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What Caused the Onset of the 1997–98 El Niño?

Geert Jan van OldenborghKNMI, De Bilt, Netherlands

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Abstract

There has been intense debate about the causes of the 1997–98 El Niño. One side sees the obvious intense westerly wind events as the main cause for the exceptional heating in summer 1997, the other emphasizes slower oceanic processes. A quantitative analysis of all factors contributing to the onset of this El Niño is presented here. Specifically, the Niño-3 index in the Hamburg Ocean Primitive Equation Model OGCM at 1 June 1997 is decomposed into contributions from the fluxes and initial state at six months’ lead time. The initial-state thermal anomalies contribute about 40% compared with an average year, and the wind stress about 50%. Compared with the previous year, in which no El Niño developed, the main difference is in the zonal wind stress. This contribution is concentrated at the time and place of the strong westerly wind events in December 1996, and March and April 1997. As westerly wind events are difficult to predict, this limited the predictability of the onset of this El Niño.

Corresponding author address: Geert Jan van Oldenborgh, KNMI, KS/OO, P.O. Box 201, NL-3730 AE De Bilt, Netherlands.

Email: oldenbor@knmi.nl

Abstract

There has been intense debate about the causes of the 1997–98 El Niño. One side sees the obvious intense westerly wind events as the main cause for the exceptional heating in summer 1997, the other emphasizes slower oceanic processes. A quantitative analysis of all factors contributing to the onset of this El Niño is presented here. Specifically, the Niño-3 index in the Hamburg Ocean Primitive Equation Model OGCM at 1 June 1997 is decomposed into contributions from the fluxes and initial state at six months’ lead time. The initial-state thermal anomalies contribute about 40% compared with an average year, and the wind stress about 50%. Compared with the previous year, in which no El Niño developed, the main difference is in the zonal wind stress. This contribution is concentrated at the time and place of the strong westerly wind events in December 1996, and March and April 1997. As westerly wind events are difficult to predict, this limited the predictability of the onset of this El Niño.

Corresponding author address: Geert Jan van Oldenborgh, KNMI, KS/OO, P.O. Box 201, NL-3730 AE De Bilt, Netherlands.

Email: oldenbor@knmi.nl

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