Sensitivity of the Simulated Annual Cycle of Sea Surface Temperature in the Equatorial Pacific to Sunlight Penetration

The annual cycle of sea surface temperature (SST) in the equatorial Pacific is compared for two simulations with a coupled atmosphere–ocean general circulation model. The simulations differ only in the optical properties of the ocean: sunlight penetrates below the topmost layer of the ocean model in one case but is completely absorbed in the top layer in the other. The simulation without the sunlight penetration produces an unrealistic annual cycle of SST with a strong semiannual component in the equatorial Pacific, whereas the simulation with sunlight penetration is more realistic. The change in the character of the annual cycle results from an increase in the effective heat capacity of the ocean associated with an increase in the depth of the mixed layer directly forced by the sunlight penetration. This produces a smaller amplitude of the annual cycle of SST at latitudes close to but off the equator. The zone of intense tropical convection then remains closer to the equator, leading to a reduced semiannual cycle of zonal wind stress at the equator. The reduction in the unrealistic semiannual wind stress forcing leads to a more realistic annual cycle in SST. The simulation of the annual mean SST is also improved by the inclusion of the sunlight penetration, with a better simulation of the warm pool in the western equatorial Pacific and associated improvements in the atmospheric circulation. This improvement is also attributed to the increase in the mixed layer depth, which changes the ocean heat flux in the western equatorial Pacific by reducing the sensitivity of SST to upwelling.