Long-Term Behaviour of Temperature in the Lower Atmosphere of Niamey a West African Tropical Station

This paper highlights the global warming of the lower atmosphere of West African tropical area using in-situ data. The study is based on the analysis of 500-m interval vertical profiles of radiosonde temperature above Niamey (13.47°N;2.16°E) a sub-Saharan meteorological station. The annual cycle of surface temperatures shows seasonally two peaks located in April/May and in October and two minimum in December/January and August respectively. In the mid-troposphere (between 5 km and 10 km height), time series of monthly mean temperatures from January 2001 to December 2018, shows an annual variability with a slight downward trend of -0.036°C per decade. In the lower stratosphere (25 - 30 km altitude) a cooling of -0.64°C/decade is observed. Temperatures time series also highlight the presence of two breaking points associated with the El Niño-Southern Oscillation (ENSO) events. When performing a separation regarding Southern Oscillation Index (SOI) time series, model parameters of the linear regression indicate a tropospheric warming during the neutral and La Niña phases and a stratospheric cooling. The analysis of the lower stratosphere zonal wind highlights different behaviours of the quasi-biennial oscillation (QBO) during the neutral and La Niña phases.