Mechanism of the Centennial Subpolar North Atlantic Cooling Trend in the FGOALS-g2 Historical Simulation

A cold blob, manifested as a centennial cooling trend in sea surface temperature (SST), is observed in the mid-latitude North Atlantic. The presence of the cold blob is hypothesized as an evidence of a slowdown of Atlantic Meridional Overturning Circulation (AMOC), based on paleoclimate proxies and global climate models (GCMs). However, the performance of GCMs in simulating the cold blob remains unsatisfactory in terms of the SST cooling rate and the spatial extent. This study investigates the forcing mechanism of the cold blob using the Flexible Global Ocean-Atmosphere-Land System Model Grid-point version 2 (FGOALS-g2), a GCM that reasonably simulates the observed cooling trend in the subpolar North Atlantic and its spatial pattern. Surface heat budget analysis suggests that the cold blob is largely a result of the imbalance between changes in the heat storage and surface turbulent heat fluxes, exhibiting a cooling and warming effect, respectively. Investigation of ocean heat content indicates that heat advection into the cold blob region has decreased, mainly due to ocean circulation changes. However, in the FGOALS-g2, both the AMOC slowdown and the reduced meridional heat transport explain a limited portion (less than 50%) of the cold blob sea surface temperature anomaly trend. Overall, complementing existing studies that attribute the cold blob to an AMOC slowdown, our results suggest that additional processes, including subpolar gyre circulation and a synergy between the atmosphere and the ocean, are at work in the formation of the cold blob.