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Class #17: Monday, February 16 Surface pressure and winds Vertical motions Jet streams aloft Class #17 Monday, February 16, 2009 1 Class #17 Monday, February 16, 2009 2 Surface pressure in the 3-cell model • High at both poles, called Polar Highs • High in the subtropics, about 30ºN and 30ºS, called Subtropical Highs • Low near the equator, called the Equatorial Low, or the Intertropical Convergence Zone (ITCZ) • Generally light winds at the Polar and Subtropical Highs, and in the ITCZ Class #17 Monday, February 16, 2009 3 Average vertical motions in the 3-cell model • Downward at the poles where surface pressure is high and the troposphere has low temperatures over ice • Downward at the subtropical highs • Upward in the ITCZ • Upward at about 60°N and S near the polar front Class #17 Monday, February 16, 2009 4 Thermal circulations in the 3-cell model • The Hadley cells have their rising branch in the ITCZ and their sinking branch in the subtropics. • The Hadley cells cover half of the surface area of Earth. • The polar cells have a rising branch near the polar front and sinking at the pole. Class #17 Monday, February 16, 2009 5 The 3-cell model’s circulation in middle latitudes • Is thermally indirect, because the air nearer the pole is rising, and the air nearer the equator is sinking. • Is an average based on smaller wind patterns in extratropical cyclones, in which the warmer air does rise, and the colder air sinks. • Has the motions required by the polar and Hadley cells. Class #17 Monday, February 16, 2009 6 Consequences of Earth’s rotation from west to east • The trade winds in the NH do not blow from the north, but are deflected to the right in the NH, so blow from the northeast. • If Earth rotated much more slowly, there would be only the Hadley cell. • If Earth rotated much more quickly, there would be more wind belts (like on Jupiter). Class #17 Monday, February 16, 2009 7 More consequences of Earth’s rotation • If it were not for the Midlatitude westerlies, Earth’s speed of rotation would slow. Easterlies alone would everywhere act to slow the rotation. • The polar easterlies blow from the pole and curve, blowing from the northeast in the NH and from the southeast in the SH. • The westerlies blow away from the equator and curve in both hemispheres, that is, they blow from the southwest in the NH, and from the northwest in the NH. Class #17 Monday, February 16, 2009 8 Complications of the real Earth • Earth has seasons – The ITCZ (sometimes called the thermal equator) shifts latitude with the seasons. – The ITCZ shifts north of the equator in NH summer, and south of the equator in SH summer (NH winter) • Earth has large land masses – Continents and oceans set up thermal circulations Class #17 Monday, February 16, 2009 9 Observed surface pressures • Vary with the seasons, requiring both a January and a July depiction • Are on average high in the sub-tropics (near 30°) and near the pole • Are on average low in the ITCZ and along the polar front (near 60°) • In summer are high over the oceans and low over the continents (thermal lows). • In winter are high over the continents and low over the oceans. Class #17 Monday, February 16, 2009 10 Seasonal shifts • The ITCZ, the subtropical highs, and the polar front all shift southward in NH winter and northward in NH summer. • Seasonal shifts are most intense over Asia, which has the largest continental air mass. • The summer monsoon is wet, with low pressure over land; the winter monsoon is dry, with high pressure over land. Class #17 Monday, February 16, 2009 11 Class #17 Monday, February 16, 2009 12 Class #17 Monday, February 16, 2009 13 Class #17 Monday, February 16, 2009 14 Class #17 Monday, February 16, 2009 15 Class #17 Monday, February 16, 2009 16 Class #17 Monday, February 16, 2009 17 Class #17 Monday, February 16, 2009 18 Class #17 Monday, February 16, 2009 19 Other monsoons • Africa, North America, and Australia have monsoon-like wind patterns, particularly in the warm season. Class #17 Monday, February 16, 2009 20 Review of temperature and surface pressure patterns • High at surface – Poles (low temperature) – Subtropical highs, especially over oceans – Land in winter, ocean in summer (low temp.) • Low at surface (thermal lows have high T) – Subpolar low at about 60º with polar front – Intertropical Convergence Zone (ITCZ) – Land in summer, ocean in winter (high temp.) Class #17 Monday, February 16, 2009 21 Winds and pressures (heights) well above the surface • Pressures and heights are on average high in the tropics and decrease to lows close to the poles. • Upper-level (500mb and above) winds are generally easterlies (blowing east to west) in the tropics and westerlies (blowing west to east) in higher latitudes. Class #17 Monday, February 16, 2009 22 Class #17 Monday, February 16, 2009 23 Class #17 Monday, February 16, 2009 24 Class #17 Monday, February 16, 2009 25 Review of upper-level height (pressure) patterns and temperature • High heights (pressure) in warm air columns with high temperature • Low heights (pressure) in cold air columns with lower temperature • Look for ridges over land in summer and oceans in winter • Look for troughs over land in winter and oceans in summer Class #17 Monday, February 16, 2009 26 Important reminder!!!! • Pressure and height patterns aloft are generally opposite to those at the surface • This opposition is necessary for thermal circulations, because the horizontal pressure gradient force must reverse aloft for a complete circulation • High at surface/low aloft; • Low at surface/high aloft Class #17 Monday, February 16, 2009 27 Jet Streams • Jet streams are regions of especially high wind speed in the atmosphere. • In the upper-level westerlies, there can be two jet streams, the Polar front jet stream, above the polar front, and the Subtropical jet stream above the polar highs. • Sometimes these jet streams merge into one. Class #17 Monday, February 16, 2009 28 Class #17 Monday, February 16, 2009 29 Class #17 Monday, February 16, 2009 30 Class #17 Monday, February 16, 2009 31 Class #17 Monday, February 16, 2009 32 Patterns in the upper-level westerlies • The pattern of upper-level westerlies has waves, with axes of high height (ridges and highs) and axes of low height (troughs and lows). These appear on day to day weather maps. • These wavelike patterns help transport energy poleward (advection) to balance the energy budget of the Earth and atmosphere. Class #17 Monday, February 16, 2009 33 Class #17 Monday, February 16, 2009 34 Class #17 Monday, February 16, 2009 35