Transcript Atmospheric and Oceanic Circulations (continued) Chapter 6

Atmospheric and Oceanic
Circulations
(continued)
Chapter 6
Lecture 14
4 February 2005
Figure Credit: “Earth’s
Climate” by W. Ruddiman
Figure Credit: “Earth’s Climate” by
W. Ruddiman
Wind
simply put, wind is the horizontal flow of air in
response to differences in air pressure
these pressure differences are usually due to
uneven solar heating at the surface
‘heat rises’
wind flows because
of pressure gradient
Credit: www.physicalgeography.net
Wind
winds are designated as direction from
not direction to
(oceanographers do it the opposite)
wind compass
so, a westerly
wind would be
coming from
what angular
direction?
Four forces that determine winds
1. Gravity - pulls gas molecules close to Earth
density & pressure decrease with height
2. Pressure gradient force - the difference in
air pressure between areas
3. Coriolis force - deflects wind from a straight
line to the right or left depending on
hemisphere
4. Friction force - the drag on air flow from the
Earth’s surface
Pressure vs. Pressure Gradient
• The value of pressure itself is NOT important
• The CHANGE in pressure over DISTANCE is
• Change over distance is a GRADIENT
• The GRADIENT in pressure gives winds &
ocean currents their “push”
Pressure Gradient Force (PGF)
isobar - a line of equal pressure gradient is 16 mb
(analogous to isotherm) (note the closer isobars)
the PGF acts at right (90º) angles to the isobars
Pressure Gradient Force
note the 1008
mb isobar
Credit:
www.physicalgeography.net
Wind speed = Const * Pressure Gradient
Here, a 4x increase in PGF corresponds to a 4x
increase in wind speed
Pressure Gradient Force and Isobars
if there were no other forces acting on wind, it
would flow in straight lines (perpendicular to
isobars) from high to low pressure zones
Coriolis Force (just the facts)
• Rotation of Earth acts to deflect any motion from
a straight line
• Deflection is to right (NH) to the left (SH)
• Coriolis “force” act on a right angle to the motion
• Coriolis Force is NOT a real “force” but is caused
by viewing motion on a rotating planet
Coriolis Force
• Show the merry-go-round video
the amount of rotation about a
vertical axis (’spinning’) is
maximum at the poles and
minimum at the equator
Figure Credit: “Earth’s
Climate” by W. Ruddiman
Earth’s Rotation
every point on earth rotates around a central axis
at 15 degrees/hour
0˚
Speed of rotation
(mph)
1041
30˚
902
50˚
670
60˚
521
90˚
0
Latitude
an object with an
initial east-west
velocity will
maintain that
velocity, even as
it passes over
surfaces with
different velocities
as a result, it
appears to be
deflected over
that surface (right
in NH, left in SH)
Coriolis Force
Coriolis Force and Deflection of Flight Path
Coriolis Force and Deflection of Flight Path
Coriolis Force and Flight Paths II.
Airplane animation
The Coriolis Force affects air flow in response to
pressure gradients in the atmosphere
Figure Credit: “Earth’s
Climate” by W. Ruddiman
geostrophic winds - PGF and Coriolis forces are
opposite and balanced
in the northern hemisphere (upper troposphere),
the CF deflects the wind to the right until wind
flows parallel to isobars
~7km
Credit: www.physicalgeography.net
Geostrophic Winds
Balance between Pressure Gradient & Coriolis Forces
Flow along isobars not across
Works for upper atmosphere winds & ocean currents
500 mb Pressure Map
PGF, CF & isobars in upper troposphere
isobars
Friction Force
surface friction reduces wind speed and reduces the
Coriolis force (remember CF increases with wind
speed)
because of this, it
causes winds to move
across isobars at an
angle
the friction force operates only
in the bottom 0.5-1 km of the
atmosphere, and it acts
opposite to the direction of
motion
Figure Credit: “Earth’s
Climate” by W. Ruddiman
PGF + Coriolis + Friction Forces
isobars
The inter-tropical convergence zone (ITCZ)
solar heating in the tropics expands air and decreases
its density - leading to increased buoyancy
It also gets more humid (adding
water vapor)
How would this change the average molecular
weight of air?
what happens
to air density
if you add
water vapor?
average density of air
is 1.3 kg/m^3
average molecular
weight of air is ~29
g/mol
Convection on your Stove
Convection on Earth
Credit:
http://ess.geology.ufl.edu/ess/Notes/Atmospheric
irculation/convect.jpeg
as this air rises, it cools and water condenses
out, leading to intense precipitation
Credit:
http://www.geog.ucsb.edu/~jeff/wallpaper/itcz_goes11_lrg.jpg
A satellite (GOES) view of the ITCZ over the
eastern Pacific
the position of the ITCZ tracks the sun (it is found in
the summer hemisphere) - the location of the ITCZ
determines the rainy season in many tropical
countries, especially those in Africa
the horizontal winds within the ITCZ are calm
- the doldrums
The C in ITCZ
the intense uplift of air creates horizontal pressure
gradients at the surface
as a result, winds converge towards the equator from
both hemispheres
Credit: NASA JPL
what about the complete cycle - where does the
uplifted air go?
Equator-to-pole cross section of circulation
Hadley cell circulation
this circulation refers to the complete circulation of
rising air in the tropics, descending air over 30 °N and
°S, and trade winds converging at the equator
the descending branch of the Hadley circulation
brings hot, dry air to the surface - leading to high
pressure areas and suppressed precipitation
Subtropical high-pressure cells
these cells occur where the tropical air descends in
either hemisphere
Figure Credit: “Earth’s
Climate” by W. Ruddiman
Monsoon Circulation
Figure Credit: “Earth’s
Climate” by W. Ruddiman
Monsoon Circulation
Monsoon Circulation
Figure Credit:
physicalgeography.net
Asian
monsoon
intense, dry winds
flow from the Asian
interior in response to
the gradient between
the continental high
pressure and the
equatorial (ITCZ) low
pressure
Asian
monsoon
in summer, the
subsolar point and
the ITCZ shift
northward,
reversing the
pressure gradient as the winds flow
over the Indian
ocean they gain
moisture
Daytime land-sea breeze
results from differential heating of land and sea not from radiation differences - but from the different
specific heats of land and water
Nighttime land-sea breeze
at night, the land cools more rapidly than the sea
and thus overlying air becomes more dense and
has a higher pressure