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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