On Northeast Brazil Orographic Enhanced Rainfall and Monsoon Dynamics

Comprehensive climatologies are essential for decision making processes in regions with recurrent droughts and increasing flood risk, such as Northeast Brazil. Therefore, our work proposes an updated rainfall climatology for Northeast Brazil (1990–2020), discussing the possible role of orography and monsoon dynamics on this climate system. We used ERA5 surface-wind and sea-level pressure, the TOPODATA altimetry and the Brazilian gridded weather BR-DWGD datasets. We also incorporate a statistical analysis of continental and oceanic surface winds and rainfall dynamics at the Araripe Plateau. Our work identifies multiple areas affected by orographic enhanced rainfall according to altitudes above 550 m above sea level. Two major orographic enhanced rainfall systems are identified at the Araripe Plateau and the Diamantina Plateau. Moreover, in southern Northeast Brazil, orography was noted to limit the incursion of the monsoon rainfall associated with the South Atlantic Convergence Zone (SACZ). In addition to the SACZ, our analysis indicates that the South American Monsoon System (SAMS) has a large influence on Northeast Brazil rainfall, reversing surface-wind fields during the summer. This monsoon influence leads to the formation of the Northeast Brazil Convergence Zone (NBCZ), an atmospheric feature which we suggest occurs zonally oriented over the southern portion of the Borborema Amphitheatre near 7.5° S, between November and April. The NBCZ acts as a dynamical barrier that limits the meridional flux of moisture between northern and southern Northeast Brazil. This is especially pronounced along the Araripe Plateau. The autumn monsoon rainfall occurs between February and April enhanced by the influence of the nearby ITCZ, which increases moisture availability on the northern coast. The drier period in most of Northeast Brazil occurs between May and October, when the region is under the influence of a high-pressure system, the South Atlantic Anticyclone, which limits rainfall predominantly to the eastern coastal region.