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

equator30°N 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: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

Icelandic LowAleutian Low

Siberian HighCanadian High

Azores Bermuda HighPacific 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

Icelandic LowAleutian Low

Siberian HighCanadian High

Azores Bermuda HighPacific 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

Monsoonal LowAzores Bermuda HighPacific High

• 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

HIGH

LOWAloft

Surface

ocean

land

Icelandic LowAleutian Low

Siberian HighCanadian High

Azores Bermuda HighPacific 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

LOW

HIGH

land

water

Monsoonal LowAzores Bermuda HighPacific High

• 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)

EastWest Pacific

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)

windSurface current

45°

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)

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 )

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

4. thermocline (17 degree C line)

Warmer than 17 degrees

Colder than 17 degrees

Western Pacific :Indonesia, Australia

Eastern Pacific:W coast S. America

LOW HIGH

Cold water

Summary of neutral conditions:

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 wetN US and Canada: warmer

Summary of El Nino conditions:

< 17°C

> 17°C

LOWHIGH

Warm water moves east

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 degreesColder than 17 degrees

Western Pacific :Indonesia, Australia

Eastern Pacific:W coast S. America

LOW HIGH

Cold water

Summary of La Nina conditions:

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

Warm (yellow, red, white)

Cool (blue, purple)

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