The Saharan Oscillation Index (1950-2024): Statistical Characterization, Persistence, and Spectral Variability

This study presents a comprehensive statistical and spectral characterization of the Saharan Oscillation Index (SaOI), an atmospheric circulation metric describing the standardized sea-level pressure difference between the Azores High and the Saharan depression over the 1950-2024 period. Using monthly SaOI values derived from ERA5 reanalysis, we assessed distributional properties, temporal persistence, spectral variability, time-frequency structure, and long-term trends using complementary time-series methods. The SaOI exhibits a near-Gaussian central distribution (µ = 0.1954, σ = 0.1137). Although the original monthly series shows moderate lag-1 autocorrelation (r ≈ 0.50), lag-1 autocorrelation is substantially reduced after removal of the climatological seasonal cycle (r₁ = 0.128), indicating weak residual month-to-month dependence. Fourier and wavelet analyses identify the annual cycle as the dominant component of variability, with a weaker semi-annual component, whereas no distinct persistent narrow-band interannual oscillation is evident. A weak positive long-term trend is detected (Sen's slope = 0.0004 units yr⁻¹), but it is not statistically significant after accounting for serial dependence (p ≈ 0.17). These results establish a reproducible statistical baseline for interpreting SaOI variability and provide a foundation for subsequent investigations of its teleconnections and regional climate impacts.