Interannual vs Decadal Impacts of Pacific Variability

Abstract

<p>Pacific Decadal Variability (PDV) is characterised by a pattern similar to the interannual El Niño Sothern Oscillation (ENSO), with anomalously warm sea surface temperatures (SSTs) in the tropical Pacific surrounded by a horseshoe of cool SSTs. PDV is one of the most influential decadal climate modes, contributing to the ‘slowdown’ of global temperature rise at the beginning of the 21<sup>st</sup> century and affecting precipitation extremes worldwide. However, differences between climate impacts of PDV and ENSO are more uncertain.</p><p>Here, the interannual and decadal impacts of PDV are investigated using ERA5 re-analyses and forced Pacific experiments from the CMIP6 Decadal Climate Prediction Project (DCPP) which impose the PDV SST pattern in the coupled Met Office climate model (HadGEM3.1). Both model and observations show a decreased Pacific Walker circulation and an increased Hadley response which combine to generate large scale Rossby waves, impacting global atmospheric circulation and precipitation patterns.</p><p>Impacts on the winter North Atlantic Oscillation (NAO) are opposite on the two timescales in both the model and observations. On interannual timescales a positive PDV produces a weakening of the stratospheric polar vortex and a negative NAO, whereas on decadal timescales a strengthening of the polar vortex and positive NAO is seen. This appears to be related to the balance between an increased meridional temperature gradient which tends to shift the jet poleward on decadal timescales, and increased  upward propagation of planetary waves which reduces the polar vortex and shifts the jet equatorward on interannual timescales. Reasons for these differences will be explored further.</p>