a challenge to the ocean and climate modelling
communities. Water mass transformations are
dependent on correctly simulating buoyancy loss
through heat and freshwater exchanges over several seasonal cycles as they participate in the gyre
circulation. Flows over sills and the related
entrainment and mixing involve processes that
take place over horizontal scales of a few kilometres and vertical scale of order 100 metres. We are
only just beginning to understand the complex
spatial structure of deep-ocean mixing and its
implications for the deep circulation. Finally, it
must be remembered that the ocean actually produces its mean property transports as a summation
of significant variability in both the wind-driven
and thermohaline circulations on time scales of
days to decades and longer.
1.2.4.3 Other buoyancy-driven flows
The North Atlantic is the saltiest of the world’s
oceans and the North Pacific is the freshest. This
results in a change in sea surface elevation across
the Arctic Ocean from the Pacific to the Atlantic.
In addition to the low-salinity water that enters
the Arctic from the Pacific through Bering Strait
(Gordon, Chapter 4.7), the considerable input
from the Siberian rivers, as well as from the
MacKenzie River in North America, creates a
low-salinity Arctic surface layer. This water exits
the Arctic through Fram Strait and the various
channels of the Canadian Archipelago and flows
into the North Atlantic as the East Greenland
and Labrador Currents, respectively. Both these
currents are trapped along the shelf break
and are driven in large part by their excess buoyancy compared with the waters found further
offshore.
River runoff at mid to high latitudes produces
low-salinity waters on continental shelves, and this
drives equatorward flows over the continental
shelves on the western sides of ocean basins, and
poleward flows on the eastern side. Global ocean
models and coupled ocean–atmosphere models do
not presently have the vertical or horizontal resolution to resolve the shelf circulations and so their
effect is usually crudely included through the various
mechanism used to control the air–sea freshwater
exchanges. The simulation of these circulations is
important for the assessment of the potential
impacts of climate change on near-coastal marine
systems.
1.2.4.4 The role of sea ice in heat and
freshwater transports
A significant element of the transfer of fresh water
(and buoyancy) between the North Pacific and the
North Atlantic via the Arctic Ocean occurs in the
form of sea ice export out of Fram Strait. Sea ice
fields are heterogeneous with ice thickness varying
from zero in leads to 10 metres or more in ice
ridges. Martin and Wadhams (1999) used satellite
estimates of sea ice extent, concentration and drift
rates to estimate the ice export through Fram Strait
as 1530 km
3 yr
91 for 1994. They estimated the ice
thickness through empirical relations between ice
thickness, ice type and geographic location. Vinje
et al. (1998) estimated the ice flux through Fram
Strait from an upward-looking sonar over 6 years
from August 1990 to August 1996. Their annual
fluxes ranged from 2050 km
3 yr
91 in 1990–91 to a
maximum of 4700 km
3 yr
91 in 1994–95 with a
mean of 2850 km
3 yr
91
. This mean value is close to
the classical estimate of 2790 km
3 yr
91 based on ice
production in the Arctic Ocean (Aagaard and
Carmack, 1989). The export of low-salinity Arctic
Surface waters through Fram Strait into the East
Greenland Current and through the Arctic Archipelago into the Labrador Current are also significant contributors to this freshwater flux. The issues
involved and particularly the response of these
fluxes to the Arctic Oscillation are discussed by
Dickson et al. (Chapter 7.3).
1.2.5 Ocean transport of heat,
fresh water and carbon
The globe is heated by short-wave solar radiation
and cooled by the emission of long wave radiation.
However, the solar input exceeds the long-wave
radiation at low latitudes and vice versa at high
latitudes, requiring a global scale redistribution
of energy. The atmosphere and the ocean together
transport about 5 PW (1 PW is 110
15 W) of energy
poleward. Bryden and Imawaki (Chapter 6.1, Fig.
6.1.3) indicate that this is approximately equipartitioned between the oceanic transport, the dry static
atmospheric transport and the latent heat transport
of fresh water (involving both ocean and atmosphere). Thus, understanding and quantifying the
ocean heat and freshwater transport is critical to
building reliable models of the climate system.
We generally think of three principal mechanisms
by which heat and fresh water are transported by
1.2 Ocean Processes and Climate Phenomena
23
Clarke, Church and Gould
communities. Water mass transformations are
dependent on correctly simulating buoyancy loss
through heat and freshwater exchanges over several seasonal cycles as they participate in the gyre
circulation. Flows over sills and the related
entrainment and mixing involve processes that
take place over horizontal scales of a few kilometres and vertical scale of order 100 metres. We are
only just beginning to understand the complex
spatial structure of deep-ocean mixing and its
implications for the deep circulation. Finally, it
must be remembered that the ocean actually produces its mean property transports as a summation
of significant variability in both the wind-driven
and thermohaline circulations on time scales of
days to decades and longer.
1.2.4.3 Other buoyancy-driven flows
The North Atlantic is the saltiest of the world’s
oceans and the North Pacific is the freshest. This
results in a change in sea surface elevation across
the Arctic Ocean from the Pacific to the Atlantic.
In addition to the low-salinity water that enters
the Arctic from the Pacific through Bering Strait
(Gordon, Chapter 4.7), the considerable input
from the Siberian rivers, as well as from the
MacKenzie River in North America, creates a
low-salinity Arctic surface layer. This water exits
the Arctic through Fram Strait and the various
channels of the Canadian Archipelago and flows
into the North Atlantic as the East Greenland
and Labrador Currents, respectively. Both these
currents are trapped along the shelf break
and are driven in large part by their excess buoyancy compared with the waters found further
offshore.
River runoff at mid to high latitudes produces
low-salinity waters on continental shelves, and this
drives equatorward flows over the continental
shelves on the western sides of ocean basins, and
poleward flows on the eastern side. Global ocean
models and coupled ocean–atmosphere models do
not presently have the vertical or horizontal resolution to resolve the shelf circulations and so their
effect is usually crudely included through the various
mechanism used to control the air–sea freshwater
exchanges. The simulation of these circulations is
important for the assessment of the potential
impacts of climate change on near-coastal marine
systems.
1.2.4.4 The role of sea ice in heat and
freshwater transports
A significant element of the transfer of fresh water
(and buoyancy) between the North Pacific and the
North Atlantic via the Arctic Ocean occurs in the
form of sea ice export out of Fram Strait. Sea ice
fields are heterogeneous with ice thickness varying
from zero in leads to 10 metres or more in ice
ridges. Martin and Wadhams (1999) used satellite
estimates of sea ice extent, concentration and drift
rates to estimate the ice export through Fram Strait
as 1530 km
3 yr
91 for 1994. They estimated the ice
thickness through empirical relations between ice
thickness, ice type and geographic location. Vinje
et al. (1998) estimated the ice flux through Fram
Strait from an upward-looking sonar over 6 years
from August 1990 to August 1996. Their annual
fluxes ranged from 2050 km
3 yr
91 in 1990–91 to a
maximum of 4700 km
3 yr
91 in 1994–95 with a
mean of 2850 km
3 yr
91
. This mean value is close to
the classical estimate of 2790 km
3 yr
91 based on ice
production in the Arctic Ocean (Aagaard and
Carmack, 1989). The export of low-salinity Arctic
Surface waters through Fram Strait into the East
Greenland Current and through the Arctic Archipelago into the Labrador Current are also significant contributors to this freshwater flux. The issues
involved and particularly the response of these
fluxes to the Arctic Oscillation are discussed by
Dickson et al. (Chapter 7.3).
1.2.5 Ocean transport of heat,
fresh water and carbon
The globe is heated by short-wave solar radiation
and cooled by the emission of long wave radiation.
However, the solar input exceeds the long-wave
radiation at low latitudes and vice versa at high
latitudes, requiring a global scale redistribution
of energy. The atmosphere and the ocean together
transport about 5 PW (1 PW is 110
15 W) of energy
poleward. Bryden and Imawaki (Chapter 6.1, Fig.
6.1.3) indicate that this is approximately equipartitioned between the oceanic transport, the dry static
atmospheric transport and the latent heat transport
of fresh water (involving both ocean and atmosphere). Thus, understanding and quantifying the
ocean heat and freshwater transport is critical to
building reliable models of the climate system.
We generally think of three principal mechanisms
by which heat and fresh water are transported by
1.2 Ocean Processes and Climate Phenomena
23
Clarke, Church and Gould