METEOROLOGY
GEL-1370
Chapter Seven
Atmospheric Circulations
Goal for this Chapter
We are going to learn answers to the following questions:
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What are eddies? How are these eddies formed?
How are sea breezes and land breezes formed
How are monsoons are formed?
What are chinook? How they are formed?
What kind of weather sea breeze and chinook bring?
Why & how winds blow around the world the way they do?
How heat is transported from equatorial regions poleward?
What are El Nino? How are they formed
Scales of Atmospheric Motion
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Winds: Workhorse of weather, moves storms and large fair weather systems around the globe; transports heat,
moisture, dust, insects/bacteria, pollen, etc.
Circulations are arranged according to their sizes; hierarchy of motion is called scales of motion --- tiny gusts to
giant storms
Microscale: Eddies constitute the smallest scale of motion; few meter in diameter; form by convection or by the
wind blowing an obstruction; short-lived (few minutes)
Mesoscale (Meso: middle): Size from few km to ~100 km in diameter; lasts from minutes to a day; include local
winds, thunderstorms, tornadoes, and small trophical storms
Scales of atmospheric motion; tiny microscale motions constitute a part of the larger mesoscale
motions and so on
Scale of atmospheric motion with the phenomena’s average size and life span
Eddies
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Synoptic scale: Weather map scale; extend from 102-103 kms; life time: days to weeks
Planetary (global) scale: Largest wind pattern; wind pattern extend over the whole earth;
Macroscale: synoptic + planetary scales
Eddies: When wind encounters a solid object, eddy forms on the object’s downwind side; size and shape of eddy
depend on the size of the object and speed of the wind; wind flowing over a building produces a larger eddies that
can be size of the building
Mountain Wave Eddy: Strong winds blowing over a mountain in stable air produce a mountain wave eddy on the
downwind sie, with a reverse flow near the ground
Eddies – contd.
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Wind Sheer: Rate of change of wind speed or wind direction over a given surface
Clear air Turbulence (CAT): Turbulence produced in a clean air
Sea breeze: A coastal local wind that blows from the ocean to the adjoining land; leading edge of the breeze is
called sea freeze front
Breeze pushes the warmer, unstable humid air to rise and condense, producing rain showers
Thermal circulations: Air circulation primarily
air
resulting from the heating and cooling of
No horizontal variation in pressure --- no pressure gradient --- no wind (Fig.a)
Air flowing past a mountain range creates eddies eddies many km downwind from the mountain
Thermal circulation produced by heating & cooling of the atmosphere near the ground
Thermal circulations
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If the atmosphere is cooled in the North & warmed to the south, isobars bunch close together in the North while in warmed south,
they spread apart (Fig.b); this dipping of the isobars produces PGF aloft that causes the air to move from higher pressure to lower
pressure
After the air aloft moves from S to N, air piles up in the northern area; surface air pressure in the south decreases and north
increases; PGF is established at the earth’s surface from north to south and surface winds begin to blow from north to south
When cool surface air flows southward, it warms & becomes less dense; warm air slowly rises, expands, cools, and flows out the
top at an elevation of ~1 km above the surface; at this level, air flows horizontally northward toward lower pressure and the
circulation is completed by sinking & flowing out the bottom of the surface high
Formation of clear air turbulence along a boundary of increasing wind speed shear
Turbulent eddies forming downwind of a mountain chain in a wind shear zone produce these billow clouds
Sea & Land Breezes
Sea Breeze is a type of thermal circulation; uneven heating of land & water causes these mesoscale coastal winds; are
strongest during the afternoon when the temperature contrast between land & ocean occurs
Sea Breeze: A coastal local wind that blows from the ocean onto the land. The leading edge of the breeze is called Sea
breeze front
Land Freeze: A coastal breeze that blows from land to sea, usually at night, when land cools more quickly than the
water; temperature contrasts are much weaker are at night hence land breezes are usually weaker than sea breeze
Development of a sea breeze and a land breeze
Land Breeze – weaker & occurs during night time
Sea & Land Breezes – contd.
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Some coastal cities experience the sea breeze by noon & their highest temperature usually occurs much earlier than
in inland cities
Sea breeze in Florida help produce state’s abundant summertime rainfall
In UP in Michigan, afternoon clouds and showers are brought to the land by breezes while lakeshore areas remains
sunny, cool and dry
Monsoon – Seasonally changing winds
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Monsoon – derived from Arabic word ‘Mausim’ means seasons
Monsoon Wind system: One that changes direction seasonally, blowing from one direction in summer and from
the opposite direction in winter
During winter, air over the continent becomes much colder than the air over the ocean; a large, shallow highpressure area develops over Siberia, producing a clockwise circulation of air that flows out over the Indian Ocean
and South China Sea; hence winter monsoon means clear skies, with winds that blow from land to sea
Annual wind flow patterns associated with winter Asian Monsoon
Monsoon – contd.
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In summer, air over the continents become much warmer than air above the water; shallow thermal low develops
over the continental interior; heated air rises; moisture bearing winds sweeping into the continent from the ocean;
humid air converges with a drier westerly flow, causing it to rise; lifting air masses cool and the air reaches the
saturation point, resulting in heavy showers and thunderstorms
Summer monsoon of southeastern Asia (June – September) is wet, rainy weather season with winds blowing from
Sea to Land
Changing annual wind flow patterns associated with summer monsoon
Monsoon – contd.
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Strength of Indian monsoon related to the reversal of surface air pressure that occurs at regular intervals about every
2-7 years at opposite ends of the tropical South Pacific Ocean
El Niño: During this event, surface water near the equator becomes much warmer over the central and eastern
Pacific; over this region near equator, we find warm rising air, convection, and heavy rain; west of the warm water
(over the region influenced by the summer monsoon) , sinking air prohibits cloud formation and convection --During El Nino period, monsoon is likely to be deficient
Monsoon – contd.
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Summer monsoon on the southern hills of the Khasi hills in northeastern India, Cherrapunji, average annual rainfall
is 1080 cm (425 inch)
Monsoon wind systems can exist if large contrasts in temperature develop between oceans and continents
Southwestern US (Arizona and New Mexico), monsoonlike circulation exists
Valley Breeze: A local wind system of a mountain valley that blows uphill during the day
Mountain Breeze: A local wind system of a mountain valley that blows downhill at night
Katabatic Wind: Any wind blowing downslope, usually cold
Valley Breeze
Mountain breeze
Mountain slopes warm during the day, air rises and often condenses into cumuliform clouds
Other wind systems
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Chinook Wind: A warm, dry wind on the eastern side of the Rocky Mountains; source of warmth for a chinook is
compressional heating, as warmer (and drier) air is brought down from aloft
Foehn: A warm, dry wind in the Alps
Santa Ana Winds: A warm, dry wind that blows into southern California from the east off the elevated desert
plateau; Its warmth is derived from compressional heating
Haboob: A dust or sandstorm that forms as cold downdrafts from a thunderstorm turbulently lift dust and sand into
the air
Other wind systems – contd.
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Haboobs are most common in the African Sudan & in the desert southwest of the US (e.g. southern Arizona)
Whirlwinds or dust devils: The spinning vortices so commonly seen on hot days in dry areas
Difference between dust devil and Tornadoes: Circulation of a tornado descends downward from the base of a
thunderstorm; circulation of a dust devil begins at the surface, normally in sunny weather, although some form
beneath convective-type clouds
City near the warm air-cold air boundary can experience sharp temperature changes
Conditions that may enhance a chinook
A chinook wall cloud forming over the Colorado Rockies
Santa Ana conditions in January; downslope winds blowing into Southern California raised temp into the
upper 80s; elsewhere much lower
Formation of a dust devil; On a hot, dry day, the atmosphere next to the ground becomes unstable; air rises, wind
blowing past an obstruction twists the rising air
A dust devil forming on a clear, hot summer day just south of Phoenix, Arizona
Global Winds
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General Circulation: It represents the average air flow around the world; caused by unequal heating of the earth’s
surface
What we have learnt:
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Incoming Solar radiation = outgoing earth radiation
Energy balance is not maintained for every latitude
Tropics experience a net gain in energy & Polar regions suffer a net loss
Atmosphere & Ocean transport warm air poleward and cool air equatorward
General Circulation of the Atmosphere
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General Circulation Models: Single-cell Model & Three cell Model
Single-cell Model Assumptions:
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Earth’s surface is uniformly covered with water (differential heating between the earth & ocean is eliminated)
Sun is always directed over the equator (winds will not shift seasonally)
Earth does not rotate (No Coriolis force and only force is PGF)
A huge thermally driven convection cell in each atmosphere
Hadley Cell: A thermal circulation proposed to explain the movement of the trade winds; consists of rising air near
the equator & sinking air near 30° latitude
General circulation of air on a nonrotating earth uniformly covered with water & with the sun directly
above the equator
Names of different regions and their latitude
Single-cell Model
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Excessive heating of the equatorial area produces a broad region of surface low pressure, while at the poles
excessive cooling creates a region of surface high pressure; closed loop with rising air near the equator, sinking air
over the poles, and equatorward flow of air near the surface, and a return flow aloft. In this manner, some of the
excess energy of the tropics is transported as sensible and latent heat to the regions of energy deficit at the poles
Limitations: Too simplistic, Coriolis force does deflect the southward-moving surface air in the Northern
Hemisphere to the right, producing easterly surface winds
Idealized wind and surface pressure distribution over a uniformly water-covered rotating earth
Three-cell Model
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Features: Tropical regions receive an excess of heat & poles a deficit
In each hemisphere, three cells redistribute energy
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Polar Cell: Circulation from the pole to ~60° {cold air aloft sinks and reaches the surface & flows back toward the polar front)
Ferrel Cell: Midlatitude cell from ~30° to ~60°
Hadley Cell: From equator to ~30°
A surface high-pressure area is located at the poles & a broad trough of surface low pressure exists at the equator
Hadley Cell is driven by latent heat released by cumulus clouds and thunderstorms produced by warm air rising in the
equatorial region
Doldrums: Region near the equator characterized by low pressure and light, shifting winds
Three-cell model contd.
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Subtropical Highs: Rising air in the equatorial region reaches the tropopause, which acts like a barrier, causing the
air to move toward the pole and this air mass gets deflected by the Coriolis force providing westerly winds aloft in
both hemispheres; this air mass converges due to radiational cooling at the midlatitudes; convergence aloft leads to
increase in the mass of air above the surface; convergence of air aloft produces of belts of high pressure called
subtropical highs
Converging dry air leads to compressional warming; subsiding air produces clear skies & warm surface temp --major deserts of the world
Three-cell model – contd.
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Horse Latitudes: Belt of latitude ~30-35° where the winds are dry & predominantly light and the weather is hot
and dry
Trade Winds: Winds that occupy most of the tropics and blow from the subtropical highs to the equatorial low
(provided an ocean route to the New World)
InterTrophical Convergence Zone (ITCZ): The boundary zone separating the northeast trade winds of the
Northern Hemisphere from the southeast trade winds of the Southern Hemisphere
Westerlies: Winds that blow in the midlatitudes on the poleward side of the subtropical high-pressure areas
Names of surface winds & pressure systems over a uniformly water-covered rotating earth
Generalized wind distribution
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From TX to Canada – commonly winds blow out of the west, than from the east
Polar Front: A semipermanent, semicontinuous front that separates tropical air masses from polar air masses
Subpolar Low: A belt of low pressure located between 50° and 70 ° (consists of Aleutian low in the North Pacific
& Icelandic low in the North Atlantic in the Northern Hemisphere)
Polar Easterlies: A shallow body of easterly winds located at high latitudes poleward of the subtropic low
Generalized Picture: At the surface, 2 major high (~30° & poles) and low pressure areas (~60° & equator)
Wind distribution – contd.
Summary contd (generalized picture of surface winds):
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Trade winds extend from subtropical high to the equator
Westerlies from the subtropical high to the polar front
Polar easterlies from the poles to the polar front
Comparison of three-cell model with observations:
Upper level winds blow from west to east
Middle cell suggests an east wind aloft as air flows equatorward – does not agree with observations
Model agrees closely with winds & pressure distribution in the surface
Average surface winds and Pressure
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Four semipermanent pressure systems in the Northern Hemisphere during January:
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Bermuda high in the Eastern Atlantic (between 30° & 35 °)
Pacific high in the Pacific (between 25° & 35 °)
Icelandic Low (in North Atlantic, covers Iceland & Southern Greenland)
Aleutian Low (over Aleutian Islands in the N. Pacific)
Other non semipermanent: Siberian high (formed because of intense cooling of the land)
Sea-level pressure & Surface wind-flow patterns in January
Sea-level pressure & Surface wind-flow patterns in July
Formation of Monsoon
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During summer, land warms --- thermal lows are formed (July map, thermal lows are seen over desert southwest of US, plateau of
Iran & north of India) --- warm, moist air from the ocean is drawn, producing the wet summer monsoon
Between January & July, maximum surface heating shifts seasonally ---major pressure systems, wind belts and ITCZ shift toward
the north in July & toward south in January
Abundant rainfall where air rises and little where air sinks --- areas of high rainfall exist in the tropics where humid air rises & at
40-55° where midlatitude storms and the polar front force air upward
Areas of low rainfall occur near 30° in the vicinity of subtropical highs and in polar regions where the air is cold & dry
Major pressure systems & idealized air motions (heavy blue arrows) & precipitation patterns (blue: abundant rainfall)
Pacific high moves northward; sinking air in eastern margin causes dry weather; in the western margin of Bermuda
high, southerly winds bring humid air leading to abundant rainfall
Average annual precipitation for Los Angeles & Atlanta
Westerly winds & Jet stream
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Jet Streams: Relatively strong winds concentrated within a narrow band in the atmosphere
Several hundred miles long, less than several hundred miles wide, less than a mile thick; wind speed can exceed 100 knots (100200 knots); usually found at the tropopause at 10-14 km
In the Northern hemisphere, situated along the boundary layer where cold, polar air lies to the north & milder, subtropical air lies
to the south; sharp contrast in temp produces rapid horizontal pressure changes --- steep pressure gradient ---- PGF causes the jet
stream
N-S temp contrast along the front is strongest in winter and weakest in summer --- seasonal variations -- Winds blow stronger in
winter and jet moves farther south; in summer, jet stream is weaker and is usually found farther north (such as southern Canada)
Jet stream – swiftly flowing current of air; colder air lies to the north & warmer air to the south
Jet streams – contd.
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There are two jet streams, located in the tropopause gaps, where mixing of tropospheric & stratospheric air takes
place
Subtropical jet stream: 13 km above the subtropical high
Polar front jet stream: 10 km above & near the polar front
Jet streams play a major role in the global transfer of heat; they tend to meander; pollutants are transported to
farther distances by jet streams
Position of polar stream & subtropical jet stream at 300-mb during March 10, 1998; solid gray lines: Isotachs Heavy
lines: position of jet stream; Heavy blue lines: direction of cold air southward; heavy red: direction of warm air
Global wind patterns & the oceans
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Wind causes the surface water to drift --- moving water piles up, creating pressure differences within water itself
In North Atlantic, Gulf Stream, a warm water current, flows northward along the east coast of US, carries warm,
tropical water into the higher latitudes; Gulf stream provides moisture and heat for developing mid latitude cyclones
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As Gulf Stream moves toward Europe, it merges with North Atlantic Drift current system; other part flows
southward as the Canary Current equatorward;
Atmospheric and ocean circulation are closely linked; wind and ocean transport heat to higher latitude; leads to
energy balance
Major ocean currents: Blue: cold currents; Red: warm currents; 1: Gulf Stream; 2: North Atlantic Drift; 6: Canary
current; 16: California current
Winds & Upwelling
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When wind blows over the ocean, surface water is set in motion; it bends slightly right due to Coriolis effect; water
drifts away from coast in California current system; cold, nutrient-rich from below rises – upwelling
Benefits of upwelling: food for fish
Link between Ocean – Atmosphere - pocketbook:
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Once 2-7 years, surface atmospheric pressure pattern break down (WHY??), as air pressure over western Pacific and
falls over the eastern Pacific ---weakens trades and during strong pressure reversals, east winds are replaced by west
winds
A warm current of nutrient-poor tropical water moves southward, replacing the cold, nutrient-rich surface water – El
Nino (spanish for boy child) referring to Christ child
Average position of the polar front jet stream & subtropical jet stream in winter; both jet streams are
flowing into the page
El Nino & Southern Oscillation
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During El Nino event, large numbers of fish & marine plants may die; dead fish & birds litter the beaches of Peru
El Nino of 1972-1973 reduced Peruvian anchovy catch from 10.3 million metric tons in 1971 to 4.6 million metric
tons in 1972 --- fishmeal production dropped in 1972 --- Animal feed prices went up --- poultry prices went up by
40%
Southern Oscillation: See-Saw pattern of reversing surface air pressure at opposite ends of the Pacific Ocean;
pressure reversals and ocean warming are more or less simultaneous --- El Nino/Southern Oscillation or ENSO
Ordinary condition - Higher pressure over the southeastern Pacific & lower pressure near Indonesia
El Nino condition: Atm pressure decreases over the eastern Pacific and rises over the W. Pacific; trade winds weaken
or reverse direction; thermocline changes
SST during non El Nino conditions – upwelling along the equator and Peru coast keeps the water cool (blue color) in
the tropical eastern Pacific
SSTs: upwelling is greatly diminished, and warm water (red color) from the Western Pacific has replaced the cool
water
Regions of climatic abnormalities due to ENSO; months in black: during the same year; red: following year
El Nino & its impacts
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In the eastern equatorial Pacific, as high as 6°C than the normal has been observed
Warm water along the coastal areas of Ecuador & Peru chokes off the upwelling that supplies cold, nutrient-rich
water to South America’s coastal region
Warm tropical water fuels the atmosphere with additional warmth and moisture --- additional storminess & rainfall
Certain regions of the world experience too much rainfall & other regions have very little
Over the warm tropical central Pacific, the frequency of typhoons increases
Effects of El Nino
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Tropical Atlantic, between Africa and Central America: fewer hurricanes
Summer monsoon conditions tend to get weaker
Drought is felt in Indonesia, southern Africa, Australia
Heavy rains & flooding in Ecuador & Peru
Storms in to California
Heavy rain into the Gulf Coast states
La Nina: Cold surface water moving to Central and eastern Pacific & warm water confined to western tropical
Pacific
What Causes El Nino
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Within the changing of seasons, especially the transition periods of spring & fall
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Winter monsoon plays a major role in triggering a major El Nino event
ENSO and monsoon system are linked
Linked to out pocket books!!
chapter –7- Summary
Micro and macro-scale motion
Wind sheer; sea breeze and land breeze
Monsoon depression; development, causes
Valley breeze, katabatic wind; chinook wind, Santa Ana winds
Dust devil
ITCZ, location of Detroit, Chicago, Barrow, Honolulu – what types of wind system
Semi-permanent high and low pressure areas
Converging/diverging along polar front
Three- and one-cell general circulation model-driest areas
Westerlies and easterlies
Hadley, Ferrel cells
Subpolar lows, doldrums, horse latitudes
Where we do see deserts
Summary – contd.
Polar front jet stream, jet stream blow direction
Upwelling
North Atlantic Drift, Gulf Stream current, California current, Labrador
Major currents that flow parallel to the coast of North America
El Nino; where does the warming occur; Southern Oscillation