Thermohahe Convection with Two Stable Regimes of Flow

Free convection between two interconnected reservoirs, due to density differences maintained by heat and salt transfer to the reservoirs, is shown to occur sometimes in two different stable regimes, and may possibly be analogous to certain features of the oseanic circulation. The density of sea-water is modified while at the surface by two distinct processes: heating and cooling, which change the temperature, and precipitation and evaporation, which change the salinity. In many important oceanic regions these two density modifying processes work contrarily to one another. For exam le, the water in and above the main thermoc P,, . e in all subtropical oceans is able to float on top of the denser deep water (we neglect here all consideration of the “salt-fountain” effect) because its density is depressed through surface heating but the excessive evaporation in these same latitudes is able to increase the salinity of the surface waters to such an extent that in general their density contrast with deep waters is only one half of what it would be on the basis of temperature alone. In some large semienclosed seas (e.g. Mediterranean and Red Seas) subject to similar surface heating and evaporation, the salinity can have a predominant influence on the density field. By qualitative comparison of these cases we form an impression that the heat transfer mechanism has a more rapid effect on the density of a newly arriving parcel of water, but that in the long run, given sufficient time, evaporation can reverse the thermal influence. We do not know enough about the details of oceanic circulation to pursue these questions in their full complexity in nature but we can explore their implications in simple idealized systems. That is all we propose to do here. First, let us consider a very simple system: a vessel of water stirred so as to maintain uniform tem erature T and salinity S. (fig. I ) . The walls o P the vessel are made of a porous substance which permits transfer of heat and salt in a simple linear fashion: dT = c ( T T ) dt