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Atmospheric Circulation-II

GEOL 1350: Introduction To Meteorology

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Overview

Primary Cause of the General Circulation

Three Cell Circulation

Important Large-scale Atmospheric Circulation Inter-tropical Convergence Zone (ITCZ) Jet Stream Walker Circulation El Nino and Southern Oscillation North Atlantic Oscillation

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Atmospheric General Circulation (global circulation) – averaged wind patterns around the globe (Actual winds at any given place and any given time may vary considerably from this average)

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Primary Cause of the General Circulation

•  Unequal heating of the earth’s surface

•  Temperature gradients --- Pressure gradients --- Winds

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Primary Cause of the General Circulation

This build-up of heat around the equator and depletion in polar regions is responsible for the poleward flow (transport) of heat energy to equalize the total energy distribution. That is one of the driving forces in the circulation patterns.

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Primary Cause of the General Circulation

•  The geographical distribution of temperature and its seasonal variability closely follows the geographical distribution of solar radiation.

Jan Jul

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Primary Cause of the General Circulation

•  Temperature plays a direct role in determining the climate of every region.

•  Temperature differences are also key in driving the global atmospheric circulation.

•  Warm air tends to rise because it is light, while cold air tends to sink because it is dense, this leads to motion in the atmosphere.

•  The tropical circulation is a good example of this.

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Primary Cause of the General Circulation

•  Surface air converges and rises at the equator, spreads laterally toward the poles, descends and flows back toward equator at the surface.

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Three Cells Circulation

•  A thermally direct circulation in the tropics (Hadley Cell), with rising motion around the equator and sinking motion at about 30 latitude.

•  A thermally indirect circulation in the middle latitudes (Ferrel Cell), with rising motion at 60 and sinking motion along with the Hadley cell at about 30.

•  Another weaker direct circulation in the polar regions (polar cell) with rising motion at about 60 and sinking motion over the pole.

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Global Air Circulation

http://www.emc.maricopa.edu/faculty/farabee/biobk/deserts.gif

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Dividing the global into several latitude bands

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Definition of the key terms

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Intertropical convergence zone (ITCZ) – the boundary where the northeasterly trade winds in NH converge with the southeasterly trade winds in the SH

Polar Front – a semi permanent front that separates tropical air masses from polar air masses

Subpolar low – a zone of low pressure near the polar front

Polar Easterlies – shallow layer of northeasterly flows formed when the southward moving cold polar air behind the polar front is deflected by the Coriolis force to the right of its path.

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Weak winds near equatorial region and 30o latitude belt

Easterly winds dominate

from equator to 30o (Trade winds)

from pole to 60o (Polar easterly)

Westerly winds predominate

between 30 to 60o (mid-latitude)

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Key features of the Three-Cell Model

Ferrel cell

Polar high

Subpolar low

Subtropical high

Tropical low

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Jet Streams –

  very strong winds concentrated within a narrow band in the atmosphere

  may be thousands of kilometers long, a few hundred meters wide, and a few kilometers thick

  are usually found at the tropopause between 10 and 15 km

  the region of the greatest wind within the jet stream is called the jet streak

  typically find two jet streams in the NH – polar front jet stream and the subtropical jet stream

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Jet Streams •  North polar jet stream meanders with movement of

polar front –  In winter may extend as far south as 30o N –  In summer average position is 50o N –  Because of this migration described as mid-latitude jet

stream

•  Semi-permanent jet stream forms over tropics in winter –  Slower than polar jet stream –  Centered at about 25o N

rst.gsfc.nasa.gov/Sect14/Sect14_1c.html

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What produces Jet Streams?

Rapid changes in pressure in a relative narrow region sets up a steep pressure gradient that intensifies the wind speed and causes the jet stream

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Pressure and wind patterns in the real world

•  The presence of continents, mountains, and ice fields alters the general circulation from the ideal 3-cell model.

•  Semi-permanent high and low pressure systems persist throughout large periods of the year –  During winter, highs form over land; lows over oceans. Vice

versa during summer.

•  The major pressure systems, wind belts, ITCZ shift toward south in January and toward north in July.

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Intertropical Convergence Zone (ITCZ)

•  Inter-tropical Convergence Zone (ITCZ): The east-west oriented band of intense convection located over the warmest regions of the tropics.

•  Inside ITCZ, the deep convection is supported by the release of latent heat when moisture condenses.

Source: NOAA GOES-11 & NASA Earth Observatory

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Intertropical Convergence Zone (ITCZ)

•  ITCZ moves back and forth across equator following the sun’s zenith point.

•  Variation in the ITCZ locations affect rainfall in the tropics.

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Four semi-permanent pressure systems:

Bermudar high and Pacific high (near latitude at 30o),

Icelandic low and Aleutian low (between 40 – 65o)

Other highs and lows that are not permanent in nature (change significantly during the course of the year):

Siberian high (becomes thermal low in July), intense, responsible for Indian monsoons

Canadian high (becomes a thermal low in July), less intense, responsible for Mexico monsoons

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Walker Circulation

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El Nino

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El Niño •  Spanish for ‘Christ Child’

•  Originally used by fishermen along the coast of Peru and Ecuador in referring to a warm ocean current appearing near Christmas and lasting a few months

•  Fish are less abundant during these warm periods because there is less ‘upwelling’ of cold nutrient-rich water

•  During some years, however, the water remains especially warm and the break in the fishing season extends into May

•  The term ‘El Niño’ has become reserved for these extreme events

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El Niño’s economic impact

•  Economic impact on Peruvian fishing is negative

•  Seabirds that feed on fish also suffer, along with other parts of the coastal ecosystem

Rough estimate of damage around the globe during 1982-83 El Nino.

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El Niño Conditions

•  Once every few years, the normal pattern ‘breaks down’

•  Pressure decreases in the eastern high and pressure increases in the western low

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•  Mean pressure is lower at Darwin than Tahiti • The SOI measures the strength of the Pacific Walker circulation

The Southern Oscillation Index

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El Niño Southern Oscillation Index

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El Niño Conditions •  The easterly trade

winds weaken (or even reverse)

•  The warm waters of the western tropical Pacific come back to the east because the trade winds are too weak to sustain the tilted sea level

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El Niño Conditions •  The upwelling or

rising of the cold nutrient-rich waters off the South American coast ceases

•  The upward motion, clouds and precipitation moves eastward into the central and eastern Pacific

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Sea-Saw of sea level pressure associated with El Niño and La Niña is shown. During an El Niño the sea level

pressure in the east Pacific decrease while that over west Pacific and Indonesia increases.

In a La Niña it is higher pressure in the east and lower pressure in the west.

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El Nino has global consequences

•  Droughts in Australia and Indonesia

•  Warm, rainy weather in the eastern Pacific and South America

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Droughts in Australia Flooding in California

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El Nino has global consequences

•  Droughts in Australia and Indonesia

•  Warm, rainy weather in the eastern Pacific and South America

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La Niña

•  Opposite to El Niño •  Higher than average pressure in the eastern Pacific

and lower than average pressure in the western Pacific

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La Niña

•  Stronger than normal easterly trade winds

•  Colder than normal ocean temperatures off of South America

•  Flooding in Australia and Indonesia

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The North Atlantic Oscillation •  The NAO is the dominant mode of winter climate

variability in the North Atlantic region ranging from central North America to Europe and much into Northern Asia.

•  The NAO is a large scale seesaw in atmospheric mass between the subtropical high and the polar low. The corresponding index varies from year to year, but also exhibits a tendency to remain in one phase for intervals lasting several years.

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The North Atlantic Oscillation

•  When the NAO is in its positive phase, low pressure anomalies over the Icelandic region and throughout the Arctic combine with high-pressure anomalies across the subtropical Atlantic to produce stronger-than-average westerlies across middle latitudes

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The Positive NAO index •  phase shows a stronger than usual

subtropical high pressure center and a deeper than normal Icelandic low.

•  The increased pressure difference results in more and stronger winter storms crossing the Atlantic Ocean on a more northerly track.

•  This results in warm and wet winters in Europe and in cold and dry winters in northern Canada and Greenland

•  The eastern US experiences mild and wet winter conditions

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The negative NAO index •  phase shows a weak subtropical high and

a weak Icelandic low. •  The reduced pressure gradient results in

fewer and weaker winter storms crossing on a more west-east pathway.

•  They bring moist air into the Mediterranean and cold air to northern Europe

•  The US east coast experiences more cold air outbreaks and hence snowy weather conditions.

•  Greenland, however, will have milder winter temperatures

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1.  The largest pattern of winds that persists around the globe is called the general circulation.

2.  At the surface in both hemispheres, winds tend to blow from east in the tropics, from the west in the middle latitudes, and from the east in polar regions.

3.  Where upper-level westerly winds tend to concentrate into narrow bands, we find jet streams.

7.  Near the equator, the intertropical convergence zone (ITCZ) is a boundary where air rises in response to the flowing together of the northeast trades and the southeast trades.

8.  When the warm water extends over a vast area of the tropical Pacific, the warming is called a major El Nino event, and the associated reversal of pressure over Pacific Ocean is called the Southern Oscillation.

SUMMARY