bluuebunny/crossref_metadata_2025 huggingface dataset:
A quasi‐one‐dimensional coupled climate‐carbon cycle model: 2. The carbon cycle component
Authors: Harvey L. D. Danny
Published: February 2004

A quasi‐one‐dimensional, coupled climate‐carbon cycle model is presented which consists of two polar domains and one nonpolar domain. The model simulates the distribution of dissolved inorganic carbon (DIC), alkalinity, phosphate, dissolved oxygen, and temperature and contains a biological pump with production of organic tissue, calcite, and arogonite. Bottom water is conditioned in one polar domain through interaction with the atmosphere and convective mixing and is injected into the lower portion of the nonpolar domain. Intermediate water flows into the other polar domain and upwells. Successful simultaneous simulation of the observed distribution of all the tracers (including isotope ratios) requires (1) an upwelling velocity in the nonpolar domain that peaks around 2 m yr−1 at a depth of 1 km, with a gradual decrease above and below this depth; (2) effective vertical diffusion coefficients for temperature and other tracers that are different in the upper 0.5 km; and (3) a carbonate carbon to organic carbon production ratio of only 0.09. These requirements are consistent with physical considerations and/or observational evidence. In particular, observational data combined with a consideration of mixing along isopycnal surfaces and model results both indicate that the effective vertical diffusion coefficient in the upper ocean should be smallest for temperature and largest for oxygen, with the values for alkalinity and phosphate modestly smaller than for DIC. The model parameters obtained by tuning the model to preindustrial tracer distributions also provide the best (and generally excellent) fit to observed transient isotope changes. Interactions between alkalinity and DIC modulate the effect on steady state atmospheric pCO2 of changes in the model parameters. However, the model uptake of anthropogenic CO2 and the computed atmospheric CO2 variation to year 2200 are remarkably insensitive to the choice of model mixing parameters, given that these are assumed to be constant during a given simulation. Finally, the sensitivity of the model atmospheric pCO2 to changes in the temperature of warm ocean surface matches that obtained by three‐dimensional ocean carbon cycle models.
Near‐inertial mixing: Modulation of shear, strain and microstructure at low latitude
Authors: Alford Matthew H. , Gregg Michael C.
Published: February 2004

We report direct, quantitative measurements of mixing associated with three cycles of a single, energetic, downward‐propagating near‐inertial wave in the Banda Sea at 6.5°S, 128°E during October 1998. The wave dominates the shear, containing 70% of the total variance. Simultaneous depth/time series of shear, strain, Froude number (Fr), and microstructure allow direct computation of their coherence and phase from 50–120 m, for 14 days. In this depth range, 72% of diapycnal diffusivity (68% of dissipation) occurs in three distinct pulses, spaced at the inertial period of 4.4 days. These are collocated with maxima of transverse shear, strain and Fr. Inertial‐band log diapycnal diffusivity, log10 Kρ, is coherent at the 95% confidence level with both components of shear and Froude number. In this data set, strain is more important than shear in modulating Fr. Owing to the low latitude, the inertial frequency (fo = 1/4.4 cycles per day) is much smaller than the diurnal and tidal frequencies. Consequently, near‐inertial motions may be studied separately from tides and other motions via time‐domain filtering. Semiempirical WKB plane‐wave solutions with observed frequency ωo = 1.02fo and vertical scale 100 m explain 66% and 42% of inertial‐band shear and strain variance, respectively. On the basis of the observed phase relationship between shear and strain, the wave is propagating equatorward, toward 295° true. Ratios of shear to strain and of parallel to transverse shear suggest that the wave's intrinsic frequency ωI ≈ 1.18feff. This indicates that background vorticity ζ has lowered the effective Coriolis frequency, feff = fo + ζ/2, relative to its planetary value, fo [Kunze, 1985]. Ray tracing suggests that the wave was generated near 6.9°S, 130.6°E, ∼20 days prior to the cruise, coincident with the end of high winds associated with the SE monsoon. A slab mixed layer model [Pollard and Millard, 1970], forced with National Center for Environmental Prediction (NCEP) model surface winds, confirms that fluxes from the wind to the ocean at this time were sufficient to generate the wave. A very simple model shows that mixing by monsoon‐generated inertial waves may add an important and strongly time‐dependent aspect to some regions' energy budgets.
Nearshore sandbar migration
Authors: Elgar Steve , Gallagher Edith L. , Guza R. T.
Published: February 2004

Field observations suggest that onshore sandbar migration, observed when breaking‐wave‐driven mean flows are weak, may be related to the skewed fluid accelerations associated with the orbital velocities of nonlinear surface waves. Large accelerations (both increases and decreases in velocity magnitudes), previously suggested to increase sediment suspension, occur under the steep wave faces that immediately precede the maximum onshore‐directed orbital velocities. Weaker accelerations occur under the gently sloping rear wave faces that precede the maximum offshore‐directed velocities. The timing of strong accelerations relative to onshore flow is hypothesized to produce net onshore sediment transport. The observed acceleration skewness, a measure of the difference in the magnitudes of accelerations under the front and rear wave faces, is maximum near the sandbar crest. The corresponding cross‐shore gradients of an acceleration‐related onshore sediment transport would cause erosion offshore and accretion onshore of the bar crest, consistent with the observed onshore migration of the bar crest. Furthermore, the observations and numerical simulations of nonlinear shallow water waves show that the region of strongly skewed accelerations moves shoreward with the bar, suggesting that feedback between waves and evolving morphology can result in continuing onshore bar migration.
Investigation of the Brazil Current front variability from altimeter data
Authors: Goni Gustavo J. , Wainer Ilana
Published: February 2004

The southwestern Atlantic Ocean is characterized by the confluence of the Brazil and Malvinas Currents, which form very strong surface and subsurface fronts that can be detected from hydrographic and remote sensing observations. Three data sets, consisting of TOPEX/Poseidon‐derived sea height anomalies, the climatologically derived depth of the 10°C isotherm, and reduced gravity, are used in conjunction with a two‐layer dynamical ocean scheme to monitor the Brazil Current front and to investigate its variability during a 6 year period (1993–1998). Results reveal that the fronts exhibit motions that are larger zonally than meridionally, showing strong interannual variability with annual mean amplitudes that range from 1° to 6°. The annual and semiannual components account for more than 75% of the variability of the frontal oscillations. In the annual cycle the frontal motions appear to be related closely to fluctuations in the baroclinic transport of the Brazil Current and are only influenced by the Malvinas Current when the Brazil Current transport is very small.
Relationship between ice decay and solar heating through open water in the Antarctic sea ice zone
Authors: Nihashi Sohey , Ohshima Kay I.
Published: February 2004

We demonstrate the importance of heat entering the open water area from the atmosphere on sea ice decay in the Antarctic Ocean. The heat budget analyses, both from the European Centre for Medium‐Range Weather Forecasts and the in situ data, show that the net heat input at the water surface reaches 100–150 W m−2 in the active ice melting season due to large solar heating, while that at the ice surface is nearly zero because of the difference in surface albedo. Thus heat input to the ice‐upper ocean system can be approximated as the product of the net heat at the water surface and the fraction of open water. Climatology data show that the total heat input to the upper ocean in the active melting season is comparable to the total latent heat required for sea ice melting in the whole Antarctic sea ice zone. The temporal variation of the heat input to the upper ocean corresponds well to the melting rate of sea ice, which is calculated from the Special Sensor Microwave Imager (SSM/I) data, in large ice extent sectors where the effect of advection is relatively small. These results suggest that melting of sea ice in the Antarctic Ocean is mostly accomplished by the heat input to the upper ocean through the open water area. On seasonal timescales the amount of heat supplied to the upper ocean is determined by the seasonal cycle of net heat input at the water surface, whereas the variability on shorter timescales and interannual differences are determined by the variation of the open water fraction.
Equatorial upwelling in the western Pacific warm pool
Authors: Helber Robert W. , Weisberg Robert H.
Published: February 2004

Vertical velocity on the equator in the western Pacific warm pool is investigated using data from the Coupled Ocean‐Atmosphere Response Experiment enhanced monitoring array (EMA) centered at 0°, 156°E. The data consist of hourly subsurface horizontal velocity time series from August 1991 until April 1994. Vertical velocity is calculated using horizontal velocity components and the application of the continuity equation. During the first year, from March 1992 until February 1993, data are available from five moorings of the EMA and thus provide nine different combinations of moorings from which to calculate vertical velocity. Four moorings were available during the remaining time period. Random errors are found to be <10−5 m s−1, while systematic errors (finite difference error, systematic instrument error, and error due to surface extrapolation) may be larger. It is suggested that errors, including finite difference errors, are not larger than the vertical velocity estimate. The estimates of vertical velocity are valid on spatial scales the size of the array (∼400 km) and timescales longer than a few days. They reveal a seasonal cycle manifested during a moderate El Niño. Results indicate upwelling, on average from 70 m down to 250 m over the 2 year time period, being slightly stronger in 1992 coincident with the stronger El Niño year. The divergence of horizontal velocity components, resulting in positive vertical velocity, is due to geostrophic divergence on the equator produced from a westward directed zonal pressure gradient force. Meridional divergence and zonal wind stress are uncorrelated, suggesting that Ekman convergence due to local westerly winds is only of partial influence. Consequently, downwelling is not found near the surface, where contributions from local winds and geostrophic divergence are in opposition. This estimate of vertical velocity indicates that water is upwelled in the warm pool from much deeper than but with comparable magnitude to the central and eastern Pacific.
Inventory of released inorganic carbon from organic matter remineralization in the deeper Arabian Sea
Authors: Hupe Axel , Thomas Helmuth , Ittekkot Venugopalan , Lendt Ralf
Published: February 2004

An attempt is made to quantify the biological carbon pump in Arabian Sea subsurface waters by estimating the total concentrations of remineralized dissolved inorganic carbon (DICrem) and its water column inventory. The influence of different sets of −ΔO2/ΔCorg ratios, which are either constant or variable with depth, on the estimate is assessed. Throughout the water column the horizontally mapped DICrem concentrations increase northward due to enlarged export fluxes of organic matter and subsequently enhanced remineralization processes as well as to the accumulation of remineralization products along the trajectory path of the water masses ventilating the Arabian Sea from the southern Indian Ocean. The choice of the remineralization ratios generates significant differences in the DICrem concentrations at specific depth horizons. The DICrem inventory of the Arabian Sea between 500 and 4500 m in the years 1995–1997 amounts to ∼39–44 Gt C depending on the applied remineralization ratios.
Stratification effects by cohesive and noncohesive sediment
Authors: Winterwerp J. C.
Published: February 2004

This study focuses on stratification effects induced by the interaction between suspensions of fine‐grained cohesive and noncohesive sediment and the turbulent flow in estuarine and coastal environments. A concise literature review reveals that this interaction may cause an appreciable modification of the vertical profiles of velocity, vertical eddy viscosity/diffusivity, and Reynolds stresses. This interaction is further studied with a one‐dimensional vertical (1DV) numerical model, which includes the standard k‐ε turbulence model with buoyancy destruction terms. Application of this model to the experimental results by Coleman [1981] shows that the measured changes in the velocity profile can be explained entirely by sediment‐induced buoyancy effects. It is argued that when the carrying capacity for cohesive sediment is exceeded, a fluid mud layer is formed upon the deposition of the cohesive sediment flocs, contrary to the case with noncohesive sediment, where the depositing grains immediately form a rigid bed. Thus a two‐layer fluid system develops causing significant damping of the vertical mixing processes, decreasing the carrying capacity further. This positive feedback results in a catastrophic collapse of the turbulence field and the concentration profile. A scaling law for this saturation behavior is derived from classical stratified flow theory. This law is sustained with a series of numerical experiments with the 1DV model. It also predicts the suspended sediment concentrations observed in the Yellow River to the right order of magnitude. It is concluded that sediment‐induced buoyancy yields appreciable stratification effects at already moderate suspended sediment concentrations. In the case of cohesive sediment these stratification effects can result in a catastrophic collapse of the turbulent flow field and the vertical profile of suspended sediment concentration.
Hydrophysical experiment “Megapolygon‐87” in the northwestern Pacific subarctic frontal zone
Authors: Maximenko Nikolai A. , Koshlyakov Mikhail N. , Ivanov Yury A. , Yaremchuk Maxim I. , Panteleev Gleb G.
Published: February 2004

The purpose of this paper is to familiarize readers with major results and findings of the biggest multi‐institutional Soviet hydrophysical experiment, “Megapolygon‐87,” conducted in the northwestern Pacific Subarctic Frontal Zone in June‐October 1987. Six large‐scale, high‐spatial‐resolution conductivity‐temperature‐depth (CTD) surveys (1653 profiles) and numerous moorings (an array of 177 buoys deployed within a 500 km × 500 km area) with current meters at four depths provided unique observations of frontal structure, mesoscale and submesoscale eddies, and their evolution and interaction with each other. We discuss errors induced in the velocity records by surface waves and show how they can be corrected to a reasonable value. Joint analysis of current meter and CTD data allowed detection of a number of interesting processes and objects in the study area. Among them are meandering of the Subarctic Front around its branching point with formation of chains of cyclonic and anticyclonic mesoscale eddies and strong baroclinicity of velocity fields with significant difference between the thermocline and abyssal flows. Statistical characteristics of corrected “Megapolygon‐87” velocities are compared with preceding measurements in the region.
Internal tides and baroclinicity in the southern Weddell Sea: 1. Model description
Authors: Robertson Robin
Published: February 2004

Near the continental shelf break in the southern Weddell Sea, Warm Deep Water and Western Shelf Water meet. Mixing mechanisms, such as internal tides, have the potential to mix these water masses and form Antarctic Bottom Water. A modified version of the Princeton Ocean Model was utilized to investigate the internal tidal fields generated by the interaction of the M2 barotropic tide with topography for transects across the continental shelf and slope in the southern Weddell Sea. Internal tides were generated over the upper continental slope as predicted by linear internal wave theory. Although the essentially two‐dimensional domain resulted in differences between the model elevations and the observations exceeding the observational uncertainties, the cross‐slope velocities agreed well with differences less than the uncertainties for 78% of the existing observations.
Role of ocean dynamics in determining the mean seasonal cycle of the South China Sea surface temperature
Authors: Qu Tangdong
Published: February 2004

The mean seasonal cycle of surface heat budget is examined using historical temperature data combined with climatological wind stress and surface heat flux in the South China Sea. In most parts of the basin, we see a negative correlation between sea surface temperature (SST) and mixed layer depth (MLD); that is, SST tends to be higher (lower) when MLD is shallower (deeper). Given the characteristics of the MLD distribution, we further partition the South China Sea into four smaller areas: one along the continental slope south of China, one off west Luzon, one in the central part of the basin, and one near the coast of Vietnam. Heat budget assessment in these smaller areas indicates that although surface heat flux is fundamental to the mean seasonal cycle of SST, the effect of ocean dynamics is not negligible. Adding the contribution of ocean dynamics better explains the SST tendency. Ekman advection forced by the northeast monsoon is the primary heating process in winter but becomes less important as the horizontal temperature gradient decreases in the following seasons. Vertical entrainment is effective at cooling when the southwest monsoon prevails, which works against the surface heating and leads to a decrease of SST by up to 5 months earlier than surface heat flux starts to cool the ocean.
Measuring the shallow porosity structure of sediments on the continental shelf: A comparison of an electromagnetic approach with cores and acoustic backscatter
Authors: Evans Rob. L.
Published: February 2004

We validate estimates of shallow sedimentary porosities made on the continental shelf by electromagnetic methods through a comparison with those measured directly in cores. Specifically, we show that an electromagnetic surveying system we have used to measure sediment resistivity in a number of seafloor locations returns porosity depth profiles that are related to those measured in cores, and that our data have good sensitivity to the structure within the uppermost few tens of centimeters of seafloor. We then consider inferences of sediment type made by observations of acoustic backscatter in the light of our porosity measurements. Our results suggest that electromagnetic methods provide complementary information that can be used to improve grain type estimates at and just below the seafloor, especially when combined with acoustic backscatter data. Because the system is pulled along the seafloor, data can be quickly collected from a large area of seafloor and can be used to interpolate facies conditions between core locations.
Holocene tidal conditions and tide‐induced sand transport in the southern North Sea
Authors: van der Molen J. , de Swart H. E.
Published: February 2004

A numerical model of Holocene tides and sand transport in the North Sea was used to establish potential tidal sand transport patterns in the Southern Bight and sand budgets for the Dutch and Belgian coast. The sand transport formulation yields magnitude and direction in addition to erosion/deposition values, in contrast to previous studies. All palaeobathymetries were obtained using the present bathymetry and sea level change results from an isostatic rebound model. The sea first invaded the Southern Bight through the Strait of Dover, and the microtidal range increased rapidly to mesotidal. The amphidromic system developed when the land bridge between Britain and the continent flooded and the northern tidal influence increased. Simultaneously, the current ellipses rotated from shore normal to shore parallel, while the flow velocities increased. The net tidal sand transport direction changed from toward the Dutch and Belgian shore to northward alongshore in the north and southward in the south. Until 7000 B.P. erosion dominated at sea, and deposition dominated near shore. Subsequently, erosion occurred in the south, and sedimentation occurred in the north. To the north of the Southern Bight, transport rates were low. Since 7500 B.P., sand has been transported mainly in suspension. Dominant transport by tidal asymmetry changed to transport by tidal residual currents and back in large areas. Comparison with previous studies illustrates that isostatic rebound is important for constructing palaeobathymetries. Most sediment was supplied to the Dutch and Belgian coast before 6000 BP. Substantial tidally induced mean sea level changes occurred in the early Holocene.
Two‐basin filling boxes
Authors: Wong A. B. D. , Griffiths R. W.
Published: February 2004

The convection induced by turbulent buoyant plumes in a rectangular “filling box” (Baines and Turner, 1969) is extended to situations where the lower depths of the box are divided into two basins by a vertical barrier. The barrier restricts horizontal spreading and allows the interbasin exchange of water to occur only in the common region above the barrier height. One‐and two‐plume systems are considered; in the two‐plume case, it is assumed that each of the plumes arises from an independent buoyancy source above each basin. The resulting two‐basin circulation is characterized by different stratifications in each basin and by an overflow from one basin into the other. The spreading depths of the plumes and ambient stratification depend on the barrier height, relative basin sizes, and, in the two‐plume case, the relative strengths of the two buoyancy fluxes. These results may be of relevance to multibasin seas and oceans where the circulation produced by near‐surface sources of intermediate and deep water is influenced by underwater ridges.
Geothermal heating and its influence on the meridional overturning circulation
Authors: Scott Jeffery R. , Marotzke Jochem , Adcroft Alistair
Published: February 2004

The effect of geothermal heating on the meridional overturning circulation is examined using an idealized, coarse‐resolution ocean general circulation model. This heating is parameterized as a spatially uniform heat flux of 50 m W m−2through the (flat) ocean floor, in contrast with previous studies that have considered regional circulation changes caused by an isolated hot spot or a series of plumes along the Mid‐Atlantic Ridge. In our model results the equilibrated response is largely advective: a deep perturbation of the meridional overturning cell on the order of several sverdrups is produced, connecting with an upper level circulation at high latitudes, allowing the additional heat to be released to the atmosphere. Rising motion in the perturbation deep cell is concentrated near the equator. The upward penetration of this cell is limited by the thermocline, analogous to the role of the stratosphere in limiting the upward penetration of convective plumes in the atmosphere. The magnitude of the advective response is inversely proportional to the deep stratification; with a weaker background meridional overturning circulation and a less stratified abyss the overturning maximum of the perturbation deep cell is increased. This advective response also cools the low‐latitude thermocline. The qualitative behavior is similar in both a single‐hemisphere and a double‐hemisphere configuration. In summary, the anomalous circulation driven by geothermal fluxes is more substantial than previously thought. We are able to understand the structure and strength of the response in the idealized geometry and further extend these ideas to explain the results ofAdcroft et al.[2001], where the impact of geothermal heating was examined using a global configuration.
Mean transport and seasonal cycle of the Kuroshio east of Taiwan with comparison to the Florida Current
Authors: Lee Thomas N. , Johns William E. , Liu Cho‐Teng , Zhang Dongxiao , Zantopp Rainer , Yang Yih
Published: February 2004

Moored observations of Kuroshio current structure and transport variability were made across the channel between northeast Taiwan and the Ryukyu Islands at 24°N from September 19, 1994, to May 27, 1996. This was a cooperative effort between the United States and Taiwan. The moored array was designated PCM‐1, for the World Ocean Circulation Experiment (WOCE) transport resolving array. The dominant current and transport variability occurred on 100‐day timescales and is shown by Zhang et al. [2001] to be caused by warm mesoscale eddys merging with the Kuroshio south of the array causing offshore meandering and flow splitting around the Ryukyu Islands. An annual transport cycle could not be resolved from our 20‐month moored record because of aliasing from the 100‐day period events. Sea level difference data were used to extend the transport time series to 7 years giving a variation in the range of the annual transport cycle of 4–10 Sv, with a mean range closer to 4 Sv. The seasonal maximum of 24 Sv occurred in the summer and the seasonal minimum of 20 Sv occurred in the fall. A weaker secondary maximum also occurred in the winter. The cycle of Kuroshio transport appears to result from a combination of local along‐channel wind forcing and Sverdrup forcing over the Philippine Sea. Our estimate of the mean transport of the Kuroshio at the entrance to the East China Sea from the moored array is 21.5±2.5 Sv. The mean transpacific balance of meridional flows forced by winds and thermohaline processes at this latitude requires an additional mean northward flow of 12 Sv with an annual cycle of ±8 Sv along the eastern boundary of the Ryukyu Islands. The mean transport and annual cycle of the Kuroshio were found to be in reasonable agreement with basin‐scale wind‐forced models. Remarkable similarities are shown to exist between the mean western boundary currents and their seasonal cycles in the Atlantic (Florida Current and Antilles Current) and Pacific (Kuroshio and boundary current east of Ryukyu Island chain) at the same latitude. However, detailed comparison shows that the mean Kuroshio is weaker and more surface intensified than the mean Florida Current, while the Kuroshio transport variability is significantly larger.
Modeled and observed impacts of the 1997–1998 El Niño on nitrate and new production in the equatorial Pacific
Authors: Radenac M.‐H. , Menkes C. , Vialard J. , Moulin C. , Dandonneau Y. , Delcroix T. , Dupouy C. , Stoens A. , Deschamps P.‐Y.
Published: February 2004

The impact of the strong 1997–1998 El Niño event on nitrate distribution and new production in the equatorial Pacific is investigated, using a combination of satellite and in situ observations, and an ocean circulation‐biogeochemical model. The general circulation model is forced with realistic wind stresses deduced from ERS‐1 and ERS‐2 scatterometers over the 1993–1998 period. Its outputs are used to drive a biogeochemical model where biology is parameterized as a nitrate sink. We first show that the models capture the essential circulation and biogeochemical equatorial features along with their temporal evolution during the 1997–1998 event, although the modeled variability seems underestimated. In particular, the model fails to reproduce unusual bloom conditions. This is attributed to the simplicity of the biological model. An analysis of the physical mechanisms responsible for the dramatic decrease of the biological equatorial production during El Niño is then proposed. During the growth phase (November 1996 through June 1997), nitrate‐poor waters of the western Pacific are advected eastward, and the vertical supply of nitrate is reduced due to nitracline deepening. These processes result in the invasion of the equatorial Pacific by nitrate‐poor waters during the mature phase (November 1997 through January 1998). At that time, the central Pacific is nitrate limited and experiences warm pool oligotrophic conditions. As a result, the modeled new production over the equatorial Pacific drops by 40% compared to the mean 1993–1996 values. Then, while El Niño conditions are still present at the surface, the nitracline shallows over most of the basin in early 1998. Therefore the strengthening of the trade winds in May 1998 efficiently switches on the nitrate vertical supply over a large part of the equatorial Pacific, leading to a rapid return of high biological production conditions. Strong La Niña conditions then develop, resulting in a biologically rich tongue extending as far west as 160°E for several months.
Modeling studies of the coastal circulation off northern California: Statistics and patterns of wintertime flow
Authors: Pullen Julie , Allen J. S.
Published: February 2004

We conduct modeling studies of the coastal circulation off northern California in the vicinity of the Eel River (40.6°N, the site of the 1995–2000 Strata Formation on Margins (STRATAFORM) marine geology observational program) using a series of nested hydrostatic primitive equation models to aid in the understanding of the shelf flow field that receives and transports sediments during the winter floods. The basic objectives of our numerical studies are to model the continental shelf and slope flow and to understand the dominant dynamical processes. We simulate the shelf and slope flow surrounding Cape Mendocino during 100 days in winter 1996–1997 using a 3 km resolution model embedded in a 9 km resolution regional model of the North Pacific Ocean (the Naval Research Laboratory's Pacific West Coast (PWC) model). The validity of the model simulations is assessed by comparison with current measurements from a STRATAFORM shelf tripod outfitted with an acoustic Doppler current profiler and with satellite altimetry. Good agreement is found between the amplitude and time variability of the 3 km resolution modeled and observed alongshore currents on the shelf. The 3 km resolution model outperforms the 9 km resolution PWC model. Using statistical maps of flow variables, strong alongshore variability in wintertime flow is documented. The role of the forcing supplied by wind and river runoff in establishing this flow asymmetry between regions north and south of Cape Mendocino is examined using empirical orthogonal functions. The evolution of a robust anticyclonic eddy over the shelf and slope adjacent to Cape Mendocino is described. The eddy forms when strong poleward winds weaken and reverse direction during winter storms. The important role of alongshore pressure gradients in the shelf circulation is documented.
Properties of the water column and bottom derived from Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data
Authors: Lee Zhongping , Carder Kendall L. , Chen Robert F. , Peacock Thomas G.
Published: February 2004

Using Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data as an example, we show in this study that the properties of the water column and bottom of a large, shallow area can be adequately retrieved using a model‐driven optimization technique. The simultaneously derived properties include bottom depth, bottom albedo, and water absorption and backscattering coefficients, which in turn could be used to derive concentrations of chlorophyll, dissolved organic matter, and suspended sediments in the water column. The derived bottom depths were compared with a bathymetry chart and a boat survey and were found to agree very well. Also, the derived bottom albedo image shows clear spatial patterns, with end‐members consistent with sand and seagrass. The image of absorption and backscattering coefficients indicates that the water is quite horizontally mixed. Without bottom corrections, chlorophyll a retrievals were ∼50 mg m−3, while the retrievals after bottom corrections were tenfold less, approximating real values. These results suggest that the model and approach used work very well for the retrieval of subsurface properties of shallow‐water environments even for rather turbid environments like Tampa Bay, Florida.
Detailed structure of the Subtropical Front over Chatham Rise, east of New Zealand
Authors: Sutton Philip
Published: February 2004

A detailed description of the Subtropical Front (STF) over Chatham Rise is presented. Information is provided by conductivity‐temperature‐depth (CTD) profiler temperature and salinity transects, high‐resolution (3–4 km spacing), near‐synoptic expendable bathythermograph surveys, shipboard acoustic Doppler current profiler measurements, a 160 km constant depth undulator tow, and satellite SST measurements in austral autumn and spring (April and October) near 178°30′E. Detailed sections across the front reveal that the STF can be divided into northern and southern fronts (NSTF and SSTF) separated by a frontal zone, consistent with observations of the STF elsewhere. Comparison with a western CTD section along 176°E reveals that this structure breaks down close to the New Zealand landmass. The dominant impact of seasonally is the formation and decay of a seasonal thermocline of fairly constant depth over the entire region. Velocity measurements indicate eastward zonal flows on the northern and southern flanks of Chatham Rise. The flow over Chatham Rise is complicated by stirring processes but has a mean southward trend consistent with the less dense, warm, salty subtropical water (STW) overriding the more dense, cold, fresh subantarctic water (SAW) to the south. Horizontal‐length‐scale analyses of the temperature field for a constant depth tow at 21 m indicate a k−2.4 spectral falloff with no proof of preferential length scales. The temperature salinity gradients are largely density compensated in this mixed layer tow. The horizontal temperature standard deviation shows subsurface maxima concentrated at the locations of the NSTF and SSTF. Vertical length scales vary across the front, with large scales (80–100 m) in the STW north of the STF and smaller scales (20–40 m) in the subtropical frontal zone and SAW to the south. The surface vertical length scales are dominated by the presence of a seasonal thermocline.
Wind and Gulf Stream influences on along‐shelf transport and off‐shelf export at Cape Hatteras, North Carolina
Authors: Savidge Dana K. , Bane John M.
Published: February 2004

Along‐shelf transports across three cross‐shelf lines on the continental shelf near Cape Hatteras have been calculated from moored current meter data over a continuous 24 month period in 1992–1994. The along‐shelf convergence has been used to infer off‐shelf export. Transport and transport convergence have been related to wind and Gulf Stream forcing and to variability in sea level at the coast. The along‐shelf transport variability is primarily wind‐driven and highly correlated with sea level fluctuations at the coast. Both winds and along‐shelf transport exhibit a near‐annual period variability. Along‐shelf transport is not well correlated with Gulf Stream offshore position. Along‐shelf transport convergence is highly correlated with Gulf Stream position offshore, with a more shoreward Gulf Stream position leading increased along‐shelf convergence by hours to a few days. Long‐period variability of 14–16 months and 1–3 months is apparent in both Gulf Stream position and transport convergence. Variability in along‐shelf convergence is poorly correlated with wind, wind convergence, or coastal sea level. A likely hypothesis accounting for the observed relationship between Gulf Stream position and along‐shelf transport convergence is that the Gulf Stream is directly influencing cross‐shelf export processes along the outer boundary of the study site. Despite predominantly convergent flow on the shelf at Cape Hatteras, brief periods of along‐shelf divergence and shoreward cross‐shelf transport exist (∼10% of the time just north of Cape Hatteras and ∼34% of the time just south of Cape Hatteras during episodes of up to 3–8 days duration). Implied onshore flows of a few cm s−1 are tentatively identified in the moored current meter data for these periods. Satellite imagery for an extended along‐shelf divergent period clearly suggests that shelf edge parcels could be advected a significant fraction of the way across the shelf.
Can general circulation models be assessed and their output enhanced with drifter data?
Authors: Toner M. , Kirwan A. D. , Kantha L. H. , Choi J. K.
Published: February 2004

Drifter data from the Gulf of Mexico are used to assess and enhance the output of a primitive equation general circulation model. The analysis is made in a 450 km × 450 km open subdomain encompassing a Loop Current ring. The model velocity field is compared with position data from four drifters at the drogue depth of 50 m using geometrical orthogonal functions (GOF). An Eulerian velocity field is reconstructed from the model velocity field and drifter velocities. This reconstructed velocity improves 8‐day numerical trajectories relative to the model field by at least an order of magnitude, as quantified by two Lagrangian error metrics referenced to the real drifter paths. An Eulerian metric that compares the two fields, however, does not exceed 7% for the 20‐day assessment period. Thus the drifter data may be reproduced with modest impact on the model velocity. Enhancement of the model velocity field is determined by two tests: the ability of the GOF velocity field to (1) improve the forecast of drifter positions using only a posteriori data and (2) improve the forecast of withheld drifter data. Using a posteriori data, the 20‐day temporal mean of the position error is improved for all drifters by 87–89% for 6‐hour and 26–38% for 30‐hour forecasts. For 6 days, a withheld drifter is 35–40 km from a drifter whose velocity is used in the reconstructed velocity field. The temporal mean of the position error during this period is improved by 20% for 6‐hour and 26% for 30‐hour forecasts.
Forcing of the Antarctic Circumpolar Wave by El Niño‐Southern Oscillation teleconnections
Authors: Cai Wenju , Baines Peter G.
Published: February 2004

We address several issues regarding the Antarctic Circumpolar Wave (ACW). These include the extent to which the ACW is related to El Niño‐Southern Oscillation (ENSO) cycles and the need to reconcile differences between the observed zonal wavenumber 2 pattern and the wavenumber 3 pattern found in coupled ocean‐atmosphere models. In the present study we revisit the sea surface temperature (SST) signal of the observed ACW. We find that the associated SST variations may be decomposed into several empirical orthogonal functions (EOFs) that are linearly independent. The first four EOFs describe 84% of the total variance on timescales of 3–7 years, and superposition of SST signals of the four EOFs reproduces SST signals of the observed ACW. Given that EOF analysis can only describe standing oscillations, we then employ complex EOF (CEOF) analysis suitable for studying propagating waves. The first two CEOFs (denoted CEOFSST1 and 2) account for 70% of the total variance, and the SST variability reconstructed from them again reproduces the observed ACW. The real part of CEOFSST1 (44% of the total variance) is similar to the first real EOF (EOFSST1). Both EOFSST1 and CEOFSST1 may be identified with ENSO and have a wavenumber 2 zonal pattern in the southern middle‐to‐high latitudes. CEOFSST1 is the major constituent of the ACW in the period of 1981–1997. The associated atmospheric circulation displays a Pacific‐South America (PSA) pattern. Statistical analyses demonstrate that the PSA pattern teleconnected to ENSO is the major mechanism that forces this principal constituent of the ACW. CEOFSST2 (26% of the total variance) has a zonal wavenumber 3 pattern in the same latitude band, similar to that simulated by several coupled models. We suggest that if ENSO is not adequately resolved in coupled models but structures with zonal wavenumber 3 pattern are, then the modeled ACW would appear to have a zonal wavenumber 3 pattern.
Modeling the exchange of the Baltic Sea from the meridional atmospheric pressure difference across the North Sea
Authors: Gustafsson Bo G. , Andersson Helén C.
Published: February 2004

The large Baltic Sea estuary has a hampered exchange with the ocean because of the topographical constriction of the transition area. Understanding the mechanisms controlling the deep water renewal and possible climatic dependencies are of primary concern in order to understand past and future changes of the Baltic Sea. In the present paper, century long time series have been used to investigate the interrelations between observed air pressure, sea level, and salinity in period bands relevant to North Sea‐Baltic Sea water and salt exchange. Strong linear relations between the north‐south air pressure difference across the North Sea and the sea level in the Kattegat were found for oscillations in the 35–200 days period band. In the same period band the surface salinity of the Kattegat covaries with variations of the Baltic sea level because of movement of the horizontal salinity gradient in Kattegat related to filling/emptying of the Baltic. Using the statistical dependencies, a model of the Baltic sea level, and a simple gradient model for the Kattegat salinity, we simulated the occurrences and strengths of large high‐saline inflows to the Baltic with the north‐south air pressure difference across the North Sea as the only forcing. The water exchange between Kattegat and the Baltic Sea have been calculated from 1902 to 1998. The computed inflow conforms with previous estimates and with observed changes seen in the Baltic Sea deep water. It is suggested that this simple model can be used to analyze the sensitivity of the Baltic Sea to climate change.
Growth and steady state stages of thermohaline intrusions in the Arctic Ocean
Authors: May Brian D. , Kelley Dan E.
Published: February 2004

Thermohaline intrusions are a widespread feature of the Arctic Ocean pycnocline and may be important for the lateral transport of heat, salt, and other tracers. In an attempt to understand the dynamics of the intrusions, we present an analysis of hydrographic observations of a front north of Svalbard. Following intrusions from profile to profile, we find that they slope upward toward the cold, fresh side of the front. However, their slope is less than that of the background isopycnals, so that the intrusions slope between horizontal and isopycnal surfaces in the “wedge” of baroclinic instability. Our analysis proceeds in two stages. First, to determine what might have caused the initial growth of the intrusions, we compare the observed intrusion properties with predictions of an instability theory. The results suggest that the intrusions developed as a form of double‐diffusive interleaving, with fluxes dominated by salt fingering and additional forcing by baroclinicity. Second, to investigate the observed finite amplitude interleaving, we apply a steady state model to the observations. A key result is that diffusive convection, not salt fingering, must be the dominant form of double diffusion in order to reach steady state. This fundamental change in dynamics is discussed in the context of adjustment from the growth stage to an eventual steady state.
Discrete particle model for sheet flow sediment transport in the nearshore
Authors: Drake Thomas G. , Calantoni Joseph
Published: February 2004

Fully three‐dimensional discrete particle computer simulations of high‐concentration sheet flow transport in oscillatory flows quantify the effect of fluid acceleration on bed load transport in highly unsteady flows typical of nearshore marine environments. A simple impulse‐momentum approach explains simulation results and forms the basis for adding an acceleration‐related term to widely used energetics sediment transport formulae. Transport predicted by the acceleration term becomes increasingly significant as wave shape approaches the sawtooth profile characteristic of surf zone bores. Simulations integrate F = ma and a corresponding set of equations for the torques for each sphere. Normal and tangential forces between contacting particles are linear functions of the distance between sphere centers and the relative tangential displacement at the contact point, respectively; particle interactions are both inelastic and frictional. Pressure gradient forces generated by the passage of surface gravity waves drive fluid and particle motion in a stack of thin horizontal fluid layers that exchange momentum and exert fluid drag, added mass, and buoyancy forces on particles. Transport properties of the simulated granular‐fluid assemblage are robust to large variations in material properties of the particles. Simulated transport rates agree with available experimental data for unsteady transport of coarse sands; the mode of bed load motion, dispersion of bed load particles, and particle segregation by size and density are qualitatively consistent with available particle‐scale observations of bed load transport of natural particles.
Generation of density fine structure by salt fingers in a spatially periodic shear
Authors: Wells Mathew G. , Griffiths Ross W. , Turner J. Stewart
Published: February 2004

Vertical shear is ubiquitous in the ocean thermocline. The temperature and salinity gradient are also often favorable to the growth of salt fingers. We investigate the implications of the superposed shear and salt fingering for the formation of fine structure and show that a depth‐dependent spatially periodic shear leads to the development of variations in the density profile on the same wavelength as the shear. Laboratory experiments were carried out in a continuous density gradient with a spatially periodic shear produced by exciting a low‐frequency baroclinic mode of vertical wavelength 60 mm. The density gradient consisted of opposing salt and sugar gradients favorable to salt fingers (an analogue to the oceanic heat/salt system). Where the shearing was large the salt finger buoyancy fluxes were smaller. Changes in salinity gradient due to the resulting flux divergence were self‐amplifying until a steady state was reached in which the spatial variations in the ratio of salt and sugar gradients were such that the flux divergence vanished. Thus, along with reducing the mean salt finger buoyancy flux a spatially varying shear can also lead to the formation of density structure.