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Wind: Global System

General Circulation of the atmosphere

• Represents "average air flow around the globe • is created by unequal heating at earth's surface • Recall:

– On global scale, earth is in radiative equilibrium, energy in equals energy out

– this is not true, however, at each latitude – general circulation's function is to transport heat pole

ward

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Single-Cell Model of the General Circulation• If you assume:

– earth is uniformly covered with water – sun is directly over equator – no rotation

• You will end up with a single-cell pattern -->>– called the Hadley Cell– warm air rises at the equator, cold air sinks at the poles

Three-Cell Model of the General Circulation

• Three-cells– Hadley cell– Ferrel Cell– Polar Cell

• Surface pressure– High and low

pressure bands• Surface winds

– Westerlies– Trade winds

• ITCZ• comparison to real

world?

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Satellites provide a global picture of winds over the oceans by converting sea roughness into wind speed and direction

Cloud movement can tell you global wind movement: http://www.nrlmry.navy.mil/sat-bin/global.cgi

Observed average surface pressure and winds during January

• Semi-permanent pressure areas:– Bermuda-Azores

High– Pacific High– Aleutian Low

• genesis region for cyclones impacting the Pacific NW

– Icelandic Low• Seasonal

pressure areas:– Siberian High– Canadian High

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Observed average surface pressure and winds during July

• Semi-permanent pressure areas:– Bermuda-

Azores High– Pacific High– Iclandic Low

• Seasonal pressure areas:– Monsoon Low– Thermal Low

over SW US.

A winter weather map depicting the main features of the general circulation over North America. Notice that the Canadian high, polar front, and subpolar lows have all moved southward into theUnited States, and that the prevailing westerlies exist south of the polar front. The arrows on the map illustrate wind direction.

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Global Precipitation Pattern Produced by the General Circulation

• Zonal distribution of precipitation around the globe (roughly speaking) – 0°N - low pressure -

cloudy – 30°N - high pressure -

sunny – 45°N-60°N - low

pressure - cloudy – polar latitudes - high

pressure - clear • Global Satellite images

often reveal cloud patterns consistent with the above general circulation

Weather associated with the Pacific and Bermuda Highs

• Pacific high- moves northward during summer – Produces strong subsidence inversion on eastern side – During winter, it moves south allowing polar front to bring precip to SW US

• Bermuda high - transports warm, moist subtropical air to US and southern Canada – This air can be unstable – Weather associated with subtropical highs can vary dramatically depending on

where you are located!

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Weather associated with the Pacific and Bermuda Highs--proof

Average Wind Flow and Pressure Patterns Aloft -- January

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Average Wind Flow and Pressure Patterns Aloft -- July

500 mb

Surface

January July

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Walker Circulation

The nonuniform distribution of land and sea and asymmetries in radiative, conductive , and latent heating that accompany it lead to Walker Circulations along the equator.

http://www.bom.gov.au/info/climate/

Jet Streams are also part of the general circulation

• Subtropical Jet– situated above the subtropical highs at about 13 km AGL– often visible as a plume of moisture extending from the tropics to

the sub-tropical regions – The jet streams exhibit a "wavy" pattern around the globe....

• Polar Jet– situated at about 10 km

AGL over the polar front

• Recall that tropopause height is proportional to the mean tropospheric temperature

Atmospheric jet streams are swiftly flowing air currents thousands of kilometers long, a few hundred kilometers wide, and a few kilometers thick.

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Jet Stream Waves:

• Often have troughs and ridges

• generally have a jet maximum (jet streak) in the base of the trough

• transport heat pole ward (cold air south and warm air north)

• Q: How are the polar and subtropical jets form????

a) Position of the polar jet stream and the subtropical jet stream at the 300-mb level on March 9, 2005. Solid lines are lines of equal wind speed (isotachs) in knots. Heavy lines show the position of the jet stream core. (b) Satellite image showing clouds and positions of the jet streams for the same day.

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Jet Stream Formation - Polar Jet

• Polar Jet:– boundary between warm

air to the south and cold air to the north

– location of a large temperature gradient near the surface

– Hence, the large temperature gradient at the surface across the polar front creates a large pressure gradient aloft

Jet Stream Formation - Polar Jet– recall that the strength of the geostrophic wind is

proportional to the magnitude of the pressure gradient force– Hence, the large pressure gradient aloft over the polar front

generates a band of strong winds– this is the jet stream!– Strong in winter, weak in summer.

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Jet Stream Formation - Subtropical Jet

• Subtropical Jet:– Forms on pole ward side of Hadley cell– Hadley cell produce the subtropoical front– Temperature contract pressure gradiance– created largely through conservation of

angular momentum• Other Jet Streams:

– Low-level Jet over Central Plains of US– Polar Stratospheric Night Jet

Jet Stream and the Conservation of angular momentum

Angular momentum is defined as the product of the mass (m) times the velocity (v) times the radial distance (r): Angular momentum = mvr

The Conservation of angular momentum:The angular momentum of the rotating system does not change if there are no external twisting forces.

How ice skater change rotating speed?

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Atmospheric-Ocean Interactions• Wind-water interactions

– Surface water draft with wind– Moving water piles up, creating pressure difference– The interaction can leads motion several hundred

meter down into the water.

• Ocean currents– Largely generated by wind- gyres – Account for about 40% of global heat transport – Water moves at 20-45 degree angle to wind dir due to

coriolis force – Move slower than winds - several km/day - km/hr

Wind

Ocean Current

Red: Warm currentsBlue: Cold currents

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Major Ocean

Currents:

Ocean Front• The boundary separating the two masses

of water with contrasting temperatures and densities

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The Ekman Spiral

1. Winds move the water, and the Coriolis force deflects the water to the right (Northern Hemisphere).

2. Below the surface each successive layer of water moves more slowly and is deflected to the right of the layer above.

3. The average transport of surface water in the Ekman layer is at right angles to the prevailing winds.

Upwelling

• Brings cold water from deeper regions to surface

• Northerly surface winds create offshore surface water flow

• Cold, nutrient-rich water rises to replace the surface water (good for fishing- Peru)

• Process is dominant when winds are parallel to coastline

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El Nino (Spanish name for the male child)

• Initially referred to a weak, warm current appearing annually around Christmas time along the coast of Ecuador and Peru (not good for their fishing industry and quano birds!)

• Can produce significant economic and atmospheric consequences worldwide

• Occur every 3-7 years, lasting about one year • Recent major events: 1982-1983 and 1997-1998 • The 97-98 event was the strongest ever

recorded!!

El Nino - Animation of warm pool• El Nino refers to the eastward

movement of warm water (shaded red) from the western equatorial pacific to the eastern equatorial pacific.

• The animation of sea surface temperature (SST) anomalies to the right shows the unusual warming that occurred during the 97-98 event.

• Note that an anomaly is a departure from some "normal" value. An anomaly can be either positive (warm) or negative (cold)

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Trade Winds and Rainfall During A Normal Year• During a normal year, the easterly trade winds push and pile the water in

the equatorial pacific into the western part of the pacific basin. • The sea level is actually higher in the western pacific • Also note the upward motion and precipitation associated with the warm

water and lower pressure in the western pacific • Also note the cooler water, high pressure and subsidence in the eastern

Pacific. • The large-scale zonal circulation in the equatorial plane is called the

Walker Circulation.

Trade Winds During an El Nino Year• During an El Nino year, the trade winds weaken • This allows the water in the western pacific to move to the

eastern pacific in the form of a "kelvin wave" • The equatorial counter current is also stronger • Notice the change in the Walker Circulation and associated

precipitation distribution.

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• NASA/JPL - El Niño/La Niña & PDO – http://topex-

www.jpl.nasa.gov/science/el-nino.html

• NOAA,SST animation– http://www.cd

c.noaa.gov/map/clim/sst_olr/sst_anim.shtml

La Nina and the current state of the

tropical pacific

• After the El Nino phase, La Nina follows

• La Nina refers to the unusually cold water that is found in the eastern pacific ocean

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These three images depict the evolution of a warm water Kelvin wave moving eastward in the equatorial Pacific Ocean during

March and April, 1997. The white areas near the equator represent ocean levels about 20 cm (8 in.) higher than average, while the red areas represent ocean levels about 10 cm (4 in.)

higher than average.

El Niño / Southern Oscillation -- ENSO

• The Southern Oscillation: The reversing surface air pressure at the opposite ends of the pacific ocean

• So, are we currently in an El Nino, La Nina, or normal phase?

ENSO index

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Typical winter weather patterns across North America during an El Niño warm event (a) and during a La Niña cold event (b).

El Niño1. Wet weather in CA and the

southern part of US2. Warmer than normal

weather over a large part of North America

La Niño1. Dry and warmer weather in

CA and the southern part of US

2. Colder than normal weather over a large part of North America

Regions of climatic abnormalities associated with El Niño–Southern Oscillation conditions.

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The Pacific Decadal Oscillation (PDO)Long-term Pacific Ocean temperature Fluctuation

Typical winter sea-surface temperature departure from normal in °C during the Pacific Decadal Oscillation’s warm phase (a) and cool phase (b).

The North Atlantic Oscillation (NAO)Refers to pressure variations associated with the Icelandic low and subtropical high pressure near the Azores

1. more and stronger storms crossing the Atlantic on a more northerly track

2. eastern US will experience mild and wet winter conditions

Stronger than normal Weaker than normal

1. fewer and weaker storms crossing the Atlantic

2. eastern US experiences more cold-air outbreaks and snowy weather conditions

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The Arctic Oscillation

Change in surface atmospheric pressure in polar regions and typical winter weather patterns associated with the (a) positive (warm) phase and the (b) negative (cold) phase of the Arctic Oscillation.