Analysis of small scale wave over the West African area

Characteristics of small scale wave (λ_v < 4 km) are analyzed in the lower stratosphere (19-23 km) above the tropical and equatorial West African zone (4°N–20°N and 20°W–15°E). First, nine years climatology of seasonal variations of small scale wave activity is derived from radiosondes data. The annual cycle of total energy density above the tropical sites confirms a peak of activity at the intense period of convection and weak values during the dry period (Kafando et al., 2008). Time series of monthly total wave energy density is linked with the phase of Quasi-Biennal Oscillation (QBO) and monsoon proxies (OLR, relative humidity,…). Wave activity during the WAM is stronger during eastward phase of the QBO. Above the equatorial area, two peaks of enhanced activity matching with the period of intense convection are observed. The analysis of wave activity during 2006 and 2007 clearly reveals a similar relationship with the phase of the QBO as observed above tropical sites. Secondly, wave spectral characteristics (period, vertical and horizontal wavelengths) are determined for the WAM period from May to September in 2006 (AMMA campaign) using the Stokes parameter technique (Chane-Ming et al., 2009). Result suggest the presence of inertia-gravity wave over the tropical area with periods of 1.1–2.1 days and horizontal wavelengths lying from 1300 km to 3900 km. Mixed tropical waves are detected over the equatorial zone with periods of 2.1–5.1 days and horizontal wavelengths of 1400–5200 km.