<P> The thermohaline circulation is mainly driven by the formation of deep water masses in the North Atlantic and the Southern Ocean caused by differences in temperature and salinity of the water . </P> <P> The great quantities of dense water sinking at high latitudes must be offset by equal quantities of water rising elsewhere . Note that cold water in polar zones sink relatively rapidly over a small area, while warm water in temperate and tropical zones rise more gradually across a much larger area . It then slowly returns poleward near the surface to repeat the cycle . The continual diffuse upwelling of deep water maintains the existence of the permanent thermocline found everywhere at low and mid-latitudes . This model was described by Henry Stommel and Arnold B. Arons in 1960 and is known as the Stommel - Arons box model for the MOC . This slow upward movement is approximated to be about 1 centimeter (0.5 inch) per day over most of the ocean . If this rise were to stop, downward movement of heat would cause the thermocline to descend and would reduce its steepness . </P> <P> The dense water masses that sink into the deep basins are formed in quite specific areas of the North Atlantic and the Southern Ocean . In the North Atlantic, seawater at the surface of the ocean is intensely cooled by the wind and low ambient air temperatures . Wind moving over the water also produces a great deal of evaporation, leading to a decrease in temperature, called evaporative cooling related to latent heat . Evaporation removes only water molecules, resulting in an increase in the salinity of the seawater left behind, and thus an increase in the density of the water mass along with the decrease in temperature . In the Norwegian Sea evaporative cooling is predominant, and the sinking water mass, the North Atlantic Deep Water (NADW), fills the basin and spills southwards through crevasses in the submarine sills that connect Greenland, Iceland and Great Britain which are known as the Greenland - Scotland - Ridge . It then flows very slowly into the deep abyssal plains of the Atlantic, always in a southerly direction . Flow from the Arctic Ocean Basin into the Pacific, however, is blocked by the narrow shallows of the Bering Strait . </P> <P> In the Southern Ocean, strong katabatic winds blowing from the Antarctic continent onto the ice shelves will blow the newly formed sea ice away, opening polynyas along the coast . The ocean, no longer protected by sea ice, suffers a brutal and strong cooling (see polynya). Meanwhile, sea ice starts reforming, so the surface waters also get saltier, hence very dense . In fact, the formation of sea ice contributes to an increase in surface seawater salinity; saltier brine is left behind as the sea ice forms around it (pure water preferentially being frozen). Increasing salinity lowers the freezing point of seawater, so cold liquid brine is formed in inclusions within a honeycomb of ice . The brine progressively melts the ice just beneath it, eventually dripping out of the ice matrix and sinking . This process is known as brine rejection . </P>

Where are the two major regions where dense water masses are created