Impacts of NAO on Western Tibetan Vortex: Summer Versus Winter Features

The North Atlantic Oscillation (NAO), a crucial low-frequency variability mode in the atmospheric circulation of the Northern Hemisphere at mid-to-high latitudes, exhibits an unclear mechanism of influence on the Western Tibetan Vortex (WTV). Based on ERA5 reanalysis data, this study elucidates the different impacts of NAO on WTV in summer versus winter. The results demonstrate that: (1) The impacts of the NAO on WTV are the strongest in summer, and stronger NAO events exert a heavier influence on the WTV. (2) The NAO modulates the WTV mainly through influencing the zonal winds north and south of the western Tibetan Plateau (TP): the positive (negative) strong NAO causes the enhanced westerlies (easterlies) on the northern flank and easterlies (westerlies) along the southern flank of the western TP, resulting in an anticyclonic (cyclonic) WTV. (3) The differences in wave energy propagation between summer and winter can explain the seasonal dependence of the impacts of the NAO on the WTV. In summer, wave energy propagates eastward along the westerly jet from the North Atlantic, reaching the western TP, generating anticyclonic/cyclonic circulation anomalies and altering the zonal winds along the western TP's northern and southern flanks, thereby modifying the WTV's circulation structure and intensity. In winter, however, the wave propagation associated with the NAO is stronger between middle and high latitudes in the North Atlantic sector, with less eastward propagation to the TP, resulting in limited influence on the WTV. This study verifies that high-latitude systems influence low-latitude climates through upstreamdownstream effects, providing a foundation for extended-range forecasting of plateau weather systems and responses to climate change.