bluuebunny/crossref_metadata_2025 huggingface dataset:
The force balance of sea ice in a numerical model of the Arctic Ocean
Authors: Steele Michael , Zhang Jinlun , Rothrock Drew , Stern Harry
Published: February 2004

The balance of forces in the sea ice model of Hibler [1979] is examined. The model predicts that internal stress gradients are an important force in much of the Arctic Ocean except in summer, when they are significant only off the northern coasts of Greenland and the Canadian Archipelago. A partition of the internal stress gradient between the pressure gradient and the viscous terms reveals that both are significant, although they operate on very different timescales. The acceleration term is generally negligible, while the sum of Coriolis plus sea surface tilt is small. Thus the seasonal average force balance in fall, winter, and spring is mostly between three terms of roughly equal magnitudes: air drag, water drag, and internal stress gradients. This is also true for the monthly average force balance. However, we find that there is a transition around the weekly timescale and that on a daily basis the force balance at a particular location and time is often between only two terms: either between air drag and water drag or between air drag and internal stress gradients. The model is in agreement with the observations of Thorndike and Colony [1982] in that the correlation between geostrophic wind forcing and the model's ice velocity field is high. This result is discussed in the context of the force balance; we show that the presence of significant internal stress gradients does not preclude high wind‐ice correlation. A breakdown of the internal stress gradient into component parts reveals that the shear viscous force is far from negligible, which casts strong doubt on the theoretical validity of the cavitating fluid approximation (in which this component is neglected). Finally, the role of ice pressure is examined by varying the parameter P*. We find a strong sensitivity in terms of the force balance, as well as ice thickness and velocity.
The chemical composition of cold events within the Eemian section of the Greenland Ice Core Project ice core from Summit, Greenland
Authors: Steffensen Jørgen Peder , Clausen Henrik B. , Hammer Claus U. , Legrand Michel , De Angelis Martine
Published: February 2004

Selected segments from the Eemian period of the Greenland Ice Core Project (GRIP) core from Summit, Greenland, have been analyzed by ion chromatography and Coulter Multisizer. The results will be discussed with special emphasis on the sudden cooling events, event 1 and event 2, in the record as observed in the δ18O profile [GRIP members, 1993; Dansgaard et al., 1993]. Whether the sudden cooling events of the Eemian represent a real climatic signal or they are a consequence of disturbed ice core st ratigraphy is still a matter of debate [Chappellaz et al., this issue]. However, several features of the chemical profiles across these cooling events are difficult to explain if they were a result of disturbed stratigraphy. We will focus our discussion on the unique profiles of nitrate, methane sulfonic acid (MSA), and ammonium across event 1 and argue that even if we allow for movement of ions by diffusion or displacement of ions by physical or chemical interactions, events 1 and 2 are not likely to consist of ice from other climatic periods which have been inserted in the Eemian strata by folding processes. Furthermore, our records do not show any layers in the vicinity of the Eemian strata which can be a possible source of the ice in event 1. We believe that the events represent a climate signal, and we will discuss other possible explanations of the profiles. Our conclusion that the cold events in the Eemian represent a real climatic signal runs counter to other evidence from gas measurements. However, whatever their cause, the unusual chemical signals require further discussion and explanation.
ERS‐1/2 orbit improvement using TOPEX/POSEIDON: The 2 cm challenge
Authors: Le Traon P.‐Y. , Ogor F.
Published: February 2004

The ERS orbit error reduction method using TOPEX/POSEIDON (T/P) data as a reference [Le Traon et al., 1995a] was applied to ERS‐1 cycles from phases C, E, F, and G and to the first 16 cycles of the ERS‐2 mission (phase A). T/P M‐GDR (geophysical data record) (version C) and ERS‐1/2 ocean product (OPR) data were used. ERS‐1/2 orbits are the D‐PAF (processing and archiving facility) orbits and, when necessary, ERS‐1/2 altimetric corrections were updated to make the T/P and ERS‐1/2 corrections homogeneous. The adjustment method has been refined, and formal error on the estimation is now calculated. The ERS‐1/2 orbit error estimation is thus estimated to be precise to within about 2 cm root‐mean‐square (rms). E‐E crossover differences are reduced from 12 to 17 cm to only 6.5 cm rms for all processed cycles. Similarly, the T/P‐E crossover differences are reduced from 11 to 14 cm to only 7 cm rms. The adjusted D‐PAF orbit error varies between 6 and 12 cm rms. The adjustment has also been performed for the Joint Gravity Model 3(JGM 3) orbits of ERS‐I phases C, E, and F. The rms difference between the corrected orbits for the D‐PAF and JGM 3 orbits is only about 1 cm rms, while it is about 11 cm before T/P orbit error correction. This shows that the adjustment is almost insensitive to the initial ERS‐I orbit used. It also confirms the 2 cm precision of the method. We also do repeat‐track analysis on the 35 day repeat cycles of ERS‐I phase C. The mean difference in sea level variance before and after orbit error correction is 34 cm2 (D‐PAF orbit) and 17 cm2 (JGM 3 orbit). The corrected ERS‐1 and T/P sea level variabilities, however, are in excellent agreement. The study thus shows that ERS‐1/2 orbit error must be corrected before analyzing large‐scale oceanic signals and combining ERS‐1/2 with T/P data. The proposed method provides a very effective correction and thus significantly enhances the quality of ERS‐1/2 data. Corresponding data sets will be distributed to the scientific community by Archiving, Validation, and Interpretation of Satellite Oceanographic Data (AVISO).
Nonhydrostatic haline convection under leads in sea ice
Authors: Smith David C. , Morison James H.
Published: February 2004

The distribution of dense brine under leads in sea ice is associated with convective sinking of individual plumes and near‐surface mixing associated with ice‐water momentum flux. The processes have been studied in the recent Lead Experiment (LeadEx) field program and previously modeled using a two‐dimensional hydrostatic numerical model. In this study these processes are reexamined using a nonhydrostatic model. It is shown that nonhydrostatic effects give rise to pressure distributions which counter hydrostatic pressure gradients in the fluid. These effects are largest in sinking plume events and can be substantial when plumes are free to sink to depths of 100 m. In the presence of the Arctic halocline (typically at 30–40 m), however, vertical accelerations of convective plumes are limited spatially and temporally, and nonhydrostatic effects are relatively small. It is the purpose of this paper to demonstrate that for realistic Arctic stratification, convective circulations under leads in sea ice are predominantly hydrostatic.
Moored observations of the current and temperature structure over the continental slope off central California: 2. The energetics of the flow off Point Sur
Authors: Tisch Timothy D. , Ramp Steven R.
Published: February 2004

The data from two current meter moorings off Point Sur, California, were analyzed to examine the energy transfers over the continental slope in the California Current system (CCS). The method used was to calculate terms in the heat equation at intermediate depths between instruments, using the thermal wind relation to estimate the horizontal temperature gradients from the vertical shear. Time series of u, v, and T were collected for 17 months (May 1989 to October 1990) at site P2 on the 800‐m isobath and for 12 months (May 1990 to May 1991) at site P3, 25 km farther offshore on the 1800‐m isobath. Instrument depths were 100, 350, and 500 m at both moorings plus 1000 m at site P3. Mean values of the energy conversion terms were computed at sites P2 and P3 at 225 m depth and at site P2 at 425 m depth. The local change of eddy potential energy (EPE) was approximately balanced (∂EPE/∂t ≈ 0) for each case but the dynamics were quite different. At P2 225, the sources of EPE were advection by the mean flow and baroclinic instability, which converted mean potential energy (MPE) to EPE. The losses came from eddy advection out of the region and the vertical eddy heat flux (VEHF), which converted EPE to eddy kinetic energy (EKE). At P2 425, baroclinic instability provided the major source of EPE. This, plus a small positive contribution from the VEHF, was balanced by advection of EPE out of the region by both the mean flow and the eddies. At P3 225, mean advection and baroclinic instability were negligible, and the downward eddy heat flux (VEHF) was balanced by advection of heat out of the region by the eddies. Time series of terms in the EPE equation were used to examine high energy transfer events at both moorings. Events at P2 involved both horizontal and vertical processes and had longer timescales (several days to weeks) than those at P3, which had much shorter timescales (2–5 days) and were dominated by vertical processes. Compared to the Gulf Stream, the mean baroclinic instability at P2 (both depths) was smaller by at least a factor of 10. A term‐by‐term comparison between the mean values off Point Sur and the Kuroshio Extension showed different processes, with the eddies gaining strength at the expense of the mean flow off Point Sur and the opposite occurring in the Kuroshio Extension.
Tidally driven residual circulation in shallow estuaries with lateral depth variation
Authors: Li Chunyan , O'Donnell James
Published: February 2004

Tidally driven residual circulation in shallow estuaries with lateral depth variation has been studied analytically using a two‐dimensional, depth‐averaged model. The solution is presented for a v‐shaped channel. Exchange flow is found to be correlated with the topography. The magnitude of this exchange flow depends mainly on four parameters: the ratio between the minimum depth on the shoal and the maximum depth in the channel, the ratio between the tidal amplitude at the mouth and the mean depth, the ratio between the length of the estuary and the tidal wave length, and the ratio between the tidal timescale and the decay timescale due to friction. Generally, a net landward flow occurs over the shoals and is balanced by a return flow in the channel. The along‐channel residual velocity changes nearly linearly across the estuary. The residual velocity decreases monotonically toward the head of the estuary. The transverse residual velocity is convergent in the channel and divergent on the shoals. The residual velocity is highly dependent on the length of the estuary. When the length of the estuary is much smaller than a quarter of the wave length, the residual velocity is relatively small. When the length of the estuary is about a quarter of the wave length, the magnitude of the residual velocity reaches its maximum. When the length of the estuary is much larger than a quarter of the wave length, the residual circulation approaches the pattern found in an infinite length estuary. On the basis of the analysis of the solution, we conclude that exchange flow generated by tides in a shallow estuary is the result of competition among several processes. An inward flux is caused by local nonlinearity, both in the bottom friction and from propagation of the tidal wave of finite amplitude. This inward flux is larger on the shoals and smaller in the channel owing to larger nonlinearity in the shallow water. The residual inward flow creates a setup (residual surface elevation) of water at the head of the estuary that produces a pressure gradient. This residual pressure gradient is approximately uniform across the estuary and drives an outward flow that is larger in the channel than on the shoals. In all of the parameter space the competition results in a net inflow on the shoals and a net outflow in the channel.
Characteristic patterns of the circulation in the Santa Barbara Channel
Authors: Harms Sabine , Winant Clinton D.
Published: February 2004

The subtidal near‐surface circulation in the Santa Barbara Channel (SBC) and on the shelf north of Point Conception is described based on observations obtained during the 3‐year period from 1993 to 1995. Near‐surface currents in the channel are a superposition of a larger‐than‐SBC scale flow and a cyclonic circulation of variable intensity located inside the channel. On seasonal timescales the larger‐than‐SBC scale flow near the surface is equatorward in spring and poleward from summer through winter. The increase in equatorward flow in spring occurs concurrently with the increase in equatorward wind stress and the decrease in near‐surface temperatures and synthetic subsurface pressures (SSPs). The flow reverses in late spring, simultaneously with the increase in the along‐channel SSP difference and months before wind stress has reached its peak. The period during which the cyclonic circulation within the SBC is strongest coincides with the period of strongest poleward flow through the eastern entrance. A synoptic description of the circulation in the SBC is presented in terms of six characteristic patterns, labeled Upwelling, Relaxation, Cyclonic, Propagating Cyclones, Flood East, and Flood West. An analysis of the 5‐ and 45‐m currents into empirical orthogonal functions (EOFs) isolates 50% (53%) of the 5‐m (45‐m) low‐frequency current variance into three (two) modes. Combining these modes with the mean current fields, the modes have spatial patterns that correspond to the characteristic flow patterns subjectively deduced from inspection of daily averages of the near‐surface currents. From late spring through fall the two largest 5‐m current modes produce a repeating pattern of circulation in which the sequence of four states (namely, Upwelling, Cyclonic, Relaxation, and Quiescent) is traversed roughly every 16 days. In addition to the large‐scale cyclonicity in the central SBC, smaller cyclonic eddies form frequently in the eastern channel and travel toward the west with an average speed of 0.06 m s−1. The patterns described here develop in response to the wind stress and the along‐channel SSP gradient.
Thermobaric effect on buoyancy‐driven convection in cold seawater
Authors: Løyning Terje Brinck , Weber Jan Erik
Published: February 2004

The equations governing buoyancy‐driven convection in a rotating horizontal fluid layer, where the thermal expansion coefficient is a function of pressure (here, depth), have been derived. The results confirm the validity of the Boussinesq approximation. Furthermore, the adiabatic lapse rate for cold seawater is shown to be so small that it can usually be neglected in comparison to the in situ temperature gradient. For purely thermal convection a linear perturbation analysis shows that a weak thermobaric effect acts as a destabilizing influence on the Rayleigh‐Bénard problem. This is also the case when Earth's rotation is taken into account. In particular, for marginal stability in the absence of rotation, it is shown that two‐dimensional convection cells are asymmetric with respect to the midlayer plane, with stronger circulation in the deeper part of the layer. This effect is also revealed by two‐dimensional, nonlinear numerical computations, when the layer is stratified by opposing gradients of heat (acting destabilizing) and salt (acting stabilizing). For thermobaric convection, i.e., in cases where the layer would have been stable in the absence of the thermobaric effect, the computed cell pattern is seen to be displaced toward the deeper part of the layer. Furthermore, the cell width increases with increasing thermobaric strength. It is suggested that thermobaric convection may act in conjunction with haline convection due to freezing in the process of vertical mixing and deep water formation in polar areas.
Fully reprocessed ERS‐1 altimeter data from 1992 to 1995: Feasibility of the detection of long term sea level change
Authors: Anzenhofer M. , Gruber T.
Published: February 2004

Global mean sea level observations are necessary to answer the urgent questions about climate changes and their impact on socio‐economy. At GeoForschungsZentrum/Geman Processing and Archiving Facility ERS altimeter data is used to systematically generate geophysical products such as sea surface topography, high‐resolution geoid and short‐ and long‐period sea surface height models. On the basis of this experience, fully reprocessed ERS‐1 altimeter data is used to generated a time series of monthly sea surface height models from April 1992 to April 1995. The reprocessing consists of improved satellite ephemerides, merging of Grenoble tidal model, and application of range corrections due to timing errors. With the new data set the TOPEX/POSEIDON prelaunch accuracy requirements are fulfilled. The 3‐year time series is taken to estimate the rate of change of global mean sea level. A careful treatment of seasonal effects is considered. A masking of continents, sea ice, and suspect sea surface heights is chosen that is common for all sea surface height models. The obtained rate of change is compared to external results from tide gauge records and TOPEX/POSEIDON data. The relation of sea level changes and sea surface temperature variations is examined by means of global monthly sea surface temperature maps. Both global wind speed and wave height maps are investigated and correlated with sea surface heights and sea surface temperatures in order to find other indicators of climate variations. The obtained rate of changes of the various global maps is compared to an atmospheric CO2 anomaly record, which is highly correlated to El Niño events. The relatively short period of 3 years, however, does not allow definite conclusions with respect to possible long‐term climate changes.
The effect of assimilating ERS‐1 fast delivery wave data into the North Atlantic WAM model
Authors: Dunlap E. M. , Olsen R. B. , Wilson L. , De Margerie S. , Lalbeharry R.
Published: February 2004

The launch of the European ERS satellites has provided a new source of wave information that is particularly suitable for use in improving wave forecasts in the open ocean. We have implemented and tested a simple system for assimilating corrections to model wave fields produced by the WAM model, where the corrections are derived from inverted synthetic aperture radar (SAR) image spectra from ERS‐1. Corrections are applied to significant wave height, mean period and direction for wave modes that are detectable in both the model and the SAR data. The system has been tested in a storm situation and in moderate conditions using buoy data and altimeter data, as well as SAR observations for verification. Overall, it is demonstrated that the net effect of assimilating SAR data is beneficial but very small. The small impact is due at least partly to relatively small spatial and temporal coverage of the SAR wave mode data. Locally larger impacts were found in the storm situation in individual cases where SAR observations were collocated with independent buoy observations.
Arabian sea surface winds and ocean transports determined from ERS‐1 scatterometer
Authors: Halpern David , Freilich Michael H. , Weller Robert A.
Published: February 2004

Satellite scatterometer wind velocity measurements are a new source of data for studies of seasonal‐to‐interannual ocean‐atmosphere interactions in the Arabian Sea, where the largest and steadiest wind speeds occur in northern summer. Three satellite wind velocity data products (named CMOD4, FD, and IFR2), all created from radar backscatter measured by the first European Remote Sensing Satellite (ERS‐1), were compared to moored‐buoy wind measurements recorded in the central Arabian Sea during October 1994 to October 1995. Orthogonal regression analysis of 74 collocations indicated IFR2 winds were more representative of buoy winds compared to CMOD4 and FD. IFR2 winds during 1995 are described, including the time of monsoon onset and the path of the Somali Jet. Monthly mean vertical transport across the bottom of the Ekman layer of the Arabian Sea north of 8°N, meridional Ekman transport along 8.5°N, and Sverdrup transport along 8.5°N were computed with IFR2 data from May 1992 to May 1996. During the 1992–1995 June–September southwest monsoon, upwelling into the Ekman layer of the Arabian Sea occurred at a mean rate of 3.4 Sv (1 Sv = 106m3s−1), which was one‐third smaller than the volume transport exiting within the Ekman layer across the southern boundary of the Arabian Sea, and the southward Sverdrup transport was 15 Sv. During the 1992–1996 December–March northeast monsoon, nearly 5 Sv sank below the bottom of the Ekman layer, 4 Sv entered the Arabian Sea in the Ekman layer, and the Sverdrup transport was negligible. Year‐to‐year transport variations were small.
Role of surface films in ERS SAR signatures of internal waves on the shelf: 2. Internal tidal waves
Authors: Ermakov S. A. , da Silva J. C. B. , Robinson I. S.
Published: February 2004

Analysis of ERS‐1 synthetic aperture radar (SAR) images over the Iberian shelf shows that they contain the signatures of large‐scale banded structures with characteristic wavelengths of 10–20 km, parallel to the shelf break. Their consistency with the structure of internal tides leads to the conclusion that they are the surface manifestations of internal tidal waves. The effect of imaging of such large‐scale phenomena by SAR operating in L, C and X bands cannot be explained by conventional hydrodynamic modulation models based on the effect of straining of surface wind waves due to the internal tidal currents. An alternative explanation is described by an analysis of modulation of centimeter‐ and decimeter‐scale surface waves caused by the redistribution of surface‐active substances by internal tidal currents. Model calculations of variations of the wind wave spectrum across the internal tide profile are presented; an empirical film pressure‐area equation and the results of thermistor chain measurements of internal tidal waves in the study area were used in the modeling. The effect of additional modulation of short surface waves due to variations of wind velocity relative to the internal tidal current is also estimated. It is demonstrated that modulation of centimeter‐decimeter wind waves in the presence of surfactant films is the dominant factor and can explain the effect of internal tidal wave imaging by SAR.
Observations of sand bar evolution on a natural beach
Authors: Gallagher Edith L. , Elgar Steve , Guza R. T.
Published: February 2004

Waves, currents, and the location of the seafloor were measured on a barred beach for about 2 months at nine locations along a cross‐shore transect extending 255 m from 1 to 4 m water depth. The seafloor location was measured nearly continuously, even in the surf zone during storms, with sonar altimeters mounted on fixed frames. The crest of a sand bar initially located about 60 m from the shoreline moved 130 m offshore (primarily when the offshore significant wave height exceeded about 2 m), with 1.5 m of erosion near the initial location and 1 m of accretion at the final location. An energetics‐type sediment transport model driven by locally measured near‐bottom currents predicts the observed offshore bar migration, but not the slow onshore migration observed during low‐energy wave conditions. The predicted offshore bar migration is driven primarily by cross‐shore gradients in predicted suspended sediment transport associated with quasi‐steady, near‐bottom, offshore flows. These strong (>50 cm/s) currents, intensified near the bar crest by wave breaking, are predicted to cause erosion on the shoreward slope of the bar and deposition on the seaward side. The feedback amoung morphology, waves, circulation, and sediment transport thus forces offshore bar migration during storms.
On the structure and evolution of ENSO‐related climate variability in the tropical Pacific: Lessons from TOGA
Authors: Wallace J. M. , Rasmusson E. M. , Mitchell T. P. , Kousky V. E. , Sarachik E. S. , von Storch H.
Published: February 2004

Improved observations in the tropical Pacific during the Tropical Ocean‐Global Atmosphere (TOGA) program have served to corroborate preexisting notions concerning the seasonally dependent relationships between sea surface temperature, sea level pressure, wind stress, rainfall, upper tropospheric circulation, and ocean thermal structure anomalies in the El Niño‐Southern Oscillation (ENSO) phenomenon. However, the paradigm of a quasiperiodic “ENSO cycle,” phase locked with the annual march, does not capture the complexity of the evolution of the anomalies. The inadequacy of this model was particularly apparent during the second half of TOGA when the variability was highly aperiodic. Also, a single modal structure or empirical orthogonal function does not appear to be capable of representing the range of spatial patterns of ocean‐atmosphere interaction in the tropical Pacific. These results suggest the need for a more inclusive phenomenological description of ENSO. Data collected during TOGA serve to confirm the influence of tropical Atlantic sea surface temperature anomalies upon rainfall in northeast Brazil.
Nature of global large‐scale sea level variability in relation to atmospheric forcing: A modeling study
Authors: Fukumori Ichiro , Raghunath Ramanujam , Fu Lee‐Lueng
Published: February 2004

The relation between large‐scale sea level variability and ocean circulation is studied using a numerical model. A global primitive equation model of the ocean is forced by daily winds and climatological heat fluxes corresponding to the period from January 1992 to January 1994. The physical nature of sea level's temporal variability from periods of days to a year is examined on the basis of spectral analyses of model results and comparisons with satellite altimetry and tide gauge measurements. The study elucidates and diagnoses the inhomogeneous physics of sea level change in space and frequency domain. At midlatitudes, large‐scale sea level variability is primarily due to steric changes associated with the seasonal heating and cooling cycle of the surface layer. In comparison, changes in the tropics and high latitudes are mainly wind driven. Wind‐driven variability exhibits a strong latitudinal dependence in itself. Wind‐driven changes are largely baroclinic in the tropics but barotropic at higher latitudes. Baroclinic changes are dominated by the annual harmonic of the first baroclinic mode and is largest off the equator; variabilities associated with equatorial waves are smaller in comparison. Wind‐driven barotropic changes exhibit a notable enhancement over several abyssal plains in the Southern Ocean, which is likely due to resonant planetary wave modes in basins semienclosed by discontinuities in potential vorticity. Otherwise, barotropic sea level changes are typically dominated by high frequencies with as much as half the total variance in periods shorter than 20 days, reflecting the frequency spectra of wind stress curl. Implications of the findings with regards to analyzing observations and data assimilation are discussed.
A fully nonlinear regional wave model for the Bight of Abaco: 1. Nonlinear‐transfer computation
Authors: Snyder Russell L. , Long Robert B. , Neu Wayne L.
Published: February 2004

This paper is the first in a series of papers describing a fully nonlinear regional wave model for the Bight of Abaco, Bahamas. It discusses this model's hybrid representation for nonlinear transfer and the numerical errors associated with this representation. This discussion extends a number of results previously reported by Snyder et al. [1993], doubling both the Boltzmann integration resolution and the spectral resolution of the resulting nonlinear‐transfer estimates and evaluating the errors associated with both resolutions. It also better resolves the structure of the negative midfrequency lobe of the nonlinear transfer for JONSWAP input [Hasselmann et al, 1973], evaluates the errors associated with the diagnostic range of the nonlinear‐transfer computation, and extends the hybrid representation to various finite depths characteristic of the Abaco Bight. It also extends the previous discussion of truncations of the hybrid representation, defining some renormalized hybrid implementations of the discrete‐interaction approximation [Hasselmann et al., 1985] and generalizing its selection algorithm to improve the accuracy of this approximation (at some cost in efficiency). Finally, it develops a systematic scheme for recursively selecting hybrid coefficients to define a family of recursively optimized renormalized hybrid truncations. Because this scheme selects coefficient groups (the interactions for which are scaled versions of one another or of mirror images of one another) rather than individual coefficients and because it optimizes over multiple spectral inputs, the resulting truncations perform well over a full range of peak frequency. The resulting truncations for a nominal spectral resolution of 16 prognostic and four diagnostic wave‐number bands and 12 angle bands include some truncations that essentially trade a factor of 10 in efficiency for a factor of 10 in accuracy (relative to the discrete‐interaction approximation). Other truncations, running only 600 times slower than the discrete‐interaction approximation, give a very accurate representation of the full nonlinear transfer. These truncations, employed in sequential fashion and extended to multiple spectral resolution, enable a relatively accurate and efficient staged inverse modeling of the action‐balance equation. By first inverse modeling to convergence at nominal spectral resolution, then inverse modeling to convergence at double spectral resolution (starting from the converged results for nominal spectral resolution), it should be possible to extrapolate the results of the inverse modeling with an error no worse than a few percent (assuming that the errors in the results of the inverse modeling generated by errors in the nonlinear‐transfer computation are no worse than these generating errors and that the impact of other numerical errors can be similarly contained).
Spatial coherence of interannual variability in water properties on the U.S. northeast shelf
Authors: Mountain David G. , Taylor Maureen H.
Published: February 2004

Interannual variability in the surface and bottom temperature and salinity on the U.S. northeast shelf is described by using hydrographic data from the Northeast Fisheries Science Center (NEFSC) MARMAP program. This 10 year data set provides the spatial resolution to describe spatial patterns in the variability of the shelf water properties. An empirical orthogonal function analysis is used to determine the primary modes of variability. All parameters investigated have significant first modes, which contain 40–50% of the variance in the winter and 25–35% in the summer period. All parameters, except the summer surface temperature, exhibit coherent variability across the entire shelf region from near Cape Hatteras to the central Gulf of Maine. The summer surface temperature appears to have two regions of coherent variability: in the central Middle Atlantic Bight and in the western Gulf of Maine. This regionalization is believed due to the effect of local sources of coastal runoff.
Model parameter estimation from data assimilation modeling: Temporal and spatial variability of the bottom drag coefficient
Authors: Ullman David S. , Wilson Robert E.
Published: February 2004

Acoustic Doppler current profiler (ADCP) data collected from a moving vessel are assimilated, using the adjoint method, into a tidal model of the lower Hudson estuary with the objective of estimating the bottom drag coefficient and residual upstream setup. An objective strategy for determining the weight given to prior estimates of the model parameters is shown to be necessary. The method of cross validation performs well for this purpose. Results of the assimilation experiments indicate that the drag coefficient increases by approximately 30% from neap tides to near‐spring tides. Hydrographic data indicate that this increase is coincident with a decrease in vertical stratification. A point model of the vertical structure in an oscillatory boundary layer shows that the damping effect of stratification on turbulent mixing causes a reduction of the bottom drag coefficient. The assimilation results also indicate relatively large spatial variability in the drag coefficient, with higher values found in shallow regions flanking the estuary channel. This agrees with the results of the boundary layer model which show that a reduction in drag coefficient is associated with an increase of the ratio of water depth to bottom roughness.
Simulation of ocean skin temperature modulation by swell waves
Authors: Wick Gary A. , Jessup Andrew T.
Published: February 2004

Three different mechanisms proposed to explain the observed modulation of the ocean skin temperature by swell waves were simulated and tested in an effort to understand and model the modulation better. The mechanisms include compression of the thermal boundary layer, localized enhancement of the wind stress along the waveform, and preferential breaking along the phase of the swell due to hydrodynamic effects. To implement the preferential‐breaking mechanism, a new model for the time evolution of the skin temperature that directly simulates disruption of the skin layer by wave breaking and surface renewal events was developed. The predicted modulation due to the different mechanisms was compared with direct observations made in open‐ocean conditions. The model incorporating preferential breaking best predicts the overall observed temperature modulation, simultaneously approximating the mean skin temperature and the amplitude of the observed temperature modulation throughout the range of observed wind speeds when the wind and swell are aligned. The model incorporating enhancement of the wind stress is unable to predict the temperature modulation at lower wind speeds, but enhanced wind stress appears to have a significant contribution at high winds. Thinning of the thermal boundary layer due to streamline compression is not sufficient to explain the observed temperature modulation. These results demonstrate that preferential wave breaking can have an important effect on both the modulation of the skin temperature and the mean bulk‐skin temperature difference.
Variability in the South Pacific Deep Western Boundary Current from current meter observations and a high‐resolution global model
Authors: Moore Mike I. , Wilkin John L.
Published: February 2004

Observations of the South Pacific Deep Western Boundary Current from the World Ocean Circulation Experiment Pacific current meter array 9 (WOCE PCM‐9) current meter array are compared with the Los Alamos National Laboratory high‐resolution global ocean model. A simple integration of PCM‐9 velocity yields a mean northward transport of water deeper than 3000 m that is some 5 times greater than the model mean transport of 3.35 × 106 m3 s−1. The low modeled abyssal transport suggests a poor simulation of the mean thermohaline circulation. However, model and observed transport variability correlate significantly. Space‐time spectral analysis shows planetary waves are responsible for most of the variability and are resolved well in the model. The details of interaction of the long waves at the western boundary are different in model and data. The model reflects these waves predominantly as planetary short waves, which decay over the region 600–800 km east of the Kermadec Ridge. PCM‐9 has a higher proportion of energy as topographic waves along the Kermadec Ridge. These two classes of waves are associated with the two observed zonal scales in the boundary current. The model results can be separated into vertical modes. Baroclinic energy is found at all timescales, including those too short for free baroclinic waves. The baroclinic flow correlates with the barotropic so as to enhance surface, and reduce abyssal, kinetic energy. This is the signature of planetary wave scattering at the submerged western boundary ridge, and results in enhanced baroclinic wave energy in the Tasman Sea.
Separation of the Kuroshio water and its penetration onto the continental shelf west of Kyushu
Authors: Lie Heung‐Jae , Cho Cheol‐Ho , Lee Jae‐Hak , Niiler Peter , Hu Jian‐Hwa
Published: February 2004

The separation of Kuroshio water west of Kyushu and its penetration onto the continental shelf of the East China Sea were investigated by analyzing trajectories of satellite‐tracked surface drifters deployed during 1989–1996. Conductivity, temperature, and depth (CTD) data collected in the eastern East China Sea in December 1993 and late April/early May 1995 were also analyzed. Composite trajectories of 172 drogued drifters provided direct evidence of the separation of the northeastward flowing Kuroshio into two parts at the western mouth of the deep trough southwest of Kyushu. The Kuroshio main stream turns to the east toward the Tokara Strait, and a northward flowing branch current penetrates onto the shelf across the continental slope west of the trough. Analysis of the CTD data and drifter trajectories, concurrently observed, shows that the branch current was part of the inshore Kuroshio, just upstream before its separation. During the periods of CTD observations, the separated, inshore Kuroshio water intruded northward in a tongue shape along the shelf break of the trough and penetrated onto the shelf after crossing obliquely the western continental slope of the trough. The water that penetrated onto the shelf continued to flow northward toward the Korea Strait. The main path of this water was easily traced by its high salinity, which is characteristic of the inshore Kuroshio water. Application of an inverse method to the observed CTD data also supports the separation and penetration of the Kuroshio and calculates the volume transport of the northward branch to be about 4.0×106 m3/s for the two different surveys. The tongue‐shaped intrusion of the inshore Kuroshio water onto the outer shelf may take place in close association with the eastward turning of the Kuroshio main stream and it might be caused by the vorticity adjustment created by the eastward turning of the Kuroshio.
Characteristics of deep currents along trenches in the northwest Pacific
Authors: Mitsuzawa Kyohiko , Holloway Greg
Published: February 2004

In the deep sea area of the Japan Trench, located about 150 km east from northern Honshu, one observes southward flow on the landward slope and the northward flow on the oceanward slope. In 1994 and 1995, moorings were deployed on the Japan Trench landward slope around 40°N. One mooring was deployed for 275 days at about 5800 m, and another mooring was deployed for 20 days at 4200 m depth. Results showed the following: (1)A deep western boundary current flows south‐southwest on the landward slope of the Japan Trench with mean speed of 3–7 cm/sec and maximum speed of 15–20 cm/s. (2) Current changes depend mainly on the M2 tide. (3) Previous measurements, as well as inferences from observed benthic fauna, imply oppositely directed flow along the oceanward slope of the Trench, forming a trench countercurrent.
The variability of the tropical Atlantic
Authors: Narvaez W. Cabos , OrtizBeviá M. J. , Oberhuber J. M.
Published: February 2004

In the present work we simulate the equatorial Atlantic variability at annual and interannual timescales using a coupled mixed layer ‐ isopycnal ocean general circulation model (OGCM) forced with observations for the period 1980–1989. Surface features such as the seasonal variations of the equatorial system of currents or of the temperature are quite satisfactorily simulated with a relatively coarse model. For the interannual variability, the model reproduces quite well the observed warmings and coolings of the Gulf of Guinea, except for the 1981 event. The anomalies appear in the simulation earlier than in observations, and their magnitude is overestimated. These characteristics of the simulated interannual variability are shared with other simulations of the tropical Atlantic using a very different OGCM and different setup. The generation of the events is monitored through the anomalous heat content field. Although this generation can be explained in a first approximation as “the displacement to the east of warm water accumulated in the west”, there are other mechanisms at work that account for the differences in the generation of the 1984 and 1988 events. Through statistical analysis the main characteristics of this field are related to other output variables and to signals in the filtered forcings. Meridional convergence in surface velocities plays a significant part in the appearance of anomalies at the Gulf of Guinea. Transition from onset stage to peak phase is connected to extraequatorial signals in the northwestern tropical Atlantic. Differences between the two simulated events (1984 and 1988) can be related to a signal in the wind forcings north of 18°N, and near the coast of Africa.
Radiant heating of the western equatorial Pacific during TOGA‐COARE
Authors: Ohlmann J. Carter , Siegel David A. , Washburn Libe
Published: February 2004

Optical, physical, and biological data collected in the western Pacific warm waterpool (WWP) as part of the Tropical Ocean‐Global Atmosphere‐Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE) are used to assess variations in the transmission of solar radiation through the water column, to investigate factors that regulate in‐water solar transmission, and to examine the sensitivity of upper ocean thermal processes to solar transmission parameterizations. Solar transmission within the upper ocean mixed layer, below 10 m, can be accurately parameterized (within 10%) by using a single exponential profile. Variations in the in‐water transmission profile are explained primarily by estimates of the upper ocean chlorophyll concentration (r2 = 0.83). Mixed layer chlorophyll concentration influences attenuation of the in‐water light field. Clouds play a secondary role by altering the shape of the incident solar spectrum. A comparison of solar transmission parameterizations indicates that use of a Jerlov type parameterization for the WWP can lead to a 15 W m−2 error in the solar flux at 30 m (based on a climatological surface irradiance of 220 W m−2). Application of a one‐dimensional ocean mixed layer model gives a mean sea surface temperature (SST) error of 0.15°C when the model is forced with a typical Jerlov type solar transmission profile. Instantaneous SST differences nearly reach 1.0°C. An increase in solar flux divergence for a mixed layer results in enhanced stratification and mixed layer shoaling. As mixed layer depth decreases, the quantity of solar radiation penetrating beyond the mixed layer increases, resulting in a destabilization of thermocline waters and a mixed layer depth increase. This feedback mechanism helps regulate both mixed layer depth and the solar irradiance lost to the mixed layer through penetration.
ENSO theory
Authors: Neelin J. David , Battisti David S. , Hirst Anthony C. , Jin Fei‐Fei , Wakata Yoshinobu , Yamagata Toshio , Zebiak Stephen E.
Published: February 2004

Beginning from the hypothesis by Bjerknes [1969] that ocean‐atmosphere interaction was essential to the El Niño‐Southern Oscillation (ENSO) phenomenon, the Tropical Ocean‐Global Atmosphere (TOGA) decade has not only confirmed this but has supplied detailed theory for mechanisms setting the underlying period and possible mechanisms responsible for the irregularity of ENSO. Essentials of the theory of ocean dynamical adjustment are reviewed from an ENSO perspective. Approaches to simple atmospheric modeling greatly aided development of theory for ENSO atmospheric feedbacks but are critically reviewed for current stumbling blocks for applications beyond ENSO. ENSO theory has benefitted from an unusually complete hierarchy of coupled models of various levels of complexity. Most of the progress during the ENSO decade came from models of intermediate complexity, which are sufficiently detailed to compare to observations and to use in prediction but are less complex than coupled general circulation models. ENSO theory in simple models lagged behind ENSO simulation in intermediate models but has provided a useful role in uniting seemingly diverse viewpoints. The process of boiling ENSO theory down to a single consensus model of all aspects of the phenomenon is still a rapidly progressing area, and theoretical limits to ENSO predictability are still in debate, but a thorough foundation for the discussion has been established in the TOGA decade.
Application of the Stommel model to shallow Mediterranean estuaries and their characterization
Authors: Hearn Clifford J.
Published: February 2004

During summer, the basins of shallow estuaries in mediterranean climates are usually vertically mixed, hypersaline, and sometimes inverse (in the sense of their densities being elevated above ocean values). These estuaries are commonly controlled by heat and salt exchange with the ocean. Because of the compensating effect of estuarine heating, the magnitude of the density inversion (due to hypersalinity) depends on the net surface buoyancy flux. The surface heat and evaporative fluxes are functions of the estuarine temperature and so the estuarine state (and its associated surface buoyancy flux) depends on the strength of the ocean exchange processes. A simple Stommel model of a summer mediterranean estuary is presented here in which this exchange is controlled by density‐driven processes. The Stommel model has two possible stable stationary states and it is shown that the estuary can be characterized by four types of steady state. They include the familiar classical and inverse estuaries plus a solution, called “quasi‐neutral”, in which the estuary is close to neutrality but just marginally inverse; that is, the net buoyancy flux is small and negative. This quasi‐neutral solution resembles the steady state in Tomales Bay (California) in late summer and shows the characteristic high level of thermal compensation of the density inversion from hypersalinity. It also has the observed very long flushing time which is responsible for some of the environmental fragility of this class of estuary. This appears to be the first detailed confirmation of the applicability of the Stommel model to estuaries. The fourth type of estuarine state arises from the simultaneous existence of both stable states. It is here called “intermittent” and contains both the classical and a quasi‐neutral Stommel solution. The type of estuarine state achieved by a narrow basin in autumn is highly dependent on its length and depth. This produces extreme sensitivity to changes in basin geometry from human influences.
Direct measurements of western boundary currents off Brazil between 20°S and 28°S
Authors: Müller Thomas J. , Ikeda Yoshimine , Zangenberg Norbert , Nonato Luiz V.
Published: February 2004

Current measurements from three moored arrays on the Brazilian continental slope between 20°S and 28°S are investigated for the existence and strength of western boundary currents from near the surface down to the North Atlantic Deep Water. The Brazil Current is found to deepen southward from 100 m to more than 670 m and to strengthen its volume transport to 16.2 × 106 m3/s. Antarctic Intermediate Water is transported in a well‐developed boundary current southward at 28°S and northward north of Cabo Frio (24°S). This result supports earlier suggestions derived from the analysis of hydrographic data that Antarctic Intermediate Water enters the Brazil Basin from the east and bifurcates as it meets the continental break off Brazil. North Atlantic Deep Water is transported southward in a weakly developed boundary current that leads to lower estimates of volume transport than expected from earlier hydrographic data analysis.