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Winds and Global Circulation • Atmospheric Pressure • Winds • Global Wind and Pressure Patterns • Oceans and Ocean Currents • El Niňo Precipitation in Montana How is Energy Transported to its “escape zones?” • Both atmospheric and ocean transport are crucial • Buoyancy-driven convection drives vertical transport • Latent heat is at least as important as sensible heat Atmospheric Circulation in a nutshell • Hot air rises (rains a lot) in the tropics • Air cools and sinks in the subtropics (deserts) • Poleward-flow is deflected by the Coriolis force into westerly jet streams in the temperate zone • Jet streams are unstable to small perturbations, leading to huge eddies (storms and fronts) that finish the job Atmospheric Circulation in a nutshell • Winds are initially generated by differences in heating at the Earth’s surface • Geostrophic winds result in rotational movement around high and low pressure centers. Ocean Circulation in a nutshell • Surface winds cause large, clockwise rotating gyres in the northern hemisphere and counterclockwise gyres in the southern hemisphere • Salinity and temperature differences in water cause sinking of water (deep water formation) in the North Atlantic and Southern Ocean (Antarctic) • EL Nino is a quasi periodic rocking of the Ocean-Atmosphere system in the tropical Pacific Atmospheric Pressure As the atmosphere is held down by gravity, it exerts a force upon every surface (pressure = force per unit area) At sea level the force is the weight of 1 kg of air that lies above each square centimeter of the surface (around 15 lbs per in2) atmospheric pressure decreases rapidly with altitude near the surface Therefore a small change in elevation will often produce a significant change in air pressure How winds are made Two columns of air– same temperature same distribution of mass 500 mb level 1000 mb 1000 mb Cool the left column; warm the right column The heated column expands The cooled column contracts 500 mb original 500 mb level 500 mb 1000 mb 1000 mb The level of the 500 mb surface changes; the surface pressure remains unchanged The level corresponding to 500 mb is displaced downward in the cooler column original 500 mb level new 500 mb level in cold air The 500 mb surface is displaced upward in the warmer column new 500 mb level in warm air The surface pressure remains the same since both columns still contain the same mass of air. 1000 mb 1000 mb A pressure difference in the horizontal direction develops above the surface The 500 mb surface is displaced upward in the warmer column The 500 mb surface is displaced downward in the cooler column original 500 mb level Low High new 500 mb level in warm air new 500 mb level in cold air 1000 mb 1000 mb The surface pressure remains the same since both columns still contain the same mass of air. Air moves from high to low pressure in the middle of the column, causing the surface pressure to change. original 500 mb level Low wind High 1003 mb 997 mb Air moves from high to low pressure at the surface… Where would we have rising motion? original 500 mb level Low High Low High 1003 mb 997 mb What have we just observed? • Starting with a uniform atmosphere at rest, we introduced differential heating • The differential heating caused different rates of expansion in the fluid • The differing rates of expansion resulted in pressure differences along a horizontal surface. • The pressure differences induced horizontal flow (wind) in the fluid • This is how the atmosphere converts heating into motion • Winds are the result of differential heating Surface Pressure Maps • Altitude-adjusted surface station pressures are used to construct sea level pressure contours Differences in air pressure = a pressure gradient The pressure gradient forces acts at right angles to the isobars (90 degrees) 820 830 820 840 850 830 860 870 840 880 890 850 860 weak pressure gradient strong pressure gradient Global Circulation 90oN Cold High Pressure 60oN But heat is transported from the Equator to the Poles - how? 30oN 0o 30oS 60oS 90oN Earth Warm Low Pressure SUN Fig. 7-6, p. 151 How is Energy Transported to its “escape zones?” • Both atmospheric and ocean transport are crucial • Buoyancy-driven convection drives vertical transport • Latent heat is at least as important as sensible heat What a single cell convection model would look like for a non-rotating earth • Thermal convection leads to formation of convection cell in each hemisphere • Energy transported from equator toward poles • What would prevailing wind direction be over N. America with this flow pattern on a rotating earth? What’s wrong with the 1-cell model? • Neglects effect of rotation - with rotation, winds would cause earth to spin down - with rotation, the upper level winds would accelerate to unphysical speeds near the pole. You would be funneling all the air from the Equator down at the Poles - It is not a stable solution for wind circulation Fig. 8-1, p. 172 Coriolis Force acts to the right in the Northern Hemisphere Physics Coriolis Effect The Coriolis Effect deflects moving objects to the right in the northern hemisphere and to the left in the southern. General Circulation of the Earth’s Atmosphere 90oN Deflection 60oN 30oN 0o No Deflection 30oS 60oS 90oN Deflection Deflection is least at the equator and greatest at the poles Wind patterns on a rotating Earth 3 circulation cells in each hemisphere Fig. 7-12, p. 154 Wind patterns on a rotating Earth 3 circulation cells in each hemisphere warm air rises at the equator producing low pressure (Intertropical Convergence Zone, ITCZ) and flows towards the poles 90oN 60oN 30oN 0o 30oS 60oS 90oN L 90oN 60oN 30oN 0o 30oS 60oS 90oN H L H Cold air sinks at 30o N and S latitude Creating high pressure (subtropical high pressure, STH) 90oN 60oN 30oN 0o 30oS 60oS 90oN Northeasterly and southeasterly surface winds flow from the subtropical high pressure belts (30o N and S) to the low pressure belt (ITCZ) at H the equator (calm winds: doldrums) L H westerly surface winds flow from the subtropical high pressure belts towards higher latitudes 90oN 60oN 30oN westerly surface winds are forced to rise around 60o N and S latitude when they encounter cold polar easterly winds from the poles L resulting in Subpolar Low pressure (SPL) belts H L 0o H 30oS 60oS 90oN L H 90oN 60oN L H 30oN L 0o H 30oS 60oS 90oN H L cold air sinks at the poles producing polar high (PH) pressure regions Figure 5.17, p. 163 H polar jet stream Jet streams are streams of fast moving air aloft subtropical that occur jet streams where atmospheric temperature gradients are strong 90oN 60oN L H 30oN L 0o H 30oS 60oS L 90oN H polar jet stream Key features of three cell model • Hadley cell (thermally direct cell) - driven by meridional gradient in heating - air rises near equator and descends near 30 degrees - explains deserts; trade winds; ITCZ • Ferrel Cell (indirect thermal cell) - driven by heat transports of eddies - air rises near 60 degrees and descends near 30 degrees - explains surface westerlies from 30-60 • Weak winds found near – Equator (doldrums) – 30 degrees (horse latitudes) • Boundary between cold polar air and mid-latitude warmer air is the polar front Geostrophic Winds Coriolis Force acts to the right in the Northern Hemisphere Physics Coriolis Effect The Coriolis Effect deflects moving objects to the right in the northern hemisphere and to the left in the southern. low pressure Coriolis Force pressure geostrophic gradient force 992 996 1000 1004 1008 1012 1016 1020 high pressure winds Gradient Wind “Geostrophic Wind” • The Geostrophic wind is flow in a straight line in which the pressure gradient force balances the Coriolis force. Lower Pressure 994 mb 996 mb 998 mb Higher Pressure Note: Geostrophic flow is often a good approximation high in the atmosphere (>500 meters) High pressure (anticyclone) Side View From above H L surrounding air is relatively low L H air descends L Low pressure (depressions, cyclone) Side View From above L H surrounding air is relatively high H L air ascends H Friction forces Near the surface, friction reduces the speed of the wind, This reduces the Coriolis Force, Which changes the direction of the geostrophic wind, The pressure gradient force over powers the Coriolis effect, As a result wind flow across the isobars. H anticyclone H L Northern Hemisphere cyclone Southern Hemisphere L Global Temperature patterns and weather Temperature Patterns • Stronger seasonal heating and cooling on land produces asymmetry • Poleward distortion of isotherms over northern high latitude oceans • Equatorward distortion over subtropics Seasonal Migration of ITCZ • Mean position is somewhat north of Equator • Strong departures from zonal mean position driven by seasonal heating over land (Especially over Asia, S. America, Africa) Monsoons In July the position of the ITCZ moves North • low pressure over land causes winds to flow off the ocean • this brings heavy rainfall Figure 5.20, p. 167 Monsoons In January high pressure over the land produces dry winds Air is flowing towards the ITCZ Figure 5.20, p. 167 Elevation of the 500 mb isobar Polar Front Jet Stream • Polar front jet stream forms along polar front where strong thermal gradient causes a strong pressure gradient • Strong pressure gradient force and coriolis force produce strong west wind parallel to contour lines • Polar jet sometimes splits into north and south branches • Fast air currents, 1000’s of km’s long, a few hundred km wide, a few km thick • Typically find two jet streams (subtropical and polar front) at tropopause in NH • When would you expect the jets to be strongest? Jet Streams Rossby Waves Smooth westward flow of upper air westerlies Develop at the polar front, and form convoluted waves eventually pinch off Primary mechanism for poleward heat transfere Pools of cool air create areas of low pressure The “dishpan” experiment • A tank of water with a hot equator and a cold pole is rotated – Troughs, ridges and eddies are produced, similar to patterns observed in earth’s general circulation movies http://jrscience.wcp.muohio.edu/coriolis/satmovies.html#anchor1386282 The Earth’s Oceans Ocean currents produced by: 1) winds 2) density differences in sea water 3) Coriolis force 4) shape of ocean basins 5) astronomical factors (TIDES) Ocean Currents driven mostly by wind blowing over the surface however, currents move slowly lag behind wind speed so often called drifts wind Ocean currents • large continuously moving loops (gyres) • produced by winds, Coriolis force and land masses Figure 5.32, p. 175 Fig. 8-2, p. 172 Each hemisphere contains a tropical and subtropical gyre N. Subtropical Gyre North Tropical Gyre EQUATOR South Tropical Gyre S. Subtropical Gyre Surface Currents redistribute heat Upwelling where cold water rises from deep ocean areas and where the Coriolis forces prompts ocean currents to diverge from coastlines Figure 5.37, p. 180 Deep-sea currents • driven by differences in temperature and salinity • much slower than surface currents Figure 5.32, p. 180 The Ocean Conveyor Belt Deep-sea currents • driven by differences in temperature and salinity • much bigger and slower than surface currents El Niño Southern Oscillation (ENSO) • Trade winds promote cold water upwelling in eastern tropical Pacific – Cool, deep water is nutrient rich and supports rich ecosystem (plankton, fish, birds,…) • Weaker trades lead to weaker upwelling. Warm nutrient-poor tropical water replaces the cold, nutrient-rich water . – • called El Niño (boy child) Every few years this El Niño (surface warming) persists and is widespread – Huge ecosystem and economic losses – Alters weather patterns over much of the world 82-83 +3 86-87 72-73 +2 57-58 65-66 97-98 76-77 El Niño +1 0 La Niña -1 -2 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Sea Surface Temperatures (oC) El Niño Normal La Niña La Niña: cold surface water moves over central and eastern Pacific. El Niño Sea Surface Temperature Anomalies oC Normal La Niña Normal conditions – equatorial Pacific ENSO (El Nino - Southern Oscillation) conditions Animations on the web Idealized ENSO wave http://www.cdc.noaa.gov/people/joseph.barsugli/anim.html For animation of most recent anomaly asee http://www.cdc.noaa.gov/map/clim/sst_olr/sst_anim.shtml Why do we care about ENSO? • Global impacts on weather. • Long timescale (months) yields improved seasonal prediction. • Provides insight into coupled behavior of oceans and atmosphere … may lead to better overall understanding of climate Weather Variation: ENSO cycle winter summer Impacts of El Niño • Droughts – Fires – Agricultural productivity – Water supply • Extreme Precipitation – – – – Floods Erosion Disease Transportation • Impacts through marine food chain – Natural ecological responses – Economic