The location change of the westerly jet core at upper troposphere in June and July is investigated by using the NCEP/NCAR reanalysis data. The results show that the location of the westerly jet core changes rapidly from 140°E to 90°E during 35th–39th pentads, which corresponds to the plum rain period over East Asia. The location change of the jet core is actually the relative intensity change of the different westerly jet centers. The meridional temperature contrast in the troposphere is associated with the rapid location change of the jet core. The diabatic heating changes are the primary factor determining the seasonal evolution of the westerly jet core over East Asia.
 In the upper troposphere and lower stratosphere, there exists a narrow and strong westerly belt with large horizontal and vertical wind shears over the subtropical East Asia, which is referred to the East Asian Subtropical Westerly Jet (EASWJ) [Sheng, 1986]. The EASWJ exhibits robust seasonal evolutions in the intensity and location. The axis and center of the EASWJ are located at 200 hPa and reach the southernmost position in March and the northernmost in August. The central intensity of the EASWJ is about 70 m s−1 in winter and 35 m s−1 in summer. From winter to summer, the axis experiences two northward jumps [Yeh et al., 1958]. Previous studies noticed that the seasonal jump of the EASWJ is closely linked to the monsoon climate in East Asia [Yin, 1949; Yeh and Zhu, 1955; Tao et al., 1958]. Recent progresses prove that the EASWJ has a stronger impact on the Asian-Pacific climate in comparison with that of ENSO [Yang et al., 2002; Liao et al., 2004]. The meridional shift of the westerly jet center also has close relationship with both the Asian monsoon onset and the interannual variability of the rainfall over China [Lau et al., 1988; Ding, 1992; Liang and Wang, 1998; Li et al., 2004; Zhou and Yu, 2005]. The orography and tropical convective heating are responsible for the formation and seasonal evolution of the EASWJ [Bolin, 1950; Smagorinsky, 1953; Krishnamurti, 1979; Yang and Webster, 1990; Dong et al., 1999, 2001]. The meridional migration of the EASWJ is essentially governed by the seasonal cycle of solar radiation and the thermal effect of the Tibetan Plateau. Previous studies mainly focused on the meridional shift of the westerly jet and its effect on eastern Asian weather and climate. The seasonal evolution of the EASWJ also experiences obvious longitudinal change, which might be related to the land-sea thermal contrast in the longitudinal direction. Unfortunately, less attention has been paid to this so far. The objective of our study is to understand the mechanism, physical processes and climate effects of the longitudinal change of the EASWJ in the early summer. The data we used is taken from the NCEP/NCAR reanalysis, and the time period covers 1961–2000 [Kalnay et al., 1996].
2. The Seasonal Evolution of the Westerly Jet at Upper Troposphere
2.1. The Climatologic Structure of the Westerly Jet
 The center of the westerly over mid-latitudes is located at 200 hPa throughout the year. The EASWJ is normally defined as the 200 hPa westerly stronger than 30 m s−1. In January, the easterly is located to the south of 10°N, and the westerly is dominant in mid-high latitudes. The EASWJ is situated between 20°N and 40°N, with its axis at 32°N and maximum zonal wind speed exceeding 70 m s−1 over the ocean to the southeast of Japan. There is another center to the west of 60°E, which is about 20 m s−1 weaker than the major one. In April, the center of the EASWJ is located at the similar position of January, but the intensity is obviously weakened. In July, the tropical easterly strongly intensifies and expands to the north of 20°N; the center of the EASWJ reaches north of 40°N, which is about 10° north to that of January. Two centers of the EASWJ are located over the northern Tibetan Plateau and the northern Iranian Plateau, respectively, having central values of 30 m s−1. In July, both the intensity and the location of the EASWJ are different from that in January. In October, the EASWJ intensifies and its center moves eastward to the location near that in April. During the seasonal evolution of the EASWJ, the south-north displacement of jet axis is less than 10 latitudes, whereas the east-west shift of the jet center over East Asia is more than 50 longitudes. Why does the EASWJ center present such strong longitudinal seasonal variations? This issue will be examined in the following section.
2.2. Longitudinal Change of the EASWJ Core
 The location of the EASWJ core is represented by the position of the maximum westerly over East Asia. The seasonal evolution of the EASWJ core is shown in Figure 1. The core of the EASWJ is located near 140°E before June and in 82°E in July, indicating a rapid seasonal east-west displacement of the EASWJ core between June and July (Figure 1a). The location change of the jet core can also be identified from the pentad mean data (Figure 1b). The location of the EASWJ core changes from 140°E to 90°E between the 35th and 39th pentad. Further analysis on the longitude-height cross section of the pentad mean zonal wind in Figure 2 indicates that the location change of the EASWJ core, as identified in Figure 1, actually reflects the relative intensity change of the different EASWJ centers. As shown in Figure 2, there are three centers over 200 hPa during 36th–37th pentads. All the three centers exhibit little changes in their position during 36th–37th pentads. In contrast, the relative intensity of the centers displays some change during pentads 35–40, which results in the location change of the EASWJ core. It is well known that the plum rain season in China starts after pentad 33 or 34 and ends after pentad 38 climatologically [Tao and Chen, 1987], this coincidence is of significance for the determination of the rainy season beginning and ending date in East Asia. To further examine the reliability of the subseasonal change of the EASWJ core, Figure 3 shows the occurrence number of the EASWJ core in the region of (30°–45°N, 60°–180°E) from pentad 32 to pentad 43. The EASWJ core occurs most frequently to the east of 140°E before pentad 35, then the large occurrence numbers oscillates between 140°–160°E and 85°–110°E during 36th–39th pentads, and after pentad 39, the EASWJ core shifts to the west of 100°E, having occurrence numbers centralized steadily at 90°E.
 East Asia is dominated by a persistent rainy season during early summer. The rapid transition time of the EASWJ core corresponds to the start and end of this rainy period.
3. Possible Mechanism
 Based on the principle of thermal wind:
the variation of zonal wind with altitude depends on the meridional gradient of air temperature. If the air temperature is decreased poleward, the westerly increases or the easterly decreases with altitude; on the contrary, if the air temperature is increased poleward, the westerly weakens or the easterly intensifies. Therefore, strong zonal winds normally appear over the frontal area of the troposphere, and the intensity of zonal winds is directly proportional to the intensity of the meridional gradient of air temperature.
Figure 4 shows the distribution of the zonal wind at 200 hPa and the meridional air temperature differences (hereinafter MTD) averaged vertically from surface to 200 hPa (values larger than 1.8°C are shaded) during 35th–40th pentads. The MTD is calculated by using the air temperature in the south minus that in the north with 2.5° latitude interval, so the positive MTD means relatively warmer in the south and colder in the north. Note that the large value of MTD from surface to 200 hPa matches well with the 200 hPa westerly jet shown in Figure 4, thus the 200 hPa westerly jet always follows the larger MTD from surface to 200 hPa. The MTD weakens over East Asia after pentad 38, then the EASWJ intensity reduces and the core disappears correspondingly. Thus the change of MTD determines the shift of the core position of the EASWJ.
 The diabatic heating has a strong impact on the atmospheric temperature change. To unravel the contribution of the diabatic heating to the temperature change in the lower-upper troposphere, and to the westerly jet core shift at 200 hPa, the total diabatic heating rate is diagnosed by using the NCEP/NCAR reanalysis data. The time-longitude variation of the diabatic heating rate averaged between 30°N and 45°N, from surface to 200hPa, are shown in Figure 5. The diabatic heating includes turbulent heating, condensation latent heating and radiative heating. The strong heating is located to the east of 130°E before pentad 24, and the westerly jet core occurs over this area coincidently. With the enhancement of the diabatic heating from 80°E to 100°E, the jet core approaches to this area. Meanwhile, the diabatic heating to the east of 120°E weakens. Thus the diabatic heating is responsible for the intensity change and location shift of the westerly jet core at the upper troposphere. Moreover, the evolution of the diabatic heating rate shown in Figure 5 represents the longitudinal thermal contrast, which is also related to the longitudinal change of the EASWJ core.
 The seasonal evolution of the EASWJ at the upper troposphere is examined by using the NCEP/NCAR reanalysis data. The feature of the location change of the westerly jet core in June and July is found from both the monthly mean data and the pentad mean data. The location of the jet core changes from 140°E to 90°E during 35th–39th pentads. Further analysis shows that the location change of the jet core actually reflects the relative intensity change of the different jet centers. As the plum rain season in China starts from pentad 34 or 35 and ends after pentad 38 climatologically, the location change of the jet core is possibly related to the rainy season commencement and ending in East Asia, which is of significance for the determination of the rainy beginning and ending date. The meridional temperature difference from surface to 200 hPa is responsible for the location and its change of the westerly jet centers. The diabatic heating variation with season is the primary factor determining both the intensity and location change of the EASWJ core.
 The reanalysis data was provided by the National Centers for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR). This work was jointly supported by the National Natural Science Foundation of China under grant 40333026, National Basic Research Program of China (2006CB400506) and Open Research Program of Key Laboratory of Regional Climate-Environment Research for Temperate East Asia (RCE-TEA), Chinese Academy of Sciences. We are grateful to Jie Song from Northern Illinois University for improving the manuscript. We also very much appreciate insightful comments and suggestions from two anonymous reviewers.