Ocean heat content

Ocean heat content (OHC) is the energy absorbed and stored by

Ocean heat content is a term used in physical oceanography to describe a type of energy that is stored in the ocean. It is defined in coordination with a particular formulation of the thermodynamic equation of state of seawater. TEOS-10 is an international standard approved in 2010 by the Intergovernmental Oceanographic Commission.^[16]

Calculation of ocean heat content is closely aligned with that of enthalpy at an ocean surface, also called potential enthalpy. OHC changes are thus made more readily comparable to seawater heat exchanges with ice, freshwater, and humid air.^[17][18] OHC is always reported as a change or as an "anomaly" relative to a baseline.

To calculate the ocean heat content, measurements of ocean temperature from sample parcels of seawater gathered at many different locations and depths are required.^[19] Integrating the areal density of ocean heat over an ocean basin, or entire ocean, gives the total ocean heat content. Thus, total ocean heat content is a volume integral of the product of temperature, density, and heat capacity over the three-dimensional region of the ocean for which data is available.^[20] The bulk of measurements have been performed at depths shallower than about 2000 m (1.25 miles).^[21]

The areal density of ocean heat content between two depths is computed as a definite integral:^[2][20]

${\displaystyle H=c_{p}\int _{h2}^{h1}\rho (z)\Theta (z)dz}$

where ${\displaystyle c_{p}}$ is the

${\displaystyle \rho (z)}$

${\displaystyle \Theta (z)}$

${\displaystyle c_{p}}$

${\displaystyle H}$

In practice, the integral can be approximated by summation using a smooth and otherwise well-behaved sequence of in-situ data; including temperature (t), pressure (p), salinity (s) and their corresponding density (ρ). Conservative temperature ${\displaystyle \Theta (z)}$ are translated values relative to the reference pressure (p0) at h0. A substitute known as potential temperature has been used in earlier calculations.^[22]

Measurements of temperature versus ocean depth generally show an

Ocean heat content measurements come with difficulties, especially before the deployment of the Argo profiling floats.^[21] Due to poor spatial coverage and poor quality of data, it has not always been easy to distinguish between long term global warming trends and climate variability. Examples of these complicating factors are the variations caused by El Niño–Southern Oscillation or changes in ocean heat content caused by major volcanic eruptions.^[10]

Argo is an international program of robotic

Starting 1992, the TOPEX/Poseidon and subsequent Jason satellite series have observed vertically integrated OHC, which is a major component of sea level rise.^[29] The partnership between Argo and Jason measurements has yielded ongoing improvements to estimates of OHC and other global ocean properties.^[26]

Causes for heat uptake

Ocean heat uptake accounts for over 90% of total planetary heat uptake, mainly as a consequence of human-caused changes to the composition of Earth's atmosphere.

Planetary heat uptake or heat content accounts for the entire energy added to or removed from the climate system.

From the perspective of land and ice covered regions, their portion of heat uptake is reduced and delayed by the dominant thermal inertia of the ocean. Although the average rise in land surface temperature has exceeded the ocean surface due to the lower inertia (smaller heat-transfer coefficient) of solid land and ice, temperatures would rise more rapidly and by a greater amount without the full ocean.^[31] Measurements of how rapidly the heat mixes into the deep ocean have also been underway to better close the ocean and planetary energy budgets.^[41]

Recent observations and changes

Numerous independent studies in recent years have found a multi-decadal rise in OHC of upper ocean regions that has begun to penetrate to deeper regions.

Studies based on Argo measurements indicate that ocean surface

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.[49] Additionally, a study from 2022 on anthropogenic warming in the ocean indicates that 62% of the warming from the years between 1850 and 2018 in the North Atlantic along 25°N is kept in the water below 700 m, where a major percentage of the ocean's surplus heat is stored.^[50]

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.^[51]

Although the upper 2000 m of the oceans have experienced warming on average since the 1970s, the rate of ocean warming varies regionally with the subpolar North Atlantic warming more slowly and the Southern Ocean taking up a disproportionate large amount of heat due to anthropogenic greenhouse gas emissions.^[5]: 1230

Deep-ocean warming below 2000 m has been largest in the Southern Ocean compared to other ocean basins.^[5]: 1230

Impacts

Warming oceans are one reason for

The ocean also functions as a sink and source of carbon, with a role comparable to that of land regions in Earth's carbon cycle.^[67][68] In accordance with the temperature dependence of Henry's law, warming surface waters are less able to absorb atmospheric gases including oxygen and the growing emissions of carbon dioxide and other greenhouse gases from human activity.^[69][70] Nevertheless the rate in which the ocean absorbs anthropogenic carbon dioxide has approximately tripled from the early 1960s to the late 2010s; a scaling proportional to the increase in atmospheric carbon dioxide.^[71]

Warming of the deep ocean has the further potential to melt and release some of the vast store of frozen

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References