Annual Cycle of Equatorial East-West Circulation over the Indian and Pacific Oceans

Abstract

Along the equator, annual mean 200-mb zonal wind is approximately in phase with annual mean outgoing longwave radiation (OLR); namely, easterlies are strongest above the convective center over the maritime continent, while westerlies reach their maximum just above the dry zone over the equatorial Pacific. This is much different from what is anticipated by theories that predict that the phase of the upper-tropospheric zonal wind is in quadrature with that of the prescribed heating. The present study provides evidence that the midlatitude-equatorial coupling is primarily responsible for the maintenance of the annual mean total 200-mb zonal winds along the equator, whereas convection contributes a great deal to the annual mean upper-level equatorial divergent winds. Annual cycles occurring over the extratropics act as a transient eddy forcing of the equatorial annual mean 200-mb zonal wind through three-dimensional convergence of localized Eliassen-Palm (E-P) fluxes. They are acting to accelerate the 200-mb annual mean westerlies (easterlies) over the equatorial eastern Pacific (Indian Ocean) where E-P fluxes are horizontally divergent (convergent). The baroclinic contribution, acting through the meridional heat flux due to annual cycles, appears to be minimal. The annual cycles differ remarkably between the equatorial Indian and eastern Pacific oceans. The annual cycle in the equatorial Indian Ocean is characterized by 1) the eastward phase propagation of monthly mean anomaly zonal winds with an inverse relationship between the surface and 200 mb (i.e., baroclinic structure in the vertical), and 2) the highest SST occurring about three (four) months prior to the strongest surface westerlies (minimum OLR). The annual cycle in the equatorial eastern Pacific exhibits coherent westward propagation of monthly mean anomaly SST and surface zonal winds, indicating the importance of planetary boundary-layer processes. On the other hand, the annual cycle of 200-mb equatorial zonal winds (the upper-level east-west circulation) is larger of standing wave character, while the annual cycle of OLR is of propagating wave character, implying that the equatorial convection contributes little to the annual cycle of the upper-level east-west equatorial circulation. It is shown that the annual cycle in the upper-level zonal winds over the equatorial eastern Pacific is largely controlled by a pronounced annual cycle of the 200-mb zonal wind occurring in the extratropics of each hemisphere.