Chapter 7 General Circulation and Secondary Circulations

CHAPTER 7 GENERAL CIRCULATION AND SECONDARY CIRCULATIONS

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 planetary-scale 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

Hadley Cells

Air rising near the equator begins moving toward the poles in the upper troposphere
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)
This air diverges from the point of impact at the surface near 30° latitude, forming the subtropical anticyclones

Polar Cells

Net energy deficits at the poles cause the air to increase in density, which initiates sinking motions through the atmosphere
This air diverges at the surface, inducing the semi-permanent high atmospheric pressure at the surface – the polar highs (one at each pole)
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°
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

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)
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)
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
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
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
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
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
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
Locations and Strength of Features in the Hadley Cells
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
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

Locations and Strength of Features in the Polar Cells
The polar highs reach their maximum strength during winter and weaken somewhat in summer
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
Locations and Strength of Surface Midlatitude Features
In the winter hemisphere, the subpolar lows are strengthened and displaced equatorward, while the subtropical anticyclones are relatively weak and displaced equatorward also
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
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 semi-permanent 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

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
By convention, counterclockwise rotation in the northern hemisphere and clockwise rotation in the southern hemisphere) is termed positive vorticity
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

Air flowing in upper-level Rossby waves conserves its absolute vorticity, thereby ensuring that ridges and troughs remain confined to the middle latitudes
If a parcel moving laterally in the mid-latitude westerlies along the level of non-divergence 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).
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

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

Rossby waves affect and are affected by baroclinic zones -- any region where air is being advected into a region with a drastically different temperature
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

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

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

The Polar Front Jet Stream
Embedded within the upper-level mid-latitude westerlies are cores of extremely fast air flow
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
The polar front jet stream’s location varies as the flow of air travels along the area of constant absolute vorticity
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
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
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