Since 2017, a plenty of gas hydrates have been drilled in a new area of Shenhu, and good heterogeneity has been found throughout the spatial distribution of the reservoir. After distinguishing different sedimentary sequence types and matching their formation with slope deposition settings, this study proposes three mass transport deposit (MTD) patterns related to canyon activity that occurred contemporaneously or epigenetically with it: well preserved MTDs, MTDs eroded by canyon migration, and MTDs dislocated by contemporaneous faults. Based on seismic reflection characteristics, this study proposed methods of quantitatively analyzing sedimentary factors, such as measuring the turbidities flow rate in the canyon, and results are interpreted with respect to canyon activity. Combining the above parameters and their relationship with gas hydrate accumulation, fine-grained seals overlapping coarse MTDs reservoirs are found to be indispensable to gas hydrate accumulation, as they prevent the release of free gas. Based on grain size data of hydrate samples from drilling wells, multi-layered gas hydrate reservoirs capped by fine-grained sediments and overlapping mud show favorable hydrate-bearing prospects. The release of gas hydrates, however, is mostly caused by the lack of mud sealing in relation to canyon activity, such as turbidities flow erosion and contemporaneous fault breaking. Canyon migration with respect to MTDs may be the actual cause of erosion of overlapping syn-sedimentary layers, and high bottom flows may contribute to an increase in the release of free gas. It is believed that contemporaneous faults caused by unstable canyon walls may break the muddy over layers and decrease the accumulation pressure of gas hydrate bearing. Thus, according to the sedimentary characteristics of MTDs and the hydrate accumulation process, three responding accumulation or releasing patterns are proposed, which respond to the different types of MTDs distinguished above: a well-preserved MTD accumulation pattern; a canyon migration eroded MTD release pattern; and a micro-contemporaneous fault dislocated MTD release pattern.

Theoretical-based ocean wave retrieval algorithms are applied by inverting a synthetic aperture radar (SAR) intensity spectrum into a wave spectrum, that has been developed based on a SAR wave mapping mechanism. In our previous studies, it was shown that the wave retrieval algorithm, named the parameterized first-guess spectrum method (PFSM), works for C-band and X-band SAR at low to moderate sea states. In this work, we investigate the performance of the PFSM algorithm when it is applied for dual-polarization c-band sentinel-1 (S-1) SAR acquired in extra wide-swath (EW) and interferometric wide-swath (IW) mode under cyclonic conditions. Strong winds are retrieved from six vertical-horizontal (VH) polarization S-1 SAR images using the c-band cross-polarization coupled-parameters ocean (C-3PO) model and then wave parameters are obtained from the image at the vertical-vertical (VV) polarization channel. significant wave height (SWH) and mean wave period (MWP) are compared with simulations from the WAVEWATCH-III (WW3) model. The validation shows a 0.69 m root mean square error (RMSE) of SWH with a –0.01 m bias and a 0.62 s RMSE of MWP with a –0.17 s bias. Although the PFSM algorithm relies on a good quality SAR spectrum, this study confirms the applicability for wave retrieval from an S-1 SAR image. Moreover, it is found that the retrieved results have less accuracy on the right sector of cyclone eyes where swell directly affects strong wind-sea, while the PFSM algorithm works well on the left and rear sectors of cyclone eyes where the interaction of wind-sea and swell is relatively poor.

The mesoscale eddy is a typical mesoscale oceanic phenomenon that transfers ocean energy. The detection and extraction of mesoscale eddies is an important aspect of physical oceanography, and automatic mesoscale eddy detection algorithms are the most fundamental tools for detecting and analyzing mesoscale eddies. The main data used in mesoscale eddy detection are sea level anomaly (SLA) data merged by multi-satellite altimeters’ data. These data objectively describe the state of the sea surface height. The mesoscale eddy can be represented by a local equivalent region surrounded by an SLA closed contour, and the detection process requires the extraction of a stable closed contour structure from SLA maps. In consideration of the characteristics of mesoscale eddy detection based on SLA data, this paper proposes a new automatic mesoscale eddy detection algorithm based on clustering. The mesoscale eddy structure can be extracted by separating and filtering SLA data sets to separate a mesoscale eddy region and non-eddy region and then establishing relationships among eddy regions and mapping them on SLA maps. This paper overcomes the problem of the sensitivity of parameter setting that affects the traditional detection algorithm and does not require a sensitivity test. The proposed algorithm is thus more adaptable. An eddy discrimination mechanism is added to the algorithm to ensure the stability of the detected eddy structure and to improve the detection accuracy. On this basis, the paper selects the Northwest Pacific Ocean and the South China Sea to carry out a mesoscale eddy detection experiment. Experimental results show that the proposed algorithm is more efficient than the traditional algorithm and the results of the algorithm remain stable. The proposed algorithm detects not only stable single-core eddies but also stable multi-core eddy structures.

To analyze the grain size and depositional environment of the foreshore sediments, a study was undertaken on wave refraction along the wide sandy beaches of central Tamil Nadu coast. The nearshore waves approach the coast at 45° during the northeast (NE) monsoon, at 135° during the southwest (SW) monsoon and at 90° during the non-monsoon or fair-weather period with a predominant wave period of 8 and 10 s. A computer based wave refraction pattern is constructed to evaluate the trajectories of shoreward propagating waves along the coast in different seasons. The convergent wave rays during NE monsoon, leads to high energy wave condition which conveys a continuous erosion at foreshore region while divergent and inept condition of rays during the SW and non-monsoon, leads to moderate and less energy waves that clearly demarcates the rebuilt beach sediments through littoral sediment transport. The role of wave refraction in foreshore deposits was understood by grain size and depositional environment analysis. The presence of fine grains with the mixed population, during the NE monsoon reveals that the high energy wave condition and sediments were derived from beach and river environment. Conversely, the presence of medium grains with uniform population, during SW and non-monsoon attested less turbulence and sediments were derived from prolong propagation of onshore-offshore wave process. These upshots are apparently correlated with the in situ beach condition. On the whole, from this study it is understood that beaches underwent erosion during the NE monsoon and restored its original condition during the SW and non-monsoon seasons that exposed the stability of the beach and nearshore condition.

The acoustic bottom backscattering strength was measured at the frequency range of 6–24 kHz on a typical sandy bottom in the South Yellow Sea by using omnidirectional sources and omnidirectional receiving hydrophones. In the experiment, by avoiding disturbances due to scattering off the sea surface and satisfying the far-field condition, we obtained values of acoustic bottom backscattering strength ranging from –41.1 to –24.4 dB within a grazing angle range of 18°–80°. In the effective range of grazing angles, the acoustic scattering strength generally increases with an increase in the grazing angles, but trends of the variation were distinct in different ranges of frequency, which reflect different scattering mechanisms. The frequency dependence of bottom backscattering strength is generally characterized by a positive correlation in the entire frequency range of 6–24 kHz at the grazing angles of 20°, 40° and 60° with the linear regression slopes of 0.222 9 dB/kHz, 0.513 0 dB/kHzand 0.174 6 dB/kHz, respectively. At the largest grazing angle of 80°, the acoustic backscattering strength exhibits no evident frequency dependence.

The present work is an analytical study of the influence of geometrical parameters, such as length, thickness and immersion of the plate, on the reflection coefficient of a regular wave for an immersed horizontal plate in the presence of a uniform current with the same direction as the propagation of the incident regular wave. This study was performed using the linearized potential theory with the evanescent modes while searching for complex roots to the dispersion equation that are neither pure real nor pure imaginary. The results show that the effects of the immersion and the relative length on the reflection coefficient of the plate are accentuated by the presence of the current, whereas the plate thickness practically does not have an effect if it is relatively small.

The response of the eastern tropical Indian Ocean (ETIO) to heat fluxes of equal amplitude but opposite sign is investigated using the Community Earth System Model (CESM). A significant positive asymmetry in sea surface temperature (SST) is found over the ETIO—the warming responses to the positive forcing exceeds the cooling to the negative forcing. A mixed layer heat budget analysis is carried out to identify the mechanisms responsible for the SST asymmetry. Results show that it is mainly ascribed to the ocean dynamical processes, including vertical advections and diffusion. The net surface heat flux, on the contrary, works to reduce the asymmetry through its shortwave radiation and latent heat flux components. The former is due to the nonlinear relationship between SST and cloud, while the latter is resulted mainly from Newtonian damping and air-sea stability effects. Changes in the SST skewness are also evaluated, with more enhanced negative SST skewness over the ETIO found for the cooling than heating scenarios due to the asymmetric thermocline-SST feedback.

This study deals with a unusual cooling event after Typhoon Mujigea passed over the northern South China Sea (SCS) in October 2015. We analyze the satellite sea surface temperature (SST) time series from October 3 to 18, 2015 and find that the cooling process in the coastal ocean had two different stages. The first stage occurred immediately after typhoon passage on October 3, and reached a maximum SST drop of –2°C on October 7 as the usual cold wake after typhoon. The second stage or the unusual extended cooling event occurred after 7 d of the typhoon passage, and lasted for 5 d from October 10 to 15. The maximum SST cooling was –4°C and occurred after 12 d of typhoon passage. The mechanism analysis results indicate that after landing and moving northwestward to the Yunnan-Guizhou Plateau (YGP), Typhoon Mujigea (2015) met the westerly wind front on October 5. The low-pressure and positive-vorticity disturbances to the front triggered meridional air flow and low-pressure trough, thus induced a katabatic cold jet downward from the Qinghai-Tibet Plateau (QTP) passing through the YGP to the northwestern SCS. The second cooling reached the maximum SST drop 4 d later after the maximum air temperature drop of –9°C on October 11. The simultaneous air temperature and SST observations at three coastal stations reveal that it is this katabatic cold jet intrusion to lead the unusual SST cooling event.

In this paper, effort is made to demonstrate the quality of high-resolution regional ocean circulation model in realistically simulating the circulation and variability properties of the northern Indian Ocean (10°S–25°N, 45°–100°E) covering the Arabian Sea (AS) and Bay of Bengal (BoB). The model run using the open boundary conditions is carried out at 10 km horizontal resolution and highest vertical resolution of 2 m in the upper ocean. The surface and sub-surface structure of hydrographic variables (temperature and salinity) and currents is compared against the observations during 1998–2014 (17 years). In particular, the seasonal variability of the sea surface temperature, sea surface salinity, and surface currents over the model domain is studied. The high-resolution model’s ability in correct estimation of the spatio-temporal mixed layer depth (MLD) variability of the AS and BoB is also shown. The lowest MLD values are observed during spring (March-April-May) and highest during winter (December-January-February) seasons. The maximum MLD in the AS (BoB) during December to February reaches 150 m (67 m). On the other hand, the minimum MLD in these regions during March-April-May becomes as low as 11–12 m. The influence of wind stress, net heat flux and freshwater flux on the seasonal variability of the MLD is discussed. The physical processes controlling the seasonal cycle of sea surface temperature are investigated by carrying out mixed layer heat budget analysis. It is found that air-sea fluxes play a dominant role in the seasonal evolution of sea surface temperature of the northern Indian Ocean and the contribution of horizontal advection, vertical entrainment and diffusion processes is small. The upper ocean zonal and meridional volume transport across different sections in the AS and BoB is also computed. The seasonal variability of the transports is studied in the context of monsoonal currents.