356
The coefficient of the first term on the right-hand side is negative and
hence stabilising for m ~ 1, and positive (destabilising) for m ~ 2. Either
way, the effect is largest for small A; weak restoring is stabilising if m ~ 1
and destabilising if m ~ 2. As a consequence, models with either fixed
surface freshwater flux (Zhang et al., 1993; Power and Kleeman, 1994;
Mikolajewicz and Maier-Reimer, 1994; Rahmstorf and Willebrand, 1995;
Pierce et al., 1996) or linear dependence of atmospheric moisture transport
on temperature gradient (MS, Lohmann et al., 1996b) are in danger of
falsely concluding that a weaker control of SST by the atmosphere means
a stabilization of the thermohaline circulation.
The two competing effects of temperature on salinity have been noted
previously by NSM and MS, without however giving a satisfactory explanation of when either effect dominates. Krasovskij and Stone (1995, pers.
comm.) have recently found a physically equivalent effect of nonlinearity
in the atmospheric heat and moisture transports, on whether a larger atmospheric equilibrium temperature gradient stabilises or destabilises the
high-latitude sinking.
Notice that all stabilising feedbacks turn destabilising and vice versa if
the low-latitude sinking equilibrium is considered (apart from the mean
flow feedback, #0, which always stabilises). Qualitatively, this can be
understood as follows. The high-latitude sinking state is dominated by
temperature effects, so ocean heat transport must be stabilising (#1) if
the steady state be stable. Temperature and salinity are antagonists, so
the effect of ocean salinity transport is destabilising (#2). Atmospheric
heat transport changes in response to a temperature change and must
counteract its cause, reducing the negative ocean heat transport feedback
( #3). Atmospheric moisture transport, finally, transports buoyancy in the
same direction as atmospheric heat transport, influences ocean density in
the same sense, and hence destabilises (#4). The low-latitude sinking state,
in contrast, is dominated by salinity, so the ocean salinity transport must
now be stabilising. Ocean heat transport opposes and destabilises, and is
in turn counteracted by both atmospheric transports, which stabilise.
The coefficient of the first term on the right-hand side is negative and
hence stabilising for m ~ 1, and positive (destabilising) for m ~ 2. Either
way, the effect is largest for small A; weak restoring is stabilising if m ~ 1
and destabilising if m ~ 2. As a consequence, models with either fixed
surface freshwater flux (Zhang et al., 1993; Power and Kleeman, 1994;
Mikolajewicz and Maier-Reimer, 1994; Rahmstorf and Willebrand, 1995;
Pierce et al., 1996) or linear dependence of atmospheric moisture transport
on temperature gradient (MS, Lohmann et al., 1996b) are in danger of
falsely concluding that a weaker control of SST by the atmosphere means
a stabilization of the thermohaline circulation.
The two competing effects of temperature on salinity have been noted
previously by NSM and MS, without however giving a satisfactory explanation of when either effect dominates. Krasovskij and Stone (1995, pers.
comm.) have recently found a physically equivalent effect of nonlinearity
in the atmospheric heat and moisture transports, on whether a larger atmospheric equilibrium temperature gradient stabilises or destabilises the
high-latitude sinking.
Notice that all stabilising feedbacks turn destabilising and vice versa if
the low-latitude sinking equilibrium is considered (apart from the mean
flow feedback, #0, which always stabilises). Qualitatively, this can be
understood as follows. The high-latitude sinking state is dominated by
temperature effects, so ocean heat transport must be stabilising (#1) if
the steady state be stable. Temperature and salinity are antagonists, so
the effect of ocean salinity transport is destabilising (#2). Atmospheric
heat transport changes in response to a temperature change and must
counteract its cause, reducing the negative ocean heat transport feedback
( #3). Atmospheric moisture transport, finally, transports buoyancy in the
same direction as atmospheric heat transport, influences ocean density in
the same sense, and hence destabilises (#4). The low-latitude sinking state,
in contrast, is dominated by salinity, so the ocean salinity transport must
now be stabilising. Ocean heat transport opposes and destabilises, and is
in turn counteracted by both atmospheric transports, which stabilise.