A detailed limited-area energy cycle for climate and weather studies: application over the West African climate during three contrasting summers 1997 (dry), 1999 (wet) and 2006 (normal).

Abstract The West African Monsoon (WAM) is an atmospheric circulation that involves a complex chain of interacting physical processes ranging from small to large temporal and spatial scales. In this work, a detailed atmospheric energy cycle is formulated for limited-area domains in order to understand the WAM intra-seasonal and interannual variability. The energy cycle is cast in terms of available enthalpy and kinetic energy reservoirs. According to the employed definition, the time-averaged (climate) energy reservoirs are decomposed in a component associated with the time-averaged atmospheric state and a component due to the time-averaged statistics of transient. With this approach, a storm energy is defined as the deviation of instantaneous energy from its climate value. The application of energy cycle on the simulated WAM climatology reveals that the available enthalpy of the time-averaged state (AM) is the largest energy reservoir, while the transient-eddy component (AE) is the smallest. On the other hand, the time-averaged and the time-variability kinetic energy reservoirs (KM and KE) are the same order of magnitude, confirming previous studies linking the African Easterly Waves with a mixed barotropic/baroclinic mechanism. A detailed analysis reveals that, in the time-mean state, there is a huge loss of energy flux due to time-averaged pressure work in boundary fluxes, with the result that only a little fraction of AM associated with the ageostrophic circulation contributes to the generation of KM. On the other hand, the loss of AE contributes almost entirely to the gain in KE, indicating negligible loss due to transient-eddy pressure work.