Increased Asian aerosols drive a slowdown of Atlantic Meridional Overturning Circulation

The Atlantic Meridional Overturning Circulation (AMOC) is a crucial component of the global climate system, transporting vast amounts of energy to the northern high latitudes. A decline in AMOC strength has been observed since the mid-1990s, although the precise causes remain a subject of ongoing debate. While increasing greenhouse gases (GHGs) are a primary suspect, anthropogenic aerosols (AAs) also play a significant role. Unlike well-mixed GHGs, AAs exhibit high spatial and temporal variability due to their shorter atmospheric residence time. The global mean AAs concentration increased until the late 1980s, primarily due to emissions from both the Eastern and Western Hemispheres. After the early 1990s, it stabilized, mainly because of emission reductions in Europe and North America (a consequence of the Clean Air Act). However, AA emissions in South and East Asia continued to increase. A growing body of research acknowledges the influence of AAs on AMOC variations. Multi-model simulations indicate a two-stage evolution: a strengthening before the mid-1990s, followed by a rapid weakening thereafter. While the decrease in AAs over North America and Europe is linked to the AMOC weakening, the effect of increased Asian AAs remains poorly understood. This study investigates the impact of increased Asian AAs on the AMOC, focusing on the underlying mechanisms and using a combination of climate simulations and multi-model analysis to assess the robustness of findings.

Several studies have investigated the role of anthropogenic aerosols in AMOC variability. Booth et al. (2012) implicated aerosols as a primary driver of 20th-century North Atlantic climate variability. Menary et al. (2013, 2020) explored the mechanisms of aerosol-forced AMOC variability in climate models. Cai et al. (2006) examined the pan-oceanic response to increasing anthropogenic aerosols, highlighting impacts on Southern Hemisphere circulation. Delworth and Dixon (2006) investigated whether anthropogenic aerosols delayed a GHG-induced AMOC weakening. Other studies, such as Hassan et al. (2021) and Robson et al. (2022), investigated the role of anthropogenic aerosol forcing in AMOC changes using CMIP6 models. However, a comprehensive understanding of the impact of increased Asian AAs on AMOC remains elusive, particularly concerning the underlying mechanisms and the robustness of the findings across different model configurations.

This study employs idealized radiative perturbation experiments using the Community Earth System Model version 1 (CESM1). A 500-year control run (CTRL) was first conducted with GHGs, AAs, and solar insolation fixed at year 2000 levels. To simulate the radiative effects of aerosol forcing, solar insolation was reduced by 10% over East Asia and South Asia (EAST simulations) and over North America and Europe (WEST simulations). The WEST simulations' sign was reversed to mimic the radiative heating effect of aerosol decrease after the 1980s. The annual-mean net radiative forcing amounted to -30 W m⁻², exceeding the actual radiative forcing to generate a robust response. Ten 100-year ensemble members were generated for each experiment (CTRL, EAST, and WEST), starting from year 350 of the CTRL run. The AMOC response was analyzed by comparing years 1-40 of the perturbed runs to the corresponding period in CTRL. Additional analyses were conducted using the first 40 and last 50 years of the simulations to assess the timescale of the AMOC response. The analysis included examining the AMOC streamfunction in density coordinates, exploring the atmospheric responses (stationary Rossby waves, westerly jet shift, sea level pressure anomalies), and analyzing winter water mass transformation (WMT) in the subpolar North Atlantic. Further validation was performed using simulations from 10 models participating in the Aerosol Chemistry Model Intercomparison Project (AerChemMIP) and the CESM2 Single Forcing Large Ensemble (CESM2-SF-LE) to test the robustness of findings across different models.

The study found a surprising result: aerosol-induced cooling over Asia led to a significant AMOC slowdown. The AMOC decreased by -2.7 Sv within the first 40 years in the EAST simulations, a robust response not attributable to internal variability. The AMOC response in density space showed reduction in the lower limb, signifying a decrease in equatorward-flowing deep water. The analysis revealed that the radiative cooling over Asia triggered stationary Rossby wave patterns, leading to an equatorward shift of the westerly jet in the northern mid-latitudes. This resulted in weakened westerlies over the subpolar North Atlantic. Winter WMT analysis showed a pronounced reduction in the subpolar North Atlantic, primarily due to suppressed heat loss in the Labrador Sea. The reduced heat loss was attributed to weakened westerlies, which reduced the transport of cold air from North America, leading to a warmer boundary layer above the Labrador Sea and thus less dense water formation. This ultimately contributed to the AMOC slowdown. Multi-model analysis using AerChemMIP and CESM2-SF-LE simulations corroborated these findings, showing similar atmospheric responses (circumglobal teleconnection, negative NAO-like trend, and suppressed westerlies over the Labrador Sea) and AMOC weakening, supporting the robustness of the results.

The study's findings directly address the research question of the impact of increased Asian AAs on the AMOC, demonstrating a previously unrecognized mechanism for AMOC weakening. The results challenge the conventional understanding that radiative cooling strengthens the AMOC. The study highlights the importance of considering the indirect effects of AA-induced atmospheric circulation changes on the AMOC. The significant slowdown observed in the simulations is consistent with observed trends in the AMOC, suggesting that the combined effects of increased Asian AAs and decreased AAs in North America and Europe contribute substantially to the observed AMOC slowdown. This underscores the need to incorporate the complex interactions between aerosol forcing, atmospheric circulation, and ocean dynamics in future climate projections.

This study demonstrates that increased Asian anthropogenic aerosols, despite their radiative cooling effect, drive a weakening of the Atlantic Meridional Overturning Circulation (AMOC). This occurs through the excitation of circumglobal Rossby waves, leading to a southward shift of the westerly jet and reduced cold air transport to the Labrador Sea. This inhibits deep water formation and contributes to AMOC slowdown. Future research should explore the interaction between other climate factors and Asian aerosol emissions to refine AMOC projections and further understand its role in global climate change.

The study primarily uses idealized radiative perturbation experiments, which simplifies the complexity of the real climate system. While multi-model validation enhances the robustness of the findings, the limited number of models and specific model configurations could still limit the generalizability of results. The study focuses on the impact of Asian aerosols; other factors contributing to AMOC variability are not explicitly considered. Further research involving more sophisticated models and observations will be necessary to refine our understanding of AMOC variability and its future behavior.