09_atmosphericcirculationppt - david m lawrence：09_atmosphericcirculationppt -戴维m劳伦斯

NAS 125: MeteorologyAtmospheres Planetary CirculationRev. 6 April 2006 1Atmospheres Planetary CirculationEl Nio, part 1 Global weather went wild in 1982-1983. Crippling droughts struck Australia, India, Indonesia, the Philippines, Mexico, Central America, and southern Africa. Huge drought-related wildfires raged across Australia and Borneo. Australias worst drought in 200 years caused $2 billion in crop damage. Floods devastated parts of western and southeastern United States, Cuba, and northwestern South America. Destructive tropical cyclones lashed French Polynesia and Hawaii.Rev. 6 April 2006 2Atmospheres Planetary CirculationEl Nio, part 2 Weather gone wild (continued): Abnormally warm waters over a 13,000-km stretch of the equatorial Pacific caused massive dieoffs of fish, seabirds, marine mammals, and corals. Collapse of plankton population led to a collapse of the anchovy population, which in turn led to a collapse of other fish populations, which in turn led to a collapse of seabird and marine mammal populations. Adult seabirds abandoned their young on Kiribati. In all, the weather-related events, called “the most disastrous in recorded history,” cost 2000 human lives, about $13 billion in damage, and vast ecological havoc).Rev. 6 April 2006 3Atmospheres Planetary CirculationEl Nio, part 3 The cause of the disaster was an unusual warming of surface waters in the eastern equatorial Pacific off the coast of Ecuador and Peru. Residents of the area had known about the weather pattern caused by the warming for some time, and called it El Nio. The rest of the world knew about it after 1982-1983.Rev. 6 April 2006 4Atmospheres Planetary CirculationGeneral circulation, part 1 According to an idealized circulation pattern (on a non-rotating earth with a uniform surface), unequal heating would create a two-cell circulation pattern. The cells, one in each hemisphere, would have rising air at the equator and descending air at the poles. Winds at the surface would blow from the poles toward the equator, and winds aloft would blow from the equator toward the poles. The rotation of the Earth and its variable surfaces create a complex atmospheric circulation pattern.Rev. 6 April 2006 5Atmospheres Planetary CirculationGeneral circulation, part 2 Only the tropical regions have complete vertical circulation cells, called Hadley cells. In a Hadley cell, heating at the equator warms the air above, causing it to rise to elevations of about 15 km, where it cools, moves poleward, then subsides. The air descends at roughly 30 north or south latitude. There are two Hadley cells. Outside the tropical and subtropical latitudes, vertical cells do not exist or are weakly and sporadically developed.Rev. 6 April 2006 6Atmospheres Planetary CirculationRev. 6 April 2006 7Atmospheres Planetary CirculationGeneral circulation, part 3 Global pressure patterns drive global wind patterns. The patterns observed migrate north and south with the patterns of solar heating produced as the Earth orbits the sun. The north-south migration of climate patterns is enhanced over the continents and reduced over the oceans.Rev. 6 April 2006 8Atmospheres Planetary CirculationRev. 6 April 2006 9Atmospheres Planetary CirculationRev. 6 April 2006 10Atmospheres Planetary CirculationRev. 6 April 2006 11Atmospheres Planetary CirculationGeneral circulation, part 4 The general circulation of the atmosphere has seven surface components: Subtropical highs Intertropical convergence zone Polar highs Subpolar lows Trade winds Midlatitide westerlies Polar easterliesRev. 6 April 2006 12Atmospheres Planetary CirculationRev. 6 April 2006 13Atmospheres Planetary CirculationGeneral circulation, part 5 Subtropical highs The subtropical latitudes serve as the “source” of the major surface winds of the planet. The subtropical highs (STHs) are large semipermanent anticyclones centered at about 30 latitude over the oceans Their average diameter is about 3,200 kilometers. They develop from the descending limbs of Hadley cells. The location of the subtropical highs are coincident with most of the worlds major desert belts. Migration of the anticyclones affects weather of midlatitudes.Rev. 6 April 2006 14Atmospheres Planetary CirculationRev. 6 April 2006 15Atmospheres Planetary CirculationGeneral circulation, part 6 Subtropical highs (continued) The Horse latitudes are areas in the subtropical highs characterized by warm, tropical sunshine and an absence of wind. They exist because the weather within a subtropical high is nearly always clear, warm, and calm. The subtropical highs serve as source for two of the worlds three major surface systems: Trade winds WesterliesRev. 6 April 2006 16Atmospheres Planetary CirculationGeneral circulation, part 7 Intertropical convergence zone The intertropical convergence zone (ITCZ) is a belt of calm air where the northeast trades and southeast trades converge, generally in the vicinity of the equator (or at least the heat equator). The zone is also called the equatorial front, the intertropical front, and the doldrums. Intertropical convergence zone thunderstorms provide the updrafts where all the rising air in of the tropics ascends. The zone often appears as a narrow band of clouds over oceans, but it is less distinct over continents.Rev. 6 April 2006 17Atmospheres Planetary CirculationGeneral circulation, part 8 Polar highs The polar highs are anticyclones centered over the polar regions. The Antarctic high is quite different from the Arctic high because it forms over an extensive, high-elevation, and very cold continent, while the Artic high forms primarily over sea ice. The Antarctic high is strong, persistent, and almost permanent, while the Arctic high is much less pronounced and more ephemeral. The polar highs are the source of the polar easterlies, which blow toward the equator and toward (not from) the west.Rev. 6 April 2006 18Atmospheres Planetary CirculationGeneral circulation, part 9 Subpolar lows The subpolar lows are a zone of low pressure situated at about 50 to 60 of latitude in both the Northern and Southern hemispheres. They often contain the polar front. The characteristics vary in either hemisphere because the continents modify circulation in the Northern hemisphere, while circulation in the Southern hemisphere is over a virtually continuous expanse of ocean, the Southern Ocean. The polar front is the meeting ground of the cold polar easterlies and the warm midlatitude westerlies, and is the site of genesis of many midlatitude weatehr systems. Rev. 6 April 2006 19Atmospheres Planetary CirculationGeneral circulation, part 10 Trade winds The trade winds are the major wind system of the tropics, originating from the equatorward sides of the subtropical highs and blowing toward the west as well toward the equator. The trades are the most reliable of all winds in terms of both direction and speed. They are named for the direction they blow from. In the Northern Hemisphere, they blow from the northeast, so are called the northeast trades. In the Southern Hemisphere, they blow from the southeast, so are called the southeast trades.Rev. 6 April 2006 20Atmospheres Planetary CirculationGeneral circulation, part 11 Trade winds (continued) The trades are warming, drying winds, but are capable of holding enormous amounts of moisture. They generally do not release moisture unless forced by a topographic barrier or a pressure disturbance. The winds typically pass over low-lying islands, drying them of moisture and turning them into desert islands. On the other hand, windward slopes exposed to the trades, as in the mountains of Hawaii, are some of the wettest places on Earth.Rev. 6 April 2006 21Atmospheres Planetary CirculationGeneral circulation, part 12 Midlatitude westerlies The westerlies are the great wind system of the midlatitudes, flowing from west to east around the world in a latitudinal zone between about 30 and 60 both north and south of the equator. They originate from the poleward side of the subtropical highs, blowing toward the poles and toward the east. There are two cores of high-speed winds at high altitudes in the westerlies: Polar front jet stream Subtropical front jet streamRev. 6 April 2006 22Atmospheres Planetary CirculationGeneral circulation, part 13 Midlatitude westerlies (continued) A major feature of the midlatitude westerlies are the Rossby waves, sweeping north-south undulations that frequently develop aloft. The undulating motion of the Rossby waves, coupled with the migratory pressure systems and storms associated with the westerlies, give the middle latitudes more short-run weather variability than any other place on Earth. Anticyclonic circulation at the surface is associated with ridges in the waves, while cyclonic circulation is associated with troughs in the waves.Rev. 6 April 2006 23Atmospheres Planetary CirculationRev. 6 April 2006 24Atmospheres Planetary CirculationGeneral circulation, part 14 Polar easterlies The polar easterlies are a global wind system that occupies most of the area between the polar highs and about 60 of latitude. The winds move generally from east to west and are typically cold and dry.Rev. 6 April 2006 25Atmospheres Planetary CirculationGeneral circulation, part 15 Vertical patterns of the general circulation Winds in the upper elevations of troposphere differ from surface winds. The most dramatic difference occurs between surface trade winds and the upper-elevation antitrade winds. Antitrade winds are tropical upper air winds that blow toward the northeast in the Northern Hemisphere and toward the southeast in the Southern Hemisphere.Rev. 6 April 2006 26Atmospheres Planetary CirculationRev. 6 April 2006 27Atmospheres Planetary CirculationGeneral circulation, part 16 Vertical patterns (continued): Trade wind inversion Forms over tropical and subtro