Download MAIN MECHANISMS THAT GENERATE THE OCEAN MOTION

MAIN MECHANISMS THAT GENERATE THE OCEAN MOTION
The ocean is permanently moving, with scales from the large
currents to the small eddies. The very beginning of all those
motions is the solar radiation and the Earth rotation
Contribuition of the solar radiant energy:
• The Sun influences the ocean circulation through the atmospheric circulation.
The energy is transfered from the wind to the upper layers of the ocean
through the frition between the atmosphere and the sea surface → wind driven
circulation
• The Sun causes variations in the temperature and salinity of the sea water.
These control the density. Variations in the temperature are caused by heat
fluxes through the air-water interface. Variations in the salinity are caused by
the addition or subtraction of fresh water through precipitation, evaporation, or
ice-water melting in the polar regions. When the surface water becomes denser
than the underlying water, it becomes unstable and it sinks.This circulation is
driven by the density → thermohaline circulation
Large scale ocean currents
•
•
Surface currents
– Affects the upper layer, above the picnocline (~10% of the ocean)
– They are consequence of the atmospheric wind gyres
Deep currents
– Affects the deep water, below the picnocline (~90% of the ocean)
– They are consequence of the density differences in the seawater
– They are broader and slower than the surface currents
The surface ocean circulation clearly follow the general circulation of the atmosphere
Wind pattern over the Atlantic
Circulation pattern in the Atlantic
Wind driven circulation
⇒
Surface current system of the World Ocean
Mean wind field at the Earth surface and the
position of the Intertropical Convergence
Zone (ITCZ) in (a) July and (b) January.
Deep circulation (thermohaline circulation)
- The origin of the deep currents are in the sub-polar regions, when the denser
waters sinks
- In the deep ocean, the waters are cold, calm, dark, non-productive, with few
living organisms and subject to high pressures.
The deep currents are
identified by the measurement
of the temperature and salinity,
from which density is derived.
.
Atlantic water
masses
Atlantic thermohaline
circulation
Meridional section of the Atlantic Ocean, showing
the motion of the main water masses
The importance of the ocean circulation in
the redistribution of energy in the Earth:
Conveyor-belt circulation
MAIN MECHANISMS THAT GENERATE THE OCEAN MOTION
Contribution of the Earth rotation:
A missile launched from the equator to the north moves not only with it firing velocity,
but also to the east, as the surface of the Earth. As it moves northward, the eastward
velocity of the earth beneath it becomes less and less, because v=Ωr, Ω=constant and r
diminishes with the latitude. As a result, in relation to the Earth, the missile moves not
only to the north, but also to the east (to its right). The same applies if the missile is
fired southward, in the northern hemisphere: in relation to the Earth, it moves not only
to the south, but also to its right (westward). The same happens to the water parcels
moving in the ocean (or air in the atmosphere) ⇒ effect of the apparent force called
Coriolis force.
The Coriolis force is an apparent force that acts on all the objects that are moving over
the surface of the Earth. It acts in a 90º angle 'cum sole‘ → to the right in the northern
hemisphere and to the left in the southern hemisphere The Coriolis force is null at the
equator and increases with the latitude, reaching the maximum value at the poles.
Horizontal component of the Coriolis force: m2ΩsinφVH=mfVH, with f = Coriolis
parameter. Notice: the Coriolis force increases with the velocity of the current!
MAIN MECHANISMS THAT GENERATE THE OCEAN MOTION
The effect of the Earth rotation
(a) A missile launched
northward from the
equator moves to the
east, as the Earth, and
to the north with the
firing velocity.
(b) The trajectory of the missile in
relation to the Earth. In the time T1 the
missile has moved to M1 and the Earth
to G1. In the time T2 the missile has
moved to M2 and the Earth to G2.
Notice that the deflection caused by the
Coriolis force (difference M1 - G1 and
M2 - G2) increases with the latitude.
The bicycle wheel does not
rotate at the equator. It starts
to rotate clockwise, in relation
to the Earth, as it is displaced
poleward. The rotating velocity
increases as it gets close to
the pole.
MAIN MECHANISMS THAT GENERATE THE OCEAN MOTION
To study the dynamics of the motions in the Ocean, we make use of the
following laws:
• Conservation of mass;
• Conservation of energy (1st law of Thermodynamics);
• Newton's 1st Law (if no force is applied to an object, it will not change its
state of motion);
• Newton's 2nd Law (the change in the velocity of an object is directly
proportional to the sum of the forces applied to that object);
• Newton's 3rd Law (when one object exerts a force on another object, this
one exerts a force of equal strength and opposite sign on the first object);
• Conservation of angular momentum;
• Newton's law of universal gravitation;
MAIN MECHANISMS THAT GENERATE THE OCEAN MOTION
Main forces to consider in the study of the motions of the Ocean:
Direct – that cause the motion:
• gravitational attraction (Sun and Moon);
• wind stress (wind friction);
• horizontal pressure gradient force;
• atmospheric pressure (∆p=1mb implies a variation of ~1cm in the
surface of the ocean);
• Seismic (result from the motion of the sea floor);
Indirect – that result from the motion:
• Coriolis force (appears due to the Earth rotation);
• friction (opposes the motion and dissipate mechanical energy that is
converted into thermal energy).
MAIN MECHANISMS THAT GENERATE THE OCEAN MOTION
The movements of the Ocean can be classified according to their driving
forces:
• Thermohaline circulation – it results from the variation of the density –
thus, the differential action of the gravity generates relative motion;
• Wind driven circulation – currents in the upper layers, surface waves
and upwelling of sub-surface waters;
• Tidal currents – essentially horizontal, direct consequence of the Law of
Gravitational Attraction;
• ‘Tsunamis’ – they result from the forces applied at the sea floor due to
movements of the submarine crust;
• Turbulent motions: they result from the shear, that means, velocity
gradients, some times close to the borders of the ocean;
• Other motions: internal waves, inertial motions, planetary Rossby
waves, etc....
THE DIFFERENT SCALES OF THE OCEAN CIRCULATION
Due to the diversity of forces that act in the Ocean, the observed motion is
a sum of motions with different scales. The motions in the Ocean range
from the small swirls, in the scales of millimeters, to the large currents,
with scales of tens of thousands kilometers, like the Gulf stream or the N.
Atlantic gyre.
Although these scales superimpose, they can be treated in an
independent way. At each scale, the relevant acting forces are different
and the pertinent laws to explain the phenomena are, in general, different
too.
The typical scales of the Ocean motions are:
• large scale;
• mesoscale;
• small scale;
• microscale – motions in the scale of centimeters or less: molecular
diffusion, boundary layer phenomena, viscosity, surface tension, etc.
THE DIFFERENT SCALES OF THE OCEAN CIRCULATION
Large scale – the large oceanic currents that determine the general circulation
of the Ocean (>1000 km).
General Circulation of the Ocean
THE DIFFERENT SCALES OF THE OCEAN CIRCULATION
Mesoscale – Local phenomena,
independent, but sometimes with
implications in the general
circulation. They result from local
forcings and their scale is from
tens to hundreds of kilometers
Examples: coastal currents and
counter-currents, eddies with
radius of tens of kilometers,
coastal upwelling’, filaments,
fronts, etc.
THE DIFFERENT SCALES OF THE OCEAN CIRCULATION
Small scale – motions in the scale of meters: internal kinematics and dynamics of
eddies and filaments, motion in the temperature fronts, motions near the sea bed in
shallow waters, motions in ports, beaches, bays, estuaries, etc.