it can switch between two equilibrium conditions:
one with the northern-driven overturning, the
other with southern-driven overturning (see
Rahmstorf, 1995, 1996). With the Agulhas leakage reduced to near zero, the Bering Strait takes on
greater significance, with relatively small variations in Bering Strait inflow (or other freshwater
sources in the north) initiating switches from one
state to the other, perhaps triggering large climate
fluctuations.
It is clear that distribution of heat and fresh
water associated with NADW formation has a
major impact on North Atlantic (and points
beyond?) climate. The process is very robust, as
even massive introductions of fresh water do not
permanently shut down the North Atlantic thermohaline overturning circulation: it’s a salty ocean
and it wants to stay that way. Why? Is it locally
maintained (certainly there is a lot of positive feedback), or might there be a global factor such as the
pattern of interocean exchange (export of fresh
water from the Atlantic by the ACC and by introduction of Indian Ocean salt through Agulhas
leakage)? NADW spreads into the ACC, entering
the southern ocean thermohaline overturning cell,
eventually contributing to AAIW, spreading into
each ocean. Most of the AAIW upwells into the
thermocline of the major oceans. Its NADWderived component must weave its way back to
the North Atlantic. Much of AAIW directly enters
the South Atlantic upwells, mostly in the tropics
(You, 1999), to enter the North Atlantic thermocline. Some AAIW no doubt upwells in the Pacific
and Indian Oceans, to be converted to thermocline
water, which then passes, with increasing salinity,
through the Indonesian Seas and around the rim of
southern Africa. Strong transients overshadowed
by other elements of the regional circulation complicate the pathway, but on the decadal and longer
scales, its impact on climate is felt.
Climate models, in their quest to simulate the
variability, must not only get mass and thermohaline fluxes correct within the major ocean basins,
but they must also simulate the fluxes in the constricted pathways connecting these oceans (see, for
example, Ribbe and Tomczak, 1997a). This is a
very challenging task because of the complex
boundary conditions within the passages, temporal
variability and lack the observational base to
enable evaluation of the model results.
Acknowledgements
The most relevant grants covering the task of preparing this chapter are: NSF grants OCE 97-29322
and OCE 95-29648, Office of Naval Research,
N00014-98-1-0270 and NASA grant NAG5-8297.
I am grateful to Will deRuijter for pointing out his
recent research with Weijer and others, concerning
the large-scale influence of Agulhas leakage on
Atlantic overturning. Comments of the editors of
this volume and of two reviewers have also greatly
improved this presentation. Lamont-Doherty Earth
Observatory contribution number 6053.
SECTION 4 THE GLOBAL FLOW FIELD
314
one with the northern-driven overturning, the
other with southern-driven overturning (see
Rahmstorf, 1995, 1996). With the Agulhas leakage reduced to near zero, the Bering Strait takes on
greater significance, with relatively small variations in Bering Strait inflow (or other freshwater
sources in the north) initiating switches from one
state to the other, perhaps triggering large climate
fluctuations.
It is clear that distribution of heat and fresh
water associated with NADW formation has a
major impact on North Atlantic (and points
beyond?) climate. The process is very robust, as
even massive introductions of fresh water do not
permanently shut down the North Atlantic thermohaline overturning circulation: it’s a salty ocean
and it wants to stay that way. Why? Is it locally
maintained (certainly there is a lot of positive feedback), or might there be a global factor such as the
pattern of interocean exchange (export of fresh
water from the Atlantic by the ACC and by introduction of Indian Ocean salt through Agulhas
leakage)? NADW spreads into the ACC, entering
the southern ocean thermohaline overturning cell,
eventually contributing to AAIW, spreading into
each ocean. Most of the AAIW upwells into the
thermocline of the major oceans. Its NADWderived component must weave its way back to
the North Atlantic. Much of AAIW directly enters
the South Atlantic upwells, mostly in the tropics
(You, 1999), to enter the North Atlantic thermocline. Some AAIW no doubt upwells in the Pacific
and Indian Oceans, to be converted to thermocline
water, which then passes, with increasing salinity,
through the Indonesian Seas and around the rim of
southern Africa. Strong transients overshadowed
by other elements of the regional circulation complicate the pathway, but on the decadal and longer
scales, its impact on climate is felt.
Climate models, in their quest to simulate the
variability, must not only get mass and thermohaline fluxes correct within the major ocean basins,
but they must also simulate the fluxes in the constricted pathways connecting these oceans (see, for
example, Ribbe and Tomczak, 1997a). This is a
very challenging task because of the complex
boundary conditions within the passages, temporal
variability and lack the observational base to
enable evaluation of the model results.
Acknowledgements
The most relevant grants covering the task of preparing this chapter are: NSF grants OCE 97-29322
and OCE 95-29648, Office of Naval Research,
N00014-98-1-0270 and NASA grant NAG5-8297.
I am grateful to Will deRuijter for pointing out his
recent research with Weijer and others, concerning
the large-scale influence of Agulhas leakage on
Atlantic overturning. Comments of the editors of
this volume and of two reviewers have also greatly
improved this presentation. Lamont-Doherty Earth
Observatory contribution number 6053.
SECTION 4 THE GLOBAL FLOW FIELD
314