상자모형에서 열염분순환의 안정도

Abstract

The sea surface thermal forcing, which acts in the direction of the present thermohaline circulation in the North Atlantic, is opposite to the haline forcing from the freshwater flux. The strong negative feedback between the sea surface temperature and the surface heat flux removes changes in the sea surface temperature rapidly. However, the freshwater flux, which arises from the local imbalance between precipitation and evaporation, is independent of the sea surface salinity. The difference in the boundary conditions for temperature and salinity may give rise to multiple equilibria of the thermohaline circulation under identical boundary conditions (see Weaver and Hughes 1992; Marotzke 1994; Whitehead 1995 for reviews). Therefore, the thermohaline circulation can switch from one equilibrium to another rapidly (thermohaline catastrophe) if the thermal or haline forcing is perturbed. Studies ranging fromsimple box models to fully developed primitive-equation numerical models have demonstrated the thermohaline catastrophe. It has been suggested that the mechanism behind the thermohaline catastrophe is a large-scale advective process (Walin 1985), but the effect this advective process has on stability of the thermohaline circulation has been neglected. Instead, many studies have been focused on boundary conditions for salinity, that is, the parameterization of the air-sea freshwater exchanges and their effects. In Stommel's (1961) two-box model, a linear mass transport relation was assumed, and the strength of the circulation through a capillary pipe connecting two boxes, Y~Dr, where Dr is the density difference between the two adjacent boxes. This linear relation has been used in most box models, which suggest that if freshwater flux to the Northern North Atlantic increases by a small amount, the present thermohaline circulation of North Atlantic would be reversed (Huang et al. 1992). A simple scaling analysis based on geostrophy and advective-diffusive buoyancy b