Season-Reliant Dominant Modes of Interannual Oscillations in the Eurasian-Pacific Westerly Jet and Their Driving Factors

The Eurasian-Pacific jet (EPJ) is a critical subsystem of the upper-tropospheric westerly jet in the Northern Hemisphere. It exerts a significant impact on mid-latitude extreme weather and climate through abnormal changes of its position and intensity. Using data from 1979 to 2022, decomposition of 200 hPa zonal wind anomalies in the EPJ region (20°N—50°N, 10°W—150°W) is conducted through Season-reliant empirical orthogonal function (S-EOF). This study systematically reveals the spatiotemporal characteristics of two major season-reliant dominant modes of the EPJ. The first mode (with a variance contribution of 17.53%) reflects the interannual oscillation features of the EPJ′s north-south oscillation across all seasons, with the most pronounced signal in autumn and winter. This mode is primarily controlled by tropical sea surface temperatures. Specifically, during the developing phase of El Niño (La Niña), anomalous tropical sea surface temperatures (SSTs) modify the meridional temperature gradient in the mid-low latitudes. This drives the southward (northward) shift of the jet. The second mode (with a variance contribution of 9.62%) primarily characterizes the interannual oscillatory features of the North Pacific jet core, which weakens and expands meridionally (strengthens and contracts meridionally) in the subsequent spring. A weakened (strengthened) winter Arctic Oscillation (AO) induces a series of anomalies in the subsequent spring over central-eastern Siberia and the Arctic Ocean coast. These anomalies include cold (warm) surface skin temperature (SKT) and above-normal (below-normal) sea ice concentration (SIC). Through associated feedback processes, they modify the meridional temperature gradient and subsequently drive changes in the jet's meridional extent and intensity.