Oceanographic Studies using Drogues – MARE 101L
Background Information
Water temperature
Water temperature is not only important to swimmers and fisherman, but also to industries and even fish and
algae. A lot of water is used for cooling purposes in power plants that generate electricity. They need cool
water to start with, and they generally release warmer water back to the environment. The temperature of the
released water can affect downstream habitats. Temperature also can affect the ability of water to hold
oxygen as well as the ability of organisms to resist certain pollutants.
Sea surface temperature (SST) is the water temperature close to the ocean's surface. The exact meaning of
surface varies according to the measurement method used, but it is between 1 millimetre (0.04 in) and 20
metres (70 ft) below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface
temperatures within a short distance of the shore. Localized areas of heavy snow can form in bands downwind
of warm water bodies within an otherwise cold air mass. Warm sea surface temperatures are known to be a
cause of tropical cyclogenesis over the Earth's oceans. Tropical cyclones can also cause a cool wake, due to
turbulent mixing of the upper 30 metres (100 ft) of the ocean. SST changes diurnally, like the air above it, but
to a lesser degree due to its higher specific heat. There is less SST variation on breezy days than on calm days.
In addition, ocean currents and the global thermohaline circulation affect average SST significantly throughout
most of the world's oceans. For SSTs near the fringe of a landmass, offshore winds cause upwelling, which can
cause significant cooling, but shallower waters over a continental shelf are often warmer. Onshore winds can
cause a considerable warm-up even in areas where upwelling is fairly constant, such as the northwest coast of
South America. Its values are important within numerical weather prediction as the SST influences the
atmosphere above, such as in the formation of sea breezes and sea fog. It is also used to calibrate
measurements from weather satellites.
Specific Conductance (Salinity)
Specific conductance is a measure of the ability of water to conduct an electrical current . It is highly
dependent on the amount of dissolved solids (such as salt) in the water. Pure water, such as distilled water,
will have a very low specific conductance, and sea water will have a high specific conductance. Rainwater often
dissolves airborne gasses and airborne dust while it is in the air, and thus often has a higher specific
conductance than distilled water. Specific conductance is an important water-quality measurement because it
gives a good idea of the amount of dissolved material in the water. Many marine organisms have specific
salinity requirements or tolerances so measuring salinity in the water column can help scientists to determine
what organisms may live there.
Turbidity
Turbidity is the amount of particulate matter that is suspended in water. Turbidity measures the scattering
effect that suspended solids have on light: the higher the intensity of scattered light, the higher the turbidity.
Material that causes water to be turbid include: clay, silt, finely divided organic and inorganic matter, soluble
colored organic compounds, plankton, microscopic organisms.
Turbidity is measured by shining a light through the water and is reported in nephelometric turbidity units
(NTU). During periods of low flow (base flow), many rivers are a clear green color, and turbidities are low,
usually less than 10 NTU. During a rainstorm, particles from the surrounding land are washed into the river
making the water a muddy brown color, indicating water that has higher turbidity values. Also, during high
flows, water velocities are faster and water volumes are higher, which can more easily stir up and suspend
material from the stream bed, causing higher turbidities.
Dissolved oxygen
You can't tell by looking at water that there is oxygen in it (unless you remember that chemical makeup of a
water molecule is hydrogen and oxygen). But, if you look at a closed bottle of a soft drink, you don't see the
carbon dioxide dissolved in that - until you shake it up and open the top. The oxygen dissolved in lakes, rivers,
and oceans is crucial for the organisms and creatures living in it. As the amount of dissolved oxygen drops
below normal levels in water bodies, the water quality is harmed and creatures begin to die off. Indeed, a
water body can "die", a process called eutrophication.
Although water molecules contain an oxygen atom, this oxygen is not what is needed by aquatic organisms
living in our natural waters. A small amount of oxygen, up to about ten molecules of oxygen per million of
water, is actually dissolved in water. This dissolved oxygen is breathed by fish and zooplankton and is needed
by them to survive.
Rapidly moving water, such as in a mountain stream or large river, tends to contain a lot of dissolved oxygen,
while stagnant water contains little. Bacteria in water can consume oxygen as organic matter decays. Thus,
excess organic material in our lakes and rivers can cause an oxygen-deficient situation to occur. Aquatic life can
have a hard time in stagnant water that has a lot of rotting, organic material in it, especially in summer, when
dissolved-oxygen levels are at a seasonal low.
Ocean Currents
Studies of ocean surface currents play a vital role in our present day understanding of weather and climate
through the dynamics of ocean-atmosphere interaction. Technology enables us to bridge vast distances, as a
result, the oceans are now accessible from every teacher’s classroom and internet user's computer. Real-life
challenges and the excitement of ocean exploration can be brought to students of all ages, in every corner of
the world, including land-locked locations.
Spurred by the need for better weather and climate forecasts, the meteorological and oceanographic
communities have expanded their monitoring of wind, temperature, currents, and density structure above and
within the ocean. These observations utilize more sophisticated techniques than visual observations from ships
of opportunity traversing limited regions of the ocean. Today, sensors on moored and drifting automated
buoys and orbiting satellites as well as ships at sea gather wind, temperature, current, and density data.
Real-time data from moored ocean buoys for improved detection, understanding and prediction of El Niño and
La Niña.
Moored buoys, deployed by various nations in their coastal waters, serve as instrumented platforms for
making automated weather and oceanographic observations. The National Data Buoy Center (NDBC), part of
the NOAA National Weather Service, operates approximately 70 moored buoys in the coastal and offshore
waters of the western Atlantic Ocean, Gulf of Mexico, and the Pacific Ocean from the Bering Sea to southern
California, around the Hawaiian Islands, and in the South Pacific, as well as the Great Lakes. Buoys are
equipped with accelerometers or inclinometers that measure the heave acceleration or the vertical
displacement provided to the buoy by waves passing during a specified time period. An onboard computer
uses statistical wave models to process these measurements and generate wind-sea and swell data that are
then transmitted to shore stations. These data include significant wave height, average wave period, and
dominant wave period during each 20-minute sampling interval. Selected buoys also measure directional wave
data, such as mean wave direction.
The modern drifter is a high-tech version of the "message in a bottle". It consists of a surface buoy and a
subsurface drogue (sea anchor), attached by a long, thin tether. The buoy measures temperature and other
properties, and has a transmitter to send the data to passing satellites. The drogue dominates the total area of
the instrument and is centered at a depth of 15 meters beneath the sea surface.
For many years, ocean currents have been estimated by how they carry objects. For example, sailors
measured the speed of their ship through the water using the ship log. They measured their absolute position
by celestial navigation (in the good old days, pre-GPS!). The difference between the absolute speed and the
speed through the water gave the speed of the currents. Very strong currents, such as the Gulf Stream of the
North American east coast, made a big difference in how long it takes to travel south versus north! Large-scale
currents were also inferred when an object dropped at one place eventually washed ashore on a distant
beach. Glass balls used by Japanese fishermen ended up on a beach in California, carried by the vast clockwiseswirling North Pacific gyre.
More recently, researchers began tracking objects while they were drifting. This tracking was first done visually
(from a coastline or anchored ship,) then using radio, and most recently using satellites. During the 1970s,
when satellite tracking became possible, many competing drifter designs were proposed, built and deployed in
various studies around the world.
In 1982 the World Climate Research Program (WCRP) recognized that a global array of drifting buoys
("drifters") would be invaluable for oceanographic and climate research, but there were large differences in
the costs and water-following properties of various designs. The WCRP declared that a standardized, low-cost,
lightweight, easily-deployed drifter should be developed.
This development took place under the Surface Velocity Program (SVP) of the Tropical Ocean Global
Atmosphere (TOGA) experiment and the World Ocean Circulation Experiment. Funding was provided by the US
Office of Naval Research, the National Oceanic and Atmospheric Administration (NOAA), and the National
Science Foundation. Competing designs were rigorously evaluated on a number of criteria including their
water-following characteristics, quantified by attaching vector-measuring current meters to the tops and
bottom of the drogues. As a result of these examinations, a uniform design for the modern SVP drifter was
proposed in 1992. The SVP drifter has a spherical surface buoy and a semirigid drogue that maintains its shape
in high-shear flows.
The modern data set of SVP drifters includes all drifters deployed during the 1979-1993 development period
that had a holey-sock drogue centered at 15 m depth. Spar-type drifters with holey-sock drogues were first
deployed by NOAA's Atlantic Oceanographic and Meteorological Laboratory in February 1979 as part of the
TOGA/Equatorial Pacific Ocean Circulation Experiment (EPOCS). Large-scale deployments of the first modern
SVP drifters took place in 1988 (WCRP, 1988) with the goal of mapping the tropical Pacific Ocean's surface
circulation. This effort was expanded to global scale as part of WOCE and the Atlantic Climate Change Program
(ACCP), in which the array of SVP drifters was extended to cover the Pacific and North Atlantic Oceans by 1992
and the Southern and Indian Oceans by 1994. The array spanned the tropical and South Atlantic Ocean by
2004.
A global array of 1250 drifting buoys was completed in September 2005 with the official launch of the 1250th
buoy during the Joint WMO-IOC Technical Commission for Oceanography and Marine Meteorology (JCOMM) II
conference in Halifax, Nova Scotia. The current status of the global drifting buoy array is shown above. The
drifting buoys represent one platform in a larger array of instrumentation monitoring the state of our oceans.
Moored buoys, tide gauge stations, profiling floats, and expendable bathythermographs provide detailed
information about the upper water column, air-sea fluxes, and surface and atmospheric conditions. The entire
array of sensors is part of a larger Global Earth Observation System of Systems, including satellites, where
global terrestrial, oceanic, and atmospheric data are collected and shared to help determine the state of our
earth and to improve climate and other predictive models.
The Atlantic Oceanographic and Meteorological Laboratory (AOML) in Miami, Florida, manages the
deployment of drifting buoys around the world, deploying some 300 new drifters annually. Using research
ships, Volunteer Observing Ships (VOS), and United States Navy aircraft, Global Lagrangian Drifters are placed
in areas of interest. Once considered operational, they are reported to AOML’s Data Assembly Center.
Incoming data from the drifters are then placed on the Global Telecommunications System for distribution to
meteorological services everywhere. The primary goal of the Global Drifter Program is to provide sea surface
temperatures (SST) and surface velocity measurements. These measurements are obtained and shared as part
of an international program designed to improve climate prediction. Climate prediction models require
accurate estimates of SST to initialize their ocean component and drifting buoys provide essential ground truth
SST data for this purpose. Models also require validation by comparison with independent data sets. Surface
velocity measurements are used for this validation.
MARE 101L - Water Quality Stations and Drogues Lab
The objectives of today’s lab will be to understand how to measure water quality and current patterns by
conducting a study in Hilo Bay. There are two parts to today’s lab: 1) Water quality stations and 2) Drogues
Water Quality Stations
We will measure standard water quality parameters using a YSI (water quality probe)(Figure 1) and Secci disc
(turbidity)(Figure 2) in the water column.
Figure 1. YSI Probe
Figure 2. Secci Disc
We will sample at up to 12 stations between the base of the Wailuku River and waters outside Hilo Bay (Figure
3).
Figure 3. Sampling stations in Hilo Bay.
Water movement analysis using Drogues
Two types of drogues will be used in the study of Hilo Bay: 1) Shallow water drogue, and 2) Subsurface drogue.
Shallow water drogues (Figure 4) are tethered closely to a flag and float support, so that surface water
movements, whether from wind driven or current driven, represent the surface movement patterns.
Subsurface drogues (Figure 5) are very similar to shallow water, except that the sea anchor portion of the
drogue is tethered several feet below the float, so that it represents subsurface currents.
Figure 4. Shallow water (surface) drogues
Figure 5. Subsurface drogue
In Hilo Bay drogues will be deployed at the same time from the same location (Figure 6). At approximately 1520 minute intervals, the drogues will be assessed for position, water quality, and time of assessment. In this
way movement patterns, direction, and velocity can be determined.
Figure 6. Drogue release and monitoring in Hilo Bay
Questions:
1)
2)
3)
4)
5)
6)
Did water quality change across the different stations in Hilo Bay or did it stay fairly constant?
Which measures were more variable and which were more consistent?
Hypothesize what factors may have been causing the patterns that you identified.
Did the shallow water and subsurface drogue follow the same movement patterns in Hilo Bay?
If there were differences, were they caused by differences in direction, velocity, or both?
Hypothesize what factors may have been causing the patterns that you identified.