Global Ocean pCO₂ Variation Regimes: Spatial Patterns and the Emergence of a Hybrid Regime

Abstract

The variability in ocean surface partial pressure of carbon dioxide (pCO2), driven by both physical and biological processes, substantially influences air‐sea carbon exchange. It is widely recognized that the pCO2 variation at a specific ocean location is primarily dominated by either thermal or nonthermal effects. However, the distinct pCO2 variation regimes, their global distribution, and the mechanisms underlying such patterns have yet to be fully determined due to a paucity of global observations. Through the use of observation‐based products and an eddy‐resolving ocean simulation, this study demonstrates the presence of three distinct regimes in the global ocean: one in which sea surface temperature (SST) is the dominant control on pCO2 variations; another in which dissolved inorganic carbon (DIC) is the primary control; and a third previously uncharacterized hybrid regime where pCO2 variations are governed by seasonally‐varying factors. This hybrid regime is generally located between SST‐ and DIC‐dominated regimes and occupies approximately 15% of the global ocean. The regimes broadly exist in zonal bands that are closely linked to the relative strengths of SST and DIC variances. Seasonally‐varying mixed‐layer depth, mesoscale variability (quantified in terms of eddy kinetic energy), and biological processes modulate local pCO2 variations and play significant roles in shaping the global pattern of regime distributions. Understanding the distribution of pCO2 regimes, including the hybrid regime revealed in this study, as well as their different oceanic drivers, is essential for future predictions of ocean carbon uptake in response to global warming.

The Role of Ocean Mesoscale Variability in Air‐Sea CO2 Exchange: A Global Perspective

“…There is some evidence for enhanced mesoscale energy (Beech et al, 2022;Martínez-Moreno et al, 2021) and associated tracer transport under global warming, suggesting a potentially larger role for mesoscale phenomena in future air-sea carbon exchange. In some parts of the global oceans, variations in pCO 2 , which directly affect changes in air-sea CO 2 flux, are predominantly influenced by temperature changes (Guo & Timmermans, 2024b;Takahashi et al, 2002). Considering the context of strengthened temperature variance and more frequent and extreme marine heatwaves (Frölicher et al, 2018) in recent decades, the thermal component of pCO 2 variations is likely undergoing modifications as a consequence, potentially altering mesoscale-induced CO 2 exchange.…”

“…Positive pCO 2 anomalies induce positive CO 2 fluxes (i.e., a source of CO 2 to the atmosphere), while negative pCO 2 anomalies correspond to negative CO 2 fluxes (i.e., a sink of CO 2 to the ocean). Changes in pCO 2 are induced by surface heating and cooling, upwelling and downwelling, as well as variations in ocean mixing and primary production (Gruber et al, 2023;Guo & Timmermans, 2024b). Since seasonality predominantly governs the temporal variability in pCO2 and CO 2 flux in most ocean regions (Takahashi et al, 2002), CO 2 flux variance is comparable between the low-resolution observation-based product and the high-resolution model output.…”

“…This biogeochemistrycoupled high-resolution POP simulation will be referred as POP-BGC-HR below, which is the same as that used by Guo and Timmermans (2024b). A more detailed description of the model is given by Long et al (2021); Guo and Timmermans (2024b). Due to computational constraints and the availability of model output, we used monthly POP-BGC-HR output from 1982 to 2000 in this study.…”

“…The ocean biogeochemical model in this simulation uses the prognostic Marine Biogeochemistry Library from CESM (Long et al, 2021). This biogeochemistrycoupled high-resolution POP simulation will be referred as POP-BGC-HR below, which is the same as that used by Guo and Timmermans (2024b). A more detailed description of the model is given by Long et al (2021); Guo and Timmermans (2024b).…”

“…Harrison et al (2018) compare global high-resolution (1/10°) and standard low-resolution (1°) climate simulations, demonstrating that ocean mesoscale variability may increase or decrease carbon export to the deep ocean by as much as 50% at regional scales. More recently, Guo and Timmermans (2024b), utilizing an eddy-resolving global biogeochemical simulation, characterized a strong link between eddy activity and ocean pCO 2 changes in mid-latitude regions. This points to the potential involvement of mesoscale processes in driving pCO 2 variations, likely modulating carbon flux across a large spatial domain.…”