(Anti)cyclonicity

The tendency for an anticyclone (or cyclone) to be located within a certain area or grid square can be quantified by counting the number of hours in a month that any high’s (or low’s) centre is in that square. Thus we may define the ‘anticyclonicity’ (or ‘cyclonicity’) of places during that month.

Leighton (1) has drawn maps for the whole of the southern hemisphere showing the distribution of these features in each month of the year. The entire domain was gridded by 5° increments. As regards the centres of highs in summer (Fig 1), they tend to linger over the oceans in four areas at about 35° S, near 0° E, 90° E, 135° E (in the Australian Bight) and 90° W. In winter anticyclonicity generally peaks at 30° S, especially over the continents. This belt of high pressures is consistent with the subsidence in the Hadley circulation cell.

Fig 1. Average anticyclonicity in January.

Low pressures tend to lie on a band at 65-70° S, just off the coast of Antarctica, apart from heat lows in northwest Australia and Paraguay in summer. That band is related to polar fronts and the ascending limb of the Ferrel cell in the Palmen-Newton model of the general circulation.

Departures from the general patterns are fairly rare. Thus, when a large high is remote from the normal position, as in a blocking pattern, one may expect unusual weather (2).

(1) Leighton, R.M. 1993. Monthly anticyclonicity and cyclonicity in the southern hemisphere; 15-year (1973-1987) averages. Tech. Report 67 (Aust. Bureau of Meteor.) 31pp.

(2) Leighton, R.M. 1994 Relationship of anomalirs of (anti) cyclonicity to some significant weather events over the Australian region. Aust. Meteor. Mag. 43, 255-61.