E2 Stratification and Buoyancy

Topic

Stratification is the vertical organization of the ocean into layers of varying density—an arrangement that resists vertical water mixing while simultaneously channeling horizontal flow along those same layers. Seawater density depends on three factors: temperature (dominant in low and mid-latitudes), salinity (dominant in polar regions), and pressure. This combined dependency is described by the international thermodynamic equation of seawater, meaning that temperature, salinity, and pressure together suffice to predict water density at any point in the ocean.

Within this stratification, the pycnocline is the transition region—located roughly between depths of 200 and 1,000 meters—where the density gradient is strongest; it is precisely this sharp gradient that clearly separates warm surface waters from the cold, dense waters of the deep ocean.

The Brunt-Väisälä frequency describes the rate at which a water parcel oscillates when vertically displaced from its equilibrium position, and its sign directly reveals the state of stratification at that point: a positive frequency indicates statically stable stratification, causing the parcel to return to its original position; a zero frequency indicates neutral stratification; and a negative frequency indicates convectively unstable stratification—a state where the parcel, rather than returning, accelerates further away from its initial position.

In practice, this same convective instability drives the deep ocean convection that occurs each winter in the North Atlantic and the Mediterranean Sea: when surface heat loss is sufficiently intense, surface water density rises until it exceeds that of the underlying deep water; this density crossover causes the now-denser surface water to sink directly to the depths instead of remaining afloat above the pycnocline. Water that sinks in this manner—along with water formed in other specific ocean regions—gives rise to water masses: bodies of water with a characteristic combination of temperature and salinity that remain reasonably intact as long as they do not mix with the surrounding water. Antarctic Bottom Water forms in the Antarctic itself; North Atlantic Deep Water forms in the North Atlantic—the same region where the previously described deep convection occurs—and Antarctic Intermediate Water forms at the Antarctic Polar Front. Each of these water masses retains its specific temperature-salinity signature throughout much of its journey through the deep ocean, making it possible to identify them even far from their regions of origin.

It is precisely this preservation of properties that has long made temperature-salinity diagrams the classic method used in physical oceanography to map deep-ocean circulation pathways: one need only recognize a water mass's characteristic temperature-salinity signature to track it—layer by layer—as it travels away from its formation site. Thus, from the density that governs stratification to the water masses that stratification ultimately produces, the deep ocean preserves, within its own vertical structure, a record of where and when each of its water masses was formed.

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