Trends of mean temperatures and warm extremes in northern tropical Africa (1961–2014) from observed and PPCA-reconstructed time series

摘要

Trends in daily maximum (TX) and minimum (TN) temperatures and indices of warm extremes are studied in tropical North Africa, west of the eastern African highlands, from 1961 to 2014. The analysis is based on the concatenation and cross-checking of two observed databases. Due to the large number of missing entries (~25%), a statistical infilling using probabilistic principal component analysis was applied. Averaged over 90 stations, the linear trends of annual mean TX and TN equal respectively +0.021 °C/yr and +0.028 °C/yr. The frequency of very hot days (TX>35°C) and tropical nights (TN>20°C), as well as the frequency of daily TX and TN above the 90th percentile (p90) (“warm days” and “warm nights”), roughly follows the variations of mean TX and TN, respectively. Heat spells of TX or TN>p90 are often short (usually <2–3 days), and the interannual variation of their mean duration is noisier than for the other indices. Nevertheless, heat spells tend to last longer, with almost constantly positive anomalies since the mid-1990s. The trends in March–June, the warmest season across the Sahelian and Sudanian belts, show similar variations as annual means. Overall, the local-scale warming in annual temperatures, and in March–June, may be viewed merely as a simple shift of the probability distribution function of daily TX and TN. The correlations between the thermal indices and the 2m temperatures suggest that the low-frequency (>8 years) variations may be viewed as a regional-scale fingerprint of the global warming, with largest correlations in the tropical Atlantic and Indian basins, while the high-frequency (<8 years) variations should be mostly viewed as a delayed remote impact of El Ni?o–Southern Oscillation (ENSO) events over the region, with warm (cold) anomalies tending to follow warm (cold) ENSO events.