What Are the El Nino and La Nina?

Review of last lecture

• Tropical cyclone genesis: Western Pacific has the highest averaged number per year. 6 necessary conditions. 4 stages.

• Tropical cyclone structure: 3 major components, rotation direction of inflow and outflow, location of maximum wind and rainfall, 3 feedbacks

• Tropical cyclone intensity scale. Category 1: 74mph, category 5: 155mph

• Tropical cyclone destruction: 4 reasons? Which side has the most intense destruction?

• Trends and variability in tropical cyclone activity

Tropical Climate

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

The TAO buoy array

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.

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

• 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

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

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

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

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”

Extratropical Climate

Mean State

• Westerly winds and jet streams

• A constant Polar Vortex: transport/mixing, important for ozone hole

The Pacific/North American Oscillation (PNA): The way El Nino/La Nina affect U.S.

The Strengthening/Weakening of Polar Vortex

• North Pole: Arctic Oscillation (AO)• South Pole: Antarctic Oscillation (AAO)

Arctic Oscillation Index

Arctic Oscillation Index: Prediction

Tropical-extratropical Interaction

• Zonal mean (3 cells)

• ENSO

• The interactions are mutual

Summary

Tropical climate:

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

• Mean state: Two types of ocean upwelling, ocean-atmosphere feedback

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

• Land-sea contrasts: seasonal monsoon

Extratropical climate:

• Mean state: westerly winds, polar vortex

• What is the primary way El Nino affect extratropics? (PNA)

• The oscillations associated with strengthening/weakening of polar vortex: AO, AAO