3.2.1 Determining currents in the ocean
Because the ocean is practically opaque to all but
acoustic radiation, knowledge of its motion below
the surface is sporadic and gained at great effort.
Most of what we know about the ocean’s circulation is based not on actual velocity observations
but rather on hydrographic measurements. These
measurements of water properties are used in two
ways. Temperature and salinity are used to calculate density that, according to the geostrophic relation, determines the vertical variations (shear) of
horizontal flow. These properties and others are
used as tracers to infer absolute flow.
Ever since the basis for the ‘dynamic method’
was laid out by Sandström and Helland-Hansen
(1903), geostrophic shear has been used to interpret hydrographic sections and develop atlases
of the large-scale ocean circulation. Numerous
methods have been advanced for converting
geostrophic shears into absolute velocities including using other hydrographic tracers to infer levels
or surfaces of no motion (e.g. Defant, 1941). Two
of the advances that provided impetus for the
World Ocean Circulation Experiment (WOCE)
were the related ‘beta-spiral’ and ‘inverse’ methods
developed, respectively, by Stommel and Schott
(1977) and Wunsch (1977). In these methods
potential density surfaces are taken as material
surfaces, allowing conservation equations for potential vorticity, and perhaps other tracers, to provide
additional constraints on absolute velocity. To a
considerable extent, the WOCE Hydrographic
Programme was designed with the intent of using
these methods and other forms of data assimilation in which dynamical and/or tracer evolution
laws are used to establish the absolute velocity of
the general circulation.
WOCE also included observations to measure
directly the velocities of the general circulation.
Current meter arrays were particularly important
in determining the structure and transport of
concentrated currents along boundaries and in
passages. Such measurements at a point are called
Eulerian in recognition of Euler’s use, in his theory
of dynamics, of coordinates that are fixed in space.
Of concern here, WOCE also included an unprecedented international project to use neutrally buoyant floats to measure mid-depth absolute velocity,
in part to establish a level of known motion with
which to reference geostrophic shears. Measurements by current-followers are often called
Lagrangian in honour of the founder of dynamical
analysis in a coordinate system that moves with
the material. The purpose of this chapter is to provide the historical context for the WOCE float
programme, to discuss the methods and limitations of analysing Lagrangian measurements, and
to present some of the results gained.
3.2.2 Historical aspects: Stommel’s vision
to the WOCE Float Programme
Some 45 years ago Bowden (1954) initiated a
serious scientific dispute by his thorough review
of contemporary knowledge and methodology of
3.2
Subsurface Lagrangian Observations
during the 1990s
Russ E. Davis and Walter Zenk
123
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
Because the ocean is practically opaque to all but
acoustic radiation, knowledge of its motion below
the surface is sporadic and gained at great effort.
Most of what we know about the ocean’s circulation is based not on actual velocity observations
but rather on hydrographic measurements. These
measurements of water properties are used in two
ways. Temperature and salinity are used to calculate density that, according to the geostrophic relation, determines the vertical variations (shear) of
horizontal flow. These properties and others are
used as tracers to infer absolute flow.
Ever since the basis for the ‘dynamic method’
was laid out by Sandström and Helland-Hansen
(1903), geostrophic shear has been used to interpret hydrographic sections and develop atlases
of the large-scale ocean circulation. Numerous
methods have been advanced for converting
geostrophic shears into absolute velocities including using other hydrographic tracers to infer levels
or surfaces of no motion (e.g. Defant, 1941). Two
of the advances that provided impetus for the
World Ocean Circulation Experiment (WOCE)
were the related ‘beta-spiral’ and ‘inverse’ methods
developed, respectively, by Stommel and Schott
(1977) and Wunsch (1977). In these methods
potential density surfaces are taken as material
surfaces, allowing conservation equations for potential vorticity, and perhaps other tracers, to provide
additional constraints on absolute velocity. To a
considerable extent, the WOCE Hydrographic
Programme was designed with the intent of using
these methods and other forms of data assimilation in which dynamical and/or tracer evolution
laws are used to establish the absolute velocity of
the general circulation.
WOCE also included observations to measure
directly the velocities of the general circulation.
Current meter arrays were particularly important
in determining the structure and transport of
concentrated currents along boundaries and in
passages. Such measurements at a point are called
Eulerian in recognition of Euler’s use, in his theory
of dynamics, of coordinates that are fixed in space.
Of concern here, WOCE also included an unprecedented international project to use neutrally buoyant floats to measure mid-depth absolute velocity,
in part to establish a level of known motion with
which to reference geostrophic shears. Measurements by current-followers are often called
Lagrangian in honour of the founder of dynamical
analysis in a coordinate system that moves with
the material. The purpose of this chapter is to provide the historical context for the WOCE float
programme, to discuss the methods and limitations of analysing Lagrangian measurements, and
to present some of the results gained.
3.2.2 Historical aspects: Stommel’s vision
to the WOCE Float Programme
Some 45 years ago Bowden (1954) initiated a
serious scientific dispute by his thorough review
of contemporary knowledge and methodology of
3.2
Subsurface Lagrangian Observations
during the 1990s
Russ E. Davis and Walter Zenk
123
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER