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 (C_ant). In subpolar and Southern Ocean biomes, however, the seasonality change for GOBMs is dominated by C_ant invasion, whereas for pCO₂ products an indeterminate combination of C_ant 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.