Amazon carbon cycle response to the exceptional drought in 2023

The year 2023 had record-high temperatures worldwide. In the Amazon region, temperature anomalies reached over 1.5C above the 1991-2020 period from September to November, and at the Amazon Tall Tower Observatory (ATTO) we saw the highest temperature from June to November since 2013. Independent assessments have shown that river levels in the basin reached record lows and a dry season that extended well into December, prolonging the fire season considerably. Recently, the World Weather Attribution attributed this drought to climate change, rather than the ongoing El Niño event, as is commonly assumed. In this study, we assess the regional anomalies in climatic conditions and show that potential cumulative water deficits (based on precipitation minus potential evapotranspiration) in some areas correspond to a drought event with a return period of more than 50 years. We then assess how the carbon cycle of the Amazon responded to this extreme event by combining multiple sources of information: simulations by Dynamic Global Vegetation Models (DGVMs), as well as continuous atmospheric CO2 mole fractions and eddy covariance CO2 flux data from ATTO. We find a consistent spatial response among DGVMs showing negative Net Biome Production (NBP) anomalies, i.e., a source anomaly, over the central Amazon and north of the main branch of the Amazon River in the Guiana Shield, along with extremely low soil moisture. These anomalies reach record lows from September to December 2023, a much larger impact than the previous high-impact drought of 2010 and the El-Niño of 2015/16. These model results are compared with the ATTO record of CO2 mole fractions and NEE fluxes. Based on this comparison, we discuss which processes contribute the most to the signals seen at ATTO and at basin-scale, and show a complete fast-track diagnostic of the Amazon carbon cycle response to last year's climate event.