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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:

Christophe MENKES, ENTROPIE
Vincent ECHEVIN, LOCEAN
Thanh NGO-DUC, USTH
Rosemary MORROW, LEGOS
Marine HERRMANN, LEGOS
Caroline ULSES, LEGOS

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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:

Mai Trong Nhuan, VNU
Vu Van Thang, IMHEN
Mai Van Khiem, VMHA
Nguyen Van Hiep, VMHA
Nguyen Tien Thanh, WRU
Pham Thi Thanh Nga, VNSC
Nguyen Thi Hien Thuan, IMHEN

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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:

Cécile Guieu, LOV
Christophe Colin, GEOPS
Sylvain Ouillon, LEGOS
Nguyen Thi Hue, IET
Catherine Jeandel, LEGOS
Sophie Cravatte, LEGOS

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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:

Robert Frouin, Scripps Institution of Oceanography, USCD, San Diego
David Doxaran, LOV, Villefranche sur Mer
Bertrand Lubac, EPOC, Univ. Bordeaux
François Schmitt, LOG, CNRS
Anne Liferman, CNES
Vincent Vantrepotte, LOG, CNRS
Hubert Loisel, LOG, ULCO
Lucile Duforêt-Gaurier LOG, ULCO

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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:

Aldo Sottolichio, EPOC, Univ. Bordeaux
Isabelle Brenon, LIENSs
Sabine Charmasson, IRSN
Robert Lafite, Université de Rouen
Romaric Verney, IFREMER
Catherine Jeandel, LEGOS
Marine Herrmann, LEGOS, IRD
Sylvain Ouillon, LEGOS, IRD / USTH, VAST

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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:

Sylvain Ouillon, LEGOS
XiXi Lu, University of Singapore
Aldo Sottolichio, EPOC
Jean-Michel Martinez, GET
Henrique LLacer ROIG, University of Brasilia
Didier Orange, IRD Montpellier
Marine Herrmann, LEGOS
Sabine Sauvage, ECOLAB
Jose-Miguel Sanchez-Perez, ECOLAB

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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:

Aldo Sottolichio, EPOC, Univ. Bordeaux
Nguyen Minh Huan, HCMO, VNU-HUS
Dinh Van Manh, Institute of Mechanics, VAST
Marine Herrmann, LEGOS, IRD / USTH, VAST
Bui Hong Long, IO, VAST
Tranh Dinh Lan, IMER, VAST
Alexei Sentchev, LOG, ULCO
Dinh Van Uu, VNU-HUS
Sylvain Ouillon, LEGOS, IRD

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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:

John Wilkin, Rutgers University
Vincent Echevin, LOCEAN / IRD
Anne Petrenko, MIO / AMU
Isabelle Dadou, LEGOS, UPS
Rosemary Morrow, LEGOS / UPS
Marine Herrmann, LEGOS / IRD
Nguyen Minh Huan, FHMO, VNU-HUS

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