a model for deformation in long valley california 1980 1983 geodetic data collected in long valley california from 1975 through 1983 define a pattern of uplift and strain which is evidently associated with a sequence of earthquakes occurring in may 1980 and subsequent swarm activity continuing until the present we have constructed a model to explain the deformation observed since may 1980 in terms of inflation of two subsurface magma chambers faulting in the south moat region of the caldera and slip on the hilton creek fault the most significant new feature of the model is the shallow magma chamber at 5 km depth located a few hundred meters to the east of the casa diablo hot spring area inflation of this chamber causes stresses which show consistency with various aspects of the seismicity in the south moat of the caldera calculations of stress across vertical planes over the magma chamber can be used together with failure criteria to estimate the inflation volume at which the rock layers intervening between the chamber and the surface will fail by extensional fracture

a simple model of interannual displacements of the gulf stream interannual variations in the latitude of the north wall of the gulf stream 1966 1999 can be hindcast by a simple model forced solely by monthly values of the north atlantic oscillation nao index the model is a modified version of the one dimensional model developed by d behringer et al and utilizes the observed tendency for the centers of the azores high and the iceland low to move north or south as the nao strengthens or weakens only one parameter the active upper layer thickness is changed from the original values although the model incorporates the thermal feedback on the wind stress used by behringer et al the results are almost unchanged without it in generating the gulf stream position the model averages the nao index over several months and introduces a time lag of at least a year these delays are associated with the time taken for the lateral advection of heat the model results are also compared with estimates of the latitude of the gulf stream made by a gangopadhyay et al 1977 1988 and by t m joyce et al 1955 1997 and with the series of gulf stream transports 1954 1997 constructed by r g curry and m s mccartney the model reproduces the observed weakness of the seasonal variations in comparison to interannual variations the nao data are used to hindcast the annual latitude of the gulf stream for the period 1825 1999 the results show that since 1970 the gulf stream may have been more consistently farther south than during any period of the past 170 years

correlation of climate cycles in middle mississippi valley loess and greenland ice two complete late wisconsin loess successions in the middle mississippi river valley reveal 39 and 41 alternating paleosol a and c horizons striking changes in soil color iron content and carbonate content define four major and two minor paleosol a horizon complexes which were interpreted to represent wisconsin interstadials 1 2 3 4 and semiinterstadials 1 5 and 2 5 respectively the timing of wisconsin interstadials matches that of corresponding greenland interstadials midcontinent loess and greenland ice records as well as rates of atmospheric 14c production have periodicities in common suggesting a solar influence only a persistent heat and moisture supply could produce prominent paleosol complexes near the continental ice margin this record suggests that el nino southern oscillation variability has amplified solar forcing and resultant tropical heat and moisture transport played a significant role in millennial and centennial scale climate cycles during the late wisconsin glaciation over the northern hemisphere

evidence of suppression of the unequal refraction error in geodetic leveling analyses of historic data indicate that procedures in force since the nineteenth century have effectively suppressed the unequal refraction error ure in geodetic leveling although theoretical considerations argue that large refraction errors should accumulate where levelings traverse gentle slopes permitting long sight lengths extensive examination of routinely developed results lends little support to this argument our analyses of the historic record consider 1 height differences based on otherwise similar levelings over the same route but characterized by substantially different sight length 2 modeled refraction error differences and actually measured elevation changes on both a section by section and a line by line basis 3 misclosures produced from levelings identified with significantly different procedural constraints in technically stable areas of comparable variation in terrain and climate and 4 alternatively and sequentially developed heights in areas of tectonic stability and areas of persisting instability all of these analyses indicate that the ure is generally small and unpredictable and hence inconsistent with both theoretical expectations and the results of two conceptually different experimental studies this inconsistency probably is attributable to the inability of the theoretical models and the failure of the experiments to incorporate in their design those procedures especially the rejection criteria at the setup section and circuit level that normally minimize refraction error traditionally invoked procedural specifications may have suppressed the ure through biasing of observations toward less refractive conditions whereas all of the theoretical models assume an averaged sampling of the atmosphere alternatively a variety of real world complexities that are disregarded in the models may take on added importance in normally constrained leveling thus refraction corrections based on simplified models and mean atmospheric and terrain conditions tend to overcompensate substantially for refraction errors associated with routinely constrained surveys

crustal structure and seismicity distribution adjacent to the pacific and north america plate boundary in southern california new three dimensional 3 d v p and v p v s models are determined for southern california using p and s p travel times from local earthquakes and controlled sources these models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the pacific north america plate boundary the models extend from the u s mexico border in the south to the southernmost coast ranges and sierra nevada in the north and have a 15 km horizontal grid spacing and an average vertical grid spacing of 4 km down to 22 km depth the heterogeneity of the crustal structure as imaged by v p and v p v s models is larger within the pacific plate than the north american plate similarly the relocated seismicity deepens and shows more complex 3 d distribution in areas of the pacific plate exhibiting compressional tectonics the models reflect mapped changes in the lithology across major geological terranes such as the mojave desert the peninsular ranges and the transverse ranges the interface between the shallow mono of the continental borderland and the deep moho of onshore california forms a broad zone to the north beneath the western transverse ranges ventura basin and the los angeles basin and a narrow zone to the south along the peninsular ranges the near surface increase in velocity from the surface to up to 8 km depth is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins at midcrustal depths a rapid increase in v p is imaged beneath the sediments of the large sedimentary basins while beneath the adjacent mountain ranges the increase is small or absent

impact of the atlantic meridional mode on gulf stream north wall position abstractthe path of the gulf stream as it leaves the continental shelf near cape hatteras is marked by a sharp gradient in ocean temperature known as the north wall previous work in the literature

seismicity and geometry of a 110 km long blind thrust fault 1 the 1985 kettleman hills california earthquake the august 4 1985 kettleman hills earthquake was the third in a sequence of moderate shocks to occur beneath the northern half of a 110 km long fold chain bounding the eastern california coast ranges the 1982 mw 5 4 new idria 1983 mw 6 5 coalinga and 1985 mw 6 1 kettleman hills events define a southward progression of seismic activity beneath the fold we use teleseismic waveforms geodetic modeling hypocenters relocated in a three dimensional velocity model and subsurface structural data to investigate the kettleman hills earthquake the main shock results from motion on a shallowly dipping thrust fault buried at 10 km depth aftershocks and coseismic fault slip extend 20 km along the fold axis nearly the full extent of the kettleman hills north dome anticline aftershocks occur primarily several kilometers in front of the fault tip and in the core of the anticline the main shock and several foreshocks occurred at a 2 km right step in the quaternary fold axis which also corresponds to the southern end of the 1983 coalinga and northern end of the 1985 kettleman hills aftershock zones from this we infer that the step in the fold is caused by an offset or tear in the underlying fault the scalar seismic moment is 1 6 1018 n m consistent with the geodetic deformation and the duration of rupture is 16 s 3 4 times greater than for the average earthquake with this scalar moment the slow rate of moment release provides an explanation for the low level of ground shaking and low local magnitude reported for the event the peak of the geodetic uplift is located 5 km perpendicular to the quaternary fold axis we argue that the fault is propagating northeast into the undeformed san joaquin valley sediments and that the overlying fold is growing at about 0 5 mm yr

topographic slope as a proxy for seismic site conditions and amplification we describe a technique to derive first order site condition maps directly from topographic data for calibration we use global 30 arc sec topographic data and v s 30 measurements here v s 30 refers to the average shear velocity down to 30 m aggregated from several studies in the u s as well as in taiwan italy and australia v s 30 values are correlated against topographic slope to develop two sets of parameters for deriving v s 30 one for active tectonic regions where topographic relief is high and one for stable shields where topography is more subdued by taking the gradient of the topography and choosing ranges of slope that maximize the correlation with shallow shear velocity observations we can recover to first order many of the spatially varying features of sitecondition maps developed for california our site condition map for the low relief mississippi embayment also predicts the bulk of the v s 30 observations in that region despite rather low slope ranges we find that maps derived from the slope of the topography is often well correlated with other independently derived regional scale site condition maps but the latter maps vary in quality and continuity and subsequently also in their ability to match observed v s 30 measurements contained therein alternatively the slope based method provides a simple approach to uniform site condition mapping

on the drivers of decadal variability of the gulf stream north wall abstractthe gulf stream is bounded to the north by a strong temperature front known as the north wall the north wall is subject to variability on a wide range of temporal and spatial scales on int

leveling precision and bench mark motions pinon flat observatory california the reliability of relative vertical motions detected by levelings of bench marks installed at pinyon flat is highly dependent on the kinds of bench marks installed for monitoring purposes the most stable marks are the end monuments for two 535 m long fluid tiltmeters nine direct measurements of average slope between tiltmeter end monuments agree to within a standard deviation about the mean of 0 2 mm 0 4 rads comparable to the slope measured by the tiltmeters themselves motions of class b steel rod marks are random with standard deviations ranging from 0 5 to 3 2 mm averaging 1 0 mm the motions of nearly all rod marks decreased greatly in the fourth year evidently because of a time dependent establishment of stability in the decomposed granite fundament other kinds of bench marks are stable between 0 5 and 0 8 mm tectonic tilt is not revealed in 15 levelings of the array in 4 years consistent with the tiltmeters but the random bench mark motions dominate the errors in the estimated tilt

a test of the parsons veronis hypothesis on the separation of the gulf stream abstract the parsons veronis model based on a two layer wind driven ocean predicts the latitude at which the western boundary current separates from the western boundary it has been tested on the gulf stream using both satellite and in situ observations the hypothesis attributes the difference in the thermocline depth from the eastern to the western side of the ocean and the corresponding northward geostrophic transport with closed northern end to the southward ekman transport integrated across the basin twelve years 1977 88 of satellite sea surface temperature data and wind data from the fleet numerical oceanography center fnoc wind database have been used for this study the satellite derived gulf stream northern edges were used to determine the latitudes of separation i e crossing the 2000 m isobath into deep water parsons model is sensitive to two free parameters namely the reduced gravity and the thermocline depth on the eastern side of the basin based on available ctd data an

a direct comparison of spirit and compensator leveling rumpf and meurisch 1981 conclude that any magnetic field including particularly that of the earth can deflect the line of sight of compensator or automatic self leveling levels since the city of los angeles has observed many lines of geodetic leveling using a zeiss ni 1 level a field investigation was designed to assess the potential impact of this error source the observations were made along a line that had been releveled several times over a period of years this generally north south line runs from tidal 8 san pedro to the vicinity of the jet propulsion laboratory in lacanada figure 1 an additional line of leveling was also observed in an east west direction our investigation seems to confirm the existence of an azimuth dependent error as suggested by rumpf meurisch

a model of variations in the north atlantic oscillation 1 the major source of interannual variability in the atmospheric circulation over the north atlantic ocean is the north atlantic oscillation nao which is associated with the strength of the winter surface westerly winds over the ocean during the last century the nao has shown pronounced multi annual and multi decadal variations fluctuations that have been accompanied by changes in the marine and terrestrial ecosystems of the north atlantic region this paper presents a simple model which reproduces the major trends in the nao index from 1866 to 2004 the model is based on climatic resonators that are forced by the global mean temperature and the southern oscillation of the pacific ocean so the ocean component of the model shows similar variations to time series from the north atlantic

millennial scale fluctuations in subtropical northwestern atlantic surface ocean hydrography during the mid pleistocene 1 we use planktonic foraminiferal globigerinoides sacculifer and neogloboquadrina dutertrei 18o records from subtropical north atlantic site 1056 to reconstruct millennial scale changes in sea surface hydrography during marine isotope stages mis 10 through 12 340 440 ka the difference in the 18o records provides a measure of the thermal gradient from the mixed layer to the seasonal thermocline tml tc which we use to constrain upper ocean stratification and infer northward ocean heat transport late holocene g sacculifer and n dutertrei 18o values place controls on how the foraminifera record recent ocean conditions at site 1056 in the down core record we find extreme excursions toward g sacculifer 18o maxima and tml tc minima throughout the entire time interval these cold events have a similar amplitude throughout but are more frequent during the glacial than the interglacial intervals during interglacial mis 11 the excursions provide evidence for repeated cooling of the sea surface contributing to existing evidence for relative climate instability during interglacial intervals during mis 10 millennial scale variability in the subtropical record is very similar to that observed in the subpolar north atlantic at site 980 perhaps related to a common forcing such as a large northern hemisphere ice sheet within the limitation of the age models the cold events of mis 11 are almost synchronous with benthic foraminiferal 13c minima at subtropical north atlantic site 1063 which suggests a general surface to deep ocean link via thermohaline circulation during this interval of time

comparison between instrumental observational and high resolution proxy sedimentary records of late holocene climatic change a discussion of possibilities abstract variability of the north atlantic region is a key factor in the workings of global climate changes related to orbital factors and to the thermohaline circulation thc occur on timescales ranging from 1000 to 100 000 years whereas subdecadal decadal and multidecadal trends are associated with changes in atmospheric circulation sea surface temperature sst and salinity long term changes are known only from proxies whereas shorter periodicities are identified in instrumental and other observational data between lies a range of less well known centennial periodicities too long to be recorded observationally and too short for all but the highest resolution proxy records such as tree rings atmospheric 14c and varved lake sediments numerous investigations of such periodicities involve comparisons between the proxy and the observational record however results depend on the length of the datasets being analysed the relative strength of the periodicities present whether data are stationary or non stationary the degree to which they have been smoothed in order to remove particular fluctuations and the methods of analysis employed ideally comparisons should be between datasets of equal length and of similar properties but these criteria are not always easy to fulfill a search for periodicities in a range of well known multidecadal datasets reveals that whilst some frequencies previously identified in the central england temperature cet record and in the koch sea ice index are statistically significant those found by several authors in the index of the north atlantic oscillation nao are not as the index approximates to white noise instead decadal periodicity of the nao described by several authors appears to be result of smoothing of the raw data periodicities of 6 1 33 9 and 213 years identified in a proxy record from laminated sediments from loch ness scotland appear respectively to be related to a subdecadal variations in sst and salinity in the north atlantic ocean b multidecadal interactions between the nao and the north atlantic thc and c a well known centennial solar periodicity

how does the deep western boundary current cross the gulf stream abstract the manner in which the deep western boundary current dwbc crosses the gulf stream is investigated using data from a hydrographic survey conducted in 1990 absolute geostrophic velocity vectors are computed using in situ float data to obtain the reference level three density layers are considered in detail two mid depth layers which together make up the shallowest water mass component of the dwbc 500 1200 m and a deep layer consisting of the norwegian greenland overflow water 2500 3500 m the shallowest layer does not make it through the crossover and is completely entrained by the gulf stream however the resulting drop in equatorward transport is almost completely replenished by offshore entrainment just south of the crossover in the intermediate layer which is denser than the gulf stream coming off the shelf part of the dwbc recirculates to the northeast while the onshoremost portion continues equatorward in the deep layer only a small amount of recirculation occurs the lateral

unified structural representation of the southern california crust and upper mantle we present a new 3d description of crust and upper mantle velocity structure in southern california implemented as a unified structural representation usr the usr is comprised of detailed basin velocity descriptions that are based on tens of thousands of direct velocity vp vs measurements and incorporates the locations and displacement of major fault zones that influence basin structure these basin descriptions were used to developed tomographic models of crust and upper mantle velocity and density structure which were subsequently iterated and improved using 3d waveform adjoint tomography a geotechnical layer gtl based on vs30 measurements and consistent with the underlying velocity descriptions was also developed as an optional model component the resulting model provides a detailed description of the structure of the southern california crust and upper mantle that reflects the complex tectonic history of the region the crust thickens eastward as moho depth varies from 10 to 40 km reflecting the transition from oceanic to continental crust deep sedimentary basins and underlying areas of thin crust reflect neogene extensional tectonics overprinted by transpressional deformation and rapid sediment deposition since the late pliocene to illustrate the impact of this complex structure on strong ground motion forecasting we simulate rupture of a proposed m 7 9 earthquake source in the western transverse ranges the results show distinct basin amplification and focusing of energy that reflects crustal structure described by the usr that is not captured by simpler velocity descriptions we anticipate that the usr will be useful for a broad range of simulation and modeling efforts including strong ground motion forecasting dynamic rupture simulations and fault system modeling the usr is available through the southern california earthquake center scec website http www scec org

uplift across long valley caldera california the results of levelings along u s highway 395 between lee vining and toms place california in 1957 1975 1980 1982 and 1983 define the development of a dome shaped uplift across the long valley caldera these levelings indicate that toms place remained virtually invariant with respect to lee vining during the full interval 1957 1983 implying that the end points of the survey route lie beyond the range of the developing dome no significant uplift occurred before 1975 but by october 1980 the uplift near casa diablo hot springs reached 0 25 m and the half width of the dome was of the order of 25 km the uplift of casa diablo hot springs increased to 0 35 m by august 1982 and to about 0 40 m by august 1983 without any appreciable increase in the lateral extent of the dome the august 1982 to august 1983 uplift apparently occurred before february 1983 with little or no uplift during the period february august 1983 the 1975 1983 uplift is attributed to the injection of 0 19 km3 of magma into a spherical chamber at a depth of 10 km beneath the southern quadrant of the ancient resurgent dome within the caldera examination of the full vertical control record through the long valley area indicates that the 1975 1983 uplift is unique within the period 1905 1983 although comparisons of the results of a 1932 leveling against a 1914 datum disclose a 1914 1932 down to the northwest tilt of about 10 rad over the reach between toms place and lee vining this tilt if real is more reasonably attributed to regional tectonic deformation than it is to the movement of magma at depth

continental shelf landscapes of the southeastern united states since the last interglacial abstract the wide sediment starved continental shelf and modern coastal areas of the southeastern united states retain well preserved but scattered remnants of a submerged paleolandscape this paper presents a conceptual model of stratigraphic deposition and landscape formation since the last interglacial on the continental shelf of south carolina with portions of north carolina georgia and florida usa data for this study include multibeam bathymetry surveys sidescan sonar mosaics high resolution subbottom profiles and ground truth surveys from 250 m to the modern tidewater region four bathymetric zones are recognized with eleven landforms and landform indicators the described zones range in depths from the modern shoreline across the shelf and over the shelf edge to 250 m msl relative sea level curves are presented for the area and discussed in conjunction with cultural and climatic events the potential for preservation of paleoamerican sites is high at the shelf edge between 130 m and 45 m with archaic and later occupations likely in depths of less than 25 m prominent vantage points for paleoamericans 11 kya would have existed at the shelf edge and tidewater resources would have been available nearby for a period of almost 6 ka rapid transgression rates 60 km ka after the sea level rose over the shelf edge make preservation of tidewater sites less likely on the outer and middle shelf searches for the earliest paleoamericans should focus on promontories at the edge of the shelf and along future discoveries of paleoincisions on the shelf mapping and delineating this paleolandscape and associated unconsolidated sedimentary deposits interspersed with rocky plains and ledges will continue to be a priority to marine archeologists coastal managers fishery scientists and marine spatial planners over the next several decades

full 3 d tomography for crustal structure in southern california based on the scattering integral and the adjoint wavefield methods we have successfully applied full 3 d tomography f3dt based on a combination of the scattering integral method si f3dt and the adjoint wavefield method aw f3dt to iteratively improve a 3 d starting model the southern california earthquake center scec community velocity model version 4 0 cvm s4 in f3dt the sensitivity frechet kernels are computed using numerical solutions of the 3 d elastodynamic equation and the nonlinearity of the structural inversion problem is accounted for through an iterative tomographic navigation process more than half a million misfit measurements made on about 38 000 earthquake seismograms and 12 000 ambient noise correlagrams have been assimilated into our inversion after 26 f3dt iterations synthetic seismograms computed using our latest model cvm s4 26 show substantially better fit to observed seismograms at frequencies below 0 2 hz than those computed using our 3 d starting model cvm s4 and the other scec cvm cvm h11 9 which was improved through 16 iterations of aw f3dt cvm s4 26 has revealed strong crustal heterogeneities throughout southern california some of which are completely missing in cvm s4 and cvm h11 9 but exist in models obtained from previous crustal scale 2 d active source refraction tomography models at shallow depths our model shows strong correlation with sedimentary basins and reveals velocity contrasts across major mapped strike slip and dip slip faults at middle to lower crustal depths structural features in our model may provide new insights into regional tectonics when combined with physics based seismic hazard analysis tools we expect our model to provide more accurate estimates of seismic hazards in southern california

wind forcing of the atlantic thermocline along 32 n at low frequencies abstract the bermuda tide gauge record extends back to the early 1930s that sea level fluctuations there are highly coherent with dynamic height from hydrographic data has two interesting implications first it should contain information about the low frequency circulation of the atlantic furthermore because dynamic height contains information on heat storage it might on the limited timescales accessible in the record also contain clues about climate a simple model of wind forcing of the atlantic from the african coast to bermuda uses the levitus mean density data to estimate the long rossby wave speed as a function of longitude sea level and thermocline variability estimated this way are in remarkably good agreement with observations at periods of more than a few years duration the peak to peak sea level signal is 18 cm which is nearly 25 of the slope across the gulf stream at this latitude the model results suggest that the variability is largest somewhat to the east of bermuda fluctuatio

hierarchical interlocked orthogonal faulting in the 2019 ridgecrest earthquake sequence a nearly 20 year hiatus in major seismic activity in southern california ended on 4 july 2019 with a sequence of intersecting earthquakes near the city of ridgecrest california this sequence included a foreshock with a moment magnitude mw of 6 4 followed by a mw 7 1 mainshock nearly 34 hours later geodetic seismic and seismicity data provided an integrative view of this sequence which ruptured an unmapped multiscale network of interlaced orthogonal faults this complex fault geometry persists over the entire seismogenic depth range the rupture of the mainshock terminated only a few kilometers from the major regional garlock fault triggering shallow creep and a substantial earthquake swarm the repeated occurrence of multifault ruptures as revealed by modern instrumentation and analysis techniques poses a formidable challenge in quantifying regional seismic hazards

wind driven response of the upper ocean along the u s west coast to tropical mjo convection in this study time lagged composites of oscar upper ocean currents from february to may of 1993 2016 were binned by active phase of the leading atmospheric mode of intraseasonal variability the madden julian oscillation mjo seven days after the convectively active phase of the mjo is present in the tropical indian ocean anomalously strong south southeastward upper ocean currents are seen along nearly the entire u s west coast seven days after the convectively active phase is present in the tropical western pacific ocean upper ocean current anomalies reverse along the u s west coast with weaker southward flow a physical pathway to the ocean was found for both of these phases a tropical mjo convection modulates upper tropospheric heights and circulation over the pacific ocean b those anomalous atmospheric heights adjust the strength and position of the aleutian low and hawaiian high c surface winds change in response to the adjusted atmospheric pressure patterns and d those surface winds project onto upper ocean currents

mantle heterogeneities and the scec reference three dimensional seismic velocity model version 3 we determine upper mantle seismic velocity heterogeneities below southern california from the inversion of teleseismic travel time residuals teleseismic p wave arrival times are obtained from three temporary passive experiments and southern california seismic network scsn stations producing good raypath coverage the inversion is performed using a damped least squares conjugate gradient method lsqr the inversion model element spacing is 20 km before the inversion the effects of crustal velocity heterogeneities represented by the southern california earthquake center scec seismic velocity model version 2 are removed from the teleseismic travel times the p wave inversion produces a variance reduction of 43 s wave velocities are determined from laboratory vp vs ratios the most prominent features imaged in the results are high p wave velocities 3 in the uppermost mantle beneath the northern los angeles basin and the previously reported tabular high velocity anomaly 3 to depths of 200 km beneath the transverse ranges crosscutting the san andreas fault we incorporate the upper mantle seismic velocity heterogeneities into the scec southern california reference seismic velocity model the prior accounting for the crustal velocity heterogeneity demonstrates the utility of the top down method of the scec seismic velocity model development

southeast greenland winter precipitation strongly linked to the icelandic low position abstractgreenland s largest precipitation flux occurs in its southeast se region during the winter controlled primarily by easterly winds and frequent cyclogenesis in the north atlantic several

on the predominant nonlinear response of the extratropical atmosphere to meridional shifts of the gulf stream abstractthe north atlantic atmospheric circulation response to the meridional shifts of the gulf stream gs path is examined using a large ensemble of high resolution hemispheric scale weather research and forecasting model simulations the model is forced with a broad range of wintertime sea surface temperature sst anomalies derived from a lag regression on a gs index the primary result of the model experiments supported in part by an independent analysis of a reanalysis dataset is that the large scale quasi steady north atlantic circulation response is remarkably nonlinear about the sign and amplitude of the sst anomaly chosen over a wide range of gs shift scenarios the nonlinear response prevails over the weak linear response and resembles the negative north atlantic oscillation nao the leading intrinsic mode of variability in the model and the observations further analysis of the associated dynamics reveals that the nonlinear responses are accompanied by the shift of the north atlantic eddy

a vs30 map for california with geologic and topographic constraints abstract for many earthquake engineering applications site response is estimated through empirical correlations with the time averaged shear wave velocity to 30 m depth v s 30 these applications therefore depend on the availability of either site specific v s 30 measurements or v s 30 maps at local regional and global scales because v s 30 measurements are sparse a proxy frequently is needed to estimate v s 30 at unsampled locations we present a new v s 30 map for california which accounts for observational constraints from multiple sources and spatial scales such as geology topography and site specific v s 30 measurements we apply the geostatistical approach of regression kriging rk to combine these constraints for predicting v s 30 for the v s 30 trend we start with geology based v s 30 values and identify two distinct trends between topographic gradient and the residuals from the geology v s 30 model one trend applies to deep and fine quaternary alluvium whereas the second trend is slightly stronger and applies to pleistocene sedimentary units the rk framework ensures that the resulting map of california is locally refined to reflect the rapidly expanding database of v s 30 measurements throughout california we compare the accuracy of the new mapping method to a previously developed map of v s 30 for california we also illustrate the sensitivity of ground motions to the new v s 30 map by comparing real and scenario shakemaps with v s 30 values from our new map to those for existing v s 30 maps online material california statewide v s 30 and shakemaps for loma prieta

automated classifications of topography from dems by an unsupervised nested means algorithm and a three part geometric signature abstract an iterative procedure that implements the classification of continuous topography as a problem in digital image processing automatically divides an area into categories of surface form three taxonomic criteria slope gradient local convexity and surface texture are calculated from a square grid digital elevation model dem the sequence of programmed operations combines twofold partitioned maps of the three variables converted to greyscale images using the mean of each variable as the dividing threshold to subdivide increasingly subtle topography grid cells sloping at less than mean gradient of the input dem are classified by designating mean values of successively lower sloping subsets of the study area nested means as taxonomic thresholds thereby increasing the number of output categories from the minimum 8 to 12 or 16 program output is exemplified by 16 topographic types for the world at 1 km spatial resolution srtm30 data the japanese islands at 270 m and part of hokkaido at 55 m because the procedure is unsupervised and reflects frequency distributions of the input variables rather than pre set criteria the resulting classes are undefined and must be calibrated empirically by subsequent analysis maps of the example classifications reflect physiographic regions geological structure and landform as well as slope materials and processes fine textured terrain categories tend to correlate with erosional topography or older surfaces coarse textured classes with areas of little dissection in japan the resulting classes approximate landform types mapped from airphoto analysis while in the americas they create map patterns resembling hammond s terrain types or surface form classes srtm30 output for the united states compares favorably with fenneman s physical divisions experiments are suggested for further developing the method the arc info aml and the map of terrain classes for the world are available as online downloads

lagrangian observations of the deep western boundary current in the north atlantic ocean part i large scale pathways and spreading rates twenty six rafos floats were deployed in the deep western boundary current dwbc of the north atlantic ocean between the grand banks and cape hatteras in 1994 95 and tracked acoustically for up to two years half of the floats were launched in the upper chlorofluorocarbon cfc maximum associated with upper labrador sea water 800 m and the other half near the deep cfc maximum that identifies the overflow water from the nordic seas 3000 m the float observations reveal the large scale pathways of these recently ventilated water masses in the subtropics the shallow float tracks show directly that upper labrador sea water is diverted away from the western boundary and into the interior at the location where the dwbc encounters the gulf stream near 368n the crossover region consistent with previous hydrographic studies east of the crossover region only one upper labrador sea water float out of seven 15 permanently crossed to the south side of the stream in two years caught in a cold core ring formation event the other shallow floats recirculated north of the gulf stream apparently confined by the mean potential vorticity gradient aligned with the stream the deep floats closely followed the topography to the crossover region then revealed a bifurcation in fluid parcel pathways one branch continues equatorward along the western boundary and the other turns first eastward along the gulf stream path then southward the deep float pathways including the bifurcation in the crossover region can be explained in terms of the deep potential vorticity distribution comparison of the float results with results from recent modeling studies suggests that the deep flow is strongly influenced by both the depth of the main pycnocline and bottom depth the effective spreading rates of upper labrador sea water and overflow water estimated directly from the float data southward at 0 6 6 0 2 cm s21 and 1 4 6 0 4 cm s21 respectively agree well with tracer derived spreading rates mean velocities in the dwbc equatorward at 2 4 cm s21 upper labrador sea water and 4 5 cm s21 overflow water are consistent with other in situ measurements one deep float drifted almost 4000 km along the western boundary in two years revealing a fast track for the spreading of overflow water in the dwbc these observations emphasize the importance of the crossover region in the spreading and mixing of recently ventilated water masses addressed in part ii of this study

postseismic deformation at el asnam algeria in the seismotectonic context of northwestern algeria summary we use a combination of seismicity tectonic features focal mechanisms seismic strain and postseismic movement to study the western part of north algeria the el asnam region and its surrounding area in particular a seismotectonic map of this part of algeria delimited by the mediterranean sea in the north and the tellian mountains in the south was built from available geological and seismological data an examination of this map shows that the most significant earthquakes are concentrated along tectonic features and quaternary basins elongated in an east west direction suggesting nnw sse compressional movements during the large el asnam earthquake of 1980 october 10 m 7 1 vertical movement was measured along a 40 km northeastsouthwest thrust fault these movements were determined geodetically in 198 1 with reference to a basic network previously measured in 1976 in order to control postseismic movement and to ensure the monitoring of the seismic area a dense geodetic network has been regularly measured since 1986 both in planemetry and altimetry the results of the altimetric remeasurements show significant vertical movements the elevation changes of the benchmarks have been deduced from precise levelling measurements a remarkable uplift 5 1 5 1 9 mm yr of the northwestern block during the 1986 91 period has been observed whereas the southeastern block is seen to be relatively stable the sar el maiirouf anticline situated along the central segment of the el asnam surface breaks appears to be growing with a maximum postseismic slip rate of 9 6 1 4 mm yr i the mean uplift rates computed for the northwestern block support the view that the 1954 earthquake did not occur on the same reverse fault as the 1980 event

loop current variability as trigger of coherent gulf stream transport anomalies abstractsatellite observations and output from a high resolution ocean model are used to investigate how the loop current in the gulf of mexico affects the gulf stream transport through the florida

the icelandic low as a predictor of the gulf stream north wall position abstractthe gulf stream s north wall east of cape hatteras marks the abrupt change in velocity and water properties between the slope sea to the north and the gulf stream itself an index of the north wall position constructed by taylor and stephens called gulf stream north wall gsnw is analyzed in terms of interannual changes in the icelandic low il pressure anomaly and longitudinal displacement sea surface temperature sst composites suggest that when il pressure is anomalously low there are lower temperatures in the labrador sea and south of the grand banks two years later warm sst anomalies are seen over the northern recirculation gyre and a northward shift in the gsnw occurs similar changes in ssts occur during winters in which the il is anomalously west resulting in a northward displacement of the gsnw 3 years later although time lags of 2 and 3 years between the il and the gsnw are used in the calculations it is shown that lags with respect to each atmospheric variable are statistica

refraction microtremors data analysis and diagnostics of key hypotheses surface wave methods are quite popular for site characterization in geotechnical earthquake engineering among these techniques a particular role is taken by passive methods for their ability to yield information on the low frequency range and consequently on large depths one such passive method the refraction microtremors remi technique has been proposed as a simple alternative to 2d array techniques to estimate surface wave dispersion by using linear arrays of geophones the technique owes its name to the use of widely available instruments also adopted for seismic refraction the basic hypotheses underlying remi are that noise is distributed isotropically in azimuth or is aligned exactly with the array these conditions often are not met and in most cases they are not verified because such analysis requires an accurate approach to data processing that is rarely applied we have developed an algorithm that verifies remi s basic hypotheses by analyzing experimental data in addition we have propo

introduction to the special section on the 2019 ridgecrest california earthquake sequence following the 16 october 1999 m w 7 1 hector mine earthquake no earthquake larger than magnitude 6 0 occurred in southern california for nearly 20 yr that lull came to an end with the occurrence of the 4 july 2019 m w 6 4 earthquake at 22 40 utc 10 33 a m local time located in a relatively sparsely populated part of the state with most of its rupture within the china lake naval air weapons station nawscl adjacent to the civilian town of ridgecrest population 26 000 the earthquake was felt widely across most of southern california immediate response efforts were mounted to investigate the rupture and assess damage stewart et al 2019 and to calculate aftershock probabilities see hardebeck 2020 also see data and resources the earthquake occurred in a region historically characterized by higher than average aftershock productivity hardebeck et al 2019 leading to a higher than average probabilities for both large aftershocks and subsequent larger events the latter odds although still small were borne out with the occurrence of 6 july m w 7 1 earthquake at 03 19 53 utc 8 19 53 p m local time on 5 july 34 hr after the initial m w 6 4 arguably the m w 7 1 could be considered an aftershock but by convention this earthquake is regarded as the mainshock whereas the initial m w 6 4 is considered a foreshock

persistence of gulf stream separation during the last glacial period implications for current separation theories 1 we present stable oxygen isotope ratio 18o measurements on deep dwelling planktonic foraminifera from the western margin of the north atlantic in order to reconstruct the latitude at which the gulf stream separated from the western boundary of the atlantic ocean in the past the modern separation latitude can be reconstructed within one degree from 18o measured on the fossil shells of deep dwelling planktonic foraminifera globorotalia truncatulinoides from the holocene sediments representing the past 10 000 years of mild climate conditions similar to today the separation latitude is captured in a sharp 18o gradient which reflects the prominent hydrographic change across the boundary between the warm waters south and east of the gulf stream and the cold slope water to the north the latitudinal 18o profile from approximately the last glacial maximum shows that the gulf stream separated from the coast near cape hatteras at almost the same latitude as it does today we assess our finding in light of existing gulf stream separation theories and conclude that one of the theories the wind induced separation mechanism is not consistent with our paleoceanographic reconstruction

intramethod variability in remi dispersion measurements and vs estimates at shallow bedrock sites the refraction microtremor remi method uses a linear array of receivers to record passive noise stress waves thereby negating the need for a strong active source such as those typically employed in other linear array surface wave methods the simplicity of such an approach is very appealing however there is a potential for significant uncertainty in shear wave velocity vs estimates obtained in this manner because much is unknown about the seismic wavefield that is being sampled this article examines intramethod variability in remi dispersion measurements and vs estimates at shallow bedrock sites the focus is not on evaluating whether remi measurements yielded the true vs profiles for the study sites but rather on evaluating how the dispersion curves and vs profiles at the sites were affected by variations in the seismic wavefield sampled by a linear array of receivers it was found that changes in the sampled seismic wavefield obtained through employing various array orientations at a site an

faster better shear wave velocity to 100 meters depth from refraction microtremor arrays current techniques of estimating shallow shear velocities for assessment of earthquake site response are too costly for use at most construction sites they require large sources to be effective in noisy urban settings or specialized independent recorders laid out in an extensive array this work shows that microtremor noise recordings made on 200 m long lines of seismic refraction equipment can estimate shear velocity with 20 accuracy often to 100 m depths the combination of commonly available equipment simple recording with no source a wavefield transformation data processing technique and an interactive rayleigh wave dispersion modeling tool exploits the most effective aspects of the microtremor spectral analysis of surface wave sasw and multichannel analysis of surface wave masw techniques the slowness frequency wavefield transformation is particularly effective in allowing accurate picking of rayleigh wave phase velocity dispersion curves despite the presence of waves propagating across the linear array at high apparent velocities higher mode rayleigh waves body waves air waves and incoherent noise two locations illustrate the application of this technique in detail coincident with a large accelerometer microtremor array in reno nevada and atop a borehole logged for shear velocity in newhall california refraction equipment could duplicate microtremor results above 3 hz but could not estimate velocities deeper than 100 m refraction microtremor cannot duplicate the detail in the velocity profile yielded by a suspension logger but can match the average velocity of 10 to 20 m depth intervals and suggest structure below the 100 m logged depth of the hole eight additional examples from southern california and new zealand demonstrate that the refraction microtremor technique quickly produces good results from a wide range of hard and soft sites

meridional gulf stream shifts can influence wintertime variability in the north atlantic storm track and greenland blocking author posting american geophysical union 2019 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geophysical research letters 46 3 2019 1702 1708 doi 10 1029 2018gl081087

gulf stream shifts following enso events over the past three decades the annual mean latitude of the gulf stream off the coast of the united states has been forecastable from the intensity of the north atlantic oscillation nao 1 the predictions accounting for more than half the variance here we show that much of the unexplained variance can be accounted for by the southern oscillation in the pacific the gulf stream being displaced northwards following el nino southern oscillation enso events this provides a link between events in the equatorial pacific and the circulation and weather conditions of the north atlantic

great salinity anomalies in the north atlantic abstract we revisited the great salinity anomaly of the 1970s gsa 70s dickson et al 1988 and documented the newly identified great salinity anomaly of the 1980s hence termed gsa 80s both propagated around the north atlantic in a similar fashion the advective mechanism initially proposed to explain the observed sequence of low salinity low temperature events during the gsa 70s apparently holds also for the gsa 80s the latter was successively observed in the west greenland current 1982 the labrador current 1983 the flemish pass 1984 near the charlie gibbs fracture zone 1984 1985 in the rockall channel 1985 south of iceland 1985 1988 in the north sea 1986 1987 norwegian sea 1987 1988 barents sea 1988 1989 and iceland sea 1989 1990 the advection speed of the gsa 80s seems to be greater than the one of the gsa 70s the 1980s anomaly reached the barents sea 6 to 7 years after peaking in the west greenland current while the 1970s anomaly traveled the same route in 8 to 10 years these anomalies however seem to be of different origin the gsa 70s was apparently boosted remotely by a freshwater sea ice pulse from the arctic via fram strait consequently the gsa 70s was accompanied by a large sea ice extent anomaly in the greenland and iceland seas which propagated into the labrador sea in contrast the gsa 80s was likely formed locally in the labrador sea baffin bay mainly because of the extremely severe winters of the early 1980s but supplemented with a possible contribution of the arctic freshwater outflow via the canadian archipelago facilitated by strong northerly winds which would have enhanced stability and ice formation this anomaly was also associated with a positive sea ice extent anomaly in the labrador sea baffin bay which however had no upstream precursor in the greenland sea thus the gsas are not necessarily caused solely by an increased export of freshwater and sea ice from the arctic via fram strait these results are corroborated by the early 1990s data when a new fresh cold anomaly was formed in the labrador sea and accompanied by a large positive sea ice extent anomaly the harsh winters of the early 1990s were however confined to the labrador sea baffin bay area while the atmospheric and oceanic conditions in the greenland iceland and irminger seas were normal the labrador sea baffin bay area appears therefore to play a key role in formation of gsas as well as in propagation of the gsas formed upstream a likely contribution of the enhanced canadian archipelago freshwater outflow to the gsa formation also seems to be significant two major modes of the gsa origin are thus identified remote generated by an enhanced arctic ocean freshwater export via either fram strait or the canadian archipelago and local resulting from severe winters in the labrador sea baffin bay both modes should be taken into account to model decadal variability of the coupled ocean ice atmosphere system in the north atlantic arctic basin

vertical surface displacements at yellowstone caldera wyoming 1976 1986 between first order leveling surveys in 1976 and 1986 the east central floor of yellowstone caldera rose 15 4 1 4 mm yr and the west central floor rose 17 0 2 0 mm yr with respect to points on the south caldera rim the same pattern has prevailed since an initial survey in 1923 except that the western caldera rose 30 faster during 1976 1986 than during 1923 1976 the 1976 1986 uplift extends beyond the northwest caldera rim near the adjacent hebgen lake fault zone site of a magnitude 7 3 earthquake in august 1959 marginally significant displacements in the eastern caldera suggest that uplift continued during 1983 1984 stopped during 1984 1985 and reversed to subsidence during 1985 1986 thus long term uplift may accumulate episodically historical uplift is especially noteworthy because regional crustal extension and thermal contraction during cooling would cause the caldera floor to subside if other processes were not acting to warp it upward the pattern and dimensions of the uplift indicate a source in the upper crust beneath yellowstone caldera probably a reservoir of rhyolite magma within a cooling granitic pluton a simple point source model would suggest a source 10 15 km deep but the reservoir is probably lens shaped and closer to the surface uplift may be caused by basaltic intrusions near the base of the reservoir or accumulation of magmatic fluids during cooling and crystallization within the reservoir subsidence might be a response to regional tectonic extension or release of trapped magmatic fluids magmatic inflation volatile accumulation and crustal extension may culminate eventually in renewed eruptive activity but there is no evidence of an imminent volcanic hazard

two modes of gulf stream variability revealed in the last two decades of satellite altimeter data monthly mapped sea level anomalies mslas of the nw atlantic in the region immediately downstream ofthegulfstream gs separationpointrevealaleadingmodeinwhichthepathshiftsapproximately 100km meridionally about a nominal latitude of 398n producing coherent sea level anomaly sla variability from 728 to 508w this mode can be captured by use of a simple 16 point index based on sla data taken along the maximum of the observed variability in the region 338 468n and 458 758w the gs shifts between 2010 and 2012 are the largest of the last decade and equal to the largest of the entire record the second group of eof modesofvariabilitydescribesgsmeanders whichpropagatemainlywestwardinterruptedbybriefperiodsof eastwardorstationarymeanders thesemeandershavewavelengthsofapproximately400kmandcanbeseen in standard eofs by spatial phase shifting of a standing meander pattern in the sla data the spectral properties of these modes indicate strong variability at interannual and longer periods for the first mode and periods of a few to several months for the meanders while the former is quite similar to a previous use of the altimeter for gs path the simple index is a useful measure of the large scale shifts in the gs path that is quickly estimatedand updatedwithoutchangesin previous estimates thetime scaleseparationallows alowpass filtered 16 point index to be reflective of large scale coherent shifts in the gs path

vertical movement and gravity change near the la grande 2 reservoir quebec leveling and precise gravity observations were made along a 50 km profile before and after the filling of the la grande 2 reservoir 53 7 n 77 w in 1979 anomalous relative vertical uplift of 4 cm and relative changes in gravity of 14 gal 140 nm s2 were observed along the profile the observed changes are anomalous with respect to theoretical results for loading of a layered elastic earth homogeneous isotropic porous elastic models with reasonable hydraulic properties are also unable to explain the observations in an effort to account for the discrepancies the accuracies of the leveling and the gravity data were investigated corrections were made to the leveling data for refraction errors rod calibrations magnetic effects and deflection of the vertical by the water mass although the corrections for systematic errors were significant the discrepancy with respect to homogeneous isotropic loading models remains the anomalous increase in gravity is thought to be the result of lateral transport of water in a high permeability formerly undersaturated zone hydraulically coupled to the reservoir the anomalous uplift is thought to be the result of elastic expansion of cracks and fractures of low aspect ratio in the high permeability zone or the result of an unexpectedly high rate of regional tilting in the lagrande 2 area

p wave seismic velocity structure derived from sonic logs and industry reflection data in the los angeles basin california 1 we present a new three dimensional seismic p wave velocity model of the los angeles basin that is based on more than 150 sonic logs and 7000 stacking velocities from industry reflection profiles the model was developed using kriging techniques implemented in gocad a commercial application specializing in the construction of surfaces the analysis of spatially distributed data and the modeling or simulation of properties and is characterized by a heterogeneous spatial varying velocity gradient with maximum velocities of 5000 m s in the sedimentary section variability of the observed p wave velocities increases with depth reflecting complex structural and sedimentological controls the model contains a new representation of the basin shape as defined by the sediment basement interface which was mapped using well control and seismic reflection profiles our model differs substantially in approach and outcome from previous representations of crustal velocity structure in southern california

slow variations in mean path of the gulf stream east of cape hatteras the gulf stream between cape hatteras and the new england seamounts varies in position over a wide range of time scales we present evidence that interannual path changes may be governed by a time varying outflow of waters from the labrador shelf region when the shelf and slope waters south of new england turn cold and fresh the stream frequently assumes a more southerly track and conversely when the waters are warm and saline these changes in path and surface water properties most likely result from large variations in shelf transport and thus greater or less dilution of the warm saline gulf stream waters that continually leak from the current into the slope sea these observations point to a thermohaline rather than winddriven mechanism for governing the path of the gulf stream after it leaves the coast

vertical deformation 1952 1987 in the salton trough california from water level recordings analysis of 36 years of water level data from three sites on the shore of the salton sea reveals a history of vertical deformation in the salton trough data from two sites allow estimation of tilt along a 36 km nw se baseline between 1952 and 1970 since 1970 the addition of a 13 km wnw ese baseline allows the full tilt vector to be estimated we find down to the southeast tilting at an average rate of about 0 01 0 01 rad yr from the early 1960s to 1987 an anomalous period of more rapid 0 2 0 04 rad yr tilting occurs between the mid 1950s and early 1960s but this coincides with a period from which we have been unable to examine all the original data a previous analysis of the 1952 1978 data suggested a precursory increase in tilt rate prior to the 1968 borrego mountain earthquake and a tilt reversal in 1972 we show that there was no significant tilt change prior to the 1968 earthquake and that the apparent 1972 reversal resulted from uncorrected offsets in the water level observations tilts derived from water level and from reliable first order leveling surveys show limited agreement but the leveling data contain evidence for nonuniform tilting recent analyses of leveling data by others using different techniques show 20 30 cm uplift and subsidence events in the salton trough between 1968 and 1978 our data constrain the relative elevations of the three water level sites to be stable within 1 2 cm during these proposed events

geochemical response on redox fluctuations in holocene lake sediments lake steisslingen southern germany abstract holocene sediments of lake steisslingen record sedimentation under different redox conditions this is proven by the changing depth of the chemocline and the resulting varying extension of laminated sediments from the deepest part of the lake to shallower water depths redox fluctuations are additionally reflected by colour changes between red and green and the quality of the nonglacial varves major and trace elements were analysed on bulk sediment samples period between 8200 6200 and 2650 2000 cal a bp to test their response to the observed changes in past depositional environments mn mo u fe and s were found to be redox sensitive elements in lake steisslingen fe and s are almost entirely fixed in pyrite because pyrite formation is limited by fe availability a parallel distribution of mo u fe and s between 8200 7200 cal a bp is interpreted as a signal for syngenetic pyrite formation from anoxic and sulfidic hypolimnetic waters since bioproductivity was low this must have been caused by conditions favouring a longterm stable stratification of the lake i e high water level high summer temperatures ice cover wind protection and or low storm activity between 6000 2000 cal a bp frequently changing redox conditions from anoxic to dysoxic are assumed mo and u curves are mirror images of the fe s and mn variations this is interpreted as showing diagenetic pyrite formation from pore waters which is enhanced during times of high fe availability under rather oxygenated conditions i e when high input of fe oxy hydroxides is possible but when u and mo cannot necessarily be fixed in the sediments changes from slightly disturbed lamination dysoxic restricted mixis to homogenous sediments oxic conditions holomixis cannot be traced within the geochemical record the observed redox fluctuations between 6000 and 2000 cal a bp are not correlated to changes in bioproductivity detrital input or cultural influences and must therefore be of a natural probably climatic origin the north atlantic oscillation is discussed as a possible cause

gulf stream excursions and sectional detachments generate the decadal pulses in the atlantic multidecadal oscillation abstractdecadal pulses within the lower frequency atlantic multidecadal oscillation amo are a prominent but underappreciated amo feature representing decadal variability of the subpolar gyre e g the great salinity anomaly of the 1970s and wielding notable influence on the hydroclimate of the african and american continents here clues are sought into their origin in the spatiotemporal development of the gulf stream s gs meridional excursions and sectional detachments apparent in the 1954 2012 record of ocean surface and subsurface salinity and temperature observations the gs excursions are tracked via meridional displacement of the 15 c isotherm at 200 m depth the gs index whereas the amo s decadal pulses are targeted through the amo tendency which implicitly highlights the shorter time scales of the amo index the gs s northward shift is shown to be preceded by the positive phase of the low frequency north atlantic oscillation lf nao and followed by a positive amo tendency by 1 25 and 2 5 yea

gulf stream variability in five oceanic general circulation models five non eddy resolving oceanic general circulation models driven by atmospheric fluxes derived from the ncep reanalysis are used to investigate the link between the gulf stream gs variability the atmospheric circulation and the atlantic meridional overturning circulation amoc despite the limited model resolution the temperature at the 200 m depth along the mean gs axis behaves similarly in most models to that observed and it is also well correlated with the north atlantic oscillation nao indicating that a northward southward gs shift lags a positive negative nao phase by 0 2 yr the northward shift is accompanied by an increase in the gs transport and conversely the southward shift with a decrease in the gs transport two dominant time scales appear in the response of the gs transport to the nao forcing a fast time scale less than 1 month for the barotropic component and a slower one about 2 yr for the baroclinic component in addition the two components are weakly coupled the gs response seems broadly consistent with a linear adjustment to the changes in the wind stress curl and evidence for baroclinic rossby wave propagation is found in the southern part of the subtropical gyre however the gs shifts are also affected by basin scale changes in the oceanic conditions and they are well correlated in most models with the changes in the amoc a larger amoc is found when the gs is stronger and displaced northward and a higher correlation is found when the observed changes of the gs position are used in the comparison the relation between the gs and the amoc could be explained by the inherent coupling between the thermohaline and the wind driven circulation or by the nao variability driving them on similar time scales in the models

the dominant influence of the icelandic low on the position of the gulf stream northwall 1 the location where the gulf stream separates from the american coast and turns eastward is called its northwall the interannual fluctuations of the northwall are significantly correlated with the north atlantic oscillation with a lag of two years when the azores high and the icelandic low pressures are taken as independent variables the latter dominates the relationship with the northwall and the influence of the azores high pressure is insignificant this is consistent with the hypothesis that the major oceanic control of the northwall is in the southward flow of labrador sea water into the slope sea the alternative mechanism that the interaction of westward propagating rossby waves with the american coast is responsible for northwall fluctuations is considered less likely because its initiation requires perturbations of the eastward winds in the mid atlantic region and they are very likely dependent on the azores high the analysis suggests that the time lag between perturbations of the icelandic low and the northwall varies between one and three years