2 Review of last lecture Know 3 Forces that affect wind speed /direction Especially work on Coriolis force, as this is the hardest to understand. Which direction is air deflected to by Coriolis force?What is the geostrophic balance? At which level is it valid? Difference between upper level and surface windsTroughs, ridges, cyclones and anticyclones. Do they correspond to high or low surface pressure? Is the air moving clockwise or counter-clockwise around them?
3 The most common atmospheric circulation structure CEHLCoolingor No HeatingHeatingFrictionHLCEImbalance of heating Imbalance of temperature Imbalance of pressure Wind
4 IntroductionWell-defined heating, temperature and pressure patterns exist across the globeThese define the general circulation of the planetIn describing wind motions:Zonal winds (east-west): flow parallel to lines of latitudeFlowing eastward: Westerly windFlowing westward: Easterly windMeridional winds (north-south): flow parallel to lines of longitudeFlowing northward: Southerly windFlowing southward: Northerly wind
5 Annual mean precipitation (heating) Extratropical stormtrackTropical rainfall
6 Primary Highs and Lows Equatorial low Subtropical high Subpolar low Polar high
7 Three-cell model Zonal mean circulation Each hemisphere is divided into 3 distinct cells:Hadley CellFerrel CellPolar Cell
8 Vertical structure and mechanisms Polar Cell (thermal):Driven by heating at 50 degree latitude and cooling at the polesFerrel Cell (dynamical): Dynamical response to Hadley and polar cellsHadley Cell (thermal): Heating in tropics forms surface low and upper level high air converges equatorward at surface, rises, and diverges poleward aloft descends in the subtropicsPolarHadley
9 Zonal mean structure of temperature Two characteristics:Horizontally uniform in the tropicsSteep gradient in the extratropics
10 Zonal mean structure of zonal wind Two characteristics:Westerly winds in the extratropical troposphereJet streams: local maximum of winds
11 Westerly winds in the extratropical troposphere The existence of the upper level pressure gradient air is being pushed toward poles Coriolis effect deflects upper air (to the right) Westerlies dominate upper troposphereStrongest during winter thermal gradient is largeExplains why storms move eastward, flight times
12 The Jet StreamsCaused by steep temperature gradients between cold and warm air massesPolar front - marks area of contact, steep pressure gradient polar jet streamLow latitudes subtropical jet streamStronger in winter, affect daily weather patterns
13 A Jet Stream seen from satellite The subtropical jet is seen as a band of clouds extending from Mexico on an infrared satellite image
15 Semipermanent Pressure Cells Instead of cohesive pressure belts circling the Earth, semipermanent cells of high and low pressure exist; fluctuating in strength and position on a seasonal basis.These cells are either dynamically or thermally created.Sinking motions associated with the subtropical highs promote desert conditions across specific latitudes.Seasonal fluxes in the pressure belts relate to the migrating Sun (solar declination).
16 1. Aleutian and Icelandic lows 2. Siberian and Bermuda-Azores highs South Pacific highSouth Atlantic highSouth Indian highFor NH winter:1. Aleutian and Icelandic lows2. Siberian and Bermuda-Azores highs3. South Pacific, Atlantic, Indian highs
17 2. Hawaiian and Bermuda-Azores highs South Pacific highSouth Atlantic highSouth Indian highFor NH summer:1. Tibetan low2. Hawaiian and Bermuda-Azores highs3. South Pacific, Atlantic, Indian highs
18 Low pressure: clouds and precipitation Extratropical stormtrackTropical rainfall
19 High pressure: warm surface temperature, drought and desert Global distribution of deserts (all near high pressure cells)
20 General circulation of the oceans Ocean surface currents – horizontal water motionsTransfer energy and influence overlying atmosphereSurface currents result from frictional drag caused by wind - Ekman SpiralWater moves at a 45o angle (right)in N.H. to prevailing wind directionDue to influence of Coriolis effectGreater angle at depth
21 Global surface currents Surface currents mainly driven by surface windsNorth/ South Equatorial Currents pile water westward, create the EquatorialCountercurrentwestern ocean basins –warm poleward moving currents (example: Gulf Stream)eastern basins –cold currents, directed equatorward
22 Summary Three precipitation (heating) belts. Primary high and lows Three-cell model. Mechanism for each cellTwo characteristics of zonal mean temperature structureTwo characteristics of zonal mean wind structure. Why does westerly winds prevail in the extratropical troposphere? What cause the jet streams?Semipermanent pressure cells. Low pressure is associated with clouds and precipitation. High pressure is associated with warm surface temperature, drought, and desert.What drives the ocean surface currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind?
23 Works cited Images http://pulleysandgears.weebly.com/gears.html