East Asian Summer High-altitude Westerly Jet Stream Variability And Its Mechanism

The East Asian upper-tropospheric jet stream (EAJS), which is one of the most important components of the East Asian summer monsoon and climate system, plays a crucial role in the weather and climate change in East Asia. By using the reanalysis data, in this study we investigated interannual and seasonal variation of the summertime EAJS, and the related circulation, sea surface temperature (SST) anomalies and possible mechanisms.Firstly, based on the EOF analysis, the results show that the interannual variations of the EAJS are characterized by the meridional displacement and intensity change, which explain about one half and one fourth total variance, respectively. Corresponding to the southward (northward) displacement of the summer EAJS, the western North Pacific summer monsoon is weakened. This meridional teleconnection over East Asia-western North Pacific is enhanced with the increase of absolute intensity of the anomalous East Asian subtropical precipitation. When the EAJS shifts southward (northward), the south Asian high moves southward (northward) in July and August and the western North Pacific subtropical high moves southward (northward) in June and August.Secondly, we calculated the lag correlation of the meridional displacement of monthly EAJS with the SST. The results show that the SST anomalies over central-eastern tropical Pacific significantly influence the variability of the sub-seasonal EAJS. The warm SST anomaly over the central tropical Pacific in the preceding winter may lead to the southward displacement of the June EAJS, possibly in a way as follows. The preceding winter warm SST anomaly firstly results in the suppressed south Asian summer monsoon in June, which induces the anticyclonic anomaly in the upper troposphere over west Asia by the baroclinic Rossby response. Then, the anticyclonic anomaly in the upper troposphere over west Asia perturbs the Asian westerly jet stream and induces the eastward propagated wave, which leads to the southward displacement of the June EAJS. While southward displacement of the EAJS in July and August are significantly related with the warm SST anomaly over eastern tropical Pacific in late spring-early summer.Finally, we pointed out that, in the seasonal evolution, the EAJS shows an abrupt northward jump after the onset of East Asian summer monsoon, which is phased-locked in the late July with the calendar year and intimately associated with the eastward propagation of disturbances in the upstream regions. By using the reanalysis data from 1958 to 2002, we found that in one half cases the EAJS jumps northward in late July, and the mean jump time is pentad 42 (July 25-29) and the standard deviation is 2.4 pentads. The northward jump of the EAJS is associated with the three zonally-oriented cells in the meridional direction over East Asia-western North Pacific. Both of the dynamical diagnosis with the reanalysis data and the linearized barotropic vorticity model reveal that the northward jump is connected with the eastward propagated wave activity flux in the upper troposphere on the north edge of Eurasian continent. In the end, we used the simulated 20th century climate data of IPCC AR4 in two coupled models to test the ability of climate models in reproducing the seasonal variation of the EAJS. Both of the two models show the northward jump of the EAJS and the associated meridional triple structure of zonal winds anomalies over East Asia-western North Pacific, but not the strong tendency of phase-locking of the northward jump with the calendar year, which suggests it is necessary to further investigate the variability and related mechanisms of the EAJS.