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

Abstract

Although both geostrophic‐balanced mesoscale eddies and unbalanced small‐scale processes have been well studied in the northeastern South China Sea (NE‐SCS), less attention has been devoted to the submesoscales in between (i.e., O(1–10 km)), which is recognized as an important conduit connecting the balanced and unbalanced motions. Based on the output from a 1/30° OGCM simulation, spatiotemporal characteristics and generation mechanisms of submesoscales in the NE‐SCS are investigated in this study. Through examining the submesoscale relative vorticity and vertical velocity, the regions southwest of Taiwan (ST) and Luzon Strait (LS) are identified as two hot spots of submesoscales in the NE‐SCS. Seasonally, the submesoscales in region ST are much stronger in winter than summer, while those in LS do not show a significant seasonality. Statistical analysis suggests that the strength of submesoscales in regions ST and LS is highly correlated with the product of mixed‐layer depth and mesoscale strain rate (MSR) and with the MSR itself, respectively. By conducting theoretical scaling and energetics analysis, the authors find that the mixed‐layer instability whose strength is determined by the combination of mixed‐layer depth and MSR and the barotropic instability associated with current‐islands interactions are the dominant generation mechanisms of submesoscales in the above two regions, respectively. Further examinations of the submesoscale energy budget indicate that, to keep a balanced state, the generated submesoscales have to be dissipated by a forward energy cascade, highlighting the important role of submesoscales in the energy balance of the NE‐SCS circulation.

On Contributions of Multiscale Dynamic Processes to the Steric Height in the Northeastern South China Sea as Revealed by Moored Observations

“…The cutoff periods of MIAO ET AL. the band-pass filtering for these multiscale components are set as 15-150 days, 2-15 days, 20-27 h, 10-14 h, and 4-10 h, respectively, following previous studies (e.g., Guan et al, 2014;Zhang et al, 2016Zhang et al, , 2020Zhang et al, , 2021.…”

“…In this study, the above objective is accomplished through quantifying the respective RMS SH of different dynamic processes based on 2-year moored temperature data in the northeastern South China Sea (NSCS). Existing observational and modeling studies suggest that the NSCS is abundant with strong ITs as well as complex mesoscale to submesoscale motions (Guan et al, 2014;Lin et al, 2020;Zhang et al, 2016Zhang et al, , 2020Zhao et al, 2016). As such, it is a good testbed to examine how feasible it is to adopt the future SWOT SSH data to quantitatively identify the quasi-geostrophically balanced smaller mesoscale eddies and larger submesoscales from a strong IGW background.…”

The Seasonality of Submesoscale Energy Production, Content, and Cascade

“…Many studies have demonstrated a significant role of submesoscales in ocean energy cascade after Capet et al (2008b). The contribution to inverse and forward energy cascades from submesoscales is studied successively in different regions based on high-resolution observations and simulations, such as the subtropical North Pacific (Qiu et al, 2014), Drake Passage , Kuroshio extension (Sasaki et al, 2017), South Indian Ocean (Schubert et al, 2019) and South China Sea (Zhang et al, 2016;Zhang et al, 2020). All of these works emphasize the importance of submesoscales in the ocean energy cascade, and some use the same model simulations analyzed in this study.…”

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

“…Given that deeper (shallower) mixed layers in winter (summer) can store more (less) available potential energy (APE) used for baroclinic instability, submesoscales are theoretically expected to be stronger in winter than summer. This mixed-layer-modulated seasonality has been widely supported by both in situ observations (e.g., Buckingham et al, 2016;Callies et al, 2015;Qiu et al, 2017;Thompson et al, 2016) and high-resolution numerical simulations (e.g., Mensa et al, 2013;Qiu et al, 2014;Sasaki et al, 2014;Su et al, 2018;Zhang et al, 2020). In addition to mixed-layer depth (MLD), the mesoscale strain, which has significant seasonality in some regions, can also modulate the seasonal variation of submesoscales through frontogenesis.…”

“…Therefore, the MLD × MSR may provide as an useful quantity to parameterize the submesoscale KE and BC in the coarse-resolution models. If we convert the BC (i.e., vertical buoyancy flux) into vertical heat flux using the density-temperature relations (Su et al, 2018), it shows that in winter, the submesoscales can cause an equivalent vertical heat flux of ∼47 W m −2 in the upper 150 m. This heat flux is comparable in magnitude with the net surface heat flux in the NESCS (Zhang et al, 2020) and supports the high-resolution simulation-derived results that submesoscales play an important role in the upper-ocean heat budget (Su et al, 2018).…”