SPRING DEHYDRATION IN THE ANTARCTIC STRATOSPHERIC VORTEX OBSERVED BY HALOE

The distribution of dehydrated air in the middle and lower stratospher e during the 1992 Southern Hemisphere spring is investigated using Hal ogen Occultation Experiment (HALOE) observations and trajectory techni ques. Comparisons between previously published Version 9 and the impro ved Version 16 retrievals on the 700-K isentropic surface show very sl ight (0.05 ppmv) increases in Version 16 CH4 relative to Version 9 wit hin the polar vortex. Version 16 H2O mixing ratios show a reduction of 0.5 ppmv relative to Version 9 within the polar night jet and a reduc tion of nearly 1.0 ppmv in middle latitudes when compared to Version 9 . The Version 16 HALOE retrievals show low mixing ratios of total hydr ogen (2CH4 + H2O) within the polar vortex on both 700 and 425 K isentr opic surfaces relative to typical middle-stratospheric 2CH4 + H2O mixi ng ratios. The low 2CH4 + H2O mixing ratios are associated with dehydr ation. Slight reductions in total hydrogen, relative to typical middle -stratospheric values, are found at these levels throughout the Southe rn Hemisphere during this period. Trajectory calculations show that mi ddle-latitude air masses are composed of a mixture of air from within the polar night jet and air from middle latitudes. A strong kinematic barrier to large-scale exchange is found on the poleward flank of the polar night jet at 700 K. A much weaker kinematic barrier is found at 425 K. The impact of the finite tangent pathlength of the HALOE measur ements is investigated using an idealized tracer distribution. This ex periment suggests that HALOE should be able to resolve the kinematic b arrier, if it exists.