late quaternary palaeoenvironment of northern guatemala evidence from deep drill cores and seismic stratigraphy of lake peten itza long sediment cores were collected in spring 2006 from lake peten itza northern guatemala in water depths ranging from 30 to 150 m as part of an international continental scientific drilling program project the sediment records from deep water consist mainly of alternating clay gypsum and carbonate units and in at least two drill sites extend back 200 kyr most of the lithostratigraphic units are traceable throughout the basin along seismic reflections that serve as seismic stratigraphic boundaries and suggest that the lithostratigraphy can be used to infer regional palaeoenvironmental changes a revised seismic stratigraphy was established on the basis of integrated lithological and seismic reflection data from the basin from ca 200 to ca 85 ka sediments are dominated by carbonate clay silt often interbedded with sandy turbidites indicating a sediment regime dominated by detrital sedimentation in a relatively humid climate at ca 85 ka an exposure horizon consisting of gravels coarse sand and terrestrial gastropods marks a lake lowstand or partial basin desiccation indicating dry climate conditions from ca 85 to ca 48 ka transgressive carbonate clay sediments overlain by deep water clays suggest a lake level rise and subsequent stabilization at high stage from ca 48 ka to present the lithology is characterized by alternating clay and gypsum units gypsum deposition correlates with heinrich events i e dry climate whereas clay units coincide with more humid interstadials

active strike slip faulting in el salvador central america several major earthquakes have affected el salvador central america during the past 100 yr as a consequence of oblique subduction of the cocos plate under the caribbean plate which is partitioned between trench orthogonal compression and strike slip deformation parallel to the volcanic arc focal mechanisms and the distribution of the most destructive earthquakes together with geomorphologic evidence suggest that this transcurrent component of motion may be accommodated by a major strike slip fault el salvador fault zone we present field geological structural and geomorphological data collected in central el salvador that allow the constraint of the kinematics and the quaternary activity of this major seismogenic strike slip fault system data suggest that the el salvador fault zone consists of at least two main e w fault segments san vicente and berlin segments with associated secondary synthetic wnw ese and antithetic nnw sse riedel shears and nw se tensional structures the two main fault segments overlap in a dextral en echelon style with the formation of an intervening pull apart basin our original geological and geomorphologic data suggest a late pleistocene holocene slip rate of 11 mm yr along the berlin segment in contrast with low historical seismicity the kinematics and rates of deformation suggested by our new data are consistent with models involving slip partitioning during oblique subduction and support the notion that a trench parallel component of motion between the caribbean and cocos plates is concentrated along e w dextral strike slip faults parallel to the volcanic arc

marginal sea overflows and the upper ocean interaction abstract marginal sea overflows and the overlying upper ocean are coupled in the vertical by two distinct mechanisms by an interfacial mass flux from the upper ocean to the overflow layer that accompanies entrainment and by a divergent eddy flux associated with baroclinic instability because both mechanisms tend to be localized in space the resulting upper ocean circulation can be characterized as a plume for which the relevant background potential vorticity is set by the slope of the topography that is a topographic plume the entrainment driven topographic plume consists of a single gyre that is aligned along isobaths the circulation is cyclonic within the upper ocean water columns are stretched the transport within one branch of the topographic plume may exceed the entrainment flux by a factor of 2 or more overflows are likely to be baroclinically unstable especially near the strait this creates eddy variability in both the upper ocean and overflow layers and a flux of momentum and e

single crystal 40ar 39ar dating of the olorgesailie formation southern kenya rift single crystal laser fusion sup 40 ar sup 39 ar analyses and several conventional bulk fusion sup 40 k sup 40 ar dates have been used to determine the age of volcaniclastic strata within the olorgesailie formation and of associated volcanic and sedimentary units of the southern kenya rift in the principal exposures along the southern edge of the legemunge plain the formation spans the interval from approximately 500 to 1 000 ka deposition continued to the east along the ol keju nyiro river where a tuff near the top of the formation has been dated at 215 ka in these exposures the formation is unconformably overlain by sediments dated at 49 ka a possible source for the olorgesailie tephra the ol doinyo nyokie volcanic complex contains as ash flow dated at approximately 1 ma extending the known age range of this complex to encompass that of virtually the entire olorgesailie formation in the legemunge plain these geologic examples illustrate the importance of the single crystal sup 40 ar sup 39 ar dating technique whereby contaminant altered or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits the authors have presented a computer modeling procedure based on an inverse isochron analysis thatmore promotes a more objective approach to trimming sup 40 ar sup 39 ar isotope data sets of this type less

geological and seismological analysis of the 13 february 2001 mw 6 6 el salvador earthquake evidence for surface rupture and implications for seismic hazard the el salvador earthquake of 13 february 2001 mw 6 6 caused tectonic rupture on the el salvador fault zone esfz right lateral strike slip surface rupture of the east west trending fault zone had a maximum surface displacement of 0 60 m no vertical component was observed the earthquake resulted in widespread landslides in the epicentral area where bedrock is composed of volcanic sediments tephra and weak ignimbrites in the aftermath of the earthquake widespread damage to houses and roads and the hazards posed by landslides captured the attention of responding agencies and scientists and the presence of surface fault rupture was overlooked additionally the tectonic context in which the earthquake took place had not been clear until mapping of the esfz was completed for the present study we identified several fault segments the distribution of surface ruptures the aftershock pattern and fault rupture scaling considerations that indicate the 21 km long san vicente segment ruptured in the 2001 event static coulomb stress transfer models for the san vicente rupture are consistent with both aftershock activity of the 2001 sequence and ongoing background seismicity in the region at mw 6 6 the 2001 earthquake was of only moderate magnitude yet there was significant damage to the country s infrastructure including buildings and roads and numerous deaths and injuries thus earthquake hazard and risk in the vicinity of the esfz which straddles the city of san salvador with a population of 2 million is high because even moderate magnitude events can result in major damage deaths and injuries in the region

amatitlan an actively resurging cauldron 10 km south of guatemala city a 14 16 km diameter collapse caldera has been recognized 10 km south of guatemala city guatemala the caldera is north of the presently active volcano pacaya and west of agua a large stratovolcano the caldera was not previously recognized because its eastern and western margins coincide with faults that outline the guatemala city graben and because the northern margin of the caldera is buried by pyroclastic rocks the existence of the northern caldera margin is now established by gravity data and a variety of geological observations including circumferential faults hot springs well log data and lithological changes in sedimentary rocks a sequence of nine silicic pyroclastic deposits totaling a volume of more than 70 km 3 dense rock were erupted from the caldera the ages of these eruptions are mainly between about 300 000 years b p to less than 23 000 years b p the rocks erupted at the caldera and its associated vents consist of domes and nonwelded pyroclastic flow surge and fall deposits mainly of rhyolitic to dacitic composition successive pyroclastic eruptions have generally become smaller in volume and more silicic with time major and minor element chemistry distinguish amatitlan pyroclastics from those of other nearby calderas the caldera lies at the intersection of an offset of the volcanic chain the palin shear and the faults along the volcanic front jalpatagua fault zone the caldera has a heavily faulted resurgent dome crosscut by an impressive longitudinal graben the graben s alignment with the jalpatagua fault zone suggests a genetic relationship the longitudinal graben and resurgent dome are morphologically youthful and are the sites of many young silicic vents available seismic data show a heavy concentration of epicenters over the northern part of the resurgent dome near a young silicic intrusion the caldera is active and will probably erupt again over 1 million people live within 20 km and would be threatened in the event of a moderate eruption suggestions for future research focus on hazard assessment and forecasting

explosive eruptions of the ayarza calderas southeastern guatemala abstract the ayarza calderas southeastern guatemala form a figure 8 shaped depression that is now occupied by laguna de ayarza the smaller eastern caldera is also the older it formed upon eruption of a mixed plinian air fall that is locally overlain by a comagmatic ash flow deposit called the mixta unit the air fall portion contains rhyolitic 75 2 wt sio 2 pumice basaltic 52 wt sio 2 blobs and compositional hybrids that are either spectacularly banded or homogeneous in color although of limited aerial extent and volume 0 1 km 3 dense rock equivalent the unit is of considerable petrologic interest because the basalt contains pristine phenocrysts and quench crystallites of calcic hornblende virtually identical 87 sr 86 sr ratios 0 70373 of both rhyolite and basalt end members suggest a common magmatic source a log from the mixta ash flow deposit has been c 14 dated at 27 000 1 600 y b p eruption of the younger pyroclastic sequence at ayarza culminated in formation of the western caldera this rhyolitic sequence also consists of a plinian air fall overlain by a comagmatic ash flow deposit the ash flow unit is quite fine grained and probably phreatomagmatic in origin a thin horizon of air fall material is locally interbedded within the ash unit volume dre of the younger sequence has been estimated at a minimum 2 km 3 a log from the younger ash flow unit has been c 14 dated at 23 100 500 y b p the age of formation of the western caldera and the last recognized activity at ayarza although major element geochemistry of the ayarzan rhyolites is very similar to that of other silicic units of northern central america minor and trace element contents permit all the rhyolites to be distinguished from each other sr isotopic ratios of the mixta rhyolite reaffirm that older radiogenic crustal rocks are not involved in the petrogenesis of rhyolites in northern central america

on the mid depth circulation in the labrador and irminger seas a numerical circulation model with 1 6 resolution and an accurate topography formulation explains details of the observed circulation in the irminger and labrador seas that were recently revealed by lavender et al 2000 we show that the recirculation pattern is established through a locally wind induced flow controlled by the bottom topography and enhanced through remote baroclinic forcing by the dense plume of denmark strait overflow water the basic circulation is a robust feature in a hierarchy of model setups it exists in the purely barotropic case driven by steady winds and is even maintained when realistic daily forcing is added the narrow recirculation zone is manifested by a sea level depression spanning from the denmark strait across the irminger into the labrador sea

neotectonic development of the el salvador fault zone and implications for deformation in the central america volcanic arc insights from 4 d analog modeling experiments the el salvador fault zone esfz is an active approximately 150 km long and 20 km wide segmented dextral strike slip fault zone within the central american volcanic arc striking n100 e although several studies have investigated the surface expression of the esfz little is known about its structure at depth and its kinematic evolution structural field data and mapping suggest a phase of extension at some stage during the evolution of the esfz this phase would explain dip slip movements on structures that are currently associated with the active dominantly strike slip and that do not fit with the current tectonic regime field observations suggest trenchward migration of the arc such an extension and trenchward migration of the volcanic arc could be related to slab rollback of the cocos plate beneath the chortis block during the miocene pliocene we carried out 4 d analog model experiments to test whether an early phase of extension is required to form the present day fault pattern in the esfz our experiments suggest that a two phase tectonic evolution best explains the esfz an early pure extensional phase linked to a segmented volcanic arc is necessary to form the main structures this extensional phase is followed by a strike slip dominated regime which results in intersegment areas with local transtension and segments with almost pure strike slip motion the results of our experiments combined with field data along the central american volcanic arc indicate that the slab rollback intensity beneath the chortis block is greater in nicaragua and decreases westward to guatemala

three thousand years of flank and central vent eruptions of the san salvador volcanic complex el salvador and their effects on el cambio archeological site a review based on tephrostratigraphy the volcanic events of the last 3 000 years at san salvador volcanic complex are reviewed using detailed stratigraphic records exposed in new excavations between 2005 and 2007 at el cambio archeological site zapotitan valley el salvador and in other outcrops on the northern and northwestern sectors of the complex the sequences that overlie tierra blanca joven cal 429 107 ad from the ilopango caldera comprise the loma caldera cal 590 90 ad and el playon 1658 1671 deposits and the san andres tuff cal 1031 29 ad related to el boqueron volcano the surge deposits within the el playon san andres tuff and overlying talpetate ii sequences indicate the significance of phreatomagmatic phases in both central vent and flank eruptions during the last 1 600 years newly identified volcanic deposits underlying tierra blanca joven at el cambio extend the stratigraphic record of the area to 3 000 years bp paleosols interstratified with those deposits contain cultural artifacts which could be associated with the middle preclassic period 900 400 bc if correct human occupation of the site during the preclassic period was more intense than previously known and volcanic eruptions must have affected prehistoric settlements the archeological findings provide information on how prehistoric populations dealt with volcanic hazards thousands of years ago in the eastern zapotitan valley where several housing projects are currently being developed the new stratigraphic and volcanological data can be used as a basis for local and regional hazard assessment related to future secondary vent activity in the san salvador volcanic complex

neotectonic deformation within an extensional stepover in el salvador magmatic arc central america implication for the interaction of arc magmatism and deformation abstract dominantly westward movement of the el salvador forearc at rates of 11 mm yr is accommodated by a series of e w to wnw oriented dextral strike slip fault zones herein referred to as the el salvador fault system esfs the geometry of the esfs defines a series of extensional step overs along the arc basaltic volcanism in the stepovers is associated with nnw oriented normal faults whereas rhyolitic volcanism is associated with strike slip fault zones of the esfs on the esfs the san salvador extensional stepover sses is bound to the south by the san vicente fault zone where the rhyolitic ilopango caldera is located in the sses tephras from ilopango the tierra blanca tb sequence track long term elongation older tb units tb5 8 contain abundant normal faults lying unconformably above these older tb units younger tb members tbj tb2 4 are generally unfaulted analyses of faults in tb5 8 indicate ne to ene oriented elongation in the sses deformation occurred between deposition of the tb4 and tb5 units during quiescence of the ilopango eruptive center using this temporal constraint minimum elongation rates of 3 50 10 15 s 1 2 06 10 14 s 1 and 4 42 10 14 s 1 were calculated for three traverses from regional geodetic data and fault kinematics throughout el salvador we interpret the sses as part of a series of pull apart structures along the arc axis the calculated paleostress orientations are consistent with a pull apart geometry resulting from forearc movement the extensional deformation occurs during a 50 k y lull in rhyolitic activity suggesting an interplay between magmatism and deformation within the arc during significant rhyolitic volcanic activity only minor elongation is observed in the sses despite ongoing translation of the salvadoran forearc we speculate that rhyolitic magmatism along upper crustal faults may facilitate strike slip movement on the esfs rather than distributing deformation throughout the extensional stepover

new information on the pectoral girdle and vertebral column in lupeosaurus reptilia pelycosauria ucla vp 1651 a new specimen from the lower permian admiral formation of archer county texas provides information on heretofore unknown portions of the postcranial skeleton of the large pelycosaur lupeosaurus presacral neural spines are elongate and have a subcircular cross section laterally directed tubercles are not present on any of the neural spines cervical vertebrae appear to have elongate neural spines and sharply keeled centra transverse processes are positioned relatively high above the bases of the centra through the length of axial column the pattern of tilting of the neural spines is similar to that in the primitive edaphosaur ianthasaurus hardestii the proximal end of the first sacral rib contributes significantly to the intervertebral articular facet usually formed by the centra the sacral ribs do not appear to fuse with one another distally the ventral clavicular plate is greatly expanded anteriorly the posterior edge of the subcoracoscapular fossa is very well defined the avail

on the three dimensional numerical modeling of the deep circulation around angel de la guarda island in the gulf of california a three dimensional nonlinear baroclinic model is used to model the circulation in the ballenas channel gulf of california mexico which was inferred from current meter observations over three sills that surround the area the suggested circulation consists of deep inflow that follows two paths the first one is a direct spill of water through san lorenzo sill into ballenas channel the second one a larger route that starts at san esteban sill then flows north of the island passing over tiburon and delfin basins and then turns to the south reaching the north ballenas channel sill and then spills into ballenas channel following the latter result a previous modeling effort to reproduce the circulation was partially obtained the long path was not reproduced and it was believed that finer horizontal resolution was needed in this work the bathymetric resolution was increased by a factor of three and the full path of this deep circulation is now obtained and corroborated

intensive surveys of the azores front 1 tracers and dynamics the hypothesis that fronts are sites of active subduction is examined using density temperature salinity and horizontal velocity data from a trio of surveys of the azores front done in may 1991 and march 1992 these surveys were made using a seasoar equipped with a conductivity temperature depth profiler and a shipboard acoustic doppler current profiler the potential density and potential vorticity indicate that dense water from the north side of the front may be sliding down beneath the surface outcrop this apparently subducting isopycnal has a great deal of temperature and salinity variability horizontal velocity is nearly parallel to isopycnals indicating that the time rate of change and vertical advection must be small the thermal wind balance is observed to be valid especially in the region of the largest horizontal density gradients shear at the base of the mixed layer is likely due to near inertial motions the potential vorticity is dominated by the planetary vorticity except at the front where vertical shears the tilting term become large the tilting term acts to reduce the magnitude of the potential vorticity at the front in agreement with simple theoretical models the magnitude of the tilting term is similar to the total vorticity in the seasonal thermocline

pacific offshore record of plinian arc volcanism in central america 2 tephra volumes and erupted masses 1 sediment gravity cores collected from the pacific seafloor offshore central america contain numerous distal ash layers from plinian type eruptions at the central american volcanic arc dating back to more than 200 ka in part 1 of this contribution we have correlated many of those ash layers between cores and with 26 tephras on land the marine ash layers cover areas of up to 106 km2 in the pacific ocean and represent a major fraction 60 90 of the erupted tephra volumes because the pacific coast lies within a few tens of kilometers downwind from the volcanic arc combining our own mapping efforts on land and published mapping results with our marine data yields erupted volumes of all major tephras along the arc that range from 1 to 420 km3 recalculated to erupted magma mass the widespread tephras account for 65 of the total magma output at the arc complementing our tephra data with published volumes of the arc volcanic edifices and volcano ages we calculate the long term average magma eruption rates for each volcano moreover we use incompatible element variations to calculate the cumulate masses that were fractionated during variable degrees of differentiation this yields a minimum estimate of long term average magma production rate at each volcano because intrusives without surface expression and losses by erosion are not accounted for peak magma production rates increase from costa rica to guatemala but there is considerable scatter within each region and large differences even between neighboring volcanoes

seasonal dynamics in the azores gibraltar strait region a climatologically based study abstract annual and seasonal mean circulations in the azores gibraltar strait region north eastern atlantic are described based on climatological data an inverse box model is applied to obtain absolute water mass transports consistent with the conservation of volume salt and heat and the equations of the thermal wind the large scale gyre circulation azores current azores counter current canary current and portugal current is well represented in climatological data the azores current annual mean transport was estimated to be 6 5 0 8 sv 1 sv 106 m3 s eastward exhibiting a seasonal signal with minimum transport in the spring 5 3 0 8 sv and maximum transport in autumn 7 3 0 8 sv the azores current transport is twice that of the azores counter current in spring and autumn and is four times higher in summer and winter the southward portugal and canary currents show similar seasonal cycles with maximum transports in spring 3 5 0 6 and 6 6 0 4 sv respectively the overturning circulation within the area has an annual mean magnitude of 2 2 0 1 sv and two seasonal extremes the highest in summer 2 6 0 1 sv and the lowest in winter 1 7 0 1 sv of the annual mean about two thirds 1 4 sv of the overturning circulation results from water mass transformation west of the strait of gibraltar the downwelling and recirculation of upper central water 0 6 sv in the intermediate layer the entrainment of central water 0 6 sv into the mediterranean outflow and the contribution of antarctic intermediate water 0 2 sv to the mediterranean outflow the remaining 0 8 sv relates to the overturning in the mediterranean sea through the two layer exchange at the gibraltar strait accordingly the density level dividing the upper inflowing and lower outflowing limbs of the overturning circulation was found to be 1 31 65 kg m 3 1 potential density referred to 1000 db which is above the isopycnal that typically separates central and mediterranean water 1 31 8 kg m 3 in terms of water masses we describe quantitatively the water mass composition of the main currents focusing on the spread of mediterranean water we found that when the northward mediterranean water branch weakens in spring and autumn the westward mediterranean water vein strengthens and vice versa the maximum net transports of mediterranean water across the northern and western sections of the box were estimated at 1 9 0 6 sv summer and 0 8 0 2 sv spring respectively within the error bar 0 2 sv we found no significant net volume transport of mediterranean water across the southern section

the ilopango tierra blanca joven tbj eruption el salvador volcano stratigraphy and physical characterization of the major holocene event of central america abstract the ilopango caldera is the source of the large tierra blanca joven tbj eruption that occurred about 1 5 ka years ago between ca ad270 and ad535 the eruption dispersed volcanic ash over much of the present territory of el salvador and pyroclastic density currents pdcs extended 40 km from the volcano in this study we document the physical characteristics of the deposits from all over el salvador to further constrain the eruption processes and the intensity and magnitude of the different phases of the eruption the succession of deposits generated by the tbj eruption is made of 8 units the eruption started with pdcs of hydromagmatic origin unit a0 followed by fallout deposits units a and b that are

instability dynamics of a subtropical jet and applications to the azores front current system eddy driven mean flow abstract the azores front is associated with an eastward flowing subtropical jet which coincides with the northern edge of the north atlantic subtropical gyre and forms a southern branch of the gulf stream an intensive study of the eddy driven mean flow induced by the nonlinear instability of the jet has been conducted using a primitive equation numerical model initialized with the summer hydrological fields typical of the azores front the authors show that the mesoscale patterns that appear after a few months are largely determined by baroclinic instability processes the jet and its associated mesoscale turbulence have a strong meridional asymmetry due to the strongly sloping thermocline in general an anticyclonic meander which grows and deforms northwestward generates a pinched off anticyclone while a cyclonic meander which grows and deforms southwestward degenerates in a thin filamental structure that will dissipate before any cyclone can be pinched off the authors propose a mechanism that

low frequency variability of the mediterranean undercurrent downstream of portimao canyon 1 data from current meters deployed during the canigo project from june 1997 to august 1998 are analyzed we focus on two meridionally aligned current meter moorings located west of the portimao canyon periodically an xbt line was performed during the period of the current meter measurements upstream of portimao canyon it preceded the release of rafos floats in each of the two cores of the mediterranean undercurrent and some of these floats drifted near the current meter moorings two flow regimes are identified at the mooring locations the first one is an oscillation of the velocity direction northwest to southwest with a regular period of about 10 days which alternates with a second regime similar to mesoscale turbulence in which the currents flow either northward or southward during several days the trajectories of the rafos floats revealed that some of them were trapped in eddies west of cape st vincent when released during the first flow regime rafos released during the second flow regime were trapped in coherent structures cyclone or anticyclone passing through or formed in the portimao canyon the regular oscillation of the currents are analyzed in the light of the propagation of rossby waves first the strong stratification above and below the undercurrent is used to diagnose the presence of topographic rossby waves along the iberian continental slope then the observed sharp potential vorticity gradients in the undercurrent are considered in order to estimate the characteristics of vortical rossby waves in a horizontally sheared flow the comparison of the wavelengths for the two types of waves shows good agreement with the size of the meddies observed in the vicinity of portimao canyon and cape st vincent the waves resonant interaction leads to baroclinic and barotropic instabilities known as mechanisms for meddy formation

tephrochronology of marine tephra adjacent to central america geochemical analyses performed on volcanic ash layers from the 3 ocean basins surrounding central america revealed 11 distinct tephra horizons during the past 300 000 yr and the distribution of each tephra was delineated the los chocoyos ash 84 000 yr old and worzel layer l ash 230 000 yr old are the 2 most widespread tephra in the region two tephra y 6 75 000 yr old and w 1 136 000 yr old are restricted to the western gulf of mexico the remaining tephra b 36 000 yr old j 1 135 000 yr old g 190 000 yr old i 212 000 yr old k 270 000 yr old i 2 270 000 yr old and i 6 300 000 yr old are primarily restricted to the eastern equatorial pacific ocean immediately adjacent to central america the age of each layer was determined by correlation to oxygen isotope and calcium carbonate stratigraphy in marine cores or by interpolation or extrapolation of ages from that stratigraphy of the marine tephra two were correlated to terrestrial exposures of silicic volcanic rocks the layer d tephra had been correlated to the los chocoyos ash from the lake atitlan caldera in guatemala the layer i 2 tephra is correlated with the t fall ash from lake amatitlan caldera in guatemala the age of the terrestrial volcanic units was determined by identifying the stratigraphic age of the marine tephra the los chocoyos ash and t fall deposit are dated at 84 000 and 270 000 yr b p respectively geochemical correlation of the marine tephra to other terrestrial sources will facilitate dating of explosive volcanic eruptions in southern mexico central america and northern south america

circulation patterns and transport of the azores front current system in this paper different types of new data sets hydrography lagrangian and eulerian current measurements and quasi geostrophic model results obtained after altimetric data assimilation are used to study the structure and variability of the azores current and frontal system the azores current was observed to transport between 11 0 and 18 0 sv eastwards and the associated counter current some 2 0 10 0 sv to the west resulting in a net value of about 8 9 sv furthermore both data and model results revealed a meandering azores current where some freely rotating eddies were also identified these hydrographic and lagrangian results exhibit space and time scales that agree fairly well with the dynamics shaped by baroclinic instability current meters moored across the azores current system delineated a mean counter current flowing westwards with a maximum subsurface core of about 2 0 cm s at a mean depth of 800 dbar this is in excellent agreement with previous studies which explains this azores counter current as the rectification process of the geostrophic turbulence occurring north of the main azores current stream a new scheme is proposed for the formation mechanism of the freely rotating cyclonic eddies observed south of the azores it is shown that the north south contrast thickness of the 17 19 c water layer across the azores front decreases downstream this will create in turn a downstream increase of the most unstable wavelength in a non linear baroclinic instability context as a consequence both large cyclonic and anticyclonic features are able to form at the eastern side of the azores current around 19 w while at the western side around 35 w only large anticyclones will survive western generated cyclones will be small enough to be quickly dissipated this means that the eastern cyclones of the azores current may live longer than the shorter western ones however because longer lived cyclonic eddies propagate westwards with a mean speed of 2 5 km day they may be observed south of azores and of the main stream several months later although they were not formed there

fluid circulation in a complex volcano tectonic setting inferred from self potential and soil co2 flux surveys the santa maria cerro quemado zunil volcanoes and xela caldera northwestern guatemala the region encompassing santa maria cerro quemado and zunil volcanoes close to quetzaltenango the second largest city of guatemala is volcanically and tectonically complex in addition the huge xela caldera about 20 km in diameter crosses this area and links up to the important zunil fault zone located between the three volcanoes two highly active geothermal sites named zunil i and zunil ii are also located between these three volcanic edifices at the southeastern boundary of xela caldera in order to determine the permeability variations and the main structural discontinuities within this complex volcano tectonic setting self potential and soil co2 flux measurements have been coupled with a step of 20 m along a 16 880 km long profile crossing the entire area two shallow hydrothermal systems with maximum lateral extensions of 1 5 km in diameter are indicated by positive self potential elevation gradients below santa maria and cerro quemado volcanoes such small hydrothermal systems cannot explain the intense geothermal manifestations at zunil i and zunil ii another minor hydrothermal system is indicated by self potential measurements on the flank of santa maria along the edge of the xela caldera co2 flux measurements display slight variations inside the caldera and decreasing values crossing outside the caldera boundary we hypothesize the presence of a magmatic body inside the southeastern border of xela caldera to explain the deeper and more intense hydrothermal system manifested by the zunil i and the zunil ii geothermal fields this magmatic system may be independent from santa maria and cerro quemado volcanoes alternatively the hypothesized xela magmatic system could have a common magmatic origin with the cerro quemado dome complex consistent with previous findings on regional gas emissions sectors bordering the cerro quemado dome complex also have high amplitude minima short wavelength anomalies in self potential interpreted as preferential rain water infiltration along faults of major permeability probably related with the most recent stages of cerro quemado dome growth

eddy formation by dense flows on slopes in a rotating fluid properties of the flow generated by a continuous source of dense fluid on a slope in a rotating system are investigated with a variety of laboratory experiments the dense fluid may initially flow down the slope but it turns under the influence of rotation to flow along the slope and initial geostrophic adjustment gives it an anticyclonic velocity profile some of the dense fluid drains downslope in a viscous ekman layer which may become unstable to growing waves provided that the viscous draining is not too strong cyclonic vortices form periodically in the upper layer and the dense flow breaks up into a series of domes three processes may contribute to the formation of these eddies first initial downslope flow of the dense current may stretch columns of ambient fluid by the taylor column process which we term capture secondly the initial geostrophic adjustment implies lower layer collapse which may stretch the fluid column and thirdly viscous drainage will progressively stretch and spin up a captured water column overall this last process may be the most significant but viscous drainage has contradictory effects in that it progressively removes dense lower layer fluid which terminates the process when the layer thickness approaches that of the ekman layer the eddies produced propagate along the slope owing to the combined effects of buoyancy coriolis balance and beta gyres this removes fluid from the vicinity of the source and causes the cycle to repeat the vorticity of the upper layer cyclones increases linearly with l d where l is the rossby deformation radius the bottom slope and d the total depth reaching approximately 2 f in the experiments presented here the frequency at which the eddy dome structures are produced also increases with while the speed at which the structures propagate along the slope is reduced by viscous effects the flow of dense fluid on slopes is a very important part of the global ocean circulation system and the implications of the laboratory experiments for oceanographic flows are discussed

the source of the canary current in fall 2009 1 the source of the canary current has been inferred from an inverse box model applied to the hydrographic data of a survey carried out in 2009 in the northeast subtropical gyre 29 37 n 9 24 w the portugal current is observed between 13 5 and 14 8 w at 37 n carrying 1 8 0 4 sv southward this current presumably merges with the eastward transport of the azores current system and partly contributes to the mediterranean inflow and partly to the northward recirculation of the azores current through the gulf of cadiz the azores current system is located in the meridional range 33 50 36 25 n at 24 50 w this system transports eastward 7 2 0 5 sv in the thermocline layers and 1 1 0 8 sv at intermediate layers the azores current intermediate water mass has the highest portion of sub arctic intermediate water saiw in the region while the azores countercurrent intermediate waters mass is mainly mediterranean water the canary current extends from 22 25 to 18 50 w at 29 n the westernmost position ever observed this current transports southward 6 2 0 6 sv in the thermocline layers and 2 0 0 8 sv in the intermediate layers this intermediate flow shows a relative maximum of oxygen and a relative minimum in nutrient concentration indicating the presence of saiw the study concludes that at least in fall 2009 the canary current extends to the intermediate waters n 27 9220 approximately 1600 dbar and that azores current feeds the canary current at surface and intermediate layers

the ilopango caldera complex el salvador stratigraphic revision of the complete eruptive sequence and recurrence of large explosive eruptions abstract ilopango caldera erupted episodically at least 13 tuff forming eruptions with a minimum estimate volume of 1 5 km3 dre per eruption reaching up to 150 km3 dre for the first caldera forming eruption all tuffs are of dacitic rhyolitic composition the complete pyroclastic sequence spans a range in time from 1 785 to 0 0015 ma and based on stratigraphy and geochronology constraints can be divided into three formations the comalapa altavista and tierras blancas formations in this work we focus on the members of the newly described altavista formation middle part of ilopango caldera volcanic sequence which consist of six consolidated pyroclastic deposits or tuffs each tuff corresponds to a specific eruption followed by a period of quiescence during which soil beds were developed on the deposits the ages of the altavista formation ranges from 918 to 257 ka based on new 40ar 39ar u pb zircon and u th zircon analyses the tuffs of this formation show similar characteristics in mineralogy and composition they are calcalkaline rhyodacitic tuffs with plagioclase clinopyroxene and hornblende from field mapping and descriptions of the deposits we have inferred the eruptive styles that include pumice fallouts pyroclastic density currents and also hydromagmatic explosions the common vent in all tuffs was the ilopango caldera and each member of the altavista formation could correspond to a caldera collapse event except for one of the six eruptions the volume of each member was estimated to be 30 km3 dre which is the same order of magnitude than that estimated for the tierra blanca joven tbj eruption at about 1 500 b p and smaller than those of the ignimbrites of the comalapa formation the first three members of the ilopango caldera reported previously the tuffs of the altavista formation are visible up to 15 20 km away from the caldera s topographic margin the recurrence interval of large explosive events at the ilopango caldera was established by integrating the stratigraphic and geochronologic data of all 13 ignimbrites and pumice fallouts erupted from ilopango caldera since the first one at 1 78 ma to the last explosive event tbj

a census of meddies in a long term high resolution simulation the output from a high resolution two decade long mediterranean outflow simulation is analysed here to provide a census of mediterranean water eddies aka meddies both anticyclones and cyclones the formation rate of meddies that survive for at least 90 days is of 12 meddies yr 1 of which 12 are cyclones the rate of formation reaches 40 meddies yr 1 30 cyclones when considering all the meddies living over 15 days about 70 of the population is born along the southwestern iberian slope but several robust meddies also originate in points of convergence of the main pathways into the open ocean the longest lived meddies propagate northwestwards but most of the anticyclones veer southwestwards after a while as the meddies drift away from their birthplace their radius tends to increase gradually from 15 to 30 km the thickness depth difference between isopycnals 27 2 and 27 5 of anticyclones born near cape st vincent contracts by approximately 100 m after travelling 1000 km from their source their mean swirl velocities range from about 21 cm s 1 at z 1000 m up to 27 cm s 1 at z 600 m mean salinity and temperature anomalies are significantly lower for cyclones which in general are also more slowly rotating shallower and thinner than anticyclones cyclones are more easily tracked at 600 m depth where longer trajectories are recorded in the vicinity of portimao canyon cyclones outnumber anticyclones while the reverse happens downstream of cape st vincent

surface circulation in the gulf of cadiz model and mean flow structure 1 the mean flow structure of the gulf of cadiz is studied using a numerical model the model consists of a set of one way nested configurations attaining resolutions on the order of 2 6 km in the region of the gulf of cadiz in the large scale configuration the entrainment of the mediterranean water is parameterized implicitly through a nudging term in medium and small scale nested configurations the mediterranean outflow is introduced explicitly the model reproduces all the known features of the azores current and of the circulation inside the gulf of cadiz a realistic mediterranean undercurrent is generated and meddies develop at proper depths on the southwest tip of the iberian slope the hypothesis that the azores current may generate in association with the mediterranean outflow plume theories is confirmed by the model results the time mean flow is dominated by a cyclonic cell generated in the gulf which expands westward and has transports ranging from 4 to 5 sv the connection between the cell and the azores current is analyzed at the scale of the gulf the time mean flow cell is composed by the westward mediterranean undercurrent and by a counterflow running eastward over the outer edge of the mediterranean undercurrent deeper vein as the latter is forced downslope this counterflow feeds the entrainment at the depths of the mediterranean undercurrent and the atlantic inflow at shallower levels coastward and upslope of this recirculation cell a second current running equatorward all the way along the northern part of the gulf is revealed this current is a very robust model result that promotes continuity between the southwestern iberian coast and the strait of gibraltar and helps explain many observations and recurrent sst features of the gulf of cadiz

an improved age framework for late quaternary silicic eruptions in northern central america five new stepwise heating 40 ar 39 ar ages and one new high sensitivity 14 c date of ash fall and ash flow deposits from late quaternary silicic volcanoes in northern central america document the eruption rates and frequencies of five major rhyodacite and rhyolite calderas atitlan amatitlan ayarza coatepe que and ilopango located north of the basalt ande site and dacite stratovolcanoes of the central ameri can volcanic front these deposits form extensive time stratigraphic horizons that intercalate regionally and knowledge of dates and stratigraphy provides a valua ble framework for age determinations of more local ized volcanic and nonvolcanic events the new data es pecially when integrated with previous stratigraphic and dating work show that all five calderas erupted several times in the past 200 ka and despite a lack of historic activity should be considered as active centers that could produce highly explosive eruptions again because of their locations near the highly vulnerable economic hearts of guatemala and el salvador the risks of eruptions from these calderas should be care fully considered along with risks of major earthquakes and volcanic front volcanoes which are much more fre quent but inflict less severe and extensive damage this

quaternary silicic pyroclastic deposits of atitlan caldera guatemala abstract atitlan caldera has been the site of several silicic eruptions within the last 150 000 years following a period of basalt andesite volcanism the silicic volcanism began with 5 10 km3 of rhyodacites erupted as plinian fall and pyroclastic flows about 126 000 yr b p at 85 000 yr b p 270 280 km3 of compositionally distinct rhyolite was erupted in the los chocoyos event which produced widely dispersed plinian fall deposits and widespread mobile pyroclastic flows in the latter parts of this eruption rhyodacite and minor dacite were erupted which compositionally resembled the earliest silicic magmas of the atitlan center as a result of this major eruption the modern atitlan iii caldera formed following this event rhyodacites were again erupted in smaller 5 13 km3 volumes partly through the lake and mafic volcanism resumed forming three composite volcanoes within the caldera the bimodal mafic silicic atitlan volcanism is similar to that which has occurred elsewhere in the guatemalan highlands but is significantly more voluminous mafic lavas are thought to originate in the mantle but rise intrude and underplate the lower crust and partly escape to the surface eventually silicic melts form in the crust possibly partly derived from underplated basaltic material rise crystallize and erupt the renewed mafic volcanism could reflect either regional magmato tectonic adjustment after the large silicic eruption or the onset of a new cycle

the eddy field associated with the azores front east of the mid atlantic ridge as observed by the geosat altimeter two years of geosat data november 1986 to december 1988 for the azores madeira region of the north atlantic are analyzed the objective is to study the eddy field associated with the terminal retroflection of the azores current the rms variability of sea level anomaly is 4 to 8 cm with maximum values between 33 degrees and 35 degrees n being associated with the azores current and located on its northern flank higher energy is observed during winter and during the year november 1987 to november 1988 analysis of velocities at crossovers reveals anisotropy with larger meridional velocity variances this is the signature of meanders and or eddies elongated in the north south direction the eddy mean flow interaction is studied through the analysis of the reynolds tensor meridional and zonal accelerations of the mean by the eddy field are consistent with retroflection of the azores current to the south and to the west spectral analyses show that the dominant signals have periods and wavelengths larger than 100 days and 300 km respectively propagation velocities are westward but slow 1 5 km d synoptic mapping of the eddy field shows the wavelike structure of the variability on the northern flank of the current it is likely that the wave generation is not directly due to an instability of the azores current but is attributable to a cause more to the east once formed the wave interacts with the current in winter and forms meanders seasonal variations of the surface circulation are finally analyzed from the 2 year long time series of synoptic maps there is a clear seasonality in the surface circulation which generally appears to be consistent with historical in situ measurements in winter there is a strong meandering and a southward branching while in summer the mean front is narrower and penetrates further to the east

evidence of time mean cyclonic cell southwest of iberian peninsula the mediterranean outflow driven β plume 1 first observational evidence of time mean cyclonic recirculation southwest of iberia is presented data sets of hydrography satellite altimetry and surface drifters velocities are analyzed jointly in order to obtain an accurate time averaged circulation in the mid latitude northeast atlantic off the gulf of cadiz a cyclonic recirculation cell with characteristics similar to those predicted by theoretical and modeling studies is detected in all computed velocity fields the cell in the upper 1000 m layer exhibits transports of 3 to 4 sv that are only slightly smaller than the model transports the cell is centered at approximately 36 n 10 w is elongated zonally and extends to 15 w westwards wind driven sverdrup transport and plume dynamics are both suggested to play a role in the generation of the cyclonic cell but the relative contribution of these effects is yet to be clarified the core of the recirculation appears compact and the magnitude of the cell fades westwards much faster than predicted by the theoretical and modeling studies considered

the 40ar 39ar and k ar dating of lavas from the hilo 1 km core hole hawaii scientific drilling project mauna kea lava flows cored in the hilo hole range in age from 200 ka to about 400 ka based on 40ar 39ar incremental heating and k ar analyses of 16 groundmass samples and one coexisting plagioclase the lavas all subaerially deposited include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic transitional tholeiitic and tholeiitic basalts the lower section has yielded predominantly complex discordant 40ar 39ar age spectra that result from mobility of 40ar and perhaps k the presence of excess 40ar and redistribution of 39ar by recoil comparison of k ar ages with 40ar 39ar integrated ages indicates that some of these samples have also lost 39ar nevertheless two plateau ages of 391 40 and 400 26 ka from deep in the hole combined with data from the upper section show that the tholeiitic section accumulated at an average rate of about 7 to 8 m kyr and has a mean recurrence interval of 0 5 kyr flow unit samples from the upper section yield relatively precise 40ar 39ar plateau and isotope correlation ages of 326 23 241 5 232 4 and 199 9 ka for depths of 415 7 m to 299 2 m within their uncertainty these ages define a linear relationship with depth with an average accumulation rate of 0 9 m kyr and an average recurrence interval of 4 8 kyr flow unit the top of the mauna kea sequence at 280 m must be older than the plateau age of 132 32 ka obtained for the basal mauna loa flow in the corehole the upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to mauna kea as it rode the pacific plate away from its magma source the hawaiian mantle plume the age depth relation in the core hole may be used to test and refine models that relate the growth of mauna kea to the thermal and compositional structure of the mantle plume

age and extent of the ilopango tbj tephra inferred from a holocene chronostratigraphic reference section lago de yojoa honduras abstract eruption of central el salvador s ilopango volcano early in the first millennium a d caused death cultural devastation and exodus of southern mesoamericans it also left a time stratigraphic marker in western el salvador and adjacent guatemala the ilopango tierra blanca joven or tbj tephra mineral suites and major element abundances identify a silicic volcanic ash in cores from lago de yojoa honduras as ilopango tbj this extends its reported range more than 150 km to the northeast analyses of glass from the tbj tephra from the chalchuapa archaeological site el salvador and from lago de yojoa honduras establish the first major element reference fingerprint for the tbj tephra the lago de yojoa cores also hold two previously undated trachyandesitic tephra layers originating from the nearby lake yojoa volcanic field one fell shortly before 11 000 14c yr b p and the other about 8600 14c yr b p

origin of silicic magmas along the central american volcanic front genetic relationship to mafic melts abstract silicic pyroclastic flows and related deposits are abundant along the central american volcanic front these silicic magmas erupted through both the non continental chorotega block to the southeast and the paleozoic continental chortis block to the northwest the along arc variations of the silicic deposits with respect to diagnostic trace element ratios ba la u th ce pb oxygen isotopes nd and sr isotope ratios mimic the along arc variation in the basaltic and andesitic lavas this variation in the lavas has been interpreted to indicate relative contributions from the slab and asthenosphere to the basaltic magmas carr m j feigenson m d bennett e a 1990 incompatible element and isotopic evidence for tectonic control of source mixing and melt extraction along the central american arc contributions to mineralogy and petrology 105 369 380 patino l c carr m j and feigenson m d 2000 local and regional variations in central american arc lavas controlled by variations in subducted sediment input contributions to mineralogy and petrology 138 3 265 283 with respect to along arc trends in basaltic lavas the largest contribution of slab fluids is in nicaragua and the smallest input from the slab is in central costa rica similar trends are observed in the silicic pyroclastic deposits data from melting experiments of primitive basalts and basaltic andesites demonstrate that it is difficult to produce high k 2 o na 2 o silicic magmas by fractional crystallization or partial melting of low k 2 o na 2 o sources however fractional crystallization or partial melting of medium to high k basalts can produce these silicic magmas we interpret that the high silica magmas associated central america volcanic front are partial melts of penecontemporaneous mantle derived evolved magmas that have ponded and crystallized in the mid crust or are melts extracted from these nearly completely crystallized magmas

three dimensional simulations of overflows on continental slopes abstract three dimensional features and instabilities of dense overflows from marginal seas onto continental slopes are investigated using a three dimensional primitive equation numerical ocean model the numerical simulations reveal important instability and three dimensional features of the overflow plumes that have not been included in previous simulations with a one dimensional streamtube model and a two dimensional plume model it is shown that the large primary plume breaks into a number of smaller subplumes on the offshore side of the plume due to instabilities manifested as growing topographic rossby waves over the slope the observed high temporal and spatial variabilities in the denmark strait overflow could be caused by the inherent dynamic instabilities as revealed by the numerical simulations the simulations indicate that the initial overflow velocity and width the properties of the source water the planetary rotation and the slope steepness play major roles in determining the scales of

role of the south china sea in regulating the north pacific double gyre system abstractthe role of the south china sea scs in regulating the north pacific circulation is investigated using a two layer quasigeostrophic qg model the double gyre circulations in the north pacific with and without the scs are compared and analyzed it is found that the scs acts as a sink of both potential vorticity pv and energy in the pacific scs system for the mean state consequently the kuroshio and the upstream kuroshio extension ke are weaker in the presence of the scs moreover the eddy activity is also lower in the north pacific ocean because the barotropic instability is suppressed for a weaker circulation in terms of low frequency variations at interannual to decadal time scale the presence of the scs is found to enhance the variability of the latitudinal position and intensity of the ke jet this is explained by a positive feedback process that is associated with the negative correlation between the inertia of the kuroshio and its intrusion into the scs

bromine and chlorine emissions from plinian eruptions along the central american volcanic arc from source to atmosphere highlights data set of br and cl emissions from 29 large cava eruptions vei 5 melt inclusions are strongly enriched in chlorine and bromine compared to their respective matrix glasses fluid partitioning is 4 to 68 times more efficient for br than for cl subducted calcareous sediments are the major control on the arc magmatic bromine contents average cava eruption would add 368 eesc of recent annual loading to stratosphere abstract large explosive volcanic eruptions inject gases aerosols and fine ashes into the stratosphere potentially influencing climate and atmosphere composition on a global scale although the potential climate effect of chlorine cl and bromine br injections into the stratosphere is known the global mass fluxes are poorly constrained in this study we focus on the magmatic degassing systematics and budgets of br and cl and on constraining the major sources of br in a subduction setting we therefore present a regional time series of br and cl emissions from 29 highly explosive eruptions throughout the central american volcanic arc cava covering the last 200 ka and a range of magmatic compositions and eruption magnitudes we have measured br and cl in matrix glasses and melt inclusions using synchrotron radiation micro x ray fluorescence spectrometry sr micro xrf and electron microprobe respectively melt inclusions of the cava tephras generally have higher br 0 9 to 17 9 ppm and cl 770 to 3800 ppm contents than the matrix glasses 0 39 to 1 5 ppm br 600 to 2800 ppm cl moreover the difference between maximum and minimum concentrations observed in melt inclusions of a given sample ranges between 9 and 90 of the maximum observed concentration for br and between 2 and 40 for cl such intra sample variations arise from variable pre eruptive degassing of these halogens into a magmatic fluid phase the relative loss of br from the melt is 4 to 68 times higher than that of cl the masses of br 2 1100 kt and cl 0 1 to 800 mt emitted by the eruptions generate instantaneous additions to the stratosphere potentially amounting to 6 5600 of the present day stratospheric annual global loading of equivalent effective stratospheric chlorine as the size of the stratospheric impact is primarily a function of eruption magnitude we use magnitude frequency relationships to estimate that eruptions adding 10 to resident eesc loading would occur every 40 years while every 200 years an eruption would double the eesc loading comparing the variations in br and cl concentrations and particularly minimum cl br ratios in melt inclusions with geochemical trace element proxies e g u la ba th and lead isotope compositions which change along the arc in response to changing subduction conditions we suggest that subducted calcareous sediment is a major source of magmatic br but also infer an important role of fluids expelled from serpentinized subducted mantle extrapolation of cava volcanic br emissions to the global subduction system thus needs to consider variations in the nature of subducted lithologies

radiocarbon and geologic evidence reveal ilopango volcano as source of the colossal mystery eruption of 539 40 ce abstract ilopango volcano el salvador erupted violently during the maya classic period 250 900 ce in a densely populated and intensively cultivated region of the southern maya realm causing regional abandonment of an area covering more than 20 000 km2 however neither the regional nor global impacts of the tierra blanca joven tbj eruption in mesoamerica have been well appraised due to limitations in available volcanological chronological and archaeological observations here we present new evidence of the age magnitude and sulfur release of the tbj eruption establishing it as one of the two hitherto unidentified volcanic triggers of a period of stratospheric aerosol loading that profoundly impacted northern hemisphere climate and society between circa 536 and 550 ce our chronology is derived from 100 new radiocarbon measurements performed on three subfossil tree trunks enveloped in proximal tbj pyroclastic deposits we also reassess the eruption magnitude using terrestrial el salvador guatemala honduras and near shore marine tbj tephra deposit thickness measurements together our new constraints on the age eruption size 43 6 km3 dense rock equivalent of magma magnitude 7 0 and sulfur yield 9 90 tg along with ilopango s latitude 13 7 n squarely frame the tbj as the major climate forcing eruption of 539 or 540 ce identified in bipolar ice cores and sourced to the tropics in addition to deepening appreciation of the tbj eruption s impacts in mesoamerica linking it to the major northern hemisphere climatic downturn of the mid 6th century ce offers another piece in the puzzle of understanding eurasian history of the period

mediterranean outflow mixing and dynamics the mediterranean sea produces a salty dense outflow that is strongly modified by entrainment as it first begins to descend the continental slope in the eastern gulf of cadiz the current accelerates to 1 3 meters per second which raises the internal froude number above 1 and is intensely turbulent through its full thickness the outflow loses about half of its density anomaly and roughly doubles its volume transport as it entrains less saline north atlantic central water within 100 kilometers downstream the current is turned by the coriolis force until it flows nearly parallel to topography in a damped geostrophic balance the mixed mediterranean outflow continues westward slowly descending the continental slope until it becomes neutrally buoyant in the thermocline where it becomes an important water mass

seasonal variability of the canary current a numerical study 1 a high resolution numerical model study of the canary basin in the northeast subtropical atlantic ocean is presented a long term climatological solution from the regional oceanic modeling system roms reveals mesoscale variability associated with the azores and canary current systems the northwest african coastal upwelling and the canary island archipelago the primary result concerns the canary current canc which in the solution transports 3 sv southward in line with observations the simulated canc has a well defined path with pronounced seasonal variability this variability is shown to be mediated by the westward passage of two large annually excited counterrotating anomalous structures that originate at the african coast the anomalies have a sea surface expression permitting their validation using altimetry and travel at the phase speed of baroclinic planetary rossby waves the role of nearshore wind stress curl variability as a generating mechanism for the anomalies is confirmed through a sensitivity experiment forced by low resolution winds the resulting circulation is weak in comparison to the base run but the propagating anomalies are still discernible so we cannot discount a further role in their generation being played by annual reversals of the large scale boundary flow that are known to occur along the african margin an additional sensitivity experiment where the azores current is removed by closing the strait of gibraltar presents the same anomalies and canc behavior as the base run suggesting that the canc is rather insensitive to upstream variability from the azores current

turbulent mixing in the red sea outflow plume from a high resolution nonhydrostatic model given the motivation that overflow processes which supply source waters for most of the deep and intermediate water masses in the ocean pose significant numerical and dynamical challenges for ocean general circulation models an intercomparison study is conducted between field data collected in the red sea overflow and a highresolution nonhydrostatic process model the investigation is focused on the part of the outflow that flows along a long narrow channel referred to as the northern channel that naturally restricts motion in the lateral direction such that the use of a two dimensional model provides a reasonable approximation to the dynamics this channel carries about two thirds of the total red sea overflow transport after the overflow splits into two branches in the western gulf of aden the evolution of the overflow in the numerical simulations can be characterized in two phases the first phase is highly time dependent during which the density front associated with the overflow propagates along the channel the second phase corresponds to that of a statistically steady state the primary accomplishment of this study is that the model adequately captures the general characteristics of the system i the gradual thickening of the overflow with downstream distance ii the advection of high salinity and temperature signals at the bottom along the channel with little dilution and iii ambient water masses sandwiched between the overflow and surface mixed layer to quantify mixing of the overflow with the ambient water masses an entrainment parameter is determined from the transport increase along the slope and is expressed explicitly as a function of mean slope angle bulk richardson numbers are estimated both from data and model and are related to the entrainment parameter the range of entrainment parameter and its functional dependence on bulk richardson number in this study are found to be in reasonable agreement with those reported from various laboratory experiments and that based on measurements of the mediterranean overflow the results reveal a complex dynamical interaction between shear induced mixing and internal waves and illustrate the high computational and modeling requirements for numerical simulation of overflows to capture at least in part turbulent transports explicitly

dissipation of turbulent kinetic energy inferred from seagliders an application to the eastern nordic seas overflows abstractturbulent mixing is an important process controlling the descent rate water mass modification and volume transport augmentation due to entrainment in the dense overflows across the greenland scotland ridge these overflows along with entrained atlantic waters form a major portion of the north atlantic deep water which pervades the abyssal ocean three years of seaglider observations of the overflows across the eastern greenland scotland ridge are leveraged to map the distribution of dissipation of turbulent kinetic energy on the iceland faroe ridge a method has been applied using the finescale vertical velocity and density measurements from the glider to infer dissipation the method termed the large eddy method lem is compared with a microstructure survey of the faroe bank channel fbc the lem reproduces the patterns of dissipation observed in the microstructure survey which vary over several orders of magnitude agreement between the inferred lem and more direct microstructure measu

near surface circulation in the southern gulf of cadiz abstract we have used several data sets expandable bathythermograph sections buoy trajectories current meter moorings and surface wind stress to investigate the temporal variation of the upper thermocline north atlantic central waters circulation patterns in the southern gulf of cadiz the main data set consists of eight expandable bathythermograph sections two per season running between the strait of gibraltar and cape beddouza just offshore the continental slope and approximately parallel to the morocco coastline a salinity temperature polynomial obtained from historical conductivity temperature depth data is used to infer dynamic properties salinity density dynamic height geostrophic velocity and transport dynamic heights are calculated referred to the 1027 25 kg m 3 neutral density surface which we justify is a good reference level for this section these observations are then combined with a process oriented one layer quasigeostrophic model for the northeastern corner of the north atlantic subtropical gyre with modified eastern boundary conditions the results indicate the existence of about 1 sv onshore transport all year long into a 300 km long coastal transition zone off morocco which must follow south as a narrow current during winter the flow is zonal towards the slope and recirculates south as a very narrow jet of less than 100 km width during summer the flow probably becomes more intense but is deflected southeastward before reaching the slope such that the onshore geostrophic transport into the coastal transition zone decreases and the recirculating band widens

silicic ignimbrites within the costa rican volcanic front evidence for the formation of continental crust abstract the origin of silicic magmas greater than 65 silica in areas without old evolved continental crust has long been an enigma in oceanic arcs silicic volcanic deposits are not common which has generally been attributed to subduction related magmas rising without stalling or fractionating through a relatively thin mafic oceanic crust the volcanic arc in costa rica is built on a thick 40 km oceanic plateau and extensive silicic magmatism dominated by silicic ignimbrites has occurred in the region in the absence of old evolved continental crust the silicic deposits display the common large ion lithophile element lile enrichment and high field strength element hfse depletion observed in magmas generated by subduction processes which are not characteristic of melts generated from oceanic plateaus key trace element ratios in the ignimbrites vary with position along the volcanic front and this variation is similar to that of the volcanic front lavas ignimbrites contain unique cumulative frequency distributions of incompatible trace element ratios that demonstrate different magma batches for each unit we propose that the caribbean oceanic plateau in costa rica is being converted to continental crust in large part by the emplacement of silicic magmas in this area the origin of these silicic magmas is due to reprocessing juvenile mantle derived subduction related magmas that have ponded in the crust

miocene to holocene marine tephrostratigraphy offshore northern central america and southern mexico pulsed activity of known volcanic complexes we studied the tephra inventory of fourteen deep sea drill sites of three dsdp and odp legs drilled offshore guatemala and el salvador legs 67 84 138 and one leg offshore mexico leg 66 marine tephra layers reach back from the miocene to the holocene we identified 223 primary ash beds and correlated these between the drill sites with regions along the volcanic arcs and to specific eruptions known from land in total 24 correlations were established between marine tephra layers and to well known quaternary eruptions from el salvador and guatemala additional 25 tephra layers were correlated between marine sites another 108 single ash layers have been assigned to source areas on land resulting in a total of 157 single eruptive events tephra layer correlations to independently dated terrestrial deposits provide new time markers and help to improve or confirm age models of the respective drill sites applying the respective sedimentation rates derived from the age models we calculated ages for all marine ash beds hence we also obtained new age estimates for eight known but so far undated large terrestrial eruptions furthermore this enables us to study the temporal evolution of explosive eruptions along the arc and we discovered five pulses of increased activity 1 a pulse during the quaternary 2 a pliocene pulse between 6 and 3 ma 3 a late miocene pulse between 10 and 7 ma 4 a middle miocene pulse between 17 11 ma and 5 an early miocene pulse 21 ma

late cenozoic tephrostratigraphy offshore the southern central american volcanic arc 1 tephra ages and provenance we studied the tephra inventory of 18 deep sea drill sites from six dsdp odp legs legs 84 138 170 202 205 206 and two iodp legs legs 334 and 344 offshore the southern central american volcanic arc cava eight drill sites are located on the incoming cocos plate and ten drill sites on the continental slope of the caribbean plate in total we examined 840 ash bearing horizons and identified 650 of these as primary ash beds of which 430 originated from the cava correlations of ash beds were established between marine cores and with terrestrial tephra deposits using major and trace element glass compositions with respect to relative stratigraphic order as a prerequisite for marine terrestrial correlations we present a new geochemical data set for significant neogene and quaternary costa rican tephras moreover new ar ar ages for marine tephras have been determined and marine ash beds are also dated using the pelagic sedimentation rates the resulting correlations and provenance analyses build a tephrochronostratigraphic framework for costa rica and nicaragua that covers the last 8 myr we define 39 correlations of marine ash beds to specific tephra formations in costa rica and nicaragua from the 4 15 ma lower sandillal ignimbrite to the 3 5 ka rincon de la vieja tephra from costa rica as well as another 32 widely distributed tephra layers for which their specific region of origin along costa rica and nicaragua can be constrained this article is protected by copyright all rights reserved

common characteristics of paired volcanoes in northern central america four pairs of active volcanoes along the northern central american volcanic front have erupted basalt andesite magmas that show consistent intrapair behavioral and compositional differences these differences are found in records of volcanic activity and complete major and minor element data on over 200 samples from northwest to southeast along the volcanic front the four volcano pairs are cerro quemado santa maria toliman atitlan acatenango fuego and santa ana izalco the volcano pair relations help explain compositional differences apart from those reflecting variation in crustal thickness of about 15 km along the volcanic front providing insight into across arc variations and closely spaced subvolcanic plumbing systems intravolcano pair spacing is less than 5 km compared with an average intervolcano spacing of 25 km along the entire volcanic front within each volcano pair the seaward volcano has had more frequent historic activity erupting magmas that are generally more mafic lower in large ion lithophile elements and higher in na2o k2o than magmas erupted from its landward counterpart each paired volcano site lies in close proximity to a rhyolitic caldera situated north or northeast of the volcano pair however rare earth element data at the toliman atitlan volcano pair imply that mixing between caldera rhyolite and the mafic magma of the paired volcanoes does not occur petrographie isotopic and other geochemical data from the toliman atitlan volcano pair suggest that separate but contemporaneous magma bodies beneath each volcano evolve and pass through the crust at different rates atitlan magmas are processed through the crust more efficiently and with greater frequency than tollman magmas which undergo longer periods of stagnation interrupted by mafic injection and rapid eruption this relation appears to hold at the other paired volcano sites and is further evidence that closely spaced volcanoes with similar subcrustal magma sources evolve over separate magmatic plumbing systems that traverse the crust the pairing pattern probably reflects the regional southward migration of the volcanic front

numerical modeling of meander and eddy formation in the azores current frontal zone abstract numerical experiments with an 11 level primitive equation finite difference model in a periodic channel are performed to analyze the properties of unstable finite amplitude disturbances in an idealized azores current release of available potential energy due to baroclinic instability occurs preferentially on scales of about 100 km with a theoretical growth time of 8 days at larger times the combined effect of friction and nonlinear transfer between internal and external depth integrated mode and the distribution of energy among different wavenumbers of the initial disturbance determine the scale of the meandering jet cold water tongues with a meridional scale of several hundred km found in satellite images and hydrographic surveys east of the azores are prescribed as initial disturbances they develop into pairs of troughs and ridges dominated by cyclonic vortices on the poleward flank of the jet phase propagation is downstream at 2 4 km day 1 extremely strong frontogenetic enhancement o

millennial scale iron fertilization of the eastern equatorial pacific over the past 100 000 years productivity in the eastern equatorial pacific is limited by the availability of iron geochemical analyses of a 100 000 year long sediment core suggest that pulses of dust deposition during heinrich stadials fuelled primary productivity

response of the subtropical north atlantic surface hydrography on deglacial and holocene amoc changes the north atlantic subtropical gyre stg circulates warm waters between 10 and 40 degrees n and is a potential area of heat storage during periods of reduced north atlantic meridional overturning circulation amoc when warm salt rich waters are retained in the subtropics in this study we investigated multicentennial to millennial scale changes in subtropical north atlantic hydrography in response to amoc changes during the last deglaciation and early holocene using sediment cores md08 3180 and geofar kf16 the coring site 38 degrees n is situated near the boundary between transitional eastern north atlantic waters and stg waters that is formed by the azores front hydrographic changes are reconstructed using new stable isotope data of benthic and subsurface dwelling planktonic foraminifera mg ca measurements on planktonic foraminifera and planktonic foraminifera abundances that are supplemented with published sea surface temperature and stable isotope data these multiproxy data indicate a close coupling between the latitudinal position of the northern stg boundary and deglacial amoc modes during weak amoc phases heinrich event 1 younger dryas yd 8 2ka event northern hemisphere subpolar water reached down to the northern stg boundary displacing the boundary southward during the bolling allerod warm period a strong warming trend of the subtropical region to 19 degrees c is observed a cooling of the sea surface temperature by 6 degrees c during the yd is accompanied by ongoing northward transport of warm subsurface water that might have contributed to the restart of amoc

the arce tephra two subsequent paroxysmal plinian eruptions from coatepeque caldera el salvador abstract the coatepeque volcanic complex in el salvador produced at least four plinian eruptions within the last 80 kyr the eruption of the 72 ka old arce tephra formed the coatepeque caldera and was one of the most powerful explosive eruptions in el salvador hitherto it was thought that the arce tephra had been emplaced only by one mostly plinian eruptive event that ended with the deposition of a thick ignimbrite however our stratigraphic geochemical and zircon data reveal a temporally closely spaced double eruption separated by a gap of only a couple of hundred years and we therefore distinguish lower and upper arce tephras both eruptions produced in the beginning a series of fallout units generated from fluctuating eruption columns and turning wind directions the final phase of the upper arce eruption produced surge deposits by several eruption column collapses before the terminal phase of catastrophic ignimbrite eruption and caldera collapse mapping of the individual tephra units including the occurrences of distal marine and lacustrine ash layers in the pacific ocean the guatemalan lowlands and the caribbean sea result in 25 6 km3 tephra volume areal distribution of 4 105 km2 and eruption column heights between 20 33 km for the lower arce eruption and 40 5 km3 tephra volume including 10 km3 for the ignimbrite distributed across 6 105 km2 and eruption column heights of 23 28 km for the upper arce eruption these values and the detailed eruptive sequence emphasize the great hazard potential of possible future highly explosive eruptions at coatepeque caldera especially for this kind of double eruption

on the reasons for the formation and variability of the azores current abstract recent studies have shown that the formation of the well defined zonally oriented azores current may be the result of water mass transformation associated with the mediterranean outflow in the gulf of cadiz as the denser mediterranean water descends down the continental slope it entrains overlying north atlantic central water it is believed that the azores current then forms as part of the horizontal recirculating gyre generated through the plume mechanism in this study the authors further explore this hypothesis by performing a series of numerical experiments these experiments are based on a high resolution general circulation model that includes the mediterranean sea and that realistically simulates the water mass exchange through the strait of gibraltar and the transport and variability of the azores current the authors show that the divergence of the relative vorticity flux and the planetary vorticity flux associated with planetary waves are the main factors determining the variab

on the origin of the azores current the azores current south of the azores archipelago is part of the subtropical north atlantic gyre using an international hydrographic data set we analyze mean and seasonal geostrophic transport fields in the upper 800 m of the ocean in order to determine the origin of the azores current in the western basin and seasonal changes in the related flow geostrophic currents are obtained by using the method applied by stramma 1984 in the eastern basin the azores current is found to originate in the area of the southwest newfoundland rise figure 10 in winter an almost uniform current connects this region of origin with the azores current while a branching into two current bands is observed in summer with the southern band forming a marked cyclonic loop within the upper 800 m all of the transport in the northern band and about 70 of the transport in the southern band recirculates in the eastern basin additionally expendable bathythermograph data from the azores current region indicate an increase of eddy potential energy from winter to summer

on the connection between the mediterranean outflow and the azores current as the salty and dense mediteranean overflow exits the strait of gibraltar and descends rapidly in the gulf of cadiz it entrains the fresher overlying subtropical atlantic water a minimal model is put forth in this study to show that the entrainment process associated with the mediterranean outflow in the gulf of cadiz can impact the upper ocean circulation in the subtropical north atlantic ocean and can be a fundamental factor in the establishment of the azores current two key simplifications are applied in the interest of producing an economical model that captures the dominant effects the first is to recognize that in a vertically asymmetric twolayer system a relatively shallow upper layer can be dynamically approximated as a single layer reduced gravity controlled barotropic system and the second is to apply quasigeostrophic dynamics such that the volume flux divergence effect associated with the entrainment is represented as a source of potential vorticity two sets of computations are presented within the 1 layer framework a primitive equation based computation which includes the divergent flow effects is first compared with the equivalent quasigeostrophic formulation the upper ocean cyclonic eddy generated by the loss of mass over a localized area elongates westward under the influence of the b effect until the flow encounters the western boundary in the steady state the circulation pattern consists of bidirectional zonal flows with a limited meridional extent eastward to the south of the sink and westward to the north of the sink the localized sink drives a horizontal circulation in the interior ocean whose strength is approximately an order of magnitude greater than the sink s strength it is demonstrated that the induced circulation in the far field from a localized sink is insensitive to the neglect of the divergent flow component a set of parameter sensitivity experiments is then undertaken with the quasigeostrophic model for an idealized midlatitude circulation driven both by wind forcing and thermohaline flow through the open southern and northern boundaries when a sink near the eastern boundary is superimposed on the idealized midlatitude circulation it is shown to alter significantly the upper ocean flow and induce an eastward zonal current which resembles the azores current in location and transport this mechanism also generates a westward current to the north of the sink location which could be associated with the azores countercurrent an extensive series of sensitivity experiments is conducted to determine the response of this current system to changes in the boundary layer processes sink strength sink distribution model resolution and wind forcing the magnitude of the current transports is found to be sensitive to the sink intensity and to its distance from the coastline

mixing and spreading of the mediterranean outflow abstract hydrographic and current profiler data taken during the 1988 gulf of cadiz expedition have been analyzed to diagnose the mixing spreading and descent of the mediterranean outflow the s properties and the thickness and width of the outflow were similar to that seen in earlier surveys the transport of pure mediterranean water i e water with s 38 4 psu was estimated to be about 0 4 106 m3 s 1 which is lower than historical estimates most of which were indirect but comparable to other recent estimates made from direct velocity observations the outflow transport estimated at the west end of the strait of gibraltar was about 0 7 106 m3 s 1 of mixed water and the transport increased to about 1 9 106 m3 s 1 within the eastern gulf of cadiz this increase in transport occurred by entrainment of fresher north atlantic central water and the salinity anomaly of the outflow was consequently reduced the velocity weighted salinity decreased to 36 7 psu within 60 km of the strait and decre

an 85 ka record of climate change in lowland central america drill cores obtained from lake peten itza peten guatemala contain a 85 kyr record of terrestrial climate from lowland central america that was used to reconstruct hydrologic changes in the northern neotropics during the last glaciation sediments are composed of alternating clay and gypsum reflecting relatively wet and dry climate conditions respectively from 85 to 48 ka sediments were dominated by carbonate clay indicating moist conditions during marine isotope stages mis 5a 4 and early 3 the first gypsum layer was deposited at 48 ka signifying a shift toward drier hydrologic conditions and the onset of wet dry oscillations during the latter part of mis 3 peten climate varied between wetter conditions during interstadials and drier states during stadials the pattern of clay gypsum wet dry oscillations during the latter part of mis 3 48 23 ka closely resembles the temperature records from greenland ice cores and north atlantic marine sediment cores and precipitation proxies from the cariaco basin the most arid periods coincided with heinrich events when cold sea surface temperatures prevailed in the north atlantic meridional overturning circulation was reduced and the intertropical convergence zone itcz was displaced southward a thick clay unit was deposited from 23 to 18 ka suggesting deposition in a deep lake and pollen accumulated during the same period indicates vegetation consisted of a temperate pine oak forest this finding contradicts previous inferences that climate was arid during the last glacial maximum lgm chronozone 21 2 ka at 18 ka peten climate switched from moist to arid conditions and remained dry from 18 to 14 7 ka during the early deglaciation moister conditions prevailed during the warmer bolling allerod 14 7 12 8 ka with the exception of a brief return to dry conditions at 13 8 ka that coincides with the older dryas and meltwater pulse 1a the onset of the younger dryas at 12 8 ka marked the return of gypsum and hence dry conditions the lake continued to precipitate gypsum until 10 3 ka when rainfall increased markedly in the early holocene

physical oceanography of the azores front abstract there is a considerable body of evidence for the existence of a permanent front sw of the azores surveys in the summer of 1981 show the structure to penetrate to a depth of 2 000 m and suggest that its meandering is related to the alignment of the atlantis gt meteor seamount chain the total volume transport above 2 000 m is between 10 and 12 106 m3 s 1 towards the east the main water mass contrasts across the front are confined to the upper 400 m of the water column only a small change in mediterranean water strength was seen across the front satellite tracked drogued buoys seeded into the front showed surface speeds of up to 40 cm sec 1 and revealed the formation of a closed cold core eddy some 100 115 km in diameter which propagated westward at 2 2 km day 1 over a 200 day period sea surface temperature contrasts across the frontal boundary are small and together with frequent cloud cover prevent the detection of the front and associated eddies by satellite infra red imagery the front apparently marks the eastward extension of the southern branch of the gulf stream recirculation and the northern limit of the 18 c water mass eddies formed at the front could propagate westward into the interior of the sargasso sea but have not yet been observed to cross the mid atlantic ridge

quasi synoptic transport budgets and water mass transformation in the azores gibraltar strait region during summer 2009 abstract we describe the circulation patterns in the azores gibraltar strait region north eastern atlantic during the 2009 caibox cruise on the basis of hydrographic and direct current velocity measurements this study offers new data for a region where importation of central waters subpolar and subtropical modes of eastern north atlantic central water and exports of mediterranean water are strongly related to large scale dynamics in the north atlantic ocean azores current mediterranean water system the description is backed up quantitatively by the results of a box inverse model which was used to obtain absolute water mass transport values consistent with thermal wind equations and with conservation of volume salt and heat the contributions of water masses were determined in an extended optimum multiparameter analysis from a quasi synoptic point of view providing detail in addition to volume salt and heat transport the surface subsurface large scale current system in the region consists of the azores current 13 1 2 5 sverdrup sv 1 sv 10 6 m 3 s 1 the azores counter current 5 2 2 1 sv the portugal current 4 5 1 4 sv and the canary current 7 1 1 1 sv broadly speaking central waters are imported into the caibox region at a rate of 1 6 0 9 sv and mediterranean water is exported at a rate of 1 5 0 4 sv the downwelling of central waters west of gibraltar strait was quantified at 1 1 sv not all this volume participates in mw formation but 0 8 sv of entrained central waters of which 0 5 sv are from central waters of subpolar origin and 0 3 sv from subtropical central waters of the 4 9 sv of subtropical central waters advected by the azores current about 0 7 sv would reach the gulf of cadiz region either to take part in central water entrainment or to flow across the gibraltar strait as part of the atlantic inflow to the mediterranean sea

mesoscale variability in denmark strait the pv outflow hypothesis abstract the outflow through denmark strait shows remarkable mesoscale variability characterized by the continuous formation of intense mesoscale cyclones just south of the sill these cyclones have a diameter of about 30 km and clear signatures at the sea surface and in currents measured near the bottom they have a remnant of arctic intermediate water aiw in their core the authors hypothesis is that these cyclones are formed by stretching of the high potential vorticity pv water column that outflows through denmark strait the light upper layer of the outflow the east greenland current remains on the surface in the irminger sea while the dense overflow water descends the east greenland continental slope the midlevel waters mostly aiw could thus be stretched by more than 100 which would induce very strong cyclonic relative vorticity the main test of this new hypothesis is by way of numerical experiments carried out with an isopycnal coordinate ocean model configured to have a marginal sea

paleoceanography of the atlantic mediterranean exchange overview and first quantitative assessment of climatic forcing 1 the mediterranean sea provides a major route for heat and freshwater loss from the north atlantic and thus is an important cause of the high density of atlantic waters in addition to the traditional view that loss of fresh water via the mediterranean enhances the general salinity of the north atlantic and the interior of the eastern north atlantic in particular it should be noted that mediterranean water outflowing at gibraltar is in fact cooler than compensating inflowing water the consequence is that the mediterranean is also a region of heat loss from the atlantic and contributes to its large scale cooling uniquely this system can be understood physically via the constraints placed on it by a single hydraulic structure the gibraltar exchange here we review the existing knowledge about the physical structure of the gibraltar exchange today and the evidential basis for arguments that it has been different in the past using a series of quantitative experiments we then test prevailing concepts regarding the potential causes of these past changes we find that 1 changes in the vertical position of the plume of mediterranean water in the atlantic are controlled by the vertical density structure of the atlantic 2 a prominent early holocene contourite gap within the gulf of cadiz is a response to reduced buoyancy loss in the eastern mediterranean during the time of sapropel 1 deposition 3 changes in buoyancy loss from the mediterranean during mis3 caused changes in the bottom velocity field in the gulf of cadiz but we note that the likely cause is reduced freshwater loss and not enhanced heat loss and 4 strong exchange at gibraltar during atlantic freshening phases implies that the gibraltar exchange provides a strong negative feedback to reduced atlantic meridional overturning given the very counterintuitive way in which the strait of gibraltar system behaves we recommend that without quantitative supporting work qualitative interpretations of how the system has responded to past external forcing are unlikely to be robust

differences between observed and a coupled simulation of north atlantic sea surface currents and temperature 1 north atlantic sea surface temperature sst distributions derived from observations and a coupled model from noaa s geophysical fluid dynamics laboratory cm2 1 are compared to evaluate the model s ability to simulate recent 1900 to the present oceanic surface characteristics the north atlantic focus will limit our analyses to spatial scales less than gyre scales usually not addressed in previous model observation comparisons identifying model differences from observations at these scales will assist modelers in identifying problems to be considered and remedies to be applied the properties compared are the mean annual sst standard deviation amplitude of the annual and semiannual harmonic decadal meridional movements of the axis of the gulf stream propagation of sst anomalies along the axis of the gulf stream and 100 year trends in sst records because of the dependence of sst on surface currents observed flow from surface drifters and simulated flow from 15 m fields are also compared the model simulates the large scale properties of all the variables compared however there are areas of differences in some variables that can be related to inadequacies in the simulated current fields for example the model gulf stream gs axis after separation from the western boundary is located some 100 km north of the observed axis which contributes to an area of warmer simulated ssts the absence of a slope current in the same region that advects colder water from the labrador sea in the observations also contributes to this area of higher model ssts the model north atlantic current nac is located to the east of the observed nac contributing to a large area of sst discrepancy the patterns of the amplitude of the annual harmonic are similar with maximum amplitude off the east coast of northern north america the semiannual harmonic exhibits relatively large amplitudes 1 c north of about 55 n a signal not found in the observations in both the model and observations a region of increased standard deviations encompasses the gs and nac the model simulates north south migrations of the gs core but at a longer period 20 years than observed the model does not simulate the sst anomalies that propagate along the observed gs and nac the model captures both the spatial and temporal characteristics of the atlantic multidecadal oscillation both model and observations exhibit a dipole in trends with positive trends in the subtropical atlantic and negative trends in the subpolar gyre the modeled region of negative trends is limited to the western subpolar atlantic the observed trends extend farther to the east

linear wind forced beta plumes with application to the hawaiian lee countercurrent abstracttwo numerical ocean models are used to study the baroclinic response to forcing by localized wind stress curl i e a wind forced plume which is a circulation cell developing to the west of the source region and composed of a set of zonal jets with implications for the hawaiian lee countercurrent hlcc an idealized primitive equation model regional ocean modeling system roms and a global eddy resolving general circulation model ocean general circulation model for the earth simulator ofes in addition theoretical ideas inferred from a linear continuously stratified model are used to interpret results in roms vertical mixing preferentially damps higher order vertical modes the damping thickens the plume to the west of the forcing region weakening the near surface zonal jets and generating deeper zonal currents the zonal damping scale increases monotonically with the meridional forcing scale indicating a dominant role of vertical viscosity over diffusion a consequence of the

distribution and circulation of water masses in the gulf of cadiz from in situ observations from data collected during golfo 2001 survey a study was conducted on the circulation and distribution of water masses in the gulf of cadiz in the spring of 2001 the general surface circulation in the gulf of cadiz is anticyclonic with

on the role of the azores current in the ventilation of the north atlantic ocean three high resolution ocean circulation models of the north atlantic differing chiefly in their description of the vertical coordinate are used to examine the ventilation of the north atlantic subtropical gyre all the models produce mode waters of realistic densities in the sargasso sea and the european basin but have azores currents of differing strengths which are categorised as strong of realistic transport intermediate and weak these differences have a critical impact upon the ventilation of the gyre most importantly the strong azores current forms an effective barrier to the southward ventilation of eastern north atlantic water from the northern european basin initially driving it southwestwards into the central gyre before turning it back eastwards again in a general cyclonic circulation north of the current the intermediate and weak azores currents instead allow the southward ventilation of this water mass near the european and african coasts the situation in nature appears to be intermediate between these two cases with the azores current acting as a partial block the study also raises the possibility of the azores current forming an advective connection between the sargasso sea mode waters in the western basin and modes of similar densities found in the eastern basin on the southern side of the current although there are high levels of variability in the extent of these lighter modes in the eastern basin in nature this postulate is supported by a number of observational studies in addition the present study also provides some support for the local production of madeira mode water in the eastern basin associated with retroflecting current branches on the southern side of the azores current overall the azores current is therefore likely to have a critical impact upon the ventilation of the subtropical gyre over a large area rather than just locally affecting the potential vorticity and density structure of the upper ocean between subtropical latitudes and the northern european basin the study also contributes to an ongoing community effort to assess the realism of our current generation of ocean models

the upper oceanic response to overflows a mechanism for the azores current abstract the oceanic response to overflows is explored using a two layer isopycnal model overflows enter the open ocean as dense gravity currents that flow along and down the continental slope while descending the slope overflows typically double their volume transport by entraining upper oceanic water the upper oceanic layer must balance this loss of mass and the resulting convergent flow produces significant vortex stretching overflows thus represent an intense and localized mass and vorticity forcing for the upper ocean in this study simulations show that the upper ocean responds to the overflow induced forcing by establishing topographic plumes that are aligned more or less along isobaths and that have a transport that is typically a few times larger than that of the overflows for the topographic plume driven by the mediterranean overflow the occurrence of eddies near cape st vincent portugal allows the topographic plume to flow across isobaths the modeled topographic plume circu

a 400 ka tephrochronological framework for central america from lake peten itza guatemala sediments highlights dating 400 ka paleoclimate record of neotropics revision and new eruptive volumes for large central american eruptions age models for peten itza sediments linking lacustrine ash inventory to eruptions from central america and mexico abstract lake peten itza northern guatemala lies within a hydrologically closed basin in the south central area of the yucatan peninsula and was drilled under the auspices of the international continental scientific drilling program icdp in 2006 at 16 55 n latitude the lake is ideally located for study of past climate and environmental conditions in the neotropical lowlands because of its great depth 160 m lake peten itza has a record of continuous sediment accumulation that extends well into the late pleistocene a key obstacle to obtaining long climate records from the region is the difficulty of establishing a robust chronology beyond 40 ka the limit of 14c dating tephra layers within the lake peten itza sediments however enable development of age depth relations beyond 40 ka ash beds from large magnitude pleistocene to holocene silicic eruptions of caldera volcanoes along the central american volcanic arc cava were found throughout drill cores collected from lake peten itza these ash beds were used to establish a robust chronology extending back 400 ka we used major and trace element glass composition to establish 12 well constrained correlations between the lacustrine tephra layers in lake peten itza sediments and dated deposits at the cava source volcanoes and with their marine equivalents in eastern pacific ocean sediments the data also enabled revision of eight previous determinations of erupted volumes and masses and initial estimates for another four eruptions as well as the designation of source areas for 14 previously unknown eruptions the new and revised sedimentation rates for the older sediment successions identify the interglacial of mis5a between 84 and 72 ka followed by a stadial between 72 and 59 ka that corresponds to mis4 we modified the age models for the lake peten itza sediment sequences extended the paleoclimate and paleoecological record for this neotropical region to 400 ka and determined the magnitude and timing of cava eruptions

alder creek sanidine acs 2 a quaternary 40ar 39ar dating standard tied to the cobb mountain geomagnetic event accurate 40ar 39ar dating of quaternary and holocene samples requires a standard neutron fluence monitor of appropriate age this standard should be isotopically homogenous at the scale of analysis and should be well intercalibrated with other standards several such standards have been proposed including sanidine from the 1 19 ma alder creek rhyolite acs turrin et al 1994 turrin b d donnely nolan j m hearn b c jr 1994 40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited geology 22 251 254 this sanidine is satisfactorily homogeneous well intercalibrated with fish canyon sanidine fcs renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 and is linked to the astronomically calibrated magnetic polarity time scale since the alder creek rhyolite lava is the type occurrence of the cobb mountain event cm we collected 408 kg of this rock from which we anticipate being able to extract more than 8 kg of 425 m sanidine crystals analyses of sanidine phenocryts from the acs 2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1 190 0 004 to 1 194 0 003 ma1 with a weighted mean age of 1 193 0 001 ma mswd 0 74 n 225 based on fcs 28 02 ma renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the value of the intercalibration factor renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 is rfcsacs 2 0 04229 0 00012 n 8 identical with that of the original acs measurements 0 04229 0 00012 renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the age of acs 2 is in agreement with the recent astronomical calibration of the cm event 1 215 to 1 190 ma channell j e t mazaud a sullivan p turner s raymo m e 2002 geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin j geophys res 107 6 10 1029 2001jb000491 the apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements aliquots from various grain sizes will be available upon request zircon is also potentially available which may be of interest to u th he researchers