Investigation of permanent eddies in Dixon Entrance, British Columbia

Abstract

A unique couplet of basin-wide baroclinic eddies was observed in Dixon Entrance. These eddies were revealed in current meter observations, drifter tracks, and contours of dynamic heights derived from hydrographic measurements. Normally, in a coastal sea strait wider than the internal Rossby radius of deformation, the outflow of brackish water is on the right, looking seaward; but in Dixon Entrance these two eddies force an outflow on the left. Several generation mechanisms for these two eddies are investigated. Calculations of the tidally rectified currents, using current ellipses observed at current meter moorings, suggest that such rectified currents are weak at these locations. However, on the shallow banks and slopes where rectification is expected to be strongest and direct measurements are absent, tidal ellipses generated by numerical models indicate sufficient rectification of currents to spin up the eddies. Current fluctuations at 2-week periods are moderately coherent with fortnightly modulations in the semidiurnal tide, also suggesting that tidal rectification may be important. Wind and estuarine forcing are also investigated. Empirical orthogonal function analysis of the current and wind data reveals that although the most spatially coherent fluctuations in subtidal currents can be attributed to the wind, the average flow cannot. Outflow of brackish water can generate currents of the approprate strength, but not the outflow along the left bank. Although a prevailing wind from the north or west at the western approach to Dixon Entrance might set up this flow, existing wind data for the period of observations indicate a prevailing wind from the southeast. Therefore, the diversion of the brackish water is likely due to the eddies set up by the process of tidal rectification.