Today s topics: Atmospheric circulation: generation of wind patterns on a rotating Earth Seasonal patterns of climate: Monsoons and Sea Breezes 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
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
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 veers 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 animations Coriolis 1, Coriolis 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
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. Circulation Animation 4
Can you predict Wet vs Arid regions based on the pattern of circulation cells? Weather animation Warm and Cold Fronts In either case, warm air is forced to rise over cold air - causing precipitation. Note: Cold front is steep, causing warm air to rise rapidly. Thus, showers are relatively brief and powerful (storms) 5
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 During 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 Monsoon ~ season long onshore breeze Heat continent over summer Continuously rising air mass Replaced by moist ocean air Continuous rain 6
Tropical Cyclones Hurricane (Atlantic) = Typhoon (Pacific) = Cyclone (Indian) Tropical Cyclones Atmospheric disturbance (in Atlantic, usually a wave from the Sahara) Low pressure cell along tropical convergence zone breaks away and flow into N or S hemisphere Heat surface water to >27ºC (~81ºF), and evaporate water to atm. Warm, moist air rises, expands, cools & Precipitates rain releasing huge amounts of E to the atm (latent heat of vaporization) and supplies additional energy for WIND Near surface flow to replace air - deflected by coriolis (spin) Wind speed > 119 km/h (74 m/h) 7
Tropical Cyclone Structure More energy contained in one cyclone than all the energy generated in US over past 20 years! Hurricanes + land = devastation Damage from Wind (exceeding 120 mph) Heavy rain (flooding) Storm surge along the coast (up to 40 feet high) Low pressure of the storm causes a bulge or hill to form on the surface of the ocean. When the storm comes ashore, this hill does too (storm surge) 8
Hurricanes produce huge waves: Hurricane Ivan (09/04) Track (green) of storm directly over wave buoy (red) Measured increasing avg storm wave H (red circle) and H max (blue x) as eye approches (green square). Max wave H = 27.7 m 24 waves with H>15 m The Max H is not true max wave H of storm, NE quad passed E of buoy Hurricanes = Major source of sediment to the coast [sufficient to account for all (in-org) sediment in healthy Rita saltmarsh wetlands] Katrina 9