Temperature in the Ocean

Ocean heat budget

According to Merriam-Webster temperature is the degree of hotness or coldness measured on a definite scale. Oxford Languages says it is the degree or intensity of heat present in a substance or object, especially as expressed according to a comparative scale and shown by a thermometer or perceived by touch.

Stewart wrote: Many physical processes depend on temperature. A few can be used to define absolute temperature T . The unit of T is the kelvin, which has the symbol K. The fundamental processes used for defining an absolute temperature scale over the range of temperatures found in the ocean include: 1) the gas laws relating pressure to temperature of an ideal gas with corrections for the density of the gas; and 2) the voltage noise of a resistance R. The measurement of temperature using an absolute scale is difficult and the measurement is usually made by national standards laboratories. The absolute measurements are used to define a practical temperature scale based on the temperature of a few fixed points and interpolating devices which are calibrated at the fixed points. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

For temperatures commonly found in the ocean, the interpolating device is a platinum-resistance thermometer. It consists of a loosely wound, strain-free, pure platinum wire whose resistance is a function of temperature. It is calibrated at fixed points between the triple point of equilibrium hydrogen at 13.8033 K and the freezing point of silver at 961.78 K, including the triple point of water at 0.060◦C, the melting point of Gallium at 29.7646◦C, and the freezing point of Indium at 156.5985◦C (Preston-Thomas, 1990). The triple point of water is the temperature at which ice, water, and water vapor are in equilibrium. The temperature scale in kelvin T is related to the temperature scale in degrees Celsius t[ ◦C] by: t [ ◦C] = T [K] − 273.15 (6.6). While oceanographers use thermometers calibrated with an accuracy of a millidegree, which is 0.001◦C, the temperature scale itself has uncertainties of a few millidegrees.

See Salinity Under CHEMISTRY OF THE OCEANS

Websites and Resources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Monterey Bay Aquarium montereybayaquarium.org

Geographical Distribution of Surface Temperature and Salinity

Ocean temperatures
According to the “Introduction to Physical Oceanography”: The distribution of temperature at the sea surface tends to be zonal, that is, it is independent of longitude. Warmest water is near the equator, coldest water is near the poles. The deviations from zonal are small. Equatorward of 40◦, cooler waters tend to be on the eastern side of the basin. North of this latitude, cooler waters tend to be on the western side. The anomalies of sea-surface temperature, the deviation from a long term average, are small, less than 1.5◦C except in the equatorial Pacific where the deviations can be 3◦C. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

The annual range of surface temperature is highest at mid-latitudes, especially on the western side of the ocean. In the west, cold air blows off the continents in winter and cools the ocean. The cooling dominates the heat budget. In the tropics the temperature range is mostly less than 2◦C.

The distribution of sea-surface salinity also tends to be zonal. The saltiest waters are at mid-latitudes where evaporation is high. Less salty waters are near the equator where rain freshens the surface, and at high latitudes where melted sea ice freshens the surface. The zonal (east-west) average of salinity shows a close correlation between salinity and evaporation minus precipitation plus river input.

Because many large rivers drain into the Atlantic and the Arctic Sea, why is the Atlantic saltier than the Pacific? Broecker (1997) showed that the water evaporated from the Atlantic does not fall as rain on land. Instead, it is carried by winds into the Pacific. Broecker points out that the quantity is small, equivalent to a little more than the flow in the Amazon River, but “were this flux not compensated by an exchange of more salty Atlantic waters for less salty Pacific waters, the salinity of the entire Atlantic would rise about 1 gram per liter per millennium.”

Oceanic Heat Budget

Robert Stewart wrote in the “Introduction to Physical Oceanography”: About half the solar energy reaching earth is absorbed by the ocean and land, where it is temporarily stored near the surface. Only about a fifth of the available solar energy is directly absorbed by the atmosphere. Of the energy absorbed by the ocean, most is released locally to the atmosphere, mostly by evaporation and infrared radiation. The remainder is transported by currents to other areas especially mid latitudes.[Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

Heat lost by the tropical ocean is the major source of heat needed to drive the atmospheric circulation. And, solar energy stored in the ocean from summer to winter helps ameliorate earth’s climate. The thermal energy transported by ocean currents is not steady, and significant changes in the transport, particularly in the Atlantic, may have been important for the development of the ice ages. For these reasons, oceanic heat budgets and transports are important for understanding earth’s climate and its short and long term variability.

Changes in energy stored in the upper ocean result from an imbalance between input and output of heat through the sea surface. This transfer of heat across or through a surface is called a heat flux . The flux of heat and water also changes the density of surface waters, and hence their buoyancy. As a result, the sum of the heat and water fluxes is often called the buoyancy flux. The flux of energy to deeper layers is usually much smaller than the flux through the surface. And, the total flux of energy into and out of the ocean must be zero, otherwise the ocean as a whole would heat up or cool down. The sum of the heat fluxes into or out of a volume of water is the heat budget.

The major terms in the budget at the sea surface are: 1) Insolation QSW , the flux of solar energy into the sea; 2) Net Infrared Radiation QLW , net flux of infrared radiation from the sea; 3) Sensible Heat Flux QS, the flux of heat out of the sea due to conduction; 4) Latent Heat Flux QL, the flux of energy carried by evaporated water; and 5) Advection QV , heat carried away by currents.

Specific heat of sea water at atmospheric pressure Cp in joules per gram per degree Celsius is a function of temperature in Celsius and salinity. Conservation of heat requires: Q = QSW + QLW + QS + QL + QV where Q is the resultant heat gain or loss. Units for heat fluxes are watts/influenced. The product of flux times surface area times time is energy in joules. The change in temperature t of the water is related to change in energy E through: E = Cp meters t where meters is the mass of water being warmed or cooled, and Cp is the specific heat of sea water at constant pressure. Cp ≈ 4.0 × 103 J · kilograms−1 · ◦C−1 Thus, 4,000 joules of energy are required to heat 1.0 kilogram of sea water by 1.0◦C.

Important Concepts in Understanding Oceanic Heat Budget

Stewart writes: To understand the importance of the ocean in earth’s heat budget, let’s make a comparison of the heat stored in the ocean with heat stored on land during an annual cycle. During the cycle, heat is stored in summer and released in the winter. The point is to show that the ocean store and release much more heat than the land. To begin, use (5.3) and the heat capacity of soil and rocks Cp(rock) = 800 J · kilograms−1 · ◦C−1 (5.4) to obtain Cp(rock) ≈ 0.2 Cp (water). [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

The volume of water which exchanges heat with the atmosphere on a seasonal cycle is 100 cubic meters per square meter of surface, i.e. that mass from the surface to a depth of 100 meters. The density of water is 1000 kilograms per cubic meter, and the mass in contact with the atmosphere is density × volume = mwater = 100, 000 kilograms. The volume of land which exchanges heat with the atmosphere on a seasonal cycle is 1 cubic meter. Because the density of rock is 3,000 kilograms per cubic meter, the mass of the soil and rock in contact with the atmosphere is 3,000 kilograms.

The seasonal heat storage values for the ocean and land are therefore: Eocean = Cp(water) mwater t t = 10◦C = (4000)(105)(10◦) Joules = 4.0 × 109 Joules Eland = Cp(rock) mrock t t = 20◦C = (800)(3000)(20◦) Joules = 4.8 × 107 Joules Eocean Eland = 100 where t is the typical change in temperature from summer to winter.

The large storage of heat in the ocean compared with the land has important consequences. The seasonal range of air temperatures on land increases with distance from the ocean, and it can exceed 40◦C in the center of continents, reaching 60◦C in Siberia. Typical range of temperature over the ocean and along coasts is less than 10◦C. The variability of water temperatures is still smaller.

Factors Influencing Insolation Incoming solar radiation is primarily determined by latitude, season, time of day, and cloudiness. The polar regions are heated less than the tropics, areas in winter are heated less than the same area in summer, areas in early morning are heated less than the same area at noon, and cloudy days have less sun than sunny days. The following factors are important: 1) The height of the sun above the horizon, which depends on latitude, season, and time of day. Don’t forget, there is no insolation at night! 2) The length of day, which depends on latitude and season. 3) The cross-sectional area of the surface absorbing sunlight, which depends on height of the sun above the horizon. 4) Attenuation, which depends on: i) Clouds, which absorb and scatter radiation. ii) Path length through the atmosphere, which varies as cscϕ, where ϕ is angle of the sun above the horizon. iii) Gas molecules which absorb radiation in some bands. H2O, O3, and CO2 are all important. iv) Aerosols which scatter and absorb radiation. Both volcanic and marine aerosols are important. And v) dust, which scatters radiation, especially Saharan dust over the Atlantic. 5) Reflectivity of the surface, which depends on solar elevation angle and roughness of sea surface. Solar inclination and cloudiness dominate. Absorption by ozone, water vapor, aerosols, and dust are much weaker.

Infrared Radiation and the Oceanic Heat Budget

According to Encyclopaedia Britannica: Infrared radiation is the portion of the electromagnetic spectrum that extends from the long wavelength, or red, end of the visible-light range to the microwave range. Invisible to the eye, it can be detected as a sensation of warmth on the skin. The infrared range is usually divided into three regions: near infrared (nearest the visible spectrum), with wavelengths 0.78 to about 2.5 micrometres (a micrometre, or micron, is 10-6 metre); middle infrared, with wavelengths 2.5 to about 50 micrometres; and far infrared, with wavelengths 50 to 1,000 micrometres. Most of the radiation emitted by a moderately heated surface is infrared; it forms a continuous spectrum. Molecular excitation also produces copious infrared radiation but in a discrete spectrum of lines or bands. [Source: Encyclopaedia Britannica]

According to Stewart there are several to consider when discussing the influence infrared flux (the action or process of flowing or flowing out): “The sea surface radiates as a blackbody having the same temperature as the water, which is roughly 290 K. The distribution of radiation as a function of wavelength is given by Planck’s equation. Sea water at 290 K radiates most strongly at wavelengths near 10 µm. These wavelengths are strongly absorbed by clouds, and somewhat by water vapor. A plot of atmospheric transmittance as a function of wavelength for a clear atmosphere but with varying amounts of water vapor shows the atmosphere is nearly transparent in some wavelength bands called windows.

The transmittance on a cloud-free day through the window from 8 µm to 13 µm is determined mostly by water vapor. Absorption in other bands, such as those at 3.5 µm to 4.0 µm depends on CO2 concentration in the atmosphere. As the concentration of CO2 increases, these windows close and more radiation is trapped by the atmosphere. Because the atmosphere is mostly transparent to incoming sunlight, and somewhat opaque to outgoing infrared radiation, the atmosphere traps radiation. The trapped radiation, coupled with convection in the atmosphere, keeps earth’s surface 33◦ warmer than it would be in the absence of a convecting, wet atmosphere but in thermal equilibrium with space. The atmosphere acts like the panes of glass on a greenhouse, and the effect is known as the greenhouse effect. See Hartmann (1994: 24–26) for a simple discussion of the radiative balance of a planet. CO2, water vapor, methane, and ozone are all important greenhouse gasses.

Sea surface temperature

The net infrared flux depends on: 1) Clouds thickness. The thicker the cloud deck, the less heat escapes to space. 2) Cloud height, which determines the temperature at which the cloud radiates heat back to the ocean. The rate is proportional to t 4, where t is the temperature of the radiating body in Kelvins. High clouds are colder than low clouds. 3) Atmospheric water-vapor content. The more humid the atmosphere the less heat escapes to space. 4) Water Temperature. The hotter the water the more heat is radiated. 5) Ice and snow cover. Ice emits as a black body, but it cools much faster than open water. Ice-covered seas are insulated from the atmosphere.

Water vapor and clouds influence the net loss of infrared radiation more than surface temperature. Hot tropical regions lose less heat than cold polar regions. Latent heat flux is influenced primarily by wind speed and relative humidity. High winds and dry air evaporate much more water than weak winds with relative humidity near 100 percent. In polar regions, evaporation from ice covered ocean is much less than from open water. In the arctic, most of the heat lost from the sea is through leads (ice-free areas). Hence the percent open water is very important for the arctic heat budget.

Measurement of Temperature in the Sea

Temperature, salinity, and pressure are measured as a function of depth using various instruments or techniques, and density is calculated from the measurements. Temperature is usually measured by a thermistor. Conductivity is measured by a conductivity cell. Pressure is measured by a quartz crystal. According to the “Introduction to Physical Oceanography”: Bathythermograph (BT) was a mechanical device that measured temperature vs depth on a smoked glass slide. The device was widely used to map the thermal structure of the upper ocean, including the depth of the mixed layer before being replaced by the expendable bathythermograph in the 1970s. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

Expendable Bathythermograph (XBT) is an electronic device that measures temperature vs depth using a thermistor on a free-falling streamlined weight. The thermistor is connected to an ohm-meter on the ship by a thin copper wire that is spooled out from the sinking weight and from the moving ship. The xbt is now the most widely used instrument for measuring the thermal structure of the upper ocean. Approximately 65,000 are used each year. The streamlined weight falls through the water at a constant velocity. So depth can be calculated from fall time with an accuracy of ±2 percent. Temperature accuracy is ±0.1◦C. And, vertical resolution is typically 65 cm. Probes reach to depths of 200 meters to 1830 meters depending on model.

Sea Temperatures during the 1997 El Nino

Image Sources: Wikimedia Commons; YouTube, NOAA

Text Sources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023