Low-Frequency Variability of Temperature in the Vicinity of the Equatorial Pacific Thermocline in SODA: Role of Equatorial Wave Dynamics and ENSO Asymmetry

B. Dewitte Laboratoire d’Etude en Géophysique et Océanographie Spatiale, Toulouse, France
Instituto del Mar del Peru, Callao, Peru

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S. Thual Instituto del Mar del Peru, Callao, Peru
ENSEEIHT, Toulouse, France

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S-W. Yeh Korea Ocean Research and Development Institute, Ansan, South Korea

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S-I. An Department of Atmospheric Sciences, Global Environmental Laboratory, Yonsei University, Seoul, South Korea

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B-K. Moon Division of Science Education, Institute of Science Education, Chonbuk National University, Jeonju, South Korea

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B. S. Giese Department of Oceanography, Texas A&M University, College Station, Texas

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Abstract

The Simple Ocean Data Assimilation (SODA) reanalysis (1958–2001) is used to investigate the decadal variability in the equatorial thermocline in the Pacific. Whereas the thermocline depth exhibits weak variation at decadal time scales, the temperature change in the vicinity of the thermocline in the western Pacific is significant and has a vertical scale of ∼150 m. Based on a modal decomposition of the model variability, it is shown that such temperature change can be interpreted to a large extent as vertical displacements of the isotherms associated with the Kelvin and first meridional Rossby waves of the first three baroclinic modes. This indicates that decadal change at the subsurface in the warm pool region may be forced by the winds, consistent with the results of a multimode linear model simulation. The decadal mode of vertical temperature can be described by the first two dominant statistical modes (EOFs): the first mode is associated with changes in the slope of the thermocline (swallowing in the western-central Pacific and deepening in the eastern Pacific), representative of the 1976/77 climate shift and ahead of the ENSO modulation; and the second mode corresponds to a basinwide uplift of the thermocline and behind the ENSO modulation. It is further shown that the subsurface temperature in the warm pool region is negatively skewed, which results from the ENSO asymmetry. The results are consistent with the hypothesis of change in mean state resulting from the residual effect of the asymmetric ENSO variability.

Corresponding author address: Boris Dewitte, IRD-Lima, 357 Calle Teruel, Miraflores, Peru. Email: boris.dewitte@gmail.com

Abstract

The Simple Ocean Data Assimilation (SODA) reanalysis (1958–2001) is used to investigate the decadal variability in the equatorial thermocline in the Pacific. Whereas the thermocline depth exhibits weak variation at decadal time scales, the temperature change in the vicinity of the thermocline in the western Pacific is significant and has a vertical scale of ∼150 m. Based on a modal decomposition of the model variability, it is shown that such temperature change can be interpreted to a large extent as vertical displacements of the isotherms associated with the Kelvin and first meridional Rossby waves of the first three baroclinic modes. This indicates that decadal change at the subsurface in the warm pool region may be forced by the winds, consistent with the results of a multimode linear model simulation. The decadal mode of vertical temperature can be described by the first two dominant statistical modes (EOFs): the first mode is associated with changes in the slope of the thermocline (swallowing in the western-central Pacific and deepening in the eastern Pacific), representative of the 1976/77 climate shift and ahead of the ENSO modulation; and the second mode corresponds to a basinwide uplift of the thermocline and behind the ENSO modulation. It is further shown that the subsurface temperature in the warm pool region is negatively skewed, which results from the ENSO asymmetry. The results are consistent with the hypothesis of change in mean state resulting from the residual effect of the asymmetric ENSO variability.

Corresponding author address: Boris Dewitte, IRD-Lima, 357 Calle Teruel, Miraflores, Peru. Email: boris.dewitte@gmail.com

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