propagation of the great salinity anomaly of the 1990s around the northern north atlantic 1 time series of t and s extending through 2001 are used to describe propagation of the great salinity anomaly of the 1990s gsa 90s comparison of the distance time relations for the gsa 70s 80s and 90s reveals a substantial intensification of the large scale circulation in the northern north atlantic especially in the subarctic gyre between newfoundland and the faroes the advection rate of the gsa 70s 80s and 90s between newfoundland and the faroe shetland channel is conservatively estimated to have been 3 5 10 and 10 cm s respectively the circulation intensification apparently occurred within a decade between the gsa 70s and 80s during the next decade the advection rate increased from 10 to 13 cm s between newfoundland and iceland basin the gsa 90s was advected towards the faroe shetland channel by the northern iceland basin s branch of the north atlantic current whereas the contribution of the southern branch via the rockall trough was minimal

lab scale ash production by abrasion and collision experiments of porous volcanic samples in the course of explosive eruptions magma is fragmented into smaller pieces by a plethora of processes before and during deposition volcanic ash fragments smaller than 2 mm has near volcano effects e g increasing mobility of pdcs threat to human infrastructure but may also cause various problems over long duration and or far away from the source human health and aviation matters we quantify the efficiency of ash generation during experimental fracturing of pumiceous and scoriaceous samples subjected to shear and normal stress fields experiments were designed to produce ash by overcoming the yield strength of samples from tenerife canary islands spain sicily and lipari islands italy with this study having particular interest in the 355 m fraction fracturing within volcanic conduits plumes and pyroclastic density currents pdcs was simulated through a series of abrasion shear and collision normal experiments an understanding of these processes is crucial as they are capable of producing very fine ash 10 m these particles can remain in the atmosphere for several days and may travel large distances 1000s of km this poses a threat to the aviation industry and human health from the experiments we establish that abrasion produced the finest grained material and up to 50 of the generated ash was smaller than 10 m in comparison the collision experiments that applied mainly normal stress fields produced coarser grain sizes results were compared to established grain size distributions for natural fall and pdc deposits and good correlation was found energies involved in collision and abrasion experiments were calculated and showed an exponential correlation with ash production rate projecting these experimental results into the volcanic environment the greatest amounts of ash are produced in the most energetic and turbulent regions of volcanic flows which are proximal to the vent finest grain sizes are produced in pdcs and can be observed as co ignimbrite clouds above density currents finally a significant dependency was found between material density and the mass of fines produced also observable in the total particle size distribution higher values of open porosity promote the generation of finer grained particles and overall greater ratios of ash while this paper draws on numerous previous studies of particle comminution processes it is the first to analyze and compare results of several comminution experiments with each other in order to characterize these mechanisms

dune bedforms produced by dilute pyroclastic density currents from the august 2006 eruption of tungurahua volcano ecuador a series of pyroclastic density currents were generated at tungurahua volcano ecuador during a period of heightened activity in august 2006 dense pyroclastic flows were confined to valleys of the drainage network while dilute pyroclastic density currents overflowed on interfluves where they deposited isolated bodies comprising dune bedforms of cross stratified ash exposed on the surface here the description measurement and classification of more than 300 dune bedforms are presented four types of dune bedforms are identified with respect to their shape internal structure and geometry length width thickness stoss and lee face angles and stoss face length 1 elongate dune bedforms have smooth shapes and are longer in the flow direction than wide or thick internal stratification consists of stoss constructional thick lensoidal layers of massive and coarse grained material alternating with bedsets of fine laminae that deposit continuously on both stoss and lee sides forming aggrading structures with upstream migration of the crests 2 transverse dune bedforms show linear crests perpendicular to the flow direction with equivalent lengths and widths internally these bedforms exhibit finely stratified bedsets of aggrading ash laminae with upstream crest migration steep truncations of the bedsets are visible on the stoss side only 3 lunate dune bedforms display a barchanoidal shape and have stratification patterns similar to those of the transverse ones finally 4 two dimensional dune bedforms are much wider than long exhibit linear crests and are organized into trains elongate dune bedforms are found exclusively in proximal deposition zones transverse lunate and two dimensional dune bedforms are found in distal ash bodies the type of dune bedform developed varies spatially within an ash body transverse dune bedforms occurring primarily at the onset of deposition zones transitioning to lunate dune bedforms in intermediate zones and two dimensional dune bedforms exclusively on the lateral and distal edges of the deposits the latter are also found where flows moved upslope elongate dune bedforms were deposited from flows with both granular based and tractional flow boundaries that possessed high capacity and competence they may have formed in a subcritical context by the blocking of material on the stoss side we do not interpret them as antidune or chute and pool structures the dimensions and cross stratification patterns of transverse dune bedforms are interpreted as resulting from low competence currents with a significant deposition rate but we rule out their interpretation as antidunes a similar conclusion holds for lunate dune bedforms whose curved shape results from a sedimentation rate dependent on the thickness of the bedform finally two dimensional dune bedforms were formed where lateral transport exceeds longitudinal transport i e in areas where currents were able to spread laterally in low velocity zones we suggest that the aggrading ash bedsets with upstream crest migration were formed under subcritical flow conditions where the tractional bedload transport was less important than the simultaneous fallout from suspension this produced differential draping with no further reworking we propose the name regressive climbing dunes for structures produced by this process a rapid decrease in current velocity possibly triggered by hydraulic jumps affecting the entire parent flows is inferred to explain their deposition this process can in principle hold for any kind of particulate density current

interpentadal variability of temperature and salinity at intermediate depths of the north atlantic ocean 1970 1974 versus 1955 1959 we have composited historical hydrographic observations for the world ocean for two pentads 1955 1959 and 1970 1974 and have objectively analyzed these data for the purpose of studying the temporal variability of temperature and salinity in the north atlantic ocean the subtropical gyre 500 1300 m of the north atlantic was colder and fresher during 1970 1974 compared to 1955 1959 the relatively large changes in temperature and salinity on constant depth surfaces in the subtropical gyre were due to upward displacements of isopycnal surfaces at intermediate depths the eastern portion of the subarctic gyre was also colder and fresher during the 1970 1974 pentad in contrast the western portion of the subarctic gyre exhibited higher temperatures and salinities during 1970 1974 compared to 1955 1959 characteristic differences in temperature and salinity for both gyres were 0 5 c and 0 025 respectively analysis of changes in potential vorticity indicates that in the subtropical gyre potential vorticity increased in all density layers above the 26 5 surface suggesting that convection in this region may have been shallower in 1970 1974 compared to 1955 1959 below the 26 5 surface potential vorticity decreased through the 27 3 surface indicating that deeper than normal convection may have formed the water masses in these layers at 250 m depth relatively large changes in temperature and salinity occurred along the line of maximum climatological mixed layer depth that extends northeast to southwest across the north atlantic ocean

surface climate variations over the north atlantic ocean during winter 1900 1989 abstract the low frequency variability of the surface climate over the north atlantic during winter is described using 90 years of weather observations from the comprehensive ocean atmosphere data set results are based on empirical orthogonal function analysis of four components of the climate system sea surface temperature sst air temperature wind and sea level pressure an important mode of variability of the wintertime surface climate over the north atlantic during this century is characterized by a dipole pattern in ssts and surface air temperatures with anomalies of one sign cast of newfoundland and anomalies of the opposite polarity off the southeast coast of the united states wind fluctuations occur locally over the regions of large surface temperature anomalies with stronger than normal winds overlying cooler than normal ssts this mode exhibits variability on quasi decadal and biennial time scales the decadal fluctuations are irregular in length averaging 9 years before 1945 and 1

mobility of pyroclastic flows and surges at the soufriere hills volcano montserrat the soufriere hills volcano on montserrat has produced avalanche like pyroclastic flows formed by collapse of the unstable lava dome or explosive activity pyroclastic flows associated with dome collapse generate overlying dilute surges which detach from and travel beyond their parent flows the largest surges partially transform by rapid sedimentation into dense secondary pyroclastic flows that pose significant hazards to distal areas different kinds of pyroclastic density currents display contrasting mobilities indicated by ratios of total height of fall h run out distance l area inundated a and volume transported v dome collapse flow mobilities characterised by either l h or a v2 3 resemble those of terrestrial and extraterrestrial cold rockfalls dade and huppert 1998 in contrast fountain fed pumice flows and fine grained secondary pyroclastic flows travel slower but for comparable initial volumes and heights can inundate greater areas

descriptive nomenclature and classification of pyroclastic deposits and fragments recommendations of the iugs subcommission on the systematics of igneous rocks

apparent changes in the climatic state of the deep north atlantic ocean determination of any long term changes in the large scale characteristics of the deep ocean circulation would be an important clue in understanding the climatic interactions of the ocean and atmosphere in the summer of 1981 the rv atlantis ii reoccupied two transatlantic sections at nominal latitudes of 24 30 n and 36 16 n with a conductivity temperature depth instrument ctd one purpose of the work was to make a comparison with previous surveys conducted during the international geophysical year igy 1 when sections were obtained in october 1957 and april may 1959 we report here that significant warming occurred in an ocean wide band from 700 m to 3 000 m with a maximum temperature difference of 0 2 c these changes are sufficient to expand the water column by several centimetres the historical temperature salinity curve was apparently unchanged interannual changes in local water mass characteristics have been proposed previously2 3 perhaps it would be most surprising if no changes were seen to occur what remains obscure is the significance of these changes and the extent to which they represent long term climate trends or merely the minor and random fluctuations to be expected in any complex fluid system

the long term hydrographic record at bermuda abstract a long by oceanographic standards time series of hydrographic observations at bermuda was begun in 1954 and continues to the present analysis of this dataset has shown the temperature and salinity variations on interannual timescales to be largely independent in the surface layer 0 200 dbar where integral timescales for salinity are 50 longer than for temperature and surface salinity changes are correlated with decadal changes in the 18 c water production temperature salinity anomalies are highly correlated in the thermocline where the interannual variability at that level is accountable to either vertical oscillations of the thermocline with amplitudes of 50 m or meridional oscillations of the horizontal gradient set up at the southern edge of the recirculation gyre of 300 kim salinity changes in the deepest layer observed at bermuda station s 1500 2500 dbar are uncorrelated with temperature masked by measurement errors in the early years of the time series inclusion of earlier

transport capacity of pyroclastic density currents experiments and models of substrate flow interaction 1 one of the more distinctive features of many ignimbrites is the presence of large lithics some greater than meter scale and pumices that have been transported great distances 10 km from the eruptive vent sometimes over steep terrain and expanses of water in many cases these particles have been transported much further than can be explained by aerodynamic forces and ballistic trajectories we examine the forces responsible for transport of large clasts and examine in detail the momentum transfer occurring when particles interact with their boundaries we performed a suite of experiments and numerical simulations to quantify the mass and momentum transfer that occurs when particles interact with a pumice bed substrate and with water substrate two geologically motivated flow end members we find that clasts transported in dilute currents are particularly sensitive to the nature of the boundary and while large particles can skip several times on a water substrate they travel less far than particles that impact pumice bed substrates all else being equal large particles in dense pyroclastic density currents are themselves relatively insensitive to the details of their boundaries however one of the most important ways boundary conditions influence large particles is not through direct interaction but by changing the local concentration of fine particles momentum transfer from fine particles to large particles appears to be required to transport large clasts great distances if initially dense flows become dilute during transport then the transport capacity of large particles in the flow is substantially decreased

fragmentation efficiency of explosive volcanic eruptions a study of experimentally generated pyroclasts abstract products of magma fragmentation can pose a severe threat to health infrastructure environment and aviation systematic evaluation of the mechanisms and the consequences of volcanic fragmentation is very difficult as the adjacent processes cannot be observed directly and their deposits undergo transport related sorting however enhanced knowledge is required for hazard assessment and risk mitigation laboratory experiments on natural samples allow the precise characterization of the generated pyroclasts and open the possibility for substantial advances in the quantification of fragmentation processes they hold the promise of precise characterization and quantification of fragmentation efficiency and its dependence on changing material properties and the physical conditions at fragmentation we performed a series of rapid decompression experiments on three sets of natural samples from unzen volcano japan the analysis comprised grain size analysis and surface area measurements the grain size analysis is performed by dry sieving for particles larger than 250 m and wet laser refraction for smaller particles for all three sets of samples the grain size of the most abundant fraction decreases and the weight fraction of newly generated ash particles up to 40 wt increases with experimental pressure potential energy for fragmentation this energy can be estimated from the volume of the gas fraction and the applied pressure the surface area was determined through argon adsorption the fragmentation efficiency is described by the degree of fine particle generation results show that the fragmentation efficiency and the generated surface correlate positively with the applied energy

origin of the mount pinatubo climactic eruption cloud implications for volcanic hazards and atmospheric impacts volcanic ash clouds can be fed by an upward directed eruption column plinian column or by elutriation from extensive pyroclastic flows coignimbrite cloud there is considerable uncertainty about which mechanism is dominant in large scale eruptions here we analyze in a novel way a comprehensive grain size database for pyroclastic deposits we demonstrate that the mount pinatubo climactic eruption deposits were substantially derived from coignimbrite clouds and not only by a plinian cloud as generally thought coignimbrite ash fall deposits are much richer in breathable 10 mm ash 5 25 wt than pure plinian ash at most distances from the source volcano we also show that coignimbrite ash clouds as at pinatubo are expected to be more water rich than plinian clouds leading to removal of more hcl prior to stratospheric injection thereby reducing their atmospheric impact

multidecadal climate variability in the greenland sea and surrounding regions a coupled model simulation pronounced oscillations of ocean temperature and salinity occur in the greenland sea in a 2000 year integration of a coupled ocean atmosphere model the oscillations involving both the surface and subsurface ocean layers have a timescale of approximately 40 80 years and are associated with fluctuations in the inten sity of the east greenland current the greenland sea temperature and salinity variations are preceded by large scale changes in near surface salinity in the arctic which appear to propagate out of the arctic through the east greenland current these anomalies then propagate around the subpolar gyre into the labrador sea and the central north atlantic these oscillations are coherent with previously identified multi decadal fluctuations in the intensity of the north atlantic thermohaline circula tion the oscillations in the g eenland sea are related to atmospheric variability negative cold anomalies of surface air temperature are associated with negative cold sea surface temperature sst anomalies in the greenland sea with amplitudes up to 2oc near green land declining to several tenths of a degree c over north western europe the c01d sst anomalies and intensified east greenland current are also associated with enhanced northerly winds over the greenland sea

localized geomagnetic field anomalies in an underground gas storage abstract to investigate possible piezomagnetic effects associated with imposed tectonic stresses repeated measurements of total magnetic field in an array 120 km 90 km in extent were performed across the hutubi underground gas storage hugs forty repeated measurement stations were established in the hugs and its surrounding areas proton precession magnetometers with a sensitivity of 0 15 nt 1 hz were used to measure the total magnetic field at each repeated station we conducted two field measurement surveys one with a minimum pressure during april 2017 and the other with a maximum pressure during november 2017 in the hugs the local magnetic field lmf of the hugs and its surrounding areas was obtained after data processing which included diurnal variation reduction dvr secular variation reduction svr and lmf acquisition the average of the mean standard deviations of the geomagnetic dvr values were 0 038 nt and 0 045 nt for the first and second field surveys respectively the amplitude of the lmf varied between about 10 nt and 90 nt in the local area negative anomalies with a maximum amplitude of 2 1 nt was observed over a seven month period april 2017 november 2017 during the loading process with a constantly increasing pressure in the hugs the wide range of negative lmf anomalies shows a negative correlation with the increasing pressure in the hugs this result constitutes powerful field observational evidence for the existence of piezomagnetic effects under middle scale experimental conditions

primary and secondary fragmentation of crystal bearing intermediate magma abstract crystal rich intermediate magmas are subjected to both primary and secondary fragmentation processes each of which may produce texturally distinct tephra of particular interest for volcanic hazards is the extent to which each process contributes ash to volcanic plumes one way to address this question is by fragmenting pyroclasts under controlled conditions we fragmented pumice samples from soufriere hills volcano shv montserrat by three methods rapid decompression in a shock tube like apparatus impact by a falling piston and milling in a ball mill grain size distributions of the products reveal that all three mechanisms produce fractal breakage patterns and that the fractal dimension increases from a minimum of 2 1 for decompression fragmentation primary fragmentation to a maximum of 2 7 by repeated impact secondary fragmentation to assess the details of the fragmentation process we quantified the shape texture and components of constituent ash particles ash shape analysis shows that the axial ratio increases during milling and that particle convexity increases with repeated impacts we also quantify the extent to which the matrix is separated from the crystals which shows that secondary processes efficiently remove adhering matrix from crystals particularly during milling abrasion furthermore measurements of crystal size distributions before using x ray computed tomography and after by componentry of individual grain size classes decompression driven fragmentation show not only that crystals influence particular size fractions across the total grain size distribution but also that free crystals are smaller in the fragmented material than in the original pumice clast taken together our results confirm previous work showing both the control of initial texture on the primary fragmentation process and the contributions of secondary processes to ash formation critically however our extension of previous analyses to characterisation of shape texture and componentry provides new analytical tools that can be used to assess contributions of secondary processes to ash deposits of uncertain or mixed origin we illustrate this application with examples from shv deposits

interpentadal variability of temperature and salinity in the deep north atlantic 1970 1974 versus 1955 1959 historical hydrographic observations have been composited and objectively analyzed for the north atlantic ocean for two pentads 1955 59 and 1970 74 for the purpose of studying the temporal variability of the thermohaline structure of the deep north atlantic ocean statistically significant differences between the two pentads are found in the deep ocean at 1750 m depth most of the north atlantic increased in temperature approximately 0 1 c and salinity 0 025 from the earlier to the later period an exception to this was a region in the eastern atlantic between 33 n and 50 n where a cooling 0 1 c and freshening 0 025 occurred changes at other depths as evidenced by difference fields of temperature along 24 5 n and 36 5 n are in agreement with differences found by roemmich and wunsch 1984 who described changes at these latitudes between sections taken in the late 1950 s and sections taken in 1981

decadal changes in water mass characteristics at 24 n in the subtropical north atlantic ocean abstract three transatlantic hydrographic sections across 24 n through the center of the subtropical gyre are analyzed for evidence of decadal changes in water mass characteristics in the main thermocline there has been a steady increase in salinity from 1957 to 1981 to 1992 the salinity at a given potential temperature between 12 c and 17 c has increased at a nearly constant rate of 0 010 per decade at constant potential density salinity and temperature both increased at rates of 0 018 per decade and 0 065 c per decade in the intermediate waters between 800 and 1500 dbar salinity has increased as well on average by as much as 0 025 from 1981 to 1992 either on constant potential temperature or constant density surfaces these changes cannot be due to vertical heaving of a constant water mass structure and they cannot be easily explained in terms of horizontal shifts in the subtropical gyre water mass structure throughout the water column down to 2000 dbar and across the zonal extent of the 24 n

granular mass flows and coulomb s friction in shear cell experiments implications for geophysical flows 1 granular mass flows of rock fragments are studied in the lab by means of a high speed video camera at 2000 frames per second these granular flows are generated using beds of pumice fragments positioned on a rough rotating disk whose angular velocity is controlled by a motor the experimental apparatus allows an understanding of the arrangement of the particles in granular mass flows with relatively small and relatively large values of the savage number the savage number represents the ratio between grain collision stresses and gravitational grain contact stresses in particular these flows develop a basal layer of agitated and colliding particles underneath a relatively rigid upper layer our experimental results suggest the validity on average of the coulomb s relationship between shear and normal forces at the base of granular mass flows irrespective of their savage number value in coulomb s equation the shear stresses do not depend on the shear rate we expect the coulomb friction law to be valid also in moving pyroclastic flows our experiments suggest that the collisions and subsequent comminution of pumice fragments in moving pyroclastic flows could provide ash for the overriding ash clouds in our experiments the amount of ash generated by particle particle and particle boundary interactions increases as the value of the savage number increases in nature part of this ash may also simply move toward the base of the flows because of kinetic sieving

a study of clast size distribution ash deposition and fragmentation in a hawaiian style volcanic eruption ash deposits from hawaiian style eruptions are generally of small volume but are important in understanding the mechanisms of these eruptions an analysis of the ash deposit from the 1959 kilauea iki eruption of kilauea volcano is presented and the whole deposit grain size distribution for the eruption is calculated the grain size distribution is found to be heavily skewed towards the coarsest clast sizes and to differ considerably from size distributions for plinian deposits

cooling and freshening of the subpolar north atlantic ocean since the 1960s little is known of the interdecadal variability in the thermohaline circulation of the world s oceans yet such knowledge is essential as a background to studies of the effects of natural and anthropogenic climate change the subpolar north atlantic is an area of extensive water mass modification by heat loss to the atmosphere lying as it does at the northern limit of the global thermohaline conveyor belt 12 changes in this region may ultimately have global consequences here we report that in august 1991 the waters between greenland and the united kingdom were on average 0 08 c and 0 15 c colder than in 1962 and 1981 respectively and slightly less saline than in 1962 the cause appears to be renewed formation of intermediate water in the labrador sea from cooler and fresher source waters and the spreading of this water mass from the west variations in the source characteristics of labrador sea water can be traced across the north atlantic with a circulation time of 18 19 years between the labrador sea and rockall trough more recently formed labrador sea water with even lower temperature and salinity should cool and freshen the north atlantic still further as it circulates around the ocean in the coming decade

climatic warming of atlantic intermediate waters abstract interdecadal temperature variability of the atlantic ocean is investigated by differencing hydrographic sections taken from the 1920s through the 1990s a comprehensive reanalysis of north atlantic sections and the inclusion of south atlantic sections show that warming seen previously in the north atlantic extends to the south atlantic the largest statistically significant changes occur on pressure surfaces between 1000 and 2000 decibars db over this pressure range and for latitudes between 32 s and 36 n temperatures have warmed by 0 5 c century 1 at 48 n a cooling of 3 c century 1 occurred between the 1950s and 1980s these isobaric temperature trends are decomposed into ones along surfaces of constant neutral density and ones due to the vertical movement of neutral surfaces the two components are associated with different processes in the southern north atlantic 8 36 n the subthermocline warming between the 1950s and 1980s appears to be due primarily to downward displacements of n

in situ production of ash in pyroclastic flows 1 abrasion and comminution of pumice clasts during the propagation of pyroclastic flows have long been recognized as a potential source for the enhanced production of volcanic ash however their relative importance has eluded quantification the amount of ash produced in situ can potentially affect runout distance deposit sorting the volume of ash introduced in the upper atmosphere and internal pore pressure we conduct a series of laboratory experiments on the collisional and frictional production of ash that may occur during different regimes of pyroclastic flow transport ash produced in these experiments is predominately 10 100 microns in size and has similar morphology to tephra fall ash from plinian events we find that collisional ash production rates are proportional to the square of impact velocity frictional ash production rates are a linear function of the velocity of the basal particle enriched bed load region of these flows using these laboratory experiments we develop a subgrid model for ash production that can be included in analytical and multiphase numerical procedures to estimate the total volume of ash produced during transport we find that for most flow conditions 10 20 of the initial clasts comminute into ash with the percentage increasing as a function of initial flow energy most of the ash is produced in the high energy regions near the flow inlet although flow acceleration on steep slopes can produce ash far from the vent on level terrain collisionally and frictionally produced ash generates gravity currents that detach from the main flow and can more than double the effective runout distance of these flows ash produced at the frictional base of the flow and in the collisional upper regions of the flow can be redistributed through the entirety of the flow although frictionally produced ash accumulates preferentially near its source in the bed load flows that descend steep slopes produce the majority of their ash in the collisonally dominant flow head and flow snouts likely develop subangular to rounded pumice during this process

abrasion in pyroclastic flows the relative size of glass rims coating crystals in the matrix ash provides a semi quantitative measure of abrasion of ash grains in pyroclastic flows median abrasion indices areacrystal areaglass rim are 8 4 to 18 5 in laacher see pyroclastic flow units but only 4 to 6 3 in assocciated fallout showing stronger abrasion of ash particles in the pyroclastic flows all pyroclasts undergo strong attrition in the vent but clasts in pyroclastic flows undergo a second major phase of abrasion during high energy near vent flow abrasion of ash particles is weaker during lower energy higher strength motion further downstream suggesting that high contents of fine ash in distal deposits are due to diminishing elutriation rather than high rate of attrition

fine ash content of explosive eruptions abstract in explosive eruptions the mass proportion of ash that is aerodynamically fine enough to cause problems with jet aircraft or human lungs 50

interdecadal variations of the thermohaline circulation in a coupled ocean atmosphere model abstract a fully coupled ocean atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the north atlantic ocean with a time scale of approximately 50 years the irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation approximately 52 n to 72 n combined with much smaller density anomalies of opposite sign in the broad rising region the spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the north atlantic the anomalies of sea surface temperature induce model surface air temperature anomalies over the northern north atlantic arctic and northwestern europe

linear trends of temperature at intermediate and deep layers of the north atlantic and the north pacific oceans 1957 1981 abstract using all available hydrographic station data on ocean temperature observations from world data center b obninsk russia investigation of temperature changes in the north atlantic and north pacific oceans within the depth range of 300 to 3000 m has been carried out for the period 1957 1981 results of statistical data analysis show that in the upper layer to about 500 m depths of both oceans on average seawater temperature declined deeper than 500 600 m in the north pacific ocean no significant temperature changes have been revealed for this 25 year time interval on average for the north atlantic ocean a statistically significant temperature rise about 0 1 c 25 yr 1 is observed in the 800 to 2500 m layer

long term hydrographic changes at 52 and 66 w in the north atlantic subtropical gyre caribbean abstract in july september 1997 two hydrographic lines were done in the western n atlantic along longitudes of 52 and 66 w as part of the woce one time hydrographic survey of the oceans each of these two lines approximately repeated earlier ones done during the international geophysical year s igy and the mid 1980s because of this repeated sampling long term hydrographic changes in the water masses can be examined in this report we focus on temperature and salinity changes within the subtropical gyre mainly between latitudes of 20 and 35 n and compare our results to those presented by bryden et al 1996 who examined changes along a zonal line at 24 n most recently occupied in 1992 since this most recent 24 n section in 1992 substantial changes have occurred in the western part of the subtropical gyre at the depths of the labrador sea water lsw in particular we see clear evidence for colder fresher labrador sea water throughout the gyre on our two recent sections that was not yet present in 1992 at similar longitudes along 24 n at shallower depths inhabited by waters that are an admixture of mediterranean mw and antarctic intermediate waters aaiw our recent survey shows an increase in salinity which can only be attributed to changes in water masses on potential temperature or neutral density surfaces furthermore waters above the mw aaiw layer and into the deeper part of the main pycnocline have continued to become saltier and warmer throughout the 40 year period spanned by our sections these latter changes have been dominantly due to a vertical sinking of density surfaces as t s changes in density surfaces are small but depths of individual t s horizons have increased with time the net change since the igy shows a mean temperature increase between 800 and 2500 m depth at a rate of 0 57 c century with a corresponding steric sea level rise of 1 mm yr and a net downward heave with small values near the top and bottom and a maximum rate of 2 7 m yr at 1800 m depth changes in the deep caribbean indicate a warming since the igy due to temperature increases of the inflowing source waters in the subtropical gyre at 1800m depth but no significant change in the deep salinity

contamination of water supplies by volcanic ashfall a literature review and simple impact modelling abstract volcanic ash is the most widely distributed product of explosive volcanic eruptions and can disrupt vital infrastructure on a large scale previous studies of effects of ashfall on natural waters and water supplies have focused mainly on the consequences of increased levels of turbidity ash suspended in water acidity and fluoride with very little attention paid to other contaminants associated with volcanic ash the aims of this paper are twofold firstly to review previous studies of the effects of volcanic ashfall on water supplies and identify information gaps and secondly to propose a simple model for predicting effects of ashfall on water supplies using available information on ash composition we reviewed reported impacts of historic eruptions on water supplies drawing on case studies from new zealand vanuatu argentina the usa costa rica montserrat iceland and guadeloupe elevated concentrations of fluoride iron sulphate and chloride as well as turbidity and acidity have been reported in water supplies from a public health perspective the two main issues appear to be 1 outbreaks of infectious disease caused by the inhibition of disinfection by high levels of suspended ash and 2 elevated fluoride concentrations we devised a simple model using volcanic ash leachate composition data to predict effects on receiving waters applying this model to the effects of ruapehu ash from the 1995 1996 eruptions suggests that the primary effects of concern are likely to be an increase in acidity decrease in ph and increases in concentrations of the metals aluminium iron and manganese these metals are not normally considered to pose health risks and are regulated only by secondary non enforceable guidelines however exceedences of guideline values for al mn fe and ph will cause water to become undrinkable due to a bitter metallic taste and dark colour and may also cause corrosion staining and scale deposition problems in water tanks and pipes therefore the main issues following volcanic ashfall of similar composition to ruapehu ash are likely to be shortages of potable water and damage to distribution systems rather than risks to public health

seasonality of the north atlantic oscillation abstract monthly sea level pressure slp data from the national centers for environmental prediction reanalysis for 1948 99 are used to develop a seasonally and geographically varying mobile index of the north atlantic oscillation naom naom is defined as the difference between normalized slp anomalies at the locations of maximum negative correlation between the subtropical and subpolar north atlantic slp the subtropical nodal point migrates westward and slightly northward into the central north atlantic from winter to summer the naom index is robust across datasets and correlates more highly than eof coefficients with historical measures of westerly wind intensity across north atlantic midlatitudes as measured by this mobile index the nao s nodes maintain their correlation from winter to summer to a greater degree than traditional nao indices based on fixed stations in the eastern north atlantic azores lisbon iceland when the naom index is extended back to 1873 its annual values during

interdecadal variations in north atlantic sea surface temperature and associated atmospheric conditions abstract evidence is presented for a distinct pattern of ocean atmosphere relationship associated with interdecadal variability in the north atlantic region using a century of surface marine observations it is shown that middle and high latitude sea surface temperature sst display a long term fluctuation with negative anomalies before 1920 and during the 1970s and 1980s positive sst conditions prevailed from about 1930 to 1960 the pattern of interdecadal sst variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years the early century warming and the more recent cooling display a similar spatial pattern in both cases the pattern is basin scale and largely of one polarity with maxima in the vicinity of iceland in the labrador sea and northeast of bermuda the corresponding differences in surface atmospheric conditions are determined by averaging and subtracting in the same manner fields of sea level pressure slp and surf

variability of the simulated meridional heat transport in the north atlantic for the period 1951 1993 a 43 year ocean model simulation for the period 1951 1993 is analyzed with a focus on the meridional heat transport mht as a proxy for the strength of meridional overturning cell moc at 25 n the surface heat flux forcing associated with the north atlantic oscillation nao pattern is related to the variability in mht both on interannual and longer timescales the manifestation of the interannual and decadal oceanic response to nao is nonlocal as evidenced by the concentrated heat content and sea level variability at the gulf stream and north atlantic current regions and shown by the comparison of the model sea level variability to the altimeter data between low mht years in mid 1980s and high mht years in early 1990s the same area is singled out by empirical orthogonal function analysis in the leading mode of the sea level mht and time series of the leading sea level mode are highly correlated which reflects the importance of the moc in determining the variability of the heat content of the whole basin also the model suggests that the mht moc has entered a very strong phase since the mid 1980s and this trend has continued up to the end of 1993 this behavior follows the general trend in the nao index

sizes and shapes of 10 ma distal fall pyroclasts in the ogallala group nebraska abstract size distributions of distal ashfall particles from correlated 10 ma layers in nebraska measured using laser diffraction methods are lognormal with mode diameters of 90 m this ashfall is 100 bubble wall shards of rhyolite glass and apparently represents a distal ashfall from an eruption 1400 km away measured terminal velocities of these ash particles are 0 2 18 cm s consistent with stokes law settling of spherical particles with diameters of 9 50 m surface area of the ash particles measured with gas adsorption is 20 30 times the surface area of equivalent stokes spheres these results highlight the effects of shape and atmospheric drag in distal ashfalls they also highlight atmospheric transport and fallout of distal ashfall particles because these deposits resemble many other ashfalls preserved in the great plains of north america throughout the tertiary and quaternary because the ashfalls preserve major mammalian death assemblages they demonstrate that deposits with modes of op

multidecadal variability of north atlantic temperature and salinity during the twentieth century abstract substantial changes occurred in the north atlantic during the twentieth century here the authors demonstrate through the analysis of a vast collection of observational data that multidecadal fluctuations on time scales of 50 80 yr are prevalent in the upper 3000 m of the north atlantic ocean spatially averaged temperature and salinity from the 0 300 and 1000 3000 m layers vary in opposition prolonged periods of cooling and freshening warming and salinification in one layer are generally associated with opposite tendencies in the other layer consistent with the notion of thermohaline overturning circulation in the 1990s widespread cooling and freshening was a dominant feature in the 1000 3000 m layer whereas warming and salinification generally dominated in the upper 300 m except for the subpolar north atlantic where complex exchanges with the arctic ocean occur the single signed basin scale pattern of multidecadal variability is evident from decadal 1000 3000 m temperature and salin

sedimentology and geomorphology of the deposits from the august 2006 pyroclastic density currents at tungurahua volcano ecuador the deposits of the pyroclastic density currents from the august 2006 eruption of tungurahua show three facies associations depending on the topographic setting the massive proximal cross stratified and distal cross stratified facies 1 the massive facies is confined to valleys on the slopes of the volcano it contains clasts of 1 m diameter to fine ash material is massive and interpreted as deposited from dense pyroclastic flows its surface can exhibit lobes and levees covered with disk shaped and vesicular large clasts these fragile large clasts must have rafted at the surface of the flows all along the path in order to be preserved and thus imply a sharp density boundary near the surface of these flows 2 the proximal cross stratified facies is exposed on valley overbanks on the upper part of the volcano and contains both massive coarse grained layers and cross stratified ash and lapilli bedsets it is interpreted as deposited from a dense pyroclastic flows that overflowed the gentle ridges of valleys of the upper part of the volcano and b dilute pyroclastic density currents created from the dense flows by the entrainment of air on the steep upper flanks 3 the distal cross stratified facies outcrops as spatially limited isolated and wedge shaped bodies of cross stratified ash deposits located downstream of cliffs on valleys overbanks it contains numerous aggrading dune bedforms whose crest orientations reveal parental flow directions a downstream decrease in the size of the dune bedforms together with a downstream fining trend in the grain size distribution are observed on a 100 m scale this facies is interpreted to have been deposited from dilute pyroclastic density currents with basal tractional boundary layers we suggest that the parental flows were produced from the dense flows by entrainment of air at cliffs and that these diluted currents might rapidly deposit through pneumatic jumps three modes are present in the grain size distribution of all samples independently of the facies which further supports the interpretation that all three facies derive from the same initial flows this study emphasizes the influence of topography on small volume pyroclastic density currents and the importance of flow transformation and flow stripping processes

freshening of the labrador sea surface waters in the 1990s another great salinity anomaly 1 the observed labrador sea surface salinities for 1951 2001 in region 55w 40w 45n 55n show freshening in the 1990s which may rival the great salinity anomaly gsa of the 1970s dickson et al 1988 the 1990s freshening is distinct in the lowest range of salinity values associated with the coastal waters the variability of the highest salinity values provided by the north atlantic current entering the labrador sea shows that the high salinity atlantic waters were not widely present in the 1970 s and 1990s this can be interpreted as weakened transport of saline waters to the area this explanation is consistent with an arctic north atlantic model results forced by ncep ncar reanalysis data for 1958 2001 which suggest that these periods are associated with a reduced meridional overturning the model also associates the coastal freshening with southward current anomalies which are well correlated with the number of icebergs crossing 48n

the contribution of experimental volcanology to the study of the physics of eruptive processes and related scaling issues a review abstract the experimental approach has become a major tool increasingly used by volcanologists in recent decades to investigate the physics of eruptive processes in complement to field and theoretical works researchers have developed various methodologies to study volcanic phenomena at reduced length scale the works involve natural or analogue materials and their types range from first order tests to identify fundamental processes and make qualitative comparison with field observations to more sophisticated experiments in which precise data obtained in a controlled environment can be used to validate outputs of theoretical models scaling is a central issue to ensure dynamic similarity between the small scale experiments and the large scale volcanic phenomena when natural conditions cannot be simulated due to inherent length scale difference and or technical limitations in this respect dimensionless numbers are used to map physical regimes and to define scaling laws which allow experimental results to be extrapolated to natural scale this review presents the variety of experimental studies conducted to investigate subterraneous and aerial volcanic phenomena involving in particular fluid particle mixtures we focus on the major scaling issues and the physical regimes investigated and we also highlight in a historical perspective some of the major advances achieved through experimental studies finally we conclude on some perspectives for future works

interpentadal variability of steric sea level and geopotential thickness of the north atlantic ocean 1970 1974 versus 1955 1959 we have composited and objectively analyzed historical hydrographic observations for the north atlantic ocean for two periods 1955 1959 and 1970 1974 difference fields of steric sea level and geopotential thickness for the north atlantic ocean in the 0 to 1500 m depth range between the 1955 1959 and 1970 1974 pentads were computed we find that in the central portion of the subtropical gyre steric sea level decreased by up to 17 5 dyn cm whereas in the western subarctic gyre north of the gulf stream steric sea level increased by an amount up to 7 5 dyn cm from the earlier to the later pentad a decrease in steric sea level 5 dyn cm occurred along the eastern boundary of the north atlantic

syneruptive sequential fragmentation of pyroclasts from fractal modeling of grain size distributions of fall deposits the cretaio tephra eruption ischia island italy abstract in this work we use fractal statistics in order to decipher the mechanisms acting during explosive volcanic eruptions by studying the grain size distribution gsd of natural pyroclastic fall deposits the method was applied to lithic rich proximal deposits from a stratigraphic section of the cretaio tephra eruption ischia island italy analyses were performed separately on bulk material juvenile and lithic fraction from each pyroclastic layer results highlight that the bulk material is characterized by a single scaling regime whereas two scaling regimes with contrasting power law exponents are observed for the juvenile and the lithic fractions on the basis of these results we infer that the bulk material cannot be considered as a good proxy for deducing eruption dynamics because it is the result of mixing of fragments belonging to the lithic and juvenile fraction both of which underwent different events of fragmentation governed by different mechanisms in addition results from fractal analyses of the lithic fraction suggest that it likely experienced a fragmentation event in which the efficiency of fragmentation was larger for the coarser fragments relative to the finer ones on the contrary we interpret the different scaling regimes observed for the juvenile fraction to be due to sequential events of fragmentation in the conduit possibly enhanced by the presence of lithic fragments in the eruptive mixture in particular collisional events generated increasing amounts of finer particles modifying the original juvenile gsds and determining the development of two scaling regimes in which the finer fragments record a higher efficiency of fragmentation relative to the coarser ones we further suggest that in lithic rich proximal fall deposits possible indications about the original gsds of the juvenile fraction might still reside in the coarser particles fraction

a tree ring based reconstruction of the atlantic multidecadal oscillation since 1567 a d 1 we present a tree ring based reconstruction of the atlantic multidecadal oscillation amo which demonstrates that strong low frequency 60 100 yr variability in basin wide 0 70 n sea surface temperatures ssts has been a consistent feature of north atlantic climate for the past five centuries intervention analysis of reconstructed amo indicates that 20th century modes were similar to those in the preceding 350 yr and wavelet spectra show robust multidecadal oscillations throughout the reconstruction though the exact relationships between low frequency sst modes higher frequency 7 25 yr atmospheric modes e g north atlantic oscillation arctic oscillation and terrestrial climates must still be resolved our results confirm that the amo should be considered in assessments of past and future northern hemisphere climates

total grain size distribution and volume of tephra fall deposits on 17 june 1996 ruapehu volcano new zealand produced a sustained andesitic sub plinian eruption which generated a narrow tephra fall deposit extending more than 200 km from the volcano the extremely detailed data set from this eruption allowed methods for the determination of total grain size distribution and volume of tephra fall deposits to be critically investigated calculated total grain size distributions of tephra fall deposits depend strongly on the method used and on the availability of data across the entire dispersal area the voronoi tessellation method was tested for the ruapehu deposit and gave the best results when applied to a data set extending out to isomass values of 1 g m 2 the total grain size distribution of a deposit is also strongly influenced by the very proximal samples and this can be shown by artificially constructing subsets from the ruapehu database unless the available data set is large all existing techniques for calculations of total grain size distribution give only apparent distributions the tephra fall deposit from ruapehu does not show a simple exponential thinning but can be approximated well by at least three straight line segments or by a power law fit on semi log plots of thickness vs area 1 2 integrations of both fits give similar volumes of about 4 106 m3 integration of at least three exponential segments and of a power law fit with at least ten isopach contours available can be considered as a good estimate of the actual volume of tephra fall integrations of smaller data sets are more problematic

generation and dispersal of fine ash and dust by volcanic eruptions abstract the volcanic eruptions which generate the greatest quantities of fine ash and dust are those of ignimbrite forming plinian vulcanian and phreatomagmatic types these are also the eruptions which produce the widest dispersal of this material attributed to the superior height attained by their eruptive columns however much of the fine ash and dust may be rapidly flushed out of the eruptive plume by water particularly in phreatomagmatic eruptions recent studies made on the dispersal and grain size of pyroclastic deposits produced by examples of plinian and phreatomagmatic types have yielded estimates of the quantities of material generated in each grain size class besides the extent of their dispersal not all of the fine volcanic particles are produced by fragmentation at the eruptive vent in ignimbrite eruptions there is good evidence for their large scale generation in and loss from the moving ash flows

decadal variability of hydrography in the upper northern north atlantic in 1948 1990 we investigate the variability of the north atlantic subarctic gyre in recent decades from time series of station temperature and salinity decadal variability stronger at the surface is identified which exhibits vertical coherence over a layer deeper than the late winter mixed layer in the northwestern atlantic it corresponds to the layer with a component of water from the arctic ocean or from the canadian arctic the spatial coherence of the signal is investigated an empirical orthogonal function decomposition of lagged time series indicates that a single pattern explains 70 of the variance in upper ocean salt content corresponding to a propagating signal from the west to the northeast in the subarctic gyre the most likely interpretation is that the salinity signal originates in the slope currents of the labrador sea and is diffused advected eastward of the grand banks over the near western atlantic in the northwestern atlantic temperature fluctuations are strongly correlated to salinity fluctuations and are aligned along the average t s characteristics this signal suggests large variations in the outflow of fresh cold water in the slope current and is strongly correlated with ice cover a basin scale atmospheric circulation of weakened westerlies at 55 n weaker northwesterlies west of greenland and weaker southerlies over the central and eastern north atlantic is associated with the high salinity and warm water phase of the first principal component this circulation pattern leads fluctuations in the northeast atlantic and lags those in the northwestern part of the basin the wind indices also suggest that the fluctuations of the fresh water outflow occur during intervals of anomalously northerly winds either east of greenland 1965 1968 1969 or off the canadian archipelago 1983 1984

great salinity anomalies in the north atlantic abstract we revisited the great salinity anomaly of the 1970s gsa 70s dickson et al 1988 and documented the newly identified great salinity anomaly of the 1980s hence termed gsa 80s both propagated around the north atlantic in a similar fashion the advective mechanism initially proposed to explain the observed sequence of low salinity low temperature events during the gsa 70s apparently holds also for the gsa 80s the latter was successively observed in the west greenland current 1982 the labrador current 1983 the flemish pass 1984 near the charlie gibbs fracture zone 1984 1985 in the rockall channel 1985 south of iceland 1985 1988 in the north sea 1986 1987 norwegian sea 1987 1988 barents sea 1988 1989 and iceland sea 1989 1990 the advection speed of the gsa 80s seems to be greater than the one of the gsa 70s the 1980s anomaly reached the barents sea 6 to 7 years after peaking in the west greenland current while the 1970s anomaly traveled the same route in 8 to 10 years these anomalies however seem to be of different origin the gsa 70s was apparently boosted remotely by a freshwater sea ice pulse from the arctic via fram strait consequently the gsa 70s was accompanied by a large sea ice extent anomaly in the greenland and iceland seas which propagated into the labrador sea in contrast the gsa 80s was likely formed locally in the labrador sea baffin bay mainly because of the extremely severe winters of the early 1980s but supplemented with a possible contribution of the arctic freshwater outflow via the canadian archipelago facilitated by strong northerly winds which would have enhanced stability and ice formation this anomaly was also associated with a positive sea ice extent anomaly in the labrador sea baffin bay which however had no upstream precursor in the greenland sea thus the gsas are not necessarily caused solely by an increased export of freshwater and sea ice from the arctic via fram strait these results are corroborated by the early 1990s data when a new fresh cold anomaly was formed in the labrador sea and accompanied by a large positive sea ice extent anomaly the harsh winters of the early 1990s were however confined to the labrador sea baffin bay area while the atmospheric and oceanic conditions in the greenland iceland and irminger seas were normal the labrador sea baffin bay area appears therefore to play a key role in formation of gsas as well as in propagation of the gsas formed upstream a likely contribution of the enhanced canadian archipelago freshwater outflow to the gsa formation also seems to be significant two major modes of the gsa origin are thus identified remote generated by an enhanced arctic ocean freshwater export via either fram strait or the canadian archipelago and local resulting from severe winters in the labrador sea baffin bay both modes should be taken into account to model decadal variability of the coupled ocean ice atmosphere system in the north atlantic arctic basin

volcanic history of lipari aeolian islands italy during the last 10 000 years abstract examination of the volcanic stratigraphy of deposits younger than 10 000 years on lipari indicates four principal periods of volcanic activity related to specific centers the products from each different volcanic center are defined as volcano stratigraphic unit vsu from the oldest these are the canneto dentro gabellotto fiume bianco forgia vecchia and monte pilato rocche rosse vsus the study of textures and dispersal of the deposits permitted the vents to be localized and the recent volcanic history of lipari to be reconstructed the oldest event formed a small explosion breccia cone with a final obsidian lava in the canneto dentro area immediately afterward a complex series of explosions produced the widespread dry surge deposits of the gabellotto fiume bianco sequence this activity ended with the extrusion of a domical lava flow the renewal of activity occurred in the pirrera area with an explosive eruption that produced explosion breccia deposits the last eruptions from this vent were coeval with the first eruption of m pilato the rim of the explosion breccia cone was partially destroyed by the forgia vecchia lava flow m pilato cone grew in a very short period of time due to a continuous swarm of explosive events after a short repose time a series of more energetic and superficial explosions occurred through a vent slightly to the south the extrusion of the rocche rosse lava flow about 729 a d ended this cycle of activity all the volcanic centers follow a quite similar stochastic pattern starting with a fall or surge eruption and ending with effusion of viscous rhyolitic lavas the four centers are aligned along either nw se or ne sw fault systems according to the structural pattern of the island they can be placed into two groups the canneto dentro gabellotto centers and the forgia vecchia m pilato centers a long quiescence 3 500 years separates the activity of these two groups while inside of each the activities were nearly contemporaneous or in rapid succession

erosion transport and segregation of pumice and lithic clasts in pyroclastic flows inferred from ignimbrite at lascar volcano chile abstract investigations have been made on the distribution of pumice and lithic clasts in the lithic rich soncor ignimbrite 26 5 ka and the 1993 pumice flow deposits of lascar volcano chile the soncor ignimbrite shows three main lithofacies which grade into one another coarse lithic breccias range from matrix poor stratified varieties irregular shaped sheets and elongate hummocks in proximal environments to breccia lenses with pumiceous ignimbrite matrix massive lithic rich facies comprise the bulk of the ignimbrite pumice rich facies are bimodal with abundant large pumice clasts often with reverse grading rare lithic clasts and occur distally and on high ground adjacent to deep proximal valleys in the 1993 pyroclastic flow deposits lithic rich facies are deposited on slopes up to 14 whereas pumice rich facies are deposited only on slopes

long term coordinated changes in the convective activity of the north atlantic the north atlantic is a peculiarly convective ocean the convective renewal of intermediate and deep waters in the labrador sea and greenland iceland sea both contribute significantly to the production and export of north atlantic deep water thus helping to drive the global thermohaline circulation while the formation and spreading of 18 degree water at shallow to intermediate depths off the us eastern seaboard is a major element in the circulation and hydrographic character of the west atlantic for as long as time series of adequate precision have been available to us it has been apparent that the intensity of convection at each of these sites and the hydrographic character of their products have been subject to major interannual change as shown by aagaard 1968 clarke et al 1990 and meincke et al 1992 for the greenland sea in the ows bravo record from the labrador sea eg lazier 1980 et seq and at the panulirus hydrostation s site in the northern sargasso off bermuda eg jenkins 1982 talley and raymer 1982 this paper reviews the recent history of these changes showing that the major convective centres of the greenland and labrador seas are currently at opposite convective extrema in our postwar record with vertical exchange at the former site limited to 1000 m or so but with labrador sea convection reaching deeper than previously observed to over 2300 m as a result greenland sea deep water has become progressively warmer and more saline since the early 70 s due to increased horizontal exchange with the arctic ocean through fram strait while the labrador sea water has become progressively colder and fresher over the same period through increased vertical exchange most recently convection has become deep enough there to reach into the more saline nadw which underlies it so that cooler but now saltier and denser lsw has resulted

particle sizes of andesitic ash fallout from vertical eruptions and co pyroclastic flow clouds volcan de colima mexico we report particle size distributions for andesitic ash fall from vertical eruptions and clouds above block and ash fl ows at volcan de colima mexico during 2004 2006 we analyzed 17 samples using laser diffraction 11 from vertical eruptions and 6 from block and ash fl ows copyroclastic fl ow ash vertical eruptions produce well sorted fall deposits whereas co pyroclastic fall deposits are poorly sorted with high proportions of very fi ne grained ash 30 m statistical analysis shows particle size distributions of vertical eruptions are more leptokurtic peaked than co pyroclastic samples deconvolution of grain size histograms shows that copyroclastic samples have at least one subpopulation with a mode of 8 3 8 7 2 4 3 1 m estimates of the number of particles in different size ranges show that co pyroclastic samples contain much greater numbers of very fi ne particles than vertical eruption samples our results provide no direct evidence that milling or comminution produces hazardous fi ne ash particles in pyroclastic fl ows but are consistent with that interpretation

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

particle size density relationships in pyroclastic deposits inferences for emplacement processes in moving pyroclastic systems particles are sorted as a function of their sizes densities and shapes the analysis of the distribution of these characteristics in particle populations of pyroclastic deposits is a major tool in evaluating properties and regimes of parent transport systems this paper reports a study of size density relationships of component subpopulations from selected quaternary pyroclastic deposits of the latera volcanic complex vulsini italy by means of integrated field and laboratory investigations including grain size and component analyses density measurements and sem crystal morphoscopy three kinds of particle size density relationships characterize the studied deposits as follows 1 a significant correlation of decreasing sizes with increasing component densities exists for direct suspension sedimentation products including plinian fall and pyroclastic surge deposits 2 a wide data dispersion with poor correlation normally occurs for the main zones of deposits from high concentration pyroclastic flows and 3 component subpopulations occur in a narrow size range irrespective of density in granular systems dominated by grain collisions such as traction zones beneath low concentration pyroclastic surges and sheared basal layers of pyroclastic flows the regression coefficients of size density relationships appear as a comparative measure of particle concentration in the parent systems consistent with that inferred from the degree of sorting and abrasion of crystal subpopulations from sequential fragmentation transport analysis of grain size data and quantitative morphoscopy the crystal component becomes less sorted and more abraded as the fit to the size density relationship decreases from plinian fall to pyroclastic surge to pyroclastic flow consistent with increasing particle concentration in the transporting current

tephra deposits associated with a large lava dome collapse soufriere hills volcano montserrat 12 15 july 2003 abstract the 12 13 july 2003 dome collapse at soufriere hills volcano montserrat was the largest event of its kind during the eruption thus far 1995 2005 involving the removal of 210 million m 3 of the lava dome complex over 18 h less than 2 of the total volume of material involved in the dome collapse was deposited on land a pyroclastic density current deposit alongshore and inland from the tar river fan was generated from a single blast originating at the shoreline the blast was caused by the interaction of pyroclastic flows with seawater we propose that at the peak of the lava dome collapse a sharp increase in the volume flux of pyroclastic flows caused substantial displacement of seawater from the shoreline followed by inrush of seawater when the flux decreased a few minutes later the tsunami allowed penetration of seawater into the main body of the pyroclastic flow at the shoreline which led to explosive fragmentation of pyroclastic blocks tephra fall deposits accumulated at a high rate on montserrat causing extensive damage to vegetation and buildings their stratigraphy recorded fallout from high co pyroclastic flow clouds from a vulcanian explosion cloud at the peak in collapse rate caused by the fragmentation of degassed lava dome and from four vulcanian explosion clouds after the dome collapse caused by fragmentation of bubbly magma in the conduit the total tephra fall volume is estimated at 10 20 million m 3

the size distribution of pyroclasts and the fragmentation sequence in explosive volcanic eruptions in an explosive eruption the atmospheric column dynamics depend strongly on the mass fraction of gas in the erupting mixture which is fixed by fragmentation in the volcanic conduit at fragmentation gas present in vesicular magmatic liquid gets partitioned between a continuous phase separating magma clasts and a dispersed phase in individual bubbles within the clasts as regards flow behavior it is the former continuous gas phase which matters and we show that its amount is determined by the fragment size analysis of 25 fall deposits and 37 flow deposits demonstrates that ash and pumice populations follow a power law size distribution such that n the number of fragments with radii larger than r is given by n r d d values range from 2 9 to 3 9 and are always larger than 3 0 in fall deposits d values for pyroclastic flow deposits are systematically smaller than those of fall deposits we show that at fragmentation the amount of continuous gas phase is an increasing function of the d value large d values cannot be attributed to a single fragmentation event and are due to secondary fragmentation processes laboratory experiments on bubbly magma and on solid pumice samples demonstrate that primary breakup leads to d values of 2 5 0 1 and that repeated fragment collisions act to increase the d value a model for size dependent refragmentation accounts for the observations we propose that in a volcanic conduit fragmentation proceeds as a sequence of events primary breakup releases a small amount of gas and is followed by fragment collisions due to refragmentation and decompression both the mass and volume fractions of continuous gas increase the final d value and hence the mass fraction of continuous gas at the vent depends on the time spent between primary fragmentation and eruption out of the vent

granular disruption during explosive volcanic eruptions volcanic eruptions can inject hazardous ash clouds into the atmosphere numerical simulations and experiments on volcanic rock samples show that clasts initially formed deep in the volcanic conduit break up during collisions in the conduit thus generating fine grained clouds of ash

runout and fine sediment deposits of axisymmetric turbidity currents we develop a model that describes the runout behavior and resulting deposit of a radially spreading suspension driven gravity current on a surface of negligible slope our analysis considers the separate cases of constant volume and constant flux sources it incorporates expressions for the conservation of volume a froude number condition at the current front and the evolution of the driving suspension due to settling of particles to the underlying bed the model captures the key features of a range of experimental observations the analysis also provides important scaling relationships between the geometry of a deposit and the source conditions for the deposit forming flow as well as explicit expressions for flow speed and deposit thickness as functions of radial distance from the source among the results of our study we find that in the absence of information regarding flow history the geometries of relatively well sorted deposits generated by flows with source conditions of constant volume or constant flux are virtually indistinguishable the results of our analysis can be used by geologists in the interpretation of some geologically important gravity surge deposits using our analytical results we consider three previously studied radially symmetric turbidites of the hispaniola caicos basin in the western atlantic ocean from gross geometry and grain size of the turbidites alone we estimate for the respective deposit forming events that upon entry into the basin the initial sediment concentrations were approximately 3 by volume and the total volumes were roughly between 30 km 3 and 100 kn1 3 each of the suspension driven flows is inferred to have spread into the basin with a characteristic speed of 3 5 m s 1 and reached its ultimate runout length of about 60 75 km while laying down a deposit over a period of about 10 12 hours

pumice pumice collisions and the effect of the impact angle 3 in nature particles have irregular shapes and their collisions are not necessarily normal to surfaces thus we present here the studies of oblique collisions of pumice cylinders on flat pumice surfaces cylinders were used to increase the complexity of the particle shape compared to that of a sphere the purpose of these experiments is to understand the effect of the impact angle on the rebound features and provide an example of their variability that can be useful in modelling pumice pumice collisions in geophysical flows the rebound features are the rebound angles the ratios of the components of velocities and the energy losses our results reveal a complexity which suggests that the description of pumice pumice impacts using a constant coefficient of restitution is an oversimplification 4 here we do not refer to the coefficient of restitution also because it applies to collisions where the energy losses are only due to plastic deformations and vibrations in geophysical flows the collisions are more complex because of for example erosion shattering and comminution of the rock fragments the coefficient of restitution e purports to describe the degree of plasticity in collisions of two bodies and is generally defined as the ratio of the final to initial components of the relative velocities normal to the contact surface goldsmith 2001 this coefficient can be measured by dropping a non rotating particle onto the horizontal surface of a large rigid body if the particle has been dropped from the height h1 and rebounds to the height h2 then e is equal to ffiffiffiffiffiffiffiffiffiffiffiffi

vesicle connectivity in pyroclasts and implications for the fluidisation of fountain collapse pyroclastic flows montserrat west indies abstract pyroclastic flows are noted for their highly mobile behaviour being able to flow on slopes as low as a few degrees and this has been attributed to fluidisation or partial fluidisation by escaping gases the three types of pyroclastic flows generated during the eruption of soufriere hills volcano on montserrat dome collapse fountain collapse and surge derived differ in their apparent mobility suggesting different degrees of fluidisation and friction reduction in the flowing granular materials several possible sources of fluidising gas operate in pyroclastic flows the purpose of this paper is to assess the feasibility of one mechanism the release during flow of pressurised gases trapped in the vesicles of juvenile clasts measurements with a helium pycnometer show that all vesicles in dense lava blocks from dome collapse pyroclastic flows are connected so that the mechanism is probably not viable for such flows on the other hand pumices from fountain collapse pyroclastic flows from soufriere hills and other volcanoes contain up to 10 vol of isolated vesicles this is confirmed by sem imagery which shows that soufriere hills pumices contain two vesicle populations with a bimodal size distribution the large vesicles are mostly interconnected whereas about two thirds of the smaller population

volcanic dilemma flow or blow volcanic eruptions can be either slow outpourings of lava or explosive ejections of fragmented rock in his perspective dingwell discusses the physical and chemical properties of magma that lead to these two styles of eruption new experiments and computer simulations are pointing to the involvement of solidlike behavior of liquid magma when it makes a transition to a glass

relating the physical properties of volcanic rocks to the characteristics of ash generated by experimental abrasion interactions between clasts in pyroclastic density currents pdcs generate volcanic ash that can be dispersed to the atmosphere in co pdc plumes and due to its small size is far travelled we designed a series of experiments to determine the effects of pyroclast vesicularity and crystal content on the efficiency and type of ash generated by abrasion two different pyroclastic materials were used 1 basaltic andesite pyroclasts from fuego volcano guatemala with 26 46 vesicularity and high groundmass crystallinity and 2 tephri phonolite avellino pumice vesuvius italy with 55 75 vesicularity and low groundmass crystallinity when milled both clast types produced bimodal grain size distributions with fine ash modes between 4 and 5 32 63 m although the vesicular avellino pumice typically generated more ash than the denser fuego pyroclasts the ash generating potential of a single pyroclast was independent of density and instead governed by heterogeneous crystal and vesicle textures one consequence of these heterogeneities was to cause the vesicular avellino clasts to split in addition to abrading which further enhanced abrasion efficiency the matrix characteristics also affected ash shape and componentry which will influence the elutriation and transport properties of ash in the atmosphere the experimental abrasion successfully replicated some of the characteristics of natural co pdc ash samples as shown by similarities in the adherence factor which measures the proportion of attached matrix on phenocrysts of both the experimentally generated ash and natural co pdc ash samples our results support previous studies which have shown that abrasion is an effective mechanism for generating fine ash that is similar in size 5 30 m to that found in co pdc deposits we further show that both the abundance and nature shape density components size distribution of those ash particles are strongly controlled by the matrix properties of the abraded pyroclasts

the respiratory health hazards of volcanic ash a review for volcanic risk mitigation studies of the respiratory health effects of different types of volcanic ash have been undertaken only in the last 40 years and mostly since the eruption of mt st helens in 1980 this review of all published clinical epidemiological and toxicological studies and other work known to the authors up to and including 2005 highlights the sparseness of studies on acute health effects after eruptions and the complexity of evaluating the long term health risk silicosis non specific pneumoconiosis and chronic obstructive pulmonary disease in populations from prolonged exposure to ash due to persistent eruptive activity the acute and chronic health effects of volcanic ash depend upon particle size particularly the proportion of respirable sized material mineralogical composition including the crystalline silica content and the physico chemical properties of the surfaces of the ash particles all of which vary between volcanoes and even eruptions of the same volcano but adequate information on these key characteristics is not reported for most eruptions the incidence of acute respiratory symptoms e g asthma bronchitis varies greatly after ashfalls from very few if any reported cases to population outbreaks of asthma the studies are inadequate for excluding increases in acute respiratory mortality after eruptions individuals with pre existing lung disease including asthma can be at increased risk of their symptoms being exacerbated after falls of fine ash a comprehensive risk assessment including toxicological studies to determine the long term risk of silicosis from chronic exposure to volcanic ash has been undertaken only in the eruptions of mt st helens 1980 usa and soufriere hills montserrat 1995 onwards in the soufriere hills eruption a long term silicosis hazard has been identified and sufficient exposure and toxicological information obtained to make a probabilistic risk assessment for the development of silicosis in outdoor workers and the general population a more systematic approach to multi disciplinary studies in future eruptions is recommended including establishing an archive of ash samples and a website containing health advice for the public together with scientific and medical study guidelines for volcanologists and health care workers

grain size characteristics of experimental pyroclasts of 1980 mount st helens cryptodome dacite effects of pressure drop and temperature using the fragmentation bomb we analysed the effects of temperature and pressure drop on the grain size characteristics of experimentally produced pyroclasts experiments performed on vesicular samples of grey dacite of the 1980 mount st helens cryptodome at t 20 900 c and initial pressure differential up to 18 5 mpa provide clear evidence of the influence of these physical conditions upon fragment size and character cylindrical dacite samples diameter 17 mm length 50 mm are placed in the high pressure temperature section of the apparatus heated and saturated by argon gas the disruption of a diaphragm located between the high and low pressure sections of the apparatus initiates the rapid depressurisation of the sample the main results may be summarised as follows 1 increasing tempera ture from 20 to 900 c results in a decrease in the fragmentation threshold value from 9 to 3 mpa and an increase in the median diameter of the experimental pyroclasts these observations imply a decrease in the dynamic tensile strength of dacite at higher temperatures which in turn influences the characteristic size of fragments 2 increasing initial pressure differential yields a decrease of the median diameter thus a higher initial elastic potential energy in the magma generates a higher degree of fragmentation 3 fragments of angular shape are observed from experiments at all investigated temperatures 20 900 c including thereby temperatures significantly higher than the classical dilatometrically or calorimetrically determined glass transition temperature determined for this dacite of 810 c thus brittle response of the dacite is observed under rapid decom pression 4 fragment size distributions do not corre spond to log normal distributions and are more closely described by rosin rammler distributions with a de crease of temperature and increase of the initial pressure differential fragment size distributions approach a rosin rammler distribution dependence 5 microscopic and grain size comparison of particle characteristics of ex perimental and natural fragments of the 1980 mount st helens cryptodome dacite demonstrate similar median diameters and shapes

linear trends in salinity for the world ocean 1955 1998 1 quality controlled oceanographic profile salinity measurements from the world ocean database 2001 wod01 were used to calculate linear trends of zonally averaged salinity anomalies for running five year periods from 1955 1959 through 1994 1998 for the world ocean and the atlantic pacific and indian ocean basins from the surface to 3000 meters depth each basin exhibits large scale coherent trends most of the pacific is freshening with the exception of the subtropical south pacific the atlantic exhibits a deep freshening in the subpolar gyre and a shallower more intense increase in salinity in the tropics and subtropics the indian ocean is becoming more saline at all latitudes in the upper 150 meter layer with a subsurface freshening between 40 s and the equator in the 250 1000 meter layer there is freshening in both the weddell and ross seas

transient two dimensional dynamics in the upper conduit of a rhyolitic eruption a comparison of closure models for the granular stress to elucidate the role of particle collisions in redistributing momentum after fragmentation a numerical study was performed comparing the behavior of an inviscid or collision less granular material with a granular material whose viscosity and pressure were modeled using kinetic theory the granular viscosity calculation is sensitive to particle size and three particle sizes were considered 0 0002 m 0 002 m and 0 02 m a critical volume fraction of gas 0 75 identifies the onset of fragmentation dividing the region of rhyolitic magma with dispersed bubbles from the region of turbulent gas with dispersed particles the transient simulations can be divided into two dynamic regimes an initial shock followed by a transition to steady state during the initial shock phase treatment of the granular pressure and viscosity led to greater particle velocities relative to the inviscid calculation due to the development of higher gas pressure at fragmentation however as steady state is approached the viscosity slows the particle phase relative to the inviscid counterpart for particles greater than a millimeter the modeled sub millimeter particle velocity was insensitive to the treatment of the granular pressure and viscosity centimeter scale particles have a much higher granular viscosity up to 10 1 pa s and the kinetic theory calculation is thus relevant in these conditions after reaching steady state the differential velocity between gas and particles at the conduit exit correlates with particle size for both the inviscid and granular viscosity calculations negligible differential velocity develops for sub millimeter particles but centimeter scale particles exit the conduit with 5 differential velocity between phases

an experimental facility for the investigation of magma fragmentation by rapid decompression a special experimental facility has been developed to investigate the fragmentation of vesicular magma undergoing rapid decompression the facility operates in a regime similar to that of shock tubes and at temperatures up to 950 c and pressures up to 200 bar cylindrical samples diameter ca 17 mm length ca 50 mm undergo rapid decompression in a high temperature high pressure section of the facility following the disruption of a diaphragm separating that section from a low pressure low temperature section actual vesicular magma samples have been experimentally fragmented at elevated temperatures and pressures corresponding to those observed during explosive volcanic eruptions and the resulting pyroclastics have been photographically resolved in flight and collected for physical characterization the results of these experiments show that the rapid decompression of highly viscous vesicular magma can generate pyroclastic ejecta via rapid and complete fragmentation of magma at high temperature this new fragmentation facility is presented as a tool for experimental volcanology under well constrained conditions