A strong control of the South American SeeSaw on the intra-seasonal variability of the isotopic composition of precipitation in the Bolivian Andes

International audienceWater stable isotopes (d) in tropical regions are a valuable tool to study both convective processes and climate variability provided that local and remote controls on d are well known. Here, we examine the intra-seasonal variability of the event-based isotopic composition of precipitation (dDZongo) in the Bolivian Andes (Zongo valley, 16°20'S-67°47'W) from September 1st, 1999 to August 31st, 2000. We show that the local amount effect is a very poor parameter to explain dDZongo. We thus explore the property of water isotopes to integrate both temporal and spatial convective activities. We first show that the local convective activity averaged over the 7-8days preceding the rainy event is an important control on dDZongo during the rainy season (~40% of the dDZongo variability is captured). This could be explained by the progressive depletion of local water vapor by unsaturated downdrafts of convective systems. The exploration of remote convective controls on dDZongo shows a strong influence of the South American SeeSaw (SASS) which is the first climate mode controlling the precipitation variability in tropical South America during austral summer. Our study clearly evidences that temporal and spatial controls are not fully independent as the 7-day averaged convection in the Zongo valley responds to the SASS. Our results are finally used to evaluate a water isotope enabled atmospheric general circulation model (LMDZ-iso), using the stretched grid functionality to run zoomed simulations over the entire South American continent (15°N-55°S; 30°-85°W). We find that zoomed simulations capture the intra-seasonal isotopic variation and its controls, though with an overestimated local sensitivity, and confirm the role of a remote control on d according to a SASS-like dipolar structure. © 2011 Elsevier B.V

Interannual variability of isotopic composition in water vapor over western Africa and its relationship to ENSO

International audienceThis study was performed to examine the relationship between isotopic composition in near-surface vapor (δ 18 O v) over western Africa during the monsoon season and El Niño-Southern Oscillation (ENSO) activity using the Isotope-incorporated Global Spectral Model. The model was evaluated using a satellite and in situ observations at daily to interannual timescales. The model provided an accurate simulation of the spatial pattern and seasonal and in-terannual variations of isotopic composition in column and surface vapor and precipitation over western Africa. Encouraged by this result, we conducted a simulation stretching 34 years (1979-2012) to investigate the relationship between atmospheric environment and isotopic signature on an interan-nual timescale. The simulation indicated that the depletion in the monsoon season does not appear every year at Niamey. The major difference between the composite fields with and without depletion was in the amount of precipitation in the upstream area of Niamey. As the interannual variation of the precipitation amount is influenced by the ENSO, we regressed the monsoon season averaged δ 18 O v from the model and annually averaged NINO3 index and found a statistically significant correlation (R = 0.56, P < 0.01) at Niamey. This relationship suggests that there is a possibility of reconstructing past western African monsoon activity and ENSO using climate proxies

Clustering mesoscale convective systems with laser-based water vapor delta O-18 monitoring in Niamey (Niger)

The isotopic composition of surface water vapor (delta(v)) has been measured continuously in Niamey along with the isotopic composition of event-based precipitation (delta(p)) since 2010. We investigate the evolution of water vapor and precipitation isotope ratios during rain events of the 2010, 2011, and 2012 monsoon periods. We establish a classification of rain systems into three types based on the delta(v) temporal evolution. We find that 51% of rain events (class A) exhibit a sharp decrease in delta O-18(v) in phase with the surface air temperature drop, leading to a depletion of water vapor by - 1.9% on average during rainfall. Twenty-nine percent of rain events (class B) show a similar decrease in delta O-18(v) in phase with the temperature drop but are characterized by a progressive enrichment of the vapor in the stratiform region, resulting in a depletion of water vapor by -1.2% on average during rainfall. The last 20% of the rain events (class C) are associated with a progressive increase in delta O-18(v) during rainfall (+0.8%). We also examine the temporal evolution of water vapor deuterium excess (d(v)) which shows a sharp increase as delta O-18(v) decreases, followed by a progressive decrease in the stratiform part for classes A and B. Using a basic box model, we examine for each class the respective roles that mesoscale subsidence and rain evaporation play on the evolution of delta O-18(v). We show that those two processes are dominant for class A, whereas other processes may exert a major role on delta O-18(v) for classes B and C

Analysis of a high level of particulate-pollution event in Paris Megacity by integrating in-situ and remote sensing aerosol / gas measurement at the SIRTA ACTRIS Observatory

A high level of particulate-pollution event has been observed in Paris Megacity between 6 and 15 March 2014. The SIRTA Observatory has documented all atmospheric variables between surface and the top of free troposphere with in-situ sensors and active/passive remote sensing instruments. We will describe the instrumental dataset (more than 150 instruments), meteorological conditions and variability of some species, such as (i) aerosol (particle mass, chemistry, size distribution and absorption/scattering), (ii) reactive gas (type, chemistry), (iii) radon and GES and (iv) isotopic species (CO2, CH4 and H2O). A specific attention will be given in this presentation to understand the processes responsible for the high concentration level of thin particles (smaller than 2.5µm with a medium radius around 0.2µm) in a thin mixing layer (night 200m and day 600-1200m). All these collocated measurements on the Saclay plateau (ACTRIS platform) is an ideal place to conduct this work. In fact, the chemical analysis shows that the major part of these particles were secondary aerosols related to anthropogenic activities such as traffic, wood burning and agriculture. And, the geochemical tracers, active and passive remote sensing instruments (lidars, cloud radar, microwave radiometer and sun-photometer) allow us to characterize the mechanisms leading to this so high particle pollution: (i) role of boundary layer dynamics (stable, neutral, convective), (ii) role of geographical origin of PM components (local, regional, European) and (iii) role of liquid phase (fog and boundary layer cloud). Finally, the impact on solar and ultra-violet downwelling fluxes will be quantified