Seasonality of wind speed, wind shears and precipitation over West Africa

Abstract

This study analysed NCEP/NCAR reanalysis datasets of wind speed components at multiple pressure levels from 1956 to 2015 with a view to examining seasonal variations in wind shears in relation to convective precipitation over West Africa. It also estimated the West African Monsoon (WAM) wind shear index and related it to the WAM dynamics during different seasons (December to February: DJF; March to May: MAM, June to August: JJA; September to November: SON). The computed wind shear indices were compared with those derived from higher-resolution datasets of ERA5. Results revealed very similar spatial and seasonal distributions of wind patterns and significant positive correlations (0.75 ≤ r ≤ 0.99; p ≤ 0.05) between NCEP and ERA5 over West African onshore region. We obtained both positive and negative trends in wind shears (−0.001 to −0.05 units per year) and precipitation (−1.5 to −3.5 mm per year) over the entire study period and region. However, the trends were found to be statistically insignificant at 95% confidence level. The WAM wind shear index (WAMI) effectively captured the northward migration of the monsoon system over West Africa with peak values in JJA season. We established significant and strong associations between wind shear indices and precipitation climatology of the Climate Research Unit. The deep layer shear (0–6 km above ground level) and WAMI gave significant and positive correlation (0.75 ≤ r ≤ 0.95) with convective precipitation while fluctuation in precipitation was explained by 50–75% variations in these indices over most parts of the monsoon region. We concluded that convective precipitation was significantly influenced by seasonal variations in wind shears over West Africa.