Early Online Release | Oceanography
From Global to Regional: Supertidal Energy
Tidal energy is mainly concentrated at the diurnal and semi-
diurnal astronomical forcing frequencies, and some of this
energy is transferred to higher (and lower) frequencies. Band-
pass fltering can separate the energy between that at semi-
diurnal tidal frequencies (Figure 2a) and that at higher, super-
tidal frequencies (Figure 2b). Diurnal and semidiurnal energy
dominate most of the internal tide spectrum, except along the
path of large amplitude internal tides near the equator. Most of
the research on IGW-IGW interactions in the open ocean has
focused on “subharmonic resonance,” a transfer of tidal energy
to lower frequencies (e.g., Ansong et al., 2018). For this project,
Solano et al. (2023) evaluated the decay of the low-mode inter-
nal tide due to superharmonic wave-wave interactions, leading
to the transfer of tidal energy to higher, supertidal frequencies.
Globally, supertidal kinetic energy (KE) accounts for about 5%
of the total IGW energy. Supertidal energy is greatest at low
latitudes. Equatorward of 25°, 9% of the total tidal energy is
transferred to supertidal KE. At generation sites of large ampli-
tude internal tides or “hotspots,” such as the Bay of Bengal,
the Amazon Shelf, and the Mascarene Ridge, 25%–50% of the
IGW KE is found at supertidal frequencies (Solano et al., 2023;
Buijsman et al., 2025).
Here, we focus on two regions with high supertidal KE: the
Amazon Shelf and the Mascarene Ridge (Figure 3). Te nonlin-
ear IGW KE transfer from primary to supertidal frequencies has
a banding pattern (Figure 3a,b) that is also present in the hor-
izontal divergence of the supertidal energy fux (Figure 3c,d),
suggesting a common mechanism for the nonlinear energy trans-
fer between length scales. Decomposing the energy into separate
modes (Figure 3e,f), the banding pattern appears when the low-
est modes (1+2) are superimposed but not for individual modes.
FIGURE 2. Time-mean and depth-integrated internal wave
kinetic energy (J m–2) band-passed at (a) semidiurnal, and
(b) supertidal frequencies. Regions with relatively high
supertidal energy indicated by the black rectangles are:
(1) the Amazon Shelf, (2) the Mascarene Ridge, and (3) the
Luzon Strait. (c) Zonal mean (averaged over seafloor depths
>2,000 m and 10° latitude bins) of the maximum number of
modes (vertical structures) resolved for various internal tide
frequency resolution criteria. K1, M2, M4 represent the domi-
nant diurnal, semidiurnal, and supertidal constituents of inter-
nal tides with decreasing wavelengths, respectively. For the
horizontal (vertical) resolution, the dark-colored polygons
(dashed lines) mark the range of the number of resolved
modes for the zonal mean, and the light-colored polygons
±1 standard deviation from this mean.
Zonal mean of the maximum
number of modes resolved