Ring shedding in the Agulhas Current System


  • Raymond C. V. Feron,

  • Wilhelmus P. M. De Ruijter,

  • Dick Oskam


After the Agulhas Current separates from the African coastline, it forms the Agulhas Retroflection and flows back into the Indian Ocean. If the current loop closes onto itself, a warm Agulhas ring is pinched off in the area south of Africa (between 15° and 20°E longitude). These rings can be monitored relatively easily with satellite altimetry when they migrate into the South Atlantic. However, the shedding process itself is difficult to extract from altimeter measurements. Geosat Exact Repeat Mission altimeter data of the period from November 1986 to September 1989 are used to analyze the ring-shedding process in the Agulhas Current System. The data set has been processed using the collinear method over the area which extends from 30°W to 90°E, and from 15°S to 50°S. The original altimeter data are statistically interpolated to a regular 1° × 1° (spatial) grid using objective analysis. The obtained sea level anomaly maps are computed at regular time steps. Combination of harmonic analysis and principal component analysis enables extraction of typical Agulhas Current frequencies from the 3-year altimetric data set. The analysis indicates that 11 or 12 dominant events took place over the 3-year period. Variations occur from year to year. February, March, and April (summer-autumn) are identified as anomalous months in which the sea level anomaly signal is not as well defined as in the other part of the year. The combined analyses showed that the first three modes of variability (explaining 25%, 20%, and 12% of the variance, respectively) are dominant. The structure of the spatial and temporal scales leads to the hypothesis that these modes can be associated with periodic Agulhas front movements, culminating in the formation of large Agulhas rings. Sharp changes (pulses) in the stability of the sea level pattern then determine the time of pinch off. This indicates a number of 18 (±2) pulses over the 3-year period in which an anomaly in the average decorrelation time occured. If these pulses can be connected to the formation of Agulhas rings, they are of great importance for the large-scale circulation because they contribute significantly to the energy and freshwater flux between the Indian Ocean and the South Atlantic.