Surface Wind Stress From Global Atmospheric Analyses

Abstract

Evaluations and comparisons of global analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) and the U.S. National Meteorological Center (NMC) have allowed an appreciation to be gained of the characteristics, problems and errors in these data sets. For surface wind, the ECMWF 1000 mb analyses are regarded as most suitable for computing surface wind stresses over the global oceans because ship data were applied directly at 1000 mb prior to September 1986. The global analyses use a four-dimensional data assimilation system and have the advantage of high spatial and temporal (twice daily) resolution. New climatologies of surface wind stress, the curl of the wind stress, and the Sverdrup transport streamfunctions have been computed based on 1980-1986 analyses. The wind stress was computed using a wind speed and surface atmospheric stability dependent drag coefficient from Large and Pond, and the effective mean drag coefficient required to convert a pseudostress into a stress has been evaluated as a function of location and time of year. Comparisons with the Hellerman and Rosenstein wind stress climatology reveal surprising similarities overall in view of the different drag coefficients used, but there are also huge differences, especially over the entire southern oceans, where the new results are believed to be the best available. As well as the long-term annual and monthly means, interannual variations have also been analyzed.