Influência dos Oceanos Pacífico E Atlântico na pré-estação chuvosa no estado do Ceará

Abstract

The pre-rainy season (PRS) in Ceará, which includes December and January, represents an intraseasonal period of high climatic and socioeconomic relevance, as it marks the transition between the dry season and the rainy season. Despite its importance, this interval still shows high temporal variability and low predictability, especially when compared to the classical months of the rainy season (RS). Therefore, this study aimed to investigate the influence of the Pacific and Atlantic Oceans on the occurrence and persistence of rainfall during the PRS. This research was mainly motivated by the recurrence of significant rainfall episodes in December and January that do not clearly fit the oceanic signals traditionally used in climate prediction, raising questions about the dominant mechanisms in this transition period. For this purpose, time series and spatial fields of precipitation and sea surface temperature (SST) were analyzed, as well as atmospheric variables in low and mid-levels, allowing an integrated assessment of ocean, atmosphere, and rainfall. The results indicate that, during the PRS, the statistical relationship between precipitation in Ceará and SST variability in the Niño 3.4 region is weak and spatially inconsistent, especially for January precipitation. This behavior suggests that, unlike what is observed during the RS, the influence of the El Niño–Southern Oscillation (ENSO) does not manifest in a direct or dominant way in this transition period. A similar behavior occurs for the tropical South Atlantic area in relation to December precipitation. In contrast, the atmospheric analyses show that the occurrence of more persistent rainfall is mainly linked to the presence of ascending motions at mid-levels and to low-level convergence over northeastern, favoring convective organization and the maintenance of precipitating systems. Additionally, the relative importance of the South Atlantic stands out in the context of its relationship with the PRS and atmospheric indicators. SST anomalies in this region showed a higher, yet still low, correlation with rainfall distribution and persistence, especially in January, which precedes the establishment of the RS. This Atlantic region is recognized as an area of origin and intensification of the South Atlantic Convergence Zone (SACZ), whose concave configuration allows the formation of a circulation flow that may extend toward the northern Northeast Brazil (nNEB), which can be clearly observed in the mid-level anomaly maps for each selected rainy year. This arrangement favors moisture channeling and contributes to the establishment of convection over Ceará, supporting positive rainfall anomalies even when the surrounding thermal environment is not favorable to sustain this scenario.

Thus, the results reinforce that this climatic transition period has a strong connection with atmospheric systems that occur through interactions with the South Atlantic and over the Brazilian territory, while the influence of SST in both oceans appears limited. This finding emphasizes the need for integrated approaches to understand and predict rainfall during the PRS, expanding the understanding of the dynamics that precede the RS in the nNEB.