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PART III: Distribution and Movement of Air
CHAPTER 8. Atmospheric Circulation and Pressure Distributions
Chapter Overview:
The general circulation of the globe is discussed. Global pressure cells and resulting winds are
emphasized through a simplified three-cell model. In addition, smaller scale wind systems are
detailed along with causes and results.
Chapter at a Glance:
Well-defined pressure patterns exist across the globe and define the general circulation of the
planet. In describing wind motions it is common to refer to zonal winds as those which blow
parallel to lines of latitude while meridional winds move along lines of longitude. <Web>
• Single-Cell Model - The general circulation single-cell model, proposed by George
Hadley, assumed a planet of only ocean and a fixed solar declination. On such a planet, a
single convection cell per hemisphere would redistribute heat from the equator to the poles.
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.
• The Three-Cell Model - In the simplified three-cell model, each hemisphere is divided
into three pressure cells. The first, occupying the tropics, is the thermally driven Hadley
cell. The second is the mid-latitude Ferrel cell, and the third a polar cell.
A. The Hadley Cell - In the tropics, air is heated through high solar angles and
constant day length such that 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). <Web> It is usually found near the vertical solar ray.
The ITCZ, or doldrums, is characterized by clouds and heavy precipitation, so
much so that it reflects the wettest areas on Earth. Air ascending in the ITCZ
diverges poleward aloft. This air gains a considerable westward motion from the
conservation of angular momentum and descends in the subtropics. The zonal
component far exceeds the meridional component of this upper atmospheric air.
Between 20o and 30o latitude, the air descends forming the subtropical highs, or
horse latitudes. Compressional warming creates clear, dry conditions near the
centers of these highs. Surface air flow is primarily from the subtropical highs
towards the ITCZ. The addition of Coriolis deflection results in the northeast trade
winds in the Northern Hemisphere and the southeast trades in the Southern
Hemisphere. The Hadley Cells are, therefore, comprised of the ITCZ, the
subtropical highs, the trade winds, and westerly motions aloft. Further, Hadley
Cell strength increases in the cool season when thermal contrasts are maximized.
B. The Ferrel and Polar Cells - Poleward of each Hadley cell lies the Ferrel cell
which circulates air between the subtropical highs and the subpolar lows. The
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subpolar lows result from surface air converging from the equatorward subtropical
high and the poleward polar high. The Ferrel cell is, therefore, an indirect cell as it
is formed from air motions initiated by the adjacent cells. Air moving from the
subtropical highs towards the subpolar lows undergoes Coriolis deflection causing
the westerlies in both hemispheres. The polar highs are thermally direct cells
formed by very cold temperatures near the poles. Air in these locations becomes
very dense resulting in the sinking motions indicative of high pressure. Air moving
equatorward is deflected by Coriolis creating the polar easterlies in both
hemispheres.
C. The Three-Cell Model vs. Reality: The Bottom Line - Pressure and winds
associated with the Hadley cells are close approximations of real world conditions.
The 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.
Also, the overturning implied by the model is false. However, the model does give
a good, simplistic approximation of an earth system devoid of continents and
topographic irregularities.
• Semi-Permanent Pressure Cells - Instead of cohesive pressure belts circling the Earth
as suggested by the three-cell model, semi-permanent pressure cells exist. These cells,
which are either thermally or dynamically produced, fluctuate in strength and position on a
seasonal basis. Semi-permanent pressure cells in the Northern Hemisphere include the
Aleutian, Icelandic, and Tibetan lows, and the Siberian, Hawaiian, and Bermuda-Azores
highs. The oceanic lows achieve maximum strength during the winter months while the
oceanic highs peak in intensity during the summer. The thermally driven continental highs
peak in winter while the continental lows achieve maximum strength during summer. The
summertime Tibetan low is an important contributor to the development of the east-Asian
monsoon. Sinking atmospheric motions associated with the subtropical highs promotes
desert conditions across affected latitudes. Seasonal fluctuations 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. This causes a poleward migration of the subtropical
highs in addition to 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. This is best exemplified in the tropics where
seasonal precipitation is closely tied to variations of the subtropical highs and the ITCZ.
• The Upper Troposphere - Upper tropospheric heights decrease poleward from lower
latitudes due to the increased density of colder air <CD7-3>. The arrangement of heights
details stronger pressure gradients for the winter hemisphere. Also, all heights are higher
during the summer as density decreases through a heated atmosphere. <CD6.6> <CD7.1>
• Westerly Winds in the Upper Atmosphere - Thermal differences corresponding to
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upper air height differences ensure lowered heights from equator to pole. Upper air
motions are, therefore, directed toward the poles but are re-directed to an eastward
trajectory due to Coriolis deflection. So westerly winds dominate the upper troposphere.
These winds are strongest during the winter months when latitudinal thermal gradients are
maximized. Also, wind speeds increase with altitude as the contours slope more steeply
with height due to latitudinal thermal differences.
A. The Polar Front and Jet Streams - Strong boundaries often 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. Although jet stream wind speeds vary they
are about twice as strong in winter as in summer. Near the equator the subtropical
jet steam, associated with the Hadley cell, exists as a mechanism that transports
moisture and energy from the tropics poleward.
B. Troughs and Ridges - Height contours meander considerably across the globe.
High heights that extend poleward are called ridges. Equatorward dipping lower
heights are known as troughs. <CD7.4>
C. Rossby Waves - Ridges and troughs comprise waves of air flow. <CD7.4>
Each Rossby wave (long wave), the largest of these configurations, has a particular
wavelength and amplitude. Although Rossby waves have preferred anchoring
positions, they also migrate eastward. The number of Rossby waves is maximized
in winter and decreases during summer. Rossby waves are instrumental to the
latitudinal transportation of energy and they also play an important role in
determining divergence and convergence areas of the upper atmosphere. <CD8.4>
• The Oceans - Oceanic waters help determine pressure center locations and associated winds.
A. Ocean Currents - Ocean currents, which represent horizontal water motions,
have a profound impact on the atmosphere that is influenced by the temperature of
the underlying waters. Ocean currents are created by wind stress but water moves
at a 45o angle to the right (N.H.) of the wind flow. Also, water speeds decrease and
the direction turns increasing 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 toward the western edges of
the ocean basins and help create the Equatorial Countercurrent. Western basin
edges are dominated by warm poleward directed currents such as the Gulf Stream.
Cold water currents, directed equatorward, occupy the eastern ocean basins.
Overlying air temperatures reflect these surface temperature differences.
B. Upwelling - Strong offshore winds drag surface waters away from coastal
locations. Colder waters from the deep ocean rise, or upwell, to replace these
waters. This is most pronounced off the western coast of South America were cold
water upwelling helps ensure the driest desert on Earth, the Atacama. Similar
processes occur off the west coast of every continent promoting cold ocean
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currents.
• Major Wind Systems - Global scale winds have already been discussed. Smaller scale
winds exist at the synoptic, meso-, and micro- scales.
A. Monsoons - The largest synoptic scale winds are the monsoons. These seasonal
reversals of wind are driven by thermal differences between landmasses and large
water bodies. They are best exemplified by the east-Asian monsoon, which is
characterized by dry (wet), offshore (onshore) flow conditions during the cool
(warm) months. The cool season situation stems from development of the
thermally driven Tibetan high and the subtropical jet stream. During warm months,
the situation reverses as a thermal low develops over the large Asian landmass.
Orographic lifting assures large precipitation amounts for locations in the
Himalayas which record some of the greatest monthly precipitation totals on Earth.
The small size of North America and the north-south alignment of mountain ranges
prohibits true monsoonal circulations. However, a strong summer thermal low
which develops over Arizona advects moist air and triggers precipitation initiating
the so-called Arizona monsoon.
B. Foehn, Chinook, and Santa Ana Winds - Winds that flow down the lee side of
mountain ranges have many localized names. All are similar in that air flowing
over the mountains undergoes compressional warming on the lee side. The name
Foehn applies to winds flowing over the Alps of Europe, and are initiated when
mid-latitude cyclones pass to the southwest. Chinooks are similar winds of the
eastern Rocky Mountains. They 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.
They occur when a high pressure cell is located to the east of California. Due to
intense compressional warming, and dry conditions indicative of high pressure,
Santa Ana winds often spread wildfires.
C. Sea and Land Breeze - Sharp interfaces between land and sea are often the sites
for land and sea breeze circulations. Both occur in relation to the differential
surface heating on a diurnal temporal scale. During day, land surfaces heat more
quickly than water and as a result a small thermal low develops. Air motion occurs
from the sea toward the land low creating a sea breeze. Air converges in the low,
ascends and induces clouds and a chance of precipitation. The sea breeze front
occurs between the cooler maritime air and the warmer continental land air being
displaced. At night, the situation reverses as the land cools more quickly than the
water. A thermal low will then be situated over the water body resulting in a land
breeze. <ME8.3>
D. Valley and Mountain Breeze - Diurnal variations similar to those initiating
land/sea breezes establish mountain/valley breezes. Solar facing mountain slopes
heat more intensely than shaded valley areas. A thermal low develops over these
locations during the day and a valley breeze is initiated as air moves from the valley
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toward these higher elevations. At night, the situation reverses as the higher
elevations cool more rapidly then the protected valley areas resulting in a mountain
breeze.
• Air-Sea Interactions - Oceans greatly influence atmospheric conditions through energy and
moisture exchanges. Oceans, due to greater density, operate on much longer time-scales than the
atmosphere. Because ocean temperatures, motions, etc. operate at slower paces and because the
influence on the atmosphere is profound, it is useful to include the oceans in long-term weather
analysis.
A. El Niño, La Niña, and the Walker Circulation - El Niño events are
characterized by unusually warm waters in the eastern equatorial Pacific Ocean.
Recent strong El Niño events have been linked to anomalous weather events across
the globe. Higher than normal water temperatures lead to increased evaporation
rates and reduced air pressure. El Niño events occur every two to five years when
trade winds, pushing equatorial waters westward, reduce in strength. Western
equatorial Pacific waters, piled by strong trade wind flow, migrate eastward. As
warmer western waters replace cooler eastern waters, a reversal occurs in the
overlying atmospheric Walker Circulation. Under normal conditions, high
atmospheric pressure is centered in the east, over cool ocean waters while low
pressure dominates the west over warm waters. As the warm water pool migrates
eastward, the overlying pressures reverse. This Southern Oscillation is inherently
linked to the oceanic variations such that most El Niño events are referred to as
ENSO (El Niño/Southern Oscillation) events. El Niño events can cause extreme
weather for preferred locations, but more research is needed to exactly quantify
relationships. <Web> <ME8.1;8.2>
After El Niño dissipation, the equatorial Pacific typically returns to a normal phase,
or a strengthened normal phase, La Niña. In this situation, anomalous cooling
occurs in the eastern Pacific waters. Distinct, but different, global teleconnection
patterns result. Research has shown that individual El Niños and La Niñas produce
different weather anomalies at the regional level. No two are ever quite alike owing
to the complexity of the air-sea interactions.
B. Katabatic Winds - Katabatic winds are also winds which flow down mountain
slopes; however, katabatic genesis occurs as cold, dense air sinks from higher
elevations. Although katabatic winds occur most frequently in Antarctica and
Greenland, the boras winds of the Balkan Mountain region and the mistral winds of
France are two widely known katabatic wind systems.
C. Pacific Decadal Oscillation - A large, long-lived oscillation pattern exists
across the Pacific Ocean. This Pacific Decadal Oscillation, or PDO, involves two
modes of sea surface temperature (SST). One exists in the northern and western
part of the basin and smaller one occupies the eastern tropical Pacific. Abrupt shifts
in SST occur along a 20 to 30 year oscillation. Research suggests that the PDO
affects climatic impacts associated with El Niño events. When the PDO is in a
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warm phase (high temperatures in the eastern tropical Pacific), El Niño’s impacts
on weather are more pronounced than when the PDO is in a cold phase.
D. Arctic Oscillation - An oscillation of atmospheric mass also exists in the North
Atlantic. The North Atlantic Oscillation (NAO), or the Arctic Oscillation (AO),
exhibits warm and cold phases. During a warm phase, low pressure anomalies exist
over the polar regions while high pressure anomalies occupy lower latitudes. A
weak polar high means cold, migratory, winter air masses have little penetrating
power to push into lower latitudes. Warm winters in the eastern US result. Cold air
masses become more frequent in Newfoundland and Greenland resulting in colder
winters there. The higher than normal pressures over the central Atlantic result in
stronger westerlies and warmer conditions for northern Europe. Opposite
conditions occur during the cold phase. The Greenland-Europe seesaw in
temperature is forced by the well-known NAO, however, the realization that the
temperature oscillation is part of a hemisphere-wide phenomenon linked to changes
in the Arctic Ocean is new. During a warm (cold) phase, warm (cold) Atlantic
(Arctic) water pushes farther into the Arctic (Atlantic) resulting in thinner (thicker)
sea ice. The AO occupies time scales of decades. The past 20 years has seen
predominantly warm phases, which may indicate anthropogenic influences on the
global climate.
Chapter Boxes:
8-1 Physical Principles: Problems with the Single-Cell Model - Conservation of
angular momentum dictates that the single-cell general circulation model will only apply to
areas equatorward of 30o. With the single-cell model, all surface winds would be easterly.
Momentum transferred from the surface to the atmosphere must equal the inverse so that
any easterly moving winds must be offset by an equal amount moving westward.
Therefore, the single-cell model is impossible for any rotating surface.
8-2 Physical Principles: The Movement of Rossby Waves - Three factors designate
Rossby wave propagation: the westerly component of internal wind speed, latitudinal
position, and wavelength. In short, Rossby waves with high wind speed and short
wavelengths propagate most rapidly. This is defined quantitatively.
8-3 Physical Principles: The Dishpan Experiment - Unequal global atmospheric
heating, Earth rotation, and the turbulent nature of gases dictates that both large and small
scale perturbations occur throughout the atmosphere. Dishpan experiments, those
consisting of a rotating fluid chilled near the center and heated along the edges, confirm the
many turbulent flows that exist in the atmosphere. Extreme temperature differences
combined with slow rotational rates cause increases in amplitude for the largest waves in
the experiment. Similar features have been observed on Earth.
8-4 Focus on the Environment: Wildfires - In 2002, during the fall and summer, the
landscape of the western US was particularly dry due to recent and persistent drought. The
amount of fuel for fires had built up for years through fire suppression. Many believe that
allowing fires to burn decreases the risk of periodic catastrophic fires. By the first day of
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summer, hundreds of thousands of acres of land had already burned across the west
marking the worst fire season in history. Fires create their own weather, which helps them
spread. Strong thermal updrafts lower pressure setting up strong pressure gradients which
feed the fires and transport burning embers over great distances.
CD Rom Unit 7 - Upper-Level Winds and Pressure:
1. Introduction - The introduction points out that many weather elements are related to
upper-level winds.
2. Geopotential Height - An animation of an object being lifted describes energy
expended against gravity and the amount of stored potential energy. Diagrams show that
pressure at any height reflects the mass of the overlying atmosphere and the associated
stored potential energy. The greater the height, the greater the potential energy.
Geopotential height, therefore, refers to the amount of potential energy held in the
atmosphere above a given pressure surface. The diagrams also depict that heights are
relative to air temperature with lower (higher) heights associated with colder (warmer) air
and lower (higher) surface pressures.
3. Height and Pressure Relationships - Diagrams show that at the same altitude, low
(high) pressure areas have lower (higher) geopotential heights. Also, heights normally
decrease with increasing latitude due to thermal inequalities as a map of heights over the
US portrays. Five pressure levels, each with particular uses, are highlighted.
4. Ridges and Troughs - Ridges are described as mountains of high heights while troughs
are referred to as valleys of lower heights. These are superimposed on heights that
decrease with increasing latitude. Heights curve poleward around ridges and equatorward
around troughs as surface cooling (warming) causes lowered (raised) heights in a trough
(ridge) over the height field, which slopes with latitude. Such lowering (raising) bends
isobars accordingly. An interactive animated map portrays this as various viewpoints may
be acquiesced.
5. Waves in the Westerlies - Ridges and troughs are described as being waves in the
upper-level height field with the largest waves termed Rossby waves. An animation of air
moving through ridges and troughs allows one to visualize air moving through the wave
form (as dictated by PGF and CF) and the wave motion. An animation of an easterly wave,
wave movement from east to west but wind movement from west to east, is also given. An
interactive portrayal of the N.H. 500 mb height field for Nov. 1993 assimilates a variety of
related topics.
Related Web Sites:
General Circulation: http://pollux.geog.ucsb.edu/~joel/g110_w99/lect19/oh99_19_3.htm
El Niño: www.elnino.noaa.gov
http://grads.iges.org/pix/pix.html
Walker Circulation:
http://nic.fb4.noaa.gov/products/analysis_monitoring/ensocycle/enso_cycle.htm
ITCZ: www.ncdc.noaa.gov/ogp/papers/houze1.html
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Weather Maps: http://weather.noaa.gov/fax/nwsfax.shtml
Media Enrichment:
ME8.1 - Movie depiction of El Niño.
ME8.2 - Movie depiction of La Niña.
ME8.3 - Image of Southern California sea breeze.
ME8.4 - Image of Algerian dust storm.
Key Terms:
general circulation
Iclandic low
single-cell model
Siberian high
zonal wind
Hawaiian high
meridional wind
land breeze
three-cell model
Tibetan low
Hadley cell
polar front
Ferrel cell
polar jet stream
upwelling
El Nino
polar cell
subtropical high
microscale
Walker circulation
ocean currents
horse latitudes
teleconnections
trade winds
La Nina
westerlies
polar high
chinook wind
Gulf Stream
Aleutian low
Equatorial countercurrent
North Atlantic Drift
katabatic wind
Canary Current
sea breeze
Labrador Current
sea breeze front
East Greenland Drift
lake breeze
Bermuda-Azores high valley breeze
West Greenland Drift
mountain breeze
Intertropical Convergence Zone
subtropical jet stream
global scale
Rossby wave
mesoscale
Southern Oscillation
ENSO
Ekman spiral
monsoon
North Equatorial Current
monsoon depressions
South Equatorial Current
foehn wind
polar easterlies
Santa Ana wind
Pacific Decadal Oscillation
North Atlantic Oscillation
semipermanent cells
Review Questions:
1. Describe the single-cell and three cell models of the general circulation.
The single-cell general circulation model was first proposed by George Hadley. The
model assumed a planet comprised of a single ocean and a fixed solar declination. A lone
convection cell exists for each hemisphere, redistributing heat across the latitudes.
Easterly surface winds would exist due to Coriolis deflection. The three-cell model
separates each hemisphere into three independent but coupled circulation cells. The cells
exist every 30o latitude and redistribute energy latitudinally. The tropical cell is dubbed the
Hadley Cell, while the mid-latitude cell is termed the Ferrel cell. The polar cell resides at
high latitudes. The three-cell model is a gross over simplification of the real world but it is
useful for a simple description of circulation aspects.
2. What is the Hadley cell and where is it found?
A Hadley cell is a convective cell formed through heated air rising near the equator. The
air cools and descends near 30o N and S. This air then diverges at the surface which causes
some air to flow back towards the equator where it converges and rises again, completing
the circulation cell. Therefore, the cell is found, in both hemispheres, between the equator
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and 30o.
3. Of the pressure and wind belts described in the three-cell model, which have the strongest basis
in reality?
The ITCZ is real and well established. Likewise, the Hadley cell circulation provides a
good account of low-latitude motions. The Ferrel and polar cells are not well represented
in reality. Regarding upper level motions, the three-cell model is not realistic at all.
4. Why do the trade winds flow from the northeast and southeast instead of directly from the east?
This is due primarily from initiation of the winds near the centers of the subtropical
anticyclones located near 30o N and S. Air moving from these regions will undergo
Coriolis deflection to the right in the N.H. and to the left in the S.H. This causes an inclined
trade wind flow.
5. What are the Ferrel and Polar cells?
Ferrel and Polar cells are cells located between 30o and 60o in each hemisphere. They are
driven partly by dynamics such as convergence, and partly by thermal properties.
6. Describe the distribution of semi-permanent cells and their seasonal changes in location and
size.
Instead of cohesive pressure belts circling Earth as the three-celled model suggests,
semi-permanent pressure belts exist. These cells are either thermally or dynamically
driven and as such they fluctuate seasonally in strength and position. Oceanic lows such as
the Aleutian and Icelandic strengthen during the winter months while oceanic highs such as
the Hawaiian, and Bermuda-Azores strengthen during the summer months. Continental
highs (Siberian) peak in winter while continental lows (Tibetan) gain maximum strength in
summer. All cells expand equatorward in winter and poleward in summer.
7. What is the Sahel? Describe its seasonal cycle of rainfall and explain its origin.
The Sahel is a region of Africa bordering the southern Sahara Desert. During summer, the
ITCZ shifts northward with the vertical solar ray, brining rain to the region. For much of
the year, the ITCZ is located south of the Sahel and the region receives little or no
precipitation as the subtropical high builds over the area.
8. Describe the average patterns of the 500 mb level for January and July. What causes the
patterns?
For both January and July, the 500 mb heights are greatest over the Tropics and decrease
with latitude. Second, the gradient in height is greater in the winter hemisphere. Third, at
all latitudes the height of the 500 mb level is greater in summer. All three changes result
from the general distribution of temperature in the lower-middle atmosphere; areas of
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warm air have greater 500 mb heights.
9. Explain why upper-atmospheric winds outside the Tropics have a strong westerly component
on average.
Upper tropospheric pressure heights decrease poleward as dictated by the increasing
density of colder air. Therefore, a gradient persists from the equator to the poles. Air
follows the gradient poleward but is deflected in both hemispheres by the Coriolis force
creating westerly flow.
10. Why is it that the equatorward bending of height contours for the 500 mb level implies the
presence of a trough?
Because this represents a region of overall lower heights. Therefore, a trough is
synonymous with a valley in the upper air flow.
11. Explain how temperature patterns lead to the development of the polar jet stream.
The polar jet stream simply represents the boundary between cold and warm air at the
surface. The cold air originates in polar latitudes while the warmer air originates near the
equator. The boundary between these air masses represents the polar front. Above this
boundary lies the polar jet stream. The greater the temperature difference between the air
masses, the stronger the jet stream winds.
12. Describe the distribution of Rossby waves and their impact on daily weather.
The largest configurations of waves in the upper air are termed Rossby waves. These
waves, represented by ridges and troughs, denote the boundary between colder and warmer
air masses at the surface. Thus, the polar jet stream is superimposed upon the Rossby
waves. When a trough exists over a particular location, cold air temperatures predominate
relative to a ridge. The waves migrate eastward in the general circulation but have
preferred anchoring nodes. Daily weather is greatly affected by these waves as the waves
determine mean air temperature in addition to defining areas of divergence and
convergence important in the creation of precipitating storms.
13. What is the Ekman spiral?
Due to relationships between wind forcing of ocean waters, and resulting Coriolis
deflection of that moving water, surface ocean currents tend to flow at an angle of 45o to
the right of the winds that drive them. This clockwise shift continues down into the water
column to a depth of 100 m where the current approaches the opposite direction of the
surface current.
14. What is upwelling? How is it caused?
When coastal waters are moved offshore either by an offshore wind or Ekman spiral,
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colder deep water is drawn up to replace the departing water. This is known as upwelling.
IT is typical over the astern portions of major oceans.
15. Describe the scope of global, synoptic, mesoscale, and microscale wind systems.
Global scale features are those such as the ITCZ, the westerlies and large Rossby waves.
Smaller features are represented at the synoptic scale by phenomena such as cyclones,
anticyclones, troughs, and ridges. These systems cover hundreds or thousands of square
km. Synoptic scale features persist for periods of days to as much as a couple of weeks
whereas global features are much larger and can persist for decades to centuries or longer.
The mesoscale is on the order of tens of square km and phenomena at this scale tend to last
for periods as brief as half and hour. The smallest exchanges of mass and energy operate at
the microscale. This might be formation of a ripple on snow or a sandy beach, or the
swirling of smoke from a cooking surface.
16. Describe the wind patterns associated with the monsoon of southern Asia.
Monsoons are simply seasonal reversals of the mean wind. During winter months, when
the continent is colder than the surrounding oceans, high pressure dominates. Relatively
speaking, low pressure dominates the ocean areas. This pressure pattern induces mean
offshore winds, which are characteristically dry. Opposite conditions prevail during the
summer months. It is during these times that moist air is advected onshore bringing with
them the copious monsoonal rains.
17. Describe Föehn winds.
Föehn (Europe), Chinook (Great Plains), and Santa Ana (southern California) winds all
describe conditions in which airflow descends the lee side of a mountain range. As air
descends, it warms through compression. Dry conditions predominate as the relative
humidity of the descending air drops as the air adiabatically warms. Specifically, Föehn
winds occur in winter when mid-latitude cyclones pass southwest of the Alps.
18. How do katabatic winds differ in origin from Föehn winds?
Katabatic winds refer to cold dense air which tends to sink into lower elevation regions.
This differs markedly from Föehn winds which blow across high elevations and
adiabatically heat as they descend the lee side.
19. What causes sea/land and mountain/valley breezes to develop?
Local thermal inequalities initiate and support these small scale circulations. During the
day, land and mountaintops absorb more radiant energy and heat more in relation to ocean
and valley regions. Low pressure cells are initiated in these locations leading sea and
valley breezes, respectively. During the night, opposite conditions initiate and prevail
causing land and mountain breezes.
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20. What is an El Niño and how is it related to the Walker circulation?
The term El Niño refers to the periodic warming of waters off the west coast of equatorial
South America. This ocean warming is related to water migrating from the western
equatorial Pacific Ocean when trade winds weaken along the equator. The Walker
circulation refers to the see saw of atmospheric pressure between the eastern and western
equatorial Pacific. Pressures are normally higher in the east than in the west. This
integrated system includes, and is supported by, cold water currents in the east and warm
water in the west. During an El Niño the pressure anomalies reverse as sea surface
temperatures increase in the east and lower in the west.
21. Describe the Pacific Decadal Oscillation and the Arctic Oscillation.
The PDO is a large, long-lived oscillation pattern across the Pacific Ocean. It involves two
modes of SST. One exists in the northern and western part of the basin and a smaller one
occupies the eastern tropical Pacific. Abrupt shifts in SST occur along a 20-30 year
oscillation. During a warm phase, El Nino impacts on weather are more pronounced. The
Arctic Oscillation, or the North Atlantic Oscillation (NAO), exhibits warm and cold phases
across the North Atlantic. During a warm phase, low pressure anomalies exist over the
polar regions while high pressure anomalies occupy lower latitudes. A weak polar high
means cold, migratory, winter air masses have little penetrating power to push into lower
latitudes.
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