African Monsoon Multidisciplinary Analysis (AMMA): An International Research Project and Field Campaign on the West African Monsoon

 

1. WHAT IS AMMA?

African Monsoon Multidisciplinary Analysis (AMMA) is a coordinated international project to improve our knowledge and understanding of the West African monsoon (WAM), its variability and impacts. AMMA will carry out the research needed to improve our ability to monitor and predict the weather and climate of West Africa and downstream tropical Atlantic. Recognizing the needs of society, AMMA also will seek to provide the observations and underpinning science needed to relate variability of the WAM to issues of health, food security and water resources.

AMMA will facilitate the multidisciplinary research required to provide improved predictions of the WAM and its impacts on daily-to-interannual timescales. This will be achieved through international coordination of ongoing activities, promoting necessary basic research, and a multi-year field campaign over West Africa and the tropical Atlantic. During 2003, AMMA received endorsement from the international CLIVAR and GEWEX projects within the WCRP and also has strong linkages with IGBP.

Planning for AMMA started in 2001, when the French community interested in West African monsoon (WAM) research produced a 5-year research plan that included a comprehensive description of scientific objectives and observational requirements. The main French funding agencies (CNES, CNRS/INSU, IFREMER, IRD, Meteo France) have since declared their support for this effort. Workshops have been held in the UK (June 2001), US (November 2001, November 2002), Niger (February 2002), Nice (April, 2003), and Cologne (July, 2003) to develop the AMMA Science Plan in an international context. A US-AMMA scientific steering group was formed in November 2002 to coordinate the US participation in AMMA.á A brief outline of the AMMA project, including the proposed US participation, is included in this summary.

2. WHY DO WE NEED AMMA?

There are multiple scientific and societal reasons why AMMA is needed at this time.

West Africa is a region that experiences marked variability in rainfall (Figure 1). The dramatic change from wet conditions in the 1950s especially and 1960s to much drier conditions in the 1970s, 1980s and 1990s in this region represents one of the strongest climate signals on the planet during last century. Superimposed on this multidecadal trend, marked interannual variations have resulted in extremely dry years with devastating environmental and socioeconomic impacts. Vulnerability of West African societies to climate variability is likely to increase in the next several decades, as demands on resources increase in association with one of the WorldÆs most rapidly growing populations. The situation also may be exacerbated by regional climate change. There is a strong societal need to develop strategies that reduce the socioeconomic impacts of WAM variability that will benefit from useful predictions of WAM variability and its impacts.

We are currently hindered in providing skilful predictions of WAM variability. This is due to a combination of factors. In addition to the large systematic errors exhibited by dynamical models used for weather and climate prediction and the sparse West African observing network, there are fundamental gaps in our knowledge of the coupled atmosphere-ocean-land 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.

Variability in West African weather and climate also impacts the rest of the world. Latent heat release in deep cumulonimbus clouds in the ITCZ over tropical Africa represents one of the major heat sources on the planet. Its annual migration and associated regional circulations impact other tropical regions, as exemplified by the known positive correlation between the interannual variability of West African rainfall and Atlantic hurricane frequency. While we know that a majority of tropical cyclones that form in the Atlantic originate from weather systems over West Africa, much less is known about the processes that account for this association and why only a small fraction of these ôseedingsö actually become tropical cyclones.

West Africa also is part of the worldÆs major source region of mineral dust aerosol. Given the great uncertainties regarding the impact of dust on weather and climate, there is an important opportunity to address aerosol issues within the AMMA project. Mobilization, transport, and impacts of aerosol on weather and climate in West African and Atlantic regions need to be investigated.

3. OBJECTIVES

AMMA has four broad internationally agreed objectives:

(i) Scientific knowledge

The major objective is to investigate the coupled atmosphere-ocean-land system processes that characterize the WAM with the aim of improving weather and climate prediction capabilities, and improving our confidence in climate change scenarios. The US will make a major contribution to this research, which will be undertaken in the following key interacting science areas:

A: Weather Systems and Processes: AMMA will strive to provide an improved understanding of the nature and variability of individual weather systems that comprise the WAM, focusing on mesoscale convective systems and African easterly waves over the continent and their fate and association with tropical cyclones downstream in the Atlantic.

B: Climate System and Processes: AMMA will investigate the key processes that influence variability and predictability of the West African monsoon on seasonal-to-interanual timescales. Along with a consideration of key global teleconnections (e.g. those associated with ENSO), special emphasis will be given to improving our understanding of the roles played by West African land surface conditions and the tropical Atlantic Ocean.

C: Aerosols: AMMA will investigate the chemical, physical, and radiative properties of aerosols, including their impact on West African regional weather and climate including the downstream tropical Atlantic. AMMA also will consider the aerosol sources and source processes.

(ii) Socioeconomic implications

A related objective of AMMA is to provide the data and underpinning science needed to characterise the impact of West African climate variability on water resources, food security, health, and to explore the feedback of human activities on climate variability.

(iii) Monitoring strategies

To achieve the above scientific and societal objectives, AMMA will recommend, and implement the multi-scale and integrated monitoring network needed to support research and prediction of WAM variability and its impacts. It is envisaged that satellites will play a strong role in this and so their potential needs to be evaluated fully.

(iv) Building capacity in partnership with African Institutions

AMMA will seek to develop blended training and education activities for African research and technical institutions as an integral part of its field operations. AMMA also intends to forge links with several education and outreach programs in the U.S. (e.g. GLOBE) and to involve these programs in the field phase in West Africa.

3. THE AMMA OBSERVING PERIODS

International discussions have led to the establishment of three interrelated observing periods in the AMMA program. It should be emphasized that enhancement of the observations during these different periods will provide a unique opportunity to evaluate models and satellite sensors and so determine future operational monitoring requirements necessary to provide improved weather and climate forecasts over West Africa and the downstream tropical Atlantic. The three planned observing periods are:

The Long Term Observing Period (LOP) is concerned with multi-year observations of the coupled atmosphere-ocean-land system. Two types of observations are envisaged: (i) unarchived historical observations, requiring data archaeology, to study interannual-to-decadal variability of the WAM and (ii) additional long-term observations (2001-2010) to document and analyze the interannual variability of the WAM and support research into its impact on health, food security, and water resources.

The Enhanced Observing Period (EOP) is designed to serve as a link between the LOP and the SOP (below). Its main objective is to strengthen observations along a climate transect in order to capture the annual cycle of the surface and atmospheric conditions and atmosphere and to study the surface memory effects at the seasonal scale. A major focus will be on improving the radiosounding network spanning the extreme climate gradient between the Gulf of Guinea and the Sahara Desert. The EOP will cover the period 2005-2007.

span>

The Special Observing Period (SOP) will provide a multiscale analysis of one monsoon season. As well as continuous monitoring of the WAM through the season, there will be more intensive observations of shorter duration that are concerned with key processes important for weather and climate. Focused process studies will take place during three key stages of the rainy season of 2006:

(i)á SOP-1 (May 15 û June 30):á Atmosphere-ocean-land interactions during the onset of the West-African monsoon, including a focus on Gulf of Guinea Ocean processes and SSTs.

(ii) SOP-2 (July 1 ûAugust 14): Mesoscale and synoptic scale weather systems over West Africa; water cycle in the atmosphere and at the land surface; continental ITCZ migration

(iii) SOP-3 (August 15 û September 15: Fate of mesoscale and synoptic scale weather systems downstream over the tropical Atlantic including their association with tropical cyclogenesis; nature of the oceanic ITCZ; role of Aerosol and the Saharan Air Layer

4. OBSERVATIONAL PLATFORMS

Current observing systems do not provide all the information needed to fully understand and quantify the multi-scale and multi-process interactions that characterize the West African Monsoon and its variability. Through international collaborations, AMMA will enhance the current observing network by coordinating extra observations over the continent and ocean. Over the continent, AMMA will take advantage of the long-term surface observations already in place there through the CATCH hydrological project (see Fig. 2). The AMMA observing strategy over the continent will provide an enhancement of the standard atmospheric observations along a north-south transect that includes the CATCH domain. For the tropical Atlantic ocean AMMA will take advantage of long-term observations already obtained from the PIRATA buoy array, ARGO floats, and XBTs (see Fig. 3). The AMMA observing strategy over the ocean will use ship platforms to provide enhanced measurements of the ocean and atmospheric conditions in the Gulf of Guinea and in the downstream tropical Atlantic where tropical cyclones typically form.

Considerable international effort is required to establish these enhanced observations, particularly for the SOP. In addition to the routine network and observations established through the EOP and LOP, the European countries have pledged remarkable contributions of observing systems to the SOP. This will include enhanced groundbased observations (e.g. surface stations, radiosoundings, wind profilers, polarimetric Doppler radars), several research aircraft (ATR, 1-2 Falcons, BAe-146 and possibly the Geophysica) and a research ship (EGEE). This will significantly supplement the US contribution to AMMA proposed in this document.

The US proposes to make significant observational contributions to al phases of the SOP. Over the continent, the US wishes to contribute to internationally agreed enhancements of the radiosonde network and will seek to provide radar (S-Pol and N-Pol) and research aircraft (NRL-P3 with ELDORA radar). For the ocean, the US wishes to provide enhanced oceanographic measurements (e.g. subsurface floats, surface drifters) and to deploy the RV Ron Brown (including radar, radiosoundings, surface fluxes, aerosol measurements) and research aircraft (NCAR C-130). The extra observations downstream of West Africa during SOP-3 will be coordinated with NOAA and NASA research aircraft. It is envisaged that aircraft will be based in Niamey during SOP-2 and Dakar or Cape-Verde during SOP-3.

5. CONTACTS

International AMMA

US AMMA