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

Large interannual atmospheric CO2 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 Cyr(-1), but half that quantity during the 2015/16 El Nino (at 0.73 Pg Cyr(-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 PgCyr(-1) during 1997/98, respectively. During 2015/16, they were 1.12 and -0.52 PgCyr(-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 PgCyr(-1)) and an increase in terrestrial ecosystem respiration (TER; 0.62 PgCyr(-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 PgCyr(-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 PgCyr(-1) during1997/98, and 1.90 and 1.45 PgCyr(-1) during 2015/16, respectively. Additionally, wildfires played a less important role during the 2015/16 than during the 1997/98 El Nino.