13Observed Distribution of Pressure and Winds (b) The real Earth has disruptionsin the zonal pattern caused by largelandmasses. These disruptionsbreak up pressure zones intosemi-permanent high and lowpressure cells.(a) An imaginary uniform Earthwith idealized zonal (continuouseast to west) pressure belts
14Semipermanent Pressure Systems: Land Sea interactions and Topography complicatethe circulation patterns.We have viewed these pressure belts as continuous systems aroundthe earth up to this point. However, because the Earth is notuniform, at most latitudes (more so in the northern hemisphere)the zonal belts are replaced by semipermanent cells of high andlow pressure.
15January Average Surface Pressure Systems and Associated Circulation Siberian High, Azores HighAleutian Low, Icelandic Low
16July Average Surface Pressure Systems and Associated Circulation Bermuda High
17Monsoons The seasonal reversal of wind direction associated with large continents, especially Asia. In the winter, the wind blows from…..the land to the sea; in the summer, it blows from the sea to theland.The Asian MonsoonThe Asian Monsoon is the result of a complex interaction betweenthe Siberian High (which is strongest in the wintertime), themigration of the ITCZ, and the topography of the region (i.e. theHimalayan mountain range and the Tibetan Plateau).
18The Asian monsoon circulation occurs in conjunction with the seasonal shift of the ITCZ and development of the Siberian High.(dry, cool, continental air)
19In the summer ITCZ migrates north and the Siberian High weakens which allows results in a reversal in wind direction.(moist, warm, maritime air)
21Monsoons The North American Monsoon Extreme summertime heating over the desert Southwest, createsa low pressure system centered over Arizona that draws inwarm, moist air from the Gulf of California.
22The Westerlies A west to east winds is a westerly wind. Pilots flying missions during World War II, collected a greatvolume of data on upper air conditions over the middle latitudes.One of the most important observations was the presence of aprevailing westerly flow (zonal) in the upper troposphere.Temperature differences between the equator and the polesdrives the westerlies.How?Winds are created and maintained by pressure differences whichare the result of temperature differences (the pressure gradient issouth to north). The Coriolis force bends the winds to the right(in the Northern Hemisphere).A west to east winds is a westerly wind.
23The WesterliesThe pressure gradient force pushes winds poleward, the Coriolisforce deflects winds to the right; a balance is reached between thepoleward-directed pressure gradient force and the Coriolis forcewhich results in a westerly wind.
25The Jet StreamsAltitude: 7500m-12,000m(25,000ft-40,000ft)Width: <100km-500km(60mi mi)Depth: ~1-3kmWind Speeds:often >200 km/hr(120mi/hr)upper limit:400 km/hr(240mi/hr)Within the westerly flow are narrow ribbons of high windspeeds known as jet streams. The most studied jet stream is the mid- latitude jet stream.
26Seasonal migration of the mid-latitude jet stream 30 day Mean Jet Stream Jan 200130 day Mean Jet Stream Jun 2001Notice the northward migration of the jet stream in thesummertime. Temperature contrasts are greatest in thewintertime (in the northern hemisphere).
27Seasonal Migration of the Polar Jet Stream Just like the ITCZand the semi-permanentpressure systems,the midlatitudejet stream migratesseasonally. Theseasonal migrationis in response to themigration of thesun’s vertical rays.
28Other visualizations of the Jet Streams The jet stream is nota continuous ribbonof fast moving air.
29Origin of the Jet Streams It is the largetemperature contrastat the surface whichcreate the greatdifferences inpressures aloft (steepgradient) which resultsin fast winds.The large temperaturecontrast occur alonglines called fronts. Themidlatitudinal jetstream occurs along amajor frontal zonecalled the polar front.
30Subtropical Jet SreamThe Sub Tropical Jet: The average level of the core of this westerlyjet lies at an altitude of about 40,000 ft (12 km) and flows just below the tropical tropopause. It occurs in the latitude range deg N/S, and is most marked during the winter and early spring of each hemisphere, but is not associated with any surface frontal structure.Midlatitude Jet StreamSubtropical Jet StreamWater Vapor Image
32Waves (Rossby Waves) in the Westerlies Rossby waves are very long wave km named after C.G.Rossby, who first explained the nature of these waves.Simplified 500-millibar height contour chart for January. The position of the jet stream core is shown in dark.Idealized flow at 500millibar level. Thefive long-wavelength undulations, called Rossby waves, compose the flow. The jetstream is the fast core of this wavy flow.
33The closer the isoheights faster the winds. A closer look at the midlatitude (polar) jet streamThe closer the isoheights faster the winds.
34Westerlies and the Earth’s Heat Budget Recall that the wind isresponsible for ~75% ofheat transfer whichmaintains the Earth’sheat budget. Based onthe three-cell theory overmuch of the Earth windsare zonal (west to east)which does not transferheat to the poles.How then does globalcirculation transferenergy?
35Global Winds and Ocean Currents Major OceanCurrents.Poleward-movingcurrents arewarm, andequatorward-moving currentsare cold.When wind blows over the ocean friction from the air transfers energy to the water. Ocean currents are reflective of the global air circulation.
36Ocean Currents and Upwelling Air moving over water pushes water in the direction of the wind causing upwelling to occur. Water from below rises up to fill the void left by the surface water. Water from deep layers is often rich in nutrients which fish feed off of. Upwelling is a very important process in the food chain.
37Seasonal variations in upwelling in the Arabian Sea. Sea-viewing Wide Field of View Sensor(SeaWiFS)Composite image of 'ocean colour' of the Indian Ocean inSeptember-October, towards the end of the Indian summermonsoon (left), and May-June, towards the end of the wintermonsoon (right) (courtesy NASA/GSFC).
38What is an El Niño?El Niño is a disruption of the ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe.
45El Niño and Southern Oscillation (ENSO) Illustration of thesee-saw pattern ofatmospheric pressurebetween the easternand western Pacific,called the SouthernOscillation.(a) Normal years(b) El Niño years
46Tracking El Niño From Space Topex/Posiden satellitePrior to El NiñoOnset of El NiñoSea Surface heighttells us much aboutthe temperature ofthe water whichgives us informationabout ENSO.Strong El NiñoDecay of El Niño