A Description of the Annual Cycle in Sea Surface Temperature and Upper Ocean Heat in the Equatorial Atlantic

Abstract

Temperature and velocity time series, obtained by surface moorings during the Seasonal Response of the Equatorial Atlantic Experiment, are used to investigate the role of ocean dynamics upon the annual cycle of equatorial sea surface temperature (SST) and upper ocean heat. The annual cycle in SST is explained by different mechanisms, each operant at different phases of the cycle. The boreal springtime decrease in SST results from upwelling in response to the seasonal intensification of easterly wind stress. This upwelling causes the seasonal formation of the cold tongue along the equator in the central and eastern portions of the basin. An early summer increase in SST is attributed to the meridional convergence of Reynolds' heat flux associated with surface current instability-generated waves. After the instability waves abate, SST and mixed layer depth remain relatively steady from late summer through fall when the advective terms are small and cancelling, suggesting that surface heating is then balanced by a diffusive flux at the base of the mixed layer. SST increases in wintertime following the seasonal relaxation in easterly wind stress, thus, completing its annual cycle. This increase is attributed to the concentration of the surface flux over a mixed layer that is shoaling due to both the basin-wide adjustment of the thermocline and the local reduction in turbulent energy production. Thus, SST variations are found to be most closely controlled by ocean dynamics during those times when the thermocline is adjusting basin-wide to the seasonal changes in wind stress; either directly by large advective fluxes (boreal spring-summer) or indirectly by mediating mixed layer depth (boreal winter). Analyses at 75 m depth show zonal and vertical advection to be important, and within a control volume over the upper 150 m all of the advective terms are important.