Influence of Biannual to Multidecadal Time-scale Forcings on Rainfall Recorded in Porto Alegre, Brazil

The city of Porto Alegre, located in the extreme south of Brazil, is one of the country’s most important urban centers. Between late April and early May 2024, it experienced the most severe flood in its history, resulting in significant infrastructural, financial, and human losses. Owing to its geographic location, several forcing factors have been identified as key modulators of the region’s climate. These include internal factors—related to oceanic and atmospheric changes—and external factors—associated with solar activity and variability in the Earth’s magnetic field. This study investigates the influence of both internal and external natural forcings on the variability of annual rainfall in Porto Alegre from 1916 to 2024. Using lag cross-correlations, spectral coherence analysis based on wavelet transforms, and partial lag cross-correlation, the research examines how climate drivers such as the El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and solar activity—including the 11-year solar cycle and the 22-year solar magnetic cycle—modulate precipitation. Results indicate that ENSO exerts the most consistent and linear antiphase influence on rainfall in the 2–7-year periodicity range. The PDO also shows persistent coherence with rainfall, particularly in multidecadal bands, often exhibiting in-phase relationships. The influence of solar cycles is more intermittent and predominantly occurs through complex mechanisms, although certain epochs demonstrate significant coherence, especially for the 22-year cycle. Additional analyses suggest that the PDO may mediate interactions between ENSO and solar activity. Overall, the findings highlight the complex interplay between terrestrial and extraterrestrial climate drivers and underscore the importance of considering both in regional hydrometeorological studies, particularly in areas affected by the South Atlantic Magnetic Anomaly. Graphical Abstract The graphical abstract represents the research entitled “Influence of Biannual to Multidecadal Time-Scale Forcings on Rainfall Recorded in Porto Alegre, Brazil.” The framework of the study is the heavy flooding that has occurred in Porto Alegre in the last one hundred years, which has caused great losses. It portrays external forcings, such as solar wind, interacting with the geomagnetic field in the region of the South Atlantic Magnetic Anomaly (SAMA). The methodology consisted of gathering data on internal forcings, represented by the Southern Oscillation Index (SOI) and Pacific Decadal Oscillation (PDO), and external forcings represented by the 11- and 22-year solar cycles, together with the annual rainfall totals of Porto Alegre. The analytical tools used were lag cross-correlation, Wavelet-based Spectral Coherence (WTC), and partial lag cross-correlation to analyze the investigated time series between the years 1916 and 2024. From the obtained results, the graphical representation leads to the conclusion that precipitation is modulated by a complex interaction between terrestrial forcing (ENSO, PDO) and extraterrestrial forcing (solar cycles).