Scale Transition From Geostrophic Motions to Internal Waves in the Northern South China Sea

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)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(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 SummaryThe 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).

Spatiotemporal Characteristics and Generation Mechanisms of Submesoscale Currents in the Northeastern South China Sea Revealed by Numerical Simulations

“…The comparison shows that both the OFES and observed KE spectra scale like k −2 at the submesoscale range (wavelength less than 80 km), which agrees with that predicted by surface quasi‐geostrophic theory modified by ageostrophic advection (Callies & Ferrari, ). The similar KE spectral features have also been reported by independent ADCP observations in the northern SCS (Cao et al, ). The above result demonstrates that the OFES simulation is able to at least capture the basic features of the submesoscale dynamics.…”

“…Based on concurrent hydrographic and microstructure observations along a high-resolution (~2 km) transect across this anticyclonic eddy, Zhong et al (2017) suggested that the submesoscales possibly produced by mesoscale strain can cause vertical transport at least 1 order of magnitude greater than that by mesoscale eddies; the study of Yang et al (2017) further pointed out that the elevated mixed-layer turbulent dissipation rate in periphery of the anticyclonic eddy was possibly furnished by these strong unbalanced submesoscales. In addition to observations, some basic features of submesoscales have also been discussed by several high-resolution simulation studies (e.g., Dong & Zhong, 2018;Li et al, 2019;Cao et al, 2019). Despite these efforts made on submesoscales in the SCS in recent years, the statistical spatiotemporal characteristics are largely unrevealed, and generation mechanisms remain elusive.…”

Role of Mixed‐Layer Instabilities in the Seasonal Evolution of Eddy Kinetic Energy Spectra in a Global Submesoscale Permitting Simulation

“…Many studies have observed a break in the KE spectrum slope ( 3 E k  in mesoscales to 2 E k  at relatively smaller wavelengths), which is due to the transition from a geostrophically balanced flow regime to a flow regime dominated by ageostrophic motions including inertia-gravity waves (Callies & Ferrari, 2013;Cao et al, 2019;Qiu et al, 2018;Sukhatme et al, 2020). However, the present study shows that geostrophic KE spectra in the upper ocean can also exhibit the coexistence of 3 E k  and 2 E k  power-laws, and this behavior is due to the seasonality in the MLI activity.…”

Seasonal Modulation of Submesoscale Kinetic Energy in the Upper Ocean of the Northeastern South China Sea

“…Yang et al, 2016;Zhang et al, 2016). In comparison, our knowledge of submesoscales in the NESCS is still poor, partially due to the difficulty to conduct targeted submesoscale observations here under the interference of strong internal tides (e.g., Cao et al, 2019;Zhao, 2014). Although efforts have been made in recent years to investigate submesoscales in the NESCS, existing studies are based on either numerical simulations (Dong & Zhong, 2018;J.…”

“…Although efforts have been made in recent years to investigate submesoscales in the NESCS, existing studies are based on either numerical simulations (Dong & Zhong, 2018;J. Li et al, 2019;Lin et al, 2020;Zhang et al, 2020) or instantaneous observations (Cao et al, 2019;Zhong et al, 2017). Due to the lack of long-term submesoscale-resolving measurements, seasonal modulation of submesoscale quantities in the NESCS remains still obscure.…”