METEOROLOGY

GEL-1370

Chapter SevenChapter Seven

Atmospheric CirculationsAtmospheric Circulations

Goal for this ChapterWe are going to learn answers to the following questions:• 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• 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

• 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

Eddies – contd.

• 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 resulting from the heating and cooling of air

• 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• 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.

• 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

• 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 high-pressure 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.

• 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.

• 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 Nińo: 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.

• 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

• 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.

• 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

• General Circulation: It represents the average air flow around the world; caused by unequal heating of the earth’s surface

• What we have learnt:– 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

• General Circulation Models: Single-cell Model & Three cell Model

• Single-cell Model Assumptions:– 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• 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

• Features: Tropical regions receive an excess of heat & poles a deficit

• In each hemisphere, three cells redistribute energy– 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. • 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.

• 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

• 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):– 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

• Four semipermanent pressure systems in the Northern Hemisphere during January:– 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

• 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

• 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 (100-200 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.

• 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

• 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

• 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

• 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:

– 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

• 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• 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

• 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

• Within the changing of seasons, especially the transition periods of spring & fall

• 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