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- Author or Editor: Sjoerd Groeskamp x
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Abstract
It might be impossible to truly fathom the magnitude of the threat that global-mean sea level rise poses. However, conceptualizing the scale of the solutions required to protect ourselves against global-mean sea level rise aids in our ability to acknowledge and understand that threat. On these grounds, we here discuss a means to protect over 25 million people and important economical regions in northern Europe against sea level rise. We propose the construction of a Northern European Enclosure Dam (NEED) that stretches between France, the United Kingdom, and Norway. NEED may seem an overwhelming and unrealistic solution at first. However, our preliminary study suggests that NEED is potentially favorable financially, but also in scale, impacts, and challenges compared to that of alternative solutions, such as (managed) migrations and that of country-by-country protection efforts. The mere realization that a solution as considerable as NEED might be a viable and cost-effective protection measure is illustrative of the extraordinary global threat of global-mean sea level rise that we are facing. As such, the concept of constructing NEED showcases the extent of protection efforts that are required if mitigation efforts fail to limit sea level rise.
Abstract
It might be impossible to truly fathom the magnitude of the threat that global-mean sea level rise poses. However, conceptualizing the scale of the solutions required to protect ourselves against global-mean sea level rise aids in our ability to acknowledge and understand that threat. On these grounds, we here discuss a means to protect over 25 million people and important economical regions in northern Europe against sea level rise. We propose the construction of a Northern European Enclosure Dam (NEED) that stretches between France, the United Kingdom, and Norway. NEED may seem an overwhelming and unrealistic solution at first. However, our preliminary study suggests that NEED is potentially favorable financially, but also in scale, impacts, and challenges compared to that of alternative solutions, such as (managed) migrations and that of country-by-country protection efforts. The mere realization that a solution as considerable as NEED might be a viable and cost-effective protection measure is illustrative of the extraordinary global threat of global-mean sea level rise that we are facing. As such, the concept of constructing NEED showcases the extent of protection efforts that are required if mitigation efforts fail to limit sea level rise.
Abstract
The deformation radius is widely used as an indication of the eddy length scale at different latitudes. The radius is usually calculated assuming a flat ocean bottom. However, bathymetry alters the baroclinic modes and hence their deformation radii. In a linear quasigeostrophic two-layer model with realistic parameters, the deep flow for a 100-km wave approaches zero with a bottom ridge roughly 10 m high, leaving a baroclinic mode that is mostly surface trapped. This is in line with published current meter studies showing a primary EOF that is surface intensified and has nearly zero flow at the bottom. The deformation radius associated with this “surface mode” is significantly larger than that of the flat bottom baroclinic mode. Using World Ocean Atlas data, the surface radius is found to be 20%–50% larger over much of the globe, and 100% larger in some regions. This in turn alters the long Rossby wave speed, which is shown to be 1.5–2 times faster than over a flat bottom. In addition, the larger deformation radius is easier to resolve in ocean models.
Abstract
The deformation radius is widely used as an indication of the eddy length scale at different latitudes. The radius is usually calculated assuming a flat ocean bottom. However, bathymetry alters the baroclinic modes and hence their deformation radii. In a linear quasigeostrophic two-layer model with realistic parameters, the deep flow for a 100-km wave approaches zero with a bottom ridge roughly 10 m high, leaving a baroclinic mode that is mostly surface trapped. This is in line with published current meter studies showing a primary EOF that is surface intensified and has nearly zero flow at the bottom. The deformation radius associated with this “surface mode” is significantly larger than that of the flat bottom baroclinic mode. Using World Ocean Atlas data, the surface radius is found to be 20%–50% larger over much of the globe, and 100% larger in some regions. This in turn alters the long Rossby wave speed, which is shown to be 1.5–2 times faster than over a flat bottom. In addition, the larger deformation radius is easier to resolve in ocean models.
Abstract
Using observationally based hydrographic and eddy diffusivity datasets, a volume budget analysis is performed to identify the main mechanisms governing the spatial and seasonal variability of Antarctic Intermediate Water (AAIW) within the density range γn = (27.25–27.7) kg m−3 in the Southern Ocean. The subduction rates and water mass transformation rates by mesoscale and small-scale turbulent mixing are estimated. First, Ekman pumping upwells the dense variety of AAIW into the mixed layer south of the Polar Front, which can be advected northward by Ekman transport into the subduction regions of lighter-variety AAIW and Subantarctic Mode Water (SAMW). The subduction of light AAIW occurs mainly by lateral advection in the southeast Pacific and Drake Passage as well as eddy-induced flow between the Subantarctic and Polar Fronts. The circumpolar-integrated total subduction yields from −5 to 19 Sv (1 Sv ≡ 106 m3 s−1) of AAIW volume loss. Second, the diapycnal transport from subducted SAMW into the AAIW layer is predominantly by mesoscale mixing (2–13 Sv) near the Subantarctic Front and vertical mixing in the South Pacific, while AAIW is further replenished by transformation from Upper Circumpolar Deep Water by vertical mixing (1–10 Sv). Last, 3–14 Sv of AAIW are exported out of the Southern Ocean. Our results suggest that the distribution of AAIW is set by its formation due to subduction and mixing, and its circulation eastward along the ACC and northward into the subtropical gyres. The volume budget analysis reveals strong seasonal variability in the rate of subduction, vertical mixing, and volume transport driving volume change within the AAIW layer. The nonzero volume budget residual suggests that more observations are needed to better constrain the estimate of geostrophic flow and mesoscale and small-scale mixing diffusivities.
Abstract
Using observationally based hydrographic and eddy diffusivity datasets, a volume budget analysis is performed to identify the main mechanisms governing the spatial and seasonal variability of Antarctic Intermediate Water (AAIW) within the density range γn = (27.25–27.7) kg m−3 in the Southern Ocean. The subduction rates and water mass transformation rates by mesoscale and small-scale turbulent mixing are estimated. First, Ekman pumping upwells the dense variety of AAIW into the mixed layer south of the Polar Front, which can be advected northward by Ekman transport into the subduction regions of lighter-variety AAIW and Subantarctic Mode Water (SAMW). The subduction of light AAIW occurs mainly by lateral advection in the southeast Pacific and Drake Passage as well as eddy-induced flow between the Subantarctic and Polar Fronts. The circumpolar-integrated total subduction yields from −5 to 19 Sv (1 Sv ≡ 106 m3 s−1) of AAIW volume loss. Second, the diapycnal transport from subducted SAMW into the AAIW layer is predominantly by mesoscale mixing (2–13 Sv) near the Subantarctic Front and vertical mixing in the South Pacific, while AAIW is further replenished by transformation from Upper Circumpolar Deep Water by vertical mixing (1–10 Sv). Last, 3–14 Sv of AAIW are exported out of the Southern Ocean. Our results suggest that the distribution of AAIW is set by its formation due to subduction and mixing, and its circulation eastward along the ACC and northward into the subtropical gyres. The volume budget analysis reveals strong seasonal variability in the rate of subduction, vertical mixing, and volume transport driving volume change within the AAIW layer. The nonzero volume budget residual suggests that more observations are needed to better constrain the estimate of geostrophic flow and mesoscale and small-scale mixing diffusivities.
Abstract
Subantarctic Mode Water (SAMW) forms in deep mixed layers just north of the Antarctic Circumpolar Current in winter, playing a fundamental role in the ocean uptake of heat and carbon. Using a gridded Argo product and the ERA-Interim reanalysis for years 2004–18, the seasonal evolution of the SAMW volume is analyzed using both a kinematic estimate of the subduction rate and a thermodynamic estimate of the air–sea formation rate. The seasonal SAMW volume changes are separately estimated within the monthly mixed layer and in the interior below it. We find that the variability of SAMW volume is dominated by changes in SAMW volume in the mixed layer. The seasonal variability of SAMW volume in the mixed layer is governed by formation due to air–sea buoyancy fluxes (45%, lasting from July to August), entrainment (35%), and northward Ekman transport across the Subantarctic Front (10%). The interior SAMW formation is entirely controlled by exchanges between the mixed layer and the interior (i.e., instantaneous subduction), which occurs mainly during August–October. The annual mean subduction estimate from a Lagrangian approach shows strong regional variability with hotspots of large SAMW subduction. The SAMW subduction hotspots are consistent with the distribution and export pathways of SAMW over the central and eastern parts of the south Indian and Pacific Oceans. Hotspots in the south Indian Ocean produce strong subduction of 8 and 9 Sv (1 Sv ≡ 106 m3 s−1) for the light and southeast Indian SAMW, respectively, while SAMW subduction of 6 and 4 Sv occurs for the central and southeast Pacific SAMW, respectively.
Abstract
Subantarctic Mode Water (SAMW) forms in deep mixed layers just north of the Antarctic Circumpolar Current in winter, playing a fundamental role in the ocean uptake of heat and carbon. Using a gridded Argo product and the ERA-Interim reanalysis for years 2004–18, the seasonal evolution of the SAMW volume is analyzed using both a kinematic estimate of the subduction rate and a thermodynamic estimate of the air–sea formation rate. The seasonal SAMW volume changes are separately estimated within the monthly mixed layer and in the interior below it. We find that the variability of SAMW volume is dominated by changes in SAMW volume in the mixed layer. The seasonal variability of SAMW volume in the mixed layer is governed by formation due to air–sea buoyancy fluxes (45%, lasting from July to August), entrainment (35%), and northward Ekman transport across the Subantarctic Front (10%). The interior SAMW formation is entirely controlled by exchanges between the mixed layer and the interior (i.e., instantaneous subduction), which occurs mainly during August–October. The annual mean subduction estimate from a Lagrangian approach shows strong regional variability with hotspots of large SAMW subduction. The SAMW subduction hotspots are consistent with the distribution and export pathways of SAMW over the central and eastern parts of the south Indian and Pacific Oceans. Hotspots in the south Indian Ocean produce strong subduction of 8 and 9 Sv (1 Sv ≡ 106 m3 s−1) for the light and southeast Indian SAMW, respectively, while SAMW subduction of 6 and 4 Sv occurs for the central and southeast Pacific SAMW, respectively.
Abstract
The small-slope approximation to the full three-dimensional diffusion tensor of epineutral diffusion gives exactly the same tracer flux as the commonly used projected nonorthogonal diffusive flux of layered ocean models and of theoretical studies. The epineutral diffusion achieved by this small-slope approximation is not exactly in the direction of the correct epineutral tracer gradient. That is, the use of the small-slope approximation leads to a very small flux of tracer in a direction in which there is no epineutral gradient of tracer. For (the tracer) temperature or salinity, the difference between the correct epineutral gradient and the small-slope approximation to it is proportional to neutral helicity. The authors also make the point that small-scale turbulent mixing processes act to diffuse tracers isotropically (i.e., the same in each spatial direction) and hence it is strictly a misnomer to call this process “dianeutral diffusion” or “vertical diffusion.” This realization also has implications for the diffusion tensor.
Abstract
The small-slope approximation to the full three-dimensional diffusion tensor of epineutral diffusion gives exactly the same tracer flux as the commonly used projected nonorthogonal diffusive flux of layered ocean models and of theoretical studies. The epineutral diffusion achieved by this small-slope approximation is not exactly in the direction of the correct epineutral tracer gradient. That is, the use of the small-slope approximation leads to a very small flux of tracer in a direction in which there is no epineutral gradient of tracer. For (the tracer) temperature or salinity, the difference between the correct epineutral gradient and the small-slope approximation to it is proportional to neutral helicity. The authors also make the point that small-scale turbulent mixing processes act to diffuse tracers isotropically (i.e., the same in each spatial direction) and hence it is strictly a misnomer to call this process “dianeutral diffusion” or “vertical diffusion.” This realization also has implications for the diffusion tensor.
Abstract
The ocean’s circulation is analyzed in Absolute Salinity S
A and Conservative Temperature Θ coordinates. It is separated into 1) an advective component related to geographical displacements in the direction normal to S
A and Θ isosurfaces and 2) into a local component, related to local changes in S
A–Θ values, without a geographical displacement. In this decomposition, the sum of the advective and local components of the circulation is equivalent to the material derivative of S
A and Θ. The sum is directly related to sources and sinks of salt and heat. The advective component is represented by the advective thermohaline streamfunction 
Abstract
The ocean’s circulation is analyzed in Absolute Salinity S
A and Conservative Temperature Θ coordinates. It is separated into 1) an advective component related to geographical displacements in the direction normal to S
A and Θ isosurfaces and 2) into a local component, related to local changes in S
A–Θ values, without a geographical displacement. In this decomposition, the sum of the advective and local components of the circulation is equivalent to the material derivative of S
A and Θ. The sum is directly related to sources and sinks of salt and heat. The advective component is represented by the advective thermohaline streamfunction
Abstract
This study provides observation-based estimates, determined by inverse methods, of horizontal and isopycnal eddy diffusion coefficients K H and K I , respectively, the small-scale mixing coefficient D, and the diathermohaline streamfunction Ψ. The inverse solution of Ψ represents the ocean circulation in Absolute Salinity S A and Conservative Temperature Θ coordinates. The authors suggest that the observation-based estimate of Ψ will be useful for comparison with equivalent diagnostics from numerical climate models. The estimates of K H and K I represent horizontal eddy mixing in the mixed layer and isopycnal eddy mixing in the ocean interior, respectively. This study finds that the solution for D and K H are comparable to existing estimates. The solution for K I is one of the first observation-based global and full-depth constrained estimates of isopycnal mixing and indicates that K I is an order of magnitude smaller than K H . This suggests that there is a large vertical variation in the eddy mixing coefficient, which is generally not included in ocean models. With ocean models being very sensitive to the choice of isopycnal mixing, this result suggests that further investigation of the spatial structure of isopycnal eddy mixing from observations is required.
Abstract
This study provides observation-based estimates, determined by inverse methods, of horizontal and isopycnal eddy diffusion coefficients K H and K I , respectively, the small-scale mixing coefficient D, and the diathermohaline streamfunction Ψ. The inverse solution of Ψ represents the ocean circulation in Absolute Salinity S A and Conservative Temperature Θ coordinates. The authors suggest that the observation-based estimate of Ψ will be useful for comparison with equivalent diagnostics from numerical climate models. The estimates of K H and K I represent horizontal eddy mixing in the mixed layer and isopycnal eddy mixing in the ocean interior, respectively. This study finds that the solution for D and K H are comparable to existing estimates. The solution for K I is one of the first observation-based global and full-depth constrained estimates of isopycnal mixing and indicates that K I is an order of magnitude smaller than K H . This suggests that there is a large vertical variation in the eddy mixing coefficient, which is generally not included in ocean models. With ocean models being very sensitive to the choice of isopycnal mixing, this result suggests that further investigation of the spatial structure of isopycnal eddy mixing from observations is required.
Abstract
The thermohaline inverse method (THIM) is presented that provides estimates of the diathermohaline streamfunction 
Abstract
The thermohaline inverse method (THIM) is presented that provides estimates of the diathermohaline streamfunction
