Upwelling-induced trace gas dynamics in the Baltic Sea inferred from 8 years of autonomous measurements on a ship of opportunity

Abstract

Autonomous measurements aboard ships of op portunity (SOOP) provide in situ data sets with high spa tial 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 en riched 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 reanal ysed 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 concentra tion 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 rela tionships between increasing trace gas levels and decreasing temperature. Upwelling relaxation is mainly driven by mix ing, modulated by air sea gas exchange, and possibly pri mary 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 re verses from CO2 sink to source, and CH4 outgassing is in tensified 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.