El Niño and La Niña El Niño is characterized by unusually warm ocean temperatures in the Equatorial Pacific, as opposed to La Niña, which characterized by unusually cold ocean temperatures in the Equatorial Pacific. El Niño is an oscillation of the ocean-atmosphere system in the tropical Pacific having important consequences for weather and climate around the globe El Niño

Walker circulation

Bjerknes’s (Nov. 2, Jul. 7, 1975, Norwegian- American meteorologist) air-sea interaction argument Walker circulation Weakened Walker circulation What causes the turnabout from warm phase to cold phase? Why do El Niños typically last only months? Why do they usually shut down rapidly and switch from warm phase to cold phases or from cold phase to warm phase?

a. Small scale Kelvin wave Kelvin wave

b. Large-scale Kelvin wave In a rotating frame, large-scale kelvin wave must be: 1. a boundary wave. 2. propagating with the boundary on its right in the North. hemisphere. 3. a trapped wave decaying from the boundary. c. Equatorial Kelvin wave NorthSouth PP C C XX

Equator downwelling upwelling thermocline downwelling upwelling Kelvin wave Rossby wave Western PacificEastern Pacific Thermocline coldwarm Normal year El Niño year

Vertical temperature anomalies along the equator during El Niño event. February W 80W January 1998

Tahiti Darwin Sea level pressure anomaly at Tahiti and Darwin Southern Oscillation

Southern Oscillation Index (SOI): defined as the pressure difference between Tahiti in the southeast Pacific and Darwin in the western Pacific (i.e., P tahiti - P darwin ). Negative SOI corresponds to El nino Positive SOI corresponds to La nina. (ENSO)

East Africa Uruguay Florida California Southern Africa Amazon Indonesia India East China Northern Australia East Europe Impact of ENSO on Global Climate Rainfall anomaly during El nino Why an anomaly, such as ENSO in the equatorial region, can affect climate thousand miles away in mid and high latitude?

Teleconnections

El Niño/La nina and hurricane 10 hurricane days during El nino years, 27 hurricane days during non-El nino years. Long running El Nio of , hurricane days of 8, 16, 10, and 7.

Chris Landsea did a statistics of hurricane damage along the three coastal locations of the US, the Gulf Coast, the Florida Peninsula, and the US East Coast. He showed that in the 10 coldest years during the recent 41 year period, the Coast hurricane spawned damage was 55 billion, while only 2.5 billion in the 10 warmest ENSO years. But Chris further pointed out that although the ENSO is typically a very strong modulator of US hurricanes when ENSO is in very cold or very warm phase, it is usually not a major player in those years of small magnitude warm or colder or neutral water conditions.