We examine the physical processes involved with the impact of stratospheric aerosols on the Subpolar North Atlantic (SPNA) and the AMOC strength in two large-ensemble experiments with two versions of a comprehensive Earth system model. The SPNA responses to increases in stratospheric aerosols are drastically different in the two experiments– one becomes warm and salty, and the other cool and fresh. The different responses in SPNA are accompanied by their diverging changes in AMOC and Arctic sea ice responses. Both ensemble simulations use the comprehensive Community Earth System Model with the Whole Atmosphere Community Climate Model as its atmospheric component (CESM-WACCM) but differ in model version and simulation design. The primary objective of this study is to investigate the physical processes involved with the influence of increased stratospheric aerosols on regional responses in the SPNA, explore their potential connections with the Arctic climate, and assess their impact on the AMOC.
This study investigates the physical processes involved with the impacts of increased stratospheric aerosols on the SPNA and the strength of the AMOC in two climate model large-ensemble sensitivity experiments. The two models used in these simulations exhibit very different AMOC behaviors, we find that differences in the base state AMOC are likely linked to the inter-model variations in the upper ocean stratification over the SPNA region and the Arctic sea ice cover. By analyzing the upper ocean freshwater budget, we find that increased stratospheric aerosols reduce the SPNA freshwater content and increase the upper ocean salinity. In both cases, we find a close connection between the SPNA and the Arctic -- the SPNA freshwater loss is mainly driven by decreased freshwater export from the Arctic, where there is a total decrease in surface freshwater fluxes. We find that the anomalous ocean deep convection induced by elevated stratospheric aerosols is mainly attributed to changes in surface heat fluxes, while the freshwater-induced buoyancy fluxes only play a small role. The response in the sensitivity experiments elucidates the regional hydrological processes in the SPNA, which affect AMOC, are largely dependent on the background climate and future greenhouse gas forcings. While the current study may not fully document all the complexities among different models, it provides valuable physical insights into the specific impacts of increased stratospheric aerosols on SPNA salinity changes and their potential connections with the AMOC and the Arctic.
The SPNA shows contrasting responses in two sensitivity experiments with increased stratospheric aerosols, offering insight into the physical processes that may impact the AMOC in a warmer climate. Both simulations use the Community Earth System Model with the Whole Atmosphere Community Climate Model component (CESM-WACCM)) but differ in model versions and stratospheric aerosol specifications. Despite both experiments using similar approaches to increase stratospheric aerosols to counteract the rising global temperature, the contrasting SPNA and AMOC responses indicate a considerable dependency on model physics, climate states, and model responses to forcings. This study focuses on examining the physical processes involved with the impact of stratospheric aerosols on the SPNA salinity changes and their potential connections with the AMOC and the Arctic. We find that in both cases, increased stratospheric aerosols act to enhance the SPNA upper ocean salinity by reducing freshwater export from the Arctic, which is closely tied to the Arctic sea ice changes. The impact on AMOC is primarily through the thermal component of the surface buoyancy fluxes, with negligible contributions from the freshwater component. These experiments shed light on the physical processes that dictate the important connections between the SPNA, the Arctic, the AMOC, and their subsequent feedbacks on the climate system.