numerical Analyses

Abstract

Moisture budget calculations for Antarctica and the Southern Ocean (40°?72deg;S) are performed using operational numerical analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF), the National Meteorological Center (NMC), and the Australian Bureau of Meteorology (ABM). The analyses are intetcompared for an 8-yr period from 1985 to 1992 and are evaluated against representative rawinsonde sites, which are considered accurate depictions of moisture transport at these sites. The comparisons to East Antarctic rawinsondes and those from Macquarie Island show the ECMWF analyses to be superior in reproducing sounding values at each level. While results are highly variable depending on the station location, agreement of the ECMWF analyses to zonally averaged sounding moisture flux values along the East Antarctic coast is very close. The zonally averaged annual meridional moisture flux, for example, is within as little as 0.03 g kg?1 m s?1, or 2% at the surface. This is particularly good considering the highly variable inflow and outflow patterns along the Antarctic perimeter. The NMC and ABM analyses generally underestimate transport at each level; error cancellation occurs during vertical integration however. A comparison of moisture convergence for East Antarctica with values calculated from rawinsonde data indicates the ECMWF analysis is within 5 mm yr?1 of the observed value, while the NMC result is severely deficient. Overall these results are not surprising given the coarse resolution and spectral nature of the analyses. The ability of the ECMWF analyses to reproduce the observed moisture transport at each level is reassuring. Comparison of the moisture transport convergence derived from the numerical analyses with previous moisture flux studies over the Southern Ocean reveals general agreement in the location of the boundary between the moisture source and sink. The ECMWF and NMC analyses place the convergence maximum slightly farther south than has been previously found. It is inferred that this results from the blocking effect of the Antarctic coastal topography. At full resolution this point is at approximately 64°S. Long-term net precipitation (precipitation minus sublimation/evaporation) derived from the numerical analyses is somewhat smaller than values determined by glaciological methods. Net precipitation varies interannually by 25%, with most of the variation concentrated in the South Pacific sector, the region of greatest poleward moisture transport. The results presented here offer a substantially more positive outlook on the prospects of determining continental precipitation trends in Antarctica through atmospheric methods than has been previously found and demonstrate that the ECMWF analyses provide generally good estimates.