Where three oceans meet : the Algulhas retroflection region

Abstract

The highly energetic Agulhas Retroflection region south of the African continent lies at the junction of the South Indian, South Atlantic, and Circumpolar Oceans. A new survey of the Agulhas Retroflection taken in March 1985, plus historical hydrographic data, allow its dynamical and water-mass characteristics, and its role in exchanging mass, tracers, and vorticity between the three oceans, to be extensively characterized. The 1985 survey is composed of three independent, synoptic elements: a grid of closely-spaced, full-water-depth hydrographic stations (the first entirely full-water-column survey in this area), including several transects of the Agulhas and Agulhas Return Currents; a continuous survey of the path of the currents (the first such survey in the Agulhas); and a contemporaneous and relatively cloud-free sea surface temperature image derived from satellite infrared measurements. Mass transport balances within the closed grid boxes of the 1985 hydrographic survey provide information about current transport, recirculation (transport in excess of estimated returning interior ocean transport), and the overall Retroflection transport pattern. The current transport values exceed by as much as a factor of 1.5 the maximum interior transport computed from observed wind-stress curl and linear theory. Agulhas Current transports ranged from 56 to 95 x 106 m s-l at four 1985 transects crossing the current. Agulhas Return Current transports at the two 1985 transects were 54 and 65 x 106 m s-l. These transports are computed relative to 2400 dbar, which lies below the deep oxygen minimum emanating from the South Indian Ocean, and above the North Atlantic Deep Water salinity maximum. The current retroflected in two distinct branches in 1985, with a cold ring and a partially isolated warm recirculation cell found between the two branches. The satellite-derived sea surface temperature (SST) image, in agreement with the in situ measurements, showed that the cold ring lacked a cold SST anomaly; that the subsurface current path, as represented by a survey of the 15 C isotherm and 200 dbar surface intersection, was closely followed by a sharp front in sea surface temperature; and that most of the Agulhas's surface warm core retroflected upstream of the second retroflection branch. Anticyclonic curvature vorticity at sharp turns in the subsurface current path was found to exceed the maximum allowed by gradient wind balance, indicating that at these locations time-dependence and cross-frontal flow are important. The current's density field is found to meet necessary conditions for baroclinic and barotropic instability. These instability mechanisms may play a role in ring formation and current meandering. Top-to-bottom cross-stream spatial and isopycnal water-mass layering in the Agulhas Current, Agulhas Return Current, and associated rings are presented in two sets of sections, one contoured with pressure and the other with potential density as vertical coordinate. Temperature, salinity, oxygen, potential density and velocity sections are shown contoured versus pressure; and pressure, salinity, oxygen, and planetary potential vorticity are shown contoured versus potential density. These sections clearly illustrate water-mass structure both in space and relative to isopycnal surfaces. Strong salt, oxygen, and potential vorticity fronts on isopycnals in the upper -300 m across the Agulhas and Agulhas Return Current are observed, as are deep western boundary filaments of (i) salty, low oxygen water at intermediate depths traceable to Red Sea Water influences, and (ii) salty North Atlantic Deep Water close round the tip of Africa. The 1985 cold-core ring is the first cold-cored isolated feature to be observed within the Retroflection itself. Its transport was 64 x 106 m s-1, its integrated kinetic and available potential energy anomalies were 8.3 and 61 x 1015 J respectively, and its integrated planetary potential vorticity anomaly was 2.8 x 10-12 m-1 s-1. The potential vorticity flux associated with the exchange of 25 warm ring/cold ring pairs per year between the South Indian and Southern Oceans would balance the potential vorticity input by the wind to the entire South Indian Ocean. Interbasin flow of warm thermocline water (warmer than 8 C) from the South Indian to the South Atlantic Ocean is reconsidered in light of the 1985 hydrographic data. Thermocline water flow from the South Indian Ocean into the South Atlantic in the 1985 and historical observations is found to range from 2.8 to <9.6 x 106 m s-I. These values are less than the S;10 x 106 m s·1 needed to balance the Atlantic Ocean export of deep water, and implies that the deep water export is balanced in part by water colder than 8 C.