Cascade of Internal Wave Energy Catalyzed by Eddy‐Topography Interactions in the Deep South China Sea. Issue 4 (14th February 2020)

• Record Type: Journal Article
• Authors: Hu, Qianwen; Huang, Xiaodong; Zhang, Zhiwei; Zhang, Xiaojiang; Xu, Xing; Sun, Hui; Zhou, Chun; Zhao, Wei; Tian, Jiwei
• Is Part Of: Geophysical research letters. Volume 47:Issue 4(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-02-14
• Subjects: deep‐ocean near inertial waves -- internal wave -- energy cascade -- eddy‐topography interactions -- nonlinear wave‐wave interactions -- South China Sea; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2019GL086510
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: The intensity of deep‐ocean mixing critically shapes the global overturning circulation, but the energy cascade participating in the elevated mixing above rough topography remains poorly understood. Using 350‐day moored observations, the energy cascade triggered by eddy‐topography interactions is for the first time revealed in the deep South China Sea. The deep‐ocean observations show bottom‐intensified near‐inertial wave (NIW) pulses during periods of eddy occurrences. Eddy‐induced NIWs appear to catalyze nonlinear wave‐wave interactions, which further cascade energies from NIWs and internal tides to higher‐frequency internal waves (HFIWs) above fk 1 frequency. Corresponding to the HFIW enhancement, the kinetic energy spectrum at 3–9 cycles per day increases by 2–9 times, and the spectral slope becomes ~ −1 rather than ~ −2, as predicted by the Garrett‐Munk oceanic internal wave spectrum. A cascade of internal waves above rough topography potentially promotes extracting energy from eddies and tides and dissipating this energy to mix deep waters. Plain Language Summary: Deep‐ocean mixing plays a pivotal role in the ocean meridional overturning circulation, ocean heat transport, and biogeochemical cycle. However, the energy cascade processes above rough topographic features and their contributions to mixing are still the key issues in oceanography. Based on 350 days of in situ mooring observations obtained over rough topography in the deep South China Sea, four pulses of near‐inertial waves (NIWs) corresponding to the passages of mesoscale eddies are revealed. Furthermore, the peaks of higher‐frequency internal waves (HFIWs) in the f interaction frequencies also stand out. NIW pulses are believed to be driven by lee wave dissipation and mixing activities resulting from eddy‐topography interactions; meanwhile, HFIW generation appears to be associated with nonlinear wave‐wave interactions. The observational results about the energy cascade induced by eddy‐topography interactions are expected to be useful for improving parameterizations of deep‐ocean mixing in models of oceanic overturning. Key Points: Deep‐ocean NIW and HFIW pulses during periods of eddy occurrences were observed over the rough topography in the SCS NIWs were bottom intensified and rotated counterclockwise, triggered by eddy‐topography interactions Deep‐ocean NIW and HFIW pulses along with the internal tide damping indicate nonlinear wave‐wave interactions over rough topography