Abrupt Northward Jump of the East Asian Upper-Tropospheric Jet Stream in Mid-Summer

In this study we document the abrupt seasonal migration of the East Asian upper-tropospheric jet stream (EAJS) in boreal mid-summer over the East Asian coast, based on the NCEP/NCAR reanalysis data from 1958 to 2002. In climatology, an abrupt northward jump of the EAJS is identified in late July. By examining each of the total 45 years there are 27 years in which the EAJS exhibits an abrupt northward jump in mid-summer, and about half of the jump cases occur in late July, suggesting the characteristic of phase locking with the calendar year. A cluster analysis on the meridional variation of the upper-tropospheric zonal wind anomalies along the eastern Asian coast associated with the northward jump of the EAJS reveals that the northward jump of the EAJS is dominated by two categories, in which the intensity of the EAJS is enhanced and weakened, respectively. For both categories, there is a pair of westerly and easterly anomalies which are located to the north and south of the EAJS's axis. However, these westerly and easterly anomalies exhibit distinct temporal variations between the two categories: The pair of westerly and easterly anomalies shifts southward from the high latitudes to the EAJS region for the first category, but appears as a quasi-stationary meridional dipole pattern for the second category.