Bottom Trapped Waves at Tidal Frequencies off Point Sur, California
Abstract
Current meter data were collected from moorings on the 800 and 1800 m isobaths on the continental slope off Point Sur, California at 100, 350, and 500 m depth and at 1000 m depth on the 1800 m isobath. Complex demodulation and spectral techniques were used to study the distribution of tidal energy at the diurnal and semidiurnal frequencies, and to study the size, rotation and orientation of the tidal current ellipses. At mooring P2 (800 m) the observed diurnal energy increased with depth for each of the 17 months of data analyzed, and the semidiurnal energy likewise increased with depth for 14 of the 17 months. The observations were studied using the theory of bottom trapped waves in a rotating stratified fluid (Rhines, 1970). Bottom trapping is expected at the diurnal and semidiurnal frequencies whenever the critical trapping frequency omega sub c = Nsin alpha sin psi exceeds 1/24 and 1/12 hours respectively, where N = the Brunt-Vaisala frequency, alpha the bottom slope, and psi is a measure of the angle the wave number vector makes with the gradient of topography. Theoretical energy decay curves matched the observed energy levels at P2 very well, indicating that the enhancement of energy with depth was likely due to the presence of bottom trapped waves. No bottom trapping was observed at P3 (1800 m) because the deepest current meters were positioned too far above the sea floor to detect bottom trapped energy, and because the density gradients near the sea floor were weaker than at site P2 and kept omega, below the tidal frequencies.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 01, 1991
- Accession Number
- ADA245611
Entities
People
- Stephen L. Sielbeck
Organizations
- Naval Postgraduate School