EAS/BIOEE 154
Lecture 16
Introduction to Oceanography
Ocean Structure and Thermohaline
Circulation
Ocean Circulation
Surface Circulation
 Driven by winds
Vertical or Thermohaline Circulation
 driven by water density differences
 Water density is controlled by temperature and salinity
Importance of Ocean Circulation
 Transports heat and affects climate
 Controls the distribution of nutrients and therefore affects abundance and
distribution of organisms
Heat and Energy
Earth receives energy from the Sun mainly as visible light.
Earth radiates energy out in to space mainly as infrared radiation.
Earth’s Surface Energy Budget
 only 47% of solar radiation entering top of atmosphere is absorbed by
surface.
 Energy lost must equal energy gain
 Energy losses from ocean: Radiation 41%, Conduction 6%, Evaporation
53%.
 Net heat gain at low latitudes
 Net heat loss at high latitudes
 Ocean and atmospheric currents transport heat from tropics to the poles.
Electromagnetic radiation is transmitted very poorly in water: gains and
losses only at surface
 Therefore, ocean water temperature can be changed only at the surface.
 For this reason, ocean temperature is said to be conservative.
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Salinity
Ways in which salinity can change:
 Dilution: rain, river water, melting sea ice
 Concentration: evaporation, formation of sea ice
 These processes occur only at the surface.
 Therefore, salinity is also a conservative property.
Salinity Variations
 Because of the way the atmosphere circulates, salinity is lowest at high
latitude and near the equator, and highest at mid-latitudes (horse
latitudes)
Ocean Water Density
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Density of ocean water is controlled by temperature and salinity. Density (in
grams per cubic centimeter) is generally designated as σ (sigma)
σ = (Density-1) × 1000
EAS/BIOEE 154
Lecture 16
Density of pure water (at 4° C) is 1.00 g/cc
On sigma scale, this would be σ = 0
Surface Density Variation
 Surface density lowest at the (climatic) equator and highest at the poles.
Density, together with winds, governs the movement of ocean water.
 Deep water motion can be predicted if density is known.
 Vertical motion of water occurs mainly at poles.
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Temperature-Salinity (T-S) Diagrams
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T-S diagrams illustrate the relationship between temperature, salinity, and
density.
Once a water mass leaves the surface, its T and S can change only by
mixing.
T-S diagrams can be used to identify water masses and mixing between
them.
Ocean Temperature and Salinity Structure
Of the 2 factors, temperature and salinity, controlling density, temperature is
the more important.
Ocean generally divided into three zones:
 Upper mixed layer
 Thermocline: Zone of rapid temperature change
 Deep water: Relatively constant temperature; cold.
Salinity Changes
 Salinity also often changes with depth
 Region of rapid salinity change is called the halocline.
Seasonal Variations in Ocean Structure
 Temperate zone seasonal changes result from seasonal changes in surface
temperature & deep mixing by waves in winter (when winds are stronger
and waves higher).
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Upwelling & Downwelling
Convergence and Divergence
 When winds and currents pile up water on the surface, a convergence is
said to occur; in this case, the water sinks or downwells.
 When winds and currents push water apart or away from coasts, a
divergence is said to occur. In this case, water will move up from below, or
upwell, to replace the surface water.
The Ekman Spiral, Ekman Transport, and Coastal Upwelling
 Because of the Earth’s rotation and the consequent Coriolis force, wind
moving over the surface of the ocean induces a surface current at 20-45˚
to the right in the northern hemisphere.
 The drag from the moving surface water induces the underlying layer to
move at 20-45˚ further right in northern hemisphere.
 The net motion, known as Ekman Transport, of the water is then 90˚ to the
right.
 When northerly winds along northern hemisphere western coasts drive water
away from coast, deep water upwells to replace it.
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EAS/BIOEE 154
Lecture 16
Everything opposite in southern hemisphere.
Biological Importance of Upwelling
 Upwelling returns nutrient-rich water to the surface
 Regions of upwelling are generally regions of high biological productivity –
and productive fisheries.
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Water Masses
A water mass is an extensive body of water which has a limited range of
temperature and salinity.
 Water masses acquires their temperature and salinity at the surface.
 After it leaves the surface, T and S can change only by mixing.
 Hence, water masses are “formed” at the surface.
Formation of Intermediate and Deep Water Masses
 Water moves up or down in the water column until it finds it equilibrium
density.
 Names reflect region of formation of water and location in water column.
e.g., Antarctic Bottom Water, Mediterranean Intermediate Water.
 Deep water masses form near poles.
 Vertical circulation of the ocean is very slow. Average ventilation time is
about 1000 yr.
Regions of Formation of Deep Water
 Mediterranean - an Intermediate Depth Water in the Atlantic. Characterized
by high salinity and moderate warmth
 North Atlantic - Deep Water found in the Atlantic; formed as salty water is
cooled by the cold atmosphere in the Labrador, Greenland Sea, and
Norwegian Seas. It then sinks and flows south, and mixes with other
dense water masses, becoming North Atlantic Deep Water (NADW).
 Antarctic Intermediate Water: AAIW forms at the Antarctic Polar Front (about
50-60˚ S) as currents and Ekman transport (westerly winds deflect water
northward) produce a convergence.
Antarctic Bottom Water
 Coldest and densest of all water masses.
 Less saline, but colder than NADW
 Forms around Antarctica in austral winter, particularly in the Weddell Sea by
strong cooling.
 Freezing of sea ice increases salinity and density.
 Leads that open in the Weddell Sea ice, called polynas, contribute to cooling
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Vertical Circulation Summary
Deep water masses formed in North Atlantic and around Antarctica
Antarctic bottom water then flows eastward, and northward into 3 oceans.
 Deep water slowly returns to the surface by slow upward mixing and
upwelling.
Biological Consequences of Global Ocean Circulation
 Atlantic exports deep water to Pacific and Indian Oceans
 Bottom water becomes increasingly rich in nutrients as it ages
 Therefore, Atlantic exports nutrients to Pacific and Indian
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EAS/BIOEE 154
Lecture 16
Thermohaline Circulation and Climate
 North Atlantic Deep Water Formation plays an important role in climate.
 Heat lost by the water in North Atlantic is gained by the atmosphere, mainly
in the North Atlantic Region.
 This keeps the North Atlantic region, particularly Europe, warmer than it
would otherwise be.
 Switching off NADW production may have contributed to the cold of the ice
ages.
Heinrich Events and NADW
 Melting icebergs in Heinrich events may have reduced salinity in the North
Atlantic surface water enough to shut down NADW. Result was cold
climate.
Some Study Questions
What are the characteristics of MIW – in other words, where would
you expect to find it on a T-S diagram? Explain why it has
these characteristics.
If a water mass is located on a T-S diagram at σ = 28.2, what
would be the density of this water in grams/cc?
What would happen to the Earth is energy losses did not balance
energy gains?
Along the coast of Peru, the prevailing wind direction is from the
south. What kind of Ekman transport do you expect will occur?
Judging from that, what would you predict about biological
productivity and fish abundance there?
Explain how polynas contribute to AABW formation.
Explain the seasonal changes in the thermocline in temperate
regions? Would you expect the same changes in
equatorial/tropical regions?
Why are deep waters of the Pacific richer in nutrients than deep
waters of the Atlantic?
In what sense is salinity a conservative property of seawater?
What is the pycnocline and water factors control it?
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