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Presentation Mode : All
Conference Day : 03/08/2021
Time Slot : PM2 16:00 - 19:00
Sections : OS - Ocean Sciences










Ocean Sciences | Tue-03 Aug


OS09-A010
Interdecadal Variation of the Northeast Cold Vortex in Recent 70 Years

Dan ZHU1#+, Xiefei ZHI2
1Nanjing University of Information Science and Technology, China, 2Nanjing University of Information Science & Technology, China


The northeast cold vortex is a synoptic scale cut-off low mainly active from Siberia to Northeast China.It plays a crucial role in the climate change over East Asia and even the whole world. Based on the reanalysis data of NCEP / NCAR and ERA5, using improved objective detection and tracking algorithm. This paper analyzes the interdecadal variation characteristics of the northeast cold vortex, and further discusses the possible physical mechanism of its transformation. The results show that the decadal variability and long term trends are important features of climate change for the northeast cold vortex. There is a significant interdecadal change of the northeast cold vortex around 1995, and then the frequency of occurrence decreased. Corresponding to this interdecadal transition, the interdecadal variability of the movement distribution center and their intensity of the northeast cold vortex can be explained by interdecadal variations of the position and intensity of the baroclinic front, which is mainly dominated by meridional temperature gradient in the lower troposphere. 

OS10-A011
Recent Increase in the Occurrences of Christmas Typhoons in the Western North Pacific

JOSEPH BASCONCILLO+, Il-Ju MOON#
Jeju National University, Korea, South


To imply the gravity of their impact on Christmas celebration, the term Christmas typhoon recently became more popular to refer to tropical cyclones (TC) in the Western North Pacific (WNP) during its less active season.  The past nine years from 2012 to 2020 saw more than 70% (210%) increases in Christmas typhoon occurrences in the WNP (Philippines). Furthermore, Mindanao Island, which is located in southern Philippines, has experienced an unprecedented 480% increase in TC passage in the same period.  Here we show that the detected recent increase in Christmas typhoons are mainly associated with the shift of the Pacific Decadal Oscillation to its positive phase in the early 2010s, which led to favorable changes in the large-scale environment for TC development such as higher relative vorticity, anomalous low-level westerlies, warmer sea surface temperatures, and extended WNP subtropical high. We also found that the poleward shift of the Intertropical Convergence Zone, warmer sea surface temperatures in the central Pacific, and possibly, the recent recovery of the Siberian High contributed to such increased occurrences.  As opposed to the more active TC season, there is a wide research gap during the less active season. We aim to fill in this knowledge gap to gain better and holistic insights on TC hazard risk reduction.

OS16-A012
Simulated Pacific Decadal Variability By An Ocean-atmosphere Coupled Model

Po-Chun CHIANG#+, Chung-Hsiung SUI, Yu-Heng TSENG
National Taiwan University, Taiwan


In this project, we analyze the simulated results of a pre-industrial control experiment (piControl) by the Community Earth System Model Version 2 (CESM2) available from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to study the features of decadal variability in Pacific Ocean and to explore the possible mechanisms for the variability. First, we analyzed the pattern of Pacific SST and subtropical cells (STCs) in the model that resemble the observed characteristics of tropical Pacific decadal variability (TPDV) and oceanic passage. The ocean temperature and STCs transport in tropical Pacific both display similar decadal oscillations at periods about 15-20 years, and the tropical Pacific SST is negatively correlated with warm water volume and interior STCs transport. Next, we selected major decadal events with clear signals and performed a composite analysis. The composite features of TPDV during the cold phase are summarized as follows: anomalous cold SST in eastern and tropical Pacific along with warm western Pacific SST in both hemispheres; anomalous positive wind stress curl extending from subtropical southern Pacific to tropics accompanied with enhanced STCs transport and deepened tropical western Pacific thermocline; enhanced equatorial undercurrent; surface cooling and interior warming. The composite features during the warm phase are similar to that of the cold phase but of opposite sign. Finally, there is asymmetry between northern and southern hemisphere in TPDV, and the results indicate that the ocean tunnel processes are most evident in the southern Pacific. Northern Pacific display a pattern similar to Pacific meridional mode (PMM), implying the mechanisms related to seasonal footprinting may also play some roles in driving TPDV.

OS16-A018
Impacts of Distinct Ocean-atmosphere Coupling Processes During the Winter Cold Surge in East Asia

Hsi-Hsien TSENG#+, Yu-Heng TSENG
National Taiwan University, Taiwan


Extreme weather condition appears more and more frequently in the future climate scenario. In the northern hemisphere, winter cold surge (CS) is the most influential weather phenomenon. In 2016 January, an unusual CS hit East Asia and caused devastating damages in many East Asia countries, potentially resulting mainly from the polar vortex ruptured under the influence of strong El Niño teleconnections. This study analyzes the associated ocean-atmosphere interaction (OAI) processes of the January 2016 CS using the reanalysis data and global coupled model experiments. Our results show that this CS event can be divided into two regimes in time: the first regime is dominated by the atmospheric forcing and the latter one is dominated by the oceanic forcing. For the atmospheric forcing-dominated regime, the strong winds and cold air associated with the CS forces the sea surface temperature change through the surface heat flux. On the other hand, for the oceanic forcing-dominated regime, the northward (southward) shift of the Kuroshio front causes an increase (decrease) in the regional sea surface temperature, which in turn dominates the surface heat flux change. Further comparing the CSs in different years, we can clarify them into fast-evolving and slow-evolving types in terms of different OAI. The fast-evolving CSs are initially dominated by the atmospheric forcing and then become dominated by the ocean, while the slow-evolving CSs are dominated by the ocean before becoming dominated by the atmosphere.



OS02-A029 | Invited
The Poleward Shift of the Edges of the Tropics Drives the Poleward Migration for the Tropical Cyclone Genesis and Its Potential Impact on Tropical Cyclone Activities in the Future

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


The known trends of poleward migration for the tropical cyclone (TC) genesis in both hemispheres are discussed from different perspectives. It is shown that the poleward migration rate of the annually averaged latitude of TC genesis in the Northern Hemisphere is significantly affected by the regional variations of TC number in the past decades, especially an increase in the North Atlantic Ocean and a decrease in the western North Pacific Ocean. The poleward migration rates of TC genesis in the two hemispheres get closer when the effect of the regional TC number variation is excluded. The poleward migration of TC genesis without the effect of regional TC number variation is found to have a good correlation with the poleward shift of the edges of the tropics in both hemispheres. A decreasing trend of the cyclonic vorticity in the lower-troposphere over the tropical ocean regions is also identified in both hemispheres, which leads to a poleward shift of the equatorward boundary for TC genesis. The effect of tropical expansion on the poleward migration of TC genesis is thus confirmed. Potential changes in TC activities over the western North Pacific at the end of the 21st century due to the tropical expansion are also assessed based on the projection experiment of global climate models. It is found that the tropical expansion plays an important role of the poleward migration of TC activities. 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.

OS02-A023 | Invited
Detection and Analysis of Tropical Cyclones and Associated Coastal Extreme Waves Within the D4PDF Mega-Ensemble Projection

Adrean WEBB#+, Tomoya SHIMURA, Nobuhito MORI
Kyoto University, Japan


Tropical cyclones (TCs) play an important role in the climate system and are a major source of natural coastal hazards worldwide, such as those due to river flooding, storm surges, and extreme waves. TC-related hazards are sensitive to properties of the storm path and while climate models can provide data to accurately model these events, samples are inadequate for analysis of extreme events on climatological time scales. The climate dataset d4PDF (Database for Policy Decision Making for Future Climate Change) was created to overcome this low-occurrence frequency limitation and spans more than 17,000 years under present, +1.5 K, +2 K, and +4 K warming conditions, making it possible to directly estimate 100 year or longer return values. Here, a tropical cyclone track dataset has been created for the d4PDF dataset. The detection method uses pressure difference, wind speed, and SST threshold parameters and is tuned to optimize annual TC count and cyclogenesis locations. Tuning and validation of the method is conducted using 30-year IBTrACS observation and JRA-55 reanalysis data. The extracted tracks are then used to dynamically model storm waves at select locations for thousands of potentially extreme wave events. Here, we will present an overview of the detection method (including validation and performance), and analyze changes in the TC tracks (such as occurrence frequency and cyclogenesis location) and their associated extreme wave heights.

OS02-A012
Development of an Optimization Method for Combining WRF Physics in Typhoon Forecasts Using Deep Neural Network and Application to Storm Surge Forecasts

Tomoki SHIRAI1+, Masashi WATANABE2, Taro ARIKAWA1#
1Chuo University, Japan, 2Nanyang Technological University, Singapore


Storm surge induced by tropical cyclones causes catastrophic damages to coastal areas. To reduce risks due to coastal hazards, an early and accurate storm surge forecast system is required. Recently, weather predictions based on numerical modeling has been used for real-time typhoon forecast. Especially, the Weather Research and Forecasting model (WRF) has widely been applied for typhoon forecasts. For numerical calculation using WRF, some physical processes (physics option) such as radiation, cloud microphysics, planetary boundary layers, and so on must be selected. A selection of optimal physics combination is not same for each typhoon because a same typhoon never occurs. Thus, it makes difficult for WRF users to determine a combination of physics options which should be used for typhoon forecast. In this study, we developed an optimization method using Deep Neural Network (hereinafter, DNN) to automatically determine optimal physics combinations in WRF. The database which includes the 200 cases of sensitivity analysis results was used for training dataset of the DNN.  This method is mainly aimed to be used for real-time typhoon forecast in the future because there is not enough time for real-time typhoon forecasting. Moreover, it is difficult to perform sensitivity analysis and determine the best combination of physics options in each time. In addition, the typhoon forecasting accuracy is expected to be improved if the combination of physics options is optimized for each typhoon using the presented method. As a result of the DNN application to typhoon forecasts, the calculated track and intensity of typhoon were reasonably estimated compared to the ones calculated based on random physics combinations, so that the accuracy of storm surge prediction is also improved. The presented results suggest that our optimization method can be applied in real-time typhoon and storm surge forecasts. 

OS02-A010
Development of an Integrated Flood Model for Coastal Urban Areas Considering Multiple Physical Processes

Junbeom JO1+, Sooyoul KIM2#, Nobuhito MORI1, Gozo TSUJIMOTO2, Hajime MASE1
1Kyoto University, Japan, 2Kumamoto University, Japan


Major typhoons/hurricanes accompany extreme rainfall when approaching the shoreline. As a result, a compound flood by multiple physical processes of surge overflow, wave overtopping/runup, rainfall, backwater flow from a sewer system and river overflow occurs in coastal urban areas. Recent studies on inundation are limited to the individual effect of either a coastal and an inland physical process. The present study proposes a coupled flood model that simultaneously calculates inundations induced by compound factors of surge overflow, wave overtopping/runup, backflow and rainfall. This model consists of three models: a coupled model of Surge, WAve and Tide (SuWAT), which calculates the surge and wave; a Integrated Formula of wave Overtopping and Runup Model (IFORM), which calculates the rate of wave overtopping and wave runup; and Storm Water Management Model (SWMM) for estimating rainfall runoff at the surface and reverse flow at the sewer. The study quantitatively evaluates the effect of the multiple causes on the inundation in a simplified coastal urban area by a series of numerical experiments: the installation location of outfall toward the coast (+1, 0, -1 m on mean sea surface level); the timing of rainfall peak and typhoon's landfall. In all conditions, we consider either the presence or absence of a flap gate of the outfall that prevents backflow. This model is validated by analyzing results of various combinations of the multiple factors, the construction conditions of drainage systems, and the timing between typhoon and rainfall. The result demonstrates that the developed model can successfully simulate and predict the inundation induced by the surge overflow, the wave overtopping/runup, the backflow, and the rainfall in the coastal urban area.

OS02-A022
Storm Surges Interacting with Wind Waves from Deep Waters to Shallow Waters

Yu-Lin TSAI1#+, Tso-Ren WU2, Philip Li-Fan LIU3
1Kyoto University, Japan, 2National Central University, Taiwan, 3National University of Singapore, Singapore


This study aims to understand storm surges interacting with wind waves where water depths change from thousand meters to a few meters. By developing a nested-grid surge-wave coupling system between a depth-integrated shallow water equation model and the wave spectral SWAN (Simulating WAves Nearshore) model, storm surges and wind waves are solved from an open ocean to nearshore regions. During the coupling procedure, water levels and depth-averaged currents provided by the shallow water equation model and wave-induced radiation stresses from SWAN are updated in between. The tidal effect is involved in both storm surge and wind wave modeling. Two severe typhoons making landfall on the east coast of Taiwan are chosen as the case studies. Integrated observation data from tide gauges (observed water-levels) and buoys (sea-level pressures, wind speed, wind directions, wave directional spectrums, significant wave heights, and mean wave periods) are used to validate input meteorological fields and model results for storm surges and wind waves. The wave-enhanced radiation stresses along the east coast of Taiwan are found to amplify the storm surge heights locally and the subsequent quantitative analysis from wind waves to storm surges is ongoing. By presenting as complete as possible observations and model results, these two typhoon events with respect to winds, sea-level pressures, storm surges, and wind waves are expected to serve as future benchmark problems for the surge-wave coupling topic in East Asia.

OS02-A003
Extreme Wave Analysis in High-order Weakly Nonlinear Wave Evolution from Deep to Shallow Water

Zuorui LYU1#+, Nobuhito MORI1, Hiroaki KASHIMA2
1Kyoto University, Japan, 2Port and Airport Research Institute, Japan


In deep-water, quasi-resonant four-wave interaction gives rise to the occurrence probability of extreme wave height, which is considered to be an important reason resulting in freak wave as a natural disaster. As water waves enter shallow water from deep-water, four-wave interaction becomes weak and wave shoaling contributes to wave propagating process, and it makes evolution of nonlinearity in wave trains become complicated. This study conducts a Monte Carlo simulation in modified Nonlinear Schrödinger equation considering bottom topography change, giving spatial evolution of high-order nonlinearity in wave trains from statistical moment. In groups of realization of wave surface elevation, maximum wave height and crest height of wave trains are extracted to give expected extreme value and distribution function. The result indicates that, slope angle plays an important role in a shallow water depth, and steep slope brings about significant enhancement of extreme events. 

OS02-A009
Effects of Land Cover Change on Atmospheric and Storm Surge Modeling During Typhoon Event

Kai YIN, Sudong XU#+, Ke YANG
Southeast University, China


Understanding the role of land cover changes on climate and hydrology during typhoon events is vital for coastal hazard mitigation, prevention, as well as the regulation of land cover. To investigate the effects of land cover changes, simulation studies using the WRF model to calculate the meteorological field combined with the Delft3D-FLOW model capable of accurately simulating storm surges were conducted. The accuracy of this modeling approach is guaranteed since the models were validated against available in-situ and satellite measurements during typhoon Rammasun (2014) period. Through comparing simulation results obtained with the MODIS 2001 and the GlobCover 2009 land cover data scenarios, it is found that land cover changes in the study region exert an apparent influence on the atmospheric and storm surge simulations. Notable disagreements exist between the selected two land cover data. Different types of land cover have disparate physical properties, such as roughness length that alters the typhoon-induced wind field simulation results, thus resulting in changes in storm surge simulation results. Determining the impact of land cover changes on atmosphere and storm surge provides clearer insight into the land cover impacts, which is of great benefit to the atmospheric and storm surge models improvement.

OS02-A004
Modeling of Wind-wave Growth Based on Boussinesq Wave Model

Shoko SATO#+, Nobuhito MORI, Tomoya SHIMURA, Takuya MIYASHITA
Kyoto University, Japan


It is becoming important to consider the growth of wind waves due to strong typhoon winds in the bay. However, wave growth in the bay is an intermediate phenomenon hardly modeled by either spectral wave model or phase resolving wave model. Therefore, it is necessary to introduce wind stress terms in the phase-resolving wave model for inner bays. In this study, we focus on the Boussinesq wave model, and consider a wave growth term by surface winds considering wave scale momentum exchange between wind and wave interactions. A spectral wave model was used to optimize the parameterization of wave growth in the Boussinesq wave model. The obtained model was then applied to the case of Kansai Airport inundation by the extreme waves of Typhoon Jebi in 2018 to estimate the overtopping volume comparing with the measured values.

OS02-A008
Modelling Storm Surge Induced Flooding in Hong Kong with Rising Sea Level Scenarios

Jie YANG#+, Meixiang CHEN
Hohai University, China


The mage-cities located in the Pearl River Estuary, including Hong Kong, Guangzhou, Macau etc., suffered from frequent flooding due to typhoons. Nevertheless, the storm-induced flooding was believed to be exacerbated due to sea level rise and intensifying strong storms by global warming. In the present study, the potential flooding risk with an increasing sea level was estimated in the case of Typhoon Mangkhut in 2018, which has caused the highest storm tide in Hong Kong since Typhoon Wanda in 1962. The future relative sea level rise (RSLR) by the end of the century is projected under the high greenhouse gas emission scenario of representative concentration pathways (RCP8.5) from the CMIP5 model resembles and is adjusted for the influence of the Glacial Isostatic Adjustment (GIA) using the ICE-6G/VM5a model. By coupling with the SCHISM–WWMIII modelling system, the corresponding oceanic surge–tide–wave processes and inland inundation processes in Hong Kong are resolved. We examine the impacts of tidal timing and typhoon track location on inundation extent, we also present the spatial distribution of nonlinear responses of storm surge influenced by RSLR, implicating the regions of exacerbation and attenuation, respectively. The present study helps identifying more vulnerable regions and providing possible upper bounds of inundation in the future.