南海中东部重磁场及构造特征的研究

通过收集的重磁资料和前人研究的成果来研究本区的构造特征。首先对南海中东部的自由空间重力异常特征、磁力ΔT异常特征进行分析;然后利用滑动平均法、垂向二阶导数法和水平方向导数等方法,得到了垂向二阶导数异常、区域异常以及剩余异常;最后对异常图进行分析解释,确定研究区的主要构造特征,同时结合南海中东部的地质资料来确定研究区的断裂构造。结果表明,该地区的断裂主要有:中部以洋盆边缘断裂和洋中脊为主;东部为海沟断裂带。研究区断裂走向为NE-NEE向、SN向和近SN向。

Distribution of heavy metals and nutrients in rainwater in coastal regions between the southern Yellow Sea and East China Sea

Rainwater samples were collected in series in Qianliyan Island (southern Yellow Sea) and Shengsi Archipelago (East China Sea) between May 2000 and May 2002, chemical analysis for pH values, concentrations of heavy metals (Cu, Pb, Zn and Cd) and nutrients (NH4+, NO3-, PO43-, SiO32-) were performed. Results indicate that concentrations of most of the heavy metals and nutrients in rainwater show clear seasonal variation, i.e. high level in winter and low level in summer. Regionally, concentrations are higher in the southern Yellow Sea than in the East China Sea, but the annual input of heavy metals into oceans by wet deposition is similar in both stations. However, the input of nutrients by wet deposition in the East China Sea is 2–3 times higher than that in the southern Yellow Sea. In individual, Pb and PO43- are input to the sea mainly by dry deposition; whereas Cu, Zn, Cd and N compounds are input dominantly by wet deposition, the N/P ratios in the rainwater from two stations are much higher than those in seawater, showing a significant impact of atmospheric wet deposition on marine production and biogeochemical circulation of nutrients in these sea regions.

Chemical and spectroscopic characterization of dissolved humic substances in a mangrove-fringed estuary in the eastern coast of Hainan Island,China

Mangrove-derived dissolved organic matter （DOM） has an important effect on estuarine and coastal area on a large scale. In order to improve the understanding of origin, composition, and fate of DOM in mangrove-fringed estuarine and coastal areas, dissolved humic substances （DHS） were isolated from one mangrove pore-water sample and one near-shore seawater sample downstream the mangrove pore-water site in the eastern coast of Hainan Island, South China. Fulvic acids, humic acids and XAD-4 fractions were obtained from the two water samples by using a two-column array of XAD-g and XAD-4 resins. Chemica~ and spectroscopic methods were used to analyze the features of these DHS. Compared to the mangrove pore-water DHS, the near-shore seawater DHS were found rich in 13C with lower C/N ratios and more aliphatic compounds and carbohydrates, but less aromatic structures and carboxyl groups. As for the three fractions of the two DHS, XAD-4 fractions contain more aliphatics, carbohydrates, carboxyl groups, and enrich in 13C with respect to both fulvic and humic acids. Photo-oxidation transformation and contribution from marine-derived DOM were considered as the main reasons resulted in the difference in compositional features for these DHS in this study.

Observations of Kuroshio intrusion into the South China Sea

To discuss the intrusion of the Kuroshio into the SCS,we examined the mixing between the North Pacific and South China Sea(SCS) waters based on in-situ CTD data collected in August and September 2008 and the moored ADCP data taken from mid September 2008 to early July 2009.The CTD survey included four meridional sections from 119°E to 122°E around the Luzon Strait,during which pressure,temperature,and salinity were measured.The CTD data show that the isopycnal surface tilted from the SCS to the North Pacific;and it was steeper in the lower layers than in the upper ones.Meanwhile,we found strong vertical mixing taken place in the areas near 121°E.The Kuroshio in high temperature and salinity intruded westward through Luzon Strait.The frequency of buoyancy was one order of magnitude greater than that of the common ones in the ocean,suggesting stronger stratification in the northeastern SCS.On the other hand,the long-term ADCP data show that before late October 2008,the direction of water flow in the SCS was eastward,and from November 2008 to late February 2009,it turned northwestward in the layers shallower than 150 m,while remained unchanged in deep layers from 200 to 450 m.From March to June 2009,the direction shifted with increasing depth from northward to southward,akin to the Ekman spiral.EOF analysis of the current time series revealed dominant empirical modes:the first mode corresponded to the mean current and showed that the Kuroshio intrusion occurred in the upper layers only from late December to early March.The temporal coefficient of the first and the second mode indicated clearly a dominant signal in a quasi-seasonal cycle.

Nonlinear interactions among internal tidal waves in the northeastern South China Sea

We used a set of 75-day long ADCP data from the northeastern South China Sea(SCS) to investigate nonlinear interactions among freely propagating internal tidal waves.The kinetic energy spectra displayed significant peaks at some higher tidal frequencies,such as O1M2(O1+M2),and M4(M2 +M2),where O1 is the lunar diurnal internal tide,M2 is the lunar semidiurnal internal tide,and M4 is the first higher harmonic frequency of M2.These higher tidal harmonic frequency peaks,as well as the fundamental tidal harmonic peaks,show a σ-2.3 spectral falloff rate with frequency.In addition,we explored the possible generation mechanism of higher tidal harmonics.Analysis on the rotary and bicoherence spectra suggests that strong forced non-resonant interaction induced by nonlinear advections was the dominant physical mechanism that induced these higher tidal harmonics.Moreover,the energetic,freely propagating semidiurnal(M2) internal tidal wave played the most crucial role in these interactions.These results indicate that strong nonlinear forced non-resonant interactions among internal tides can be one of the processes responsible for the redistribution of energy in the internal wave spectrum.

Response of Asian Summer Monsoon to CO_2 Doubling

Based on simulations of the IPCC 20C3M and SRES A1B experiments in ten coupled models, the Asian summer mon-soon (ASM) response to CO2 doubling and the different responses among models are examined. Nine models show the similar results that the weakening of land-ocean thermal contrast caused by the CO2 doubling contributes to a weaker large-scale ASM circulation. Further analysis in this study also shows that the major ASM components,such as the Somali cross-equatorial flow,the low level India-South China Sea monsoon trough,and the upper level tropical easterly jet stream, weaken as CO2 doubles.However,the ASM rainfall increases as a result of the increased moisture from the warmer Indian Ocean and the South China Sea, and the enhanced northward moisture transport over the ASM region.For the response of enhanced northward moisture transport over South Asia, the positive contribution of moisture content increase in the Indian Ocean is dominant and the negative contribution of the weaker mon-soon circulation is secondary at 850 hPa,but both have positive contribution to the total moisture transport along the East China coast.The paradox of the weaker ASM circulation and the increasing precipitation in CO2 doubling is confirmed.It is found that strengthening of northward moisture transport could intensify the precipitation and atmospheric heat source over the north Arabian Sea and East China,and result in enhanced southwesterly at 850 hPa as global warming occurs.All ten models show significant enhanced southwesterly response over the north Arabian Sea,and six of them show enhanced southwesterly response along the East China coast.

Impact of assimilating Taiwan＇s coastal radar radial velocity on forecasting Typhoon Morakot (2009) in southeastern China using a WRF-based EnKF

This study explores for the first time the impact of assimilating radial velocity(Vr)observations from a single or multiple Taiwan's coastal radars on tropical cyclone(TC)forecasting after landfall in the Chinese mainland by using a Weather Research and Forecasting model(WRF)-based ensemble Kalman filter(EnKF)data assimilation system.Typhoon Morakot(2009),which caused widespread damage in the southeastern coastal regions of the mainland after devastating Taiwan,was chosen as a case study.The results showed that assimilating Taiwan's radar Vr data improved environmental field and steering flow and produced a more realistic TC position and structure in the final EnKF cycling analysis.Thus,the subsequent TC track and rainfall forecasts in southeastern China were improved.In addition,better observations of the TC inner core by Taiwan's radar was a primary factor in improving TC rainfall forecast in the Chinese mainland.

Dongsha erosive channel on northern South China Sea Shelf and its induced Kuroshio South China Sea Branch

Based on a data base of multi-channel seismic profiles covered over Dongsha plateau of the northern South China Sea margin, we found that the sea bed morphology of northern South China Sea margin had been changed dramatically after Dongsha uplifting, that sedimentary layer since Miocene age had been eroded with maximum eroded thickness more than 1000 m, and that an erosive channel had been formed of 20 km in width and 200 km in length and several hundreds meters in depth on the outer shelf of northern South China Sea. The erosive channel is parallel to the 600 m isobath line, stretching from northeast to the southwest north of Dongsha uplift. The Kuroshio intrudes the South China Sea through Luzon Strait both in winter time and summer time, and in the northern South China Sea margin area, the intruded Kuroshio Branch takes the form of Pacific-Indian Ocean Through Flow (PITH) in winter time, while the Luzon Strait Subsurface Inflow (LSSIF) in summer time, the routes of both PITH and LSSIF coincide well with the distribution of the erosive channel. After climbing from the northern slope up to the northern shelf, and after joined by the southward flow from the middle northern shelf of South China Sea, the Kuroshio Branch is strengthened and thus is able to erode the sea floor, and the shape of the erosive channel is a result of the long-term interaction between the Kuroshio South China Sea Branch and the Dongsha outer shelf sea floor.

Morphology, sedimentary characteristics, and origin of the Dongsha submarine canyon in the northeastern continental slope of the South China Sea

The Dongsha submarine canyon is a large canyon belonging to a group of canyons on the northeastern South China Sea margin Investigation of the Dongsha canyon is important for understanding the origin of this canyon group as well as the transport mechanism of sediments on the margin, and the evolution of the Taixinan foreland basin and the associated Taiwan orogenic belt. In this study, the morphology, sedimentary characteristics, and origin of the Dongsha canyon were investigated by inte- grating high-resolution multi-channel seismic reflection profiles and high-precision multibeam bathymetric data. This is a slope-confined canyon that originates in the upper slope east of the Dongsha Islands, extends downslope in the SEE direction, and finally merges with the South Taiwan Shoal canyon at a water depth of 3000 m. The total length and average width of the canyon are around 190 and 10 km, respectively. Eleven seismic sequence boundaries within the canyon fills were identified and interpreted as incision surfaces of the canyon. In the canyon fills, four types of seismic facies were defined： parallel onlap fill, chaotic fill, mounded divergent facies, and migrated wavy facies. The parallel onlap fill facies is interpreted as altemating coarser turbidites or other gravity-flow deposits and fine hemipelagic sediments filling the canyon valley. The chaotic fill faci- es is presumed to be debrites and/or basal lag deposits filling the thalwegs. The mounded divergent and migrated wavy seismic facies can be explained as canyon levees consisting mainly of overspilled fine turbidites and sediment waves on the levees or on the canyon-mouth submarine fans. Age correlation between the sequence boundaries and the ODP Site 1144 data suggests that the Dongsha canyon was initiated at approximately 0.9 Ma in the middle Pleistocene. Mapping of the canyon indicates that the canyon originated at the upstream portion of the middle reach of the modem canyon, and has been continuously expanding both upstream and downstream by retrogressive erosion, incision, and deposition of turbidity currents and other gravity transport processes. The ages of the sequence boundaries representing major canyon incision events are in good agreement with those of global sea-level lowstands, indicating that sea-level changes may have played an important role in the canyon＇s development. The Dongsha canyon developed in a region with an active tectonic background characterized by the Taiwan up- lifting and the development of the Taixinan foreland basin. However, no evidence suggests that the canyon formation is directly associated with local or regional faulting and magmatic activities. Turbidity currents and other gravity transport processes （includ- ing submarine slides and slumps） may have had an important influence on the formation and evolution of the canyon.

A thinned lithosphere beneath coastal area of southeastern China as evidenced by seismic receiver functions

During Mesozoic to Cenozoic time, the large-scale tectono-magmatism had strongly modified the lithosphere beneath the southeastern continent of China, leaving the present-day lithosphere as a new one evolving from the ancient lithosphere that was largely removed and replaced. But this model proposed from geochemical and petrological research is urgently in need of support from seismic observational evidence. In this paper, based on the dataset recorded by the dense stations of two NE ori- ented broadband seismic profiles deployed in the coastal area of southeastern China （SE China）, both P-wave （P-RF） and S-wave （S-RF） receiver functions were isolated. We identified Pls phase converted from the Lithosphere-Asthenosphere Boundary （LAB） in P-RFs of individual stations. Migrated Pls phase indicated a depth of 60-70 km for LAB. Inver- sions/comparisons of P-RF （Pls phase） and S-RF （Sip phase） waveforms together with Ps and Sp imaging for the crust and up- per mantle structure further confirmed this result. P-RF and S-RF migrated images exhibit that a flat LAB is positioned at the depth of 60-70 km spreading along the profile, whereas a distinct structural change of lithospheric base appears at the Min River estuary. Both Ps and PpPs migrated images of P-RFs present an abrupt Moho drop across the Min River fault from south to north, which is consistent with previous result obtained from deep seismic sounding. By taking into consideration other ge- ological and geophysical features such as locally high anomalies of crustal Poisson＇s ratios and heat flow at the Min River es- tuary, we infer that the Min River fault penetrates down to the Moho and may, furthermore, interfere in the deeper lithospheric structure.

Climatic variation of tropical cyclones affecting China during the past 50 years

Trends of the tropical cyclones (TCs) influence on China and its four subregions,namely the South China (SC),East China (EC),Northeast China (NEC),and China's inland area (CI),are detected by applying quantile regression to the CMA-STI tropical cyclone best track and related severe wind and precipitation observation datasets.The results indicate that in the past 50 years,the number of TCs affecting China and its four subregions has remained steady,except that the frequency in extremely active years has decreased not only in China as a whole,but also in NEC.In addition,TC activity is found to have weakened over the northwest South China Sea,Guangdong,and Shandong Peninsula.However,the most important changes in seasonality are found in the first quartiles of the number of days of TCs affecting CI.While the extreme values of sustained winds all have decreasing trends,the extreme values of wind gusts are completely different not only among different orders of extreme values,but also among different subregions.However,the trends of extreme TC rainfall,namely the maximum storm precipitation and the maximum 1-h precipitation,are not significant.