Chapter 11- Global Wind Systems Review- When do clouds form? Low Pressure? High Pressure? Upper Level? Surface? The important criterion is rising air, not pressure. Clear skies Clouds Sinking air Rising air L H Wind V Lake Wind speeds up Less dense More dense Here, a surface low gives fair weather. Often a surface low gives clouds and rain. (See Fig. 7.13) In all cases, rising air gives clouds, regardless of pressure! Fig. 11.2 represents a model of atmospheric circulation assuming: 1) No land 2) No tilt of the Earth’s axis, however the Earth does rotate. This is a depiction of surface pressures and wind directions. General Features: 1) Heat at equator causes air to rise - Surface low forms, clouds and rain. - Surface air flows in from higher latitudes. - Coriolis force causes NE winds in the Northern hemisphere and SE winds in the Southern hemisphere, respectively. 2) Cold at poles causes sinking air. - Winds flow away from the poles on surface. - Coriolis deflects these winds eastward. 3) Another cell forms between 30° and 60° latitude that opposes the polar and Hadley (subtropical) cells. - This Ferrel cell has northward flowing surface winds in the Northern Hemisphere (Southward winds in the Southern Hemisphere) and, - Coriolis causes westerlies in the Northern and Southern Hemispheres. Easterlies above and below equator are called trade winds. Winds are light near equator and subtropical high (+/- 30°) – doldrums and horse latitudes. ITCZ – trade winds converge. Fig. 11.3 – The real world. Average sea-level pressures and winds. Observations: 1) Semi-permanent highs and lows. 2) Thermal (seasonal) cyclones (lows) and anticyclones (highs). 3) Seasonal movement of ITCZ and other features. 4) Sinking (stable) air near +/- 30° associated with clear weather, shifts with seasons. Why are winds light near highs and lows? H L H ∆P d d P ∆P distance Generally, ∆P is small near highs and lows and largest in between highs and lows. d Fig. 11.8 Upper level pressure and wind. 1) Semi-permanent lows moved westward (Why?) – Coriolis? 2) Mostly westerlies aloft – small easterly flow equatorward of highs. 3) Contours closer in winter steeper pressure gradients, higher winds. Also, recall from Chapter 9, PGF = m ∆P ρ d As you go higher in altitude, air density decreases ⇒ PGF increases so wind blows faster at higher altitudes. N Westerly winds have v > 1000 knots Easterly winds have v < 1000 knots Equator on surface v = 1000 knots v As viewed by stationary observer above earth S “Calm” air would be fixed relative to the surface of earth, so to the stationary observer, would be moving at 1000 knots. Note: Speed at surface of earth gets smaller toward the poles because you go less far in 24 hours. Example: Standing 1foot from the North Pole, you would move 2π feet in 24 hours! However, westerly winds move faster towards the east than the earth’s surface does and so appear to us on earth to move toward the east. Winds that appear to us to move towards the west are actually moving toward the east to a stationary observer! Jet Stream: Fast moving air currents, long and thin. Rising air flattens out at the tropopause because the atmosphere in the stratosphere is very stable (an inversion). Jet streams are caused by steep pressure gradients aloft. (See Fig 11.10) At locations where adjacent cells meet (Hadley, Ferrel), warm air meets cold air and strong temperature and pressure gradients develop. Jet streams are also helped by angular momentum, “L”. Smaller r L = mvr larger v As air aloft moves from the equator to the poles and is deflected east by coriolis force, r is also decreasing, which speeds wind up. Why do winds follow curvature the curvature of the earth as they blow over long distances? N PGF FG wind equator As the earth curves away from the wind, PGF decreases, so FG > PGF and FG –PGF provides the centripetal force required, curving the wind in an arc. It is northward and southward blowing winds that are responsible for pumping momentum away from the equator. These north and south winds are generally associated with highs and lows. Winds blowing away from the equator are turned into westerlies by coriolis force. Equator Winds blowing toward the equator are turned into easterlies by coriolis force. Equator Ocean currents tend to follow air currents. Closed ocean current loops are called gyres (the same root as gyration). Large, high pressure cells dominate the world’s major bodies of water. Winds blow clockwise and outward in the northern hemisphere and counterclockwise and outward in the southern hemisphere around these highs. Air High High Isobars Northern Hemisphere Southern Hemisphere Water responds to these more-or-less constant winds, through the frictional force. Air f a,w Wind f w,a Water Surface Newton’s 3rd Law: For every action there is an equal and opposite reaction. fw,a fa,w Result: Water is pushed in direction of air. However, coriolis force is around, too: in northern hemisphere, water deflects to the right and in the southern hemisphere, water deflects to the left. High High Water Northern Hemisphere Southern Hemisphere As you go below the surface of the water, the layers above push each lower layer, so water direction deflects more from wind direction with depth. Fig. 11.17 Ekman Spiral. wind N. America S. America Up welling Up welling Water wind Water El Niño and the Southern Oscillation. Falling air, Clear, Dry Rising air, Rain High Low Peru Indonesia Warm Water – South Equatorial Current “Normal” Conditions For some reason, the high and low pressures weaken, or even reverse sets off a chain of events that effect weather worldwide. Cool Water – Peru Current Upwelling El Niño Event. This Since ~ 1975, El Niño events have been more frequent, longer lasting and stronger. We don’t know why. Weather in Florida: 1997-1998 – very wet winter followed by 1198 very dry summer ( La Niña).