Abstract
Lens-shaped structures within thermocline potentially play a significant role in subsurface transport of mass, heat, and salt in the global ocean. Whilst such structures have been documented in many oceanic regions, none has been observed in the China Seas. This study reports on observations of a lens-shaped structure within thermocline in the southwestern South China Sea in September 2007. This structure had a maximum thickness of approximately 60 m and a horizontal extent exceeding 220 km. This lens was peculiar in that its size is larger than most similar structures documented in the literature. The lens core was characterized by well-mixed water with higher temperature (~28.8 °C), lower salinity (~33.3) and lower potential vorticity (PV) compared to the surrounding waters. Based on an ocean reanalysis, possible generation mechanism of the lens is explored by examining the evolution of surface and subsurface thermohaline properties, and an analysis of vertical PV flux. The lens was likely generated by a mixture of the local mixed-layer water and the water from the coastal jet separation site.
Highlights
Lens-shaped structures within thermocline potentially play a significant role in subsurface transport of mass, heat, and salt in the global ocean
Along transect A, the thermocline, halocline and pycnocline are tilted downward from west to east (Fig. 2a–c). Such a downward tilt toward east is in agreement with the sea level distribution (Supplementary Fig. S1d), which shows transect A extending from the periphery of a cyclonic eddy to an anticyclonic eddy
The western half of transect C cuts across the center of cyclonic eddy; a larger uplift of thermocline is observed compared to that at transect A. This leads to a steeper pycnocline near station C03 (Fig. 2f), implying a larger horizontal density gradient and a stronger vertical shear of the velocity according to the thermal wind relation
Summary
Lens-shaped structures within thermocline potentially play a significant role in subsurface transport of mass, heat, and salt in the global ocean Whilst such structures have been documented in many oceanic regions, none has been observed in the China Seas. This study reports on observations of a lens-shaped structure within thermocline in the southwestern South China Sea in September 2007 This structure had a maximum thickness of approximately 60 m and a horizontal extent exceeding 220 km. Within the China Seas, Zhang et al.[13] reported two subsurface lens-shaped water bodies in the South China Sea (SCS), and Xie et al.[14] found a subsurface anticyclonic eddy in the Luzon Strait These eddies were situated in the intermediate layer beneath the thermocline. An eastward jet separates from the coast of Vietnam[18], and an eddy pair associated with the jet is formed
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