Institutional Repository of South China Sea Institute of Oceanology, CAS
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Future changes in coastal upwelling ecosystems with global warming: The case of the California Current System
No full textCoastal upwelling ecosystems are among the most productive ecosystems in the world, meaning that their response to climate change is of critical importance. Our understanding of climate change impacts on marine ecosystems is largely limited to the open ocean, mainly because coastal upwelling is poorly reproduced by current earth system models. Here, a high-resolution model is used to examine the response of nutrients and plankton dynamics to future climate change in the California Current System (CCS). The results show increased upwelling intensity associated with stronger alongshore winds in the coastal region, and enhanced upper-ocean stratification in both the CCS and open ocean. Warming of the open ocean forces isotherms downwards, where they make contact with water masses with higher nutrient concentrations, thereby enhancing the nutrient flux to the deep source waters of the CCS. Increased winds and eddy activity further facilitate upward nutrient transport to the euphotic zone. However, the plankton community exhibits a complex and nonlinear response to increased nutrient input, as the food web dynamics tend to interact differently. This analysis highlights the difficulty in understanding how the marine ecosystem responds to a future warming climate, given to range of relevant processes operating at different scales
Development of Wind Speed Retrieval from Cross-Polarization Chinese Gaofen-3 Synthetic Aperture Radar in Typhoons
No full textThe purpose of our work is to determine the feasibility and effectiveness of retrieving sea surface wind speeds from C-band cross-polarization (herein vertical-horizontal, VH) Chinese Gaofen-3 (GF-3) SAR images in typhoons. In this study, we have collected three GF-3 SAR images acquired in Global Observation (GLO) and Wide ScanSAR (WSC) mode during the summer of 2017 from the China Sea, which includes the typhoons Noru, Doksuri and Talim. These images were collocated with wind simulations at 0.12 degrees grids from a numeric model, called the Regional Assimilation and Prediction System-Typhoon model (GRAPES-TYM). Recent research shows that GRAPES-TYM has a good performance for typhoon simulation in the China Sea. Based on the dataset, the dependence of wind speed and of radar incidence angle on normalized radar cross (NRCS) of VH-polarization GF-3 SAR have been investigated, after which an empirical algorithm for wind speed retrieval from VH-polarization GF-3 SAR was tuned. An additional four VH-polarization GF-3 SAR images in three typhoons, Noru, Hato and Talim, were investigated in order to validate the proposed algorithm. SAR-derived winds were compared with measurements from Windsat winds at 0.25 degrees grids with wind speeds up to 40 m/s, showing a 5.5 m/s root mean square error (RMSE) of wind speed and an improved RMSE of 5.1 m/s wind speed was achieved compared with the retrieval results validated against GRAPES-TYM winds. It is concluded that the proposed algorithm is a promising potential technique for strong wind retrieval from cross-polarization GF-3 SAR images without encountering a signal saturation problem
Remote impact of North Atlantic sea surface temperature on rainfall in southwestern China during boreal spring
No full textAs an important oceanic signal, the North Atlantic sea surface temperature (SST) affects not only the climate variability over East China and Northeast China but also can affect climate variability over southwestern China (SWC). Based on station rainfall data and reanalysis datasets, the present study investigates the relationship of North Atlantic SST with SWC rainfall during boreal spring for the period 1979-2016. The results show that there is a significant positive correlation between North Atlantic SST and SWC rainfall during boreal spring. The atmospheric circulation over southern Asia associated with North Atlantic SST is favorable for positive rainfall anomalies. Further analyses show that North Atlantic SST can induce a North Atlantic-western Russia-western Tibetan Plateau-SWC (NRTC) teleconnection wave train from upper level to low level. At low level, two anomalous anticyclones are found over the mid-high latitude of North Atlantic and the western Tibetan Plateau, and two anomalous cyclones are observed over the western Russia and Bay of Bengal (BOB), respectively. The NRTC teleconnection wave train plays a bridging role between the North Atlantic SST and SWC rainfall during boreal spring. Both the observational analysis and two numerical experiments suggest that the North Atlantic SST during boreal spring can induce an anomalous cyclone over BOB by the NRTC teleconnection pattern. The anomalous cyclone over BOB favors moisture transport to SWC, accompanying with significant anomalous ascending motion, and thus results in positive rainfall anomalies in SWC during boreal spring
Elasto-plastic deformation and plate weakening due to normal faulting in the subducting plate along the Mariana Trench
No full textWe investigated variations in the elasto-plastic deformation of the subducting plate along the Mariana Trench through an analysis of flexural bending and normal fault characteristics together with geodynamic modeling. Most normal faults were initiated at the outer-rise region and grew toward the trench axis with strikes mostly subparallel to the local trench axis. The average trench relief and maximum fault throws were measured to be significantly greater in the southern region (5 km and 320 m, respectively) than the northern and central regions (2 km and 200 m). The subducting plate was modeled as an elasto-plastic slab subjected to tectonic loading at the trench axis. The calculated strain rates and velocities revealed an array of normal fault-like shear zones in the upper plate, resulting in significant faulting-induced reduction in the deviatoric stresses. We then inverted for solutions that best fit the observed flexural bending and normal faulting characteristics, revealing normal fault penetration to depths of 21,20, and 32 km beneath the seafloor for the northern, central, and southern regions, respectively, which is consistent with the observed depths of the relocated normal faulting earthquakes in the central Mariana Trench. The calculated deeper normal faults of the southern region might lead to about twice as much water being carried into the mantle per unit trench length than the northern and central regions. We further calculated that normal faulting has reduced the effective elastic plate thickness L by up to 52% locally in the southern region and 33% in both the northern and central regions. The best-fitting solutions revealed a greater apparent angle of the pulling force in the southern region (51-64 degrees) than in the northern (22-35 degrees) and central (20-34 degrees) regions, which correlates with a general southward increase in the seismically-determined dip angle of the subducting slab along the Mariana Trench
Acoustic characteristics of seafloor sediments in the abyssal areas of the South China Sea
No full textDeep-sea sediment cores were obtained from abyssal areas (water depth > 4000 m) in the South China Sea (SCS) to derive acoustic characteristics by using a WSD-3 digital acoustic instrument at a frequency of 100 kHz. The compressional wave velocity (Vp) of the seafloor sediments in the central basin is differentiated into 3 provinces on the basis of the sediment properties (esp. mean grain size and velocity). Provinces 1.11 and H2 have a high compressional wave velocity and coarse particles, while the middle province L. has a low compressional wave velocity and fine particles. The results show that the distribution of Vp is related to the sediment physical properties, sediment sources and ocean current. Vertical variations in the Vp indicate that it increases with burial depth in the high-velocity province, but is complicated in the low-velocity province, reflecting that the more active or complicated sedimentary environment in province L may be affected by the topography. We proposed a laboratory sound velocity measurement method based on Hamilton sound velocity ratio method. In addition, the measurement values from this method are close to the in situ sound velocity determined by adjusting the temperature of the sediment samples
Coral-Derived Western Pacific Tropical Sea Surface Temperatures During the Last Millennium
No full textReconstructions of ocean temperatures prior to the industrial era serve to constrain natural climate variability on decadal to centennial timescales, yet relatively few such observations are available from the west Pacific Warm Pool. Here we present multiple coral-based sea surface temperature reconstructions from Yongle Atoll, in the South China Sea over the last similar to 1,250 years (762-2013 Common Era [CE]). Reconstructed coral Sr/Ca-sea surface temperatures indicate that the "Little Ice Age (1711-1817 CE)" period was similar to 0.7 degrees C cooler than the " Medieval Climate Anomaly (913-1132 CE)" and that late 20th century warming of the western Pacific is likely unprecedented over the past millennium. Our findings suggest that the Western Pacific Warm Pool may have expanded (contracted) during the Medieval Climate Anomaly (Little Ice Age), leading to a strengthening (weakening) of the Asian summer monsoon, as recorded in Chinese stalagmites. Plain Language Summary Tropical Pacific climate variability influences global climate system on interannual and longer timescales. In contrast to the eastern and central tropical Pacific, where paleo-sea surface temperature data are relatively abundant, such data are fairly sparse in the western tropical Pacific, which limits our understanding of Pacific Warm Pool and its role in low-frequency climate variability. Here we present a multiple coral-based estimates of monthly resolved sea surface temperature from the Yongle Atoll, South China Sea, that span segments of the last 1,250 years. We demonstrate significant variations in mean sea surface temperature over this period-with warmer conditions during the so-called " Medieval Climate Anomaly" and cooler conditions during the so-called " Little Ice Age," in keeping with compilations of Northern Hemisphere temperature over this period. Our data also reveal unprecedented warming of sea surface temperatures in the late 20th century and early 21st century that are unprecedented in the past 1,250 years. We believe that our manuscript will be of broad interest to geologists, climatologists, and coral reef scientists, given the paucity of climate records from the region
Two-Step Inversion of Geoacoustic Parameters with Bottom Reverberation and Transmission Loss in the Deep Ocean
No full textThe parameters of deep ocean sediments are relevant for accurately predicting the sound field; however, it is difficult to measure the parameters in situ. Most inversion methods used in shallow water are inapplicable in the deep ocean because of the considerable differences in propagation characteristics. At present, no method for simultaneously obtaining sound speed, density, and attenuation that considers the sensitivity of sediment parameters is yet available. This study proposes a two-step inversion of geoacoustic parameters in the deep ocean. On the basis of the half-space model, the decline tendency of bottom reverberation level with travel time is used for the inversion of sound speed and density, whereas transmission loss is used for inversion of attenuation. Inversion results can be practical for acoustic applications when this method is used. Experimental data from the South China Sea in the summer of 2014 are processed during the inversion process. The sediment parameters obtained from the inversion process are close to the laboratory-measured sampling values and may be used to predict the sound field in various applications, such as in transmission loss in the deep ocean
The stable isotope fingerprint of chemosymbiosis in the shell organic matrix of seep-dwelling bivalves
No full textChemosymbiotic bivalves harboring endosymbiotic, chemotrophic bacteria have been investigated from a variety of hydrocarbon seeps worldwide. It has been shown that carbon, nitrogen, and sulfur isotopic compositions of the animal soft body parts are excellent indicators for evaluating energy transfer and food sources for the respective deep-sea habitats. However, recognition of chemosymbiosis has proven to be difficult for bivalves that dwelled at ancient seeps due to the lack of soft tissue. Here, we investigated delta C-13, delta N-15, and delta S-34 signatures of the tissue (mantle) and the shell organic matrix (SOM) of the same specimens of three bathymodiolin mussel species with different chemotrophic symbionts (methanotrophs in Bathymodiolus platifrons and B. childressi and thiotrophs in B. aduloides) and one vesicomyid clam (Calyptogena sp.) from a variety of hydrocarbon seeps from the South China Sea and the Gulf of Mexico. The data obtained demonstrate that all seep bivalves regardless of species or locations reveal overall small differences in delta C-13 (<= + 4 parts per thousand), delta N-15 (<= + 1 parts per thousand), and delta S-34 (<= + 5 parts per thousand) values between SOM and mantle (Delta(SOM-mantle)) of the specimens. Relatively larger Delta(SOM-mantle) for d13C values (as high as + 10 parts per thousand)in B. platifrons and larger Delta(SOM-mantle) for delta S-34 values (up to 16 parts per thousand) in B. aduloides and Calyptogena sp. might be due to different symbionts in their gills. Since SOM can be extracted from fossil bivalve shells, the proxy can be used as a fingerprint of chemosynthesis-based food chains, although its utility will depend on the quality of preservation of the shell organic matter. Despite this uncertainty, the new proxy has great potential to reconstruct energy flow through different types of chemosynthesis-based ecosystems
Seismogenic structures of the 2006 M(L)4.0 Dangan Island earthquake offshore Hong Kong
No full textThe northern margin of the South China Sea, as a typical extensional continental margin, has relatively strong intraplate seismicity. Compared with the active zones of Nanao Island, Yangjiang, and Heyuan, seismicity in the Pearl River Estuary is relatively low. However, a M(L)4.0 earthquake in 2006 occurred near Dangan Island (DI) offshore Hong Kong, and this site was adjacent to the source of the historical M5.8 earthquake in 1874. To reveal the seismogenic mechanism of intraplate earthquakes in DI, we systematically analyzed the structural characteristics in the source area of the 2006 DI earthquake using integrated 24-channel seismic profiles, onshore-offshore wide-angle seismic tomography, and natural earthquake parameters. We ascertained the locations of NW- and NE-trending faults in the DI sea and found that the NE-trending DI fault mainly dipped southeast at a high angle and cut through the crust with an obvious low-velocity anomaly. The NW-trending fault dipped southwest with a similar high angle. The 2006 DI earthquake was adjacent to the intersection of the NE- and NW-trending faults, which suggested that the intersection of the two faults with different strikes could provide a favorable condition for the generation and triggering of intraplate earthquakes. Crustal velocity model showed that the high-velocity anomaly was imaged in the west of DI, but a distinct entity with low-velocity anomaly in the upper crust and high-velocity anomaly in the lower crust was found in the south of DI. Both the 1874 and 2006 DI earthquakes occurred along the edge of the distinct entity. Two vertical cross-sections nearly perpendicular to the strikes of the intersecting faults revealed good spatial correlations between the 2006 DI earthquake and the low to high speed transition in the distinct entity. This result indicated that the transitional zone might be a weakly structural body that can store strain energy and release it as a brittle failure, resulting in an earthquake-prone area
The mitochondrial genome of the deep-sea tubeworm Paraescarpia echinospica (Siboglinidae, Armada) and its phylogenetic implications
No full textParaescarpia echinospica is a conspicuous annelid living in the cold seeps and hydrothermal vents of the Western Pacific region and relying on their endosymbiont bacteria as a source of energy and organic carbon. We report the complete mitochondrial genome of P. echinospica, which is 15,280 bp in length, containing 13 protein-coding genes, two ribosomal RNA genes, 22 tRNA genes and a putative control region. The overall base composition is AT-biased. The control region contains repeated nucleotide motifs. Phylogenetic analyses of the concatenated mitochondrial genes strongly support a sister relationship of P. echinospica with a Glade containing Escarpia and Seepiophila