3 d inhomogeneities in the upper mantle abstract the method developed by aki christofferson and husebye for determining a 3 d seismic image of the earth s interior from the travel time data obtained at a 2 d array has been applied to seventeen arrays around the world by various authors the most outstanding results from these works are that small scale lateral velocity anomalies exist in the lower crust and upper mantle everywhere studied the reality of these deep seated anomalies is supported by 1 1 significant reduction of residual variance 2 2 resolution analysis for decoupling from shallower anomalies and 3 3 error analysis for the significance of anomalies in this paper we try to strengthen the credibility of these deep seated anomalies by drawing attention to those found where they were expected namely low velocity bodies under several geothermal areas and high velocity anomalies associated with the subducting plate

redistribution of dynamic stress during coseismic ruptures evidence for fault interaction and earthquake triggering we investigate the spatiotemporal evolution of dynamic stress outside a rupturing extended fault the dynamic stress variations caused by a coseismic rupture in a half space are computed by using the discrete wavenumber and reflectivity methods after a transient phase the stress time history evolves to the final static stress value we compare the static stress changes resulting from this model with those computed from a static dislocation model we have applied this method to study the interactions between the first two normal faults which ruptured during the 1980 ms 6 9 irpinia earthquake these two subevents are separated in time by nearly 20 s while the third and last subevent occurred 40 s after the rupture onset we compute the dynamic stress changes caused by the rupture of the first subevent our modeling results show that the dynamic stress peak on the second subevent fault plane is reached between 7 s and 8 s after the rupture initiation on the main fault on the average the static stress level on the second subevent 20 s fault plane is reached nearly after 14 s the dynamic rupture did not jump from a rupturing segment to the adjacent one immediately but the triggering of the 20 s subevent is delayed by roughly 10 s with respect to the instant of occurrence of the dynamic stress peak induced by the 0 s event the dynamic stress pulse propagates along the strike direction of the second subevent fault plane at an average velocity of nearly 3 4 km s the delayed triggering of the second subevent can be interpreted in terms of the frictional properties of the faults in particular rate and state dependent frictional law can explain a delayed instability after a sudden change in stress using the estimated values of the subevent triggering delay and the shear stress change we attempt to constrain the parameter a on the 20 s fault the values here inferred agree well with those resulting from previous studies

the linked stress release model for spatio temporal seismicity formulations procedures and applications summary the linked stress release model is based on the build up of stress through elastic rebound and its dissipation in the form of earthquakes in addition stress can be transferred between large scale geological or seismic features the model can be statistically fitted to both historical and synthetic seismicity catalogues and through simulation can be used to create probabilistic forecasts of earthquake risk we review the genesis of the model provide some observations on forecasting using the model and follow with a comprehensive review of applications to date a systematic procedure for identification of the best model is illustrated by data from the persian region we then consider the evaluation of fitted models using residual point processes and information gains implications of the use of benioff strain rather than seismic moment are discussed the sensitivity of the model to regionalization magnitude errors catalogue incompleteness catalogue size and declustering magnitude cut off is then considered in detail with reference to data from north china the latter data are also used to illustrate the model evaluation techniques introduced earlier some technical material on numerical fitting simulation and calculation of the information gain is given in an appendix

importance of small earthquakes for stress transfers and earthquake triggering 1 we estimate the relative importance of small and large earthquakes for static stress changes and for earthquake triggering assuming that earthquakes are triggered by static stress changes and that earthquakes are located on a fractal network of dimension d this model predicts that both the number of events triggered by an earthquake of magnitude m and the stress change induced by this earthquake at the location of other earthquakes increase with m as similar to 10 dm 2 the stronger the spatial clustering the larger the influence of small earthquakes on stress changes at the location of a future event as well as earthquake triggering if earthquake magnitudes follow the gutenberg richter law with b d 2 small earthquakes collectively dominate stress transfer and earthquake triggering because their greater frequency overcomes their smaller individual triggering potential using a southern california catalog we observe that the rate of seismicity triggered by an earthquake of magnitude m increases with m as 10 alpha m where alpha 1 05 0 05 we also find that the magnitude distribution of triggered earthquakes is independent of the triggering earthquake s magnitude m when alpha approximate to b small earthquakes are roughly as important to earthquake triggering as larger ones we evaluate the fractal correlation dimension d of hypocenters using two relocated catalogs for southern california the value of d measured for distances 0 1 r 5 km is d 1 54 for the shearer et al catalog and d 1 73 for the hauksson et al catalog the value of d reflects both the structure of the fault network and the nature of earthquake interactions by considering only those earthquake pairs with interevent times larger than 1000 days we can largely remove the effects of short term clustering then d approximate to 2 close to the value d 2 alpha 2 1 predicted by assuming that earthquake triggering is due to static stress the value d approximate to 2b implies that small earthquakes are as important as larger ones for stress transfers between earthquakes and that considering stress changes induced by small earthquakes should improve models of earthquake interactions

a simplified ice ocean coupled model for the antarctic ice melt season abstract in the antarctic ocean sea ice melts mostly by warming of the ocean mixed layer through heat input mainly solar radiation in open water areas a simplified ice upper ocean coupled model is proposed in which sea ice melts only by the ocean heat supplied from the air the model shows that the relationship between ice concentration i e fraction c and mixed layer temperature t converges asymptotically with time c t relationship which agrees with observed c t plots during summer in the sector 25 45 e this relationship can be used for estimating the bulk heat transfer coefficient between ice and ocean by fitting to observations and a value of 1 2 10 4 m s 1 is obtained the model shows that the ratio of the heat used for melting to the heat input through open water is inclined to be determined as a function of ice concentration for typical conditions in the antarctic ice melt season the ratio ranges mostly between 0 7 and 0 9 when the model is extended to two dimensions in the meri

modeling deep convection in the greenland sea the development of deep convective events in the high latitude ocean is studied using a three dimensional coupled ice ocean model oceanic mixing is described according to the level 2 5 turbulence closure scheme in which convection occurs in a continuous way i e convective adjustment is not invoked the model is forced by strong winds and surface cooling strong upwelling at the multiyear ice edge and consequent entrainment of warm atlantic waters into the mixed layer is produced by winds parallel to the ice edge concomitant cooling drives deep convection and produces chimneylike structures inclusion of a barotropic mean flow over topography to the model provides important preconditioning and selects the location of deep convection the most efficient preconditioning occurs at locations where the flow ascends a slope in a stratified environment similar to the greenland sea with a 12 m s 1 wind the model simulations show that localized deep convection takes place after about 10 days to depths of 1000 m

accelerating seismic release from a self correcting stochastic model 1 we investigate the conditions under which the stress release model a stochastic version of the elastic rebound model produces synthetic earthquake sequences characterized by accelerating seismic release asr in this model the level or stress of the process accumulates linearly with time through tectonic input and decreases as the result of earthquakes these stress drops correspond to some power of the energy released in the earthquakes either e0 5 benioff strain or e seismic moment earthquakes occur in a point process with rate controlled by the level of the process we hypothesize that the critical factor in the appearance of asr is the manner in which the event sizes depend on the level of the process this is modeled by the square root of energy released following either a tapered pareto or truncated gutenberg richter distribution with maximum earthquake size controlled by a tail off or truncation point as the tail off point becomes large so does the average size corresponding to an acceleration to criticality of the system we found that those cases where the underlying level of the process corresponded to accumulated seismic moment produced numerous asr sequences whereas those cases using accumulated benioff strain as the level did not these results suggest that the occurrence of asr is strongly dependent on how large earthquakes affect the dynamics of the fault system in which they are embedded and hopefully provide some insight into the mechanics of acceleration to criticality i e on the possible causes of occurrence nonoccurrence of asr

the sea ice thickness distribution in the northwestern weddell sea we present new data on distribution of snow and sea ice thicknesses in the northwestern weddell sea the data were obtained through direct measurements along 19 profiles each approximately 100 m long on 17 different floes located between 54 46 w and 59 64 s the overall probability density functions pdfs for ice thicknesses reflect the complex mixture of first second and multi year ice to be expected in the outflowing branch of the weddell gyre further differentiation of the data reveals four distinct thickness classes which reflect differences in the formation and subsequent histories of the ice encountered these classes i iv represent strongly deformed first year ice less deformed first and second year ice and deformed second or multi year ice respectively each of the classes is characterized by a specific set of quantities related to ice texture and surface snow characteristics and by distinct pdfs for snow and ice thicknesses in addition geometric surface and bottom roughness characteristics differ significantly for each of the floe classes

an ice ocean coupled model an ice model an ocean model and a method of coupling the models are described the ice model is a synthesis with variations and extensions of previous modeling ideas ice thickness concentration velocity and internal energy are prognostic variables the ice thermodynamics are represented by temperatures at the snow surface ice surface the interior and the bottom surface melting and freezing rates are calculated at the ice atmosphere ice ocean and atmosphere ocean interfaces a prescribed portion of summer meltwater can be stored on the surface and refrozen in the fall the ocean model includes a second moment turbulence closure submodel and enables one to solve for oceanic heat flux the interfacial stress and subsurface properties in this paper the model is applied to one dimensional simulations but the equations are cited in a form for implementation by two and three dimensional models in a companion paper kantha and mellor this issue the model is used for two dimensional vertical plane simulations in the bering sea several one dimensional sensitivity studies are performed in the case where the ice model is decoupled from the ocean here the oceanic heat flux and sea surface temperature are prescribed constants the studies reveal the role and sensitivity of surface trapped meltwater ice concentration and ice divergence with the coupled ice ocean model the seasonally varying oceanic heat flux and mixed layer properties are determined by the model some comparisons with observations in the central arctic ocean are possible the role of the molecular sublayer immediately adjacent to the ice is examined frazi ice production is related to the large disparity in the molecular diffusivities for temperature and salinity the mixed layer model contains empirical constants which are known from turbulence data the molecular sublayer parameterization requires one empirical parameter b which is uncertain but from this study is assuredly greater than zero the value implicit in previous models the ice model requires the empirical parameters ci f and ci u to quantitatively account for freezing or melting processes in open leads their values are also uncertain but we present reasoning and sensitivity studies to suggest specific values finally an empirical parameter g is introduced it is the ratio of the value of the ice thickness used to represent average ice volume in the dynamic and thermodynamic equations to the value of the thickness needed in the heat conduction equation estimates of g are made from observed thickness distribution functions sensitivity studies show it to be an important parameter

images from galileo of the venus cloud deck images of venus taken at 418 violet and 986 near infrared nir nanometers show that the morphology and motions of large scale features change with depth in the cloud deck poleward meridional velocities seen in both spectral regions are much reduced in the nir in the south polar region the markings in the two wavelength bands are strongly anticorrelated the images follow the changing state of the upper cloud layer downwind of the subsolar point and the zonal flow field shows a longitudinal periodicity that may be coupled to the formation of large scale planetary waves no optical lightning was detected

seismological monitoring on the 2003 tokachi oki earthquake derived from off kushiro permanent cabled obss and land based observations abstract japan marine science and technology center installed a cabled geophysical observatory system off kushiro hokkaido island in july 1999 this observatory system comprises three ocean bottom seismographs obss two tsunami gauges and a geophysical geochemical monitoring system 4 years and 2 months after the installation a megathrust earthquake the 2003 tokachi oki earthquake 26th september in japan standard time jst mjma 8 0 occurred along a plate boundary underneath a forearc basin where the system is located the system recorded clear unsaturated seismograms just at 28 6 km from the epicenter this paper demonstrates advantages brought by the cabled observatory to record the megathrust earthquake showing how earthquake detectability is improved dramatically combining permanent obs and land based observations around the region and importance of the in situ monitoring on the seismogenic zone in the present study processing obss and land based network together and comparing magnitudes of common observed earthquakes with national authorized network event detection level improved down to m 1 5 which is much lower than the previously designed as down to m 2 comparing detection level before and after installing obss we found dramatic improvement of the earthquake detection level in the interesting region real time continuous observations of microearthquakes since 1999 have brought us tremendous findings first a seismic quiescence started about 10 days before the 2003 tokachi oki earthquake second aftershock distribution is not uniform over the focal area and can be divided into several sub regions which might indicate an existence of several asperities we think that the geophysical observations helped to understand the initiation process of the rupture of the 2003 tokachi oki earthquake and that observations including seismological geodynamic hydrogeological and the other multidisciplinary observations would provide a clue to future understanding of seismogenic processes at subduction zones

long term variation in the cloud tracked zonal velocities at the cloud top of venus deduced from venus express vmc images 1 we present observational evidence of the variation of the cloud tracked zonal velocity by 20 m s 1 with a timescale of a few hundred days in the southern low latitude region based on an analysis of cloud images taken by the venus monitoring camera on board venus express a spectral analysis suggests that the variation has a periodicity with a period of about 255 days although cloud features are not always passive tracers the periodical variation of the dynamical state is a robust feature superposed on this long term variation of the zonal velocity kelvin wave like disturbances tend to be observed in periods of relatively slow background velocity while rossby wave like disturbances tend to be observed in periods of fast background velocity since the momentum deposition by these waves can accelerate and decelerate the mean flow these waves may contribute to the suggested long term oscillation

implications of diverse fault orientations imaged in relocated aftershocks of the mount lewis ml 5 7 california earthquake 1 we use seismic waveform cross correlation to determine the relative positions of 2747 microearthquakes near mount lewis california that have waveforms recorded from 1984 to 1999 these earthquakes include the aftershock sequence of the 1986 ml5 7 mount lewis earthquake approximately 90 of these aftershocks are located beyond the tips of the approximately north striking main shock defining an hourglass with the long axis aligned approximately with the main shock surprisingly our relocation demonstrates that many of these aftershocks illuminate a series of near vertical east west faults that are 0 5 1 km long and separated by as little as 200 m we propose that these structures result from the growth of a relatively young fault in which displacement across a right lateral approximately north striking fault zone is accommodated by slip on secondary left lateral approximately east striking faults we derive the main shock induced static coulomb failure function f on the dominant fault orientation in our study area using a three dimensional 3 d boundary element program to bound viable friction coefficients we measure the correlation between the rank ordering of relative amplitudes of f and seismicity rate change we find that likely friction coefficients are 0 2 0 6 and that the assumed main shock geometry introduces the largest uncertainties in the favored friction values we obtain similar results from a visual correlation of calculated f contours with the distribution of aftershocks viable rate and state constitutive parameters bound the observed relationship between magnitude of f and seismicity rate change and for our favored main shock model a maximum correlation is achieved when f is computed with friction coefficients of 0 3 0 6 these values are below those previously cited for young faults

solar locked and geographical atmospheric structures inferred from a venus general circulation model with radiative transfer abstract solar locked and geographical atmospheric structures of daily averaged wind and temperature on venus were investigated using an atmospheric general circulation model with venusian topography and a two stream radiative code and were compared with wind fields obtained from the akatsuki ultraviolet imager the horizontal wind fields simulated around the subsolar region are similar to the observed ones at the cloud top mid latitude jets of 120 m s 1 and an equatorial fast flow of 90 m s 1 are formed around the cloud top a poleward flow of 8 m s 1 is formed above the cloud layer where solar heating is strong around the cloud top a poleward flow of 1 m s 1 is confined within the equatorward flank of the jet core whereas an indirect circulation is formed in the jet core by the eddy heat fluxes owing to the thermal tide and baroclinic waves in solar fixed coordinates the subsolar to antisolar circulation is predominant around the cloud top thus differences are significant between the zonal and dayside averages of the meridional wind and its related fluxes within the cloud layer this suggests the zonal mean meridional wind of the hadley circulation eddy momentum and heat fluxes from the one side hemisphere must be estimated carefully in the experiment including topography a near surface subrotation is formed in latitudinal zones over high land and mountains a weakly stable layer is formed at 10 20 km at low latitudes and the zonal wind is weakened at the cloud top over the aphrodite terra regional stationary modification of the atmospheric structure due to topographical waves appears in the cloud layer

the dynamics of large scale wind driven variations in the antarctic circumpolar current a nonlinear model is devloped and analytical results obtained to discuss the response of the antarctic circumpolar current to wind forcing over a wide range of frequencies the main results are as follows i the nonlinear equations of motion can be conveniently separated into one baroclinic and one barotropic mode ii for forcing with period t equal to less than a few years wind driven fluctuations in the antarctic circumpolar current are barotropic and governed by the linearized laplace tidal equations theory suggests that fluctuations in the transport should lag and be most strongly correlated with the circumpolar averaged wind stress these theoretical results are consistent with recent measurements made in drake passage an interested untested theoretical prediction is that the sea level fluctuations measured at the southern side of drake passage with t between one month and a few years should be coherent at zero lag with sea level fluctuations at the same latitude around the earth iii for longer period forcing baroclinic fluctuations are important the baroclinic pressure current and associated density variations all decrease exponentially with depth exponential depth decay of these baroclinic fields is in fact observed the decay scale being about 1 km iv the theory indicates thatmore significant large scale wind driven fluctuations in the strength of the baroclinic antarctic circumpolar current can only occur at frequencies with periodicity or approx 70 years climatic changes associated with such variability must therefore consist of oscillations of similar or longer period this is consistent with limited observations which suggest that wind and sea surface temperature in the region of the antarctic circumpolar current have fluctuated through one cycle over the last 100 years less

coastal southern ocean a strong anthropogenic co2 sink 1 large scale estimates of the southern ocean co2 sink do not adequately resolve the fluxes associated with antarctic continental shelves using a mechanistic three dimensional biogeochemical model of the ross sea we show that antarctic shelf waters are a strong sink for co2 due to high biological productivity intense winds high ventilation rates and extensive winter sea ice cover net primary production npp in these waters is 0 055 pg c yr 1 some of this carbon sinks to depth driving an influx of co2 of 20 50 g c m 2 yr 1 although currently unaccounted for the total atmospheric co2 sink on the ross sea continental shelf of 0 013 pg c yr 1 is equivalent to 27 of the most recent estimate of the co2 sink for the entire southern ocean given these results these and other highly productive waters around the antarctic continent need to be included in future budgets of anthropogenic co2

physical and stochastic models of earthquake clustering abstract the phenomenon of earthquake clustering i e the increase of occurrence probability for seismic events close in space and time to other previous earthquakes has been modeled both by statistical and physical processes from a statistical viewpoint the so called epidemic model etas introduced by ogata in 1988 and its variations have become fairly well known in the seismological community tests on real seismicity and comparison with a plain time independent poissonian model through likelihood based methods have reliably proved their validity on the other hand in the last decade many papers have been published on the so called coulomb stress change principle based on the theory of elasticity showing qualitatively that an increase of the coulomb stress in a given area is usually associated with an increase of seismic activity more specifically the rate and state theory developed by dieterich in the 90s has been able to give a physical justification to the phenomenon known as omori law according to this law a mainshock is followed by a series of aftershocks whose frequency decreases in time as an inverse power law in this study we give an outline of the above mentioned stochastic and physical models and build up an approach by which these models can be merged in a single algorithm and statistically tested the application to the seismicity of japan from 1970 to 2003 shows that the new model incorporating the physical concept of the rate and state theory performs not worse than the purely stochastic model with two free parameters only the numerical results obtained in these applications are related to physical characters of the model as the stress change produced by an earthquake close to its edges and to the a and parameters of the rate and state constitutive law

secondary aftershocks and their importance for aftershock forecasting the potential locations of aftershocks which can be large and damaging are often forecast by calculating where the mainshock increased stress we find however that the mainshock induced stress field is often rapidly altered by aftershock induced stresses we find that the percentage of aftershocks that are secondary aftershocks or aftershocks triggered by previous aftershocks increases with time after the mainshock if we only consider aftershock sequences in which all aftershocks are smaller than the mainshock the percentage of aftershocks that are secondary also increases with mainshock magnitude using the california earthquake catalog and monte carlo trials we estimate that on average more than 50 of aftershocks produced 8 or more days after m 5 mainshocks and more than 50 of all aftershocks produced by m 7 mainshocks that have aftershock sequences lasting at least 15 days are triggered by previous aftershocks these results suggest that previous aftershock times and locations may be important predictors for new aftershocks we find that for four large aftershock sequences in california an updated forecast method using previous aftershock data and neglecting mainshock induced stress changes can outperform forecasts made by calculating the static coulomb stress change induced solely by the mainshock manuscript received 20 november 2002

meridional transport across a zonal channel topographic localization experiments are performed using a two layer isopycnic numerical model in a zonal channel with a large meridional topographic ridge in the lower layer the model is forced only by a steady meridional volume transport in the upper layer and develops a current structure similar to the antarctic circumpolar current meridional volume flux across time mean geostrophic streamlines is found to be due to a combination of the geostrophic eddy bolus flux and the lateral reynolds stress the proportion of each depends on the strength of the forcing the reynolds stress increases with the forcing while the bolus flux is relatively constant topography localizes the eddy fluxes at and downstream of the topography where eddy energies are greatest the strength of the zonal transport is governed by the onset of baroclinic instability and so is relatively insensitive to the strength of the meridional transport

triggering of seismicity at short timescales following californian earthquakes 1 a method for measuring seismicity rate changes caused by the occurrence of major earthquakes in the surrounding crust is proposed it is based on a nonstationary poisson modeling of earthquake activity an estimate of the seismicity rate change and a probability measure of seismicity triggering following a large shock are derived and the removal of the influence of independent aftershock sequences of other nearby previous earthquakes is attempted without using more traditional declustering techniques the seismicity rate change estimator is corrected for its natural bias in favor of positive changes at short timescales that has so far strongly hindered measures of seismicity quiescences three californian earthquakes are examined using this method the 1989 m7 1 loma prieta the 1992 m7 3 landers and the 1994 m6 7 northridge earthquakes positive triggering is very commonly observed as expected but quiescence is however more seldomly obtained for timescales up to 100 days after the main shock this relative absence of quiescence as compared to typical predictions of coulomb modeling for example is also independently observed through direct correlation analyses of californian seismicity the possibility that the stresses caused by the main shock are spatially highly variable may explain these observations

the effect of snow on antarctic sea ice simulations in a coupled atmosphere sea ice model the effect of a snow cover on sea ice accretion and ablation is estimated based on the zero layer version sea ice model of semtner and is examined using a coupled atmosphere sea ice model including feedbacks and ice dynamics effects when snow is disregarded in the coupled model the averaged antarctic sea ice becomes thicker when only half of the snowfall predicted by the atmospheric model is allowed to land on the ice surface sea ice gets thicker in most of the weddell and ross seas but thinner in east antarctic in winter with the average slightly thicker when twice as much snowfall as predicted by the atmospheric model is assumed to land on the ice surface sea ice also gets much thicker due to the large increase of snow ice formation these results indicate the importance of the correct simulation of the snow cover over sea ice and snow ice formation in the antarctic our results also illustrate the complex feedback effects of the snow cover in global climate models in this study we have also tested the use of a mean value of 0 16 wm 1 k 1 instead of 0 31 for the thermal conductivity of snow in the coupled model based on the most recent observations in the eastern antarctic and bellingshausen and amundsen seas and have found that the sea ice distribution changes greatly with the ice becoming much thinner by about 0 2 m in the antarctic and about 0 4 m in the arctic on average this implies that the magnitude of the thermal conductivity of snow is of considerable importance for the simulation of the sea ice distribution an appropriate value of the thermal conductivity of snow is as crucial as the depth of the snow layer and the snowfall rate in a sea ice model the coupled climate models require accurate values of the effective thermal conductivity of snow from observations for validating the simulated sea ice distribution under the present climate conditions

a frictional population model of seismicity rate change 1 we study models of seismicity rate changes caused by the application of a static stress perturbation to a population of faults and discuss our results with respect to the model proposed by dieterich 1994 these models assume a distribution of nucleation sites e g faults obeying rate state frictional relations that fail at constant rate under tectonic loading alone and predicts a positive static stress step at time t0 will cause an immediate increased seismicity rate that decays according to omori s law we show one way in which the dieterich model may be constructed from simple general ideas illustrated using numerically computed synthetic seismicity and mathematical formulation we show that seismicity rate changes predicted by these models 1 depend on the particular relationship between the clock advanced failure and fault maturity 2 are largest for the faults closest to failure at t0 3 depend strongly on which state evolution law faults obey and 4 are insensitive to some types of population heterogeneity we also find that if individual faults fail repeatedly and populations are finite at timescales much longer than typical aftershock durations quiescence follows a seismicity rate increase regardless of the specific frictional relations for the examined models the quiescence duration is comparable to the ratio of stress change to stressing rate which occurs after a time comparable to the average recurrence interval of the individual faults in the population and repeats in the absence of any new load perturbations this simple model may partly explain observations of repeated clustering of earthquakes

seasonality of southern ocean sea ice the sea ice cover of the southern ocean undergoes a rapid decrease from mid november to mid january the atmosphere to ocean heat flux is insufficient to account for this melting even in the presence of the high percentage of open water characteristic of the southern ocean sea ice it is estimated that sea air heat exchange in the 60 to 70 s zone can account for roughly 50 of the required spring heating the remainder must be supplied by the relatively warm deep water residing below the southern ocean pycnocline deep to surface water heat flux is accomplished by upwelling of the pycnocline due to the regional ekman divergence of the surface layer and by cross pycnocline mixing it is estimated that the required vertical mixing coefficient for the pycnocline is 1 5 cm2 s this value is considered as realistic in view of the relatively weak pycnocline which is a consequence of low levels of fresh water input to the surface layer it is suggested that the magnitude of fresh water input is a major factor in determining the degree of seasonality of southern ocean sea ice

relation between mainshock rupture process and omori s law for aftershock moment release rate summary we compare the source time functions i e moment release rates of three large california mainshocks with the seismic moment release rates during their aftershock sequences aftershock moment release rates computed by summing aftershock moments in time intervals follow a power law time dependence similar to omori s law from minutes to months after the mainshock furthermore in contrast to the previously observed saturation in numbers of aftershocks shortly after the mainshock rupture no such saturation is seen in the aftershock moment release rates which are dominated by the largest aftershocks we argue that the observed saturation in aftershock numbers described by the time offset parameter c in omori s law is likely an artefact due to the underreporting of small aftershocks which is related to the difficulty of detecting large numbers of small aftershocks in the mainshock coda we further propose that it is more natural for c to be negative i e singularity follows the onset of mainshock rupture than positive singularity precedes onset of rupture to make a more general comparison of mainshock rupture process and aftershock moment rates we then scale mainshock time functions to equalize the effects of the varied seismic moments for the three california mainshocks we compare the scaled time functions with similarly scaled aftershock moment rates finally we compare global averages of scaled time functions of many shallow events to the average scaled aftershock moment release rate for six california mainshocks in each of these comparisons the extrapolation using omori s law of the aftershock moment rates back in time to the onset of the mainshock rupture indicates that the temporal intensity of the aftershock moment release is about 1 5 orders of magnitude less than the maximum reached by the mainshock rupture this may be due to the differing amplitudes and relative importance of static and dynamic stresses in aftershock initiation compared to mainshock rupture propagation

covariance of optimal parameters of an arctic sea ice ocean model abstractthe uniqueness of optimal parameter sets of an arctic sea ice simulation is investigated a set of parameter optimization experiments is performed using an automatic parameter optimization

estimating tectonic stress rate and state with landers aftershocks a new method of analyzing the location and timing of aftershocks that makes possible estimates of the rate at which faults in the area are being loaded with stress has been developed this new method is an application of a theory of aftershocks proposed by dieterich 1994 the stress step from the mainshock triggers the aftershocks which release energy stored by tectonic loading and the mainshock and the tectonic stressing rate is responsible for the background activity thus a comparison of the number of aftershocks with the rate of background activity as a function of stress step can provide an estimate of tectonic stress rate this method requires analysis of a catalog which includes background activity as well as aftershocks the method has been applied to the aftershocks of the 1992 landers california earthquake the tectonic stress rate estimated for the mojave block is 1 2 pa d which corresponds to a repeat time for large events of 14 000 years and suggests that much of the observed strain in the mojave is anelastic the southern san andreas fault system is found to have a stress fate of 100 pa d which corresponds to a repeat time of 270 years and is consistent with primarily elastic strain accommodation inversions for the best fitting background stress state are consistent with fits of focal mechanisms and support an effective friction coefficient near 0 6 fits to the aftershock decay provide estimates of the dieterich friction parameter ad ranging from 0 02 if lithostatic pore fluids are present to 0 0003 if they are not

satellite derived maps of arctic and antarctic sea ice motion 1988 to 1994 standard image processing methods applied to 85 5 ghz special sensor microwave imager ssm i data provide previously unseen details about sea ice motion in the arctic and southern oceans means calculated from daily ice motions for 1988 1994 confirm the basic circulation patterns established from drifting stations and buoys but extend these observations to illustrate less well known transport patterns in the arctic coastal zones and sub arctic seas in the antarctic the gridded motion fields show a nearly continuous westward transport along the antarctic coast with well defined regions of exchange between this east wind drift and the antarctic circumpolar current this detailed view of mean ice motion in the southern ocean is unique and presents a comprehensive overview not previously available for antarctic ice motions

the use of earthquake rate changes as a stress meter at kilauea volcano stress changes in the earth s crust are generally estimated from model calculations that use near surface deformation as an observational constraint but the widespread correlation of changes of earthquake activity with stress has led to suggestions that stress changes might be calculated from earthquake occurrence rates obtained from seismicity catalogues although this possibility has considerable appeal because seismicity data are routinely collected and have good spatial and temporal resolution the method has not yet proven successful owing to the non linearity of earthquake rate changes with respect to both stress and time here however we present two methods for inverting earthquake rate data to infer stress changes using a formulation for the stress and time dependence of earthquake rates application of these methods at kilauea volcano in hawaii yields good agreement with independent estimates indicating that earthquake rates can provide a practical remote sensing stress meter

a reanalysis of venus winds at two cloud levels from galileo ssi images abstract the global circulation of the venus atmosphere is characterized at cloud level by a zonal super rotation studied over the years with data from a battery of spacecrafts orbiters balloons and probes among them the galileo spacecraft monitored the venus atmosphere in a flyby in february 1990 in its route toward jupiter since the flyby was almost equatorial published analysis of zonal winds obtained from displacements of cloud elements on images obtained by the ssi camera belton m j s and 20 colleagues 1991 science 253 1531 1536 stop at latitudes 50 north and south in this paper we present new results on venus winds based on a reanalysis of an extended set of images obtained at two wavelengths 418 nm violet and 986 nm near infrared that sense different altitude levels in the upper cloud our main result is that we have been able to extend the zonal wind profile up to the polar latitudes 70 n and 70 s at 418 nm and 70 n at 986 nm binned and smoothed profiles are given in tabular form we show that the zonal winds drop in their velocity poleward of latitudes 45 n and 50 s where an intense meridional wind shear develops at the two cloud levels our data confirm the magnitude of this shear retrieved previously from radio occultation data but disagrees with it in the latitudinal location of the sheared region the new wind data can be used to recalibrate the zonal winds retrieved from the previous measurements of the temperature field and the cyclostrophic balance assumption the meridional profiles of the zonal winds at the two cloud levels are used to assess the vertical wind shear in the upper cloud layer as a function of latitude and locate the most unstable region

toggling of seismicity by the 1997 kagoshima earthquake couplet a demonstration of time dependent stress transfer 1 two m 6 well recorded strike slip earthquakes struck just 4 km and 48 days apart in kagoshima prefecture japan in 1997 providing an opportunity to study earthquake interaction aftershocks are abundant where the coulomb stress is calculated to have been increased by the first event and they abruptly stop where the stress is dropped by the second event this ability of the main shocks to toggle seismicity on and off argues that static stress changes play a major role in exciting aftershocks whereas the dynamic coulomb stresses which should only promote seismicity appear to play a secondary role if true the net stress changes from a sequence of earthquakes might be expected to govern the subsequent seismicity distribution however adding the stress changes from the two kagoshima events does not fully capture the ensuing seismicity such as its rate change temporal decay or migration away from the ends of the ruptures we therefore implement a stress transfer model that incorporates rate state friction in which seismicity is treated as a sequence of independent nucleation events that are dependent on the fault slip slip rate and elapsed time since the last event the model reproduces the temporal response of seismicity to successive stress changes including toggling decay and aftershock migration nevertheless the match of observed to predicted seismicity is quite imperfect due perhaps to inadequate knowledge of several model parameters however to demonstrate the potential of this approach we build a probabilistic forecast of larger earthquakes on the expected rate of small aftershocks taking advantage of the large statistical sample the small shocks afford not surprisingly such probabilities are highly time and location dependent during the first decade after the main shocks the seismicity rate and the chance of successive large shocks are about an order of magnitude higher than the background rate and are concentrated exclusively in the stress triggering zones

simulated interannual variability of the greenland sea deep water formation and its connection to surface forcing a fully prognostic arctic ice ocean model is used to study the interannual variability of deepwater formation in the greenland sea gyre based on the simulations for the arctic ice ocean system for the period 1955 and 1960 1985 the model uses monthly climatology for thermodynamic forcing components such as air temperature and cloudiness together with constant annual net precipitation and river runoff the daily wind forcing is derived from analyzed sea level air pressures from the national center for atmospheric research in summary the model shows that the occurrence of deep convection in the greenland sea gyre is controlled by the extensive fram strait ice export and or local wind conditions in the greenland sea in the latter case the weakening of the local wind curl allows the polar front to move eastward the movement of the polar front causes adverse ice conditions often together with much larger than normal ice export from the arctic such as in 1968 which can block convection in the gyre the density difference between upper and lower layers is investigated as an indication of water mass formation through convection occurring as strong diffusion in the model the model simulated density difference between the average top 100 m and deep levels reveals that the period 1960 1985 had only a few distinct years with weak stratification and especially the model predicts no deep convection since the mid 1970s the common factor for the years of the weakest stratification is extreme wintertime wind events deep convection is also evident as a decrease of the model predicted heat content of the upper 2000 m which can to a high degree be explained by local heat loss the salt content changes in the greenland gyre are independent of the heat content changes and are determined by the advection of fresh or salty anomalies either from the arctic or from the frontal movement the model indicates that the surface salinity flux favors freshening even in wintertime so that brine rejection is less important in buoyancy loss than heat loss in leading to deep convection the model shows that the system has a memory deep convection in one year will precondition the following year

inferring rate and state friction parameters from a rupture model of the 1995 hyogo ken nanbu kobe japan earthquake we consider the applicability of laboratory derived rate and state variable friction laws to the dynamic rupture of the 1995 kobe earthquake we analyze the shear stress and slip evolution of ide and takeo s 1997 dislocation model fitting the inferred stress change time histories by calculating the dynamic load and the instantaneous friction at a series of points within the rupture area for points exhibiting a fast weakening behavior the dieterich ruina friction law with values of dc 0 01 0 05 m for critical slip fits the stress change time series well this range of dc is 10 20 times smaller than the slip distance over which the stress is released dc which previous studies have equated with the slip weakening distance the limited resolution and low pass character of the strong motion inversion degrades the resolution of the frictional parameters and suggests that the actual dc is less than this value stress time series at points characterized by a slow weakening behavior are well fitted by the dieterich ruina friction law with values of dc 0 01 0 05 m the apparent fracture energy gc can be estimated from waveform inversions more stably than the other friction parameters we obtain a gc 1 5 l06 j m 2 for the 1995 kobe earthquake in agreement with estimates for previous earthquakes from this estimate and a plausible upper bound for the local rock strength we infer a lower bound for dc of about 0 008 m

a coupled sea ice mixed layer pycnocline model for the weddell sea a dynamic thermodynamic sea ice model is coupled to a one dimensional model of the oceanic mixed layer and pycnocline and is applied to the weddell sea this model prognostically determines the vertical oceanic heat flux from the mixed layer dynamics in contrast to earlier sea ice modeling where the oceanic heat flux was prescribed in addition to the standard simulation polynya and paleoclimate experiments were performed to investigate the effects of sea ice dynamics furthermore the mixed layer pycnocline model is compared to the original kraus turner approach

earthquake hazard after a mainshock in california after a strong earthquake the possibility of the occurrence of either significant aftershocks or an even stronger mainshock is a continuing hazard that threatens the resumption of critical services and reoccupation of essential but partially damaged structures a stochastic parametric model allows determination of probabilities for aftershocks and larger mainshocks during intervals following the mainshock the probabilities depend strongly on the model parameters which are estimated with bayesian statistics from both the ongoing aftershock sequence and from a suite of historic california aftershock sequences probabilities for damaging aftershocks and greater mainshocks are typically well constrained after the first day of the sequence with accuracy increasing with time

seasonal simulation of the southern ocean coupled ice ocean system the southern ocean ice ocean system is investigated using a coupled model developed at princeton university the ocean model uses the primitive equations and solves the second moment closure for turbulent mixing the snow ice model uses a three level thermodynamic scheme resembling semtner s 1976 model the focus here is to study the seasonal variability of the ice covered areas and water mass formation the model simulates reasonably well the seasonal cycle of the ice cover in the southern ocean the annual rapid advance of the ice extent is tied to the ice advection away from the coast toward the antarctic circumpolar current acc where it melts rapidly with the increase of radiative fluxes in the austral spring the european center for medium range weather forecasts climatology tends to produce too little ice in the pacific sector for which the primary cause is a weak meridional wind stress the oceanic heat fluxes are up to 100 w m 2 in open water areas and along the coastal areas except in the weddell and ross seas which are occupied by a year round sea ice cover and have average heat flux of 20 w m 2 or less however there is a small region in the ross sea with high heat loss suggesting deep convection the meridional and zonal heat fluxes are in reasonable agreement with observational estimates the atlantic meridional heat flux is zero at 34 s northern boundary of the model which is an inadequacy in the model in the indian ocean sector the flux is southward while in the pacific sector it is weak and equatorward from the water mass balance we can deduce that the antarctic bottom water aabw t 27 85 annual production is 12 sv in the atlantic sector and 5 sv in the ross sea the total production of 17 sv is in excellent agreement with the observational estimates of 10 20 sv intermediate water types are produced in the indian ocean sector where some of the aabw is lost during that conversion in the atlantic sector 6 sv of the annually formed aabw is advected north across the acc in the pacific sector the northward aabw flow is 7 5 sv which is in excess of the ross sea production suggesting that a considerable portion of the aabw formed in the weddell sea is contributing to that flux

transport heat and freshwater fluxes within a diagnostic numerical model fram abstract results are presented from the integration of a fine resolution numerical model of the ocean operating in a diagnostic mode the region covered lies south of 24 s as depicted in the fram atlas here transports heat salt and freshwater fluxes are examined at 60 s and near 30 s in all three oceans results are found to be generally realistic at midlatitude the meridional heat flux is largely determined by the structure of the mean meridional motions and to a lesser degree by the gyre scale horizontal motions these roles are reversed for freshwater fluxes at a fixed high latitude the freshwater flux is determined by the mean meridional motions and the heat flux principally by the large scale wandering of the circumpolar current across the latitude only in this latter case namely for the heat flux at 60 s do mesoscale motions contribute to a significant extent the model underestimates the production and export of abyssal water the climatological state with which it is initialized is ident

the ice thickness distribution across the atlantic sector of the antarctic ocean in midwinter during the first leg of the winter weddell sea project antarktis v 2 cruise of f s polarstern the entire width of the antarctic sea ice zone was traversed in the vicinity of 0 longitude in the period july 18 to september 10 1986 ice thicknesses were measured by direct drilling and by helicopter profiling using an exstar 100 mhz impulse radar system in addition aerial photography of the ice cover was done from 100 to 2000 m altitude using a 70 mm aerial camera mounted in the helicopter the results of the point measurements drilling are reported in this paper together with an indication of how the radar and photography data will be used to extend them so as to yield area averaged ice thickness distributions it was found that the main ice type across the entire width of the ice cover was consolidated pancake ice occurring in vast floes this formed out of a 250 km wide band at the advancing ice edge which comprised a concentrated field of individual pancakes in a matrix of frazil ice preferred thicknesses of undeformed floes were 40 60 cm of ice covered with 5 15 cm of snow the individual pancakes attained almost all of this thickness before consolidation subsequent congelation growth was slow estimated at 0 4 cm d 1 the floes contained much small scale roughness on the upper and lower surfaces due to rafting of pancakes at the time of consolidation but pressure ridging was modest except in the far south a few very thick 8 11 m multiyear floes were observed embedded in the pack at latitudes beyond 66 s

evidence from the ad 2000 izu islands earthquake swarm that stressing rate governs seismicity magma intrusions and eruptions commonly produce abrupt changes in seismicity far from magma conduits that cannot be associated with the diffusion of pore fluids or heat such swarm seismicity also migrates with time and often exhibits a dog bone shaped distribution the largest earthquakes in swarms produce aftershocks that obey an omori type exponential temporal decay but the duration of the aftershock sequences is drastically reduced relative to normal earthquake activity here we use one of the most energetic swarms ever recorded to study the dependence of these properties on the stress imparted by a magma intrusion a 1 000 fold increase in seismicity rate and a 1 000 fold decrease in aftershock duration occurred during the two month long dyke intrusion we find that the seismicity rate is proportional to the calculated stressing rate and that the duration of aftershock sequences is inversely proportional to the stressing rate this behaviour is in accord with a laboratory based rate state constitutive law suggesting an explanation for the occurrence of earthquake swarms any sustained increase in stressing rate whether due to an intrusion extrusion or creep event should produce such seismological behaviour

improved automatic estimation of winds at the cloud top of venus using superposition of cross correlation surfaces abstract accurate wind observation is a key to study atmospheric dynamics a new automated cloud tracking method for the dayside of venus is proposed and evaluated by using the ultraviolet images obtained by the venus monitoring camera onboard the venus express orbiter it uses multiple images obtained successively over a few hours cross correlations are computed from the pair combinations of the images and are superposed to identify cloud advection it is shown that the superposition improves the accuracy of velocity estimation and significantly reduces false pattern matches that cause large errors two methods to evaluate the accuracy of each of the obtained cloud motion vectors are proposed one relies on the confidence bounds of cross correlation with consideration of anisotropic cloud morphology the other relies on the comparison of two independent estimations obtained by separating the successive images into two groups the two evaluations can be combined to screen the results it is shown that the accuracy of the screened vectors are very high to the equatorward of 30 degree while it is relatively low at higher latitudes analysis of them supports the previously reported existence of day to day large scale variability at the cloud deck of venus and it further suggests smaller scale features the product of this study is expected to advance the dynamics of venusian atmosphere

a numerical model of the weddell sea large scale circulation and water mass distribution a circumpolar model for regional studies of the wind driven and thermohaline circulation of the southern ocean including the major sub ice shelf areas is described a first series of numerical experiments focusing on the weddell sea reveals a pronounced and persistent double cell structure of the weddell gyre with a maximum transport of 60 sv in agreement with observations experiments with artificial passive tracers point to the shallow shelf areas off the filchner ronne ice shelf as main locations for bottom water production the trajectories of lagrangian floats are used to determine the pathways and timescales of water mass spreading in the model in addition the effect of sub ice shelf forcing on the water mass characteristics is evaluated it is shown that water modified in the sub ice cavities contributes significantly to the deep and bottom water formation along the continental slope and affects the water mass characteristics throughout the weddell sea by increasing the stability of the near surface stratification and preventing deep convection

strategies for nested and eddy permitting state estimation 1 both ocean process and prediction studies increasingly rely on state estimation i e data assimilation to provide the most complete representation of how the ocean circulates this study applies the formalism and methodology of state estimation recently developed for the global coarse resolution problem to eddy permitting state estimation in regional domains two major challenges exist for a state estimate that is nested inside a global state estimate 1 estimation of open boundary conditions consistent with information interior and exterior to the regional domain and 2 estimation with the higher resolution models of regional studies here a least squares cost function defines the problem of minimizing the misfit between a north atlantic regional general circulation model and actual observations including those of the subduction experiment a first experiment using a novel multiscale method to constrain the large scale regional circulation shows that the use of the adjoint of both an eddy permitting model and its coarse resolution twin leads to a solution of the least squares problem in a computationally practical way therefore no fundamental obstacle exists to constraining the large scale regional circulation nested within a global circulation a second experiment in the north atlantic shows that the model circulation can be constrained to the full observational signal including eddy variability as observed at selected point locations both experiments in this study produce eddy permitting state estimates which are exactly self consistent with the equations of motion as embodied by a general circulation model therefore dynamical balances can be diagnosed and easily interpreted in particular a companion paper uses the state estimates to determine eddy subduction rates in the north atlantic

further applications of the stochastic stress release model to historical earthquake data abstract the stochastic stress release model provides a stochastic version of the elastic rebound model which may be used to fit simulate and forecast occurrence of large earthquakes within a seismic region this paper reviews its applications to historical earthquakes from china iran and japan thereby extending and supporting earlier work on the chinese historical data the model generally provides an improvement in fit over poisson models with either constant rate or linear trend a crucial factor is the identification of suitable subregions which can act as largely independent seismic units in northern china divisions into two and four such units are considered based on recognized geophysical subdivisions in iran the best results are obtained from a subdivision into three such units the situation in japan seems more complex a preliminary model with two units gives a significant improvement of fit in one out of the two cases in all regions treating the subregions separately gives marked improvements over treating the whole region as a single unit however the quality of fit within subregions differs sharply from case to case pointing to substantial differences in the seismic regimes as reflected in the historical data

the deacon cell and the other meridional cells of the southern ocean abstract the meridional circulation cells of the southern ocean are investigated using the results from a fine resolution primitive equation model zonal integration along depth levels shows the classical series of meridional cells but integration along density layers shows a number of differences including the virtual disappearance of the deacon cell to investigate the differences the meridional transport is calculated as a function of both density and depth the results show that the deacon cell is associated with systematic changes in the depth of density surfaces between the western boundary current region off south america and the return flow in the interior of the ocean water flowing on each density surface produces a meridional cell with a vertical excursion of a few hundred meters thew cells combine without water crossing density surfaces to produce a single integrated deacon cell extending from the surface to below 2000 m the results also show that at each latitude water on each of the

cloud level winds from the venus express monitoring camera imaging abstract six years of continuous monitoring of venus by european space agency s venus express orbiter provides an opportunity to study dynamics of the atmosphere our neighbor planet venus monitoring camera vmc on board the orbiter has acquired the longest and the most complete so far set of ultra violet images of venus these images enable a study the cloud level circulation by tracking motion of the cloud features the highly elliptical polar orbit of venus express provides optimal conditions for observations of the southern hemisphere at varying spatial resolution out of the 2300 orbits of venus express over which the images used in the study cover about 10 venus years out of these we tracked cloud features in images obtained in 127 orbits by a manual cloud tracking technique and by a digital correlation method in 576 orbits total number of wind vectors derived in this work is 45 600 for the manual tracking and 391 600 for the digital method this allowed us to determine the mean circulation its long term and diurnal trends orbit to orbit variations and periodicities we also present the first results of tracking features in the vmc near ir images in low latitudes the mean zonal wind at cloud tops 67 2 km following rossow w b del genio a t eichler t 1990 j atmos sci 47 2053 2084 is about 90 m s with a maximum of about 100 m s at 40 50 s poleward of 50 s the average zonal wind speed decreases with latitude the corresponding atmospheric rotation period at cloud tops has a maximum of about 5 days at equator decreases to approximately 3 days in middle latitudes and stays almost constant poleward from 50 s the mean poleward meridional wind slowly increases from zero value at the equator to about 10 m s at 50 s and then decreases to zero at the pole the error of an individual measurement is 7 5 30 m s wind speeds of 70 80 m s were derived from near ir images at low latitudes the vmc observations indicate a long term trend for the zonal wind speed at low latitudes to increase from 85 m s in the beginning of the mission to 110 m s by the middle of 2012 vmc uv observations also showed significant short term variations of the mean flow the velocity difference between consecutive orbits in the region of mid latitude jet could reach 30 m s that likely indicates vacillation of the mean flow between jet like regime and quasi solid body rotation at mid latitudes fourier analysis revealed periodicities in the zonal circulation at low latitudes within the equatorial region up to 35 s the zonal wind show an oscillation with a period of 4 1 5 days 4 83 days on average that is close to the super rotation period at the equator the wave amplitude is 4 17 m s and decreases with latitude a feature of the kelvin wave the vmc observations showed a clear diurnal signature a minimum in the zonal speed was found close to the noon 11 14 h and maxima in the morning 8 9 h and in the evening 16 17 h the meridional component peaks in the early afternoon 13 15 h at around 50 s latitude the minimum of the meridional component is located at low latitudes in the morning 8 11 h the horizontal divergence of the mean cloud motions associated with the diurnal pattern suggests upwelling motions in the morning at low latitudes and downwelling flow in the afternoon in the cold collar region

repeated intermittent earthquake cycles in the san francisco bay region forecasts of future earthquake hazard in the san francisco bay region sfbr are dependent on the distribution used for the possible magnitude of future events based on the limited observed data it is not possible to statistically distinguish between many distributions with very different tail behavior these include the modified and truncated gutenberg richter distributions and a composite distribution assembled by the working group on california earthquake probabilities there is consequent ambiguity in the estimated probability of very large and hence damaging events a related question is whether the energy released in earthquakes is a small or large proportion of the stored energy in the crust corresponding loosely to the ideas of self organized criticality and intermittent criticality respectively however the sfbr has experienced three observed accelerating moment release amr cycles terminating in the 1868 hayward 1906 san andreas and 1989 loma prieta events a simple stochastic model based on elastic rebound has been shown to be capable of producing repeated amr cycles in large synthetic catalogs we propose that such catalogs can provide the basis of a test of a given magnitude distribution via comparisons between the amr properties of the real and synthetic data our results show that the truncated gutenberg richter distribution produces amr behavior closest to the observed amr behavior the proviso is that the magnitude parameters b and m max are such that a sequence of large events that suppresses activity for several centuries is unlikely to occur repeated simulation from the stochastic model using such distributions produces 30 year hazard estimates at various magnitudes which are compared with the estimates from the 2003 working group on california earthquake probabilities

suppression of large earthquakes by stress shadows a comparison of coulomb and rate and state failure stress shadows generated by california s two most recent great earthquakes 1857 fort tejon and 1906 san francisco substantially modified 19th and 20th century earthquake history in the los angeles basin and in the san francisco bay area simple coulomb failure calculations which assume that earthquakes can be modeled as static dislocations in an elastic half space have done quite well at approximating how long the stress shadows or relaxing effects should last and at predicting where subsequent large earthquakes will not occur there has however been at least one apparent exception to the predictions of such simple models the 1911 m 6 0 earthquake near morgan hill california occurred at a relaxed site on the calaveras fault we examine how the more complex rate and state friction formalism based on laboratory experiments might have allowed the 1911 earthquake rate and state time to failure calculations are consistent with the occurrence of the 1911 event just 5 years after 1906 if the calaveras fault was already close to failure before the effects of 1906 we also examine the likelihood that the entire 78 years of relative quiet only four m 6 earthquakes in the bay area after 1906 is consistent with rate and state assumptions given that the previous 7 decades produced 18 m 6 earthquakes combinations of rate and state variables can be found that are consistent with this pattern of large bay area earthquakes assuming that the rate of earthquakes in the 7 decades before 1906 would have continued had 1906 not occurred these results demonstrate that rate and state offers a consistent explanation for the 78 year quiescence and the 1911 anomaly although they do not rule out several alternate explanations

deep and bottom water of the weddell sea s western rim oceanographic observations from the ice station weddell 1 show that the western rim of the weddell gyre contributes to weddell sea bottom water a thin 300 meters highly oxygenated benthic layer is composed of a low salinity type of bottom water overlying a high salinity component this complex layering disappears near 66 s because of vertical mixing and further inflow from the continental margin the bottom water flowing out of the western rim is a blend of the two types additionally the data show that a narrow band of warmer weddell deep water hugged the continental margin as it flowed into the western rim providing the continental margin with the salt required for bottom water production

cumulative coulomb stress triggering as an explanation for the canterbury new zealand aftershock sequence initial conditions are everything using 2 years of aftershock data and three fault plane solutions for each of the initial m7 1 darfield earthquake and the larger m 6 aftershocks we conduct a detailed examination of coulomb stress transfer in the canterbury 2010 2011 earthquake sequence moment tensor solutions exist for 283 of the events with m ge 3 6 while 713 other events of m ge 3 6 have only hypocentre and magnitude information available we look at various methods for deciding between the two possible mechanisms for the 283 events with moment tensor solutions including conformation to observed surface faulting and maximum delta cff transfer from the darfield main shock for the remaining events imputation methods for the mechanism including nearest neighbour kernel smoothing and optimal plane methods are considered fault length width and depth are arrived at via a suite of scaling relations a large 50 70 proportion of the faults considered were calculated to have initial loading in excess of the final stress drop the majority of faults that accumulated positive delta cff during the sequence were encouraged by the main shock failure but on the other hand of the faults that failed during the sequence more than 50 of faults appeared to have accumulated a negative delta cff from all preceding failures during the sequence these results were qualitatively insensitive to any of the factors considered we conclude that there is much unknown about how coulomb stress triggering works in practice

structural constraints on the spatial distribution of aftershocks 1 calculations of static stress changes due to large earthquakes have shown that the spatial distribution of aftershocks is predictable to first order with aftershocks primarily occurring in areas experiencing positive stress changes delineation of these areas relies on resolving the stress perturbation onto planes with known orientations common practice is to use poorly constrained regional stress information to compute optimally oriented failure planes assuming that they exist everywhere here we show that this assumption is not supported by observation but rather that aftershock failure planes are controlled by geological structure we argue that useful aftershock hazard estimates are better made by replacing information on regional stress with statistical measures of structural orientations

can coseismic stress variability suppress seismicity shadows insights from a rate and state friction model 1 the paucity of detectable seismicity shadows in the days months following a main shock has raised the question as to whether dynamic rather than static triggering could be the main mechanism driving the seismicity at these timescales the lack of correlation between the stress unloading of the main fault and the typically vigourous aftershock production taking place on it however suggests that the spatial heterogeneity of the coseismic stress change could also be a factor in the apparent suppression of seismicity shadows at least on the main fault here we study whether this stress variability can indeed be an important aspect in the stress modeling of earthquake sequences a rate and state friction model of seismicity is used in conjunction with realistic levels of stress heterogeneity at the earthquake nucleation scale 1 10 m as predicted from these friction laws based on scale invariant models of coseismic slip distribution in this model a relatively weak on fault quiescence is delayed by months to years because of the high stress heterogeneity delayed quiescences due to slip heterogeneity are similarly predicted at distances of up to about half the rupture length away from the fault we also postulate that off fault seismicity can be significantly controlled by stress variability that originates from small scale crustal heterogeneity and the complexity in fault geometries rather than mapping stress changes by providing a single stress value at every grid point these results suggest that an estimate of the likely stress variability acting at the nucleation scale is also needed especially when trying to account for the seismicity occurring at relatively short timescales

second order moment of central california seismicity 1969 1982 the second order moment cross correlation function of earthquakes in the u s geological survey central california catalog between 1969 and 1982 was calculated with respect to a magnitude threshold m 4 0 over interevent distances up to 80 km and interevent times up to 320 days the statistical procedure results in a representation of the spatial temporal structure of the catalog associated with m 4 0 earthquakes and is capable of revealing patterns too weak to be detected in the space time distribution of seismicity for individual earthquake sequences a method is introduced for identifying aftershocks based on a physical two parameter model of the earthquake interaction process the results show that the aftershock process dominates the second order moment and may even obscure the statistical expression of a precursory process a concentration of foreshocks within 15 km and 3 days of m 4 0 main shocks exhibits an apparent migration toward the main shock loci with velocity 2 6 5 3 km d this concentration may be related to an observed tendency for m 4 0 events to cluster auto correlate over this interevent range with the identified aftershocks removed the residual catalog is poissonion in space and time when two m 4 0 earthquakes occur within 80 km and 40 days of each other aftershock productivity appears to be relatively enhanced in the earlier sequence this suggests that aftershock populations are not solely dependent on their main shocks and that unusually productive aftershock sequences may be predictors of future moderate earthquakes

onto what planes should coulomb stress perturbations be resolved 1 coulomb stress maps are produced by computing the tensorial stress perturbation due to an earthquake rupture and resolving this tensor onto planes of a particular orientation it is often assumed that aftershock fault planes are optimally oriented in other words the regional stress and coseismic stress change are used to compute the orientation of planes most likely to fail and the coseismic stress is resolved onto these orientations this practice assumes that faults capable of sustaining aftershocks exist at all orientations an assumption contradicted by the observation that aftershock focal mechanisms have strong preferred orientations consistent with mapped structural trends here we systematically investigate the best planes onto which stress should be resolved for the landers hector mine loma prieta and northridge earthquakes by quantitatively comparing observed aftershock distributions with stress maps based on optimally oriented planes two and three dimensional main shock orientation and regional structural trend we find that the best model differs between different tectonic regions but that in all cases models that incorporate the regional stress field tend to produce stress maps that best fit the observed aftershock distributions although not all such models do so equally well our results suggest that when the regional stress field is poorly defined or in structurally complex areas the best model may be to fix the strike of the planes upon which the stress is to be resolved to that of the main shock but allow the dip and rake to vary

venus cloud top winds from tracking uv features in venus monitoring camera images 1 to date dynamical observations of the venus clouds have delivered mainly either only short term or long term averaged results with the venus monitoring camera vmc it finally became possible to investigate the global dynamics with a relatively high resolution in space and time on a long term basis our findings from manual cloud feature wind tracking in vmc uv image sequences so far show that the details of the mesospheric dynamics of venus appear to be highly variable although the general rotation of the atmosphere remained relatively stable since mariner 10 more than 30 years ago by now there are indications of short term variations in the general circulation pattern of the venus atmosphere at cloud top level in some cases significant variations in the zonal wind properties occur on a timescale of days in other cases we see rather stable conditions over one atmospheric revolution or longer at cloud top level it remains an interesting question whether the irregularly observed midlatitude jets are indeed variable or simply become shielded from view by higher h2so4 haze layers for varying time intervals winds at latitudes higher than 60 s are still difficult to obtain track because of low contrast and scarcity of features but increasing data is being collected over all it was possible to extend latitudinal coverage of the cloud top winds with vmc observations thermal tides seem to be present in the data but final confirmation still depends on synthesis of visible and infrared thermal imaging spectrometer and vmc observations on night and dayside although poorly resolved meridional wind speed measurements agree mainly with previous observations and with the presence of a hadley cell spanning between equatorial region and about 45 s latitude

geostrophic circulation in a rectangular basin with a circumpolar connection abstract a simple theory is presented for steady geostrophic circulation of a stratified fluid in a rectangular basin with a circumpolar connection the interior flow obeys the plane sverdrup vorticity balance and the circulation is closed by geostrophic boundary currents the circulation is forced by surface thermal gradients and wind driven ekman transport near the latitudes of the circumpolar connection a thermal circumpolar current arises in response to imposed surface thermal gradients and northward ekman transport across the gap latitudes the transport of this model circumpolar current depends on the imposed surface thermal gradients and the gap geometry but not on the strength of the wind forcing in contrast the circulation induced in a related reduced gravity model by sverdrup transport into the gap latitudes has zero zonally integrated zonal transport the thermal current arises as a consequence of the geostrophic constraint which requires that the northern region fill with warm fluid un

numerical simulation of the weddell sea pack ice to investigate the role of ice dynamics on the seasonal cycle of antarctic sea ice a series of 2 year simulations of the weddell sea pack ice are carried out the simulations employ the dynamic thermodynamic model developed by hibler 1979 and use a 1 day time step on an 18 15 grid with a resolution of 222 km daily atmospheric data from 1979 are used to drive the simulations with atmospheric temperature fields modified to reflect buoy observations the simulations yield a seasonal cycle of ice with maximum and minimum extents close to that observed except for portions of the western weddell the advance of the ice is found to be primarily thermodynamic in nature while the rapid decay depends critically on the presence of both leads and lateral ice advection the average fraction of open water is substantial even in winter and varies from a minimum of 10 in september to a maximum of 35 in march these values are in general agreement with estimates from satellite microwave data in early summer the ice motion causes a residual tongue of ice to extend eastward from the antarctic peninsula in agreement with observations mean ice thicknesses are consistent with observations and vary from about 3 m in the perennial ice in the western weddell to 1 m in first year ice in the eastern weddell possible feedback between the ice extent and atmosphere are investigated by simulations with the surface air temperatures constrained to below freezing values in the presence of ice these coupled simulations yield a more realistic rapid decay in summer with dynamics included but an unrealistically small decay without dynamics simulated ice drift results yield mean drift rates of about 5 km day in good agreement with buoy drift observations but with slightly inadequate northward transport in the western weddell near the ice edge the drift rates are relatively insensitive to the ice strength near the coast however lower strengths are found to yield a decrease in northward drift rates

zonal mean circulation at the cloud level on venus spring and fall 1979 ocpp observations abstract cloud motions were obtained from a number of images acquired in reflected solar ultraviolet light during spring and fall of 1979 from the pioneer venus orbiter cloud photopolarimeter ocpp to determine the zonal mean circulation of the atmosphere of venus at the cloud top level the meridional profile of the zonal component of motion is somewhat different from that previously obtained from mariner 10 and preliminary pioneer venus observations although the equatorial magnitude is about the same 94 m sec the mean meridional motion is toward the south pole south of about 5 south latitude and toward the north pole north of this latitude with peak mean magnitudes of about 7 m sec polewards of 20 north and 40 south latitudes in the respective hemispheres from the few measurements obtained at higher latitudes the magnitude of the mean meridional component appears to decrease although it is still directed toward the respective poles due to the evolution of the cloud patterns over the duration of the images from which the cloud velocities are obtained the uncertainties in the mean zonal and meridional components may be as large as 5 10 and 2 4 m sec respectively preliminary estimates of meridional momentum transport show that the mean circulation dominates the eddy circulation transport completely in agreement with the estimates obtained from mariner 10 data although the uncertainties in both the mean and eddy circulation transports are large the momentum transports are polewards and their peak magnitudes occur at latitudes between 20 and 40 in both the hemispheres

southern ocean winter mixed layer austral winter 1986 observations from the polarstern along the greenwich meridian from the ice edge to the antarctic margin show the mixed layer beneath the winter sea ice cover to be significantly depressed in oxygen saturation incorporation of weddell deep water wdw into the winter mixed layer responsible for this undersaturation also introduces heat and salinity into the surface layer which strongly influences the mixed layer sea air exchanges and sea ice formation processes the total wdw transfer into the mixed layer averages 45 myr 1 implying a residence time for the surface water of 2 5 years the associated winter heat flux is 41 w m 2 which limits ice thickness to about 0 55 m agreeing quite well with observations the air temperatures during the cruise are just sufficient to remove the wdw heat input in the presence of observed ice thickness and concentration this suggests that the sea ice cover and wdw heat input into the mixed layer are in approximate balance by midwinter the annual heat flux from wdw to the surface layer and hence into the atmosphere is estimated as 16 w m 2 extrapolation of the greenwich meridian wdw entrainment value to the full circumpolar 60 70 s belt yields total up welling of 24 106 m3 s 1 similar extrapolation of the heat flux value gives a circumpolar total of 2 8 1014 w as a consequence of circulation topography interaction the maud rise water column stands out as an anomaly relative to the surrounding region with a significantly more saline and dense mixed layer below the mixed layer the water column over the crest of the rise is identical to that over the flanks if the latter water column is upwelled by 400 m this uplifting is believed to be a response of the upstream flow encountering the rise increased upstream flow would be expected to increase maud rise upwelling and the dependent salinity density of the mixed layer slight increases in the mixed layer density could trigger a convective mode and generation of a polynya it is hypothesized that spin up of the weddell gyre s barotropic circulation induced by an increase of the regional wind stress curl would enhance the probability of polynya development over maud rise

sensitivity of thermal tides in the venus atmosphere to basic zonal flow and newtonian cooling 1 a numerical model is constructed to investigate sensitivity of thermal tides in the venus atmosphere to distributions of basic mean zonal wind and newtonian cooling rate it is found that the amplitude and spatial structure of the diurnal tide are strongly influenced by the midlatitudinal jets and the newtonian cooling rates in all cases examined in this study the diurnal tide propagates downward in the lower atmosphere below the cloud levels and reaches the ground the semidiurnal tide is generally less dependent on these parameters but its zonal winds are increased in the upper jet regions

interannual variability of monthly southern ocean sea ice distributions the region of the southern ocean exhibiting interannual variability in monthly averaged sea ice distributions has been mapped and analyzed using data from the nimbus 5 electrically scanning microwave radiometer esmr and the nimbus 7 scanning multichannel microwave radiometer smmr over the 13 year record from the two instruments 1973 1976 and late 1978 through mid 1987 interannual variability is particularly high for the early summer month of december when 10 106 km2 of the ocean had ice covered conditions in some years and ice free conditions in other years this area of december variability is almost three times the ocean area consistently having ice covered conditions in the 13 esmr smmr decembers in contrast in mid winter the area of consistent ice coverage far exceeds the area of variability which in spite of the much greater ice coverage overall is not as large as the area of variability in december the mid winter variability is confined to 1 a ring of variable width around the edge of the ice pack and 2 the location of the 1974 1976 weddell polynya separation of the results to the esmr and smmr time periods shows that ice in the west indian ocean and the ross sea tended to be more expansive in the earlier esmr years than in the later smmr years but that ice in the weddell sea tended to be less expansive in the esmr years in the bellingshausen and amundsen seas the esmr years tended to have less summer ice than the smmr years but not less winter ice the images and plots provide a succinct summary of the range of spatial distributions of southern ocean sea ice during the esmr and smmr time periods they can hence be used as a convenient base for comparisons with future sea ice distributions in the effort to identify occurrences of climate change

estimating rate and state friction parameters using a two node stochastic model for aftershocks abstract the well known omori and modified omori omori utsu empirical formulae describe the decay in the rate of occurrence of aftershocks dieterich s rate and state dependent constitutive law for aftershock production produces a similar temporal formula with physically interpretable parameters such as the shear stressing rates the normal stress and a fault constitutive parameter this naturally leads to an interest in estimating these parameters from earthquake catalogs which requires a simpler stochastic model an alternative to the epidemic type aftershock sequence etas model is the two node seismic transfer and release model where one node represents the mainshock process and a secondary node represents the aftershock zone the aftershock decay rate in the two node model equals that derived from dieterich s rate and state dependent constitutive law and the inclusion of aseismic slip in the aftershock zone allows for values of p 1 in the best matching modified omori law the two node model can be easily fitted to aftershock data and has an application to aftershock stacking for estimating p values the equivalence between the rate and state and two node aftershock rates forms a bridge between a formula derived from physical considerations and a phenomenological stochastic model we show how this can be exploited by identifying physical units for the parameters in the latter to estimate the fault constitutive parameters and the stressing rates in dieterich s formula through a best fit inversion on real aftershock sequences the procedure is illustrated using data from four japanese earthquakes

the open boundary condition in the united kingdom fine resolution antarctic model abstract the united kingdom fine resolution antartic model fram project it a community program to study the southern ocean central to this is an eddy resolving three dimensional primitive equation ocean general circulation model the open boundary condition at the northern boundary is described here the boundary condition is based on that of stevens 1990

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

a two dimensional coupled ice ocean model of the bering sea marginal ice zone a two dimensional coupled ice ocean model has been formulated and applied to the wintertime bering sea marginal ice zone the oceanic component is a multilevel model that incorporates second moment closure for turbulent mixing in the water column the ice cover is modeled as a viscous plastic continuum melting at the ice ocean interface is computed using well known lawof the wall concepts in a turbulent boundary layer with particular attention to the disparate momentum and scalar transfer resistance coefficients over rough walls the thermodynamic and dynamical interactions between the ocean and the ice cover and the energy balances at the air ice and air sea interfaces are modeled according to the companion paper mellor and kantha this issue the model incorporates barotropic tides both diurnal and semidiurnal for application to the bering shelf double diffusive fluxes across the interface between the colder fresher layer beneath the melting ice and the warmer more saline water underneath are prescribed from laboratory data on double diffusive convection during winter sea ice in the central bering sea is transported toward the shelf break by off ice winds where it encounters northward flowing warmer north pacific waters and melts it is this situation to which the two dimensional model has been applied by neglecting all variations in the along ice edge direction the water conditions downstream of the ice edge the ice conditions upstream and the wind stress are the primary inputs to the model the model simulates transition from ice covered to open ocean conditions and the associated ice edge front and the two layer circulation underneath the ice cover sensitivity studies indicate that the density structure and the circulation beneath the ice and the position of the ice edge are rather sensitive to the parameters affecting the dynamics and the thermodynamics of the coupled ice ocean system even small changes in the relevant parameters can cause a substantial retreat or advance of the ice edge which may help explain why marginal ice zones are such dynamically active regions

global omori law decay of triggered earthquakes large aftershocks outside the classical aftershock zone 1 triggered earthquakes can be large damaging and lethal as evidenced by the1999 shocks in turkey and the 2001 earthquakes in el salvador in this study earthquakes with ms 7 0 from the harvard centroid moment tensor cmt catalog are modeled as dislocations to calculate shear stress changes on subsequent earthquake rupture planes near enough to be affected about 61 of earthquakes that occurred near defined as having shear stress change 0 01 mpa the ms 7 0 shocks are associated with calculated shear stress increases while 39 are associated with shear stress decreases if earthquakes associated with calculated shear stress increases are interpreted as triggered then such events make up at least 8 of the cmt catalog globally these triggered earthquakes obey an omori law rate decay that lasts between 7 11 years after the main shock earthquakes associated with calculated shear stress increases occur at higher rates than background up to 240 km away from the main shock centroid omori s law is one of the few time predictable patterns evident in the global occurrence of earthquakes if large triggered earthquakes habitually obey omori s law then their hazard can be more readily assessed the characteristic rate change with time and spatial distribution can be used to rapidly assess the likelihood of triggered earthquakes following events of ms 7 0 i show an example application to the m 7 7 13 january 2001 el salvador earthquake where use of global statistics appears to provide a better rapid hazard estimate than coulomb stress change calculations

immediate and updated forecasting of aftershock hazard 1 it is well known that the detection rate of aftershocks is extremely low during the period immediately after a large earthquake due to the contamination of arriving seismic waves this has led to considerable difficulties in obtaining estimates of the empirical laws of aftershock decay and magnitude frequency immediately after main shocks this paper presents an estimation method for predicting the underlying occurrence rate of aftershocks of any magnitude range based on a magnitude frequency model that combines the gutenberg richter law with the detection rate function this procedure enables real time probability forecasting of aftershocks immediately after the mainshock when the majority of large aftershocks are likely to occur

stress transferred by the 1995 mw 6 9 kobe japan shock effect on aftershocks and future earthquake probabilities the kobe earthquake struck at the edge of the densely populated osaka kyoto corridor in southwest japan we investigate how the earthquake transferred stress to nearby faults altering their proximity to failure and thus changing earthquake probabilities we find that relative to the pre kobe seismicity kobe aftershocks were concentrated in regions of calculated coulomb stress increase and less common in regions of stress decrease we quantify this relationship by forming the spatial correlation between the seismicity rate change and the coulomb stress change the correlation is significant for stress changes greater than 0 2 1 0 bars 0 02 0 1 mpa and the nonlinear dependence of seismicity rate change on stress change is compatible with a state and rate dependent formulation for earthquake occurrence we extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of kobe and calculating the change in probability caused by these stress changes transient effects of the stress changes are incorporated by the state dependent constitutive relation which amplifies the permanent stress changes during the aftershock period earthquake probability framed in this manner is highly time dependent much more so than is assumed in current practice because the probabilities depend on several poorly known parameters of the major faults we estimate uncertainties of the probabilities by monte carlo simulation this enables us to include uncertainties on the elapsed time since the last earthquake the repeat time and its variability and the period of aftershock decay we estimate that a calculated 3 bar 0 3 mpa stress increase on the eastern section of the arima takatsuki tectonic line attl near kyoto causes fivefold increase in the 30 year probability of a subsequent large earthquake near kyoto a 2 bar 0 2 mpa stress decrease on the western section of the attl results in a reduction in probability by a factor of 140 to 2000 the probability of a mw 6 9 earthquake within 50 km of osaka during 1997 2007 is estimated to have risen from 5 6 before the kobe earthquake to 7 11 afterward during 1997 2027 it is estimated to have risen from 14 16 before kobe to 16 22

cloud tracked winds from pioneer venus ocpp images abstract analysis of ultraviolet image sequences obtained from the pioneer venus orbiter cloud photopolarimeter and covering five 80 day periods from 1979 1985 provides the first climatological description of the cloud top circulation on venus the average zonal winds can be characterized as a 5 day retrograde rotation of the whole cloud level atmosphere with weak jets at middle to high latitudes both the midlatitude and equatorial zonal winds vary by about 5 8 m s 1 over time spans of 1 6 years the average meridional circulation is poleward in both hemispheres up to at least 60 latitude consistent with the presence of a thermally direct hadley circulation associated with the clouds the strength of the hadley circulation also varies with time four wave modes are clearly identified a diurnal solar tide a semi diurnal solar tide a 4 day equatorial wave and a 5 day midlatitude wave the semidiurnal tide appears to have an amplitude of about 5 m s 1 and to be approximately constant with time

volume transport of the antarctic circumpolar current from bottom pressure measurements abstract meausrements from bottom pressure gauges located at the north and south sides of drake passage are used to extend the one year time series of volume transport of the antarctic circumpolar current acc given previously by whitworth a small error in that paper is corrected and a revised transport time series is presented which shows the importance of including the transport in the northern and southern margins of drake passage direct measurements of vertical shear averaged across the passage are in good agreement with geostrophic shear and suggest that the acc is in geostrophic balance although most of the acc transport is in the baroclinic field transport fluctuations are mainly barotropic transport estimates based on pressure difference alone differ from the estimates of whitworth by less than 10 106 m3 s 1 fluctuations in transport of almost half of the mean value occur over periods as short as two weeks

on the development and cause of the weddell polynya in a sea ice simulation abstract occurrence of the weddell polynya a large open water area in the midst of the wintertime pack ice of the southern ocean is examined through a sequence of numerical simulations of the sea ice cover using mean climatological input data the numerical model realistically simulates the emergence of the polynya through the encircling of an open water region by ice in addition with only the wind fields altered the model successfully simulates winter seasons totally lacking a weddell polynya since the vertical ocean heat flux inserted into the model is spatially invariant these results call into question without actually refuting those explanations of the polynya based exclusively on oceanographic factors on the other hand the failure of the modeled polynya to survive the winter season without significant weakening as the observed polynya did in each of the years 1974 76 suggests that the oceanography and or the ocean atmosphere feedbacks are essential to the polynya maintenance even if not

an eddy permitting southern ocean state estimate abstract an eddy permitting general circulation model of the southern ocean is fit by constrained least squares to a large observational dataset during 2005 06 data used include argo float profiles ctd synoptic sections southern elephant seals as oceanographic samplers seaos instrument mounted seal profiles xbts altimetric observations envisat geosat jason 1 and ocean topography experiment topex poseidon and infrared and microwave radiometer observed sea surface temperature an adjoint model is used to determine descent directions in minimizing a misfit function each of whose elements has been weighted by an estimate of the observational plus model error the model is brought into near agreement with the data by adjusting its control vector here consisting of initial and meteorological boundary conditions although total consistency has not yet been achieved the existing solution is in good agreement with the great majority of the 2005 and 2006 southern ocean observations and better repr