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Baroclinic seamount-trapped waves are thought to influence their surrounding ecosystem; however, trapped waves are not well-studied in near-equatorial settings, where stratification is strong and Burger numbers (...) are large. Motivated by observations, we use daily output (2005-2009) from the global Parallel Ocean Program Model (POP) to examine topographically trapped baroclinic waves around the Seychelles Plateau (S > 400) in the tropical Indian Ocean. These trapped waves are associated with velocity and temperature oscillations at periods of 15-16 days, similar to the dominant period of some equatorial Yanai waves. Energy flux maps using POP output suggest that quasi-biweekly equatorial Yanai waves excite trapped waves on the western and south-western flanks of the Seychelles Plateau, near the surface. The anticyclonic energy flux associated with the trapped wave extends vertically throughout the water column and around most of the plateau circumference, diminishing on the eastern flank of the plateau. This work highlights the role that equatorial planetary waves and trapped waves play in facilitating energy redistribution, dissipation, and mixing in the tropical ocean. [ABSTRACT FROM AUTHOR]

The ocean circulation around and over the Seychelles Plateau (SP) is characterized using 35 months of temperature and velocity measurements along with a numerical model. The results here provide the first documented description of the ocean circulation atop the SP. The SP is an unusually broad (~200 km), shallow (~50 m) plateau, dropping off steeply to the abyss. It is situated in a dynamic location (3.5-5.5°S, 54-57°E) in the south-western tropical Indian Ocean where northwesterly winds are present during austral summer and become southeasterly in austral winter, following the reversal of the western Indian ocean monsoon winds. Measurements around the Inner Islands, on the SP, have been carried out since 2015. Velocity measurements show that most of the depth-averaged current variance on the SP arises from near-inertial oscillations and lower-frequency variability. Lower-frequency variability encompasses seasonal and intraseasonal variability, the latter of which includes the effects of mixed Rossby-gravity waves and mesoscale eddies. A global 0.1° numerical ocean simulation is used in conjunction with these observations to describe the regional circulation around the SP. Atop the SP, circulation is dominated by ageostrophic processes consistent with Ekman dynamics, while around the SP, both geostrophic and ageostrophic processes are important and vary seasonally. Stratification responds to the sea surface height semiannual signal which is due to Ekman pumping-driven upwelling (related to the Seychelles-Chagos Thermocline Ridge) and the arrival of an annual downwelling Rossby wave. Plain Language Summary This study characterizes the Seychelles Plateau (SP) circulation using 35 months of temperature and velocity measurements and a global numerical model. The SP is an unusually broad shallow plateau (~50 m), situated in the south-western tropical Indian Ocean. In this region, ocean and climate dynamics are strongly modulated by the western Indian Ocean monsoon signal, with northwesterly winds from December to March, and southeasterly winds from April to November. Model results show that the seasonal circulation is dominated by local wind-driven processes at the peak of the monsoons (December to February and June to August), while the rest of the year, circulation is controlled by a combination of local and remote atmospheric and oceanic processes. Atop the Plateau, oscillations with periods similar to the rotation rate of the earth at this latitude (~6 days) and planetaryscale waves traveling along the equator are important contributors to the ocean circulation. Temperature observations taken near the island of Mahé, show the warmest water in April and May (>29° C) and the coldest water in July and August (<26° C). The results here will aid regional navigation and will contribute to an improved understanding of regional models and biogeochemical cycles and fisheries over the SP region. [ABSTRACT FROM AUTHOR]

Existing momentum budgets over coral reefs have predominantly focused on cross‐reef dynamics, lacking analysis of alongshore processes. To complement existing cross‐reef research and enhance our understanding of forcing variability at the semidiurnal period, this study examines the σ–coordinate, depth‐averaged alongshore momentum budget over a fore‐reef as a function of tidal phase. The observations were gathered over a 3‐week timespan, between the 12‐ and 20‐m isobaths of a Hawaiian fringing reef system, focusing on two moorings on the 12‐m isobath, where median drag coefficients estimated from log fits are CD=0.0080[−0.002,+0.004] and CD=0.0023[−0.0006,+0.0009]. Analysis at one location shows that the unsteadiness, barotropic pressure gradient, and bottom drag are equally important, and their combination is sufficient to close the momentum budget. However, bottom drag is less important at the second mooring; the difference between unsteadiness and pressure gradient suggests that advective acceleration plays a significant role. Plain Language Summary: Coral reefs are important, productive ocean ecosystems that are highly influenced by hydrodynamic forcing. Although a lot of research has been done to understand what forces drive the flow across tropical reefs (from offshore to onshore), less is known about the forces that drive flow parallel to the shoreline (alongshore). Here we study the physical dynamics over a coral reef in Hawai'i and determine that two primary forces drive the alongshore flow acceleration. One of the dominant forces is the drag exerted by the bottom reef, since coral are rougher than typical sandy coastal beds. The other dominant force is the pressure gradient, associated with the difference in sea level set up by the tide. The tidal cycle and the resulting flow response has important implications for the reef environment, with relevance for reef biology and, eventually, for ecosystem management policies. Key Points: A tidal alongshore momentum budget is estimated over a tropical fringing coral reef systemUnsteadiness, barotropic pressure gradient, and bottom drag play a dominant role on the fore‐reefAdvective acceleration can be of similar magnitude and necessary for budget closure [ABSTRACT FROM AUTHOR]