bluuebunny/crossref_metadata_2025 huggingface dataset:
Transfer functions of the well‐aquifer systems response to atmospheric loading and Earth tide from low to high‐frequency band
Authors: Lai Guijuan , Ge Hongkui , Wang Weilai
Published: April 2013

The transfer functions of the well‐aquifer systems response to atmospheric loading and Earth tide can be used to calculate the well‐aquifer properties. Due to the low signal‐to‐noise ratio, the study on barometric response at frequencies higher than 8 cycles per day (cpd for short) is almost blank. Using the recorded water level and barometric pressure as well as the corresponding theoretical tidal volumetric strain at 17 well stations in China, we obtained continuous barometric and tidal responses of the well‐aquifer systems by cross‐spectra estimation. It shows that the barometric responses are stable at low frequencies (0.1–0.5 cpd), while fluctuate at tidal frequencies (0.5–8 cpd). In the high‐frequency band (8–30 cpd), by stacking the transfer functions to suppress the noise, we obtained stable barometric responses for the first time. According to the low‐ and high‐frequency barometric responses, we can better judge whether the aquifers are confined in the timescale that we focused and whether the wellbore storage effect can be ignored, and expect to determine the fluid flow properties of the aquifers more reliably. For the three aquifers whose water table drainage and wellbore storage effects are ignored, we used the barometric and tidal responses to estimate their formation properties, which are consistent with previous results. The tidal strain sensitivities are related to the aquifer lithology, which are mainly controlled by the compressibility of the porous matrixes with different porosities and different aspect ratio fractures.
Experimental constraints on the thermodynamics and sound velocities of hcp‐Fe to core pressures
Authors: Murphy Caitlin A. , Jackson Jennifer M. , Sturhahn Wolfgang
Published: April 2013

We report the high‐pressure thermoelastic and vibrational thermodynamic parameters for hexagonal close‐packed iron (ε‐Fe), based on nuclear resonant inelastic X‐ray scattering and in situ X‐ray diffraction experiments at 300 K. Long data collection times, high‐energy resolution, and quasi‐hydrostatic sample conditions produced a high‐statistical quality data set that comprises the volume‐dependent phonon density of states (DOS) of ε‐Fe at eleven compression points. From the integrated phonon DOS, we determine the Lamb‐Mössbauer factor (fLM), average force constant (Φ), and vibrational entropy (Svib) of ε‐Fe to pressures relevant to Earth's outer core. We find fLM = 0.923 ± 0.001 at 171 GPa, suggesting restricted thermal atomic motion at large compressions. We use Φ to approximate ε‐Fe's pressure‐ and temperature‐dependent reduced isotopic partition function ratios (β‐factors), which provide information about the partitioning behavior of iron isotopes in equilibrium processes involving solid ε‐Fe. In addition, we use the volume dependence of Svib to determine the product of ε‐Fe's vibrational thermal expansion coefficient and isothermal bulk modulus, which we find to be pressure‐independent and equal to 5.70 ± 0.05 MPa/K at 300 K. Finally, from the low‐energy region of each phonon DOS, we determine the Debye sound velocity (vD), from which we derive the compressional (vP) and shear (vS) sound velocities of ε‐Fe. We find vD = 5.60 ± 0.06, vP = 10.11 ± 0.12, and vS = 4.99 ± 0.06 km/s at 171 GPa, thus providing a new tight constraint on the density dependence of ε‐Fe's sound velocities to outer core pressures.
Rayleigh wave constraints on the structure and tectonic history of the Gamburtsev Subglacial Mountains, East Antarctica
Authors: Heeszel David S. , Wiens Douglas A. , Nyblade Andrew A. , Hansen Samantha E. , Kanao Masaki , An Meijan , Zhao Yue
Published: April 2013

The Gamburtsev Subglacial Mountains (GSM), located near the center of East Antarctica, remain one of the most enigmatic mountain ranges on Earth. A lack of direct geologic samples renders their tectonic history almost totally unconstrained. We utilize teleseismic Rayleigh wave data from a 2 year deployment of broadband seismic stations across the region to image shear velocity structure and analyze the lithospheric age of the GSM and surrounding regions. We solve for 2‐D phase velocities and invert these results for 3‐D shear velocity structure. We perform a Monte Carlo simulation to improve constraints of crustal thickness and shear velocity structure. Beneath the core of the GSM, we find crustal thickness in excess of 55 km. Mantle shear velocities remain faster than global average models to a depth of approximately 250 km, indicating a thick lithospheric root. Thinner crust and slower upper mantle velocities are observed beneath the Lambert Rift System and the Polar Subglacial Basin. When compared with phase velocity curves corresponding to specific tectonothermal ages elsewhere in the world, average phase velocity results for the GSM are consistent with regions of Archean‐Paleoproterozoic origin. Combined with radiometric ages of detrital zircons found offshore, these results indicate a region of old crust that has undergone repeated periods of uplift and erosion, most recently during the Mesozoic breakup of Gondwana. Lower crustal seismic velocities imply a moderately dense lower crust beneath the core of the GSM, but with lower density than suggested by recent gravity models.
Recovery of plate coupling at a ruptured asperity
Authors: Yoshida Shingo , Nakatani Masao , Kato Naoyuki
Published: April 2013

Earlier earthquake cycle simulations using rate‐ and state‐dependent friction (RSF) law have revealed that frictional coupling at a ruptured asperity starts immediately after dynamic slip because of logarithmically time‐dependent healing. However, some Global Positioning System (GPS) inversion studies of the 2011 great Tohoku‐Oki earthquake suggest that afterslip continued for 7–8 months or longer in the deep seismogenic interface including asperities where M7 earthquakes repeatedly occurred. Nakatani and Scholz [] introduced the intrinsic cutoff time tcx in the logarithmically time dependent healing and pointed out that a long tcx could lead to delayed start of frictional coupling. Assuming RSF laws into which tcx was incorporated, we conducted a numerical simulation using a block spring model. We defined the time required to restore frictional coupling as Tcpl measured from the instant of dynamic slip. The value of Tcpl for tcx = 10−3 s was almost the same as Tcpl in the conventional law, which was on the order of 10 min in the case of the aging law. It increased approximately in proportion to tcx for short tcx (≤1 s), whereas Tcpl was not so sensitive to tcx for a long tcx (>1 s). Moreover, a long tcx cannot be reasonably assumed because the maximum slip velocity during dynamic slip decreased with increasing tcx, limiting the largest possible Tcpl to be months at most. Friction laws with two state variables, each having a different cutoff time, were also investigated. The maximum slip velocity was determined by the smaller cutoff time, and Tcpl was affected by the larger cutoff time. As a result, longer Tcpl became possible, although the values are still on time scale of months.
Earthquake clusters in southern California II: Classification and relation to physical properties of the crust
Authors: Zaliapin Ilya , Ben‐Zion Yehuda
Published: April 2013

This is a second paper in a study of statistical identification and classification of earthquake clusters using a relocated catalog of 1981–2011 seismicity in southern California and synthetic catalogs produced by the Epidemic Type Aftershock Sequence model. Here we focus on classification of event families—statistically significant clusters composed of foreshocks, mainshocks, and aftershocks—that are detected with the methodology discussed in part I of the study. The families are analyzed using their representation as time oriented tree graphs. The results (1) demonstrate that the clustering associated with the largest earthquakes, m > 7, is statistically different from that of small‐to‐medium earthquakes; (2) establish the existence of two dominant types of small‐to‐medium magnitude earthquake families—burst‐like and swarm‐like sequences—and a variety of intermediate cluster forms obtained as a mixture of the two dominant types; (3) suggest a simple new quantitative measure for identifying the cluster type based on its topological structure; (4) demonstrate systematic spatial variability of the cluster characteristics on a scale of tens of kilometers in relation to heat flow and other properties governing the effective viscosity of a region; and (5) establish correlation between the family topological structure and a dozen of metric properties traditionally considered in the literature (number of aftershocks, duration, spatial properties, b‐value, parameters of Omori‐Utsu and Båth law, etc.). The burst‐like clusters likely reflect highly brittle failures in relatively cold regions, while the swarm‐like clusters are likely associated with mixed brittle‐ductile failures in regions with relatively high temperature and/or fluid content. The results of this and paper I may be used to develop improved region‐specific hazard estimates and earthquake forecasts.
Earthquake clusters in southern California I: Identification and stability
Authors: Zaliapin Ilya , Ben‐Zion Yehuda
Published: April 2013

We use recent results on statistical analysis of seismicity to present a robust method for comprehensive detection and analysis of earthquake clusters. The method is based on nearest‐neighbor distances of events in space‐time‐energy domain. The method is applied to a 1981–2011 relocated seismicity catalog of southern California having 111,981 events with magnitudes m ≥ 2 and corresponding synthetic catalogs produced by the Epidemic Type Aftershock Sequence (ETAS) model. Analysis of the ETAS model demonstrates that the cluster detection results are accurate and stable with respect to (1) three numerical parameters of the method, (2) variations of the minimal reported magnitude, (3) catalog incompleteness, and (4) location errors. Application of the method to the observed catalog separates the 111,981 examined earthquakes into 41,393 statistically significant clusters comprised of foreshocks, mainshocks, and aftershocks. The results reproduce the essential known statistical properties of earthquake clusters, which provide overall support for the proposed technique. In addition, systematic analysis with our method allows us to detect several new features of seismicity that include (1) existence of a significant population of single‐event clusters, (2) existence of foreshock activity in natural seismicity that exceeds expectation based on the ETAS model, and (3) dependence of all cluster properties, except area, on the magnitude difference of events from mainshocks but not on their absolute values. The classification of detected clusters into several major types, generally corresponding to singles, burst‐like and swarm‐like sequences, and correlations between different cluster types and geographic locations is addressed in a companion paper.
Multiple transition zone seismic discontinuities and low velocity layers below western United States
Authors: Tauzin B. , van der Hilst R. D. , Wittlinger G. , Ricard Y.
Published: April 2013

With P‐to‐S converted waves recorded at seismic stations of the U.S. Transportable Array, we image the fine structure of upper mantle and transition zone (TZ) beneath the western U.S. We map the topographies of seismic discontinuities by stacking data by common conversion points along profiles. Systematic depth and amplitude measurements are performed not only for the well‐known “410” and “660” interfaces but also for minor seismic discontinuities identified around 350, 590, and 630 km depths. The amplitude of conversion suggests shear wave velocity (Vs) increase by 4% at the 410 and the 660. The observed 660 velocity contrast is smaller than expected from the 6% in IASP91 but consistent with a pyrolitic model of mantle composition. The Gorda plate, subducted under northern California, is tracked to the TZ where it seems to flatten and induce uplift of the 410 under northern Nevada. Maps of 410/660 amplitude/topography reveal that the TZ is anomalous beneath the geographical borders of Washington, Oregon, and Idaho, with (1) a thickened TZ, (2) a sharp change in depth of the 660, (3) a reduced 410 conversion amplitude in the North, and (4) a positive “630” discontinuity. Such anomalous structure might be inherited from the past history of plate subduction/accretion. A thinned TZ under the Yellowstone suggests higher‐than‐average temperatures, perhaps due to a deep thermal plume. Both the “350” and the “590” negative discontinuities extend over very large areas. They might be related either to an increased water content in the TZ, a significant amount of oceanic material accumulated through the past 100 Myr, or both.
Melt and shear interactions in the lithosphere: Theory and numerical analysis of pure shear extension
Authors: Mohajeri Arash , Finzi Yaron , Muhlhaus Hans , Rosenbaum Gideon
Published: April 2013

We present a linear instability analysis and numerical simulations describing deformation and melt patterns in pure shear extension of a partly molten rock. Our models implement numerical techniques that enable strong strain localization and are applied to study melt‐strain interactions during continental rifting. Our results show that instabilities can initiate with either strain localization or melt localization, followed by a coupled evolution of melt and shear bands driven by a strong melt‐viscosity‐shear feedback. This indicates that a local increase in melt fraction due to segregation and/or local melting promotes strain localization and may lead to the formation of large shear bands. Melt‐shear interactions can therefore enable rifting where tectonic forces are not sufficient to induce melt‐free rifting, resulting in lubricated faults, but not necessarily observed volcanism. Finally, our simulations reveal significant asymmetry in melt segregation around localized shear bands, providing new insights into melt distribution across rift boundary faults and other extensional structures.
Damage and seismic velocity structure of pulverized rocks near the San Andreas Fault
Authors: Rempe Marieke , Mitchell Thomas , Renner Jörg , Nippress Stuart , Ben‐Zion Yehuda , Rockwell Thomas
Published: April 2013

A combination of seismic refraction tomography, laboratory ultrasonic velocity measurements, and microstructural observations was used to study the shallow velocity structure of a strand of the San Andreas fault (SAF) just south of Littlerock, California. The examined site has a strongly asymmetric damage structure with respect to the SAF core. The conglomerates to the southwest show little to no damage, whereas a ~100 m wide damage zone exists to the northeast with a ~50 m wide zone of pulverized granite adjacent to the fault core. Seismic P‐wave velocities of the damaged and pulverized granite were investigated over a range of scales. In situ seismic velocity imaging was performed on three overlapping profiles normal to the SAF with lengths of 350 m, 50 m, and 25 m. In the laboratory, ultrasonic velocities were measured on centimeter‐ to decimeter‐sized samples taken along the in situ profiles. The samples were also investigated microstructurally. Micro‐scale fracture damage intensifies with increasing proximity to the fault core, allowing a subdivision of the damage zone into several sections. Laboratory‐derived velocities in each section display varying degrees of anisotropy, and combined with microfracture analysis suggest an evolving damage fabric. Pulverized rocks close to the fault exhibit a preferred fault‐parallel orientation of microfractures, resulting in the lowest P‐wave velocity orientated in fault‐perpendicular direction. Closest to the fault, pulverized rocks exhibit a gouge‐like fabric that is transitional to the fault core. Comparison of absolute velocities shows a scaling effect from field to laboratory for the intact rocks. A similar scaling effect is absent for the pulverized rocks, suggesting that they are dominated by micro‐scale damage. Fault‐parallel damage fabrics are consistent with existing models for pulverized‐rock generation that predict strong dynamic reductions in fault‐normal stress. Our observations provide important constraints for theoretical models and imaging fault damage properties at depth using remote methods.
Extracting surface wave attenuation from seismic noise using correlation of the coda of correlation
Authors: Zhang Jian , Yang Xiaoning
Published: April 2013

Extracting surface wave travel time information from the cross‐correlation (CC) of seismic ambient noise has been a great success and remains fast growing. However, it is still challenging to exploit the amplitude content of the noise CC. Although spatial average is able to constrain somewhat meaningful attenuation using noise CC amplitudes, clear bias is observed when spatially varying attenuation is estimated with the traditional noise CC calculation methods. Perhaps the key lies in the development of novel techniques that can mitigate the effect of the uneven distribution of natural noise sources. In this paper, we propose a new method to use the correlation of the coda of correlation of noise (C3) for amplitude measurement. We examine the ability of the method to retrieve surface wave attenuation using data from selected line array stations of the USArray. By comparing C3‐derived attenuation coefficients with those estimated from earthquake data, we demonstrate that C3 effectively reduces bias and allows for more reliable attenuation estimates from noise. This is probably because of the fact that the coda of noise correlation contains more diffused noise energy, and thus, the C3 processing effectively makes the noise source distribution more homogeneous. When selecting auxiliary stations for C3 calculation, we find that stations closer to noise sources (near the coast) tend to yield better signal‐to‐noise ratios. We suggest to preprocess noise data using a transient removal and temporal flattening method, to mitigate the effect of temporal fluctuation of the noise source intensity, and to retain relative amplitudes. In this study, we focus our analysis on 18 s measurements.
Numerical and experimental investigation of buoyancy‐driven dissolution in vertical fracture
Authors: Oltéan Constantin , Golfier Fabrice , Buès Michel Antoine
Published: April 2013

Alteration and dissolution resulting from reactive fluid flows in vertical fracture are investigated from numerical and laboratory experiments. Due to fluid density contrast, buoyancy effects are observed leading to significant changes in fracture geometry. Buoyant and forced convection forces act here in the same direction. The experiments were carried out at two different flow rates. When buoyancy forces are preponderant (low injection flow rate), the dissolution rate increases with the vertical distance. By contrast, for convection‐dominated transport (high injection flow rate), a uniform dissolution is observed. Using numerical simulations, four dissolution regimes were identified. The fracture patterns observed strongly depend on the characteristic dimensionless numbers of the process, respectively, the Richardson, Damköhler, and Péclet numbers. The good agreement between numerical simulations and experimental results in terms of fracture patterns highlights the capability of the numerical model to describe the complex coupling between flow dynamics, buoyancy, and chemical reaction. Finally, a 3‐D behavior diagram is constructed to illustrate these interactions and as a means of relating the appropriate dimensionless parameters to the morphological changes observed.
Systematic relocation of seismicity on Hawaii Island from 1992 to 2009 using waveform cross correlation and cluster analysis
Authors: Matoza Robin S. , Shearer Peter M. , Lin Guoqing , Wolfe Cecily J. , Okubo Paul G.
Published: April 2013

The analysis and interpretation of seismicity from mantle depths to the surface play a key role in understanding how Hawaiian volcanoes work. We present results from a comprehensive and systematic re‐analysis of waveforms from 130,902 seismic events recorded by the U.S. Geological Survey Hawaiian Volcano Observatory permanent seismic network from January 1992 to March 2009. We compute high‐precision relative relocations for 101,390 events (77% of all events considered) using waveform cross correlation and cluster analysis, resulting in a multiyear systematically processed catalog of seismicity for all of Hawaii Island. The 17 years of relocated seismicity exhibit a dramatic sharpening of earthquake clustering along faults, streaks, and magmatic features, permitting a more detailed understanding of fault geometries and volcanic and tectonic processes. Our relocation results are generally consistent with previous studies that have focused on more specific regions of Hawaii. The relocated catalog includes crustal seismicity at Kilauea and its rift zones, seismicity delineating crustal detachment faults separating volcanic pile and old oceanic crust on the flanks of Kilauea and Mauna Loa, events along inferred magma conduits, and events along inferred mantle fault zones. The relocated catalog is available for download in the supporting information.
The role of viscous magma mush spreading in volcanic flank motion at Kīlauea Volcano, Hawai‘i
Authors: Plattner C. , Amelung F. , Baker S. , Govers R. , Poland M.
Published: April 2013

Multiple mechanisms have been suggested to explain seaward motion of the south flank of Kīlauea Volcano, Hawai‘i. The consistency of flank motion during both waxing and waning magmatic activity at Kīlauea suggests that a continuously acting force, like gravity body force, plays a substantial role. Using finite element models, we test whether gravity is the principal driver of long‐term motion of Kīlauea's flank. We compare our model results to geodetic data from Global Positioning System and interferometric synthetic aperture radar during a time period with few magmatic and tectonic events (2000–2003), when deformation of Kīlauea was dominated by summit subsidence and seaward motion of the south flank. We find that gravity‐only models can reproduce the horizontal surface velocities if we incorporate a regional décollement fault and a deep, low‐viscosity magma mush zone. To obtain quasi steady state horizontal surface velocities that explain the long‐term seaward motion of the flank, we find that an additional weak zone is needed, which is an extensional rift zone above the magma mush. The spreading rate in our model is mainly controlled by the magma mush viscosity, while its density plays a less significant role. We find that a viscosity of 2.5 × 1017–2.5 × 1019 Pa s for the magma mush provides an acceptable fit to the observed horizontal surface deformation. Using high magma mush viscosities, such as 2.5 × 1019 Pa s, the deformation rates remain more steady state over longer time scales. These models explain a significant amount of the observed subsidence at Kīlauea's summit. Some of the remaining subsidence is probably a result of magma withdrawal from subsurface reservoirs.
Global teleconnections in the oceanic phosphorus cycle: Patterns, paths, and timescales
Authors: Holzer Mark , Primeau François W.
Published: April 2013

Nutrient transport and productivity teleconnections with the Southern Ocean are diagnosed in a data‐assimilated circulation model coupled to a jointly optimized simple phosphorus cycling model. The North Atlantic has the strongest extratropical teleconnections with the Southern Ocean: phosphate (PO4) last utilized in the Southern Ocean sustains 29 ± 6% of the subpolar and 14 ± 6% of the subtropical, new production in the North Atlantic. A PO4 path‐density diagnostic shows that these teleconnections are mediated by thermocline paths and reveals that most paths to anywhere north of 40°S lie in the deep Pacific. Forcing nearly complete Southern‐Ocean nutrient utilization increases the overall number of paths to anywhere north of 40°S, but reduces the number of paths from the Pacific to the North Atlantic by trapping nutrients in return paths to the Southern Ocean. At the same time, the mean export‐to‐uptake transit times to anywhere north of 40°S increase, while the mean transit times to the North Atlantic decrease. Correspondingly, the amount of North‐Atlantic production sustained by Southern‐Ocean export increases in spite of decreased total production in response to Southern‐Ocean nutrient trapping. The distributions of export‐to‐uptake, export‐to‐surface, and surface‐to‐surface transit times are computed and summarized in terms of their mean transit times and their mean interior residence times. The combined particle and advective–diffusive transport of nutrients is characterized by broad, skewed transit‐time distributions, which result in mean residence times much longer than the mean transit times, in turn much longer than the most probable transit times.
Small‐scale variability of the cross‐shelf flow over the outer shelf of the Ross Sea
Authors: Kohut Josh , Hunter Elias , Huber Bruce
Published: April 2013

The importance of cross‐shelf transport across the Ross Sea on local and remote processes has been well documented. In the Ross Sea, mid‐water intrusions of Circumpolar Deep Water (CDW) are modified by shelf water near the shelf break to form Modified Circumpolar Deep Water (MCDW). In 2010–2011, we deployed multi‐platform technologies focused on this MCDW intrusion in the vicinity of Mawson and Pennell Banks to better understand its role in ecosystem processes across the shelf. The high‐resolution time and space sampling provided by an underwater glider, a short‐term mooring, and a ship‐based survey highlight the scales over which these critical cross‐shelf transport processes occur. MCDW cores were observed as small‐scale well‐defined features over the western slopes of Pennell and Mawson Banks. The mean transport along Pennell Bank was estimated to be about 0.24 Sv but was highly variable in time (hours to days). The observations suggest that the core of MCDW is transported by a predominately barotropic flow that follows topography around the banks toward the south until the slope of the bank flattens and the warmer water moves up and over the bank. This pathway is shown to link the source MCDW with an area of high productivity over the shallows of Pennell Bank.
Pathways of the North Pacific Intermediate Water identified through the tangent linear and adjoint models of an ocean general circulation model
Authors: Fujii Yosuke , Nakano Toshiya , Usui Norihisa , Matsumoto Satoshi , Tsujino Hiroyuki , Kamachi Masafumi
Published: April 2013

This study develops a strategy for tracing a target water mass, and applies it to analyzing the pathway of the North Pacific Intermediate Water (NPIW) from the subarctic gyre to the northwestern part of the subtropical gyre south of Japan in a simulation of an ocean general circulation model. This strategy estimates the pathway of the water mass that travels from an origin to a destination area during a specific period using a conservation property concerning tangent linear and adjoint models. In our analysis, a large fraction of the low salinity origin water mass of NPIW initially comes from the Okhotsk or Bering Sea, flows through the southeastern side of the Kuril Islands, and is advected to the Mixed Water Region (MWR) by the Oyashio current. It then enters the Kuroshio Extension (KE) at the first KE ridge, and is advected eastward by the KE current. However, it deviates southward from the KE axis around 158°E over the Shatsky Rise, or around 170°E on the western side of the Emperor Seamount Chain, and enters the subtropical gyre. It is finally transported westward by the recirculation flow. This pathway corresponds well to the shortcut route of NPIW from MWR to the region south of Japan inferred from analysis of the long‐term freshening trend of NPIW observation. Copyright © 2013 John Wiley & Sons, Ltd.
Enhanced gas fluxes in small sea ice leads and cracks: Effects on CO 2 exchange and ocean acidification
Authors: Steiner N. S. , Lee W. G. , Christian J. R.
Published: April 2013

Earth system models generally partition grid cells into an ice‐covered and an open‐water area and view the ice‐covered area as a barrier to gas fluxes. However, observations suggest that exchange in cracks and small leads can be much higher in localized areas than expected under similar conditions in open water. While these models project a significant retreat in sea ice cover, affecting air‐sea CO 2 exchange and consequently ocean acidification, the simple grid cell partitioning might underestimate the actual CO 2 exchange. A sensitivity study with the Canadian Earth System Model (CanESM2) shows that enhanced CO 2 exchange in sea ice areas in the Arctic Ocean increases the uptake in fall and winter, allowing more continuous equilibration and hence reduced uptake in summer. The reduction in summer also hints at a limited CO 2‐uptake capacity of Arctic surface waters. Retreating sea ice in the future leads to a similar shift in the seasonal cycle. The annual mean carbon uptake of the Arctic Ocean north of 68°N changes only slightly with the enhanced flux parameterization ( < 3%). For the central Arctic (north of 80°N) the change is up to 21% and accelerates ocean acidification, e.g. locally, surface waters could reach aragonite undersaturation 1–2 decades earlier. The differences between standard and enhanced exchange test runs become less pronounced in the future due to the reduced ice cover and extended open water season. For the Antarctic sea ice zone where the CO 2 flux is out of the ocean, the enhanced exchange increases outgassing, slightly slowing down acidification.
On the dynamics of the South China Sea deep circulation
Authors: Lan Jian , Zhang Ningning , Wang Yu
Published: April 2013

In the South China Sea (SCS), the deep water is confined to a bowel‐type trench, and the maximum depth is approximately 4700 m. The Luzon Strait is the only deep connection between the SCS and the Pacific Ocean, with the deepest sill at about 2400 m in the Bashi Channel. Using the Hybrid Coordinate Ocean Model, this study gives a good description of the SCS deep circulation. The most obvious features are basin‐scale cyclonic gyre and western intensification. The gyre is elliptical shaped, and its major axis is northeast‐southwest. A numerical experiment is designed to investigate what could be possibly responsible for the SCS deep circulation. The results reveal that the deepwater overflow through the Luzon Strait controls over the basin‐scale circulation structure in the deep SCS basin. The driving mechanism is elucidated based on the potential vorticity (PV) integral constraint, which means the PV inflow across the boundary is balanced by the net PV dissipation along the boundary.
A new observationally motivated Richardson number based mixing parametrization for oceanic mesoscale flow
Authors: Forryan Alexander , Martin Adrian P. , Srokosz Meric A. , Popova Ekaterina E. , Painter Stuart C. , Renner Angelika H. H.
Published: April 2013

Ocean models require subgrid‐scale parametrizations of vertical mixing expressed in terms of a quantity that is easily diagnosable from model output, such as the Richardson number. To date parametrizing mixing for low (<1) Richardson number flows, such as the Equatorial Undercurrent, has received the most attention. Here a new Richardson number parametrization is proposed that provides estimates of vertical turbulent diffusivity in the high Richardson number stratified shear flow that is associated with mesoscale ocean features such as eddies and fronts. This parametrization is based on direct observations of vertical turbulent diffusivity from three separate ocean regions in the North Atlantic and Southern Ocean and is found to be robust for values of the Richardson number greater than 1 at depths below the ocean surface boundary layer. The new parametrization gives substantially improved agreement with the observed mixing in the presence of mesoscale ocean features compared to existing Richardson number parametrizations.
Effects of transient bottom water currents and oxygen concentrations on benthic exchange rates as assessed by eddy correlation measurements
Authors: Holtappels Moritz , Glud Ronnie N. , Donis Daphne , Liu Bo , Hume Andrew , Wenzhöfer Frank , Kuypers Marcel M. M.
Published: April 2013

Eddy correlation (EC) measurements in the benthic boundary layer (BBL) allow estimating benthic O2 uptake from a point distant to the sediment surface. This noninvasive approach has clear advantages as it does not disturb natural hydrodynamic conditions, integrates the flux over a large foot‐print area and allows many repetitive flux measurements. A drawback is, however, that the measured flux in the bottom water is not necessarily equal to the flux across the sediment‐water interface. A fundamental assumption of the EC technique is that mean current velocities and mean O2 concentrations in the bottom water are in steady state, which is seldom the case in highly dynamic environments like coastal waters. Therefore, it is of great importance to estimate the error introduced by nonsteady state conditions. We investigated two cases of transient conditions. First, the case of transient O2 concentrations was examined using the theory of shear flow dispersion. A theoretical relationship between the change of O2 concentrations and the induced vertical O2 flux is introduced and applied to field measurements showing that changes of 5–10 μM O2 h−1 result in transient EC‐fluxes of 6–12 mmol O2 m−2 d−1, which is comparable to the O2 uptake of shelf sediments. Second, the case of transient velocities was examined with a 2D k‐ε turbulence model demonstrating that the vertical flux can be biased by 30–100% for several hours during changing current velocities from 2 to 10 cm s−1. Results are compared to field measurements and possible ways to analyze and correct EC‐flux estimates are discussed.
Gap‐leaping Kuroshio and blocking westward‐propagating Rossby wave and eddy in the Luzon Strait
Authors: Lu Jiuyou , Liu Qinyu
Published: April 2013

Based on analysis of both observational data and data‐assimilation model product, it is shown that there exists a gap‐leaping path of the Kuroshio in the Luzon Strait. Numerical results of two sets of Hybrid Coordinate Ocean Model twin experiments indicate that bottom topography and islands in Luzon Strait exert control of Kuroshio's gap‐leaping behavior, especially existence of sharp northeast cape of Philippine Island is one of the most important factors for the gap‐leaping Kuroshio path in model simulation. Corresponding to the gap‐leaping Kuroshio, there is steep westward shoaling of thermocline in Luzon Strait. The enhanced upper‐layer stratification in South China Sea due to westward shoaling thermocline results in strong zonal potential vorticity (PV) gradient (one order higher than ß‐induced planetary PV gradient), and PV isolines are always parallel to the gap‐leaping path of the Kuroshio. This PV front acts as a dynamic barrier in the Luzon Strait, blocking the westward propagating Rossby waves and eddies from Pacific. This blocking effect is verified through sea‐surface height anomaly spectrum analysis, Radon Transform based Feature‐Tracking method, and eddy identification and tracking method. Numerical twin experiments demonstrate from another point of view that the gap‐leaping Kuroshio can efficiently block the westward propagating Rossby wave and eddy energy, while the fluctuating Kuroshio is less efficient for blocking.
Comment on “Current separation and upwelling over the southeast shelf of Vietnam in the South China Sea” by Chen et al.
Authors: Dippner Joachim W. , Bombar Deniz , Loick‐Wilde Natalie , Voss Maren , Subramaniam Ajit
Published: April 2013

In a recent paper, Chen et al. (2012) showed that the offshore current in front of the Vietnamese upwelling area in the South China Sea (SCS) is caused by an encounter of southward buoyancy‐driven coastal current and tidal rectified currents from the southwest. These findings seem not in agreement with in‐situ observations. The mechanism for the formation of the offshore current has its origin in the inter‐annual variability of atmospheric forcing. El Niño Southern Oscillation events modulate the northern position of Inter‐Tropical Convergence Zone and the intensity of both, SW monsoon and upwelling. Strong upwelling influences the spatial distribution of characteristic water masses which results in a blocking of the near coastal northward propagation of the plume of River Mekong and the formation of an offshore current.
Bathymetric effects on estuarine plume dynamics
Authors: Lee Jungwoo , Valle‐Levinson Arnoldo
Published: April 2013

The influence of bathymetry on an estuary plume at an estuary‐shelf transition is studied with a three‐dimensional ocean circulation model. To understand the response of the plume to bathymetry, several types of estuarine shapes and shelf geometries were adopted in this numerical study. The channel's shape and its width‐to‐depth aspect ratio affected the fate of the plume by determining flow characteristics inside the estuary. Moreover, the bathymetry of the shelf such as the shelf slope and the direction of a submarine channel defined the plume characteristics on the shelf. An estuarine channel with a triangular cross section generated relatively stronger exchange flows at midestuary than a rectangular cross section, which resulted in a larger surface plume over the shelf. The extension of the submarine channel onto the shelf favored increased plume water transport out to the shelf, a result of reduced frictional effects on the shelf. The orientation of the submarine channel changed the direction of the plume over the shelf, with no additional external forces. Two fronts developed at the edges of the submarine channel because of enhanced lateral shears in the flow. When the estuary was relatively wide compared to the internal Rossby radius (Kelvin number Ke ≥ 5), or when the relative strength of the freshwater discharge compared to the estuary width was weak (Rossby number Ro ≤ 0.05), the coastal plume did not expand up‐shelf. In fact, results indicated that freshwater up‐shelf transport in a coastal current, moving against Coriolis’ accelerations, was proportional to Ro.
Three‐dimensional mixture simulations of flow over dynamic rippled beds
Authors: Penko A.M. , Calantoni J. , Rodriguez‐Abudo S. , Foster D.L. , Slinn D.N.
Published: April 2013

A three‐dimensional mixture theory model for flow and sediment transport in the seafloor boundary layer, SedMix3D, simulated the flow over and the resulting sediment entrainment and evolution of rippled beds. SedMix3D treats the fluid‐sediment mixture as a continuum of varying density and viscosity with the concentration of sediment and velocity of the mixture simulated by the Navier‐Stokes equations coupled with a sediment flux equation for the mixture. Model validation was performed by comparing simulated time‐dependent flow quantities and bulk flow statistics with measurements obtained in the laboratory under scaled forcing conditions. Two‐dimensional planes extracted from a three‐dimensional simulation were compared to observations made using planar Particle Image Velocimetry (PIV) in a laboratory flume. The simulated results of time‐averaged velocities and time‐dependent quantities of vorticity and swirling strength were in good agreement with the observations. The model was used to analyze the three‐dimensionality of vortex formation and ejection produced by oscillatory flow over vortex ripples, a process that cannot be observed in the laboratory with planar PIV measurements. The three‐dimensional simulated results showed that the swirling strength varied significantly in the cross‐flow direction, indicating that the vortices formed and dissipated non‐uniformly due to random fluctuations. Subsequently, an order of magnitude difference in offshore sediment flux was observed using two different methods to calculate sediment fluxes (spatially averaging and at a point). The results suggest that while a two‐dimensional plane may be sufficient to examine the general hydrodynamics over ripples, three‐dimensional analysis is necessary for a complete understanding of sediment transport.
Yanai waves in the western equatorial Indian Ocean
Authors: Chatterjee A. , Shankar D. , McCreary J.P. , Vinayachandran P. N.
Published: April 2013

Observations and models have shown the presence of intraseasonal fluctuations in 20–30‐day and 10–20‐day bands in the equatorial Indian Ocean west of 60°E (WEIO). Their spatial and temporal structures characterize them as Yanai waves, which we label low‐frequency (LFYW) and high‐frequency (HFYW) Yanai waves, respectively. We explore the dynamics of these intraseasonal signals, using an ocean general circulation model (Modular Ocean Model) and a linear, continuously stratified model. Yanai waves are forced by the meridional wind τy everywhere in the WEIO most strongly during the monsoon seasons. They are forced both directly in the interior ocean and by reflection of the interior response from the western boundary; interference between the interior and boundary responses results in a complex surface pattern that propagates eastward and has nodes. Yanai waves are also forced by instabilities primarily during June/July in a region offshore from the western boundary (52–55°E). At that time, eddies, generated by barotropic instability of the Southern Gyre, are advected southward to the equator. There, they generate a westward‐propagating, cross‐equatorial flow field, veq, with a wave number/frequency spectrum that fits the dispersion relation of a number of Yanai waves, and these waves are efficiently excited. Typically, Yanai waves associated with several baroclinic modes are excited by both wind and eddy forcing; and typically, they superpose to create beams that carry energy vertically and eastward along ray paths. The same processes generate LFYWs and HFYWs, and hence, their responses are similar; differences are traceable to the property that HFYWs have longer wavelengths than LFYWs for each baroclinic mode.
Effects of stretching on maintaining the Kuroshio meander
Authors: Kurogi Masao , Hasumi Hiroyasu , Tanaka Yukio
Published: April 2013

Using a two‐way nested‐grid ocean general circulation model driven by a repeat annual cycle forcing, the Kuroshio path variation is examined by varying the strength of wind forcing. For climatological or 10% weaker wind forcing, both large‐meander (LM) and non‐large‐meander (NLM) paths alternately appear, with each type of path continuing for a few years to a decade. This timescale and path transition processes are generally consistent with observations. For the LM path, the main balance in the depth‐integrated vorticity equation for the upper ocean is shown to be between the beta, advection, and stretching terms. The stretching term is comparable to and has the same sign as the beta term at the western side of the meandering part, indicating that the stretching term has the effects of shortening the wavelength and stabilizing the LM path. The stretching term is mainly determined by downwelling caused by crossing of the upper and deep flows. Contrary to the above two cases, only the NLM path appears for 10% stronger wind forcing. It is suggested that the strength of climatological wind forcing is near the upper limit that allows the LM path to occur. In this study, the control experiment is carried out by choosing parameterizations and parameter values to reproduce the Kuroshio path variation as realistic as possible. Therefore, the results from the two sensitivity experiments are expected to represent a realistic response of the Kuroshio path variation.
Water and suspended sediment division at a stratified tidal junction
Authors: Buschman F. A. , van der Vegt M. , Hoitink A. J. F. , Hoekstra P.
Published: April 2013

Tidal junctions play a crucial role in the transport of water, salt, and sediment through a delta distributary network. Water, salt and sediment are exchanged at tidal junctions, thereby influencing the transports in the connecting branches and the overall dynamics of the system. This paper presents observations of water, salt and sediment transports in three channels that connect at a stratified tidal junction. Flow variation in one channel was found to lag behind flow variation in a connected channel by more than 1 h, which is largely attributed to channel length differences from the junction to the sea. The water columns in the three channels were periodically stratified during spring tide, whereas the salinity structure represented a salt wedge during neap tide. Salinity differences between the three channels were substantial. The channels contain water bodies of different salinity and act largely independently. Flow velocities in the upper and lower layers differed substantially. Flow in the lower layer was generally in the direction of acceleration produced by the baroclinic pressure gradient. Interestingly, baroclinic pressure gradients were sometimes directed landward, indicating the presence of saltier water at the land side of the estuary. In sharp channel bends close to the junction, secondary flow was strongest at the highest axial flow velocity during spring tide. In one channel bend, these circulations steered the suspended sediment toward the inner bend, which affected the suspended sediment division.
Surf zone cross‐shore boundary layer velocity asymmetry and skewness: An experimental study on a mobile bed
Authors: Berni C. , Barthélemy E. , Michallet H.
Published: April 2013

An experimental study on a physical model of a beach in a two‐dimensional wave flume was designed to investigate velocity nonlinearities in the wave boundary layer. The cross‐shore velocity was measured in the surf zone along a vertical profile every 3mm from free‐stream elevation down to the still bed level. The skewness and the asymmetry of the phase averaged velocity were computed at each elevation. Observations indicate that the free‐stream asymmetry transforms into bottom velocity skewness. A linear experimental relation between free‐stream asymmetry to skewness ratio and bottom skewness to free‐stream skewness ratio is established. A theoretical linear relationship is discussed, which predicts the phase lead of the bottom velocity. This phase lead is also determined by Fourier analyzing the velocity time series. The first two Fourier components yield the same phase lead at the bed that is found to be about 30° and nearly constant over all the experiments made.