The Amazon River’s hydrological cycle has become more extreme over the past two decades with increasing seasonal precipitation across much of the basin despite drier conditions in the southern parts of Earth’s largest rainforest, finds a new study published in Geophysical Research Letters.

The research, led by Emanuel Gloor of the University of Leeds, analyzed monthly Amazon River discharge at Óbidos, a point that drains 77 percent of the Amazon Basin, and compared it with regional precipitation patterns. Suspecting that temperatures in the tropical Atlantic might be influencing rainfall — as has been suggested in other studies — the authors also assessed sea surface temperature data from the National Oceanic and Atmospheric Administration (NOAA).

The researchers found a significant intensification in the Amazon’s hydrological cycle, with increased discharge during the rainy season punctuated by occasional episodes of severe drought.

“The catchment of the world’s largest river is experiencing a substantial wetting trend since approximately 1990,” the authors write. “This intensification of the hydrological cycle is concentrated overwhelmingly in the wet season driving progressively greater differences in Amazon peak and minimum flows.”



A tributary of the Amazon River in Peru. Photo by Rhett A. Butler





In other words, the wet season is getting wetter across much of the Amazon. But how does that square with the apparent increased incidence of severe drought — including the two worst on record in 2005 and 2010 — in the region? The authors say the answer may lie in the complex relationship between Atlantic sea temperatures and rainfall in the Amazon.

When Atlantic sea surface temperatures spike, the effect in the Amazon is a drop in rainfall, as was the case in 2005 and 2010. This “anti-correlation” may be linked to temporary shifts in the position of the Inter-tropical-Convergence-Zone (ITCZ), which normally delivers moisture the region. When the ITCZ shifts northward into the Caribbean, much of the Amazon — especially the southern fringe — is left parched. Meanwhile the increased moisture in the Caribbean creates enabling conditions for stronger Atlantic hurricanes. El Niño Southern Oscillation (ENSO) and La Niña events can also affect rainfall in the Basin: El Niño tends of depress rainfall, La Niña increases it.



(a) Annual mean, maximum and minimum monthly Amazon River discharge at Óbidos. Arrows mark the last four severe drought events in 1995, 1998, 2005, 2010; (b)

Amazon Basin wide annual mean and monthly mean low pass filtered precipitation; (c)

tropical equatorial Atlantic mean sea surface record. Image and text courtesy of Gloor et al (2013).

However Gloor and colleagues put these short-term influences aside and focus on the broader trend of rising ocean temperatures, which increase the rate of evaporation.

“The onset of the trend coincides with the onset of an upward trend in tropical Atlantic sea surface temperatures (SST),” they write. “This positive longer-term correlation contrasts with the short-term, negative response of basin-wide precipitation to positive anomalies in tropical North Atlantic SST, which are driven by temporary shifts in the Inter-tropical-Convergence-Zone (ITCZ) position. We propose that the Amazon precipitation changes since 1990 are instead related to increasing atmospheric water vapor import from the warming tropical Atlantic.”

“The coincidence of the onset of the trends suggests that the intensification of the hydrological cycle might simply be due to the increase of water vapor input by the airstream entering the basin from the tropical Atlantic and feeding the Amazon hydrological cycle.”

The findings are significant because they suggest that the world’s largest river is becoming less predictable. More severe flood and drought cycles will have implications for transportation, hydroelectric projects, and urban infrastructure. In a biome that is already experiencing rapid change due to deforestation, dam construction, and overfishing, an intensified hydrological cycle could also have substantial impacts on ecological processes like fish migration, nutrient cycling and seed dispersal,



Dams in the Amazon. Courtesy of InfoAmazonia.

CITATION: Emanuel Gloor et al (2013). Intensification of the Amazon hydrological cycle over the last two decades. Geophysical Research Letters DOI: 10.1002/grl.50377

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