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- Abrahamsen, E. Povl, et al.
(author)
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ANTARCTICA AND THE SOUTHERN OCEAN
- 2020
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In: BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY. - 0003-0007 .- 1520-0477. ; 101:8
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Journal article (peer-reviewed)
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- Cronin, M. F., et al.
(author)
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Developing an Observing Air-Sea Interactions Strategy (OASIS) for the global ocean
- 2022
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In: Ices Journal of Marine Science. - : Oxford University Press (OUP). - 1054-3139 .- 1095-9289. ; 80:2, s. 367-73
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Journal article (peer-reviewed)abstract
- The Observing Air-Sea Interactions Strategy (OASIS) is a new United Nations Decade of Ocean Science for Sustainable Development programme working to develop a practical, integrated approach for observing air-sea interactions globally for improved Earth system (including ecosystem) forecasts, CO2 uptake assessments called for by the Paris Agreement, and invaluable surface ocean information for decision makers. Our "Theory of Change" relies upon leveraged multi-disciplinary activities, partnerships, and capacity strengthening. Recommendations from >40 OceanObs'19 community papers and a series of workshops have been consolidated into three interlinked Grand Ideas for creating #1: a globally distributed network of mobile air-sea observing platforms built around an expanded array of long-term time-series stations; #2: a satellite network, with high spatial and temporal resolution, optimized for measuring air-sea fluxes; and #3: improved representation of air-sea coupling in a hierarchy of Earth system models. OASIS activities are organized across five Theme Teams: (1) Observing Network Design & Model Improvement; (2) Partnership & Capacity Strengthening; (3) UN Decade OASIS Actions; (4) Best Practices & Interoperability Experiments; and (5) Findable-Accessible-Interoperable-Reusable (FAIR) models, data, and OASIS products. Stakeholders, including researchers, are actively recruited to participate in Theme Teams to help promote a predicted, safe, clean, healthy, resilient, and productive ocean.
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- du Plessis, Marcel, 1990, et al.
(author)
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The Daily-Resolved Southern Ocean Mixed Layer: Regional Contrasts Assessed Using Glider Observations
- 2022
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In: Journal of Geophysical Research-Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 127:4
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Journal article (peer-reviewed)abstract
- Water mass transformation in the Southern Ocean is vital for driving the large-scale overturning circulation, which transports heat from the surface to the ocean interior. Using profiling gliders, this study investigates the role of summertime buoyancy forcing and wind-driven processes on the intraseasonal (1-10 days) mixed layer thermohaline variability in three Southern Ocean regions southwest of Africa important for water mass transformation-the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), and Marginal Ice Zone (MIZ). At intraseasonal time scales, heat flux was shown as the main driver of buoyancy gain in all regions. In the SAZ and MIZ, shallow mixed layers and strong stratification enhanced mixed layer buoyancy gain by trapping incoming heat, while buoyancy loss resulted primarily from the entrainment of cold, salty water from below. In the PFZ, rapid mixing linked to Southern Ocean storms set persistently deep mixed layers and suppressed mixed layer intraseasonal thermohaline variability. In the polar regions, lateral stirring of meltwater from seasonal sea-ice melt dominated daily mixed layer salinity variability. We propose that these meltwater fronts are advected to the PFZ during late summer, indicating the potential for seasonal sea-ice freshwater to impact a region where the upwelling limb of overturning circulation reaches the surface. This study reveals a regional dependence of how the mixed layer thermohaline properties respond to small spatiotemporal processes, emphasizing the importance of surface forcing occurring between 1 and 10 days on the mixed layer water mass transformation in the Southern Ocean.
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- Narayanan, Aditya, 1988, et al.
(author)
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Zonal Distribution of Circumpolar Deep Water Transformation Rates and Its Relation to Heat Content on Antarctic Shelves
- 2023
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In: Journal of Geophysical Research-Oceans. - 2169-9275. ; 128:6
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Journal article (peer-reviewed)abstract
- We analyze 15-year of observational data and a 5-year Southern Ocean model simulation to quantify the transformation rates of Circumpolar Deep Water (CDW) and the associated heat loss to the surface. This study finds that over the continental shelves of East Antarctica and the Weddell and Ross Seas, surface buoyancy fluxes transform similar to 4.4 Sv of surface waters into CDW, providing a path for CDW to lose heat to the surface. In addition, similar to 6.6 Sv of CDW are mixed with surface waters in the Weddell and Ross subpolar gyres. In contrast, enhanced stratification inhibits the outcropping of CDW isopycnals, reducing their transformation rates by a factor of similar to 8 over the continental shelf and by a factor of similar to 3 over the deeper ocean in the Amundsen and Bellingshausen Seas. The CDW retains its offshore warm properties as it intrudes over the continental shelves, resulting in elevated bottom temperatures there. This analysis demonstrates the importance of processes in subpolar gyres to erode CDW and to facilitate further transformation on the continental shelves, significantly reducing the heat able to access ice shelf fronts. This sheltering effect is strongest in the western Weddell Sea and tends to diminish toward the east, which helps explain the large zonal differences in continental-shelf bottom temperatures and the melt rates of Antarctic ice shelves. Plain Language Summary The continental slope around Antarctica acts as a barrier to deep and warmer offshore waters that can bring heat to the glaciers along the coastline, enhancing their melt rate and contributing to global sea level rise. Around the Antarctic continent these offshore waters, the so-called Circumpolar Deep Waters, differ in their ability to cross this barrier while retaining their heat, explaining to a large extent why West Antarctic glaciers melt much faster than other Antarctic ice sheets. We study the properties of the warm waters over the continental shelf and offshore regions and contrast them across regions. We show that in East Antarctica, the Ross Sea, and the Weddell Sea, deep warm waters are brought to the surface where they lose heat and mix with surface waters. However, in the Amundsen and Bellingshausen Seas, the warm water is insulated from the surface by land run-off of fresher and lighter waters that occupy the surface. These results highlight the importance of the subpolar gyres in sheltering Antarctic glaciers.
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- Nicholson, Sarah‐Anne, et al.
(author)
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Storms drive outgassing of CO2 in the subpolar Southern Ocean
- 2022
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 13
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Journal article (peer-reviewed)abstract
- The subpolar Southern Ocean is a critical region where CO2 outgassing influences the global mean air-sea CO2 flux (FCO2). However, the processes controlling the outgassing remain elusive. We show, using a multi-glider dataset combining FCO2 and ocean turbulence, that the air-sea gradient of CO2 (∆pCO2) is modulated by synoptic storm-driven ocean variability (20 µatm, 1–10 days) through two processes. Ekman transport explains 60% of the variability, and entrainment drives strong episodic CO2 outgassing events of 2–4 mol m−2 yr−1. Extrapolation across the subpolar Southern Ocean using a process model shows how ocean fronts spatially modulate synoptic variability in ∆pCO2 (6 µatm2 average) and how spatial variations in stratification influence synoptic entrainment of deeper carbon into the mixed layer (3.5 mol m−2 yr−1 average). These results not only constrain aliased-driven uncertainties in FCO2 but also the effects of synoptic variability on slower seasonal or longer ocean physics-carbon dynamics.
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- Oelerich, Ria, et al.
(author)
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Stirring across the Antarctic Circumpolar Current's southern boundary at the prime meridian, Weddell Sea
- 2023
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In: Ocean Science. - 1812-0784 .- 1812-0792. ; 19:5, s. 1465-1482
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Journal article (peer-reviewed)abstract
- At the southern boundary of the Antarctic Circumpolar Current (ACC), relatively warm ACC waters encounter the colder waters surrounding Antarctica. Strong density gradients across the southern boundary indicate the presence of a frontal jet and are thought to modulate the southward heat transport across the front. In this study, the southern boundary in the Weddell Sea sector at the prime meridian is surveyed for the first time in high resolution over 2 months during an austral summer with underwater gliders occupying a transect across the front on five occasions. The five transects show that the frontal structure (i.e. hydrography, velocities and lateral density gradients) varies temporally. The results demonstrate significant, transient (a few weeks) variability of the southern boundary and its frontal jet in location, strength and width. A mesoscale cold-core eddy is identified to disrupt the southern boundary's frontal structure and strengthen lateral density gradients across the front. The front's barrier properties are assessed using mixing length scales and potential vorticity to establish the cross-frontal exchange of properties between the ACC and the Weddell Gyre. The results show that stronger lateral density gradients caused by the mesoscale eddy strengthen the barrier-like properties of the front through reduced mixing length scales and pronounced gradients of potential vorticity. In contrast, the barrier-like properties of the southern boundary are reduced when no mesoscale eddy is influencing the density gradients across the front. Using satellite altimetry, we further demonstrate that the barrier properties over the past decade have strengthened as a result of increased meridional gradients of absolute dynamic topography and increased frontal jet speeds in comparison to previous decades. Our results emphasise that locally and rapidly changing barrier properties of the southern boundary are important to quantify the cross-frontal exchange, which is particularly relevant in regions where the southern boundary is located near the Antarctic shelf break (e.g. in the West Antarctic sector).
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- Patmore, Ryan D., et al.
(author)
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Evaluating Existing Ocean Glider Sampling Strategies for Submesoscale Dynamics
- 2024
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In: JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY. - 0739-0572 .- 1520-0426. ; 41:7, s. 647-663
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Journal article (peer-reviewed)abstract
- Mixing in the upper ocean is important for biological production and the transfer of heat and carbon be-tween the atmosphere and deep ocean, properties commonly targeted by observational campaigns using ocean gliders. Weassess the reliability of ocean gliders to obtain a robust statistical representation of submesoscale variability in the oceanmixed layer of the Weddell Sea. A 1/488regional simulation of the Southern Ocean is sampled with virtual"bow-tie"gliderdeployments, which are then compared against the reference model output. Sampling biases of lateral buoyancy gradientsassociated with the arbitrary alignment between glider paths and fronts are formally quantified, and the magnitude of thebiases is comparable to observational estimates, with a mean error of 52%. The sampling bias leaves errors in the retrieveddistribution of buoyancy gradients largely insensitive to deployment length and the deployment of additional gliders. Nota-ble sensitivity to these choices emerges when the biases are removed by sampling perpendicular to fronts at all times.Detecting seasonal change in the magnitude of buoyancy gradients is sensitive to the glider-orientation sampling bias butthe change in variance is not. We evaluate the impact of reducing the number of dives and climbs in an observational cam-paign andfind that small reductions in the number of dive-climb pairs have a limited effect on the results. Lastly, examin-ing the sensitivity of the sampling bias to path orientation indicates that the bias is not dependent on the direction of travelin our deep ocean study site.
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- Sallee, J. B., et al.
(author)
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Southern ocean carbon and heat impact on climate
- 2023
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In: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. - 1364-503X .- 1471-2962. ; 381:2249
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Journal article (peer-reviewed)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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- Swart, Sebastiaan, 1983, et al.
(author)
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Submesoscale Fronts in the Antarctic Marginal Ice Zone and Their Response to Wind Forcing
- 2020
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In: Geophysical Research Letters. - 0094-8276. ; 47:6
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Journal article (peer-reviewed)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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