retrograding wintertime low frequency disturbances over the north pacific ocean abstract retrograding westward moving features over the middle and high latitudes of the northern hemisphere during winter are examined in observations and data from a gcm simulation using complex principal component analysis the first eigenvector of the low frequency middle troposphere geopotential height over the north pacific has a zonal scale of 120 160 deg longitude and a dipole like meridional structure with maximum amplitude over the gulf of alaska and the bering sea these disturbances propagate westward and go through a life cycle of growth and decay over a period of about three weeks the temporal evolution of the energy and its conversions during the life cycle of the simulated pacific disturbances are composited using the time dependent coefficient of the principal component analysis as an index for their occurrence the composite energy cycle indicates that the kinetic and available potential energy of these disturbances grow and decay together both baroclinic and barotropic conversion

strain analysis and vorticity of flow in the northern sardinian variscan belt recognition of a partitioned oblique deformation event abstract a field example of strain partitioning has been analysed along the nurra asinara transect of the nw sardinian variscan chain italy the section in the nurra asinara area is in a continuous sequence of tectono metamorphic complexes made of low to high grade metamorphic rocks affected by a polyphase tectonic history the principal fabric of the area is controlled by a d2 progressive deformation phase in which the strain is partitioned into folds and shear zone domains the d2 stretching lineation and shear sense show a clear change from south to north the principal meso and micro structures vorticity gauges and a quantitative kinematic analysis of local strain suggest that the d2 kinematic history could be envisaged as an oblique heterogeneous deformation similar to the transpressive systems described in ancient and modern settings elsewhere using a simple kinematic model we also propose that both a transpressive system followed by thrusting or a partitioned transpressive system could be responsible for the fabric distribution and strain accumulation described in the study transect

the atmospheric angular momentum cycle during the tropical madden julian oscillation abstract global scale wind and outgoing longwave radiation anomalies are documented during the atmospheric angular momentum aam cycle associated with the madden julian oscillation cross spectral and compositing techniques are applied to 150 and 850 mb national meteorological center wind data and outgoing longwave radiation olr data during ten northern winter and summer seasons coherent wind and olr signals are found in the tropics and in the winter hemisphere and are dominated by zonal wavenumbers 0 2 at the time of zero aam anomaly anomalous zonal winds are found over the western hemisphere tropics during northern winter while convection anomalies tend to be collocated with the seasonal mean australasian convection zonal mean olr anomalies are small as the aam anomaly increases the anomalous zonal winds move eastward and poleward while dipole convection anomalies become established over the oceanic warm pool and result in an east west shift of the australasian monsoon zonal mean olr ano

mountain torques and atmospheric oscillations theoretical work and general circulation model gcm experiments suggest that the midlatitude jet stream s interaction with large scale topography can drive intrasea sonal oscillations in large scale atmospheric circulation pat terns in support of this theory we present new observa tional evidence that mountain induced torques play a key role in 15 30 day oscillations of the northern hemisphere circulation s dominant patterns the affected patterns in clude the arctic oscillation ao and the pacific north american pna pattern positive torques both accelerate and anticipate the midlatitude westerly winds at these peri odicities moreover torque anomalies anticipate the onsets of weather regimes over the pacific as well as the break ups of hemispheric scale regimes

multiple flow regimes in the northern hemisphere winter part i methodology and hemispheric regimes abstract recurrent and persistent flow patterns are identified by examining multivariate probability density functions pdfs in the phase space of large scale atmospheric motions this idea is pursued systematically here in the hope of clarifying the extent to which intraseasonal variability can be described and understood in terms of multiple flow regimes bivariate pdfs of the northern hemisphere nh wintertime anomaly heights at 700 mb are examined in the present paper using a 37 year dataset the two dimensional phase plane is defined by the two leading empirical orthogonal functions eofs of the anomaly fields pdfs on this plane exhibit synoptically intriguing and statistically significant inhomogeneities on the periphery of the distribution it is shown that these inhomogeneities are due to the existence of persistent and recurrent anomaly patterns well known as dominant teleconnection patterns that is the pacific north american pna pattern its reverse and zonal and blocked phases of the

a study of the low frequency inertio gravity waves observed during the pyrenees experiment the purpose of this paper is to analyze and interpret the onset of some well characterized transient inertio gravity waves observed during the pyrex pyrenees experiment campaign the data used are the high resolution soundings launched every 6 hours on both sides of the ridge in agreement with other observations made during the field campaign certain among those soundings often show steady mountain waves it appears that those waves are often transients and are followed by well characterized inertio gravity waves propagating upward and downward from the level where the initial mountain waves are large to interpret those observations it is suggested that the observed inertio gravity waves result from the geostrophic adjustment of the large scale flow that follows the transit and eventually the breaking of the main mountain wave this hypothesis is supported by an analytical calculation of inertio gravity waves emission from a momentum deposit that lasts a finite time the spatial extent of this forcing are also finite and its maximum amplitude is a fraction of the measured mountain drag

on the source of midlatitude low frequency variability part i a statistical approach to persistence abstract the forcing of low frequency variability by synoptic transient traveling perturbations is investigated within a quasi geostrophic channel forced by a localized baroclinic jet spontaneously generated baroclinic perturbations grow and decay along a storm track at the end of the track a maximum of low frequency variability is obtained in agreement with atmospheric observations the structure of low frequency variability is studied with a combination of statistical methods using a multivariate red noise model as a random reference we show that the anomalies are preferentially linked with local stationary structures or long wave vacillations according to their location and their sign a systematic study of persistence is conducted with a criterion based on rms of the streamfunction variations the interesting quantity is the probability of persistence which shows a very inhomogeneous distribution in phase space and several separated maxima the composites based on these maxima exhibit the charact

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

experimental and numerical studies of an eastward jet over topography motivated by the phenomena of blocked and zonal flows in earth s atmosphere we conducted laboratory experiments and numerical simulations to study the dynamics of an eastward jet flowing over wavenumber two topography the laboratory experiments studied the dynamical behaviour of the flow in a barotropic rotating annulus as a function of the experimental rossby and ekman numbers two distinct flow patterns resembling blocked and zonal flows in the atmosphere were observed to persist for long time intervals earlier model studies had suggested that the atmosphere s normally upstream propagating rossby waves can resonantly lock to the underlying topography and that this topographic resonance separates zonal from blocked flows in the annulus the zonal flows did indeed have super resonant mean zonal velocities while the blocked flows appear subresonant low frequency variability periodic or irregular was present in the measured time series of azimuthal velocity in the blocked regime with dominant periodicities in the range of 6 25 annulus rotations oscillations have also been detected in zonal states with smaller amplitude and similar frequency in addition over a large region of parameter space the two flow states exhibited spontaneous intermittent transitions from the one to the other we numerically simulated the laboratory flow geometry in a quasi geostrophic barotropic model over a similar range of parameters both flow regimes blocked and zonal were reproduced in the simulations with similar spatial and temporal characteristics including the low frequency oscillations associated with the blocked flow the blocked and zonal flow patterns are present over wide ranges of forcing topographic height and bottom friction for a significant portion of parameter space both model flows are stable depending on the initial state either the blocked or the zonal flow is obtained and persists indefinitely showing the existence of multiple equilibria

on the interpretation of eddy fluxes during a blocking episode abstract using twice daily synoptic charts objectively analyzed at the national meteorological centre horizontal eddy fluxes of temperature and quasi geostrophic potential vorticity are computed for the month of july 1976 when a blocking anticyclone was centered over western europe the local time averaged eddy variance equations are used to provide a dynamical basis for interpreting the spatial pattern of eddy fluxes and their relation to mean gradients it is shown that a rotational non divergent flux can be identified the cross gradient component of which balances the mean flow advection of eddy variance the remaining flux is the dynamically significant one which helps maintain the block and can be understood in terms of a response to sources and sinks of eddy variance

multiple regimes and low frequency oscillations in the southern hemisphere s zonal mean flow abstract this paper studies multiple regimes and low frequency oscillations in the northern hemisphere zonal mean zonal flow in winter using 55 yr of daily observational data the probability density function estimated in the phase space spanned by the two leading empirical orthogonal functions exhibits two distinct statistically significant maxima the two regimes associated with these maxima describe persistent zonal flow states that are characterized by meridional displacements of the midlatitude jet poleward and equatorward of its time mean position the geopotential height anomalies of either regime have a pronounced zonally symmetric component but largest amplitude anomalies are located over the atlantic and pacific oceans high frequency synoptic transients participate in the maintenance of and transitions between these regimes significant oscillatory components with periods of 147 and 72 days are identified by spectral analysis of the zonal flow time series these oscillations are described b

nonlinear orographically forced planetary waves abstract traditionally stationary wave models have been linearized about a zonal mean flow and the response calculated to various fixed orographic and thermal forcings in this paper it is shown that the inclusion of nonlinear interactions can significantly modify the solution for orographic forcing this arises primarily from the improved boundary condition that allows the flow to be deflected around the mountain as well as over it in this context a useful conceptual model is obtained by linearization based on the smallness of the latitudinal extent of a mountain the nonlinear model is considerably less sensitive to the zonal mean surface flow and in some instances the perturbation amplitude decreases with increasing surface flow the nonlinear hemispheric solutions for full orography and wintertime basic state are shown for both the northern and southern hemispheres they suggest that the direct effect of orographic forcing alone accounts for less than one half of the observed time mean asymmetries

life cycles of persistent anomalies ii the development of persistent negative height anomalies over the north pacific ocean abstract the present study extends our previous work on the life cycles of persistent anomalies by providing more comprehensive analyses of the synoptic and dynamical characteristics associated with the developments of the anomalies we focus here on the developments of major cases of persistent negative height anomalies over the extratropical central north pacific pac region during wintertime these ewes are generally manifested at the surface by an anomalously intense and eastward displaced aleutian low and at upper levels by an abnormally strong zonal jet that extends across most of the western and central pacific at midlatitudes the associated flow anomalies usually resemble particularly strong realizations of the pacific north american pna teleconnection pattern the large scale flow anomalies are typically preceded by a buildup of anomalously cold air over asia and an intensification of the upper level jet over southeastern asia and the fox western pacific a few days prior to the larger scal

wintertime blocking and mountain forcing of the zonally averaged flow a cross spectral time series analysis of observed data abstract nine winters of northern hemisphere observations are used for a statistical analysis of the relation between the mountain forcing of the zonally averaged barotropic wind and blocking thereby the temporal variability of the phenomenon blocking is expressed by a blocking index lime series it is demonstrated that the zonally averaged wind exhibits statistically significant negative anomalies at midlatitudes 40 n 53 n for the early blocking stage the mountain drag is anomalously negative corresponding to a slow down of the zonal mean wind at high 53 n 70 n and at middle 40 n 53 n latitudes these significant negative mountain drag anomalies are found before the onset or at the very first stage of blocking in terms of a signal to noise ratio the strongest impact of the mountain drag on the zonal mean wind tendency occurs at the latitudes 30 n 40 n where the main mountain ranges of the earth are located

transitions between blocked and zonal flows in a rotating annulus with topography the mid latitude atmosphere is dominated by westerly nearly zonal flow occasionally this flow is deflected poleward by blocking anticyclones that persist for 10 days or longer experiments in a rotating annulus used radial pumping to generate a zonal jet under the action of the coriolis force in the presence of two symmetric ridges at the bottom of the annulus the resulting flows were nearly zonal at high forcing or blocked at low forcing intermittent switching between blocked and zonal patterns occurs because of the jet s interaction with the topography these results shed further light on previous atmospheric observations and numerical simulations

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

mountain torques and northern hemisphere low frequency variability part i hemispheric aspects the ncep ncar reanalysis dataset for 1958 97 is used to analyze intraseasonal variations in mountain torques and the large scale atmospheric circulation patterns associated with them spectral analysis of the atmospheric angular momentum aam budget shows that the dominant variations of mountain torque have periodicities near 30 days and shorter while the dominant aam variations occur in the 40 60 day band this difference is due to the 40 60 day aam variations being primarily related to equatorial processes while mountain torque variations are associated mostly with extratropical processes the northern hemisphere nh mountain torque has enhanced power and significant spectral peaks in the 20 30 day band the signal in this band accounts for 33 of the nh mountain torque variance once the seasonal cycle has been removed lag composites of the nh 700 hpa geopotential heights based on the 20 30 day mountain torque signal show the latter to be associated with coherent large scale patterns that resemble lowfrequency oscillations identified in this band by previous authors the composite patterns that are in phase quadrature with the 20 30 day nh mountain torque have a pronounced zonally symmetric component these patterns are associated with substantial aam variations arguably driven by the nh mountain torque in this band principal component pc analysis of the nh 700 hpa heights shows that in the 20 30 day band the mountain torque is also in phase quadrature with the two leading pcs the first corresponds to changes in the midlatitude jet intensity near the subtropics while the second corresponds to the arctic oscillation the relationships with aam of the latter essentially occurs through the mass term mountain torques are furthermore nearly in phase with dominant patterns of low frequency variability that exhibit substantial pressure gradients across the rockies and the tibetan plateau

the environment of the ethiopian highlands at the mid pleistocene transition fauna flora and hominins in the 850 700 ka sequence of gombore ii melka kunture abstract environment climatic change and human evolution have been debated over the last 50 years giving special attention to the plio pleistocene sites of the rift valley in this paper we discuss the environment and the limits of hominin adaptability based on evidence from melka kunture at 2000 m asl on the ethiopian highlands and specifically on the 850 ka to 700 ka sequence at sub site gombore ii human fossils and multiple acheulean occurrences as well as hippo remains and footprints combined with palynological analysis provide a highly detailed chronological resolution of the changing local environmental conditions during the last 150 ka of the mpt mid pleistocene transition including the sequence of events after a volcanic eruption layers containing footprints and fossils are evidence of near continuous occupation by hippos and their recolonization of the area after a disruptive volcanic eruption conversely acheulean implements and human fossils suggest that peopling by hominins occurred at a different and discontinuous pace even when the flora and fauna were re established and the environment was rather stable most notably the assembled evidence points to the limits of homo erectus s l adaptability apparently this hominin could no longer live at 2000 m asl when the climate deteriorated during glacial isotopic stage 20 becoming markedly colder than it is today but re colonized the area when the climate turned warmer again during isotopic stage 19

a striking example of the atmosphere s leading traveling pattern abstract conventional and complex empirical orthogonal function eof techniques show that for at least four months during the fall and winter of 1979 80 a large amplitude large scale traveling flow anomaly existed in the troposphere and stratosphere this feature evolved in a cyclic manner exhibiting an average period of about 23 days its phase propagation was toward the west while its amplitude was concentrated in a broad envelope centered over canada the phenomenon was equivalent barotropic in the troposphere and increased in amplitude with altitude it represented about 25 of the variance of geopotential heights in the troposphere and stratosphere for the months of november through march the meridional and zonal structure of the zonal wavenumber one component of the traveling anomaly is similar to that of the theoretical 16 day wave however it is distinct from that theoretical mode in that its period is longer and in the troposphere zonal wave two makes a large contribution to its structur

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

statistics and dynamics of persistent anomalies abstract persistent anomalies with recurrent spatial patterns play an important role in the atmosphere s low frequency variability we establish a connection between statistical and dynamical methods of description and prediction of persistent anomalies this is done by computing and analyzing the empirical orthogonal functions eofs in a simple deterministic model on the one hand and in southern hemisphere geopotential heights on the other the dynamical model is governed by the fully nonlinear equivalent barotropic vorticity equation on the sphere with simplified forcing dissipation and topography model solutions exhibit persistent anomalies identifiable with blocked zonal and wave train anomalies in northern hemisphere atmospheric data flow structures similar to the patterns above occur as high variance eofs of this nonlinear model the southern hemisphere data we analyze consist in gridded daily maps of 500 mb heights from june 1972 to july 1983 two types of persistent anomalies appear in t

alleviation of stationary biases in a gcm through a mountain drag parameterization scheme and a simple representation of mountain lift forces abstract the problem of the representation of the orientation of mountain forces in a gcm is examined first a series of winter simulations is presented with the laboratoire de meteorologie dynamique gcm where the model ground is flat and where the mountains are represented by applying forces to the flow that are either strictly opposite to the local wind i e a drag or strictly perpendicular to it i e a lift these experiments show that the drag forces improve the zonal wind but do not improve the steady planetary wave everywhere while the lift forces scarcely affect the zonal wind but force a steady planetary wave that is everywhere realistic the drag force is formulated following the realistic subgrid scale orographic sso scheme developed recently at ecmwf the lift force essentially enhances vortex stretching over large scale mountains to address issues concerning the representation of sso in gcms experiments with mean orography the ecmwf sso scheme and enhanced lift forces over mounta

the shape propagation and mean flow interaction of large scale weather systems abstract for a zonal average operator the eliassen palm flux provides a diagnostic of both eddy behavior and the feedback of the eddies onto the mean flow this paper addresses the diagnosis problem for other averaging operators and in particular for time averaging which has proved in recent years such a powerful means of viewing the three dimensional tropospheric flow the horizontal velocity correlation tensor gives a measure of the characteristic horizontal eddy shape at a point it also implies the direction of the group velocity relative to the mean flow in cases where such a concept is valid however the major emphasis here is on the mean flow feedback of eddies in this respect the eddy vorticity flux is determined by derivatives of the components of the anisotropic part of the tensor making a reasonable approximation allows the eddy vorticity flux convergence to be written in terms of e v 2 u 2 u v a simple interpretation of the mean flow feedback of eddies is then possible a sligh

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

on the existence of storm tracks abstract given that middle latitude weather systems transport heat in a manner such as to weaken the baroclinicity that is thought to be crucial to their growth it is perhaps surprising that concentrated regions of such eddy activity i e storm tracks are found in the northern hemisphere winter the existence and possible self maintenance of storm tracks is investigated using a linear stationary wave model with storm track region forcings taken from data averaged over a number of winters it is found that the direct thermal effect of the eddies does indeed act against the existence of the storm track their vorticity fluxes lead to some reduction of this effect it is argued that the mean diabatic heating in the storm track region is an indirect eddy effect this heating is found to maintain the mean maximum in baroclinicity in the region further the mean low level flow induced by the eddy effects is such as to enhance the warm western oceanic boundary currents that are crucial to the existence of t

regional sources of mountain torque variability and high frequency fluctuations in atmospheric angular momentum abstract the sources of high frequency 14 day fluctuations in global atmospheric angular momentum aam are investigated using several years of surface torque and aam data the midlatitude mountain torque associated with the rockies himalayas and andes is found to be responsible for much of the high frequency fluctuations in aam whereas the mountain torque in the tropics and polar regions as well as the friction torque play a much lesser role on these timescales a maximum in the high frequency mountain torque variance occurs during the cool season of each hemisphere though the northern hemisphere maximum substantially exceeds that of the southern this relationship indicates the seasonal role played by each hemisphere in the high frequency fluctuations of global aam a case study reveals that surface pressure changes near the rockies and himalayas produced by mobile synoptic scale systems as they traversed these mountains contributed to a large fluctuation in mountain torque and a notable high fre

the reddest atmospheric modes and the forcings of the spectra of these modes abstract the reddest atmospheric modes are studied using a 500 yr integration performed with the coupled echam1 lsg general circulation model by fitting a first order autoregressive process to the considered time series a simple measure of the spectral shape is obtained that allows an objective intercomparison of the spectra and the time series two modes the tropical and the southern hemisphere modes are identified as the reddest modes of the model atmosphere both are strongly anisotropic the southern hemisphere mode characterized by a dipole of zonal wind anomalies at southern hemispheric mid and high latitudes is related to a mass redistribution and a well defined meridional circulation whereas no significant anomalies of surface pressure and meridional velocity are found for the tropical mode which is characterized by a maximum of zonal wind anomalies in the upper tropical troposphere on timescales longer than a month the two modes make relative to other motions the largest contribution