Synoptic‐scale forcing of coastal lows: Forced double kelvin waves in the atmosphere

Gill (1977) has interpreted the coastal lows of southern Africa as atmospheric coastal Kelvin waves. the lows are trapped horizontally next to the steep slopes of the Great Central Plateau by Coriolis forces and vertically by a low‐level inversion. This hypothesis is tested and extended using two linear models. A barotropic primitive‐equation analysis on an equatorial beta‐plane models the generation of these waves. an impermeable eastern boundary is included to reflect the blocking of the low‐level flow below the coastal inversion by the orography. Results of the linear analysis suggest that the formation of the coastal lows may be explained in terms of the scattering of eastward‐propagating synoptic‐scale disturbances at the meridional boundary. the boundary response is structurally similar to a Kelvin wave. At the frequencies of interest, no zonally propagating Rossby waves are excited. Further, in contrast to the case of a western boundary, the equatorial response is negligible. A two‐layer, f‐plane model enables the dynamics of the layer of fluid below the interior inversion to be incorporated. At the period of the synoptic forcing (six days), the boundary response is a maximum along the escarpment and decays exponentially from it. the e‐folding distance is slightly less than the appropriate Rossby radius of deformation. These results, together with observations, suggest that the coastal low and its companion low over the interior may be interpreted as forced internal double Kelvin waves. The possible application of these results to foehn (chinook) winds and lee cyclogenesis is mentioned.