The Sources and Mixing Characteristics of the Agulhas Current

Abstract

Abstract Recent observations taken at four principal latitudes in the Agulhas Current show that the watermass properties on either side of its dynamical core are significantly different. Inshore of its velocity core are found waters of predominantly Arabian Sea, Red Sea, and equatorial Indian Ocean origin, while offshore waters are generally from the Atlantic Ocean, the Southern Ocean, and the southeast Indian Ocean. For the most part, the inshore waters approach the Agulhas Current through the Mozambique Channel, while those offshore are circulated within the southern Indian Ocean subtropical gyre before joining the current. These disparate water masses remain distinct during their 1000-km journeys along the South African continental slope, despite the convergence, extreme velocity shears, and high eddy kinetic energies found within the Agulhas Current. Both potential vorticity conservation and kinematic arguments are discussed as potential inhibitors of along-isopycnal mixing. It is concluded that a high cross-stream gradient of potential vorticity is the dominant mechanism for watermass separation near the surface, while the kinematic steering of water particles by the current is dominant at intermediate depths, where cross-stream potential vorticity is homogeneous. Hence, three lateral mixing regimes for the Agulhas Current are suggested. The surface and thermocline waters are always inhibited from mixing, by the presence of both a strong, cross-frontal potential vorticity gradient and kinematic steering. At intermediate depths mixing is inhibited by steering alone, and thus in this regime periodic mixing is expected during meander events (such as Natal pulses), when the steering level will rise and allow cross-frontal exchange. Below the steering level in the deep waters, there is a regime of free lateral mixing. The deep waters of the Agulhas Current are homogeneous in the cross-stream sense, being from the same North Atlantic source, and their salinity steadily (and rather rapidly) decreases to the north. Here, it is suggested that mixing must be dominated by vertical processes and a large vertical mixing coefficient of order 10 cm2 s−1 is estimated.