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- Author or Editor: Sarah T. Gille x
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Abstract
The momentum balance of the Antarctic Circumpolar Current is investigated using both output from a high-resolution primitive equation model and sea surface height measurements from the Geosat altimeter. In the Semtner–Chervin general circulation model, run with approximately one-quarter degree resolution and time-varying ECMWF winds, topographic form stress is the dominant process balancing the surface wind forcing. Detailed examination of form stress in the model indicates that it is due to three large topographic obstructions located at Kerguelen Island, Campbell Plateau, and Drake Passage. The difference between wind stress and form stress represents the lateral transfer of momentum into and out of the ACC. It is examined both in zonal coordinates to conform to the model architecture and along mean streamlines in order to reduce the effects of standing eddies. In this particular model, in stream coordinates, biharmonic friction dominates the lateral transfer of momentum. Since biharmonic friction is a parameterization of subgrid-scale transient eddy processes, this indicates that the unresolved transient eddy processes play a critical role in fluxing momentum across the ACC in this model. Although the relative importance of individual terms in the momentum balance does not vary substantially along streamlines, elevated levels of eddy kinetic energy are associated with the three major topographic features. In contrast, altimeter data show elevated energy levels at many more topographic features of intermediate scales, suggesting that smaller topographic effects are better able to communicate with the surface in the real ocean than in the model.
Abstract
The momentum balance of the Antarctic Circumpolar Current is investigated using both output from a high-resolution primitive equation model and sea surface height measurements from the Geosat altimeter. In the Semtner–Chervin general circulation model, run with approximately one-quarter degree resolution and time-varying ECMWF winds, topographic form stress is the dominant process balancing the surface wind forcing. Detailed examination of form stress in the model indicates that it is due to three large topographic obstructions located at Kerguelen Island, Campbell Plateau, and Drake Passage. The difference between wind stress and form stress represents the lateral transfer of momentum into and out of the ACC. It is examined both in zonal coordinates to conform to the model architecture and along mean streamlines in order to reduce the effects of standing eddies. In this particular model, in stream coordinates, biharmonic friction dominates the lateral transfer of momentum. Since biharmonic friction is a parameterization of subgrid-scale transient eddy processes, this indicates that the unresolved transient eddy processes play a critical role in fluxing momentum across the ACC in this model. Although the relative importance of individual terms in the momentum balance does not vary substantially along streamlines, elevated levels of eddy kinetic energy are associated with the three major topographic features. In contrast, altimeter data show elevated energy levels at many more topographic features of intermediate scales, suggesting that smaller topographic effects are better able to communicate with the surface in the real ocean than in the model.
Abstract
Potential vorticity (PV) is used as an indicator of the forcing processes and dissipation at work in the Southern Ocean. Output from the Semtner–Chervin model run with quarter-degree resolution is considered on isopycnal surfaces along Montgomery streamfunctions. Numerical results are compared with hydrographic measurements. Although simple hypotheses might suggest that subsurface PV should be unaffected by wind forcing and constant along streamlines, these results indicate that even at about 1000-m depth, PV varies along mean streamlines in both the numerical model output and in the in situ observations. The changes in PV are largely represented by stratification changes rather than shifts in the Coriolis parameter or in relative vorticity. In the numerical model output, a combination of mechanisms is responsible for these changes in PV, including transient tracer fluxes, transient momentum fluxes, diffusive processes, and long-term tracer drift.
Abstract
Potential vorticity (PV) is used as an indicator of the forcing processes and dissipation at work in the Southern Ocean. Output from the Semtner–Chervin model run with quarter-degree resolution is considered on isopycnal surfaces along Montgomery streamfunctions. Numerical results are compared with hydrographic measurements. Although simple hypotheses might suggest that subsurface PV should be unaffected by wind forcing and constant along streamlines, these results indicate that even at about 1000-m depth, PV varies along mean streamlines in both the numerical model output and in the in situ observations. The changes in PV are largely represented by stratification changes rather than shifts in the Coriolis parameter or in relative vorticity. In the numerical model output, a combination of mechanisms is responsible for these changes in PV, including transient tracer fluxes, transient momentum fluxes, diffusive processes, and long-term tracer drift.
Abstract
Autonomous Lagrangian Circulation Explorer (ALACE) floats are used to examine mean flow and eddy fluxes at 900-m depth in the Southern Ocean. Mean temperature and dynamic topography from float data are consistent with earlier estimates from hydrographic surveys, although floats imply warmer temperatures and narrower frontal structures than do atlas data. Differences between hydrographic and ALACE dynamic topography suggest the presence of eastward bottom velocities of about 2 cm s−1 below the eastward-flowing jets of the Antarctic Circumpolar Current. Flow is steered by bathymetry and can be represented as an equivalent barotropic system with an e-folding depth of about 700 m.
Abstract
Autonomous Lagrangian Circulation Explorer (ALACE) floats are used to examine mean flow and eddy fluxes at 900-m depth in the Southern Ocean. Mean temperature and dynamic topography from float data are consistent with earlier estimates from hydrographic surveys, although floats imply warmer temperatures and narrower frontal structures than do atlas data. Differences between hydrographic and ALACE dynamic topography suggest the presence of eastward bottom velocities of about 2 cm s−1 below the eastward-flowing jets of the Antarctic Circumpolar Current. Flow is steered by bathymetry and can be represented as an equivalent barotropic system with an e-folding depth of about 700 m.
Abstract
Autonomous Lagrangian Circulation Explorer (ALACE) floats are used to examine eddy fluxes in the Southern Ocean. Eddy fluxes are calculated from differences between ALACE float data and mean fields derived from hydrographic atlas data or objectively mapped float observations. Heat fluxes indicate an average poleward eddy heat transport across the Antarctic Circumpolar Current (ACC) of about 3–7 kW m−2 at 900-m depth. Because analysis of current meter data suggests that ALACE's 9–25-day averaging underestimates the total heat flux, the initial ALACE estimates are rescaled to account for this undersampling. This results in a total corrected heat flux of 5–10 kW m−2 at 900 m, depending on the mean field used for the calculations. If the cross-ACC heat flux is assumed to vary exponentially through the water column with an e-folding depth of 1000 m, then the implied net poleward heat flux across the ACC is between 0.3 ± 0.1 and 0.6 ± 0.3 (×1015 W). These estimates are in agreement with previous Southern Ocean eddy flux estimates, which have suggested a cross-ACC heat fluxes ranging between 0.05 and 0.9 (×1015 W). Cross-stream fluxes vary geographically, with the largest fluxes occuring in the Indian Ocean sector, near the Agulhas Retroflection. Statistically significant poleward fluxes also occur along the core of the ACC. Along-stream fluxes are comparable in size to cross-stream fluxes. Momentum fluxes observed by ALACE are isotropic and do not indicate statistically significant eddy–mean flow interactions.
Abstract
Autonomous Lagrangian Circulation Explorer (ALACE) floats are used to examine eddy fluxes in the Southern Ocean. Eddy fluxes are calculated from differences between ALACE float data and mean fields derived from hydrographic atlas data or objectively mapped float observations. Heat fluxes indicate an average poleward eddy heat transport across the Antarctic Circumpolar Current (ACC) of about 3–7 kW m−2 at 900-m depth. Because analysis of current meter data suggests that ALACE's 9–25-day averaging underestimates the total heat flux, the initial ALACE estimates are rescaled to account for this undersampling. This results in a total corrected heat flux of 5–10 kW m−2 at 900 m, depending on the mean field used for the calculations. If the cross-ACC heat flux is assumed to vary exponentially through the water column with an e-folding depth of 1000 m, then the implied net poleward heat flux across the ACC is between 0.3 ± 0.1 and 0.6 ± 0.3 (×1015 W). These estimates are in agreement with previous Southern Ocean eddy flux estimates, which have suggested a cross-ACC heat fluxes ranging between 0.05 and 0.9 (×1015 W). Cross-stream fluxes vary geographically, with the largest fluxes occuring in the Indian Ocean sector, near the Agulhas Retroflection. Statistically significant poleward fluxes also occur along the core of the ACC. Along-stream fluxes are comparable in size to cross-stream fluxes. Momentum fluxes observed by ALACE are isotropic and do not indicate statistically significant eddy–mean flow interactions.
Abstract
Sea surface height anomalies measured by the Ocean Topography Experiment (TOPEX)/Poseidon satellite altimeter indicate high values of skewness and kurtosis. Except in a few regions, including the Gulf Stream, the Kuroshio Extension, and the Agulhas Retroflection, that display bimodal patterns of sea surface height variability, kurtosis is uniformly greater than 1.5 times the squared skewness minus an adjustment constant. This relationship differs substantially from what standard Gaussian or double-exponential noise would produce. However, it can be explained by a simple theory in which the noise is assumed to be multiplicative, meaning that a larger background state implies larger random noise elements. The existence of multiplicative noise can be anticipated from the equations of motion, if ocean dynamics are split into a slowly decorrelating deterministic component and a rapidly decorrelating contribution that is approximated as noise. Such a model raises the possibility of predicting the probabilities of extreme sea surface height anomalies from first physical principles and may provide a useful null hypothesis for non-Gaussian sea surface height variability.
Abstract
Sea surface height anomalies measured by the Ocean Topography Experiment (TOPEX)/Poseidon satellite altimeter indicate high values of skewness and kurtosis. Except in a few regions, including the Gulf Stream, the Kuroshio Extension, and the Agulhas Retroflection, that display bimodal patterns of sea surface height variability, kurtosis is uniformly greater than 1.5 times the squared skewness minus an adjustment constant. This relationship differs substantially from what standard Gaussian or double-exponential noise would produce. However, it can be explained by a simple theory in which the noise is assumed to be multiplicative, meaning that a larger background state implies larger random noise elements. The existence of multiplicative noise can be anticipated from the equations of motion, if ocean dynamics are split into a slowly decorrelating deterministic component and a rapidly decorrelating contribution that is approximated as noise. Such a model raises the possibility of predicting the probabilities of extreme sea surface height anomalies from first physical principles and may provide a useful null hypothesis for non-Gaussian sea surface height variability.
Abstract
Coarse-resolution numerical models of ocean circulation rely on parameterizations of unresolved mesoscale eddy effects. In order to investigate the role of eddy-flux divergences in the density equation, the GFDL Modular Ocean Model (MOM) has been configured as a simple flat-bottomed channel model with sufficient resolution to represent mesoscale eddies. Eady-type baroclinic instability and a wind-forced channel have been considered. As an analog to the large-scale components addressed by low-resolution models, the influence of eddy fluxes on the zonal-mean density field was evaluated. Results show that eddy-flux divergences are larger than mean-flux divergences. The effect of mesoscale eddies on the mean density field is often hypothesized to take an advective form that conserves mean density so that eddy effects are adiabatic in the zonal mean. However, in both of the examples studied a significant component of the mesoscale eddy effect on the zonal mean is diabatic and makes mean density nonconservative. The associated diapycnal fluxes result from zonally averaging terms representing processes that are locally adiabatic.
Subgrid-scale parameterizations (such as eddy diffusion) represent the unresolved eddy-flux divergence as a function of the resolved density field. The authors computed optimal coefficients for a variety of parameterizations and evaluated their skill. When the model output is time-averaged, quasi-adiabatic parameterizations, such as the one proposed by Gent and McWilliams, are able to explain as much as 43% of the mean-squared eddy-flux divergence. However, for shorter averaging periods or instantaneous snapshots, even for the spatially averaged model fields, parameterization skill drops.
Abstract
Coarse-resolution numerical models of ocean circulation rely on parameterizations of unresolved mesoscale eddy effects. In order to investigate the role of eddy-flux divergences in the density equation, the GFDL Modular Ocean Model (MOM) has been configured as a simple flat-bottomed channel model with sufficient resolution to represent mesoscale eddies. Eady-type baroclinic instability and a wind-forced channel have been considered. As an analog to the large-scale components addressed by low-resolution models, the influence of eddy fluxes on the zonal-mean density field was evaluated. Results show that eddy-flux divergences are larger than mean-flux divergences. The effect of mesoscale eddies on the mean density field is often hypothesized to take an advective form that conserves mean density so that eddy effects are adiabatic in the zonal mean. However, in both of the examples studied a significant component of the mesoscale eddy effect on the zonal mean is diabatic and makes mean density nonconservative. The associated diapycnal fluxes result from zonally averaging terms representing processes that are locally adiabatic.
Subgrid-scale parameterizations (such as eddy diffusion) represent the unresolved eddy-flux divergence as a function of the resolved density field. The authors computed optimal coefficients for a variety of parameterizations and evaluated their skill. When the model output is time-averaged, quasi-adiabatic parameterizations, such as the one proposed by Gent and McWilliams, are able to explain as much as 43% of the mean-squared eddy-flux divergence. However, for shorter averaging periods or instantaneous snapshots, even for the spatially averaged model fields, parameterization skill drops.
Abstract
This study addresses the response of the Southern Ocean to high-frequency wind forcing, focusing on the impact of several barotropic modes on the circumpolar transport. A suite of experiments is performed with an unstratified model of the Southern Ocean, forced with a stochastic wind stress that contains a large range of frequencies with synoptic time scales. The Southern Ocean adjustment displays a different character for frequencies below and above 0.2 cpd. The low-frequency range is dominated by an “almost-free-mode” response in the region where contours of f /H are obstructed by only a few bathymetric features; the truly free mode only plays a minor role. Topographic form stress, rather than friction, is the dominant decay mechanism of the Southern Mode. It leads to a spindown time scale on the order of 3 days. For the high-frequency range, the circumpolar transport is dominated by the resonant excitation of oscillatory modes. The “active” response of the ocean leads to strong changes and even discontinuities in the phase relation between transport and wind stress.
Abstract
This study addresses the response of the Southern Ocean to high-frequency wind forcing, focusing on the impact of several barotropic modes on the circumpolar transport. A suite of experiments is performed with an unstratified model of the Southern Ocean, forced with a stochastic wind stress that contains a large range of frequencies with synoptic time scales. The Southern Ocean adjustment displays a different character for frequencies below and above 0.2 cpd. The low-frequency range is dominated by an “almost-free-mode” response in the region where contours of f /H are obstructed by only a few bathymetric features; the truly free mode only plays a minor role. Topographic form stress, rather than friction, is the dominant decay mechanism of the Southern Mode. It leads to a spindown time scale on the order of 3 days. For the high-frequency range, the circumpolar transport is dominated by the resonant excitation of oscillatory modes. The “active” response of the ocean leads to strong changes and even discontinuities in the phase relation between transport and wind stress.
Abstract
The Argo array provides a unique dataset to explore variability of the subsurface ocean interior. This study considers the subtropical North Pacific Ocean during the period from 2004 to 2011, when Argo coverage has been relatively complete in time and space. Two distinct patterns of Argo dynamic height transport function (
The limited temporal range of the Argo observations does not allow analysis of the correlation of ocean transport and wind forcing in the basin for the multiyear time scale (6–8-yr period) typical of the dominant gyre patterns. The meridional geostrophic transport anomaly between 180° and 150°E is computed both from Argo data (0–2000 db) and from the Sverdrup relation (using reanalysis winds): similarities are observed in a latitude–time plane, consistent with local forcing playing an important role. A forcing contribution from the eastern subtropics will also reach the region of interest, but on a longer time scale, and it is not analyzed in this study.
Compared with the 8-yr Argo record, the longer 19-yr time series of satellite altimetry shows a similar but somewhat modified pattern of variability. A longer Argo record will be needed to observe the decadal-scale fluctuations, to separate interannual and decadal signals, and to ensure statistical confidence in relating the wind forcing and the oceanic response.
Abstract
The Argo array provides a unique dataset to explore variability of the subsurface ocean interior. This study considers the subtropical North Pacific Ocean during the period from 2004 to 2011, when Argo coverage has been relatively complete in time and space. Two distinct patterns of Argo dynamic height transport function (
The limited temporal range of the Argo observations does not allow analysis of the correlation of ocean transport and wind forcing in the basin for the multiyear time scale (6–8-yr period) typical of the dominant gyre patterns. The meridional geostrophic transport anomaly between 180° and 150°E is computed both from Argo data (0–2000 db) and from the Sverdrup relation (using reanalysis winds): similarities are observed in a latitude–time plane, consistent with local forcing playing an important role. A forcing contribution from the eastern subtropics will also reach the region of interest, but on a longer time scale, and it is not analyzed in this study.
Compared with the 8-yr Argo record, the longer 19-yr time series of satellite altimetry shows a similar but somewhat modified pattern of variability. A longer Argo record will be needed to observe the decadal-scale fluctuations, to separate interannual and decadal signals, and to ensure statistical confidence in relating the wind forcing and the oceanic response.
Abstract
Probability density functions (pdfs) are employed to evaluate the distribution of velocities in the global ocean. This study computes pdfs of ocean surface velocity using altimetric data from the TOPEX/Poseidon satellite. Results show that the shape of the observed pdfs changes with the size of the domain over which they are calculated: if data are drawn from a small region of the ocean, the pdfs are Gaussian. As the area of the ocean considered increases, the pdfs take on more exponential shapes. The appearance of exponential pdfs is particularly pronounced when data are drawn from a large range of latitudes, while data drawn from constant latitude tend to have a more Gaussian pdf. The authors show that this distinction between zonal and meridional regions is also observed in acoustic Doppler current profiler measurements.
The authors propose a simple statistical model to explain the observed velocity pdfs. This explanation depends on the fact that root-mean-squared velocity (or the width of velocity pdf) varies throughout the ocean. The velocity pdf is predicted from the distribution of the mean-squared velocity. The model matches the observations in predicting a pdf that is parabolic for small velocities with generalized exponential decay for large velocities.
Abstract
Probability density functions (pdfs) are employed to evaluate the distribution of velocities in the global ocean. This study computes pdfs of ocean surface velocity using altimetric data from the TOPEX/Poseidon satellite. Results show that the shape of the observed pdfs changes with the size of the domain over which they are calculated: if data are drawn from a small region of the ocean, the pdfs are Gaussian. As the area of the ocean considered increases, the pdfs take on more exponential shapes. The appearance of exponential pdfs is particularly pronounced when data are drawn from a large range of latitudes, while data drawn from constant latitude tend to have a more Gaussian pdf. The authors show that this distinction between zonal and meridional regions is also observed in acoustic Doppler current profiler measurements.
The authors propose a simple statistical model to explain the observed velocity pdfs. This explanation depends on the fact that root-mean-squared velocity (or the width of velocity pdf) varies throughout the ocean. The velocity pdf is predicted from the distribution of the mean-squared velocity. The model matches the observations in predicting a pdf that is parabolic for small velocities with generalized exponential decay for large velocities.
Abstract
The barotropic intraseasonal variability in the Australia–Antarctic Basin (AAB) is studied in terms of the excitation and decay of topographically trapped barotropic modes. The main objective is to reconcile two widely differing estimates of the decay rate of sea surface height (SSH) anomalies in the AAB that are assumed to be related to barotropic modes. First, an empirical orthogonal function (EOF) analysis is applied to almost 15 years of altimeter data. The analysis suggests that several modes are involved in the variability of the AAB, each related to distinct areas with (almost) closed contours of potential vorticity. Second, the dominant normal modes of the AAB are determined in a barotropic shallow-water (SW) model. These stationary modes are confined by the closed contours of potential vorticity that surround the eastern AAB, and the crest of the Southeast Indian Ridge. For reasonable values of horizontal eddy viscosity and bottom friction, their decay time scale is on the order of several weeks. Third, the SW model is forced with realistic winds and integrated for several years. Projection of the modal velocity patterns onto the output fields shows that the barotropic modes are indeed excited in the model, and that they decay slowly on the frictional
Abstract
The barotropic intraseasonal variability in the Australia–Antarctic Basin (AAB) is studied in terms of the excitation and decay of topographically trapped barotropic modes. The main objective is to reconcile two widely differing estimates of the decay rate of sea surface height (SSH) anomalies in the AAB that are assumed to be related to barotropic modes. First, an empirical orthogonal function (EOF) analysis is applied to almost 15 years of altimeter data. The analysis suggests that several modes are involved in the variability of the AAB, each related to distinct areas with (almost) closed contours of potential vorticity. Second, the dominant normal modes of the AAB are determined in a barotropic shallow-water (SW) model. These stationary modes are confined by the closed contours of potential vorticity that surround the eastern AAB, and the crest of the Southeast Indian Ridge. For reasonable values of horizontal eddy viscosity and bottom friction, their decay time scale is on the order of several weeks. Third, the SW model is forced with realistic winds and integrated for several years. Projection of the modal velocity patterns onto the output fields shows that the barotropic modes are indeed excited in the model, and that they decay slowly on the frictional
