By Luciana Constantino
The sea surface temperature in the North Atlantic Ocean can be a predictor up to three months in advance of extreme climate events involving reduced rainfall and intense drought in the Northeast region of Brazil. This is one of the main findings of a study by researchers in Brazil, China, Australia and Germany, according to an article published in Geophysical Research Letters.
Using a novel methodology based on the concept of precipitation deficit, the study showed that in recent years the influence of the North Atlantic has become more persistent than that of the tropical Pacific, hitherto considered one of the main factors in the intensity of the droughts suffered by the Northeast. At the same time, atmospheric connections between the Pacific and North Atlantic have become more frequent, suggesting that interactions between tropical ocean basins have reinforced the droughts occurring in the region in recent decades.
"The study was motivated by the severe drought that lasted from 2012 until 2015. This long period led us to think, from the meteorological standpoint, about how tropical ocean temperatures influence the climate. The difference now is the innovative methodology exploring the contrasts between the Pacific and Atlantic, and the pattern of drought in the Northeast of Brazil. The findings can be used as a management tool for weather forecasts in advance of events with this potential," Lincoln Muniz Alves, a scientist at Brazil's National Space Research Institute (INPE) and a co-author of the article, told Agência FAPESP.
The 2012–15 drought in a part of the Northeast already classified as semi-arid was so intense that it destroyed crops and left cities and villages without water. Other studies had already identified changes in atmospheric circulation as the main cause, suggesting that an active role was played by the Atlantic Ocean's surface temperature, which was warmer than usual, and by El Niño, a climate phenomenon involving abnormally warm temperatures in the Pacific.
This El Niño was considered one of the most impactful (after those recorded in 1982-83 and 1997-98) and caused losses in different parts of the world. In Brazil, the adverse effects included intense drought in the Northeast and the Amazon, a far longer dry season than usual in the North and several parts of central Brazil (northern areas of Minas Gerais and Goiás states as well as the Federal District), and floods in the South.
"This type of El Niño, known as 'canonical' because the anomalous warming occurs in the same specific area of the Pacific Ocean, has changed in terms of both location and intensity. In parallel, we've seen anomalous warming in the tropical Atlantic in recent decades. Based on our multifaceted analysis, the article provides ample evidence for forecasters to monitor the signs coming from the tropical Atlantic several months in advance. The Pacific's influence is undeniable, but the Atlantic has more," Alves said.
New parameters
As explained by the authors, the study used methods such as non-linear phase coherence and generalized event synchronization analysis to understand the cause-and-effect mechanisms underlying the climate phenomena investigated. To this end, the relationships between sea surface temperature variability and the standard precipitation index were interpreted as direct interactions, while those between the oceans were interpreted as indirect effects on rainfall levels.
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