The Role of Daily Surface Forcing in the Upper Ocean over the Tropical Pacific: A Numerical Study

Abstract

The impacts of high-frequency surface forcing in the upper ocean over the equatorial Pacific are investigated using a nonlinear reduced-gravity isopycnal ocean circulation model forced by daily and monthly mean forcing. The simulated sea surface temperature (SST) in the daily forcing experiment is colder than that in the monthly forcing experiment near the equator. A mixed layer heat budget calculation shows that the net surface heat flux is primarily responsible for the SST difference in the western Pacific, while zonal advection accounts for the SST difference in the eastern Pacific where other budget terms are large but canceling each other. The daily forcing primarily enhances vertical mixing that reduces the vertical shear of the upper ocean. It also changes the net heat into the ocean through two contrasting processes: one is the increased surface latent heat loss induced by transient winds and the other is colder SST due to stronger mixing, which further reduces heat loss at the surface. As a result, the annual mean net surface heat flux into the ocean is reduced and the meridional thermal advection is weaker. The daily forcing also impacts the variation of the thermocline through a changing mixed layer depth so that the temperature in the simulation with the daily forcing is warmer around the thermocline.