Transient Characteristics of Residual Meridional Circulation During Stratospheric Sudden Warming
【摘要】：The residual meridional circulation derived from the thermodynamic equation can be separated into two parts, as Gille et al. (1987) did: the slowly varying diabatic circulation and the transient circulation. We calculated the transient and diabatic circulation. For the transient residual meridional circulation, it was calculated both including and omitting the eddy heat transport term in the transformed Eulerian-mean thermodynamic equation and continuity equation. We used daily European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data for 14 stratospheric sudden warming (SSW) events from 1979 to 2002. The results showed that calculations of transient residual meridional circulation present rapid variations during SSWs, with or without the eddy heat transport term. In the mid-SSW periods, the transient residual meridional circulation showed two cells at high northern latitudes: an upper one, with upward motion at the pole and descending motion in midlatitudes, and a lower one with circulation in the opposite sense. Totally different characteristics were observed in the pre-SSW and post-SSW periods. The daily variations in transient residual meridional circulation in the high-latitude regions from the pre-SSW period to the post-SSW period are also described in this paper. The transient residual meridional circulation results including and omitting the eddy heat transport term were carefully compared. Results indicated that although the patterns of transient residual meridional circulation including the eddy heat transport term were similar to that of the omitted eddy heat transport during SSW, the magnitudes in the upper stratosphere and high-latitude regions differed. Specifically, the transient residual meridional circulation including the eddy heat transport term was stronger than that of the omitted-eddy heat transport, and its variations at SSW onset were more rapid. Therefore, the eddy heat transport term should be included in the upper stratosphere of the high-latitude regions.As for the diabatic circulation, its daily variations were small during SSW events, and its patterns are in agreement with its monthly average.