1. Spatial and Temporal Variability of Hydrography in the Vicinity of the Main Endeavour Field Scott Veirs, Christian Sarason, Russell McDuff, Fritz Stahr, Ko-ichi Nakamura, and Richard Thomson School of Oceanography, University of Washington, Box 357940, Seattle, WA, 98195-7940 | scottv@ocean.washington.edu | www2.ocean.washington.edu/flowmow/ Spatial variations in plume distribution : During a 24-hr vertically oscillating tow between 1850m and 2080m, we completed 5 circuits around the perimeter of the MEF. Viewing each side of the MEF perimeter “wall” from a southeast perspective, successive circuits reveal how quickly the near-field plume distribution evolves. Regional erosion of background stratification: On a regional scale, potential density profiles reveal a weakening of the Northeast Pacific deep ocean stratification within about 300m of the bottom. As the MEF is approached, a 50m-thick mixed-layer is often present near the axial valley floor. The heat and salt contamination alters the background potential temperature (  ) – salinity (S) curve, spreading the isopycnal surfaces above the ridge. Study site: During the August, 2000, Flow Mow program we examined how the Northeast Pacific is altered by hydrothermal plumes above the Endeavour Segment of the Juan de Fuca Ridge. Our goal was to define a background hydrography that would allow meaningful calculation of thermal anomalies and associated heat fluxes from measurements made above the Main Endeavour Field (MEF). The MEF perimeter is shown in black at right; red dots are known vents. Currents at 5 depths: Current measurements were made throughout the hydrographic survey on a nearby mooring (purple circle on the map above) that supported meters at ~50, 100, 150, 200, and 250m above the seafloor. Strong flow above the ridge crest is predominantly to the southwest, while in the axial valley the fastest currents usually flow to the north. Mean current speeds are ~5cm/s above the ridge crest and ~2cm/s within the axial valley, though intermittent peak flow velocities are ~5cm/s faster. Instrumentation: We used a navigated instrument package to measure temperature, conductivity, pressure, light transmission, optical backscatter, and altitude. Attached redox potential sensors and Niskin bottles characterized fluid chemistry. Additionally, we separated our 2 sets of temperature and conductivity sensors vertically to detect vertical instabilities. T1,C1 T2,C2 Temporal variations in plume distribution: North (station 28) and south (station 33) of the MEF, we monitored the effects of tidal currents on hydrothermal plume distributions by vertically cycling the package 5 to 500m above the seafloor for 10-14 hrs. Conclusions: 1) Both the vertical stations and the perimeter survey indicate that the hydrography above the MEF is highly variable. 2) Near the MEF, equilibrated plumes are intermixed with a complex assemblage of buoyant, chemically-active plumes. In such areas the background stratification is eroded, and plume distribution appears consistent with the current meter measurements. 3) The fluid in the bottom 100m is usually contaminated, but occasionally is influenced by background Northeast Pacific deep water. Time series of potential temperature anomaly [oC] Depth, [m] UTM X, [m] UTM Y, [m] Hodographic histograms of hourly currents Spatial series of potential temperature anomaly [oC] W E Successive circuits South (33) North (28)