the multidecadal variability of the asymmetric mode of the boreal autumn hadley circulation and its link to the atlantic multidecadal oscillation abstractprevious studies show that the first principal mode of the variability of the seasonal mean hadley circulation hc is an equatorial asymmetric mode am with long term trend this study demonstrates that the variability of the boreal autumn september november son hc is also dominated by an am but with multidecadal variability the son am has ascending and descending branches located at approximately 20 n and 20 s respectively and explains about 40 of the total variance further analysis reveals that the am is closely linked to the atlantic multidecadal oscillation amo which is associated with a large cross equatorial sea surface temperature sst gradient and sea level pressure slp gradient the cross equatorial thermal contrast further induces an equatorial asymmetric hc anomaly numerical simulations conducted on an atmospheric general circulation model also suggest that amo associated sst anomalies can also induce a cross equatorial slp gradient and anomalous vertical shear of the

atlantic meridional overturning circulation and the southern hemisphere supergyre 1 the ocean s role in climate manifests itself through its high heat capacity its own rich internal dynamics and its ability to transport vast quantities of heat and freshwater of particular interest is the global ocean circulation associated with the atlantic meridional overturning amoc because observations are sparse the detailed global structure of the amoc remains poorly understood particularly the pathways along which water returns to the atlantic to compensate the export of north atlantic deep water nadw here we provide the first quantitative 3 dimensional global view of the amoc using a lagrangian reconstruction which integrates hundred of thousands water particle trajectories in an ocean model the resulting pattern elucidates the role of the wind in structuring the pathways of the amoc in particular the lagrangian analysis reveals a strong link with the three subtropical gyres of the southern hemisphere sh which merge into a supergyre spanning the three ocean basins the coupling between the upper ocean wind driven circulation and the overturning in the model suggests that changes in the sh winds can alter the pathways of the amoc and therefore the water properties and the associated heat and freshwater transports

impacts of two types of la nina on the nao during boreal winter the present work identifies two types of la nina based on the spatial distribution of sea surface temperature sst anomaly in contrast to the eastern pacific ep la nina event a new type of la nina central pacific or cp la nina is featured by the sst cooling center over the cp these two types of la nina exhibit a fundamental difference in sst anomaly evolution the ep la nina shows a westward propagation feature while the cp la nina exhibits a standing feature over the cp the two types of la nina can give rise to a significantly different teleconnection around the globe as a response to the ep la nina the north atlantic na western european we region experiences the atmospheric anomaly resembling a negative north atlantic oscillation nao pattern accompanied by a weakening atlantic jet it leads to a cooler and drier than normal winter over western europe however the cp la nina has a roughly opposing impact on the na we climate a positive nao like climate anomaly is observed with a strengthening atlantic jet and there appears a warmer and wetter than normal winter over western europe modeling experiments indicate that the above contrasting atmospheric anomalies are mainly attributed to the different sst cooling patterns for the two types of la nina mixing up their signals would lead to difficulty in seasonal prediction of regional climate since the la nina related sst anomaly is clearly observed during the developing autumn the associated winter climate anomalies over western europe could be predicted a season in advance

mode and timing of terrane accretion in the forearc of the andes in ecuador the volcanic basement of the ecuadorian western cordillera pallatanga formation and san juan unit is made up of mafi c and ultramafi c rocks that once formed an oceanic plateau radiometric ages from these rocks overlap with a hornblende 40 ar 39 ar plateau age of 88 1 6 ma age obtained for oceanic plateau basement rocks of the pinon formation in coastal ecuador and with ca 92 88 ma ages reported for oceanic plateau sequences in the caribbean and western colombia these results suggest that the oceanic plateau rocks of the western cordillera and fl at forearc in ecuador are derived from the late cretaceous caribbean colombia oceanic plateau intraoceanic island arc sequences rio cala group overlie the plateau in the western cordillera and yield crystallization ages that range between ca 85 and 72 ma the geochemistry and radiometric ages of island arc lavas from the rio cala group combined with the age range and geochemistry of their turbiditic volcaniclastic products indicate that the arc was initiated by westward subduction beneath the caribbean plateau they are coeval with island arc rocks of coastal ecuador las orquideas san lorenzo and cayo formations and colombia ricaurte arc these island arc

indian ocean capacitor effect on indo western pacific climate during the summer following el nino significant climate anomalies persist through the summer june august after el nino dissipates in spring over the equatorial pacific they include the tropical indian ocean tio sea surface temperature sst warming increased tropical tropospheric temperature an anomalous anticyclone over the subtropical northwest pacific and increased mei yu baiu rainfall over east asia the cause of these lingering el nino effects during summer is investigated using observations and an atmospheric general circulation model gcm the results herein indicate that the tio warming acts like a capacitor anchoring atmospheric anomalies over the indo western pacific oceans it causes tropospheric temperature to increase by a moist adiabatic adjustment in deep convection emanating a baroclinic kelvin wave into the pacific in the northwest pacific this equatorial kelvin wave induces northeasterly surface wind anomalies and the resultant divergence in the subtropics triggers suppressed convection and the anomalous anticyclone the gcm results support this kelvin wave induced ekman divergence mechanism in response to a prescribed sst increase over the tio the model simulates the kelvin wave with low pressure on the equator as well as suppressed convection and the anomalous anticyclone over the subtropical northwest pacific an additional experiment further indicates that the north indian ocean warming is most important for the kelvin wave and northwest pacific anticyclone a result corroborated by observations these results have important implications for the predictability of indo western pacific summer climate the spatial distribution and magnitude of the tio warming rather than simply whether there is an el nino in the preceding winter affect summer climate anomalies over the indo western pacific and east asia

contrasting impacts of two types of enso on the boreal spring hadley circulation the possible influences of two types of enso i e the canonical enso and enso modoki em on hadley circulation hc during the boreal spring are investigated during 1979 2010 el ni no events are featured with a symmetric pattern in equatorial zonal mean sea surface temperature anomalies ssta with a maximum around the equator in contrast the zonal mean ssta associated with el ni modoki events shows an asymmetric structure with a maximum around 108n the contrasting underlying thermal structures corresponding with enso and em have opposite impacts on the simultaneous hc in el ni years a symmetric anomalous meridional circulation is seen with enhanced rising around the equator and anomalousdescentatabout158nand208s incontrast anasymmetricequatorialmeridionalcirculationisobserved for el ni modoki years with anomalous ascent around 108n and descent at about 108s and 208n the contrasting meridional circulation anomalies within enso and em are caused by their different meridional ssta structure this result is theoretically explained indicating that anomalous meridional circulation is subject to the meridional ssta gradient moreover the observed results are reproduced in numerical experiments driven by anomalous warming in the eastern and central pacific ocean thus the authors conclude that the anomalous hc linked to enso and em is induced by the accompanying meridional gradient in zonal mean ssta

mesoscale and submesoscale processes in the southeast atlantic and their impact on the regional thermohaline structure the turbulent processes in the cape basin the southeasternmost gate of the atlantic ocean play a key role in the transport and mixing of upper to intermediate water masses entering the area from the indian ocean making them especially relevant for the indo atlantic transfer of heat and salt in this paper two numerical simulations at different horizontal resolutions are used to study meso and submesoscale dynamics their phenomenology their evolution and their impact on the local water masses submesoscale processes seasonally affect both the upper and intermediate layers but there are clear dynamical differences between the two layers several types of instabilities underline this spatial and temporal variability near the surface mixed layer instabilities occur during winter while mesoscale driven instabilities as the symmetric type prevail in summer the connection between these two seasonal regimes is ensured in anticyclonic eddies and within the mixed layers by charney baroclinic instabilities involved in the local formation and subduction of mode water that we have dubbed as agulhas rings mode water intermediate depths are instead characterized by mesoscale mechanisms of density compensation and lateral stirring of the tracer variance triggering a significant filamentogenesis whose vertical scales are comparable to those mentioned in previous studies this leads to a particularly efficient mixing of antarctic intermediate waters of indian and atlantic origins lagrangian estimates highlight the new and significant role of fine scale structures in setting the water masses properties of upper and lower thermocline waters materializing the indo atlantic exchange and therefore potentially affecting the global ocean circulation

the petrology and geochemistry of cretaceous to recent volcanism in colombia the magmatic history of an accretionary plate margin the pacific margin of colombia records a complex history of subduction related magmatism from at least cretaceous times cretaceous lavas from the western cordillera and part of the central cordillera of the colombian andes exhibit some chemical characteristics typical of island arc tholeiites including low lil element abundances and flat to light depleted ree patterns but also have chemical and geological affinities with deposits from marginal basins and with transitional type ocean floor basalts an appraisal of the geological and chemical evidence leads to the conclusion that these rocks were erupted in an oceanic environment in close proximity to the continent a suite of basic rocks from the coastal cordillera of upper cretaceous to tertiary age has been interpreted elsewhere as abyssal tholeiites following accretion of the cretaceous sequences on to cratonic s america volcanism continued in the upper miocene with the eruption of basalts and basaltic andesites on the eastern flanks of the western cordillera this tholeiitic suite is lree and lil element enriched compared with the cretaceous lavas and represents the trenchward portion of an ensialic island arc sequence subduction related magmatism continued from the miocene with the formation of composite calc alkaline strato volcanoes mostly along the axis of the central cordillera these miocene to recent lavas are dominantly andesitic with strong lil and lree enrichment and 87 sr 86 sr ratios of 0 7042 0 7047

characteristics energetics and origins of agulhas current meanders and their limited influence on ring shedding abstractthe agulhas current intermittently undergoes dramatic offshore excursions from its mean path because of the downstream passage of mesoscale solitary meanders or natal pulses new observations and analyses are presented of the variability of the current and its meanders using mooring observations from the agulhas current time series experiment act near 34 s using a new rotary eof method mesoscale meanders and smaller scale meanders are differentiated and each captured in a single mode of variance during mesoscale meanders an onshore cyclonic circulation and an offshore anticyclonic circulation act together to displace the jet offshore leading to sudden and strong positive conversion of kinetic energy from the mean flow to the meander via nonlinear interactions smaller meanders are principally represented by a single cyclonic circulation spanning the entire jet that acts to displace the jet without extracting kinetic energy from the mean flow synthesizing in situ observations with altimeter

dynamics of agulhas retroflection and ring formation in a numerical model part ii energetics and ring formation abstract an energetics analysis of several numerical experiments on an idealized south atlantic indian ocean basin is presented the model used in the experiments is the quasi isopycnic coordinate model of bleck and boundra forced by wind and configured with two or three layers the region of focus is the most dynamically active one the agulhas current retroflection south of africa and the dynamical mechanisms associated with formation of agulhas rings are given special attention whether rings form in the model and their frequency depend on two primary factors the shape of africa and southward inertia baroclinicity in the overshooting agulhas the boundary condition on africa no slip free slip and horizontal resolution are also important experiments in which rings form exhibit considerably larger values of km to ke transfer than those in which no rings form in three of the experiments ring formation is studied in detail with the help of instantaneous top and bottom layer flow patterns and time se

the retroflection paradox abstract the classical question of what happens when a warm western boundary current such as the north brazil current nbc retroflects is addressed analytically using a reduced gravity nonlinear model the traditional view is that the northwestward flowing current separates from the wall turns to the right looking offshore and forms a zonal boundary current that flows eastward integration of the steady inviscid momentum equation along the boundary gives the longshore momentum flux or flow force and shows that such a scenario leads to a paradox to resolve the paradox the separated current must constantly shed anticyclones which propagate to the northwest due to and an interaction with the boundary this new eddy shedding mechanism which is not related to the traditional instability of a zonal jet may explain why the nbc must produce rings a nonlinear analytical solution to the problem is constructed with the aid of a powerful theoretical approach based on the idea that nonlinear periodic

new geological and geochronological data from the colombian andes continental growth by multiple accretion the regional geology of colombia consists of a composite palaeozoic to cenozoic sedimentary basin overlying precambrian shield rocks which are exposed in the eastern cordillera magdalena valley and at least part of the extensive llanos plains a palaeozoic metamorphic schist belt with abundant post metamorphic igneous rocks comprises the central cordillera a mesozoic volcanic suite extends from the western flank of the central cordillera and accounts for the bulk of the western cordillera a cenozoic sedimentary sequence probably overlying a late cretaceous oceanic crust and fore arc complex forms the pacific coastal plain the continental edge of central and southern colombia is interpreted as a composite margin made up of successively accreted oceanic island arc related sequences of palaeozoic early mesozoic and late cretaceous ages the present day cycle which began in the miocene is not associated with island arc volcanism

mesozoic evolution of exotic terranes and marginal seas western north america abstract regional terrane analysis has been combined with a global evaluation of plate kinematics to produce a new tectonic model for the mesozoic evolution of western north america and its associated marginal seas the model employs a two tiered data reliability ranking system to resolve conflicts within data sets the lower tier of the ranking system involves the assignment of a numerical rank to paleomagnetic and or paleobiogeographic data based on each data set s reliability the upper tier of the ranking system places the paleomagnetic and paleobiogeographic data in perspective by assigning a relative order of importance to different types of data the most reliable data are considered to be departure rift and arrival collision times that are tightly constrained by independent data sets evidence of subduction and or strike slip motion also ranks high in the master ranking system paleomagnetic and vertebrate paleobiogeographic data come next in the hierarchy followed by invertebrate and floral paleobiogeographic data application of this approach to a case study the baja british columbia controversy has resulted in a coherent model for the entire region results derived from the application of this modelling approach to the problem of the mesozoic assembly of western north america suggest that its evolution may be modeled using a minimum of 57 large terranes oblique convergence may have been the dominant tectonic process along the western north american margin during the mesozoic causing large scale latitudinal transport of many exotic terranes the motions of the terranes produced a number of marginal seas yukonia etc that were short lived but important components of western margin tectonics a combination of collision events side swiping episodes and changes in plate kinematics led to the initiation of several regional tectonic upheavals including the nevadan and sevier orogenies

k ar ages of basic rocks in the patia valley southwest colombia abstract the patia valley situated between the western and central cordilleras of the southwest colombian andes contains two areas in which mesozoic basic and ultrabasic rocks crop out in abundance late cretaceous diabase group pillow basalts which make up much of the western cordillera are at least 81 5 ma in the e1 tambo e1 penol area 105 97 ma hornblende and whole rock dates from the ophiolitic los azules complex indicate an albian age of formation although many dates are lower 65 62 ma owing the low grade ocean floor metamorphism the metamorphic age distribution here supports an end of cretaceous emplacement for the complex rather than the early mid cretaceous emplacement suggested for north colombian ophiolites similarly aligned along the romeral fault system tertiary dacites intruding the low azules complex are 36 15 ma

agulhas ring formation as a barotropic instability of the retroflection 1 agulhas leakage is an important link in the global ocean circulation as it transfers a significant volume of relatively warm and salty water from the indian ocean to the atlantic ocean the main route of this transfer is through the shedding of large agulhas rings from the agulhas retroflection in this paper we study the dynamics of the ring formation process by analyzing the stability of the indian atlantic supergyre in a reduced gravity model we show that the ring shedding process results from a barotropic instability of the steady circulation in the agulhas retroflection region the destabilizing mode appears to be linked to a rossby basin mode of the combined south indian atlantic basin which is localized in the retroflection region by the background flow

the early cretaceous san juan plutonic suite ecuador a magma chamber in an oceanic plateau sections through an oceanic plateau are preserved in tectonic slices in the western cordillera of ecuador south america the san juan section is a sequence of mafic ultramafic cumulates to establish that these plutonic rocks formed in an oceanic plateau setting we have developed criteria that discriminate intrusions of oceanic plateaus from those of other tectonic settings the mineralogy and crystallization sequence of the cumulates are similar to those of intra plate magmas clinopyroxene predominates throughout and orthopyroxene is only a minor component rocks of intermediate composition are absent and hornblende is restricted to the uppermost massive gabbros within the sequence the ultramafic cumulates are very depleted in light rare earth elements lree whereas the gabbros have flat or slightly enriched lree patterns the composition of the basaltic liquid in equilibrium with the peridotite calculated using olivine compositions and ree contents of clinopyroxene contains between 16 and 8

regional changes in the annual mean hadley circulation in recent decades previous studies suggest that the zonally averaged hadley circulation zahc has experienced a robust poleward expansion and its trend in intensity displays inconsistency among different data sets this study examines changes in regional hc intensity and poleward edge using six reanalyses outgoing longwave radiation and precipitation data sets hcs in six regions including africa afhc the indian ocean iohc the western pacific wphc the eastern pacific ephc south america sahc and the atlantic athc are investigated intensity trends in the northern hemisphere nh wphc and athc and the southern hemisphere sh ephc and athc are in agreement with each other in the six reanalyses furthermore regional hcs in these domains appear to be intensifying although not all of the reanalyses show statistically significant trends for the poleward edge its trend in the nh afhc iohc ephc sahc and athc is significantly larger than zero and the northern hc poleward edge exhibits uniform poleward migrations in these five regions in the sh only the trend in the sahc poleward edge is significantly different from zero furthermore the trend in the sh sahc poleward edge is significantly larger than those in the sh afhc iohc and athc the results indicate that the poleward migration of the southern zahc poleward edge during recent decades that has been identified by previous studies may be attributed mainly to the poleward migration of the southern sahc poleward edge further analyses suggest that changes in regional hc poleward edges could have a significant impact on regional precipitation anomalies

variability of boreal spring hadley circulation over the asian monsoon domain and its relationship with tropical sst the variability of boreal spring hadley circulation hc over the asian monsoon domain over the last four decades is explored the climatological distribution of the regional hc is symmetric of the equator with the ascending branch around the equator and sinking branch around the subtropics in each hemisphere the first dominant mode eof1 of the regional hc is equatorial asymmetric with the main body in the southern hemisphere sh and the ascending branch to the north of the equator this mode is mainly characterized by interannual variation and is related to el nino southern oscillation enso significant negative sea surface temperature sst anomalies are observed over the tropical indian ocean tio along with the development of la nina events however the magnitude of sst anomalies in the southern indian ocean is greater than that in the northern counterpart contributing to eof1 formation the spatial distribution of the second dominant mode eof2 is with the main body lying in the northern hemisphere nh and the ascending branch located to the south of the equator the temporal variation of this mode is connected to the warming of the tio the warming rate of the southern tio sst is faster than that in the northern counterpart resulting in the southward migration of the rising branch the above result indicates the critical role of the meridional distribution of sst on the variability of the regional hc

tobago west indies a fragment of a mesozoic oceanic island arc petrochemical evidence the pre cenozoic rocks of tobago west indies represent an exceptional cross section through a portion of a composite mesozoic arc system other parts of which may be found in the villa de cura klippe the aruba blanquilla island chain and perhaps in the western cordillera of colombia and ecuador the geochemical data from tobago presented in this paper provide a basis from which a regional view of the petrologic character of the accreted mesozoic arc of the southern caribbean mountain system can be deduced the mesozoic rocks of tobago can be divided into four main groups the north coast schist the ultramafic to mafic plutonic complex the tobago volcanic group and late mafic dykes rocks of the north coast schist served as wallrock for the plutonic volcanic dyke complex which evolved during the mid cretaceous geochemical and nd and sr isotopic data presented demonstrate the oceanic island arc character of all the pre cenozoic rocks of the island all suites span the calc alkaline and tholeiitic regions on discrimination diagrams rare earth element ree patterns for the north coast schist tobago volcanic group and plutonic suite are essentially identical and are moderately lree enriched la yb 1 1 2 7 the dykes exhibit the most unevolved ree patterns and are interpreted as most closely representative of parental liquid compositions initial nd 6 6 to 9 4 and 87 sr 86 sr 0 70303 0 70364 for all samples except north coast schist and tobago volcanic group argillites are similar in value and overall range to other oceanic island arcs limited isotopic data from argillite samples from tobago suggest an important change in the continental derived sediment contribution during mesozoic arc growth nd isotopic data from north coast schist siliceous argillite indicate virtually no continental detritus in this neocomian sediment while nd isotopic data from the tobago volcanic group argillite suggest a definite continental contribution during albian arc growth

impacts of the boreal spring indo pacific warm pool hadley circulation on tropical cyclone activity over the western north pacific abstractthis study investigated the impacts of the interannual variability in the boreal spring regional hadley circulation over the indo pacific warm pool ipwp on the tropical cyclone tc activity over the western north pacific wnp the principal modes of the interannual variability in the ipwp hadley circulation were calculated using empirical orthogonal function eof analysis the leading mode eof 1 features cross equatorial southerly wind anomalies over the indian ocean and maritime continent and has an evident impact on wnp tc activity during summer in the summer following a positive phase of the eof 1 a cyclonic circulation anomaly with upward motion positive relative vorticity anomalies and weak sea level pressure dominates the wnp and this favors increased tc genesis however large positive vertical wind shear anomalies over the south china sea and philippine sea inhibit the tc intensification a positive wind sea surface temperature sst precipitation feedback was found to facili

high mg extrusive rocks from the romeral zone ophiolites in the southewestern colombian andes abstract in the southwestern colombian andes high mg extrusive rocks occur within the mesozoic late cretaceous ophiolitic fragments from the romeral fault zone a major suture between the continental domain of the central cordillera and the oceanic domain of the western cordillera the high mg rocks include picrites and picritic basalts and are associated in places with peridotites showing transitional to extrusive textures plagioclase free picrites characterized by quench textures have been found in two localities and occur as pillow lava rio boloblanco or as clasts of monogenic and polygenic volcanic breccias el encenillo they show similar olivine phyric micro spinifex textures with groundmass consisting of hopper and plate olivine or ca poor pyroxene at el encenillo skeletal clinopyroxene and glass hyaloporphyritic texture occurs in the rio boloblanco picrites at pillow rims chemically these picrites are characterized by mgo contents of 20 and are distinguished by the content of tio 2 and incompatible elements in a high ti type rio boloblanco and a very high ti type el encenillo the latter particularly remarkable for the high contents of ti zr and nb which give evidence of an alkaline tendency phase chemistry is consistently different between the two picrite types but comparably high al contents up to 14 of skeletal pyroxenes suggest that the magmas were erupted and cooled in similar conditions olivine phyric picritic rocks characterized by microlitic non quenched groundmasses occur as clasts of volcanic breccias at el encenillo picritic basalts and are present with comparatively high frequency within major los azules and minor upper chapungo valley ophiolitic fragments from the romeral zone either as lavas and volcanigenic breccias these rocks are mostly richer in olivine than the quench textured picrites contain modal plagioclase and relatively low al pyroxenes as regards chemistry a high ti and very low al group los azules upper chapungo valley and a very high ti group el encenillo can also be distinguished within these picrites a comparison with the ultramafic volcanic rocks from gorgona island in the nearby pacific domain off colombia among which the unique phanerozoic komatiites were discovered indicates close similarities between the rio boloblanco picrites and some gorgona island komatiites olivine phyric komatiites with micro spinifex groundmass regarding their petrochemical features it is suggested that they record a unique event related to the same tectonic environment and consequently that they belong to the same structural unit developed in the late cretaceous and extending from continental colombia to the pacific domain this is in contrast with previous subdivisions of the ophiolitic complexes from southwestern colombia into distinct age groups progressively younger from the central cordillera toward the pacific ocean

alluvial lithofacies recognition in a humid tropical setting abstract cobble gravel deposits in the antigua formation accumulated on a large alluvial fan or braid plain west of the cordillera occidental in southwest colombia this formation was probably deposited during the pleistocene in a very wet tropical climate 500 cm yr rainfall fining upwards sequences of clast supported imbricated boulders and cobbles dominate with maximum clast sizes between 30 and 300 cm the sand matrix in the antigua gravels and the minor 10 sand facies are weathered to clay at depths of up to 20 m the sand facies contains abundant drift logs and leaf mats except for the absence of debris flows and the very coarse nature of the gravel the antigua gravels have lithofacies similar to the glacial outwash braid plain in the proximal area of the scott type model gravels and sands of the younger panambi formation were deposited by a braided stream that was smaller confined by valley walls and flowing at a lower gradient than the river that deposited the antigua gravels we recognize no sedimentologic characteristics of these deposits as diagnostic of a humid tropical environment except for textural and compositional changes in matrix sediments caused by deep and rapid chemical weathering

a numerical study of the agulhas retroflection the role of bottom topography abstract the oceanic circulation near the eastern margin of the african continent is dominated by the poleward flow of the agulhas current the agulhas current separates from the african coast near the southern tip of the continent makes an abrupt turn to the east known as the agulhas retroflection and flows into the indian ocean as a meandering jet called the agulhas return current the object of this study is to investigate possible causes for the agulhas retroflection a series of process oriented experiments are conducted with the princeton ocean model a primitive equation numerical model in sigma coordinates the model is started from rest forced at its surface with different wind stress distributions and run until dynamical equilibria are reached after which the steady states of the model are analysed in these experiments the effects of inertia wind stress distribution and realistic bottom topography on the agulhas retroflection are analyzed the results of these experiments indicate tha

the andean ophiolitic megastructures on the buga buenaventura transverse western cordillera valle colombia abstract a structural study of the buga buenaventura transverse in the central part of the western cordillera of colombia has shown the presence of three structural units which from bottom to top and from west to east are the rio dagua unit the rio calima unit and the loboguerrero window unit all three units comprise strata between 120 and 80 m y old overlying a basement of green rocks showing the characteristics of submarine flows the bolivar ultrabasic and basic massif is geographically linked to the rio calima unit in which green rocks predominate and is separated from it by a tectonic contact the upper part of the massif on the other hand shows high temperature metamorphic rocks formed during its emplacement the rio dagua unit shows two tectonic phases with isoclinal folding and development of schistosity the first phase is contemporaneous with low grade metamorphism equivalent to lower greenschist facies conditions the loboguerrero window unit shows a large recumbent fold oriented towards the southeast deformation in the rio calima unit is weaker and appears to correspond to a higher structural level than in the two other units as the attitudes of the s1 schistosity in the rio dagua unit and of the shear zones located at the green rocks sediments contact in this same unit are similar to that of the overthrust at the base of the rio calima unit we are led to postulate that the overthrust belongs to the first phase of deformation as also does the recumbent fold in the loboguerrero window unit we are thus led to propose a southeastward direction of emplacement for the nappes in the western cordillera of colombia in conclusion the authors extend their observations and propose a new structural interpretation of the occidente colombiano

paleostress evolution of the northern andes eastern cordillera of colombia implications on plate kinematics of the south caribbean region 1 new tectonic field data and paleostress determinations in the eastern cordillera of colombia enable us to identify stress regimes that prevailed in the northern andes since late cretaceous times until present day these regimes were characterized by an e w to wsw ene active contraction from late cretaceous to late paleocene this direction subsequently changed to nw se and finally became wnw ese during the andean tectonic phase the characteristics and relative chronology of these regimes were deduced from field structures and stratigraphic criteria using stress inversion of fault slip data sets to evaluate our model we used the rotation poles of the south america caribbean and nazca plates relative to north america and paleogeographic reconstructions of the caribbean region to derive a kinematic model of the northern andes assuming a general correspondence between the regional convergence directions and the regional compression we built a theoretical model of the regional stress regimes based on kinematics of the northwestern south america since late cretaceous times this model reveals a major variation at the end of the paleocene when relative divergence shifted to convergence between the americas this is in agreement with our tectonic model based on independent tectonic field data inversion in the eastern cordillera this regional kinematic and tectonic change which affected the caribbean at the end of the paleocene was responsible for and consistent with the major change in stress patterns and structural style in the eastern cordillera it is also coeval with the development of a major regional unconformity in colombia

development of the gulf of guayaquil ecuador during the quaternary as an effect of the north andean block tectonic escape 1 interpretation of industrial multichannel seismic profiles and well data are used to identify the main tectonic features of the gulf of guayaquil gg area these include two e w trending major detachments the posorja and the jambeli detachment systems which represent half grabens with oppositely dipping detachments to the south and to the north respectively the ne sw trending puna santa clara fault system developed as a transfer fault system between the posorja and the jambeli detachments the esperanza and the jambeli basins exhibit 3 4 km of sediment that accumulated during the past 1 6 1 8 myr the puna santa clara fault system bounds the esperanza and the jambeli basins evidencing that the evolution of these basins is tightly controlled by the two detachments at depth to the west the n s trending domito fault system bounding the posorja detachment system and the esperanza basin to the west acted as a transfer zone between the shelf area and the continental slope the pliocene series show no significant variations in thickness throughout the gulf of guayaquil area suggesting that no important tectonic deformation occurred from 5 2 to 1 8 1 6 ma the major period of tectonic deformation in the gulf of guayaquil area occurred during the pleistocene times three main tectonic steps are identified from early pleistocene to 180 ka major subsidence occurred along the esperanza and jambeli basins from 180 to 140 ka most of the gulf of guayaquil area was above sea level during the isotope substage 6 low stand from 140 ka to present tectonic activity is restricted along the normal faults bounding the esperanza basin the tenguel fault and the puna santa clara and domito fault systems a n s trending tensional stress regime characterizes the pleistocene times throughout the northward drifting of the north andean block is proposed to control the tectonic evolution and associated subsidence of the gulf of guayaquil area it is also accepted that the collision of the carnegie ridge with the trench axis has to play a major role in controlling the north andean block northward drift because the carnegie ridge subduction is possibly an ongoing process which began prior to the pliocene we postulate the along strike morphology of the ridge at the origin of interplate coupling variations the subduction of an along strike positive relief of the ridge is proposed at the origin of the major tectonic reorganization of the gg area occurring at 1 8 1 6 ma

la nina diversity and northwest indian ocean rim teleconnections the differences in tropical pacific sea surface temperature sst expressions of el nino southern oscillation enso events of the same phase have been linked with different global atmospheric circulation patterns this study examines the dynamical forcing of precipitation during october december ond and march may mam over east africa and during december march djfm over central southwest asia for 1950 2010 associated with four tropical pacific sst patterns characteristic of la nina events the cold phase of enso the self organizing map method along with a statistical distinguishability test was used to isolate la nina events and seasonal precipitation forcing was investigated in terms of the tropical overturning circulation and thermodynamic and moisture budgets recent la nina events with strong opposing sst anomalies between the central and western pacific ocean phases 3 and 4 force the strongest global circulation modifications and drought over the northwest indian ocean rim over east africa during mam and ond subsidence is forced by an enhanced tropical overturning circulation and precipitation reductions are exacerbated by increases in moisture flux divergence over central southwest asia during djfm the thermodynamic forcing of subsidence is primarily responsible for precipitation reductions with moisture flux divergence acting as a secondary mechanism to reduce precipitation eastern pacific la nina events in the absence of west pacific sst anomalies phases 1 and 2 are associated with weaker global teleconnections particularly over the indian ocean rim the weak regional teleconnections result in statistically insignificant precipitation modifications over east africa and central southwest asia

ophiolite complex from la tetilla southwestern colombia south america a disrupted ophiolitic complex of cretaceous age from the romeral fault zone in the southwestern colombian andes occurs in the la tetilla area near popayan it represents a peculiar association of low ti and high ti tholeiitic basalts among the ophiolitic complexes from the romeral zone it includes 1 a main sequence with incipient metamorphism under prehnite pumpellyite to greenschist facies conditions consisting in ascending order of cumulus wehrlite and gabbro massive gabbro and basaltic and doleritic lavas and breccias the basalts are characterized by the crystallization of clinopyroxene prior to plagioclase cpx phyric basalt group and have a primitive low ti tholeiite composition mg numbers between 70 6 and 65 4 the clinopyroxene chemistry is consistent with this magmatic affinity the plutonic rocks can be related to a similar low ti tholeiitic magma 2 a sequence of pervasively metamorphosed basalts with greenschist facies metamorphism of oceanic type structurally underlies the mai

cretaceous basaltic terranes in western colombia elemental chronological and sr nd isotopic constraints on petrogenesis accreted terranes comprising mid to late cretaceous picrites basalts and dolerites occur in three north south trending belts in western colombia in the central cordillera western cordillera and along the pacific coast the geochemistry of these rocks is consistent with an oceanic plateau plume related origin and they most probably formed in the pacific as part of the caribbean oceanic plateau these igneous rocks display small but significant inter cordillera variations being younger and more depleted in incompatible trace element ratios and with more positive end values to the west the igneous rocks of the pacific coast serrania de baudo are dated at 73 78 ma 40ar 39ar and those of the western cordillera at 90 ma whereas the volcanics of the central cordillera are believed to be older than 100 ma most of the igneous rocks are basaltic and it is suggested that they have fractionated from picritic primary magmas generated by partial melting within a hot mantle plume variable and positive end values reveal that the plume must have been heterogeneous originating from a mantle source with a long term history of depletion partial melt modelling suggests that the composition of the basalts requires at least some input from a mantle source region containing garnet and that the extent of partial melting required to reproduce the composition of the erupted basalts is of the order of 20 mixing of melts from different depths either in the mantle melting column or during fractionation in lithospheric magma chambers can explain the relative homogeneity of basaltic lavas erupted to form this and other oceanic plateaux the caribbean colombian oceanic plateau may have formed at an oceanic spreading centre and valuable comparisons can be made between iceland and the caribbean colombian plateau

metamorphic pattern of the cretaceous celica formation sw ecuador and its geodynamic implications abstract the volcanic rocks of the cretaceous celica formation of southern ecuador are affected by a weak although widespread alteration the chemical study of the secondary chemical phases present in andesitic and basaltic lava flows reveals that this alteration corresponds to very low grade metamorphism comprising the zeolite and the prehnite pumpellyite facies main features of this metamorphism are weak lithostatic pressure moderate to steep thermal gradient high o 2 low value of the seawater rock ratio and total absence of deformation these characteristics are typically present in other volcanic suites of similar age and composition along the andes and correspond to the pattern of metamorphism developed in extensional settings diastathermal metamorphism linked to various degrees of thinning of the continental crust based on this metamorphic pattern a geodynamic model is proposed in which the celica formation is interpreted as an ensialic aborted marginal basin developed on strongly attenuated continental crust at the border of the south american plate the relationship between the ecuadorian and colombian volcanic suites of cretaceous age present along the western cordillera is discussed in the light of the model suggested

substratum transverse faults in kuqa foreland basin northwest china and their significance in petroleum geology abstract the importance of transverse faults in basin evolution and petroleum geology has been often emphasized however the transverse faults in the kuqa foreland basin the most important gas producing area in china have rarely been studied interpreted seismic sections and earthquake focal mechanism solutions as well as other geological and geophysical data allow us to identify a few widely spaced approximately ns trending transverse strike slip faults separating major structural units and to clarify the geometry and kinematics of these transverse faults hidden below thrust faults in the kuqa foreland basin two major structural domains can be distinguished transverse faults in the northern kuqa foreland basin are mainly nne trending sinistral indicating clockwise rotation of fault bounded blocks in contrast the predominant nw trending dextral faults in the southern kuqa foreland basin imply counterclockwise rotation of fault bounded blocks we propose a tectonic model in which crustal blocks are bounded by strike slip faults in a zone of simple shear rotation about vertical axis the strike slip faulting and thrust faulting in the kuqa foreland basin suggest that some of the convergence between south tianshan and tarim blocks may have been accommodated not only by obvious crustal shortening and thickening along thrust faults but also by rotation and possible lateral movement of the crust along transverse faults controlled by the remote collision of indian block with eurasian block since the miocene these reactivated substratum faults which may inherit from paleozoic deformation control various elements relevant to gas accumulation in the kuqa foreland basin which should expect to be paid more attention in the future these elements include maturity of tertiary and jurassic source rocks a difference in the regional cap of kumugelimu salt beds from east to west reservoir bed properties gas migration channels and traps formation in addition the strike slip faulting of the transverse faults may affect deformation of the sedimentary cover including making the western qiulitag structural belt to form an arcuated structure curved to the ssw

on sverdrup discontinuities and vortices in the southwest indian ocean abstract the southwest indian ocean is distinguished by discontinuities in the wind driven sverdrup circulation these connect the northern and southern tips of madagascar with africa and the southern tip of africa with south america in an analytical barotropic model with a flat bottom the discontinuities produce intense westward jets those off the northern tip of madagascar and the southern tip of africa are always present while the strength of that off southern madagascar depends on the position of the zero curl line in the indian ocean the jet is strong if the line intersects madagascar but weak if the line is north of the island all three jets are barotropically unstable by the rayleigh kuo criterion the authors studied the development of the instability using a primitive equation model with a flat bottom and realistic coastlines the model produced westward jets at the three sites and these became unstable after several weeks generating 200 300 km scale eddies the eddies generated west of

interdecadal changes in the major modes of asian australian monsoon variability strengthening relationship with enso since the late 1970s abstract the present paper develops an integral view of the year to year variability across the entire asian australian monsoon a am system which covers one third of the global tropics between 40 and 160 e using season reliant empirical orthogonal function s eof analysis the authors identified two major modes of variability for the period 1956 2004 the first exhibits a prominent biennial tendency and concurs with the turnabout of el nino southern oscillation enso providing a new perspective of the seasonally evolving spatiotemporal structure for tropospheric biennial oscillation the second mode leads enso by one year the remote el nino forcing the monsoon warm pool ocean interaction and the influence of the annual cycle are three fundamental factors for understanding the behavior of the first mode the monsoon ocean interaction is characterized by a positive feedback between the off equatorial convectively coupled rossby waves and the underlying sea surface temperature sst dipole anoma

interannual eddy kinetic energy modulations in the agulhas return current interannual variability in the mesoscale eddy field in the agulhas return current arc of 32 42 degrees s and 15 35 degrees e is investigated based on satellite altimeter observations and state estimate from the estimating the circulation and climate of the ocean phase ii from 1993 to 2016 it is found that the interannual modulation of eddy kinetic energy in the arc region is externally mediated by the wind stress forcing that generates the westward propagating sea surface height anomalies across the south indian ocean subtropical gyre the wind forced sea surface height anomalies influence the upstream agulhas current volume transports by modulating the intensity of barotropic instability of the arc mean flow centered around the retroflection region the agulhas current inflow variability leads to the downstream interannual eddy kinetic energy fluctuations in the arc region

mechanisms of hemispherically symmetric climate variability inspired by paleoclimate evidence that much past climate change has been symmetric about the equator the causes of hemispherically symmetric variability in the recent observational record are examined using the national centers for environmental prediction national center for atmospheric research reanalysis dataset and numerical models it was found that the dominant cause of hemispherically symmetric variability is the el nino southern oscillation during an el nino event the tropics warm at all longitudes and the subtropical jets in both hemi spheres strengthen on their equatorward flanks poleward of the tropical warming there are latitude belts of marked cooling extending from the surface to the tropopause in both hemispheres at all longitudes and in all seasons the midlatitude cooling is caused by changes in the eddy driven mean meridional circulation changes in the transient eddy momentum fluxes during an el nino event force upper tropospheric ascent in midlatitudes through a balance between the eddy fluxes and the coriolis torque the eddy driven ascent causes anomalous adiabatic cooling which is primarily balanced by anomalous diabatic heating using a quasigeostrophic spherical model forced by an imposed surface eddy disturbance of chosen wave number and frequency it is shown that the anomalous eddy momentum fluxes are caused by the impact that the changes in the tropically forced subtropical jets have on the propagation in the latitude height plane of transient eddies changes in zonal winds and associated changes in the meridional gradient of potential vorticity create an anomalous region of low meridional wavenumber in the midlatitudes that refracts waves away both poleward and equatorward tropical forcing of variability in the eddy driven mean meridional circulation is another way in addition to rossby wave teleconnections whereby the tropics can influence extratropical climate unlike teleconnections this mechanism causes climate variability that has strong zonally and hemispherically symmetric components and operates throughout the seasonal cycle

a critical review of tectonic processes at continental margin orogens abstract a cordilleran type orogen has previously been defined as contractional deformation of a continental margin due to subcontinental subduction the plate tectonic setting of cordilleran type orogenesis is fundamentally different from that of collisional orogenesis intracontinental orogens such as the himalayas and alps form by collision between a continental margin and a continental margin subduction zone orogens at continental margins may form due to subcontinental subduction cordilleran type orogeny or due to collision of a continental margin with an intra oceanic subduction zone which is typically followed by a reversal of subduction polarity in the latter case collisional orogenesis may proceed along with the reversed subcontinental subduction modern arcs are mostly extensional or neutral so subduction hangingwall contraction is not a necessary consequence of subduction crustal contraction in known collisional orogens lasts for tens of millions of years so collisional deformation at a continental margin may last for tens of millions of years after arc continent collision and subduction reversal collisional orogenesis is a predictable consequence of subduction and collisional deformation is kinematically linked to the kinematics of subduction noncollisional cordilleran type orogenesis is not predicted by plate theory a geologic history that would demonstrate noncollisional orogenesis is a magmatic arc built into a passive margin sequence with strong deformation but without an earlier arc or ophiolite accretion event this history has not been recognized in the geologic record the classic examples of cordilleran type orogens including the cretaceous cenozoic of the andes and the jurassic early tertiary of the western united states may be interpreted as arc continent collisions however the youngest increments of deformation in these two orogens occurred 100 million years after the youngest collisions and are thus difficult to ascribe to collisional processes notwithstanding the long time scales these deformations occurred at the end of space time continua of probable collisional orogenies alternatively these deformations may represent lithosphere scale fault bend folding duplex folding backthrusting and or out of sequence forethrusting during shallow subduction

atmospheric circulation cells associated with the el nino southern oscillation abstract atmospheric circulation cells associated with the el nino southern oscillation enso are described and examined using the ncep ncar reanalysis field and the ncep sea surface temperature sst from january 1950 to december 1999 the divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells the warm phase of enso shows positive sst anomalies in the equatorial eastern pacific and along the east coast of asia and the west coast of north america and negative sst anomalies in the off equatorial western pacific and in the central north pacific associated with this sst anomaly distribution are variations of atmospheric zonal and meridional circulation cells over the pacific the equatorial zonal walker circulation cell is weakened consistent with previous schematic diagrams the anomalous meridional hadley circulation cell in the eastern pacific shows the air rising in the tropics flowing poleward in the upper troposphere sinking in the subt