Journal of Physical Oceanography

Published by American Meteorological Society
Online ISSN: 1520-0485
Print ISSN: 0022-3670
(left) Trajectories on the period 1 Feb 2004 through 1 Apr 2004 for simulated drifting floats deployed outside the 'forbidden zone' on the WFS as identified by Yang et al. (1999), which is indicated by the cross-hatched region. (right) A snapshot on 1 Mar 2004 of surface ocean velocity field as produced by a HYCOM simulation of the WFS, which was used to produce the trajectories in the plot on the left. Maximum velocity magnitude is roughly 2 m s −1 . Indicated bathymetry contours are in meters.  
Application of dynamical systems tools has recently revealed in surface ocean currents produced by a Hybrid-Coordinate Ocean Model (HYCOM) simulation the presence of a persistent large-scale Lagrangian coherent structure (LCS) on the southern portion of the west Florida shelf (WFS). Consistent with satellite-tracked drifter trajectories, this LCS constitutes a cross-shelf barrier for the lateral transport of passive tracers. Because of the constraints that the above LCS, as well as smaller-scale LCSs lying shoreside, can impose on pollutant dispersal and its potentially very important biological consequences, a study was carried out on the nature of the surface ocean Lagrangian motion on the WFS. The analysis is based on the same simulated surface ocean velocity field that has been able to sustain the aforementioned persistent cross-shelf transport barrier. Examination of several diagnostics suggests that chaotic stirring dominates over turbulent mixing on time scales of up to two months or so. More specifically, it is found on those time scales that tracer evolution at a given length scale is governed to a nonnegligible extent by coarser-scale velocity field features, fluid particle dispersion is spatially inhomogeneous, and the Lagrangian evolution is more irregular than the driving Eulerian flow.
A finite-difference model of the North Atlantic is constructed for the purpose of making an estimate of the circulation through an inverse calculation. The data base is eclectic, and includes hydrography, oxygen, nutrients, current meter and float records, atmospheric momentum, heat and water vapor transfers, as well as estimates of certain integral fluxes. Owing to the available hydrographic database, the model resolution is restricted to 1 deg at best, and is much coarser in many aspects. This limited resolution is a major obstacle to accurate estimates of climatological fluxes. In its final form, there are about 9000 constraints in 29,000 formal unknowns plus 9000 noise unknowns. The system is solved as a tapered least-squares system by a sparse conjugate gradient algorithm. With the exception of a few float velocities, all constraints are found to be consistent within error estimates. The model produces estimates of large-scale fluxes and flux divergences for all conventional properties including heat and nutrients as well as carbon dioxide and alkalinity. Meridional fluxes of carbon are found to be indistinguishable from zero, whereas the North Atlantic tends to export nutrients to the south, but carry heat to the north. Traditional oceanographic depictions of the circulation through combination of nonsynoptic data into steady models may have reached their useful limit in the present calculation, as the conflicts between the data and physical requirements become quantitatively apparent.
The absolute dynamic topography of the world ocean is estimated from the largest scales to a short-wavelength cutoff of about 6700 km for the period July through September, 1978. The data base consisted of the time-averaged sea-surface topography determined by Seasat and geoid estimates made at the Goddard Space Flight Center. The issues are those of accuracy and resolution. Use of the altimetric surface as a geoid estimate beyond the short-wavelength cutoff reduces the spectral leakage in the estimated dynamic topography from erroneous small-scale geoid estimates without contaminating the low wavenumbers. Comparison of the result with a similarly filtered version of Levitus' (1982) historical average dynamic topography shows good qualitative agreement. There is quantitative disagreement, but it is within the estimated errors of both methods of calculation.
The sea level differences between the Sargasso Sea and the slope waters across the Gulf Stream region, averaged between 73 and 61 deg W, and the comparable areas across the Kuroshio extension region, averaged between 143 and 156 deg E, were estimated using the Geosat altimeter data obtained between November 1986 and December 1988. The sea-level differences between the two regions showed a strong correlation between the northwest Atlantic and Pacific, dominated by annual cycles that peak in late-September to mid-October, with about 9 cm (the Gulf Stream region) and about 6.9 cm (Kuroshio region) amplitudes.
This paper examines the results of assimilating Geosat sea level variations relative to the November 1986-November 1988 mean reference, in a nonlinear reduced-gravity model of the Indian Ocean, Data have been assimilated during one year starting in November 1986 with the objective of optimizing the initial conditions and the yearly averaged reference surface. The thermocline slope simulated by the model with or without assimilation is validated by comparison with the signal, which can be derived from expandable bathythermograph measurements performed in the Indian Ocean at that time. The topography simulated with assimilation on November 1986 is in very good agreement with the hydrographic data. The slopes corresponding to the South Equatorial Current and to the South Equatorial Countercurrent are better reproduced with assimilation than without during the first nine months. The whole circulation of the cyclonic gyre south of the equator is then strongly intensified by assimilation. Another assimilation experiment is run over the following year starting in November 1987. The difference between the two yearly mean surfaces simulated with assimilation is in excellent agreement with Geosat. In the southeastern Indian Ocean, the correction to the yearly mean dynamic topography due to assimilation over the second year is negatively correlated to the one the year before. This correction is also in agreement with hydrographic data. It is likely that the signal corrected by assimilation is not only due to wind error, because simulations driven by various wind forcings present the same features over the two years. Model simulations run with a prescribed throughflow transport anomaly indicate that assimilation is rather correcting in the interior of the model domain for inadequate boundary conditions with the Pacific.
Air-sea transfers of sensible heat, latent heat and momentum are computed from 25 years of middle-latitude and subtropical ocean weather ship data in the North Atlantic and North Pacific using the bulk aerodynamic method. The results show that monthly averaged wind speeds, temperatures and humidities can be used to estimate the monthly averaged sensible and latent heat fluxes from the bulk aerodynamic equations to within a relative error of approximately 10%. The estimates of monthly averaged wind stress under the assumption of neutral stability are shown to be within approximately 5% of the monthly averaged nonneutral values.
A model is presented which shows that, for a fully developed sea driven by the wind with speeds above 5 m/sec the air in the transition zone immediately above the air-sea interface is mixed with sea water droplets from bursting air bubbles. The absorptive droplet concentration in the zone is assumed to have a profile tapering off from the interface to zero at a certain height. The dielectric constant of the absorptive inhomogeneous droplet profile is thus both a function of the wind speed and the height above the interface. Both the inhomogeneity effect and the absorption-emission effect of the droplet concentration have been considered. Theoretical calculations show that the presence of the absorptive inhomogeneous droplet transition zone significantly increases the sea brightness temperature as wind speed rises. Combined effects of both the droplet areas and the foam areas on sea surface have also been considered.
Climatic dynamic topography variations in the Alaska gyre during the period 1968-1990 are described with an objective analysis of more than 12000 STD and XBT stations, and COADS wind stress data Interannual the dynamic height and SST variations were correlated and were consistent with recently described large-scale climatic shifts in the North Pacific. The gyre was centered more to the east, circulation appeared stronger, and SST was lower during the early to mid-1970s than during the 1980s. The Aleutian low (NP and PNA indices) intensified during the interim, but the response did not appear as a gyre spinup. Instead, the associated wind stress anomalies forced a slowly varying dynamic height anomaly across the eastern and northern part of the gyre through Ekman convergence, which had the effect of displacing the gyre's low somewhat to the WSW in the 1980s. The wind curl spectrum was white, and the slow oceanic response was modeled as stochastic-forced climate variability with a simple first-order Markov autoregression process. Forcing was assumed to be Ekman pumping of the pycnocline, and the damping coefficient was estimated from the data to be approx. 1 yr. A hindcast with observed winds gave estimated dynamic height patterns similar to those observed, with a canonical correlation of 0.79 at 99% confidence. This response was weak in the western half of the gyre, where slow baroclinic variability may have been influenced by long Rossby wave propagation. A simple autoregression simulation using artificial white noise forcing shows the evolution of decadal variations similar in nature to those observed. This result, along with the low frequency correlation between dynamic height and SST, suggests that the upper-ocean climatic variability in this region is primarily wind forced.
The forced circulation over a continental shelf generated by the alongshore wind stress is studied within the frictional regime. The model alongshore wind stress has a finite extent in the alongshore direction and oscillates monochromatically, resembling a series of anticyclones traveling across the coastline. Both the shelf-wave response and the localized non-wavelike response exist within the wind band. The numerical results show that the interaction of shelf waves with locally wind-forced response generates a large increase in the phase speed within the wind band. Given an alongshore variation in the alongshore wind stress, a right-bounded phase propagation is possible even in the absence of continental shelf waves. Given a reasonable friction currently accepted for the east coast of the United States, the resonance mechanism at the cutoff frequency may not be important, and lower frequency wind events generate larger amplitude continental shelf circulation. By reducing the friction, energy at cutoff fequencies leaks out of the wind band in both directions effectively. It also is shown that non-wavelike response depends only on the local wind stress and is not affected significantly by friction.
A technique is presented that applies modal decomposition to estimate dynamic height (0-450 db) from Expendable BathyThermograph (XBT) temperature profiles. Salinity-Temperature-Depth (STD) data are used to establish empirical relationships between vertically integrated temperature profiles and empirical dynamic height modes. These are then applied to XBT data to estimate dynamic height. A standard error of 0.028 dynamic meters is obtained for the waters of the Gulf of Alaska- an ocean region subject to substantial freshwater buoyancy forcing and with a T-S relationship that has considerable scatter. The residual error is a substantial improvement relative to the conventional T-S correlation technique when applied to this region. Systematic errors between estimated and true dynamic height were evaluated. The 20-year-long time series at Ocean Station P (50 deg N, 145 deg W) indicated weak variations in the error interannually, but not seasonally. There were no evident systematic alongshore variations in the error in the ocean boundary current regime near the perimeter of the Alaska gyre. The results prove satisfactory for the purpose of this work, which is to generate dynamic height from XBT data for coanalysis with satellite altimeter data, given that the altimeter height precision is likewise on the order of 2-3 cm. While the technique has not been applied to other ocean regions where the T-S relation has less scatter, it is suggested that it could provide some improvement over previously applied methods, as well.
Snapshots and time-mean streamfunction for simulation S1: (a) 80-yr time mean; (b), (c) typical snapshots.
The P diff field for the snapshot in Fig. 1b plotted on a log scale: (a) only values above 0.1 times the maximum are shown; (b) values above 0.001 times the maximum are shown. The portion of the field visible in (b) but not in (a) accounts for about 30% of the total.
Wind stress curl [k · ( 1 )] for the snapshot corresponding to Fig. 1b. Lighter shades correspond to cyclonic forcing.
Convolution of u o · diff for simulations S1 and S2, plotted as a function of horizontal wavenumber. Values are multiplied by the horizontal wavenumber so that the area under the curve corresponds to the power sink. Units are arbitrary.
The improvement in the climatological behavior of a numerical model as a consequence of the assimilation of surface data is investigated. The model used for this study is a quasigeostrophic (QG) model of the Gulf Stream region. The data that have been assimilated are maps of sea surface height that have been obtained as the superposition of sea surface height variability deduced from the Geosat altimeter measurements and a mean field constructed from historical hydrographic data. The method used for assimilating the data is the nudging technique. Nudging has been implemented in such a way as to achieve a high degree of convergence of the surface model fields toward the observations. Comparisons of the assimilation results with available in situ observations show a significant improvement in the degree of realism of the climatological model behavior, with respect to the model in which no data are assimilated. The remaining discrepancies in the model mean circulation seem to be mainly associated with deficiencies in the mean component of the surface data that are assimilated. On the other hand, the possibility of building into the model more realistic eddy characteristics through the assimilation of the surface eddy field proves very successful in driving components of the mean model circulation that are in relatively good agreement with the available observations. Comparisons with current meter time series during a time period partially overlapping the Geosat mission show that the model is able to 'correctly' extrapolate the instantaneous surface eddy signals to depths of approximately 1500 m. The correlation coefficient between current meter and model time series varies from values close to 0.7 in the top 1500 m to values as low as 0.1-0.2 in the deep ocean.
The paper considers temporal variations in regional models of the Sargasso sea from GEOS-3 telemetry. The methods of regional models and the analysis of overlapping passes are utilized, and short-wave maxima and minima in the regional surface models are examined for correlations with surface and remote sensed infrared temperature data supplemented with subsurface expendable bathythermograph data (XBT). The analysis of overlapping passes provide a better picture of instanteneous sea surface height (SSH) variability through wavelengths greater than 30 km. Correlation studies with cyclonic and anticyclonic ocean eddies from infrared imagery and XBT data indicate satisfactory agreement with equivalent SSH features 98% of the time if the time varying factors are allowed for.
Altimeter data obtained from GEOS-3 during the three year period 1975-78 for a region of the western North Atlantic which includes a portion of the Gulf Stream system and part of the open ocean area of the subtropical gyre are analyzed by a new technique which utilizes all the points along the satellite tracks. The physical phenomenon studied are the time-variable but almost geostrophic currents, or mesoscale eddies, so that geoid errors contaminate the scientific signal minimally and the dynamical interpretation is direct. Results presented include the spatial distribution of geostrophic eddy kinetic energy and examples of a synoptic map of the eddy field (April 1977) and of a time series at a point. These results are compared to and synthesized with a diverse and select set of existing measurements and observations obtained in situ by a variety of instrumental techniques. The agreement is generally good, and the altimeter data analyzed provides new information on features in the map of mean eddy kinetic energy. The implications are that satellite altimetry will serve as a powerful quantitative tool in eddy current research and that even presently archived data contains further useful scientific information.
Direct estimation of the absolute dynamic topography from satellite altimetry has been confined to the largest scales (basically the basin-scale) owing to the fact that the signal-to-noise ratio is more unfavorable everywhere else. But even for the largest scales, the results are contaminated by the orbit error and geoid uncertainties. Recently a more accurate Earth gravity model (GEM-T1) became available, providing the opportunity to examine the whole question of direct estimation under a more critical limelight. It is found that our knowledge of the Earth's gravity field has indeed improved a great deal. However, it is not yet possible to claim definitively that our knowledge of the ocean circulation has improved through direct estimation. Yet, the improvement in the gravity model has come to the point that it is no longer possible to attribute the discrepancy at the basin scales between altimetric and hydrographic results as mostly due to geoid uncertainties. A substantial part of the difference must be due to other factors; i.e., the orbit error, or the uncertainty of the hydrographically derived dynamic topography.
For a well-developed sea at equilibrium with a constant wind, the energy-containing range of the wavenumber spectrum for wind-generated gravity waves is approximated by a generalized power law involving the angular spread function and mu, interpreted as a fractal codimension of a small surface patch. Dependence of mu on the wave age is estimated, and the 'Phillips constant', beta, along with the low-wavenumber boundary, k0, of the inertial subrange are analyzed on the basis of the wave action and energy conservation principles. The resulting expressions are employed to evaluate various non-Gaussian statistics of a weakly nonlinear sea surface, which determine the sea state bias in satellite altimetry. The locally accelerated decay of the spectral density function in a high-wavenumber dissipation subrange is pointed out as an important factor of wave dynamics and the geometrical optics treatment of the sea state bias. The analysis is carried out in the approximation of a unidirectional wave field and confined to the case of a well-developed sea.
Linear statistical estimators are used to examine 29 years of nonseasonal, monthly-mean, tide-gauge sea-level data along the west coast of North America. The objective is exploration of the structure, and causes of nearshore ocean variability over time scales of months to years at 20 stations from Alaska to Mexico. North of San Francisco, 50–60% of the sea-level variability reflects a simple inverse barometric response to local atmospheric pressure. These inverted barometer effects account for only 10–15% of the variance at stations to the south. The dominant signal of inverse-barometer-corrected sea level represents a nearly uniform rise or fall of sea level everywhere along the eastern rim of the North Pacific. The interannual aspects of this large-scale sea-level variability are closely related to El Nino occurrences in the eastern tropical Pacific which appear to propagate poloward with phase speeds of ∼40 cm s−1. Higher frequency aspects of this large-scale sea-level variability appear to re...
The bifurcation diagram of the Stommel two-box model for η 1 = 3.0 and η 3 = 0.2. The points labelled A and B represent the thermally-driven steady state and salinity-driven steady state, respectively, considered in section 3. The points labelled P and Q represent the bifurcation points of the model, respectively. Solid curves indicate linearly stable steady states, whereas the states on the dashed curve are unstable. There are thermally-driven (TH) stable steady states ( ¯ Ψ > 0) and salinity-driven (SA, ie., the circulation is salinity-dominated) stable steady states ( ¯ Ψ < 0).
The magnitude of perturbations J obtained at t e = 2.5 for the the evolutions of perturbations of the thermally-driven state in the tangent linear model and nonlinear model. The initial perturbations have the form (T ′ (0), S ′ (0)) = (δ cos θ, δ sin θ) with δ = 0.2.
The critical value of δ (δ c ) versus the parameter controlling the salinitydriven state near the saddle-node bifurcation at η 2 = 0.6.
Within a simple model context, the sensitivity and stability of the thermohaline circulation to finite amplitude perturbations is studied. A new approach is used to tackle this nonlinear problem. The method is based on the computation of the so-called Conditional Nonlinear Optimal Perturbation (CNOP) which is a nonlinear generalization of the linear singular vector approach (LSV). It is shown that linearly stable thermohaline circulation states can become nonlinearly unstable and the properties of the perturbations with optimal nonlinear growth are determined. An asymmetric nonlinear response to perturbations exists with respect to the sign of finite amplitude freshwater perturbations, on both thermally dominated and salinity dominated thermohaline flows. This asymmetry is due to the nonlinear interaction of the perturbations through advective processes.
Recent measurements of the earth's radiation budget from satellites, together with extensive atmospheric energy transport summaries based on rawinsonde data, allow a new estimate of the required poleward energy transport by Northern Hemisphere oceans for the mean annual case. In the region of maximum net northward energy transport (30–35N), the oceans transport 47% of the required energy (1.7×1022 cal year−1). At 20N, the peak ocean transport accounts for 74% at that latitude; for the region 0–70N the ocean contribution averages 40%.
Based on the best presently available satellite radiation, atmospheric and oceanic data sets, the long-term mean heat balance of the earth and its normal seasonal variation are investigated over the Northern Hemisphere. Quantitative estimates for the various flux and storage terms in the atmospheric and terrestrial branches of the heat balance are given for 10-deg-wide latitude belts and for each calendar month. The results are presented in both graphical and tabular form. As was known before, the storage of heat in the oceans is found to dominate the energy storage in the combined atmosphere-ocean-land-cryosphere system. In the tropics, large changes in oceanic heat storage are found in the 10 N-20 N belt with a maximum in spring and a minimum in late summer. The main new finding of this study is that the inferred oceanic heat transports appear to undergo very large seasonal variations especially in the tropics.
A deep-water approximation to the Stokes drift velocity profile is explored as an alternative to the monochromatic profile. The alternative profile investigated relies on the same two quantities required for the monochromatic profile, viz the Stokes transport and the surface Stokes drift velocity. Comparisons with parametric spectra and profiles under wave spectra from the ERA-Interim reanalysis and buoy observations reveal much better agreement than the monochromatic profile even for complex sea states. That the profile gives a closer match and a more correct shear has implications for ocean circulation models since the Coriolis-Stokes force depends on the magnitude and direction of the Stokes drift profile and Langmuir turbulence parameterizations depend sensitively on the shear of the profile. The alternative profile comes at no added numerical cost compared to the monochromatic profile.
In this paper we investigate mixing and transport in correspondence of a meandering jet. The large-scale flow field is a kinematically assigned streamfunction. Two basic mixing mechanisms are considered, first separately and then combined together: deterministic chaotic advection, induced by a time dependence of the flow, and turbulent diffusion, described by means of a stochastic model for particle motion. Rather than looking at the details of particle trajectories, fluid exchange is studied in terms of markovian approximations. The two-dimensional physical space accessible to fluid particles is subdivided into regions characterized by different Lagrangian behaviours. From the observed transitions between regions it is possible to derive a number of relevant quantities characterizing transport and mixing in the studied flow regime, such as residence times, meridional mixing, correlation functions. These estimated quantities are compared with the corresponding ones resulting from the actual simulations. The outcome of the comparison suggests the possibility of describing in a satisfactory way at least some of the mixing properties ot the system through the very simplified approach of a first order markovian approximation, whereas other properties exhibit memory patterns of higher order. Comment: 31 pages laTeX, 19 figures postscript
An attempt is made to determine the three-dimensional ocean circulation from satellite altimeter measurements by assimilating Geosat sea surface height data into an eddy-resolving QuasiGeostrophic (QG) model of the eastern North Atlantic Ocean. Results are tested against independent information from hydrographic field observations and moored current meter data collected during the Geosat ERM. The comparison supports the concept of inferring aspects of the three-dimensional flow field from sea surface height observations by combining altimetric measurements with the dynamics of ocean circulation models. A Holland-type QG model with open boundaries is set up on a 2000 km X 2000 km domain of the eastern North Atlantic between 25 deg. and 45 deg. N, 32 deg. and 8 deg. W. By using a simple nudging technique, about two years of Geosat altimeter data are assimilated into the model every five days as space-time objective analyses on the model grid. The error information resulting from the analysis is used during the assimilation procedure to account for data uncertainties. Results show an intense eddy field, which in the surface layer interacts with a meandering Azores Front. Compared to Geosat, the model leads to smoothed fields that follow the observations. Model simulations are significantly correlated with hydrographic data from March 1988 and June 1989, both close to the surface and in the subsurface. Good agreement is also found between the model velocity fields and moored current meter data in the top two model layers. The agreement is visually weak in the bottom layer, although a coherence analysis reveals an agreement between the model simulation and current meter data over the full water column at periods exceeding 80 days.
In the framework of Monitoring by Ocean Drifters (MONDO) Project, a set of Lagrangian drifters were released in proximity of the Brazil Current, the western branch of the Subtropical Gyre in the South Atlantic Ocean. The experimental strategy of deploying part of the buoys in clusters offers the opportunity to examine relative dispersion on a wide range of scales. Adopting a dynamical systems approach, we focus our attention on scale-dependent indicators, like the finite-scale Lyapunov exponent (FSLE) and the finite-scale (mean square) relative velocity (FSRV) between two drifters as function of their separation, and compare them with classic time-dependent statistical quantities like the mean square relative displacement between two drifters and the effective diffusivity as functions of the time lag from the release. We find that, dependently on the given observable, the quasigeostrophic turbulence scenario is overall compatible with our data analysis, with discrepancies from the expected behavior of 2D turbulent trajectories likely to be ascribed to the non stationary and non homogeneous characteristics of the flow, as well as to possible ageostrophic effects. Submesoscale features of O(1) km are considered to play a role, to some extent, in determining the properties of relative dispersion as well as the shape of the energy spectrum. We present, also, numerical simulations of an OGCM of the South Atlantic, and discuss the comparison between experimental and model data about mesoscale dispersion.
The subtidal frequency response of sea level to atmospheric forcing along the coastal region between Cape Hatteras and Charleston is investigated for a four-month period: 1 September-31 December, 1974. It is found that low-frequency sea level fluctuations are preferentially forced by wind stress components which are aligned with the local topography. Also, a localized, one-dimensional model of sea surface response to a clockwise rotating wind for the Charleston coastal regime is developed. The phase spectrum of the alongshore wind component versus sea level as predicted by the model is shown to compare favorably to that derived from actual observations at Charleston, an open ocean coastal site. The model results and observations also suggest that wind-induced fluctuations of coastal sea level are trapped within 40 km of the coast by the combined effects of friction, Coriolis force and bottom topography. The outer shelf is dominated by fluctuations which are less related to wind stress and are attenuated rapidly in the shoreward direction. A reasonable estimate of bottom frictional coefficient, r = 0.05 cm/s, is also established.
A simple statistical technique is described to determine monthly mean marine surface-layer humidity, which is essential in the specification of surface latent heat flux, from total water vapor in the atmospheric column measured by space-borne sensors. Good correlation between the two quantities was found in examining the humidity soundings from radiosonde reports of mid-ocean island stations and weather ships. The relation agrees with that obtained from satellite (Seasat) data and ship reports averaged over 2 deg areas and a 92-day period in the North Atlantic and in the tropical Pacific. The results demonstrate that, by using a local regression in the tropical Pacific, total water vapor can be used to determine monthly mean surface layer humidity to an accuracy of 0.4 g/kg. With a global regression, determination to an accuracy of 0.8 g/kg is possible. These accuracies correspond to approximately 10 to 20 W/sq m in the determination of latent heat flux with the bulk parameterization method, provided that other required parameters are known.
The average wind speeds from the scatterometer (SASS) on the ocean observing satellite SEASAT are found to be generally higher than the average wind speeds from ship reports. In this study, two factors, sea surface temperature and atmospheric stability, are identified which affect microwave scatter and, therefore, wave development. The problem of relating satellite observations to a fictitious quantity, such as the neutral wind, that has to be derived from in situ observations with models is examined. The study also demonstrates the dependence of SASS winds on sea surface temperature at low wind speeds, possibly due to temperature-dependent factors, such as water viscosity, which affect wave development.
Recent studies indicate that altimetric observations of the ocean's mesoscale eddy field reflect the combined influence of surface buoyancy and interior potential vorticity anomalies. The former have a surface-trapped structure, while the latter have a more grave form. To assess the relative importance of each contribution to the signal, it is useful to project the observed field onto a set of modes that separates their influence in a natural way. However, the surface-trapped dynamics are not well-represented by standard baroclinic modes; moreover, they are dependent on horizontal scale. Here we derive a modal decomposition that results from the simultaneous diagonalization of the energy and a generalisation of potential enstrophy that includes contributions from the surface buoyancy fields. This approach yields a family of orthonomal bases that depend on two parameters: the standard baroclinic modes are recovered in a limiting case, while other choices provide modes that represent surface and interior dynamics in an efficient way. For constant stratification, these modes consist of symmetric and antisymmetric exponential modes that capture the surface dynamics, and a series of oscillating modes that represent the interior dynamics. Motivated by the ocean, where shears are concentrated near the upper surface, we also consider the special case of a quiescent lower surface. In this case, the interior modes are independent of wavenumber, and there is a single exponential surface mode that replaces the barotropic mode. We demonstrate the use and effectiveness of these modes by projecting the energy in a set of simulations of baroclinic turbulence.
A case study of the ocean radar backscatter dependence on near-surface wind and wind stress is presented using the data obtained on February 18, 1986 during the Frontal Air-Sea Interaction Experiment. The particular wind-wave conditions and their variations across a sharp sea surface temperature front are described. The small change in wind speed across the front cannot account for the large change in wind stress implying significant changes in the drag coefficient and surface roughness length. The results strengthen the hypothesis that radar backscatter is closely correlated to wind stress, and therefore, could be used for remote sensing of the wind stress itself over the global oceans.
The stability of baroclinic flows with horizontal shear over sloping topography is analyzed with special emphasis on the structure and energetics of the unstable perturbations. The study is conducted by using a linearized two-layer quasi-geostrophic channel model for different topography profiles and distributions of the basic velocity field. Interactions between the two fluid layers and the energy conversions by the unstable perturbations are described. It is found that topography sloping as (opposed to) the fluid interface contributes to enhance the perturbation amplitude in the upper (lower) layer relative to the lower (upper) layer. The results for bottom topography with differing characteristics across the flow indicate pronounced localized effects on the energy conversions over the slopes and the meridional scale of the perturbations in the lower layer.
Seasonal heat transport is examined in a simple, linear, shallow-water model on the equatorial beta plane. It is found in this model that meridional transport by the seasonally varying western boundary current is of the same magnitude but opposite phase to the seasonally varying interior transport and therefore tends to cancel.
The problem of a small-amplitude wave propagating over a flat porous bed is reanalyzed subject to the bottom boundary condition. The boundary condition is of the form of a radiation-type condition commonly encountered in heat conduction problems. The important physical quantities (velocity, velocity potential, streamfunction, shear stress and energy dissipation) have been derived and are presented, subject to natural conditions. The bottom boundary layer is represented by the linearized Navier-Stokes equations under the usual boundary layer approximation. It is found that the boundary layer velocity distribution and shear stress can be greatly altered from impermeable bed predictions. Theoretical results for energy dissipation and shear stress are compared to existing data and are found to agree very well. The predictions of classical small-amplitude wave theory are not appreciably modified away from the boundary.
Soft muddy bottoms have significant effects on properties of water waves which propagate over them. The wave dispersion equation is modified and wave energy is dissipated by the coupling between the waves in water and those induced in the mud layer. These effects are theoretically determined by assuming a viscoelastic mud layer. A boundary-value problem is solved for the water-mud system with sinusoidal waves. The theoretical dissipation rates are compared favourably with field measurements. (A)
A mean vorticity budget analysis is presented of Holland's (1978) numerical ocean general circulation experiment. The stable budgets are compared with classical circulation theory to emphasize the ways in which the mesoscale motions of the model alter (or leave unaltered) classical vorticity balances. The basinwide meridional transports of vorticity by the mean flow and by the mesoscale flow in the mean are evaluated to establish the role(s) of the mesoscale in the larger scale equilibrium vorticity transports. The vorticity equation for this model fluid system is presented and the budget analysis method is described. Vorticity budgets over the selected regions and on a larger scale are given, and a summary of budget results is provided along with remarks about the utility of this type of analysis.
Previous analyses of satellite-tracked drifting buoy data (30 m drogue depth) and Fleet Numerical Ocean Center (FNOC) surface wind stress in the midlatitude North Pacific during autumn/winter have shown near-surface current vectors 25° to the right of the surface wind stress vector (i.e., approximately parallel to sea level air pressure isobars). In the present study, the complex coherence between time series of the two vectors, near-surface current and surface wind stress, is examined using the vector cross-spectral analysis technique developed by Mooers, yielding the frequency response of surface current to wind stress from the inertial frequency down to one cycle per 16 days. The analysis shows that during summer the near-surface a few days. In this season, the rotary spectrum of the near-surface current is dominated by anticyclonic motion, with periods of approximately 8 to 32 days, that is not locally wind-forced. In contrast, during autumn/winter, the two vectors are highly coherent over th...
We show that long-term memory effects, present in the chaotic dispersion process generated by a meandering jet model, can be nonetheless taken into account by a first order Markov process, provided that the states of the phase space partition, chosen to describe the system, be appropriately defined.
NASA Lewis Res. Cen The three-dimensional time-dependent flow in the Western Basin of Lake Erie has been calculated numerically by means of both a two-mode free-surface model and a rigid-lid model. Detailed comparisons of results from these two models are presented for two wind conditions; a constant wind suddenly imposed, and an actual wind which is variable in both space and time. For relatively short time intervals, significant differences between the results of the two models occur since the seiche motion is eliminated in the rigid-lid model. Long-term time-averaged circulation computed by the two models agree well in periods of strong wind, but differ appreciably in periods of light wind and active seiching. (A)
The variation of salinity gradient at the estuary mouth ( ¯ Σ X | 0 ) with the estuary control variables -F R and F T . The solid lines represent isocontours of ¯ Σ X | 0 .  
Estuary classification diagram with lines representing isocontours of F 0. The three regions are three types of estuaries: (a) light gray is well mixed, (b) white is partially mixed, and (c) dark gray is highly stratified or salt wedge. For data and expansion of abbreviations, see Table 1.
List of coefficients used in different equations.
This paper presents the governing equations of a tidally-averaged, width-averaged, rectangular estuary in completely nondimensionalized forms. Subsequently, we discover that the dynamics of an estuary is entirely controlled by only two variables: (i) the Estuarine Froude number, and (ii) a nondimensional number related to the Estuarine Aspect ratio and the Tidal Froude number. Motivated by this new observation, the problem of estuary classification is re-investigated. Our analysis shows that the two control variables are capable of completely determining the stratification at the estuary mouth, and therefore can specify the estuary type. The theoretical estuary classification scheme proposed in this paper is validated against real estuarine data collected from existing literature. Our classification scheme on comparison with the state-of-the-art theory shows significant improvement.
Thermal scanner imagery acquired during a field experiment designed to study an upwelling event in Lake Superior is investigated. Temperature data were measured by the thermal scanner, with a spatial resolution of 7 m. These data were correlated with temperatures measured from boats. One- and two-dimensional Fourier transforms of the data were calculated and temperature variances as a function of wavenumber were plotted. A k-to-the-minus-three dependence of the temperature variance on wavenumber was found in the wavenumber range of 1-25/km. At wavenumbers greater than 25/km, a k-to-the-minus-five-thirds dependence was found.
A study is presented of the geophysical algorithms relating the Seasat-A scatterometer (SASS) backscatter measurements with a wind parameter. Although these measurements are closely related to surface features, an identification with surface layer parameters such as friction velocity or the roughness length is difficult. It is shown how surface truth in the form of wind speeds and coincident stability can be used to derive friction velocity or the equivalent neutral wind at an arbitrary height; it is also shown that the derived friction velocity values are sensitive to contested formulations relating friction velocity to the roughness length, while the derived values of the equivalent neutral wind are not. Examples of geophysical verification are demonstrated using values obtained from the Gulf of Alaska Seasat Experiment; these results show very little sensitivity to the type of wind parameter employed, suggesting that this insensitivity is mainly due to a large scatter in the SASS and surface truth data.
An analysis is presented for computing the diffraction of barotropic Kelvin waves by a localized topographical irregularity on flat-bottom ocean with an arbitrary vertical stratification. It was shown that all baroclinic Kelvin waves will be generated downstream of the bump, with the first baroclinic mode having the largest amplitude. The Poincare waves predominate in the lowest modes, and are more directionally anisotropic. It was concluded that baroclinic Poincare waves radiating offshore from the bump topography could contribute to the internal wave field in the open ocean and provide an alternative mechanism to dissipate the barotropic tides.
A general circulation model of the Indian Ocean is fitted to monthly averaged climatological temperatures, salinities, and surface fluxes using the adjoint method. Interannual variability is minimized by penalizing the temporal drift from one seasonal cycle to another during a two-year integration. The resultant meridional overturning and heat transport display large seasonal variations, with maximum amplitudes of 18 and 22 (x 10(exp 6) cubic m/s) for the overturning and 1.8 and 1.4 (x 10(exp 15) W) for heat transport near 10 S and 10 N, respectively. A dynamical decomposition of the overturning and heat transport shows that the time-varying Ekman How plus its barotropic compensation can explain a large part of the seasonal variations in overturning and heat transport. The maximum variations at 10 deg N and 10 deg S are associated with monsoon reversal over the northern Indian Ocean and changes of the easterlies over the southern Indian Ocean. An external mode with variable topography has a moderate contribution where the Somali Current and the corresponding gyre reverse direction seasonally. Contribution front vertical shear (thermal wind and ageostrophic shear) is dominant near the southern boundary and large near the Somali Current latitudes. The dominant balance in the zonally integrated heat budget is between heat storage change and heat transport convergence except south of 15 S. Optimization with seasonal forcings improves estimates of sea surface temperatures, but the annual average overturning and heat transport are very similar to previous results with annual mean forcings. The annual average heat transport consists of roughly equal contributions from time-mean and time-varying fields of meridional velocities and temperatures in the northern Indian Ocean. indicating a significant rectification to the heat transport due to the time-varying fields. The time-mean and time-varying contributions are primarily due to the overturning and horizontal gyre, respectively. Inclusion of TOPEX data enhances the seasonal cycles of the estimated overturning and heat transport in the central Indian Ocean significantly and improves the estimated equatorial zonal flows but leads 10 unrealistic estimates of the velocity structure near the Indonesian Throughflow region, most likely owing to the deficiencies in the lateral boundary conditions.
A simple second-order model is developed to describe the transformation between surface wave slope and radiance incident on an optical sensor. The model includes azimuth as well as elevation variations in the sky-radiance distribution and their effects on the imaging of wave slopes approaching the sensor azimuth from various directions. The model also includes upwelling radiance. Comparison between the exact transformation from wave slope to radiance and the second-order model suggests that they yield quantitatively similar results. The model is used in physical interpretations of how wave slopes are imaged and in indicating a way of optimizing the linearity and contrast of the transfer function relating wave slope to radiance for various sensor geometries, thereby minimizing sources of error. One-dimensional numerical simulations of the imaging of an analytically generated wave-slope profile demonstrate the utility of this technique in obtaining slope spectra.
Determination of sea surface slopes distribution and wind velocity using sun glitter viewed from synchronous satellite
The values of sea surface stress determined with the dissipation method and those determined with a surface-layer model from observations on F.S. Meteor during the Joint Air-Sea Interaction (JASIN) Experiment are compared with the backscatter coefficients measured by the scatterometer SASS on the satellite Seasat. This study demonstrates that SASS can be used to determine surface stress directly as well as wind speed. The quality of the surface observations used in the calibration of the retrieval algorithms, however, is important. This sample of measurements disagrees with the predictions by the existing wind retrieval algorithm under non-neutral conditions and the discrepancies depend on atmospheric stability.
New empirical estimates of the long-period fortnightly (Mf) tide obtained from TOPEX/Poseidon (T/P) altimeter data confirm significant basin-scale deviations from equilibrium. Elevations in the low-latitude Pacific have reduced amplitude and lag those in the Atlantic by 30 deg or more. These interbasin amplitude and phase variations are robust features that are reproduced by numerical solutions of the shallow-water equations, even for a constant-depth ocean with schematic interconnected rectangular basins. A simplified analytical model for cooscillating connected basins also reproduces the principal features observed in the empirical solutions. This simple model is largely kinematic. Zonally averaged elevations within a simple closed basin would be nearly in equilibrium with the gravitational potential, except for a constant offset required to conserve mass. With connected basins these offsets are mostly eliminated by interbasin mass flux. Because of rotation, this flux occurs mostly in a narrow boundary layer across the mouth and at the western edge of each basin, and geostrophic balance in this zone supports small residual offsets (and phase shifts) between basins. The simple model predicts that this effect should decrease roughly linearly with frequency, a result that is confirmed by numerical modeling and empirical T/P estimates of the monthly (Mm) tidal constituent. This model also explains some aspects of the anomalous nonisostatic response of the ocean to atmospheric pressure forcing at periods of around 5 days.
Storm-relative representation of all the wave components observed during the landfall flight of 26 Aug 1998 (blue) and during the open ocean flight on 24 Aug 1998 (red). The short green radials show the average of the HRD surface wind analyses on the two days with a length of 10 km representing a wind speed of 50 m s 1 .
Landfall data from Fig. 12 plotted vs distance from shore in Onslow Bay. The bottom panel plots the approximate bathymetric profiles for Onslow Bay, the northward flight line toward Cape Hatteras (Fig. 5), and the Banzai Pipeline on the northwest shore of Oahu, Hawaii.
plots the same landfall data shown in Fig. 12 but versus the distance from shore. A piecewise linear approximation to the shoreline of Onslow Bay, located between Cape Fear and Cape Lookout, was used to compute the minimum distance to the shoreline for each observation point. In this representation, the data show a much less erratic variation. The contours of constant water depth in Onslow Bay approximately follow the shoreline. The solid curve in the bottom panel of Fig. 14 shows the water depth variation along a line perpendicular to the shoreline starting from about 34.53N, 77.33W. shows that the wavelength begins to shorten as soon as the waves encounter the continental shelf, and the effect becomes more pronounced as the depth decreases near shore. The wave height continues to grow on the shelf until about 50 km from shore, where the bathymetry gradient steepens, and the strong winds can no longer overcompensate for the dissipation effects. The data gaps in the 20-40 km and 75-100 km distances from shore occurred because the flight lines generally tended to parallel the shoreline during the landfall flight. For perspective, two other bathymetric profiles are shown in the bottom panel of Fig. 14. The dashed curve shows the depth variation along the northward flight
On 26 August 1998, the SRA at 2.2 km height documented the directional wave spectrum in the region between Charleston, SC, and Cape Hatteras, NC, as Hurricane Bonnie was making landfall near Wilmington, NC. The storm was similar in size during the two flights, but the maximum speed in the NOAA Hurricane Research Division surface wind analysis was 15% lower prior to landfall (39 m/s) than it had been in the open ocean (46 m/s). This was compensated for by its faster movement prior to landfall (9.5 m/s) than when it was encountered in the open ocean (5 m/s), significantly increasing the effective fetch and duration of waves near the peak of the spectrum which propagated in the direction of the storm track. The open ocean wave height variation indicated that Hurricane Bonnie would have produced waves of 11 m significant wave height on the shore northeast of Wilmington had it not been for the continental shelf. The bathymetry distributed the steepening and breaking process across the shelf so that the wavelength and wave height were reduced gradually as the shore was approached. The wave height 5 km from shore was about 4 in.
FIG. A1. Grayscale-coded topography for 180-line segment measured with the SRA when the aircraft was about 75 km north and 125 km east of the eye and traveling toward the southwest. The outline of the uniform grid the data are interpolated to is indicated by the square box.
The sea surface directional wave spectrum was measured for the first time in all quadrants of a hurricane in open water using the NASA airborne scanning radar altimeter (SRA) carried aboard one of the NOAA WP-3D hurricane hunter aircraft at 1.5 km height. The SRA measures the energetic portion of the directional wave spectrum by generating a topographic map of the sea surface. At 8 Hz, the SRA sweeps a radar beam of 1 deg half-power width (two-way) across the aircraft ground track over a swath equal to 0. 8 of the aircraft height, simultaneously measuring the backscattered power at its 36 GHz (8.3 mm) operating frequency and the range to the sea surface at 64 positions. These slant ranges are multiplied by the cosine of the incidence angles to determine the vertical distances from the aircraft to the sea surface. Subtracting these distances from the aircraft height produces the sea surface elevation map. The sea surface topography is interpolated to a uniform grid, transformed by a two-dimensional FFT, and Doppler corrected. The data presented were acquired on 24 August 1998 when hurricane Bonnie was east of the Bahamas and moving slowly to the north. Wave heights up to 18 m were observed and the spatial variation of the wave field was dramatic. The dominant waves generally propagated at significant angles to the downwind direction and at times there were wave fields traveling at right angles to each other. The NOAA aircraft spent over five hours within 180 km of the hurricane Bonnie eye, and made five eye penetrations. A 2-minute animation of the directional wave spectrum spatial variation over this period will be shown.
The influence of the directional distribution of wave energy on the dispersion relation is calculated numerically using various directional wave spectrum models. The results indicate that the dispersion relation varies both as a function of the directional energy distribution and the direction of propagation of the wave component under consideration. Furthermore, both the mean deviation and the random scatter from the linear approximation increase as the energy spreading decreases. Limited observational data are compared with the theoretical results. The agreement is favorable.
Free, equatorially trapped sinusoidal wave solutions to a linear model on an equatorial beta plane are used to fit the Geosat altimetric sea level observations in the tropical Pacific Ocean. The Kalman filter technique is used to estimate the wave amplitude and phase from the data. The estimation is performed at each time step by combining the model forecast with the observation in an optimal fashion utilizing the respective error covariances. The model error covariance is determined such that the performance of the model forecast is optimized. It is found that the dominant observed features can be described qualitatively by basin-scale Kelvin waves and the first meridional-mode Rossby waves. Quantitatively, however, only 23 percent of the signal variance can be accounted for by this simple model.
Top-cited authors
Fabrice Ardhuin
  • Institut Français de Recherche pour l'Exploitation de la Mer
Jonathan Gula
  • Université de Bretagne Occidentale
Jim Moum
  • Oregon State University
William David Smyth
  • Oregon State University
David Ferreira
  • University of Reading