Dynamical Components of Observational Pacific Decadal Oscillation

The Pacific Decadal Oscillation (PDO) is a robust pattern of sea surface temperature (SST) variability of the global ocean and the Pacific Ocean. It emerges as a leading pattern in several different kinds of analysis: e.g., as the leading principal component (PC) of North Pacific SST, as the leading PC of 6-yr or longer low-pass filtered pan-Pacific SST and as one of the leading modes of pairwise-rotated global SST. There is still considerable debate about the nature and causes of the PDO and, in particular, whether it should be viewed as the "debris of ENSO" (a term coined by Vimont, 2005), as a North Hemisphere extratropical phenomenon that exists independently of ENSO, or as something in between.

However, determining the relative role played by each process remains challengeable because the surface atmosphere variability over the North Pacific acts as both the response to tropical ENSO through "atmosphere bridge" and the forcing to drive the PDO variability. Based on the empirical orthogonal function analysis of the tendency of SST, which represents the integrated response of SST to the surface atmospheric forcing and oceanic processes, we quantitatively decompose the observed PDO into three orthogonal components, respectively. First component is associated with the ENSO bridged from tropical Pacific, second a pure local stochastic atmospheric forcing which is independent of ENSO, and third the residual. This statistical decomposition of the observational PDO help us to identify that the local stochastic atmospheric forcing can only explain about 20% variance of PDO in the North Pacific, while about 40% variance are associated with tropical ENSO teleconnection. Our finding provides an observational constrain on the tropical-extratropical connection for the climate model evaluation.