Seasonal variations of the synoptic-scale transient eddy activity (STEA) and the jet streams over East Asia are examined through analysis of the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data. Extracted from the 6-hourly upper-level wind fields, the distribution of the jet core numbers exhibits a distinct geographical border for the East Asian subtropical jet (EASJ) and the East Asian polar front jet (EAPJ) at the latitudes of the northern Tibetan Plateau (TP). In the cool seasons, two branches of the STEA and low-level baroclinicity exist over the East Asian landmass, accompanied by the two-jet state of the EASJ and EAPJ. In the warm seasons, a single jet pattern of the EASJ along the north flank of the TP is accompanied by the weakened STEA over the mid- to high latitudes of East Asia. Further analysis shows two distinct features of the seasonal variations of the STEA over East Asia, compared with that over the North Pacific. First, during the transitional period of April–June, the main STEA band over East Asia migrates northward dramatically, in conjunction with the EAPJ shifting in the same direction. Second, both the upper-level STEA and the lower-level baroclinicity poleward of the TP are prosperous in spring. The relationship between the STEA, baroclinicity, vertical wind shear, and static stability in the EAPJ region in different seasons is further investigated. It is found that in addition to the time-mean wind fields, the rapid increase in the sensible heat flux poleward side of the TP region in spring and the associated boundary layer processes are partially responsible for the spring prosperity of the local baroclinicity and the STEA.

A set of statistical analyses of the fields from the European Center for Medium‐Range Weather Forecasts reanalysis (ERA‐40) are carried out to investigate the connections between three selected fields, namely, the wintertime western Pacific jet stream (WPJS), as represented by the zonal wind at 200 hPa; the oceanic surface heating represented by the latent and sensible heat fluxes; and the synoptic‐scale transient eddy activity (STEA) over the North Pacific. First, empirical orthogonal function (EOF) analysis is performed to characterize the wintertime WPJS variability. Further analyses yield the following conclusions. (1) The most dominant relationship among the three selected fields is strongly linked to the El Niño‐Southern Oscillation (ENSO). During ENSO events, anomalous zonal wind at 200 hPa downstream of the WPJS main body, shown in the first EOF, is accompanied by STEA anomalies around the region of the anomalous midlatitude zonal wind. (2) The strengthening (weakening) of the wintertime WPJS main body, shown in the second EOF, is connected to the surface heating anomalies in the Kuroshio and its extension region as well as to STEA anomalies in the region downstream of the WPJS main body. (3) The northward (southward) shift of the wintertime WPJS, represented in the third EOF, is closely related to the south‐north dipolar structure of the surface heating anomalies in the western North Pacific. The STEA spatial displacement is in conjunction with the shift of the jet stream in the same direction due to the dynamical connection between the winds and the eddies. Furthermore, the STEA anomalies in winter may cause the WPJS anomalies on the submonthly timescale. As a consequence, the northward (southward) shift of the STEA is harmoniously in favor of the similar jet shift.

This study examines the features and dynamical processes of subseasonal zonal oscillation of the western Pacific subtropical high (WPSH) during early summer, by performing a multivariate empirical orthogonal function (MVEOF) analysis on daily winds and a diagnosis on potential vorticity (PV) at 500 hPa for the period 1979–2016. The first MV-EOF mode is characterized by an anticyclonic anomaly occupying southeastern China to subtropical western North Pacific regions. It has a period of 10–25 days and represents zonal shift of the WPSH. When the WPSH stretches more westward, the South Asian high (SAH) extends more eastward. Above-normal precipitation is observed over the Yangtze–Huaihe River (YHR) basin. Suppressed convection with anomalous descending motion is located over the subtropical western North Pacific. The relative zonal movement of the SAH and the WPSH helps to establish an anomalous local vertical circulation of ascending motion with upper-level divergence over the YHR basin and descending motion with upper-level convergence over the subtropical western Pacific. The above local vertical circulation provides a dynamic condition for persistent rainfall over the YHR basin. An enhanced southwest flow over the WPSH’s western edge transports more moisture to eastern China, providing a necessary water vapor condition for the persistent rainfall over the YHR basin. A potential vorticity diagnosis reveals that anomalous diabatic heating is a main source for PV generation. The anomalous cooling over the subtropical western Pacific produces a local negative PV center at 500 hPa. The anomalous heating over the YHR basin generates a local positive PV center. The above south–north dipolar structure of PV anomaly along with the climatological southerly flow leads to northward advection of negative PV. These two processes are conducive to the WPSH’s westward extension. The vertical advection process is unfavorable to the westward extension but contributes to the eastward retreat of the WPSH.

A regional climate model is coupled to a regional ocean model in this study, its performance in simulating the East Asian summer monsoon in 1998 is discussed, and the cold drift of the simulated sea-surface temperature (SST) that appears in the coupled model system and the reasons for its existence are found. The atmospheric component of the coupled model system is a regional climate model named P–σRCM, and the oceanic component of the coupled model is based on the Princeton ocean model (POM). By simulation-to-observation comparison, it is shown that the coupled model has a certain ability to simulate the seasonal propagation of the monsoon rain belt, the associated low-level southwest wind and the western Pacific subtropical high from May to August in 1998. It can reproduce the East Asian summer monsoon onset during the third dekad of May and its evolution process after the onset. From the coupled simulation results, it is also found that the south-to-north inflow in the southern South China Sea (SCS) and the summer monsoon current to the south of Sri Lanka do not occur until after mid-May, indicating that the ocean surface currents in the SCS and the Bay of Bengal regions are closely related with the SCS summer monsoon. The coupled simulation results show some differences with the individual uncoupled P–σRCM and POM. The obvious difference between the coupled and the uncoupled POM (also observation) is a cold tendency of simulated SST in the coupled model system. Two experimental groups are performed to investigate the reasons for the ‘cold drift’. The results confirm that the ‘cold drift’ is mainly caused by the disagreement of the surface heat fluxes produced by the atmosphere model with those required by the POM. Therefore, a more accurate description and parameterization of the physical process in the atmosphere model is an important way to decrease the differences. Copyright © 2005 Royal Meteorological Society.