2020 Vol. 39, No. 11
This study investigates the submesoscale fronts and their dynamic effects on the mean flow due to frontal instabilities in the wind-driven summer offshore jet of the western South China Sea (WSCS), using satellite observations, a 500 m-resolution numerical simulation, and diagnostic analysis. Both satellite measurements and simulation results show that the submesoscale fronts occupying a typical lateral scale of O(~10) km are characterized with one order of Rossby (Ro) and Richardson (Ri) numbers in the WSCS. This result implies that both geostrophic and ageostrophic motions feature in these submesoscale fronts. The diagnostic results indicate that a net cross-frontal Ekman transport driven by down-front wind forcing effectively advects cold water over warm water. By this way, the weakened local stratification and strong lateral buoyancy gradients are conducive to a negative Ertel potential vorticity (PV) and triggering frontal symmetric instability (SI) at the submesoscale density front. The cross-front ageostrophic secondary circulation caused by frontal instabilities is found to drive an enhanced vertical velocity reaching O(100) m/d. Additionally, the estimate of the down-front wind forcing the Ekman buoyancy flux (EBF) is found to be scaled with the geostrophic shear production (GSP) and buoyancy flux (BFLUX), which are the two primary energy sources for submesoscale turbulence. The large values of GSP and BFLUX at the fronts suggest an efficient downscale energy transfer from larger-scale geostrophic flows to the submesoscale turbulence owing to down-front wind forcing and frontal instabilities. In this content, submesoscale fronts and their instabilities substantially enhance the local vertical exchanges and geostrophic energy cascade towards smaller-scale. These active submesoscale processes associated density fronts and filaments likely provide new physical interpretations for the filamentary high chlorophyll concentration and frontal downscale energy transfer in the WSCS.
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.
This study presents a Lagrangian view of upper water exchanges across the Luzon Strait based on the finite-time Lyapunov exponents (FTLE) fields computed from the surface geostrophic current. The Lagrangian coherent structures (LCSs) extracted from the FTLE fields well identify the typical flow patterns and eddy activities around the Luzon Strait. In addition, they reveal the intricate transport paths and fluid domains, which are validated by the tracks of satellite-tracked surface drifters and cannot be visually recognized in the velocity maps. The FTLE fields indicate that there are mainly four types of transport patterns near the Luzon Strait; among them, the Kuroshio northward-flowing “leaping” pattern and the clockwise rotating “looping” pattern occur more frequently than the “leaking” pattern of the direct Kuroshio branch into the SCS and the “outflowing” pattern from the SCS to the Pacific. The eddy shedding events of the Kuroshio at the Luzon Strait are further analyzed, and the importance of considering LCSs in estimating transport by eddies is highlighted. The anticyclonic eddy (ACE) shedding cases reveal that ACEs mainly originate from the looping paths of Kuroshio and thus could effectively trap the Kuroshio water before eddy detachments. LCSs provide useful information to predict the positions of the upstream waters that finally enter the ACEs. In contrast, LCS snapshots indicate that during the formation of cyclonic eddies (CEs), most CEs are not connected with the pathways of Kuroshio water. Hence, the contribution of CEs to the surface water exchanges from the Pacific into the SCS is tiny.
Effects of currents on winter wind waves in the tide-dominated Qiongzhou Strait (QS) were numerically evaluated via employing the coupled ocean-atmosphere-wave-sediment transport (COAWST) modeling system. Validations showed satisfactory model performance in simulating the intense tidal currents in the QS. Different effects of sea level variations and tidal currents on waves were examined under the maximum eastward (METC) and westward (MWTC) tidal currents. In the east entrance area of the QS, the positive sea levels under the MWTC deepened the water depth felt by waves, benefiting the further propagation of wave energy into the inner strait and causing increased wave height. The METC and the MWTC could both enhance the wave height in the east entrance area of the QS, mainly through current-induced convergence and wavenumber shift, respectively. By current-induced refraction, the METC (MWTC) triggered counterclockwise (clockwise) rotation in peak wave directions in the northern part of the QS while clockwise (counterclockwise) rotation in the southern part.
The mode-2 internal solitary waves (ISWs) generated by mode-2 internal tide (IT) are identified by mooring observations in the northern South China Sea (SCS) from 2016 to 2017. Two mode-2 ISWs with a re-appearance period of 24.9 h observed on 29 and 30 July 2016 are characterized by type-b ISWs. They occurred when the isotherms compressed obviously in the vertical direction. Modal decomposition of IT horizontal currents shows that the vertical compression of the isotherms is mainly caused by diurnal mode-2 IT. The analysis of the role of the density stratification reveals that a deeper and thinner pycnocline is favorable for generation of mode-2 ISWs rather than pycnocline intensity. By comparing the mode-2 nonlinear, dispersion coefficients and the Ursell numbers calculated based on the stratification associated with different kinds of ITs with the observation results, it is shown that the diurnal mode-2 IT plays a crucial role in the generation of the mode-2 ISWs. When the diurnal mode-2 IT interacts with the semidiurnal IT and causes a deeper and thinner pycnocline, the mode-2 ISWs are easily excited.
The Kuroshio intrusion in a quasi-global eddy-resolving model (LICOMH) and a fully air-sea coupled model (LICOMHC) was evaluated against observations. We found that the Kuroshio intrusion was exaggerated in the former, while biases were significantly attenuated in the latter. Luzon Strait transport (LST) in winter was reduced from –8.8×106 m3/s in LICOMH to –6.0×106 m3/s in LICOMHC. Further analysis showed that different LST values could be explained by different large-scale and local surface wind stresses and the eddies east to the Luzon Strait as well. The relatively stronger cyclonic eddies in LICOMH northeast of the Luzon Island led to weak Kuroshio transport and strong intrusion through the Luzon Strait. The summed transport of all three factors was approximately 2.0×106 m3/s, which was comparable with the difference in LST between the two experiments. The EKE budget showed that strong EKE transport and the baroclinic transformation term led to strong cyclonic eddies east of the Kuroshio in LICOMH, while surface winds contributed little to the differences in the eddies.
Upper turbulent mixing in the interior and surrounding areas of an anticyclonic eddy in the northern South China Sea (SCS) was estimated from underwater glider data (May 2015) in the present study, using the Gregg-Henyey-Polzin parameterization and the Thorpe-scale method. The observations revealed a clear asymmetrical spatial pattern of turbulent mixing in the anticyclonic eddy area. Enhanced diffusivity (in the order of 10–3 m2/s) was found at the posterior edge of the anticyclonic mesoscale eddy; on the anterior side, diffusivity was one order of magnitude lower on average. This asymmetrical pattern was highly correlated with the eddy kinetic energy. Higher shear variance on the posterior side, which is conducive to the triggering of shear instability, may be the main mechanism for the elevated diffusivity. In addition, the generation and growth of sub-mesoscale motions that are fed by mesoscale eddies on their posterior side may also promote the occurrence of strong mixing in the studied region. The results of this study help improve our knowledge regarding turbulent mixing in the northern SCS.
SST fronts at the mesoscale eddy edge (ME fronts) were investigated from 2007–2017 in the northern South China Sea (NSCS) based on an automatic method using satellite sea level anomaly (SLA) and SST data. The relative probabilities between the number of anticyclonic/cyclonic ME fronts (AEF/CEF) and the number of anticyclones/cyclones reached 20%. The northeastern and southwestern parts of these anticyclones had more fronts than the northwestern and southeastern parts, although CEFs were nearly equally distributed in all directions. The number of ME fronts had remarkable seasonal variations, while the eddy kinetic energy (EKE) showed no seasonal variations. The total EKE at the ME fronts was three times of that within the MEs, and it was much stronger in AEFs than in CEFs. The interannual variability in the number of ME fronts and EKE had no significant correlation with the El Niño-Southern Oscillation (ENSO) index. Possible mechanisms of ME fronts were discussed, but the contributions of mesoscale eddies to SST fronts need to be quantified in future studies.
Using mesoscale eddy trajectory product derived from satellite altimetry data from 1993 to 2017, this study analyzes the statistical characteristics of spatiotemporal distribution of mesoscale eddy propagation velocities ( C ) in the South China Sea (SCS) deep basin with depths >1 000 m. Climatologically, the zonal propagation velocities (cx) are westwards in the whole basin, and the meridional velocities (cy) are southwards in the northwestern basin, and northwards in the southeastern basin. The variation of cy with longitude is consistent with that of the background meridional currents with correlation coefficient R2 of 0.96, while the variation of cx is related both to the background zonal currents and β effect. The propagation velocities characterize significant seasonality with the minimum magnitude occurring in summer and the maximum in winter for cx and C . Interannually, larger values of cx and cy mostly occurred in La Niña years in the negative phase of the Pacific Decadal Oscillation (PDO). Mesoscale eddies move fast at the beginning and end of their life span, i.e., at their growth and dissipation periods, and slowly during their stable “midlife” period. This trend is negatively correlated with the rotating tangential velocity with R2 of –0.93. Eddies with extreme propagation velocities are defined, which are slower (faster) than 1.5 cm/s (15.4 cm/s) and take 1.5% (1.9%) of the total eddies. The extremely slow-moving (fast-moving) eddies tend to appear in the middle (on the edge) of the basin, and mostly occur in summer (winter). The mechanism analysis reveals that the spatiotemporal distributions of the propagation velocities of mesoscale eddies in the SCS are modulated by the basin-scale background circulation.
Based on satellite remote sensing dataset and survey data during upwelling season of 2015, the spatial structures of phytoplankton biomass and community for the first time in the eastern Hainan upwelling (EHU) and its adjacent area, the eastern Leizhou Peninsula upwelling (ELPU) were illustrated. It is found that a significant cold tongue with high salinity and low temperature along the eastern Hainan coast driven by upwelling-favorable summer monsoon. The ELPU was relative weaker than the EHU because of its wide and gentle continental slope. Due to mixing by tides and waves, DO concentration with high value (>6.0 mg/L) were almost homogenous from surface to 30 m depth at the EHU. Beneath that, low DO water (<6.0 mg/L, anoxia) were pumped upward from bottom by the upwelling. The ELPU has worse DO condition compared with the EHU where bottom DO were lower than 3.5 mg/L owing to abundant DO consumption. The phytoplankton biomass reached maximal value about 1.5 mg/m3 at 30 m depth layer rather than surface layer at the EHU indicating the impact limit of upwelling on phytoplankton growth and DO distribution. Nourished by rich nutrient input, the phytoplankton biomass at the ELPU were much higher than the EHU where the maximal value can reach about 4.0 mg/m3. The phytoplankton biomass were reduced to about 0.2–0.3 mg/m3 at the offshore areas of the EHU and ELPU which were close to the value at open sea. At the inshore of the EHU, the phytoplankton community was dominated by diatom which accounted for about 50% of phytoplankton biomass. And prokaryotes (about 40%), green algae (about 20%) and prochlorococcus (about 20%) became main species at the offshore of the EHU. At the ELPU, diatom accounted for about 80% of phytoplankton biomass followed by green algae, indicating a different ecosystem at this region compared with the EHU.