Non-intuitive differences in Ninos-driven CO2 flux variability and long-term changes in the tropical Pacific and Atlantic

Both the tropical Pacific and Atlantic upwelling systems are modulated by their respective Ninos (ENSO and Atlantic Nino), which significantly affect the regional and global climate variability. Coincidentally, two of largest ocean carbon outgassing systems are also located in these domains. As a result, the interannual variability of ocean CO2 fluxes in these regions have predominant imprint on the globally integrated variations (Landschutzer et al., 2016). In contrast to the effect of anomalously cold surface temperature, the upwelling of deep-water rich in dissolved inorganic carbon is understood to be the main driver for the mean CO2 outgassing. In the tropical Pacific, El Nino (La Nina) leads to a suppressed (stronger) upwelling condition and an anomalously weaker (stronger) carbon outgassing. On the other hand, the Atlantic Nino and Nina exert considerable variability in the surface freshwater and temperature, which leads to spatially heterogeneous responses in the contemporary CO2 fluxes. In both systems, we discover a critical role of subsurface alkalinity in regulating the observed variability, primarily through altering the surface buffering capacity (Koseki et al., 2023). We show that bias in CMIP6 Earth system models in simulating the mean contemporary alkalinity state in the tropical Pacific leads to contrasting future impacts (Vaittinada Ayar et al., 2022) and could have ramifications on the climate carbon cycle feedback.    References Koseki, S., J. Tjiputra, F. Fransner, L. R. Crespo, and N. S. Keenlyside (2023), Disentangling the impact of Atlantic Nino on sea-air CO2 fluxes, Nature Communications, 14, 3649, https://doi.org/10.1038/s41467-023-38718-9. Landschützer, P., N. Gruber, and D. C. E. Bakker (2016), Decadal variations and trends of the global ocean carbon sink, Global Bio- geochem. Cycles, 30, 1396–1417, http://doi.org/10.1002/2015GB005359. Vaittinada Ayar, P., L. Bopp, J. R. Christian, T. Ilyina, J. P. Krasting, R. Séférian, H. Tsujino, M. Watanabe, A. Yool, and J. Tjiputra (2022), Contrasting projections of the ENSO-driven CO2 flux variability in the equatorial Pacific under high-warming scenario, Earth Syst. Dynam., 13, 1097–1118, https://doi.org/10.5194/esd-13-1097-2022.