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Presentation Mode : All
Conference Day : 05/08/2021
Time Slot : AM2 11:00 - 13:00
Sections : OS - Ocean Sciences










Ocean Sciences | Thu-05 Aug




OS02-A015 | Invited
A 1921 Western Australian Tropical Cyclone Underscores the Utility of Historical Records for Coastal Hazard Analysis in Sites of Marginal Cyclone Influence.

Adam SWITZER1#+, Joseph CHRISTENSEN2, Joanna ALDRIDGE3, David TAYLOR4, Jim CHURCHILL4, Holly WATSON4, Matthew FRASER2, Jenny SHAW5
1Nanyang Technological University, Singapore, 2University of Western Australia, Australia, 3Insurance Australia Group Limited (IAG), Australia, 4Baird Australia, Australia, 5 Western Australian Marine Science Institution, Australia


Shark Bay Marine Park is a UNESCO World Heritage Area in a region of marginal tropical cyclone influence in Western Australia. Sustainable management of this unique environment as the climate changes requires a quantified understanding of its vulnerability to natural hazards. Analysis of historical archives has uncovered reports of an extreme storm surge associated with a Tropical Cyclone in 1921 that generated remarkable overland flow that left fish and sharks stranded up to 9.66 km (6 miles) inland. The information from the historical archives provide inputs to detailed modelling of this event which improves the understanding of its magnitude and provides records of the impacts of what occurred on that day and notably also in the years following. The plausible tracks that reproduce the historical data contextualise its return interval as equal as or marginally greater than the current 500 year annual return interval (ARI) planning level for the region. The outcome underscores the importance of understanding the for risk management in areas of marginal cyclone influence where vulnerable ecosystems or vital regional infrastructure of key economic importance are located, and the need to factor in TC risk in marine conservation and planning in the Shark Bay World Heritage Area. 

OS02-A019
Seashells on the Mountaintop – High-elevation Holocene Waterspout Sediments at Langkawi Islands, Malaysia

Miklos KAZMER1#+, Mohd Shafeea LEMAN2, Kamal Roslan MOHAMED2, Che Aziz ALI2
1Eötvös Loránd University, Hungary, 2Universiti Kebangsaan Malaysia, Bangi, Malaysia


Molluscan sand with unusually well-preserved shells was found at three sites in the Langkawi Islands, Peninsular Malaysia (Pulau Tanjung Dendang, Gua Pinang, and Pulau Singa), in altitudes up to 65 metres above sea level. At the Pulau Tanjung Dendang site the fossil bed is exposed in 2.5 m thickness. It is weakly cemented sand, rich in mollusc shells, arranged horizontally. The fauna is of mixed origin. Components are derived from coral reef, from mangrove, and from shallow, sandy bottom environments, inhabited both by epifaunal and infaunal groups. Most valves are single, some of them are double. Double valves indicate that these animals were transported and entombed while still living. Shells are often intact, unbroken, and not abraded either (unlike what one would expect on a beach). There are Saccostrea valves, narrow V-shaped ones – these probably grew on mangrove roots adjacent to each other, deforming each other’s shape. Other, palm-sized oysters overgrew coral colonies. Neither of these were attached to a rock surface. Many of the shells are oriented with the convex side upwards, indicating deposition from a high-energy medium. Curiously, a soda straw stalactite is also embedded among the molluscs. Radiocarbon data of the 23 m level are between 5000 and 6000 years, excluding high Mid-Holocene sea level or tectonic uplift. We suggest that waterspouts repeatedly siphoned sediment from nearby shallow sea and deposited it along their route at various elevations. Most of the deposits might have been washed back to the shore, while those landed on protected ground below the overhanging limestone cliffs were secured from surface run-off. Those deposited within cave openings, and those overgrown speleothems got additional protection. Resedimentation by waterspouts is a mechanism to be considered when discussing the formation of various marine sedimentary deposits found in elevated position in coastal and nearshore settings. 

OS02-A024
Morphology and Emplacement of Coastal Boulders in San Julian, Eastern Samar, Philippines

Robelyn MANGAHAS1#+, Lyndon NAWANAO JR.2, Jelian REYES2, Romer Carlo GACUSAN3, Deo Carlo LLAMAS1,3, Bryan MARFITO4, Kurt Joshua TIZON5, Carl Reiner PADUA5, Noelynna RAMOS2
1Philippine Institute of Volcanology and Seismology, Philippines, 2University of the Philippines Diliman, Philippines, 3University of the Philippines, Philippines, 4Nanyang Technological University, Singapore, 5Mines and Geosciences Bureau, Philippines


The distribution and morphology of large coastal boulders provide important insights on the kinematics and magnitude of extreme wave events such as tsunamis and storm surges. In eastern Samar Island, clusters of coastal boulders are generally observed on top of the intertidal fossil reef platforms. Morphological characteristics of 61 large boulders in San Julian were analyzed through field measurements and interpretation of drone-generated orthophotos. Zingg shape classification reveals that the majority of the boulders are oblate (51%) and the rest are bladed (19.3%), equant (15.6%), or rod (14.1%). Boulders with a long axis of > 3 m are generally oriented NNW-SSE, almost parallel to the coastline configuration. The distance of the boulders from the reef edge ranges from 100 m to 400 m. The largest boulder in the study area weighs approximately 35 tons and is observed almost 200 m away from the reef edge. Wave height and wave velocity to transport and emplace the boulders were estimated from hydrodynamic equations. Calculated minimum tsunami and storm surge heights to initiate transport of subaerial boulders are 0.1 m, and 0.4 m, respectively. Minimum wave velocities to mobilize the boulders by sliding, rolling, and lifting are 1.6 m/s, 1.1 m/s, and 5.1 m/s, respectively.  Wave height and wave velocity values were analyzed vis-à-vis historical extreme events that affected Eastern Samar such as storm surges of the 2013 Supertyphoon Haiyan, local tsunamis of the 1975 and 1995 earthquakes from the Philippine Trench, and even possibly, tsunami of the 1960 Valdivia and 1700 Cascadia earthquakes. While the source of these boulders is yet to be ascertained, these initial results highlight the importance of coastal boulders in evaluating an area’s vulnerability to coastal hazards.

OS02-A025
Tracing the Possible Tsunami Deposit Distribution in the Nanao Island, Northern South China Sea Using a Forward Modeling Approach

Fating LI+, Linlin LI#
Sun Yat-sen University, China


Tsunami deposits provide crucially important information to estimate the frequency, size and potential sources of paleotsunami. Some geological records have been reported in the offshore islands inside the South China Sea (SCS), indicating that large tsunami may have occurred 1000 years ago. As the SCS is located in the northwest Pacific region, a region most frequently affected by typhoons, how to differentiate sediment deposit generated by tsunamis and storm surges is always an issue. So whether the SCS has experienced an ocean-wide tsunami is still a pending question. Here, using a forward modeling approach, we choose the NanAo Island, a location with reported tsunami deposit, as an example location to investigate the potential tsunami deposit distribution generated by two groups of representative earthquakes. One group represents the 1918 M7.5 NanAo earthquake which is one of the few earthquakes with confirmed tsunami records in northern SCS. Another group of earthquakes represents megathrust earthquakes originated from the Manila trench. We use COMCOT-SED to investigate the physical process of tsunami inundation and sediment transport in some locations with reported tsunami deposit at the NanAo Island, Guangdong, China. Our preliminary results show that the tsunami wave heights generated by the 1918 NanAo Earthquake can reach ~6 m at the southern coast of NanAo Island. The first tsunami wave may hit the coast of Qingaowan in less than 30 minutes with maximum amplitude of ~ 3 m. The extreme scenario with earthquake magnitude of ~Mw9.0 can produce ~19 m high tsunami waves in the NanAo surrounding area. The first tsunami wave may arrive at Qingaowan about 104 minutes with the maximum amplitude of ~ 10 m. This study aims to understand whether tsunami waves generated by representative earthquakes are capable of transporting large amount of sediment and leaving traceable tsunami deposits.

OS02-A030
Multiple Drivers and Controls of Fluid Escape Across the Canterbury Margin, New Zealand

Aaron MICALLEF1,2#+, Tanita AVERES3, Jasper HOFFMANN4, Gareth CRUTCHLEY5, Joshu MOUNTJOY6, Mark PERSON7, Denis COHEN2, Susi WOELZ6, Sarah BURY6, Sebastian KRASTEL8
1GEOMAR - Helmholtz Centre for Ocean Research Kiel, Germany, 2University of Malta, Malta, 3Institute of Geosciences, Christian-Albrechts-Universität zu Kiel, Germany, 4Department of Geology, University of Otago, Dunedin, New Zealand, New Zealand, 5GNS Science, New Zealand, 6National Institute of Water and Atmospheric Research, New Zealand, 7New Mexico Tech, United States, 8Christian-Albrechts-Universität zu Kiel, Germany


We document >6800 pockmarks in the middle to outer shelf and lower continental slope of the Canterbury Margin, off New Zealand. Biogenic methane, generated by in situ microbial degradation of organic matter deposited during sea-level lowstands, appears to be the most common fluid involved, although a mixture of saline and freshened groundwater may also be seeping across the slope. The drivers of fluid flow and seepage include overpressure due to sediment loading (the effect of which is most pronounced across the continental slope), methane generation, and lower sea-levels, as well as low confining pressures away from rapid sedimentation during the late Quaternary. Past methane expulsion when sea-levels were lower is likely responsible for pockmark formation across the shelf and slope, whereas seepage of saline and freshened groundwater may be forming pockmarks across the slope at present. Pockmarks are more common in muddy seafloor sediments, whereas bottom currents (e.g. Southland Current, STF and the Antarctic Intermediate water flow) are responsible for pockmark elongation. Our study demonstrates how pockmark occurrence, characteristics and formation can be controlled by a fine balance between fluid type, overpressures, confining pressures, seafloor sediment type, stratigraphy, and oceanography. Investigations of pockmark formation across a margin would thus benefit from an approach combining geophysics, sedimentology, geochemistry and numerical modelling, and considering a wide range of hydrogeological, sedimentological and oceanographic factors.

OS02-A027
Assessing the Quality of GNSSInterferometric Reflectometry for Measuring Water Level: A Case Study from Quarry Bay, Hong Kong

Mai YE1+, Linlin LI2#, Dongju PENG3
1South China Sea Institute of Oceanology, Chinese Academy of Sciences, China, 2Sun Yat-sen University, China, 3Nanyang Technological University, Singapore


GNSS signals reflected off the water surface can be indirectly used to measure water levels by analyzing the signal-to-noise ratio (SNR) data. Such GNSS interferometric reflectometry (GNSS-IR) technique has been proved to be able to measure sea level changes (e. g. astronomical tides, sea states, storm surges). Compared with conventional tide gauges, GNSS stations placed at high ground near the coast have the advantage of being more resistant to extreme waves induced by typhoons/tsunamis, and at the same time, recording absolute vertical land motions simultaneously. In this study, we assess the accuracy of the sea levels estimated from GNSS interferometric reflectometry in GNSS station HKQT which is located near Quarry Bay, Hong Kong. We first obtain 5-year water levels by using the multi-mode and multi-frequency GNSS-IR signals. The retrieved data is corrected by accounting for dynamic sea surface and signal propagation delay by the troposphere. We then assess the quality of 5-year water level by comparing them with the data from a collocated traditional tide gauge in Quarry Bay. The purpose of this study is to understand the capability of GNSS-IR recording long-term water levels in this specific site. Such assessment provides helpful information in designing GNSS tide gauges which are dedicated to sea level monitoring in future.

OS02-A007
Tsunami Wave Height Prediction Along the Coast Based on Tsunami Observation and Data Assimilation by Using Oceanographic Radar on the Southern Coast of Java, Indonesia

Muhammad Irham SAHANA1#+, Ryotaro FUJI2, Hirofumi HINATA1
1Ehime University, Japan, 2Kokusai Kogyo Co., Ltd., Japan


The conventional tsunami prediction consists of a combination of seismic wave observation and numerical simulation database of tsunami scenarios. However, it is insufficient to ensure accurate prediction of tsunami if the earthquake that occurred cannot be represented by any previously calculated tsunami scenarios. Thus, we examined a data assimilation method designed to enable oceanographic radar to predict the tsunami wave height along the coast. Instead of using seismic source parameters or initial sea surface height as the initial conditions, we predicted tsunami wave height by assimilating the tsunami radial velocity data virtually observed by the oceanographic radar at 64 observational stations into a numerical simulation. We performed tsunami simulation by JAGURS with the scenario of the 8.9 Mw Java trench megathrust earthquake. We verified the accuracy of tsunami wave height prediction along the southern coast of Java by data assimilation based on variance reduction (VR) from the time window of 10 minutes, 15 minutes, and 20 minutes. As a result of comparing tsunami wave heights from the data assimilation and JAGURS for 2 hours at the point between two radar installation sites (Purworejo and Bantul site), the accuracy was shown to increase related to the time window used, with the VR of 16.4%, 47.7%, and 77.5%, respectively. While, the maximum and minimum accuracy (VR) on the area between two radar sites are 32.2% and 77.5%, respectively, by applying for 20 minutes time window.

OS15-A017
Sea Level Observations from the Korea Ocean Research Stations

MyeoungHee HAN#+, Yong Sun KIM, Hyoun-Woo KANG, Jin-Young JEONG
Korea Institute of Ocean Science and Technology, Korea, South


Observed time series from three Korea ocean research stations (KORSs)–Ieodo ORS (I-ORS), Gageocho ORS (G-ORS), and Socheongcho ORS (S-ORS)–have been analyzed in oceanographic and atmospheric fields since their constructions in 2005.  Albeit its broad implication, sea level data from the KORSs has not been investigated as much as other key environmental variables such as air-sea turbulent flux, wind, wave, and water properties. Sea level rises estimated at the I-ORS (5.6 mm yr-1) from both satellite and tide-gauge observations were higher than those in the northeast Asian marginal seas (5.4 mm yr-1) and the globe (4.6 mm yr-1) from satellite observation from 2009 to 2018. The thermosteric, halosteric, and their summed (steric) sea level rise rates were 2.7~4.8, −0.7~2.6, and 2.3~7.4 mm yr-1, respectively. The rising rate of the steric sea level was higher than that of the total sea level even after additional data quality control processing. A non-steric component of sea level, which is estimated by subtracting the steric component from the total sea level, was not found to yield a significant trend. These results indicate a considerable uncertainty in water properties time series. This uncertainty was not likely to arise from the data analysis but from a lack of adequate data, even though the time series of sea level, temperature, and salinity were strictly processed based on two levels of the quality control procedure. The sea level time series observed from the KORSs suggests that the maintenance management of observation systems, equipment, and data quality control should be improved to facilitate data use from those stations. 

OS15-A007
Sea Level Observations in Singapore

Trina NG#+
Centre for Climate Research Singapore/NTU, Singapore


Singapore, a low lying densely populated island state, is highly vulnerable to sea level rise in a warming climate and hence, estimates of sea level rise are important for adaptation purposes. In this study I analyze the sea level change around Singapore’s coast using 13 tide gauge records, most starting during the 1980s. Continuous records dating back to the 1970s at tide gauges Sembawang, Sultan Shoal and Raffles Light House show sea level rise rates of 1.84±0.14mm/yr, 3.15±0.14mm/yr and 2.20±0.18mm/yr respectively. My results provide evidence that rate of mean sea level rise at Sembawang in the north of the island is at least 0.35mm/yr slower than the other stations in the south for the period 1973-2018. Many reasons have been considered to explain this disparity, including geographical locations, local vertical land movement, and the role of the regional ocean variability, but none of which are strongly conclusive. The larger exposure to open seas in the southern coastline as compared to the narrow channel Sembawang faces could be one factor attributing to this difference. However, due to the relatively short time scale of these trends, the effects of multi-decadal and regional variability cannot be ruled out. Hence, one of the key findings here highlighted the importance of long tide gauge records of at least 50 years for long-term sea level trends to be robustly estimated. 

OS15-A006
On the Role of Seasonal Signal in Explaining Regional Sea Level Variability Over 25 Years with Focus on South East Asian Maritime Region

Hindumathi PALANISAMY#+, Svetlana JEVREJEVA
Centre for Climate Research Singapore, Singapore


Oceans absorb about 93% of the excess energy accumulated in the climate system due to anthropogenic greenhouse gas emissions. Regionally, sea level change is far from being uniform and exhibit a time-space varying signature due to different processes such as ocean dynamic processes, changes in Earth’s gravity and rotation, land mass change induced visco-elastic solid Earth deformation. Therefore understanding regional sea level rise and variability and its underlying causes is crucial as human activities are increasingly concentrated on the coastal zones of the world’s oceans. In recent years, most of the studies have focused on studying sea level variability at interannual, decadal and multi-decadal time scales. However the seasonal cycle playing a crucial role in coastal infrastructure and adaptation planning, nevertheless, is currently not included in state-of-the art sea level projections and is not covered adequately by the uncertainties. This study provides an investigation on the seasonal sea level variability over the past 25 years between 1993 and 2018. We first study the spatial features of seasonal sea level signal as observed by satellite altimetry, how much of the total observed sea level variability is contributed by seasonal signal, and the role of steric component in explaining the seasonal variability. We then move our focus towards South East Asian Maritime region where we identify that most of the total sea level variability is contributed by the seasonal signal. Using tide gauge records in conjunction with altimetry and ocean reanalysis data, we quantify and then explore the causes of seasonal sea level oscillations in this region.

OS16-A019
Evaluation of the Impact of Future SST Conditions on Future Tropical Cyclone Characteristics

Jung-A YANG1#+, Nobuhito MORI2
1Korea University, Korea, South, 2Kyoto University, Japan


Tropical cyclones (TC) generating extreme winds, rainfall and storm surge are one of the elements that pose hazards to low lying coastal areas. As future changes in TC characteristics due to climate change are anticipated, their future projections on a national scale are required to respond to coastal disasters. When projecting future TC properties by using the atmospheric general circulation model (AGCM), there are several uncertainties due to model resolution, model physics parameterization, given sea surface temperature (SST) under future climate condition, and global warming scenarios. The uncertainties stemming from physics and numerical modelling configuration can be reduced by improving the accuracy of AGCMs, while those from the global warming scenario and future SST condition are unable to be. This study assessed uncertainties in projecting future change to typhoon properties such as tracks, frequency and intensity depending future SST condition. To this end, TC data which were extracted from the results of climate experiments with six SST conditions by applying the TC detection technique was employed. The different future SST patterns were derived from six models of CMIP5 models: CCSM4, GFDL-CM3, HadGEM2-AO, MIROC5, MPI-ESM-MR, MRI-CGCM3. They were selected based on a cluster analysis of geographical patterns of SST changes so that the six patterns cover the most part of the uncertainty of the patterns in all the CMIP5 models. This study focused on TC occurring in the North Western Pacific and passing around the Korean Peninsula which is defined as the region of 122°E - 132°E and 32°N - 40°N. It was expected the number of the typhoons affecting KP will decrease while their strength will increase in most SST conditions. However, the extent of their change varied depending on the future SST.

OS17-A011
Variations in Dissolved Oxygen Induced by a Tropical Storm within an Anticyclone in the Northern South China Sea

Zhenhui YI1+, Dongxiao WANG1, Chunhua QIU1#, Huabin MAO2, Jiancheng YU2, Shumin LIAN2
1Sun Yat-sen University, China, 2Chinese Academy of Sciences, China


Tropical storms (TSs) can induce sea surface cooling, freshening and phytoplankton blooms. The dissolved oxygen concentration (DO) response to TS within an anticyclone is still unclear due to the rarity of in situ observations. We investigated the variations in DO attributed to TS ‘Haitang’ within an anticyclonic eddy in the northern South China Sea (NSCS) based on Chinese underwater gliders data. DO has higher value at the edge of eddy thant that at the eddy core. Influenced by TS, DO concentration decreased significant in the subsurface layer in all three regions (inside, edge and outside of the anticyclonic eddy). The mean DO decreased more at the edge of anticyclone that that inside the anticyclone. The recovery time of DO after TS was around one week at the edge of anticyclone, and is > 10 days within the eddy. DO variations were dominated by horizontal advection and vertical convection terms in subsurface layer. The enhancement of outward radial velocity was lagged behind upward velocity, indicating that the decline of DO underwent two processes: upwelling and divergence. Subsurface chlorophyll-a maximum (SCM) increased during TS and deceased quickly after TS inside eddy, while it changes quickly at both the eddy edge and outside of eddy. This may induce the decrease (increase) of DO in oversuaturated (unsuaturated) water through the respiration (photosynthesis) and particle remineralization process.

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