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

Abstract

Large interannual atmospheric CO2 variability is dominated by the response of the terrestrial biosphere to El Ninõ-Southern Oscillation (ENSO). However, the behavior of terrestrial ecosystems differs during different El Ninõs in terms of patterns and biological processes. Here, we comprehensively compare two extreme El Ninõs (2015/16 and 1997/98) in the context of a multi-event "composite" El Ninõ. 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 (FTA) anomaly during the 1997/98 El Ninõ was 1.64 PgCyr-1, but half that quantity during the 2015/16 El Ninõ (at 0.73 PgCyr-1). Moreover, FTA showed no obvious lagged response during the 2015/16 El Ninõ, 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 Ninõ. 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 FTA. 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 Ninõ. These warmer conditions enhanced GPP and TER over Eurasia during the 2015/16 El Ninõ, 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 Ninõ.