Influence of Abyssal Mixing on the Multilayer Circulation in the South China Sea

Abstract

By parameterizing the abyssal mixing as the exchange velocity (entrainment/detrainment) between the middle and deep layers of the South China Sea (SCS), its effects on the multilayer circulation are examined. Results indicate that the cyclonic circulation in the deep SCS appears only when the mixing induces an entrainment of at least 0.72 Sv (1 Sv ≡ 106 m3 s−1) from the deep to the middle layer, which is equivalent to a diapycnal diffusivity of 0.65 × 10−3 m2 s−1 or a net input rate of gravitational potential energy (GPE) of 6.89 GW, respectively. It is also found that tidal mixing in the SCS is stronger than the threshold for the generation of the cyclonic abyssal circulation, but the pattern and evolution of the deep circulation and meridional overturning circulation also depend on the spatiotemporal variability of the mixing. Moreover, the abyssal mixing is able to intensify the anticyclonic circulation in the middle layer but weaken the cyclonic circulation in the upper layer. Vorticity analysis suggests that the upward net flux induced by the abyssal mixing leads to vortex stretching (squeezing) and modulates the pressure gradient by redistributing the layer thickness, hence affects the pattern and strength of the circulation in the middle (deep) layer of the SCS, respectively. The depth-integrated effect of the thickness variation can modulate the pressure gradient across all layers and hence influence the upper-layer circulation.

Structure and Variability of Abyssal Current in Northern South China Sea Based on CPIES Observations

“…mixing (approximately 10 −3 m 2 /s) in the deep ocean intensifies the abyssal circulation and the overturning circulation in the SCS (Quan & Xue, 2019;Tian et al, 2009; X. Wang et al, 2017).…”

Influence of the Kuroshio Intrusion on Deep Flow Intraseasonal Variability in the Northern South China Sea

“…For example, interactions between the TRWs and planetary Rossby waves probably account for the deep upwelling associated with meridional overturning circulation in the abyssal SCS (Shu et al., 2014). Moreover, TRWs and wave‐related mixing play a vital role in the generation and evolution of deep circulation in the SCS (Quan & Xue, 2019), as well as in the vertical coupling of the SCS circulation between the upper and middle layers (Quan & Xue, 2018). All these factors may lead to more active SCS circulation that could modulate the Indo‐Pacific exchange, thereby affecting the regional climate and ecosystem.…”

Deep-Current Intraseasonal Variability Interpreted as Topographic Rossby Waves and Deep Eddies in the Xisha Islands of the South China Sea

“…The Luzon Strait is the only deep channel connecting the SCS and western Pacific, with 2400 m maximum water depth. Cyclonic deep circulation is driven by the Luzon Strait overflow and intensified mixing in the deep basin of the SCS (Tian et al 2009;Wang et al 2011;Lan et al 2015;Gan et al 2016;Quan and Xue 2019). Previous studies have shown that the deep currents in the SCS exhibit strong intraseasonal variability, which is thought to be related to the Luzon Strait intrusion in the deep layer (Zhao et al 2014;Zhang et al 2014;Zhou et al 2017Zhou et al , 2020Ye et al 2019).…”