Topographic Rossby Waves in the Abyssal South China Sea

Abstract

Topographic Rossby waves (TRWs) in the abyssal South China Sea (SCS) are investigated using observations and high-resolution numerical simulations. These energetic waves can account for over 40% of the kinetic energy (KE) variability in the deep western boundary current and seamount region in the central SCS. This proportion can even reach 70% over slopes in the northern and southern SCS. The TRW-induced currents exhibit columnar (i.e., in-phase) structure in which the speed increases downward. Wave properties such as the period (5–60 days), wavelength (100–500 km), and vertical trapping scale (102–103 m) vary significantly depending on environmental parameters of the SCS. The TRW energy propagates along steep topography with phase propagation offshore. TRWs with high frequencies exhibit a stronger climbing effect than low-frequency ones and hence can move further upslope. For TRWs with a certain frequency, the wavelength and trapping scale are dominated by the topographic beta, whereas the group velocity is more sensitive to the internal Rossby deformation radius. Background circulation with horizontal shear can change the wavelength and direction of TRWs if the flow velocity is comparable to the group velocity, particularly in the central, southern, and eastern SCS. A case study suggests two possible energy sources for TRWs: mesoscale perturbation in the upper layer and large-scale background circulation in the deep layer. The former provides KE by pressure work, whereas the latter transfers the available potential energy (APE) through baroclinic instability.

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

“…In accordance with Quan et al. (2021), we set three criteria to identify the typical TRWs in the NSCS: (a) the in situ deep KE falling into the TRWs period range (5–90 days) is significant at the 95% confidence level; (b) the spatiotemporal structures of the deep‐current fluctuations match the characteristics of TRWs (e.g., vertical coherence and bottom‐intensified amplitude); and (c) the frequency and wavenumber conform to the dispersion relation of TRWs (i.e., to leading order). TRWs are identified widely over the continental slopes of the NSCS in the model (Figure 4c) and would require more observations for validation in future studies.…”

“…With a time step of 1 h, the backward integrations of the ray tracing model for 60 days at the six sites in Figure 4b show that the TRWs identified over the entire NSCS can, in general, be traced back to the east of the Dongsha Islands (green rays in Figure 7b), which is consistent with the bottom flow shown in Figure 4b and the forward tracing by Quan et al. (2021). Therefore, the TRWs found to the west of the Dongsha Islands are primarily formed on the eastern side.…”

“…The relevant mechanism, which is discussed in detail by Quan et al. (2021), can be interpretated as PV conservation or being energized through pressure work in terms of energetics. This is consistent with our findings, as well as a number of other observational and modeling studies in the SCS (Shu et al., 2016; Wang et al., 2019; H. Yang et al., 2013) and Gulf of Mexico (Maslo et al., 2020; Y. Yang et al., 2020), in which energy transfers due to instabilities are largely confined to the upper 1,000 m and pressure work dominates the energetics at depth.…”

“…According to Rhines (1970), the group speed of TRWs is written as where is the topographic beta and θ is the angle between the wavenumber vector and topographic gradient. In the NSCS, for a long wave and overwhelms on the continental slopes (Quan et al., 2021). Therefore, the normalized deep‐layer K 1 anomaly propagated westward along the TRW raypath at a quicker speed of 0.32 m s −1 and it took about 20 days to cross the NSCS (Figure 9c), which is consistent with the time lag in the correlation.…”

“…Recently, Quan et al. (2021) revealed the presence of energetic topographic Rossby waves (TRWs) in the abyssal SCS (>2,000 m), which contribute greatly to the intraseasonal variability of the deep flow over continental slopes. In this study, we show that the Kuroshio intrusion and related eddies serve as an important energy source for the TRWs in the northern South China Sea (NSCS), which are energized via pressure work and propagate along continental slopes to drive the deep‐flow variability (Figure 1).…”