NATS 101 Lecture 21 AtmoOcean Interactions El NioSouthern Oscillation

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NATS 101Lecture 21Atmo-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
• Wet Regions in Rising Branches of Cells
• 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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H
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WET
DRY
DRY
WET
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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, almost the same amount of heat as the
  wind.

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Ocean Currents of World
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L
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Ahrens Fig. 7.24
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Primary Ocean Gyres
http//fermi.jhuapl.edu/student/currents/figures/s
urface_current_map.gif
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Upwelling

• Now lets turn our attention to an important ocean
  concept upwelling.

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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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Recall that ocean temperatures decrease with
depth. If bottom water is brought to the surface,
then surface becomes colder.
Wallace and Hobbs, p348
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Ekman Spiral
Wind causes surface water to move Coriolis force
deflects surface current to the right
45? Subsurface water moves at angles 45?. Net
transport of surface layer is 90? to the right.

Ahrens, Older Ed.
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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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Wind Patterns and Oceans

• Now that we understand the concept behind
  upwelling and ocean circulations, we now move
  onto another topic of great interest, El Niño/La
  Niña.

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El Nino 3.4
Upwelling Regions
Current SST http//www.ssec.wisc.edu/data/sst/l
atest_sst.gif
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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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
Earlier onset of monsoon
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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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Other Atmospheric Indices

• PDO - Pacific Decadal Oscillation
• PNA Pacific North American pattern
• NAO North Atlantic Oscillation
• AO Arctic Oscillation
• MJO Madden-Julian Oscillation

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PDO - Pacific Decadal Oscillation
Decadal variability in the Indo-Pacific sector.
Timescale (events 20-30 yrs)
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PDO - Pacific Decadal Oscillation
Negative Phase 1950s - mid 1970s (La Niña
like) Positive Phase mid 1970s - 1990s (El
Niño like)
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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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Assignment for Next Lecture

• Topic - Air Masses, Fronts
• Reading - Ahrens pg 201-212, 212-219
• Problems - 8.1, 8.11, 8.12, 8.13