To observe the deep current in the SCS, a current meter mooring, M1, was deployed in the deep western boundary zone of the SCS (16.6°N, 115.6°E; red dot in Fig. 1a) from June 2017 to September 2018. The water depth at the mooring location was 4 149 m and Recording Current Meters (RCMs) were mounted at 2 226 m, 3 226 m and 3 726 m to monitor the horizontal velocity of the deep flow.
The U.S. Navy GDEM V3.0 monthly climatology of observed temperature and salinity, with a horizontal resolution of 0.25° × 0.25°, was also used in this study. This data set has 78 standard depths from the surface down to 6 600 m, with a vertical resolution varying from 2 m at the surface to 200 m below 1 600 m (Carnes, 2009). The geostrophic velocity was calculated from the GDEM V3.0 using the thermal wind relation assuming the depth of no motion to be at 2 400 m. The idea of selecting a level of no motion has been used extensively in study of abyssal circulation (Stommel and Arons, 1959–1960a; Speer et al., 1993; Wang et al., 2011).
For the circulation of deep basin, the flow near the bottom has been observed to be faster than that above it (Section 3.1). Therefore, an approximation can be made that the upper layer pressure gradient is negligible, so that we can set up an inverse reduced-gravity model on a β-plane:
where x and y are zonal and meridional coordinates and u and v are the horizontal velocity components, respectively; f=βy is the Coriolis parameter; g′=0.03 is the reduced gravity; R/h0, ah are the parameters for the bottom friction and horizontal momentum dissipation, respectively, which are set to R = 10–3 m/s, ah = 300 m2/s. The thickness of the water column h is defined as:
where h0 = 3 000 is the vertical thickness of the deep-water column; hb is the prescribed bathymetry; and
$ \eta $is the deviation of the interface. The inverse reduced-gravity model has been widely used in deep ocean circulation studies (Stommel and Arons, 1959–1960a; Speer et al., 1993; Wang et al., 2018).
The model domain comprises the SCS central basin (9°–22°N, 111°–122°E). The resolution is (1/6)° ×(1/6)°. The deep SCS circulation is forced by a uniform upwelling, w, and the deep Luzon overflow on a β-plane. The transport in the Luzon overflow has been estimated to be between 0.7×106–3.0×106 m3/s based on different observations and model results (Wang, 1986; Liu and Liu, 1988; Qu et al., 2006; Tian et al., 2006; Zhao et al., 2014). In this study, based on in-situ observational results from Zhao et al. (2014) where we set the average Luzon overflow to be 1.6×106 m3/s with 0.7×106 m3/s entering through the northern inlet and 0.9×106 m3/s entering through the southern inlet (CTRL case). The wall boundary condition was used except for the two inflows through the Taltung Canyon (0.4×106 m3/s) and the Bashi Channel (1.2×106 m3/s) in the model domain. In order to satisfy basin-wide mass conservation, a spatially uniform upwelling of 1×10–6 m/s is set, so that the total upwelling transport over the area of model domain matches the total transport of the deep Luzon overflow (1.6×106 m3/s).
In order to investigate the influence of the Luzon overflow through the two inlets, we also consider two groups of experiments, one group has two cases where only 0.7×106 m3/s (upwelling is 4.38×10–7 m/s to satisfy the mass conservation, inflows of Taltung Canyon and Bashi Channel are 0.175×106 m3/s and 0.525×106 m3/s) of the Luzon overflow occurs through the northern inlet (Case-N1) and only 0.9×106 m3/s (upwelling is 5.62×10–7 m/s to satisfy the mass conservation, inflows of Taltung Canyon and Bashi Channel are 0.225×106 m3/s and 0.675×106 m3/s) of the Luzon overflow occurs through southern inlet (Case-S1), the other group has two cases where all 1.6×106 m3/s of the Luzon overflow occurs through the northern (Case-N2) or southern (Case-S2) inlets. In all the cases, the transport ratio between the northern and southern inlets are controlled by adjusting the inlet topography. The simple model is spun-up for 10 years to reach a quasi-steady state, and then run for an additional 5 years, and the average result of the last 5 years is computed and analyzed. The details of these experimental cases are listed in Table 1.
Case Overflow Other settings CTRL 1.6×106 m3/s, 0.7×106 m3/s from northern inlet
and 0.7×106 m3/s southern inlet
β-plain, uniform upwelling and with bottom topography Case-N1 only 0.7×106 m3/s, all from northern inlet Case-S1 only 0.9×106 m3/s, all from southern inlet Case-N2 1.6×106 m3/s, all from northern inlet Case-S2 1.6×106 m3/s, all from southern inlet
Table 1. Details of the numerical experiment
The Lagrangian trajectory model, TRACMASS (Döös et al., 2013) was used on the inverse reduced-gravity model results to calculate the particle trajectories. The theory behind the original scheme for steady state velocities was derived for rectangular and curvilinear grids using different vertical coordinates for the oceanic and atmospheric circulation models. The TRACMASS trajectories are exact solutions to differential equations and can therefore be integrated both forward and backward with unique solutions. The TRACMASS has been effectively used for the deep meridional overturning circulation (MOC) in the SCS (Shu et al., 2014). In this study, we only use the horizontal velocities in the TRACMASS as the inverse reduced-gravity model has no vertical velocity component.
Influence of two inlets of the Luzon overflow on the deep circulation in the northern South China Sea
- Received Date: 2020-01-21
- Accepted Date: 2020-04-02
- Available Online: 2020-12-28
- Publish Date: 2020-11-25
Abstract: An inverse reduced-gravity model is used to simulate the deep South China Sea (SCS) circulation. A set of experiments are conducted using this model to study the influence of the Luzon overflow through the two inlets on the deep circulation in the northern SCS. Model results suggest that the relative contribution of these inlets largely depends on the magnitude of the input transport of the overflow, but the northern inlet is more efficient than the southern inlet in driving the deep circulation in the northern SCS. When all of the Luzon overflow occurs through the northern inlet the deep circulation in the northern SCS is enhanced. Conversely, when all of the Luzon overflow occurs through the southern inlet the circulation in the northern SCS is weakened. A Lagrangian trajectory model is also developed and applied to these cases. The Lagrangian results indicate that the location of the Luzon overflow likely has impacts upon the sediment transport into the northern SCS.
|Citation:||Muping Zhou, Changlin Chen, Yunwei Yan, Wenhu Liu. Influence of two inlets of the Luzon overflow on the deep circulation in the northern South China Sea[J]. Acta Oceanologica Sinica, 2020, 39(11): 13-20. doi: 10.1007/s13131-020-1621-1|