Calculation of ocean heat content follows that of
enthalpy referenced to the ocean surface, also called
potential enthalpy. OHC changes are thus made more readily comparable to seawater heat exchanges with ice, freshwater, and humid air. OHC is always reported as a change or as an "anomaly" relative to a baseline. Positive values then also quantify ocean heat uptake (OHU) and are useful to diagnose where most of planetary energy gains from global heating are going. To calculate the ocean heat content, measurements of
ocean temperature from sample
parcels of seawater gathered at many different locations and depths are required.
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. H= c_p \int_{h2}^{h1} \rho(z) \Theta(z) dz where c_p is the
specific heat capacity of
sea water, h_2 is the lower depth, h_1 is the upper depth, \rho(z) is the in-situ
seawater density profile, and \Theta(z) is the
conservative temperature profile. c_p is defined at a single depth h0 usually chosen as the ocean surface. In
SI units, H has units of
Joules per square metre (J·m−2). 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 \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. Measurements of temperature versus ocean depth generally show an
upper mixed layer (0–200 m), a
thermocline (200–1500 m), and a
deep ocean layer (>1500 m). These boundary depths are only rough approximations. Sunlight penetrates to a maximum depth of about 200 m; the top 80 m of which is the
habitable zone for photosynthetic marine life covering over 70% of Earth's surface. Wave action and other surface
turbulence help to equalize temperatures throughout the upper layer. Unlike
surface temperatures which decrease with latitude, deep-ocean temperatures are relatively cold and uniform in most regions of the world. About 50% of all ocean volume is at depths below 3000 m (1.85 miles), with the
Pacific Ocean being the largest and deepest of five oceanic divisions. The thermocline is the transition between upper and deep layers in terms of temperature, nutrient flows, abundance of life, and other properties. It is semi-permanent in the tropics, variable in
temperate regions (often deepest during the summer), and shallow to nonexistent in polar regions.
Measurements Ocean heat content is derived from ocean temperature measurements. Ocean temperature measurements come with difficulties, especially before the deployment of the
Argo profiling floats. The program's initial 3000 units had expanded to nearly 4000 units by year 2020. At the start of each 10-day measurement cycle, a float descends to a depth of 1000 meters and drifts with the current there for nine days. It then descends to 2000 meters and measures temperature, salinity (conductivity), and depth (pressure) over a final day of ascent to the surface. At the surface the float transmits the depth profile and horizontal position data through
satellite relays before repeating the cycle. Starting 1992, the
TOPEX/Poseidon and subsequent
Jason satellite series altimeters have observed vertically integrated OHC, which is a major component of sea level rise. Since 2002,
GRACE and GRACE-FO have remotely monitored ocean changes using
gravimetry. The partnership between Argo and satellite measurements has yielded ongoing improvements to estimates of OHC and other global ocean properties. ==Causes for heat uptake==