dynamics of wind forced coherent anticyclones in the open ocean we numerically study the dynamics of coherent anticyclonic eddies in the ocean interior for the hydrostatic rotating stably stratified turbulence we use a high resolution primitive equation model forced by small scale winds in an idealized configuration many properties of the horizontal motions are found to be similar to those of two dimensional and quasi geostrophic turbulence major differences are a strong cyclone anticyclone asymmetry linked to the straining field exerted by vortex rossby waves which is also found in shallow water flows and the complex structure of the vertical velocity field which we analyze in detail locally the motion can become strongly ageostrophic and vertical velocities associated with vortices can reach magnitudes and levels of spatial complexity akin to those reported for frontal regions transport and mixing properties of the flow field are further investigated by analyzing lagrangian trajectories particles released in the pycnocline undergo large vertical excursions because of the vertical velocities associated to the vortices with potentially important consequences for marine ecosystem dynamics

major pathways of atlantic water in the northern north atlantic and nordic seas toward arctic 1 the major pathways of near surface atlantic water in the northern north atlantic and nordic seas are identified as current speeds above 30 cm s using 1014 lagrangian drifters combined with previously published hydrography the inflow over the scotland greenland ridge and establishment of the two branch norwegian atlantic current nwac are described in light of the circulation in the northern north atlantic the western branch of the nwac appears as a jet in the polar front topographically guided from the iceland faroe front through the nordic seas toward fram strait the eastern branch starts as a shelf edge current above the irish scottish continental shelf and after passing through the faroe shetland channel it continues northward along the norwegian shelf edge toward the arctic with a branch bifurcating into the barents sea the nwac appears to maintain its two branch structure throughout the nordic seas with the atlantic water confined to a 200 600 km wide wedge

paleozoic mesozoic tectono sedimentary evolution and magmatism of the egersund basin area norwegian central north sea abstract this study focuses on the tectonic evolution of the greater egersund basin in the norwegian central north sea with special emphasis on late paleozoic extensional tectonics following caledonian collapse and the variscan orogeny and the impact of the basement structural grain on this evolution the caledonian collapse likely resulted in development rejuvenation of the deepest e w trending structures depocenters by late devonian time thus a late devonian early carboniferous age can be assigned to the initial extension which was associated with the development of an e w striking basin system to be overprinted by n s extensional structures of similar age a phase of regional magmatism at the carboniferous permian transition 300 ma may be associated with a large igneous province centred on the skagerrak area faulting during late carboniferous early permian was minor within the study area as reflected by uniform sedimentary thicknesses of the uppermost carboniferous and lower permian sequences major normal faults mainly trending n s were active during a late permian early triassic rift phase affecting large parts of the central and northern north sea area a later phase of extension was initiated in late middle jurassic time and the egersund basin proper formed during the late jurassic early cretaceous the depocenters that developed during this phase was influenced by the deep late paleozoic sub salt structural grain including strike slip movements along the sorgenfrei tornquist zone later events include mild inversion along the northern flank of the egersund basin possibly as a late cretaceous response to far field alpine compression and cenozoic regional tilting

the turbulent oscillator a mechanism of low frequency variability of the wind driven ocean gyres intrinsic low frequency variability is studied in the idealized quasigeostrophic midlatitude wind driven ocean gyres operating at large reynolds number a robust decadal variability mode driven by the transient mesoscale eddies is found and analyzed the variability is a turbulent phenomenon which is driven by the competition between the eddy rectification process and the potential vorticity anomalies induced by changes of the intergyre transport

the kuroshio extension northern recirculation gyre profiling float measurements and forcing mechanism abstract middepth time mean circulation in the western north pacific ocean 28 45 n 140 165 e is investigated using drift information from the profiling floats deployed in the kuroshio extension system study kess and the international argo programs a well defined cyclonic recirculation gyre rg is found to exist north of the kuroshio extension jet confined zonally between the japan trench 145 e and the shatsky rise 156 e and bordered to the north by the subarctic boundary along 40 n this northern rg which is simulated favorably in the eddy resolving ogcm for the earth simulator ofes hindcast run model has a maximum volume transport at 26 4 sv across 159 e and its presence persists on the interannual and longer time scales an examination of the time mean x momentum balance from the ofes hindcast run output reveals that horizontal convergence of reynolds stresses works to accelerate both the eastward flowing kuroshio extension jet and a westward mean flow north of the meandering jet

salt influenced normal fault growth and forced folding the stavanger fault system north sea abstract displacement ratio d r is the ratio between salt thickness t v and sub salt normal fault displacement d d r t v d and it is typically used to predict the degree of geometric and kinematic linkage between sub and supra salt fault populations and the overall structural style in salt influenced extensional settings however we currently lack natural examples of how d r and the underlying geological controls vary and how these may control the three dimensional geometry and evolution of salt influenced normal fault systems furthermore it is currently unknown if kinematic coherence in salt influenced extensional settings can be maintained over relatively long length scales 10 1 10 3 m and for relatively long timeframes and how this may impact the growth and geometry of large throw 500 m salt influenced normal fault systems in this paper we use a 3600 km 2 high quality 3d seismic reflection dataset and borehole data from the stavanger fault system sfs egersund basin eastern north sea basin to investigate i how pre rift salt thickness t v and sub salt fault throw t control the structural style and evolution of a basin bounding salt influenced normal fault system and ii the role salt plays in maintaining kinematic coherence in normal fault systems we demonstrate that i pre rift salt distribution t v specifically its presence in the proto footwall i e when t v 0 is the primary control on partitioning of faulting and forced folding along the fault system and the style of linkage i e hard or soft linkage between sub and supra salt fault populations and ii sub and supra salt fault populations represent brittle elements of a single geometrically and kinematically coherent structure the geometry and evolution of which is related to the ductile translation of strain on a scale up to 8 km and duration c 65 myr that believe is significantly greater and longer than previously documented

on low frequency variability of the midlatitude ocean gyres the first two authors are thankful to the natural environment research council for the support of this work through the grant ne j006602 1 and the use of archer the uk national supercomputing service we express our gratitude to s burbidge and m harvey for their help with imperial college london cluster as well as to a thomas for his help with managing and maintaining the data storage the last two authors were supported by the spanish ministry of economy and competitiveness under grant tin2013 41049 p and this support is gratefully acknowledged we would also like to thank unknown referees for valuable comments and suggestions which helped us to improve the paper

stochastic variability of oceanic flows above topography anomalies 1 we describe a stochastic variability mechanism which is genuinely internal to the ocean i e not due to fluctuations in atmospheric forcing the key ingredient is the existence of closed contours of bottom topography surrounded by a stirring region of enhanced eddy activity this configuration leads to the formation of a robust but highly variable vortex above the topography anomaly the vortex dynamics integrates the white noise forcing of oceanic eddies into a red noise signal for the large scale volume transport of the vortex the strong interannual fluctuations of the transport of the zapiola anticyclone 100 sv in the argentine basin are argued to be partly due to such eddy driven stochastic variability on the basis of a 310 years long simulation of a comprehensive global ocean model run driven by a repeated year forcing

planetary wave response to surface forcing and instability in the presence of mean flow and topography the local response of an ocean with slowly varying mean flow stratification and topography to two sources of disturbance is examined concentrating on whether the resulting surface elevations are observable the first is the ocean response to surface forcing ekman pumping or buoyancy forcing for typical amplitudes of random forcing while much of the ocean response is small surface elevations less than 1 mm there are sufficient near resonances or pseudoresonances involving a critical layer to produce elevations of 1 cm or more in much of the ocean the second source is baroclinic instability the fastest linear growth rate as well as those for specific wavelengths is computed globally almost all of the ocean is baroclinically unstable and the most unstable waves are found to possess a small wavelength often less than 10 km with a disturbance concentrated near the surface e folding times o 20 days are frequently found however the phase speed for the disturbances is almost everywhere slower westward than free planetary waves with mean flow and topography since the free waves propagate at speeds similar to observations instability may be a good source of variability but is probably not responsible directly for observed wave propagation

kuroshio extension variability and forcing of the pacific decadal oscillations responses and potential feedback abstract a forcing mechanism is sought for the large scale circulation changes in the kuroshio extension region of the western north pacific ocean as inferred by topex poseidon sea surface height ssh data the low frequency signal of the kuroshio extension over the last decade was characterized by a modulation in its zonal mean flow intensity the mean kuroshio extension jet weakened progressively from 1993 to 1996 and this trend reversed after 1997 the ability to simulate the major trends in the observed ssh signals with linear vorticity dynamics leads the authors to conclude that the modulation in the zonal mean jet was remotely forced by wind stress curl anomalies in the eastern north pacific ocean related to the pacific decadal oscillations pdos to be specific the weakening strengthening trend in 1993 96 1997 2001 was caused by westward expansions of negative positive ssh anomalies south of the kuroshio extension and positive negative ssh anomalies north of the kuroshio extension emerg

sequence stratigraphy of carbonate evaporite basins models and application to the upper permian zechstein of northeast england and adjoining north sea the successions of carbonate evaporite basins which are periodically cut off from the world ocean are considered in terms of their sequence stratigraphy two basin types are distinguished those where incomplete drawdown takes place and those showing complete drawdown and wholesale basin desiccation it is argued that following relative sea level falls sequences begin with the evaporites precipitated as marginal lowstand gypsum wedges in the first basin type and as lowstand basin centre halite fills in the second the evaporites pass up into carbonate sediments which are mainly deposited on shallow water platforms around the basin during transgressive and highstand systems tracts the concepts and models developed are applied to the zechstein upper permian strata of northeast england and adjoining north sea seven sequences are proposed with some containing thin parasequences the sequence approach to carbonate evaporite basins which considers both basin margin and basin centre facies gives a more encompassing perspective of sedimentation than the traditional cycle concept with its emphasis on the basin centre

global bifurcation of shilnikov type in a double gyre ocean model abstract the dynamics of an idealized wind driven double gyre circulation in an ocean basin are studied from a dynamical systems point of view in an effort to better understand its variability while previous analyses of this circulation have mostly dealt with local bifurcations of steady states and limit cycles this study demonstrates the importance of considering global bifurcations as well in one case a coherent picture of the global dynamics spanning a range of parameters from where there are only stable steady state solutions to where there is chaotic eddy shedding is presented a simple but novel use of power spectra along with dynamical projections of the dynamics suggests that just beyond the regime in which there are only stable steady states the system exhibits a complicated global bifurcation known as the shilnikov phenomenon

a frequency dependent description of propagating sea level signals in the kuroshio extension region abstracthilbert empirical orthogonal function analysis is used to provide a frequency dependent description of observed sea level variability in the kuroshio extension region 1993 2012 inclusive the dominant high frequency mode periods between 140 and 350 days describes signals that propagate westward with the largest amplitudes in the vicinity of the shatsky rise and emperor seamounts based on the close correspondence between the variance of the high frequency variability and the underlying bathymetry it is speculated that this mode is driven by jet bathymetry interactions the dominant low frequency mode periods longer than 350 days is explained in terms of wind forced jet trapped rossby waves that propagate along the mean kuroshio extension jet one of the most surprising findings of this study is that sea level changes north of the jet in the meander region anticipate changes south of the jet by about 3 yr based on correlations of observed sea level with the pacific decadal oscillation and

the effects of mesoscale ocean atmosphere coupling on the large scale ocean circulation abstract small scale variation in wind stress due to ocean atmosphere interaction within the atmospheric boundary layer alters the temporal and spatial scale of ekman pumping driving the double gyre circulation of the ocean a high resolution quasigeostrophic qg ocean model coupled to a dynamic atmospheric mixed layer is used to demonstrate that despite the small spatial scale of the ekman pumping anomalies this phenomenon significantly modifies the large scale ocean circulation the primary effect is to decrease the strength of the nonlinear component of the gyre circulation by approximately 30 40 this result is due to the highest transient ekman pumping anomalies destabilizing the flow in a dynamically sensitive region close to the western boundary current separation the instability of the jet produces a flux of potential vorticity between the two gyres that acts to weaken both gyres

structural style and early stage growth of inversion structures 3d seismic insights from the egersund basin offshore norway abstract high quality three dimensional 3d seismic reflection and borehole data from the egersund basin offshore norway are used to characterise the structural style and determine the timing of growth of inversion related anticlines adjacent to a segmented normal fault system two thick skinned normal faults which offset permian clastics and evaporites delineate the north eastern margin of the basin these faults strike nnw sse have up to 1900 m of displacement and are separated by an ese dipping c 10 km wide relay ramp both of these faults display exclusively normal separation at all structural levels and tip out upwards into the upper part of the lower cretaceous succession at relatively shallow structural levels in the hangingwalls of these faults a series of open low amplitude fault parallel anticlines are developed these anticlines which are asymmetric and verge towards the footwalls of the adjacent faults are interpreted to have formed in response to mild inversion of the egersund basin the amplitude of and apparent shortening associated with the anticlines vary along strike and these variations mimic the along strike variations in throw observed on the adjacent fault segments we suggest that this relationship can be explained by along strike changes in the propensity of the normal faults to reactivate during shortening wider damage zones and lower angles of internal friction coupled with higher pore fluids pressures at the fault centre mean that reactivation is easier at this location than at the fault tips or in the undeformed country rock seismic stratigraphic analysis of growth strata indicate that the folds initiated in the latest turonian to earliest coniacian c 88 6 ma and santonian c 82 6 ma the control on this c 6 myr diachroneity in the initiation of fold growth is not clear but it may be related to strain partitioning during the early stages of shortening anticline growth ceased in the maastrichtian and the inversion event is therefore interpreted to have lasted at least c 20 myr this study indicates that 3d seismic reflection data is a key tool to investigate the role that normal fault segmentation can play in controlling the structural style and timing of inversion in sedimentary basins furthermore our results highlight the impact that this structural style variability may have on the development of structural and stratigraphic hydrocarbon traps in weakly inverted rifts

a kuroshio extension system model study decadal chaotic self sustained oscillations abstract a model study of the kuroshio extension system in which forcing is provided by a time independent climatological wind yields a mean meandering path and a decadal variability of the jet in significant agreement with in situ and altimetric measurements a reduced gravity primitive equation ocean model is implemented in a box spanning the whole north pacific including a schematic coastline at the western side and an analytical wind forcing is determined according to the ecmwf and comprehensive ocean atmosphere data set coads climatologies the modeled time averaged kuroshio extension shows meanders northern and southern recirculation regions and a jet penetration that are in good agreement with the corresponding observed climatological features this result suggests that intrinsic nonlinear mechanisms are likely to play a major role in determining the meander pattern of the mean flow the internal low frequency variability is found to be a chaotic bimodal self sustained oscillation between a

supra salt normal fault growth during the rise and fall of a diapir perspectives from 3d seismic reflection data norwegian north sea abstract normal faulting and the deep subsurface flow of salt are key processes controlling the structural development of many salt bearing sedimentary basins however our detailed understanding of the spatial and temporal relationship between normal faulting and salt movement is poor due to a lack of natural examples constraining their geometric and kinematic relationship in three dimensions to improve our understanding of these processes we here use 3d seismic reflection and borehole data from the egersund basin offshore norway to determine the structure and growth of a normal fault array formed during the birth growth and decay of an array of salt structures we show that the fault array and salt structures developed in response to i late triassic to middle jurassic extension which involved thick skinned sub salt and thin skinned supra salt faulting with the latter driving reactive diapirism ii early cretaceous extensional collapse of the walls and iii jurassic to neogene active and passive diapirism which was at least partly coeval with and occurred along strike from areas of reactive diapirism and wall collapse our study supports physical model predictions showcasing a three dimensional example of how protracted multiphase salt diapirism can influence the structure and growth of normal fault arrays

interdecadal variability of the southern ocean the intrinsic variability of the antarctic circumpolar current is investigated using an idealized wind driven model the model uses three quasigeostrophic layers with steady wind stress forcing and no diabatic effects despite the idealized nature of the model the simulations display a robust mode of low frequency variability in the flow it is demonstrated that this variability is dependent upon the explicit simulation of the dynamics of mesoscale eddies as such the variability is sensitive to stratification horizontal viscosity bottom stress and topography the energetic balance of the variability is diagnosed and a driving mechanism is proposed that involves positive feedback between the generation of eddies through baroclinic instability and the dynamics of the mean circulation

decadal variability of the kuroshio extension observations and an eddy resolving model hindcast abstract low frequency variability of the kuroshio extension ke is studied using observations and a multidecadal 1950 2003 hindcast by a high resolution 0 1 eddy resolving global ocean general circulation model for the earth simulator ofes in both the ofes hindcast and satellite altimeter observations low frequency sea surface height ssh variability in the north pacific is high near the ke front an empirical orthogonal function eof analysis indicates that much of the ssh variability in the western north pacific east of japan is explained by two modes with meridional structures tightly trapped along the ke front the first mode represents a southward shift and to a lesser degree an acceleration of the ke jet associated with the 1976 77 shift in basin scale winds the second mode reflects quasi decadal variations in the intensity of the ke jet both the spatial structure and time series of these modes derived from the hindcast are in close agreement with observations a linear rossby wave

a stratified model of the inertial recirculation abstract an inertial gyre with characteristics very similar to the recirculation observed in eddy resolving general circulation models is obtained with a simple analytically tractable two layer model the recirculating gyre is contained in a box of simple geometry which isolates it from the sverdrup interior the gyre is forced by prescribing anomalous values of potential vorticity northward in the subtropical gyre or can be though of as a rough parametrization of diabatic forcing in both cases the forcing is confined to the water above the thermocline which is represented by the upper layer and is transmitted to the abyssal ocean through interfacial friction the condition for the abyssal water to be set in motion is derived and for oceanic values the recirculation goes all the way to the bottom when this occurs the center of the gyre is dominated by a barotropic flow while the baroclinic flow is confined to the edges of the gyre the width and strength of the gyre can be easily calculated in the

reactivation of intrabasement structures during rifting a case study from offshore southern norway abstract pre existing structures within crystalline basement may exert a significant influence over the evolution of rifts however the exact manner in which these structures reactivate and thus their degree of influence over the overlying rift is poorly understood using borehole constrained 2d and 3d seismic reflection data from offshore southern norway we identify and constrain the three dimensional geometry of a series of enigmatic intrabasement reflections through 1d waveform modelling and 3d mapping of these reflection packages we correlate them to the onshore caledonian thrust belt and devonian shear zones based on the seismic stratigraphic architecture of the post basement succession we identify several phases of reactivation of the intrabasement structures associated with multiple tectonic events reactivation preferentially occurs along relatively thick c 1 km relatively steeply dipping c 30 structures with three main styles of interactions observed between them and overlying faults i faults exploiting intrabasement weaknesses represented by intra shear zone mylonites ii faults that initiate within the hangingwall of the shear zones inheriting their orientation and merging with said structure at depth or iii faults that initiate independently from and cross cut intrabasement structures we demonstrate that large scale discrete shear zones act as a long lived structural template for fault initiation during multiple phases of rifting

the dynamics of a simple baroclinic model of the wind driven circulation the authors study the dynamics of a two layer approximation to the steadily forced baroclinic circulation in a closed ocean basin with the aim of understanding its temporal variability and the onset of low frequency variability it is found that for a range of dissipation that includes values used in a number of ocean modeling studies in the past five years if one waits a sufficient length of time the asymptotic behavior of the system is characterized by only a very small number of degrees of freedom by varying the dissipation as a control parameter the authors identify abrupt transitions in the form of the long term circulation exhibited by the model one type of transition from a time varying circulation dominated by two frequencies to a chaotic circulation is accompanied by the appearance of low frequency variability this constitutes an internal mechanism for the production of variability at climatological timescales the model used is a two layer quasigeostrophic model forced by a steady wind stress with a uniform cyclonic curl dissipation is modeled by a lateral diffusion of momentum with a uniform eddy viscosity in two sets of experiments with two internal deformation radii and layer depth ratios the eddy viscosity is varied and the types of circulation that result are reported the stable steady circulation seen in the viscous limit gives way to time dependent circulations of increasing temporal complexity spatially the circulations fall into two types those dominated by large recirculating gyres in the western part of the basin and those with a strong and strongly meandering peripheral current reminiscent of that seen in the black sea from an examination of the linear eigenmodes of the steady circulation the initial transition to time dependence may be characterized as a baroclinic instability of the western recirculation gyre

dynamics of the gulf stream deep western boundary current crossover part ii low frequency internal oscillations abstract a low frequency oscillation in the gulf stream deep western boundary current dwbc system is identified and its influences on several important aspects of the basin scale circulation are investigated an eddy resolving regional primitive equation model is used to demonstrate that feedbacks between the gulf stream with its associated northern and southern recirculation gyres and the upper core of the dwbc can lead to self sustaining large amplitude internal oscillations of roughly decadal frequency the oscillator cycle is described as follows the upper core of the dwbc is entrained under the gulf stream through interaction with the eddy driven northern and southern recirculation gyres as described in part i of this study once entrained the low potential vorticity dwbc water stabilizes the gulf stream and suppresses the eddy fluxes that maintained the interior recirculation gyres this causes the upper dwbc to switch to a southward path along the western boundary thus removing the source o

origin of an anhydrite sheath encircling a salt diapir and implications for the seismic imaging of steep sided salt structures egersund basin northern north sea the ingress of meteoric fluids into the crests of salt structures typically results in the formation of anhydrite dominated caprock the migration of connate fluids up the margins of salt structures has also been documented although the products related to this are rarely sampled in the deep subsurface we use 3d seismic reflection and borehole data to document the geometry and lithology of a salt diapir seismic data suggest that a weld is developed along the diapir stem although borehole data indicate that the stem consists of an inner c 1500 m thick halite dominated zone and an outer c 250 m thick anhydrite dominated sheath the anhydrite may have formed early in the basin history as a depositional anhydrite or crestal caprock an alternative interpretation is that the anhydrite represents lateral caprock which formed late in the basin history in response to the migration of nacl poor fluid up the margins of the diapir and dissolution of halite the possibility that lateral caprock may form adjacent to salt structures has implications for understanding the patterns and vigour of groundwater flow in sedimentary basins furthermore our study shows the margins of steep sided salt structures may be misidentified by several hundred metres if time migrated seismic reflection data are used

a model of the inertial recirculation driven by potential vorticity anomalies abstract some essential features of a recirculating inertial gyre the recirculation can be analyzed with a very simple analytically tractable model in wind driven eddy resolving general circulation models the recirculation appears as a strong sub basin scale inertial flow with homogeneous potential vorticity the constant value of potential vorticity decreases with increasing forcing dissipation ratio while the size and the strength of the recirculating gyre increases in the subtropical gyre the recirculating gyre might be driven by anomalous values of low potential vorticity carried northward by the western boundary current we have modeled this process using a barotropic model and prescribing the values of potential vorticity at the edge of the gyre our model gyre is contained in a rectangular box in an attempt to simplify the geometry as much as possible and to isolate the processes occurring in the recirculating region with weak diffusion the prescribed boundary forcing induces a flow with co

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

arctic warming evolution and spreading of the 1990s warm event in the nordic seas and the arctic ocean 1 observations in the arctic ocean revealed changes in oceanic temperature salinity and ice cover of the 1990s as compared with earlier data with a numerical model we favorably reproduce the development and subsequent propagation of temperature anomalies in water of atlantic origin in the 1980s and 1990s these propagated into the arctic ocean via the barents sea and the fram strait two warm anomalies entered the arctic ocean through these passages while the first smaller anomaly only warmed up the western eurasian basin the second large anomaly spread far into the eastern eurasian basin and across the lomonossov ridge into the western arctic basins intensified boundary currents during the high nao state in the first half of the 1990s significantly influenced the amplitude and speed of propagation of the temperature anomalies inside the arctic ocean in contrast to the notion of a continuous warming process during the 1990s our model results suggest the warming of the atlantic layer in the arctic ocean occurred in the form of events the event with the largest anomalous heat input during the modeled period entered the arctic between 1989 and 1994 it is possible to trace back the additional heat input into the arctic to an increased volume inflow via the faroer scotland passage and reduced heat loss to the atmosphere in the early 1990s after a weaker warm inflow in the second half of the 1990s the most recent observations and the model results point to a recurring warm anomaly in the inflow from 1999 onward

the influence of structural inheritance and multiphase extension on rift development the northern north sea the northern north sea rift evolved through multiple rift phases within a highly heterogeneous crystalline basement the geometry and evolution of syn rift depocentres during this multiphase evolution and the mechanisms and extent to which they were influenced by pre existing structural heterogeneities remain elusive particularly at the regional scale using an extensive database of borehole constrained 2d seismic reflection data we examine how the physiography of the northern north sea rift evolved throughout late permian early triassic rp1 and late jurassic early cretaceous rp2 rift phases and assess the influence of basement structures related to the caledonian orogeny and subsequent devonian extension during rp1 the location of major depocentres the stord and east shetland basins was controlled by favorably oriented devonian shear zones rp2 shows a diminished influence from structural heterogeneities activity localises along the viking sogn graben system and the east shetland basin with negligible activity in the stord basin and horda platform the utsira high and the devonian lomre shear zone form the eastern barrier to rift activity during rp2 towards the end of rp2 rift activity migrated northwards as extension related to opening of the proto north atlantic becomes the dominant regional stress as rift activity in the northern north sea decreases through documenting the evolving syn rift depocentres of the northern north sea rift we show how structural heterogeneities and prior rift phases influence regional rift physiography and kinematics controlling the segmentation of depocentres as well as the locations styles and magnitude of fault activity and reactivation during subsequent events

velocity field of the western entrance to the barents sea 1 by the use of 4 year long records from moored current meters between 71 30 n and 73 30 n of the western entrance to the barents sea the velocity field of the atlantic inflow is examined the mean velocity field shows the atlantic inflow as a wide core that occupies most of this section but the general picture is a velocity field that is dominated by large and frequent fluctuations the key parameter that to a large degree determines the spatial distribution of the velocity field is sea level changes within the section that are induced by the local wind field the main process is the ekman transport through its ability to accumulate water thereby creating strong gradients of barotropic pressure and associated currents southwesterly winds along the norwegian coast and southeasterly winds farther north will in general create wide inflows while northerly winds will result in wide outflows these flow regimes may be persistent for up to 2 3 weeks and are related to the relative strength and lateral extension of the icelandic low and the arctic high although the alignment of the local isobars must be considered in order to be able to describe the details of the flow

ocean eddy dynamics in a coupled ocean atmosphere model the role of mesoscale oceanic eddies is analyzed in a quasigeostrophic coupled ocean atmosphere model operating at a large reynolds number the model dynamics are characterized by decadal variability that involves nonlinear adjustment of the ocean to coherent north south shifts of the atmosphere the oceanic eddy effects are diagnosed by the dynamical decomposition method adapted for nonstationary external forcing the main effects of the eddies are an enhancement of the oceanic eastward jet separating the subpolar and subtropical gyres and a weakening of the gyres the flow enhancing effect is due to nonlinear rectification driven by fluctuations of the eddy forcing this is a nonlocal process involving generation of the eddies by the flow instabilities in the western boundary current and the upstream part of the eastward jet the eddies are advected by the mean current to the east where they backscatter into the rectified enhancement of the eastward jet the gyre weakening effect which is due to the time mean buoyancy component of the eddy forcing is a result of the baroclinic instability of the westward return currents the diagnosed eddy forcing is parameterized in a non eddy resolving ocean model as a nonstationary random process in which the corresponding parameters are derived from the control coupled simulation the key parameter of the random process its variance is related to the large scale flow baroclinicity index it is shown that the coupled model with the non eddy resolving ocean component and the parameterized eddies correctly simulates climatology and low frequency variability of the control eddy resolving coupled solution

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

on ocean dynamics in midlatitude climate abstract several recent models of midlatitude climate have speculated on the role of the north atlantic ocean in modulating the north atlantic oscillation nao here this role is examined by means of numerical experimentation with a quasigeostrophic ocean model underneath a highly idealized atmosphere it is argued that the dominant midlatitude oceanic influence is due to the so called inertial recirculations rather than linear baroclinic waves as have previously been studied in these experiments the forced response of the inertial recirculations dominates the leading order ocean spatial mode but that mode is energized by oceanic intrinsic variability the oceanic signals are amplified relative to those predicted by wave models the primary oceanic role of the coupling is to damp sea surface temperature sst at longer timescales and the interdecadal atmospheric variability is placed under the control of the ocean the sst damping reflects competition between intrinsically driven intergyre heat flu

physiography of the ne margin of the permian salt basin new insights from 3d seismic reflection data three dimensional seismic reflection data from the egersund basin offshore norway image geomorphological features that record mid permian footwall degradation of basin bounding fault systems this ancient landscape was subsequently flooded during the pan european late permian transgression of the north permian salt basin and was fossilized beneath zechstein supergroup evaporites we provide the first conclusive evidence for pre zechstein normal faulting in the egersund basin indicating that extensional strain was shared between permian and late jurassic early cretaceous rift events supplementary materials uninterpreted versions of the seismic sections shown in figure 2 are available are available at www geolsoc org uk sup18678

decadal shifts of the kuroshio extension jet application of thin jet theory abstractmeridional shifts of the kuroshio extension ke jet on decadal time scales are examined using a 1960 2004 hindcast simulation of an eddy resolving ocean general circulation model for the earth simulator ofes the leading mode of the simulated ke represents the meridional shifts of the jet on decadal time scales with the largest southward shift in the early 1980s associated with the climate regime shift in 1976 77 a result confirmed with subsurface temperature observations the meridional shifts originate east of the date line and propagate westward along the mean jet axis a trajectory inconsistent with the traditionally used linear long rossby waves linearized in cartesian coordinates although the phase speed is comparable to that in the traditional framework the zonal scale of these westward propagation signals is about 4000 km and much larger than their meridional scale to understand the mechanism for the westward propagation of the ke jet shifts the authors consider the limit of a thin

boundary current separation in a quasigeostrophic eddy resolving ocean circulation model abstract the response of a rectangular flat bottom eddy resolving quasigeostrophic ocean to a steady double gyre wind stress is studied to assess the sensitivity of the solutions to a partial slip lateral boundary condition in which tangential stress is proportional to tangential velocity the constant of proportionality has limiting values of zero and infinity corresponding to free slip no stress and no slip conditions respectively seven numerical solutions corresponding to the values 0 0 2 0 3 5 5 0 6 5 8 0 and 100 0 are obtained which span the free slip and no slip limits significant qualitative changes in the time mean behavior of the solutions are observed to occur with increasing these changes include a gradual retreat of the separation points of the western boundary currents in the subtropical and subpolar gyres a dramatic reduction in the basin integrated reservoirs of mean and eddy kinetic energy a weakening of bottom dissipation and its replacement by lateral dissipat

multidecadal mobility of the north atlantic oscillation abstractthe north atlantic oscillation nao is one of the most important modes of variability in the global climate system and is characterized by a meridional dipole in the sea level pressure field with centers of action near iceland and the azores it has a profound influence on the weather climate ecosystems and economies of europe greenland eastern north america and north africa it has been proposed that around 1980 there was an eastward secular shift in the nao s northern center of action that impacted sea ice export through fram strait independently it has also been suggested that the location of its southern center of action is tied to the phase of the nao both of these attributes of the nao have been linked to anthropogenic climate change here the authors use both the one point correlation map technique as well as empirical orthogonal function eof analysis to show that the meridional dipole that is often seen in the sea level pressure field over the north atlantic is not purely the

quantitative analysis of triassic extension in the northern viking graben it has been recognized for over a decade that large displacement pre jurassic faults are present in the northern viking graben part of the north sea rift these faults define a series of major fault blocks below the more obvious jurassic rift basin we attempt here to quantify the amount of extension associated with this older rift event which is probably of early triassic age quantitative modelling of the triassic rift and the succeeding period of thermal subsidence has been undertaken using a combination of flexural backstripping and flexural cantilever forward modelling these techniques suggest that triassic extension across the horda platform norwegian sector reached c 40 s 1 4 the consequences of this extension were deposition of a thick 3 km triassic upper jurassic syn rift and post rift sequence prior to renewed but minor extension in the late jurassic earliest cretaceous the thickness of the viking group reservoirs in the troll area appears to have been almost entirely controlled by sediment loading during post triassic thermal subsidence jurassic extension on the horda platform was in the uk sector of the viking graben triassic structures are less obvious than those below the horda platform because of jurassic overprinting they are however still present average triassic extension across the east shetland basin was c 15 comparable with the magnitude of jurassic extension in the same area we believe that the tern eider and cormorant fault blocks with proven shallow basement comprised a large eroded horst during the early triassic uplifted in the footwalls of major faults flanking the magnus and statfjord half graben during the triassic the magnus half graben was contiguous with the unst basin now situated in the western footwall of the younger jurassic basin the presence of the unst basin suggests that triassic extension occurred across the area that is now the northern shetland platform continuing into the west shetland area although the more obvious structures in the viking graben are jurassic in age the earlier triassic event was equally as important in controlling the structural and stratigraphic history of the basin

evolution of the nw margin of the north permian basin uk north sea mapping of depositional zones within the zechstein supergroup using semi regional 2d and 3dseismic data constrains the geological evolution of the northwestern margin of the north permian basinin uk offshore quadrants 14 15 19 20 and 21 four depositional zones are identified on the basis of mappable seismic facies which are assumed to vary with halite content well control characterizes the most reflective seismic facies as an interbedded carbonate and anhydrite shelf succession which grades laterally through clastic rich sequences to the least reflective a thick halite basinal equivalent the trendsof the depositional zones illustrate the geometry of the north permian basin and may have been locally controlled by basement lineaments within the north permian basin a n s trending axis devil s hole high characterized by thick reflective facies separates the central north sea from the forth approaches basin basement fault blocks control facies type and thickness along the northwest margin of the north permian basin in the moray firth area permian tilt of the zechstein shelf related to regional flexure played a role in precipitating gravity driven tectonics within the seismically reflective facies on the east flank of the devil s hole high this permian phase of salt tectonics resulted in numerous intra zechstein structures which can be interpreted as rafted and folded zones of interbedded halite carbonate sulphate and clastics the rafts initially slid downdip towards the main halite basin then their evolution became dominated by differential subsidence of the interbeds which forced salt layers into adjacent passive diapirs this episode predates deposition of the triassic smith bank shales which rest upon the zechstein with local angular unconformity the spatial distribution of zechstein facies influenced the tectonostratigraphy of the triassic jurassic and cretaceous systems boundaries between zechstein facies correspond to the limit of triassic halokinesis on the west central shelf and the limit of detachment of jurassic and lower cretaceous thin skinned tectonics in moray firth half graben

salt dissolution and salt related deformation of the forth approaches basin uk north sea abstract this paper addresses the potential role of regional scale salt dissolution as a mechanism responsible for salt related deformation this analysis is based on an investigation of the late permian to triassic succession of the forth approaches basin on the nw margin of the north permian basin and makes use of a regional 2d seismic grid tied to five key exploration wells this basin is ideal for this study because it consists of a deformed central region with a ne sw trending axis and an undeformed periphery which provides a template and key datums for reconstruction of the adjacent zone of deformation structural styles in this zone include simple salt cored anticlines salt walls salt diapirs detachment faults and unusually for the north sea intra zechstein salt cored folds pre triassic detailed mapping was carried out in uk block 26 10 and this shows that the dominant structural trend is ne sw suggesting that basement tilting influenced structural development the stratal geometry of the numerous triassic mini basins is used to constrain the way in which accommodation space in the mini basins was created by depletion of mobile salt the spatial pattern of salt depletion can be accounted for by two contrasting mechanisms or some combination thereof 1 deformation through downbuilding during a phase of regional subsidence or 2 flow induced by regional salt dissolution volumetric arguments necessary to discriminate between these two mechanisms are inconclusive due to later deformation of the top zechstein datum selective preservation of z3 and z4 cycles of the zechstein over large parts of the block 26 10 study area suggests that much of the salt depletion process preferentially removed z2 halite this observation is compatible with a process of subjacent dissolution possibly resulting from low salinity groundwater circulating in the pre zechstein aquifers

the influence of mechanically weak layers in controlling fault kinematics and graben configurations examples from analog experiments and the norwegian continental margin fault systems in extensional basins commonly display geometries that vary with depth reflecting depth and lithology dependent mechanical strength using an experimental approach we investigate this relationship by deploying physical analog models with stratified sequences consisting of brittle ductile sand silicone polymer sequences subject to single and polyphase deformation the experiments were used as analogs for a sandstone sequence interlayered by beds of evaporates or overpressured or unconsolidated mudstone in nature the latter being representative of decollement horizons experiments series 1 s1 using homogeneous and stratified quartz and feldspar sand produced asymmetric composite single grabens with diverse fault frequencies and fault styles for the graben margin faults for the mechanically stratified experiments with one decollement level series 2 contrasting graben configurations were produced in that the lowermost sequence was characterized by graben geometries of similar type to that of the s1 experiments whereas the sequence above the decollement was characterized by large fault blocks delineated by steepened or oversteepened faults the experiments with two decollements series 3 were displayed similarly but included graben geometries that widened upward with each level being characterized by independent fault systems the results can be used to explain strata bound fault patterns and depth dependent extension as seen in several places along the norwegian continental margin and elsewhere

mechanisms of decadal variability of the wind driven ocean circulation eddy resolving quasigeostrophic simulations of wind driven circulation in a large ocean basin are presented the results show that strong modes of low frequency variability arise in many parameter regimes and that the strength of these modes depends upon the presence of inertial recirculations in the flow field the inertial recirculations arise through advection of anomalous potential vorticity by the western boundary current and are barotropized by the effect of baroclinic eddies in the flow the mechanism of low frequency oscillations is explored with reference to previous studies and it is found that the observed mode can be linked to the gyre mode but is strongly modified by the effect of eddies

multiple equilibria and low frequency variability of the wind driven ocean circulation abstract the link between low frequency time dependent variability and the existence of multiple unstable steady state solutions in a reduced gravity quasigeostrophic ocean model for the midlatitude wind driven circulation is investigated it is shown that a sequence of successive symmetry breaking pitchfork bifurcations lead to multiple equilibria that differ from each other primarily in the elongation of the recirculation cell in the amount of meandering present in the intergyre jet and in a north south shift in the eastward jet the elongation of the recirculation cells and the meandering of the jet play compensating roles in the establishment of the global energy and vorticity balance the solutions also have distinct energy levels but general agreement between them and the bumps in a histogram of the total energy obtained from a 1200 yr time dependent simulation is not found nevertheless a substantial fraction of the variance 30 can be accounted for by four coherent structures that capture th