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Air-Sea Interaction (Chapter 8) The Solar Connection
The Coriolis Effect The Winds
Hurricane Wilma Observation Device: GOES-12 1 km visible imagery. Visualization Date: October 24, 2005 15:02:23 http://www.nnvl.noaa.gov/cgi-bin/index.cgi?page=items&ser=109929
Atmospheric Layers
The troposphere (air we breathe) is approximately: 78% Nitrogen 21% Oxygen <1% Argon <0.1% Carbon dioxide 0.01% other gases
Reasons for seasons… In addition to the Earth’s orbit around the sun, there are three main reasons for uneven heating of Earth: 1 - The spherical shape of Earth 2 - The tilted axis of rotation 3 - Seasonal changes in distance from sun to Earth
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As the Earth revolves around the sun, the north pole will tilt toward the sun for part of the year (summer), and away from the sun for part of the year (winter).
When the northern hemisphere has summer, the southern hemisphere has winter
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As the Earth revolves around the sun, the angle at which we receive sunlight changes. In summer, we receive high angle sunlight.
Higher angle sunlight means more concentrated heat and hotter summer weather.
Notice how the lower angle sunlight spreads out, heating a larger area and therefore giving less concentrated heat.
…so solar heating is uneven Uneven warming
causes convection Convection occurs
when warm air becomes less dense and rises, then cool, dense air comes in to replace the rising warm air = circulation
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Idealized Convection
Warm air rises at equator, cooler air flows from poles to replace warm air at surface level. High, warmer air flows toward poles, then cools and sinks = circular pattern.
Circulation cells and air pressure
http://www.ucar.edu/learn/1_1_1.htm
Coriolis(1835) Effect…
• The path of a moving object deflects to the right in the northern hemisphere and to the left in the southern hemisphere.
• … due the Earth’s rotation,
wind flows east and west as well as north and south.
http://www-history.mcs.st-andrews.ac.uk/history/PictDisplay/Coriolis.html
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Relative motion
The linear velocity of points on the Earth’s surface decreases with increasing latitude. At the equator (0°), the eastward speed is about 1,620 km/hr, but at the north pole (90°N) the eastward speed is zero.
Earth’s rotation and shape causes the Coriolis effect
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http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml
Real Life Example: This movie of a ball rolling across the surface of a rotating merry-go-round is a helpful demonstration of the Coriolis force.
• http://www.classzone.com/books/earth_science/terc/content/visualizations/es1904/es1904page01.cfm?chapter_no=visualization
Winds and the Coriolis effect Convection plus Coriolis effect gives us prevailing wind patterns. Air moves horizontally from areas of higher pressure to areas of lower pressure = wind. Coriolis steers wind to the right in the northern hemisphere (but hurricanes spin counterclockwise in the northern hemisphere! - why??)
General Circulation http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/global_winds.rxml
Polar Easterlies: From 60-90 degrees latitude.
Prevailing Westerlies: From 30-60 degrees latitude (aka Westerlies).
Tropical Easterlies: From 0-30 degrees latitude (aka Trade Winds). The easterly trade winds of both hemispheres converge at an area near the equator called the "Intertropical Convergence Zone (ITCZ)", producing a narrow band of clouds and thunderstorms that encircle portions of the globe.
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How do hurricanes form?
Forcasting intensity…
http://oceanexplorer.noaa.gov/edu/learning/welcome.html - lesson14
Tropical Cyclone Formation Tropical cyclones form in one air mass
Air starts moving toward
a zone of low pressure
and veers off course
to right
Core of tropical cyclone rotating counterclockwise
Air sta
rts m
oving
towar
d a zo
ne of
low
pres
sure
and v
eers
off c
ours
e to r
ight
N
Equator
Fig. 8-25, p. 220 Stepped Art
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Conditions Required for Tropical Depression Formation
• Weather disturbance with low pressure that draws in air from all directions (usually 5° - 20° latitude)
• Warm surface water (0-200m), ca. 80°F (27°C) or warmer
Hurricane Development • Tropical Depression: organized system of
clouds and thunderstorms; wind speed LE 38 mph
• Tropical Storm: cyclonic circulation develops; wind speed 39-73 mph; storm is named
• Tropical Cyclone (aka hurricane in the North Atlantic): intense cyclonic winds surrounding a well defined eye; wind speed GE 74 mph
Saffir-Simpson Scale (NHC)
• Category 1: 74-95 mph; minimal damage • Category 2: 96-110 mph; moderate damage • Category 3: 111-130 mph; extensive damage • Category 4: 131-155 mph; extreme damage • Category 5: GE 156 mph; catastrophic
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Common tropical cyclone tracks. The breeding grounds of tropical cyclones are shown as orange-shaded areas. The storms follow curving paths: first they move westward with the trade winds and usually away from the equator. Then they either die over land or lose power over the cooler ocean of mid-latitudes.
Wilma
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Hurricane Andrew
Hurricane Andrew 1992
Hurricane Mitch
http://news.bbc.co.uk/2/hi/americas/202395.stm http://www.fortogden.com/fant-8.jpg
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Are hurricanes getting more intense due to warming sea-
surface temperatures?
…probably
What about Monsoons? From the Arabic mausim meaning season…
(Honjo and Weller, 1997)
Summer Southwest monsoon Winter Northeast monsoon
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Seasonal cycles are critical to biological cycles too…
• Biological communities have adapted to seasonal cycles:
• Gray whales migrate to Arctic waters in summer to feed on krill and other plankton
• Subsistence hunters in Alaska depend on annual migrations of marine mammals