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

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

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

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

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

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

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

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

The Three-Cell Model

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

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

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

January

July

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

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

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

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

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

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

Sequence of Rossby Wave Migration

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

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

Infrared Satellite Image of the Gulf Stream

Ekman Spiral Global Ocean Circulation

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

The winter monsoon The summer monsoon

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

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

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

Development of a Valley and Mountain Breeze

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

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 20-30 yr SST oscillation

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

SSTs during an ENSO event SSTs associated with the PDO

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