Subtropical cyclogenesis

E. Linacre and B. Geerts


The lifecycle of frontal disturbances was discussed in Note 13.B. A frontal low develop as a kink forms in the jet stream and the further evolution is closely associated with the air flow aloft.

Only the southern margins of Australia come close to frontal lows which are usually centered around 60° S. Classical occlusion rarely happens near Australia. Extratropical lows near Australia are connected with either weaker fronts or polar lows within masses of cold air, not with the interface of cold and warm airmasses, which coincides with the jet stream aloft. This 'subtropical' cyclogenesis (Note 13.C) is rare, simply because a major equatorward meander in the jet stream is rare. Subtropical lows may be associated with a band of cloud and rain, little temperature gradient horizontally at low levels, but a tongue of warm air aloft, above a lower-than-normal tropopause. Rainfall is generally stratiform, however conditional instability is fairly common, leading to embedded thunderstorms. Subtropical lows are more likely to form off the southeast coasts of Australia or South Africa, in part because of vortex stretching (Note 13.D), in part because of the relatively high sea surface temperature.