The relationship of sea surface temperature to thermocline depth at annual and interannual time scales in the tropical Atlantic Ocean

Abstract

Expendable bathythermograph data derived from SEQUAL/FOCAL and TOGA sponsored volunteer observing ship programs in the tropical Atlantic Ocean are used to study the relationship between the sea surface temperature (SST) and the underlying thermocline. Time series up to 8 years long allow investigation of both the annual and interannual variability between 2N and 2S along selected shipping lanes. For the annual harmonic a warm SST is correlated with a shallow thermocline everywhere except in the eastern equatorial region. Near the intertropical convergence zone (ITCZ) the 180° phase shift in the annual harmonic of both the SST and the thermocline depth occurs at slightly different latitudes. Over most of the tropical Atlantic the annual cycle of SST is linked to the phase of the net surface heat flux and only in the eastern equatorial and coastal upwelling regions does heat flux across the thermocline become a significant factor for the annual cycle of the SST. Canonical correlation analysis of the interannual variability reveal two distinct modes with structures similar to the annual cycle. The first, in which a warm SST is correlated with a shallow thermocline, is a dipole with centers of maximum amplitude off the equator at approximately 8S at 12N and a node at the ITCZ. The second mode, for which a warm SST is correlated with a deep thermocline, peaks at the equator. The predominance of the first mode suggests that for most of the tropical Atlantic Ocean the dynamical state beneath the surface mixed layer has only a limited influence on SST fluctuations.