SEASONAL-VARIATION OF ISENTROPIC TRANSPORT OUT OF THE TROPICAL STRATOSPHERE

The seasonal variation of the isentropic transport out of the tropical stratosphere is examined using contour advection calculations with wi nds from meteorological analyses for the period July 1991 to June 1994 . Calculations using winds from three different meteorological analyse s are compared. There is good qualitative agreement between the differ ent calculations for extratropical structures but not for structures i n the tropics. There is also reasonable agreement between area diagnos tics (for transport out of the tropics) derived from these calculation s. Transport out of the tropics occurs in Rossby wave breaking events in which filaments of tropical air are drawn into middle latitudes. In all years these wave breaking events occur during the fall to spring period when there are westerlies throughout middle latitudes (late Sep tember to early May in the northern hemisphere). This includes periods before the formation of, and after the breakup of, the polar vortex. In the northern hemisphere the rate of transport out of the tropics du ring fall to spring fluctuates around a constant value, whereas in the southern hemisphere there is a relatively quiet midwinter period, wit h maxima in the transport in early and late winter.