deconstructing atlantic intertropical convergence zone variability influence of the local cross equatorial sea surface temperature gradient and remote forcing from the eastern equatorial pacific 1 we investigate causes of interannual variability in atlantic intertropical convergence zone itcz convection using a monthly mean global precipitation data set spanning 1979 1999 starting from the hypothesis of two dominant influences on the itcz namely the cross equatorial gradient in tropical atlantic sea surface temperature sst and the anomalous walker circulation due to the rearrangement of tropical pacific convection associated with the el nino southern oscillation we analyze anomaly composites over the 1979 1999 period that best isolate the effects of each mechanism our results suggest that to first order a strong anomalous walker circulation suppresses precipitation over the tropical atlantic whereas an anomalous warm north cool south sst gradient shifts the meridional location of maximum itcz convection anomalously north we examined the processes underlying each of the two mechanisms for the anomalous walker circulation we find consistency with the idea of suppression of convection through warming of the tropical troposphere brought about by anomalous convective heating in the eastern equatorial pacific for the sst gradient mechanism our results confirm previous studies that link convection to cross equatorial winds forced by meridional sst gradients we find that positive surface flux feedback brought about through the cross equatorial winds is weak and confined to the deep tropics on the basis of the results of this and other studies we propose an expanded physical picture that explains key features of atlantic itcz variability including its seasonal preference its sensitivity to small anomalous sst gradients and its role in the context of tropical atlantic sst gradient variability index terms 4215 oceanography general climate and interannual variability 3309 3339 meteorology and atmospheric dynamics ocean atmosphere interactions 0312 4504 3354 meteorology and atmospheric dynamics precipitation 1854 3374 meteorology and atmospheric dynamics tropical meteorology 4522 oceanography physical ocean optics keywords tropical atlantic precipitation climate variability el nino southern oscillation ocean atmosphere interaction

north south variations of tropical storm genesis locations in the western hemisphere in the western hemisphere tropical storms or hurricanes form in the north atlantic and eastern north pacific previous studies have focused on storm variability in the frequency duration and intensity in each basin here we find that the tropical storm genesis location in one ocean basin ties to the other one on both interannual and multidecadal timescales a northward southward shift of the tropical storm genesis location is associated with a southward northward variation in the other ocean basin the change of cross central america wind in the upper troposphere which induces an out of phase relation of vertical wind shear bridges storm activity in the two ocean basins sea surface temperatures in both the tropical pacific and north atlantic can induce the zonal wind change across central america it implies that hurricane outlooks can be improved by considering the two ocean basins together and thus helping reduce the damage caused by hurricane landfall

westward shift of western north pacific tropical cyclogenesis tropical cyclones tcs in the western north pacific or typhoons account for one third of all tcs in the world and the change of the mean tc genesis location can affect billions of people in pacific islands and asian countries the annual mean tc genesis longitude is generally controlled by the east west shift of the tropical upper tropospheric trough tutt a pronounced westward shift in the tutt is found in all of the available reanalysis data sets during 1979 2012 suppressing tc genesis in the eastern portion east of 145 e of the western north pacific basin due to the enhanced vertical wind shear associated with the tutt shift as a result the annual mean tc genesis longitude has significantly shifted westward since 1979 the westward shifting trends in the tutt and tc genesis are associated with the enhanced tropical tropospheric warming which is consistent with the response of the tropospheric temperature to global warming

adjustment of the atmospheric circulation to tropical pacific sst anomalies variability of transient eddy propagation in the pacific north america sector nas a pattern that is shifted southward northward compared with climatology paths of coherent phase propagation of transient eddies and of the propagation of wave packets are analyzed a coherent path of propagation across the pacific towards north america is identified that is more zonal during el niwinters and during la nihas a dominant path heading northeastward to the pacific northwest a second path heading southeastward from the central pacific to the tropical east pacific is more accentuated during la ni nas than el ni nos these changes in wave propagation are reproduced in an ensemble of seasonal integrations of a general circulation model forced by a tropical pacific sea surface temperature pattern confirming that the changes are forced by changes in the mean atmospheric state arising from changes in tropical sea surface temperature a simplified model with a specified basic state is used to model the storm tracks for el ni no and la ni na winters the results suggest that the changes in transient eddy propagation and the eddy statistics can be understood in terms of the refraction of transient eddies within different basic states copyright c 2010 royal meteorological society

the enigma of prokaryotic life in deep hypersaline anoxic basins deep hypersaline anoxic basins in the mediterranean sea are a legacy of dissolution of ancient subterranean salt deposits from the miocene period our study revealed that these hypersaline basins are not biogeochemical dead ends but support in situ sulfate reduction methanogenesis and heterotrophic activity a wide diversity of prokaryotes was observed including a new abundant deeply branching order within the euryarchaeota furthermore we demonstrated the presence of a unique metabolically active microbial community in the discovery basin which is one of the most extreme terrestrial saline environments known as it is almost saturated with mgcl2 5 m

classifying north atlantic tropical cyclone tracks by mass moments a new method for classifying tropical cyclones or similar features is introduced the cyclone track is considered as an open spatial curve with the wind speed or power information along the curve considered to be a mass attribute the first and second moments of the resulting object are computed and then used to classify the historical tracks using standard clustering algorithms mass moments allow the whole track shape length and location to be incorporated into the clustering methodology tropical cyclones in the north atlantic basin are clustered with k means by mass moments producing an optimum of six clusters with differing genesis locations track shapes intensities life spans landfalls seasonal patterns and trends even variables that are not directly clustered show distinct separation between clusters a trend analysis confirms recent conclusions of increasing tropical cyclones in the basin over the past two decades however the trends vary across clusters

methodology for bounding calculations of nuclear criticality of fissile material accumulations external to a waste container at yucca mountain nevada abstract the possibility of nuclear criticality however remote in the vicinity of the proposed repository at yucca mountain nevada generates justified concerns and may impact the performance of the repository a heuristic approach is presented here for determining the amount spatial distribution and other characteristics of fissile material accumulation in the rock beneath a waste package that could contribute to such an event this study is concerned primarily with waste packages containing special spent fuel from the department of energy and high level nuclear waste glass mixing with less alkaline waters and the subsequent drop in ph is the mechanism that is most efficient for precipitating fissile material from the waste package internal leachate in contrast to natural deposits in which redox changes are the main precipitation driver external accumulation size is determined by 1 computing the chemical composition of the leachate leaving a package as its internal materials degrade with the batch geochemical code eq3 6 2 determining precipitation of fissile material into mineral phases using the 1d geochemical code phreeqc as the effluent mixes with percolation water and 3 heuristically scaling results to a 3d volume and computing the criticality coefficient using the code mcnp loci for accumulation are the multiple lithophysal cavities and the fracture system a bounding conservative approach is used by necessity in step 3 nuclear criticality is sensitive to small variations in the distribution of fissile material and parameters of natural systems vary by orders of magnitude because the most likely combinations of parameters are not conducive to nuclear criticality this study focuses on extreme values of parameter probabilistic distributions such as limited flow into the package associated with a large percolation rate combinations of material degradation rates favoring actinide release and very high host rock porosity values by considering these combinations most favorable to criticality but unlikely it was concluded that external nuclear criticality is not a concern at the proposed repository

baroclinic eddies and the extent of the hadley circulation an idealized gcm study thehadleycirculationhaswidenedoverthepast30years thiswideninghasbeenqualitativelyreproduced in general circulation model gcm simulations of a warming climate comprehensive gcm studies suggest this widening may be caused by a poleward shift in baroclinic eddy activity yet the limited amplitude of the climate change signals analyzed so far precludes a quantitative comparison with theories this study uses two idealized gcms one with and one without an active hydrologic cycle to investigate changes in the extent of the hadley circulation over a wide range of climates the climates span global mean temperatures from 243 to 385k and equator to pole temperature contrasts from 12 to 100k baroclinic eddies control the extent of the hadley circulation across most of these climates a supercriticality criterion that quantifies the depth of baroclinic eddies relative to that of the troposphere turns out to be a good indicator of where baroclinic eddies become deep enough to terminate the hadley circulation the supercriticality depends on meridional temperature gradients and an effective stability that accounts for the effect of convective heating on baroclinic eddies astheequator to poletemperaturecontrastweakensortheconvectivestaticstabilityincreases convective heating increasingly influences the thermal stratification of the troposphere and the supercriticality consistent with the supercriticality criterion the hadley circulation contracts as meridional temperature gradients increase and it widens as the effective static stability increases the former occurs during el nino and may account for the observed hadley circulation contraction then the latter occurs during global warming

convective self aggregation and tropical cyclogenesis under the hypohydrostatic rescaling abstractthe behavior of rotating and nonrotating aggregated convection is examined at various horizontal resolutions using the hypohydrostatic or reduced acceleration in the vertical rave rescaling this modification of the equations of motion reduces the scale separation between convective and larger scale motions enabling the simultaneous and explicit representation of both types of flow in a single model without convective parameterization without the rave rescaling a dry bias develops when simulations of nonrotating radiative convective equilibrium are integrated at coarse resolution in domains large enough to permit convective self aggregation the rescaling reduces this dry bias and here it is suggested that the rescaling moistens the troposphere by weakening the amplitude and slowing the group velocity of gravity waves thus reducing the subsidence drying around aggregated convection separate simulations of rotating radiative convective equilibrium exhibit tropical cyclogenesis as horizo

tropical sea surface temperature vertical wind shear and hurricane development 1 the anomalously strong hurricane activity in the atlantic sector during the recent years led to a controversy about the impact of global warming on hurricane activity in the atlantic sector here we show that the temperature difference between the tropical north atlantic and the tropical indian and pacific oceans indo pacific is a key parameter in controlling the vertical wind shear over the atlantic an important quantity for hurricane activity the stronger warming of the tropical north atlantic relative to that of the indo pacific during the most recent years drove reduced vertical wind shear over the atlantic and is thus responsible for the strong hurricane activity observed in 2006 however the temperature difference between the tropical north atlantic and the tropical indian and pacific oceans is much reduced which explains the relatively weak hurricane season

tropical cyclone recurvature an intrinsic property the typical track of a tropical cyclone tc in the northern hemisphere is an initial northwestward movement followed by an eventual turning toward the east such turning is referred to as recurvature and often explained by the change of the environmental flow that steers the tc here we show that even in the absence of background flow a tc initiated at a high enough latitude can recurve itself differential horizontal advection of the planetary vorticity by the tc circulation at different vertical levels leads to the development of vertical wind shear upper tropospheric anticyclone and asymmetric distribution of convection the flow associated with the upper tropospheric anticyclone on the equatorward side of the tc and the diabatic heating associated with the asymmetric convection combine to cause the tc to recurve such knowledge an intrinsic recurvature property of the tc is important in forecasting the tc track when the environmental flow is weak

probabilistic clustering of extratropical cyclones using regression mixture models a probabilistic clustering technique is developed for classification of wintertime extratropical cyclone etc tracks over the north atlantic we use a regression mixture model to describe the longitude time and latitude time propagation of the etcs a simple tracking algorithm is applied to 6 hourly mean sea level pressure fields to obtain the tracks from either a general circulation model gcm or a reanalysis data set quadratic curves are found to provide the best description of the data we select a three cluster classification for both data sets based on a mix of objective and subjective criteria the track orientations in each of the clusters are broadly similar for the gcm and reanalyzed data they are characterized by predominantly south to north s n west to east w e and southwest to northeast sw ne tracking cyclones respectively the reanalysis cyclone tracks however are found to be much more tightly clustered geographically than those of the gcm for the reanalysis data a link is found between the occurrence of cyclones belonging to different clusters of trajectory shape and the phase of the north atlantic oscillation nao the positive phase of the nao is associated with the sw ne oriented cluster whose tracks are relatively straight and smooth with cyclones that are typically faster more intense and of longer duration the negative nao phase is associated with more erratic w e tracks with typically weaker and slower moving cyclones the s n cluster is accompanied by a more transient geopotential trough over the western north atlantic no clear associations are found in the case of the gcm composites the gcm is able to capture cyclone tracks of quite realistic orientation as well as subtle associated features of cyclone intensity speed and lifetimes the clustering clearly highlights though the presence of serious systematic errors in the gcm s simulation of etc behavior

is the poleward migration of tropical cyclone maximum intensity associated with a poleward migration of tropical cyclone genesis a recent study showed that the global average latitude where tropical cyclones achieve their lifetime maximum intensity has been migrating poleward at a rate of about one half degree of latitude per decade over the last 30 years in each hemisphere however it does not answer a critical question is the poleward migration of tropical cyclone lifetime maximum intensity associated with a poleward migration of tropical cyclone genesis in this study we will examine this question first we analyze changes in the environmental variables associated with tropical cyclone genesis namely entropy deficit potential intensity vertical wind shear vorticity skin temperature and specific humidity at 500 hpa in reanalysis datasets between 1980 and 2013 then a selection of these variables is combined into two tropical cyclone genesis indices that empirically relate tropical cyclone genesis to large scale variables we find a shift toward greater smaller average potential number of genesis at higher lower latitudes over most regions of the pacific ocean which is consistent with a migration of tropical cyclone genesis towards higher latitudes we then examine the global best track archive and find coherent and significant poleward shifts in mean genesis position over the pacific ocean basins

flow pattern variability in natural fracture intersections we use numerical simulations to examine the variability of flow patterns in representative fracture intersection geometries in contrast to existing studies of perfectly orthogonal intersections we demonstrate that more realistic geometries lead to a rich spectrum of flow patterns moreover numerical solutions of the navier stokes equations in these fracture intersections indicate that non linear inertial effects become important for reynolds numbers as low as 1 100 such reynolds numbers often exist in naturally fractured formations particularly in karst systems and in the vicinity of wells during pump tests

climate modulation of north atlantic hurricane tracks the variability of north atlantic tropical storm and hurricane tracks and its relationship to climate variability is explored tracks from the north atlantic hurricane database for the period 1950 2007 are objectively separated into four groups using a cluster technique that has been previously applied to tropical cyclones in other ocean basins the four clusters form zonal and meridional separations of the tracks the meridional separation largely captures the separation between tropical and more baroclinic systems while the zonal separation segregates gulf of mexico and cape verde storms general climatologies of the seasonality intensity landfall probability and historical destructiveness of each cluster are documented and relationships between cluster membership and climate variability across a broad spectrum of time scales are identified composites with respect to cluster membership of sea surface temperature and other environmental fields show that regional and remote modes of climate variability modulate the cluster members in substantially

mechanisms of enso forcing of hemispherically symmetric precipitation variability summary the patterns of precipitation anomalies forced by the el nino southern oscillation during northern hemisphere winter and spring are remarkably hemispherically symmetric and in the mid latitudes have a prominent zonally symmetric component observations of global precipitation variability and the moisture budget within atmospheric reanalyses are examined to argue that the zonally symmetric component is caused by interactions between transient eddies and tropically forced changes in the subtropical jets during el nino events the jets strengthen in each hemisphere and shift equatorward changes in the subtropical jet influence the transient eddy momentum fluxes and the eddy driven mean meridional circulation during el nino events eddy driven ascent in the mid latitudes of each hemisphere is accompanied by low level convergence and brings increased precipitation this changes in the transient eddy and stationary eddy moisture fluxes almost exactly cancel each other and in sum do not contribute to the zonal mean precipitation anomalies propagation of anomalous stationary waves disrupts the zonal symmetry flow around the deeper aleutian low and the eastward extension of the pacific jet stream supply the moisture for increased precipitation over the eastern north pacific and the western seaboard of the united states while transient eddy moisture convergence supplies the moisture for increased precipitation over the southern united states in each case increased precipitation is fundamentally caused by anomalous ascent forced by anomalous heat and vorticity fluxes

interdecadal changes in eastern pacific itcz variability and its influence on the atlantic itcz the eastern pacific and atlantic intertropical convergence zones itcz exhibit the largest year to year variations in boreal spring we show evidence suggesting that atlantic itcz april may variability is linked to that for the eastern pacific through the walker circulation as it respond to changes in equatorial pacific convection analysis of itcz proxy indices shows the link appears to be strong in the 1980 90 s and 1920 30 s but virtually nonexistent in the 1950 60 s we argue that this apparent nonstationarity results from the nonlinear relationship between sea surface temperature sst and convection in the eastern equatorial pacific and its consequent effect on the walker circulation and the atlantic itcz this mechanism was modulated over 1856 1998 by interdecadal changes in peak sst attained in the eastern equatorial pacific during boreal spring

the 1995 and 1996 north atlantic hurricane seasons a return of the tropical only hurricane hurricane activity over the north atlantic basin during 1995 and 1996 is compared to the combined hurricane activity over the previous four years 1991 94 the earlier period produced a total of 15 hurricanes compared to a total of 20 hurricanes over the latter period despite this similarity in numbers the hurricanes of 1995 and 1996 were generally of the tropical only variety which marks a substantial departure from activity during the early 1990s the return of tropical only hurricanes to the atlantic basin is likely the result of several global and local factors including cool sst conditions in the equatorial central and eastern pacific and warm ssts in the tropical atlantic the hurricane activity of 1995 and 1996 is more reminiscent of activity of some seasons during the early and mid 1950s

local partitioning of the overturning circulation in the tropics and the connection to the hadley and walker circulations conceptually it is useful to partition the three dimensional tropical circulation into meridional and zonal components namely the hadley and walker circulations the averaging involved in their definitions can introduce ambiguities these problems can be circumvented by first partitioning the total vertical mass flux into components associated with overturning in the meridional and zonal directions respectively called here the local hadley and local walker circulations defining the local hadley and local walker circulations this way ensures the pair of two dimensional overturning circulations can be added to give the original three dimensional circulation even when the averages are taken over limited domains the method is applied to the vertical motion from the era interim reanalysis for the period 1979 to 2009 one important result is that the local hadley circulation responds much more strongly to enso than the local walker circulation even though the local walker circulation in the central pacific weakens during el nino years and strengthens and widens during la nina years

some atmospheric processes governing the large scale tropical circulation in idealized aquaplanet simulations abstract the large scale tropical atmospheric circulation is analyzed in idealized aquaplanet simulations using an atmospheric general circulation model idealized sea surface temperatures ssts are used as lower boundary conditions to provoke modifications of the atmospheric general circulation results show that 1 an increase in the meridional sst gradients of the tropical region drastically strengthens the hadley circulation intensity 2 the presence of equatorial zonal sst anomalies weakens the hadley cells and reinforces the walker circulation and 3 a uniform sst warming causes small and nonsystematic changes of the hadley and walker circulations in all simulations the jet streams strengthen and move equatorward as the hadley cells strengthen and become narrower some relevant mechanisms are then proposed to interpret the large range of behaviors obtained from the simulations first the zonal momentum transport by transient and stationary eddies is shown to modulate the eddy driven jets whic

cluster analysis of typhoon tracks part ii large scale circulation and enso abstract a new probabilistic clustering method based on a regression mixture model is used to describe tropical cyclone tc propagation in the western north pacific wnp seven clusters were obtained and described in part i of this two part study in part ii the present paper the large scale patterns of atmospheric circulation and sea surface temperature associated with each of the clusters are investigated as well as associations with the phase of the el nino southern oscillation enso composite wind field maps over the wnp provide a physically consistent picture of each tc type and of its seasonality anomalous vorticity and outgoing longwave radiation indicate changes in the monsoon trough associated with different types of tc genesis and trajectory the steering winds at 500 hpa are more zonal in the straight moving clusters with larger meridional components in the recurving ones higher values of vertical wind shear in the midlatitudes also accompany the straight moving tracks compared to

concurrent changes to hadley circulation and the meridional distribution of tropical cyclones abstractpoleward trends in seasonal mean latitudes of tropical cyclones tcs have been identified in direct observations from 1980 to the present paleoclimate reconstructions also indicate poleward equatorward migrations over centennial millennial time scales hadley circulation hc is often both implicitly and explicitly invoked to provide dynamical linkages to these shifts although no direct analysis of concurrent changes in the recent period has been presented here the observational tc record 1981 2016 and era interim jra 55 and merra 2 are studied to examine potential relationships between the two a zonally asymmetric hc is defined by employing helmholtz theory for vector decomposition and this permits the derivation of novel hc diagnostics local to tc basins coherent variations in both long term linear trends and detrended interannual variability are found tc genesis and lifetime maximum intensity latitudes share trend sign and magnitude with shifts in local hc extent with rates being a

cluster analysis of downscaled and explicitly simulated north atlantic tropical cyclone tracks abstracta realistic representation of the north atlantic tropical cyclone tracks is crucial as it allows for example explaining potential changes in u s landfalling systems here the authors present a tentative study that examines the ability of recent climate models to represent north atlantic tropical cyclone tracks tracks from two types of climate models are evaluated explicit tracks are obtained from tropical cyclones simulated in regional or global climate models with moderate to high horizontal resolution 1 0 25 and downscaled tracks are obtained using a downscaling technique with large scale environmental fields from a subset of these models for both configurations tracks are objectively separated into four groups using a cluster technique leading to a zonal and a meridional separation of the tracks the meridional separation largely captures the separation between deep tropical and subtropical hybrid or baroclinic cyclones while the zonal separation segregates gulf of mexico and ca

transport behavior in three dimensional fracture intersections 1 flow and transport patterns in three dimensional fracture intersections are investigated with emphasis on the occurrence and effects of local flows around each intersection flow and transport simulations indicate that local flow circulations referred to as local flow cells arise because of an interplay between intersection geometry and corresponding boundary conditions for flow along the fracture boundaries local flow cells are thus unique features in three dimensional fracture networks the analysis here suggests that local flow cells have little effect on the overall fluid flow in a fracture network but may constitute an effective mechanism which contributes to the long breakthrough tailing and retardation often observed in transport through discrete fracture networks

experimental mixing variability in intersecting natural fractures laboratory experiments of flow through epoxy replicas of intersecting natural fractures show that solute transport deviates significantly from predictions of two dimensional streamline routing through parallel plate intersections surface roughness in intersecting fractures causes two major discrepancies between mixing experiments and the parallel plate predictions first the fluids exiting the intersection are not uniformly mixed but consist of ribbons with varied solute concentrations which tend to follow streamlines in the flow some of these streams maintain a nearly pure inlet fluid composition as they exit the intersection and traverse the outlet fracture second when the outlet fluids are collected and homogenized we find that this complex redirection of streamlines within natural rough walled fracture intersections results in more total mixing than is predicted by the parallel plate model

seasonal to multidecadal variability of the width of the tropical belt 1 an expansion of the tropical belt has been extensively reported in observations reanalyses and climate model simulations but there is a great deal of uncertainty in estimates of the rate of widening as different diagnostics give a wide range of results this study critically examines robust diagnostics for the width of the tropical belt to explore their seasonality interannual variability and multidecadal trends the width based on the latitudes of the maximum tropospheric dry bulk static stability measuring the difference in potential temperature between the tropopause and the surface is found to be closely coupled to the width based on the subtropical jet cores on all time scales in contrast the tropical belt width and northern hemisphere edge latitudes based on the latitudes at which the vertically averaged stream function vanishes a measure of the hadley circulation s poleward edges lag those of the other diagnostics by approximately 1month the tropical belt width varies by up to 10 latitude among the diagnostics with trends in the tropical belt width ranging from 0 5 to 2 0 per decade over the 1979 2012 period nevertheless in agreement with previous studies nearly all diagnostics exhibit a widening trend although the stream function diagnostic exhibits a significantly stronger widening than either the jet or dry bulk stability diagnostics finally gps radio occultation observations are used to assess the ability of the reanalyses to reproduce the tropical belt width finding that they better situate the latitudes of maximum bulk stability versus those of the subtropical jets

trends in the local hadley and local walker circulations the linear trend in the local hadley and walker circulations from 1979 to 2009 is calculated these local circulations are defined through a decomposition of the vertical mass flux into its zonal and meridional components defining the local circulation this way ensures that the two orthogonal circulations the local hadley and walker circulations sum to the original circulation even after averaging the circulations regionally large regional differences in changes in the local hadley and walker circulations over a 31 year period are found for example the local hadley circulation has shifted southward over africa the maritime continent and the western and central pacific by about 1 over the americas and the atlantic the local hadley circulation has strengthened by about 1 5 the zonal component of the vertical mass flux has increased by about 10 20 in the tropics over all continents and decreased over the adjacent oceans by about 10 20 although the local walker circulations in the indian ocean and the atlantic have weakened the circulation in the pacific has changed little about 1 2 the local walker circulations in all ocean basins have shifted westward by about 1 2 on average