very important. A ‘telegraphic’ summary would be
as listed in Table 2.1.1.
I am acutely aware that the debate as to the
oceanographic future is ongoing, and that many
organizational developments are taking place.
Here I run the risk of preaching to the choir; perhaps by the time this book appears, the community will regard what I say here as obvious, and
will have moved to accommodate these views. I
hope that this will be true.
2.1.3 What do we know?
WOCE has completed its field phase, and the serious analysis of the data has only begun. It is
nonetheless possible to draw some immediate conclusions. Much of the data, but especially the
TOPEX/POSEIDON altimetric data, now 7; years
long (as of early 2000), shows the extremely active
time dependence of the oceanic circulation. (Wunsch
and Stammer (1998) review the altimetric results.)
Other widespread data sets, e.g. the neutrally buoyant floats (Davis, 1998a,b; Hogg and Owens,
1999), current meter records, surface drifters (e.g.
Niiler and Paduan, 1995), etc., all confirm the turbulent nature of the flow. This extreme variability
is confirmed by the high-resolution general circulation models that have been developed as part of
and alongside WOCE (Semtner and Chervin, 1992;
Stammer et al., 1996; Smith et al., 2000). In parallel with WOCE and its preparations, it was shown
that tracers such as tritium and chlorofluorocarbons penetrated to the abyssal seafloor from the
surface, on time scales of 10 years, rather than the
hundreds to thousands of years (e.g. Östlund and
Rooth, 1990) suggested by the ‘historical’ view.
The palaeoceanography core records show that
major climate shifts occurred in the ocean on time
scales of order a decade and even less (e.g. Boyle,
1990). Furthermore, even the coarse-resolution
models were suggesting that the ocean circulation
could undergo dramatic shifts on very short time
scales (e.g. Manabe and Stouffer, 1994; Marotzke
and Willebrand, 1991; Weaver et al., 1993), much
shorter than the historical view would lead one to
expect.
A major component of the climate role of the
ocean involves its transports in three dimensions
of the scalar fields of heat (temperature), fresh
water (salt), carbon, etc., whose large-scale structures have pervaded the discussion of the ocean
circulation for more than a century. It is too soon
in the WOCE analysis phase to quantify the degree
to which these property transports and their variability through space and time, seasonally to interannually to decadal and beyond, are the result of
(a) a simple time-dependent version of a laminar
conveyor belt; (b) the result of a complex integration
2.1 Global Problems and Global Observations
51
Wunsch
Table 2.1.1 Frameworks (paradigms) of the ocean circulation
Historical
WOCE
Ocean is nearly steady, fundamentally large-scale, and laminar
Ocean is unsteady on all space and time scales and
fundamentally turbulent
Modelling consequences
¥ Coarse resolution adequate
¥ Extremely high resolution probably required
¥ Quasi-steady adequate
(:1/12° in boundary currents)
¥ Integration over very long periods is accurate
¥ Highly non-steady (turbulent fluid flow)
¥ Systemic errors in physics may well accumulate over long
periods of time and swamp the results
Observation consequences
¥ Greatly simplified: keep coarse track of upper branch
¥ Very serious and demanding problem
of ‘global conveyor belt’
¥ Small spatial scales and long temporal scales must be
observed
¥ Not possible to rule out any oceanic depth or region
as irrelevant
Schematic of the consequences of the two different views of the ocean circulation – ranging from the ‘historical’ one of a basically steady,
laminar system, and the view that one is dealing with a constantly changing intrinsically turbulent fluid flow. If either view were completely
correct, the modelling and observational consequences would be quite extreme. In practice, both are partially valid, but the existence of any
turbulent component forces one towards the consequences of the fully turbulent system.
as listed in Table 2.1.1.
I am acutely aware that the debate as to the
oceanographic future is ongoing, and that many
organizational developments are taking place.
Here I run the risk of preaching to the choir; perhaps by the time this book appears, the community will regard what I say here as obvious, and
will have moved to accommodate these views. I
hope that this will be true.
2.1.3 What do we know?
WOCE has completed its field phase, and the serious analysis of the data has only begun. It is
nonetheless possible to draw some immediate conclusions. Much of the data, but especially the
TOPEX/POSEIDON altimetric data, now 7; years
long (as of early 2000), shows the extremely active
time dependence of the oceanic circulation. (Wunsch
and Stammer (1998) review the altimetric results.)
Other widespread data sets, e.g. the neutrally buoyant floats (Davis, 1998a,b; Hogg and Owens,
1999), current meter records, surface drifters (e.g.
Niiler and Paduan, 1995), etc., all confirm the turbulent nature of the flow. This extreme variability
is confirmed by the high-resolution general circulation models that have been developed as part of
and alongside WOCE (Semtner and Chervin, 1992;
Stammer et al., 1996; Smith et al., 2000). In parallel with WOCE and its preparations, it was shown
that tracers such as tritium and chlorofluorocarbons penetrated to the abyssal seafloor from the
surface, on time scales of 10 years, rather than the
hundreds to thousands of years (e.g. Östlund and
Rooth, 1990) suggested by the ‘historical’ view.
The palaeoceanography core records show that
major climate shifts occurred in the ocean on time
scales of order a decade and even less (e.g. Boyle,
1990). Furthermore, even the coarse-resolution
models were suggesting that the ocean circulation
could undergo dramatic shifts on very short time
scales (e.g. Manabe and Stouffer, 1994; Marotzke
and Willebrand, 1991; Weaver et al., 1993), much
shorter than the historical view would lead one to
expect.
A major component of the climate role of the
ocean involves its transports in three dimensions
of the scalar fields of heat (temperature), fresh
water (salt), carbon, etc., whose large-scale structures have pervaded the discussion of the ocean
circulation for more than a century. It is too soon
in the WOCE analysis phase to quantify the degree
to which these property transports and their variability through space and time, seasonally to interannually to decadal and beyond, are the result of
(a) a simple time-dependent version of a laminar
conveyor belt; (b) the result of a complex integration
2.1 Global Problems and Global Observations
51
Wunsch
Table 2.1.1 Frameworks (paradigms) of the ocean circulation
Historical
WOCE
Ocean is nearly steady, fundamentally large-scale, and laminar
Ocean is unsteady on all space and time scales and
fundamentally turbulent
Modelling consequences
¥ Coarse resolution adequate
¥ Extremely high resolution probably required
¥ Quasi-steady adequate
(:1/12° in boundary currents)
¥ Integration over very long periods is accurate
¥ Highly non-steady (turbulent fluid flow)
¥ Systemic errors in physics may well accumulate over long
periods of time and swamp the results
Observation consequences
¥ Greatly simplified: keep coarse track of upper branch
¥ Very serious and demanding problem
of ‘global conveyor belt’
¥ Small spatial scales and long temporal scales must be
observed
¥ Not possible to rule out any oceanic depth or region
as irrelevant
Schematic of the consequences of the two different views of the ocean circulation – ranging from the ‘historical’ one of a basically steady,
laminar system, and the view that one is dealing with a constantly changing intrinsically turbulent fluid flow. If either view were completely
correct, the modelling and observational consequences would be quite extreme. In practice, both are partially valid, but the existence of any
turbulent component forces one towards the consequences of the fully turbulent system.