Amazonian rainfall increases in wet season: New research

The cold colors here represent the cooling of sea surface temperature, the increased precipitation and enhanced moisture transport divergence; warm colors represent the warming of sea surface temperature, the reduced precipitation and weakened moisture transport divergence. | Credit: WANG Xinyue

Since it covers more than two million square miles in South America, the Amazonian rainforest plays a vital role in regulating level of carbon dioxide and thus affects global climate. In the Amazonian rainforest, strong transpiration along with the sea-land interaction, produce the classic tropical rainforest climate. The growth of the rainforest is influenced by precipitation, especially during its wet season, which lasts from December to May.

Studies in the past have also indicated that the interannual variability of the Amazonian precipitation could be because of the variabilities in the South American monsoon and the El Nino/Southern Oscillation (ENSO). However, not much is known about the trend of the wet season tropical Amazonian precipitation during the recent times. Has the Amazonian rainfall in wet season varied in the past decades?

The study was published recently in the journal Environmental Research Letters. Professor Zhu Jiang, LI Xichen and their Ph.D. student Wang Xinyue from the Institute of Atmospheric Physics, Chinese Academy of Sciences and Professor Clemente A. S. Tanajura from the Institute of Physics, Federal University of Bahia have shown that Amazonian precipitation in wet season has largely increased during the past three decades i.e., from 1979 to 2015. The research team used the atmospheric general circulation model to find out in depth what caused enhanced rainfall.

The results, based on model simulation indicate that the tropical sea surface temperature (SST) variability may control the precipitation, particularly in the Atlantic and Pacific. In the past three decades, the Atlantic has experienced the positive phase of the Atlantic Multileader Oscillation, which is characterized by a strong warm anomaly over the North and tropical Atlantic; while the Pacific showed signs of the negative phase of the Pacific Decadal Oscillation, which is marked by a cooling anomaly over the tropical east Pacific.

“The SST changes over both the Atlantic and the Pacific contribute to the precipitation increase over the Amazonian basin, through atmospheric teleconnections. And we find the Atlantic SST forcing plays a comparatively more important role (than the Pacific SST)”, said ZHU, also the corresponding author of the study. These results give new perspectives for examining the long-term changes of the wet-season Amazonian precipitation.

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