Asian dust storm activity and its association with atmospheric circulation from 1995 to 2006

摘要

In this paper, 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 birthplace for severe dust events in Asia, accounting for approximately 58% of the total percentage, 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 provides a favorable environment for the frequent occurrences of dust events and subsequent dust transport across Asia. To provide a quantitative 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 westward (e.g., 2001), an east-west oriented low-high dipole appeared over Asia. This anomalous pressure dipole enhanced the prevailing northwesterly flows over Mongolia and northern China, resulting in a drier-than-normal atmosphere in favor of dust storm activity. On the contrary, in a year when the EAT was shifted eastward (e.g., 1997), a reverse dipole occurred, resulting in a wetter-than-normal atmosphere 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 derived from the NOAA HYSPLIT model further suggests that dust aerosols arisen from the Gobi Desert and the Loess Plateau are likely to propagate eastward along the strong westerly flows. On the contrary, easterly flows occurring over the Taklamakan Desert incline dust aerosols there to slow westward propagation. Different geographical locations relative to the mean circulation and topography effect are responsible for such different patterns.