A thermal front within the marine atmospheric boundary layer over the Agulhas Current south of Africa: Composite aircraft observations

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Abstract
Aircraft cross‐section observations across the northern edge of the warm Agulhas Current to the south of Africa are used to form composite results of mean and turbulent meteorological parameters over a 4‐day period characterized by winter zonal westerlies. Across a sea surface temperature (SST) front of 8°C (40 km)−1 near 35°S, 23°E, surface heat fluxes and wind stress increase fivefold. The moist unstable marine atmospheric boundary layer is 400 m deeper on the warm side of the SST front, and turbulent variances increase with SST in a logarithmic correlation (r = +0.76). A jetlike core of accelerated winds (17 m s−1) over the axis of warmest water is found. Its existence is thought to be due to secondary circulations set up by enhanced pressure gradients and baroclinic effects. The sharp thermal front is quasi‐stationary and semipermanent during synoptic prefrontal conditions and zonal flows and lies approximately over the 200‐m isobath in association with the Agulhas Current.