Intensification of the Amazon hydrological cycle over the last two decades

Abstract

The Amazon basin hosts half the planet's remaining moist tropical forests, but they may be threatened in a warming world. Nevertheless, climate model predictions vary from rapid drying to modest wetting. Here we report that the catchment of the world's largest river is experiencing a substantial wetting trend since approximately 1990. This intensification of the hydrological cycle is concentrated overwhelmingly in the wet season driving progressively greater differences in Amazon peak and minimum flows. The onset of the trend coincides with the onset of an upward trend in tropical Atlantic sea surface temperatures (SST). 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 intertropical convergence zone position. We propose that the Amazon precipitation changes since 1990 are instead related to increasing atmospheric water vapor import from the warming tropical Atlantic.

Land-use and climate change risks in the Amazon and the need of a novel sustainable development paradigm

“…These wetting trends (up to 80 mm·mo −1 in the period 2000-2009 compared with period 1981-1990) are not homogeneously distributed around the basin, and they tend to occur during the wet season over the northwestern part of the basin. On the other hand, the decreasing wet season rainfall trend (−20 mm·mo −1 for the same period) was observed in the southern and southeastern parts of the basin (79).…”

“…Precipitation reductions observed in the last two decades over southern and southeastern Amazon cannot, as yet, be fully explained, but one possible explanation could be the effects of extensive land-use change in this region and relative changes in the albedo (79,86). The increasing precipitation trend observed during the rainy season was attributed to a gradual warming of the tropical North Atlantic SST and the resulting increase of water vapor supply from the ocean toward the continent (79).…”

“…This is due to the large natural variability, relatively small number of weather stations, and important gaps in time series (72,78). To complement these studies, some assessments have been based on river discharge data, rather than precipitation, due to their better integrative nature and dataset consistency (79,80). Although some studies indicated a decreasing trend in rainfall (drying) in the Amazon, in recent decades (80)(81)(82)(83), others pointed to a wetting trend for the region, as a result of an intensification of the hydrological cycle (79,84,85).…”

“…To complement these studies, some assessments have been based on river discharge data, rather than precipitation, due to their better integrative nature and dataset consistency (79,80). Although some studies indicated a decreasing trend in rainfall (drying) in the Amazon, in recent decades (80)(81)(82)(83), others pointed to a wetting trend for the region, as a result of an intensification of the hydrological cycle (79,84,85). These wetting trends (up to 80 mm·mo −1 in the period 2000-2009 compared with period 1981-1990) are not homogeneously distributed around the basin, and they tend to occur during the wet season over the northwestern part of the basin.…”

“…The increasing precipitation trend observed during the rainy season was attributed to a gradual warming of the tropical North Atlantic SST and the resulting increase of water vapor supply from the ocean toward the continent (79). The influence of the tropical North Atlantic SST anomalies, however, is more gradual and has a different pattern from that observed during 2005 and 2010 droughts, when North Atlantic SST anomalies resulted in changes in the Intertropical Convergence Zone (ITCZ) position (79). The important point here is that, even for wetter conditions, the southern and southeastern Amazon forest presented drying trends in the last two decades.…”