6.2.1 The importance of freshwater
transport
The majority of water cycling through the atmosphere derives from the ocean, causing either distillation or freshening of ocean surface waters. The
steady-state portion of the salinity distribution in
the ocean reflects a balance between the exchange
of fresh water at the surface and compensating
mixing and advective ocean processes. Hence, in
the long-term, the transport and circulation of
fresh water (and salinity) by the ocean mirrors the
transport of latent heat by the atmosphere, which
comprises as much as 1.5 PW of the total of 4 PW
of poleward atmospheric energy transport (Rosen,
1999). Oceanic freshwater transport is therefore a
fundamental parameter in the planetary energy
budget (see Grassl, Chapter 1.1 and Clarke et al.,
Chapter 1.2).
As both atmospheric, ocean and coupled climate
models improve, the need for accurate estimates of
the components of the planetary energy budget
becomes increasingly acute. Without good estimates of the partition of energy transport between
the ocean and the atmosphere, and between diabatic and adiabatic processes, diagnosing errors in
the models or assessing the effects of new physical
parameterizations is made more difficult. For oceanonly models in particular, the lack of accurate estimates of surface freshwater forcing has fuelled the
use of unphysical model boundary conditions, the
effect of which will be discussed below.
Freshwater forcing imposes a vertical velocity
at the ocean’s surface, resulting in a Sverdruplike dynamical response that can be significant
(Huang and Schmitt, 1993). In addition, the ocean
thermohaline circulation, as modelled in coarseresolution climate models, is also sensitive to the
strength of freshwater forcing, especially at high
latitudes (Weaver et al., 1993; Rahmstorf, 1996).
The stability of the thermohaline circulation, in
particular, is sensitive to both the freshwater forcing and the strength of ocean vertical mixing
(Zhang et al., 1999b).
Historically, as has been the case for ocean heat
transport (see Bryden and Imawaki, Chapter 6.1),
most estimates of oceanic freshwater transport
derive indirectly from measurements of atmospheric
vapour transport (for a recent example, see Trenberth and Guillemot, 1998) or from surface observations (ship and island) of rainfall rates and the
variables required to estimate evaporation (e.g. da
Silva et al., 1994). Since the 1980s, estimates based
on in-situ ocean data started to appear. These direct
estimates of oceanic fresh-water transport provide
both an independent check of the atmospherically
based numbers as well as insight into the oceans’
response to the surface forcing.
6.2.2 Indirect estimates of oceanic
freshwater transport
In recent years, many new estimates of both
atmospheric and surface fluxes of fresh water have
appeared, based on the development of dataassimilating atmospheric general circulation models and the availability of new satellite data sets
that measure variables used to deduce evaporation, the radiometric signature of precipitation or
6.2
Ocean Transport of Fresh Water
Susan E. Wijffels
475
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
transport
The majority of water cycling through the atmosphere derives from the ocean, causing either distillation or freshening of ocean surface waters. The
steady-state portion of the salinity distribution in
the ocean reflects a balance between the exchange
of fresh water at the surface and compensating
mixing and advective ocean processes. Hence, in
the long-term, the transport and circulation of
fresh water (and salinity) by the ocean mirrors the
transport of latent heat by the atmosphere, which
comprises as much as 1.5 PW of the total of 4 PW
of poleward atmospheric energy transport (Rosen,
1999). Oceanic freshwater transport is therefore a
fundamental parameter in the planetary energy
budget (see Grassl, Chapter 1.1 and Clarke et al.,
Chapter 1.2).
As both atmospheric, ocean and coupled climate
models improve, the need for accurate estimates of
the components of the planetary energy budget
becomes increasingly acute. Without good estimates of the partition of energy transport between
the ocean and the atmosphere, and between diabatic and adiabatic processes, diagnosing errors in
the models or assessing the effects of new physical
parameterizations is made more difficult. For oceanonly models in particular, the lack of accurate estimates of surface freshwater forcing has fuelled the
use of unphysical model boundary conditions, the
effect of which will be discussed below.
Freshwater forcing imposes a vertical velocity
at the ocean’s surface, resulting in a Sverdruplike dynamical response that can be significant
(Huang and Schmitt, 1993). In addition, the ocean
thermohaline circulation, as modelled in coarseresolution climate models, is also sensitive to the
strength of freshwater forcing, especially at high
latitudes (Weaver et al., 1993; Rahmstorf, 1996).
The stability of the thermohaline circulation, in
particular, is sensitive to both the freshwater forcing and the strength of ocean vertical mixing
(Zhang et al., 1999b).
Historically, as has been the case for ocean heat
transport (see Bryden and Imawaki, Chapter 6.1),
most estimates of oceanic freshwater transport
derive indirectly from measurements of atmospheric
vapour transport (for a recent example, see Trenberth and Guillemot, 1998) or from surface observations (ship and island) of rainfall rates and the
variables required to estimate evaporation (e.g. da
Silva et al., 1994). Since the 1980s, estimates based
on in-situ ocean data started to appear. These direct
estimates of oceanic fresh-water transport provide
both an independent check of the atmospherically
based numbers as well as insight into the oceans’
response to the surface forcing.
6.2.2 Indirect estimates of oceanic
freshwater transport
In recent years, many new estimates of both
atmospheric and surface fluxes of fresh water have
appeared, based on the development of dataassimilating atmospheric general circulation models and the availability of new satellite data sets
that measure variables used to deduce evaporation, the radiometric signature of precipitation or
6.2
Ocean Transport of Fresh Water
Susan E. Wijffels
475
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER