CHAPTER 7 GENERAL CIRCULATION AND SECONDARY CIRCULATIONS
General
 The general circulation redistributes heat (energy) that arrives at the earth in greater
quantities near the equator than near the poles
 Secondary circulations characterized by traveling high- and low-pressure systems also
affect climate, and are influenced in the mid-latitudes by their position relative to planetaryscale waves of motion
Circulation of a Nonrotating Earth
 Tropical air should warm, become less dense, rise in the area near the equator, travel
poleward far above the surface (in both the northern and southern hemispheres), and by the
time it reached the polar areas, it would cool, sink, diverge at the surface in the vicinity of the
poles, and move back equatorward across the surface to begin the process anew
 Two main complications: First, the various topographic and land/water surface variations
upset the formation of a single hemispheric convection cell in each hemisphere; second, the
rotation of the earth initiates trajectory changes in moving fluids through the Coriolis effect
Idealized General Circulation on a Rotating Planet
 The Hadley Cells
o Air rising near the equator begins moving toward the poles in the upper
troposphere
o Most of the air flowing away from the equator aloft does not actually travel all the
way to the poles, but instead cools while it is high above the surface and sinks in
the vicinity of 30° latitude (both north and south of the equator)
o This air diverges from the point of impact at the surface near 30° latitude, forming
the subtropical anticyclones
 The Polar Cells
o Net energy deficits at the poles cause the air to increase in density, which initiates
sinking motions through the atmosphere
o This air diverges at the surface, inducing the semi-permanent high atmospheric
pressure at the surface – the polar highs (one at each pole)
o Upon reaching the surface, the diverging air travels equatorward toward lower
latitudes, is deflected by the Coriolis effect, and eventually converges at
approximately 60° latitude with air that was diverging from the subtropical
anticyclones at 30°
o The converging air near 60° latitude is forced to rise, creating low atmospheric
pressures centered on 60° latitude in each hemisphere, called the subpolar lows
 Planetary Wind Systems
o Surface winds moving between 30°N and the equator take on a motion that is
essentially from the northeast – the northeast trade winds (or northeast trades)
o Surface winds moving between 30°S and the equator take on a motion that is
essentially from the southeast – the southeast trade winds (or southeast trades)
o On the poleward sides of the subtropical anticyclones, the surface pressure
gradient force pushes air from the high-pressure core toward the subpolar lows, as
it is being deflected to the right (in the northern hemisphere) or to the left (in the
southern hemisphere) by the Coriolis effect – the midlatitude westerlies
o As air travels from the region of the north (or south) pole toward lower latitudes it is
deflected strongly (because of the high latitude) to the right (or left in the southern
hemisphere), creating the surface wind belt known as the polar easterlies
o Aloft over the Hadley cells, the upper-level pressure gradient force pushes air from
the equatorial region toward the area over the subtropical anticyclones where it is
deflected (slightly, because of the low latitudes) by the Coriolis Effect – results in
southwesterly flow over the northern hemisphere Hadley cell and northwesterly
flow over the southern hemisphere Hadley cell
o Aloft over the polar cells, the upper-level pressure gradient force pushes the air
from the area over the subpolar lows to the area over the polar highs, where it is
deflected strongly by the Coriolis effect – resulting in westerlies aloft over both
hemispheres
o Because of the difference in temperatures from the equator to pole, the difference
in a given geopotential height level between the equator and pole increases with
increasing height in the troposphere, and when upper-tropospheric air moves
poleward in response to this mid-latitude height gradient, the Coriolis effect
deflects it, resulting in upper-level westerly winds in both hemispheres
Modifications to the Idealized General Circulation: Observed Surface Patterns
 Land-Water Contrasts
o Because air temperatures over land have greater seasonal and diurnal ranges
than air temperatures over oceans, and because of the variations in surface
elevations associated with continents, continents disrupt the circulation in the
idealized, homogeneous earth model
o Locations and Strength of Features in the Hadley Cells
o In the “real world”, the ITCZ and trade winds move poleward into the summer
hemisphere in response to the displacement of the vertical solar rays, but the ITCZ
does not migrate much over places affected by cold ocean currents, even if these
locations are in the path of the sun’s vertical rays
o The subtropical anticyclones migrate somewhat from their summer position,
latitudinally with solar declination and longitudinally depending on where the
coldest water lies, within the subtropical oceans
o Locations and Strength of Features in the Polar Cells
o The polar highs reach their maximum strength during winter and weaken
somewhat in summer
o The relatively warm ocean surface in winter (compared to adjacent land areas)
promotes low-level atmospheric instability, thereby reinforcing the tendency for
rising motion as suggested by the idealized general circulation; this supports the
growth of the subpolar lows
o Locations and Strength of Surface Midlatitude Features
o In the winter hemisphere, the subpolar lows are strengthened and displaced
equatorward, while the subtropical anticyclones are relatively weak and displaced
equatorward also
o In the summer hemisphere, the subtropical highs are strengthened and displaced
poleward while the subpolar lows simultaneously shrink and retreat poleward
 Putting It All Together: Surface Pressure Patterns and Impacts
o The waxing and waning of these pressure systems affect and are affected by the
changing of the seasons in the high- and mid-latitudes, and changes in the semipermanent circulation cells trigger direct precipitation regime changes for many
high- and mid-latitude locations
Modifications to the Idealized General Circulation: Upper-Level Airflow and Secondary
Circulations
 Although flow in the upper-level mid-latitudes is primarily westerly, these upper-atmospheric
air currents typically exhibit flow characteristics that meander northward and southward as
they move in the general west-to-east direction – creating ridges and troughs
 Vorticity
o Absolute vorticity is the sum of relative vorticity – the spin that occurs because
the object itself is turning – and planetary vorticity – rotation imparted on any
object simply because it is on a rotating earth
o By convention, counterclockwise rotation in the northern hemisphere and
clockwise rotation in the southern hemisphere) is termed positive vorticity
o Air flowing in upper-level Rossby waves conserves its absolute vorticity, thereby
ensuring that ridges and troughs remain confined to the middle latitudes
 Constant Absolute Vorticity Trajectory
o Air flowing in upper-level Rossby waves conserves its absolute vorticity, thereby
ensuring that ridges and troughs remain confined to the middle latitudes
o If a parcel moving laterally in the mid-latitude westerlies along the level of nondivergence begins to move higher in latitude at all, say from southwest to
northeast in the northern hemisphere (or northwest to southeast in the southern
hemisphere), the increase in Coriolis deflection causes planetary vorticity to
increase, so to conserve absolute vorticity the relative vorticity must decrease as it
moves poleward, thereby creating a ridge (in either hemisphere).
o When the parcel reaches the area of maximum negative vorticity (and maximum
planetary vorticity) – the ridge axis – its clockwise (in the northern hemisphere)
flow begins to take it toward the southeast (or northeast in the southern
hemisphere) and its planetary vorticity decreases, so relative vorticity must
increase, counterclockwise flow in the northern hemisphere (clockwise in the
southern) results, and continued counterclockwise flow (clockwise in the southern)
will eventually take the parcel to a point at which is it as far equatorward as it will
go – the trough axis
 Flow Over Mountainous Terrain
o As a west-east-flowing air column of air moves over a mountain, its vertical
constriction on ascent of the slope causes it to acquire negative relative vorticity
(and form a ridge) and positive relative vorticity on its descent (and form a trough),
in order to conserve its potential vorticity
 Baroclinicity
o Rossby waves affect and are affected by baroclinic zones -- any region where air
is being advected into a region with a drastically different temperature
o Baroclinic zones are favored where steep thermal gradients exist, such as at
coastlines, where there is a differential heating of land masses versus ocean
surfaces at a given line of latitude
 Rossby Wave Divergence and Convergence
o The trough-to-ridge side of the Rossby wave supports surface cyclones and
cyclogenesis, while the ridge-to-trough side of the wave supports surface
anticyclones and anticyclogenesis
 Rossby Wave Diffluence and Confluence
o Areas of diffluence along the wave tend to support surface cyclones and
cyclogenesis, while areas of confluence along the wave support surface
anticyclones and anticyclogenesis
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The Polar Front Jet Stream
o Embedded within the upper-level mid-latitude westerlies are cores of extremely
fast air flow
o A sharp thermal gradient occurs between warm and cold air at the surface with the
boundary extending through a vertical profile of the atmosphere, and the polar
front jet stream exists over this gradient
o The polar front jet stream’s location varies as the flow of air travels along the area
of constant absolute vorticity
o The thermal boundary associated with the polar front jet stream is generally much
more prominent in winter because the latitudinal thermal gradient is much greater
during that season as compared to summer
Mean Patterns of Rossby Wave Flow
o The presence of topographical barriers and baroclinic zones cause persistence in
the long-term jet pattern, such as a ridge over the western mountain cordillera of
North America and a trough over the east
o The circumpolar vortex and in particular the embedded polar front jet stream,
exists over the zone of the sharpest temperature contrasts and the strongest
baroclinic zone and can exert a significant influence on weather and climates
beneath it