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Diapycnal: definition.
In the ocean, density always increases with increasing depth. However,
surfaces of constant density (called "isopycnals")
are not always level. Wind, the rotation of the Earth, and other processes cause
density surface to slope. For example, a wind blowing along the coast can cause
cold water which is usually deep to rise up to the surface, leading to water that
is too cold to swim in, even in summer. The direction at right angles to the local
isopycnal surface is called the “diapycnal” (i.e., across-isopycnal)
direction. The angle between the diapycnal direction and vertical (i.e., directly
out from the Earth) is always very small, a fraction of a degree at most. However,
the difference between the two coordinates is very important to oceanographers,
because the slope of isopycnals tells us a great deal about how the ocean is moving,
and how it can interact with the coast and the atmosphere.
Who cares? Numerical models of the ocean are used to predict how the
ocean and the coupled ocean/atmosphere will change in the future. Models depend
on how mixing in the ocean is represented. Mixing occurs at scales of an inch
or less and on time scales of a few seconds, whereas climate models only resolve
space and time scales of 10-100 miles and hours-to-days, respectively. So, we
need to devise ways to include mixing in our climate models, even though it can’t
be resolved. We call this process “parameterization”. The natural
coordinates for mixing are diapycnal and “isopycnal” (diapycnal mixing
requires energy to keep it going, whereas isopycnal mixing doesn’t), yet
many models are coded in terms of the more familiar “vertical” and
“horizontal” coordinates. Unless the method used to relate diapycnal/isopycnal
mixing to the model’s coordinate system is accurate, the model will produce
incorrect results when used to predict the future state of the ocean.