FY 1991

Eastern equatorial Pacific response to three composite westerly wind types

Giese, B.S., and D.E. Harrison

J. Geophys. Res., 96(Suppl.), 3239–3248, doi: 10.1029/90JC01861 (1991)

It has been suggested recently that episodes of westerly wind near the international date line in the western Pacific tend to fall into one of four types. Three of the types have enough equatorial zonal wind variation to be able to induce an eastern Pacific response by forcing equatorial waves. In this note we examine the central and eastern Pacific waveguide response to idealized representations of these three types of westerly wind episodes, using a combination of linear theory and results from an ocean general circulation model. The idealized westerly wind types are the C type (maximum anomalies centered on the equator), the N type (maximum anomalies just north of the equator), and the S type (maximum anomalies just south of the equator). The composite C-type event excites equatorially trapped Kelvin pulses that alter sea surface temperature in the central and eastern Pacific. Warm anomalies caused by the C-type episode have approximately equal contributions from a zonal advection of heat by the Kelvin pulses and a meridional advection of heat by an altered instability wave field. Because the center of forcing of the S-type event is displaced off of the equator, the Kelvin pulses excited by the S-type episode have smaller amplitude than those of the C-type episode, even though the maximum wind anomaly is the same in the two cases. The S-type anomaly also excites weak Rossby-gravity waves that can propagate into the eastern Pacific. Because the Rossby-gravity waves are dispersive, their amplitude diminishes as the waves propagate into the eastern Pacific. The N-type episode, as observed in winds from the western tropical Pacific, is much weaker and of shorter duration than the C and S types, and therefore excites only a very weak eastern Pacific response. Although all types of westerly wind excite a local western Pacific response, only the C- and S-type events excite a significant eastern Pacific response.