Formation of the quasi-stationary Baiu front to the south of the Japan Islands in early may of 1979

The formation of the “quasi-stationary” Baiu front at the beginning of May 1979 and its relation to the seasonal transition of the large-scale baroclinicity in East Asia was examined by using the observational data. The frontal zone to the south of the Japan Islands, corresponding to the southern branch of the middle-level westerly jet around the Tibetan Plaeau, is characterized by the alternative passages of synoptic-scale extratropical cyclones and anticyclones in April. On the other hand, the cloud belt associated with the stationary front is sustained at ~25°N in May, even after the passage of the synoptic-scale (or larger meso-α-scale) cyclone at ~30°N. In other words, the frontal zone there becomes quasi-stationary at the beginning of May (the formation of the “quasi-stationary Baiu front” around Japan). The analysis of the location of the front on twice-daily surface weather maps in 1985, 1986, 1987 and 1988, as well as 1979, shows that the change into the quasi-stationary frontal zone occurred in early May of those other years. The two baroclinic zones, corresponding to the southern and the northern branches of the westerly jet, respectively, are separate from each other around the Japan Islands in May. This results in the weakening of baroclinicity in May just to the north of the Baiu front as well as across the Baiu front. Thus the development of a migratory anticyclone is suppressed, which provides a favorable condition for sustaining the “quasi-stationary” Baiu front in May. It is noted that the change into the “quasi-stationary” frontal zone at the beginning of May 1979 mentioned above is a different event from the abrupt disappearance of the temperature gradient across the Baiu front in China in late May pointed out by Kato (1985a, 1987).