What Are the El Nino and La Nina?

Review of last lecture

1.1. The developmental stages and vertical structure of The developmental stages and vertical structure of middle latitude cyclones (boundary between middle latitude cyclones (boundary between northern cold air and southern warm air, upper level northern cold air and southern warm air, upper level low to the west of surface low)low to the west of surface low)

2.2. How upper level longwaves and shortwaves may How upper level longwaves and shortwaves may enhance cyclonic development at the surface (upper enhance cyclonic development at the surface (upper level low to the west of surface low)level low to the west of surface low)

3.3. The three regions of cyclogenesis and typical tracksThe three regions of cyclogenesis and typical tracks4.4. Lake effectLake effect5.5. Anticyclones, heat waves and droughtsAnticyclones, heat waves and droughts6.6. Climate change: Climate change: move the tracks of the midlatitude

cyclones towards the poles

The Tropical Atmosphere Ocean (TAO) buoy array by NOAA

Tropical mean state: Sea surface temperature (SST)

Indo-Pacific warm pool

Eastern Pacific cold tongue

2 basic regions

Tropical mean state: Precipitation

GPCP Annual Mean Precipitation for 1979-2005 (mm/day)

Inter-tropical convergence zone (ITCZ)

Strong rainfall (heating)

Weak rainfall

Tropical mean State: Walker Circulation

• An atmospheric circulation cell oriented along the equator with rising motion in the west and sinking motion in the east, which is induced by the SST contrast between the Indo-Pacific warm pool and eastern Pacific cold tongue

• is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water.

Ocean upwelling

• Equatorial upwelling: Due to Coriolis effect

• Coastal upwelling: Due to Coriolis effect

Tropical mean State: Ocean-atmosphere feedback

• Interacts with underlying Pacific Ocean with stronger upwelling (cooling) in the east, leading to a positive feedback.

Video: Chasing El Ninohttps://www.youtube.com/watch?v=TYR3GcSLHco

El Nino/Southern Oscillation (ENSO): The 4-year oscillation

• Southern Oscillation: The atmospheric oscillation associated with the El Nino-La Nina cycle.

• The whole phenomena is now called El Nino/Southern Oscillation (ENSO)

• El Nino: Very warm sea surface temperature over central and eastern tropical Pacific, which occurs every 3-7 years. The Walker Circulation becomes disrupted during El Niño events, which weakens upwelling in eastern Pacific.

• La Nina: the opposite condition to El Nino

Typical ENSO period is 3-7 years, but with significant irregularity

Evolution of the 1997-1998 El Nino event

The dramatic impacts of ENSO around the globe

Flood in Lakeport, California as a result of the 1998 El Nino event

Bushfire in Australia as a result of the 1998 El Nino event

Disastrous effects of 1982-1983 El Nino:1.Australia-Drought and devastating brush fires 2.Indonesia, Philippines-Crops fail, starvation follows3.India, Sri Lanka-Drought,fresh water shortages4.Tahiti-6 tropical cyclones5.South America-Fish industry devastated - decrease in nutrients off Peru- fewer fish (anchovy)6.Across the Pacific-Coral reefs die7.Colorado River basin-Flooding, mud slides8.Gulf states-Downpours cause death, property damage9.Peru, Ecuador-Floods, landslides10.Southern Africa-Drought, disease, malnutrition

• T over land > water in summer

Seasonal temperature distributions:

Land-sea Contrast: Seasonal “Monsoon”

• T over land < water in winter

• A seasonal reversal of wind due to seasonal thermal differences between landmasses and large water bodies

• Orographic lifting often enhances precipitation totals

The Seasonal “Monsoon”

World’s major monsoon systems

From Jay Fein 1987

Seasonal variation of monsoon rainfall

Impacts of monsoons:(1) Normal year

Dry season

Wet season

Impacts of monsoons:(2) Failure of monsoon and droughts

Drought affected area in Karnataka, India, 2012, image from Wikipedia

Impacts of monsoons:(3) Excessive monsoon and floods

Image from Doctors without Borders

Summary

• Mean state: The two basic regions of SST? Which region has stronger rainfall? What is the Walker circulation?

• El Nino and La Nina: Which region has warm SST anomaly during El Nino? 4-year period.

• Global impacts of ENSO

• Effects of El Nino on U.S. winter temperature and precipitation: Warm temperature around Great Lakes, Heavy rainfall in California and Gulf coast

• Land-sea temperature contrasts: seasonal monsoon