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Abstract
The rainfall variability for the state of Alagoas, Northeast of Brazil, was evaluated based on the Standardized Precipitation Index (SPI). Harmonic decomposition was applied to 31 years-long (1960-1990) series of SPI, from 33 stations, to relate their modes of variability to El Niño-Southern Oscillation (ENSO) and the Atlantic Ocean sea surface temperature (SST). The most important harmonics identified by the spectral analysis had periods of 10-15 and 2-3 years, followed by other oscillations with smaller periods. The 10-15 years harmonic was associated with the Atlantic interhemispheric SST gradient (AITG), a cross-equatorial dipole which impacts the northeast region of Brazil by influencing the position of the intertropical convergence zone (ITCZ), leading to dry or wet conditions. The 2-3 years harmonic was consistent with the variability of ENSO events. The harmonic analysis is a powerful tool to identify the principal modes of variability of SPI. Although the magnitude of SPI is underestimated in some cases, this tool significantly increases the knowledge of the main drivers of rainfall and droughts in the region.