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Seawater and beaches are the wonders of nature. Populations
from all parts of the world flock to the beaches for
bathing, etc., and enjoy with this mighty nature. After
bathing, however, one feels like having a freshwater
shower to remove the sticky feeling that is due to formation
of a thin layer of salt over the body. While one wishes
to remove it from the body, the salt is an important
constituent in our food that originates from the seawater.
How much is the salt in seawater? Will this resource
also be scarce like other natural resources? Forget
it! There is so much of salt in the water that it is
impossible to really become 'scarce'. It is said, "if
the oceans around continents get dried up completely,
enough salt would be left behind to build a 180-mile-tall,
one- mile-thick wall around the equator!" So no
worry at all. This is because the seawater consists
on an average 3.5% of the salt. Chemically speaking,
90% of this salt is Sodium Chloride - the ordinary table
salt.
Although, the salt concentration in seawater (or salinity
in scientific terminology) is 3.5% as indicated, it
varies slightly from place to place and season to season.
This variation in a given oceanic region is mainly due
to the net effect of evaporation, precipitation, river
runoff and melting of ice. While evaporation of water
increases the salt content, the precipitation (ie. rainfall
over the water body), river runoff into the sea and
melting of sea ice dilute the saltwater and lowers the
salinity. In the earliest years, salinity was measured
using chemical titration of the water sample with silver
nitrate. The best measurements of salinity from titration
give salinity with an accuracy of ± 0.02 psu
(practical salinity units - in which salt concentration
is measured). In this unit, ocean water has an average
salinity of 35 psu.
The advancement in the technology helped developing
an instrument that replaced the titration method of
measuring salinity of seawater. This instrument is called
'CTD profiler'. Each of the alphabets in the 'CTD' stands
for measuring the properties of seawater Conductivity
(C) and Temperature (T) with Depth (D) in the sea. This
instrument is lowered from ships in the seawater to
a desired depth to get profiles of temperature and salinity.
Interestingly, this instrument does not measure salinity
directly, but by measuring conductivity (how easily
electric current passes through the seawater), scientists
can get a measurement of that water sample's salinity
because electric current passes much more easily through
water with a higher salt content. So, if we know the
conductivity of the water, we can calculate how much
salt is there in the water.
Having confirmed that there is enough salt available
for us for future consumption why do we worry of measuring
salt content of seawater at different places? Yes, the
difference in the salinity in the seawater, and its
understanding is very important to all of us. Because,
the circulation of seawater from one place to other
place, formation of watermasses in specific oceanic
regions, the occurrence of EI Nino's in Pacific, even
the monsoon rainfall over India - all that have an important
bearing on the salinity - especially sea surface salinity
(SSS) - of the water at a particular place in the oceans.
In order to know / predict the likely changes in salinity
and particularly the SSS, scientists build models and
simulate the conditions in the sea as salinity is an
important variable along with temperature and pressure.
Though it was possible since long to measure sea surface
temperature (SST) from satellite remote sensing techniques,
the data on salinity poses a problem always and block
in the development of prediction models at micro-level.
In development of models for circulation, etc., the
salinity was chosen from the climatological atlases.
It is no doubt that the World Ocean Atlas does provide
some data on salinity collected in a place by ships
for years. But it is also very scanty. If we check how
much observations existed for 1 x1 grid over global
oceans, one ends up in knowing that only 70% of them
(grids) have some data and some have not been observed
more than once for reconfirmation!
Salinity is especially an important variable for simulating
the tropical sea dynamics. It is no doubt that the American
and European satellite missions that are planned for
the year 2008 would then start providing salinity data.
But this does not prevent enthusiastic scientists to
stop thinking on other alternatives.
The scientists at NIO, along with their colleagues
in other research laboratories, had a breakthrough in
estimating the sea surface salinity using the existing
satellite images data. The new technique uses a set
of algorithms (based on statistical relationships) relating
the atmospheric convection parameter, the Outgoing Longwave
Radiation (OLR), the geopotential thickness of the oceanic
near surface stratified layer (also called effective
oceanic layer - the EOL), freshwater flux (precipitation
minus evaporation), and the climatological sea surface
salinity. They developed algorithms for four sub-regions
in the tropical Indian Ocean. Using all the available
data at hand, they have been able to estimate salinity
at 2.5 x 2.5 grid in the tropical Indian Ocean.
Figure 1 Comparison of the observed
(WOCE), climatological (WOA98), estimated sea surface
salinity, and estimated sea surface salinity with 1-month
lag (a, b) along WOCE I1 section (~8°N in the AS
(left) and ~10°N in the BOB (right)) during 13 September
to 14 October 1995, (c) along WOCE I2 section (~8°S)
during 5 December 1995 to 20 January 1996, and (d) along
WOCEI3 section (~20°S) during 27 April to 5 June
1996. The data points represent the center of the 2.5°
x 2.5° grids along the sections.
The sea surface salinity was estimated for the period
1995-2000 using the OLR data for 22 years (1979-2000).
This was then compared with the simultaneously available
salinity data (Figure 1), collected during World Ocean
Circulation Experiment (WOCE), during Bay of Bengal
Monsoon Experiment (BOMMEX), etc. On the basis of the
results, the seasonal and interannual variability of
the estimated sea surface salinity was also arrived
at.
The technique was found to be of great value in estimating
sea surface salinity in future in the tropical oceans.
The monthly maps of estimated sea surface salinity were
found to be accurate than available model simulations
or maps made from conventional methods. In long run,
this method would be useful to the researchers and modelers
ultimately to understanding the circulation of oceanic
waters on different time scales.
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