flow and sediment transport induced by tide propagation 2 the wavy bottom case 1 a model is formulated to describe the flow field locally induced by the propagation of a tidal wave and to evaluate the sediment transport the three dimensional reynolds averaged momentum equations are considered and coriolis terms are retained since they affect the tide propagation turbulent reynolds stresses are described by means of the boussinesq assumption and the eddy viscosity is assumed to be an assigned function of the distance from the sea bed the effects of wind waves on sediment transport are taken into account and both the bed load and the suspended load are computed a comparison of the obtained results with laboratory data and field measurements shows that the model provides a reliable and accurate description of both the velocity field and sediment transport and does not require high computational costs

a coupled decadal prediction of the dynamic state of the kuroshio extension system abstractbeing the extension of a wind driven western boundary current the kuroshio extension ke has long been recognized as a turbulent current system rich in large amplitude meanders and energetic pinched off eddies an important feature emerging from recent satellite altimeter measurements and eddy resolving ocean model simulations is that the ke system exhibits well defined decadal modulations between a stable and an unstable dynamic state here the authors show that the decadally modulating ke dynamic state can be effectively defined by the sea surface height ssh anomalies in the 31 36 n 140 165 e region by utilizing the ssh based ke index from 1977 to 2012 they demonstrate that the time varying ke dynamic state can be predicted at lead times of up to 6 yr this long term predictability rests on two dynamic processes 1 the oceanic adjustment is via baroclinic rossby waves that carry interior wind forced anomalies westward into the ke region and 2 the low frequency ke variability influenc

sand ripples under sea waves part 3 brick pattern ripple formation an oscillatory flow over a cohesionless bottom can produce regular three dimensional bedforms known as brick pattern ripples characterized by crests perpendicular to the direction of fluid oscillations joined by equally spaced bridges shifted by half a wavelength between adjacent sequences a photo of brick pattern ripples is shown in sleath 1984 p 141 in the present paper brick pattern ripple formation is explained on the basis of a weakly nonlinear stability analysis of a flat cohesionless bottom subject to an oscillatory flow in which three dimensional perturbations are considered

impact of aleutian low activity on the stmw formation in the kuroshio recirculation gyre region 1 to understand the formation of north pacific subtropical mode water stmw in the kuroshio recirculation gyre region the cause of stmw thickness variation is investigated using temperature profiles in a historically archived data set the thickness variation is predominantly controlled by the main thermocline depth mtd when the main thermocline deepens shoals the wintertime mixed layer depth can develop not develop and consequently thicker thinner stmw is observed in summer the large scale atmospheric forcing controlling the mtd is explored using a wind driven hindcast ocean model the mtd variation stems primarily from a baroclinic response in the ocean to the aleutian low al activity especially the meridional movement of the al exerts a remarkable influence

marked freshening of north pacific subtropical mode water in 2009 and 2010 influence of freshwater supply in the 2008 warm season 1 changes in salinity of the north pacific subtropical mode water stmw were investigated using profiles for 2003 2011 in 2009 and 2010 the stmw freshened markedly s 34 7 psu with salinity 0 1 psu lower than other years freshening in 2009 was caused by excess rainfall in the 2008 warm season which was related to an increase in the number of low pressures passing over the stmw formation region associated with a southeastward shrinkage of the summertime north pacific subtropical high the freshening signal persisted under the seasonal pycnocline during the 2009 warm season this resulted in the freshening of stmw in 2010

climatology and decadal variations in multicore structure of the north pacific subtropical mode water the pycnostad of the north pacific subtropical mode water stmw often displays multiple vertical minima in the potential vorticity profile argo profile data from 2004 to 2015 are used to investigate inter annual to decadal variations of the multicore structure climatologically about 24 pycostads of stmw have multicore structure and most of them distribute in the region west of 150 e stmw cores are classified into three sub modes the cold middle and warm ones with potential temperatures of 16 0 17 c 17 18 c and 18 19 5 c respectively the kuroshio extension ke varies between stable and unstable states the unstable ke with large meanders increases the subsurface stratification and shoals the winter mixed layer east of 150 e with warmer temperatures there the dominant stmw type varies from the cold single type in stable ke years making up 72 of the profiles with stmw to the middle single one 53 in unstable years the variation of the dominant stmw type in the region east of 150 e subsequently affects the multicore structure of stmw west of 150 e in a broad region between 130 e and 180 e profiles with stmw are fewer in unstable years but the proportion of multicore profiles increases among stmw profiles this might be due to the split recirculation gyre with a chaotic ke

long bed waves in tidal seas an idealized model an idealized model is proposed to explain the appearance of the long bed waves that have been recently observed in shallow tidal seas the model assumes that these bedforms grow due to tide topography interaction the water motion is described by means of the depth averaged shallow water equations and the bottom evolution is governed by conservation of sediment mass the sediment transport formulation includes a critical bottom stress below which no sediment moves also anisotropic sediment transport due to local bottom slopes in the longitudinal and transverse directions is taken into account a linear stability analysis of the flat bottom configuration reveals that different bottom patterns can emerge in accordance with previous analyses for strong tidal currents the fastest growing modes are sand banks however if the tidal currents are elliptical and the maximum bottom stress is just above its threshold value for the initiation of sediment motion the model shows the presence of further growing modes which resemble the long bed waves observed in the field

revisiting the linear theory of sand dune formation a linear stability theory for dune and antidune formation is presented that implements a rotational two dimensional flow model as in previous linear theories the phase lag between sediment transport and bed elevation remains the main mechanism driving instability however it is shown that this phase lag varies significantly in a neighbourhood of the bed moreover since the layer in which sediments are moving has a finite though small thickness it is assumed that the perturbations of the fluid stress driving bedload transport should be evaluated at the top of the layer itself it is shown that such an apparently minor modification of the classical approach alters remarkably the balance between stabilizing and destabilizing effects that drives the instability process this also allows a clarification of the debated role of bedslope on the formation of dunes and antidunes following the above ideas antidunes are shown to form in the absence of suspended sediment load and without any effect associated with sediment inertia the present analysis ultimately allows a successful unification of the theories of dune and antidune formation

classification of large scale subaqueous bedforms a new look at an old problem sepm bedforms and bedding structures abstract a symposium entitled classification of large scale flow transverse bedforms was convened at the 1987 mid year meeting of sepm in austin texas with the purpose of examining the problems involved in classifying large subaqueous flow transverse bedforms developed in fluvial intertidal and marine environments and recommending changes in nomenclature the consensus of the participants is that despite the wide spectrum of morphologies of large scale flow transverse bedforms excluding antidunes they all occupy a similar position in the lower flow regime sequence between ripples and upper plane bed the wide variety of forms is a reflection of secondary effects such as channelization fluctuating water levels and unsteady and reversing flows the bedforms appear not to fall into size classes with naturally occurring boundaries but rather form a continuum with spacing from just under 1 m to over 1 000 m the symposium panel proposes therefore that they should have only one name dune dune is preferred as it has historical precedence over other terms in use such as megaripple and sand wave the term dune should be modified by rimary descriptors of shape i e 2 d or 3 d and size based on spacing small 0 6 5 m medium 5 10 m large 10 100 m or very large 100 m and the adjective subaqueous when it is important to distinguish them from eolian dunes the panel recommends a morphologically based classification that is descriptive with an underlying genetic rationale second order descriptors such a sediment size and bedform superposition may be used to describe more thoroughly the variety of subaqueous dunes in nature

on the modeling of sand wave migration 1 we describe a simple mathematical model capable of reproducing the main features of sand wave inception and growth in particular we focus on the prediction of the migration rates that sand waves undergo because of tidal and residual currents the model adequately predicts migration rates even for the cases of upstream propagating sand waves i e for sand waves which migrate in the direction opposite to that of the residual current we show that upstream downstream propagation is mainly controlled by the relative strength of the residual current with respect to the amplitude of the quarter diurnal tide constituent and by the phase shift between the semi diurnal and quarter diurnal tide constituents therefore to accurately predict field cases a detailed knowledge of the direction strength and phase of the different tide constituents is required

modeling sand wave characteristics on the belgian continental shelf and in the calais dover strait 1 the capability of the model of besio et al 2006 to predict the main geometrical characteristics crest orientation wavelength of tidal sand waves is tested by comparing the theoretical predictions with field data in particular the field observations carried out by mouchet 1990 and van lancker et al 2005 along the continental shelf of belgium are used additional comparisons are carried out against the field measurements described by le bot 2001 and le bot and trenteseaux 2004 which were carried out in an adjacent region attention is focused on the prediction of the wavelength of the bottom forms indeed the capability of a linear stability analysis to predict the occurrence of sand waves has been already tested by hulscher and van den brink 2001 and more recently by van der veen et al 2006 the obtained results show that the theoretical predictions fairly agree with field observations even though some of the comparisons suggest that the accuracy of the predictions depends on the accurate evaluation of the local current and sediment characteristics

sand ripples under sea waves part 2 finite amplitude development in the present paper the authors formulate a theory to predict the time development of sand ripples characterised by small but finite amplitude under the action of surface gravity waves the theory is based on a weakly nonlinear stability analysis of a flat sandy bottom subject to viscous oscillatory flow the parameters of the problem namely the reynolds number of the flow and the reynolds and froude numbers of sediments are assumed to fall within a neighbourhood of the critical conditions determined in blondeaux 1990 the analysis can predict the actual ripple height wavelength and profile when flow separation is absent i e for the case of rolling grain ripples assuming sleath s 1984 criterion for separation the values of the relevant parameters at which transition from rolling grain ripples to vortex ripples occurs are predicted a comparison between theoretical findings and experimental data supports the validity of the present theory

north pacific subtropical mode water volume decrease in 2006 09 estimated from argo observations influence of surface formation and basin scale oceanic variability abstractanalysis of argo temperature and salinity profiles gridded at 0 5 0 5 resolution for 2005 12 shows a strong north pacific subtropical mode water npstmw volume and density decrease during 2006 09 in this time period upper ocean temperature stratification and potential vorticity pv all increased within the region in and around the npstmw low pv pool contributing to the npstmw volume decrease in two ways i the volume of water satisfying the low pv constraint that is part of the mode water definition decreased and ii some water that was initially in the npstmw density range 25 0 25 5 kg m 3 was transformed into lighter water both changes in density and in pv in the npstmw region were a manifestation of basinwide changes a positive pv anomaly started to propagate westward from the central pacific in 2005 followed by a negative density anomaly in 2007 which caused a dramatic npstmw volume and density decrease a walin estimate of surface formation in the npstmw density ran

volume and potential vorticity budgets of eighteen degree water abstractmode waters are a distinctive baroclinic feature of the world ocean characterized by relatively weak vertical stratification they correspond dynamically to low potential vorticity pv in the north atlantic subtropical gyre the mode waters have become known as eighteen degree water their dynamics involves air sea interaction diapycnal and isopycnal mixing and subduction understanding mode water dynamics is therefore both challenging and important since it connects several aspects of the ocean circulation mass and pv budget of the mode water s core evaluated in a realistic primitive equation north atlantic model are used to characterize mode water maintenance it is shown that the surface pv flux has very little impact on mode water the surface buoyancy flux in combination with eddy mass flux is the most important control on mode water structure a mean pv formalism is used to show that the pv and water mass formation budgets are intrinsically linked a decomposition of the budget demons

faults strengthening and seismicity induced by geothermal exploitation on a spreading volcano mt amiata italia abstract seismogenic structures such as faults play a primary role in geothermal system generation recharge and output they are also the most susceptible to release seismic energy over fluid injection extraction operations during anthropic exploitation we describe the microseismic activity recorded in 2000 2001 in the piancastagnaio geothermal field on the se flank of mt amiata volcano southern tuscany italy from our field observations we find that a relatively high percentage i e about 5 of the recorded events are of hydro fracturing origin and have a distinct waveform seismic signature when compared to the recorded events of tectonic shear fracturing origin while hydrofracturing events are mostly concentrated around the geothermal fields the spatial distribution of hypocenters shows a deepening and a density increase of the micro seismic activity from the volcanic axis toward the exploited geothermal reservoir suggesting that volcanic spreading at amiata is still active the study of different data sets from different time periods together with the knowledge from terzaghi s law that production of large quantity of pore fluid with the associated fluid pressure reduction could augment the stress normal to faults surfaces and thus their resistance to slip make us argue that the process of volcanic spreading affecting the edifice of amiata may allow augmented accumulation of stresses on faults eventually leading to the release of higher stress drops once ruptures occur the gutenberg richter magnitude frequency distribution shows that the strongest events on record have a local magnitude in the 5 5 5 m l range for 100 year recurrence time in conclusions we infer that geothermal exploitation at mt amiata should be closely monitored in order to understand how fluid injection production is responsible for the hydrofracturing seismic activity and affects stress accumulation on and rupture of faults within and in the neighborhood of the geothermal fields this understanding may allow a geothermal field management that will hopefully reduce the risk for inducing larger seismic events in the area

variability of the kuroshio extension jet recirculation gyre and mesoscale eddies on decadal time scales abstract twelve years of sea surface height ssh data from multiple satellite altimeters are used to investigate the low frequency changes and the interconnections of the kuroshio extension ke jet its southern recirculation gyre and their mesoscale eddy field the dominant signal is characterized by the steady weakening of the ke jet recirculation gyre from 1993 to 1996 followed by a gradual strengthening after 1997 during the weakening period of 1993 96 the ke path migrated southward in general and this path migration reversed in direction during the strengthening period of the ke jet and recirculation gyre after 1997 by hindcasting the ssh signals using linear vorticity dynamics it was found that weakening strengthening in the ke jet and recirculation gyre is consistent with westward propagation of negative positive ssh anomalies generating in the eastern north pacific and strengthening during their westward propagation when the ke jet and recirculation gyre were in a weak mode during 19

the relationship between water mass formation and the surface buoyancy flux with application to phillips red sea model abstract a buoyancy flux across the sea surface between the outcropping of isopycnals must be balanced by a subsurface diapycnal buoyancy flux if this flux were only advective its derivative with respect to buoyancy would provide a direct estimate of the buildup of volume between isopycnals or rate of water mass formation a diapycnal velocity however requires diapycnal mixing which also causes a diapycnal buoyancy flux and it is shown that there is no reason to expect a simple relationship between the advective and diffusive fluxes for a surface layer with vigorous vertical mixing and weak horizontal mixing however the diapycnal diffusive flux of buoyancy is small and the flow through the base of the mixed layer can be derived from the derivative of the surface buoyancy flux with respect to buoyancy these points are illustrated by examination of the phillips similarity solution for the convective circulation driven in a channel by a uniform surface buoyancy loss

influence of kuroshio path variation south of japan on formation of subtropical mode water abstractdistributions of subtropical mode water stmw in the northwestern part of the north pacific subtropical gyre were investigated using temperature salinity profiles from 2005 to 2011 with particular reference to the kuroshio meander and non meander path states south of japan in spring of meander years warm stmw with a potential temperature of 19 20 c potential density anomaly of 24 6 24 9 kg m 3 was found in the shikoku basin whereas cold stmw below 19 c was distributed throughout the southern region of japan in non meander years the warm stmw was formed in a spatially isolated and warm winter mixed layer ml in the shikoku basin where a local recirculation developed in association with the kuroshio meander path both the absence of horizontal mixing with a cold ml south of the kuroshio extension because of the spatially isolated ml and an increase in horizontal heat advection due to the westward flow associated with this local recirculation caused the ml warming in the shikoku basin a

eddy mean flow interaction in the decadally modulating kuroshio extension system abstract satellite altimeter sea surface height ssh data of the past 16 years are used to investigate the decadal changes of the kuroshio extension ke system that oscillated between a stable and an unstable dynamic state during the stable state of 10 1992 06 1995 and 01 2002 12 2004 the ke jet was intense and had a northerly zonal mean path and a well defined southern recirculation gyre during the unstable state of 07 1995 12 2001 and 01 2005 present the ke jet had a reduced eastward transport and a more southerly flow path transitions between the two dynamic states are caused by the basin scale wind stress curl forcing in the eastern north pacific related to the pacific decadal oscillations pdos or the north pacific gyre oscillations npgos during the positive pdo or negative npgo phase the intensified aleutian low generates negative ssh anomalies in the eastern north pacific through ekman divergence as these wind induced negative ssh anomalies propagate to the west as baroclinic rossby waves they weaken the zonal ke jet and shift its path southward as its path is pushed southward 32 n the deep reaching ke jet has to ride over the shallow shatsky rise generating localized disturbances that lead to their subsequent development along the ke jet west of the shatsky rise the sequence opposite to that listed above occurs when the pdo npgo changes to its negative positive phase after the ke system transitions from a stable to unstable state the enhanced eddy interaction is found to strengthen both the southern recirculation gyre and quasi stationary meanders along the ke s upstream path this nonlinear eddy interaction is important in determining the amplitude of the observed decadal ke variability

dunes and alternate bars in tidal channels a simple idealized model is proposed to predict the appearance of alternate bottom forms in tidal channels the model is based on the linear stability analysis of the flat seabed driven by tidal currents the hydrodynamics is described by means of the full three dimensional continuity and reynolds equations to quantify turbulent stresses the boussinesq assumption is introduced and an algebraic model is used for the eddy viscosity the morphodynamics is described by the exner equation and a simple sediment transport predictor when applied to tidal channels the model predicts the appearance of alternate bottom forms if the channel width is larger than a critical value this finding agrees with previous analyses however the results obtained show the existence of two different modes close to the conditions of incipient sediment motion or when the suspended load is intense the model suggests the appearance of an alternate sequence of shoals and pools the first mode characterized by a wavelength which might be comparable with the horizontal excursion of the tide however under such circumstances to provide accurate quantitative results the model should be extended to include the effects of the local acceleration and of the possible variations of the depth and width of the channel which are neglected in the analysis in all the other conditions the model predicts the appearance of a second mode presently termed tidal alternate bars this mode is geometrically similar to the first mode i e it is characterized by depositional and erosional areas which are found in an alternate arrangement but it has significantly shorter wavelengths in this case the wavelength of the bedforms scales with the water depth the physical mechanism generating tidal alternate bars appears to be the same as that generating tidal dunes and it cannot be described by means of a depth averaged approach

on the formation of sand waves and sand banks a fully three dimensional model is proposed for the generation of tidal sand waves and sand banks from small bottom perturbations of a flat seabed subject to tidal currents the model predicts the conditions leading to the appearance of both tidal sand waves and sand banks and determines their main geometrical characteristics a finite wavelength of both sand waves and sand banks is found around the critical conditions thus opening the possibility of performing a weakly nonlinear stability analysis able to predict the equilibrium amplitude of the bottom forms as shown by previous works on the subject the sand wave crests turn out to be orthogonal to the direction of the main tidal current the present results also show that in the northern hemisphere sand bank crests are clockwise or counter clockwise rotated with respect to the main tidal current depending on the counter clockwise or clockwise rotation of the velocity vector induced by the tide only for unidirectional or quasi unidirectional tidal currents are sand banks always counter clockwise rotated the predictions of the model are supported by comparisons with field data finally the mechanisms leading to the appearance of sand waves and sand banks are discussed in the light of the model findings in particular it is shown that the growth of sand banks is not only induced by the depth averaged residual circulation which is present around the bedforms and is parallel to the crests of the bottom forms a steady drift of the sediment from the troughs towards the crests is also driven by the steady velocity component orthogonal to the crests which is present close to the bottom and can be quantified only by a three dimensional model while the former mechanism appears to trigger the formation of counter clockwise sand banks only the latter mechanism can give rise to both counter clockwise and clockwise rotated sand banks

a note on tidally generated sand waves the process leading to the formation of sand waves in tide dominated coastal areas is investigated by means of the linear stability analysis of a flat sandy bottom subject to oscillatory tidal currents the conditions for the decay or amplification of small bottom perturbations are determined for arbitrary values of the parameters of the problem according to field observations the initial growth of sand waves requires a minimum amplitude of the tidal current even when the critical bed shear stress for the initial motion of sediment is set equal to zero moreover the minimum amplitude depends on sediment characteristics in particular the analysis shows that sand waves appear only for a sandy bottom and their growth does not take place when a coarse sediment covers the sea bed the solution procedure extends the truncation method which is often used to describe the flow generated by the interaction of bottom perturbations with the oscillatory tidal current the obtained results show that the truncation method describes the mechanism inducing the growth of sand waves but values of the parameters exist for which its results are not quantitatively accurate finally the asymptotic approach for large values of both r w hich isthe ratio between the amplitude of the horizontal tidal excursion and the wavelength of the bottom perturbations and of the stress parameter s is modified in the bottom boundary layer to describe cases characterized by values of s of order one which is the order of magnitude suggested by an analysis of field data

decadal variability of north pacific eastern subtropical mode water a significant decadal variability of the north pacific eastern subtropical mode water npestmw related to the pacific decadal oscillation is identified from the gridded argo and en4 observational data sets to investigate the decadal variability of the npestmw volume the high resolution estimating the circulation and climate of the ocean phase ii simulation is used to diagnose the volume budget of the npestmw density layer in the npestmw formation region 20 degrees 36 degrees n 160 degrees 120 degrees w between 1992 and 2016 results show that npestmw is formed through subduction of low potential vorticity pv water this low pv water subduction is mainly determined by mixed layer depth change and lateral induction which account for approximately 58 and 32 respectively the decadal npestmw volume anomaly is originated from the pacific decadal oscillation induced decadal anomaly of air sea buoyancy flux this decadal buoyancy flux anomaly drives the decadal anomaly of mixed layer depth change then causes anomalous subduction and leads to anomalous low pv water subduction the decadal npestmw volume anomaly is finally generated by this anomalous low pv water subduction

annual and interannual variability in the kuroshio current system abstract individual seasonal 300 m temperature maps were constructed over the kuroshio current system from 130 e to 170 w for a 4 year period from summer 1976 through spring 1980 using transpac xbt data and jodc temperature depth data quasi stationary meanders in the kuroshic current system occurred at 137 c i e kuroshio meander at 144 e and 150 e i e lee wave meanders and near 160 e i e meander over the shatsky rise a composite of the paths of the kuroshio i e the 12 c isotherm from the individual seasonal maps and the total variance map finds nodes i e minima and anti nodes i e maxima of variability to have existed along the mean kuroshio path the anti nodes coincided with the location of the quasi stationary meanders the nodes in between zonal propagation of temperature anomalies accounted for 20 30 of the total interannual variance these temperature anomalies propagated eastward at 0 5 1 5 cm s 1 in the region 140 155 e and westward at 1 to 2 cm s 1 in the re

a mechanism for the recurrence of wintertime midlatitude sst anomalies abstract in the early 1970s namias and born speculated that ocean temperature anomalies created over the deep mixed layer in winter could be preserved in the summer thermocline and reappear at the surface in the following fall or winter this hypothesis is examined using upper ocean temperature observations and simulations with a mixed layer model the data were collected at six ocean weather stations in the north atlantic and north pacific concurrent and lead lag correlations are used to investigate temperature variations associated with the seasonal cycle in both the observations and the model simulations concurrent correlations between the surface and subsurface temperature anomalies in both the data and the model indicate that the penetration of temperature anomalies into the ocean is closely tied to the seasonal cycle in mixed layer depth high correlations extend to relatively deep shallow depths in winter summer lead lag correlations in both the data and the model at some of the stations

effect of mesoscale eddies on subtropical mode water variability from the kuroshio extension system study kess abstract forty eight profiling floats have been deployed in the kuroshio extension ke region since may 2004 as part of the kuroshio extension system study kess project by combining the float temperature salinity measurements with satellite altimetry data this study investigates the role played by mesoscale eddies in controlling the property changes in north pacific subtropical mode water stmw following a 3 yr period of low eddy activity in 2002 04 the ke showed a transition to a high eddy kinetic energy state in 2005 this transition is the result of delayed oceanic response to the 2002 shift in the basin scale surface wind forcing in connection with the pacific decadal oscillation the high eddy kinetic energy state of the ke is characterized by successive shedding of strong cold core rings into the recirculation gyre resulting from the interaction of the ke jet with the shatsky rise or the preexisting cutoff rings by transporting northern origin high potential vorticity pv ke water into t

a linear stability analysis of tidally generated sand waves a linear stability analysis is carried out to examine the initial stage of sand wave growth under tidal flows and the occurrence of a preferred length scale the fact that these bedforms typically have length scales small compared to the tidal excursion is exploited by adopting an asymptotic approach to solve the hydrodynamic part of the problem i e to find the hydrodynamic response to an initially small bed perturbation this method is shown to have important advantages over previously used methods since it allows an exploration of the complete sand wave regime whereas other methods fail for short sand waves and in general it is also more accurate it is found that the selection of a preferred length scale depends mainly on only two parameters the bed slope coefficient and the ratio of friction velocity to eddy viscosity whereas there appears to be almost no dependence on the water depth