In oceanography and climatology, ocean heat content (OHC) is a term for the energy absorbed by the ocean, which is stored for indefinite time periods as internal energy or enthalpy. Ocean warming accounts for about 90% of Earth's energy accumulation from global warming since year 1970. About one third of this added thermal energy has propagated to depths below 700 meters as of 2020. Changes in ocean heat content have far-reaching consequences for the planet's marine and terrestrial ecosystems; including multiple impacts to coastal ecosystems and communities.
The more abundant equatorial solar irradiance which is absorbed by Earth's tropical surface waters drives the overall poleward propagation of ocean thermal energy. Warming oceans are directly responsible for coral bleaching and contribute to the migration of marine species. Redistribution of the planet's internal energy by atmospheric circulation and ocean currents produces internal climate variability, often in the form of irregular oscillations, and helps to sustain the global thermohaline circulation. Marine heat waves are regions of life-threatening and persistently dense ocean heat. Releases of OHC to the atmosphere occur primarily via evaporation and enable the planetary water cycle. Concentrated releases in association with high sea-surface temperatures help drive tropical cyclones, atmospheric heat waves and other extreme weather events.
The increase in OHC accounts for 30-40% of global sea-level rise from 1900 to 2020 because of thermal expansion. It is also an accelerator of sea ice, iceberg, and tidewater glacier melting. The resulting ice retreat has been most consistent and pronounced for Arctic sea ice, and within northern fjords such as those of Greenland and Canada. Impacts to Antarctic sea ice and the vast Antarctic ice shelves which terminate into the Southern Ocean have been more varied.
Definition and measurementEdit
where is seawater density, is the specific heat of sea water, h2 is the lower depth, h1 is the upper depth, and is the temperature profile. In SI units, has units of J·m−2. Integrating this density over an ocean basin, or entire ocean, gives the total heat content, as indicated in the figure to right. Thus, the total heat content is the product of the density, specific heat capacity, and the volume integral of temperature over the three-dimensional region of the ocean in question.
Ocean heat content can be estimated using temperature measurements obtained by a Nansen bottle, an ARGO float, or ocean acoustic tomography. Sea surface temperatures are also measured by the Global Drifter Program. The World Ocean Database Project is the largest database for temperature profiles from all of the world’s ocean.
The upper ocean heat content in most North Atlantic regions is dominated by heat transport convergence (a location where ocean currents meet), without large changes to temperature and salinity relation.
Several studies in recent years have found a multi-decadal rise in OHC of the deep and upper ocean regions. The studies attribute the heat uptake to anthropogenic warming which is equivalently expressed as a change to Earth's energy balance.
Studies based on ARGO indicate that ocean surface winds, especially the subtropical trade winds in the Pacific Ocean, change ocean heat vertical distribution. This results in changes among ocean currents, and an increase of the subtropical overturning, which is also related to the El Niño and La Niña phenomenon. Depending on stochastic natural variability fluctuations, during La Niña years around 30% more heat from the upper ocean layer is transported into the deeper ocean.
Model studies indicate that ocean currents transport more heat into deeper layers during La Niña years, following changes in wind circulation. Years with increased ocean heat uptake have been associated with negative phases of the interdecadal Pacific oscillation (IPO). This is of particular interest to climate scientists who use the data to estimate the ocean heat uptake.
A study in 2015 concluded that ocean heat content increases by the Pacific Ocean were compensated by an abrupt distribution of OHC into the Indian Ocean.
This animation uses Earth science data from a variety of sensors on NASA Earth observing satellites to measure physical oceanography parameters such as ocean currents, ocean winds, sea surface height and sea surface temperature. These measurements can help scientists understand the ocean's impact on weather and climate. (in HD)
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