Abstract
Abstract. Large interannual atmospheric CO2 variability is dominated by the response of the terrestrial biosphere to El Niño–Southern Oscillation (ENSO). However, the behavior of terrestrial ecosystems differs during different El Niños in terms of patterns and biological processes. Here, we comprehensively compare two extreme El Niños (2015/16 and 1997/98) in the context of a multi-event “composite” El Niño. 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 Niño was 1.64 Pg C yr−1, but half that quantity during the 2015/16 El Niño (at 0.73 Pg C yr−1). Moreover, FTA showed no obvious lagged response during the 2015/16 El Niño, 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 Niño. 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 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 Niño. These warmer conditions enhanced GPP and TER over Eurasia during the 2015/16 El Niño, 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 Niño.
Highlights
The atmospheric CO2 growth rate has significant interannual variability, greatly influenced by the El Niño–Southern Oscillation (ENSO) (Bacastow, 1976; Keeling et al, 1995)
The interannual variability of the atmospheric CO2 growth rate principally originates from the terrestrial ecosystems (Fig. 1c)
In order to evaluate the performance of the VEGAS simulation on the interannual timescale, we present CAMS, MACC, and CarbonTracker inversion results
Summary
The atmospheric CO2 growth rate has significant interannual variability, greatly influenced by the El Niño–Southern Oscillation (ENSO) (Bacastow, 1976; Keeling et al, 1995). Clark et al (2003) and Doughty et al (2008) concluded, based on in situ observations, that warming anomalies can reduce tropical tree growth and CO2 uptake Considering this strong emergent linear relationship, these studies (Clark et al, 2003; Doughty et al, 2008; Cox et al, 2013; Wang et al, 2013, 2014; Anderegg et al, 2015) have suggested that temperature dominates the interannual variability of the FTA or CO2 growth rate.
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