bluuebunny/crossref_metadata_2025 huggingface dataset:
Polar boundary layer plumes and bottom water formation: A missing element in ocean general circulation models
Authors: Harvey L. D. Danny
Published: February 2004

Much of the deep water in the world oceans forms as relatively dense water on shelves in polar regions, followed by downslope flow of the dense water as a boundary layer plume. This process is omitted from current global ocean general circulation models (GCMs) which, instead, form bottom water only as a result of the large‐scale thermohaline overturning and as a result of deep convection. Here the role of polar boundary layer plumes is investigated by coupling a two‐dimensional plume model to a two‐dimensional (latitude‐depth) dynamical ocean model, which, in turn, is coupled to sea ice and surface climate models. The polar plume mass flux at the shelf brink is parameterized in terms of the rate of sea ice formation, net evaporation, and surface cooling over polar shelves. The presence of plumes has only a modest impact on the high‐latitude temperature response to a greenhouse gas increase and the projected sea level rise due to thermal expansion because, in the present model, the overall response is strongly conditioned by changes in the relative strength of northern hemisphere and southern hemisphere bottom waters which occur whether or not plumes are included. These changes, in turn, appear to depend critically on the spin‐up flow field and density fields produced by the coupled model. It is concluded that polar plume processes should be incorporated in coupled atmosphere‐ocean GCMs but that correct simulation of the ocean density field and surface forcing conditions is a more important priority.
Structure and evolution of interannual variability of the tropical Pacific upper ocean temperature
Authors: Zhang Rong‐Hua , Levitus Sydney
Published: February 2004

Yearly in situ temperature anomaly data for the period 1961–1990 are analyzed to reveal the dominant structure and evolution of interannual variability of the tropical Pacific upper ocean. We use multivariate empirical orthogonal function (EOF) analyses to detect the principal three‐dimensional structure related to El Niño. There are well‐defined subsurface thermal patterns, characterized by a prominent sea‐saw structure with opposite anomaly polarity in the equatorial and off‐equatorial tropical North Pacific regions. During an El Niño year a positive temperature anomaly is found in the eastern and central tropical upper ocean. This is accompanied by a corresponding negative anomaly at subsurface depths in the west, with a maximum at 100–150 m off the equator. Unlike sea surface temperature, whose variations are confined largely to the east with one dominant polarity, temperature variations at 50–200 m depths have a dipole pattern with out‐of‐phase oscillations in the central equatorial Pacific and in the western tropical North Pacific. A reverse pattern of these anomalies is observed during a La Niña year. Evolution between El Niño and La Niña involves a significant zonal transfer of anomaly phase across the equatorial basin as well as across the off‐equatorial tropical North Pacific, showing consistent and coherent variations from west to east, from subsurface to sea surface, and from on the equator to off the equator. This phase propagation is more evident at subsurface depths than that at the sea surface, suggesting a continual movement of anomaly pattern in succession, eastward along the equator and westward off the equator of the tropical North Pacific, with spatial inhomogeneities of zonal propagation in longitude. Our analyses present evidence of the manner in which temperature anomalies evolve at subsurface ocean depths in the tropical Pacific, thus providing an observational basis for evaluating theoretical studies and model simulations. The dynamical implication of these results is also discussed.
Isotope data from Ice Station Weddell: Implications for deep water formation in the Weddell Sea
Authors: Weppernig R. , Schlosser P. , Khatiwala S. , Fairbanks R. G.
Published: February 2004

Helium isotope data (3He/4He ratios and 4He concentrations) and H218O/H216O ratios obtained from stations occupied during the drift of Ice Station Weddell (February to June 1992) are used, together with hydrographic data, to study formation of deep and bottom water in the western Weddell Sea. The data indicate deep and bottom water formation along the entire track of the ice station (71.4 to 65.8°S, ≈53°W). Ice Shelf Water (ISW) seems to contribute significantly to the formation of Weddell Sea Deep Water (WSDW) and Weddell Sea Bottom Water (WSBW) in the southern part of the drift track. Toward the north, the fraction of ISW contained in WSDW/WSBW decreases. This trend is overlaid by high ISW fractions in the deep and bottom waters found in the vicinity of the Larsen Ice Shelf. The fraction of Western Shelf Water (WSW) in WSBW shows the opposite trend, increasing from south to north. The combined fraction of ISW and WSW in waters with potential temperatures below 0°C is about 20%, corresponding to a roughly 200 m thick layer. Overall, WSW seems to contribute approximately 2 to 3 times more water than ISW to the water column below the 0°C isotherm. Using the estimated flow of ISW over the sill north of the Filchner Depression of 1 Sv [Foldvik et al., 1985a] together with the ratio of WSW plus Winter Water (WW) to ISW, we calculate a value of about 5 Sv for the formation rate of WSBW with a potential temperature of −0.7°C. About one third of this flux represents near‐surface waters (WSW/WW) which have recently been equilibrated with the atmosphere, whereas pure ISW contributes about 10%.
Ice‐coupled wave propagation across an abrupt change in ice rigidity, density, or thickness
Authors: Barrett Murray D. , Squire Vernon A.
Published: February 2004

The model of Fox and Squire [1990, 1991, 1994], which discusses the oblique propagation of surface gravity waves from the open sea into an ice sheet of constant thickness and properties, is augmented to include propagation across an abrupt transition of properties within a continuous ice sheet or across two dissimilar ice sheets that abut one another but are free to move independently. Rigidity, thickness, and/or density may change across the transition, allowing, for example, the modeling of ice‐coupled waves into, across, and out of refrozen leads and polynyas, across cracks, and through coherent pressure ridges. Reflection and transmission behavior is reported for various changes in properties under both types of transition conditions.
Evolution model of the vertical structure of the turbulent active layer of the sea
Authors: Võsumaa Üllar , Heinloo Jaak
Published: February 2004

An one‐dimensional differential model of the vertical structure of the turbulent active layer of the sea based on the theory of the rotationally anisotropic turbulence is proposed. The present article consists of two parts. The first part gives the definition of the class of rotationally anisotropic turbulence (RAT) and presents the basics of the theory of RAT. In the second part the theory is used to describe the evolution of the vertical structure of the turbulent active layer of the ocean within the framework of the traditional model assumption about the horizontal homogeneity of all the fields under consideration. A set of submodels (including the analogue to the classical mixing length model) is derived and discussed. The model is used under different initial and boundary conditions. Depending on the actual conditions the model describes various effects: the appearance of the upper mixed layer with a steep thermocline below it, the switching between the convective and diffusive mixing regimes, the staircase‐like structure of the thermocline (caused by unsteady shear flow or by double‐diffusion instabilities), and the modulation of the velocity field by the period of the oscillation of the heat flux. The set of the model coefficients is determined on the basis of sea surface temperature data from the ocean weather station (OWS) Papa. The results of the following computations of temperature distribution show a reasonable similarity between the model results and the corresponding data from the OWS Papa and the Long‐Term Upper Ocean Study (LOTUS).
The effect of wind‐wave enhancement of bottom stress on the circulation induced by tropical cyclones on continental shelves
Authors: Tang Yong Ming , Grimshaw Roger
Published: February 2004

The effect of increased bottom friction due to wind‐wave current interaction on continental shelves is examined with a two‐dimensional shallow water numerical model using an adaptation of the Grant and Madsen [1979] bottom‐boundary layer theory. In our formulation we allow for the wind‐wave amplitude to depend on the wind stress and bottom topography. We find that under strong tropical cyclones, the Grant and Madsen theory may break down in very shallow water where the wind‐wave amplitudes can become quite large. Since this is a region where the wind‐generated waves are particularly likely to be breaking, we introduce a wave‐breaking criterion into our bottom friction formulation. The effects of this wind‐wave‐induced bottom friction law on the wind‐driven circulation are then examined.
Local ocean response to a multiphase westerly wind burst: 2. Thermal and freshwater responses
Authors: Smyth W. D. , Hebert D. , Moum J. N.
Published: February 2004

A westerly wind burst observed in the warm pool of the western equatorial Pacific Ocean cooled the ocean's surface layer by about 0.8°C. Turbulent entrainment at the base of this layer caused cooling but also heating due to the reversal of the vertical temperature gradient during rain events. Consequently, the cumulative effect of turbulent entrainment was minimized. Following the wind burst, a sustained eastward surface current contributed to high current shear and turbulent dissipation rates at the top of the thermocline. As a result, most of the heat transfer into the thermocline occurred after the wind burst had ended. The cruise‐averaged turbulent flux into the thermocline was 17 ± 10 W m−2, which suggests that the annual mean is only a few watts per square meter. The restratification of the upper ocean in the aftermath of the wind burst is accounted for partly (but not wholly) by local turbulent entrainment. Despite heavy precipitation, upper ocean salinity generally increased during the cruise. Advection appears to have been the dominant factor governing local salinity changes.
Assessment of new production at the upwelling center at Point Conception, California, using nitrate estimated from remotely sensed sea surface temperature
Authors: Dugdale Richard C. , Davis Curtiss O. , Wilkerson Frances P.
Published: February 2004

Remotely sensed sea surface temperature (SST) and a model originally developed for Cap Blanc, northwest Africa [Dugdale et al., 1989], are used to estimate new production (i.e., nitrate uptake, in the sense of Dugdale and Goering [1967]) for the persistent coastal upwelling feature at Point Conception, California. Parameters required to initialize the model and remotely sensed SST (from advanced very high resolution radiometer images) were available for spring 1983, from data collected as part of the Organization of Persistent Upwelling Structures (OPUS) study. Some examples of the spatial extent of new production are illustrated using false color images, and temporal variability is shown by the time series of depth‐ and area‐integrated nitrate uptake obtained from eight images. The model results are compared with shipboard data for three different upwelling conditions that occurred during OPUS‐83, along with the model results and data published for Cap Blanc. These two regions, Point Conception and Cap Blanc, represent two ends of a spectrum of coastal upwelling performance, with low new production at Point Conception and less effective conversion of available nitrate into particulate nitrogen biomass in contrast to the high levels of new production at Cap Blanc. The daily new production at the Point Conception upwelling center is about 10% of the Cap Blanc new production, both estimated from the remote‐sensing model and satellite‐derived SSTs. The model is shown to work well for both extremes and should therefore be suitable for intermediate situations. The long‐term objective is to produce a model which can be used for coastal upwelling systems globally to provide a estimate of new production from remotely sensed data in these important areas and to assist in understanding the role of these coastal systems in the air‐sea exchange of biogeochemical elements.
Ice shelf basal melting at the grounding line, measured from seismic observations
Authors: Smith A. M.
Published: February 2004

Analysis of seismic data provides a new technique to measure basal melting of an ice shelf close to the grounding line. Internal reflections have been observed within the ice on a seismic reflection profile at the grounding line of Ronne Ice Shelf. The changes in ice thickness above and below these reflections have been used in a steady state model to calculate the basal mass flux of the ice shelf soon after the ice begins to float. Strain rates within the ice, calculated during the modeling, agree with values from nearby survey data. The calculated melt rates range between 0 and 7 m yr−1 with an estimated standard error of ±2.4 m yr−1, in reasonable agreement with earlier estimates based on surface glaciological observations. Limitations in the method include the difficulties in determining reflector geometry from a single seismic section and also the limitations imposed by the assumption of steady state. Additional seismic data would greatly reduce these limitations.
A lumped parameter model for the atmosphere‐to‐snow transfer function for hydrogen peroxide
Authors: McConnell Joseph R. , Bales Roger C. , Winterle James R. , Kuhns Hampden , Stearns Charles R.
Published: February 2004

Of the main atmospheric oxidants, only hydrogen peroxide (H2O2) is preserved in polar ice cores. To make use of the peroxide record, however, requires a quantitative understanding of the “transfer function” or relation between atmospheric concentrations of H2O2 and those preserved in the ice core. Snow‐pit H2O2 profiles adjacent to three automatic snow‐depth gages from Summit, Greenland were used to estimate parameters and evaluate the performance of a lumped parameter model to relate concentrations in the atmosphere with those in surface snow and shallow firn. Three of the model parameters define an equilibrium partitioning coefficient between snow and atmosphere as a nonlinear function of depositional temperature. Model parameters yielded a function that closely matched previous laboratory estimates [Conklin et al., 1993]. A fourth parameter reflects the disequilibrium that may be preserved during periods of rapid accumulation. The final model parameter describes the exchange of H2O2 between near‐surface snow and the atmosphere, allowing already buried snow to either take up or release H2O2 as conditions in and above the snowpack change. We simulated snow pit profiles by combining this transfer function model with a finite‐difference model of gas‐phase diffusion in the snowpack. Two applications for this transfer function are (1) to estimate the local seasonal or annual atmospheric H2O2 concentration in the past from snow‐pit and ice‐core records and (2) to invert snow‐pit and ice‐core H2O2 profiles to obtain estimates of the seasonal or annual accumulation time series. In the first case, an independent estimate of snow accumulation is needed, and in the second application, an independent estimate of the annual H2O2 atmospheric cycle is needed.
Wave transformation across the inner surf zone
Authors: Raubenheimer B. , Guza R. T. , Elgar Steve
Published: February 2004

Sea and swell wave heights observed on transects crossing the mid and inner surf zone on three beaches (a steep concave‐up beach, a gently sloped approximately planar beach, and a beach with an approximately flat terrace adjacent to a steep foreshore) were depth limited (i.e., approximately independent of the offshore wave height), consistent with previous observations. The wave evolution is well predicted by a numerical model based on the one‐dimensional nonlinear shallow water equations with bore dissipation. The model is initialized with the time series of sea surface elevation and cross‐shore current observed at the most offshore sensors (located about 50 to 120 m from the mean shoreline in mean water depths 0.80 to 2.10 m). The model accurately predicts the cross‐shore variation of energy at both infragravity (nominally 0.004 < f < 0.05 Hz) and sea swell (here 0.05 < f ≤ 0.18 Hz) frequencies. In models of surf zone hydrodynamics, wave energy dissipation is frequently parameterized in terms of γs, the ratio of the sea swell significant wave height to the local mean water depth. The observed and predicted values of γs increase with increasing beach slope β and decreasing normalized (by a characteristic wavenumber k) water depth kh and are well correlated with β/kh, a measure of the fractional change in water depth over a wavelength. Errors in the predicted individual values of γs, are typically less than 20%. It has been suggested that infragravity motions affect waves in the sea swell band and hence γs, but this speculation is difficult to test with field observations. Numerical simulations suggest that for the range of conditions considered here, γs is insensitive to infragravity energy levels.
Remotely sensed sea surface thermal patterns in the Gulf of Cadiz and the Strait of Gibraltar: Variability, correlations, and relationships with the surface wind field
Authors: Folkard Andrew M. , Davies Peter A. , Fiúza Armando F. G. , Ambar Isabel
Published: February 2004

Satellite image sequences (covering periods of a few days throughout the annual cycle) of the waters off southern Iberia have been analyzed in conjunction with concurrent surface wind speed data from coastal stations. Qualitative analysis reveals a large degree of temporal and spatial variability in the thermal signature of the sea surface over periods of both a few days and several months. During the summer, a cool seasurface temperature signature extends from the western Iberian coast around Cape St. Vincent and eastward as far as Faro. At the same time, a warm signature originating on the Iberian coast between Faro and Cadiz extends into the Strait of Gibraltar. These two features are shown to sometimes adopt more westerly positions, and the strait experiences regions of cool thermal signature originating at its southern side. During winter, the surface flow into the Mediterranean through the Strait of Gibraltar is anomalously warm and appears to come from the interior of the Gulf of Cadiz. Quantitative measurements show that temporal variability over timescales of a few days at individual sites is maximum in midsummer. Spatial thermal variability over the whole region is found to peak toward the end of the summer. Statistical analyses of the data reveal the coupling between the surface wind field in the Gulf of Cadiz and the surface thermal pattern (especially during the summer). Wind‐induced, across‐stream upwelling in the Strait of Gibraltar, although dynamically subordinate to tidal and density‐driven processes, is shown to occasionally dominate the surface thermal signature.
Residence times in the upper Arctic Ocean
Authors: Becker P. , Björk G.
Published: February 2004

We use passive tracers in a one‐dimensional numerical model of the Arctic Ocean to determine the residence time in the mixed layer and the cold halocline. When run to a steady state, the model successfully reproduces most of the observed distribution of salinity and temperature in the Arctic above the Atlantic layer. Comparison of model‐calculated tritium concentrations with observational data also indicates that the transient properties are correct. An important component of the model is the implementation of a “shelf circulation” of about 0.8 Sv that simulates the observed production and interleaving of cold, highly saline shelf waters. We use the model to derive bulk residence times, which are about 25 years in the mixed layer and about 100 years in the halocline. These values are higher than those published in the literature. We explain how the model residence time is related to different tracer ages by generating age distributions with the model. It is shown that the weighted mean of these distributions corresponds well with published tracer age data. The model dynamics that correctly reproduce Arctic mixed‐layer and cold halocline vertical structure in salinity, temperature, density, and various tracers also appear to simulate the natural processes that filter out interannual fluctuations in the freshwater influx from runoff and Bering Strait flow. Under these conditions, the period of the variation must be over 30 years to get 50% of the signal through to Fram Strait. In accord with other investigations, this would suggest that short‐term fluctuations in Arctic river runoff are not the direct cause of freshwater anomalies in the northern North Atlantic.
Distribution of Ca, Fe, K, and S between soluble and insoluble material in the Greenland Ice Core Project ice core
Authors: Laj Paolo , Ghermandi Grazia , Cecchi Rodolfo , Maggi Valter , Riontino Carlo , Hong Sungmin , Candelone Jean‐Pierre , Boutron Claude
Published: February 2004

We have developed an original method coupling particle induced X ray emission and scanning electron microscope/X ray dispersive analysis in order to characterize the partitioning of Fe, Ca, K, and S between the soluble and the insoluble phases in wind‐blown deposits in the Greenland ice. We applied this technique to several sections of the Greenland Ice Core Project ice core. We found that the dominant fraction (from 25 to 100%) of Ca and S deposited in Greenland is soluble, while the proportion of soluble material is lower for K and Fe (between 10 and 80%). For all elements the distribution between soluble and insoluble material varies according to the ice‐core depth. The distribution appears to be linked to either meteorological factors, such as temperature or pH of the precipitation (in the case of Ca or S), or the alterability of the mineral assemblages found in the ice (in the case of Fe and K). The fraction of soluble material is therefore linked to the characteristics of source origin (S is predominantly emitted in very soluble forms) and to the dissolution of certain minerals (such as calcium carbonate or Fe‐oxides) during transport in the atmosphere. The dynamics of alteration processes is, however, still uncertain, especially for K‐containing minerals.
A finite‐volume, incompressible Navier Stokes model for studies of the ocean on parallel computers
Authors: Marshall John , Adcroft Alistair , Hill Chris , Perelman Lev , Heisey Curt
Published: February 2004

The numerical implementation of an ocean model based on the incompressible Navier Stokes equations which is designed for studies of the ocean circulation on horizontal scales less than the depth of the ocean right up to global scale is described. A “pressure correction” method is used which is solved as a Poisson equation for the pressure field with Neumann boundary conditions in a geometry as complicated as that of the ocean basins. A major objective of the study is to make this inversion, and hence nonhydrostatic ocean modeling, efficient on parallel computers. The pressure field is separated into surface, hydrostatic, and nonhydrostatic components. First, as in hydrostatic models, a two‐dimensional problem is inverted for the surface pressure which is then made use of in the three‐dimensional inversion for the nonhydrostatic pressure. Preconditioned conjugate‐gradient iteration is used to invert symmetric elliptic operators in both two and three dimensions. Physically motivated preconditioners are designed which are efficient at reducing computation and minimizing communication between processors. Our method exploits the fact that as the horizontal scale of the motion becomes very much larger than the vertical scale, the motion becomes more and more hydrostatic and the three‐dimensional Poisson operator becomes increasingly anisotropic and dominated by the vertical axis. Accordingly, a preconditioner is used which, in the hydrostatic limit, is an exact integral of the Poisson operator and so leads to a single algorithm that seamlessly moves from nonhydrostatic to hydrostatic limits. Thus in the hydrostatic limit the model is “fast,” competitive with the fastest ocean climate models in use today based on the hydrostatic primitive equations. But as the resolution is increased, the model dynamics asymptote smoothly to the Navier Stokes equations and so can be used to address small‐scale processes. A “finite‐volume” approach is employed to discretize the model in space in which property fluxes are defined normal to faces that delineate the volumes. The method makes possible a novel treatment of the boundary in which cells abutting the bottom or coast may take on irregular shapes and be “shaved” to fit the boundary. The algorithm can conveniently exploit massively parallel computers and suggests a domain decomposition which allocates vertical columns of ocean to each processing unit. The resulting model, which can handle arbitrarily complex geometry, is efficient and scalable and has been mapped on to massively parallel multiprocessors such as the Connection Machine (CM5) using data‐parallel FORTRAN and the Massachusetts Institute of Technology data‐flow machine MONSOON using the implicitly parallel language Id.
Impact of a surfactant on a turbulent shear layer under the air‐sea interface
Authors: Tsai Wu‐Ting
Published: February 2004

The effects of surface active material (surfactant) on a turbulent shear layer near the air‐sea interface are studied by direct numerical simulation of the flow. The study is motivated by the observation of the reduction of the scalar exchange rate across the air‐sea interface by a surfactant film. Though the model, based on the concept of surface renewal by turbulent eddies, explains the observation well, there has been little experimental or numerical validation of the detailed hydrodynamic mechanism. The present work aims to identify the differences in the underlying turbulence structures between clean and contaminated interfaces and to provide a basis for parameterizing the scalar transfer rate across a contaminated interface. The simulation results show that the inviscid blocking effect of a clean interface, the attenuation of vertical fluctuating velocities and the increase in horizontal turbulence intensities, is significantly reduced by surfactant contamination. The viscous sublayer adjacent to the interface is thickened with the presence of a surfactant. Within this viscous sublayer, both horizontal and vertical fluctuating turbulent intensities are attenuated. The upwelling (splatting) events, which are responsible for renewing the fluids in contact with the interface, are reduced drastically. In addition, the vortex connection onto the interface, which frequently occurs on a clean interface, is also diminished. The impact of a surfactant on the sheared turbulence is explained by the interaction between the transport of the surfactant and the underlying coherent horseshoe vortical flow. For turbulence beneath a clean interface, the impingement of the horseshoe vortex causes upwelling and, subsequently, a vortex connection onto the interface. If the surface is covered with a surfactant, surfactant concentration gradients are formed by the impinging turbulent eddies. Surface shear stresses are induced, which retard the approaching of the horseshoe vortices toward the interface, diminish the renewal of near‐surface fluids, and, consequently, reduce the scalar transfer rate.
Relation between sea level and barometric pressure determined from altimeter data and model simulations
Authors: Gaspar Philippe , Ponte Rui M.
Published: February 2004

The relation between sea level and barometric pressure and, specially, the validity of the inverted barometer (IB) approximation is examined over the global oceans, using nearly 2 years of TOPEX‐POSEIDON altimeter measurements. Both crossover differences and collinear differences between consecutive cycles are utilized in this study. Linear regressions between barometric pressure and sea level time series yield coefficients between 0.8 and 1 cm/mbar poleward of 20° and as low as 0.5 cm/mbar in the equatorial regions. Such deviations from the IB value of 1 cm/mbar can be due to the presence of data errors or to correlations between pressure and adjusted sea level (i.e., sea level corrected for IB effect). A simple error model for the pressure fields and a number of sensitivity tests are used to evaluate the changes in the regression coefficient possibly induced by data errors (pressure errors, altimeter measurements errors, and radial orbit errors). The combined (root‐mean‐square) effect of the different errors amounts to 0.8 mm/mbar poleward of 20° and 1.8 mm/mbar within 20° of the equator, in general smaller than the observed deviations from the IB value. Regression coefficients thus imply a correlation between adjusted sea level and pressure. Results from a shallow‐water, global ocean model forced by realistic wind and pressure fields corroborate this finding. The model is able to explain the observed coefficients, within measurement errors, with wind‐driven effects being most important in accounting for differences from the simple IB model. Pressure‐forced dynamical signals cause maximum deviations of only 1 mm/mbar. The analyses point to the general validity of the IB approximation over the deep oceans but also highlight the complex relation between sea level and barometric pressure resulting from correlations between various sea level signals.
The origin of the North Pacific Intermediate Water
Authors: Yasuda Ichiro
Published: February 2004

Possible source regions of the North Pacific Intermediate Water (NPIW) were investigated with synoptic conductivity‐temperature‐depth surveys. In the Okhotsk Sea Kuril Basin a pycnostad (referred to as the Okhotsk Sea Mode Water (OSMW)) was found in the density range of 26.6–27 σθ. The pycnostad is not seen in the Western Subarctic Gyre (WSAG); thus it is an indicative water peculiar to the Okhotsk Sea. The maximum thickness is at 26.7–26.9 σθ (centered at 26.8 σθ) which coincides with the NPIW density range in the North Pacific Subtropical Gyre. In the Oyashio area east of the southern Kuril Islands and Hokkaido, waters that have intermediate temperature, salinity, and potential vorticity between the OSMW and the WSAG water were distributed, suggesting a following transport route of the NPIW source water. The thick (thus low‐Q; Q is potential vorticity) and low‐salinity water mass outflowing from the Okhotsk Sea mixes with the relatively warm, saline, and high‐Q WSAG water to form the Oyashio water. A coastal part of the Oyashio water flows southwestward along the east coast of the southern Kuril Islands and Hokkaido, retaining the pycnostad (Q minimum). A large part of the low‐Q Oyashio water flows further southward and merges with the Kuroshio Extension and then flows eastward, forming new NPIW. In this process a salinity minimum develops and the Q minimum disappears because of the mixing with a saline Kuroshio water (old NPIW) without a pycnostad in the NPIW density range. These results suggest that the origin of NPIW is the water mass in the Okhotsk Sea (OSMW) and the density of the water mass primarily determines the NPIW density centered at around 26.8 σθ.
Assimilation of wave spectra from pitch‐and‐roll buoys in a North Sea wave model
Authors: Voorrips A. C. , Makin V. K. , Hasselmann S.
Published: February 2004

A technique for the assimilation of spectral wave observations in wave models is presented and tested. The method uses the concept of spectral partitioning to project the entire wave spectrum onto a few essential mean parameters. Model and observed partition parameters are assimilated using an optimal interpolation (OI) technique. After data reduction, obtained by the partitioning, the cost of the assimilation is negligible compared to the cost of the model run itself. Therefore the optimal interpolation of partitions (OI‐P) method is a very attractive assimilation technique for operational wave forecasting. The paper focuses on the assimilation of pitch‐and‐roll buoy spectra in a North Sea version of the WAM wave model. Treatment of the (non‐fully two‐dimensional) buoy spectra is discussed. Appropriate choices for the OI weight functions are made. The problem of correlating wave partitions in different spectra is addressed, which is essential for obtaining a robust and efficient system. In order to assess the influence of spectral wave observations on the analysis of the sea state, the method is compared to a second scheme, optimal interpolation of integral parameters (OI‐I), which can only be used to assimilate observations of significant wave height and mean wave period. First, tests with synthetic data are described, which illustrate advantages of the partitioning method over the OI‐I scheme. Also, the inherent limitations of OI are shown in both methods. Experiments with buoy observations for actual North Sea conditions show the benefits of the system, especially when several wave systems are present at the same time.
Inherent optical property inversion of ocean color spectra and its biogeochemical interpretation: 1. Time series from the Sargasso Sea
Authors: Garver Sara A. , Siegel David A.
Published: February 2004

A nonlinear statistical method for the inversion of ocean color spectra is used to determine three inherent optical properties (IOPs), the absorption coefficients for phytoplankton and dissolved and detrital materials, and the backscattering coefficient due to particulates. The inherent optical property inversion model assumes that (1) the relationship between remote‐sensing reflectance and backscattering and absorption is well known, (2) the optical coefficients for pure water are known, and (3) the spectral shapes of the specific absorption coefficients for phytoplankton and dissolved and detrital materials and the specific backscattering coefficient for particulates are known. This leaves the magnitudes for the three unknown coefficients to be determined. A sensitivity analysis is conducted to determine the best IOP model configuration for the Sargasso Sea using existing bio‐optical models. The optical and biogeochemical measurements used were collected as part of the Bermuda Bio‐Optics Project and the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time Series (BATS). The results demonstrate that the IOP model is most sensitive to changes in the exponential decay constant used to model absorption by dissolved and detrital materials. The retrieved chlorophyll a estimates show excellent correspondence to chlorophyll a determinations (r2=81%), similar to estimates from standard band ratio pigment algorithms, while providing two additional retrievals simultaneously. The temporal signal of retrieved estimates of absorption by colored dissolved and detrital materials is mirrored in ratios of Kd(410) to Kd(488), a qualitative indicator for nonalgal light attenuation coefficients. The backscatter coefficient for particles is nearly constant in time and shows no correspondence with the temporal signal observed for chlorophyll a concentrations. Last, the IOP model is evaluated using only those wavelengths which closely match the Sea Viewing Wide Field of View Sensor wave bands. This results in only a 1 to 6% decrease in hindcast skill with the BATS biogeochemical data set. This is encouraging for the long‐range goal of applying the IOP model to data from upcoming ocean color satellite missions.
Dynamic impact of oscillating sources on the North Atlantic deep circulation: A process study
Authors: Karcher Michael J.
Published: February 2004

In this process study the dynamic effects of fluctuating deep water sources on the deep circulation of the North Atlantic are examined by means of a 1½‐layer model. The model is used with a realistic topography of the Atlantic Ocean between 65°N and 25°S and has an open southern boundary. According to observations, the northern inflow of deep water is subject to variability on timescales between days and decades. The same holds for the Deep Western Boundary Current (DWBC), which acts as a link between the deep water inflow and the interior circulation. The timescales of the observed variability motivate the forcing functions used in the model. They determine the nature of the models response. The flow response relative to the local mean flow can be considerably larger than the original perturbation at the source. Normalizing the perturbed flow with the local steady state flow leads to the identification of sensitive areas. Their location and shape are determined by the difference between the characteristic travel paths of the perturbation and the streamlines of the steady state circulation. In the eastern equatorial Atlantic a forcing with periods between 1 and 3 months gives rise to larger flow perturbations than a forcing with periods between half a year and 1 year. Forcing with the latter periods leads to a stronger response in the western equatorial Atlantic and in the interior ocean. Changes in the direction of the deep equatorial flow are driven by low‐frequency oscillations of the DWBC in the model. This is consistent with observations in the area. Perturbations radiate into the interior away from the eastern boundary as long Rossby waves. As this propagation is along geostrophic contours, consequently large parts of the abyssal interior are inaccessible. The results support the notion that the area where the DWBC encounters the equator is not only a key region for the “slow” interhemispheric exchange of heat and tracers but also for the transport of information by “fast” internal waves along the equator.
Seasonal variation of the three‐dimensional mean circulation over the Scotian Shelf
Authors: Han Guoqi , Hannah Charles G. , Loder John W. , Smith Peter C.
Published: February 2004

The seasonal‐mean circulation over the Scotian Shelf is studied numerically by computing mean and tidal current fields for winter, spring, and summer using a three‐dimensional nonlinear diagnostic model. The mean current fields are forced by seasonal‐mean baroclinic pressure gradients, tidal rectification, uniform wind stresses, and associated barotropic pressure gradients. A historical hydrographic database is used to determine the climatological mean baroclinic forcing. Upstream open boundary conditions are estimated from the density fields to give no normal geostrophic bottom flow and are specified as either along‐boundary elevation gradients or depth‐integrated normal velocities. The numerical solutions for nominal bimonthly periods (January–February, April–May, and July–August) reveal the dominant southwestward nearshore and shelf‐break flows of relatively cool and fresh shelf water from the Gulf of St. Lawrence and Newfoundland Shelf, with speeds up to about 20 cm/s. The seasonal intensification of the southwestward flows is reproduced by the model, with the transport increasing from 0.3 Sv in summer to 0.9 Sv in winter on the inner Halifax section. There are also pronounced topographic‐scale influences of submarine banks, basins, and cross‐shelf channels on the circulation, such as anticyclonic gyres over banks and cyclonic gyres over basins. Baroclinicity is the dominant forcing throughout the domain, but tidal rectification is comparable on the southwestern Scotian Shelf (e.g., about 0.2 Sv recirculating transport around Browns Bank for all the periods). The mean wind stress generates offshore surface drift in winter. The solutions are in approximate agreement with observed currents and transports over the Scotian Shelf, although there are local discrepancies.
Vertical motion in the Gulf Stream and its relation to meanders
Authors: Lindstrom Scott S. , Qian Xiaoshu , Watts D. Randolph
Published: February 2004

Vertical motion at 700 m in the Gulf Stream main thermocline is computed using the adiabatic heat equation and a combination of data from inverted echo sounders and moored current meter measurements of T and (u, v) at 400, 700, 1000, and 3500 m. Daily maps of all data on a regular grid were created using optimal estimation; from these maps the tendency and advection terms in the heat equation were calculated. The mapped vertical motions w agree well with w observed contemporaneously along isopycnal RAFOS float tracks. Strong events occur frequently within the Gulf Stream: diagnosed vertical motions greater than 2 mm s−1 that persist for days are common, occurring roughly every week within the 250‐km mapped region. Upwelling and downwelling regions have horizontal scales of about 100 km both along and across stream. Case studies and animation elucidate the relationship between meander properties and the structure, phasing, and amplitude of the w fields: typically, the largest w is positioned near the steepest thermocline slope, downwelling/upwelling occurs entering/exiting troughs, strongest vertical motions occur in slowly propagating features, and vertical motions are usually weaker for rapidly propagating features. This observed dependence upon phase speed is opposite to predictions of recent kinematic and conceptual models. The main contribution to observed w (and the factor missing in those models) is the existence of strong abyssal flow, serving as a barotropic reference for the entire water column, with a component which may flow orthogonal to the baroclinic front along sloping isopycnals. The strongest vertical and cross‐frontal components are usually associated with slowly moving, or stalled, steep meanders, associated with which abyssal cyclones or anticyclones have developed. The vertical motion which is forced in these cases can significantly alter the vertical vorticity and/or frontal strength through tilting of the strong vertical shear and of the strong vertical stratification. Vertical and cross‐frontal transports of 5 Sv occur in steep meanders.
Multiyear variability in the near‐surface temperature structure of the midlatitude western North Atlantic Ocean
Authors: Molinari Robert L. , Mayer Dennis A. , Festa John F. , Bezdek Hugo F.
Published: February 2004

Between 1966 and 1995, subsurface temperature data have been collected in the western North Atlantic Ocean using expendable bathythermographs. Data coverage is sparse in both time and space, but evidence for decadal variability in the upper 400 m of the water column is found. The data were averaged by month onto a 2° of latitude by 4° of longitude grid. Thirty‐one quadrangles in the region bounded by 17°N and 43°N and 78°W and 66°W have sufficient data to provide consistent results. Anomaly time series at 0, 100, 200, 300, and 400 m were estimated by subtracting a mean monthly climatology. The individual records were detrended and filtered to highlight the longer‐period signals. The analysis resulted in 25‐year records (1969–1993) for study. Within the thermocline of the subtropical gyre and the Gulf Stream at 100 and 200 m, periods of predominately positive temperature anomaly end in 1971, 1982, and 1990, while periods of negative anomaly end in 1976 and 1985. Only the events ending in 1971, 1976, and 1990 are in the majority of the records at 300 and 400 m. Most of the events also appear in the sea surface temperature (SST) records but are somewhat masked by significant noise at the surface. Meridional‐vertical temperature sections through the subtropical gyre show that transitions from negative to positive anomaly events are characterized by a deepening of the isotherms throughout the section and transitions from positive to negative events by a rising of the isotherms. Significant lateral migration of the axis of the Gulf Stream, although possibly masked by the 2° averaging, is not necessary to explain either type of event. The transitions in the SST and 100‐m temperature time series occur at essentially the same time as the transitions in an index of the North Atlantic Oscillation (NAO) that has also been detrended (i.e., 1971, 1976, 1980, 1984, 1988). The 1971, 1976, and 1988 NAO events are also observed at 300 and 400 m as described earlier. Periods of positive subsurface temperature anomaly are coincidental with periods of positive NAO index, and periods of negative subsurface temperature anomaly are coincidental with periods of negative NAO index. Thus earlier results showing connections between the NAO and western Atlantic SST at decadal timescales are now extended to at least 400 m in the water column. Trends were computed from the individual 25‐year records. The trends at all depths are predominately negative north of 38°N and positive south of 38°N. Inferences from the horizontal distribution of the trends and results from earlier studies suggest that the 1969–1993 period may be a phase of a 30‐ to 50‐year signal observed in the northern Atlantic since the beginning of the century.
Dynamics of mesoscale vortices generated by turbulent convection at large aspect ratios
Authors: Narimousa Siavash
Published: February 2004

Turbulent convection from a localized circular top surface into a rotating, homogeneous layer has been investigated in a cylindrical laboratory tank. The initial conditions for the experiments were selected so that the aspect ratio R/H≫1 and a three‐dimensional turbulent layer penetrated with speed u≈(0.6±0.1) (B0h)1/3 and then spread radially in the form of a gravity front upon reaching the bottom of the tank (here B0 is the buoyancy flux, h is the local depth, H is the total depth, and R is the radius of the source). This front later underwent a baroclinic instability generating mesoscale vortices with maximum density g′≈(10±1) (B0R)2/3/H. Measurements and theoretical arguments have enabled us to scale the number of vortices N≈(1.5±0.3) (R0,R)−2/3 (R/H)−3/5, their mean diameter D/R≈8 ((R0,R)2/3, swirl velocity ν≈(B0R)1/3, and relative voracity ƒ′/ƒ≈0.5 (here (R0,R) = (B0ƒ3R2)1/2 is a Rossby number based on R, and ƒ is the Coriolis parameter). An application of the present study to deep convective events observed in the Golfe du Lions compares favorably with the field observations.
Sensitivity of bottom stress and bottom roughness estimates to density stratification, Eckernförde Bay, southern Baltic Sea
Authors: Friedrichs C. T. , Wright L. D.
Published: February 2004

Thermohaline density stratification may significantly alter the classic near‐bottom logarithmic velocity profile in many weak to moderately energetic, partially mixed estuaries. Results from Eckemförde Bay suggest fits to log profiles which neglect thermohaline stratification may lead to overestimates of bottom stress and roughness of the order of 130% and 600%, respectively. Measurements of velocity obtained at four heights within 1 m of the seabed are input to theoretical models for velocity shear derived via dimensional arguments for the “overlap” layer. Previous investigators applying dimensional arguments to thermohaline stratification in estuaries have assumed buoyancy flux to be independent of height within the overlap layer. This may be a poor assumption since there is no significant source or sink of thermohaline buoyancy at the sediment‐water interface. In this paper, dimensional arguments which do not assume constant buoyancy flux are used to reduce estimates of the drag coefficient and bottom roughness to below the unreasonably high values predicted by simple log profiles. Formulations assuming very weak and relatively strong stratification are applied, and estimates of buoyancy frequency derived from fits to velocity profiles are compared with independent estimates of stratification. Estimates of bottom stress and roughness derived from velocity profiles are also found to be sensitive to fluid acceleration, uncertainties in instrument settling, and limitations in current meter accuracy, but these latter effects appear secondary in Eckernförde Bay to the impacts of thermohaline stratification.