Oceanography | Vol. 38, No. 2
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studies considered IGW-IGW interactions to be the dominant
processes. One mechanism, called “induced diffusion,” involves
the interaction of near-inertial and tidal IGWs. Induced diffu
sion is thought to be very important in transferring KE across
length scales. However, most studies have not considered
IGW-eddy interactions in the same manner.
Skitka et al. (2024) used a framework to diagnose IGW-eddy
interactions with IGW-IGW interactions in a regional MITgcm
(1/48°) ocean simulation of the North Pacific. They found that
IGW-eddy interactions induce a downscale KE flux in a man
ner analogous to IGW-IGW interactions. At this grid spacing,
the “eddy-induced diffusion” was the dominant mechanism of
energy exchange within the IGW supertidal continuum, and
comparable to the wave-induced diffusion achieved by regional
models with 250 m (1/192°) horizontal grid spacing. Thus, finer
vertical and horizontal grid spacing is expected to change the
details of the IGW cascade in simulations, including the mecha
nisms and rate of energy transfer and its dissipation.
ACOUSTICS
Tidally Forced Simulations and Sound Speed
We first examined how tidal forcing affects sound speed and
acoustic properties using a series of global HYCOM (1/25°)
simulations with or without tidal (T) forcing and with or with
out data assimilation (DA), four simulations in all. Each simu
lation was forced by wind and had 41 layers. Hourly output was
recorded from May to June 2019. Temperature and salinity were
interpolated from the native grid to a uniform 2 m vertical grid
and then used to compute sound speed.
As an initial comparison, the sound speed variability in each
of the four simulations was compared to glider observations
over a small geographic area in the North Pacific (Figure 5a;
Rudnick, 2016). A mean and standard deviation of sound speed
was computed from May 20 to May 26, using three-hour out
put from the simulation and averaged over the region covered
by the glider track. The glider profiled from the surface to 500 m
depth roughly every three hours. Although this is not a region
of large tidal energy, the simulations with tidal forcing still had
FIGURE 5. (a) Standard deviation of sound speed for May 20–26, 2019, from Global HYCOM simulations with and without tides and with and without
data assimilation (DA) at the location indicated on the map off the coast of California. Simulations were compared to standard deviation computed from
glider observations over the same week and location. (b,c) The depth of the 1,510 m s–1 sound speed along 20°N, extending from the coast of Hainan
Island eastward (111.16°E–160°E) for global HYCOM simulations. Bathymetry is overlaid on each, with the Luzon Strait located at 1,000 km distance from
the coast. (d,e) SLD and BLG for global HYCOM simulation with tides (Exp 19.0) and for a nonhydrostatic regional MITgcm simulation at the Mascarene
Ridge near the island of Madagascar (see Figure 3b).