OCEAN CIRCULATION, AND EROSION AND SEDIMENT DEPOSITION
. Surface circulation: surface heating and wind . Deepwater (thermohaline) circulation: density differences of water masses
Transport of heat, salt and oxygen in oceans occurs via the ocean circulation
Ocean circulation is therefore important for: (a) climate of the earth (b) Oxygenetion of deep waters 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:
waves, currents, sediment supply tectonics
(2) Sediment transport: the sediment supplied to the coast by rivers and produced by wave 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 mass 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 latitude. Facts: 1) Cold water is heavy and warm water is light 2) Saline water is heavy 3) Sea ice formation increase in salinity and density Surface Circulation
Factors
1. Solar Heating 2. Winds 3. Gravity 4. Coriolis
Solar heating causes water to expand. Near the equator the water is about 8 cm higher than that in middle latitudes.
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. Friction 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 speed.
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 pressure gradient.
But the Coriolis Force intervenes and cause the water to move to the right (in the northern hemisphere) around the mound of water. AtmosphericAtmospheric circulationcirculation
Atmosphere-Ocean interaction Weather - 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 energy
Coriolis Force The Earth’s rotational velocity 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 motion in Northern Hemisphere and left in the Southern Hemisphere 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) Geostrophic current
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) Coriolis force b) Gravity
Ekman transport
Surface circulation of oceans driven by wind: Atmospheric circulationAtmospheric circulation Winds driven by solar heating and Earth’s rotation Subtropical gyres Equatorial currents Western & estern Boundary currents Dynamic topography of ther ocean surface: subtropical gyres
Surface circulation of the Atlantic Ocean: two gyres with warm and cold currents NE Atlantic:
Sea Surface Temperature (SST), showing the meandering Gulf Stream Upwelling and Downwelling Upwelling in Equatorial Divergence Zone
Equatorial Divergence Zone driven by trade winds and deflected by the Coriolis effect, which pulls surface waters away. This results in the upwelling Upwelling and downwelling in coastal seas 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 thermohaline circulation 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