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to renew convection (it may be noted that two-dimensional models lack
these gyre mode circulations). The details of this process vary between
particular oscillations - in some oscillations convection renews abruptly
as the destabilizing effect of the subsurface warming is enhanced by the
nonlinearity of the equation of state (Winton, 1993); in other oscillations,
an intermediate water forming cell deepens and strengthens breaking down
the halo cline by advective processes (Winton and Sarachik, 1993). In the
subsequent deep- coupled phase, heat is flushed out of the basin and the
initial burst of poleward heat transport slowly weakens. Eventually the
fixed freshening at high latitudes dominates the reduced upward heat flux
to stabilize the water column and reform the halo cline. The warming of the
basin in the deep- decoupled phase is associated with a storage of potential
energy which is later converted to kinetic energy in the deep-coupled phase
(Winton, 1995a).
Although diffusive warming of the deep ocean and associated poleward
advective mixing of heat is the fundamental cause of the transition to the
coupled phase as shown by heat and salt budget calculations, there are
three important processes through which this transition can be triggered.
The first is the maintenance of shallow convection and intermediate water
formation while the deep ocean is warming during the decoupled phase; the
convection and overturning strengthen and deepen gradually, importing
more salt into the sinking regions and helping the onset of convection. The
second is related to the nonlinearity of sea water density with temperature.
As the deep ocean warms, the decrease in density grows nonlinearly with
temperature, which often leads to an abrupt destabilization of the polar
stratification. The third is associated with the convective eddies which
depart from a salty western boundary current and propagate eastward
through the polar halo cline, and deposit salt in the sinking region. After
a transition into the coupled phase, heat is flushed out of the basin over
high latitudes, and poleward heat transport slowly weakens after an initial
burst. Eventually the fixed surface freshening at high latitudes dominates
the reduced upward heat flux to stabilize the water column and reform the
halo cline (Winton and Sarachik, 1993; Winton, 1993).
Note that the transition between the coupled and decoupled phases happens on a decadal time scale. The transition corresponds to rapid changes
in convection at high latitudes associated with the formation and destruction of a halocline. Presumably the time scale of transition is the dynamic
adjustment time of the polar ocean to a sudden onset of convection, which
to renew convection (it may be noted that two-dimensional models lack
these gyre mode circulations). The details of this process vary between
particular oscillations - in some oscillations convection renews abruptly
as the destabilizing effect of the subsurface warming is enhanced by the
nonlinearity of the equation of state (Winton, 1993); in other oscillations,
an intermediate water forming cell deepens and strengthens breaking down
the halo cline by advective processes (Winton and Sarachik, 1993). In the
subsequent deep- coupled phase, heat is flushed out of the basin and the
initial burst of poleward heat transport slowly weakens. Eventually the
fixed freshening at high latitudes dominates the reduced upward heat flux
to stabilize the water column and reform the halo cline. The warming of the
basin in the deep- decoupled phase is associated with a storage of potential
energy which is later converted to kinetic energy in the deep-coupled phase
(Winton, 1995a).
Although diffusive warming of the deep ocean and associated poleward
advective mixing of heat is the fundamental cause of the transition to the
coupled phase as shown by heat and salt budget calculations, there are
three important processes through which this transition can be triggered.
The first is the maintenance of shallow convection and intermediate water
formation while the deep ocean is warming during the decoupled phase; the
convection and overturning strengthen and deepen gradually, importing
more salt into the sinking regions and helping the onset of convection. The
second is related to the nonlinearity of sea water density with temperature.
As the deep ocean warms, the decrease in density grows nonlinearly with
temperature, which often leads to an abrupt destabilization of the polar
stratification. The third is associated with the convective eddies which
depart from a salty western boundary current and propagate eastward
through the polar halo cline, and deposit salt in the sinking region. After
a transition into the coupled phase, heat is flushed out of the basin over
high latitudes, and poleward heat transport slowly weakens after an initial
burst. Eventually the fixed surface freshening at high latitudes dominates
the reduced upward heat flux to stabilize the water column and reform the
halo cline (Winton and Sarachik, 1993; Winton, 1993).
Note that the transition between the coupled and decoupled phases happens on a decadal time scale. The transition corresponds to rapid changes
in convection at high latitudes associated with the formation and destruction of a halocline. Presumably the time scale of transition is the dynamic
adjustment time of the polar ocean to a sudden onset of convection, which