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.

Internal tides can have a signature of several centimeters at the ocean surface and need to be corrected from altimeter measurements. We present a detailed validation of several internal-tide models using existing satellite altimeter databases. The analysis focuses on the main diurnal and semidiurnal tidal constituents. Results show the interest of the methodology proposed, the quality of the internal-tide models tested and their positive contribution for estimating an accurate sea level.

As we warm our planet, oceans expand, ice on land melts, and sea levels rise. On century timescales, we find that the sea level response to warming can be characterized by a single metric: the transient sea level sensitivity. Historical sea level exhibits substantially higher sensitivity than model-based estimates of future climates in authoritative climate assessments, implying recent projections could well underestimate the likely sea level rise by the end of this century.

In this paper, we applied FVCOM to perform the numerical simulation of salinity in Liao River estuary waters, with consideration of the effects of river discharge cutoff and shoreline variation. Decrease in runoff was demonstrated to significantly increase the mean salinity values of the estuary. The shoreline change caused by the port construction was observed to have an obvious influence on salinity distribution patterns, with the mean salinity value increased in wetland waters near the port.

Because the density of seawater depends on both temperature (T) and salinity (S), it is possible for seawater samples of the same density to have widely different T and S characteristics ranging from hot and salt (spicy) to fresh and cold (minty). For several decades, oceanographers have been debating how to best construct a variable for quantifying the spiciness of seawater. This work discusses the relative merits and drawbacks of existing approaches and proposes a new way forward.

Data from the ARKTIKA-2018 expedition and new satellite data help us to follow rapid changes in the upper layer of the Laptev and East Siberian seas (LS, ESS) in summer 2018. With satellite-derived surface temperature, an improved SMOS salinity, and wind, we study how the fresh river water is mixed with cold sea water and ice-melted water at small time and spatial scales. The wind pushes fresh water northward and northeastward, close to and under the ice, forcing it into the deep Arctic Ocean.

We conducted an underway measurement of dissolved O₂ / Ar using membrane inlet mass spectrometry and estimated net community production (NCP) in the South China Sea (SCS) in both summer and autumn. We found that nutrient content, especially nitrogen, is the dominant factor affecting primary production in the SCS. In the summer, we observed a rapid response of the ecosystem to episodic nutrient supply induced by eddies. This is the first report on NCP estimation based on O₂ / Ar data in the SCS.

The Atlantic Meridional Overturning Circulation (AMOC) is an important component of the climate system. Understanding its structure and variability is a key priority for many scientists. Here, we present the first estimate of AMOC variations for the tropical South Atlantic from the TRACOS array at 11° S. Over the observed period, the AMOC was dominated by seasonal variability. We investigate the respective mechanisms with an ocean model and find that different wind-forced waves play a big role.

The RAPID array has observed the Atlantic meridional overturning circulation (AMOC) since 2004, but the AMOC was directly calculated only five times from 1957–2004. Here we create a statistical regression model from RAPID data, relating AMOC changes to density changes within the different water masses at 26°  N, and apply it to historical hydrographic data. The resulting 1981–2016 record shows that the AMOC from 2008–2012 was its weakest since the mid-1980s, but it shows no overall decline.

Changes in ocean temperature may affect vertical density stratification, which may hamper turbulent exchange and thus nutrient availability for phytoplankton growth. To quantify varying physical conditions, we sampled the upper 500  m along 17 ± 5° W between [30, 63]°  N in summer. South to north, temperature decreased with stratification while turbulence and nutrient fluxes remained constant, likely due to internal waves breaking and little affected by the physical process of global warming.

Environmental issues arising due to coastal modification and future sea level scenarios are a major environmental hazard facing the Maldives today. Here, we carry out high-resolution tidal modelling of a Maldivian atoll for the first time and show that coastal modification in the island scale is capable of driving large-scale change in the wider atoll basin in a short time, comparable to that of long-term sea level rise scenarios and on par with observations.

The Maud Rise polynya is a hole in the sea ice surrounding Antarctica that occurs during winter. It appeared in 2016 and 2017. Our study concludes that heat and salt accumulation around 1000 m depth are likely to be important for polynya formation. The heat is mixed upward to the surface where it is able to melt the sea ice and, thus, create a polynya. How often the polynya forms depends largely on the variation in the time of the heat and salt accumulation.

The classical theories of the western boundary currents, proposed in the first half of the 20th century, are extended to include cases of zonally elongated and meridionally narrow ocean basins. Results show for the first time that in basins that are sufficiently narrow meridionally, the equatorward wind-driven transport away from the western boundary is lower than that in meridionally wide basins. Our theoretical results are employed to explain the low transport in the East Australian Current.

The Arctic Ocean is a major sink for heat and salt for the global ocean. Ocean mixing contributes to this sink by mixing the Atlantic and Pacific waters with surrounding waters. We investigate the drivers of ocean mixing north of Svalbard based on observations collected during two research cruises in 2018 as part of the Nansen Legacy project. We found that wind and tidal forcing are the main drivers and that 1 % of the Atlantic Water heat loss can be attributed to vertical turbulent mixing.

Marine electromagnetic (EM) signals largely depend on three factors: the direction and speed of ocean flow, the strength of Earth’s main magnetic field, and seawater’s electrical conductivity (which depends on the local temperature and salinity). Because of this, there is interest in using marine EM signals to monitor and study ocean circulation. Our study investigates using voltage data from retired seafloor telecommunication cables in the Pacific Ocean to monitor large-scale flows.

In this study, we found that the interannual subsurface temperature and salinity variability of the Bay of Bengal (BoB) shows a remarkable delayed correlation with the Indian Ocean Dipole mode. We employed a regional model and determined the contributions of the coastal Kelvin waves and the westward-moving Rossby waves to this correlation. An analysis of the salinity budget revealed that the advection terms dominate the subsurface salinity changes in the BoB.