MET 10 1 The General Circulation of the Atmosphere

MET 10 2 General Circulation of the Atmosphere  Large scale flow of the atmosphere  Focus on both upper level and lower level winds  Definitions: –Zonal winds: –Meridional winds: –Westerly winds; come from the west –Southwest winds, come from the southwest East-West North-south

MET 10 3 Atmospheric Scales of Motion ScaleTime ScaleDistance ScaleExamples Macroscale -PlanetaryWeeks to yearsWesterlies, trade winds -SynopticDays to weeksCyclones, anticyclones and hurricanes MesoscaleMinutes to daysLand-sea breeze, thunderstorms and tornadoes MicroscaleSeconds to minutesTurbulence, dust devils and gusts 500 – 25,000 miles 50 – 3,000 miles 1 – 50 miles < 1 mile

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5 Conservation of Angular Momentum  Describes motion of air/earth on a rotating planet  Says if all winds blew from one direction (east/west), planet’s rotation rate would have to change  If the atmosphere speeds up (stronger westerly winds) then the solid Earth must slow down (length-of-day increases).  So, winds are westerly some places, easterly at others.

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7 Single Cell Model  Early description of general circulation  George Hadley ( ) developed this model  Assumptions: –Earth is primarily heated in the tropics –Thermally direct circulation results from heating differences –Low pressure at equator, high-pressure at the poles  Surface heat imbalance produces air movement to balance. Not realistic, because it violates COAM

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9 Three Cell Model  Proposed to explain how the Earth’s heat balance is maintained  Good simple model of global circulation  Terms: –Hadley Cell: The tropical circulation  ITCZ - intertropical convergence zone  Horse Latitudes:

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MET Global wind patterns  Must conserve angular momentum, so direction varies  Westerlies: what we experience here in U.S. –Weather in east coast usually starts here.  Trade Winds: from NE in NH, SE in SH –E.g. Hawaii –El Nino: when trade winds reverse directions, which cools W. Pacific and warms E. Pacific oceans  Doldrums –Area of no wind (ITCZ) where only air movement in up!

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MET Semi-permanent pressure systems  Part of 3-cell model  Exist due to seasonal changes and land-sea differences (specific heat)  Change with the seasons.  Land: –predominantly high pressure in winter –predominantly low pressure in summer  Water: –predominantly low pressure in winter –predominantly high pressure in summer  Think of cold areas as having the high pressure. Warm areas/low pressure

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MET ITCZ Seasonal variations Intertropical Convergence Zone (ITCZ) Is a semi-permanent low-pressure system where the trade winds meet and converge, forcing rising motion ITCZ changes with season This is a reflection of the changing location of the Hadley Cell The ITCZ follows the sun   To the north in June   To the south in December

Positions of intertropical convergence zone in January and July

MET Westerly winds in the upper atmosphere  The Jet Stream  Caused by differences in temperatures at the surface, or uneven heating of the surface  Higher heights (of pressure levels) exist in the tropics.  Pressure gradient exists across middle latitudes  Pressure gradient force is stronger in winter than summer – larger temperature gradient.  Upper atmosphere winds are predominately –westerly in both hemispheres.

MET The Jet Stream  Due to large differences in temperature at the surface –Strongest in winter  Flow from west to east in a wavy pattern  ~35,000 feet above sea level  Illustrates that atmosphere is full of waves that –Bring heat from equator to poles –Bring cold air from poles to equator –Control our weather here in the mid-latitudes  Fronts, low-pressure troughs, and high-pressure ridges

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