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

Abstract

Four 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.

Sea level anomalies affect the ocean circulation at abyssal depths

“…For example, reversal currents are observed in the Kane Gap connecting deep basins in the Equatorial Atlantic 9 . Similar variability at synoptic time scales is found in the abyssal layer of the Northeast Pacific 11 and Southwest Indian Ocean 40 . Repeated moorings deployed in the Vema Channel since 1979 28 do not show significant long-term trends in the abyssal velocity.…”

The Size Fractionation and Speciation of Iron in the Longqi Hydrothermal Plumes on the Southwest Indian Ridge

“…Another possible reason may be the currents in the studied area that are quite fast and prohibit the aggregation and deposition of colloids and particles. It has been observed that the current velocity was over 6 cm/s with the maximal value of 19.1 cm/s near the Longqi field (Liao et al ).…”

“…The possible sources include diffuse flow adjacent to the high-temperature vents and/or in situ biological activities. Since the Longqi hydrothermal field was found in 2007 (Tao et al, 2012), various studies, including the physical, chemical, and biological characteristics of the hydrothermal fluids, plumes, and the diffuse flow, have been carried out (Ji et al, 2017;Li et al, 2014Li et al, , 2016Liao et al, 2016;Pan et al, 2016;Peng et al, 2011;Tao et al, 2012Tao et al, , 2014Wang et al, 2012Wang et al, , 2015. It was found that there were no significant differences in microbial compositions between the hydrothermal plume samples and the ambient seawater (Li et al, 2016).…”

Intensified Deep Ocean Variability Induced by Topographic Rossby Waves at the Pacific Yap‐Mariana Junction

“…Theoretical studies have demonstrated that the presence of sloping topography can derive bottom‐trapped topographic Rossby waves (TRWs; Rhines, ). TRWs have been observed by field measurements in different ocean basins (e.g., Hamilton, , , ; Hogg, , ; Kolodziejczyk et al, ; Liao et al, ; Louis et al, ; Pickart, ; Shu et al, ; Timmermans et al, ; Zhao & Timmermans, ). In these measurements, the vertical structure and variability of deep current and/or isotherm displacement conform to TRW theories of intensification with depth and highly vertical coherence throughout the deep‐water column.…”