General information about ocean observing systems, WOCE, CLIVAR observational networks, etc compiled by the Ocean Information Center (OCEANIC) at http://www.cms.udel.edu/
Goals of measurements for large-scale circulation: to describe the flow and variability of the large-scale, long time-average circulation, at all depths and in three dimensions. To describe the property distributions at the same scales (heat, salinity, oxygen, nutrients, carbon, trace gases) to help deduce the mean circulation, and also for transports and distributions of the properties themselves.
Surface drifters with drogues below the surface ("parachutes") follow the current just below the surface with minimum windage problems.
TOGA and WOCE drifters are drogued at 15 m, and use a drogue design which was chosen for its minimum slippage. A portion of drifters are also drogued at about 100-150 m, but it is not clear what they are measuring.
Drifter maps from the
Data information and availability:
Concentrated deployments of acoustically-tracked floats
have been made over the years in the Gulf Stream region
and in the North Atlantic Current.
Permanent data source, with information about data and availability:
Data information and availability
ADCP's are used in ship mountings, on lowered instrument
packages and on moorings as current meters.
The acoustic doppler current profiler data assembly center
at the U. Hawaii
provides online information and data.
The JODC (Japan Oceanographic Data Center) is the world data
center for archiving and distributing ADCP data.
Data information and availability
Temperature and salinity are measured to provide density
profiles, which can then be used to compute the vertical
shear of geostrophic currents perpendicular to
the line connecting a station pair. With
an assumption, measurement, or inference of the absolute
velocity at at least one level for that station pair, the
velocity profile can be contructed for the station pair.
Inferences come from mapping of various properties, along
vertical cross-sections, or on maps (usually isopycnal
surfaces). Tracers with independent sources and sinks
are the most useful - these include various salinity and
temperature themselves, nutrients, oxygen, chlorofluorocarbons,
tritium, helium-3 (with deep hydrothermal sources as
well as surface sources), carbon-14, and other tracers.
These types of measurements are made from research
ships. Temperature profiling is also done regularly
from ships of opportunity (including many merchant vessels),
using XBT's (see below), providing information on temporal
variability.
Direct velocity measurements could be those from a large
enough set of subsurface floats, or suitably averaged
acoustic doppler current profiling simultaneous with the
geostrophic measurement, and hopefully and eventually
from satellite altimetry.
Data information and availability:
Examples of the products available using XBT data in
the coarse resolution mode:
Data information and availability
Other primary source information:
Sea level measurements, although scattered in space, are used
as long time series to indicate overall change, and have
some limited use to calibrate alimetry measurements.
Permanent data sources:
Acoustic Thermometry of Ocean Climate (ATOC) website.
The El Nino theme page listed here is a window to many of the NOAA,
Navy and other governmental analyses.
Subsurface floats.
Subsurface floats are either tracked acoustically (SOFAR
floats which are sound sources and which are tracked by
moored receivers, or RAFOS floats floats which receive
sound from moored sound sources) or are tracked periodically
by satellite navigation when they pop to the surface (ALACE).
Global deployments for WOCE are concentrating on the 800-
1000 meter level.
Current meters.
Current meters are deployed on fixed moorings. Most use
a rotor or vane (or pair) and a compass. Some use acoustics
(acoustic doppler current measurements). Information on
current meter moorings recently deployed, and for historical
information can be obtained from:
WOCE Current Meter Data Assembly Center
at Oregon State University. It includes maps, data,
and a pointer to a
set of averages and statistics maintained in the U.K.
Acoustic Doppler Current profiling
Acoustic Doppler Current profilers measure the speed of
the instrument relative to the particles that the sound
scatters from. If the instrument is mounted in a ship, then
it measures the speed of the ship relative to the particles.
There are generally several beams at
angles to each other. ADCP's originated as doppler
speed logs for ships - to measure the speed of the ship
through the water. With very precise information from
navigation about the ship's speed, heading, and motion,
the ship's motion relative to the earth can be subtracted and
the speed of the water measured. The range of an ADCP
is about 300 meters, depending on the frequency and efficiency
of scattering.
Other direct current measurements - electric fields
Measurements of water properties and indirect
measurements of circulation.
Hydrographic observations
from research ships.
Some online resources:
XBT and XCTD observations
from ships of opportunity.
XBT's (expendable bathythermographs) measure temperature
as a function of time. Using a calibrated fall rate, the
depth is calculated. At this time, XBT's are deployed
in two modes for global observations: a coarse space
resolution, from many different ships, usually only to 400
meters, and a high spatial resolution from a sparse network
of ships, to 800-1000 meters.
Surface topography from altimetry
Satellite altimetry provides a measure of the sea surface
height relative to the earth's geoid.
The sea surface height measurement is directly
related to the pressure and hence to the geostrophic
currents at the sea surface.
It is sufficiently
accurate to provide a measure of the variability of geostrophic
currents, and may eventually provide a measure of the mean
flow. More complete information
can be obtained starting at the
< href="http://diu.cms.udel.edu/woce/dacs.html#satellite>
WOCE Data Information
Unit's page on satellite measurements
Sea level from island and coastal stations
Data information and availability:
Acoustic tomography
Acoustic tomography maps changes in ocean temperature using
changes in sound speed along paths between acoustic
sources and receivers. It is currently used in two somewhat
different modes - in concentrated regional experiments where
an attempt is made to reconstruct the full three-dimensional
temperature field, and over global paths to monitor changes
in the average temperature along very long paths. The first
has been and is being used to good effect in winter convection regions,
where in situ ship observations have been very difficult to
obtain due to the small size of convection features and the poor
weather in the interesting part of the year. The second
is being used for global climate monitoring.
Integrated observing systems
In the past few years, a large amount of information has become
more readily available online, and in near real-time. These
systems are primarily satellite based, but also include the
ambitious integrated observing system of the tropical Pacific
(TAO array - Tropical Ocean and Atmosphere).