Actualité scientifique
Scientific news
April 2012
Water vapour is the primary
greenhouse gas, and it is
still an unknown factor
(along with its associated
cloud processes) when
creating climate projections
for 2100. A new technique,
developed by IRD
researchers and their
partners1, enables
measurement of the
isotopic composition2 of
atmospheric water vapour.
It will enable study of such
diverse elements as the
movement and the origin of
masses of air, or the
formation of clouds, in a
more precise fashion than
had previously been offered
by standard meteorological
variables.
Isotopic data collected over
a full year at the IRI1 in
Niger has enabled
researchers to deconstruct
the mechanisms behind the
Sahel water cycle. It has
revealed that humidity plays
its part, even during the dry
season, and that this
regular influx of humid air
comes from the
Mediterranean. This
humidity is present before
the monsoon, and probably
plays an important role in
triggering rainfall3.
Now routinely used in
Africa, this technique can
be extended to other
tropical regions such as the
Andes, where the
topography casts even
more doubt over projected
changes in precipitation.
A new method
for tracing the Sahel water cycle
© IRD / J-L Rajot
N° 401
Actualidad cientifica
The isotopic measurement of atmospheric water vapour will enable an understanding, and better predictions as a result, of the triggers for the African monsoon
(here, the squall line arrives at an IRD measurement site in Niger).
Water vapour is the primary greenhouse gas, itself
causing around 60% of the effect. Nevertheless, the
cloud processes linked to it are one of the primary
sources of uncertainty in climate projections, particularly for tropical and subtropical regions such as
the Sahel. As a result, it is crucial to understand the
transport, the sources and the reserves of atmospheric humidity. Researchers from the HSM unit at
the IRD and their French and Nigerian partners from
the LSCE and the IRI1 have developed a new measurement technique to further this understanding.
Field observation
This innovative method records the isotopic composition2 of water vapour in situ. It consists of a laser technique, based on the absorption spectra of the different
water molecules that make up atmospheric vapour.
The stable water isotopes are a well-known geochemical tracer for reconstituting historic climate variations, using archives such as ice cores. But their
capacity to teach us about current variability has not
been established. Using a new technique enabling
the observation of these isotopes in atmospheric
vapour, the researchers have demonstrated that they
are far more sensitive to physical atmospheric
processes than to basic meteorological variables
such as humidity or pressure. They thus offer a
means of precisely studying elements and phenomena as diverse as the origin of masses of air, cloud
formation or the different types of humidity transport.
This technique has been used by the research team
at the IRI campus in Niamey, with financial support
from the IRD. Under the leadership of this Nigerian
institute, isotopic measurements have been taking
place over the Sahel continually since 2010.
Water from the Mediterranean
Climatologists have analysed these measurements
over the course of a full year, including the monsoon
period in 2010 and the dry season that followed.
Unexpectedly, the isotopic date showed a very
strong variability in the availability of atmospheric
FOR MORE INFORMATION
water over the course of the year. And this was true
even during the dry season, where it was understood that the absence of rain meant that there was
no activity over the desert. The study also shows
that this water vapour is due to regular intrusions of
humid air from North Africa, with an eruption during
January and February - the height of the dry
season. This result suggests that the Sahel climate
depends on very slight changes in atmospheric
circulation at a regional level, particularly in the
Mediterranean. In addition, this humidity, already
present when the monsoon arrives in the Sahel
several weeks later, has an important role to play in
triggering rainfall. The research is also shedding
new light on the variability of the tropical atmosphere outside of the monsoon period, scarcely
documented until now.
Analysing the squall line
The temporal resolution of the data acquired (a few
seconds) has enabled the specialists to deconstruct
the genesis and the propagation of typical squall
lines for monsoon thunderstorms. They have closely
studied hydration processes (such as the evaporation of water droplets) and the inverse dehydration
processes (such as the dry air caused by descending vertical air currents) that directly control the
availability of water in the atmosphere. Water
isotopes have varying characteristics depending on
the process that created them, and researchers
have demonstrated that both of these processes
are strongly present in the Sahel, capable of generating gust fronts that can lead to thunderstorms.
Contacts
Françoise VIMEUX,
IRD researcher
Tel: +33 (0)1 69 08 57 71
francoise.vimeux@ird.fr
UMR 50 HydroSciences Montpellier –
HSM (IRD, CNRS, UM1, UM2)
Address
LSCE, CEA Saclay
Orme des Merisiers, bât. 701
91191 Gif-sur-Yvette Cedex
The newly-obtained data will allow an improvement
in the development projections for the precipitation
regime in the Sahel, currently based on models in
which the strong variability in water levels observed
has not been fully represented**.
Guillaume TREMOY,
doctoral student
Tel: +33 (0)1 69 08 94 63
guillaume.tremoy@lsce.ipsl.fr
Laboratoire des sciences du climat et de
l’environnement – LSCE (CEA, CNRS, UVSQ)
Address
LSCE, CEA Saclay
Orme des Merisiers, bât. 701
91191 Gif-sur-Yvette Cedex
Routinely used by the IRI, this technique is being
made available to other African laboratories for the
whole continent. It can also be extended to other
tropical regions where the precipitation development could reduce the availability of water
resources, such as the Andes. Projections there are
even more uncer tain than in the Sahel. The
complexity of the tropical atmospheric water cycle
is coupled with another major problem: relief, also
poorly represented in climate models…
Salla MAYAKI,
lecturer at Abdou Moumouni
universityi
Tel: +227 20 31 58 50
mayakisalla@gmail.com
Institut des RadioIsotopes, IRI
Address
IRI, Université Abdou Moumouni
BP 10727 - Niamey, Niger
*see sheet no.300– Patagonian glacier yields clues for improved
understanding of global climate change
and sheet no.232– - New light on the tropical climate of the past:
20 000 years ago the Amazon basin was wetter than predicted
References
Tremoy G., Vimeux Françoise, Mayaki S.,
Souley I., Cattani O., Risi C., Favreau Guillaume,
Oï Monique. A 1-year long 18O record of water
vapor in Niamey (Niger) reveals insightful
atmospheric processes at different timescales,
Geophysical Research Letters, 2012, in press. http://
dx.doi:10.1029/2012GL051298.
By Françoise Vimeux and Gaëlle Courcoux
Tremoy G., Vimeux Françoise, Cattani O., Mayaki S.,
Souley I., Favreau Guillaume. Measurements of
water vapor isotope ratios with wavelength-scanned
cavity ring-down spectroscopy technology: new
insights and important caveats for deuterium excess
measurements in tropical areas in comparison
with isotope-ratio mass spectrometry. Rapid
Communications in Mass Spectrometry, 2011, 25
(23), p. 3469-3480. fdi:010054250
1. This research forms part of the African Monsonn Multidisciplinary Analyses (AMMA), and has been carried out by researchers from
the Montpelier HydroSciences unit (IRD-CNRS-UM1-UM2) and the Climate and Environmental Sciences laboratory (CEA-CNRSUVSQ) and their partners from the Institut des RadioIsotopes (Université Abdou Moumouni).
2. Hydrogen and oxygen have several stable isotopes which differ in their number of neutrons. These result in different forms of H2O
molecules, varying in weight. The isotopic composition is the relative quantity of these different molecules in water vapour.
3. The Sahel monsoon is formed by flows of humidity originating in the south-southwest which appear at the start of the summer.
4e Conférence internationale AMMA,
du 2 au 6 juillet 2012, Toulouse, France.
Keywords
Climate, water, West Africa, isotopes
**DID YOU KNOW?
Currently more than two thirds of climate models do not even agree on the amplitude and direction (positive
or negative) of changes in precipitation at the end of the 21st century in the Sahel and in the Amazon.
Coordination
Gaëlle Courcoux
Information and Culture Department
Tel: +33 (0)4 91 99 94 90
Fax: +33 (0)4 91 99 92 28
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indigo@ird.fr
IRD photographs on this topic, free for media
reproduction without additional permission:
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© IRD/DIC, april 2012 - Design and graphics: L. CORSINI
The laser instrument (left, in the IRI laboratory in Niger) measures the isotopic composition of atmospheric vapour (centre, vapour sampling on the IRI roof), enabling
study of cloud formation or even squall line propagation (right).
© IRD / J-L Rajot
© LSCE / G. Tremoy
© LSCE / G. Tremoy
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