Contrasting projection of the ENSO-driven CO<sub>2</sub> flux variability in the Equatorial Pacific under high warming scenario

Abstract

Abstract. The El Niño Southern Oscillation (ENSO) widely modulates the global carbon cycle, in particular, by altering the net uptake of carbon in the tropical ocean. Indeed, over the tropics less carbon is released by oceans during El Niño while it is the opposite for La Niña. Here, the skill of Earth System Models (ESM) from the latest Coupled Model Intercomparison Project (CMIP6) to simulate the observed tropical Pacific CO2 flux variability in response to ENSO is assessed. The temporal amplitude and spatial extent of CO2 flux anomalies vary considerably among models, while the surface temperature signals of El Niño and La Niña phases are generally well represented. Under historical conditions followed by the high warming Shared Socio-economic Pathway (SSP5-8.5) scenarios, about half the ESMs simulate a reversal in ENSO-CO2 flux relationship. This gradual shift, which occurs as early as the first half of the 21st century, is associated with a high CO2-induced increase in Revelle factor that leads to stronger sensitivity of partial pressure of CO2 (pCO2) to changes in surface temperature between ENSO phases. At the same time, uptake of anthropogenic CO2 substantially increases upper ocean dissolved inorganic carbon (DIC) concentrations, reducing its vertical gradient in the thermocline, and weakening the ENSO-modulated surface DIC variability. The response of ENSO-CO2 flux relationship to future climate change is sensitive to the contemporary mean state of the carbonate ion concentration in the tropics. Models that simulate shift in ENSO-CO2 flux relationship simulate positive bias in surface carbonate concentration.

Equatorial Pacific pCO2 Interannual Variability in CMIP6 Models

“…However, an alkalinity bias alone does not explain all the models that underestimate pCO 2,nonT , relative to pCO 2,T , as we analyze here. For example, Vaittinada Ayar et al (2022) shows that IPSL-CM6A-LR doesn't have a strong alkalinity bias, however, we find that its pCO 2,nonT : pCO 2,T variance ratio is weaker than the ratio in MRI-ESM2-0 (Figure 7b), which is a model they show with a strong alkalinity bias.…”

“…Previous studies of historical simulations from the earlier CMIP5 found that FCO 2 IAV were also underestimated in models (Dong et al, 2016(Dong et al, , 2017. Results from another CMIP6 study also find that most models simulate weak FCO 2 anomalies while overestimating SST IAV (Vaittinada Ayar et al, 2022).…”

“…However, this amount of damping is not enough to explain the insufficient pCO 2,nonT variability. Vaittinada Ayar et al (2022) find that models with strong alkalinity biases have weak surface DIC biases (i.e., weak surface DIC variability), which leads to a reduction in pCO 2,nonT variability. They find that for some models (CanESM5, GFDL-CM4 and MRI-ESM2-0), pCO 2,nonT variability is weak enough that pCO T variability can dominate total pCO 2 anomalies.…”

“…Vaittinada Ayar et al (2022) proposed that models without a strong alkalinity bias may be better predictors of future ENSO-CO 2 flux dynamics. However, we find that these models underestimate equatorial Pacific pCO 2 IAV and ENSO-related covariability.…”

“…The latest generation of ESMs from CMIP6 have demonstrated progress in representing the mean state of ocean biogeochemistry (Séférian et al, 2020). However, as in CMIP5, weak FCO 2 IAV were also found in CMIP6 (Vaittinada Ayar et al, 2022). Identifying sources of model biases in FCO 2 IAV for the contemporary period, where some data constraints exist, is a first step toward model improvements.…”