What the average Brazilian knows.

In Brazil, we first learn about the Northeast (NEB) semi-arid region in our childhood. The histories of resistance and suffering of its population are many. In the same proportion, there are literary novels, poems and songs  using the semi-arid constraints upon life as inspiration. From Vidas Secas ('Barren Lives,' a book by Graciliano Ramos) and Morte e Vida Severina ('Severine Life and Death' by João Cabral de Melo Neto), to Asa branca, a song composed by Luis Gonzaga, the struggle of people living in arid regions is well known to every Brazilian. On the other side of the Atlantic, the West Africa (WAF) semi-arid is part of the Sahel, which may be the most notorious arid region of our planet.

What Physical Oceanographers know.

The Ocean modes are known to influence decadal climate variability. Several studies have shown how particular sea surface temperature anomalies can cause pressure and wind anomaly fields, thereby forcing decadal precipitation anomalies in continents adjacent to these modes and, sometimes, far away through teleconnections.

What our study unravels.

Our work started when, looking at different precipitation data sets, we found an anti-correlation of decadal precipitation between NE and WAF from 1979 to 2005. Then we started looking at the Ocean separating these regions and found a low frequency  climate variability that creates an anti-correlation of decadal precipitation between them.

Using the different Atlantic Ocean variables from 1979 to 2015 to generate a Self Organised Map (SOM), an unsupervised neural network algorithm, we created a correspondence map of the Atlantic Ocean data with the SOM feature space, which we can cluster. Each cluster corresponds to a particular snapshot of the Atlantic Ocean decadal variability (a.k.a system state). Studying how the Atlantic variables evolves in this feature space, we discuss the evolution of the different Atlantic Ocean modes and how they control the decadal precipitation anti-correlation between NEB and WAF.

Finally, using sea surface temperature reanalysis data from 1870 to 2019, we created another SOM, which was used to study the decadal modes of variability during the last one and a half centuries. The cluster of this SOM points to 50-year cycles of sea surface temperature anomaly patterns, indicating that the NEB and WAF precipitation anti-correlation seen in the satellite era may be a cyclic feature of the Atlantic Ocean decadal variability.