6D.4 VARIABILITY OF ATMOSPHERIC MOISTURE DURING THE BOREAL SPRING IN WEST AFRICA Roberto J. Mera1,*, Arlene Laing2, and Fredrick H.M. Semazzi1 1North Carolina State University, Raleigh, North Carolina 2National Center for Atmospheric Research, Boulder, Colorado 1. INTRODUCTION explain 25% of the year-to-year differences in meningitis outbreaks in Niger. Thomson et al. (2006) found that 1.1 Motivation anomalies in annual meningitis incidence at the district level in Burkina Faso, Niger, and Mali were related to Weather and climate in the African Sahel are monthly climate anomalies. Although questions remain dominated by two major wind systems, the about the initiation of outbreaks, a clear pattern has southwesterly West African Monsoon (WAM) and the emerged in the cessation of the disease, brought about northeasterly (Harmattan) trade winds. As the primary by the advent of moist conditions. driver for precipitation, the WAM is a vital component of While there is no consensus on which measure of the socio-economic environment of the region. In humidity and other environmental variables correlate addition to the agricultural benefit of the rains, the public best with meningitis incidence, a few studies provide health sector is affected due to the relationship between some guidance. For example, Greenwood et al. (1979) the onset of monsoon moisture and end of meningitis used absolute humidity and mean maximum outbreaks (Molesworth et al., 2003). Knowledge and temperature while Thomson et al. (2006) examined prediction of moisture distribution during the boreal rainfall, satellite-derived dust loading, and vegetation spring is vital to the mitigation of meningitis by providing indices. Besancenot et al. (1997) identified three guidance for vaccine dissemination. Yet the boreal meteorological conditions associated with the spring has received little attention in the literature; in Harmattan regime and meningitis epidemics: minimal contrast the summer monsoon has been the subject of mean temperature of no more than 20 °C, a mean numerous studies (Reed et al. 1977, Sultan and Janicot relative humidity of no more than 40%, and the 2003, Kiladis et al. 2006) and field campaigns such as presence of at least three days of dust haze. The the Global Atmospheric Research Program (GARP) present study uses the 40% surface relative humidity Atlantic Tropical Experiment (GATE, Kuettner and (RH) as a threshold for alleviation of epidemic Parker 1976), and the African Monsoon Disciplinary conditions. Analysis (AMMA, Redelsperger et al., 2006). Providing information on the predictability and Much of what is known about the outlook for meteorological conditions that have an climate/environmental factors relevant to meningitis has appreciable correlation with the cessation of disease been based on annual and seasonal data. Meningitis epidemics will guide allocation of scarce vaccines to epidemics tend to occur in sectors of West Africa that regions likely to remain dry. Our study complements exhibit particular environmental characteristics. Ongoing ongoing research at the University Corporation for research indicates that along with moisture (relative and Atmospheric Research (UCAR) by evaluating the skill of absolute humidity), incidence of the disease can also be the Weather Research and Forecasting Model (WRF) at affected by land-cover type (e.g. arid, semi-arid, tropical spatial resolutions (district level) and timescales that are forest), dust loading in the atmosphere and surface relevant to meningitis management in the region. temperature. The most robust and actionable climate/meningitis relationship arises from the strong 1.2 The pre-onset of the monsoon and the correlation between the start of the rainy season and the Intertropical Front (ITF) abrupt decline in the transmission of the disease (Molesworth et al., 2003). The critical variables Meteorological signals prior to the start of the associated with meningitis outbreaks are very low WAM are weak at a regional scale because rainfall humidity and dusty conditions (Besancenot et al 1997); initiation over the Sudano–Sahelian zone is seldom while cessation occurs with the onset of rains abrupt in nature and it is usually preceded by a (Molesworth et al. 2003). Sultan et al. (2005) found a succession of isolated precipitation systems of uncertain strong correlation between the timing of the epidemic intensity interspersed with dry periods of varying onset in Mali and the winter wind maximum but could duration (Omotosho et al., 2000; Ati et al. 2002). In not differentiate low and high incidence years. Yaka et order to understand the persistence, strength and al. (2008) found that variations in surface winds can variability of the early season WAM rains, it is important to analyze the behavior of the ITF. As introduced by *Corresponding author address: Roberto J Mera, North Hamilton and Archbold (1954), Eldridge (1957), Hare Carolina State University, Dept. of Marine, Earth and (1977), and later expanded on by Sultan and Janicot Atmospheric Sciences, Raleigh, NC 27695; e-mail: (2003) and Lélé and Lamb (2009), the ITF is an integral [email protected] part of the buildup and retreat of the WAM. Lélé and Lamb (2009) postulated that the location of the ITF in The aim of this study is to describe the variability April could be used as a predictor for the advancement and prediction of surface moisture in West Africa using of the leading edge of the southwest monsoon flow simulations, global model analyses, satellite during the May-June period. Sultan and Janicot (2003) observations, and local station observations. We will diagnosed this critical feature as one where the first diagnose some fundamental dynamical characteristics isolated precipitation systems occur ahead of the WAM. of the boreal spring in West Africa under certain critical The definition of the ITF, in terms of the variables synoptic and climatological states. From this we will used to derive its location and behavior, is calculated infer the meteorological factors pertinent to meningitis differently in recent studies. Lélé and Lamb (2009) used mitigation at intraseasonal timescales. daily temperature, humidity, and rainfall data at 10-day (dekad) resolution to calculate concurrent monthly ITF- 2. DATA AND METHODS rainfall relations. In Sultan and Janicot (2003), the ITF is defined by the 925 hPa zero isoline of the zonal wind 2.1 Data, Domain, and Period of Study (where the westerly monsoon winds begin). Another marker of the ITF is the 15°C dewpoint temperature The study domain is centered on West Africa and (Pospichal et al. 2006). This paper uses mostly the large enough to encompass cross-equatorial and Sultan and Janicot’s (2003) definition of the ITF since midlatitude influences that may affect regional moisture they also identify short rains at the northern boundary as variability (Fig. 1). The simulation periods are January to an essential feature in the progression of the WAM. The August of 2000 to 2009. Synoptic climatologies for April Intertropical Convergence Zone (ITCZ) is most often and May of each year were developed using the identified by the band of precipitation to the south of the National Centers for Environmental Prediction (NCEP)/ ITF. National Center for Atmospheric Research (NCAR) The ITF, which is also defined as the Intertropical Reanalysis or NNRP (Kalnay et al., 1996). We chose Discontinuity (ITD), is strongly affected by the variability 2006 because of the significant amount of station of the Saharan Heat Low (SHL). The SHL is an observations during AMMA (Redelsperger et al., 2006) extension of the equatorial trough and the pressure and we use the station in Niamey, Niger (13°27'N, gradient between the SHL and the subtropical high 02°06'E) to analyze model results at a specific location. controls the strength of the Harmattan. Recent studies Standard meteorological surface observations are suggest that the monsoon onset is partly controlled by used to diagnose the position of the ITF, the shift to the the SHL dynamics (Sultan and Janicot, 2003; Drobinski monsoon moisture regime, and for validation of the et al., 2005; Ramel et al., 2006; Sijikumar et al., 2006). model simulations. Daily mean outgoing longwave Unusual dry season rain events are a potentially- radiation (OLR) data with a spatial resolution of 2.5° predictable source of precipitation during the dry season (Liebmann and Smith 1996) are used as a proxy for that could affect the progression of the ITF and WAM. convective activity. Rainfall observations are the In Knippertz and Fink (2008), their case study of a Tropical Rainfall Measurement Mission (TRMM) Multi- January precipitation event in West Africa associated satellite Precipitation Analysis, a gauge calibrated 3- with a mid-latitude disturbance in 2004 occurs in a year ° ° that saw the ITF arrive at 15°N 23 days in advance of hourly dataset with a 0.25 x 0.25 grid (Huffman et the climatological arrival of this dynamical feature. The al. 2007). Mixing ratio at 925 and 850 hPa from the connection between the arrival of ITF (and the Atmospheric Infrared Sounder (AIRS) instrument associated short convective systems) and feedbacks (Aumann et al. 2003) are used to validate simulated from short term events during the dry season can have lower-tropospheric moisture. A parcel back-trajectory important impacts on the initiation and evolution of analysis utilizing the horizontal and vertical wind convection during the dry-to-rainy season transition. components, as well as RH (%) from the NNRP, is Moisture and precipitation over West Africa is employed to compute the sources of air parcels for the associated with synoptic disturbances including African points bounded by 10°W-10°E and 10°N-15°N for the easterly waves, the dominant synoptic system during years 2000-2008. The 925 hPa level was chosen as the the boreal summer (Reed et al. 1977; Diedhiou et al. endpoint surface to circumvent noise generated in the 1999; Kiladis et al. 2006; Mekonnen et al. 2006). reanalysis below 925 hPa, where parcels can intercept Convection is also modulated by intraseasonal the ground.