Scale Transition From Geostrophic Motions to Internal Waves in the Northern South China Sea. Issue 12 (18th December 2019)

• Record Type: Journal Article
• Authors: Cao, Haijin; Jing, Zhiyou; Fox‐Kemper, Baylor; Yan, Tong; Qi, Yiquan
• Is Part Of: Journal of geophysical research. Volume 124:Issue 12(2019)
• Page Start: 9364
• Page End: 9383
• Publication Date: 2019-12-18
• Subjects: Balanced to unbalanced transition -- Wave‐vortex decomposition -- Spectral analysis -- Northern South China Sea; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2019JC015575
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: The eddy‐abundant circulation in the northern South China Sea (NSCS) tends to be dynamically complex due to monsoon forcing, Kuroshio intrusion, and emitted internal waves from the Luzon Strait. This study uses 13‐year shipboard acoustic Doppler current profiler measurements (2004–2016), orbital altimeter data, and high‐resolution model output to perform wave‐vortex decompositions and investigate the scale of transition from dominantly geostrophic flows to internal wave motions in the northern South China Sea. The upper ocean kinetic energy spectra transition on scales exceeds 200 km. This large scale of transition is attributed to the energetic low‐mode internal waves (e.g., internal tides and inertia‐gravity waves). However, inconsistencies in the decomposition reveal that the assumptions of homogeneity and isotropy required for the 1‐D decomposition (Bühler et al., 2014) are sufficiently violated at smaller scales to affect the subdominant member of the decomposition on scales below 100 km. A method for direct quantification of the consequences by degree of violation using bootstrapping of the 2‐D model data is developed and illustrated. Observed and modeled sea surface height spectra flatten at scales smaller than 125 km, which is found in the model to be due to the coherent, semidiurnal internal waves. The large scale of transition between geostrophic and wave motions in the South China Sea is an irreducible uncertainty for altimeter velocities (e.g., the Surface Water and Ocean Topography mission). Plain Language Summary: The scale transition from balanced geostrophic flows to unbalanced motions is of great importance for understanding the circulation system of the South China Sea where both eddies and internal waves are particularly active. Based on observations and high‐resolution simulation, this paper investigates the wave number spectra of kinetic energy and sea surface height variance in the northern South China Sea. The large transition length scale (>200 km) from balanced geostrophic flows to unbalanced motions is due to strong low‐mode internal waves in the upper ocean. The violations of the assumptions for the 1‐D decomposition (stationarity, homogeneity, and horizontal isotropy) are also assessed using the 2‐D model data. With the model output, the low‐mode semidiurnal internal tides are found to have significant impacts on the sea surface height variance at scales of ~125 km. These results are directly relevant to the efficiency of diagnosing geostrophic flows from altimeters (e.g., the upcoming Surface Water and Ocean Topography mission). Key Points: The transition of KE from geostrophic flows to unbalanced motions is found to be larger than 200 km in the upper ocean of the NSCS The frequency‐wavenumber analysis reveals the dominated processes for the transition The semidiurnal internal tides are found to have a significant contribution to the variance of sea surface height in the NSCS