Multiscale Temporal Variability of the Global Air‐Sea CO₂ Flux Anomaly

Abstract

The global air‐sea CO2 flux (F) impacts and is impacted by a plethora of climate‐related processes operating at multiple time scales. In bulk mass transfer formulations, F is driven by physico‐ and bio‐chemical factors such as the air‐sea partial pressure difference (∆pCO2), gas transfer velocity, sea surface temperature, and salinity–all varying at multiple time scales. To de‐convolve the impact of these factors on variability in F at different time scales, time‐resolved estimates of F were computed using a global data set assembled between 1988 and 2015. The F anomalies were defined as temporal deviations from the 28‐year time‐averaged value. Spectral analysis revealed four dominant timescales of variability in F–subseasonal, seasonal, interannual, and decadal with relative amplitude differences varying across regions. A second‐order Taylor series expansion was then conducted along these four timescales to separate drivers across differing regions. The analysis showed that on subseasonal timescales, wind speed variability explains some 66% of the global F anomaly and is the dominant driver. On seasonal, interannual, and decadal timescales, the ∆pCO2 effect controlled by the ∆pCO2 anomaly, explained much of the F anomaly. On decadal timescales, the F anomaly was almost entirely governed by the ∆pCO2 effect with large contributions from high latitudes. The main drivers across timescales also dominate the regional F anomaly, particularly in the mid‐high latitude regions. Finally, the driver of the ∆pCO2 effect was closely connected with the relative strength of atmospheric pCO2 and the nonthermal component of oceanic pCO2 anomaly associated with dissolved inorganic carbon and alkalinity.

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

“…We selected four sites in different parts of the global ocean (the Kuroshio, Gulf Stream and Agulhas currents and the Equatorial Pacific, Figure 1a) chosen as they are typical regions where both mesoscale energy and induced transport are strong (following Martínez-Moreno et al, 2021). CO 2 flux variations at these representative locations within these regions are synchronized with changes in ocean pCO 2 (in both observation-based data set and POP-BGC-HR, Figures 1e-1h), indicating the dominant influence of pCO 2 variations in setting CO 2 flux anomalies (Gu et al, 2023;Mongwe et al, 2018). 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).…”

“…Globally, the ocean has the capacity to uptake more than 2,000 megatonnes of carbon (MtC, 10 6 tC) annually through CO 2 flux at the surface (Friedlingstein et al, 2023). This air-sea CO 2 flux exhibits variations across a range of spatial and temporal scales (Gruber et al, 2023), predominantly governed by the difference in partial pressure of CO 2 (pCO 2 ) between the atmosphere and the ocean (Gu et al, 2023;Mongwe et al, 2018). Notably, high-quality in-situ measurements of CO 2 flux obtained through the eddy covariance method are mostly limited to local scales (Miller et al, 2010(Miller et al, , 2024.…”