PhD thesis at LMD

Preliminary version of my thesis.

Presentation for my defense (in French).

PhD subject and supervisors

My PhD at the Laboratoire de Météorologie Dynamique (LMD) in Paris started in September 2006. My directors were Sandrine Bony et Jean Jouzel. The subject is: "Analysis and modeling of the siganture of the variations of the tropical climate in the isotopic composition of water (O18 et D)".
I was supervised mainly by Sandrine Bony (LMD). I also work with Françoise Vimeux and Amaelle Landais (LSCE).

Thesis summary

Different forms ow the water molecules can be found: H₂¹⁶O is the most frequent, but there is also H₂¹⁸O, HDO, and some H₂¹⁷O. The isotopic composition of water is the relative proportion of these water forms.

The stable isotopic composition of water is a promising tracer of the present day water cycle and past climates. While the isotopic composition recorded in polar ice core have long been used to reconstruct past temperatures, however, what controls the isotopic composition of the tropical precipitation is more complex. The goal of this thesis is thus to better understand the processes that affect the isotopic composition of tropical precipitation and atmospheric water, more particularly in the tropics. Since most of the tropical precipitation arises from atmospheric convection, and most isotopic archives are on land, we focus more particularly on the impact of convective and land surface processes. In turn, what can be learned about convection and land surface processes using isotopic measurements? Can they help constrain their representation in models? At the inter-annual to climate change scale, what information about the tropical climate variability is recorded in isotopic signals observed in archives?

1) Water stable isotopes and atmospheric convection

First, we investigate the influence of convection on water stable isotopes. We use both:

numerical modeling, with a hierarchy of models (single column model, two-dimensional model of squall lines, general circulation model),
data analysis, using isotopic data from rain collected in the Sahel during the AMMA (African Monsoon Multidisciplinary Analysis) campaign, at the event and intra-event scales (e few pictures from the campaign).

These studies highlight the strong impact of convection on the precipitation composition, and stress the importance of rain evaporation and convective or meso-scale subsidence in controlling the rain isotopic composition. Convection also plays an important role on isotopic profiles in the upper troposphere-lower stratosphere.

2) Water stable isotopes and climate variations

Second, we study what information about climatic variability is recorded by water stable isotopes in precipitation. We analyze simulations of present day and past climates with LMDZ, and evaluate to what extent isotopic archives of tropical precipitation can be used to reconstitute past precipitation changes. We also explore the added value of a new but promising isotopic tracer: H₂¹⁷O.

3) Water stable isotopes and land surface processes

Third, we analyse the influence of land surface processes on the isotopic composition of precipitation and land surface waters, using simulations with the land surface model ORCHIDEE in which we also introduced the isotopes. The isotopic composition of the different water pools of the land-atmosphere system may inform about the partitionning of precipitation into runoff, drainage, transpiration and bare soil evaporation.