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Volume 14, issue 5 | Copyright
Ocean Sci., 14, 1283-1301, 2018
https://doi.org/10.5194/os-14-1283-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 24 Oct 2018

Research article | 24 Oct 2018

Spectral signatures of the tropical Pacific dynamics from model and altimetry: a focus on the meso-/submesoscale range

Michel Tchilibou1, Lionel Gourdeau1, Rosemary Morrow1, Guillaume Serazin1, Bughsin Djath2, and Florent Lyard1 Michel Tchilibou et al.
  • 1Laboratoire d'Etude en Géophysique et Océanographie Spatiales (LEGOS), Université de Toulouse, CNES, CNRS, IRD, UPS, Toulouse, France
  • 2Helmholtz-Zentrum Geesthacht Max-Planck-Straße, Geesthacht, Germany

Abstract. The processes that contribute to the flat sea surface height (SSH) wavenumber spectral slopes observed in the tropics by satellite altimetry are examined in the tropical Pacific. The tropical dynamics are first investigated with a 1∕12° global model. The equatorial region from 10°N to 10°S is dominated by tropical instability waves with a peak of energy at 1000km wavelength, strong anisotropy, and a cascade of energy from 600km down to smaller scales. The off-equatorial regions from 10 to 20° latitude are characterized by a narrower mesoscale range, typical of midlatitudes. In the tropics, the spectral taper window and segment lengths need to be adjusted to include these larger energetic scales. The equatorial and off-equatorial regions of the 1∕12° model have surface kinetic energy spectra consistent with quasi-geostrophic turbulence. The balanced component of the dynamics slightly flattens the EKE spectra, but modeled SSH wavenumber spectra maintain a steep slope that does not match the observed altimetric spectra. A second analysis is based on 1∕36° high-frequency regional simulations in the western tropical Pacific, with and without explicit tides, where we find a strong signature of internal waves and internal tides that act to increase the smaller-scale SSH spectral energy power and flatten the SSH wavenumber spectra, in agreement with the altimetric spectra. The coherent M2 baroclinic tide is the dominant signal at  ∼ 140km wavelength. At short scales, wavenumber SSH spectra are dominated by incoherent internal tides and internal waves which extend up to 200km in wavelength. These incoherent internal waves impact space scales observed by today's along-track altimetric SSH, and also on the future Surface Water Ocean Topography (SWOT) mission 2-D swath observations, raising the question of altimetric observability of the shorter mesoscale structures in the tropics.

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This paper is motivated by the next SWOT altimetric mission dedicated to the observation of mesoscale and submesoscale oceanic features. It focuses on tropical areas with a strong discrepancy in the spectral signature between altimetry and models. The paper reviews the spectral signature of tropical turbulence which presents a rich variety of phenomena depending on the latitudinal dependence of the Coriolis force. Internal tides observed by altimetry explain the discrepancy with the model.
This paper is motivated by the next SWOT altimetric mission dedicated to the observation of...
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