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A climate shift in mid-1970's in Northwest Amazonia and Southern Brazil
Guillermo O. Obregón, Center for Weather Forecasting and Climate Studies/INPE, Cachoeira Paulista, Brazil; and C. A. Nobre
Based on rainfall data analysis for the period 1951-1990 it is found that the observed climate shift that occurred in the mid-1970's had opposite impacts on the rainfall regime in two extreme regions of Brazil. In relation to the period prior to the mid-1970's, rainfall over Southern Brazil (Northwest Amazonia) increased (decreased) in almost all months after about 1975. These trends were more significant at the end of austral summer in Northwest Amazonia and austral spring in Southern Brazil. By examining the PDF's of the monthly data clustered prior to and after 1975, we note that the distribution of the data after 1975 experienced a marked shift towards higher (lower) values in Southern Brazil (Northwest Amazonia), and in the two periods the variance of Northwest Amazonia rainfall is smoother in comparison to Southern Brazil rainfall. Also, the PDF of monthly rainfall prior to 1975 for Northwest Amazonia exhibits two distinct peaks, at around 290 mm and 400 mm, and it changes to a PDF with a single peak at 240 mm for the post-1975 period. Multivariate analysis between the rainfall data for South America, global NCEP/NCAR reanalysis data and SST for the global oceans shows that the rainfall shift that occurred in the mid-1970's over both regions are strongly associated to SST changes in the Pacific Ocean, mainly during the austral summer months. Additionally, the rainfall shift found in Southern Brazil seems to be related to the Antarctic Oscillation.
Session 4, Interdecadal and Longer Term Climate Variability I
Tuesday, 25 March 2003, 8:30 AM-1:30 PM
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