Zonally asymmetric mode of anomalous activity in summer Asian subtropical westerly jet and its possible sources

Abstract

The ERA-Interim (1979–2016) and National Center for Environmental Prediction (NCEP) (1958–2016) reanalysis data are applied in an empirical orthogonal function (EOF) decomposition of the summer zonal wind anomaly in the upper troposphere (at 200 hPa) over Asia. Consistent spatio-temporal modes are derived from the two datasets on different time scales, with the first three dominant modes representing the principal features of the summer Asian subtropical westerly jet (ASWJ). The first and third modes respectively describe the meridional position deviation and the overall strength variations of ASWJ, and both exhibit zonally symmetric distributions, which have been extensively investigated. The second mode describes a zonally asymmetric distribution of the jet over Asia, which reflects coordinated variations in the upstream and downstream systems and has not yet been systematically studied. In this study, we focus on the analysis of the second mode of ASWJ. The observation results show that the second mode is closely related to external forcing factors as well as the large-scale circulation at mid-high latitudes. Through the interaction of the preceding North Atlantic sea surface temperature (SST), the simultaneous soil moisture, and the atmospheric circulation that propagates zonally via the waveguide effect of the jet, the ocean–land–atmosphere coupled system can produce coordinating impacts on the variability of ASWJ. These results aid in a more complete and comprehensive understanding of the physical characteristics of the summer westerly jet and provide a new perspective for the study of the variability of ASWJ.