Lecture 9 ATMOSPHERIC CIRCULATION Chapter 8 OVERVIEW Atmosphere

Lecture 9: ATMOSPHERIC CIRCULATION Chapter 8

OVERVIEW • Atmosphere and ocean are one interdependent system • Uneven distribution of solar energy creates winds • Winds drive surface ocean currents and waves • Examples of interactions: – El Niño-Southern Oscillation – Greenhouse effect

OCEAN-ATMOSPHERE INTERACTION • Atmosphere – volume of gases, water vapor, and airborne particles enveloping the Earth • Ocean-atmosphere interacts, gases dissolve over sea surface • Little extremes in global surface temperature due to movement of evaporated water by wind • Weather – state of atmosphere at specific place and time • Climate – long term average of weather in an area

COMPOSITION OF THE ATMOSPHERE Dry air, by volume • Homogeneous mixture of gases • Air never dry, contains water vapor (up to 4% by vol. ) • Water vapor = clouds/fog/invisible • Residence time = 10 days • Condenses into dew, rain or snow Fig. 8. 1 78% Nitrogen, 21% Oxygen 1% everything else

AIR HAS MASS Fig. 8. 2 • A mass of warm air occupies more space • Warm air less dense • Humid air less dense than dry at same temperature – water molecules weigh less than N and O molecules • at surface air densely packed by own weight • expands where less pressure • Warm air holds more water vapor, rises, cools, condenses into clouds, forms water droplets, precipitation

HEAT BUDGET • 51% of incoming short-wave radiation (light) absorbed by land water. Converted into long-wave infrared radiation (heat), and transferred back into the atmosphere • Heat budget – accounting for heat input and heat output • Over time: total incoming heat = total outgoing heat so Earth in thermal equilibrium • Heat budget for Earth as a whole is in balance, but for different latitudes is not • Due to solar heating varying with latitude

EARTH’S HEAT BUDGET Fig. 8. 3 Earth maintained a nearly constant average temperature because of equal rates of heat gain and heat loss

ATMOSPHERE HAS UNEVEN SOLAR HEATING. THIS IS PROMOTED BY……. . • Angle of incidence of solar rays per area - high angle in equatorial region = more heat - low angle in polar region = less heat • Albedo = radiation reflected back into space • Thickness of atmosphere – light passes thru more atmosphere near poles, less reaches Earth • Day/night = heating/cooling of water vs land • Seasons = regional heating/cooling

SOLAR HEATING VARIES WITH LATITUDE High Latitudes Light penetration is shallow, light flux/m 2 is lower, longer path through atmosphere Low Latitudes Light penetration is deeper, light flux/m 2 is higher, shorter path through atmosphere Fig. 8. 4

SOLAR HEATING VARIES WITH LATITUDE • High latitudes: more heat lost than gained - Due to albedo of ice and low angle of incidence of solar rays • Low latitudes: more heat gained than lost - Due to vast ocean’s heat capacity, high solar angle of incidence, lower albedo

AREAS OF HEAT GAIN AND HEAT LOSS Fig. 8. 5 Heat Budget Balance Heat gained between 35°N and 35°S is balanced by heat lost at higher latitudes by heat transfer through atmosphere and ocean circulation.

SOLAR HEATING VARIES WITH SEASONS • Seasons caused by variations in the amount of incoming solar energy as Earth makes its annual rotation around the sun on axis tilted at 23. 5° • N. hemisphere winter = S. hemisphere tilted towards the sun, N. hemisphere has less light and heat • N. hemisphere – lean towards sun in June, away in December • Summer – sun higher in the sky, days are longer • Mid-latitudes – 3 x as much solar energy in summer than winter

SEASONS • Earth’s axis of rotation tilted with respect to its eliptical orbit • Tilt (not distance from sun) responsible for seasons - Vernal (spring) equinox - Summer solstice - Autumnal equinox - Winter solstice • Seasonal changes and day/night cause unequal solar heating of Earth’s surface

THE SEASONS

THE SEASONS Fig. 8. 6

UNEVEN SOLAR HEATING RESULTS IN LARGE-SCALE ATMOSPHERIC CIRCULATION • Remember – warm air rises, cool air sinks • Think air circulation with radiator opposite cold closed window • Convection current/cell

CONVECTION CURRENT • Warm air, less dense (rises) • Cool air, more dense (sinks) • Moist air, less dense (rises) • Dry air, more dense (sinks) • Analogous to atmospheric and ocean circulation Fig. 8. 7

MOVEMENT IN THE ATMOSPHERE sinks wind rises • Air (wind) always moves from regions of high pressure to low • Cool dense air, higher surface pressure • Warm less dense air, lower surface pressure

AIR CIRCULATION IF UNEVEN SOLAR HEATING WAS THE ONLY FACTOR • Air heated in tropics – less dense and rises • Travel to poles – less dense, sinks • Turns equatorwards Fig. 8. 8

CORIOLIS EFFECT • Coriolis effect causes deflection in path of moving body across Earth’s surface • Due to Earth’s rotation to east and fact that rotational velocity at equator is ~1000 mph and 0 mph at the poles • Most pronounced on objects that move long distances across latitudes • Deflection to right in Northern Hemisphere • Deflection to left in Southern Hemisphere • Maximum Coriolis effect at poles • No Coriolis effect at equator

CORIOLIS EFFECT Movement of air on a rotating Earth

CORIOLIS EFFECT INFLUENCES ATMOSPHERIC CIRCULATION • Coriolis effect does not cause the wind, it influences wind direction • Convection circuits called atmospheric circulation cells (3 cells per hemisphere) • Wind belts – deflected by coriolis • Starts at Equator – warm air rises, loses moisture through ppt (expansion and cooling), becomes denser and sinks at 30°N and 30°S

GLOBAL ATMOSPHERIC CIRCULATION • High pressure zones (air sinks) - Subtropical highs ~ 30°N & 30°S - Polar highs ~ 90°N & 90°S - Clear skies, dry • Low pressure zones (air rises) - Equatorial low - Subpolar lows ~ 60°N & 60°S - Overcast skies with lots of precipitation

GLOBAL ATMOSPHERIC CIRCULATION • Air circulation organized as convective circulation cells driven by changes in density due to: - Changes in air temperature - Changes in water vapor content • Major circulation cells - Hadley Cells (0 o to 30 o N and S) - Ferrel Cells (30 o to 60 o N and S) - Polar Cells (60 o to 90 o N and S)

GLOBAL ATMOSPHERIC CIRCULATION Fig. 8. 13

ATMOSPHERIC CIRCULATION GENERATES LARGE-SCALE SURFACE WIND PATTERNS • Trade Winds - Northeast trades in Northern Hemisphere - Southeast trades in Southern Hemisphere • Prevailing westerlies – moderately deflected winds from west to east • Polar easterlies – strongly deflected winds from east to west • Boundaries between wind belts - Doldrums or Intertropical Convergence Zone (ITCZ) = rainy & unpredictable winds (0°) - Horse latitudes = dry & light winds (30°) - Polar fronts = cloudy & lots of precip. (60°)

ATMOSPHERIC CIRCULATION AND WIND Polar BELTS Cell Ferrel Cell Hadley Cells theoretical wind paths if Earth didn’t rotate

METEOROLOGICAL EQUATOR • Geographical equator at 0° • Convergence zone lies at meteorological equator (thermal equator) • Situated about 5°N of geographical equator • Moves with seasons • Winds shift with this movement • Convergence zone = Intertropical Convergence Zone (ITCZ) – low pressure associated with Hadley Cells

INTERTROPICAL CONVERGENCE ZONE (ITCZ) Fig. 8. 14 • Moves north in northern summer, towards equator in southern summer

MONSOONS • Seasonal prevailing wind linked to specific heats of land water, and movement of ITCZ • January (NE monsoon) - land cools faster than ocean - air cools and sinks over land - dry surface wind moves seaward • July (SW monsoon) - land heats faster than ocean - air over land warmer so rises - cool air flows from ocean to land - continued heating causes humid air to rise, condenses forming clouds and heavy rain Fig. 8. 16

SEA BREEZES AND LAND BREEZES Arise from uneven surface heating Fig. 8. 17

STORMS • Regional atmospheric disturbances characterized by strong winds and often accompanied by precipitation • Cyclones/hurricanes

TROPICAL CYCLONES (HURRICANES) • Large rotating mass of low pressure (warm, humid) • Strong winds, torrential rain • Classified by maximum sustainable wind speed ( >74 mph) • Flooding from torrential rain • Storm surge most damaging

INTERNAL STRUCTURE • Can be 1000 km in diameter, normally < 200 km • Calm center (eye), surrounded by rain and wind bands • Air moves across ocean surface towards low pressure center and drawn upwards around the ‘eye’ Fig. 8. 23

TROPICAL CYCLONES Fig. 8. 24 Develop in zones of high humidity and warm air, between 10 – 25° latitude (red color)

CYCLONES AND THE CORIOLIS EFFECT • Air turns counterclockwise in N. hemisphere • Coriolis causes rightward deflection of approaching air – causes storm to spin counterclockwise Fig. 8. 25

WIND DIRECTION AROUND LOW PRESSURE REGION • Northern hemisphere winds move counterclockwise (cyclonic) around a low pressure region • Southern hemisphere winds move clockwise (anticyclonic) around a low pressure region

TROPICAL CYCLONE TRACKS HURRICANES CYCLONES TYPHOONS Fig. 8. 26

STORM SURGE • Responsible for 90% of hurricane deaths • Low pressure center produces low hill of water. • Hill moves over ocean, as reaches shore can increase sea level by 12 m “stacks” water up on shore, some of the greatest damage

2005 ATLANTIC HURRICANE SEASON • Most destructive year ever recorded, hottest year on record • 1 st June to 30 th November = hurricane season • 28 tropical cyclones, 15 became hurricanes • 3 reached Category 5 • $100 billion damage • 1777 lives lost

HURRICANE KATRINA AND RITA • Katrina: - formed over Bahamas, hit Fl. as Category 1 - increased to Category 5 (175 mph) over Gulf of Mexico - hit land as Category 4 (125 mph) • Rita: - hit as a Category 3 less than a month later • New Orleans most impacted, below sea level, levees weakened Fig. 8. 27

HURRICANE WILMA • Hit Mexico as a Category 4 Fig. 8. 33