A simple model of El Niño and the southern oscillation

A model of tropical ocean-atmosphere interaction is used to study the El Nino-Southern Oscillation phenomenon. The model ocean consists of the single baroclinic mode of a two-layer ocean. The thermodynamics of the upper layer are highly parameterized; sea surface temperature is assigned one of two v...

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Bibliographic Details
Published in:Monthly weather review 1984-01, Vol.112 (5), p.934-946
Main Authors: MCCREARY, J. P. JR, ANDERSON, D. L. T
Format: Article
Language:English
Subjects:
Earth, ocean, space
Exact sciences and technology
External geophysics
Physics of the oceans
Sea-air exchange processes
Online Access:Get full text
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Summary:A model of tropical ocean-atmosphere interaction is used to study the El Nino-Southern Oscillation phenomenon. The model ocean consists of the single baroclinic mode of a two-layer ocean. The thermodynamics of the upper layer are highly parameterized; sea surface temperature is assigned one of two values, warm or cool, according to whether the interface is shallower or deeper than an externally specified depth. The model atmosphere consists of two wind patches of zonal stress that are idealizations of the annual cycle of the equatorial trades, tau sub(s) , and of Bjerknes' Walker circulation, tau sub(w) . When the eastern ocean is in its cool state, both patches drive the ocean; when it is warm, tau sub(w) is switched off. Solutions compare favorably with observations in several ways. Most important, for reasonable choices of parameters, solutions oscillate at the long time scales associated with the Southern Oscillation. The response of the ocean to tau sub(w) introduces positive feedback into the system, with the result that the system can adjust to one or the other of two equilibrium states: a state with tau sub(w) switched on, and another with it switched off. The annual wind tau sub(s) is the trigger that switches tau sub(w) off or on, and thereby prevents the system from ever reaching either equilibrium state. When tau sub(w) switches on, equatorial Kelvin waves swiftly propagate from the wind patch into the eastern ocean and raise the interface there to a shallow level. Rossby waves, also generated by the wind, subsequently reflect from the western boundary as a second set of equatorial Kelvin waves. The arrival of this second set in the eastern ocean begins a gradual deepening of the interface there toward its equilibrium value. It is this overshoot together with slow relaxation of the interface in the eastern ocean that permits the model to oscillate at long time scales. Essentially, the ocean must be sufficiently relaxed toward an equilibrium state before tau sub(s) can act to switch tau sub(w) off or on.
ISSN:0027-0644
1520-0493
DOI:10.1175/1520-0493(1984)112<0934:ASMOEN>2.0.CO;2