Modal Characteristics Of Summer Asian Subtropical Westerly Jet Anomalous Activity And Its Possible Causes

The subtropical upper-tropospheric jet stream is an important component of the planetary-scale circulation and plays a vital role in the transportation and equilibration of atmospheric momentum and energy. First of all, by using theoretical analysis and dynamic diagnostic methods, we analyzed the physical characteristics and the connection of external forcing and large-scale circulations with the dominant modes of summer Asian subtropical westerly jet (ASWJ). Secondly, through version 5.1 of the NCAR Community Atmosphere Model (CAM5.1), numerical experiments are designed to explore the specific contribution of different regions SST anomaly. These results aid in a more comprehensive understanding of the changing characteristics of summer ASWJ and provide a new basis for the study of the jet variation mechanism. The main conclusions are as follows:(1) We applied the ERA-Interim (1979-2014) and National Center for Environmental Prediction (NCEP) (1958-2014) re-analysis data in an empirical orthogonal function (EOF) decomposition of the summer zonal wind anomaly in the upper troposphere (at 200hPa) over Asia. Consistent spatio-temporal modes were derived from the two datasets, with the two dominant modes representing the principal features of the summer ASWJ. The first mode describes the meridional displacement of position of ASWJ, which has been the focus of a number of previous studies; the second mode describes the zonally asymmetric distribution of the summer jet over East and West Asia, which partly reflects coordinated variations in the upstream and downstream systems and has not been widely studied.(2) Further analysis of the physical characteristics of two jet modes showed the stable quasi-barotropic conditions of composited wind and geo-potential height field, the upper-level wind and pressure fields dominate the distribution of the circulation throughout the whole atmospheric layer. Besides, the anomalous distribution of the jet is closely related to the atmospheric thermodynamic anomaly. It is also evident that the zonal wind anomaly is significantly affected by the meridional temperature gradient, which complies with the thermal wind theorem.(3) The variation mechanism of different ASWJ modes has been preliminary investigated, tropical SST is considered to be the dominant factor of the first jet mode. Thermal forcing that is closely related to the second jet mode mainly originates from the continent and the ocean at middle- and high-latitude regions. Moreover, the effect of large-scale circulation adjustment seems to be nonnegligible.(4) As an important external forcing factor of atmospheric circulation, SST has outstanding contributions to both ASWJ modes. Various of numerical experiments are designed to verify the specific effects of different regions SST anomaly for the observation. Results show that, the first mode is intimately related to the similar influences of the Pacific and Indian Ocean SSTA; while the upstream and downstream wind fields of the second mode are affected by different sea surface heat source.