<P> In the deep ocean, the predominant driving force is differences in density, caused by salinity and temperature variations (increasing salinity and lowering the temperature of a fluid both increase its density). There is often confusion over the components of the circulation that are wind and density driven . Note that ocean currents due to tides are also significant in many places; most prominent in relatively shallow coastal areas, tidal currents can also be significant in the deep ocean . There they are currently thought to facilitate mixing processes, especially diapycnal mixing . </P> <P> The density of ocean water is not globally homogeneous, but varies significantly and discretely . Sharply defined boundaries exist between water masses which form at the surface, and subsequently maintain their own identity within the ocean . But these sharp boundaries are not to be imagined spatially but rather in a T-S - diagram where water masses are distinguished . They position themselves above or below each other according to their density, which depends on both temperature and salinity . </P> <P> Warm seawater expands and is thus less dense than cooler seawater . Saltier water is denser than fresher water because the dissolved salts fill interstices between water molecules, resulting in more mass per unit volume . Lighter water masses float over denser ones (just as a piece of wood or ice will float on water, see buoyancy). This is known as "stable stratification" as opposed to unstable stratification (see Bruunt - Väisälä frequency) where denser waters are located over less dense waters (see convection or deep convection needed for water mass formation). When dense water masses are first formed, they are not stably stratified, so they seek to locate themselves in the correct vertical position according to their density . This motion is called convection, it orders the stratification by gravitation . Driven by the density gradients this sets up the main driving force behind deep ocean currents like the deep western boundary current (DWBC). </P> <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>

In one word what is the primarily driving force for deep currents