Variability of Atmospheric Moisture during the Boreal Spring in West Africa

The present study seeks to understand the variability of the atmospheric moisture during the boreal spring over West Africa. The West Africa Monsoon (WAM) is a vital source of rainfall for the Sahelian region of Africa and an important aspect of the socio-economic environment of the region. In addition to the agricultural benefit incurred by the rains, there is also an important interaction with the public health sector due to the monsoon onset's relationship with the cessation of meningitis outbreaks in West Africa. Understanding the variability of moisture during the boreal spring is currently unmet in West Africa and is the focus of a wider Google.org/ University Corporation for Atmospheric Research (UCAR) project. This study will serve as a compliment to work currently under development at UCAR by characterizing the sources of moisture, as well as circulation patterns and relative influences from the tropics and mid-latitude systems that affect the variability of the region.

Given the influence that the onset of monsoon rains has on meningitis epidemics, the present study explores the main characteristics of Sahelian climate leading up to the influx of moisture. The preonset period of the WAM is defined as the arrival in the intertropical front (ITF) at 15 N, where the confluence line between moist southwesterly monsoon winds and dry northeasterly Harmattan occurs. In particular, the role of sea surface temperatures (SSTs) adjacent to the region is investigated, as well how teleconnections with mid-latitude systems, equatorial waves, and global circulation patterns modulate the moisture content of the region on an interannual scale. Further, this study explores the role of regional dynamics of the Saharan heat low and the African Easterly Jet. along with transient disturbances such as mid-latitude systems and westward-propagating convective systems have in dictating intraseasonal variability of the moisture front dynamics.

Initial results using parcel back-trajectory analysis has provided new insight of the pathways for atmospheric humidity over the Sahel during the dry season, the transition period, and the early monsoon. Air masses can be traced to upper levels of the atmosphere from the mid-latitudes during the Harmattan, and to lower level parcels traversing the South Atlantic and Gulf of Guinea regions during the monsoon.

Preliminary results for this study include model configuration and optimization over West Africa, real-time ensemble forecasts for the relevant variables during the 2009 season, parcel back-trajectory analysis for a matrix of end points over West Africa, as well as the analysis of important predictors of surface moisture such as winds at 925mb, SSTs in the Gulf of Guinea and the Eastern Tropical Atlantic Ocean, convectively-coupled equatorial waves and Mid-Latitude influences. The model is able to capture intraseasonal as well as diurnal variability, both which are highly important in the study of the WAM.

We find that westward-propagating systems in late spring and boreal winter mid-latitude systems have an important influence in the modulation of moisture regime at latitudinal and temporal scales. The implication of transient convective systems on surface moisture is evident in our analysis and WRF allows us to trace a precipitation event on May 7-8, 2009 at Niamey, Niger as well as the associated spike in moisture to a westward-propagating disturbance. This disturbance is also evident in the NCEP/NCAR reanalysis as well as observation reports from Kano, Nigeria and Bamako, Mali. Further, initial results also show that this feature is associated with a Madden-Julian Oscillation (MJO) event. As each one of the transient disturbances progresses, the net amount of moisture at the surface increases to a higher regime. Similarly, dry-season precipitation associated with forcing from the extratropics tends to modulate surface moisture and can have an impact on the onset of higher humidity as spring transitions into summer. Ongoing efforts are aimed to further explore these mechanisms along with other aspects of the intraseasonal variability of the moisture front.