Interannual and longer‐term variability of upper air circulation in the northeast Brazil‐tropical Atlantic sector

Abstract

The NCEP‐NCAR 1958–1997 upper air data set along with surface ship observations have been analyzed for the March‐April extremum of the annual cycle in the tropical Atlantic sector and core of the Northeast Brazil (Nordeste) rainy season. (1) In the long‐term mean, the Intertropical Convergence Zone (ITCZ) overlies a band of warmest surface waters and is accompanied by ascending motion and upper tropospheric divergent outflow directed mainly into a band of upper tropospheric convergence and subsidence over the tropical North Atlantic. (2) During years of extreme drought in the Nordeste the band of warmest surface waters is displaced northward, and concomitant with this is a northward shift of the strongest lower tropospheric convergence, ascending motion, and upper tropospheric divergent outflow. (3) During the low/warm as compared to the high/cold (SO) phase the bands of warmest surface waters, most intense convection, and largest lower tropospheric convergence, ascending motion, and upper tropospheric outflow are all shifted northward, similar to the pattern characteristic of the Nordeste drought years. (4) Long‐term trends are documented by a southward displacement of the bands of most intense convection, largest lower tropospheric convergence, ascending motion, and upper tropospheric divergence, and increasing cloudiness over the equatorial Atlantic. The study thus substantiates the upper air processes instrumental for the interannual and decadal‐scale variability in the surface climate of the region.