MEAN HYDROGRAPHIC AND VELOCITY SECTIONS NEAR PIPE ORGAN VENT AT JUAN-DE-FUCA RIDGE

A composite cross-ridge hydrographic and velocity section near the Pip e Organ vent site on the Juan de Fuca Ridge is presented. The hydrogra phic section is composed of surveys obtained from 1990 to 1994. The po tential temperature field shows pronounced downward sloping of the iso therms below 1.9 degrees (centered at about 150 m above the bottom) on either side of the ridge crest. The horizontal scale is about 2 km, c omparable to the local Rossby deformation radius. The maximum potentia l temperature anomaly is greater than 0.03 degrees C. Significant ligh t attenuation anomalies are also present. The mean along-ridge velocit y section is constructed from the records of 85 current meters. The me an flow is southward at and to the east of the vent and northward to t he west. Largest velocities occur at the depth of the neutrally buoyan t plume. The cross-ridge shear is thus anti-cyclonic with a characteri stic value of -0.05 f. Geostrophic velocity fields obtained from the d ensity section differ significantly from the field produced from the c urrent meter data. An estimate of the net mean flux of anomalous heat gives 780 (+/-400) MW to the south.