The South Westerly Buster is a well known wind event on the KwaZulu Natal coast, characterised by a sudden shift of wind from north easterly to south westerly, a rapid drop in temperature, a sharp rise in pressure and often gusty conditions with a squally leading edge.

Its formation is directly linked to the sequence of a coastal low migrating along the southern and eastern coast, typically preceded by hot Berg wind conditions and warm north easterly flow. Once the coastal low has passed a given location, the South Atlantic High (an anti cyclonic system) ridges in behind the low, which dramatically changes the local pressure pattern. The anti cyclonic circulation of this high, combined with the pressure increase on the western side of the departing low, drives a cool, moist south westerly flow towards the coast. Because this south westerly flow arrives suddenly and replaces a warm north easterly flow, the transition is abrupt, often accompanied by line squalls, low cloud and cool temperatures, hence the name "buster". 

The phenomenon is therefore caused by the anti cyclonic flow of air from a high pressure system ridging in behind a coastal low.

The anti cyclonic flow of air from a high pressure system ridging in behind a coastal low → CORRECT. This description captures the precise mechanism of the South Westerly Buster. A coastal low moves eastward along the coast and, once past KwaZulu Natal, the South Atlantic High ridges in behind it. The anti cyclonic flow of this high drives a cool moist south westerly wind towards the coast, replacing the pre existing warm north easterly. The rapid transition, combined with the pressure gradient between the departing low and the advancing high, produces the characteristic gusty, squally onset of the Buster.

The cyclonic rotation of a cut off low positioned over the interior of the country → INCORRECT. Cut off lows over the interior are significant weather makers in South Africa, particularly associated with heavy rainfall over the southern and eastern interior. However, they are not the driving mechanism of the South Westerly Buster. The Buster is a coastal phenomenon tightly linked to the passage of a surface coastal low, not to an upper level cut off low over the interior.

A steep pressure gradient between a tropical low and the Kalahari High → INCORRECT. A gradient between a tropical low and the Kalahari High can produce strong winds over the northern and central interior of Southern Africa, but this configuration does not correspond to the South Westerly Buster. The Buster is a coastal wind generated by the interaction between a coastal low and the ridging South Atlantic High, not by interior tropical to continental pressure contrasts.

The convergence of the warm Agulhas current with a cold berg wind → INCORRECT. The Agulhas Current and Berg winds are real features of the South African environment, but their convergence is not the cause of the Buster. Berg winds actually occur ahead of the coastal low (hot, dry, offshore flow), while the Buster happens after the low, with an abrupt south westerly onshore flow. The mechanisms are chronologically and dynamically distinct.