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Modulation of Tropical Instability Wave Intensity by Equatorial Kelvin Waves

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  • 1 Earth System Science, Stanford University, Stanford, California
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Abstract

Tropical instability waves (TIWs) and equatorial Kelvin waves are dominant sources of intraseasonal variability in the equatorial Pacific Ocean, and both play important roles in the heat and momentum budgets of the large-scale flow. While individually they have been well studied, little is known about how these two features interact, although satellite observations suggest that TIW propagation speed and amplitude are modulated by Kelvin waves. Here, the influence of Kelvin waves on TIW kinetic energy (TIWKE) is examined using an ensemble set of 1/4° ocean model simulations of the equatorial Pacific Ocean. The results suggest that TIWKE can be significantly modified by 60-day Kelvin waves. To leading order, TIWs derive kinetic energy from the meridional shear and available potential energy of the background zonal currents, while losing TIWKE to friction and the radiation of waves. The passage of Kelvin waves disrupts this balance. Downwelling (upwelling) Kelvin waves induce decay (growth) in TIWKE through modifications to the background currents and the TIWs’ Reynolds stresses. These modulations in TIWKE affect eddy heat fluxes and the downward radiation of waves, with implications for the variability of SST and the energetics of abyssal flows in the eastern equatorial Pacific.

Current affiliation: ARC Centre of Excellence for Climate System Science, Climate Change Research Centre, and School of Mathematics and Statistics, University of New South Wales, Sydney, New South Wales, Australia.

Corresponding author address: R. M. Holmes, Climate Change Research Centre, Level 4, Matthews Building, University of New South Wales, Sydney, NSW 2052, Australia. E-mail: ryan.holmes@unsw.edu.au

Abstract

Tropical instability waves (TIWs) and equatorial Kelvin waves are dominant sources of intraseasonal variability in the equatorial Pacific Ocean, and both play important roles in the heat and momentum budgets of the large-scale flow. While individually they have been well studied, little is known about how these two features interact, although satellite observations suggest that TIW propagation speed and amplitude are modulated by Kelvin waves. Here, the influence of Kelvin waves on TIW kinetic energy (TIWKE) is examined using an ensemble set of 1/4° ocean model simulations of the equatorial Pacific Ocean. The results suggest that TIWKE can be significantly modified by 60-day Kelvin waves. To leading order, TIWs derive kinetic energy from the meridional shear and available potential energy of the background zonal currents, while losing TIWKE to friction and the radiation of waves. The passage of Kelvin waves disrupts this balance. Downwelling (upwelling) Kelvin waves induce decay (growth) in TIWKE through modifications to the background currents and the TIWs’ Reynolds stresses. These modulations in TIWKE affect eddy heat fluxes and the downward radiation of waves, with implications for the variability of SST and the energetics of abyssal flows in the eastern equatorial Pacific.

Current affiliation: ARC Centre of Excellence for Climate System Science, Climate Change Research Centre, and School of Mathematics and Statistics, University of New South Wales, Sydney, New South Wales, Australia.

Corresponding author address: R. M. Holmes, Climate Change Research Centre, Level 4, Matthews Building, University of New South Wales, Sydney, NSW 2052, Australia. E-mail: ryan.holmes@unsw.edu.au
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