Evolution Dynamics of Tropical Ocean-Atmosphere Annual Cycle Variability

Abstract

The structure of ocean-atmosphere annual cycle variability is extracted from the revised Comprehensive Ocean-Atmosphere Data Set SSTs, surface winds, and the latent heat (LH) and net shortwave (SW) surface fluxes using the covariance-based rotated principal component analysis method. The coupled annual cycle variability is concisely described using two modes that are in temporal quadrature. The first, peaking in June/July (and Dec/Jan), represents monsoonal flow onto Indochina, Central America, and western Africa. The second mode peaks in September/October and March/April when it represents the extreme phases of the SST annual cycle in the eastern oceans. Analysis of the surface momentum balance in the Pacific cold tongue core shows the equatorial flow, and in particular the zonal wind, to be dynamically consistent with the SST gradient during both the cold tongue's nascent (Jun/Jul) and mature (Sep/Oct) phases; the dynamical consistency improves when the impact of near-surface static stability variation on horizontal momentum dissipation is also considered. Evolution structure of the extracted annual cycle, moreover, shows the easterly wind tendency to lead SST cooling in the off-coastal zone. Taken together, these findings suggest that the Pacific cold tongue westward expansion results from local interaction of the zonal wind and zonal SST gradient, as encapsulated in the proposed ?westward expansion hypothesis? -a simple analytic model of which is also presented. Although positive LH flux tendency leads SST cooling in the off-coastal zone, its modest magnitude (?5 W m?2/mo) indicates that its direct impact on SSTs, while additive, is secondary to the impact of equatorial upwelling. Comparison of the open ocean and coastal annual evolutions reveals that the northward expansion of the Pacific cold tongue likely results from the positive feedback between coastal meridional winds and the upwelled meridional SST gradient, but suggests that the reason for the nonobservance of equatorially antisymmetric SSTs is the counter LH-flux impact northward of the equator. The comparatively modest SST annual cycle in the northern equatorial Indian Ocean is forced by the Asian-monsoon-driven (i.e., nonlocally forced) surface winds through coastal upwelling along the Somali coast and from the monsoon-cloudiness-impacted net SW surface flux and wind-speed-influenced LH flux in the off-coastal sector.