Chapter 7
General Circulation and Secondary Circulations
2. Describe the key pressure and wind features in the idealized circulation model.
Answer: In the idealized circulation model there are alternating regions of high and low surface pressure
in each hemisphere at 30o intervals of latitude from the poles to the equator. Specifically, low surface
pressure exists in a broad belt near the equator, known as the ITCZ, with high surface pressure near 30
degrees north and south latitude (the subtropical highs), low surface pressure near 60 degrees north and
south latitude (the subpolar lows), and high surface pressure at the poles (the polar highs). These pressure
differences set the atmosphere in motion and create the planetary wind systems. The circulation between
the equator and 30o latitude is largely convective, with rising motion near the equator and sinking motion
near 30o latitude. The trade winds in each hemisphere flow toward lower pressure at the equator and form
the ITCZ. The sinking air near 30o latitude forms the subtropical highs. The weak descent of air in these
regions provides very calm conditions. Air flowing on the poleward side of the subtropical highs in each
hemisphere forms the midlatitude westerlies in response to pressure gradient force and Coriolis deflection.
The polar easterlies are formed in both hemispheres as air moves from the polar high toward lower
latitudes. Areas of relatively low pressure, termed subpolar lows, are centered on 60o latitude in each
hemisphere due to the convergence of the midlatitude westerlies and the polar easterlies
4. Detail the role of surface thermal contrasts relative to “real-world” general circulation features.
Answer: Surface thermal contrasts between land and water surfaces complicate the general circulation of
the atmosphere. Due to the high specific heat of water large water bodies maintain relatively constant
temperatures throughout the year compared to inland areas. Continents heat and cool much more rapidly
than water surfaces, resulting in wide temperature variations. These factors combined with local
topography cause major disruptions in the circulation features.
6. Describe the waxing and waning of the semipermanent pressure cells through the course of the year.
Answer: The polar highs weaken and contract during the summer hemisphere and the Intertropical
Convergence Zone (ITCZ) moves poleward in response to the migration of the sun’s vertical rays. The
subtropical anticyclones over the ocean basins are strongest in the summer when the ocean surface is
colder than the continental surface in the subtropics. The polar highs strengthen and expand equatorward
during the winter hemisphere while the subtropical anticyclones weaken and retreat equatorward.
8. Identify and compare relative and absolute vorticity.
Answer: Relative vorticity is the spin that occurs because the object itself is turning. Planetary vorticity is
the spin that occurs because of the spin of the earth. The amount of planetary vorticity is proportional to
the Coriolis effect. Absolute vorticity is the sum of the relative and planetary vorticity.
10. Describe the role of vorticity in the development of zonal and meridional Rossby wave patterns.
Answer: Changes in vorticity impact the development of zonal and meridional Rossby wave patterns. Air
near a trough axis will experience the maximum amount of positive relative vorticity. Relative vorticity
will begin to decrease as the air moves toward a region of negative vorticity. Zonal patterns have only
small changes in relative vorticity between the ridge and trough axis. Rossby waves conserve their
absolute vorticity, ensuring that ridges and troughs remain confined to the middle latitudes.
12. Discuss the role of constant absolute vorticity trajectory relative to atmospheric motions aloft.
Answer: At the level of nondivergence, where neither convergence nor divergence is occurring, air
motion follows a constant absolute vorticity trajectory. If a parcel in the midlatitudes begins to move
poleward at all, it will acquire an increase in Coriolis deflection. As this happens, its relative vorticity
must decrease so that absolute vorticity can be conserved. The result of the decrease in relative vorticity is
the formation of a Rossby wave ridge. On the downstream side of the resulting ridge, the Coriolis
deflection decreases, which induces positive relative vorticity in order to conserve absolute vorticity. The
result is the formation of a trough.
14. Discuss the role of baroclinicity in the development of upper-atmospheric flow patterns.
Answer: Baroclinicity is the intermixing of steep thermal gradients across small regions. Baroclinic zones
are regions of active and sometimes severe weather in the midlatitudes. Baroclinic zones can enhance the
development of upper-level troughs when areas of strong thermal advection are oriented nearly
perpendicular to upper-atmospheric flow. Cyclonic development is supported as air moves into an upperlevel trough. The jet stream is located over the strongest baroclinic zones with and the jet stream winds
vary according to the temperature contrasts.