ON THE ANNUAL CYCLE OF THE EASTERN EQUATORIAL PACIFIC

Although the sun ''crosses'' the equator twice a year, the eastern equ atorial Pacific has a pronounced annual cycle, in sea surface temperat ure and in both components of the surface winds for example. (This is in contrast to the Indian Ocean and western Pacific where a semiannual oscillation of the zonal wind is the dominant signal on the equator.) Calculations with a relatively simple coupled ocean-atmosphere model indicate that the principal reason for this phenomenon is the marked a symmetry, relative to the equator, of the time-averaged climatic condi tions in the eastern tropical Pacific. The important asymmetries are i n surface winds, oceanic currents, and sea surface temperature: The ti me-averaged winds and currents have northward components at the equato r and the warmest waters are north of the equator. Because of those as ymmetries, seasonally varying solar radiation that is strictly antisym metric relative to the equator can force a response that has a symmetr ic component. The amplitude of the resultant annual cycle at the equat or depends on interactions between the ocean and atmosphere, and on po sitive feedbacks that involve low-level stratus clouds that form over cold surface waters.