The Indian Ocean is the important channel of #cod#x0201c;Maritime Silk Road#cod#x0201d; in the 21st century, and is the main origins of the two sub-systems (the Indian monsoon and the East Asian monsoon) of the Asian monsoon, which plays an important role in the global change. Tephra in marine sediments as a stratigraphic tool not only records the information of source area, transporting medium and mechanism, but also can effectively make isochronal dating. The content and composition of tephra has great significance to study sedimentary and tectonic activity. In this paper, we summarize progress of applied research of tephra in the Indian Ocean sediments, including: 1) the definition of tephra and its isochronal dating in the Indian Ocean sediments; and 2) the analysis method of tephra (volcanic glass) and the effect on trace material sources, volcanic structure and magmatism, the Indian monsoon evolution, etc. Our future work on tephra in the Indian Ocean mainly includes in two tasks: 1) tephra and its application in surface sediments; and 2) tephra and high-resolution paleoenvironment records. Finally, regional climate change and driving factors are discussed by means of tephra isochronal dating.

Based on the sediment grain size analysis of the gravity core ADM-C1 from the Andaman Sea, grain size populations were partitioned using the method of grain size versus standard deviation. It was found that two sensitive grain size populations (1.5~11.9 #cod#x003bc;m and 11.9~74 #cod#x003bc;m) had significant fluctuations, and were mainly controlled by the circulation dynamics in the sea area, which was closely related to the Indian Summer Monsoon (ISM). Evolution history of the ISM in the Andaman Sea during the Holocene is reconstructed based on sensitive grain size populations. The results show that the evolution history of the ISM generally can be divided into three stages: 10.4~8.8 ka BP, when the intensity of the ISM was the weakest of the three stages; 8.8~4.7 ka BP, when percentage and average grain size of the sensitive grain size populations had significant increase, indicating that the intensity of the ISM was at its strongest stage during this period; and 4.7~0 ka BP, when sharp decreases of percentage and average grain size of the sensitive grain size populations indicated that the intensity of the ISM was obviously weaker than those of the previous two stages. The reconstructed evolution history of the ISM shown in this paper is consistent with many other records during the Holocene, and further verifies the feasibility that sensitive grain size can be used as a reliable proxy of the ISM in the Andaman Sea.

The Indian Ocean is critical for China's economy and climate changes. Coral-based climatic and environmental changes can effectively prolong recorded time series, longer than instrumental records, because reef corals are widely distributed in the Indo-Pacific Ocean, their aragonite skeletons have an annual banding structure and are suitable for U/Th dating, and their geochemical records are reliable. In this paper, we first introduced the coral reef distribution in the Indian Ocean, and its current status and importance. Then, we reviewed coral-based past climatic and environmental change reconstructions based on four aspects including environmental pollution, paleo-storms, paleo-sea level variation, and climate changes (such as Asian monsoon, rainfall, atmospheric circulation, sea surface temperature, Indian Ocean Dipole, and ocean currents). We ended the paper with a summary and discussion.

Recent investigations of the ultraslow-spreading (full spreading rate: 12~18mm#cod#x000b7;yr^-1) Southwest Indian Ridge revealed two kinds of crustal structure: Magmatic and amagmatic accretionary crust. Magmatic accretionary segments appear as the axial rise, relatively low mantle Bouguer gravity anomaly, strong magnetization and thick crust. Amagmatic accretionary segments feature detachments and abundant high-angle normal faults, lack of transform faults, deep water, relatively high mantle Bouguer gravity anomaly and weak magnetization. There are also significant amount of serpentinized peridotites exposed on the seafloor, and the igneous crust is thin, even absent. The Southwest Sub-basin of the South China Sea (SWSB) has relatively slow-spreading rates (full spreading rate: 50~35mm#cod#x000b7;yr^-1). The central part of SWSB also presents thin crust and there might be some serpentinized peridotites in the basin area, which are similar to the characteristics of the amagmatic accretionary crust in the ultraslow-spreading Southwest Indian Ridge.

The frequency of tsunamis occurred in the Indian Ocean is much lower than that of the Pacific Ocean; in the past fifteen years, however, three out of ten major tsunamis triggered by the earthquake occurred in the Indian Ocean region. The Makran and the Sumatra subduction zones are the two active regions in the North Indian Ocean for tsunamigenic earthquakes. In the northern Sumatra subduction zone, two earthquakes with Mw 9.0 and 8.6 occurred on December 26, 2004 and March 28, 2005, respectively, and they were ranked as the second and fourth largest earthquakes in the past half century. The 2004 event generated a tsunami disaster with the largest wave runup of 50.9m, and resulted in the most devastating historical disaster, while the 2005 event only generated a maximum wave runup of 4m. What caused the completely different tsunami scenarios by the two earthquakes with similar location and focal mechanism is worthy of study. Recent studies showed that the seismic activities along the Makran subduction zone were divided into two neighboring sections: the seismic activity of the eastern section is significantly stronger than the western section, and the 1945 tsunami was located in this section. Whether the western section, or the whole Makran subduction zone has the potential to rupture together and thus generate major tsunamis requires further investigations.

The discovery of cold hydrocarbon seeps has been one of the most important achievements of marine geology besides hydrothermal vents during the last half century. Gravitational and tectonic forces are common in the northern Indian Ocean, which results in methane seepages on the seafloor. Recent studies of cold hydrocarbon seeps from Makran and Bay of Bengal areas have made important progresses and these findings have opened a window for understanding the resource and environmental issues of submarine cold seeps. An overview about the present knowledge of the northern Indian Ocean is provided, which concentrates on 1) sedimentary records of the cold seep activities in the Makran area both inside and outside of the oxygen minimum zone and the cold seep activity triggered by earthquake activity, and 2) the dissociation of gas hydrate in the Bay of Bengal that recorded in the carbonate rocks. Furthermore, challenges of seep research in the northern Indian Ocean and scientific problems in the future are discussed.

In this study we used the high-resolution shipboard multibeam bathymetry and gravity data to investigate the tectonic and magmatic characteristics of the Southwest Indian Ridge (SWIR) between 14#cod#x000b0;and 25#cod#x000b0;E. First, we filtered the original bathymetry to obtain a short-wavelength bathymetry map (wavelength less than 20 km), which was used together with the topographic slope map to identify surface normal faults. We also obtained a long-wavelength bathymetry map (wavelength more than 20 km) that was used to calculate across-ridge axis topographic relief. We also calculated the fraction of plate separation accommodated by magmatic accretion, i.e., the M factor. We then calculated the Residual Mantle Bouguer Anomaly (RMBA) by removing from the free-air gravity anomaly the gravitational effects of water/crust and crust/mantle interfaces as well as lithospheric plate cooling, assuming a reference crustal thickness of 6 km. Finally, we calculated the M factor, the mean values of RMBA, and fault throws within 10-km- wide running windows along profiles across the ridge axis and investigated the correlations among these parameters. We found that the magma supply varied significantly in time and space at the SWIR between 14#cod#x000b0;and 25#cod#x000b0;E and the axial relief showed strong asymmetry between conjugate ridge flanks that seemed to be controlled by the mean M factor near the ridge axis. Regionally-averaged tectonic extensional strains (i.e., 1-M) were about 20%~50% and the southern flank underwent greater average tectonic extensional strains. Areas with thicker crust (i.e., more negative RMBA) are often associated with greater M values and smaller fault throws, indicating episodes of increased local 3D magma supply at this ultraslow spreading ridge.

Dragon Flag Hydrothermal Field (DFHF, 49º39'E), located on the ultraslow-spreading Southwest Indian Ridge (SWIR), was the first active hydrothermal vent discovered in this area; and it is an essential location for seafloor exploration of polymetallic sulfide resources and deep ocean studying for China. A three-dimensional tomographic study successfully revealed the deep structure characteristics of the DFHF, but it only provided the static velocity information. For comparison, seismic anisotropic study is an effective method for investigating the dynamic mechanism of the DFHF. In this paper, we briefly introduce the oceanic bottom seismometer (OBS) surveys of both active and passive sources. Our preliminary analysis shows a cosine relationship between travel-time residuals and azimuths based on the velocity structure obtained from previous three-dimensional tomography, indicating that there is an anisotropy on the velocity structure at the DFHF; but, the source of the anisotropy is unclear until now. Seismic anisotropic study at the DFHF will contribute greatly to hydrothermal circulation mechanism and dynamic evolution process of hydrothermal field on the SWIR. Hence, we intend to conduct seismic anisotropic study by combining azimuthal seismic anisotropy and shear-wave splitting techniques based on the OBS data generated by the active (airguns) and passive (earthquakes) sources. Combined with three-dimensional velocity model and regional geological background data, anisotropic parameters of fast-wave direction and travel-time difference between fast and slow waves are used to depict the distribution of crustal cracks, stress variation and mantle flow. In addition, hydrothermal circulation mechanism, lithospheric deformation and deep dynamics process are further revealed.

Based on the monthly mean data of Argo and Aquarius satellite observations, we analyze the seasonal variation of sea surface salinity (SSS) in the tropical South Indian Ocean. The results show that the SSS has significant seasonal variation characteristics, that is, the SSS is lower in winter and higher in summer in the region of 60°-80°E, 5°-15°S. However, the center of anomalous SSS does not correspond to the center of anomalous precipitation. The seasonal variation of precipitation cannot explain the seasonal variation of SSS. Salinity budget analysis indicates that the ocean dynamics contribute to the seasonal variation of SSS. During the summer half of the year, the SSS increasing is attributed to the meridional advection transport high salinity from the equatorial region to the South Indian Ocean, as well as the entrainment strengthening from April to May. In the winter half of the year, precipitation increases, the northward currents transport low-salinity water induced by increasing precipitation to the study region, which favors the accumulation of low-salinity water in the region; at the same time, the westward zonal advection transports the low-salinity water from the Southeastern Indian Ocean to the west, both of which have important contributions to the decreasing SSS.

Based on the All India Rainfall data, NCEP/NCAR reanalysis data and HadISST data, we demonstrated a delayed effect of the El Nino-Southern Oscillation (ENSO) and Indian Ocean Basin (IOB) mode on Indian summer monsoon rainfall (ISMR). The results showed that decreased Indian rainfall in the early summer (Jun-Jul) and increased rainfall in the late summer (Aug-Sep) are influenced by the anti-symmetric mode and the second warming in the northern Indian Ocean (NIO). The responses of ISMR to El Niño are distinct between the developing and decaying years. In the developing year of El Niño, the ISMR decreases due to the change of Walker circulation in the tropical ocean. In the decaying year, the IOB mode decreases (increases) the ISMR in early (late) boreal summer. The anti-symmetric pattern of atmospheric anomalies with northeasterly (northwesterly) wind anomalies north (south) of the equator happens in spring. The northeasterly anomalies weaken the Indian Summer Monsoon and force the second anomalous sea surface temperature (SST) peak in the NIO by reducing wind speed and surface evaporation. The southwest monsoon brings more moisture (Q°), which is produced by the warm NIO SST to the India subcontinent, leading to increased monsoon rainfall in the late summer.

The dynamic characteristics of time-mean meridional overturning circulation in the Indian Ocean was examined using six reanalysis datasets. The results showed consistent time-mean features of the deep meridional overturning circulation, which is an anticlockwise overturning cell with inflows in the bottom and deep layers and outflows in the intermediate and upper layers. Dynamic decomposition of meridional overturning circulation was applied to examine the similarities and differences of every dynamic component. The structure of Ekman component is an anticlockwise overturning cell in the South Indian Ocean with maximum strength at ~10°S. In the region south of 10°S, geostrophic and external components show clockwise and anticlockwise overturning cells, respectively, they both reach maximum strengths at ~27°S. Based on different products of heat and momentum fluxes used, the dynamic components resulted from the six datasets show some inconsistent features as follows. The overall structures of Ekman component are similar since the wind fields of the six datasets have few differences. The discrepancies of the geostrophic component in the six datasets are due to the strength of the baroclinic flows in the interior ocean and the structure of the western boundary current: the greater the baroclinic flows in the interior ocean, the stronger the strength of the geostrophic component; the wider the western boundary current, the greater impact on the baroclinic flows in the interior ocean, and then on the strength of the geostrophic component. The strength of the external component is affected by the intensity of the western boundary current: the greater the western boundary current, the stronger the strength of the external component.

With extensive development of marginal basins and subduction zones, the western Pacific is a key area in scientific ocean drilling. This paper intends to show the current status of scientific ocean drilling and discuss potential future breakthroughs, through summarizing scientific ocean drilling results in geodynamics over the past 40 years in the western Pacific. Drilling results documented the evolution of the marginal basins, including the Japan Sea, the Philippine Sea and the South China Sea. Deep sea sediments and geochemical analysis of basalts provided important information for evolution of basins and mantle processes. Ocean drilling results verified that the dip of a subducting slab not only has an effect on dynamic mechanism of the subduction factory but also controls plate coupling at the subduction zone. A record depth of 3056 mbsf had been drilled into the forearc of Nankai Trough subduction zone and retrieval of rock samples from the seismogenic zone is expected in the next few years. Ocean drilling results support more than one hypothesis of formation of the oceanic plateaus in the western Pacific, including the Shatsky Rise and the Ontong Java Plateau. Pelagic brown claystone occurred in the southwestern Pacific marginal basin, and it’s formation was controlled by seafloor spreading. In both the South China Sea and the Celebes Sea, pelagic brown claystone lie directly above the basement basalt units. Because of the structural complexity and diversity of the western Pacific, many scientific problems still need to be resolved despite a large number of ocean drilling expeditions.

Using the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) and Simple Ocean Data Assimilation (SODA) reanalysis, this study investigates the decadal variability of sea surface temperature (SST) over the South China Sea (SCS) associated with El Niño-Southern Oscillation (ENSO) and Indian Ocean Basin (IOB) mode. Four epochs during 1870-2007 were identified with significant differences. The SST anomalies showed single peak of warming during the periods before the 1950s, and double peaks of warming after the 1950s. The results of ocean heat budget indicated that the oceanic advection anomalies contributed little to the SCS warming in an ENSO developing year during Epochs 2, 3 and 4. The SCS warming was mainly due to the latent heat flux and shortwave radiation anomalies associated the ENSO. However, the SCS warming in the summer of an ENSO decay year could attribute to oceanic advection during the periods after the 1950s. The different contributions of oceanic advection to the SST warming in the past 138 years indicated that the ENSO teleconnection to the SCS climate experienced decadal changes.

Based on the simple ocean data assimilation (SODA) reanalysis and satellite observations, we analyzed the seasonal variation of the meridional salt exchange between the Bay of Bengal (BOB) and the equatorial eastern Indian Ocean (EEIO). We estimated the annual-mean freshwater transport along the 6°N section at the mouth of the BOB. Results showed that the salt exchange between BOB and EEIO is dominated by the currents, and its direction is almost opposite between the east and west sides at the bay mouth. During the southwest monsoon (northeast monsoon), the salt flux is northward (southward) in the west while it is southward (northward) in the east of the bay mouth. Furthermore, the freshwater flows out of (into) the bay during April to October (November to the following March). The freshwater transport from the BOB to the EEIO happens in the whole depth. The annual-mean freshwater transport is 1.0×10⁵m³·s^-1.

We analyzed the interannual variability of the Wyrtki Jet and its salinity transport associated with Indian Ocean Dipole (IOD) in fall season using observations and Simple Ocean Data Assimilation (SODA). During negative IOD events, the Wyrtki Jet strengthened in the equatorial Indian Ocean, forced by enhanced equatorial westerly wind, which favors a stronger eastward transport of high-salinity water. During positive IOD events, the above-mentioned processes reversed. In addition, both the Wyrtki Jet and associated salinity transport anomalies on the strength and spatial distribution displayed significant asymmetric characteristics between the positive and negative IOD events. The amplitudes of zonal velocity and salinity anomalies were stronger during the positive IOD events. The anomalies centered in the central equatorial Indian Ocean during the positive IOD events, but they moved to the east in the negative IOD events. Moreover, the velocity anomalies reached a deeper depth during the negative IOD events, even though the salinity anomalies occurred at a rather shallow depth.

We estimated the Rossby wave phase speed in the tropical Indian Ocean (5°~20°S, and 5°~20°N) based on two- dimensional Radon Transform method, using monthly average sea level anomaly data (1993~2001) from multi-altimeters. We also calculated the first baroclinic gravity-wave phase speed, in combination with Rossby wave theoretical model, and the density by using climatologically averaged temperature and salinity profiles from the Simple Ocean Data Assimilation (SODA) reanalysis data (1993~2008). We found that Rossby wave phase speeds from model simulation and altimeter observations are in good agreement in 5°~20°S, but are quite different in 5°~20°N. The difference is possibly caused by 1) Rossby waves are diffracted and reflected by salient topographic features, and 2) interaction of Rossby waves generated by different boundaries. We computed the Rossby wave deformation radius utilizing the estimated phase speed, and further presented an analytic quadratic formula between latitude and deformation radius.

Chaetognaths, a unique group of marine zooplankton, play an important role in the trophic web of the pelagic realm. This study examined the species composition, abundance distribution and community structure of chaetognaths based on the data obtained from the tropical northeastern Indian Ocean in April-May, 2011. A total of 14 species of chaetognath were identified, characterized by the ecological groups with widespread tropical species. The dominant species were mainly represented by Sagitta enflata, Sagitta pacifica, Pterosagitta draco, and Sagitta ferox. The study area could be divided into three transects: eastern, equatorial and western transects based on the locations of sampling stations and marine environment. The range of species richness was from five to 14, with the high diversity at the equatorial and western transects, and below 10 at the eastern transect. An average of chaetognath abundance was 5.35(±2.82) ind·m^-3, with an uneven distribution characterized by high values at the western transect and low values at the equatorial and eastern transects. Results of species cluster analysis showed that the species of chaetognaths in the study area could be divided into three groups at the similarity level of 50%, with one group including 11 species and the other two groups only including three species each. The similarity percentage of community structure at the western transect was higher than at the other transects. The high diversity and abundance of chaetognaths at the western transect could be due to the influence of both coastal waters and upwelled waters.

Using the data collected during 2010~2012 Eastern Indian Ocean (EIO) cruises, we analyzed the hydrographical features in the upper layer of the southern Bay of Bengal (BoB) and eastern equatorial Indian Ocean (EEIO) during spring monsoon transition. We also studied the variations of meridional geostrophic transport and the thermocline at the mouth of the BoB. The results show that the equatorial westerly wind bursts become the dominant force in this region during spring intermonsoon transition around March-May. They change the equatorial pressure gradient force from eastward to westward by transporting the Arabian Sea water to the east, and weaken the equatorial undercurrent. At the BoB mouth, the wave propagation in the equator enhances the northward water volume transport, mixes the water masses form the Arabian Sea and BoB, and increase the salinity gradient. In addition, the remote forcing of equatorial waves leads to the formation of a cyclonic eddy at the BoB mouth. The remote forcing also deepens the thermocline at the west end in the southern BoB, while the upper-layer low salinity water keeps it shallow at the east end even though the thermocline in the EEIO is deepened.

Seasonal variation of the surface-layer circulation in the eastern tropical Indian Ocean (ETIO) is analyzed using nearly 20 years of surface absolute dynamic topography (ADT) data derived from satellite observations, ocean surface current analysis-realtime (OSCAR) data and the trajectories of Argos surface drifters. The results show that the seasonal variations of the surface circulation and the monsoons in the ETIO are almost coinstantaneous. The seasonal variations are more significant north of the equator. With this seasonal cycle of large-scale circulation, the circulation at the mouth of the Bay of Bengal changes accordingly. During May-September, the eastern mouth of the bay is dominated by the southward meridional current, which extends further southward off Sumatra, while it reverses to flow northward during the rest months. The meridional current in the western bay mouth is generally opposite to that in the east. The Argos drifting buoy trajectories further reveal that the water exchange pathways in and out the bay change seasonally. During the summer monsoon, the drifting buoys from the southern India Ocean and the Arabian Sea are mainly drifted into the bay from the west side of the bay mouth, while those from the bay are drifted out from the east side. The pathway reverses during the winter monsoon. This study also shows that the maximum variance of the zonal currents is caused by the monsoon currents including the Wyrtki Jet, while the maximum variance of the meridional currents is due to the East Indian Coastal Current (EICC) and Lakshadweep High (Lakshadweep Low).

Based on the NCEP/NCAR reanalysis data and the sea surface temperature (SST) data from the NOAA, the influence of SST anomalies (SSTAs) associated with the ENSO and IOD (Indian Ocean Dipole) events on low-level anomalous anticyclone in the Philippine Sea (named PSAC) was investigated using diagnosis methods and simulation experiments. The results are as follows. There is a more significant relationship between the IOD events in preceding autumn and the PSAC from winter to spring than that between other Indian SSTA events and PSAC. In a pure El Niño year, there is a notable PSAC appearing over the Philippine Sea from November to the following April. Compared to the El Niño events, the influence of pure positive IOD events on the PSAC is much weaker based on the reanalysis data and has shorter lifecycle from November to the following April. In the co-occurring year of El Niño and IOD, the PSAC tends to enhance and remain until the following August, which indicates that these SSTA events reinforce the PSAC jointly.

The spectral absorption properties of colored dissolved organic matter (CDOM) is important for building bio-optical model and for quantitative ocean color remote sensing. During the Eastern Indian Ocean Integrated Scientific Investigation Cruise from April 1 to mid-May, 2011, CDOM was sampled at nine stations along 6°N. Based on the dataset of 36 samples, we examined the relationship between spectral absorption coefficients and the slope of absorption curve (S). Over the whole transect, a_CDOM (355) spanned the range of 0.131-0.524 m^-1 with an average value of 0.272 m^-1, and a_CDOM(375) varied between 0.081 and 0.453 m^-1 with an average value of 0.242 m^-1. The concentration of CDOM was lower than that of nearshore waters, similar to the findings along 18°N of the South China Sea. There exists low correlation between chlorophyll a and absorption coefficient of CDOM. S_300—500nm varied between 0.008 and 0.019 nm^-1, averaging 0.012 nm^-1; S_300—350nm was a good indication to M, the molecular relative weight of CDOM. Absorption coefficient and Sof CDOM showed a good negative correlation, so did Mand absorption coefficient. By analyzing CTD data along the 6^oN section, water mass with high salinity at depth 30-90 m was found to have an impact on horizontal and vertical distribution of Mand a_CDOM(355).

This paper reviews the studies on the southern Indian Ocean (SIO) air-sea interaction and summarizes the climate variability in the SIO at the inter-annual time scale. Variance and correlation analysis of the Indian Ocean sea surface temperature (SST) indicates that a strong dipole oscillation occurs in the SIO, the so-called Southern Indian Ocean Dipole (SIOD). Large-scale atmospheric circulation in the midlatitude of the Southern Hemisphere plays a role in the formation of SIOD. A subtropical anticyclonic anomaly contributes to the tropical easterly anomalies and midlatitude westerly anomalies; this results in anomalies in latent heat flux, upwelling, and Ekman heat transport, and then changes the SST. Through changing the heat distribution in the atmosphere in the South Asia and tropical Pacific, the SIOD has an impact on tropical atmospheric circulation. In particular, it remotely influences the Asian summer monsoon rainfall; for example, a close relationship is found between SIOD and rainfall over China. In addition, the anomaly induced by the SIOD can result in anticyclonic atmosphere anomalies over the South China Sea and Philippine Islands.

The Sea Surface Temperature (SST) warming in the North Indian Ocean (NIO) is examined in terms of its association with ENSO using ICOADS monthly data for the period 1979?2007. The results indicate there are two SST warming periods in the NIO, occurring in El Ni?o developing year and decay year, respectively. The responses in the Arabian Sea and the Bay of Bengal are different. The change in surface heat fluxes related to ENSO is an important forcing to the NIO warming. The influences of shortwave radiation and latent heat flux are not simultaneous: the former is two or three months earlier than the latter. When the SST warms up in the NIO, the shortwave radiation turns to be a damping effect to the change in SST. Furthermore, the responses of SST in the NIO to El Ni?o and La Ni?a are asymmetric. During the El Ni?o, the SST increase displays two or three peaks. During the La Ni?a, only one significant cooling is formed over the NIO.

Aiming at the rich marine resource, important strategic status and potential natural disaster as well as man-made disaster such as terrorism and military conflict in the South China Sea and Indian Ocean, assessment indices were selected for risk evaluation modeling from disaster-forming environment, menace of disaster-causing factor and disaster-bearing substance based on risk assessment theory. Then, natural disaster risk, man-made disaster risk and synthetic risk were calculated and zoned, respectively, using GIS platform. The results suggested that in winter the regions with the highest synthetic risk are located in the Gulf of Aden, the top of the Bay of Bengal, around Malacca and southeastern offshore of Vietnam, with the second-highest synthetic risk regions located in the northeastern South China Sea, west of the Spratly Islands, Sunda Strait, Makassar Strait, Laccadive Sea, the coast of Somalia and the Hormuz Strait. In summer, the Gulf of Aden, the top of the Bay of Bengal, and Malacca are the highest synthetic risk regions, while the central and western Arabian Sea, northeastern waters off Somalia, southwest coast of Sri Lanka, west coast of Luzon, and region near the Bashi Channel are the second-highest synthetic risk regions.

The diatom mat deposits are giant “shade flora” diatoms bloom and quickly deposit to the ocean bottom. At the same time, some other species of diatoms also bloom. A total of 101 diatom taxa (including variation) belonging to 40 genera has been identified from the 155 samples based on the two cores of WPD03 and WPD12. It was found that the relative percentage of Thalassionema frauenfeldii is the highest, and that of Thalassionema nitzschioides, Azpeitia nodulifera, Nitzschia marina, and Hemidiscus cuneiformis takes the second place. The five species take about 85% of all diatom species’ relative percentage. The result indicated the five species are the easiest to coexist and bloom with the diatoms forming mats in the diatom mats’ deposit process of the surface water in this area. We estimated that the period for forming mats was a tropical environment of open-ocean circulation.

Three-dimensional (3D) ocean bottom seismometer (OBS) survey provides a significant foundation for the deep crustal and upper mantle structure of the hydrothermal field (49°39′E) (Area A) in the Southwest Indian Ridge. OBS data processing is the basic step of obtaining the 3D seismic velocity structure. The flow steps for data processing of three types of OBS (domestic, French and Germanic OBS) were firstly introduced, containing the decompilation, cutting and seismic signal visualization. Taking the shot 2790th for example, waveforms and frequency spectrums of three types of OBSs were then analyzed, which were related to frequency band, sensor and seismograph for different OBSs. Domestic and French OBSs recorded long-periodic and short-periodic noises, and Germanic OBS only recorded short-periodic noises. However, air-gun signals were highlighted and noises were suppressed for all the OBSs after using a band-passed filter. Moreover, several seismic phases, e.g., Pg, PmP and Pn, were clearly revealed in the recorded seismic sections of three types of OBSs (OBS04, OBS08 and OBS23) along the profile X1X2. These phases will provide a strong data base for 3D tomography for studying Area A.

The spatial and temporal distribution of the upper (0 - 700 m) ocean heat content (UOHC) over the tropical western Pacific and the probable causes for the variation are studied, using gridded monthly mean Argo profiling dataset from the JMESTC (Japan Marine-Earth Science and Technology Center), zonal wind data, SST data and SSH (Sea Surface Height) data. The research tools include cyclostationary empirical orthogonal function (CSEOF) analysis, Maximum Entropy Method (MEM) and correlation analysis. The main results indicate that the UOHC anomaly of the tropical western Pacific has an east-west anti-phase oscillation and remarkable seasonal and bi-annual variation. Moreover, the UOHC anomaly displayed a sharper “negative-positive-negative” undulation , which had obvious seasonal and inter-annual (~4a) changes. With further analysis, we find that the bi-annual oscillation was highly related to the ENSO (El Nino-Southern Oscillation) events and displayed a lagged response of about 1 - 2 months to the local zonal wind anomaly.

The impact of springtime heat content in the Indian Ocean on the South China Sea (SCS) summer monsoon onset is
investigated using Empirical Orthogonal Function (EOF). The author uses 0-400 m upper-ocean heat content data from the
Scripps Institution of Oceanography and meteorological data from the National Centers for Environmental Prediction/National
Center for Atmospheric Research (NCEP/NCAR). The results show that under the influence of El Niño-Southern Oscillation
(ENSO) EOF1 of the heat content has a pattern of sea-saw variation between the eastern and western Indian Ocean. When the
heat content is positively anomalous in the eastern Indian Ocean and negatively anomalous in the western Indian Ocean, the
SCS summer monsoon has an early onset; otherwise the onset is delayed. The mechanism is that the heat content pattern in the
Indian Ocean affects the vertical movement and divergence field of upper and lower atmosphere over the tropical Indian Ocean
as well as the strength of zonal wind, leading to early or late onset of the SCS summer monsoon. The spatial pattern of condi-
tional EOF1 (CEOF1) of the heat content is similar to that of EOF1, while the spatial pattern of CEOF2 displays a uniform
pattern except for a small region. The relative importance of the two modes gives uncertainty of their influence on the SCS
summer monsoon onset.

Based on the daily-averaged data derived from the NCEP/NCAR reanalysis dataset and using the single-layer (500hPa, and 850hPa) and the whole-layer (four layers) EOF methods, the leading modes of the subtropical high with vertical structure are extracted. It is found that the modes obtained by different ways are the same, which implies that there truly exist the same spatial distribution and temporal-change features from high to low of the subtropical high. Further, by applying the wavelet energy spectrum, it is found that there exist marked low-frequency oscillations of 20−30 days and 30−60 days. Considering the weather in 1995, the possible mechanism that the low-frequency potential wave at different latitude restricted the subtropical high in 1995 is diagnosed, i.e., the low-frequency potential wave of sub-tropical areas restricted the medium-term movement with a period of 20 days of the main body of the subtropical high, while the short-term process with a period of 5−10 days of the western subtropical high was related to the accumulated disturbance energy of the low-frequency potential wave coming from high latitude and tropical areas in the western Pacific region.

The authors study the relationship between the Western Pacific Subtropical High (WPSH) and sea-surface temperature (SST) anomaly in each selected key area based on lag correlation analysis. The lag correlation indicates that the eastern Pacific SST in winter correlates with the WPSH anomaly two or three months later to the highest degree; the SST in the tropical Indian Ocean and the WPSH anomaly in the corresponding period significantly correlates with each other in winter; and the SST in the western Pacific in winter and spring negatively correlates to the WPSH anomaly in the corresponding period. In addition, the SST in the northern Pacific in winter and spring negatively correlates with the WPSH anomaly one to two months earlier. The largest positive correlation between the Atlantic Warm Pool and the WPSH anomaly in the corresponding period occurs in June. Regarding to the southern Pacific, the WPSH anomaly in winter negatively correlates with the SST anomaly from last autumn to next spring. To sum up, in winter and spring, SST anomaly is almost fully responsible to the annual variation of the WPSH anomaly, but in summer and autumn, the influence of SST on the WPSH is constrained to a small scale.