ageostrophic geopotential fluxes in downstream and upstream development of baroclinic waves abstract with the use of a simple primitive equation model it is demonstrated that the convergence divergence of ageostrophic geopotential fluxes can be a major source sink of kinetic energy for both downstream and upstream development of baroclinic waves and can play a dominant role during the early stages of wave development it is also shown that both surface friction and effects lead to an asymmetry in the upstream versus downstream development with downstream development much stronger a total group velocity is defined based on ageostrophic fluxes and its relationship to the rate of wave packet spreading and to convective and absolute instability is discussed

objective classification of extratropical cyclogenesis extratropical cyclones experience vastly different genesis conditions at the first point of their tracks a novel method is introduced to characterize this variability and classify genesis events by computing 30 diagnostic variables that describe the synoptic scale environment of 16029 genesis events in the northern hemisphere extratropics using era interim reanalyses from 2000 2011 these variables are referred to as precursors and include parameters characterizing upper level forcing low level baroclinicity thermodynamic stability surface fluxes and moist processes the genesis events spread over a large portion of the 30 dimensional precursor phase space and no obvious clusters occur which highlights the high variability of cyclogenesis processes and indicates that they form in a continuum rather than a few distinct categories a projection of the genesis events to the first two principle components pc of the precursor phase space allows reducing the dimensionality and introducing a meaningful segmentation of the genesis events in five classes the first two pcs are characterized by upper level forcing e g the amplitude of the upper level potential vorticity pv anomaly and low tropospheric diabatic processes e g precipitation and diabatically produced low level pv respectively the first of the five identified classes constitutes the center of the pc1 pc2 phase space and represents average conditions composites reveal that the four classes of events characterized by large positive or negative scores of pc1 and pc2 occur in distinct and strongly differing flow regimes characterized by the strength of the upper level forcing the structure of the upper level jet and the amplitude of low level moist processes and baroclinicity the four classes also have clearly differing geographical distributions many well known cyclogenesis events fall within classes characterized by strong low level moist processes with or without strong upper level forcing also discussed are the robustness of the method and the linkage to classical concepts of cyclone classifications

new c isotope stratigraphy from southwest china implications for the placement of the precambrian cambrian boundary on the yangtze platform and global correlations a high resolution c isotope profile from a fossiliferous section spanning the precambrian cambrian pc c boundary on the yangtze platform southwest china is used to constrain c isotope chemostratigraphy and its global correlation the discovery of a strong negative c isotopic anomaly just below the china marker a indicates that this negative anomaly can be correlated globally and demonstrates that the pc c boundary of the chinese successions should be close to marker a on the basis of c isotope chemostratigraphic constraints and biostratigraphic data we find that c isotopic peak s2 also first reported here from the chinese successions can be correlated with c isotopic peak i in northern siberia and c isotope feature d in southwest mongolia by integrating the data from archaeocyaeothan fauna the c isotopic peak s3 in the middle dahai carbonates may be correlative with feature g in the mongolian section

tropopause folds and synoptic scale baroclinic wave life cycles abstract the linear stability of three midlatitude zonal mean states of varying baroclinicity and barotropy is examined using a primitive equation stability analysis under the anelastic approximation each of these basic states includes both tropospheric and stratospheric regions with realistic properties compared with those of the earth s atmosphere in midlatitudes in all three cases the fastest growing synoptic scale modes delivered by the linear stability analysis have wavelengths of approximately 4000 km the nonlinear evolution of these synoptic scale modes is explored using a three dimensional anelastic finite difference model whose zonal scale is chosen to be equal to the wavelength of the fastest growing mode of linear theory during the nonlinear evolution of the wave deep tropopause folds are shown to form generically and the depth to which the fold penetrates the troposphere is seen to increase with the baroclinicity of the mean state these folds descending along the sloping frontal zone

a characteristic life cycle of upper tropospheric cyclogenetic precursors during the experiment on rapidly intensifying cyclones over the atlantic erica abstract this paper documents a characteristic life cycle of upper tropospheric precursors to surface cyclogenesis observed during the field phase of the experiment on rapidly intensifying cyclones over the atlantic erica december 1988 february 1989 this life cycle begins with the development of an elongated region of lower dynamic tropopause that forms in association with an intensifying midtropospheric jet front over central north america the elongated disturbance subsequently compacts into a more circular configuration prior to crossing the east coast of north america and frequently is associated with rapid surface cyclogenesis offshore a representative example of the life cycle outlined above is documented through a detailed case study of the upper precursor associated with the second erica intensive observation period iop 2 cyclone emphasis is placed upon i description of the tropopause structure and evolution during the upper precursor life cycle ii diagnosis of mechanisms leading to t

the possible influence of upstream upper level baroclinic processes on the development of the qe ii storm abstract an analysis of the qe ii storm of 9 11 september 1978 presents evidence for the existence of upper level baroclinic processes upstream of the rapidly developing cyclone the analysis shows that a deepening shortwave trough was located 400 to 500 km upstream of the site of the storm 12 h prior to rapid cyclogenesis the trough was associated with 1 a polar jet marked by 65 m s 1 winds in its core and significant vertical and horizontal wind shear 2 positive vorticity advection and divergence at the 300 mb level and 3 an intense frontal zone that extended from 300 mb down to the surface it also appears that a tropopause fold likely extruded stratospheric air down to the 700 800 mb level 400 500 km upstream of the surface low and 12 h prior to the explosive development phase of the cyclone these findings raise questions about gyakum s assertion that the qe ii storm developed in an area in which the baroclinic support was confined to the lower troposphere and the related assertion by anthes e

evaluation of δ13c chemostratigraphy for intrabasinal correlation vendian strata of northeast siberia integrated sedimentologic stratigraphic and geochemical 13 c 18 o fe mn and sr data from vendian cambrian carbonate ramp deposits of the northeast siberian platform olenek uplift and kharaulakh mountains region are used to constrain primary time dependent oscillations in the carbon isotope record and to evaluate 13 c chemostratigraphy for high resolution intrabasinal correlation the vendian 13 c record of northeast siberia reflects global variations seen elsewhere by displaying in ascending order a strong positive isotopic shift to values near 6 herein named the p interval an intermediate interval of relatively little isotopic change i interval which shows in some sections a monotonic decrease in 13c from 2 at the base to near 0 at the top and a negative excursion to 4 n interval just beneath the vendian cambrian boundary in addition to secular isotopic shifts these strata exhibit local small scale signals related to intrabasinal variations in subsidence erosion and diagenetic alteration e g degradation of organic matter dolomitization and hydrothermal and burial effects these local intrabasinal processes in some cases have modified both the magnitude and form of the primary isotopic excursions which are used as the basis for correlation in all studies carbon isotope profiles provide an important method to evaluate intrabasinal variations in subsidence erosion and stratigraphic completeness these profiles reflect increasing subsidence along the platform to basin transition whereas progressive truncation of isotopic profiles along this trend illustrate pronounced uplift and erosion of ramp strata along unconformity surfaces

northern hemisphere 500 hpa trough merger and fracture a climatology and case study abstract the results of an objective climatology of 500hpa trough merger defined as the amalgamation of two initially separate vorticity maxima and trough fracture defined as the splitting of a single vorticity center into two separate vorticity centers are presented for the northern hemisphere the data source is the gridded national meteorological center nmc now the national centers for environmental prediction analyses available on cd rom on the nmc operational octagonal grid 381 km resolution at 60 n for the period september 1957 to may 1989 a total of 2152 3053 merger fracture events are identified merger and fracture events tend to occur preferentially in the band of middle latitude westerlies merger events exhibit a tendency to avoid major mountainous regions and show a somewhat higher frequency of occurrence downstream of these regions fracture events cluster somewhat over eastern ocean basins the southwestern united states and northwestern africa while also avoiding major mount

stratification in the northern bering sea in early summer of 2017 and 2018 abstract we investigated spatial and interannual variation in the physical environment in the northern bering sea focusing on stratification which is one factor affecting biological production in arctic subarctic regions in particular we analyzed in situ data obtained onboard the training ship oshoro maru in early summer in 2017 and 2018 we found that stratification in the areas just north of st lawrence island around 64 5 n and west of 168 5 w and south southwest of st lawrence island was significantly weaker in 2018 than in 2017 these results are consistent with the extremely low sea ice extent present in the winter of 2017 2018 which would have resulted in less freshwater being supplied to the surface layers and a warmer and less saline bottom water conversely stratification was as strong in 2018 as in 2017 in the area close to the alaska mainland including the bering strait area suggesting that the alaskan coastal water dominates stratification in this area in early summer moreover we found that the weakly stratified water column in the bering strait area stratified quickly shortly after the occurrence of strong northerly winds likely because of the ekman transport of warm and low salinity alaskan coastal water from the east

nitrogen isotope and trace element composition characteristics of the lower cambrian niutitang formation shale in the upper middle yangtze region south china abstract the early cambrian is a key interval in the global development of biological evolution in which occurred many important environmental events and organic rich sedimentary layers were deposited which formed important hydrocarbon source rocks the lower cambrian niutitang formation is one of the most important marine sedimentary layers in the yangtze region south china and it records abundant important geological information that can be used for the reconstruction of the lower cambrian paleo climates and paleo environments nitrogen isotope compositions record and reflect variations in the original sedimentary environment and the redox conditions of marine ecosystems trace elements are also reliable indicators of sedimentary environments in this study we collected samples from the lower cambrian niutitang formation shale in the upper middle yangtze region of south china and measured the nitrogen isotope values of bulk sediments 15nbulk as well as their organic carbon isotope values 13corg total organic carbon toc contents total nitrogen tn contents and trace element concentrations the 15nbulk values of the niutitang shale range from 0 6 to 2 8 they show a certain positive excursion from bottom to top in the longitudinal direction and they exhibit a significant correlation with the longitudinal variations in trace elements these trace element characteristics suggest that the lower section of the niutitang formation existed in a strongly reduced sedimentary environment and was located in a hydrothermally active layer during the deposition of the lower section of black shale the nitrogen cycle was dominated by anaerobic nitrogen fixation thus its nitrogen isotope values are relatively lighter in contrast during the deposition of the upper section of the niutitang formation the redox conditions of water column were unstable the oxygen contents increased and oxygenated reactions were intermittent or localized thus producing relatively strong isotopic fractionation and causing the 15nbulk values in the upper section of black shale to exhibit a positive excursion we conclude that the sedimentary environment is the key factor affecting nitrogen isotopes and that the redox conditions of the water column exhibit a significant correlation with the nitrogen isotope values

baroclinic development within zonally varying flows previous idealized modelling studies have shown the importance of across jet barotropic shear to the resulting evolution of cyclones anticyclones surface based fronts and upper level fronts meanwhile many observational studies of cyclones have shown the importance of along jet variations in the horizontal wind speed i e confluence and diffluence this study investigates the importance of these along jet zonal for zonally oriented jets variations in the horizontal wind speed to the resulting structures and evolutions of baroclinic waves using idealized models of growing baroclinic waves an idealized primitive equation channel model is configured with growing baroclinic perturbations embedded within confluent and diffluent background flows when the baroclinic perturbations are placed in background confluence the lower tropospheric frontal structure and evolution initially resemble the shapiro keyser cyclone model with a zonally oriented cyclone strong warm front and bent back warm front later as the baroclinic wave is amplified in the stronger downstream baroclinicity the warm sector of the cyclone narrows becoming more reminiscent of the norwegian cyclone model the upper level frontal structure develops with a southwest northeast orientation and becomes strongest at the base of the trough where geostrophic cold advection is occurring in contrast when the baroclinic perturbations are placed in background diffluence the lower tropospheric frontal structure and evolution resemble the norwegian cyclone model with a meridionally oriented cyclone strong cold front and occluded front the upper level frontal structure is initially oriented northwest southeast on the western side of the trough before becoming zonally oriented weak geostrophic temperature advection occurs along its length these results are compared to those from previous observational and idealized modelling studies copyright 2007 royal meteorological society

new c isotope stratigraphy from southwest china implications for the placement of the precambrian cambrian boundary on the yangtze platform and global correlations comment and reply shen and schidlowski 2000 present a c isotope pro le from theprecambrian cambrian pc c interval in east yunnan for the placementof the pc c boundary on the yangtze platform and for global correlation we here challenge shen and schidlowski s 2000 chemostratigraphic cor relation and point out important omissions and biostratigraphic errors intheir paper omissions shen and schidlowski 2000 p 623 state that no dis tinct shift has yet been reported for the isotope composition of carbonacross the china marker a however the authors fail to take into accountthe work of brasier et al 1990 and references therein who report anegative

late winter water column and sea ice conditions in the northern bering sea 1 seasonal sea ice concentration and thickness were evaluated on a weekly basis during two years with contrasting ice coverage 1998 1999 and 2000 2001 using data provided by the u s national ice center ice in the bering sea during 1998 1999 was extensive and thick but by contrast in 2000 2001 winter sea ice formed late with thin ice and ice melt proceeded earlier during spring the presence and timing of a winter polynya an area of relatively open water or thin ice surrounded by heavier ice located south of st lawrence island also varied between these two winters shipboard measurements south of st lawrence island during two late winter early spring cruises in 1999 and 2001 showed that some brine injected water was present resulting in localized areas of bottom water with salinities approaching or exceeding 33 psu the mean salinity of bottom water was significantly higher in 2001 32 6 psu than in 1999 32 3 psu these varying degrees of brine injection associated with large differences in ice conditions heavy in 1998 1999 light in 2000 2001 influenced bottom water salinity and density but there were indications that variation in water mass structure also can explain differences in salinity observed between the two winters the mean 18oh2o values of bottom seawater were significantly higher in 2001 0 78 0 16 sd than in 1999 0 98 0 20 sd consistent with a more saline nutrient rich water being present in 2001 consistent with these indications inorganic nutrients nitrate phosphate and silicate in bottom water were significantly higher in 2001 than in 1999 although it is seemingly paradoxical that salinities were higher in 2001 light ice than in 1999 heavy ice when brine injection might have been more prevalent the sampling period was approximately 1 month earlier in 2001 when ice formation may have been more active in addition 18oh2o values indicate a higher proportion of anadyr water with higher salinity and nutrients in 2001 despite these differences in both ice conditions and water mass nutrient chemistry water column chlorophyll was uniformly low in both years this indicates that changes in bering sea ice regimes during the late winter months such as may occur under various climate change scenarios will not necessarily lead to any higher productivity or an earlier onset of seasonal biological production despite relatively high inorganic nutrient concentrations in both years a well mixed water column was observed with low water column chlorophyll a chl a concentrations these observations suggest that there was little primary production during either cruise in spite of the relatively open water and high water column nutrient concentrations during 2001 strong southerly winds during winter likely impeded ice formation vertically mixed the water column and prevented early spring open water primary production

factors influencing physical structure and lower trophic levels of the eastern bering sea shelf in 2005 sea ice tides and winds abstract in spring and fall 2005 cross and along shelf transects were sampled to evaluate the influence of physical forcing including sea ice tides and winds on the lower trophic levels of the bering sea ecosystem the hydrography nutrients chlorophyll and zooplankton abundance and species composition were all affected by the presence or absence of sea ice on a north south transect along the 70 m isobath in may shelf waters between 59 n and 62 n were cold and relatively fresh and benthic invertebrate larvae and chaetognaths were a significant fraction of the zooplankton community while to the south the water was warmer saltier and the zooplankton community was dominated by copepods the position of the transition between ice affected and ice free portions of the shelf was consistent among temperature salinity nutrients and oxygen this transition in the hydrographic variables persisted through the summer but it shifted 150 km northward as the season progressed while a transition also occurred in zooplankton species composition it was farther north than the physical chemical transition and did not persist through the summer mooring data demonstrated that the change in the position of the transition in physical and chemical properties was due to northward or eastward advection of water onto and across the shelf from south to north along the 70 m isobath tidal energy decreased resulting in a less sharply stratified water column on the northern portion of the middle shelf as opposed to a well defined two layered system in the southern portion this more gradual stratification in the north permitted a greater response to mixing from winds which were homogeneous from north to south thus the physical and biological structure at any one location over the middle shelf is dynamic over the course of a year and results from a combination of in situ processes and climate mediated regional forcing which is dominated in most years by sea ice

the effect of large scale flow on low level frontal structure and evolution in midlatitude cyclones abstract observational and modeling studies documented in the literature indicate that the large scale flow has an important effect on the structure and evolution of low level fronts in midlatitude cyclones the purpose of this paper is to address the role of the large scale flow on low level cyclone frontal structure and evolution through a combined observational and idealized modeling approach analyses of two observed cyclone cases embedded in large scale diffluence and confluence respectively are presented to illustrate two possible cyclone frontal structures and evolutions specifically the cyclone moving into a diffluent high amplitude ridge becomes meridionally elongated and possesses a strong meridionally oriented cold front and a weak warm front the cold front rotates into the warm front forming an occluded front in the manner of the norwegian cyclone model as indicated by the narrowing of the thermal ridge connecting the warm sector to the cyclone center in contrast the cyclone moving i

postlandfall tropical cyclone reintensification in a weakly baroclinic environment a case study of hurricane david september 1979 abstract an analysis is conducted from a potential vorticity pv perspective of the reintensification of tropical storm david over the northeastern united states in september 1979 david a major long lived hurricane originated near the cape verde islands in late august 1979 it made final landfall in georgia on 4 september 1979 and weakened rapidly thereafter the noteworthy aspect of david was its subsequent reintensification approximately 27 h after landfall as a warm core disturbance in a weakly baroclinic environment in this regard the redevelopment of david is unlike the classical extratropical transformation of a tropical storm in a strongly baroclinic environment that has been documented in the literature the authors analysis of the evolution of the dynamical tropopause subsequent to storm landfall revealed that david reintensified in response to tropopause lifting upward displacement of the dynamic tropopause ahead of a nondeepening and otherwise very weak upper tropospheric disturbance

a diagnostic study of jet streaks kinematic signatures and relationship to coherent tropopause disturbances abstract a diagnostic study is conducted of the kinematics and evolution of upper level jet streaks representative of three of the four phases in the shapiro conceptual model of a jet streak progressing through a synoptic scale baroclinic wave over north america the three phases selected for consideration apply to those segments of the wave pattern where jet streaks are relatively straight the 1200 utc 2 december 1991 case trough over ridge considers a strong jet streak located over eastern north america and constitutes the bulk of the study the other two cases which also concern jet streaks over north america are from 0000 utc 11 november 1995 northwesterly flow and 0000 utc 28 october 1995 southwesterly flow kinematic signatures consistent with the classic four quadrant conceptual model for a straight jet streak are evident in all three cases although flow curvature and thermal advection lead to significant departures from this conceptual model the position of the jet streak within the s

seasonal variations of sea ice and ocean circulation in the bering sea a model data fusion study 1 a 9 km coupled ice ocean model ciom was implemented in the entire bering sea to investigate seasonal cycles of sea ice and ocean circulation under atmospheric forcing sea ice cover with a maximum of 0 6 106 km2 in february to late march was reasonably reproduced by the bering ciom and validated by special sensor microwave imager ssm i measurements the model also captures some important spatial variability and downscaling processes such as polynyas and ridging which the ssm i measurements cannot reproduce because of their coarse 25 km resolution there are two distinct surface ocean circulation patterns in winter and summer on the bering shelves because of the dominant winds which are northeasterly in winter and southwesterly in summer summer low temperature high salinity water mass 3 c on the bering shelf is formed locally during winter because of strong vertical convection caused by salt injection when ice forms wind and wind wave mixing on the shelf the northward volume transport across the 62 5 n line with an annual mean of 0 8 0 33 sv 1 sv 106 m3 s 1 that is consistent with the measurements in the bering strait has barotropic structure which transports heat flux with an annual mean of 7 74 tw 1 tw 1012 w northward the anadyr current advects warmer saltier water northward during summer nevertheless it reverses its direction to southward during winter because of predominant northeasterly and northerly wind forcing therefore the anadyr current advects cold salty water southward the volume transport on the broad midshelf is northward year round advecting heat 3 3 2 4 tw and freshwater 8 10 104 practical salinity unit psu m3 s 1 northward one important finding is that the anadyr current and the midshelf current are out of phase in volume and heat transports the alaskan coastal current also transports heat and freshwater northward on an annual basis the bering ciom also captures the winter dense water formation along the siberian coast which is promoted by the downwelling favorable northeasterly wind and the summer upwelling due to the basin scale upwelling favorable southwesterly wind which brings up the cold salty and nutrient rich water from the subsurface to the surface within a narrow strip along the west coast this upwelling found in the model was also confirmed by satellite measurements in this study

large scale antecedent conditions associated with the 12 14 march 1993 cyclone superstorm 93 over eastern north america abstract the results of a multiscale analysis of the 12 14 march 1993 superstonn ss93 over eastern north america are presented a time sequence of overlapping 10 day time mean 5oo hpa geopotential height and anomaly composites shows that the northern hemisphere nh flow pattern from 18 february to 15 march 1993 is characterized by 1 three persistent troughs situated over eastern asia and the northwestern pacific over eastern north america and over northwestern africa and southwestern europe eastward to central russia and 2 a massive blocking anticyclone located over the central and eastern atlantic beginning 8 9 march 1993 the planetary scale flow amplifies substantially the explosive ss93 cyclogenesis and the transport of cold air to very low latitudes occurs a few days later as the nh available potential energy content after peaking on 9 march 1993 decreases by about 6 7 a dynamical tropopause analysis is used to track coherent transient potential vorticity pv anomalies and show their q

some controls on flow and salinity in bering strait 1 during 1993 1994 steric forcing of flow through bering strait represented a northward sea level drop of 0 7 m from the bering sea basin to the adjacent deep arctic ocean of which 2 3 was due to the salinity difference between the basins seasonal variability of steric forcing appears small 0 05 m in contrast to large seasonal wind effects interannual changes in steric forcing may exceed 20 however and warm inflow from the north atlantic accumulation of freshwater in the southwest canada basin and temperature and salinity changes in the upper bering sea have all contributed to recent changes the mean salinity balance in bering strait is primarily maintained by large runoff to the bering shelf dilute coastal inflow from the gulf of alaska and on shelf movement of saline and nutrient rich oceanic waters from the bering sea basin in bering strait therefore both the throughflow and its salinity are affected by remote events

a potential vorticity based study of the role of diabatic heating and friction in a numerically simulated baroclinic cyclone abstract a particularly intense case of western atlantic baroclinic cyclogenesis was investigated in this study specifically the roles of latent heat of condensation and surface friction were examined from the potential vorticity or pv thinking perspective the methodology used for this study involves three key components 1 a full physics mesoscale model which provides a continuous and dynamically consistent dataset and provides full user control over physical processes 2 a partitioned pv integration which temporally integrates the accumulation of pv due to various physical processes in the model s eulerian framework and 3 the piecewise inversion method of davis and emanuel which calculates the balanced wind and mass field associated with particular pv anomalies potential vorticity features obtained through the partitioned integration technique were inverted to yield their direct contributions to the total circulation in addition sensitivity studies were carried out to determine the overal

life history of mobile troughs in the upper westerlies abstract increasing evidence indicates that surface cyclogenesis is predominantly a response to the approach of a preexisting trough at upper levels a question then arises about the origin of the upper level predecessor as an initial approach to this question mobile troughs in the major band of westerlies were crudely tracked in daily northern hemispheric 500 mb analysis during nine recent cold seasons these troughs were identified only in the 552 dam height contour between 8 and 15 of them were present on a given day study of a particular cold season showed a median duration of 12 days and a mode of 5 days average zonal phase speed was 13 m s 1 locations or origin and termination of individual troughs were distributed over all longitudes but births greatly exceeded deaths over and east of the rocky mountains in north america and the highlands of central asia trough terminations dominated over the eastern portions of the oceans within the quasi steady planetary waves origins and terminations o

upper level frontogenesis associated with the birth of mobile troughs in northwesterly flow previous studies have shown that 500 hpa mobile trough births or genesis occur preferentially in northwesterly flow during upper level frontogenesis and that cold advection assists in and is a product of mobile trough intensification this study focuses on the synoptic environments and thermal advection patterns of upperlevel fronts associated with mobile trough births in northwesterly flow a climatology of 186 such events derived from an earlier study by sanders shows that most births tend to occur within uniform or diffluent flow and that most tend to be associated with relatively weaker 500 hpa thermal advection most mobile trough births in diffluence however tend to be associated with increasing 500 hpa cold advection typically indicated by a cyclonic rotation of isentropes whereas most mobile trough births in confluence tend to be associated with weaker 500 hpa thermal advection two cases of upper level frontogenesis associated with mobile trough genesis one in diffluence and one in confluence are compared to determine the processes acting to produce the differing thermal advection patterns at 500 hpa a thermal advection tendency equation is developed and shows that the magnitude of the temperature advection can be changed by accelerating the advecting wind speed or by changing the temperature gradient i e vector frontogenesis the latter can be accomplished either by changing the magnitude of the temperature gradient the frontogenetical component fn also known as scalar frontogenesis or by rotating the direction of the temperature gradient relative to the flow the rotational component fs the dominant processes acting on fn for the diffluence and confluence cases are tilting and deformation frontogenesis respectively the dominant process acting on fs for the diffluence case is rotation of the isentropes due to the vorticity term whereas rotation of the isentropes due to the vorticity and tilting terms are both important for the confluence case the rotational component of frontogenesis is cyclonic downstream of the vorticity maximum for both cases favoring increasing cold advection downstream of the vorticity maximum for both cases the rate of rotation of the isentropes at a point due to horizontal advection is large and that due to vertical advection is negligible since advection can only transport the existing isentrope angle and cannot change the isentrope angle the rotational component of frontogenesis normalized by the temperature gradient is the only term that can increase the isentrope angle following the flow this term dominates in the diffluence case but is small in the confluence case this diagnosis suggests the following reasoning in diffluent flow the vorticity associated with the incipient trough is compacted into a more circular shape and intensifies the potent vorticity maximum leads to robust isentrope rotation in confluent flow however the vorticity is deformed into an elongated maximum inhibiting both strong isentrope rotation and increasing cold advection thus the rotational frontogenesis component explains the rotation of the isentropes that is responsible for the differing thermal advection patterns diagnosis of these cases supports the results from the climatology indicating a strong relationship between the synoptic environment and the upper tropospheric thermal advection pattern nevertheless current conceptual models of upper level frontogenesis do not fully explain the variety of these features in the real atmosphere in particular mobile trough genesis and its associated upper level frontogenesis can occur in weak 500 hpa thermaladvection patterns in contrast to the confluence and cold advection that have been previously identified as important to upper level frontal intensification this result provides further support for the possibility that generation and intensification of mobile troughs can occur by barotropic processes

barotropic regeneration of upper level synoptic disturbances in different configurations of the zonal weather regime abstract barotropic dynamics of upper tropospheric midlatitude disturbances evolving in different configurations of the zonal weather regime i e in different zonal like large scale flows were studied using observational analyses and barotropic model experiments the contraction stage of upper level disturbances that follows their elongation stage leads to an increase of eddy kinetic energy that is called the barotropic regeneration process in this text this barotropic mechanism is studied through notions of barotropic critical regions btcrs and effective deformation that have been introduced in a previous paper the effective deformation field is equal to the difference between the square of the large scale deformation magnitude and the square of the large scale vorticity regions where the effective deformation is positive correspond to regions where the large scale flow tends to strongly stretch synoptic disturbances a btcr is an area separating two large scale regions of positive effective defo

diagnosis of ageostrophic circulations in a two dimensional primitive equation model of frontogenesis abstract diagnoses are presented of the transverse ageostrophic circulation patterns for two cases from a two dimensional primitive equation model of frontogenesis forced by a combination of confluence and horizontal shear the cold advection case the specified alongfront potential temperature gradient results in differential cold advection by the upper level jet realistically simulates upper level frontogenesis in response to tilting by a cross front gradient of subsidence the warm advection case features a frontal system spanning the troposphere which develops in response to differential horizontal advection the diagnoses are performed by numerically solving three versions of the two dimensional sawyer eliassen equation for the streamfunction of the transverse ageostrophic circulation the quasi geostrophic qg and geostrophic momentum gm versions are based on the approximation of cross front geostrophic balance while the primitive equation pe version includes the possibility of an alongfront

initiation and evolution of an intense upper level front abstract within confluent northwesterly flow of an intensifying baroclinic wave over north america in late october 1963 an intense frontal zone developed in 12 h near the inflection point in the middle and upper troposphere by 24 h after its initial appearance the zone extended roughly from 400 to 700 mb and from the inflection point to just beyond the downstream trough horizontal gradients of potential temperature reached 15 20 k 100 km 1 air within the frontal zone was extremely dry as the accompanying trough approached the east coast of he united states surface frontogenesis occurred offshore remaining distinct from the upper level front a region of subsidence elongated in the direction of the upper level flow displayed maximum descent on the warm edge of the frontal zone and played a frontogenetical role through tilting of the isentropic surfaces analysis of isentropic potential vorticity showed significant increase of this quantity near the cold base and a probable decrease near the top

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

climatology of cyclogenesis mechanisms in the mediterranean abstract a general climatology of the main mechanisms involved in mediterranean cyclogenesis is presented a diagnostic study of both composite means and case studies is performed to analyze processes occurring in different seasons and in different cyclogenetic regions within the same season it is shown that cyclones that developed over the three most active areas in winter the gulf of genoa the aegean sea and the black sea are essentially subsynoptic lows triggered by the major north atlantic synoptic systems being affected by local orography and or low level baroclinicity over the northern mediterranean coast it is also suggested that cyclones in two or all three of these regions often occur consecutively linked to the same synoptic system in spring and summer thermally induced lows become progressively more important despite the existence of other factors such as the atlas mountains contributing to lee cyclogenesis in northern africa or the extension of the asian monsoon into the eastern

new carbon isotope stratigraphy of the ediacaran cambrian boundary interval from sw china implications for global correlation the yangtze platform preserves relatively thick carbonate successions and excellent fossil records across the ediacaran cambrian boundary interval the intensely studied meishucun section in east yunnan was one of the global stratotype section candidates for the precambrian cambrian boundary however depositional breaks were suspected in the section and the first appearance of small shelly fossils could not be verified the laolin section located in ne yunnan is more continuous and shows great potential for global correlation of carbon isotope features across the precambrian cambrian boundary however the stratigraphic framework and correlations were controversial we studied and systematically sampled the laolin section and present here new carbon isotope data for this section the laolin section consists of in ascending order the baiyanshao dolostone of the dengying formation the daibu siliceous dolostone zhongyicun dolomitic phosphorite lower dahai dolostone and upper dahai limestone of the zhujiaqing formation and the black siltstone of the shiyantou formation our data reveal a large negative 13 c excursion 7 2 l1 in the daibu member which matches the previously published data for the laolin section and a large positive excursion 3 5 l4 in the dahai member which was not shown in the published data the excursion l1 correlates well with the similarly large negative excursion near the first appearance of small shelly fossils in siberia and mongolia similar magnitude excursions are also known from morocco and oman for which there are no robust fossil constraints but from where volcanic ash beds have been dated precisely at 542 ma thus confirming a global biogeochemical event near the ediacaran cambrian boundary our data also indicate that deposition was more continuous at the laolin section compared with the meishucun section where there are no records of a comparable negative excursion near the ediacaran cambrian boundary nor any comparable positive excursion in the dahai member therefore the laolin section has proven potential to be a supplementary global stratotype section for the ediacaran cambrian boundary on the yangtze platform

the abundance and distribution of euphausiids and zero age pollock on the inner shelf of the southeast bering sea near the inner front in 1997 1999 abstract acoustic data and net samples were collected during late spring and early fall 1997 1999 to assess zooplankton and micronekton abundance and distribution relative to the inner front at three sampling grids port moller cape newenham and nunivak island on the inner shelf of the southeast bering sea epibenthic scattering layers were observed during may june and august september in all three years acoustic data were scaled to euphausiid biomass using target strength models mean euphausiid biomass determined acoustically for each transect line was 0 7 21 g m 2 with most values below 5 g m 2 there was no consistent relationship between the distribution and biomass of euphausiids and the location of the inner front zero age pollock were observed on the inner shelf in august september during all years but were confined primarily to the stratified side of the inner front and to the frontal regime the acoustic data for pollock were scaled to biomass using laboratory measurements of gas bladder dimensions and target strength models acoustic determinations of mean transect biomass for euphausiids did not differ from literature values for the inner shelf of the southeast bering sea and pollock biomass on the inner shelf did not differ from that around the pribilof islands despite recent anomalies in climate and oceanographic conditions on the inner shelf and high mortality of shorttail shearwaters during 1997 we found no evidence of significant interannual differences in the biomass of euphausiids or zero age pollock on the inner shelf of the southeast bering sea

stratification on the eastern bering sea shelf revisited abstract the timing and magnitude of stratification can have profound influences on the marine ecosystem on the eastern bering sea shelf in the absence of strong wind mixing stratification can be initiated by the melting of seasonal sea ice or by springtime warming of the surface temperature and salinity both influence the stratification of the eastern bering sea shelf with their relative importance varying spatially and temporally in the northern middle shelf domain north of 60 n salinity stratification is often as important as temperature stratification on the southern middle shelf while the influence of temperature on stratification dominates during summer the influence of salinity stratification plays a role in the interannual variability mooring 2 m2 56 9 n 164 1 w has been deployed at 70 m depth in the southern middle shelf domain since 1995 data from this mooring show that stratification typically begins to set up in may and to break down in september october but these dates can vary by 30 d while no trend is found in the timing of the spring setup the fall stratification breakdown exhibited a trend toward later breakdown 2 d later per year from 1996 to 2009 results suggest that it may be difficult to forecast stratification on the eastern bering sea shelf from climate models as simple indices of wind mixing or heat fluxes are not correlated with stratification contrary to intuition the strength of summer stratification is not correlated with depth averaged temperature warm years such as 2000 and 2001 can have low stratification and cold years such as 2007 can have very high stratification this decoupling of stratification and temperature has implications for forecasting the ecosystem in the face of climate change as we cannot assume that projections of a warmer climate simply imply higher stratification in the future

an analysis of frontogenesis in numerical simulations of baroclinic waves abstract using a primitive equation pe model we revisit two canonical flows that were previously studied using a semigeostrophic equation sg model in a previous paper the authors showed that the pe and the sg models can have significantly different versions of the large scale dynamics here they report on the implications of this difference for frontogenesis the program for the study of frontogenesis developed by b j hoskins and collaborators is followed to show how in the pe version of the canonical cases the surface warm front develops before the cold front and why the upper level front is a long nearly continuous feature going from ridge to trough the frontogenesis experienced by an air parcel is computed following the parcel to illustrate better the mechanisms involved as the present calculations are carried out longer than most previous ones the relation of the upper frontogenesis to the formation of the upper level cutoff cyclone is also examined trajectory and three dimensional g

piecewise tendency diagnosis of dynamical processes governing the development of an upper tropospheric mobile trough abstract the intensification and evolution of midlatitude upper tropospheric mobile troughs may be viewed in terms of the isentropic advection and deformation of the tropopause potential vorticity gradient the potential vorticity viewpoint allows one to qualitatively assess observed events in the context of existing theories of mobile trough genesis such as baroclinic instability or downstream development in order to quantitatively determine the role of distinct dynamical process the method of piecewise tendency diagnosis or ptd is developed ptd is an extension of piecewise potential vorticity inversion applied to height tendencies with the forcing terms in the quasigeostrophic height tendency equation partitioned into potential vorticity advection associated with distinct dynamical processes a particular case of mobile trough genesis which occurred during 1 4 december 1980 over north america is diagnosed using ptd although about 20 of the intensification of the trough was due to superposition

conceptual models of upper level frontogenesis in south westerly and north westerly flow the conceptual model proposed by shapiro as it is applied to the evolution of an upper level frontal zone within a baroclinic wave is reviewed and its limitations are investigated through previous literature and two casestudies presented in this paper the early stages in the evolutions of these two cases are used to examine specific limitations of this conceptual model 1 upper level frontogenesis in south westerly flow that evolves from a state of equivalent barotropy to a state of cold advection along the front and 2 upper level frontogenesis in northwesterly flow with along front variation in the sign of the thermal advection such that warm advection occurs upstream of cold advection in the thermal trough vector frontogenesis diagnostics for the lagrangian rate of change of the magnitude and direction of the horizontal potential temperature gradient including tilting due to vertical motion are derived these diagnostics are applied to the two cases to examine the maintenance of the potential temperature gradient and the development of cold advection along each upper level front the upper level front in south westerly north westerly flow was maintained primarily by deformation tilting frontogenesis in agreement with previous research the increasing cold advection along the upper level front in both cases was related to an upstream vorticity maximum for the case in south westerly flow the pre existing vorticity maximum approached a downstream equivalent barotropic upper level front in a manner similar to an instant occlusion resulting in cold advection along the length of the upper level front for the case in north westerly flow an intensifying vorticity maximum concentrated the cold advection in the base of the thermal trough as warm advection developed upstream these two cases are compared to upper level fronts in previous literature and a climatology of upper level fronts associated with landfalling cyclones over the eastern north pacific ocean the results indicate that these two cases are typical of early evolutions of upper level fronts that can occur in south westerly and north westerly flow therefore a revised version of the shapiro conceptual model is presented that represents more accurately the early evolutions exhibited in the present and previous studies

the ohio valley wave merger cyclogenesis event of 25 26 january 1978 part i multiscale case study abstract the long standing observational view of cyclogenesis involves an interaction between tropopause and surface based finite amplitude disturbances there are however instances where more than one upper level disturbance contributes to low level development a subset of these events involves wave or trough merger which has been defined as the amalgamation of two or more distinct 500 hpa vorticity maxima an example of this phenomenon involving a case of very intense continental cyclogenesis 25 26 january 1978 over the eastern united states is selected to elucidate the origin and evolution of precursor disturbances from the planetary scale to the mesoscale the analysis of this event reveals that the two well defined tropopause based disturbances that contribute to cyclogenesis have distinctly different origins and are brought together by confluent planetary scale flow one of these disturbances originates over the western north pacific ocean 10 days prior to cyclogenesis and tracks eastward

a model aided study of the origin and evolution of the anomalously high potential vorticity in the inner region of a rapidly deepening marine cyclone abstract high spatial and temporal resolution fields generated by a mesoscale prediction model are used to study a case of rapid marine cyclogenesis 26 mb in 12 h within the context of potential vorticity pv thinking the case which occurred on 23 february 1987 near the east coast of the united states was well simulated by the model as was verified by ship reports and satellite imagery three components of the cyclone were investigated 1 a surface thermal anomaly or surrogate pv anomaly 2 a low level diabatically produced pv anomaly in and near the frontal cloud band and 3 a dry pv anomaly of upper tropospheric and lower stratosphoric origin trajectory tracing revealed that the surface thermal anomaly was caused primarily by northward transport of warm air into the central region of the low although surface sensible heat flux also contributed to the warm anomaly a sensitivity experiment run with the flux withheld showed that it had little impact on the depth of the storm furthermore

the march 1993 superstorm cyclogenesis incipient phase synoptic and convective scale flow interaction and model performance abstract the incipient stages of the 12 14 march 1993 superstorm ss93 cyclogenesis over the gulf of mexico are examined noteworthy aspects of ss93 include 1 it is the deepest extratropical cyclone ever observed over the gulf of mexico during the 1957 96 period and 2 existing operational prediction models performed poorly in simulating the incipient cyclogenesis over the northwestern gulf of mexico a dynamic tropopause dt analysis shows that ss93 is triggered by a potent potential vorticity pv anomaly as it crosses extreme northern mexico and approaches the gulf of mexico the low level environment over the western gulf of mexico is warmed moistened and destabilized by a persistent southerly flow ahead of the approaching pv anomaly ascent and a lowering of the dt associated with a lowering of the potential temperature ahead of the pv anomaly contributes to further destabilization that is realized in the form of a massive convective outbreak an examination of the national centers for envi

a comparison between late summer 2012 and 2013 water masses macronutrients and phytoplankton standing crops in the northern bering and chukchi seas abstract survey data from the northern bering and chukchi sea continental shelves in august september 2012 and 2013 reveal inter annual differences in the spatial structure of water masses along with statistically significant differences in thermohaline properties chemical properties and phytoplankton communities we provide a set of water mass definitions applicable to the northern bering and chukchi continental shelves and we find that the near bottom bering chukchi summer water bcsw was more saline in 2012 and alaskan coastal water acw was warmer in 2013 both of these water masses carried higher nutrient concentrations in 2012 supporting a larger chlorophyll a biomass that was comprised primarily of small

an amplification mechanism of medium scale tropopausal waves abstract the mechanism for the amplification of medium scale tropopausal waves with horizontal wavelengths of 2000 3000 km and wave periods of 1 2 days that are basically neutral is investigated using a three dimensional wave activity flux distinctive upward wave activity fluxes are found in the midtroposphere 400 hpa upstream of the region where the medium scale waves are dominant that is where the horizontal components of the wave activity flux are large these upward fluxes converge in the upper troposphere 300 hpa this convergence is a major source of medium scale waves producing large values of the horizontal components of the wave activity flux in the active regions examination of the spatial structure of the heat flux shows that the upward fluxes are due to occasional baroclinic interactions between upper level medium scale waves and lower tropospheric disturbances with the same temporal scales the seasonal and geophysical variations of the waves depend mostly on the amount of baroclin

the vendian cambrian δ13c record north iran evidence for overturning of the ocean before the cambrian explosion continuous fossilliferous successions across the precambrian cambrian pc c boundary in the elburz mountains of northern iran show a remarkable negative 13c excursion just below the pc c boundary high concentrations of manganese phosphorus barium and high abundances of fossil phytoplankton and black shale coincide with the excursion worldwide stratigraphic correlation shows that the isotopic anomaly is a global event the initial metazoan diversification coupled with13c enrichment occurs stratigraphically just above the excursion we propose the following scenario for oceanic environmental changes before the cambrian faunal explosion based on new data from iran a global warm climate following the last precambrian glaciation resulted in a generally stagnant oceanic condition so that surface water was oxic deep water was dysoxic depleted in13c and enriched in nutrients massive upwelling of deep water vertical advection of nutrients and13c depleted co2 caused enhanced phytoplankton productivity and a sharp drop in 13c in shallow water carbonate and organic carbon we conclude that latest cryptozoic overturning of ocean stratification preceded the cambrian explosion

a technique for diagnosing three dimensional ageostrophic circulations in baroclinic disturbances on limited area domains abstract the methodology developed by keyser et al for representing and diagnosing three dimensional vertical circulations in baroclinic disturbances using a two dimensional vector streamfunction referred to as the psi vector is restricted to f plane channel model geometry the vertical circulation described by the psi vector consists of the irrotational or divergent part of the ageostrophic wind and the vertical velocity a key property of the psi vector is that its projections onto arbitrarily oriented orthogonal vertical planes yield independent vertical circulations allowing separation of a three dimensional vertical circulation into two two dimensional components and thus objective assessment of the extent to which a three dimensional vertical circulation is oriented in a preferred direction here the methodology for determining the psi vector is modified to be suitable for real data applications the modifications consists of reformulating the diagnostic equations to apply to conformal map pr

a technique for representing three dimensional vertical circulations in baroclinic disturbances abstract the problem of representing vertical circulations in frontal zones is reexamined with the objective of devising a methodology sufficiently general to apply in situations where these circulations are no longer confined to the cross front transverse vertical plane and therefore must be viewed as fully dimensional the proposed methodology which builds upon the earlier work of hoskins and draghici and of eliassen consists of adopting a vector streamfunction that describes the vertical velocity and the horizontal irrotational flow this generalized streamfunction referred to as the psi vector may be determined uniquely from the vertical velocity field over a limited region provided that suitable lateral boundary conditions on the velocity potential for the irrotational part of the horizontal velocity can be specified a key property of the psi vector is that its projections onto arbitrary orthogonal vertical planes yield two independent vertical circulations providing an objective means for se

time variation of 500 mb height fluctuations with long intermediate and short time scales as deduced from lag correlation statistics abstract the time variation of northern hemisphere wintertime 500 mb height fluctuations with short intermediate and long time scales is investigated using lag correlation patterns derived from time filtered data fluctuations with short 2 5 6 day periods time scales propagate eastward at a rate consistent with the notion of a steering level around 700 mb which supports an interpretation in terms of baroclinic waves the mobile teleconnection patterns associated with the intermediate 10 30 day periods time scales exhibit a pattern of time variation suggestive a rossby wave dispersion with a predominance of southward dispersion from middle latitudes into the tropics the geographically fixed teleconnection patterns characteristic of the longer time scales do not show a well defined pattern of time variation but their horizontal structure resembles that of the fastest growing normal mode associated with barotropic instability of the climatological mean wintertime flow

origins of the subsurface ammonium maximum in the southeast bering sea abstract in the bering sea it has long been argued that ammonium rich bottom water from the middle shelf of bristol bay is tidally diffused seaward resulting in a mid depth ammonium tongue over the outer shelf weak horizontal mean flows in the region relative to an especially strong tidal component support this contention we examined the distribution of ammonium further north in the vicinity of the pribilof islands on the middle shelf bottom waters had concentrations of 4 7 mol kg 1 and over the outer shelf there was a mid depth ammonium tongue optimal multiparameter analysis of hydrographic data suggested that bottom waters from the middle shelf were prevalent across the outer shelf and could account for this ammonium tongue drifter tracks demonstrated that middle shelf water was incorporated into a westward flow along the shelf break south of st george island and mean flows derived from several decades of drifter tracks also show prominent cross shelf advection in the region this was consistent with a scalar argument suggesting that in the vicinity of the pribilof islands the seaward movement of middle shelf water and loss of nitrogen over the middle shelf was the result of advection rather than tidally driven lateral diffusion

the presidents day cyclone of 18 19 february 1979 influence of upstream trough amplification and associated tropopause folding on rapid cyclogenesis abstract a diagnostic analysis of an amplifying polar jet trough system and associated tropopause fold which preceded the 19 february 1979 presidents day cyclone is presented the analysis is based on conventional radiosonde data infrared and visible satellite imagery 6 7 m water vapor measurements from the temperature humidity infrared radiometer thir and ozone measurements from the total ozone mapping spectrometer toms the case study indicates that dynamically forced meso scale vertical circulations played an important role in the extrusion of stratospheric air along the axis of a polar jet and the subsequent development of the storm system specific findings include 1 tropopause folding accompanying an amplifying polar jet trough system occurred along the axis of the intensifying polar jet in response to subsidence forced by geostrophic deformation patterns associated with the jet streak 2 the folding process extruded dry stratosphere air marked by high values of potential vorticity dow

explosive cyclogenesis in the west central north atlantic ocean 1981 84 part i composite structure and mean behavior abstract tracks and central values of surface low pressure centre and 500 mb absolute vorticity maxima were gathered from operational analysis for 48 casts of expressive cyclogenesis in the west central north atlantic the cases were stratified into relatively strong moderate and weak categories tracking was done from 36 h before to 24 h after the time of most rapid surface deepening the surface center often first appeared only 12 to 24 h before maximum deepening but the upper vorticity maximum was present 36 h or more in advance often many days mean motion of the surface low was rapidly northeastward from the southeastern states or just offshore to newfoundland intense deepening occurred in a period of no more than 24 36 h strong cyclones moved most rapidly and most meridionally traveled farther over warm water and deepened dramatically in a single 12 h period while crossing the closely spaced isotherms of sea surface temperature north of the gulf stream the upper vorticity center moved rapid

planetary and synoptic scale characteristics of explosive wintertime cyclogenesis over the western north atlantic ocean abstract the planetary and synoptic scale environment for explosive wintertime cyclogenesis over the western north atlantic ocean is documented using a series of 6 day composites based on a 12 season sample of 42 cyclones this research extends earlier studies in that it focuses on planetary scale flow features and their relationship to synoptic scale upper tropospheric precursors to surface cyclogenesis notable features of the explosive cyclone composite include i a negative 500 hpa geopotential height anomaly over the north pacific indicating strengthening and a southward shift of the pacific jet stream relative to a 25 yr climatology ii a quasi stationary troposphere deep ridge over western north america iii two predecessor upper troughs that cross the east coast of north america approximately 72 and 36 h prior to the onset of surface deepening iv a cyclogenetic mobile upper trough that becomes organized in northwesterly flow over central north america 24 36 h prior to surface deepening

the presidents day cyclone of 18 19 february 1979 synoptic overview and analysis of the subtropical jet streak influencing the pre cyclogenetic period abstract the presidents day cyclone of 18 19 february 1979 was an intense and rapidly developing storm which produced heavy snowfall along the east coast of the united states an analysis of the cyclone is presented which isolates three jet streaks that appear to have played important roles in the development of two separate areas of heavy snow one area of heavy snow developed prior to cyclogenesis and is linked in part to an increasingly unbalanced subtropical jet streak stj and a noticeably ageostrophic low level jet the second area of heavy snow developed in conjunction with the explosive cyclogenesis off the east coast as a polar jet streak and midtropospheric trough propagated toward the coastal region from the north central united states this paper examines the stj in detail the maximum wind speeds associated with the stj increased by 15 to 20 m s 1 between 1200 gmt 17 and 1200 gmt 18 february 1979 as the jet propagated from the south central toward the eastern united states during the 24

variability of physical and chemical characteristics along the 70 m isobath of the southeastern bering sea abstract from observations of ice cover temperature salinity currents and nitrate it is evident that along shelf variability was significant over the middle shelf of the eastern bering sea but less distinct than that observed in the cross shelf domains along the 70 m isobath three zones were evident in the summer the southeastern cold pool centered at 57 n an intermediate zone consisting of warmer water with weaker stratification and the northern cold pool extending northward from 58 n small scale 20 km horizontal features that persisted for months were common nutrient concentrations were related to salinity and were replenished more uniformly over the southern shelf than north of the pribilof islands although mean currents were weak 1 cm s 1 short energetic advective events impacted the temperature and salinity structure

type b cyclogenesis in a zonally varying flow abstract it is hypothesized that surface cyclogenesis in the northern hemisphere storm track regions can be described by the structural modification of baroclinic wave packets traversing a zonally varying flow field we test this hypothesis using a linear quasigeostrophic model with a zonally varying basic state and zonally varying ekman layer eddy viscosity at midchannel the basic state consists of a region of strong low level baroclinicity and weak ekman dissipation surrounded by regions of weak low level baroclinicity strong ekman dissipation and enhanced low level static stability eigenanalyses and initial value integrations support this model of type b cyclogenesis the results can be summarized as follows 1 a disturbance initiated upstream of the midchannel baroclinic zone rapidly evolves into a wave packet with maximum amplitude near the tropopause the wave packet undergoes a structural modification upon entering the low level baroclinic zone developing maximum amplitude at the surface t

characteristics and variability of the inner front of the southeastern bering sea the inner front of the southeastern bering sea shows marked spatial variability in frontal characteristics created by regional differences in forcing mechanisms differences in forcing mechanisms sea ice advance retreat and storm strength and timing and early spring water properties result in strong interannual variability in biological chemical and physical features near the front we have developed a simple model based on surface heat flux and water column mixing to explain the existence of cold belts cont shelf res 19 14 1999 1833 associated with such fronts hydrography fluorescence and nutrient observations show that pumping of nutrients into the euphotic zone occurs and this can prolong primary production at the inner front the effectiveness of this process depends on two factors the existence of a reservoir of nutrients in the lower layer on the middle shelf and the occurrence of sufficient wind and tidal energy to mix the water column

potential vorticity anomalies of the lowermost stratosphere a 10 yr winter climatology abstract inspection of the potential vorticity pv distribution on isentropic surfaces in the lowermost stratosphere reveals the ubiquitous presence of numerous subsynoptic positive pv anomalies to examine the space time characteristics of these anomalies a combined identification and tracking tool is developed that can catalog each individual anomaly s effective amplitude location overall spatial structure and movement from genesis to lysis a 10 yr winter climatology of such anomalies in the northern hemisphere is derived for the period 1991 2001 based upon the 40 yr european centre for medium range weather forecasts ecmwf re analysis era 40 the climatology indicates that the anomalies are frequently evident above high topography and in a quasi annular band at about 70 n are long lived days to weeks and that their effective amplitude is typically 2 pv units pvu higher than that of the ambient environment in addition the derived climatologies and associated composites pose questions

tessera deformation and the contemporaneous thermal state of the plateau highlands venus abstract geodynamic modeling of tessera deformation at the ancient venusian plateau highlands implies lithospheric thermal gradients exceeding 17 k km 1 regularly spaced contractional ridges in the highlands manifest the unstable growth of perturbations at a dominant wavelength of 20 km controlled by a mechanical layer of relatively uniform thickness we model the deformation as the shortening of a plastic layer over a viscous substrate a layer thickness of less than 6 km is consistent with the observed ridge spacings assuming an anhydrous diabase crustal rheology and a minimum strain rate of 10 16 s 1 the geotherm was at least 17 k km 1 higher strain rates consequently higher thermal gradients are more probable we rule out the possibility that the plateau highlands were locally anomalously hot at the time ridges formed in the tessera that the highlands are dominantly composed of rock more felsic than diabase or that the crust was weak because it was hydrated therefore the average global lithospheric heat flux when the highlands tesserae were deforming was considerably higher than it has been since

a case of downstream baroclinic development over western north america abstract numerical simulations have been made of the initiation of a strong ridge trough system over western north america and the eastern pacific the terminus of the pacific storm track with the objective of determining the extent to which downstream development contributed to its growth and the possible influence of topography on the energetics of the storm while a control simulation demonstrated considerable skill in reproducing the storm a simplified simulation in which topography surface that fluxes and latent heating were removed not only reproduced the primary features of the ridge trough system permitting a clearer interpretation of the factors contributing to its growth but actually generated a stronger system suggesting that these effects as a whole inhibited storm development application of an energy budget that distinguishes between energy generation via baroclinic processes and generation via the convergence of geopotential fluxes revealed that early growth of studies that have sh

correlation of the cambrian evolutionary radiation geochronology evolutionary stasis of earliest cambrian terreneuvian small shelly fossil ssf taxa and chronostratigraphic significance early faunas with watsonella crosbyi with or without aldanella spp have been equated with the siberian tommotian stage uppermost terreneuvian and used to define a proposed cambrian stage 2 base much earlier terreneuvian occurrences are now shown by recovery of these micromolluscs below the i carbon excursion in the siberian nemakit daldynian stage and comparable 13 c excursions in the middle meishucunian china and middle chapel island formation avalonia this 13 c excursion a reliable stage 2 marker lies in a c 10 ma interval in the cambrian radiation in which long ranged small shelly fossil taxa provide limited biostratigraphic resolution

a potential vorticity diagnostic approach to upper level frontogenesis within a developing baroclinic wave the process of tropopause folding is studied in the context of the life cycle of baroclinic waves previous studies of upper level frontogenesis have emphasized the role of the vertical circulation in driving stratospheric air down into the midtroposphere here a potential vorticity based approach is adopted that focuses on the generation of a folded tropopause to facilitate comparison of the two approaches the diagnosis is applied to the upper level front previously simulated and studied by rotunno et al the potential vorticity approach clarifies the primary role played by the horizontal nondivergent wind in producing a fold and explains why folding should be a common aspect of baroclinic development between the trough and upstream ridge prolonged subsidence within a region of weak system relative flow generates a tropopause depression oriented at an angle to the large scale flow the large scale vertical shear then locally increases the slope of the tropopause eventually leading to a tropopause fold in contrast tropopause folding in the base of the trough is caused by the nondivergent cyclonic circulation associated with the surface thermal wave the winds associated with the thermal wave amplify the potential vorticity wave aloft and these winds which decrease with height rapidly generate a tropopause fold within the trough

a review of the structure and dynamics of upper level frontal zones abstract this article presents a review of upper level fronts with the intent of synthesizing observational and modeling studies into a conceptual and dynamical description of these fronts and their evolution relative to the life cycle of midlatitude baroclinic waves the discussion begins by tracing present day concepts concerning the structure of upper level frontal systems which are based on composite analyses of radiosonde and aircraft data from their origins in the pioneering analyses of upper air data in the 1930s perspectives from scales both smaller and larger than upper level frontal systems are provided respectively by considering the effects of turbulent processes on frontal structure and dynamics and by relating variations in frontal structure to the evolution of the baroclinic waves that provide the dynamical environment for upper level frontogenesis the dynamics of upper level fronts are shown to comprise the interactions between the primary geostrophic and secondary ageostrophic cir

the relationship between sea ice break up water mass variation chlorophyll biomass and sedimentation in the northern bering sea the northern bering sea shelf is dominated by soft bottom infauna and ecologically significant epifauna that are matched by few other marine ecosystems in biomass the likely basis for this high benthic biomass is the intense spring bloom but few studies have followed the direct sedimentation of organic material during the bloom peak in may satellite imagery water column chlorophyll concentrations and surface sediment chlorophyll inventories were used to document the dynamics of sedimentation to the sea floor in both 2006 and 2007 as well as to compare to existing data from the spring bloom in 1994 an atmospherically derived radionuclide 7be that is deposited in surface sediments as ice cover retreats was used to supplement these observations as were studies of light penetration and nutrient depletion in the water column as the bloom progressed chlorophyll biomass as sea ice melted differed significantly among the three years studied 1994 2006 2007 the lowest chlorophyll biomass was observed in 2006 after strong northerly and easterly winds had distributed relatively low nutrient water from near the alaskan coast westward across the shelf prior to ice retreat by contrast in 1994 and 2007 northerly winds had less northeasterly vectors prior to sea ice retreat which reduced the westward extent of low nutrient waters across the shelf additional possible impacts on chlorophyll biomass include the timing of sea ice retreat in 1994 and 2007 which occurred several weeks earlier than in 2006 in waters with the highest nutrient content late winter brine formation and associated water column mixing may also have impacts on productivity that have not been previously recognized these observations suggest that interconnected complexities will prevent straightforward predictions of the influence of earlier ice retreat in the northern bering sea upon water column productivity and any resulting benthic ecosystem re structuring as seasonal sea ice retreats in the northern bering sea