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.

<p>Variability in the Aleutian Low is a known contributor to North Pacific sea surface temperature (SST) variability, but its role in forcing the basin-wide SST anomalies that characterise Pacific Decadal Variability (PDV) is unclear owing to the difficulty of disentangling coupled atmosphere-ocean processes. Here we perform a large ensemble experiment with an intermediate complexity GCM where the winter-time Aleutian Low is nudged to an anomalously strong state during successive winters. This ensemble is compared to a free-running simulation to isolate the impacts of the anomalous Aleutian Low. The nudged experiment produces a basin-scale SST response that closely resembles PDV in the free running simulation, confirming that the Aleutian Low can force PDV-like variability. Tropical Pacific sea surface temperatures (SSTs) are significantly warmer in response to the strong Aleutian Low, demonstrating that extratropical atmospheric forcing can impart a signature in tropical SSTs. The largest tropical Pacific warming is manifest in the season following nudging (boreal spring), though anomalies persist year-round. We use the Bjerknes Stability Index to attribute the drivers of the tropical Pacific SST response and find that the thermocline feedback is key, which itself is most dominant in summer. The results lend new understanding to the potential for extratropical atmospheric forcing of tropical ocean variability.</p>

<p>There is an incomplete understanding of the mechanisms that govern the Pacific Decadal Oscillation (PDO), a major mode of climate variability that plays a key role in the evolution of global climate on decadal time-scales. Recent research has suggested that regional anthropogenic aerosol (AA) emissions could modulate the behaviour of the PDO, including the transition to a negative PDO phase starting in the late 1990s (Smith et al., 2016). However, other studies have questioned whether this connection is robust (Oudar et al., 2018). East Asia is a region of particular focus, where AA emissions having increased in recent decades (Bartlett et al., 2017). Here we combine analysis of an ensemble of coupled climate models running idealised AA perturbation experiments and a steady-state primitive equation model (LUMA) forced by diabatic heating anomalies to examine whether AA emissions influence the behaviour of the Aleutian low - a climate feature closely associated with the PDO  - and if so, test the posited teleconnection mechanisms proposed by Smith et al. (2016). We further compare the response of the Aleutian low to well mixed greenhouse gases to examine if AAs and GHGs influence the Aleutian low in a similar manner.</p>

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