<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> 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>
in one word what is the primarily driving force for deep currents
in one word what is the primarily driving force for deep currents
convection
convection