Notes

Outline

Lecture 8: El Nino-Southern Oscillation The Southern Oscillation Ocean Adjustment Interaction Between Atmosphere and Ocean Predicting El Nino Modulation of El Nino

Global Impacts of El Nino Devastating floods in Ecuador and Peru Disappearing of the usually abundant fish along the South American coast Disastrous droughts in the Southeast Asia Disastrous droughts in northern Australia Poor monsoons over India Low rainfall over Southeastern Africa Unusual weather patterns over North and South America

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El Nino and Southern Oscillation

Coupled Atmosphere-Ocean System

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1997-98 El Nino

1982-83 El Nino

“Measuring” ENSO

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Subsurface Ocean Observation

Ocean Memory The atmosphere responds to altered sea surface temperature patterns within a matter of days      è little memory in the atmosphere The ocean has far more inertia and takes months to respond to the wind pattern changes in the atmosphere      è Oceans have a long memory. The state of the ocean at any time is not simply determined by the winds at that time because the ocean is still adjusting to and has a memory of earlier winds. Ocean memory is carried by wave propagation along the thermocline.

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Delayed Oscillator: Wind Forcing The delayed oscillator suggested that oceanic Rossby and Kevin waves forced by atmospheric wind stress in the central Pacific provide the phase-transition mechanism (I.e. memory) for the ENSO cycle. The propagation and reflection of waves, together with local air-sea coupling,  determine the period of the cycle.

Wave Propagation and Reflection

Why Only Pacific Has ENSO? Based on the delayed oscillator theory of ENSO, the ocean basin has to be big enough to produce the “delayed” from ocean wave propagation and reflection. It can be shown that only the Pacific Ocean is “big” (wide) enough to produce such delayed for the ENSO cycle. It is generally believed that the Atlantic Ocean may produce very weak and short-period ENSO-like oscillation. The Indian Ocean is considered too small to produce ENSO.

Decadal Changes of ENSO

Pacific Decadal Oscillation “Pacific Decadal Oscillation" (PDO) is a decadal-scale climate variability that describe an oscillation in northern Pacific sea surface temperatures (SSTs). PDO is found to link to the decadal variations of ENSO intensity.

PDO Index

How El Nino Changes When Climate Warms?