Interannual variability of Tropical Atlantic and its influence on drought and flood events in the Amazon Basin

The Tropical Pacific and Tropical Atlantic Ocean modulate the interannual precipitation over the Amazon region and the decadal and interdecadal variation as well. During El Niño Southern Oscillation (ENSO), below-average rainfall is recorded in the North and Northeast of the Basin, while deficit of precipitation is observed in the West and South. On the other hand, during La Niña years, rainfall is above of normal in the North and Northeast of Amazon Basin. However, there are also drought events, such as in 1964 and 2005, unrelated to the El Niño event, but influenced by warm conditions in the Tropical North Atlantic. In fact, the exceptional drought recorded in 2010 was influenced by a combined effect of the El Niño event during the peak of rainy season, followed by warm conditions in the Tropical North Atlantic during final of rainy season and dry season.

Therefore, the main aim of this study is exploring the Atlantic Sea Surface Temperature (SST) condition in modulating patterns that influence the development of drought and flood events in the Amazon Basin. First of all, the Atlantic Ocean is divided into Tropical North Atlantic (TNA), Tropical South Atlantic (TSA) and Subtropical South Atlantic (STSA), to analyze the behavior of each region separately. Atlantic Index, in each region, is the first principal component (PC1) time series, which comes from the empirical orthogonal function (EOF) analysis applied to Hadley Center Global Sea Ice and Sea Surface Temperature (HadISST) dataset for the 1870-2107 period. The Tropical North Atlantic, Tropical South Atlantic and Subtropical South Atlantic indices show the main years when drought and flood events reaching the Amazon Basin (droughts in 2005, 2010 and 2015, and floods in 2009 and 2012, mainly), and 5-years moving correlations indicate that these three ocean basin have been coupled and decoupled periodically each other in the last century.

The equatorial Pacific, North Atlantic and South Atlantic indices were also correlated with rainfall over the Amazon for three databases: the Tropical Rainfall Mission Measurements (TRMM), the Global Precipitation Climatology Centre (GPCC) and the HyBAm Observed Precipitation. All three databases showed the same results. An increase of the SST in Eastern Pacific influences in low precipitation over the central and west of the Amazon Basin during the rainy season (December to February), increase of the SST in Central Pacific influences for droughts over the northeast region and the TSA influences in the central Amazon. Increase of the SST in TNA and STSA influences mainly in the dry season (May to September), intensifying it. TNA is responsible for precipitation below normal over the central and west Amazon Basin, while STSA only influences in the central region of the basin. Finally, analysis of extreme events indicate that droughts and floods in the Amazon are intensified (de-intensified) if we consider warm (cold) phases of the AMO (Atlantic Multidecadal Oscillation) and the PDO (Pacific Decadal Oscillation).