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

Abstract

Interactions between the open ocean and marginal seas have been suggested to be critical to the redistribution and dissipation of global energy. Here, we propose a mechanism for the upper open ocean influencing the deep flow in marginal seas that hinges on the formation and propagation of topographic Rossby waves (TRWs). Observations and high‐resolution simulations suggest substantial intraseasonal variability with periods of 5–60 days associated with the deep flow over continental slopes of the northern South China Sea (NSCS). These fluctuations generally account for over 40% of the total deep kinetic energy variability and the number can reach 70% over the slopes to the west of the Luzon Strait, southwest of the Dongsha Islands, and northeast of the Xisha Islands. By examining the spatiotemporal features of the fluctuations, we demonstrate that the intraseasonal variability of the deep flow in the NSCS is closely associated with TRWs. Using a recently developed multiscale energetics analysis in combination with a ray tracing model, we show that the energy sources of TRWs can be traced back to the east of the Dongsha Islands, where the Kuroshio intrusion and related eddies energize the TRWs primarily through pressure work. These waves propagate westward across the NSCS and drive the intraseasonal variability of the deep flow over continental slopes. Our findings highlight an energy pathway from the open ocean western boundary current to the abyssal marginal sea that could modulate regional circulation as well as exchanges between major ocean basins.

Coherent Interannual–Decadal Potential Temperature Variability in the Tropical–North Pacific Ocean and Deep South China Sea

“…Since the variabilities of deep‐water exchange between marginal sea and open ocean have often been linked to upper‐layer processes through vertical coupling (Cai et al., 2023; Hamilton et al., 2019; Quan et al., 2021; Tenreiro et al., 2018), we further computed the deep‐layer LST using satellite‐based ocean bottom pressure difference (∆OBP) between the east (21°N–23°N, 122°E–123°E) and west (19°N–21°N, 120°E–121°E) of the Luzon Strait. The above three modes account for about 92% of the total variance of ∆OBP, and the time series of PC1 (M1) largely correlates (Figure 4e; r = 0.76) with ∆OBP, suggesting that the coherent modes of SSTAs in the NPO and TPO potentially influence the deep‐layer processes in the SCS.…”

“…Since the variabilities of deep-water exchange between marginal sea and open ocean have often been linked to upper-layer processes through vertical coupling (Cai et al, 2023;Hamilton et al, 2019;Quan et al, 2021; ∆OBP contains barotropic components associated with sea level differences in the upper layer and baroclinic components related to cross-strait density differences in the lower layer. Previous studies have confirmed that both the barotropic and baroclinic components of pressure contribute to the variability of the deep-layer LST (Cai et al, 2023;Song, 2006;Zhou et al, 2023;Zhu, Yao, et al, 2022).…”

On the Genesis of the South China Sea Mesoscale Eddies

“…Note that the amplitude of −∇•Q 1 P is comparable with, and even larger than, the canonical transfers in most areas. Recently, Quan et al [40] reported that the dominant energy source for the intraseasonal variability in the deep layer of the SCS was pressure work. The results from [40] and this study highlight that nonlocal eddy generation by pressure work is an important factor that should be considered when investigating eddy dynamics in the SCS.…”

“…Recently, Quan et al [40] reported that the dominant energy source for the intraseasonal variability in the deep layer of the SCS was pressure work. The results from [40] and this study highlight that nonlocal eddy generation by pressure work is an important factor that should be considered when investigating eddy dynamics in the SCS. Different from other regions, the contributions of −∇•Q 1 P and canonical transfers are of opposite signs in the Kuroshio loop region, suggesting that pressure work acts to redistribute the mesoscale eddy energy generated by instabilities there.…”

Formation of the Layered Circulation in South China Sea With the Mixing Stimulated Exchanging Current Through Luzon Strait

“…In the deep and middle layers, the exchanging currents are related to the density difference between the two basins (Qu et al., 2006; Y. T. Song, 2006), such as the numerical simulations (X. Wang et al., 2017; Zhao et al., 2014) showed that the intensified diapycnal mixing in the SCS basin induced the density gradient across the LS and maintained the deep intrusion. It is also noted that the variabilities of deep water exchange between marginal sea and open ocean have often been linked to the upper layer processes through vertical coupling (Hamilton et al., 2019; Quan et al., 2021; Tenreiro et al., 2018), such as the deep exchanging flow through the Yucatan Channel was well correlated with the variations in the upper Loop Current in the Gulf of Mexico (Bunge et al., 2002; Chang & Oey, 2011; Ezer et al., 2003).…”