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VERTICAL STRUCTURE OF THE OCEAN – from Knauss Chapters 1 and 2
Oceanographers divide the ocean into zones
Biological oceanographers refer to the ocean environment, in its totality, as the
pelagic zone. They divide that into the surf, or neritic zone, and the oceanic
zone. Further, the ocean bottom is referred to as benthic zone.
Biological oceanographers also classify the ocean based on light intensity:
euphotic zone = surface to ~200 m (abundant light)
disphotic zone = 200 m to 1000 m (minimal light)
aphotic zone = deeper than 1000 m (no light)
Light attenuation, via the processes of absorption and scattering, is
responsible for decreasing light intensity with increasing depth
At the same depth, light intensity in productive seawater (lots of
plankton, and thus lots of scattering) is smaller compared to that in
nonproductive seawater
THE DENSITY PROFILE (FIGURE 2.4 IN KNAUSS)
Density increases with depth, this stabilizes the water column
and inhibits vertical transport of water and its components
(salt, nutrients, suspended particles (phytoplankton), etc.)
IMPORTANT POINTS ABOUT SEAWATER DENSITY STRATIFICATION
1) Solar absorption enforces the density stratification by increasing water
temperature in the topmost layer and thus decreasing its density
OTHER IMPORTANT POINTS ABOUT SEAWATER DENSITY STRATIFICATION
3) Rain (precipitation) decreases salinity, this enforces the density stratification
4) Emitted longwave radiation cools the surface water, increasing its density, and
can thus defeats the stratification
5) Evaporation decreases the surface water temperature, increases salinity, and
also defeats the stratification. When acting with emitted longwave, e.g.
during wintertime, a relatively deep mixed layer is produced
6) The base of this mixed layer is characterized by a region where the temperature
decreases rapidly with increasing depth. This is called the thermocline (see
Figure 1.5a in Knauss)
ANOTHER POINT ABOUT SEAWATER DENSITY STRATIFICATION
8) Below the thermocline water temperature is nearly constant, both with
season and latitude
HOW WE KNOW DENSITY – THE SEAWATER EQUATION OF STATE
Oceanographers routinely measure salinity, temperature and pressure (S,t and p)
Seawater density is derived from S,t and p via the equation of state
Conceptually, the equation of state is simple:
Density is increased by decreased temperature or by increased pressure and salinity
Quantitatively, the equation of state is a pain.
More quantitatively1)The pressure effect on density (a.k.a., compressibility) is dominant and relatively
easy to account for (hydrostatics).
2)The temperature effect on density (a.k.a., thermal expansion) is smaller, and is
variable in space and time
3)The salt effect on density (a.k.a., saline contraction) is smallest, and can also be
dependent on location and time of year
The Equation of State - This describes the functional
dependence of density (r), salinity (S), temperature (t) and
pressure (p)
r S ,t , p  f ( S , t , p )
Sigma or the density anomaly -
 S ,t , p  r S ,t , p  1000
(2.2)
Integration of the equation of state along an adiabatic path, starting at the
actual state defined by S,t and p and ending at the surface, yields the
Potential temperature of seawater -

Potential temperature is a virtual property, it represents what the
temperature would be if water at depth ascended adiabatically.
Potential density - Like potential temperature, this is a virtual property. It
represents what the density would be if water defined by S,t and p
ascended to the surface adiabatically
r   f ( S , ,0 )
Potential density anomaly - This follows from the definition of the density
anomaly (2.2)
   r   1000
Surface density and surface density anomaly are defined similar to
potential density and potential density anomaly.
r t  f ( S , t ,0 )
 t  r S ,t , 0  1000
(2.3)
Note:
1)The discussed properties are dependent on the thermodynamic state of
the water which is defined by measurements of S,t and p
2)Since S,t and p vary vertically, so do the derived properties (density,
density anomaly, potential temperature, etc.)
3)Surface density anomaly is only slightly smaller than potential density
anomaly (see Figure 2.4 of Knauss)
4)The difference between temperature and potential temperature
decreases with decreasing depth (see Figure 2.4)
1)
Density anomaly decreases with decreasing depth, from S,t,p=54 kg/m3 at 5.5 km to  S,t,p
=24 kg/m3 at the surface
2)
Potential density (  ) and surface density ( t ) anomalies are uniform below 0.4 km.
These properties represent the density of seawater independent of the compressibility
3)
Above 0.4 km two opposing effects determine the density profile (increased temperature
and increased salinity; the former usually dominates)
Summarizing 1.
The oceans are stably-stratified (density increases with increasing
depth)
2.
Because of density stratification, upwelling/downwelling only occurs
if there is "forcing" (i.e., wind, evaporation, radiative cooling).
3.
Instabilities also supplement vertical transport via upwelling and
downwelling. These instabilities are isopycnal mixing instability
and double diffusion instability.
4.
Above the thermocline, water properties vary seasonally in response
to solar absorption, thermal emission, precipitation, etc