Dynamical study of a tropical cut-off low over South Africa, and its impact on tropospheric ozone

The structure and evolution of an intense tropical cut-off low (COL) occurring over South Africa is documented, by combining meteorological and ozone data from a number of different sources, such as in situ radiosoundings, Measurement of ozone and water vapor by airbus in-service aircraft data, satellite (Meteosat) and modelled European Center for Medium-Range Weather Forecast data. The COL extends to latitude 10°S, with a lifetime of approximately 2 weeks, and a horizontal size of about 10°. A distinguishing feature of this case study is that it becomes detached from the stratospheric reservoir in both the vertical and horizontal planes, in an irreversible way and as such is different in structure to a mid-latitude or a polar COL. Consequently, even though tropical COLs may occur infrequently, it is likely that the tropospheric ozone enhancement induced by this irreversible intrusion could be significant.