Activities For Episode Five, Over The Edge

Activities for Episode Eleven: Going with the Flow Activity 11.3: Measuring Ocean Currents Acoustically (an audio activity)

Learning Objectives As technology advances, so does science and this is no less true for the measurement of ocean currents. The development of acoustic techniques for measuring currents provided a means for oceanographers (not to mention fishermen and sailors) to directly measure currents over large temporal and spatial scales from a variety of platforms, including ships and buoys. This tool, the Acoustic Doppler Current Profiler (ADCP), revolutionized our understanding of the nature in which the ocean moves.

In this activity, you’ll learn more about this exciting new technique and dive into the research of a physical oceanographer using ADCP and other emerging technologies for measuring California coastal currents. By the end of this activity, you should be able to:  describe the principles of acoustic Doppler current profiling  explain the ways that it is being applied in the ocean  discuss the benefits of ADCP and other types of current measurements for understanding the coastal environment and human impacts

I. What do the scientists say? In this activity, we’ll focus on the work of a physical oceanographer, Libe Washburn, at the University of California, Santa Barbara. Dr. Washburn’s research focuses on coastal circulation, mesoscale processes, air-sea interaction, and other types of interdisciplinary oceanography. Let’s hear what he has to say first, then we’ll learn more about his research.

LIBE WASHBURN “Current measurements are among the most difficult oceanographic measurements to make, and the reason for this is that the current speeds are small. They require instrumentation that has to sit out in the ocean for long periods of time, or it requires that instruments be made from ships that are moving. And these ships are typically moving at speeds that are much greater than the ocean currents, which are the object of the measurement. Originally, current measurements were made with drifters or particles that were placed in the water, and their position was tracked over time. And this could give an estimate of a current speed. Ship drift was an early method for measuring ocean current.”

“Now, we use acoustic techniques primarily in ocean current measurement. And one of the revolutionary devices is called the Acoustic Doppler Current Profiler, which is a means of measuring ocean currents over the water column of the ocean over depths of a few hundred…up to a few hundred meters using sound energy. And these techniques have the advantage that they do not require instruments to be placed at several places in the water column, but often a single instrument can be moored or can be placed on the bottom, and currents can be determined hundreds of meters above…in the water column extending hundreds of meters above the instrument.”

“Ocean currents are very important for delivering nutrients, especially along coastlines. A good example is the California current system which has a region of very strong upwelling along the coast, and the cool, nutrient-rich water is brought to the surface and is distributed along the coastline by the California current system. Organisms also depend upon currents for moving around. Many organisms have a planktonic phase where they drift freely in the ocean or swim very weakly, and they rely upon ocean currents for their movement as they disperse passively in ocean currents.”

II. What is Acoustic Doppler Current Profiling ADCP relies on a phenomenon well-known to most of us, the Doppler effect. Ever hear an ambulance or fire engine car as it roars towards or away from us? How about a train going by while waiting at a crossing gate? caltraindopplerstereo.mp3 Refer to the audio file on the website.

Formally described by the Austrian mathematician and physicist, Christian Doppler, the Doppler effect results from a shift in the frequency of sound (or light) resulting from the compression or expansion of sound (or light) waves as an object moves towards or away from an observer, respectively. An oceanographer can use this principle to measure the speed and direction of water.

In this activity, you will explore several web sites (and find your own) that feature descriptions and explanations of ADCP. You’ll also explore Dr. Washburn’s research to see how ADCP can be applied to understanding the effects of currents on planktonic organisms and to solving problems associated with human impacts in the coastal environment. Then you will design a small study of ocean currents in a location of your choosing using three instruments (you choose the make and model) on a platform (or platforms) of your choice.

Your research design using ADCP’s must include the following:  the location of your study site and a brief description of its hydrography, including type of water body (estuarine, near-shore, coastal, oceanic, etc), major water column characteristics (temperature, salinity, general circulation), proximity to bathymetric features and human sources of water-borne particles or chemicals and any other distinguishing features that you deem important  a statement of what exactly you intend to study (what type of currents, i.e. surface, near- bottom, etc) and why it would be important to study currents at your chosen location (i.e. role of currents in distributing larvae or kelp spores, role of currents in distributing pollutants)  the instrument type and model you have chosen, how it works and why it’s particularly suited for the environment of your chosen location  how, when, and where you will deploy your three ADCP instruments and for how long  what you expect to learn from this experiment

You may design your ADCP Research Proposal as a written paper, an oral presentation, a Powerpoint Presentation, a web site or some other means, as approved by your instructor. Be sure to include figures, where possible. You may even want to sketch your instrument and show how it will be deployed at your site. Use your imagination! This is the one time you get to propose research without worrying about a budget!

Recommended web sites:

ADCP – Acoustic Doppler Current Profiler Overview http://www.sontek.com/adcp.htm

An ADCP for every use http://www.rdinstruments.com/applications.html

Broadband Acoustic Doppler Current Profiler http://www.whoi.edu/coastalresearch/instrumentation/html/adcp.html

Set Sail to Learn About the Sea and the Sky (Science from a Cruise Ship) http://www.oar.noaa.gov/spotlite/archive/spot_explorer.html

Explorer of the Seas Acoustic Doppler Current Profiler http://www.rsmas.miami.edu/rccl/adcp.html

Shipboard ADCP Data Collection http://ilikai.soest.hawaii.edu/sadcp/metho.html

The Southern Central California CODAR Home Page (use menu choices in left column, i.e., Scientific Details, Related links); see also Poster 2 in Recent News http://www.icess.ucsb.edu/iog/codar.htm

Libe Washburn’s Current Projects Page http://www.icess.ucsb.edu/iog/Washburn.html

UPPER OCEAN CURRENTS IN THE INTENSIVE FLUX ARRAY DURING TOGA COARE http://www.soest.hawaii.edu/COARE/abstracts/exabcurr.html Ocean Research Benefits Air Force, Guam http://www.af.mil/news/Feb2001/n20010206_0174.shtml

Currents and Suspended Particulate Matter http://www.af.mil/news/Feb2001/n20010206_0174.shtml

The TAO Project: Mooring Information (see Current Meter moorings) http://www.pmel.noaa.gov/tao/proj_over/mooring.shtml

The 2000 Great Chesapeake Bay Swim (employing ADCP to help swimmers) http://noaa.chesapeakebay.net/BaySwim/BaySwim.htm