Metocean1_ Ocean.pdf

7/27/2019 Metocean1_ Ocean.pdf

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Oceans


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Oceans & Numbers

The earth can be considered as a spherod with a total volume of about 1021m3and a total surface of about 5.1 1014m2. It is slightly flattened at the poles and

bellied around the equator. Polar radius (6357 km) is 21 km smaller than theequatorial radius (6378 km).

Oceans cover an area of about 3.6 1014m2, that is 70.8%, while continents cover about 1.5 1014m2 (29.2%).The distribution of oceans is not even. They account for about 60.7% of the total surface in the northern hemisphere and for about80.9% in the southern hemisphere.

LatitudeSouth

LatitudeNorth

EARTH OCEAN

106 km2

Tchernia, 1978


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The average depth of the oceans is 3.8 km. That is about 1/1700th of the earths radius (6400 km).Oceans total volume is about 1.4 1018 m3 . It is about 14 times the volume of the emerging continents.

The four main oceanic basins are Pacific, Atlantic, Indian and Arctic. In 2000, the International Oceanographic Association added a fifth

one, the Austral ocean, including all waters south of the 60S parallel.


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Structure of the Oceanic Basins

Continental Shelf: submerged area close to the continent, with

about the same slope as the neighbouring emerging area andlimited off-shore by a sharp change of slope. Their maximumdepth is in a range between 100 and 300/400 metres (150 m onaverage). They can extend to a distance of about 600 km to 800km from the shore. Average extension is 350 km with a slope of0.4%.The relief of the shelf is often similar to the one of the

neighbouring continent. It can be very narrow along shoreslimited by mountains.

Continental slope : Topographic break between continentalshelf and deep ocean bottom ( 3000 to 6000 m depths).With an average slope of about 3 to 3.5 % (locally up to 25%).

Continental rise : Region with milder slope in which thecontinental slope grades into the abyssal plain.

Deep oceanic floor (Abyssal plain) : The bottom surface ofthe ocean, with a very mild slope (0.05% ~1m/km). Mostlycovered with sediments their average depth varies between 3000and 6000 m.

Oceanic Trenches : Below 7000 m (The deepest : 11035 mMarianes Trenches).Limited areas usually near continents or areas with underwatervolcanic activity.

Mid-Ocean ridges : At the boundaries of divergent tectonicplates, rising above the abyssal plain.


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Depth (m) Percentage of total oceans area Name

0 to 200 7.6% Continental Shelf

200 to 10001000 to 2000

2000 to 3000

4.3%4.2%

6.8%

Continental Rise (15.3%)

3000 to 40004000 to 50005000 to 60006000 to 7000

19.6%33.0%23.3%1.1%

Deep Ocean Floor (77.0%)

> 7000 0.1% Oceanic Trenches

Distribution of oceanic depths (Tchernia, 1978)

West-East section of the surface of the earth from Pacific to Atlantic

Vertical amplification X100 (The Open University, 1989)

Note : One has to remember that, in spite of this static description, shape and structure of oceans basins are evolving in time. Shapealteration is mostly due to the slow motions of the lithosphere plates (Indian and Atlantic Oceans are enlarging while Pacific is shrinking).Bathymetry is also changing, because of sediment layering and erosion induced by currents and also because of isostatic adjustments.


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CONTINENTAL SHELF IN THE BAY OF BISCAY


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Sea Water

97% of the water on earth is found in the oceans.Average water temperature in the oceans is 3.6CAverage salinity is 34.7 g/l.

Density of sea water is modified by salinity and, mostly, by temperature. It is also affected by pressure so that these three parameters areresponsible for a large part of the oceans circulation and mostly vertical motions (more dense cold waters will sink towards the bottomwhile warmer and less salty waters, having less density, will stay near the surface).

Sea Water salinity

Sea water salinity is measured in Practical Salinity Unit (PSU) which corresponds to the mass of salt (in grams) per kilogram or litre ofwater.Main dissolved salts are : NaCl (78%), MgCl2 (11%), MgSO4 (5%), CaSO4 (4%), K22SO4 (2%). One also finds rare elements, (Si, Sr, Al,

Au) as well as dissolved gas (O2,N2,CO2)

Process of salts dissolution in oceans waters took place over hundreds of millions of years. Major sources for these dissolved elements

are:- Erosion of soils and rocks,- Volcanic gas and hydrothermal activity,- Biological processes of consumption, alteration and sedimentation on the bottom of oceans.


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The vertical structure in the ocean basins, can be globally dividedinto three layers, except at the upper latitudes in the north andsouth. This structure is mostly driven by the density of the water.

In the upper layer, or mixed layer, which extends at depthbetween ~100m and ~300m, water is moved under the action ofwinds creating surface currents.In the intermediate layer (pycnocline), down to about 1000m, thewater density increases rapidly with depth as water temperaturelowers.

The lower layer or deep layer, which represents the mostimportant part of the ocean, is more stable and cold.

DEPTH (m)

NORTH SOUTHLATITUDE60 40 20 0 20 40 60

0

400

800

120

0

160

0

200

0

Mixed Layer

Pycnocline

Deep Layer


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Oceans circulation

Oceans are continually in motion under the combined action ofvarious elements :

- Solar radiation,- Earths rotation,- Water density, salinity and temperature,- Gravitational forces

This circulation can by divided into two main components : the

surface circulation, mostly wind driven, and the deep oceanscirculation due to density, temperature and salinity variations(thermohaline).

Wind driven Currents

Solar radiation above earths surface is uneven yielding uneven distribution of temperatures, hence atmosphere density and pressure.Balance between the high and low pressures is the main cause for winds.


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Friction of the air on the surface of the sea creates ripples (waves) which in turn induce enough roughness to create a coupling betweenwater and wind. If the wind blows for a long time, its action will initiate displacement of a volume of water which will keep travelling evenafter the wind stops blowing. This is purely inertial motion.

Since the earth is rotating, this volume of water is submitted to the Coriolis force and the equation of motion can be written in the form :

guz

p

tw

uy

p

tv

vx

p

tu

cos21

sin21

sin21

where p is the pressure, is the latitude and (rad/s) is the rotation of the earth.

We assume that :

- The coriolis force is the only force acting on the water, hence there is no horizontal pressure gradient and 0

y

p

x

p

- The flow is horizontal

So that equations become :

Cuutv

Cvvtu

sin2sin2

where sin2C is the Coriolis parameter

Solving these coupled differential equations, one obtains a solution in the form :

vuV

CtVvCtVu

22

cossin


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This solution of the inertial current is actually the parametric equation for a circle of diameterCVD 2 and period

CT 2 so that such inertial

currents will be observed as large eddies travelling in the ocean. They are deflected to the right of the wind direction in the NorthernHemisphere (clockwise rotation) and to the left of the wind direction in the Southern Hemisphere (counter-clockwise rotation).

Such currents which are created by rather quick wind changes will also decay rapidly (within a few days).


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We now take friction into account, and consider that the volume of water is placed under the combined action of friction forces andCoriolis force.

Introducing the wind stress in the x and y directions :

z

u

KT zxz

,z

v

KT zyz

where Kz is the vertical eddy viscosity, which we

assume here to be constant, and is the water density and assuming that the flow is horizontal, homogeneous and steady so that

0

yxt, momentum equations reduce to :

0

0

2

2

2

2

z

vCu

z

uCv

K

K

z

z

where C is the Coriolis parameter

Solutions of this set of equations will be in the form :

4cos

4sinexp2

4sin

4cosexp2

a

z

a

z

a

z

aCv

az

az

az

aCu

TT

TT

yzxz

yzxz

whereC

aK

z2

From this results, it can be seen that:

- on the surface (z=0), the current travels at an angle of 45

to the wind direction (to the right of the wind in the

Northern Hemisphere, to the left in the SouthernHemisphere),

- the velocity of the current decays exponentially with depth,- the orientation of the current varies with depth: a layer will

deflect to the right of the overlying layers movement in theNorthern Hemisphere (to the left in the SouthernHemisphere)

EKMAN Spiral


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Depth=6m Depth=550 m

Mercator Model

The total mass transport in the Ekman layer is obtained bycomputing the integral of the Ekman velocity from the surface toa depth dat the lower limit of the layer. Under this theoretical

approach and with the given assumptions, it can be shown thatthis mass transport has an angle of 90 to the wind direction.

As a result from Ekman transport surface water is piled up insome areas of the ocean and removed from other areas,producing variations in the height of the sea surface, causing it toslope gradually. One consequence of a sloping ocean surface isthe generation of horizontal gradients in water pressure. Thesepressure gradients, in turn, give rise to geostrophic current.


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Geostrophic Equations

Under the assumption of a stationary flow 0

tw

tv

tu and neglecting viscous effects (friction), there is a balance between pressure

forces and Coriolis forces on an horizontal level. This balance is called the Geostrophic balance and will induce a flow oriented to the rightof the pressure gradient force in the Northern Hemisphere and to the left of the pressure gradient force in the southern Hemisphere :

x

p

Cv

ypCu

1

1

where C is the Coriolis parameter and P the pressure gdzzph

0

Credits Ssalto/Duacs.


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Deep Ocean circulation

The density of ocean water is not globally homogeneous, but varies significantly . As cold wind blows across oceans at high latitudes, itproduces evaporation which lowers the water temperature while increasing its salinity (only pure water is evaporated). These largemasses of cold and salty waters, denser, will sink to lower depths in the oceans. At lower latitudes, mixing with warmer less salty massesof water will create overturning that will bring back this cold denser water to the surface. This circulation once referred to as thermohalineis now defined as meridional overturning circulation. It is not just mass circulation, it also carries heat, salt, oxygen

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