Impacts of ENSO on Atmospheric Circulation over the Western Tibetan Plateau in Autumn

Abstract As the leading mode of atmospheric circulation variability over the western Tibetan Plateau (TP), the western Tibetan vortex (WTV) exerts an important influence on the climate over the western TP. However, it is still unclear how the WTV is affected by El Niño–Southern Oscillation (ENSO). Using the fifth major global reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ERA5), we find that ENSO and the WTV are most linked in autumn. La Niña (El Niño) episodes are associated with an anticyclonic (a cyclonic) WTV. Moreover, the autumn coupling is stronger with a stronger ENSO. Sea surface temperature (SST)-forcing experiments validate this influence of ENSO on the autumn WTV although with a slightly weaker intensity. Importantly, by conducting a set of runs with ENSO-related SST forcing in three key tropical regions, we identify the tropical western Pacific as the key area from which ENSO exerts its influence. Finally, we propose a possible mechanism. During La Niña (El Niño) episodes, the strengthened (weakened) Walker circulation in the Indian Ocean causes large-scale easterly (westerly) wind anomalies at the upper troposphere over the tropical Indian Ocean. These anomalies extend to the subtropical area near the southern edge of the western TP. To maintain the quasigeostrophic balance, the geopotential height over the western TP increases (decreases), resulting in an anticyclonic (a cyclonic) WTV. A stronger ENSO would cause a stronger large-scale zonal wind anomaly near the southern edge of the western TP, leading to a stronger WTV. These results contribute to the understanding of the coupling processes between the tropical and Tibetan climates, with great implications for the major drivers of the Tibetan climate.