Chapter 6a Atmospheric circulation
Hypothetical atmospheric circulation without
rotation of the Earth
driven by density differences between air masses
Low pressure
cell rising air (reduces atmospheric pressure at the Earth surface)
High pressure
cell sinking air (increases atmospheric pressure at the Earth
surface)
Coriolis is an apparent
deflection caused by the rotation of the Earth
in the Northern
Hemisphere, an object in motion will bend to the right; reverse in Southern
Hemisphere
throwing a ball between
two riders on a merry-go-round
the apparent trajectory
depends upon the perspective of the viewer:
·
If the viewer is on
the merry-go-round, the ball appears to bend and miss the intended target.
·
If the viewer is
stationary above the merry-go-round, the ball travels in a straight line and
the merry-go-round moves out from under the ball.
same angular velocity
but different tangential velocities
at pole: 0 km/hr
at equator: 1600 km/hr
The effect of Coriolis varies
with latitude:
no Coriolis effect at the equator
highest at the poles
with speed of the object:
slow little Coriolis effect
fast significant Coriolis effect
most noticeable over long
distances
the Coriolis effect can be
offset by friction
Understand the difference between the geographic equator, which is fixed
and the thermal equator, which moves with the seasons
The thermal equator
is the position on the Earth surface that receives direct rays from the Sun,
and consequently the most intense solar heating
The global
wind pattern is driven by heating at the thermal equator and cooling at the
poles
The
circulation of air masses (of different densities) is organized into three main
circulation cells in each hemisphere (north & south)
Low-latitude atmospheric circulation
driven by heating at the equator, this causes strong
convection, with rising, hot, moisture-laden air
the Doldrums
are the area at the thermal equator where most of the surface winds are going
straight up (vertical flow because of the convection)
Hadley cell hot air in the lower atmosphere near the equator
rises to the upper troposphere, spreads laterally away from the equator, cools
and sinks between 20 and 30 degrees latitude north & south of the equator
The descending air has had most of the moisture
removed (by condensation and precipitation), and heats adiabatically as it
descends; by the time this air reaches the Earth surface, it has extremely low
humidity; most of the major deserts are in this zone, and the oceans have very
salty surface water
Trade Winds strong, continuous easterly
winds (blowing from east to west) 10 degrees north and south of the
Equator; the surface air is being drawn
toward the Equator by the intense convection; Coriolis acts on the surface
flow, and bends the winds toward the west
driven by cooling over the poles
cold, dense air sinks and flows radially away from
the poles
Coriolis bends the surface winds to the west (same
as with the Trade Winds)
the boundary of this cold, polar air mass is the Polar Front
the Jet Stream
forms along this boundary
Ferrel cells develop in the mid latitudes in response to the
circulation caused by the low-latitude Hadley cells and the high-latitude Polar
cells
surface winds in the Ferrel cells flow from low
latitude (higher than about 30 degrees north & south) to high latitude (the
Polar front)
Coriolis bends the surface flow to the east,
creating the Westerlies, which is a zonal wind that blows from west to east,
and carries most of the weather systems of the mid latitudes
the map in your textbook shows the long-term average
atmospheric pressure
organized into a bands of low pressure at the
Equator, and along the north and south Polar Fronts
(in the
Southern Ocean, this area is known as the Roaring
Forties [40 degrees south latitude], because of the strong low
pressure systems that develop, and the nearly continuous, very strong winds
that produce large waves)
large high-pressure systems centered at about 25-30
degrees latitude develop over most of the ocean basins, for example, the Bermuda High in the North Atlantic
similarly, large low-pressure systems develop at
about 45-50 degrees latitude, for example, the Aleutian
Low in the North Pacific
these dominant, persistent high- and low-pressure
systems steer the Jet Streams and produce many of the weather systems, they are
also directly involved with events such as ENSO (the El Niρo Southern
Oscillation)
the large-scale patterns of atmospheric circulation
produce the forces that set surface water in the oceans in motion
the North Atlantic Gyre
dynamic topography refers to changes in the surface elevation of the
ocean, which is responding to density differences caused by heat and salt, and
wind stress
the highest bulge in the North Atlantic is in the Sargasso Sea, off the Bahamas and Antilles
Islands
shows changes in ocean-surface
elevation as much as ± 1 meter
For the Northern Hemisphere:
net transport is 90 degrees to right of the wind
3-4% of wind velocity is
transferred to the water
winds blowing
parallel or oblique to the shoreline can induce upwelling or downwelling,
depending upon the wind direction and the resulting Ekman transport
large, persistent high-pressure system off Peru
produces winds that blow SE to NW, almost parallel to the coastline
Ekman transport is to the left of wind direction in
the Southern Hemisphere
net transport of surface water is away from the Peru
coast, this water must be replaced from mid depths (200 to 400 m), which is upwelling
the upwelling water has high concentrations of
nutrients, which causes high phytoplankton productivity
in the center of the ocean gyres, Ekman transport causes
convergence and downwelling (equivalent to an atmospheric high-pressure system)
the Trade Winds blowing along the Equator cause
Ekman transport both to the north and south, away from the Equator, which
causes divergence and upwelling
zone of high phytoplankton productivity in the
Equatorial East Pacific
(remember the
calcareous-siliceous ooze produced here)