CIRCULATION Global Wind Circulation Winds generated by… Pressure differences due to… • Unequal heating of Earth’s surface Winds blow to balance inequalities… Air Pressure… • Surface Temperature differences Wind patterns seasonally migrate N & S… Latitudes of maximum Solar Radiation Global Wind Circulation Equator – Low Poles – High Without Land or Rotation… Surface… •Poles Æ Equator Aloft… •Equator Æ Poles Hadley (1735) Global Wind Circulation Air Aloft… Some Æ Poles Most Subsides… • 30° Latitude Global Wind Circulation 3-Cell Model (1920s) Hadley Cell •0-30° Mid-Latitude • 30-60° Polar • 60-90° Global Wind – ITC Equator Intense Sunlight •Warm Air Rises “Equatorial Low” •AKA –InterTropical Convergence Zone (ITCZ) Weak Winds • =Doldrums Clouds & Rain (Daily) Global Wind – STH 30° Latitude Air Aloft flows from Equator Æ 30° Latitude Pile-up Aloft at 30° Air Sinks… • Coriolis stronger… – Earth’s Rotation • Radiation Cooling… – Air Density Increases Global Wind – STH 30° Latitude Creates High Pressure Sub-Tropical High •AKA “Horse Latitudes” Weak Winds Clear Skies & Dry Air Global Wind – Trade Winds Trade Winds Surface Air pile-up at 30° Latitude •Air Diverges Sub-Tropical High Æ Equatorial Low • Northeast Trades • Southeast Trades Global Wind – Trade Winds Trade Winds Strong reliable Winds Æ Equator • Sailing Ships… –Exploration –Trade Winds Converge at Equator… • Air Rises… Global Wind – Hadley Cells Between... Equator 30° Latitude Both N & S… Antitrade Winds Aloft Global Wind – Westerlies Westerlies Sub-Tropical High Æ 60° Latitude • More deflection due to Rotation – Stronger Coriolis Not continuous… • Series of Mid-Latitude Cyclonic Storms migrating… Global Wind – Polar Polar High Cold, Dense Air Sinks Wind blows away from High at Poles • “Polar Easterlies” –Weak Winds Global Wind – Polar 60° Latitude Convergence of… • Polar Easterlies • Westerlies Polar Front Air Rises •Low Pressure • Cold & Rain, Snow AKA “Subpolar Low” Global Wind Circulation Polar High Polar Easterlies Polar Front Subpolar Low Westerlies ST High Horse… Trade Winds Equatorial Low Doldrums ITCZ Idealized Circulation Only ideal for… Subpolar Low in So. Hemisphere ITCZ Changes… Seasons Land in No. Hemisphere • Monsoons Pressure Cells Pressure Cells in No. Hemisphere ST High • Hawaiian – AKA “Pacific” •Azores Subpolar Low • Aleutian • Icelandic Sub-Tropical Highs 30°N Latitude… Hawaiian & Azores Trade Winds 30° Latitude Æ Equator From East • NE Trades in No. Hemisphere Trade Winds & Rain Leeward side Windward side SW & Dryer NE & Wet Intertropical Convergence ITCZ – Equatorial Low Straddles Equator Bends Poleward over Land Satellite – ITC & STH ITCZ (Low) Clouds… •Equator ST High Clear… • Sahara •SW Africa Westerlies 30° Æ 60° Latitude From West •Europe Polar Highs North Pole South Pole Antarctica Polar Easterlies Poles Æ 60° Latitude From Northeast Global Wind Circulation Idealized Winds & Pressure Composite by Latitude Westerlies Westerlies strong Aloft – Balance… Pressure Gradient Æ Pole Coriolis Force Æ Equator Jet Streams High Speed Winds Aloft 120-240 mph Due to steep Temperature Gradient between… •Surface •Aloft Jet Streams Polar Jet Stream Polar Front Strongest in Winter Subtropical Jet Stream Subtropical High Weak in Summer Jet Streams Polar Jet Stream Center of Storms… • Polar Easterlies • Westerlies Subtropical Jet Not Permanent Weaker Polar Jet Stream Changes seasonally… North in Summer South in Winter •35 mph •75 mph Rossby Waves Waves in Polar Jet Stream Fastest flow… •“Core” Stronger dip Southward in Winter • Cold Polar Air Æ South • Cyclonic Storms Polar Jet Stream Jet Stream Core – January Wind & Ocean Currents Winds drive Ocean Currents High Pressure at 30° drives Trade Winds • Trade Winds drive Equatorial Currents ST Highs & Trade Winds 30° Latitude Highs… NE & SE Trades Wind & Ocean Currents Equatorial Currents hit Land Masses… Circulate Clockwise in Northern Hemisphere • Counterclockwise in Southern Hemisphere Wind & Ocean Currents North Atlantic Ocean Equatorial Current Æ Gulf Stream Wind & Ocean Currents North Atlantic Ocean Gulf Stream Æ North Atlantic Current (Warm) • È Canaries (Cold) Æ Equatorial Current Wind & Ocean Currents Equatorial current Warm – Flows West Other Currents… Warm on West side Cold on East side Wind & Ocean Currents

Northern Hemisphere

Southern Hemisphere Wind & Ocean Currents Affects Weather… North Atlantic Current warms Europe • Above normal for High Latitude Wind & Ocean Currents Cold Currents cool Air Æ Fog West Coast Deserts • Baja CA •Atacama (Peru) •Namib (SW Africa) Wind & Ocean Currents Upwelling in Cold Currents… Coriolis deflects Water from Coasts • Deeper Cold water rises to replace it Wind & Ocean Currents Maintain Earth’s Heat Balance… Redistribute Excess from Equator Æ Poles • Remedy Deficit at Poles Wind & Ocean Currents Heat Transfer for Earth’s Heat Balance… ¼from Ocean ¾ from Wind El Niño Normally very strong Equatorial Current Build-up of Warm Water & Pressure in West Reverses with counter-current in December El Niño Normally Cold Peru Current... Becomes Warm with El Niño •Lose Fishing Southern Oscillation Normally… W Pacific – Warm Water, Rainy & Low Pressure E Pacific – Cool Water, Dry & High Pressure Southern Oscillation El Niño reversal… W Pacific – Cool Water, Dry & High Pressure E Pacific – Warm Water, Rainy & Low Pressure El Niño Late year reversal becomes long-lasting… Irregularly ~ 3-7 years Affects climate outside region El Niño Affects Subtropical & Westerly Jet Streams Wetter in Peru Dryer in Australia & Indonesia El Niño Affects U.S.A. weather… Wetter Winter in NW & SE Less Hurricanes Warmer Winter in NE El Niño & La Niña El Niño… Warmer ocean at Equator La Niña… Colder ocean at Equator • Above average El Niño & La Niña ENSO Index – El Niño/Southern Oscillation Cycles between El Niño & La Niña

El Niño

La Niña La Niña La Niña affects U.S.A. weather… Cool & Snowy in NW Cold Winters in Great Plains Dry in SW & SE More Hurricanes Season Changes Seasons shift Solar Radiation… Pressure Cells… • Migrate N & S •Strengthen & weaken other cells Season Changes Season Changes InterTropical Convergence Zone (ITCZ)… Ç North in July È South in January January Surface Pressure Pressure Cells È South ITCZ Low Pacific & Azores Highs Subpolar Lows… Aleutian & Icelandic July Surface Pressure Pressure Cells… ITCZ Low Ç North Azores High Æ Bermuda High Aleutian & Icelandic Lows disappear California Seasons Summer Winter Monsoons = Seasonal Winds Due to changes… • Temperature •Pressure Asia – Interior Winter Monsoon Summer Monsoon Monsoons Winter Monsoon Cold in Siberia Subsiding Air •High Pressure Wind from Land Æ Sea •North È South DRY Monsoons Summer Monsoon Hot on Land • Rising Air •Low Pressure Wind from Sea Æ Land • North South Ç Monsoons Summer Monsoon Moist Air from Sea Æ Land Orographic Lifting up mtns ITCZ Ç North •Low Pressure WET – RAINY Monsoons – South Asia Wet Summer Cherrapunjee, India 1,042” Rain in 1 year Dry Winter Monsoons – East Asia World Monsoon Areas ½ World’s Population Eurasia South Asia Southeast Asia Monsoon – SW USA Summer Heat… Air Rises Low Pressure Moist Air… Gulf of California Monsoon – SW USA Rainy Summer July, August • Tucson, AZ World Zonal Precipitation Sea Level Precipitation… Matches Air Pressure zones • Differential Solar Radiation – Latitude & Seasons World Zonal Precipitation 4 Major Zones: Polar High •Dry Subpolar Low •Wet –(Westerlies) Subtropical High •Dry ITCZ •Wet World Zonal Precipitation Transitional Latitudes for Solar Seasons Between ITCZ & Subtropical High •Summer Wet •Winter Dry World Zonal Precipitation Transitional Latitudes for Solar Seasons Between Subtropical High & Subpolar Low •Summer Dry •Winter Wet Subtropical Highs East Coasts West Coasts Wet – Warm ocean Dry – Sinking Air Hypothetical Continent Hypothetical Continent Northern Hemisphere Continent Precipitation at Sea Level if no Topography •7 Regions Annual Precipitation Annual Precipitation Wet Regions 0-10° Latitude Patterns of Precipitation Wet Regions 10-25° Latitude East Coasts • Trade Winds Patterns of Precipitation Wet Regions 25-40° Latitude East Coasts • Warm Ocean Currents Patterns of Precipitation Wet Regions 40-60° Latitude West Coasts •Westerly Winds Patterns of Precipitation Dry Regions 20-30° Latitude •Subtropical High Patterns of Precipitation Dry Regions 20-30° Latitude West Coasts •Subtropical High Patterns of Precipitation Dry Regions Rainshadow areas Patterns of Precipitation Dry Regions Polar Areas… •Polar High Precipitation Variability Wet areas – Less Variability between years Dry areas – More Variability between years