Institutional Repository of South China Sea Institute of Oceanology, CAS
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Atmospheric deposition of trace elements to Daya Bay, South China Sea: Fluxes and sources
No full textThis study was conducted from October 2015 to March 2017, with the aim of providing the first data on the fluxes and sources of wet and dry deposition of trace elements (TEs) in. Daya Bay, South China Sea. Wet deposition flux of TEs was always preponderant and orders of magnitude higher than that of dry deposition owing to the high rainfall frequency in Daya Bay. The total deposition fluxes of TEs in the target area were higher than in most places worldwide, but at a moderate level within China. Wet deposition was highest in summer and lowest in winter, whereas dry deposition showed an opposite seasonal trend. The main sources of TEs in wet deposition were seasalt/dust, fossil fuel combustion, and crustal sources, and in dry deposition, they were dust/metallurgic, fossil fuel, petrochemical industry and crustal sources
Deep Sea Currents Driven by Breaking Internal Tides on the Continental Slope
No full textMooring data collected on the continental slope of the South China Sea show that along-slope deep sea bottom currents are generated when large spring internal tides (internal waves with tidal frequency) are observed, with the maximum velocity amplitude exceeding 0.15m/s. The observations are consistent with predictions that near-bottom breaking of internal waves can result in generation of along-slope flows when these waves obliquely approach the slope. A linear internal tide model in one horizontal dimension with realistic topography and stratification is used to show that the breaking of internal tides is likely due to near-critical reflection on the slope. Combining the mooring observations and the model simulation, an along-slope near-bottom transport of similar to 0.5Sv is estimated. Along-slope bottom flows caused by breaking internal waves potentially provide a significant way to deform continental slopes and affect deep water exchange between the marginal sea and open ocean
Model-based assessment of a Northwestern Tropical Pacific moored array to monitor intraseasonal variability
No full textThe Northwestern Tropical Pacific Ocean (NWTPO) moorings observing system, including 15 moorings, was established in 2013 to provide velocity profile data. Observing system simulation experiments (OSSEs) were carried out to assess the ability of the observation system to monitor intraseasonal variability in a pilot study, where ideal "mooring-observed" velocity was assimilated using Ensemble Optimal Interpolation (EnOI) based on the Regional Oceanic Modeling System (ROMS). Because errors between the control and "nature" runs have a mesoscale structure, a random ensemble derived from 20-90-day bandpass-filterednine-year model outputs is proved to be more appropriate for the NWTPO mooring array assimilation than a random ensemble derived from a 30-day running mean. The simulation of the intraseasonal currents in the North Equatorial Current (NEC), North Equatorial Countercurrent (NECC), and Equatorial Undercurrent (EUC) areas can be improved by assimilating velocity profiles using a 20-90-day bandpass-filtered ensemble. The root mean square errors (RMSEs) of the intraseasonal zonal (U) and meridional velocity (V) above 500m depth within the study area (between 0 degrees N-18 degrees N and 122 degrees E-147 degrees E) were reduced by 15.4% and 16.9%, respectively. Improvements in the downstream area of the NEC moorings transect were optimum where the RMSEs of the intraseasonal velocities above 500m were reduced by more than 30%. Assimilating velocity profiles can have a positive impact on the simulation and forecast of thermohaline structure and sea level anomalies in the ocean
Eastern Pacific ITCZ Dipole and ENSO Diversity
No full textThe eastern tropical Pacific features strong climatic asymmetry across the equator, with the intertropical convergence zone (ITCZ) displaced north of the equator most of time. In February- April (FMA), the seasonal warming in the Southern Hemisphere and cooling in the Northern Hemisphere weaken the climatic asymmetry, and a double ITCZ appears with a zonal rainband on either side of the equator. Results from an analysis of precipitation variability reveal that the relative strength between the northern and southern ITCZ varies from one year to another and this meridional seesaw results from ocean-atmosphere coupling. Surprisingly this meridional seesaw is triggered by an El Nino-Southern Oscillation (ENSO) of moderate amplitudes. Although ENSO is originally symmetric about the equator, the asymmetry in the mean climate in the preceding season introduces asymmetric perturbations, which are then preferentially amplified by coupled ocean-atmosphere feedback in FMA when deep convection is sensitive to small changes in cross-equatorial gradient of sea surface temperature. This study shows that moderate ENSO follows a distinct decay trajectory in FMA and southeasterly cross-equatorial wind anomalies cause moderate El Nino to dissipate rapidly as southeasterly cross-equatorial wind anomalies intensify ocean upwelling south of the equator. In contrast, extreme El Nino remains strong through FMA as enhanced deep convection causes westerly wind anomalies to intrude and suppress ocean upwelling in the eastern equatorial Pacific
Estimation of suspended particulate matter in turbid coastal waters: application to hyperspectral satellite imagery
No full textAn empirical algorithm is proposed to estimate suspended particulate matter (SPM) ranging from 0.675 to 25.7 mg L-1 in the turbid Pearl River estuary (PRE). Comparisons between model predicted and in situ measured SPM resulted in R(2)s of 0.97 and 0.88 and mean absolute percentage errors (MAPEs) of 23.96% and 29.69% by using the calibration and validation data sets, respectively The developed algorithm demonstrated the highest accuracy when compared with existing ones for turbid coastal waters. The diurnal dynamics of SPM was revealed by applying the proposed algorithm to reflectance data collected by a moored buoy in the PRE. The established algorithm was implemented to Hyperspectral Imager for the Coastal Ocean (HICO) data and the distribution pattern of SPM in the PRE was elucidated. Validation of HICO-derived reflectance data by using concurrent MODIS/Aqua data as a benchmark indicated their reliability. Factors influencing variability of SPM in the PRE were analyzed. which implicated the combined effects of wind, tide. rainfall. and circulation as the cause. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Ageement
Origins of Eddy Kinetic Energy in the Bay of Bengal
No full textBy analyzing satellite observational data and ocean general circulation model experiments, this study investigates the key processes that determine the spatial distribution and seasonality of intraseasonal eddy kinetic energy (EKE) within the Bay of Bengal (BOB). It is revealed that a complicated mechanism involving both local and remote wind forcing and ocean internal instability is responsible for the generation and modulation of EKE in this region. High-level EKE mainly resides in four regions: east of Sri Lanka (Region 1), the western BOB (Region 2), northwest of Sumatra (Region 3), and the coastal rim of the BOB (Region 4). The high EKE levels in Regions 1 and 2 are predominantly produced by ocean internal instability, which contributes 90% and 79%, respectively. Prominent seasonality is also observed in these two regions, with higher EKE levels in boreal spring and fall due to enhanced instability of the East Indian Coast Current and the Southwest Monsoon Current, respectively. In contrast, ocean internal instability contributes 49% and 52% of the total EKE in Regions 3 and 4, respectively, whereas the atmospheric forcing of intraseasonal oscillations (ISOs) also plays an important role. ISOs produce EKE mainly through wind stress, involving both the remote effect of equatorial winds and the local effect of monsoonal winds. Equatorial-origin wave signals significantly enhance the EKE levels in Regions 3 and 4, in the form of reflected Rossby waves and coastal Kelvin waves, respectively. The local wind forcing effect through Ekman pumping also has a significant contribution in Regions 3 and 4 (24% and 22%, respectively)
Plate interactions, crustal deformation and magmatism along the eastern margins of the Tibetan Plateau
No full textIn order to investigate the deep structure and dynamics of the triangular colliding system of the eastern Tibetan Plateau (TP), as well as the Yangtze Platform and the Qinling-Dabie Fold Belt, a large number of P- and S-wave arrival times from both local and teleseismic earthquakes were used to generate high-resolution tomographic images beneath the eastern margins of TP. The results of this study show that the western margin of the Minjiang fault is characterized by low velocity (low-V) anomalies in sharp contrast to the high velocity (high-V) anomalies that are derived from the eastern part. This means that the Minshan uplift zone is imaged as high-V and acts as a barrier to the eastward escape crustal flow, which leads to obvious segmentation of the Longmenshan fault (LMSF) zone between velocity belts that are concatenated into the Minjiang fault. Because high-V anomalies are clearly imaged in this study to extended depths of ca. 450 km, we consider the Yangtze Platform (containing the Sichuan Basin) to comprise craton-like lithosphere. This study reveals the presence of a low-V belt extending to a depth of ca. 250 km between the Sichuan basin and the Ordos Block which probably represents the migration pathway of upwelling asthenosphere materials from Tibet. At the same time, low-V zone with a downdip depth of ca. 500 km accompanied by an approximately 400 km rectangular high-V body is imaged under the Tengchong volcano. We interpret the high-V body and the low-V zone as evidence for the subducted Indian plate and the volcanic magma source, respectively. The results of this study reveal a number of new features of structural heterogeneities relative to deep coupling between the Tibet and the Yangtze platform, as well as the relationship between the Sichuan Basin and the Ordos Block, and the original source of the Tengchong volcano. The results presented here enable us a better understanding of the plate interaction, crustal deformation, and magmatism of the Tengchong volcano along the eastern margins of TP
The Characteristics of Microseisms in South China Sea: Results From a Combined Data Set of OBSs, Broadband Land Seismic Stations, and a Global Wave Height Model
No full textIt has long been recognized that ocean gravity waves can generate microseisms through their interactions and coupling with the solid earth. Their generation mechanisms, wave types, and propagation have been studied and debated intensively. In this study, we focus on the characteristics of microseisms in South China Sea (SCS) and compare them with globally observed microseisms. We use data from six ocean bottom seismometers, 10 publicly available broadband land seismometers near SCS, and a reanalyzed global wave height model from 1 May 2012 to 20 August 2012 (UTC), the time span of ocean bottom seismometers' data. We apply three techniques including power spectral density, correlation, and frequency dependent polarization analysis. Our results show that (1) microseisms observed in SCS are dominated by sources from adjacent oceans, instead of the common global sources from the Pacific, Atlantic, or Indian Ocean; (2) the split of double-frequency microseisms (DFMs) is observed in SCS, especially on land stations: the sources are located near the central sea basin for long-period DFMs (0.1-0.25Hz) and are local near stations for short-period DFMs (0.25-0.5Hz); (3) typhoons both strengthen microseisms and affect the source locations of microseisms with the biggest influence on the short-period DFMs; and (4) microseisms in or near SCS are a mixture of Love and Rayleigh waves and the relative dominance of Love and Rayleigh waves changes with locations
Cenozoic tectonic subsidence in the Qiongdongnan Basin, northern South China Sea
No full textA number of major controversies exist in the South China Sea, including the timing and pattern of seafloor spreading, the anomalous alternating strike-slip movement on the Red River Fault, the existence of anomalous post-rift subsidence and how major submarine canyons have developed. The Qiongdongnan Basin is located in the intersection of the northern South China Sea margin and the strike-slip Red River fault zone. Analysing the subsidence of the Qiongdongnan Basin is critical in understanding these controversies. The basin-wide unloaded tectonic subsidence is computed through 1D backstripping constrained by the reconstruction of palaeo-water depths and the interpretation of dense seismic profiles and wells. Results show that discrete subsidence sags began to form in the central depression during the middle and late Eocene (45-31.5Ma). Subsequently in the Oligocene (31.5-23Ma), more faults with intense activity formed, leading to rapid extension with high subsidence (40-90mMyr(-1)). This extension is also inferred to be affected by the sinistral movement of the offshore Red River Fault as new subsidence sags progressively formed adjacent to this structure. Evidence from faults, subsidence, magmatic intrusions and strata erosion suggests that the breakup unconformity formed at ca. 23Ma, coeval with the initial seafloor spreading in the southwestern subbasin of the South China Sea, demonstrating that the breakup unconformity in the Qiongdongnan Basin is younger than that observed in the Pearl River Mouth Basin (ca. 32-28Ma) and Taiwan region (ca. 39-33Ma), which implies that the seafloor spreading in the South China Sea began diachronously from east to west. The post-rift subsidence was extremely slow during the early and middle Miocene (16mMyr(-1), 23-11.6Ma), probably caused by the transient dynamic support induced by mantle convection during seafloor spreading. Subsequently, rapid post-rift subsidence occurred during the late Miocene (144mMyr(-1), 11.6-5.5Ma) possibly as the dynamic support disappeared. The post-rift subsidence slowed again from the Pliocene to the Quaternary (24mMyr(-1), 5.5-0Ma), but a subsidence centre formed in the west with the maximum subsidence of ca. 450m, which coincided with a basin with the sediment thickness exceeding 5500m and is inferred to be caused by sediment-induced ductile crust flow. Anomalous post-rift subsidence in the Qiongdongnan Basin increased from ca. 300m in the northwest to ca. 1200m in the southeast, and the post-rift vertical movement of the basement was probably the most important factor to facilitate the development of the central submarine canyon
Intra- and intertrench variations in flexural bending of the Manila, Mariana and global trenches: implications on plate weakening in controlling trench dynamics
No full textWe conducted detailed analyses of a global array of trenches, revealing systematic intra-and intertrench variations in plate bending characteristics. The intratrench variations of the Manila and Mariana Trenches were analysed in detail as end-member cases of the relatively young (16-36 Ma) and old (140-160 Ma) subducting plates, respectively. Meanwhile, the intertrench variability was investigated for a global array of additional trenches including the Philippine, Kuril, Japan, Izu-Bonin, Aleutian, Tonga-Kermadec, Middle America, Peru, Chile, Sumatra and Java Trenches. Results of the analysis show that the trench relief (W-0) and width (X-0) of all systems are controlled primarily by the faulting-reduced elastic thickness near the trench axis (T-e(m)) and affected only slightly by the initial unfaulted thickness (T-e(M)) of the incoming plate. The reduction in T-e has caused significant deepening and narrowing of trench valleys. For the cases of relatively young or old plates, the plate age could be a dominant factor in controlling the trench bending shape, regardless the variations in axial loadings. Our calculations also show that the axial loading and stresses of old subducting plates can vary significantly along the trench axis. In contrast, the young subducting plates show much smaller values and variations in axial loading and stresses