Temperature Advection by Tropical Instability Waves

A numerical model of the tropical Pacific Ocean is used to investigate the processes that cause the horizontal temperature advection of tropical instability waves (TIWs). It is found that their temperature advection cannot be explained by the processes on which the mixing length paradigm is based. Horizontal mixing of temperature across the equatorial SST front does happen, but it is small relative to the “oscillatory” temperature advection of TIWs. The basic mechanism is that TIWs move water back and forth across a patch of large vertical entrainment. Outside this patch, the atmosphere heats the water and this heat is then transferred into the thermocline inside the patch. These patches of strong localized entrainment are due to equatorial Ekman divergence and due to thinning of the mixed layer in the TIW cyclones. The latter process is responsible for the zonal temperature advection, which is as large as the meridional temperature advection but has not yet been observed. Thus, in the previous observational literature the TIW contribution to the mixed layer heat budget may have been underestimated significantly.