Undergraduate-level survey of the General Circulation of the Atmosphere
This is a brief summary. For an easy-to-read text, you may consult a textbook, e.g. Ackerman and Knox
2003, Chapter 7, or Wallace and Hobbs, 1977, Chapter 9 (more advanced)
1. Definition
The hemispheric or global circulation typically averaged for a period of one month or longer.
2. Surface or lower atmosphere
(1). 4 Pressure zones
A. The Equatorial Trough or ITCZ: latent heat release in the ascending convection produces the low.
(also: heat lows over summer subtropical continents, like the Sonora low, Sahara low, Pakistani
low, N/W Australia, Paraguay)
B. The Subtropical High (the horse latitudes, Bermuda/Azores/North-Pacific, southern oceans): 30 ºN
or ºS, mainly over the oceans in summer (warm High, subsidence produces the surface
divergence, subsidence heating is offset by radiational cooling)
C. The Arctic Low (Icelandic, Aleutian): 60 ºN, mainly over the oceans in winter, cold core and
therefore deep vertical structure (graveyard of baroclinic systems, resulting in surface convergence
of the air currents from the north Pole and from the subtropical High)
D. The Polar High: the Arctic ocean in summer, Antarctica year-round, boreal winter continents, cold
core and therefore quite shallow (very stable, baroclinic disturbances generally ride along the
edge).
(2). 3 Prevailing (most frequent) low-level wind zones
A. The tropical easterlies or trade winds (the winds with an east component, NE, E, SE winds)
B. The temperate (middle latitude) westerlies.
C. The polar easterlies.
(3). 3-cell meridional circulation
A. Hadley cell: low latitude, equator to 30 degree latitudes, thermally direct, driven by deep
convection, strongest in the summer hemisphere.
B. Ferrel cell: middle latitude, 30 to 60 degree latitudes, weak, thermally indirect, meridional eddies
transport heat poleward
C. Polar cell: high latitude, 60 degree latitudes to poles, expanding over the winter boreal continents,
thermally direct but shallow.
(5) Causes of surface pressure variation: think of SLP as the vertical integral of air mass, i.e. temperature,
per unit area.
A. Winter
The ocean surface is relatively warm compared to the nearby land at the same latitude. The warmer
ocean surface supports lower SLP. The shallow cold highs have a high SLP because of the low
vertically-integrated temperature. But cold air is so thin that the 500 mb height is low.
B. Summer
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The ocean surface is relatively cool compared to the nearby land at the same latitude, especially in
the lowest 1-3 km. The result is offshore high pressure. Subsidence occurs above these subtropical
highs, making the air above the trade wind inversion as warm as that over adjacent land. Very
stable conditions.
(6) The permanent centers of action
A. The subtropical Highs (both Pacific High and Bermuda High) exist year-round, but they intensify in
summer due to the relatively cooler ocean surface air that subsides aloft, sustaining the surface
divergence.
B. The high-midlatitude lows are most persistent year-round around Antarctica, supporting strong
circumpolar westerlies over the southern ocean.
(7) The semi-permanent centers of action
A. The Arctic Lows (both Aleutian and Icelandic Lows)
B. The Arctic Lows weaken and even disappear in summer.
(8) The seasonal variations of the strength of the centers of action
A. The subtropical Highs
(A) Summer: strengthen, expand, and their centers move over the ocean and poleward (to about 40
ºN or ºS), following the poleward migration of the zenithal sun.
(B) winter: weaken, shrink, and their centers displace toward land and the equator (about 20 ºN or
ºS), following the southward migration of the sun.
B. The Arctic Lows
(A) Summer: weaken or disappear, relatively cool oceans favors the formation of thermal Highs, and
thus weakens the Lows (thermal effect).
(B) Winter: intensify, vorticity sustained by advection of mature/decaying baroclinic systems.
3. Middle and Upper Atmospheres (700 mb and higher)
(1) 3 pressure or elevation (on an isobaric surface) zones
A. The equatorial Low (up to ~ 700 mb)
B. The subtropical High (700 mb), esp in summer over land, eg Sahara, producing an easterly jet at
~700 mb (thermal wind – warmer at 20-25º than at the equator)
(A). The subtropical High slopes equatorward, toward the ITCZ as the elevation increases. At 300
mb the ridge is over the ITCZ
a. The mean location of the surface subtropical High 30 ºN, over oceans
b. The mean location of the 500 mb subtropical High: 25 ºN, over land
(B). The subtropical High becomes a continuous ridge, instead of separating into the Hawaiian and
the Bermuda Highs.
C. The polar Low (the circumpolar vortex center) The surface polar High is replaced by a Low at the 700
mb and higher levels because the cold air is associated with an upper level low (lower thickness).
In winter, the low center moves, with increasing height, from the oceans (eg Iceland) to the
eastern boreal continents (E. Siberia, E Canada)
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(2). 2 prevailing wind zones
A. The tropical easterlies. South of the subtropical High to the equatorial Low. In summer, easterlies
are found up to ~30º near the surface, and up to ~10º at 200 mb.
B. The polar westerlies
(A). The circumpolar vortices.
(B). From the subtropical High to the North Pole.
(C). The 1000-300 mb cold air towards the poles (&boreal continents in winter) establishes an
upper-level polar Low. The low-level warm air in the south (the subtropical warm Highs) establishes
an upper-level High (the hydrostatic equation).
(D). Stronger UL westerlies in winter than in summer: the low-level horizontal temperature gradient
is greater in winter than in summer.
(E). The westerlies expand toward the equator in winter and shrink toward the North Pole in
summer, following the seasonal migration of the sun.
(3). The Arctic Low.
A.
Slopes upward toward the North Pole. Cold air column is associated with an upper Low (the
hydrostatic equation).
B.
Mean location of the surface Arctic Low: 60ºN or ºS.
C.
Location of the 700 mb (or higher) Arctic Low in summer: the Pole.
(4). Long waves or Planetary waves: quasistationary
A.
Wavelength >5000 km.
Wavelength: the distance between two subsequent ridges or troughs, determined by the contours
on an isobaric surface.
B.
3 mean winter Rossby-wave troughs
(A).
The eastern North American seaboard: 80ºW.
From the Hudson Bay to the east coast of N. America
(B).
(strongest) The eastern Asiatic coast: 140ºE. over Japan.
(C).
(weakest) The east Mediterranean sea: 50 ºE.
C. Causes of the three troughs
(A).
Troughing in lee of the Tibet Plateau and the Rocky mountains (PV conservation)
(B).
Strong horizontal temperature gradient between ocean (warm boundary current) and
continent.
(6) Long wave
A.
Stationary (slow-speed)
B.
Responsible for weekly or monthly weather variations.
Examples: heat wave, flood, or drought lasts for a week or longer.
(7) Short waves
A.
Wavelength ~ 1000 km  most obvious in the vorticity field
B.
propagate with the flow (only slightly slower)
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C.
causes departures from geostrophic balance. This in turn produces ageostrophic flow, including
vertical motion, even though the atmosphere generally is stably stratified. This ageostrophic flow
attempts to restore geostrophic balance. Vertical motion is responsible for daily weather variations.
(8) Jet Stream
A.
A fast-moving upper-level air current (strong wind), confined vertically by a rapid reversal of
temperature gradient (sloping tropopause), and horizontally by a sharp LL temperature gradient
(front). The wind speed generally exceeds 30 m/s at the 300-mb level.
B.
The thermal wind theory
(A).
The upper-level height gradient is determined by the low-level horizontal temperature
gradient (the hydrostatic equation).
(B).
A strong low-level horizontal temperature gradient creates a strong upper-level height
gradient, and hence strong wind (a jet stream).
C. A westerly jet separated the cold air to the pole and the warm air to the equator
D. An easterly jet separates the warm air poleward from cooler air equatorward (in the tropics).
E.
Confluence Theory for frontogenesis over the ocean (proposed by Namias and Clapp). For
instance: the north Atlantic
(A).
cold air advection from northwest around the Icelandic Low (sometimes continental polar
cold-air outbreaks)
(B).
warm air advection from southwest, diverging from the Bermuda High.
(C).
confluence leads to frontogenesis between the High and the Low (polar front), resulting a
strong wind zone in the upper level. Small perturbations may amplify (cyclogenesis), they usually
mature before they reach W. Europe.
F.
Types of Jet
(A).
The polar front Jet.
mean location: 47ºN (35 to 60ºN).
jet core (the elevation of the highest wind speed): 250-300 mb.
occurs all year round, but strongest in winter and near eastern shores of 2 boreal
continents
discontinuous, unsteady
variable, often high vorticity in trof of on cyclonic shear side (>f).
associated with a surface polar front.
is distorted in association with LL cyclogenesis and cold/warm advection.
tropopause sloped or folded: middle-latitude tropopause much higher than the polar
tropopause.
(B).
The subtropical jet
mean location: 27 ºN (20 to 35 ºN).
jet core: 150 mb.
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occurs in winter only (lack of strong horizontal temperature gradient in the lower
atmosphere in summer)
north of a subtropical High, near a weaker tropopause slope aloft
often merged with the polar jet, esp on the east side of the large continents
(9). Palmen and Newton's General Circulation Model
A. The tropopause is broken into 3 sections.
(A).
Tropical tropopause (16-18 km, sharply defined esp above the ITCZ).
(B).
Middle latitude tropopause (12 km).
(C).
Polar tropopause (6-8 km, becomes ill-defined in winter [polar night westerly jet near 20 km]).
B. The subtropical jet: @ gap between the tropical and middle-latitude tropopauses.
C. Polar front jet;
(A).
The gap between the middle-latitude and polar tropopause.
(B).
The polar front extends to the ground surface from the jet core [observations indicate that
front is weak above ~800 mb and below the tropause fold]
(10). Conservation of absolute vorticity (wave patterns of air flows)
A. Absolute vorticity = (relative vorticity + Coriolis parameter) = constant if DIV=0.
B. Vorticity: the tendency of an air parcel to rotate in the same direction as the earth
(positive vorticity).
C. Relative vorticity: shear vorticity + curvature vorticity
D. Conservation of absolute vorticity can be used to explain Rossby waves (restoring force: )
(A).
northerly (equatorward) flow feels decreasing plan vort, leads to increased relative vorticity.
(B).
An air parcel tends to change from clockwise circulation to counterclockwise circulation
(The air parcel changes the direction, moving back toward north).
(11). Vanishing storms over the Rocky Mountains, lee cyclogenesis
A. conservation of the potential vorticity: (Absolute vorticity/Depth of an air column) = constant
(A).
A ridge tends to generate or intensify over a large mountain due to the shrinking of the air
column.
(B).
A trough tends to generate or intensify in the lee of a large mountain due to the stretching of
the air column.
B. The Great Plains (Alberta - Colorado): cyclogenesis (cyclone formation or intensification).
4. Trade wind Inversion
(1). Most intense in the east part of a subtropical high (Hawaiian High for example)
A. San Francisco in summer (or Concepcion, Chile, or Walvis Bay, Namibia):
(A). Average inversion base height: 500 m.
(B). Average inversion magnitude (inversion top temperature- inversion base temperature) 10-15ºC.
B. closer to the ITCZ (eg Honolulu):
(A). Average inversion base height: 2000 m.
(B). Average inversion magnitude: 5ºC.
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(2). Causes
A. Compressional heating due to the subsidence (sinking) of the upper level air originating from the
equator.
B. Cool surface marine layer
(A). Cold ocean current: travels from north to south.
(B). Upwelling:
a. Rising of ocean water from below.
b. Ekman spiral: Ocean surface water flows to the right (left) of along-shore equatorward winds,
resulting in flow away from the continent inducing the rising of ocean water from below to
replace the offshore surface transport.
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