an interannual link between arctic sea ice cover and the north atlantic oscillation this work investigates links between arctic surface variability and the phases of the winter djf north atlantic oscillation nao on interannual time scales the analysis is based on era reanalysis and model data from the ec earth global climate model our study emphasizes a mode of sea ice cover variability that leads the nao index by 1 year the mechanism of this leading is based on persistent surface forcing by quasi stationary meridional thermal gradients associated thermal winds lead a slow adjustment of the pressure in the following winter which in turn feeds back on the propagation of sea ice anomalies the pattern of the sea ice mode leading nao has positive anomalies over key areas of south davis strait labrador sea the barents sea and the laptev ohkostsk seas associated to a high pressure anomaly over the canadian archipelago baffin bay and the laptev east siberian seas these anomalies create a quasi annular quasi steady positive gradient of sea ice anomalies about coastal line when leading the positive nao phase and force a cyclonic vorticity anomaly over the arctic in the following winter during recent decades in spite of slight shifts in the modes spectral properties the same leading mechanism remains valid encouraging actual models appear to reproduce the same mechanism leading model s nao relative to model areas of persistent surface forcing this indicates that the link between sea ice and nao could be exploited as a potential skill source for multi year prediction by addressing the key problem of initializing the phase of the nao ao arctic oscillation

a seasonal climatology of rossby wave breaking in the 320 2000 k layer abstract differential advection in rossby waves can lead to potential vorticity pv p contours on isentropic surfaces folding over in latitude py 0 in a process called rossby wave breaking rwb exploring the properties of rwb may shed light on underlying dynamics and enable quantification of irreversible transport a seasonal climatology of py and rwb statistics is presented for the 320 850 k layer using ncep reanalysis data during 1979 2005 and for the 320 2000 k layer using the met office ukmo data during 1991 2003 a primary goal is to depict the spatial extent and seasonality of rwb maxima this analysis shows seven distinct rwb regimes poleward and equatorward of the subtropical westerly jets poleward and equatorward of the stratospheric polar night jets flanking the equator in the stratosphere and mesosphere equatorward of subtropical monsoon anticyclones and the summertime polar stratosphere a striking pv gradient maximum exists at the equator throughout the layer 360 2000 k flanke

characteristics of the atlantic storm track eddy activity and its relation with the north atlantic oscillation abstract this study focuses on feedbacks of the high frequency eddy activity onto the quasi stationary circulation particularly with regard to the north atlantic oscillation nao the methodology consists of analyzing ncep ncar reanalysis data and sensitivity runs from a high resolution nonhydrostatic regional model consistent with recent studies results show that the jet displacement characteristic of the nao phenomenon depends strongly on the dynamics of the synoptic scale waves and the way they break positive and negative phases of the nao are closely related to anticyclonic and cyclonic wave breaking respectively indeed the high frequency momentum flux whose sign is directly related to the type of wave breaking is correlated with the nao index over the atlantic the peak of the momentum flux signal precedes that of the nao by a few days suggesting that wave breaking is triggering nao events two examples illustrate the significant impact of single storms in particular those occurring in the e

the link between the north pacific climate variability and the north atlantic oscillation via downstream propagation of synoptic waves abstractthe north atlantic oscillation nao response to the northeast pacific climate variability is examined using the era 40 dataset the main objective is to validate a mechanism involving downstream wave propagation processes proposed in a recent idealized companion study a low frequency planetary scale ridge trough anomaly located in the eastern pacific north american sector induces more equatorward poleward propagation of synoptic scale wave packets on its downstream side which favors the occurrence of anticyclonic cyclonic wave breakings in the atlantic sector and the positive negative nao phase the mechanism first provides an interpretation of the canonical impact of the el nino southern oscillation on the nao in late winter the wintertime relationship between the pacific north american oscillation pna and the nao is also investigated for out of phase fluctuations between the pna and nao indices i e the most recurrent situation in late winter the eastern pacific pna ridge trou

effect of the hadley circulation on the reflection of planetary waves in three dimensional tropospheric flows the nonlinear behavior of quasi stationary planetary waves excited by midlatitude orographic forcing is considered in a three dimensional primitive equation model that includes a representation of the hadley cir culation the hadley circulation is forced by newtonian cooling to a zonally symmetric reference temperature and vertical diffusion on the zonally symmetric component of the flow to quantify the effect of the hadley circulation on wave propagation breaking and nonlinear reflection an initial state with no meridional flow but with the same zonal flow as the hadley state is also considered in order to allow the propagation of large scale waves over extended periods rayleigh friction is applied at low levels to delay the onset of baroclinic instability as in the absence of a hadley circulation the waves in the hadley state propagate toward low latitudes where the background flow is weak and the waves are therefore likely to break potential vorticity fields on isentropic surfaces are used to diagnose wave breaking nonlinear pseudomomentum conservation relations are used to quantify the absorption reflection behavior of the wave breaking region in the presence of a hadley circulation representative of winter conditions the nonlinear reflection requires more forcing to get established but a reflected wave train is still present in the numerical simulations both for a longitudinally symmetric forcing and for the more realistic case of an isolated forcing the effect of the thermal damping on the waves is more severe in the current three dimensional simulations than in the shallow water case considered in an earlier study both the directly forced wave train and the reflected wave train are quite barotropic in character however in the shallow water case one is essentially assuming an infinite vertical scale

the role of synoptic waves in the formation and maintenance of the western hemisphere circulation pattern abstractprevious studies have demonstrated that the nao the leading mode of atmospheric low frequency variability over the north atlantic could be linked to northeast pacific climate variability via the downstream propagation of synoptic waves in those studies the nao and the northeast pacific climate variability are considered as two separate modes that explain the variance over the north atlantic sector and the east pacific north american sector respectively a newly identified low frequency atmospheric regime the western hemisphere wh circulation pattern provides a unique example of a mode of variability that accounts for variance over the whole north atlantic north american north pacific sector the role of synoptic waves in the formation and maintenance of the wh pattern is investigated using the ecmwf reanalysis datasets persistent wh events are characterized by the propagation of quasi stationary rossby waves across the north pacific north american north atlantic regions and by associated s

a feature based approach to classifying summertime potential vorticity streamers linked to rossby wave breaking in the north atlantic basin abstractthis study examines climatological potential vorticity streamer pvs activity associated with rossby wave breaking rwb which can impact tc activity in the subtropical north atlantic na

tropospheric rossby wave breaking and the nao nam objective analysis of several hundred thousand anticyclonic and cyclonic breaking rossby waves is performed for the northern hemisphere nh winters of 1958 2006 a winter climatology of both anticyclonic and cyclonic rossby wave breaking rwb frequency and size zonal extent is presented for the 350 k isentropic surface over the nh and the spatial distribution of rwb is shown to agree with theoretical ideas of rwb in shear flow composites of the two types of rwb reveal their characteristic sea level pressure anomalies upper and lower tropospheric velocity fields and forcing of the upper tropospheric zonal flow it is shown how these signatures project onto the centers of action and force the velocity patterns associated with the north atlantic oscillation nao and northern hemisphere annular mode nam previous studies have presented evidence that anticyclonic cyclonic breaking leads to the positive negative polarity of the nao and this relationship is confirmed for rwb over the midlatitudes centered near 50 n however an opposite and statistically significant relationship in which cyclonic rwb forces the positive nao and anticyclonic rwb forces the negative nao is shown over regions 20 to the north and south centered at 70 and 30 n respectively on a winter mean basis the frequency of rwb over objectively defined regions covering 12 of the area of the nh accounts for 95 of the nao index and 92 of the nam index a 6 hourly analysis of all the winters indicates that rwb over the objectively defined regions affects the nao nam without a time lag details of the objective wave breaking analysis method are provided in the appendix

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

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

nonlinear planetary wave reflection in an atmospheric gcm the nonlinear behavior of planetary waves excited by midlatitude topography is considered in an atmospheric gcm the gcm is run at standard resolution t42 and includes all of the complexity normally associated with a gcm only two simplifications are made to the model first it is run in perpetual january mode so that the solar radiation takes the diurnally varying value associated with 15 january second the lower boundary is simplified so that it is entirely ocean with zonally symmetric ssts planetary waves are excited by gaussian shaped topography centered at 45 n 90 w as in earlier studies the excited wave train propagates toward low latitudes where for sufficiently large forcing amplitude i e height of topography the wave will break several different experiments are run with different mountain heights each experiment is run for a total of 4015 days the response of the model depends on the height of the mountain for the small amplitude mountain 500 m the wave is dissipated at low latitudes near its critical latitude for large amplitude mountains 2000 3000 and 4000 m wave breaking and nonlinear reflection out of the wave breaking region is observed the spatial character of the reflected wave train is similar to that detected in earlier studies with more idealized models

coupling between greenland blocking and the north atlantic oscillation pattern 1 through the adoption of a bidimensional blocking index based on geopotential height it is shown that the blocking frequency over greenland is not only a key element in describing the north atlantic oscillation nao index but it also operates as an essential element in modulating its pattern when greenland blocking is lower than average the first empirical orthogonal function eof of the 500 hpa geopotential height over the euro atlantic region no longer resembles the nao moreover the typical trimodal variability observed in the atlantic eddy driven jet stream is reduced to a bimodal variability consistent with this result we link the eastward displacement of the nao pattern observed in recent years to the decreasing frequency of greenland blocking considering the large bias seen in the simulated blocking frequency in climate models such strong coupling might have important consequences in the analysis of the nao in climate simulations

bidimensional diagnostics variability and trends of northern hemisphere blocking abstractin this paper northern hemisphere winter blocking is analyzed through the introduction of a set of new bidimensional diagnostics based on geopotential height that provide information about the occurrence the duration the intensity and the wave breaking associated with the blocking this analysis is performed with different reanalysis datasets in order to evaluate the sensitivity of the index and the diagnostics adopted in this way the authors are able to define a new category of blocking placed at low latitudes that is similar to midlatitude blocking in terms of the introduced diagnostics but is unable to divert or block the flow furthermore over the euro atlantic sector it is shown that it is possible to phenomenologically distinguish between high latitude blocking occurring over greenland north of the jet stream and dominated by cyclonic wave breaking and the traditional midlatitude blocking localized over europe and driven by anticyclonic wave breaking these latter events are uniform

on the use of the norwegian geotechnical institute s prototype seabed coupled shear wave vibrator for shallow soil characterization i acquisition and processing of multimodal surface waves summary pure shear wave data are only very rarely acquired for offshore site investigations and exploration here we present details of a novel seabed coupled shear wave vibrator and field data recorded by a densely populated multicomponent ocean bottom cable to improve shallow soil characterization the prototype shear wave vibrator uses vibroseis technology adopted for marine environments through its instalment on top of a suction anchor assuring seabed coupling in combination with self weight penetration the prototype is depth rated to 1500 m water depth and can be rotated while installed in the seabed the philosophy is to acquire fully complementary seismic data to conventional p and p to s converted waves in particular for 2 d profiling vsp vertical seismic profiling or monitoring purposes thereby exploiting advantages of shear waves over compressional waves for determining for example anisotropy small strain shear modulus and excess pore pressures effective stress the source was primarily designed for reservoir depths however significant energy is emitted as surface waves which provide detailedgeotechnicalinformationthroughmappingofshearwavevelocitiesinpotentiallyhigh resolution of the upper soil units to fully utilize pure shear wave content a proper analysis of surface waves is paramount due to the proximity of surface wave propagation speed with shear wave velocities the experiment was carried out in the northern north sea in 364 m water depth cable dragging was necessary to obtain close receiver spacing 2 5 m effective spacing with total line length of 600 m frequency waveform transforms reveal both scholte and love waves up to six surface wave modes are identified that is fundamental mode and several higher surface wave modes the occurrence of these two different dispersive surface wave types with well resolved higher modes allows for a unique analysis and inversion scheme for highresolution mapping of physical properties in the shallow subsurface as well as anisotropy which is discussed in an accompanying paper the data presented in this paper are thus a unique long and densely populated receiver array allows for multimodal love and scholte surface waves from the marine environment but challenging marine operations marine data set

seismic inversion of soil damping and stiffness using multichannel analysis of surface wave measurements in the marine environment determination of soil material damping is known to be difficult and uncertain especially in the offshore environment using an advanced inversion methodology based on multichannel spectral analysis scholte and love wave measurements are used to characterize subsea soil from a north sea site after normalization a determinant based objective function is used in a genetic algorithm optimization to estimate the soil shear modulus the inverted shear modulus profile is comparable to previously published results for the same data although a higher degree of certainty is achieved in the near surface layers the half power bandwidth method is used for extracting the attenuation curve from the measurements and efficient reference data points are chosen based on wavelet compression the material damping ratio inversion is performed using a modified stochastic optimization algorithm accounting for measurement errors the material damping ratio profile is retrieved from the fundamental mode scholte wave with a high degree of certainty furthermore a method is proposed for identifying the frequency dependence of the material damping ratio from in situ measurements no evidence for frequency dependence is found and the small strain soil material damping ratio at this site can be said to be frequency independent for the measured conditions

seismic attenuation from recordings of ambient noise abstractwe applied seismic interferometry to data from an ocean bottom survey offshore norway and found that ambient seismic noise can be used to constrain subsurface attenuation on a reservoir scale by crosscorrelating only a few days of recordings by broadband ocean bottom seismometers we were able to retrieve empirical green s functions associated with surface waves in the frequency range between 0 2 and 0 6 hz and acoustic waves traveling through the sea water between 1 0 and 2 5 hz we discovered that the decay of these surface waves cannot be explained by geometrical spreading alone and required an additional loss of energy with distance we quantified this observed attenuation in the frequency domain using a modified bessel function to describe the cross spectrum in a stationary field we averaged cross spectra of equally spaced station couples and sorted these azimuthally averaged cross spectra with distance we then obtained frequency dependent estimates of attenuation by minimizing the misfit

how rossby wave breaking over the pacific forces the north atlantic oscillation 1 anticyclonic rossby wave breaking rwb over a well defined limited area region of the east pacific leads to the positive polarity of the north atlantic oscillation nao by locally piling up wave activity where it may be advected downstream resulting in increased wave activity flux and anticyclonic rwb over the subtropical atlantic a composite time series shows that pacific rwb occurs several days prior to the atlantic rwb and the peak of the nao index following pacific rwb a channel of increased pseudomomentum flux extends from the pacific wave breaking region northeastward toward midlatitudes of eastern north america where pseudomomentum density accumulates for several days prior to moving eastward and leading to anticyclonic rwb over the atlantic

phase speed spectra of transient eddy fluxes and critical layer absorption abstract tropospheric zonal mean eddy fluxes of heat and momentum and the divergence of the eliassen palm flux are decomposed into contributions from different zonal phase speeds data analyzed are ecmwf operational global analyses covering 1980 87 eastward moving medium scale waves zonal waves 4 7 dominate the spectra of lower tropospheric heat fluxes in both hemispheres and all seasons upper tropospheric wave flux spectra are similar to the low level spectra in midlatitudes but shift to slower zonal phase speeds as low latitudes are approached the cause of this shift is the selective absorption of faster moving components in midlatitudes as the waves propagate meridionally latitude phase speed distributions of eddy fluxes are constructed and compared to the zonal mean wind structure these results demonstrate that upper tropospheric eddies break and decelerate the zonal mean flow approximately 10 20 in latitude away from their critical line where phase speed equals zonal wind speed compari

retrieving surface waves from ambient seismic noise using seismic interferometry by multidimensional deconvolution retrieving virtual source surface waves from ambient seismic noise by cross correlation assumes among others that the noise field is equipartitioned and the medium is lossless violation of these assumptions reduces the accuracy of the retrieved waves a point spread function computed from the same ambient noise quantifies the associated virtual source s spatial and temporal smearing multidimensional deconvolution mdd of the retrieved surface waves by this function has been shown to improve the virtual source s focusing and the accuracy of the retrieved waves using synthetic data we tested mdd on data recorded during the batholiths experiment a passive deployment of broadband seismic sensors in british columbia canada the array consisted of two approximately linear station lines using 4 months of recordings we retrieved fundamental mode rayleigh waves 0 05 0 27 hz we only used noise time windows dominated by waves that traverse the northern line before reaching the southern 2 5 of all data compared to the conventional cross correlation result based on this subset the mdd waveforms are better localized and have significantly higher signal to noise ratio furthermore mdd corrects the phase and the spatial deconvolution fills in a spectral f k domain gap between the single frequency and double frequency microseism bands frequency whitening of the noise also fills the gap in the cross correlation result but the signal to noise ratio of the mdd result remains higher comparison of the extracted phase velocities shows some differences between the methods also when all data are included in the conventional cross correlation

climatology of intrusions into the tropical upper troposphere regions of upper tropospheric equatorial westerly winds observed over the pacific and atlantic oceans during northern fall to spring are important for extratropical tropical interactions this paper focuses on one feature of these westerly ducts that has received relatively little attention to date the occurrence of rossby wave breaking events that transport tongues of extratropical air deep into the tropics mix tropical and subtropical air and can affect deep convection a climatology of these intrusion events formed from 20 years of meteorological analyses shows a strong dependence on the basic state flow notably intrusion events are found to occur almost exclusively within westerly ducts with more events in the presence of stronger equatorial westerlies it is also found that there is strong interannual variability in the frequency of pacific events with fewer events during the warm phases of enso consistent with the changes in the basic flow since these intrusion events laterally mix trace constituents and have been linked to tropical convection their spatial and temporal variability may cause related variability in the distribution of trace constituents and tropical convection

the connection between extreme stratospheric polar vortex events and tropospheric blockings the variations of tropospheric blockings and their connections with the stratospheric polar vortex during different stages of the life cycle of extreme stratospheric polar vortex events i e strong vortex sv events and weak vortex wv events are investigated the blocking frequency decreases over the euro atlantic sector and increases over the western north pacific during the onset and maturation stages of sv events there are more blocking days over the western north pacific and weakened upward planetary wave fluxes in the stratosphere during the maturation stage of shorter time scale sv events than during longer time scale sv events and the weakening of the upward planetary wave fluxes mainly results from the linear wave interference effect the blocking frequency is increased over the euro atlantic sector during the decline stage of sv events this increase is found to be related to the descending stratospheric zonal wind anomalies and the poleward displacement of the eddy driven jet stream which lasts for about 10 days and is associated with the descending positive stratospheric northern annular mode nam anomalies the increased blocking frequency leads to an enhancement of the planetary wave fluxes in the stratosphere via both the linear wave interference for shorter time scale sv events and nonlinear wave interference for longer time scale sv events effect the changes of the blocking frequency during the growth stage of wv events are almost opposite to those observed during the onset and maturation stages of sv events the blocking frequency decreases over the euro atlantic sector after the central dates of wv events which are related to the descending stratospheric zonal wind anomalies and the equatorward displacement of the eddy driven jet stream preceded by the descending negative stratospheric nam anomalies about 10 days we found that the nonlinear wave interference begins to make a considerable contribution to the weakening of the upward wave fluxes in the stratosphere after the central date of wv events and this is not emphasized in previous literature

the role of tropospheric rossby wave breaking in the pacific decadal oscillation abstract the leading pattern of extratropical pacific sea surface temperature variability the pacific decadal oscillation pdo is shown to depend on observed variability in the spatiotemporal distribution of tropospheric rossby wave breaking rwb where rwb is the irreversible overturning of potential vorticity on isentropic surfaces composite analyses based on hundreds of rwb cases show that anticyclonic cyclonic rwb is associated with a warm moist cool dry column that extends down to a surface anticyclonic cyclonic circulation and that the moisture and temperature advection associated with the surface circulation patterns force turbulent heat flux anomalies that project onto the spatial pattern of the pdo the rwb patterns that are relevant to the pdo are closely tied to el nino southern oscillation the pacific north american pattern and the northern annular mode these results explain the free troposphere to surface segment of the atmospheric bridge concept wherein el nino anomalies eme

estimation of seabed shear wave velocity profiles using shear wave source data this paper estimates seabed shear wave velocity profiles and their uncertainties using interface wave dispersion curves extracted from data generated by a shear wave source the shear wave source generated a seismic signature over a frequency range between 2 and 60 hz and was polarized in both in line and cross line orientations low frequency scholte and love waves were recorded dispersion curves of the scholte and love waves for the fundamental mode and higher order modes are extracted by three time frequency analysis methods both the vertically and horizontally polarized shear wave velocity profiles in the sediment are estimated by the scholte and love wave dispersion curves respectively a bayesian approach is utilized for the inversion differential evolution a global search algorithm is applied to estimate the most probable shear velocity models marginal posterior probability profiles are computed by metropolis hastings sampling the estimated vertically and horizontally polarized shear wave

vertical and latitudinal variation of the intertropical convergence zone derived using gps radio occultation measurements abstract using gps radio occultation refractivity data collected over the period of 2002 2013 we present a new method for identification of the intertropical convergence zone itcz the itcz is identified by estimating the maximum in the monthly meridional refractivity and specific humidity field by applying a gaussian fit at each longitude the interannual variability and climatology of the itcz is presented from 12 years of refractivity data this new method captures all the general features of itcz extent and its variability we also examine the effects of the itcz vertically during different seasons the itcz is observed mostly at eastern pacific in may month and it is zonally distributed in the september and october months of each year the zonal variability is large between lower and higher levels particularly over the indian monsoon and western pacific the latitudinal difference in the vertical extent between 850 hpa and higher levels is larger during the northern hemisphere nh summer than nh winter

in situ measurements of shear wave velocity in sediments with higher mode rayleigh waves a seismic survey was carried out on a tidal flat in the sw netherlands in order to determine shear wave velocities in sediments by means of higher mode rayleigh waves the dispersion properties of these rayleigh waves were measured in the 2 d amplitude spectrum or f k spectrum and resulted in phase velocities for six different modes as a function of frequency 5 30 hz these observed phase velocities were inverted for a nine layer model for the shear wave velocity to a depth of 50 m

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

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

planetary wave breaking and nonlinear reflection seasonal cycle and interannual variability abstract forty six years of daily averaged ncep ncar reanalysis data are used to identify the occurrence of planetary wave breaking pwb in the subtropical upper troposphere as large amplitude waves propagate into the subtropics where the zonal flow is weak they may break pwb is diagnosed by observing the large scale meridional overturning of potential vorticity pv contours on isentropic surfaces near the subtropical tropopause pwb occurs most often during summer and almost exclusively over the subtropical ocean basins in the northern hemisphere the seasonal evolution of the zonal flow and the associated latitudinal pv gradient regulates the location and frequency of pwb significant interannual variability in pwb is associated with well known modes of climate variability one of the most interesting dynamical consequences of pwb is the possibility of nonlinear reflection poleward out of the wave breaking region modeling studies have found nonlinear reflection following pwb observations show

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

on the teleconnectivity of the arctic oscillation the term arctic oscillation ao has recently been introduced to describe the leading structure of slp variability over the northern hemisphere a key feature of the ao is its zonally symmetric appearance with a primary center of action over the arctic and opposing anomalies in midlatitudes does the ao s annular appearance result from significant temporal correlations between slp anomalies at distant longitudes the results presented indicate that the temporal coherence between the arctic and midlatitudes is strongest over the atlantic sector with weak correlations between the atlantic and pacific midlatitudes both on intraseasonal and interannual time scales during the past 50 yrs hence the annular character of the ao is more a reflection of the dominance of its arctic center of action than any coordinated behavior of the atlantic and pacific centers of action in the slp field the ao is nearly indistinguishable from the leading structure of variability in the atlantic sector e g the north atlantic oscillation their temporal correlation is 0 95 for monthly data

arctic oscillation or north atlantic oscillation abstract the definition and interpretation of the arctic oscillation ao are examined and compared with those of the north atlantic oscillation nao it is shown that the nao reflects the correlations between the surface pressure variability at its centers of action whereas this is not the case for the ao the nao pattern can be identified in a physically consistent way in principal component analysis applied to various fields in the euro atlantic region a similar identification is found in the pacific region for the pacific north american pna pattern but no such identification is found here for the ao the ao does reflect the tendency for the zonal winds at 35 and 55 n to anticorrelate in both the atlantic and pacific regions associated with the nao and pna because climatological features in the two ocean basins are at different latitudes the zonally symmetric nature of the ao does not mean that it represents a simple modulation of the circumpolar flow an increase in the ao or nao implies stro

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

identification and era 15 climatology of potential vorticity streamers and cutoffs near the extratropical tropopause abstract a novel approach is introduced to identify potential vorticity pv streamers and cutoffs as indicators of rossby wave breaking near the extratropical tropopause and to compile climatologies of these features on different isentropic surfaces the method is based on a contour searching algorithm that identifies the dynamical tropopause 2 potential vorticity units pvu pvu 1 10 6 k kg 1 m2 s 1 isoline on isentropic surfaces the contour is then analyzed to search for cutoffs and filament like streamers whereas the identification of cutoffs is unambiguous the one for streamers requires the specification of two parameters that determine the width and length of the contour feature to be classified as a streamer this technique has been applied to the pv distribution in the northern hemisphere on isentropes from 295 to 360 k during the time period from 1979 to 1993 using the 15 yr european centre for medium range weather forecasts ecmwf re analysis era 15 the climatology reveals a pronou

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

tropospheric rossby wave breaking and the sam an objective analysis of tropospheric anticyclonic and cyclonic breaking rossby waves is performed for the southern hemisphere in austral summer december february of 1979 2009 the climatology of both anticyclonicandcyclonic rossbywavebreaking rwb frequencyispresented thefrequencyofanticyclonic rwb is highest in an extended region of theeastern hemisphere on the anticyclonic side of the jet whilethat of cyclonic rwb is highest on the cyclonic side of the jet a composite analysis of anticyclonic and cyclonic rwb shows howtheycontributetoapositiveandnegativesouthernannualmode sam index respectively thetimeseries of austral summer anticyclonic rwb occurrence has a trend that closely matches the trend in the sam index regions of rwb that are significantly correlated with the sam index are objectively determined even thoughseveralsuchregionsareidentified onlytworegions anticyclonicandcyclonic covering17 ofthearea of the hemisphere are required in a linear regression model of the sam index the anticyclonic rwb region is zonally extended at 458s and explains 78 of the variability of the summer mean sam index the cyclonic region is located at high latitudes somewhat decoupled from the jet in the longitudinal sector of the indian ocean on synoptic time scales transitions of the sam index respond to rwb without time lag ensocyclespresentaninterestingzonalasymmetrytothedistributionofsouthernhemisphericrwbinthe

local mixing events in the upper troposphere and lower stratosphere part ii seasonal and interannual variability the lyapunov diffusivity is used to investigate local isentropic mixing events in the upper troposphere lower stratosphere utls region the diagnostic highlights the seasonal cycle of the longitudinally varying mixing properties in particular those associated with the monsoon circulations and the westerly ducts in the subtropics the results are broadly consistent with studies of rossby wave breaking frequencies the mixing structure is shown to be modulated by modes of atmospheric variability el nino southern oscillation enso is found to strongly influence the mixing structure throughout the tropics and subtropics the sub tropical jet is associated with longitudinal bands of mixing minima isentropic mixing barriers separated by synoptic scale regions of strong mixing activity the greatest enso modulation in december february is confined to the pacific sector where the barriers associated with the subtropical jets extend farther into the eastern pacific and in the western pacific a barrier is found at the equator during the positive phase during june august the influence is seen to extend beyond the pacific region with the barrier at the subtropical jet in the southern hemisphere increasing in strength at all longitudes and with an increase in strength and isolation of monsoon related mixing over asia and north america the local influence of the north atlantic oscillation in wintertime is investigated during the positive phase a double barrier structure is associated with the subtropical jet the northern branch crosses the atlantic toward scandinavia and the southern branch tends toward north africa the antarctic oscillation is shown to influence whether the subvortex region is isolated from midlatitudes

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

estimation of wave phase speed and nearshore bathymetry from video imagery a new remote sensing technique based on video image processing has been developed for the estimation of nearshore bathymetry the shoreward propagation of waves is measured using pixel intensity time series collected at a cross shore array of locations using remotely operated video cameras the incident band is identified and the cross spectral matrix is calculated for this band the cross shore component of wavenumber is found as the gradient in phase of the first complex empirical orthogonal function of this matrix water depth is then inferred from linear wave theory s dispersion relationship full bathymetry maps may be measured by collecting data in a large array composed of both cross shore and longshore lines data are collected hourly throughout the day and a stable daily estimate of bathymetry is calculated from the median of the hourly estimates the technique was tested using 30 days of hourly data collected at the sandyduck experiment in duck north carolina in october 1997 errors calculated as the difference between estimated depth and ground truth data show a mean bias of 35 cm rms error 91 cm expressed as a fraction of the true water depth the mean percent error was 13 rms error 34 excluding the region of known wave nonlinearities over the bar crest the accuracy of the technique improved and the mean rms error was 20 cm 75 cm additionally under low amplitude swells wave height h 1 m the performance of the technique across the entire profile improved to 6 29 of the true water depth with a mean rms error of 12 cm 71 cm

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

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

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

on the use of the norwegian geotechnical institute s prototype seabed coupled shear wave vibrator for shallow soil characterization ii joint inversion of multimodal love and scholte surface waves summary seismic data generated by a novel seabed coupled shear wave vibrator prototype and recorded by a densely populated multicomponent ocean bottom cable allowed several modes of propagation of love and scholte waves to be retrieved in a relatively wide frequency band both global dispersion curves and local dispersion curves are extracted in the frequency wavenumber f k domain and inverted with a multimodal joint scholte and love wave inversion algorithm monte carlo inversion is used for a estimating the global s wave velocity profile of the seabed sediments whereas laterally constrained inversion is used to detect lateral variations of the layer interface depths the results are in agreement and allowed consistent full waveform simulation to be performed the investigation depth is limited to around 40 m due to the low shear wave velocities within the shallow clay layer

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

a new rossby wave breaking interpretation of the north atlantic oscillation this paper proposes the hypothesis that the low frequency variability of the north atlantic oscillation nao arises as a result of variations in the occurrence of upper level rossby wave breaking events over the north atlantic these events lead to synoptic situations similar to midlatitude blocking that are referred to as high latitude blocking episodes a positive nao is envisaged as being a description of periods in which these episodes are infrequent and can be considered as a basic unblocked situation a negative nao is a description of periods in which episodes occur frequently a similar but weaker relationship exists between wave breaking over the pacific and the west pacific pattern evidence is given to support this hypothesis by using a two dimensional potential vorticity based index to identify wave breaking at various latitudes this is applied to northern hemisphere winter data from the 40 yr ecmwf re analysis era 40 and the events identified are then related to the nao certain dynamical precursors are identified that appear to increase the likelihood of wave breaking these suggest mechanisms by which variability in the tropical pacific and in the stratosphere could affect the nao