Transcript Tropical Circulation and El Niño

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