A coupled ocean-atmosphere box model of the Atlantic Ocean: a bimodal climate response

A coupled ocean-atmosphere box model is presented which, although extremely simplified, permits study of the time domain response of a model of the Atlantic Ocean coupled to an atmosphere. In particular the model can be used to systematically examine questions of multiple equilibria and steady state solutions of the earth's climate. The ocean model includes differential surface heating and evaporation, horizontal and vertical exchange of heat and salt between boxes, and a imply parameterized thermohaline circulation. Surface heat fluxes and evaporation are determined through the coupled ocean and energy-balance atmosphere models which treat fluxes of long and short wave radiation and sensible and latent heat. In this paper only circulations symmetric about the equator are presented. Two parameters represent the most important physics: mu. controls the magnitude of the thermohaline circulation; epsilon controls the strength of the hydrological cycle. For fixed mu, two regimes are distinguished. One, associated with relatively small values of epsilon, has weak latitudinal water vapor transport in the atmosphere, a strong thermohaline circulation with sinking in high latitudes, upwelling in low latitudes and strong latitudinal transport of heat by the ocean. The second regime is characterized by strong latitudinal water vapor transport which, by reducing th surface salinity in high latitudes, nearly shuts down the thermohaline circulation and has relatively low latitudinal heat transport by the ocean. Although many important physical processes are not present in the model, the strong circulation regime corresponds roughly to a relatively warm interglacial climate and the weak circulation regime is associated with reduced ocean and atmosphere temperatures in high latitudes, i.e. with such events as the Younger Dryas or the last glacial maximum.