Showing papers by "Camille Risi published in 2017"

Rainforest-initiated wet season onset over the southern Amazon.

Abstract: Although it is well established that transpiration contributes much of the water for rainfall over Amazonia, it remains unclear whether transpiration helps to drive or merely responds to the seasonal cycle of rainfall. Here, we use multiple independent satellite datasets to show that rainforest transpiration enables an increase of shallow convection that moistens and destabilizes the atmosphere during the initial stages of the dry-to-wet season transition. This shallow convection moisture pump (SCMP) preconditions the atmosphere at the regional scale for a rapid increase in rain-bearing deep convection, which in turn drives moisture convergence and wet season onset 2–3 mo before the arrival of the Intertropical Convergence Zone (ITCZ). Aerosols produced by late dry season biomass burning may alter the efficiency of the SCMP. Our results highlight the mechanisms by which interactions among land surface processes, atmospheric convection, and biomass burning may alter the timing of wet season onset and provide a mechanistic framework for understanding how deforestation extends the dry season and enhances regional vulnerability to drought.

Evaluating the skills of isotope‐enabled general circulation models against in situ atmospheric water vapor isotope observations

Abstract: The skills of isotope-enabled general circulation models are evaluated against atmospheric water vapor isotopes. We have combined in situ observations of surface water vapor isotopes spanning multiple field seasons (2010, 2011, and 2012) from the top of the Greenland Ice Sheet (NEEM site: 77.45°N, 51.05°W, 2484 m above sea level) with observations from the marine boundary layer of the North Atlantic and Arctic Ocean (Bermuda Islands 32.26°N, 64.88°W, year: 2012; south coast of Iceland 63.83°N, 21.47°W, year: 2012; South Greenland 61.21°N, 47.17°W, year: 2012; Svalbard 78.92°N, 11.92°E, year: 2014). This allows us to benchmark the ability to simulate the daily water vapor isotope variations from five different simulations using isotope-enabled general circulation models. Our model-data comparison documents clear isotope biases both on top of the Greenland Ice Sheet (1–11‰ for δ18O and 4–19‰ for d-excess depending on model and season) and in the marine boundary layer (maximum differences for the following: Bermuda δ18O = ~1‰, d-excess = ~3‰; South coast of Iceland δ18O = ~2‰, d-excess = ~ 5‰; South Greenland δ18O = ~4‰, d-excess = ~7‰; Svalbard δ18O = ~2‰, d-excess = ~7‰). We find that the simulated isotope biases are not just explained by simulated biases in temperature and humidity. Instead, we argue that these isotope biases are related to a poor simulation of the spatial structure of the marine boundary layer water vapor isotopic composition. Furthermore, we specifically show that the marine boundary layer water vapor isotopes of the Baffin Bay region show strong influence on the water vapor isotopes at the NEEM deep ice core-drilling site in northwest Greenland. Our evaluation of the simulations using isotope-enabled general circulation models also documents wide intermodel spatial variability in the Arctic. This stresses the importance of a coordinated water vapor isotope-monitoring network in order to discriminate amongst these model behaviors

The isotopic composition of near-surface water vapor at the Maïdo observatory (Reunion Island, southwestern Indian Ocean) documents the controls of the humidity of the subtropical troposphere

Abstract: We present a 1 year long record of the isotopic composition of near-surface water vapor (δ18Ov) at the Maido atmospheric observatory (Reunion Island, Indian Ocean, 22°S, 55°E) from 1 November 2014 to 31 October 2015, using wavelength-scanned cavity ring down spectroscopy. Except during cyclone periods where δ18Ov is highly depleted (−20.5‰), a significant diurnal variability can be seen on both δ18Ov and qv with enriched (depleted) water vapor (mean δ18Ov is −13.4‰ (−16.6‰)) and moist (dry) conditions (mean qv is 9.7 g/kg (6.4 g/kg)) during daytime (nighttime). We show that δ18Ov diurnal cycle arises from mixing processes for 65% of cases with two distinct sources of water vapor. We suggest that δ18Ov diurnal cycle is controlled by an interplay of thermally driven land-sea breezes and upslope-downslope flows, bringing maritime air to the observatory during daytime, whereas at night, the observatory is above the atmospheric boundary layer and samples free tropospheric air. Interestingly, δ18Ov record also shows that some nights (15%) are extremely depleted (mean δ18Ov is −21.4‰). They are among the driest of the record (mean qv is 2.9 g/kg). Based on different modeling studies, we suggest that extreme nocturnal isotopic depletions are caused by large-scale atmospheric transport and subsidence of dry air masses from the upper troposphere to the surface, induced by the subtropical westerly jet.

Modelling tree ring cellulose δ 18 O variations in two temperature-sensitive tree species from North and South America

Abstract: . Oxygen isotopes in tree rings (δ18OTR) are widely used to reconstruct past climates. However, the complexity of climatic and biological processes controlling isotopic fractionation is not yet fully understood. Here, we use the MAIDENiso model to decipher the variability in δ18OTR of two temperature-sensitive species of relevant palaeoclimatological interest (Picea mariana and Nothofagus pumilio) and growing at cold high latitudes in North and South America. In this first modelling study on δ18OTR values in both northeastern Canada (53.86° N) and western Argentina (41.10° S), we specifically aim at (1) evaluating the predictive skill of MAIDENiso to simulate δ18OTR values, (2) identifying the physical processes controlling δ18OTR by mechanistic modelling and (3) defining the origin of the temperature signal recorded in the two species. Although the linear regression models used here to predict daily δ18O of precipitation (δ18OP) may need to be improved in the future, the resulting daily δ18OP values adequately reproduce observed (from weather stations) and simulated (by global circulation model) δ18OP series. The δ18OTR values of the two species are correctly simulated using the δ18OP estimation as MAIDENiso input, although some offset in mean δ18OTR levels is observed for the South American site. For both species, the variability in δ18OTR series is primarily linked to the effect of temperature on isotopic enrichment of the leaf water. We show that MAIDENiso is a powerful tool for investigating isotopic fractionation processes but that the lack of a denser isotope-enabled monitoring network recording oxygen fractionation in the soil–vegetation–atmosphere compartments limits our capacity to decipher the processes at play. This study proves that the eco-physiological modelling of δ18OTR values is necessary to interpret the recorded climate signal more reliably.

Importance of the advection scheme for the simulation of water isotopes over Antarctica by general circulation models: a case study with LMDZ-iso (LMDZ5a revision 1750)

Abstract: . Atmospheric general circulation models (AGCMs) are known to have a warm and isotopically enriched bias over Antarctica. We test here the hypothesis that these biases are consequences of a too diffusive advection. Using the LMDZ-iso model, we show that a good representation of the advection, especially on the horizontal, is very important to reduce the bias in the isotopic contents of precipitation above this area and to improve the modelled water isotopes – temperature relationship. A good advection scheme is thus essential when using GCMs for paleoclimate applications based on polar water isotopes.

Importance of the Saharan Heat Low on the control of the North Atlantic free tropospheric humidity deduced from IASI δD observations

Abstract: The isotopic composition of water vapour in the North Atlantic free troposphere is investigated with IASI measurements of the D/H ratio ( δ D) above the ocean. We show that in the vicinity of West Africa, the seasonality of δ D is particularly strong (160 ‰), which is related with the installation of the Saharan Heat Low (SHL) during summertime. The SHL indeed largely influences the dynamic in that region by producing deep turbulent mixing layers, yielding a specific water vapor isotopic footprint. The influence of the SHL on the isotopic budget is analysed at various time and space scales and is shown to be large, highlighting the importance of the SHL dynamics on the moistening and the HDO-enrichment of the free troposphere over the North Atlantic. We also report important inter-annual variations of δ D above Izana (Canary Islands) that we interpret, using backward trajectory analyses, in terms of the ratio of air-masses coming from the North Atlantic and air-masses coming from the African continent. Finally, we present spatial distributions of δ D and humidity above the North Atlantic and we show that the different sources and dehydration pathways controlling the humidity can be disentangled thanks to the added value of δ D observations.