A major peturbation in the Agulhas retroflection area in 1986

L.V. Shannon, J.j Agenbag, N.D. Walker and J.R.E. Lutjeharms

During late 1985 and in 1986 major peturbation 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 element of the Agulhas Current aroung the Cape of Good Hope, during the seconds halkf of 1985 and the first part of 1986, and offshoot of the Current developed in June as a glow into the Atlantic around the edge of the Agulhas Bank. Subsequently a major change in the retroflection occured, and in August 1986 a large body of warm water joined to the Current moved northwards and was situation withing 20 km of the Cape Peninsula and Cape Columbine. Boundary thermal gradients associated with this warm intrustion were 3-4 degrees celcius in 25 km. This intrustion moved progressively northwards and westwards at 4.5-95 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 subantarctic water moved northwards, flooding ,uch 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 south of Africa, are consistant with published numerical modelling results which predict a substantial flow of Agulhas water around the Cape of Godd Hope under certain conditions.

Ref: Deep Sea Research, Vol. 37, No. 3, pp. 493-512, Jan. 1, 1990

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