Etudes et évaluation de processus océaniques par des hiérarchies ...

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Chapter 2
Observing the Ocean
tel-00545911, version 1 - 13 Dec 2010
A systematic determination of the bathimetry (depth structure) of the world ocean and its
observation started with the HMS Challenger expedition (1872–1876). Besides biological and
geological observations, the temperature was measured at different depths and locations of the
worlds ocean and water samples were taken which were then analysed to determine the salinity
and the composition of seawater. Current measurements in the open sea were more difficult to
perform from a ship subject to current and wind-forces and could only be estimated.
Today the ocean is observed from research vessels which take measurements along a well
defined trajectory and at given depth, from moorings which are attached to the ocean floor
and take observations at one location during a certain period of time, usually a few years, and
from floating buoys which are transported by the current but change their depth following a
predefined schedule and communicate the measurements by satellite. These devices measure
the velocity, temperature and chemical and biological composites of the oceanic water and
provide us with a spatio-temporal, that is, a four dimensional picture of the dynamics and the
composition of the worlds ocean. This picture is, however very patchy. At every instance in time
large areas of the ocean go unobserved. Starting from the 1980’s satellite observations measure
the height of the sea surface, the sea surface temperature, the sea surface salinity and the
ocean color of the sea surface at a spacial and temporal density and continuity unknown from
previous observations. Satellites can, however, only provide us with data from the sea surface
as electro-magnetic waves do not penetrate into the deep ocean. Much of today’s knowledge
of the worlds oceans is due to satellite observations and many efforts go to extrapolating this
surface measurements into the deep ocean.
2.1 Geometry of the Ocean
The world ocean has a surface of 361 × 10 6 km 2 , and an average depth of 3.8km. The average
depth is approximately the same in the Pacific, Atlantic and Indian Ocean.
Exercise 1: Search the Internet for maps of the bathimetry (depth) of the worlds ocean.
2.2 Variables measured
Using oceanic currents to accelerate and facilitate sea voyages is a concept as old as navigation
itself. On the open sea ocean currents were however hard to detect. Sea men using the gulf
stream to travel from the east coast of the US, realised quiet early that the gulf-stream water was
warmer than the surrounding waters and they used temperature measurements to determine
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