Impacts of the extremes of the Southern Oscillation in Amazonia. Part 2. Circulation, convection and Sea Surface Temperatures
Jose A. Marengo, Alice M. Grimm, Paulo Zaratini
Abstract
The influence of the extremes of the Southern Oscillation (SO) in the precipitation over Amazonia, is assessed through the NCEP reanalysis, OLR fields and sea surface temperatures in both tropical Pacific and Atlantic. The spatial and temporal distribution of this influence along the cycles of these events is studied. Part 2 deals with circulation, SST and OLR aspects. It has been observed that during the El Niño, the near surface Atlantic trader winds are weaker, consistent with an anomalously northward displaced Intertropical Convergence Zone (ITCZ), thus the moisture input from the Atlantic into the Amazon basin is weak, the moisture convergence and convection is also weak over Amazonia, and lower rainfall is observed specially in the central and mouth of Amazon River regions. The SST dipole in the tropical Atlantic indicates anomalously warmer surface water to the north.
During La Niña, these patterns are opposite, with a southward displaced ITCZ, strong Atlantic trades and moisture transport into Amazonia, and an inverted SST Atlantic dipole. On western part of the basin, there is a descending branch of the Walker cell during El Niño, with subsidence affecting a zonal band from the Andes to the Atlantic. The large positive SST anomalies from the Pacific NIÑO 3 region, is also accompanied by strong upward flow and convection over northern Peru (west of the Andes), implying compensatory subsidence and reduced rainfall on the other side of the Andes (western Amazonia). The pattern is somewhat slightly inverted during La Niña, but not as intense as during El Niño. Southeast Amazonia rainfall seems independent from the extremes of the SO, and the moisture from the Amazon basin feeds convection and rainfall on this region, which is also an integral components of the South American monsoon.
At upper levels, there are signals of a weaker Bolivian tropospheric high during El Niño, while the subtropical westerly jet were accelerated and located anomalously northward of its average position, while during La Niña, the jet is weaker and anomalously southward of its average position.
Key words: Southern Oscillation, circulation, El Niño, convection, Amazonia