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Anthropogenic-aerosol (AA) radiative forcing modulates multidecadal greenhouse radiative forcing. However, decadal climate responses to AA are poorly characterized given AA forcing uncertainty and internal climate variability. This motivates revisiting a recent claim that AA drove a negative trend in the Pacific Decadal Oscillation and an associated cooling influence in the 10-15years following the late-1990's El Nino. The average of a 50-member initial condition ensemble of the second generation Canadian Earth System Model version 2 that was forced only with AA does not exhibit the negative-Pacific Decadal Oscillation/slowdown response. However, spurious responses of this kind, that are artifacts of subsetting the large ensemble (LE) in a manner consistent with published literature, can readily be found. This illustrates the caution needed in interpreting regional- and decadal-scale responses to AA and suggests that improved characterization of model uncertainty in AA over the recent period is required.

Plain Language Summary Pollution associated with the emission of aerosols from industry and transportation have been considered to be important to explain the long-term behavior of global mean temperature over the past half century. On shorter timescales of a couple of decades or less, recent work has suggested that these so called anthropogenic aerosols (AAs) have also limited the amount of global warming that has taken place in recent years. The sources and processes associated with AA are highly complex and uncertain, and so we here are revisiting a recent claim that AA played a detectable role in modulating global temperatures in the 10-15 years after the late 1990's El Nino. We use a large number of simulations of the Canadian Earth System Model version 2 that sample the chaotic variability of climate, and this ensemble shows that AA does not lead to a strong signal over this period. However, we show that if we look at smaller samples of this ensemble, a spurious signal can arise that falsely gives the appearance that AA is driving large signals. Although the required ensemble is only available for version 2 of the Canadian Earth System Model, this result suggests that caution is needed in saying that AA has caused recent unexplained trends in the temperature record.