Pacific Decadal Variability: Structure and Mechanisms

Unlike El Niño/Southern Oscillation (ENSO), spatiotemporal structures of Pacific decadal variability (PDV) remains to be robustly defined, due, in part, to the relative short lengths of available data sets; subsurface observations relevant to PDV are especially few and sparse. The resulting ambiguity in PDV characterization, among others, leads to difficulties in analyzing the mechanisms operative in PDV. The first part of the proposed work is to improve characterization of PDV structures and evolution. The primary analysis will be based on rotated extended empirical orthogonal function (EOF) of a variety of ocean-atmosphere data sets; the analysis will be designed such that all major modes of Pacific low-frequency variability (e.g., biennial variability, ENSO, PDV, etc.) are extracted in a single step. Features of the analysis include using of unfiltered data, a Pan-Pacific analysis domain and inclusion of all four seasons in the analysis; all for the purpose of a consistent way of modal extraction. The results from rotated extended EOF will be corroborated with those from other analysis (e.g., canonical correlation analysis). Stability/physicality of the extracted modes is one major concern, which will be assessed via sensitivity tests and/or correlations with non-physical data, say, biological productivity. Expectation is to provide a robust and physically meaningful PDV modal extraction, which proves to be promising according to a pilot analysis of a century-long sea surface temperature (SST) data set. The second part of the work involves exploration of PDV mechanisms, via data analysis and diagnostic modeling. Lead/lag relationships between key oceanic and atmospheric variables will be analyzed in observations and in coupled simulations, in context of PDV; of interest is SST anomalies in the Pacific Tropics and midlatitudes that are connected via atmospheric and/or oceanic “bridges”. Diagnostic modeling is proposed to help understand the influence of SST anomalies on local and remote near-surface circulation and fluxes at different stages of PDV evolution. The role of upper ocean, on the other hand, will be investigated by analyzing the PDV-covarying heat content. Volumetric heat balance in the upper ocean will be evaluated in an effort to understand the relative importance of surface heat fluxes and subsurface processes in PDV context.