A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Abstract for the 2 nd International Scientific Conference of the Large Scale Biosphere-Atmosphere (LBA) Experiment, Manaus, July 7-10 2002.Inundations in the Llanos de Mojos (Bolivia, south western Amazon) andassociated atmospheric circulation features in South America.Josyane Ronchail, Luc Bourrel, Gérard Cochonneau, Eurides de Oliveira et Jean-LoupGuyot.HiBAm (Hidrology and Geodynamics of the Amazon basin) – Institut de Recherche pour leDéveloppement (IRD) / Agência Nacional de Energia Elétrica (ANEEL).IRD, CP 7091, Lago Sul 71619-970, Brasilia (DF), Brasil.josyane@aneel.gov.brInundations in the region of Trinidad (Llanos de Mojos, Bolivia) are associated with intenserainy episodes in the lowlands of Bolivia, as those of February 1992 (400 mm in 5 days),and with sustained positive rainfall anomalies as during the February 1992-January 1993period.Outgoing long-wave radiation (OLR), and low and high level geopotential height and windfields from NCEP-NCAR Reanalysis are used to compute composites for the days withintense rainfall in Trinidad (55 days with rainfall above 20 mm during the February 1992-January 1993 period, with exception of the winter episodes).At 200hPa, a wave-ridge pattern is observed with negative geopotential height anomaliesover the South Pacific Convergence Zone (SPCZ) and positive ones eastward, at mid andsubtropical latitudes. In the tropics, a negative geopotential height is observed overnortheastern Brazil while the Bolivian High is slightly enhanced.Near the ground (850hPa), the south Atlantic subtropical High is stronger than usuallywhile negative height anomalies prevail over a great portion of the South Americancontinent and especially over the southwestern part of the Amazon basin and in centralBrazil. Over these regions, westward and northward abnormal winds, consistent with thegeopotential height anomalies, show that an enhancement of the perturbations from theAtlantic and from the extra-tropics may give rise to strong convection and heavy rainfallover the southwestern region of the Amazon basin. This is confirmed by the analysis of theOLR anomalies.