193
The only candidate for a truly coupled decadal mode that we know of is
Latif and Barnett (1994). Fig. 11 shows the SST in the North Pacific for
this mode. In view of our previous discussion of atmospheric forcing of the
ocean, the question naturally arises whether or not this (model) mode is
in fact a mode or is it the result of forcing the ocean by natural variability
in the atmosphere. One way to decide is to examine the natural variability
of the atmosphere in the absence of interactive SST, i.e. in response to
climatological SST and seeing whether or not the coupling significantly
enhances the atmospheric variability. Another way would be to see if the
prediction skill exceeds the persistence skill. Since neither of these has yet
been done, we will simply remark the claim of a coupled mode and reserve
judgment.
In a coupled model, Delworth et al. (1994) analyzed a 600 year simulation with interdecadal irregular oscillations in meridional overturning.
One would have expected this to be a coupled mode yet the dynamics turns
out to be very much like ocean internal variability. The power spectrum
of SST shows a peak at a broad time scale around 50 years. A regression analysis reveals that the oscillation is driven by density anomalies in
the confined sinking region of the thermohaline circulation combined with
much smaller density anomalies of opposite sign in the more extensive rising regions. They proposed the following mechanism: beginning with a
weak THe, the reduced heat transport associated the this weak THe results in the development of a cold, dense pool through the top 1 km in
the middle of the North Atlantic; the cold, dense pool has an associated
cyclonic circulation; the anomalous flow across the mean salinity contours
associated with this gyre circulation results in an enhanced salt transport
into the sinking region, thereby increasing the salinity and density of the
sinking region; this processes is associated with a strengthening of the THe
until it reaches its maximum. The result of this strengthening of the THe
is that the northward advection of heat is enhanced, thereby generating a
pool of warm, less dense water throughout the top 1 km in the middle of
the North Atlantic; as this warm pool develops, an anomalous anticyclonic
gyre circulation (i.e. a weakened cyclonic circulation) is created, which
acts to decrease the salt transport into the sinking region and weakening
the THe.
The essence of this mechanism is the 10 year lead of saline to heat tran~ports and associated "gyre mode" circulation (deviation from the zonal
mean). It is not clear why there should be an anomalous anticyclonic gyre
The only candidate for a truly coupled decadal mode that we know of is
Latif and Barnett (1994). Fig. 11 shows the SST in the North Pacific for
this mode. In view of our previous discussion of atmospheric forcing of the
ocean, the question naturally arises whether or not this (model) mode is
in fact a mode or is it the result of forcing the ocean by natural variability
in the atmosphere. One way to decide is to examine the natural variability
of the atmosphere in the absence of interactive SST, i.e. in response to
climatological SST and seeing whether or not the coupling significantly
enhances the atmospheric variability. Another way would be to see if the
prediction skill exceeds the persistence skill. Since neither of these has yet
been done, we will simply remark the claim of a coupled mode and reserve
judgment.
In a coupled model, Delworth et al. (1994) analyzed a 600 year simulation with interdecadal irregular oscillations in meridional overturning.
One would have expected this to be a coupled mode yet the dynamics turns
out to be very much like ocean internal variability. The power spectrum
of SST shows a peak at a broad time scale around 50 years. A regression analysis reveals that the oscillation is driven by density anomalies in
the confined sinking region of the thermohaline circulation combined with
much smaller density anomalies of opposite sign in the more extensive rising regions. They proposed the following mechanism: beginning with a
weak THe, the reduced heat transport associated the this weak THe results in the development of a cold, dense pool through the top 1 km in
the middle of the North Atlantic; the cold, dense pool has an associated
cyclonic circulation; the anomalous flow across the mean salinity contours
associated with this gyre circulation results in an enhanced salt transport
into the sinking region, thereby increasing the salinity and density of the
sinking region; this processes is associated with a strengthening of the THe
until it reaches its maximum. The result of this strengthening of the THe
is that the northward advection of heat is enhanced, thereby generating a
pool of warm, less dense water throughout the top 1 km in the middle of
the North Atlantic; as this warm pool develops, an anomalous anticyclonic
gyre circulation (i.e. a weakened cyclonic circulation) is created, which
acts to decrease the salt transport into the sinking region and weakening
the THe.
The essence of this mechanism is the 10 year lead of saline to heat tran~ports and associated "gyre mode" circulation (deviation from the zonal
mean). It is not clear why there should be an anomalous anticyclonic gyre