South Atlantic convergence zone and ENSO occurrence in the 2000–2021 period

Abstract

In this study we defined the association of SACZ episodes with ENOS phases during the 2000–2021 summer seasons, considering November to March months, and the circulation associated patterns. Each SACZ episode was classified when OLR values were below 220 W m^− 2 and precipitable water values above 45 kg m^− 2 for more than three consecutive days. The association between ONI and the annual number of days with SACZ and the number of SACZ episodes shows linear correlation of -0.44 and − 0.34, respectively, showing the prevalence of SACZ episodes during the La Niña phase. Analysis of the entire series shows a linear annual mean increase of ~ 16 days with SACZ episodes from 2000 up to 2021. Sea level pressure anomalies between El Niño and La Niña periods present meridional dipole patterns between northern and southern South Atlantic, including the southamerican continent. OLR anomalies fields present negative (positive) values during LN (EN) periods over the northern South America extending to SACZ areas, helping to explain the higher number of SACZ episodes in LN (63 episodes) than in EN (29 episodes) periods. Both SLP and OLR anomalous patterns are associated with higher moisture convergence in the SACZ area in LN than in EN periods. Analysis of very dry (2014–2015) and very rainy (2020–2021) summer seasons over southeastern South America, illustrating El Niño and La Niña periods, respectively, shows strengthened upward movement over southwestern South America and weakened upward movement over northeastern in the former summer season and the opposite signals in the second one. These patterns were associated with typical circulation at low and high tropospheric levels: the upper level cyclonic vortex and the subtropical South Atlantic high pressure displacement to continental areas during the very dry period, 2014–2015, and the displacement of both systems, in upper and low levels, to the ocean during the very rainy period, 2020–2021.