VARIATIONS IN MEAN CURRENTS AFFECTING HYDROTHERMAL PLUMES ON THE JUAN-DE-FUCA RIDGE

A summary of current meter observations near the Juan de Fuca Ridge sh ows new features of topographic steering of flow that determine the fa te of plumes from hydrothermal vents. An anticyclonic circulation exis ts over the ridge with north and south mean flows along the west and e ast flanks. Speeds are up to 2-3 cm s(-1) near the ridge and decrease to zero about 25 km from the crest. Contributions from hydrothermal fo rcing and tidal rectification may generate the along-ridge flows. Aver age anomalous heat fluxes associated with the north and south flows we re estimated at 1400 and 2600 MW, respectively, and they are larger th an previous estimates based on single-point current measurements. Vari ability of flow occurs on several time scales and space scales. Daily average currents over a 4-day period can rotate over the entire ridge with sufficient cross-axis speeds to move plumes away from the vents i nto the stronger north-south flows over the flanks. Currents along the west side of North Cleft diverge with near-ridge flow (<10 km from th e ridge) turning eastward around the north end of the segment and offs hore flow turning westward. Farther west in winter. off-axis currents can be several times as large as the long-term mean and could advect p lume water relatively long distances. Significant cross-axis flow occu rs on the north side of Axial Volcano that may be the result of intera ction between the general circulation and topography generating eddy c irculations around or near Axial or the entire Axial-Cobb massif.