crustal structure and surface zonation of the canadian appalachians implications of deep seismic reflection data in 1986 1181 km of marine seismic reflection data was collected to 18 20 s of two way traveltime in the gulf of st lawrence area the seismic profiles sample all major surface tectono stratigraphic zones of the canadian appalachians they complement the 1984 deep reflection survey northeast of newfoundland together the seismic profiles reveal the regional three dimensional geometry of the orogen three lower crustal blocks are distinguished on the seismic data they are referred to as the grenville central and avalon blocks from west to east the grenville block is wedge shaped in section and its subsurface edge follows the form of the appalachian structural front the grenville block abuts the central block at mid crustal to mantle depths the avalon block meets the central block at a steep junction that penetrates the entire crust consistent differences in the seismic character of the moho help identify boundaries of the deep crustal blocks the moho signature varies from uniform over extended di

antarctic ozone depletion causes an intensification of the southern ocean super gyre circulation 1 recent climate trends over the southern hemisphere sh summer feature a strengthening of the circumpolar westerly and a weakening of the midlatitude westerly extending from the stratosphere to earth s surface much of the change is attributable to antarctic ozone depletion however the consequential ocean circulation changes are unknown here i demonstrate that the observed surface wind changes have forced a southward shift and spin up of the super gyre which links the subtropical south pacific indian and atlantic ocean circulation advecting more warm water southward the circulation change includes a strengthening of the east australian current eac flow passing through the tasman sea the southward shift may be responsible for the observed unusually large warming in the sh midlatitude ocean and may contribute to the reported range extension to the south of many marine species in the south west pacific

variability and forcing of the east australian current 1 the spatial and temporal variability of the east australian current eac is investigated using 6 years 1993 1998 of surface geostrophic stream function from an optimal interpolation of altimeter sea surface heights and velocities derived from tracking thermal features in satellite imagery variability appears as a series of cyclones and anticyclones propagating southwestward and westward with periods between 90 and 180 days the behavior of the variability changes over the 6 years energy in the mesoscale frequencies moves slowly south and diminishes with more westward propagation in the region where the current separates from the coast we find no evidence for a consistent forcing of the eac by mesoscale signals propagating westward from the south pacific basin we suggest that the observations are consistent with variability originating between 32 s and 35 s through intrinsic instabilities of the current

characterizing frontal eddies along the east australian current from hf radar observations the east australian current eac dominates the ocean circulation along south eastern australia however little is known about the submesoscale frontal instabilities associated with this western boundary current one year of surface current measurements from hf radars in conjunction with mooring and satellite observations highlight the occurrence and propagation of meanders and frontal eddies along the inshore edge of the eac eddies were systematically identified using the geometry of the high spatial resolution 1 5 km surface currents and tracked every hour cyclonic eddies were observed irregularly on average every 7 days with inshore radius 10 km among various forms of structures frontal eddies associated with eac meanders were characterized by poleward advection speeds of 0 3 0 4 m s migrating as far as 500 km south based on satellite imagery flow field kinematics show that cyclonic eddies have high rossby numbers 0 6 1 9 and enhance particle dispersion patches of intensified surface divergence at the leading edge of the structures are expected to generate vertical uplift this is confirmed by subsurface measurements showing temperature uplift of up to 55 m over 24 h and rough estimates of vertical velocities of 10s of meters per day while frontal eddies propagate through the radar domain independently of local wind stress upfront wind can influence their stalling and growth and can also generate large cold core eddies through intense shear such coherent structures are a major mechanism for the transport and entrainment of nutrient rich coastal or deep waters influencing physical and biological dynamics and connectivity over large distances

paleozoic laurentia gondwana interaction and the origin of the appalachian andean mountain system laurentia the rift bounded precambrian nucleus of north america may have broken out from a neoproterozoic supercontinent between east and west gondwana several lines of evidence suggest that the appalachian margin of laurentia subsequently collided with the proto andean margin of the amalgamated gondwana supercontinent in different relative positions during early and mid paleozoic time in route to final docking against northwest africa to complete the assembly of pangea hence the appalachian and andean orogens may have originated as a single mountain system the overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the paleozoic era laurentia gondwana collisions may help to explain contemporaneous unconformities in the paleozoic sedimentary cover of the laurentian gondwanan and baltic cratons

nature and evolution of the eastern margin of lapetus geochemical and isotopic constraints from siluro devonian granitoid plutons in the new brunswick appalachians in the gander zone ordovician to devonian metaluminous to slightly peraluminous plutons with within plate affinities intrude a lower paleozoic metasedimentary package during the siluro devonian end t and 18o values of the metasedimentary rocks would have been about 7 to 8 and 10 to 14 respectively granitoid end t values range from 0 3 to 4 2 18o values lie between 6 6 and 10 4 and published pb isotopic compositions are radiogenic 207pb 204pb 15 65 end t values of associated silurian gabbros are somewhat positive 1 2 to 2 3 some plutons exhibit evidence for comingling with this mafic component potential source materials include enriched mantle supracrustal and infracrustal sources although some constraints can be put on the first two sources the nature of infracrustal material is difficult to evaluate heterogeneous data obtained from silurian plutons can be explained by crust mantle mixing if some plutons contain a significant mantle component then mesoproterozoic o

decadal variability of east australian current transport inferred from repeated high density xbt transects a ctd survey and satellite altimetry a time series of the net geostrophic transport through the tasman sea representing the flow of the east australian current eac extension is determined from a full depth ctd section 15 years of high density xbt transects and satellite altimetry data a section between sydney and wellington px34 has been occupied four times per year since 1991 with high resolution xbt sampling two methods to infer baroclinic transport from proxy data along the section are presented the first uses shallow xbt transects to derive geostrophic transport relative to a deep 2000 m reference level in the second approach synts the subsurface temperature and salinity structure are inferred from satellite surface height and temperature fields using a model developed from historical in situ observations the baroclinic transport is then computed in the usual manner the methods are validated using both a full depth ctd occupation of the px34 section and further transects crossing the eac in the northern tasman sea there is close agreement between the 49 xbt and synts px34 transport estimates obtained between 1992 and 2006 the time series of transport through the sydney wellington section shows a range of temporal signals from eddyscale seasonal interannnual to decadal in particular we note that the net eac flow ranges from 5 sv in 1995 to a maximum of 16 sv in 2000 2001 this decadal variation confirms the eac response to a spin up of the south pacific circulation forced by changes in the basin wide winds and matches the changes in oceanic properties observed in the tasman sea

granite petrogenesis in the gander zone ne newfoundland mixing of melts from multiple sources and the role of lithospheric delamination silurian to devonian granites within the gander zone of the appalachian central mobile belt in northeastern newfoundland formed adjacent to the former gondwanan continental margin following terminal closure of the iapetus ocean comparison of geochemical and isotopic characteristics of the granites with their host migmatites and metasediments amphibolite and orthogneiss constrain their potential source nd and sr isotopic compositions indicate that no single source or binary mixing product could have produced the granites instead we show that they result from multicomponent mixing involving a contribution from unexposed crystalline basement mantle or underplate and variable contamination by supracrustal host rocks the timing and composition of granite magmatism do not exhibit collisional orogenic subduction related or continental rifting characteristics hence we relate magmatism to lithospheric melting following delamination of an orogenic keel this process provides the influx of mantle derived magma into fertile crust and hence promotes lower crustal melting and primary magma mixing

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

modes of mesoscale sea surface height and temperature variability in the east australian current 1 mesoscale variability where the east australian current eac separates from the coast is studied using sea surface temperature and surface velocity streamfunction observed by satellite and a regional numerical model the mean circulation simulated by the model the regional ocean modeling system roms is compared to a high resolution regional climatology and the realism of the simulated mesoscale variability is tested by comparison to statistical analyses of the satellite data both roms and data show spectral peaks in the mesoscale energy band at periods between 90 and 180 days complex empirical orthogonal function eof analysis identifies two significant modes of mesoscale variability in the data an eddy mode for which the variability propagates southwestward along the coast and a wave mode for which phase propagation is predominantly onshore the regional model open boundary conditions include only annual and semiannual harmonics of variability so remote mesoscale forcing is absent the eddy mode is represented well in the model indicating this aspect of the circulation results from local instabilities of the flow and that its underlying dynamical process is simulated well while the observed and modeled wave modes have some similarities their differences suggest the model is deficient in representing westward propagation of mesoscale period variability in the region whatever the source of this energy the orthogonality property of the eof analysis indicates the wave mode does not interact significantly with eddy processes in the eac separation

east australian current volume transports at 30 s estimates from the world ocean circulation experiment hydrographic sections pr11 p6 and the pcm3 current meter array between september 1991 and march 1994 the oceanic region off the east coast of australia at 30 s was the subject of an intense observational program part of the world ocean circulation experiment the objective was to improve our understanding of the dynamics of the east australian current eac and to measure its volume and energy flux the two main components were the pacific current meter array 3 pcm3 with six moorings spanning a total distance of 120 km and repeat high resolution hydrographic surveys the long term average frompcm3 shows the eac as a narrow swift and highly variable current centered at 40 km from the coast with strong shear in the upper 1000 m and mean southward velocities of 0 6 m s 1 in the core the measurements also revealed a thick countercurrent underneath the poleward flow the mean volume transport for the entire section covered by the array between the coast and 154 4 e was 22 1 4 6 sv toward the south with an rms variability of 30 sv combining the data from the pcm3 array with the hydrographic sections generated nine detailed snapshots of the absolute geostrophic current field the snapshots were used to evaluate the reliability of the pcm3 data for determining volume transports the rms difference between transports derived from direct current observations and from hydrographie data was 3 8 sv indicating that the pcm3 array can be used to describe the current variability despite the sparse spatial distribution of velocity measurements transport variability was dominated by periods between 90 and 140 days which are attributed to eddy shedding in the region occasional intense northward flow events were observed where total transports reached up to 50 sv northward on those occasions the southward boundary current changed from being surface intensified and mainly baroclinic to northward and more barotropic related to the northward migration past the array of the eac coastal separation point

a numerical modeling study of the east australian current encircling and overwashing a warm core eddy 1 warm core eddies wces often form in the meanders of western boundary currents wbcs wces are frequently overwashed with less dense waters sourced from the wbc we use the regional ocean modelling system to investigate the ocean state during the overwashing of one such wce in october 2008 in the east australian current eac comparisons of model outputs with satellite sea surface temperature and vertical profiles show that the model provides a realistic simulation of the eddy during the period when the eac encircled and then overwashed the eddy during the encircling stage an eddy with closed circulation persisted at depth in the surface eac water entered from the north encircled the eddy and exited to the east the overwashing stage was initiated by the expulsion of cyclonic vorticity for the following 8 days after the expulsion waters from the eac washed over the top of the eddy transferring heat and anticyclonic vorticity radially inward after approximately one rotation period of overwashing the eddy separated the overwashing creates a two layer system that forms a subsurface maximum velocity at the interface of the two layers analysis of water mass properties eulerian tracer dynamics and lagrangian particle tracks show that the original eddy sinks 10 50 m during the overwashing period overwashing has been observed in many wbcs and occurs in most wces in the western tasman sea

the east australian current and property transport at 27 s from 2012 to 2013 abstractthe east australian current eac is the complex and highly energetic poleward western boundary current of the south pacific ocean a full depth current meter and property temperature and salinity mooring array was deployed from the continental shelf to the abyssal waters off brisbane australia 27 s for 18 months from april 2012 to august 2013 the eac mooring array is an essential component of the australian integrated marine observing system imos during this period the eac was coherent with an eddy kinetic to mean kinetic energy ratio of less than 1 the 18 month mean poleward only mass transport above 2000 m is 22 1 7 5 sverdrups sv 1 sv 106 m3 s 1 the mean poleward only heat transport and flow weighted temperature above 2000 m are 1 35 0 42 pw and 15 33 c respectively a difference in the poleward only and net poleward mass and heat transports above 2000 m of 6 3 sv and 0 24 pw reflects the presence of an equatorward eac retroflection at the eastern offshore end of the

sm nd studies of archaean metasediments and metavolcanics from west greenland and their implications for the earth s early history metasedimentary and metavolcanic rocks from the archaean of west greenland have been examined for evidence of crustal components greater than 3 8 ga in age and for their compatibility with the presently adopted bulk earth sm nd parameters sm nd isotopic data have been obtained for the garbenschiefer metagabbro unit metasediments from the isua supracrustal belt gneisses interior to the isua belt and metasediments from the malene supracrustal belt using estimates of emplacement age t of between 3 77 and 3 67 ga for the parental volcanics to the garbenschiefer unit initial143nd 144nd ratios yield positiveendt values between 1 0 and 3 1 relative to the chur parameters for seven out of eight samples model sm nd ages for the isua gneisses and metasediments are only compatible with their estimated stratigraphic ages if their sources were ca 2end relative to chur at those times similarly model sm nd ages for the malene samples are only compatible with stratigraphic age constraints when based on a source evolution with positiveendt implications of these results for the early development of the earth s mantle are discussed

mechanisms of the 40 70 day variability in the yucatan channel volume transport the yucatan channel connects the caribbean sea with the gulf of mexico and is the main outflow region of the caribbean sea moorings in the yucatan channel show high frequent variability in kinetic energy 50 100 days and transport 20 40 days but the physical mechanisms controlling this variability are poorly understood in this study we show that the short term variability in the yucatan channel transport has an upstream origin and arises from processes in the north brazil current to establish this connection we use data from altimetry and model output from several high resolution global models a significant 40 70 day variability is found in the sea surface height in the north brazil current retroflection region with a propagation toward the lesser antilles the frequency of variability is generated by intrinsic processes associated with the shedding of eddies rather than by atmospheric forcing this sea surface height variability is able to pass the lesser antilles it propagates westward with the background ocean flow in the caribbean sea and finally affects the variability in the yucatan channel volume transport

cyclonic and anticyclonic motion in the upper ocean 1 upper ocean variability is highly energetic and contributes to key processes such as heat transport and water mass formation here the distribution of ocean surface cyclonic and anticyclonic motion is computed from global drifter observations for scales from large eddies to submesoscale two zonal bands of small scale motion are recovered a known anticyclonic band at 30 40 latitude mostly wind induced and an unexpected cyclonic band at 10 20 latitude it is suggested that this is due to submesoscale processes related to salinity front instabilities these results provide a first global view of the upper ocean including these motions

geochemical and isotopic characteristics of granitoids of the avalon zone southern new brunswick possible evidence for repeated delamination events nd o and pb isotope data from southern new brunswick suggest that 625 to 360 ma granitoid plutons contain a major juvenile component and small amounts of middle proterozoic or older crustal material plutons fall into four major age groups 600 630 540 560 422 430 and 367 396 ma the first and third groups can be related to subduction in other parts of the appalachian orogen but most of the second and fourth magmatic pulses have lithologic associations and chemistry not readily reconciled to subduction the timing and volume of magmatism presence of mafic end members and juvenile signatures in the plutons suggest that crustal delamination may have been involved little evidence exists for extensive middle proterozoic or older continental crust beneath the avalon zone which consists mainly of late proterozoic continental margin arc material intruded by younger granitoid plutons that include a major mantle component this differs from contiguous gander and meguma zones in which plutons contain a ma

oceanic turbulence and stochastic models from subsurface lagrangian data for the northwest atlantic ocean the historical dataset provided by 700 m acoustically tracked floats is analyzed in different regions of the northwestern atlantic ocean the goal is to characterize the main properties of the mesoscale turbulence and to explore lagrangian stochastic models capable of describing them the data analysis is carried out mostly in terms of lagrangian velocity autocovariance and cross covariance functions in the gulf stream recirculation and extension regions the autocovariances and cross covariances exhibit significant oscillatory patterns on time scales comparable to the lagrangian decorrelation time scale they are indicative of sub and superdiffusive behaviors in the mean spreading of water particles the main result of the paper is that the properties of lagrangian data can be considered as a superposition of two different regimes associated with looping and nonlooping trajectories and that both regimes can be parameterized using a simple first order lagrangian stochastic model with spin parameter v the spin couples the zonal and meridional velocity components reproducing the effects of rotating coherent structures such as vortices and mesoscale eddies it is considered as a random parameter whose probability distribution is approximately bimodal reflecting the distribution of loopers finite v and nonloopers zero v this simple model is found to be very effective in reproducing the statistical properties of the data

quantitative mapping of the east australian current encroachment using time series himawari 8 sea surface temperature data the east australian current eac is a highly dynamic western boundary current that exerts significant influences on the marine ecosystem off south east margin of australia for the first time a quantitative mapping of the eac system was conducted using himawari 8 sea surface temperature data and a topographic position index tpi based image processing technique the validation using bluelink reanalysis data suggested good reliability of our mapping results subsequent quantitative examination provides new insights into the eac s cross shelf movement which is a driving force of adjacent slope shelf circulations along the coast of new south wales nsw large scale and high frequency eac encroachment was observed being every 60 80 days upstream 30 32 s and every 90 100 days downstream 33 35 s which is associated with the eac s intrinsic oscillation and eddy shedding downstream the eac encroachment exhibits a 20 day longer period and a double amplitude such dephasing is most likely due to the abrupt change of regime at the eac separation point 32 33 s upstream the eac encroachment also exhibits seasonality being 10 15 km closer to the coast in austral summer which is due to the seasonal eac broadening higher frequency 16 32 days and smaller magnitude eac intrusion was observed along the entire nsw coast 28 37 s which is possibly associated with eac s meanders and frontal eddies in the extension zone 37 30 44 s we observed maximum eac encroachment in summer which is an expression of the seasonal boundary flow off eastern tasmania a dataset of eac encroachment 2015 2017 is available see data availability

the fraser gyre a cyclonic eddy off the coast of eastern australia this paper examines the on shelf circulation of the eastern australian continental shelf for a region off southeast queensland we identify a characteristic seasonally reoccurring wind driven cyclonic flow it influences the cross shelf exchange with the east australian current eac which is the western boundary current of the south pacific ocean we refer to this cyclonic circulation as the fraser gyre it is located south of fraser island between about 25 s and 27 s the region is adjacent to the intensification zone of the eac where the current accelerates and establishes a swift albeit seasonally variable southward boundary flow through the analysis of several data sets including remotely sensed sea surface temperature and sea surface height anomaly satellite tracked surface drifters ocean and atmospheric reanalysis data as well as geostrophic currents from altimetry we find that the on shelf fraser gyre develops during the southern hemisphere autumn and winter months the gyre is associated with a longshore near coast northward flow maximum northward on shelf depth averaged velocities are estimated with about 0 15 0 26 ms 1 the flow turns eastward just to the south of fraser island and joins the persistent southward eac flow along the shelf break the annual mean net cross shelf outward and inward flow associated with the gyre is about 1 17 0 23 sv in the north and 0 23 0 13 sv 1 sv 106 m3s 1 in the south mean seasonal water renewal time scales of the continental shelf are longest during austral winter with an average of about 3 3 days due to the fraser gyre retaining water over the shelf however monthly estimates range from 2 to 8 days with the longer timescale during the austral autumn and winter the southerly wind during austral autumn and winter is identified as controlling the on shelf circulation and is the principal driver of the seasonally appearing fraser gyre the conceptual model of the fraser gyre is consistent with general physical principals of the coastal shelf circulation a southerly wind is associated with surface layer flow toward the coast a near coast positive ssha with a current in the direction of the wind down welling and export of shelf water the fraser gyre influenced cross shelf exchanges are possibly facilitating the offshore transport of fish larvae sediments nutrients river discharges and other properties across the shelf break and into the southward flowing eac during the austral autumn and winter

an avenue of eddies quantifying the biophysical properties of mesoscale eddies in the tasman sea 1 the tasman sea is unique characterised by a strong seasonal western boundary current that breaks down into a complicated field of mesoscale eddies almost immediately after separating from the coast through a 16 year analysis of tasman sea eddies we identify a region along the southeast australian coast which we name eddy avenue where eddies have higher sea level anomalies faster rotation and greater sea surface temperature and chlorophyll a anomalies the density of cyclonic and anticyclonic eddies within eddy avenue is 23 and 16 higher respectively than the broader tasman sea we find that eddy avenue cyclonic and anticyclonic eddies have more strongly differentiated biological properties than those of the broader tasmansea as a result of larger anticyclonic eddies formed from coral sea water depressing chl a concentrations and for coastal cyclonic eddies due to the entrainment of nutrient rich shelf waters cyclonic eddies within eddy avenue have almost double the chlorophyll a 0 35 mg m 3 of anticyclonic eddies 0 18 mg m 3 the average chlorophyll a concentration for cyclonic eddies is 16 higher in eddy avenue and 28 lower for anticyclonic eddies when compared to the tasman sea with a strengthening east australian current the propagation of these eddies will have significant implications for heat transport and the entrainment and connectivity of plankton and larval fish populations citation everett j d m e baird p r oke and i m suthers 2012 an avenue of eddies quantifying the biophysical properties of mesoscale eddies in the tasman sea geophys res lett 39 l16608 doi 10 1029 2012gl053091

eddy shedding and energy conversions in the east australian current 1 mesoscale variability and eddy shedding in the tasman sea particularly of the east australian current eac is studied through the analysis of remotely sensed observations and outputs from a global ocean model previous observations of the western boundary current separation from the coast showed strong variability at periods ranging between 90 and 140 days we show from satellite altimetric observations that rapid northward migration of the separation point of the eac follows the formation of large eddies at periods of 100 days after an eddy separation event the normally southward flowing current swiftly assumes a more zonal configuration near the separation latitude with a cyclonic circulation developing inshore the formation of large separation eddies is preceded by the southward propagation of sea level anomalies along the east australian continental slope from 25 s sea level anomalies grow as they travel south eventually being pinched off in the form of large anticyclones at 32 s in the current retroflection area energy conversion terms in a global ocean model and in altimetric data suggest both barotropic and baroclinic instability may account for the growth of these anomalies as they propagate south east of the main eac jet there is evidence that eddies may be feeding potential energy back to the mean flow

sm nd isotopic geochemistry of precambrian to paleozoic granitoid suites and the deep crustal structure of the southeast margin of the newfoundland appalachians in the eastern central mobile belt of the newfoundland appalachians late precambrian basement inliers have end from 3 to 2 but cambro ordovician metasedimentary rocks have initial end below 7

on the cross shelf exchange driven by frontal eddies along a western boundary current during austral winter 2007 western boundary currents are characterized by high mesoscale eddy activity including that of cyclonic frontal eddies cfes these form frequently adjacent to the shelf break export coastal water and affect biological productivity here we study the physical properties of eddies identified within the intensification zone of the east australian current eac through the analysis of remotely sensed data the application of an ocean circulation model and the evaluation of lagrangian simulations during austral winter 2007 we examine the spatial and temporal evolution of identified eddies investigate their formation mechanism and quantify the contribution made to cross shelf exchanges the mesoscale eddies are tracked from generation to decay using an eddy detection and tracking method in this case study we identify two cfes referred to as c1 and c2 with lifetimes of 11 and 38 days and radii of 45 and 70 km respectively and one anticyclonic eddy ace with a lifetime of 62 days and a radius of 60 km both cfes interact with the ace leading to quasi stationary dipole eddies located in the vicinity of the shelf this is the first time dipole eddies are reported for the intensification zone of the eac the location of cfes characterised by negative sea surface height anomaly ssha and clockwise rotation coincides with remotely sensed sea surface temperature sst and chlorophyll a chl a anomalies cfes c1 and c2 appear to contribute in different ways to the cross shelf transport along the shelf break cfe c1 drives a stronger onshore transport while cfe c2 appears to enhance the offshore transport the estimated daily cross shelf export of shelf water associated with cfe c2 is estimated with about 1 23 sv 106 km3 day 1 it renews the regional shelf water in about 7 days the lifetime of cfe c2 is about 38 days thus the eddy had the potential to flush the shelf at least five times the transient cfes were found for the first time to intensify the cross shelf exchange that is due to the wind driven cyclonic circulation of the fraser gyre this seasonally occurring gyre dominates the local shelf during austral autumn and winter the results from this study demonstrate the importance of cfes in exporting cooler and chl a enriched shelf water into the warmer and oligotrophic waters of the southward flowing eac

influence of a western boundary current on shelf dynamics and upwelling from repeat glider deployments the separation zone of a dynamically important western boundary current wbc is resolved through a series of sustained glider deployments along the coastal edge of the jet the comprehensive data set from 23 missions 2008 2014 provides a new high resolution hydrographic climatology which is exploited to understand the spatial extent of dense water uplift and the depth averaged momentum balances across the east australian current eac separation zone the predominantly geostrophic shelf circulation and temperature fields are least most variable upstream downstream where encroachment separation dominates for the first time we resolve the nonlinear advection terms which are considerable in the along shelf momentum balance near bottom water masses indicate dense water uplift as a result of the eac encroachment and separation the data provide both new insight into and a climatology of separation induced uplift and demonstrate a successful model for repeat glider missions in a dynamic wbc environment

granite magma generation ascent and emplacement within a transpressional orogen crustal thickening during transpressive orogenesis may produce anatectic granites which may then localize deformation leading to transcurrent movement granites may be transported from sites of generation through the mid crust in dyke like channelways within relatively narrow strike slip shear zones which link to major fault zones in the upper crust extensional jogs within fault systems provide developing sites for the assembly of plutons from magma arriving from below the model is based upon observations from the cadomian belt of nw france which exposes sections through middle and upper levels of the late precambrian crust within different elements of the orogen the mechanism provides a favourable alternative to diapirism and explains the common collocation of granites and shear zone fault systems within orogenic belts

the response of the southern annular mode the east australian current and the southern mid latitude ocean circulation to global warming 1 climate models predict an upward trend of the southern annular mode sam in response to increasing atmospheric co2 concentration however the consequential impact of this change on oceanic circulation has not been explored here we analyse the outputs of a series of global warming experiments from the csiro mark 3 climate model we show that although for the zonal wind stress change the maximum is located at approximately 60 s in terms of the change in surface wind stress curl the maximum is situated at approximately 48 s this change in the wind stress curl causes a spin up of the entire southern midlatitude ocean circulation including a southward strengthening of the subtropical gyres particularly the east australia current eac the intensified eac generates a warming rate in the tasman sea that is the greatest in the southern hemisphere sh with significant implications for sea level rise the pan southern ocean scale suggests a broad impact on the marine ecosystem of the entire southern midlatitude ocean

drivers of decadal variability in the tasman sea in this study we compare optimally interpolated monthly time series tasman sea xbt data and a comprehensive set of ocean data assimilation models forced by atmospheric reanalysis to investigate the stability of the tasman sea thermocline and the transport variability of the east australian current eac the tasman front and eac extension we find that anomalously weaker eac transport at 25 s corresponds to an anomalously weaker tasman front and anomalously stronger eac extension we further show that post about 1980 and relative to the previous 30 years the anomalously weaker eac transport at 25 s is associated with large scale changes in the tasman sea specifically stronger stratification above the thermocline larger thermocline temperature gradients and enhanced energy conversion significant correlations are found between the maria island station sea surface temperature sst variability and stratification thermocline temperature gradient and baroclinic energy conversion suggesting that nonlinear dynamical responses to variability in the basin scale wind stress curl are important drivers of decadal variability in the tasman sea we further show that the stability of the eac is linked via the south caledonian jet to the stability of the pan basin subtropical south pacific ocean storm track

lithoprobe onshore seismic reflection transects across the newfoundland appalachians vibroseis seismic reflection data have been recorded to 18 s two way traveltime along three transects across the island of newfoundland the upper crust has both steep and subhorizontal reflectors consistent with a ramp flat style of deformation whereas the middle and lower crust are largely free of regional flats reflectors descend through ca 20 km of vertical section in the middle and lower crust to flatten into the moho or perhaps cut through it in places the moho is interpreted to be no younger than the dipping reflectors reflection fabrics interpreted to be indicators of dominantly mid ordovician to mid silurian strain show consistent orientations among the transects and divide the crust into two blocks a northwestern block is characterized by upper and middle crustal reflectors dipping mostly southeast at variable angles this block is underlain to the southeast by supposedly younger and dominantly northwesterly dipping reflectors that define a northwest tapering wedge shaped block floored

recent freshening of the east australian current and its eddies the east australian current eac has a relatively weak mean flow and an energetic eddy field that dominates the circulation the properties of the mean flow have been studied in detail but the changes in the eddy field have received little attention we analyze argo temperature and salinity profiles for 2005 2012 to construct a picture of the time mean and time varying properties of eac eddies we find that eddies and the surrounding waters of the western tasman sea are freshening at a rate of 0 017 0 025 practical salinity unit yr over the top 100 m with no significant temperature change consistent with the observations fields from an eddy resolving ocean model show freshening with no temperature trend moreover the model results indicate that observed changes are significant in the context of the variability over the last 20 years and may be part of a multiyear perhaps decadal cycle we attribute the freshening of the region to increased precipitation off eastern australia

deep crustal and local rheological controls on the siting and reactivation of fault and shear zones northeastern newfoundland the siting of a c 25 km wide transpressive high strain zone at the eastern margin of the gander zone of ne newfoundland corresponds to the trace of a fundamental contact between two gondwanan basement blocks displaced sinistrally relative to one another during silurian orogenesis changes in plate motion during the devonian led to kinematic reversal and reactivation of the shear zone and at high structural levels the development of a major brittle ductile fault system at a local scale within the silurian ductile high strain zone the focus of deformation and shifts in siting of shear were closely related to magma presence we consider that granite magmas exploited shear zones within the crust to aid ascent and in doing so enhanced local deformation cessation of magma supply and or cooling of magmas within conduits caused deformation to relocate elsewhere ne newfoundland hence provides an example of how fault shear zone siting and reactivation may be controlled by regional scale pre existing basement configuration and more localised processes affecting rheology notably plutonism

southern ocean origin of multidecadal variability in the north brazil current analysis of model data from a long 200 years simulation of a high resolution version of the parallel ocean program indicates a connection between a mode of multidecadal variability in the southern ocean the so called southern ocean mode and multidecadal variability in the north brazil current the multidecadal sea surface height variability in the southern ocean propagates northward and submerges at about 40 s northward propagating anomalies in ocean heat content are found between 5 and 40 s at depths down to 1 km and affect the north brazil current similar variability and connections between southern ocean and north brazil current are also found in a 200 years simulation of a high resolution global version of the community earth system model the results provide a new mechanism for the low frequency variability of the north brazil current

on the variability of the east australian current jet structure meandering and influence on shelf circulation given the importance of western boundary currents over a wide range of scales in the ocean it is crucial that we understand their dynamics to accurately predict future changes for this we need detailed knowledge of their structure and variability here we investigate the jet structure of the east australian current eac using observations from hf radars and moorings deployed at 30 s 31 s meandering core velocity width and eddy kinetic energy eke are quantified from 4 years of hourly 1 5 km resolution surface current maps 2012 2016 to obtain the most detailed representation of the surface eac jet to date the eac flows predominantly over the 1 500 m isobath 50 km offshore but makes large amplitude displacements eastward every 65 100 days the time scale associated with mesoscale eddy shedding at the eac separation smaller amplitude higher frequency meanders occur every 20 45 days using a coordinate frame that follows the jet we show core velocity and eke exhibit seasonality in both magnitude and variance being maximum in summer 1 55 m s 1 mean core velocity minimum in winter 0 8 m s 1 however it is the eddy shedding time scale that dominates jet variability as the eac moves shoreward shelf temperature and along stream velocity vary linearly with jet movement within 35 km of the core the eac is within this range 75 of the time demonstrating its importance to the shelf circulation temperature and velocity fluctuations at the 70 m 100 m isobath are more influenced by wind eac encroachment with the strongest response occurring when wind and eac act constructively

a tectonic window in central newfoundland geological evidence that the appalachian dunnage zone may be allochthonous in the through hill area of central newfoundland mafic to ultramafic complexes which preserve varying amounts of ophiolite stratigraphy mark the trace of a major fault zone that outcrops in a roughly elliptical pattern the major northeast trending axis is about 60 km and the shorter axis is about 30 km in length the most complete ophiolite stratigraphy is preserved in the coy pond and pipestone pond complexes which have steep dips and face east and west respectively outwards from the centre of the ellipse the ophiolitic rocks are bounded on the outside by rocks typical of the dunnage zone of central newfoundland principally lower to middle ordovician volcanic and volcaniclastic rocks interpreted to be of island arc affinity davidsville victoria lake and baie d espoir groups cold spring pond formation and upper ordovician to silurian clastic sediments botwood group the volcano sedimentary sequences are interpreted to have been deposited on the ophiolite conformably but the contacts are n

are crustal thickness variations in old mountain belts like the appalachians a consequence of lithospheric delamination comparison of moho geometry and reflection character of active collision zones like the alps with those of fossil collision zones like the appalachians lends support for the view that lithospheric delamination is both a common and fundamentally important component of collisional orogeny in particular the relative crustal thinning commonly observed beneath the internides of old collisional orogens is an expected consequence of delamination neither precollisional crustal thickness variations nor an unrelated postcollision extensional event is required to produce this feature acceptance of the delamination hypothesis in turn fundamentally alters one s view of the tectonic evolution of old collisional orogens in the case of the appalachians delamination has direct implications for the interpretation of alleghanian granites the direction of subduction leading to final closure between laurentia and gondwana and the thermal state of the lithosphere at the onset of atlantic rifting a corollary of the delamination hypothesis is that lateral reflectivity variations cannot be used to define paleogeographic terranes in the deep crust of old orogens conversely the observation that delamination leaves a distinctive geophysical imprint on the crust itself provides an important additional clue for deciphering the tectonic evolution of such regions

the separation of the east australian current a lagrangian approach to potential vorticity and upstream control the east australian current eac is the western boundary current flowing along the east coast of australia separating from the coast at approximately 34 s after the separation two main pathways can be distinguished the eastward flowing tasman front and the extension of the eac flowing southward the area south of the separation latitude is eddy rich and the separation latitude of the eac is variable little is known of the properties of the water masses that separate at the bifurcation of the eac this paper presents new insights from the lagrangian perspective where the water masses that veer east and those that continue south are tracked in an eddy permitting numerical model the transport along the two pathways is computed and a 1 3 ratio between transport in the eac extension and transport in the tasman front is found the results show that the fate of the particles is to first order already determined by the particle distribution within the eac current upstream of the separation latitude where 85 of the particles following the eac extension originate from below 460 m and 90 of the particles following the tasman front originate from the top 460 m depth at 28 s the separation and pathways are controlled by the structure of the isopycnals in this region analysis of anomalies in potential vorticity show that in the region where the two water masses overlap the fate of the water depends on the presence of anticyclonic eddies that push isopycnals down and therefore enable particles to travel further south

wind forced variability in eddy formation eddy shedding and the separation of the east australian current the east australian current eac like many other subtropical western boundary currents is believed to be penetrating further poleward in recent decades previous observational and model studies have used steady state dynamics to relate changes in the westerly winds to changes in the separation behavior of the eac as yet little work has been undertaken on the impact of forcing variability on the eac and tasman sea circulation here using an eddy permitting regional ocean model we present a suite of simulations forced by the same time mean fields but with different atmospheric and remote ocean variability these eddy permitting results demonstrate the nonlinear response of the eac to variable nonstationary inhomogeneous forcing these simulations show an eac with high intrinsic variability and stochastic eddy shedding we show that wind stress variability on time scales shorter than 56 days leads to increases in eddy shedding rates and southward eddy propagation producing an increased transport and southward reach of the mean eac extension we adopt an energetics framework that shows the eac extension changes to be coincident with an increase in offshore upstream eddy variance via increased barotropic instability and increase in subsurface mean kinetic energy along the length of the eac the response of eac separation to regional variable wind stress has important implications for both past and future climate change studies

observed and simulated lagrangian and eddy characteristics of the east australian current and the tasman sea new insights into the lagrangian and eddy dynamical processes within the east australian current eac and the tasman sea are presented we briefly discuss the past campaigns undertaken to observe the eac and the tasman sea eddies as well as the motivation to renew the deployment of drifting buoys into the eac and the tasman sea the specific features discussed are motivated by the recent observing campaigns using drifting buoys and the availability of high spatial and temporal resolution estimates of the ocean state and circulation from eddy resolving models the interpretation of these features is also aided by other components of the ocean observing system the dynamics presented includes a transient eac separation through a vortex dipole b stratified vortex mergers and secondary circulation of eac eddies c eddy networks in the tasman sea and d formation and propagation of the eac separation point the importance of these dynamical features to the eac and the tasman sea and their implications for the observing system and modelling are discussed

observations of submesoscale variability and frontal subduction within the mesoscale eddy field of the tasman sea abstractsubmesoscale lenses of water with anomalous hydrographic properties have previously been observed in the east australian current eac system embedded within the thermocline of mesoscale a

relationships between syn tectonic granite fabrics and regional pttd paths an example from the gander avalon boundary of ne newfoundland abstract syn tectonic granitic plutons and their host rocks not only provide a potential wealth of information about the timing and nature of regional kinematics but also record information about thermal conditions successive syn tectonic plutons emplaced into the gander zone of ne newfoundland preserve fabric overprinting formed during cooling to regional ambient thermal conditions ambient conditions have been approximated for each pluton by careful analysis of microstructures and consideration of potential cooling histories and have illustrated regional exhumation during a tectonic reversal from silurian devonian sinistral transpression to devonian dextral transpression because the time period of emplacement is relatively short by comparison to orogenic events typically by more than an order of magnitude individual plutons may record a snap shot of the regional history by combining information from a number of plutons emplaced sequentially during a period of regional orogenesis a picture may be built up defining time temperature deformation ttd paths such information may compliment regional metamorphic pt studies in helping to establish orogenic pttd paths

decadal spinup of the south pacific subtropical gyre abstract an increase in the circulation of the south pacific ocean subtropical gyre extending from the sea surface to middepth is observed over 12 years datasets used to quantify the decadal gyre spinup include satellite altimetric height the world ocean circulation experiment woce hydrographic and float survey of the south pacific a repeated hydrographic transect along 170 w and profiling float data from the global argo array the signal in sea surface height is a 12 cm increase between 1993 and 2004 on large spatial scale centered at about 40 s 170 w the subsurface datasets show that this signal is predominantly due to density variations in the water column that is to deepening of isopycnal surfaces extending to depths of at least 1800 m the maximum increase in dynamic height is collocated with the deep center of the subtropical gyre and the signal represents an increase in the total counterclockwise geostrophic circulation of the gyre by at least 20 at 1000 m a comparison of woce and

mesoscale structure of the mean east australian current system and its relationship with topography abstract we document the mean circulation of the east australian current eac system which is the major western boundary current in the southwest pacific ocean a new high resolution climatology of the region csiro atlas of regional seas cars is used to produce regional fields of steric height and sections of geostrophic velocity the realism of these fields has been enhanced by the high spatial resolution allowance for bathymetric control and the influence of land barriers the increased detail in the maps and sections reveal the major influence that the complex regional topography has on the circulation the results also provide the appropriate resolution for comparison with recent model studies of the region we have been able to both resolve well known features with greater detail and identify previously unrecognized aspects of the circulation important results include 1 the zonal inflow north of 30 s breaks into a series of individual jets after flowing through the island chains as suggested by recent model results 2 the interaction of the zonal inflow with the local topography as it undergoes bifurcation is documented 3 a detailed picture of the evolution of the eac at the western boundary is presented including surface currents and transports 4 separation and recirculation of the eac within the tasman abyssal basin are shown to occur as a double cell structure constrained by bathymetry 5 the eac outflow is resolved as a series of eastward and northeastward currents 6 the path of the tasman front is defined as it interacts with the tasman ridge systems 7 a quasi permanent eddy is identified for the first time adjacent to the norfolk ridge northwest of new zealand

development of high thermal gradients by coeval transpression and magmatism during the variscan orogeny insights from the cap de creus eastern pyrenees abstract the cap de creus massif in the eastern pyrenees is a fragment of the variscan mid crust geological mapping and meso and micro scale analysis of the relationships between fold structures metamorphic mineral assemblages and fabrics affecting igneous bodies suggest the interaction of three processes deformation metamorphism and magmatism low pressure prograde metamorphism grades laterally from greenschist to amphibolite facies over a distance of 5 km zones of intense metamorphism coincide with transpressional deformation and with the intrusion of small sheets of diorite and granitoid sillimanite schists and small migmatite pods are distributed around the small intrusions of basic intermediate composition the inferred maximum thermal gradient is 80 c km this thermal pattern is similar to patterns observed in other variscan massifs in the pyrenees where high metamorphic gradients are interpreted in different ways in the ne cap de creus the type and distribution of prograde metamorphism was caused or at least enhanced by the intrusion of diorites and granitoids alternatively deformation developed at prograde conditions and synchronously with magmatism displays a highly heterogeneous distribution producing a sub vertical zone of high strain that coincides with domains of highest grade metamorphism and igneous activity this zone of high strain developed in a broad transpressive regime could have favoured the injection of magmatic sheets from deep to mid crustal levels progressive folding from prograde to retrograde metamorphic conditions might account for the narrowing of metamorphic zones

long term trends in the east australian current separation latitude and eddy driven transport an observed warming of the tasman sea in recent decades has been linked to a poleward shift of the maximum wind stress curl and a strengthening of the poleward flow along the coast of southeastern australia however changes in the east australian current eac separation latitude as well as in the contribution of the eac the eac extension and its eddy field to the total southward transport due to such a strengthening remain unknown this study uses 30 years 1980 2010 of the ocean forecast for the earth simulator ofes sea surface height and velocity outputs to obtain a three decade long time series of i the eac separation latitude ii the southward transport along the coast of southeastern australia 28 s 39 s and iii the southward transport across the eac separation latitude a lagrangian approach is implemented and the spin parameter is used to provide a quantitative distinction between the transports occurring outside and inside cyclonic and anticyclonic eddies significant positive trends of the low pass southward transports indicate that the intensification of the poleward flow has occurred both within the eac and in the eac extension in addition a significant increase in southward transport inside and outside eddies is found importantly the contribution of eddy driven transport has a large temporal variability and shows a sharp increase from 2005 onward finally our results show that the eac has not penetrated further south but it has separated more frequently at the southernmost latitudes within the region where it typically turns eastward

the cold core eddy and strong upwelling off the coast of new south wales in early 2007 abstract during the austral summer of 2006 07 a series of extreme oceanic events occurred in the tasman sea following a series of strong wind driven upwelling events an intense cold core eddy developed off sydney australia a data assimilating eddy resolving ocean model is used to create a three dimensional time varying reanalysis of these events the reanalysis indicates that the cold anomalies associated with the upwellings were in excess of 5 c near the coast where sea level decreased by as much as 0 2 m the reanalysed three dimensional structure of the cold core eddy shows the eddy leaning to the west north west in towards the continental shelf the diameter of the eddy is about 100 km and the sea level anomaly at the eddy centre peaks at around 1 m with an associated sub surface temperature anomaly in excess of 8 c at 200 m depth corresponding to an upward isotherm excursion of 600 m the circulation around the cyclonic eddy is ageostrophic with upwelling in the southern sector of the eddy where flow is onshore and climbing the continental slope and downwelling in the northern sector where flow is descending off the slope three dimensional trajectories of water parcels around the eddy involve 50 100 m vertical excursions based on the reanalysed circulation and composite satellite images of chlorophyll a we hypothesise that the circulation around the eddy led to significant nutrient enrichment in the euphotic zone around the perimeter of the eddy

a tale of two eddies the biophysical characteristics of two contrasting cyclonic eddies in the east australian current system mesoscale cyclonic eddies are known to be highly productive less well known are the dynamics and productivity of smaller cyclonic eddies known as frontal eddies that form on the landward side of western boundary currents in this study we investigate the physical and biogeochemical properties of two contrasting cyclonic eddies in the east australian current eac the first murphy a mesoscale cyclonic eddy that formed at 28 s with a diameter of 160 km and high surface chlorophyll a concentrations which lived 47 days the second freddy a smaller frontal eddy 35 km diameter that formed from a shelf water billow 7 days prior to sampling at 31 5 s and was advected off the shelf along the eac front from 200m to 4000m of water both eddies were at least 1000m deep with a similar steric height anomaly we introduce and employ the method of closest approach using shipboard adcp velocities to estimate the eddy centers which reveals significant tilting through the water column we estimate rotation rates of 4 10 days and 1 9 days and rossby numbers 0 25 0 1 and 0 6 0 1 from the surface to 600m for murphy and freddy respectively high resolution altimetry measurements from the saral altika satellite provide estimates of the ageostrophic component of rotation our results show that the frontal eddy is significantly more ageostrophic energetic and productive than the mesoscale cyclone despite its small size and short life 4 weeks we suggest that frontal eddies have potential to contribute significantly to the net productivity of the tasman sea region this article is protected by copyright all rights reserved

tasman leakage in a fine resolution ocean model tasman leakage the westward flow of thermocline waters south of australia from the pacific to the indian ocean is one of the lesser studied of the inter ocean exchanges here some of the properties of the tasman leakage are inferred from lagrangian particles integrated using the three dimensional velocity fields of the 1 10 degree resolution ofes model the mean tasman leakage in this model is 4 2 sv with a standard deviation of 4 3 sv the heat flux associated with this leakage lies in the range 0 08 0 18 pw there is large variability in the tasman leakage on both sub weekly and inter annual scales but no trend over the 1983 1997 period despite the large weekly variability with peaks of more than 20 sv it appears that less than half of the tasman leakage is carried within eddies

topographically induced upwelling off eastern australia abstract a high resolution numerical study of an idealized western boundary current flow over variable topography is presented with application to the east australian current eac the results indicate that alongshelf topographic variations off australia s east coast cause the eac to accelerate over the narrowing continental shelf near cape byron this acceleration is sufficient to hinder the geostrophic adjustment in the bottom boundary layer bbl which would usually cause the eac driven bbl to shut down consequently a region of persistent high bottom stress was established off cape byron which was responsible for driving an upwelling bbl it is shown that the enhanced vertical mixing associated with a low richardson number flow beneath the eac reduced the local stratification consequently the burger number is decreased resulting in a long shutdown timescale of the bbl which enables a nearshore thermal to be established and maintained such fronts are commonly observed in the region as a p

revisiting the circulation of the east australian current its path separation and eddy field abstract the traditional view of the east australian current eac as depicted in many schematics is of a continuous boundary current that flows along the shelf off eastern australia between approximately 18 s and about 32 5 s where it separates from the coast and continues either towards new zealand along the tasman front or towards tasmania as the eac extension additionally it is widely recognised that eddies are prevalent in the eac region particularly south of the eac separation we revisit this long standing paradigm and suggest that the eac is perhaps better viewed as a continuous meandering stream flowing adjacent to the coast that feeds a field of mesoscale eddies observations show that eac eddies are prevalent over a broad region of the western tasman sea as far north as 25 s typically with maximum intensity between 30 and 35 s at any instant in time the eac jet is usually evident as a poleward stream adjacent to the continental shelf edge other commonly accepted components of the eac system including the tasman front and eac extension are rarely seen as distinct identifiable features rather these features are evident only in time mean fields when the eddy variability is filtered out it is also common for the eac to be spatially discontinuous due to the presence of eddies often with multiple short streams that sometimes separate and re attach to the coast recognition of the eac as an eddy dominated current system has seen many recent studies focus on various aspects of eddies in the eac system providing new insights into mesoscale ocean dynamics recent studies of individual eddies have shown that the circulation within eddies including tilting and vertical motion is more complex than previously understood a summary of these studies along with a review of the eac system particularly its path separation and eddy field is presented here

the effect of the south pacific convergence zone on the termination of el nino events and the meridional asymmetry of enso abstractduring large el nino events the westerly wind response to the eastern equatorial pacific sea surface temperature anomalies sstas shifts southward during boreal winter and early spring reaching latitudes of 5 7 s the resulting meridional asymmetry along with a related seasonal weakening of wind anomalies on the equator are key elements in the termination of strong el nino events using an intermediate complexity atmosphere model it is demonstrated that these features result from a weakening of the climatological wind speeds south of the equator toward the end of the calendar year the reduced climatological wind speeds which are associated with the seasonal intensification of the south pacific convergence zone spcz lead to anomalous boundary layer ekman pumping and a reduced surface momentum damping of the combined boundary layer lower troposphere surface wind response to el nino this allows the associated zonal wind anomalies to shift south of the equator furthermore using a linear sh