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Abstract
The deep channel north of New Guinea (NG) is the choke site for the upper deep branches of the Pacific meridional overturning circulation (U-PMOC). The U-PMOC is a crucial element of the ocean’s climate and biogeochemical systems. It carries the mixed water of the Upper Circumpolar Water and North Pacific Deep Water with a potential temperature over 1.2°–2.2°C. The pathway and volume transport of U-PMOC through the deep channel north of NG are revealed by mooring measurements from 2014 to 2019. Mean U-PMOC is located at ∼2000–3500 m with a velocity core at 2550 m and is directed eastward. The U-PMOC shows a strong seasonal variability with a direction reversal from June to September. The oceanic reanalysis product GLORYS12V1 well reproduces the observed U-PMOC and is thus used to estimate the mean and standard deviation of U-PMOC’s volume transport as 2.19 ± 11.4 Sv (1 Sv ≡ 106 m3 s−1) and to explore the underlying dynamics of the U-PMOC. The seasonality of U-PMOC is induced by the vertical propagation of the Rossby energy through the upper ocean in the eastern Pacific to the deep ocean in the western Pacific. The mean eastward U-PMOC transport is forced by the zonal deep pressure gradient, which is mainly determined by the local upper-ocean processes above 500 m.
Abstract
The deep channel north of New Guinea (NG) is the choke site for the upper deep branches of the Pacific meridional overturning circulation (U-PMOC). The U-PMOC is a crucial element of the ocean’s climate and biogeochemical systems. It carries the mixed water of the Upper Circumpolar Water and North Pacific Deep Water with a potential temperature over 1.2°–2.2°C. The pathway and volume transport of U-PMOC through the deep channel north of NG are revealed by mooring measurements from 2014 to 2019. Mean U-PMOC is located at ∼2000–3500 m with a velocity core at 2550 m and is directed eastward. The U-PMOC shows a strong seasonal variability with a direction reversal from June to September. The oceanic reanalysis product GLORYS12V1 well reproduces the observed U-PMOC and is thus used to estimate the mean and standard deviation of U-PMOC’s volume transport as 2.19 ± 11.4 Sv (1 Sv ≡ 106 m3 s−1) and to explore the underlying dynamics of the U-PMOC. The seasonality of U-PMOC is induced by the vertical propagation of the Rossby energy through the upper ocean in the eastern Pacific to the deep ocean in the western Pacific. The mean eastward U-PMOC transport is forced by the zonal deep pressure gradient, which is mainly determined by the local upper-ocean processes above 500 m.
Abstract
The spectral description of the energy of oceanic internal gravity waves is generally represented by the Garrett–Munk (GM) model, a function with a power-law decrease of spectral energy in wavenumber–frequency space. Besides the slopes of these power laws, the spectrum is expressed as a function of energy and a bandwidth parameter that fixes the range of vertical modes excited in the respective state. Whereas concepts have been developed and agreed upon of what processes feed the wave spectrum and what dissipates energy, there is no explanation of what shapes the spectral distribution, i.e., how the power laws come about and what sets the bandwidth. The present study develops a parametric spectral model of energy and bandwidth from the basic underlying energy balance in terms of forcing, propagation, refraction, spectral transfer, and dissipation. The model is an extension of the IDEMIX (Internal Wave Dissipation, Energy and Mixing) models where bandwidth was taken as a constant parameter. The current version of the model is restricted to single-column mode and the slopes of the spectral power laws are fixed. A coupled system of predictive equations for energy and bandwidth (for up- and downward propagating waves) results. The equations imply that bandwidth relates to energy by a power law with an exponent given by the dynamical parameters. It agrees favorably with energy, bandwidth, and slope data from previously published fits of the GM model to Argo float observations. Numerical solutions of the coupled energy–bandwidth model in stand-alone modus are presented.
Abstract
The spectral description of the energy of oceanic internal gravity waves is generally represented by the Garrett–Munk (GM) model, a function with a power-law decrease of spectral energy in wavenumber–frequency space. Besides the slopes of these power laws, the spectrum is expressed as a function of energy and a bandwidth parameter that fixes the range of vertical modes excited in the respective state. Whereas concepts have been developed and agreed upon of what processes feed the wave spectrum and what dissipates energy, there is no explanation of what shapes the spectral distribution, i.e., how the power laws come about and what sets the bandwidth. The present study develops a parametric spectral model of energy and bandwidth from the basic underlying energy balance in terms of forcing, propagation, refraction, spectral transfer, and dissipation. The model is an extension of the IDEMIX (Internal Wave Dissipation, Energy and Mixing) models where bandwidth was taken as a constant parameter. The current version of the model is restricted to single-column mode and the slopes of the spectral power laws are fixed. A coupled system of predictive equations for energy and bandwidth (for up- and downward propagating waves) results. The equations imply that bandwidth relates to energy by a power law with an exponent given by the dynamical parameters. It agrees favorably with energy, bandwidth, and slope data from previously published fits of the GM model to Argo float observations. Numerical solutions of the coupled energy–bandwidth model in stand-alone modus are presented.
Abstract
The Arctic Ocean is characterized by an ice-covered layer of cold and relatively fresh water above layers of warmer and saltier water. It is estimated that enough heat is stored in these deeper layers to melt all the Arctic sea ice many times over, but they are isolated from the surface by a stable halocline. Current vertical mixing rates across the Arctic Ocean halocline are small, due in part to sea ice reducing wind–ocean momentum transfer and damping internal waves. However, recent observational studies have argued that sea ice retreat results in enhanced mixing. This could create a positive feedback whereby increased vertical mixing due to sea ice retreat causes the previously isolated subsurface heat to melt more sea ice. Here, we use an idealized climate model to investigate the impacts of such a feedback. We find that an abrupt “tipping point” can occur under global warming, with an associated hysteresis window bounded by saddle-node bifurcations. We show that the presence and magnitude of the hysteresis are sensitive to the choice of model parameters, and the hysteresis occurs for only a limited range of parameters. During the critical transition at the bifurcation point, we find that only a small percentage of the heat stored in the deep layer is released, although this is still enough to lead to substantial sea ice melt. Furthermore, no clear relationship is apparent between this change in heat storage and the level of hysteresis when the parameters are varied.
Abstract
The Arctic Ocean is characterized by an ice-covered layer of cold and relatively fresh water above layers of warmer and saltier water. It is estimated that enough heat is stored in these deeper layers to melt all the Arctic sea ice many times over, but they are isolated from the surface by a stable halocline. Current vertical mixing rates across the Arctic Ocean halocline are small, due in part to sea ice reducing wind–ocean momentum transfer and damping internal waves. However, recent observational studies have argued that sea ice retreat results in enhanced mixing. This could create a positive feedback whereby increased vertical mixing due to sea ice retreat causes the previously isolated subsurface heat to melt more sea ice. Here, we use an idealized climate model to investigate the impacts of such a feedback. We find that an abrupt “tipping point” can occur under global warming, with an associated hysteresis window bounded by saddle-node bifurcations. We show that the presence and magnitude of the hysteresis are sensitive to the choice of model parameters, and the hysteresis occurs for only a limited range of parameters. During the critical transition at the bifurcation point, we find that only a small percentage of the heat stored in the deep layer is released, although this is still enough to lead to substantial sea ice melt. Furthermore, no clear relationship is apparent between this change in heat storage and the level of hysteresis when the parameters are varied.
Abstract
Freshwater from the Greenland Ice Sheet is routed to the ocean through narrow fjords along the coastline where it impacts ecosystems both within the fjord and on the continental shelf, regional circulation, and potentially the global overturning circulation. However, the timing of freshwater export is sensitive to the residence time of waters within glacial fjords. Here, we present evidence of seasonal freshwater storage in a tidewater glacial fjord using hydrographic and velocity data collected over 10 days during the summers of 2012 and 2013 in Saqqarleq (SQ), a midsize fjord in west Greenland. The data revealed a rapid freshening trend of −0.05 ± 0.01 and −0.04 ± 0.01 g kg−1 day−1 in 2012 and 2013, respectively, within the intermediate layer of the fjord (15–100 m) less than 2.5 km from the glacier terminus. The freshening trend is driven, in part, by the downward mixing of outflowing glacially modified water near the surface and increasingly stratifies the fjord from the surface downward over the summer melt season. We construct a box model that recreates the first-order dynamics of the fjord and describes freshwater storage as a balance between friction and density-driven exchange outside the fjord. The model can be used to diagnose the time scale for this balance to be reached, and for SQ we find a month lag between subglacial meltwater discharge and net freshwater export. These results indicate a fjord-induced delay in freshwater export to the ocean that should be represented in large-scale models seeking to understand the impact of Greenland freshwater on the regional climate system.
Abstract
Freshwater from the Greenland Ice Sheet is routed to the ocean through narrow fjords along the coastline where it impacts ecosystems both within the fjord and on the continental shelf, regional circulation, and potentially the global overturning circulation. However, the timing of freshwater export is sensitive to the residence time of waters within glacial fjords. Here, we present evidence of seasonal freshwater storage in a tidewater glacial fjord using hydrographic and velocity data collected over 10 days during the summers of 2012 and 2013 in Saqqarleq (SQ), a midsize fjord in west Greenland. The data revealed a rapid freshening trend of −0.05 ± 0.01 and −0.04 ± 0.01 g kg−1 day−1 in 2012 and 2013, respectively, within the intermediate layer of the fjord (15–100 m) less than 2.5 km from the glacier terminus. The freshening trend is driven, in part, by the downward mixing of outflowing glacially modified water near the surface and increasingly stratifies the fjord from the surface downward over the summer melt season. We construct a box model that recreates the first-order dynamics of the fjord and describes freshwater storage as a balance between friction and density-driven exchange outside the fjord. The model can be used to diagnose the time scale for this balance to be reached, and for SQ we find a month lag between subglacial meltwater discharge and net freshwater export. These results indicate a fjord-induced delay in freshwater export to the ocean that should be represented in large-scale models seeking to understand the impact of Greenland freshwater on the regional climate system.
Abstract
The generation of broadband wave energy frequency spectra from narrowband wave forcing in geophysical flows remains a conundrum. In contrast to the long-standing view that nonlinear wave–wave interactions drive the spreading of wave energy in frequency space, recent work suggests that Doppler-shifting by geostrophic flows may be the primary agent. We investigate this possibility by ray tracing a large number of inertia–gravity wave packets through three-dimensional, geostrophically turbulent flows generated either by a quasigeostrophic (QG) simulation or by synthetic random processes. We find that, in all cases investigated, a broadband quasi-stationary inertia–gravity wave frequency spectrum forms, irrespective of the initial frequencies and wave vectors of the packets. The frequency spectrum is well represented by a power law. A possible theoretical explanation relies on the analogy between the kinematic stretching of passive tracer gradients and the refraction of wave vectors. Consistent with this hypothesis, the spectrum of eigenvalues of the background flow velocity gradients predicts a frequency spectrum that is nearly identical to that found by integration of the ray tracing equations.
Abstract
The generation of broadband wave energy frequency spectra from narrowband wave forcing in geophysical flows remains a conundrum. In contrast to the long-standing view that nonlinear wave–wave interactions drive the spreading of wave energy in frequency space, recent work suggests that Doppler-shifting by geostrophic flows may be the primary agent. We investigate this possibility by ray tracing a large number of inertia–gravity wave packets through three-dimensional, geostrophically turbulent flows generated either by a quasigeostrophic (QG) simulation or by synthetic random processes. We find that, in all cases investigated, a broadband quasi-stationary inertia–gravity wave frequency spectrum forms, irrespective of the initial frequencies and wave vectors of the packets. The frequency spectrum is well represented by a power law. A possible theoretical explanation relies on the analogy between the kinematic stretching of passive tracer gradients and the refraction of wave vectors. Consistent with this hypothesis, the spectrum of eigenvalues of the background flow velocity gradients predicts a frequency spectrum that is nearly identical to that found by integration of the ray tracing equations.
Abstract
Energetic internal tides in the Pacific Ocean generated from multiple sources are the subject of many studies, although the subpolar North Pacific (SNP) is known as a high-latitude hotspot that remains less explored. The present study is the first detailed investigation on M2 internal tide energetics and dynamics in the SNP by high-resolution numerical simulations. The M2 internal tides in the SNP mainly originate from the Aleutian Ridge (area-integrated 5.51 GW and averaged ∼10−3 W m−2 of barotropic-to-baroclinic conversion rate), wherein the Amukta Pass is the most significant source. The Amukta Pass radiates northward 0.55 GW (averaged 2.3 kW m−1) to the Bering Sea and southward 1.40 GW (averaged 3.7 kW m−1) to the North Pacific. The subsequent south–north asymmetric radiation pattern is consistent with satellite altimeter detection. In the Bering basin, multiwave superposition in the near field between the Amukta Pass and adjacent sources generates two standing wave patterns. After approaching the Bering Sea slope, remote internal tides from Aleutian Ridge enhance local generation and dissipation. The dissipation field is relatively similar to the generation map, which is explained by the higher local dissipation efficiency q (>1/2) and the faster energy attenuation than in the midlatitudes. The simulated dissipation rates compare favorably with the estimate from fine-scale parameterization, indicating the dominance of internal tidal mixing. The averaged dissipation rate in SNP is O(10−10) W kg−1, and the depth-integrated dissipation rates reach O(10−1) W m−2 near the Amukta Pass. It is important to understand the unique physics and dissipative process of high-latitude internal tides to fully characterize the redistribution of global tidal energy and associated mixing.
Abstract
Energetic internal tides in the Pacific Ocean generated from multiple sources are the subject of many studies, although the subpolar North Pacific (SNP) is known as a high-latitude hotspot that remains less explored. The present study is the first detailed investigation on M2 internal tide energetics and dynamics in the SNP by high-resolution numerical simulations. The M2 internal tides in the SNP mainly originate from the Aleutian Ridge (area-integrated 5.51 GW and averaged ∼10−3 W m−2 of barotropic-to-baroclinic conversion rate), wherein the Amukta Pass is the most significant source. The Amukta Pass radiates northward 0.55 GW (averaged 2.3 kW m−1) to the Bering Sea and southward 1.40 GW (averaged 3.7 kW m−1) to the North Pacific. The subsequent south–north asymmetric radiation pattern is consistent with satellite altimeter detection. In the Bering basin, multiwave superposition in the near field between the Amukta Pass and adjacent sources generates two standing wave patterns. After approaching the Bering Sea slope, remote internal tides from Aleutian Ridge enhance local generation and dissipation. The dissipation field is relatively similar to the generation map, which is explained by the higher local dissipation efficiency q (>1/2) and the faster energy attenuation than in the midlatitudes. The simulated dissipation rates compare favorably with the estimate from fine-scale parameterization, indicating the dominance of internal tidal mixing. The averaged dissipation rate in SNP is O(10−10) W kg−1, and the depth-integrated dissipation rates reach O(10−1) W m−2 near the Amukta Pass. It is important to understand the unique physics and dissipative process of high-latitude internal tides to fully characterize the redistribution of global tidal energy and associated mixing.
Abstract
Breaking internal tides contribute substantially to small-scale turbulent mixing in the ocean interior and hence to maintaining the large-scale overturning circulation. How much internal tide energy is available for ocean mixing can be estimated by using semianalytical methods based on linear theory. Until recently, a method resolving the horizontal direction of the internal waves generated by conversion of the barotropic tide was lacking. We here present the first global application of such a method to the first vertical mode of the principal lunar semidiurnal internal tide. We also show that the effect of supercritical slopes on the modally decomposed internal tides is different than previously suggested. To deal with this the continental shelf and the shelf slope are masked in the global computation. The global energy conversion obtained agrees roughly with the previous results by Falahat et al. if the mask is applied to their result, which decreases their energy conversion by half. Thus, around half of the energy conversion obtained by their linear calculations occurs at continental slopes and shelves, where linear theory tends to break down. The barotropic-to-baroclinic energy flux at subcritical slopes away from the continental margins is shown to vary substantially with direction depending on the shape and orientation of topographic obstacles and the direction of the local tidal currents. Taking this additional information into account in tidal mixing parameterizations could have important ramifications for vertical mixing and water mass properties in global numerical simulations.
Abstract
Breaking internal tides contribute substantially to small-scale turbulent mixing in the ocean interior and hence to maintaining the large-scale overturning circulation. How much internal tide energy is available for ocean mixing can be estimated by using semianalytical methods based on linear theory. Until recently, a method resolving the horizontal direction of the internal waves generated by conversion of the barotropic tide was lacking. We here present the first global application of such a method to the first vertical mode of the principal lunar semidiurnal internal tide. We also show that the effect of supercritical slopes on the modally decomposed internal tides is different than previously suggested. To deal with this the continental shelf and the shelf slope are masked in the global computation. The global energy conversion obtained agrees roughly with the previous results by Falahat et al. if the mask is applied to their result, which decreases their energy conversion by half. Thus, around half of the energy conversion obtained by their linear calculations occurs at continental slopes and shelves, where linear theory tends to break down. The barotropic-to-baroclinic energy flux at subcritical slopes away from the continental margins is shown to vary substantially with direction depending on the shape and orientation of topographic obstacles and the direction of the local tidal currents. Taking this additional information into account in tidal mixing parameterizations could have important ramifications for vertical mixing and water mass properties in global numerical simulations.
Abstract
A long-standing challenge in dynamical oceanography is to distinguish nonlinearly intermingled dynamical regimes of oceanic flows. Conventional approaches focus on time-scale or space-scale decomposition. Here, we pursue a dynamics-based decomposition, where a mean flow is introduced to extend the classic theory of wavy and vortical modes. Mainly based on relative magnitudes of the relative vorticity and the modified horizontal divergence in spectral space, the full flow is decomposed into wavy and vortical motions. The proposed approach proves simple and efficient and can be used particularly for online disentangling vortical and wavy motions of the simulated flows by ever-popular tide-resolving high-resolution numerical models. This dynamical approach, combined with conventional time-scale- or space-scale-based approaches, paves the way for online mixing parameterizations using model simulated vortical (for isopycnal mixing) and wavy (for diapycnal mixing) motions and for understanding of multiregime and multiscale interactions of oceanic flows.
Abstract
A long-standing challenge in dynamical oceanography is to distinguish nonlinearly intermingled dynamical regimes of oceanic flows. Conventional approaches focus on time-scale or space-scale decomposition. Here, we pursue a dynamics-based decomposition, where a mean flow is introduced to extend the classic theory of wavy and vortical modes. Mainly based on relative magnitudes of the relative vorticity and the modified horizontal divergence in spectral space, the full flow is decomposed into wavy and vortical motions. The proposed approach proves simple and efficient and can be used particularly for online disentangling vortical and wavy motions of the simulated flows by ever-popular tide-resolving high-resolution numerical models. This dynamical approach, combined with conventional time-scale- or space-scale-based approaches, paves the way for online mixing parameterizations using model simulated vortical (for isopycnal mixing) and wavy (for diapycnal mixing) motions and for understanding of multiregime and multiscale interactions of oceanic flows.
Abstract
A combination of in situ and remotely sensed observations are used to constrain the imprint of submesoscale turbulence in the sea surface height (SSH) field. The distribution of SSH variance across frequencies and wavenumbers is estimated by comparing an empirical model spectrum to two sets of observations. First, submesoscale SSH variance is constrained using a pair of GPS buoys spaced at about 10 km. From these data, one can estimate frequency spectra not only of SSH variance but also of the variance in the SSH difference between the buoys. The ratio between these two spectral estimates is sensitive to how much SSH variance is present in the submesoscale range and thus constrains the spectral roll-off of SSH variance in wavenumber space. Second, a combination of moored current meters and nadir altimetry is used to obtain an independent constraint. This constraint is enabled by geostrophy and the nonseparability of the wavenumber–frequency spectrum of SSH variance revealed by the GPS data. The frequency spectra of kinetic energy and SSH variance follow different power laws, and the difference constrains the spectral content in wavenumber space, allowing for a constraint without the need to actually resolve the submesoscales in space. In all four locations studied, spanning the midlatitude and subtropical ocean, these constraints indicate that the wavenumber spectral roll-off of submesoscale SSH variance is between about k− 4 and k− 5, where k is the wavenumber. These estimates are consistent with previous observations, model results, and theoretical predictions. They provide for a strong prior for the interpretation of upcoming high-resolution satellite data.
Abstract
A combination of in situ and remotely sensed observations are used to constrain the imprint of submesoscale turbulence in the sea surface height (SSH) field. The distribution of SSH variance across frequencies and wavenumbers is estimated by comparing an empirical model spectrum to two sets of observations. First, submesoscale SSH variance is constrained using a pair of GPS buoys spaced at about 10 km. From these data, one can estimate frequency spectra not only of SSH variance but also of the variance in the SSH difference between the buoys. The ratio between these two spectral estimates is sensitive to how much SSH variance is present in the submesoscale range and thus constrains the spectral roll-off of SSH variance in wavenumber space. Second, a combination of moored current meters and nadir altimetry is used to obtain an independent constraint. This constraint is enabled by geostrophy and the nonseparability of the wavenumber–frequency spectrum of SSH variance revealed by the GPS data. The frequency spectra of kinetic energy and SSH variance follow different power laws, and the difference constrains the spectral content in wavenumber space, allowing for a constraint without the need to actually resolve the submesoscales in space. In all four locations studied, spanning the midlatitude and subtropical ocean, these constraints indicate that the wavenumber spectral roll-off of submesoscale SSH variance is between about k− 4 and k− 5, where k is the wavenumber. These estimates are consistent with previous observations, model results, and theoretical predictions. They provide for a strong prior for the interpretation of upcoming high-resolution satellite data.
Abstract
It is now well established that changes in the zonal wind stress over the Antarctic Circumpolar Current (ACC) do not lead to changes in its baroclinicity nor baroclinic transport, a phenomenon referred to as “eddy saturation.” Previous studies provide contrasting dynamical mechanisms for this phenomenon: on one extreme, changes in the winds lead to changes in the efficiency with which transient eddies transfer momentum to the sea floor; on the other extreme, structural adjustments of the ACC’s standing meanders increase the efficiency of momentum transfer. In this study the authors investigate the relative importance of these mechanisms using an idealized, isopycnal channel model of the ACC. Via separate diagnoses of the model’s time-mean flow and eddy diffusivity, the authors decompose the model’s response to changes in wind stress into contributions from transient eddies and the mean flow. A key result is that holding the transient eddy diffusivity constant while varying the mean flow very closely compensates for changes in the wind stress, whereas holding the mean flow constant and varying the eddy diffusivity does not. This implies that eddy saturation primarily occurs due to adjustments in the ACC’s standing waves/meanders, rather than due to adjustments of transient eddy behavior. The authors derive a quasigeostrophic theory for ACC transport saturation by standing waves, in which the transient eddy diffusivity is held fixed, and thus provides dynamical insights into standing wave adjustment to wind changes. These findings imply that representing eddy saturation in global models requires adequate resolution of the ACC’s standing meanders, with wind-responsive parameterizations of the transient eddies being of secondary importance.
Abstract
It is now well established that changes in the zonal wind stress over the Antarctic Circumpolar Current (ACC) do not lead to changes in its baroclinicity nor baroclinic transport, a phenomenon referred to as “eddy saturation.” Previous studies provide contrasting dynamical mechanisms for this phenomenon: on one extreme, changes in the winds lead to changes in the efficiency with which transient eddies transfer momentum to the sea floor; on the other extreme, structural adjustments of the ACC’s standing meanders increase the efficiency of momentum transfer. In this study the authors investigate the relative importance of these mechanisms using an idealized, isopycnal channel model of the ACC. Via separate diagnoses of the model’s time-mean flow and eddy diffusivity, the authors decompose the model’s response to changes in wind stress into contributions from transient eddies and the mean flow. A key result is that holding the transient eddy diffusivity constant while varying the mean flow very closely compensates for changes in the wind stress, whereas holding the mean flow constant and varying the eddy diffusivity does not. This implies that eddy saturation primarily occurs due to adjustments in the ACC’s standing waves/meanders, rather than due to adjustments of transient eddy behavior. The authors derive a quasigeostrophic theory for ACC transport saturation by standing waves, in which the transient eddy diffusivity is held fixed, and thus provides dynamical insights into standing wave adjustment to wind changes. These findings imply that representing eddy saturation in global models requires adequate resolution of the ACC’s standing meanders, with wind-responsive parameterizations of the transient eddies being of secondary importance.
