WHAT OCEAN MEASUREMENTS CAN BE MADE?
T
he immense size of Earth’s oceans present a great challenge to scientists making ocean
measurements. To develop a comprehensive theory of ocean structure and dynamics, a wide variety of measurements--including water temperature, salinity, wind
speed, and ocean current speed--must be made in many places and at many times. Earthorbiting satellites have made this task much easier and more effective than the relatively
slow and sporadic ship and aircraft techniques required before the advent of space flight.
However, satellites can make only surface measurements.
This theme (Oceans - Measurements) describes the kinds of quantitative measurements that
can be made at the ocean’s surface, within our ocean waters, and below the ocean floor.
Related Themes:
• Information on ocean features that change over time, as measured by ocean instruments can be found in Oceans - Process and Change.
• Why measuring ocean currents provides benefit to humankind is addressed in
Oceans - Human Interactions.
• How the interaction between the oceans and the atmosphere affects the dynamics of
both systems is examined in Oceans - Systems and Interactions.
• The effect of salinity, temperature, and density on the oceans’ vertical structure is
featured in Oceans - Scale and Structure.
• The importance of global satellite measurements for understanding climate is presented in the Climate - Measurements.
Related Activities:
• Fathometer in a Box
• Sea Surface Topography
• How Level is Sea Level?
INTRODUCTION
Quantitative measurements form the foundation of modern science. Measuring the oceans in
a manner that helps scientists develop useful theories has proven to be one of the greatest challenges of physical science — for example, monitoring the dynamics of the oceans to discover
how they interact with and affect land masses and the atmosphere.
Since the earliest days of ocean exploration, mariners have attempted to measure the direction and speed of ocean winds and currents for ship navigation purposes. Shortly after its inception in 1660, the Royal Society of London for Improving Natural Knowledge established a program to investigate the nature of sea water, the movement of ocean currents, and the manner in
which wind and sea affect each other.
One of the early mysteries investigated by the Royal Society concerned the strong surface
ocean current that was known to flow from the Atlantic ocean through the Strait of Gibraltar into
the Mediterranean Sea. What made the current such a mystery is the fact that so much water
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could flow ceaselessly into a land-locked sea without
raising its level.
In 1679, the mystery was solved by an Italian scientist named Luigi Ferdinando Marsigli by experimenting in the eastern part of the Mediterranean, in the
Bosporus strait near Istanbul, Turkey [Fig. 1]. Turkish
fishermen had noticed that their nets tended to drift
toward the Mediterranean when cast just below the
surface waters, but in the opposite direction when cast
deep below the surface. Marsigli strung a series of
white corks on a long rope and lowered the line into
the sea. He noticed that the deeper corks drifted in the
opposite direction of the corks near the top of the line.
He speculated that a current of dense, salty water
flowed at the lower depth from the Mediterranean into
the Black Sea. The surface currents consisting of less
salty (and therefore less dense) water flowed into the
Mediterranean.
Figure 1. View of Bosporus from space.
Marsigli began his experiment in the narrow channel between the eastern Mediterranean Sea and the Black Sea. The top of
this image is approximately north. The Sea
of Marmara (eastern Mediterranean) is
found in the south and the Black Sea is the
bright body of water in the north part of the
image. The city of Istanbul is located on both
the eastern and western sides of the channel, near the mouth of the Sea of Marmara.
Marsigli constructed a clever demonstration of how
differences in water density can lead to opposing currents at different depths. He built a transparent, watertight tank separated into two equal parts by a vertical
divider. The divider contained two holes, one near the
top, and one near the bottom, which could be sealed
with covers [Fig. 2]. Marsigli filled one half of the tank
with fresh water, and the other half with salt water,
which is more dense than fresh water. Upon removing the covers from the holes, he observed
that the heavier salt water flowed through the bottom hole toward the side of the tank containing
fresh water, forcing the fresh water to flow through the top hole in the opposite direction. Marsigli’s
experiment convinced the Royal Society that the same processes were occurring at Gibraltar.
Robert Plot confirmed that an undercurrent existed at the Straits of Gilbraltar in 1684.
A variety of important measurements can be made at the surface of the ocean. For hundreds
of years, sea-going vessels have kept records of various local conditions during their voyages
including: local current velocity which is a measure of the speed and direction of surface currents;
water temperature, which is a function of ocean depth as well as other factors; and wind velocity at
the sea surface.
Surface measurements, usually made from ships and buoys, are extremely important for ocean
science but are very limited in space and time. To understand the oceans as a global system,
numerous measurements made over time at a variety of locations must be correlated. Earthorbiting satellites can give scientists a synoptic view. From Earth’s orbit, the conditions over an
entire ocean can be seen simultaneously.
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Figure 2. Marsigli’s experiment. When the divider covers are removed, water can flow from the one side
of the tank to the other. Denser water will flow through the bottom hole, forcing less dense water to flow
through the upper hole in the opposite direction.
SHIPBOARD MEASUREMENTS
The earliest shipboard measurements were primarily concerned with charting the surface of the ocean.
Locations of land masses and long-term currents,
winds, and waves were considered the most important surface measurements. In 1768 the British Admiralty commissioned Captain James Cook to lead an
expedition to the then-largely unexplored regions of
the South Pacific. Cook spent nearly ten years exploring the Pacific, making detailed observations of winds,
currents, and water temperatures. He was the first
mariner to circumnavigate the globe at high southern
latitudes in 1772 - 1775.
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Figure 3. H.M.S. Challenger. The first largescale voyage with the express purpose of
studying the marine environment was on
board this ship.
In 1872 a research team led
by Sir Charles Wyville Thomson
began a forty-one month scientific mission aboard the H.M.S.
Challenger [Fig. 3]. A re-fitted
warship, sail-powered with an
auxiliary steam engine, her goal
was to extend human knowledge of the ocean from the surface to great depths. Challenger
was outfitted with laboratories
and workrooms equipped with
the finest scientific equipment
available, including microscopes, thermometers, hydrometers (devices for measuring the
density of sea water), analytical
chemistry sets, Bunsen burners,
and reels of rope up to eight
miles long for retrieving samples
from the deep ocean floor.
In addition to discovering
numerous new forms of sea life
(715 genera and 4417 species of
previously unknown organisms), Challenger charted some
of the world’s major surface currents. Among many important
findings, they discovered that
sea water is always colder at
greater depths (independent of
the season or surface water temperature), and that deep water Figure 4. Glomar Challenger deep-drilling rig and core sample.
becomes gradually warmer as This specially designed ship was able to drill the ocean floor while
one travels from the Antarctic floating on the surface three miles above. It was succeeded in 1985
northward. This was the first in- by the larger JOIDES Resolution.
dication that water from the Antarctic slowly flows north toward the tropics. Challenger’s scientific journals and logs were ultimately compiled into a comprehensive fifty volume treatise on Earth’s oceans.
Nearly one hundred years later, another global-ranging vessel set out to explore another dimension of the world’s oceans, what lies beneath the oceans’ floor. The Glomar Challenger, managed by the Scripps Oceanographic Institute, featured a deep-sea drilling rig for obtaining core
samples from beneath the ocean floor at depths up to six kilometers [Fig. 4]. One of the tasks of
the Glomar Challenger was to test the theory of plate tectonics, which asserts that the continents
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float on lithospheric plates that slowly move on Earth’s mantle, spreading away from mid-oceanic
ridges at a rate roughly equal to the rate at which human fingernails grow. By measuring the
physical and chemical properties of sediments and rocks in core samples taken near the midoceanic ridges by Glomar Challenger, scientists confirmed that the tectonic plates are indeed
moving.
SATELLITE MEASUREMENTS
Earth-orbiting satellites have revolutionized our ability to collect ocean data. Satellites can
provide frequent data over very large areas, greatly improving upon the infrequent, highly localized data collected by ships. However, not all important ocean data can be collected by satellites.
Science research ships continue to play an important role in understanding oceans because they
can observe what is happening below the sea surface. Moreover, ship and buoy measurements
are compared to satellite measurements to ensure that the satellite sensors are in proper calibration. Satellites orbit far above Earth, the data they collect tend to have relatively low resolution as
opposed to the pinpoint measurements made by ships.
Satellites measure various things about the oceans. For example, satellites measure sea surface temperature by monitoring thermal infrared wavelengths over the oceans. Other satellites
measure the amount of phytoplankton in our oceans, a significant link in the ocean food chain.
Some satellites measure winds using a scatterometer, which detects wind speed and direction
over the ocean by analyzing the backscatter from the small wind-caused ripples. Here we focus on
one satellite, TOPEX/Poseidon, which will illustrate the complexity of one such spaceborne mission. (See the Mission section on this CD for more detailed information.)
TOPEX/POSEIDON’S SCIENCE MEASUREMENTS
TOPEX/Poseidon is a joint U.S.-France program to observe Earth’s oceans from space.
Launched on August 10, 1992 aboard a French Ariane rocket, the TOPEX/Poseidon satellite [Fig.
5] bounces radar signals off the ocean’s surface to get precise measurements of the distance between the satellite and the sea surface. These data are combined with measurements from other
instruments that target the satellite’s exact location in space.
Sea level measurements from TOPEX/Poseidon are accurate to within 4.3 centimeters (1.7
inches). From its orbital vantage point 1,330 kilometers (about 830 miles) above Earth, the satellite can produce complete maps of ocean surface currents every ten days. TOPEX/Poseidon data
are also being used to help scientists better understand how Earth’s oceans interact with the
atmosphere, which will eventually improve our ability to predict long-term global climate.
The TOPEX/Poseidon satellite’s main scientific instrument is the radar altimeter. It emits microwave radar pulses that are reflected by the ocean surface and then return to the satellite.
Under ideal conditions (i.e., in a vacuum), the speed of each radar pulse is a known value, the
speed of light. By accurately measuring the amount of time it takes the pulse to travel from the
satellite to the ocean and back, the distance the pulse travels can be calculated. However, two
factors can slow each radar pulse as it travels along its path: charged particles in the ionosphere
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Figure 5. The TOPEX/Poseidon satellite. This model shows the TOPEX/Poseidon spacecraft and its
instruments. Data from all the instruments must be combined to generate the extremely accurate sea surface measurements needed to study currents and their effect on climate.
and water vapor in the lower regions of the atmosphere. Instruments aboard TOPEX/Poseidon
measure both of these factors so that scientists can properly adjust the data .
Knowing the satellite-to-ocean distance is only useful for determining sea surface height when
the position of the satellite is known accurately. Although they are not directly used for science
purposes, there are three other instruments onboard TOPEX/Poseidon that determine satellite
position [Fig. 5]:
• DORIS Antenna: A ground-based beacon sends radio signals to TOPEX/
Poseidon’s onboard receiver that measures the Doppler effect. The Doppler effect is
what causes the distinctive sound pattern you hear from a passing car. As the distance between your ear and the car changes so does the frequency of the sound
waves generated by the car.
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• Laser Retroreflectors: Light from ground-based lasers is reflected off mirrors on the
TOPEX/Poseidon satellite.
• Global Positioning System (GPS) Antenna: The GPS network, which is operated by
the United States Department of Defense, has 24 Earth-orbiting satellites. When
TOPEX/Poseidon’s onboard receiver tracks several GPS satellites simultaneously, its
position can be precisely calculated.
By measuring sea level, scientists can derive the speed of ocean currents. The speed of some
surface currents are directly related to the slope of the oceans’ surface. The shape of the sea
surface and associated currents change over time, therefore maps showing such variability are
very useful for studying seasonal and other changes. TOPEX/Poseidon can also determine the
height of waves and speed of ocean winds by analyzing the radar signal reflected from the sea
surface. Here are some of the data products derived from TOPEX/Poseidon satellite measurements:
• Ocean topography: The overall shape of the ocean surface after the effects of gravity and tides have been removed. Sea surface height that is caused by ocean circulation is shown. However, there are slight deviations from this long-term, basic pattern that are shown in maps of sea-surface variability.
• Sea-surface variability: The manner and rate at which surface topography changes
over time. Variability data help scientists easily see how short-term conditions vary
from long-term patterns. These deviations are mostly caused by short-term changes
in ocean heat storage and ocean winds.
• Significant wave height: The height of waves on the ocean is determined by how
the radar pulse is spread out when it is received at the satellite. A calm sea returns a
sharp pulse. A rough sea returns broad pulses. This is because high waves spread
out the pulses along their wave crests and troughs.
• Wind speed: Speed of surface winds can be determined from the strength of the
bounced radar signal when it returns to the satellite. A calm sea returns a strong
pulse. A wind-blown sea returns a weak pulse because much of the radar energy
gets scattered away from the satellite by the rough, choppy ocean surface. Not surprisingly, wind speed and significant wave height are correlated (compare with
significant wave height).
• Precipitable water vapor: This measurement is used to correct for delays in the path
of the radar pulse caused by water vapor in the lower atmosphere.
• Electron content: The data show the amount of free electrons in the ionosphere,
high above Earth’s atmosphere. These measurements are used to adjust TOPEX/
Poseidon’s altimeter data.
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CONCLUSION
The process of making careful measurements is fundamental to science. Ocean scientists on
ships can make such measurements in specific locations from the sea surface to the sea floor (and
even below the sea floor). Ocean measurements that have been made over hundreds of years
include temperature, salinity, density, current speed, and wind speed. Now, satellites can be
used to measure large areas of Earth’s surface at one time. These data allow scientists to calculate
derived quantities, such as the speed and direction of global currents. The ultimate goal of collecting these data is to better understand our global ocean and its influence on our climate and
lives.
VOCABULARY
backscatter
derive
lithospheric
phytoplankton
resolution
species
thermal infrared
calibration
genus
local current velocity
plate tectonics
scatter
synoptic view
variability
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circumnavigate
ionosphere
mantle
radar altimeter
scatterometer
temperature
wind velocity