Mean and Time-Dependent Motions in the Blake Escarpment Region

Author

David Y. Lai, University of Rhode Island

Date of Award

1983

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Department

Oceanography

First Advisor

Mark Wimbush

Abstract

An array of current meters, temperature sensors and bottom-pressure gauges was deployed along and across the Blake Escarpment for three and a half months to investigate the dynamical effects of this step-like topographic feature on oceanic motions. The mean and sub-inertial motions, and M₂ tides are discussed.

The Western Boundary Undercurrent was observed to flow southward along the escarpment with a mean volume transport of 24 Sverdrups. It reaches a maximum speed of 22 cm/sec at 2500 m of depth and extends to 50 km east of the escarpment. In two occasions the mean current switched off and even reversed its direction. A tilt in the sea surface above the Undercurrent opposite to that of the density surface can result in these events, as explained in a simple three-layer model.

Current meter measurements suggest that motions at 10 to 20-day periods are consistent with the dynamics of linear bottom-intensified topographic waves which are trapped vertically to the bottom and horizontally to the junction of topographic change. Oceanic pressure oscillations at 6.7 and 10-day periods along the Blake Escarpment and at San Juan can be explained by southward propagating Kelvin waves. Coastal sea level data, much influenced by local atmospheric conditions, are not useful for the study of open ocean oscillations.

The M₂ barotropic and baroclinic tides in the horizontal velocity measurements are separated by a method using the phase stability and the vertical structure of vertical standing modes. The internal modes have very different phase stability time scales, indicative of the absence of intense beams of internal tides generated from the escarpment. While the first internal mode on top of the escarpment propagates in an east-west direction (perpendicular to the topography), it propagates southward east of the escarpment. A much larger percentage (51 versus 23) of the baroclinic energy at M₂ tidal frequency is found in the nontidal continuum at the top of the escarpment than east of it. Regional bottom-pressure records indicate that the surface elevation of the M₂ tide propagates southward at 2870 km/sec and its amplitude decreases southward and eastward.

Recommended Citation

Lai, David Y., "Mean and Time-Dependent Motions in the Blake Escarpment Region" (1983). Open Access Dissertations. Paper 1594.
https://digitalcommons.uri.edu/oa_diss/1594