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

the role of stationary waves in the maintenance of the northern annular mode as deduced from model experiments abstract the influence of stationary waves on the maintenance of the tropospheric annular mode am is examined in a simple global circulation model with perpetual january conditions the presented model experiments vary in the configurations of stationary wave forcing by orography and land sea heating contrasts all simulations display an am like pattern in the lower troposphere the zonal momentum budget shows that the feedback between eddies with periods less than 10 days and the zonal mean zonal wind is generally the dominating process that maintains the am the kinetic energy of the high frequency eddies depends on the stationary wave forcing where orographic forcing reduces and thermal forcing enhances it the ams in the model experiments differ in the superposed anomalous stationary waves and in the strength of the zonally symmetric component if only orographic stationary wave forcing is taken into account the mountain torque decelerates the barotropic wind anomaly and thus acts to weaken the a

multiple regimes and low frequency oscillations in the northern hemisphere s zonal mean flow 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 by singular spectrum analysis and the multitaper method the 147 day oscillation involves zonal flow anomalies that propagate poleward while the 72 day oscillation only manifests northward propagation in the atlantic sector both modes mainly describe changes in the midlatitude jet position and intensity in the horizontal plane though the two modes exhibit synchronous centers of action located over the atlantic and pacific oceans the two persistent flow regimes are associated with slow phases of either oscillation

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

characteristics of wave packets in the upper troposphere part i northern hemisphere winter abstract gridded data produced by the ecmwf reanalysis project have been analyzed to document the properties of wave packets in the northern hemisphere winter midlatitude upper troposphere based on results from earlier investigations 300 hpa meridional wind variations were chosen for analysis wave packet envelopes were also defined by performing complex demodulation on the wind data the properties of the wave packets are mainly illustrated using time lagged one point correlation maps performed both on and wave packet envelopes the results show that over most regions in the northern hemisphere winter with the exception of the deep tropics and near the aleutian low medium scale waves dominant wavenumber 5 8 exhibit the characteristics of downstream development and occur within wave trains that propagate with eastward group velocities much faster than the phase speeds of individual phases of the waves their group velocity is highly correlated with the local time mean 200 400 hpa wind while the

teleconnections in the geopotential height field during the northern hemisphere winter abstract contemporaneous correlations between geopotential heights on a given pressure surface at widely separated points on earth referred to as teleconnections in this paper are studied in an attempt to identify and document recurrent spatial patterns which might be indicative of standing oscillations in the planetary waves during the northern hemisphere winter with time scales on the order of a month or longer a review of existing literature on the subject reveals the existence of at least four such patterns the north atlantic and north pacific oscillations identified by walker and bliss 1932 a zonally symmetric seesaw between sea level pressures in polar and temperature latitudes first noted by lorenz 1951 and what we will refer to as the pacific north american pattern which has been known to operational long range forecasters in this country since the 1950 s a data set consisting of nmc monthly mean sea level pressure and 500 mb height analyses for a 15 year period is used as a basis fo

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

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

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

lessons learned from oxygen isotopes in modern precipitation applied to interpretation of speleothem records of paleoclimate from eastern asia abstract variability in oxygen isotope ratios collected from speleothems in chinese caves is often interpreted as a proxy for variability of precipitation summer precipitation seasonality of precipitation and or the proportion of 18o to 16o of annual total rainfall that is related to a strengthening or weakening of the east asian monsoon and in some cases to the indian monsoon we use modern reanalysis and station data to test whether precipitation and temperature variability over china can be related to changes in climate in these distant locales we find that annual and rainy season precipitation totals in each of central china south china and east india have correlation length scales of 500 km shorter than the distance between many speleothem records that share similar long term time variations in 18o values thus the short distances of correlation do not support though by themselves cannot refute the idea that apparently synchronous variations in 18o values at widely spaced 500 km caves in china are due to variations in annual precipitation amounts we also evaluate connections between climate variables and 18o values using available instrumental measurements of 18o values in precipitation these data from stations in the global network of isotopes in precipitation gnip show that monthly 18o values generally do not correlate well with either local precipitation amount or local temperature and the degree to which monthly 18o values do correlate with them varies from station to station for the few locations that do show significant correlations between 18o values and precipitation amount we estimate the differences in precipitation amount that would be required to account for peak to peak differences in 18o values in the speleothems from hulu and dongge caves assuming that 18o scales with the monthly amount of precipitation or with seasonal differences in precipitation insofar as the present day relationship between 18o values and monthly precipitation amounts can be applied to past conditions differences of at least 50 in mean annual precipitation would be required to explain the 18o variations on orbital time scales which are implausibly large and inconsistent with published gcm results similarly plausible amplitudes of seasonal cycles in amounts or in seasonal variations in 18o values can account for less than half of the 4 5 difference between glacial and interglacial 18o values from speleothems in china if seasonal cycles in precipitation account for the amplitudes of 18o values on paleoclimate timescales they might do so by extending or contracting the durations of seasons a frequency modulation of the annual cycle but not by simply varying the amplitudes of the monthly rainfall amounts or monthly average 18o values amplitude modulation allowing that several processes can affect seasonal variability in isotopic content we explore the possibility that one or more of the following processes contribute to variations in 18o values in chinese cave speleothems different source regions of the precipitation which bring different values of 18o in vapor different pathways between the moisture source and the paleorecord site along which exchange of 18o between vapor surface water and condensate might differ a different mix of processes involving condensation and evaporation within the atmosphere or different types of precipitation each may account for part of the range of 18o values revealed by speleothems and each might contribute to seasonal differences between past and present that do not scale with monthly or even seasonal precipitation amounts

eddy zonal flow interactions and the persistence of the zonal index abstract an idealized atmospheric general circulation model is used to investigate the factors controlling the time scale of intraseasonal 10 100 day variability of the extratropical atmosphere persistence on these time scales is found in patterns of variability that characterize meridional vacillations of the extratropical jet depending on the degree of asymmetry in the model forcing patterns take on similar properties to the zonal index annular modes and north atlantic oscillation it is found that the time scale of jet meandering is distinct from the obvious internal model time scales suggesting that interaction between synoptic eddies and the large scale flow establish a separate intraseasonal time scale a mechanism is presented by which eddy heat and momentum transport couple to retard motion of the jet slowing its meridional variation and thereby extending the persistence of zonal index and annular mode anomalies the feedback is strong and quite sensitive to model parameters when the mod

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

is the north atlantic oscillation a breaking wave abstract given the recent observational evidence that the positive negative phase of the north atlantic oscillation nao is the remnant of anticyclonic cyclonic wave breaking this study uses a multilevel primitive equation model to investigate important dynamical attributes of the above wave breaking behavior for this purpose a hierarchy of different basic states two and three dimensional and initial perturbations are used with the three dimensional climatological flow as the basic state it is found that initial perturbations located equatorward poleward and upstream of the climatological atlantic jet lead to wave breaking similar to that of the positive negative nao phase consistently analysis of observational data indeed shows that the pacific storm track is displaced equatorward poleward prior the onset of the positive negative nao phase this result suggests that the latitudinal position of the pacific storm track plays an important role for determining the phase of the nao sen

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

reconstructing sea level pressure variability via a feature tracking approach abstractthe aim of this paper is to quantify the contribution of synoptic transients to the full spectrum of space time variability of sea level pressure slp in middle latitudes in previous work by the authors it was shown that tracking cyclones and anticyclones in an idealized atmospheric model allows one to reconstruct a surprisingly large fraction of the model s variability including not only synoptic components but also its large scale low frequency component motivated by this result the authors performed tracking of cyclones and anticyclones and estimated cyclone and anticyclone size and geometry characteristics in the observed slp field using the 1948 2008 ncep ncar reanalysis dataset the reconstructed synoptic field was then produced via superimposing radially symmetrized eddies moving along their actual observed trajectories it was found that similar to earlier results for an idealized model the synoptic reconstruction so obtained accounts for a major fraction of the full observed slp v

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

bifurcation in eddy life cycles implications for storm track variability by analyzing a number of very high resolution nonhydrostatic experiments of baroclinic lifecycles it was concluded that the intensity of the near surface baroclinic development influences the upper level wave to such an extent that it could produce cyclonic or anticyclonic wave breaking since the final jet position is equatorward or poleward the position depends on whether the waves break cyclonically or anticyclonically respectively the low level baroclinicity plays a very important role in the outcome of the wave and feedback to the mean circulation using a shallow water model the hypothesis that the intensity of the eddy forcing from the lower layers of the atmosphere can have a profound effect on the disturbances of the upper layers is tested from these experiments the following is concluded for weak intensities the strong effective beta asymmetries due to the earth s sphericity produce anticyclonic wave breaking and a poleward shift of the zonal jet will occur for moderate forcing anticyclonic wave breaking occurs and consequently as before a poleward shift of the zonal jet will occur however there is an important distinction between weak and moderate forcing in the latter case the eddy anticyclonic centers are very intense the influence of the two anticyclones produces a difluence field that will strain the cyclonic vortex along the sw ne direction consequently the meridional vorticity flux y9z9 is positive in the north and negative in the south this process has two effects thinning the cyclone and producing positive vorticity fluxes on the north negative fluxes on the south and moving the jet poleward by increasing the forcing the cyclone centers become considerably more intense than the anticyclones cvc and they are able to deform and thin the anticyclones thus moving the jet equatorward this transition is very abrupt above a threshold amplitude the life cycle bifurcates to a cyclonic wave breaking the implications for storm track variability are quite direct in normal years at the entrance of the storm track intense baroclinicity produces cvcs with a slight shift of the jet equatorward at the last half of the storm track due to much weaker baroclinicity anticyclonic wave breaking occurs avcs displacing the jet poleward the eddies at the entrance of the storm track develop from the baroclinicity of the subtropical jet downstream fluxing and weaker surface baroclinicity make the upper level waves more aloft and barotropic by the middle of the storm track these waves normally break anticyclonically enhancing the subpolar eddy driven jet in the warm phase of enso more baroclinicity and subtropical moisture flux is present in the eastern pacific ocean this enhanced baroclinicity could support more cvcs in the eastern basin maintaining the subtropical jet further east

synoptic view of the north atlantic oscillation abstract this article investigates the synoptic characteristics of individual north atlantic oscillation nao events by examining the daily evolution of the potential temperature field on the nominal tropopause the 2 pvu surface this quantity is obtained from the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis dataset for the winter season for both phases the nao is found to originate from synoptic scale waves as these waves evolve into the low frequency nao pattern they break anticyclonically for the positive phase and cyclonically for the negative phase the results of this analysis suggest that it is the remnants of these breaking waves that form the physical entity of the nao throughout the nao events for both phases the nao is maintained by the successive breaking of upstream synoptic scale waves when synoptic scale disturbances are no longer present mixing processes play an important role in the nao decay as in other recent

sensitivity of the position and variability of the eddy driven jet to different sst profiles in an aquaplanet general circulation model abstractthe response of the storm track and eddy driven jet to various steady sea surface temperature sst forcings is analyzed using the aquaplanet configuration of the atmospheric general circulation model action de recherche petite echelle grande echelle climat arpege climat the sst profiles are zonally homogeneous and piecewise linear functions of the latitude it allows for modifying the tropical component that controls the intensity of the subtropical jet without changing the midlatitude sst front and vice versa sensitivities to the position width and intensity of the midlatitude sst front as well as to the tropical ssts are investigated the baroclinicity and the storm track eddy activity both intensify for a stronger a wider or a more equatorward sst front the eddy driven jet always lies on the poleward side of the sst front and its relative distance to the front is interpreted in terms of upper tropospheric rossby wave breakings a focus is then made on the eddy driven jet variability t

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

forced annular mode patterns in a simple atmospheric general circulation model abstract previous studies using simplified general circulation models have shown that annular modes arise as the dominant mode of variability a simple gcm is used here to explore to what extent these modes are also the preferred response of the system to generic forcing a number of trials are conducted under which the model is subjected to an artificial zonally symmetric angular momentum forcing and the climatologies of these trials are compared to that of the control the forcing location is varied among the several trials it is found that the changes in the model s climatology are generally annular mode like as long as the imposed forcing projects strongly upon the annular modes of the unforced model the role of changes to the eddy zonal flow feedback versus the action of direct forcing is also considered through the use of a zonally symmetric version of the model it is found that the direct responses to forcing are insufficient to capture either the strength or the structure of the annular mo

a stochastic model for the spatial structure of annular patterns of variability and the north atlantic oscillation abstract meridional dipoles of zonal wind and geopotential height are found extensively in empirical orthogonal function eof analysis and single point correlation maps of observations and models notable examples are the north atlantic oscillation and the so called annular modes or the arctic oscillation minimal stochastic models are developed to explain the origin of such structure in particular highly idealized analytic purely stochastic models of the barotropic zonally averaged zonal wind and of the zonally averaged surface pressure are constructed and it is found that the meridional dipole pattern is a natural consequence of the conservation of zonal momentum and mass by fluid motions extension of the one dimensional zonal wind model to two dimensional flow illustrates the manner in which a local meridional dipole structure may become zonally elongated in eof analysis producing a zonally uniform eof even when the dynamics is not particularly zonally coherent on hemispheric length scales

zonal asymmetry of the annular mode and its downstream subtropical jet an idealized model study abstractthis study investigates linkages between the zonal asymmetry of the annular mode am zonal pattern and the subtropical jet stj over its downstream regions of the storm track by using an idealized model observational analyses show that the am zonal patterns are more zonally asymmetric during days when the stj downstream of the storm track is unusually strong and vice versa in the idealized model the stj downstream of the storm track is varied by introducing an additional zonally localized tropical heating the model s am variability exhibits a nearly zonally uniform structure when there is no or only weak tropical heating however the signatures of the am are locally strengthened in the heating sector thus the am zonal pattern is zonally asymmetric when the tropical heating is large enough to create a strong stj the model results also show that the percentage of the variance explained by the am the persistence of the am index and the intensity of eddy feedback are also increased when t

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

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

important role for ocean warming and increased ice shelf melt in antarctic sea ice expansion changes in sea ice significantly modulate climate change because of its high reflective and strong insulating nature in contrast to arctic sea ice sea ice surrounding antarctica has expanded1 with record extent2 in 2010 this ice expansion has previously been attributed to dynamical atmospheric changes that induce atmospheric cooling3 here we show that accelerated basal melting of antarctic ice shelves is likely to have contributed significantly to sea ice expansion specifically we present observations indicating that melt water from antarctica s ice shelves accumulates in a cool and fresh surface layer that shields the surface ocean from the warmer deeper waters that are melting the ice shelves simulating these processes in a coupled climate model we find that cool and fresh surface water from ice shelf melt indeed leads to expanding sea ice in austral autumn and winter this powerful negative feedback counteracts southern hemispheric atmospheric warming although changes in atmospheric dynamics most likely govern regional sea ice trends4 our analyses indicate that the overall sea ice trend is dominated by increased ice shelf melt we suggest that cool sea surface temperatures around antarctica could offset projected snowfall increases in antarctica with implications for estimates of future sea level rise

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

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

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

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