North Atlantic Sea Surface Salinity as a Predictor of Sahel Rainfall

Water evaporating from the subtropical oceans sustains precipitation on land. Moisture exiting the ocean surface leaves an imprint on sea surface salinity (SSS). Thus, the question arises of whether variations in subtropical SSS can provide mechanistic insight into, and predictability of, terrestrial precipitation. Using a series of observational and reanalysis products, here we provide evidence that springtime SSS in the subtropical North Atlantic is a robust, physically meaningful predictor of terrestrial precipitation during the subsequent monsoon season. Specifically, abnormally high springtime SSS in the central to eastern subtropical North Atlantic tends to be followed by a wetter-than-normal monsoon season in the African Sahel. This linkage between SSS and precipitation is established through ocean-to-land moisture transport, and a proposed delay mechanism provided by soil moisture. Evaluation of the atmospheric moisture budget suggests that high SSS in the subtropical North Atlantic is associated with regional moisture flux divergence from the ocean, which converges over the African Sahel. This springtime moisture supply can trigger a positive local feedback between soil moisture and additional atmospheric moisture flux, leading to stronger monsoon precipitation. Due to the observed close relationship between SSS and Sahel precipitation, significantly improved seasonal forecasts of monsoonal precipitation can be achieved by incorporating North Atlantic SSS into statistical prediction models. Thus, expanded monitoring of ocean salinity should contribute to more skillful predictions of precipitation in vulnerable subtropical regions including the Sahel.