5 The Three-Cell Model Polar cell -- northeasterly winds at surface Ferrel cell -- southwesterly winds at surfaceSubtropical high -- Air subsides (dry climate)Hadley cell -- tropical convection cellIntertropical convergence zone (ITCZ) -- surface low pressure with clouds and rain
6 The three-celled model vs. reality: Hadley cells are close approximations of real world equatorial windsFerrel and polar cells do not approximate the real world winds very well at allModel is unrepresentative of westerly flow aloftContinents and topographic irregularities cause significant variations in real world wind patterns compared to the model
7 Northern hemisphere semi-permanent cells Semi-Permanent Pressure Cells are large areas of higher or lower atmospheric pressure than the surface averageThey may be thermally induced (rising warm air or subsiding cold air) or they may be caused dynamically by converging or diverging wind patterns)They fluctuate seasonallyNorthern hemisphere semi-permanent cellsThe Aleutian, Icelandic, and Tibetan lowsSiberian, Hawaiian, and Bermuda-Azores highsITCZ (low)
8 Average atmospheric air pressure and wind patterns in January
9 Average atmospheric air pressure and wind patterns in July
10 The Sahel reflects seasonal migration of the ITCZ The Sahel is rainy during northern hemisphere summer and dry during northern hemisphere winter as the ITCZ shifts north and south through the year. It is the reason for annual flooding of the Nile River.
12 Mid-latitude winds in the middle and upper troposphere are controlled b the pressure gradient force and the Coriolis force, giving the westerly windsStronger pressure gradients in winter give stronger westerlies
13 Westerly winds in the upper atmosphere Air motions directed towards polesRedirected by Coriolis deflectionWesterly winds aloft resultThe polar front and jet streamsFast stream of air in upper troposphereAbove polar frontStronger in winter
14 Polar jet profile Strongest pgf here 300 mb 500 mb 800 mb The fastest middle and upper troposphere mid-latitude winds (the jet stream) are at polar front. There is a sharp temperature contrast and horizontal pgf to the south and north of the polar front.
15 The subtropical jet transports low latitude moisture and energy Subtropical front and jet
18 Rossby waves are the wave-like pattern of ridges and troughs in the upper troposphere winds. Ridges and troughs (Rossby waves) will migrate either east or west with time
19 Sequence of Rossby wave migration -- the dashed line shows the migration and evolution of a trough with time across the country
20 Ocean currents are driven by wind stresses and are deflected by the Coriolis force. Thus, the water moves at about a 45° angle to the winds. The diection of movement of the ocean changes with depth following a pattern called the Ekman spiral.Western basins usually experience warm ocean currentsEastern basins usually experience cold ocean currents
25 Major Wind Systems of the Earth MonsoonsThermal induced seasonal wind patterns associated with shifts of the ITCZMonsoons are characterized by dry offshore winter flow and wet onshore summer flowThe monsoon in East Asia experiences orographic enhancement
29 Foehn winds are strong, downslope winds that adiabiatically compress, raising the air temperature. Foehn winds are associated with hot, dry, clear weatherChinook winds are foehn winds along the east slope of the Rockies (“snow eaters”)Santa Ana winds are foehn winds that blow from the deserts and over the mountains into the valleys of southern and central CaliforniaKatabatic winds are cold, dense winds that flow down mountain slope. They warm as they descend, but they are still colder than the surrounding air.Boras and mistral winds are forms of katabatic winds in Europe
31 Sea and land breezes form due to temperature differences over land and sea. Sea breezes form during the day, and land breezes form at night.Valley and mountain breezes form due to heating and cooling on mountain sides. Valley breezes form during the day, and mountain breezes form at night (similar to katabatic winds)
35 Air-Sea Interactions in the Equatorial Pacific El Niño, La Niña, and the Walker circulationEl Niño eventsUnusually warm water in the eastern equatorial Pacific OceanLinked to global weather anomalies2 to 5 year recurrenceLa Niña events -- wind and temperature patterns reversed of El Niño patternsWalker circulationVertical and horizontal tropospheric flow in the equatorial Pacific that controls areas of heavy rainfall
36 The “Normal” Walker Circulation (no El Niño conditions)
37 ENSO (El Niño/Southern Oscillation) events -- oscillations between El Niño and La Niña conditions ENSO results in global teleconnection patterns (weather effects far from the equatorial Pacific)
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