bluuebunny/crossref_metadata_2025 huggingface dataset:
Barotropic and baroclinic semidiurnal tidal currents in two contrasting coastal upwelling zones of Chile
Authors: Bravo L. , Ramos M. , Sobarzo M. , Pizarro O. , Valle‐Levinson A.
Published: April 2013

Current velocity observations from the continental shelves of Coquimbo (~30°S) and Concepcion (~36°30'S), central Chile, were analyzed to evaluate the role of water column stratification and shelf width on baroclinic semidiurnal tidal currents. Semidiurnal barotropic currents off both zones were typically < 5 cm/s, but depth‐dependent semidiurnal flows could exceed 10 cm s‐1 during stratified conditions. Both zones are recognized as pronounced upwelling centers, with maximum upwelling‐favorable winds in spring and summer, respectively. At the northern zone, stratification was mainly controlled by temperature differences between surface and bottom waters with maximum stratification during summer. The southern zone showed more stratification during winter because of freshwater input from local rivers. Consequently, greater variability in the baroclinic semidiurnal currents was observed during summer at the northern continental shelf and in winter at the south. In both regions, much of the semidiurnal variability was consistent with an internal wave's first baroclinic mode of wavelengths of ~10–13 km. Nevertheless, during the period of maximum energy fluxes off the north, the second baroclinic mode (wavelength ~7 km) was also important and matched periods of low upwelling index (relaxation of upwelling favorable winds). Typical energy fluxes during summer integrated in the water column, related to the semidiurnal internal tides were 0.12 W/m of the northern site and 0.1 W/m off the southern site. Possible sites of internal wave generation off the south were the Biobío submarine canyon and the slope/shelf break, while off the north, the generation site was the slope/shelf break.
Low‐frequency sea level variability in the southern Indian Ocean and its impacts on the oceanic meridional transports
Authors: Zhuang Wei , Feng Ming , Du Yan , Schiller Andreas , Wang Dongxiao
Published: April 2013

Sea levels in the southern Indian Ocean (SIO) display significant interannual to decadal variability. Off the northwest Australian coast, it has been demonstrated that sea level variability is mostly modulated by remote wind forcing from the tropical Pacific through equatorial and coastal waveguides. In this study, a linear reduced gravity model is used to investigate relative contributions of local wind forcing and remote forcing from the Pacific to the sea level variability of the SIO, with a focus on the western SIO. North of the South Equatorial Current bifurcation latitude (17ºS), model simulated sea levels are well correlated with altimeter observations at the dissipation timescale of about 3 years, suggesting that sea level variability on interannual‐to‐decadal timescales could well be explained by nondispersive baroclinic Rossby wave adjustment. The large sea level variability of the western SIO is primarily caused by westward‐propagating Rossby waves driven by wind stress curl in 70°E‐95°E, with a minor influence from the remote Pacific forcing. To the south, sea level variability at around 20ºS displays lower amplitude due to weaker wind variations at this latitude band, and the modeled sea level variability is weaker than observations. There is a close linkage between the cross‐basin sea level difference at 15ºS and the interior meridional ocean transport across this latitude on decadal timescales, as assessed with outputs from a data‐assimilation model. Thus, the meridional overturning cell of the SIO is influenced by both remote forcing from equatorial Pacific and local winds in the SIO.
Sensitivity and uncertainty analysis of model hypoxia estimates for the Texas‐Louisiana shelf
Authors: Mattern Jann Paul , Fennel, Katja , Dowd Michael
Published: April 2013

Numerical ocean models are becoming increasingly important tools for marine research and for management of marine resources. It is therefore crucial that uncertainty in model predictions and model sensitivity to errors in the model inputs be quantified. We performed a combined sensitivity and uncertainty analysis for a realistic physical‐biological model of the Texas‐Louisiana shelf in the northern Gulf of Mexico. The model simulates the major physical and biological processes involved in the formation of the hypoxic zone that develops on the shelf every summer. With the help of a statistical emulator technique, we introduced uncertainty in selected model inputs and assessed the effects of these uncertainties on the predicted development and spatial distribution of bottom hypoxia. The uncertain inputs we examined belong to two categories: (i) biological inputs including river nutrient concentration, phytoplankton growth rate and initial and boundary conditions of biological variables, and (ii) physical inputs including freshwater river runoff, wind forcing, and mixing coefficients. We show that uncertainty in different inputs has distinct effects on model output which vary in magnitude, time, and space. Uncertainty in physical inputs was found to have a strong impact on estimates of hypoxia, e.g., hypoxic area estimates vary by more than 40%, due to a 20% variation in the freshwater river runoff.
Mesoscale eddies in the southern Gulf of California during summer: Characteristics and interaction with the wind stress
Authors: Lavín M. F. , Castro Rubén , Beier Emilio , Godínez Victor M.
Published: April 2013

The hydrodynamic characteristics of mesoscale eddies of the southern Gulf of California are described from observations made in August 2004. Vertical profiles to ~1000 m were made with a conductivity‐temperature‐depth (CTD) probe with dissolved oxygen and fluorescence sensors, and a lowered acoustic Doppler profiler. The four sampled eddies were aligned along the Gulf axis and had alternating sense of rotation. The Okubo‐Weiss parameter was used to calculate the eddy core radii, and it also revealed that the current field of almost all the sampled area was eddy dominated. The mean radii of the eddy cores varied between 32 and 36 km. Maximum surface velocities were 0.4–0.5 m/s. The anticyclonic eddies were 500–700 m deep, while the cyclones were 450–500 m deep. The depression of the isolines associated with the anticyclonic eddies was ~100 m at depths between 200 and 400 m; however, the thermocline/pycnocline, which spanned from ~20 to 75 m, was domed by ~10 m. The cyclonic eddies lifted the isolines by ~70 m at depths above 500 m, but the isolines were flat above ~60 m. This suggests that different dynamics operate in the layer above the strong and shallow pycnocline. In particular, the wind stress curl can affect circulation and pycnocline topography and, therefore, the patterns that appear in chlorophyll satellite images. In our case, the eddies were detectable in those images because of the chlorophyll‐enhanced streamers along their edges and, for anticyclones, probably because the chlorophyll maximum found at 40–50 m depth was domed following the pycnocline.
Assessment of Southern Ocean water mass circulation and characteristics in CMIP5 models: Historical bias and forcing response
Authors: Sallée J.‐B. , Shuckburgh E. , Bruneau N. , Meijers A. J. S. , Bracegirdle T. J. , Wang Z. , Roy T.
Published: April 2013

The ability of the models contributing to the fifth Coupled Models Intercomparison Project (CMIP5) to represent the Southern Ocean hydrological properties and its overturning is investigated in a water mass framework. Models have a consistent warm and light bias spread over the entire water column. The greatest bias occurs in the ventilated layers, which are volumetrically dominated by mode and intermediate layers. The ventilated layers have been observed to have a strong fingerprint of climate change and to impact climate by sequestrating a significant amount of heat and carbon dioxide. The mode water layer is poorly represented in the models and both mode and intermediate water have a significant fresh bias. Under increased radiative forcing, models simulate a warming and lightening of the entire water column, which is again greatest in the ventilated layers, highlighting the importance of these layers for propagating the climate signal into the deep ocean. While the intensity of the water mass overturning is relatively consistent between models, when compared to observation‐based reconstructions, they exhibit a slightly larger rate of overturning at shallow to intermediate depths, and a slower rate of overturning deeper in the water column. Under increased radiative forcing, atmospheric fluxes increase the rate of simulated upper cell overturning, but this increase is counterbalanced by diapycnal fluxes, including mixed‐layer horizontal mixing, and mostly vanishes.
Solute dispersion in the coastal boundary layer of southern Lake Michigan
Authors: Thupaki Pramod , Phanikumar Mantha S. , Whitman Richard L.
Published: April 2013

We evaluate a three‐dimensional, nested‐grid nearshore model of Lake Michigan for its ability to describe key aspects of hydrodynamics and solute transport using data from a field study conducted in summer 2008. Velocity comparisons with observations from five bottom‐mounted ADCPs at different depths in the coastal boundary layer (CBL) show that the numerical model was able to simulate currents and flow reversals accurately within the inertial boundary layer, however model accuracy reduced close to the shoreline. Power spectra of observed and simulated velocity time series at different locations showed that the hydrodynamic model was able to describe the energy contained in the inertial scales, but over‐predicted turbulent dissipation rates. As a result, model‐predicted values of energy in the smaller, dissipation scales were lower compared to values calculated from observations in the CBL. Differences between energy contained in the observed and simulated velocity spectra increased as the shoreline is approached. Observations showed that vertical variations in the alongshore and cross‐shore velocities were dominated by inertial waves. Inaccuracies in representing energy dissipation rates and processes not explicitly described in the hydrodynamic model (e.g., anisotropy, waves) could potentially contribute to errors in describing transport in the CBL. Measurements from a continuous dye release experiment from a riverine outfall were described using a nearshore model with a mean horizontal dispersion coefficient of 5.6 m2/s. Improved representations of physical processes (such as turbulence, internal waves and wave‐current interactions) can be expected to provide better descriptions of solute transport in the CBL.
Estimation of phytoplankton size fractions based on spectral features of remote sensing ocean color data
Authors: Li Zuchuan , Li Lin , Song Kaishan , Cassar Nicolas
Published: April 2013

Through its influence on the structure of pelagic ecosystems, phytoplankton size distribution (pico‐, nano‐, and micro‐plankton) is believed to play a key role in “the biological pump.” In this paper, an algorithm is proposed to estimate phytoplankton size fractions (PSF) for micro‐, nano‐, and pico‐plankton (fm,fn, andfp, respectively) from the spectral features of remote‐sensing data. From remote‐sensing reflectance spectrum (Rrs(λ)), the algorithm constructs four types of spectral features: a normalizedRrs(λ), band ratios, continuum‐removed spectra, and spectral curvatures. Using support vector machine recursive feature elimination, the algorithm ranks the constructed spectral features andRrs(λ) according to their sensitivities to PSF which is then regressed against the sensitive spectral features through support vector regression. The algorithm is validated with (1) simulatedRrs(λ) and PSF, and (2)Rrs(λ) obtained by Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) and PSF determined from High‐Performance Liquid Chromatography (HPLC) pigments. The validation results show the overall effectiveness of the algorithm in estimating PSF, withR2of (1) 0.938 (fm) for the simulated SeaWiFS data set; and (2) 0.617 (fm), 0.475 (fn), and 0.587 (fp) for the SeaWiFS satellite data set. The validation results also indicate that continuum‐removed spectra and spectral curvatures are the dominant spectral features sensitive to PSF with their wavelengths mainly centered on the pigment‐absorption domain. Global spatial distributions offm,fn, andfpwere mapped with monthly SeaWiFS images. Overall, their biogeographical distributions are consistent with our current understanding that pico‐plankton account for a large proportion of total phytoplankton biomass in oligotrophic regions, nano‐plankton in transitional areas, and micro‐plankton in high‐productivity regions.
Evaluation and optimization of bio‐optical inversion algorithms for remote sensing of Lake Superior's optical properties
Authors: Mouw Colleen B. , Chen Haidi , McKinley Galen A. , Effler Steven , O'Donnell David , Perkins Mary Gail , Strait Chris
Published: April 2013

Satellite remote sensing offers one of the best spatial and temporal observational approaches. However, well‐validated satellite imagery has remained elusive for Lake Superior. Lake Superior's optical properties are highly influenced by colored dissolved organic matter (CDOM), which has hindered the retrieval of chlorophyll concentration through band‐ratio algorithms. This study evaluated seven existing inversion algorithms. The top‐performing inversion algorithm was tuned to a Lake Superior optical data set and applied to satellite imagery. The retrieval of chlorophyll concentration via inversion algorithms was not possible due to errors in derived CDOM absorption being greater than phytoplankton absorption values and the very small contribution of phytoplankton absorption to the overall absorption budget. However, the retrieval of absorption due to CDOM from satellite imagery was encouraging. To ensure that the best satellite remotely sensed reflectance estimates were used in the retrieval of absorption due to CDOM, several atmospheric correction schemes were evaluated. The absorption due to CDOM was greatest in the western arm of Lake Superior and near river mouths and decreased with distance offshore. The absorption due to CDOM had a bimodal distribution over the annual cycle with the greatest peak in fall and a smaller peak in spring.
A near‐inertial mode observed within a Gulf Stream warm‐core ring
Authors: Joyce Terrence M. , Toole John M. , Klein Patrice , Thomas Leif N.
Published: April 2013

Layering of ocean velocity “fine structure” has been coherently observed across the entire extent of a Gulf Stream warm‐core ring using a shipboard acoustic Doppler current profiler system in September 2009 and independently sampled as the ring transited a moored array. Lines of constant velocity phase generally followed isopycnals as they deepened within the ring center. We also observed a clear separation of the vertical structure of the flows associated with the ring (of order 0.5 m/s) with the shorter (200 m) and less energetic (~0.2 m/s) flows of the velocity fine structure, which was further observed to rotate clockwise with increasing depth, consistent with downward propagating near‐inertial waves (NIWs). Observations are consistent with a ring‐scale NIW packet, probably wind forced, that shows enhanced NIW energy within the sloping pycnocline at depths of 300–700 m. Evidence of wind‐forced NIWs within anticylonic eddies in a numerical simulation shows some similar features to our observations, which we try to understand physically with basic WKB‐type wave/current dynamics along the lines of previously published work and a new calculation of NIW trapping within an isolated, baroclinic vortex.
Subantarctic mode water in the southeast Pacific: Effect of exchange across the Subantarctic Front
Authors: Holte J. W. , Talley L. D. , Chereskin T. K. , Sloyan B. M.
Published: April 2013

This study considered cross‐frontal exchange as a possible mechanism for the observed along‐front freshening and cooling between the 27.0 and 27.3 kg m − 3 isopycnals north of the Subantarctic Front (SAF) in the southeast Pacific Ocean. This isopycnal range, which includes the densest Subantarctic Mode Water (SAMW) formed in this region, is mostly below the mixed layer, and so experiences little direct air‐sea forcing. Data from two cruises in the southeast Pacific were examined for evidence of cross‐frontal exchange; numerous eddies and intrusions containing Polar Frontal Zone (PFZ) water were observed north of the SAF, as well as a fresh surface layer during the summer cruise that was likely due to Ekman transport. These features penetrated north of the SAF, even though the potential vorticity structure of the SAF should have acted as a barrier to exchange. An optimum multiparameter (OMP) analysis incorporating a range of observed properties was used to estimate the cumulative cross‐frontal exchange. The OMP analysis revealed an along‐front increase in PFZ water fractional content in the region north of the SAF between the 27.1 and 27.3 kg m − 3 isopycnals; the increase was approximately 0.13 for every 15° of longitude. Between the 27.0 and 27.1 kg m − 3 isopycnals, the increase was approximately 0.15 for every 15° of longitude. A simple bulk calculation revealed that this magnitude of cross‐frontal exchange could have caused the downstream evolution of SAMW temperature and salinity properties observed by Argo profiling floats.
Characteristics of interfacial signatures on a wind‐driven gravity‐capillary wave
Authors: Tsai Wu‐ting , Chen Shi‐ming , Lu Guan‐hung , Garbe Christoph S.
Published: April 2013

Direct numerical simulation of a wind‐driven gravity‐capillary wave and the underlying turbulent flow is conducted to identify the characteristic signatures of various surface parameters, including temperature, gas flux, velocities, and roughness, and to reveal the impacts of the nonbreaking surface waves on these surface flow and tracer parameters. Three characteristic surface signatures and the corresponding flow processes are identified: the carrier gravity wave, the parasitic capillary wavelets, and the elongated streaks. The elongated streaks are induced by both the coherent streamwise vortices formed within the turbulent shear layer and the Langmuir circulations arising from nonlinear interaction between the carrier gravity wave and the drift current. All three surface signatures can be observed in the distributions of various quantities, although some are more apparent than the others. Image‐processing techniques, employing empirical mode decomposition and phase averaging, are developed to decompose the distinct signatures thus to quantify the contributions by the responsible flow processes. It is found that elongated streaks prevail the distribution of surface temperature and gas flux, indicating that Langmuir cells and the coherent eddies contribute to the major interfacial heat and gas transport. These eddies also induced strong cross‐stream velocity divergence at the water surface, which exhibits resemblant elongated distribution as that of gas flux (correlation coefficient ≈ 0.6). High correlation between the surface distributions of temperature and gas flux is observed (correlation coefficient ≈ 0.8 to 0.9), suggesting that the spatial and temporal distribution of surface temperature is a good proxy tracer of interfacial gas transfer.
Wave‐current interactions in a wave‐dominated tidal inlet
Authors: Dodet Guillaume , Bertin Xavier , Bruneau Nicolas , Fortunato André B. , Nahon Alphonse , Roland Aron
Published: April 2013

Wave‐current interactions play a major role in the dynamics of shallow tidal inlets. This study investigates these interactions at a natural inlet, with a strong focus on current‐induced changes on wave propagation. The analysis of hydrodynamic data collected at the Albufeira lagoon, Portugal, revealed spatiotemporal variations of water levels and wave heights along the inlet, attributed to wave‐current interaction processes. We compared the simulations of a coupled wave‐circulation modeling system, computed with and without waves, and propagated with and without current feedback. The wave‐induced setup inside the lagoon represented 7%–15% of the offshore significant wave height. The accuracy of the wave's predictions improved when current feedback was included. During ebb, the currents increased the wave height at the mouth of the inlet (up to 20%) and decreased the wave height in the inlet (up to 40%), due to current‐induced refraction, steepness dissipation, and partial blocking. During flood, the currents decreased the wave height in the inlet (up to 10%) and increased the wave height at the exterior parts of the ebb shoal (up to 10%), due to current‐induced refraction. These effects significantly attenuate seaward sediment fluxes during ebb and contribute to the sediment accretion in the inlet.
Tidal currents in the Adriatic as measured by surface drifters
Authors: Poulain Pierre‐Marie
Published: April 2013

Velocities of surface drifters are analyzed to study tidal currents throughout the Adriatic Sea. Spectral and harmonic analyses indicate that the M2, S2, and K1 constituents dominate. Maps of tidal characteristics show that M2 and S2 are rectilinear currents (reversing tides) aligned with the main axis of the Adriatic basin with maximum amplitude (~7 cm/s for M2 and ~4 cm/s for S2) in the northern area off the Istrian Peninsula. Near the northern coast, semidiurnal tidal currents decrease in amplitude and rotate in the counterclockwise sense. Near the Po River delta, M2 (S2) motions rotate in the counterclockwise (clockwise) sense. S2 rotation is also counterclockwise near the northeastern coast. M2 phases increase from about 130° on the eastern Croatian coast to 190° on the western Italian side. S2 phases range from 150° to 200°. In the middle and southern Adriatic, the semidiurnal tides are small (~1 cm/s). The diurnal tidal currents (K1) are strong across the basin at the levels of Monte Conero and the Gargano Peninsula with speed larger than 5 cm/s and mainly clockwise rotation, and also in coastal areas (e.g., on the Albanian shelf and close to the Otranto Channel). Phases increase from the east to the west coasts (by as much as 150°). These new results compare satisfactorily with previous observations and numerical simulations, although tidal amplitudes are under‐estimated with respect to mooring measurements. They extend for the first time the description of the Adriatic tidal currents to the entire basin based on direct velocity observations.
Formation of salinity maximum water and its contribution to the overturning circulation in the North Atlantic as revealed by a global general circulation model
Authors: Qu Tangdong , Gao Shan , Fukumori Ichiro
Published: April 2013

The formation of salinity maximum water in the North Atlantic is investigated using a simulated passive tracer and its adjoint. The results reveal that most salinity maximum water in the North Atlantic comes from the northwestern part of the subtropical gyre, and direct contribution from the evaporation‐precipitation maximum region via the surface Ekman current is minor. Water originating from the evaporation‐precipitation maximum region has to recirculate in the subtropical gyre before entering the sea surface salinity maximum region from the northwest. Once subducted, some portion (~10%) of the salinity maximum water enters the equatorial region in the shallow subtropical cell, but most (~70%) of it appears to turn northward to join the North Atlantic Deep Water. The latter pathway involves a three‐dimensional circulation. When the warm, fresh surface water flows northward along the western boundary, it turns eastward in the northern subtropical gyre. As a result of the large excess of evaporation over precipitation, this water gradually gains its salinity on the route, until it reaches the sea surface salinity maximum region in the central subtropical gyre. From there, the salinity maximum water is subducted and flows back to the western boundary in the depth range of the thermocline. With its high‐salinity nature, a major portion of this water penetrates into the subpolar region and directly contributes to the deep thermohaline circulation.
Seismic reflection imaging of shallow oceanographic structures
Authors: Piété Helen , Marié Louis , Marsset Bruno , Thomas Yannick , Gutscher Marc‐André
Published: April 2013

Multichannel seismic (MCS) reflection profiling can provide high lateral resolution images of deep ocean thermohaline fine structure. However, the shallowest layers of the water column (z < 150 m) have remained unexplored by this technique until recently. In order to explore the feasibility of shallow seismic oceanography (SO), we reprocessed and analyzed four multichannel seismic reflection sections featuring reflectors at depths between 10 and 150 m. The influence of the acquisition parameters was quantified. Seismic data processing dedicated to SO was also investigated. Conventional seismic acquisition systems were found to be ill‐suited to the imaging of shallow oceanographic structures, because of a high antenna filter effect induced by large offsets and seismic trace lengths, and sources that typically cannot provide both a high level of emission and fine vertical resolution. We considered a test case, the imagery of the seasonal thermocline on the western Brittany continental shelf. New oceanographic data acquired in this area allowed simulation of the seismic acquisition. Sea trials of a specifically designed system were performed during the ASPEX survey, conducted in early summer 2012. The seismic device featured: (i) four seismic streamers, each consisting of six traces of 1.80 m; (ii) a 1000 J SIG sparker source, providing a 400 Hz signal with a level of emission of 205 dB re 1 μPa @ 1 m. This survey captured the 15 m thick, 30 m deep seasonal thermocline in unprecedented detail, showing images of vertical displacements most probably induced by internal waves.
Assessment of Southern Ocean mixed‐layer depths in CMIP5 models: Historical bias and forcing response
Authors: Sallée J.‐B. , Shuckburgh E. , Bruneau N. , Meijers A. J. S. , Bracegirdle T. J. , Wang Z.
Published: April 2013

The development of the deep Southern Ocean winter mixed layer in the climate models participating in the fifth Coupled Models Intercomparison Project (CMIP5) is assessed. The deep winter convection regions are key to the ventilation of the ocean interior, and changes in their properties have been related to climate change in numerous studies. Their simulation in climate models is consistently too shallow, too light and shifted equatorward compared to observations. The shallow bias is mostly associated with an excess annual‐mean freshwater input at the sea surface that over‐stratifies the surface layer and prevents deep convection from developing in winter. In contrast, modeled future changes are mostly associated with a reduced heat loss in winter that leads to even shallower winter mixed layers. The mixed layers shallow most strongly in the Pacific basin under future scenarios, and this is associated with a reduction of the ventilated water volume in the interior. We find a strong state dependency for the future change of mixed‐layer depth, with larger future shallowing being simulated by models with larger historical mixed‐layer depths. Given that most models are biased shallow, we expect that most CMIP5 climate models might underestimate the future winter mixed‐layer shallowing, with important implications for the sequestration of heat, and gases such as carbon dioxide, and therefore for climate.
Sea ice motion and open water area at the Ronne Polynia, Antarctica: Synthetic aperture radar observations versus model results
Authors: Hollands T. , Haid V. , Dierking W. , Timmermann R. , Ebner L.
Published: April 2013

This study deals with observations and simulations of the evolution of coastal polynias focusing on the Ronne Polynia. We compare differences in polynia extent and ice drift patterns derived from satellite radar images and from simulations with the Finite Element Sea Ice Ocean Model, employing three atmospheric forcing data sets that differ in spatial and temporal resolution. Two polynia events are analyzed, one from austral summer and one from late fall 2008. The open water area in the polynia is of similar size in the satellite images and in the model simulations, but its temporal evolution differs depending on katabatic winds being resolved in the atmospheric forcing data sets. Modeled ice drift is slower than the observed and reveals greater turning angles relative to the wind direction in many cases. For the summer event, model results obtained with high‐resolution forcing are closer to the drift field derived from radar imagery than those from coarse resolution forcing. For the late fall event, none of the forcing data yields outstanding results. Our study demonstrates that a dense (1–3 km) model grid and atmospheric forcing provided at high spatial resolution ( < 50 km) are critical to correctly simulate coastal polynias with a coupled sea‐ice ocean model.
Field observations of wave‐induced streaming through a submerged seagrass (Posidonia oceanica) meadow
Authors: Luhar Mitul , Infantes Eduardo , Orfila Alejandro , Terrados Jorge , Nepf Heidi M.
Published: April 2013

This paper reports the findings of a 2 week field campaign designed to study wave‐induced flows within a meadow of Posidonia oceanica at water depth 9 m. Previous laboratory experiments suggest that waves induce a mean mass drift in the direction of wave propagation (“streaming”) through submerged canopies of vegetation. This paper provides the first field measurements of this wave‐induced streaming. During periods of high wave activity, streaming flows with magnitudes as high as 20% of the near‐bed oscillatory velocity were measured within the meadow. In addition to presenting field measurements of wave‐induced streaming, this paper also considers the damping of wave‐induced oscillatory flow within the seagrass meadow. Oscillatory velocities measured within the meadow were reduced by less than 30% relative to those above the meadow over the duration of the study. This is in agreement with previous laboratory and field measurements which show that oscillatory flows are damped less within submerged canopies compared to unidirectional flows. Existing analytical models underpredict the magnitude of the streaming flow and overpredict oscillatory velocity reductions. These discrepancies are thought to arise because the drag generated by flexible seagrasses moving with wave‐induced flow is not well described.
Sediment‐generated noise and bed stress in a tidal channel
Authors: Bassett Christopher , Thomson Jim , Polagye Brian
Published: April 2013

Tidally driven currents and bed stresses can result in noise generated by moving sediments. At a site in Admiralty Inlet, Puget Sound, Washington State (USA), peak bed stresses exceed 20 Pa. Significant increases in noise levels are attributed to mobilized sediments at frequencies from 4–30 kHz with more modest increases noted from 1–4 kHz. Sediment‐generated noise during strong currents masks background noise from other sources, including vessel traffic. Inversions of the acoustic spectra for equivalent grain sizes are consistent with qualitative data of the seabed composition. Bed stress calculations using log layer, Reynolds stress, and inertial dissipation techniques generally agree well and are used to estimate the shear stresses at which noise levels increase for different grain sizes. Regressions of the acoustic intensity versus near‐bed hydrodynamic power demonstrate that noise levels are highly predictable above a critical threshold despite the scatter introduced by the localized nature of mobilization events.
Two modes of sea‐ice gravity drainage: A parameterization for large‐scale modeling
Authors: Turner Adrian K. , Hunke Elizabeth C. , Bitz Cecilia M.
Published: April 2013

We present a new one‐dimensional parameterization of gravity drainage implemented in an all‐new thermodynamic component of the Los Alamos Sea Ice Model (CICE), based on mushy layer theory. We solve a set of coupled, nonlinear equations for sea‐ice temperature (enthalpy) and salinity using an implicit Jacobian‐free Newton‐Krylov method. Time resolved observations of gravity drainage show two modes of desalination during growth. Rapid drainage occurs in a thin region just above the ice/ocean interface, while slower drainage occurs throughout the ice. Parameterizations are designed to represent each of these modes and work simultaneously. Near the interface, desalination occurs primarily via the fast drainage, while slow drainage continues to desalinate ice above the interface. The rapid desalination is convectively driven and is parameterized based on a consideration of flow driven upward within the mush and downward in chimneys, modified by the Rayleigh number. The slow desalination is represented as a simple relaxation of bulk salinity to a value based on a critical porosity for sea‐ice permeability. It is shown that these parameterizations can adequately reproduce observational data from laboratory experiments and field measurements.
Annual maximum water levels from tide gauges: Contributing factors and geographic patterns
Authors: Merrifield Mark A. , Genz Ayesha S. , Kontoes Christopher P. , Marra John J.
Published: April 2013

Hourly time series from a quasi‐global set of 145 tide gauges are used to investigate annual maximum water levels at each station. High water levels are deconstructed into (1) a predicted tidal component, (2) a seasonal component, (3) a low‐frequency nontidal residual that accounts for sea level variability at time scales greater than a month but less than a year, and (4) a high‐frequency nontidal residual that captures variability particularly associated with storms at time scales greater than a month. The time‐averaged annual maximum water level correlates significantly with, and scales as 2.5 times, the water level standard deviation at the tide gauge stations. This relationship is used to estimate time‐averaged annual maximum water level on a nearly continuous global scale (excluding ice‐covered polar regions) by specifying variance maps of the tides from a tide model, the seasonal and low‐frequency residual components from satellite altimetry sea surface height, and the high‐frequency residual component from an atmospheric reanalysis product. The variance fields are combined to estimate time‐averaged annual maximum water levels that compare well with observed values at the tide gauge stations. Spatial patterns of annual maximum water levels and relative contributions from the tides and nontidal residual components are considered.
Seasonal variability of water masses and transport on the Antarctic continental shelf and slope in the southeastern Weddell Sea
Authors: Graham Jennifer A. , Heywood Karen J. , Chavanne Cédric P. , Holland Paul R.
Published: April 2013

An array of five moorings was deployed from February 2009 to February 2010 across the Antarctic shelf and slope in the southeastern Weddell Sea (~18°W). Observations demonstrate the key processes responsible for variability in water masses and transport in the region. Rapid fluctuations in temperature and salinity throughout the year are linked with variability in wind stress over the array. This causes the deepening or shoaling of the pycnocline, past the depth of the moorings. In the upper 500 m, the seasonal cycle in salinity shows freshening in autumn, with the strongest freshening at the shallowest mooring (~250 m), furthest on‐shelf. The sea ice concentration over the array exceeds 90% during this period and contributes a positive salt flux into the ocean during autumn. Freshening begins during strong along‐shore (easterly) winds in late April 2009. This demonstrates that variations in Ekman transport and wind‐driven mixing play a key role in determining the salinity of shelf waters around Antarctica. Transport of the Antarctic Slope Current also shows a seasonal cycle with a maximum during late April. Model simulations show the importance of along‐shore advection, as the arrival of a fresh anomaly from upstream determines the timing of the salinity minimum at the array. These processes are likely to be important for other regions around the Antarctic continent.
Structure and surface properties of eddies in the southeast Pacific Ocean
Authors: Holte James , Straneo Fiamma , Moffat Carlos , Weller Robert , Farrar J. Thomas
Published: April 2013

A number of studies have posited that coastally generated eddies could cool the southeast Pacific Ocean (SEP) by advecting cool, upwelled waters offshore. We examine this mechanism by characterizing the upper‐ocean properties of mesoscale eddies in the SEP with a variety of observations and by estimating the surface‐layer eddy heat flux divergence with satellite data. Cyclonic and anticyclonic eddies observed during two cruises featured deep positive salinity anomalies along the 26.5 kg m−3isopycnal, indicating that the eddies had likely trapped and transported coastal waters offshore. The cyclonic eddies observed during the cruises were characterized by shoaling isopycnals in the upper 200 m and cool near‐surface temperature anomalies, whereas the upper‐ocean structure of anticyclonic eddies was more variable. Using a variety of large‐scale observations, including Argo float profiles, drifter records, and satellite sea surface temperature fields, we show that, relative to mean conditions, cyclonic eddies are associated with cooler surface temperatures and that anticyclonic eddies are associated with warmer surface temperatures. Within each data set, the mean eddy surface temperature anomalies are small and of approximately equal magnitude but opposite sign. Eddy statistics drawn from satellite altimetry data reveal that cyclonic and anticyclonic eddies occur with similar frequency and have similar average radii in the SEP. A satellite‐based estimate of the surface‐layer eddy heat flux divergence, while large in coastal regions, is small when averaged over the SEP, suggesting that eddies do not substantially contribute to cooling the surface layer of the SEP.
Mechanisms of aerosol‐forced AMOC variability in a state of the art climate model
Authors: Menary Matthew B. , Roberts Christopher D. , Palmer Matthew D. , Halloran Paul R. , Jackson Laura , Wood Richard A. , Müller Wolfgang A. , Matei Daniela , Lee Sang‐Ki
Published: April 2013

Mechanisms of sustained multidecadal changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) are investigated in a set of simulations with a new state‐of‐the‐art Earth system model. Anthropogenic aerosols have previously been highlighted as a potential mitigator of AMOC weakening. In this study, we explain the oceanic mechanisms behind how anthropogenic aerosols force a strengthening of the AMOC by up to 20% in our state‐of‐the‐art Earth system model. This strengthening is driven via atmospheric circulation changes which subsequently modulate the salinity budget of the North Atlantic subpolar gyre. Gradual salinification occurs via increased evaporation and decreased fluxes of ice through the Fram Straits. A component of the salinification is a positive feedback from the AMOC bringing more saline water northwards from the subtropical Atlantic. Salinification of the subpolar gyre results in increased deep convection and a strengthening of the AMOC. Following a reduction in aerosol concentrations, the AMOC rapidly weakens, approximately 3 times faster than in the case where anthropogenic aerosol concentrations had never been increased. Similarities and differences with available observational records and long term reanalysis products are also discussed.
Southern Ocean nutrient trapping and the efficiency of the biological pump
Authors: Primeau François W. , Holzer Mark , DeVries Timothy
Published: April 2013

We present a data‐assimilated model of the ocean's phosphorus cycle that is constrained by climatological phosphate, temperature, salinity, sea‐surface height, surface heat and freshwater fluxes, as well as chlorofluorocarbon‐11(CFC‐11) and natural Δ14C. Export production is estimated to be 5.8±2.0×1012 mol P/yr of which (26±6)% originates in the Southern Ocean (SO) south of 40°S. The biological pump efficiency, defined as the proportion of the ocean's phosphate inventory that is regenerated, is (39±7)%. Dividing the SO south of 40°S into a sub‐Antarctic zone (SANTZ) and an Antarctic zone (ANTZ) separated by the latitude of maximum Ekman divergence, we estimate that the SANTZ and ANTZ account, respectively, for (23±5)% and (3±1)% of global export production, (17±4)% and (3±1)% of the regenerated nutrient inventory, and (31±1)% and (43±5)% of the preformed nutrient inventory. Idealized SO nutrient depletion experiments reveal a large‐scale transfer of nutrients into circumpolar and deep waters and from the preformed to the regenerated pool. In accord with the concept of the biogeochemical divide, we find that nutrient drawdown in the ANTZ is more effective than in the SANTZ for increasing the efficiency of the biological pump, while having a smaller impact on production in regions north of 40°S. Complete SO nutrient drawdown would allow the biological pump to operate at 94% efficiency by short circuiting the transport of nutrients in northward Ekman currents, leading to a trapping of nutrients in circumpolar and deep waters that would decrease production outside the SO by approximately 44% while increasing it in the SO by more than 725%.
Relationships between inherent optical properties and the depth of penetration of solar radiation in optically complex coastal waters
Authors: Cunningham Alex , Ramage Leanne , McKee David
Published: April 2013

The attenuation of downward planar irradiance can be quantified by
, the diffuse attenuation coefficient calculated from the surface to the depth where the irradiance Ed at wavelength λ falls to 10% of its surface value. Theoretical studies by Gordon (1989) and Lee et al. (2005a) suggest that
can be derived from the absorption coefficient, a(λ) and the backscattering coefficient, bb(λ), using equations incorporating either the solar zenith angle (θa) or the subsurface distribution function (D0) and empirical coefficients derived by radiative transfer modeling. These results have not, however, been validated against in situ measurements. We have therefore assessed the performance of both models using measurements of a(λ), bb(λ), and
for 100 stations in UK coastal waters. Best results were obtained from the Lee et al. (2005a) model, for which over 90% of the predicted
values in the 440 nm to 665 nm range were within ±0.1 m−1 of those measured in situ. A strong linear relationship (R2> 0.95, mean relative difference 5.4%) was found between
at 490 nm and the reciprocal of the depth of the midpoint of the euphotic zone (z10%, PAR). This allowed (z10%, PAR) to be predicted from measured values of a(490 nm), bb(490 nm) and θa, using the Lee et al. model as an intermediate step, with an RMS error of 1.25 m over the 2.5–25.0 m range covered by our data set.
Temporal changes and impacts of submarine fresh groundwater discharge to the coastal environment: A decadal case study in Toyama Bay, Japan
Authors: Hatta Mariko , Zhang Jing
Published: April 2013

Recent dramatic changes of freshwater systems in high latitudes will allow Submarine Fresh Groundwater Discharge (SFGD) to play a more important role in the coastal environment; especially in that SFGD will directly effect heat flux. Toyama Bay, along the central western Japan coast, is a suitable and representative case study area to estimate SFGD flux using hydrographic properties since multiple high‐flow rate SFGD sites exist there. Salinities averaged over the water column (depth range 10–100 m) measured during research cruises in June 2003 and May 2005 show lower levels in the eastern than the western area in this bay. Together with monthly hydrographic properties over a 10 year period (1987–1998), the low‐salinity water mass in the eastern area exists consistently but distinctly and varies systematically, as does nutrient flux, affected by SFGD more than riverine input. SFGD fluxes in June 2003 (1 × 108 m3 month−1) and May 2005 (<1 × 108 m3 month−1) were estimated using a box model, which is divided into a shallow box (0–40 m) and a deeper box (40–100 m). The monthly flux ratio between the SFGD and the river inputs is 13% in June, comparable to higher values reported in other global studies. Our results demonstrate that the box model analysis, based on hydrographic observations in coastal areas, is an efficient approach that can be used to estimate SFGD fluxes between the land and ocean.
Observed near‐surface flows under all tropical cyclone intensity levels using drifters in the northwestern Pacific
Authors: Chang Yu‐Chia , Chen Guan‐Yu , Tseng Ruo‐Shan , Centurioni Luca R. , Chu Peter C.
Published: April 2013

Data from drifters of the surface velocity program and tropical cyclones (TCs) of the Joint Typhoon Warning Center during 1985–2009 were analyzed to demonstrate strong currents under various storm intensities such as category‐4 to −5, category‐2 to −3, and tropical storm to category‐1 TCs in the northwestern Pacific. Current speeds over 2.0 m s−1 are observed under major TCs with the strongest mean currents to the right of the storm track. This study provides the characterization of the near‐surface velocity response to all recorded TCs, and agrees roughly with Geisler's theory (1970). Our observations also verify earlier modeling results of Price (1983).
Quantifying the length‐scale dependence of surf zone advection
Authors: Wilson Greg W. , Özkan‐Haller H. Tuba , Holman Robert A.
Published: April 2013

We investigate the momentum balance in the surf zone, in a setting which is weakly varying in the alongshore direction. Our focus is on the role of nonlinear advective terms. Using numerical experiments, we find that advection tends to counteract alongshore variations in momentum flux, resulting in more uniform kinematics. Additionally, advection causes a shifting of the kinematic response in the direction of flow. These effects are strongest at short alongshore length scales, and/or strong alongshore‐mean velocity. The length‐scale dependence is investigated using spectral analysis, where the effect of advective terms is treated as a transfer function applied to the solution to the linear (advection‐free) equations of motion. The transfer function is then shown to be governed by a nondimensional parameter which quantifies the relative scales of advection and bottom stress, analogous to a Reynolds Number. Hence, this parameter can be used to quantify the length scales at which advective terms, and the resulting effects described above, are important. We also introduce an approximate functional form for the transfer function, which is valid asymptotically within a restricted range of length scales.