on the wave number spectrum of oceanic mesoscale variability observed by the seasat altimeter variations of sea surface height measured by the seasat altimeter in nearly repeat orbits during the last 24 days of the mission are used to compute the wave number spectrum of mesoscale variability in various parts of the oceans the instrument noise level 150 cm2 cycles km has limited the resulting oceanic spectrum to wavelengths longer than 100 km we found that the characteristics of the oceanic spectrum are dependent on the energy level of the mesoscale variability in the high energy areas close to major current systems most of the energy is contained at wavelengths longer than 250 km at wavelengths shorter than 250 km the spectrum basically follows a k 5 k is wave number dependence in the low energy areas remote from major current systems the spectrum follows a k 1 dependence at wave lengths from 100 to 1000 km based on the assumption of horizontal isotropy of mesoscale variability scalar wave number spectra of sea surface height and geostrophic kinetic energy are also presented dynamical implications of these spectra are discussed the effects of residual geoid and atmospheric water vapor on the computed oceanic spectra have been rigorously examined it was found that the general characteristics of the oceanic spectra were not significantly affected by them

the poleward motion of extratropical cyclones from a potential vorticity tendency analysis abstractthe poleward propagation of midlatitude storms is studied using a potential vorticity pv tendency analysis of cyclone tracking composites in an idealized zonally symmetric moist gcm a detailed pv budget reveals the important role of the upper level pv and diabatic heating associated with latent heat release during the growth stage the classic picture of baroclinic instability emerges with an upper level pv to the west of a low level pv associated with the cyclone this configuration not only promotes intensification but also a poleward tendency that results from the nonlinear advection of the low level anomaly by the upper level pv the separate contributions of the upper and lower level pv as well as the surface temperature anomaly are analyzed using a piecewise pv inversion which shows the importance of the upper level pv anomaly in advecting the cyclone poleward the pv analysis also emphasizes the crucial role played by latent heat release in the poleward motion of the cyclone the l

abrupt stratospheric vortex weakening associated with north atlantic anticyclonic wave breaking the sudden stratospheric warming ssw of 12 february 2018 was not forecast by any extended range model beyond 12 days from early february all forecast models that com prise the subseasonal to seasonal s2s database abruptly transitioned from indicating a strong stratospheric polar vortex spv to a high likelihood of a major ssw we demonstrate that this forecast evolution was associated with the track and intensity of a cyclone in the north east atlantic with an associated anticyclonic rossby wave break which was not well forecast the wave break played a pivotal role in building the ural high which existing literature has shown was a precursor of the 2018 ssw the track of the cyclone built an anomalously strong sea level pressure dipole between scandinavia and greenland termed the s g dipole which we use as a diagnostic of the wave break forecasts which did not capture the magnitude of this event had the largest errors in the spv strength and did not show enhanced vertical wave activity a composite of similarly strong wintertime november march s g dipoles in reanalysis shows associated anticyclonic wave breaking leading to significantly enhanced vertical wave activity and a weakened spv in the following days which occured in 35 of the 15 day periods preceding observed major ssws our results indicate a particular transient trigger for weakening the spv complementing existing results on the importance of tropospheric blocking for disruptions to the northern hemisphere extratropical stratospheric circulation

observations of the vertical structure of the eddy field in the kuroshio extension normalized kinetic energy frequency distributions at a site in the kuroshio extension along 152 e are independent of depth as a good first approximation the lowest frequencies are strongly vertically coherent from 350 to 6000 m depths nominal none of the other vertical sets of time series previously examined possess these characteristics to such a degree coherence being typically much lower between the thermocline and abyssal depths with spectral shape changing significantly as well the vertical distributions of eddy kinetic energy at this kuroshio extension site and for the gulf stream along 55 w are very similar if a depth independent contribution of approximately 100 cm2 s 2 be added to the former the latter observation suggests the existence of a more energetic weakly depth dependent component for the eddy field in the north atlantic than for the north pacific at special locations in the vicinity of their mid latitude jets possibly a signature of distinct instability mechanisms

geologic history of the mead impact basin venus the geologic history of the mead impact basin on venus a basin similar in size to chicxulub mexico may be a guide of what to expect from future exploration of chicxulub during the collapse phase of crater formation in the mead basin radar bright impact melt material was deposited as a topographically flat surface within a large central area burying the transient cavity rim and other underlying structures the central area is not flat now and has been modified by viscous relaxation and thermal cooling effects substantial parts of the ejecta deposits have been covered by postimpact volcanic flows that are not obvious without the topographic data previous global surveys of venusian impact craters using only image data may have underestimated the number of craters embayed by volcanism

the isostatic state of mead crater abstract we have analyzed high resolution magellan doppler tracking data over mead crater using both line of sight and spherical harmonic methods and have found a negative gravity anomaly of about 4 5 mgal at spacecraft altitude 182 km this is consistent with no isostatic compensation of the present topography the uncertainty in the analysis allows perhaps as much as 30 compensation at shallow depths 25 km this is similar to observations of large craters on earth which are not generally compensated but contrasts with at least some lunar basins which are inferred to have large moho uplifts and corresponding positive bouguer anomalies an uncompensated load of this size requires a lithosphere with an effective elastic lithosphere thickness greater than 30 km in order for the crust mantle boundary not to have participated in the deformation associated with the collapse of the transient cavity during the creation of the crater the yield strength near the top of the mantle must have been significantly higher on earth and venus than on the moon at the time of basin formation this might be due to increased strength against frictional sliding at the higher confining pressures within the larger planets alternatively the thinner crusts of earth and venus compared to that of the moon may result in higher creep strength of the upper mantle at shallower depths

the integrated effect of condensation in numerical simulations of extratropical cyclogenesis abstract by combining traditional sensitivity studies with techniques that focus on the conservation and invertibility properties of ertel s potential vorticity pv we illustrate the effect of latent heating on the structure and evolution of three simulated extratropical cyclones the cases include one continental cyclone development 15 december 1987 which we examine extensively and two cyclones over the western atlantic ocean 6 january 1983 and 5 january 1985 of somewhat greater intensity which are diagnosed to assess the generality of our findings for the continental case each storm featured a weaker cyclonic low level circulation when latent heating was removed from the simulation but the magnitude of the effect varied greatly in all cases the difference in intensity was attributed to velocities associated with a positive condensation produced pv anomaly above the warm front the amplification of the surface thermal perturbations was not strongly altered in even the case most affected by

crustal thickness and support of topography on venus united states national aeronautics and space administration nasa planetary geology and geophysics program grant

cleopatra crater on venus venera 15 16 data and impact volcanic origin controversy cleopatra structure is a 100 km diameter feature having a morphology similar to that of double ring basins of the moon and mercury and dissimilar to that of volcanic calderas on mars earth and venus the 2 4 km depth of cleopatra is anomalously large compared to venusian and terrestrial impact craters of equivalent diameters an impartial summary of the situation is as follows if cleopatra is a volcanic caldera it is a strange caldera if cleopatra is an impact crater it is a strange crater

elastoviscoplastic relaxation of impact crater topography with application to ganymede and callisto 1 we use an elastoviscoplastic rheological model to study the long term relaxation of impact crater topography on ganymede and callisto we employ a complete set of rheological parameters for ductile creep in ice ih and implement a new shallower initial shape model under high heat flow conditions stresses are relieved in deeper less viscous material and the remainder concentrates as flexural bending stresses in a viscously stiffer near surface layer essentially a mechanical lithosphere forms that focuses stresses but with uplifts and stress levels that are always greater than those obtained for a purely viscous half space we discuss how classic elastic flexure and purely viscous relaxation are special cases of this more generalized rheological model stress concentration in the flexing i e bending layer can exceed the plastic yield strength of ice for high enough heat flows weakening the layer somewhat and increasing the total amount of relaxation these plastic strains tend to be limited to small magnitudes 1 mm permits both retention and relaxation of topography at the lower and higher ends respectively of the anticipated heat flow range consideration of relaxed craters in marius regio and of the palimpsests and penepalimpsests suggests ancient heat flows in excess of 10 mw m 2 consistent with other estimates

mechanisms controlling the downstream poleward deflection of midlatitude storm tracks abstractthe atlantic and pacific storm tracks in the northern hemisphere are characterized by a downstream poleward deflection which has important consequences for the distribution of heat wind and precipitation in the midlatitudes in this study the spatial structure of the storm tracks is examined by tracking transient cyclones in an idealized gcm with a localized ocean heat flux the localized atmospheric response is decomposed in terms of a time and zonal mean background flow a stationary wave and a transient eddy field the lagrangian tracks are used to construct cyclone composites and perform a spatially varying pv budget three distinct mechanisms that contribute to the poleward tilt emerge transient nonlinear advection latent heat release and stationary advection the downstream evolution of the pv composites shows the different role played by the stationary wave in each region in the region where the tilt is maximized all three mechanisms contribute to the poleward propagation of the

a consistent poleward shift of the storm tracks in simulations of 21st century climate 1 a consistent poleward and upward shift and intensification of the storm tracks is found in an ensemble of 21st century climate simulations performed by 15 coupled climate models the shift of the storm tracks is accompanied by a poleward shift and upward expansion of the midlatitude baroclinic regions associated with enhanced warming in the tropical upper troposphere and increased tropopause height the poleward shift in baroclinicity is augmented in the southern hemisphere and partially offset in the northern hemisphere by changes in the surface meridional temperature gradient the poleward shift of the storm tracks also tends to be accompanied by poleward shifts in surface wind stress and precipitation and a shift towards the high index state of the annular modes these results highlight the integral role that the storm tracks play in the climate system and the importance of understanding how and why they will change in the future

stationary and traveling mesoscale perturbations in the kuroshio extension current abstract several hundred xbt observations in the western north pacific were collected each month over a 2 year period 1976 78 they have been assembled into a monthly map sequence of thermocline temperature in the kuroshio extension current having 100 km spatial resolution the overall time men of the maps exhibits mesoscale 200 600 km perturbations which correlate with several major bathymetric features especially the shatsky rise time variability about the mean decreases significantly cast of the shatsky rise in addition time variable mesoscale disturbances propagate zonally westward at 3 8 cm s 1 attempts to explain the observed propagation phase velocity through simple analytic baroclinic rossby wave theory lead to the implication that there exists in the region an eastward deep mean flow of 3 or 4 cm s 1 direct current measurements of long duration are required to help resolve and explain these observations

the role of stationary eddies in shaping midlatitude storm tracks transient and stationary eddies shape the extratropical climate through their transport of heat moisture and momentum in the zonal mean the transports by transient eddies dominate over those by stationary eddies but this is not necessarily the case locally in particular in storm track entrance and exit regions during winter stationary eddies and their interactions with the mean flow dominate the atmospheric energy transport here it is shown that stationary eddies can shape storm tracks and control where they terminate by modifying local baroclinicity simulations with an idealized aquaplanet gcm show that zonally localized surface heating alone e g ocean heat flux convergence gives rise to storm tracks which have a well defined length scale that is similar to that of earth s storm tracks the storm tracks terminate downstream of the surface heating even in the absence of continents at a distance controlled by the stationary rossby wavelength scale stationary eddies play a dual role within about half a rossby wavelength downstream of the heating region stationary eddy energy fluxes increase the baroclinicity and therefore contribute to energizing the storm track farther downstream enhanced poleward and upward energy transport by stationary eddies reduces the baroclinicity by reducing the meridional temperature gradients and enhancing the static stability transports both of sensible and latent heat water vapor play important roles in determining where storm tracks terminate

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

meanders of the gulf stream downstream from cape hatteras 1975 1978 abstract weekly gulf stream paths within 1000 km downstream of cape hatteras were obtained for 1975 78 from the navy s weekly eofa charts based on satellite ir imagery they displayed two dominant meander modes first a standing meander energetic over periods between 4 months and at least 4 years and second down stream propagating meanders that were most energetic at periods of several weeks the long period standing meander was confined between nodes located at the separation point near cape hatteras i e where the stream s mean path turns seaward and at a point about 600 km farther downstream the rms amplitude was 36 km at the antinode the amplitude of propagating meanders increased rapidly in the first 200 km downstream of the separation point where the capture of warm core eddies was common farther down stream the predominant meanders had a wavelength averaging 330 km a period averaging 1 5 month a phase speed averaging 8 cm s 1 a downstream group speed averaging 17 cm s 1 and downstrea

estimates of the geostrophic stream function in the western north pacific from xbt surveys the upper ocean temperature and dynamic height fields are correlated in the western north pacific ocean this relationship can be used to map a relative geostrophic stream function of the kuroshio extension using temperature data from expendable bathythermograph xbt surveys we have examined the ability of the north pacific ship of opportunity xbt program known as transpac to carry out this procedure estimates of 600 1200 dbar geostrophic shear were generated from the xbt data utilizing objective mapping techniques these estimates were compared with measured currents at 600 m relative to 1200 m along 152 e from 27 n to 42 n the estimated and measured relative currents are positively correlated with an rms error of 6 cm s over a measured dynamic range of 30 cm s however the xbt estimated shears consistently underestimate the measured values by a factor of 2 to 3 the underestimate of the amplitude is the result of both undersampling and mapping produres that eliminate high wavenumber contributions of the dynamic height variability these factors appear to be responsible for previous differences in amplitude between directly measured and transpac xbt estimated values of relative eddy kinetic energy and reynolds stress the transpac xbt survey can only monitor qualitatively the dynamic topography of the kuroshio extension meanders and rings the variability outside of the extension with a temperature range of 3 c cannot be resolved because of inaccuracies in the temperature dynamic height relationship to monitor the kuroshio extension variability in the future it is recommended that transpac xbt resources in the western north pacific concentrate on obtaining a denser sample of the field during the fall and winter seasons because of incomplete ship track coverage in spring and summer

horizontal eddy fluxes of momentum and kinetic energy in the near surface of the kuroshio extension abstract sequential monthly mean temperature fields in the mid latitude western north pacific 30 45 n 140 180 e constructed from transpac xbt expendable bathythermograph data over a two year period are employed to analyze the interaction between mesoscale variability and the mean flow of the kuroshio extension an empirical temperature dynamic height regression and the geostrophic approximation lead to the computation of mean and eddy horizontal velocity components over the region from which horizontal eddy processes in both the mean momentum and the mean kinetic energy balances of the kuroshio extension are estimated the kuroshio extension generally loses mean kinetic energy as it travels from the coast of japan from 140 to 180 e with much larger eddy kinetic energy west of the shatsky rise 160 e than east of there concentrating upon zonally averaged quantities in both subregions east and west of the shatsky rise horizontal eddy momentum fluxes due to transient mesoscale variability i e

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

geological evolution of venus rises plains plumes and plateaus crustal plateaus and volcanic rises major physiographic features on venus both formed over mantle plumes crustal plateaus were produced by large degrees of plume melting beneath thin lithosphere the oldest tectonic features in crustal plateaus are ribbon like troughs indicating early uplift and tensile stretching their shallow depths suggest that surface temperature there was higher in the past widespread volcanic plains derived from the broad upwellings of internally heated mantle convection were continually erupted during the time of crustal plateau formation discrete volcanic rises younger than crustal plateaus formed over a thicker lithosphere with less plume partial melting the presence of only one transitional form indicates that the lithosphere thickened rapidly thermal and magmatic models show that the formation of these major features can be tied to an expected change in mantle convective style about 1 billion years ago

phase speed spectra and the recent poleward shift of southern hemisphere surface westerlies 1 the poleward shift of the southern hemisphere surface westerlies in recent decades is examined in reanalysis data and in the output of coupled atmosphere ocean and uncoupled atmospheric models the space time spectra of the eddy momentum fluxes in the upper troposphere reveal a trend that marks an increase in the eastward phase speed of the tropospheric eddies accompanied by a poleward displacement of the region of wave breaking in the subtropics a dynamical mechanism is suggested that may help explain the connections among the lower stratospheric wind anomalies the increased eastward propagation of tropospheric eddies and the poleward shift of the tropospheric circulation

comments on using the viscoelastic relaxation of large impact craters to study the thermal history of mars karimi et al 2016 icarus 272 102 113 and studying lower crustal flow beneath mead basin implications for the thermal history and rheology of venus karimi and dombard 2017 icarus 282 34 39 abstract two recent papers by karimi et al 2016 icarus 272 102 113 and karimi and dombard 2017 icarus 282 34 39 tried to deduce paleo heat flows for respectively mars and venus from modeling the viscoelastic relaxation of large impact craters indeed crater relaxation would be consequence of the flow of the lower crust and uppermost mantle this flow is dependent on temperature permitting the link with the calculation of thermal profiles and heat flows both papers used conductive thermal profiles and constant thermal conductivities for both the crust and upper mantle equivalent to using linear thermal gradients given there were no mention to crustal or mantle heat sources and appropriate rheological laws in the present discussion we show that the background heat flows contemporaneous to impact craters formation and relaxation obtained by karimi and co workers when used along with the assumptions made by these authors lead to temperatures that produce massive even total lower crust melting in all and at least a substantial part of the cases for respectively venus and mars it is clear that the heat flow results presented by karimi and co workers suffer of inconsistency between model requirements explicitly indicated a crust free of melting and the implications of the obtained basal heat flows a lowermost crust partially or totally molten thus we consider that the papers by karimi and co workers do not give reliable information on the thermal history of mars and venus

postimpact modification by volcanic or tectonic processes as the rule not the exception for venusian craters 1 we present three investigations that use the venusian impact crater population to constrain the planet s resurfacing history we evaluate stereo derived topography for 91 venusian craters that have a diameter d greater than 15 km craters with radar bright floors have greater rim floor depths and rim heights than craters with radar dark floors for the bright floored craters rim floor depths are dbf 0 483 d0 165 and rim heights are rhbf 0 056 d0 483 trends for dark floored craters are ddf 0 424 d0 108 and rhdf 0 181 d 0 025 for a 60 km crater this represents differences of 290 m in rim floor depth and 240 m in rim height we interpret these results to indicate that dark floored craters have experienced postimpact volcanic embayment and filling we examine the population of craters with d 20 km that have radar dark halos surrounding their continuous ejecta 114 craters we find that a portion of the halo has been removed for almost all dark floored craters consistent with our interpretation that dark floored craters have been affected by postimpact volcanism finally we assessed geologic histories of 12 large impact structures with stereo coverage all but one of these structures has experienced postimpact volcanism or tectonic deformation often in multiple episodes in summary widespread volcanic and tectonic activity occurred throughout the time period of emplacement of the crater population postulated resurfacing histories that consider the majority of craters to be at the top of the stratigraphic column are invalid and the mean surface age of venus is young 150 my

on the possibility of viscoelastic deformation of the large south polar craters and true polar wander on the asteroid vesta the asteroid vesta located within the inner asteroid belt is a differentiated body with a prominent rotational bulge nasa s dawn mission revealed the presence of two large relatively shallow impact craters in the south polar region one with a high standing central peak the shallowness and prominent central peak are reminiscent of large craters on some icy satellites that may have experienced strong topographic relaxation the location of these basins near the south pole is also unusual and suggests true polar wander which requires relaxation of the rotational bulge thus we use the finite element method and a viscoelastic rheology to examine the feasibility of relaxation processes operating on vesta given the plausible thermal state of vesta by the decay of long lived radioactive elements we find that the lithosphere is not compliant enough to allow strong relaxation of the large south polar craters and thus the peculiar morphology is possibly a product of the formation of these large basins at a planetary scale additionally the asteroid has not been warm enough to permit the relaxation of the rotational bulge consequently these craters both happened to form near the south pole as unlikely as that is

a comparison of the mid latitude eddy fields in the western north atlantic and north pacific oceans abstract latitudinal distributions of estimates of eddy kinetic energy and off diagonal component of the horizontal reynolds stresses are demonstrated to have similar shapes at corresponding generally western locations but at much different depths in the mid latitude north atlantic and north pacific oceans this comparison is based on direct current measurements at abyssal depths in the north atlantic along 55 w and on indirect estimates of upper level velocity differences in the vertical for the north pacific between the izu or izu ogasawara or south honshu ridge and the shatsky rise these sections span the more energetic segments of the kuroshio extension and gulf stream systems

nonlinear baroclinic dynamics of surface cyclones crossing a zonal jet mechanisms leading a synoptic surface cyclone to cross an upper level zonal jet and its subsequent deepening are investigated using a two layer model on a b plane the baroclinic interaction of a low level circular cyclonic perturbation with an upper level one is first studied in vertical and horizontal cyclonic or anticyclonic uniform shears a first nonlinear effect acting on the shape and energetics of the perturbations is analyzed if the background shear is anticyclonic the perturbations are stretched horizontally they lose energy barotropically but gain it baroclinically by a well maintained westward tilt with height conversely if the shear is cyclonic perturbations remain quite isotropic but they do not keep a favorable vertical tilt with time and the baroclinic interaction is thus only transient the latitudinal motion of the perturbations also results from a nonlinear effect it is found to depend strongly on the background potential vorticity pv gradient this effect is a baroclinic equivalent of the so called nonlinear barotropic b drift and combines the nonlinear advection and vertical stretching terms these results are confirmed when the anomalies are initially located south of a confined westerly jet the poleward shift of the lower cyclonic anomaly occurs faster when the vertically averaged pv gradient is strongly positive which happens when the jet has a large barotropic component the lower anomaly crosses the jet from the warm to the cold side and deepens afterward after a detailed description of this regeneration process with the help of an energy budget it is shown that linear dynamics are not able to reproduce such behavior

recent long term moored instrument observations in the western north pacific selected results based on the first deployment duration was 298 324 days depending on specific location of an array of 38 current temperature meters on 10 moorings along 152 e from 28 n to 41 n are discussed abyssal eddy kinetic energies along 152 e near the kuroshio extension are a factor of 2 to possibly 5 lower than those previously observed at similar depths close to the energetic regions of the gulf stream the thermocline kinetic energy frequency distribution at 28 n 152 e is similar in shape to that observed at a site near 31 n 70 w but not 28 n 70 w although the total energy levels are quite different however the distance from the corresponding strong current regimes is nearly the same the largest scale latitudinal distribution of the off diagonal component of horizontal reynolds stress has a characteristic general shape that is negative to the south and positive to the north of the gulf stream or kuroshio extension or mid latitude jet at all depths for all longitudes where such data are available in both oceans and in the numerical experiments examined

the poleward shift of storm tracks under global warming a lagrangian perspective comprehensive models of climate change projections have shown that the latitudinal band of extratropical storms will likely shift poleward under global warming here we study this poleward shift from a lagrangian storm perspective through simulations with an idealized general circulation model by employing a feature tracking technique to identify the storms we demonstrate that the poleward motion of individual cyclones increases with increasing global mean temperature a potential vorticity tendency analysis of the cyclone composites highlights two leading mechanisms responsible for enhanced poleward motion nonlinear horizontal advection and diabatic heating associated with latent heat release our results imply that for a 4 k rise in the global mean surface temperature the mean poleward displacement of cyclones increases by about 0 85 of latitude and this occurs in addition to a poleward shift of about 0 6 in their mean genesis latitude changes in cyclone tracks may have a significant impact on midlatitude climate especially in localized storm tracks such as the atlantic and pacific storm tracks which may exhibit a more poleward deflected shape

water weakening of clinopyroxene in the dislocation creep regime we performed a series of triaxial compressive creep experiments at two different water fugacities to investigate the effect of water on the creep strength of a natural clinopyroxenite samples were deformed under water saturated conditions at temperatures between 1373 and 1473 k confining pressures of 150 and 300 mpa and differential stresses from 34 to 261 mpa strain rates were in the range 10 7 to 10 5 s 1 water fugacity was controlled at either 140 or 280 mpa the creep results yield a stress exponent of 2 7 0 3 and an activation energy of 670 40 kj mol compared to dry clinopyroxene wet samples creep over 100 times faster at a given temperature confining pressure water fugacity and differential stress the creep rate of clinopyroxene is proportional to the water fugacity to the 3 0 0 6 power with an activation volume of 0 m 3 mol one possible water weakening mechanism is an enhancement of the rate of dislocation climb associated with increases in the concentration of jogs and the diffusivity of silicon ions compared to other major minerals in earth s lower crust specifically olivine and plagioclase the water weakening effect is most significant for clinopyroxene under hydrous conditions the strengths of clinopyroxene and anorthite are comparable over the investigated stress range and both phases are weaker than olivine since the mineral assemblages in earth s lower continental crust are often dominated by plagioclase and pyroxene in places where a wet flow law applies the mechanical behavior of clinopyroxene will have a substantial effect on creep strength

studying lower crustal flow beneath mead basin implications for the thermal history and rheology of venus abstract mead the largest crater on venus has low topographic relief at the surface and at the crust mantle boundary due to high surface temperatures viscous deformation could play an important role in crustal structure using the finite element method we simulate the long term viscoelastic deformation of mead crater and investigate the role of lower crustal flow in the evolution of the surface and subsurface topography we examine the thermal states that allow this evolution to occur and determine the background heat flux our study constrains the background heat flux in the vicinity of mead basin to 55 90 mw m 2 this surface heat flow is generally higher than the average venusian global values suggested by recent thermal models in addition by applying hydrous and anhydrous creep rheological parameters we demonstrate that the venus s interior is rheologically dry and that the crust near mead could be relatively high in plagioclase

two year moored instrument results along 152 e results from a 2 year nominal deployment of a moored array along 152 e from 28 n to 41 n are presented with emphasis on similarities and differences between settings approximately year to year ten moorings and 38 instruments were involved from the fall of 1980 to 1982 current temperature meters spanned the water column from 500 to 4000 m depths with occasional instruments in the vicinity of 300 m and near the bottom 6000 m upper level features located in the center of the array near 35 36 n were generally stable relative to the axis of the kuroshio extension varying from deployment to deployment with meanders in this flow regime some properties of the abyssal 4000 m currents were comparatively more variable with these changes not connected to meandering of the kuroshio extension in an obvious way the vertical structure and frequency distributions of eddy kinetic energy were similar in shape and to some extent in amplitude from deployment to deployment especially relative to meanders of the kuroshio extension but translated in the opposite direction

constraints on the composition of the martian south polar cap from gravity and topography abstract the polar caps of mars have long been acknowledged to be composed of unknown proportions of water ice solid co 2 dry ice and dust gravity and topography data are here analyzed over the southern cap to place constraints on its density and hence composition using a localized spectral analysis combined with a lithospheric flexure model of ice cap loading the best fit density of the volatile rich south polar layered deposits is found to be 1271 kg m 3 with 1 limits of 1166 and 1391 kg m 3 the best fit elastic thickness of this geologically young deposit is 140 km though any value greater than 102 km can fit the observations the best fit density implies that about 55 dry ice by volume could be sequestered in these deposits if they were completely dust free alternatively if these deposits were completely free of solid co 2 the dust content would be constrained to lie between about 14 and 28 by volume the bulk thermal conductivity of the polar cap is not significantly affected by these maximum allowable concentrations of dust however even if a moderate quantity of solid co 2 were present as horizontal layers the bulk thermal conductivity of the polar cap would be significantly reduced reasonable estimates of the present day heat flow of mars predict that dry ice beneath the thicker portions of the south polar cap would have melted depending on the quantity of solid co 2 in these deposits today it is even possible that water ice could melt where the cap is thickest if independent estimates for either the dust or co 2 content of the south polar cap could be obtained and if radar sounding data could determine whether this polar cap is presently experiencing basal melting or not it would be possible to use these observations to place tight constraints on the present day heat flow of mars

viscoelastic evolution of lunar multiring basins 1 we investigate the evolution of multiring basins on the moon the orbital geophysical and geochemical data collected by lunar prospector allow three distinct groups of basins to be identified group 1 basins show well preserved topography a bull s eye gravity signature and little mare basalt fill group 2 basins contain large amounts of mare basalt and show mare dominated gravity and topography signatures group 3 basins are characterized by low amplitude topographic relief and weak gravity anomalies while the crustal structures of group 1 and 2 basins appear to date from the collapse of the transient cavity group 3 basins are highly degraded and show very little crustal thinning using a viscoelastic model we explore the evolution of the lunar multiring basins with the goal of explaining the degraded condition of group 3 basins our results show that while viscous relaxation is a plausible explanation for the state of group 3 farside basins requiring heat flux 25 40 mw m 2 the thin nearside crust renders subsolidus ductile flow in the crust unlikely the existence of many degraded basins on the nearside points to a weakened and possibly partially molten lower crust that south pole aitken appears not to have relaxed suggests that similar conditions did not exist there and that the nearside was hotter than the farside early on this is likely a consequence of the nearside farside crustal thickness dichotomy which would have caused the farside to reach full crystallization first leaving the nearside with the remaining liquid and an excess of heat producing elements

the origin of lunar mascon basins high resolution gravity data from the gravity recovery and interior laboratory spacecraft have clarified the origin of lunar mass concentrations mascons free air gravity anomalies over lunar impact basins display bull s eye patterns consisting of a central positive mascon anomaly a surrounding negative collar and a positive outer annulus we show that this pattern results from impact basin excavation and collapse followed by isostatic adjustment and cooling and contraction of a voluminous melt pool we used a hydrocode to simulate the impact and a self consistent finite element model to simulate the subsequent viscoelastic relaxation and cooling the primary parameters controlling the modeled gravity signatures of mascon basins are the impactor energy the lunar thermal gradient at the time of impact the crustal thickness and the extent of volcanic fill

relaxation of compensated topography and the evolution of crustal plateaus on venus 1 crustal plateaus dominant physiographic features on venus likely originate through dynamic mantle processes although a debate exists on whether they formed by mantle upwellings or downwellings regardless of the mode of formation several observations led to the hypothesis that viscous relaxation may be the driving force behind the apparent evolutionary sequence from a high standing plateau to a low standing plateau with elevated margins we apply analytic and finite element models to test this hypothesis for isostatically compensated topography as modeling of gravity data suggests that crustal plateaus are presently supported by crustal roots geotherm values 5 k km 1 combined with a surface temperature of 740 k preclude relaxation within 109 years while geotherm values 20 k km 1 can yield relaxation times of 108 years or less hence significant relaxation requires hot conditions in order to occur within the appropriate 1 gyr timescale set by crustal plateau ages we also show that a compensated plateau can either retain its shape as it relaxes or become more domical in appearance mantle temperatures 1400 k allow strong crust mantle coupling that hinders flow of crustal material from the center of the plateau and produces domical relaxed profiles higher temperatures lead to a relatively inviscid mantle and relaxation that is largely insensitive to topographic wavelength thus preserving the original topographic shape during relaxation in either case relaxation of compensated plateaus does not yield elevated rims we propose that the state of compensation must be considered variable

lunar multiring basins and the cratering process abstract numerous studies of the lunar gravity field have concluded that the lunar moho is substantially uplifted beneath the young multiring basins this uplift is presumably due to the excavation of large quantities of crustal material during the cratering process and subsequent rebound of the impact basin floor using a new dual layered crustal thickness model of the moon the excavation cavities of some nearside multiring basins grimaldi and larger and younger than tranquillitatis were reconstructed by restoring the uplifted moho to its preimpact location the farside south pole aitken spa basin was also considered due to its importance in deciphering lunar evolution restoring the moho to its preimpact position beneath these basins resulted in a roughly parabolic depression from which the depth and diameter of the excavation cavity could be determined using these reconstructed excavation cavities the basin forming process was investigated excavation cavity diameters were generally found to be on the small side of most previous estimates for orientale the modeled excavation cavity lies within the inner rook ring additionally with the exception of the three largest basins serenitatis imbrium and south pole aitken the depth diameter ratios of the excavation cavities were found to be 0 115 0 005 a value consistent with theoretical and experimental results for impact craters orders of magnitude smaller in size the three largest basins however appear to have significantly shallower depths of excavation compared to this trend it is possible that this may reflect a different physical process of crater formation e g nonproportional scaling special impact conditions or postimpact modification processes the crustal thickness model also shows that each basin is surrounded by an annulus of thickened crust we interpret this thickened crust as representing thick basin ejecta deposits and we show that the radial variation in the thickness of these deposits is consistent with scaling laws obtained from small scale experimental studies if multiring basins ever possessed a terraced main crater rim this terraced zone may be presently unrecognizable at the surface due to the emplacement of ejecta deposits that exceed a few kilometers in thickness exterior to the excavation cavity rim we also show that the interiors of many basins were superisostatic before mare volcanism commenced those basins that were closest to approaching a premare isostatic state lie close to or within an anomalous geochemical province rich in heat producing elements

climate change as a regulator of tectonics on venus tectonics volcanism and climate on venus may be strongly coupled large excursions in surface temperature predicted to follow a global or near global volcanic event diffuse into the interior and introduce thermal stresses of a magnitude sufficient to influence widespread tectonic deformation this sequence of events accounts for the timing and many of the characteristics of deformation in the ridged plains of venus the most widely preserved volcanic terrain on the planet

farside gravity field of the moon from four way doppler measurements of selene kaguya the farside gravity field of the moon is improved from the tracking data of the selenological and engineering explorer selene via a relay subsatellite the new gravity field model reveals that the farside has negative anomaly rings unlike positive anomalies on the nearside several basins have large central gravity highs likely due to super isostatic dynamic uplift of the mantle other basins with highs are associated with mare fill implying basalt eruption facilitated by developed faults basin topography and mantle uplift on the farside are supported by a rigid lithosphere whereas basins on the nearside deformed substantially with eruption variable styles of compensation on the near and farsides suggest that reheating and weakening of the lithosphere on the nearside was more extensive than previously considered

using the viscoelastic relaxation of large impact craters to study the thermal history of mars we simulate the long term deformation of martian craters and investigate the role of lower crustal flow in the evolution of surface and subsurface topography using the finite element method and a viscoelastic rheological model we model the deformation of more than 30 large craters and quasi circular depressions qcds in the diameter range of 200 500 km in both the northern lowlands and southern highlands we determine the most appropriate background heat fluxes that produce the current topography beneath the impacts at the crust mantle boundary ranges from 40 to 90 mw m 2 our study shows that a higher background heat flux leads to more relaxation at the surface and subsurface by applying various viscous creep parameters for hydrous and anhydrous rheologies we demonstrate that mars s interior is wet to a certain degree which is consistent with other estimates since craters and qcds are distributed fairly equally on the surface of the red planet this study provides a less regionally biased picture of the thermal history of early mars than in previous studies based on our results the ancient average background heat flux in the northern lowlands was higher than that of the southern highlands which could indicate that whatever process formed the crustal dichotomy had a thermal signature at least through the middle noachian

midlatitude fronts and variability in the southern hemisphere tropical width abstractthis study examines the relationship between midlatitude synoptic activity and variations in the width of the tropics in the southern hemisphere for the period 1979 2016 the edge of the tr

expansion of the hadley cell under global warming 1 a consistent weakening and poleward expansion of the hadley circulation is diagnosed in the climate change simulations of the ipcc ar4 project associated with this widening is a poleward expansion of the subtropical dry zone simple scaling analysis supports the notion that the poleward extent of the hadley cell is set by the location where the thermally driven jet first becomes baroclinically unstable the expansion of the hadley cell is caused by an increase in the subtropical static stability which pushes poleward the baroclinic instability zone and hence the outer boundary of the hadley cell

venus crater distribution and plains resurfacing models detailed analysis of the distribution of craters on venus using mth nearest neighbor analysis coupled with models based upon surface morphology constraints indicates that the hypothesis of complete spatial randomness csr cannot be rejected but is not a unique model of the observed crater distribution based on morphologic mapping the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0 5t the mean surface age of the planet this four unit plains model along with its derivatives produce test statistics that indicate such models also cannot be rejected further the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis model corresponds to venus is true is lowest for the csr model there is no particular reason to pick a csr model along with its implications for catastrophic resurfacing as a constraint on the evolution of venus and there are geological reasons to choose the multiage models we find that we cannot distinguish statistically among models that have two three or four distinct production ages within the plains however the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons first the binomial probability that such a process could exist within each of the plains units is 0 05 except the smallest and youngest unit pl1 second using a chi squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10 4 indicating confident rejection of the hypothesis thus csr cannot be used as a constraint on models of resurfacing or planetary evolution of venus because of the non uniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age geological and geophysical constraints provide our best clues for understanding venus

the lunar crust global structure and signature of major basins new lunar gravity and topography data from the clementine mission provide a global bouguer anomaly map corrected for the gravitational attraction of mare fill in mascon basins most of the gravity signal remaining after corrections for the attraction of topography and mare fill can be attributed to variations in depth to the lunar moho and therefore crustal thickness the large range of global crustal thickness 20 120 km is indicative of major spatial variations in melting of the lunar exterior and or significant impact related redistribution the 61 km average crustal thickness constrained by a depth to moho measured during the apollo 12 and 14 missions is preferentially distributed toward the farside accounting for much of the offset in center of figure from the center of mass while the average farside thickness is 12 km greater than the nearside the distribution is nonuniform with dramatic thinning beneath the farside south pole aitken basin with the global crustal thickness map as a constraint regional inversions of gravity and topography resolve the crustal structure of major mascon basins to half wavelengths of 150 km in order to yield crustal thickness maps with the maximum horizontal resolution permitted by the data the downward continuation of the bouguer gravity is stabilized by a three dimensional minimum slope and curvature algorithm both mare and non mare basins are characterized by a central upwarped moho that is surrounded by rings of thickened crust lying mainly within the basin rims the inferred relief at this density interface suggests a deep structural component to the surficial features of multiring lunar impact basins for large 300 km diameter basins moho relief appears uncorrelated with diameter but is negatively correlated with basin age in several cases it appears that the multiring structures were out of isostatic equilibrium prior to mare emplacement suggesting that the lithosphere was strong enough to maintain their state of stress to the present

viscous relaxation of impact crater relief on venus constraints on crustal thickness and thermal gradient because of the high surface temperature on venus and the strong temperature dependence of strain rate in silicates solid state creep may be an important mechanism for the reduction of topographic relief on that planet venera 15 16 radar altimetric profiles of impact craters 30 140 km in diameter however indicate crater depths of several hundred meters suggesting that substantial relaxation has not occurred for these features on the basis of models for the viscous relaxation of impact crater topography we derive constraints on the thickness of the crust and the mean lithospheric thermal gradient beneath the craters from the mean and variance of observed crater depth as a function of crater diameter on venus and from estimates of the initial depths of fresh impact craters on venus obtained by scaling from lunar observations we formulate a statistical test for the maximum amount of viscous relaxation that is consistent with the observations at a specified confidence level we develop a general formulation for gravity driven flow in a linearly viscous fluid incorporating the densities and temperature dependent effective viscosities of distinct crust and mantle layers the statistical comparison of predicted and observed depths yields linked upper limits to crustal thickness h and thermal gradient dt dz since dt dz can be estimated from global heat loss arguments or thermal models an upper bound to h may be derived the preserved relief of the largest craters constrain h to be less than or equal to 10 20 km because the craters with measured depths appear to be representative of a larger population of impact structures in terms of regional elevation geological unit and degree of preservation of morphological detail extrapolation of this result to lowlands and rolling plains regions on a global basis is not unreasonable such an extrapolation together with isostatic considerations yields an upper limit to the crustal volume of venus of 1010 km3 this value is an order of magnitude less than the time integrated volume of crust produced on earth and implies either that the average rate of crustal generation has been much smaller on venus than on earth or that some form of crustal recycling has occurred on venus

mid latitude mesoscale temperature variability in six multiship xbt surveys abstract we discuss the spatial character of mesoscale temperature variability as observed in six midlatitude multiship expendable bathythermograph xbt surveys two from the north pacific and four from the north atlantic these surveys sample mid ocean regions as well as regions adjacent to the western boundary currents of both subtropical gyres mesoscale characteristics exhibit substantial changes between surveys between ship tracks within each survey and between different portions of individual ship tracks these results suggest that a single xbt section across a region is unlikely to represent the range of variability within that region satisfactorily fluctuations below the mixed layer are strongly vertically correlated but surface temperatures are not well correlated with those below the mixed layer we concentrate on the horizontal variability of the temperature at 450 m and present rms thermal variability values and zonal autocorrelation functions as conventional measures of the variability

global mapping of crustal and lithospheric thickness on venus 1 we have produced a high resolution global admittance map of venus using a spatiospectral localization technique calculating 64 800 admittance spectra evenly spaced in latitude and longitude we grouped the admittance spectra into 35 classes that we inverted for elastic crustal and lithospheric thickness using bottom loading top loading and short wavelength top loading models with long wavelength bottom loading hot spot models our best fit models identify 17 classes consistent with top loading 40 of the surface 7 with bottom loading 39 and 4 with a long wavelength bottom loading model 15 the remaining seven classes 6 of the surface generally had large amplitude top loading signatures that could not be fit with our models for the classes that were successfully modeled the estimated range of crustal 0 90 km and elastic thickness 0 100 km is larger than results for most previous studies primarily because of the incorporation of bottom loading nearly half of the planet has elastic thickness 20 km which we cannot distinguish from isostasy suggesting that these regions are tectonically inactive a key finding is that the lithospheric properties typically vary over scales of 1000 km or less except in some plains and crustal plateau regions both the scale and range of crustal and elastic thickness variations indicate that venus remains an active planet and that geologic processes may be more complex than previously recognized in many regions the correlation with surface geology is not obvious and this suggests that deformation of the lower lithosphere and crust may occur with little surface manifestation

a magmatic loading model for coronae on venus 1 models for the formation of coronae quasi circular volcanotectonic features on venus must explain four critical characteristics coronae display 1 a wide range of diameters 2 complex varied topography 3 fracture annuli and 4 sometimes extensive volcanism previous models have difficulty simultaneously satisfying all four constraints on the basis of observations and interpretations of features on venus and earth and experiments in geophysical fluid dynamics we propose that coronae form in response to magmatic loading of the crust over zones of partial melting at the tops of thermally buoyant heads of transient mantle plumes that impinge on the base of the thermal lithosphere by tying corona formation to a melt zone and not directly to an impinging upwelling our conceptual model can account for the wide range in corona diameters lateral crustal flow facilitated by magmatic heating may lead to central depressions characteristic of many coronae a thermomechanical finite element simulation with a dissipating thermal anomaly leads to crustal thinning and predicts an elevation profile consistent with one of the more populous topographic classes of coronae deformation concentrated at the transition from thicker to thinner lithosphere above the thermal anomaly yields a narrow annulus of enhanced differential stress consistent with formation of fracture annuli estimates of excess heat in an upwelling exceed the thermal energy in the melt that comprises the corona band pass filtered maps of the topography and gravity fields in the beta atla themis region are consistent with upwellings now impinging on the lithosphere beneath seven coronae previously inferred to be active

an algorithm to detect isolated anomalies in three dimensional stratified data fields with an application to density fields from four deep basins of the baltic sea 1 a new approach to describe the mesoscale variability caused by eddies in terms of the integral properties of all eddies contained in a certain ocean volume is suggested for this purpose a pattern recognition algorithm to detect isolated anomalies in three dimensional stratified data fields was developed and is introduced in detail an application to 12 eddy resolving density fields acquired in the framework of the german russian project mesodyn in four deep basins of the baltic sea namely the arkona basin the bornholm basin the stolpe furrow and the eastern gotland basin demonstrates that the algorithm works properly the most remarkable finding of this application is that the volume fraction occupied by the isolated anomalies or eddies is constant it amounts to about 12 of the total volume for each data field no matter whether the stratification corresponds to a summer or a winter situation many possible applications of the pattern recognition are proposed to document its general usefulness finally the main features of the algorithm and possible extensions are discussed extensively

mechanisms for lithospheric heat transport on venus implications for tectonic style and volcanism the tectonic and volcanic characteristics of the surface of venus are poorly known but these characteristics must be closely related to the mechanism by which venus rids itself of internal heat on the other solid planets and satellites of the solar system lithospheric heat transport is dominated by one of three mechanisms 1 plate recycling 2 lithospheric conduction and 3 hot spot volcanism we evaluate each mechanism as a candidate for the dominant mode of lithospheric heat transfer on venus and we explore the implications of each mechanism for the interpretation of venus surface features despite claims made to the contrary in the literature plate recycling on venus cannot be excluded on the basis of either theoretical arguments or present observations on topography and radar backscatter landforms resulting from plate convergence and divergence on venus would differ substantially from those on the earth because of the high surface temperature and the absence of oceans on venus the lack of free or hydrated water in subducted material the possibility that subduction would more commonly be accompanied by lithospheric delamination and the rapid spreading rates that would be required if plate recycling removes a significant fraction of the internal heat if plate recycling occurs on venus the rolling plains and lowlands provinces would be approximate analogs to terrestrial ocean basins in terms of age igneous rock type and formative process highlands on venus would be roughly analogous to terrestrial continents the hypothesis that lithospheric conduction dominates shallow heat transfer on venus leads to the prediction that the lithosphere is thin if venus has a global heat loss per mass equal to that for earth then temperatures marking the base of the thermal lithosphere on earth would be reached on venus at an average depth of about 40 km unless the mantle convective planform can maintain lithospheric regions of persistently low heat flow or unless the present atmospheric greenhouse on venus is geologically recent then such a lithospheric thickness leads to the conclusion that the topographic features contributing to the 13 km of relief on the planet must be geologically young the hypothesis that hot spot volcanism dominates lithospheric heat transfer on venus leads to the prediction that the surface must be covered with numerous active volcanic sources in particular if a typical venus hot spot has a volcanic flux equal to the average flux for the hawaiian hot spot for the last 40 m y then 104 such hot spots are necessary to remove the venus internal heat by volcanism such a number would produce enough volcanic material to resurface the entire planet to a depth of 1 km every 2 m y few areas of the planet would escape resurfacing for geologically long periods of time we find that none of the mechanisms for lithospheric heat transfer on venus can be excluded as unimportant at present it is likely that as on earth a combination of mechanisms operates on venus the strongest conclusion to emerge from this evaluation is that most of the major topographic features and probably many of the surface geological units on venus are young by comparison with the surfaces of the smaller terrestrial planets

role of moist processes in the tracks of idealized midlatitude surface cyclones abstractthe effects of moist processes on the tracks of midlatitude surface cyclones are studied by performing idealized mesoscale simulations in each simulation a finite amplitude surface cyclone is initialized on the warm air side of a zonal baroclinic jet for some simulations an upper level cyclonic anomaly upstream of the surface cyclone is also added initially sensitivities to the upper level perturbation and moist processes are analyzed by both performing a relative vorticity budget analysis and adopting a potential vorticity pv perspective whatever the simulation there is a systematic crossing of the zonal jet by the surface cyclone occurring after roughly 30 h a pv inversion tool shows that it is the nonlinear advection of the surface cyclone by the upper level pv dipole which explains the cross jet motion of the surface cyclone the simulation with an initial upper level cyclonic anomaly creates a stronger surface cyclone and a more intense upper level pv dipole than the simulation with

abyssal eddy kinetic energy levels in the western north pacific abstract an earlier observation of about 50 cm2 s 2 for the maximum abyssal 4000 m depth eddy kinetic energy ke in the western north pacific along 152 e occurring in the vicinity of the kuroshio extension is now supported by nearly two years of observation with moored instruments deployed from 28 to 41 n the maximum occurred at the same site each year nominal from one array setting to the next near 35 n and the composite or two year average is 45 cm2 s 2 at analogous longitudes in the western north atlantic one finds abyssal kinetic energies as high as 100 150 cm2 s 2 in the vicinity of the gulf stream however the abyssal western north pacific could be more energetic at longitudes other than 152 e eddy intensities at abyssal depths near the gulf stream do drop off to smaller values approaching either cape hatteras or longitudes in the vicinity of the grand banks of newfoundland abyssal ke values at specific sites along 152 e were observed to vary by as low as a few percent and as high a

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