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Physical Oceanographic Measurements

Contents

Background

  • This very brief description is not intended to provide fundamental instruction in physical oceanography, as with all OceanTeacher materials, we leave basic marine education to the colleges and universities. This article merely provides a conceptual framework and terminology for data managers who may have a slightly different specific background, but who have marine data responsibilities.

Principal Physical Measurements

The first three parameters listed here are classical hydrochemistry measurements used by physical oceanographers historically for water mass identification and inferences about large-scale movements and mixing processes.

Temperature

Salinity

Oxygen

  • Oxygen, a gas, has a curious history of measurements, because the units used to report it have changed greatly over the years.  In the first half of the 20th century it was always reported as volume-per-volume, or ml/l to be exact.  It has also been reported as millimoles per liter and millimoles per kg seawater.  See History of Chemical Measurement Units for further information.
  • Oxygen measurements are most accurate when obtained with the Winkler titration method, seen here on the right.  Probes capable of measuring oxygen at depth are available for deployment on CTD devices.
WinklerTitration.jpg











From NOAA Photograph Library

Density

CTD

  • Because salinity and temperature are measured so often, and are so important in oceanography, from the early 1960's engineers began developing submersible probes to measure them in the ocean water column.  Naturally, these instruments also needed pressure gauges to complete the three variables needed to calculate density.  These instruments are the workhorses of hydrography today, universally called CTD's for Conductivity (the proxy measurement for salinity), Temperature and Depth.  CTD frames can accomodate other probes, such as oxygen.

Currents

  • Wikipedia: Ocean current
  • Current measurements quantify the directional flow of ocean waters, whether due to named large ocean current systems, transient wind events, tides or other factors.
    • The earliest known ocean "data" were the results of ad hoc ocean current drifter experiments by the ancient Greeks, known today as "Lagrangian measurements".
    • For much of the modern history of oceanography instruments have been available to place in the sea for recording instantaneous currents at one location, known as "Eulerian measurements."
    • A large fraction of the total current can also be estimated from density distribution by the "geostrophic approximation" formula
  • Currents were measured historically in units of "knots" which is one-sixtieth of a degree of geographic latitude in one hour, or one nautical mile per hour. One knot is equal to approximately 51 cm/sec.
  • Current velocities, measured today in centimeters per second, range from very slow "residual" (or average) currents on the order of a few cm per sec, up to measured "gusts" of several hundred cm/sec.
  • Lagrangian versus Eulerian Tracers
  • Ocean Surface Currents Glossary
  • Ocean Surface Current Climatology in the Northern Gulf of Mexico - The largest known synthesis of both Lagrangian and Eulerian current data for one region

Lagrangian Measurements

  • Historically, Lagrangian current data were obtained by casting marked objects into the sea and awaiting either the return of the object or of an included postal card, if they were found ashore by persons willing to assist in their return. Many thousands of drift objects, worldwide, resulted in begin point-end point type datasets that proved quite interesting prior to the age of satellite tracking. Since the 1970's however, satellite tracking of the precise path of the drifting object has been possible. The international System Argos has been a leader in such services, where "smart tracking of dumb drifters" was needed. With the advent of the GPS, "smart drifters" can now telemeter their own positions to other systems as well. The major global Lagrangian activity today is the Global Drifter Program, involving hundreds of surface drifters deployed by many nations
  • Lagrangian data take the form of coordinate locations versus time, with reporting intervals from minutes to days, depending on conditions, and positional accuracy depending on the system involved (i.e. 100s of meters to better than one meter)
  • Point-to-point Lagrangian data can be used to calculate the mean surface drift speed (typically as cm/s) and direction, thus yielding a current vector conventionally assigned to the midpoint of the segment. Ensemble datasets of many such surface current "instaneous" vectors have been used to create grids of surface currents. Work is underway in many groups to find other ways to use Lagrangian data more directly in ocean modeling.
  • Other measures of ocean energy, such as dispersion values, can be calculated.
  • System Argos - The French satellite tracking system used to capture Lagrangian trejectory data from many surface drifters
  • Global Drifter Program (GDP) - Satellite-tracked surface drifting buoy program using drifters whose design has been shown to track surface water particles extremely closely, due to the use of a "holey sock" drogue that anchors the buoy to a surface water mass.
  • Argo Project - "Argo is a global array of 3,000 free-drifting profiling floats that measures the temperature and salinity of the upper 2000 m of the ocean. This allows, for the first time, continuous monitoring of the temperature, salinity, and velocity of the upper ocean, with all data being relayed and made publicly available within hours after collection."  [From the Argo website] - The surface trajectories are not as detailed as GDP, due to near-continuous submergence, but the project complements GDP with closely related data.

Eulerian Measurements

  • Defined as currents measured at a single point in the sea
  • Over the past century numerous devices hae been invented to make Eulerian current measurements, all collectively called "current meters."  They range in complexity from simple mechanical devices, lowered into the sea on hydrographic cable for short periods of time, to complex, moored electronic devices that measure currents throughout the entire water column for long periods of time.  We present here a representative sampling of major devices you will encounter in various databases.
Instrument Name
 Operating Principle
 Image
Ekman Current Meter
 See Wikipedia: Ekman current meter


EkmanCurrentMeter.jpg

From [NOAA Photograph Library]

Impeller Current Meters
 The impeller is a vaned rotor, acting like an airplane propeller in reverse: it derives its speed of rotation from the current.  Many different designs have been used.  The model shown here has a umbrella-like guard of light bars to keep trash away, and the impeller rotor itself is just barely visible at the top of the missile-like electronic housing.  Typically these units were directed into the current by fins, as you see her on the right.  This unit is about to be deployed on a mooring that is held upright in the sea by the air-filled, white steel sphere, above.  The cable disappearing into the water has a substantial anchor at the bottom to hold the mooring in place; used railroad wheels are often used, due to their availability and cheap cost.
ImpellerCurrentMeter.jpg
From NOAA Photograph Library
Niskin Winged Current Meter
 "Winged housing is turned in the direction of the flow and swept away from the vertical by the water current; a measurement of the magnitude and direction of the tilt provides speed and direction of the current." From General Oceanics website.
NiskinWingedCurrentMeter.jpg
From General Oceanics website
Electromagnetic Current Meters "Electromagnetic Current Meter measures the voltage resulting from the motion of a conductor (water flow velocity) through a magnetic field according to Faraday's law of electromagnetic induction. Simply stated, Faraday's law defines the voltage produced in a conductor as the product of the speed of the conductor (water flow velocity) times the magnitude of the magnetic field times the length of the conductor....Two orthogonal pairs of electrodes and an internal flux gate compass provide the current vector."  From InterOcean Systems, Inc. website.
ElectromagneticCurrentMetersS4.jpb.jpg
From InterOcean Systems, Inc website
Acoustic Doppler Current Meters
AcousticDopplerCurrentProfiler.jpg
From NOAA Photograph Library


Sea Surface Height

  • Sea surface height (SSH) is defined as the height of the instantaneous (i.e. synoptic) sea surface compared to the long term average surface level.

Components of SSH

  • Long-term changes associated with changes in seawater density (i.e. temperature rise due to global warming). Amounts to a few millimeters rise per year.
  • Medium-term changes associated with ocean circulation (i.e. density distribution due to currents). Amounts to a total "signal" as large as approximately 20 cm (+/-) with fluctuations on the order of days to seasons.
  • Short-term changes associated with storm surges. Amounts to 1- to few-day perturbations on the order of meters, along a storm track up to a few hundred kilometers in width.
  • Short-term changes associated with tides. Amounts to cm- to m-scale cyclic changes over near-daily periods.
  • Very short-term changes associated with a tsunami. Amounts to rapid sea leavel rise of up to a few meters, over minutes to hours, affecting entire ocean basin coasts in the worst cases; at sea, tsunami perturbations are only tens of centimeters high.

Measuring SSH

  • Tide gauges - Devices have been in place for decades in some locations, and data accuracies on the order of mm are routinely obtained
  • Satellite altimeters - Several satellite platforms are providing altimeter data today, with accuracies on the order of 2 cm globally

Waves

  • Wikipedia: Ocean surface waves
  • Wikipedia: Significant wave height
  • Wikipedia: Swell (ocean)
  • Measured quantities:
    • Wave height (from trough to crest) - Measured in meters; the average of the highest third of the waves is called the significant wave height, and this is often the reported value; values over 10 m can occur
    • Wavelength (from crest to crest) - Measured in meters
    • Period (time interval between arrival of consecutive crests at a stationary point) - Measured in seconds
    • Wave propagation direction (with respect to north) - Degrees true

Physical Parameter Lists

There are many lists of physical measurements in oceanography. By far, the most comprehensive list is that developed by the BODC (below).

  • See Parameter Name Standards for Marine Data for the current status of discussions to adopt a standard parameter list
  • Resources on the UK NERC Data Grid Vocabulary Server - The Vocabulary Server is a Web Service API implemented both as SOAP and pseudo-restful HTTP-POX interfaces, containing many vocabularies gathered by UK scientists
    • BODC Use Metadata Parameters List - List P011 contains all measurement terms, including physical measurements. This list is enormous (~19,000 terms), so should not normally be downloaded. The surprising number is due to the fact that the BODC register includes both parameters and methods to measure the parameters. Chemical, biological and geological parameters are included, so a simple count of physical parameters is impossible, but it is probably the largest category of parameters.[Similarly named lists contain working datasets, and should not be consulted]
    • BODC Discovery Metadata Parameters List - List P021 contains all measurement terms, including physical measurements. This list is relatively manageable in size, because the terms are more generalized than the use metadata link above. [Similarly named lists contain working datasets, and should not be consulted]

Additional Resources



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Information about this article

Short title: Physical Measurements

Description: This article is intended to provide a brief introduction to the types of data and products involved in this discipline. It is not a general introduction to the discipline itself.

Expertise level: beginner

Author: Murray.Brown

Approval status: approved

Approved by: Murray.Brown

Last change: 2011-8-6

Subsection of: Marine Data Concepts

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