Abstract
Relationships between simultaneous occurrences of distinctive atmospheric easterly wave (EW) signatures that cross the south-equatorial Atlantic, intense mesoscale convective systems (lifespan > 2 hour) that propagate westward over the western south-equatorial Atlantic, and subsequent strong rainfall episodes (anomaly > 10 mm·day−1) that occur in eastern Northeast Brazil (ENEB) are investigated. Using a simple diagnostic analysis, twelve cases with EW lifespan ranging between 3 and 8 days and a mean velocity of 8 m·s−1were selected and documented during each rainy season of 2004, 2005, and 2006. These cases, which represent 50% of the total number of strong rainfall episodes and 60% of the rainfall amount over the ENEB, were concomitant with an acceleration of the trade winds over the south-equatorial Atlantic, an excess of moisture transported westward from Africa to America, and a strengthening of the convective activity in the oceanic region close to Brazil. Most of these episodes occurred during positive sea surface temperature anomaly patterns over the entire south-equatorial Atlantic and low-frequency warm conditions within the oceanic mixing layer. A real-time monitoring and the simulation of this ocean-atmosphere relationship could help in forecasting such dramatic rainfall events.
Highlights
The Northeast region of Brazil (NEB) is located between the parallels 01◦S and 18◦S and the meridians 35◦ W and 47◦W (Figure 1(a))
This study details a relationship between westward positive unfiltered relative vorticity over the southern tropical Atlantic, mesoscale convective systems (MCSs) occurrences over the western southern tropical Atlantic, and strong rainfall events over the eastern Northeast Brazil (ENEB)
These processes, named here “true alarms”, include a combination of atmospheric variables initially based on easterly wave (EW)/positive vorticity anomalies which initiate at east of 20◦W and propagate westwards with a mean velocity of about 8 m·s−1 reaching the eastern longitude of ENEB in a few days
Summary
The Northeast region of Brazil (NEB) is located between the parallels 01◦S and 18◦S and the meridians 35◦ W and 47◦W (Figure 1(a)). Crossing the south-equatorial Atlantic basin from West Africa to South America [9,10,11], the atmospheric easterly waves (EWs) constitute an important process in the seasonal rainfall regime over NEB These waves are often associated with clusters of well-defined mesoscale convective systems (MCSs) [12, 13] that propagate westward above the oceanic basin reaching the NEB coast generating large amount of rainfall. A simple diagnostic analysis was undertaken to estimate the relationship between strong precipitation episodes over ENEB and preconditioning occurrences of atmospheric and oceanic behaviors This preliminary investigation was conducted over the rainy season of a three-year period (2004 to 2006) that included a representative number of strong rainfall episodes (hereafter defined as rainfall anomaly >+10 mm·day−1).
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