A numerical simulation of flow at Fieberling Guyot

Abstract

A numerical sigma‐coordinate ocean circulation model is used to investigate tidally forced flow near Fieberling Guyot. The aim is to reproduce the observed currents and to identify the dominant physical mechanisms that lead to the complex three‐dimensional flow fields at this tall and steep seamount in the northeast Pacific. Our very high resolution simulation (with 500 m horizontal and less than 20 m vertical grid spacing in the vicinity of the seamount summit) was performed with the newest version of the terrain‐following sigma‐coordinate primitive equation model (SPEM). As in previous more idealized studies, the seamount was placed in the center of a uniformly rotating periodic channel and forced by diurnal barotropic tides. The exponential initial stratification, the tidal forcing amplitude, and its orientation were chosen based on observations from the Fieberling area. Three major characteristics of the flow observed at Fieberling Guyot are reproduced both qualitatively and quantitatively: diurnal currents reach about 20 cm s−1, a twentyfold amplification relative to the tidal flow away from the seamount; the time‐mean anticyclonic flow speeds are close to the observed 10 cm s−1 maximum azimuthal velocity; and the spatial structure of this vortex shows a maximum at about 6 km radius between 400 and 600 m depth, clearly lifted off the bottom. The time‐mean flows are found to be maintained by diurnal waves of a mixed type: the net motion shows characteristics of both bottom‐intensified seamount trapped waves and vertically propagating vortex trapped waves. While the former are mainly responsible for setting up a time‐mean anticyclonic flow along the upper flanks at the bottom, the latter are needed to redistribute the momentum and time‐mean density, thereby reproducing the observed structure with a flow maximum off the bottom. An analysis of the energy, momentum, and density fluxes shows that rectification depends critically on the eddy fluxes that balance the time‐mean downwelling over the seamount summit and upper flanks. The results of sensitivity and parameter studies are utilized to further interpret the role of individual physical mechanisms and the time‐mean dynamical balances.