Season-Dependent Atmosphere-Ocean Coupled Processes Driving SST Seasonality Changes in a Warmer Climate

Abstract

Amplification of sea surface temperature (SST) seasonality in response to global warming is a robust feature in climate model projections but season-dependent regional disparities in this amplification and the associated mechanisms are not well addressed. Here, by analyzing large ensemble simulations using Community Earth System Model version 2, we investigate detailed spatiotemporal characteristics of the amplification of SST seasonality focusing on the North Pacific and North Atlantic, where robust changes are projected to emerge around 2050 under SSP3-7.0 scenario. Our results indicate that atmosphere-ocean coupled processes shape regional changes in SST seasonality differently between warm (MAMJJAS) and cold seasons (ONDJF). During the warm season, the projected warming tendency is mainly due to increased net surface heat flux and weakening of vertical mixing. On the other hand, in the cold season, the projected cooling tendency is driven by strengthened vertical mixing over the North Pacific associated with the northward shift of storm tracks but weakened horizontal advection and mixing due to changes in ocean currents over the North Atlantic. In addition to the increase in annual mean sea surface temperature (SST), the amplitude of SST seasonal cycle is projected to intensify in response to greenhouse warming over many parts of the global ocean due to more warming during local summer than winter. By analyzing large ensemble simulations, we show how different atmospheric and ocean processes influence spatiotemporal differences in SST seasonality changes in the North Pacific and North Atlantic. Our findings indicate that during local summer, the projected warming tendency intensifies with the increased heat input into the ocean and weakening of ocean mixing. Meanwhile, during local winter, the projected cooling tendency intensifies with the strengthening of vertical mixing associated with the enhanced storm track activities in the North Pacific, but the weakening of ocean horizontal advection and mixing attributable to the weakened ocean circulation over the North Atlantic. They together contribute to the amplification of SST seasonality, which could have implications for marine ecosystems, including the plankton phenology. Using CESM2 large ensemble simulations, this study explores the robustness of future sea surface temperature (SST) seasonality changes and the associated atmospheric and surface ocean processes In the warm season, the future SST warming is mainly driven by the reduced upper ocean mixing and the increased net surface heat influx into the ocean In the cold season, the future SST cooling is attributable to strengthened vertical mixing over the North Pacific but weakened horizontal advection and mixing in the North Atlantic