a mechanism for the low frequency variability of the mid latitude troposphere low frequency variability of the zonal mean zonal wind in long runs of a simplified global circulation model is dominated by bands of wind anomaly which originate in the tropics and propagate slowly poleward with speeds of 0 2 m s 1 such propagation could be the result of the interaction between the zonal wind anomaly and an equatorward flux of rossby wave activity originating in middle or high latitude a barotropic weakly nonlinear model of this process has been derived it suggests propagation of zonal wind anomalies similar to those produced in the global circulation model and leads to comparable propagation speeds

eddy zonal flow feedback in the southern hemisphere abstract the variability of the zonal mean zonal wind in the southern hemisphere is studied using eof analysis and momentum budget diagnostics of ncep reanalysis data 1978 97 the leading eof of the zonal mean zonal wind is well separated from the remaining eofs and represents the north south movement of the midlatitude jet analysis of the momentum budget shows that a positive feedback between the zonal mean wind anomalies and the eddy momentum fluxes accounts for the unusual persistence of eof1 and plays an important role in the selection of the leading eof of midlatitude variability further analysis also shows a propagating feedback common to both eof1 and eof2 which is responsible for the poleward drift of wind anomalies with time the observations support the following feedback mechanism anomalous baroclinic wave activity is generated at the latitude of anomalous temperature gradient that by thermal wind coincides with the latitude of the anomalous zonal jet the net propagation of baroclinic

the dynamics of intraseasonal atmospheric angular momentum oscillations abstract the global and zonal atmospheric angular momentum aam budget is computed from seven years of national centers for environmental prediction data and a composite budget of intraseasonal 30 70 day variations during northern winter is constructed regressions on the global aam tendency are used to produce maps of outgoing longwave radiation 200 hpa wind surface stress and sea level pressure during the composite aam cycle the primary synoptic features and surface torques that contribute to the aam changes are described in the global budget the friction and mountain torques contribute about equally to the aam tendency the friction torque peaks in phase with subtropical surface easterly wind anomalies in both hemispheres the mountain torque peaks when anomalies in the midlatitude northern hemisphere and subtropical southern hemisphere are weak but of the same sign the picture is different for the zonal mean budget in which the meridional convergence of the northward relative angular moment

mantle and crustal processes in the magmatism of the campania region inferences from mineralogy geochemistry and sr nd o isotopes of young hybrid volcanics of the ischia island south italy ischia one active volcano of the phlegraean volcanic district prone to very high risk is dominated by a caldera formed 55 ka bp followed by resurgence of the collapsed area over the past 3 ka the activity extruded evolved potassic magmas only a few low energy explosive events were fed by less evolved magmas a geochemical and sr nd o isotope investigation has been performed on minerals and glass from products of three of such eruptions molara vateliero and cava nocelle 2 6 ka bp data document strong mineralogical geochemical and isotopic heterogeneities likely resulting from mingling mixing processes among mafic and felsic magmas that already fed the ischia volcanism in the past detailed study on the most mafic magma has permitted to investigate its origin the mantle sector below ischia underwent subduction processes that modified its pristine chemical isotopic and redox conditions by addition of 1 of sediment fluids melts similar processes occurred from southeast to northwest along the apennine compressive margin with addition of up to 2 5 of sediment derived material this is shown by volcanics with poorly variable typical 18o mantle values and 87sr 86sr progressively increasing toward typical continental crust values multiple partial melting of this modified mantle generated distinct primary magmas that occasionally assimilated continental crust acquiring more 18o than 87sr at ischia 7 of hercynian granodiorite assimilation produced isotopically distinct k basaltic to latitic magmas a sw ne regional tectonic structure gave these magmas coming from large depth the opportunity to mingle mix with felsic magmas stagnating in shallower reservoirs eventually triggering explosive eruptions

observational evidence for the mechanism of the poleward propagation of zonal wind anomalies over the north atlantic this study examines the physical mechanisms that drive the poleward propagation of zonal mean flow anomalies over the north atlantic during the boreal winter with ecmwf era40 reanalysis data that spans the years 1958 2001 an empirical orthogonal function eof analysis is performed on the zonal wind anomalies for the north atlantic domain based on time lag correlations between the first two eofs 14 poleward propagation events are identified a series of lagged composite calculations of the poleward propagation events reveals the following processes midlatitude synoptic scale rossby waves propagate equatorward and break when they encounter their critical latitudes in the tropics and subtropics a new critical latitude is generated slightly poleward of the previous critical latitude and the subsequent wave breaking occurs at this new critical latitude this interaction between the wave breaking and the critical latitude drives the poleward propagation of zonal mean zonal wind anomaly a comparison between the poleward propagation events and zonal index events indicates that the poleward propagation is associated with a relatively weak potential vorticity gradient over the subtropics these north atlantic poleward propagation events are found to be preceded by anomalously strong transient eddy activity over the subtropical north pacific and by anomalously weak tropical convection over the tropical atlantic ocean and the amazon copyright c 2012 royal meteorological society

a northern hemisphere volcanic chemistry record 1869 1984 and climatic implications using a south greenland ice core the effect of volcanic emission of acidic aerosols on climate is well documented the presence of acid droplets in the stratosphere can reduce transmissivity and hence decrease surface temperatures since the amount and chemical composition of erupted material has important effects on regional climate knowledge of past volcanic events is of extreme importance detailed glaciochemical records provide the only milieu wherein the geochemistry of paleovolcanic events can be fully documented we present a detailed sulfate and chloride record from an ice core drilled at site 20 d 40 km sw of dye 3 in southern greenland the record spans the time period 1869 1984 with chemical analyses of approximately eight samples per year time series decomposition and locally weighted scatter plot smoothing techniques were used to extract long term trends from the data so that individual volcanic eruptions could be documented a number of events identified here have been unnoticed previously and a high percentage of the major chemical signatures documenting these events is associated with large decreases in temperature in the latitudinal zone 60 90 on many authors have pointed out that the amount of volcanic acids such as hci and h2s04 injected into the atmosphere has a very important influence on global climate yet this volcanic input has been difficult to quantify prior to 1960 our data help to alleviate this problem these individual events can be compared to available frost tree ring data from north america further establishing a volcanism climatic linkage

circumglobal teleconnections and wave packets associated with israeli winter precipitation this investigation performs diagnostic analyses with ncep ncar reanalysis data and israeli precipitation data to examine the dynamical processes that drive the teleconnection pattern associated with israeli winter precipitation anomalies a dipole teleconnection pattern with one centre of action over western europe and another centre of action over the eastern mediterranean em is found to be associated with the winter precipitation anomalies in israel this pattern is referred to as the southern levant sl pattern the positive negative phase of the sl pattern is found to be closely associated with enhanced reduced precipitation in israel composites of the 300 hpa stream function field show that both phases of the sl pattern are associated with eastward propagating wave packets that traverse about three quarters of the distance across the globe these wave packets which have a phase speed that is close to zero originate over the northeast pacific the east coast of north america for the positive negative sl phase and propagate eastward across the north atlantic western europe the em and southern asia until they eventually decay over the northwest pacific as these wave packets pass over western europe and the em the sl pattern is triggered and anomalies in israeli winter precipitation occur the sl pattern is found to have an e folding time scale of 4 days which is determined by the group velocity of the wave packet the time averaged spatial structure of the circumglobal wave packet closely resembles the first empirical orthogonal function of the hemispheric 300 hpa meridional wind field this suggests that the circumglobal wave packet may be a fundamental pattern of variability which is relevant for the entire northern hemisphere copyright 2008 royal meteorological society

a 220 year continuous record of volcanic h2so4 in the antarctic ice sheet continuous h2so4 profiles observed in snow from several antarctic locations reveal four major volcanic events of the past two centuries agung krakatoa tambora and another large scale event not recorded historically acid deposition and interhemispheric distribution mechanisms are quantified and then used to obtain an order of magnitude estimate for the h2so4 emissions from these eruptions

water speciation and sr isotopic exchange during water melt interaction a combined nmr tims study on the cretaio tephra ischia island south italy abstract in order to improve the microscopic understanding of the water magma interaction process during explosive volcanism volcanic glasses representative of deposits with sedimentological characteristics suggesting different water melt ratios were studied by a combination of the nuclear magnetic resonance nmr and tims methods the glasses were separated from pumices of two surge layers and one fallout bed of the cretaio tephra ischia island italy which is the product of an explosive eruption that occurred at ischia in the second century bc the 29si cp mas nmr experiments indicate the occurrence of 1h 29si dipolar couplings in glasses from the phreatomagmatic activity suggesting the presence of hydrogen atoms in proximity of silicon atoms this feature is not detected in the glass from the deposit of the magmatic explosion 1h mas nmr spectra reveal different peaks attributed to different hydrous species characterized by different motional properties these include rigid h2o groups isolated in the glass structure more mobile water species and possibly structural hydroxyl groups 1h mas nmr spectra recorded after deuteration experiments of the glass at a temperature up to 300 c revealed that the exchange reactions of the d2o vapor with hydrogen were limited to the most mobile water species possibly on vesicle surfaces or in channels the hydrogen concentration linearly correlates with the 87sr 86sr isotope ratio in glasses suggesting isotopic tracer exchanges between the sr dissolved in the water vapor and the sr in the silicon oxygen network during hydration it is proposed that the uprising melt interacted with a hydrothermal system of seawater derived fluids characterized by relatively high sr isotopic composition

a comprehensive study of pumice formation and dispersal the cretaio tephra of ischia italy the cretaio tephra is a small volume 0 02 km3 pumice deposit on the island of ischia in italy it was erupted during historical time and is associated with a tuff ring dome complex extruded along a n s trending normal fault system bordering the east side of the mt epomeo resurgent block 14c ages of the deposit average 1860 a b p which is in agreement with the potsherds found in paleosols underlying and overlying the deposit the eruption sequence consists of 7 members most of these members are pumice fall deposits one is an ash and pumice fallout and another is a surge bed the tephra have a trachytic phonolitic composition all chemical characteristics including major and trace elements and sr isotope composition are homogeneous throughout the sequence the vesicularity of pumice clasts does not show any systematic change the erupted magma had the same amount of volatiles the same flux and vesicle growth behavior throughout the eruption sequence grain size analyses of 90 tephra samples representative of all the members at variable distance from the supposed vent have been processed using the sequential fragmentation transport theory in order to define the effects of eruptive fragmentation and subsequent transport on the particle sizes distribution four significant subpopulations have been identified and related to transport mechanisms

a mechanism for the poleward propagation of zonal mean flow anomalies abstract observational studies have shown that tropospheric zonal mean flow anomalies frequently undergo quasi periodic poleward propagation a set of idealized numerical model runs is examined to investigate the physical mechanism behind this poleward propagation this study finds that the initiation of the poleward propagation is marked by the formation of negative zonal wind anomalies in the tropics these negative anomalies arise from meridional overturning breaking of waves that originate in midlatitudes this wave breaking homogenizes the potential vorticity pv within the region of negative zonal wind anomalies and also leads to the formation of positive zonal wind anomalies in the subtropics subsequent equatorward radiation of midlatitude waves is halted which results in wave breaking at the poleward end of the homogenized pv region this in turn generates new positive and negative zonal wind anomalies which enables a continuation of the poleward propagation the shielding of the homogenized

annular variability and eddy zonal flow interactions in a simplified atmospheric gcm part i characterization of high and low frequency behavior abstract experiments have been performed using a simplified newtonian forced global circulation model to investigate how variability of the tropospheric jet can be characterized by examining the combined fluctuations of the two leading modes of annular variability eddy forcing of this variability is analyzed in the phase space of the leading modes using the vertically integrated momentum budget the nature of the annular variability and eddy forcing depends on the time scale at low frequencies the zonal flow and baroclinic eddies are in quasi equilibrium and anomalies propagate poleward the eddies are shown primarily to reinforce the anomalous state and are closely balanced by the linear damping leaving slow evolution as a residual at high frequencies the flow is strongly evolving and anomalies are initiated on the poleward side of the tropospheric jet and propagate equatorward the eddies are shown to drive this evolution strongly eddy location and amplitude reflect the past baroclinicity while

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

wave driven zonal flow vacillation in the southern hemisphere the variability of the zonal mean flow in the southern hemisphere during the period 1985 94 is studied using european centre for medium range forecasts analyses the dominant mode of variability has approximately equivalent barotropic variations of opposite signs centered at 40 8 and 608s this structure is dominant in all seasons and has similar variance in all seasons the temporal variance of the amplitude of this mode is well modeled as gaussian red noise with a correlation e folding time of about 10 days zonal wind anomalies are maintained against frictional drag by variations in the zonal flow accelerations driven by transient eddies and associated mean meridional circulations the eddy structures suggest that equatorward propagation is favored when the jet is displaced poleward and zonal propagation is favored when the jet is displaced equatorward

indices of the southern hemisphere zonal wind abstract the time and spatial variation of the southern hemisphere 500 hpa zonal wind has been examined using spectral analysis bandpass filtering and empirical orthogonal function eof analysis applied to a 15 year sample of australian hemispheric analyses spectral analysis of the entire 15 year record of the 500 hpa zonal mean westerly wind shows significant peaks only on the seasonal and interannual time scales the mean segmented spectra for each season failed to show any additional significant peaks at higher frequencies or any significant differences between seasons the primary pattern of variation revealed by eof analysis shows similar behavior on synoptic to seasonal time scales with compensating departures occurring at intervals of 20 latitude the first eof has opposing peaks near 40 and 60 s and shows an equivalent barotropic pattern when correlated with individual grid point values at 500 hpa and mean sea level pressure correlations of the zonal pattern with regional anomalies reveal

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

preferred modes of variability and their relationship with climate change spatial structure of annular modes shows a remarkable resemblance to that of the recent trend in the observed circulation thompson et al this study performs a series of multilevel primitive equation model simulations to examine the extent to which the annular mode is capable of predicting changes in the zonal mean flow response to external heat perturbations each of these simulations represents a statistically steady state and differs from each other in the values of the imposed tropical heating h and high latitude cooling c defining the annular mode as the first empirical orthogonal function eof1 of zonal mean tropospheric zonal wind it is found that the climate predictability is generally high in the small c large h region of the parameter space but is markedly low in the large c small h region in the former region eof1 represents meridional meandering of the midlatitude jet while in the latter region eof1 and eof2 combine to represent coherent poleward propagation of zonal mean flow anomalies it is also found that the climate predictability tends to be higher with respect to changes in c than to changes in h the implications of these findings for the southern hemisphere climate predictability are also presented

an observational study of the intraseasonal poleward propagation of zonal mean flow anomalies abstract the poleward propagation of zonal mean relative angular momentum mr anomalies is examined using ncep ncar reanalysis data for both the winter and summer seasons of the northern and southern hemisphere this analysis is performed with a regression analysis using base latitudes in the subtropics midlatitudes and high latitudes it is found that the poleward mr anomaly propagation occurs at all latitudes with the propagation speed being greater in the subtropics and high latitudes compared to midlatitudes other fields such as eddy angular momentum flux convergence eddy heat flux friction torque and 300 mb streamfunction are regressed for the northern hemisphere winter and the southern hemisphere summer the main finding is that in the subtropics and midlatitudes the poleward mr anomaly propagation is primarily due to high frequency 10 day transient eddy angular momentum flux convergence and in high latitudes the propagation is mostly due to the summation of cross frequency and low fr

relations between annular modes and the mean state southern hemisphere winter in a zonally symmetric climatology with a single eddy driven jet such as prevails in the southern hemisphere summer the midlatitude variability is dominated by fluctuations of the jet around its mean position as described by the southern hemisphere annular mode sam to study whether this result holds for a zonally asymmetric climatology the observed variability of the southern hemisphere winter is analyzed the mean state in this case is characterized by relatively weak stationary waves yet there exist significant zonal variations in the mean strength and meridional structure of the subtropical jet stream as in summer the winter sam signature is annular in shape and the corresponding wind anomalies are dipolar but it is associated with two different behaviors of the eddy driven jet in different longitudinal ranges over the indian ocean the sam is associated primarily with a latitudinal shift of the jet around its mean position over the pacific sector it is instead characterized by a seesaw in the wind speed between two distinct latitudes corresponding to the positions of the midlatitude and subtropical jets composites of eddy forcing and baroclinicity over both sectors appear consistent with the two different behaviors as in the zonal mean case high frequency eddies both force and maintain the low frequency wind anomalies associated with the sam the positive feedback by eddies is however not local changes in the eddy forcing are influenced most strongly by zonal wind anomalies located upstream

wave zonal flow interaction and ultra low frequency variability in a simplified global circulation model the interactions between baroclinic waves and the zonal mean mid latitude flow are investigated using a multilevel primitive equation model of moderate resolution with idealized surface friction and diabatic heating terms integrations corresponding to 100 years were performed under perpetual solstice conditions empirical orthogonal function eof analyses were carried out on the zonal mean zonal wind u adjustments to the winter hemisphere zonal jet due to the growth and decay of baroclinic waves can be described in terms of the first two eof patterns which exhibit substantial interannual variability for a single lifecycle such adjustments take the form of a circuit of a phase space whose axes are defined by the amplitudes of the two eofs the cumulative effects of multiple lifecycles lead to a low frequency circulation in the phase space with a timescale of about 150 days we suggest that this effect may be intrinsically related to the internal generation of ultra low frequency variability in the model normal mode lifecycle calculations including dissipation reveal similar phase space properties the growth rates of normal mode disturbances are shown to be highly sensitive to the phase of the first eof

volcanological study of the great tambora eruption of 1815 the 1815 eruption of tambora is one of the largest explosive volcanic events of the past 10 000 yr by a conservative estimate 175 km3 of nepheline normative trachyandesitic pyroclastic material equivalent to about 50 km3 of dense rock was erupted in 24 h major activity began with a brief plinian phase followed by pyroclastic flows much of the ignimbrite that was produced shows evidence of deposition by turbulent flows a 6 7 km caldera about 1 km deep formed at the end of the pyroclastic flow phase caldera volume closely matches that of magma ejected plinian and co ignimbrite ash fall 1 cm thick covered 500 000 km2 of the java sea and surrounding islands accounting for more than two thirds of the magma volume erupted this distal tephra is known only from 1815 reports indicating that some great eruptions especially those from island volcanoes may be virtually undetectable in the geologic record without supporting information such as deep sea core data land based volume estimates may be insufficient to characterize eruption magnitude in many cases

spatial variability of sea level pressure and 500 mb height anomalies over the southern hemisphere abstract the spatial variability of mean sea level pressure slp and 500 mb height anomalies over the southern hemisphere during summer djf and winter jja is determined using eigenvector analysis based on daily synoptic maps from 1972 to 1979 the patterns of spatial distribution of pressure and height anomalies are further verified and examined by means of station data and the eigenvectors are compared between the seasons and to those found for the northern hemisphere the first eigenvector shows that midlatitude anomalies of slp and 500 mb height are of an opposite sign to those found over and around antarctica the pattern is highly barotropic and suggests strengthening and weakening of the zonal wind in alternating latitude belts the 500 mb height differences are calculated for five midlatitude to antarctic station pairs using data from the late 1950 s onward these latitudinal height differences are also used to describe the association between the hemispheric westerlies and the southern oscil

historic eruptions of tambora 1815 krakatau 1883 and agung 1963 their stratospheric aerosols and climatic impact abstract decreases in surface temperatures after the eruptions of tambora 1815 krakatau 1883 and agung 1963 were of similar magnitude even though the amount of material dust and volatiles injected into the stratosphere by these three events differed greatly large amounts of fine ash and volatiles were dispersed into the upper atmosphere by krakatau and tambora the agung eruption in 1963 was a much smaller vulcanian type eruption which injected dust and volatiles into the stratospheric aerosol layer more directly analyses of magmatic volatiles indicate that the agung eruption was proportionately richer in so 2 and cl than either tambora or krakatau relative amounts of fine ash produced by the tambora krakatau and agung eruptions are estimated at about 150 20 1 whereas the masses of atmospheric sulfate aerosols produced were on the order of 7 5 3 1 decreases in surface temperature of a few tenths of a degree c for several years following volcanic eruptions are primarily a result of the sulfate aerosols rather than of the silicate dust the similarity in the atmospheric response after these three eruptions supports the idea of limiting mechanisms on volcanic stratospheric aerosol loading which is suggested by microphysical processes of aerosol particles fluctuations in stratospheric aerosol optical depth seem to be controlled to a large degree by high intensity sulfur rich eruptions e g agung 1963 which may however be relatively small in total ejecta volume such eruptions leave little geologic record but appear as acidity peaks in polar ice cores

reconciliation of experimental results on h2o speciation in rhyolitic glass using in situ and quenching techniques in order to resolve the controversy over how to interpret experimental h2o speciation results using in situ and quenching techniques we have carried out an infrared spectroscopic study to determine whether the molecular h2o 5230 cm 1 and oh 4520 cm 1 band intensity variation with measurement temperature below glass transition is owing to species interconversion or to the temperature dependence of molar absorptivities by comparing rhyolitic glasses with different total h2o content from 0 18 to 0 76 wt we show that the peak height of the 4520 cm 1 band increases by a similar relative amount about 2 if the baseline is fit with a flexicurve and 10 if the baseline is fit by a straight line from 25 to 400oc independent of the total h2o content the results show that 1 the molar absorptivities do indeed change with temperature and 2 in our experiments below the glass transition temperature species concentrations do not change noticeably with temperature and the band intensity variations are caused mainly by changes in the shape of the absorbance bands with temperature the absence of unquenchable species reaction in the glass state on our experimental time scale confirms that speciation data can be obtained using the quench technique from 400 to 600oc on the other hand the temperature dependence of the molar absorptivities must be quantified for the full potential of the in situ technique to be realised 1999 elsevier science b v all rights reserved

relation between annular modes and the mean state southern hemisphere summer abstract the annular modes emerge as the leading mode of extratropical month to month climate variability in both hemispheres here the influence of the background state on the structure and dynamics of the southern hemisphere annular mode sam during the austral summer when the climatology is characterized by a single well defined eddy driven jet is studied subsets of the climatology are constructed for early and late summer and for contrasting polarities of the enso cycle the analysis is based both on observations and on perpetual state gcm experiments the main differences between the subsets involve variations of the latitude of the mean jet it is found that in all the cases the sam is characterized by latitudinal shifts of the jet about its mean position reinforced by a positive momentum flux feedback from baroclinic waves this result is consistent with previous studies of the dynamics of the zonally averaged circulation but is found here to hold over all longitudes and for different positi

quantifying the pinatubo volcanic signal in south polar snow recent snow and firn core samples from south pole contain increased sulfate so42 concentrations during 1992 1994 as a result of the june 1991 pinatubo eruption and the august 1991 cerro hudson eruption in chile traces of pinatubo tephra volcanic ash were identified in the 1993 and 1994 snow layers supporting the conclusion that increased so42 in 1993 1994 is from the pinatubo eruption although the pinatubo eruption preceded hudson its so42 signal in south polar snow follows and is resolved from that of hudson the deposition of the pinatubo so42 aerosol was delayed due to the long transport to the high southern latitudes and its initial existence at high altitudes in the antarctic atmosphere multi year multi site sampling demonstrates that the volcanic signals are well preserved and spatially consistent measurements on 2 firn cores show that the south pole so so42 flux from pinatubo is 10 9 1 2 kg km 2 over 2 2 years while the hudson flux is 3 2 0 6 kg km 2 in 1 1 years these results when combined with satellite determined pinatubo sulfur dioxide so2 emission make it possible to link quantitatively the atmospheric aerosol mass loading from a low latitude volcanic eruption to its signal in polar ice cores

effect of upper and lower level baroclinicity on the persistence of the leading mode of midlatitude jet variability abstractthe sensitivity of the variability of an eddy driven jet to the upper and lower level baroclinicity of the mean state is analyzed using a three level quasigeostrophic model on the sphere

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

time scale and feedback of zonal mean flow variability the physical processes that determine the time scale of zonal mean flow variability are examined with an idealized numerical model that has a zonally symmetric lower boundary in the part of the parameter space where the time mean zonal flow is characterized by a single double jet the dominant form of zonalmean flow variability is the zonal index poleward propagation and the time mean potential vorticity gradient is found to be strong and sharp weak and broad the e folding time scale of the zonal index is found to be close to 55 days much longer than the observed 10 day time scale the e folding time scale of the poleward propagation is about 40 days the long e folding time scales for the zonal index are found to be consistent with an unrealistically strong and persistent eddy zonal mean flow feedback a calculation of the refractive index indicates that the background flow supports eddies that are trapped within midlatitudes undergoing relatively little meridional propagation additional model runs are performed with an idealized mountain to investigate whether zonal asymmetry can disrupt the eddy feedback for single jet states the time scale is reduced to about 30 days if the mountain height is 4 km or less the reduction in the time scale occurs because the stationary eddies excited by the mountain alter the background flow in a manner that leads to the replacement of zonal index events by shorter time scale poleward propagation with a 5 km mountain the time scale reverts and increases to 105 days this threshold behavior is again attributed to a sharpening of the background zonal jet which arises from an extremely strong stationary wave momentum flux convergence in contrast for double jet states the time scale changes only slightly and the poleward propagation is maintained in all mountain runs

a baroclinic mechanism for the eddy feedback on the zonal index a baroclinic mechanism for the positive eddy feedback on the zonal index is proposed the author considers the quasigeostrophic response of the zonally averaged flow to forcing by baroclinic eddies the transient response to eddy forcing is a largely barotropic acceleration of the westerly flow and a reduction of the baroclinicity at the latitudes of eddy generation but over time the action of surface drag leads to enhanced baroclinicity at these latitudes the steady state response has positive baroclinicity at the latitude of eddy generation if the eddies propagate away from this latitude before they dissipate this reinforcement of the low level baroclinicity provides a positive feedback if it is assumed that baroclinic eddies are generated more vigorously in regions of stronger low level baroclinicity the proposed mechanism explains observed and modeled features of zonal index variations the frequency and drag dependence of eddy feedback the bandedness in latitude of zonal wind variations and the tendency for anomalies in the zonally averaged zonal wind to drift poleward over time

petrogenesis and tectonic setting of the roman volcanic province italy abstract the volcanism in the roman province of italy can be modelled by the partial melting of heterogeneously enriched mantle sources the heterogeneity was created by materials derived from a subducted slab which can still be traced geophysically beneath the central apennines new petrographical and chemical data are presented for the high k calc alkaline and the shoshonitic volcanics of the campania region primary magmas are present only locally the existence of spatial zonation in the volcanism of campania is documented for the first time the shoshonitic leucite basanitic and leucititic volcanics of the phlegraean fields procida ischia and the somma vesuvius areas are at similar degrees of evolution about two times richer in nb and ba than those of northwestern campania and the latium part of the roman province accordingly distinct north western and south eastern subprovinces can be defined the evaluation of enrichment factors that is the abundance ratio between the average contents of each element in the relatively primitive lavas of the low k and the high k suites shows that the mantle sources prior to k metasomatism were different in the two sub provinces of the roman province in the north western one they resembled the sources of ocean island tholeiites and moderately enriched mor basalts in the south east they were similar to those of ocean island alkaline lavas and enriched morb s modelling based on k p ce sr rb ba th sm eu gd y nb and 87 sr 86 sr was carried out it indicates that the range of mantle sources of the volcanics in northwestern campania and latium can be modelled by the addition of 3 to 20 of materials derived by partial melting of carbonaceous pelites to a sr enriched mantle wedge comparable to the honolulu mantle source least enriched in nb the production of sr enriched mantle wedge requires either the action of fluids produced by dehydration of subducted oceanic crust or a small amount of metasomatism caused by the presence of carbonatite melts the near absence of ti ta nb yb and the highly fractionated ree in the metasomatizing component requires the presence of residual garnet and accessory ti rich minerals during the partial melting of the subducted sediments the writers propose that the mantle wedge overlying the subducted slab was hybridized by melts produced by partial fusion of subducted material derived from the continental crust probably sediments this process played a dominant role in the generation of the mantle sources from which the high k calc alkaline shoshonitic leucite basanitic and leucititic magmas of the roman province were derived

14c geochronological constraints for the volcanic history of the island of ischia italy over the last 5000 years abstract the chronology of the most significant eruptions at the island of ischia in the last 5000 years has been studied by means of accelerator mass spectrometry to obtain 14 c data the results are in good agreement with stratigraphical and archeological constraints this allows interpretation of the measured dates as ages of the eruptions which produced the deposits overlying or incorporating the sampled paleosols and charcoal fragments respectively these ages define the timing of volcanism in the last 5000 years suggesting that the recent volcanic activity at ischia is characterized by periods of very intense volcanism alternating to periods of quiescence this timing of the volcanism is correlated with the behaviour of the magmatic system and dynamics of resurgence of the mt epomeo block

zonal flow vacillation and eddy forcing in a simple gcm of the atmosphere abstract zonal flow vacillation with very long time scales is observed in a 3070 day simple gcm simulation with zonally symmetric forcing the long lasting zonal wind anomalies suggest that zonal flow vacillation is self maintained wave mean flow interactions are investigated by composite analysis and transform eulerian momentum budget analysis nonlinear life cycle simulations are conducted to demonstrate that each extreme phase of the zonal flow vacillation is a quasi stable state and is self maintained by the embedded synoptic eddies the firm eof mode of zonal mean wind shows an out of phase relation between anomalies at 60 s and at 40 s with a barotropic structure this structure is similar to the dominant vacillation pattern observed in the southern hemisphere the composite jet stream in the high low index phase of zonal flow vacillation shifts poleward equatorward from the time mean location and becomes broader narrower and weaker stronger composite eddies in the high index phase tilt nw

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

is the atmospheric zonal index driven by an eddy feedback abstract the authors address the question of whether or not eddy feedback plays an important role in driving the anomalous relative angular momentum associated with the zonal index zi in the atmosphere for this purpose composites of anomalous relative angular momentum and anomalous eddy angular momentum flux convergence eddy forcing are examined with national centers for environmental prediction national center for atmospheric research reanalysis data by using an empirical orthogonal function analysis it is found that zi behavior dominates the summer season of both hemispheres and also the winter season of the southern hemisphere for the summer season the zi is characterized by meridional displacements of the midlatitude eddy driven jet and for the southern hemisphere winter it is characterized by a simultaneous movement of the subtropical and eddy driven jets in the opposite direction for the zi of each of the above seasons unfiltered eddy forcing did not exhibit a prominent eddy feedback h

on the midlatitude thermal response to tropical warmth 1 observations reveal a global scale thermal response to el nino and la nina featuring a band of tropospheric anomalies opposite in sign to those in the tropics when an idealized dynamical model is forced by increased tropical heating a similar response with an enhanced subtropical thermal gradient results in the idealized model this response is produced by changes in transient eddy fluxes of heat and momentum and consequent changes in the mean meridional circulation the similarity between the two level model results and those from observations as well as the zonally averaged behavior of a gcm suggests that the same dynamics operate in nature this represents a tropical midlatitude teleconnection distinct from and in addition to that mediated by stationary rossby wave trains

a mechanism and simple dynamical model of the north atlantic oscillation and annular modes a simple dynamical model is presented for the basic spatial and temporal structure of the large scale modes of intraseasonal variability and associated variations in the zonal index such variability in the extratropical atmosphere is known to be represented by fairly well defined patterns and among the most prominent are the north atlantic oscillation nao and a more zonally symmetric pattern known as an annular mode which is most pronounced in the southern hemisphere these patterns may be produced by the momentum fluxes associated with large scale midlatitude stirring such as that provided by baroclinic eddies it is shown how such stirring as represented by a simple stochastic forcing in a barotropic model leads to a variability in the zonal flow via a variability in the eddy momentum flux convergence and to patterns similar to those observed typically the leading modes of variability may be characterized as a mixture of wobbles in the zonal jet position and pulses in the zonal jet strength if the stochastic forcing is statistically zonally uniform then the resulting patterns of variability as represented by empirical orthogonal functions are almost zonally uniform and the pressure pattern is dipolar in the meridional direction resembling an annular mode if the forcing is enhanced in a zonally localized region thus mimicking the effects of a storm track over the ocean then the resulting variability pattern is zonally localized resembling the north atlantic oscillation this suggests that the north atlantic oscillation and annular modes are produced by the same mechanism and are manifestations of the same phenomenon the time scale of variability of the patterns is longer than the decorrelation time scale of the stochastic forcing because of the temporal integration of the forcing by the equations of motion limited by the effects of nonlinear dynamics and friction for reasonable parameters these produce a decorrelation time of the order of 5 10 days the model also produces some long term 100 days or longer variability without imposing such variability via the external parameters except insofar as it is contained in the nearly white stochastic forcing

magnetostratigraphy of the late cretaceous to eocene sverdrup basin implications for heterochroneity deformation and rotations in the canadian arctic archipelago the temporal and spatial patterns of sedimentation in the sverdrup basin provide clues to how deformation in the canadian arctic accommodated late cretaceous eocene relative motion between greenland and north america although the sediments contain a rich assemblage of mammal and megafloral fossils dating of the sequence has been controversial some work suggests a dramatic faunal and floral heterochroneity with species appearing in the arctic 2 18 m y prior to their appearance at lower latitudes to obtain a chronostratigraphic framework for these sediments a 2 6 km section of the eureka sound group and kanguk formation on western axel heiberg island was sampled for magnetostratigraphy after removal of a pervasive modern field overprint with thermal and alternating field demagnetization a characteristic remanent magnetization chrm is isolated despite high directional dispersion the chrms form 11 distinct polarity intervals which can be correlated to chrons 34 to 24r this correlation indicates that some of these sediments are 10 m y younger than thought previously reducing the need for large scale heterochroneity sedimentation rates derived from the magnetostratigraphy suggest that an increase in basin subsidence is recorded near the middle of the section sampled a similar pattern has been reported from the eureka sound group exposed on ellesmere island we interpret this increased sedimentation as a response to crustal flexure caused by lithospheric loading during the middle paleocene c26r the loading may be related to a blind thrust system to the west of axel heiberg island that marks compression between north america and greenland driven by rapid seafloor spreading in the labrador sea the new data together with prior results indicate that most of the cretaceous canadian arctic archipelago has undergone a counterclockwise vertical axis rotation the new data are more consistent with this rotation being related to events during chron 26r rather than marking block rotations associated with the terminal eocene phases of eurekan deformation

mechanisms of hemispherically symmetric climate variability inspired by paleoclimate evidence that much past climate change has been symmetric about the equator the causes of hemispherically symmetric variability in the recent observational record are examined using the national centers for environmental prediction national center for atmospheric research reanalysis dataset and numerical models it was found that the dominant cause of hemispherically symmetric variability is the el nino southern oscillation during an el nino event the tropics warm at all longitudes and the subtropical jets in both hemi spheres strengthen on their equatorward flanks poleward of the tropical warming there are latitude belts of marked cooling extending from the surface to the tropopause in both hemispheres at all longitudes and in all seasons the midlatitude cooling is caused by changes in the eddy driven mean meridional circulation changes in the transient eddy momentum fluxes during an el nino event force upper tropospheric ascent in midlatitudes through a balance between the eddy fluxes and the coriolis torque the eddy driven ascent causes anomalous adiabatic cooling which is primarily balanced by anomalous diabatic heating using a quasigeostrophic spherical model forced by an imposed surface eddy disturbance of chosen wave number and frequency it is shown that the anomalous eddy momentum fluxes are caused by the impact that the changes in the tropically forced subtropical jets have on the propagation in the latitude height plane of transient eddies changes in zonal winds and associated changes in the meridional gradient of potential vorticity create an anomalous region of low meridional wavenumber in the midlatitudes that refracts waves away both poleward and equatorward tropical forcing of variability in the eddy driven mean meridional circulation is another way in addition to rossby wave teleconnections whereby the tropics can influence extratropical climate unlike teleconnections this mechanism causes climate variability that has strong zonally and hemispherically symmetric components and operates throughout the seasonal cycle

salt tectonics as a self organizing process a reaction transport and mechanics model salt tectonics is placed within the theory of nonlinear dynamical systems features such as waves diapirs and tears are viewed as natural consequences of the symmetry breaking instabilities and related self organized dynamics of the deforming salt body coupled to the reaction transport and mechanics of the surrounding sediments the fundamental nonlinearities are in the surrounding rock and salt rheology our findings are based on a coupled rtm model simulated using finite element techniques the centerpiece of the rheology of both rocks and salt is a nonlinear incremental stress formulation that integrates poroelasticity continuous irreversible mechanical deformation with yield behavior pressure solution and fracturing in contrast to previously presented studies in our approach the descriptive variables of all solid and fluid phases stress velocity concentrations etc and porous media texture i e volume fractions composition etc are solved from rtm equations accounting for interactions and interdependencies between them the role of the coupling between the spatial distribution of sediment input rate and diapir growth and stalling is examined as is the creation of an array of salt tectonic minibasins

reassessment of crete greenland ice core acidity volcanism link to climate change previous comparisons of the crete greenland ice core acidity record with climate variations of the last 1 400 years suggested that changes in volcanism may have significantly influenced climate on this time scale however the ice core acidity record consists of volcanically induced spikes superimposed on a background acidity of nonvolcanic origin that varied significantly in amplitude herein the authors produce a record of crete volcanism based solely on an objective definition of individual volcanic events in the ice core record there are 92 acidity peaks in the 1 420 year record since at least 26 of the post 1 600 eruptions are of local icelandic origin they estimate a mean recurrence interval of 20 8 years for hemispheric scale eruptions rich in sulphate on decadal time scales volcanism has varied by a factor of three four over the last 1 400 years however the climate volcanism correlation is considerably less impressive r minus 0 23 than previously concluded r minus 0 52 the good correspondence between volcanism and climate previously reported are due to background acidity levels that show a significant little ice age increase the background increase may reflect changes in ocean productivity 24 refs 5 figs 1 tab