To text describing this figure   To MLS home page    To MLS scientific results page This figure shows MLS maps on four selected days which illustrate the evolution of lower stratospheric ClO and ozone during the 1992 southern hemisphere winter. The data have been vertically-interpolated to the 465 K potential temperature isentropic surface (approximately 20 km). The thick black contours concentric with the pole on the ClO maps show the daylight edge of MLS measurements, and the thin white contour concentric with the pole indicates the edge of polar night. The green contours indicate temperatures of 195 K (outer contour) and 190 K (inner contour). Irregular white contours indicate the approximate edge of the polar vortex (potential vorticity contours of -2.5 and -3.0 x 10^-5 K m² kg^-1 s^-1). These were the first measurements of Antarctic ClO made during early and mid winter. They show that enhanced ClO can appear by the first of June, and remain enhanced throughout the winter in the sunlit regions of the vortex. Both the outer and inner edges of enhanced ClO move poleward during mid to late winter. Poleward retreat of the outer edge is expected due to quenching of ClO by NO₂ produced by the increased photolysis of HNO₃ as the sun rises higher in the sky. Poleward retreat of the inner edge of enhanced ClO is expected due to the returning sun's photolysis of ClOOCl producing ClO. Note that the enhanced ClO extends, at times, equatorward of 60^o latitude. During early winter (note the 1 June and 11 July maps) ozone increases in the lower stratospheric vortex, as can be explained by descent of ozone-rich air from above. This descent brings in more ozone than is destroyed by the enhanced ClO during this period. However, by mid-August the chlorine destruction in this layer dominates the increase due to influx: ozone has decreased between 11 July and 14 August in the region where there is largest ClO. Further ozone depletion occurs between mid-August and mid-September, leading to the formation of the ozone hole. These data indicate that loss of ozone occurs in the southern vortex before development of the ozone hole, which is generally taken to start in September. The low ozone abundances in the tropics at this altitude are normal, and are due to the rising air motions which bring ozone-poor air up from below (which was likely enhanced by effects of the Pinatubo volcano). See Waters, et al. GRL 20, 1219-1222 [1993] for more discussion.

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