Long-term observations of the variability of abyssal plain currents from Ocean Networks Canada’s seafloor observatory.

Steven F Mihaly, University of Victoria, Victoria, BC, Canada, Manman Wang, Ocean Networks Canada, Victoria, BC, Canada and Richard Thomson, Inst Ocean Sciences, Sidney, BC, Canada
Ocean Networks Canada operates the 800 km long NEPTUNE seafloor observatory in the Northeast Pacific. The cable crosses the Cascadia Basin separating the active tectonic spreading centre of Juan de Fuca ridge from the Cascadia subduction zone. The basin is an abyssal plain at a nominal depth of about 2700m at the observation site. Most of the plain is sedimented, but there are a few outcrops of bedrock where heat and fluid flux can occur between the ocean and the oceanic lithosphere.

A NEPTUNE node is located at a central site on the abyssal plain over the Juan de Fuca plate. At this site there is an upward-looking 75kHz acoustic Doppler current profiler and a CTD/O2 sensor. The current profiler provides currents over the bottom 500 m of the water and the CTD/O2 provides temperature, salinity, dissolved oxygen and pressure a few metres above the bottom.

Here we present continuous observations of currents collected over six years. The rich data set shows variability from interannual to tidal time scales. Low-frequency variability of currents that can be modelled as a passage of deep eddies are observed. Water property variability suggest that bottom water heating can initiate the influx of denser water into the basin in a yet to determined periodicity.

The results are preliminary but suggest a much more dynamic setting than expected that can be contrasted dynamics of flow over rough topography at the nearby Juan de Fuca Ridge.