Impact Of Wind-Stress To The Nonlinear Stability And Equilibria Transition Of Thermohaline Circulation

The thermohaline circulation (THC) mainly originated from the North Atlantic, which plays an important role in maintaining the balance of water and heat, and influences the climate change. It has been shown from many researches that the abrupt climate change has a close relation with the change of THC. Now, both "Past Global Changes" (PAGES) and "Climate Variability and Predictability" (CLIVAR), which are two of the most important international plans in the global climate research, give emphasis to the research of global THC.There are a lot of papers about THC, which make investigations from different aspects. This paper studies the impact of wind-stress to the THC on the basis of Stommel's box model, where a way is used in which wind is considered in the form of diffusion. This problem has been tackled in Longworth et al. (2005). So far, many works have been done to investigate the effect of wind-stress, such as GCM model and box model. However, they mainly handled the nonlinear problem with numerical integration method, and there was no general theoretical prove. In this paper, we make a different access to tackle this problem, which use a new method called conditional nonlinear optimal perturbation (CNOP). CNOP is issued first in Mu et al. (2003), which is an optimal way to study nonlinear problem and is proved to be efficient and effective. This paper will be launched from four aspects: multi-equilibria, evolution of optimal initial perturbation, critical value of optimal initial perturbation and decadal variability about THC. The conclusion is that wind-stress will increase the northern THC flow, which will result in the strengthened overturning in the state of thermal-driven (TH) THC and the weakened overturning in the state of saline-driven (SA) THC. Under the law that the stronger overturning is, the more stable THC is, the wind-stress will stablize the TH-driven THC and destablize the SA-driven THC. We also give the mathematical prove to support the conclusion, and point out that there is a mechanics that wind-stress can result in more decrement of salt differences in the north-south direction than that of temperature differences. Moreover, this mechanics plays different roles when THC is in the different equilibria.