Topographically Enhanced Subinertial Currents at Endeavour Ridge

We use velocity records collected from moored current meters to examine the effects of seafloor topography and hydrothermal venting on near-bottom (\~ 2000 m depth) currents flowing over the Endeavour Segment of Juan de Fuca Ridge. Focus is on the 50-m vertical resolution records collected from July-October 2000 near the main Endeavour vent field and on the 2-km lateral resolution records collected from July-October 2001 at three sites within the 100 m deep axial valley. Semidiurnal currents are found to be marginally more energetic than diurnal currents, and flow above the ridge crest is often dominated by wind-generated inertial events (periods \~ 16 hrs at 48N) and low-frequency (O(10 day) period) clockwise rotary motions. Observations, supported by numerical modeling, reveal marked topographic amplification of subinertial motions within 100 m of the ridge crest. Motions within the diurnal, inertial, and wind-forced frequency bands undergo especially pronounced above-ridge amplification but attenuate equally rapidly within the confines of the narrow (\~ 1 km) axial valley. Semidiurnal currents are much less affected by the ridge topography and have approximately uniform amplitudes with depth within the first 250 m of the bottom. Within a few tens of meters of the valley floor, the flow is dominated by \~ 5 cm/s along-axis semidiurnal oscillations and a surprisingly strong (2 to 4 cm/s), persistently northward up-valley flow. The up-valley flow appears to be independent of, and generally counter to, the prevailing flow in the overlying water column. Initial findings suggest that the enhanced near-bottom flow is maintained by an along-valley pressure gradient created by turbulent entrainment of cold (\~ 2 C) ambient water by the superheated (\~ 350 C) hydrothermal plumes and low-temperature diffuse flow. If so, the mean-flow dynamics may be analogous to the summer sea-breeze in coastal fjords, with hydrothermal convection playing the role of summertime heating of the land. In analogy with coastal inlets, the steep sides of the axial valley confine the thermally driven flow, forcing cold, high salinity bottom water to enter the valley from the south. Regardless of the forcing mechanism, the observed topographically enhanced circulation at Endeavour Ridge is likely to have significant implications for space-time variability in biotic and abiotic fluxes from the axial valley.