1. Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt
Anthony J. Vega

2. Part 3. Distribution and Movement of Air
Chapter 8
Atmospheric Circulation and Pressure Distributions

3. Introduction Well-defined pressure patterns exist across the globe
These define the general circulation of the planet
In describing wind motions:
Zonal winds are those which blow parallel to lines of latitude
Meridional winds move along lines of longitude
The general circulation of the atmosphere may be examined through a single-cell or three-cell model

4. Single-Cell Model
This model, proposed by George Hadley, assumed an ocean-only planet with a fixed solar declination
A single convection cell per hemisphere would redistribute heat from the equator to the poles under such conditions
Coriolis deflection would cause surface winds to be primarily easterly
Although incomplete, Hadley’s single-cell model was essential in identifying consequences of a thermally direct circulation
Single-Cell Model

5. Three-Cell Model Each hemisphere is divided into three pressure cells
The first is the thermally driven:
Hadley cell
In the tropics air is heated through high solar angles and constant day length
Air becomes heated, expands, and diverges toward higher latitudes
The equatorward boundary of the Hadley cell is characterized by expanding and ascending surface air that forms the equatorial low, or intertropical convergence zone (ITCZ)
The ITCZ (or doldrums) is usually found near the vertical solar ray
It is usually characterized by clouds and heavy precipitation
Reflects some of the wettest areas on Earth
Ascending air diverges poleward aloft
Air gains considerable westward momentum and descends in the subtropics
The zonal component far exceeds the meridional component

6. The ITCZ is observable
as a band of clouds
extending from northern
South America into the
Pacific

7. Between 20 and 30o latitude, air descends forming the subtropical highs, or horse latitudes
Compressional warming creates clear, dry conditions near the centers of the highs
Surface air flow is primarily from the subtropical highs towards the ITCZ
The addition of Coriolis deflection results in the northeast (southeast) trade winds in the northern (southern) hemisphere
Hadley cell strength increases during the cool season when thermal contrasts are maximized
Ferrel and Polar Cells
Constitute the remaining hemispheric cells
Ferrel cells lie poleward of each Hadley cell
They circulate air between the subtropical highs and the subpolar lows

8. The subpolar lows result from surface air converging from the equatorward subtropical high and the poleward polar high
The Ferrel cell is an indirect cell as it is formed from air motions initiated by adjacent cells
Air moving from the subtropical highs towards the subpolar lows undergoes Coriolis deflection causing the westerlies in both hemispheres
Polar Highs
Thermally direct cells formed by very cold temperatures near the poles
Air in these locations becomes very dense resulting in sinking motions indicative of high pressure
Air moving equatorward is deflected by Coriolis creating the polar easterlies in both hemispheres

9. The Three-Cell Model

10. The Three-Celled Model vs. 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 poleward of about 30o do not show the persistence of the trade winds, however, long-term averages do show a prevalence indicative of the westerlies and polar easterlies
For upper air motions, the three-cell model is unrepresentative
The Ferrel cell implies easterlies in the upper atmosphere where westerlies dominate
Overturning implied by the model is false
The model does give a good, simplistic approximation of an earth system devoid of continents and topographic irregularities

11. Semipermanent Pressure Cells
Instead of cohesive pressure belts circling the Earth, semipermanent cells exist
Cells are either dynamically or thermally produced
Fluctuate in strength and position on a seasonal basis
For the northern hemisphere they include:
The Aleutian, Icelandic, and Tibetan lows
The oceanic (continental) lows achieve maximum strength during winter (summer) months
Siberian, Hawaiian, and Bermuda-Azores highs
The oceanic (continental) highs achieve maximum strength during summer (winter) months
The summertime Tibetan low is important to the east-Asia monsoon
Sinking motions associated with the subtropical highs promotes desert conditions across the affected latitudes
Seasonal fluxes in the pressure belts relate to the migrating vertical ray of the Sun

12. The ITCZ lags slightly behind the vertical solar ray into the summer hemisphere which causes a poleward migration of the subtropical highs and a weakening of the higher latitude oceanic lows
In the winter hemisphere, opposite conditions result as the oceanic lows strengthen and the subtropical highs weaken and migrate equatorward
Such migrations greatly influence temperature and precipitation regimes across the globe
Especially evident concerning the Sahel region of Africa
The situation is best exemplified in the tropics where seasonal precipitation is closely tied to variations of the subtropical highs and ITCZ

13. January

14. July

15. The Sahel, a region bordering the
southern Sahara Desert
Shifts in the ITCZ bring rain to
the Sahel in summer.
During most of the year the ITCZ
and the rain is located south of
the region

16. The Upper Troposphere
Upper tropospheric heights decrease poleward from lower latitudes due to the increased density of colder air
The arrangement of heights details stronger pressure gradients for the winter hemisphere
Heights are also higher during summer as density decreases with heating
Decreasing heights
with latitude

17. Westerly Winds in the Upper Atmosphere
Thermal differences corresponding to upper air height differences ensure lowered heights from equator to pole
Upper air motions are directed towards the poles but are redirected to an eastward trajectory due to Coriolis deflection
Westerly winds, therefore, dominate the upper troposphere
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
The Polar Front and Jet Streams
Strong boundaries occur between warm and cold air
In the mid-latitudes, the polar front marks this thermal discontinuity at the surface
The polar jet stream, a fast stream of air, exists in the upper troposphere above the polar front
Winds are about 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 poleward

18. Profile of the polar jet
The subtropical jet is seen as a
band of clouds extending from
Mexico on an infrared satellite
image

19. Troughs and Ridges Rossby Waves
Height contours meander considerably across the globe
High heights extending poleward are called ridges
Equatorward dipping lower heights are known as troughs
Rossby Waves
Ridges and troughs comprise waves of air flow
Rossby, or long-waves, are the largest of these configurations
Each has a particular 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

20. Profile of ridges and troughs
A trough over the mid-continent is
depicted in b

21. Sequence of Rossby Wave Migration

22. Rossby waves in the upper air
advect cold or warm air (left),
evident by surface temperatures
(below)

23. The Oceans Ocean Currents Represent horizontal water motions
Profound impact on the atmosphere which is influenced by the ocean temperatures
Currents are created by wind stress but water moves at a 45o angle to the right (N.H.) from the wind flow
Current speeds decrease and the direction turns increasingly towards the right (N.H.) with depth
This Ekman Spiral, initiated by Coriolis force, becomes negligible at a depth of about 100 m
The North and South Equatorial Currents pile water westward and help create the Equatorial Countercurrent
Western basin edges are dominated by warm poleward directed currents (example: Gulf Stream) while cold currents, directed equatorward, occupy the eastern basins
Overlying air temperatures reflect these surface temperatures

24. Infrared Satellite Image of the Gulf Stream

25. Ekman Spiral
Global Ocean Circulation

26. Major Wind Systems Upwelling Monsoons
Strong offshore winds drag surface waters away from coastal locations
Colder waters from the deep ocean rise, or upwell, to replace these waters
Most pronounced off the western coast of South America where cold water upwelling helps ensure the driest desert on Earth, the Atacama
Major Wind Systems
Monsoons
The largest synoptic scale winds on Earth
A seasonal reversal of wind due to seasonal thermal differences between landmasses and large water bodies
Best exemplified by the east-Asian monsoon which is characterized by dry (wet), offshore (onshore) flow conditions during cool (warm) months
Orographic lifting assures large precipitation amounts for locations in the Himalayas which record some of the greatest precipitation amounts on Earth

27. The winter monsoon
The summer monsoon

28. Foehn, Chinook, and Santa Ana Winds
Winds which flow down the lee side of mountain ranges
Air undergoes compressional warming
Foehn winds are initiated when mid-latitude cyclones pass to the southwest of the Alps
Chinooks are similar winds on the eastern side of the Rocky Mtns.
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
Occur when high pressure is located to the east
Often spread wildfires
Katabatic Winds
Cold, dense air flows sinking down mountain sides from high elevations
Boras winds of the Balkan Mountains and the Mistral winds of France are examples

29. Valley and Mountain Breeze
Sea and Land Breeze
Sharp interfaces between land and sea may produce a land and sea breeze circulation
Occur in relation to the differential surface heating on a diurnal scale
During the day (night) land (water) surfaces are hotter (cooler) than large water (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, land breezes blow out to sea
Valley and Mountain Breeze
Diurnal variation similar to a land/sea breeze occur in mountainous areas
Solar facing mountain slopes heat more intensely than shaded valley areas developing a thermal low during the day which produces a valley breeze
At night the situation reverses producing a mountain breeze

30. Sea breeze development
Sea breezes initiate clouds
and precipitation on the
island of Hawaii

31. Development of a Valley and Mountain Breeze

32. Air-Sea Interactions El Niño, La Niña, and the Walker Circulation
El Niño events are characterized by unusually warm waters in the eastern equatorial Pacific Ocean
These events have been linked to anomalous global weather events
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
Western waters, piled previously by strong trade wind flow migrates eastward
Cooler waters in the east are replaced by warmer waters causing a reversal of the Walker Circulation
Under normal conditions, high atmospheric pressure dominates the east while low pressure dominates the west
As the warm water pool migrates eastward, the pressures reverse

33. Pacific Decadal Oscillation
The Southern Oscillation is inherently linked to the oceanic variations such 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
Anomalous cooling occurs in the eastern Pacific during these events
Distinct, but different, global teleconnection patterns result
Individual ENSO and La Niña events produce different regional weather anomalies
Pacific Decadal Oscillation
A large, long-lived oscillation pattern exists across the Pacific Ocean
The Pacific Decadal Oscillation (PDO) involves two modes of sea surface temperatures (SST)
One exists in the northern and westerly part of the basin, the other in the eastern tropical Pacific
The PDO describes a yr SST oscillation

34. The “Normal” Walker Circulation
The PDO affects climatic impacts associated with El Niño events
When the PDO is in a warm phase (high temperatures in the eastern tropical Pacific), El Niño impacts on weather are more pronounced than when the PDO is in a cool phase
The “Normal” Walker Circulation

35. SSTs during an ENSO event
SSTs associated with the PDO

36. End of Chapter 8 Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt