Response of ocean acidification to ENSO in the California Current System in two fully coupled climate models

We use two fully coupled global climate models with varying resolution in the ocean (GFDL ESM2M and ESM2.6) coupled to a state-of-the-art ocean biogeochemistry model (COBALT) to investigate the influence of natural variability on ocean acidification in the California Current System (CCS). Using Niño3.4 indices defined for each model run, we determine the magnitude of the response in pH and the aragonite saturation state (Ω_arag) to strong ENSO events. Both simulations are forced with constant atmospheric CO₂ conditions and thus allow us to consider the effect of natural variability without the effect of rising CO₂ on ocean acidification.

We find that during strong El Niños/La Niñas, both models simulate anomalies of up to ±0.02 in pH and of ±0.1 in Ω_arag during the winter and spring seasons with respect to the climatological state. In the case of the fine resolution model (ESM2.6-COBALT, 10 km ocean resolution), this area of maximum anomalies is limited to the first 100 km offshore for pH and extends roughly 300-500 km offshore for Ω_arag. The coarser model (ESM2M-COBALT, ~100 km ocean resolution) on the other hand, lacks this strong cross-shore gradient in pH and Ω_arag and exhibits a weaker response to La Niña than ESM2.6-COBALT.

The magnitudes of the nearshore signals in pH and Ω_arag in ESM2.6-COBALT are comparable to atmospheric CO₂-driven decadal decreases in pH and Ω_arag found in a recent regional modeling study of the CCS. Thus, ENSO has the potential to significantly modify the purely anthropogenically driven signals in pH and Ω_arag, which are on the order of −0.015/decade and −0.05/decade, respectively, and should be taken into consideration in studies investigating the evolution of ocean acidification in the CCS.