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R. T. Pollard

Abstract

Inverse analysis (Wunsch, 1978) has been applied to two 10-day surveys of an area 15 km consisting of nearly 100 CTD stations, in order to determine the velocity field with a horizontal resolution of 45 km.

Reference levels above the main thermocline (1100 m) lead to physically implausible circulations below the thermocline. The preferred solution conserves mass with bottom velocities not significantly different from zero. All casts were repeated after 6 h, and the surveys were 20 days apart, allowing errors in the reference velocities due to internal waves, tides and changes of mass between surveys to be estimated at less than 1 cm s−1.

Further spatial resolution is obtainable by classical geostrophic analysis, contouring dynamic heights with an assumed bottom level of no motion from casts 15 km apart forming triangles 45 km apart. Two regions of anticyclonic circulation are revealed, one a meander the other an eddy. The meander and eddy have different water properties, and interact as the eddy, whose diameter is 80–100 km, drifts northwestward at 2 cm s−1. The circulations themselves have velocities of 10–20 cm s−1 and vary only weakly with depth from the surface to 1000 m, decreasing to near zero at 1500 m.

Observations from moored current meters are generally in good agreement with the geostrophic estimates, but reveal that there may be some times and places for which there is no level of no motion. sub-thermocline flows reaching 10 cm s−1 or more.

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R. T. Pollard

Abstract

Inertial oscillations in current records in the top 100 m of the ocean collected on three moorings 50 and 70 km apart are examined. At 12 and 32 m depth the records are horizontally coherent at inertial frequencies and wavelengths between 700 and 1700 km are suggested. At 52 and 72 m depth, records are not horizontally coherent. Vertical wavelengths in the seasonal thermocline lie between 100 and 200 m. Significant differences between the 12 m currents are related to differences in wind over the 50 km spacing, and can be modeled by forcing the Pollard and Millard (1970) model with the local wind. Analysis of the inertial oscillations generated by a well-developed traveling depression suggests that their amplitudes can fall to zero within a few tens of kilometers of the storm track. The waves propagate along paths parallel to the storm track, but form a system of standing waves perpendicular to it. During periods of strong inertial wave generation up to two-thirds of the horizontal kinetic energy in the mixed layer is at near-inertial frequencies. The vertical component of the group velocity in the top 100 m is downward, confirming that the flow of energy is down from the surface.

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R. T. Pollard and L. A. Regier

Abstract

Density and velocity data with 4-km horizontal resolution from a survey of a front during FASINEX (Frontal Air-Sea Interaction Experiment) are combined to describe the structure of the top 300 m of the ocean.

The geostrophic velocity field is derived and is used to examine the relative importance of stratification, relative vorticity, and twisting terms in Ertel's potential vorticity Q. Tenfold isopycnic changes in Q are found across a horizontal scale of only 10 km. These changes are confined to isopycnals that outcrop from the seasonal thermocline into the mixed layer. The ageostrophic velocity field is quantified by solution of the omega equation, and vertical velocities of up to 40 m day−1 are found. Small (40 km) surface-trapped (top 200 m) eddies are found to play a crucial role in the transport and effective diffusion of properties across the thermocline out of the mixed layer.

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J. F. Read and R. T. Pollard

Abstract

No abstract available.

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P. G. Challenor, J. F. Read, R. T. Pollard, and R. T. Tokmakian

Abstract

This paper describes a new method for combining altimetry data with hydrography in order to produce absolute surface geostrophic currents from altimetry. This method is then applied to data from the Drake Passage allowing surface currents to be monitored every 35 days during the second half of 1992. The resulting currents show several regions of strong currents with water flowing to the east and other places where the currents are either zero or flowing to the west After comparison with a model it is suggested that this structure is a result of the bathymetry.

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