3.1.1 The role of hydrographic
measurements
Such was the transformation in shipboard hydrographic work during WOCE, that it is difficult to
think back to the pre-WOCE era, and to recall
what then represented the state-of-the-art in
knowledge, expectation and procedures. Indeed, a
generation of oceanographers has been trained
whose only experience has been of the demanding
standards achieved by the WOCE programme.
There had been high-quality campaigns on a
smaller scale before, but these depended on the
expertise of a relatively small number of Principal
Investigators (PIs). Standards and protocols
adopted and promoted during WOCE have now
become the target for many groups outside the
WOCE community. The Scientific Plan for WOCE
(WCRP, 1986) is a valuable reminder of the background against which these standards were set,
and we make no apology for leaning heavily on
that document for this introduction. Volumes I
and II of the WOCE Implementation Plan (WCRP,
1988a,b) are a source of further details.
The earliest information about ocean currents
came from measurements of ship drift – the
difference between the average velocity of the
ship determined by celestial navigation and the
average velocity that would have been expected
(from dead-reckoning) in the absence of currents.
Compilation of ship-drift estimates provided
navigational charts of the world’s mean surface
currents, but no information about the flow
beneath the surface, and hence little idea of current transports and the general three-dimensional
circulation.
Direct measurements of subsurface currents
have always been few and far between, so ocean
circulation has been inferred primarily from measurements of water properties: temperature, salinity, and the concentrations of oxygen and nutrients,
and other tracers (Sverdrup et al., 1942). The classical mode of inference was based on the intuitive
notion that flow must be from the source of a
water property towards the sink, hence along
‘tongues’ of extreme values of the property. A
variation of this idea is that in the limit of weak
diffusion, flow should be along lines of constant
value of a conservative property such as salinity.
With improved understanding of ocean dynamics,
these qualitative modes of inference became
increasingly supplemented, and often supplanted,
by quantitative calculations of the vertical structure of horizontal currents. Except in the top
100 m and over full depth near the equator, the
slowly changing components of ocean circulation
are almost perfectly in geostrophic balance: that is,
horizontal components of the pressure gradient are
balanced by the Coriolis force, which is to the
right of the velocity in the northern hemisphere
and to the left in the southern. The horizontal
pressure gradient cannot be measured directly, but
its rate of change in the vertical is proportional to
the horizontal gradient of density, and this can be
estimated from a hydrographic section – a line of
vertical profiles of temperature and salinity.
Hydrographic sections, then, provide estimates
of the velocity component perpendicular to the section at any depth, relative to the unknown velocity
at a single reference depth. Most quantitative
estimates of ocean circulation come from these
3.1
Shipboard Observations during WOCE
B. A. King, E. Firing and T. M. Joyce
99
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
measurements
Such was the transformation in shipboard hydrographic work during WOCE, that it is difficult to
think back to the pre-WOCE era, and to recall
what then represented the state-of-the-art in
knowledge, expectation and procedures. Indeed, a
generation of oceanographers has been trained
whose only experience has been of the demanding
standards achieved by the WOCE programme.
There had been high-quality campaigns on a
smaller scale before, but these depended on the
expertise of a relatively small number of Principal
Investigators (PIs). Standards and protocols
adopted and promoted during WOCE have now
become the target for many groups outside the
WOCE community. The Scientific Plan for WOCE
(WCRP, 1986) is a valuable reminder of the background against which these standards were set,
and we make no apology for leaning heavily on
that document for this introduction. Volumes I
and II of the WOCE Implementation Plan (WCRP,
1988a,b) are a source of further details.
The earliest information about ocean currents
came from measurements of ship drift – the
difference between the average velocity of the
ship determined by celestial navigation and the
average velocity that would have been expected
(from dead-reckoning) in the absence of currents.
Compilation of ship-drift estimates provided
navigational charts of the world’s mean surface
currents, but no information about the flow
beneath the surface, and hence little idea of current transports and the general three-dimensional
circulation.
Direct measurements of subsurface currents
have always been few and far between, so ocean
circulation has been inferred primarily from measurements of water properties: temperature, salinity, and the concentrations of oxygen and nutrients,
and other tracers (Sverdrup et al., 1942). The classical mode of inference was based on the intuitive
notion that flow must be from the source of a
water property towards the sink, hence along
‘tongues’ of extreme values of the property. A
variation of this idea is that in the limit of weak
diffusion, flow should be along lines of constant
value of a conservative property such as salinity.
With improved understanding of ocean dynamics,
these qualitative modes of inference became
increasingly supplemented, and often supplanted,
by quantitative calculations of the vertical structure of horizontal currents. Except in the top
100 m and over full depth near the equator, the
slowly changing components of ocean circulation
are almost perfectly in geostrophic balance: that is,
horizontal components of the pressure gradient are
balanced by the Coriolis force, which is to the
right of the velocity in the northern hemisphere
and to the left in the southern. The horizontal
pressure gradient cannot be measured directly, but
its rate of change in the vertical is proportional to
the horizontal gradient of density, and this can be
estimated from a hydrographic section – a line of
vertical profiles of temperature and salinity.
Hydrographic sections, then, provide estimates
of the velocity component perpendicular to the section at any depth, relative to the unknown velocity
at a single reference depth. Most quantitative
estimates of ocean circulation come from these
3.1
Shipboard Observations during WOCE
B. A. King, E. Firing and T. M. Joyce
99
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER