398
A complementary study with a global model was done by Weisse et al.
(1993). A stochastic freshwater flux perturbation excites century (320
yr) and decadal scale oscillations. The latter are localized, again, in the
Labrador Sea area, have periods in the range of 10-40 years, but the
mechanism is distinctly different from that above. The relatively isolated
marginal sea integrates the stochastic freshwater flux perturbations and
sends salinity anomalies into the North Atlantic on a time scale of about
10-40 years. This time scale is determined by the flushing time of the
upper 250 m of the basin, since the stratification is quite stable. Once the
perturbations arrive in the North Atlantic, where the stratification between
50 0 N and 60
0
N is weak, they strongly influence the deep water formation
rates and create the variability observed.
These two mechanisms differ distinctly from each other in that in the first
study, the Labrador Sea itself generates the variability by changing rates
of local deep water formation, whereas in the other case the same region
merely appears as a storage of perturbations which, once accumulated, act
outside the basin. An increased resolution and, with it, a better representation of the water masses in the Labrador Sea will refine our understanding
of its role in controlling the natural variability in the North Atlantic region.
3.4 Basin-Scale Thermohaline Circulation
Basin-scale variations of the Atlantic meridional overturning and hence
meridional heat flux are possible mechanisms for century time scale variability. The Hamburg global OGCM was run under mixed boundary conditions including a stochastic freshwater flux perturbation (Mikolajewicz
and Maier-Reimer 1990). Large fluctuations with a time scale of 320 years
are found in the mass transport through Drake Passage and the meridional heat flux and overturning in the North Atlantic. Amplitudes would
be large enough to be detected in paleoclimatic archives (the heat budget
in the Southern Ocean shows peak-to-peak amplitudes of up to 3 X 10 15 W);
events are not regular cycles but appear in the time series as distinct events.
The mechanism is associated with the long residence time of SSS anomalies.
The random freshwater flux perturbations create local salinity anomalies
that are advected northward by the near-surface circulation in the Atlantic.
Depending on the spatial structure of the mean freshwater fluxes they are
enhanced or removed before they reach the deep water formation area in
the high-latitude Atlantic. If they are enhanced, they tend to accelerate
A complementary study with a global model was done by Weisse et al.
(1993). A stochastic freshwater flux perturbation excites century (320
yr) and decadal scale oscillations. The latter are localized, again, in the
Labrador Sea area, have periods in the range of 10-40 years, but the
mechanism is distinctly different from that above. The relatively isolated
marginal sea integrates the stochastic freshwater flux perturbations and
sends salinity anomalies into the North Atlantic on a time scale of about
10-40 years. This time scale is determined by the flushing time of the
upper 250 m of the basin, since the stratification is quite stable. Once the
perturbations arrive in the North Atlantic, where the stratification between
50 0 N and 60
0
N is weak, they strongly influence the deep water formation
rates and create the variability observed.
These two mechanisms differ distinctly from each other in that in the first
study, the Labrador Sea itself generates the variability by changing rates
of local deep water formation, whereas in the other case the same region
merely appears as a storage of perturbations which, once accumulated, act
outside the basin. An increased resolution and, with it, a better representation of the water masses in the Labrador Sea will refine our understanding
of its role in controlling the natural variability in the North Atlantic region.
3.4 Basin-Scale Thermohaline Circulation
Basin-scale variations of the Atlantic meridional overturning and hence
meridional heat flux are possible mechanisms for century time scale variability. The Hamburg global OGCM was run under mixed boundary conditions including a stochastic freshwater flux perturbation (Mikolajewicz
and Maier-Reimer 1990). Large fluctuations with a time scale of 320 years
are found in the mass transport through Drake Passage and the meridional heat flux and overturning in the North Atlantic. Amplitudes would
be large enough to be detected in paleoclimatic archives (the heat budget
in the Southern Ocean shows peak-to-peak amplitudes of up to 3 X 10 15 W);
events are not regular cycles but appear in the time series as distinct events.
The mechanism is associated with the long residence time of SSS anomalies.
The random freshwater flux perturbations create local salinity anomalies
that are advected northward by the near-surface circulation in the Atlantic.
Depending on the spatial structure of the mean freshwater fluxes they are
enhanced or removed before they reach the deep water formation area in
the high-latitude Atlantic. If they are enhanced, they tend to accelerate