Interannual Variability of the Tropospheric Circulation and its Relation to the Stratosphere in the Southern Hemisphere

摘要

The interannual variability of the seasonal evolution of the zonal mean wind and wave activity in the southern hemisphere troposphere is investigated, using a global analysis data-set for 1979-1993 provided by the National Meteorological Center (NMC), to understand the dynamical coupling between the troposphere and the stratosphere. The interannual variance of the zonal mean wind at 300 hPa calculated for each calendar day has maxima at 60°S and 40°S almost all year round. We found that a zonal wind index defined by the difference of the zonal wind speed between 60°S and 40°S is a sensitive indicator of two typical latitudinal profiles of the zonal mean wind in the southern hemisphere troposphere: the single-jet and the double-jet profiles. The seasonal evolution of zonal wind indices is separated into two groups, i.e., DS (double-single) years and SD (single-double) years. DS years are characterized by a double-jet structure in early winter and a single-jet in late winter, while in SD years the zonal mean wind has a single-jet structure in early winter and changes its profile into a double-jet in late winter. The appearance of DS and SD years is similar to that of the two categories of the westerly jet profile in the southern hemisphere upper stratosphere in Shiotani et al. (1993). Wave activity is diagnosed by the Eliassen-Palm (E-P) flux. Clear differences are seen in May-June between DS and SD years in the horizontal component of E-P flux. Variation of the horizontal component precedes change in the zonal mean wind, indicating its importance in transitions between the single- and double-jet. The zonal wind structure has influence on planetary wave 1 activity. The vertical component of wave 1 E-P flux is small (large) at 60°S and 500 hPa when the zonal wind has a double-jet (single-jet) structure during winter. The weak flux at 500 hPa in the years with a double-jet structure yields weak wave 1 activity in April-June but gives strong wave 1 activity through more efficient propagation in August-November at 100 hPa relative to the years with a single-jet structure.