Coupled Sea-Ice Models of Interdecadal Variability
Climate variability on the time scales of decadal -to-millennial years has occupied climate dynamicists since 1950s and is important in understanding global change. The key role of the ocean's overturning or thermohaline circulation (THC) in this variability, especially on interdecadal time scale, has just been realized. The purpose of this work is to understand the physical mechanisms by which the THC affects climate variability on time scales of decades-to -centuries. Numerical models of different complexity are applied in our research. An idealized North Atlantic ocean model is used first to investigate low-frequency variability in the climate system. We found that the model sustains an interdecadal oscillation for a realistic range of parameters. The interdecadal oscillation is characterized by a pair of vortices of opposite signs, that grow and decay in quadrature with each other in the ocean's upper layer; their centers follow each other anticlockwise through the northwestern quadrant of the model domain. It is caused by surface-density variations in northern high latitudes; these variations are due to either net evaporation or constant cooling there from the applied surface fluxes. This interdecadal variability is different from the one found when the prescribed surface freshwater flux is characterized by strong freshening in northern high latitudes. A hybrid-coupled ocean-atmosphere model is then constructed by coupling the ocean model with a Budyko-Sellers -North, two-dimensional energy-balance model. A unique feature of the model so coupled is the ocean density's exclusive dependence on temperature. An interdecadal oscillation with a period of 40-50 years is found in the hybrid coupled model. The interdecadal oscillation's physical mechanism resembles that of the interdecadal oscillation analyzed in the earlier, uncoupled model, in which density depends on both salinity and temperature. Central to the mechanism is the prescribed component in the surface heat fluxes. In this coupled model, the prescribed forcing component comes from solar radiation. Third, hydrological cycles of different forms are included into the hybrid coupled ocean-atmosphere model. The interdecadal variability in the coupled ocean-atmosphere model still sustains in the model, unless the hydrological cycle is given by constant freshwater fluxes of zonally averaged forms. Millennial oscillations are favored by the later. Finally, a simple thermodynamic sea-ice model is added to the hybrid coupled ocean-atmosphere model. None of the oscillations analyzed before could survive when sea ice is included. Various oscillations ranging from decadal to centennial or even millennial years are found in the model in which salinity is fixed and no hydrological cycle is considered. The major physical process responsible for the oscillations is the ice-insulation feedback--the ice extent has a positive feedback on the underlying sea surface temperature, while the temperature has a negative feedback on the overlying ice cover.
- Pub Date:
- January 1995
- CLIMATE VARIABILITY;
- Physics: Atmospheric Science; Physical Oceanography