Análise da precipitação pluvial da pré-estação e da estação chuvosa de Roraima e do leste do Nordeste do Brasil.

Abstract

The aim of this study was to evaluate the precipitations of the RS and PRS of the ENEB and Roraima, in addition to the relation between this variable in the two study areas for the sake of getting a better understanding of the precipitating systems that cause RS, which generally occurs in the quarter of April, May and June, thus outside the monsoon period in SA. Additionally the study aimed to verify the relation of the precipitation of the ENEB and Roraima with the SST of the Equatorial Pacific and Tropical Atlantic. In order to do so, daily precipitation data from 1979 to 2013 from the NCEP CFSR were used, apart from Era-Interim data from the ECMWF project of the same period and, finally, monthly ocean data from the El Niño regions and the Atlantic sectors from 1982 to 2013. At first, the 5-days cycles of the precipitation were calculated, in which the time series of study areas and the atmospheric patterns associated were analyzed. Ward's agglomerative hierarchical clustering method was used to classify the homogeneous precipitation regions of both study areas. The link between the SST and precipitation in ENEB and Roraima and between precipitations in these areas was estimated by the Pearson correlation coefficient. The results obtained showed three homogeneous groups of precipitation in Roraima and two in the ENEB. On average, the RS for the study areas starts at the 17 pentad and at the end at the 40 pentad. The PRS for ENEB and Roraima appeared to inconsistent, because among the five precipitation groups, four of them presented a number of PRS occurrences below half of the historical series. The high sea-level pressure of the Atlantic sectors play a fundamental role contributing to the ENEB and Roraima RS. It was also verified that the ENSO has a relevant role in the interannual variability of the ENEB and Roraima precipitation, both in the annual totals and in the RS. The rainfall in Roraima shows a statistically significant correlation with the SST of the North Atlantic, and those of the Agreste region of the ENEB with the SST of the South Atlantic. We observed that the interannual variabilities of the annual and seasonal precipitation totals of Roraima and ENEB are in-phase, that is, in general, the rainy years in Roraima are the same as in the ENEB, in addition to the dry ones. The most relevant conclusions were: the RS in Roraima has a monsoon type regime with abrupt onset and slow ending, while the ENEB has a gradual transition, forming a regime different from that of the monsoons; the meteorological systems producing precipitation in the two regions are different, however acting at the same time of year; the annual marching of the pentad totals of precipitation throughout the year are the same and are influenced by the ENSO and the intensity and position of the high pressure of the Atlantic, resulting in interannual variabilities of the precipitation in phase in the two regions.