NATS 101-05Lecture 15
Atmo-Ocean InteractionsEl Niño-Southern Oscillation
Summary
• Global CirculationDifferential Heating Between Tropics and Poles Three Cells: Hadley-Ferrel-PolarMid-Latitude WesterliesPatterns shift slightly with seasons
• Precipitation
Major Deserts occur under Sub-Tropical High
Mid-latitude storms occur along Polar Front
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
Hadley Ferrel Polar
Eq
uat
or P
ole
Global Circulation - Precipitation
Ahrens Fig 13-2
Prevailing Winds
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.
Ocean Currents of World
Ahrens Fig. 7.24
Summer SST Along West Coast
Ahrens Fig 7.24
Prevailing Prevailing WindsWinds
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.
Coastal Coastal Surface WaterSurface Water
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.
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.
weather.unisys.com
Upwelling RegionsUpwelling Regions
El Nino 3.4
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
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
Walker Circulation
Pushes water westward
DarwinDarwin TahitiTahiti
Walker Circulation oscillates with a quasi-period of every few years. Oscillation is very evident in SLP records for Darwin and Tahiti.
Aguado & Burt, p230
Southern Oscillation Index (SOI)
SLPSLP
SSTSST
See current conditions
El Nino Precipitation Extremes
La Nina Precipitation Extremes
El Nino Precipitation Extremes
La Nina Precipitation Extremes
ENSO-Atmosphere Feedback
Ocean temperature pattern
SLP, winds, storms, etc.Ocean currents, upwelling
By observing SST’s 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).
Summary
• Major Ocean CurrentsDriven by prevailing wind
• Upwelling Regions
Occurs along west coasts of continents
Cold water rises from below to surface
Nutrient rich, excellent fishing regions
Summary
• El Nino-Southern OscillationOccurs every few yearsCentral equatorial Pacific warmsLow SLP, T-storms move with warm waterUpwelling weakens along Peru coastCan be predicted up to one-year in advanceModulates global patterns of wind, temp, rain
Assignments
• Following Lecture
Topic - Air Masses
Reading - Ahrens pg 201-212
Problems - 8.1, 8.11