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Tides significantly affect polar coastlines by modulating ice shelf melt and modifying shelf water properties through transport and mixing. However, the effect of tides on the marine carbonate chemistry in such regions, especially around Antarctica, remains largely unexplored. We address this topic with two case studies in a coastal polynya in the south-eastern Weddell Sea, neighbouring the Ekström Ice Shelf. The case studies were conducted in January 2015 (PS89) and January 2019 (PS117), capturing semi-diurnal oscillations in the water column. These are pronounced in both physical and biogeochemical variables for PS89. During rising tide, advection of sea ice meltwater from the north-east created a fresher, warmer, and more deeply mixed water column with lower dissolved inorganic carbon (DIC) and total alkalinity (TA) content. During ebbing tide, water from underneath the ice shelf decreased the polynya's temperature, increased the DIC and TA content, and created a more stratified water column. The variability during the PS117 case study was much smaller, as it had less sea ice meltwater input during rising tide and was better mixed with sub-ice shelf water. The contrasts in the variability between the two case studies could be wind and sea ice driven, and they underline the complexity and highly dynamic nature of the system. The variability in the polynya induced by the tides results in an air–seaCO2 flux that can range between a strong sink (−24 mmol m−2 d−1) and a small source (3 mmol m−2 d −1) on a semi-diurnal timescale. If the variability induced by tides is not taken into account, there is a potential risk of overestimating the polynya's CO2 uptake by 67 % or underestimating it by 73 %, compared to the average flux determined over several days. Depending on the timing of limited sampling, the polynya may appear to be a source or a sink of CO2. Given the disproportionate influence of polynyas on heat and carbon exchange in polar oceans, we recommend future studies around the Antarctic and Arctic coastlines to consider the timing of tidal currents in their sampling strategies and analyses. This will help constrain variability in oceanographic measurements and avoid potential biases in our understanding of these highly complex systems.

The decline in ocean oxygen content is one of the most alarming consequences of anthropogenic-driven climate change. A key challenge is that global climate models do not currently reproduce observed changes in deoxygenation, showing high inter-model variability and uncertainty. This uncertainty is partially due to the models’ inability to resolve features smaller than their computational grid cells resulting in large biases in ventilation. The Persian Gulf Water outflow has been pointed out by several studies as one of the sources of ventilation in the Arabian Sea Oxygen Minimum Zone (OMZ). This oxygenated water mass flows eastward along the shelf edge of the northern Omani coast at 200m depth and is fragmented by the mesoscale eddy field and rough topography, generating small “peddies”. These peddies and their relatively high oxygen concentrations have potential to ventilate the OMZ, yet this has been poorly investigated due to a lack of adequate observations. We use multi-month glider campaigns off the coast of Oman with a SeaExplorer glider equipped with an ADCP to resolve the contribution of the Persian Gulf Water outflow to oxygen supply within the Arabian Sea OMZ. We characterize its properties, seasonality, and spatial distribution and estimate mixing rates from double diffusion, salt-fingering, and shear-driven mixing to understand water mass transformations and oxygen fluxes into the OMZ.

We present year-long, vertically-resolved, high-resolution observations of phytoplankton community and primary productivity in the Bornholm Basin of the Baltic Sea. Sustained high-resolution monitoring reveals how the small-scale environmental dynamics regulate the phytoplankton community composition and primary productivity in the Bornholm Basin. We combine data from the Swedish National Monitoring Program, collecting phytoplankton (abundance, biomass and productivity) and environmental (e.g. nutrients concentration) samples with remote sensing data and sustained observations from an Ocean Glider Observatory from March 2021 to 2022.The Ocean Glider Observatory continuously collected high-resolution physical (e.g.temperature, salinity) and biochemical (chlorophyll a, dissolved oxygen) data (vertical resolutions at 10-centimetre scale and vertical profiles with a mean temporal resolution of ~37 min). Glider data are calibrated against remote sensing data, while in situ measurements of photosynthetic yield and community composition allow us to model community composition and primary productivity across the dataset.Accurate estimation of primary productivity reveals the importance of short term and rapid changes in near-surface stratification and the deep chlorophyll maximum for the Bornholm ecosystem. Vertically resolved glider estimates also highlight limitations of remote sensing methods in this cloud covered region. As phytoplankton plays a crucial role in the transfer of energy through the food web via zooplankton up to fish, the importance of sustained, continuous high-resolution observations is required to fully capture the magnitude and variability of ecosystem processes and consequences to productivity.

Funding: This work is from the MELT project, a component of the International Thwaites Glacier Collaboration (ITGC). Support from the National Science Foundation (NSF, grant no. 1739003) and the Natural Environment Research Council (NERC, grant no. NE/S006656/1). Logistics provided by NSF U.S. Antarctic Program and NERC British Antarctic Survey. The ship-based CTD data were supported by the ITGC TARSAN project (NERC grant nos. NE/S006419/1 and NE/S006591/1; NSF grant no. 1929991). ITGC contribution no. ITGC 047. Thwaites Glacier is one of the fastest-changing ice–ocean systems in Antarctica1,2,3. Much of the ice sheet within the catchment of Thwaites Glacier is grounded below sea level on bedrock that deepens inland4, making it susceptible to rapid and irreversible ice loss that could raise the global sea level by more than half a metre2,3,5. The rate and extent of ice loss, and whether it proceeds irreversibly, are set by the ocean conditions and basal melting within the grounding-zone region where Thwaites Glacier first goes afloat3,6, both of which are largely unknown. Here we show—using observations from a hot-water-drilled access hole—that the grounding zone of Thwaites Eastern Ice Shelf (TEIS) is characterized by a warm and highly stable water column with temperatures substantially higher than the in situ freezing point. Despite these warm conditions, low current speeds and strong density stratification in the ice–ocean boundary layer actively restrict the vertical mixing of heat towards the ice base7,8, resulting in strongly suppressed basal melting. Our results demonstrate that the canonical model of ice-shelf basal melting used to generate sea-level projections cannot reproduce observed melt rates beneath this critically important glacier, and that rapid and possibly unstable grounding-line retreat may be associated with relatively modest basal melt rates. Publisher PDF

Shelf seas play a key role in both the global carbon cycle and coastal marine ecosystems through the draw-down and fixing of carbon, as measured through phytoplankton net primary production (NPP). Measuring NPP in situ and extrapolating this to the local, regional, and global scale presents challenges however because of limitations with the techniques utilised (e.g. radiocarbon isotopes), data sparsity, and the inherent biogeochemical heterogeneity of coastal and open-shelf waters. Here, we introduce a new data set generated using a technique based on the synergistic use of in situ glider profiles and satellite Earth observation measurements which can be implemented in a real-time or delayed-mode system (https://doi.org/10.5285/e6974644-2026-0f94-e053-6c86abc00109; Loveday and Smyth, 2022). We apply this system to a fleet of gliders successively deployed over a 19-month time frame in the North Sea, generating an unprecedented fine-scale time series of NPP in the region. At a large scale, this time series gives close agreement with existing satellite-based estimates of NPP for the region and previous in situ estimates. What has not been elucidated before is the high-frequency, small-scale, depth-resolved variability associated with bloom phenology, mesoscale phenomena, and mixed layer dynamics.

Tides significantly affect polar coastlines by modulating ice shelf melt and modifying shelf water properties through transport and mixing. However, the effect of tides on the marine carbonate chemistry in such regions, especially around Antarctica, remains largely unexplored. We address this topic with two case studies in a coastal polynya in the south-eastern Weddell Sea, neighbouring the Ekström Ice Shelf. The case studies were conducted in January 2015 (PS89) and January 2019 (PS117), capturing semi-diurnal oscillations in the water column. These are pronounced in both physical and biogeochemical variables for PS89. During rising tide, advection of sea ice melt water from the north-east created a fresher, warmer, more deeply mixed water column with lower dissolved inorganic carbon (DIC) and total alkalinity (TA) content. During ebbing tide, water from underneath the ice shelf decreased the polynya's temperature, increased the DIC and TA content, and created a more stratified water column. The variability during the PS117 case study was much smaller, as it had less sea ice melt water input during rising tide and was better mixed with sub-ice shelf water. The contrasts in the variability between the two case studies could be wind and sea ice driven, and underline the complexity and highly dynamic nature of the system. The variability in the polynya induced by the tides results in an air-sea CO2 flux that can range between a strong sink (-20 mmol m-2 day-1) and a small source (7 mmol m-2 day-1) on a semi-diurnal time scale. If the variability induced by tides is not taken into account, there is a potential risk of overestimating the polynya's CO2 uptake by 98 % or underestimating it by 108 % (mistaking it for a source instead of a variable sink), compared to the average flux determined over several days. Given the disproportionate influence of polynyas on heat and carbon exchange in polar oceans, we recommend that future studies around the Antarctic and Arctic coastlines consider the timing of tidal currents in their sampling strategies and analyses. This will help constrain variability in oceanographic measurements and avoid potential biases in our understanding of these highly complex systems.

High-resolution underwater glider data collected in the Gulf of Oman (2015-16), combined with reanalysis datasets, describe the spatial and temporal variability of the mixed layer during winter and spring. We assess the effect of surface forcing and submesoscale processes on upper ocean buoyancy and their effects on mixed layer stratification. Episodic strong and dry wind events from the northwest (Shamals) drive rapid latent heat loss events which lead to intraseasonal deepening of the mixed layer. Comparatively, the prevailing southeasterly winds in the region are more humid, and do not lead to significant heat loss, thereby reducing intraseasonal upper ocean variability in stratification. We use this unique dataset to investigate the presence and strength of submesoscale flows, particularly in winter, during deep mixed layers. These submesoscale instabilities act mainly to restratify the upper ocean during winter through mixed layer eddies. The timing of the spring restratification differs by three weeks between 2015 and 2016 and matches the sign change of the net heat flux entering the ocean and the presence of restratifying submesoscale fluxes. These findings describe key high temporal and spatial resolution drivers of upper ocean variability, with downstream effects on phytoplankton bloom dynamics and ventilation of the oxygen minimum zone.