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Teleconnection and dynamical influences on the intraseasonal rainfall variability over Vietnam

12 mai 2023 @ 9h30 - 12h00

– Hong Hanh Le, LEGOS, Toulouse –

 

Résumé :

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.

 

Jury :

  • Andreas Fink – Prof. Karlsruhe, Germany – Rapporteur
  • Satoru Yokoi – DR, JAMSTEC, Japan – Rapporteur
  • Francis Codron – Prof. Sorbonne Univ., LOCEAN, Paris – Rapporteur
  • Jean-Philippe Duvel – DR, ENS, LMD, Paris – Examinateur
  • Pascal Roucou – MC, Univ Bourgogne, CRC -Examinateur
  • Isabelle Dadou – Prof., Univ. Toulouse, LEGOS – Examinatrice
  • Nicholas Hall – Prof., Univ. Toulouse, LEGOS -Directeur de Thèse
  • Thanh Ngo Duc – Prof., USTH, Hanoi – Co-Directeur de Thèse

 

Lien Zoom :

https://univ-tlse3-fr.zoom.us/j/99360344988

ID de réunion : 993 6034 4988

 

Détails

Date :
12 mai 2023
Heure :
9h30 - 12h00
Catégories d’Évènement:
,

Organisateur

LEGOS

Lieu

Salle Coriolis

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