LONG-TERM VARIATIONS IN EQUATORIAL CIRCULATION AND RAINFALL

Abstract

The large-scale air-sea interaction over the equatorial Pacific proposed by Bjerknes is investigated. It was found from a study of the Canton Island record that ocean temperature, rainfall, trade wind flow, and equivalent potential temperature are related and undergo long-term variations with periods in excess of a year. Similar changes occur in the high troposphere. Satellite cloud observations, however, indicate important longitudinal variations near the Equator. During the abnormal rainy season of 1965?66 at Canton Island, the amount of cloudiness remained low over the eastern equatorial Pacific despite above-normal sea-surface temperatures. This suggests a continuation of the widespread subsidence usually present over this region, which is apparently part of a large-scale semipermanent zonal circulation. Satellite observations further show that there are three major ?centers of action? (standing eddies) in the vicinity of the Equator. Probably the major part of the condenstion heating necessary for the Hadley circulation occurs in these areas. This study also indicates a possible relation between equatorial rainfall in the central Pacific and the strength of the Northern Hemisphere westerlies as suggested by Bjerknes. In addition, rainfall appears to vary inversely with the eddy kinetic energy over the Northern Hemisphere suggesting an inverse relation with the large-scale planetary waves in the Northern Hemisphere.