NATS 101-06 Lecture 16 Atmo-Ocean Interactions El Ni

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NATS 101-06Lecture 16Atmo-Ocean InteractionsEl
Niño-Southern Oscillation

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Summary

• Global Circulation
• Differential Heating Between Tropics and Poles
• Three Cells Hadley-Ferrel-Polar
• Mid-Latitude Westerlies
• Patterns shift slightly with seasons
• Precipitation
• Major Deserts occur under Sub-Tropical High
• Mid-latitude storms occur along Polar Front

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Triple Cell (Real World)

• Equator-to-Pole temperature difference and
  rotation of Earth produce 3 circulation cells
• Hadley Cell (Thermally Direct)
• Ferrel Cell (Indirect Forced by Hadley Polar)
• Polar Cell (Thermally Direct)

Shift with Seasons
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Global Circulation - Precipitation
Ahrens Fig 13-2
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Prevailing Winds
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Ocean Currents

• Drag from wind exerts a force on the ocean
  surface in the same direction as the wind.
• Currents of upper ocean are due to wind.
• Tend to flow in the direction of prevailing wind.
• Poleward currents are warm equatorward currents
  are cold.
• Thus, oceans transport heat from the tropics to
  the poles, about the same amount of heat as the
  wind.

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Ocean Currents of World
Ahrens Fig. 7.24
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Summer SST Along West Coast

• Sea surface temperatures (SST) along West Coast
  are quite cold during summer, especially off
  Northern California.
• Due to upwelling of cold, nutrient rich water by
  prevailing N winds.

Prevailing Winds
Coastal Surface Water
Ahrens Fig 7.24
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Ekman Spiral
Ahrens, Older Ed.
Surface water moves 45? to the right of
prevailing wind. Subsurface water moves at angles
greater than 45?. Net transport of surface
layer is 90? to the right. Coriolis force is
responsible for the rightward rotation.
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Upwelling from Alongshore Winds
Ahrens Fig 7.25
Wind pushes surface water southward. Coriolis
force deflects water to the right. Cold water
from below rises to surface. Fog persists over
the cold water.
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El Nino 3.4
Upwelling Regions
weather.unisys.com
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weather.unisys.com
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El Nino-Southern Oscillation (ENSO)

• An important atmospheric-ocean feedback
• Normal conditions in tropical Pacific
• -Warm SST, low SLP, and T-storms in W Pacific
• -Strong subtropical highs in E Pacific
• -Easterly winds and cool upwelling water along
  equator in East Pacific
• -Prevailing southerly winds off of Peru produce
  cold upwelling and excellent fishing

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El Niño-Southern Oscillation (ENSO)

• Every few years (4-5 years)
• -Equatorial Central Pacific warms
• -Low SLP, T-storms shift to Central and East
  Pacific (the Southern Oscillation)
• -Trades and southerly winds off Peru weaken
• -Upwelling ceases off Peru, warming leads to
  massive kill off of fish. Typically occurs around
  Christmas (an El Niño event)
• ?Alters global patterns of wind, temp and rain

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Walker Circulation
Pushes water westward
Darwin
Tahiti
Aguado Burt, p230
Walker Circulation oscillates with a quasi-period
of every few years. Oscillation is very evident
in SLP records for Darwin and Tahiti.
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Southern Oscillation Index (SOI)
SST
SLP
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See current conditions
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El Niño Anomalies
Ahrens Fig. 7.28
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El Nino Precipitation Extremes
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La Nina Precipitation Extremes
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El Nino Precipitation Extremes
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La Nina Precipitation Extremes
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ENSO-Atmosphere Feedback
Ocean temperature pattern
SLP, winds, storms, etc.
Ocean currents, upwelling
By observing SSTs and surface winds in
equatorial Pacific, we are able to forecast ENSO
events with considerable skill. ENSO forecasts
lead to skillful seasonal forecasts for the US
several months in advance (e.g. 1997-1998 winter).
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Summary

• Major Ocean Currents
• Driven by prevailing wind
• Upwelling Regions
• Occurs along west coasts of continents
• Cold water rises from below to surface
• Nutrient rich, excellent fishing regions

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Summary

• El Nino-Southern Oscillation
• Occurs every few years
• Central equatorial Pacific warms
• Low SLP, T-storms move with warm water
• Upwelling weakens along Peru coast
• Can be predicted up to one-year in advance
• Modulates global patterns of wind, temp, rain

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Assignments

• Following Lecture
• Topic - Air Masses
• Reading - Ahrens pg 201-212
• Problems - 8.1, 8.11