Background to Oceans

• 71% of the Earth is covered by oceans
• 60% Northern Hemisphere, 80% Southern
  

Depth

• Deepest point is ~11 km deep (where?)
• Average depth is 3.8 km
  

The ocean as a 2-layer system

• Top: thin, warm and less dense
• Bottom: thick, cold, and more dense
  • Thermocline
• very stable (consequences?)

Composition

Why the difference?

Circulation of these masses are either

• wind-driven for surface circulation
• thermohaline (density) for circulation at depth
  

Vertical Structure of the Oceans

• temperature vs. depth
• density vs. depth
  • Density ranges from 1.02200 to 1.03000 g/cm³ in open ocean
  • Density increases with depth
    
  • The insitu density in deep trenches is about 5% greater than at the ocean surface.
    

Surface Currents

• Surface currents are driven by energy from sun
  
• Effects to about 0.2 km depth
  
• Horizontal variation in temperature, salinity, and density
  
• The movement and circulation of the oceans is tied very closely to the circulation of the atmosphere, both interact with each other and both feedback to affect Earth's climate.
  
• Like the atmosphere, the circulation of the oceans is ultimately driven by solar energy:
  • The distribution of solar energy over space and time results in the formation of the global wind belts.
    
  • These roughly latitudinal wind patterns in turn produce the ocean currents that determine the circulation patterns of the upper ocean.

Ekman Spiral

• Imagine the upper level of the ocean operating as a series of very thin individual layers of water
  
• Friction between the wind and the upper layer of the ocean causes the top layer of water to move
  
• The Coriolis effect results in that top layer being deflected to the right of the windfield (in the Northern Hemisphere)
  
• Friction between the top water layer and the next one down causes the second layer to move, and again the Coriolis Effect causes that layer to be deflected further to the right of the one above
• As you move deeper and deeper through that upper part of the ocean the water is deflected further and further to the right of the original windfield
  
• At some depth below the surface, this wind-driven ocean movement will actually be in the opposite direction to the wind that is causing the movement

• Seen in three dimensions, the water movement takes on a spiral shape. The net flow (when adding together the effects of each individual layer) is approximately at right angles to the windfield
  
• These currents move west parallel to the equator.
  
• Due to Coriolis effect and deflection along continental margins currents move away from equator as warm boundary currents.
  
• Motion is caused by dominant surface feature of each major ocean basins: the subtropical gyres.
  
• Gyres rotate clockwise in Northern hemsiphere and counterclockwise in southern hemisphere.
  
• North equatorial Current of the Indian Ocean and South Equatorial current in the Pacific ocean are driven by easterly winds in a westerly direction.
  
• Net effect is water deficiency at surface between two currents.
  
• Water from deep within upper-water mass comes to surface to fill the void.
  
• Upwelling predominates west coast of South America.
  
• Replacement water comes from lower portions of the upper water (the Peru Current).
  
• This area is characterized by low surface temperatures and high concentrations of nutrients making resulting in high biological productivity.

Thermohaline Circulation

• Thermohaline circulation accounts for thorough mixing of deep ocean water
  
• Thermohaline circulation is initiated at the ocean surface in high latitudes
  • A very cold, highly saline, body of water that forms at the surface primarily in the Norwegian/Greenland Seas and in the Weddell Sea (Antarctica).

• Caused by temperature and salinity conditions that produce a high density mass which sinks and spreads slowly beneath the surface waters
  
• Some surface masses sink to the ocean bottom
  
• Some deep sea masses rise to the surface
  
• A deep ocean layer in which density increases very slowly with depth. The deep ocean contains about 80% of the volume of the oceans
  
• This vertical structure is inherently stable and as horizontal density differences (due to temperature or salinity gradients) are small, movement is relatively slow
  
• The time-scale for the complete replacement of the Bottom Water throughout the oceans is on the order of 1000 years.