Multiple-scale temporal variations and fluxes near a hydrothermal vent over the Southwest Indian Ridge

doi:10.1007/s11707-015-0529-0

Front. Earth Sci.

2015, Vol. 9

Issue (4) : 691-699 https://doi.org/10.1007/s11707-015-0529-0

RESEARCH ARTICLE

Multiple-scale temporal variations and fluxes near a hydrothermal vent over the Southwest Indian Ridge

Xiaodan CHEN¹,Chujin LIANG^1,^*(

),Changming DONG^2,³,Beifeng ZHOU¹,Guanghong LIAO¹,Junde LI¹

1. State Key Lab of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Hangzhou 310012, China
2. School of Marine Sciences, Nanjing University of Information Science and Technology, Nanjing 210044, China
3. Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90001, USA

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Abstract

A deep-ocean mooring system was deployed 100 m away from an active hydrothermal vent over the Southwest Indian Ridge (SWIR), where the water depth is about 2,800 m. One year of data on ocean temperature 50 m away from the ocean floor and on velocities at four levels (44 m, 40 m, 36 m, and 32 m away from the ocean floor) were collected by the mooring system. Multiple-scale variations were extracted from these data: seasonal, tidal, super-tidal, and eddy scales. The semidiurnal tide was the strongest tidal signal among all the tidal constituents in both currents and temperature. With the multiple-scale variation presented in the data, a new method was developed to decompose the data into five parts in terms of temporal scales: time-mean, seasonal, tidal, super-tidal, and eddy. It was shown that both eddy and tidal heat (momentum) fluxes were characterized by variation in the bottom topography: the tidal fluxes of heat and momentum in the along-isobath direction were much stronger than those in the cross-isobath direction. For the heat flux, eddy heat flux was stronger than tidal heat flux in the cross-isobath direction, while eddy heat flux was weaker in the along-isobath direction. For the momentum flux, the eddy momentum flux was weaker than tidal momentum flux in both directions. The eddy momentum fluxes at the four levels had a good relationship with the magnitude of mean currents: it increased with the mean current in an exponential relationship.

Keywords multiple-scale analysis tidal flux eddy flux

Corresponding Author(s): Chujin LIANG

Just Accepted Date: 26 August 2015 Online First Date: 30 September 2015 Issue Date: 30 October 2015

Cite this article:

Xiaodan CHEN,Chujin LIANG,Changming DONG, et al. Multiple-scale temporal variations and fluxes near a hydrothermal vent over the Southwest Indian Ridge[J]. Front. Earth Sci., 2015, 9(4): 691-699.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-015-0529-0
https://academic.hep.com.cn/fesci/EN/Y2015/V9/I4/691

Fig.1 Location of the hydrothermal vent (red star). The active hydrothermal field is located at (37°47′S, 49°39′E) along the SWIR. The mooring system is about 100 m away from the hydrothermal vent, which is not distinctive from the vent in the figure.

Fig.2 The structure of the mooring system: a downward-looking 300-kHz ADCP, with a temperature sensor (not shown), is fixed at 50 m above the ocean bottom.

Fig.3 Daily-averaged time series of current speed (a), direction (b) and temperature (c) (at 50 m above the ocean floor). Blue and red curves denote daily-averaged and 30-day-moving-smoothed values, respectively.

Fig.4 Spectral density of (a) seawater temperature and (b, c) vertically-averaged currents (b: along-isobath velocity U; c: cross-isobath velocity V). Obviously, semidiurnal tidal signals dominate in both currents and temperature.

Fig.5 Tidal ellipses of the two leading tidal constituents: (a) M2: at 44 m; (b) M2: at 40 m; (c) M2: at 36 m; (d) M2: at 32 m; (e) S2: at 44 m; (f) S2: at 40 m; (g) S2: at 36 m; (h) S2: at 32 m. Tidal vectors turn clockwise.

Fig.6 Heat fluxes at 50 m away from the sea floor: (a) along-isobath direction and (b) cross-isobath direction. Blue indicates tidal heat flux, red represents eddy heat flux and the remaining colors denote cross terms.

Fig.7 Momentum fluxes at four levels: (a) along-isobath, (b) cross-isobath and (c) along-cross-isobath. Blue indicates tidal momentum flux, red represents eddy momentum flux and the remaining colors denote cross terms.

Fig.8 Yearly-averaged eddy flux vs. mean current velocity. (a) Eddy momentum flux in the along-isobath direction. (b) Eddy momentum flux in the cross-isobath direction. The red line is a regression line.

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