surface geostrophic currents across the antarctic circumpolar current in drake passage from 1992 to 2004 the southern ocean plays an important role in the global overturning circulation as a significant proportion of deep water is converted into intermediate and deeper water masses in this region recently a secular trend has been reported in wind stress around the southern ocean and it is thought theoretically that the strength of the acc is closely related to wind stress so one consequence should be a corresponding increase in acc transport and hence changes in the rate of the global overturning there are no long term data sets of acc transport and so we must examine other data that may also respond to changing wind stress here we calculate surface currents in drake passage every seven days over 11 25 years from 1992 to 2004 we combine surface velocity anomalies calculated from satellite altimeter sea surface heights with measured surface currents since 1992 the uk has regularly occupied woce hydrographic section sr1b across the acc in drake passage from seven hydrographic sections surface currents are estimated by referencing relative geostrophic velocities from ctd sections with current measurements made by shipboard and lowered acoustic doppler current profilers combining the seven estimates of surface currents with the altimeter data reduces bias in the estimates of average currents over time through drake passage and we show that surface current anomalies estimated by satellite and in situ observations are in good agreement the strongest surface currents are found in the subantarctic and polar fronts with average speeds of 50 cm s and 35 cm s respectively and are inversely correlated so that maximum westward flow in one corresponds to minimum westward flow in the other the average cross sectional weighted surface velocity from 1992 to 2004 is 16 7 0 2 cm s a spectral analysis of the average surface current has only weakly increasing energy at higher frequencies and there is no dominant mode of variability the standard deviation of the seven day currents is 0 68 cm s and a running 12 month average has only a slightly smaller standard deviation of 0 52 0 16 cm s the southern annular mode sam measures the circumpolar average of wind stress and like the surface currents its spectrum has slightly increased energy at frequencies greater than 1 cpy a cospectral analysis of these averaging cospectra of five slightly overlapping 36 month segments improve statistical reliability suggests that there is coherence between them at 1 cpy with the currents leading changes in the southern annular mode we conclude that the sam and average drake passage surface currents are weakly correlated with no dominant co varying modes and hence predicting southern ocean transport variability from the sam is not likely to give significant results and that secular trends in surface currents are likely to be masked by weekly and interannual variability

seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

observed basin scale response of the north atlantic meridional overturning circulation to wind stress forcing abstractthe response of the north atlantic meridional overturning circulation moc to wind stress forcing is investigated from an observational standpoint using four time series of overturning transports below and relative to 1000 m overlapping by 3 6 yr these time series are derived from four mooring arrays located on the western boundary of the north atlantic the rapid western atlantic variability experiment wave array 42 5 n the woods hole oceanographic institution line w array 39 n rapid moc mocha 26 5 n and the meridional overturning variability experiment move array 16 n using modal decompositions of the analytic cross correlation between transports and wind stress the basin scale wind stress is shown to significantly drive the moc coherently at four latitudes on the time scales available for this study the dominant mode of covariance is interpreted as rapid barotropic oceanic adjustments to wind stress forcing eventually forming two counterrotating ekman overturning cells c

significant sink of ocean eddy energy near western boundaries ocean eddies generated through instability of the mean flow are a vital component of the energy budget of the global ocean modelling combined with satellite altimetry data suggests that the energy from westward propagating eddies is scattered and eventually dispersed when they reach the western boundary of an ocean basin

basinwide integrated volume transports in an eddy filled ocean the temporal evolution of the strength of the atlantic meridional overturning circulation amoc in the subtropical north atlantic is affected by both remotely forced basin scale meridionally coherent climate relevant transport anomalies such as changes in high latitude deep water formation rates and locally forced transport anomalies such as eddies or rossby waves possibly associated with small meridional coherence scales which can be considered as noise the focus of this paper is on the extent to which local eddies and rossby waves when impinging on the western boundary of the atlantic affect the temporal variability of the amoc at 26 5 degrees n continuous estimates of the amoc at this latitude have been made since april 2004 by combining the florida current ekman and midocean transports with the latter obtained from continuous density measurements between the coasts of the bahamas and morocco representing respectively the western and eastern boundaries of the atlantic at this latitude within 100 km of the western boundary there is a threefold decrease in sea surface height variability toward the boundary observed in both dynamic heights from in situ density measurements and altimetric heights as a consequence the basinwide zonally integrated upper midocean transport shallower than 1000 m as observed continuously between april 2004 and october 2006 varies by only 3 0 sv 1 sv 10 6 m 3 s 1 rms instead upper midocean transports integrated from western boundary stations 16 40 and 500 km offshore to the eastern boundary vary by 3 6 6 0 and 10 7 sv rms respectively the reduction in eddy energy toward the western boundary is reproduced in a nonlinear reduced gravity model suggesting that boundary trapped waves may account for the observed decline in variability in the coastal zone because they provide a mechanism for the fast equatorward export of transport anomalies associated with eddies impinging on the western boundary an analytical model of linear rossby waves suggests a simple scaling for the reduction in thermocline thickness variability toward the boundary physically the reduction in amplitude is understood as along boundary pressure gradients accelerating the fluid and rapidly propagating pressure anomalies along the boundary the results suggest that the local eddy field does not dominate upper midocean transport or amoc variability at 26 5 degrees n on interannual to decadal time scales

the observed north atlantic meridional overturning circulation its meridional coherence and ocean bottom pressure abstractanalyses of meridional transport time series from the rapid climate change meridional overturning circulation rapid moc array at 26 n and from argo float and altimetry data at 41 n reveal that at semiannual and longer time scales the contribution from the western boundary dominates the variability of the north atlantic meridional overturning circulation moc defined as the transport in the upper 1000 m of the ocean because the variability of the western boundary contribution is associated with a geostrophic overturning it is reflected in independent estimates of transports from gradient of ocean bottom pressure obp relative to and below 1000 m on the continental slope of the western boundary at three nominal latitudes 26 39 and 42 5 n time series of western meridional transports relative to and below 1000 m derived from the obp gradient or equivalently derived from the transport shear profile exhibit approximately the same phase relationship between 26 and 39 42 5 n as the wes

the vertical partition of oceanic horizontal kinetic energy to produce an interpretation of the surface kinetic energy as measured by altimeters a survey is made of the vertical structure of kinetic energy profiles in a large number of globally distributed long current meter records although the data are geographically confined primarily to a latitude band in the north pacific to the north atlantic and to a few moorings in the south atlantic the results show generally speaking that most regions are dominated by the barotropic and first baroclinic modes because of the near surface intensification of baroclinic modes altimeters primarily reflect the first baroclinic mode and thus the motion of the main thermocline there is good quantitative agreement with a few exceptions with estimates of the surface kinetic energy obtained from the topex poseidon altimeter and from vertical extrapolations to the surface of the mooring profiles these results are consistent with previous suggestions that barotropic models have little skill in depicting variability as seen in the altimeter data an eof analysis is shown to produce fictitious mode coupling unless the dynamical modes have very different energy levels

eddy impacts on the florida current the gulf stream in the atlantic carries warm water northwards and forms both the return closure of the subtropical gyre as well as the upper limb of the meridional overturning circulation recent time series recorded east of the bahamas at 26 n indicate that from may 2009 to april 2011 in contrast with past observations the northward flowing antilles current covaried with the gulf stream in the florida straits the florida current even though the florida and antilles currents are separated by banks and islands spanning 150 km the peak to trough amplitude of transport variations during this period was 15 106 m3 s 1 for the florida current and 12 106 m3 s 1 for the antilles current at time scales of 50 days to a year from satellite observations we show that the fluctuations in both the florida and antilles currents between may 2009 and april 2011 are driven by eddy activity east of the bahamas since the florida current time series is a critical time series for the state of the oceans and often compared to climate models this newly identified source of variability needs careful consideration when attributing the variability of the florida current to changes in the larger scale circulations e g gyre and overturning or wind forcing

the accuracy of estimates of the overturning circulation from basin wide mooring arrays previous modeling and observational studies have established that it is possible to accurately monitor the atlantic meridional overturning circulation amoc at 26 5 n using a coast to coast array of instrumented moorings supplemented by direct transport measurements in key boundary regions the rapid mocha wbts array the main sources of observational and structural errors have been identified in a variety of individual studies here a unified framework for identifying and quantifying structural errors associated with the rapid array based amoc estimates is established using a high resolution eddy resolving at low mid latitudes eddy permitting elsewhere ocean general circulation model which simulates the ocean state between 1978 2010 we define a virtual rapid array in the model in close analogy to the real rapid array and compare the amoc estimate from the virtual array with the true model amoc the model analysis suggests that the rapid method underestimates the mean amoc by 1 5 sv 1 sv 106 m3 s 1 at 900m depth however it captures the variability to high accuracy we examine three major contributions to the streamfunction bias i due to the assumption of a single fixed reference level for calculation of geostrophic transports ii due to regions not sampled by the array and iii due to ageostrophic transport a key element in i and iii is use of the model sea surface height to establish the true or absolute geostrophic transport in the upper 2000m we find that the reference level bias is strongest and most variable in time whereas the bias due to unsampled regions is largest below 3000m the ageostrophic transport is significant in the upper 1000m but shows very little variability the results establish for the first time the uncertainty of the amoc estimate due to the combined structural errors in the measurement design and suggest ways in which the error could be reduced our work has applications to basin wide circulation measurement arrays at other latitudes and in other basins as well as quantifying systematic errors in ocean model estimates of the amoc at 26 5 n

moored observations of western boundary current variability and thermohaline circulation at 26 5 n in the subtropical north atlantic abstract a 5 8 year time series of moored current meter observations is used with hydrographic section data cme model results and gridded wind fields over the north atlantic to describe the mean structure and variability of circulation and volume transports east of abaco bahamas at 26 5 n a mean antilles current with 5 sv of northward transport is confined against the bahamas boundary in the upper 800 m and combines with approximately 19 sv of florida current transport to balance the sverdrup interior circulation and does not contribute to interhemispheric exchange the mean transport of the deep western boundary current dwbc off the bahamas is approximately 40 sv of which 13 sv compensates the upper branch of the thermohaline circulation requiring a 27 sv deep recirculation robust annual and semiannual cycles of meridional are found in both moored observations and model results with remarkable agreement in amplitude 13 sv and phase maximum northward transports occur in winter and summer

variations in earthquake rupture properties along the gofar transform fault east pacific rise mid ocean ridge transform faults experience more foreshocks than continental faults yet the mainshock rarely ruptures the entire fault analysis of seismic data from the gofar transform fault at the east pacific rise indicates that the foreshock region has different material properties from the mainshock region and acts as a barrier to rupture propagation on a global scale seismicity on oceanic transform faults that link mid ocean ridge segments is thermally controlled1 2 however temperature cannot be the only control because the largest earthquakes on oceanic transform faults rupture only a small fraction of the area that thermal models predict to be capable of rupture3 4 5 instead most slip occurs without producing large earthquakes3 4 6 when large earthquakes do occur they often repeat quasiperiodically7 8 moreover oceanic transform faults produce an order of magnitude more foreshocks than continental strike slip faults7 9 here we analyse a swarm of about 20 000 foreshocks recorded on an array of ocean bottom seismometers which occurred before a magnitude 6 0 earthquake on the gofar transform fault east pacific rise we find that the week long foreshock sequence was confined to a 10 km long region that subsequently acted as a barrier to rupture during the mainshock the foreshock zone is associated with a high porosity and undergoes a 3 decrease in average shear wave speed during the week preceding the mainshock we conclude that the material properties of fault segments capable of rupturing in large earthquakes differ from those of barrier regions possibly as a result of enhanced fluid circulation within the latter we suggest that along strike variations in fault zone material properties can help explain the abundance of foreshocks and the relative lack of large earthquakes that occur on mid ocean ridge transform faults

propagation of slow slip leading up to the 2011 mw 9 0 tohoku oki earthquake many large earthquakes are preceded by one or more foreshocks but it is unclear how these foreshocks relate to the nucleation process of the mainshock on the basis of an earthquake catalog created using a waveform correlation technique we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude mw 9 0 tohoku oki earthquake in japan the time history of quasi static slip along the plate interface based on small repeating earthquakes that were part of the migrating seismicity suggests that two sequences involved slow slip transients propagating toward the initial rupture point the second sequence which involved large slip rates may have caused substantial stress loading prompting the unstable dynamic rupture of the mainshock

eastern boundary contribution to the residual and meridional overturning circulations a model of the thermocline linearized around a specified stratification and the barotropic linear winddriven stommel solution is constructed the forcings are both mechanical the surface wind stress and thermodynamical the surface buoyancy boundarycondition the effects of diapycnal diffusivity and of eddy fluxesofbuoyancy parameterizedintermsofthelarge scalebuoyancygradient areincluded theeddyfluxes of buoyancy are especially important near the boundaries where they mediate the transport in and out of the narrow ageostrophic down upwelling layers the dynamics of these narrow layers can be replaced by effective boundary conditions on the geostrophically balanced flow the effective boundary conditions state that the residual flow normal to the effective coast vanishes the separate eulerian and eddy induced components may be nonzero this formulation conserves the total mass and the total buoyancy while permitting an exchangebetweentheeulerianandeddytransportofbuoyancy withinthedown upwellinglayers inturn this exchange allows buoyancy gradients along all solid boundaries including the eastern one a special focus is on the buoyancy along the eastern and western walls since east west buoyancy difference determines the meridional overturning circulation the inclusion of advection of buoyancy by the barotropic flow allows a meaningful distinction between the meridional and the residual overturning circulations while retaining the simplicity of a linear model the residual flow in both meridional and zonal directions reveals how the subsurface buoyancy distribution is established and in particular how the meridional buoyancy gradient is reversed at depth in turn the horizontal buoyancy gradient maintains stacked counterrotating cells in the meridional and residual overturning circulations quantitative scaling arguments are given for each of these cells which show how the buoyancy forcing the wind stress and the diapycnal and eddy diffusivities as well as the other imposed parameters affect the strength of the overturn

mass and volume transport variability in an eddy filled ocean eddy activity in the north atlantic ocean produces fluctuations in ocean wide volume transport on the order of 20 106 cubic metres per second on multi year timescales such background noise makes it impossible to detect possible trends in the ocean circulation due to a changing climate without multi decadal observations in three spatial dimensions the possibility that the oceanic general circulation is undergoing changes as part or the cause of major climate shifts is being intensely discussed1 with some published results relying on data from moorings spanning the north atlantic ocean2 3 the circulation is however extremely noisy here i use existing estimates of the frequency and wavenumber content of geostrophic eddies in the ocean4 to show that variations in ocean wide integrated transport must appear even in the absence of a true long term trend expected fluctuations exceed 20 109 kg s 1 or 20 106 m3 s 1 and exhibit multi year timescales existing knowledge of the eddy field allows predictions of observed variability and produces lower bounds on the multi decadal timescale required to detect true trends of a large magnitude detecting and understanding the effect of climate change on the ocean circulation requires observations in three dimensions over long periods of time

the structures stratigraphy and evolution of the gulf of corinth rift greece summary a multichannel seismic and bathymetry survey of the central and eastern gulf of corinth goc greece reveals the offshore fault geometry seismic stratigraphy and basin evolution of one of earths most active continental rift systems active right stepping en echelon north dipping border faults trend ese along the southern gulf margin significantly overlapping along strike the basement offsets of three akrata derveni sithas and xylocastro are linked the faults are biplanar to listric typically intermediate angle 35 in the centre and 45 48 in the east near the surface but decreasing in dip and or intersecting a low or shallow angle 15 20 in the centre and 19 30 in the east curvi planar reflector in the basement major s dipping border faults were active along the northern margin of the central gulf early in the rift history and remain active in the western gulf and in the subsidiary gulf of lechaio but unlike the southern border faults are without major footwall uplift much of the eastern rift has a classic half graben architecture whereas the central rift has a more symmetric w or u shape the narrower and shallower western gulf that transects the 40 km thick crust of the hellenides is associated with a wider distribution of overlapping high angle normal faults that were formerly active on the peloponnesus peninsula the easternmost sector includes the subsidiary gulfs of lechaio and alkyonides with major faults and basement structures trending ne e w and nw the basement faults that control the rift architecture formed early in the rift history with little evidence other than the vrachonisida fault along the northern margin in the marine data for plan view evolution by subsequent fault linkage several have maximum offsets near one end crestal collapse graben formed where the hanging wall has pulled off the steeper onto the shallower downdip segment of the derveni fault the dominant strikes of the corinth rift faults gradually rotate from 090 120 in the basement and early rift to 090 100 in the latest rift reflecting a 10 rotation of the opening direction to the 005 presently measured by gps the sediments include a locally 1 5 km thick early rift section and a late rift section also locally 1 5 km thick that we subdivide into three sequences and correlate with seven 100 ka glacio eustatic cycles the gulf depocentre has deepened through time currently 700 mbsl as subsidence has outpaced sedimentation we measure the minimum total horizontal extension across the central and eastern gulf as varying along strike between 4 and 10 km and estimate full values of 6 11 km the rift evolution is strongly influenced by the inherited basement fabric the regional nnw structural fabric of the hellenic nappes changes orientation to ese in the parnassos terrane facilitating the focused north south extension observed offshore there the basement penetrating faults lose seismic reflectivity above the 4 14 km deep seismogenic zone multiple generations and dips of normal faults some cross cutting accommodate extension beneath the goc including low angle 15 20 interfaces in the basement nappes the thermally cool forearc setting and cross orogen structures unaccompanied by magmatism make this rift a poor analogue and unlikely precursor for metamorphic core complex formation

under the hood of the earthquake machine toward predictive modeling of the seismic cycle advances in observational laboratory and modeling techniques open the way to the development of physical models of the seismic cycle with potentially predictive power to explore that possibility we developed an integrative and fully dynamic model of the parkfield segment of the san andreas fault the model succeeds in reproducing a realistic earthquake sequence of irregular moment magnitude m w 6 0 main shocks including events similar to the ones in 1966 and 2004 and provides an excellent match for the detailed interseismic coseismic and postseismic observations collected along this fault during the most recent earthquake cycle such calibrated physical models provide new ways to assess seismic hazards and forecast seismicity response to perturbations of natural or anthropogenic origins

observations inferences and mechanisms of the atlantic meridional overturning circulation a review this is a review about the atlantic meridional overturning circulation amoc its mean structure temporal variability controlling mechanisms and role in the coupled climate system the amoc plays a central role in climate through its heat and freshwater transports northward ocean heat transport achieved by the amoc is responsible for the relative warmth of the northern hemisphere compared to the southern hemisphere and is thought to play a role in setting the mean position of the intertropical convergence zone north of the equator the amoc is a key means by which heat anomalies are sequestered into the ocean s interior and thus modulates the trajectory of climate change fluctuations in the amoc have been linked to low frequency variability of atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses on intra annual timescales variability in amoc is large and primarily reflects the response to local wind forcing meridional coherence of anomalies is limited to that of the wind field on interannual to decadal timescales amoc changes are primarily geostrophic and related to buoyancy anomalies on the western boundary a pacemaker region for decadal amoc changes is located in a western transition zone along the boundary between the subtropical and subpolar gyres decadal amoc anomalies are communicated meridionally from this region amoc observations as well as the expanded ocean observational network provided by the argo array and satellite altimetry are inspiring efforts to develop decadal predictability systems using coupled atmosphere ocean models initialized by ocean data

assimilation of rapid array observations into an ocean model experiments assimilating the rapid dataset of deep temperature and salinity profiles at 26 5 n on the western and eastern atlantic boundaries into a 1 global nemo ocean model have been performed the meridional overturning circulation moc is then assessed against the transports calculated directly from observations the best initialization found for this short period was obtained by assimilating the en3 upper ocean hydrography database prior to 2004 after which different methods of assimilating 5 day average rapid profiles at the western boundary were tested the model moc is strengthened by 2 sv giving closer agreement with the rapid array transports when the western boundary profiles are assimilated only below 900 m the approximate depth of the florida straits which are not well resolved and when the t s observations are spread meridionally from 10 to 35 n along the deep western boundary the use of boundary focused covariances has the largest impact on the assimilation results otherwise using more conventional gaussian covariances has a very local impact on the moc at 26 n with strong adverse impacts on the moc stream function at higher and lower latitudes even using boundary focused covariances only enables the moc to be strengthened for 2 years after which the increased transport of warm waters leads to a negative feedback on water formation in the subpolar gyre which then reduces the moc this negative feedback can be mitigated if en3 hydrography data continue to be assimilated along with the rapid array boundary data copyright 2012 royal meteorological society and crown in the right of canada

sea surface height signals as indicators for oceanic meridional mass transports numerical models are used to test whether the sea surface height ssh can be used as an indicator for the variability of atlantic meridional oceanic mass transports the results suggest that if the transports over the western boundary current region and those in the eastern part of the basin are considered separately significant correlations 0 3 0 9 are found between zonal ssh differences and the meridional transports in the top 1100 m much weaker correlations are found for the basinwide transport which corresponds to the surface branch of the meridional overturning circulation moc for the eastern and western branches of the meridional transport combining the ssh signal with the baroclinic structure obtained from rossby wave theory enables calculation of a quantitative estimate of the transport variability in the top 1100 m the results of the method are less convincing for the variability of the moc the reason for this is that even small relative errors in the variability of the eastern and western branches can be large compared with the moc variability these errors project onto the sum of the eastern and western transports and therefore onto the surface branch of the moc nevertheless being able to infer transport anomalies from ssh signals in the eastern and western parts of the atlantic might prove useful in interpreting moc observations from the u k natural environment research council rapid climate change rapid mooring array at 268n which show a large subannual variability that is mainly due to changes at the western boundary transports inferred from the ssh could help to identify the origin of this variability and whether transport anomalies propagate into the western boundary region from the basin interior or from other latitudes

temporal variability of the meridional overturning circulation at 34 5 s results from two pilot boundary arrays in the south atlantic data from two boundary arrays deployed along 34 5 s are combined to produce the first continuous in situ time series observations of the basin wide meridional overturning circulation moc in the south atlantic daily estimates of the moc between march 2009 and december 2010 range between 3 sv and 39 sv 1 sv 106 m3 s 1 after a 10 day low pass filter is applied much of the variability in this 20 month record occurs at periods shorter than 100 days approximately two thirds of the moc variability is due to changes in the geostrophic baroclinic plus barotropic volume transport with the remainder associated with the direct wind forced ekman transport when low pass filtered to match previously published analyses in the north atlantic the observed temporal standard deviation at 34 5 s matches or somewhat exceeds that observed by time series observations at 16 n 26 5 n and 41 n for periods shorter than 20 days the basin wide moc variations are most strongly influenced by ekman flows while at periods between 20 and 90 days the geostrophic flows tend to exert slightly more control over the total transport variability of the moc the geostrophic shear variations are roughly equally controlled by density variations on the western and eastern boundaries at all time scales captured in the record the observed time mean moc vertical structure and temporal variability agree well with the limited independent observations available for confirmation

active faulting at the western tip of the gulf of corinth greece from high resolution seismic data abstract the gulf of corinth is one of the fastest spreading intra continental rifts on earth gps data indicate that the rift is currently opening in a nne ssw direction with a rate of extension reaching up to 16 mm yr 1 in its westernmost part although the rest of the offshore rift has been well studied the western tip of the rift is still poorly explored we present an accurate map of submarine faults in this area based on two high resolution seismic reflection surveys single channel sparker in the eastern part of the studied area the sedimentary infill is affected by the known north eratini south eratini and west channel faults further to the west the seafloor is mostly flat and is bounded to the north by the normal south dipping trizonia fault to the north the shallower part of the gulf shows to the east a diffuse pattern of normal and strike slip deformation which is replaced to the west by a 7 5 km long se striking strike slip fault zone called the managouli fault zone to the westernmost tip of the gulf in the nafpaktos basin two fault sets with different strikes are encountered the one with a ne sw strike exhibits a clear strike slip component the western tip of the gulf of corinth is the only part of the corinth rift where convincing evidence for strike slip movement has been found this fault pattern is likely related to the complex deformation occurring at the diffuse junction at the western tip of the rift between three crustal blocks continental greece peloponnese and the ionian island akarnania block

variability of shallow and deep western boundary currents off the bahamas during 2004 05 results from the 26 n rapid moc array data from an array of six moorings deployed east of abaco bahamas along 26 5 n during march 2004 may 2005 are analyzed these moorings formed the western boundary array of a transbasin observing system designed to continuously monitor the meridional overturning circulation and meridional heat flux in the subtropical north atlantic under the framework of the joint u k u s rapid climate change rapid meridional overturning circulation moc program important features of the western boundary circulation include the southward flowing deep western boundary current dwbc below 1000 m and the northward flowing antilles current in the upper 1000 m transports in the western boundary layer are estimated from direct current meter observations and from dynamic height moorings that measure the spatially integrated geostrophic flow between moorings the results of these methods are combined to estimate the time varying transports in the upper and deep ocean over the width of the western boundary layer to a distance of 500 km offshore of the bahamas escarpment the net southward transport of the dwbc across this region inclusive of northward deep recirculation is 26 5 sv sv 10 6 m 3 s 1 which is divided nearly equally between upper 13 9 sv and lower 12 6 sv north atlantic deep water nadw in the top 1000 m 6 0 sv flows northward in a thermocline intensified jet near the western boundary these transports are found to agree well with historical current meter data in the region collected between 1986 and 1997 variability in both shallow and deep components of the circulation is large with transports above 1000 m varying between 15 and 25 sv and deep transports varying between 60 and 3 sv much of this transport variability associated with barotropic fluctuations occurs on relatively short time scales of several days to a few weeks upon removal of the barotropic fluctuations slower baroclinic transport variations are revealed including a temporary stoppage of the lower nadw transport in the dwbc during november 2004

covariances and linear predictability of the atlantic ocean abstract the problem of understanding linear predictability of elements of the ocean circulation is explored in the atlantic ocean for two disparate elements 1 sea surface temperature sst under the storm track in a small region east of the grand banks and 2 the meridional overturning circulation north of 30 5 s to be worthwhile any nonlinear method would need to exhibit greater skill and so a rough baseline from which to judge more complex methods is the goal a 16 year ocean state estimate is used under the assumption that internal oceanic variability is dominating externally imposed changes no evidence exists of significant nonlinearity in the bulk of the system over this time span linear predictability is the story of time and space correlations and some predictive skill exists for a few months in sst with some minor capability extending to a few years sixteen years is however far too short for an evaluation for interannual much less decadal variability although orders of magnitude are likely stably estimated the meridional structure of the meridional overturning circulation moc defined as the time varying vertical integral to the maximum meridional volume transport at each latitude shows nearly complete decorrelation in the variability across about 35 n the gulf stream system if a time scale exists displaying coherence of the moc between subpolar and subtropical gyres it lies beyond the existing observation duration and that has consequences for observing system strategies and the more general problem of detectability of change

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

a window on the deep ocean the special value of ocean bottom pressure for monitoring the large scale deep ocean circulation we show how by focusing on bottom pressure measurements particularly on the global continental slope it is possible to avoid the fog of mesoscale variability which dominates most observables in the deep ocean this makes it possible to monitor those aspects of the ocean circulation which are most important for global scale ocean variability and climate we therefore argue that such measurements should be considered an important future component of the global ocean observing system to complement the present open ocean and coastal elements our conclusions are founded on both theoretical arguments and diagnostics from a fine resolution ocean model that has realistic amplitudes and spectra of mesoscale variability these show that boundary pressure variations are coherent over along slope distances of tens of thousands of kilometres for several vertical modes we illustrate the value of this in the model atlantic by determining the time for boundary and equatorial waves to complete a circuit of the northern basin 115 and 205 days for the first and second vertical modes showing how the boundary features compare with basin scale theoretical models and demonstrating the ability to monitor the meridional overturning circulation using these boundary measurements finally we discuss applicability to the real ocean and make recommendations on how to make such measurements without contamination from instrumental drift

leaky slope waves and sea level unusual consequences of the beta effect along western boundaries with bottom topography and dissipation abstractcoastal trapped waves ctws carry the ocean s response to changes in forcing along boundaries and are important mechanisms in the context of coastal sea level and the meridional overturnin

variability of the deep western boundary current at 26 5 n during 2004 2009 five years of data from a line of dynamic height moorings dhm bottom pressure recorders bpr and pressure equipped inverted echo sounders pies near the atlantic ocean western boundary at 26 51n are used to evaluate the structure and variability of the deep western boundary current dwbc during 2004 2009 comparisons made between transports estimated from the dhm bpr and those made by the pies demonstrate that the two systems are collecting equivalent volume transport information correlation coefficient r 0 96 root mean square difference 6 sv 1 sv 10 6 m 3 s 1 integrated to 450 km off from the continental shelf and between 800 and 4800 dbar the dwbc has a mean transport of approximately 32 sv and a standard deviation during these five years of 16 sv both the barotropic full depth vertical mean and baroclinic flows have significant variability changes exceeding 10 sv on time scales ranging from a few days to months with the barotropic variations being larger and more energetic at all time scales the annual cycle of the deep transport is highly dependent on the horizontal integration distance integrating 100 km offshore yields an annual cycle of roughly similar magnitude but shifted in phase relative to that found from current meter arrays in the 1980 1990s while the annual cycle becomes quite weak when integrating 450 km offshore variations in the dwbc transport far exceed those of the total basin wide meridional overturning circulation standard deviations of 16 sv vs 5 sv transport integrated in the deep layer out to the west side of the mid atlantic ridge still demonstrates a surprisingly high variance indicating that some compensation of the western basin deep variability must occur in the eastern basin published by elsevier ltd

two decades of the atlantic meridional overturning circulation anatomy variations extremes prediction and overcoming its limitations the zonally integratedmeridional volumetransport in the north atlantic atlantic meridionaloverturning circulation amoc is described in a 19 yr long ocean state estimate one consistent with a diverse global dataset apart from a weak increasing trend at high northern latitudes the amoc appears statistically stable over the last 19yr with fluctuations indistinguishable from those of a stationary gaussian stochastic process this characterization makes it possible to study using highly developed tools extreme values predictability and the statistical significance ofapparenttrends gaussianbehaviorisconsistentwith the central limittheorem for a process arising from numerous independent disturbances in this case generators include internal instabilities changes in wind and buoyancy forcing fields boundary waves the gulf stream and deep western boundary current transports the interior fraction in sverdrup balance and all similar phenomena arriving as summation effects from long distances and times as a zonal integral through the sum of the large variety of physical processes in the three dimensional ocean circulation understanding of the amoc if it is of central climate importance requires breaking it down into its unintegrated components over the entire basin

the frictional behavior of serpentinite implications for aseismic creep on shallow crustal faults serpentine is common in many active faults and may be responsible for aseismic creep along segments of these faults to test this we have conducted a series of velocity stepping experiments to determine the frictional velocity dependence of serpentinite we slid initially bare rough surfaces of antigorite serpentinite at room temperature with velocities from 0 0032 to 10 0 m s 1 0 102 to 3 2 105 mm yr and normal stresses from 25 to 125 mpa we find that the velocity dependence of serpentinite undergoes a transition from velocity weakening at fast loading velocities to velocity strengthening at slow velocities and that this change is accompanied by other changes in the constitutive behavior these results suggest that serpentinite should not be the site of instability initiation during sliding at plate velocities but may permit propagation of unstable slip initiated elsewhere

vertical structure of eddies and rossby waves and their effect on the atlantic meridional overturning circulation at 26 5 n the meridional overturning circulation moc at inline image in the atlantic has a standard deviation of 4 9 sv and contains large fluctuations at subannual periods the geostrophic component of the moc is believed to be influenced on subannual time scales by eddies and rossby waves to quantify this effect the vertical structure and surface characteristics of westward propagating signals are studied using altimetric data and full depth mooring measurements from the rapid array at inline image westward propagating features are observed in the western north atlantic in both data sets and have periods of 80 250 days in the first baroclinic mode these features are still observed by the rapid moorings 20 km offshore of the western boundary the western boundary also exhibits deep variability characterized by enhanced energy in higher baroclinic modes the effect of eddies and rossby waves on the geostrophic transport is quantified by representing their vertical structure with the first baroclinic mode in total 42 of the variance of the transbasin thermocline transport inferred from geostrophic calculations at inline image can be attributed to first mode variability which is associated with eddies and rossby waves at periods of 80 250 days the standard deviation of the transbasin thermocline transport due to eddies and rossby waves is estimated to be 2 6 sv

interseismic coupling segmentation and mechanical behavior of the central chile subduction zone global positioning system gps measurements carried out in chile over the last two decades showed that an entire portion of the nazca south america subduction zone 38 s 24 s was locked over this period of time the induced accumulation of elastic deformation in the upper plate was not released until the recent maule earthquake of 27 february 2010 mw 8 8 that ruptured the southern part of this section locking or coupling between the two plates varies both with depth and along strike here we use our own gps data an updated solution of our extended network in central chile combined with other published data sets to quantify the spatial variations of the coupling that prevailed before the maule earthquake using a simple elastic model based on the back slip assumption we show that coupling variations on the subduction plane are sufficient to explain the observed surface deformation with no need of a sliver in central chile we identify four segments characterized by higher coupling and separated by narrow areas of lower coupling this segmentation is in good agreement with historical and recent seismicity in chile in particular the narrow zones of lower coupling seem to have stopped most large seismic ruptures including maule s these zones are often associated with irregular bathymetric or coastal features fracture zones or peninsulas finally coseismic and early post seismic slip distribution of the maule earthquake occurring either in previously highly or weakly coupled zones map a complex distribution of velocity weakening and velocity strengthening patches on the subduction interface

basinwide response of the atlantic meridional overturning circulation to interannual wind forcing an eddy resolving ocean general circulation model for the earth simulator ofes and a simple wind driven two layer model are used to investigate the role of momentum fluxes in driving the atlantic meridional overturning circulation amoc variability throughout the atlantic basin from 1950 to 2010 diagnostic analysis using the ofes results suggests that interior baroclinic rossby waves and coastal topographic waves play essential roles in modulating the amoc interannual variability the proposed mechanisms are verified in the context of a simple two layer model with realistic topography and only forced by surface wind the topographic waves communicate high latitude anomalies into lower latitudes and account for about 50 of the amoc interannual variability in the subtropics in addition the large scale rossby waves excited by wind forcing together with topographic waves set up coherent amoc interannual variability patterns across the tropics and subtropics the comparisons between the simple model and ofes results suggest that a large fraction of the amoc interannual variability in the atlantic basin can be explained by wind driven dynamics

a numerical model study of the effects of interannual time scale wave propagation on the predictability of the atlantic meridional overturning circulation we investigate processes leading to uncertainty in forecasts of the atlantic meridional overturning circulation amoc a climate model is used to supply initial conditions for ensemble simulations in which members initially have identical ocean states but perturbed atmosphere states baroclinic transports diverge on interannual timescales even though the ocean is not eddy permitting interannual fluctuations of the model amoc in the subtropical gyre are caused by westward propagating rossby waves divergence of the predicted amoc with time occurs because the waves develop different phases in different ensemble members predominantly due to differences in eastern boundary windstress curl these windstress fluctuations communicate with interior ocean transports via modifications to the vertical velocity and the vortex stretching term dw dz consequently errors propagate westwards resulting in longer predictability times in the interior ocean compared with the eastern boundary another source of divergence is transport anomalies propagating along the gulf stream and other boundary currents the propagation mechanism seems to be predominantly advection by mean currents and we show that the arrival of westward propagating waves can trigger development of these anomalies the mean state of the amoc has a small effect on interannual predictability in the subtropical gyre most likely because eastern boundary windstress curl predictability is not strongly dependent on the state of the amoc in the subtropics eastern boundary windstress curl was more predictable at 45 degree sign n when the amoc was in a strongly decreasing state but unlike at 30 degree sign n no mechanism was found linking windstress curl fluctuations with deep transports

magmatic and tectonic extension at mid ocean ridges 1 controls on fault characteristics we use 2 d numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction m of the total spreading rate i e m 1 for fully magmatic spreading heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust we examine a range of intrusion rates and axial thermal structures by varying m spreading rate and the efficiency of crustal cooling by conduction and hydrothermal circulation fault development proceeds in a sequential manner with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis fault spacing and heave are primarily sensitive to m and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis contrary to what is often cited in the literature but consistent with prior results of mechanical modeling we find that thicker axial lithosphere tends to reduce fault spacing and heave in addition fault spacing and heave are predicted to increase with decreasing rates of off axis lithospheric thickening the combination of low m particularly when m approaches 0 5 as well as a reduced rate of off axis lithospheric thickening produces long lived large offset faults similar to oceanic core complexes such long lived faults produce a highly asymmetric axial thermal structure with thinner lithosphere on the side with the active fault this across axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes

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

reconstructing monitoring and predicting multidecadal scale changes in the north atlantic thermohaline circulation with sea surface temperature sea surface temperature sst observations in the north atlantic indicate the existence of strong multidecadal variability with a unique spatial structure it is shown by means of a new global climate model which does not employ flux adjustments that the multidecadal sst variability is closely related to variations in the north atlantic thermohaline circulation thc the close correspondence between the north atlantic sst and thc variabilities allows in conjunction with the dynamical inertia of the thc for the prediction of the slowly varying component of the north atlantic climate system it is shown additionally that past variations of the north atlantic thc can be reconstructed from a simple north atlantic sst index and that future anthropogenically forced changes in the thc can be easily monitored by observing ssts the latter is confirmed by another state ofthe art global climate model finally the strong multidecadal variability may mask an anthropogenic signal in the north atlantic for some decades

the velocity and vorticity structure of the agulhas current at 32 s the full depth velocity field of the agulhas current the western boundary current of the southwest indian ocean has been directly measured using a lowered acoustic doppler current profiler ladcp fifteen combined conductivity temperature depth ctd and ladcp stations were occupied across the current at 32 s off the east coast of south africa in february march 1995 as part of the world ocean circulation experiment indian ocean program the deep velocity structure of the agulhas current was found to be very different than previously described using geostrophic estimates in particular ladcp results reveal a v shaped pattern for the level of no motion across the agulhas current and an agulhas undercurrent is observed flowing equatorward below 800 m depth directly beneath the surface core of the poleward flowing agulhas current comparisons of direct and geostrophic velocities suggest that the velocity structure of the agulhas current is essentially geostrophic below about 200 m depth where differences are generally less than estimated errors above these depths the ladcp measured shears and geostrophic shears exhibit differences and velocities can diverge shipboard adcp data support ladcp results near the surface indicating that both instruments are measuring real signals the geostrophic and ladcp measured shears are well matched away from the surface and a depth averaged fit below 200 m between them exhibits small standard deviations the geostrophic volume transport of the agulhas current as referenced to ladcp is 73 sv and is just 3 different from the direct ladcp transport estimate the most recent full depth estimate of agulhas current transport in the literature is given by toole and warren 1993 who estimate a geostrophic volume transport of 85 sv the baroclinic velocity structures from their section and from this work are similar indicating that the difference in the transport estimates is due to the choice of geostrophic reference level and ultimately to the presence of the previously unobserved agulhas undercurrent combined ctd ladcp data are very useful for potential vorticity analysis since velocity and density measurements are simultaneous and coincident the vorticity structure of the agulhas current suggests the presence of a mixing boundary at middepths similar to features observed in the gulf stream the feature appears to be associated with a sharp upturn in isopycnals coincident with the foot of the continental slope the boundary inhibits cross stream mixing of intermediate water masses and may help explain the appearance of a discrete filament of red sea water inshore of antarctic intermediate waters in the same density layer despite intense cross stream shears

exploiting the potential of an improved multimission altimetric data set over the coastal ocean until now most satellite altimetry studies of the coastal ocean have been based on along track data from a single mission whereas up to four missions were operative in 2002 2005 here to monitor the coastal ocean we have applied specialized corrections and dedicated processing strategies to compute a multimission data set at a mean distance of 32 km of the coast the resulting altimetric data set is compared with sea level data from three in situ stations over a coastal zone of the northwestern mediterranean the mean rms difference between this data set and the sea level stations is 2 9 cm against 3 7 cm when using the aviso altimetric product comparison of altimeter derived geostrophic velocities with a mooring also shows that the spatial and temporal variability of the surface current field is well reproduced the agreement with in situ measurements extends to intraseasonal time scales showing a significant improvement compared to previous studies in the 50 km coastal band

critical frequencies at ocean boundaries past theoretical work has shown that for low frequency flow at a given ocean boundary location a critical frequency c exists such that for c unforced motion is trapped to the coast while for c energy leaves the coast alternatively for motion of a given frequency on the boundary there exists a critical latitude such that the motion is trapped to the coast poleward of that latitude but equatorward it is not this theory is discussed physically for both meridional and nonmeridional boundaries the simple formula for critical frequency along a nonmeridional boundary is used to document critical periods for the eastern and western atlantic pacific and indian ocean boundaries the theory occasionally breaks down when boundary curvature is too large

true to milankovitch glacial inception in the new community climate system model the equilibrium solution of a fully coupled general circulation model with present day orbital forcing is compared to the solution of the same model with the orbital forcing from 115 000 years ago the difference in snow accumulation between these two simulations has a pattern and a magnitude comparable to the ones inferred from reconstructions for the last glacial inception this is a major improvement over previous similar studies and the increased realism is attributed to the higher spatial resolution in the atmospheric model which allows for a more accurate representation of the orography of northern canada and siberia the analysis of the atmospheric heat budget reveals that as postulated by milankovitch s hypothesis the only necessary positive feedback is the snow albedo feedback which is initiated by reduced melting of snow and sea ice in the summer however this positive feedback is almost fully compensated by an increased meridionalheat transportin theatmosphereandareducedconcentrationoflow arcticclouds in contrasttosimilar previous studies the ocean heat transport remains largely unchanged this stability of the northern north atlanticcirculation is explainedby the regulatingeffect of thefreshwaterimportthroughthe naresstrait and northwest passage and the spiciness import by the north atlantic current

an integrated perspective of the continuum between earthquakes and slow slip phenomena slow slip a mechanism by which faults can relieve stress was thought to be distinct from earthquakes however a global review of slow slip phenomena suggests that instead there is a continuum between the two types of event

wind forced interannual variability of the atlantic meridional overturning circulation at 26 5 n the observed atlantic meridional overturning circulation amoc at 26 5 n shows interannual variability consisting of an increase from early 2004 to late 2005 and a following downtrend which reaches a minimum in the winter of 2009 2010 these interannual amoc fluctuations are dominated by changes in the upper mid ocean geostrophic flow except during the winter of 2009 2010 when the anomalous wind driven ekman transport also has a significant contribution the physical mechanisms for the interannual changes of the amoc are proposed and evaluated in a two layer model while the ekman transport is linked to the north atlantic oscillation nao the anomalous geostrophic transport involves the oceanic adjustment to surface wind forcing in particular the intensification and weakening of the southward interior geostrophic flow is modulated by the internal rossby wave adjustment to the surface wind forcing the gulf stream on the other hand is controlled by both topographic waves along the us coast and westward propagating planetary waves our study suggests that a large part of the observed amoc interannual variability at 26 5 n can be explained by wind driven dynamics

satellite altimetry the marine geoid and the oceanic general circulation for technical reasons the general circulation of the ocean has historically been treated as a steady laminar flow field the recent availability of extremely highaccuracy and high precision satellite altimetry has provided a graphic demonstration that the ocean is actually a rapidly time evolving turbulent flow field to render the observations quantitatively useful for oceanographic purposes has required order of magnitude improvements in a number of fields including orbit dynamics gravity field estimation and atmospheric variability with five years of very high quality data now available the nature of oceanic variability on all space and time scales is emerging including new findings about such diverse and important phenomena as mixing coefficients the frequency wavenumber spectrum and turbulent cascades because the surface elevation is both a cause and consequence of motions deep within the water column oceanographers soon will be able to provide general circulation numerical models tested against and then combined with the altimeter data these will be complete three dimensional timeevolving estimates of the ocean circulation permitting greatly improved estimates of oceanic heat carbon and other property fluxes

mechanisms of eddy driven variability of the florida current abstractin this study mechanisms causing year to year changes in the florida current seasonality are investigated using controlled realistic numerical experiments designed to isolate the western b

strain accommodation by slow slip and dyking in a youthful continental rift east africa continental rifts begin and develop through repeated episodes of faulting and magmatism but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented in highly evolved rifts tensile stresses from far field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions1 2 3 these rifting crises are rarely observed in thick lithosphere during the initial stages of rifting here we show that most of the strain during the july august 2007 seismic crisis in the weakly extended natron rift tanzania was released aseismically deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping this event provides compelling evidence for strain accommodation by magma intrusion in addition to slip along normal faults during the initial stages of continental rifting and before significant crustal thinning

quaternary evolution of the corinth rift and its implications for the late cenozoic evolution of the aegean summary we present geological and morphological observations at different scales to constrain rates of faulting and the distribution of deformation in the seismically active aegean region we focus first on the 130 km long corinth rift an asymmetric graben where a flight of terraces of marine origin are uplifted we show that the edges of the terraces lie in the footwall of the normal fault bounding the corinth rift and correspond to sea level highstands of laic pleistocene age using a detailed analysis of aerial and spot imagery supported by field observations we have mapped 10 terrace platforms and strandlines ranging in elevation from 10 to 400 m over distances of 2 to 20 km from the fault the elevation of the terraces inner edges was estimated at 172 sites with an error of 5m this data set contains a precise description of the uplift and flexure of 10 different palaeohorizontal lines with respect to the present sea level to date the deformation we correlate the corinth terraces with late pleistocene oxygen isotope stages of high sea level stands and with global sea level fluctuations using a thick elastic plate model consistent with our current understanding of the earthquake cycle and a boundary element technique we reproduce the geometry of the shorelines to constrain both mechanical parameters and the slip on the fault we show that the seismogenic layer behaves over the long term as if its elastic modulus were reduced by a factor of about 1000 all the terraces are fitted for fault slip increasing in proportion to terrace age and the component of regional uplift is found to be less than 0 3 mm yr 1 the best fits give a slip rate of 11 3 mm yr 1 on the main rift bounding fault over the last 350 kyr other geological and morphologic information allows us to estimate the total age of the main fault 1 ma and to examine the mechanical evolution of the corinth rift the minimum observed sediment thickness in the gulf places an extreme check on the results of the modelling and a lower bound on slip rate of 6 7 mm yr 1 40 per cent less than estimated with modelling even this slip rate is nearly 10 times higher than for comparable features in most of the aegean and elsewhere in the world at a larger scale the spacing and asymmetry of the rift systems in the aegean suggest strain localization in the upper mantle with slow extension starting 15 myr ago or earlier the more recent 1 myr rapid phase of rifting in corinth partly reactivated this earlier phase of extension the younger faulting in corinth appears to result from its present location in the inhomogeneous stress field process zone of the south westward propagating tip of the southern branch of the north anatolian fault we extend these relations to propose a mechanical model for the late cenozoic evolution of the aegean as the arabia europe collision progressed in eastern turkey it caused anatolia to move to the west and the north anatolian fault to propagate into the aegean where the early slow extension started to be modified about 5 ma ago the process of propagation dramatically increased the activity of some but not all of the earlier rifts the model we present is compatible with tectonic observations as well as with the seismicity the palaeomagnetic rotations and the displacement field now observed with gps and slr

scaling of small repeating earthquakes explained by interaction of seismic and aseismic slip in a rate and state fault model because of short recurrence times and known locations small repeating earthquakes present a rare predictable opportunity for detailed field observations they are used to study fault creeping velocities earthquake nucleation stress drops and other aspects of tectonophysics earthquake mechanics and seismology an intriguing observation about repeating earthquakes is their scaling of recurrence time with seismic moment which is significantly different from the scaling based on a simple conceptual model of circular ruptures with stress drop independent of seismic moment and no aseismic slip here we show that a model of repeating earthquakes based on laboratory derived rate and state friction laws reproduces the observed scaling in the model a small fault patch governed by steady state velocity weakening friction is surrounded by a much larger velocity strengthening region long term slip behavior of the fault is simulated using a methodology that fully accounts for both aseismic slip and inertial effects of occasional seismic events the model results in repeating earthquakes with typical stress drops and sizes comparable with observations for a fixed set of friction parameters the observed scaling is reproduced by varying the size of the velocity weakening patch in simulations a significant part of slip on the velocity weakening patches is accumulated aseismically even though the patches also produce seismic events the proposed model supplies a laboratory based framework for interpreting the wealth of observations about repeating earthquakes provides indirect evidence that rate and state friction acts on natural faults and has important implications for possible scenarios of slip partition into seismic and aseismic parts

a simple model of the response of the atlantic to the north atlantic oscillation the response of an idealized atlantic ocean to wind and thermohaline forcing associated with the north atlantic oscillation nao is investigated both analytically and numerically in the framework of a reducedgravity model the nao related wind forcing is found to drive a time dependent leaky gyre circulation that integrates basinwide stochastic wind ekman pumping and initiates low frequency variability along the western boundary thisissubsequentlycommunicated togetherwith the stochasticvariability induced bythermohaline forcing at high latitudes to the remainder of the atlantic via boundary and rossby waves at low frequencies the basinwide ocean heat content changes owing to nao wind forcing and thermohaline forcing are found to oppose each other the model further suggests that the recently reported opposing changes of the meridional overturning circulation in the atlantic subtropical and subpolar gyres between 1950 70 and 1980 2000 may be a generic feature caused by interplay between the nao wind and thermohaline forcing

periodic slow earthquakes from the cascadia subduction zone continuous geodetic measurements from convergent margins have shown that deep transient creep events can release large amounts of strain energy without detectable seismic shaking and they are thus known as slow or silent earthquakes because subduction zones generate the largest earthquakes

seismic and geodetic constraints on cascadia slow slip 1 automatically detected and located tremor epicenters from episodic tremor and slip ets episodes in northern cascadia provide a high resolution map of washington s slow slip region thousands of epicenters from the past four ets events from 2004 to 2008 provide detailed map view constraints that correlate with geodetic estimates of the simultaneous slow slip each of these ets events exhibits remarkable similarity in the timing and geographic distribution of tremor density and geodetically inferred slip analysis of the latest 15 month inter ets period also reveals ageodetic tremor activity similar both in duration and extent to ets tremor epicenters from both ets and inter ets tremor are bounded between the 30 and 45 km plate interface depth contours and locate approximately 75 km east of previous estimates of the locked portion of the subducting juan de fuca plate inter ets tremor overlaps but is generally downdip of ets tremor and does not yet correlate with geodetically observed slip but this is likely because the slip is below current gps detection levels based on the tremor and slip correlation and the tremor duration and slip magnitude relationship we suggest that the well resolved sharp updip edge of tremor epicenters reflects a change in plate interface coupling properties the region updip of this boundary may accumulate stress with the potential for coseismic shear failure during a megathrust earthquake alternatively plate convergence in this region could be accommodated by continuous slow slip with no detectable tremor or by slow slip events with sufficiently long recurrence intervals that none have been detected during the past 10 years of gps observations

seismic cycles and earthquake predictability on east pacific rise transform faults the concept of a seismic cycle where the stress on a fault repeatedly builds up over a long period of time and then is rapidly released in a large earthquake influences studies of both the basic physics of faulting and applied research aimed at estimating earthquake hazards this hypothesis suggests that large earthquakes might be quasi periodic and that the probability of a particular portion of a fault rupturing twice in quick succession should be low however this basic hypothesis has been difficult to verify owing to the long repeat times of the largest earthquakes on most faults east pacific rise epr transform faults are an advantageous location to evaluate the seismic cycle hypothesis owing to their fast slip rates and the moderate size m w 6 of their largest earthquakes using surface wave based determinations of the relative separations between earthquake centroids i document 16 pairs of m w 5 5 events that had overlapping ruptures the distribution of interevent times for these pairs is tightly clustered around 5 yr with a coefficient of variation 0 2 indicating that quasi periodicity may be prevalent for the largest events on these faults moreover i find no pairs of overlapping m w 5 5 6 2 earthquakes separated by less than 50 cm of elapsed plate motion indicating that the two basic features of the seismic cycle hypothesis are evident in the timing of large epr transform mainshocks i have also confirmed earlier results demonstrating a high degree of short term predictability of epr mainshocks by combining teleseismic and hydroacoustic earthquake catalogs thus there appears to be a high degree of both short and long term predictability on epr transforms

monitoring the atlantic meridional overturning circulation the rapid climate change programme rapid has established a prototype system to continuously observe the strength and structure of the atlantic meridional overturning circulation moc at 26 5 n here we provide a detailed description of the rapid moc monitoring array and how it has evolved during the first four deployment years as well as an overview of the main findings so far the rapid moc monitoring array measures 1 gulf stream transport through florida strait by cable and repeat direct velocity measurements 2 ekman transports by satellite scatterometer measurements 3 deep western boundary currents by direct velocity measurements 4 the basin wide interior baroclinic circulation from moorings measuring vertical profiles of density at the boundaries and on either side of the mid atlantic ridge and 5 barotropic fluctuations using bottom pressure recorders the array became operational in late march 2004 and is expected to continue until at least 2014 the first 4 years of observations april 2004 april 2008 have provided an unprecedented insight into the moc structure and variability we show that the zonally integrated meridional flow tends to conserve mass with the fluctuations of the different transport components largely compensating at periods longer than 10 days we take this as experimental confirmation of the monitoring strategy which was initially tested in numerical models the moc at 26 5 n is characterised by a large variability even on timescales as short as weeks to months the mean maximum moc transport for the first 4 years of observations is 18 7 sv with a standard deviation of 4 8 sv the mechanisms causing the moc variability are not yet fully understood part of the observed moc variability consists of a seasonal cycle which can be linked to the seasonal variability of the wind stress curl close to the african coast close to the western boundary fluctuations in the gulf stream and in the north atlantic deep water nadw coincide with bottom pressure variations at the western margin thus suggesting a barotropic compensation ongoing and future research will put these local transport variations into a wider spatial and climatic context

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

earthquake scaling relations for mid ocean ridge transform faults 1 a mid ocean ridge transform fault rtf of length l slip rate v and moment release rate can be characterized by a seismic coupling coefficient ae at where ae v is an effective seismic area and at l3 2v 1 2 is the area above an isotherm tref a global set of 65 rtfs with a combined length of 16 410 km is well described by a linear scaling relation 1 ae at which yields 0 15 0 05 for tref 600 c therefore about 85 of the slip above the 600 c isotherm must be accommodated by subseismic mechanisms and this slip partitioning does not depend systematically on either v or l rtf seismicity can be fit by a truncated gutenberg richter distribution with a slope 2 3 in which the cumulative number of events n0 and the upper cutoff moment mc dcac depend on at data for the largest events are consistent with a self similar slip scaling dc ac1 2 and a square root areal scaling 2 ac at1 2 if relations 1 and 2 apply then moment balance requires that the dimensionless seismic productivity 0 0 atv should scale as 0 at 1 4 which we confirm using small events hence the frequencies of both small and large earthquakes adjust with at to maintain constant coupling rtf scaling relations appear to violate the single mode hypothesis which states that a fault patch is either fully seismic or fully aseismic and thus implies ac ae the heterogeneities in the stress distribution and fault structure responsible for relation 2 may arise from a thermally regulated dynamic balance between the growth and coalescence of fault segments within a rapidly evolving fault zone

bottom dissipation of subinertial currents at the atlantic zonal boundaries 1 estimates of the dissipation of subinertial currents due to bottom boundary layer drag at the eastern and western boundaries of the north atlantic ocean between 15 n and 60 n are computed using data from the world s largest archive of ocean current meter time series we show from these data that a significant proportion of such loss in this region is due to dissipation at the western boundary ocean floor via quadratic bottom boundary layer drag with an estimated 40 60 31 47 gw of the wind input power across the whole basin dissipated by this method we further show that the majority of this dissipation occurs at shallow depths 500 m this has significant implications for the power available for abyssal mixing

atlantic water variability on the se greenland continental shelf and its relationship to sst and bathymetry 1 interaction of warm atlantic origin water aw and colder polar origin water pw advecting southward in the east greenland current egc influences the heat content of water entering greenland s outlet glacial fjords here we use depth and temperature data derived from deep diving seals to map out water mass variability across the continental shelf and to augment existing bathymetric products we compare depths derived from the seal dives with the ibcao version 3 bathymetric database over the shelf and find differences up to 300 m near several large submarine canyons in the vertical temperature structure we find two dominant modes a cold mode with the typical aw pw layering observed in the egc and a warm mode where aw is present throughout the water column the prevalence of these modes varies seasonally and spatially across the continental shelf implying distinct aw pathways in addition we find that satellite sea surface temperatures sst correlate significantly with temperatures in the upper 50 m r 0 54 but this correlation decreases with depth r 0 22 at 200 m and becomes insignificant below 250 m thus care must be taken in using sst as a proxy for heat content as aw mainly resides in these deeper layers sample unit level copyright

buoyancy driven circulation in an ocean basin with isopycnals intersecting the sloping boundary abstract the dynamics that govern the spreading of a convectively formed water mass in an ocean with sloping boundaries are examined using an isopycnal model that permits the interface between the layers to intersect the sloping boundaries the simulations presented here use a two layer configuration to demonstrate some of the pronounced differences in a baroclinically forced flow between the response in a basin with a flat bottom and vertical walls and a more realistic basin bounded by a sloping bottom each layer has a directly forced signal that propagates away from the forcing along the potential vorticity pv contours of that layer paired opposed boundary currents are generated by refracted topographic rossby waves rather than kelvin waves it is impossible to decompose the flow into globally independent baroclinic and barotropic modes topography causes the barotropic i e depth averaged response to buoyancy forcing to be just as strong as the baroclinic response because layer pv contours di

global observations of large oceanic eddies 1 ten years of sea surface height ssh fields constructed from the merged topex poseidon t p and ers 1 2 altimeter datasets are analyzed to investigate mesoscale variability in the global ocean the higher resolution of the merged dataset reveals that more than 50 of the variability over much of the world ocean is accounted for by eddies with amplitudes of 5 25 cm and diameters of 100 200 km these eddies propagate nearly due west at approximately the phase speed of nondispersive baroclinic rossby waves with preferences for slight poleward and equatorward deflection of cyclonic and anticyclonic eddies respectively the vast majority of the eddies are found to be nonlinear

monitoring the transport of the antarctic circumpolar current at drake passage abstract data from an extensive moored instrument array and three hydrographic cruises are used to calculate a time series of the net transport through the upper 2500 m of the drake passage the baroclinic geostrophic transport was monitored with heavily instrumented moorings at each side of the passage pressure gauge measurements at the northern and southern ends of the passage provided a reference speed at 500 m to adjust the relative transport the net transport of the upper 2500 m for the 370 day record ending in january 1980 averaged 121 106 m3 s 1 the total transport through the entire cross sectional area of drake passage is probably between 118 and 146 106 m3 s 1 fluctuations in the relative transport are related to changes in latitude of the northernmost front in the passage the subantarctic front variability of the net transport is coherent with the zonally averaged eastward wind stress

deep water formation the subpolar gyre and the meridional overturning circulation in the subpolar north atlantic on interannual to decadal times scales model simulations suggest a strong relationship between anomalies in the deep water formation rate the strength of the subpolar gyre and the meridional overturning circulation in the north atlantic whether this is valid can only be confirmed by continuous long observational time series several measurement components are already in place but crucial arrays to obtain time series of the meridional volume and heat transport in the subpolar north atlantic are still missing here we summarize the recent developments of the deep water formation rates and the subpolar gyre transports we discuss how existing observational components in the subpolar north atlantic could be supplemented to provide long term monitoring of the meridional heat and volume transport through a combined analysis of observations and model results the temporal and spatial scales that had to be covered with instruments are discussed together with the key regions with the highest variability in the velocity and temperature fields

vertical structure of midlatitude mesoscale instabilities abstract instability processes in frontal jet regions outside the western boundary currents are examined for their ability to produce mesoscale variability comparable to that observed in the open ocean the important of sufficient numerical resolution for a correct modeling of these processes is considered using a local plane quasi geostrophic multilevel model with periodic horizontal boundary conditions the combined effects of special density and current profiles typical for the midlatitude eastern north atlantic are studied the linearized approach yields a set of vertical shear modes with different vertical structure depending on the zonal wavelength of the perturbation some of these shear modes are unstable the mesoscale range 50 500 km reveals two different types a surface intensified shear mode and a deep sea intensified one the growth rates of the former are usually largely exceeding those of the latter due to the larger velocities and shears their exponential amplification time scale ra

rossby rip currents oceanic rossby waves and eddies flux energy and fluid westward the latter through the stokes drift or bolus transport while the wave energy is largely dissipated at the western boundary mass conservation requires that the fluid be returned offshore through rossby rip currents the form and magnitude of these rip currents are investigated through linear rossby wave theory a nonlinear numerical model and analysis of sea surface height satellite observations the net eastward volume transport by rossby rip currents over the global ocean is estimated to be of order 10 sv 1 sv 106 m3 s 1 in an eddying ocean both the westward stokes drift and eastward rip currents can assume the form of banded quasi zonal jets

a means of estimating the intrinsic and atmospherically forced contributions to sea surface height variability applied to altimetric observations drawing on a 50 member ocean ensemble hindcast the magnitude and characteristic temporal and spatial scales of intrinsic and forced sea surface height ssh variability are evaluated over a 37 year period the intrinsic and forced contributions derived from the ensemble are found to have similar temporal spectra but different characteristic spatial scales these results suggest that with an appropriate choice of cutoff scales simple spatial filtering can be used to estimate the forced and intrinsic contributions given either a single model run or an observational data set the method is tested using a single member drawn from the ensemble before being applied to the observed altimetric record two sample applications with relevance to large scale climate are used to illustrate the method s potential utility firstly the long term trends calculated from the total and recreated forced components using the altimetric record are compared and local differences highlighted second the recreated forced ssh is shown to covary with the north atlantic oscillation at seasonal time scales in regions where no such influence can be found using the original ssh signal some limitations and uses for which the method may prove unsuitable are also briefly considered

western boundary current structure and variability east of abaco bahamas at 26 5 n abstract during april 1987 to june 1988 a four mooring transport resolving current meter array was deployed in a section extending 70 km eastward from abaco bahamas at 26 5 n mean currents in the upper layer 800 m showed a clockwise rotation that appears to be associated with a small scale quasi permanent anticyclonic gyre centered just northeast or the bahamas deep mean currents were persistenly southward and indicated a strong deep jet the deep western boundary current dwbc reaching core speeds of 20 cm s 1at 2500 m depth about 25 km seaward of the western boundary vertically integrated meridional volume transport over the section showed a surprisingly large variability ranging from approximately 20 sv northward to 70 sv southward 1 sv 106m3s 1 the mean meridional volume transport was 30 sv to the south with about 3 sv flowing northward above 800 m which could be produced by a weak antilles current and 33 sv flowing southward below 800 m the deep southward transport of 33 sv i

the kuroshio east of taiwan moored transport observations from the woce pcm 1 array observations from the woce pcm 1 moored current meter array east of taiwan for the period september 1994 to may 1996 are used to derive estimates of the kuroshio transport at the entrance to the east china sea three different methods of calculating the kuroshio transport are employed and compared these methods include 1 a direct method that uses conventional interpolation of the measured currents and extrapolation to the surface and bottom to estimate the current structure 2 a dynamic height method in which moored temperature measurements from moorings on opposite sides of the channel are used to estimate dynamic height differences across the current and spatially averaged baroclinic transport profiles and 3 an adjusted geostrophic method in which all moored temperature measurements within the array are used to estimate a relative geostrophic velocity field that is referenced and adjusted by the available direct current measurements the first two methods are largely independent and are shown to produce very similar transport results the latter two methods are particularly useful in situations where direct current measurements may have marginal resolution for accurate transport estimates these methods should be generally applicable in other settings and illustrate the benefits of including a dynamic height measuring capability as a backup for conventional direct transport calculations the mean transport of the kuroshio over the 20 month duration of the experiment ranges from 20 7 to 22 1 sv 1 sv 106 m3 s 1 for the three methods or within 1 3 sv of each other the overall mean transport for the kuroshio is estimated to be 21 5 sv with an uncertainty of 2 5 sv all methods show a similar range of variability of 10 sv with dominant timescales of several months fluctuations in the transport are shown to have a robust vertical structure with over 90 of the transport variance explained by a single vertical mode the moored transports are used to determine the relationship between kuroshio transport and sea level difference between taiwan and the southern ryukyu islands allowing for long term monitoring of the kuroshio inflow to the east china sea

surface estimates of the atlantic overturning in density space in an eddy permitting ocean model 1 a method to estimate the variability of the atlantic meridional overturning circulation amoc from surface observations is investigated using an eddy permitting ocean only model orca 025 the approach is based on the estimate of dense water formation from surface density fluxes analysis using 78 years of two repeat forcing model runs reveals that the surface forcing based estimate accounts for over 60 of the interannual amoc variability in 0 coordinates between 37 n and 51 n the analysis provides correlations between surface forced and actual overturning that exceed those obtained in an earlier analysis of a coarser resolution coupled model our results indicate that in accordance with theoretical considerations behind the method it provides a better estimate of the overturning in density coordinates than in z coordinates in subpolar latitudes by considering shorter segments of the model run it is shown that correlations are particularly enhanced by the method s ability to capture large decadal scale amoc fluctuations the inclusion of the anomalous ekman transport increases the amount of variance explained by an average 16 throughout the north atlantic and provides the greatest potential for estimating the variability of the amoc in density space between 33 n and 54 n in that latitude range 70 84 of the variance is explained and the root mean square difference is less than 1 sv when the full run is considered

a theory for the surface atlantic response to thermohaline variability abstract the response of the upper warm limb of the thermohaline circulation in the north atlantic to a rapid change in deep water formation at high latitudes is investigated using a reduced gravity ocean model changes in deep water formation rate initiate kelvin waves that propagate along the western boundary to the equator on a timescale of months the response in the north atlantic is therefore rapid the southern hemisphere response is much slower limited by a mechanism here termed the ldquo uatorial buffer since to leading order the flow is in geostrophic balance the pressure anomaly decreases in magnitude as the kelvin wave moves equatorward where the coriolis parameter is lower together with the lack of sustained pressure gradients along the eastern boundary this limits the size of the pressure field response in the southern hemisphere interior adjustment is by the westward propagation of rossby waves but only a small fraction of the change in thermohaline circulation strength is commun

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

seismicity of oceanic and continental rifts a geodynamic approach abstract two major kinds of divergent structures oceanic and intracontinental rifts were compared in principal seismic and tectonic characteristics first the role of main components of the mid oceanic ridges mor was estimated for the whole earth we considered two levels of the mor segmentation the first order structures are the segments of mor between triple junctions and the second order structures are a transform faults and rift parts of mor the seismic catalogues neic and cmt were used to assess the seismic moment release the seismic moment release was calculated another way using the global plate tectonic model nuvel 1 and brune s formulae comparison of these two values shows that the seismic coupling coefficient varies from 1 to 10 for most of mor and is always higher for transform faults most of the deformation therefore is aseismic slip most seismicity of mor is confined to transform faults the energy contribution of transform faults is one to two orders magnitude higher than that of the rift and increases with the spreading rate there is a strong correlation between the seismic moment release of strike slip faults and their total lengths the correlation shows that the seismic moment release depends on the total transform area and confirms the simple thermal model of transform seismicity that was given by burr and solomon the seismic moment release and the spreading rate have opposite patterns for the rifts there is an inverse correlation between the seismic moment of normal faults and spreading velocity while it seems for transforms that these parameters are independent finally these results show that the seismicity of transforms and rifts depends first of all on the thermal structure of oceanic lithosphere in the case of continental rifts one can distinguish in the degree of seismic activity depending on the stage of rifting hence analysis of the continental seismicity requires the consideration of factors of a geological evolution that play practically no role in the case of oceanic lithosphere the comparison of geological and seismic data for the east african region has allowed us to outline the regular changes of the seismic regime during development of the rift zone from the stage of incipient rift to mature oceanic rift in the evolutionary series intracontinental incipient rift intracontinental mature rift intercontinental rift oceanic slow spreading rift oceanic fast spreading rift there is a gradual decrease of the role of rifts sensu stricto as tension structures and increase of the role of strike slip faults epicenters concentrate along major faults as well the level of seismic energy becomes lower although the rate of deformation increases

temporal variability of the atlantic meridional overturning circulation at 26 5 n the vigor of atlantic meridional overturning circulation moc is thought to be vulnerable to global warming but its short term temporal variability is unknown so changes inferred from sparse observations on the decadal time scale of recent climate change are uncertain we combine continuous measurements of the moc beginning in 2004 using the purposefully designed transatlantic rapid climate change array of moored instruments deployed along 26 5 n with time series of gulf stream transport and surface layer ekman transport to quantify its intra annual variability the year long average overturning is 18 7 5 6 sverdrups sv range 4 0 to 34 9 sv where 1 sv a flow of ocean water of 106 cubic meters per second interannual changes in the overturning can be monitored with a resolution of 1 5 sv

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses

seasonal variations in the south atlantic meridional overturning circulation from observations and numerical models monthly climatologies of temperature and salinity from observations and numerical models are used to estimate the atlantic meridional overturning circulation amoc at 34 s observational estimates suggest that the geostrophic transport plays an equal role to the ekman transport in the amoc seasonal variations at this latitude whereas in the models the ekman transport controls the amoc seasonality the seasonality of the geostrophic transport from observations is largely controlled by the seasonal density variations at the western boundary but in the models the eastern boundary dominates the observed density seasonality at the western boundary is linked to the intensity of the malvinas current which is poorly reproduced in the models the results indicate that the weak seasonal cycle in the model geostrophic transport can primarily be attributed to excessively strong baroclinicity below the surface mixed layer whereas the observations show a strong vertical coherence in the velocity down to 1200 m