Modulation of the internal wave regime over a tropical seamount ecosystem by basin-scale oceanographic processes

Shallow seamounts are becoming increasingly recognised as key habitats for conservation due to their role as
biological refuges, particularly throughout oligotrophic oceans. Traditionally, Taylor caps have been invoked as
the mechanism driving biomass aggregation over seamounts but emerging evidence based on higher resolution
measurements highlights the importance of internal waves (IW) to the local ecosystem. These waves can flush the
benthic habitat with cool water from depth and impact on nutrient supply over short time scales through turbulent
mixing that may also influence fish behaviour. They are dependent on the regional stratification, however,
and thus influenced by planetary-scale variability in oceanographic conditions. We present here detailed observations
of the internal wave regime over a shallow seamount, called Sandes, in the central Indian Ocean
throughout different phases of the Indian Ocean Dipole (IOD) that modulated the regional stratification. A deep
thermocline, caused by the 2019 IOD event precluded internal wave activity over the summit, whereas a thermocline
collocated with the summit during 2020 when the IOD reversed polarity resulted in a 30 m amplitude
internal tide signal (t ~ 12.5 h). A shallow thermocline, observed during 2022, resulted in propagation of IWs
over the summit with less visible internal tide. Harmonic analysis shows the presence of high frequency waves (t
~ 15 min) on both flanks of the seamount during 2020 & 2022, which are likely a result of local shear instability,
whereas 2019 shows an asymmetric response, potentially due to the strong background current and suppression
of the thermocline beneath the depth of the summit. The potential importance of the waves over the summit to
the local ecosystem may be attributed to the elevated turbulence measured at the thermocline during internal
wave propagation, with ε > 10-5 W kg-1 routinely observed. Our results highlight the ability of thermocline
depth to act as a gating condition for internal wave evolution over the summit. These results show that, whilst
the water column exhibits variability at short spatiotemporal scales compared to the frequently cited Taylor cap
dynamics, it is also regulated by the wider basin scale processes. Thus, a more integrated approach is needed
when assessing these dynamic and environmentally critical habitats to include the effects of physical oceanographic
controls across multiple spatiotemporal scales.