Abstract
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 1000 km wavelength, strong anisotropy, and a cascade of energy from 600 km 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 ∼140 km wavelength. At short scales, wavenumber SSH spectra are dominated by incoherent internal tides and internal waves which extend up to 200 km 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.
Highlights
Recent analyses of global sea surface height (SSH) wavenumber spectra from along-track altimetric data (Xu and Fu, 2011, 2012; Zhou et al, 2015) have found that while the midlatitude regions have spectral slopes consistent with quasi-geostrophic (QG) theory or surface quasigeostrophic (SQG) theory, the tropics were noted as regions with very flat spectral slopes (Fig. 1a)
Using the fixed wavelength band leads to a slight reduction in the low-frequency SSH spectral slope estimate but without a drastic modification. These results indicate that if the internal balanced dynamics of our 1/12◦ model were the main contribution to the altimetric SSH, we would expect a k−4 (SQG) slope in the equatorial band and closer to k−5 (QG) in the off-equatorial band
The processes that could contribute to the flat SSH wavenumber spectral slopes observed in the tropics by satellite altimetry have been examined in the tropical Pacific
Summary
Recent analyses of global sea surface height (SSH) wavenumber spectra from along-track altimetric data (Xu and Fu, 2011, 2012; Zhou et al, 2015) have found that while the midlatitude regions have spectral slopes consistent with quasi-geostrophic (QG) theory or surface quasigeostrophic (SQG) theory, the tropics were noted as regions with very flat spectral slopes (Fig. 1a). The objective of this paper is to better understand the processes specific to the tropics that contribute to the SSH wavenumber spectral slopes observed by satellite altimetry, in the “mesoscale” range at scales < 600 km and 90 days (Tulloch et al, 2009). A few studies have addressed the tropical dynamics at spatial scales smaller than this 600 km cutoff wavelength. Different studies have clearly distinguished the tropical regions dominated by linear planetary waves from the midlatitudes dominated by non-linear regimes (Fu, 2004; Theiss, 2004; Chelton et al, 2007). Close to the Equator, baroclinic instability is inhibited, while barotropic instability becomes more important (Qiu and Chen, 2004), and mesoscale structures arise from the baroclinic and barotropic instabilities associated with the vertical and horizontal shears of the up-
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