Mechanisms for the climatological characteristics and interannual variations of the guinea coast precipitation: Early summer west african monsoon

Abstract

This study presents the climatological characteristics and physical mechanisms of Guinea Coast precipitation in June. Traditionally, the low-tropospheric air temperature and equivalent potential temperature (θe) play crucial roles in the generation of monsoon precipitation through the following mechanisms: 1. Near-surface atmospheric front, depicted by steep δθe/δy, corresponds to the vertical motion in the lower troposphere. 2. Strong easterly wind in the middle troposphere (600-500 hPa), generated by a steep δT/δy near the surface at 12° N, induces a positive vorticity to the south and vertical motion over the Guinea Coast (~5° N). Meanwhile, the strong Guinea Coastal precipitation, in association with the interannual variability, is mainly determined by the sea-surface temperature (SST) anomaly in the eastern equatorial Atlantic Ocean. In years of warm SST in the eastern equatorial Atlantic, δθe/δy in the lower troposphere is less pronounced than in normal years. However, the atmospheric buoyancy (moist static instability) increases, owing to a strong vertical θe gradient (δθe/δp) arising from the increase in moisture and warm temperature in the lower troposphere over the warm SST area. Consequently, the eastern equatorial Atlantic warm SST modulates the Guinea Coastal thermodynamic structure, causing deep convection that increases precipitation south of the Guinea Coast. Forced by the eastern equatorial Atlantic warm SST anomaly, the strong precipitation and corresponding atmospheric structures are successfully simulated from the Geophysical Fluid Dynamics Laboratory global atmosphere model 2.1.