The climatological Rossby wave source over the STCZs in the summer Northern Hemisphere

Abstract

Using the twice-daily data for the 10-year period from 1989 to 1998, from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis, the authors analyze the climatological Rossby wave sources at 200 hPa in summer, focusing on the upper-tropospheric circulation related to the subtropical convergence zones (STCZs) in the Northern Hemisphere, namely, the North Pacific convergence zone (NPCZ, including the Meiyu/Baiu front) and the North Atlantic convergence zone. In the June-July-August (JJA) period, a Rossby wave sink clearly appears along the upper-tropospheric westerly jet streams over East Asia and the extratropical North Pacific, but is very weak in the extratropical North Atlantic. For the JJA mean, or in early summer (June), the Rossby wave sink over East Asia, and the extratropical North Pacific, is much stronger than that in the tropical western North Pacific. The latter sink, however, becomes stronger in late summer (August), corresponding to the enhancement and poleward shift of the atmospheric convection, and becomes comparable with the former sink. The components of the Rossby wave source, namely, vortex stretching and advection of absolute vorticity by divergent flow, are also examined and used to judge the roles of tropical and extratropical heatings. Despite the integrated appearance of the sink along the NPCZ, this sink results dominantly from the advection of absolute vorticity by divergent flow in East Asia, and the western Pacific, and from vortex stretching over the mid-Pacific. These results reveal that the tropical heating is responsible for the maintenance of upper-tropospheric circulation in East Asia, and the extratropical western North Pacific, through the advection of absolute vorticity by divergent flow, while the diabatic heating in the NPCZ plays a dominant role in maintaining the upper-tropospheric circulation over the mid-Pacific. © 2004, Meteorological Society of Japan.