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Physical and Dynamical Oceanography: Centre For Atmospheric Sciences Indian Institute of Technology Delhi

The document discusses stratified layers in the ocean. It describes how the ocean typically has a surface mixed layer that overlies a pycnocline layer where density increases rapidly with depth. Below this lies the deep, cold, dense water layer. The mixed layer is typically 10-200m thick in tropical and mid-latitude regions and varies in depth and temperature from day to day and seasonally due to wind and heat/freshwater fluxes. Temperature generally decreases with depth, with the thermocline being the layer of rapid decrease. Thermoclines vary in depth and intensity by latitude and location.

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Abhishek Pratap Singh
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128 views11 pages

Physical and Dynamical Oceanography: Centre For Atmospheric Sciences Indian Institute of Technology Delhi

The document discusses stratified layers in the ocean. It describes how the ocean typically has a surface mixed layer that overlies a pycnocline layer where density increases rapidly with depth. Below this lies the deep, cold, dense water layer. The mixed layer is typically 10-200m thick in tropical and mid-latitude regions and varies in depth and temperature from day to day and seasonally due to wind and heat/freshwater fluxes. Temperature generally decreases with depth, with the thermocline being the layer of rapid decrease. Thermoclines vary in depth and intensity by latitude and location.

Uploaded by

Abhishek Pratap Singh
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Physical and Dynamical Oceanography

(ASL 737)

Dr. Vimlesh Pant

Centre for Atmospheric Sciences


Indian Institute of Technology Delhi
Module - 6
Stratified Layers in Ocean

[Garrison, 2012]

In most of the ocean, a surface zone (or mixed layer) of relatively


warm, low-density water overlies a layer called the pycnocline.
Density increases rapidly with depth in the pycnocline. Below the
pycnocline lies the deep zone of cold, dense water
Oceanic Mixed Layer (ML)

Wind blowing on the ocean


stirs the upper layers
leading to a mixed layer
having constant
temperature and salinity
from the surface down to a
depth.

The mixed layer is roughly 10–200 m thick over most of the


tropical and mid-latitude belts.

The depth and temperature of the mixed layer varies from day
to day and from season to season
Layers in the Ocean (Continued)
ILD (Isothermal Layer Depth) is defined as the depth at
which temperature decreases by 0.5oC from SST.

MLD (Mixed Layer Depth) is defined as the depth where


density has increased from its surface value by an amount
that corresponds to a temperature drop of 0.5oC.

Sometimes MLD also considered the depth where the


density has increased by 0.125 kg/m3 from its sea surface
value.

The depth of MLD is determined by wind speed and the flux


of heat and freshwater through the sea surface
Temperature profile in ocean
Temperature decreases with
increasing depth. The layer in
which temperature falls rapidly is
known as ‘Thermocline’.

Figure: Typical temperature


profiles at polar, tropical, and
middle (temperate) latitudes.
Note that polar waters lack a
thermocline.
Thermocline depth and intensity
also vary with season, local
conditions (storms, for example),
currents, and many other factors.
[Garrison, 2012]
Thermoclines are not identical in all areas or latitudes.

Because the ocean is heated to a greater depth, the tropical


thermocline is deeper than thermoclines at higher latitudes.
It is also much more pronounced

The transition to the colder, denser water below is more


abrupt in the tropics than at high latitudes.

Polar waters, which receive relatively little solar warmth, are


not stratified by temperature and generally lack a
thermocline because surface water in the polar regions is
nearly as cold as water at great depths.
[Talley et al, 2011]
Typical potential temperature (°C)/depth (m) profiles for the open
ocean in (a) the tropical western North Pacific (5°N), (b) the western
and eastern subtropical North Pacific (24°N), and (c) the western
subpolar North Pacific (47°N).
Seasonal and permanent thermocline

Seasonal
thermocline varies
slightly with the
seasons ( 20 and
200 m)
Permanent
thermocline is from
below the seasonal
thermocline
Density and sigma-t of sea water
Calculation of water movement requires measurements of density
with an accuracy of a few parts per million
In practice, density is not measured, it is calculated from in situ
measurements of pressure, temperature, and conductivity using
the equation of state for sea water.
At the sea surface, density is typically 1027 kg/m3
Generally we express density anomaly or Sigma (S,t,p), where
S,t,p are salinity, temperature and pressure, respectively.
σ(S, t, p) = ρ(S, t, p) − 1000 kg/m3

If we are studying surface layers of the ocean, we can ignore


compressibility, and we use a new quantity sigma-t
σt = σ(S, t, 0)
Salinity and Density Profiles in ocean

Density increases with


depth.
Pycnocline is the layer
(beloe mixed layer) where
density rapidly increases
with depth.

Salinity profile is not well


defined. It generally
increases with depth BUT
not necessarily (depends
on the location)

Halocline is the layer


where salinity rapidly
increases with the depth
Courtesy: Sagar booklet, NIO, Goa

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