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 20 o and 30 o 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 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
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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 30 o
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 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.
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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 45 o
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 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
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(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 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.
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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 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
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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 30 o
. 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 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.
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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
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
Hadley cell
Ferrel cell upwelling polar cell microscale ocean currents teleconnections
Tibetan low polar front
El Nino subtropical high Rossby wave
Walker circulation Southern Oscillation horse latitudes trade winds polar jet stream
North Atlantic Drift
Canary Current
Labrador Current
East Greenland Drift katabatic wind sea breeze sea breeze front lake breeze
Bermuda-Azores high valley breeze
West Greenland Drift
Intertropical Convergence Zone subtropical jet stream
Ekman spiral
North Equatorial Current mountain breeze global scale mesoscale
ENSO monsoon monsoon depressions
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La Nina polar high
Gulf Stream westerlies chinook wind
Aleutian low
Equatorial countercurrent
South Equatorial Current polar easterlies
Pacific Decadal Oscillation
North Atlantic Oscillation
Review Questions:
1. Describe the single-cell and three cell models of the general circulation. foehn wind
Santa Ana wind semipermanent cells
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 30 o
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 30 o 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 and 30 o .
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 30 o
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 30 o
and 60 o
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.
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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 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
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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 45 o
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, 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
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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.
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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