- 18th December 2023, Nguyen Thi Thanh Hue, PhD student at LEGOS and USTH
- 12th May 2023, Le Hong Hanh, PhD student at LEGOS and USTH
- 10th November 2022, Tran Manh Cuong, PhD student at LOG
- 11th May 2022, Nguyen Duy Tung, PhD student at LEGOS and USTH
- 24th February 2022, To Duy Thai, PhD student at LEGOS, Researcher at IO
- 9th December 2020, Trinh Bich Ngoc, PhD student at LEGOS, Researcher at USTH
- 7th October 2020, Nguyen Thi Tuyet, PhD student at IMHEN
- 24th June 2020, Pham Quoc Viet, PhD student at LEGOS
- 16th June 2020, Tran Trung Kien, PhD student at LOG
- 16th December 2019, Violaine Piton, PhD student at LEGOS
- 2nd December 2019, Wei Xi, Phd Student at ECOLAB
- 05th October 2018, Vu Duy Vinh, Phd Student at USTH, Researcher at IMER
- 18th May 2018, Nguyen Dac Da, Phd Student at LEGOS
18th December 2023, LEGOS, Toulouse, France - Nguyen Thanh Hue, LEGOS PhD student
Title: Air-sea interactions in Southeast Asia: Evaluating the impact of ENSO/ENSO Modoki on rainfall variability and characterizing historical and future surface air-sea heat exchanges
Summary: Southeast Asia (SEA) gathers 10% of the world's population and is subject to a wide range of climate factors and hazards: typhoons, monsoons, El Niño Southern Oscillation (ENSO), climate change… At the interface between the Indian Ocean, the Pacific Ocean and the atmosphere, the SEA region, which includes the maritime continent, is moreover key to the functioning of global oceanic and atmospheric circulation. The general objective of this thesis is to better understand the functioning and impact of air-sea interactions in the SEA climate. This is of primary importance for in-depth knowledge and a better prediction capacity of climate variability at all scales in the region, from extreme events and interannual variability to future projections, but also to better understand, model and forecast global climate. We focused on two processes that play an important role in SEA climate: El Niño Southern Oscillation (ENSO) and air-sea heat exchanges. First, the impact on SEA rainfall variability of ENSO and its variant, ENSO Modoki, were investigated for the period 1979-2019. The observed decrease (increase) in rainfall over SEA during Modoki events compared to the canonical ENSO events was explained by a reduced (enhanced) moisture transport into the region and a weakening (strengthening) of the ascending branch of the Walker circulation. Second, we analyzed available observational and numerical datasets and conducted sensitivity simulations to explore and assess the range of estimates of air-sea heat fluxes in the SEA region. This revealed a huge uncertainty in estimates from various datasets, with values of net heat flux varying from approximately -30 to +40 W.m-2. The SYMPHONIE numerical model was used with two methods of surface heat flux forcing (bulk formulae vs. prescribed fluxes from atmospheric datasets) to investigate the sensitivity of the model's sea surface temperature to those fluxes. Results provided a +12.5 W.m-2 estimate of net heat gain for the ocean over 2009-2018, and suggested that ERA5, the fifth generation of the European Centre for Medium-Range Weather Forecasts reanalysis, can be used as a reference though a slight overestimation of net heat flux. Last, air-sea fluxes produced by 30 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations were evaluated against ERA5. Over the historical period, the CMIP6 ensemble average reproduces well the spatial variability of heat fluxes but underestimates the net heat gain for the ocean by two-thirds compared to ERA5. The primary contributors to the net bias are shortwave radiation (SW) and latent heat flux (LH). The net heat gain is projected to increase during the XXIst century, resulting from an increase of SW gain and LH loss and a decrease of longwave radiation (LW) and sensible heat (SH) losses. Models with higher projected sea surface warming exhibit larger changes in heat fluxes. Heat fluxes are predicted to change the most under the SSP5-8.5 scenario (+3.7, +1.0, -8.4, +9.2, and +1.9 W.m-2, respectively for Qnet, SW, LH, LW, and SH), followed by the SSP2-4.5 scenario, and finally the SSP1-2.6 scenario.
Committee:
12th May2023, LEGOS, Toulouse, France - Le Hong Hanh, LEGOS PhD student
Title: Teleconnection and dynamical influences on the intraseasonal rainfall variability over Vietnam
Summary: The intraseasonal variability of Vietnam rainfall has a large amplitude and considerable complexity, with much variation between the subregions. The objective of this study is first to assess the contribution of moisture budget terms to regional events for positive and negative rainfall anomalies; then to analyse these opposite regional events, Wet and Dry, to identify large-scale dynamical precursors and their pathways of influence. Particular attention is paid to nonlinearity or asymmetry of the influence and to the sensitivity to the choice of region, threshold and composited quantity. A combination of observation and modelling approaches is used at large and regional scales.
A reanalysis dataset (ERAi, 1979-2016) is used to construct wet and dry composites over Vietnam using the vertically integrated moisture flux convergence (VIMC) as a proxy for rainfall. At regional scales VIMC correlates well with reanalysed rainfall. The large-scale dynamics associated with opposing North and South events show asymmetrical large-scale precursors and different pathways of influence. The exact nature of the precursors is sensitive to the definition of the composite index. Two extratropical pathways and one tropical pathway emerge that are distinct for Wet and Dry events and North and South regions.
The global modelling study uses the global model DREAM in stationary wave configuration. Basic states derived from summer ERAi reanalysis have been used to further investigate the pathways. Two target regions, North and South Vietnam, are investigated from a set of heating experiments to find the initial perturbation location that gives the most influence. A nudging technique is then applied in that region to simulate realistic precursors. We found that for North Vietnam at a range of 2 weeks, there is only extratropical influence. Tropical sources influence both North and South in a limit of 9 days. The model response to 15-day lag European precursors shows two distinct pathways consistent with observations, especially for Wet events. Although the jet pathway is more reproducible on the climatological basic state, for different basic states the model response is dependent on the conjunction between precursors and the basic state jet including both Atlantic and Asian jets.
Analysis of Rossby wave ray-tracing on different states is also used to reveal high-latitude pathways that are consistent with observed composites. High-latitude pathways emanate from the Europe-Eastern Asia region and arrive over Vietnam in 1-2 weeks. The positions of Rossby wave sources over Europe-Eastern Asia are dependent on the basic state. Another preferred location for Rossby wave propagation to Vietnam is Mongolia. Wavenumbers K=1,3 take only 1-2 days to arrive over Vietnam, and this is not sensitive to the basic state. This direct source study indicates the way anomalies might grow along the jet and then directly influence Vietnam via Rossby wave propagation.
A downscaling study is then undertaken using the regional model RegCM4 over the Southeast Asia domain, forced by ERAi dataset. The composite of RegCM output is assembled by the regional index from the observational analysis. Extratropical forcing leads to large-scale rainfall anomaly patterns over the whole domain for all events. NVN events and CVN wet events were successfully reproduced, but local rainfall over SVN and CVN (dry) was not. The reproduction of moisture budget terms by RegCM shows a consistent simulation of TIMC for all events and large-scale patterns of rainfall that correspond to the VIMC anomaly, but there is a reversed sign for Evaporation. A lack of signal over the South China Sea suggests that the regional model has difficulty propagating some tropical signals, and that large-scale dynamics, including extratropical and tropical influences, should be attributed to the wet and dry events over Vietnam subregions differently.
Committee:
10th November 2022, ULCO, Wimereux, France - Tran Manh Cuong, LOG PhD student
Title: Characterization of the coastal dynamics and turbulent dispersion in the Gulf of Tonkin from HF radar measurements and modeling: the effect of fine-scale dynamics on the spatial structuring of phytoplankton
Summary: The Gulf of Tonkin (GoT), located on the shelf in the northwestern part the Vietnam East Sea, / South China Sea (VNES/SCS), is very rich in natural resources. Despite its strategic location, the regional circulation and scales of its variability are only poorly understood preventing a progress in environmental studies. A limited knowledge of the circulation in the Gulf is tightly related to the lack of observations. To make up the gap, a network of High-Frequency (HF) radars was deployed in the region providing with an opportunity to monitor the coastal circulation continuously via the remote sensing of surface currents. The 2.5-year-long velocity time series, derived from radar measurements in the southern Gulf, allowed characterization of the flow variability and identification of key process governing this variability. It was shown that tidal motions account for a major part of this variability (60% of the total variance). The freshwater input strongly affects the seasonal variability of currents by modifying the polarization of tidal current ellipses and causing the coastal current intensification in late summer and autumn. The monsoon wind controls the large-scale dynamics and modulates the spatial extension of the coastal current. Transport patterns of passive tracers and turbulent dispersion regimes were investigated in the Lagrangian framework using the velocity fields from radar measurements and high-resolution SYMPHONIE model simulations. Lagrangian diagnostics (relative dispersion and Finite Size Lyapunov Exponents), revealed a significant change in dispersion rate and transport pathways caused by the monsoon wind and freshwater input. The effect of turbulence and horizontal steering on particulate material distribution was found particularly strong in four regions of the gulf. It was shown that fine-scale dynamics plays a major role in structuring the phytoplankton distribution there.
Committee:
11th May 2022, University of Toulouse, Toulouse, France - Nguyen Duy Tung, LEGOS/USTH PhD student
Title: Study of the Red River plume in the Gulf of Tonkin from cluster analysis and ensemble simulations
Summary: This study aims to better understand the variability of the Red River plume in the Gulf of Tonkin (GOT) in the area near the mouth and further offshore, using numerical modeling. Understanding plume variability and the fate of delta waters is of paramount importance for a better understanding and better predictive ability of ocean circulation and hydrology in the GOT, as well as for better management of coastal waters. and monitoring of coastal ecosystems.
In the first part of the thesis, a configuration of the SYMPHONIE model is implemented with realistic forcings and a variable high-resolution grid, based on the configuration of V. Piton (2019). A simulation over a period of 6 years (2011-2016) is carried out to study the daily to interannual variability of the plume of the Red River and of three rivers whose mouth is nearby. The simulation is then compared to several sources of observations. Then, the plume is identified using passive tracers injected into the simulation. Using an unsupervised machine learning algorithm (K-means), the main plume regimes and their evolution over time are classified and analyzed according to four clusters and then related to different environmental conditions. In winter, the plume is narrow and mostly stays along the coast due to the coastal current and northeast wind. In early summer, the southwest monsoon wind blows the plume offshore. The plume reaches its greatest coverage in September, after the peak in flow. On the vertical, the thickness of the plume also shows seasonal variations. In winter, the plume is mixed throughout the water column, while in summer, the plume can be detached both from the bottom and from the coast. The plume may deepen offshore in summer, due to strong winds (in May, June) or specifically due to a recurring eddy occurring near 19°N (in August). This first part was the subject of a publication in 2021.
The cluster analysis above shows that, whatever the cluster, the plume is strongly affected by the wind. Therefore, in the second part of this thesis, I use a set of simulations to evaluate the response of the model to disturbances added to the forcing wind. The sensitivity of the simulation presented in the first part is statistically evaluated by calculating the dispersion and the distribution of the variables of interest from a set of 50 members. Due to computing and memory constraints, this study is carried out over a short period, from June to August 2015, corresponding to the high flow season. First, the error on the forcing wind is estimated by comparison with a satellite product. Then, its impact on the model is evaluated for surface and subsurface variables. For surface temperature and salinity, the uncertainty is higher near the Vietnamese coast and the Red River Delta. On the vertical, the uncertainty is greatest at the surface for salinity and at the sub-surface for temperature. I then analyze the sensitivity of the river plume. The dispersion of the plume surface is greatest in August, which is also the period when the plume surface is greatest. The cluster analysis shows some cluster changes between different members of the ensemble, but the cluster most likely to occur is always that of the reference simulation (with the wind undisturbed). These limited changes suggest that the Part I results are indeed robust to wind forcing errors. Finally, the set is verified using the available sets of observations.
Committee:
24th February 2022, University of Toulouse, Toulouse, France - To Duy Thai, LEGOSPhD student, IO researcher
Title: Interannual and intraseasonal variability of the South Vietnam upwelling: the role of high-frequency atmospheric forcing, offshore and coastal ocean dynamics, and intrinsic oceanic variability
Summary: The South Vietnam Upwelling (SVU) develops off the Vietnamese coast (South China Sea, SCS) under the influence of southwest summer monsoon winds. A high resolution configuration (1 km at the coast) of the SYMPHONIE model was developed to study the functioning, variability and influence of the SVU. A simulation was first performed over the period 2009-2018. The realism of the simulation in terms of representation of ocean dynamics and water masses, from daily to interannual, and coastal to regional scales, was assessed in detail by comparison with available satellite data and four sets of in-situ observations. The interannual variability of the SVU is examined over its main areas of development: the southern (BoxSC) and northern (BoxNC) coasts, and the offshore area (BoxOF). For BoxSC and BoxOF, our results confirm the driving role of the summer regional mean wind and induced circulation. They moreover reveal that the spatial and temporal organization of mesoscale ocean structures and high frequency atmospheric forcing modulate this interannual variability. For BoxNC, the upwelling interannual variability is mainly determined by coastal circulation and mesoscale structures: similar summer wind conditions can be associated with very contrasting upwelling intensities, and vice versa, depending on the circulation in the BoxNC area. We then perform an ensemble of 10 twin simulations with perturbed initial conditions to examine the mechanisms involved in the daily to intraseasonal variability of upwelling. This ensemble reveals the role of the daily to intraseasonal chronology of wind forcing, but also the strong influence of Ocean Intrinsic Variability (OIV), related to the influence of coastal and mesoscale circulation.
Committee:
9th December 2020, University of Toulouse, Toulouse, France - Trinh Bich Ngoc, LEGOS PhD student, Researcher at USTH
Title: Water, heat and salt cycles in the South China Sea, from seasonal variation to interannual variability: a high-resolution closed balance modelling approach
Summary: The South China Sea (SCS) ocean dynamics play an important role at the local scale for the regional climate system, but also in global ocean circulation and climate. Surface waters of the global thermohaline circulation indeed transit from the Pacific to the Indian
Oceans across the SCS through several interocean straits (the South China Sea Throughflow, SCSTF), and are significantly modified during this transit. Ocean dynamics moreover influences the SCS marine life through its role in the transport and mixing of the pelagic planktonic ecosystems’ components. The general objective of this thesis is to contribute to the understanding of the SCS ocean dynamics and of their interactions with the other compartments of the regional system, by focusing on the functioning and variability of the SCS water, heat and salt budgets with the perspective to study their impact on the pelagic planktonic ecosystems. For that, a high resolution (4 km) configuration of a regional physical-biogeochemical ocean model covering the SCS with rigorously closed budgets is developed and used to perform and analyze simulations over the recent period 2009 – 2018.
We first show by comparison with available satellite data and in-situ observations the ability of our physical simulation to reproduce the surface water masses and circulation characteristics as well as thermohaline vertical distribution, at the climatological, seasonal and interannual scales.
We then examine the climatological average and seasonal cycle of all components involved in the water volume, heat and salt budgets over the SCS: internal variations and lateral, atmospheric and river fluxes. Water and salt inputs to the SCS are mostly related to the lateral inflow of Pacific water through the Luzon strait. About ½ of those inputs is released through the Mindoro strait to the Sulu sea, ¼ through the Taiwan strait to the East China Sea and ¼ through the Karimata strait to the Java Sea. Heat gain mostly comes for the Luzon lateral input (~ ¾) and from the atmosphere (~¼), and is equivalently released through the Mindoro, Taiwan and Karimata straits. Over the studied period, the
SCS stores respectively 0.3% and 2.5% of the total salt and heat inputs. The seasonal cycle of water and salt budgets is mainly driven by the net lateral water flux through interocean straits, whereas the seasonal cycle of heat budget is mainly governed by the atmospheric heat flux.
On the interannual time scale, water, heat and salt fluxes at Luzon and Mindoro straits are highly correlated together and show the strongest variability of all straits transports, and high correlations with ENSO (El Niño Southern Oscillation) and PDO (Pacific Decadal Oscillation). The annual atmospheric water flux governs the interannual variability of SCS water budget: a variation of annual atmospheric freshwater input induces a mirror variation of lateral outflow so that the SCS volume hardly varies at the interannual scale. The SCS salt budget is regulated at the first order by the interannual variability of net lateral water flux, then by the salinity of the in/outflowing interocean waters. In particular, we show that the recent SCS saltening was mainly induced by the increase in the inflow of salty Pacific water which compensated a deficit of rainfall freshwater over the area. The heat budget interannual variability is driven first by the total lateral heat flux, itself driven by the variability of the temperature of the out/inflowing waters, then by the variability of the lateral water flux and surface heat flux. Water and salt budgets as well as the SCSTF are strongly affected by ENSO and PDO, whereas the heat budget is only affected by ENSO.
Committee:
7th October 2020, Institute of Meteorology, Hydrology and Climate Change, Vietnam - Nguyen Thi Tuyet, IMHEN PhD student,
Title: A study on climate change projection and climate analog in Southeast Asia
Summary: In the thesis, the 2-m temperature and rainfall variables were evaluated and projected over Southeast Asia (SEA) and Viet Nam. Climate analog, disappearing and novel climate analysis over SEA and Viet Nam were also implemented. The RegCM4.3 model was used to downscale six CMIP5 Global Climate Models (GCMs) under the framework of the SEACLID/CORDEX-SEA project. Results showed that: i) regional downscaling allowed a more accurate representation of temperature but displayed a higher variability of rainfall over SEA compared to the results of the GCMs; ii) The ensemble product (ENS) had advantages in reproducing temperature and rainfall variations compared to the individual GCM and RCM experiments in SEA and Viet Nam; iii) A modified version of the existing formulation to estimate climate distance was introduced with weighting factors for temperature and precipitation, and for the ensemble; iv) A common tendency of climatic relocation for the six big cities in SEA including Ha Noi, Manila, Kuala Lumpur, Bangkok, Jakarta and Hinthada towards warmer regions is prominent with the regional ENS experiment; v) The percentages of novel climate areas in SEA at the end of the 21st century were projected to be 24% (RCM ENS) and 21% (GCM ENS) under the RCP8.5; vi) Novel climate are mainly located in coastal areas and islands, especially near equatorial areas and disappearing climate are found in mountainous areas; vii) In Viet Nam, the projection results of the present study were also compared to those in the previous study with a high agreement in the temperature changes but a remarkable uncertainty in rainfall trend; viii) 2.39% of Viet Nam land, mainly located in the Northern and Southern Central Highlands, was projected to experience disappearing climate by the ENS experiment under the RCP8.5.
The results of the thesis would provide worthwhile inputs for climate change impact assessment, adaptation and mitigation research. The results of novel climate and disappearing climate in Southeast Asia and Viet Nam could be linked to various sectors such as agriculture, infrastructure, urban, health, immigration, etc. to help people better adapt to and mitigate climate change.
Committee:
24th June 2020, University of Toulouse, Toulouse, France - Pham Quoc Viet, PhD student at LEGOS
Title:Tracing the lithogenic footprint in Coral and Solomon Seas: contribution of rare elements and the isotopic composition of neodymium
Summary: The aim of this thesis was to better constrain the land-ocean fluxes using geochemical tracers, Rare Earth Element (REE) and Neodymium Isotopic Composition (Nd-IC). The studied area was the Coral and Solomon Seas, a part of the Southwest Pacific Ocean (SPO) where the waters feeding the Equatorial Pacific Ocean are circulating which confers an important role in the climate regulation to this area. The SPO is also a place where important hydrological and chemical modifications of the water masses occur.
A new dREE analytical protocol was developed to replace the one used for years in my research team. This “seaSLOW” manifold allows the simultaneous treatment of 8 seawater samples, dramatically reducing the analytical time, with a low matrix blank signal - less than 0.5% for dREE signal. Affordable detection limits (equal ~1/40 of the average seawater concentration) and high quantitative recoveries (>90%) were obtained during the blank and performance tests, respectively. The strong points of seaSLOW are that it is low-cost (equal one fifteenth of the commercial product seaFAST) and easy to fabricate, making it accessible for other users, including developing countries.
Vertical profiles of dissolved REE (dREE) concentrations for 143 samples collected in the Coral and Solomon Seas as part of the PANDORA cruise in summer 2012 were analyzed. dREE concentration –except the insoluble Ce– indicates the nutrient like profiles, i.e. low values at the surface increasing at depth (e.g. Nd concentrations range from ~5 pmol/kg at the surface to ~ 25 pmol/kg in the deepest layers). Local dREE enrichments are observed along the coast and in the three northern exiting straits of the Solomon Sea (a.k.a: Indispensable, Solomon and Vitiaz Straits). dREE variations in the different water layers were quantified using box models, pointing out a net enrichment of 145 ± 46 tNd/year in the lower thermocline. The basaltic material imprint was brought to light by positive Eu and Ce anomalies, especially in surface layer. The better correlation between heavy REE (HREE) and Si cycle than that observed for the LREE in the oceanic water was also confirmed. These results are published in Chemical Geology (Pham et al, 2019)
Dissolved Nd-IC were also measured on the same samples. The distributions of both Nd parameters (Nd concentration and Nd-IC) between the the Coral Sea, the entrance and the exit of the Solomon Sea were established as vertical profiles and as maps representing the layers transporting the different water bodies. All the vertical profiles show similar patterns with more radiogenic Nd-IC in the surface layer (up to 𝜀𝑁𝑑 = +3), minimal ε_Nd values at Intermediate depths (~1100m) before increasing again toward the bottom. On a regional scale, higher Nd-IC signals are observed at the northern exits than at the southern entrance of the Solomon Sea, likely reflecting the release of radiogenic material to the water masses during their pathways across the Solomon Sea. The occurrence of boundary exchange (BE) processes were suggested at many place within the sea, either at a local scale (e.g: surface water passing through Indispensable strait) or at the regional scale (e.g: deep water across the Solomon Sea). Quantification of BE via box models was conducted for the lower part of the Lower thermocline (LLTW) and the upper layer of the deep water layers (UCDW). As an illustration, the external input of Nd required to balance both the Nd-IC and Nd concentrations of the upper deep layer (UCDW) between the southern entrance and the northern exits is of 105±50 TNd/y. Applying Nd both parameters helped to better identify the sources of material and to reduce the uncertainties affecting the processes. Future studies of these parameter distributions on the suspended particles collected at the same stations will help to get a full picture of the processes governing the dissolved-particulate exchanges.
Committee:
16th June 2020, ULCO, Wimereux, France - Tran Trung Kien, PhD student at LOG
Title: Observations of particulate organic carbon and particulate assemblages from remote sensing in contrasted coastal waters
Summary: The inversion of data from water color allows the estimation of a variety of biogeochemical parameters from space. Past studies have demonstrated, for example, the possibility of estimating the particulate organic carbon (POC) concentration in the open ocean with satisfactory accuracy. Estimating POC in coastal waters is a more difficult task than in the open ocean because of the complexity of contrasting environments characterized by high particulate and dissolved matter content from a variety of sources (terrestrial and oceanic). The first objective of this thesis was to develop an algorithm to estimate the concentration of POC in coastal waters. A new approach was thus developed; it is based on the maximum reflectance ratio in two spectral bands. The performance of the new algorithm is superior to existing methods previously documented. The applicability of the new method has been illustrated at the regional scale on the Louisiana coast. The POC concentrations estimated with the new algorithm, using MERIS data, are consistent with those previously estimated using a regional algorithm (Le et al., 2016). The second objective of this work was to examine the classification of coastal waters through two approaches. Firstly, a classification based on the shape of the hyperspectral reflectance spectrum was proposed from a complete in situ data set. Six water classes were defined, providing a finer representation of the optical properties of coastal waters than had been established in previous studies using the same method (e.g. 4 classes by Vantrepotte et al., 2012). These 6 classes can be associated with a dilution gradient from turbid to clear water. Secondly, another classification of coastal waters based on the ratio of POC concentration and suspended particulate matter, as proposed by Wozniak et al. 2010, was re-examined. In practice, new thresholds have been established on the POC/SPM ratio to provide a more accurate partitioning of the coastal ocean in terms of 3 simple categories. These three classes are mineral-dominated waters, phytoplankton-dominated waters and mixed waters. Both classification methods have been applied to the MERIS data for the Channel/North Sea. The spatio-temporal dynamics of the water types are consistent with the observed seasonal changes.
Committee:
16th December 2019, University of Toulouse, Toulouse, France - Violaine Piton, PhD student at LEGOS
Title: From the Red River to the Gulf of Tonkin : hydrodynamics and sediment transport along the estuary-coastal area continuum
Summary: Deltas and coastal regions deliver the largest inputs of freshwater and sediments to the shelf and open ocean, understanding water and sediment dynamics and variability in those regions is therefore crucial. The spatio-temporal variability of estuarine and ocean dynamics under the influence of natural forcings and their impact on sediment transport and fate was assessed along the Red River estuary - coastal ocean - Gulf of Tonkin continuum. First, in-situ estuarine observations evidenced the seasonal and tidal variabilities of flow and suspended matter, and showed in particular the role of tidal pumping in the estuary siltation. Second, a 3D realistic hydrodynamic model was set up and calibrated with various observations and satellite data. Beforehand, a high-resolution model configuration was implemented and optimized with sensitivity tests of the Gulf of Tonkin’s tidal components to bathymetry and various bottomfriction parameterizations. Third, the resulting optimized configuration was used to study the large scale Gulf of Tonkin circulation at daily, seasonal and interannual scales, and to identify the drivers of their variabilities. Ekman transport variability due tomonsoon winds reversal drives the seasonal circulation, which can be reversed in summer by episodic typhoon events and intensified in winter. ENSO, strong typhoon activity and Arctic Oscillation have been identified as drivers of the interannual circulation variability. Lastly, preliminary tests with a sediment transport module coupled with the hydrodynamics model revealed the importance of the seabed composition and of the parameterization of the erosion coefficients.
Committee:
2nd December 2019, University of Toulouse, Toulouse, France - Wei Xi, Phd Student at ECOLAB
Title: A modelling approach to diagnose the impacts of global changes on hydrology, suspended sediment and organic carbon in an Asian tropical basin: the case of the Red River (China and Vietnam)
Summary: The Asian river basins are great contributors to sediments and organic carbon to the seas. However, these river basins are subject to the influence of climate variability and human activities, which alters the transport and fate of water and associated matter in rivers, and then modifies the coastal biochemical processes. The Red River is a representative Asian river basin and plays an important role in the economy and agriculture in China and Vietnam. However, lack of data sharing between countries and difficulty in in-situ observations and samplings, make the study through the whole basin difficult both spatially and temporally. In order to overcome these issues and better understand the water resources and matters transfer dynamics, interactive use of in-situ measurements, remote sensing observations and
numerical modellings are necessary.
This work proposed a modelling approach to simulate the transfer dynamics of water, suspended sediment (SS) and organic carbon at a daily scale in the Red River, and to understand and quantify their responses to the impacts of climate variability and dam constructions. The physical-based SWAT model, combining the remote sensing data, was used in this study to simulate the water regime and suspended sediment. Six dams (two were operated before the study period and the other four started operation since 2008) were implemented in this model. The model was calibrated based on observed discharge (Q) and suspended sediment concentration (SSC) data from 2000 to 2013 at five gauge stations (the outlets of the main tributaries and of the continent basin) at a daily time step. After Q and SSC calibrated under actual conditions, a scenario of natural conditions (without any dams inside the basin) was modelled to disentangle and quantify the impacts of climate variability and dams on Q and sediment fluxes (SF). Dissolved and particulate organic carbon (DOC, POC) were calibrated based on observed Q, SSC and in-situ organic carbon sampling data. According to these relationships, the organic carbon concentrations and fluxes under actual and natural conditions are calculated, in order to further quantify the impacts of climate variability and dams on DOC and POC
transfer. This study highlighted the strong impacts of dams on sediment fluxes (-80%) and organic carbon (POC, -85%; DOC, -13%), and the impacts of climate variability on Q (-9%). Without dams, the Red River basin would have a high specific sediment yield (779 t km-2 yr-1) compared to other Asian river basins, though its sediment export was low compared to them. The high soil erosion due to precipitation, slope and agricultural practice in the middle part of the basin is the main factor contributing to the specific sediment yield. The specific yields of DOC (1.62 t km2 yr-1) and POC (2.96 t km2 yr-1) of the Red River basin were more than twice those of other Asian basins. Soil organic carbon content and high soil erosion and leaching were the main influencing factors. The percentage of POC in total organic carbon (TOC) decreased from 86% to 74% until 2007 then to 47% with new dams. Dam constructions altered the TOC yield and POC/TOC ratio. Furthermore, simple rating curves between monthly mean Q and SF were established in this study for estimating SF at the outlet of the tributaries and the Red River, which enables stakeholders to estimate the monthly SF without using the SWAT model. Future studies on other nutrients and contaminants transfer and global changes can be carried on based on this modelling.
Committee:
05th October 2018, USTH, Hanoi, Vietnam - Vu Duy Vinh, Researcher IMER
Title: Suspended sediment dynamics in Red River distributaries and along the Red River delta: Focus on estuarine processes and recent balances
Summary:
Estuaries and adjacent coastal area are natural highly dynamic and rapidly changing systems, constrained by a complex combination of riverine and marine processes. Due to their favourable condition for waterway developments like harbors, ports and navigational channels, these regions are attractive places where socio- economics activities grow. However, estuaries and their adjacent coastal areas are sensitive places with influences of human activities as well as land-sea interation processes. Among them, sediment dynamics are attracting increasing attention by coastal researchers.
The Red River Delta (RRD) area, located in the western coast of the Gulf of Tonkin, is the second largest delta in Vietnam. RRD region has a very high population density, with about 994 inhabitants per km2 (2016), twice the one of the Mekong river Delta (443 inhabitants per km2). This region encompasses coastal industry zones and ports system, that have constitued the main gate to connect the North Vietnam to the outside world. RRD coastal area is a typical place to study sediment dynamics, with moderate diurnal tidal range (3-4m) as well as a high seasonal variation of fluvial flow. There, deposition in river mouths, siltation on the navigation channels, erosion along some parts of the coastal line, and impacts of modified fluvial fluxes to the coastal zones are occuring. During previous boat trips between Haiphong city and the bay, we observed that the suspended particulate concentration at the surface seemed to increase in the downstream portion of the estuary before decreasing further at sea, which foreshadowed the existence of an Estuarine Turbidity Maximum (ETM) zone in the distributaries of the Red River. The asymmetry of the tide during its propagation in the estuary, studied at three stations, reinforced this hypothesis. In this context, the general aim of this thesis is to improve our knowledge of the sediment processes and sediment fluxes involved, to document and analyze their dynamics, and to improve our capacity to monitor and simulate them, for scientific and management purposes. This study was organized around three main questions: 1) What are the present and recent water and sediment supplies of the Red River basin to the coastal zones? 2) How do the suspended particles transform in the estuary, and is it possible to document the characteristic parameters of Estuarine Turbidity Maxima (ETM) – if any – in the Cam-Nam Trieu estuary? and 3) Is it possible to estimate the longshore sediment fluxes along the RRD coastal area by numerical simulations?
Committee:
18th May 2018, LEGOS, Univ. Toulouse, France - Nguyen Dac Da, LEGOS (IRD ARTS fellowship)
Manuscript available here
Title : The interannual variability of the South Vietnam Upwelling : contributions of atmospheric, oceanic, hydrologic forcing and the ocean intrinsic variability
Summary :
The summer South Vietnam Upwelling (SVU) is a major component of the South China Sea circulation that also influences the ecosystems. The objectives of this thesis are first to quantitatively assess the interannual variability of the SVU in terms of intensity and spatial extent, second to quantify the respective contributions from different factors (atmospheric, river and oceanic forcings; ocean intrinsic variability OIV; El-Niño Southern Oscillation ENSO) to the SVU interannual variability, and third to identify and examine the underlying physical mechanisms. To fulfill these goals we use a set of sensitivity eddy-resolving simulations of the SCS circulation performed with the ROMS_AGRIF ocean regional model at 1/12° resolution for the period 1991-2004. The ability of the model to realistically represent the water masses and dynamics of the circulation in the SCS and SVU regions was first evaluated by comparison with available satellite and in-situ observations. We then defined a group of sea-surface-temperature upwelling indexes to quantify in detail the interannual variability of the SVU in terms of intensity, spatial distribution and duration. Our results reveal that strong SVU years are offshore-dominant with upwelling centers located in the area within 11-12°N and 110-112°E, whereas weak SVU years are coastal-dominant with upwelling centers located near the coast and over a larger latitude range (10-14°N). The first factor that triggers the strength and extent of the SVU is the summer wind curl associated with the summer monsoon. However, its effect is modulated by several factors including first the OIV, whose contribution reaches 50% of the total SVU variability, but also the river discharge and the remote ocean circulation. The coastal upwelling variability is strongly related to the variability of the eastward jet that develops from the coast. The offshore upwelling variability is impacted by the spatio-temporal interactions of the ocean cyclonic eddies with the wind stress curl, which are responsible for the impact of the OIV. The ocean and river forcing also modulate the SVU variability due to their contribution to the eddy field variability. ENSO has a strong influence on the SVU, mainly due to its direct influence on the summer wind. Those results regarding the interannual variability of the SVU are robust to the choice of the surface bias correction method used in the model. We finally present in Appendix-A2 preliminary results about the impacts of tides.
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