Moored Current Meter Experiments

Model residual flows are northward to the west and southward to the east at speeds upward of 3 cm s-1. The off-axis pattern and magnitude are comparable to residual flows measured across the JdFR. (Click for larger image.)

Current meter observations show the general long-term average flow along and near the Juan de Fuca Ridge. Topographic features of the ridge and seamount chains steer flow in a variety of ways that determine the fate of plumes from hydrothermal vents. Mean flows at ridge-crest depths are relatively large for the deep ocean, and their magnitudes are comparable to upper ocean currents. Because of topographic differences, important differences can be expected on different segments and on different parts of the same segment. For example, a major cross-ridge flow occurs at the Coaxial site on the north side of Axial Volcano, and it is speculated that the Axial-Cobb massif induces a clockwise circulation about itself. Early observations at South Cleft showed weak and variable mean flow over the crest and in the rift valley, but observations from North Cleft, 1990 at showed southward flow on the east side extending over the vent field right up to the west wall, probably due to the absence of the east side wall. Other effects of topography at North Cleft show various features that can affect distributions of plumes from the vent field. In 1992-93 the north and south flows along the west and east flanks of Cleft decreased away from the crest to zero at the outer edge of the ridge, and in 1993 there was eastward turning of flow around the north end of the west wall.

Rotary current spectra show the frequency content of currents measured at Axial seamount. A sizeable spectral peak occurs in A at 0.01 cph (4days) but not in B, though data come from two mooring sites separated by just 25 km laterally and 275 m vertically. That difference in the spectra suggests topographic influence. Tidal and inertial motions are dominant at both sites.

Time variability in flow also is important in determining plume distributions and occurs at various frequencies. Off-axis currents south of Vance segment of the Juan de Fuyca, show much stronger westward flow during winter. Storm induced 4-day oscillations can cause plumes to rotate, and they may or may not be present at a specific site on a specific day. Larger scale variations can cause the whole flow field to rotate over the ridge. Finally, observations around Pipe Organ vent (on the Cleft segment), with WHOI showed the possible existence of plume induced vortex flows . A vortex can become unstable, detach from the vent source, and carry hydrothermal signatures for at least 1000 km to the southwest.

Simultaneous monitoring of flow at North and South Cleft began in 1995 and will continue during the Ridge Observatory.

Physical Oceanography Methods: Current Meters | CTD | RAFOS