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Presentation Mode : All
Conference Day : 02/08/2021
Time Slot : PM1 13:30 - 15:30
Sections : OS - Ocean Sciences










Ocean Sciences | Mon-02 Aug


OS02-A018
Tsunami Resonance Characterization in Japan from Trans-pacific Sources: Response on the Bay and Continental Shelf

Yuchen WANG1#+, Natalia ZAMORA2, Marco QUIROZ3, Kenji SATAKE4, Rodrigo CIENFUEGOS3
1Japan Agency for Marine-Earth Science and Technology, Japan, 2Barcelona Computing Center, Spain, 3Pontificia Universidad Católica de Chile, Chile, 4The University of Tokyo, Japan


The tsunami resonance leads to the amplification of wave heights and extends the duration of wave activity. For tsunami warning and coastal planning, it is important to understand resonance behavior. We characterized the tsunami resonance of trans-Pacific events in the bays and continental shelves of Japan, focusing on the Hokkaido and Sanriku coasts. We considered five tsunami sources in the Peru–Chile subduction zone and the Alaska–Aleutian subduction zone: 1730 Valparaiso earthquake (Mw 9.1), 1906 Ecuador–Colombia earthquake (Mw 8.8), 1960 Valdivia earthquake (Mw 9.5), 1964 Alaska earthquake (Mw 9.1), and 2010 Maule earthquake (Mw 8.8). Spectral analysis was applied to observed and synthetic waveforms at coastal tide gauges and offshore GPS gauges to study the frequency content of tsunami energy, and modal analysis was conducted to investigate the eigen-modes of the natural oscillation affected by regional and local bathymetry. The results of spectral analysis showed that the signatures of trans-Pacific tsunami sources are lost in the spectra at most coastal gauges. But they partly remain in the spectra at offshore gauges, which are affected less significantly by topography or bathymetry. The predominant periods in the spectra at most coastal gauges match with the eigen-periods of natural oscillation calculated by modal analysis. Hence, we confirmed that regional (~ 100 km) or local (~ 10 km) bathymetry is the dominant factor in the resonance behaviors of trans-Pacific tsunami events, rather than the source location or geometry. These findings are based on the systematic assessment of trans-Pacific sources, and have implications for tsunami warning and coastal planning along the Pacific coast of Japan. In future work, we will extend our analysis to other areas of the Japanese coast, and study the contribution of internal structure of tsunami sources.

OS03-A002
The Relations Between Eddy Field Vorticity and Tidal Critical Latitude Shifts

Tianyu ZHOU1+, YUANZHI LI2, Peien XU2, Litong YANG2, Yifan GU2, Muzi WU2, Yuyao LI2, Qi WANG2, Jialin LI2, Robin ROBERTSON2#
1Xiamen University, China, 2Xiamen University Malaysia, Malaysia


The tidal critical latitudes have been found to influence the behavior of internal tides and waves and the resultant energy cascade and mixing. Relative vorticity from eddies or currents can locally shift the critical latitude either equatorward or poleward of its location due only to planetary vorticity. But the degree of this shift and its potential impact are poorly understood and are presently ignored in coarse-resolution ocean general circulation and climate models. Using a series of numerical simulations with the Regional Ocean Modeling System (ROMS), the effect of various strengths of vorticity on the shift of the critical latitude and the impact on diffusivity, viscocity, tidal fields, temperature and salinity fields, and energy transfers were investigated. Relations between the amount of vorticity and various factors were developed. These factors include the shift of the planetary critical latitude to the effective critical latitude, diurnal and semidiurnal tidal responses, temperature and salinity field responses, and the frequency content of the velocities, viscosities, and diffusivities. These relations will enable larger scale ocean circulation and climate models to account for critical latitude effects in an efficient and effective manner. Come see our poster and learn the relations.

OS03-A009
Spatial Variation of Bottom Mixed Layer in the South China Sea and Its Associated Mechanism

Jianing LI#+, Qingxuan YANG, Wei ZHAO, Jiwei TIAN
Ocean University of China, China


Based on the full depth CTD and ADCP data at 201 stations, the characteristics and spatial variation of the bottom mixed layer in the South China Sea are investigated. The stratification of the bottom mixed layer in the South China Sea is about 6×10-7 s-2 with the thickness ranging from 16 m to 570 m, which is much thicker and less stratified than the upper mixed layer. Luzon Strait and Zhongsha island chain are found to be two hotspots of thick bottom mixed layer, where the mean thickness (347 m) is three times the other areas (114 m). A 2D MITgcm numerical simulation is run for 15 days to explore the thick bottom mixed layer and the corresponding enhanced bottom mixing at Zhongsha island chain. The simulation indicate that the interaction of the background current and the special topography feature would generate energetic lee wave. The breaking of lee wave causes strong mixing (3.6×10-2 s-2), which forms a 350 m thick bottom mixed layer at 10 km far from the seamount, consistent with the observed result.

OS08-A008
A New Automated Quality Control system for Ocean In-situ Temperature Profile Observations

Zhetao TAN1#+, Lijing CHENG2, Jiang ZHU3
1Institute of Atmospheric Physics, Chinese Academic of Sciences, China, 2Institute of Atmospheric Physics, Chinese Academy of Sciences, China, 3Chinese Academy of Sciences, China


Improving the Automated Quality Control (AutoQC) is one of the basic tasks in oceanography. The quality-controlled datasets are often used for marine scientific research (e.g., numerical models, earth energy imbalance analysis). However, previous studies have found that the performance of the AutoQC systems are not well robust to identify the bad profiles correctly (e.g., WOD-QC system, GTSPP-QC system). In addition, the quality-controlled datasets have serious quality problems (e.g., WOD18). In this study, we developed a new AutoQC system for ocean In-situ temperature observations, which includes 6 main checks (Crude Range Check, Maximum Depth Check, Spike Check, Constant Value Check, Gradient check, and Local Climatology Check). Moreover, a new historical temperature climatology for Local Climatology Check was reconstructed based on temperature profiles in WOD18. This new climatology is statistically analyzed on a one-degree grid for temperature with 119 depth levels from the surface to 6,000 m, including 7 statistical variables (mean, median, standard deviation, 95% quantile, 99% quantile, skewness, and kurtosis) and two time periods (1940-2020 and 2005-2017). The performance of the AutoQC system was also evaluated by comparing it with other AutoQC systems. Results confirm that the true positive rate (TPR) of this AutoQC system was increased after updating this new local climatology archive. This study shows another try to provide the best oceanic quality-controlled in-situ observational dataset for oceanography in the future.

OS08-A017
Studies on Stratification in the North Port, Incheon, Korea, and Its Effect on Currents with Fvcom

Jae-Soon JEONG1#+, Han Soo LEE1, Seung-Buhm WOO2
1Hiroshima University, Japan, 2Inha University, Korea, South


In ports developed in estuaries with strong tides and high river discharge, research on nearby currents is important for the safety of ships and maintenance of port facilities. The unstructured-grid Finite Volume Community Ocean Model (FVCOM) was used to study the baroclinic effect in the North Port of Incheon, Korea. The port is developed perpendicular to the main channel and forced by river discharge from the north and tides with the range over 9 m. This model with 59,757 nodes and 113,918 cells considered open boundaries for tide and river discharge, and atmospheric forcing for surface wind and pressure from Local Data Assimilation and Prediction System (LDAPS). The highest horizontal resolution of the unstructured meshes was 50 m around the port. Currents and salinity were observed at the main channel and within the port in February and July 2019. In the main channel, the salinity difference between the sea surface and bottom increased during neap tide by weakening currents, resulting in the enhanced density stratification. However, at the port, the stratification was strengthened during spring tide in both model results and observations. During spring tide, surface fresh water from the upper estuary descended further southward at low tide than during neap tide. The fresh water flowed into the port during flood tide and was trapped in the semi-closed shape of the port. The horizontal saline gradient between the trapped fresh water and the saltier water at the main channel increased until the flood tide finished: high tide. As a result, density-driven currents, which induce bottom currents flowing into the port, are maintained for more than 3 hours after high tide even if the surface currents started to ebb. Those density-driven circulations in the port are well simulated in the numerical model results and agreed well with the field observations.

OS08-A020
Understanding the Current Status of Seawater pH Changes Near Scallop Aquaculture Facilities in Northern Japan

Ayumi MORI1#+, Takeshi YOSHIMURA1, Isao KUDO1, Koichi SAKAGUCHI2
1Hokkaido University, Japan, 2Aquaculture Fishery Cooperative of Saroma Lake, Japan


In recent years, ocean acidification has gained attention, and there has been growing concern regarding the decrease of ocean pH. Although seasonal and diurnal changes in pH may be greater in coastal areas than in the open ocean, the changes are not well understood. As pH decreases, the saturation state of calcite (ΩCa), a type of calcium carbonate, decreases, affecting the survival and growth of calcifying organisms. Calcite-forming scallops (Patinopecten yessoensis) are one of the most important fishery resources in northern Japan. Their growth may be inhibited by decreased pH. However, the current situation remains unclear. Therefore, in this study we conducted surveys to understand the current state of pH decrease near scallop aquaculture facilities in Lake Saroma and Funka Bay in Hokkaido, as well as Mutsu Bay in Aomori, Japan. Furthermore, the observed values were used to predict pH and ΩCa values at the end of this century (2081-2100) under different RCP scenarios. The results show that seasonal pH changes were similar in the three areas, although the magnitude of change differed among sites. The largest range of pH changes (7.5-8.3) was observed in Lake Saroma, and the lowest values were observed in the bottom layer in summer when organic matter decomposition occurs. In addition, ΩCa in Lake Saroma decreased with decreasing pH, as well as a decrease in water temperature and salinity in all layers in winter. Thus, we demonstrate that the bottom layer in summer and all layers in winter should be considered for scallop growth inhibitions in Lake Saroma. Predictions based on these results showed that scallop cultivation would be still possible under RCP 2.6, but may be impossible due to significantly reduced growth in all areas and seasons under RCP 8.5.



OS10-A001
Pacific Subsurface Ocean Temperature as a Long-range Predictor of South China Tropical Cyclone Landfall

Ralf TOUMI#+, Nathan SPARKS
Imperial College London, United Kingdom


Seasonal forecasts of the tropical cyclones which frequently make landfall along the densely populated South China coast are highly desirable. Here, we analyse observations of landfalling tropical cyclones in South China and of subsurface ocean temperatures in the Pacific warm pool region, and identify the possibility of forecasts of South China tropical cyclone landfall a year ahead. Specifically, we define a subsurface temperature index, subNiño4, and build a predictive model based on subNiño4 anomalies with a robust double cross-validated forecast skill against climatology of 23%, similar in skill to existing forecasts issued much later in the spring. We suggest that subNiño4 ocean temperatures precede the surface El Niño/Southern Oscillation state by about 12 months, and that the zonal shifts in atmospheric heating then change mid-level winds to steer tropical cyclones towards landfall in South China. We note that regional subsurface ocean temperature anomalies may permit atmospheric predictions in other locations at a longer range than is currently thought possible.

OS10-A004
Sea Surface Current Response Patterns to Tropical Cyclones

Han ZHANG#+
Second Institute of Oceanography, Ministry of Natural Resources, China


Tropical cyclones (TCs) are strong synoptic systems which induce strong sea surface currents. This paper first cross-checks a detailed surface current response to specific TCs based on buoy and mooring observations as well as a three-dimensional numerical model and a one-dimensional semi-analytical model which simplifies the driving forcing into wind stress and Coriolis force, then estimates the impact of all possible tropical cyclones using the semi-analytical model. The results show that the sea surface current response to the kinetic energy input of TCs, which is dependent on the TC configurations (translation speed, size and intensity) and the environmental configurations (Coriolis frequency and upper ocean stratification), can be represented by two simple parameters, namely the TC nondimensional translation speed (S) and the TC wind force parameter (Vc) or TC wind energy parameter (E).  represents a combined effect of TC translation speed, size and Coriolis frequency, determines the structure of surface current response. Vc or E represents a combined effect of TC intensity, mixed layer depth and Coriolis frequency, determines the intensity of surface current response or wind energy input into surface currents. Ekman-like divergence dominates the sea surface current response when S is small (0<S≤0.4), an inertial oscillation in the lee that bias to the right rear part of a TC dominates when S is medium (0.4<S≤5), and an impulse with forward (backward) current on the right (left) side of the track dominates when is large (S>5). This work provides a simple and easy-to-use method to estimate the surface current response pattern to TCs when TCs and associated environmental configurations are given, which may help to improve the parameterization of TCs in regional and climate modelling. It also suggests that S is a better index than TC translation speed to classify TCs when studying the oceanic response.

OS10-A010
Uncertainty of Tropical Cyclone Wind Radii on Sea Surface Temperature Cooling

Iam-Fei PUN1#+, John KNAFF2, Charles SAMPSON3
1National Central University, Taiwan, 2NOAA STAR, United States, 3Naval Research Laboratory, United States


The sea surface temperature (SST) beneath a tropical cyclone (TC) is of great importance to its dynamics; therefore, understanding and accurately estimating the magnitude of SST cooling is of vital importance.  Existing studies have explored important influences on SST such as TC translation speed, maximum surface winds, ocean thermal condition and ocean stratification.  But the influence of the TC wind radii (or collectively called the TC size) on SST has been largely overlooked.  In this study we assess the influence of wind radii uncertainty on SST cooling by a total of 15,983 numerical simulations for the western North Pacific during the 2014-2018 seasons.  Results show a 6-20% SST cooling error induced using wind radii from the Joint Typhoon Warning Center official forecast and a 35-40% SST cooling error using wind radii from the operational runs of the Hurricane Weather Research and Forecasting (HWRF) model.  Our results indicate that SST cooling is most sensitive to the radius of 64 kt winds (R64) due to its effects on the integrated kinetic energy of the TC and subsequent mixing of the ocean surface layer.  It is also found that the correlation between SST cooling induced by the TC and its size is 0.49, which is highest among all the parameters tested.  This suggests that it is extremely important to get TC size correct in order to predict the SST cooling response, which then impacts TC evolution in numerical weather prediction models.

OS10-A005
Warm Ocean Accelerating Tropical Cyclone Hagibis (2019) Through Interaction with a Mid-latitude Westerly Jet

Kosuke ITO1#+, Hana ICHIKAWA2
1Kyoto University, Japan, 2University of the Ryukyus, Japan


One of the remarkable environmental characteristics of tropical cyclone (TC) Hagibis (2019) was the positive sea surface temperature (SST) anomaly observed in the western North Pacific Ocean. In this study, an ensemble-based sensitivity experiment was conducted with a nonhydrostatic model, focusing on the impact of SST on TC motion. The TC with the analyzed SST (warm run) moved faster near mainland Japan than with the lowered SST (cold run), as the TC in the warm run was embedded earlier in the mid-latitude westerly jet located to the north than that in the cold run. The TC displacement was consistent with the large decrease of geopotential height at 500-hPa (Z500) in the north of TC Hagibis during the warm run. Further investigation showed that the approach to the westerly jet presumably induced the low local inertial stability as well as the southwesterly vertical wind shear enhancing the upward mass flux in the north of the TC. They led the enhanced upper-tropospheric northward outflow from the TC energized by the warm SST, and it resulted in the decrease of the Z500 in the north. This study suggests that warm SST can affect TC tracks through interaction with mid-latitude westerly jets.

OS10-A012
The Effects of Air-sea Interaction on Eyewall Replacement Cycle of Typhoon Trami (2018)

Xiangcheng LI1, Xiaoping CHENG1#+, Jiangfang FEI1, Xiaogang HUANG2, Juli DING1
1College of Meteorology and Oceanography, National University of Defense Technology, China, 2National University of Defense Technology, China


The duration of the eyewall replacement cycle (ERC) in typhoons is determined by the rate of dissipation (intensification) of the inner (outer) eyewall, and is an important indicator for predicting changes in the intensity and structure of typhoons. Previous results suggest that an ERC depends on internal dynamical processes that are governed by the interaction between concentric eyewalls. On the one hand, the substantial subsidence induced by the secondary eyewall over the inner eyewall region can suppress updrafts in the latter. On the other hand, the outer eyewall cuts off the moisture and momentum supply of the inner eyewall by blocking the inflow in the boundary layer. These simulations were almost conducted with uncoupled atmospheric-only numerical models, without considering 3-D ocean responses. However, the dissipation caused by energy outages from the ocean have not been adequately investigated. Using the coupled atmosphere-ocean model, the simulation for Typhoon Trami generates an ERC that matches observations, whereas an unrealistic long-lived ERC is produced in the uncoupled simulation. Compared with classic ERC theory, the dramatic drop of the ocean’s energy supply largely contributed to the demise of the inner eyewall, thus speeding up the ERC. The results show that the typhoon-induced non uniform cold wake can not only weaken the typhoon, but can also modulate the duration of the ERCs. A diagnostic analysis of the bulk budget equation for the equivalent potential temperature indicates that the reduction in surface heat fluxes was the driving force that triggered the weakening of the radial and the vertical processes successively in the boundary layer. As a result of the thermal adjustment, the inner eyewall collapsed due to the reduction in the air-sea heat fluxes. The results show that the effect of modulation owing to the cold wake is essential for the accurate forecasting of ERCs. 

OS10-A007
Dynamics and Thermodynamics of Sea Spray Under Tropical Cyclones

Alexander SOLOVIEV1#+, Breanna VANDERPLOW1, Roger LUKAS2, Bian HAUS3, Muhammad SAMI4, Isaac GINIS5
1Nova Southeastern University, United States, 2University of Hawaii, United States, 3University of Miami, United States, 4Ansys Inc., United States, 5University of Rhode Island, United States


There are serious questions regarding the appropriate methods for the incorporation of the air-sea interaction in the models capable of forecasting rapid intensification and rapid decay of tropical cyclones. Dynamics and thermodynamics of sea spray remain a serious challenge, involving air-sea interface, airflow with Mach up to 0.25, pressure fluctuations due to surface waves, surfactants, entrainment of spray particles in the turbulent airflow, evaporation and condensation, sea-salt particles inducing condensation and cloud formation, etc. Our approach for getting an insight into this extremely complicated dynamics and thermodynamics is comprised of computer modeling in ANSYS Fluent, laboratory verifications, and the extrapolation of the results to the open ocean conditions. We have implemented a Volume of Fluid to Discrete Phase model with dynamic remeshing, which resolves spray particles ranging in size from tens of micrometers to a few millimeters. The water particles that satisfy the condition of asphericity are converted into Lagrangian particles, which are involved in a two-way interaction with the airflow. This model has been partially verified at the University of Miami Surge Structure Atmosphere Interaction facility. The Fluent Evaporation-Condensation model provides the opportunity to model spray evaporation/condensation and related heat and enthalpy fluxes. The spray particles entrained in the turbulent airflow evaporate and contribute less to the enthalpy flux because of the effect of negative feedback. A complication is that negative feedback depends on background air humidity in the marine boundary layer, which is not well-known parameter under tropical cyclones. Spray generation in the model increases with wind speed; while the maximum size of the spray particles suspended in the turbulent airflow also increases. As a result, the relative contribution of sea spray in the enthalpy flux may not increase, which can explain the nearly constant enthalpy exchange coefficient observed in laboratory and oceanic experiments on tropical cyclones.

OS10-A013
A Simple Trajectory Model for Climatological Studies of Tropical Cyclone and Its Potential to Be Used for Assessing Possible Impact of Climate Change on Tropical Cyclone

Kaiyue SHAN1#+, Xiping YU2
1Tsinghua University, China, 2Southern University of Science and Technology & Tsinghua University, China


The establishment of a tropical cyclone (TC) trajectory model that can represent the basic physics and is practically applicable considering both accuracy and computational cost is essential to the climatological studies of global TC activities. In this study, we propose a simple deterministic model based on a newly developed semi-empirical formula for the beta drift under known conditions of the environmental steering flow. To verify the proposed model, all historical TC tracks in the western North Pacific and the North Atlantic Ocean basins during the period 1979-2018 are simulated and statistically compared with the relevant results derived from observed data. The proposed model is shown to well capture the spatial distribution patterns of the TC occurrence frequency in the two ocean basins. Prevailing TC tracks as well as the latitudinal distribution of the landfall TC number in the western North Pacific Ocean basin are also shown to agree better with the results derived from observed data, as compared to the existing models that took different strategies to include the effect of the beta drift. It is then confirmed that the present model is advantageous in terms of not only the accuracy but also the capacity to accommodate the varying climate. We also use the TC trajectory model to assess the potential changes in TC activities over the western North Pacific and the potential threat to China at the end of the 21st century. It is suggested that the TC occurrence in southern China will decrease, while the more landfalling TCs further north will lead potential threat to Zhejiang Province, Jiangsu Province, and northern coastal provinces. Reducing greenhouse gas emissions can effectively reduce the threat of TCs to high-latitude regions. 

OS10-A002
Development of a Simplified-Ocean Model Coupled with an Atmospheric Global Climate Model and Their Climate Change Impacts on Tropical Cyclones

Tomoharu OKADA1#+, Tomoya SHIMURA1, Adrean WEBB1, Takuya MIYASHITA1, Nobuhito MORI1, Ryo MIZUTA2
1Kyoto University, Japan, 2Meteorological Research Institute, Japan


Climate change can alter the characteristics of extreme phenomena in addition to the mean climate state. Recent IPCC reports have summarized the possibility of long-term changes in tropical cyclone (TC) characteristics under future warmer climate conditions, yet uncertainty still remains large. Atmosphere Ocean coupled Global Climate models (AOGCMs) from the Coupled Model Inter-comparison Project (CMIP) have been widely used for climate change impact assessments. However, current AOGCMs do not sufficiently account for air-sea interactions, such as sea surface cooling (SSC) due to strong winds. It is important to consider the effects of cooler sea surface temperatures (SSTs) during the TC development process. This study develops a simplified ocean model coupled with an AGCM to reproduce TCs while considering air-sea interactions. Furthermore, climate simulations based on CMIP6 experiments are conducted with this model, and their climate change impacts on TC characteristics are estimated. The Japanese Meteorological Research Institute climate model, MRI-AGCM, is coupled with an ocean slab model and optimized based on atmospheric and oceanic analysis datasets. Projected SST and sea ice conditions under climate change are obtained from CMIP6 and multi-model mean SST and sea ice fields are estimated in comparison with observations. The multi-model means are then used as base boundary conditions for the MRI-AGCM climate simulations, which are regionally modified by the ocean model when strong wind-induced upper ocean mixing or SSC occurs at the sea surface. We have conducted two types of global climate simulations, typical and newly proposed experiments, to estimate the ocean coupled effects and climate change impacts on TC characteristics. The ocean model which considers SSC significantly contributes to a reduction in TC intensity. Moreover, we have confirmed that climate change can reduce the TC frequency and enhance the TC intensity, which is consistent with IPCC reports.

OS10-A003
Rapid Intensification of Super Typhoon Hagibis (2019)

I-I LIN1#+, Rogers ROBERT2, Hsiao-Ching HUANG1, Yi-Chun LIAO1, Jin-Yi YU3, Christina M PATRICOLA4, Jun ZHANG5, Derrick HERNDON6, Ya-Ting CHANG1, Iam-Fei PUN7, Chun-Chi LIEN1
1National Taiwan University, Taiwan, 2NOAA's Atlantic Oceanographic and Meteorological Laboratory, United States, 3University of California, Irvine, United States, 4Lawrence Berkeley National Laboratory, United States, 5University of Miami, United States, 6 University of Wisconsin, United States, 7National Central University, Taiwan


Category-5 Super typhoon Hagibis is one of the highest impact typhoons in recent years. In October 2019, Japan’s densely-populated Kanto region (including the capital Tokyo and surrounding area) were hit by Hagibis. Though having already weakened to a moderate category-2 typhoon at landfall, torrential rain and wind from Hagabis still caused significant loss of life and widespread damage. While heavy rainfall was the main cause of damage at landfall, one of the most striking characteristics of Hagibis was its rapid intensification (RI, Kaplan and DeMaria 2003) over the ocean. In 24 h, Hagibis intensified from Tropical Storm (TS) to Category-5 (i.e. 55 kt to 140 kt) intensity. This ‘explosive’ intensification of 85 kt in 24 h is 283% of the RI threshold of 30 kt in 24 h, making Hagabis one of the most rapidly-intensifying typhoons on record. This work aims to study Hagibis’s RI by examining the structure of Hagabis and its surrounding environment.