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

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

Bin Wang* and Xiouhua Fu

Department of Meteorology and International Pacific Research Center**,

School of Ocean and Earth Science and Technology,

University of Hawaii at Manoa, Honolulu, Hawaii 96822

Abstract

The annual reestablishment of the equatorial Pacific cold tongue (ECT) starts with a sharp turnaround from a warming in March to a cooling in May. The processes responsible for this dramatic transition 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 ocean mixed layer (ML) with a variable depth.

The budget study indicates that the rapid boreal spring cooling in the ML of the ECT (4^oS-2^oN, 120^oW-90^oW) is primarily attributed to turbulent entrainment, surface evaporation, and meridional advection. The spring shallowness of the ML is also a significant implicit contributor. Annually, the ML depth in the ECT varies nearly 180^o out of phase with the SST while in phase with the ML heat content. Competing effects of the Ekman transport and turbulent entrainment determine the annual variation of the ML depth. From March to July, the increase of the meridional wind component dominates that of the zonal component; thereby, the effect of entrainment exceeds the upwelling effect, leading to mixed layer deepening. The mechanism governing annual variation of the ML heat content is essentially the same as those governing the ML depth variation. Accurate modeling of the ML turbulent mixing may hold a key to realistic simulation of the annual cycle of the ECT.

Beneath the intertropical convergence zone (ITCZ) (8^oN-12^oN, 100^oW-120^oW), the rapid spring warming owe its origin to increased surface heat flux, while entrainment and thermal advection play a minor role. From February to May, the increased downward shortwave radiation and reduced surface latent heat fluxes, along with concurrent equatorial cooling, results in a northward progression of the annual warming and promotes an active ITCZ-ECT interaction (including evaporation-wind feedback and cloud-radiation-SST interaction).