Tropical Ocean Decadal Variability and Resonance of Planetary Wave Basin Modes. Part II: Numerical Study

Haijun Yang Department of Atmospheric Science, School of Physics, Peking University, Beijing, China

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Zhengyu Liu Center for Climatic Research, University of Wisconsin—Madison, Madison, Wisconsin

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Qiong Zhang LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

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Abstract

Oceanic response to stochastic wind forcing is studied in a tropical–extratropical basin using two shallow water models: a periodically forced model and a time-forward model. Consistent with theory, extratropical stochastic wind forces a decadal spectral peak in the tropical and eastern boundary ocean as a resonant response of the planetary wave basin mode. This resonant response is characterized by a rather uniform amplitude and phase in the equatorial and eastern boundary region. In comparison, away from the eastern boundary, the extratropical ocean is affected significantly by the local Ekman pumping forcing, with spectral peaks varying with location. A complex EOF (CEOF) analysis of the time-forward model simulation further suggests that these resonant responses are robust, and can be extracted as the leading CEOF modes. Thus, the resonance of the planetary wave basin mode gives a physically based guideline for the interpretation of decadal oceanic variability in the tropical–extratropical ocean.

Corresponding author address: Z. Liu, University of Wisconsin—Madison, 1225 W. Dayton St., Madison, WI 53706. Email: zliu3@wisc.edu

Abstract

Oceanic response to stochastic wind forcing is studied in a tropical–extratropical basin using two shallow water models: a periodically forced model and a time-forward model. Consistent with theory, extratropical stochastic wind forces a decadal spectral peak in the tropical and eastern boundary ocean as a resonant response of the planetary wave basin mode. This resonant response is characterized by a rather uniform amplitude and phase in the equatorial and eastern boundary region. In comparison, away from the eastern boundary, the extratropical ocean is affected significantly by the local Ekman pumping forcing, with spectral peaks varying with location. A complex EOF (CEOF) analysis of the time-forward model simulation further suggests that these resonant responses are robust, and can be extracted as the leading CEOF modes. Thus, the resonance of the planetary wave basin mode gives a physically based guideline for the interpretation of decadal oceanic variability in the tropical–extratropical ocean.

Corresponding author address: Z. Liu, University of Wisconsin—Madison, 1225 W. Dayton St., Madison, WI 53706. Email: zliu3@wisc.edu

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