North Atlantic Salinity as a Predictor of Sahel Precipitation

Abstract:

Water evaporating from the subtropical oceans sustains precipitation on land, playing a vital role in the terrestrial water and energy balances. This net ocean to land moisture exchange leaves its imprint on sea surface salinity (SSS), suggesting that the variation of SSS can provide insight into terrestrial precipitation. Here, we provide evidence that higher springtime SSS in the central to eastern subtropical North Atlantic is followed by abnormally high Sahel precipitation during monsoon seasons. This relationship is established through the changes of moisture flux that diverges away from the subtropical North Atlantic and converges into the African Sahel. The enhanced moisture flux divergence in the subtropical North Atlantic is mainly due to the intensification and northward movement of the North Atlantic subtropical high. The stronger high-pressure system results in excessive loss of freshwater from the ocean surface and increases the SSS. At the same time, the northward movement of the subtropical high strengthens this SSS anomaly through the salinity advection by Ekman flow. The SSS signal in the subtropical North Atlantic propagates into the Sahel region through the convergence of oceanic moisture flux. It is further amplified in the subsequent seasons through a positive local feedback between soil moisture content and atmospheric moisture flux, leading to stronger monsoon precipitation. Due to the close relationship between subtropical North Atlantic SSS, subtropical high circulation, and moisture exchange between the North Atlantic and the African continent, seasonal forecasts of Sahel precipitation can be improved by incorporating the North Atlantic SSS into prediction models. Thus, expanded monitoring of ocean salinity is likely to contribute to more skillful continental precipitation predictions in vulnerable subtropical regions, such as the Sahel.