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Impact of vertical mixing induced by small vertical scale structures above and within the equatorial thermocline on the tropical Pacific in a CGCM

Abstract

Oceanic vertical mixing is known to influence the state of the equatorial ocean which affects the climate system, including the amplitude of El Niño/Southern Oscillation (ENSO). Recent measurements of ocean currents at high vertical resolution capture numerous small vertical scale structures (SVSs) within and above the equatorial thermocline that contribute significantly to vertical mixing but which are not sufficiently resolved by coarse resolution ocean models. We investigate the impact of the vertical mixing induced by the SVSs on the mean state and interannual variability in the tropical Pacific by using a coupled general circulation model. The vertical mixing induced by the SVSs is represented as an elevated vertical diffusivity from the surface down to the 20 °C isotherm depth, a proxy for the depth of the thermocline. We investigate different forms for the elevated mixing. It is found that the SVS-induced mixing strongly affect the mean state of the ocean leading to a warming of sea surface temperature (SST) and associated deepening and sharpening of the thermocline in the eastern equatorial Pacific. We find that the SST warming induced by the elevated mixing is further strengthened through the Bjerknes feedback and SST-shortwave flux feedback. We also find a reduction in the number of large amplitude ENSO events and in certain cases an increase in the skewness of ENSO.

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Acknowledgments

The authors would like to acknowledge Dr. P. Sandery for the English correction. This study was supported by the Japan Science and Technology Agency/ Japan International Cooperation Agency through Science and Technology Research Partnership for Sustainable Development (SATREPS). All experiments were performed on the Earth Simulator 2.

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Correspondence to Wataru Sasaki.

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Sasaki, W., Richards, K.J. & Luo, JJ. Impact of vertical mixing induced by small vertical scale structures above and within the equatorial thermocline on the tropical Pacific in a CGCM. Clim Dyn 41, 443–453 (2013). https://doi.org/10.1007/s00382-012-1593-8

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Keywords

  • Small vertical scale structures
  • Vertical mixing
  • Coupled general circulation model
  • ENSO