The Effect of Anthropogenic Aerosols on the Aleutian Low

Abstract

Past studies have suggested that regional trends in anthropogenic aerosols can influence the Pacific Decadal Oscillation (PDO) through modulation of the Aleutian Low. However, the robustness of this connection is debated. This study analyses changes to the Aleutian Low in an ensemble of climate models forced with large, idealised global and regional black carbon (BC) and sulphate aerosol perturbations. To isolate the role of ocean feedbacks, the experiments are performed with an interactive ocean and with prescribed sea surface temperatures. The results show a robust weakening of the Aleutian Low forced by a global 10-fold increase in BC in both experiment configurations. A linearised steady-state primitive equation model is forced with diabatic heating anomalies to investigate the mechanisms through which heating from BC emissions influences the Aleutian Low. The heating from BC absorption over India and east Asia generates Rossby wave trains that propagate into the North Pacific sector, forming an upper tropospheric ridge. Sources of BC outside of east Asia enhance the weakening of the Aleutian Low. The responses to a global 5-fold and regional 10-fold increase in sulphate aerosols over Asia show poor consistency across climate models, with a multi-model mean response that does not project strongly onto the Aleutian Low. These findings for a large, idealised step increase in regional sulphate aerosol differ from previous studies that suggest the transient increase in sulphate aerosols over Asia during the early 21st century weakened the Aleutian Low and induced a transition to a negative PDO phase.

Drivers of Recent North Pacific Decadal Variability: The Role of Aerosol Forcing

“…Consistency of the response pattern shown here with previous studies suggests that the response is likely to be driven by changes in Asian aerosol emissions, which induce local circulation changes and initiate a Rossby wave train from the midlatitudes as shown in Smith et al (2016) and Wilcox et al (2019). Assessment of the role of different aerosol species by Dow et al (2021) suggests an important role for black carbon emissions over Asia in particular, with emissions from elsewhere enhancing the response. It is possible that other pathways exist by which anthropogenic aerosols could affect Pacific Decadal Variability.…”

“…Assessment of the role of different aerosol species by Dow et al. (2021) suggests an important role for black carbon emissions over Asia in particular, with emissions from elsewhere enhancing the response. It is possible that other pathways exist by which anthropogenic aerosols could affect Pacific Decadal Variability.…”

“…Furthermore, the 1981-2012 period is also the period with the strongest black carbon emission increases (Figure 1). A recent study has suggested that the increase in NP SLP is primarily a response to black carbon aerosols rather than sulfate aerosols (Dow et al, 2021), but we do not differentiate between different species of anthropogenic aerosols here.…”

Greenhouse Gas Forcing a Necessary Causation for Marine Heatwaves Over the Northeast Pacific Warming Pool

“…We attribute the pattern of increased SST over the warming pool primarily to increased GHG concentrations, with anthropogenic aerosols playing a secondary role. In addition, regional trends in anthropogenic aerosols over North Pacific with aerosols-cloud interaction being the dominant aerosols forcing 42 , can influence the PDO through modulation of the Aleutian low [42][43][44][45] . Thus, it is plausible that the external forcings (GHG and aerosols forcing) have influenced both North Pacific SSTs and the PDO index itself.…”

“…In the 21 st Century warming climate, however, PDO trends are clearly distinguishable from internal climate variability 39,40 , with the amplitude reduced and the decadal cycle shifted toward a higherfrequency band 41 . In addition, regional trends in anthropogenic aerosols over North Pacific with aerosols-cloud interaction being the dominant aerosols forcing 42 , can influence the PDO through modulation of the Aleutian low [42][43][44][45] . Thus, it is plausible that the external forcings (GHG and AER forcing) have influenced both North Pacific SSTs and the PDO index itself.…”