the equatorial response to higher latitude forcing abstract the classic view following charney and webster and holton is that significant midlatitude forcing of the tropics can be expected only in regions with westerly winds in the upper troposphere because it is only in these regions that stationary rossby waves will be able to propagate toward the equator here it is shown that higher latitude forcing can project directly onto equatorial waves and give a significant tropical response in both easterly and westerly tropical flow the equatorial response to higher latitude forcing is considered in the context of a dry atmosphere and a localized higher latitude forcing with eastward or westward phase speed previous ideas of the doppler shifting of equatorial waves by zonal flows are extended to include consideration of a forcing involving a range of zonal wavenumbers a gill type model suggests that there can be significant forcing of equatorial waves by either vorticity forcing or heating in higher latitudes in agreement with the theory the kelvin wav

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

mm5 modeling of the madden julian oscillation in the indian and west pacific oceans model description and control run results abstract a new methodology to study the madden julian oscillation mjo is introduced while previous mjo studies typically have involved highly simplified mathematical models or general circulation models this new approach seeks to reproduce the mjo by using a regional model with prescribed boundary conditions this paper reports initial control run results for this methodology using the fifth generation pennsylvania state university psu national center for atmospheric research ncar mesoscale model mm5 for a domain extending from the western indian ocean to the date line the control run boundaries are forced using the ncep ncar reanalysis nra dataset for a 24 month time period the climatology for the 24 month period is examined to establish the robustness of mm5 for this region results indicate good agreement in the mean winds between the model and the forcing dataset the primary differences are an easterly bias at 850 hpa and altered flow patterns in the indian monsoon region mean outgoing

cross equatorial response to middle latitude forcing in a zonally varying basic state abstract a nonlinear model based on the shallow water equations is used to study cross equatorial propagation of forced waves in the presence of a longitudinally varying time mean basic state zonal wind field it is found that global scale planetary waves are unable to propagate past a critical latitude where the mean zonal wind speed vanishes however if the longitudinally asymmetric basic state includes a duet in which the zonal winds are westerly waves of zonal scale less than the zonal scale of the westerly duct may propagate from one hemisphere to the other even though the zonally symmetric mean zonal wind remains easterly in the equatorial region the amplitude of the response in one hemisphere to forcing in the opposite hemisphere increases strongly with the magnitude of the westerlies in the equatorial duct the existence and annual variations of a westerly duct region in the upper troposphere in the eastern pacific appear to account for some features of the low frequency variability in the no

coherent fluctuations of extratropical geopotential height and tropical convection in intraseasonal time scales abstract coherent fluctuations between extratropical circulation and tropical conviction in the intraseasonal time scale are studied possible relationships between 500 mb height field and outgoing longwave radiation olr are examined using correlation complex eof and composite techniques results show that in the 20 70 day bands data there is a systematic evolution of extratropical wavetrains from eurasia across the pacific to north america and the north atlantic in the time scale of 5 to 10 days over the tropics the dominant mode of intraseasonal variation in convection is an east west dipole like feature which propagates from the western indian ocean eastward to the dateline with a quasi period of 40 50 days the space time evolution of these extratropical wavetrains is found to he coherent with the tropical dipolar convection and with the strongest convection over indonesia central pacific in approximate quadrature with the peak phase of the eurasia and pacific north america wavetrains while the

the tropical extratropical interaction between high frequency transients and the madden julian oscillation abstract the interaction between high frequency transient disturbances and convection and the madden julian oscillation mjo is investigated using ncep ncar reanalysis and satellite outgoing longwave radiation data for 15 northern winters during the phase of the mjo with enhanced convection over the east indian ocean and indonesia and suppressed convection over the south pacific convergence zone both the asian pacific jet and the region of upper tropospheric tropical easterlies over the warm pool are displaced westward these changes in the basic state lead to a weaker or leakier waveguide in the asian pacific jet with a westward displaced forbidden region of tropical easterlies such that high frequency transient waves propagate equatorward into the deep tropics over the central pacific near the date line as these waves induce convection in the region of ascent and reduced static stability ahead of the upper level cyclonic disturbances there is an enhancement of high frequency convective var

evolution of the madden julian oscillation in two types of el nino abstractevolution characteristics of the madden julian oscillation mjo during the eastern pacific ep and central pacific cp types of el nino have been investigated mjo activities are strengthened over the western pacific during the predeveloping and developing phases of ep el nino but suppressed during the mature and decaying phases in contrast mjo activities do not show a clear relationship with cp el nino before their occurrence over the western pacific but they increase over the central pacific during the mature and decaying phases of cp el nino lag correlation analyses further confirm that mjo activities over the western pacific in boreal spring and early summer are closely related to ep el nino up to 2 11 months later but not for cp el nino ep el nino tends to weaken the mjo and lead to a much shorter range of its eastward propagation anomalous descending motions over the maritime continent and western pacific related to el nino can suppress convection and moisture flux convergence the

tropical convection and precipitation regimes in the western united states abstract the authors have documented the relationship between tropical convection and precipitation regimes in the western united states circulation patterns associated with precipitation regimes are described and physical mechanisms are proposed contributions from intraseasonal and interannual bands are examined when enhanced convection is located in the western pacific dry conditions in the southwest sw and wet conditions in the pacific northwest pnw are observed fluctuations in both intraseasonal and interannual bands contribute to the rainfall variability enhanced convection in the western pacific is accompanied by suppressed convection in the central pacific the associated rossby wave vorticity source s anomalies keep the pacific jet west of 150 w a westward shift of the storm track to the north pacific also contributes to dry conditions in the sw and wetness in the pnw when enhanced tropical convection is located near 150 e substantial contributions to outgoing longwave radiation ano

simulation of the madden julian oscillation in a coupled general circulation model part ii the role of the basic state abstract in part i of this study it was shown that air sea coupling had a positive impact on some aspects of the simulation of the madden julian oscillation mjo by a gcm however errors in the basic state climate of that gcm appeared to be preventing the mjo related convection from propagating into the west pacific in this paper the actual impact of these errors will be addressed an integration of a flux adjusted version of the coupled model has been performed which has reduced basic state errors in the west pacific in this version of the coupled gcm the mjo does propagate into the west pacific the simulation of the mjo by a coupled model with the same atmospheric component but a different ocean gcm is also analyzed this coupled gcm has similar systematic errors in low level zonal wind and precipitation to the model studied in part i but with warmer ssts results from this experiment together with the other available evidence suggest that it is the errors in the low level zonal wind component

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

a case study of the mechanics of extratropical influence on the initiation of the madden julian oscillation abstract mechanisms for extratropical influences on the initiation of the madden julian oscillation mjo are investigated using numerical simulations and a global reanalysis product previous simulations by a tropical channel model captured the timing and gross features of the initiations of two mjo events and suggested that the initiations were due to influences from the extratropics in this study latitudinal transport of momentum from the extratropics is found to be crucial in generating the lower tropospheric westerlies in the tropics associated with the mjo initiation the diagnoses of the zonal momentum budget for the mjo initiation region revealed that the advection by meridional winds could be important prior to the initiation of the mjo the time evolution of the wave activity identifies its source over the southern indian ocean where it grows by extracting kinetic energy from the mean flow the time scale of the lateral boundary conditions that is responsible for the mjo initiation is also inv

a numerical case study on the initiation of the madden julian oscillation a mesoscale tropical channel model is used to study the long standing problem of the initiation of the madden julian oscillation mjo with initial and lateral boundary conditions provided by a global reanalysis this model is able to reproduce the initiation and gross features of two observed mjo events up to 2 months after the start of simulations this leads to a conjecture that these two mjo events are generated by the influences from the lateral boundaries this conjecture is supported by a series of sensitivity tests these sensitivity tests demonstrate that the simulated mjo initiation does not critically depend on detailed characteristics of sea surface temperature varying versus constant in time mean distribution from boreal spring versus winter initial conditions within a 10 day period the latitudinal location of the lateral boundaries 218 388n and s or even latent heating and moist processes the only factor found critical to the reproduction of the mjo initiation is time varying lateral boundary conditions from the reanalysis when such lateral boundary conditions are replaced by time independent conditions the model fails to reproduce the mjo initiation these results support the idea that extratropical influences can be an efficient mechanism for mjo initiation implications of these results are discussed

stress permutations three dimensional distinct element analysis accounts for a common phenomenon in brittle tectonics 1 using three dimensional 3 d distinct element modeling we explored a variety of simulations to characterize and interpret the stress permutations in brittle tectonics stress inversions of fault slip data or earthquake focal mechanisms often revealed such permutations the main aim of our study is to produce simple mechanically consistent 3 d models that account for these switches between the principal stress axes 1 2 or 2 3 even with simple boundary conditions the stress changes induced by variations in rheology are large enough to modify the local tectonic behavior and to produce permutations of principal stress axes rather than simple directional changes of stress axes which exist but often remain limited the relative variations in principal stress magnitudes are the major cause of permutations 1 2 and 2 3 in nature permutations being left apart the orientations of axes often remain tightly clustered in our experiments we adopted a ratio 2 3 1 3 of 0 5 which makes permutations difficult low favors 2 3 permutations high favors 1 2 permutations and we explored a variety of tectonic situations involving compression extension and strike slip our experiments indicate that the major causes of stress permutations are the heterogeneity of the brittle deformation e g intact rock massifs between heavily faulted deformation zones and the anisotropy of the mechanical properties that results from fracturing and faulting which concur to modify the mechanical balance inside the analyzed volume and to produce stress permutations our models show that contrasts and anisotropy in rock properties favor stress permutations of major importance is the existence of relatively resistant zones at tips of the deformed zones acting as channels where stress concentrates and switches occur because in nature such zones move in time and space it is not surprising that stress permutations are so pervasive

an observational study of tropical midlatitude interaction on intraseasonal time scales during winter abstract eight northern hemisphere winters of five and ten day average midlatitude 500 mb heights and tropical outgoing ir are used in a correlative study of tropical midlatitude interaction the seasonal cycle and interannual variability are removed so that only intraseasonal variability remains results indicate that energy predominantly propagates from midlatitudes to the tropics for both five and ten day averaged data although the propagation is more apparent in five day averaged data this is evidenced by the fact that the largest tropical ir patterns are southeastward of the 500 mb point with which the ir field is correlated the result is interpreted in terms of a quasi stationary rossby wave which has an eastward component of group velocity the southwest northeast tilt of the 500 mb height correlation patterns indicating poleward momentum transport or equatorward wave propagation also supports the hypothesis that midlatitude flow drives the tropics lead and lag correlations show that when 5

mm5 modeling of the madden julian oscillation in the indian and west pacific oceans implications of 30 70 day boundary effects on mjo development abstract the results of an experiment designed to isolate the initiation phase of the madden julian oscillation mjo from 30 70 day boundary effects is presented the technique used to accomplish this involves employing the fifth generation pennsylvania state university national center for atmospheric research psu ncar mesoscale model mm5 as first presented in the companion paper to this paper two runs each 2 yr long are integrated forward from 1 june 1990 the first run called the control uses the unmodified national centers for environmental prediction ncep ncar reanalysis nra dataset for boundary conditions the second run called the notched uses the same nra dataset for the boundary conditions with the exception that all signals with periodicities in the 30 70 day range have been removed any signals in the 30 70 day range subsequently generated by the notched run are then solely due to signals generated from within the model domain or from signals entering through the domain boundar

some aspects of western hemisphere circulation and the madden julian oscillation abstractalthough the greatest variance in convection associated with the madden julian oscillation mjo occurs over the indo pacific warm pool the mjo is associated with substantial circulation patterns in the tropics and the extratropics of the western hemisphere reanalysis data suggest that upper tropospheric easterly wind anomalies on the equator between 40 and 140 w precede 86 of active convective phases of mjo events greater than one standard deviation in amplitude over the indian ocean basin during the northern hemisphere winter composites of those mjo events that are preceded by westerly wind anomalies and those events preceded by easterly wind anomalies are compared results show that those events that are preceded by westerly wind anomalies fail to thrive and do not yield the amplitude in convection or the canonical atmospheric circulation response that is associated with those preceded by easterly wind the composite of events preceded by easterly winds reveals that these winds amplify coi

barotropic wave propagation and instability and atmospheric teleconnection patterns abstract a global barotropic model linearized about the 300 mb climatological mean january flow is perturbed by applying a series of localized forcings distributed throughout the tropics and subtropics structures which resemble the observed pacific north american and east atlantic teleconnection patterns noted by wallace and gutzler 1981 tend to recur in the responses similar patterns are found to result from the dispersion of isolated initial perturbations placed at a variety of locations in the tropics and midlatitudes it is shown that these structures are related to the most rapidly growing mode associated with barotropic instability of the zonally varying climatological basic state in the absence of damping this mode has an e folding time of about a week and a period close to 50 days in localized regions the instantaneous growth rates can be competitive with those of baroclinic instability these episodes of rapid local barotropic growth are interspersed with intervals in which the local

organized convective systems in the tropical western pacific as a process in general circulation models a toga coare case study we examine the large scale effects of organized convective systems in the tropical western pacific observed during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare in a case study approach we examine realizations of a supercluster associated with the onset of the december 1992 westerly wind burst in the t213 operational medium range weather forecasting model of the european centre for medium range weather forecasts ecmwf we idealize a supercluster as a hierarchy of three interacting scales namely organized cumulonimbus c 1 mesoscale convective systems c 2 and the supercluster component c 3 it is shown that the ecmwf model represents this hierarchy as a c 3 like surrogate whose influence dominates the effect of parametrized convection this causes over prediction of the model tendencies which in the case of zonal momentum is explained in elementary terms the structure of the resolved scale momentum flux is explained by moncrieff s 1992 archetypal theory of organized convection which has been verified against observations and cloud resolving model data sets the parametrization of subgrid scale convective momentum flux in the ecmwf model based on a momentum mixing concept produces subgrid scale tendencies that are physically different from transports associated with cumulonimbus convection in a shear flow we outline a strategy for parametrizing the momentum flux by the c 1 component based on the archetypal model the c 2 component which is part resolved and part parametrized is at odds with the assumptions of scale separation underpinning parametrization it is argued that this component should be represented as part of the prognostic treatment of convectively generated cirrus finally we suggest cloud resolving modelling studies to further quantify the structure and large scale impact of superclusters in a westerly wind burst environment ranging from idealized models to models having data assimilation capability

the 1985 86 intraseasonal oscillation and the role of the extratropics abstract an intraseasonal oscillation that occurred in the 1985 86 northern winter is documented in this study the tropical convection of this event is dominated by the mixture of a standing oscillation over the maritime continent and an eastward moving feature from the indian ocean into the central pacific the time evolution of the upper tropospheric circulation patterns instead of propagating eastward along the equator as suggested in the existing composites of the intraseasonal oscillation is characterized by a series of wave patterns in the northern hemisphere and does not complete the cycle around the globe the familiar moist kelvin wave explanation for the intraseasonal oscillation receives little support from diagnosis of this event using zonal wind height field streamfunction and potential vorticity only in the lower troposphere near the date line is the convincing evidence for its existence found a scenario for the intraseasonal oscillation which is suggested by the analysis includes

1 f noise and pulse like events in the tropical atmospheric surface variabilities this study discusses the 1 f noise behaviour and its connection with pulse like events in tropical surface meteorological data the 1 f noise withf being frequency refers to the unique spectrum shape of signals that represents the same degree of variability for a whole spectrum range with very long lag correlations the pulse like events are characterized by their high localizations in time and a lack of a distinct spectrum peak approximate 1 f noise behaviour is found in tropical surface temperature moisture and wind speed for periods of 1 hour to beyond 10 days and in oceanic wind stress up to 2 year periods the origin of the 1 f noise can be closely tied to pulse like events with a highly intermittent nature in the time series examples for such pulse like events include the cumulus convection events leading to convective downdraughts and intraseasonal westerly wind events associated with the madden julian oscillations mjo a method based on the wavelet is introduced to extract individual pulse like events these events are characterized by a 1 f spectrum extending up to 1 2 decades of frequency centred around their characteristic durations their successive occurrences are mutually uncorrelated and the individual pulse like events themselves constitute a 1 f spectrum for a frequency range around their durations the superposition of these events of various durations yields the 1 f noise spectrum for much wider ranges physical implications of these findings to convective quasi equilibrium and mjo el nino southern oscillation relationship are discussed copyright 2004 royal meteorological society

extratropical forcing of tropical wave disturbances along the indian ocean itcz the role of extratropical waves in the excitation of tropical waves on submonthly timescales is explored along the indian ocean intertropical convergence zone itcz during the austral summer using japanese reanalysis products and noaa outgoing longwave radiation data the analysis period is december february of 1979 1980 to 2008 2009 the submonthly tropical waves are regarded as a type of rossby wave propagating along the mean monsoon westerly flow they play an important role in modulating the indian ocean itcz convection the linkage between the tropical and extratropical waves which is responsible for the formation and strengthening of tropical waves is examined composite analysis results linked to the tropical wave train development show that the midlatitude rossby wave train progresses eastward and northeastward from the south atlantic into the subtropical indian ocean as a trough and ridge that form part of the midlatitude wave train approach the southern africa southwest indian ocean swio region a southwest northeast oriented wave train is subsequently established originating from this feature and is strengthened across the tropical indian ocean the midlatitude wave propagation toward the subtropics induces the growth of the trough and ridge over the swio wave activity flux diagnostics indicate that the amplified trough and ridge over the swio act as an energy source for northeastward amplification of the tropical waves through the wave energy dispersion process the results suggest that the propagation of the midlatitude wave toward the swio is the fundamental mechanism behind the development of the tropical waves along the indian ocean itcz

the global response to tropical heating in the madden julian oscillation during the northern winter a life cycle of the madden julian oscillation mjo was constructed based on 21 years of outgoing long wave radiation data regression maps of ncep ncar reanalysis data for the northern winter show statistically significant upper tropospheric equatorial wave patterns linked to the tropical convection anomalies and extratropical wave patterns over the north pacific north america the atlantic the southern ocean and south america to assess the cause of the circulation anomalies a global primitive equation model was initialized with the observed three dimensional 3d winter climatological mean flow and forced with a time dependent heat source derived from the observed mjo anomalies a model mjo cycle was constructed from the global response to the heating and both the tropical and extratropical circulation anomalies generally matched the observations well the equatorial wave patterns are established in a few days while it takes approximately two weeks for the extratropical patterns to appear the model response is robust and insensitive to realistic changes in damping and basic state the model tropical anomalies are consistent with a forced equatorial rossby kelvin wave response to the tropical mjo heating although it is shifted westward by approximately 20 longitude relative to observations this may be due to a lack of damping processes cumulus friction in the regions of convective heating once this shift is accounted for the extratropical response is consistent with theories of rossby wave forcing and dispersion on the climatological flow and the pattern correlation between the observed and modelled extratropical flow is up to 0 85 the observed tropical and extratropical wave patterns account for a significant fraction of the intraseasonal circulation variance and this reproducibility as a response to tropical mjo convection has implications for global medium range weather prediction copyright 2004 royal meteorological society

regional simulation of the october and november mjo events observed during the cindy dynamo field campaign at gray zone resolution abstractthis study investigates the october and november mjo events observed during the cooperative indian ocean experiment on intraseasonal variability in the year 2011 cindy dynamics of the mjo dynamo field campaign through cloud permitting numerical simulations the simulations are compared to multiple observational datasets the control simulation at 9 km horizontal grid spacing captures the slow eastward progression of both the october and november mjo events in surface precipitation outgoing longwave radiation zonal wind humidity and large scale vertical motion the vertical motion shows weak ascent in the leading edge of the mjo envelope followed by deep ascent during the peak precipitation stage and trailed by a broad second baroclinic mode structure with ascent in the upper troposphere and descent in the lower troposphere both the simulation and the observations also show slow northward propagation components and tropical cyclone like vortices after the passage of the mjo active phase

large scale distinctions between mjo and non mjo convective initiation over the tropical indian ocean abstractin this study the authors seek large scale signals that may distinguish mjo from non mjo convective events before they start over the indian ocean three such signals were found low level easterly anomalies extend from the surface to the midtroposphere and move from the western to eastern indian ocean surface pressure anomalies exhibit a zonal structure of wavenumber 1 with an equatorial low pressure surge penetrating eastward from africa through the indian ocean and reaching the maritime continent negative temperature anomalies in the middle to upper troposphere start over the indian ocean and move eastward all of them emerge 20 days before convective initiation of the mjo and move eastward at speeds close to that of the mjo without any direct connection to mjo convection they are not obviously related to the extratropics in any discernible way or any preceding mjo events they are absent in non mjo convective events these signals provide useful information for forecasting mjo initiation o

intraseasonal oscillations in the global atmosphere i northern hemisphere and tropics abstract we have examined systematically oscillatory modes in the northern hemisphere and in the tropics the 700 mb heights were used to analyze extratropical oscillations and the outgoing longwave radiation to study tropical oscillations in convection all datasets were band pass filtered to focus on the intraseasonal is band of 10 120 days leading spatial patterns of variability were obtained by applying eof analysis to these is data the leading principal components pcs were subjected to singular spectrum analysis ssa ssa is a statistical technique related to eof analysis but in the time domain rather than the spatial domain it helps identify nonlinear oscillations in short and noisy time series in the northern hemisphere there are two important modes of oscillation with periods near 48 and 23 days respectively the 48 day mode is the most important of the two it has both traveling and standing components and is dominated by a zonal wavenumber two the 23 day mode has the spatial stru

development characteristics and dynamic structure of tropical intraseasonal convection anomalies abstract the development and dynamical structure of intraseasonal low frequency convection anomalies in the equatorial region are investigated using 10 years 1975 85 of outgoing longwave radiation olr and 7 years 1979 85 of 200 and 850 mb wind data the composite olr anomalies for 36 cases show a four stage development process initiation over equatorial africa rapid intensification when passing through the indian ocean mature evolution characterized by a weakening in the maritime continent and redevelopment over the western pacific and dissipation near the date line in moderate events or emanation from the equator toward north america and southeastern pacific in strong events a noticeable feature in vertical structure is that the 850 mb convergence leads convection and midtropospheric upward motion by about 30 degrees longitude in both developing and mature phases equatorial upper lower level easterly westerly anomalies and associated twin anomalous anticyclonic cyclonic circulation an

palaeoproductivity and environmental changes during the holocene in central italy as recorded in two crater lakes albano and nemi abstract three cores from two crater lakes in central italy albano and nemi spanning the last ca 11 kyr bp are discussed here they were analysed for organic matter dry density algal and photosynthetic bacteria pigments diatoms and cladocera these palaeolimnological records show great internal consistency in their response to distinct but inter related aspects of the lake catchment system and reflect major changes in these systems particularly in lake productivity the early holocene was characterised by very high productivity at both sites trends in productivity are similar for many proxies in the holocene records of lake albano and lake nemi trophic levels throughout much of the period of human activity ca 4000 yr bp onward are lower but have remained relatively high and stable at both lakes it is only in the last few decades that inferred productivity levels have exceeded those of the early holocene statistical analysis constrained incremental sum of square cluster analysis coniss showed five distinct periods during the holocene characterised by major variations in the abundance and community structure of biological remains and geochemistry a comparison of results from the different taxonomic groups has enabled the reconstruction of phases of increased productivity and the recognition of signals of anthropogenic impact on the lakes and their catchments from published pollen diagrams nine major deforestation events are recognised during the holocene these events in particular the one which occurred in both lakes at 1800 yr bp and identifies the roman period plus another at 3000 3500 yr bp related to agricultural activity during the bronze age produced profound changes in the pelagic community structure of diatoms and cladocera which in some cases were even more marked than the accelerated eutrophication seen in recent times the mid holocene climatic transition is also clearly evident and is characterised by a strong decrease in the concentrations of the biological proxies most of the observed lacustrine environmental changes are almost synchronous in the two lakes indicating the importance of regional rather than local changes

the role of global circumnavigating mode in the mjo initiation and propagation a series of convection permitting regional model simulations are used to study the influences of intraseasonal forcings from the periphery of the tropical indian ocean on the october 2011 mjo event well observed during the dynamics of the madden julian oscillation dynamo field campaign these sensitivity experiments demonstrate the predominant role of a global circumnavigating mode in the initiation and propagation of this mjo event although the regional framework would not completely rule out influences from the extratropics through the western boundary of the model domain it is found that filtering out intraseasonal forcing leads to the absence of kelvin waves which may reduce moisture transport and weakened lower level convergence at the leading edge of the westerlies and further leads to diminishing mjo signatures in these sensitivity experiments meanwhile for the same regional domain initiated two weeks before the onset of the mjo convection there is little impact of whether or not the global mjo signals retained in the initial conditions on the other hand the removal of the global mjo signals from the lateral boundary conditions will not affect the propagation of the mjo after it has been initiated in the regional model domain

intraseasonal variability in a dry atmospheric model abstract a long integration of a primitive equation dry atmospheric model with time independent forcing under boreal winter conditions is analyzed a variety of techniques such as time filtering space time spectral analysis and lag regressions are used to identify tropical waves it is evident that oscillations with intraseasonal time scales and a kelvin wave structure exist in the model tropical atmosphere coherent eastward propagations in the 250 hpa velocity potential and zonal wind are found with a speed of about 15 m s 1 the oscillation is stronger in the eastern hemisphere than in the western hemisphere interactions between the tropical and extratropical flows are found to be responsible for the simulated intraseasonal variability wave activity flux analysis reveals that a tropical influence occurs in the north pacific region where a northeastward wave activity flux is found associated with the tropical divergent flow in the western and central pacific in the north atlantic sector on the ot

numerical study of convection observed during the winter monsoon experiment using a mesoscale two dimensional model abstract a two dimensional version of the pennsylvania state university mesoscale model has been applied to winter monsoon experiment data in order to simulate the diurnally occurring convection observed over the south china sea the domain includes a representation of part of borneo as well as the sea so that the model can simulate the initiation of convection also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle this allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck which typically occupies several hundred kilometers of the domain frank and cohen s cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area the major conclusions are ice phase processes are important in determining the level of maximum large scale heating and vertical motion because there is a strong anvil componen

primary and successive events in the madden julian oscillation conventional analyses of the mjo tend to produce a repeating cycle such that any particular feature cannot be unambiguously attributed to the current or previous event we take advantage of the sporadic nature of the mjo and classify each observed madden julian mj event as either primary with no immediately preceding mj event or successive which does immediately follow a preceding event 40 of mj events are primary events precursor features of the primary events can be unambiguously attributed to that event a suppressed convective anomaly grows and decays in situ over the indian ocean prior to the start of most primary mj events an associated mid tropospheric temperature anomaly destabilises the atmosphere leading to the generation of the active mj event hence primary mj events appear to be thermodynamically triggered by a previous dry period although stochastic forcing may also be important other theories predict that boundary layer convergence humidity propagation of dynamical structures around the equator sea surface temperatures and lateral forcing by extratropical transients may all be important in triggering an event although precursor signals from these mechanisms are diagnosed from reanalysis and satellite observational data in the successive mj events they are all absent in the primary mj events hence it appears that these apparent precursor signals are part of the mjo once it is established but do not play a role in the spontaneous generation of the mjo the most frequent starting location of the primary events is the indian ocean but over half of them start elsewhere from the maritime continent to the western pacific copyright 2008 royal meteorological society

the tropical madden julian oscillation and the global wind oscillation abstract the global wind oscillation gwo is a subseasonal phenomenon encompassing the tropical madden julian oscillation mjo and midlatitude processes like meridional momentum transports and mountain torques a phase space is defined for the gwo following the approach of wheeler and hendon for the mjo in contrast to the oscillatory behavior of the mjo two red noise processes define the gwo the red noise spectra have variance at periods that bracket 30 60 or 30 80 days which are bands used to define the mjo the correlation between the mjo and gwo is 0 5 and cross spectra show well defined coherent phase relations in similar frequency bands however considerable independent variance exists in the gwo a basic dynamical distinction occurs in the direction of midlatitude wave energy dispersion being predominantly meridional during a mjo and zonal during the gwo this is primarily a winter season feature centered over the pacific ocean a case study during april may 2007 focuses on the gwo and two

distinct manifestations of austral summer tropical intraseasonal oscillations 1 event to event intraseasonal convection is found to be significantly different from a canonical madden julian oscillation mjo nearly half of all austral summer events do not show the same initiation or propagation characteristics of canonical mjos although they meet some mjo criteria during their life cycles variations of intraseasonal convection fall within three distinct forms the canonical mjo an eastward decaying mode and an eastward intensifying mode this distinction offers important insights into the overall dynamics and predictability of tropical intraseasonal variability sea surface temperature anomalies generated during convective breaks are fundamental for canonical propagation and result from radiative forcing whose modulation is a consequence of large scale wave dynamics eastward propagation seems to rely more on the state of the tropical atmosphere ocean system before convective triggering than on the trigger itself eastward decaying events exhibit weaker wave related anomalies alternatively mechanisms driving intensification over indonesia are very different from the first two categories