The Potential of using Remote Sensing data to estimate Air–Sea CO<sub>2</sub> exchange in the Baltic Sea

Abstract. In this article, we present the first climatological map of air–sea CO2 flux over the Baltic Sea, based on remote-sensing data: satellite imaging derived estimates of pCO2 and remotesensed wind estimates. The pCO2 estimates estimated using self-organizing maps classifications along with class-specific linear regressions (SOMLO methodology). The estimates have a spatial resolution of 4-km both in latitude and longitude and a monthly temporal resolution from 1998 to 2011. The CO2 fluxes are estimated using two types of wind products, i.e. reanalysis winds and satellite wind products, the higher-resolution wind product generally leading to higher-amplitude fluxes estimations. Furthermore, the CO2 fluxes were also estimated using two methods: the method of Wanninkhof et al. (2012) and the method of Rutgersson et al. (2010), i.e. reanalysis winds and satellite wind products, the higher-resolution wind product generally resulting in higheramplitude fluxes. The seasonal variation in fluxes reflects the seasonal variation in pCO2 and stays similar throughout the Baltic Sea, with high CO2 emissions in winter and high CO2 uptake in summer. All basins act as a source for the atmosphere, with a higher degree of emission in the southern regions (mean source of 1.6 mmol m−2 d−1 for the South Basin and 0.9 for the Central Basin) than in the northern regions (mean source of 0.1 mmol m−2 d−1 and the coastal areas act as a larger sink (annual uptake of −4.2 mmol m−2 d−1 than does the open sea (−4 mmol m−2 d−1). In this study, we find that the Baltic Sea acts as a small source of 1.2 mmol m−2 d−1 on average and that annual uptake has increased from 1998 to 2012.