Modeling the Variability of the Greater Agulhas Current System

Abstract

An eddy-permitting, regional ocean model has been used to examine the variability in the source regions of the Agulhas Current on a range of time scales. These source regions are the East Madagascar Current, the flow through the Mozambique Channel, and the recirculation of the southwest Indian Ocean. The effect of variability in these source regions on the interocean leakage at the Agulhas retroflection south of Africa has been quantified using a retroflection index. On the annual mean, the recirculation in the southwest Indian Ocean subgyre is by far the dominant contribution to the volume transport of the Agulhas Current in the model. On average the recirculation also contributes the largest amount of heat, although the difference between the three sources is not as great as that seen in the volume flux since the water in the Mozambique Channel may be warmer than in the recirculation. Local winds seem to be the dominant forcing mechanism of variability in this recirculation, although it is also partly in phase with the zonally averaged wind stress curl over the subtropical Indian Ocean. A strong relationship was found between the transport of the recirculation and that of the Agulhas Current, particularly on interannual time scales. Consistent with observations, the model flow through the Mozambique Channel is dominated by eddies, with a strong annual cycle, lagging the South Equatorial Current by 1 month and a weaker semiannual cycle. The southern limb of the East Madagascar Current also shows an annual and semiannual variation in transport at 20°S, partly in phase with the local winds. South of about 24°S, the East Madagascar Current breaks up into eddies. An investigation into the sensitivity of the flows in the source regions and in the retroflection index to a 2° southward shift in the mean winds was conducted. In the model run with the shifted winds, the transport strengthened in the recirculation subgyre together with increased mesoscale activity as well as reduced leakage into the southeast Atlantic Ocean when the winds were shifted south by 2°.