Mesoscale variations in summer winds over the Cape Columbine — St Helena Bay region, South Africa

Abstract

Spatial and temporal variations in the near-surface wind field and their relation to upwelling in the lee of the Cape Columbine headland were investigated during the summers of 1979–1980. Time series of meteorological data were collected at an automatic weather station situated on the northern tip of the headland. Intensive data collection by means of pilot balloons and hand-held sensors permitted description of thermally-driven wind oscillations and, on a larger scale, gradient wind cycles during January and February 1979. Results of aerial surveys were used to document wind shear and upwelling over St Helena Bay in November and December 1980. Synoptic weather systems further south controlled the depth and trajectory of wind flow. During cool westerly flow, homogeneous patterns prevailed with limited Seabreeze enhancement along the coast north of St Helena Bay. The prevailing equatorward wind was concentrated in a "jet" over the Cape Columbine headland. This wind jet reached maximum intensity in the 100–300 m layer just before sunset at 19h00, fanning out and decelerating with cyclonic curvature downstream over St Helena Bay. The influence of these mesoscale variations in wind stress is spatially correlated to coastal upwelling patterns in case studies.