Oceanic Internal Waves and Internal Tides in the East Asian Marginal Seas

Oceanic internal waves (IWs) at frequencies from local inertial (e.g., near-inertial internal waves) to buoyancy frequencies (nonlinear internal waves or internal solitary waves), sometimes including diurnal and semidiurnal tidal frequencies, play an important role in redistributing heat, momentum, materials, and energy via turbulent mixing. IWs are found ubiquitously in many seas, including East Asian marginal seas (Indonesian Seas, South China Sea, East China Sea, Yellow Sea, and East Sea or Japan Sea), significantly affecting underwater acoustics, coastal and offshore engineering, submarine navigation, biological productivity, and the local and global climate. Despite decades of study on the IWs in some regions, our understanding of the IWs in the East Asian marginal seas is still in a primitive state and the mechanisms underlying every stage (generation, propagation, evolution, and dissipation) of IWs are not always clear. This Special Issue includes papers related to all fields of both low- and high-frequency IW studies in the specified region, including remote sensing, in situ observations, theories, and numerical models

Characterizing the nonlinear internal wave climate in the northeastern South China Sea

Four oceanographic moorings were deployed in the South China Sea from April 2005 to June 2006 along a transect extending from the Batanes Province, Philippines in the Luzon Strait to just north of Dong-Sha Island on the Chinese continental slope. The purpose of the array was to observe and track large-amplitude nonlinear internal waves (NIWs) from generation to shoaling over the course of one full year. The basin and slope moorings observed velocity, temperature (<i>T</i>) and salinity (<i>S</i>) at 1–3 min intervals to observe the waves without aliasing. The Luzon mooring observed velocity at 15 min and <i>T</i> and <i>S</i> at 3 min, primarily to resolve the tidal forcing in the strait. <br><br> The observed waves travelled WNW towards 282–288 degrees with little variation. They were predominantly mode-1 waves with orbital velocities exceeding 100 cm s<sup>−1</sup> and thermal displacements exceeding 100 m. Consistent with earlier authors, two types of waves were observed: the a-waves arrived diurnally and had a rank-ordered packet structure. The b-waves arrived in between, about an hour later each day similar to the pattern of the semi-diurnal tide. The b-waves were weaker than the a-waves, usually consisted of just one large wave, and were often absent in the deep basin, appearing as NIW only upon reaching the continental slope. The propagation speed of both types of waves was 323±31 cm s<sup>−1</sup> in the deep basin and 222±18 cm s<sup>−1</sup> over the continental slope. These speeds were 11–20% faster than the theoretical mode-1 wave speeds for the observed stratification, roughly consistent with the additional contribution from the nonlinear wave amplitude. The observed waves were clustered around the time of the spring tide at the presumed generation site in the Luzon Strait, and no waves were observed at neap tide. A remarkable feature was the distinct lack of waves during the winter months, December 2005 through February 2006. <br><br> Most of the features of the wave arrivals can be explained by the tidal variability in the Luzon Strait. The near-bottom tidal currents in the Luzon Strait were characterized by a large fortnightly envelope, large diurnal inequality, and stronger ebb (towards the Pacific) than flood tides. Within about ±4 days of spring tide, when currents exceeded 71 cm s<sup>−1</sup>, the ebb tides generated high-frequency motions immediately that evolved into well-developed NIWs by the time they reached mooring B1 in the deep basin. These waves formed diurnally and correspond to the a-waves described by previous authors. Also near spring tide, the weaker flood tides formed NIWs which took longer/further to form, usually not until they reached mooring S7 on the upper continental slope. These waves tracked the semidiurnal tide and correspond to the b-waves described by previous authors. These patterns were consistent from March to November. During December–February, the structure of the barotropic tide was unchanged, so the lack of waves during this time is attributed to the deep surface mixed layer and weaker stratification along the propagation path in winter

Advanced Geoscience Remote Sensing

Nowadays, advanced remote sensing technology plays tremendous roles to build a quantitative and comprehensive understanding of how the Earth system operates. The advanced remote sensing technology is also used widely to monitor and survey the natural disasters and man-made pollution. Besides, telecommunication is considered as precise advanced remote sensing technology tool. Indeed precise usages of remote sensing and telecommunication without a comprehensive understanding of mathematics and physics. This book has three parts (i) microwave remote sensing applications, (ii) nuclear, geophysics and telecommunication; and (iii) environment remote sensing investigations

Ocean Remote Sensing with Synthetic Aperture Radar

The ocean covers approximately 71% of the Earth’s surface, 90% of the biosphere and contains 97% of Earth’s water. The Synthetic Aperture Radar (SAR) can image the ocean surface in all weather conditions and day or night. SAR remote sensing on ocean and coastal monitoring has become a research hotspot in geoscience and remote sensing. This book—Progress in SAR Oceanography—provides an update of the current state of the science on ocean remote sensing with SAR. Overall, the book presents a variety of marine applications, such as, oceanic surface and internal waves, wind, bathymetry, oil spill, coastline and intertidal zone classification, ship and other man-made objects’ detection, as well as remotely sensed data assimilation. The book is aimed at a wide audience, ranging from graduate students, university teachers and working scientists to policy makers and managers. Efforts have been made to highlight general principles as well as the state-of-the-art technologies in the field of SAR Oceanography

IMPROVED SOUND SPEED CONTROL THROUGH REMOTELY DETECTING STRONG CHANGES IN THE THERMOCLINE

Internal waves are a common phenomenon associated with stratification developed in shallow tidal seas during summer time. From a hydrographic point of view, they result in very rapid undulations in the main velocline which, if not accounted for, will result in significant refraction errors in multibeam echo sounder data. Mechanical sound speed profiling, both static and mobile, cannot sample this structure adequately (Hughes Clarke, 2017). Thus, an alternate means of detecting and accounting for that variability is needed. Within the oceanographic community, it has long been recognized that a distinct acoustic volume scattering layer is often associated within or close to major oceanographic boundaries. This has been noted to reflect a combination of temperature/salinity microstructure and zooplankton around the pycnocline depth. Several weeks of multibeam survey on the Irish continental shelf were undertaken during which multispectral acoustic scattering data from a Simrad EK60 scientific echo sounder were acquired together with profiles from a Moving Vessel Profiler. This thesis proposes and implements a method to determine the evolving sound speed structure by processing the images derived from the EK60 echo sounder. This is done by extracting the scattering layer depth and finding the correlations with the velocline found in each of the discrete MVP profiles. Thus, a continuously evolving estimate of the local sound speed structure is derived. From this, by calculating the associated depth bias in the seafloor modeling caused by the difference in sound speed structure between the last actual profile and the estimate at each ping, a visual indication of the need for a new in situ measurement is made. In this manner, the sound speed structure may be monitored to adjust the spatial and temporal resolutions of the profile casts more efficiently

Earth resources: A continuing bibliography with indexes (issue 61)

This bibliography lists 606 reports, articles, and other documents introduced into the NASA scientific and technical information system between January 1 and March 31, 1989. Emphasis is placed on the use of remote sensing and geophysical instrumentation in spacecraft and aircraft to survey and inventory natural resources and urban areas. Subject matter is grouped according to agriculture and forestry, environmental changes and cultural resources, geodesy and cartography, geology and mineral resources, oceanography and marine resources, hydrology and water management, data processing and distribution systems, and instrumentation and sensors, and economic analysis

Oceanography

How inappropriate to call this planet Earth when it is quite clearly Ocean (Arthur C. Clarke). Life has been originated in the oceans, human health and activities depend from the oceans and the world life is modulated by marine and oceanic processes. From the micro-scale, like coastal processes, to macro-scale, the oceans, the seas and the marine life, play the main role to maintain the earth equilibrium, both from a physical and a chemical point of view. Since ancient times, the world's oceans discovery has brought to humanity development and wealth of knowledge, the metaphors of Ulysses and Jason, represent the cultural growth gained through the explorations and discoveries. The modern oceanographic research represents one of the last frontier of the knowledge of our planet, it depends on the oceans exploration and so it is strictly connected to the development of new technologies. Furthermore, other scientific and social disciplines can provide many fundamental inputs to complete the description of the entire ocean ecosystem. Such multidisciplinary approach will lead us to understand the better way to preserve our "Blue Planet": the Earth

The development of satellite derived nitrate and stratification indices for the southern Benguela ecosystem

An earth observation based study was conducted in the southern Benguela upwelling system, aimed at developing remotely sensed proxies to determine ecological conditions conducive to the formation of harmful algal blooms (HAB) which are endemic to the system. The aim of this study was to identify the relationship between nutrient availability, turbulence and phytoplankton community assemblages using remotely sensed data. Certain phytoplankton functional groups are adapted to a particular environment within an ecological space conceptually based on the nutrient availability and turbulence (Margalef, 1978). Two proxies, representing the nutrient availability, and turbulence or stratification, were created using satellite-derived surface nitrate (NO3) concentrations and the 12ºC isotherm (Iso12) depths, and used to define the ecosystem state through Margalef's Mandala (Margalef, 1978). The approach involved the development of robust algorithms using in situ data collected in the greater St Helena Bay region to estimate the surface NO3 concentrations and the depth of the Iso12 for the southern Benguela, using remotely sensed sea surface temperatures (SST) and wind data. The derivation of the nutrient proxy was based on a model initially developed by Dugdale et al., (1989) then modified by Silió-Calzada et al., (2008) for use in the Benguela. The turbulence proxy was derived using a simple linear regression model to estimate the depth of the Iso12 which was utilized as a proxy for the thermocline depth in the system. The performance of the nutrient and turbulence proxies were assessed on local, meso- and synoptic scales for their ability to resolve the event and seasonal scale variations in the inner shelf environment of the southern Benguela. The derived NO3 and Iso12 products were sufficiently able to resolve the event and mesoscale variability of the system in 2005, 2006 and 2007. The performance of the products at capturing the annual and intra-seasonal variability of the system was satisfactory, displaying an ability to resolve the ecosystem upwelling variability. Using the NO3 and Iso12 products as the nutrient and turbulence proxies was satisfactory as a first attempt at using earth observation to classify the ecosystem according to Margalef's Mandala (Margalef, 1978). The proxies were able to model the ecosystem inner-shelf environment for multiple years and thus create the ability to hypothesize ecosystem sub-habitats occupied by particular life forms of phytoplankton. There were however two concerns that needed further consideration in the approach: 1).The large warm bias discovered between the in situ and remotely sensed temperatures which had a direct influence on the validity of the algorithms in the ecosystem and 2.) The temporal and spatial disconnects between the physical forcing and biological response of the ecosystem and subsequent impact upon the utility of the remotely sensed proxies

Proceedings of the XXVIIIth TELEMAC User Conference 18-19 October 2022

Hydrodynamic

Characterising the subtropical front and associated water masses offshore Otago using seismic oceanography

The Subtropical Front is a global ocean boundary separating warm, salty Subtropical Water from relatively cool, fresh Subantarctic Water. Near Dunedin, on the east coast of the South Island of New Zealand, the front is located in the vicinity of the continental shelf break, just 20–40 km offshore. At this boundary, mixing processes are important in the transfer of heat, salt, and nutrients between the two water masses. Seismic oceanography involves the acquisition of marine seismic reflection data normally used for subseafloor imaging, repurposed to image the water column. Seismic reflections come from temperature and salinity contrasts within the ocean. Four different applications of the seismic oceanography method were examined in this study, including the first dedicated seismic oceanographic cruises in Australasia. First, seismic data acquired over decades of petroleum exploration were reprocessed, revealing significant seismic reflectivity associated with Subtropical, Subantarctic, and Antarctic Intermediate Waters. The reflectivity patterns were interpreted based on synthetic seismograms calculated from historical oceanographic data, and the location of the Subtropical Front was confirmed using satellite sea-surface temperatures as well as near-surface temperatures calculated from the seismic data themselves. High-frequency electro-acoustic seismic data were acquired along with conductivity-temperature-depth profiles (CTDs) on four cruises along the historically well-studied Munida Transect. While the seismic data did not produce discernible water-column reflections above the background noise field, synthetic seismograms produced from the CTDs contributed to a better understanding of the water masses in the region, including an examination of temporal variability in reflectivity. A larger-scale cruise was also carried out, where high-frequency generator-injector air gun seismic data were acquired, accompanied by expendable temperature-depth probes. These data produced seismic images with reflections clearly associated with the temperature gradients at the Subtropical Front. Repeat acquisition over a period of days showed the dynamic nature of the boundary. Finally, CTD data were collected by a chase boat during the acquisition of a petroleum industry 3D seismic survey. These data definitively connect strong seismic reflections in the seismic image to offshore waters with high temperatures and salinities, and particularly allow for the interpretation of a lens-like reflective feature as an eddy. The swath seismic data also allow the three-dimensionality of reflections to be examined. This study demonstrates the feasibility of investigating oceanographic features in this region using seismic oceanography and provides a methodological comparison to guide future projects. Legacy seismic data are a vast data source, well-suited for mapping water mass boundaries throughout the water column, helping to determine the regional distribution and variability of oceanographic features. High-resolution seismic acquisition produces cross-sectional subsurface images of the Subtropical Front at scales typically unachievable using conventional oceanographic methods. These images reveal characteristic differences in the expression of the front at the surface compared to the subsurface. Combining seismic images with in-situ oceanographic data corroborates the identification of high-temperature, high-salinity waters found well offshore of the Subtropical Front, masked by the presence of a surface mixed layer. These waters appear in features resembling meanders and eddies that could represent a significant mechanism for the mixing of water masses