Seasonal Asymmetry in the Evolution of Surface Ocean pCO₂ and pH Thermodynamic Drivers and the Influence on Sea‐Air CO₂ Flux

Abstract

It has become clear that anthropogenic carbon invasion into the surface ocean drives changes in the seasonal cycles of carbon dioxide partial pressure (pCO2) and pH. However, it is not yet known whether the resulting sea‐air CO2 fluxes are symmetric in their seasonal expression. Here we consider a novel application of observational constraints and modeling inferences to test the hypothesis that changes in the ocean's Revelle factor facilitate a seasonally asymmetric response in pCO2 and the sea‐air CO2 flux. We use an analytical framework that builds on observed sea surface pCO2 variability for the modern era and incorporates transient dissolved inorganic carbon concentrations from an Earth system model. Our findings reveal asymmetric amplification of pCO2 and pH seasonal cycles by a factor of two (or more) above preindustrial levels under Representative Concentration Pathway 8.5. These changes are significantly larger than observed modes of interannual variability and are relevant to climate feedbacks associated with Revelle factor perturbations. Notably, this response occurs in the absence of changes to the seasonal cycle amplitudes of dissolved inorganic carbon, total alkalinity, salinity, and temperature, indicating that significant alteration of surface pCO2 can occur without modifying the physical or biological ocean state. This result challenges the historical paradigm that if the same amount of carbon and nutrients is entrained and subsequently exported, there is no impact on anthropogenic carbon uptake. Anticipation of seasonal asymmetries in the sea surface pCO2 and CO2 flux response to ocean carbon uptake over the 21st century may have important implications for carbon cycle feedbacks.

Processes Driving Global Interior Ocean pH Distribution

“…No attempt has been made to correct for this bias in this present work, although we note that this bias is primarily an issue for surface measurements and this analysis focuses on the ocean interior. We nevertheless note that previous studies (e.g., Fassbender et al, 2018 ;Landschützer et al, 2018 ;Lenton et al, 2012) have indicated that there is a nonnegligible difference between summertime and wintertime trends in surface ocean pCO 2 , and it is possible that this is also the case for pH. Therefore, the anthropogenic CO 2 impacts on pH (and Ω) estimated here, which align with previous estimates (e.g., Caldeira & Wickett, 2003), should be considered representative of the summertime.…”

“…Therefore, the anthropogenic CO 2 impacts on pH (and Ω) estimated here, which align with previous estimates (e.g., Caldeira & Wickett, ), should be considered representative of the summertime. Relatedly, warming of the ocean also affects the amplitude of the seasonal cycling and ocean stratification, which has been shown to affect ocean p CO 2 (Fassbender et al, ; Landschützer et al, ). The same effect is likely for ocean acidification, but the GLODAPv2 data product, due to the seasonal bias, cannot be used to assess this effect.…”

Quantifying the Effects of Nutrient Enrichment and Freshwater Mixing on Coastal Ocean Acidification

“…To account for correlation between ALK and DIC and strong dilution with freshwater (e.g., Fassbender et al, ) at inner harbor sampling stations, we use freshwater mixing (∆Ω mix ) and biogeochemical (∆Ω BGC ) terms rather than ∆Ω DIC and ∆Ω ALK , as has been done in a number of other studies (e.g., Doney, Lima et al, for pCO 2 ; Wang et al, ; Xu et al, for Ω) or a combined biophysical term (e.g., Fassbender et al, for pCO 2 ). The mixing term reflects the effects on Ω of changes of DIC and ALK as a function of salinity for two‐endmember conservative mixing determined for Buzzards Bay: …”

Complexity of Marine CO2 System Highlighted by Seasonal Asymmetries

“…This means that the temperature component of pH does not grow in constant proportion to the annual mean value, while pCO 2 does. The result is that when viewed as pH, in some areas the amplitude of pH decreases over time, but when viewed as [H + ] the amplitude increases (Fassbender et al, , figure 5). The cause of this apparent discrepancy is that the log scale of pH conceals the magnitude of the change in [H + ], giving the impression that the changes are smaller than they actually are.…”

“…Separating and attributing different drivers to carbon system changes will not necessarily be easy, but it will be important. Fassbender et al () make a great contribution to helping us better understand seasonal changes in the marine carbon cycle.…”