Summer Upper-Level Vortex over the North Pacific

During northern warm seasons, the overhead sun moves northward across the equator to the Northern Hemisphere. Because of the decrease in the meridional thermal gradients, particularly in midlatitudes, the Northern Hemisphere summer circulation becomes less rigorous in its intensity. In spite of this seasonal change, certain circulation elements still distinctly stand out. As shown in Fig. 1a, the upper troposphere exhibits the major Tibetan high and the smaller Mexican high, both separated by the strongly tilted oceanic troughs over the North Pacific and the North Atlantic (Krishnamurti 1971). Northwest of these two subtropical midocean troughs are the weak midlatitude troughs along the east coasts of East Asia and North America. Corresponding to these upper midlatitude troughs are the surface lows over the Bering Sea and the Labrador Sea. In contrast, beneath the two subtropical oceanic troughs are oceanic anticyclones divided by the Indian monsoon trough and the North American heat low (White 1982). In the deep Tropics adjacent to these subtropical circulation elements are equatorial troughs, for example, the intertropical convergence zone. As summarized by Krishnamurti (1979) and Nitta (1982), tropical synoptic disturbances embedded in the Asian summer monsoon system consist of equatorial waves in the western tropical Pacific (e.g., Yanai et al. Summer Upper-Level Vortex over the North Pacific