Drivers of Air‐Sea CO₂ Flux in the Subantarctic Zone Revealed by Time Series Observations

Abstract

The subantarctic zone is an important region in the Southern Ocean with respect to its influence on air‐sea CO2 exchange and the global ocean carbon cycle. However, understanding of the magnitude and drivers of the flux are still being refined. Using observations from the Southern Ocean Time Series (SOTS) station (∼47°S, 142°E) and auxiliary data, we developed a multiple linear regression model to compute the sea surface partial pressure of CO2 (pCO2) over the past two decades. The mean amplitude of the pCO2 seasonal cycle between 2004 and 2021 was 44 μatm (range 30–54 μatm). Summer minima ranged from 310 to 370 μatm and winter maxima were near equilibrium with the atmosphere. The non‐thermal (i.e., biological processes and mixing) contribution to the seasonal variability in pCO2 was several times larger than the thermal contribution. The SOTS region acted as a net carbon sink at annual time scales, with mean magnitude of 6.0 mmol m−2 d−1. The positive phase of the Southern Annular Mode (SAM) increased ocean carbon uptake primarily through an increase in wind speed at zero time lag. Increased surface pCO2 was correlated with a positive SAM with a lag of 4 months, mainly due to reduced biological uptake and increased mixing. During the autotrophic season, pCO2 was predominantly impacted by primary productivity, whereas water mass movement, inferred by temperature and salinity anomalies, had a larger impact on the heterotrophic season. In general, mesoscale processes such as eddies and frontal movement impact the local biogeochemical features more than the SAM.

Carbon Export in the Subantarctic Zone Revealed by Multi‐Year Observations From Biogeochemical‐Argo Floats and Sediment Traps

“…In the SOTS region, the E MLP shows a slight enhancement in April (Figure 3 and Figure S3 in Supporting Information S1). This is likely related to the movement of the STF (Yang et al, 2024; their Figure 6). The STF passing through the SOTS region introduces more variability to the ML bio depth, thus affecting the E MLP .…”

“…Only two events were captured in the SOTS region in autumn 2018. This shows that the SOTS area is affected by the eddies associated with the STF migration (Yang et al, 2024). Given the similarity in the magnitude and seasonality of the carbon pumps in the SOTS and broader SAZ region, hereafter they will be discussed as a whole.…”