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 shows new features of topographic steering of flow that determine the fate of plumes from hydrothermal vents. An anticyclonic circulation exists over the ridge with north and south mean flows along the west and east 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 forcing and tidal rectification may generate the along-ridge flows. Average anomalous heat fluxes associated with the north and south flows were estimated at 1400 and 2600 MW, respectively, and they are larger than previous estimates based on single-point current measurements. Variability 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 into the stronger north-south flows over the flanks. Currents along the west side of North Cleft diverge with near-ridge flow (<10 km from the ridge) turning eastward around the north end of the segment and offshore flow turning westward. Farther west in winter. off-axis currents can be several times as large as the long-term mean and could advect plume water relatively long distances. Significant cross-axis flow occurs on the north side of Axial Volcano that may be the result of interaction between the general circulation and topography generating eddy circulations around or near Axial or the entire Axial-Cobb massif.