The wind stress over the ocean from observations in the trades

Abstract The method of geostrophic departure is used to determine the surface drag of the ocean in the Trade‐wind belt, this region being particularly suitable for the technique because the opposition of the thermal wind to the surface geostrophic wind leads to a maximum in wind speed at a few hundred metres elevation. This pilot study is based on balloon winds at three very small Pacific islands and the U.S.W.B. Northern Hemisphere Charts of surface pressure. A drag coefficient of 1·0 × 10 −3 is found from two of the stations, and 1·5 × 10 −3 at the third and there is no systematic variation with surface‐wind speed from 3 m/sec to 12 m/sec. The higher value of drag coefficient may be due to excessive roughness caused by reefs, or possibly to thermal instability. A similar analysis for San Juan in the West Indies shows the disturbing effect of a larger land area on the geostrophic departure so that only in high surface winds does the apparent drag tend to the probable value. Some other features of the Trade‐wind regime are briefly described.