Thermohaline circulation stability: a box model study - Part II: coupled atmosphere-ocean model

A thorough analysis of the stability of a coupled version of an inter-hemispheric 3-box model of Thermohaline Circulation (THC) is presented. This study follows a similarly structured analysis on an uncoupled version of the same model presented in Part I. The model consists of a northern high latitudes box, a tropical box, and a southern high latitudes box, which respectively can be thought as corresponding to the northern, tropical and southern Atlantic ocean. We study how the strength of THC changes when the system undergoes forcings representing global warming conditions. Since we are dealing with a coupled model, a direct representation of the radiative forcing is possible, because the main atmospheric physical processes responsible for freshwater and heat fluxes are formulated separately. Each perturbation to the initial equilibrium is characterized by the total radiative forcing realized, by the rate of increase, and by the North-South asymmetry. Although only weakly asymmetric or symmetric radiative forcings are representative of physically reasonable conditions, we consider general asymmetric forcings, in order to get a more complete picture of the mathematical properties of the system. The choice of suitably defined metrics allows us to determine the boundary dividing the set of radiative forcing scenarios that lead the system to equilibria characterized by a THC pattern similar to the present one, from those that drive the system