Sea surface pCO2 variability on different time scales in the East China Sea based on high-frequency time-series observations

We examined the sub-seasonal to interannual variability and multi-year trend of sea surface CO₂ partial pressure (pCO₂) and air-sea CO₂ flux at a coastal site of the East China Sea (31⁰N, 122.8⁰E) based on high-frequency time-series data collected by a buoy since 2013. Seasonal average sea surface pCO₂ was highest in autumn, but the lowest value can appear in winter or spring, depending on the biological productivity in spring. The seasonal amplitude of pCO₂ was up to 123 μatm. Based on property-property relationships and a simple mass budget model, we found that temperature change, biological activity, water mixing and air-sea CO₂ exchange all made significant contributions to the seasonal variation of pCO₂. From winter to summer, seasonal warming and atmospheric CO₂ uptake elevated the pCO₂, while net biological production, weakened vertical mixing and the retreat of the Yellow Sea Coastal Water (YSCW) lowered the pCO₂. Conversely, from summer to winter, seasonal cooling and CO₂ emission lowered the pCO₂, while respiration, enhanced vertical mixing and the YSCW intrusion raised them up. Over short-term timescale, biological production and respiration frequently drew down or elevated the pCO₂ by 150-400 μatm within 5-10 days during warm months. When biological activity was suppressed during cold months, such short-term variations were dominated by water mixing with a smaller pCO₂ amplitude of 5-60 μatm within 2-6 days. This site was a sink of atmospheric CO₂ in winter and spring, but a CO₂ source in summer and autumn. Annually, it was a moderate CO₂ source in 2014 (air-sea CO₂ flux was 2.88 ± 11.02 mmol m⁻² d⁻¹), a weak CO₂ sink in 2016 (-0.21 ± 12.23 mmol m⁻² d⁻¹), and a weak CO₂ source in the combined year of the first half of 2017 and the second half of 2018 (0.40 ± 9.11 mmol m⁻² d⁻¹). The relatively high CO₂ source in 2014 was likely due to the weaker biological production in spring and more typhoon passage in autumn. From 2013 to 2019, the wintertime sea surface pCO₂ didn’t follow the increasing trend of the atmospheric pCO₂, leading to an enhancing carbon sink in winter.