are also poleward eastern boundary currents associated with the northern hemisphere subpolar
gyres. In the North Atlantic, a poleward undercurrent helps to transport Mediterranean water
northwards (Candela, Chapter 5.7). In the North
Pacific, there is a continuous subsurface current jet
carrying 0.810
6 m
3 s
91 poleward just offshore
of the shelf break between 33 and 51°N (Pierce
et al., 2000). Issues concerning eastern boundary
currents, and particularly those flowing poleward,
were summarized by Neshyba et al. (1989).
Many eastern boundary currents were instrumented during the WOCE period, thus enabling
their transports to be estimated. These observations include the Benguela Current (Garzoli et al.,
1996), the Leeuwin Current (Domingues et al.,
1999), the California Current (Strub et al., 1997)
and the poleward eastern boundary current in the
North Atlantic (Fiuza et al., 1998).
1.2.4.2 The thermohaline circulation
In addition to the wind-driven circulation, the
ocean exhibits a large meridional overturning circulation referred to as the thermohaline circulation. More than 60% of the surface ocean has
temperatures warmer than 20°C. Below the main
thermocline, the deep ocean is cold and weakly
stratified (Figs 1.2.4 and 1.2.5, see Plates 1.2.4 and
1.2.5, p. 44). More than 80% of the ocean volume
is colder than 4°C (calculation based on Levitus,
1982). In fact, 25% of the ocean volume has a
temperature between 0.5 and 1.7°C and a salinity
between 34.65 and 34.74 (Worthington, 1981).
The properties of these cold, deep watermasses
are set in the winter mixed layers in the high-latitude
North Atlantic and Southern Oceans. There, intense
ocean heat loss to the atmosphere combined with
salt rejection from the formation of sea ice, produces cold dense water that convects to form the
deep and bottom waters of the global ocean (Lazier
et al., Chapter 5.5). These water masses then spread
to fill the deep ocean. Volume conservation requires
surface waters to flow poleward into these regions
to replace this spreading deep water.
This coupling of the surface and deep ocean
within a global circulation system has been
expressed by the greatly simplified ‘icon’ of the
global conveyor belt (Broecker, 1991). While this
‘icon’ has captured the public’s and politicians’
attention, it greatly oversimplifies a complex but
vital component of the climate system.
Deep and intermediate water mass formation
Deep and intermediate water masses are formed
through convective processes both in the open
ocean and on continental shelves.
In the open ocean, deep convection occurs
within localized regions of less than 200 km diameter which have very special characteristics. Each
of these locations is within a cyclonic circulation
that reduces the stability of the water column; all
are subject to intense air–sea exchange; and contain warmer and saltier subsurface water masses.
These conditions are set by the large-scale gyre
circulation and by the previous winter’s cooling
cycle that will have already removed most of the
buoyancy from the water column as the water is
advected towards these special convection regions.
Open-ocean deep convection has been directly
observed in both the Mediterranean and Labrador
Seas; indirect evidence indicates that it also occurs
in the Greenland (Schott et al., 1993b; Rudels et al.,
1989; Watson et al., 1999) and Weddell Seas (e.g.
the Weddell Sea polynya; Gordon, 1982). Marshall
and Schott (1999) have published a comprehensive
review of open-ocean deep convection from a
North Atlantic perspective.
Deep-water formation also occurs through convection over the Antarctic and Arctic continental
shelves. Very intense air–sea–ice interaction in
coastal polynyas again leads to large oceanic heat
loss and to an increase of salinity through the formation of sea ice and the rejection of salt. Because
this takes place in shallow water, these exchanges
result in the formation of very dense waters that
flow along the sea bed to the edge of the shelf and
descend the continental slopes into the ocean
depths. The major site for formation of Antarctic
Bottom Water (AABW) is the Weddell Sea but
there is also significant AABW formation elsewhere around Antarctica. For example, recent
observations have revealed the presence of cold,
dense plumes (with high concentrations of chlorofluorocarbons, CFCs) cascading down the slope
(Rintoul et al., Chapter 4.6). This type of convection also occurs at low latitudes where evaporation in inshore bays produces dense saline brines
that drain to deeper water.
Circulation of water masses
From their high-latitude source regions, the dense
waters sink into the ocean interior and participate
in the global-scale overturning (the thermohaline)
1.2 Ocean Processes and Climate Phenomena
21
Clarke, Church and Gould
gyres. In the North Atlantic, a poleward undercurrent helps to transport Mediterranean water
northwards (Candela, Chapter 5.7). In the North
Pacific, there is a continuous subsurface current jet
carrying 0.810
6 m
3 s
91 poleward just offshore
of the shelf break between 33 and 51°N (Pierce
et al., 2000). Issues concerning eastern boundary
currents, and particularly those flowing poleward,
were summarized by Neshyba et al. (1989).
Many eastern boundary currents were instrumented during the WOCE period, thus enabling
their transports to be estimated. These observations include the Benguela Current (Garzoli et al.,
1996), the Leeuwin Current (Domingues et al.,
1999), the California Current (Strub et al., 1997)
and the poleward eastern boundary current in the
North Atlantic (Fiuza et al., 1998).
1.2.4.2 The thermohaline circulation
In addition to the wind-driven circulation, the
ocean exhibits a large meridional overturning circulation referred to as the thermohaline circulation. More than 60% of the surface ocean has
temperatures warmer than 20°C. Below the main
thermocline, the deep ocean is cold and weakly
stratified (Figs 1.2.4 and 1.2.5, see Plates 1.2.4 and
1.2.5, p. 44). More than 80% of the ocean volume
is colder than 4°C (calculation based on Levitus,
1982). In fact, 25% of the ocean volume has a
temperature between 0.5 and 1.7°C and a salinity
between 34.65 and 34.74 (Worthington, 1981).
The properties of these cold, deep watermasses
are set in the winter mixed layers in the high-latitude
North Atlantic and Southern Oceans. There, intense
ocean heat loss to the atmosphere combined with
salt rejection from the formation of sea ice, produces cold dense water that convects to form the
deep and bottom waters of the global ocean (Lazier
et al., Chapter 5.5). These water masses then spread
to fill the deep ocean. Volume conservation requires
surface waters to flow poleward into these regions
to replace this spreading deep water.
This coupling of the surface and deep ocean
within a global circulation system has been
expressed by the greatly simplified ‘icon’ of the
global conveyor belt (Broecker, 1991). While this
‘icon’ has captured the public’s and politicians’
attention, it greatly oversimplifies a complex but
vital component of the climate system.
Deep and intermediate water mass formation
Deep and intermediate water masses are formed
through convective processes both in the open
ocean and on continental shelves.
In the open ocean, deep convection occurs
within localized regions of less than 200 km diameter which have very special characteristics. Each
of these locations is within a cyclonic circulation
that reduces the stability of the water column; all
are subject to intense air–sea exchange; and contain warmer and saltier subsurface water masses.
These conditions are set by the large-scale gyre
circulation and by the previous winter’s cooling
cycle that will have already removed most of the
buoyancy from the water column as the water is
advected towards these special convection regions.
Open-ocean deep convection has been directly
observed in both the Mediterranean and Labrador
Seas; indirect evidence indicates that it also occurs
in the Greenland (Schott et al., 1993b; Rudels et al.,
1989; Watson et al., 1999) and Weddell Seas (e.g.
the Weddell Sea polynya; Gordon, 1982). Marshall
and Schott (1999) have published a comprehensive
review of open-ocean deep convection from a
North Atlantic perspective.
Deep-water formation also occurs through convection over the Antarctic and Arctic continental
shelves. Very intense air–sea–ice interaction in
coastal polynyas again leads to large oceanic heat
loss and to an increase of salinity through the formation of sea ice and the rejection of salt. Because
this takes place in shallow water, these exchanges
result in the formation of very dense waters that
flow along the sea bed to the edge of the shelf and
descend the continental slopes into the ocean
depths. The major site for formation of Antarctic
Bottom Water (AABW) is the Weddell Sea but
there is also significant AABW formation elsewhere around Antarctica. For example, recent
observations have revealed the presence of cold,
dense plumes (with high concentrations of chlorofluorocarbons, CFCs) cascading down the slope
(Rintoul et al., Chapter 4.6). This type of convection also occurs at low latitudes where evaporation in inshore bays produces dense saline brines
that drain to deeper water.
Circulation of water masses
From their high-latitude source regions, the dense
waters sink into the ocean interior and participate
in the global-scale overturning (the thermohaline)
1.2 Ocean Processes and Climate Phenomena
21
Clarke, Church and Gould