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5 3D Level Modelling
5.7 The Thermohaline Circulation
5.7.1 The Abyssal Circulation
The abyssal circulation of the ocean is far less well known than the wind-driven
surface circulation. Knowledge about the deep circulation of the oceans has been
derived from spatial distributions of chemical tracers (Broecker and Peng, 1982)
and water mass properties; that is, temperature and salinity. Water-mass analysis
is a technique that allows to determine the relative distributions of distinct source
water masses along their path in the deep ocean. Tomczak and Godfrey (2003) give
a general overview of the method of water-mass analysis.
The abyssal circulation, also called thermohaline circulation, is driven by convective formation of dense water in particular regions of the world ocean. The
key players in this deep and bottom circulation are North Atlantic Deep Water
(abbreviated as NADW), formed in the Labrador and Greenland Seas in the northern
North Atlantic Ocean, and Antarctic Bottom Water, formed in the Weddell Sea and
the Ross Sea of Antarctica. Influenced by the Coriolis force, dense water leaving
these regions appears as bottom-arrested gravity plumes.
The lower limb or cold branch of the thermohaline circulation in the north
Atlantic Ocean is the so-called Deep Western Boundary Current (abbreviated as
DWBC). It moves dense water formed by open-ocean convection in the Labrador
and Greenland Seas southward as a narrow subsurface flow along the western continental slope. The Deep Western Boundary Current can cross the equator and some of
its water eventually merges with the Antarctic Circumpolar Current to flow around
Antarctica and into the Indian and Pacific Oceans as North Atlantic Deep Water
(NADW). The pathway of NADW is associated with a time scale of 1,500 years,
which can been derived from radiocarbon dating, a method first described by Arnold
and Libby (1949).
In order to conserve volume, southward spreading of the DWBC must be accompanied by a northward return flow. If the volume transport associated with the
DWBC increases during its travel owing to entrainment of ambient water, the volume transport of the return flow has to increase as well. Open scientific questions
are where this return flow occurs and what the forcing mechanisms of this return
flow are. These questions are difficult to answer given that deep-ocean currents are
generally very weak and difficult to measure and given that currents of the upper
1,000–2,000 m of the water column are predominantly driven by winds which overshadows other flow contributions.
5.7.2 The Stommel-Arons Model
The permanent thermocline is a zone of rapid decrease of temperature with depth
in the tropics and subtropics reaching from the base of the seasonal thermocline
5 3D Level Modelling
5.7 The Thermohaline Circulation
5.7.1 The Abyssal Circulation
The abyssal circulation of the ocean is far less well known than the wind-driven
surface circulation. Knowledge about the deep circulation of the oceans has been
derived from spatial distributions of chemical tracers (Broecker and Peng, 1982)
and water mass properties; that is, temperature and salinity. Water-mass analysis
is a technique that allows to determine the relative distributions of distinct source
water masses along their path in the deep ocean. Tomczak and Godfrey (2003) give
a general overview of the method of water-mass analysis.
The abyssal circulation, also called thermohaline circulation, is driven by convective formation of dense water in particular regions of the world ocean. The
key players in this deep and bottom circulation are North Atlantic Deep Water
(abbreviated as NADW), formed in the Labrador and Greenland Seas in the northern
North Atlantic Ocean, and Antarctic Bottom Water, formed in the Weddell Sea and
the Ross Sea of Antarctica. Influenced by the Coriolis force, dense water leaving
these regions appears as bottom-arrested gravity plumes.
The lower limb or cold branch of the thermohaline circulation in the north
Atlantic Ocean is the so-called Deep Western Boundary Current (abbreviated as
DWBC). It moves dense water formed by open-ocean convection in the Labrador
and Greenland Seas southward as a narrow subsurface flow along the western continental slope. The Deep Western Boundary Current can cross the equator and some of
its water eventually merges with the Antarctic Circumpolar Current to flow around
Antarctica and into the Indian and Pacific Oceans as North Atlantic Deep Water
(NADW). The pathway of NADW is associated with a time scale of 1,500 years,
which can been derived from radiocarbon dating, a method first described by Arnold
and Libby (1949).
In order to conserve volume, southward spreading of the DWBC must be accompanied by a northward return flow. If the volume transport associated with the
DWBC increases during its travel owing to entrainment of ambient water, the volume transport of the return flow has to increase as well. Open scientific questions
are where this return flow occurs and what the forcing mechanisms of this return
flow are. These questions are difficult to answer given that deep-ocean currents are
generally very weak and difficult to measure and given that currents of the upper
1,000–2,000 m of the water column are predominantly driven by winds which overshadows other flow contributions.
5.7.2 The Stommel-Arons Model
The permanent thermocline is a zone of rapid decrease of temperature with depth
in the tropics and subtropics reaching from the base of the seasonal thermocline