Moored observation of abyssal flow and temperature near a hydrothermal vent on the Southwest Indian Ridge

Abstract

AbstractFour moorings were deployed near “Dragon Flag,” an active hydrothermal vent in the valley of the Southwest Indian Ridge. The goal was to examine the variability of currents and temperature, which will guide the trajectory of spreading plumes. The mean current was cross‐isobath, and the circulation was characterized by a submesoscale circulation. Observed currents also showed fluctuations with periods of 1–15 days. The inferred phase speed and wavelength for the wave with a period of 4.4 day are 10.4 km d−1 and 45.8km, respectively, which are consistent with the topographic Rossby wave theory. The persistent warming tendency with corresponding variation of salinity based on background θ‐S properties may be caused by background circulation and divergence of the water column. The warming or cooling episodes were most likely as signatures of isopycnal surface depression or uplifting induced by the moving of mesoscale eddies. Well‐resolved rotary spectra exhibited important nonlinear interactions between inertial and semidiurnal tide in the velocity and temperature records. Amplification of near‐inertial currents in the near bottom is also exposed. These discoveries provided new evidence for the nonlinear interaction and trapped near‐inertial waves by the ridge, which occurred in the deep ocean of the Southern Hemisphere. Such nonlinear interaction may represent a significant energy loss pathway for the internal waves, and part of the decay of such motion would likely result in increased mixing to maintain the abyssal stratification. Enhanced near‐inertial motions can play a major role for the local advection of hydrothermal plumes.