Seawater
The global ocean is filled with water whose average density is 1.03 g/cm3.
The density is not uniform: it varies in the vertical, with denser water always
below lighter water. The temperature, the quantity of dissolved salts (also
known as salinity), and the pressure to which a parcel of seawater is
exposed determine its density. 75% of the water in the oceans has
temperatures ranging between 0° and 6°C; the average temperature is
3.5°C. Water at depth and near the poles is cold. It gets warmer towards
the surface and towards the equator (Fig. 12). Surface temperature in the
Bay of Bengal is usually between 22°C and 31ºC. It is cooler by 1-2ºC in the Arabian Sea. This difference
has major implications for the
atmosphere above the two
basins.
Salinity is measured as the ratio
of weight of dissolved salts to
total weight; the ratio is usually
expressed as parts per thousand
(ppt). 75% of seawater has a
salinity ranging between 34-35
ppt (Fig. 13). The average
salinity in the 
oceans is 34.7 ppt,
i.e., on an average there is 34.7
g of salt in every kg of seawater.
Salinity near the surface in the
northern Bay of Bengal can be
as low as 31 ppt (Fig. 14)
because the bay receives lots of
freshwater in the form of rain and
from runoff of surrounding rivers
(Ganga, Brahmaputra,
Irrawaddy, Godavari, and
others). If all the freshwater that
the bay receives during a year
is accumulated and spread
uniformly over its entire surface,
it would form a layer over a metre
thick. Salinity near the surface in
the Arabian Sea is much higher
than in the Bay of Bengal
because evaporation over the
Arabian Sea is much greater and
it receives relatively less river
runoff.
We live at 1 atmosphere
pressure. In the ocean, pressure
increases by 1 atmosphere for
every 10 m increase in depth.
This means that the pressure at
the bottom of the ocean of an average depth of 3700 m will
be 370 atmospheres.
Pressure has an influence on
physical (density), chemical,
and biological (decomposition
of shells) properties of the
ocean.
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