2.2.1 The improving realism of ocean
models
The influence of numerical ocean modelling has
grown dramatically over the last few decades. The
use of ocean general circulation models during the
1970s and 1980s contributed to our fundamental
understanding of the physical mechanisms governing ocean dynamics by extending theory beyond
the limits of analytical methods. However, with
the exception of certain phenomena mainly associated with wind-driven variations at low latitudes,
the models were notoriously limited in their ability
to simulate the observed intricacies of the global
ocean circulation, rendering any quantitative confrontation with data sets beyond gross climatologies premature. In fact, the lingering disparity
between the early ocean models and the highprecision, deep ocean data sets meticulously gathered by individual oceanographic expeditions may
be regarded as a characteristic of pre-WOCE (World
Ocean Circulation Experiment) oceanography.
This distinguished oceanography from the situation in atmospheric research where models must
routinely stand the test of a wealth of synoptic
data and provide a primary means of obtaining
dynamical insight into physical processes.
Due to significant improvements in numerical
methods, advances in computer capabilities, and
better data sets for initialization and forcing,
ocean circulation models have become much more
realistic in recent years. Simulations with realistic
forcing, basin geometry, and resolutions fine
enough to retain the energetic flow features with
horizontal scales of the first mode Rossby radius
of deformation have been ground-breaking in the
sense that they provided, for the first time, solutions of planetary scale that permitted direct and
quantitative comparison with ocean observations.
Having fine resolution also allows the numerical
diffusion coefficients in a model to be chosen small
enough to avoid fictitious non-physical damping;
it also facilitates the correct transport of properties
such as heat and salt by adequately representing
the correlation of property maxima with the core
velocities of strong narrow currents. With models
improving in so many respects, an ensuing, more
effective interaction between the observational and
modelling communities during the WOCE decade
has ultimately led to a significant expansion in the
utilization of numerical models, not only for applications in physical oceanography, but increasingly
also in related fields such as biogeochemistry and
palaeoceanography.
As a consequence, ocean modelling activities
have acquired a role in WOCE that goes much
beyond the primary aim ‘to develop models useful
for predicting climate change and to collect the
data necessary to test them’. Irrespective of the
continuing need of critical testing against observed
ocean behaviours, there is a growing appreciation
of the critical importance of ocean models as a key
means for an understanding of a natural fluid system that, given its vast spectrum of variability, can
never be comprehensible on the basis of observations alone. In particular, the great challenge of
‘state estimation’ (Talley et al., Chapter 7.1), of
2.2
High-Resolution Modelling of the
Thermohaline and Wind-Driven Circulation
Claus W. Böning and Albert J. Semtner
59
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
models
The influence of numerical ocean modelling has
grown dramatically over the last few decades. The
use of ocean general circulation models during the
1970s and 1980s contributed to our fundamental
understanding of the physical mechanisms governing ocean dynamics by extending theory beyond
the limits of analytical methods. However, with
the exception of certain phenomena mainly associated with wind-driven variations at low latitudes,
the models were notoriously limited in their ability
to simulate the observed intricacies of the global
ocean circulation, rendering any quantitative confrontation with data sets beyond gross climatologies premature. In fact, the lingering disparity
between the early ocean models and the highprecision, deep ocean data sets meticulously gathered by individual oceanographic expeditions may
be regarded as a characteristic of pre-WOCE (World
Ocean Circulation Experiment) oceanography.
This distinguished oceanography from the situation in atmospheric research where models must
routinely stand the test of a wealth of synoptic
data and provide a primary means of obtaining
dynamical insight into physical processes.
Due to significant improvements in numerical
methods, advances in computer capabilities, and
better data sets for initialization and forcing,
ocean circulation models have become much more
realistic in recent years. Simulations with realistic
forcing, basin geometry, and resolutions fine
enough to retain the energetic flow features with
horizontal scales of the first mode Rossby radius
of deformation have been ground-breaking in the
sense that they provided, for the first time, solutions of planetary scale that permitted direct and
quantitative comparison with ocean observations.
Having fine resolution also allows the numerical
diffusion coefficients in a model to be chosen small
enough to avoid fictitious non-physical damping;
it also facilitates the correct transport of properties
such as heat and salt by adequately representing
the correlation of property maxima with the core
velocities of strong narrow currents. With models
improving in so many respects, an ensuing, more
effective interaction between the observational and
modelling communities during the WOCE decade
has ultimately led to a significant expansion in the
utilization of numerical models, not only for applications in physical oceanography, but increasingly
also in related fields such as biogeochemistry and
palaeoceanography.
As a consequence, ocean modelling activities
have acquired a role in WOCE that goes much
beyond the primary aim ‘to develop models useful
for predicting climate change and to collect the
data necessary to test them’. Irrespective of the
continuing need of critical testing against observed
ocean behaviours, there is a growing appreciation
of the critical importance of ocean models as a key
means for an understanding of a natural fluid system that, given its vast spectrum of variability, can
never be comprehensible on the basis of observations alone. In particular, the great challenge of
‘state estimation’ (Talley et al., Chapter 7.1), of
2.2
High-Resolution Modelling of the
Thermohaline and Wind-Driven Circulation
Claus W. Böning and Albert J. Semtner
59
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER