Free falling Probe Current and Temperature Measurement System

Ocean currents all over the globe contribute significantly to the state of the world climate. Knowledge of how they vary with time can provide a useful information on future states of the climate system. While satellites provide excellent spatial coverage, they measure only surface currents. To determine mass- and heatfluxes and their change over time accurately requires regular and repeat measurement of subsurface currents and thermal structure. An attractive way to do this is from merchant marine vessels that traffic the same route on a regular basis. Experience with ADCPs installed on the CMV Oleander and MS Nuka Arctica demonstrate how effective this approach is. However,ADCPs reach only down into the main thermocline. There is an emerging need to include the deep waters in such ocean observation programs. Here we describe a system to profile both velocity and temperature to great depths from vessels underway. The key element of the system is an acoustic probe, similar to an expendable bathythermograph that is dispensed into the water from a ship and falls at a constant velocity. During its descent it continually measures temperature. The measured data, along with a clock signal, are transmitted to a small baseline array of hydrophones in the hull of the vessel such that the probe-hydrophone travel times are continuously known. This information, together with an accurate vessel attitude or inertial navigation system, allows us to determine the probe descent trajectory and hence the current profile. Such systems, operated on merchant marine vessels in repeat and regular traffic, could provide valuable information about ocean currents and water properties along the ship´s route and how these change over time.