tropical circulation and el ni no
DESCRIPTION
Tropical Circulation and El Ni no. 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, - PowerPoint PPT PresentationTRANSCRIPT
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Tropical Circulation and El Nino
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ITCZ Intertropical Convergence Zone
Low pressure in equatorial regions due to warm air rising
(air rises to tropopause)
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Just below tropopause:air moves poleward to subtropics, where it sinks, spreads out at surface, splits: poleward or equatorward: Hadley Cell
tropopause
equator30°N 30°S
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• Trade winds at surface converge at equator
• Band of clouds in satellite image represents ITCZ (middle of Hadley Cell)
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Trade winds
Surface arms of the Hadley Cells make up the trade winds:trade winds:
– Steady winds from 0 – 30° N and S– In Northern Hemisphere: NE– In Southern Hemisphere: SE
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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
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Northern Hemisphere Winter:
Icelandic Low
Aleutian Low
Canadian High
Siberian High
Azores Bermuda High
Pacific High
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Icelandic LowAleutian Low
Siberian HighCanadian High
Azores Bermuda HighPacific High
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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
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Southern Hemisphere
Analogous except continental variations not as strong: no large landmasses:
– Strongly developed oceanic Highs in summer
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Icelandic LowAleutian Low
Siberian HighCanadian High
Azores Bermuda HighPacific High
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Southern Hemisphere
Analogous except continental variations not as strong: no large landmasses:
– Strongly developed oceanic Highs in summer– Strongly developed oceanic lows in winter
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Monsoonal LowAzores Bermuda HighPacific High
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• ITCZ migrates with seasons because zone of maximum heating migrates with subsolar point
• Zone of maximum heating varies depending on surface( land or water)
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monsoon
• Caused by seasonal shifting of ITCZ
• Most pronounced over Asia :– large Asian landmass – Himalayas enhance orographic precipitation
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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
HIGH
LOWAloft
Surface
ocean
land
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Icelandic LowAleutian Low
Siberian HighCanadian High
Azores Bermuda HighPacific High
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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
LOW
HIGH
land
water
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Monsoonal LowAzores Bermuda HighPacific High
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• 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
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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
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2. Low pressure over warm western Pacific
– More rainfall over western Pacific
– Convective cell develops: Walker Circulation (Low in west; High in east)
EastWest Pacific
Low High
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3. Along west coast of south America, upwelling of cold, nutrient-rich water replaces warm surface water, cools surface
– Due to Ekman Spiral:
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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)
windSurface current
45°
Net transport of water
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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)
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ITCZ
NE, SE trades
Ekman transport
Result of Ekman spiral: bulk of water transport is 90
degrees from wind direction
(converging wind, diverging water, moving west )
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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 Southequator
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4. thermocline (17 degree C line)
Warmer than 17 degrees
Colder than 17 degrees
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Western Pacific :Indonesia, Australia
Eastern Pacific:W coast S. America
LOW HIGH
Cold water
Summary of neutral conditions:
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El Nino
1. relaxing of trade winds
(may even flow eastward)
in central and western Pacific
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2. warmer water than usual in eastern Pacific
warm water sloshes eastward to coast of Americas
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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
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4. thermocline (17 degree C line)
depressed in east Pacific; elevated in west Pacific
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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 wetN US and Canada: warmer
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Summary of El Nino conditions:
< 17°C
> 17°C
LOWHIGH
Warm water moves east
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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
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2. Exceptionally strong trade winds
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3. thermocline (17 degree C line)
Warmer than 17 degreesColder than 17 degrees
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Western Pacific :Indonesia, Australia
Eastern Pacific:W coast S. America
LOW HIGH
Cold water
Summary of La Nina conditions:
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La Nina’s effects:
• Very heavy rain, flooding in western Pacific
• Strengthening of Atlantic hurricane season
• Wetter Pacific northwest
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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
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Return of El Nino
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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
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Cool, Negative Phase
• “Horseshoe” of warm water in west and cool water in east
Warm (yellow, red, white)
Cool (blue, purple)
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Warm, Positive Phase• West Pacific is cool, east warms• 1977 – 1999
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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