Climate variability in Southern South America ssociated with El Niño and La Niña events.
Part I: Precipitation anomalies.
Alice M. Grimm
Department of Physics, Federal University of Parana, Caixa Postal 19081 - CEP 80531-990 Curitiba-Brazil; e-mail: grimm@fisica.ufpr.br
Moira E. Doyle and Vicente R. Barros
Department of Atmospheric Sciences - University of Buenos Aires
ABSTRACT
A comprehensive view is given of the precipitation anomalies associated with the various stages of El Niño (EN) and La Niña (LN) events all over Southern South America (SSA). This view comprises the delineation of the coherent regions with respect to these anomalies, the identification of the seasons of maximum anomalies and the assessment of their magnitude and consistency during those events. Part of the methodology is based on Ropelewski and Halpert?s global study. The larger data set used in this study allowed the disclosure of regional differences in the precipitation anomalies and the delineation of more than one coherent region. Besides, the spatial and temporal variability of these anomalies is detailed by calculating their consistency and the expected precipitation percentiles for each station and each month during EN and LN cycle.
Southern Brazil presents the strongest average signal in EN events. The general behavior towards opposite signals in the precipitation anomalies over SSA during almost the same periods of the EN and LN cycles indicates a large degree of linearity in the precipitation response to these events. The timing of the anomalies changes throughout SSA, leading to the identification of eight different coherent regions in the EN case and six in the LN case. All these regions show a significant response in some part of each cycle. Its magnitude and consistency show a large spatial variability and some areas present very strong and consistent anomalies sometimes not disclosed when large coherent regions are analyzed. In spite of the differences in timing, some features of the precipitation anomalies are rather uniform throughout the region during EN and LN events. In EN cycle, there is a tendency to lower than median precipitation in the year before the event, which continues until March of the year of the event. In a vast region, east of Andes, the strongest positive rainfall anomalies occur in spring of this year. During January of the mature stage the positive anomalies disappear or even reverse in some regions and then reappear in February. From then on the response shows a larger spatial variability, although there are still strong and consistent anomalies in large areas. This description holds for the LN cycle, but with opposite signs