Apparent Changes in Pacific Decadal Variability Caused by Anthropogenically Induced Mean State Modulations

Pacific decadal variability (PDV), reflected in low-frequency Pacific sea surface temperature (SST) changes, impacts global climate. Disentangling anthropogenic effects upon PDV is challenging because PDV and anthropogenic forcing vary on similar time scales. Using single-forcing climate model large ensembles, we find that anthropogenic forcing drives a spatially varying pattern of mean-state change in Pacific SST that projects onto PDV patterns, principally the Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO). When the trend is removed by subtracting the ensemble mean, there is no forced change of either PDV mode. However, analysis of individual ensemble members, where the mean-state trend cannot be cleanly removed, yields apparent anthropogenic changes in PDO and NPGO decadal variability. This suggests that observed PDV responses to anthropogenic forcing may be erroneously convolved with the background trend pattern. Therefore, correctly determining the mean-state trend is a necessary precursor for identifying possible forced changes to PDV.