bluuebunny/crossref_metadata_2025 huggingface dataset:
Physical mechanisms for vertical‐CLVD earthquakes at active volcanoes
Authors: Shuler Ashley , Ekström Göran , Nettles Meredith
Published: April 2013

Many volcanic earthquakes large enough to be detected globally have anomalous focal mechanisms and frequency content. In a previous study, we examined the relationship between active volcanism and the occurrence of a specific type of shallow, non‐double‐couple earthquake. We identified 101 earthquakes with vertical compensated‐linear‐vector‐dipole (vertical‐CLVD) focal mechanisms that took place near active volcanoes between 1976 and 2009. The majority of these earthquakes, which have magnitudes 4.3 ≤ MW ≤ 5.8, are associated with documented episodes of volcanic unrest. Here we further characterize vertical‐CLVD earthquakes and explore possible physical mechanisms. Through teleseismic body‐wave analysis and examination of the frequency content of vertical‐CLVD earthquakes, we demonstrate that these events have longer source durations than tectonic earthquakes of similar magnitude. We examine the covariance matrix for one of the best‐recorded earthquakes and confirm that the isotropic and pure vertical‐CLVD components of the moment tensor cannot be independently resolved using our long‐period seismic data set. Allowing for this trade‐off, we evaluate several physical mechanisms that may produce earthquakes with deviatoric vertical‐CLVD moment tensors. We find that physical mechanisms related to fluid flow and volumetric changes are incompatible with seismological, geological, and geodetic observations of vertical‐CLVD earthquakes. However, ring‐faulting mechanisms explain many characteristics of vertical‐CLVD earthquakes, including their seismic radiation patterns, source durations, association with volcanoes in specific geodynamic environments, and the timing of the earthquakes relative to volcanic activity.
On robust and reliable automated baseline corrections for strong motion seismology
Authors: Melgar Diego , Bock Yehuda , Sanchez Dominga , Crowell Brendan W.
Published: April 2013

Computation of displacements from strong motion inertial sensors is to date an open problem. Two distinct methodologies have been proposed to solve it. One involves baseline corrections determined from the inertial data themselves and the other a combination with other geophysical sensors such as GPS. Here we analyze a proposed automated baseline correction algorithm using only accelerometer data and compare it to the results from the real‐time combination of strong motion and GPS data. The analysis is performed on 48 collocated GPS and accelerometers in Japan that recorded the 2011 Mw 9.0 Tohoku‐oki earthquake. We study the time and frequency domain behavior of both methodologies. We find that the error incurred from automated baseline corrections that rely on seismic data alone is complex and can be large in both the time and frequency domains of interest in seismological and engineering applications. The GPS/accelerometer combination has no such problems and can adequately recover broadband strong motion displacements for this event. The problems and ambiguities with baseline corrections and the success of the GPS/accelerometer combination lead us to advocate for instrument collocations as opposed to automated baseline correction algorithms for accelerometers.
Influence of offset weak zones on the development of rift basins: Activation and abandonment during continental extension and breakup
Authors: Chenin Pauline , Beaumont Christopher
Published: April 2013

We use numerical modelling to investigate reactivation of inherited Offset Weak Zones (OWZ) in continental crust and Mantle Weak Zones (MWZ) to form offset rift basins during continental rifting and breakup. Offset rift basins are basins that are set off/offset from the main rift/locus of breakup. Weak zones embedded in a stiff layer are preferentially and rapidly reactivated, whereas the same zones are either ignored or slowly reactivated when embedded in pliable layers. Here stiff implies a nonlinear flow law with a high stress exponent (n  > ∼  10,000), a plastic material, and pliable means a low stress exponent (n ∼ 2–5) as in ductile, power‐law creep of rocks. Whether offset rift basins form during rifting of a composite lithosphere (i.e., comprising stiff and pliable layers) depends on the competition between necking instabilities that develop at the weak zones in the stiff layers, and the coupling between the stiff and pliable layers. Stiff/cratonic lithosphere results in early localization of the deformation at the MWZ, rapid necking and breakup without developing offset rift basins. In contrast, warm pliable lithosphere develops significant offset basins and has protracted rifting because the MWZ is now embedded in a pliable layer. We also investigate the influence of OWZ dip, sedimentation, and the sensitivity of reactivation to the distance from OWZ to the MWZ, and to the size of the MWZ. A tectonic rifting styles diagram is used to show that the model results agree with natural examples.
Tectonic development of the Samail ophiolite: High‐precision U‐Pb zircon geochronology and Sm‐Nd isotopic constraints on crustal growth and emplacement
Authors: Rioux Matthew , Bowring Samuel , Kelemen Peter , Gordon Stacia , Miller Robert , Dudás Frank
Published: April 2013

New high‐precision single grain U‐Pb zircon geochronology and whole rock Nd isotopic data provide insight into the magmatic and tectonic development of the Samail ophiolite. The analyzed rocks can be broadly divided into two groups based on their structural position, dates, and isotopic composition: an older group related to on‐axis magmatism and a younger group of post‐ridge dikes, sills, and stocks. On‐axis gabbros, tonalites and trondhjemites yielded Th‐corrected 206Pb/238U dates from 96.441 ± 0.062 to 95.478 ± 0.056 Ma. These dates, combined with dates from Rioux et al. (2012), suggest that most of the ophiolite crust formed at an oceanic spreading center in <1 Ma. The post‐ridge intrusions come from all depths in the crust, the upper mantle, and the metamorphic sole. Post‐ridge gabbros, tonalites, and trondhjemites from the crust and mantle yielded Th‐corrected 206Pb/238U dates of 95.405 ± 0.062 to 95.077 ± 0.062 Ma. A small trondhjemitic pod from the metamorphic sole yielded younger Th‐corrected 206Pb/238U dates of 94.90 ± 0.38 to 94.69 ± 0.12 Ma. Isotopic data suggest two distinct sources for the post‐ridge magmas: five of the gabbros and tonalites from the crust have εNd(96 Ma) = 6.90 ± 0.12 to 7.88 ± 0.16, and two trondhjemites from the upper mantle and metamorphic sole have εNd(96 Ma) = −7.77 ± 0.08 and −7.01 ± 0.16. The negative εNd(t) and U‐Pb dates from the mantle dike require that subduction or thrusting was established below the ophiolite ≤0.25–0.5 Ma after formation of the crust. The bimodal isotopic composition of post‐ridge magmas may reflect coeval decompression and/or fluid fluxed melting of the mantle and melting, dehydration, or assimilation of sediment in the down going plate at this time. The new data place temporal constraints on mid‐ocean ridge and supra‐subduction zone models for ophiolite formation.
Observing Iceland's Eyjafjallajökull 2010 eruptions with the autonomous NASA Volcano Sensor Web
Authors: Davies Ashley Gerard , Chien Steve , Doubleday Joshua , Tran Daniel , Thordarson Thorvaldur , Gudmundsson Magnús T. , Höskuldsson Ármann , Jakobsdóttir Steinunn S. , Wright Robert , Mandl Daniel
Published: April 2013

Between 24 March and 5 June 2010, the Hyperion hyperspectral imager and Advanced Land Imager (ALI) on NASA's Earth Observing 1 (EO‐1) spacecraft obtained an unprecedented sequence of 50 observation pairs of the eruptions at Fimmvörðuháls and Eyjafjallajökull, Iceland. This high acquisition rate was possible only through the use of data flow streamlined by using the autonomously operating NASA Volcano Sensor Web (VSW). The VSW incorporates notifications of volcanic activity from multiple sources to retask EO‐1 and process Hyperion data to extract eruption parameters from high spatial and spectral resolution visible and short‐wavelength infrared data. Physical changes in eruption style and magnitude were charted as the eruptions ran their course. Rapid data downlink and automatic data‐processing algorithms generated a variety of products which are compared with estimates from ground‐based observations and post‐eruption in situ measurements. Estimates of effusion rate from heat loss measurements underestimate actual effusion rate (while still following broad eruption rate trends) but are closer to in situ estimates for effusive eruptions (Fimmvörðuháls) than explosive, ash‐rich eruptions (Eyjafjallajökull). During the later stages of the 2010 eruption, VSW‐generated products were rapidly delivered to end‐users in Iceland to aid in the assessment of risk and hazard. The success of the VSW led to Icelandic Meteorological Office (IMO) in situ sensors being incorporated into the VSW, and in May 2011 an IMO seismic alert autonomously triggered EO‐1 observations of a new eruption at Grímsvötn volcano. Finally, the VSW demonstrates an autonomy‐driven, multi‐asset, spacecraft retasking and data processing system that maximizes science return, a desirable capability for future NASA missions.
Interaction of ascending magma with pre‐existing crustal fractures in monogenetic basaltic volcanism: an experimental approach
Authors: Le Corvec Nicolas , Menand Thierry , Lindsay Jan
Published: April 2013

Magma transport through dikes is a major component of the development of monogenetic volcanic fields. These volcanic fields are characterized by numerous volcanic centers, each typically resulting from a single eruption. Therefore, magma must be transported from source to surface at different places, which raises the question of the relative importance of (1) the self‐propagation of magma through pristine rock and (2) the control exerted by pre‐existing fractures. To address this issue, we have carried out a series of analogue experiments to constrain the interaction of a propagating dike through a medium with pre‐existing fractures. The experiments involved the injection of air into an elastic gelatin solid, which was previously cut into its upper part to simulate pre‐existing fractures. The volume of the dikes, their distance from the fractures, and the ambient stress field were systematically varied to assess their influence on potential dike‐fracture interactions. The results show that distance and angle between dikes and fractures influence these interactions and the dike trajectory. Dike geometry and dynamics are also affected by both the presence of the fractures and the dike volume; dikes propagating in between fractures tend to decelerate. In nature, interactions are expected for dikes and fractures separated by less than about 200 m, and dikes with a volume less than about 10−2 km3 would experience a velocity decrease. These results highlight the influence of pre‐existing fractures on the mechanics and dynamics of dikes. These heterogeneities must be considered when studying the transport of magmas within the crust.
Late‐stage magma flow in a shallow felsic reservoir: Merging the anisotropy of magnetic susceptibility record with numerical simulations in La Gloria Pluton, central Chile
Authors: Gutiérrez F. , Payacán I. , Gelman S. E. , Bachmann O. , Parada M. A.
Published: April 2013

La Gloria Pluton is a 10 Myr old epizonal intrusion located in the southern Andes. We present anisotropy of magnetic susceptibility data that indicate a magnetic fabric that is mainly oblate. We find that lineations are weak and have a N‐NW trend with a nearly horizontal dip, while foliations are more pronounced, have NW trends, and have dips that vary from vertical at the walls of the intrusion to horizontal at the center and under the roof of the chamber. To interpret these magmatic fabrics, we developed a time‐dependent 2‐D magmatic fluid dynamic numerical simulation. Our model is calibrated with MELTS and accounts for the coupled processes of cooling, crystallization, and degassing of a magma chamber. Simulations indicate that the resulting convective flow pattern in the crystallizing reservoir is consistent with the magnetic fabric, which is largely produced in the shear zone between the convecting liquid‐dominated core and the growing solidification fronts adjacent to the walls. The magnetic fabric records the last increment of strain induced by convective magmatic flow in the cooling reservoir during crystallization at the rheological magma locking point along solidification fronts. Despite the small size of the pluton, the core of the chamber remains thermally insulated from the colder host rocks, surviving up to 20 kyr above the solidus, which allows enough time for the extraction of residual leucogranitic melt and partial late magmatic reactive recrystallization. The results of the simulations are also consistent with the previously determined compositional and mineralogical zonation patterns in the pluton.
Evaluation of Wasatch fault segmentation and slip rates using Lake Bonneville shorelines
Authors: Jewell Paul W. , Bruhn Ronald L.
Published: April 2013

Analysis of Lake Bonneville shorelines using lidar digital elevation data challenges accepted models of Wasatch fault deformation since the late Pleistocene. While footwall deformation of the Weber segment of the Wasatch fault is consistent with back‐rotation of the footwall block and greatest displacement rate toward the center of the segment, shorelines along the footwall of the Salt Lake City segment decrease in elevation toward the interior and are highest at the segment boundaries, an opposite pattern of footwall deformation than predicted for boundaries arresting or strongly inhibiting displacement during earthquakes. The spatial pattern of footwall rebound implies that some of the proposed persistent fault segment boundaries do not stop earthquake ruptures that originate on adjacent fault segments, nor constrain ruptures initiated within the Salt Lake City segment. Net vertical fault displacement at the boundary between the Salt Lake and Provo segments is 16–20 m over the past 16.3–18.5 ka, corresponding to a vertical displacement rate of 0.8–1.2 mm/yr, a net fault slip rate of 2.0–2.8 mm/yr, and horizontal extension rate of 1.8–2.6 mm/yr on the 25° west‐southwest dipping fault that forms the southern Salt Lake City segment boundary. Shoreline analysis suggests isostatic rebound caused by a drop in lake level was concentrated during a relatively short (~2000 year) time period following the Bonneville flood at ~16 ka. Lidar‐derived topography in conjunction with robust geomorphic datums improves our ability to map deformation associated with lithospheric flexure and faulting while demonstrating the limitation of lacustrine shorelines in this type of analysis.
The role of farfield tectonic stress in oceanic intraplate deformation, Gulf of Alaska
Authors: Reece Robert S. , Gulick Sean P. S. , Christeson Gail L. , Horton Brian K. , van Avendonk Harm , Barth Ginger
Published: April 2013

An integration of geophysical data from the Pacific Plate reveals plate bending anomalies, massive intraplate shearing and deformation, and a lack of oceanic crust magnetic lineaments in different regions across the Gulf of Alaska. We argue that farfield stress from the Yakutat Terrane collision with North America is the major driver for these unusual features. Similar plate motion vectors indicate that the Pacific plate and Yakutat Terrane are largely coupled along their boundary, the Transition Fault, with minimal translation. Our study shows that the Pacific Plate subduction angle shallows toward the Yakutat Terrane and supports the theory that the Pacific Plate and Yakutat Terrane maintain coupling along the subducted region of the Transition Fault. We argue that the outboard transfer of collisional stress to the Pacific Plate could have resulted in significant strain in the NE corner of the Pacific Plate, which created pathways for igneous sill formation just above the Pacific Plate crust in the Surveyor Fan. A shift in Pacific Plate motion during the late Miocene altered the Yakutat collision with North America, changing the stress transfer regime and potentially terminating associated strain in the NE corner of the Pacific Plate. The collision further intensified as the thickest portion of the Yakutat Terrane began to subduct during the Pleistocene, possibly providing the impetus for the creation of the Gulf of Alaska Shear Zone, a > 200 km zone of intraplate strike‐slip faults that extend from the Transition Fault out into the Pacific Plate. This study highlights the importance of farfield stress from complex tectonic regimes in consideration of large‐scale oceanic intraplate deformation.
Geological CO2 sequestration in multi‐compartment reservoirs: Geomechanical challenges
Authors: Castelletto N. , Gambolati G. , Teatini P.
Published: April 2013

Sequestration of large amounts of CO2 within deep underground reservoirs has been proposed as a potential approach for reducing atmospheric emissions of greenhouse gases. A CO2 sequestration project should address the associated environmental and safety issues and, in this respect, the importance of geomechanics has recently been widely recognized. Geomechanics is even more important when fluid injection is planned in faulted reservoirs. How much CO2 can be safely injected into multi‐compartment reservoirs? Are geomechanical constraints more restrictive than flow‐dynamic constraints? These and other questions are addressed in the present study using a three‐dimensional finite element‐interface element geomechanical model. We simulate the possible mechanical failure in both the injected formation and the caprock, the fault/thrust reactivation, and the ground surface displacement in a faulted reservoir of the offshore northern Italy, where seismic surveys provided an accurate characterization of the faulted geological structure. Based on reliable petrophysical/geomechanical properties from well logs and pore overpressure as predicted by a fluid‐dynamic model, the results show that the injection of 1 × 106 ton/a of CO2 may be performed over a few years only. Thereafter, part of the injected formation fails by shear stress. A number of parametric scenarios are investigated to address the major uncertainties on the geomechanical response to CO2 injection. The modeling outcome suggests that shear failure and faults/thrusts reactivation can occur much before attaining the hydraulic fracturing pressure, hence representing two major constraints for a safe and permanent containment.
Interactions and triggering in a 3‐D rate‐and‐state asperity model
Authors: Dublanchet P. , Bernard P. , Favreau P.
Published: April 2013

We present a 3‐D continuous quasi‐dynamic rate‐and‐state model of multiple seismic asperities forced by surrounding aseismic creep and motivated by observations of coplanar multiplets. Our model allows to study the physics of interactions among a set of asperities. First, we show that the amount of interactions and clustering, characterized by the Omori law and interevent time distribution, depends on how far the system is from a critical density of asperities, which is related to the friction properties of the barriers separating the sources. This threshold controls the ability of a population of asperities to destabilize the creeping barriers between them and therefore determines whether dynamic sequences including several asperities in the same event might occur, in agreement with what is expected from observed magnitude‐frequency distributions. Therefore, the concept of critical density of asperity provides a mechanical interpretation of statistical properties of seismicity. As an illustration, we used our numerical results in the specific case of Parkfield in the period preceding the Mw 6, 2004 earthquake, in order to infer the steady state friction parameter (a−b) characterizing the creep of this part of the San Andreas Fault. We estimate a value of (a−b) that locally exceeds 0.001, which is in the upper range of what has already been proposed for the postseimic period of the Mw 6, 2004 Parkfield earthquake.
Intraseasonal Kelvin wave in Makassar Strait
Authors: Pujiana K. , Gordon A. L. , Sprintall J.
Published: April 2013

Time series observations during 2004–2006 reveal the presence of 60–90 days intraseasonal events that impact the transport and mixing environment within Makassar Strait. The observed velocity and temperature fluctuations within the pycnocline reveal the presence of Kelvin waves including vertical energy propagation, energy equipartition, and nondispersive relationship. Two current meters at 750 and 1500 m provide further evidence that the vertical structure of the downwelling Kelvin wave resembles that of the second baroclinic wave mode. The Kelvin waves derive their energy from the equatorial Indian Ocean winds, including those associated with the Madden‐Julian oscillations, and propagate from Lombok Strait to Makassar Strait along the 100‐m isobath. The northward propagating Kelvin waves within the pycnocline reduce the southward Makassar Strait throughflow by up to 2 Sv and induce a marked increase of vertical diffusivity.
Sea level and heat content changes in the western North Pacific
Authors: Moon Jae‐Hong , Tony Song Y.
Published: April 2013

Altimetry‐observed sea level rise (SLR) over the western North Pacific (WNP), including the South China Sea, Yellow Sea, East China Sea, and East/Japan Sea, has a rate of ~5 mm/year over 1993–2010, which is about 1.5 times the rate of the global mean. Here we have examined sea level changes and related ocean heat content (OHC) in the WNP by comparing results from a non‐Boussinesq ocean general circulation model (mass‐conserved) with data sets from altimeters, the Gravity Recovery and Climate Experiment (GRACE), and in situ profiles. Our model reproduces the altimetry regional trends as well as their seasonal/interannual variations. Adding a GRACE‐estimated mass to the model result further explains the altimetry SLR in a way that mass‐induced effect contributes more in the midlatitudes than in the tropical WNP over the GRACE period. In addition, interannual variability and linear trend of regional sea levels are explained mainly by changes in the OHC due to heat convergence and divergence by ocean circulations, while seasonal variability is caused mainly by surface air‐sea fluxes. To understand the underline physics, a comparative experiment was carried out, showing that the recent strengthening trends of SLR and OHC in the tropical regions are significantly attributed to the heat and water mass redistribution in the upper ocean caused by the intensified easterly trade wind over the past two decades.
Water column iron dynamics in the subarctic North Pacific Ocean and the Bering Sea
Authors: Uchida Ren , Kuma Kenshi , Omata Aya , Ishikawa Satoko , Hioki Nanako , Ueno Hiromichi , Isoda Yutaka , Sakaoka Keiichiro , Kamei Yoshihiko , Takagi Shohgo
Published: April 2013

We measured water‐column iron concentrations from west to east along 47°N in the subarctic North Pacific, and in the Bering Sea. In the North Pacific dissolved Fe (D‐Fe) showed surface depletion, mid‐depth maxima at 1000–1500 m (west, 1.3–1.6 nM; east, 0.9–1.1 nM), and a gradual decrease with depth below 3500–4000 m depth (west, 1.1–1.4 nM; east, 0.6–0.7 nM). D‐Fe and total soluble Fe (T‐Fe) in deep water showed a decreasing trend eastward. The higher iron concentrations in western deep waters probably result from higher inputs of dissolved Fe through atmospheric deposition or lateral transport. In contrast, D‐Fe throughout the Bering Sea showed a consistent depth regime characterized by a rapid increase with depth to mid‐depths, a gradual increase with depth in intermediate water to a maximum of 1.6–1.7 nM at 1500–2250 m, and a gradual decrease with depth to 1.3–1.4 nM at 3700 m. Higher iron concentrations and deeper D‐Fe maxima in the Bering Sea are likely due to higher biological productivity and greater and deeper D‐Fe input from the decomposition of sinking particulate organic matter in deep water. We suggest that the higher concentrations and deeper input of D‐Fe as well as PO4 and humic‐type fluorescent dissolved organic matter in the Bering Sea probably results from the longer time for the accumulation of decomposition products resulting from iron supply from the organic‐rich downslope sediment along the steep continental slopes and slow replacement of the deep water in the Bering Sea Basin.
Observations of spatial flow patterns at the coral colony scale on a shallow reef flat
Authors: Hench James L. , Rosman Johanna H.
Published: April 2013

Although small‐scale spatial flow variability can affect both larger‐scale circulation patterns and biological processes on coral reefs, there are few direct measurements of spatial flow patterns across horizontal scales <100 m. Here flow patterns on a shallow reef flat were measured at scales from a single colony to several adjacent colonies using an array of acoustic Doppler velocimeters on a diver‐operated traverse. We observed recirculation zones immediately behind colonies, reduced currents and elevated dissipation rates in turbulent wakes up to 2 colony diameters downstream and enhanced Reynolds stresses in shear layers around wake peripheries. Flow acceleration zones were observed above and between colonies. Coherent flow structures varied with incident flow speeds; recirculation zones were stronger and wakes were more turbulent in faster flows. Low‐frequency (<0.03 Hz) flow variations, for which water excursions were large compared with the colony diameters (Keulegan‐Carpenter number, KC >1), had similar spatial patterns to wakes, while higher‐frequency variations (0.05–0.1 Hz, KC < 1) had no observable spatial structure. On the reef flat, both drag and inertial forces exerted by coral colonies could have significant effects on flow, but within different frequency ranges; drag dominates for low‐frequency flow variations and inertial forces dominate for higher‐frequency variations, including the wave band. Our scaling analyses suggest that spatial flow patterns at colony and patch scales could have important implications for both physical and biological processes at larger reef scales through their effects on forces exerted on the flow, turbulent mixing, and dispersion.
Inter‐annual to decadal sea‐level variability in the coastal zones of the Norwegian and Siberian Seas: The role of atmospheric forcing
Authors: Calafat F. M. , Chambers D. P. , Tsimplis M. N.
Published: April 2013

Inter‐annual to decadal sea‐level variations from tide gauge records in the coastal zones of the Norwegian and Siberian Seas are examined for the period 1950–2010 using a combination of hydrographic observations, wind data, and theory. We identify two large areas of highly coherent sea‐level variability: one that includes the Norwegian, Barents, and Kara Seas, and another one that includes the Laptev, East Siberian, and Chukchi Seas. We provide evidence of a new contribution to the sea‐level variability along the Norwegian coast associated with the poleward propagation of sea‐level fluctuations along the eastern boundary of the North Atlantic. When this propagating signal is combined with the local wind, we are able to explain over 70% of the variance along the Norwegian coast. The steric component explains ~61% of the sea‐level (corrected for the inverse barometer) variability along the Norwegian coast. The high coherency between the sea level along the Norwegian coast and that in the Barents and Kara Seas suggests that part of the Norwegian signal propagates further north into these regions. We introduce an atmospheric vorticity index that explains much of the sea‐level variability in the Laptev, East Siberian, and Chukchi Seas with correlations ranging from 0.73 to 0.81. In the East Siberian Sea, we identify a sea‐level increase of ~22 cm between 2000 and 2003, which is partly explained by the vorticity index, and a decline of ~15 cm after 2003, which we relate to the strengthening of the Beaufort Gyre.
Linking synoptic forcing and local mesoscale processes with biological dynamics off Ningaloo Reef
Authors: Rossi Vincent , Feng Ming , Pattiaratchi Charitha , Roughan Moninya , Waite Anya M.
Published: April 2013

A hydrographic survey offshore Ningaloo reef, north‐west Australia, in austral autumn 2010 revealed relatively stable subsurface water masses in the region, despite the influence of interannual variability of the Leeuwin Current (LC). The surface water mass seems slightly more variable at seasonal time scale, probably due to various contributions of the geographically distinct source waters of the LC. A subsurface nitrate maximum (~110–230 m), a prominent feature of the Ningaloo area during autumn, was consistently observed within different water masses. Tightly coupled variations of subsurface nitrate and oxygen at small vertical scale suggest it is due to local in situ remineralization of organic matter likely to accumulate along sharp physical interfaces and possibly favored by injections of oxygenated subsurface waters. Offshore, enhanced levels of surface chlorophyll a within the downwelling‐favorable LC are associated with deeper mixed layer depth, eroding the shallow source of nutrients. Close to the continental shelf, these nutrients are observed to be efficiently uplifted within the core of quasi‐persistent topographically trapped submesoscale cyclonic eddies dominated by nonlinear effects. A wind‐driven coastal upwelling event occurred in autumn and was characterized by a relatively deep source (~100–150 m), coinciding with the subsurface maximum of nutrients, thus promoting coastal productivity locally and farther north by alongshore advection within the Ningaloo current.
Large‐eddy simulation and low‐order modeling of sediment‐oxygen uptake in a transitional oscillatory flow
Authors: Scalo C. , Boegman L. , Piomelli U.
Published: April 2013

We have tested a dissolved oxygen (DO) transport model based on large‐eddy simulation (LES) of a transitional oscillatory flow observed in the bottom boundary layer of Lake Alpnach, Switzerland. The transition from a quasi‐laminar to a fully turbulent state makes this flow difficult to study with a Reynolds‐averaged Navier‐Stokes equation (RANSE) model. By resolving the full range of governing transport processes, LES provides a reliable prediction of the sediment‐oxygen uptake (SOU). The model biogeochemical and flow parameters have been calibrated against DO and velocity measurements from published in situ data at the earliest phase available in the cycle. The fully developed flow thus obtained is used as an initial condition for the imposed oscillatory forcing. Numerical predictions show that transport in the outer layer is in equilibrium with the main current throughout most of the cycle and that nonequilibrium effects are limited to the diffusive sublayer response to the external forcing. During flow deceleration, the concentration boundary layer slowly expands as turbulence decays; later, during re‐transition, mixing is restored by rapid and intense turbulent production events enhancing the SOU with a well‐defined time lag. An algebraic model for the SOU is proposed for eventual inclusion in RANSE biogeochemical management‐type models developed based on parameterizations used in turbulent mass transfer and with the support of published numerical data and the present simulation. The only input parameters required are the sediment oxidation rate, bulk temperature and DO concentration, and friction velocity.
MIMOC: A global monthly isopycnal upper‐ocean climatology with mixed layers
Authors: Schmidtko Sunke , Johnson Gregory C. , Lyman John M.
Published: April 2013

A monthly, isopycnal/mixed‐layer ocean climatology (MIMOC), global from 0 to 1950 dbar, is compared with other monthly ocean climatologies. All available quality‐controlled profiles of temperature (T) and salinity (S) versus pressure (P) collected by conductivity‐temperature‐depth (CTD) instruments from the Argo Program, Ice‐Tethered Profilers, and archived in the World Ocean Database are used. MIMOC provides maps of mixed layer properties (conservative temperature, Θ, absolute salinity, SA, and maximum P) as well as maps of interior ocean properties (Θ, SA, and P) to 1950 dbar on isopycnal surfaces. A third product merges the two onto a pressure grid spanning the upper 1950 dbar, adding more familiar potential temperature (θ) and practical salinity (S) maps. All maps are at monthly 0.5° × 0.5° resolution, spanning from 80°S to 90°N. Objective mapping routines used and described here incorporate an isobath‐following component using a “Fast Marching” algorithm, as well as front‐sharpening components in both the mixed layer and on interior isopycnals. Recent data are emphasized in the mapping. The goal is to compute a climatology that looks as much as possible like synoptic surveys sampled circa 2007–2011 during all phases of the seasonal cycle, minimizing transient eddy and wave signatures. MIMOC preserves a surface mixed layer, minimizes both diapycnal and isopycnal smoothing of θ‐S, as well as preserves density structure in the vertical (pycnoclines and pycnostads) and the horizontal (fronts and their associated currents). It is statically stable and resolves water mass features, fronts, and currents with a high level of detail and fidelity.
Seasonal variability and coastal upwelling near Cape Santa Marta (Brazil)
Authors: Campos Paula C. , Möller Osmar O. , Piola Alberto R. , Palma Elbio D.
Published: April 2013

The Santa Marta Grande Cape (CSM) area, a relatively steep and narrow continental shelf off southeast Brazil, is under the influence of the Brazil Current and intense seasonal variations of the wind field that affect the composition of water masses and their circulation. Hydrographic data, satellite‐derived wind stress, and numerical simulations are used to characterize the seasonal and shorter period variability of the oceanographic conditions off CSM, in the area between 27°S and 30°S. The study is focused on the upwelling episodes and the dynamical mechanisms associated with these events. Field data and numerical results indicate significant seasonal changes in the oceanographic structure, the water mass composition, and the dynamical balance. During fall and winter, when southwesterly winds prevail and the frequency of meteorological frontal systems increases, the region is characterized by waters of continental origin, the Plata Plume Water (PPW). During spring and summer, deeper waters reach the surface layer due to the action of northeasterly winds. Coastal upwelling events are revealed by surface temperature minima observed south of CSM. Synoptic data show that these events occur in pulses associated with changes in the wind direction. Analysis of the numerical results shows that this particular location of the upwelling band results from the synergy between shelf‐break upwelling and the regional shelf circulation induced by the northeasterly winds, both of which are modulated by changes in the coastline orientation and shelf width.
On the seasonal freshwater stratification in the proximity of fast‐flowing tidewater outlet glaciers in a sub‐Arctic sill fjord
Authors: Mortensen J. , Bendtsen J. , Motyka R. J. , Lennert K. , Truffer M. , Fahnestock M. , Rysgaard S.
Published: April 2013

The Greenland Ice Sheet releases large amounts of freshwater into the fjords around Greenland and many fjords are in direct contact with the ice sheet through tidewater outlet glaciers. Here we present the first seasonal hydrographic observations from the inner part of a sub‐Arctic fjord, relatively close to and within 4–50 km of a fast‐flowing tidewater outlet glacier. This region is characterized by a dense glacial and sea ice cover. Freshwater from runoff, subglacial freshwater (SgFW) discharge, glacial, and sea ice melt are observed above 50–90 m depth. During summer, SgFW and subsurface glacial melt mixed with ambient water are observed as a layered structure in the temperature profiles below the low‐saline summer surface layer (<7 m). During winter, the upper water column is characterized by stepwise halo‐ and thermoclines formed by mixing between deeper layers and the surface layer influenced by ice melt. The warm (T > 1°C) intermediate water mass is a significant subsurface heat source for ice melt. We analyze the temperature and salinity profiles observed in late summer with a thermodynamic mixing model and determine the total freshwater content in the layer below the summer surface layer to be between 5% and 11%. The total freshwater contribution in this layer from melted glacial ice was estimated to be 1–2%, while the corresponding SgFW was estimated to be 3–10%. The winter measurements in the subsurface halocline layer showed a total freshwater content of about 1% and no significant contribution from SgFW.
Chlorophyll bloom development and the subtropical front in the North Pacific
Authors: Wilson Cara , Villareal Tracy A. , Brzezinski Mark A. , Krause Jeffrey W. , Shcherbina Andrey Y.
Published: April 2013

In late summer, satellite ocean color data consistently show localized chlorophyll blooms in the oligotrophic NE Pacific. Based on historical data and the results from recent cruises, these blooms are associated with elevated diatom abundance. However, the physical dynamics that stimulate the blooms remain unknown. Mechanisms suggested to be driving the blooms include mixing at the subtropical front, breaking of internal waves at the critical latitude, shoaling of the mixed layer depth, eddy interactions, and winter mixing of nutrients. To examine these hypotheses, we use data from four summer cruises (2002, 2007, 2008, and 2009) in this region that sampled near a bloom temporally and/or spatially. Conditions associated with five blooms (two blooms were sampled in 2009) are examined. Each area was sampled at a different stage in bloom development, including prebloom, initiation, full bloom, decline, and postbloom conditions. No one variable is found which can explain unequivocally the development of a chlorophyll bloom at a certain location. We describe a set of conditions that could result in the injection of nutrients into the surface water to stimulate a bloom. This “perfect storm” of conditions requires a subsurface stratification minimum layer that intersects the nutricline and that this minimum is close to the base of the mixed layer. These conditions are not predictable in the sense of an annual climatology; however, they do occur often enough to create a reasonably certain, if spatially variable, summer NE Pacific bloom.
Kuroshio‐induced wake in the lee of Green Island off Taiwan
Authors: Chang Ming‐Huei , Tang Tswen Yung , Ho Chung‐Ru , Chao Shenn‐Yu
Published: April 2013

The wake of Green Island was investigated in the alongshore flowing Kuroshio east of Taiwan, where current speeds range from 1 to 1.5 ms–1. Vertical profiling with shipboard lowered acoustic Doppler current profiler and conductivity‐temperature‐depth profiler revealed sizable anomalies in flow and water characteristics in the lee of Green Island. Two different stages of wake evolution were observed from two shipboard surveys. In the first stage, a recirculation developed leeward of the island, followed by a wavy (wave‐like) tail that resembled a weak vortex street. In the second stage, a cold eddy probably originating from the leeward side of the island showed up 14 km downstream of the island. The wake water was colder, saltier, higher in chlorophyll‐a concentration, and produced isopycnal doming up to 60 m. In the recirculation area, the relative vorticity, either positive or negative, was 10 times of planetary vorticity, and the horizontal divergence or convergence was O (10–4 s–1) on average. Flow divergence and convergence in the wake were expected to form upwelling and downwelling zones, producing a vertical circulation that vertically displaces isotherms. High inverse Richardson number, produced by strong vertical shear of horizontal currents, was associated with intense overturning events in the wake. High vertical shear of horizontal currents drove the mixing. The dissipation rate of turbulent kinetic energy in the overturn regions is O (10–7–10–5) W Kg–1; the corresponding eddy diffusivity is O (10–3–10–1) m2 s–1. The wake water properties are vertically diffused via upwelling and turbulence and can be delivered downstream through eddy shedding or advection. The extent of downstream influence remains to be investigated.
Formation and entrainment of fluid mud layers in troughs of subtidal dunes in an estuarine turbidity zone
Authors: Becker Marius , Schrottke Kerstin , Bartholomä Alexander , Ernstsen Verner , Winter Christian , Hebbeln Dierk
Published: April 2013

The formation and entrainment of fluid mud layers in troughs of subtidal dunes were investigated in the Weser Estuary, North Sea, Germany, based on hydroacoustic measurements. Near‐bed suspension layers were found to consist of a suspension of large mud flocs of variable concentration, ranging from 25 g/L below the lutocline to 70 g/L at the river bed, whereas the gelling concentration was below 70 g/L. Sites of fluid mud formation coincided with the location of the estuarine turbidity zone during slack water. On average, near‐bed density gradients were initially observed in dune troughs 1.2 h before slack water, and all fluid mud layers were entrained 2.3 h after slack water. No shear instabilities occurred until 1.8 h after slack water. While the flow was oriented in the dune direction, rapid entrainment was related to the development of the turbulent flow field behind dunes and is explained to be induced by advection of strong turbulence during accelerating currents. Fluid mud layers in dune troughs were entrained at an earlier point in time after slack water, compared to adjacent layers formed on a comparatively flat bed, where dune crests did not protrude from the lutocline.
Influence of Amur River discharge on phytoplankton photophysiology in the Sea of Okhotsk during late summer
Authors: Isada Tomonori , Iida Takahiro , Liu Hongbin , Saitoh Sei‐Ichi , Nishioka Jun , Nakatsuka Takeshi , Suzuki Koji
Published: April 2013

We investigated the photosynthetic parameters of phytoplankton in the Sea of Okhotsk during the late summer of 2006 to characterize their spatiotemporal variability and to test the hypothesis that discharge from the Amur River could influence the algal photophysiology. The highest maximum quantum yield of carbon fixation in photosynthesis (Φcmax; 0.098 mol C mol photons‐1) was found near the Amur River mouth, where nitrate was depleted. However, none of the photosynthetic parameters, including primary productivity (PP) at the surface, were correlated with temperature, daily photosynthetically available radiation (PAR), or ambient nutrient concentrations. Variations in Φcmax depended on the variations in not only the mean chlorophyll a specific absorption coefficient of phytoplankton (ā*ph) but also the slope index of the absorption coefficient of phytoplankton (aph slope), an indicator for the ratio of nonphotosynthetic carotenoids to photosynthetic carotenoids. These results indicated that the phytoplankton assemblages acclimated to the ambient light conditions by regulating their cellular pigments. Additionally, ā*ph and euphotic depth (Zeu) were significantly correlated with salinity, suggesting that photoacclimation of the phytoplankton assemblages observed in this study could be induced by discharge of Amur River. Because spatiotemporal variations in PP were concomitant with Φcmax, ā*ph, and the chlorophyll a concentration, PP models based on inherent optical property (IOP) were suitable for estimating PP in the Sea of Okhotsk. This study is the first to investigate the factors controlling phytoplankton photophysiology in the Sea of Okhotsk, one of the highest primary production areas in the world.
Mesoscale eddies in the northwestern subtropical Pacific Ocean: Statistical characteristics and three‐dimensional structures
Authors: Yang Guang , Wang Fan , Li Yuanlong , Lin Pengfei
Published: April 2013

Mesoscale eddy properties in the northwestern subtropical Pacific Ocean are investigated by analyzing 22,567 cyclonic eddies (CEs) and 26,365 anticyclonic eddies (AEs) detected from 19 year altimetric sea level records. Eddy occurrence frequency and kinetic energy are prevailingly high in the Subtropical Countercurrent zonal band between 19°N and 26°N and further elevated near the Luzon‐Taiwan coast. A general superiority of AEs is observed at most latitudes except between 19°N and 22°N, where the CE number is larger. The modal radius and mean lifespan of the CEs (AEs) are 134 km and 11.2 weeks (121 km and 10.9 weeks), respectively. After generation, most eddies propagate westward with a mean speed of 7.2 cm s−1 and deflect northward following the Kuroshio along the Luzon‐Taiwan coast. Three‐dimensional eddy structures are further explored with composite eddy images in five subregions constructed by surfacing Argo temperature/salinity data into altimeter‐detected eddy areas. Due to the existence of mode waters in the main thermocline, eddy‐induced temperature anomaly exhibits a double‐core vertical structure which is especially evident in CE images. Because of the vertical water mass distribution, salinity anomaly features a sandwich‐like pattern which is more evident in AE images. Also revealed is the significant structure difference in these five subregions. Eddies are greatly intensified as they approach the western boundary, inducing larger temperature and salinity anomalies and influencing deeper ocean. Along the Luzon‐Taiwan coast, AEs are preferentially strengthened by the northward background flow.
Modeling Antarctic ice shelf responses to future climate changes and impacts on the ocean
Authors: Kusahara Kazuya , Hasumi Hiroyasu
Published: April 2013

We investigate basal melting of all Antarctic ice shelves by a circumpolar ice shelf‐sea ice‐ocean coupled model and estimate the total basal melting of 770–944 Gt/yr under present‐day climate conditions. We present a comparison of the basal melting with previous observational and modeling estimates for each ice shelf. Heat sources for basal melting are largely different among the ice shelves. Sensitivities of the basal melting to surface air warming and to enhanced westerly winds over the Antarctic Circumpolar Current are investigated from a series of numerical experiments. In this model the total basal melting strongly depends on the surface air warming but is hardly affected by the change of westerly winds. The magnitude of the basal melting response to the warming varies widely from one ice shelf to another. The largest response is found at ice shelves in the Bellingshausen Sea, followed by those in the Eastern Weddell Sea and the Indian sector. These increases of basal melting are caused by increases of Circumpolar Deep Water and/or Antarctic Surface Water into ice shelf cavities. By contrast, basal melting of ice shelves in the Ross and Weddell Seas is insensitive to the surface air warming, because even in the warming experiments there is high sea ice production at the front of the ice shelves that keeps the water temperature to the surface freezing point. Weakening of the thermohaline circulation driven by Antarctic dense water formation under warming climate conditions is enhanced by basal melting of ice shelves.