global upper mantle structure from finite frequency surface wave tomography 1 we report global shear wave velocity structure and radial anisotropy in the upper mantle obtained using finite frequency surface wave tomography based upon complete three dimensional born sensitivity kernels because wavefront healing effects are properly taken into account finite frequency surface wave tomography improves the resolution of small scale mantle heterogeneities especially for deep anomalies that are constrained by the longest period surface waves in our finite frequency model ffsw1 the globally averaged radial anisotropy shows a transition from positive sh sv to negative anisotropy sv sh at about 220 km consistent with a change in the dominant mantle circulation pattern from predominantly horizontal flow at shallow depths to vertical flow at greater depths the radial anisotropy beneath cratons and the old pacific plate agrees well with previous studies however our model exhibits a strong negative radial anisotropy at depths greater than 120 km beneath mid ocean ridges a feature that is not present in previous upper mantle models more interestingly the depth extent of the ridge anomalies is distinctly different beneath fast and slow spreading centers anomalies beneath fast spreading centers are stronger but the strength decreases rapidly below 250 km in contrast beneath slow spreading centers such as the northern mid atlantic ridge and the red sea anomalies extend down at least to the top of the transition zone the different depth extent of the ridge anomalies suggests that the primary driving force of slow spreading seafloor may be different from that of fast spreading seafloor and that active upwelling beneath slow spreading ridges may play a major role in the opening of the seafloor

on mantle chemical and thermal heterogeneities and anisotropy as mapped by inversion of global surface wave data 1 we invert global observations of fundamental and higher order love and rayleigh surface wave dispersion data jointly at selected locations for 1 d radial profiles of earth s mantle composition thermal state and anisotropic structure using a stochastic sampling algorithm considering mantle compositions as equilibrium assemblages of basalt and harzburgite we employ a self consistent thermodynamic method to compute their phase equilibria and bulk physical properties p s wave velocity and density combining these with locally varying anisotropy profiles we determine anisotropic p and s wave velocities to calculate dispersion curves for comparison with observations models fitting data within uncertainties provide us with a range of profiles of composition temperature and anisotropy this methodology presents an important complement to conventional seismic tomography methods our results indicate radial and lateral gradients in basalt fraction with basalt depletion in the upper and enrichment of the upper part of the lower mantle in agreement with results from geodynamical calculations melting processes at mid ocean ridges and subduction of chemically stratified lithosphere compared with preliminary reference earth model prem and seismic tomography models our velocity models are generally faster in the upper transition zone tz and slower in the lower tz implying a steeper velocity gradient while less dense than prem density gradients in the tz are also steeper mantle geotherms are generally adiabatic in the tz whereas in the upper part of the lower mantle stronger lateral variations are observed the retrieved anisotropy structure agrees with previous studies indicating positive as well as laterally varying upper mantle anisotropy while there is little evidence for anisotropy in and below the tz

geodynamics layer cake or plum pudding whether convection in the earth s mantle extends through its entire depth or if the mantle is layered has long been debated recent research suggests that spatially and temporally intermittent or partial layering is the most likely solution

a three dimensional model of the mean and seasonal circulation of the gulf of california 1 the mean and seasonal circulation of the gulf of california is simulated with a three dimensional numerical model forced by the pacific ocean through specifying the sea level the temperature and the salinity fields at its entrance at the sea surface the wind and the heat and freshwater fluxes are specified the model reproduces the observed mean and seasonal variability of sea level of heat and salt balances and the sea surface temperature sst climatology it also reproduces the general surface circulation of the northern gulf which consists of a seasonally reversing basin wide gyre it is found that tides and heat fluxes are both indispensable to reproduce the spatial structure and temporal evolution of the sst tides provide the mixing to upwell the cooler subsurface waters in the large islands area and the heating from the surface raises the sst the general circulation of the southern part of the gulf is due to the wind and the pacific forcing in the ekman layer two periods of anticyclonic circulation and one cyclonic per year occur below this layer two cyclonic periods develop in the northern part the tides play an important role in producing mean residual currents and both tides and winds compete against the pacific forcing to produce a cyclonic and anticyclonic circulation once a year contrary to conclusions of previous studies the thermohaline circulation is found to be unimportant in the gulf

seasonal gyres in the northern gulf of california abstract the circulation pattern in the northern gulf of california based on drifting buoys and hydrographic observations can be explained using the results of a linear two layer primitive equations model forced at the annual frequency by the pacific ocean wind stress and heat flux through the surface the modeled surface circulation consists of a cyclonic gyre from june to october and an anticyclonic gyre from december to april both located in the central region of the northern gulf of california which includes angel de la guarda island the maximum intensities of the gyres occur in august and february respectively with values of surface velocities of 65 cm s 1 in agreement with the observations and very low opposite velocities in the bottom layers may and november are transition months in which both gyres can be observed finally in june july or december january the growing gyre is still connected with the rest of the gulf of california through the narrows between tiburon island san este

sst thermohaline structure and circulation in the southern gulf of california in june 2004 during the north american monsoon experiment 1 the thermohaline structure circulation and heat fluxes in the gulf of california entrance during june 2004 are described based on conductivity temperature depth and lowering acoustic doppler current profiler data collected in a 14 day survey supported by satellite data the avhrr images show extensive mesoscale structures in the region the most striking being 1 a cool filament extending from the california current domain and 2 a warm intrusion along the mainland shelf on the warm side of the thermal front created by the cool filament there was a strong current flowing into the gulf with speeds up to 0 70 ms 1 in the surface this current which the sst images suggest was associated with a decaying eddy carried 6 sv into the gulf associated with the second structure there was an ingoing coastal current on the mainland shelf with weak surface currents but with speeds 0 25 ms 1 at its core between 70 and 200 m this coastal current transported 2 sv into the gulf the two ingoing currents appear to join inside the gulf forming a very strong speeds 0 40 0 80 ms 1 narrow 30 km coastal current between the surface and 500 m depth changes in the thermohaline structure of the upper layers observed by repeat sampling of three cross sections were dominated by advection however it was found that the advective heat flux is very variable in space and time for the period of observation it was estimated that the lateral heat input was 4 8 3 0 105 wm 2 as estimated with ladcp currents and 5 7 2 20 105 wm 2 with geostrophic velocities

internal structure of sst features in the central gulf of california abstract closely spaced xbt and ctd data were used to study the sub surface structure of features observed in satellite avhrr sea surface temperature sst images of the gulf of california gc such as fronts jets and gyres the lowest ssts in the gc are found around the midriff islands and especially over the sills because of strong mixing from tidal currents and breaking internal waves this minimum sst area is limited to the south and north by sharp sst fronts which frequently show convolutions gyres and filaments a vertical section of temperature along the gulf length shows that about 300 km south of the sills the isotherms begin to spread out in the vertical reducing stratification the thermocline isotherms begin surfacing about 100 km south of the sill forming the sst minimum area which has a maximum temperature difference of 6 c with waters to the south and a horizontal sst gradient of 0 05 c km 1 the evolution and subsurface structure of a cool filament apparently originated as an instability of the sst front was intensely sampled satellite images show that its head advanced due south at about 0 5 ms 1 along the edge of a pool of warm surface water about 110 km in diameter the thermocline below the warm pool was raised at its edge and some isotherms reached the surface producing the cool filament the tilt of the temperature salinity and density isolines at the edge of the warm pool produced a geostrophic jet adjacent to the cool filament and just inside the warm pool with maximum speed 0 5 ms 1 and extending down to 80 m the filament lasted about 10 days its width was 30 km and it reached a maximum length of 120 km the volume transport of the jet in the top 100 m was around 0 7 sv

tidal and atmospheric forcing of the upper ocean in the gulf of california 1 sea surface temperature variability two years of satellite infrared imagery 1984 1986 are used to examine the sea surface temperature sst variability in the northern gulf of california empirical orthogonal functions eofs of the temporal and spatial sst variance for 20 monthly mean images show that the dominant sst patterns are generated by spatially varying tidal mixing in the presence of seasonal heating and cooling these patterns are modified in the fall and winter when shelf temperatures south of tiburon island cool in response to upwelling favorable winds these same winds bring cold dry air from off the continental united states causing local cooling of the shallow northern shelf during the rest of the year the broad shallow shelves are warmer than offshore the seasonally reversing temperature patterns are consistent with recent hydrographic observations which show a cyclonic surface circulation in the summer and a weaker anticyclonic circulation during the rest of the year atmospheric forcing of the northern gulf appears to occur over large spatial scales area averaged ssts for the guaymas basin island region and northern basin show significant fluctuations which are highly correlated these fluctuations in sst correspond to similar fluctuations in the air temperature which are related to synoptic weather events over the gulf during periods of particularly low wind speeds the air temperature over the gulf increases dramatically by afternoon intense heating of the sea surface results in the appearance of warm sst anomalies in the satellite data these ssts are approximately 2 c warmer than surrounding ssts and most likely occur as a result of a spatially varying wind field a regression analysis of the sst relative to the fortnightly tidal range shows that tidal mixing occurs over the sills in the island region as well as on the shallow norther shelf mixing over the sills however occurs as a result of large breaking internal waves or internal hydraulic jumps which mix water over the upper 300 500 m this mixing pumps heat away from the surface deep into the water column there by maintaining the cool ssts since mixing occurs over greater depths in the island region the temperatures there are much colder than those generated by tidal mixing on the shallow shelves resulting in the persistent pool of cool water evident in the satellite data this cooler water is mixed horizontally by the basin scale circulation lowering the ssts over much of the northern gulf these reduced ssts have a large impact on the surface heat flux by lowering the saturation vapor pressure of the air as a result the amount of heat lost to the atmosphere through the latent or evaporative heat flux is reduced this may explain why the gulf of california gains heat on an annual average rather than losing heat as occurs in the mediterranean and red seas where tidal mixing is not significant

anisotropic shear wave velocity structure of the earth s mantle a global model 1 we combine a new large data set of surface wave phase anomalies long period waveforms and body wave travel times to construct a three dimensional model of the anisotropic shear wave velocity in the earth s mantle our modeling approach is improved and more comprehensive compared to our earlier studies and involves the development and implementation of a new spherically symmetric reference model simultaneous inversion for velocity and anisotropy as well as discontinuity topographies and implementation of nonlinear crustal corrections for waveforms a comparison of our new three dimensional model s362ani with two other models derived from comparable data sets but using different techniques reveals persistent features 1 strong 200 km thick high velocity anomalies beneath cratons likely representing the continental lithosphere underlain by weaker fast anomalies extending below 250 km which may represent continental roots 2 weak velocity heterogeneity between 250 and 400 km depths 3 fast anomalies extending horizontally up to 2000 3000 km in the mantle transition zone beneath subduction zones 4 lack of strong long wavelength heterogeneity below 650 km suggesting inhibiting character of the upper mantle lower mantle boundary and 5 slow velocity superplumes beneath the pacific and africa the shear wave radial anisotropy is strongest at 120 km depth in particular beneath the central pacific lateral anisotropic variations appreciably improve the fit to data that are predominantly sensitive to the uppermost and lowermost mantle but not to the waveforms that control the transition zone and midmantle depths tradeoffs between lateral variations in velocity and anisotropy are negligible in the uppermost mantle but noticeable at the bottom of the mantle

a numerical investigation of the annual variability in the gulf of california the observations of sea level at the annual frequency in the gulf of california are reproduced both in amplitude and phase with a horizontal two dimensional linear two layer model the main forcing agents through which variability is explained are wind stress and the action of the pacific ocean which excites an internal wave in the mouth of the gulf the surface heating is shown to play a secondary role the response of the basin is qualitatively similar to that observed that is an energetic circulation in the upper layer cyclonic in summer and anticyclonic in winter compared to a weaker and opposite circulation in the bottom layer as well as a transversely averaged horizontal heat flux equal both in amplitude and phase to that calculated with historical hydrographic data the results of the simulation show that variability across the gulf is as important as the longitudinal variability

high and low resolution images of the earth s mantle implications of different approaches to tomographic modeling recently published images of the earth s mantle are characterized by a nominal resolution much higher than that used in previous studies where substantially different techniques were employed the agreement between such high resolution and low resolution images often seems very poor in an attempt to determine the reason for this discrepancy we analyze how the choice of inversion algorithm exact or iterative regularization norm or roughness minimization and parameterization spherical harmonics up to a variable degree blocks affects a tomographic model we also investigate the effects of the varying density of the data coverage on the final solution in our experiments we employ two seismic data sets rayleigh wave phase velocity at 75 s period and p wave travel times we construct a new model of p velocity in the mantle bdp98 based on the international seismological center bulletins 1964 1992 we use our findings in an evaluation of recent mantle models including our own focusing on similarities and discrepancies between models of different nominal resolution in all the models the long wavelength component is the most stable however consistent high resolution details probably corresponding to features of the real earth are also seen in general we conclude that most of the differences between existing tomographic models derive from the arbitrary choices made in the process of defining and solving the inverse problem rather than from actual errors or approximations

a modeling study of coastal trapped wave propagation in the gulf of california part ii response to idealized forcing abstract the evolution of remotely forced coastal trapped waves in the gulf of california is studied using a hydrostatic primitive equation model the sea level time variability at a remote station south of the gulf is assumed to propagate northward into the gulf as a mode 1 coastal trapped wave ctw to validate this assumption observations and model results are compared in general sea level fluctuations are reasonably well represented by the model with model data correlations decreasing from 0 76 at topolobampo close to the entrance of the gulf to 0 52 at santa rosalia in the central gulf model data correlations of velocity are lower 0 6 in the gulf ctws propagate northward along the east side with no significant changes south of the sill which is 600 km north of the entrance when incident waves propagating northward in the gulf along the east side arrive at the sill a small fraction of the wave energy enters the northern gulf and is dissipated most of the wave energy is steered at the

internal waves in the gulf of california observations from a spaceborne radar pronounced signatures of internal waves were detected repeatedly in the gulf of california by the seasat synthetic aperture radar sar a series of nine images with exactly repeating ground coverage was used to study the temporal variability of the internal wave field in the area it was found that the number of observed wave groups was highly correlated with the strength of the local tides the maximum number occurred during spring tides and the minimum number occurred during neap tides indicating that the internal waves were tidally forced most of the wave activity was found to the north of 28 n where the tides were the strongest in the gulf the application of a simple nonlinear internal wave model to the observations indicated that the peak to peak amplitude of the observed waves was about 50 m with an uncertainty of a factor of 2 the estimated upper bound for the rate of the loss of tidal energy to internal waves was about 5 1015 erg s representing only 10 of the rate of the dissipation of the dominant m2 tide in the gulf

global event location with full and sparse data sets using three dimensional models of mantle p wave velocity in order to improve on the accuracy of event locations at teleseismic distances it is necessary to adequately correct for lateral variations in structure along the ray paths either through deterministic model based corrections empirical path station corrections or a combination of both approaches in this paper we investigate the ability of current three dimensional models of mantle p wave velocity to accurately locate teleseismic events we test four recently published models two are parameterized in terms of relatively long wavelength spherical harmonic functions up to degree 12 and two are parameterized in terms of blocks of constant velocity which have a dimension of a few hundreds of km these models together with detailed crustal corrections are used to locate a set of 112 global test events consisting of both earthquakes and explosions with p wave travel time data compiled by the international seismological centre isc the results indicate that the supposedly higher resolution block models do not improve the accuracy of teleseismic event locations over the longer wavelength spherical harmonic models for some source locations the block models do not predict the range of observed travel time residuals as well as the longer wavelength models the accuracy of the locations largely varies randomly with geographic position although events in central asia are particularly well located we also tested the effect of reduced data sets on the locations multiple location iterations using 30 p wave travel times indicate that teleseismic events may be located within an area of 1000 km of the true location 66 of the time with only the model based corrections and increasing to 75 if calibration information is available if as few as 8 phases are available then this is possible only 50 of the time further refinement in models and or procedure such as the addition ofp phases azimuth data and consideration of p wave anisotropy may provide further improvement in the teleseismic location of small events

the effect of the seasonal variation of stratification on the circulation of the northern gulf of california abstract direct current measurements reveal that the circulation of the northern gulf of california in the annual timescale consists of a cyclonic basinwide gyre 0 35 m s 1 that lasts from june to september 4 months and an anticyclonic gyre 0 35 m s 1 lasting from november to april 6 months the transitions between regimes take about three weeks each the hypothesis that the difference in duration of the two circulation regimes is due to the seasonal variation of stratification of the water column is explored by simulations with a nonlinear two layer numerical model of circulation and thermodynamics that includes vertical mixing parameterized as an entrainment velocity the model results agree remarkably well with the observations considering its simplified vertical structure in addition the model predicts a net circulation consisting of an anticyclonic gyre of 0 05 m s 1 with a corresponding average concavity of the interface and a two layer exchange through the main channels of the arc

tidal dissipation and stratification in the gulf of california the effect of tidal mixing upon the distribution of stratification and sea surface temperature sst in the gulf of california is studied by direct observation and by numerical modeling data from conductivity temperature depth surveys made in september and december of 1986 are used to describe the distribution of stratification and sst a vertically integrated finite differences numerical model was used to obtain the geographical distribution of available turbulent kinetic energy tke from the m2 tide the following three areas were found to be strongly affected by tidal mixing the shallow coastal area at the head of the gulf the shelf south of tiburon island and the area of the midgulf archipelago the first two are vertically well mixed and are therefore similar to other shelf sea frontal areas dissipation occurs all around the islands of the archipelago and the sills are foci of the most intense tidal dissipation stratification in this area was reduced with respect to the background but complete vertical mixing does not occur therefore the strong sst fronts that occur in this area do not mark the boundary between well mixed and stratified water the simpson hujiter criterion simpson and hunter 1974 for stratification and frontal position produced 2 75 crit 3 0 for the frontal position in the shallow northern gulf this value of crit is larger than in other shelf sea fronts which may be due to horizontal advection which is not included in the criterion we found that the effect of advection may be of the same order of magnitude as that of buoyancy flux in the area over the sills criti the available tke is not enough to produce complete mixing over the 400 m of water over the sills

ocean currents across the entrance to the gulf of california observations of transport and currents in april may and december 1992 and january 1993 were made across the entrance to the gulf of california with an acoustically tracked dropsonde flow was into the gulf along sinaloa and out of the gulf along baja california the transports were 5 10 sv and the currents were deep with 10 cm s 1 flow extending to depths greater than 1000 m the currents were intensified in the upper 300 m geostrophic flows compared well with observed currents when smoothed to 2 3 times the rossby radius salinity in the upper 300 m was higher on the baja california side of the gulf indicating modification of subtropical subsurface waters within the gulf as well as the presence of surface and near surface gulf waters the salinity minimum associated with california current waters at 50 m had narrow cores that can be resolved only with closely spaced conductivity temperature depth casts mass and heat fluxes for the upper 300 m were estimated as 280 t s 1 and 0 1 1012 w in may and 170 t s 1 and 2 0 1012 w in december

a map of topography on the 410 km discontinuity from pp precursors we derive a new map of global topography on the 410 km discontinuity from observations of precursors to pp obtained by stacking almost 25 000 long period seismograms the inferred 410 topography exhibits average peak to peak amplitude of about 30 km has a strong degree one component and is highly correlated with previous results obtained from ss precursors flanagan and shearer 1998 spatial variations in 410 topography appear unrelated to ocean continent differences suggesting that continental roots are not a significant factor in observed global temperature variations at 410 km depth

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

role of tides and mixing in the formation of an anticyclonic gyre in san pedro martir basin gulf of california abstract a three dimensional numerical model of the seasonal evolution of the hydrography and circulation of the gulf of california predicts a quasi permanent anticyclonic gyre in the central part of the gulf over san pedro martir basin spmb just south of the mid gulf archipelago a historical hydrographic data bank is used to search for evidence of the predicted gyre despite 363 useful profiles in 15 cruises from 1939 1996 sampling in the area of interest only allowed climatological average distributions of temperature and salinity to be obtained from which dynamic height and geostrophic currents were calculated the average distributions of temperature salinity and density all show a concave shape over spmb which translate into a dynamic height dome and therefore into and anticyclonic gyre vertical sections across and along the gulf axis of geostrophic velocity show that the gyre occupies most of the water column which is 800 m deep in the center of spmb experiments with the numerical model indicate that stratification is necessary and that the tidal currents are responsible for the formation of the gyre this occurs through nonlinear interactions friction and advection and vertical mixing mechanisms that are enhanced in spmb due to the proximity of the sills and channels of the midriff archipelago where internal tides are generated

constraining seismic velocity and density for the mantle transition zone with reflected and transmitted waveforms we examine stacks of several seismic phases having different sensitivities to mantle transition zone structure when analyzed separately underside p and s reflections pdp and sds are suggestive of very different structures despite similar raypaths and data coverage by stacking the radial component of pdp rather than the vertical pdp we show that this difference does not result from interference from other more steeply inclined phases such as pkp and ppdpdiff in general stacks of p to s converted phases pds appear to lack evidence of a 520 km discontinuity when examined without other phases when these phases and stacked topside p reflections ppdp are analyzed jointly using a nonlinear inversion method consistent but nonunique seismological models emerge these models show that a discontinuity at 653 km depth has smaller contrasts in density and velocity than found in most previous studies a sub 660 gradient can account for the majority of this difference a 1 6 0 5 p velocity contrast and a 2 2 0 3 density contrast at 518 km depth without a s velocity contrast can explain the lack of a p520s together with robust pp520p and s520s phases for models parameterized with a finite thickness for each discontinuity the 410 km discontinuity is consistently 3 times thicker than the 660 km discontinuity

whole earth tomography from delay times of p pcp and pkp phases lateral heterogeneities in the outer core or radial anisotropy in the mantle we perform tomographic inversions of compressional wave travel time measurements in order to image the laterally varying radial anisotropy of p wave velocity in the entire mantle in order to achieve an adequate sensitivity to both horizontal and vertical p velocity throughout the mantle we include in our data set travel time measurements corresponding to ray paths of very different geometries specifically we use data associated with the phases p pcp core reflected and pkpbc pkp df core refracted all extracted from the bulletins of the international seismological centre 1964 1993 pcp pkpbc and pkpdf data are also sensitive to the topography of the core mantle boundary cmb and pkp only to the velocity structure of the earth s core these circumstances require that we also allow for cmb topographic anomalies as free parameters in our inversions and investigate the possibility of the existence of lateral heterogeneities in the outer core e g ritzwoller et al 1986 stevenson 1987 wahr and de vries 1989 vasco and johnson 1998 the solution models that we propose include alternatively a laterally heterogeneous outer core a radially anisotropic mantle or both so that trade offs between those features can also be evaluated we conclude that only minor and very localized radially anisotropic anomalies are present in the middle and lower mantle the most prominent being located in the deepest 400 km under the western pacific we find a discrepancy particularly evident by a comparison between different models of the cmb topography between the travel time measurements that are sensitive to the earth s core and those that are not we show that it is not possible to explain this discrepancy only in terms of radial anisotropy of the mantle and or lateral heterogeneity of the outer core

lagrangian surface circulation in the gulf of california from a 3d numerical model the surface circulation of the gulf of california was studied from a lagrangian point of view using the results of a 3d numerical model the advection of particles whose depth is fixed was carried out for annual and monthly periods with 710 particles homogeneously distributed over the gulf an anticyclonic circulation was found at the northern region from october to may and in the central and southern regions during may and from october to november cyclonic circulation was found in the entire gulf from july to august this circulation pattern agrees with direct measurements of drifters in the northern region during the cyclonic period the particles from the central and southern regions travel to the northern basin where they are trapped by a gyre for more than a month on the other hand during the anticyclonic period the particles flow to the southern part of the gulf some particles escape to the pacific ocean and others remain in the gulf and return to the northern part during the cyclonic period the principal gyre that appears on the center of the northern gulf in both cyclonic and anticyclonic periods traps particles for more than 30 days in a radius of 50 km in the central and southern regions a trapping zone was found in the peninsula side where the particles travel less than 100 km in 30 days on the other hand over the continental coast there is a rapid transit zone the particles escape only during the anticyclonic period

gulf of california biogeographic regions based on coastal zone color scanner imagery topographically the gulf of california is divided into a series of basins and trenches that deepen to the south maximum depth at the mouth is greater than 3000 m most of the northern gulf is less than 200 m deep the gulf has hydrographic features conducive to high primary productivity upwelling events have been described on the basis of temperature distributions at the eastern coast during winter and spring and at the western coast during summer tidal amplitude may be as high as 9 m in the upper gulf on the basis of discrete phytoplankton sampling the gulf was previously divided into four geographic regions this division took into consideration only the space distribution taxonomic composition and abundance of microphytopla nkton with the availability of the coastal zone color scanner czcs imagery we were able to in clude the time variability of pigments to make a more detailed biogeographic division of the gulf with weekly composites of the imagery we generated time series of pigment concentrations for 33 locations throughout the gulf and for the whole life span of the czcs the time series show a clear seasonal variation with maxima in winter and spring and minima in summer the effect of upwelling at the eastern coast is clearly evident with high pigment concentrations the effect of the summer upwelling off the baja california coast is not evident in these time series time series from locations on the western side of the gulf also show maxima in winter and spring that are due to the eddy circulation that brings upwelled water from the eastern side principal component analysis was applied to define 14 regions ballenas channel between angel de la guarda and baja california and the upper gulf always appeared as very distinct regions some of these 14 regions relate to the geographic distributions of important faunal groups including the benthos or their life cycles for example the upper gulf is a place for reproduction and the nursery of many fish species marine mammals and birds are specially abundant in ballenas channel sardine spawning mostly occurs in the central gulf in spring and shrimp are abundant off mainland mexico

surface heat flux in the northern gulf of california recent coastal meteorological data 1979 1986 are used to evaluate the seasonal cycle of surface heat fluxes in the northern gulf of california although net heat loss occurs from october to january the annual mean of the net surface heat flux was found to be into the sea 69 w m 2 near the coast and 83 w m 2 offshore with a mean evaporation rate 0 9 m yr 1 these results require an oceanic export of heat and salt out of the northern gulf to achieve a balance and imply that the annual mean thermohaline circulation must have an inflowing component at depth thus enriching the central and northern parts of the gulf these results differ from previous meteorological estimates which suggested a net surface heat loss but confirm those based on hydrographic data

lagrangian observations of the circulation in the northern gulf of california argos drifters deployed in the northern gulf of california in september 1995 showed the presence of a cyclonic gyre while a second deployment in march 1996 revealed an anticyclonic gyre a circulation pattern consisting of a seasonally reversing gyre had been proposed before on the basis of satellite images geostrophic calculations and numerical models but so far no direct observations have been made to test its existence in september the gyre was cyclonic baroclinic very well defined stable and strong its mean speed and rotation time were 0 3 m s21 and 7 days in march the gyre had the same mean speed but it was anticyclonic and displaced to the northwest of the summer position the march gyre has barotopic and baroclinic characteristics but the observed speeds are stronger than in numerical simulations these data and a data bank analysis suggest that the summer gyre is a persistent summer feature but the winter spring situation remains ill defined and requires further research

internal tides in the northern gulf of california 1 the characteristics of the internal tide in the northern gulf of california are described using data from two moored arrays of temperature and current sensors one for summer and one for winter located between angel de la guarda island and the mainland from the summer six sensor mooring it was found that 1 the current fluctuations are dominated by the semidiurnal frequency band while the quarterdiurnal frequency dominated the temperature fluctuations 2 the baroclinic semidiurnal horizontal current fluctuations are aligned with the gulf axis and have amplitudes of 10 15 cm s 1 the vertical displacements reached 4 m in this frequency band 3 the vertical modal structure for the temperature and velocity oscillations was dominated by the first and third modes 4 the energy of the semidiurnal internal tide is 45 of that of the barotropic tide 5 vertical wave number spectra showed slightly asymmetric peaks in the high wave number components indicating that their downflowing energy is larger than that flowing upward from the winter two sensor mooring it was found that the vertical oscillations were mainly semidiurnal with root mean square amplitudes of 7 m

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

global azimuthal seismic anisotropy and the unique plate motion deformation of australia differences in the thickness of the high velocity lid underlying continents as imaged by seismic tomography have fuelled a long debate on the origin of the roots of continents1 2 3 4 5 some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth with horizontally polarized shear waves travelling faster than vertically polarized ones2 this azimuthally averaged anisotropy could arise from present day deformation at the base of the plate as has been found for shallower depths beneath ocean basins6 such deformation would also produce significant azimuthal variation owing to the preferred alignment of highly anisotropic minerals7 here we report global observations of surface wave azimuthal anisotropy which indicate that only the continental portion of the australian plate displays significant azimuthal anisotropy and strong correlation with present day plate motion in the depth range 175 300 km beneath other continents azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans we infer that the fast moving australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents

love rayleigh wave incompatibility and possible deep upper mantle anisotropy in the iberian peninsula love and rayleigh wave phase velocities are analyzed with the goal of retrieving information about the anisotropic structure of the iberian lithosphere the cross correlation method is used to measure the interstation phase velocities between diverse stations of the iliha network at periods between 20 and 120 s despite the 2 d structure of the network the love wave data are too few to enable an analysis of phase velocity azimuthal variations azimuthal averages of love and rayleigh wave phase velocities are calculated and inverted both in terms of isotropic and anisotropic structures realistic isotropic models explain the rayleigh wave and short period love wave phase velocities therefore no significant anisotropy needs to be introduced in the crust and down to 100 km depth in the upper mantle to explain our data a discrepancy is observed only at long periods where the data are less reliable love wave data at periods between 80 and 120 s remain 0 15 km s faster than predicted by isotropic models explaining the long period rayleigh wave data possibilities of biases in the measurements due to interferences with higher modes are examined but seem unlikely a transversely isotropic model with 8 of s wave velocity anisotropy in the upper mantle at depths larger than 100 km can explain the whole set of data in terms of a classical model of mantle anisotropy this corresponds to 100 of the crystals perfectly oriented in the horizontal plane in a pyrolitic mantle this is a rather extreme model which predicts at time delay between 0 and 2 seconds for split sks

where can seismic anisotropy be detected in the earth s mantle in boundary layers during the last 30 years considerable evidence of seismic anisotropy has accumulated demonstrating that it is present at all scales but not in all depth ranges we detail which conditions are necessary to detect large scale seismic anisotropy firstly minerals must display a strong anisotropy at the microscopic scale and or the medium must be finely layered secondly the relative orientations of symmetry axes in the different crystals must not counteract in destroying the intrinsic anisotropy of each mineral and there must be efficient mechanisms of orientation of minerals and aggregates finally the strain field must be coherent at large scale in order to preserve long wavelength anisotropy part of shallow anisotropy can be related to the past strain field frozen in anisotropy however the deep anisotropy is due to the present strain field all these conditions are fulfilled only in boundary layers of convective mantle we review in this paper the seismic data sets which provide insight into the location at depth of large scale anisotropy from the d layer up to the lithosphere in addition to the well documented seismic anisotropy in the lithosphere and asthenosphere there is new evidence of seismic anisotropy in the upper 400 660 km and lower 660 900 km transition zones and in the d layer nonetheless the bulk of the lower mantle seems close to isotropy if we assume the hypothesis that seismic anisotropy is associated with boundary layers in convective systems these observations strongly suggest that the transition zone is a boundary layer which makes the pasage of matter between the upper and the lower mantle difficult however this general statement does not rule out flow circulation between the upper and lower mantles finally the geophysical mineral physics and geological applications are briefly reviewed an intercomparison between surface wave anisotropy and body wave anisotropy data sets is presented we discuss the scientific potential of seismic anisotropy and how it makes it possible to gain more insight into continental root deformation and geodynamics processes

shelf circulation in the gulf of california a description of the variability the first long term measurements 1983 1984 of winds temperature bottom pressure and currents made in the gulf of california are used to describe the dynamically important spatial and temporal scales and to compare and contrast the shelf circulation on opposite sides of the guaymas basin in addition a qualitative assessment of possible forcing mechanisms of gulf shelf circulation and an account of an energetic coastal trapped wave event are presented winds over the gulf are weak and variable with maximum wind speeds of 10 15 m s and sustained wind events of less than a week low frequency 3 cpd fluctuations in the dominant along gulf component of the wind field which are largely coherent over the study area are directed upgulf during summer and downgulf for the remainder of the year low frequency winds are not significantly correlated 95 confidence level with currents temperature and pressure in part because of weak local winds and nonlocally forced coastal trapped waves on longer time scales seasonal upwelling on both sides of the gulf and guaymas basin mean currents are related to local winds low frequency current fluctuations on the two facing shelf regions guaymas on the mexican mainland and santa rosalia on the baja california peninsula are not significantly correlated along gulf currents in the basin however are negatively correlated with alongshelf currents at guaymas 0 54 due to sporadically occurring eddylike features and santa rosalia 0 61 due to seasonal flow patterns low frequency fluctuations on the mainland shelf are significantly correlated during the summer e g 0 67 in temperature between topolobampo and guaymas separation of 375 km such that topolobampo leads guaymas by approximately 1 5 days this lagged signal is associated with coastal trapped waves that are generated by tropical storms and hurricanes south of the gulf and propagate poleward along the mainland shelf at speeds of 200 250 km day near guaymas current speeds of approximately 50 cm s were measured at the 100 m isobath on the mainland shelf during an event with a 20 cm coastal sea level rise the offshore decay of these waves is of the order of 50 km on the guaymas shelf low frequency temperature or equivalently density and sea level fluctuations are correlated 0 78 such that a 10 cm rise in sea level corresponds to a 20 m drop in isotherm depths over much of the water column this sea surface isotherm relationship is such that the amplitude of pressure fluctuations decays rapidly with depth e folding scale 160 m a similar relationship is only weakly apparent correlation 0 39 on the santa rosalia shelf cross gulf geostrophy is observed with 35 of the along gulf current averaged across the gulf related to dp dx measured between guaymas and santa rosalia on the guaymas and santa rosalia shelves dp dx measured across each shelf explains approximately 20 of the variability in alongshelf currents in the alongshelf momentum balance dp dy accounts for approximately 40 of the low frequency variability in dv dt at the guaymas shelf break consistent with the dynamics of freely propagating coastal trapped waves the same balance is only weakly apparent 10 of variance explained at santa rosalia at higher frequencies temperature fluctuations at guaymas are coherent and in phase to 200 m depth coherent 0 74 with the alongshelf near surface current at the shelf break m8 and exhibit a spring neap modulation consistent with internal tides

inferences on flow at the base of earth s mantle based on seismic anisotropy we applied global waveform tomography to model radial anisotropy in the whole mantle we found that in the last few hundred kilometers near the core mantle boundary horizontally polarized s wave velocities vsh are on average faster by 1 than vertically polarized s wave velocities vsv suggesting a large scale predominance of horizontal shear this confirms that the d region at the base of the mantle is also a mechanical boundary layer for mantle convection a notable exception to this average signature can be found at the base of the two broad low velocity regions under the pacific ocean and under africa often referred to as superplumes where the anisotropic pattern indicates the onset of vertical flow

mesoscale eddies in the southern gulf of california during summer characteristics and interaction with the wind stress 1 the hydrodynamic characteristics of mesoscale eddies of the southern gulf of california are described from observations made in august 2004 vertical profiles to 1000 m were made with a conductivity temperature depth ctd probe with dissolved oxygen and fluorescence sensors and a lowered acoustic doppler profiler the four sampled eddies were aligned along the gulf axis and had alternating sense of rotation the okubo weiss parameter was used to calculate the eddy core radii and it also revealed that the current field of almost all the sampled area was eddy dominated the mean radii of the eddy cores varied between 32 and 36 km maximum surface velocities were 0 4 0 5 m s the anticyclonic eddies were 500 700 m deep while the cyclones were 450 500 m deep the depression of the isolines associated with the anticyclonic eddies was 100 m at depths between 200 and 400 m however the thermocline pycnocline which spanned from 20 to 75 m was domed by 10 m the cyclonic eddies lifted the isolines by 70 m at depths above 500 m but the isolines were flat above 60 m this suggests that different dynamics operate in the layer above the strong and shallow pycnocline in particular the wind stress curl can affect circulation and pycnocline topography and therefore the patterns that appear in chlorophyll satellite images in our case the eddies were detectable in those images because of the chlorophyll enhanced streamers along their edges and for anticyclones probably because the chlorophyll maximum found at 40 50 m depth was domed following the pycnocline

global variability of transition zone thickness 1 we analyze recordings from more than 3000 shallow earthquakes and measure s410s s660s differential travel times on a global scale these differential times are obtained using a novel cross correlation technique that eliminates effects associated with a priori assumptions of crustal thickness and velocity variations above the upper mantle transition zone our measurements show absolute perturbations up to 8 s from the global average 242 km which imply transition zone thickness variations of 20 km on a length scale of 2000 km the transition zone under major subduction zones e g the western pacific and south america is significantly thicker than the global average this could be associated with large scale depressions of the 660 km discontinuity due to the accumulation of cold oceanic lithosphere at the base of the upper mantle a narrow transition zone is observed under the central pacific and northern atlantic oceans we also observe a modest anticorrelation between s410s s660s times and the delay times of shear waves predicted using s12 wm13 su et al 1994 in the transition zone which suggests considerable thermal influence on the large scale topography of the 410 and 660 km discontinuities the lack of a stronger anticorrelation suggests that 1 compositional heterogeneities may be present and 2 a significant fraction of thermal anomalies near the transition zone do not extend through the transition zone and influence both the 410 and the 660 km discontinuities a regional analysis shows that the transition zone is on average 6 8 km thicker under continents than under oceans this implies a temperature difference of 50 70 c however we do not observe a clear correlation between thickness and the age of the crust

grain size sensitive seismic wave attenuation in polycrystalline olivine 1 in order to investigate the processes responsible for the attenuation of seismic shear waves in the earth s upper mantle four olivine polycrystals ranging in mean grain size d from 3 to 23 m have been fabricated characterized and mechanically tested in torsion at high temperatures and seismic frequencies both the shear modulus which governs the shear wave speed vs and the dissipation of shear strain energy q 1 have been measured as functions of oscillation period to temperature t and for the first time grain size at sufficiently high t all four specimens display similar absorption band viscoelastic behavior adequately represented for 1000 t 1200 or 1300 c and 1 to 100 s by the expression q 1 a tod 1 exp e rt with a 7 5 102 s m 0 26 and e 424 kj mol 1 this mildly grain size sensitive viscoelastic behavior of melt free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding the latter becoming progressively more important with increasing t and or to extrapolation to the larger mm cm grain sizes expected in the earth s upper mantle yields levels of dissipation comparable with those observed seismologically implying that the same grain size sensitive processes might be responsible for much of the observed seismic wave attenuation the temperature sensitivity of vs is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations

the projected demise of barnes ice cap evidence of an unusually warm 21st century arctic as a remnant of the laurentide ice sheet barnes ice cap owes its existence and present form in part to the climate of the last glacial period the ice cap has been sustained in the present interglacial climate by its own topography through the mass balance elevation feedback a coupled mass balance and ice flow model forced by coupled model intercomparison project phase 5 climate model output projects that the current ice cap will likely disappear in the next 300 years for greenhouse gas representative concentration pathways of 2 6 to 8 5 wm 2 the projected ice cap survival times range from 150 to 530 years measured concentrations of cosmogenic radionuclides 10be 26al and 14c at sites exposed near the ice cap margin suggest the pending disappearance of barnes ice cap is very unusual in the last million years the data and models together point to an exceptionally warm 21st century arctic climate

ocean color observations of eddies during the summer in the gulf of california 1 using seawifs ocean color satellite images collected during 1997 2001 a series of eddies have been observed in the gulf of california these eddies are not clearly seen in the summertime sea surface temperature images the surface circulation between the midriff islands and the mouth of the gulf appears to be dominated by this series of eddies that have an alternating sense of rotation an eddy pair is observed to extend from cabo lobos in each of the three years suggesting that the eddies are topographically locked the formation mechanism of the eddies remains unknown however we suspect the most likely mechanism includes baroclinic instabilities along the coast produced by interactions between coastally trapped waves and local wind driven currents

seasonal evolution of the geostrophic circulation in the northern gulf of california abstract an improved description of the seasonal variability of the geostrophic circulation in the northern gulf of california is obtained from a historical hydrographic database larger than in previous studies the evolution of the geostrophic circulation consists of a cyclonic summer period an anticyclonic winter period and two transition periods the cyclonic summer period lasts from june to september four months and is characterized by a strongly baroclinic cyclonic gyre which dominates the circulation the anticyclonic winter period starts in november and persists until april six months the anticyclonic geostrophic circulation is less robust than that in summer the transition periods take of the order of one month occurring around october and april may and are characterized by the simultaneous presence of both types of gyres empirical orthogonal function analysis shows a strong seasonal signal in the dynamic height for the first mode 97 2 of the total variance whose spatial structure is a closed gyre which oscillates between cyclonic during the summer months and anticyclonic the rest of the year the results strengthen and extend those from previous work based on analysis of hydrographic data on limited direct observations on satellite image analysis and on numerical modelling

new images of the earth s upper mantle from measurements of surface wave phase velocity anomalies 1 the sensitivity of a surface wave to the lateral structure of the earth s mantle depends on the local structure and thickness of the crust particularly at short periods the strong lateral heterogeneity of the crust has a significant nonlinear effect on the surface wave sensitivity functions thus far this effect has not been quantified and tomographic studies of the upper mantle based on surface wave observations have not taken it into account on the basis of a jeffreys wentzel kramers brillouin jwkb description of surface wave propagation we find and discuss the lateral variations of the sensitivity of love and rayleigh waves to mantle seismic anomalies associated with a realistic model of the crust mooney et al 1998 we then use the resulting laterally varying sensitivity kernels to determine a new tomographic image of horizontally and vertically polarized shear velocity in the upper mantle from love and rayleigh wave phase velocity measurements periods from 35 to 300 s in terms of large scale patterns of high and low velocity our model is similar to previously published ones confirming among other features the anomalous anisotropy underlying the central pacific ekstrom and dziewonski 1998 we find however that the lateral dependence of the surface wave sensitivity kernels has a significant effect on the absolute amplitude of the anomalies which should not be neglected as we attempt to obtain higher resolution images of the upper mantle