While physical oceanographers have been making measurements of near-surface ocean temperatures, currents, and salinity for more than a century, probing the depths to determine the vertical structure of the ocean has been even more challenging than efforts by meteorologists to probe the upper atmosphere. Accurate measurement of deep currents, and water temperature and salinity at various depths not only requires instruments that can withstand the tremendous pressures exerted by ocean water, but also appropriate platforms that provide a means of getting the instruments to the desired depth and retrieving the data once measurements are made.
The term sounding, used by both oceanographers and meteorologists to denote a sequence of observations in the vertical, appears to have originated from the ancient practice of river or ocean depth determination by means of a measured line.
Measurement of ocean currents can be made by either the float or the flow method. The float method for determining the speed and direction of ocean currents involves tracing the movement of a free-floating object, such as a floating drift bottle for surface currents or a parachute drogue for currents typically within 100 m of the surface. Neutrally buoyant floats (e.g., Swallow floats, named for their inventor) sink to a predetermined depth. These floats contain an electronic system called a "pinger" that emits "pings" or short sound pulses that can be detected by a receiver onboard ship. The flow method involves measurement of the speed and direction of a current past a fixed object, such as a mechanical current meter, where a rotor or propeller measures current speed and the orientation of the device indicates current direction. Because this type of instrument must be anchored at a fixed location, it is impractical for monitoring deep ocean currents.
Some of the first temperature and salinity soundings in the early 20th century employed an instrument rack that contained reversing thermometers to measure ocean temperature and collection bottles that permit the determination of salinity at various depths. Reversing thermometers were mercury-recording thermometers that indicate the temperature at the desired depth. However, this type of sounding method was time consuming because the instrument rack had to be lowered on a cable to a predetermined depth, a water sample collected at depth, and the rack had to be retrieved before analysis could be made. Sampling at various depths was needed to produce profiles of temperature, salinity, and ultimately, water density.
Subsequently, an instrument called a bathythermograph (BT) was developed that could measure a nearly continuous temperature profile with depth from a moving ship. As this torpedo-shaped device sank, a temperature-sensing element (typically a deformation or bimetallic thermometer) inside the device caused a stylus to scratch a trace on a metal-coated slide. The resulting graphical temperature profile could only be interpreted after the BT was retrieved.
Since the 1970s, newer electronic technologies have permitted more rapid soundings that produce an almost continuous profile of temperature. An expendable bathythermograph (XBT) is a non-recoverable device that is deployed from a ship and measures a nearly continuous ocean temperature profile to a depth of approximately 1800 m. This torpedo-shaped device consists of a thermistor, or electronic thermometer, placed within an expendable casing. A thin conducting wire connecting the XBT to the ship serves as the means for transmitting the temperature signal to an onboard recorder. As the device sinks into the ocean at a known rate, data are transmitted electronically via the wire at regular intervals, permitting the recording of the ocean temperature as a function of depth. A slightly different profiling instrument is an expendable conductivity-temperature-depth profiler (XCTD) that provides essentially continuous profiles of ocean conductivity and salinity in addition to temperature and pressure. With direct measurements of the conductivity and the temperature at various depths, the salinity can be calculated directly.
NOAA's Atlantic Oceanographic and Meteorological Laboratory in Miami, FL monitors the XBT program. Volunteer commercial ships deploy as many as four XBTs daily along selected shipping lanes. The data are compiled by a computer onboard ship and then transmitted by satellite relay to the Laboratory for global distribution. More than 70 volunteer ships produce 26 monthly transects across the three major ocean basins.
For several decades, orbiting satellites have provided improved observations of sea surface temperatures, sea-level topography, and surface winds. In recent years, major international efforts have been undertaken to develop global systems for observing, analyzing, and modeling the ocean environment, with the ultimate goal of improving climate modeling and prediction. Various international programs include the Global Climate Observing System (GCOS), the Global Ocean Observing System (GOOS), and the Climate Variability and Predictability Experiment (CLIVAR). An attempt is being made to complement satellite data with an array of unattended-instrumented platforms analogous to the worldwide network of radiosonde stations that probe the atmosphere to altitudes of approximately 30,000 m.
In 1985, a ten-year international Tropical Ocean Global Atmosphere (TOGA) program commenced. One of the TOGA projects was the deployment of the TAO (Tropical Atmosphere/Ocean Project) array of moored buoys in the tropical Pacific Ocean. Information from this array has been extremely useful for detecting and predicting such atmospheric and oceanic episodes as El Niño and La Niña. This array, renamed TAO/TRITON array in 2000, presently consists of approximately 70 deep-sea moorings that measure several meteorological variables (wind, relative humidity, and air temperature) as well as oceanic parameters (sea surface and subsurface temperatures at 10 depths in the upper 500 m of the ocean). Several newer moorings also have salinity sensors, along with additional meteorological sensors. Five moorings along the equator also measure ocean velocity using a Subsurface Acoustic Doppler Current Profiler. The data are collected and relayed in a nearly real-time mode to shore via orbiting satellites. Real time data displays from the TAO/TRITON array are available from NOAA's Pacific Marine Environmental Laboratory (PMEL) in Seattle, WA.
In the last decade, a variety of autonomous (free-drifting) instrumented profilers have been developed and tested to measure large-scale subsurface currents and make repeated vertical measurements of ocean variables. Early versions of these free-drifting profilers were identified with the term PALACE (an acronym for Profiling Autonomous Lagrangian Circulation Explorer), while a subsequent version is called APEX (Autonomous Profiling Explorer). These subsurface floats, approximately one meter in length and less than 20 cm in diameter, can be deployed from either ships or aircraft. Once deployed, a PALACE/APEX profiler is designed to sink to a depth where it is neutrally buoyant, drift for approximately 10 days at depth, and then slowly rise back to the surface. The maximum drift depth to which the profiler can sink is 2000 m. In order to change its vertical position by a change in buoyancy, hydraulic fluid is pumped from an internal reservoir to an external bladder. During its ascent, the environmental temperature, pressure, and salinity are recorded nearly continuously by onboard sensors. Upon return to the ocean surface, the float telemeters these data to an orbiting satellite for subsequent relay to data collection stations. The float's position is also determined by satellite. Ocean current information is inferred from the horizontal displacement of the float from one surfacing to the next. Following a programmed interval at the surface, the float returns to depth for the next 10 to 14 day cycle. The anticipated lifetime of one of the PALACE/APEX floats is 100 cycles.
In order to monitor the climate of the upper ocean on a denser spatial and temporal basis, a widely spaced array of 3000 such instrumented floating profilers is to be deployed across the ocean at a 3-degree latitude-longitude spacings within the next several years. The large-scale average oceanic flow is expected to be mapped by the long-term observations provided by the array that is called ARGO (for Array for Real-time Geostrophic Oceanography). While this global-scale array is to be international, approximately half of the floats will be deployed by various U.S. institutions. Other countries participating include Australia, Canada, Japan, France and the United Kingdom. As of the end of last week (24 October 2008), 3190 floats were operational in a global array. The U.S. portion of the ARGO program includes floats that are under the auspices of the University of Washington, Scripps Institution of Oceanography, and Woods Hole Oceanographic Institution. The ARGO Float Profilers link found on the DataStreme WES website displays near real-time data from the University of Washington ARGO/PALACE Program. NOAA's Atlantic Oceanographic and Meteorological Laboratory in Miami is archiving the ARGO data.
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Prepared by WES Central Staff and Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2008, The American Meteorological Society.
