delimiting oceanographic provinces to determine drivers of mesoscale patterns in benthic megafauna a case study in the barents sea abstract communities of benthic megafauna in the deep waters of continental shelves 100 m are important components of marine ecosystems in high latitude ecosystems this fauna is increasingly impacted by human activities and climate variability in this study we provide baseline knowledge on the oceanographic conditions affecting its distribution in the barents sea in the vicinity of the polar front an oceanic front occurring at the transition zone between the atlantic and arctic water masses we used fields of temperature and currents from an ocean circulation model regional ocean modelling system roms to derive variables divided into 3 groups relevant to bottom fauna temperature water column structure and bottom currents expressing either mean conditions or temporal variability over 10 years 2001 2010 benthic megafauna was surveyed in summer 2011 at 139 sites to analyze the relationship between spatial variability in the composition of benthic megafauna i e diversity and oceanographic conditions we 1 used generalized dissimilarity modelling gdm and 2 delimited oceanographic provinces i e regions of similar conditions for each group of variables using principal component analysis pca followed by cluster analysis turnover in benthic megafauna was explained by 7 oceanographic variables temperature 4 water column structure 2 bottom currents 1 depth and geographic distance 56 7 of total deviance explained concurrently patterns in oceanographic provinces among the 3 groups of variables coincided with results from the gdm where provinces derived from temperature were sharply delimited relative to the other groups we concluded that the spatial structure of the environment is important in the relationship between spatial variability of benthic megafauna and oceanographic conditions in shelf deep waters ocean models are powerful tools to study this relationship but the way in which their inherent uncertainty affects the conclusions of ecological models should be assessed more thoroughly

cyclogenesis in frontal zones abstract observational evidence has demonstrated that localized baroclinic zones in the atmosphere are unstable to disturbances with wavelengths of approximately 1000 km examples of such disturbances include polar front cyclones baiu cyclones polar lows and comma clouds in an attempt to understand the dynamical processes responsible for the generation of these diverse but apparently related phenomena a two dimensional baroclinic zone is tested for stability against small amplitude three dimensional perturbations the baroclinic zone employed in this analysis is the hoskins bretherton semigeostrophic model of a synoptic scale front the fronts am found to be highly unstable and the fastest growing disturbances compatible with an origin through secondary instability in the longwave are those with horizontal length scales on the order of 1000 km the energetics of these disturbances demonstrate that they grow by a new cyclone scale mode of baroclinic instability the structure of the most unstable cyc

on the role of the mesoscale circulation on an idealized coastal upwelling ecosystem 1 an idealized circulation model of coastal upwelling coupled to an ecosystem model has been built to address the impact of features such as eddies and filaments emerging from mesoscale dynamics on a marine ecosystem the model mimics coastal upwelling along an infinite straight coast with north south cyclic boundary conditions thanks to the parametrization of the geostrophic onshore flow in the thermocline the circulation captures the typical characteristics of a coastal upwelling region an equatorward coastal jet a poleward undercurrent along the continental slope and mesoscale eddies and filaments this eddying three dimensional simulation is compared to a two dimensional simulation using the averaged velocity field of the first simulation as velocity field this approach allows us to compare simulations having similar upwelling and nutrient input but differing in the nature of the flow an offshore spreading of the phytoplankton bloom is found in the eddying simulation the width of the productive coastal band is increased from 80 km to 200 km by the mesoscale activity a biogeochemical budget carried out in a 300 km wide coastal band provides evidence that mesoscale activity decreases the total phytoplankton content mainly by exporting a significant part of the surface phytoplankton below the euphotic layer in presence of mesoscale activity the downward and offshore export of phytoplankton zooplankton and detritus significantly contributes to total export of organic matter out of the surface coastal ocean whereas their contribution to export is weak in the two dimensional case

wind and buoyancy induced transport of the norwegian coastal current in the barents sea 1 the focus of this study is on the fluxes and forcing of the norwegian coastal current ncc at the entrance to the barents sea the structure and dynamics of the ncc which consists of a slope branch and an offshore branch are investigated using 1 a recent 1 year full depth current meter record in the core of the slope region 2 repeated hydrographic data at an inshore monitoring station 3 broader regional hydrographic surveys and 4 atmospheric reanalysis data the total fluxes are estimated to 1 8 sv for volume and 26 msv for freshwater relative to a reference salinity of 34 8 with the largest contributions from baroclinic offshore branch the heat flux calculated for the slope branch only is 34 tw these estimates are higher compared to earlier estimates but are based on more comprehensive data the major mode of variability in the slope branch is consistent with a continental shelf wave on time scales in the range of 3 16 days forced by the along coast component of the wind stress maximum along slope currents during fall winter corresponding to the stronger wind forcing during these seasons suggest that the observed seasonality in the ncc can be attributed to a similar mechanism

modification of polar low development by orography and sea ice abstractthe life cycles of intense high latitude mesoscale cyclones and polar lows are strongly shaped by their ambient environments this study focuses on the influence of the orography of svalbar

short baroclinic waves of small amplitude in a moist environment baroclinic instability at short wavelengths is investigated using a primitive equation non hydrostatic two dimensional numerical model in which the vertical shear is uniform and in which moisture is present in the lower part of the troposphere the height of the transition between saturated and unsaturated air determines the vertical and horizontal scales of the most unstable modes as the transition region is raised the most unstable wavelength increases until the large scale moist eady wave extending to the tropopause prevails

fuzzy clustering for seafloor classification in order to develop quantitative seafloor sediment classification techniques it is important to acknowledge that by nature the boundaries between soft sediments are characterized by transition zones and therefore are indeterminate and gradual a fuzzy clustering method fuzzy c means fcm was used to identify these transition zones within a subset of the data used to generate the australian seascapes classification model the overlapping classes and gradual boundaries resulting from the fuzzy c means algorithm provided estimates of sediment boundaries that are a closer model of reality than sharp boundaries fcm output is given in the form of membership layers for each class hard classes for each grid cell based on the maximum membership value and a confusion index layer quantifying uncertainty in class attribution the confusion index layer provided a spatial representation of transition zones and overlap between seafloor classes and highlighted areas of greatest uncertainty we extended the standard fcm algorithm by applying the new fmle fuzzy clustering algorithm that takes into account spatial relationships in the data in addition we implemented and applied new cluster validity techniques pcaes pbmf and xb to determine the optimal number of clusters in the data which is a novel pattern recognition application for seabed mapping the 5 class fcm classification provided the most reliable result the results of this research were tested and validated on a simulated dataset and then the clustering and validation algorithms were applied to marine sediment data to identify seascapes the new results were compared with previously published seascapes classes identified with hard isodata clustering techniques from geoscience australia s seascapes classification result with the increasing use of physical surrogates to explain marine biodiversity this research plays a crucial role in the development of techniques to identify habitat zones on the seabed

a family of frontal cyclones over the western atlantic ocean part ii parameter studies in this study a series of sensitivity experiments is performed to study the relative influence of latent heating surface friction and surface heat fluxes on the development of a family of frontal cyclones that occurred over the western atlantic ocean using the simulation presented in part i as a control run it is shown that dry dynamics determines the initiation and track of all the frontal cyclones and it accounts for about 59 of the deepening of a major frontal cyclone vorticity budget calculations reveal that in the absence of latent heating preexisting upper level cyclonic vorticity associated with a ring of potential vorticity provides the necessary forcing for the initiation and movement of the frontal cyclones whereas the low level thermal advection is responsible for a large portion of their amplifications as well as for their shallow circulations the impact of surface sensible and latent heat fluxes on the frontal cyclogenesis depends on the cyclones location with respect to the warm water surface in the absence of latent heating the surface fluxes have very weak impact through modifying the low level baroclinicity on the evolution and final intensity of the frontal cyclones when latent heating is allowed however the surface fluxes could result in more rapid cyclogenesis as a result of reduced static stability and increased moisture content in the maritime boundary layer the impact is as pronounced as the latent heating it is found that dry frontal cyclogenesis could still occur over a vast continental surface although it is the slowest moving and deepening system among all the sensitivity tests being conducted the results reveal that i the frontal cyclones in the present case are baroclinically driven in nature although they are markedly modulated by diabatic heating and surface fluxes and ii the rapid frontal cyclogenesis phenomena tend to occur more frequently over a warm ocean surface due to its associated weak surface friction and its generated weak static stability in the maritime boundary layer

baroclinic instability of semi geostrophic fronts with uniform potential vorticity ii comparison of analytic and numerical solutions the structure of an asymptotic analytic solution for linear baroclinic waves on a frontal basic state is analysed and compared with finite difference solutions the accuracy of the geostrophic fields is good for fronts with widths less than about 400 km there is direct cancellation between along front advection of the disturbance by the strong frontal jet and disturbance cross front advection of the sharp basic state gradients this cancellation of small scale effects leaves the larger scales to control the evolution of the disturbance it follows that the structure is very similar to that of the classic meridionally uniform eady model secondary features such as the ageostrophic circulation are more sensitive to the frontal width and hence less accurately reproduced in the asymptotic solution the analysis illustrates significant differences between the ageostrophic circulation associated with deformation frontogenesis and that which occurs in the linear stage of the instability forced by along front curvature in the latter case the horizontal ageostrophic wind associated with the rising motion is characterized by along front convergence and cross front divergence i e ageostrophic frontolysis

evolution of a mesoscale upper tropospheric vorticity maximum and comma cloud from a cloud free two dimensional potential vorticity anomaly a diagnostic case study is presented in which output from the limited area forecast version of the meteorological office operational unified model was used together with water vapour and cloud imagery from the meteosat satellite the case chosen for study was an archetypally simple example in which a long narrow strip of air with high potential vorticity pv in the upper troposphere strictly a tropopause depression became unstable and rolled up into fairly ling lived mesoscale vortices for much of the time the vortices were cloud free and they were dynamically pure examples of very small upper air pv anomalies moving relative to the strata beneath as such they provided a good test of theoretical deductions from the pv invertability principle and of the performance of the dynamical components of the forecast model the evolution of the mesoscale vortices as revealed in the satellite imagery was well handled by the unified model indicating that the small scales were reproduced well by the model dynamics even though they were not fully defined by the routine observations fed into the model as the vortices began to form the stretching deformation along the axis of the initial pv strip was very small and the 900 km spacing of the vortices was broadly consistent with theoretical expectations for the wavelength of instabilities growing on such a pv strip a detailed analysis of one of the mesoscale vortices throughout its 3 day lifetime showed that its maximum potential vorticity and absolute vorticity decreased with time as it travelled within the circulation of large scale anticyclonic gyre by the end of the second day when cloud first formed the vortex had become a weak upper tropospheric shear line 500 km long sandwiched between a pair of anticyclonic mesoscale vortices 900 km apart the meteosat imagery revealed a characteristic two component structure both in the water vapour channel and when the cloud eventually formed in the infrared channel too this structure which resembled that seen on other occasions is explained in terms of a particular combination of vertical motion and advection in the region of sharp humidity gradients alongside dry stratospheric air that had intruded into the upper troposphere

fragmentation of stratospheric intrusions evidence is presented pointing to the existence of rich and coherent subsynoptic and mesoscale flow features at tropopause levels these features are related to and evolve from the classical v shaped intrusions of stratospheric air down to tropospheric elevations it is shown that intrusions can develop into elongated 2000 3000 km and slender 200 km streamers and that thereafter such a streamer can roll up to form a train of stalactite shaped vortex subentities with an accompanying substantial thinning of the intervening filament in addition there are indications that the vortices themselves can develop a spirallike interior structure of interleaved stratospheric and tropospheric air these inferences are based upon two independent but complementary sources analysis of the potential vorticity distribution on tropopause transcending isentropic surfaces derived from the analysis fields of the european centre for medium range weather forecasts either directly or indirectly using a contour advection technique and imagery from the water vapor channel of the european space agency meteosat 4 satellite streamers were observed to occur with a frequency of approximately one per week over central and southern europe during the winter of 1991 1992 the fragmentation is linked to the instability or self development of a filament of enhanced potential vorticity and it can modify or instigate surface weather systems moreover by inducing a substantial and rapid enlargement of the intrusion s surface area it greatly enhances the potential for local irreversible mixing of stratospheric and tropospheric air

dynamical characterization of the fastex cyclogenesis cases summary a series of diagnostics based on the automatic tracking of cyclones is applied to the 25 life cycles captured during the fastex field experiment the tracking enables a setting of the various cases into a common frame of reference that moves with the system time filtering is used to further separate the events into a basic state and perturbations information is obtained on environmental properties such as strain or baroclinicity combining these diagnostics it is shown that instability mechanisms based on the existence of potential vorticity strips or the control of these instabilities by deformation may be involved in the initial genesis of a number of cases but do not seem to characterize specific types nor to determine a systematic behaviour on the other hand it is shown that rapid deepening phases leading eastern atlantic systems to their maximum amplitude happens suddenly and at the same time as crossing the jet stream even in the absence of a clear additional upper level feature for cold air systems development means meeting a diffluent zone this is unlike the american east coast systems that are associated with an exit entrance complex another remarkable feature is the generation of a frontal waveas a re sult of the parent vorticity maximum being apparently stretched and split this has occurred in 25 of cases a number of other properties of the fastex cases are shown such as several occurrences of large

linking benthic hydrodynamics and cold water coral occurrences a high resolution model study at three cold water coral provinces in the ne atlantic observations from numerous cold water coral locations in the ne atlantic show energetic near bottom flow dynamics along the european continental margin at individual coral mounds and mound clusters dynamics are largely controlled by tide topography interaction generating and enhancing periodic motions such as trapped waves freely propagating internal tides and internal hydraulic jumps in this study linkages between key abiotic parameters and cold water coral occurrences are explored across entire cold water coral mound provinces using an integrated modelling and observational approach the 3 d ocean circulation model roms agrif was applied to simulate near bottom hydrodynamic conditions at three provinces in the ne atlantic logachev mounds arc mounds and belgica mounds adopting a nested model setup with a central grid resolution of 250 m simulations were carried out with a focus on accurate high resolution topography and tidal forcing the central model bathymetry was taken from high resolution inss irish national seabed survey seafloor mapping data the model was integrated over a full one year reference period starting from the 1st january 2010 interannual variability was not considered tidal forcing was obtained from a global solution of the oregon state university osu inverse tidal model modelled fields of benthic currents were validated against available independent in situ observations coral assemblage patterns presence and absence locations were obtained from benthic surveys of the eu fp7 coralfish programme and supplemented by data from additional field surveys modelled near bottom currents temperature and salinity were analysed for a 1 month subset 15th april to 15th may 2010 corresponding to the main coralfish survey period the model results show intensified near bottom currents in areas where living corals are observed by contrast with coral absence and random background locations instantaneous and time mean current speeds at mound clusters in the logachev province exceeded typical values in non coral areas by up to a factor of three currents at cold water coral locations in the arc and belgica mound provinces were less energetic but still elevated compared to non coral locations an analysis of dynamical processes associated with oscillatory flow interacting with topography suggests that these motions are locally important food supply mechanisms to cold water corals by promoting large amplitude local vertical mixing and organic matter fluxes it is shown that their presence varies considerably between provinces based on the interplay of topographic slope flow magnitude and ambient stratification

quantifying the influence of atlantic heat on barents sea ice variability and retreat abstractthe recent arctic winter sea ice retreat is most pronounced in the barents sea using available observations of the atlantic inflow to the barents sea and results from a regional ice ocean model the authors assess and quantify the role of inflowing heat anomalies on sea ice variability the interannual variability and longer term decrease in sea ice area reflect the variability of the atlantic inflow both in observations and model simulations during the last decade 1998 2008 the reduction in annual july june sea ice area was 218 103 km2 or close to 50 this reduction has occurred concurrent with an increase in observed atlantic heat transport due to both strengthening and warming of the inflow modeled interannual variations in sea ice area between 1948 and 2007 are associated with anomalous heat transport r 0 63 with a 70 103 km2 decrease per 10 tw input of heat based on the simulated ocean heat budget it is found that the heat transport into the western barents sea sets the bou

frontal wave cyclogenesis in the north atlantic a climatological characterisation the ambient conditions during cyclogenesis on fronts in the north atlantic na are characterised climatologically in the era interim dataset these secondary cyclones which grow from frontal waves are typically small in scale shallow and deepen rapidly and pose a challenge to forecasters and numerical weather prediction models frontal wave development has been studied intensively in the past based on observations e g fastex real case studies or analytical models in this study frontal wave cyclogenesis is studied from a climatological perspective and is detected by a combination of two automated identification methods for the period between 1979 and 2014 a climatology of frontal wave cyclogenesis is presented horizontal composites provide an estimate of the climatological mean of conditions during frontal wave cyclogenesis which may prove useful in the assessment of the realism of proposed idealized models of frontogenesis the findings are also compared with assumptions made in earlier idealized studies of frontal waves the composites reveal coherent structures of diabatically generated low level pv in the lower troposphere neutral moist static stability at cyclogenesis and an ambient lower troposphere close to saturation the vertical pv structure in the east na is more vertically aligned than in the west na and pv anomalies from the upper and lower levels reach deeper into the mid troposphere which indicates an enhanced phase lock between the two anomalies previous studies suggest the existence of an upper limit of along frontal stretching above which frontal wave development is strongly suppressed the presented multi decadal analysis confirms its existence in a range between 0 6 and 0 8 10 5 s 1 finally a climatology of along frontal stretching for the na is presented and compared to the fraction of cyclogenesis forming on frontal waves

using tropopause maps to diagnose midlatitude weather systems the use of potential vorticity pv allows the efficient description of the dynamics of nearly balanced atmospheric flow phenomena but the distribution of pv must be simply represented for ease in interpretation representations of pv on isentropic or isobaric surfaces can be cumbersome as analyses of several surfaces spanning the troposphere must be constructed to fully apprehend the complete pv distribution following a brief review of the relationship between pv and nearly balanced flows it is demonstrated that the tropospheric pv has a simple distribution and as a consequence an analysis of potential temperature along the dynamic tropopause here defined as a surface of constant pv allows for a simple representation of the upper tropospheric and lower stratospheric pv the construction and interpretation of these tropopause maps which may be termed isertelic analyses of potential temperature are described in addition techniques to construct dynamical representations of the lower tropospheric pv and near surface potential temperature which complement these isertelic analyses are also suggested case studies are presented to illustrate the utility of these techniques in diagnosing phenomena such as cyclogenesis tropopause folds the formation of an upper trough and the effects of latent heat release on the upper and lower troposphere

the arctic front in the greenland sea during february 1989 hydrographic and biological observations high resolution hydrographic and biological observations in the arctic frontal regime were made in february 1989 along 74 45 n the frontal zone was found to contain four individual frontal interfaces these fronts were a result of the large scale convergence of the currents around the greenland sea gyre bringing together arctic and polar surface waters and two modes of atlantic surface water the distribution of arctic and atlantic copepod species as well as the composition of their developmental stages showed a strong correlation with the distribution of water masses whereby the frontal interfaces acted as biogeographic boundaries whereas the fronts bounding the polar water reached only to a depth of about 100 m the fronts bordering the atlantic water masses could be observed to depths of 600 m at subsurface levels a core of atlantic intermediate water from the jan mayen current was observed the properties of this water type can be explained quantitatively from double diffusive exchange with the overlying water the mixing across the fronts was found to be dominated by fluxes of heat salt and copepods toward the central greenland sea gyre

frontal wave development by interaction between a front and a cyclone application to fastex iop 17 the way new cyclones appear along a baroclinic zone is central to the problem of the prediction of mid latitude cyclones a new barotropic mechanism for frontal wave development is proposed in the present paper the ingredients are a frontal environment characterized by horizontal wind shear and a finite amplitude initial wind perturbation orthogonal to the main deformation axis associated with the front it is found that the initial wind perturbation can be provided by a low level vortex maximum located north westward with respect to the position of the new cyclone interactions between these synoptic features are documented using local energetics the theory is only valid at the very early stage of development after a few hours the main growing phase which is mostly baroclinic will take place a wide set of experiments is performed in order to find the preferred location of the vortex at the initial time in the frame of the barotropic vorticity equation the result is found to be consistent with the fronts and atlantic storm track experiment case studied in this paper frontogenetic forcing provided by a deformation field does not inhibit the perturbation amplification it is also found that the impact of the vortex is significantly larger than potential instabilities developing along the front where the unstable feature is defined as the most unstable singular vector copyright 2004 royal meteorological society

modelling the quasi equilibrium dynamics of the atmosphere the prime need in forecasting atmospheric motions is to predict the synoptic pattern accurately and to fill in as much mesoscale detail on it as possible the problem is difficult because the atmospheric fields contain almost discontinuous structures such as fronts and inversions and the large scale flow may separate around mountains this paper demonstrates that such phenomena may be described as quasi equilibrium structures where the fluid remains close to a minimum energy state this minimum is shown to correspond to geostrophic and hydrostatic balance and the implied quasi equilibrium evolution to correspond to the lagrangian form of the geostrophic momentum equations examples of flows that can be treated include boundary layers separating at surface fronts and mountain blocking with associated drag by including diabatic effects the inland penetration of sea breezes can also be studied the quasi equilibrium idea can be applied globally but describes only a limited subset of tropical motions primitive equation models attempt to produce these structures by relying on explicit geostrophic adjustment this paper presents evidence that this is not always adequate especially when the structures contain discontinuities implicit algorithms are described which may be more effective

a family of frontal cyclones over the western atlantic ocean part i a 60 h simulation despite marked improvements in the predictability of rapidly deepening extratropical cyclones many operational models still have great difficulties in predicting frontal cyclogenesis that often begins as a mesoscale vortex embedded in a large scale parent cyclone system in this paper a 60 h simulation and analysis of a family of frontal cyclones that were generated over the western atlantic ocean during 13 15 march 1992 are performed using the pennsylvania state university national center for atmospheric research mesoscale model with a fine mesh grid size of 30 km although it is initialized with conventional observations the model reproduces well the genesis track and intensity of the frontal cyclones their associated thermal structure and precipitation pattern as well as their surface circulations as verified against the canadian meteorological centre analysis and other available observations it is shown that each frontal cyclone is initiated successively to the southwest of its predecessor in the cold sector first appearing as a pressure trough superposed on a baroclinically unstable basic state in the lowest 150 300 hpa then it derives kinetic energy from the low level available potential energy as it moves over an underlying warm ocean surface with weak static stability toward a leading large scale frontal zone and deepens rapidly by release of latent heat occurring in its own circulations one of the frontal cyclones originating in the cold air mass deepens 44 hpa in 42 h and overwhelms the parent cyclone after passing over the warm gulf stream water into the leading frontal zone these cyclones have diameters ranging from 500 to 1100 km as denoted by the last closed isobar and are spaced 1000 1400 km apart between their circulation centers during the mature stage they begin to establish their own cold warm frontal circulations once their first closed isobars appear thus distorting the leading large scale frontal structures and altering the distribution and type convective versus stratiform of precipitation it is found that the frontal cyclones accelerate and experience their central pressure drops as they move from high to low pressure regions toward the parent cyclone center and then they decelerate and fill as they travel away from the parent cyclone their spatial and temporal scales vertical structures as well as deepening mechanisms are shown to differ significantly from those typical extratropical cyclones as previously studied

the itcz in the central and eastern pacific on synoptic time scales the itcz in the central and eastern pacific on synoptic time scales is highly dynamic the active season extends roughly from may through october during the active season the itcz continuously breaks down and re forms and produces a series of tropical disturbances the life span of the itcz varies from several days to 3 weeks sixty five cases of itcz breakdown have been visually identified over five active seasons 1999 2003 in three independent datasets itcz breakdown can be triggered by two mechanisms 1 interaction with westward propagating disturbances wpds and 2 the vortex rollup vr mechanism results show that the frequency of occurrence of itcz breakdown from these two mechanisms is the same the vr mechanism may have been neglected because the produced disturbances are rather weak and they may dissipate quickly the itcz shows a strong tendency to re form within 1 2 days in the same location the itcz may break down via the vr mechanism without any other support and thus it may continuously generate numerous tropical disturbances throughout the season there are two main differences between the two mechanisms 1 the wpds induced itcz breakdown tends to create one or two vortices that may be of tropical depression strength the vr induced itcz breakdown generates several nearly equal sized weak disturbances 2 the wpds tend to disturb the itcz in the eastern pacific only disturbances generally move along the mexican coast after shedding off from the itcz and do not further disturb the itcz in the central pacific therefore the vr mechanism is observed more clearly and is the dominating mechanism for itcz breakdown in the central pacific

itcz breakdown in three dimensional flows the intertropical convergence zone itcz is observed to undulate and at times break down into a series of tropical disturbances in several days some of these disturbances may develop into tropical cyclones and move to higher latitudes while others dissipate and the itcz may reform in the original region it has been proposed that the itcz may break down because of its heating induced potential vorticity pv anomalies here this process is examined in three dimensional simulations using a primitive equation model a simulation of the itcz in a background state of rest is compared to simulations in different background flows the effect of different vertical structures of the prescribed heating is also examined deep heating induces a positive pv anomaly in the lower troposphere leading to a reversal of the pv gradient on the poleward side of the heating while the induced pv anomaly at upper levels is negative leading to a reversal of the pv gradient on the equatorward side of the heating the response at upper levels leads to a weaker pv gradient change but the response is greater in areal extent than the lowertropospheric response for shallow heating the lower tropospheric pv response is greater than that for deep heating and there is no upper tropospheric pv response the itcz lasts longer before breaking in this case than in the deep heating case effects of the background flow are mainly felt in the deep heating cases when the background flow enforces the pv induced wind field itcz breakdown occurs more rapidly whereas when the background flow is opposite to the pv induced flow itcz breakdown takes longer and the itcz may dissipate before breakdown

rossby wave breaking the upper level jet and serial clustering of extratropical cyclones in western europe winter 2013 14 was the stormiest on record for the uk and was characterized by recurrent clustering of extratropical cyclones this clustering was associated with a strong straight and persistent north atlantic jet and was also associated with rossby wave breaking rwb on both flanks pinning the jet in place the occurrence of rwb and cyclone clustering is further studied in 36 years of the era interim reanalysis clustering at 55 n is associated with an extended and anomalously strong eddy driven jet flanked on both sides by rwb however clustering at 65 45 n has a dominance of rwb to the south north of the jet deflecting the jet northward southward a positive correlation was found between clustering and rwb occurrence to the north and south of the jet however there is considerable spread in these relationships

warm and occluded fronts in two dimensional moist baroclinic instability the two dimensional eady mode of baroclinic instability exhibits a strong cold front structure after a period of development using semi geostrophic equations excluding moist processes indeed the surface wave has a single centre of cyclonic vorticity on the cold side of the warm air anomaly here results are presented from a semi geostrophic numerical model including moist effects which predicts an eady wave with a cold and a warm front associated with two centres of surface vorticity located on either side of the warm anomaly the ascent regions are taken to be saturated and close to moist neutral which in this baroclinic environment corresponds to a condition of zero equivalent potential vorticity the descent is taken to be unsaturated this development of a structure resembling a warm front occurs only when the moist effects become dominant within a pre existing weak dry eady mode using the same condensation representation starting from an extremely small and random amplitude disturbance a pure moist eady mode develops with a single vorticity maximum in the form of a cold front in nature it is likely that often the moisture becomes dominant after a period albeit sometimes a short period of essentially unsaturated growth although observational evidence is limited therefore the process of warm frontogenesis described here may be a contributory factor to those minority of mid latitude cyclones which exhibit a strong warm front the model indicates that the warm front moves more slowly than the cold front resulting in a cold occlusion process these investigations require a detailed examination of the initial growth of small amplitude disturbances near the limit of neutral moist ascent i e pure moist modes results of previous such calculations using an analytical 2 level model are confirmed here using a numerical multi level model the more accurate estimates obtained here show that the finite growth rate in the limit of zero equivalent potential vorticity has a value of 1 9 times the dry value compared with the 2 4 fold increase given by the 2 level model

semigeostrophic flow over orography in a stratified rotating atmosphere part i steady three dimensional solutions over finite ridges abstract three dimensional steady and inviscid flow over orography is examined by means of a semigeostrophic model a constant coriolis parameter f uniform potential vorticity and a uniform basic flow characterize this model first used by merkine and kalnay rivas it is demonstrated that the neglect of ageostrophic accelerations which characterize a semigeostrophic model essentially requires low rosssby number flow ro 0 3 and relatively small values of the nondimensional mountain height d 0 5 where d 3 103 m is the deformation depth in this parameter range the disturbance potential is a solution of laplace s equation the atmosphere is semi infinite and the finite amplitude lower boundary is an isentropic surface the basic solution expressed in prolate spheroidal coordinates provides disturbance flows over an isolated mountain finite ridges and the limiting flow over a two dimensional ridge a comparison between the quasi geostrophic and semigeostrophic responses to flow over the isola

secondary cyclogenesis the initiation phase of a frontal wave observed over the eastern atlantic the initiation phase of a secondary frontal wave in the wake of a primary cyclogenesis event over the atlantic is documented north of the trailing surface cold front and south west of the primary cyclone an intense upper level jet generates a direct and transverse ageostrophic circulation in its entrance region evidence is shown for two conflicting roles of this circulation first stabilization of the front through frontogenetic cross frontal low level convergence and second an increase of the potential for instability of the front through diabatic e g latent heat release potential vorticity pv redistribution in the upward branch of the circulation following saturation of the primary cyclogenesis and the associated along front north eastwards shift of the upper level jet the low level branch of the ageostrophic circulation behaves like a zipper closing on the front while in the upward branch an anomalous strip of pv builds along the cold front isentropic pv maps show that parcles of stratospheric origin embedded in the downward branch of the ageostrophic circulation a tropopause fold may have ended in the vicinity of the strip of anomalous low level pv perhaps contributing to an increase in the potential for instability of the surface front using the domain independent attribution method of bishop it is shown that the frontal wave forms on a part of the cold front where first the along front stretching rate due to the environmental flow decays below the critical threshold value 0 6 10 5 s 1 prescribed by theoretical studies and second the across front frontogenetic component due to the ageostrophic circulation decays this is consistent with semi geostrophic theory where the cross frontal convergence and the stretching deformation are linked through the sawyer eliassen equation results add to other recent observational findings that the potential for instability the evolving environmental flow and the induced ageostrophic convergence each have a crucial role in differentiating between frontal waves that will grow and those that will be suppressed

secondary frontal waves in the north atlantic region a dynamical perspective of current ideas the current understanding of the dynamics of secondary cyclogenesis on frontal systems in the north atlantic region is reviewed these secondary cyclones are often poorly forecast and in extreme cases can have damaging consequences but because of their small scales and potential for rapid growth detailed observations and analyses are limited in this paper recent work is described and compared it is argued that on the synoptic scale the development of primary baroclinic waves is well understood but on the subscales of these systems where the secondaries develop there is a wider variety of mechanisms which may cause or modify cyclone growth this variety may be reflected in a wider range of resulting systems various dynamical models have been postulated secondary frontal waves are seen as the instability of a low level potential vorticity strip or warm band at a front with possible finite amplitude triggering by upper level features latent heating is thought to provide a suitable unstable potential vorticity strip but the degree to which the latent heating contributes to the instability itself is a matter of dissent secondary systems are often shallow so boundary layer processes are important but the influence of these is not well understood laboratory experiments have suggested that the boundary layer will act to suppress instability in general frontogenetic strain flow while acting to intensify the basic state front suppresses wave development in contrast recent work suggests that a frontolytic strain flow may be important in enhancing the development of one class of waves the fastex experiment an international field study of north atlantic secondary cyclogensis has recently attempted to observe these systems in detail it is hoped that the data which have been obtained will help to resolve some of the outstanding issues in this field

mesoscale dynamics of cold fronts structures described by dropsoundings in fronts 87 the fronts 87 project was a european experiment to make mesoscale observations to deduce the dynamics of active cold fronts approaching north west europe with a minimum of orographic influences in this paper data from dropsoundings made by the meteorological office s c 130 research aircraft are analysed the aircraft flew a pattern with 4 or 5 runs oriented approximately at right angles to the front but with each run displaced in the along front direction by about 100 km the dropsondes gave soundings to a height of approximately 7 km with a cross frontal spacing of 20 km at best and 60 km on average between 30 and 50 soundings were made in each event over an area of 500 500 km2 these data are unique in their mesoscale resolution over a synoptic scale region this was made possible by the ability to track up to 5 sondes in the air at any time several new aspects of cold front dynamics have been identified from these data the approach here is to treat each run as an instantaneous cross frontal section the discussion is centred on the hypotheses postulated before the experiment these were intended to allow verification or refutation of aspects of models of frontal dynamics such as those embodied in semi geostrophic theory a minimal and purely objective analysis has been performed on the data presented here but the high resolution of the dropsonde observations permits evaluation of differentiated quantities such as potential vorticity pv with some confidence issues raised here include the degree of thermal wind balance the structure of conserved variables such as equivalent potential temperature and absolute momentum the cross frontal circulation and the role of diabatic processes such as the evaporation of snow and the potential vorticity structure on the mesoscale and its interpretation the intention is to present an integrated view of the dynamical structure of fronts in the light of theoretical rather than conceptual or airflow models

the role of tropical cyclones in the formation of tropical upper tropospheric troughs abstract tropical upper tropospheric troughs tutts also known as midoceanic troughs are elongated troughs that appear in summer monthly averaged maps of the upper tropospheric flow over the oceans the transient part of these climatological features is composed of tutt cells and their origin is the subject of this study tutt cells often occur to the east of tropical cyclones a nonlinear shallow water model on the sphere was used in a simplified study of the interactions between tropical cyclones and the circumpolar vortex based on the results of these simulations and keeping in mind their limitations it is proposed that dispersion of short rossby wave energy is a possible mechanism to explain the formation of tutt cells to the east of tropical cyclones the model simulations suggest that two types of tutt cells may form to the east of tropical cyclones when embedded in cyclonic or weak anticyclonic shear the trough to the east of the tropical cyclone may broaden resulting in the formation of an

the role of secondary cyclones and cyclone families for the north atlantic storm track and clustering over western europe secondary cyclones are those that form in association with a pre existing primary cyclone typically along a trailing cold front in previously studied cases they have been shown to cause extreme damage across europe particularly when multiple cyclones track over the same location in rapid succession known as cyclone clustering to determine the dynamical relationship between primary and secondary cyclones over the north atlantic a frontal identification algorithm is partnered with a cyclone identification method to objectively identify secondary cyclones in 35 extended winter periods using reanalysis data cyclones are grouped into cyclone families consisting of a single primary cyclone and one or more secondary cyclones this paper aims to quantify the differences between secondary and primary cyclones over the north atlantic and how cyclone families contribute to episodes of cyclone clustering across western europe secondary cyclones are shown to occur most frequently in the central and eastern north atlantic whereas primary cyclones are commonly found over the western north atlantic cyclone families have their strongest presence over the north atlantic ocean and contribute more than 50 of cyclones over the main north atlantic storm track a final category solo cyclones which are not associated with cyclogenesis on any connected fronts are most commonly identified over continental regions as well as the mediterranean sea primary cyclones are associated with the development of an environment that is favourable for secondary cyclone growth enhanced rossby wave breaking following primary cyclone development leads to an increase in the upper level jet speed and a decrease in low level stability secondary cyclogenesis commonly occurs in this region of anomalously low stability close to the european continent during periods of cyclone clustering secondary cyclones are responsible for approximately 50 of the total number of cyclones the increase in jet speed and decrease in static stability initiated by the primary cyclones acts to concentrate the genesis region of secondary cyclones and direct the cyclones that form along a similar track while there is an increase in the secondary cyclogenesis rate near western europe during periods of european clustering the basin wide secondary cyclogenesis rate decreases during these periods thus the large scale environment redistributes secondary cyclones during periods of clustering rather than increasing the total number of secondary cyclones

forward and reverse shear environments during polar low genesis over the northeast atlantic abstractthe synoptic and subsynoptic environments associated with polar low genesis are examined ambient pre polar low environments are classified as forward or reverse shear conditions based on the angle between the thermal and mean wind forward shear environments are associated with a synoptic scale ridge over scandinavia featuring a zonally oriented baroclinic zone extending throughout the troposphere with a wind speed maximum at the tropopause similar to typical midlatitude cyclogenesis concurrent wavelike development occurs both in the lower and upper troposphere along the baroclinic zone and the mean propagation direction is eastward parallel to isolines of sea surface temperature reverse shear environments exhibit a distinctly different structure and are characterized by a trough over scandinavia associated with a synoptic scale occluded cyclone the genesis area exhibits strong cold air advection on its right hand side and polar low development occurs on the warm side of an intense low le

role of salient potential vorticity elements in an event of frontal wave cyclogenesis extratropical cyclogenesis is usually accompanied by both an upper level trough and a surface front with its cloud diabatic accoutrements in the present study these distinctive structures are interpreted as salient elements of the atmosphere s potential vorticity pv distribution and their role examined for one particular event of frontal wave cyclogenesis the strategy adopted is a form of diagnosis by pv dismemberment it entails adopting a successful forecast with a limited area model as a surrogate for the actual event and then performing a sequence of simulations with modified initial conditions that correspond in each case to the removal of one or more of the salient pv elements this approach provides an assessment of the relative significance of the individual elements permits an insightful comparison of various extant theories of cyclogenesis and pinpoints the particular pv elements influencing the event s predictability it is shown that the replication of the simulated cyclone requires an adequate representation both of specific fine scale pv features at tropopause levels and of the diabatically maintained low level pre frontal pv band thus a tenable theory of cyclogenesis that incorporates this not atypical event should both recognize and help clarify the dependence of cyclogenesis upon the relative locations movement scales and structures of the aforementioned salient pv elements

breakdown and reformation of the intertropical convergence zone in a moist atmosphere abstract the effects of moisture on the intertropical convergence zone itcz over the eastern pacific on the synoptic time scale are investigated using an intermediate complexity atmospheric circulation model the quasi equilibrium tropical circulation model qtcm1 on an aquaplanet the dry simulation shows results consistent with those of simple dynamic models except that a slightly stronger heating rate is needed owing to different model designs in the moist simulations the most important result is the formation of a tail southwest of a vortex during and after the itcz breakdown this tail may extend zonally more than 60 longitude and last for more than two weeks in an idealized simulation in the eastern north pacific this phenomenon is often observed in cases that involve easterly waves in a sense the formation of the tail suggests a possible mechanism that forms an itcz efficiently this study shows that the surface convergent flow induced by a disturbance initializes a positive wind evapor

objective automated classification technique for marine landscape mapping in submarine canyons abstract this study proposes a fully automated and objective technique to map marine landscapes in submarine canyons the method is suitable for broad and regional scale mapping derived from sonar data using multivariate statistical analysis the method is divided into two main parts the terrain analysis and the multivariate statistical analysis the first part aims to optimise the sonar data and comprises three steps 1 data resampling 2 determination of length scale and 3 multiple scale analysis the second part covers the actual marine landscape classification and consists of 1 principal component analysis pca 2 k means clustering and 3 cluster determination in addition a confidence map is presented based on cluster membership derived from cluster distance in attribute space the technique was applied in the lisbon setubal and cascais canyons offshore portugal the area was classified into 6 marine landscapes that represent the geomorphological features present in submarine canyons the main findings from the study are 1 the transferability of a tool from geomorphometric analysis estimation of scale parameter esp to detect the length scale of potential patterns in bathymetric grids 2 multiple scale terrain analysis allows an appropriate discrimination of local and broad scale geomorphic features in marine landscape mapping 3 the method not only delineates geomorphic seafloor features but also points out properties that might influence biodiversity in a complex terrain

frontal instability generated by tropospheric potential vorticity anomalies the development of secondary disturbances on a two dimensional front is described using a linear stability analysis a two stage process is envisaged in which first strong frontogenesis produces condensation at the front this leads to the generation of a lower tropospheric zone of high potential vorticity pv lying along the front as frontogenesis weakens the second stage is entered in which an essentially steady front is susceptible to the growth of linear normal modes along the front owing to the existence of the pv anomaly or equivalently the frontal rainband thus for a typical band of width 160 km a frontal wave of some 800 km is produced with a growth rate of about 1 day 1 the dynamics of these modes is examined by considering both the realistic frontal flow as well as idealized strips of pv anomalies the idealized case is potentially applicable to a variety of meteorological situations such as upper jet streaks useful insight is gained from an analysis of the energetics of the modes this shows that for narrow frontal pv anomalies the waves main energy source is the basic kinetic energy and further they have a negative vertical heat flux recent observations suggest that frontal waves are indeed characterized by such a heat flux as most frontal zones involve significant condensation and therefore are likely to exhibit a lower tropospheric pv maximum it is stressed that studies of frontal waves need to include this aspect of the frontal structure

large impact basins and the mega impact origin for the crustal dichotomy on mars the origin of the fundamental crustal dichotomy on mars remains the outstanding unsolved problem in martian geologic evolution the hypothesis that this dichotomy is due to a single giant mega impact early in martian history is tested by comparing the observed number of large impact basins on mars with the number expected from a d 2 distribution this test is based on the assumption that the mega impact borealis basin is the largest member of such a distribution the difference between observed and expected basins is the number of missing basins that might have been obscured by subsequent geologic events if the borealis basin were the largest member of a d 2 impact population there would be a very large number of missing basins on mars 1 larger than 5000 km 10 larger than 2000 km and 40 50 larger than about 1000 km diameter the area occupied by these missing basins allowing for overlap is too large to be hidden by the younger surface units a large population of presently unrecognized large impact basins would have to be located in the well preserved cratered highlands by contrast if chryse is the largest member of a d 2 impact population the more modest number of missing basins could in principle be confined to areas of mars that have been resurfaced or reworked by subsequent geologic processes a possible alternative to the mega impact hypothesis is that the crustal dichotomy is due to the cumulative effects of multiple overlapping large but not giant impacts

frontogenesis in the presence of small stability to slantwise convection abstract it is often observed that despite the existence of near neutrality to slantwise convection rainbands and snowbands can persist for long periods with narrow intense updrafts producing large quantities of precipitation in many cases this is probably due to the presence of active frontogenesis which as shown here maintains such structures in order to investigate this process we use two dimensional semigeostrophic theory to solve the deformation forced frontogenesis problem for a circumstance in which the stability to moist slantwise convection is small but positive in this case numerical simulation is necessary to determine the evolution of the front several previous studies have described the role of diabatic forcing in modifying the cross frontal circulation by use of the sawyer eliassen equation in particular analytic solutions of that equation for the case of small moist symmetric stability show that a narrow updraft should occur ahead of the maximum geostrophic frontogenesis numeri

multiple polar mesocyclones over the japan sea on 11 february 1997 abstract the characteristics and development mechanism of four meso scale polar mesocyclones m pcs which appeared as a vortex train over the japan sea from 0000 to 1700 utc on 11 february 1997 are studied by means of observational analyses and numerical simulations these four m pcs were first observed at 0000 utc along the southern edge of a meso scale cloud system that looked like an occluded extratropical cyclone in a japanese geostationary meteorological satellite 5 gms 5 infrared satellite image detailed evolutionary processes of these m pcs were described based on satellite and radar images the horizontal scales of these m pcs were on the order of 100 km and the spatial intervals between their centers were about 200 km their lifetimes were around 10 h the occlusion like features of the meso scale cloud system and the synoptic situation prior to the development of these m pcs are examined by using regional objective analysis ranal data of the japan meteorological agency jma in o

baroclinic instability in an environment of small stability to slantwise moist convection part i two dimensional models abstract in the semigeostrophic system the growth rate of baroclinic waves varies with the inverse square root of the potential vorticity which acts as the effective static stability recent observations in the ascent regions of middle latitude cyclones show that the effective potential vorticity for saturated air is very near zero in this paper we examine the structure and rate of growth of baroclinic cyclones when the effective potential vorticity is small for upward saturated displacements but large in regions of descent analytic solutions for two dimensional disturbances in a two layer semigeostrophic model and numerical simulations using a multilevel semigeostrophic model show that when the effective potential vorticity is small in regions of upward motion growth rates are modestly increased and the region of ascent intensifies and collapses onto a thin ascending sheet in the limit of zero moist potential vorticity the fastest growing wave has a finite growth rate which is about 2 5 times the