Observational Inferences of Lateral Eddy Diffusivity in the Halocline of the Beaufort Gyre

Meneghello, Gianluca; Marshall, John; Cole, Sylvia T.; Timmermans, Mary‐Louise

doi:10.1002/2017gl075126

Cited by 50 publications

(90 citation statements)

References 38 publications

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“…Additionally, Yang et al () showed that the main balance of the time mean circulation is between the curl of the wind stress and eddy fluxes. Both Manucharyan and Spall () and Meneghello et al () showed that mesoscale eddies control variations in FWC by opposing the tendency of the Ekman pumping to deepen the halocline. Based on ice‐tethered profiler data, Zhao et al () showed that the greatest numbers of eddies were along the south and western boundaries of the BG, with the number of lower halocline eddies reaching a maximum in 2013–2014.…”

Section: Introductionmentioning

confidence: 99%

Pacific Water Pathway in the Arctic Ocean and Beaufort Gyre in Two Simulations With Different Horizontal Resolutions

Myers

2019

JGR Oceans

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A set of numerical simulations (with horizontal resolutions of 1/4° and 1/12°) is conducted to study the Pacific Water pathway in the Arctic Ocean and the freshwater content in Beaufort Gyre. Passive tracer tags the Pacific Water entering through Bering Strait into the Arctic Ocean and further reveals its circulation routes and spatial distribution. Both the 1/4° and 1/12° simulations show Pacific Water mainly follows the Transpolar Drift over the integration period of 2002–2016, with a limited amount being able to flow eastward along the Alaskan coast to enter the Canadian Arctic Archipelago. However, the circulation pattern of Pacific Water within the Beaufort Gyre is quite different with a stronger and tighter anticyclonic circulation in the 1/12° simulation corresponding to the difference in freshwater content. The 1/12° simulation successfully reproduces the overall recent increasing trend in the freshwater content in the Beaufort Gyre, while the 1/4° simulation fails to maintain the high freshwater content state after 2007. Budget analysis suggests that this difference in Beaufort Gyre freshwater storage is mainly caused by lateral advection. The lateral freshwater flux is decomposed into two components due to the slow‐varying circulation and mesoscale eddies. The difference in the capability to resolve eddies in the two simulations causes the difference in the temporal evolution of both components of the lateral flux.

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Section: Introductionmentioning

confidence: 99%

Pacific Water Pathway in the Arctic Ocean and Beaufort Gyre in Two Simulations With Different Horizontal Resolutions

Myers

2019

JGR Oceans

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“…Eddy diffusivity in the BG can vary significantly depending on the mean flow, stratification and as we will demonstrate in this study, continental slopes. Observational evidence from mooring data suggest that eddy diffusivity reaches values of about 600 m 2 /s (Meneghello et al, ), which is sufficient to counteract the Ekman pumping of about 10 m/year given the observed magnitudes of the halocline slopes. In the idealized eddy‐resolving model of Manucharyan and Spall (), which is characterized by a balance between eddy transport and Ekman pumping, an eddy diffusivity of 300 ± 200 m 2 /s were diagnosed, with higher values near gyre boundaries that have large halocline slopes.…”

Section: Introductionmentioning

confidence: 99%

Critical Role of Continental Slopes in Halocline and Eddy Dynamics of the Ekman‐Driven Beaufort Gyre

Manucharyan

Isachsen

2019

JGR Oceans

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The Beaufort Gyre (BG) is a large‐scale bathymetrically constrained circulation driven by a surface Ekman convergence that creates a bowl‐shaped halocline and stores a significant portion of the Arctic Ocean's freshwater. Theoretical studies suggest that in the gyre interior, the halocline is equilibrated by a balance between Ekman pumping and counteracting mesoscale eddy transport energized by baroclinic instability. However, the strongest anticyclonic flows occur over steep continental slopes, and, despite bathymetric slopes being known to influence baroclinic instability, their large‐scale impacts on BG halocline remain unexplored. Here we use an idealized eddy‐resolving BG model to demonstrate that the existence of continental slopes dramatically affects key gyre characteristics leading to deeper halocline, stronger anticyclonic circulation, and prolonged equilibration. Over continental slopes, the magnitude of the Eulerian mean circulation is dramatically reduced due to the Ekman overturning being compensated by the eddy momentum‐driven overturning. The eddy thickness flux overturning associated with lateral salt transport is also weakened over the slopes, indicating a reduction of eddy thickness diffusivity despite the isopycnal slopes being largest there. Using a theoretical halocline model, we demonstrate that it is the localized reduction in eddy diffusivity over continental slopes that is critical in explaining the halocline deepening and prolonged equilibration time. Our results emphasize the need for observational studies of eddy overturning dynamics over continental slopes and the development of slope‐aware mesoscale eddy parameterizations for low‐resolution climate models.

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“…The surface stress for the dashed blue line has been calculated using the ensemble average ice‐ocean relative velocity in equation . The gray shaded region on the x axis highlights the estimated range of 100–600 m 2 /s for eddy diffusivity in the BG (Meneghello et al, ).…”

Section: Analysis Of a General Circulation Modelmentioning

confidence: 99%

“…This mechanism is dubbed the Ice‐Ocean Governor by analogy with mechanical governors that regulate the speed of engines and other devices through dynamical feedbacks (see, e.g., Maxwell, ). The ocean velocity has long been included when calculating the ice‐ocean stress in numerical models (see, e.g., Hibler, ), but appreciation of the importance of this effect has been very recent (Dewey et al, ; Kwok & Morison, ; Meneghello et al, ; Meneghello, Marshall, Timmermans, et al, ; Meneghello, Marshall, Campin, et al, ; Zhong et al, ) following the publication of a data set that estimates sea surface height and surface geostrophic velocity in sea ice‐covered areas (Armitage et al, ).…”

Section: Introductionmentioning

confidence: 99%

“…The Ice‐Ocean Governor does not require mesoscale eddies to equilibrate the BG. However, mesoscale eddies are commonly found in the Arctic Ocean (Dmitrenko et al, ; Hunkins, ; Manley & Hunkins, ; Meneghello et al, ; Timmermans et al, ; Zhao et al, , ). It is therefore important to extend the theory of the Ice‐Ocean Governor presented by Meneghello, Marshall, Campin, et al () to include the effect of eddy diffusivity.…”

Section: Introductionmentioning

confidence: 99%

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A Three‐Way Balance in the Beaufort Gyre: The Ice‐Ocean Governor, Wind Stress, and Eddy Diffusivity

et al. 2019

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The Beaufort Gyre (BG) is a large anticyclonic circulation in the Arctic Ocean. Its strength is directly related to the halocline depth, and therefore also to the storage of freshwater. It has recently been proposed that the equilibrium state of the BG is set by the Ice‐Ocean Governor, a negative feedback between surface currents and ice‐ocean stress, rather than a balance between lateral mesoscale eddy fluxes and surface Ekman pumping. However, mesoscale eddies are present in the Arctic Ocean; it is therefore important to extend the Ice‐Ocean Governor theory to include lateral fluxes due to mesoscale eddies. Here, a nonlinear ordinary differential equation is derived that represents the effects of wind stress, the Ice‐Ocean Governor, and eddy fluxes. Equilibrium and time‐varying solutions to this three‐way balance equation are obtained and shown to closely match the output from a hierarchy of numerical simulations, indicating that the analytical model represents the processes controlling BG equilibration. The equilibration timescale derived from this three‐way balance is faster than the eddy equilibration timescale and slower than the Ice‐Ocean Governor equilibration timescales for most values of eddy diffusivity. The sensitivity of the BG equilibrium depth to changes in eddy diffusivity and the presence of the Ice‐Ocean Governor is also explored. These results show that predicting the response of the BG to changing surface forcing and sea ice conditions requires faithfully capturing the three‐way balance between the Ice‐Ocean Governor, wind stress, and eddy fluxes.

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Observational Inferences of Lateral Eddy Diffusivity in the Halocline of the Beaufort Gyre

Cited by 50 publications

References 38 publications

Pacific Water Pathway in the Arctic Ocean and Beaufort Gyre in Two Simulations With Different Horizontal Resolutions

Pacific Water Pathway in the Arctic Ocean and Beaufort Gyre in Two Simulations With Different Horizontal Resolutions

Critical Role of Continental Slopes in Halocline and Eddy Dynamics of the Ekman‐Driven Beaufort Gyre

A Three‐Way Balance in the Beaufort Gyre: The Ice‐Ocean Governor, Wind Stress, and Eddy Diffusivity

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