Modeling the effects of fresh water rain flux on convective coupling in the Eastern Tropical Pacific

Abstract:

Upper ocean stratification in the Eastern Tropical Pacific is strongly affected by short duration convective rain events. Cloud resolving simulations coupled to a mixed layer ocean model indicate that fresh water pools last 1-2 days and can effectively limit the thermal boundary layer depth that defines the sea-surface temperature (SST). In this study, we examine if this short-term change in upper ocean stratification affects subsequent convection by contrasting two coupled experiments; one with fine vertical resolution in the ocean surface boundary layer, and a second with the boundary layer averaged over 10 m depth, eliminating fresh water pooling. Compared with the surrounding ocean, fresh water pools can either be warmer during the day from trapping of solar insolation, or cooler at night from suppressed mixing that confines the effects of surface cooling over a limited depth. Over a 10-day period, we found that the effect of rainfall lowers the average surface salinity ~0.5 psu and decreases the SST by ~0.25 ^oC, suggesting that night time cooling is the dominant control. We did not find substantial changes in the feedback of cooler SST on subsequent convection over the short duration of these experiments. However, our results indicate that resolving near-surface ocean vertical salinity and temperature structure, especially in the convective warm pool regions, is critical for predicting SST for longer term, seasonal forecasts. We will discuss the impacts of these conclusions for needed measurements in Tropical Pacific process studies and the longer-term TPOS moorings.