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ABSTRACT
A three-box model of haline and thermal mode overturning is developed to study thermohaline oscillations found in a number of ocean general circulation models and that might have occurred in warm equable paleoclimates. By including convective adjustment modified to represent the localized nature of deep convection, the box model shows that a steady haline mode circulation is unstable. For certain ranges of freshwater forcing/ vertical diffusivity, a self-sustained oscillatory circulation is found in which haline-thermal mode switching occurs with a period of centuries to millennia. It is found that mode switching is most likely to occur in warm periods of earth's history with, relative to the present climate, a reduced Pole-equator temperature gradient, an enhanced hydrological cycle, and somewhat smaller values of oceanic diffusivities.
1. Introduction
In past climates the thermohaline circulation (THC) of the ocean may have been quite different from that of today. Paleoclimatic records (e.g., Railsback et al. 1990) suggest that in warm periods of earth's history the abyssal ocean was very much warmer than that of today. A recurring theme of the paleoclimatic literature is the speculation that in these warm climates ocean deep water formation could have been triggered by evaporation from the subtropics resulting in a "haline mode" accounting for abnormal warmth in the subsurface ocean (e.g., Brass et al. 1982). This is very different from today's climate in which deep water formation at high latitudes brings cold water to depth in a "thermal mode." Such extremes of ocean circulations have very different implications for climate and biogeochemical cycles [see, e.g., Zhang et al. (2001)].
In certain parameter regimes, ocean general circulation models (OGCMs) exhibit "mode switching" in which the meridional overturning circulation (MOC) switches between thermal and haline modes. Self-sustained thermohaline oscillations have been found in several OGCM studies using an idealized single basin configuration (Marotzke 1989; Wright and Stocker 1991; Weaver and Sarachik 1991 a,b; Weaver et al. 1993; Winton and Sarachik 1993; Huang 1994). In a study of possible modes of the late Permian ocean circulation with a coarse-resolution OGCM (Zhang et al. 2001), we found that the haline mode (HM) was inherently unstable for fixed external forcing, periodically switching in to a transient thermal mode (TM) in which deep water formed in polar...