The Break in the Mongolian Rainy Season and Its Relation to the Stationary Rossby Wave along the Asian Jet

Abstract

Seasonal and interannual variation of rainfall over Mongolia was investigated using 10-day rainfall data from 92 stations during 1993–2001 and NCEP–NCAR reanalysis data from 1979 to 2001. The break in the rainy season was found in the middle of July, and the meteorological stations with a clear break period were concentrated in eastern Mongolia where the plain prevails relative to western Mongolia without regard to the difference in annual precipitation. Clear breaks in the rainy season were recognized in 5 yr among an analysis period of 9 yr. In the break period, the stationary Rossby wave trapped in the Asian jet was predominant at 200 hPa, and a barotropic ridge associated with the Rossby wave developed over Mongolia. Furthermore, interannual variation of the break also corresponded to the variation of the stationary Rossby wave. It is considered that the break of the Mongolian rainy season is caused by the stationary Rossby wave trapped in the Asian jet. The stationary Rossby wave was climatologically phase locked in seasonal evolution and, as a result, the break period was also concentrated around the middle of July.