Cruise Summary Report RRS DISCOVERY D276

8 – 21 December 2003 Fortaleza / Brazil and Rio de Janeiro / Brazil The cruise was a German contribution to the CLIVAR programme. The intention was to build upon the data gathered during the World Ocean Circulation Experiment. The spreading of Antarctic Bottom Water in the South Atlantic is seen as an integral limb of the global thermohaline circulation. During the last 30 years, there has been a marked rise in bottom water temperature which is thought to be significantly relevant for climate change. It was the purpose of the cruise to deploy two subsurface moorings at the entrance of the Vema Channel. They are designed to monitor physical property fluctuations during the coming 16 months

The Mid-Ocean Dynamics Experiment

The Mid-Ocean Dynamics Experiment (MODE-1) was designed to investigate mid-ocean mesoscale eddies. An intensive and extensive program of measurements in three spatial dimensions and time was undertaken in an area southwest of Bermuda from March through mid-July 1973. Principal components of the experiment were an array of moored current meters and temperature-pressure recorders, hydrographic stations, drifting neutrally buoyant floats at 1500 m tracked by SOFAR, and acoustic and electromagnetic profilers. During MODE-1 a smaller scale survey relying on ship-tracked neutrally buoyant floats, a conductivity-temperature (CTD) survey, and a moored current meter array, MINIMODE, was carried out. The experiment was preceded by MODE-0, consisting of measurements by a series of moored current meters and other instruments in the general area selected for MODE-1. MODE-1 observations were generally within a 300-km radius circle centered at 26°N, 69° 40′W, with a greater concentration of observations in the interior of the circle. The region covers varied topography, with a flat a byssal plain sloping upward to the continental rise in the western half and rough topography in the eastern half. Descriptive, dynamical, numerical results of the experiment are presented. It is concluded that mid-ocean eddies are part of an energetic and structured variability field superimposed on the weaker gyre-scale mean circulation. In the western North Atlantic there is a band of eddy variability of around 100-day period and 70-km scale in which currents are horizontally nearly isotropic; vertical scales are of the order of the depth. The experiment provided conclusive evidence of the existence of mid-ocean eddies and serves as the basis for future experiments, such as POLYMODE, to extend our knowledge of these systems

Fahrtbericht POSEIDON-Reise Nr. 127/3

St. John's, NF/Kanada- Plymouth/England 11.4. - 20.4.198

Detection of Overflow Events in the Shag Rocks Passage, Scotia Ridge

During an almost yearlong period of observations made with a current meter in the fracture zone between the Falkland Islands (Islas Malvinas) and South Georgia, several overflow events were recorded at a depth of 3000 meters carrying cold bottom water from the Scotia Sea into the Argentine Basin. The outflow bursts of Scotia Sea bottom water, a mixing product of Weddell Sea and eastern Pacific bottom water, were associated with typical speeds of more than 28 centimeters per second toward the northwest and characteristic temperatures below 0.6°C. The maximum 24-hour average speed of 65 centimeters per second, together with a temperature of 0.29°C, was encountered on 14 November 1980 at a water depth of 2973 meters, 35 meters above the sea floor

Intercomparing drifts from eddy-resolving and cycling floats in the deep western boundary current along the Mid-Atlanic Ridge

The Lagrangian nature of cycling floats is evaluated in the framework defined by the Deep Western Boundary Current of the Mid-Atlantic Ridge. In a statistical approach, speeds and drifts are estimated for an APEX cycling float and compared with the velocities inferred from a park ensemble of four eddy-resolving RAFOS floats. They were deployed at the same location and ballasted for drifting at the same mission depth. Displacement errors induced by geostrophic shear and wind forced currents are analyzed. We observe that the velocity estimated from the RAFOS floats is not statistically different from the velocity estimated from the APEX float. Likewise, the initial separation between the cycling float and a simultaneously deployed RAFOS float has been studied in terms of the turbulent diffusivity. Though the performance of this study in comparable cases without a mean current field may be limited, these oceanic observations support exploiting the Lagrangian nature of the cycling floats