Seasonal Variability of the Surface Ocean Carbon Cycle: A Synthesis

Abstract

The seasonal cycle is the dominant mode of variability in the air‐sea CO2 flux in most regions of the global ocean, yet discrepancies between different seasonality estimates are rather large. As part of the Regional Carbon Cycle Assessment and Processes phase 2 project (RECCAP2), we synthesize surface ocean pCO2 and air‐sea CO2 flux seasonality from models and observation‐based estimates, focusing on both a present‐day climatology and decadal changes between the 1980s and 2010s. Four main findings emerge: First, global ocean biogeochemistry models (GOBMs) and observation‐based estimates (pCO2 products) of surface pCO2 seasonality disagree in amplitude and phase, primarily due to discrepancies in the seasonal variability in surface DIC. Second, the seasonal cycle in pCO2 has increased in amplitude over the last three decades in both pCO2 products and GOBMs. Third, decadal increases in pCO2 seasonal cycle amplitudes in subtropical biomes for both pCO2 products and GOBMs are driven by increasing DIC concentrations stemming from the uptake of anthropogenic CO2 (Cant). In subpolar and Southern Ocean biomes, however, the seasonality change for GOBMs is dominated by Cant invasion, whereas for pCO2 products an indeterminate combination of Cant invasion and climate change modulates the changes. Fourth, biome‐aggregated decadal changes in the amplitude of pCO2 seasonal variability are largely detectable against both mapping uncertainty (reducible) and natural variability uncertainty (irreducible), but not at the gridpoint scale over much of the northern subpolar oceans and over the Southern Ocean, underscoring the importance of sustained high‐quality seasonally‐resolved measurements over these regions.

An Assessment of CO2 Uptake in the Arctic Ocean From 1985 to 2018

“…Although differences in the biogeochemistry in the models may cause the discrepancy, there is insufficient observational data available to evaluate this. A similar discrepancy in the seasonal cycle of p CO 2w between the products and the models has been found in the global ocean (Rogers et al., 2023).…”

Magnitude, Trends, and Variability of the Global Ocean Carbon Sink From 1985 to 2018

“…However, the coarse spatial resolution and lack of seasonal variability and biological carbon cycling in the OCIM could still bias its estimates of F ant,CO2 . Seasonality is known to influence rates of anthropogenic CO 2 uptake (Rodgers et al, 2008), and this variability will become more important as the ocean acidifies and the Revelle buffer factor increases, enhancing the biologically and temperature-driven seasonal variations in seawater chemistry (Fassbender et al, 2018(Fassbender et al, , 2022Hauck & Völker, 2015;Rodgers et al, 2023). The influence of missing seasonality, small-scale circulation features, and biology on anthropogenic CO 2 uptake by the OCIM has not been quantified, but here we adopt an ad hoc estimate of about 10%-15%, or an additional 0.3 PgC yr −1 uncertainty to the OCIM estimates of F ant,CO2 .…”

Assessment of Global Ocean Biogeochemistry Models for Ocean Carbon Sink Estimates in RECCAP2 and Recommendations for Future Studies

“…Furthermore, a study with ESMs has shown that AMOC-biases are strongly correlated to sea surface temperature biases in the North Atlantic (Wang et al, 2014). While we did not analyze sea surface temperature biases in the North Atlantic, Rodgers et al (2023) found that the seasonal cycle of pCO 2 in the subpolar Atlantic is thermally driven in the GOBMs while that of the pCO 2 -products is non-thermally driven. This might lead to the F net of the GOBMs being more sensitive to warming than that based on pCO 2 products (Goris et al, 2018), which may contribute to the increasing gap between GOBMs and pCO 2 -products with time.…”