Climatology of air mass transport to Cape Point.

Abstract

Air transport climatology to Cape Point (South Africa; 34.21ºS, 18.29ºE) was examined by analyzing 5-day kinematic back trajectories at both 300 and 5000 m above ground level for the period 2001-2003. Trajectories were computed using the Hybrid Single-Particle Lagrangian Integrated Trajectory model version 4 (HYSPLIT-4). A multivariate statistical procedure, known as cluster analysis was performed separately for the whole data set, and for yearly and seasonal trajectories, in this way trajectories were classified into distinct transport pathways showing interannual and seasonal variations. The influence of weather systems such as semi-permanent subtropical continental anticyclones, transient ridging anticyclones originating in the midlatitude westerlies, and baroclinic disturbances in the westerlies on major transport patterns were also discussed. Cluster analysis results identified seven transport flow patterns to Cape Point (northerly, north-easterly, south-easterly, southerly, south-westerly, westerly and north-westerly). Similar year to year gross features of atmospheric transport patterns were observed at the 5000 m level, however, variability in transport patterns was observed at the 300 m level. Both spring and summer mean clusters exhibit a south-easterly transport associated with trade winds, while autumn and winter show recirculation associated with anticyclones dominating over the interior of South Africa during this period. The influence of the 2002/2003 ENSO (El Ninõ-Southern Oscillation) proved to be insignificant on flow characteristics of Cape Point. Other features such as recirculation of air across southern Africa were also observed. The flow height characteristics of the 5-day cluster means to Cape Point affirmed that South Africa lies in the descending component of the Hadley cell circulation. The autumn and winter cluster means show greater wind shear between the 300 and 5000 m levels.