NATS 101-06Lecture 16

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