Observations and Modeling of Ocean Circulation in the Seychelles Plateau Region

Abstract

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.

Generation of Low‐Latitude Seamount‐Trapped Waves: A Case Study of the Seychelles Plateau

“…Equatorial intraseasonal fluctuations in the monsoonal wind stress can generate quasi-biweekly (period around 14 days) Yanai waves in the western basin (  50 -60 E; e.g., Arzeno et al, 2020;Chatterjee et al, 2013;Miyama et al, 2006;Murty et al, 2006;Sengupta et al, 2004). This is noteworthy, since Castillo-Trujillo et al (2020b) observe that most of the current variability atop the Seychelles Plateau is found at periods of 10-40 days, consistent with the quasi-biweekly Yanai wave period.…”

“…Our research is motivated by instrument and satellite data, as well as model output in the equatorial Indian Ocean. Current velocities on the Seychelles Plateau show energy in the 10-40 day band (Castillo-Trujillo et al, 2020b) and, more specifically, in the 10-20 day band (Figure 2). Energy at similar periods is found in satellite data.…”

“…Following Castillo‐Trujillo et al. (2020b), we divided and averaged our profiles into four seasons: December–February (DJF), March–May (MAM), June–August (JJA), and September–November (SON). Profiles were also averaged radially and among the cross‐sections.…”

“…The goal of this work, carried out as part of the larger North Arabian Sea Circulation autonomous research (NASCar) project (Centurioni et al, 2017) funded by the Oce of Naval Research (ONR), is to use the Parallel Ocean Program (POP) numerical model to examine the generation of trapped waves around the Seychelles Plateau by equatorial Yanai waves, driven by observations in both Castillo-Trujillo et al (2020b) and Arzeno et al (2020). This paper is organized as follows: in Section 2, we provide information on the POP numerical ARZENO-SOLTERO ET AL.…”

“…Using both datasets, Wang et al (2018) saw good agreement between the seasonal cycles of SSH in the altimetry and POP. Castillo-Trujillo et al (2020b) validated the region of the south-western tropical Indian Ocean by comparing seasonal climatologies of AVISO SSH and geostrophic velocities with those found in the POP model, focused specifically on the Seychelles Plateau. The model showed stronger geostrophic velocities than those derived from AVISO SSH, and there were slight spatial differences in the location of the SSH anomaly zero-crossings; however, there was overall agreement between the model and observations on the seasonal timescale (Castillo-Trujillo et al, 2020b).…”