West African Monsoon Modeling and Evaluation

(WAMME): A CIMS/CEOP initiative

 

1.  Introduction

 

The Coordinated 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.

 

As 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: 

 

(a) 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.

 

(b) 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

 

Aerosols, both 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).

 

The 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.

 

2.  The West African Monsoon (WAM)

 

WAM is one of the major monsoon systems in the world that has experienced the most severe long-term drought during the late 20^th 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.

 

We 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 relationship.  However, 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.

 

This is a community-interest driven activity. The goals and approaches will finally be determined by the participants.  Contact Persons:  Yongkang Xue (yxue@geog.ucla.edu), William Lau (lau@climate.gsfc.nasa.gov), Kerry Cook (khc6@cornell.edu).

 

 

Reference

 

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, DOI 10.1007/s00382-006-0141-9.

 

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, 2006:  Asian 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 AMMA.  http://www.amma-international.org/science/index

 

Prospero, J.M. and P. J. Lamb, 2003: African Droughts and Dust Transport to the Caribbean: Climate Change Implications, Science 302, 1024 – 1027.

 

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, 2782-2792.

 

Xue, Y., 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.  J. Geophy.  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 Heidelberg, P59-77.

 

Zeng, N., 2003, Drought in the Sahel, Science, 302, 999– 1000.