SYNOPTIC CLIMATOLOGY, AND INTRAHEMISPHERIC ASSOCIATIONS, OF COLD-AIR MESOCYCLONES IN THE AUSTRALASIAN SECTOR

Satellite IR images for seven months in 1992 are interpreted for cold air mesoscale cyclones (mesocyclones) occurring in the Australasian se ctor (similar to 70 degrees E-150 degrees W) of the Southern Ocean. Ti me-averaged (monthly, seasonal) distributions of mesocyclogenesis, mes ocyclolysis, and tracks of movement, along with statistical summaries of mesocyclone attributes (e.g., cloud vortex size, speeds of movement ), are presented and discussed in the context of the larger-scale atmo spheric circulation. Maximum frequencies of mesocyclones occurred in t he transitional months of April and October 1992, with a secondary pea k in July. Statistically significant differences in mesocyclone track length between months appear related dominantly to changes in speed of the background flow, associated with the semiannual oscillation (SAG) of tropospheric pressure/height. The associations of mesocyclone ''ou tbreaks'' with composite anomaly fields of pressure and height are ide ntified for three subareas of the Australasian sector suggested by the analysis of mesocyclone spatial patterns. Outbreaks occur in the stro ng southerly geostrophic airflow located between pressure and height a nomalies that are negative (positive) to the eastward (westward). When outbreaks occurred in the New Zealand subarea in 1992, a similarly st rong couplet of pressure/height anomalies developed in the southern So uth America/Antarctic Peninsula sector but not when outbreaks occurred south of Australia. The mesocyclone remote association that is sugges ted is evaluated by using polar orbiter IR imagery for the southeaster n Pacific region. Frequencies of mesocyclones increase (decrease) west of Chile but decrease (increase) through Drake Passage when mesocyclo ne outbreaks occur near New Zealand (south of Australia). These long d istance associations of mesocyclone outbreaks are consistent with the connectivity of the baroclinic waves and might prove useful in the dev elopment of techniques to forecast mesocyclones over the Southern Ocea n.