Processes Determining the Rapid Reestablishment of the Equatorial Pacific Cold Tongue/ITCZ Complex

Abstract

The annual reestablishment of the equatorial cold tongue (ECT) in the Pacific is signified by a remarkably rapid reversal of the warming trend from March to May. The processes responsible for this dramatic turnabout are investigated using the outputs generated by a coupled ocean?atmosphere model, which simulates realistic tropical Pacific climate. A new diagnostic equation is put forward for a budget study of the temperature tendency in a mixed layer (ML) with a variable depth. The budget study reveals that the rapid boreal spring cooling in the ML of the ECT (4°S?2°N, 120°?90°W) is primarily attributed to turbulent entrainment (54%), surface evaporation (21%), and meridional advection (14%). The spring shallowness of the ML is also a significant?implicit? contributor. Annually, the ML depth in the ECT varies nearly 180° out of phase with the SST while in phase with the ML heat content. The annual variation of the ML depth is determined by competing effects of the Ekman transport and turbulent entrainment. From March to July, the increase of the meridional wind component dominates that of the zonal component; thereby, the effect of entrainment surpasses that of upwelling, leading to mixed layer deepening. The mechanism governing the annual variation of the ML heat content is essentially the same as those governing the ML depth variation. The results suggest that accurate modeling of the ML turbulent mixing holds the key to realistic simulation of the annual cycle of the ECT. In contrast, beneath the ITCZ (8°?12°N, 100°?120°W), the rapid spring warming is attributable to increased surface heat flux, while entrainment and thermal advection play minor roles. From February to May, the downward shortwave radiation and the surface latent heat fluxes, along with concurrent equatorial cooling, result in a northward progression of the annual warming and promote an active ITCZ?ECT interaction (including evaporation?wind feedback and cloud?radiation?SST interaction).