- 13th December 2024, Pauline MARTINOT, PhD student at MIO and USTH
- 11th December 2024, Juliette PENICAUD, PhD student at LEGOS and USTH
- 10th December 2024, Adrien GARINET, PhD student at LEGOS
- 7th May 2024, Quentin DESMET, PhD student at LEGOS and USTH
- 13th February 2024, Pham Thi Thanh Nga, PhD student at ENSIACET, Toulouse INP
- 18th December 2023, Nguyen Thi Thanh Hue, PhD student at LEGOS and USTH
- 30th June 2023, Tran Manh Duy, PhD student at LOG
- 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
- 24th April 2021, Dinh Ba Duy, PhD student at VNU HUS
- 9th December 2020, Trinh Bich Ngoc, PhD student at LEGOS, Researcher at USTH
- 7th October 2020, Nguyen Thi Tuyet, PhD student at IMHEN
- 17th July 2020, Do Minh Huy, PhD student at LCA
- 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
- 6th February 2018, Pham Duc Binh, Phd Student at LERMA
13th December 2024, MIO, Marseille, France - Pauline MARTINOT, MIO PhD student
Title:Distribution and reactivity of dissolved Black Carbon in coastal environments: Application to the Bay of Marseille (France) and the Red River Delta - Gulf of Tonkin (Vietnam).
Summary:
The aim of this thesis was to gain a better understanding of the distribution, fluxes and biological and chemical reactivity of dissolved black carbon (DBC) in relation to dissolved organic matter (DOM) in coastal environments. Field campaigns were carried out in the Bay of Marseille (1 campaign), the Van Uc estuary (2 campaigns) and the main branch of the Red River delta in Vietnam (3 campaigns). In the laboratory, the optical properties of DOM_BC solutions were compared with those of marine and terrestrial DOM. The biological and chemical reactivities of DOM_BC and DBC were assessed via a biodegradation experiment involving marine heterotrophic prokaryotes and fluorescence quenching experiments with polycyclic aromatic hydrocarbons (PAHs).
In the Bay of Marseille, the mean concentration of DBC was 15.0 ± 5.1 μg C L-¹, with high values near shipping lanes, reflecting the influence of maritime traffic. In the Van Uc estuary, the mean DBC concentration was 27.9 ± 12.7 μg C L-¹, and maximum in November, after the rainy season. Several local sources were identified, and DBC concentrations decreased from the river towards the estuary, reflecting the impact of phototransformation during transport to the ocean. In the Red River, DBC concentrations ranged from 29.0 ± 10.2 μg C L-¹ in March to 65.6 ± 12.4 μg C L-¹ in September. The decreasing DBC concentrations from Hanoi city towards the estuary in June were consistent with the majority of the river's input to the estuary. Conversely, the increasing concentrations of DBC towards the estuary in March and September appeared to be attributable to groundwater intrusion phenomena around Hanoi, diluting the DBC upstream in the dry season, and to the release of larger quantities of DBC washed out by the rains and transported in patches along the river in September. We have estimated that the Red River delta is responsible for around 0.14% of the global flow of DBC to the ocean.
The study of the optical properties of DOM_BC led to the proposal of a fluorescence index, COX, which is suitable for detecting combustion products in the marine environment. The biodegradation experiment showed a decrease in DBC of 27 to 45% in 3 months, depending on the origin of the BC, accompanied by a decrease in the DOM_BC content (DOC, PAHs, FDOM). The bioavailability of DBC was influenced by the origin and oxidation of the BC and the dynamics of the prokaryotic communities. The partition coefficients between DOM_BC and PAHs (phenanthrene and benzo[a]pyrene), measured by fluorescence quenching, were 18 and 265 × 10³ L kg C-¹, i.e. higher than those of natural DOM.
This thesis work confirmed DBC as a non-refractory component of coastal and oceanic DOM. Further molecular analyses, such as FT-ICR-MS, would clarify the sources and increase our knowledge of how BCD is modified by biotic and abiotic processes.
Committee:
11th December 2024, LEGOS, Toulouse, France - Juliette PENICAUD, LEGOS PhD student
Title: Hydrodynamics and sedimentary processes in a tropical estuary: the case of the Van Uc River, Red River delta, Vietnam. Joint influences of tide and flow at different temporal scales
Summary:
Estuaries, which are transition zones between rivers and oceans, are crucial for transporting large amounts of sediments and nutrients essential for coastal ecosystems. They are influenced by tidal forces and river flows. These external forces affect the hydrodynamics, and their complex interactions determine the sedimentary and saline structures, which evolve according to tidal cycles, seasonal changes, and at interannual scales. In the case of the Red River Delta in Vietnam, the estuarine dynamics are particularly intricate due to the influence of the monsoon regime, typhoons, and variations in river flow across seasons and years, making this system highly vulnerable to global changes.
This thesis focuses on the response of the Van Uc estuary, a branch of the delta, to various external forces and at different temporal scales, utilizing complementary methods of in situ observations and numerical modeling. Three measurement campaigns have been conducted and have contributed to the study of the evolution of salinity, flow speeds, and turbidity at high temporal and spatial frequency during spring tides and under a wide range of flow conditions. In parallel, a three-dimensional hydrodynamic modeling was performed using the SYMPHONIE model, specifically adapted to the Van Uc estuary. Additionally, the sediment transport model MUSTANG was used to simulate the transport of fine sediments, evaluating their response to various environmental forces. The Van Uc estuary is dominated by the inflow, and the amplitude of the tidal wave remains constant downstream of the estuary but is attenuated upstream. On a tidal scale, salinity progresses from the mouth to upstream with the tidal cycle, reaching a maximum at the beginning of the ebb tide. During neap tides, an estuarine circulation develops, accompanied by strong stratification up to the limit of saline intrusion. During spring tides, the mixing in the water column increases, reducing stratification and limiting estuarine circulation downstream. The flood limit (position where the reversal of currents produced by the tide vanishes) occurs further upstream in the surface layer than in the bottom layer. During neap tides, it is the opposite due to the established estuarine circulation. Spring tides and high river flows favor tidal asymmetry and attenuation of the tidal wave. During spring tides, the maximum turbidity, although relatively small (below 500 mg/l), was measured during the arrival of salinity. The initial sedimentary simulations were generally able to reproduce the variations between neap and spring tides. Saline intrusion is heavily influenced by river flow, with high flows pushing salinity back towards the mouth, while low flows favor its extension upstream. The measurement campaigns allowed to track the evolution of saline intrusion and stratification, showing a reduction in the length of saline intrusion (measured at 4 km upstream of the mouth) and the stratified region during high flows compared to lower flows (intrusion measured at 13 km upstream). The flood limit is also greatly influenced by river flow, being pushed further downstream during high flows. Finally, the estuary is situated in a region where river flows and tides vary greatly on an interannual scale, due to El Niño-Southern Oscillation (ENSO) and variation of the lunar nodal cycle (18.6 years), respectively. This indicates a significant interannual modulation of the balance between these forces. At a larger scale, impacts exacerbated by climate change and human activities (dams, dredging) are expected. This evolving dynamics places the Van Uc estuary at the center of contemporary environmental and scientific issues, combining physical, human, and ecological challenges.
Committee:
10th December 2024, LEGOS, Toulouse, France - Adrien GARINET, LEGOS PhD student
Title: Evidence and reduction of spurious numerical mixing in a numerical model including internal tides : a case study on the South-East Asian seas
Summary: Because they attempt to represent the complexity of continuous processes on discrete grids, computer simulations used to study the ocean have a fundamental tendency to generate numerical inaccuracies that must be controlled in order for the output to be realistic. Among these, spurious numerical mixing between layers of different densities is particularly troublesome. Indeed, physical mixing in this direction, though of major importance, is quite subtle, still poorly understood, and can easily be overwhelmed by numerical errors. In numerical models that use fixed levels to represent the vertical physics of the ocean, transport across these levels - and errors associated with their discretization - is the main source of such spurious cross-isopycnal mixing.
This work focuses first on showing that since these errors scale with velocities, explicitly resolving the strong vertical displacements generated by internal tides propagating in the interior of the ocean can lead to intense levels of such spurious mixing if care is not taken in how vertical advection is implemented. Using the Symphonie ocean model, and taking the simulation of the South-East Asian Seas as a realistic case study, we show that while internal tides are essential for a correct representation of the water masses in the region, numerical errors generated by using an overly diffusive vertical advection scheme can severely bias the output.
Investigating the choice of a better scheme for handling vertical transport, we then recognize that the reduction of numerical mixing cannot be properly achieved without taking into account the properties of the time-stepping algorithm. We thus propose a new way to design advection schemes aiming at counteracting its spurious effect. Overall, this work offers a deep look into the way vertical advection is handled in the fixed coordinate model and suggests ways to improve it.
Committee:
7th May 2024, LEGOS, Toulouse, France - Quentin DESMET, LEGOS PhD student
Title: Exploring the keys to advance air–sea coupled regional modeling for deeper insights into Southeast Asian climate
Summary: Southeast Asia (SEA) is a tropical region composed of a multitude of mountainous islands and seas of varying depths. Air–sea interactions occur at multiple spatio-temporal scales, e.g., in tropical cyclones or under the influence of large oscillatory phenomena such as the El Niño–Southern Oscillation. With the long-term vision of studying more precisely how these various atmospheric and oceanic features interact in the SEA climate system, we develop in this thesis an air–sea coupled regional climate model for the region, and provide innovative solutions to the fundamental problems surrounding such developments. The RegCM5 (atmosphere) and SYMPHONIE (ocean) models are coupled via the OASIS coupler. Initially, we strive to select the most suitable configuration of RegCM5 for our coupled system, among thirty-six combinations of schemes covering the following a spects: radiative transfer, planetary boundary layer, cumulus convection, resolved-scale microphysics, and cloud fraction. To address this classic yet underexplored problem in air–sea coupled models, we develop a reproducible strategy aimed at facilitating the execution of such efforts for the community at large. The procedure we follow consists of reducing the set of configurations considered, by applying three successive filters based on (1) statistical analysis of the relative performance of configurations compared to the ensemble in the framework of uncoupled simulations; (2) specific analysis of the best simulations found in (1); and (3) specific analysis of coupled simulations implementing the best configurations based on (2). Point (1) relies on the use of an objective ensemble ranking method developed for this thesis, based on a universal normalization function capable of assigning a score to any result from any evaluation metric. Our results indicate that no configuration of RegCM5 outperforms all others indisputably, with, e.g., the simulation of realistic surface radiative fluxes being rarely associated with a satisfactory representation of turbulent heat fluxes. Nevertheless, it appears that better capabilities to represent oceanic precipitation in 'uncoupled' mode are determinants for driving the most optimal coupled simulations. For this aspect and surface wind simulation, the choice of cumulus convection scheme absolutely dominates the performance obtained, with Tiedtke (Kain–Fritsch) outperforming (underperforming) the ensemble. Ultimately, we select RRTM, Holtslag, Tiedtke, Nogherotto–Thompkins, and Xu–Randall schemes (for the aspects of the model mentioned in the same order as above), which produce the best performance for precipitation in 'uncoupled' mode, and for surface temperature when we activate air–sea coupling. Finally, we develop a novel coupling algorithm, combining conservation, synchronicity, and reasonable computational cost. We show that near-synchronous coupling at high frequency is entirely affordable in the context of regional modeling, if one adapts to the time steps of the surface and radiation modules within the atmospheric component. Our approach improves the consistency between the two components of the model, which now see the same interfacial fluxes for the same durations. Preliminary results are presented regarding the assessment of its added value in realistic simulations.
Committee:
13th February 2024, LCA ENSIACET, Toulouse, France - Pham Thi Thanh Nga, ENSIACET PhD student
Title: Study of extraction and purification of bio-based polymers, and their use in copper removal
Summary:Over the past few decades, copper (Cu) has been widely used in various applications. In the absence of proper treatment prior to discharging industrial and domestic wastewater with high Cu concentration into the environment, it becomes very toxic on both aquatic ecosystems and human health. Lignocellulosic materials have been proved as a flexible, easily operational, and cost-effective approach for contaminant removal.
The objective of this work was to optimize the production of different lignocellulosic sorbents from sugarcane bagasse (SCB), and understand the Cu sorption mechanisms on these extracted materials. The extraction by introducing alkaline solution allowed to solubilize lignin and hemicellulose, and to remain cellulose in the residue. A purification step for the liquid fraction was also investigated.
The SCB fractionation was carried out following the Doehlert design, with three parameters: NaOH concentration (0.25–0.75 M), temperature (50–80 ºC), and time (1–3 h). Response surface methodology was used to assess the impact of parameters on extraction/recovery yield, extraction mechanisms, and to optimize the recovery yield. Comparison between extraction and recovery yield confirmed the degradation phenomenon during the extraction process. The optimal condition to produce sorbents (cellulose and acid-insoluble lignin) for copper removal, and high recovery yield of xylose in the extract was 0.5 M NaOH, at 65 ºC, for 3 h.
The extract was then purified by membrane filtration, with tested parameters: transmembrane pressure (0.5–3.0 bar), shear rates (2,831–22,696 s⁻1), temperature (20 and 40 °C), membrane molecular weight cut-off (5 and 10 kDa), and membrane material (PES and PS). The higher temperature corresponds to the higher permeate flux, and the impact of shear rate was more significant at high temperature. Although the 5 kDa PES membrane could slightly retain larger molecules, the 10 kDa one demonstrated a greater capacity for removing phenolic acids. For the 10 kDa PS membrane, the polarization layer started to form at a TMP lower than 0.5 bar. The PS membrane had lower retention rate for all molecules compared to the PES one.
In addition to the raw SCB, the residue after extraction, and to lignin precipitated from SCB extract, a commercial lignin was also used for Cu removal. The specific surface area of these sorbents was different by H2O and N2 BET sorption analysis, at 298 and 77 K respectively, due to different interactions of the target molecules with the material. The scanning electron microscopy (SEM) analysis showed the effect of temperature on material morphology. From pH-edge sorption experiments, it comes that pH 6 was the optimal one for all sorbents. The hydroxyl (–OH) group and certain functional groups found in lignin can form monodentate or bidentate complexes with Cu ions. SCB lignin gave the highest Cu sorption capacity at 2.2×10–2 mmol m–2, followed by commercial lignin, raw SCB, and residue, reference to specific surface water determined with H2O. Equilibrium sorption was achieved within 3 h for both raw SCB and its residue, 3 d for the commercial lignin and 30 min for SCB lignin. The good fit of kinetic study with the pseudo-second model suggested the occurrence of chemical reactions between Cu ions and sorbents. The Langmuir-type model was suitable for describing the Cu sorption behaviour for all sorbents, while the Freundlich-type model was good only for SCB lignin. The Cu presence on all sorbents after sorption was confirmed by SEM coupled to energy dispersive X-ray spectroscopy (EDS) techniques.
In conclusion, the study optimized the extraction of SCB, extract purification and fixation of Cu on lignocellulosic materials. We also showed some possible mechanisms occurring during these processes, and that lignocellulose could be considered a promising sorbent to remove metals, such as Cu, from aqueous medium
Committee:
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:
30th June 2023, LOG, Wimereux, France - Tran Manh Duy, LOG PhD student
Title: Monitoring of the quality of marine waters in French Guiana by remote sensing and in-situ measurements
Summary:French Guiana marine waters hold unique marine environments and habitats representing areas of high ecological, societal, and economical importance. These interface regions (land/Amazon-ocean) are vulnerable to changes in environmental conditions of natural or anthropogenic origins acting on both land and ocean geosystems. Monitoring these coastal waters appears therefore as a priority for supporting the development of sustainable, ecosystem-based environmental policies. Up to now, the lack of in-situ observation in the area represents a limitation to assess the variability of these marine ecosystems. In that context, satellite observation can represent a valuable and cost-effective tool to synoptically depict the biogeochemichal spatio-temporal variability of these ecosystems at sufficient spatial and temporal resolutions to also describe local and episodic features. However, ocean color remote sensing application in optically complex waters such as the ones of French Guiana requires the use of 1) adapted inversion methods to deliver valuable information on key biogeochemichal descriptors and 2) adapted statistical approaches for extracting insights into the spatiotemporal dynamics of the water masses. This PhD has been built in this context and divided into 5 chapters with the principal aims of 1) developing a set of ocean color products (Chlorophyll-a, Chl-a, Suspended Particulate Matter, SPM, Colored Dissolved Organic Matter, CDOM, Particulate and Dissolved Organic Carbon, POC and DOC, respectively) to monitor water quality in French Guiana, 2) describing the biogeochemical dynamics at the regional scale to assess the main factors driving this variability as well as the key biogeochemical descriptors to monitor at the region scale, and 3) proposing a partition of the water masses in order to support the development of regional monitoring policies. Chapter 1 provides background information introducing the rationale of this work. Chapter 2 introduces the datasets and statistical approaches used in the frame of this PhD. Chapter 3 presents the methological developments performed to improve a set of inversion algorithms at the regional scale for diverse high (e.g., Sentinel-2 with 10m) and medium (e.g., MERIS and Sentinel-3 with 1 km and 300m, respectively) spatial resolution sensors. The most adapted inversion models for delivering information on phytoplankton, particulate, and dissolved matter were defined from regional optimization/validation exercices. Our results further emphasized the challenges in estimating Chl-a over optically complex waters and a general framework based on optical classification was established to combine different Chl-a models for global applications. Chapter 4 describes the spatio-temporal variability of the considered biogeochemichal parameters using time series analysis performed on the generated ocean color archives. Regions under the influence of the Amazon River's plume have been well identified from multispectral satellite data (i.e., mudbank migration, North Brazil Current (NBC) retrofection). Our results show no evidence of significant long-term changes has been detected except the fingerprint of mudbank migration along the coast. It is also suggested that POC/SPM ratio might be an appropriate descriptor to monitor French Guiana's coastal water quality. Chapter 5 focuses on the partition of the French Guiana water masses considering different approaches (i.e., optical and temporal classification). In particular, an optical classification scheme can provide a comprehensive and dynamic view of the water masses characteristics in the French Guiana coastal domain. An illustration of the potential offered by temporal variation-based classifications for delimiting French Guiana waters is further illustrated.
Committee:
12th May 2023, 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:
24th April 2021, VNU Hanoi University of Science, Hanoi, Vietnam - Dinh Ba Duy, VNU HUS PhD student, VN government employee
Title: Characteristics of the activity and seasonal forecasting ability of tropical cyclones in East Sea, Vietnam
Summary: This thesis presents a comprehensive analysis of tropical cyclones (TC) over the Vietnam East Sea, examining various aspects such as the number of TCs, the number of TC days, TC frequency, TC active areas, decadal variations in TC characteristics, and the relationship between TCs and various climate indicators, utilizing the most updated datasets from 1979 to 2019. The thesis could identify suitable predictive factors and develop statistical prediction equations (for a lead time of three months) to forecast the number of TCs over the Vietnam East Sea in the first three months (June, July, and August) and the last three months (September, October, and November) of the TC season. The forecast results for the number of TCs in were found to be superior to climatological forecasts, with errors and forecasting skills comparable to results for other areas previously published both regionally and globally. Consequently, the thesis recommends further investigation to incorporate the prediction schemes into an ensemble and operational forecasting system. Additionally, the correlations between TCs accumulated over 3 months and 6 months with climate indicators and the climatological forecasting products of the CFSv2 model highlight the potential for including additional forecasting factors. These factors include, among others, the number of TC days accumulated over 3 months, the number of TCs accumulated over 6 months, and the number of TC days accumulated over 6 months. Incorporating these factors is expected to improve the seasonal forecasting results of TC activity over the Vietnam East Sea for lead times of 3 to 6 months.
Committee:
Supervisors:
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:
17th July 2020, INP Toulouse, France - Do Minh Huy, LCA PhD student (USTH fellowship)
Title: Development and characterization of nanocomposites and molecularly imprinted polymers based on chitosan for glyphosate detection in water
Summary: Glyphosate, classified as probably carcinogenic to humans, has being the world's most widely applied herbicide in agriculture. Therefore, glyphosate concentration is strictly regulated in drinking water: the permissible limit is 0.1 μg/L (0.6 nM) in Europe and 700 μg/L (4142 nM) in USA. Due to its physical-chemical properties, the detection of glyphosate at a laboratory scale is rather costly, time-consuming, and requires highly skillful persons. Consequently, the sensor technology has been proposed as an alternative analytical technique for glyphosate in-situ monitoring in water. The main objective of the thesis was to develop an optical chemical sensor for glyphosate detection. Different kinds of sensing layers based on chitosan were studied by means of surface plasmon resonance (SPR) technology. In a first approach, thin films of chitosan (CS) and its nanocomposites as chitosan/zinc oxide (CS/ZnO) and chitosan/graphene oxide (CS/GO) on the Au-layer as plasmonic material of the sensor transducer were prepared using a spin coating method. The thin films were characterized by Fourier-transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and contact angle measurement. The performances toward glyphosate sensing were studied in several conditions (crosslinking, mobile phase properties) by SPR measurement. Results showed that the thin-film of CS/ZnO composite provided the highest sensitivity for glyphosate sensing in the range of 0.03-0.60 μM and the detection limit of 0.008 μM. Furthermore, selectivity was investigated towards the sorption of glyphosate metabolite (aminomethylphosphonic acid, AMPA), glufosinate, and 3-(methylphosphinico) propionic acid (MPPA). In a second step, the molecular imprinting technique was used to improve sensitivity and selectivity of the chitosan layer in the presence of glyphosate as the template molecule. The physical-chemical conditions of sensor preparation, i.e. cross-linking, template extraction, pH and salt concentration, were optimized. Morphology of the films was investigated and characterized as during the first approach. For the optimal experimental conditions, the sensor showed a good linear response vs. glyphosate concentration (0.003-0.0295 μM). The limit of detection and limit of quantitation values were estimated to be 0.001 μM and 0.003 μM, respectively. The selectivity was evaluated with non-imprinted polymers SPR and other compounds. At the same concentration, the sensor response of glyphosate was higher 1.5 times than the one of MPPA, while the sensor was not sensitive to AMPA and glufosinate. In conclusion, the confrontation of the two types of sensitive layer for the detection of glyphosate by using the SPR method showed that the introduction of molecularly imprinting technique increased the sensitivity and selectivity, which should make it possible to develop a future sensor for the detection of this herbicide in water.
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.
Committee:
6th February 2018, LERMA, Sorbonne Univ. Paris, France - Pham Duc Binh, LERMA (USTH fellowship)
Manuscript available here
Title : Satellite remote sensing of the variability of the continental hydrology cycle in the lower Mekong basin over the last two decades
Summary :Surface water is essential for all forms of life since it is involved in almost all processes of life on Earth. Quantifying and monitoring surface water and its variations are important because of the strong connections between surface water, other hydrological components (groundwater and soil moisture, for example), and the changing climate system. Satellite remote sensing of land surface hydrology has shown great potential in studying hydrology from space at regional and global scales. In this thesis, different techniques using several types of satellite estimates have been made to study the variation of surface water, as well as other hydrological components in the lower Mekong basin (located in Vietnam and Cambodia) over the last two decades. This thesis focuses on four aspects. First, the use of visible/infrared MODIS/Terra satellite observations to monitor surface water in the lower Mekong basin is investigated. Four different classification methods are applied, and their results of surface water maps show similar seasonality and dynamics. The most suitable classification method, that is specially designed for tropical regions, is chosen to produce regular surface water maps of the region at 500 m spatial resolution, from January 2001 to present time. Compared to reference data, the MODIS-derived surface water time series show the same amplitude, and very high temporal correlation for the 2001-2007 period (> 95%). Second, the use of SAR Sentinel-1 satellite observations for the same objective is studied. Optical satellite data are replaced by SAR satellite data to benefit the ability of their microwave wavelengths to pass through clouds. Free-cloud Landsat-8 satellite imagery are set as targets to train and optimize a Neural Network (NN). Predicted surface water maps (30 m spatial resolution) are built for the studied region from January 2015 to present time, by applying a threshold (0.85) to the output of the NN. Compared to reference free-cloud Landsat-8 surface water maps, results derived from the NN show high spatial correlation (_90%), as well as true positive detection of water pixels (_90%). Predicted SAR surface water maps are also compared to floodability maps derived from topography data, and results show high consistency between the two independent maps with 98% of SAR-derived water pixels located in areas with a high probability of inundation (>60%). Third, the surface water volume variation is calculated as the product of the surface water extent and the surface water height. The two components are validated with other hydrological products, and results show good consistencies. The surface water height are linearly interpolated over inundated areas to build monthly maps at 500 m spatial resolution, then are used to calculate changes in the surface water volume. Results show high correlations when compared to variation of the total land surface water volume derived from GRACE data (95%), and variation of the in situ discharge estimates (96%). Fourth, two monthly global multi-satellite surface water products (GIEMS & SWAMPS) are compared together over the 1993-2007 period at regional and global scales. Ancillary data are used to support the analyses when available. Similar temporal dynamics of global surface water are observed when compared GIEMS and SWAMPS, but _50% of the SWAMPS inundated surfaces are located along the coast line. Over the Amazon and Orinoco basins, GIEMS and SWAMPS have very high water surface time series correlations (95% and 99%, respectively), but SWAMPS maximum water extent is just a half of what observed from GIEMS and SAR estimates. SWAMPS fails to capture surface water dynamics over the Niger basin since its surface water seasonality is out of phase with both GIEMS- and MODIS-derived water extent estimates, as well as with in situ river discharge data.
Committee: