3.4.1 Forcing the ocean
The ocean circulation is forced at the surface
through exchanges with the atmosphere of
momentum, heat and water. The wind exerts a
stress on the surface, which alone can produce
many features of ocean currents in observations
and in numerical model analyses. The buoyancy
due to cooling and heating at the surface, and salt
concentration or dilution due to evaporation or
precipitation, likewise can also generate much of
the observed circulation pattern. The real ocean
combines these varying forces in non-linear ways
with the tidally driven motions to produce a complex and varying circulation pattern. Understanding the fundamentals of these processes lies behind
the World Ocean Circulation Experiment (WOCE).
The variability and predictability of the consequences of varying forcing, natural or due to
anthropogenic effects and the resulting feedback
on the atmospheric circulation are central to the
Climate Variability and Predictability (CLIVAR)
programme.
The oceanic storage of radiatively active (greenhouse) gases depends on surface wind and upper
ocean temperature, so is intimately connected to
all the other air–sea interaction processes and their
consequences. Joanne Malkus once lamented that
the observations at large scales and the interactions on all scales were almost impossible ever to
achieve and take account of (Malkus, 1962).
However, that was before the advent of satellite
meteorology and oceanography. Today, adequate
observations at significant temporal and spatial
scales can be achieved from the vantage point
of space; however, strong support from in-situ
measurements is required, particularly from
sensors such as the Argo depth-profiling array
(Wilson, 2000). An illustration of the coverage of
the global ocean today by three operating surface
wind-sensing satellites is found in Fig. 3.4.1 (see
Plate 3.4.1, p. 172). Advances in satellite technologies, better algorithms, and detailed sampling in
time and space promises that sufficient measurements to understand climate variability and predictability can be obtained in the near future.
In this chapter, we discuss current methods to
evaluate the air–sea fluxes from space and point to
some promising trends.
The momentum flux is the result of wind shear.
Heat flux can be divided into four components:
1 sensible heat resulting from thermal gradient;
2 latent heat carried by evaporation;
3 short-wave radiation from the sun; and
4 long-wave radiation from the atmosphere and
the ocean.
Hydrologic forcing is the difference between
precipitation (rain) and evaporation. Momentum
flux, sensible heat flux and evaporation are turbulence transports (Liu, 1990). The turbulence fluxes
are not measured routinely over the global oceans.
They are parameterized, through bulk aerodynamic formulae, in terms of the mean measurements provided in ship reports or by space-based
sensors.
3.4.2 Bulk parameterization
The bulk formulae relate the fluxes of momentum
(surface stress ), sensible heat (SH), and latent
3.4
Air–Sea Fluxes from Satellite Data
W.Timothy Liu and Kristina B. Katsaros
173
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
The ocean circulation is forced at the surface
through exchanges with the atmosphere of
momentum, heat and water. The wind exerts a
stress on the surface, which alone can produce
many features of ocean currents in observations
and in numerical model analyses. The buoyancy
due to cooling and heating at the surface, and salt
concentration or dilution due to evaporation or
precipitation, likewise can also generate much of
the observed circulation pattern. The real ocean
combines these varying forces in non-linear ways
with the tidally driven motions to produce a complex and varying circulation pattern. Understanding the fundamentals of these processes lies behind
the World Ocean Circulation Experiment (WOCE).
The variability and predictability of the consequences of varying forcing, natural or due to
anthropogenic effects and the resulting feedback
on the atmospheric circulation are central to the
Climate Variability and Predictability (CLIVAR)
programme.
The oceanic storage of radiatively active (greenhouse) gases depends on surface wind and upper
ocean temperature, so is intimately connected to
all the other air–sea interaction processes and their
consequences. Joanne Malkus once lamented that
the observations at large scales and the interactions on all scales were almost impossible ever to
achieve and take account of (Malkus, 1962).
However, that was before the advent of satellite
meteorology and oceanography. Today, adequate
observations at significant temporal and spatial
scales can be achieved from the vantage point
of space; however, strong support from in-situ
measurements is required, particularly from
sensors such as the Argo depth-profiling array
(Wilson, 2000). An illustration of the coverage of
the global ocean today by three operating surface
wind-sensing satellites is found in Fig. 3.4.1 (see
Plate 3.4.1, p. 172). Advances in satellite technologies, better algorithms, and detailed sampling in
time and space promises that sufficient measurements to understand climate variability and predictability can be obtained in the near future.
In this chapter, we discuss current methods to
evaluate the air–sea fluxes from space and point to
some promising trends.
The momentum flux is the result of wind shear.
Heat flux can be divided into four components:
1 sensible heat resulting from thermal gradient;
2 latent heat carried by evaporation;
3 short-wave radiation from the sun; and
4 long-wave radiation from the atmosphere and
the ocean.
Hydrologic forcing is the difference between
precipitation (rain) and evaporation. Momentum
flux, sensible heat flux and evaporation are turbulence transports (Liu, 1990). The turbulence fluxes
are not measured routinely over the global oceans.
They are parameterized, through bulk aerodynamic formulae, in terms of the mean measurements provided in ship reports or by space-based
sensors.
3.4.2 Bulk parameterization
The bulk formulae relate the fluxes of momentum
(surface stress ), sensible heat (SH), and latent
3.4
Air–Sea Fluxes from Satellite Data
W.Timothy Liu and Kristina B. Katsaros
173
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER