Atmospheric and seawater carbon dioxide at time series station DYFAMED

A time series of fCO2, SST, and fluorescence data was collected between 1995 and 1997 by a CARIOCA buoy moored at the DyFAMed station (Dynamique des Flux Atmospheriques en Mediterranée) located in the northwestern Mediterranean Sea. On seasonal timescales, the spring phytoplankton bloom decreases the surface water fCO2 to approximately 290 µatm, followed by summer heating and a strong increase in fCO2 to a maximum of approximately 510 µatm. While the DELTA fCO2 shows strong variations on seasonal timescales, the annual average air-sea disequilibrium is only 2 µatm. Temperature-normalized fCO2 shows a continued decrease in dissolved CO2 throughout the summer and fall at a rate of approximately 0.6 µatm/d. The calculated annual air-sea CO2 transfer rate is -0.10 to -0.15 moles CO2 m-2 y-1, with these low values reflecting the relatively weak wind speed regime and small annual air-sea fCO2 disequilibrium. Extrapolating this rate over the whole Mediterranean Sea would lead to a flux of approximately -3 * 10**12 to -4.5 * 10**12 grams C/y, in good agreement with other estimates. An analysis of the effects of sampling frequency on annual air-sea CO2 flux estimates showed that monthly sampling is adequate to resolve the annual CO2 flux to within approximately ±10 - 18% at this site. Annual flux estimates made using temperature-derived fCO2 based on the measured fCO2-SST correlations are in agreement with measurement-based calculations to within ± 7-10% (depending on the gas transfer parameterization used), and suggest that annual CO2 flux estimates may be reasonably well predicted in this region from satellite or model-derived SST and wind speed information.

1995年至1997年间，系泊于地中海西北部DyFAMed站（全称：地中海大气通量动力学站，Dynamique des Flux Atmospheriques en Mediterranée）的CARIOCA浮标采集了二氧化碳分压（fCO2）、海表温度（Sea Surface Temperature, SST）及荧光数据。在季节时间尺度上，春季浮游植物水华会将表层海水fCO2降至约290微大气压（µatm），随后夏季升温使得fCO2显著升高，峰值可达约510微大气压。尽管ΔfCO2在季节尺度上存在强烈波动，但年平均海-气二氧化碳不平衡度仅为2微大气压。经温度归一化的fCO2显示，整个夏季至秋季溶解态二氧化碳持续降低，速率约为0.6微大气压/天。计算得到的年海-气CO₂传输速率为-0.10至-0.15摩尔CO₂·米⁻²·年⁻¹，该低值反映了该区域较弱的风速环境与较小的年际海-气fCO2不平衡度。将该速率外推至整个地中海海域，可得到约-3×10¹²至-4.5×10¹²克碳/年的通量，这与其他估算结果吻合良好。针对采样频率对年海-气CO₂通量估算影响的分析表明，在该站点采用月度采样即可将年CO₂通量的估算误差控制在±10%至±18%以内。基于实测fCO2与SST的相关性，利用海表温度反推得到的fCO2所计算的年通量估算值，与基于实测数据的计算结果偏差仅为±7%至±10%（取决于所采用的气体传输参数化方案），这表明该区域的年CO₂通量可通过卫星或模型反演得到的海表温度与风速信息进行较为可靠的预测。