annular modes and apparent eddy feedbacks in the southern hemisphere lagged correlation analysis is often used to infer intraseasonal dynamical effects but is known to be affected by nonstationarity we highlight a pronounced quasi 2 year peak in the anomalous zonal wind and eddy momentum flux convergence power spectra in the southern hemisphere which is prima facie evidence for nonstationarity we then investigate the consequences of this nonstationarity for the southern annular mode and for eddy momentum flux convergence we argue that positive lagged correlations previously attributed to the existence of an eddy feedback are more plausibly attributed to nonstationary interannual variability external to any potential feedback process in the midlatitude troposphere the findings have implications for the diagnosis of feedbacks in both models and reanalysis data as well as for understanding the mechanisms underlying variations in the zonal wind

formation of two dimensional circulation in response to unsteady wave forcing in the middle atmosphere abstractin previous studies a steady state assumption has been frequently used for the analysis of wave induced meridional circulation in general however the wave forcing is not constant and thus induced circulation can vary in time thus to understand such transient behaviors time evolutions of a slow variable describing balanced flows and two fast variables describing gravity waves and flows that are slaved to balanced flows are investigated a boussinesq equation system is used to examine zonal mean flow responses to unsteady zonally uniform forcing green s function is used to analytically obtain the evolution of meridional circulation responses to zonal wave forcing are mainly examined although responses to a diabatic heating and to wave forcing are discussed in brief for forcing with a step function shape in time gravity waves are radiated as a transient response the time needed to form the circulation depends on the aspect ratio i e latitudinal to vertical lengths of wave forcing whi

meridional and downward propagation of atmospheric circulation anomalies part ii southern hemisphere cold season variability abstract as in the northern hemisphere there exists a simultaneous poleward propagation of temperature anomalies in the stratosphere and equatorward propagation in the troposphere in the southern hemisphere s cold season it takes about 110 days for anomalies of one polarity to propagate from the equator to the pole or half the period of the complete cycle nearly twice as long as in the northern hemisphere the earlier poleward propagation of temperature anomalies in upper levels compared with those in lower levels results in an apparent downward propagation in the stratosphere accompanying the poleward and downward propagating warm cold anomalies is a successive leveling steepening of isentropic surfaces reflecting a simultaneous reduction strengthening of the meridional temperature gradient and increase decrease of the vertical static stability following changes in the thermal fields are poleward and downward propagating zonal wind anomalies of the opposite sign the arrival of the pole

influence of antarctic ice sheet lowering on the southern hemisphere climate modeling experiments mimicking the mid miocene a coupled global atmosphere ocean model is used to study the influence of the antarctica ice sheet in a configuration that mimics that of the early miocene on the atmospheric and oceanic circulations based on different climate simulations of the present day ctr and conducted with distinct antarctic ice sheet topography ais exp it is found that the reduction of the antarctic ice sheet topography ais induces warming of the southern hemisphere and reduces the meridional thermal gradient consequently the atmospheric transient low level eddy heat flux overline v prime t prime and the eddy momentum flux overline u prime v prime are reduced causing the reduced transport of heat from the mid latitudes to the pole the stationary flow and transient wave anomalies generate changes in the ssts which modify the rate of deep water formation strengthening the formation of the antarctic bottom water substantial changes are predicted to occur in the atmospheric and oceanic heat transport and a comparison between the total heat transport of the atmosphere ocean system as simulated by the ais exp and the ctr runs shows that the reduction of the ais height leads to reduced southern hemisphere poleward and increased equatorward heat transport these results are in agreement with reduced storm track activities and baroclinicity

zonal mean dynamics of extended recoveries from stratospheric sudden warmings abstractthe recovery of the arctic polar vortex following stratospheric sudden warmings is found to take upward of 3 months in a particular subset of cases termed here polar night jet oscillation pjo events the anomalous zonal mean circulation above the pole during this recovery is characterized by a persistently warm lower stratosphere and above this a cold midstratosphere and anomalously high stratopause which descends as the event unfolds composites of these events in the canadian middle atmosphere model show the persistence of the lower stratospheric anomaly is a result of strongly suppressed wave driving and weak radiative cooling at these heights the upper stratospheric and lower mesospheric anomalies are driven immediately following the warming by anomalous planetary scale eddies following which anomalous parameterized nonorographic and orographic gravity waves play an important role these details are found to be robust for pjo events as opposed to sudden warmings in general in that ma

the non geometric p s wave in high resolution seismic data observations and modelling summary in data from a high resolution seismic survey conducted over a near surface environment consisting of homogeneous soft clay we consistently observe a distinct seismic phase arriving between the direct compressional wave and the rayleigh wave this phase is characterized by high amplitudes at near offsets and a phase velocity corresponding to about twice the shear wave velocity based on analytical and numerical analyses this signal could be unambiguously identified as a non geometric wave which is excited if the source is located near the earth s surface and the poisson s ratio in the vicinity of the source is unusually high to date there are only a few speculative observations of this particular non geometric seismic wave phenomenon however given the commonly very high poisson s ratio of surficial materials we expect this phase to be present albeit unidentified in many near surface seismic surveys the presence of this non geometric wave increases the complexity of the seismic record and failure to identify it may result in misinterpretations particularly of high resolution seismic reflection data as well as of shear wave and surface wave data potential applications of this seismic phase may arise from its high sensitivity to the shear wave velocity in the immediate source region

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

ice shelf melt response to changing winds and glacier dynamics in the amundsen sea sector antarctica it has been suggested that the coastal southern ocean subsurface may warm over the 21st century in response to strengthening and poleward shifting winds with potential adverse effects on west antarctic glaciers however using a 1 128 ocean regional model that includes ice shelf cavities we find a more complex response to changing winds in the amundsen sea simulated offshore subsurface waters get colder under strengthened and poleward shifted winds representative of the sam projected trend the buoyancy driven circulation induced by ice shelf melt transports this cold offshore anomaly onto the continental shelf leading to cooling and decreased melt below 450 m in the vicinity of ice shelf fronts ekman pumping contributes to raise the isotherms in response to changing winds this effect overwhelms the horizontal transport of colder offshore waters at intermediate depths between 200 and 450 m and therefore increases melt rates in the upper part of the ice shelf cavities which reinforces the buoyancydriven circulation and further contributes to raise the isotherms then prescribing an extreme grounding line retreat projected for 2100 the total melt rates simulated underneath thwaites and pine island are multiplied by 2 5 such increase is explained by a larger ocean ice interface exposed to cdw which is then amplified by a stronger melt induced circulation along the ice draft our main conclusions are that 1 outputs from ocean models that do not represent ice shelf cavities e g cmip5 models should not be directly used to predict the thermal forcing of future ice shelf cavities 2 coupled ocean ice sheet models with a velocity dependent melt formulation are needed for future projections of glaciers experiencing a significant grounding line retreat

the importance of atmospheric dynamics in the northern hemisphere wintertime climate response to changes in the earth s orbit milankovitch proposed that variations in the earth s orbit cause climate variability through a local thermodynamic response to changes in insolation this hypothesis is tested by examining variability in an atmospheric general circulation model coupled to an ocean mixed layer model subjected to the orbital forcing of the past 165 000 yr during northern hemisphere summer the model s response conforms to milankovitch s hypothesis with high low insolation generating warm cold temperatures throughout the hemisphere however during northern hemisphere winter the climate variations stemming from orbital forcing cannot be solely understood as a local thermodynamic response to radiation anomalies instead orbital forcing perturbs the atmospheric circulation in a pattern bearing a striking resemblance to the northern annular mode the primary mode of simulated and observed unforced atmospheric variability the hypothesized reason for this similarity is that the circulation response to orbital forcing reflects the same dynamics generating unforced variability these circulation anomalies are in turn responsible for significant fluctuations in other climate variables most of the simulated orbital signatures in wintertime surface air temperature over midlatitude continents are directly traceable not to local radiative forcing but to orbital excitation of the northern annular mode this has paleoclimate implications during the point of the model integration corresponding to the last interglacial eemian period the orbital excitation of this mode generates a 1 2 c warm surface air temperature anomaly over europe providing an explanation for the warm anomaly of comparable magnitude implied by the paleoclimate proxy record the results imply that interpretations of the paleoclimate record must account for changes in surface temperature driven not only by changes in insolation but also by perturbations in atmospheric dynamics

cross seasonal influence of the december february southern hemisphere annular mode on march may meridional circulation and precipitation abstractnew evidence suggests that interannual variability in zonal mean meridional circulation and precipitation can be partially attributed to the southern hemisphere annular mode sam the dominant mode of climate variability in the southern hemisphere sh extratropics a cross seasonal correlation exists between the december february djf sam and march may mam zonal mean meridional circulation and precipitation this correlation is not confined to the sh it also extends to the northern hemisphere nh subtropics when the preceding djf sam is positive counterclockwise and clockwise meridional cells accompanied by less and more precipitation occur alternately between the sh middle latitudes and nh subtropics in mam in particular less precipitation occurs in the sh middle latitudes the sh tropics and the nh subtropics but more precipitation occurs in the sh subtropics and the nh tropics a framework is built to explain the cross seasonal impact of sam related sst anomalies evidence indic

mechanisms of zonal index variability in an aquaplanet gcm abstract data from a 2025 day geophysical fluid dynamics laboratory aquaplanet gcm integration are used to examine the temporal evolution of the zonal index defined as the principal component of the first eof of the zonally and vertically integrated absolute angular momentum this eof represents meridional displacements of the subtropical jet positive negative values of the zonal index correspond to poleward equatorward displacements of the subtropical jet and are referred to as the high low zonal index composites of various quantities are used to examine the temporal evolution of both the zonal mean zonal winds and the eddy fields associated with either index the high index is initiated by an enhancement in the equatorward wave activity propagation which causes the subtropical jet to move in a poleward direction the low index is led by a weakening in the equatorward propagation which results in an equatorward displacement of the subtropical jet more importantly for the high index the corr

two paradigms of baroclinic wave life cycle behaviour two idealized baroclinic wave 6 life cycles examined here suggest a framework of opposite extremes a in which to view the behaviour of real synoptic scale disturbances in middle latitudes and b in which to interrelate the synoptic and wave theoretic viewpoints using the saturation propagation saturation sps picture of wave 6 life cycle behaviour the two life cycles dented by lc1 and lc2 are higher resolution versions of the simmons hoskins basic and anomalous cases showing strong and weak late decay of eddy kinetic energy eke they illustrate in varying degrees two extreme types of behaviour here disignated anticyclonic and cyclonic and epitomized by strongly contrasting upper air trough behaviour anticyclonic behaviour dominates the late stages of lc1 and is characterized by backward tilted thinning troughs being advected anticyclonically and equatorward as in the commoner cases of planetary scale mid stratospheric rossby wave breaking cyclonic behaviour dominates lc2 and is characterized by forward tilted broadening troughs wrapping themselves up cyclonically and poleward producing major cut off cyclones in high latitudes these morphologies are visualized by upper air maps of potential temperature on the nominal tropopause defined as a constant potential vorticity surface some atmospheric mid latitude disturbances examined here using the same visualization applied to operational analyses show the same two extreme types of trough behaviour together with intermediate cases the sps picture is re examined using eliassen palm and refractive index cross sections it is shown in particular by reference to a wave activity theorem of haynes that the late stages of lc2 can be looked upon as a remarkably clear and morphologically novel large amplitude counterpart of the nonlinear reflection scenario of rossby wave critical layer theory the late stages of lc1 by constrast look more akin to an nonlinear critical layer absorption scenario lc2 exhibits region of largely undular pv contours adjacent to a nonlinear of irreversiby deformen pv contous in the latter region pv rearrangement and hence absorption of rossby wave activity has largely ceased this accounts for the more persistent ekee in the lc2 case

observed relationships between the el nino southern oscillation and the extratropical zonal mean circulation abstract there is increasing evidence indicating that the climate response to variations in the el nino southern oscillation enso includes not only thermally forced zonal wind anomalies in the subtropics but also eddy driven zonal wind anomalies that extend into the mid high latitudes of both hemispheres in this study new insights into the observed seasonally varying signature of enso in the extratropical zonal mean circulation are provided and the associated linkages with the dominant patterns of extratropical variability are examined the zonal mean extratropical atmospheric response to enso is characterized by two principal features an equivalent barotropic dipole in the southern hemisphere sh zonal mean zonal flow with centers of action located near 40 and 60 during austral summer and a weaker but analogous dipole in the northern hemisphere nh with centers of action located near 25 and 45 during early and late boreal winter both structures are accompanied by eddy momentum flux ano

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

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

observed signatures of the barotropic and baroclinic annular modes in cloud vertical structure and cloud radiative effects abstractthe signatures of large scale annular variability on the vertical structure of clouds and cloud radiative effects are examined in vertically resolved cloudsat and other satellite and reanalysis data products the northern and southern barotropic annular modes the nam and sam have a complex vertical structure both are associated with a meridional dipole in clouds between subpolar and middle latitudes but the sign of the anomalies changes between upper middle and lower tropospheric levels in contrast the northern and southern baroclinic annular modes have a much simpler vertical structure both are linked to same signed anomalies in clouds extending throughout the troposphere at middle to high latitudes the changes in cloud incidence associated with both the barotropic and baroclinic annular modes are consistent with dynamical forcing by the attendant changes in static stability and or vertical motion the results also provide the first observational estimates of the vertically resolved a

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

a strengthened impact of november arctic oscillation on subsequent tropical pacific sea surface temperature variation since the late 1970s the connection between november arctic oscillation ao and the following spring summer tropical pacific sea surface temperature sst experienced a pronounced enhancement around the late 1970s the connection was weak before but strong after the late 1970s the present study investigates the plausible reasons for the connection change after the late 1970s significant anomalous cyclone appears over subtropical north pacific and pronounced anomalous westerly winds occur over tropical western pacific in the positive phase of november ao as such november ao could exert a significant influence on the subsequent spring summer tropical pacific sst variation by contrast before the late 1970s anomalous cyclone and associated anomalous westerly winds to its south are weak and shift northward thereby the connection between november ao and tropical pacific sst is weak further analyses suggest that the interdecadal change in the anomalous cyclone over the subtropical north pacific may be related to the change in the november ao s pacific component and the strength of feedback of synoptic scale eddy to mean flow specifically after the late 1970s november ao s pacific component displays a zonally elongated structure which is favorable for the formation of easterly wind anomalies over the mid latitude north pacific and related wave mean flow interaction in addition feedback of synoptic eddy to mean flow is enhanced after the late 1970s which is partly related to the intensification of the north pacific storm track this may also contribute to enhancement of the anomalous cyclone over the subtropical pacific

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

variability of sea ice in the northern weddell sea during the 20th century the record of winter fast ice in the south orkney islands northern weddell sea antarctica is over a century long and provides the longest observational record of sea ice variability in the southern hemisphere here we present analyses of the series of fast ice formation and breakout dates from 1903 to 2008 we show that over the satellite era post 1979 the timing of both final autumn formation and complete spring breakout of fast ice is representative of the regional sea ice concentrations sic in the northern weddell sea and associated with atmospheric conditions in the amundsen sea region to the west of the antarctic peninsula variation in the fast ice breakout date is influenced by the intensity of the westerly north westerly winds associated with the southern annular mode sam in contrast the date of ice formation displays correlations with regional oceanic and sea ice conditions over the previous 18 months which indicate a preconditioning during the previous summer and winter and exhibits variability associated with variation in tropical pacific sea surface temperature i e the el nino sothern oscillation enso a reduction in fast ice duration at the south orkney islands around the 1950s was associated with both later formation and earlier breakout however there were marked changes in variability with periodicities of 3 5 year 7 9 year and 20 year in each of the series and in their relationships with enso and sam indicating the need for caution in interpreting changes in ice conditions based on shorter term satellite series

downward migration of extratropical zonal wind anomalies 1 we show in confirmation of previous work using one and three dimensional models that extratropical zonal wind anomalies produced by fluctuating rossby wave forcing in the troposphere appear first in the stratosphere and migrate downward into the troposphere by systematically eliminating wave reflection and downward control through an induced meridional circulation it is shown that the downward migration is dependent on neither process rather the mechanism appears to rely on local wave mean flow interaction just as in the similar downward migration evident in the tropical quasi biennial oscillation in particular these results imply that the similar downward migration observed in the arctic oscillation should not be taken to indicate any controlling influence of the stratosphere on the troposphere

propagation of planetary waves between the troposphere and stratosphere abstract the propagation of planetary waves between the troposphere and stratosphere is investigated using a linear time dependent primitive equation model it is found that the tropopause acts like a valve for the propagation of planetary waves the key parameters controlling the valve are the vertical gradient of buoyancy frequency and the vertical shear of the zonal winds at the tropopause for a given wind profile smaller gradient of buoyancy frequency enhances the propagation of planetary waves for a given profile of buoyancy frequency the larger the shear the more the wave activity is trapped in the troposphere and therefore there is less left to propagate into the stratosphere the transmission across the tropopause is however not sensitive to zonal winds in the upper stratosphere the propagation of planetary waves is very sensitive to transience more transient waves propagate more vertically within the troposphere and for more transient waves more wave activity is transferred into the

boreal winter links between the madden julian oscillation and the arctic oscillation abstract there is increasing evidence that the madden julian oscillation mjo modifies the mid to high latitude circulation and in particular appears to have a relationship to the leading mode of extratropical variability the arctic oscillation ao in this study new insights into the observed similarities between the mjo and the ao are explored it is shown that the eastward progression of the convectively active phase of the mjo is associated with a corresponding shift in the tendency and sign of the ao index moreover the ao and the mjo share several analogous features not only in the global circulation but also in surface temperature fields also the ao is linked to a pattern of eastward propagating mjo like variability in the tropics that is partially reproduced in free runs of the ncep climate forecast system cfs model finally it is shown that the structure of the ao as defined by the leading mode in the 1000 hpa geopotential height field is significantly altered based on the phase o

understanding anomalous synoptic eddy vorticity forcing in pacific north american teleconnection pattern events abstractutilizing a decomposition of anomalous eddy vorticity forcing evf proposed by song in 2016 and a modified geophysical fluid dynamics laboratory gfdl dynamical core atmospheric model th

meridional circulation and the surface pressure change associated with the southern annular mode comparison with the arctic oscillation 1 eddy forced meridional circulation and corresponding surface pressure change associated with the month to month variability of the southern annular mode sam are examined in the framework of the eulerian mean dynamics and they are compared with those of the arctic oscillation ao effect of wave forcings on the surface pressure was diagnosed by means of a zonal mean quasi geostrophic model on the sphere it is found that the surface pressure change associated with the sam is mainly produced by the meridional circulation driven by mechanical and thermal eddy forcings of zonal wave number 1 and mechanical forcing by high frequency transient eddies this contrasts with the surface pressure change associated with the ao which is produced by the mechanical forcing by zonal wave number 2 or 3 and high frequency transient eddies close relationship between the meridional circulation and surface pressure was found not only for the month to month variability but also for the decadal variability as also observed for the ao

role of medium scale waves on the southern annular mode 1 we examined the role of short period waves on the southern annular mode sam through analysis of 6 hourly reanalysis data of the european centre for medium range weather forecasts ecmwf era interim we focused on medium scale waves those with periods shorter than about 6 days although medium scale waves generally have much smaller amplitudes than synoptic waves they have a much higher correlation with the sam index as a result about one third of wave driven meridional circulation and zonal wind acceleration associated with the sam is generated by medium scale waves their contribution is large in spite of their climatologically smaller amplitude moreover two thirds of sam related meridional circulation and zonal wind acceleration comes from waves of periods shorter than about 3 days our analysis demonstrates a positive feedback between westerly winds around 60 s and amplification of synoptic and medium scale waves which sustains hemispheric sam variability for the feedback process between zonal wind and waves baroclinic conversion is a key process to both synoptic waves and medium scale waves our analysis also shows that wave forcings calculated from daily data explain only half of the total forcings and that the contribution from short period waves is very significant around 50 s

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

barotropic and baroclinic annular variability in the southern hemisphere abstractthe leading patterns of large scale climate variability in the southern hemisphere are examined in the context of extratropical kinetic energy it is argued that variability in the southern hemisphere extratropical flow can be viewed in the context of two distinct and largely independent structures both of which exhibit a high degree of annularity 1 a barotropic structure that dominates the variance in the zonal mean kinetic energy and 2 a baroclinic structure that dominates the variance in the eddy kinetic energy the former structure corresponds to the southern annular mode sam and has been extensively examined in the literature the latter structure emerges as the leading principal component time series of eddy kinetic energy and has received seemingly little attention in previous work the two structures play very different roles in cycling energy through the extratropical troposphere the sam is associated primarily with variability in the meridional propagation of wave activity has a s

suppression of baroclinic instability in horizontally sheared flows abstract baroclinic instability in the presence of a meridionally sheared barotropic component to the basic flow is studied using a two level model the inclusion of a linear shear results in merldionally confined normal modes with growth rates much reduced compared to the unsheared case similar results hold for more complicated situations such as a baroclinic jet with barotropic shear the results are applied to life cycle calculations in which it is shown that the nonlinear decay phase of a baroclinic disturbance sets up barotropic shears whichtend to suppress further baroclinic developments an example of the southern hemisphere winter mean flow is found to be strongly stabilized by virtue of a strong sheared barotropic part of the zonal flow

variability of the polar night jet in the northern and southern hemispheres the spatial and temporal characteristics of the month to month variability of the polar night jet and its relationship with tropospheric circulation is investigated for both the northern and southern hemispheres the variabilities of the hemispheres have many common characteristics of the polar night jet oscillation pjo these common characteristics include the following 1 the anomalous zonal mean zonal winds shift poleward and downward 2 the anomalous polar temperatures propagate downward from the stratopause to the upper troposphere and 3 they are made through a wave mean flow interaction with mainly the planetary wave of zonal wave number one annular modes associated with the pjos appear in both hemispheres when the zones of maximum polar temperature anomaly descend to the lowermost stratosphere and upper troposphere the major difference in the pjos of the two hemispheres is found in their temporal characteristics in the southern hemisphere the phase of the pjo is closely locked to the annual cycle while in the northern hemisphere it exhibits quasiperiodic variability with its envelope controlled by the annual cycle the origin of the differences between the pjos is discussed based on the theory of the wave mean flow interaction

coupled ocean atmosphere ice response to variations in the southern annular mode abstract the coupled ocean atmosphere ice response to variations in the southern annular mode sam is examined in the national center for atmospheric research ncar community coupled climate model version 2 the model shows considerable skill in capturing the predominantly zonally symmetric sam while regional deviations between model and observation sam winds go a long way in explaining the generally small differences between simulated and observed sam responses in the ocean and sea ice systems vacillations in the position and strength of the circumpolar winds and the ensuing variations in advection of heat and moisture result in a dynamic and thermodynamic forcing of the ocean and sea ice both meridional and zonal components of ocean circulation are modified through ekman transport which in turn leads to anomalous surface convergences and divergences that strongly affect the meridional overturning circulation and potentially the pathways of intermediate water ventilation a heat budget analysis de

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

statistical characterization of arctic polar night jet oscillation events abstracta novel diagnostic tool is presented based on polar cap temperature anomalies for visualizing daily variability of the arctic stratospheric polar vortex over multiple decades this visualization illustrates the ubiquity of extended time scale recoveries from stratospheric sudden warmings termed here polar night jet oscillation pjo events these are characterized by an anomalously warm polar lower stratosphere that persists for several months following the initial warming a cold anomaly forms in the middle stratosphere as does an anomalously high stratopause both of which descend while the lower stratospheric anomaly persists these events are characterized in four datasets microwave limb sounder mls temperature observations the 40 yr ecmwf re analysis era 40 and modern era retrospective analysis for research and applications merra reanalyses and an ensemble of three 150 yr simulations from the canadian middle atmosphere model the statistics of pjo events in the model are found t

sensitivity of the brewer dobson circulation and polar vortex variability to parameterized nonorographic gravity wave drag in a high resolution atmospheric model abstractthe role of parametrized nonorographic gravity wave drag nogwd and its seasonal interaction with the resolved wave drag in the stratosphere has been extensively studied in low resolution coarser than 1 9 2 5 climate models but is comparatively unexplored in higher resolution models using the european centre for medium range weather forecasts integrated forecast system at 0 7 0 7 resolution the wave drivers of the brewer dobson circulation are diagnosed and the circulation sensitivity to the nogw launch flux is explored nogws are found to account for nearly 20 of the lowerstratospheric southern hemisphere sh polar cap downwelling and for less than 10 of the lower stratospheric tropical upwelling and northern hemisphere nh polar cap downwelling despite these relatively small numbers there are complex interactions between nogwd and resolved wave drag in both polar regions seasonal cycle analysis reveals a temporal offset in the resolved and parametrized wave interaction the n

fine scale structure of the extratropical tropopause region 1 a vertically high resolved climatology of the thermal and wind structure of the extratropical tropopause region is presented the climatology is based on data from 80 u s radiosonde stations covering the period 1998 2002 time averages for each radiosonde station are computed using the tropopause as a common reference level for all vertical profiles within the mean a strong inversion at the tropopause in the mean vertical temperature gradient is uncovered that is temperature strongly increases with altitude within the lowermost stratosphere this tropopause inversion layer exists on average throughout the investigated extratropics about 30 n to 70 n accordingly the static stability parameter shows considerably enhanced values within the lowermost extratropical stratosphere compared to typical extratropical stratospheric values further aloft conventional averages are not able to capture the tropopause inversion layer mean profiles of the horizontal wind show behavior qualitatively corresponding to thermal wind balance winter and summer exhibit distinctly different climate states in the extratropical tropopause region an approximated potential vorticity is considered and found to be close to well mixed within the troposphere as well as within the tropopause inversion layer this suggests the view of the tropopause inversion layer as representing a dynamically active atmospheric layer some potential implications are discussed

dynamical feedbacks and the persistence of the nao the persistence of the north atlantic oscillation nao is studied using observations of the threedimensional vorticity budget in the atlantic sector analysis of the relative vorticity tendency equation shows that convergence of eddy vorticity flux in the upper troposphere counteracts the effect of anomalous largescale divergence at the upper level at low levels the convergence associated with this large scale vertical circulation cell maintains the relative vorticity anomaly against frictional drag the eddy vorticity flux convergence thus acts to sustain the vorticity anomaly associated with the nao against drag and increases the persistence of the nao vorticity anomaly the adiabatic cooling associated with the rising motion in the vorticity maximum sustains the thermal structure of the nao anomaly enhancing the baroclinicity and thus eddy generation this constitutes a positive eddy feedback that helps maintain the nao the positive eddy feedback occurs only in the midlatitude region and is strongest during the negative phase of the nao when the atlantic jet is displaced toward the equator with a high pressure anomaly to the north and a low pressure anomaly to the south the stronger feedback demonstrated during the negative phase is consistent with the greater persistence observed for this phase of the nao the positive feedback appears to be associated with anomalous northward eddy propagation away from the jet

seasonal to interannual response of southern ocean mixed layer depth to the southern annular mode from a global 1 10 ocean model abstractthe relationship between the southern annular mode sam and southern ocean mixed layer depth mld is investigated using a global 0 1 resolution ocean model the sam index is defined as t

the palette of fronts and cyclones within a baroclinic wave development abstract a study is undertaken of the concurrent development of fronts and cyclones within a growing baroclinic wave the dynamical framework for the study is that of the semigeostrophic adiabatic flow of a uniform potential vorticity atmosphere that is sandwiched between two horizontal plates on an f plane a three dimensional spectral model is used to examine the growth to finite amplitude of the normal modes of this system for basic states that comprise of a symmetric jetlike baroclinic flow with and without a superposition of a uniform barotropic shear of either cyclonic or anticyclonic vorticity it is demonstrated that the presence of such lateral shear in the ambient environment exerts a profound influence upon the ensuing palette of fronts and cyclones on the synoptic scale this flow component influences the scale and strength of the individual surface highs and lows and also modifies the extent of their lateral drift away from and their relative movement along the baroclinic zone aloft t

the pacific center of action of the northern hemisphere annular mode real or artifact the leading empirical orthogonal function eof of the sea level pressure slp field referred to as the arctic oscillation ao or northern hemisphere annular mode nam consists of a dipole between the polar cap region and the surrounding zonal ring centered along 458n embedded within the outer ring are centers of action over the euro atlantic and pacific sectors in which slp fluctuates in phase that the observed slp fluctuations at these two centers of action are virtually uncorrelated raises the question of whether the pacific center in the annular mode could be an artifact of eof analysis it is argued that sea level pressure fluctuations at the pacific and euro atlantic centers of action of the ao nam would be more strongly correlated were it not for the fact that slp variability over the north pacific is dominated by a pattern in which fluctuations over the north atlantic and north pacific are inversely related evidence of the coexistence of such a pattern which resembles an augmented version of the pacific north american pattern is presented

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

converted waves in a shallow marine environment experimental and modeling studies seismic waves converted from compressional to shear mode in the shallow subsurface can be useful not only for obtaining shear wave velocity information but also for improved processing of deeper reflection data these waves generated at deep seas have been used successfully in hydrocarbon exploration however acquisition of good quality converted wave data in shallow marine environments remains challenging we have looked into this problem through field experiments and synthetic modeling a high resolution seismic survey was conducted in a shallow water canal using different types of seismic sources data were recorded with a four component water bottom cable observed events in the field data were validated through modeling studies compressional waves converted to shear waves at the water bot tom and at shallow reflectors were identified the shear waves showed distinct linear polarization in the horizontal plane and low velocities in the marine sediments modeling results indicated the presence of a nongeometric shear wave arrival excited only when the dominant wavelength exceeded the height of the source with respect to the water sediment interface as observed in air gun data this type of shear wave has a traveltime that corresponds to the raypath originating not at the source but at the interface directly below the source thus these shear waves excited by the source water bottom coupled system kinematically behave as if they were generated by an s wave source placed at the water bottom in a shallow water environment the condition appears to be favorable for exciting such shear waves with nongeometric arrivals these waves can provide useful information of shear wave velocity in the sediments

atmospheric teleconnections involving the southern ocean 1 i review the characteristic patterns of low frequency interannual and subdecadal variation in atmospheric circulation over the southern hemisphere sh extratropics and their climatic associations e g temperature precipitation winds sea surface temperatures ssts and sea ice conditions for the southern ocean the considerable spatial and temporal limitations of the observational database have strongly conditioned the pace of our understanding of sh teleconnections particularly for the pacific sector where the tropical el nino southern oscillation enso exerts a dominant influence an enso signal appears in the intensity interannual variations of the amundsen sea mean low pressure asl jet streams and long waves these variations comprise the so called pacific south america psa teleconnection patterns of low frequency variability in sh circulation that are mostly extratropical in origin but which can interact with enso are dominated by the zonally symmetric high latitude mode or antarctic oscillation aao the aao involves an alternation of atmospheric mass between middle and high southern latitudes a zonally asymmetric mode of wave number 1 is represented by an oscillation in pressure height between australian and south american sectors depicted by a trans polar circulation index a significant wave number 2 oscillation in the coupled atmosphere ocean ice system or antarctic circumpolar wave acw has a similar periodicity to enso and is strongest in the pacific sw atlantic sectors atmospheric teleconnections involving the southern ocean including those to enso show evidence of recent changes and the climatic implications of these are discussed

global influence of the northern hemisphere second mode of the zonal average of the zonal wind 1 the authors examine the impact of two zonal wind modes in the northern hemisphere northern annular mode nam and the second zonal wind mode on the global domain during the normal non el nino southern oscillation enso northern winter from 1981 2002 it is found that both modes are in association with tropical zonal wind anomalies but only the second zonal wind mode has significant linkage with the southern annular mode sam particularly leading about 6 19 days this kind of linkage can also be found in extended datasets but only evident since 1974

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

the relationship between zonal mean flow and quasi stationary waves in the midtroposphere abstract the degree to which quasi stationary midtropospheric flow is consistent with linear potential vorticity conservation is investigated the linear theory suggests there should be a well defined relationship between the zonal mean component of steady flow and departures from the zonal mean a set of 45 monthly mean winter states is examined to determine if there is a zonal eddy relationship month to month variations in zonal mean winds are represented in terms of an eof analysis correlation and compositing techniques isolate two dimensional patterns which appear to be associated with each zonal eof solutions to the nondivergent barotropic vorticity equation linearized about a zonal mean state suggest that the two dimensional flow patterns associated with several of the leading zonal mean eofs are consistent with the dynamical mechanism rossby first proposed as an explanation for a zonal eddy relationship it is suggested that the zonal eddy relationship examined here may in fact play a role in l

theoretical analysis of non geometric ps waves recorded at the water bottom interface when a seismic source is placed in the water at a height less than a wavelength from the water solid interface a prominent s wave arrival can be observed it travels kinematically as if it was excited at the projection point of the source on the interface this non geometric s wave has been investigated before mainly for a free surface configuration however as was shown in a field experiment the non geometric swave can also be excited at a fluid solid configuration if the s wave speed in the solid is less than the sound speed in the water the amplitude of this wave exponentially decreases when the source is moved away from the interface revealing its evanescent character in the fluid in the solid this particular converted mode is propagating as an ordinary s wave and can be transmitted and reflected as such there is a specific region of horizontal slownesses where this non geometric wave exists depending on the ratio of the s wave velocity and the sound speed of water only for ratios smaller than 1 this wave appears lower ratios result in a wider region of appearance due to this property this particular p s converted mode can be identified and filtered from other events in the radon domain

coherent wave zonal mean flow interactions in the troposphere abstract spatial structure and temporal evolution of synoptic time scale variations of the tropospheric zonal mean flow are studied by extensive cross correlation analyses and the degree to which observations agree with two dimensional adiabatic theory is determined observational data from seven years of operational daily global analyses are studied along with data from the ncar general circulation model observed zonal mean zonal wind and temperature tendencies are compared with analyzed adiabatic forcing terms although significant correlations are found throughout the extratropics there are significant equation residuals in both operational analyses and model data model momentum residuals result from calculational inaccuracies interpolation to pressure surfaces and spectral aliasing of nonlinear terms and biases introduced by once daily sampling diabatic terms are also important for the daily thermodynamic balance coherent wave zonal mean flow interactions are revealed via cross correlation a

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

growth of planetary waves and the formation of an elevated stratopause after a major stratospheric sudden warming in a t213l256 gcm 1 recovery processes after a major stratospheric sudden warming ssw with the formation of an elevated stratopause and a strong polar night jet are investigated using a gravity wave resolving gcm the major ssw that occurred in the gcm bears a strong resemblance to observations in january 2006 and january 2009 the recovery phase of the ssw in the gcm is divided into two stages in the first stage during about five days just after the ssw a large positive eliassen palm e p flux divergence associated with the growth of planetary waves contributes to the quick recovery of eastward wind above 2 hpa about 42 km which is likely due to baroclinic and or barotropic instabilities in the second stage over the next three weeks a prolonged westward wind in the lower stratosphere blocked upward propagation of gravity waves with westward intrinsic phase velocities it reduces the deceleration of eastward wind in the upper mesosphere and raises the breaking height of gravity waves since the height of westward gravity wave forcing also rises the polar stratopause created by the gravity wave driven meridional circulation is formed at an elevated height about 75 km compared to that before the ssw 55 65 km in addition the weaker westward gravity wave forcing in the upper mesosphere drives weaker downwelling around 1 hpa and forms a cold layer consequently the strong polar night jet forms at a higher altitude than before the ssw as a result of adjustment toward the thermal wind balance this indicates that these two stages provide eastward acceleration in different ways

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

response of air sea carbon fluxes and climate to orbital forcing changes in the community climate system model 1 a global general circulation model coupled to an ocean ecosystem model is used to quantify the response of carbon fluxes and climate to changes in orbital forcing compared to the present day simulation the simulation with the earth s orbital parameters from 115 000 years ago features significantly cooler northern high latitudes but only moderately cooler southern high latitudes this asymmetry is explained by a 30 reduction of the strength of the atlantic meridional overturning circulation that is caused by an increased arctic sea ice export and a resulting freshening of the north atlantic the strong northern high latitude cooling and the direct insolation induced tropical warming lead to global shifts in precipitation and winds to the order of 10 20 these climate shifts lead to regional differences in air sea carbon fluxes of the same order however the differences in global net air sea carbon fluxes are small which is due to several effects two of which stand out first colder sea surface temperature leads to a more effective solubility pump but also to increased sea ice concentration which blocks air sea exchange and second the weakening of southern ocean winds that is predicted by some idealized studies occurs only in part of the basin and is compensated by stronger winds in other parts

downward propagating temperature anomalies in the preconditioned polar stratosphere abstract dynamical links of the northern hemisphere stratosphere and troposphere are studied with an emphasis on whether stratospheric changes have a direct effect on tropospheric weather and climate in particular downward propagation of stratospheric anomalies of polar temperature in the winter spring season is examined based upon 22 years of ncep ncar reanalysis data it is found that the polar stratosphere is sometimes preconditioned which allows a warm anomaly to propagate from the upper stratosphere to the troposphere and sometimes it prohibits downward propagation the arctic oscillation ao is more clearly seen in the former case to understand what dynamical conditions dictate the stratospheric property of downward propagation the upper stratospheric warming episodes with very large anomalies such as stratospheric sudden warming are selected and divided into two categories according to their downward propagating features eliassen palm e p diagnostics and wave propagation theories are u

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

vertical propagation of information in a middle atmosphere data assimilation system by gravity wave drag feedbacks 1 the mesospheric response to the 2002 antarctic stratospheric sudden warming ssw is analysed using the canadian middle atmosphere model data assimilation system cmam das where it represents a vertical propagation of information from the observations into the data free mesosphere the cmam das simulates a cooling in the lowest part of the mesosphere which is accomplished by resolved motions but which is extended to the mid to upper mesosphere by the response of the model s non orographic gravity wave drag parameterization to the change in zonal winds the basic mechanism is that elucidated by holton consisting of a net eastward wave drag anomaly in the mesosphere during the ssw although in this case there is a net upwelling in the polar mesosphere since the zonal mean mesospheric response is shown to be predictable this demonstrates that variations in the mesospheric state can be slaved to the lower atmosphere through gravity wave drag

stationary atmospheric responses to an idealized sea surface temperature anomaly in the southern ocean abstractthe stationary atmospheric response to an idealized sea surface temperature anomaly ssta is studied with a quasigeostrophic atmospheric model of the southern hemisphere sensitivity of the stationary response to the midlatitude ssta location is determined and responses are decomposed on vertical modes the ssta almost directly forces baroclinic responses inducing warm air anomalies 40 50 downstream eastward to the ssta these baroclinic responses arise from an equilibrium between the ssta induced anomalous vortex stretching and i advection by the quasi stationary flow and ii dissipation by high frequency eddies the barotropic response consists of a midlatitude ridge trough and a south pole trough ridge for sstas localized from the drake passage to the western indian ocean from south of australia to the center of the pacific ocean this response can be further decomposed into i a zonally asymmetric component a quasi stationary wave train forced by a barotropic ridge downstream of

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

case studies of the mesospheric response to recent minor major and extended stratospheric warmings 1 we have studied the mesospheric response to two recent stratospheric warmings by performing short term forecasts at medium 1 5 and high 0 5 spatial resolution under different gravity wave drag gwd scenarios we validated our models with our high altitude analysis that extends from 0 to 90 km for the minor warming of january 2008 reduced upper level orographic gwd weakened the downward residual circulation and cooled the mesosphere parameterized nonorographic gwd increased the simulated mesospheric cooling for the prolonged major warming of 2006 heavily attenuated orographic gwd led to pronounced cooling near 50 km during the extended phase of this event an unusually strong westerly polar vortex reformed in the lower mesosphere which allowed westward propagating nonorographic gravity waves to reach the mesosphere and break with net westward accelerations of over 50 m s 1 d 1 this in turn forced a strong residual circulation yielding descent velocities over 2 cm s 1 between 65 n and 85 n consistent with previous reports of enhanced downward transport of trace constituents the resulting adiabatic heating as evidenced by the unusually vertically displaced stratopause at 80 km is likely a direct consequence of this enhanced gravity wave driven descent high resolution simulations without parameterized gwd were closer to the analysis than medium resolution simulations with parameterized orographic gwd only but still did not fully simulate the mesospheric thermal response specifically the 80 km temperature enhancement was still underestimated in these simulations this suggests that higher spatial resolution is needed to adequately resolve extratropical gravity wave momentum fluxes

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