Dynamical Processes Controlling the Distribution of Southern Hemisphere Column Ozone

The monthly mean distribution of column ozone in the southern hemisphere winter and spring usually exhibits an elongated maximum centered near 50 S, stretching eastward from the Indian Ocean over the Pacific Ocean in a croissant shape. This paper explores dynamical causes underlying its geographical distribution. It is argued that tropical convection exerts a first order control on the distribution of southern hemisphere column ozone by influencing midlatitude synoptic waves and stratospheric planetary waves, through upward-amplifying, poleward tilting ridges, commonly near Australia.

Daily global meteorological analyses, TOMS column ozone, and 3D assimilated ozone from the GMAO were examined in detail for 2000-2003, spanning a La Nina to El Nino transition. Daily distributions exhibit remarkable variability compared to monthly means. We hypothesize that the location of the ozone croissant is fundamentally linked to the tropopause-level outflow from the northern summer monsoon into the southern Indian Ocean. The resulting ``Australian High" exerts a strong control on midlatitude Rossby waves 4-5 near the tropopause, and on wave 1-2 structures in the stratosphere, creating a preferred region where ozone converges and descends, the ozone croissant. The geographical distribution of Rossby wave breaking (RWB) at 350 K is very similar to column ozone. Both are displaced eastward along with the tropical Pacific convection from La Nina (2000) to neutral (2001) to El Nino (2002).

Film loops and Hovmoller diagrams of case studies will be shown which illustrate how: 1) southwestward flow pulses out of the Tibetan High excite anticyclones over the southern Indian Ocean, leading to enhanced column ozone maxima just downstream; 2) travelling planetary-scale ridges in the stratosphere amplify when aligned with the Australian High; and 3) the largest column ozone maxima occur when a planetary wave ridge aloft is aligned with a synoptic ridge near Australia. A preferred ozone transport pathway is identified, where ozone-rich air just to the east of a planetary wave ridge sinks equatorward into the synoptic trough near the tropopause. Subsequent mixing dilutes the downstream end of the croissant near South America, yielding its tropospheric counterpart, the wave one ozone maximum in the upper troposphere over the Atlantic.