Transcript Atmospheric and Oceanic Circulations (continued) Chapter 6

Atmospheric and Oceanic
Circulations
(continued)
Chapter 6
Lecture 14
7 February 2005
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
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 Gradient Force
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
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
the CF deflects the wind to the right until upper
troposphere wind flows parallel to isobars
~7km
Credit: www.physicalgeography.net
Friction Force
Surface friction reduces wind speed and the effects
of the Coriolis force
Friction 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
PGF + Coriolis + Friction Forces
Figure Credit: “Earth’s
Climate” by W. Ruddiman
Show shockwave 27_WindPatDev.swf
Useful things to remember
• Coriolis Force
Acts to the right of motion (left in SH)
Not a real force – matter of perspective
• Geostrophic wind – upper troposphere/ocean
Horizontal pressure & Coriolis forces balance
Winds go ALONG isobars
• Surface friction – lower troposphere
Enables CROSS isobar flow
Useful things to remember
• Low pressure cells
Less dense –humid
Circulation is counter-clockwise (NH) – cyclonic
Convergent near ground – rising air masses
• High pressure cells
More dense – dry
Circulation is clockwise (NH) – anticyclonic
Divergent near ground – descending air masses
Heating & Humidity in Tropics
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
Warm, moist air rises and is replaced by cooler
drier air from other sites
Credit:
http://ess.geology.ufl.edu/ess/Notes/AtmosphericCirculation/
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
InterTropical Convergence Zone
the position of the ITCZ tracks the sun (it is 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 warm, dry air to the surface
leading to high pressure & reduced precipitation
Subtropical high-pressure cells
these cells occur where the tropical air descends in
either hemisphere
Equator-to-pole cross section of circulation
Jet
Streams
Figure 6.18
Show shockwave 04_GlobalWinds.swf
Figure Credit: http://www.geog.ucsb.edu/~joel/g110_w03/chapt10/vorticity/agburt2_10_07.jpg
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 & equatorial
(ITCZ) low pressure
Asian
monsoon
in summer, the
ITCZ shifts north,
reversing the
pressure gradient –
winds flow from the
Indian ocean & 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
What time of day would you go jogging in
Los Angeles?
WHY?