Variability in the characteristics of cut-off low pressure systems over subtropical southern Africa

Variability in the characteristics of cut-off low pressure systems over subtropical southern Africa is examined for the 1973-2002 period. These characteristics include their seasonality, frequency, duration, location and size. It is found that on average 11 cut-off lows occur over southern Africa south of 20 degrees S per year and are most common in the March-May season. Potential relationships between the number of cut-off lows over southern Africa with the El Nino Southern Oscillation (ENSO), the Pacific South America pattern, the wave number 3 pattern and the semi-annual oscillation (SAO) are discussed. La Nina years appear to be associated with above average annual frequencies of cut-off lows but the reverse is generally not true for El Nino years. There was a shift in the preferred season for cut-off lows from March-May to June-August in the 1980s, which coincided with a weakening of the SAO and a shift in zonal wave number 3. This period also showed a change in the preferred location of these systems from southwestern subtropical southern Africa to the northeast of the region. The results suggest that there may be a relationship between cut-off lows over subtropical southern Africa, the wave number 3 pattern in the Southern Hemisphere and the SAO. Copyright (C) 2006 Royal Meteorological Society.