the corresponding tropospheric environments during downward extending and nondownward extending events of stratospheric northern annular mode anomalies abstractusing the ncep ncar reanalysis dataset this study classifies stratospheric northern annular mode nam anomalies during the negative or positive phase into two categories anomalies extendi

a detached eddy simulation model for the study of lateral separation zones along a large canyon bound river lateral flow separation occurs in rivers where banks exhibit strong curvature in canyon bound rivers lateral recirculation zones are the principal storage of fine sediment deposits a parallelized three dimensional turbulence resolving model was developed to study the flow structures along lateral separation zones located in two pools along the colorado river in marble canyon the model employs the detached eddy simulation des technique which resolves turbulence structures larger than the grid spacing in the interior of the flow the des 3d model is validated using adcp flow measurements taken during the 2008 controlled flood release from glen canyon dam a point to point validation using a number of skill metrics often employed in hydrological research is proposed here for fluvial modeling the validation results show predictive capabilities of the des model the model reproduces the pattern and magnitude of the velocity in the lateral recirculation zone including the size and position of the primary and secondary eddy cells and return current the lateral recirculation zone is open having continuous import of fluid upstream of the point of reattachment and export by the recirculation return current downstream of the point of separation differences in magnitude and direction of near bed and near surface velocity vectors are found resulting in an inward vertical spiral interaction between the recirculation return current and the main flow is dynamic with large temporal changes in flow direction and magnitude turbulence structures with a predominately vertical axis of vorticity are observed in the shear layer becoming three dimensional without preferred orientation downstream

shallow water turbulence modeling and horizontal large eddy computation of river flow by introducing the concept of sds subdepth scale turbulence to model three dimensional 3d turbulence with length scales less than the water depth and treating it explicitly with a proper separate modeling an sds 2dh model has been developed to simulate the evolution of horizontal large scale eddies in shallow water applying this model to river flows with transverse shear due to vegetation drag the horizontal large scale hls eddies were found to dominate horizontal momentum mixing the bottom friction and vegetation drag acting as sinks of vorticity play the key roles in the development of the horizontal large scale eddies and in reynolds stress generation the sds 2dh model can directly describe effects of flow geometry such as vegetation layer width on the large scale eddy development and hence predicts turbulence mixing better than the k e model

geological modelling from field data and geological knowledge part ii modelling validation using gravity and magnetic data inversion the analysis of multiple data sets is required to select a realistic 3d geological model among an infinite number of possibilities an inverse method that aims at describing the 3d geometry of geological objects is presented the method takes into account the geology and the physical properties of rocks while respecting the topological properties of an a priori model the a priori model is built from the geological data set and its geometry is largely dependent upon assumptions about inaccessible geology at depth this method referred to as total litho inversion is a generalized 3d inversion that results in quantifying the lithology and the distribution of rock property in a probabilistic way its application is demonstrated through i a simple synthetic case and ii the relative distribution characterization of granites and diorites in an orogenic domain

3d stochastic inversion of gravity data using cokriging and cosimulation a new application has been developed based on geostatistical techniques of cokriging and conditional simulation for the 3d inversion of gravity data including geologic constraints the necessary gravity density and gravity density covariance matrices are estimated using the observed gravity data then the densities are cokriged or simulated using the gravity data as the secondary variable the model allows noise to be included in the observations the method is applied to two synthetic models a short dipping dike and a stochastic distribution of densities then some geologic information is added as constraints to the cokriging system the results show the ability of the method to integrate complex a priori information the survey data of the matagami mining camp are considered as a case study the inversion method based on cokriging is applied to the residual anomaly to map the geology through the estimation of the density distribution in this region the results of the inversion and simulation methods are in good agreement with the surface geology of the survey region

eddies and the annular modes of climate variability the dominant modes of month to month varia tion in the troposphere are realistically simulated in a gen eral circulation model whose lower boundary is prescribed with realistic topography and seasonally varying climato logical sea surface temperatures in both hemispheres these internal modes describe a nearly zonally symmetric north south shifting of the zonal jets as anomalous westerlies vac illate between high 50 60 a nd low 30 40 latitudes the eddy structures evolve with the jets and the corre sponding eddy momentum forcings support the shifts in jet position stationary synoptic wave variations mainly ac count for the total eddy forcings in the northern south ern hemisphere pronounced eddy variations occur over the north atlantic sector

ellipsoid geoid gravity geodesy and geophysics geophysics uses gravity to learn about the density variations of the earth s interior whereas classical geodesy uses gravity to define the geoid this difference in purpose has led to some confusion among geophysicists and this tutorial attempts to clarify two points of the confusion first it is well known now that gravity anomalies after the free air correction are still located at their original positions however the free air reduction was thought historically to relocate gravity from its observation position to the geoid mean sea level such an understanding is a geodetic fiction invalid and unacceptable in geophysics second in gravity corrections and gravity anomalies the elevation has been used routinely the main reason is that before the emergence and widespread use of the global positioning system gps height above the geoid was the only height measurement we could make accurately i e by leveling the gps delivers a measurement of height above the ellipsoid in principle in

seismic reflections from the basal contacts of batholiths although seismic reflection data in batholithic terranes are scarce datasets available to us from three different regions show surprising similarities all three profiles one across precambrian rocks in texas and two across mesozoic batholiths in california and nevada show strong subhorizontal reflections at depths of 6 10 km these reflections are interpreted to originate from the base of the batholiths and indicate that these batholiths are tabular in shape the large amplitudes of the reflections require a marked contrast in acoustic impedance product of velocity and density between the batholith and the underlying rocks two of the profiles show reflective zones having appreciable thicknesses 0 5 2 5 km while the third profile shows a single continuous reflection the four most favored candidates for sub batholithic reflections are 1 underplated gabbro 2 cumulate layers 3 migmatites and 4 thrust faults cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance

history and controls of subsidence in the late cretaceous tertiary great valley forearc basin california abstract analysis of the upper cretaceous tertiary fill of the great valley forearc basin of california reveals a complicated history of subsidence partitioned in time and space western oceanward parts of the basin record subsidence then uplift in apparent response to the angle and rate of descent of the underlying subducting plate specifically uplift in the forearc basin corresponds to the onset of the laramide orogeny and inferred low angles of subduction in contrast eastern arcward parts of the basin display subsidence histories suggestive of thermal contraction and compatible with continentward migration of sierran arc magmatism in the late cretaceous these results imply that subsidence histories of forearc basins are likely to be much more complicated and less predictable than those of rifted continental margins and foreland basins where single subsidence mechanisms predominate and time varying subduction is not an underlying controlling factor

sediment eddy diffusivity in meandering turbulent jets implications for levee formation at river mouths 1 depositional patterns characteristic of river mouths are controlled by the sediment laden turbulent jet exiting from rivers here we show that jet instability developing at river mouths with small width to depth ratio can affect these patterns numerical simulations indicate that sediment eddy diffusivity which is a measure of the spreading of sediments out of the jet core depends on the interplay between sediment settling and large coherent flow structures which are associated with unstable jets our results suggest that optimal conditions for eddy diffusivity are met when the time scale of sediment settling is equal to the characteristic eddy time scale in this condition sediments are deposited at the farthest distance from the jet centerline thus promoting the formation of lateral deposits settling time scales much greater or much smaller than the eddy time scale is associated with low eddy diffusivity which promotes the formation of a central deposit such eddy mediated sediment transport is more evident in the zone of established flow than in the zone of flow establishment since the former is characterized by fully developed vortices a simplified model is proposed to estimate the settling and eddy time scales and to predict the optimal condition for levee formation

an exact solution for the gravity curvature bullard b correction the complete bouguer reduction includes in addition to the simple bouguer slab correction bullard a both curvature bullard b and terrain bullard c corrections a new closed form formula for the curvature correction is derived for which the calculated values differ from those published by swick by more than 0 5 mgal for high elevations these corrections reduce those of an infinite slab bullard a to that of a spherical cap having a surface radius of 166 7 km the spherical cap produces a lesser effect than the infinite slab because of the truncation of that part of the slab above the earth and extending to infinity but it produces a greater effect than the slab because of subslab earth resulting from curvature the physical significance of the correction lies in the combination of these two differences which are each a function of elevation the bullard b surface radius 166 7 km outer radius of the hayford bowie system is reaffirmed by the exact formula to be appropriate for exploration su

model for the tectonic evolution of the mississippi embayment and its contemporary seismicity aeromagnetic and gravity surveys of the northern mississippi embayment have delineated a northeast trending basement depression about 70 km wide and more than 300 km long the feature the mississippi valley graben is remarkably linear over its known length has nearly parallel sides and has estimated depths relative to surrounding basement of more than 2 km both gravity and magnetic anomalies show an alignment of dense magnetic plutons along the boundaries of the graben estimated magnetization directions for the plutons indicate a mesozoic age the graben of probable earliest paleozoic age underlies the northwest side of the embayment and contains the area of major seismicity in particular much of the instrumentally located contemporary seismicity and the estimated locations of the large magnitude 1811 1812 earthquakes are in the central part of the graben we interpret the early paleozoic graben the mesozoic plutonism and the sedimentation cycles as evidence of stretching events seismicity is attributed to strain in and below the central graben area which would appear to be a focal point of deformation in the stretching model we suggest that the sequence of events can be explained by the action of a rotating horizontal stress field on a crustal flaw that persisted well back in pre cambrian time the postulated rotating stress field is presumably controlled by the system that drives the global plates

role of the quasi biennial oscillation in the downward extension of stratospheric northern annular mode anomalies using the jra 55 reanalysis datasets and radiosonde observations the impact of the quasi biennial oscillation qbo on the downward extension of stratospheric northern annular mode nam signals in its positive and negative phases positive and negative nam hereafter is examined the results show that the average number of days per month on which stratospheric negative nam signals extend downward into the troposphere is more than twice as large during the easterly phase of the qbo eqbo as those during the westerly phase of the qbo wqbo the increasing likelihood of downward extension of stratospheric negative nam signals during the eqbo is related to three factors enhanced upward wave 1 fluxes entering the stratosphere anomalous poleward wave fluxes below 100 hpa and eqbo induced residual circulation it is also revealed that the eqbo induced subtropical tropospheric easterly anomalies lead to positive height anomalies over the subtropical north atlantic which are amplified by transient eddies and maintained by convection these subtropical positive height anomalies further stimulate an anomalous wave train forcing subpolar positive height anomalies over the euro atlantic sector and enhancing wave 1 over the pacific the eqbo could also induce positive height anomalies in the tropical troposphere and these anomalies are related to convection and vertical motion the positive height anomalies can induce anomalous poleward propagating waves forcing negative height anomalies over the north pacific at mid high latitudes and the enhancing wave 1 fewer stratospheric positive nam signals extend downward into the troposphere during the eqbo than during the wqbo with mechanisms similar to those of negative nam

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

mechanism of subsidence of ancient cratonic rift basins cratonic basins commonly occur over ancient rift zones these inactive rift basins are recognizable by a positive linear bouguer gravity anomaly that may correspond to the axial gravity high found in modern rift valleys many of these basins undergo discrete periods of increased subsidence rates or reactivations long after the mass excess responsible for the linear gravity high was emplaced furthermore the reactivation of many cratonic basins occurs simultaneously with large scale compressional tectonics it is suggested that the driving force for subsidence is the isostatically uncompensated ancient mass excess the subsidence of these basins is modelled by a lithospheric flexure model with a nonlinear maxwell viscoelastic rheology solutions to this model indicate that basins may experience a low subsidence rate throughout geologic time the subsidence of a basin will stop only when isostatic compensation of the mass excess is achieved since ancient rift mass excesses may be uncompensated over long geologic time intervals the early thermal and structural evolution of rifts may not significantly influence later basin subsidence the models suggest that basins may be reactivated by any mechanism which lowers the effective viscosity of the lithospheric material allowing the uncompensated basin to settle toward an isostatic compensation depth faster than normal since viscosity is a strong function of temperature reactivation by a world wide increase in heat flow is suggested as a possible mechanism for the synchroneity of basin subsidence throughout a continent an increase of 15 in the geothermal gradient for example from 16 5 18 9 k km will cause about a 5 increase in subsidence this increase in heat flow however seems unlikely of producing by itself the magnitude of basin subsidence during a reactivation phase that is observed in the geologic record where up to 100 increase in subsidence might occur since the rheology of the lithosphere is nonlinear effective viscosity is also a strong nonlinear function of stress the presence of a regional compressive stress during periods of tectonism of 1 1 108 pa about 2 8 of the buckling strength of the lithosphere produces a short period of reactivated subsidence 105 yr during the reactivated subsidence the newly imposed regional stress relaxes sufficiently in the lower lithosphere to restore the effective viscosity to values similar to that before reactivation this suggests that reactivated subsidence caused by regional compression can be maintained as long as the stress level remains high in the lower lithosphere this may be accomplished by an intermittent application of the regional stress over time

recent breakdown of the seasonal linkage between the winter north atlantic oscillation northern annular mode and summer northern annular mode abstractthe summer northern annular mode nam and the winter north atlantic oscillation nao winter nam have a positive correlation from the mid 1960s to the 1980s namely when the winter nao na

abrupt evolution of the summer northern hemisphere annular mode and its association with blocking 1 using reanalysis data from the national centers for environmental prediction national center for atmosperic research boulder colorado for the period from 1958 to 2005 we statistically analyzed the relationships of the summer northern hemisphere annular mode summer nam with hemispheric scale anomalous summer weather and the occurrence of blocking highs the anomalous positive nam low pressure anomaly in the arctic and high pressure anomaly in midlatitudes accounts well for the hemispheric scale weather associated with anomalous blocking between the polar and subtropical jets whereas blocking rarely occurs during negative nam periods the double jet stream structure is more evident during periods of anomalous positive nam than during periods of negative nam the surface temperatures associated with the anomalous positive nam clearly show europe to be hot and east asia to be cool as was the case during the anomalous summer of 2003 the occurrence of a positive summer nam is therefore consistent with the hemispheric scale anomalous summer weather associated with blocking in 2003 we investigated the abrupt evolution of atmospheric patterns and the geographic distribution of blocking highs associated with the development maintenance and decay periods of an anomalous positive nam during the development period blocking tends to occur over europe and the atlantic ocean but no significant blocking signature is evident over eastern eurasia during the maintenance stage blocking tends to occur in the far east during the decay stage blocking over the pacific region is obvious this longitudinal migration of blocking phenomena may be used to predict the evolution through time of the nam

forward modeling of gravity data using geostatistically generated subsurface density variations abstractusing geostatistical models of density variations in the subsurface constrained by geologic data forward models of gravity anomalies can be generated by discretizing the subsurface and calculating the cumulative effect of each cell pixel the results of such stochastically generated forward gravity anomalies can be compared with the observed gravity anomalies to find density models that match the observed data these models have an advantage over forward gravity anomalies generated using polygonal bodies of homogeneous density because generating numerous realizations explores a larger region of the solution space the stochastic modeling can be thought of as dividing the forward model into two components that due to the shape of each geologic unit and that due to the heterogeneous distribution of density within each geologic unit the modeling demonstrates that the internally heterogeneous distribution of density within each geologic unit can contribute significantly to the resulting calculat

evidence for wave resonance as a key mechanism for generating high amplitude quasi stationary waves in boreal summer several recent northern hemisphere summer extremes have been linked to persistent high amplitude wave patterns e g heat waves in europe 2003 russia 2010 and in the us 2011 floods in pakistan 2010 and europe 2013 recently quasi resonant amplification qra was proposed as a mechanism that when certain dynamical conditions are fulfilled can lead to such high amplitude wave events based on these resonance conditions a detection scheme to scan reanalysis data for qra events in boreal summer months was implemented with this objective detection scheme we analyzed the occurrence and duration of qra events and the associated atmospheric flow patterns in 1979 2015 reanalysis data we detect a total number of 178 events for wave 6 7 and 8 and find that during roughly one third of all high amplitude events qra conditions were met for respective waves our analysis reveals a significant shift for quasi stationary waves 6 and 7 towards high amplitudes during qra events lagging first qra detection by typically one week the results provide further evidence for the validity of the qra hypothesis and its important role in generating high amplitude waves in boreal summer

the summertime annular mode in the northern hemisphere and its linkage to the winter mode 1 the seasonal variations of the northern hemisphere annular mode nam are investigated through empirical orthogonal function analysis of the zonally averaged geopotential height fields for each individual calendar month patterns of the winter and summer nams differ not only in the geopotential height fields but also in the mean meridional circulation and eddy structure the summer nam has a smaller meridional scale and is displaced poleward as compared to the winter nam the antinode on the lower latitude side in the summer nam is at the nodal latitude of the winter nam the summer nam is more strongly related to surface air temperatures over eurasia than the original arctic oscillation the summer nam is a wave driven internal atmospheric mode that is maintained by both stationary and transient waves the summer nam is associated with the arctic front polar jet and storm track around the arctic ocean the winter to summer linkage described by m ogi et al can be interpreted as a preferred transition from one polarity of the winter annular mode to the same polarity of the summer annular mode the spring cryosphere i e snow in eurasia and sea ice in the barents sea plays a supporting role in this transition index terms 1620 global change climate dynamics 3309 3309 meteorology and atmospheric dynamics climatology 1620 3319 meteorology and atmospheric dynamics general circulation 3349 meteorology and atmospheric dynamics polar meteorology keywords northern hemisphere s annular mode arctic oscillation winter to summer link

rift structure of the northern mississippi embayment from the analysis of gravity and magnetic data gravity and magnetic data have been simultaneously inverted to derive a crustal model of the northern mississippi embayment which is compatible with geologic and seismic refraction data the results indicate that this region is a site of a late precambrian early paleozoic rift system an anomalously dense layer is present at the base of the crust and thickens beneath a broad northeast trending graben that formed during the initial phases of rifting the thickest part of anomalous crust occurs beneath the region of greatest seismic activity which lies within the geographic limits of the graben it is suggested from this correlation that strain accumulates within the anomalous crust layer which acts as an inhomogeneity in a relatively homogeneous lithosphere based on the results of this study and previous geophysical studies the complex tectonic evolution of the upper mississippi embayment since late precambrian time is rift formation possibly along a pre existing shear zone graben development in cambrian time reactivations in permian and cretaceous time and subsequent formation of the present mississippi embayment and fault zones with large influences from rift structures

mechanisms of intraseasonal amplification of the cold siberian high mechanisms of intraseasonal amplification of the siberian high are investigated on the basis of composite anomaly evolution for its strongest events at each of the grid points over siberia at each location the amplification of the surface high is associated with formation of a blocking ridge in the upper troposphere over central and western siberia what may be called wave train atlantic origin type is common where a blocking ridge forms as a component of a quasi stationary rossby wave train propagating across the eurasian continent a cold air outbreak follows once anomalous surface cold air reaches the northeastern slope of the tibetan plateau it is found through the potential vorticity pv inversion technique that interaction between the upperlevel stationary rossby wave train and preexisting surface cold anomalies is essential for the strong amplification of the surface high upper level pv anomalies associated with the wave train reinforce the cold anticyclonic anomalies at the surface by inducing anomalous cold advection that counteracts the tendency of the thermal anomalies themselves to migrate eastward as surface thermal rossby waves the surface cold anomalies thus intensified in turn act to induce anomalous vorticity advection aloft that reinforces the blocking ridge and cyclonic anomalies downstream of it that constitute the propagating wave train the baroclinic development of the anomalies through this vertical coupling is manifested as a significant upward flux of wave activity emanating from the surface cold anomalies which may be interpreted as dissipative destabilization of the incoming external rossby waves

a vortex based model of velocity and shear stress in a partially vegetated shallow channel 1 this paper presents a method for predicting the distributions of velocity and shear stress in shallow channels with a boundary of emergent vegetation experiments in a laboratory channel with model vegetation show that the velocity profile exhibits a distinct two layer structure consisting of a rapidly varying shear layer across the vegetation interface and a more gradual boundary layer in the main channel in addition coherent vortices are observed which span both layers and are the dominant contributors to lateral momentum fluxes from these observations we propose a model for the vortex induced exchange and find expressions for the width of momentum penetration into the vegetation the velocity and shear stress at the vegetation edge and the width of the boundary layer in the main channel these variables along with a momentum balance in the main channel comprise a modeling framework which accurately reproduces the observed velocity and shear stress distributions the predictions for the velocity and shear stress can provide a basis for modeling flood conveyance overbank sediment transport and scalar residence time in the vegetated layer

wave maintained annular modes of climate variability abstract the leading modes of month to month variability in the northern and southern hemispheres are examined by comparing a 100 yr run of the geophysical fluid dynamics laboratory gcm with the ncep ncar reanalyses of observations the model simulation is a control experiment in which the ssts are fixed to the climatological annual cycle without any interannual variability the leading modes contain a strong zonally symmetric or annular component that describes an expansion and contraction of the polar vortex as the midlatitude jet shifts equatorward and poleward this fluctuation is strongest during the winter months the structure and amplitude of the simulated modes are very similar to those derived from observations indicating that these modes arise from the internal dynamics of the atmosphere dynamical diagnosis of both observations and model simulation indicates that variations in the zonally symmetric flow associated with the annular modes are forced by eddy fluxes in the free troposphere while

northern hemisphere snow cover and atmospheric blocking variability 1 the interseasonal relationship between northern hemisphere nh snow cover and regional blocking patterns is explored for a 31 year data set it is found that snow cover exerts an important influence on regional atmospheric blocking which in turn modulates snow cover extent at subcontinental scales observational results provide strong evidence of two primary linkages in the seasonal snow cover blocking relationship that support an interannual persistence cycle the first one links winter blocking over the atlantic and the subsequent spring summer eurasian north american snow cover anomalies the second one implies that spring summer eurasian north american snow cover precedes an anomalous winter atlantic blocking activity we describe the temporal stages of the snow cover blocking relationship in the framework of a six step conceptual model according to that an enhanced atlantic blocking activity in winter favors a later spring snow disappearance through an enhanced cold advection toward western eurasia the resulting snow cover anomalies partially force an opposite sign blocking response over west and central pacific which is sustained through spring and early summer presumably because of the persistence of snow cover anomalies this anomalous pattern seems to play a role in the propagation of snow cover anomalies from eurasia in spring to the hudson s bay region of north america in summer the excessive snow cover over this region induces an asymmetrical temperature distribution which in turn favors blocking activity over europe and the west pacific the connection between autumn and winter climates is not clear but it could be related with the ability of autumn high atl blocking activity to determine an early snow cover appearance in october over western eurasia this linkage completes a snow cover blocking cycle of interactions which identifies snow cover as a candidate for the recently observed blocking trends and a contributor to the interannual persistence of winter climate

atmospheric blocking space time links to the nao and pna in the northern hemisphere regions characterized by an enhanced frequency of atmospheric blocking overlap significantly with those associated with the major extra tropical patterns of large scale climate variability namely the north atlantic oscillation nao and the pacific north american pna pattern there is likewise an overlap in the temporal band width of blocks and these climate patterns here the nature of the linkage between blocks and the climate patterns is explored by using the era 40 re analysis data set to examine 1 their temporal and spatial correlation and 2 the interrelationship between blocks and the nao pna it is shown that a strong anti correlation exists between blocking occurrence and the phase of the nao pna in the north atlantic western north pacific and that there are distinctive inter basin differences with a clear geographical over north atlantic and quantitative over north pacific separation of typical blocking genesis lysis regions during the opposing phases of the climate patterns an empirical orthogonal function eof analysis points to a significant influence of blocking upon the nao pattern identifiable as the leading eof in the euro atlantic and a temporal analysis indicates that long lasting blocks are associated with the development of negative nao pna index values throughout their life time in addition an indication of a cause and effect relationship is set out for the north atlantic linkage

plate tectonics model for proterozoic continental accretion in the southwestern united states proterozoic crustal provinces in the southwestern united states decrease in age south from the archean wyoming province supracrustal rocks define three major age provinces from north to south 1 72 to 1 80 1 65 to 1 72 and 1 1 to 1 2 b y successions within each province are similar and define two assemblages a bimodal volcanic assemblage commonly overlain by a quartzite shale assemblage precambrian granitic plutons intrude all three provinces geochemical and sr isotope data indicate a variably depleted upper mantle source for basalts and a short lived these observations can be explained by a plate tectonics model involving successive marginal basin closures and andean type orogenies associated with southward migrating arcs the net result is to accrete 1 300 km of continental crust to the southwestern margin of north america between 1 8 and 1 1 b y ago

evolution and dynamics of summertime blocking over the far east and the associated surface okhotsk high time evolution and dynamics of an upper level blocking ridge over the far east and the associated surface high over the sea of okhotsk that give rise to abnormal summertime coolness over eastern japan are investigated through a composite analysis applied to distinct surface high events the formation mechanism and vertical structure of the blocking differ fundamentally between may and july attributable to seasonal changes in the jet structure storm track activity over the far east and the land sea thermal contrast within the okhotsk region in may forcing from migratory eddies is strong enough for precursory anticyclonic anomalies retrograding slowly over the north pacific to develop into a blocking ridge the ridge remains equivalent barotropic under the weak land sea thermal contrast in the okhotsk region in contrast the eddy forcing is no longer the primary factor for the blocking formation in july in the presence of the weakened and split westerlies rather the propagation of a stationary rossby wave packet that has emanated from precursory anticyclonic anomalies over northern europe stagnates over the far east and the subsequent local breaking of the packet leads to the blocking formation in a particular case in july 1993 a wave packet involved in blocking formation could be traced back as far upstream as the east coast of north america over the two previous weeks acting on the strong thermal contrast in july between the cool sea surface and a warm land mass to the west the anomalous surface easterlies induced by the blocking ridge can build up a cold surface anticyclone through cold advection rendering the blocking anomalies distinctly baroclinic copyright 2004 royal meteorological society

identification of a precambrian rift through missouri by digital image processing of geophysical and geological data digital free air and bouguer gravity anomaly images have been constructed from approximately 287 000 station readings for the region bordered by 25 49 n latitude and 80 110 w longitude the technique used to interpolate between station locations was based on a two dimensional spatial filter where the average of the anomaly values located within the filter area was computed the images contain as many as 256 contours values in byte variable so that subtle anomaly patterns can be identified and traced with much greater certainty than on most contour maps a newly discovered feature in the midcontinent is a gravity low that begins at a break in the midcontinent gravity high in se nebraska extends across missouri in a nw se direction and intersects the mississippi valley graben to form the pascola arch the anomaly varies from 120 to 160 km in width extends about 700 km and is best expressed in southern missouri where it has a bouguer amplitude of approximately 34 mgal the magnitude of the anomaly cannot be explained on the basis of a thickened section of paleozoic sedimentary rock the gravity data and the sparse seismic refraction data for the region are consistent with an increased crustal thickness beneath the gravity low some of the discrete positive magnetic anomalies in missouri are located along the borders of the gravity low digitally enhanced thermal infrared images from the heat capacity mapping mission show a distinct alignment of linear structures with the gravity feature the linear features in some cases correspond to mapped high angle normal faults to drape folds over relief within the precambrian basement and in some cases to extensions of mapped structures the gravity anomaly also cuts across the major precambrian boundary in se missouri marking the change from older sheared granites and metasedimentary rocks to younger granites and rhyolites given the cumulative evidence the gravity anomaly is probably the present expression of a failed arm of a rifting event perhaps one associated with the spreading that led to or preceded formation of the granite and rhyolite terrain of southern missouri

a new isostatic residual gravity map of the conterminous united states with a discussion on the significance of isostatic residual anomalies to display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust a new colored isostatic residual gravity map of the conterminous united states has been prepared using the gravity data set compiled for the gravity anomaly map of the united states society of exploration geophysicists 1982 the new isostatic residual gravity map is based on an airy heiskanen model of local compensation in which the surface load requiring compensation is defined by 5 min topographic and bathymetric data sets a colored first vertical derivative map of isostatic residual gravity further enhances the short wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field for the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features anomalies on isostatic residual gravity maps should not be casually interpreted in terms of undercompensation or overcompensation because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies we present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment in general highs correlate with intruded or accreted mafic material or with upthrusted crustal sections whereas lows occur over low density sedimentary or volcanic sections felsic intrusive bodies or down warped crustal sections although some longer wavelength anomalies such as the broad gravity high centered over montana could be manifestations of density contrasts deep in the mantle many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers so that their source bodies need not be deep the fact that certain of these broader anomalies have well defined boundaries which correlate with near surface geologic features increases the likelihood that their sources lie entirely within the lithosphere if so then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions we have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the united states these fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories

the recent shift in early summer arctic atmospheric circulation 1 the last six years 2007 2012 show a persistent change in early summer arctic wind patterns relative to previous decades the persistent pattern which has been previously recognized as the arctic dipole ad is characterized by relatively low sea level pressure over the siberian arctic with high pressure over the beaufort sea extending across northern north america and over greenland pressure differences peak in june in a search for a proximate cause for the newly persistent ad pattern we note that the composite 700 hpa geopotential height field during june 2007 2012 exhibits a positive anomaly only on the north american side of the arctic thus creating the enhanced mean meridional flow across the arctic coupled impacts of the new persistent pattern are increased sea ice loss in summer long lived positive temperature anomalies and ice sheet loss in west greenland and a possible increase in arctic subarctic weather linkages through higheramplitude upper level flow the north american location of increased 700 hpa positive anomalies suggests that a regional atmospheric blocking mechanism is responsible for the presence of the ad pattern consistent with observations of unprecedented high pressure anomalies over greenland since 2007 2012 american geophysical union all rights reserved

dynamics of shallow lateral shear layers experimental study in a river with a sandy bed shallow lateral shear layers forming between flows with different velocities though essential for mixing processes in natural streams have been examined only in laboratory settings using smooth fixed bed channels this paper reports the results of an experimental study of a shear layer in a straight reach of a natural river where the layer in contrast to the two dimensional structure observed in the laboratory is highly three dimensional the study included pronounced transverse pressure gradients which influenced shear layer structure compared to flume experiments it also introduces an analysis that complements conventional theory on mixing layers the lateral velocity gradient between the flows downstream from a splitter plate placed in the river the principal controlling factor was adjusted for three experimental runs to determine the influence of different gradients on shear layer dynamics in each run detailed three dimensional measurements of mean and turbulent characteristics were obtained at five cross sections downstream from the splitter plate although experimental results agreed with conventional mixing layer theories with respect to turbulence the dynamics of the shear layers were dominated by the mean lateral fluxes of momentum after re examining the governing equations we developed a parabolic equation describing the shear layer evolution and several scaling relations for essential terms of the energy budget mean and turbulent lateral fluxes of momentum turbulent kinetic energy and dissipation rates the study also provides insight into the spectral dynamics of turbulence in the shear layer and clarifies previous observations reported for confluences in natural streams

precambrian basement geology of north and south dakota combined analysis of drill hole gravity and magnetic data indicates that the buried precambrian basement rocks of the dakotas can be divided into several lithotectonic terranes eastern north dakota and northeastern south dakota are underlain by archean gneiss except for the black hills region of south dakota where archean rocks are also exposed the western third of both dakotas is underlain mainly by early proterozoic gneiss and metasedimentary rocks part of this region is underlain by archean crust with an early proterozoic tectonic overprint a broad transition zone of strongly overprinted archean crust occurs between the proterozoic rocks to the west and the archean rocks to the east south central south dakota is underlain by an early proterozoic batholith early proterozoic felsic volcanic rocks occur in southeast south dakota the bootheel portion of south dakota contains a diverse assemblage of basement rocks that are partly archean in age churchill province rocks of the trans hudson foldbel

bouguer gravity in northeastern georgia a buried suture a surface suture and granites a bouguer gravity map of an area of northeastern georgia encompassing parts of the inner piedmont charlotte belt and carolina slate belt has been constructed from gravity stations spaced 2 km apart anomalies in the charlotte carolina slate belts are due to metamorphosed mafic rocks in the upper crust depth extent

analogous tectonic evolution of the ordovician foredeeps southern and central appalachians new stratigraphic data suggest that the diachronous evolution of the ordovician foredeeps in the southern and central appalachians was remarkably similar stratigraphic features that characterize the middle ordovician sevier basin in tennessee and the middle and late ordovician martinsbuirg basin in pennsylvania are in identical ascending order 1 disconformity on the knox group beekmantown group 2 shelf carbonates 3 slope deposits 4 submarine fan turbidites and 5 contourites and muddy turbidites we propose that diachronous attempted subduction of the north american craton beneath southeastern microplates and or volcanic arcs resulted in uplift and erosion of the western shelf followed by its rapid subsidence basinward migration of eastern and northeastern terrigenous source areas and associated submarine fan turbidites resulted from continued convergence and filled the basins finally tectonic stabilization and lowering of the source area is recorded by contourites and muddy turbidites the evolutionary model proposed for the sevier and martinsburg basins closely resembles present day tectonics of the timor foredeep and the adjoining sahul shelf north of australia similar comparisons have also been made for ordovician basins in the northern appalachians analogous tectonic mechanisms therefore appear to have operated diachronously along the eastern margin of north america from tennessee to new england and possibly to newfoundland during ordovician time

exchange processes induced by large horizontal coherent structures in floodplain vegetated channels mangrove forests along the mekong delta estuaries are usually observed to degrade together with the increasing extension of fish farms in this limited width condition the formation of coherent structures their interactions with mangroves the role of the width of the mangrove forest and their effects on the exchange processes between the open channel and the adjacent floodplain are still not well examined in order to obtain more insight a unique laboratory experiment of a vegetated compound channel mimicking estuarine mangroves has been conducted the results show that in a compound vegetated channel with a very gentle transverse slope the vegetated shear layer dynamics resembled that associated with vegetation rather than that associated with a depth differential furthermore the flow field under the effect of large horizontal coherent structures lhcss shows a spatially and temporally cycloid motion with associated flow events namely sweep ejection stagnant and reverse flows it is also suggested that the coherent structures can have an influence on a broader area than the vegetation area into which the eddy structures can penetrate in terms of the exchange processes the momentum transfer and the intensity of transverse fluctuations induced by the lhcss can be related to this phenomenon consequently decreasing the mangrove width can significantly affect the pattern of the lhcss disturb the transverse exchange processes induced by these structures and thereby changing the shear layer creating unfavorable conditions for sedimentation inside forests and for river bank stability

the arctic frontal zone as seen in the ncep ncar reanalysis abstract calculations of a thermal front parameter using ncep ncar reanalysis data over the period 1979 98 reveal a relative maximum in frontal frequencies during summer along northern eurasia from about 60 to 70 n best expressed over the eastern half of the continent a similar relative maximum is found over alaska which is present year round although best expressed in summer these high latitude features can be clearly distinguished from the polar frontal zone in the midlatitudes of the pacific basin and collectively resemble the summertime arctic frontal zone discussed in several early studies while some separation between high and midlatitude frontal activity is observed in all seasons the summer season is distinguished by the development of an attendant mean baroclinic zone aligned roughly along the arctic ocean coastline and associated wind maxima in the upper troposphere the regions of maximum summer frontal frequency correspond to preferred areas of cyclogenesis and to where the summertime

eddy zonal flow feedback in the northern hemisphere winter abstract the variability of the zonal mean zonal wind in the northern hemisphere winter december march is studied using eof analysis and momentum budget diagnostics of ncep ncar reanalysis data 1976 2001 the leading eof of the zonal mean zonal wind is well separated from the remaining eofs and represents the north south movement of the midlatitude westerlies analysis of the momentum budget shows that a positive feedback between the zonal mean wind anomalies and the eddy momentum fluxes selects the leading eof of midlatitude variability like the southern hemisphere the baroclinic eddies reinforce the zonal wind anomalies while external rossby waves damp the wind anomalies in the northern hemisphere the quasi stationary eddies also reinforce the zonal wind anomalies but the baroclinic eddies are most important for the positive eddy zonal flow feedback the observations support the following feedback mechanisms 1 above normal baroclinic wave activity is generated in the region of enhanced wester