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Lecture 13

General Circulation

Jet streamEl Nino and La

Nina

Hadley cell Thermal cell

Hadley cellLight, variable winds at the equator are

known as DoldrumsSimilarly, little wind at 30o N and S is

known as the Horse Latitudes

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Polar cell Thermal cell

What is difference between two convergent Lows: ITCZ and subpolar Low?

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Ferrel cell Dynamic cell

Ferrel cell is not successful in explaining the westerlies aloft.

Scales of Atmospheric Motions

Time and space scale of atmospheric motions

Global scale 5000 km

Synoptic scale 2000 km

Mesoscale 20 km

Microscale 2m

Typical size

Typical life span

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Where is dominated by high pressure in winter between land

and ocean? Why?

Fig. 7-26, p. 190

Observed surface pressure winterSemi-permanent pressure areas

and seasonal pressure areas

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Observed surface pressure January

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Semi-perman

entPacific

High

Semi-perman

ent Bermuda

High

Observed surface pressure January

Semi-permane

nt Aleutian

Low

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Semi-permane

nt Icelandic

Low

Observed surface pressure January

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Seasonal pressure

areas:Siberian High

Seasonal pressure

areas:Canadian High

Fig. 7-26, p. 190

Observed surface pressure summerSemi-permanent pressure areas

and seasonal pressure areas

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at the center of a surface low, the air converges, and then must rise

at the center of a surface high, the air diverges, and must come from aloft due to sinking motion

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H L

Global precipitation pattern

Produced by the general circulation

Zonal distribution of precipitation:

0oN---low pressure cloudy

30oN---high pressure sunny

45-60oN---low pressure cloudy

Polar latitudes---high pressure clear

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Weather associated with The Pacific and Bermuda Highs

Pacific High (1) moves northward during summer produced strong subsidence inversion on eastern side (2) during winter, it moves south allowing polar fronts to bring precipitation to SW US

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Bermuda High transports moist, warm subtropical air to US and southern Canada This air can be unstable

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Weather associated with The Pacific and Bermuda Highs

Jet Steams

Jet streamsA part of the general circulation

Polar Jet Situated at about 10 km AGL over the polar front (30-70oN)

Subtropical Jet Situated above the subtropical highs at about 13 km AGL(20-50oN)

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Jet streamsWind speed > 60 knots (111

km/h / 69 mph)

1 knot = 1.151 miles per hour

often have troughs and ridges generally have a maximum (jet streak) in the base of the trough transport heat pole ward (cold air south and warm air north)

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Jet stream waves

Jet stream formationPolar Jet

Large temperatureGradient at surface

45oN

5 km

EQ

2

4

6

8

10

0

NP

700 mb

500 mb

Tropopause

200 mb

Warm Cold

km

PGF

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Jet stream formationPolar Jet

Q: How is the

subtropical jet

formed? 21

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Which position is fast?

A B

Jet stream formationSubtropical Jet

-formed on pole ward side of Hadley cell-created largely through the conservation of angular momentum

-angular momentum = mVrm=massV=velocityr=radius

-conservation of angular momentum

1r

2r

1m

2m

1V

2V1 1 1 2 2 2 constantmV r m V r

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El Nino and La Nina

Coastal Upwelling

• Coastal divergence results in upwelling as cold water rises to replace surface water

brings cold water from deep ocean to the surface

cold, nutrient-rich water rises to replace the surface water (good for fishing)

Walker Circulation East-west circulations caused by continent and topography

Typical Walker Circulationrising air is over the western Pacific, and sinking

air is over the eastern Pacific

PGF

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Circulation during an El Nino

Abnormal Walker Circulation El Nino year occurs with an abnormal Walker circulation

PGF

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El Nino refers to eastern movement of warm water 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)

El Nino related to the abnormal Walker circulationSpanish name for a little boy

during certain years, the coastal waters near Peru were abnormally warm, causing unfavorable fishing conditions. This would occur during the Christmas period

occur every 3-7 years, lasting about one year

La Nina related to the strong Walker circulation Spanish name for a little girl

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Previous El Niño Years1902-1903 1905-1906 1911-1912 1914-1915

1918-1919 1923-1924 1925-1926 1930-1931

1932-1933 1939-1940 1941-1942 1951-1952

1953-1954 1957-1958 1965-1966 1969-1970

1972-1973 1976-1977 1982-1983 1986-1987

1991-1992 1994-1995 1997-1998 2002-2003

2009-

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Fig. 7-32, p. 196

Sea surface temperature (SST) in a normal year

Low pressure dominates the western Pacific over warm water, and high pressure dominates the eastern Pacific over cold water.

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Fig. 7-32, p. 196

SST in an El Nino yearduring an El Nino year, the SSTs in the eastern Pacific become more warmer

than in a normal yearhigh pressure shifts from the eastern Pacific to the western Pacific low pressure shifts from the western Pacific to the eastern Pacific this shift in surface pressure is called El Nino Southern Oscillation (ENSO)

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El Nino can have a dramatic effect on weather around the world, creates both

precipitation and temperature anomalies. These changes are referred

to as the teleconnections

ONIOceanic Nino Index

Nino Region 3.4

Oceanic Niño Index (ONI)

• The ONI is based on SST departures from average in the Niño 3.4 region, and is a principal measure for monitoring, assessing, and predicting ENSO.

• Defined as the three-month running-mean SST departures in the Niño 3.4 region. Departures are based on a set of improved homogeneous historical SST analyses (Extended Reconstructed SST – ERSST.v3b). The SST reconstruction methodology is described in Smith et al., 2008, J. Climate, vol. 21, 2283-2296.)

• NOAA’s operational definitions of El Niño and La Niña are keyed to the ONI index.

NOAA Operational Definitions for El Niño and La Niña

El Niño: characterized by a positive ONI greater than or equal to +0.5°C.

La Niña: characterized by a negative ONI less than or equal to -0.5°C.

By historical standards, to be classified as a full-fledged El Niño or La Niña episode, these thresholds must be exceeded for a period of at least 5 consecutive overlapping 3-month seasons.

CPC considers El Niño or La Niña conditions to occur when the monthly Niño3.4 OISST departures meet or exceed +/- 0.5°C along with consistent atmospheric features. These anomalies must also be forecasted to persist for 3 consecutive months.

ONI (oC): Evolution since 1950

El Niño

La Niña

neutral

The most recent ONI value (August – October 2011) is -0.4oC.