The Trade Wind Field Over the Pacific Ocean. Part I. The Mean Field and the Mean Annual Variation,

The trade wind field over the Pacific Ocean between 30N and 30S has been studied on the basis of five million wind observations made by ships. Data were averaged for areas of 2 degrees latitude and 10 degrees longitude to resolve northsouth gradients in the wind field adequately. Mean monthly maps of the wind vector and of the wind stress vector were produced and the development of the trade wind field throughout the year is discussed. Either trade wind system is largest and strongest during the winter of the respective hemisphere. It is shown that the area covered by the northeast trades is smaller than the area covered by the southeast trades, but the northeast trades have a stronger mean wind stress and a larger seasonal variation both in area and mean stress. Bimonthly mean maps of the divergence of the wind and of the wind stress curl are also produced. The wind divergence shows clear relations to cloud cover; the maps of wind stress curl show considerable detail and more structure and seasonal variation than expected.