variations of the east asian jet stream and asian pacific american winter climate anomalies in this study the authors apply the ncep ncar reanalysis and other observations to depict the association of the asian pacific american climate with the east asian jet stream eajs with an emphasis on boreal winter seasons and on interannual timescales they analyze the variations of the eajs and their relationships with el nino southern oscillation enso and extratropical north pacific sea surface temperature sst and assess the relative connections of the eajs and enso to the anomalies of atmospheric circulation surface temperature and precipitation in the asian pacific american region it is found that the eajs is coupled to a teleconnection pattern spanning the entire asian pacific american region with the strongest signals over east asia and the western pacific this pattern differs significantly from that associated with enso which influences the earth s climate extensively with a strongest impact on the climate over the central pacific and east a strong eajs is associated with an intensification of the weather and climate systems in asia and over the pacific such as deepening of the east asian trough and the aleutian low and strengthening of the east asian winter monsoon it is linked to colder and drier conditions in east asia and stronger convection over the tropical asia australia sector compared with enso the eajs seems to link to the climate signals of asia and the pacific more strongly an intensified eajs is also associated with anomalies of temperature and precipitation in north america due to the related changes in stationary wave patterns while the eajs does not strongly link to the tropical central eastern pacific sst it is significantly associated with the extratropical north pacific sst more specifically the second most dominant mode of the empirical orthogonal function analysis of the sst in addition a strong weak eajs seems to follow a large small meridional gradient of the western pacific sst associated with warming cooling in the tropics subtropics and cooling warming in the extratropics

three eurasian teleconnection patterns spatial structures temporal variability and associated winter climate anomalies the eurasian eu pattern is a distinct teleconnection pattern observed in boreal winter since the eu pattern was first identified three types have been reported in the literature the conventional eu pattern the type 1 eu pattern or scandinavian scand pattern and the type 2 eu pattern or east atlantic west russia eatl wrus pattern based on several reanalysis and observational datasets the three eu patterns are extracted using the rotated empirical orthogonal function method in order to provide a further distinction and understanding of the three eu patterns a comprehensive side by side comparison is performed among them including their temporal variability horizontal and vertical structure related stationary rossby wave activity impact on climate and possible driving factors associated with external forcing the results reveal that all three eu patterns are characterised by a clear quasi barotropic wave train structure but each has a distinct source and centre of action accordingly their impacts on the precipitation and surface air temperature also differ from one other further evidence suggests that the conventional eu pattern is likely driven by anomalous sea surface temperatures sst over the north atlantic in which process the transient eddies are actively involved the scand pattern is partly maintained by the vorticity source over western europe which arises from the anomalous convergence divergence over the mediterranean and is efficiently driven by the tropical and southern indian ocean sst via divergent circulation the eatl wrus pattern shows some linkage to the north american snow cover and the involved process remains unclear and needs further investigation

connections between wintertime jet stream variability oceanic surface heating and transient eddy activity in the north pacific 1 a set of statistical analyses of the fields from the european center for medium range weather forecasts reanalysis era 40 are carried out to investigate the connections between three selected fields namely the wintertime western pacific jet stream wpjs as represented by the zonal wind at 200 hpa the oceanic surface heating represented by the latent and sensible heat fluxes and the synoptic scale transient eddy activity stea over the north pacific first empirical orthogonal function eof analysis is performed to characterize the wintertime wpjs variability further analyses yield the following conclusions 1 the most dominant relationship among the three selected fields is strongly linked to the el nino southern oscillation enso during enso events anomalous zonal wind at 200 hpa downstream of the wpjs main body shown in the first eof is accompanied by stea anomalies around the region of the anomalous midlatitude zonal wind 2 the strengthening weakening of the wintertime wpjs main body shown in the second eof is connected to the surface heating anomalies in the kuroshio and its extension region as well as to stea anomalies in the region downstream of the wpjs main body 3 the northward southward shift of the wintertime wpjs represented in the third eof is closely related to the south north dipolar structure of the surface heating anomalies in the western north pacific the stea spatial displacement is in conjunction with the shift of the jet stream in the same direction due to the dynamical connection between the winds and the eddies furthermore the stea anomalies in winter may cause the wpjs anomalies on the submonthly timescale as a consequence the northward southward shift of the stea is harmoniously in favor of the similar jet shift

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

upstream subtropical signals preceding the asian summer monsoon circulation in this study the authors address several issues with respect to the antecedent signals of the large scale asian summer monsoon that were earlier identified by webster and yang in particular they revisit the changes in the subtropical upper tropospheric westerlies preceding the monsoon depict the detailed structure of the monsoon s antecedent signals and investigate the physical processes from the signals to the monsoon they also explore the teleconnection of these signals to various large scale climate phenomena and emphasize the importance of the upstream location of the signals relative to the tibetan plateau and the monsoon before a strong weak asian summer monsoon the 200 mb westerlies over subtropical asia are weak strong during the previous winter and spring a significant feature of these signals is represented by the variability of the middle east jet stream whose changes are linked to the arctic oscillation north atlantic oscillation el

turbulence structure above a vegetation canopy we compare the turbulence statistics of the canopy roughness sublayer rsl and the inertial sublayer isl above in the rsl the turbulence is more coherent and more efficient at transporting momentum and scalars and in most ways resembles a turbulent mixing layer rather than a boundary layer to understand these differences we analyse a large eddy simulation of the flow above and within a vegetation canopy the three dimensional velocity and scalar structure of a characteristic eddy is educed by compositing using local maxima of static pressure at the canopy top as a trigger the characteristic eddy consists of an upstream head down sweep generating hairpin vortex superimposed on a downstream head up ejection generating hairpin the conjunction of the sweep and ejection produces the pressure maximum between the hairpins and this is also the location of a coherent scalar microfront this eddy structure matches that observed in simulations of homogeneous shear flows and channel flows by several workers and also fits with earlier field and wind tunnel measurements in canopy flows it is significantly different from the eddy structure educed over smooth walls by conditional sampling based only on ejections as a trigger the characteristic eddy was also reconstructed by empirical orthogonal function eof analysis when only the dominant sweep generating head down hairpin was recovered prompting a re evaluation of earlier results based on eof analysis of wind tunnel data a phenomenological model is proposed to explain both the structure of the characteristic eddy and the key differences between turbulence in the canopy rsl and the isl above this model suggests a new scaling length that can be used to collapse turbulence moments over vegetation canopies

zonal jet structure and the leading mode of variability an observational analysis of the northern hemisphere circulation during winter reveals that the leading mode of variability depends on longitude in particular the first eof of the zonal mean circulation differs over the atlantic and pacific sectors these results provide motivation for a series of model runs where a simple gcm is used to investigate the effects of the zonal jet structure on the leading mode of variability in the northern hemisphere model results indicate that the leading mode of variability depends on the distance between the eddy driven and subtropical jets when the jets are well separated the leading mode of variability describes latitudinal shifting of the eddy driven jet however when the two jets are nearly collocated pulsing of the combined jet dominates the variability this change coincides with a weakening of the positive feedback between the eddies and zonal flow anomalies these results provide a possible explanation for the reduced amplitude of the northern annular mode in the pacific sector relative to that in the atlantic sector during northern hemisphere winter

circumglobal teleconnections the jet stream waveguide and the north atlantic oscillation abstract monthly and seasonally averaged upper tropospheric northern hemisphere winter fields are examined to determine whether the waveguiding effect of the time averaged tropospheric jets on low frequency disturbances that is predicted by theory does affect the behavior of these disturbances it is found that indeed disturbances in the vicinity of the mean jets particularly the jet that stretches across south asia are fundamentally different from those that reside in regions where the mean winds have weaker meridional gradients like the mid pacific patterns of variability in the jets tend to be smaller scale and to consist of zonally oriented chains of anomalies while variability in the mid pacific is composed of patterns with distinct meridional orientation because they are meridionally trapped and zonally elongated patterns associated with the jet stream waveguide connect activity at points that are much farther apart than do patterns in other regions of the globe within the south asian waveg

neutral turbulent flow over forested hills numerical simulations of neutral turbulent flow over periodic forested hills are carried out using a first order turbulence closure scheme the forest is represented by the inclusion in the model of a canopy near the surface where additional drag is exerted on the flow as a result of the trees simulations with a more traditional roughness length parametrization of the surface which does not explicitly model the flow within the forest are also carried out for comparison comparison of simulations with existing wind tunnel data shows that the simple mixing length canopy closure scheme provides a good estimate of the mean flow over a forested hill and is a significant improvement on a traditional roughness length parametrization in this case the numerical results are seen to be in close agreement with existing linear analytical solutions for flow over a low and wide forested hill the model is used to investigate the breakdown of the analytical solution when the hill becomes steep and the perturbations to the flow are no longer linear even in the case of a gentle slope the analytical solution can break down when the hill is narrow or the canopy is deep and the vertical velocities near the top of the canopy become large this leads to a downwind shift in the surface pressure perturbation and an enhanced drag compared to a roughness length model model results also show enhanced flow separation in the presence of a canopy these results are important for a range of meteorological problems including the interpretation of flux measurements parametrization of orographic drag predicting wind damage to trees and calculating the yield from wind turbines royal meteorological society 2005 s b vosper s contribution is crown copyright

the linkage between upper level jet streams over east asia and east asian winter monsoon variability abstractthis study investigates the linkage between east asian winter monsoon eawm variability and upper level jets with particular focus on the east asian polar front jet pj and its concurrent variation with the subtropical jet located to the south of the tibetan plateau tsj the winter upper level zonal wind variations over the asian landmass 70 120 e are dominated by two principal modes i e meridional displacement of the pj and out of phase variation in the intensity of the tsj and pj and they are closely linked to the eawm northern mode and southern mode respectively southward shifting of the pj concurs with northwestward displacement of the siberian high sh an enhanced northern east asian trough leading to cold winter in northern east asia meanwhile the simultaneous tsj intensification and pj weakening is linked to an amplified sh a southward shift of the aleutian low al a strengthened southern east asian trough and a wavelike anomaly pattern extending from western barents s

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

inversion of small basins effects on structural variations at the leading edge of the axial zone antiformal stack southern pyrenees spain the southern axial zone of the pyrenees consists of a foreland dipping system of thrusts and backthrusts forming the leading edge of a basement involved antiformal stack several small stephanian permian extensional basins were strongly inverted during the pyrenean cainozoic orogeny and controlled thrust geometry and kinematics causing lateral structural variation palinspastic restoration reveals three main grabens erill castell estac and gramos basins capped by undeformed triassic strata in the lower orri and erta thrust sheets the overlying nogueres thrust sheet partitioned into several slices contains smaller basins northern basins the erill castell half graben with a steeply dipping sedimentary infill was inverted along its border fault and extruded by bedding parallel backthrusts the narrow thick estac basin which was less deformed internally was a heterogeneity causing major lateral variation in the system the gramos basin originally wide and covered by a gentle unconformity angle was also inverted along its border fault but caused less lateral variation in the thrust system the northern basins appear only in the western part of the present day nogueres thrust sheet where minor units are strongly stacked and rotated in the east thrust slices are large and shallow dipping the study case is a good example of complex 3d thrust development influenced by the initial geometry of a heterogeneous basin system

evolution of the meridional shift of the subtropical and subpolar westerly jet over the southern hemisphere during the past 21 000 years abstract deciphering the variations in the subtropical westerly jet stj and the subpolar westerly jet spj over the southern hemisphere in earth s history will help us to better understand how the westerlies may respond to various external forcings in the future using simulation of transient climate evolution over the past 21 000 years simulations we demonstrate that the stj migrated equatorward in the last glacial maximum relative to the present which followed by abrupt poleward equatorward and poleward shifts during the heinrich stadial i antarctic cold reversal and younger dryas respectively these large amplitude fluctuations are controlled by changes in meltwater fluxes and polar ice sheet topography during the holocene the stj shows no obvious long term trend due to the offsetting effect between increased orbital insolation in austral summer and decreased antarctic ice sheet topography the meridional shift of the spj shows an in phase change with the stj during the past 21 kyr in terms of the dynamic mechanisms the meridional shift of the stj and spj is induced by changes in the meridional temperature gradient and the maximum eddy growth rate respectively and is also linked to changes in the mean meridional circulations that is the hadley and ferrel cells the modeled meridional shift of the southern westerlies is supported by the results from the paleoclimate modelling intercomparison project phase and is consistent with several reconstructions but it is difficult to constrain robustly by geological evidence as the reconstructed westerlies vary across sites and proxies nevertheless our study provides a possible scenario for the evolution of the southern westerlies during the past 21 kyr and identifies the dominant forcing and the associated mechanisms which may advance our knowledge on the future behavior of the southern westerlies

geometry and kinematics of the mosha fault south central alborz range iran an example of basement involved thrusting abstract field investigation of the western part of the mosha fault in several structural sections in the south central alborz range showed that the fault has a high angle of dip to the north and emplaces precambrian to cenozoic rocks over the eocene karaj formation study of the kinematics of the mosha fault in this area based on s c fabric and microstructures demonstrates that it is a deep seated semi ductile thrust strain analysis on rock samples from different sections across the mosha fault shows a flattening pattern of deformation in which the long axis of the strain ellipsoid is aligned in the fault shear sense the mosha fault is associated with a large hanging wall anticline cored by precambrian rocks and series of footwall synclines formed of late tertiary rocks this geometry together with several low angle short cut thrusts in the fault footwall implies that the mosha fault is an inverted normal fault which has been reactivated since the late tertiary in the study area the reverse fault mechanism was associated with the rapid uplift and igneous activity in the central alborz range during the late tertiary unlike in the eastern portion of the fault where the fault kinematics was replaced by a strike slip mechanism in the late miocene

observations of flow and turbulence in the nocturnal boundary layer over a slope measurements were conducted on an eastern slope of the salt lake basin slb as a part of the vertical transport and mixing experiment vtmx conducted in october 2000 of interest was the nocturnal boundary layer on a slope in particular katabatic flows in the absence of significant synoptic influence extensive measurements of mean flow turbulence temperature and solar radiation were made from which circulation patterns on the slope and the nature of stratified turbulence in katabatic winds were inferred the results show that near the surface 25 50 m the nocturnal flow is highly stratified and directed downslope but at higher levels winds strongly vary in magnitude and direction with height and time implying the domination of upper levels by air intrusions these intrusions may peel off from different slopes surrounding the slb have different densities and flow at their equilibrium density levels the turbulence was generally weak and continuous but sudden increases of turbulence levels were detected as the mean gradient richardson number dropped to rig about unity with a short timescale fluctuated on the order of a few tens of seconds while modulating with rig a longer along slope internal waves sloshing timescale of about half an hour the mixing efficiency or the flux richardson number of the flow was found to be a strong function of similar to that found in laboratory rig experiments with inhomogeneous stratified shear flows the eddy diffusivities of momentum and heat were evaluated and they showed a systematic variation with when scaled with the shear length scale and the rms rig vertical velocity of turbulence

asian jet waveguide and a downstream extension of the north atlantic oscillation abstract anomalous atmospheric fields associated with the north atlantic oscillation nao are analyzed on interannual and intraseasonal time scales in order to examine the extent to which the nao is a regional phenomenon analyses on the interannual time scale reveal that the nao signal is relatively confined to the euro atlantic sector in december while it extends toward east asia and the north pacific in february the difference is most clearly seen in the meridional wind anomaly which shows a wave train along the asian jet collocated with an anomalous vorticity source near the jet entrance diagnoses using a linear barotropic model indicate that this wave train is interpreted as quasi stationary rossby waves trapped on the asian jet waveguide and effectively excited by the anomalous upper level convergence over the mediterranean sea it is found that when the nao accompanies the mediterranean convergence mc anomaly most frequently seen in february the nao indeed has a much wider horizontal str

observed trends in the global jet stream characteristics during the second half of the 20th century 1 in this paper we propose a new method based on the detection of jet cores with the aim to describe the climatological features of the jet streams and to estimate their trends in latitude altitude and velocity in the national centers for environmental prediction ncep national center for atmospheric research ncar and 20th century reanalysis data sets due to the fact that the detection method uses a single grid point to define the position of jet cores our results reveal a greater latitudinal definition allowing a more accurate picture of the split flow configurations and double jet structures to the best of our knowledge these results provide the first multiseasonal and global trend analysis of jet streams based on a daily resolution 3 d detection algorithm trends have been analyzed over 1958 2008 and during the post satellite period 1979 2008 we found that in general trends in jet velocities and latitudes have been faster for the southern hemisphere jets and especially for the southern polar front jet which has experienced the fastest velocity increase and poleward shift over 1979 2008 during the austral summer and autumn results presented here show an acceleration and a poleward shift of the northern and southern winter subtropical jets over 1979 2008 that occur at a faster rate and over larger zonally extended regions during this latter period than during 1958 2008

structural controls on palaeozoic basin evolution and inversion in southwest wales the tectonic evolution of sw dyfed is presented in terms of a model of basin evolution and subsequent destruction by reactivation of the basin bounding faults in contractional mode movements along long lived palaeozoic extensional faults were reversed during variscan compression with the pre existing faults accommodating the contraction in addition to the development of a thin skinned thrust and fold system basin analysis of the pre variscan extensional system uses sedimentological and stratigraphical evidence to constrain the timing of movements of the basin bounding faults thus allowing division of the stratigraphy into depositional megasequences related to these extensional movements field examples on all scales illustrate active extension during the palaeozoic structural cross sections through sw dyfed demonstrate the interrelationship of the extensional and contractional deformations with palinspastic restorations of the cross sections illustrating the complexity of the pre variscan template

the different configurations of the east asian polar front jet and subtropical jet and the associated rainfall anomalies over eastern china in summer abstractto investigate the concurrent impacts of the east asian polar front jet eapj and subtropical jet easj on the summer rainfall over eastern china positive strengthened eapj with weakened easj and negative weakened eapj with strengthened easj configurations are identified in the positive configuration rainfall decreases in the northern part of eastern china and increases in the southern part vice versa in the negative configuration the possible mechanisms maintaining the two jet configurations are further proposed from the perspectives of sea surface temperature sst and synoptic scale transient eddy activities stea in the positive configuration meridional distributed cold warm sst anomalies over the eastern north pacific may induce regional circulation and meridional temperature gradient anomalies which can strengthen the eapj and weaken the easj the central pacific la nina like sst anomalies are related with the arctic vortexlike anomalies in the stratosphere which may strength

influence of a transverse basement fault on along strike variations in the geometry of an inverted normal fault case study of the mosha fault central alborz range iran abstract inverted normal faults show variations in structural geometry from footwall shortcut thrusts to hanging wall break back thrusts in the present study we evaluated along strike variations in the geometry of the mosha fault central alborz range iran detailed structural mapping within the taleqan mountains revealed that the mosha fault in the eastern part of the study area is associated with a large basement cored hanging wall anticline thrust over tertiary rocks that occur as a syncline or cut by footwall shortcut thrusts in the western part of the study area however the hanging wall consists of break back thrust sheets of paleozoic mesozoic rocks these thrust sheets occur as a duplex system and have allochthonous masses gravitationally emplaced in the foothills of the taleqan mountains curved fold axial traces and scattered intrusive bodies are observed in the valian valley area where the mosha fault is displaced across a transverse basement fault this basement fault controlled along strike variations in the geometry of the mosha fault during its evolution from a basin bounding fault to an inverted normal fault inversion of the fault in the eastern part of the study area where the fault is relatively steep involved basement rocks in the western part of the study area where the fault is relatively shallowly dipping deformation in the hanging wall is accommodated by thin skinned break back thrust sheets

interannual variations of east asian trough axis at 500 hpa and its association with the east asian winter monsoon pathway interannual variations of the east asian trough eat axis at 500 hpa are studied with the european centre for medium range weather forecasts 40 yr reanalysis data the associated circulation pattern and pathway of the east asian winter monsoon eawm with the eat axis tilt are specially investigated with a trough axis index which is closely related to the midlatitude baroclinic process and mainly represents the intensity of the eddy driven jet over the east asia north pacific sector when the tilt of eat is smaller than normal the eawm prefers to take the southern pathway and less cold air moves to the central north pacific however the eawm prefers the eastern pathway and brings more cold air to the north pacific when the tilt of eat is larger than normal these differences induce pronounced changes in both the precipitation and the surface air temperature over east and southeast asia furthermore the tilt status of the eat has a significant modulation effect on the regional climate anomalies related to the intensity of the eawm the findings suggest an increase in the temperature anomaly associated with the eawm intensity and a clear northward southward shift in its pattern in anomalous tilt phase of the eat in addition the modulation tends to be confined mainly to east asia and expanded to a larger area during the weak and the strong eawm winters respectively the possible reasons for interannual variations of the eat tilt are discussed and it is speculated that the midlatitude air sea interaction in the north pacific plays a dominant role this study on the eat tilt may enrich knowledge of the east asian winter monsoon beyond the conventional intensity index and may be helpful to improve regional climate prediction in east asia

thermally driven and eddy driven jet variability in reanalysis two important dynamical processes influence the extratropical zonal wind field angular momentum transport by the thermally direct hadley circulation thermal driving t and momentum flux convergence by atmospheric waves eddies that develop in regions of enhanced baroclinicity eddy driving e the relationship between extratropical zonal wind variability and these driving processes is investigated using 40 yr european centre for medium range weather forecasts re analysis era 40 data indices representing the processes it and ie are defined based on vertically integrated diabatic heating and meridional convergence of the meridionalflux of zonal momentum by eddies respectively zonal wind signatures associated with these indices are identified via composite analysis in the atlantic sector zonal wind variability is mainly associated with momentum flux convergence by baroclinic eddies supporting the established view that the atlanticjetis primarilyeddydriven inthe pacificsector zonal windvariability isassociatedwith bothdriving processes evidence that the pacific jet is both thermally driven and eddy driven the thermally driven pacific signature reflects changes in jet strength intensity and longitudinal extent with some resemblance to the zonal wind anomalies of the pacific north america pna pattern the eddy driven signature reflects a latitudinal shift of the jet exit region in both sectors that resembles the zonal wind anomalies of the north atlantic oscillation nao or west pacific wp patterns

influence of low arctic sea ice minima on anomalously cold eurasian winters 1 influence of low arctic sea ice minima in early autumn on the wintertime climate over eurasia is investigated observational evidence shows that significant cold anomalies over the far east in early winter and zonally elongated cold anomalies from europe to far east in late winter are associated with the decrease of the arctic sea ice cover in the preceding summer to autumn seasons results from numerical experiments using an atmospheric general circulation model support these notions the remote response in early winter is regarded as a stationary rossby wave generated thermally through an anomalous turbulent heat fluxes as a result of anomalous ice cover over the barents kara seas in late autumn which tends to induce an amplification of the siberian high causing colder conditions over the far east the late winter cold anomalies over eurasia are also reproduced in our experiment which is associated with the negative phase of the north atlantic oscillation

pacific east asian teleconnection how does enso affect east asian climate observational evidence is presented to show a teleconnection between the central pacific and east asia during the extreme phases of enso cycles this pacific east asian teleconnection is confined to the lower troposphere the key system that bridges the warm cold events in the eastern pacific and the weak strong east asian winter monsoons is an anomalous lower tropospheric anticyclone cyclone located in the western north pacific the western north pacific wind anomalies develop rapidly in late fall of the year when a strong warm or cold event matures the anomalies persist until the following spring or early summer causing anomalously wet dry conditions along the east asian polar front stretching from southern china northeastward to the east of japan kuroshio extension using atmospheric general circulation and intermediate models the authors show that the anomalous philippine sea anticyclone results from a rossby wave response to suppressed convective heating which is induced by both the in situ ocean surface cooling and the subsidence forced remotely by the central pacific warming the development of the anticyclone is nearly concurrent with the enhancement of the local sea surface cooling both the anticyclone and the cooling region propagate slowly eastward the development and persistence of the teleconnection is primarily attributed to a positive thermodynamic feedback between the anticyclone and the sea surface cooling in the presence of mean northeasterly trades the rapid establishment of the philippine sea wind and sst anomalies implies the occurrence of extratropical tropical interactions through cold surge induced exchanges of surface buoyancy flux the central pacific warming plays an essential role in the development of the western pacific cooling and the wind anomalies by setting up a favorable environment for the anticyclone sst interaction and midlatitude tropical interaction in the western north pacific

sandbox model studies of inversion tectonics abstract scaled plane strain sandbox models have been used to simulate basement controlled structural inversion in a sedimentary overburden two basement configurations were tested in the first one reversal of movement along vertical and normal faults that controlled a graben relief induced only a partial recovery of displacement along the pre existing overburden faults the inversion produced new reverse faults in the footwall of the pre existing overburden faults the reactivated normal faults and the new reverse faults defined wedge shaped fault blocks at the edges of the inverted structure in the second configuration the relationship between inversion and tilted fault blocks was tested when a tilting basement block does not move parallel to its bounding fault a compression is induced in the overburden which generates an inverted structure bounded by new reverse faults both experiments show that structural inversion not only reactivates pre existing faults but also generates new faults in the overburden faults related to inversion are the result of horizontal compression either associated with rotation of one of the fault blocks or with vertical uplift of basement blocks

analytical models for the mean flow inside dense canopies on gentle hilly terrain simplifications and scaling arguments employed in analytical models that link topographic variations to mean velocity perturbations within dense canopies are explored using laboratory experiments laser doppler anemometry lda measurements are conducted in a neutrally stratified boundary layer flow within a large recirculating flume over a train of gentle hills covered by a dense canopy the hill and canopy configuration are such that the mean hill slope is small and the hill is narrow in relation to the canopy h l 1 and lc l 1 where h is the hill height l the half length and lc the canopy adjustment length scale the lda data suggest that the often criticized linearizations of the advective terms turbulent shear stress gradients and drag force appear reasonable except in the deep layers of the canopy as predicted by a previous analytical model the lda data reveal a recirculation region within the lower canopy on the lee slope adjusting the outer layer pressure perturbations by a virtual ground that accounts for the mean streamline distortions induced by this recirculation zone improves this model s performance for the velocity perturbations in the deeper layers of the canopy a new analytical model which retains a balance between mean horizontal advection mean pressure gradient and mean drag force but neglects the turbulent shear stress gradient is developed the proposed model reproduces the lda measurements better than the earlier analytical model which neglected advection but retained the turbulent shear stress gradient in the lower layers of the canopy and near the hill top this finding is consistent with the fact that the earlier model was derived for tall hills in which advection inside the canopy remains small in essence the newly proposed model for the narrow hill studied here assumes that in the deeper layers of the canopy the spatial features of the mean flow perturbations around their background state can be approximated by the inviscid mean momentum equation we briefly discuss how to integrate all these findings with recent advances in canopy lidar remote sensing measurements of general topography and canopy height copyright 2008 royal meteorological society

storm tracks and zonal mean flow variability a comparison between observed and simulated data a statistical technique is used to analyze the relation between monthly mean zonal flow and storm tracks activity in the observations and numerical simulations echam4 model the singular value decomposition technique svd has been used to correlate storm tracks and monthly mean wintertime anomaly fields the analysis has been performed on data from january 1980 to december 1989 nmc analyses and on an ensemble of agcm simulations with prescribed sst for the same period separately in the euro atlantic and pacific sectors we found good correlation between storm tracks activity and zonal flow in both regions in both data and simulations the dominant svd modes show that the storm tracks spatial displacement is in conjunction with jet shifts in the same direction our analysis suggests that the model is highly sensitive to the equatorial ocean forcing although the model produces an excessive response to el nino and la nina phases it shows good capability of simulating the dynamical connection between storm tracks and jet

seasonality and interannual variability of the westerly jet in the tibetan plateau region abstract in this study 40 yr ecmwf re analysis era 40 data are used for the description of the seasonal cycle and the interannual variability of the westerly jet in the tibetan plateau region to complement results based on the analysis of monthly mean horizontal wind speeds an occurrence based jet climatology is constructed by identifying the locations of the jet axes at 6 hourly intervals throughout 1958 2001 thus a dataset describing the highly transient and localized features of jet variability is obtained during winter and summer the westerly jet is located respectively to the south and north of the tibetan plateau during the spring and autumn seasons there are jet transitions from south to north and vice versa the median dates for these transitions are 28 april and 12 october the spring transition is associated with large interannual variations while the fall transition occurs more reliably within a 3 week period the strength of the jet exhibits a peculiar seasonal cycle during northw

spring arctic oscillation east asian summer monsoon connection through circulation changes over the western north pacific in the present study the links between spring arctic oscillation ao and east asian summer monsoon easm was investigated with focus on the importance of the north pacific atmospheric circulation and sea surface temperature sst to reduce the statistical uncertainty we analyzed high pass filtered data with the inter annual time scales and excluded the el nino southern oscillation signals in the climate fields using a linear fitting method the significant relationship between spring ao and easm are supported by the changes of multi monsoon components including monsoon indices precipitation and three dimensional atmospheric circulations following a stronger positive spring ao an anomalous cyclonic circulation at 850 hpa appears in southeastern asia and the western north pacific in summer with the easterly anomalies spanning from the pacific to asian continent along 25 n 30 n and the westerly anomalies south of 15 n at the same time the summer western north pacific subtropical high becomes weaker consistently the positive precipitation anomalies are developed over a broad region south of 30 n stretching from southern china to the western pacific and the negative precipitation anomalies appear in the lower valley of the yangtze river and southern japan the anomalous cyclone in the western north pacific persisting from spring to summer plays a key role in modulating easm and monsoon precipitation by a positive air sea feedback mechanism during spring the ao associated atmospheric circulation change produces warmer ssts between 150 e 180 near the equator the anomalous sensible and latent heating in turn intensifies the cyclone through a gill type response of the atmosphere through this positive feedback the tropical atmosphere and sst patterns sustain their strength from spring to summer that consequently modifies the monsoon trough and the western north pacific subtropical high and eventually the easm precipitation moreover the sst response to ao circulation is supported by the numerical simulations of an ocean model and the anomalous atmospheric circulation over the western north pacific is also reproduced by the dedicated numerical simulations using the coupled atmosphere ocean model the observation evidence and numerical simulations suggest the spring ao can impact the easm via triggering tropical air sea feedback over the western north pacific

optimal control of katabatic flows within canopies what slope angle favours the development of katabatic flow is still an open question some studies have clarified that katabatic winds are stronger on steep slopes while others have demonstrated that katabatic winds are stronger on gentle slopes here we explore the control mechanisms of katabatic flow using a simplified theoretical model in an attempt to clarify the causes of the paradoxical findings our results indicate that optimal conditions for katabatic flows within canopies are synergistically controlled by terrain angle canopy structure and thermal condition through a simple equation where is terrain slope lc 1 cda is canopy length scale cd is drag coefficient a is leaf area density vt rc is a thermal factor rc is a cooling rate is ambient lapse rate and b is a constant this theoretical prediction implies that gentle slopes are optimal for katabatic flow developments in stably stratified air while steep slopes are optimal in weak or near neutral stratification canopy effect is significant in the control of katabatic flows only on gentle slopes copyright 2012 royal meteorological society

regionality of record breaking low temperature events in china and its associated circulation extreme cold events frequently occur around the world in recent several years and arouse widespread concern in this study 17 record breaking event processes rbeps of low temperature during 1981 2012 are identified by using daily minimum temperature at 1897 meteorological stations in china these rbeps are classified into two types based on the occurring area at northern or southern china to compositely examine the associated circulations although the correspondence between arctic oscillation ao and rbeps is not linearly stable there still exist relationship between them i e under ao negative phase the rbeps tend to occur at northern china nor the southern part where the rbeps prefer to happen under ao positive phase in the rbeps occurring at southern china the continent high pressure over mongolia area is extremely intensified and the east asian polar front jet stream is enhanced accompanied with strong synoptic scale eddy kinetic energy transports correspondingly the cold air masses break out and unobstructed southward intrude to low latitudes causing severe cooling effect in southern china in the rbeps occurring at northern china however the extremely intensified high pressure over northern siberian area combining with the northward enhanced subtropical jet stream lead to the cold air mass accumulation and blockage at mid latitudes and therefore rbeps of low temperature at this area further study implies that interdecadal change of the ao phase and differences of synoptic scale eddy activity might synthetically attribute to the different regional preference of those rbeps of low temperature that are mostly located at southern china in 1990s but concentrated in northern china in 2000s

the winter concurrent meridional shift of the east asian jet streams and the associated thermal conditions abstractin this study the concurrent meridional shift of the east asian polar front jet eapj and the east asian subtropical jet easj and the associated thermal conditions have been investigate

structural inheritance in mountain belts an alpine apennine perspective abstract the geological structure of continental lithosphere shows complex variety that is inherited into orogenic belts and influences the localization and amplification of contractional structures during mountain building in the alpine apennine sector together with other sectors of the tethyan orogenic system the pre orogenic crustal template can include arrays of extensional faults other faults can form adjacent to the evolving mountain belt and subsequently become incorporated as the thrust belts migrate into their forelands while in some areas these inherited features may simply reactivate under inversion more commonly faults show complex partial reactivation structures in volumes of distributed strain faults may serve to nucleate large scale buckle folds for example along basement cover interfaces these different patterns of basement reactivation may reflect spatially varying strength depth profiles in continental lithosphere that are themselves inherited from spatially distinct geological histories even when not themselves reactivating basement faults can control deformation in the overlying sedimentary cover by offsetting otherwise regionally extensive detachment horizons the 3d form of thrust systems can be strongly compartmentalized by pre existing cross faults such as the oblique lineaments of the apennines on a large scale the distribution of pre existing faults and other weaknesses may affect the propensity for orogenic contraction in basement and therefore directly control larger scale tectonic processes in the central mediterranean the evolution of slab roll back and the related growth of overlying extensional basins e g tyrrhenian sea may be strongly modulated by the distribution of rift related weak zones in the adjacent continental crust the subduction of continental crust will strongly depend on the inherited structure of this crust specifically the distribution of deep crust of basic composition this develops relatively higher densities associated with eclogite metamorphism which act in turn to reduce the buoyancy of thickened continental crust that otherwise serves to inhibit further shortening investigating all these aspects from the scale of bulk crustal compositions to the geometry timing and strength of earlier fault zones preserved in orogenic belts requires the integration of substantial multidisciplinary geological data sets the extent to which continental orogenic belts represent the amplification of inherited geological heterogeneities as opposed to self ordered phenomena modulated by the syntectonic environment remains unclear

remote response of the east asian winter monsoon to tropical forcing related to el nino southern oscillation 1 the mechanism of the east asian winter monsoon variability in response to el nino southern oscillation enso related tropical forcing is investigated using japanese long term reanalysis project data additionally aided by the japan meteorological agency climate data assimilation system there are at least two different responses zonally symmetric and asymmetric of the asian jet over south asia to the enso related tropical convective forcing during the northern hemisphere winter the zonally symmetric response induced by zonally extended anomalous convection from the philippine sea through southern india and sri lanka is pronounced at the mature phase of enso the zonally asymmetric response is intimately associated with anomalous convection localized in the vicinity of the philippine and south china seas accompanied by an anomalous walker circulation cell between the maritime continent and tropical indian ocean when this asymmetric response is prominent enso related anomalous convection can give rise to a change in the east asian winter monsoon system through stationary rossby wave propagation along the south asian waveguide the north atlantic oscillation nao related extratropical forcing is also a crucial factor and contributes not only to the downstream development of subpolar teleconnections across northern eurasia but also to the reinforcement of the zonally asymmetric pattern of the asian jet over south asia resulting in a significant effect on the east asian winter monsoon circulation a combination of the enso and nao related forcing plays a vital role in triggering the occurrence of extraordinary anomalous monsoon circulations such as extremely heavy snowfall in the 2005 2006 winter in japan

interannual variability of the east asian winter monsoon and related modulations of the planetary waves abstractinterannual variability of the east asian winter monsoon is investigated through composite analysis applied to observational data for 50 recent years although the monsoon activity itself is confined into the lower troposphere its midwinter variability tends to accompany upper tropospheric geopotential height anomalies similar to the eurasian eu and western pacific wp teleconnection patterns the eu like pattern is characterized by a wavy signature over the eurasian continent and the north atlantic with surface temperature anomalies over the far east and north america in the wp like pattern a meridional dipole of upper level height anomalies is evident over the far east these anomaly patterns related to the anomalous winter monsoon activity are found to accompany marked modulations of the climatological development of the upper tropospheric planetary waves from late autumn to midwinter enhanced monsoon activity in january associated with the wp like pattern involves anomalous seasonal

concurrent variation between the east asian subtropical jet and polar front jet during persistent snowstorm period in 2008 winter over southern china 1 the concurrent variation features between the east asian subtropical jet and polar front jet were investigated during persistent snowstorm period in 2007 2008 winter over southern china the east asian subtropical jet was divided into two parts 1 the plateau jet located along the southern side of the tibetan plateau and 2 the ocean jet situated at the southeastern japan island the concurrent intensity variation among the polar front jet plateau jet and ocean jet and the associated atmospheric anomalous signals were examined a possible mechanism for concurrent variation among the three jets was also investigated from a perspective of synoptic scale transient eddy activities stea the enhanced plateau jet was simultaneously correlated with the weakened polar front jet while the variation of the ocean jet lagged the variation of the plateau jet polar front jet about 5 days the concurrent variation between the plateau jet and the polar front jet acted as an important bridge that linked the snowstorm to the atmospheric anomalous signals associated with the cold and warm air activities due to the opposite trends of stea variation over the southern and northern sides of the tibetan plateau the plateau jet and the polar front jet exhibited a significant concurrent variation feature the stea anomalies over the plateau jet and polar front jet regions propagated downstream to the east asian coast as a wave train along the southern and northern sides of the tibetan plateau respectively resulting in a 5 day lag variation relationship between the ocean jet and the plateau jet polar front jet

persistent severe drought in southern china during winter spring 2011 large scale circulation patterns and possible impacting factors 1 severe drought persisted in southern china from january to may in 2011 in this study a statistical analysis is carried out to discuss the multiple possible impacting factors including la nina the north atlantic oscillation nao and the thermal condition of the tibetan plateau tp the la nina event in 2010 11 excited a lower tropospheric anomalous cyclone over the northwestern pacific weakening the northwestern pacific subtropical high and caused an eastward shift of the high as a result transportation of wet and warm moisture from tropical oceans to southern china decreased the la nina event also strengthened the upper tropospheric east asian jet stream and deepened the east asian trough favoring a southward intrusion of dry northerly flow from the siberia the la nina condition in the previous two seasons also seemed to provide precursory signals for the drought moreover in january may 2011 the nao was in a positive phase and it tended to excite stationary rossby waves that were distributed along the sub polar and subtropical waveguides respectively the sub polar one induced an anomalous anticyclone over the siberia favoring a southward intrusion of high latitude northerly flow to southern china the subtropical one associated possibly with the enhanced convection over the broad region from the mediterranean to sahara was favorable for an influence of upper tropospheric flow on southern china the tp might also exert an influence on the drought by weakening the westerly flow to the southern flank of tp and reducing water vapor transport from the bay of bengal to southern china

inversion tectonics in central alborz iran abstract structural analyses carried out in the southern central alborz iran have shown that the evolution of this belt has been strongly conditioned by the inversion of pre existent extensional faults inversion tectonics has been identified especially in the southern part of the belt and it is related to the reactivation of grabens formed in the foreland of the late triassic eo cimmerian orogen resulting from the accretion of the iranian block to eurasia three main associations of structures active during the late cenozoic have been distinguished 1 e w thrust faults and folds followed by the activation of 2 e w right lateral strike slip faults associated to large ene wsw trending en echelon folds and finally 3 ese wnw to se nw thrust faults and left lateral strike slip faults inverting in some cases previous e w right lateral faults central alborz is strongly controlled by the geometry of pre existing tectonic discontinuities which are responsible for strain partitioning between strike slip and reverse faults during convergence these results provide new insights on the style and evolution of this complex intracontinental belt which can be considered a significant example of transpressive tectonics

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

basin inversion and fault reactivation in laboratory experiments abstract a series of analogue models have been performed to get insights into the relationships between the reactivation of pre existing normal faults and the orientation of a successive compressive stress field the models were deformed in two phases 1 a first extension phase h 3 generating a graben and 2 a successive compressive phase with the orientation of the shortening direction h 1 trending between 0 and 90 obliquity angle to the faults bounding the graben the deepening of this graben was accompanied by syn tectonic sedimentation that included a basal ductile layer simulating salt in nature overlain by a brittle sand pack the experimental results suggest that the development of compressive structures is strongly controlled by the pre existing normal faults and by the obliquity angle the experiments have invariably shown that the extensional faults were reactivated during shortening whatever the value of the angle whereas strike slip reactivation of major normal faults is favoured by low obliquity angles mostly dip slip reactivation of pre existing structures is instead promoted for high obliquity angles the analysis of model results highlights the important role played by a ductile layer at the base of the basin fill and the obliquity angle in controlling both strain partitioning and brittle ductile decoupling

can a simple dynamical system describe the interplay between drag and buoyancy in terrain induced canopy flows abstractunder nonneutral stratification and in the presence of topography the dynamics of turbulent flow within a canopy is not yet completely understood this has among other consequences serio

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

seasonal variations of the synoptic scale transient eddy activity and polar front jet over east asia abstract seasonal variations of the synoptic scale transient eddy activity stea and the jet streams over east asia are examined through analysis of the 40 yr european centre for medium range weather forecasts ecmwf re analysis era 40 data extracted from the 6 hourly upper level wind fields the distribution of the jet core numbers exhibits a distinct geographical border for the east asian subtropical jet easj and the east asian polar front jet eapj at the latitudes of the northern tibetan plateau tp in the cool seasons two branches of the stea and low level baroclinicity exist over the east asian landmass accompanied by the two jet state of the easj and eapj in the warm seasons a single jet pattern of the easj along the north flank of the tp is accompanied by the weakened stea over the mid to high latitudes of east asia further analysis shows two distinct features of the seasonal variations of the stea over east asia compared with that over the north pacific first during the transit

cretaceous tertiary convergence and continental collision sanandaj sirjan zone western iran abstract the sanandaj sirjan zone contains the metamorphic core of the zagros continental collision zone in western iran the zone has been subdivided into the following from southwest to northeast an outer belt of imbricate thrust slices radiolarite bisotun ophiolite and marginal sub zones which consist of mesozoic deep marine sediments shallow marine carbonates oceanic crust and volcanic arc respectively and an inner complexly deformed sub zone late palaeozoic mesozoic passive margin succession rifting and sea floor spreading of tethys occurred in the permian to triassic but in the sanandaj sirjan zone extension related successions are mainly of late triassic age subduction of tethyan sea floor in the late jurassic to cretaceous produced deformation metamorphism and unconformities in the marginal and complexly deformed sub zones deformation climaxed in the late cretaceous when a major southwest vergent fold belt formed associated with greenschist facies metamorphism and post dated by abundant palaeogene granitic plutons in the southwest of the zone a late cretaceous island arc passive margin collision occurred with ophiolite emplacement onto the northern arabian margin similar to that in oman final closure of tethys was not completed until the miocene when central iran collided with the northeast arabian margin

seasonal evolution of the upper tropospheric westerly jet core over east asia the location change of the westerly jet core at upper troposphere in june and july is investigated by using the ncep ncar reanalysis data the results show that the location of the westerly jet core changes rapidly from 140 e to 90 e during 35th 39th pentads which corresponds to the plum rain period over east asia the location change of the jet core is actually the relative intensity change of the different westerly jet centers the meridional temperature contrast in the troposphere is associated with the rapid location change of the jet core the diabatic heating changes are the primary factor determining the seasonal evolution of the westerly jet core over east asia

flow over a hill covered with a plant canopy we develop an analytical model for atmospheric boundary layer flow over a hill that is covered with a vegetation canopy the slope of the hill is assumed to be small enough that the flow above the canopy can be treated within the linear framework of hunt perturbations to the flow within the canopy are driven by the pressure gradient associated with the flow over the hill in the upper canopy this pressure gradient is balanced by downwards turbulent transport of momentum and the canopy drag the flow there can be calculated from linearized dynamics which show that the maximum streamwise winds are where the perturbation pressure is at a minimum i e near the crest of the hill deep within the canopy the pressure gradient associated with the flow over the hill is balanced by the canopy drag here the nonlinear canopy drag this nonlinear balance shows how the streamwise winds are largest where the perturbation pressure gradient is largest i e on the upwind slope of the hill in the lee of the hill this nonlinear solution shows how the pressure gradient decelerates the wind deep within the canopy leading to separation with a region of reversed flow when the canopy is sufficiently deep coupling between the out of phase flows within and above the canopy means that the maximum velocity is further upwind of the hill crest than in flow over a rough hill while the extra turbulent mixing caused by the canopy significantly reduces the magnitude of the velocity speed up over the hill finally we find that there is no formal limit process where the solutions with a canopy yield the well known solutions for flow over a rough hill this finding calls into question the very use of a roughness length in accelerating or decelerating turbulent boundary layers copyright 2004 royal meteorological society

concurrent variations in the location and intensity of the asian winter jet streams and the possible mechanism this study investigates the concurrent variations in the location and intensity of the east asian subtropical jet easj and polar front jet eapj with a focus on the relationship between the variations and the atmospheric circulation in the mid latitude region the possible mechanisms for the concurrent variations of the easj and eapj are explored the wintertime upper level zonal wind variation over the landmass 50 100 e is characterized by two principal modes i e the out of phase variations in the intensity and the meridional displacements of the easj eapj the easj and eapj are intensified weakened when they are located close to far away from each other corresponding to a weakened intensified east asian trough over the northern pacific region and a decreased increased geopotential height in the mid latitude over central asia the spatial pattern of 500 hpa geopotential height anomaly is similar to the eurasian teleconnection pattern while the pattern of sea level pressure anomaly shows a feature similar to the arctic oscillation analyses of related thermal dynamical processes indicate that the increased decreased diabatic heating and horizontal heat transport lead to a larger smaller meridional temperature gradient over the easj eapj regions which subsequently intensifies weakens the east asian jets severe weak convective activities accompanied with warmer colder sst in the tropics also contribute to the intensification weakening of the easj and eapj dynamically the strong weak divergence of the eddy vorticity flux along the northern tibetan plateau and that of the eliassen palm flux of the synoptic scale stationary waves are favorable for the intensification weakening of zonal winds through the wave mean flow interactions as a result the easj and eapj are intensified weakened

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