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Latest Observation Results on Abyssal Current in South China Sea

June 23, 2021      Author:

Recently, Ph.D. student Zheng Hua from SJTU School of Oceanography had the team's latest observation results on abyssal current and seasonal variability in the deep South China Sea published in Journal of Geophysical Research: Oceans and Journal of Physical Oceanography, titled "Structure and Variability of Abyssal Current in Northern South China Sea Based on CPIES Observations" and "Propagation of Topographic Rossby Waves in the Deep Basin of the South China Sea Based on Abyssal Current Observations", respectively. The first author of the two papers is Zheng Hua and the corresponding author is Zhu Xiaohua, a researcher at SJTU School of Oceanography. SJTU School of Oceanography is the first affiliation.

Researcher Zhu Xiaohua's team deployed 11 CPIESs (Current-Pressure Equipped Inverted Echo Sounders) 4000m below water in the western boundary current area of the deep South China Sea (SCS) deep basin and obtained large-scale and long-term deep-sea observation data. The study based on these data further reveals the fine structure, seasonal variation characteristics and temporal variability of the flow field in the deep basin of SCS, which provides a reference for studying the complex multi-scale dynamic process in this region. The research results were published in Journal of Geophysical Research: Oceans in April 2021.

The data demonstrates that the deep current in SCS has obvious seasonal variation characteristics, and the study clearly shows for the first time that the low-frequency Topographic Rossby waves (TRWs) from northeast to southwest in the deep basin of SCS, which fills the void in previous studies and is of great significance to research on the deep current in SCS and its relationship with the oceanic dynamic process in the middle and upper layers. The result was published in Journal of Physical Oceanography in June 2021.

Author: Zheng Hua

Source: School of Oceanography, SJTU

Translated by Zhou Rong

Proofread by Xiao Yangning, Fu Yuhe

 

Structure and Variability of Abyssal Current in Northern South China Sea Based on CPIES Observations

ABSTRACT:

The abyssal current is important for water renewal and energy conversion in the deep South China Sea (SCS), and it potentially contributes to the upper and middle circulations. A mooring array including 11 current and pressure-recording inverted echo sounders was deployed in the northern SCS between July 2016 and April 2019 to observe large-scale and long-term abyssal currents. The yearly averaged current flows southwestward following the boundary with a maximum velocity of 2.25-2.52 cm/s. The spatial structure exhibits a weaker and wider current in the north, where topography is subdued, whereas the current enhances and narrows on the steep slopes. The southwestward current is strong in the summer and autumn but is insignificant in the winter and spring. In the autumn, the current is narrower and closer to the boundary than that in the summer, and a northeastward countercurrent is found ∼60 km away from the western boundary. The southwestward and northeastward currents constitute the cyclonic circulation. Drastic temporal variability is observed in the deep ocean. The abyssal current is dominated by semidiurnal tides, diurnal tides, and near-inertial waves (NIWs) in the high-frequency band. Bursts of NIWs are observed in the deep ocean after two typhoons, and the NIWs propagate southwestward at a velocity of 2.4 m/s. The low-frequency band is dominated by a westward-propagating 70-day fluctuation following the characteristics of topographic Rossby waves.

 

Propagation of Topographic Rossby Waves in the Deep Basin of the South China Sea Based on Abyssal Current Observations

ABSTRACT:

Topographic Rossby waves (TRWs) are oscillations generated on sloping topography when water columns travel across isobaths under potential vorticity conservation. Based on our large-scale observations from 2016 to 2019, near 65-day TRWs were first observed in the deep basin of the South China Sea (SCS). The TRWs propagated westward with a larger wavelength (235 km) and phase speed (3.6 km/day) in the north of the array and a smaller wavelength (80 km) and phase speed (1.2 km/day) toward the southwest of the array. The ray-tracing model was used to identify the energy source and propagation features of the TRWs. The paths of the near 65-day TRWs mainly followed the isobaths with a slightly downslope propagation. The possible energy source of the TRWs was the variance of surface eddies southwest of Taiwan. The near 65-day energy propagated from the southwest of Taiwan to the northeast and southwest of the array over ~100-120 and ~105 days, respectively, corresponding to a group velocity of 4.2-5.0 and 10.5 km/day, respectively. This suggests that TRWs play an important role in deep-ocean dynamics and deep current variation, and upper ocean variance may adjust the intraseasonal variability in the deep SCS.