Tree-ring oxygen isotopes record a decrease in Amazon dry season rainfall over the past 40 years

Extant climate observations suggest the dry season over large parts of the Amazon Basin has become longer and drier over recent decades. However, such possible intensification of the Amazon dry season and its underlying causes are still a matter of debate. Here we used oxygen isotope ratios in tree rings (delta O-18(TR)) from six floodplain trees from the western Amazon to assess changes in past climate. Our analysis shows that delta O-18(TR) of these trees is negatively related to inter-annual variability of precipitation during the dry season over large parts of the Amazon Basin, consistent with a Rayleigh rainout model. Furthermore delta O-18(TR) increases by approximately 2%c over the last four decades (similar to 1970-2014) providing evidence of an Amazon drying trend independent from satellite and in situ rainfall observations. Using a Rayleigh rainout framework, we estimate basin-wide dry season rainfall to have decreased by up to 30%. The delta O-18(TR) record further suggests such drying trend may not be unprecedented over the past 80 years. Analysis of delta O-18(TR) with sea surface temperatures indicates a strong role of a warming Tropical North Atlantic Ocean in driving this long-term increase in delta O-18(TR) and decrease in dry season rainfall.