FORMATION OF LOW-OZONE POCKETS IN THE MIDDLE STRATOSPHERIC ANTICYCLONE DURING WINTER

Microwave limb sounder observations of midstratospheric ozone during s tratospheric warmings show tongues of high ozone drawn up from low lat itudes into the developing anticyclone. Several days later, an isolate d pocket of low ozone mixing ratios appears, centered in the anticyclo ne, and extending in the vertical from approximate to 15 to 5 hPa, wit h higher mixing ratios both above and below. These low ozone mixing ra tios during northern hemisphere warmings are comparable to values well inside the vortex and are approximate to 3 parts per million by volum e lower than typical midlatitude extra-vortex mixing ratios. This type of feature is seen whenever the anticyclone is strong and persistent, including during relatively strong minor warmings in the southern hem isphere. Three-dimensional back trajectory calculations indicate that the air in the region of the low-ozone pockets originates at higher al titudes and low latitudes, where ozone mixing ratios are much higher. The air parcels studied here are typically confined together for 1 to 3 weeks before the lowest ozone mixing ratios are observed. The trajec tory calculations and comparisons with passive tracer data confirm tha t the observed low-ozone regions in the midstratosphere could not resu lt solely from transport processes.