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- Author or Editor: Rui M. Ponte x
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Abstract
Quantitative analysis of the energetics of the ocean is crucial for understanding its circulation and mixing. The power input by fluctuations in atmospheric pressure pa resulting from the S1 and S2 air tides and the stochastic continuum is analyzed here, with a focus on globally integrated, time-mean values. Results are based on available 1° × 1° near-global pa and sea level fields and are intended as mainly order-of-magnitude estimates. The rate of work done on the radiational and gravitational components of the S2 ocean tide is estimated at 14 and −60 GW, respectively, mostly occurring at low latitudes. The net extraction of energy at a rate of −46 GW is about 10% of available estimates of the work rates by gravity on the S2 tide. For the mainly radiational S1 tide, the power input by pa is much weaker (0.25 GW). Based on daily mean quantities, the stochastic pa continuum contributes ∼3 GW to the nontidal circulation, with substantial power input being associated with the pa -driven dynamic response in the Southern Ocean at submonthly time scales. Missing contributions from nontidal variability at the shortest periods (≤ 2 days) may be substantial, but the rate of work done by pa on the general circulation is likely to remain < 1% of the available wind input estimates. The importance of pa effects when considering local, time-variable energetics remains a possibility, however.
Abstract
Quantitative analysis of the energetics of the ocean is crucial for understanding its circulation and mixing. The power input by fluctuations in atmospheric pressure pa resulting from the S1 and S2 air tides and the stochastic continuum is analyzed here, with a focus on globally integrated, time-mean values. Results are based on available 1° × 1° near-global pa and sea level fields and are intended as mainly order-of-magnitude estimates. The rate of work done on the radiational and gravitational components of the S2 ocean tide is estimated at 14 and −60 GW, respectively, mostly occurring at low latitudes. The net extraction of energy at a rate of −46 GW is about 10% of available estimates of the work rates by gravity on the S2 tide. For the mainly radiational S1 tide, the power input by pa is much weaker (0.25 GW). Based on daily mean quantities, the stochastic pa continuum contributes ∼3 GW to the nontidal circulation, with substantial power input being associated with the pa -driven dynamic response in the Southern Ocean at submonthly time scales. Missing contributions from nontidal variability at the shortest periods (≤ 2 days) may be substantial, but the rate of work done by pa on the general circulation is likely to remain < 1% of the available wind input estimates. The importance of pa effects when considering local, time-variable energetics remains a possibility, however.
Abstract
Forcing by freshwater fluxes implies variable surface loads that are not treated in volume-conserving ocean models. A similar problem exists with the representation of volume changes implied by surface heat fluxes. Under the assumption of an equilibrium response, such surface loads merely lead to spatially uniform sea level fluctuations, which carry no dynamical significance. A barotropic model forced by realistic freshwater fluxes is used to test the validity of the equilibrium assumption on seasonal to daily time scales. The simulated nonequilibrium signals have amplitudes much weaker than those of the forcing, with standard deviations well below 1 mm over most of the deep ocean. Larger values (up to ∼1 cm) can be found in shallow and semienclosed coastal areas, where the equilibrium assumption can lead to substantial errors even at monthly and longer time scales. Forcing by mean seasonal river runoff yields similar results, and heat flux effects lead to weaker nonequilibrium signals. In contrast, nonequilibrium signals driven by atmospheric pressure loading are at least an order of magnitude larger than those forced by freshwater fluxes. The exceptions occur for some shallow, coastal regions in the Tropics and at the longest time scales, in general, where forcing by freshwater flux is much stronger than by pressure.
Abstract
Forcing by freshwater fluxes implies variable surface loads that are not treated in volume-conserving ocean models. A similar problem exists with the representation of volume changes implied by surface heat fluxes. Under the assumption of an equilibrium response, such surface loads merely lead to spatially uniform sea level fluctuations, which carry no dynamical significance. A barotropic model forced by realistic freshwater fluxes is used to test the validity of the equilibrium assumption on seasonal to daily time scales. The simulated nonequilibrium signals have amplitudes much weaker than those of the forcing, with standard deviations well below 1 mm over most of the deep ocean. Larger values (up to ∼1 cm) can be found in shallow and semienclosed coastal areas, where the equilibrium assumption can lead to substantial errors even at monthly and longer time scales. Forcing by mean seasonal river runoff yields similar results, and heat flux effects lead to weaker nonequilibrium signals. In contrast, nonequilibrium signals driven by atmospheric pressure loading are at least an order of magnitude larger than those forced by freshwater fluxes. The exceptions occur for some shallow, coastal regions in the Tropics and at the longest time scales, in general, where forcing by freshwater flux is much stronger than by pressure.
Abstract
For a dynamical interpretation of sea level records, estimates are needed of the isostatic, or so-called inverted barometer, signals (η ib) associated with the ocean response to atmospheric loading. Seasonal and longer-period η ib signals are evaluated over the global ocean for the period 1958–2000 using monthly sea level pressure fields from two different atmospheric reanalyses. Variability and linear trends in η ib agree well for the two reanalyses in most regions but less so over the Southern Ocean, where uncertainties in η ib seem to be largest. The standard deviation of η ib ranges from <1 cm in equatorial regions to >7 cm in the regions of the Aleutian and Iceland lows and parts of the Southern and Arctic Oceans. When compared to a global tide gauge dataset, both seasonal and interannual η ib signals are found to contribute importantly to the sea level variance in many mid- and high-latitude records, with seasonal signals important as well in tropical records from India and Southeast Asia. For these records, subtracting η ib from the data can lead to changes in variance of 40% or more. Over the period of study, linear trends in η ib are mostly negative at low and midlatitudes and can cause negative biases in tide gauge estimates of global mean sea level rise that are comparable in magnitude to the effects of postglacial rebound. In agreement with previous findings, η ib signals are found to introduce anomalous behavior in local records (e.g., substantially weaker upward trends in the Mediterranean), and their removal can also reduce formal trend uncertainties. Accounting for η ib effects can be even more important when analyzing relatively short (decadal) records, such as those available from satellite altimetry.
Abstract
For a dynamical interpretation of sea level records, estimates are needed of the isostatic, or so-called inverted barometer, signals (η ib) associated with the ocean response to atmospheric loading. Seasonal and longer-period η ib signals are evaluated over the global ocean for the period 1958–2000 using monthly sea level pressure fields from two different atmospheric reanalyses. Variability and linear trends in η ib agree well for the two reanalyses in most regions but less so over the Southern Ocean, where uncertainties in η ib seem to be largest. The standard deviation of η ib ranges from <1 cm in equatorial regions to >7 cm in the regions of the Aleutian and Iceland lows and parts of the Southern and Arctic Oceans. When compared to a global tide gauge dataset, both seasonal and interannual η ib signals are found to contribute importantly to the sea level variance in many mid- and high-latitude records, with seasonal signals important as well in tropical records from India and Southeast Asia. For these records, subtracting η ib from the data can lead to changes in variance of 40% or more. Over the period of study, linear trends in η ib are mostly negative at low and midlatitudes and can cause negative biases in tide gauge estimates of global mean sea level rise that are comparable in magnitude to the effects of postglacial rebound. In agreement with previous findings, η ib signals are found to introduce anomalous behavior in local records (e.g., substantially weaker upward trends in the Mediterranean), and their removal can also reduce formal trend uncertainties. Accounting for η ib effects can be even more important when analyzing relatively short (decadal) records, such as those available from satellite altimetry.
Abstract
The nature of the sea surface adjustment to atmospheric loading in a stratified ocean is examined for both midlatitude and equatorial regions, using simple analytical solutions to the quasigeostrophic and equatorial β-plane equations. While the interior response can have vertical structures ranging from oscillatory to surface trapped, depending on the temporal and spatial scales of the forcing, the sea surface reacts as an inverted barometer at most scales. The inverted barometer or isostatic approximation breaks down only for narrow ranges of frequency and horizontal wavenumber values, where the vertical dependence of solutions approaches that of the oceanic normal modes. In these regions, the sea surface adjustment can be both smaller and larger than the isostatic limit and is sensitive to frequency and wavenumber. The vertical stratification introduces a number of nonisostatic regimes (particularly in the equatorial regions) not possible in a constant density ocean, but the importance of these effects in the real ocean is likely to be small.
Abstract
The nature of the sea surface adjustment to atmospheric loading in a stratified ocean is examined for both midlatitude and equatorial regions, using simple analytical solutions to the quasigeostrophic and equatorial β-plane equations. While the interior response can have vertical structures ranging from oscillatory to surface trapped, depending on the temporal and spatial scales of the forcing, the sea surface reacts as an inverted barometer at most scales. The inverted barometer or isostatic approximation breaks down only for narrow ranges of frequency and horizontal wavenumber values, where the vertical dependence of solutions approaches that of the oceanic normal modes. In these regions, the sea surface adjustment can be both smaller and larger than the isostatic limit and is sensitive to frequency and wavenumber. The vertical stratification introduces a number of nonisostatic regimes (particularly in the equatorial regions) not possible in a constant density ocean, but the importance of these effects in the real ocean is likely to be small.
Abstract
Deep lateral boundary processes (e.g., western boundary currents) are hypothesized as an alternative energy source exciting the equatorial wave guide at long time scales. A linear, continuously stratified model is used to study the equatorial zonal currents generated by a time dependent, short vertical scale deep zonal jet located at the meridional walls and centered at the equator. Examples of solutions with periodic, transient and spectral forcing are presented. For low frequency forcing at the western or eastern boundaries, energy travels from the source along ray paths associated with Kelvin and long Rossby waves, respectively. Linearly damped solutions look similar in both cases.
Solutions show in general a rich baroclinic structure and a complex time dependence (e.g., periodic solutions can exhibit both upward and downward phase propagation and standing mode oscillations at different depths in the water column), with the vertical structure depending, among other factors, on the vertical scale and frequency composition assumed for the boundary jet. Results suggest the potential importance of deep forcing mechanisms to the existence of long time scale, deep baroclinic currents in the equatorial ocean. Solutions are qualitatively similar to observations of the equatorial deep jets, but any detailed comparison between model results and data is premature, given the lack of observational knowledge about the time scales, strength and spatial distribution of deep energy sources.
Abstract
Deep lateral boundary processes (e.g., western boundary currents) are hypothesized as an alternative energy source exciting the equatorial wave guide at long time scales. A linear, continuously stratified model is used to study the equatorial zonal currents generated by a time dependent, short vertical scale deep zonal jet located at the meridional walls and centered at the equator. Examples of solutions with periodic, transient and spectral forcing are presented. For low frequency forcing at the western or eastern boundaries, energy travels from the source along ray paths associated with Kelvin and long Rossby waves, respectively. Linearly damped solutions look similar in both cases.
Solutions show in general a rich baroclinic structure and a complex time dependence (e.g., periodic solutions can exhibit both upward and downward phase propagation and standing mode oscillations at different depths in the water column), with the vertical structure depending, among other factors, on the vertical scale and frequency composition assumed for the boundary jet. Results suggest the potential importance of deep forcing mechanisms to the existence of long time scale, deep baroclinic currents in the equatorial ocean. Solutions are qualitatively similar to observations of the equatorial deep jets, but any detailed comparison between model results and data is premature, given the lack of observational knowledge about the time scales, strength and spatial distribution of deep energy sources.
Abstract
A linear, continuously stratified model is used to investigate the flows generated by a midlatitude, eastern boundary zonal inflow representing the flux of Mediterranean Water into the North Atlantic. The model allows for time dependence and vertical mixing of density and meridional momentum and assumes a geostrophic balance for zonal momentum. Rossby and Kelvin wave propagation and dissipation away from the inflow region determine the character of the analytical solutions. For both periodic and steady forcing and for a wide range of mixing coefficients, currents have a significant boundary signature. Inflows generate poleward currents at the depth of the forcing and weaker countercurrents above and below. The amplitude of meridional coastal flows can be substantially larger than the amplitudes of the forcing jet, and interior flows are generally weaker. Zonal decay scales depend on the amount of mixing and the relative importance of Kelvin and Rossby wave dynamics in the solutions.
Abstract
A linear, continuously stratified model is used to investigate the flows generated by a midlatitude, eastern boundary zonal inflow representing the flux of Mediterranean Water into the North Atlantic. The model allows for time dependence and vertical mixing of density and meridional momentum and assumes a geostrophic balance for zonal momentum. Rossby and Kelvin wave propagation and dissipation away from the inflow region determine the character of the analytical solutions. For both periodic and steady forcing and for a wide range of mixing coefficients, currents have a significant boundary signature. Inflows generate poleward currents at the depth of the forcing and weaker countercurrents above and below. The amplitude of meridional coastal flows can be substantially larger than the amplitudes of the forcing jet, and interior flows are generally weaker. Zonal decay scales depend on the amount of mixing and the relative importance of Kelvin and Rossby wave dynamics in the solutions.
Abstract
The response of the global ocean to the surface pressure signal associated with the well-known 5-day Rossby–Haurwitz atmospheric mode is explored using analytical and numerical tools. Solutions of the Laplace tidal equations for a flat-bottom, globe-covering ocean, point to a depth-independent nonequilibrium response related to the near-resonant excitation of the barotropic oceanic mode. Numerical experiments with a shallow-water model illustrate the effects of realistic continental boundaries, topography, and dissipation on the solutions. The character of the oceanic adjustment and the structure of resonances changes substantially, but a nonequilibrium response occurs in all cases studied. Besides the excitation of large-scale vorticity modes or waves, which becomes less important when topography and strong dissipation are present, basin-scale nonequilibrium signals are associated with gravity wave dynamics and the process of interbasin mass adjustment in the presence of global-scale forcing and continents that require interbasin mass fluxes to occur through the Southern Ocean. Solutions with forcing most representative of the observed atmospheric wave agree qualitatively with the results of analyses of Pacific and Atlantic tide gauge records by Luther and Woodworth et al. The observed nonequilibrium signals thus seem related to the Rossby–Haurwitz forcing mode.
Abstract
The response of the global ocean to the surface pressure signal associated with the well-known 5-day Rossby–Haurwitz atmospheric mode is explored using analytical and numerical tools. Solutions of the Laplace tidal equations for a flat-bottom, globe-covering ocean, point to a depth-independent nonequilibrium response related to the near-resonant excitation of the barotropic oceanic mode. Numerical experiments with a shallow-water model illustrate the effects of realistic continental boundaries, topography, and dissipation on the solutions. The character of the oceanic adjustment and the structure of resonances changes substantially, but a nonequilibrium response occurs in all cases studied. Besides the excitation of large-scale vorticity modes or waves, which becomes less important when topography and strong dissipation are present, basin-scale nonequilibrium signals are associated with gravity wave dynamics and the process of interbasin mass adjustment in the presence of global-scale forcing and continents that require interbasin mass fluxes to occur through the Southern Ocean. Solutions with forcing most representative of the observed atmospheric wave agree qualitatively with the results of analyses of Pacific and Atlantic tide gauge records by Luther and Woodworth et al. The observed nonequilibrium signals thus seem related to the Rossby–Haurwitz forcing mode.
Abstract
Analyses of large-scale (>750 km) ocean bottom pressure p b fields, derived from the Gravity Recovery and Climate Experiment (GRACE) and from an Estimating the Circulation & Climate of the Ocean (ECCO) state estimate, reveal enhanced interannual variability, partially connected to the Antarctic Oscillation, in regions of the Australian–Antarctic Basin and the Bellingshausen Basin, with p b magnitudes comparable to those of sea level and good correlation between the GRACE and ECCO p b series. Consistent with the theory of Gill and Niiler, the patterns of stronger p b variability are partly related to enhanced local wind curl forcing and weakened gradients in H/f, where H is ocean depth and f is the Coriolis parameter. Despite weaker H/f gradients, motions against them are sufficiently strong to play a role in balancing the local wind input. Topographic effects are as or more important than changes in f. Additionally, and contrary to the dominance of barotropic processes at subannual time scales, baroclinic effects are not negligible when balancing wind input at periods of a few years. Results highlight the emerging capability to accurately observe and estimate interannual changes in large-scale p b over the Southern Ocean, with implications for the interpretation of low-frequency variability in sea level in terms of steric height and heat content.
Abstract
Analyses of large-scale (>750 km) ocean bottom pressure p b fields, derived from the Gravity Recovery and Climate Experiment (GRACE) and from an Estimating the Circulation & Climate of the Ocean (ECCO) state estimate, reveal enhanced interannual variability, partially connected to the Antarctic Oscillation, in regions of the Australian–Antarctic Basin and the Bellingshausen Basin, with p b magnitudes comparable to those of sea level and good correlation between the GRACE and ECCO p b series. Consistent with the theory of Gill and Niiler, the patterns of stronger p b variability are partly related to enhanced local wind curl forcing and weakened gradients in H/f, where H is ocean depth and f is the Coriolis parameter. Despite weaker H/f gradients, motions against them are sufficiently strong to play a role in balancing the local wind input. Topographic effects are as or more important than changes in f. Additionally, and contrary to the dominance of barotropic processes at subannual time scales, baroclinic effects are not negligible when balancing wind input at periods of a few years. Results highlight the emerging capability to accurately observe and estimate interannual changes in large-scale p b over the Southern Ocean, with implications for the interpretation of low-frequency variability in sea level in terms of steric height and heat content.
Abstract
Linear models of dynamical ocean adjustment to wind field changes, local atmospheric driving, and eastern boundary forcing are often invoked to explain observed patterns of interannual regional sea level variability. While skillful in some regions, these processes alone cannot explain low levels of interannual sea level variability observed in the tropical Atlantic. In this study, through a set of modeling approaches, interannual sea level changes in the tropical South Atlantic are attributed and the dynamical influence of buoyancy forcing is elucidated. Similar to recent findings in the southeast tropical Pacific, sea level patterns in the tropical South Atlantic (as estimated from a data-constrained ocean general circulation model) are found to result from the action of both surface wind and buoyancy forcing; in addition to static local effects, the buoyancy-driven changes comprise important nonlocal ocean dynamical processes. It is shown that the buoyancy-driven sea level changes can be understood within the framework of a linear first baroclinic mode Rossby wave model forced by atmospheric fields and variability along the eastern boundary. To lowest order, the linear model framework also reproduces qualitative patterns of basinwide compensation between wind- and buoyancy-driven sea level changes, which are mostly tied to the anticorrelation of both surface and boundary forcing. Results suggest that the ocean’s dynamical adjustment to buoyancy forcing exerts an important influence on interannual sea level changes across all tropical oceans.
Abstract
Linear models of dynamical ocean adjustment to wind field changes, local atmospheric driving, and eastern boundary forcing are often invoked to explain observed patterns of interannual regional sea level variability. While skillful in some regions, these processes alone cannot explain low levels of interannual sea level variability observed in the tropical Atlantic. In this study, through a set of modeling approaches, interannual sea level changes in the tropical South Atlantic are attributed and the dynamical influence of buoyancy forcing is elucidated. Similar to recent findings in the southeast tropical Pacific, sea level patterns in the tropical South Atlantic (as estimated from a data-constrained ocean general circulation model) are found to result from the action of both surface wind and buoyancy forcing; in addition to static local effects, the buoyancy-driven changes comprise important nonlocal ocean dynamical processes. It is shown that the buoyancy-driven sea level changes can be understood within the framework of a linear first baroclinic mode Rossby wave model forced by atmospheric fields and variability along the eastern boundary. To lowest order, the linear model framework also reproduces qualitative patterns of basinwide compensation between wind- and buoyancy-driven sea level changes, which are mostly tied to the anticorrelation of both surface and boundary forcing. Results suggest that the ocean’s dynamical adjustment to buoyancy forcing exerts an important influence on interannual sea level changes across all tropical oceans.
Abstract
The seasonal monsoon drives a dynamic response in the southern tropical Indian Ocean, previously observed in baroclinic Rossby wave signatures in annual sea level and thermocline depth anomalies. In this paper, monthly mass grids based on Release-05 Gravity Recovery and Climate Experiment (GRACE) data are used to study the annual cycle in southern tropical Indian Ocean bottom pressure. To interpret the satellite data, a linear model of the ocean’s response to wind forcing—based on the theory of vertical normal modes and comprising baroclinic and barotropic components—is considered. The model is evaluated using stratification from an ocean atlas and winds from an atmospheric reanalysis. Good correspondence between model and data is found over the southern tropical Indian Ocean: the model explains 81% of the annual variance in the data on average between 10° and 25°S. Model solutions suggest that, while the annual baroclinic Rossby wave has a seafloor signature, the annual cycle in the deep sea generally involves important barotropic dynamics, in contrast to the response in the upper ocean, which is largely baroclinic.
Abstract
The seasonal monsoon drives a dynamic response in the southern tropical Indian Ocean, previously observed in baroclinic Rossby wave signatures in annual sea level and thermocline depth anomalies. In this paper, monthly mass grids based on Release-05 Gravity Recovery and Climate Experiment (GRACE) data are used to study the annual cycle in southern tropical Indian Ocean bottom pressure. To interpret the satellite data, a linear model of the ocean’s response to wind forcing—based on the theory of vertical normal modes and comprising baroclinic and barotropic components—is considered. The model is evaluated using stratification from an ocean atlas and winds from an atmospheric reanalysis. Good correspondence between model and data is found over the southern tropical Indian Ocean: the model explains 81% of the annual variance in the data on average between 10° and 25°S. Model solutions suggest that, while the annual baroclinic Rossby wave has a seafloor signature, the annual cycle in the deep sea generally involves important barotropic dynamics, in contrast to the response in the upper ocean, which is largely baroclinic.
