171
several hundred year period will be discussed in Sec. 5. First, the global
world ocean model was spun up, driven by Hellerman and Rosenstein wind
stress, and was restored to COADS air temperature and Levitus salinity
(the model included sea ice). Then the fresh water flux was diagnosed, and
a "white" noise component with standard deviation of 16 mm/month was
added.
Weisse et al. (1994) analyzed the same run using Principal Oscillation Patterns (POPs). They found that the dominant POP mode showed
decadal to interdecadal frequencies (10 to 40 year time scales) and is
centered near the Labrador Sea. Salt anomalies accumulate first in the
Labrador Sea, and then are advected into the northern North Atlantic
Ocean. The decadal time scale seems to be set by the flushing time of
the upper layers of the Labrador Sea. Once the salt anomalies leave the
enclosed Labrador Sea, their properties and lifetimes are governed by advection and convection in the open Atlantic. It is interesting that the
Labrador Sea seems to be chosen both by oceanic processes and, as we will
see in the next subsection, by atmospheric processes.
3.3 Atmospheric Variability
Because the atmosphere has a great deal of high frequency variability,
there is no question that it can force a slowly responding ocean (or land
or ice) and generate lower frequency variability. In analogy to the cointossing paradigm, it is clear that sudden events in the atmosphere (squalls,
weather, fronts, etc.) produces energy at all lower frequencies and will
therefore generate a white spectrum on periods longer than a few days. It
has been indicated that atmospheric motions can, through non-linear interactions, transfer energy directly to slower time scale (James and James,
1989) tending to redden the spectrum at the lower frequencies. As we have
seen, the Hasselmann mechanism also gives energy at low frequencies due
to the action of short impulsive weather events on the oceanic mixed layer
but, in this case, the spectrum would be white. In either case, the question
naturally arises as to whether the atmosphere can organize the variability
spatially and, in particular, whether the spatial distribution of these low
frequency variations, induced by topography and orography, can explain
ocean decadal variability. The Hasselmann theory does not explain spatial
patterns directly: something else must be going on.
Explicit calculations (G. Nitsche, personal communication) using the
several hundred year period will be discussed in Sec. 5. First, the global
world ocean model was spun up, driven by Hellerman and Rosenstein wind
stress, and was restored to COADS air temperature and Levitus salinity
(the model included sea ice). Then the fresh water flux was diagnosed, and
a "white" noise component with standard deviation of 16 mm/month was
added.
Weisse et al. (1994) analyzed the same run using Principal Oscillation Patterns (POPs). They found that the dominant POP mode showed
decadal to interdecadal frequencies (10 to 40 year time scales) and is
centered near the Labrador Sea. Salt anomalies accumulate first in the
Labrador Sea, and then are advected into the northern North Atlantic
Ocean. The decadal time scale seems to be set by the flushing time of
the upper layers of the Labrador Sea. Once the salt anomalies leave the
enclosed Labrador Sea, their properties and lifetimes are governed by advection and convection in the open Atlantic. It is interesting that the
Labrador Sea seems to be chosen both by oceanic processes and, as we will
see in the next subsection, by atmospheric processes.
3.3 Atmospheric Variability
Because the atmosphere has a great deal of high frequency variability,
there is no question that it can force a slowly responding ocean (or land
or ice) and generate lower frequency variability. In analogy to the cointossing paradigm, it is clear that sudden events in the atmosphere (squalls,
weather, fronts, etc.) produces energy at all lower frequencies and will
therefore generate a white spectrum on periods longer than a few days. It
has been indicated that atmospheric motions can, through non-linear interactions, transfer energy directly to slower time scale (James and James,
1989) tending to redden the spectrum at the lower frequencies. As we have
seen, the Hasselmann mechanism also gives energy at low frequencies due
to the action of short impulsive weather events on the oceanic mixed layer
but, in this case, the spectrum would be white. In either case, the question
naturally arises as to whether the atmosphere can organize the variability
spatially and, in particular, whether the spatial distribution of these low
frequency variations, induced by topography and orography, can explain
ocean decadal variability. The Hasselmann theory does not explain spatial
patterns directly: something else must be going on.
Explicit calculations (G. Nitsche, personal communication) using the