394
The table is grouped into six categories of mechanisms that will be discussed below and ordered according to associated time scales. The latter
is not rigorous, of course, since periods depend on model parameters. Nevertheless, it contains information which processes in the climate system
might be important on a given time scale. This should contribute to the
identification of the most robust and fundamental modes of variability and
change and distinguish them from more model-specific fluctuations.
3.1 Gyre-Thermohaline Circulation
Weaver and Sarachik (1991a) report self-sustained oscillations in a hemispheric 3-D OGCM under mixed boundary conditions. Formed in the
western boundary current, warm and saline anomalies travel eastward and
are picked up by the sub-polar gyre which transports them into the region
of deep water formation; this journey takes about 8 to 9 years. There,
they influence the basin-scale overturning and so feedback to the surface
advection of these anomalies. The role of the latitudinal structure of the
freshwater forcing was also studied (Weaver et al. 1993). When precipitation in the high latitude is increased internal variability on the decadal
(as before) and the interdecadal (15-20yr) time scales is generated. For
sufficiently strong forcing sequences of violent overturnings and little deep
water formation could be excited. As these are connected to diffusive processes in the ocean interior during the time of reduced ventilation, time
scales are on the order of 500 years and are decreasing with increasing
amplitude of the stochasting forcing.
Similar oscillations were found by Winton (1993) using a frictional-geostrophic
model. Important in these models is the fact that anomalies are advected
by the gyre circulation near the surface which determines the decadal time
scale. These oscillations affect the entire water column in the high latitudes by turning on and off deep water formation, and the amplitude of
the changes of the meridional heat flux are of order 0.2 X 10 15 W. While
these models allow us to isolate and investigate various mechanisms of internal variability, immediate application to the real world is limited because
of the simple parameterization of ocean-atmosphere interaction, simplified
geometry, and their still fairly coarse resolution.
The table is grouped into six categories of mechanisms that will be discussed below and ordered according to associated time scales. The latter
is not rigorous, of course, since periods depend on model parameters. Nevertheless, it contains information which processes in the climate system
might be important on a given time scale. This should contribute to the
identification of the most robust and fundamental modes of variability and
change and distinguish them from more model-specific fluctuations.
3.1 Gyre-Thermohaline Circulation
Weaver and Sarachik (1991a) report self-sustained oscillations in a hemispheric 3-D OGCM under mixed boundary conditions. Formed in the
western boundary current, warm and saline anomalies travel eastward and
are picked up by the sub-polar gyre which transports them into the region
of deep water formation; this journey takes about 8 to 9 years. There,
they influence the basin-scale overturning and so feedback to the surface
advection of these anomalies. The role of the latitudinal structure of the
freshwater forcing was also studied (Weaver et al. 1993). When precipitation in the high latitude is increased internal variability on the decadal
(as before) and the interdecadal (15-20yr) time scales is generated. For
sufficiently strong forcing sequences of violent overturnings and little deep
water formation could be excited. As these are connected to diffusive processes in the ocean interior during the time of reduced ventilation, time
scales are on the order of 500 years and are decreasing with increasing
amplitude of the stochasting forcing.
Similar oscillations were found by Winton (1993) using a frictional-geostrophic
model. Important in these models is the fact that anomalies are advected
by the gyre circulation near the surface which determines the decadal time
scale. These oscillations affect the entire water column in the high latitudes by turning on and off deep water formation, and the amplitude of
the changes of the meridional heat flux are of order 0.2 X 10 15 W. While
these models allow us to isolate and investigate various mechanisms of internal variability, immediate application to the real world is limited because
of the simple parameterization of ocean-atmosphere interaction, simplified
geometry, and their still fairly coarse resolution.