The role of ocean processes in future Northern Hemisphere mid-latitude winter precipitation changes

In response to anthropogenic emissions winter net precipitation (precipitation minus evaporation) in the Northern Hemisphere mid-latitudes is projected to considerably intensify by the end of this century. Previous studies argued for the importance of surface quantities in setting precipitation changes. Thus, to elucidate the future changes in precipitation, it is crucial to examine the changes in ocean processes, as they play a fundamental part in the surface response to anthropogenic emissions. In this study, we use a hierarchy of ocean coupling simulations to quantify the relative roles of the ocean, and its dynamic and thermodynamic components, in the projected intensification of Northern Hemisphere mid-latitude winter net precipitation. We find that the increase in mid-latitude net precipitation over land and the Pacific Ocean stems from the impact of thermodynamic ocean processes, which mostly act to warm the surface, increase the meridional moisture gradient, and thus the moisture flux convergence by mid-latitude eddies. In contrast, over the Atlantic Ocean, dynamic ocean processes modify the zonal moisture gradient, therefore increasing the convergence of moisture over that region. Our results highlight the importance of better investigating and monitoring ocean-atmosphere coupling processes to improve our preparedness for future large-scale climate change.