understanding weather and climate · worldwide unusual weather events. –after an enso event, the...

Atmospheric

Circulation

and Pressure

Distributions

Chapter 8 Lecture

Redina L. Herman

Western Illinois University

Understanding

Weather and

Climate

Seventh Edition

Frode Stordal, University of Oslo

• Some atmospheric features cover large portions of

Earth and are maintained over extensive time

period, referred to as global scale.

• High and low pressure patterns over large parts of

continents (hundreds or thousands of square km)

occur at what is called synoptic scale.

– Mesocale covers just a few square km to hundreds of

square km.

– Microscale refers to a very small scale, like ripples that

form on snow or a sandy beach.

The Concept of Scale

Single-Cell and Three-Cell Models

Single-Cell Model

• The single-cell model describes the

general movement of the atmosphere

and was proposed by George

Hadley.

• Zonal winds move in an east/west or

west/east direction, while meridional

winds move in north/south or

south/north direction.

• Hadley thought heating at the equator

caused a circulation pattern in which

air expands upwards and diverges

toward the poles, sinks to the

surface, and returns to the equator.

Single-Cell Model

• Coriolis deflection would cause

surface winds to be primarily

easterly.

• Although incomplete, Hadley’s

single-cell model was essential in

identifying the consequences of a

thermally direct circulation.

Single-Cell Model

The Three-Cell Model

• The three-cell model

was proposed by William

Ferrel.

• This model divides each

hemisphere into three

cells.

– Hadley cell: circulates air

between the tropics and

subtropics

– Ferrel cell: circulates air in

the middle latitudes

– Polar cell: circulates air at

the poles

• Each cell consists of

rising air with low surface

pressure, a zone of

sinking air with surface

high pressure, a surface

wind zone with air flowing

from high to low

pressure, and an airflow

in the upper atmosphere

from the rising and

sinking air.

• The Hadley Cell

– Intense heating at the

equator creates a zone of

low pressure called the

equatorial low, or the

Intertropical Convergence

Zone (ITCZ).

– The ITCZ is the rainiest

latitude zone in the world.

– The Hadley cell sinks

toward the surface about

20–30° latitude to form the

subtropical highs (large

band of high pressure).

• The Hadley Cell

– The NE trade winds in

the Northern Hemisphere

and the SE trade winds in

the Southern Hemisphere

are deflected to the right

and left.

– The Hadley cell is

strongest in the winter

season, when temperature

gradients are the

strongest.

• The Ferrel Cell • Ferrel cell circulates air

between the subtropical highs and the subpolar lows (areas of low pressure).

• Air moving from the subtropical highs toward the subpolar lows is deflected by Coriolis, causing the westerlies in both hemispheres.

• The Polar Cell • Polar cell circulates surface

air from the polar highs (areas of high pressure) to the subpolar lows.

• Thermally direct circulations are formed by very cold temperatures near the poles.

• Air moving toward the equator is deflected by Coriolis, creating the polar easterlies in both hemispheres.

• The Three-Cell Model versus

Reality: The Bottom Line

– Pressure and winds associated

with Hadley cells are close

approximations of real-world

conditions.

– Ferrel and Polar cells do not

approximate the real world as well.

– Surface winds of about 30

degrees and above do not show

the persistence of the trade winds;

however, long-term averages do

show a prevalence indicative of

the westerlies and polar easterlies.

• The Three-Cell Model

versus Reality: The

Bottom Line

– For upper-air motions, the

three-cell model is

unrepresentative.

– The model does give a

good, simplistic

approximation of an earth

system devoid of

continents and topographic

irregularities.

Semipermanent Pressure Cells

• Instead of cohesive pressure belts

circling Earth, semipermanent

cells of high and low pressure

exist, fluctuating in strength and

position on a seasonal basis.

• These cells are either dynamically

or thermally created.

• For the Northern Hemisphere they

include:

– The Aleutian, Icelandic, and Tibetan

– Siberian, Hawaiian, and Bermuda-

Azores highs

• Sinking motions associated

with the subtropical highs

promote desert conditions

across specific latitudes.

• Seasonal fluxes in the

pressure belts relate to the

migrating Sun (solar

declination).

• Westerly Winds in the

Upper Atmosphere

– Thermal differences

correspond to upper-air

height differences.

– Upper-air motions are

directed toward the poles but

are redirected to an eastward

trajectory due to Coriolis

deflection.

The Upper Troposphere

• Westerly Winds in the

Upper Atmosphere

– Westerly winds dominate the

upper troposphere and are

strongest during winter when

latitudinal thermal gradients

are maximized.

– Speeds also increase with

altitude as contours slope

more steeply with height due

to latitudinal thermal

differences.

• Westerly Winds in the Upper Atmosphere

• The Polar Front and Jet Streams

– Polar fronts are strong boundaries that occur between

warm and cold air.

– In the midlatitudes, the polar front marks this thermal

discontinuity at the surface.

– The polar jet stream, a fast stream of air sometimes called

“rivers,” exists in the upper troposphere.

• Winds are twice as strong in winter as summer.

– Near the equator, the subtropical jet stream exists as a

mechanism to transport moisture and energy from the

tropics toward the poles.

• The Polar Front and Jet Streams

• Troughs and Ridges

– Height contours meander considerably across the globe.

– The bulges of heights extending toward the poles are

called ridges.

– The valley of low heights extending toward the equator is

known as troughs.

• Troughs and Ridges

• Rossby Waves

– The largest of the atmospheric long waves is called the

Rossby wave.

– Three to seven Rossby waves circle the globe at any one

time, and each has its own wavelength and amplitude.

– Although they have preferred anchoring positions, they do

migrate eastward.

– The number of Rossby waves is maximized in winter and

decreases in summer.

– They are instrumental to meridional transport of energy

and also play an important role in determining areas of

divergence and convergence important to storm

development.

• Rossby Waves

• Atmospheric Rivers

• Monsoons

– Monsoon indicates a seasonal reversal in surface winds.

– Monsoon occur due to seasonal thermal differences

between landmasses and large water bodies.

– The East Asian monsoon is characterized by dry, offshore

flow conditions during cool months and wet, onshore flow

conditions during warm months.

– Orographic lifting brings larger precipitation amounts for

locations in the Himalayas, which record some of the

highest precipitation amounts on Earth.

Major Wind Systems

• Monsoons

Major Wind Systems

• Foehn, Chinook, and Santa Ana Winds

– Foehn winds flow down the side of mountain slopes. Air

undergoes compressional warming. They are initiated

when midlatitude cyclones pass to the southwest of the

– Chinooks are similar winds on the eastern side of the

Rocky Mountains and form when low pressure systems

occur east of the mountains.

– Both Foehn and Chinook winds are most common in

winter.

– Santa Ana winds occur in California during the transitional

seasons, especially autumn, when high pressure is located

to the east. The Santa Ana winds often contribute to the

spread of wildfires.

Major Wind Systems

• Katabatic Winds

– Katabatic winds warm by compression but originate when

air is locally chilled over high elevations. The air becomes

dense (with low temperature) and flows downslope.

– Common along Antarctica and Greenland ice sheets.

– Also referred to as Boras winds of the Balkan Mountains

and the Mistral winds of France.

Major Wind Systems

• Sea and Land Breezes

– Temperature differences between land and sea produce a

land and sea breeze circulation.

– During the day, land surfaces are hotter than large water

surfaces. During the night, water surfaces are hotter than

land surfaces.

– A thermal low develops over the warmest region.

– Air converges into the low, ascends, and produces clouds

and possibly precipitation.

– Sea breezes blow from the sea to land, while land breezes

blow out to sea from the land.

Major Wind Systems

• Sea and Land Breezes

Major Wind Systems

• Valley and Mountain Breezes

– Diurnal variation similar to a land–sea breeze occurs in

mountainous areas and are called valley and mountain

breezes.

– Mountains facing the Sun heat more intensely than shaded

valley areas. This develops a thermal low during the day

which produces a valley breeze.

– At night, the situation reverses producing a mountain

breeze.

Major Wind Systems

• Valley and Mountain Breezes

Major Wind Systems

• El Niño, La Niña, and the Walker Circulation

Ocean–Atmosphere Interactions: ENSO

– El Niño events are characterized by unusually warm

waters in the eastern equatorial Pacific Ocean.

• Higher water temperatures lead to increased evaporation

rates and reduced air pressure.

• Occur every two to five years when trade winds, pushing

equatorial waters westward, reduce in strength.

– Cooler waters in the east are replaced by warmer waters

causing a reversal of the Walker Circulation.

– As the warm water pool migrates eastward, the pressures

reverse.

– The Southern Oscillation is inherently linked to the

oceanic variations that most El Niño events are dubbed

ENSO (El Niño/Southern Oscillation) events.

– The offsetting of atmospheric pressures contributes to

worldwide unusual weather events.

– After an ENSO event, the equatorial Pacific returns to a

normal phase, or a strengthened normal phase, La Niña.

– Individual El Niño and La Niña events produce different

regional weather anomalies.

Normal / LaNina situation

El Nino situation partly reversed

• Positive and negative feedbacks (Box 8.5)

– Internal processes

– Positive feedback explains appearance of

both El Niño and La Niña events

• Trade winds push ocean currents

• Ocean currents impact the SST, surface pressure,

Walker circulation and trade winds

• Well understood, first by Jacob Bjerknes

– Negative feedbacks break down both El

Niño and La Niña events

• Lag behind the positive feedbacks

• Not well understood

• Many unanswered questions, e.g. why irregular?

Based on pressure

difference

Azores - Reykjavik

Ocean–Atmosphere Interactions: NAO

Positive NAO Index

• 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

http://www.ldeo.columbia.edu/res/pi/NAO/

http://www.ldeo.colum

bia.edu/res/pi/NAO/

Negative NAO Index

• 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.

http://www.ldeo.columbia.edu/res/pi/NAO/

http://www.ldeo.colum

bia.edu/res/pi/NAO/

• Arctic Oscillation and North Atlantic Oscillation

– The oscillations of the Atlantic Ocean are known as the Arctic

Oscillation (AO) and the North Atlantic Oscillation (NAO).

– The NAO is in a positive phase when the pressure gradient is

greater than normal and negative when it is less than normal.

NAO+ NAO-

NAO: Atmospheric variability vs trend

under global warming (IPCC/AR5)

In the Northern Hemisphere, the NAO exhibit considerable

variability comparable in magnitude to anthropogenically

forced trends.

Hence, while the NAO is likely to exhibit a small trend towards

its positive polarity, there will continue to be considerable

variability on all time scales.

The Oceans STARTS HERE Not shown

• Causes of Ocean Currents

– Ocean currents are horizontal water motions of surface

water that are often found along the rims of the major

basins.

– Ocean currents greatly impact the atmosphere.

– Currents are created by wind stress but water moves at a

45° angle to the right (N.H.) from the wind flow.

– Current speeds decrease and the direction turns

increasingly toward the right (N.H.) with depth.

The Oceans

– The Ekman Spiral, initiated by Coriolis force, becomes

negligible at a depth of about 100 m.

The Oceans

– The North and South Equatorial Currents turn water

westward and help to create the Equatorial

Countercurrent.

The Oceans

– Western basin edges are dominated by warm poleward-

directed currents (for example, Gulf Stream), while cold

currents, directed equatorward, occupy the eastern basins.

– Overlying air temperatures reflect these surface temperatures.

The Oceans

• Upwelling and Downwelling

– Upwelling occurs when strong offshore winds along a

coastal region drag warmer surface waters seaward.

– Upwelling draws up cooler waters from below.

– Upwelling is most pronounced off the western coast of

South America, where cold water upwelling helps to create

the driest desert on Earth, the Atacama.

The Oceans

• Upwelling and Downwelling

– Downwelling occurs when surface waters cool, and also

when they lose moisture through evaporation. Salt is left

behind, which makes the water denser than the previously

fresher water.

– A good example of downwelling is in the North Atlantic,

where the warm North Atlantic Drift loses huge quantities

of heat and moisture to the atmosphere above.

The Oceans

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