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| Lozier, M. Susan; Bacon, Sheldon; Bower, Amy S.; Cunningham, Stuart A.; De Jong, M. Femke; De Steur, Laura; Deyoung, Brad; Fischer, Juergen; Gary, Stefan F.; Greenan, Blair J. W.; Heimbach, Patrick; Holliday, Naomi P.; Houpert, Loic; Inall, Mark E.; Johns, William E.; Johnson, Helen L.; Karstensen, Johannes; Li, Feili; Lin, Xiaopei; Mackay, Neill; Marshall, David P.; Mercier, Herle; Myers, Paul G.; Pickart, Robert S.; Pillar, Helen R.; Straneo, Fiammetta; Thierry, Virginie; Weller, Robert A.; Williams, Richard G.; Wilson, Chris; Yang, Jiayan; Zhao, Jian; Zika, Jan D.. |
| A new ocean observing system has been launched in the North Atlantic in order to understand the linkage between the meridional overturning circulation and deep water formation. For decades oceanographers have understood the Atlantic Meridional Overturning Circulation (AMOC) to be primarily driven by changes in the production of deep water formation in the subpolar and subarctic North Atlantic. Indeed, current IPCC projections of an AMOC slowdown in the 21st century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep water formation. The motivation for... |
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| Ano: 2017 |
URL: http://archimer.ifremer.fr/doc/00358/46881/46764.pdf |
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| Macgilchrist, Graeme A.; Johnson, Helen L.; Marshall, David P.; Lique, Camille; Thomas, Matthew; Jackson, Laura C.; Wood, Richard A.. |
| A substantial fraction of the deep ocean is ventilated in the high-latitude North Atlantic. Consequently, the region plays a crucial role in transient climate change through the uptake of carbon dioxide and heat. However, owing to the Lagrangian nature of the process, many aspects of deep Atlantic Ocean ventilation and its representation in climate simulations remain obscure. We investigate the nature of ventilation in the high latitude North Atlantic in an eddy-permitting numerical ocean circulation model using a comprehensive set of Lagrangian trajectory experiments. Backwards-in-time trajectories from a model-defined ‘North Atlantic DeepWater’ (NADW) reveal the locations of subduction from the surface mixed layer at high spatial resolution. The major... |
| Tipo: Text |
Palavras-chave: North Atlantic Ocean; Atmosphere-ocean interaction; Lagrangian circulation/transport; Ocean circulation; Boundary currents; Diapycnal mixing. |
| Ano: 2020 |
URL: https://archimer.ifremer.fr/doc/00646/75833/76825.pdf |
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| Lique, Camille; Johnson, Helen L.. |
| The Atlantic Water (AW) layer in the Arctic Basin is isolated from the atmosphere by the overlaying surface layer, yet observations have revealed that the velocities in this layer exhibit significant variations. Here analysis of a global ocean/sea ice model hindcast, complemented by experiments performed with an idealized process model, is used to investigate what controls the variability of AW circulation, with a focus on the role of wind forcing. The AW circulation carries the imprint of wind variations, both remotely over the Nordic and Barents Seas where they force the AW inflow variability, and locally over the Arctic Basin through the forcing of the wind-driven Beaufort Gyre, which modulates and transfers the wind variability to the AW layer. The... |
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| Ano: 2015 |
URL: http://archimer.ifremer.fr/doc/00313/42420/41755.pdf |
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| Cornish, Sam B.; Kostov, Yavor; Johnson, Helen L.; Lique, Camille. |
| The freshwater content (FWC) of the Arctic Ocean is intimately linked to the stratification—a physical characteristic of the Arctic Ocean with wide relevance for climate and biology. Here, we explore the relationship between atmospheric circulation and Arctic FWC across 12 different Coupled Model Intercomparison Project Phase 5 control run simulations. Using multiple lagged regression, we seek to isolate the linear response of Arctic FWC to a step change in the strength of the Arctic Oscillation (AO), as well as the second and third orthogonal modes of SLP variability over the Arctic domain. There is broad agreement amongst models that a step change to a more anticyclonic AO leads to an increase in Arctic FWC, with an e-folding timescale of five to ten... |
| Tipo: Text |
Palavras-chave: Arctic; Arctic Oscillation; Freshwater; Statistical techniques; Regression. |
| Ano: 2020 |
URL: https://archimer.ifremer.fr/doc/00601/71270/69668.pdf |
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| Macgilchrist, Graeme A.; Marshall, David P.; Johnson, Helen L.; Lique, Camille; Thomas, Matthew. |
| Ventilation of the upper ocean plays an important role in climate variability on interannual to decadal timescales by influencing the exchange of heat and carbon dioxide between the atmosphere and ocean. The turbulent nature of ocean circulation, manifest in a vigorous mesoscale eddy field, means that pathways of ventilation, once thought to be quasi-laminar, are in fact highly chaotic. We characterize the chaotic nature of ventilation pathways according to a nondimensional filamentation number, which estimates the reduction in filament width of a ventilated fluid parcel due to mesoscale strain. In the subtropical North Atlantic of an eddy-permitting ocean model, the filamentation number is large everywhere across three upper ocean density... |
| Tipo: Text |
Palavras-chave: Ventilation; North Atlantic; Thermocline; Chaos; Mesoscale eddies; Lagrangian trajectories. |
| Ano: 2017 |
URL: https://archimer.ifremer.fr/doc/00410/52104/52807.pdf |
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| Cimoli, Laura; Caulfield, Colm‐cille P.; Johnson, Helen L.; Marshall, David P.; Mashayek, Ali; Naveira Garabato, Alberto C.; Vic, Clement. |
| There have been recent advancements in the quantification of parameters describing the proportion of internal tide energy being dissipated locally and the “efficiency” of diapycnal mixing, i.e. the ratio of the diapycnal mixing rate to the kinetic energy dissipation rate. We show that oceanic tidal mixing is non‐trivially sensitive to the co‐variation of these parameters. Varying these parameters one at the time can lead to significant errors in the patterns of diapycnal mixing driven upwelling and downwelling, and to the over and under estimation of mixing in such a way that the net rate of globally‐integrated deep circulation appears reasonable. However, the local rates of upwelling and downwelling in the deep ocean are significantly different when both... |
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| Ano: 2019 |
URL: https://archimer.ifremer.fr/doc/00595/70675/68873.pdf |
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| Muilwijk, Morven; Ilicak, Mehmet; Cornish, Sam B.; Danilov, Sergey; Gelderloos, Renske; Gerdes, Rüdiger; Haid, Verena; Haine, Thomas W.n.; Johnson, Helen L.; Kostov, Yavor; Kovács, Tamás; Lique, Camille; Marson, Juliana M.; Myers, Paul G.; Scott, Jon; Smedsrud, Lars H.; Talandier, Claude; Wang, Qiang. |
| Multi‐model Arctic Ocean ``Climate Response Function” (CRF) experiments are analyzed in order to explore the effects of anomalous wind forcing over the Greenland Sea (GS) on poleward ocean heat transport, Atlantic Water (AW) pathways, and the extent of Arctic sea ice. Particular emphasis is placed on the sensitivity of the AW circulation to anomalously strong or weak GS winds in relation to natural variability, the latter manifested as part of the North Atlantic Oscillation (NAO). We find that anomalously strong (weak) GS wind forcing, comparable in strength to a strong positive (negative) NAO index, results in an intensification (weakening) of the poleward AW flow, extending from south of the North Atlantic Subpolar Gyre, through the Nordic Seas, and all... |
| Tipo: Text |
Palavras-chave: Arctic Ocean; Atlantic Water; Sea ice; Wind forcing; Model intercomparison; FAMOS. |
| Ano: 2019 |
URL: https://archimer.ifremer.fr/doc/00510/62126/66338.pdf |
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| Lique, Camille; Johnson, Helen L.; Plancherel, Yves. |
| The appearance of winter deep mixed layers in the Arctic Ocean under a warming climate is investigated with the HiGEM coupled global climate model. In response to a four times increase of atmospheric CO2 levels with respect to present day conditions, the Arctic Basin becomes seasonally ice-free. Its surface becomes consequently warmer and, on average, slightly fresher. Locally, changes in surface salinity can be far larger (up to 4 psu) than the basin-scale average, and of a different sign. The Canadian Basin undergoes a strong freshening, while the Eurasian Basin undergoes strong salinification. These changes are driven by the spin up of the surface circulation, likely resulting from the increased transfer of momentum to the ocean as sea ice cover is... |
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| Ano: 2018 |
URL: http://archimer.ifremer.fr/doc/00395/50609/51451.pdf |
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