An upper ocean moored current and density profiler applied to winter conditions near Bermuda

Abstract

A new moored instrument which makes repeated high vertical resolution profiles of current, temperature, and salinity in the upper ocean over extended periods was used to observe midwinter conditions near Bermuda. The operation and performance of the instrument, called the profiling current meter (PCM), in the surface wave environment of winter storms is reported here. The PCM profiles along the upper portion of a slightly subsurface mooring by adjusting its buoyancy under computer control. This design decouples the instrument from vertical motions of the mooring induced by surface waves, so that its electromagnetic current sensor operates in a favorable mean‐to‐fluctuating flow regime. Current, temperature, and electrical conductivity are (vector) averaged into contiguous preselected bins several meters wide over the possible profile range of 20‐ to 250‐m depth. The PCM is capable of collecting 1000–4000 profiles in a 6‐ to 12‐month period, depending on depth range and ambient currents. A variety of baroclinic motions are evident in the Bermuda observations. Upper ocean manifestations of both Kelvin and superinertial island‐trapped waves dominate longshore currents. Vertical coherences of onshore current and temperature suggest that internal wave vertical wave number energy distribution is independent of frequency but modified by island bathymetry. Kinetic energy in shear integrated over a 115.6‐m‐thick layer in the upper ocean is limited to values less than or equal to the potential energy required to mix the existing stratification. Mixing events occur when kinetic energy associated with shear drives the bulk Richardson number (defined by the ratio of energy integrals over the range profiled) to unity, where it remains while shear and stratification disappear together.

Observations of amplified flows atop a large seamount

“…show evidence for various island-trapped modes [Wunsch, 1972;Hogg, 1980;Eriksen, 1982a;Luther, 1985], so it is natural to expect analogous wave modes to be trapped to seamounts. Hints that seamount trapped waves exist were provided by two short current meter array records at different seamounts' Fieberling Guyot [Genin et al, 1989] and Dowd Seamount (13ø28'N, 119ø50'W) (P. Lonsdale, personal communication, 1988).…”

Ocean frontal variability in the Frontal Air‐Sea Interaction Experiment

“…These instruments were developed by C. S. Draper Laboratory and are called profiling current meters (PCMs). Details of the performance of the engineering prototype (the instrument set at F9) are given by Eriksen et al [1982]. The other three instruments were built to obtain better performance and be simpler to maintain.…”

Variability and seasonality of physical and biological fields at the Great Meteor Tablemount (subtropical NE Atlantic)

“…At large spatial scales, seamounts can induce a deflection of current (Roden and Taft, 1985;Vastano et al, 1985). At shorter scales, reflected internal gravity waves (Eriksen, 1991;Eriksen et al, 1982), seamount-trapped topographic waves (Brink, 1990;Codiga, 1997a, b;Codiga and Eriksen, 1997), reflection, amplification and distortion of internal waves (Eriksen, 1982(Eriksen, , 1985(Eriksen, , 1991Kaneko et al, 1986) and amplification of diurnal and semidiurnal tides (Genin et al, 1989;Kunze and Toole, 1997;Noble and Mullineaux, 1989) have been reported. Seamounts have also found to be sites of amplified turbulent mixing (Kunze and Toole, 1997;Lueck and Mudge, 1997;Navatov and Ozmidov, 1988).…”