Observing the tropical carbon balance sensitivity to memory and climate extremes.

The trajectory of the tropical carbon balance is in part regulated by the size and frequency of extreme weather events. Specifically, understanding the cumulative impact of ENSO events remains challenging, largely due to uncertainties in the integrated response of carbon cycle processes to climate variability. Ultimately, an integrated understanding of both climate forcings and legacy effects (or "ecosystem memory") on the terrestrial carbon balance is needed to understand the net impact of ENSO events on the tropical C balance. Here we use the CARbon DAta-MOdel fraMework (CARDAMOM) diagnostic model-data fusion approach - constrained by an array of C cycle satellite surface observations, including MODIS leaf area, biomass, OCO-2 and GOSAT solar-induced fluorescence, as well as "top-down" atmospheric inversion estimates of CO2 and CO surface fluxes from the NASA Carbon Monitoring System Flux (CMS-Flux) - to constrain and predict spatially-explicit tropical carbon state variables during 2010-2016. The combined assimilation of land surface and atmospheric datasets into CARDAMOM places key constraints on the temperature sensitivity and first order carbon-water feedbacks throughout the tropics and combustion factors within biomass burning regions. Our terrestrial carbon cycle analysis indicates that a quantitative disentanglement of direct forcing and memory effects is key for understanding the impact of individual ENSO events on the tropical C balance, and provides a critical insight on the cumulative impact of ENSO events in future projections of the tropical C balance. For the 2015 ENSO event, we found that (a) lagged effects amounted to a substantial component of the observed 2015 net biospheric exchange (NBE) anomaly, and (b) lagged effects attributable to both extreme and nominal years substantially impacted 2015 NBE. Based on the 2010-2016 observationally constrained carbon cycle states and their associated climate sensitivities, CARDAMOM terrestrial C balance projections indicate that an increase in climate variability will likely lead to a net positive impact on cumulative NBE throughout the 21^st century.