The potential of using remote sensing data to estimate air–sea CO2 exchange in the Baltic Sea

In this article, we present the first climatological map of air–sea CO2 flux over the Baltic Sea based on remote sensing data: Estimates of pCO2 derived from satellite imaging using self-organizing map classifications along with class-specific linear regressions (SOMLO methodology) and remotely sensed wind estimates. The estimates have a spatial resolution of 4km 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 flux estimations. Furthermore, the CO2 fluxes were also estimated using two methods: The method of Wanninkhof et al. (2013) and the method of Rutgersson and Smedman (2009). The seasonal variation in fluxes reflects the seasonal variation in pCO2 unvaryingly over the whole Baltic Sea, with high winter CO2 emissions and high pCO2 uptakes. All basins act as a source for the atmosphere, with a higher degree of emission in the southern regions (mean source of 1.6mmolm-2d-1 for the South Basin and 0.9 for the Central Basin) than in the northern regions (mean source of 0.1mmolm-2d-1) and the coastal areas act as a larger sink (annual uptake of -4.2mmolm-2d-1) than does the open sea (-4mmolm-2d-1). In its entirety, the Baltic Sea acts as a small source of 1.2mmolm-2d-1 on average and this annual uptake has increased from 1998 to 2012.