Ocean Salinity as a Predictor of Terrestrial Precipitation
[23-May-2017] Li, L., Schmitt, R., Ummenhofer, C., and Karnauskas, K.
Presented at the Global Ocean Salinity and the Water Cycle Workshop
The global water cycle is fueled by water evaporating from the oceanic source regions. Over the global oceans, these sources are primarily located in the subtropics, where evaporation exceeds precipitation resulting in a net moisture export from the local oceans. About a third of the subtropical water is transported to the land and becomes an indispensable water source for terrestrial precipitation. Thus, the amount of terrestrial precipitation is significantly modulated by the availability of oceanic moisture. At the same time, the net ocean-to-land moisture transport leaves an imprint on sea surface salinity (SSS), suggesting that the variation of subtropical SSS can provide predictive value for terrestrial precipitation. In this presentation, we provide observation-based evidence that springtime salinity in the subtropical North Atlantic can be a very useful predictor of terrestrial precipitation with a one season lead. Specifically, high springtime SSS in the northeastern portion of the subtropical North Atlantic is followed by excessive monsoon precipitation in the African Sahel, whereas high SSS in the western North Atlantic is indicative of extreme summer precipitation in the US Midwest. The physical mechanism is established through an increased ocean-to-land moisture transport, and a positive soil moisture feedback process which preserves the initial oceanic moisture inflow for 3 months. In the African Sahel, the soil moisture influences monsoon-season precipitation by enabling the region to draw more moisture from the ambient oceans. In the US Midwest, the soil moisture exerts both thermodynamic and dynamic effects on summer precipitation. Thermodynamically, soil moisture itself is a moisture source for summer precipitation. Dynamically, the spatial distribution of soil moisture intensifies the Great Plains Low-level jet to transport more moisture into the Midwest. Due to the close relationship between subtropical North Atlantic SSS and ocean-to-land moisture transport, and the active role of soil moisture in the regional water cycle, seasonal forecasts of Sahel and Midwest precipitation can be improved by incorporating the North Atlantic SSS into prediction models. We will further show that the improvement in the precipitation is most significant for extreme precipitation. In addition, SSS is shown to provide superior predictive value than preseason SST for these regions. Our studies suggest that expanded monitoring of ocean salinity is likely to contribute to more skillful continental precipitation predictions.