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Tropical Circulation and El Nino ITCZ Intertropical Convergence Zone Low pressure in equatorial regions due to warm air rising (air rises to tropopause) Just below tropopause: air moves poleward to subtropics, where it sinks, spreads out at surface, splits: poleward or equatorward: Hadley Cell tropopause 30°N equator 30°S • Trade winds at surface converge at equator • Band of clouds in satellite image represents ITCZ (middle of Hadley Cell) Trade winds Surface arms of the Hadley Cells make up the trade winds: – Steady winds from 0 – 30° N and S – In Northern Hemisphere: NE – In Southern Hemisphere: SE Subtropical highs • Winds aloft in Hadley cells are flowing poleward • Coriolis effect prevents them from flowing further north • Therefore air begins to pile up at 30 degrees – surface pressure rises; • air sinks and diverges; • warms as it sinks; • clear skies Northern Hemisphere Winter: Icelandic Low Aleutian Low Canadian High Siberian High Azores Bermuda High Pacific High Canadian High Siberian High Icelandic Low Aleutian Low Azores Bermuda High Pacific High Northern Hemisphere Summer: • 2 continental highs (Canada and Siberian) gone • Monsoonal low over Asia • Oceanic lows weaker due to relatively cold ocean (compared to land) • Azores Bermuda High and Pacific High closer to poles and have higher central pressures Southern Hemisphere Analogous except continental variations not as strong: no large landmasses: – Strongly developed oceanic Highs in summer Canadian High Siberian High Icelandic Low Aleutian Low Azores Bermuda High Pacific High Southern Hemisphere Analogous except continental variations not as strong: no large landmasses: – Strongly developed oceanic Highs in summer – Strongly developed oceanic lows in winter Pacific High Azores Bermuda High Monsoonal Low • ITCZ migrates with seasons because zone of maximum heating migrates with subsolar point • Zone of maximum heating varies depending on surface( land or water) monsoon • Caused by seasonal shifting of ITCZ • Most pronounced over Asia : – large Asian landmass – Himalayas enhance orographic precipitation January: very cold air over Asia causes surface pressure to rise strong HIGH develops (Siberian High); southwest wind off continent: dry warm air Result: Winter monsoon: dry Aloft LOW land HIGH ocean Surface Canadian High Siberian High Icelandic Low Aleutian Low Azores Bermuda High Pacific High July: heating of continent leads to reversal of surface and upper level winds. Divergence aloft, promotes uplift; Onshore winds, bringing moist stable air from Indian Ocean to Asian continent Cloud formation enhanced as air rises over Himalayas Result: summer monsoon: very heavy rain HIGH land LOW water Pacific High Azores Bermuda High Monsoonal Low • ENSO (El Nino / Southern Oscillation) • a Cycle with 3 phases: – El Nino – La Nina – Neutral (“normal”) – 3-7 yr intervals (average = 40 months) – Glacial cores suggest this cycle has been going on for 1500 years Neutral (normal) 1. Easterly trade winds move warm surface water west over equatorial Pacific – Sea surface temperature 8 C higher in Indonesia than Ecuador – Sea surface is 0.5 m higher in Indonesia than Ecuador 2. Low pressure over warm western Pacific – More rainfall over western Pacific – Convective cell develops: Walker Circulation (Low in west; High in east) West Pacific East Low High 3. Along west coast of south America, upwelling of cold, nutrient-rich water replaces warm surface water, cools surface – Due to Ekman Spiral: Ocean water moves with wind; as it moves, it is deflected by the Coriolis effect Surface water movement transfers momentum to layer of water immediately underneath, which is deflected to right of motion of layer above; etc. Creates a spiral to a depth of about 100 m (Ekman Spiral) 45° Surface current wind Net transport of water Result of Ekman spiral: bulk of water transport is 90 degrees from wind direction (converging wind, diverging water, moving west ) As warm surface water moves west, diverging cold subsurface water upwells to replace it; (rich in nutrients, part of aquatic food chain) NE, SE trades Ekman transport ITCZ Result of Ekman spiral: bulk of water transport is 90 degrees from wind direction (converging wind, diverging water, moving west ) As warm surface water moves west, diverging, cold subsurface water upwells to replace it; (rich in nutrients, part of aquatic food chain) Upwelling water North equator South 4. thermocline (17 degree C line) Warmer than 17 degrees Colder than 17 degrees Summary of neutral conditions: Western Pacific : Indonesia, Australia LOW Eastern Pacific: W coast S. America HIGH Cold water El Nino 1. relaxing of trade winds (may even flow eastward) in central and western Pacific 2. warmer water than usual in eastern Pacific warm water sloshes eastward to coast of Americas 3. no longer have strong convectional low over west Pacific – Low over east : rising air: • rain: flooding in Peru, California – High over west: sinking air: • drought in Indonesia, Australia – “Southern Oscillation”: inverted pattern of west and east Pacific pressures 4. thermocline (17 degree C line) depressed in east Pacific; elevated in west Pacific Global effects of El Nino: hard to predict due to other interfering factors, But, in general, places that are vulnerable to droughts and flooding are most likely to be damaged by El Nino Polar front jet shifts north of its usual place Low over Pacific draws warm air into Canada and N. US higher than normal temp in west Canada and upper plains of US; southern US: low pressure draws cold moist air in, bringing lower than usual temps and rain US weather: SE US wet N US and Canada: warmer Summary of El Nino conditions: HIGH LOW Warm water moves east > 17°C < 17°C La Nina (hyper-neutral) (Does not always follow an El Nino) 1. Cold water replaces warm pool in eastern Pacific; COLDER than normal/neutral water in east Pacific 2. Exceptionally strong trade winds 3. thermocline (17 degree C line) Warmer than 17 degrees Colder than 17 degrees Summary of La Nina conditions: Western Pacific : Indonesia, Australia LOW Eastern Pacific: W coast S. America HIGH Cold water La Nina’s effects: • Very heavy rain, flooding in western Pacific • Strengthening of Atlantic hurricane season • Wetter Pacific northwest TOPEX/Poseidon and Jason-1 satellite missions • Records sea surface height • Colors show sea level height relative to average • Higher heights correspond to warmer temps Return of El Nino Pacific Decadal Oscillation (PDO) Discovered in 1996 Long-term ocean fluctuation of the Pacific: switches between 2 phases every 20 – 30 years: 1. Cool, negative phase 2. Warm, positive phase Cool, Negative Phase • “Horseshoe” of warm water in west and cool water in east Cool (blue, purple) Warm (yellow, red, white) Warm, Positive Phase • West Pacific is cool, east warms • 1977 – 1999 PDO’s effect on climate: Change in warm and cold masses in Pacific changes path of jet stream We are in cool phase, which will steer jet north over western US Dry conditions on west coast North America in the phase we have just entered