Impact of Horizontal Diffusion on the Nonlinear Stability of Thermohaline Circulation in a Modified Box Model

Abstract

Abstract This note studies the impact of horizontal diffusion (HD) on the thermohaline circulation (THC) within a modified Stommel’s box model, which was introduced by Longworth et al. HD may arise as a result of mesoscale eddies from the instability of wind-driven gyres. Focuses are on the multi-equilibriums’ existence and nonlinear stability of the THC. A nonlinear approach called conditional nonlinear optimal perturbation (CNOP) is adopted. Both numerical and analytical analyses suggest that there exists a physical mechanism, which makes HD result in more decrement of salinity difference in the meridional direction than that of temperature difference. Consequently, the effects of HD on the thermally and salinity-driven equilibriums of THC are different: HD stabilizes the former but destabilizes the latter.