Instabilities and Multiple Steady States of the Thermohaline Circulation

Abstract

It is well established by observations that the thermohaline circulation in the Atlantic Ocean is strongly asymmetric with respect to the equator. Salinities at high latitudes are much higher in the North Atlantic than in the South Atlantic. The deep water is mainly formed in the North Atlantic and moves southward across the equator, while warmer water in the thermocline crosses the equator northward, causing a heat flux of approximately 0.8 PW (Isemer et al. 1989). The question arises if this behaviour can have causes other than the obvious asymmetry in land-sea distribution. Posed differently, the question is, if ocean models with symmetric geometry and surface forcing can have several steady states, especially asymmetric ones, with deep water formation in one hemisphere only. The answer is clearly ‘yes’, for models of widely varying complexity. Rooth (1982), Walin (1985) and Welander (1986) investigated box models, Marotzke et al. (1988) a 2-dimensional model with T-S advection and diffusion, a simplified momentum balance and purely thermo haline forcing. Bryan (1986) found that the 3-dimensional GFDL model, with symmetric thermohaline and wind forcing, has an asymmetric steady state.