Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.
Physical oceanography is one of several sub-domains into which oceanography is divided. Others include biological, chemical and geological oceanography. Like the study of atmospheric physics, physical oceanography is founded upon principles of thermodynamics and fluid mechanics.
Physical oceanography may be subdivided into descriptive and dynamical physical oceanography.
Descriptive physical oceanography seeks to research the ocean through observations and complex numerical models that describe temperature, salinity, density, currents and other ocean features as accurately and completely as possible.
Dynamical physical oceanography focuses primarily upon the processes that govern the motion of fluids with emphasis upon theoretical research and numerical models. These are part of the large field of Geophysical Fluid Dynamics (GFD) that is shared together with meteorology. GFD is a sub field of fluid dynamics describing flows occurring on spatial and temporal scales that are greatly influenced by the Coriolis force.
Physical setting
Roughly 100% of the planet's water is in its oceans, and the oceans are the source of the vast majority of water vapor that condenses in the atmosphere and falls as rain or snow on the continents. The tremendous heat capacity of the oceans moderates the planet's climate, and its absorption of various gases affects the composition of the atmosphere. The ocean's influence extends even to the composition of volcanic rocks through seafloor metamorphism, as well as to that of volcanic gases and magmas created at subduction zones.
From sea level, the oceans are far deeper than the continents are tall; examination of the Earth's hypsographic curve shows that the average elevation of Earth's landmasses is only , while the ocean's average depth is . Though this apparent discrepancy is great, for both land and sea, the respective extremes such as mountains and trenches are rare.
+ Area, volume plus mean and maximum depths of oceans (excluding adjacent seas) Body Area (106km2) Volume (106km3) Mean depth (m) Maximum (m) Pacific Ocean 165.2 707.6 4282 -11033 Atlantic Ocean 82.4 323.6 3926 -8605 Indian Ocean 73.4 291.0 3963 -8047 Southern Ocean 20.3 -7235 Arctic Ocean 14.1 1038 Caribbean Sea 2.8 -7686
Temperature, salinity and density
Because the vast majority of the world ocean's volume is deep water, the mean temperature of seawater is low; roughly 75% of the ocean's volume has a temperature from 0° – 5 °C (Pinet 1996). The same percentage falls in a salinity range between 34 and 35 ppt (3.4–3.5%) (Pinet 1996). There is still quite a bit of variation, however. Surface temperatures can range from below freezing near the poles to 35 °C in restricted tropical seas, while salinity can vary from 10 to 41 ppt (1.0–4.1%).
The vertical structure of the temperature can be divided into three basic layers, a surface mixed layer, where gradients are low, a thermocline where gradients are high, and a poorly stratified abyss.
In terms of temperature, the ocean's layers are highly latitude-dependent; the thermocline is pronounced in the tropics, but nonexistent in polar waters (Marshak 2001). The halocline usually lies near the surface, where evaporation raises salinity in the tropics, or meltwater dilutes it in polar regions. These variations of salinity and temperature with depth change the density of the seawater, creating the pycnocline.
Temperature The temperature of ocean water varies significantly across different regions and depths. As mentioned, the vast majority of ocean water (around 75%) lies between and , mostly in the deep ocean, where sunlight does not penetrate. The surface layers, however, experience far greater variability. In polar regions, surface temperatures can drop below freezing, while in tropical and subtropical regions, they may reach up to . This thermal stratification results in a vertical temperature gradient that divides the ocean into distinct layers.
Surface Mixed Layer: This uppermost layer is well-mixed due to wind and wave action, resulting in minimal temperature variation with depth. The thickness of this layer varies depending on location and season but can extend from . Thermocline: Below the mixed layer lies the thermocline, a zone where temperature decreases rapidly with increasing depth. The thermocline is especially pronounced in tropical and temperate regions but is absent in polar waters where surface temperatures are already near freezing. The depth and sharpness of the thermocline can shift with seasonal changes and ocean currents, playing a critical role in regulating heat exchange between the ocean and the atmosphere. Abyssal Zone: Beneath the thermocline is the deep ocean or abyssal zone, where temperatures remain relatively uniform, hovering just above freezing, . This cold, dense water originates from polar regions, where surface water cools, sinks, and spreads towards the equator along the ocean floor, forming the deep ocean circulation system.
Salinity Salinity, a measure of the concentration of dissolved salts in seawater, typically ranges between 34 and 35 parts per thousand (ppt) in most of the world’s oceans. However, localized factors such as evaporation, precipitation, river runoff, and ice formation or melting cause significant variations in salinity. These variations are often most evident in coastal areas and marginal seas.
Surface Salinity: In the open ocean, salinity is generally highest in subtropical regions where high evaporation rates dominate, and lowest in regions of high precipitation or freshwater influx from rivers, such as the mouths of the Amazon and Congo Rivers. Tropical and subtropical seas, such as the Red Sea and the Mediterranean, can experience salinities as high as 40-41 ppt due to intense evaporation and restricted water exchange with the open ocean. Halocline: The halocline is a layer within the ocean where salinity changes rapidly with depth. This stratification can be influenced by surface processes like evaporation (which increases salinity) and freshwater input (which decreases it). The halocline often coincides with the thermocline, particularly in tropical and subtropical regions, contributing to overall water column stability. Polar Regions: In polar areas, surface salinity is generally lower due to freshwater input from melting ice. During sea ice formation, however, brine rejection increases the salinity of surrounding waters, contributing to the sinking of dense water masses and the formation of deep ocean currents that drive global circulation patterns. The salt in the oceans originates from runoff from terrestrial sources as well as hydrothermal vents. It has been estimated that the salinity of oceans was greater in the distant past than it is today.
Density and the Pycnocline The combination of temperature and salinity variations leads to changes in seawater density. Seawater density is primarily influenced by both these factors—colder, saltier water is denser than warmer, fresher water. This variation in density creates stratification in the ocean and is key to understanding ocean circulation patterns.
Pycnocline: The pycnocline is a layer within the ocean where the density increases rapidly with depth. It typically coincides with the thermocline and halocline in tropical and subtropical waters, forming a sharp boundary between the less dense surface waters and the denser deep ocean waters. This density stratification acts as a barrier to vertical mixing, limiting the exchange of heat, gases, and nutrients between the surface and deep ocean layers. Thermohaline Circulation: Density differences drive the thermohaline circulation, also known as the global "conveyor belt," which plays a crucial role in regulating Earth's climate. Cold, dense water formed in the polar regions sinks and moves along the ocean floor toward the equator, while warmer surface waters flow poleward to replace it. This global circulation helps redistribute heat and maintains the ocean's dynamic equilibrium. Regional Variations: In areas of upwelling or downwelling, the density structure of the water column can be disrupted. Upwelling brings cooler, nutrient-rich water to the surface, while downwelling pushes surface waters to greater depths, impacting local ecosystems and global climate.
Understanding the complex interactions between temperature, salinity, and density is essential for predicting ocean circulation patterns, climate change effects, and the health of marine ecosystems. These factors also influence marine life, as many species are sensitive to the specific temperature and salinity ranges of their habitats.
Circulation
Energy for the ocean circulation (and for the atmospheric circulation) comes from solar radiation and gravitational energy from the Sun and Moon.Munk, W. and Wunsch, C., 1998: Abyssal recipes II: energetics of tidal and wind mixing. Deep-Sea Research Part I, 45, pp. 1977--2010. The amount of sunlight absorbed at the surface varies strongly with latitude, being greater at the equator than at the poles, and this engenders fluid motion in both the atmosphere and ocean that acts to redistribute heat from the equator towards the poles, thereby reducing the temperature gradients that would exist in the absence of fluid motion. Perhaps three quarters of this heat is carried in the atmosphere; the rest is carried in the ocean.
The atmosphere is heated from below, which leads to convection, the largest expression of which is the Hadley circulation. By contrast the ocean is heated from above, which tends to suppress convection. Instead ocean deep water is formed in polar regions where cold salty waters sink in fairly restricted areas. This is the beginning of the thermohaline circulation.
Oceanic currents are largely driven by the surface wind stress; hence the large-scale atmospheric circulation is important to understanding the ocean circulation. The Hadley circulation leads to Easterly winds in the tropics and Westerlies in mid-latitudes. This leads to slow equatorward flow throughout most of a subtropical ocean basin (the Sverdrup balance). The return flow occurs in an intense, narrow, poleward western boundary current. Like the atmosphere, the ocean is far wider than it is deep, and hence horizontal motion is in general much faster than vertical motion. In the southern hemisphere there is a continuous belt of ocean, and hence the mid-latitude westerlies force the strong Antarctic Circumpolar Current. In the northern hemisphere the land masses prevent this and the ocean circulation is broken into smaller gyres in the Atlantic and Pacific basins.
Coriolis effect The Coriolis effect results in a deflection of fluid flows (to the right in the Northern Hemisphere and left in the Southern Hemisphere). This has profound effects on the flow of the oceans. In particular it means the flow goes around high and low pressure systems, permitting them to persist for long periods of time. As a result, tiny variations in pressure can produce measurable currents. A slope of one part in one million in sea surface height, for example, will result in a current of 10 cm/s at mid-latitudes. The fact that the Coriolis effect is largest at the poles and weak at the equator results in sharp, relatively steady western boundary currents which are absent on eastern boundaries. Also see secondary circulation effects.
Ekman transport