IB Marine Science Standard Level (SL) Practice Exam

Density differences in seawater created by temperature and salinity variations drive a global, deep-ocean circulation known as thermohaline circulation. Cold, salty water becomes dense enough to sink in polar regions, pulling water from above and spreading it as a deep current that travels across basins. As this dense water moves poleward or equatorward, it is upwelled elsewhere and warmed or diluted, becoming less dense and rising to close the loop. This slow, large-scale overturning transports heat and nutrients through the ocean and operates independently of the wind-driven surface currents. The other options describe surface or near-surface flows driven by winds, wave effects, or general subsurface movement, not the density-driven process tied to temperature and salinity.