Contributions of Internal Variability and External Forcing to the Recent Pacific Decadal Variations

There is substantial uncertainty in the relative contributions of internal variability and external forcing to the recent Pacific decadal variability, especially regarding their linkage with the Interdecadal Pacific Oscillation. By analyzing observations and large ensembles of coupled climate model simulations, here we show that observed Pacific decadal variations since 1920 resulted primarily from internal variability, although greenhouse gas (GHG) and other external forcing did modulate decadal variations in Pacific sea surface temperatures (SSTs) significantly, especially for the period since the early 1990s. Specifically, the GHG-induced warming and the recovery from the volcanic cooling caused by the 1991 Pinatubo eruption led to large warming in the tropical Pacific during 1993-2012, while recent anthropogenic aerosols contributed to Pacific regional SST variations on multiyear to decadal scales, causing a La Nina-like cooling pattern in the Pacific since 1998 in some of the models. Our results provide new evidence that both internal variability and external forcing have contributed to the recent decadal variations in Pacific SSTs since the early 1990s, although large uncertainties exist among the model-simulated effects of anthropogenic aerosols. Plain Language Summary The Interdecadal Pacific Oscillation (IPO), the dominant mode of decadal to multidecadal variations of sea surface temperatures (SSTs) in the whole Pacific basin, has major impacts on global and regional climate. Given the large anthropogenic climate change during recent decades, it is natural to ask the following question: Are the recent IPO cycles also linked to anthropogenic forcing? Here we examine and attribute the roles of internal variability and external forcing in the recent IPO cycles through analyses of large ensembles of simulations by coupled climate models. Results suggest that while internal variability played a dominant role for the IPO cycles since 1920, the external forcing from volcanic and anthropogenic aerosols also contributed to the recent decadal variations in Pacific SSTs since the early 1990s. Our study contributes to the ongoing debate on the role of external forcing in recent decadal climate variability and improves current understanding of Pacific decadal variations, especially the relative role of external forcing and internal variability.