Physics of the Sea

the division of geophysics that deals with the study of physical processes in the ocean. The term “physical oceanography” is sometimes used as a synonym of the physics of the sea. However, in a narrow sense, physical oceanography is the subdivision of the physics of the sea that is concerned with the description of the geographic distributions of the physical properties of the ocean. The physics of the sea includes marine thermodynamics, hydrodynamics, acoustics, and optics, the nuclear hydrophysics of the ocean, and the study of electromagnetic fields in the ocean.

Marine thermodynamics studies the thermodynamic properties of ocean water, such as temperature, salinity, density, the speed of sound, electrical conductivity, refractive index, enthalpy, and internal and potential energy. It also examines the processes by which the distributions of the thermodynamic properties in horizontal planes and with respect to depth are established—including the oceanic heat and water balances, the mixing of waters, and the freezing and thawing of ice—and the diurnal, synoptic, seasonal, and annual variations of the distributions.

Marine hydrodynamics investigates the various forms of motion of ocean water, namely, ocean currents and ocean waves of different origins. Ocean currents are divided into quasi-steady currents and irregular eddies, of which the latter give rise to synoptic variability. The quasi-steady currents include the very large-scale Antarctic Circumpolar Current (seeWEST WIND DRIFT), trade-wind drifts, equatorial undercurrents, and subtropical anticyclonic gyres; the Gulf Stream and the Kuroshio Current are the strong western components of two subtropical gyres. The following types of ocean waves are distinguished: surface gravity waves, for example, wind waves (seeSEA WAVES) and tidal waves (seeTIDE); inertia waves, which are generated by inertial forces as the earth rotates; and internal waves, which are generated in the water, as a result of stratification (seeSTRATIFICATION OF WATER), by tidal forces, variations in atmospheric pressure, or surface waves.

Marine hydrodynamics also deals with vertical microstructure and turbulence. Vertical microstructure is the characteristic stratification of ocean water into quasi-homogeneous layers having thicknesses of tens of meters to 1 mm and separated by surfaces at which discontinuities in temperature and salinity occur. Turbulence (seeTURBULENCE IN THE ATMOSPHERE AND HYDROSPHERE) is the main mechanism of vertical mixing in the ocean and, in particular, the primary mechanism for the exchange of momentum and heat with the atmosphere.

Marine acoustics investigates the propagation of sound waves in the ocean; the scattering and absorption of sound waves in the water (especially by air bubbles), at the surface, and at the bottom; and natural noise, including sounds made by fishes and crustaceans.

Marine optics studies the propagation, scattering, and absorption of light of different wavelengths and polarizations in the ocean, as well as natural light fields, such as the fields produced by sunlight and by bioluminescence.

Nuclear hydrophysics investigates the natural and anthropogenic radioactivity of ocean water and variations in the radioactivity.

The physics of the sea also deals with quasi-steady electric and magnetic fields in the ocean, the propagation of low-frequency electromagnetic disturbances, and magnetohydrodynamic effects that may occur because of the electrical conductivity of ocean water.

The most important problem of the physics of the sea is the interaction of the atmosphere and ocean; the interaction determines the thermodynamic state of the ocean and produces most of the types of motion of ocean water. For example, momentum fluxes from the atmosphere to the ocean, including both turbulent fluxes and fluxes produced by variations in atmospheric pressure, play a major role in the generation of ocean currents, wind waves, and internal waves. The interaction of the atmosphere and ocean is a primary factor in the formation of climate and of long-term weather anomalies.

The physics of the sea is of great practical importance, especially for navigation safety and weather forecasting.