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Ocean Climate Drift and Interdecadal Oscillation Due to a Change in Thermal Damping

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  • 1 CSIRO Division of Atmospheric Research, Aspendale, Victoria, Australia
  • | 2 Department of Oceanography, Naval Postgraduate School, Monterey, California
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Abstract

The authors investigate the effect of a change in the rate of thermal damping upon the climate of an ocean general circulation model. Initially, the thermal forcing condition is that proposed by Haney, that is, restoring the model surface temperature to a climatology. The restoring condition represents a strong damping. When a steady state is reached, the thermal damping is switched to a weaker one, but the atmosphere-ocean heat exchange is adjusted so that at the moment of the switch the heat flux is identical to that prior to the switch. It is found that interdecadal oscillations and climate drift occur as a result of the switch, regardless of the forcing condition for salinity. The cause for the variability and drift can be traced to the spinup. During the spinup, the surface climatology of the model ocean is forcefully “nudged” toward that of the climatology, regardless of whether or not the internal dynamics of the model ocean can maintain the climatology. This leads to intermittent convections in the spinup state. When the thermal damping becomes weaker, the system chooses a convective pattern (the location and intensity of the convection) more compatible with the internal dynamics. An implication of these results is that drift and variability in a coupled model may be caused by the mechanism. Effects of flux corrections in coupled models are discussed.

Abstract

The authors investigate the effect of a change in the rate of thermal damping upon the climate of an ocean general circulation model. Initially, the thermal forcing condition is that proposed by Haney, that is, restoring the model surface temperature to a climatology. The restoring condition represents a strong damping. When a steady state is reached, the thermal damping is switched to a weaker one, but the atmosphere-ocean heat exchange is adjusted so that at the moment of the switch the heat flux is identical to that prior to the switch. It is found that interdecadal oscillations and climate drift occur as a result of the switch, regardless of the forcing condition for salinity. The cause for the variability and drift can be traced to the spinup. During the spinup, the surface climatology of the model ocean is forcefully “nudged” toward that of the climatology, regardless of whether or not the internal dynamics of the model ocean can maintain the climatology. This leads to intermittent convections in the spinup state. When the thermal damping becomes weaker, the system chooses a convective pattern (the location and intensity of the convection) more compatible with the internal dynamics. An implication of these results is that drift and variability in a coupled model may be caused by the mechanism. Effects of flux corrections in coupled models are discussed.

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