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  • Author or Editor: Min Chen x
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Hua Zheng
,
Xiao-Hua Zhu
,
Min Wang
,
Juntian Chen
,
Feng Nan
, and
Fei Yu

Abstract

Abyssal vorticity balance in the northeast South China Sea was assessed for over a year based on observations from 28 current- and pressure-recording inverted echo sounders distributed west of the Luzon Strait. The regional first-order balance was dominated by the planetary vorticity flux and bottom pressure torque, which reflect the external and internal dynamics of abyssal circulation. Vertical motion considerably contributed to the planetary vorticity flux, whereas the contribution of horizontal motion was negligible. Positive and negative planetary vorticity fluxes dominate the areas along the eastern and western boundaries, indicating upward and downward vertical transport, respectively. The opposite planetary vorticity fluxes in the different areas were accompanied by different current patterns; regional anticyclonic and cyclonic characteristics appeared near the western and eastern boundaries, respectively, owing to the deep topography as the abyssal current followed the boundary. The planetary vorticity flux near the eastern boundary was substantial in spring and autumn; in contrast, along the western boundary it was enhanced only in spring. Deep eddies played important roles in planetary vorticity flux and regional vorticity balance. The results of this study reveal the formation dynamics of abyssal circulation in the South China Sea as well as its spatiotemporal distributions, providing a more detailed description of abyssal circulation.

Significance Statement

The deep South China Sea (SCS) is a nearly enclosed basin characterized by cyclonic abyssal circulation. Based on the observations from 28 current- and pressure-recording inverted echo sounders distributed west of the Luzon Strait, the vorticity balance in the deep SCS was clarified. The planetary vorticity flux and bottom pressure torque maintain a first-order balance of vorticity, which acts as the external and internal dynamics of the abyssal circulation. The study describes the temporal variability and spatial distribution of vorticity terms in the deep ocean west of the Luzon Strait, which may contribute to a more detailed understanding of abyssal circulation formation and its evolution.

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Hua Zheng
,
Xiao-Hua Zhu
,
Ruixiang Zhao
,
Juntian Chen
,
Min Wang
,
Qiang Ren
,
Yansong Liu
,
Feng Nan
,
Fei Yu
, and
Jae-Hun Park

Abstract

Typhoon Mangkhut crossed the northeastern South China Sea (SCS) in September 2018 and induced energetic near-inertial waves (NIWs) that were captured by an array of 39 current- and pressure-recording inverted echo sounders and two tall moorings with acoustic Doppler current profilers and current meter sensors. The array extended from west of the Luzon Strait to the interior SCS, with the path of the typhoon cutting through the array. NIWs in the interior SCS had lower frequency than those near the Luzon Strait. After the typhoon crossed the SCS, Mangkhut-induced near-inertial currents in the upper ocean reached over 50 cm s−1. NIWs traveled southward for hundreds of kilometers, dominated by modes 2 and 3 in the upper and deep ocean. The horizontal phase speeds of mode 2 were ∼3.9 and ∼2.5 m s−1 north and south of the typhoon’s track, respectively, while those of mode 3 were ∼2.1 and ∼1.7 m s−1, respectively. Mode 5 was only identified in the north with a smaller phase speed. Owing to different vertical group velocities, the energy of mode-2 NIWs reached the deep ocean in 20 days, whereas the higher-mode NIWs required more time to transfer energy to the bottom. NIWs in the north were trapped and carried by a westward-propagating anticyclonic eddy, which enhanced the near-inertial kinetic energy at ∼300 m and lengthened the duration of energetic NIWs observed in the north.

Significance Statement

Near-inertial waves (NIWs), generally caused by wind (e.g., typhoons and monsoons) in the upper ocean, are one of the two types of energetic internal waves widely observed in the ocean. After their generation near the surface, energetic NIWs propagate downward and equatorward, thereby significantly contributing to turbulent mixing in the upper and deep ocean and acting as a mechanism of energy transfer from the surface to the deep ocean. The unprecedented NIW observations in the South China Sea describe the generation, propagation, and vertical normal modes of typhoon-induced NIWs in the upper and deep oceans, and contribute to knowledge regarding the dynamic responses of abyssal processes to typhoons.

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Min Wang
,
Xiao-Hua Zhu
,
Hua Zheng
,
Juntian Chen
,
Zhao-Jun Liu
,
Qiang Ren
,
Yansong Liu
,
Feng Nan
,
Fei Yu
, and
Qiang Li

Abstract

Using a large-scale observation array of 27 simultaneous pressure-recording inverted echo sounders (PIESs), the standing wave features of the mode-1 M2 internal tide west of the Luzon Strait (LS) were identified. These features exhibited nonmonotonic spatial phase shifts and half-wavelength amplitude modulation, resulting in spatially varying amplitudes under PIES observations, which have not been previously observed in field observations west of the LS. Satellite altimeter measurements also identified standing-wave patterns consistent with the PIES observations. These patterns emanated from interference between the northwestward and southeastward beams from the LS and the slope of the southern Taiwan Strait, respectively. Near the LS, the two beams superimposed into partial standing waves, whereas the superimposed waves tended to become perfect standing waves near the slope of the southern Taiwan Strait. The nodes and antinodes of the wave shifted under the influence of an anticyclonic eddy. The eddy-induced background current modified the phase speed of the internal tides, and the superimposed standing-wave nodes and antinodes deflected clockwise. The node shifted during three anticyclonic eddy events, and two stations on two sides of the wave node showed opposite variations in amplitude.

Significance Statement

The internal tidal constituent (M2) propagating in opposite directions can result in standing waves, which have been frequently observed in global oceans but were absent west of the Luzon Strait (LS). Our observations (based on a large-scale array west of the LS) discovered a standing M2 internal tide, which stems from interference between the northwestward beams emanating from the LS and southeastward beams from the slope of the southern Taiwan Strait. Anticyclonic eddies play important roles in adjusting the amplitude of internal tides by deflecting the standing-wave nodes and antinodes clockwise. The study facilitates the understanding of the energy distribution and mixing processes west of the LS and provides a fresh perspective on the dynamic relationship between mesoscale perturbations and internal tides.

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Hua Zheng
,
Xiao-Hua Zhu
,
Juntian Chen
,
Min Wang
,
Ruixiang Zhao
,
Chuanzheng Zhang
,
Ze-Nan Zhu
,
Qiang Ren
,
Yansong Liu
,
Feng Nan
, and
Fei Yu

Abstract

Topographic Rossby waves (TRWs) play an important role in deep-ocean dynamics and abyssal intraseasonal variations. Observational records from 15 current- and pressure-recording inverted echo sounders (CPIESs) and two moorings deployed in the northern Manila Trench (MT), South China Sea (SCS), for over 400 days were utilized to analyze the widely existing near-21-day bottom-trapped TRWs in the trench. The TRWs were generally generated in winter and summer, dominated by perturbations in the upper ocean. Kuroshio intrusion and its related variabilities contributed to the perturbations in winter, whereas the perturbations generated north of Luzon Island dominated in summer. Eddies north of Luzon propagated northwestward in the summer of 2018; however, these eddies caused the Kuroshio meanderings in the Luzon Strait (LS) in the summer of 2019. The variations in the Kuroshio path and the Kuroshio-related eddies induced TRWs in the deep ocean in regions with steep topography. However, the spatiotemporal distributions of TRWs were complex owing to the propagation of the waves. Some cases of TRWs showed no relation to the local upper-layer perturbations but propagated from adjacent regions. Some of these TRWs were induced by perturbations in the upper ocean in adjacent regions, and propagated anticlockwise in the MT with shallow water to their right, while others may be related to the intraseasonal variations in deep-water overflow in the LS and propagated northward. This study suggests that the Kuroshio and Kuroshio-related eddies significantly contribute to the dynamic processes associated with intraseasonal variations in the deep SCS through the generation of TRWs.

Significance Statement

Topographic Rossby waves (TRWs) are fluctuations generated when water columns travel across sloping topography under potential vorticity conservation. Based on observations of 15 current- and pressure-recording inverted echo sounders (CPIESs) and two moorings in the northern Manila Trench (MT) in the South China Sea (SCS), TRWs with periods of approximately 21 days were observed and analyzed. This study describes the generation, propagation, and spatiotemporal distribution of TRWs west of the LS and confirms that regional Kuroshio meanderings and upper eddies play important roles in the dynamic processes associated with intraseasonal variations in the deep SCS; the study may thus contribute to knowledge on the dynamic response of the abyssal current to mesoscale perturbations in the upper ocean.

Free access
Min Wang
,
Xiao-Hua Zhu
,
Hua Zheng
,
Juntian Chen
,
Ruixiang Zhao
,
Zhao-Jun Liu
,
Qiang Ren
,
Yansong Liu
,
Feng Nan
,
Fei Yu
,
Jianfeng Wang
, and
Qiang Li

Abstract

Energetic internal tides (ITs) are generated from the Luzon Strait (LS) and propagate westward into the South China Sea (SCS). Owing to the lack of large-scale synchronous measurements, the propagation features and seasonal variations of diurnal ITs remain unclear. From 2018 to 2019, mode-1 diurnal ITs west of the LS were continuously observed using a large-scale moored array of 27 pressure-recording inverted echo sounders (PIESs) and a thermistor chain. Measurements confirmed that diurnal ITs radiate from the LS with a north–south asymmetrical pattern, with the most energetic channel located in the middle and south of the LS. The total energy radiated into the SCS across 120°E is 2.67 GW for the K1 ITs and 1.54 GW for the O1 ITs, approximately 2 times larger than those inferred from satellite observations. K1 dominates among the diurnal ITs, with its maximum isopycnal displacement (amplitude) and energy input to the SCS being the strongest in summer (i.e., 16.3 m and 2.81 GW, respectively). The propagation speed of K1 is higher in summer and autumn along the main channel (i.e., 4.33and 4.36 m s−1, respectively). Seasonal stratification and circulation play important roles in the seasonal variation of amplitude and propagation speed of the K1 ITs. The seasonal variability of diurnal-band ITs, which includes all diurnal constituents, is location-dependent and primarily results from the superposition of the K1 and P1 ITs. In particular, vertical displacement is strong in summer and winter along the main channel of the K1 and P1 ITs. The seasonal amplitude of K1 can modulate this seasonal feature.

Significance Statement

Internal tides (ITs) are internal waves at tidal frequencies. The Luzon Strait (LS) is one of the most energetic sites to generate large-amplitude ITs. The ITs propagate into the South China Sea (SCS), interact with mesoscale eddies, large-scale circulations, etc., and influence local hydrodynamics as well as ecosystem and sediment transport. This motivated an observation plan to investigate the ITs at the western entrance of the LS. From June 2018 to August 2019, an array of 28 PIESs was deployed in the northeastern SCS, almost covering the western entrance of the LS, to investigate the propagation properties of ITs including their amplitude, phase speed, wavelength, propagation direction, and energy fluxes and their annual and seasonal variations. Here, we primarily focus on the mode-1 diurnal ITs. The new insights enrich our understanding of IT dynamics and seasonal variations and support further improvements in numerical simulations.

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