Bottom Currents Derived from a Shipborne ADCP on WOCE Cruise A11 in the South Atlantic

Abstract

This paper is divided into two parts: the first describing an enhancement of the shipborne ADCP system and the second describing information about bottom currents that have been obtained therefrom on WOCE cruise A11 in the South Atlantic, Punta Arenas to Cape Town. RRS Discovery has been fitted with a GPS3DF receiver and an array of antennas about its bridge, which determine the instantaneous attitude of the ship. The heading record from this instrument has been compared with the ship's gyrocompass throughout the 40-day cruise. Differences have been calculated and examined according to heading, speed, and movements. Underway differences are found insensitive to heading except in a southerly sector, but on station, differences exhibit a sinusoidal dependence on heading with range 3°. Steaming from a station led to transient behavior with recognizable effects for 1 hour. The results are generally consistent with earlier reports and are attributed to gyro error. An improved heading measurement has a marked effect on currents determined from the ADCP when under way. The cruise data from this instrument has been reprocessed, replacing the observations of the gyrocompass with those from the GPS3DF unit. We describe and implement the procedure for dealing with two difficulties, namely, gaps in the data and misalignment of the two pieces of equipment. An estimate of the current error from known causes is found to be only 1 cm s?1 for 4-h steaming or station averages. A comparison of the ADCP cross-track component with geostrophic estimates referenced to the bottom for 91 stations shows excellent overall agreement, both for underway and station-pair measurements. Differences are principally due to transient ageostrophic currents, tides, and inertial oscillations, which are estimated at about 5 cm s?1. Two rings in the Cape Basin and an intense vortex in the Argentine Basin exhibit significant centripetal contributions to the measured flow; when they are removed, the comparisons with geostrophy improve. Differences, or cross-track bottom currents, are significantly above noise only in the Argentine Basin; three regions are identified there. In the Falkland Current strong barotropic components (16 to 40 cm s?1 northward) augment the baroclinic transport fourfold to 50 Sv (Sv &equiv 106 m3 s?1), in accordance with the arguments of Peterson and the predictions of a numerical ocean model (FRAM). It follows that this will have a large impact on heat, salt, and tracer fluxes across the section, though these are not discussed here. The second region of significant bottom flow is in the confluence zone (Brazil and Falkland Current Extension), where intense surface circulations penetrate in attenuated form (+12 to ?18 cm s?1) to the seabed at 6000-m depth. Extremely turbid bottom mixed layers were detected there with transmittance values of 10% m?1, verifying the existence of strong currents. The third region of significant bottom flows is contiguous with and east of the confluence zone, where a broad region of southerly flow is succeeded by a similar region of northerly flow (?15 to +8 cm s?1). These lie over a shallow abyssal ridge, with axis along the cruise track (the Zapiola Drift), which exhibited mud waves throughout and was draped with a turbid bottom layer. An anticyclonic flow about the Zapiola Drift has been inferred from the morphology of the mud waves seen on its flanks, and this circulation is shown to be contemporary by two current meter records from the region. Our measurements appear to confirm this circulation and suggest that the strength exceeds 100 Sv. Is this a recirculation such as seen near the Gulf Stream extension? No evidence for it is found in numerical models and further observations of its existence are needed. The authors believe that the bottom currents over the deepest parts of the Argentine Basin are strong over a wide area. The value of combining the GPS3DF and ADCP observations in order to identify regions of strong bottom flows is demonstrated.