Contrasting terrestrial carbon cycle responses to the 1997/98 and 2015/16 extreme El Niño events

Large interannual atmospheric CO 2 variability is dominated by the response of the terrestrial biosphere to El Nino–Southern Oscillation (ENSO). However, the behavior of terrestrial ecosystems differs during different El Ninos in terms of patterns and biological processes. Here, we comprehensively compare two extreme El Ninos (2015/16 and 1997/98) in the context of a multi-event composite El Nino. We find large differences in the terrestrial carbon cycle responses, even though the two events were of similar magnitude. More specifically, we find that the global-scale land–atmosphere carbon flux ( F TA ) anomaly during the 1997/98 El Nino was 1.64 Pg C yr −1 , but half that quantity during the 2015/16 El Nino (at 0.73 Pg C yr −1 ). Moreover, F TA showed no obvious lagged response during the 2015/16 El Nino, in contrast to that during 1997/98. Separating the global flux by geographical regions, we find that the fluxes in the tropics and extratropical Northern Hemisphere were 1.70 and −0.05 Pg C yr −1 during 1997/98, respectively. During 2015/16, they were 1.12 and −0.52 Pg C yr −1 , respectively. Analysis of the mechanism shows that, in the tropics, the widespread drier and warmer conditions caused a decrease in gross primary productivity (GPP; −0.73 Pg C yr −1 ) and an increase in terrestrial ecosystem respiration (TER; 0.62 Pg C yr −1 ) during the 1997/98 El Nino. In contrast, anomalously wet conditions occurred in the Sahel and East Africa during 2015/16, which caused an increase in GPP, compensating for its reduction in other tropical regions. As a result, the total 2015/16 tropical GPP and TER anomalies were −0.03 and 0.95 Pg C yr −1 . GPP dominance during 1997/98 and TER dominance during 2015/16 accounted for the phase difference in their F TA . In the extratropical Northern Hemisphere, the large difference occurred because temperatures over Eurasia were warmer during the 2015/16, as compared with the cooling seen during the 1997/98 and the composite El Nino. These warmer conditions enhanced GPP and TER over Eurasia during the 2015/16 El Nino, while these fluxes were suppressed during 1997/98. The total extratropical Northern Hemisphere GPP and TER anomalies were 0.63 and 0.55 Pg C yr −1 during1997/98, and 1.90 and 1.45 Pg C yr −1 during 2015/16, respectively. Additionally, wildfires played a less important role during the 2015/16 than during the 1997/98 El Nino.