Ocean heat content
Oceanic heat content (OHC) is the heat stored in the ocean. Oceanography and climatology are the science branches which study ocean heat content. Changes in the ocean heat content play an important role in the sea level rise, because of thermal expansion. It is with high confidence that ocean warming accounts for 90% of the energy accumulation from global warming between 1971 and 2010.
Definition and measurement
The areal density of ocean heat content between two depth levels is defined as:
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. Multiplying this quantity by the area of an ocean basin, or entire ocean, gives the total heat content, as indicated in the figure to right.
Ocean heat content can be computed using temperature measurements obtained by a Nansen bottle, an ARGO float, or ocean acoustic tomography. The World Ocean Database Project is the largest database for temperature profiles from all of the world’s oceans.
Several studies in recent years have found a multi-decadal oscillation increase in OHC of the deep and upper ocean regions and attribute the heat uptake to anthropogenic warming. 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.
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