Polona Itkin

The thickness and consolidation state of pressure ridges are variables relevant for sea ice mass balance, melt processes and ecosystem habitat. We show how both variables can be detected by the multi-frequency electromagnetic induction (EMI) sounding, based on data collected during Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) in the central Arctic Ocean between November 2019 and July 2020. We validate the EMI results by collocating them to sea ice topography from the airborne laser scanner, underwater topography from multi-beam sonar, and sea ice thickness and structure from drill holes. Selected channels of low frequency give good estimates of total thickness, while selected channels of high frequency give good estimates of consolidated layer thickness. The MOSAiC EMI dataset was collected over a large number of ridge systems formed between freeze-up and break-up. Nine individual ridge transects can be used to track the seasonal development. The footprint size and sensitivity of the method make the EMI sounding appropriate for the detection of ridges with up to 10 m total thickness. Where available, the ridge structure data from other methods are consistent with our results. The superior temporal and spatial coverage of the MOSAiC EMI data permits further analysis that indicates a slow reduction of the total thickness towards the consolidated layer thickness. Interestingly, the consolidated layer thickness exceeds the level ice thickness by a factor of 1.6 to 2 and also shows, at least for individual ridges, a seasonal decrease. This may be a feature of the thin snow cover at MOSAiC. Multi-frequency EMI is a promising method for non-intrusive pressure ridge surveying with a large potential for pressure ridge dataset extension.

The Arctic is the fastest changing region in the world. Its atmosphere is warming 3–4 times the global average; the ocean is warming through increased advection of heat from the Atlantic and Pacific; the sea ice cover is disappearing, thinning, and becoming more mobile. The interactions between ocean, ice and atmosphere are also evolving. These changes are manifest in polynyas, openings in the sea ice that are crucial for exchanges of heat, moisture, momentum and gases between ocean and atmosphere, but also for ecosystems, Indigenous communities, and navigation. We here review how polynyas and their impact on the climate system have changed throughout the satellite record, focussing on the last twenty years. The trend is of fewer but larger polynyas that open for longer, unless they depend on ice arches (in which case they are shorter-lived). Many regions have switched from ''latent heat'' polynyas, only driven by dynamics, to ''hybrid'', where the heat from ocean or atmosphere matters. Consistent with that switch and with an overall warming and wetting of the atmosphere, moisture fluxes from polynyas in winter seem to be decreasing despite an increase in polynya area. How this will impact the low altitude clouds, generated by polynyas and that help maintain polynyas, has not been studied. Similarly, sea ice production in polynyas is decreasing at the pan-Arctic scale. In the ocean's Eurasian basin, process studies suggest a conflict between this switch to hybrid polynyas (suppressing dense water formation) and Atlantification (favouring it). In the Amerasian basin, where upwelling of warm water is necessary for dense water formation, this should be enhanced, but it also facilitates the inflow of warm water into the Greenland glacial fjords. In all components of the climate system, large uncertainties remain because of the lack of direct observations and the lack of continuous time series in the atmosphere, ice, and ocean.

Pan-Arctic sea ice thickness estimates are routinely produced from Ku-band radar altimetry observations. State-of-the-art waveform retracking algorithms rely on the uncertain assumption that Ku-band radar waves penetrate through the snow and that the dominant return originates from surface scattering at the snow/ice interface. However, growing evidence suggests that Ku-band radar altimetry freeboards may not always accurately trace the snow/ice interface elevation. We investigate this by analyzing the evolution of spaceborne radar and laser freeboards over immobile regions of landfast first-year ice (FYI) and multi-year ice (MYI) off Greenland's coast from mid-winter to mid-summer 2022. Our results suggest that the radar freeboards over FYI trace the snow/ice interface during most of the cold season (up to mid-May, at this location), providing empirical support for the validity of the assumption of full snow penetration and a dominant return from the snow/ice interface at Ku-band frequency. Over MYI, the retracked heights correspond to locations well below the air/snow interface most of the time, at least 40 % deep in the snow but likely much deeper, although the exact depth could not be reliably assessed. However, these data also provide evidence for short periods of positively biased Ku-band radar freeboards throughout the winter and the melt season. In particular, our results suggest that, during winter, the Ku-band radar freeboards tend to be biased high (i) for ice with a saline snow cover during a warming event (brine volume > 1 %), and (ii) during and immediately after strong snowfall events. Winter warming events are often accompanied by snowfall, leading to a cumulative bias for the areas with saline snow – i.e. most FYI in the Arctic. During winter, biases are up to 4 cm for FYI and 10 cm for MYI, and periods with biased freeboards are short, typically lasting 7–10 d. In the period of snow melt onset in May/June, biased radar freeboards appear to be related to saline snow only (i.e. FYI), but biases are on average larger (up to 10 cm) than during the winter period. Though our results indicate a positive bias in satellite radar freeboards under specific snow conditions, the winter periods with biased freeboards are short, in total accounting for only ∼ 15 % of the time. Our findings therefore generally support the assumption of a dominant return originating from the snow/ice interface for Ku-band sea ice thickness and dual-altimetry snow depth retrievals during winter, such as planned for the Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL) mission.

The Eurasian Basin Transect 2026 cruise (EBT2026; ID 2026007011) was conducted 23 July–10 September 2026. Its geographical coverage included the Nansen and Amundsen Basins, the adjacent Lomonosov Ridge and Southeastern Makarov Basin, including the full width of the Transpolar Drift current. Throughout the cruise, a comprehensive set of physical, chemical, and biological observations was collected from multiple components of the Arctic system, including the atmosphere, ocean, sea ice, and sea floor.

The availability of nutrients to sea-ice microbial communities varies as drifting pack ice traverses distinct hydrographic regimes and experiences episodic mixing events. Vertical turbulent fluxes generated by processes such as ice movement and wind can enhance nutrient resupply at the ice–ocean interface, potentially influencing community composition and net community production. In this study, we evaluate the relationships between bottom sea-ice algal communities and oceanic conditions during a sea-ice drift study north of Svalbard, which crossed different branches of the Atlantic Water inflow to the Arctic over the Yermak Plateau and the Sofia Deep. Despite elevated nitrate concentrations in under-ice waters associated with the Svalbard Branch of Atlantic Water inflow, the largest nutrient and heat fluxes to the bottom ice occurred during a storm event. These fluxes coincided with increased biomass-specific net community productivity but also enhanced algal biomass loss through melt. At the same time, snow redistribution during the storm increased snow depths and reduced light availability at the bottom ice, affecting net community production. Independent of nutrient conditions, the transition from net heterotrophy (oxygen consumption) to net autotrophy (oxygen production) occurred when under-ice transmitted photosynthetically active radiation exceeded a daily range of 1.1 to 3.5 µmol m^–2 s^–1. Although nutrient supply remains important, our results suggest that light availability may exert a stronger control on sea-ice net community production than nutrient supply when nutrients are not critically depleted. We further highlight the importance of episodic storm events in modifying nutrient and heat vertical fluxes, and light availability at the ice–ocean interface, thereby contributing to regional variability in ice algal bloom dynamics.

The availability of nutrients to sea-ice microbial communities varies as drifting pack ice traverses distinct hydrographic regimes and experiences episodic mixing events. Vertical turbulent fluxes generated by processes such as ice movement and wind can enhance nutrient resupply at the ice–ocean interface, potentially influencing community composition and net community production. In this study, we evaluate the relationships between bottom sea-ice algal communities and oceanic conditions during a sea-ice drift study north of Svalbard, which crossed different branches of the Atlantic Water inflow to the Arctic over the Yermak Plateau and the Sofia Deep. Despite elevated nitrate concentrations in under-ice waters associated with the Svalbard Branch of Atlantic Water inflow, the largest nutrient and heat fluxes to the bottom ice occurred during a storm event. These fluxes coincided with increased biomass-specific net community productivity but also enhanced algal biomass loss through melt. At the same time, snow redistribution during the storm increased snow depths and reduced light availability at the bottom ice, affecting net community production. Independent of nutrient conditions, the transition from net heterotrophy (oxygen consumption) to net autotrophy (oxygen production) occurred when under-ice transmitted photosynthetically active radiation exceeded a daily range of 1.1 to 3.5 µmol m^–2 s^–1. Although nutrient supply remains important, our results suggest that light availability may exert a stronger control on sea-ice net community production than nutrient supply when nutrients are not critically depleted. We further highlight the importance of episodic storm events in modifying nutrient and heat vertical fluxes, and light availability at the ice–ocean interface, thereby contributing to regional variability in ice algal bloom dynamics.

The Arctic is the fastest changing region in the world. Its atmosphere is warming 3–4 times the global average; the ocean is warming through increased advection of heat from the Atlantic and Pacific; the sea ice cover is disappearing, thinning, and becoming more mobile. The interactions between ocean, ice and atmosphere are also evolving. These changes are manifest in polynyas, openings in the sea ice that are crucial for exchanges of heat, moisture, momentum and gases between ocean and atmosphere, but also for ecosystems, Indigenous communities, and navigation. We here review how polynyas and their impact on the climate system have changed throughout the satellite record, focussing on the last twenty years. The trend is of fewer but larger polynyas that open for longer, unless they depend on ice arches (in which case they are shorter-lived). Many regions have switched from ''latent heat'' polynyas, only driven by dynamics, to ''hybrid'', where the heat from ocean or atmosphere matters. Consistent with that switch and with an overall warming and wetting of the atmosphere, moisture fluxes from polynyas in winter seem to be decreasing despite an increase in polynya area. How this will impact the low altitude clouds, generated by polynyas and that help maintain polynyas, has not been studied. Similarly, sea ice production in polynyas is decreasing at the pan-Arctic scale. In the ocean's Eurasian basin, process studies suggest a conflict between this switch to hybrid polynyas (suppressing dense water formation) and Atlantification (favouring it). In the Amerasian basin, where upwelling of warm water is necessary for dense water formation, this should be enhanced, but it also facilitates the inflow of warm water into the Greenland glacial fjords. In all components of the climate system, large uncertainties remain because of the lack of direct observations and the lack of continuous time series in the atmosphere, ice, and ocean.

The availability of nutrients to sea-ice microbial communities varies as drifting pack ice traverses distinct hydrographic regimes and experiences episodic mixing events. Vertical turbulent fluxes generated by processes such as ice movement and wind can enhance nutrient resupply at the ice–ocean interface, potentially influencing community composition and net community production. In this study, we evaluate the relationships between bottom sea-ice algal communities and oceanic conditions during a sea-ice drift study north of Svalbard, which crossed different branches of the Atlantic Water inflow to the Arctic over the Yermak Plateau and the Sofia Deep. Despite elevated nitrate concentrations in under-ice waters associated with the Svalbard Branch of Atlantic Water inflow, the largest nutrient and heat fluxes to the bottom ice occurred during a storm event. These fluxes coincided with increased biomass-specific net community productivity but also enhanced algal biomass loss through melt. At the same time, snow redistribution during the storm increased snow depths and reduced light availability at the bottom ice, affecting net community production. Independent of nutrient conditions, the transition from net heterotrophy (oxygen consumption) to net autotrophy (oxygen production) occurred when under-ice transmitted photosynthetically active radiation exceeded a daily range of 1.1 to 3.5 µmol m^–2 s^–1. Although nutrient supply remains important, our results suggest that light availability may exert a stronger control on sea-ice net community production than nutrient supply when nutrients are not critically depleted. We further highlight the importance of episodic storm events in modifying nutrient and heat vertical fluxes, and light availability at the ice–ocean interface, thereby contributing to regional variability in ice algal bloom dynamics.

The Arctic is the fastest changing region in the world. Its atmosphere is warming 3–4 times the global average; the ocean is warming through increased advection of heat from the Atlantic and Pacific; the sea ice cover is disappearing, thinning, and becoming more mobile. The interactions between ocean, ice and atmosphere are also evolving. These changes are manifest in polynyas, openings in the sea ice that are crucial for exchanges of heat, moisture, momentum and gases between ocean and atmosphere, but also for ecosystems, Indigenous communities, and navigation. We here review how polynyas and their impact on the climate system have changed throughout the satellite record, focussing on the last twenty years. The trend is of fewer but larger polynyas that open for longer, unless they depend on ice arches (in which case they are shorter-lived). Many regions have switched from ''latent heat'' polynyas, only driven by dynamics, to ''hybrid'', where the heat from ocean or atmosphere matters. Consistent with that switch and with an overall warming and wetting of the atmosphere, moisture fluxes from polynyas in winter seem to be decreasing despite an increase in polynya area. How this will impact the low altitude clouds, generated by polynyas and that help maintain polynyas, has not been studied. Similarly, sea ice production in polynyas is decreasing at the pan-Arctic scale. In the ocean's Eurasian basin, process studies suggest a conflict between this switch to hybrid polynyas (suppressing dense water formation) and Atlantification (favouring it). In the Amerasian basin, where upwelling of warm water is necessary for dense water formation, this should be enhanced, but it also facilitates the inflow of warm water into the Greenland glacial fjords. In all components of the climate system, large uncertainties remain because of the lack of direct observations and the lack of continuous time series in the atmosphere, ice, and ocean.

Pan-Arctic sea ice thickness estimates are routinely produced from Ku-band radar altimetry observations. State-of-the-art waveform retracking algorithms rely on the uncertain assumption that Ku-band radar waves penetrate through the snow and that the dominant return originates from surface scattering at the snow/ice interface. However, growing evidence suggests that Ku-band radar altimetry freeboards may not always accurately track this elevation. We investigate this question by analyzing the evolution of spaceborne radar and laser freeboards over immobile regions of landfast first-year ice (FYI) and multi-year ice (MYI) off Greenland's coast from mid-winter to mid-summer 2022. Our results suggest that the radar freeboards over FYI trace the snow/ice interface during most of the cold season (up to mid-May, at this location), providing empirical support for the validity of the assumption of full snow penetration at Ku-band frequency. Over MYI, the retracked heights correspond to locations well below the air/snow interface most of the time, at least 60 % deep in the snow, but the exact depth could not be reliably assessed. However, these data also provide evidence for a positive bias in Ku-band radar freeboards during short intervals throughout the winter and the melt season, for both ice types. In particular, our results suggest that, during winter, the Ku-band radar freeboards tend to be biased high: i) for ice with a saline snow cover during a warming event (brine volume>1 %), and ii) during and immediately after strong snowfall events. Winter warming events are often accompanied by snowfall, leading to a cumulative bias for the areas with saline snow – i.e. most FYI in the Arctic. In the period of snow melt onset in May/June, biased radar freeboards appear to be related to saline snow only, but biases are on average larger (up to 10 cm) than during the winter period (under 4 cm). Though our results indicate a positive bias in satellite radar freeboards under specific snow conditions, periods of biased freeboard are short-term during the winter – in total accounting for approximately 15 % of the time. Our findings therefore generally support the assumption of full snow penetration for Ku-band sea ice thickness and dual-altimetry snow depth retrievals during winter, such as planned for the Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL) mission.