What Causes the Winds? Katrina As a Model Katrina a Classic Hurricane

Home , Horse latitudes

Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington What Causes the Winds? Katrina as a Model • Processes that cause air to start moving • Counterclockwise spiral • Processes that affect how the wind moves – Damage worse to the east – Moves E to W • Two categories of wind – Prevailing Winds • Grows over – Transient & storm winds the ocean Global patterns of prevailing winds – Pressure • decreases • Local/regional patterns of transient & storm – Winds winds increase

www.nhc.noaa.gov/pastall.shtml • Starting point: Katrina & hurricanes • Weakens www.nhc.noaa.gov/pdf/TCR-AL122005_Katrina.pdf www.aoml.noaa.gov/hrd/Storm_pages/katrina2005/sat.html 1 2 over land www.nhc.noaa.gov/archive/2005/KATRINA.shtml?

Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Katrina a Classic Hurricane Katrina & the Ocean • Forms over warm water (Orange color) • Warn surface water intensifies the hurricane – Heat drives • Evaporation is a major component convection in – Water vapor the atmosphere less dense – Air spirals from than air Earth’s rotation – Rises & cools • Moves off to – Moisture con- NE at higher denses latitudes – Releases heat – Wilma October – Causes more 2005 air to rise 3 4 www.srh.weather.gov/jetstream/tropics/tc.htm Garrison Fig. 8.23 p. 193 Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Cyclone Basics A Model for Global Winds • An exaggerated case of a very common • ALL winds driven by surface heating atmospheric phenomenon – Warm air rises – Low-pressure system or cyclone – Air drawn along the surface to replace • Cyclones driven by air heated at the surface rising air • Wind! – Warm air rises – What goes up, Air is drawn along – must come down the surface to replace rising air • Cooler air sinks somewhere – Air spirals counter- • Uneven heating clockwise because • Creates of Earth’s rotation 5 http://earthobservatory.nasa.gov/Library/Hurricanes/ 6 convection cells Garrison Fig. 8.7 p. 182

• Low latitudes are warmer than high • Both ocean & atmosphere transport heat latitudes – Movement of air = convection & wind – But temperature difference is not nearly as • Water vapor also transports heat great as the – Movement of water = convection & currents difference in solar heating. – Heat is transported from low latitudes to high latitudes.

7 Garrison Fig. 8.4 p. 180 8 Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Atmospheric Convection Atmospheric Convection

• At low latitudes the sea surface is heated • Not all this moisture condenses over the – Tremendous evaporation from the oceans equator, however. – Warm, moist air rises – Some of it rises and spreads north and south toward higher latitudes. – The air cools as it rises – Eventually this air cools and sinks back to Cool air can hold less water vapor – the surface. – So as the air rises, moisture condenses – As it does, more moisture condenses and – Thus there is a belt of high precipitation causes rain (and snow) at these higher (rain) near the equator latitudes.

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Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Atmospheric Convection Atmospheric Convection • Heat is thus transported by both the • A simple model of the atmosphere warm air and the latent heat of – Assumes all ocean and not rotating vaporization. – Large convection cells N & S of equator – Energy required to convert water to vapor – Cooler, drier air sinks at poles at same temperature – Returns to low latitudes – LHOV is transferred from water vapor to along the surface air when the moisture condenses again – Replaces air that rises – Either over the equator or at higher near the equator latitudes. Fig. 6.15

11 12 Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Atmospheric Convection Atmospheric Pressure • Actually 3 convection cells in each • Rising warm, moist air is less dense hemisphere. – Creates lower atmospheric pressure – Created by Earth’s rotation – Air aloft sinks at about 30˚ N and 90˚ N – Surface air rises at about 60˚ N.

Fig. 6.19 Fig. 6.19 13 14

Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Atmospheric Pressure Convergence & divergence • Sinking dry air is more dense • At surface where air along the surface • Creates higher atmospheric pressure comes together & rises: convergence. – Low pressure at 0˚ and 60˚ N • At surface where air sinks & spreads out: divergence – High pressure at 30˚ & 90˚ N

15 16 Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Atmospheric Symmetry Atmospheric Symmetry • Convection • Belts of low and pressure & high high & low precipitation at pressure 0˚ and about patterns are 60˚N & S symmetrical in latitudes Northern & • Latitudes of Southern tropical and hemispheres temperate rain forests

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Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Atmospheric Symmetry A Model for Climate • Belts of high • Rising air in the tropics & condensation pressure & low – Global belt of wet climate at tropical latitudes precipitation • Another wet belt at temperate - subpolar at about 30˚ & latitudes 90˚ N & S – Seattle latitudes. – Nov. • Latitudes of 1979 major deserts average – Rain forests

19 20 http://earthobservatory.nasa.gov/Observatory/Datasets/rainfall.gpcp.html Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington A Model for Climate Winds • Desert belts indicate sinking & drying air • Driven by differences in pressure – Sahara, Kalahari, Gobi, Australian outback – From High to Low pressure – Poles are deserts too! • In this convection model, all winds • But moisture accumulates as snow & ice would be directly from North or South

21 http://earthobservatory.nasa.gov/Observatory/Datasets/rainfall.gpcp.html 22

Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Winds Coriolis Effect • Note: winds are named for the direction • Some people call it Coriolis “force.” from which they blow – As if it diverts moving objects from a • Prevailing winds come mainly from E & W straight line. • Coriolis Effect • It is not a real force. – Moving objects – Objects actually travel in a straight line. turn right in – It’s the Earth that is turning. N. Hemi- – It is simply an effect of being on a rotating sphere body. – Left in S. hemisphere 23 24 Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Formula for Coriolis Atmospheric Zonal Circulation • Coriolis Effect f=2! sin " v. • Coriolis effect + 3 convection cells • 2! + rotation rate of the earth (a • Equator to 30˚ N constant) – Surface air is moving toward south • sin " = 0 to 1 depending on latitude – So it turns right, toward the west – Coriolis Effect is zero at the equator and – Forms a belt of easterly winds (from the maximum at the poles east) • V = speed of moving object – Trade Winds. – Greater effect at higher speeds

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Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Atmospheric Zonal Circulation Atmospheric Zonal Circulation • 30˚ N to 60˚ N • 60˚ N to 90˚ N – Surface air is moving toward north – Surface air is moving toward south – So it turns right, toward the east – So it turns right, toward the west – Forms a belt of westerly winds (from the – Forms a belt of easterly winds (from the west) east) – Prevailing Westerlies. – Polar Easterlies.

27 28 Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Atmospheric Zonal Circulation Atmospheric Zonal Circulation • Direction of winds is symmetrical in S. • Equator is a transition zone Hemisphere – Air movement is vertical more than – Coriolis is opposite horizontal to the left – Region of weak surface winds • Trade Winds – Doldrums from the East • Prevailing Westerlies • Polar Easterlies

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Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 11 © 2006 University of Washington Atmospheric Zonal Circulation Atmospheric Zonal Circulation • 30˚ N & S also transition zones • 60˚ N & S are regions of strong winds – Air movement is vertical more than – “The polar front” & jet stream horizontal – Strong temperature contrast between – Region of weak surface winds converging temperate and polar air masses – Horse Latitudes – No such contrast at lower latitudes.

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