What Are the El Nino and La Nina?

Review of last lecture The developmental stages and vertical structure of middle latitude cyclones (boundary between northern cold air and southern warm air, upper level low to the west of surface low) How upper level longwaves and shortwaves may enhance cyclonic development at the surface (upper level low to the west of surface low) The three regions of cyclogenesis and typical tracks Lake effect Anticyclones, heat waves and droughts 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 Inter-tropical convergence zone (ITCZ) Strong rainfall (heating) Weak rainfall GPCP Annual Mean Precipitation for 1979-2005 (mm/day)

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

Ocean upwelling 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. 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 Nino https://www.youtube.com/watch?v=TYR3GcSLHco

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

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 follows 3.India, Sri Lanka-Drought,fresh water shortages 4.Tahiti-6 tropical cyclones 5.South America-Fish industry devastated - decrease in nutrients off Peru- fewer fish (anchovy) 6.Across the Pacific-Coral reefs die 7.Colorado River basin-Flooding, mud slides 8.Gulf states-Downpours cause death, property damage 9.Peru, Ecuador-Floods, landslides 10.Southern Africa-Drought, disease, malnutrition

Land-sea Contrast: Seasonal “Monsoon” Seasonal temperature distributions: T over land > water in summer T over land < water in winter

The Seasonal “Monsoon” A seasonal reversal of wind due to seasonal thermal differences between landmasses and large water bodies Orographic lifting often enhances precipitation totals

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