The Mean Annual Cycle in Global Ocean Wind Stress

Abstract

The mean annual cycle in surface wind stress over the global oceans from surface wind analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) for seven years (1980–86) is presented. The drag coefficient is a function of wind speed and atmospheric stability, and the density is computed for each observation. Annual and seasonal mean climatologies of wind stress, wind stress and Sverdrup transport and the first two annual harmonies of the wind stress are presented. The Northern and Southern hemispheres are contrasted as an the Pacific and Atlantic basins. The representativeness of the climatology is also assessed. The main shortcomings with the current results are in the topics. The wind stress statistics over the southern ocean are believed to be the moon reliable because of the paucity of direct wind observations. Annual mean values exceed 2 dyn cm−2 over the eastern hemisphere near 50°S and locally exceed 3 dyn cm−2 in the southern Indian Ocean; values much larger than in previous climatologies. The 12 month variations dominate the annual cycle over most of the globe and are strongest in the Arabian Sea, North Pacific and North Atlantic. But strong semiannual components occur especially over the Southern Ocean and in the North Pacific. The former are associated with semiannual increases in the strength of the southern westerlies whereas in the North Pacific, the semiannual cycle occurs locally largely because of the annual variations in intensity and meridional movement of the Aleutian low and subtropical high. The wind stress considerably from year to year. Over most of the world's ocean the mean annual cycle explains less then 45% of the monthly variance in each of the wind stress components and the curl of wind stress. In addition, mean values for the climatology differ significantly from those of previous periods. There is good reason to believe that these differences in the Northern Hemisphere an mostly real and represent climate variations on interannual and decadal time scales that have major implications for the circulation of the oceans. A related factor is that this period included two Pacific Warm Events (El Niños), but no Cold (La Niña) Events.