Far below the wind-driven surface layer, dense water masses creep through the ocean basins at speeds of only a few centimeters per second. These deep currents are powered by differences in temperature and salinity that create density gradients. Cold, salty water formed in polar regions sinks and spreads equatorward, forming the lower limb of the global overturning circulation.

Deep currents transport heat, carbon, oxygen, and nutrients over thousands of kilometers and on timescales of centuries. They are difficult to observe directly; most knowledge comes from water-mass properties, deep moorings, and the trajectories of neutrally buoyant floats. Major pathways include the deep western boundary currents that hug the continental slopes.

Changes in the formation rate of dense water, possibly linked to freshwater input from melting ice, could alter the strength of deep currents and their climate influence. Monitoring these slow but massive flows remains a priority for understanding long-term ocean and climate variability.

• Density-driven flow in the deep ocean
• Part of the global overturning circulation
• Transports heat, carbon, and oxygen
• Moves slowly but carries enormous volume