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Coupled Ocean–Atmosphere Response to Idealized Freshwater Forcing over the Western Tropical Pacific

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  • 1 Physical Oceanography Laboratory, Ocean University of China, Qingdao, China
  • | 2 Department of Natural History Science, Graduate School of Science, Hokkaido University, Hokkaido, Japan
  • | 3 Physical Oceanography Laboratory, Ocean University of China, Qingdao, China
  • | 4 Department of Natural History Science, Graduate School of Science, Hokkaido University, Hokkaido, Japan
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Abstract

The coupled ocean–atmosphere responses to idealized freshwater forcing in the western tropical Pacific are studied using a fully coupled climate model. The model explicitly demonstrates that freshwater forcing in the western tropical Pacific can lead to a basinwide response with the pattern resembling the Pacific decadal oscillation. In the tropics, a negative (positive) freshwater forcing over the western tropical Pacific decreases (increases) sea surface height locally, and sets up a positive (negative) zonal pressure gradient anomaly, which accelerates (decelerates) the meridional overturning circulation and equatorial surface westward flow. This leads to an intensification (reduction) of meridional heat divergence and vertical cold advection, and thus a development of La Niña (El Niño)–like responses in the tropics. The tropical responses are further substantiated by the positive Bjerknes feedback, and subsequently force significant changes in the extratropical North Pacific through atmospheric teleconnection. The local freshwater response also reinforces the imposed forcing, forming a positive feedback loop. Applications to Pacific climate changes are discussed.

Corresponding author address: Dr. Lixin Wu, 5 Yushan Road, Physical Oceanography Laboratory, Ocean University of China, Qingdao 266003, China. Email: lxwu@ouc.edu.cn

Abstract

The coupled ocean–atmosphere responses to idealized freshwater forcing in the western tropical Pacific are studied using a fully coupled climate model. The model explicitly demonstrates that freshwater forcing in the western tropical Pacific can lead to a basinwide response with the pattern resembling the Pacific decadal oscillation. In the tropics, a negative (positive) freshwater forcing over the western tropical Pacific decreases (increases) sea surface height locally, and sets up a positive (negative) zonal pressure gradient anomaly, which accelerates (decelerates) the meridional overturning circulation and equatorial surface westward flow. This leads to an intensification (reduction) of meridional heat divergence and vertical cold advection, and thus a development of La Niña (El Niño)–like responses in the tropics. The tropical responses are further substantiated by the positive Bjerknes feedback, and subsequently force significant changes in the extratropical North Pacific through atmospheric teleconnection. The local freshwater response also reinforces the imposed forcing, forming a positive feedback loop. Applications to Pacific climate changes are discussed.

Corresponding author address: Dr. Lixin Wu, 5 Yushan Road, Physical Oceanography Laboratory, Ocean University of China, Qingdao 266003, China. Email: lxwu@ouc.edu.cn

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