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CIRCULATION, AND EROSION AND SEDIMENT DEPOSITION

. Surface circulation: surface heating and . Deepwater (thermohaline) circulation: density differences of

Transport of heat, salt and oxygen in occurs via the ocean circulation

Ocean circulation is therefore important for: (a) climate of the (b) Oxygenetion of deep that is itself imprortant for life in oceans

Ocean circulation occurs by surface and deep currents that are important for:

(1) Morphology of the shelves that is often itself determined by:

, currents, sediment supply tectonics

(2) Sediment transport: the sediment supplied to the coast by rivers and produced by erosion is transported by waves and longshore currents.

Currents and waves leave their marks on the seafloor as depositional and erosional features. The ocean circulation is achieved by transport of water masses. The cause of water transport and, thus ocean circulation, is the density differences.

The density differences is the result of :

1) Salinity difference 2) Temperature difference.

Salinity and temperature depends on the climate belt and thus the . Facts: 1) Cold water is heavy and warm water is light 2) Saline water is heavy 3) ice formation increase in salinity and density Surface Circulation

Factors

1. Solar Heating 2. 3. 4.

Solar heating causes water to expand. Near the the water is about 8 cm higher than that in middle .

This causes a very slight slope, and water tends to flow down the slope. Surface ocean circulation by wind Winds blowing on the surface of the ocean push the water. is the coupling between the wind and the water's surface

Wind blowing for 10 hours across the ocean will cause the surface waters to flow at about 2% of the wind .

Water will pile up in the direction the wind is blowing. Gravity will tend to pull the water down the "hill" or pile of water against the .

But the Coriolis intervenes and cause the water to move to the right (in the ) around the mound of water. AtmosphericAtmospheric circulationcirculation

Atmosphere-Ocean interaction - Climate

Tilt of the Earth’s axis and its result on Solar radiation and formation of seasons The earth’s surface and oceans are not heated uniformly by the solar

Coriolis Force The Earth’s rotational increases from zero at the poles to 1600 km/hour at the equator

Moving objects appear to be deflected to the right of the direction of in Northern Hemisphere and left in the An air mass at 30º lat., heading southward towards equator at 32 km/h will arrive 100 h later, and 20.000 km west of the point directly south of the air mass when it first started moving (200 kmx100h=20.000 km; earth moves 200 km faster than the air mass)

Circular gyres are produced by wind piling up the water in central gyres, with its apex close to west margin.

Water flows diagionally from the pile under the effect of : a) b) Gravity

Ekman transport

Surface circulation of oceans driven by wind: Atmospheric circulationAtmospheric circulation Winds driven by solar heating and Earth’s Subtropical gyres Equatorial currents Western & estern Boundary currents Dynamic topography of ther ocean surface: subtropical gyres

Surface circulation of the : two gyres with warm and cold currents NE Atlantic:

Sea Surface Temperature (SST), showing the meandering and Upwelling in Equatorial Divergence Zone

Equatorial Divergence Zone driven by and deflected by the Coriolis effect, which pulls surface waters away. This results in the upwelling Upwelling and downwelling in coastal On western margin of continents Deep water circulation

Deep water circulation of oceans take place because of density differences of water masses caused by

 Salinity  Temperature differences

The result is Sverdrups (Sv) is106 m3/s (0.001 km3/s) Deep water circulation

Effects of wave and currents

Erosional features:

 Wave-cut terraces,  Scourmarks  Clean-swept banks  Submarine plateaus

Depositional features:

 Ripples,  Sand waves,  Bars,  Berms,  Sediment drifts

Sediment transport

Important parameters are

Grain size, Porosity, Cohesiveness (function of clay and organic matter content) Current velocity Sediment or seafloor surface characteristics: smooth vs. rough

The role of velocity:

The velocity of current decreases towards zero near the surface. Bottom shear stresses developed by the flow regime The effect of current on the surface depends on roughness of the surface and the turbulence it creates. Turbulence causes sudden changes in the impact of the water on grains at the surface

As the current velocity increases, the frequency and the force of impact pulses increase and some grains start to move.

The movement of sediment takes two forms:

 Bed load  Suspension load

Wave base level Coarse material due to winnowing by currents

Stokes Law for settling of grains