zircon 4he 3he thermochronometry abstract multiple thermochronometric methods are often required to constrain time continuous rock exhumation for studying tectonic processes or development of km scale topography at earth s surface here we explore 4he 3he thermochronometry of zircon as a method for constraining continuous time temperature t t paths of individual samples through a temperature range that is complementary to methods such as 40ar 39ar thermochronometry of k feldspar and 4he 3he thermochronometry of apatite for different cooling rates and diffusion domain size the temperature sensitivity of zircon 4he 3he thermochronometry ranges from slightly less than 100 c to slightly greater than 250 c a typical sample provides continuous thermal constraints over 100 c within that range outside these temperatures 4he in zircon will either be quantitatively retained or completely lost by volume diffusion as proof of concept we present stepwise release 4he 3he spectra and associated u and th concentration maps measured by laser ablation icp ms analysis of individual crystal aliquots of fish canyon tuff fct zircon and of a more complex setting in the sierra nevada batholith that experienced reheating from a proximal basaltic intrusion the little devil s postpile ldp the fct zircon 4he 3he release spectra are consistent with a 4he spatial distribution dominated by alpha ejection from crystal surfaces the spatial distributions of u and th measured in the same crystals do not substantially influence 4he 3he release spectra that are predicted for the known thermal history even when incorporating spatially variable diffusivity due to accumulation of radiation damage conversely the ldp 4he 3he release spectra are strongly influenced by the observed parent nuclide zonation a three dimensional 3d numerical model of 4he production and diffusion which incorporates crystal geometry u and th zonation and spatially variable he diffusion kinetics substantially improves the fit between measured and modeled 4he 3he release spectra for the independently known thermal history of the sample we conclude that zircon 4he 3he thermochronometry provides accurate thermal constraints through geologic time but certain applications of this technique may require 3d numerical modeling approaches

past present and future of thermochronology in one form or another geochronologists have been practicing thermochronology 1 the use of radioisotopic dating to constrain thermal histories of rocks and minerals for over 40 years building from lessons learned over these four decades thermochronology continues to evolve due to technical developments increasingly sophisticated theoretical models and an expanding range of applications in geologic and planetary science most recently interest in earth surface processes and interactions between tectonics erosion and climate has drawn attention to techniques that can address the timing and rates of processes operating at temperatures below about 300 c the purpose of this rimg volume is to assess the current state of thermochronology as of circa 2005 which is coincidentally the 100th anniversary of the first radioisotopic date rutherford 1905 1906 excellent review papers and books on specific topics within this field have been published but no single volume has yet provided a comprehensive review of current practices basic theory and illustrative examples the motivation for this volume stems from these considerations knowing that in a fast developing field a book like this can quickly become dated we tried to include sufficient review of fundamentals and the literature to offer students and new users a useful introduction to thermochronology that may have some staying power in this chapter we first review the salient points of thermochronology s history before assessing our current capabilities and challenges and then taking the risk of suggesting where the field is headed we do not provide a comprehensive history of the method that does full justice to the work of the large and growing cohort of thermochronologists in this short space we instead opted to give our perspectives on where the intellectual and technological roots of the discipline lie which run deeper and go back farther than is sometimes appreciated

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

atlantic multidecadal oscillation and northern hemisphere s climate variability proxy and instrumental records reflect a quasi cyclic 50 80 year climate signal across the northern hemisphere with particular presence in the north atlantic modeling studies rationalize this variability in terms of intrinsic dynamics of the atlantic meridional overturning circulation influencing distribution of sea surface temperature anomalies in the atlantic ocean hence the name atlantic multidecadal oscillation amo by analyzing a lagged covariance structure of a network of climate indices this study details the amo signal propagation throughout the northern hemisphere via a sequence of atmospheric and lagged oceanic teleconnections which the authors term the stadium wave initial changes in the north atlantic temperature anomaly associated with amo culminate in an oppositely signed hemispheric signal about 30 years later furthermore shorter term interannual to interdecadal climate variability alters character according to polarity of the stadium wave induced prevailing hemispheric climate regime ongoing research suggests mutual interaction between shorter term variability and the stadium wave with indication of ensuing modifications of multidecadal variability within the atlantic sector results presented here support the hypothesis that amo plays a significant role in hemispheric and by inference global climate variability with implications for climate change attribution and prediction

a damped decadal oscillation in the north atlantic climate system a simple stochastic atmosphere model is coupled to a realistic model of the north atlantic ocean a north south sst dipole with its zero line centered along the subpolar front influences the atmosphere model which in turn forces the ocean model by surface fluxes related to the north atlantic oscillation the coupled system exhibits a damped decadal oscillation associated with the adjustment through the ocean model to the changing surface forcing the oscillation consists of a fast wind driven positive feedback of the ocean and a delayed negative feedback orchestrated by overturning circulation anomalies the positive feedback turns out to be necessary to distinguish the coupled oscillation from that in a model without any influence from the ocean to the atmosphere using a novel diagnosing technique it is possible to rule out the importance of baroclinic wave processes for determining the period of the oscillation and to show the important role played by anomalous geostrophic advection in sustaining the oscillation

interannual variability in a tropical atmosphere ocean model influence of the basic state ocean geometry and nonlinearity abstract the behavior of a tropical coupled atmosphere ocean model is analyzed for a range of different background states and ocean geometries the model is essentially that of cane and zebiak for the tropical pacific except only temporally constant background states are considered here for realistic background states and ocean geometry the model solutions feature oscillations of period of 3 5 yr by comparing the full model solution with a linearized version of the model it is shown that the basic mechanism of the oscillation is contained within linear theory a simple linear analog model is derived that describes the nature of the interannual variability in the coupled tropical atmosphere ocean system the analog model highlights the properties that produce coupled atmosphere ocean instability in the eastern ocean basin and the equatorial wave dynamics in the western ocean basin that are responsible for a delayed negative feedback into this instability growth the growth rate of the local instabil

response of the separated western boundary current to harmonic and stochastic wind stress variations in a 1 5 layer ocean model abstract a time dependent version of the parsons model geostrophic 1 5 layer model of the ventilated thermocline has been developed to investigate the response of the midlatitude ocean to wind stress variations in a simple configuration in this model the total amount of water is kept constant and the eastern boundary thermocline depth can vary in time so as to maintain mass balance here basin modes are not investigated in contrast to many recent studies but the emphasis is on the line where the motionless second layer outcrops which represents the separated western boundary current it is shown that the position of this line only depends on the wind stress the earth rotation and the thermocline interior solution the position is not influenced by the parameterization of the dissipative processes this generalizes previous results established in the stationary case the displacement of the outcrop line in the case of harmonic or stochastic wind stress variations is computed numerically showing

long term tectonothermal history of laramide basement from zircon he age eu correlations abstract the long term 1 ga thermal histories of cratons are enigmatic with geologic data providing only limited snapshots of their evolution we use zircon u th he zircon he thermochronology and age composition correlations to understand the proterozoic phanerozoic thermal history of archean wyoming province rocks exposed in the northern laramide ranges of western north america zircon he ages from the wind river range 54 dates and bighorn mountains 32 dates show negative correlations with effective uranium eu a proxy for radiation damage zircon dates from the bighorns are between 960 ma low eu and 20 ma high eu whereas samples from the wind rivers are between 582 ma low eu and 33 ma high eu we applied forward modeling using the zircon radiation damage and annealing model zrdaam to understand this highly variable dataset a long term t t path that is consistent with the available geologic constraints successfully reproduced age eu correlations the best fit to the wind rivers data involves two phases of rapid cooling at 1800 1600 ma and 900 700 ma followed by slower cooling until 525 ma during the phanerozoic these samples were heated to maximum temperatures between 160 and 125 c prior to laramide cooling to 50 c between 60 and 40 ma data from the bighorn mountains were successfully reproduced with a similar thermal history involving cooler phanerozoic temperatures of 115 c and earlier laramide cooling between 85 and 60 ma our results indicate that age eu correlations in zircon he datasets can be applied to extract long term thermal histories that extend beyond the most recent cooling event in addition our results constrain the timing magnitude and rates of cooling experienced by archean wyoming province rocks between recognized deformation events including the 1 ga period represented by the regionally extensive great unconformity

observations inferences and mechanisms of the atlantic meridional overturning circulation a review this is a review about the atlantic meridional overturning circulation amoc its mean structure temporal variability controlling mechanisms and role in the coupled climate system the amoc plays a central role in climate through its heat and freshwater transports northward ocean heat transport achieved by the amoc is responsible for the relative warmth of the northern hemisphere compared to the southern hemisphere and is thought to play a role in setting the mean position of the intertropical convergence zone north of the equator the amoc is a key means by which heat anomalies are sequestered into the ocean s interior and thus modulates the trajectory of climate change fluctuations in the amoc have been linked to low frequency variability of atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses on intra annual timescales variability in amoc is large and primarily reflects the response to local wind forcing meridional coherence of anomalies is limited to that of the wind field on interannual to decadal timescales amoc changes are primarily geostrophic and related to buoyancy anomalies on the western boundary a pacemaker region for decadal amoc changes is located in a western transition zone along the boundary between the subtropical and subpolar gyres decadal amoc anomalies are communicated meridionally from this region amoc observations as well as the expanded ocean observational network provided by the argo array and satellite altimetry are inspiring efforts to develop decadal predictability systems using coupled atmosphere ocean models initialized by ocean data

radiation effects in zircon the widespread distribution of zircon in the continental crust its tendency to concentrate trace elements particularly lanthanides and actinides its use in age dating and its resistance to chemical and physical degradation have made zircon the most important accessory mineral in geologic studies because zircon is highly refractory it also has important industrial applications including its use as a lining material in high temperature furnaces however during the past decade zircon has also been proposed for advanced technology applications such as a durable material for the immobilization of plutonium or when modified by ion beam irradiation as an optic waveguide material in all of these applications the change in properties as a function of increasing radiation dose is of critical importance in this chapter we summarize the state of knowledge on the radiation damage accumulation process in zircon

the modeled atmospheric response to midlatitude sst anomalies and its dependence on background circulation states abstract the atmospheric response to a midlatitude sst anomaly in the north pacific and its dependence on background flow are examined in a gcm experiments are conducted using the same warm sst anomalies but two different model states perpetual january and perpetual february the atmospheric responses to the sst anomalies are statistically significant in both january and february but are completely different the anomalous circulation in january is characterized by a trough decaying with height immediately downstream of the sst anomalies in february the anomalous circulation is dominated by a downstream ridge growing with height the patterns of the anomalous heights in the two months are nearly orthogonal vorticity and thermodynamic budgets are diagnosed to illustrate how the anomalous circulations are maintained over the sst anomalies low level convergence and ascent are observed in both months in january the anomalous convergence is balanced by a residual due primarily to the forcing by submont

radiation damage in zircon a single zoned sri lankan zircon exhibits a range of microstructures from crystalline to nearly amorphous that are the result of radiation damage over a dose range of 2 1 10 1 1015 decay events mg 0 16 0 47 dpa the zones in the crystal were examined at a variety of length scales using optical microscopy micro raman spectroscopy electron microprobe analysis and transmission electron microscopy birefringence varies linearly with dose birefringence 4 71 10 18 g d 4 86 10 2 full width at half maximum fwhm measurements of the b1g 3 peak as determined by micro raman spectroscopy were used to estimate the extent of radiation damage in each zone the radiation dose calculated on the basis of u and th concentrations and sample age vs damage from the fwhm measurements of the b1g 3 peak relationship among zones was consistent with results for a suite of sri lankan single crystals suggesting that all of these zircon crystals have undergone a similar thermal history based on a comparison with zircon crystals that are considered to have undergone minimal annealing e g lunar zircon the sri lankan zircon crystals have accumulated less damage than is expected based on their calculated dose consistent with previous evidence for annealing to estimate the extent of annealing in sri lankan samples an equivalent damage dose i e the dose required to produce the same amount of damage in an unannealed sample was calculated for a given u and th concentration by determining the time of damage accumulation required to create an equivalent amount of damage in an unannealed zircon the dose vs damage relationship in sri lankan zircon crystals 560 ma is equivalent to that of a 375 ma unannealed zircon suggesting that approximately one third of the damage has been lost due to annealing the dose vs damage relationship is consistent with the direct impact model of damage accumulation transmission electron microscopy revealed zoning on a scale finer than could be identified optically 0 5 m and the presence of abundant zro2 nano particles at zone boundaries

mechanisms of decadal sea level variability in the eastern north atlantic and the mediterranean sea decadal sea level variations from tide gauge records along the western european coast and in the mediterranean sea commencing in the late 19th and early 20th centuries are examined relative to large scale atmospheric forcing recent studies have provided evidence for a link between sea level in the eastern north atlantic and atmospheric forcing however the nature of this relationship is still unclear here the outputs of a regional barotropic model and a nearly global baroclinic model are used in conjunction with wind stress and heat flux data to explore the physical mechanisms responsible for the observed sea level variability all tide gauge records show significant decadal variability up to 15 cm and are highly correlated with the nao and among themselves at decadal periods there is a coherent sea level signal that affects the eastern boundary of the north atlantic northward of 25 n and is limited to a narrow band of the order of a few hundred kilometers along the coast this band tends to become narrower towards higher latitudes we find that longshore wind and wave propagation along the boundary are the major contributors to coastal sea level variability but no significant contribution from mass redistribution linked to changes in the strength of the subtropical gyre is observed the mass component dominates sea level in the mediterranean and is mainly driven by mass exchanges with the atlantic which explains the correlation between both regions southward of 25 n sea level changes are mainly driven by heat advection through ekman fluxes

simultaneous evolution of gyre and atlantic meridional overturning circulation anomalies as an eigenmode of the north atlantic system abstractthe authors identify an interdecadal oscillatory mode of the north atlantic atmosphere ocean system in a general circulation model gfdl cm2 1 via a linear inverse model lim the oscillation mechanism is mostly embedded in the subpolar gyre anomalous advection generates density anomalies in the eastern subpolar gyre which propagate along the mean gyre circulation and reach the subpolar gyre center around 10 years later when associated anomalous advection of the opposite sign starts the other half cycle as density anomalies reach the labrador sea deep convection region atlantic meridional overturning circulation amoc anomalies are also induced both the gyre and amoc anomalies then propagate equatorward slowly following the advection of density anomalies the oscillation is further demonstrated to be more likely an ocean only mode while excited by the atmospheric forcing in particular it can be approximated as a linearly driven damped oscillator that is partly excited by the north atla

simulated variability of the circulation in the north atlantic from 1953 to 2003 abstract the variability of the circulation in the north atlantic and its link with atmospheric variability are investigated in a realistic hindcast simulation from 1953 to 2003 the interannual to decadal variability of the subpolar gyre circulation and the meridional overturning circulation moc is mostly influenced by the north atlantic oscillation nao both circulations intensified from the early 1970s to the mid 1990s and then decreased the monthly variability of both circulations reflects the fast barotropic adjustment to nao related ekman pumping anomalies while the interannual to decadal variability is due to the baroclinic adjustment to ekman pumping buoyancy forcing and dense water formation consistent with previous studies an original characteristic of the oceanic response to nao is presented that relates to the spatial patterns of buoyancy and wind forcing over the north atlantic anomalous ekman pumping associated with a positive nao phase first induces a decrease of the southern sub

diffusion of helium in radiation damaged zircon abstract diffusion of helium was characterized in natural zircon that had been implanted with kr at a range of energies to simulate the effects of alpha recoil in natural radiation damage polished slabs of zircon oriented either normal or parallel to c were implanted with kr ions at a series of energies to produce a damaged region 1 m in thickness then implanted with 100 kev 3he at a dose of 1 1015 3he cm2 which placed the 3he within the region damaged by the kr ion implantation the implanted zircons were annealed isothermally in pt capsules in 1 atm furnaces at temperatures ranging from 51 to 252 c and times ranging from 15 min to 3 weeks 3he distributions following experiments were measured with nuclear reaction analysis using the reaction 3he d p 4he for diffusion in zircon normal to c we obtain the following arrhenius relation d 1 7 10 8 exp 71 2 kj mol 1 rt m2 sec 1 this activation energy for diffusion is about half of the value for he diffusion in zircon from the same sample locality but without kr implantation measured with an otherwise similar experimental and analytical approach cherniak et al 2009 he diffusivities in the kr implanted material are also significantly faster further he diffusivities normal and parallel to c in the kr implanted zircon are similar in contrast to the marked diffusional anisotropy with differences of two orders of magnitude for diffusion normal and parallel to c observed for he diffusion in this zircon in samples not implanted with kr these data indicate that heavily damaged zircon will have very limited retentivity of helium only those zircons larger than a few hundred m will have mean closure temperatures exceeding typical earth surface temperatures mean closure temperatures for he in highly damaged zircon will be about 200 c lower than those for relatively undamaged zircon at 200 c equivalent amounts of he will be lost from severely damaged zircon at a rate about a million times faster than from relatively undamaged zircon for retentivity of he comparable to that for low damage zircon at 200 c the severely damaged zircon would require residence temperatures of 10 c

variations in the difference between mean sea level measured either side of cape hatteras and their relation to the north atlantic oscillation we consider the extent to which the difference in mean sea level msl measured on the north american atlantic coast either side of cape hatteras varies as a consequence of dynamical changes in the ocean caused by fluctuations in the north atlantic oscillation nao from analysis of tide gauge data we know that changes in msl difference and nao index are correlated on decadal to century timescales enabling a scale factor of msl difference change per unit change in nao index to be estimated changes in trend in the nao index have been small during the past few centuries when measured using windows of order 60 120 years therefore if the same scale factor applies through this period of time the corresponding changes in trend in msl difference for the past few centuries should also have been small it is suggested thereby that the sea level records for recent centuries obtained from salt marshes adjusted for long term vertical land movements should have essentially the same nao driven trends south and north of cape hatteras only differing due to contributions from other processes such as changes in the meridional overturning circulation or geophysical fingerprints the salt marsh data evidently support this interpretation within their uncertainties for the past few centuries and perhaps even for the past millennium recommendations are made on how greater insight might be obtained by acquiring more measurements and by improved modelling of the sea level response to wind along the shelf

mechanisms of decadal north atlantic climate variability and implications for the recent cold anomaly decadal sea surface temperature sst fluctuations in the north atlantic ocean influence climate over adjacent land areas and are a major source of skill in climate predictions however the mechanisms underlying decadal sst variability remain to be fully understood this study isolates the mechanisms driving north atlantic sst variability on decadal time scales using low frequency component analysis which identifies the spatial and temporal structure of low frequency variability based on observations large ensemble historical simulations and pre industrial control simulations we identify a decadal mode of atmosphere ocean variability in the north atlantic with a dominant time scale of 13 18 years large scale atmospheric circulation anomalies drive sst anomalies both through contemporaneous air sea heat fluxes and through delayed ocean circulation changes the latter involving both the meridional overturning circulation and the horizontal gyre circulation the decadal sst anomalies alter the atmospheric meridional temperature gradient leading to a reversal of the initial atmospheric circulation anomaly the time scale of variability is consistent with westward propagation of baroclinic rossby waves across the subtropical north atlantic the temporal development and spatial pattern of observed decadal sst variability are consistent with the recent observed cooling in the subpolar north atlantic this suggests that the recent cold anomaly in the subpolar north atlantic is in part a result of decadal sst variability

role of the gulf stream and kuroshio oyashio systems in large scale atmosphere ocean interaction a review abstract ocean atmosphere interaction over the northern hemisphere western boundary current wbc regions i e the gulf stream kuroshio oyashio and their extensions is reviewed with an emphasis on their role in basin scale climate variability sst anomalies exhibit considerable variance on interannual to decadal time scales in these regions low frequency sst variability is primarily driven by basin scale wind stress curl variability via the oceanic rossby wave adjustment of the gyre scale circulation that modulates the latitude and strength of the wbc related oceanic fronts rectification of the variability by mesoscale eddies reemergence of the anomalies from the preceding winter and tropical remote forcing also play important roles in driving and maintaining the low frequency variability in these regions in the gulf stream region interaction with the deep western boundary current also likely influences the low frequency variability surface heat fluxes damp the low frequency sst anomalies ove

simulation of north atlantic decadal multidecadal winter sst anomalies driven by basin scale atmospheric circulation anomalies the north atlantic winter sea surface temperature anomaly tsa response to anomalous surface atmospheric circulation anomalies that vary over decadal and short term inter decadal periods is simulated for 1950 through 1992 anomalous ocean variability is driven by adding monthly coads derived anomalous fields of vector wind stress and wind speed to the climatological annual cycle forcing a simple model is derived relating winter tsa to the integrated atmospheric forcing present earlier in time that is responsible for its existence the basinscale structure of forced winter tsa patterns depends on the structure of the atmospheric forcing along with regional differences in the dominant mixed layer processes that generate tsa for example when the atmospheric subtropical high and subpolar low pressure systems are simultaneously strong enhanced flow around and baroclinic adjustments within the subtropical gyre results in anomalously warm water in the gulf stream region off the u s east coast at the same time however the open ocean generally cools because the westerlies and trades are anomalously strong by analyzing tsa variability not driven by the atmosphere an anomalously cold decade is identified characterized by rapid onset and termination that both occur within one year the onset during 1968 coincides with the appearance of the great salinity anomaly while the termination during 1977 coincides with an abrupt northern hemisphere climate shift that is particularly evident in the pacific

nonseasonal mass fluctuations in the midlatitude north atlantic ocean measurements from the gravity recovery and climate experiment reveal spatially coherent nonseasonal fluctuations in bottom pressure pb in the midlatitude north atlantic ocean the spatial structure of the pb anomalies is centered on the north atlantic current along the intergyre region separating subtropical and subpolar ocean basins and their temporal behavior is tied to indices of climate variability namely the north atlantic oscillation and the arctic oscillation correspondence to variations in wind stress curl suggests an interpretation of the midlatitude north atlantic pb variability in terms of a barotropic sverdrup balance anomalous mass in the midlatitude north atlantic ocean covaries with mass anomalies in abutting marginal seas including the mediterranean sea and the arctic ocean as well as the north sea and hudson bay results have implications for diagnosing ocean meridional heat transports due to depth independent gyre circulation changes

an ocean model s response to north atlantic oscillation like wind forcing the response of the atlantic ocean to north atlantic oscillation nao like wind forcing was investigated using an ocean only general circulation model coupled to an atmospheric boundary layer model a series of idealized experiments was performed to investigate the interannual to multi decadal frequency response of the ocean to a winter wind anomaly pattern overall the strength of the sst response increased slightly with longer forcing periods in the subpolar gyre however the model showed a broad response maximum in the decadal band 12 16 years

a study of the interaction of the north atlantic oscillation with ocean circulation observed patterns of wind stress curl and air sea heat flux associated with the north atlantic oscillation nao are used to discuss the response of ocean gyres and thermohaline circulation to nao forcing and their possible feedback on the nao the observations motivate and are interpreted in the framework of a simple mathematical model that couples ekman layers ocean gyres and thermohaline circulation to the atmospheric jet stream meridional shifts in the zero wind stress curl line are invoked to drive anomalies in ocean gyres and north south dipoles in air sea flux drive anomalous thermohaline circulation both gyres and thermohaline circulation play a role in modulating sea surface temperature anomalies and hence through air sea interaction the overlying jet stream the model which can be expressed in the form of a delayed oscillator with ocean gyres and or thermohaline circulation providing the delay identifies key nondimensional parameters that control whether the ocean responds passively to nao forcing or actively couples it suggests that both thermohaline circulation and ocean gyres can play a role in coupled interactions on decadal timescales

4he 3he thermochronometry using classical diffusion theory we present a mathematical technique for the determination of 4he concentration profiles in minerals this approach should prove useful for constraining the low temperature cooling histories of individual samples and for correcting u th he ages for partial diffusive loss the calculation assumes that the mineral of interest contains an artificially produced and uniform distribution of 3he obtained by proton irradiation shuster et al earth planet sci lett 217 2004 19 32 in minerals devoid of natural helium this isotope allows measurement of he diffusion coefficients in minerals with measurable radiogenic he it permits determination of 4he profiles arising during ingrowth and diffusion in nature the 4he profile can be extracted from stepwise degassing experiments in which the 4he 3he ratio is measured the evolution of the 4he 3he ratio as a function of cumulative 3he released can be compared with forward models to constrain the shape of the profile alternatively we present a linear inversion that can be used to directly solve for the unknown 4he distribution the inversion incorporates a standard regularization technique to filter the influence of random measurement errors on the solution using either approach we show that stepwise degassing data can yield robust and high resolution information on the 4he profile profiles of radiogenic he are a sensitive function of the time temperate t t path that a cooling sample experienced thus by step heating a proton irradiated sample it is possible to restrict the sample s acceptable t t paths the sensitivity of this approach was explored by forward modeling 4he profiles resulting from a range of realistic t t paths using apatite as an example results indicate that 4he profiles provide rich information on t t paths especially when the profiles are coupled with u th he cooling ages on the same sample samples that experienced only moderate diffusive loss have 4he concentration profiles that are rounded at the edge but uniform in the core of the diffusion domain such profiles can be identified by nearly invariant 4he 3he ratios after the first few to few tens of percent of 3he have been extracted by step heating we show how such data can be used to correct u th he ages for partial diffusive loss

on the nature of decadal anomalies in north atlantic sea surface temperature north atlantic sea surface temperature data from the comprehensive ocean atmosphere data set were used to investigate the behavior of temperature anomalies on multiple year timescales during the period 1948 1992 monthly anomaly time series for each 2 square from the equator to 70 n were low pass filtered at 4 years and normalized by the local standard deviation attention is focused on the extreme events the upper and lower deciles of the anomaly time series a 45 year sequence of january maps shows the already familiar phenomena of generally cold conditions prior to 1951 a long warm interval from 1951 through 1967 and again a cold period from 1968 through 1977 the years 1978 through 1982 were largely devoid of persistent strong anomalies but moderate cold conditions returned during 1983 1986 warm conditions dominated the north atlantic from 1987 onward within these thermal epochs however a total of five cold anomaly features and nine warm anomaly features have been identified these features have individual lifetimes of 3 to 10 years a typical size is 20 of latitude or longitude but they range from barely detectable to spanning the width of the basin the latter especially in lower latitudes most of the anomalies move long distances along certain preferred paths these paths generally follow the routes of the subarctic and subtropical gyres anomalies originating off north america along the boundary between the gyres move northeastward toward the norwegian sea along the approximate route of the north atlantic current midlatitude anomalies originating at the eastern boundary tend to spread both northward and southward along the coast the speed of these movements 1 3 km d 1 is generally less than the expected speed of the near surface ocean circulation simple ideas about the effects of beta dynamics and air sea heat exchanges are briefly considered but do not provide a satisfactory explanation for the movements of the anomalies the long timescale of these extreme events and the continuity of their movements suggest a useful degree of predictability of sea surface temperature based on persistence and propagation of features

solar modulation of north atlantic central water formation at multidecadal timescales during the late holocene article i nfo understanding natural climate variability in the north atlantic region is essential not only to assess the sensitivity of atmosphere ocean climate signal exchange and propagation but also to help distinguish between natural and anthropogenic climate change the north atlantic oscillation is one of the controlling modes in recent variability of atmosphere ocean linkages and ice freshwater fluxes between the polar and north atlantic ocean through these processes the nao influences water mass formation and the strength of the atlantic meridional overturning circulation and thereby variability in ocean heat transport however the impact of the nao as well as other forcing mechanisms on multidecadal timescales such as total solar irradiance on eastern north atlantic central water production central water circulation and climate signal propagation from high to low latitudes in the eastern subpolar and subtropical basins remains uncertain here we use a 1200 yr long benthic foraminiferal mg ca based temperature and oxygen isotope record from a 900 m deep sediment core off northwest africa to show that atmosphere ocean interactions in the eastern subpolar gyre are transferred at central water depth into the eastern boundary of the subtropical gyre further we link the variability of the nao over the past 165 yrs and solar irradiance late holocene and their control on subpolar mode water formation to the multidecadal variability observed at mid depth in the eastern subtropical gyre our results show that eastern north atlantic central waters cooled by up to 0 8 0 7 c and densities decreased by 0 3 0 2 during positive nao years and during minima in solar irradiance during the late holocene the presented records demonstrate the sensitivity of central water formation to enhanced atmospheric forcing and ice freshwater fluxes into the eastern subpolar gyre and the importance of central water circulation for cross gyre climate signal propagation during the late holocene 2011 elsevier b v all rights reserved

localization of abrupt change in the north atlantic thermohaline circulation 1 recent climate model experiments as well as paleoclimate records suggest that the meridional overturning circulation or thermohaline circulation in the atlantic ocean could change abruptly as a result of global warming and that this could have a significant impact on european climate we use a reduced gravity model to investigate the response of the atlantic overturning circulation to changes in forcing we find that variability at decadal and higher frequencies is confined to a single hemisphere this implies that a overturning variability resulting from high frequency changes in buoyancy forcing in the labrador and greenland seas will be limited to the north atlantic and b any observed decadal and higher frequency fluctuations in north atlantic overturning can only result from changes in the surface fluxes within the north atlantic basin itself these results suggest that southern ocean wind forcing is not important for north atlantic overturning on decadal and shorter timescales

coherency of late holocene european speleothem δ18o records linked to north atlantic ocean circulation speleothem 18o records provide valuable information about past continental environmental and climatic conditions although their interpretation is often not straightforward here we evaluate a compilation of late holocene speleothem 18o records using a monte carlo based principal component analysis mc pca method that accounts for uncertainties in individual speleothem age models and for the variable temporal resolution of each 18o record the mc pca approach permits not only the identification of temporally coherent changes in speleothem 18o it also facilitates their graphical depiction and evaluation of their spatial coherency the mc pca method was applied to 11 holocene speleothem 18o records that span most of the european continent apart from the circum mediterranean region we observe a common shared mode of speleothem 18o variability that suggests millennial scale coherency and cyclicity during the last 4 5 ka these changes are likely caused by variability in atmospheric circulation akin to that associated with the north atlantic oscillation reflecting meridionally shifted westerlies we argue that these common large scale variations in european speleothem 18o records are in phase with changes in the north atlantic ocean circulation indicated by the vigour of the iceland scotland overflow water isow the strength of the subpolar gyre spg and an ocean stacked north atlantic ice rafted debris ird index based on a recent modelling study we conclude that these changes in the north atlantic circulation history may be caused by wind stress on the ocean surface driven by shifted westerlies however the mechanisms that ultimately force the westerlies remain unclear

a simple model of the decadal response of the ocean to stochastic wind forcing abstract a simple linear model is used to estimate the decadal response of the extratropical ocean to wind stress forcing assuming a flat bottom a mean state at rest and no dissipation the barotropic fields are governed by a time dependent sverdrup balance the baroclinic ones by the long rossby wave equation the ocean is bounded by a coast in the east and a radiation condition is used in the west at each frequency the baroclinic response consists of a forced response plus a rossby wave generated at the eastern boundary for zonally independent forcing the response propagates westward at twice the rossby phase speed the wind stress is assumed to be stochastic with a white frequency spectrum so the model represents the continuous excitation of the ocean interior by the weather fluctuations the model predicts the shape and level of the frequency spectra of the oceanic pressure field and their variation with longitude and latitude the baroclinic response is spread over a continuum of frequencies

effects of u th rich grain boundary phases on apatite helium ages abstract single grain apatite u th sm he ages he ages from non detrital samples sometimes show larger dispersions than can be explained by known sources of age dispersion such as grain size radiation damage parent zonation fracturing or intracrystalline inclusions we present observations and model age bias effects of an additional source of apatite he age dispersion observed in some cases u th rich grain boundary phases gbps precipitated on apatite crystal surfaces apatite grains from several samples with dispersed apatite he ages are coated or partially coated by reddish orange gbps rich in fe u and th these gbps are heterogeneous in thickness grain coverage and composition and have effective u concentrations eu up to 1000 ppm these phases may have large effects on the bulk eu and 4he compositions of apatite grains and can produce significant he age biases the direction and magnitude of this age bias is primarily a function of four factors 1 the host grain s size 2 the gbps thickness and eu relative to the host grain 3 the timing of gbp formation relative to the cooling age of the host grain and 4 whether the gbps are preserved and analyzed with the grain during he dating some of the most severe effects occur when gbps formed before or near the time of the apatite cooling age but were lost just prior to analysis for example during mineral separation in this case gbps of commonly observed thicknesses 1 10 m with eu 2 10 times that of the host apatite grain implant enough 4he into typical sized apatite grains to produce hundreds of percent positive age biases in contrast when u th rich gbps are preserved and analyzed with apatite grains the resulting he ages can be negatively biased though the heterogeneity variable preservation and ambiguous formation ages of gbps preclude a general quantitative and practical solution to this problem our model demonstrates that observed age dispersions in some samples are consistent with the effects of u th rich grain boundary phases and illustrates the conditions under which and by how much they will bias he ages

mechanisms underlying recent decadal changes in subpolar north atlantic ocean heat content the subpolar north atlantic spna is subject to strong decadal variability with implications for surface climate and its predictability in 2004 2005 spna decadal upper ocean and sea surface temperature trends reversed from warming during 1994 2004 to cooling over 2005 2015 this recent decadal trend reversal in spna ocean heat content ohc is studied using a physically consistent observationally constrained global ocean state estimate covering 1992 2015 the estimate s physical consistency facilitates quantitative causal attribution of ocean variations closed heat budget diagnostics reveal that the spna ohc trend reversal is the result of heat advection by midlatitude ocean circulation kinematic decompositions reveal that changes in the deep and intermediate vertical overturning circulation cannot account for the trend reversal but rather ocean heat transports by horizontal gyre circulations render the primary contributions the shift in horizontal gyre advection reflects anomalous circulation acting on the mean temperature gradients maximum covariance analysis mca reveals strong covariation between the anomalous horizontal gyre circulation and variations in the local wind stress curl suggestive of a sverdrup response results have implications for decadal predictability

helium diffusion in rutile and titanite and consideration of the origin and implications of diffusional anisotropy abstract diffusion of helium has been characterized in natural sphene titanite and synthetic rutile polished slabs of sphene and rutile were implanted with 100 kev 3he at a dose of 5 1015 3he cm2 and annealed in 1 atm furnaces 3he distributions following experiments were measured with nuclear reaction analysis using the reaction 3he d p 4he for diffusion in rutile we obtain the following arrhenius relations d a 2 26 1 0 1 0 exp 126 11 kj mol 1 rt m 2 sec 1 d c 1 75 1 0 8 exp 12 0 7 kj mol 1 rt m 2 sec 1 although activation energies for diffusion parallel to the c and a axes are comparable there is marked diffusional anisotropy with diffusion parallel to the c axis about 2 orders of magnitude faster than transport parallel to the a axis these diffusivities bracket the values determined for he diffusion in rutile in bulk release experiments stockli et al 2005 stockli et al 2007 wolfe 2009 although the role of anisotropy could not be directly evaluated in those measurements in titanite the following arrhenius relation was obtained over the temperature range 252 550 c for diffusion parallel to the a axis d 2 14 1 0 6 exp 148 8 kj mol 1 rt m 2 sec 1 in contrast to rutile and zircon cherniak et al 2009 titanite shows little evidence of anisotropy as diffusivities parallel to the a and c axes are similar and diffusivities for titanites from two different localities are similar he diffusion coefficients obtained in this study are similar to those measured through bulk release of he by step heating shuster et al 2003 over the investigated temperature range diffusion of he in titanite is similar to that of he diffusion in rutile parallel to the c axis but much faster than diffusion parallel to the a axis since the diffusion of he in rutile exhibits such pronounced anisotropy we model diffusional loss of he with a recently developed finite element code watson et al 2010 created to simulate diffusion in cylindrical geometry with differing radial and axial diffusion coefficients we present example applications to evaluate helium losses from rutile grains as a function of grain size and length to diameter ratios in efforts to better understand the occurrence of pronounced anisotropy for he diffusion in some crystals e g rutile zircon but not in others e g apatite titanite we consider the density and distribution of interstitial apertures in the crystal structure that might permit he migration these determinations an extension of the concept of ionic porosity are consistent with observations of relative diffusivities and the existence or absence of significant anisotropy for the minerals rutile zircon apatite and titanite

diffusion of helium in zircon and apatite abstract diffusion of helium has been characterized in natural zircon and apatite polished slabs of zircon and apatite oriented either normal or parallel to c were implanted with 100 kev 3he at a dose of 5 1015 3 he cm2 diffusion experiments on implanted zircon and apatite were run in pt capsules in 1 atm furnaces 3he distributions following experiments were measured with nuclear reaction analysis using the reaction 3he d p 4he for diffusion in zircon we obtain the following arrhenius relations d c 2 3 10 7 exp 146 11 kj mol 1 r t m 2 s 1 d c 1 7 10 5 exp 148 17 kj mol 1 r t m 2 s 1 although activation energies for diffusion normal and parallel to c are comparable there is marked diffusional anisotropy with diffusion parallel to c nearly 2 orders of magnitude faster than transport normal to c these diffusivities bracket the range of values determined for he diffusion in zircon in bulk release experiments although the role of anisotropy could not be directly evaluated in those measurements in apatite the following arrhenius relation was obtained over the temperature range of 148 449 c for diffusion normal to c d 2 10 10 6 exp 117 6 kj mol 1 r t m 2 s 1 in contrast to zircon apatite shows little evidence of anisotropy he diffusivities obtained in this study fall about an order of magnitude lower than diffusivities measured through bulk release of he through step heating and within an order of magnitude of determinations where ion implantation was used to introduce helium and he distributions measured with elastic recoil detection since the diffusion of he in zircon exhibits such pronounced anisotropy helium diffusional loss and closure cannot be modeled with simple spherical geometries and the assumption of isotropic diffusion a finite element code cylmod has recently been created to simulate diffusion in cylindrical geometry with differing radial and axial diffusion coefficients we present some applications of the code in evaluating helium lost from zircon grains as a function of grain size and length to diameter ratios and consider the effects of shape anisotropy where diffusion is isotropic as in the case of apatite but shapes of crystal grains or fragments may depart significantly from spherical geometry

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

tracking labrador sea water property signals along the deep western boundary current observations of the deep western boundary current dwbc at line w on the western north atlantic continental slope southeast of cape cod from 1995 to 2014 reveal water mass changes that are consistent with changes in source water properties upstream in the labrador sea this is most evident in the cold dense and deep class of labrador sea water dlsw that was created and progressively replenished and deepened by recurring winter convection during the severe winters of 1987 1994 the arrival of this record cold fresh and low potential vorticity anomaly at line w lags its formation in the labrador sea by 3 7 years complementary observations along the path of the dwbc provide further evidence that this anomaly is advected along the boundary and indicate that stirring between the boundary and the interior intensifies south of the flemish cap finally the consistency of the data with realistic advective and mixing time scales is assessed using the waugh and hall 2005 model framework the data are found to be best represented by a mean transit time of 5 years from the labrador sea to line w with a leading order role for both advection by the dwbc and mixing between the boundary flow and interior waters

on the structure and teleconnections of north atlantic decadal variability abstractdecadal variability in the north atlantic has been associated in the literature with a tripolar pattern of sea surface temperature sst anomalies that show one sign in the western midlatitudinal north atlantic and the opposite in the subpolar and tropical north atlantic the present analysis of observed sst from 1870 to 2009 leads to the dissection of the sst tripole into two components each with a different time scale in the decadal band and different teleconnections in the atlantic basin while the subpolar and tropical poles present quasi decadal variations with a period of about 9 years essentially uncorrelated with other parts of the basin the center of action in the western midlatitudes is characterized by a longer time scale of about 14 years and significant correlations with the tropical south atlantic and the norwegian and north sea s the 9 yr period variations are associated with an atmospheric configuration resembling the east atlantic pattern whereas the 14 yr period fluctuation

inverse solution for pentadal variability in the north atlantic 1 seven pentads 1960 1964 to 1990 1994 of the north atlantic climatology by lozier et al 1995 are analyzed with the help of an inverse finite element ocean circulation model the model solves for temperature and salinity fields that are close to observations and respect stationary dynamical balances and produces estimates of the circulation the first two eof s of transport streamfunction variability 39 and 26 of the explained variance are found to have a high correlation with the north atlantic oscillation nao index averaged over pentads and lagged negatively by three years first eof and between zero and one years second eof the spatial pattern of the first eof corresponds to an anticyclonic anomaly in the positive nao phase located between 20 and 45 n with the largest variability between 30 and 45 n and much weaker cyclonic anomalies in the subpolar gyre the second eof shows a sharp anticyclonic anomaly along the gulf stream and its extension and a wide cyclonic cell over most of subtropical gyre the anomaly in the meridional overturning streamfunction is characterized by a single positive cell which poorly correlates with the nao index

a modelling and observational study of the relationship between sea surface temperature in the north west atlantic and the atmospheric general circulation results from four pairs of 50 day wintertime integrations of the meteorological office s 5 level general circulation model with warm and cold sea surface temperature s s t anomalies of about 3 k in the northwest atlantic are described difference fields between the warm and cold integrations are statistically significant at the 1 level with positive geopotential height over the central north atlantic and weaker negative height over europe the storm track over the atlantic is displaced from its normal position results from four further pairs of integrations with halved s s t anomalies are also described the response is approximately linear with systematic differences in 500 mb geopotential height over the atlantic parts of which are just significant at the 10 level with half the full s s t anomaly overall however the model s response is weaker than could be obtained with tropical s s t anomalies of the same magnitude results from the model integrations are compared with results from an observational study of the relationship between wintertime s s t in the north west atlantic and mean sea level pressure and 500mb height two independent 30 year periods were chosen for study thus minimizing the influence of long term trends in s s t over the atlantic and europe the model results compared well with the observations with s s t data lagging the atmosphere by one month the observational study appears to show that the s s t anomalies are initially forced by perturbations in the atmospheric circulation with s s t data leading the atmosphere by one month results show that atmospheric and s s t anomalies are most persistent in the period october to december throughout the winter these lagged relationships are much weaker and not statistically significant diagnostics of e vector divergence from the gcm experiments are used to suggest that anomalous baroclinic wave activity over the atlantic is important as a momentum forcing for the anomalous time mean flow pattern on the other hand the role of thermal forcing provided by anomalous diabatic heating and transient eddy heat flux convergence may be important to substantiate this statement a simple linear steadystate two layer model of the response to extratropical thermal forcing is described with a suitable basic state flow and a mid tropospheric heat source given mainly by the transient eddy heat flux convergence the response is shown to be equivalent barotropic with a downstream ridge and ascent over the thermal source conversely results from an ocean mixed layer model are discussed which suggest that warm s s t anomalies could be maintained by a positive surface pressure perturbation positioned downstream of the anomaly through anomalous northward advection of warm ocean water by ekman drift currents this northward advection would balance the sensible and latent heat loss into the atmosphere over the s s t anomaly hence it is possible that some positive ocean atmosphere feedback may account for the persistence of such atmospheric and oceanic anomalies

frequency domain multimodel analysis of the response of atlantic meridional overturning circulation to surface forcing the dynamics of the atlantic meridional overturning circulation amoc vary considerably among different climate models for example some models show clear peaks in their power spectra while others do not to elucidate these model differences transfer functions are used to estimate the frequency domain relationship between surface forcing fields including sea surface temperature salinity and wind stress and the resulting amoc response these are estimated from the outputs of the coupled model intercomparison project phase 5 cmip5 and phase 3 cmip3 control runs for eightdifferent models with a specificfocus on geophysical fluid dynamics laboratory climate model version 2 1 gfdl cm2 1 and the community climate system model version 4 ccsm4 which exhibit rather different spectral behavior the transfer functionsshowvery littleagreement amongmodelsforany ofthe pairsofvariablesconsidered suggestingthe existence of systematic model errors and that considerable uncertainty in the simulation of amoc in current climate models remains however a robust feature of the frequency domain analysis is that models with spectral peaks in their amoc correspond to those in which amoc variability is more strongly excited by high latitude surface perturbations that have periods corresponding to the frequency of the spectral peaks this explains why different models exhibit such different amoc variability these differences would not be evident without using a method that explicitly computes the frequency dependence rather than a priori assuming a particular functional form finally transfer functions are used to evaluate two proposed physical mechanisms for model differences in amoc variability differences in labrador sea stratification and excitation by westward propagating subsurface rossby waves

on the mechanism of north atlantic decadal variability abstract north atlantic decadal climate variability is studied with a coupled atmosphere ocean sea ice model ecbilt after having reached an approximate statistical equilibrium in coupled mode without applying flux corrections a subsequent 1000 yr integration is performed and analyzed compared to the current climate the surface temperatures are 2 c warmer in the tropics to almost 8 c warmer in the polar regions the covariability between the atmosphere and ocean is explored by performing a singular value decomposition svd of boreal winter sst anomalies and 800 hpa geopotential height anomalies the first svd pair shows a red variance spectrum in sst and a white spectrum in 800 hpa height the second mode shows a peak in both spectra at a timescale of about 16 18 yr the geopotential height pattern is the model s equivalent of the north atlantic oscillation nao pattern the sst anomaly pattern is a north south oriented dipole additional experiments have revealed that the decadal oscillation in ec

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

heat budget in the north atlantic subpolar gyre impacts of atmospheric weather regimes on the 1995 warming event abstract in the mid 1990s the north atlantic subpolar gyre has shown a dramatic warming event that has been thoroughly investigated from observations and numerical simulations some studies suggest that it was due to an interannual wind driven weakening and shrinking of the gyre that facilitated the penetration of warm atlantic water the weakening of the gyre being attributed to changes in the north atlantic oscillation nao and in the east atlantic pattern which are the two dominant modes of atmospheric variability in the north atlantic however other studies suggest that the warming event was due to a decadal buoyancy driven strengthening of the meridional overturning circulation and subsequent intensification of the poleward heat transport in response to the positive nao conditions of 1988 1995 to reconcile this discrepancy the heat budget in the north atlantic subpolar gyre is reconstructed from four ocean hindcast simulations sharing the same modelling platform but using different settings the novelty of this work is the decomposition of the subpolar gyre into a western and an eastern subregion which is motivated by water mass distribution around reykjanes ridge and by the fact that deep convection only occurs in the western subpolar gyre in the western subpolar gyre the 1995 warming event is the decadal baroclinic ocean response to positive nao conditions from 1988 to 1995 the latter induced increased surface heat loss in the labrador sea that intensified deep convection hence strengthened the meridional overturning circulation and the associated poleward heat transport in the eastern subregion a concomittant warming is induced by an interannual barotropic adjustment of the gyre circulation to an abrupt switch from positive nao conditions in winter 1995 to negative nao conditions in winter 1996 indeed the gyre response to negative nao conditions is a cyclonic intergyre gyre that increases northward volume and heat transports at the southeastern limit of the subpolar gyre therefore the discrepancies found in the literature about the 1995 warming event of the north atlantic subpolar gyre are reconciled in the present work which suggests that the atmospheric drivers the mechanisms at stake and the associated timescales are different to the east and to the west of reykjanes ridge

wind feedback mediated by sea ice in the nordic seas air sea interactions play a critical role in the climate system this study investigates wind induced changes in the ocean surface temperature and sea ice cover feeding back onto the atmospheric circulation this interaction was modeled in the nordic seas using a partial coupling method to constrain the ocean with prescribed wind forcing in an otherwise fully coupled earth system model this enabled the evaluation of not only the direct oceanic but also the indirect atmospheric response to idealized forcing scenarios of perturbed winds over the nordic seas the results show that an anticyclonic wind anomaly forcing leads to significant surface cooling in the greenland sea mostly due to anomalous drift of sea ice during winter the cooling reduces the net surface heat flux to the atmosphere and increases sea level pressure the pressure gradients result in anomalous geostrophic southerly winds which locally are comparable both in direction and in velocity to the prescribed forcing anomalies suggesting a positive feedback

routes to long term atmospheric predictability in reduced order coupled ocean atmosphere systems impact of the ocean basin boundary conditions the predictability of the atmosphere at short and long time scales associated with the coupling to the ocean is explored in a new version of the modular arbitrary order ocean atmosphere model maooam based on a 2 layer quasi geostrophic atmosphere and a 1 layerreduced gravity quasi geostrophic ocean this version features a new ocean basin geometry with periodic boundary conditions in the zonal direction the analysis presented in this paper considers a low order version of the model with 40 dynamical variables first the increase of surface friction and the associated heat flux with the ocean can either induce chaos when the aspect ratio between the meridional and zonal directions of the domain of integration is small or suppress chaos when it is large this reflects the potentially counter intuitive role that the ocean can play in the coupled dynamics second and perhaps more importantly the emergence of long term predictability within the atmosphere for specific values of the friction coefficient occurs through intermittent excursions in the vicinity of a long period unstable periodic solution once close to this solution the system is predictable for long times i e a few years the intermittent transition close to this orbit is however erratic and probably hard to predict this new route to long term predictability contrasts with the one found in the closed ocean basin low order version of maooam in which the chaotic solution is permanently wandering in the vicinity of an unstable periodic orbit for specific values of the friction coefficient the model solution is thus at any time influenced by the unstable periodic orbit and inherits from its long term predictability

palaeoclimatic records of the loess palaeosol sequences of the chinese loess plateau whether during past climate stages or into a progressively warming world changes in precipitation constitute a key component of climatic change quantitative proxies for palaeo precipitation are relatively rare the magnetic properties of the windblown loess units and interbedded palaeosols of the famous chinese loess plateau provide key palaeo precipitation data for this populous monsoon dominated region the loess palaeosol sediments record rainfall totals directly complementing the oxygen isotope records of chinese speleothems these isotopic records predominantly reflect moisture source and hence large scale atmospheric circulation changes the two major asian monsoon systems appear to display antiphase behaviour dominance of the indian summer monsoon system seems associated with minimum precession maximum northern hemisphere summer heating dominance of the east asian summer monsoons with maximum precession at 2 8 ma more intense development of the east asian winter monsoon initiated major increases in dust deposition rates and formation of relatively unweathered loess layers glacial stage loess units then interleaved with interglacial interstadial stage palaeosols throughout the quaternary period decoupling of the loess palaeosol rainfall records from the chinese cave records of moisture source shows that the indian and east asian monsoon winds were continuously driven by precessional forcing while summer monsoonal rainfall was greatly suppressed during cool glacial stages the timing of these east asian climatic transitions the subsequent intensification of northern hemisphere glaciations and the association between monsoon circulation changes and north atlantic temperatures indicates a possibly leading global role for these monsoonal changes via alterations in the poleward distribution of heat and moisture

patterns of low frequency monthly sea level pressure variability 1899 1986 and associated wave cyclone frequencies abstract spatial patterns of low frequency sea level pressure slp variability are identified by performing rotated principal component analysis rpca on a long term 1899 1986 northern hemisphere gridded dataset the analysis is limited to the region 160 e eastward to 40 e due to missing data early in the century the objective is to identify a comprehensive set of highly recurrent slp teleconnection patterns to examine some aspects of their seasonality and to identify the associated mean winter pressure fields and cyclone frequencies occurring at times of opposite eigenvector polarity the results are further described in the context of the southern oscillation and known midtropospheric teleconnection patterns four low frequency variability patterns are identified over the atlantic european sector including i the north atlantic 0scillation nao and spatial patterns with slp variability centers over ii the eastern atlantic eatl iii southern europe and the northern mediterranean basin s

dry annealing of metamict zircon a differential scanning calorimetry study we report the results of a differential scanning calorimeter dsc study of the annealing of a metamict sri lankan zircon raman measurements on most chips of the powdered zircon starting material sri lankan zircon wz19 showed no evidence of a crystalline structure whereas a few chips retained residual raman bands typical of highly radiation damaged zircon dsc runs on aliquots of the powdered sample were heated to 850 and 1000 c at rates of 2 and 10 c min and to 1500 c at a rate of 10 c min raman spectroscopy was used to investigate the crystallinity of grains at selected temperature stages exothermal peaks were observed at about 910 and 1260 c during the dsc run to 1500 c the 910 c peak was demonstrated by raman spectroscopy to mark the crystallization of tetragonal zirconia and the exothermic peak at about 1260 c was demonstrated to represent the reaction of zirconia and amorphous silica to form crystalline zircon the degree of crystallinity of these grains was almost identical to that of highly crystalline zircons from recent gem gravels from new south wales a small number of experimental chips from dsc analyses under 1000 c were found to have zircon raman bands that indicated they had undergone partial annealing the present experimental results suggest that reconstitution of amorphous zircon to the crystalline state by dry annealing will rarely occur in terrestrial geological settings even under extreme metamorphic conditions

crystal surface integrity and diffusion measurements on earth and planetary materials abstract characterization of diffusion behavior in minerals is key to providing quantitative constraints on the ages and thermal histories of earth and planetary materials laboratory experiments are a vital source of the needed diffusion measurements but these can pose challenges because the length scales of diffusion achievable in a laboratory time are commonly less than 1 m an effective strategy for dealing with this challenge is to conduct experiments involving inward diffusion of the element of interest from a surface source followed by quantification of the resulting diffusive uptake profile using a high resolution depth profiling technique such as rutherford backscattering spectroscopy rbs nuclear reaction analysis nra or ion microprobe sims the value of data from such experiments is crucially dependent on the assumption that diffusion in the near surface of the sample is representative of diffusion in the bulk material historical arguments suggest that the very process of preparing a polished surface for diffusion studies introduces defects in the form of dislocations and cracks in the outermost micrometer of the sample that make this region fundamentally different from the bulk crystal in terms of its diffusion properties extensive indirect evidence suggests that in fact the near surface region of carefully prepared samples is no different from the bulk crystal in terms of its diffusion properties a direct confirmation of this conclusion is nevertheless clearly important here we use transmission electron microscopy to confirm that the near surface regions of olivine quartz and feldspar crystals prepared using careful polishing protocols contain no features that could plausibly affect diffusion this finding does not preclude damage to the mineral structure from other techniques used in diffusion studies e g ion implantation but even in this case the role of possible structural damage can be objectively assessed and controlled while all evidence points to the reliability of diffusivities obtained from in diffusion experiments we do not recommend experiments of this type using a powder source as a means of obtaining diffusant solubility or partitioning information for the mineral of interest

equilibrium atmospheric response to north atlantic sst anomalies abstract the equilibrium general circulation model gcm response to sea surface temperature sst anomalies in the western north atlantic region is studied a coarse resolution gcm with realistic lower boundary conditions including topography and climatological sst distribution is integrated in perpetual january and perpetual october modes distinguished from one another by the strength of the midlatitude westerlies an sst anomaly with a maximum of 4 c is added to the climatological sst distribution of the model with both positive and negative polarity these anomaly runs are compared to one another and to a control integration to determine the atmospheric response in all cases warming cooling of the midlatitude ocean surface yields a warming cooling of the atmosphere over and to the east of the sst anomaly center the atmospheric temperature change is largest near the surface and decreases upward consistent with this simple thermal response the geopotential height field displays a baroclinic

meddies and decadal changes at the azores front from 1980 to 2000 twenty years of time series observations from the deep sea mooring kiel276 are used to obtain information on the frequency and propagation of meddies mediterranean water eddies on long term changes in flow properties and on a possible relation to the north atlantic oscillation the mooring was set at the nominal position 33 n 22 w at a water depth of more than 5200 m in the northern canary basin it is located near the southern boundary of the azores current ac which is part of the north atlantic subtropical gyre and also in the large scale mediterranean water mw tongue originating from the strait of gibraltar the amplitudes of time varying vertical quasi geostrophic modes and the correlation of current and temperature changes at levels in the mw and the north atlantic central water above are used to identify meddies a total of 10 meddies passed the mooring during the period 1980 2000 half of the events can be related to earlier observations directional changes in meddy related velocities are used to estimate speeds and directions of meddy propagation directions of propagation are very homogeneous with all the 10 meddies observed propagating with a southward velocity component within a sector of 90 and typical speeds are 2 3 cm s meddy occurrence was uneven in time with six meddies found during the first four years and only four meddies during the remaining 16 years decadal changes show the annual mean and the fluctuating kinetic energy levels at the site changing from lower values in the 1980s to high values in the 1990s this change appears to be correlated with variations in the north atlantic oscillation nao index with a delay in oceanic response of about 3 years a conceptual model of ac meanders is used to identify meander signals in the upper layer time series the ac axis appears to be closer to the site during the 1990s than during the preceding decade and indicates a southward or southwestward displacement of the ac with increasingly positive values of the nao index meddy frequency is lower when the ac gets closer from the north a reduction in meddy occurrence in the region just south of the ac is possibly caused by the shear induced blocking of some meddies crossing the front from the north

geodetic exploration of strain along the el pilar fault in northeastern venezuela we use global navigation satellite systems observations in northeastern venezuela to constrain the el pilar fault epf kinematics and to explore the effects of the variable elastic properties of the surrounding medium and of the fault geometry on inferred slip rates and locking depth the velocity field exhibits an asymmetric velocity gradient on either side of the epf we use five different approaches to explore possible models to explain this asymmetry first we infer a 1 6 km locking depth using a classic elastic half space dislocation model second we infer a 1 5 km locking depth and a 0 33 asymmetry coefficient using a heterogeneous asymmetric model including contrasting material properties on either side of a vertical fault suggesting that the igneous metamorphic terranes on the northern side are 2 times more rigid than the sedimentary southern side third we use a three dimensional elastostatic model to evaluate the presence of a compliant zone suggesting a 30 reduction of rigidity in the upper 3 km at the depth of a 1 to 5 km wide fault zone fourth we evaluate the distribution of fault slip revealing a widespread partial creep pattern in the eastern upper segment while the upper western segment exhibits a partially locked area which coincides with the rupture surface of the 1797 and 1929 earthquakes to supplement these models we upgrade the previously published displacement simulation method using nonvertical dislocations with data acquired between 2003 and 2013 the localized aseismic displacement pattern associated with creeping or partially creeping fault segments could explain the low level of historic seismicity

the variability of the atlantic meridional overturning circulation the north atlantic oscillation and the el nino southern oscillation in the bergen climate model abstract the link between the interannual to interdecadal variability of the atlantic meridional overturning circulation amoc and the atmospheric forcing is investigated using 200 yr of a control simulation of the bergen climate model where the mean circulation cell is rather realistic as is also the location of deep convection in the northern north atlantic the amoc variability has a slightly red frequency spectrum and is primarily forced by the atmosphere the maximum value of the amoc is mostly sensitive to the deep convection in the irminger sea which it lags by about 5 yr the latter is mostly forced by a succession of atmospheric patterns that induce anomalous northerly winds over the area the impact of the north atlantic oscillation on deep convection in the labrador and greenland seas is represented realistically but its influence on the amoc is limited to the interannual time scale and is primarily associated with wind forcing the tropical pacific shows a strong variability in the model

a theory of interdecadal climate variability of the north pacific ocean atmosphere system abstract a linear coupled model for the atmosphere upper ocean system is proposed to highlight the mechanisms of decadal to interdecadal climate variability in the north pacific in this model wind stress anomalies over the north pacific are related to anomalies in the meridional temperature gradient of the upper ocean the latter depends upon air sea thermodynamical feedbacks and meridional heat transport by upper ocean currents slow adjustment of the oceanic gyre circulation to the change in wind stress is accomplished by the forced baroclinic oceanic rossby waves which carry out the meridional heat transport uncoupled ocean dynamic adjustment can produce a weak decadal to interdecadal peak in the power spectrum of the meridional transport under temporal white noise wind stress forcing with organized spatial structure coupled dynamics produce a basin scale interdecadal oscillatory mode this mode arises from the dynamic coupling and the memory of the system residing in the slow gyre circulation ad

incomplete retention of radiation damage in zircon from sri lanka a suite of 18 zircon gemstones from placers in the highland southwestern complex sri lanka were subjected to a comprehensive study of their radiation damages and ages the investigation included x ray diffraction raman and pl spectroscopy electron microprobe pixe and hrtem analysis as well as u th he and shrimp u th pb age determinations zircon samples described in this study are virtually homogeneous they cover the range from slightly metamict to nearly amorphous generally concordant u th pb ages averaging 555 11 ma were obtained late ordovician zircon u th he ages scattering around 443 9 ma correspond reasonably well with previously determined biotite rb sr ages for rocks from the hswc slightly to moderately metamict zircon has retained the radiogenic he whereas only strongly radiation damaged zircon calculated total fluences exceeding 3 5 10 18 events g has experienced significant he loss when compared to unannealed zircon from other localities sri lanka zircon is about half as metamict as would correspond to complete damage accumulation over a 555 m y lasting self irradiation period suggesting significant annealing of the structural radiation damage insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon we suggest to estimate effective doses for sri lanka zircon by multiplying total fluences which were calculated using the zircon u th pb age by a correction factor of 0 55 this conversion may be applied to literature data as well because all gem zircon samples from sri lanka this work and previous studies seem to reveal the same general trends of property changes depending on the radiation damage the use of effective doses for sri lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation damaged zircon

oceanic influence on the north atlantic oscillation and associated northern hemisphere climate variations 1959 1993 the north atlantic oscillation nao ex hibits variations at interannual to multidecadal time scales and is associated with climate variations over eastern north america the north atlantic europe and north africa therefore it is very important to understand causes of these nao variations and assess their predictability it has been hypothesized based on observations that sea surface temperature sst and sea ice variations in the north atlantic ocean influ ence the nao we describe results of an ensemble of sixteen experiments with an atmospheric general cir culation model in which we used observed sst and sea ice boundary conditions globally during 1949 1993 we show that multiyear nao and associated climate variations can be simulated reasonably accurately if re sults from a large number of experiments are averaged we also show that the ambiguous results of previous nao modeling studies were strongly influenced by the ensemble size which was much smaller than that in the present study the implications of these results for understanding and predictability of the nao are dis cussed