evolution of midplate hotspot swells numerical solutions the evolution of midplate hotspot swells on an oceanic plate moving over a hot upwelling mantle plume is amenable to numerical simulation in our model the plume supplies to the lithosphere a gaussian shaped thermal perturbation and dynamic support dominated by thermal buoyancy we consider the two fundamental mechanisms of transferring heat conduction and convection during the interaction of the lithosphere with the mantle plume the transient heat transfer equations with boundary conditions varying in both time and space are solved in cylindrical coordinates using the alternating direction implicit finite difference method on a 100 100 grid the topography geoid anomaly and heat flow anomaly of the hawaiian swell and the bermuda rise are used to constrain the models our results confirm the conclusion from previous work that the hawaiian swell can not be explained by conductive heating alone even if an extremely high thermal perturbation is allowed on the other hand the model of convective thinning predicts successfully the topography geoid anomaly and the heat flow anomaly around the hawaiian islands as well as the changes in the topography and anomalous heat flow along the hawaiian volcanic chain the model constrains the hawaiian plume to have a half wavelength of about 500 km a center heat flux 5 6 times the background value and a convective current velocity 3 10 times that of the background convective current comparatively the mantle plume for the bermuda rise is much weaker conductive heating is probably the dominant mechanism for the evolution of the bermuda rise

spruce trees from a melting ice patch evidence for holocene climatic change in the colorado rocky mountains usa in september 2006 we found the remains of timber sized spruce trees picea engelmannii on the floors of melting ice patches at altitudes of 3465 3480 m in the mummy range of north central colorado the ice patches occupy northeast facing recesses in which windblown snow scoured from a tundra upland to the southwest accumulates deeply we hypothesize that the upland was timbered during the early to middle holocene dense forest vegetation intercepted snow preventing it from blowing to the recesses and allowing trees to become established there neoglacial cooling led to gradual deforestation of the upland renewed transport and deposition of snow by wind and death of the ice patch trees radiocarbon dates show that the trees died between 3860 15 and 3780 20 14c yr bp c 4200 cal yr bp the trunks show decay similar to that of historic log structures built above timberline during the late nineteenth and early twentieth centuries suggesting that they have been protected by ice for all but a

geographic variability in the export of moist static energy and vertical motion profiles in the tropical pacific 1 column integrated moist static energy mse budgets were calculated using daily data from three reanalyses 1998 2001 and isccp radiative cooling the mse export by the circulation was separated into vertical and horizontal mse advection components in order to examine how observationally derived vertical motion profiles affect gross moist stability calculations in a 2000 km wide region in the central eastern pacific itcz vertical motions import mse implying a negative gross moist stability horizontal mse advection is negative and of similar magnitude to vertical mse advection in rainy regions geographic differences in export of mse by the vertical circulation are primarily due to differences in the shape of the vertical motion profile in the west pacific warm pool rainy regions mean horizontal convergence extends up to 300 mb while in parts of the pacific itcz where meridional sst gradients are strong the vertical motion profile is bottom heavy with convergence below 800 mb and divergence above on daily timescales the mode of vertical motion variability has little dependence on precipitation rate and is consistent with observed regional differences in the vertical structure of trmm pr reflectivities

the madden julian oscillation barotropic dynamics and north pacific tropical cyclone formation part i observations low level barotropic dynamics may help to explain the modulation of eastern and western north pacific tropical cyclones by the madden julian oscillation mjo during northern hemisphere summer the mjo is characterized by alternating periods of westerly and easterly 850 mb zonal wind anomalies across the tropical pacific ocean when mjo 850 mb wind anomalies are westerly small scale slow moving eddies grow through barotropic eddy kinetic energy eke conversion from the mean flow these growing eddies together with strong surface convergence 850 mb cyclonic shear and high mean sea surface temperatures create a favorable environment for tropical cyclone formation periods of strong mjo easterlies over the pacific are characterized by lesser eke and negligible eddy growth by barotropic conversion the term 2 x is a leading contributor to low level barotropic eke conversion during mjo westerly 2 u9 u periods across the pacific indicating the importance of zonal variations in the westerly jet for producing concentrations of eddy energy this mechanism can be described as wave accumulation associated with variations of the low level zonal flow the conversion term 2 y contributes a smaller portion of the total conversion u9y9 u over the eastern pacific but is of comparable importance to 2 x during westerly mjo events in the 2 u9 u western pacific

thermomechanical erosion of the lithosphere by mantle plumes a simplified semianalytic theory is presented that describes the process of lithosphere thinning through thermomechanical erosion by a hot mantle upwelling or plume the theory compares well with published numerical models but predicts that the rate of thinning is very dependent on the lateral scale of the upwelling and on the viscosity through the lithosphere asthenosphere transition a plume tail is likely to cause rapid local thinning reducing the lithosphere thickness substantially within a few million years plume heads or broadly spreading plume tail material are unlikely to cause significant thinning by this mechanism alone these results are consistent with geophysical evidence for localized thinning under the hawaiian volcanic chain but not under the broader hawaiian swell they are also qualitatively consistent with the distribution of basaltic volcanism in oceanic and continental hotspot tracks the theory predicts that thin continental lithosphere may be further thinned sufficiently by a plume tail to cause crustal melting within about 20 m y thus this mechanism may account for some delayed silicic magmatism on some slow continental plates higher thinning rates would have occurred in the archean

zonal and vertical structure of the madden julian oscillation abstract a statistical study of the three dimensional structure of the madden julian oscillation mjo is carried out by projecting dynamical fields from reanalysis and radiosonde data onto space time filtered outgoing longwave radiation olr data mjo convection is generally preceded by low level convergence and upward motion in the lower troposphere while subsidence cooling and drying prevail aloft this leads to moistening of the boundary layer and the development of shallow convection followed by a gradual and then more rapid lofting of moisture into the middle troposphere at the onset of deep convection after the passage of the heaviest rainfall a westerly wind burst region is accompanied by stratiform precipitation where lower tropospheric subsidence and drying coincide with continuing upper tropospheric upward motion the evolution of the heating field leads to a temperature structure that favors the growth of the mjo the analysis also reveals distinct differences in the vertical structure

the madden julian oscillation in a warmer world global warming s impact on the madden julian oscillation mjo is assessed using one of the few models capable in reproducing its key features in a warmer climate predicted for the end of the century the mjo increases in amplitude by 30 and frequency showing a more circumglobal propagation tendency the mjo spatial extent becomes enhanced deeper and more zonally extended but meridionally confined a stronger vertical tilting structure in diabatic heating moisture and convergence fields is seen our findings indicate that these changes result from an intensification of the frictional wave conditional instability of the second kind mechanism via the coupling of dynamical and thermodynamic response to the warming the warming and moistening of the mean state contribute to the enhanced deep convective heating driving a stronger forced kelvin wave like perturbation this reinforces the frictional low level convergence leading to larger shallow convective heating and therefore to a faster development and enhancement of the deep convection in the mjo

impacts of vertical structure of large scale vertical motion in tropical climate moist static energy framework abstractinteractions between cumulus convection and its large scale environment have been recognized as crucial to the understanding of tropical climate and its variability in this study the moist static energy mse budget is employed to investigate the potential impact of the vertical structure of large scale vertical motion in tropical climate based on results from both reanalysis data and model simulation two domains are selected over the western and eastern pacific with vertical motion profiles that are dominated by top heavy and bottom heavy structures respectively the bottom heavy structure is climatologically associated with more shallow convection while the top heavy structure is related to more deep convection the column integrated vertical mse advection of top heavy vertical motion is positive while that of bottom heavy vertical motion tends to be negative controlling factors responsible for the above vertical mse advection contrast are discussed based on a simple decomposition of the

the role of rheology in lithospheric thinning by mantle plumes lithospheric thinning by mantle plumes is an important planetary heat transfer process resulting in the broad topographic uplift that characterizes the volcanic rises on venus the tharsis rise on mars and several hotspot swells on earth we present a suite of time dependent three dimensional numerical calculations of a plume impinging upon the lithosphere in a temperature dependent viscosity mantle efficient lithospheric thinning is found to depend on the formation of convective instabilities in the plume lithosphere boundary layer these instabilities are non axisymmetric time dependent and have horizontal scales of a few tens of kilometers these instabilities depend on the temperature dependence of viscosity and occur when the plume s viscosity is about an order of magnitude less than the background mantle as predicted by boundary layer theory thus in planetary mantles plumes with excess temperatures of 100 to 200 k will efficiently thin the lithosphere via small scale convective instabilities

on the accuracy of digital bathymetric data the global 5 arcmin gridded topography data etopo 5 are based on contour maps rather than original soundings and have large errors the artificial statistical distribution generated by digitizing contours makes these data unsuitable for use in regression models for depth age variations their amplitude spectrum is bounded by a frequency 4 power law so that they should not be used when the gravity topography transfer function is important such as in studies of flexural isostasy and mantle convection i assess the accuracy of 14 491 069 digital ship soundings in 2253 cruise surveys collected between 1955 and 1992 in the lamont doherty earth observatory on line data base by analyzing 329 058 crossover errors coes at intersecting ship tracks five percent of cruises with internal coes yield root mean square coe amplitudes exceeding 500 m all of these have errors in digitizing two way travel time from analog precision depth recorder traces twenty eight cruises were found which had errors caused by misinterpretation of the nominal sound velocity used when travel times were reported as nominal depths two nominal sound velocities in common use differ by 2 5 an amount which is often undetectable producing uncertainties in depth of this magnitude ship data have been acquired at different rates over time with the peak of activity in the early 1970s although present technologies can yield very accurate data these are acquired at a rate which is small with respect to the total available data the cumulative median global coe has remained constant at 26 m since the late 1970s most recent data acquisition has been in the northern hemisphere oceans and the oldest and least accurate data are in the southern oceans where median coes are 100 250 m the majority of the data in the south pacific were acquired before the advent of satellite navigation

role of the boundary layer moisture asymmetry in causing the eastward propagation of the madden julian oscillation the moisture budget associated with the eastward propagating madden julian oscillation mjo was diagnosed using 1979 2001 40 yr ecmwf re analysis era 40 data a marked zonal asymmetry of the moisture relative to the mjo convection appears in the planetary boundary layer pbl below 700 hpa creatingapotentiallymoreunstablestratificationtotheeastofthemjoconvectionandfavoringtheeastward propagation of mjo the pbl integrated moisture budget diagnosis indicates that the vertical advection of moisture dominates the low level moistening ahead of the convection a further diagnosis indicates that the leading term in the vertical moisture advection is the advection of the background moisture by the mjo ascending flow associated with pbl convergence the cause of the zonally asymmetric pbl convergence is further examined it is found that heating induced free atmospheric wave dynamics account for 75 90 of the total pbl convergence while the warm sst anomaly induced by air sea interaction contributes 10 25 of the total pbl convergence the horizontal moisture advection also plays a role in contributing to the pbl moistening ahead of the mjo convection the leadingterm in the moistureadvectionis the advectionacross the backgroundmoisture gradient by the mjo flow in the western indian ocean maritime continent and western pacific the meridional moisture advection by the mjo northerly flow dominates while in the eastern indian ocean the zonal moisture advection is greater the contribution of the moisture advection by synoptic eddies is in generalsmall ithas a negativeeffect overthetropicalindianoceanand westernpacificandbecomespositive in the maritime continent region

reconstruction of holocene high altitude vegetation cover in the french southern alps evidence from soil charcoal the study presented here from the southern french alps demonstrates the reliability of soil charcoal analysis for the study of holocene past treeline positions the occurrence of charcoal in mineral soils along transects from 1950 m up to 2920 m demonstrates the role of fire in the establishment of the present vegetation patterns the abrupt decrease of charcoal concentration at about 240 0 2 600 m which varies across the study sites corresponds to the modern transition between subalpine forest and alpine tundra charcoal particles formedinsitu providehighspatial resolutiondataforthereconstructionofpastforestandtreelinechanges soil charcoal analysis indicated that 1 treeline was 300 m higher around 6800 cal bp than today and 2 the uppermost forest belt up to 2810 m was colonized by larch larix decidua mill and arolla pine pinus cem bra l thispineispresenttodaybutpatchilydistributed itisabsentfromthethreeareasstudied radiocarbon dates ranging from c 6800 cal bp to the modern period along with historical and archaeological data sug gest that the present pattern of the uppermost forest belt and the patchy distribution of arolla pine in the stud ied area are the results of anthropogenic fire mainly agropastoral activities the question of global warming consequences on treelines in this part of the french alps is discussed

lithospheric thinning by midplate mantle plumes and the thermal history of hot plume material ponded at sublithospheric depths mantle plumes supply hot material to the asthenospheric channel which spreads beneath initially normal oceanic lithosphere at midplate hotspots the fate of this material and the thermal structure of the lithosphere depend on the vigor of secondary convection through the steep thermal gradient above the ponded plume material it has been proposed either that 1 secondary convection is so vigorous that material hotter than the normal mantle adiabat is cooled a short distance downstream of the hotspot such that the geotherm resembles that of younger lithosphere or 2 that the plume material retains its excess temperature for a significant time and the lithosphere thins slowly a parameterized method of modeling transient secondary convection is developed to examine this question calculations indicate that secondary convection is likely to be sluggish unless partial melting greatly lowers viscosity in addition vigorous secondary convection would greatly retard upstream asthenospheric flow of plume material beneath the nose of the hawaiian swell and asthenospheric flow of hot material away from hotspots on slowly moving plates like cape verde this conclusion is compatible with limited seismic evidence that indicates that the lithosphere is not detectably thinned beneath the hawaiian swell

lithospheric thinning and chemical buoyancy beneath the hawaiian swell the source of the buoyancy that supports the hawaiian swell is not well understood the swell may be supported by replacement of negatively buoyant lithosphere with buoyant mantle or by emplacement of buoyant material beneath lithosphere of normal thickness these mechanisms can be distinguished by examining the quadratic relationship between geoid height and bathymetry at the hawaiian swell the curvature of the geoid height vs swell topography relationship is negative indicating that the swell is supported by thinned lithosphere the magnitude of the curvature suggests that the mantle filling in the region of thinned lithosphere is both thermally and chemically buoyant

two station measurements of rayleigh wave group velocity along the hawai ian swell two station measurements of rayleigh wave group velocity between midway atoll and o ahu provide a useful constraint on lithospheric thickness along the hawai ian swell comparison of the observed dispersion curve with age dependent regionalized dispersion curves suggests that the swell acts as 50 110 m y old lithosphere and not as 20 50 m y old lithosphere as is predicted by the currently favored hypothesis for the swell s formation a preliminary isotropic shear velocity model for the swell suggests that the lithosphere may be as thick as 100 km this value is difficult to reconcile with the lithosphere being substantially thinned during its passage over the hawai ian hotspot

compensation of swells and plateaus in the north pacific no direct evidence for mantle convection at intermediate and long wavelengths the ratio of geoid height to topography is sensitive to the depth and mode of compensation a low geoid topography ratio 8 m km in conjunction with a poor correlation between geoid height and topography is evidence of mantle convection after subtracting a reference geoid from the observed geoid previous studies have found a regular pattern of geoid highs and lows with a characteristic wavelength of 3000 4000 km since these geoid highs and lows were poorly correlated with topography and resulted in very high geoid topography ratios 10 20 m km they were believed to reflect the planform of mantle convection we show that the regular pattern of geoid highs and lows is an artifact caused by truncating the reference geoid at spherical harmonic degree 10 since the geoid spectrum is red the residual geoid is dominated by degree 11 when the harmonics of the reference geoid are rolled off gradually the regular pattern of geoid highs and lows disappears in the northeast pacific the new residual geoid reflects the lithosphere age offsets across the major fracture zones in the northwest pacific the residual geoid corresponds to isostatically compensated swells and plateaus we have calculated the geoid topography ratio for 10 swells and plateaus and have found a range of compensation depths the highest geoidaopography ratio of 5 5 m km occurs on the flanks of the hawaiian swell intermediate ratios occur in four areas including the midway swell these intermediate ratios reflect a linear combination of the decaying thermal swell and the increasing volume of airy compensated seamounts low geoid topography ratios occur over the remaining five areas e g emperor seamounts reflecting the absence of a thermal swell our findings do not support the hypothesis that the planform of mantle convection is evident in the geoid we see only indirect evidence of thermal plumes reheating the lower lithosphere

realistic initiation and dynamics of the madden julian oscillation in a coarse resolution aquaplanet gcm 1 the main mechanisms for the initiation and propagation of the madden julian oscillation mjo are still widely debated the capacity of operational global climate models gcms to correctly simulate the mjo is hindered by the inadequacy of the underlying cumulus parameterizations here we show that a coarse resolution gcm coupled to a simple multicloud model parameterization mimicking the observed dynamics and physical structure of organized tropical convection simulates the mjo in an idealized setting of an aquaplanet without ocean dynamics we impose a fixed nonhomogeneous sea surface temperature replicating the indian ocean western pacific warm pool this results in a succession of mjos with realistic phase speed amplitude and physical structure each mjo event is initiated at a somewhat random location over the warm pool and dies sometimes near the eastern boundary of the warm pool and sometimes at a random location way beyond the warm pool also occasionally the mjo events stall at the center of maximum heating this is reminiscent of the fact that in nature some mjos stall over the maritime continent while others reach the central pacific ocean and beyond the initiation mechanism in the model is believed to be a combination of persistent intermittent convective events interacting with observed large scale flow patterns and internal tropical dynamics the large scale flow patterns are associated with planetary scale dry kelvin waves that are triggered by preceding mjo events and circle the globe while congestus cloud decks on the flanks of the warm pool are believed to force rossby gyres which then funnel moisture toward the equatorial region

resolving the upper ocean warm layer improves the simulation of the madden julian oscillation here we show that coupling a high resolution one column ocean model to an atmospheric general circulation model dramatically improves simulation of the madden julian oscillation mjo to have realistic strength period and propagation speed the mechanism for the simulated mjo involves both frictional wave convective conditional instability of the second kind frictional wave cisk and air sea convective intraseasonal interaction ascii in particular better resolving the fine structure of upper ocean temperature especially the warm layer produces more vigorous atmosphere ocean interaction and strengthens intraseasonal variations in both sst and atmospheric circulation this helps organize and strengthen deep convection inducing a stronger kelvin wave like perturbation and frictional near surface convergence to the east in addition the warmer sst ahead of the mjo also acts to destabilize the boundary layer and enhance frictional convergence these lead to a more realistic eastward propagating mjo a suite of sensitivity experiments were performed to show the robustness of the mechanisms and to demonstrate 1 that mean state differences are not the main contributors to the improved simulation of our coupled model 2 the role of sst variability in enhancing frictional convergence and intraseasonal variations in precipitation and 3 that the simulation is significantly degraded when the first ocean model layer is thicker than 10 m our coupled model results are consistent with observations and demonstrate a simple but effective means to significantly improve mjo simulation and potentially also forecasts

past and present day madden julian oscillation in cnrm cm5 madden julian oscillation mjo in the past nineteenth century and present day twentieth century is examined using preindustrial and historical experiments of centre national de recherches meteorologiques coupled models version 5 cnrm cm5 in coupled model intercomparison project phase 5 cmip5 the present day mjo is stronger than the past mjo by 33 and it is 10 more frequent in particular the mjo phases 4 7 signifying deep convection situated over the maritime continent and western pacific wp are considerably enhanced these changes are due mainly to greenhouse gas forcing with little impact from nature forcing dynamical mechanisms for this change are investigated a peculiar strengthening of mjo over wp comes from increased basic state sea surface temperature sst over the central pacific cp and ep the increase in precipitation over wp results from both the response to enhanced sst over cp and the inverted walker circulation induced by the ep and cp sst increase the latter causes a pair of anticyclonic rossby waves straddling the equator leading to moisture convergence over wp

mechanical erosion and reheating of the lithosphere a numerical model for hotspot swells it is currently debated if either thermal erosion of the lithosphere or dynamical support is the source of topography and geoid anomalies the origin of tins controversy lies probably in the difficulty to model simultaneously these two effects for this purpose we have studied the time dependent behavior of two dimensional convection with a temperature and pressure dependent viscosity the use of a control volume method allows us to define a rigid zone simulating the mechanical lithosphere the interface between the lithosphere and the convective mantle is determined by a viscosity cutoff first some experiments model the rise of a plume below the lithosphere in order to observe the evolution of the uplift and thus to appreciate the various processes involved in the swell formation before the plume reaches the base of the thermal lithosphere an uplift a few hundred meters in amplitude develops which can only be ascribed to a pure dynamical support the major uplift occurs when the ductile part of the lithosphere the convective boundary layer is squeezed by the plume the reheating of the mechanical lithosphere takes place after this transient stage of dynamical erosion however this late process is very slow but can magnify the amplitude of the swell if the lithosphere stays long enough above the plume these results shed some light on the different mechanisms occurring during the swell formation but the configuration modeled does not correspond to the one expected for actual hotspot swells they feature plume rising up to the lithosphere while natural situations correspond to lithosphere drifting above preexisting plumes an experiment with a moving lithosphere was run and shows that thermal erosion does not affect significantly a moving lithosphere even for relatively slow drifting velocities few centimeters year indeed the thermal structure of the lithosphere is not modified above the 800 c isotherm except for a motionless plate in this case the resulting swell should be greater this could explain why azores crozet or cap verde swells are so high on the other hand the shape of a swell over a moving lithosphere is strikingly reminiscent of the hawaiian swell

seasonality in the madden julian oscillation abstract this study using observational data and global reanalysis products synthesizes existing knowledge on the seasonality in the madden julian oscillation mjo and discusses its possible dependence on the mean background state the seasonality in the mjo is documented in terms of its components in low level zonal wind and precipitation a single peak season boreal winter exists near the equator in the indian and western pacific oceans for the broad tropical region however the seasonality in the mjo is featured by a latitudinal migration across the equator between two peak seasons the primary peak season is in boreal winter december march during which mjo signals are mainly confined to the indian and western pacific ocean and reach their maxima in the south pacific convergence zone the secondary peak season is boreal summer june september during which the strongest mjo occurs north of the equator from the bay of bengal to the south china sea and another separated region of strong mjo

modeling tropical convergence based on the moist static energy budget abstract the vertically integrated moist static energy equation provides a convenient starting point for the construction of simple models of the time mean low level convergence in the tropics a vertically integrated measure of the moist static stability the gross moist stability proves to be of central importance minima in this quantity mark the positions of the tropical convergence zones we argue that the positions of these minima are determined by the time mean moisture field which is in turn closely tied to the time mean surface temperature

sclerotia as indicators of mid holocene tree limit altitude colorado front range usa studies in the front range of north central colorado have shown that sclerotia of cenococcum geophilum a common ectomycorrhizal fungus are larger beneath spruce fir forest and tree island vegetation than beneath tundra grasses and herbs the presence in alpine tundra soils of c geophilum sclerotia with median diameters that approach or exceed 1 0 mm is evidence that tree limit formerly stood higher than it does today a sediment sample collected 116 m above tree limit on rowe mountain in rocky mountain national park contained charred sclerotia with a median diameter of 1 1 mm wood charcoal particles too large to have been transported by wind were associated radiocarbon dates of 4770 25 yr bp charred sclerotia and 4900 15 yr bp charred spruce twig apply to the wildfire that destroyed forest or tree island vegetation at this locality lack of evidence for post fire regeneration suggests that growing season temperatures had already begun to decline from elevated middle holocene levels

interpreting palaeofire evidence from fluvial sediments a case study from santa rosa island california with implications for the younger dryas impact hypothesis fluvial sequences from the late pleistocene to the holocene are exposed in arlington canyon santa rosa island northern channel islands california usa including one outcrop that features centrally in the controversial hypothesis of an extra terrestrial impact at the onset of the younger dryas the fluvial sequence in arlington canyon contains a significant quantity and range of organic material much of which has been charred the purpose of this study was to systematically describe the key outcrop of the arlington sequence provide new radiocarbon age control and analyse organic material in the arlington sediments within a rigorous palaeobotanical and palaeo charcoal context these analyses provide a test of previous claims for catastrophic impact induced fire in arlington canyon carbonaceous spherular materials were identified as predominantly fungal sclerotia carbon elongates are predominantly arthropod coprolites including termite frass glassy carbon formed from the precipitation of tars during charcoalification none of these materials indicate high temperature formation or combustion charcoal and other materials in arlington canyon document widespread and frequent fires both before and after the onset of the younger dryas recording predominantly low temperature surface fires in summary we find no evidence in arlington canyon for an extra terrestrial impact or catastrophic impact induced fire

what happens to the ocean surface gravity waves when enso and mjo phases combine during the extended boreal winter the safety of vulnerable coastal and offshore infrastructures requires an in depth understanding of wave variability and climate drivers we investigate the association of significant wave height hs and peak wave period tp with the co occurrence of el nino southern oscillation enso and the madden julian oscillation mjo at the global scale we calculate composites of daily anomalies in modelled hs tp and surface wind for periods of enso mjo phase combinations calculations spanned november march seasons over the period 1979 2018 wave anomalies are widespread across the world s oceans with remarkable strength during several enso mjo phase combinations demonstrating strong tropic tropic and tropic extratropic teleconnections positive hs anomalies are strongest in the pacific ocean during el nino mjo phase 8 in the atlantic ocean during enso neutral mjo phase 3 and in the indian ocean during enso neutral mjo phase 4 positive tp anomalies are strongest in the pacific ocean during la nina mjo phase 8 in the atlantic ocean during el nino mjo phase 1 and in the indian ocean during el nino mjo phase 8 in the southern ocean the strongest hs anomalies occur during el nino mjo phase 8 whereas in the maritime continent they appear during enso neutral mjo phases 5 6 despite previous studies finding low correlations of enso indices with wave parameters in the north atlantic our results suggest that enso related conditions play a significant role in the area when combined with certain mjo related conditions this study also reveals that the wave anomalies associated with enso mjo phase combinations can be twice as strong as those found in previous work related only to the mjo therefore considering multiple concurrent climate patterns in the analysis of wave anomalies is essential to developing more reliable coastal management plans

numerical modelling of a mantle plume the plume head lithosphere interaction in the formation of an oceanic large igneous province abstract the thermomechanical processes associated with formation of large igneous provinces lips remain poorly understood owing to fundamental difficulties in simulating plume lithosphere interactions in current numerical models these models which aim to simulate the rise of mantle plume and the spread of plume head material imply a mechanically over simplified lithosphere and commonly a flat lithosphere zero vertical displacement as the upper boundary condition we propose a new numerical model derived from lithospheric scale models it has a high numerical resolution in the lithospheric domain and explicitly accounts for 1 free upper surface boundary condition 2 elastic plastic ductile lithospheric rheology including surface faulting and 3 vertical strength variations in the lithosphere we study the final stages of plume ascent and we focus on surface and lithospheric evolution and intra plate strain localisations the experiments predict that the first surface elevation occurs in less than 0 2 ma after plume initiation at 400 km depth variation of rheological parameters results in different surface elevations 500 2500 m ascent 2 10 m yr and base plate strain rates 10 12 10 15 s 1 fast 0 2 0 3 m yr plume head flattening starts at the moment when the plume head reaches the base of the lithosphere it leads to large scale extension and deep normal faulting at the centre of the plateau and to strong thermomechanical erosion at its base the erosion is maximal not under the plume centre as was predicted before but in two large bordering zones our study locally is the igneous province of the caribbean plate where the pre existing farallon lithosphere has been affected by the galapagos hotspot activity that generated thermal perturbations and crustal thickening with two main episodes of volcanism and underplating

comprehensive analysis of nanodiamond evidence relating to the younger dryas impact hypothesis during the end of the last glacial period in the northern hemisphere near 12 9k cal a bp deglacial warming of the bolling allerod interstadial ceased abruptly and the climate returned to glacial conditions for an interval of about 1300 years known as the younger dryas stadial the younger dryas impact hypothesis proposes that the onset of the younger dryas climate reversal pleistocene megafaunal extinctions and disappearance of the clovis paleoindian lithic technology were coeval and caused by continent wide catastrophic effects of impact bolide events in north america while there are no known impact structures dated to the younger dryas onset physical evidence of the impact bolide events is argued to be present in sediments spanning several continents at stratigraphic levels inferred to date to the bolling allerod younger dryas boundary ydb reports of nanometer to submicron sized diamonds in ydb sediments in particular the rare 2h hexagonal polytype of diamond lonsdaleite have been presented as strong evidence for shock processing of crustal materials we review the available data on diamonds in sediments and provide new data we find no evidence for lonsdaleite in ydb sediments and find no evidence of a spike in nanodiamond concentration at the ydb layer to support the impact hypothesis

relative importance of climatic and autogenic controls on holocene carbon accumulation in a temperate bog in southern ontario canada peat cores from wylde lake bog in temperate southern ontario canada were examined to determine factors affecting holocene ecological change and long term rate of carbon accumulation lorca by 1

observational constraints on models of the hawaiian hot spot swell a detailed analysis of the topography and geoid associated with the hawaii emperor seamount chain reveals that while geoid topography ratios are high over the southeastern part of the chain they fall off with distance away from the current hot spot location the topographic expression of the swell has two maxima near the intersections between the seamount chain and the murray and molokai fracture zones respectively both geoid and topography rapidly decrease westward of the intersection with the murray fracture zone it appears that the region of the pacific plate between the two fracture zones has been more susceptible to reheating and hot spot penetration than elsewhere along the chain alternatively the observed pattern may be indicative of time variation in the strength of the hawaiian hot spot

the effect of a shallow low viscosity zone on the apparent compensation of mid plate swells abstract a simple model for mid plate swells is that of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region finite element calculations have been used to determine the effects of the viscosity contrast the layer thickness and the rayleigh number on the flow and on the perceived compensation mechanism for the resulting topographic swell as the viscosity decreases in the low viscosity zone the effective local rayleigh number for the top boundary layer of the convecting cell increases also because the lower viscosity facilitates greater velocities in the low viscosity zone the low viscosity layer produces proportionally greater horizontal flow near the conducting lid causing the base of the conducting lid to appear like a free boundary the change in the local rayleigh number and in the effective boundary condition both cause the top boundary layer to thin through a green s function analysis we have found that the low viscosity zone damps the response of the surface topography to the temperature anomalies at depth whereas it causes the gravity and geoid response functions to change sign at depth counteracting the positive contributions from the shallower temperature variations by increasing the viscosity contrast the conbined effects of the thinning of the boundary layer and the behaviour of the response functions allow the apparent depth of compensation to become arbitrarily small therefore shallow depths of compensation cannot be used to argue against dynamic support of mid plate swells furthermore we compared the distribution of the effective compensating densities which is used to obtain the geoid to that of pratt compensation which is often used to calculate the depth of compensation from geoid and topography data for mid plate swells for all of our calculations including those with no low viscosity layer the effective gravitational mass distribution is more complex than assumed in simple pratt models so that the pratt models are not an appropriate gauge of the compensation mechanism

impacts of vertical structure of convection in global warming the role of shallow convection abstractglobal warming induced changes in regional tropical precipitation are usually associated with changes in the tropical circulation which is a dynamic contribution this study focuses on the mechanisms of the dynamic contribution that is related to the partition of shallow convection in tropical convection to understand changes in tropical circulation and its associated mechanisms 32 coupled global climate models from cmip3 and cmip5 were investigated the study regions are convection zones with positive precipitation anomalies where both enhanced and reduced ascending motions are found under global warming an upward shift structure of ascending motion is observed in the entire domain implying a deepening of convection and a more stable atmosphere which leads to a weakening of the tropical circulation in a more detailed examination areas with enhanced weakened ascending motion are associated with more less import of moist static energy by a climatologically bottom heavy top heavy str

lithospheric thickness and mantle lithosphere density contrast beneath beta regio venus the spatial variation of the geoid topography ratio over the large venusian volcanic highland beta regio is suggestive of thermal compensation i e support of the highland s topography by lithospheric thinning both the thickness of the lithosphere and the density contrast at its base can be inferred from a quadratic regression of suitably filtered 600 km wavelength 4000 km geoid vs topography data the regression yields a mean lithospheric thickness of 270 km and a density contrast of magnitude 2 5 to 3 0 simple isostatic balance of the long wavelength topography at beta regio requires thinning of the lithosphere by 50 60 beneath the rise

the madden julian oscillation in echam6 and the introduction of an objective mjo metric this study presents a quantitative evaluation of the simulated madden julian oscillation mjo in an ensemble of 42 experiments performed with echam6 and previous echam versions the echam6 experiments differ in their parameter settings resolution and whether the atmosphere is coupled to an ocean or not the analysis concentrates on a few basic features of the mjo namely the signatures of convection precipitation coupled with the circulation system and the eastward propagation strength of outgoing longwave radiation olr and 850 and 200 hpa zonal winds within the mjo related frequency wavenumber range it also examines whether precipitation and olr show similar signatures in the mjo as simulated by echam the experiments reveal an mjo however to different degrees and in different aspects so that a sound assessment requires a multivariate approach in particular the convective rainfall signatures are decoupled from the dynamic signature of the mjo in the simulations herein which eventually leads to the introduction of a new mjo diagram and metric that incorporate olr and the zonal winds in 850 and 200 hpa the analysis here confirms the importance of the convection scheme only with the nordeng modifications to the tiedtke scheme can realistic mjo features be simulated high resolution coupled experiments better represent the mjo as compared to low resolution amip experiments this is shown to follow from two more general findings namely that 1 air sea interaction mainly increases the convective signature and 2 increased resolution enhances eastward propagation

late holocene paleoclimate and treeline fluctuation in wyoming s wind river range usa this study presents evidence for treeline advance in the central rocky mountains united states between 1800 and 800 cal yr bp this evidence was generated by systematic survey sampling and acc

a simple model of climatological rainfall and vertical motion patterns over the tropical oceans abstract a simple model is developed that predicts climatological rainfall vertical motion and diabatic heating profiles over the tropical oceans given the sea surface temperature sst using statistical relationships deduced from the 40 yr ecmwf re analysis era 40 the model allows for two modes of variability in the vertical motion profiles a shallow mode responsible for all boundary layer convergence between 850 hpa and the surface and a deep mode with no boundary layer convergence the model is based on the argument expressed in the authors companion paper that boundary layer convergence can be usefully viewed as a forcing on deep convection not just a result thereof the shallow mode is either specified from satellite observations or modeled using a simple mixed layer model that has sst as well as 850 hpa geopotential height winds and temperature as boundary conditions the deep mode amplitude is empirically shown to be proportional to a simple measure of conditional instability in conve

convective thinning of the lithosphere a mechanism for the initiation of continental rifting thinning of the lithosphere by heat carried convectively to its base is a possible mechanism for the initiation of continental rifting the lithosphere asthenosphere boundary marks the transition from a quasi rigid behavior of the mantle on geological time scales to a viscous one the boundary lies at or close to the solidus of the mantle material and its position is a function of temperature and pressure additional heat carried to the boundary will cause it to rise and the lithosphere to thin a one dimensional model is used to study the upward movement of the boundary from an initial equilibrium position as a consequence of an increase in the heat flux from the convecting mantle the ratio of the amount of thinning to the initial thickness is directly proportional to the increase in heat flux from the asthenosphere and inversely proportional to the difference between the final subcrustal geothermal gradient and the slope of the transition temperature profile the rate of thinning is determined by the initial and final equilibrium thicknesses the thermal diffusivity the transition temperature profile and the plume temperature profile it depends only slightly on any latent heat associated with the lithosphere asthenosphere transition during a large fraction of the time between the initial and final equilibrium states the lithosphere thins at a rate which is inversely proportional to the square root of time t even 300 km thick lithospheres may be thinned to 50 km or crustal thicknesses in tens of million years by the heat advected by mantle plumes this heat contributes to an enhanced surface heat flow because of the constant level of radioactive heat production in the crust the ratio of the final to the initial surface heat flow increases much less than the ratio of the final to the initial heat flow from the asthenosphere for large increases in asthenospheric heat flow the lithosphere is almost thinned to the crust before any significant change of surface heat flow occurs uplift due to thermal expansion upon lithospheric thinning is of the order of 102 103 m a rayleigh taylor instability acting on the thinning lithosphere may further increase thinning rates and cause magmatic intrusions of the lithosphere and surface volcanism

active lithospheric thinning abstract this paper starts with a discussion of the physical mechanisms capable of inducing a rapid upward migration of the lithosphere asthenosphere boundary two models based on a thermomechanical destabilization of the lower lithosphere are proposed first we consider the enhancement of small scale sublithospheric convection modes by a hot thermal plume a perturbation of a few hundred degrees is found to thin the lithosphere and generate uplift within a few tens of million years this efficient erosion from below only stops when it reaches a layer of low density such as the crust or some depleted mantle material second we show that passive lithospheric stretching leads to an unstable lithospheric configuration on the sides of the rift the denser unstretched lithosphere is subject to a lateral convective penetration by the hotter asthenospheric mantle material coming from the rift zone this lateral destabilization generates uplift of the shoulders natural examples can be found along passive continental margins a good example is given by the uplift of the scandinavian caledonides following the break up in the north atlantic

cool skin simulation by a one column ocean model 1 a one column ocean model is used to simulate the cool skin effect of sea surface temperatures four schemes for determining the thickness of the cool skin are compared it is established that all four schemes can capture the cool skin for wind speeds 7 5 m s 1 however for wind speeds 7 5 m s 1 artale et al s 2002 scheme performs better this study demonstrates that the cool skin effect can be reproduced by the one column model while a proper vertical resolution in the top 1 mm is embedded

estimating the compensation depth of the hawaiian swell with linear filters broad areas of anomalously shallow seafloor surround regions of active or recently active hot spot volcanism the cause of these midplate swells has been modeled both as elevated temperatures in a convecting layer and as thermal expansion within the conducting portion of the lithosphere according to the former explanation we would expect the low density material compensating the swell to lie beneath the conducting lid whereas the latter model allows compensation at shallower depths we present a technique based on the application of linear filters for determining the compensation depth of midplate swells given observations of topography and geoid or gravity anomalies our method does not require an a priori estimate of swell height or width nor does it assume complete spatial or spectral separation between the swell topography and the signal from the hot spot volcanoes which are compensated by flexure of the oceanic moho rather we exploit the difference in the predicted sign and amplitude of the moho deflection beneath elevated topography produced by loading above versus loading below the elastic plate in order to separate the two effects an application of these linear filters to residual topography and geoid data from the region surrounding the hawaiian hot spot yields a compensation depth of 70 10 km within the lower lithosphere

interpreting gravity geoid and topography for convection with temperature dependent viscosity application to surface features on venus while venus and earth are similar in size venus does not appear to have developed the same system of mobile surface plates which dominates convective heat transfer from the interior of earth numerical models with temperature dependent viscosity do not produce plate boundaries and are more closely reminiscent of venus than earth we demonstrate that many puzzling aspects of surface features on venus are also found in the analysis of the surface observables in these models despite having no strong lithosphere our calculations exhibit very high geoid topography ratios and uncompensated short wavelength topography surface heat flux values are not required to be very different from earth scaled values and the mantle remains below the likely solidus

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006