The Physical Oceanography is an essential part of the study in oceanography. It is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.

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PHYSICAL OCEANOGRAPHY
By
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science,
University of Mysore,
Mysore-6

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Introduction:
Oceanography is the scientific study of distribution
of ocean waters, marine resources, oceanic
processes, dynamics and their role on global
climate. It encompasses geological, physical,
chemical and biological components and aspects.
The Physical Oceanography is an essential part of
the study in oceanography. It is the study of
physical conditions and physical processes within
the ocean, especially the motions and physical
properties of ocean waters.

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The fundamental role of the oceans in shaping the
Earth is continuously studied by the ecologists,
geologists, meteorologists, climatologists,
geographers and oceanographers.
Great Oceanographic expeditions and explorations
helped to understand the oceans to some extent.
Oceans offer a lot of marine resources, fisheries,
mineral deposits, food and energy resources.
It is necessary to understand the integrated
aspects of oceanography with all scientific facts
and figures.

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In this episode, the following aspects of physical
oceanography are highlighted:
1. The Physical Setting of Oceans
2. Ocean-water properties
3. Fluid mechanics and Ocean dynamics
4. Equations of Motions with Viscosity
5. Numerical Models and land-sea inrefcae

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1. THE PHYSICAL SETTING OF OCEANS:
The Earth has only one larger marine water
mass. It is divided into various oceans, seas, basins
and bays. There are five major oceans on earth as
the Pacific, the Atlantic, the Indian, the Arctic and
the Antarctic Oceans. All these cover about 70.8%
of the surface of the earth.

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The dimensions of oceans may range from 1500
Km for the minimum width of the Atlantic to more
than 13000 Km for its North-South extension. The
surface area of
a) The Pacific Ocean is 181.34 million Sq.Km
b) The Atlantic Ocean is 106.57 million Sq.Km
c) The Indian Ocean is 74.12 million Sq.Km
The oceans are very thin layer of water. The
vertical scale is very small when compared to the
horizontal scale.

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Hence, for all analysis the vertical scale is to be
exaggerated.
The ratio of depth to width of the ocean basins,
is very small. It is very important for
understanding ocean currents.
Vertical velocities must be much smaller than
horizontal velocities.
The small amount of vertical velocities have
great influence on turbulence.

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Bathymetrically, the profile of the oceans are
divided into 3 distinctive regions as
a) Continental Shelf
b) Continental Slope and
c) The deep ocean basins
The depths of the oceans are measured using
acoustic echo-sounders on ships and altimeters
housed in Satellites. Today, all echo-sounder data
are digitized and combined to make maps and
charts, instantaneously.

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Relief Features of the Sea-Floor
The lithosphere of the earth is divided into
continental crust and oceanic crust. The oceanic
crust is thin and dense. It is 10 Km in its thickness.
The Continental Crust is a thick crust and light in
density. It is 40 Km in its thickness. The deep,
lighter continental crust floats higher on the
denser mantle than the oceanic crust. Due to this,
the continents have a mean elevation of 1100m
and the oceans have a mean depth of -4300m.

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The total volume of water in the ocean exceeds
the volume of the ocean basins.
The crust is broken into lithospheric plates that
more relative to each other. These relative
motions create distinctive morphological features
on the ocean floors. They include the mid-ocean
ridges, deep sea trenches, seamounts, rift-valleys,
troughs and island (arcs).

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Almost all these underwater features strongly
influence the ocean water circulations, turbulence
and interrupt ocean currents.
In physical oceanography, sound is used to
evaluate the properties of the sea floor, depth of
the ocean, temperature of oceanic waters and the
dynamics of ocean currents. The speed of sound in
the ocean varies with temperature, salinity and
pressure. Typical sound speed in the ocean is
1480 m/second.

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Mapping of the ocean floor is effectively done
using all the modern tools and techniques. Almost
all the oceans have been thoroughly evaluated to
find out the maximum number of relief features.
AIR-SEA INTERFACE is a very unique zone on
earth.
This includes the atmospheric influences, waves
and their impacts, and the heat budget of the
oceans.

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Atmospheric Influences:
All dynamic processes in the oceans are directly
or indirectly controlled by the Sun’s radiant
energy and the atmosphere. The dominant
mechanisms are the role of Sun light, evaporation
of sea water, infrared emissions from the sea
surface and the cooling (or) warming of ocean
waters. The atmospheric circulations is controlled
by oceans which are the dominant sources of heat.

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The loss and gain of heat by the oceans
produces winds in the atmosphere. Evaporation
and condensation are two more mechanisms. The
uneven Solar heating controls the strength and
direction of winds in the atmosphere. Due to
these, high and low atmospheric pressure zones
are created. Warm and cold fronts are generated
out of these changes. The atmosphere within
100m of the sea surface is influenced by the
turbulent drag of the wind on the sea.

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This is called as the atmospheric boundary
layer. Wind measurements, in seas and oceans,
have been made for several centuries.
Wind reports were made as early as 1855.
Wind speeds were referred to by Beaufort Scale, as
proposed by Sir. F. Beaufort in 1806.
Later, the state of the sea was identified using
the modified Beaufort Scale, of Kent and Taylor.

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Beaufort Wind Scale gives the state of the sea.
Weather observations on ships including the
Beaufort force are measured, four times a day,
everywhere in the world. They are 0000Z, 0600Z,
1200Z and 1800Z where Z is the GMT.
Today, winds at sea, waves, their amplitudes,
orientation speed and direction are measured
using weather buoys Anemometers,
Scatterometeres, microwave radiometers and
special Sensors housed in Ships and Satellites.

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The force of the wind (or) the work done by the
wind are important in oceanographic studies. The
horizontal force of the wind on the ocean surface is
called as the Wind Stress. It is calculated based on
the density of air, wind speed at 10m and the drag
coefficient.
II. OCEAN WATER PROPERTIES:
The ocean water has certain properties which play
a very significant role in the dynamics and on
marine conditions.

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They are the
a) Temperature,
b) Salinity
c) Density
d) Light penetration.
The Heat Budget of the Oceans is an essential
aspect.
About 50% of the Solar energy reaching the
Planet Earth is absorbed by the ocean and land.

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One-fifth is absorbed by the atmosphere of the
energy absorbed by the ocean, most of it is
released to the atmosphere, through evaporation
and infra-red radiation. The balance is transt us
seeported by currents.
Heat Flux is the transfer of heat across the ocean
and the sea surface. The flux of heat and water,
change the density of water, and its buoyancy. It is
estimated that about 4000 joules of energy are
required to heat 1.0 Kg of sea water by 1o C.

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Isolation is the term denoting the incoming
solar radiation. It is determined by latitude,
season, time of the day and cloudiness. The Polar
regions are heated less than the tropics. Areas in
winter are heated less than the same area in
summer and the morning is less than the Noon.
There is no insolation at night.

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Earth gains heat at the top of the tropical
atmosphere, and it loses heat at the top of the
polar atmosphere. The North-South movement is
called as meridonial heat transport.
Flow of heat, evaporation, rain, river water
inflow, freezing and melting of sea ice, all control
the distribution of temperature and salinity of the
oceanic waters. Changes in these two properties
can increase (or) decrease the density of water at
the surface, which can lead to convection.

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When the surface water sinks into the deeper
ocean, it creates pressure variations and generate
ocean currents.
Salinity is the total amount of dissolved solids in
gm per Kg of sea water. It is a dimensionless
quantity. It ranges from 34.6 to 34.8 parts per
thousand. It is also determined based on
conductivity and chlorinity.

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Sea water contains higher concentrations of all
cations and anions, especially sodium and chloride.
Many physical processes happening inside the
oceans are dependent on temperature.
The oceanographers use thermometers calibrated
with an accuracy of a milli degree, which is
0.0010C. The distribution of temperature at the
sea surface tends to be varying and independent of
longitude. Warmest water is near the equator and
the coldest water is near the poles.

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The mean temperature of oceans’ waters is 3.50C
and the mean salinity is 34.7.
The wind movements on the ocean stirs the upper
layers creating a thin mixed layer. Changes in
density generates vertical movement. Turbulence
in the mixed layer mixes the heat downward.
Below the mixed layer, water temperature
decreases rapidly with depth.
The density of water at the sea surface is
typically 1027 Kg/m3.

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The temperature in the oceans is measured
using Thermistors, mercury thermometers, infra
Red radiometers, and Advanced Very High
Resolution Radiometers.
Conductivity and Salinity are measured using
conductivity meters which use Platinum
electrodes and by passing currents.

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Temperature and Salinity measurements with
reference to depth using Bathythermography,
Expendable Bathythermography and Nansen
Bottles.
Light penetration and Absorption of light by
oceanic waters are very important due to various
reasons.
Sunlight heats sea water, Warms the surface
layers, it provides the energy for phytoplanktons
and helps in navigation of animals.

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Phytoplankton change the color of the sea water
which can be measured from the space.
III. FLUID MECHANICS AND OCEAN DYNAMICS:
Ocean waters are subjected to internal and
external forces.
Fluid mechanics, Newtonian mechanics,
conservation of Mass, Conservation of momentum,
angular momentum and conservation of energy all
are involved in the ocean mater dynamics.

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Many equations of Fluid motion are to be derived
from the Conservation Laws.
The approaches employed under these concepts
are:
1. Conservation of mass leads to Continuity
Equation.
2. Conservation of Heat Energy leads to Heat
Budgets
3. Conservation of Mechanical Energy leads to
Wave Equation.

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4. Conservation of Momentum leads to Navier
Stokes Momentum Equation.
5. Conservation of Angular Momentum leads to
Conservation of Vorticity.
The forces which are essential to understand the
Physical Oceanographic processes are:
a) Gravity
b) Friction and
c) Coriolis force.

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Forces are vectors and hence have magnitude
and direction. Gravity is the dominant force. The
weight of water in the ocean generates pressure.
Due to solar lunar influences, tides, tidal currents
and tidal mixing in the oceanic waters happen,
regularly.
Buoyancy is another parameter. It is the
upward (or) downward force due to gravity. It
acts on a parcel of water. Density plays a catalytic
role for enacting the Buoyancy.

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Horizontal Pressure Gradients are also created
due to the varying weight of water in different
parts of the oceans.
Friction of wind blowing over waves, force of
water movement or air movement over water may
create frictional forces. Wind stress is also created
due to this motion.
In addition to these, Pseudo-forces and Coriolis
Force are also acting on oceanic waters.

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Coriolis Force results from motion in a rotating
co-ordinate system.
Next, the Types of Water Flow in the Ocean are to
be understood.
Ocean water circulation includes, origin, and
movements of Waves and Currents.
The following are the types of flows in the oceans:
1. The General water circulation is the
permanent, time-averaged circulation.

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2. The Deep water circulation is driven by
mixing in the deep ocean.
3. The wind-driven circulation is caused by local
winds on the surface of oceans.
4. The Gyres are wind-driven cyclonic (or) anti-
cyclonic currents of giant magnitude.
5. The Boundary Currents are the currents
flowing parallel to the coast.

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6. The Jet streams (or) Squirts are long narrow
currents with dimension of few 100 Km
perpendicular to West Coasts.
7. The mesascale Eddies are turbulent or
Spinning flows, running for a few 100 Km.
In addition to these flows, due to ocean currents,
there are many oscillatory flows due to wave
motion. They are
1. Planetary waves depend on the rotation of
originating due to Restoring force.

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2. The Earth surface waves called gravity waves
originating due to large density contrast
between air and water at the Sea Surface.
3. Internal waves are subsea waves. These are
generated by the restoring force due to change
in density with depth.
4. Tsunamis are surface waves generated by
earthquakes.
5. Tiddal Currents are horizontal currents
associated with internal waves.

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6. Edge waves are surface waves confined to
shallow regions near the shore.
The other essential part of study in Physical
oceanography is the Conservation of Mass and
Salt. This helps in deriving the continuity
equation and understanding the dynamics of
mass and force.

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IV. EQUATIONS OF MOTION WITH VISCOSITY.
The role of friction in fluid flows and the
stability of the flows to small changes in Velocity
(or) density are to be understood in Physical
Oceanography. Viscosity is the tendency of a fluid
to resist shear.

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The influence of viscosity, especially the molecular
viscosity which comes due to striking of water
with solid mass, is to be analyzed. The
components of Stress at a point in a fluid having
pressure include normal stress and other shear
stresses.
Turbulence is an important aspect in oceanic
waters. Molecular viscosity is also effected by
turbulence.

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Turbulence in the oceans lead to mixing. Because
the ocean has stable stratification, vertical
displacement must work against the buoyancy
force. Vertical mixing requires more energy than
horizontal mixing. The vertical mixing is very
important in oceans. As it brings up the deep
waters in some regions upward.
Vertical Mixing is driven by the deep-ocean tidal
currents.

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In oceanic waters, stability is yet another
parameter to be understood. Many kinds of
instability occur in the water masses of oceans.
The kinds of stability observed are:
a) Static Stability-associated with change of
density with depth.
b) Dynamic Stability –associated with velocity
shear and

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c) Double –diffusion associated with Salintiy
and temperature gradients in the oceans.
Application of Ekman Theory is an important
aspect in this context.
Steady winds blowing on the sea surface produce a
thin, horizontal boundary layer called Ekman
Layer. This layer may be of few hundred meters
thick.

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Because steady winds blowing on the oceanic
surface create an Ekman Layer that transports
water at right angles to the wind direction and
subsequently generate upwelling. This upwelling
enhances biological productivity to feed the fishers
and other marine life. There is also a Deep Ocean
Ekman layer near the sear floor. A current that
spiral around an axis parallel to the wind direction
is called as Langmuir circulation.

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The role of Geostrophic currents is yet another
mechanism in physical oceanography.
In the mid oceans, the horizontal pressure
gradients exactly balance the coriolis force
resulting from horizontal currents. This is known
as geostrophic balance. The forces acting on the
vertical dimensions are the pressure gradient and
the weight of the water. The geostrophic balance
requires that the coriolis force balance the
horizontal pressure gradient.

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If the ocean is stratified, the horizontal pressure
gradient may include
a) pressure gradient due to the slope at the sea
surface and the
b) pressure gradient due to the horizontal density
differences.
Satellite altimetric observations of the oceanic
topography can give the time-variant data surface
geostrophic currents.

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Hydrographic data are used to calculate the
internal geostrophic currents in the ocean. Flow in
the ocean that is independent of depth is called
barotropic flow and the flow that depends on
depth is called as baroclinic flow.
Baroclinic flow can be determined by using
hydrographic data.
The next aspect is the Wind Driven Ocean
circulation.

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It is customary to think that ocean currents are
driven mainly by the winds. Currents and ocean
circulations are driven by not only the winds but
also other factors and forces caused by site,
location and space on the globe.
The theory for wind-driven geostrophic currents
was studied by the scientists Sverdrup, Stommel
and Munk between 1947 and 1951.

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There are several types of ocean currents existing
in the oceans in addition to Gulf stream, which are
accelerated by turbulence.
Most of the fluid flows from bathtubs to swimming
pools are not rotating.
In the oceanic waters, rotations and the
conservation of vorticity stongly influence flow
over distances exceeding a few tones of kilometers.

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Vorticity is the rotation of the fluid. There are two
types of vorticity as Planetary vorticity and
Relative vorticity.
The sum of these two is known as Absolute
vorticity.
Vorticity strongly influences ocean dynamics.
The curl of the wind stress adds relative vorticity
to central Gyres of each ocean basin.

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The Deep circulation in the ocean is also an aspect
to be understood.
The direct forcing of the ocean water circulation by
wind is effective only in the upper part of the
oceanic water column. Below this layer, there is a
vast water mass extending upto 4 to 5 km. This
water mass is cold with a temperature range of
4°C. Deep mixing pulls the water up through the
thermocline over large regions.

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It is this upward mixing that produces deep
circulation in the ocean. Since it happens in the
abyssal plain, it is called as abyssal circulation.
Deep circulation circulation carries heat, salinity,
oxygen, Co2 and other properties from high
latitudes in winter to lower latitude throughout
the world. It has many effects on oceans.

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The contrasting nature of cold deep water and
warm surface water determines the stratification
of the ocean. This strongly effects the ocean
circulation.
The volume of deep ocean water is far longer in
volume than the volume of surface water.
All these factors have a great role to play in
controlling the global climate and the earth’s heat
budget.

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The Equatorial zone in the oceans experience
certain unique impacts and processes.
Role of solar radiation on oceanic water masses is
enormous. The global weather patterns are
controlled by equatorial processes. Evaporation of
water from the oceans and that water condenses
as rain. During these processes energy is playing
its dominant role.

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Solar heat is the driving engine for evaporation of
ocean surface waters.
It is seen that oceans receive 5 meters of rainfall
per year. Five m rain/year releases an average of
400 w/m2 of heat to the atmosphere.
The Equatorial currents moderate the air-sea
interactions, through the phenomena known as El-
Nino (the child Jesus), which are found to be
counter currents along Peruvian coasts.

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The east-west temperature gradient on the
equator drives a zonal circulation in the
atmosphere. This is called as Walker circulation.
El Nino denotes a disruption of the entire
equatorial system along the Pacific which causes
the changing weather patterns around the globe.
The term El Nino is related to the southern
oscillation (ENSO).

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The term La Nina referes to the positive phase of
the Oscillation when trade winds are strong and
equatorial water temperature is cold, along
eastern regions. El Nino is a sea-level pressure
anomaly in the eastern equatorial Pacific Region.
The tropical and equatorial Pacific is a vast ocean.
It is a remote area which is seldom visited by
ships.
El Nino causes the biggest changes in equatorial
dynamics.

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During El Nino, trade winds weaken in the western
Pacific, the thermocline becomes less deep in the
west. This generates the Kelvin wave eastward
along the equator. These are the largest sources of
annual fluctuations in global weather patterns.
Due to these drought occurs in the Indonesian area
and Northern Australia. Similarly, floods occur in
Western, tropical South America. [Ocean currents,
winds and weather patterns are all related to each
other, along the equator in the Pacific ocean].

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V. NUMERICAL MODELS & LAND-SEA
INTERFACE
Numerical models are employed to understand the
ocean dynamics.
Normally, it is very difficult to describe the ocean
dynamics from mere measurements and display of
data. Numerical models may provide useful and
global view of ocean dynamics with some level of
accuracy and reliability.

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Numerical models of ocean currents can help
simulate the flows in realistic ocean basins.
Numerical models use algebraic approximations to
the differential equations governing the oceanic
parameters.
There are several kinds of Numerical models
applied to solve and analyse oceanographic
problems.

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The mechanistic models are used to analyse the
Planetary waves, interaction of the flow with sea-
floor features, or the response of the surface of the
ocean to the wind blow.
Simulation models help analyzing the changes in
density due to fluxes of heat and water and the
thermodynamic processes.
Ocean and atmospheric models are used to analyse
the air-sea interaction with reference to space and
time.

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Climate models are used to analyse the large-scale
hydrographic conditions, climate dynamics and
water-mass function.
Coastal models evaluate the processes of coastal
zones, currents, tides and storm surges.
The storms which drive large changes of sea level
at the coast, are called as storm surges. Surges can
cause great damage to coasts and coastal
structures.

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Storm-surge models analyse the storms coming
ashore across wide, shallow continental shelves.
Many models produce output in terms of data on
current velocity or surface topography. These are
called as Assimilation models.
There are also coupled ocean and Atmosphere
models developed to study the climate, its
variability and the impacts of external forces.

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Numerical models can be used to handle real-time
oceanographic data from ships and satellites to
produce forecasts of oceanic conditions, including
El Nino in the pacific and the Gulf streams of the
Atlantic.
Models are forming the Prototype tools to
understand the situation effectively and may help
in reasonal conclusions.
Ocean waves are powerful systems shaping and
reshaping the coastal zones.

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Waves have energy.
When we look at the sea from above, we can see
waves and waves on the sea surface. These are
undulations of the sea surface with a height of
around a meter. Each wave has a wave height
which the vertical distance between the bottom of
the through the top of the wave called crest. The
wave length is the distance between two
prominent adjacent crests. The wavelength may
range from 50 to 100 m.

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When we watch these waves for a few minutes, we
can notice that the wave height and wave length
are not constant. They vary in space and time..
Wave period is the time it takes two successive
wave crests (or) troughs to pass a fixed point.
All offshore waves are generated by wind. It is
also true that the sea level changes from hour to
hour. Sea level increases and decreases during a
day. The slow rise or fall of the sea level is due to
the tides.

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Tides have wavelengths of 1000s of km. They are
generated by the slow, very small changes in
gravity as influenced by the moon and motion of
the sun.
In physical oceanography, a detailed study of the
ocean waves, tides and their impacts on the coastal
zone, is needed.
Waves are the natural and outstanding phenomena
of the oceans.

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Wave phenomena involve the transmission of
energy and monsentum by means of vibratory
impulses.
For ocean-surface waves, the direction of
propagation is perpendicular to the wave crests in
the positive direction. Ocean waves propagating
over great distance are dispersive.
Ocean waves are produced by wind. The faster the
wind, the longer the wind blows.

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The bigger the area over which the wind blows, the
bigger the area over which the wind blows, the
bigger the oceanic waves generated.
The turbulence in the wind produces random
pressure fluctuations at the sea surface. These
produce small waves. Their wavelengths may be
of a few centimeters. Then the wind acts on these
small waves causing them to become larger.
Gradually, the waves grow bigger and bigger.

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Finally, these waves began to interact among
themselves to produce longer waves. Sometimes,
these waves may be moving faster than the wind
also.
Observations by mariners on ships and satellites
using sensors and altimeters help to prepare the
global charts and maps of wave heights. In
addition to the wave heights and movements, the
directions are also detected by the modern
sensors.

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In addition to these, it is also necessary to
understand the Coastal Processes and Tides.
In Physical oceanography, the following are
interesting and important observations prompting
us to study in detail:
a) Transformation of waves as they come nearer
to the shore and break.
b) Currents and edge waves generated by the
interaction of waves with coasts
c) Impacts of Tsunamis, storm surges and tides.

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Oceanic waves have phases, velocities, wave
periods and wave frequencies. Waves change
direction due to refraction. Waves break if the
water is sufficiently shallow. The broken waves
pour water into the surf-zone, creating long-shore
and rip currents. Rip currents are dangerous
things for the swimmers.
Edge Waves are produced by the variability of
wave energy reaching the shore.

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Tsunamis are yet another issues in oceans and
coastal regions. The impacts of tsunamis are very
devastating. The recent Tsunamis have made the
whole world population to think very seriously
about natural disasters.
Tsunamis are low-frequency ocean waves
generated by underwater earthquakes. They are
called as seismic-sea-waves.

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Sudden motion of sea floor over distances of a
hundred or more km generated Tsunamis with
periods of 15-40 minutes. These may propagate at
a speed of 180 m/s with a wave length of 130 km
in water. Tsunami’s may be 3.6 km deep and not
noticeable at the seal surface. After nearing the
coast, the surge may rise upto 10 or more meters
above the sea level.

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A Tsunami can travel thousands of kilometers and
do serious damage. The first wave of a Tsunami is
not likely to be the biggest.
Many a time, Storm Surges also play a very unique
role.
Storms blowing over the shallow continental shelf
regions pile water against the coast. The increase
in sea level due to this pile up is known as storm
surge.

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The factors responsible for storm surges are:
1. Ekman Transport by winds parallel to the
coast.
2. Winds blowing towards the coast
3. Edge waves traveling along the coast
4. Low pressure in the region.

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Ocean Tides:
Oceanic Tides produce strong currents in many
parts of the ocean. Their speed may go upto 5
m//s in the coastal waters.
Oceanic tides have a great energy. Waves dissipate
much tidal energy.
The study of physical oceanography helps in
understanding all these aspects in detail.