The largest CPIES array in the marginal sea: abundant dynamics in the northeast South China Sea

The largest CPIES array in the marginal sea: abundant dynamics in the northeast South China Sea

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Hua Zheng¹, Xiao-Hua Zhu²^,³^,⁴^,^*, Min Wang⁵, Ruixiang Zhao²^,⁴^,⁶^,⁷, Chuanzheng Zhang²^,⁴, Feng Nan⁶^,⁷^,⁸, Fei Yu⁶^,⁷^,⁸^,⁹

Acta Oceanologica Sinica | 2024, 43(1) : 135 - 137

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Acta Oceanologica Sinica | 2024, 43(1): 135-137

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The largest CPIES array in the marginal sea: abundant dynamics in the northeast South China Sea

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Hua Zheng¹, Xiao-Hua Zhu²^,³^,⁴^,^*, Min Wang⁵, Ruixiang Zhao²^,⁴^,⁶^,⁷, Chuanzheng Zhang²^,⁴, Feng Nan⁶^,⁷^,⁸, Fei Yu⁶^,⁷^,⁸^,⁹

Affiliations

¹ Institute of Polar and Ocean Technology, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China

² State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China

³ School of Oceanography, Shanghai Jiao Tong University, Shanghai 200030, China

⁴ Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519082, China

⁵ National Marine Environmental Monitoring Center, Dalian 116023, China

⁶ Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China

⁷ University of Chinese Academy of Sciences, Beijing 100049, China

⁸ Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China

⁹ Laboratory for Ocean and Climate Dynamics, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China

Published: 2024-01-25 doi: 10.1007/s13131-024-2293-z

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Hua Zheng, Xiao-Hua Zhu, Min Wang, Ruixiang Zhao, Chuanzheng Zhang, Feng Nan, Fei Yu. The largest CPIES array in the marginal sea: abundant dynamics in the northeast South China Sea[J]. Acta Oceanologica Sinica, 2024 , 43 (1) : 135 -137 . DOI: 10.1007/s13131-024-2293-z

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The South China Sea (SCS) is a marginal sea connecting the Pacific and Indian oceans and has gained much attention in recent decades. The dynamics in the northeast SCS are considerably influenced by topography, monsoons, tropical cyclones, the Kuroshio intrusion, and water exchange through the Luzon Strait (LS). Recently, an array of 38 current and pressure-recording inverted echo sounders (CPIES) and two moorings are deployed in the northeast SCS from 2016 to 2019 (Fig. 1), constituting the largest CPIES array in the marginal sea to date. We report scientific observations of fundamental dynamics in the northeast SCS based on this large CPIES array, including circulations, eddies, internal waves, and intraseasonal variabilities, as well as their connections and interactions (Fig. 2).

The upper northeast SCS is characterized by the Kuroshio intrusion and active eddies. Studies have reported that the Kuroshio intrudes the SCS via looping, leaking, and leaping paths which contribute to the regional oceanic environment (Nan et al., 2011). Recently, a new kind of summer Kuroshio water intrusion is revealed based on observations. Wind-induced anticyclonic eddies north of Luzon Island known as North Luzon warm eddies (NLWEs) move northwestward in summer 2018 and carry Kuroshio water into the SCS (Zhao et al., 2023). NLWEs contribute to nearly half of the westward LS transport during summer and are critical for regional circulation. Historical statistics indicate that NLWEs occur only from April to October and are strongly related to local winds. Monsoon provides favorable conditions for eddy growth, whereas strong negative wind stress curl is directly responsible for eddy generation. The eddy heat transport (EHT) across the LS was determined based on observational data, and a novel methodology was developed to extend the time series of EHT from sea surface height (Zhao et al., 2024). Additionally, the variability and underlying mechanisms of EHT on both seasonal and interannual time scales were examined.

Internal tides (ITs) and near-inertial waves (NIWs) are two types of energetic internal waves. Semi-diurnal and diurnal ITs, generated by energetic barotropic tides traveling across steep ridges in the LS, have been analyzed in detail (Wang et al., 2023b, c). Most energetic diurnal ITs radiate from the middle and southern LS and extend southwestward. The energy of K₁ and O₁ ITs radiate into the SCS is 2.67 GW and 1.54 GW, respectively—roughly twice as large as the values obtained via satellite observations. Additionally, the M₂ ITs are characterized by the presence of standing waves formed by a northwestward beam from the LS and a southeastward beam from the southern slope of the Taiwan Strait. Influenced by the eddy-induced background flow, two oppositely propagating beams undergo clockwise rotation upon entering the anticyclonic eddy. Such a rotation causes deflection of the nodes and antinodes of standing waves, leading to variability in the standing wave amplitude. Typhoon Mangkhut (2018) crosses the SCS and induces energetic NIWs that travel equatorward for hundreds of kilometers and propagate downward, dominated by Modes 2 and 3 (Zheng et al., 2023). The wavelengths and phase speeds of different modes, which are difficult to obtain from traditional observations, are captured. The NIWs southwest of Taiwan are trapped by a westward-propagating eddy. The bottom-reaching NIWs contribute considerably to the upper and deep dynamics.

Deep-water overflow driven by a pressure gradient across the LS contributes to SCS abyssal circulation. Based on these observations, abyssal circulations in the northeast SCS have been described (Zheng et al., 2021a, 2022b, 2024), and measurements from the Gravity Recovery and Climate Experiment have been evaluated (Wang et al., 2023a). Deep water flows northward after entering the SCS and then turns southwestward following the deep boundary, forming a cyclonic circulation. The current is narrow and strong near the steep eastern boundary and near the Zhongsha Islands, whereas the current near the subdued northwestern boundary is wide and weak. The current near the LS is enhanced in autumn and early winter, whereas that in the interior SCS is stronger in summer. These different seasonal patterns are caused by the propagation of seasonal variations. Planetary vorticity flux and bottom pressure torque maintain the first-order deep vorticity balance, reflecting the outer and inner forcing of the abyssal circulation.

Abundant abyssal intraseasonal variability has been attributed to topographic Rossby waves (TRWs), with dominant periods increasing from the LS to the interior SCS (Zheng et al., 2021b, 2022a). Nearly 65-day TRWs east of the Zhongsha Islands propagate along the isobaths with a slightly downslope group velocity, showing the dynamic response of abyssal currents to mesoscale perturbations southwest of Taiwan Island. Additionally, nearly 21-day TRWs driven by the Kuroshio meandering and upper eddies are trapped by the Manila Trench. These findings suggest that TRWs act as a mechanism that connects upper-ocean mesoscale perturbations and deep-ocean dynamics.

In conclusion, the dynamics of ITs and abyssal circulation are analyzed in detail based on the large CPIES array. Active upper-layer mesoscale perturbations are found to contribute to the Kuroshio intrusion, adjust the propagation of ITs and NIWs, and induce TRWs. This paper reports the systematic evaluation of northeast SCS dynamics based on unprecedented observations that have great scientific significance for improving our understanding of the spatiotemporal circulation structure and multi-scale dynamic processes in the SCS. In addition, these successful observations in the northeast SCS indicate that a CPIES array is economical and practical for large-scale and long-term oceanic observations.

Funding

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The National Natural Science Foundation of China under contract Nos 41920104006; the Scientific Research Fund of Second Institute of Oceanography, Ministry of Natural Resources under contract Nos JZ2001, XRJH2410, and QNYC2102; the Oceanic Interdisciplinary Program of Shanghai Jiao Tong University under contract No. SL2021MS021; the Global Climate Changes and Air-sea Interaction Program under contract No. GASI-02-PAC-ST-Wwin; the Taishan Scholars Program under contract No. tsqn202306282.

References

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Nan Feng, Xue Huijie, Chai Fei, et al. 2011. Identification of different types of Kuroshio intrusion into the South China Sea. Ocean Dynamics, 61(9): 1291–1304, doi: 10.1007/s10236-011-0426-3

Wang Xuecheng, Zheng Hua, Zhu Xiaohua, et al. 2023a. Validation and evaluation of GRACE-FO estimates with in situ bottom pressure array measurements in the South China Sea. Remote Sensing, 15(11): 2804, doi: 10.3390/rs15112804

Wang Min, Zhu Xiaohua, Zheng Hua, et al. 2023b. Direct evidence of standing internal tide west of the Luzon Strait observed by a large-scale observation array. Journal of Physical Oceanography, 53(9): 2263–2280, doi: 10.1175/JPO-D-23-0043.1

Wang Min, Zhu Xiaohua, Zheng Hua, et al. 2023c. Propagation features of diurnal internal tides west of the Luzon strait revealed by a large PIES array. Journal of Physical Oceanography, 53(12): 2823–2846, doi: 10.1175/JPO-D-22-0206.1

Zhao Ruixiang, Yu Fei, Nan Feng, et al. 2024. Eddy heat transport across the Luzon Strait derived from current- and pressure-recording inverted echo sounder observations. Progress in Oceanography, 222: 103234, doi: 10.1016/j.pocean.2024.103234

Zhao Ruixiang, Zhu Xiaohua, Zhang Chuanzheng, et al. 2023. Summer anticyclonic eddies carrying Kuroshio waters observed by a large CPIES array west of the Luzon Strait. Journal of Physical Oceanography, 53(1): 341–359, doi: 10.1175/JPO-D-22-0019.1

Zheng Hua, Zhang Chuanzheng, Zhao Ruixiang, et al. 2021a. Structure and variability of abyssal current in northern South China Sea based on CPIES observations. Journal of Geophysical Research: Oceans, 126(4): e2020JC016780, doi: 10.1029/2020JC016780

Zheng Hua, Zhu Xiaohua, Chen Juntian, et al. 2022a. Observation of bottom-trapped topographic Rossby waves to the west of the Luzon strait, South China Sea. Journal of Physical Oceanography, 52(11): 2853–2872, doi: 10.1175/JPO-D-22-0065.1

Zheng Hua, Zhu Xiaohua, Wang Min, et al. 2024. Regional abyssal vorticity balance in the northeast South China Sea: External and internal dynamics of abyssal circulation. Journal of Physical Oceanography, 54(1): 265–279, doi: 10.1175/JPO-D-23-0060.1

Zheng Hua, Zhu Xiaohua, Zhang Chuanzheng, et al. 2021b. Propagation of topographic rossby waves in the deep basin of the South China Sea based on abyssal current observations. Journal of Physical Oceanography, 51(9): 2783–2791, doi: 10.1175/JPO-D-21-0051.1

Zheng Hua, Zhu Xiaohua, Zhang Chuanzheng, et al. 2022b. Observation of abyssal circulation to the west of the Luzon strait, South China Sea. Journal of Physical Oceanography, 52(9): 2091–2109, doi: 10.1175/JPO-D-21-0284.1

Zheng Hua, Zhu Xiaohua, Zhao Ruixiang, et al. 2023. Near-inertial waves reaching the deep basin in the South China Sea after typhoon Mangkhut (2018). Journal of Physical Oceanography, 53(10): 2435–2454, doi: 10.1175/JPO-D-22-0136.1

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doi: 10.1007/s13131-024-2293-z

Receive Date：2024-01-01
Online Date：2025-11-15
Published：2024-01-25

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Received：2024-01-01
Accepted：2024-01-25

Funding

The National Natural Science Foundation of China under contract Nos 41920104006; the Scientific Research Fund of Second Institute of Oceanography, Ministry of Natural Resources under contract Nos JZ2001, XRJH2410, and QNYC2102; the Oceanic Interdisciplinary Program of Shanghai Jiao Tong University under contract No. SL2021MS021; the Global Climate Changes and Air-sea Interaction Program under contract No. GASI-02-PAC-ST-Wwin; the Taishan Scholars Program under contract No. tsqn202306282.

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