MIDDEPTH FLOW NEAR HYDROTHERMAL VENTING SITES ON THE SOUTHERN JUAN DE FUCA RIDGE

Long-term moored current measurements around the southern end of the Juan de Fuca Ridge show that flow parallels the ridge and may be intensified by its presence. The mean flow is northward on the west side and southward on the east. Thus plumes emanating from the ridge probably are advected along the ridge before flowing away from the ridge. Other topographic features may be responsible for westward or eastward turning. Along the west side of the ridge, we hypothesize a regional flow pattern with southward flow north of Axial Seamount, northward flow south of Axial, and a convergence somewhere south of Axial resulting in westward flow. Along the east side, the flow may be southward everywhere. Above the ridge crest the long-term mean flow is weaker and more variable, and it includes tidal, inertial, and a dominant broadband 4-day oscillation. This 4-day energy is maximum in the rift valley (where the inertial is least), decreases upward and away from the ridge, and is more energetic in winter. This oscillation is predominately a clockwise rotating feature which, along with the presence of inertial oscillations, suggests that it may be storm-induced from the sea surface. These observations along with those by Cannon and Pashinski (1990) suggest northward propagation. Flow variations across the ridge lasting one to a few months have mean speeds of 1-3 cm/s and are capable of transporting plumes off axis to the west or east into the northerly or southerly flows, respectively.