PHYSICS

Physical oceanography is the study of the many forms of currents, waves, tides, and dynamical processes that govern the motion of seawater. Solar energy is unevenly distributed over the surface of the ocean, decreasing strongly with increasing latitude. Therefore, the ocean seeks to redistribute heat from the Equator by moving warm equatorial water toward the poles. As warm, saline water moves north- and southward, it is gradually cooled until its density has increased to the point where it becomes heavier than its surroundings and starts to sink. The return flow toward the Equator happens at depth, shielded from the impact of the sun. This is the global overturning circulation, and takes approximately 800-1000 years to complete a circuit. However, this process is heavily influenced by the Earth’s rotation, the presence of land masses and the atmospheric conditions, which leads to a number of smaller scale phenomena such as basin-wide ocean gyres, eddies, and small-scale local effects. 

 

The relationship between atmosphere and ocean is complex, as the energy transfer between the two domains drive dynamical processes ranging from large scale wind and current patterns to small scale ocean wave heights in coastal areas. In the North Atlantic Ocean, the large scale atmospheric wind pattern sets up currents that flow clockwise around the whole basin, dominated by the strong, northward-flowing Gulf Stream originating in the Gulf of Mexico. The Canary Current is the weaker but broader southward return flow, following the coast of southern Europe and Africa toward the Equator. A westward flow along the Equator toward the Caribbean (the North Equatorial Current) completes the loop. We find a similar gyre in all the ocean basins, although because of the Coriolis effect the gyres are counter-clockwise in the southern hemisphere.

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Another energy source is the gravitational pull from the Moon and the Sun, leading to daily, localized variations in sea levels and current directions due to tidal effects. Ocean tides are either diurnal (one high tide and one low tide during one lunar day) or semi-diurnal (two high tides and two low tides during one lunar day). The Earth rotates once during 24 hours, but during this time the moon has shifted position relative to a fixed point on Earth, so a lunar day is longer: 24 hours and 50 minutes. This is why high and low tides does not appear at the same time each day in a specific location. Tides are measured by tide gauges, either visually using a measuring rod or by a pressure gauge. Both types must be fixed to the seabed or to a structure such as a jetty.

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Data collection is typically performed using CTD probes (Conductivity, Temperature, Depth) and current meters (mechanical or acoustic). CTD probes take advantage of the fact that electrical conductivity of seawater is correlated with salinity. A CTD probe is lowered by winch from a ship at rest, from the ocean surface to the desired depth, measuring continuously as it descends the water column at a speed of 1-2 m/s. For deep measurements (5000 m or more), a ship can spend over 2 hours on station. Some probes are equipped with water sample bottles that can be closed at set depths by the CTD operator, and analyzed after being brought onboard. These samples can be used to verify measurements and calibrate the CTD probe, but may also yield information on ocean chemistry and suspended biological matter. 

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Temperature and salinity data are also collected at fixed stations using instruments moored to land or anchored to the seabed. For example, NOAA maintains a large network of moored buoys with CTD probes mounted at specific depth intervals that measure temperature and salinity at regular intervals. These measurements provide invaluable time series of long term changes, and are used to study climate change and to predict the onset of El Nino and La Nina phases. Ongoing trials using inverse methods to convert acoustic signal travel times to temperature and salinity properties have shown promise, but need further verification.

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There are several other types of instruments that yield information about ocean physics, such as satellite sensors (altimeters, ocean color, temperature sensors) and radar scatterometers that use radar backscatter to measure surface waves. Satellite altimeters measure the two-way travel time of radar signals between the satellite and the surface, and gives information on the large-scale surface shape of the ocean. This can be used to monitor sea level rise, and also to calculate current flows on scales of order 100-1000 km. Ocean color data from satellite-borne sensors are designed to monitor algal blooms in the surface ocean, but the structures of these blooms yield secondary information on the nature of current flows, eddies and other ocean phenomena. Finally, sea surface temperature can be measured by infrared sensors on satellites, and with daily global coverage, high accuracy and spatial coverage. For more information on how satellites are used to monitor the ocean, please see NOAA National Ocean Service.