Rainfall Response in Northeast Brazil from Ocean Climate Variability during the Second Half of the Twentieth Century

Abstract

he authors investigated the rainfall variability response in northern Northeast Brazil (NNEB) from El Niño/La Niña (EN/LN) events and from the meridional sea surface temperature gradient (MGRAD) over the tropical Atlantic during the period 1948?97. The diagnostic analysis was stratified according to four climatic series of scenarios associated with EN, LN, and positive and negative MGRAD. During ENs, which were more numerous and more intense after the 1970s, the MGRAD was generally not noticeable, and the drought impact in NNEB was mainly due to the warm Pacific influence. Conversely, during LNs, the MGRAD signal was important, but there was an inverse relationship between the third and the fourth quarters of the twentieth century. Thus, before the 1970s the LNs were associated with positive MGRAD, which led to an inverse influence inducing minor changes in seasonal rainfall in NNEB. After the 1970s the LNs were linked to negative MGRAD, which induced a cumulative wet influence in NNEB. The positive MGRADs were generally associated with ENs, which reinforced the drought impact in NNEB. The well-marked negative MGRADs, which all occurred after the beginning of the 1970s, were generally linked with large LNs that induced very consistent wet episodes in NNEB. Interestingly, the two low-frequency variations in the tropical oceans observed during the second half of the twentieth century (i.e., from a few to several strong ENs and from none to numerous strong negative MGRADs) occurred concomitantly with symmetric long-term changes in the Pacific decadal oscillation (PDO) and the North Atlantic Oscillation (NAO). This symmetrical long-term climate behavior during the second half of the twentieth century could have lead to an inverse influence on the climate over the north Northeast Brazil, in agreement with a quasi-null long-term trend of the rainfall observed in that region all along this period. Such symmetrical behavior seems to have been unique during the last 150 years.