Atmospheric Circulation. Recall Vertical Circulation

Transcription

1 Today s topics: Atmospheric circulation: generation of wind patterns on a rotating Earth Seasonal patterns of climate: Monsoons and El Niño Tropical Cyclones: Hurricanes and typhoons Atmospheric Circulation Recall Vertical Circulation Recall vertical circulation driven by surface heating and moisture transfer? What global circulation pattern would result if Earth was not rotating? Near surface: air must travel over surface to replace updraft Aloft: air must move away as new updraft arrives 1

2 Atm. Circulation on non-rotating Earth Heating and rising at Equator - air rushes over surface toward equator cooling and sinking at pole - aloft, air rushes toward poles Is this model realistic? Remember, Earth Rotates! Rotation toward East Faster at low latitude This results in Coriolis effect Which in turn results in atm. circulation pattern 2

3 Coriolis Effect Plane leaves equator North velocity But also, initial eastward velocity due to rotation of the Earth (1674 km/h at equator) Viewing from surface Plane appears to veer east as it travels northward because it has this initial East velocity that is greater than the rotation speed of the earth at higher latitude. Coriolis. Effect deflects paths to the right in N. hemisphere and left in S. hemisphere Show Web Link1, Animation, and Link 2 Atm. Circulation & the Coriolis effect Vertical circulation model combined with Coriolis effect produce the pattern of observed circulation cells. Equatorial heating Near surface air masses heat, rise, expand, cool, and precipitate over tropics Air flows over surface to replace rising air mass Easterlies = surface flow deflected by Coriolis 3

4 Circulation at Polar cells Polar cooling Air sinks Flow over surface toward lower latitude, deflected westward by Coriolis Polar easterlies = deflection Heats as it flow over surface and rises at border with temperate cell Temperate or Mid-latitude (30-60º ) cells Flow deflected by Coriolis = westerlies Result from neighboring cells: sinks at subtropic high pressure belt due to sinking of subtropical cell Flows over surface, heating and Rises at 60º due to meeting air mass from polar cell. 4

5 Can you predict Wet vs Arid regions based on the pattern of circulation cells? Effect of land on Circulation: Sea Breezes During day - on-shore breeze Heat land (low heat capacity) Rising air mass draws in cooler air from over the ocean Suring night = off-shore breeze Warm ocean relative to land (heat capacity again) Warm air over ocean rises as cool air over land sinks and flows out to ocean 5

6 Monsoon ~ season long onshore breeze Heat continent over summer Continuously rising air mass Replaced by moist ocean air Continuous rain Global weather pattern: El Niño - Southern Oscillation Normal Conditions: High pressure over South America, and low pressure over Western Pacific Warm Surface water flows with Easterlies (SE trade winds) to the far western Pacific. This builds the Pacific Warm Pool bulge in W. Pacific. Cool upwelling of deep ocean water on coast of South America Rich fishery - 25% of world catch 6

7 El Niño - Southern Oscillation Begin with Southern Oscillation Reverse atmospheric pressure pattern over Pacific ocean. Why? not yet known Reversal may cause Pacific Warm Pool bulge to flow East El Niño = arrival of warm bulge off South America Prolonged warm ocean T (~12 to 18 months) La Niña = reverse of El Niño and follow after El Niño periods Normal El Niño La Niña 7

8 Results of El Nino: Global effects Abnormally warm sea surface Temp off coast of SA Disrupts fisheries and agriculture of SA and NA Abnormal rainfall (too much in SA - landslides etc.) W coast SA & NA Mild Winter in NA Disrupts hurricane development Tropical Cyclones Hurricane = typhoon Cause: Heat surface water to >27ºC (~81ºF) Warm, moist air rises, expands, cools & Precipitates rain Near surface flow to replace air - deflected by coriolis (spin) Precipitation releases Heat to atm = additional E for wind Wind speed > 119 km/h (74 m/h) 8

9 Hurricanes + land = devastation October 29, 1998 October 30, 1998 October 29, 1998 October 31,

10 Precipitation La Ceiba Total Daily Precipitation, in mm /26/98 10/26/98 Tegucigalpa 10/27/98 Choluteca 10/27/98 10/28/98 10/28/98 10/29/98 10/29/98 10/30/98 10/30/98 10/31/98 10/31/ /26/98 10/27/98 10/28/98 10/29/98 10/30/98 10/31/98 Tegucigalpa 10

11 Landslides El Berrinche Landslide Flooding Rio Choluteca at Mollol Bridge Rio Chiquito Dredging operation Extreme sediment aggradation Tegucigalpa Stadium Location of destroyed neighborhood Right bank downstream of El Chile Bridge 11