In this pa per, Asian dust storm activity from 1995 to 2006 and the associated atmospheric circulation are examined using SYNOP data and the NCEP/NCAR reanalysis atmospheric data. Observations show that the Gobi Desert is the most frequent birth place for severe dust events in Asia, accounting for pproximately 58% of the total percent age, followed by about 32% from the Taklamakan Desert and nearly 10% from the Loess Plateau. Climatologically, the existence of a large-scale dry zone over mid-latitudes of Asia during the Spring pro vides a favor able environment for the frequent occurrences of dust events and subsequent dust transport across Asia.
To provide aquantitative measure of Asian dust storm activity, a "dust activity index" (DAI) is defined in this study. The time series of yearly DAI shows that Asian dust storm activity manifested marked interannual variations during 1995 ~ 2006. For an active year such as 2001, the magnitude of DAI (26986) is about a factor of 5 ~ 6 larger than that in 1997 (4569). Our analyses show that such variations are closely connected to the position of EAT (East Asian Trough), rather than to its strength. In a year when the EAT was shifted west ward (e.g., 2001), an east-west oriented low-high dipole appeared over Asia. This anomalous pressure dipole enhanced the prevailing north westerly flows over Mongolia and northern China, resulting in a drier-than-nor mal at mo sphere in favor of dust storm activity. On the contrary, in a year when the EAT was shifted east ward (e.g., 1997), a reverse dipole occurred, resulting in a wetter-than-nor mal at mo sphere disfavoring dust storm activity. A SVD (singular value decomposition) analysis of the Asian synoptic circulation has shown that the connection between the pressure dipoles and the position of EAT is rather robust when dust storm activity is particularly strong (e.g., 2001, 2002, and 2006) or weak (e.g., 1997 and 1999).
Analyzing a large number of trajectories de rived from the NOAA HYSPLIT model further suggests that dust aerosols arisen from the Gobi Desert and the Loess Plateau are likely to prop a gate east ward along the strong westerly flows. On the contrary, easterly flows occurring over the Taklamakan Desert in cline dust aero sols there to slow west ward propagation. Different geographical locations relative to the mean circulation and topography effect are responsible for such different patterns.