ALBERT

Description: Based on direct surface pCO2 observation and a model-based extrapolation technique, we established a regional pCO2 climatology of the Baltic Sea. Observations from June 2003 to Dec. 2021 are obtained from the SOCAT version 2022 data collection and largely based on ICOS DE-SOOP Finnmaid data. The extrapolation technique uses model-based patters of variability to create observational data-constrained, gap- and discontinuity-free mapped fields including local error estimates without the need for or dependence on ancillary data (like, e.g., satellite sea surface temperature maps). Details on the pCO2 climatology and the model-based extrapolation technique are found in Bittig et al. (2023). Here we make the corresponding dataset available with monthly climatological pCO2 value as well as a linear pCO2 time trend for the Baltic Sea domain. Both value and trend are provided with their error estimate and are centered on the 15th of each month. Besides, the long-term trend 2003-2021 in pCO2 as well as its error estimate is given.

Description: Autonomous measurements aboard ships of opportunity (SOOP) provide in situ data sets with high spatial and temporal coverage. In this study, we use 8 years of carbon dioxide (CO2) and methane (CH4) observations from SOOP Finnmaid to study the influence of upwelling on trace gas dynamics in the Baltic Sea. Between spring and autumn, coastal upwelling transports water masses enriched with CO2 and CH4 to the surface of the Baltic Sea. We study the seasonality, regional distribution, relaxation, and interannual variability in this process. We use reanalysed wind and modelled sea surface temperature (SST) data in a newly established statistical upwelling detection method to identify major upwelling areas and time periods. Large upwelling-induced SST decrease and trace gas concentration increase are most frequently detected around August after a long period of thermal stratification, i.e. limited exchange between surface and underlying waters. We found that these upwelling events with large SST excursions shape local trace gas dynamics and often lead to near-linear relationships between increasing trace gas levels and decreasing temperature. Upwelling relaxation is mainly driven by mixing, modulated by air–sea gas exchange, and possibly primary production. Subsequent warming through air–sea heat exchange has the potential to enhance trace gas saturation. In 2015, quasi-continuous upwelling over several months led to weak summer stratification, which directly impacted the observed trace gas and SST dynamics in several upwelling-prone areas. Trend analysis is still prevented by the observed high variability, uncertainties from data coverage, and long water residence times of 10–30 years. We introduce an extrapolation method based on trace gas–SST relationships that allows us to estimate upwelling-induced trace gas fluxes in upwelling-affected regions. In general, the surface water reverses from CO2 sink to source, and CH4 outgassing is intensified as a consequence of upwelling. We conclude that SOOP data, especially when combined with other data sets, enable flux quantification and process studies addressing the process of upwelling on large spatial and temporal scales.