Tropospheric Circulation Features During Wet and Dry Years over West Africa.

Tropospheric Circulation Features During Wet and Dry Years over West Africa

Volume 7:1
Ewanlen* Rufus, Shou Yixuan**, Shou Shaowen*
*Key Laboratory of Meteorological Disaster, Nanjing University of Information Science & Technology, Nanjing 210044, P. R. China
**Key Laboratory of Radiometric Calibration and Validation for Environmental Satellites, China Meteorological Administration (LRCVES/CMA) National Satellite
Meteorological Center, Beijing, 100081, P. R. China.
(Manuscript received 20 October 2012, in final form 27 May 2013)


This paper describes a composite study comparing some features of the tropospheric circulation over the region of West Africa in the monsoon season for “dry” (1981-85) and “Wet” (1998-2002) selected years. Zonal and meridional components of the monthly mean wind are computed directly with NCEP reanalysis data for the main rainy season of May, June, July and August. The potential temperature and moisture variables are also examined for the periods. The results of our analysis show major contrasts between the characteristics of tropical troposphere in West Africa during the two scenarios. In the (dry) years 1981-85, the upper and lower troposphere are characterized by weaker flows in coexistent with stronger Africa easterly jet (AEJ) and a southward shift of the major circulation patterns as well as numerous dynamical parameters. The observed weak tropical easterly jet (TEJ) and vertical alignment of the AEJ and TEJ axes enhance the rainfall deficit in dry years. In contrast, during the (wet) years 1998-2002, the troposphere is dominated by stronger flows at lower and upper levels in concomitant with a weaker AEJ and intense northward shift of the zonal and meridional cells circulations. The observed strong vertical coupling between the zonal component anomalies at lower levels (monsoonal flow) and upper levels (TEJ), reflect the opposition in direction of mean winds at these levels. Over the region, the latitudinal location and intensity of the AEJ seems to be imperative, and tend to regulate the instability mechanisms. The instability is observed to be greater in August than in May in all the years. The contrasting convergence of monsoonal flow and the upper divergence associated with TEJ is linked with the activeness of the walker-type cell during the periods. At 925hPa lower level, the Inter-Tropical Discontinuity (ITD) surface positions in the region generally appear to be almost in same locations for both dry and wet years. As consequence, the length of the rainy season becomes unvarying. Instead, the wet period 1998-2002 are well dominated by more extreme rainfall. Finally, it is evident that the convective process over the tropical region of West Africa is dynamically influenced by strong upper-level divergence at TEJ level associated with ascent in the lower troposphere, where the disturbances build-up. Contrasts in the monsoon layer between wet and dry years may be related to these fluctuations. The results from this study serve as bases for further work on the tropospheric circulation features over the region of West Africa for an enhance understanding of the regional climate.
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