Oceanic Circulation

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Oceanic Circulation

• I. How oceans work
• II. Surface currents
• III. Deep Currents
• IV The Air-Sea Interactions (ENSO)

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The Role of Oceans in Global Climate Variability
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The Role of Oceans in Hydrological Cycle
97 of the Earths free water 86 of the global
evaporation 78 of global precipitation
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I. How oceans work

• Area covers 70 of the Earths surface
• Volume 97 of all the water on the Earth
• Depth 4 kilometers
• Density 1034-1035 kg/m3 (Pure water 1000 kg/m3)
  over 90 of the ocean. Depends on temperature and
  salinity.
• cold water ? high density
• loss of water by evaporation ? increase salinity
  ? high density
• precipitation and river discharge ? decrease
  salinity ? low density
• Heat capacity high
• Temperature less variable than in the atmosphere
• Freezing point 1.9C, not at 0C because of
  salinity
• Surface is not level due to currents, waves,
  atmospheric pressure differences, and variations
  in gravity.
• Two main forms of circulation
• wind-driven circulation (horizontal, surface
  waters, fast)
• thermohaline circulation (vertical, deep waters,
  slow)

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I. How oceans work (Contd)
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Relative proportions of dissolved salts in
seawater
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Annual Mean Ocean Surface Temperature
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Annual Zonal Mean Ocean Surface Temperature
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Annual Mean Ocean Surface Salinity
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Annual Zonal Mean Ocean Salinity
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Ocean Meridional Overturning (Global)
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Ocean Meridional Overturning in Atlantic
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I. How oceans work (Contd)
A. Heated primarily by Sun, largely at the
Equator, with global heat transfer by ocean
currents ? profound effect of oceans on
climate B. Two overall layers 1. Thin, warm, less
dense surface layer well mixed by turbulence
generated by wind 2. Thick, cold, more dense deep
layer that is calm and marked by slow currents 3.
Thermocline is the boundary between the layers
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Mixed Layer Processes
Vigorous mixing processes lead to uniform
conditions within the surface mixed layer.
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Mixed Layer Processes
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I. How oceans work (Contd)
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Quiz
1. Which of the following is NOT true about the
oceans? A. Approximately 97 of the total
water on Earth is located in the oceans B.
The oceans cover about 70 of Earths surface
C. The average depth of the oceans is about 4
km. D. The sea surface is level for all the
oceans. E. Because it takes far more energy
to change the temperature of water than land or
air, water warms up and cools off much more
slowly than either. 2. The three largest
reservoirs of water at the earths surface in
decreasing order of volume are A. oceans, ice
caps/glaciers, and ground water B. oceans, ice
caps/glaciers, and soils C. oceans,
lakes/rivers, and soils D. oceans, atmosphere,
and biosphere E. oceans, ground water, and
soils 3. What is the primary salt in the ocean?
A. methane B. CO2 C. FeO2 D. CaSO4 E.
NaCl 4. Which of the following increase
salinity? A. Evaporation B. Precipitation. C.
Formation of sea ice. D. River runoff. E. A
and C only.
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II. Surface currents
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II. Surface currents

• A. Three primary forces
• Global wind patterns cause surface ocean currents
  because of frictional drag
• 2. Coriolis effect
• 3. Pressure gradients

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B. Global wind patterns cause surface ocean
currents because of frictional drag
July

• Creation of waves
• 2. Creation of hemisphere-scale gyres
• 3. Decrease in current speed with depth

January
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C. Coriolis effect

• 1. Earth rotation speed is greatest at Equator
  falling to zero at Poles
• 2. Conservation of angular momentum
• a. Deflection to the right for component of
  Equator-to-Pole flow in Northern Hemisphere
• b. Deflection to the left for component of
  Equator-to-Pole flow in Southern Hemisphere

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C. Coriolis effect (Contd)

• 3. Creation of Ekman Spiral
• a. To depth of 100 m
• b. Surface current moves 20-45o from the wind
  direction (45o in theory)
• c. Deflection increases with depth, forming a
  spiral
• d. Net transport of water is 90o from the wind
  direction

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C. Coriolis effect (Contd)

• 3. Creation of Ekman Spiral
• a. To depth of 100 m
• b. Surface current moves 20-45o from the wind
  direction (45o in theory)
• c. Deflection increases with depth, forming a
  spiral
• d. Net transport of water is 90o from the wind
  direction

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D. Pressure gradients

• 1. Differences in water height (i.e., piling of
  water against a continent because of the wind)
• 2. Density differences because of temperature or
  salinity
• 3. Atmospheric pressure differences
• E. Geostrophic currents - balance of Coriolis
  force by pressure gradient
• F. Land forms barriers to global ocean currents

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G. Resultant overall current pattern

• 1. Currents converge toward Equator following
  Trade Winds and ITCZ
• 2. Westward flow along Equator (i.e., North and
  South Equatorial Currents)
• 3. Equatorial Currents turn poleward where they
  encounter land barriers (e.g., Gulf Stream)
• 4. Eastward flow of currents is enhanced by the
  Westerlies
• 5. Currents turn toward the Equator where they
  encounter land barriers, completing the gyres

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Effects of surface currents

• 6. Significant heat transfer
• a. More solar heating and evaporation at Equator,
  less at Poles ? salinity and temperature
  Equator/Pole gradient
• b. Warm and salty water carried poleward

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Effects of surface currents

• 7. Zones of upwelling
• a. Deflection of water away from continent
• b. Upwelling of deeper water to replace surface
  water
• c. Commonly nutrient rich

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7. Zones of upwelling
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III. Deep Currents

• A. Thermohaline-driven currents
• 1. Greater salinity ? greater density
• 2. Lower temperature ? greater density
• 3. Equilibrium is sought in global-scale deep
  currents (conveyor belt)

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B. Global Conveyor Belt (Contd)

• 1. Warm and salty shallow water from Tropics is
  carried northward through Atlantic
• 2. Water cools in North Atlantic and sinks east
  of Greenland as cold and salty dense water
• 3. North Atlantic Deep Water flows toward the
  Equator and into the South Atlantic, where it
  meets yet colder and denser Antarctic Bottom
  Water flowing toward the Equator

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1
3
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B. Global Conveyor Belt (Contd)

• 4. Combined flow eastward below Africa and into
  the Pacific Ocean
• 5. Deflected by Asia
• 6. Journey through Pacific causes warming and
  slow rise to surface
• 7. Warm and less dense Pacific surface waters
  move south and west to converge toward Equator as
  shallow surface water to continue warming

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B. Global Conveyor Belt (Contd)

• 8. 1,000 years for a complete cycle
• 9. Conveyor Belt aided by imbalance between loss
  of water by evaporation in Atlantic and gain of
  water by precipitation and runoff
• 10. Bering Strait prevents free exchange between
  Arctic and Pacific Oceans ? inhibits
  Arctic-Pacific Conveyor Belt

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The Role of Oceans

• I. How oceans work
• II. Surface currents
• III. Deep Currents
• IV The Air-Sea Interactions (ENSO)

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The Air-Sea Interactions
A. Inter-Tropical Convergence Zone (ITCZ) B. El
Niño - Southern Oscillation, aka ENSO El
Niño Spanish for boy child Christ child
referring to warm water conditions and associated
consequences, frequently occurring around
Christmas Southern Oscillation A seesaw
pattern of reversing surface air pressures at
opposite ends of the Pacific Ocean Walker
Circulation (zonal convection) refers to the
localized zonal (west-east) convection cell of
atmospheric circulation above the Pacific Ocean.
The easterly trade winds are part of the
low-level component of the Walker circulation.
C. La Niña (girl child) Opposite extreme of
El Niño
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A. Inter-tropical Convergence Zone (ITCZ)
1.What is ITCZ? Region near the equator where
the wind systems (trade winds) of the Northern
Hemisphere and Southern Hemisphere meet. 2.
Position changes with season. North of equator in
July, (generally) south of equator in
January. 3. Broad trough of low pressure,
typical of cloudiness, thunderstorm, and heavy
precipitation. 4. More intensely developed in
the western Pacific because of warm water pool of
surface seawater (T 31 C) and Asian
monsoons. 5. Influences the ocean currents and
salinity.
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El NinoHow itWorks
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Known for over 400 years ...
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Occurs during Christmas season ...

• Christ Child
• The Boy
• The Little One

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Food Chain
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B. El Niño - Southern Oscillation, a.k.a. ENSO
1.What is ENSO? ENSO is a periodic climatic
phenomenon caused by ocean-atmosphere dynamics in
the tropical Pacific Ocean.
ENSO Process Every two to seven years, strong
westward-blowing SE trade winds subside. Warm
water moves back eastward across the Pacific,
like water shifting in a giant bathtub. The warm
water and shifting winds interrupt the upwelling
of cool, nutrient-rich water.
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El Niño and SOI
El Niño Noticed at Christmas time by Peruvian
fisherman warm coastal waters leading to poor
fishing
El Niño / Southern Oscillation an oscillation
in the surface pressure (atmospheric mass)
between the SE tropical Pacific and the
Australian-Indonesian regions.
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Reading the Index
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2. Importance of understanding ENSO
a. Severe rapid climate variations caused by El
Niño are natural b. El Niño has important
consequences for famine and economics around the
globe.
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3. Basics of ENSO
Western equatorial Pacific has some of the
world's warmest ocean water. To the east, cool
water wells up, carrying nutrients (see Oceans
lecture) that support large fish populations
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Oscillation of surface water in the Pacific
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What causes ENSO? large-scale internal waves
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What Causes ENSO?
During the warm phase of ENSO (El Niño phase) the
West Pacific Warm Pool and its associated low
pressure atmospheric center migrate to the east
along the equator. The water is transported by
these internal waves. Kelvin waves cross the
Pacific in 2-3 months and lead to a warming of
the surface waters and rise in sea level along
Peru.
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What Causes ENSO?
As sea level rises and warm water accumulates in
the eastern Pacific, Rossby waves are generated
that move west across the Pacific. The time it
takes for these waves to cross the Pacific is
strongly dependent on the latitude at which the
wave is traveling (near the equator, they take a
9 month journey 12 degrees from the equator,
they take 4 years). The Rossby waves reach the
Western Pacific, travel along the coast as Kelvin
waves to the equator and turn east and begin
another crossing of the Pacific.
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5. If its a tropical phenomenon, why does it
affect us in Texas?
Teleconnection during ENSO, warmer, moister air
than usual is generated in the eastern part of
the ITCZ in the Pacific. This changes
atmospheric circulation patterns and deflects the
jet stream that goes west-east across US at a
more easterly longitude than during non-ENSO
year. As a result, different amounts of moisture
and heat than usual are delivered to different
parts of the world
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El Nino and Texas
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El Nino and Texas

• December - March TX wetter and cooler
• More wet and cool in S. Texas
• Experience flooding, high number of storms
  originating from Gulf of Mexico and tropical
  Pacific because of the strengthened subtropical
  jet stream
• Protect your property homeowners insurance

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6. Whats good and bad about the effects of
ENSO?

1. This depends on where you live and what your
   lifestyle is

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b. 1997-1998 event and the medias response
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7. Measuring El Nino
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Measuring El NinoWeather Stations

• Stevenson Screen
• Temperature
• Precipitation

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Weather Station Colorado
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Measuring El NinoBuoys
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Measuring El NinoSatellites
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Satellites Temperatures
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Measuring El NinoSea Surface Temperatures
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Measuring El NinoCoral Bleaching
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