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

climatology of anticyclonic and cyclonic rossby wave breaking on the dynamical tropopause in the southern hemisphere abstract the characteristics of the climatological distribution of the anticyclonic lc1 and cyclonic lc2 rossby wave breaking rwb in the southern hemisphere sh are investigated by calculating the occurrence frequency of the lc1 and lc2 like stratospheric potential vorticity pv streamers in the sh during the austral summer december february djf and wintertime june august jja on several isentropic surfaces by using the 40 yr european centre for medium range weather forecasts ecmwf re analysis era 40 daily dataset the results show that 1 on the equatorward flank of the climatological midlatitude jet mlj the lc1 like pv streamers are frequently found over the central oceanic regions whereas the lc2 like pv streamers are almost absent on the poleward flank of the climatological mlj both types of pv streamers are frequently observed and the lc2 like pv streamers predominate 2 the regions where the occurrences of the pv streamers are frequent overlap the weak zonal wind regions

dansgaard oeschger and heinrich event imprints in alboran sea paleotemperatures past sea surface temperature sst evolution in the alboran sea western mediterranean during the last 50 000 years has been inferred from the study of c37 alkenones in international marine global change studies md952043 core this record has a time resolution of 200 years allowing the study of millennial scale and even shorter climatic changes the observed sst curve displays characteristic sequences of extremely rapid warming and cooling events along the glacial period comparison of this alboran record with 18o from greenland ice greenland ice sheet project 2 core shows a strong parallelism between these sst oscillations and the dansgaard oeschger events five prominent cooling episodes standing out in the sst profile are accompanied by an anomalous high abundance of neogloboquadrina pachyderma sinistral which is confined to the duration of these cold intervals these features and the isotopic record reflect drastic changes in the surface hydrography of the alboran sea in association with heinrich events hl 5

atmospheric response to zonal variations in midlatitude sst transient and stationary eddies and their feedback midwinter storm track response to zonal variations in midlatitude sea surface temperatures ssts has been investigated using an atmospheric general circulation model under aquaplanet and perpetual january conditions zonal wavenumber 1 sst variations with a meridionally confined structure are placed at various latitudes having these sst variations centered at 308n leads to a zonally localized storm track while the storm track becomes nearly zonally uniform when the same sst forcing is moved farther north at 408 and 508n large small baroclinic energy conversion north of the warm cold sst anomaly near the axis of the storm track near 408n is responsible for the large small storm growth the equatorward transfer of eddy kinetic energy by the ageostrophic motion and the mechanical damping are important to diminish the storm track activity in the zonal direction significant stationary eddies form in the upper troposphere with a ridge trough northeast of the warm cold sst anomaly at 308n heat and vorticity budget analyses indicate that zonally localized condensational heating in the storm track is the major cause for these stationary eddies which in turn exert a positive feedback to maintain the localized storm track by strengthening the vertical shear near the surface these results indicate an active role of synoptic eddies in inducing deep tropospheric scale response to midlatitude sst variations finally the application of the model results to the real atmosphere is discussed

eddies and the annular modes of climate variability the dominant modes of month to month varia tion in the troposphere are realistically simulated in a gen eral circulation model whose lower boundary is prescribed with realistic topography and seasonally varying climato logical sea surface temperatures in both hemispheres these internal modes describe a nearly zonally symmetric north south shifting of the zonal jets as anomalous westerlies vac illate between high 50 60 a nd low 30 40 latitudes the eddy structures evolve with the jets and the corre sponding eddy momentum forcings support the shifts in jet position stationary synoptic wave variations mainly ac count for the total eddy forcings in the northern south ern hemisphere pronounced eddy variations occur over the north atlantic sector

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

the last glacial interglacial transition lgit in the western mid latitudes of the north atlantic abrupt sea surface temperature change and sea level implications abstract high resolution reconstructions of sea surface temperature uk 37 sst coccolithophore associations and continental input total organic carbon higher plant n alkanes n alkan 1 ols in core d13882 from the shallow tagus mud patch are compared to sst records from deep sea core md03 2699 and other western iberian margin cores results reveal millennial scale climate variability over the last deglaciation in particular during the lgit in the iberian margin heinrich event 1 h1 and the younger dryas yd represent two extreme episodes of cold sea surface condition separated by a marine warm phase that coincides with the bolling allerod interval b a on the neighboring continent following the yd event an abrupt sea surface warming marks the beginning of the holocene in this region ssts recorded in core d13882 changed however faster than those at deep sea site md03 2699 and at the other available palaeoclimate sequences from the region while the sst values from most deep sea cores reflect the latitudinal gradient detected in the iberian peninsula atmospheric temperature proxies during h1 and the b a the tagus mud patch core d13882 experienced colder ssts during both events this is most certainly related to a supplementary input of cold freshwater from the continent to the tagus mud patch a hypothesis supported by the high contents of terrigenous biomarkers and total organic carbon as well as by the dominance of tetra unsaturated alkenone c37 4 observed at this site the comparison of all western iberia sst records suggests that the sst increase that characterizes the b a event in this region started 1000 yr before meltwater pulse 1a mwp 1a and reached its maximum values during or slightly after this episode of substantial sea level rise in contrast during the yd holocene transition the sharp sst rise in the tagus mud patch is synchronous with meltwater pulse 1b the decrease of continental input to the mud patch confirms a sea level rise in the region thus the synchronism between the maximum warming in the mid latitudes off the western iberian margin the adjacent landmasses and greenland indicates that mwp 1b and the associated sea level rise probably initiated in the northern hemisphere rather than in the south

links between rossby wave breaking and the north atlantic oscillation arctic oscillation in present day and last glacial maximum climate simulations upper tropospheric rossby wave breaking processes are examined in coupled ocean atmosphere simulations of the last glacial maximum lgm and of the modern era lgm statistics of the northern hemisphere in winter computed from the paleoclimate modeling intercomparison project phase ii pmip2 dataset are compared with those from preindustrial simulations and from the 40 yr ecmwf re analysis era 40 particular attention is given to the role of wave breaking events in the north atlantic oscillation nao for each simulation anticyclonic awb and cyclonic cwb wave breaking events during lgm are shown to be less and more frequent respectively than in the preindustrial climate especially in the pacific this is consistent with the slight equatorward shift of the eddy driven jets in the lgm runs the most remarkable feature of the simulated lgm climate is that it presents much weaker latitudinal fluctuations of the eddy driven jets this is accompanied by less dispersion in the wave breaking events a physical interpretation is provided in terms of the fluctuations of the low level baroclinicity at the entrance of the storm tracks the nao in the preindustrial simulations and in era 40 is characterized by strong latitudinal fluctuations of the atlantic eddy driven jet as well as by significant changes in the nature of the wave breaking during the positive phase the eddy driven jet moves to the north with more awb events than usual and is well separated from the subtropical african jet the negative phase exhibits a more equatorward atlantic jet and more cwb events in contrast the lgm nao is less well marked by the latitudinal vacillation of the atlantic jet and for some models this property disappears entirely the lgm nao corresponds more to acceleration deceleration or extension retraction of the atlantic jet the hemispheric point of view of the arctic oscillation exhibits similar changes

zonal flow vacillation and bimodality of baroclinic eddy life cycles in a simple global circulation model abstract a global primitive equation model of the atmosphere is used to study the relationship between the temporal variations of the zonal mean zonal flow and baroclinic eddies nonperiodic low frequency vacillation of the mean zonal flow is found in longtime integrations of the model under a perpetual condition the zonal mean jet in the extratropics changes its position nearly barotropically a potential vorticity potential temperature pv analysis is performed for two extreme periods of the zonal flow vacillation anticyclonic breakings of upper troughs are dominant in the period of a high latitude jet while cyclonic breakings are dominant in the period of a low latitude jet a statistically significant relationship between the zonal flow vacillation and the morphology of life cycles of baroclinic eddies is obtained for the entire period analyzed an index of the life cycles which is introduced in this study shows clear bimodality in its frequency distribution function the relationship is also

a new perspective toward cataloging northern hemisphere rossby wave breaking on the dynamic tropopause abstractrossby wave breaking rwb events are a common feature on the dynamic tropopause and act to modulate synoptic scale jet dynamics these events are characterized on the dynamic tropopause by

two types of baroclinic life cycles during the southern hemisphere summer abstract baroclinic eddy life cycles of the southern hemisphere sh summer are investigated with ncep ncar reanalysis data a composite analysis is performed for the years 1980 through 2004 individual life cycles are identified by local maxima in synoptic scale eddy energy two types of baroclinic life cycles are examined each defined by the strength of the barotropic energy conversion 2 days prior to the maximum baroclinic growth for one life cycle the barotropic conversion is anomalously weak before the maximum baroclinic growth for the other the barotropic conversion is anomalously strong these two life cycles are referred to as the weak barotropic wb and strong barotropic sb life cycles the analyses for the wb life cycle find that a poleward anomalous wave activity flux is observed within the sh tropics and subtropics just before the initial growth of the synoptic scale eddies in contrast the sb life cycle exhibits an equatorward anomalous wave activity flux prior to the initial wave de

the relationship between the madden julian oscillation and the north atlantic oscillation in this study the relationship between the madden julian oscillation mjo and the north atlantic oscillation nao is investigated by analyzing nao events both when the mjo is active and inactive the european centre for medium range weather forecasts re analysis era interim dataset is used for the years 1979 2006 and the months of december february when the mjo is active negative positive nao events are preceded by enhanced mjo convection over the tropical western and central pacific ocean tropical indian ocean and maritime continent the accompanying sea level pressure slp anomaly fields reveal patterns that closely resemble the northern annular mode nam the mjo related negative nao events are preceded by an increase in the vertical propagation of planetary scale wave activity into the stratosphere followed by a marked weakening of the zonally symmetric component of the lower stratospheric zonal wind the opposite behaviour is found for the mjo related positive nao events the mjo related nao events are also long lived persisting for approximately 30 days in contrast when the mjo is inactive there is little change to the zonally symmetric component of the lower stratospheric zonal wind these events are relatively short lived lasting for about 10 days and the nao corresponds to dipole that is confined to the north atlantic

the structure and predictability of the high latitude mode in the csiro9 general circulation model abstract the csiro9 general circulation model shows a zonally symmetric mode of variability which closely resembles the high latitude mode hlm in middle and high latitudes of the southern hemisphere the leading eof of the zonal mean zonal wind between 30 and 68 s whose amplitude has been taken as an index of the hlm shows opposing variations centered near 40 and 60 s accounting for 43 of the daily variance analysis has concentrated on composites for periods when the index changed quickly between significant peaks of the opposite sign or persisted with a large amplitude for an extended period the momentum flux variations are small at the northern and southern boundaries and the principal variations are centered near 49 s between the maxima in the zonal wind the changes in angular momentum content are around 30 smaller in the southern band eddy heat fluxes are less coherent but help in maintaining the zonal wind anomalies against friction a simple model of the zonal wind index with stochastic

atmosphere sea ice low frequency variability with a simple model of the southern hemisphere summary a simple primitive equation general circulation model sgcm is used as a representation of an aquaplanet with no orography to model constant southern hemisphere winter conditions the statistical signie cance of the decadal and quasi biennial variability is erst assessed in a long time sgcm run by means of a frequencydomain statistical decomposition the modelled long wave atmospheric pattern is then associated with the observed southern hemisphere annular mode as suggested by previous studies using the same sgcm a simple linear relationship between the low level wind and the ice velocity is then introduced into the sgcm to investigate the feedbacks between the sea ice cover and the baroclinic waves the zonal mean state reached by the sea ice cover in a long coupled run a few hundred years exhibits characteristics similar to the observations investigation of the low frequency variability in the coupled simulation leads to the following results 1 statistically signie cant variability is observed on interdecadal decadal interannual and quasi biennial time scales 2 interdecadal decadal and quasi biennial signals are associated with the atmospheric annular mode that forces changes in the northward extent of the sea ice cover at all southern ocean longitudes 3 the interannual signal 3 6 year period exhibits spatio temporal patterns and propagation speed similar to the wellknown antarctic circumpolar wave

fragmentation of stratospheric intrusions evidence is presented pointing to the existence of rich and coherent subsynoptic and mesoscale flow features at tropopause levels these features are related to and evolve from the classical v shaped intrusions of stratospheric air down to tropospheric elevations it is shown that intrusions can develop into elongated 2000 3000 km and slender 200 km streamers and that thereafter such a streamer can roll up to form a train of stalactite shaped vortex subentities with an accompanying substantial thinning of the intervening filament in addition there are indications that the vortices themselves can develop a spirallike interior structure of interleaved stratospheric and tropospheric air these inferences are based upon two independent but complementary sources analysis of the potential vorticity distribution on tropopause transcending isentropic surfaces derived from the analysis fields of the european centre for medium range weather forecasts either directly or indirectly using a contour advection technique and imagery from the water vapor channel of the european space agency meteosat 4 satellite streamers were observed to occur with a frequency of approximately one per week over central and southern europe during the winter of 1991 1992 the fragmentation is linked to the instability or self development of a filament of enhanced potential vorticity and it can modify or instigate surface weather systems moreover by inducing a substantial and rapid enlargement of the intrusion s surface area it greatly enhances the potential for local irreversible mixing of stratospheric and tropospheric air

analysis and modeling of the arctic oscillation using a simple barotropic model with baroclinic eddy forcing abstract in this study a numerical simulation of the arctic oscillation ao is conducted using a simple barotropic model that considers the barotropic baroclinic interactions as the external forcing the model is referred to as a barotropic s model since the external forcing is obtained statistically from the long term historical data solving an inverse problem the barotropic s model has been integrated for 51 years under a perpetual january condition and the dominant empirical orthogonal function eof modes in the model have been analyzed the results are compared with the eof analysis of the barotropic component of the real atmosphere based on the daily ncep ncar reanalysis for 50 yr from 1950 to 1999 according to the result the first eof of the model atmosphere appears to be the ao similar to the observation the annular structure of the ao and the two centers of action at pacific and atlantic are simulated nicely by the barotropic s model therefore the atmospheric low frequency variabilities

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

the influence of the quasi biennial oscillation on the troposphere in winter in a hierarchy of models part i simplified dry gcms abstracta dry primitive equation model is used to explain how the quasi biennial oscillation qbo of the tropical stratosphere can influence the troposphere even in the absence of tropical convection anomalies and a variable stratospheric polar vortex qbo momentum anomalies induce a meridional circulation to maintain thermal wind balance this circulation includes zonal wind anomalies that extend from the equatorial stratosphere into the subtropical troposphere in the presence of extratropical eddies the zonal wind anomalies are intensified and extend downward to the surface the tropospheric response differs qualitatively between integrations in which the subtropical jet is strong and integrations in which the subtropical jet is weak while fluctuation dissipation theory provides a guide to predicting the response in some cases significant nonlinearity in others particularly those designed to model the midwinter subtropical jet of the north pacific prevents its universal application when the ext

the signature of the annular modes in the moisture budget abstract the mid and high latitude variations of mass and momentum variously termed the arctic and antarctic oscillations and or the northern and southern annular modes also have a signature in the moisture budget this is investigated using associated regression pattern analysis in the control simulation of the canadian centre for climate modelling and analysis cccma coupled climate model and in ncep reanalysis data the analyzed behavior is similar in the two hemispheres and is characterized when pressure is low in polar regions by anomalous westerlies and a thermodynamically indirect meridional cell poleward of midlatitudes and by anomalous easterlies and a thermodynamically direct meridional cell equatorward of midlatitudes moisture transports are associated primarily with advection by the lower branches of these anomalous meridional circulation cells and secondarily with anomalous eddy transports giving in this phase of the mode moisture divergence and decreased precipitation at midlatitu

internal variability of the troposphere stratosphere coupled system simulated in a simple global circulation model internal variability of a troposphere stratosphere coupled system is investigated in a series of numerical experiments with a simple global circulation model under a perpetual winter condition in order to examine the relative importance of forced planetary waves in the interaction with the zonal mean zonal flow and baroclinic disturbances amplitude of a sinusoidal surface topography of zonal wavenumber 1 or 2 is swept as an experimental parameter 1000 day integrations are performed for 110 combinations of the external parameters intraseasonal variability of the extratropical winter stratosphere in the parameter sweep experiment of the zonal wavenumber 1 topography is classified into four regimes dependent on the topographic amplitude i a nearly radiative equilibrium state ii small undulations of the polar vortex iii intermittent breakdowns of the polar vortex or occurrence of stratospheric sudden warming events and iv a usually weak and warm state of the polar vortex the behavior of planetary waves in the stratosphere is characterized by linear propagation in regime i and by quasi linear or weakly nonlinear wave mean flow interaction in regime ii in regimes iii and iv on the other hand it is in a highly nonlinear state reflecting the occurrence of sudden warming events and the usually distorted polar vortex respectively the northern hemisphere winter stratosphere corresponds to regime iii in the intraseasonal variations while the southern counterpart is close to regime ii a lag correlation analysis shows that the dynamical linkage between the stratosphere and the troposphere is also dependent on the regimes the vertical linkage is primarily the upward control in regime i planetary waves are generated by nonlinear interaction of baroclinic disturbances in the troposphere and propagate into the stratosphere in regime ii stratospheric variations are largely confined in the stratosphere although planetary waves are generated in the troposphere the linkage in regime iii on the other hand is inevitably two way planetary wave variability has upward influence from the troposphere to the stratosphere as well as the zonal mean zonal wind shows preconditioning in the troposphere before stratospheric sudden warming events downward propagation of signals to the upper troposphere are also seen in the zonal mean temperature in high latitudes it is higher than normal at the final stage of the sequence of sudden warming events the linkage is also two way in regime iv

importance of midlatitude oceanic frontal zones for the annular mode variability interbasin differences in the southern annular mode signature abstracta midlatitude oceanic frontal zone is a confluent region of warm and cool ocean currents characterized by a strong meridional gradient in both sea surface temperature sst and surface air temperature sat while recent observational and modeling studies indicate potential impacts of midlatitude oceanic fronts on the extratropical climatological circulation including storm tracks and eddy driven westerlies their impacts on the atmospheric dominant low frequency variability i e the annular mode still remain to be understood this study explores possible impacts of midlatitude oceanic frontal zones on annular mode signatures in the wintertime southern hemisphere sh to mimic the sh sets of idealized aquaplanet experiments are conducted with zonally symmetric distributions of sst prescribed globally at the lower boundary of an atmospheric general circulation model by systematically changing the latitude of frontal gradient in the sst profile the experiments reveal that the characteristic

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

roles of low and high frequency eddies in the transitional process of the southern hemisphere annular mode abstract the effects of low and high frequency eddies time scales longer and shorter than 10 days respectively on the transitional processes of the southern hemisphere annular mode are investigated based on ncep ncar daily reanalysis data for the period 1979 2001 special attention is focused on the zonal symmetry asymmetry and the temporal evolution of the eddy forcing for the poleward transitional process the effects of low frequency eddies precede those of high frequency eddies in driving the jet transition quasi stationary rossby waves propagating along the polar jet with wavelengths of 7000 km play an important role the waves originally come from the indian ocean through the waveguide associated with the polar jet dissipate equatorward over the eastern pacific ocean this anomalous equatorward dissipation of wave activity induces an anomalous poleward momentum flux which is responsible for changes in the polar jet over the pacific ocean during the beginning stage following the low freq

continuous growth line sequences in gastropod shells abstract studies of gastropod shell growth at the level of fine growth layers are scarce because coiling of gastropod shells hinders the observation of continuous growth series spanning across whorls we propose here a new method to obtain continuous growth patterns of gastropod shells over whorls with terebralia palustris l as an example comparison of vertical and horizontal sections of the shell of this species reveals that growth patterns can be observed continuously for several whorls on the vertical section of the columella in addition an obvious tidal growth pattern preserved in this species enables us to trace its growth with a precision of 12 4 h and indicates that it takes about two years for a shell to grow from 3 cm to 9 5 cm in height and add 5 complete whorls

zonal flow regime changes in a gcm and in a simple quasigeostrophic model the role of stratospheric dynamics the atmospheric general circulation is characterized by both single and double jet patterns the doublejet structure of the zonal mean zonal wind is analyzed in southern hemisphere observations for the two calendar months of november and april the observed features are studied further in an idealized quasigeostrophic and a simplified general circulation model gcm results suggest that capturing the bimodality of the zonal mean flow requires the parameterization of momentum and heat fluxes associated with baroclinic instability of the three dimensional fields the role of eddy heat fluxes in generating the observed double jet pattern is ascertained by using an analytical eady model with stratospheric easterlies in which a single wave disturbance interacts with the mean flow in this model the dual jets are generated by the zonal mean flow correction sensitivity of the results to the tropospheric vertical wind shear or equivalently the meridional temperature gradient in the basic state s troposphere is also studied in the eady model and compared to related experiments using the simplified gcm

the relationship between the north atlantic jet and tropical convection over the indian and western pacific oceans abstractboreal winter jet variability over the north atlantic is investigated using 40 yr european centre for medium range weather forecasts re analysis era 40 data where the variability is defined by the first eof of the zonal wind on seven vertical levels the principal component time series of this eof is referred to as the jet index a pattern correlation analysis indicates that the jet index more accurately describes intraseasonal north atlantic zonal wind variability than does the north atlantic oscillation nao a series of composite calculations of the jet index based on events of intraseasonal convective precipitation over the tropical indian and western pacific oceans reveals the following statistically significant relationships 1 negative jet events lead enhanced indian ocean precipitation 2 positive jet events lag enhanced indian ocean precipitation 3 positive jet events lead enhanced western pacific ocean precipitation and 4 negative jet events lag enhanced western pacific ocean p

intraseasonal variations of the british baikal corridor pattern abstractthe british baikal corridor bbc pattern is the dominant waveguide mode trapped along the summertime polar front jet over northern eurasia it consists of four geographically fixed centers

tropospheric double jets meridional cells and eddies a case study and idealized simulations the observed low frequency variability of the zonally averaged atmospheric circulation in the winter hemisphere is found to be amenable to an interpretation where the subtropical jet is flanked by a secondary midlatitude one observations also suggest that the link between the stratosphere and the troposphere modulates the variability of the tropospheric double jet structure moreover the summer hemisphere is characterized by a strong midlatitude jet sided by an intermittent subtropical one and easterly winds in the stratosphere this work addresses the question about the role of eddies in generating and maintaining these key features of the general circulation by means of a simplified general circulation model model solutions for different parameter settings and external radiative forcings in the stratosphere are studied with and without eddies active on the system the following main findings are noted 1 eddy dynamics alone through the baroclinic instability processes in an atmosphere subjected to radiative forcing and dissipation may account for the observed meridional variance of the tropospheric jets 2 the hadley cell can extend to the pole overlying the ferrel cell a feature supported by observations in the summer hemisphere 3 the meridional temperature gradient reversal in the summer stratosphere contributes to the observed lowfrequency variability introducing an intermittent formation of a subtropical jet and the occurrence of easterlies in the tropical stratosphere 4 poleward propagation of the zonal wind anomaly is when it occurs related to the activity of synoptic eddies

sensitivity of the latitude of the surface westerlies to surface friction the sensitivity to surface friction of the latitude of the surface westerlies and the associated eddy driven midlatitude jet is studied in an idealized dry gcm the westerlies move poleward as the friction is reduced in strength an increase in the eastward phase speed of midlatitude eddies is implicated as playing a central role in this shift this shift in latitude is mainly determined by changes in the friction on the zonal mean flow rather than the friction on the eddies if the friction on the zonal mean is reduced instantaneously the response reveals two distinctive adjustment time scales in the fast adjustment over the first 10 20 days there is an increase in the barotropic component of zonal winds and a substantial decrease in the eddy kinetic energy the shift in the surface westerlies and jet latitude occurs in a slower adjustment the space time eddy momentum flux spectra suggest that the key to the shift is a poleward movement in the subtropical critical latitude associated with the faster eastward phase speeds in the dominant midlatitude eddies the view is supported by simulating the upper tropospheric dynamics in a stochastically stirred nonlinear shallow water model

opening of drake passage gateway and late miocene to pleistocene cooling reflected in southern ocean molluscan dispersal evidence from new zealand and argentina thirty two benthic marine molluscan genus group taxa are newly recognised as common to new zealand and argentina previously suggested examples including panis athlopecten austrovenus large cirsotrema lucinoma aulacomya and zygochlamys are confirmed from new collections the total of 46 cenozoic dispersed taxa now recognised falls into four main groups a oligocene early miocene dispersals from south america to new zealand 8 taxa b oligocene early miocene dispersals from new zealand to south america 22 c latest miocene pliocene dispersals from south america to new zealand 9 and d pleistocene holocene dispersals from south america to new zealand at least 7 taxa dispersal seems to have occurred in bursts both to and from south america when the antarctic circumpolar current acc initiated i e when drake passage opened late in the oligocene while dispersal from south america to new zealand only seems to have occurred again at two later times 1 during the latest miocene pliocene enhancement of acc circulation caused by the initiation of west antarctic glaciation and 2 during pleistocene glaciations however the incompleteness of the argentinian record exaggerates the size of the initial burst and allows no assessment of post middle miocene dispersal to south america formation of the acc through the opening of a passage south of australia at ca 43 40 ma and of drake passage at ca 23 ma not only led to the glaciation of antarctica but also greatly altered southern ocean biogeography

the modulation of the subtropical and extratropical atmosphere in the pacific basin in response to the madden julian oscillation abstract the 40 yr ecmwf re analysis era 40 data are combined with a number of novel climatologies to conduct a comprehensive examination of the response of the subtropical and extratropical atmosphere over the pacific basin to an evolving madden julian oscillation mjo event the adopted approach constitutes a symbiosis of a climatological analysis during the northern hemisphere winter from 1979 to 2002 and a case study analysis of a distinct mjo event that occurred in january february 1993 the former is designed to obtain the general characteristics observed during a composite mjo life cycle while the latter is used to provide insight into the instantaneous mechanisms responsible for the observed composite evolution a primary component of the study involves the diagnosis of anomalous wave breaking activity in response to mjo forcing in the form of tropical convection and or upper level divergence wave breaking events are separated by their characteristic life cycles lc1 anticyclonic and lc2 c

anatomy of synoptic eddy nao interaction through eddy structure decomposition abstracta method of eddy structure decomposition is proposed to detect how low frequency flow associated with the north atlantic oscillation nao organizes systematically synoptic eddy se activity to generate in phase and upstream feedbacks in this method a statistical eddy streamfunction ses field defined by the three point covariance of synoptic scale streamfunction is introduced to characterize spatiotemporal se flow structures the ses field is decomposed into basic and anomalous parts to represent the climatological se flow structure and its departure these two parts are used to calculate the basic and anomalous eddy velocity eddy vorticity and thus eddy vorticity flux fields in order to elucidate those two se feedbacks onto the nao this method is validated by the fact that the observed anomalous eddy vorticity flux field can be reproduced well by two linear terms the basic eddy velocity field multiplied by anomalous eddy vorticity field and the anomalous eddy velocity field multiplied

synchronous variability in the southern hemisphere atmosphere sea ice and ocean resulting from the annular mode zonally symmetric fluctuations of the midlatitude westerly winds characterize the primary mode of atmospheric variability in the southern hemisphere during all seasons this is true not only in observations but also in an unforced 15 000 yr integration of a coarse resolution r15 coupled ocean atmosphere model here it is documented how this mode of atmospheric variability known as the southern annular mode sam generates ocean circulation and sea ice variations in the model integration on interannual to centennial timescales that are tightly in phase with the sam the positive phase of the sam is associated with an intensification of the surface westerlies over the circumpolar ocean around 608s and a weakening of the surface westerlies farther north this induces ekman drift to the north at all longitudes of the circumpolar ocean and ekman drift to the south at around 308s through mass continuity the ekman drift generates anomalous upwelling along the margins of the antarctic continent and downwelling around 458s the anomalous flow diverging from the antarctic continent also increases the vertical tilt of the isopycnals in the southern ocean so that a more intense circumpolar current is also closely associated with positive sam in addition the anomalous divergent flow advects sea ice farther north resulting in an increase in sea ice coverage finally positive sam drives increases in poleward heat transport at about 308s while decreases occur in the circumpolar region ocean and sea ice anomalies of the opposite sign occur when the sam is negative the ocean and sea ice fluctuations associated with the sam constitute a significant fraction of simulated ocean variability poleward of 308s year round the robustness of the mechanisms relating the sam to oceanic variability suggests that the sam is likely an important source of large scale variability in the real southern hemisphere ocean

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

breaking waves at the tropopause in the wintertime northern hemisphere climatological analyses of the orientation and the theoretical lc1 2 classification abstract breaking waves on the tropopause are viewed as potential vorticity pv streamers on middle world isentropic levels a northern hemisphere winter climatology of the streamers spatial distribution and meridional orientation is derived from the 40 yr ecmwf re analysis era 40 dataset and used to assess the nature and frequency of occurrence of breaking synoptic scale waves the streamers are grouped into two classes related to the so called cyclonic lc2 and anticyclonic lc1 patterns and the ambient wind strength and wind shear is also noted it is shown that the occurrence of cyclonic and anticyclonic pv streamers exhibits a distinct spatial variability in the horizontal and the vertical the majority of cyclonic pv streamers are found on lower isentropic levels that intersect the tropopause at more poleward latitudes whereas anticyclonic streamers predominate at higher elevations in the subtropics an analysis of the streamer patterns for the two phases of the north atlantic oscillation

deconstructing terminations i and ii revisiting the glacioeustatic paradigm based on deep water temperature estimates benthic and planktonic oxygen isotope 18occ and mg ca analyses in two cores from the northeast atlantic have permitted the reconstruction of surface and deep water temperature tdw and 18o 18ow variations across the last two deglaciations these records allow the timing of de glacial melt water pulses reaching the northeast atlantic to be compared with the evolution of local deep water tdw 18ow conditions although each glacial termination is unique in detail a similar pattern of hydrographic change is reconstructed for both deglaciations with the first major decrease in deep water 18ow due to sea level and or purely local deep water change occurring in parallel with the onset of intensely cold glacial surface water temperatures and prior to a terminal ice rafting and melt water event the evolution of deep water across both de glaciations involved two transient incursions of cold low 18o water into the deep northeast atlantic the second of which was particularly pronounced each time these pulses of cold deep water are interpreted to reflect the incursion of water directly analogous to modern antarctic bottom water aabw and containing a significant component of brine rejected during sea ice formation the results presented here show that the same type of transient changes in deep water circulation that occurred across termination i also occurred across termination ii and that as a result of these deep ocean changes the timing of each benthic 18o termination cannot precisely reflect the timing of de glacial sea level change as many palaeoceanographic interpretations and some controversies are prone to assume such imprecision in timing especially may well extend to marine isotope stage mis boundaries in general as a principle of hydrographic variability and its expression in the geological record

variability of the western mediterranean sea surface temperature during the last 25 000 years and its connection with the northern hemisphere climatic changes sea surface temperature sst profiles over the last 25 kyr derived from alkenone measurements are studied in four cores from a w e latitudinal transect encompassing the gulf of cadiz atlantic ocean the alboran sea and the southern tyrrhenian sea western mediterranean the results document the sensitivity of the mediterranean region to the short climatic changes of the north atlantic ocean particularly those involving the latitudinal position of the polar front the amplitude of the sst oscillations increases toward the tyrrhenian sea indicating an amplification effect of the atlantic signal by the climatic regime of the mediterranean region all studied cores show a shorter cooling phase 700 years for the younger dryas yd than that observed in the north atlantic region 1200 years this time diachroneity is related to an intra yd climatic change documented in the european continent minor oscillations in the southward displacement of the north atlantic polar front may also have driven this early warming in the studied area during the holocene a regional diachroneity propagating west to east is observed for the sst maxima 11 5 10 2 kyr b p in the gulf of cadiz 10 9 kyr b p in the alboran sea and 8 9 8 4 kyr b p in the thyrrenian sea a general cooling trend from these sst maxima to present day is observed during this stage which is marked by short cooling oscillations with a periodicity of 730 40 years and its harmonics

dynamics of synoptic eddy and low frequency flow interaction part ii a theory for low frequency modes abstract amidst stormy atmospheric circulation there are prominent recurrent patterns of variability in the planetary circulation such as the antarctic oscillation aao arctic oscillation ao or north atlantic oscillation nao and the pacific north america pna pattern the role of the synoptic eddy and low frequency flow self feedback in the formation of these dominant low frequency modes is investigated in this paper using the linear barotropic model with the self feedback proposed in part i it is found that the ao like and aao like leading singular modes of the linear dynamical system emerge from the stormy background flow as the result of a positive self feedback this self feedback also prefers a pna like singular vector as well among other modes under the climatological conditions of northern winters a model with idealized conditions of basic mean flow and activity of synoptic eddy flow and a prototype model are also used to illustrate that there is a natural scale selection for the aao

fourier analysis and the extinction of unionoid bivalves near the cretaceous tertiary boundary of the western interior usa pattern causes and ecological significance abstract the williston basin in the western interior of the usa is known for its highly diverse and well preserved fauna of unionoid bivalves this fauna undergoes a significant turnover near the cretaceous tertiary k t boundary a quantitative study based on multivariate morphometrics has been carried out to better understand the pattern and causes of the extinction of unionoid bivalves in the k t transition the decrease in taxonomic diversity in the interval spanning the k t boundary is associated with a significant shift in morphospace occupation this shift indicates a major decrease in habitat stability four events and processes are discussed as possible causes of this change in habitat stability 1 the chicxulub impact on the yucatan peninsula 2 global climate changes to some extent due to deccan volcanism 3 tectonic changes in the emerging rocky mountains and 4 eustatic changes in the western interior seaway we conclude that the bolide impact was not the major killing agent for unionoid bivalves the tectonic and eustatic processes are much more important to explain the unionoid faunal turnover at the end of the cretaceous the results and conclusions of the study are considered to be helpful to better understand the present day distribution of unionoid bivalves

response of baroclinic life cycles to barotropic shear abstract cyclonic barotropic shear of incrementally increasing magnitude is imposed on an idealized midlatitude jet and the life cycles of baroclinically unstable wavenumber 6 perturbations growing on these jets are studied when the barotropic shear parameter passes a critical value the life cycle makes an abrupt transition from anticyclonic to cyclonic behavior the abrupt transition in behavior is most evident in the barotropic decay of eddy kinetic energy in the structure of the eddies as seen in potential vorticity maps and in the nature of the zonal flow accelerations produced during the life cycle it is suggested that the abrupt transition can be interpreted as arising from a positive feedback between eddy propagation and eddy induced zonal flow accelerations dependences on the zonal scale of the eddy are also investigated wavenumber 8 exhibits cyclonic behavior while wavenumber 4 exhibits anticyclonic behavior for all values of cyclonic shear considered this dependence on zonal scale is c

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

southern annular modes simulated by a climate model patterns mechanisms and uses abstract both high latitude hlm and low latitude modes llm of variability of zonal wind in the southern hemisphere have been identified through an analysis of a simulation for 1871 2200 by the csiro mark 3 climate model the extent to which these might both be described as annular modes based on their statistical patterns physical mechanisms and usefulness in climate study is assessed the modes are determined as eof1 and eof2 of vertically integrated zonal and monthly mean zonal wind for 1871 1970 these match well those from ecmwf re analysis era data and also from the earlier mark 2 model the mode index time series relate to largely annular patterns of local wind and surface pressure anomalies with hlm giving the familiar southern annular mode sam and other simulated quantities while modes calculated from 90 sectors are only moderately correlated mostly in the polar region for hlm the link increases with time scale there is little such relationship for llm a momentum equation

dynamics of synoptic eddy and low frequency flow interaction part i a linear closure abstract the interaction between synoptic eddy and low frequency flow self has been recognized for decades to play an important role in the dynamics of the low frequency variability of the atmospheric circulation in this three part study a linear framework with a stochastic basic flow capturing both the climatological mean flow and climatological measures of the synoptic eddy flow is proposed based on this linear framework a set of linear dynamic equations is derived for the ensemble mean eddy forcing that is generated by anomalous time mean flows by assuming that such dynamically determined eddy forcing anomalies approximately represent the time mean anomalies of the synoptic eddy forcing and by using a quasi equilibrium approximation an analytical nonlocal dynamical closure is obtained for the two way self feedback this linear closure directly relating time mean anomalies of the synoptic eddy forcing to the anomalous time mean flow becomes an internal part of a new linear dynamic system for an

antarctic oscillation signal on precipitation anomalies over southeastern south america 1 the relation between the antarctic oscillation aao and the precipitation in southeastern south america sesa is examined the aao influence is particularly strong during winter and late spring although of opposite sign in particular during spring aao positive negative phases are associated with the intensification of an upper level anticyclonic cyclonic anomaly weakened enhanced moisture convergence and decreased increased precipitation over sesa the combined influence of both enso and aao on sesa precipitation was also explored and significant correlation values between both oscillations are only observed during spring results show that during that particular season aao activity produces a strong modulation of the enso signal on sesa precipitation

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

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

size all it s shaped up to be evolution of shape through the lifespan of the cenozoic bivalve spissatella crassatellidae

effect of latitudinal variations in low level baroclinicity on eddy life cycles and upper tropospheric wave breaking processes abstract an analysis of the potential vorticity gradient and the refractive index in quasigeostrophic qg flows on the sphere reveals that the absolute vorticity and the stretching parts have two contradictory effects on the horizontal shape of the baroclinic waves when the full variations of the coriolis parameter are taken into account in each term the absolute vorticity effect favors the anticyclonic southwest northeast tilt and anticyclonic wave breaking awb and is stronger in the upper troposphere in contrast the stretching effect promotes the cyclonic northwest southeast tilt and cyclonic wave breaking cwb and is more efficient at lower levels a positive eddy feedback acting on the latitudinal variations of the zonal winds is deduced because the absolute vorticity and the stretching effects are respectively more and less efficient with increasing latitude a more northward southward jet renders awb more less probable and cwb less more probable the jet is pushed or maintained mor

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

opposing effects of reflective and nonreflective planetary wave breaking on the nao planetary wave breaking pwb over the subtropical north atlantic is observed over 45 winters december 1958 arch 2003 using ncep ncar reanalysis data pwb is manifested in the rapid large scale and irreversible overturning of potential vorticity pv contours on isentropic surfaces in the subtropical upper troposphere as breaking occurs over the subtropical north atlantic an upper tropospheric pv tripole anomaly forms with nodes over the subtropical midlatitude and subpolar north atlantic the northern two nodes of this tripole are quite similar to the spatial structure of the north atlantic oscillation nao with positive polarity nonlinear reflection is identified in approximately a quarter of all pwb events following breaking two distinct circulation regimes arise one in response to reflective events and the other in response to nonreflective events for reflective events anomalies over the north atlantic rapidly propagate away from the breaking region along a poleward arching wave train over the eurasian continent the quasi stationary wave activity flux indicates that wave activity is exported out of the atlantic basin at the same time the regional poleward eddy momentum flux goes through a sign reversal as does the polarity of the nao for nonreflective events the dipole anomaly over the north atlantic amplifies diagnostics for nonreflective events suggest that wave activity over the azores gets absorbed allowing continued enhancement of both the regional poleward eddy momentum flux and the positive nao

diabatic damping of zonal index variations abstractzonal index variations or north south shifts of the midlatitude jet are the dominant mode of zonal wind variability in the southern hemisphere previous studies have shown that synoptic time scale eddy momentum flux provides a positive feedback and acts to increase the persistence and low frequency variance of the zonal index however the impact of diabatic heating due to the precipitation associated with these eddies has not been investigated in this study regression analyses have been conducted to demonstrate that a robust precipitation anomaly can be found to accompany the jet and eddy momentum flux anomalies associated with a poleward shift of the jet with enhanced precipitation on the poleward flank of the jet and reduced precipitation on the equatorward flank diabatic heating associated with such a precipitation anomaly is expected to reduce the temperature gradient across the jet anomaly thus decreasing eddy generation and damping the anomaly this expectation is confirmed by three

baroclinic wave breaking and the internal variability of the tropospheric circulation abstract a simple model of the tropospheric circulation based on a 10 level primitive equation model is forced by linearly relaxing the potential temperature toward an idealized zonally symmetric equilibrium field the model equations are integrated in time until a statistically steady state is obtained the local relationship between the state of the background flow the direction of wave propagation and subsequent wave breaking at the tropopause level is then investigated maps of potential vorticity pv on isentropic surfaces are analyzed and all four different types of wave breaking described recently by peters and waugh are shown to occur it is found that cyclonic wave breaking events are usually initiated by poleward fluxes of wave activity and anticyclonic events by equatorward fluxes composites are then used to show that equatorward fluxes are associated with a jet that is locally broad and weak with relatively strong isentropic pv gradients to its equatorward flank by contrast poleward

eddy zonal flow feedback in the northern hemisphere winter abstract the variability of the zonal mean zonal wind in the northern hemisphere winter december march is studied using eof analysis and momentum budget diagnostics of ncep ncar reanalysis data 1976 2001 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 westerlies analysis of the momentum budget shows that a positive feedback between the zonal mean wind anomalies and the eddy momentum fluxes selects the leading eof of midlatitude variability like the southern hemisphere the baroclinic eddies reinforce the zonal wind anomalies while external rossby waves damp the wind anomalies in the northern hemisphere the quasi stationary eddies also reinforce the zonal wind anomalies but the baroclinic eddies are most important for the positive eddy zonal flow feedback the observations support the following feedback mechanisms 1 above normal baroclinic wave activity is generated in the region of enhanced wester

a pv view of zonal flow vacillation abstract it is suggested here that the zonal flow vacillation of the southern hemisphere is caused by a mutual interaction between the barotropic shear of the zonal flow and the evolution of baroclinic eddies during the later stages of their lifecycle an index of the zonal wind anomaly difference between 40 and 60 s is defined for a 13 yr record of european centre for medium range weather forecasts ecmwf analyses periods are selected during which the zonal flow is at the extremes of its characteristic vacillation which are distinguished by a broad westerly jet with minor maxima near 30 and 60 s and a narrower jet peaked near 40 s the narrower 40 s jet has stronger cyclonic shear across the baroclinic zone these changes are accompanied by shifts in poleward eddy momentum flux on the 500 hpa surface and potential vorticity fluxes on the theta surfaces that lie in the midtroposphere which are both consistent with the maintenance of the zonal wind anomalies against friction series of synoptic maps