Institutional Repository of South China Sea Institute of Oceanology, CAS
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Atlantic Meridional Overturning Circulation at 14.5 degrees N in 1989 and 2013 and 24.5 degrees N in 1992 and 2015: volume, heat, and freshwater transports
No full textThe Atlantic Meridional Overturning Circulation (AMOC) is analyzed by applying a box inverse model to hydrographic data from transatlantic sections along 14.5 degrees N, occupied in 1989 and 2013, and along 24.5 degrees N, occupied in 1992 and 2015. Direct comparison of water mass properties among the different realizations at the respective latitudes shows that the Antarctic Intermediate Water (AAIW) became warmer and saltier at 14.5 degrees N, and the densest Antarctic Bottom Water became lighter, while the North Atlantic Deep Water freshened at both latitudes. The inverse solution shows that the intermediate layer transport at 14.5 degrees N was also markedly weaker in 2013 than in 1989, indicating that the AAIW property changes at this latitude may be related to changes in the circulation. The inverse solution was validated using the RAPID and MOVE array data, and the GECCO2 ocean state estimate. Comparison among these datasets indicates that the AMOC has not significantly weakened over the past 2 decades at both latitudes. Sensitivity tests of the inverse solution suggest that the overturning structure and heat transport across the 14.5 degrees N section are sensitive to the Ek-man transport, while freshwater transport is sensitive to the transport-weighted salinity at the western boundary
Nonlinear Meridional Moisture Advection and the ENSO-Southern China Rainfall Teleconnection
No full textIn the boreal cooler months of 2015, southern China (SC) experienced the largest rainfall since 1950, exceeding 4 times the standard deviation of SC rainfall. Although an El Nino typically induces a positive SC rainfall anomaly during these months, the unprecedented rainfall increase cannot be explained by the strong El Nino of 2015/2016, and the dynamics is unclear. Here we show that a nonlinear meridional moisture advection contributes substantially to the unprecedented rainfall increase. During cooler months of 2015, the meridional flow anomaly over the South China Sea region, which acts on an El Nino-induced anomalous meridional moisture gradient, is particularly large and is supported by an anomalous zonal sea surface temperature gradient over the northwestern Pacific, which recorded its largest value in 2015 since 1950. Our study highlights, for the first time, the importance of the nonlinear process associated with the combined impact of a regional sea surface temperature gradient and large-scale El Nino anomalies in forcing El Nino rainfall teleconnection. Plain Language Summary El Nino affects regional climate extremes, but the teleconnection is not robust and a linear relationship often underestimates the impact, as in 2015 over Southern China. We found that nonlinear meridional moisture advection, involving the combined impact of a regional SST gradient over the northwest Pacific and large-scale El Nino-induced moisture anomalies, significantly modulates the teleconnection. The nonlinear process is important and must be taken into account in evaluating and predicting El Nino's impact. This result is of interest to El Nino modelers, seasonal forecasters, and broader communities in general
Land surface air temperature variations over Eurasia and possible causes in the past century
No full textIn this study, the variations of annual land surface air temperature (SAT) over Eurasia and the northern part of Africa (0 degrees-180 degrees E, 0 degrees-90 degrees N) were investigated using monthly SAT data from the Climatic Research Unit, University of East Anglia for 1901-2014 and the simulations from the Geophysical Fluid Dynamics Laboratory coupled model. The observed results suggested that the SAT variations exhibited robust non-uniform spatial features at multi-time scales. For the variations in inter-annual to decadal time scales (IDV), the intensity generally increased from south to north, with the strongest intensity being around Siberia and four times that of the weakest intensity found around China. The IDV leading pattern showed a north-south dipole across 40 degrees N. The simulated results suggested that the north-south dipole and the northwards increase of the IDV were due to internal interactions within the complex nonlinear climate system, but the natural and greenhouse gas forcings could intensify the IDV. The warming trend of the SAT was generally homogeneous, but it showed distinctive multi-decadal fluctuations in different regions. The linear secular trends and robust multi-decadal variation around Siberia and China corresponded to the considerable acceleration and deceleration in the warming over the two regions, respectively. The warming around Siberia was mainly caused by greenhouse gases but its modulation due to natural forcing was also considerable because of the robust multi-decadal variations. Around China, the multi-decadal variation, contributed by the natural forcing, can explain more than half the variances in the warming. The warming trend around central Asia was intense and parabolic, and the multi-decadal variation over there was weak and showed few modulating effects
Global trends in phytoplankton research of river ecosystems during 1991-2016: A bibliometric analysis
No full textRivers are representative lotic freshwater ecosystems and are regarded as the lotic belt for transportation of nutrients and pollutants from the land to the sea. To understand the global research profile and fronts of river phytoplankton, 13,434 publications regarding river phytoplankton indexed by Web of Science during 1991-2016 were analyzed using a bibliometric approach. The EU Water Framework Directive presents a good case for promoting not only frequent cooperation between European countries but also the development of river phytoplankton studies. Such directives are anticipated to promote global consistency of the water evaluations and management in the future. Phytoplankton play important roles in bio-assessments and energy flow of river ecosystems. The trait-based method has many advantages compared with traditional methods based on community composition and metric indices. The trait-based method considers a complex mixture of stressors by combining qualitative and quantitative parameters but also transfers different geographic taxonomic compositions to a similar complement of traits. Moreover, phytoplankton-based food chains are generally considered to transfer energy and carbon more efficiently to higher trophic levels than bacterial-based food chains. Algae are a high-quality food for higher trophic levels, compared with bacteria and terrestrial material. The algal component of periphyton has been demonstrated to be the major basal component of stream food webs
Deep Hydrography of the South China Sea and Deep Water Circulation in the Pacific Since the Last Glacial Maximum
No full textThe oxygen (O-18) and carbon (C-13) isotopic compositions of benthic foraminifer Cibicidoides wuellerstorfi as reliable proxies of deep water properties were compiled from 50 core-top and down-core sites in the South China Sea (SCS) for two time slices of the late Holocene and Last Glacial Maximum (LGM) to reconstruct the glacial deep hydrographic structure. The bathymetric profiles of both O-18 and C-13 in the SCS show similar trends between the LGM and the late Holocene, but the O-18 gradients between intermediate (approximate to 500-1,000 m) and deep (>1,500 m) waters obviously increased during the LGM, indicating an intensified bathyal stratification and weakened vertical mixing between intermediate and deep waters in the SCS during the glacial period. A spatial comparison of bathymetric O-18 and C-13 profiles was also made for the Southern Ocean (Indo-Pacific sector) and southwest Pacific, western equatorial Pacific (i.e., Ontong Java Plateau), off-Japan margin as well as the SCS. The meridional C-13 profiles indicate a decreasing trend of C-13 (aging effect) in the deep layer (approximate to 1,200-2,600 m) from south to north, reflecting a northward flow pathway of deep waters from the southwest Pacific (upstream) to the low-latitude northwest Pacific and the SCS (downstream) during the LGM. The gradients of deep water O-18 (approximate to 1,200-2,600 m) between these upstream and downstream regions were higher during the LGM relative to the Holocene, implying that a greater contribution of the northern-sourced North Pacific Intermediate Water (NPIW) with a relatively low O-18 signature to the deep waters in the low-latitude Pacific during the LGM
The Role of Nanograins in Ductile Shear Zones: An Example from Hainan Island, Northern Margin of South China Sea
No full textNanograins are commonly observed in slip faults and ductile shear zones, and it is more and more widely accepted that nanograins can lubricate the fault, but the role of nanograins in the ductile shear zone is unclear. In order to study this problem, samples taken from the Xiaomei ductile shear zone, Hainan Island, were carefully observed with SEM and TEM. Our observations show that nanograins found in our samples can be divided into three types based on their morphologies: spherical nanograins (S-grain), rod-like nanograins (R-grain) and nanoclays. We find that nanograins are localized in the boundaries of the large minerals, and are likely to localize strain and decrease shear strength in the ductile shear zone. We think S-grains are mainly originated from milling process, and they can also come from solidification of melts and devitrification of amorphous materials. The mirror surfaces can decrease the friction coefficient, and the formation of layered materials is likely to represent the steady state creep
Abnormal data retrieval of three-dimensional OBS survey at the Bashi Channel area of the South China Sea
No full textA three-dimensional seismic survey was carried out in the Bashi Channel area of the South China Sea from April to July 2016 by R/V "Shiyan2" in order to explore the crustal structure of the northern Manila trench subduction zone. Forty-eight Chinese portable OBSs were deployed and finally 41 OBSs were recovered. A total 10,800 shots were fired with shooting lines up to 2300 km long. Abundant seismic data were collected, but those recorded by 25 new types of Chinese OBSs were abnormal. No seismic phases can be identified from these 25 seismic record sections, leading to a serious problem in subsequent structural simulation. After checking data format, verifying shooting time, comparing shooting signals with adjacent normal OBSs, and analyzing the external and internal OBS data file time, we finally found that the sample intervals of abnormal OBSs have been changed from 4. 0 ms to 4. 5 ms. The OBS internal time was calculated wrong due to the sample interval change, and no seismic phases were identified during data format conversion. Decreasing CPU frequency of the new type OBS in order to save energy is the main reason to change the sample interval. By use of adjusting sample interval and resampling, the abnormal seismic data of 25 OBSs were retrieved eventually and clear seismic phases can be identified in these 25 seismic record sections. This research not only retrieved the valuable OBS data which are the basis for further structural simulation, making the maritime mission of NSFC major project more complete, but also promoted the scientific significance of this cruise and provided an important reference for the development and usage of Chinese OBSs in the future
Postseafloor Spreading Volcanism in the Central East South China Sea and Its Formation Through an Extremely Thin Oceanic Crust
No full textP wave velocity models were obtained by forward and inverse modeling from 38 ocean bottom seismometers deployed in the central East subbasin of the South China Sea (SCS). Four types of crust have been defined: (a) thin oceanic crust (<5 km), (b) typical oceanic crust (5-6 km), (c) thick oceanic crust hosting postspreading volcanoes (>6 km) with significant intrusive roots, and (d) thick oceanic crust with enhanced spreading features (>6 km) but without significant roots. Within the central East subbasin, the thin oceanic crust, only identified inside an 80 km wide zone, is located within an overall 150 km wide domain characterized by N055 degrees seafloor spreading trends. The postspreading volcanoes were formed during a N-S tensional episode around 6-10 Ma, several millions of years after seafloor spreading ceased in the SCS. Seafloor spreading (N055 degrees and N145 degrees) and postspreading (N000 degrees and N090 degrees) features are observed in the morphology of some of these volcanoes. The rupture of the brittle thin oceanic crust was focused where the crust was the weakest, i.e., at the intersection of the extinct spreading ridge with former fracture zones. From geological and geophysical arguments, we suggest that the postspreading volcanism might have been influenced by the Hainan plume activity through a buoyancy-driven partial melting mechanism
Seismotectonics of the Taiwan Shoal region in the northeastern South China Sea: Insights from the crustal structure
No full textA cluster of earthquakes occurred in the Taiwan Shoal region on the outer rise of the Manila Trench. Although most were of small to medium magnitudes, one strong earthquake occurred on September 16, 1994. Several previous studies have provided important information to progress our understanding of this single earthquake. However, little is currently known about the earthquake cluster, and it is necessary to investigate the deep crustal structure of the Taiwan Shoal region to understand the mechanisms involved in controlling and generating it. This study presents a two-dimensional seismic tomographic image of the crustal structure along the OBS2012 profile based on ocean-bottom seismograph (OBS) data, which exhibits a high-velocity anomaly flanked by low-velocity anomalies in the upper crust beneath the Taiwan Shoal. In this study, 765 earthquakes (Richter magnitude M-L > 1.5) occurring between 1991 and 2015 were studied and analyses of earthquake epicenters, regional faults, and the crustal structure provides an improved understanding of the nature of active tectonics in this region. Results of analyses indicate firstly that the high-velocity area represents major asperities that correspond to the location of the earthquake cluster and where stress is concentrated. It is also depicted that the earthquake cluster was influenced by fault interactions. However, the September 1994 earthquake occurred independently of these seismic activities and was associated with reactivation of a preexisting fault. It is also determined that slab pull is resisted by the exposed precollision accretionary prism, and the resistive force is causing accumulation of inplane compressive-stress. This may trigger a future damaging earthquake in the Taiwan Shoal region
红树林微生物DH-2胞外蛋白酶的性质及产酶条件优化
No full text从大亚湾红树林土壤样品中分离得到产蛋白酶菌株,鉴定所产胞外蛋白酶的酶学性质以及菌株的最佳发酵培养条件。采用平板透明圈法筛选菌株,福林酚显色法测定蛋白酶的酶活,通过单因素和正交试验确定其最佳发酵培养基以及发酵条件。从壤样品中分离得到一株产蛋白酶的枯草芽孢杆菌DH-2,该菌株分泌的蛋白酶最适反应pH和温度分别为8.0和65℃,50℃保温处理60 min后,剩余酶活仍保留80%以上。该蛋白酶对多种金属离子、有机溶剂及表面活性剂均有较好的耐受性。确定该菌株产蛋白酶的最适条件:1%(m/V,下同)可溶性淀粉,1%胰蛋白胨、1%NaCl,初始pH 5.5及7%的接种量,40℃培养36 h。在最适条件下测得其发酵液的酶活为236.30 U/mL,约为初筛时的酶活的8倍。该蛋白酶具有较为广阔的作用温度和pH范围,金属离子、有机溶剂及表面活性剂耐受性好,酶的性质比较稳定