4.3
The Tropical Ocean Circulation
J. S. Godfrey, G. C. Johnson, M. J. McPhaden, G. Reverdin and Susan E.Wijffels
215
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
4.3.1 Flow and water mass
transformation patterns
Most of the heat absorption into the global
oceans, and much of the freshwater absorption,
occurs in the tropics. Furthermore, Sea Surface
Temperature (SST) is sufficiently high in the tropics that deep atmospheric convection can and does
occur over it. Movements of deep convection patterns, which affect climate globally, depend sensitively on small changes in SST. Thus it is critically
important for the improvement of global climate
models that their ocean components – when driven
by accurate surface fluxes – should accurately simulate water mass changes, circulation, and above
all SST, not only on long-term mean but on a variety of time scales. To achieve this one must do
more than simply locate where water mass conversion takes place; it is necessary also to identify and
parameterize the mechanisms by which it occurs.
These aims are particularly relevant in the tropics.
The top-to-bottom vertical sections obtained in
the World Ocean Circulation Experiment (WOCE)
provide a resource for learning how to make such
model improvements. At least 21 of the meridional
and 10 of the zonal WOCE one-time hydrographic
sections were occupied partly or fully within the
tropics throughout the Pacific, Atlantic and Indian
Oceans. These WOCE sections, together with
other data and models, are the subject of active
study by many scientists. Their research builds on
a long history of earlier work on the tropical
circulation. The primary aim of this chapter is to
review the present understanding of flow and
water mass transformation patterns within the
tropics of all three oceans.
Unfortunately, it is a very complex subject.
With regard to the ‘traditional’ use of hydrographic data for evaluating transports of heat and
other quantities, particular care has to be taken
in analysing cross-equatorial or near-equatorial
sections; geostrophy cannot be assumed for flow
through such sections, unless several repeats are
undertaken (e.g. Lukas and Firing, 1984). More
importantly, especially in the Pacific, near-equatorial
currents and temperatures have major inter-annual
variability associated with the El Niño phenomenon, so that (for example) the time-averaged product (vT) of temperature and meridional velocity
at a given location may have large eddy-flux contributions from variations on El Niño-Southern
Oscillation (ENSO) time scales. These must be
allowed for in order to achieve the WOCE goals of
quantifying the long-term mean heat and fresh
water transports in the global ocean. There are
also significant variations due to the seasonal
cycle, the Madden–Julian Oscillations of 40- to
60-day period, and Tropical Instability Waves of
20- to 30-day period. To elucidate these, it will be
necessary to supplement WOCE observations with
others that resolve these variations. The Tropical
Ocean and Global Atmosphere (TOGA) Observing System can resolve many of them in the upper
few hundred metres of the Pacific, from about
1985 onwards. It consists of the Tropical Atmosphere Ocean (TAO) mooring array, expendable
bathythermograph (XBT) sections including surface salinity data, surface drifters and tide gauge
data, supplemented with remotely sensed SST, wind
stresses and sea level. These data, and relevant
results from them, are reviewed by McPhaden et al.
The Tropical Ocean Circulation
J. S. Godfrey, G. C. Johnson, M. J. McPhaden, G. Reverdin and Susan E.Wijffels
215
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
4.3.1 Flow and water mass
transformation patterns
Most of the heat absorption into the global
oceans, and much of the freshwater absorption,
occurs in the tropics. Furthermore, Sea Surface
Temperature (SST) is sufficiently high in the tropics that deep atmospheric convection can and does
occur over it. Movements of deep convection patterns, which affect climate globally, depend sensitively on small changes in SST. Thus it is critically
important for the improvement of global climate
models that their ocean components – when driven
by accurate surface fluxes – should accurately simulate water mass changes, circulation, and above
all SST, not only on long-term mean but on a variety of time scales. To achieve this one must do
more than simply locate where water mass conversion takes place; it is necessary also to identify and
parameterize the mechanisms by which it occurs.
These aims are particularly relevant in the tropics.
The top-to-bottom vertical sections obtained in
the World Ocean Circulation Experiment (WOCE)
provide a resource for learning how to make such
model improvements. At least 21 of the meridional
and 10 of the zonal WOCE one-time hydrographic
sections were occupied partly or fully within the
tropics throughout the Pacific, Atlantic and Indian
Oceans. These WOCE sections, together with
other data and models, are the subject of active
study by many scientists. Their research builds on
a long history of earlier work on the tropical
circulation. The primary aim of this chapter is to
review the present understanding of flow and
water mass transformation patterns within the
tropics of all three oceans.
Unfortunately, it is a very complex subject.
With regard to the ‘traditional’ use of hydrographic data for evaluating transports of heat and
other quantities, particular care has to be taken
in analysing cross-equatorial or near-equatorial
sections; geostrophy cannot be assumed for flow
through such sections, unless several repeats are
undertaken (e.g. Lukas and Firing, 1984). More
importantly, especially in the Pacific, near-equatorial
currents and temperatures have major inter-annual
variability associated with the El Niño phenomenon, so that (for example) the time-averaged product (vT) of temperature and meridional velocity
at a given location may have large eddy-flux contributions from variations on El Niño-Southern
Oscillation (ENSO) time scales. These must be
allowed for in order to achieve the WOCE goals of
quantifying the long-term mean heat and fresh
water transports in the global ocean. There are
also significant variations due to the seasonal
cycle, the Madden–Julian Oscillations of 40- to
60-day period, and Tropical Instability Waves of
20- to 30-day period. To elucidate these, it will be
necessary to supplement WOCE observations with
others that resolve these variations. The Tropical
Ocean and Global Atmosphere (TOGA) Observing System can resolve many of them in the upper
few hundred metres of the Pacific, from about
1985 onwards. It consists of the Tropical Atmosphere Ocean (TAO) mooring array, expendable
bathythermograph (XBT) sections including surface salinity data, surface drifters and tide gauge
data, supplemented with remotely sensed SST, wind
stresses and sea level. These data, and relevant
results from them, are reviewed by McPhaden et al.