A MAJOR PERTURBATION IN THE AGULHAS RETROFLECTION AREA IN 1986

During late 1985 and in 1986 a major perturbation in the retroflection of the Agulhas Current occurred. A combination of climatological data and satellite-derived sea surface temperature observations are used to describe the evolution of the disturbance which resulted in 1986 being the warmest year on record in the southeast Atlantic. Possible causes of the changes in the retroflection and the associated input of Agulhas water into the southern Benguela Current region are discussed. Results show that, following intermittent leakage of elements of the Agulhas Current around the Cape of Good Hope, during the second half of 1985 and the first part of 1986, an offshoot of the Current developed in June as a flow into the Atlantic around the edge of the Agulhas Bank. Subsequently a major change in the retroflection occurred, and in August 1986 a large body of warm water joined to the Current moved northwards and was situated within 20 km of the Cape Peninsula and Cape Columbine. Boundary thermal gradients associated with this warm intrusion were 3-4°C in 25 km. This intrusion moved progressively northwards and westwards at 4.5-9.5 cm s-1 during the winter and spring of 1986 and had a typical zonal width of 240 km. The intrusion was evidently terminated when filaments of subtarctic water moved northwards, flooding much of the area with cold water during December 1986 and January 1987. Although intermittent leakage of Agulhas water into the southeast Atlantic continued during 1987, it was not substantial. By late 1987 the situation in the retroflection area had returned to normal. The observations of the Agulhas intrusion, when viewed in the light of changes in wind stress in the southwest Indian Ocean and south of Africa, are consistent with published numerical modelling results which predict a substantial flow of Agulhas water around the Cape of Good Hope under certain conditions. © 1990.