ECMWF Analyses and Reanalyses Depiction of ENSO Signal in Antarctic Precipitation*

Abstract

The El Nino-Southern Oscillation (ENSO) signal in Antarctic precipitation is evaluated using European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses and ECMWF 15-yr (1979-93) reanalyses. Operational and reanalysis datasets indicate that the ENSO teleconnection with Antarctic precipitation is man- ifested through a close positive correlation between the Southern Oscillation index and West Antarctic sector (758-908S, 1208W-1808) precipitation from the early 1980s to 1990, and a close negative correlation after 1990. However, a comparison between the operational analyses and reanalyses shows significant differences in net precipitation (P 2 E) due to contrasts in the mean component of moisture flux convergence into the West Antarctic sector. These contrasts are primarily due to the mean winds, which differ significantly between the operational analyses and the reanalyses for the most reliable period of overlap (1985-93). Some of the differences in flow pattern are attributed to an error in the reanalysis assimilation of Vostok station data that suppresses the geopotential heights over East Antarctica. Reanalysis geopotential heights are also suppressed over the Southern Ocean, where there is a known cold bias below 300 hPa. Deficiencies in ECMWF reanalyses result in a weaker ENSO signal in Antarctic precipitation and cause them to miss the significant upward trend in precipitation found in recent operational analyses. Ice-core analyses reflect both an upward trend in ice accumulation and the ENSO teleconnection correlation pattern seen in the operational analyses. This study confirms the results of a previous study using ECMWF operational analyses that was the first to find a strong correlation pattern between the moisture budget over the West Antarctic sector and the Southern Oscillation index.