Implications of North Atlantic Sea Surface Salinity for Summer Precipitation over the US Midwest: Mechanisms and Predictive Value

Abstract:

Moisture originating from the subtropical North Atlantic feeds precipitation throughout the Western Hemisphere. This ocean-land moisture exchange leaves its imprint on sea surface salinity (SSS), enabling SSS over the subtropical oceans to be an indicator of terrestrial precipitation. This study demonstrates that springtime SSS over the northwestern portion of the subtropical North Atlantic significantly correlates with summertime precipitation over the United States (US) Midwest. The linkage between springtime SSS and the Midwest summer precipitation is established through ocean-to-land moisture transport followed by a soil moisture feedback over the southern US. In the spring, high SSS over the Northwestern subtropical Atlantic coincides with a local increase in moisture flux divergence. The moisture flux is then directed toward and converges over the southern US, which experiences increased precipitation and soil moisture. The increased soil moisture influences the regional water cycle both thermodynamically and dynamically, leading to excessive summer precipitation in the Midwest. Thermodynamically, the increased soil
moisture tends to moisten the lower troposphere and enhances the meridional humidity gradient north of 36N. Thus, more moisture converges into the Midwest transported by the climatological low-level wind. Dynamically, the increases in soil moisture over the Southern US enhance the west-to-east soil moisture gradient eastward of the Rocky Mountains, which intensifies the Great Plains Low-level jet in the summer favoring moisture flux convergence over the Midwest. Due to these robust physical linkages, the springtime SSS outweighs the leading SST modes in predicting the Midwest summer precipitation and significantly improves rainfall prediction in this region.