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Oscillations with Two Feedback Processes in a Coupled Ocean–Atmosphere Model

Shang-Ping XieDepartment of Geophysics, Tohoku University, Sendai, Japan

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Atsushi KubokawaDepartment of Geophysics, Tohoku University, Sendai, Japan

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Kimio HanawaDepartment of Geophysics, Tohoku University, Sendai, Japan

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Abstract

A one-layer reduced-gravity ocean model and a linear single-mode baroclinic atmosphere model are coupled to investigate the ocean-atmosphere interaction in the tropics. The original Kraus-Turner formula is adopted to parameterize the entrainment in the ocean mixed layer and an external wind is imposed to sustain a basic state of the ocean. By examining the linear stability of the model, we show that the existence of ocean upwelling is one of the necessary conditions for eastward propagating instability. According to the upwelling's strength, the tropical ocean is separated into two regions, i.e., a stable region in the west and an unstable one in the cast.

In the fully nonlinear experiment, oscillation with a period of several years appears, which is asymmetric between the warm and cold phases because of the existence of two different coupled instabilities. The eastward unstable mode is responsible for the warm phase, while the cold phase is attributed to the unstable Rossby mode, which advects westward the cold water upwelled by a Kelvin wave in the east. The unstable Rossby mode radiates a free Rossby wave into the western stable region, which reflects at the western boundary into a Kelvin wave. The reflected Kelvin wave suppresses the westward temperature advection in the central region and triggers the next warm phase. The model's results are found to compare favorably with observations and GCM simulations in several respects.

Abstract

A one-layer reduced-gravity ocean model and a linear single-mode baroclinic atmosphere model are coupled to investigate the ocean-atmosphere interaction in the tropics. The original Kraus-Turner formula is adopted to parameterize the entrainment in the ocean mixed layer and an external wind is imposed to sustain a basic state of the ocean. By examining the linear stability of the model, we show that the existence of ocean upwelling is one of the necessary conditions for eastward propagating instability. According to the upwelling's strength, the tropical ocean is separated into two regions, i.e., a stable region in the west and an unstable one in the cast.

In the fully nonlinear experiment, oscillation with a period of several years appears, which is asymmetric between the warm and cold phases because of the existence of two different coupled instabilities. The eastward unstable mode is responsible for the warm phase, while the cold phase is attributed to the unstable Rossby mode, which advects westward the cold water upwelled by a Kelvin wave in the east. The unstable Rossby mode radiates a free Rossby wave into the western stable region, which reflects at the western boundary into a Kelvin wave. The reflected Kelvin wave suppresses the westward temperature advection in the central region and triggers the next warm phase. The model's results are found to compare favorably with observations and GCM simulations in several respects.

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