| 1. |
- Heuzé, Céline, 1988, et al.
(författare)
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Spaceborne infrared imagery for early detection of Weddell Polynya opening
- 2021
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Ingår i: Cryosphere. - : Copernicus GmbH. - 1994-0416. ; 15:7, s. 3401-3421
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Tidskriftsartikel (refereegranskat)abstract
- Knowing when sea ice will open is crucial, notably for scientific deployments. This was particularly obvious when the Weddell Polynya, a large opening in the winter Southern Ocean sea ice, unexpectedly re-appeared in 2016. As no precursor had been detected, observations were limited to chance autonomous sensors, and the exact cause of the opening could not be determined accurately. We investigate here whether the signature of the vertical ocean motions or that of the leads, which ultimately re-open the polynya, are detectable in spaceborne infrared temperature before the polynya opens. From the full historical sea ice concentration record, we find 30 polynyas starting from 1980. Then, using the full time series of the spaceborne infrared Advanced Very High Resolution Radiometer, we determine that these events can be detected in the 2 weeks before the polynya opens as a reduction in the variance of the data. For the three commonly used infrared brightness temperature bands, the 15 d sum and 15 d standard deviation of their area median and maximum are systematically lower than the climatology when a polynya will open. Moreover, by comparing the infrared brightness temperature to atmospheric reanalysis, hydrographic mooring data, and autonomous profilers, we find that temporal oscillations in one band and the decrease in the difference between bands may be used as proxies for upwelling of warm water and presence of leads, respectively, albeit with caution. Therefore, although infrared data are strongly limited by their horizontal resolution and sensitivity to clouds, they could be used for studying ocean or atmosphere preconditioning of polynyas in the historical record.
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| 2. |
- Mohrmann, Martin, et al.
(författare)
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Observed Mixing at the Flanks of Maud Rise in the Weddell Sea
- 2022
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 49:8
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Tidskriftsartikel (refereegranskat)abstract
- Maud Rise is a seamount in the eastern Weddell Sea and the location of polynyas and a persistent halo of reduced sea ice. We present novel in situ data from two profiling floats with up to daily resolved hydrographic profiles in this region. The water properties below the mixed layer of the Maud Rise region are significantly correlated with bathymetric depth; thus, the Maud Rise flank defines the front between the Warm Deep Water of the abyssal ocean and the colder Taylor cap over Maud Rise. We analyze the spiciness curvature in density space to quantify the observed frequency or magnitude of intrusions, which are substantially increased along the flanks of Maud Rise. These intrusions are indicative of enhanced lateral and vertical mixing along heavily sloping isopycnals, creating favorable conditions for thermobaric and double diffusive convection and likely facilitating the formation of the Maud Rise halo and polynyas.
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| 3. |
- Mohrmann, Martin
(författare)
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Ocean mixing and polynyas at Maud Rise, Weddell Sea
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Weddell Sea Polynya is an intermittent, ice free area in the marginal ice zone with an extent of up to 350 000 km². It was first observed by satellites in the winter seasons of 1974-1976. In 2016 and 2017, an open-ocean polynya opened over the Maud Rise oceanic plateau in the eastern Weddell Sea, which was the largest since the 70’s. Polynyas have an important role in ocean-atmosphere heat exchange, deep water and sea-ice formation. A deep layer of relatively warm Circumpolar Deep Water below the mixed layer provides the potential heat source to keep the polynya open during winter, but it is not yet fully understood how this heat is transported towards the surface. Due to its rare occurrence, most of what we know about the Weddell and Maud Rise Polynyas is based on modelling studies. This thesis is delineated into two main themes related to the Maud Rise Polynya. Firstly, this work assesses the presence and magnitude of Southern Ocean polynyas in global climate models. For this purpose, a novel algorithm to detect polynyas in satellite observational products and climate model output is applied to sea-ice concentration and thickness data. We find that both coastal and open-ocean polynyas are not well represented in climate models in terms of extent or frequency. This part discusses methods to improve the models towards a more realistic representation of polynyas. The second theme of the thesis uses new hydrographic observations at Maud Rise and the regional vicinity from autonomous profiling floats programmed to profile at high-frequency (1-3 days), that I deployed and managed. These unique observations are used in two subsequent studies. In the first, salinity and temperature profiles collected over several annual cycles indicate strong spatial gradients between relatively cold and fresh water over Maud Rise and warmer, saltier water surrounding it (Maud Rise Halo). These spatial patterns are tightly correlated with the Maud Rise bathymetry. At the transition between those two water masses at the flank of Maud Rise, interleaving is shown to occur, which causes double diffusive and thermobaric mixing to depths of 800 m. The second study focuses on the upper ocean mixed layer dynamics. The deepest wintertime mixed layers occurred over Maud Rise, but polynyas usually form over the Maud Rise Halo - a region of warm water flow surrounding the plateau region. We find that the winter water over Maud Rise is substantially thicker, and that entrainment of this winter water in autumn makes the mixed layer comparatively cold and fresh compared to the halo region. In this study a comparison with earlier profiling float observations during the 2016 and 2017 polynyas reveals that the mixed layer is significantly saltier in the autumn season. This allows for the mixed layer to deepen more rapidly and by doing so entrain warmer water from below into the mixed layer. This results in a delayed onset of sea-ice formation. In conclusion, this thesis contributes to an improved understanding of the Maud Rise oceanography and related drivers of polynya formation, by focusing on (1) large-scale forcing seen in climate models, (2) intermediate depth water mass interleaving and mixing processes and (3) mixed layer processes as a regulator to polynya occurrence.
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| 4. |
- Mohrmann, Martin, et al.
(författare)
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Southern Ocean polynyas in CMIP6 models
- 2021
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Ingår i: The Cryosphere. - : Copernicus GmbH. - 1994-0416 .- 1994-0424. ; 15:9, s. 4281-4313
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Tidskriftsartikel (refereegranskat)abstract
- Polynyas facilitate air–sea fluxes, impacting climate-relevant properties such as sea ice formation and deep water production. Despite their importance, polynyas have been poorly represented in past generations of climate models. Here we present a method to track the presence, frequency and spatial distribution of polynyas in the Southern Ocean in 27 models participating in the Climate Model Intercomparison Project Phase 6 (CMIP6) and two satellite-based sea ice products. Only half of the 27 models form open-water polynyas (OWPs), and most underestimate their area. As in satellite observations, three models show episodes of high OWP activity separated by decades of no OWP, while other models unrealistically create OWPs nearly every year. In contrast, the coastal polynya area is overestimated in most models, with the least accurate representations occurring in the models with the coarsest horizontal resolution. We show that the presence or absence of OWPs is linked to changes in the regional hydrography, specifically the linkages between polynya activity with deep water convection and/or the shoaling of the upper water column thermocline. Models with an accurate Antarctic Circumpolar Current transport and wind stress curl have too frequent OWPs. Biases in polynya representation continue to exist in climate models, which has an impact on the regional ocean circulation and ventilation that should be addressed. However, emerging iceberg discharge schemes, more adequate vertical grid type or overflow parameterisation are anticipated to improve polynya representations and associated climate prediction in the future.
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| 5. |
- Sallee, J. B., et al.
(författare)
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Southern ocean carbon and heat impact on climate
- 2023
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Ingår i: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. - 1364-503X .- 1471-2962. ; 381:2249
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Tidskriftsartikel (refereegranskat)abstract
- The Southern Ocean greatly contributes to the regulation of the global climate by controlling important heat and carbon exchanges between the atmosphere and the ocean. Rates of climate change on decadal timescales are therefore impacted by oceanic processes taking place in the Southern Ocean, yet too little is known about these processes. Limitations come both from the lack of observations in this extreme environment and its inherent sensitivity to intermittent processes at scales that are not well captured in current Earth system models. The Southern Ocean Carbon and Heat Impact on Climate programme was launched to address this knowledge gap, with the overall objective to understand and quantify variability of heat and carbon budgets in the Southern Ocean through an investigation of the key physical processes controlling exchanges between the atmosphere, ocean and sea ice using a combination of observational and modelling approaches. Here, we provide a brief overview of the programme, as well as a summary of some of the scientific progress achieved during its first half. Advances range from new evidence of the importance of specific processes in Southern Ocean ventilation rate (e.g. storm-induced turbulence, sea-ice meltwater fronts, wind-induced gyre circulation, dense shelf water formation and abyssal mixing) to refined descriptions of the physical changes currently ongoing in the Southern Ocean and of their link with global climate.This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.
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| 6. |
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| 7. |
- Swart, Sebastiaan, 1983, et al.
(författare)
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Submesoscale Fronts in the Antarctic Marginal Ice Zone and Their Response to Wind Forcing
- 2020
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Ingår i: Geophysical Research Letters. - 0094-8276. ; 47:6
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Tidskriftsartikel (refereegranskat)abstract
- Submesoscale flows in the ocean are energetic motions, O(1–10 km), that influence stratification and the distributions of properties, such as heat and carbon. They are believed to play an important role in sea‐ice‐impacted oceans by modulating air‐sea‐ice fluxes and sea‐ice extent. The intensity of these flows and their response to wind forcing are unobserved in the sea‐ice regions of the Southern Ocean. We present the first submesoscale‐resolving observations in the Antarctic marginal ice zone (MIZ) collected by surface and underwater autonomous vehicles, for >3 months in austral summer. We observe salinity‐dominated lateral density fronts occurring at sub‐kilometer scales. Surface winds are shown to modify the magnitude of the mixed‐layer density fronts, revealing strongly coupled atmosphere‐ocean processes. We posture that these wind‐front interactions occur as a continuous interplay between front slumping and vertical mixing, which leads to the dispersion of submesoscale fronts. Such processes are expected to be ubiquitous in the Southern Ocean MIZ.
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| 8. |
- Zhou, Lu, 1993, et al.
(författare)
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Early Winter Triggering of the Maud Rise Polynya
- 2022
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 49:2
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Tidskriftsartikel (refereegranskat)abstract
- What triggers Maud Rise polynya, a large opening in the winter Antarctic sea ice, is still debated. We show that the upcoming opening of all Maud Rise polynyas can be detected in early winter up to four months ahead, especially since the 2002 expansion in satellite observations. In all polynya years, continuous anomalous sea ice thinning begins in May, caused by atmospheric and oceanic forcings. Dynamically, an anomalous cyclonic circulation in the atmosphere and the ocean strengthens the Weddell Gyre and exerts anomalously intense stresses on the ice. Thermodynamically, the warm water advected by the intensified circulation, and most importantly entrained into the mixed layer, thins the ice from below at the beginning of the freezing season, preconditioning the region for a polynya event months later. This four-month-ahead pattern enables early predictions of the polynya, and improved expedition planning and sensor deployment.
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| 9. |
- Zhou, Lu, 1993, et al.
(författare)
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Sea Ice Production in the 2016 and 2017 Maud Rise Polynyas
- 2023
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Ingår i: Journal of Geophysical Research: Oceans. - 2169-9275 .- 2169-9291. ; 128:2
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Tidskriftsartikel (refereegranskat)abstract
- Sea ice production within polynyas, an outcome of the atmosphere-ice-ocean interaction, is a major source of dense water and hence key to the global overturning circulation, but is poorly quantified over open-ocean polynyas. Using the two recent extensive open-ocean polynyas within the wider Maud Rise region of the Weddell Sea in 2016 and 2017, we here explore the sea ice energy budget and estimate their sea ice production based on satellite retrievals, in-situ hydrographic observations and the Japanese 55-year Reanalysis. We find that the oceanic heat flux amounts to 36.1 and 30.7Wm−2 within the 2016 and 2017 polynyas, respectively. Especially the 2017 open-ocean polynya produced nearly 200km3 of new sea ice, which is comparable to the production in the largest Antarctic coastal polynyas. Finally, we determine that ice production is highly correlated with and sensitive to skin temperature and wind speed, which affect the turbulent fluxes. It is also strongly sensitive to uncertainties in the sea ice concentration and 1,000hPa temperature, which all urgently need to be better monitored at high latitudes. Lastly, more process-oriented campaigns are required to further elucidate the role of open-ocean polynya on the local and global ocean circulations.
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