This project aims to understand how natural low-frequency modes of Pacific variability will respond to anthropogenic climate change. Recent work by the PIs has shown that decadal climate variability of the Pacific is only partially explained by the Pacific Decadal Oscillation (PDO). A better representation of the Pacific decadal climate dynamics must include the North Pacific Gyre Oscillation (NPGO), which is associated with basin- and regional-scale changes in ocean circulation, sea-surface temperatures, and ecosystem transitions. The El Niño Southern Oscillation (ENSO) drives an important fraction of both the PDO and NPGO variability. While the canonical eastern Pacific warming (EPW) El Niño drives a PDO response, new evidence suggests that the non-canonical central Pacific warming (CPW) El Niño (or Modoki) is dynamically linked to the NPGO. The IPCC AR4 climate model projections suggest that the frequency of CPW El Niño events will increase in response to climate change, suggesting that the NPGO variance will increase at the expense of the PDO. In the proposed research, we examine the dynamical relationship between these modes of low-frequency Pacific climate variability and anthropogenic climate change in both observations and climate models. The first research goal of this proposal is to assess how the dynamics and statistics of the EPW/PDO and CPW/NPGO modes are represented in the IPCC AR4 and AR5 coupled climate models during the 20th century, and how these dynamics and statistics are projected to evolve under continued greenhouse forcing during the 21st century. The second research goal of this proposal is to identify and quantify the statistical significance of any anthropogenic changes in CPW/NPGO variance with respect to natural variability, and identify the specific dynamics responsible for the CPW/NPGO’s response to climate change. This project brings together a diverse group of climate scientists that will pursue these research goals using a wide range of methodologies designed to isolate, quantify and diagnose the effects of anthropogenic forcing on Pacific decadal climate variability. These methodologies include (a) linear inverse statistical techniques to examine the IPCC models (Di Lorenzo, Anderson, Schneider), (b) annually-resolved multi-proxy climate reconstructions over the last 300 years to examine the range of decadal and secular variations of the climate modes (Cobb, Di Lorenzo), and (c) targeted coupled climate model experiments to isolate specific mechanisms of the low-frequency modes responses to climate change (Vimont, Alexander). Overall, improved understanding of the historical, current, and future evolution of the NPGO and PDO will provide significantly enhanced predictability of decadal-scale variations that influence global weather and climate patterns, as well as marine ecosystems.