Chapter 10 - Geostrophic Currents

10.9 Eulerian Measurements of Currents

Eulerian techniques have used many different types of current meters attached to many types types of moorings or ships.

Moorings (Figure 10.18) areplaced on the seafloor by ships. The moorings may last for months to longer than a year. Because the mooring must be deployed and recovered by deep-sea research ships, the technique is expensive. Subsurface mooring shown on the right in the figure is preferred for several reasons: the surface float is not forced by high frequency, strong, surface currents; the mooring is out of sight and it does not attract the attention of fishermen; and the floatation is usually deep enough to avoid being caught by fishing nets. Measurements made by moorings have errors due to:

1. Mooring motion. Subsurface moorings move least. Surface moorings in strong currents move most, and are seldom used.
2. Inadequate Sampling. Moorings tend not to last long enough to give accurate estimates of mean velocity or interannual variability of the velocity.
3. Fouling of the sensors by marine organisms, especially instruments deployed for more than a few weeks close to the surface.

Figure 10.18 Left: An example of a surface mooring of the type deployed by the Woods Hole Oceanographic Institution’s Buoy Group. Right: An example of a subsurface mooring deployed by the same group. From Baker (1981).

Acoustic-Doppler Current Meters and Profilers
The most common Eulerian measurements of currents are made using sound. Typically, the current meter or profiler transmits sound in three or four narrow beams pointed in different directions. Plankton and tiny bubbles reflect the sound back to the instrument. The Doppler shift of the reflected sound is proportional to the radial component of the velocity of whatever reflects the sound. By combining data from three or four beams, the the horizontal velocity of the current is calculated assuming the bubbles and plankton do not move very fast relative to the water.

Two types of acoustic current meters are widely used. The Acoustic-Doppler Current Profiler, called the ADCP, measures the Doppler shift of sound reflected from water at various distances from the instrument using sound beams projected into the water just as a radar measures radio scatter as a function of range using radio beams projected into the air. Data from the beams are combined to give profiles of current velocity as a function of distance from the instrument. On ships, the beams are pointed diagonally downward at 3–4 horizontal angles relative to the ship's bow. Bottom-mounted meters use beams pointed diagonally upward.

Ship-board instruments are widely used to profile currents within 200 to 300 m of the sea surface while the ship steams between hydrographic stations. Because a ship moves relative to the bottom, the ship's velocity and orientation must be accurately known. GPS data have provided this information since the early 1990s.

Acoustic-Doppler current meters are much simpler than the ADCP. They transmit continuous beams of sound to measure current velocity close to the meter, not as a function of distance from the meter. They are placed on moorings and sometimes on a CTD. Instruments on moorings record velocity as a function of time for many days or months. The Aanderaa current meter (figure 10.19) in the figure is an example of this type. Instruments on CTDs profile currents from the surface to the bottom at hydrographic stations.

Figure 10.19 An example of a moored current meter, the RCM 9 produced by Aanderaa Instruments. Two components of horizontal velocity are measured by an acoustic current meter, and the directions are referenced to north using an internal Hall-effect compass. The electronics, data recorder, and battery are in the pressure-resistant housing. Accuracy is ± 0.15 cm/s and ±5°. (Courtesy Aanderaa Instruments)

Acoustic Tomography
Another acoustic technique uses acoustic signals transmitted through the sound channel to and from a few moorings spread out across oceanic regions. The technique is expensive because it requires many deep moorings and loud sound sources. It promises, however, to obtain information difficult to obtain by other means. The number of acoustic paths across a region rises as the square of the number of moorings. And, the signal propagating along the sound channel has many modes, some that stay near the axis of the channel, others that propagate close to the sea surface and bottom (See Figure 3.16). The various modes give the vertical temperature structure in the ocean, and the many paths in the horizontal give the spatial distribution of temperature. If one mooring retransmits the signal it receives from another mooring, the time for the signal to propagate in one direction minus the time for the signal to propagate in the reverse direction, the reciprocal travel time, is proportional to current component parallel to the acoustic path.

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