The Trade Wind Field Over the Pacific Ocean

The trade wind field over the Pacific Ocean between 30°N and 30°S has been studied on the basis of five million wind observations made from ships. Data were sorted by quadrangles of 2° latitude and 10° longitude to resolve north-south gradients in the wind field adequately. Maps of the surface wind stress vector for February and August are presented and the development of the field throughout the year is discussed. The trade wind regime in each hemisphere is largest and strongest during the respective winter and spring. The area covered by northeast trades is smaller than the area covered by southeast trades, but the northeast trades have a stronger mean wind stress and a larger annual variation both in area and mean stress. The computed divergence of the wind velocity revealed a little known area of convergence in the southeastern Pacific near the equator. The curl of the wind stress and the meridional profile of zonal wind stress vary considerably during the year. The minimum in zonal stress between the northeast and the southeast trades is a poorly developed feature from January to March while the maximum northeast trades are south of their mean position. Averaging the zonal stress over the South Pacific is deceptive because of large zonal variations. Time series of the Pacific Ocean area covered by northeast and southeast trades, the mean zonal wind stress within the area, and the mean zonal wind stress in large, fixed areas are presented for the period from 1947 to 1972. The southeast trades covered an anomalously large area during 1955–56, 1964, 1966–67 and 1970–71. Interannual variations of the northeast trades are smaller than those of the southeast trades. There is no apparent relationship between fluctuations in the strength and areal extent of the northeast and southeast trades, except with the annual cycle.