This paper describes challenges around the direct measurement of CO₂ in the ocean using ocean gliders. We discuss our method of using multiple sensor platforms as test beds to carry out observing experiments and highlight the implications of our study for future glider missions. We also show high-resolution measurements and discuss challenges and lessons learned in the context of future ocean gas measurements.

We investigated the long-term changes of the principal tidal component M₂ along North Atlantic coasts, from 1846 to 2018. We analysed 18 tide gauges. We found that M₂ variations are consistent at all the stations in the North-East Atlantic, whereas some discrepancies appear in the North-West Atlantic. The similarity between the North Atlantic Oscillation and M₂ variations in the North-East Atlantic suggests a possible influence of the large-scale atmospheric circulation on the tide.

Vertical land motion (VLM) significantly contributes to relative sea level change. Here, we improve the accuracy and precision of VLM estimates, which are based on the difference of altimetry tide gauge observations. Advanced coastal altimetry and an improved coupling procedure of along-track altimetry data and high-frequency tide gauge observations are key factors for a greater comparability of altimetry and tide gauges in the coastal zone and thus for more reliable VLM estimates.

Dense waters sinking by Antarctica and in the North Atlantic control global ocean currents and carbon storage. We need to know how these change with climate change, and thus we need accurate climate models. Here we show that dense water sinking in the latest models is better than in the previous ones, but there is still too much water sinking. This impacts how well models represent the deep ocean density and the deep currents globally. We also suggest ways to improve the models.

The proposed method of data assimilation is capable of effectively correcting basin-scale mismatch of oceanographic models when the domain is under nearly coherent external forcing. The method uses basin-scale EOF modes, calculated from the long-term model statistics. These modes are used to reconstruct gridded fields from point observations, which are further fed to the model using relaxation. Tests with sea surface temperature and salinity in the NE Baltic Sea were successful.

Atlantic Water that carries heat and anthropogenic carbon into the Arctic Ocean plays an important role in the Arctic sea-ice cover decline, but its pathways and travel times remain unclear. Here we used two radionuclides of anthropogenic origin (¹²⁹I and ²³⁶U) to track Atlantic-derived waters along their way through the Arctic Ocean, estimating their travel times and mixing properties. Results help to understand how future changes in Atlantic Water properties will spread through the Arctic.