The Effect of Anthropogenic Aerosols on the Aleutian Low

Abstract:

Past studies have suggested that regional changes in anthropogenic aerosols can influence the Pacific Decadal Oscillation through modulation of the Aleutian Low. However, the robustness of this connection is debated. This study analyses changes to the Aleutian Low in boreal winter in an ensemble of climate models forced with idealised global and regional black carbon (BC) and sulphate aerosol perturbations. To investigate how ocean feedbacks help regulate the climate response, each experiment is carried out with an interactive ocean and with prescribed sea surface temperatures (SST). The results show a robust weakening of the Aleutian Low forced by a global 10-fold increase in BC in both coupled and prescribed SST experiments. A linearised steady-state primitive equation model is forced by diabatic heating anomalies to investigate the mechanisms through which BC emissions influence the Aleutian Low. The direct heating from BC absorption over both India and China is found to generate Rossby wave trains that propagate north-east into the North Pacific sector, forming an upper tropospheric ridge. Conversely, the response to a global 5-fold increase in sulphate aerosols and a regional 10-fold increase in sulphate aerosols over Asia shows poor consistency across models, with a multi-model mean response that does not project strongly onto the climatological Aleutian Low structure. This is in contrast to some earlier studies that suggested the transient increase in sulphate aerosol over Asia during the period 1998-2012 caused a weakening of the Aleutian Low and a transition to a negative phase of the PDO.