116
Chapter 8: Longer Term Responses: From Seasons to Centuries
Scales of External Forcing
We need not concern ourselves directly with events at the millennial scale, but it will be
useful to remember what has occurred in the not-so-distant past, and the consequences
of these events for what we observe at the present time. Two lessons stand out from
retrospective studies: the first is how fundamentally and rapidly the ocean circulation
may change, and the second is the extent to which it is in constant flux—so that during
no two consecutive years are ocean conditions identical.
Self-evidently, the hydrosphere and atmosphere together form a closely coupled system,
the thermodynamic state of each being determined by transfers of heat, moisture, and
momentum between these two “fluids,” which must remain in equilibrium. This balance
is, of course, modified by secondary effects induced by the cryosphere and the biosphere:
these four systems act out their play on the stage set by the arrangement and topography
of the land masses of the geosphere. Energy is transferred within the two “fluids” by
the planetary wind systems and by the wind-driven surface currents of the ocean, each
of which transfers sensible heat from low to high latitudes. Exchange of latent heat
between ocean and atmosphere occurs by means of rainfall and evaporation, balancing
the heat capacities of each. Vertical exchange of latent heat between lower atmosphere
and stratosphere occurs by convective processes, and between surface and deep oceans
by the density-driven flux of the global thermohaline circulation (THC).
We should briefly consider the impermanence of the THC. This major oceanic flux
was understood only much later than the wind-driven current system (Stommel and
Arons, 1960; Gordon, 1966). The THC is forced by the formation of an extremely dense
(cold, highly saline) surface water mass by evaporation during winter in small regions of
the Norwegian-Greenland and Labrador Seas, and also in the Weddell Sea. Very recently,
it has been shown that it is also formed in the Irminger Sea, beneath the atmospheric “tipjet” that forms intermittently in winter off southern Greenland (Dickson, 2003). These
dense surface waters sink by deep convective flow, in the Atlantic forming the North
Atlantic Deep Water, which can be traced south through the Indian and Pacific Oceans,
to be progressively and eventually returned surfaceward. Were the THC to be interrupted,
the distribution of heat within the ocean, and its exchange with the atmosphere, would
be strongly modified and there must be major consequences for global climate.
And, of course, it can be interrupted—as has happened frequently in the past—at the
origin of its descending branch. All that is required is an accelerated release of freshwater
from glaciers or from sea ice, sufficient to increase the stability of the surface layer
in small, critical areas of polar seas and so to cap the deep convective process. More
fundamentally, the same may be accomplished by an extension of sea-ice cover, which
may completely shut down the process of deep convection. There is evidence that each of
these processes occurred in the (climatically) recent past, during the Holocene glaciations.
But, closer to our times, it is postulated that the rapid cooling of the Younger Dryas
event (c.11 ky BP) was forced by a shutdown of the THC, which entered an alternative
steady state. Such a process would result in a complete rearrangement and relocation
of the oceanographic features that I have suggested may be useful to indicate the limits
of characteristic marine ecosystems in the present ocean. The THC appears to exist in
three modes (Clark et al., 2002). The Normal mode is what we see today, whereas during
the Glacial mode deep convection reaches only to <2500 m. During the Heinrich mode,
northern deep convection is absent and the Atlantic basin is filled, to within 1000 m
of the surface, with very cold water originating in the Antarctic deep convective zone.
Dansgaard-Oeschger events in the paleoceanographic record represent switches from one
state to another, occasioned by rapid (years to decades) warming, followed by a very slow
cooling trend. These events have a characteristic return period of 1500 ± 500 years. Note
Chapter 8: Longer Term Responses: From Seasons to Centuries
Scales of External Forcing
We need not concern ourselves directly with events at the millennial scale, but it will be
useful to remember what has occurred in the not-so-distant past, and the consequences
of these events for what we observe at the present time. Two lessons stand out from
retrospective studies: the first is how fundamentally and rapidly the ocean circulation
may change, and the second is the extent to which it is in constant flux—so that during
no two consecutive years are ocean conditions identical.
Self-evidently, the hydrosphere and atmosphere together form a closely coupled system,
the thermodynamic state of each being determined by transfers of heat, moisture, and
momentum between these two “fluids,” which must remain in equilibrium. This balance
is, of course, modified by secondary effects induced by the cryosphere and the biosphere:
these four systems act out their play on the stage set by the arrangement and topography
of the land masses of the geosphere. Energy is transferred within the two “fluids” by
the planetary wind systems and by the wind-driven surface currents of the ocean, each
of which transfers sensible heat from low to high latitudes. Exchange of latent heat
between ocean and atmosphere occurs by means of rainfall and evaporation, balancing
the heat capacities of each. Vertical exchange of latent heat between lower atmosphere
and stratosphere occurs by convective processes, and between surface and deep oceans
by the density-driven flux of the global thermohaline circulation (THC).
We should briefly consider the impermanence of the THC. This major oceanic flux
was understood only much later than the wind-driven current system (Stommel and
Arons, 1960; Gordon, 1966). The THC is forced by the formation of an extremely dense
(cold, highly saline) surface water mass by evaporation during winter in small regions of
the Norwegian-Greenland and Labrador Seas, and also in the Weddell Sea. Very recently,
it has been shown that it is also formed in the Irminger Sea, beneath the atmospheric “tipjet” that forms intermittently in winter off southern Greenland (Dickson, 2003). These
dense surface waters sink by deep convective flow, in the Atlantic forming the North
Atlantic Deep Water, which can be traced south through the Indian and Pacific Oceans,
to be progressively and eventually returned surfaceward. Were the THC to be interrupted,
the distribution of heat within the ocean, and its exchange with the atmosphere, would
be strongly modified and there must be major consequences for global climate.
And, of course, it can be interrupted—as has happened frequently in the past—at the
origin of its descending branch. All that is required is an accelerated release of freshwater
from glaciers or from sea ice, sufficient to increase the stability of the surface layer
in small, critical areas of polar seas and so to cap the deep convective process. More
fundamentally, the same may be accomplished by an extension of sea-ice cover, which
may completely shut down the process of deep convection. There is evidence that each of
these processes occurred in the (climatically) recent past, during the Holocene glaciations.
But, closer to our times, it is postulated that the rapid cooling of the Younger Dryas
event (c.11 ky BP) was forced by a shutdown of the THC, which entered an alternative
steady state. Such a process would result in a complete rearrangement and relocation
of the oceanographic features that I have suggested may be useful to indicate the limits
of characteristic marine ecosystems in the present ocean. The THC appears to exist in
three modes (Clark et al., 2002). The Normal mode is what we see today, whereas during
the Glacial mode deep convection reaches only to <2500 m. During the Heinrich mode,
northern deep convection is absent and the Atlantic basin is filled, to within 1000 m
of the surface, with very cold water originating in the Antarctic deep convective zone.
Dansgaard-Oeschger events in the paleoceanographic record represent switches from one
state to another, occasioned by rapid (years to decades) warming, followed by a very slow
cooling trend. These events have a characteristic return period of 1500 ± 500 years. Note