the madden julian oscillation barotropic dynamics and north pacific tropical cyclone formation part i observations low level barotropic dynamics may help to explain the modulation of eastern and western north pacific tropical cyclones by the madden julian oscillation mjo during northern hemisphere summer the mjo is characterized by alternating periods of westerly and easterly 850 mb zonal wind anomalies across the tropical pacific ocean when mjo 850 mb wind anomalies are westerly small scale slow moving eddies grow through barotropic eddy kinetic energy eke conversion from the mean flow these growing eddies together with strong surface convergence 850 mb cyclonic shear and high mean sea surface temperatures create a favorable environment for tropical cyclone formation periods of strong mjo easterlies over the pacific are characterized by lesser eke and negligible eddy growth by barotropic conversion the term 2 x is a leading contributor to low level barotropic eke conversion during mjo westerly 2 u9 u periods across the pacific indicating the importance of zonal variations in the westerly jet for producing concentrations of eddy energy this mechanism can be described as wave accumulation associated with variations of the low level zonal flow the conversion term 2 y contributes a smaller portion of the total conversion u9y9 u over the eastern pacific but is of comparable importance to 2 x during westerly mjo events in the 2 u9 u western pacific

stratigraphic numerical modelling of a carbonate platform on the romanche transverse ridge equatorial atlantic abstract the romanche transverse ridge equatorial atlantic is located in the northern flank of the fracture zone opposite the eastern ridge transform intersection rti it constitutes a major positive topographic anomaly that reaches a height of over 4 km above the level predicted by the thermal subsidence curve a series of e w aligned peaks located on the crest of the transverse ridge were at sea level during early and middle miocene times they are presently capped by a 300 m thick shallow water carbonate platform that grew on a subsiding oceanic crust basement surface flattened by erosion at sea level these structures are now about 1 km below sea level high resolution seismic reflection surveys and multibeam morphobathymetry as well as study of samples recovered from the carbonate platform allowed a reconstruction of the paleoenvironment and of the vertical movements of the peaks starting from the lower miocene ages derived from microfossils suggest that the base of the carbonate platform dates from 17 25 m y ago and the sinking of the platform started between 18 and 13 m y ago these data were included in a numerical simulation model that takes into account thermal and tectonic subsidence growth potential of the carbonates subaerial and submarine erosion rates and mio pliocene absolute sea level fluctuations the results outline the subsidence history of the romanche transverse ridge and suggest post miocene subsidence faster than that predicted by the thermal cooling model

tracking and mean structure of easterly waves over the intra americas sea abstract easterly waves ews are prominent features of the intertropical convergence zone itcz found in both the atlantic and pacific during the northern hemisphere summer and fall where they commonly serve as precursors to hurricanes over both basins a large proportion of atlantic ews are known to form over africa but the origin of ews over the caribbean and east pacific in particular has not been established in detail in this study reanalyses are used to examine the coherence of the large scale wave signatures and to obtain track statistics and energy conversion terms for ews across this region regression analysis demonstrates that some ew kinematic structures readily propagate between the atlantic and east pacific with the highest correlations observed across costa rica and panama track statistics are consistent with this analysis and suggest that some individual waves are maintained as they pass from the atlantic into the east pacific whereas others are generated locally in the caribbean a

geologic evaluation of plate kinematic models for the north american caribbean plate boundary zone heubeck ch and mann p 1991 geologic evaluation of plate kinematic models for the north american caribbean plate boundary zone tectonophysics 191 l 26 four published plate kinematic models for the present day north american caribbean plate boundary predict significantly different fault behavior along this dominantly strike slip plate boundary zone pbz using a computer graphics system we compare the deformational pattern predicted by the four kinematic models with geologic and seismic observations along the entire length of the pbz from central america to the northern lesser antilles our results indicate that none of the kinematic models predict all the geologic and seismic observations across the entire pbz however all four models predict mapped fault character and observed earthquake focal mechanisms along certain segments of the pbz we conclude that the present day deformation within the north american caribbean plate boundary zone cannot be described by rotation about a single pole instead the caribbean plate appears to be segmented into at least three major sub plates or blocks each of which requires an individual pole of rotation to describe its motion relative to the north american plate sub plate boundaries correspond to two poorly studied zones of diffuse deformation within the caribbean plate honduras depression of central america and beata ridge of the caribbean sea

extreme mantle uplift and exhumation along a transpressive transform fault earth s crust diverges and extends along mid ocean ridges analyses of gravity and seismic data from the equatorial atlantic show that propagation of ridge segments can compress the crust and create sufficient uplift to create small islands

regional scale morphological pattern of the tyrrhenian sea new insights from emodnet bathymetry abstract a large body of literature exists on the morphology of the tyrrhenian sea the youngest back arc basin of the eastern mediterranean however most of these studies are focused on local areas without considering the broad regional setting in this paper we consider the morphology of the entire tyrrhenian sea at a regional scale based on a dem analysis of middle resolution bathymetry downloaded from the emodnet website based on these data a first morpholineaments map has been created identifying 500 features 10 km in length without considering their tectonic settings and age furthermore a tyrrhenian regional tectonic map has been digitized from literature and the faults divided in pre pliocene pliocene and plio quaternary morpholineaments and tectonic lineaments have been grouped in six tyrrhenian sea areas plotted in rose diagrams in order to obtain the main trends in each region and to detect relationships between morphology and tectonics rose diagrams show that most of the identified morpholineaments have a tectonic origin although they are not coeval as they developed during different phases of the opening of the tyrrhenian back arc basin moreover morpholineaments data suggest that some regions of the tyrrhenian sea may be dominated by a single main trend whereas in other regions the occurrence of more trends testifies a major tectonic complexity

depth and age in the cayman trough abstract caribbean plate boundaries are almost everywhere either strike slip or convergent so oceanic crust accreted at the spreading axis within the cayman trough provides information critical to determining the relative motions and tectonic interactions among the caribbean and surrounding plates the tectonic evolution of the trough as inferred from magnetic anomaly identifications is not entirely compatible with either its crustal subsidence or the geological evidence for time of opening and rate of offset movement along the north american caribbean plate boundary a close examination of basement topography and depth within the trough suggests that the total opening of the cayman trough is about 1100 km and that this opening occurred in two stages crust generated by the present ongoing stage of very slow spreading has extensive relief and shows a distinct increase of crustal depth with increasing distance from the spreading center crust generated by an earlier episode of faster spreading is characterized by reduced topographic relief and no appreciable differential subsidence and is separated from the younger crust by distinct topographic breaks the rate of spreading of the current slow spreading episode is15 5mm yr based upon a new crustal subsidence curve which accommodates lateral as well as vertical cooling of the accreting slab the trough has opened at this rate for about 30 ma the rate of opening prior to 30 ma cannot be determined with any reliability from crustal subsidence a reexamination of magnetic anomalies in the central part of the trough shows that these anomalies can be generated at a much slower spreading rate than previously thought this revised interpretation of cayman trough evolution brings magnetic subsidence and geological evidence into much better agreement

oceanic broad multifault transform plate boundaries oceanic transform plate boundaries consist of a single narrow a few kilometers wide strike slip seismic zone offsetting two mid ocean ridge segments however we define here a new class of oceanic transform boundaries with broad complex multifault zones of deformation similar to some continental strike slip systems examples are the 750 km long 120 km wide andrew bain transform on the southwest indian ridge and the romanche transform where the mid atlantic ridge is offset by a lens shaped 900 km long 100 km wide sliver of deformed lithosphere bound by two major transform valleys one of the valleys is seismically highly active and constitutes the present day principal transform boundary however strike slip seismic events also occur in the second valley and elsewhere in the deformed zone some of these events may be triggered by earthquakes from the principal boundary numerical modeling predicts the development of wide multiple transform boundaries when the age offset is above a threshold value of 30 m y i e in extra long 500 km slow slip transforms multiple boundaries develop so that strike slip ruptures avoid very thick and strong lithosphere

the nature of the crust under cayman trough from gravity abstract considerable crustal thickness variations are inferred along cayman trough a slow spreading ocean basin in the caribbean sea from modeling of the gravity field the crust to a distance of 50 km from the spreading center is only 2 3 km thick in agreement with dredge and dive results crustal thickness increases to 5 5 km at distances between 100 and 430 km west of the spreading center and to 3 5 6 km at distances between 60 and 370 km east of the spreading center the increase in thickness is interpreted to represent serpentinization of the uppermost mantle lithosphere rather than a true increase in the volume of accreted ocean crust serpentinized peridotite rocks have indeed been dredged from the base of escarpments of oceanic crust rocks in cayman trough laboratory measured density and p wave speed of peridotite with 40 50 serpentine are similar to the observed speed in published refraction results and to the inferred density from the model crustal thickness gradually increases to 7 8 km at the far ends of the trough partially in areas where sea floor magnetic anomalies were identified basement depth becomes gradually shallower starting 250 km west of the rise and 340 km east of the rise in contrast to the predicted trend of increasing depth to basement from cooling models of the oceanic lithosphere the gradual increase in apparent crustal thickness and the shallowing trend of basement depth are interpreted to indicate that the deep distal parts of cayman trough are underlain by highly attenuated crust not by a continuously accreted oceanic crust

flexural uplift of a lithospheric slab near the vema transform central atlantic timing and mechanisms abstract the vema transverse ridge vtr is a prominent long and narrow topographic anomaly that runs for over 300 km along a sea floor spreading flow line south of the vema transform at 11 n in the atlantic it rises abruptly about 140 km from the axis of the mid atlantic ridge mar in 10 myr old crust and runs continuously up to 25 myr old crust it reaches over 3 km above the predicted lithospheric thermal contraction level it is absent in crust younger than 10 myr thus the uplift of the vtr must have ended roughly 10 ma the vtr is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere that was generated at an 80 km long mar segment based on satellite gravimetry imagery this mar segment was born roughly 50 ma and increased its length at an average rate of 1 6 mm yr multibeam data show that the mar parallel sea floor fabric south of the vtr shifts its orientation by 5 to 10 clockwise in 11 12 myr old crust indicating a change at that time of the orientation of the mar axis and of the position of the euler rotation pole this change caused extension normal to the transform followed between 12 and 10 ma by flexure of the edge of the lithospheric slab uplift of the vtr at a rate of 2 to 4 mm yr and exposure of a lithospheric section vema lithospheric section or vls at the northern edge of the slab parallel to the vema transform ages of pelagic carbonates encrusting ultramafic rocks sampled at the base of the vls at different distances from the mar axis suggest that the entire vtr rose vertically as a single block within the active transform offset a 50 km long portion of the crest of the vtr rose above sea level subsided was truncated at sea level and covered by a carbonate platform subaerial and submarine erosion has gradually removed material from the top of the vtr and has modified its slopes spreading half rate of the crust south of the transform decreased from 17 2 mm yr between 26 and 19 ma to 16 9 mm yr between 19 and 10 ma to 13 6 mm yr from 10 ma to present the slowing down of spreading occurred close in time to the change in ridge transform geometry suggesting that the two events are related a numerical model relates lithospheric flexure to extension normal to the transform suggesting that the extent of the uplift depends on the thickness of the brittle layer consistent with the observed greater uplift of the older lithosphere along the vtr

rhodalgal foramol facies in equatorial carbonates insights from miocene tectonic islands of the central atlantic abstract shallow water carbonates ranging in thickness from 250 to 500 m have been deposited during the miocene on the summit of transverse ridges flanking the romanche and vema transforms that offset the equatorial mid atlantic ridge these carbonate platforms are dominated by the same biota assemblages consisting mainly of perforate larger benthic foraminifera and red algae corals are present occasionally as minor components we discuss several factors i e temperature of sea water elevated co 2 levels hydrothermal brine fluxes trophic conditions biological induced precipitation processes water energy substratum stability or instability and platform geometry that could have played a role in favoring specific features that characterize our atlantic carbonates relative to other known examples from the equatorial realm we use the knowledge on the biota association of st paul rocks small islets from the equatorial atlantic the only modern example of oceanic tectonic islands to compare present day with ancient assemblages we propose that equatorial upwelling and humid conditions co 2 input and iron addition related to the mid ocean ridge water energy and substratum instability may have created the conditions suitable for the predominance of calcitic organisms over aragonitic corals in the miocene tectonic islands of equatorial atlantic moreover we do not exclude for the vema paleoislands the influence of the fresh water plumes from south american rivers

propagation and maintenance mechanism of the tc submonthly wave pattern and tc feedback in the western north pacific abstractpropagation and maintenance mechanisms of the tropical cyclone submonthly wave pattern in the western north pacific are explored the wave pattern exhibited an equivalent barotropic structure with maximum vorticity and kinetic energy in the lower troposphere and propagated northwestward in the philippine sea in the intraseasonal oscillation iso westerly phase and north northeastward near the east asian coast in the easterly phase the mean flow advection played a dominant role in the propagation in both phases barotropic energy conversion is the dominant process in maintaining the kinetic energy of the pattern the wave pattern tended to occur in the confluent zone between the monsoon trough and the anticyclonic ridge where the kinetic energy could be efficiently extracted from the westerly mean flow associated with the monsoon trough the individual circulation circuit embedded in the pattern was oriented northeast southwest east west to have optimal growth and propagation during the iso wes

fault array evolution in extensional basins insights from statistical analysis of gravel deposits in the cecina river tuscany italy two statistical analyses of gravel clasts from the lower pleistocene deposits in the lower cecina valley tuscany italy have been combined to unravel changes in the palaeo drainage system data from 16 outcrops were collected and 6400 clasts described facies analysis micro palaeontology and macro palaeontology and petrographic characteristics of the gravel deposits have highlighted the presence of three allostratigraphic units clast lithology is the main discriminator among these units cluster and principal component analyses of the 6400 clasts have improved understanding of the stratigraphy of the lower pleistocene deposits and constrain the re routing of the lower palaeo cecina river from a supposedly south east to north west direction to the present east to west direction short rivers feeding small fan deltas represented by the oldest allostratigraphic units were abandoned in the lower pleistocene when the re routing of the cecina river caused the capture of these streams this evolution suggests a change in the tectonic regime of the area the fan deltas developed on the hanging wall of normal faults sub parallel to the coast a change to a transtensile tectonic regime caused the deviation of the main river channel toward the present coast and the formation of a pull apart basin which is now exploited by the cecina river this study illustrates the value of lithological analyses of gravel deposits for understanding the tectonic evolution of an area

rossby waves in zonally opposing mean flow behavior in northwest pacific summer monsoon abstract the interactions between monsoon circulations and tropical disturbances in the northwest pacific where the low level mean flow is westerly in the west and easterly in the east are studied with a barotropic model the authors model results suggest that the scale contraction by the confluent background flow the nonlinear dynamics the effect and the large scale convergence are important for the energy and enstrophy accumulation near the region where the zonal flow reverses the energy enstrophy accumulation can be maintained with a continuous rossby wave emanation upstream the largest accumulation occurs when the emanating zonal wavelength is around 2000 km longer rossby waves experience less scale contraction and nonlinear effects while shorter rossby waves cannot hold a coherent structure against dispersive effects the nonlinear energy enstrophy accumulation mechanism is significantly different from previous linear energy accumulation theories in the linear theories this is primarily a

a climatology of central american gyres abstractcentral american gyres cags are large closed cyclonic circulations that occur during the rainy season may november which can yield exceptional rainfall leading to catastrophic flooding and large societal impacts a reanalysis based climatology of cags is developed from an algorithm that distinguishes cag cases from other systems this algorithm identified cag cases based on circulation intensity a broad radius of maximum winds and the existence of closed earth relative cyclonic flow based on these criteria 47 cag cases were identified from 1980 to 2010 featuring a bimodal distribution of cases with maxima in may june and september november cag cases are composited into two categories based on their upper tropospheric pv structure nonbaroclinic cags are more common n 42 and characterized by an upper tropospheric anticyclone while baroclinic cags are less common n 5 and characterized by an upper tropospheric trough whereas a nonbaroclinic cag has anomalous moisture and preci

the interaction of planetary scale tropical easterly waves with topography a mechanism for the initiation of tropical cyclones abstract the interaction of a basic state flow consisting of a planetary scale easterly wave superimposed on a uniform easterly zonal wind with an isolated topographic feature is examined by numerical integration of the shallow water equations in an equatorial beta plane channel the topographic parameters are chosen to represent the sierra madre mountains of mexico the zonal wavelength and phase speeds of the basic state easterly waves are chosen to correspond with previously published observations the topographic modification of a slow rossby mode in which the basic state vorticity maximum is located near 15 latitude is characterized by the generation of a cyclonic vorticity maximum in the lee of the mountain which extends the entire length of the ridge and a secondary maximum which propagates downstream a fast mixed rossby gravity mode in which the vorticity maximum is located at the equator interacts less strongly with the mountain with a lee vorticity maximum that is confined near the southern

diagnosis of the mjo modulation of tropical cyclogenesis using an empirical index the modulation of tropical cyclone activity by the madden julian oscillation mjo is explored using an empirical genesis potential gp index composite anomalies of the genesis index associated with the different mjo phases are consistent with the composite anomalies in tc genesis frequency that occur in the same phases indicating that the index captures the changes in the environment that are at least in part responsible for the genesis frequency changes of the four environmental variables that enter the genesis potential index the midlevel relative humidity makes the largest contribution to the mjo composite gp anomalies the second largest contribution comes from the low level absolute vorticity and only very minor contributions come from the vertical wind shear and potential intensity when basin integrated mjo composite anomalies of the gp index are regressed against basin integrated composite anomalies of tc genesis frequency the results differ quantitatively from those obtained from the analogous calculation performed on the annual climatologies in the two quantities the gp index captures the mjo modulation of tc genesis to a lesser degree than the climatological annual cycle of genesis to which it was originally tuned this may be due to weaknesses of the reanalysis or indicative of the importance of precursor disturbances not well captured in the gp index computed from weekly data to the intraseasonal tc genesis frequency fluctuations

tropical storm formation in a baroclinic environment abstract an analysis is presented of the large scale conditions associated with the initial development of tropical storm diana september 1984 in a baroclinic environment ordinary extratropical wave cyclogenesis began along an old frontal boundary east of florida after 0000 utc 7 september and culminated in tropical cyclogenesis 48 h later water vapor satellite imagery showed that the initial cyclogenesis and incipient tropical storm formation was nearly indistinguishable from a classical midlatitude development cyclogenesis occurred in three stages a large scale cold trough and associated frontal system crossed the atlantic coast while a small potential vorticity maximum aloft fractured off the main trough and stalled over central florida in the first stage as the main trough sheared off eastward cyclogenesis began along the southwestern end of the stalled frontal zone east of florida anticyclogenesis to the north in the wake of the shearing trough allowed a surge of cooler and drier air to flo

the lifecycle of mid ocean ridge seamounts and their prodigious flank collapses abstract volcanic seamounts are spectacular bathymetric features representing undersea mountains comparable in size to volcanic islands they are often sites of active volcanism represent major marine habitats and contain potentially globally significant commodities of trace metals however we know comparatively little about how they form the extent of their volcanic lifecycle and what risks they pose from periodic flank collapse here we study a uniquely extensive 43 myr record of volcaniclastic and calciclastic turbidites accumulated on the madeira abyssal plain in the ne atlantic which originate from large submarine landslides from the adjacent great meteor cruiser seamount complex on the flanks of the mid atlantic ridge these turbidites present the first long term temporal record of the recurrence magnitude and emplacement mechanism of seamount flank collapses from a single seamount complex throughout its lifecycle landslides from the great meteor cruiser seamount complex are comparable size to those from the canary islands with average volumes of 30 40 km3 but can be as large as 130 km3 comparable to many volcanic island landslides these seamount landslides were predominantly multi stage failures similar to those form neighbouring volcanic islands but in contrast occurred preferentially during sea level lowstands we show that volcanic seamounts above mantle plumes have long protracted lifecycles and that substantial flank failures are capable of occurring at all stages of development temporal change in turbidite composition has for the first time enabled reconstruction of the inception around 43 45 myrs ago ascension taking around 20 25 myrs emergence after 30 myrs and later recession last 9 myrs cessation in volcanism last 4 myrs and subsidence of the seamounts from 2 myrs onwards these seamounts above a mantle plume have taken around 26 myrs to emergence above sea level from inception while their volcanic island contemporaries only took 2 to 5 myrs here we use the history of turbidites derived from landslides from the seamounts to show how the seamounts developed and suffered mass wasting during their life cycles

north central pacific tropical cyclones impacts of el nino southern oscillation and the madden julian oscillation abstractboth el nino southern oscillation enso and the madden julian oscillation mjo have been documented in previous research to impact tropical cyclone tc activity around the globe this study examines the relationship of each mode individually along with a combined index on tropical cyclone activity in the north central pacific approximately twice as many tropical cyclones form in the north central pacific in el nino years compared with la nina years these differences are attributed to a variety of factors including warmer sea surface temperatures lower sea level pressures increased midlevel moisture and anomalous midlevel ascent in el nino years when the convectively enhanced phase of the mjo is located over the eastern and central tropical pacific the north central pacific tends to have more tropical cyclone activity likely because of reduced vertical wind shear lower sea level pressures and increased vertical motion the convectively enhanced phase of the mjo is also responsible for

tropical cyclogenesis associated with kelvin waves and the madden julian oscillation abstractthe madden julian oscillation mjo influences tropical cyclone formation around the globe convectively coupled kelvin waves are often embedded within the mjo but their role in tropical cyclogenesis remains uncertain this case study identifies the influences of the mjo and a series of kelvin waves on the formation of two tropical cyclones typhoons rammasun and chataan developed in the western north pacific on 28 june 2002 two weeks earlier conditions had been unfavorable for tropical cyclogenesis because of uniform trade easterlies and a lack of organized convection the easterlies gave way to equatorial westerlies as the convective envelope of the madden julian oscillation moved into the region a series of three kelvin waves modulated the development of the westerlies cyclonic potential vorticity pv developed in a strip between the growing equatorial westerlies and the persistent trade easterlies farther poleward rammasun and chataan emerged from the apparent breakdown of this strip the

mjo and tropical cyclogenesis in the gulf of mexico and eastern pacific case study and idealized numerical modeling the role of the madden julian oscillation mjo in modulating the frequency and location of tropical cyclogenesis over the eastern pacific and the gulf of mexico during august september 1998 is examined during the nonconvective phase of the mjo convection and low level cyclonic vorticity occurred primarily in conjunction with the intertropical convergence zone itcz during the convective phase convection low level cyclonic vorticity and convergence expanded into the northeastern pacific and the gulf of mexico this was accompanied by enhanced eddy kinetic energy and barotropic energy conversions as compared to the nonconvective phase consistent with previous research during the nonconvective phase of the mjo vertical shear was relatively weaker but tropical cyclones tended to form mainly within the itcz on the contrary during the convective phase vertical wind shear exceeded 10 m s 1 over much of this region and tropical cyclone development occurred north of the itcz near the mexican pacific coast and the gulf of mexico idealized numerical experiments are conducted using a barotropic model with time invariant basic states representative of the nonconvective and convective phases of the mjo the simulations indicate that the propagation paths as well as the amplification of the eddies differ substantially between the two phases during the nonconvective phase the waves tend to propagate westward into the eastern pacific during the convective phase stronger southerlies steer the waves into the gulf of mexico the mjo related modulation of tropical cyclogenesis in the eastern pacific and gulf of mexico thus appears to involve anomalous convergence cyclonic vorticity vertical wind shear and differing tracks of easterly waves

vertical crustal movements at the vema fracture zone in the atlantic evidence from dredged limestones abstract a crustal block shallower by over 3 km than the level predicted by the thermal contraction model for the ocean crust is located on the southern side of the vema transform in the atlantic lithology magnetic and gravity data suggest the anomalous block is an uplifted piece of crust and upper mantle and not a constructional volcanic feature the crest and upper slopes of the anomalous block are capped by shallow water reef limestones determination of paleofacies and ages of samples of these limestones suggest that 1 a the crest of the anomalous crustal block reached sea level and was exposed subaerially in middle pliocene time about 3 m y ago and subsequently sank to its present depth at an average rate of about 0 3 mm year that is a rate one order of magnitude faster than that predicted by the thermal contraction model for crust of the same age 2 b the anomalous block reached at or above sea level either continuously or intermittently throughout the late miocene from roughly 10 m y ago 3 c paleocene sediments dated at about 55 58 m y have been recovered from the uplifted block where assuming a spreading rate of 1 1 cm year the crust should not be more than about 35 m y old vertical crustal movements at the vema transform provide an example of vertical tectonism in the large oceanic transform fracture zones

modulation of eastern north pacific hurricanes by the madden julian oscillation abstract hurricane and tropical storm statistics verify the modulation of eastern pacific tropical systems by the madden julian oscillation mjo as hypothesized by maloney and hartmann over twice as many named tropical systems hurricanes and tropical storms accompany equatorial 850 mb westerly anomalies than accompany equatorial easterly anomalies and the systems that do exist are stronger hurricanes are over four times more numerous during westerly phases of the mjo than during easterly phases the current study constructs a composite life cycle of the mjo during may november 1979 95 using an index based on the 850 mb equatorial zonal wind equatorial kelvin waves propagating eastward from convective regions of the western pacific ocean alter dynamical conditions over the eastern pacific ocean westerly easterly equatorial 850 mb wind anomalies are accompanied by enhanced suppressed convection over the eastern pacific hurricane region convection locally amplifies the wind anomalies over the ea

neogene development of the swan islands restraining bend complex caribbean sea we present seamarc ii and multichannel seismic data describing deformation at an active restraining bend complex formed along a left lateral strike slip fault in the western caribbean sea the bend complex consists of three distinct strike slip fault traces that form two right stepping sharp restraining bends or push ups at its western end and form a right stepping gentle restraining bend at its eastern end deformation at the sharp restraining bends is characterized by local folding between overlapping strike slip faults deformation at the much larger gentle restraining bend consists of a large anticline adjacent to a gently curving strike slip fault a small active accretionary wedge forms the northern flank of the bend complex we speculate that the 30 km wide right stepping bend complex formed as a consequence of southward propagation of the mid cayman spreading center during oligocene to early miocene time active faults along the southern edge of the bend complex may have formed after the initial uplift and folding of the complex and may currently serve to straighten the strike slip fault by bypassing the bend complex

are multiple tropical cyclone events similar among basins abstractthe present study intercompares multiple tropical cyclone event mtce characteristics among each global tropical cyclone tc basin using best track data specific focus is placed on examining the number of mtces and tcs during mtces the zonal distance between tcs during mtces and the spatiotemporal separation between genesis events during mtces the results suggest that the ratio of mtces relative to single tcs is substantially higher in the eastern north pacific enp western north pacific wnp and south indian ocean si basins compared to the north atlantic na and south pacific sp the prolific nature of enp wnp and si mtce activity results in approximately half of tcs occurring during mtces during new tc genesis the majority of preexisting tcs are generally located westward at a consistent zonal distance from new tc genesis for mtces within each basin with median values between 1620 and 1961 km tc induced rossby wave dispersion may set this zonal length scale as implied by it

morphotectonics of the gorringe bank summit eastern atlantic ocean based on high resolution multibeam bathymetry abstract high resolution swath bathymetry of the gorringe bank located along the europe africa obliquely convergent plate boundary offshore sw portugal allowed investigation of its shallow depth morphotectonic structure as a contribution to understanding the interplay of sea level changes and active deformation of the region dominant ene wsw to nne wsw and subordinate nw se trending morphologic alignments were mapped on the digital terrain model dtm built from multibeam data for both the gettysburg and ormonde summits of the bank based on appraisal of regional structural and seismological data the dominant and subordinate sets are interpreted to reflect dip slip to left oblique reverse faults and normal faults or dyke like joints respectively several terrace remnants were mapped on the dtm and were grouped into five orders at depths ranging from 40 to 120 m morphological features suggest that the terraces were formed in a coastal environment likely during middle late pleistocene sea level changes the terraces are probably polycyclic implying minor vertical tectonic motion however the whole terrace set except the deeper and younger one which was mainly shaped during the lgm lowstand is consistently up to 10 m lower at ormonde in the east relative to gettysburg in the west suggesting displacement across a nne ssw striking fault mapped in the saddle between the two seamounts the structure assemblage is argued to represent the shallow splay system of a crustal scale fault ramp underlying the bank the ene wsw reverse faults have a southward polarity and together with the overall northwest tilt of the bank point to displacement toward the horseshoe abyssal plain to the south at least at upper crustal levels

a statistical analysis of relationships between western north pacific tropical cyclones and extratropical circulation patterns accompanying the madden julian oscillation abstractrelationships between the madden julian oscillation mjo and the extratropical circulation have been studied extensively and applied in operational settings to improve subseasonal predicti

effect of enso and the mjo on western north pacific tropical cyclones prior work has suggested that rossby wave accumulation in the lower troposphere may be an important mechanism for the development of synoptic scale disturbances in the western north pacific in northern summer this work examines the intraseasonal and interannual variability of wave accumulation by computing the barotropic wave activity flux divergence at 850 hpa for different phases of the madden julian oscillation mjo and el nino southern oscillation enso indirectly and without ruling out other influences the results suggest a physical mechanism behind previously identified relationships between tropical cyclogenesis statistics and these two modes of variability mjo and enso

holocene tectonic uplift of the st peter and st paul rocks equatorial atlantic consistent with emplacement by extrusion abstract st peter and st paul rocks spspr are emergent portions of a peridotitic mass within the st paul fracture zone of the equatorial mid atlantic ridge where local seismicity is dominated by strike slip mechanisms we used subtidal red algae in living position death assemblages in intertidal conglomerates and marine terraces as sea level markers the highest deposits correspond to a marine terrace with a shoreline angle at 8 1 m asl that formed ca 6400 6200 cal yr bp after correction for the sea level changes derived by glacioisostatic modelling ams and radiometric 14 c age measurements on the holocene sea level markers indicate emergence during the last 6600 a at a minimum average rate of 1 5 mm a we propose that uplift resulted from peridotite emplacement by extrusion rather than volcanic activity or fault slip with the implication that the ridge has been subject to compression during the holocene

deformation and structural evolution of mantle peridotites during exhumation on transform faults a forced transition from ductile to brittle regime abstract the exhumation of peridotite rocks in oceanic transform zones passes by the rheological transition between the ductile and brittle deformation until the complete emplacement in the oceanic lithosphere the sao pedro and sao paulo archipelago spspa in the equatorial atlantic records the deformational products of ductile brittle and the rocks fluid interaction generating specific structures in each domain the deformational stages are related to the transpressional and transtensional geodynamics of sao paulo transform fault firstly during transpression exhumation occurs associated with the ductile domain causing intense mylonitization in temperatures between 700 and 800 c defined by olivine and orthopyroxene recrystallization the interaction with fluids initially originated from the mantle generates amphibole and oxide rich layers marking the passage to a semi brittle deformation the continuation of peridotite exhumation associated with an nw se shortening and transpressional led to a higher availability of hydrothermal fluids as a consequence four serpentinization episodes are recorded which are associated with semi brittle to brittle transition under temperatures between 300 and 400 c finally the complete exhumation and establishment of brittle mechanisms led to carbonatation phase near the surface with temperatures ranging from 300 to 150 c the active nw se tectonic stress generated e w strike slip faults that were filled by carbonates recording the final exhumation stage

african easterly jet barotropic instability waves and cyclogenesis abstract this study investigates the structure of the african easterly jet focusing on instability processes on a seasonal and subseasonal scale with the goal of identifying features that could provide increased predictability of atlantic tropical cyclogenesis the modern era retrospective analysis for research and applications merra is used as the main investigating tool merra is compared with other reanalyses datasets from major operational centers around the world and was found to describe very effectively the circulation over the african monsoon region in particular a comparison with precipitation datasets from the global precipitation climatology project shows that merra realistically reproduces seasonal precipitation over that region the verification of the generalized kuo barotropic instability condition computed from seasonal means is found to have the interesting property of defining well the location where observed tropical storms are detected this property does not appear to be an arti

on relationships between nonrecurving western north pacific tropical cyclones the madden julian oscillation and the east asian subtropical jet abstracta 200 hpa zonal momentum budget is performed to examine the role that western north pacific tropical cyclones tcs play in helping to organize intraseasonal extratropical circulation anoma

oceanic transform earthquakes with unusual mechanisms or locations relation to fault geometry and state of stress in the adjacent lithosphere on oceanic transforms most earthquakes are expected to occur on the principal transform displacement zone ptdz and to have strike slip mechanisms consistent with transform parallel motion we conducted a search for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the harvard centroid moment tensor catalogue and the bulletin of the international seismological centre respectively events with unusual mechanisms occur on several transforms we have determined the source mechanisms and centroid depths of 10 such earthquakes on the st paul s marathon owen heezen tharp menard and rivera transforms from inversions of long period body waveforms relative locations of earthquakes along these transforms have been determined with a multiple event relocation technique much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the ptdz or the presence of large structural features that may influence slip on the fault reverse faulting earthquakes occur at a compressional bend in the owen transform in the area of mount error and at the st paul s transform near st peter s and st paul s rocks a normal faulting earthquake on the heezen transform is located at the edge of a pull apart basin marking an extensional offset of the fault normal faulting earthquakes along the tharp menard and rivera transforms may also be related to extensional offsets some events with unusual mechanisms occur outside of the transform fault zone however and do not appear to be related to fault zone geometry for instance earthquakes with mechanisms indicating reverse faulting on ridge parallel fault planes are located near the ridge transform intersections of the st paul s and the marathon transforms possible additional contributors to the occurrence of anomalous earthquakes include recent changes in plate motion differential lithospheric cooling and the development of a zone of weakness along the fault zone but we do not find strong evidence to confirm the influence of these processes

modulation of hurricane activity in the gulf of mexico by the madden julian oscillation the madden julian oscillation mjo is a large scale episodic modulation of tropical winds and precipitation that travels eastward from asia to america with a characteristic repeat time of 30 to 60 days here it is shown that when mjo wind anomalies in the lower troposphere of the eastern pacific are westerly gulf of mexico and western caribbean hurricane genesis is four times more likely than when the mjo winds are easterly accurate predictions of the mjo may lead to improved long range forecasts of tropical cyclone activity

planetary and synoptic scale influences on eastern pacific tropical cyclogenesis abstract the structure and evolution of lowpass filtered background flow and synoptic scale easterly waves were examined during the 1991 eastern pacific hurricane season active and inactive cyclogenesis periods conformed well to the sign of the near equatorial lowpass filtered 850 mb zonal wind anomaly consistent with the recent results of maloney and hartmann this behavior emphasizes the importance of westerly wind bursts associated with the madden julian oscillation mjo in creating an environment favorable for eastern pacific tropical cyclogenesis synoptic scale easterly waves reached the western caribbean and eastern pacific regularly from upstream usually from africa the amplitude of waves leaving africa had little correlation with the likelihood of a wave producing an eastern pacific storm rather easterly waves intensified and tropical depressions formed during the convectively active phase of the mjo in the western caribbean and eastern pacific wave growth measured by strengthening o

transform migration and vertical tectonics at the romanche fracture zone equatorial atlantic the romanch transform offsets the mid atlantic ridge mar axis by about 950 km in the equatorial atlantic multibeam and high resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the riv akademik strakhov as part of the russian italian mid atlantic ridge project primar morphobathymetric data show the existence on the northern side of the transform of a major 800 kin long aseismic valley oriented 10 o to 15 o from the active valley it disappears about 150 km from the western mar segment the aseismic valley marks probably the former location of the romanche transform paleoromanche that was active up to roughly 8 10 ma when the transform boundary migrated to its present position a temporary microplate developed during the migration and reorientation of the transform this microplate changed its sense of motion as it was transferred from the south american to the african plate a prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side reaching its shallowest part shallower by over 4 km than the predicted thermal contraction depth in a zone opposite the eastern mar axis transform intersection rti flat top peaks on the summit of the transverse ridge are capped by acoustically transparent weakly stratified shallow water platform lagunal reef limestones this limestone unit is a few hundred meters thick and overlies igneous basement evaluation of the seismic reflection data as well as study of samples of carbonates ventifact basaltic pebbles and gabbroic peridofitic and basaltic rocks recovered at different sites on the transverse ridge suggest that 1 the summit of the transverse ridge was above sea level at and before about 5 ma 2 the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate its summit was flattened by erosion at sea level during subsidence 3 the transverse ridge is an uplifted sliver of lithosphere and not a volcanic construcfional feature and 4 transtensional and transpressional tectonics have affected the transverse ridge hypotheses on the origin of the romanche transverse ridge include 1 lateral heat conduction across the rti 2 shear heating 3 lithospheric flexure due to thermal stresses in the cooling lithosphere 4 viscoelastic deformation of the lithosphere 5 hydration dehydration of mantle peridotites and 6 longitudinal flow of melt and igneous activity across the rti these processes cannot by themselves explain the transverse ridge although some of them could contribute to its formation to a small extent vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former paleoromanche and the younger transform boundaries before and during the transition to the present boundary after migration of the transform boundary to its present position transpression was replaced by transtension and by subsidence of the transverse ridge an aseismic axial rift valley impacting against the transform valley about 80 km west of the present rti suggests eastward ridge jumping that probably followed transform migration localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements such as those that formed an ultradeep 7800 m pull apart basin along the transform valley

the madden julian oscillation barotropic dynamics and north pacific tropical cyclone formation part ii stochastic barotropic modeling abstract a stochastic barotropic model linearized about the 850 mb flow is used to investigate the relationship between wind variations associated with the madden julian oscillation mjo and eddy kinetic energy variations in the tropics such a model is successful in predicting the observed location of eddy kinetic energy maxima during the westerly phase of the mjo and the suppression of eddy activity during the easterly phase of the mjo the concentration of eddy energy during the westerly phase results from the strong east west and north south gradients of the large scale wind fields the model shows that barotropic wave propagation and wave mean flow interaction tend to concentrate small scale rossby wave energy in regions of convergence which may be an important mechanism for organizing convection into tropical cyclones the structure and barotropic energetics of the wave activity are similar to those observed but the modeled eddies are smaller in scale and do not move westward as do the observed e

late quaternary reorientation of stress field and extension direction in the southern gulf of suez egypt evidence from uplifted coral terraces mesoscopic fault arrays and borehole breakouts uplifted pleistocene coral terraces and modern earthquakes show that several large normal faults are presently active in the southern gulf of suez rift basin the footwall of one of these faults is exposed at gebel el zeit where terraces at elevations of 10 18 and 42 m have been radiometrically dated as 125 ka and 426 ka respectively after correcting for eustatic sea level changes this results in net tectonic uplift rates of about 0 1 m kyr published interpretations and our own observations indicate that the average extension direction during the miocene to pliocene synrift history was 055 analysis of borehole breakouts and published earthquake fault plane solutions however suggests that the present day stress field in the southern gulf has a 010 020 shmin orientation detailed structural observations show that a change in extension direction occurred in the late pleistocene with rotation of the stress field beginning prior to formation of the 125 ka terraces but after formation of older pleistocene terraces whose ages are less tightly constrained using a horizontal slip direction of 015 and our observed net footwall uplift rate we calculate a separation velocity between sinai and africa of 0 8 1 2 m kyr the proposed pleistocene change in extension direction in the gulf of suez corresponds closely with the post 500 ka change in extension direction documented in the kenyan rift system and a similar change in extension direction recorded in the central red sea these regional similarities in tectonic history suggest that the underlying causes of these events may be a plate scale phenomenon affecting the entire afro arabian rift system rather than local changes in the quaternary stress field

nonvolcanic tectonic islands in ancient and modern oceans 1 most oceanic islands are due to excess volcanism caused by thermal and or compositional mantle melting anomalies we call attention here to another class of oceanic islands due not to volcanism but to vertical motions of blocks of oceanic lithosphere related to transform tectonics sunken tectonic islands capped by carbonate platforms have been previously identified along the vema and romanche transforms in the equatorial atlantic we reprocessed seismic reflection lines did new facies analyses and 87sr 86sr dating of carbonate samples from the carbonate platforms a 50 km long narrow paleoisland flanking the vema transform underwent subsidence erosion and truncation at sea level it was then capped by a 500 m thick carbonate platform dated by 87sr 86sr at 11 10 ma three former islands on the crest of the romanche transverse ridge are now at 900 m bsl they show horizontal truncated surfaces of oceanic crust capped by 300 m thick carbonate platforms with 10 6 ma sr isotopic ages these sunken islands formed due to vertical tectonics related to transtension transpression along long offset slow slip transforms another tectonic sunken island is atlantis bank an uplifted gabbroic block along the atlantis ii transform sw indian ridge 700 m bsl a modern tectonic island is st peter and st paul rocks a rising slab of upper mantle located at the st paul transform equatorial atlantic cold tectonic islands contrast with hot volcanic islands related to mantle thermal and or compositional anomalies along accretionary boundaries and within oceanic plates or to supra subduction mantle melting that gives rise to islands arcs

influence of the monsoon trough on westward propagating tropical waves over the western north pacific part ii energetics and numerical experiments abstractpart i of this study examined the modulation of the monsoon trough mt on tropical depression td type mixed rossby gravity mrg and equatorial rossby er waves over the western north pacific based on observations this part investigates the interaction of these waves with the mt through a diagnostics of energy conversion that separates the effect of the mt on td mrg and er waves it is found that the barotropic conversion associated with the mt is the most important mechanism for the growth of eddy energy in both td mrg and er waves the large rotational flows help to maintain the rapid growth and tilted horizontal structure of the lower tropospheric waves through a positive feedback between the wave growth and horizontal structure the baroclinic conversion process associated with the mt contributes a smaller part for td mrg waves but is of importance comparable to barotropic conversion for er waves as it can produce the tilted vertical structure the growth rates of the waves are much larg

potential vorticity easterly waves and eastern pacific tropical cyclogenesis abstract a significant sign reversal in the meridional potential vorticity gradient was found during the summer of 1991 on the 310 k isentropic surface near 700 mb over the caribbean sea the charney stern necessary condition for instability of the mean flow is met in this region it is speculated that the sign reversal permits either invigoration of african waves or actual generation of easterly waves in the caribbean during the same season a correlation existed between the strength of the negative potential vorticity gradient in the caribbean and subsequent cyclogenesis in the eastern pacific the meridional pv gradient convective heating measured by outgoing longwave radiation data and eastern pacific cyclogenesis all varied on the timescale of the madden julian oscillation mjo it is hypothesized that upstream wave growth in the dynamically unstable region provides the connection between the mjo or any other convective forcing and the associated enhanced downstream tropical cyclogenesis

contributions of tropical waves to tropical cyclone genesis over the western north pacific the present study investigates the relationship between the tropical waves and the tropical cyclone tc genesis over the western north pacific wnp for the period 1979 2011 five wave types are considered in this study it is shown that the tc genesis is strongly related to enhanced low level vorticity and convection of tropical waves and significant difference are detected in the tc modulation by dynamic and thermodynamic components of the waves more tcs tend to form in regions of waves with overlapping cyclonic vorticity and active convection about 83 2 of tcs form within active phase of tropical waves mainly in a single wave and two coexisting waves each wave type related genesis accounts for about 30 of all tc geneses except for the kelvin waves that account for only 25 2 of tc geneses the number of each wave type related tc genesis consistently varies seasonally with peak in the tc season july november which is attributed to a combined effect of active wave probability and intensity change the interannual variation in the tc genesis is well reproduced by the tropical wave related tc genesis especially in the region east of 150 e an eastward extension of the enhanced monsoon trough coincides with increased tropical wave activity by accelerated wave mean flow interaction

development of synoptic scale disturbances over the summertime tropical northwest pacific abstract this study addresses the origin of the synoptic scale disturbances that occur in the tropical western north pacific ocean wp region in northern hemisphere summer these have been called easterly waves and tropical depression type td disturbances this analysis uses the national center for environmental prediction national center for atmospheric research reanalysis dataset by performing a regression analysis on several terms in the vorticity equation at 850 hpa it is shown that the td disturbances propagate approximately as barotropic rossby waves at 850 hpa given this ray tracing calculations and the wave activity diagnostic introduced by plumb are used to show that wave accumulation is a promising candidate for the initial development mechanism of the td disturbances the expected local growth rate from this mechanism is simply the convergence of the group velocity which reaches values corresponding to a growth timescale of 3 days this convergence is dominated by but somewhat lar

stratigraphic and tectonic implications of uranium series dated coral reefs from uplifted red sea islands abstract corals from raised reef terraces on two islands zabargad and northern brother considered to have been tectonically uplifted in connection with the red sea rifting were dated with the u series method at zabargad there are at least three systems of raised coral reefs the oldest terrace 290 000 300 000 yr b p is found at 10 to 15 m a 200 000 yr b p high sea stand is recorded by a terrace relict at 17 m on peridotite bedrock the youngest system 125 000 138 000 yr b p is very well represented around the island with terraces at about 6 to 8 m corals from northern brother yield ages of 132 000 135 000 and 204 000 yr b p suggesting the existence of two systems of interglacial raised reefs both islands appear to have been tectonically quite stable since at least 125 000 yr b p

zabargad and the isotopic evolution of the sub red sea mantle and crust abstract zabargad island in the red sea exposes ultramafic bodies in contact with partially retrograded granulite facies lower crustal rocks the contact could represent the sub red sea moho prior to the opening of the red sea the ultramafic rocks are largely protogranular spinel lherzolites that grade locally to porphyroclastic to mylonitic amphibole peridotites some of the peridotites are infiltrated by high mg gabbros troctolites the peridotites and the lower crustal rocks are cut by diabase dikes the zabargad peridotites are isotopically heterogeneous on a scale of meters or less sr and nd isotopic ratios of clinopyroxenes cpx from spinel lherzolites lie in the depleted portion of the 143 nd 144 nd 87 sr 86 sr mantle array whereas cpx and amphiboles from amphibole peridotites fall off the array towards the low 143 nd 144 nd and sol 87 sr 86 sr field that may characterize portions of the sub continental mantle cpx from troctolites plot in the middle of the 143 nd 144 nd 87 sr 86 sr mantle array and have higher 87 sr 86 sr ratios and more depleted lree patterns than the intruded spinel lherzolites and amphibole peridotites all whole rock lherzolites and peridotites and most diabase dikes from zabargad as well as a variety of rocks from other parts of the red sea area morb glasses a diabase dike from the brothers island basanites and clinopyroxenes from associated nodules from the ataq diatreme in yemen define a linear array on a sm nd isochron diagram with a best fit age of ca 675 ma and an initial nd of 6 7 this array is probably a mixing line between depleted morb type mantle and a low 143 nd 144 nd 87 sr 86 sr metasomatic component the age defined by the linear array may date either an episode of pan african mantle homogenization or in our preferred model the pan african segregation of a previously homogeneous mantle into two or more reservoirs of identical 143 nd 144 nd but different sm nd ratios the subsequent partial remixing of these isotopic reservoirs during metasomatism associated with the opening of the red sea did not perturbed the slope of the array the lree enriched reservoir may have been the pan african lower crust since there is field and textural evidence that the metasomatic fluids were derived from or equilibrated with the adjacent crustal complex alternatively the lree enriched component may have been a mantle reservoir similar to that which generated basalts and nodules from the nearby ataq diatreme the troctolites are isotopically distinct from the host peridotites and retain igneous textures suggesting they intruded the peridotites during the opening of the red sea their isotopic patterns are similar to those of recent volcanics from afar portions of the ataq diatreme and some zabargad dikes indicating derivation from a similar mantle source their very lree depleted patterns suggest they are either re melted fractions of previously very depleted spinel lherzolites or early formed mineral aggregates that became enmeshed in the peridotites as fractionating melt ascended to higher levels our results indicate that at least two major processes metasomatism and intrusion have modified the isotopic pat terns of the spinel lherzolites of zabargad had the resultant chemically and isotopically heterogeneous bodies stayed in the mantle instead of being uplifted to their present positions they would have proven to be enigmatic sources for future basalts and xenoliths

barotropic aspects of itcz breakdown abstract in satellite images the itcz intertropical convergence zone is sometimes observed to undulate and break down into a series of tropical disturbances tropical cyclones may later develop within these disturbances and move into higher latitudes allowing the itcz to reform it has been proposed that itcz breakdown results from a convectively modified form of combined barotropic and baroclinic instability of the mean flow an unstable mean flow can be produced by itcz convection in just a couple of days in this sense the itcz produces favorable conditions for its own instability and breakdown in this study a nonlinear shallow water model on the sphere is used to simulate barotropic aspects of itcz breakdown in the shallow water model the itcz is simulated by a prescribed zonally elongated mass sink near the equator the mass sink produces a cyclonic potential vorticity pv anomaly that has a reversed meridional pv gradient on its poleward side according to the ripa theorem a flow that has a