Statistical characterization of non-thermal open ocean pCO2 variability

The dynamics of the oceanic partial pressure of CO₂ (pCO₂) are governed by the combined influence of thermal effects, driven by temperature variability, and non-thermal processes related to several processes such as biology, and air–sea gas exchange. The relative contribution of these components can be quantified using the decomposition framework proposed by Takahashi et al. (1993, 2002, 2009). Here, this methodology is applied to high-frequency in situ pCO₂ observations from 17 fixed-position open-ocean moorings (Sutton et al., 2019), providing an Eulerian view of surface ocean carbon variability across a range of oceanic regions. This approach allows us to isolate the non-thermal component of pCO₂ variability and to investigate its statistical properties beyond mean or seasonal signals. The impact of non-thermal processes is examined using probability density function (PDF) analyses and PDF-quotient diagnostics (Xu et al., 2007). These analyses reveal that non-thermal forcing plays a key role in shaping the distribution of pCO₂ variability, with a particularly strong influence on extreme values relative to the core of the distribution. Such extremes are often underestimated when variability is characterized using low-frequency or climatological approaches. Despite the generally lower variability of open-ocean environments compared to coastal regions, our results demonstrate that non-thermal processes significantly contribute in these environments to short-term pCO₂ fluctuations and extremes. This highlights the importance of sustained, high-frequency pCO₂ observations for improving air–sea CO₂ flux estimates and for reducing uncertainties in regional and global ocean carbon budgets.

 

References:

Takahashi et al. (1993), Seasonal variation of CO₂ and nutrients in the high-latitude surface oceans: A comparative study. Global Biogeochemical Cycles, 7 (4), 843–878. doi: 10.1029/93GB02263

Takahashi et al. (2002), Global sea–air CO₂ flux based on climatological surface ocean pCO₂, and seasonal biological and temperature effects. Deep Sea Research Part II: Topical Studies in Oceanography, 49 (9), 1601–1622. doi: 10.1016/S0967-0645(02)00003-6308

Takahashi et al. (2009), Climatological mean and decadal change in surface ocean pCO₂, and net sea–air CO₂ flux over the global oceans. Deep Sea Research Part II: Topical Studies in Oceanography, 56 (8), 554–577. doi: 10.1016/j.dsr2.2008.12.009313

Sutton et al. (2019), Autonomous seawater pCO₂ and pH time series from 40 surface buoys and the emergence of anthropogenic trends. Earth System Science Data, 11 (1), 421–439. doi: 10.5194/essd-11-421-2019295

Xu et al. (2007), Curvature of Lagrangian Trajectories in Turbulence. Physical Review Letters, 98 (5), 050201. doi: 10.1103/PhysRevLett.98.050201324