D334: Monitoring ocean circulation in the Atlantic


Cruise diary

19 November 2008

Craig writes...

"In the last update we mentioned the constant logging of data by the ship’s onboard sensors even during transits between mooring stations. As well as the meteorological sensors, the ship also carries an ‘ADCP’ unit to help measure ocean currents. Gerard explains more..."

The glories of the Vessel-Mounted ADCP
"Hi, my name is Gerard and on this cruise I’m responsible for the ship-mounted ADCP. The first thing that you’re wondering about must be what A-D-C-P stands for! I can tell you straight away that it stand for an Acoustic Doppler Current Profiler -- and you’re probably still no wiser. The ADCP is a machine that measures the ocean currents. And I’ll try to explain to you how.

This clever machine uses an effect called the Doppler effect to measure ocean currents. The Doppler effect is something that may well be familiar to you. Have you ever heard sirens change pitch as the vehicle speeds past? Or maybe you’ve heard a train’s horn change sound as the train zooms past? These are all examples of the Doppler effect. It is a change in frequency (pitch) due to the speed of the object emitting the sound. The pitch rises as the vehicle approaches and lowers as the vehicle moves away.

On Discovery, we have an ADCP fitted to the bottom of the hull of the ship. It works by emitting four beams of sound and listening for echoes. The echoes in the ocean come mainly from zooplankton -- tiny microscopic animals that live throughout the ocean -- that move with the same speed as the ocean currents. When the beam that the ADCP produces hits the zooplankton, the frequency (pitch) changes. The ADCP measures this change in frequency and, because the frequency and speed of sound are related, it can determine the speed of the ocean currents.

See, simple really! It is a very useful instrument as it gives the full ocean current in the top 800 metres of the ocean. This is not an easy thing to measure. Often in oceanography, we can only measure relative currents. That is, there could be a constant velocity through the whole of the ocean that we may not know about. The ADCP can tell us about the full current. The ADCP works constantly while the ship is at sea and by the end of the cruise it should provide us with an idea of the ocean currents all the way from the Canaries to the middle of the Atlantic..."

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