flood volcanism in the northern high latitudes of mercury revealed by messenger messenger observations from mercury orbit reveal that a large contiguous expanse of smooth plains covers much of mercury s high northern latitudes and occupies more than 6 of the planet s surface area these plains are smooth embay other landforms are distinct in color show several flow features and partially or completely bury impact craters the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement these characteristics as well as associated features interpreted to have formed by thermal erosion indicate emplacement in a flood basalt style consistent with x ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites the plains formed after the caloris impact basin confirming that volcanism was a globally extensive process in mercury s post heavy bombardment era

explosive volcanic eruptions on mercury eruption conditions magma volatile content and implications for interior volatile abundances images obtained by the messenger spacecraft have revealed evidence for pyroclastic volcanism on mercury because of the importance of this inference for understanding the interior volatile inventory of mercury we focus on one of the best examples determined to date a shield volcano like feature just inside the southwestern rim of the caloris impact basin characterized by a near central irregularly shaped depression surrounded by a bright deposit interpreted to have a pyroclastic origin this candidate pyroclastic deposit has a mean radius of 24 km greater in size than the third largest lunar pyroclastic deposit when scaled to lunar gravity conditions from the extent of the candidate pyroclastic deposit we characterize the eruption parameters of the event that emplaced it including vent speed and candidate volatile content the minimum vent speed is 300 m s and the volatile content required to emplace the pyroclasts to this distance is hundreds to several thousands of parts per million ppm of the volatiles typically associated with pyroclastic eruptions on other bodies e g co co2 h2o so2 h2s for comparison measurements of the exsolution of volatiles h2o co2 s from basaltic eruptive episodes at kilauea volcano hawaii indicate values of 1300 6500 ppm for the terrestrial mantle source evidence for the presence of significant amounts of volatiles in partial melts derived from the interior of mercury is an unexpected result and provides a new constraint on models for the planet s formation and early evolution

signatures of induced vertical air motion accompanying quasi horizontal roll up of stratospheric intrusions the quasi horizontal roll up of unstable stratospheric intrusions into isolated vortices is known to result in specific structures on satellite water vapor images that are characterized by intermingling dark and light filaments the current paper investigates how these features are generated and how they relate to partly similar features found on concurrent maps of the tropopause height or potential vorticity pv the roll up of a stratospheric intrusion is simulated numerically with an idealized quasigeostrophic model which focuses on the dynamics induced by anomalies in the height of the tropopause the upper tropospheric adiabatic vertical wind is calculated explicitly and is used to simulate water vapor images in the model these images show qualitatively the same characteristic features as observed they are generated through a combination of horizontal advection of initial moisture anomalies and the creation of additional moisture anomalies resulting from the upper tropospheric vertical air motion the latter is in turn induced by the quasi horizontal motion of the tropopause anomaly it is suggested that a substantial portion of the spiral like structures on the water vapor images is likely to reflect the vertical wind induced by the evolution of the intrusion itself when the tropopause is defined through a fairly low value of pv it may acquire similar spiraling structures as it is being advected almost like a passive tracer on the other hand for the dynamically active core part of the intrusion which is located at higher values of pv one may expect an evolution leading to more compact vortex cores and less structure overall

intrusions into the tropical upper troposphere three dimensional structure and accompanying ozone and olr distributions abstract the evolution and structure of stratospheric intrusions into the upper troposphere ut over the northern tropical pacific is examined in terms of both potential vorticity pv and ozone o3 analysis of 20 years of ncep ncar reanalysis pv shows that the intrusion events have remarkably similar evolution and structure at 350 k with all events producing narrow tongues of high pv that have an almost north south orientation and last around 3 days nearly all events extend up into the lower stratosphere but only for a small percentage is there deep downward penetration the intrusions explain a large amount of the observed variability in upper tropospheric o3 above hilo hawaii with large values occurring when a tongue of high pv passes over hilo and low values when hilo is just upstream of a high pv tongue there is also an increase in total column ozone within the pv tongues but for most intrusions the increase is relatively small the relationship between deep convection as diagnosed by sate

climatology of the stratospheric polar vortex and planetary wave breaking abstract we use the distribution of ertel s potential vorticity pv on the 850 k isentropic surface to establish a climatology for the transient evolution of the planetary scale circulation in the northern hemisphere winter midstratosphere we compute pv distribution from gridded nmc daily temperature and height maps for the 10 and 30 mb levels and show that a very good approximation for 850 k pv can be derived from 10 mb heights and temperatures alone we assume that reversals of the latitudinal gradient of pv localized in longitude and latitude may be regarded as signatures of planetary wave breaking wave breaking identified by such signatures tends to occur mainly in the vicinity of the aleutian anticyclone with a secondary maximum over europe the area of the polar vortex defined as the area enclosed by pv contours greater than a certain critical value is strongly influenced by wave breaking erosion of the polar vortex due to transport and mixing of pv leads to a preconditioned state when def

wave breaking and mixing at the subtropical tropopause abstract this paper discusses rossby wave breaking on the isentropic surfaces that intersect the subtropical tropopause using winds and isentropic potential vorticity from high resolution meteorological analysis the focus is both on particular aspects of individual wave breaking events as well as on more general aspects such as the spatial and temporal distribution of the mixing associated with these events the direction and intensity of wave breaking is shown to exhibit the same dependence on stagnation points in the wind field as that seen in previous highly idealized numerical model studies wave breaking that results in stratospheric intrusions into the troposphere can be categorized as weak or strong depending on the development of filaments or larger coherent vortices or cutoff lows the events presented show a deep vertical structure that approximately spans the region between the 330 k and 370 k isentropic surfaces where the tropopause is steeply sloping through the subtropical jets this i

observations of 250 km wavelength clear air eddies and 750 km wavelength mesocyclones associated with a synoptic scale midlatitude cyclone abstract satellite images of a decaying synoptic scale cyclone over the north pacific reveal two distinct types of multiple mososcale cyclonic disturbances the approximate positions of these disturbances within the synoptic scale cyclone are determined using analyses from an operational global scale numerical model one type a set of four 250 km wavelength eddies occurred in clear air and represent perturbations within the cyclonic shear side of a 40 45 m s 1 upper level jet front system their propagation at 28 5 3 5 m s 1 roughly matched the wind speed and direction within the jet at their position and provided evidence of stretching deformation along their axis their growth is documented over 18 h and is measured in terms of horizontal displacements of a preexisting moisture boundary in water vapor imagery their e folding time increased from 6 to 9 h as horizontal displacements exceeded 100 km and horizontal billows indicative of wave breaking formed the billow like structures most likely repr

break up of a stratospheric streamer observed by mst radar the break up of a stratospheric streamer over the british isles is described using satellite mesosphere stratosphere troposphere mst radar and in situ observations together with european centre for medium range weather forecasts analyses the streamer started as a thin ribbon of high potential vorticity to the west of the british isles as this was strained by the background flow it broke up into a series of mesoscale vortices mst radar and ozonesonde observations delineated a tropopause fold to the west of the initial streamer taking the form of a thin layer of stratospheric air embedded in a deep layer of descending upper tropospheric air the fold appears to have been mixed out by small scale turbulence as the streamer broke up this break up also generated strong inertia gravity waves which broke just above the tropopause introducing extensive mixing to the lowermost stratosphere one of the mesoscale vortices returned over the mst radar showing a very indistinct tropopause typical of a synoptic scale cut off low this vortex was also responsible for the development of a rain band on its eastern flank which brought significant rainfall to the british isles at a time when the synoptic flow pattern was strongly anticyclonic

a feature based approach to classifying summertime potential vorticity streamers linked to rossby wave breaking in the north atlantic basin abstractthis study examines climatological potential vorticity streamer pvs activity associated with rossby wave breaking rwb which can impact tc activity in the subtropical north atlantic na

the influence of the 1997 99 el niňo southern oscillation on extratropical baroclinic life cycles over the eastern north pacific the el nino southern oscillation enso strongly influences the interannual and seasonal atmospheric circulation over the north pacific the present study shows that the meridional displacement of the time mean upper level jet associated with enso modulates the time mean barotropic meridional wind shear over the central and eastern north pacific storm track earlier theoretical and observational studies established the influence of barotropic wind shear on the life cycles of extratropical cyclones and their upper level potential vorticity pv waves the present study suggests that differences in the time mean flow associated with the 1997 99 enso cycle had a similar impact on tropopause pv structure meridional eddy fluxes of momentum and temperature and predictability

two paradigms of baroclinic wave life cycle behaviour two idealized baroclinic wave 6 life cycles examined here suggest a framework of opposite extremes a in which to view the behaviour of real synoptic scale disturbances in middle latitudes and b in which to interrelate the synoptic and wave theoretic viewpoints using the saturation propagation saturation sps picture of wave 6 life cycle behaviour the two life cycles dented by lc1 and lc2 are higher resolution versions of the simmons hoskins basic and anomalous cases showing strong and weak late decay of eddy kinetic energy eke they illustrate in varying degrees two extreme types of behaviour here disignated anticyclonic and cyclonic and epitomized by strongly contrasting upper air trough behaviour anticyclonic behaviour dominates the late stages of lc1 and is characterized by backward tilted thinning troughs being advected anticyclonically and equatorward as in the commoner cases of planetary scale mid stratospheric rossby wave breaking cyclonic behaviour dominates lc2 and is characterized by forward tilted broadening troughs wrapping themselves up cyclonically and poleward producing major cut off cyclones in high latitudes these morphologies are visualized by upper air maps of potential temperature on the nominal tropopause defined as a constant potential vorticity surface some atmospheric mid latitude disturbances examined here using the same visualization applied to operational analyses show the same two extreme types of trough behaviour together with intermediate cases the sps picture is re examined using eliassen palm and refractive index cross sections it is shown in particular by reference to a wave activity theorem of haynes that the late stages of lc2 can be looked upon as a remarkably clear and morphologically novel large amplitude counterpart of the nonlinear reflection scenario of rossby wave critical layer theory the late stages of lc1 by constrast look more akin to an nonlinear critical layer absorption scenario lc2 exhibits region of largely undular pv contours adjacent to a nonlinear of irreversiby deformen pv contous in the latter region pv rearrangement and hence absorption of rossby wave activity has largely ceased this accounts for the more persistent ekee in the lc2 case

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

tropopause folds in era interim global climatology and relation to extreme weather events tropopause folds are intimately linked to upper level frontogenesis and jet stream dynamics they play an important role for stratosphere troposphere exchange the dynamical coupling of upper and lower tropospheric levels and for generating severe weather events this study presents a global climatology of tropopause folds using era interim reanalysis data from 1979 to 2012 and a refined version of a previously developed 3 d labeling and fold identification algorithm this algorithm objectively separates stratospheric and tropospheric air in complex situations e g in regions with strong low level inversions and in extratropical cyclones where diabatically generated potential vorticity anomalies typically occur three classes of tropopause folds are defined shallow medium and deep and their geographical distribution vertical extent and seasonal cycle are investigated most shallow folds occur along the subtropical jet stream in agreement with previous studies hot spots of medium and deep tropopause folds are found west of australia and along the coast of antarctica in the southern hemisphere and around the east coast of north america in the northern hemisphere seasonal cycles show maxima in winter for all fold classes medium and deep folds are frequently associated with surface wind gust and precipitation extremes as quantified for folds over the southern indian ocean wind gust extremes occur mainly in an elongated band upstream and equatorward of folds whereas precipitation extremes occur mainly east and poleward of folds overall in the considered region about 20 of medium folds and 33 of deep folds are associated with surface wind or precipitation extremes in the vicinity of the fold

a northern hemispheric climatology of cross tropopause exchange for the era15 time period 1979 1993 1 a 15 year climatology of cross tropopause exchange in the extratropical northern hemisphere is presented based upon a previously developed lagrangian methodology and reanalysis data from the european centre for medium range weather forecasts ecmwf for the years 1979 1993 for this time period no apparent trends exist for upward troposphere to stratosphere tst and downward stratosphere to troposphere stt cross tropopause transport the seasonal cycles of hemispherically integrated tst and stt and the zonally integrated meridional exchange distributions show relatively little interannual variability the 15 year averaged geographical distribution of stt reveals significant deviations from zonal symmetry and pronounced seasonal variability stt occurs predominantly over the atlantic and pacific storm track regions during winter spring and autumn and also over the mediterranean in winter and spring in contrast during summer stt maxima are located over the continents particularly over southeastern europe and central asia tst is less variable and is strongest near the southern tip of greenland and the aleutian islands during all seasons aspects of the interannual variability of the geographical exchange distributions are investigated over the atlantic stt varies significantly in line with the changing storm tracks during different phases of the north atlantic oscillation nao whereas tst does less so special consideration is given to vertically deep exchange where the exchange air parcels transfer rapidly from the lowermost stratosphere to the boundary layer or vice versa these processes occur mainly during winter and exhibit pronounced maxima over the north american west deep stt and east deep tst coasts

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

a new perspective of the climatological features of upper level cut off lows in the southern hemisphere this study presents a detailed view of the seasonal variability of upper level cut off lows cols in the southern hemisphere the cols are identified and tracked using data from a 36 year period of the european centre for medium range weather forecast reanalysis era interim the objective identification of the cols uses a new approach which is based on 300 hpa relative vorticity minima and three restrictive criteria of the presence of a cold core stratospheric potential vorticity intrusion and cut off cyclonic circulation the highest col activity is in agreement with previous studies located near three main continental areas australia south america and africa with maximum frequencies usually observed in the austral autumn the col mean intensity values show a marked seasonal and spatial variation with maximum minimum values during the austral winter summer a unique feature that has not been observed previously in studies based on the geopotential the link between intensity and lysis is examined and finds that weaker systems are more susceptible to lysis in the vicinity of the andes cordillera associated with the topographic rossby wave lysis and genesis regions are close to each other confirming that cols are quasi stationary systems also cols tend to move eastward and are faster over the higher latitudes the mean growth decay rates coincide with the major genesis and lysis density regions such as the significant decay values across the andes all year as a consequence of using vorticity for the tracking method a longer lifetime of cols is detected than in other studies but this does not affect the total frequency of occurrence comparisons with other studies suggest that the differences in seasonality are due to uncertainties in the reanalyses and the methods used to identify cols

depth of faulting on mercury implications for heat flux and crustal and effective elastic thickness 1 topographic profiles across a lobate fault scarp on mercury have been used to constrain the depth of faulting to 30 40 km here we use this depth to place constraints on the crustal thickness and heat flow into the base of the crust with no crustal heat production the mantle heat flux on mercury at the time of scarp formation was 30 50 mwm 2 however higher crustal heat production rates allow significantly lower mantle heat fluxes in all cases the mantle heat flux exceeds the likely radiogenic heat flux 4 gyr ago it is likely that secular cooling accounted for the remainder irrespective of crustal heat generation a crustal thickness of 140 km is required to satisfy both the faulting observations and the requirement that the base of the crust does not melt the effective elastic thickness of the lithosphere at the time of faulting is predicted to be 25 30 km

evidence for two stages of compressive deformation in a buried basin of mercury the surface of mercury shows abundant compressive tectonic landforms including lobate scarps wrinkle ridges and high relief ridges which are different manifestations of thrust faults and long wavelength topography variations which could be the expression of large scale folding these landforms probably relate to planetary cooling although other factors such as mantle convection tidal despinning or true polar wander could affect the distribution expression and orientation patterns of compressive features in this work we show that an area of smooth plains including a buried 500 km diameter impact basin experienced two different stages of deformation the younger deformation stage is characterized by a set of nw se oriented wrinkle ridges affecting the smooth plains and having the same approximate orientation as the wrinkle ridges and lobate scarps deforming the surrounding terrains the older set of tectonic structures consists of ne sw oriented closely spaced subparallel quasi rectilinear and low relief ridges partially buried by the smooth plains material and crossed by the wrinkle ridges therefore our results suggest that several events occurred between both deformation stages at least one stage of basin filling a change in stress orientation an increasing in the avelength and amplitude of deformation and maybe an increasing of the thickness of the deformed layer our observations imply a complex stress history for compressive deformation maybe influenced by the internal and or orbital rotational history of mercury and are illustrative of the complexity of tectonic history likely to have affected many or most other regions of the planet

flexure of the continental lithosphere with multilayered rheology summary in this paper a model of flexure of the continental lithosphere is derived taking into account crustal and mantle rheologies bending of the continental lithosphere is modelled with a double yield stress envelope three layers brittle elastic and ductile for the crust and three analogous layers for the mantle portion the deformation of the layers is controlled by the rheological properties of quartz rich crustal rocks and olivine rich mantle rocks the influence of various factors such as the depth of moho strain rates thermal structure of the lithosphere boundary conditions and topographic load is examined results show that the mechanical strength of the continental lithosphere in the horizontal and vertical directions is primarily controlled by the present thermal structure of the plate boundary forces and moments and the applied topographic load this explains why mountainous regions may be more locally compensated than adjacent regions we also thus are able to explain why many continental plates have apparent effective rigidities much smaller than those predicted on the basis of their geological ages the model is then applied to the tien shan tarim area central asia and original topography and gravity data are used to constrain parameters of the model we found that the model satisfactorily matches the data and is also able to predict the thermal state of the plate and the location of the deep seismicity

influence of barotropic shear on the poleward advection of upper tropospheric air abstract the characteristics of the poleward advection of upper tropospheric air are investigated using meteorological analyses and idealized numerical models isentropic deformations of the tropopause during northern hemisphere winter are examined using maps of ertel s potential vorticity together with contour advection calculations large poleward excursions of upper tropospheric air are observed during rossby wave breaking events these poleward breaking events occur in regions of diffluence over the eastern atlantic ocean europe region and over the eastern pacific ocean north america region and the evolution of the tropospheric air depends on the local meridional shear in anticyclonic or weak cyclonic shear the tropospheric air tilts downstream broadens and wraps up anticyclonically whereas in cyclonic shear the tropospheric air tilts upstream thins and is advected cyclonically the role of ambient barotropic flow is further examined by considering the flow in two numerical models a pl

insolation driven variations of mercury s lithospheric strength mercury s coupled 3 2 spin orbit resonance in conjunction with its relatively high eccentricity of 0 2 and near zero obliquity results in both a latitudinal and longitudinal variation in annual average solar insolation and thus equatorial hot and cold regions this results in an asymmetric temperature distribution in the lithosphere and a long wavelength lateral variation in lithosphere structure and strength that mirrors the insolation pattern we employ a thermal evolution model for mercury generating strength envelopes of the lithosphere to demonstrate and quantify the possible effects the insolation pattern has on mercury s lithosphere we find the heterogeneity in lithosphere strength is substantial and increases with time we also find that a crust thicker than that of the moon or mars and dry rheologies for the crust and mantle are favorable when compared with estimates of brittle ductile transition depths derived from lobate scarps regions of stronger and weaker compressive strength imply that the accommodation of radial contraction of mercury as its interior cooled manifest as lobate scarps may not be isotropic imparting a preferential orientation and distribution to the lobate scarps

surface cyclones in the era 40 dataset 1958 2001 part i novel identification method and global climatology abstract a novel method is introduced to generate climatological frequency distributions of meteorological features from gridded datasets the method is used here to derive a climatology of extratropical cyclones from sea level pressure slp fields a simple and classical conception of cyclones is adopted where a cyclone is identified as the finite area that surrounds a local slp minimum and is enclosed by the outermost closed slp contour this cyclone identification procedure can be applied to individual time instants and climatologies of cyclone frequency fc are obtained by simple time averaging therefore unlike most other climatologies the method is not based on the application of a tracking algorithm and considers the size of cyclones in combination with a conventional cyclone center tracking algorithm that allows the determination of cyclone life times and the location of cyclogenesis and cyclolysis additional frequency fields can be obtained for special categories of cyclones that are gener

dynamics and geometry of extratropical cyclones in the upper troposphere by a neighbor enclosed area tracking algorithm abstractthis study shows that the morphological characteristics of upper tropospheric extratropical eddies are closely related to the background flow in the northern hemisphere winter enclosed surfaces of 300 hpa relative vorticity are identified by using the neighbor enclosed area tracking algorithm and the periphery of these surfaces are approximated by ellipses eddies are classified into five categories according to the approximate ellipse eddies having an oblateness of less than 0 6 are classified as near circle or are otherwise classified as northeast southwest ne sw northwest southeast nw se north south or west east according to the direction of the major axis in the wintertime climatology ne sw oriented cyclones are collocated with the jet stream while nw se oriented cyclones mostly reside north of the jet in interannual variability moreover the frequency of ne sw cyclones is slightly correlated with the arctic oscillation ao index while the frequency of nw se cyclones is high

influence of a predecessor rain event on the track of tropical cyclone isaac 2012 abstractanalysis of a predecessor rain event pre over the straits of florida ahead of tropical cyclone tc isaac on 25 august 2012 is presented this pre is unique compared to previously documented pres in midlatitudes because it occurred over the oceanic subtropics and impacted the track of an approaching tc the isaac pre developed in conjunction with a tropical moisture plume with precipitable water values over 60 mm that intersected a region of mid and upper level frontogenesis and warm air advection on the southeast flank of an upper level trough the pre occurred in an environment with more abundant tropical moisture and weaker synoptic scale forcing for ascent compared to the environments in which midlatitude pres developed the isaac pre contributed to the fracture of the upper level trough through negative potential vorticity advection by convectively driven divergent outflow fracture of the upper level trough and midlevel cyclonic vorticity amplification associated with the pre acted to stee

tropical plumes and extreme precipitation in subtropical and tropical west africa three cases of extreme precipitation in october january and march april over subtropical north western and tropical west africa have been selected in order to study the moisture transports precipitation generation mechanisms and large scale dynamics involved all cases show strong tropical extratropical interactions and elongated cloud bands extending from the tropics into the subtropics usually referred to as tropical plumes tps investigations are based on observational data and high resolution output from simulations using the university of wisconsin nonhydrostatic modeling system trajectory analyses show that moisture is transported from the tropical easterly mid level flow the monsoonal southerlies and the north easterly trade winds to provide precipitable water and potential instability at mid tropospheric levels the instability is released through ascent related both to weak quasi geostrophic forcing to the east of an upper level trough and to the strong inertially unstable and highly divergent upper level subtropical jet stj accompanying the tp in the january and march cases the passage of a precursor upper level trough over the same location precedes the development and initiates the moisture transport from the deep tropics the associated north westerly cold advection supports the generation of precipitation by triggering convection over the tropical atlantic ocean whose outflow feeds into the stj over africa and by enhancing the vertical mass transport within the intertropical discontinuity over and near west africa in october when the still active african monsoon facilitates the extraction of tropical moisture a wave in the tropical easterlies and strong trade winds from the southern hemisphere are dominant factors low level cyclogenesis and frontogenesis only play roles in the late stages of the developments the elongated mostly positively tilted potential vorticity pv streamers associated with the observed troughs form as a result of an equatorward transport of high pv air downstream of a large ridge over the north atlantic the rapid amplification of the ridge is achieved through a combination of negative horizontal pv advection and diabatic reduction of upper level pv through latent heating within a cloud band that forms in connection with an explosive baroclinic development near the east coast of north america copyright 2005 royal meteorological society

the tectonics of mercury the view after messenger s first flyby article i nfo during its first flyby of mercury messenger imaged many tectonic landforms most of which are contractional in nature and include lobate scarps high relief ridges and wrinkle ridges wrinkle ridges are found on the smooth plains that partially fill the interior and surround the exterior of the caloris basin and also on smooth plains that fill the interiors of smaller impact basins and larger craters messenger revealed a radial graben complex pantheon fossae nearlyco centered with the caloris basin pantheon fossae and a network of mostly basin concentric graben intheouterportionsofthecalorisbasininteriorformapatternofextensionnotseenelsewhereonmercury the first clear example of extensional faults outside of the caloris basin has been documented on smooth plains inside the peak ring of the relatively young raditladi basin a map of the distribution of tectonic landforms imaged by messenger and mariner 10 shows that lobate scarps are the most widespread type of feature estimates of the horizontal shortening associated with lobate scarps that crosscut and overthrust small impact craters imaged by messenger range from 1 to 3 km previously unrecognized lobate scarps detected in areas imaged by mariner 10 indicate thatearlierestimatesof contractional strainarelow anewestimate of theaverage arealcontractionalstrain 0 06 accommodated byscarps is at leastonethirdgreater thancomparableprevious estimates andcorresponds to a decrease in mercury s radius of at least 0 8 km since the end of heavy impact bombardment of the inner solar system these figures are lower bounds because of the likelihood that not all lobate scarps have been identified even in areas imaged to date crosscutting and embayment relations revealed by messenger suggest that lobate scarps formed before the end of smooth plains emplacement and continued to be active after the emplacement of the youngest smooth plains deposits relatively recent activity on lobate scarps may be the expression of slow but continuous radial contraction that accompanied cooling of mercury s interior and the growth of the planet s solid inner core published by elsevier b v

climatology of intrusions into the tropical upper troposphere regions of upper tropospheric equatorial westerly winds observed over the pacific and atlantic oceans during northern fall to spring are important for extratropical tropical interactions this paper focuses on one feature of these westerly ducts that has received relatively little attention to date the occurrence of rossby wave breaking events that transport tongues of extratropical air deep into the tropics mix tropical and subtropical air and can affect deep convection a climatology of these intrusion events formed from 20 years of meteorological analyses shows a strong dependence on the basic state flow notably intrusion events are found to occur almost exclusively within westerly ducts with more events in the presence of stronger equatorial westerlies it is also found that there is strong interannual variability in the frequency of pacific events with fewer events during the warm phases of enso consistent with the changes in the basic flow since these intrusion events laterally mix trace constituents and have been linked to tropical convection their spatial and temporal variability may cause related variability in the distribution of trace constituents and tropical convection

deep water hexactinellid sponge mounds from the upper triassic of northern sichuan china a belt of upper triassic carnian sponge mounds was discovered in northern sichuan in the transition zone between a shallow shelf sea in the se chuan dian shelf and a deeper marine trough ganzi aba trough in the nw individual sponge mounds are up to 60 m high and 150 m wide and interfinger laterally with dark shales main frame builders of the mounds are lychniscosa and lyssacinosa accompanied by much rarer hexactinosa lithistida annelid worms occasional crinoids and bryozoa the reef dwelling fauna consists of brachiopods ostracods foraminifera pelecypods gastropods and ammonites typical shallow water reef builders scleractinian corals and calcareous algae are lacking or extremely rare coralline sponges this supports the interpretation of the mounds as buildups in deeper water of several tens up to one hundred meters the sponge mounds of northern sichuan are thus an equivalent of similar buildups of late jurassic age in central and western europe they provide the first well established record of lychniscosa in the triassic and their first reported pre tertiary occurrence outside of europe

influence of an insulating megaregolith on heat flow and crustal temperature structure of mercury mercury is covered by a megaregolith layer which constitutes a poor thermally conducting layer that must have an influence on the thermal state and evolution of the planet although most thermal modeling or heat flow studies have overlooked it in this work we have calculated surface heat flows and subsurface temperatures from the depth of thrust faults associated with several prominent lobate scarps on mercury valid for the time of the formation of these scarps by solving the heat equation and taking into account the insulating effects of a megaregolith layer we conclude that megaregolith insulation could have been an important factor limiting heat loss and therefore interior cooling and contraction of mercury as mercurian megaregolith properties are not very well known we also analyze the influence of these properties on the results and discuss the consequences of imposing the condition that the total radioactive heat production must be lower than the total surface heat loss this is the urey ratio ur must be lower than 1 in a cooling and thermally contracting planet such as mercury at the time of scarp emplacement our results show that satisfying the condition of ur 1 implies that the average abundances of heat producing elements silicate layer is 0 4 times or less the average surface value placing an upper bound on the bulk content of heat producing elements in mercury s interior

tropopause folds and cross tropopause exchange a global investigation based upon ecmwf analyses for the time period march 2000 to february 2001 1 a new methodology is proposed to identify folds of the dynamical tropopause taken as the 2 potential vorticity pv units pvu isosurface from global analysis data sets from the european centre for medium range weather forecasts ecmwf it consists of a three dimensional subdivision of the atmosphere into stratospheric and tropospheric parts and a subsequent examination of multiple tropopause crossings in vertical profiles of the analyses the method is applied to a 1 year period starting in march 2000 seasonal mean fold frequency distributions show that folds occur preferentially in the subtropics with maximum frequencies of about 30 pronounced maxima are located in the subtropical bands that extend from 20 e to 120 e on both hemispheres during summer generally subtropical folds are comparatively shallow deep folds occur most frequently in midlatitudes during winter over eastern north america and the western north atlantic with maximum frequencies of about 1 detailed investigation of the relationship between individual folds and cross tropopause exchange events reveals that on the average individual folds in the extratropics are associated with larger exchange mass fluxes compared to the subtropics however due to the much higher frequency of subtropical folds the percentage of exchange events that are associated with tropopause folds is larger near 20 40 latitude 50 70 than further poleward 20 30 during winter and 10 20 during summer this indicates that tropopause folds are the key feature for cross tropopause exchange in the subtropics whereas in the extratropics other tropopause structures are at least of equal importance

heat flows through the ice lithosphere of europa some of the geological features on the surface of europa s ice shell suggest the existence of a brittle lithosphere that in some regions is no more than 2 km thick the surface heat flow needed to put the brittle ductile transition in ice at a depth of 2 km is at least 100 mw m 2 much higher than predicted by tidal heating models for a purely conductive ice shell a possible explanation lies in the probable existence of a convective layer beneath the brittle lithosphere which owing to tidal dissipation in warm ice could contribute to higher heat flows if a convective subsurface layer and grain size dependent flow behavior of ice are admitted the grain size at brittle ductile transition depth cannot be 1 mm which is consistent with the required value for the onset of convection in an ice shell thinner than 20 km

the palette of fronts and cyclones within a baroclinic wave development abstract a study is undertaken of the concurrent development of fronts and cyclones within a growing baroclinic wave the dynamical framework for the study is that of the semigeostrophic adiabatic flow of a uniform potential vorticity atmosphere that is sandwiched between two horizontal plates on an f plane a three dimensional spectral model is used to examine the growth to finite amplitude of the normal modes of this system for basic states that comprise of a symmetric jetlike baroclinic flow with and without a superposition of a uniform barotropic shear of either cyclonic or anticyclonic vorticity it is demonstrated that the presence of such lateral shear in the ambient environment exerts a profound influence upon the ensuing palette of fronts and cyclones on the synoptic scale this flow component influences the scale and strength of the individual surface highs and lows and also modifies the extent of their lateral drift away from and their relative movement along the baroclinic zone aloft t

a climatology of rossby wave breaking along the subtropical tropopause ten years 1986 95 of global analyses from the european centre for medium range weather forecasts are used to investigate the temporal and spatial distributions of rossby wave breaking rwb at 350 k along the tropopause herein defined by the 61 5 potential vorticity pv unit 10 26 km 2 s21 kg21 contours though many studies acknowledge rwb as an important contributor to the complex of mixing processes in the atmosphere there exists no prior climatological study of its distribution near the tropopause as in previous studies rwb is identified in the global analyses by southward directed pv gradients at 350 k rwb along the tropopause occurs preferentially during summer over the midoceans in relative proximity to the planetary scale high pressure systems in the subtropics isentropic trajectories at 350 k show that outflow from the tops of these subtropical highs directly participates in rwb over the adjacent downstream oceanic regions two regions are highlighted in this study the north pacific during boreal summer and the south atlantic during austral summer synoptic maps of breaking rossby waves in these regions are provided to reveal the acute tropopause folding in the meridional plane which characteristically accompanies rwb the rich interaction between the tropical flow and the extratropical westerly current exhibited by these cases suggests that the subtropical highs serve as important agents in the coupling between the tropical troposphere and the extratropical stratosphere as expected from theoretical considerations the locations where rwb occurs most frequently known as surf zones are shown to coexist with regionally weak time mean wind speeds and horizontal gradients of pv at 350 k

the link between cut off lows and rossby wave breaking in the southern hemisphere the climatological link between cut off low col pressure systems that occur from 20 to 50 s and rossby wave breaking rwb in the southern hemisphere sh is examined for 1979 2008 it is shown that cols are associated with either rwb events 89 or with potential vorticity pv intrusions where there is north south advection of high pv air 11 in the vast majority of cols the rwb events occur upstream on or before the day of the col formation the evolution of the pv geopotential heights static stability absolute vorticity and temperature advection fields during the cols are consistent with the formation of high pv anomalies that induce cyclonic circulations as predicted by theory rwb plays a key role in producing the split flow associated with cols in the sh which in turn produces absolute vorticity anomalies by shear curvature vorticity conversion and creates static stability anomalies the cols associated with rwb at 330 k are deeper and more persistent than those associated with 350 k rwb and surface processes differ depending on the isentropic surface on which the associated rwb occurs the seasonality of the rwb and cols are similar and is linked to the seasonal march of the westerly jets copyright 2010 royal meteorological society

the extratropical transitions of hurricanes felix and iris in 1995 abstract the extratropical transitions of hurricanes felix and iris in 1995 are examined and compared both systems affected northwest europe but only iris developed significantly as an extratropical system in both cases the hurricane interacts with a preexisting extratropical system over the western atlantic the remnants of the exhurricanes can be identified and tracked across the atlantic as separate low level potential vorticity pv anomalies the nature of the baroclinic wave involved in the extratropical transition is described from a pv perspective and shown to differ significantly between the two cases the role of vertical shear in modifying the hurricane structure during the early phase of the transition is investigated iris moved into a region of strong shear the high pv tower of iris developed a marked downshear tilt felix moved into a vertically sheared environment also but the shear was weaker than for iris and the pv tower of felix did not tilt much iris maintained its warm core struc

paleo heat flows radioactive heat generation and the cooling and deformation history of mercury estimates of lithospheric strength for mercury based on the depth of thrust faults associated with large lobate scarps which were most probably formed previously to similar to 3 ga or on the effective elastic thickness of the lithosphere supporting a broad rise in the northern smooth plains whose formation is poorly constrained but posterior to 3 8 ga serve as a basis for the calculation of paleo heat flows referred to the time when these structures were formed the so obtained paleo heat flows can give information on the urey ratio ur the ratio between the total radioactive heat production and the total surface heat loss by imposing the condition ur 1 corresponding to a cooling mercury consistent with the observed widespread contraction we obtain an upper limit of 0 4 times the average surface value for the abundance of heat producing elements in the outer solid shell of mercury we also find that if the formation of the northern rise occurred in a time posterior to 3 ga then in that time the urey ratio was lower and the cooling more intense than when most of large lobate scarps were formed thus because largest lobate scarps deform older terrains suggesting more intense contraction early in the mercurian history we conclude that the northern rise was formed previously to similar to 3 ga if the age of other smooth plains large wavelength deformations is similar then tectonic activity in mercury would have been limited in the last 3 billion of years

near surface temperatures on mercury and the moon and the stability of polar ice deposits abstract in order to assess the thermal stability of polar ice deposits we present model calculated temperatures of flat surfaces and surfaces within bowl shaped and flat floored polar impact craters on mercury and the moon our model includes appropriate insolation cycles realistic crater shapes multiple scattering of sunlight and infrared radiation and depth and temperature dependent regolith thermophysical properties unshaded water ice deposits on the surface of either body are rapidly lost to thermal sublimation a subsurface water ice deposit is stable within 2 latitude of the moon s poles meter thick water ice deposits require billions of years to sublime if located in the permanently shaded portions of flat floored craters within 10 latitude of the poles of mercury and 13 latitude of the poles of the moon results for craters associated with radar features on mercury are consistent with the presence of stable water ice deposits if a thin regolith layer thermally insulates deposits at lower latitudes and within smaller craters a regolith cover would also reduce losses from diffusion ion sputtering impact vaporization and h ly and is implied independently by the radar observations permanently shaded areas near the moon s poles are generally colder than those near mercury s poles but the moon s obliquity history its orbit through earth s magnetospheric tail and its radar opaque regolith may limit the volume and radar detectability of ice deposits there

synoptic view of the north atlantic oscillation abstract this article investigates the synoptic characteristics of individual north atlantic oscillation nao events by examining the daily evolution of the potential temperature field on the nominal tropopause the 2 pvu surface this quantity is obtained from the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis dataset for the winter season for both phases the nao is found to originate from synoptic scale waves as these waves evolve into the low frequency nao pattern they break anticyclonically for the positive phase and cyclonically for the negative phase the results of this analysis suggest that it is the remnants of these breaking waves that form the physical entity of the nao throughout the nao events for both phases the nao is maintained by the successive breaking of upstream synoptic scale waves when synoptic scale disturbances are no longer present mixing processes play an important role in the nao decay as in other recent

a new method to objectively classify extratropical cyclones for climate studies testing in the southwest pacific region abstract extratropical cyclones can vary widely in their configuration during cyclogenesis development mechanisms spatial and temporal characteristics and impacts an automated method to classify extratropical cyclones identified in era interim data from 1979 to 2010 in the australia and new zealand region has been developed the technique uses k means clustering on two upper tropospheric flow fields at the time of cyclogenesis and identifies four distinct clusters composites of these clusters are investigated along with their life cycles and their spatial and temporal variability the four clusters are similar to a previous manual classification cluster 1 develops in the equatorward entrance region of the subtropical jet clusters 2 and 4 develop in the poleward exit region of the subtropical jet but with different relative positions of the upper level trough and jet streak and cluster 3 resembles secondary cyclogenesis on a preexisting front far poleward of the subtropical jet the clusters have

the modulation of the subtropical and extratropical atmosphere in the pacific basin in response to the madden julian oscillation abstract the 40 yr ecmwf re analysis era 40 data are combined with a number of novel climatologies to conduct a comprehensive examination of the response of the subtropical and extratropical atmosphere over the pacific basin to an evolving madden julian oscillation mjo event the adopted approach constitutes a symbiosis of a climatological analysis during the northern hemisphere winter from 1979 to 2002 and a case study analysis of a distinct mjo event that occurred in january february 1993 the former is designed to obtain the general characteristics observed during a composite mjo life cycle while the latter is used to provide insight into the instantaneous mechanisms responsible for the observed composite evolution a primary component of the study involves the diagnosis of anomalous wave breaking activity in response to mjo forcing in the form of tropical convection and or upper level divergence wave breaking events are separated by their characteristic life cycles lc1 anticyclonic and lc2 c

tropical and subtropical north atlantic vertical wind shear and seasonal tropical cyclone activity abstractgiven recent insights into the role of anticyclonic rossby wave breaking awb in driving subseasonal and seasonal north atlantic tropical cyclone tc activity this study further examines

breaking waves at the tropopause in the wintertime northern hemisphere climatological analyses of the orientation and the theoretical lc1 2 classification abstract breaking waves on the tropopause are viewed as potential vorticity pv streamers on middle world isentropic levels a northern hemisphere winter climatology of the streamers spatial distribution and meridional orientation is derived from the 40 yr ecmwf re analysis era 40 dataset and used to assess the nature and frequency of occurrence of breaking synoptic scale waves the streamers are grouped into two classes related to the so called cyclonic lc2 and anticyclonic lc1 patterns and the ambient wind strength and wind shear is also noted it is shown that the occurrence of cyclonic and anticyclonic pv streamers exhibits a distinct spatial variability in the horizontal and the vertical the majority of cyclonic pv streamers are found on lower isentropic levels that intersect the tropopause at more poleward latitudes whereas anticyclonic streamers predominate at higher elevations in the subtropics an analysis of the streamer patterns for the two phases of the north atlantic oscillation

substructure of a map streamer a study is undertaken of the substructure of an upper level potential vorticity streamer that approached the european alps during the field phase of the mesoscale alpine programme map the diagnosis is based upon a lagrangian forward projection lfp technique which can provide a spatial refinement of the operational european centre for medium range weather forecasts analysis fields the reconstructed fields capture several notable fine scale features of the streamer s structure that include spiral arms of a vortex like feature and a richly structured western flank to the streamer comprising a deep primary fold surmounted by a significant secondary fold with an intermediate distinctive striation that extends back into the stratosphere these features are not at variance with independent high resolution satellite water vapour images and measurements gathered during a flight mission using an airborne water vapour differential absorption lidar dial instrument and hence lend a measure of credence to both the lfp approach and the dial measurements moreover the analysis sheds light on the evolution and dynamics of the streamer s vortices the origin of the folds the mismatch between the satellite observations and fields from other sources and also has implications for stratosphere troposphere exchange and the accompanying chemical mixing copyright 2003 royal meteorological society

the early thermal and magnetic state of the cratered highlands of mars surface heat flows are calculated from elastic lithosphere thicknesses for the heavy cratered highlands of mars in terms of the fraction of the surface heat flow derived from crustal heat sources previous heat flow estimations for mars used linear thermal gradients which is equivalent to ignoring the existence of heat sources within the crust we compute surface heat flows following a methodology that relates effective thickness and curvature of an elastic plate with the strength envelope of the lithosphere and assuming crustal heat sources homogeneously distributed in a radioactive element rich layer 20 or 60 km thick the obtained results show that the surface heat flow increases with the proportion of heat sources within the crust and with the decrease of both radioactive element rich layer thickness and surface temperature also the results permit us to calculate representative temperatures for the crust base rock strength for the upper mantle and lower and upper limits to the crustal magnetization depth and intensity respectively for terra cimmeria an effective elastic thickness of 12 km implies between 30 and 80 of heat sources located within the crust in this case the uppermost mantle would be weak at the time of loading and temperatures in the lower crust cold enough to favor unrelaxed crustal thickness variations and to permit deep curie depths in the highlands as suggested by the observational evidence d 2005 elsevier b v all rights reserved

identification and era 15 climatology of potential vorticity streamers and cutoffs near the extratropical tropopause abstract a novel approach is introduced to identify potential vorticity pv streamers and cutoffs as indicators of rossby wave breaking near the extratropical tropopause and to compile climatologies of these features on different isentropic surfaces the method is based on a contour searching algorithm that identifies the dynamical tropopause 2 potential vorticity units pvu pvu 1 10 6 k kg 1 m2 s 1 isoline on isentropic surfaces the contour is then analyzed to search for cutoffs and filament like streamers whereas the identification of cutoffs is unambiguous the one for streamers requires the specification of two parameters that determine the width and length of the contour feature to be classified as a streamer this technique has been applied to the pv distribution in the northern hemisphere on isentropes from 295 to 360 k during the time period from 1979 to 1993 using the 15 yr european centre for medium range weather forecasts ecmwf re analysis era 15 the climatology reveals a pronou

heavy precipitation on the alpine southside an upper level precursor analyses of four separate autumnal events of very heavy precipitation on the southern slopes of the european alps indicate that each was accompanied by a strikingly similar anomaly at upper tropospheric levels it took the form of a narrow 500km deep 4km and elongated 2000 km streamer of intruded stratospheric air sic high potential vorticity air extending north south from the british isles to the western mediterranean in each case the streamer translated comparatively slowly eastward and the storm event ensued as its forward flank approached the alpine ridge empirical indicators and theoretical considerations support the thesis that the streamer is an ubiquitous dynamically significant and distinctive precursor of storms on the alpine southside

influence of upstream diabatic heating upon an alpine event of heavy precipitation abstract narrow extended troughs associated with elongated streamers of potential vorticity pv are a frequent feature of upper tropospheric flow and they can instigate or modify surface weather systems in this study consideration is given to the generation of one particular pv streamer that was itself a dynamical precursor of a heavy precipitation event in the alpine region it is shown that the streamer s parturition over the eastern north atlantic is linked to a prior event of cyclo and frontogenesis upstream over the western north atlantic diagnostic trajectory analysis and heuristic simulations with a limited area nwp model suggest that the streamer s origin is influenced crucially by the following chain of physical processes cloud diabatic heating associated with the upstream cyclogenesis enhances a downstream negative pv anomaly in the upper troposphere and the flow associated with this anomaly subsequently helps transform a farther downstream preexisting broad positive pv anomaly into an elo

does mantle convection currently exist on mercury abstract the full equations of convective motion over the age of the solar system can be solved in a few hours 2d to several weeks 3d with numerical simulations in cartesian or spherical geometry using the 2d cartesian geometry code conman we model the thermal evolution of mercury our preferred calculation has an initial rayleigh number of 8 10 4 no mantle internal heating and a constant core heat flux of 9 75 mw m 2 in a non newtonian rheology these values all fall within the range of previously estimated parameters from 1d calculations and allow for weak mantle convection to persist until present supporting the outer core dynamo explanation of mercury s intrinsic magnetic field further the peak dynamic topography and geoid anomalies from our model are about 179 m and 37 m respectively values that are not unrealistic and should be well within the range of observation by the messenger spacecraft future spacecraft observations yielding new and more accurate data will help us better understand the thermal evolution of mercury and the origin of its magnetic field

topography of the northern hemisphere of mercury from messenger laser altimetry laser altimetry by the messenger spacecraft has yielded a topographic model of the northern hemisphere of mercury the dynamic range of elevations is considerably smaller than those of mars or the moon the most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains within this lowland is a broad topographic rise that experienced uplift after plains emplacement the interior of the 1500 km diameter caloris impact basin has been modified so that part of the basin floor now stands higher than the rim the elevated portion of the floor of caloris appears to be part of a quasi linear rise that extends for approximately half the planetary circumference at mid latitudes collectively these features imply that long wavelength changes to mercury s topography occurred after the earliest phases of the planet s geological history

rossby wave breaking and transport between the tropics and extratropics above the subtropical jet abstractrossby wave breaking is an important mechanism for the two way exchange of air between the tropical upper troposphere and lower stratosphere and the extratropical lower stratosphere the authors present a 30 yr climatology 1981 2010 of anticyclonically and cyclonically sheared wave breaking events along the boundary of the tropics in the 350 500 k potential temperature range from ecmwf interim re analysis era interim lagrangian transport analyses show net equatorward transport from wave breaking near 380 k and poleward transport at altitudes below and above the 370 390 k layer the finding of poleward transport at lower levels is in disagreement with previous studies and is shown to largely depend on the choice of tropical boundary in addition three distinct modes of transport for anticyclonic wave breaking events are found near the tropical tropopause 380 k poleward equatorward and symmetric transport associated with cyclonic wave breaking events however is predominantly poleward

linkages between extreme precipitation events in the central and eastern united states and rossby wave breaking abstractlinkages between extreme precipitation events epes in the central and eastern united states and synoptic scale rossby wave breaking are investigated using 1979 2015 climatologies of epes

the mechanical and thermal structure of mercury s early lithosphere 1 insight into the mechanical and thermal structure of mercury s early lithosphere has been obtained from forward modeling of the largest lobate scarp known on the planet our modeling indicates the structure overlies a thrust fault that extends deep into mercury s lithosphere the best fitting fault parameters are a depth of faulting of 35 to 40 km a fault dip of 30 to 35 and a displacement of 2 km the discovery rupes thrust fault probably cut the entire elastic and seismogenic lithosphere when it formed 4 0 gyr ago on earth the maximum depth of faulting is thermally controlled assuming the limiting isotherm for mercury s crust is 300 to 600 c and it occurred at a depth of 40 km the corresponding heat flux at the time of faulting was 10 to 43 mw m 2 this is less than old terrestrial oceanic lithosphere but greater than the present heat flux on the moon

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

gravity field and internal structure of mercury from messenger radio tracking of the messenger spacecraft has provided a model of mercury s gravity field in the northern hemisphere several large gravity anomalies including candidate mass concentrations mascons exceed 100 milli galileos mgal mercury s northern hemisphere crust is thicker at low latitudes and thinner in the polar region and shows evidence for thinning beneath some impact basins the low degree gravity field combined with planetary spin parameters yields the moment of inertia c mr2 0 353 0 017 where m and r are mercury s mass and radius and a ratio of the moment of inertia of mercury s solid outer shell to that of the planet of cm c 0 452 0 035 a model for mercury s radial density distribution consistent with these results includes a solid silicate crust and mantle overlying a solid iron sulfide layer and an iron rich liquid outer core and perhaps a solid inner core

rossby wave breaking in the southern hemisphere wintertime upper troposphere abstract the characteristics of rossby wave propagation and breaking in the southern hemisphere upper troposphere during winter are examined although the southern hemisphere subtropical jet is more zonally symmetric than that of the northern hemisphere there are still significant zonal variations in the upper tropospheric flow in particular the flow within a given sector 120 longitude can generally be characterized into one of four different configurations i a single jet ii a broken subtropical jet iii a polar jet at the upstream end of the subtropical jet or iv a polar jet at the downstream end of the subtropical jet using potential vorticity thinking and barotropic wind shear arguments it is argued that the characteristics of the rossby wave propagation and breaking will differ between each flow configuration consistent with these arguments examination of potential vorticity maps and contour advection calculations show differing wave breaking characteristics in particular t

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

potential vorticity of flow along the alps abstract observations from the german front experiment are presented here that show the existence in conditions with a dominant flow component parallel to the main alpine chain of a mesoscale region to the north of the alps where the absolute and potential vorticity pv are substantially negative these structures exist before the front arrives to the alps and appear to be affected little by the passage of the front a dynamical explanation for these and other mesoscale structures is sought by considering a simple unsheared airflow impinging on the alps from the west a linear frictionless model for the steady state response is used as well as a full nonlinear numerical model with and without friction a vastly simplified alpine orography is considered as well as one that adequately describes its mesoscale detail the results show that the frictionless linear dynamics lead to a zone north of the alps with anticyclonic vorticity but with uniform positive potential vorticity with boundary layer processe

quantification of filaments penetrating the subtropical barrier a method based on contour advection is introduced that aims to quantify the formation of filaments both equatorward and poleward of the subtropical barrier it is applied to diagnosed potential vorticity fields for every day of january and february in 1997 and 1998 an isosurface of modified potential vorticity is found to represent the region of largest isentropic gradients of potential vorticity and is hence assumed as the tropopause tropopause penetrating structures filaments forming in contour advected potential vorticity fields are identified and statistics of the abundance of such structures are derived filamentation as measured by this method exhibits a large temporal variability on the scale of days to weeks for example tropospheric cutoff systems developing in the wake of rossby wave breaking events cause strong filamentation to occur in the lowermost stratosphere the timescales governing filamentation are a function of altitude reflecting the differing types and amplitudes of waves inducing filamentation at different isentropic levels zonal asymmetries arise as rossby waves favorably break near the end of the north atlantic storm track a difference in the intensities of filamentation in january and february of 1997 and 1998 suggests that the relatively low values of ozone in the lowermost stratosphere observed in 1997 are related to increased filamentation induced stratosphere troposphere exchange compared to 1998 for example around 30 more filaments are found in the hemispheric and bimonthly mean at the 330 k isentropic surface in january and february of 1997 than during the same months of 1998 single events can represent most of the filamentation occurring in a month and hence interannual variability in the frequency of filamentation can be considerable