spatio temporal structure of the pentadecadal variability over the north pacific abstract using a multi taper frequency domain singular value decomposition mtm svd a pentadecadal oscillation was detected in the winter spring sea level pressure slp field over the north pacific and surface air temperature in north america which was significant at the 95 confidence level the mtm svd captured the different slp and air temperature distributions between the winter and spring seasons in a consistent manner the pentadecadal slp signature in the spring season is centered nearer the west coast of north america than in the winter season this zonal displacement is consistent with the prominent springtime pentadecadal air temperature variability in mid latitude western north america a wavelet analysis of the pacific decadal oscillation index pdoi showed that the regime shifts in the 1920s 1940s and 1970s involved simultaneous phase reversals of the bidecadal and pentadecadal variations the two interdecadal variations are synchronized with one another such that a half period of the pentadecadal oscillation one epoch of an individual regime corresponds to one and half periods of the bidecadal oscillation these results are consistent with the wavelet analysis of the north pacific index npi similar resonance between the bidecadal and pentadecadal variations is evident in air temperatures over alaska the bidecadal and pentadecadal signals have different seasonality in these time series suggesting that although the two interdecadal variations arise from two different physical mechanisms they interact with each other the most distinct seasonal difference was observed in mid latitude western north america where the bidecadal variation prevails only in the winter season and the pentadecadal variation only in the spring season alaska air temperatures in the winter and winter spring of 1999 were the coldest since 1977 as were springtime air temperatures in mid latitude western north america in contrast to the warm anomalies that prevailed in this region during 1977 98 the npi and pdoi also exhibited an opposite polarity in 1999 to the respective regime mean polarities these anomalous conditions in winter and spring seasons of 1999 may signify a major regime shift in 1998 1999 in order to verify whether or not a regime shift did occur in 1998 1999 a careful examination of additional data in coming ten or so years will be necessary

topographic meandering of antarctic circumpolar current and antarctic circumpolar wave in the ice ocean atmosphere system 1 topographic meandering of antarctic circumpolar current acc is found to be an impediment in the propagation of antarctic circumpolar wave acw in the indian ocean sector of antarctica reasons for this are attributed to the southward advection of the acw anomalies associated with the topographic meandering of the acc the southward meandering of acc facilitates warming up of the region east of 20 e by about 1 c during winter thereby reducing the sea ice these processes interfere with the eastward propagating positive sea ice anomalies and reduce its strength warming of ocean induced by topographic meandering leads to upward vertical velocities between 40 60 e where the ocean surface velocities are weak and southward and the vertical meridional advection of temperature dominates the zonal advection in the atmosphere this results in the decoupling of the acw in the region east of 40 e in regions out side the indian ocean sector vertical advection is minimum and zonal velocity of acc becomes positive which facilitates the acw propagation in the central pacific ross and weddell seas citation nuncio m a j luis and x yuan 2011 topographic meandering of antarctic circumpolar current and antarctic circumpolar wave in the ice ocean atmosphere system geophys res lett 38 l13708

role of the ocean in the decadal climate change in the north pacific 1 i have investigated features of decadal climate variability in the north pacific and clarified the role of the ocean in controlling them by analyzing historical data sets and executing experiments with a low order model detailed investigation of the variability of the wintertime aleutian low and water temperature in the subarctic frontal region implies a new scenario for the mechanism of the decadal variability the decadal variability can be interpreted as the transition process between two stable states cold and warm period in midlatitude atmosphere when the transition to a cold period occurs sea surface temperature in the subarctic frontal region begins to be lowered not only through the surface processes but by westward propagation of cold anomaly from eastern subarctic region as a baroclinic rossby wave as a result the meridional gradient of sea surface heat flux yq0 decreases its magnitude and then the forcing of the aleutian low is weakened when this weakening exceeds a threshold the transition from a cold to a warm period occurs abruptly after the transition to a warm period yq0 in turn increases its magnitude through the opposite processes to cause the reverse transition the timescale of this cycle is mainly determined by the propagation speed of a baroclinic rossby wave the el nino and southern oscillation variability both in the atmosphere and ocean can also affect the timescale by causing the thresholds for transitions to be temporarily exceeded experiments with a four variable low order model support the proposed mechanism of the decadal variability

structure of the california coast ranges and san andreas fault at safod from seismic waveform inversion and reflection imaging 1 a seismic reflection and refraction survey across the san andreas fault saf near parkfield provides a detailed characterization of crustal structure across the location of the san andreas fault observatory at depth safod steep dip prestack migration and frequency domain acoustic waveform tomography were applied to obtain highly resolved images of the upper 5 km of the crust for 15 km on either side of the saf the resulting velocity model constrains the top of the salinian granite with great detail steep dip reflection seismic images show several strong amplitude vertical reflectors in the uppermost crust near safod that define an 2 km wide zone comprising the main saf and two or more local faults another prominent subvertical reflector at 2 4 km depth 9 km to the northeast of the saf marks the boundary between the franciscan terrane and the great valley sequence a deep seismic section of low resolution shows several reflectors in the salinian crust west of the saf two horizontal reflectors around 10 km depth correlate with strains of seismicity observed along strike of the saf they represent midcrustal shear zones partially decoupling the ductile lower crust from the brittle upper crust the deepest reflections from 25 km depth are interpreted as crust mantle boundary

the shape of mars before global surveyor results from reanalysis of the viking control point network three dimensional coordinates of 3739 globally distributed control points derived from photogrammetric analysis of viking orbiter image data are studied with respect to the shape and large scale morphology of mars spheres spheroids spherical functions and a digital terrain model dtm of 50 km grid spacing are fitted to the data prominent topographic features in the terrain model include the tharsis volcanoes olympus mons alba patera and prominent mare and highland regions as well as the major impact basins while the global dichotomy is clearly visible in our data our model does not reveal any distinct signature associated with the presumed dichotomy boundary at this global scale rather it suggests a smooth topographic transition from southern to northern hemisphere this model is unique among existing topographic data sets for mars as it combines global coverage a spatial resolution sufficient to resolve regional topography and absolute elevation data more accurate than in previous control point network analyses formal errors and a comparison with the first released topographic profiles obtained by mars global surveyor s laser altimeter mola suggest that 70 and 90 of the dtm grid elements represent the topography of the planet to better than 1000 m and 2000 m respectively

anatomy of north pacific decadal variability abstract a systematic analysis of north pacific decadal variability in a full physics coupled ocean atmosphere model is executed the model is an updated and improved version of the coupled model studied by latif and barnett evidence is sought for determining the details of the mechanism responsible for the enhanced variance of some variables at 20 30 yr timescales the possible mechanisms include a midlatitude gyre ocean atmosphere feedback loop stochastic forcing remote forcing or sampling error decadal variability in the model is expressed most prominently in anomalies of upper ocean streamfunction sea surface temperature sst and latent surface heat flux in the kuroshio oyashio extension koe region off japan the decadal signal off japan is initiated by changes in strength and position of the aleutian low the atmospheric perturbations excite sst anomalies in the central and eastern north pacific with opposing signs and canonical structure the atmospheric perturbations also change the ekm

the effect of north pacific sea surface temperature anomalies on the january climate of a general circulation model abstract four perpetual january integrations of an atmospheric general circulation model have been performed in each of which a different sea surface temperature sst anomaly was specified in the north pacific the observed sst anomaly for the 1976 77 winter was chosen as the basic anomaly and 1200 day runs were carded out in which this anomaly was multiplied by 1 and 2 a fifth run was performed which combined the basic midlatitude sst anomaly from 1976 77 with a tropical pacific sst anomaly representative of the mature phase of a warm el nino southern oscillation enso episode an ensemble of eight independent 90 day averaged realizations was extracted from each simulation maps of ensemble mean differences from the model climatology are presented in this paper together with estimates of the statistical significance of some of the features which appear on these maps the model response to the basic sst anomaly and to twice the basic sst anomaly is a midiatitude teleconnection pattern the pacifi

characteristic variations of sea surface temperature with multiple time scales in the north pacific abstract temporal evolution and spectral structure of sea surface temperature sst anomalies in the north pacific over the last 37 years are investigated on the three characteristic time scales shorter than 24 months hf 24 60 months es and longer than 60 months dc the leading empirical orthogonal function eof for the dc time scale is characterized by a zonally elongated monopole centered at around 40 n 180 the leading eof for the hf time scale is somewhat similar to that for the dc time scale although there are two centers of action with the same polarity at the mid and western pacific the leading eof for the es time scale however exhibits a different pattern whose center of action at the mid pacific is located farther southeastward in the time evolution of the sst anomalies associated with the leading eof of the dc time scale several anomaly periods can be identified that last five years or longer the transition from a persistent period to another with the opposite polarity is gene

southern hemisphere circulation features associated with el nino southern oscillation events abstract composite seasonal mean and anomaly fields prepared from operational numerical analyses have been used to describe the southern hemisphere sh circulation features associated with el nino southern oscillation enso events the period of analyses available 1972 83 has limited the composites to include only three enso events the reliability and stability of the composites has been tested using multiple permutation methods and by comparison with the results obtained using a longer period 1950 79 of sh rawinsonde station data in the sh winter a weak equivalent barotropic wavetrain pattern of anomalies extends over australia and the south pacific ocean to south america this wavetrain pattern is quite variable in amplitude and location between enso events although it is more stable over the subtropical pacific in the sh summer the circulation anomalies are more zonally symmetric with increased height at low and high latitudes and decreased height in middle latitudes the circulation anoma

maximal wavelet filter and its application to bidecadal oscillation over the northern hemisphere through the twentieth century based on a wavelet transform a new method referred to as maximal wavelet filter mwf is proposed to extract temporal structure changes of a climatic oscillation which varies its pattern corresponding to the changes of the oscillation period the mwf is a bandpass filter having a narrow pass band the central frequency of which temporally varies according to the periods of maximal wavelet amplitudes for a specific region mwf is applied to wintertime sea level pressures slps in the northern hemisphere from 1899 to 2000 to extract slp changes associated with the bidecadal oscillation bdo which distributes globally but has the strongest amplitudes in the north pacific in the pacific sector the bdo center of action captured by the mwf was located over alaska in the first few decades of the record and then moved southward to the central north pacific from 1920 to 1950 with maximal bdo amplitudes in the middle of the century the southward migration was accompanied by the previously reported increase of the oscillation period from 15 to 20 years on the other hand atlantic slp variations coherent with the pacific bdo had large amplitudes in midlatitudes high latitudes in the early late part of the twentieth century in association with these spatial structure changes the pattern of the recent bdo resembles the pattern of the arctic oscillation the analysis of the sea surface temperatures ssts gridded from the comprehensive ocean atmosphere data set coads and the newly digitized kobe collections suggests that bdo pattern in the ssts also shifted toward the south between the first and last few decades of the twentieth century furthermore covariability between the land air temperatures and aleutian low strength is observed through the twentieth century for alaska but only after 1940 for the midlatitudes of western north america and hawaii indicating that the bdo influence was limited to the high latitudes in the first few decades of the twentieth century in these regions consistent with the spatial structure changes in the slp field over the north pacific

stratigraphic relationships within martian polar cap deposits abstract deposits of layered ice and dust accumulate on the smooth and banded terrains of the north and south polar cap deposits while erosion of equator facing slopes of layered terrain expose these layers banded terrain occurs at exposures of irregular layers resulting from slight erosion of the feather edges of layers deposited on pole facing trough walls these deposits unconformably overlap onto adjacent layered terrain stripping of layers occurs in areas of strong multidirectional wind forming striped terrain local wind scour has also produced steep scarps with crescentic planform often associated with dune fields positioned immediately downwind unconformities are common within layered deposits in areas of complex topography at the north pole these indicate that episodes of deposition of 5 10 layers alternate with slight erosion of pole facing trough walls troughs that strike north south alternate between erosional widening and deepening and depositional infilling creating multiple unconformities troughs near the equatorward limit of polar deposits increase in relief by deposition on adjacent smooth terrain and locally by erosional deepening troughs may migrate poleward replacing the relief increase by depositional infilling when they near the poles

teleconnections in the southern hemisphere abstract teleconnections are calculated from monthly mean anomalies of sea level pressure and 500 mb geopotential height for the southern hemisphere 10 90 s for five month winter and summer seasons the monthly means were calculated from australian analyses for the period from june 1972 to november 1980 zonally averaged anomalies at both sea level and 500 mb display an out of phase relation between low and high latitudes and in midlatitudes are negatively correlated with anomalies in the subtropics and polar regions in winter a striking zonal wavenumber 3 pattern is found over the southern ocean anomalies in 500 mb geopotential heights at 50 s 95 e 58 s 150 w and 38 s 15 w exhibit strong positive correlations while showing weaker negative correlations with heights over antarctica and in low latitudes a similar pattern appears in sea level pressure in summer anomalies in 500 mb geopotential height over the three subtropical continents appear to occur out of phase with anomalies over the su

2d finite difference viscoelastic wave modelling including surface topography we have pursued two dimensional 2d finite difference fd modelling of seismic scattering from free surface topography exact free surface boundary conditions for the particle velocities have been derived for arbitrary 2d topographies the boundary conditions are combined with a velocity stress formulation of the full viscoelastic wave equations a curved grid represents the physical medium and its upper boundary represents the free surface topography the wave equations are numerically discretized by an eighth order fd method on a staggered grid in space and a leap frog technique and the crank nicholson method in time in order to demonstrate the capabilities of the surface topography modelling technique we simulate incident point sources with a sinusoidal topography in seismic media of increasing complexities we present results using parameters typical of exploration surveys with topography and heterogeneous media topography on homogeneous media is shown to generate significant scattering we show additional effects of layering in the medium with and without randomization using a von karman realization of apparent anisotropy synthetic snapshots and seismograms indicate that prominent surface topography can cause back scattering wave conversions and complex wave patterns which are usually discussed in terms of inter crust heterogeneities

thermodynamic mechanisms responsible for the tropospheric response to sst anomalies in the antarctic circumpolar wave abstract tropospheric temperature and vorticity budgets for the antarctic circumpolar wave acw are diagnosed utilizing the national centers for environment prediction national center for atmospheric research reanalysis datasets from 1983 to 1992 focusing on the eastern atlantic indian and western and central pacific sectors of the southern ocean where remote forcing from the tropics has been observed to be weak there warm sea surface temperature sst anomalies are found in the acw propagating eastward together with anomalous upward latent heat flux positive precipitation low level convergence upper level divergence midlevel ascent and poleward surface wind diagnosing the anomalous temperature budget finds sst induced latent heat flux instigating anomalous mid to upper level diabatic heating and low level diabatic cooling in the absence of significant eddy heat flux divergence this diabatic heating profile is balanced by a combination of vertical and horizontal heat advection giving rise t

first high resolution stratigraphic column of the martian north polar layered deposits 1 this study achieves the first high spatial resolution layer scale measured stratigraphic column of the martian north polar layered deposits using a 1m posting dem the marker beds found throughout the upper north polar layered deposits range in thickness from 1 6 m 16 0 m 1 4 m and 6 of 13 marker beds are separated by 25 35 m thin layer sets have average layer separations of 1 6 m these layer separations may account for the spectral power peaks found in previous brightness profile analyses marker bed layer thicknesses show a weak trend of decreasing thickness with depth that we interpret to potentially be the result of a decreased accumulation rate in the past for those layers however the stratigraphic column reveals that a simple rhythmic or bundled layer sequence is not immediately apparent throughout the column implying that the relationship between polar layer formation and cyclic climate forcing is quite complex

meter scale morphology of the north polar region of mars mars north pole is covered by a dome of layered ice deposits detailed 30 centimeters per pixel images of this region were obtained with the high resolution imaging science experiment on board the mars reconnaissance orbiter mro planum boreum basal unit scarps reveal cross bedding and show evidence for recent mass wasting flow and debris accumulation the north polar layers themselves are as thin as 10 centimeters but appear to be covered by a dusty veneer in places which may obscure thinner layers repetition of particular layer types implies that quasi periodic climate changes influenced the stratigraphic sequence in the polar layered deposits informing models for recent climate variations on mars

crustal seismic imaging from multifold ocean bottom seismometer data by frequency domain full waveform tomography application to the eastern nankai trough 1 in offshore surveys the deep crust is generally investigated by traveltime tomography applied to sparse ocean bottom seismometer data the inferred velocity models are of limited resolution precluding a quantitative analysis of deep tectonic discontinuities if dense arrays of ocean bottom seismometers can be deployed the resulting data sets become amenable to full waveform processing which should lead to a significant improvement in the resolution of structures such acquisition and processing were achieved on the eastern nankai subduction zone full waveform inversion is entirely implemented in the frequency domain enabling efficient finite difference wave modeling and allowing to limit the inversion to a few discrete frequencies of increasing value such a hierarchical procedure defines a multiresolution approach to seismic imaging the data set was recorded by 93 instruments deployed along a 105 km long profile thirteen frequencies ranging from 3 to 15 hz were inverted sequentially wavelengths imaged by the full waveform inversion typically range between 0 5 and 8 km full waveform tomography reveals an intense compression in the upper prism and underlying backstop as evidenced by several thrusts the frontal ones still active corresponding to negative velocity anomalies the subducting paleo zenisu ridge is also reconstructed above which stands the decollement velocities in the upper mantle beneath the ridge are rather low 7 5 km s 1 suggesting partially serpentinized mantle beneath the ridge this paper demonstrates the feasibility of full waveform tomography based on dense ocean bottom seismometer data sets and its ability to quantitatively image the entire crust with a significantly improved resolution compared to what is usually achieved through traveltime tomography

effects of surface scattering in full waveform inversion in full waveform inversion of seismic body waves often the free surface is ignored on grounds of computational efficiency asyntheticstudywasperformedtoinvestigatetheeffectsofthissimplification intermsofsizeandfrequency the testmodelanddataconformtoareallong offsetsurveyofthe upper crust across the san andreas fault random fractal variations are superimposed on a background model with strong lateral and vertical velocity variations ranging from 1200to6800m s syntheticdatawerecomputedandinverted forthismodelanddifferenttopographies afullyviscoelastic time domain code was used to synthesize the seismograms and a viscoacoustic frequency domain code was utilized to invert them the inversion was focused on early arrivals which are dominated by p waves but also contain strong p rayleigh wave conversions from the near field of the receiver resulting waveform models show artifacts and a loss of resolution from neglecting the free surface in the inversion but the inversions are stable and they still improve the resolution of kinematic models the extent of deterioration dependsmoreonthesubsurfacethanonthesurfacestructure inversion results were improved at no additional expense by introducing a weak contrast along a staircase function above shotsandreceivers

influence of the antarctic circumpolar wave on australian precipitation from 1958 to 1997 abstract year to year changes are found in australian precipitation app covarying with those in sea surface temperature sst and troposphere moisture flux mf over the three oceans surrounding australia for 40 yr from 1958 to 1997 australia s wet dry years are associated with warm cool sst anomalies surrounding australia and convergent divergent mf anomalies directly overhead differences in app sst between wet and dry years can reach 0 75 m 1 2 c in northeast australia subtropical indian ocean wet dry years often occur during la nina el nino but significant differences in covarying sst mf and app anomalies from one el nino to the next are found indicating that regional climate changes also influence app covarying sst and mf anomalies on basin space scales and interannual timescales are found to take 2 3 yr to propagate eastward from africa to australia this propagation occurs in association with the antarctic circumpolar wave acw in the southern ocean the north branch of th

antarctic circumpolar waves an indication of ocean atmosphere coupling in the extratropics recent observational studies have detected coherent oceanic and atmospheric antarctic circumpolar waves acws which propagate eastward at speeds of 6 8 cm s 1 with a circumpolar wavenumber of 2 analysis of global wind data suggests that the acws are not remotely forced by tropical el nino activity through atmospheric teleconnection rather a coupled instability of the antarctic circumpolar current and its overlying atmospheric motion is proposed to be responsible for the generation of the acws the phase relationship among the observed oceanic and atmospheric variables supports the coupled instability theory this study provides one clear example that extratropical ocean atmosphere dynamical coupling is essential in generating climate variations on time scales of several years and longer

observations of the north polar region of mars from the mars orbiter laser altimeter elevations from the mars orbiter laser altimeter mola have been used to construct a precise topographic map of the martian north polar region the northern ice cap has a maximum elevation of 3 kilometers above its surroundings but lies within a 5 kilometer deep hemispheric depression that is contiguous with the area into which most outflow channels emptied polar cap topography displays evidence of modification by ablation flow and wind and is consistent with a primarily h2o composition correlation of topography with images suggests that the cap was more spatially extensive in the past the cap volume of 1 2 3 10 6 to 1 7 3 10 6 cubic kilometers is about half that of the greenland ice cap clouds observed over the polar cap are likely composed of co 2 that condensed out of the atmosphere during northern hemisphere winter many clouds exhibit dynamical structure likely caused by the interaction of propagating wave fronts with surface topography

on the stochastic forcing of modes of interannual southern ocean sea surface temperature variability abstract a simple linearized transport model of anomalous southern ocean sea surface temperature sst is studied to determine whether it can sustain anomalies of realistic amplitudes under a physically based stochastic forcing as noted in previous studies eigenmodes of this system with zonal wavenumbers 2 and 3 share key propagation characteristics with the sst anomalies associated with the antarctic circumpolar wave acw the system is solved on a grid that follows the path of the antarctic circumpolar current acc and is forced by a stochastic heat flux the forcing is white in space and time and represents the advection of the mean sst gradient by high frequency variations in the cross acc velocity due to mesoscale eddy variability the magnitude of the stochastic forcing is determined from a global eddy permitting ocean model anomalous ocean surface velocity variability 8 cm s 1 coupled to a mean cross acc sst gradient of 0 8 c latitude 1 sustains anomalous interannual sst variability at l

detailed stratigraphy and bed thickness of the mars north and south polar layered deposits the mars polar layered deposits pld likely hold an extensive record of recent climate during a period of high amplitude orbit and obliquity cycles previous work has detected limited evidence for orbital signatures within pld stratigraphy but data from the high resolution imaging science experiment hirise permit renewed analysis of pld stratigraphy at sub meter scale topography derived from hirise images using stereogrammetry resolves beds previously detectable only as alternating light and dark bands in visible images we utilize these data to measure the thickness of individual beds within the pld corrected for non horizontal bed orientation stratigraphic columns and bed thickness profiles are presented for two sites within the npld and show several sets of finely bedded units 1 2 m thick isolated marker beds 3 4 m thick and undifferentiated sections bed thickness measurements for three sites within the spld exhibit only one bed type based on albedo and morphology and bed thicknesses have a larger mean and variance compared to measurements for the npld power spectra of brightness and slope derived along the measured stratigraphic sections confirm the regularity of npld fine bed thickness and the lack of a dominant spld bed thickness the regularity of fine bed thickness of the npld is consistent with quasiperiodic bed formation albeit with unknown temporal period the spld thickness measurements show no such regularity

north polar stratigraphy and the paleo erg of mars an accurate self consistent way of coregistering the imaging and topographic data sets of the mars global surveyor mission was developed and used to begin a stratigraphic analysis of the northern polar region a distinct change in the layering style exists at a definite stratigraphic horizon near the base of the north polar layered deposits occurrences of the contact between two distinct layered units can be mapped hundreds of kilometers apart at nearly the same mars orbiter laser altimeter mola elevation the lower layered unit has a consistent association with sand dunes leading to the conclusion that it is an eroding sand rich deposit that predates most of the overlying north polar layered deposits which exhibits the expected features of a dust ice mixture these results suggest that an areally extensive erg was in existence before the present ice cap and that the present circumpolar erg is likely composed of material reworked from this older deposit the volume of this lower unit is estimated to be on the order of 10 5 km 3 the presence of this deposit implies that there existed a period in mars history when there was no icy polar cap a dramatic climatic change leading to the deposition of the upper layered icy unit in the present day polar layered deposits represents a major event in mars history however owing to uncertainties in the mechanics of layered deposits formation such an event cannot be dated at this time

new curvilinear scheme for elastic wave propagation in presence of curved topography we study a new curvilinear scheme for wave propagation modelling in presence of topography the discrete scheme takes advantage of recent developments in areoacoustics our new scheme relies on the conventional grid coupled with optimized filters to remove numerical noise in case of strong material heterogeneity we used non centred stencils for free surface implementation and optimized the explicit runge kutta scheme for the time differencing we performed a complete theoretical stability and dispersion analysis of the discrete scheme finally we illustrate the numerical accuracy of the new scheme by intensive experiments

the role of sublimation for the formation of the northern ice cap results from the mars orbiter laser altimeter abstract the evolutionary history of the martian ice caps is practically unknown and the caps are key to understanding the climatic history of mars sublimation and ice flow are two hypotheses that have recently been suggested to explain the observed form of the northern ice cap here we present a sublimation model and compare the shape of the cap obtained from this model with mars orbiter laser altimeter mola observations of the north polar region this model can reproduce the shape of the cap and individual troughs sublimation is an important process as are flow and wind erosion we were able to constrain the time scales of sublimation only to about two orders of magnitude 1 100 million years this estimate depends on many unknown parameters such as friction velocity and wind speed over the ice caps these can be determined only by direct measurement over the surface

winter to winter recurrence of sea surface temperature salinity and mixed layer depth anomalies abstract the mean seasonal cycle of mixed layer depth mld in the extratropical oceans has the potential to influence temperature salinity and mixed layer depth anomalies from one winter to the next temperature and salinity anomalies that form at the surface and spread throughout the deep winter mixed layer are sequestered beneath the mixed layer when it shoals in spring and are then re entrained into the surface layer in the subsequent fall and winter here we document this re emergence mechanism in the north pacific ocean using observed ssts subsurface temperature fields from a data assimilation system and coupled atmosphere ocean model simulations observations indicate that the dominant large scale sst anomaly pattern that forms in the north pacific during winter recurs in the following winter the model simulation with mixed layer ocean physics reproduced the winter to winter recurrence while model simulations with observed ssts specified in the tropical pacific and a 50 m slab in the north pacific did not this difference between the model results indicates that the winter to winter sst correlations are the result of the re emergence mechanism and not of similar atmospheric forcing of the ocean in consecutive winters the model experiments also indicate that sst anomalies in the tropical pacific associated with el nino are not essential for re emergence to occur the recurrence of observed sst and simulated sst and sss anomalies are found in several regions in the central north pacific and are quite strong in the northern 50 n part of the basin the winter to winter autocorrelation of sss anomalies exceed those of sst since only the latter are strongly damped by surface fluxes the re emergence mechanism also has a modest influence on mld through changes in the vertical stratification in the seasonal thermocline

topography and stratigraphy of the northern martian polar layered deposits using photoclinometry stereogrammetry and mola altimetry we present two photoclinometric profiles across a trough in the martian northern polar layered terrain complications caused by albedo variations were avoided by using an early springtime viking image with a thin cover of seasonal co2 frost the topographic profiles were constrained with stereogrammetric elevations derived from summertime viking images of the same region we find that the photoclinometric profiles are consistent with a nearby mola mars orbiter laser altimeter track crossing the same polar trough the trough is asymmetric with higher relief and a steeper slope on the equatorward facing wall individual layers are subdued and difficult to observe in the profiles a decrease in both relief and elevation toward the eastern end of the trough suggests that layers become thinner to the east declining equatorward slopes in the eastern portion of the trough imply that erosion rates have varied along the trough the variation in erosion rate may be linked to the change in layer thickness along the trough layers have an average thickness of 19 8 m in the center of the trough and 59 32 m on the northern wall the northern wall is most likely composed of thinner layers that are obscured to first order we find that a 19 m layer requires 16 000 years of deposition to form although this timescale does not coincide with orbital variation periods of 10 5 and 10 6 years deposition rates may not be constant and thus the 16 000 year layer formation time does not preclude layer formation during part of each orbital oscillation c 2000 academic press

martian north polar layered deposits stratigraphy implications for accumulation rates and flow 1 we have used mars orbiter camera moc images and mars observer laser altimeter mola data to correlate specific layers within the upper 500 m of the martian north polar layered deposits in many troughs using these correlation results we derive relative accumulation rates across the polar layered deposits and through time we identify two major layer sequences one lying stratigraphically above the other each consistent with an overall curving downward layer structure with shallow slopes but with significant localized variations in layer height major disruptions of stratigraphy such as angular unconformities do not appear within the correlated layer sequences on local and broad scales each layer sequence exhibits a different relative accumulation rate pattern indicating that this rate has not remained constant through time and has been affected by localized processes the overall pattern of the lower layer sequence is similar to that of a classic terrestrial ice sheet with a decreasing relative accumulation rate away from the center of the polar layered deposits southward the upper layer sequence exhibits no obvious overall trends we also compare our correlation results to predictions of the effects of simple ice flow on layer structure finding that mass balance patterns have overprinted any effects of large scale ice flow flow has not been significant compared to mass balance in forming the overall structure of the correlated layers

interdecadal variations in north atlantic sea surface temperature and associated atmospheric conditions abstract evidence is presented for a distinct pattern of ocean atmosphere relationship associated with interdecadal variability in the north atlantic region using a century of surface marine observations it is shown that middle and high latitude sea surface temperature sst display a long term fluctuation with negative anomalies before 1920 and during the 1970s and 1980s positive sst conditions prevailed from about 1930 to 1960 the pattern of interdecadal sst variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years the early century warming and the more recent cooling display a similar spatial pattern in both cases the pattern is basin scale and largely of one polarity with maxima in the vicinity of iceland in the labrador sea and northeast of bermuda the corresponding differences in surface atmospheric conditions are determined by averaging and subtracting in the same manner fields of sea level pressure slp and surf

positive feedbacks between the antarctic circumpolar wave and the global el nino southern oscillation wave atmospheric and oceanic teleconnections link the antarctic circumpolar wave acw in the southern ocean white and peterson 1996 and the global el nino southern oscillation enso wave gew in the tropical indo pacific ocean white and cayan 2000 both signals characterized by eastward phase propagation and 3 to 5 year period variability we extend the tropical standing mode of enso into the extratropics by regressing the nino 3 sea surface temperature sst index against sea level pressure slp anomalies over the globe finding the pacific south america psa pattern in slp anomaly cai and baines 2001 straddling drake passage in the southern ocean the amplitude of this psa pattern is similar to1 3 that of the acw in this domain and thus cannot be considered its principal driver on the other hand suppressing the tropical standing mode of enso in interannual st surface temperature and slp anomalies over the globe allows the gew to be observed much more readily whereupon its eastward phase propagation across the warm pool is found to remotely force the acw in the eastern pacific and western atlantic sectors of the southern ocean through atmospheric teleconnections sardeshmukh and hoskins 1988 which propagate along with it subsequently the acw propagates this imposed gew signal throughout the remainder of the southern ocean as a coupled wave in covarying st and slp anomalies whereupon entering the indian sector 1 5 to 2 5 years later it spawns a northern branch which takes another 1 5 to 2 5 years to propagate the acw signal equatorward into the warm pool south of indonesia there it interferes constructively with the gew thus the two forms of teleconnection one fast and directed from the tropics to the high southern latitudes via the atmosphere and the other slow and directed from the high southern latitudes to the tropics via the ocean complete a global circuit of 3 to 5 year duration that reinforces both the acw and gew and influences the tropical standing mode of enso

simple coupled midlatitude climate models a set of simple analytical models is presented and evaluated for interannual to decadal coupled ocean atmosphere modes at midlatitudes the atmosphere and ocean are each in sverdrup balance at these long timescales the atmosphere s temperature response to heating determines the spatial phase relation between sst and sea level pressure slp anomalies vertical advection balancing heating produces high low slp lying east of warm cold sst anomalies as observed in the antarctic circumpolar wave acw the decadal north pacific mode and the interannual north atlantic mode zonal advection in an atmosphere with a rigid lid produces low slp east of warm sst however if an ad hoc equivalent barotropic atmospheric response is assumed high slp lies east of warm sst relaxation to heating produces behavior like the observed north atlantic decadal pattern with low slp over warm sst meridional advection in the atmosphere cannot produce the observed sst slp patterns the dominant balance in the ocean s temperature equation determines the phase speed of the modes the coupled mode is nondispersive in all models examined here indicating the need for additional processes for modes with an sst slp offset as observed in the acw and north pacific ekman convergence acting as a heat source causes eastward propagation relative to the mean ocean flow sverdrup response to ekman convergence acting on the mean meridional temperature gradient causes westward propagation relative to the mean ocean flow when the ocean temperature adjusts through surface heat flux alone the mode is advected by the mean ocean flow and is damped relaxation to heating in the atmosphere when operating with sverdrup response in the ocean produces the only complete solution presented here that exhibits growth with an e folding timescale of order 100 days this solution appears appropriate for the north atlantic decadal mode in northern hemisphere basins with meridional boundaries the same sets of dynamics create the observed sst slp phase relation an additional factor is the creation of sst anomalies through variations in the western boundary current strengths which are related to the zonally integrated wind stress curl over the whole basin if barotropic and hence fast adjustment is assumed the resulting positive feedback can maintain or strengthen the coupled anomalies in the north pacific and interannual north atlantic modes

why are there rossby wave maxima in the pacific at 10 s and 13 n observations indicate the existence of two bands of maximum thermocline depth variability centered at 108s and 138n in the tropical pacific ocean the analysis of a numerical integration performed with the national center for atmospheric research ocean general circulation model ogcm forced with observed fluxes of momentum heat and freshwater over the period from 1958 to 1997 reveals that the tropical centers of thermocline variability at 108s and 138n are associated with first mode baroclinic rossby waves forced by anomalous ekman pumping in this study the factors that may be responsible for the rossby wave maxima at 108s and 138n including the amplitude and spatial coherency of the forcing at those latitudes are systematically investigated a simple rossby wave model is used to interpret the ogcm variability and to help to discriminate between the different factors that may produce the tropical maxima these results indicate that the dominant factor in producing the maximum variability at 108s and 138 ni s thezonal coherency of the ekman pumping a characteristic of the forcing that becomes increasingly more pronounced at low frequencies maximizing at timescales in the decadal range local maxima in the amplitude of the forcing while not explaining the origin of the centers of variability at 108s and 138n appear to affect the sharpness of the variability maxima at low frequencies although the rossby wave model gives an excellent fit to the ogcm some discrepancies exist the amplitude of the thermocline variance is generally underestimated by the simple model and the variability along 13 8 ni s westward intensified in the wave model but reaches a maximum in the central part of the basin in the ogcm short rossby waves excited by small scale ekman pumping features or the presence of higher order rossby wave modes may be responsible for the differences in the zonal variance distribution along 138n

multiscale imaging of complex structures from multifold wide aperture seismic data by frequency domain full waveform tomography application to a thrust belt summary an application of full waveform tomography to dense onshore wide aperture seismic data recorded in a complex geological setting thrust belt is presented the waveform modelling and tomography are implemented in the frequency domain the modelling part is solved with a finite difference method applied to the visco acoustic wave equation the inversion is based on a local gradient method only the p wave velocity is involved in the inversion the inversion is applied iteratively to discrete frequency components by proceeding from low to high frequencies this defines a multiscale imaging in the sense that high wavenumbers are progressively incorporated in images the linearized waveform tomography requires an accurate starting velocity model that has been developed by first arrival traveltime tomography after specific pre processing of the data 16 frequency components ranging between 5 4 and 20 hz were inverted ten iterations were computed per frequency component leading to 160 tomographic models the waveform tomography has successfully imaged southwest dipping structures previously identified from other geophysical data as being associated with high resistivity bodies the relevance of the tomographic images is locally demonstrated by comparison of a velocity depth function extracted from the waveform tomography models with a coincident vertical seismic profiling vsp log available on the profile moreover comparison between observed and synthetic seismograms computed in the starting traveltime and waveform tomography models demonstrates unambiguously that the waveform tomography successfully predicts for wide angle reflections from southwest dipping geological structures this study demonstrates that the combination of first arrival traveltime and frequency domain full waveform tomographies applied to dense wide aperture seismic data is a promising approach to quantitative imaging of complex geological structures indeed wide aperture acquisition geometries offer the opportunity to develop an accurate background velocity model for the subsequent waveform tomography this is critical because the building of the macromodel remains an open question when only near vertical reflection data are considered

cold jets in the martian polar caps 1 mars seasonal polar caps display dark ice local darker spots aligned elongate patches and radially dendritic forms that reverse albedo contrast the unexpected variety and sequence of these features are explained on the basis of processes involving co2 dust sand and h2o these processes are largely related to the atmosphere being near its saturation temperature and they have few terrestrial analogies in the simplest case the 1 m thick seasonal cap initially dusty cleans itself and becomes translucent after sunrise and is impermeable over extensive regions except for local vents the slab ice sublimates at the base and is levitated on high pressure gas causing humidity exchange with deeper layers subslab gas converging toward the vents erodes channels in the soil and ejects this material in high velocity jets recent spectral observations indicate great variety in the details

high resolution topography and albedo of the south polar layered deposits on mars using a new photoclinometric technique with high resolution mariner 9 images maximum slopes of 10 20 are found to occur on an exposure of layered deposits within the south polar residual cap of mars stereophotogrammetry is used to constrain the photoclinometric solutions which resolve layer thicknesses of 100 300 m albedo variations are correlated with slope indicating that frost is present on level areas there is evidence for temporal changes in frost distribution in the 7 days 4 of l s between the two images used in this study the magnitude of the slopes derived here and consideration of the stability of water ice at the surface of the layered deposits strongly suggest the presence of a competent weathering rind the weathered surface may be composed of dark filamentary sublimation residue particles that protect the underlying ice from solar heating this hypothesis is consistent with previous studies of the regional color and albedo of the layered deposits which indicate that the deposits are slightly darker and less red than the bright dust that mantles much of the south polar region furthermore the proposed weathering mechanism provides a plausible source of dark saltating material for the martian polar dune fields

color and albedo of the south polar layered deposits on mars five color albedo units including polar frost have been recognized and mapped in the southern layered deposits on mars atmospheric dust scattering was measured in shadows and modeled in order to remove the component of brightness in mars images due to the atmosphere and quantify the albedo and color of the surface the layered deposits appear to be mantled by red dust except where eolian stripping has exposed the underlying bedrock frost and bare ground are mixed below the resolution of the images in many areas adjacent to the polar cap some of which appear to be younger than the surrounding layered terrain dark material has been deposited in topographic depressions in much of the south polar region including the layered deposits the available observational data suggest that the layered deposits are composed of bright dust ice and a small amount of dark material if the dark material is sand a periodic change in polar winds seems required in order to transport the sand poleward into the layered terrain in any case the observations are not consistent with the layered deposits being composed only of bright dust and ice the mars observer camera and infrared instruments should be particularly useful in this investigation

surface ages and resurfacing rates of the polar layered deposits on mars interpretation of the polar stratigraphy of mars in terms of global climate changes is complicated by the significant difference in surface ages between the north and south polar layered terrains inferred from crater statistics we have reassessed the cratering record in both polar regions using viking orbiter and mariner 9 images no craters have been found in the north polar layered terrain but the surface of most of the south polar layered deposits appears to have been stable for many of the orbital axial cycles that are thought to have induced global climate changes on mars the inferred surface age of the south polar layered deposits about 10 ma is two orders of magnitude greater than the surface age of the north polar layered deposits and residual cap at most 100 ka similarly modeled resurfacing rates are at least 20 times greater in the north than in the south these results are consistent with the hypotheses that polar layered deposit resurfacing rates are highest in areas covered by perennial ice and that the differences in polar resurfacing rates result from the 6 4 km difference in elevation between the polar regions deposition on the portion of the south polar layered deposits that is not covered by the perennial ice cap may have ceased about 5 million years ago when the obliquity of mars no longer exceeded 40

density of martian north polar layered deposits implications for composition mariner 9 and viking orbiter observations of the topography and mass of layered deposits in the north polar region of mars suggest a density of about 1 gm cm consideration of the self compaction of these deposits further suggests that this low density cannot be explained by a high porosity sediment rather a significant component of ice is probably required to account for the low value the density further implies that the silicate fraction regardless of its density i e its composition represents a volumetrically almost negligible component of the deposit the most likely values 5 silicate dust and 95 ice agree well with a theoretical model for the formation of the deposits through cyclic incorporation of the perennial polar ice cap additional tests of this model may be posed in advance of future spacecraft missions to mars

on the relationship between tropical and north pacific sea surface temperature variations abstract empirical orthogonal function analysis of winter sea surface temperature sst anomalies over the pacific domain 60 n 20 s reveals an el nino southern oscillation enso mode that is linked to the eastern north pacific and a north pacific mode that is linearly independent of enso the north pacific mode exhibits maximum amplitude and variance explained along 40 n west of 170 w ssts in this region have decreased by 1 5 c from 1950 to 1987 the cooling in winter has been associated with a strengthening of the overlying westerly winds

upper ocean thermal variations in the north pacific during 1970 1991 abstract a newly available extensive compilation of upper ocean temperature profiles was used to study the vertical structure of thermal anomalies between the surface and 400 m depth in the north pacific during 1970 1991 a prominent decade long perturbation in climate occurred during this time period surface waters cooled by 1 c in the central and western north pacific and warmed by about the same amount along the west coast of north america from late 1976 to 1988 comparison with data from coads suggests that the relatively sparse sampling of the subsurface data is adequate for describing the climate anomaly the vertical structure of seasonal thermal anomalies in the central north pacific shows a series of cold pulses beginning in the fall of 1976 and continuing until late 1988 that appear to originate at the surface and descend with time into the main thermocline to at least 400 m depth individual cold events descend rapidly 100 m yr 1 superimposed upon a slower cooling 15 m yr 1 the inter

dynamics of sea surface temperature anomalies in the southern ocean diagnosed from a 2d mixed layer model we analyze the processes responsible for the generation and evolution of sea surface temperature anomalies observed in the southern ocean during a decade based on a 2d diagnostic mixed layer model in which geostrophic advection is prescribed from altimetry anomalous air sea heat flux is the dominant term of the heat budget over most of the domain while anomalous ekman heat fluxes account for 20 40 of the variance in the latitude band 40 60 s in the acc pathway lateral fluxes of heat associated with anomalous geostrophic currents are a major contributor dominating downstream of several topographic features reflecting the influence of eddies and frontal migrations a significant fraction of the variability of large scale sst anomalies is correlated with either enso or the sam each mode contributing roughly equally the relation between the heat budget terms and these climate modes is investigated showing in particular that anomalous ekman and air sea heat fluxes have a co operating effect with regional exceptions hence the large sst response associated with each mode it is further shown that enso or sam locked anomalous geostrophic currents generate substantial heat fluxes in all three basins with magnitude comparable with that of atmospheric forcings for enso and smaller for the sam except for limited areas enso locked forcings generate sst anomalies along the acc pathway and advection by mean flows is found to be a non negligible contribution to the heat budget exhibiting a wavenumber two zonal structure characteristic of the antarctic circumpolar wave by contrast sam related forcings are predominantly zonally uniform along the acc hence smaller zonal sst gradients and a lesser role of mean advection except in the southwest atlantic while modeled sst anomalies are significantly correlated with observations over most of the southern ocean the analysis of the data model discrepancies suggests that vertical ocean physics may play a significant role in the nonseasonal heat budget especially in some key regions for mode water formation

structure and cycle of decadal variability of upper ocean temperature in the north pacific abstract yearly upper ocean in situ temperature anomaly data for the period 1961 90 are analyzed to reveal spatial structure and evolution of decadal variability in the north pacific ocean an eof analysis has been performed on individual temperature anomaly fields at upper ocean standard levels as well as simultaneously on the entire upper ocean data to depict the combined three dimensional structure in a coherent manner time evolution of anomaly fields is depicted by using a regression analysis the analyses detect the principal basin scale structure of decadal warm period dwp and decadal cold period dcp there is a well defined subsurface thermal anomaly pattern characterized by a prominent seesaw structure with opposite anomaly polarity between the midlatitude north pacific and the subtropical regions during a dwp a positive temperature anomaly is found in the central midlatitude upper ocean with the maximum at about 100 m depth this is accompanied by a corresponding negative anomaly in the

evaluating the meaning of layer in the martian north polar layered deposits and the impact on the climate connection abstract using data from the high resolution imaging science experiment hirise aboard the mars reconnaissance orbiter we reassess the methods by which layers within the north polar layered deposits npld can be delineated and their thicknesses measured apparent brightness and morphology alone are insufficient for this task high resolution topographic data are necessary from these analyses we find that the visible appearance of layers depends to a large degree on the distribution of younger mantling deposits which in turn is partially influenced by inherent layer properties and on the shape and location of the particular outcrop this younger mantle partially obscures layer morphology and brightness and is likely a cause of the gradational contacts between individual layers at this scale high resolution images reveal that there are several layers similar in appearance to the well known marker bed discovered by malin m edgett k 2001 j geophys res 106 23429 23570 the morphology thicknesses 4 8 2 m and separation distances 5 32 2 m of these marker beds as gleaned from a high resolution stereo digital elevation model lend insight into the connection between stratigraphy and climate

subduction of decadal north pacific temperature anomalies observations and dynamics abstract observations of oceanic temperature in the upper 400 m reveal decadal signals that propagate in the thermocline along lines of constant potential vorticity from the ventilation region in the central north pacific to approximately 18 n in the western pacific the propagation path and speed are well described by the geostrophic mean circulation and by a model of the ventilated thermocline the approximate southward speed of the thermal signal of 7 mm s 1 yields a transit time of approximately eight years the thermal anomalies appear to be forced by perturbations of the mixed layer heat budget in the subduction region of the central north pacific east of the date line a warm pulse was generated in the central north pacific by a series of mild winters from 1973 to 1976 and reached 18 n around 1982 after 1978 a succession of colder winters initiated a cold anomaly in the central north pacific that propagated along a similar path and with a similar speed as the warm anomaly then arrived in the west

introduction to the 4th mars polar science and exploration conference special issue five top questions in mars polar science abstract as an introduction to this icarus special issue for the 4th mars polar science and exploration conference we discuss five key questions in mars polar science gleaned from plenary discussions and presentations held at the conference these questions highlight major unknowns in the field 1 what are the physical characteristics of the polar layered deposits pld and how are the different geologic units within beneath and surrounding the pld related 2 how old are the pld and what are their glacial fluvial depositional and erosional histories 3 what are the mass and energy budgets of the pld and what processes control these budgets on seasonal and longer timescales 4 what chronology compositional variability and record of climatic change is expressed in the stratigraphy of the pld 5 how have volatiles and dust been exchanged between polar and non polar reservoirs and how has this exchange affected the past and present distribution of surface and subsurface ice

topographic information of sand dunes as extracted from shading effects using landsat images topographic variations affect the reflectance properties of the earth s surface and are often removed in remote sensing studies especially when significant terrain variations exist in this study however we show that shading effects assessed by landsat can be treated as a signal that stores important topographic information especially when the spectral characteristics of a surface are homogenous the coastal transverse dunes of the ashdod area and the desert linear dunes of nizzana both located in israel were selected to investigate the abovementioned idea the dune heights in these areas are 10 m on average relative to their surroundings and have maximum slopes of 33 an innovative method for extracting slope aspect and height data for sand dunes using landsat thematic mapper tm and enhanced thematic mapper plus etm images was developed based on the regularity and periodicity of dunes landscapes using two landsat images representing different sun zenith and azimuth angles reflectance values of each image were converted to cos i values i incident angle between the surface normal and the solar beam radiation applying histogram matching methods the slope and aspect of each pixel were determined as those that give the best prediction of the observed value of cos i height profiles were then extracted using simple trigonometric relationships the accuracies of heights and slopes along selected profile lines were to the order of 1 m and 3 respectively at a spatial resolution of 15 m best results were obtained when the images included one from the summer and the other from the winter corresponding to maximum difference in solar zenith and azimuth angles errors in heights were attributed to surface heterogeneity e g presence of biogenic soil crusts in the rainy season geometric correction errors cast shadows and bidirectional reflectance distribution function brdf effects comparison to advanced thermal emission and reflection radiometer aster 3d information showed that the proposed method is better in representing the topographic variation of the area than the digital elevation model dem produced by aster

the antarctic circumpolar wave a beta effect in ocean atmosphere coupling over the southern ocean the antarctic circumpolar wave acw is a nominal 4 yr climate signal in the ocean atmosphere system in the southern ocean propagating eastward at an average speed of 6 8 cm s21 composed of two waves taking approximately 8 years to circle the globe the acw is characterized by a persistent phase relationship between warm cool sea surface temperature sst anomalies and poleward equatorward meridional surface wind msw anomalies recently white and chen demonstrated that sst anomalies in the southern ocean operate to induce anomalous vortex stretching in the lower troposphere that is balanced by the anomalous meridional advection of planetary vorticity yielding msw anomalies as observed in the present study the authors seek to understand how this atmospheric response to sst anomalies produces a positive feedback to the ocean i e an anomalous sst tendency displaced eastward of sst anomalies that both maintains the acw against dissipation and accounts for its eastward propagation to achieve this we couple a global equilibrium climate model for the lower troposphere to a global heat budget model for the upper ocean in the absence of coupling the model antarctic circumpolar current acc advects sst anomalies from initial conditions to the east at speeds slower than observed taking 12 14 years to circle the globe with amplitudes that become insignificant after 6 8 years in the presence of coupling eastward speeds of the model acc are matched by those due to coupling together yielding a model acw of a nominal 4 yr period composed of two waves that circle the globe in approximately 8 years as observed feedback from atmosphere to ocean works through the anomalous zonal surface wind response to sst anomalies yielding poleward ekman flow anomalies in phase with warm sst anomalies as such maintenance of the model acw is achieved through a balance between anomalous meridional ekman heat advection and anomalous sensible plus latent heat loss to the atmosphere this balance requires the alignment of covarying sst and msw anomalies to be tilted into the southwest northeast direction which accounts for the spiral structure observed in global sst and sea level pressure anomaly patterns around the southern ocean eastward coupling speeds of the model acw derive from a beta effect in coupling that displaces a portion of the anomalous meridional ekman heat advection and its corresponding anomalous sst tendency to the east of sst anomalies therefore the acw is an example of self organization within the global ocean atmosphere system depending upon the spherical shape of the rotating earth for its propagation and the mean meridional sst gradient for its maintenance and producing a net poleward eddy heat flux in the upper ocean that tends to reduce this mean gradient

an antarctic circumpolar wave in surface pressure wind temperature and sea ice extent the southern ocean is the only oceanic domain encircling the globe it contains the strong eastward flow of the antarctic circumpolar current and is the unifying link for exchanges of water masses at all depths between the world s major ocean basins1 as these exchanges are an important control on mean global climate the southern ocean is expected to play an important role in transmitting climate anomalies around the globe interannual variability has been often observed at high southern latitudes and observations of sea ice extent suggest that such features propagate eastwards around the southern ocean2 3 here we use data from a variety of observational techniques to identify significant interannual variations in the atmospheric pressure at sea level wind stress sea surface temperature and sea ice extent over the southern ocean these anomalies propagate eastward with the circumpolar flow with a period of 4 5 years and taking 8 10 years to encircle the pole this system of coupled anomalies which we call the antarctic circumpolar wave is likely to play an important role in climate regulation and dynamics both within and beyond the southern ocean

periodic climate change on mars review of evidence and effects on distribution of volatiles abstract the polar regions of mars preserve in both their layering and their topography a record of recent climate changes because of the coincidence of the growth of the northern seasonal cap with global dust storms dust may be currently accumulating on the northern cap but conditions at the poles will alternate with the precessional cycle deposition is also modulated by changes in eccentricity and obliquity which interact complexly affecting initiation of global dust storms the stability of volatiles at the surface and global wind regimes formation of spiral valleys and low undulations on the surface of the layered deposits may result from prefential sublimation of volatiles on sunward facing slopes and condensation on the adjacent flats with the rates also modulated by astronomically caused insolation variations lack of impact craters on the surface and lack of interruption of the layers by impact scars suggest that the polar deposits are no more than a few million years old older deposits may have been periodically removed as indicated by etch pitted terrain at the south pole and by superposition relations around the periphery of the present layered deposits evidence of ancient periodic climate changes that occurred before formation of the present layered terrain is fragmentary but includes pedestal craters parallel moraine like ridges and etched ground at high latitudes perturbation of the orbital motions also results in adsorption and desorption of volatiles in the regolith which leads to variations in atmospheric pressure and partial dehydration of the equatorial near surface materials

inverse theory applied to multi source cross hole tomography part 1 acoustic wave equation method1 frequency domain methods are well suited to the imaging of wide aperture cross hole data however although the combination of the frequency domain with the wavenumber domain has facilitated the development of rapid algorithms such as diffraction tomography this has also required linearization with respect to homogeneous reference media this restriction and association restrictions on source receiver geometries are overcome by applying inverse techniques that operate in the frequency space domain in order to incorporate the rigorous modelling technique of finite differences into the inverse procedure a nonlinear approach is used to reduce computational costs the method of finite differences is applied directly to the frequency domain wave equation the use of high speed high capacity vector computers allow the resultant finite difference equations to be factored in place in this way wavefields can be computed for additional source positions at minimal extra cost allowing inversions to be generated using data from a very large number of source positions synthetic studies show that where weak scatter approximations are valid diffraction tomography performs slightly better than a single iteration of non linear inversion however if the background velocities increase systematically with depth diffraction tomography is ineffective whereas non linear inversion yields useful images from one frequency component of the data after a single iteration further synthetic studies indicate the efficacy of the method in the time lapse monitoring of injection fluids in tertiary hydrocarbon recovery projects

subsurface structure of interannual temperature anomalies in the australian sector of the southern ocean 1 a 7 year time series of austral summer expendable bathythermograph xbt sections between tasmania and antarctica is used to describe the subsurface structure of southern ocean interannual temperature anomalies comparison of the discontinuous xbt record with a continuous weekly average satellite based sst data set confirms that the xbt sampling is adequate to resolve interannual variability significant correlations are found between changes in surface and subsurface temperature along much of the section in the subantarctic zone surface and subsurface changes are coherent because deep convection in winter homogenizes the upper 600 m and the summer mixed layer temperature reflects the preconditioning of the previous winter at other latitudes the regions of high correlation generally extend well beyond the depth of direct atmospheric influence i e the depth of the winter mixed layer negative positive temperature anomalies at interannual timescales are coherent with poleward equatorward displacements of fronts the simplest explanation for the correspondence between deep vertically coherent temperature anomalies and the location of acc fronts is that the observed features reflect meridional shifts of temperature gradients associated with the fronts the link between temperature anomalies and front displacements suggests southern ocean sea surface temperature sst anomalies primarily reflect a dynamical response of the ocean to wind or other forcing rather than surface trapped features produced by anomalous air sea heat flux the xbt record contains both interannual and longer unresolved period signals which cannot be isolated in the short discontinuous xbt time series spectral analysis of the 17 year sst data set reveals roughly equal variability in 2 7 year and 7 year bands

seismic waveform inversion in the frequency domain part 2 fault delineation in sediments using crosshole data a crosshole experiment was carried out in a layered sedimentary environment in which a normal fault is known to cut through the section initial traveltime inversions produced stable but low resolution images from which the fault could be only vaguely inferred to image the fault wavefield inversion was used to produce a velocity model consistent with the detailed phase and amplitude of the data at a number of frequencies our wavefield inversion scheme uses a classical descent type algorithm for decreasing the data misfit by iteratively computing the gradient of this misfit by repeated forward and backward propagations our propagator is a full wave equation frequency domain acoustic finite difference method the use of the frequency space domain yields computational advantages for multisource data and allows an easy incorporation of viscous effects by running wavefield inversion on the field data a quantitative velocity image was produced that yielded a significantly improved image of the fault when compared with the original traveltime inversions because the original field data were noisy and contained a high degree of multiple scattering from the layering of the sediments the transmitted arrivals were selectively windowed to enhance the image the sediments at the site were strongly attenuating we therefore used a viscoacoustic model during the modeling and the inversion that correctly simulated the observed decrease in amplitude with increasing frequency and source receiver offset furthermore since the traveltime inversion indicated a high degree of anisotropy at the site a fixed homogeneous level of anisotropy was used during the inversion tests at varying levels of anisotropy confirmed the improvement in image quality and in data fit when anisotropy was incorporated the final image was verified by examining the distribution of the residuals in the frequency domain by comparing time domain modeled wavefields with the observed data and by direct comparison with borehole logs

3 d finite difference elastic wave modeling including surface topography three dimensional finite difference fd modeling of seismic scattering from free surface topography has been pursued we have developed exact 3 d free surface topography boundary conditions for the particle velocities a velocity stress formulation of the full elastic wave equations together with the boundary conditions has been numerically modeled by an eighth order fd method on a staggered grid we give a numerical stability criterion for combining the boundary conditions with curved grid wave equations where a curved grid represents the physical medium with topography implementation of this stability criterion stops instabilities from arising in areas of steep and rough topographies we have simulated scattering from teleseismic p waves using a plane vertically incident wavefront and real topography from a 40 40 km area centered at the noress array of seismic receiver stations in southeastern norway synthetic snapshots and seismograms of the wavefield show clear conversion from p waves to rg sh

coherent interannual and decadal variations in the atmosphere ocean system 1 we investigate the source of poleward propagating atmospheric zonal wind anomalies originating at the equator and penetrating to high latitudes in both hemispheres in conjunction with enso dickey et al 1992 and report the discovery of similar variability on decadal and longer timescales since atmospheric dissipation times are generally on the order of a month or less we examine the ocean as a memory source for these globally coherent anomalies this hypothesis is substantiated by the observation of complementary oscillation in the sea surface temperature sst field further we detect a robust decadal variability 10 12 yrs in both the sst and contemporaneous atmospheric angular momentum aam series analyzing gisst sst data beginning in 1902 we confirm this decadal mode and find signatures of longer multidecadal sst variability centered in the equatorial and north pacific

two dimensional velocity models from wide angle seismic data by wavefield inversion summary modern wide angle surveys are often multi fold and multi channel with densely sampled source and receiver spacings such closely spaced data are potentially amenable to multi channel techniques involving wavefield propagation methods such as those commonly used in reflection data processing however the wide angle configuration requires techniques capable of handling very general wave types including those not commonly used in reflection seismology this is a situation analogous to that faced in cross borehole seismics where similar wave types are also recorded in a real cross borehole example we compare pre stack migration traveltime tomography and wavefield inversion we find that wavefield inversion produces images that are quantitative in velocity as are the tomograms but are of significantly higher resolution the wavefield inversion results have a resolution comparable to that of the qualitative pre stack migration images we seek to extend this novel development to the larger scale problem of crustal imaging an essential element of the approach we adopt is its formulation entirely within the temporal frequency domain this has three principal advantages 1 we can choose to decimate the data by selecting only a limited number of frequency components to invert thus making inversion of data from large numbers of source positions feasible 2 we can mitigate the notorious non linearity of the seismic inverse problem by progressing from low frequency components in the data to high frequency components and 3 we can include in the model any arbitrary frequency dependence of inelastic attenuation factors q and indeed solve for the spatial distribution of q an initial synthetic test with an anomaly located within the middle crust yields a velocity image with the correct structural features of the anomaly and the correct magnitude of velocity anomaly this is related to the fact that the reconstruction is obtained from forward scattered waves under these conditions the method thus behaves much like tomography a second test with a deeper more extensive anomaly yields an image with the correct velocity polarity and the correct location but with a deficiency in low and high wavenumbers in this case this is because the reconstruction is obtained from backscattered waves under these conditions the method behaves not like tomography but like migration a more extensive test based on a large wide angle survey in south eastern california and western arizona demonstrates a real potential for high resolution imaging of crustal structures although our results are limited by the acoustic approximation and by the relatively low frequencies that we can model today the images are sufficiently encouraging to warrant future research the problem of local minima in the objective function is the most significant practical problem with our method but we propose that appropriate layer stripping methods can handle this problem

photoclinometric determination of the topography of the martian north polar cap abstract photoclinometry is useful for the determination of topography for areas which have a uniform albedo the technique is applied to early spring viking images of the martian north polar cap taken when the surface was covered by a nearly uniform frost cover unlike earlier approaches the topographic profiling can be used for surfaces with any photometric function but the strike of the planetary surface relative to the illumination angle must be specified along the profile the resultant profiles are relatively insensitive to misestimation of the photometric functin and slope orientation but are quite sensitive to the assumed values of the reflectance of an equivalent level surface and the atmospheric opacity if it is large

links between tropical convection and variations of the extratropical circulation during toga coare abstract data from the intensive observation period of the tropical ocean global atmosphere coupled ocean atmosphere response experiment november 1992 february 1993 have been used to investigate the links between intraseasonal variations in tropical convection and those in forcing of upper tropospheric rossby waves in the extratropics the primary databases are geostationary meteorological satellite imagery and tropical wind analyses from the bureau of meteorology australia a number of 5 day periods showing convection in different locations were chosen for each period mean fields of divergence cloud top temperature and upper tropospheric rossby wave source are presented vorticity budgets are used to demonstrate the processes responsible for the rossby wave source patterns the approach follows earlier studies of links between interannual variations of tropical convection associated with the southern oscillation and variations of the extratropical circulation it is shown that the regions of tropi

is climate variability over the north pacific a linear response to enso abstract indices of the dominant spatial patterns of wintertime northern hemisphere 500 mb height and north pacific sea surface temperature are strongly correlated with one another on interannual and interdecadal timescales and both am correlated with indices of the el nin southern oscillation one possible interpretation of these relationships is that the tropical sst anomalies associated with enso force the atmospheric circulation over the north pacific and these atmospheric anomalies in turn give rise to the observed sst anomalies over the extratropical north pacific in this study linear relationships between enso and extratropical variables are examined in two different ways first the component of the observed extratropical variability that is linearly dependent upon enso is removed the dominant spatial patterns in the residual variability of 500 mb height and sst anomalies over the north pacific are shown to be similar to their counterparts in the total fields and remain strongly coupled on

the mars observer laser altimeter investigation the primary objective of the mars observer laser altimeter mola investigation is to determine globally the topography of mars at a level suitable for addressing problems in geology and geophysics secondary objectives are to characterize the 1064 nm wavelength surface reflectivity of mars to contribute to analyses of global surface mineralogy and seasonal albedo changes to assist in addressing problems in atmospheric circulation and to provide geodetic control and topographic context for the assessment of possible future mars landing sites the principal components of mola are a diode pumped neodymium doped yttrium aluminum garnet laser transmitter that emits 1064 nm wavelength laser pulses a 0 5 m diameter telescope a silicon avalanche photodiode detector and a time interval unit with 10 ns resolution mola will provide measurements of the topography of mars within approximately 160 m footprints and a center to center along track foot print spacing of 300 m along the mars observer subspacecraft ground track the elevation measurements will be quantized with 1 5 m vertical resolution before correction for orbit and pointing induced errors mola profiles will be assembled into a global 0 2 deg x 0 2 deg grid that will be referenced to mars center of mass with an absolute accuracy of approximately 30 m other data products will include a global grid of topographic gradients corrected individual profiles and a global 0 2 deg x 0 2 deg grid of 1064 nm surface reflectivity

topography and stratigraphy of martian polar layered deposits abstract topography and stratigraphy of young sediments in the martian north polar region are being studied from high resolution viking orbiter images to test and constrain theories of terrain evolution driven by cyclical climate change several steepened slopes upon which layers are exposed in section were found to have local relief of 200 to 800 m and slopes ranging from 1 to 8 first attempts to calculate average layer thicknesses from portions of sections yielded results ranging from 14 to 46 m linearly correlated with slope in some cases under the assumption of short range lateral continuity a greater number of layers was inferred to be present than was actually resolved on a particular section when apparent layer thickness are revised accordingly the correlation with slope is destroyed we conclude that images of the shallowest sloping sections of layered deposits should yield the most stratigraphic information and layers thinner than 14 m may occur indeed they may even be common or typical because of the sensitivity of summer frost distribution to local slope and other properties of exposed strata interseasonal comparisons of images promise to enhance the capability to detect layers and differentiate them by material properties

interdecadal variability of the pacific ocean model response to observed heat flux and wind stress anomalies variability of the pacific ocean is examined in numerical simulations with an ocean general circulation model forced by observed anomalies of surface heat flux wind stress and turbulent kinetic energy tke over the period 1970 88 the model captures the 1976 77 winter time climate shift in sea surface temperature as well as its monthly seasonal and longer term variability as evidenced in regional time series and empirical orthogonal function analyses examination of the surface mixed layer heat budget reveals that the 1976 77 shift was caused by a unique concurrance of sustained heat flux input anomalies and very strong horizontal advection anomalies during a multi month period preceding the shift in both the central pacific region where cooling occurred and the california coastal region where warming occurred in the central pacific the warm conditions preceding and the cold conditions following the shift tend to be maintained by anomalous vertical mixing due to increases in the atmospheric momentum flux tke input into the mixed layer which deepens in the model after the shift from the early 1970s to the late 1970s and 1980s since the ocean model does not contain feedback to the atmosphere and it succeeds in capturing the major features of the 1976 77 shift it appears that the midlatitude part of the shift was driven by the atmosphere although effects of midlatitude ocean atmosphere feedback are still possible the surface mixed layer heat budget also reveals that in the central pacific the effects of heat flux input and vertical mixing anomalies are comparable in amplitude while horizontal advection anomalies are roughly half that size in the california coastal region in contrast where wind variability is much weaker than in the central pacific horizontal advection and vertical mixing effects on the mixed layer heat budget are only one quarter the size of typical heat flux input anomalies

accumulation of sedimentary debris in the south polar region of mars and implications for climate history abstract stratigraphic units of the south polar region of mars were mapped relative chronology determined and detailed modeling of the observed crater populations was used to set absolute constraints on the age of emplacement of materials significant secular variation in the net debris accumulation rate over history is evident an episode of enhanced crater obliteration at about 3 7 ga ago lasting a few hundred ma is inferred for south polar cratered terrains a similar peak in low latitude obliteration rates suggests that the event may have been global in scale whether the debris is volcanic or aeolian in origin the event suggests a possible enhancement in atmospheric density at the time modeling results imply that cratered terrains poleward of 65 south latitude have subsequently experienced steady state net accumulation of material at a rate of about 0 1 km ga while equatorial cratered terrains have been retained in relatively pristine form fifteen craters of impact origin were discovered on the south polar layered terrain formerly thought to be almost devoid of craters their presence implies that the surface is at least a few 100 ma old and that the net accumulation rate is no more than 10 km ga if layer formation is modulated by periodic climatic effects either the period of oscillation is a few ma or longer or the layer deposition mechanism ceased operating at least several 100 ma ago

2d finite difference elastic wave modelling including surface topography1 a 2d numerical finite difference algorithm accounting for surface topography is presented higher order dispersion bounded cost optimized finite difference operators are used in the interior of the numerical grid while non reflecting absorbing boundary conditions are used along the edges transformation from a curved to a rectangular grid achieves the modelling of the surface topography we use free surface boundary conditions along the surface in order to obtain complete modelling of the effects of wave propagation it is important to account for the surface topography otherwise near surface effects such as scattering are not modelled adequately even if other properties of the medium for instance randomization can improve numerical simulations inclusion of the surface topography makes them more realistic

the antarctic circumpolar wave in a coupled ocean atmosphere gcm abstract a phenomenon called the antarctic circumpolar wave acw suggested earlier from fragmentary observational evidence has been simulated realistically in an extended integration of a max planck institute coupled general circulation model the acw both in the observations and in the model constitutes a mode of the coupled ocean atmosphere sea ice system that inhabits the high latitudes of the southern hemisphere it is characterized by anomalies of such climate variables as sea surface temperature sea level pressure meridional wind and sea ice that exhibit intricate and evolving spatial phase relations to each other the simulated acw signal in the ocean propagates eastward over most of the high latitude southern ocean mainly advected along in the antarctic circumpolar current on average it completes a circuit entirely around the southern ocean but is strongly dissipated in the south atlantic and in the southern indian ocean just marginally maintaining statistical significance in these areas

topography of the northern hemisphere of mars from the mars orbiter laser altimeter the first 18 tracks of laser altimeter data across the northern hemisphere of mars from the mars global surveyor spacecraft show that the planet at latitudes north of 50 is exceptionally flat slopes and surface roughness increase toward the equator the polar layered terrain appears to be a thick ice rich formation with a non equilibrium planform indicative of ablation near the periphery slope relations suggest that the northern tharsis province was uplifted in the past a profile across ares vallis channel suggests that the discharge through the channel was much greater than previously estimated the martian atmosphere shows significant 1 micrometer atmospheric opacities particularly in low lying areas such as valles marineris

equilibrium atmospheric response to north atlantic sst anomalies abstract the equilibrium general circulation model gcm response to sea surface temperature sst anomalies in the western north atlantic region is studied a coarse resolution gcm with realistic lower boundary conditions including topography and climatological sst distribution is integrated in perpetual january and perpetual october modes distinguished from one another by the strength of the midlatitude westerlies an sst anomaly with a maximum of 4 c is added to the climatological sst distribution of the model with both positive and negative polarity these anomaly runs are compared to one another and to a control integration to determine the atmospheric response in all cases warming cooling of the midlatitude ocean surface yields a warming cooling of the atmosphere over and to the east of the sst anomaly center the atmospheric temperature change is largest near the surface and decreases upward consistent with this simple thermal response the geopotential height field displays a baroclinic

recent formation and evolution of northern martian polar layered deposits as inferred from a global climate model 1 we present a time marching model which simulates the exchange of water ice between the martian northern cap the tropics and a high latitude surface reservoir net annual exchange rates of water and their sensitivity to variations in orbital rotational parameters are examined using the martian water cycle modeled by the lmd three dimensional global climate model these rates are propagated over the last 10 myr to follow the thickness of the reservoirs the effect of a sublimation dust lag is taken account to test simple models of layer formation periods of high mean polar summer insolation 5 10 ma ago lead to a rapid exhaustion of a northern polar cap and a prolonged formation of tropical glaciers the formation of a northern cap and of a high latitude icy mantle may have started 4 ma ago with the average decrease of polar insolation tropical ice may have disappeared around 2 7 ma ago but small glaciers could have formed during the last peaks of polar summer insolation over the last 4 myr most of the present cap may have formed at the expense of tropical and high latitude reservoirs forming distinct layers at almost each 51 kyr 120 kyr insolation cycle layers thickness ranges from 10 to 80 m variations being produced by the modulation of the obliquity with 2 4 and 1 3 myr periods because only 30 insolation cycles have occurred since 4 ma ago we found an inconsistency between the recent astronomical forcing the observed number of layers and simple astronomically based scenarios of layers formation

low frequency variability in the southern ocean region in a simplified coupled model 1 patterns of interannual variability of the ocean atmosphere coupled system in the southern hemisphere extratropics are studied with a simple dynamical model in order to determine the basic physical processes of interaction independently of tropical forcing the model used is an atmospheric quasi geostrophic model coupled to a slab oceanic mixed layer which includes mean geostrophic advection by the antarctic circumpolar current acc the ocean atmosphere coupling occurs through surface heat fluxes and ekman current heat advection in a fully coupled simulation the atmospheric part of the model which includes high frequency transient eddies at midlatitudes exhibits a strong southern annular mode sam as the first mode of variability at interannual timescales the sam related wind anomalies induce ekman currents in the mixed layer which produce sea surface temperature anomalies these are then advected along by the acc a forced mechanism where the ocean role is reduced to advect the sea surface temperature sst appears sufficient to reproduce the main features of the variability nevertheless a positive feedback of the ocean was also found it operates through anomalous ekman currents heat advection and contributes to the maintenance of the sst anomaly

a theory of interdecadal climate variability of the north pacific ocean atmosphere system abstract a linear coupled model for the atmosphere upper ocean system is proposed to highlight the mechanisms of decadal to interdecadal climate variability in the north pacific in this model wind stress anomalies over the north pacific are related to anomalies in the meridional temperature gradient of the upper ocean the latter depends upon air sea thermodynamical feedbacks and meridional heat transport by upper ocean currents slow adjustment of the oceanic gyre circulation to the change in wind stress is accomplished by the forced baroclinic oceanic rossby waves which carry out the meridional heat transport uncoupled ocean dynamic adjustment can produce a weak decadal to interdecadal peak in the power spectrum of the meridional transport under temporal white noise wind stress forcing with organized spatial structure coupled dynamics produce a basin scale interdecadal oscillatory mode this mode arises from the dynamic coupling and the memory of the system residing in the slow gyre circulation ad

crosshole seismic waveform tomography i strategy for real data application summary the frequency domain version of waveform tomography enables the use of distinct frequency components to adequately reconstruct the subsurface velocity field and thereby dramatically reduces the input data quantity required for the inversion process it makes waveform tomography a computationally tractable problem for production uses but its applicability to real seismic data particularly in the petroleum exploration and development scale needs to be examined as real data are often band limited with missing low frequencies a good starting model is necessary for waveform tomography to fill in the gap of low frequencies before the inversion of available frequencies in the inversion stage a group of frequencies should be used simultaneously at each iteration to suppress the effect of data noise in the frequency domain meanwhile a smoothness constraint on the model must be used in the inversion to cope the effect of data noise the effect of non linearity of the problem and the effect of strong sensitivities of short wavelength model variations in this paper we use frequency domain waveform tomography to provide quantitative velocity images of a crosshole target between boreholes 300 m apart due to the complexity of the local geology the velocity variations were extreme between 3000 and 5500 m s 1 making the inversion problem highly non linear nevertheless the waveform tomography results correlate well with borehole logs and provide realistic geological information that can be tracked between the boreholes with confidence

reflection seismic waveform tomography 1 in seismic waveform tomography if using reflection data with limited source receiver offsets it is difficult to reconstruct the deep part of the subsurface velocity model we present two approaches to tackle this problem layer stripping and weighted updating in a layer stripping procedure we replace the top portion of seismic data with synthetics generated from the previous layer inversion and make the current inversion focus on the minimization of the data misfit corresponding to the deep part of the model to improve efficiency we use only sparsely sampled frequency data in the deeper layer inversions unlike the first layer inversion where we use densely sampled frequency data as usual the sparsely sampled frequencies together have the full wave number coverage for effective imaging combined use of dense and sparse sampling in frequency is a compromise between resolution and efficiency as it reduces the number of iterations needed in layer stripping inversion while still producing a good image in the second scheme we apply depth dependent weights to model updates in order to improve the convergence in an iterative solution the weighting is inversely proportional to the ray density variation along the depth and is mathematically equivalent to the application of an inverse hessian matrix which sharpens the gradient vector for model updating for real seismic data we transfer point source shot records to line source records by partial amplitude compensation and phase adjusting before inputting it to the waveform tomography we perform traveltime inversion to generate a reliable layered velocity model and then waveform tomography to produce a high resolution image of the subsurface model through frequency domain iteration

sand dune materials and polar layered deposits on mars abstract the relationship of dark sand dunes to the polar layered deposits of mars is investigated by mapping the colors and morphologic settings of the dunes in and near the polar deposits in the north polar region the dunes have restricted sources in the layered deposits that are associated with are arcuate scarps 10 50 km in length and

a modelling and observational study of the relationship between sea surface temperature in the north west atlantic and the atmospheric general circulation results from four pairs of 50 day wintertime integrations of the meteorological office s 5 level general circulation model with warm and cold sea surface temperature s s t anomalies of about 3 k in the northwest atlantic are described difference fields between the warm and cold integrations are statistically significant at the 1 level with positive geopotential height over the central north atlantic and weaker negative height over europe the storm track over the atlantic is displaced from its normal position results from four further pairs of integrations with halved s s t anomalies are also described the response is approximately linear with systematic differences in 500 mb geopotential height over the atlantic parts of which are just significant at the 10 level with half the full s s t anomaly overall however the model s response is weaker than could be obtained with tropical s s t anomalies of the same magnitude results from the model integrations are compared with results from an observational study of the relationship between wintertime s s t in the north west atlantic and mean sea level pressure and 500mb height two independent 30 year periods were chosen for study thus minimizing the influence of long term trends in s s t over the atlantic and europe the model results compared well with the observations with s s t data lagging the atmosphere by one month the observational study appears to show that the s s t anomalies are initially forced by perturbations in the atmospheric circulation with s s t data leading the atmosphere by one month results show that atmospheric and s s t anomalies are most persistent in the period october to december throughout the winter these lagged relationships are much weaker and not statistically significant diagnostics of e vector divergence from the gcm experiments are used to suggest that anomalous baroclinic wave activity over the atlantic is important as a momentum forcing for the anomalous time mean flow pattern on the other hand the role of thermal forcing provided by anomalous diabatic heating and transient eddy heat flux convergence may be important to substantiate this statement a simple linear steadystate two layer model of the response to extratropical thermal forcing is described with a suitable basic state flow and a mid tropospheric heat source given mainly by the transient eddy heat flux convergence the response is shown to be equivalent barotropic with a downstream ridge and ascent over the thermal source conversely results from an ocean mixed layer model are discussed which suggest that warm s s t anomalies could be maintained by a positive surface pressure perturbation positioned downstream of the anomaly through anomalous northward advection of warm ocean water by ekman drift currents this northward advection would balance the sensible and latent heat loss into the atmosphere over the s s t anomaly hence it is possible that some positive ocean atmosphere feedback may account for the persistence of such atmospheric and oceanic anomalies

efficient waveform tomography for lithospheric imaging implications for realistic two dimensional acquisition geometries and low frequency data summary we provide a series of numerical experiments designed to test waveform tomography under i a reduction in the number of input data frequency components efficient waveform tomography ii sparse spatial subsampling of the input data and iii an increase in the minimum data frequency used these results extend the waveform tomography results of a companion paper using the same third party 2 d wide angle synthetic viscoelastic seismic data computed in a crustal geology model 250 km long and 40 km deep with heterogeneous p velocity s velocity density and q factor structure accurate velocity models were obtained using efficient waveform tomography and only four carefully selected frequency components of the input data 0 8 1 7 3 6 and 7 0 hz this strategy avoids the spectral redundancy present in full waveform tomography and yields results that are comparable with those in the companion paper for an 88 per cent decrease in total computational cost because we use acoustic waveform tomography the results further justify the use of the acoustic wave equation in calculating p wave velocity models from viscoelastic data the effect of using sparse survey geometries with efficient waveform tomography were investigated for both increased receiver spacing and increased source spacing sampling theory formally requires spatial sampling at maximum interval of one half wavelength 2 5 km at 0 8 hz for data with receivers every 0 9 km conforming to this criterion artefacts in the tomographic images were still minimal when the source spacing was as large as 7 6 km three times the theoretical maximum larger source spacings led to an unacceptable degradation of the results when increasing the starting frequency image quality was progressively degraded acceptable image quality within the central portion of the model was nevertheless achieved using starting frequencies up to 3 0 hz at 3 0 hz the maximum theoretical sample interval is reduced to 0 67 km due to the decreased wavelengths the available sources were spaced every 5 0 km more than seven times the theoretical maximum and receivers were spaced every 0 9 km 1 3 times the theoretical maximum higher starting frequencies than 3 0 hz again led to unacceptable degradation of the results