West African Monsoon Modeling and Evaluation
(WAMME): A CIMS/CEOP initiative
Enhanced Observing Period (CEOP), initiated in mid-2001, was designed to bring
together current and planned Earth observing satellites, existing suite of
operational satellites, and observational and modeling assets of the GEWEX
continental scale experiments and the field observations of CLIVAR and other
WCRP activities, to support key science objectives in climate prediction and
monsoon system studies.
Phase-I (2002-2005) for CEOP is completing, two full annual cycles (2003-2004)
of research-quality data sets from satellites, reference sites, and model output
location time series (MOLTS) have been processed and made available for data
analyses and model validation studies.
Phase-II (2006-2010) will continue
the data collection and processing, while providing increasing focus on
scientific research using CEOP data, and coordination with planned observational
and modeling programs. In Phase-II,
CEOP Inter-Monsoon Model Study
(CIMS) will continue the original focus
(Lau et al. 2004), but including a new science thrust on Aerosol- Monsoon Water
Cycle Interactions (Lau 2005) with the objectives:
To provide better understanding of the mechanisms of extreme events that affect
water availability in monsoon regions, and their relationships to oceanic, land,
atmospheric (including aerosols) forcings.
To unravel the effects of natural and anthropogenic aerosols on the monsoon
water cycle and their interaction with the atmosphere-land-ocean system, from
diurnal, intraseasonal to interannual time scales
natural and man-made, as well as floods and droughts are the two most urgent
societal problems confronting monsoon regions around the world. Dust transported
from central Asia, and the Middle East deserts combined with aerosols from from
industrial pollution, and biomass burning are severely afflicting the climate
and human health in the South Asian and East Asian monsoon regions. As a matter
of fact, North Africa is the world’s major source of mineral dust aerosols,
which affect not only the climate of the West Africa monsoon regions but
possibly weather and climate of the Middle East, Europe and the east coast of North America (Prospero
and Lamb, 2003). The new thrust of CIMS will study the regional features
and teleconnections, associated with fluctuation of the monsoon water cycles and
interaction with aerosols, by comparing and contrasting 6 major monsoon systems
around the world, i.e., East Asian monsoon, South Asian, West Africa, Australia,
North America/Mexico, and South America (Lau
and Kim 2005).
Studies have indicated that we are
currently hindered in providing skillful predictions of WAM variability and its
impacts. For example, the African
Monsoon Interdisciplinary Analysis (AMMA) project indicated that there are still
fundamental gaps in our knowledge of the coupled atmosphere-land-ocean system at
least partly arising from lack of appropriate observational datasets but also
because of the complex scale interactions between the atmosphere, biosphere and
hydrosphere that ultimately determine the nature of the WAM.
Dynamical models used for prediction suffer from large systematic errors
in the West African and tropical Atlantic regions; current models have problems
simulating fundamental characteristics of rainfall such as the diurnal, seasonal
and annual cycles. More research is required to validate and exploit the
observational data to improve the WAM prediction. (Lebel et al., 2005).
present initiative is proposed in the interests of increasing coordination and
collaboration between CIMS and AMMA. Research
proposed under this initiative will have the benefit of intercomparison with
other monsoon systems where similar initiatives focusing on aerosol-monsoon
water cycle are being pursued under CIMS.
The West African Monsoon (WAM)
is one of the major monsoon systems in the world that has experienced the most
severe long-term drought during the late 20th century with
devastating impacts on humanity. Despite
of some progress in understanding the effect of boundary forcing on WAM
variability, however, the monsoon precipitation and associated important
features, such as African Easterly Jet (AEJ), and impacts of oceanic, and land
processes, and aerosols are not well understood, due in part to lack of detailed
observations, and to inability of GCM to simulate these features at different
temporal and spatial scales, including diurnal cycle, intraseasonal evolution,
and interannual variability (Xue and Shukla, 1998; Xue et al., 2004a; Cook and
Vizy, 2005). CEOP in conjunction
with the AMMA field observations will provide a unique set of multi-scale
observations to evaluate models. Testing physics
dependencies for the diurnal cycle and for multi-scale interactions represents a
“Grand Challenge” problem to the modeling community in terms of scientific
effort and computing.
plan to initiate a West African Monsoon
Modeling and Evaluation project (WAMME) using GCMs and
regional climate models (RCMs) to address issues regarding the role of
land-ocean-atmosphere interaction, land-use and water-use change, vegetation
dynamics, as well as aerosol, particularly dust, on WAM development. Since
preliminary studies have demonstrated the utility of RCMs in improving the
WAM’s simulation (e.g., Druyan et al., 2006), we will apply both GCMs and
nested RCMs for this project. In
the WAMME project, we plan (1) to
evaluate the performance of current GCMs and RCMs in simulating WAM
precipitation and relevant processes at diurnal, intraseasonal, to interannual
scales, as well as its onset and withdrawal, (2) to identify the common
discrepancies and provide better understanding of fundamental physical
processes in WAM; (3) to conduct sensitivity
experiments to isolate important key physical processes for interannual and
interdecadal variations of WAM, (4) to demonstrate the utility and
synergy of CEOP and AMMA field data in providing a pathway for model physics
evaluation and improvement, and (5) to evaluate the nested RCMs’ ability of
downscaling West African regional climate simulations.
In Item (3), we will
focus on the mechanism of the recovery of the Sahelian drought (although it
may still be in historical low, but has recovered from the worst drought in late
70s and early 80s). A study (Lau
and Kim 2005) finds that there is an increase in the "continentality"
of the WAM, reflected by a shift of the WAM convection towards land from
the 1980s to 1990s. SSTs in the
last decade in the Indian Ocean and Atlantic have been increasing, which
seems to contradict with general perception regarding SST/Sahel rainfall
a possible link between
Mediterranean SSTs and Sahelian rainfall is consistent with the latest Sahel
rainfall recovery (Rowell, 2003). On
other hand, a preliminary GCM study suggests that dust-radiation-atmosphere
feedback could cause the WAM to shift inland, and may provide a missing
mechanism for the recovery of the Sahel rainfall. This mechanism is
analogous to the “elevated heat pump” effect caused by dust and black carbon
stacking up the southern slopes of the Tibetan Plateau, which leads to increased
rainfall over northern India, and reduced rainfall in regions to the south (Lau
et al 2006). In addition, satellite
data indicate the leaf area index in Sahel is increasing during the last decade.
The land/atmosphere feedback process and/or improvement in land
management may also contribute to this recovery (Zeng, 2003; Xue et al., 2004b).
The testing of this issue will be carried out after initial model
evaluation and comparisons.
is a community-interest driven activity. The goals and approaches will finally
be determined by the participants. Contact
Persons: Yongkang Xue (firstname.lastname@example.org),
William Lau (email@example.com),
Kerry Cook (firstname.lastname@example.org).
Cook K.-H. and E.K. Vizy,
2006: Coupled Model Simulations of the West African Monsoon System: 20th century
Simulations and 21st Century Predictions. J.
Climate, 19, 3681-3703.
Druyan, L.M., M. Fulakeza, P. Lonergan, 2006: Mesoscale analyses of West
African summer climate: focus on wave disturbances, Climate Dynamics, 26,
Lau, K. M., J. Matsumoto, M. Ballasinno,
and H. Berbery, 2004: Validation of
diurnal variability over monsoon regions: a CIMS pilot study.
CEOP Newsletter 5, 2-4.
Lau, K.M. 2005:
Aerosol-water cycle interaction: A
new challenge in monsoon climate research.
GEWEX Newsletter, 15, 1, 7-9.
Lau, K.M., M. K. Kim, and K. M. Kim,
summer monsoon anomalies induced by aerosol direct forcing: the role of the
Tibetan Plateau. Climate
Dynamics, 26, 855-864.
Lau K. M., and K. M. Kim, 2006:
Diurnal and seasonal cycles in monsoon systems.
J. Meteor. Soc. Japan, CEOP Special Issues (submitted).
Lebel, T., J.-L.
Redelsperger, C. Thorncroft, et al., 2005: The International Science Plan for
Prospero, J.M. and P. J.
Lamb, 2003: African Droughts and Dust Transport to the Caribbean: Climate Change
Implications, Science 302, 1024 –
Rowell, D. P., 2003: The Impact of Mediterranean SSTs on the Sahelian
Rainfall Season, 16, 849-8610.
Xue, Y. and J. Shukla, 1998:
model simulation of the influence of global SST anomalies on the Sahel rainfall.
Mon. Wea. Rev., 126,
H.-M. H. Juang, W. Li, S.
Prince, R. DeFries, Y. Jiao, R. Vasic, 2004a: Role of land surface processes in
monsoon development: East Asia and West Africa.
Res., 109, D03105, doi:10.1029/2003JD003556.
Xue, Y., R.W.A. Hutjes, R.J. Harding,
M. Claussen, S. Prince, E. F. Lambin, S. J. Allen, P. Dirmeyer, T. Oki, 2004b:
The Sahelian Climate (Chapter A5) in Vegetation, Water, Humans and the Climate,
Eds, P. Kabat, M. Claussen, P. A. Dirmeyer, et al., Springer-Verlag, Berlin
2003, Drought in the Sahel, Science, 302,