Variability of Surface Currents over Central Juan de Fuca Strait

Four years of surface currents from a two-element array of high frequency (HF) radars are used to study variability of the regional estuarine circulation over central Juan de Fuca Strait. Surface currents in the long term mean are directed in a seaward outflow to the continental shelf. This outflow is subject to substantial variability on time scales ranging from tidal to interannual, and includes a pronounced seasonal cycle. The integrated along-strait surface-layer transport is used characterize the strength of the sub-tidal circulation. Variability of the transport includes marked reversals of the estuarine outflow that occur primarily during winter and are related to downwelling winds over the outer coast with a time lag of 3 to 4 days. Notably, the surface-layer transport is subject to a seasonal modulation with an approximately three-fold increase in the outflow from winter to summer. Over a wide range of frequencies, the transport is coherent with cross-strait sea-level differences derived from tide gauge data. At the lowest resolved frequencies, including the annual cycle, the surface-layer transport is in approximate geostrophic balance with cross-strait sea-level differences. On interannual time scales, the integrated surface-layer transport during summer appears to respond to variations in the net fresh water runoff due to the spring freshet of the Fraser River, although the record length is too short to draw firm conclusions regarding this dependence.