representation of the carbon cycle in box models and gcms 1 solubility pump 1 bacastow 1996 broecker et al 1999 and archer et al 2000 have called attention recently to the fact that box models and general circulation models gcms represent the thermal partitioning of co2 between the warm surface ocean and cold deep ocean in different ways they attribute these differences to mixing and circulation effects in gcms that are not resolved in box models the message that emerges from these studies is that box models have overstated the importance of the ocean s polar regions in the carbon cycle a reduced role for the polar regions has major implications for the mechanisms put forth to explain glacial interglacial changes in atmospheric co2 in parts 1 and 2 of this paper a new analysis of the ocean s carbon pumps is carried out to examine these findings this paper part 1 shows that unresolved mixing and circulation effects in box models are not the main reason for box model gcm differences the main factor is very different kinds of restrictions on gas exchange in polar areas polar outcrops in gcms are much smaller than in box models and they are assumed to be ice covered in an unrealistic way this finding does not support a reduced role for the ocean s polar regions in the cycling of organic carbon the subject taken up in part 2

circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

compartmentalization of the coso east flank geothermal field imaged by 3 d full tensor mt inversion author s lindsey nj kaven jo davatzes n newman ga abstract the authors 2016 published by oxford university press on behalf of the royal astronomical society previous magnetotelluric mt studies of the high temperature coso geothermal system in california identified a subvertical feature of lowresistivity 2 5ohmm and appreciable lateral extent g1 km in the producing zone of the east flank field however these models could not reproduce gross 3 d effects in the recorded data we perform 3 d full tensor inversion and retrieve a resistivity model that out performs previous 2 d and 3 d off diagonal models in terms of its fit to the complete 3 d mt data set as well as the degree of modelling bias inclusion of secondary zxx and zyy data components leads to a robust east dip 60 to the previously identified conductive east flank reservoir feature which correlates strongly with recently mapped surface faults downhole well temperatures 3 d seismic reflection data and local microseismicity we perform synthetic forward modelling to test the best fit dip of this conductor using the response at a nearby mt station we interpret the dipping conductor as a fractured and fluidized compartment which is structurally controlled by an unmapped blind east flank fault zone

lack of bipolar see saw in response to southern ocean wind reduction 1 a cessation of the atlantic meridional overturning circulation amoc significantly reduces northward oceanic heat transport in response to anomalous freshwater flux this leads to the classic bipolar see saw pattern of northern cooling and southern warming in surface air and ocean temperatures by contrast as shown here in a coupled climate model both northern and southern cooling are observed for an amoc reduction in response to reduced wind stress in the southern ocean so for very weak so wind stress not only the overturning circulation collapses but sea ice export from the so is strongly reduced consequently sea ice extent and albedo increase in this region the resulting cooling overcompensates the warming by the reduced northward heat transport the effect depends continuously on changes in wind stress and is reversed for increased winds it may have consequences for abrupt climate change the last deglaciation and climate sensitivity to increasing atmospheric co2 concentration

salinity driven thermocline transients in a wind and thermohaline forced isopycnic coordinate model of the north atlantic abstract an isopycnic coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation the model performance is illustrated by a 30 year spinup run with coarse horizontal resolution 2 mesh in a domain with north atlantic topography extending from 10 to 60 n latitude the vertical structure encompasses 10 isopycnic layers in steps of 0 2 units capped by a thermodynamically active mixed layer from an initially isohaline state with isopycnals prescribed by zonally averaged climatology the model is forced by seasonally varying wind stress radiative and freshwater fluxes and by a thermal relaxation process at the surface after a mechanical spinup time of about 15 years a quasi stationary pattern of mean circulation and annual variability ensues characterized by pronounced subtropical mode water formation

extending the vostok ice core record of palaeoclimate to the penultimate glacial period the ice core record of local temperature dust accumulation and air composition at vostok station antarctica now extends back to the penultimate glacial period 140 200 kyr ago and the end of the preceding interglacial this yields a new glaciological timescale for the whole record which is consistent with ocean records temperatures at vostok appear to have been more uniformly cold in the penultimate glacial period than in the most recent one concentrations of co2 and ch4 correlate well with temperature throughout the record

a continuum mechanical formulation for shallow polythermal ice sheets this paper is concerned with a new theoretical approach to model grounded ice sheets in three dimensions these are considered as polythermal i e there will be regions with temperatures below the

dansgaard oeschger and heinrich event imprints in alboran sea paleotemperatures past sea surface temperature sst evolution in the alboran sea western mediterranean during the last 50 000 years has been inferred from the study of c37 alkenones in international marine global change studies md952043 core this record has a time resolution of 200 years allowing the study of millennial scale and even shorter climatic changes the observed sst curve displays characteristic sequences of extremely rapid warming and cooling events along the glacial period comparison of this alboran record with 18o from greenland ice greenland ice sheet project 2 core shows a strong parallelism between these sst oscillations and the dansgaard oeschger events five prominent cooling episodes standing out in the sst profile are accompanied by an anomalous high abundance of neogloboquadrina pachyderma sinistral which is confined to the duration of these cold intervals these features and the isotopic record reflect drastic changes in the surface hydrography of the alboran sea in association with heinrich events hl 5

abrupt climate changes from the past to the future a review a review of climatic variability is given with a focus on abrupt changes during the last glacial evidence from palaeoclimatic archives suggests that these were most likley due to reorganisations of the atmosphere ocean system the mechanisms responsible for these changes are presented their implication for future climate changes is discussed in light of recent climate model simulations

climate records covering the last deglaciation the oxygen 18 oxygen 16 ratio of molecular oxygen trapped in ice cores provides a time stratigraphic marker for transferring the absolute chronology for the greenland ice sheet project gisp ii ice core to the vostok and byrd ice cores in antarctica comparison of the climate records from these cores suggests that near the beginning of the last deglaciation warming in antarctica began approximately 3000 years before the onset of the warm bolling period in greenland atmospheric carbon dioxide and methane concentrations began to rise 2000 to 3000 years before the warming began in greenland and must have contributed to deglaciation and warming of temperate and boreal regions in the northern hemisphere

the tectonic crustal stress field and style of faulting along the pacific north america plate boundary in southern california we invert for the state of stress in the southern california crust using a catalogue of high quality earthquake focal mechanisms 1981 2010 the stress field is best resolved where seismicity rates are high and sufficient data are available to constrain the stress field across most of the region from the stress field we determine the maximum horizontal compressive stress s hmax orientations and the style of faulting across southern california the trend of s hmax exhibits significant regional and local spatial heterogeneities the regional trend of s hmax varies from north along the san andreas system to nne to the east in the eastern california shear zone as well as to the west within the continental borderland and the western transverse ranges the transition zones from one state of stress to the other occur over a distance of only a few kilometres following a trend from yucca valley to imperial valley to the east and the western edge of the peninsular ranges to the west the local scale heterogeneities in the s hmax trend include nnw trends along the san andreas fault near cajon pass tejon pass and the cucapah range as well as nne trends near the northern san jacinto fault and the wheeler ridge area the style of faulting exhibits similar complexity ranging from predominantly normal faulting in the high sierra nevada to strike slip faulting along the san andreas system to three consecutive bands of thrust faulting in the wheeler ridge area and the western transverse ranges the local variations in the style of faulting include normal faulting at the north end of the san jacinto fault and scattered areas of thrust faulting the regional variations in the shmax trends are very similar to the pattern of the gps measured maximum shortening axes of the surface strain rate tensor field although the strain field tends to be smoother and appears to capture some of the upper mantle deformation field the mean trend of s hmax departs about approximately 14 to the east from the trend of the maximum shortening directions derived from anisotropy in the upper mantle

the 8 2 ka event abrupt transition of the subpolar gyre toward a modern north atlantic circulation 1 climate model simulations of the 8 2 ka event show an abrupt strengthening of the atlantic subpolar gyre that allows us to connect two major but apparently contradictory climate events of the early holocene the freshwater outburst from proglacial lakes and the onset of labrador sea water formation the 8 2 ka event is the largest climatic signal of our present interglacial with a widespread cooling in the north atlantic region about 8200 years before present it coincides with a meltwater outburst from north american proglacial lakes that is believed to have weakened the atlantic meridional overturning circulation and northward heat transport followed by a recovery of the deep ocean circulation and rising temperatures after a few centuries marine proxy data however date the onset of deep water formation in labrador sea to the same time the subsequent strengthening of the slope current system created a regional signal recorded as an abrupt and persistent surface temperature decrease although similarities in timing are compelling a mechanism to reconcile these apparently contradictory events was missing our simulations show that an abrupt and persistent strengthening of the atlantic subpolar gyre provides a plausible explanation the intense freshwater pulse triggered a transition of the gyre circulation into a different mode of operation stabilized by internal feedbacks and persistent after the cessation of the perturbation as a direct consequence deep water formation around its center intensifies this corresponds to the modern flow regime and stabilizes the meridional overturning circulation possibly contributing to the holocene s climatic stability

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

transtensional model for the sierra nevada frontal fault system eastern california active strike slip and normal faults along the eastern margin of the sierra nevada primarily accommodate northwestward translation of the sierra nevada central valley i e sierran microplate with respect to stable north america strike slip faults bordering the eastern sierran microplate are subparallel to small circles about the sierra nevada north american euler pole normal faults of the sierra nevada frontal fault system strike 45 clockwise of the small circle trajectories and exhibit well defined left stepping en echelon patterns consistent with formation in dextral transcurrent regime major graben bordering the northeastern sierran microplate are located in regions where the locus of range front deformation steps abruptly eastward in a releasing geometry relative to sierra nevada north american motion crustal shortening occurs at the northern end of the sierran microplate where a component of northwest dextral shear steps westward in a left restraining geometry across the sierran crest to the sacramento valley kinematic inversions of earthquake focal mechanisms from the walker lane belt bordering the eastern sierra nevada indicate that seismogenic deformation primarily is characterized by horizontal shearing and oblique crustal thinning directions of macroscopic dextral shear inferred from the inversions are subparallel to the trajectories of small circles about the sierra nevada north american euler pole normal faulting along most of the eastern sierran range front thus appears to primarily accommodate microplate translation rather than sierran uplift or regional basin and range extension

surface slip during large owens valley earthquakes data sets and expanded results contributing to this study are available in the supporting information the earthscope southern and eastern california lidar project available online at http opentopo sdsc edu involved data acquisition and processing for the plate boundary observatory pbo by ncalm http www ncalm org unavco operates the pbo for earthscope http www earthscope org supported by the national science foundation ear 0350028 and ear 0732947 funding for this study was provided by the southern california earthquake center scec project 12140 the geological society of america graduate student research fund the community foundation of san bernardino county and the western washington university geology department we thank g seitz m price and k morgan for assistance in the field and s bacon j arrowsmith r weldon k scharer j unruh c madden madugo and d haddad for helpful discussions constructive reviews by d schwartz r briggs e schermer d clark and one anonymous reviewer substantially improved the paper we also thank the staff at the uc white mountain research center for facilitating this work any use of trade firm or product names is for descriptive purposes only and does not imply endorsement by the u s government

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

thermo mechanically coupled ice sheet response cold polythermal temperate classical mixture concepts are the appropriate vehicle for describing the dynamics of ice masses containing some water we review and derive respectively the theoretical formulations of cold polythermal and temperate ice masses emphasize the peculiarities of the model equations and point to difficulties that were encountered with the proposed models the focus is both on the adequate physical motivation of the models and the consistency of their mathematical representation

gisp2 oxygen isotope ratios the gisp2 oxygen isotope record with its high resolution detail yields crucial information on past climate change the glacial 18o oscillations of the gisp2 core with their very fast onsets are templates of a prototype oscillation of variable duration with an amplitude of 3 9 the halfway mark of the cold warm transition is reached in 2 years the top is reached in 50 years the 18o time gradient of the leading front is about 7 8 per 100 yr after reaching the top 18o slowly declines by 0 14 per 100 yr the duration of 18o decline varies from a couple of centuries for fast oscillations to about 4000 yr for slower ones the subsequent 18o downturn during the warm cold transition has a 18o time gradient of 3 2 per 100 yr and lasts about 80 yr

history of metamorphic fluids along outcrop scale faults in a paleogene accretionary prism sw japan implications for prism scale hydrology 1 synkinematic quartz crystals collected from late stage syn to post cleavage faults in the paleogene shimanto accretionary complex contain fluids that provide constraints on fluid flow within the accretionary wedge in particular the trapped fluids provide a glimpse of intraprism flow during a phase of widespread prism thickening carbonic ch4 co2 rich and aqueous h2o rich inclusions occur intermixed within single crystals that nucleated primarily in releasing stepovers along faults that deform well indurated and or cleaved wallrock invariably the carbonic inclusions contain an aqueous liquid as a perimeter phase and the aqueous inclusions contain a carbonic vapor phase suggesting coeval trapping of unmixed phases from a miscible parent fluid these observations are interpreted to reflect crystal growth following deformation driven dilitation and consequent unmixing of the parent fluid at one location cape oyama the occurrence of ch4 and h2o in the absence of co2 enables explicit constraints to be placed on pressure and temperature conditions during crystal growth these conditions suggest geothermal gradients of 50 c km during late stage faulting our results suggest local intraprism flow that was driven by deformation the scale of flow that is suggested is quite restricted and this leads us to hypothesize that the paleogene shimanto accretionary prism acted largely as a barrier to fluid flow during late stage deformation as a consequence the paleogene rocks probably served to focus fluid escape elsewhere most likely trenchward and arcward the combination of the elevated geothermal gradient which would have contributed to increasing the relative strength of this package of rocks and the lack of fluid pathways within it would have favored the development of new out of sequence thrust faults outboard of the paleogene prism or the reactivation of older more landward structures

on the relationships between features of the depth latitude meridional overturning streamfunctions across global coupled climate models a comparative analysis of the state and response of the latitude depth meridional overturning streamfunctions in the climate model inter comparison project 3 cmip3 model set is presented simulated overturning strengths of the north atlantic cell tend to converge towards observational estimates the models whose simulations of the north atlantic cell are closest to observational estimates indicate a 29 5 13 decrease in the maximum intensity of that cell by 2 100 in contrast agreement with regard to the state and the response to anthropogenic radiative forcing of the global southern ocean abyssal cell is poor among the models a weak relationship between the mean state and the response of the abyssal cell can be used to constrain the reduction of the southern abyssal cell by 2 100 to 29 3 20 7 in rough agreement with the decrease predicted in the northern cell the biases across the cmip3 models in the northern deep cell and southern abyssal cell cannot be related dynamically by a buoyancy based seesaw like argument the absence or presence of characteristic relationships between the state and evolution of different features of the overturning streamfunction indicate that the main reasons for across model spread are how each model deals with subgrid scale processes and viscosity this highlights the fact that subgrid scale parameterizations and resolution improvements should be a priority of model development these factors are able to explain qualitatively the inter model differences between the northern overturning cells of the different models across model differences in the winds over the southern ocean are responsible for much of the disparity in the overturning circulation cells of the southern ocean

preface to special section on arctic ocean model intercomparison project aomip studies and results more than 130years ago theaustrian arctic explorer carl weyprecht weyprecht and ihne 1913 who discov ered franz josef land an archipelago north of russia andwho advanced a successful scheme for international coop eration in polar science the international polar year con cept postulated that a number of fundamental problems ofmeteorology and geophysics could be solved near theearth spoles thishypothesisisstillvalidinthe21stcenturybecause the arctic is recognized as a region where globalclimate change signals are amplified the model basedconclusion of manabe and stouffer 1994 that the firstsigns of climate change could be detected in the arctic hasbeen corroborated by numerous other model results e g holland and bitz 2003 symon et al 2005 on the otherhand it has been demonstrated that model conclusions maybe highly uncertain and that model estimates of futureclimate change differ significantly from model to model in order to reduce these uncertainties it is important tovalidate and to improve models the latter is the first majorgoal of the arctic ocean model intercomparison project aomip the second major aomip goal is to investigatevariability of the arctic ocean and sea ice at seasonal todecadal timescales and identify mechanisms responsible forthe observed changes some of the latest aomip activitiesassociated with these project goals arereflected inthe papersof this special section

application of a polythermal three dimensional ice sheet model to the greenland ice sheet response to steady state and transient climate scenarios abstract steady state and transient climate change computations are performed with the author s three dimensional polythermal ice sheet model simulation code for polythermal ice sheets for the greenland ice sheet the distinctive feature of this model is the detailed consideration of the basal temperate ice layer in which the water content and its impact on the ice viscosity are computed its transition surface to the cold ice region is accounted for by continuum mechanical jump conditions on this interface the simulations presented include steady states subject to a range of physical parameters and two different climates present and glacial conditions as well as three types of transient scenarios namely i sinusoidal milankovic period forcing ii paleoclimatic forcing from the greenland ice core project core reconstruction and iii future greenhouse warming forcing

representation of the carbon cycle in box models and gcms 2 organic pump 1 box models of the ocean atmosphere co 2 system rely on mechanisms at polar outcrops to alter the strength of the ocean s organic carbon pump gcm based carbon system models are reportedly less sensitive to the same processes here we separate the carbon pumps in a three box model and the gcm based princeton ocean biogeochemistry model to show how the organic pumps operate in the two kinds of models the organic pumps are found to be quite different in two respects deep water in the three box model is relatively well equilibrated with respect to the pco 2 of the atmosphere while deep water in the gcm tends to be poorly equilibrated this makes the organic pump inherently stronger in the gcm than in the three box model the second difference has to do with the role of polar nutrient utilization the organic pump in the gcm is shown to have natural upper and lower limits that are set by the initial po 4 concentrations in the deep water formed in the north atlantic and southern ocean the strength of the organic pump can swing between these limits in response to changes in deep water formation that alter the mix of northern and southern deep water thus unlike the situation in the three box model the organic pump in the gcm can become weaker or stronger without changes in polar nutrient utilization

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

a new tracer advection scheme for bryan and cox type ocean general circulation models in regions with large tracer gradients and or velocities the advection scheme that is used in many ocean models leads to significant under and overshoot of the tracer values in the case of the u k fine resolution antarctic model fram this lead to unphysical negative surface temperatures in some regions and overheating in others in this paper a new advection scheme is proposed and tested in the ocean model context using a limited area model centered on a region where problems occurred in fram

multiple regimes and low frequency variability in the quasi adiabatic overturning circulation 2015 american meteorological society when interior mixing is weak the ocean can support an interhemispheric overturning circulation on isopycnals that outcrop in both the northern hemisphere and a high latitude southern circumpolar channel this overturning cell participates in a salt advection feedback that counteracts the precipitation induced surface freshening of the northern high latitudes the net result is an increase in the range of isopycnals shared between the two hemispheres which strengthens the overturning circulation however if precipitation in the northern hemisphere sufficiently exceeds that in the southern hemisphere the overturning cell collapses and is replaced by a cell circulating in the opposite direction whose southern end point is equatorward of the channel this reversed cell is shallower and weaker than its forward counterpart and is maintained diffusively for a limited range of parameters freshwater hysteresis occurs and multiple overturning regimes are found for the same forcing these multiple regimes are by definition unstable with regard to finite amplitude disturbances since a sufficiently large perturbation can affect a transition from one regime to the other both overturning regimes show pronounced nearly periodic thermohaline variability on multidecadal and multicentennial time scales the multidecadal oscillation is expressed in the north hemisphere gyre and driven by a surface thermohaline instability the multicentennial oscillation has the character of an interhemispheric loop oscillation these oscillations mediate transitions between overturning regimes by providing an internal source of finite amplitude disturbances as the diffusivity is reduced the reverse cell becomes weaker and thus less stable to a given perturbation amplitude this causes the width of the hysteresis to decrease with decreasing diffusivity

waxing and waning of the perennial north polar h2o ice cap of mars over obliquity cycles abstract the perennial north polar h2o ice cap of mars is investigated with the dynamic thermodynamic ice sheet model sicopolis computational results for flow velocities ice temperatures and surface accumulation ablation rates are presented for the steady state with present conditions as well as for transient scenarios over idealized obliquity cycles with periods of 1 3 myr and 125 kyr the transient simulations lead to a stop and go type dynamics with a cold and almost stagnant modern ice cap and a warmer flowing ice cap for past large obliquity periods the most likely scenario for the present ice cap comprises an ice volume of 1 2 million km3 a maximum thickness of 3 km the absence of pronounced local isostasy h2o ice accumulation rates of the order of tenth mm water equivalent w e per earth year yr flow velocities of the order of mm yr and an ice base far below pressure melting

seismotectonics of the coso range indian wells valley region california transtensional deformation along the southeastern margin of the sierran microplate

a 25 000 year record of earthquakes on the owens valley fault near lone pine california implications for recurrence intervals slip rates and segmentation models seven trenches in eastern california across the owens valley fault near lone pine expose two episodes of faulting since early holocene time in the form of 1 m throw in lacustrine beds with liquefaction that were buried and then faulted again 1 m by the m 7 5 to 7 75 a d 1872 owens valley earthquake geomorphic maps applications of sequence stratigraphy and analyses of radiocarbon from charcoal and tufa deposits indicate that the paleoearthquake the penultimate event here occurred between 10 200 200 and 8800 200 cal yr b p the cumulative vertical displacement from these last two earthquakes in three trenches averages 2 4 0 3 m 2 and the penultimate event has slightly larger displacements a synthesis of available data indicates that the antepenultimate event was probably as large and occurred between ca 24 000 and 14 000 cal yr b p 2 thus the two interseismic intervals between the last three surface faulting earthquakes on the southern owens valley fault are each 10 000 yr this 25 000 year record indicates that the two event normal oblique slip rate on the owens valley fault near lone pine is 1 0 0 5 m k y this result is similar to that of several previous geological studies here yet it is still slower than slip rates on the northern owens valley fault and several factors slower than contemporary geodetic measurements this study attempts to account for different dating methods and interpretational uncertainties to acknowledge how little is known about the slip history of the owens valley fault and adjacent faults and to consider the role of segmentation as well as splay and distributed faulting in comparisons of displacement data among different sites along the entire 100 10 km length of the owens valley fault

the role of meridional density differences for a wind driven overturning circulation experiments with the coupled climate model climber 3 which contains an oceanic general circulation model show deep upwelling in the southern ocean to be proportional to the surface wind stress in the latitudinal band of drake passage at the same time the distribution of the southern ocean upwelling onto the oceanic basins is controlled by buoyancy distribution the inflow into each basin being proportional to the respective meridional density difference we observe approximately the same constant of proportionality for all basins and demonstrate that it can be directly related to the flow geometry for increased wind stress in the southern ocean the overturning increases both in the atlantic and the indo pacific basin for strongly reduced wind stress the circulation enters a regime where atlantic overturning is maintained through pacific upwelling in order to satisfy the transports set by the density differences previous results on surface buoyancy and wind stress forcing obtained with different models are reproduced within one model in order to distill a consistent picture we propose that both southern ocean upwelling and meridional density differences set up a system of conditions that determine the global meridional overturning circulation

large scale instabilities of the laurentide ice sheet simulated in a fully coupled climate system model 1 heinrich events related to large scale surges of the laurentide ice sheet represent one of the most dramatic types of abrupt climate change occurring during the last glacial here using a coupled atmosphere ocean biosphereice sheet model we simulate quasi periodic large scale surges from the laurentide ice sheet the average time between simulated events is about 7 000 yrs while the surging phase of each event lasts only several hundred years with a total ice volume discharge corresponding to 5 10 m of sea level rise in our model the simulated ice surges represent internal oscillations of the ice sheet at the same time our results suggest the possibility of a synchronization between instabilities of different ice sheets as indicated in paleoclimate records index terms 1827 hydrology glaciology 1863 1620 global change climate dynamics 3309 3344 meteorology and atmospheric dynamics paleoclimatology 5416 planetology solid surface planets glaciation citation calov r a ganopolski v petoukhov m claussen and r greve largescale instabilities of the laurentide ice sheet simulated in a fully coupled climate system model geophys res lett 29 24 2216 doi 10 1029 2002gl016078 2002

collision versus sliver transport in the hanging wall at the middle america subduction zone constraints from background seismicity in central costa rica earthquake focal mechanism solutions for 135 small magnitude events are inverted for best fitting partial strain rate tensors that characterize contemporary strain in five areas that span the western margin of the panama microplate in central costa rica the results indicate the predominance of subhorizontal maximum stretching subparallel to the middle america trench mat and provide constraints on the role of cocos ridge collision at the mat the trajectory of maximum stretching changes 25 45 over several tens of kilometers from the central costa rica deformed belt ccrdb where it is nearly e w to the area inboard of the cocos ridge where it is nw se this change suggests that background seismogenic deformation reflects the transition from the trailing edge of a fore arc sliver to an area of the upper plate affected by ridge collision this diffuse deformation may be localized in part on conjugate strike slip faults of the ccrdb

oceanic isopycnal mixing by coordinate rotation abstract current numerical models of oceanic circulation differentiate between the eddy diffusion and viscosity transport along the geopotential horizontal and vertical directions only in order to model the effect of anisotropic turbulence as diffusive transport along and across density surfaces the isopycnal mixing tensor has been transformed from a diagonal second rank tensor in the isopycnal coordinate system to a tensor containing off diagonal elements in the geopotential coordinate system

seismogenic strain across the transition from fore arc slivering to collision in southern taiwan tectonic slivers have profound implications on the mechanical behavior of fore arcs and subduction zones southern taiwan is characterized by precollision fore arc slivering that likely exerts important controls on the evolution of collision inversion of focal mechanism solutions for seismogenic strain reveals spatially partitioned plate motion south of taiwan with dip slip faulting nearer the manila trench and strike slip faulting nearer the luzon arc to the north these kinematics are less clearly segregated and both types of faults appear to occur in the same volume of crust further north deformation is dominated by oblique slip on potentially reactivated steep reverse faults in the collision zone existing work on the active fault zones in taiwan suggests that the subduction to collision transition is marked by a releasing step between the strike slip fault that bounds the fore arc sliver and the oblique thrusts of the collision zone this configuration may help to explain the lack of fore arc basement and cover exposed in the suture zone marked by the longitudinal valley

reconstructing monitoring and predicting multidecadal scale changes in the north atlantic thermohaline circulation with sea surface temperature sea surface temperature sst observations in the north atlantic indicate the existence of strong multidecadal variability with a unique spatial structure it is shown by means of a new global climate model which does not employ flux adjustments that the multidecadal sst variability is closely related to variations in the north atlantic thermohaline circulation thc the close correspondence between the north atlantic sst and thc variabilities allows in conjunction with the dynamical inertia of the thc for the prediction of the slowly varying component of the north atlantic climate system it is shown additionally that past variations of the north atlantic thc can be reconstructed from a simple north atlantic sst index and that future anthropogenically forced changes in the thc can be easily monitored by observing ssts the latter is confirmed by another state ofthe art global climate model finally the strong multidecadal variability may mask an anthropogenic signal in the north atlantic for some decades

temporal geographical meltwater influences on the north atlantic conveyor implications for the younger dryas the temporal and geographical roles of meltwater discharge from the laurentide ice sheet on north atlantic deep water nadw production are investigated utilizing a global realistic geometry coupled climate model which does not require the use of flux adjustments model results suggest that preconditioning by meltwater discharge to the mississippi prior to the younger dryas yd is capable of pushing nadw beyond the limit of its sustainability the diversion of meltwater to the st lawrence then merely serves to completely inhibit nadw production the modeled change in surface air temperature generally agrees with the global pattern and magnitude of temperature change seen in paleoclimatic reconstructions of the yd and is intimately linked to changes in nadw formation the global thermohaline circulation provides an interhemispheric teleconnection with the southern oceans while changes in the atmospheric heat transport reacting to a global redistribution of oceanic heat transport provide a mechanism for interbasin teleconnection although the primary thermodynamic and hydrological cycle feedback processes are included within the atmospheric model in the absence of additional feedbacks an equilibrium without the presence of nadw is possible the inclusion of the wind stress speed feedback is found to significantly contribute to the resumption of nadw production as suggested by previous studies contrary to these same studies however the coupled model indicates an advective spin up timescale is required for resumption of nadw production and hence the termination of the modeled yd like climate event as opposed to a decadal century timescale the reason for the discrepancy is unclear but may be associated with the use of fixed salt flux fields applied in previous studies or the duration strength and geographical location of the imposed meltwater applied

formation of most of our coal brought earth close to global glaciation the bulk of earth s coal deposits used as fossil fuel today was formed from plant debris during the late carboniferous and early permian periods the high burial rate of organic carbon correlates with a significant drawdown of atmospheric carbon dioxide co2 at that time a recent analysis of a high resolution record reveals large orbitally driven variations in atmospheric co2 concentration between 150 and 700 ppm for the latest carboniferous and very low values of 100 80 ppm for the earliest permian here i explore the sensitivity of the climate around the carboniferous permian boundary to changes in earth s orbital parameters and in atmospheric co2 using a coupled climate model the coldest orbital configurations are characterized by large axial tilt and small eccentricities of earth s elliptical orbit whereas the warmest configuration occurs at minimum tilt maximum eccentricity and a perihelion passage during northern hemisphere spring global glaciation occurs at co2 concentrations

large scale ice sheet modelling as a means of dating deep ice cores in greenland the three dimensional ice sheet model sicopolis is used to simulate the dynamic thermodynamic behaviour of the entire greenland ice sheet from 250 000 a bp until today external forcing consists of a surface temperature history constructed from 18 o data of the grip core a snowfall history coupled linearly to that of the surface temperature a piecewise linear sea level scenario and a constant geothermal heat flux the simulated greenland ice sheet is investigated in the vicinity of summit the position where the maximum elevation is taken and where the two drill sites grip and gisp 2 are situated 28 km apart from each other in the region the aggreement between modelled and observed topography and ice temperature tuns out to be very good computed age depth profiles for grip and gisp2 are presented which can be used to complete the dating of these cores in the deeper regions where annual layer counting is not possible however artififical diffusion influences the computed ages in a near basel boundary lar of approximately 15 of theice thickness so that the age at the bottom of the cores cannot be predicted yet

coastal zone color scanner pigment concentrations in the southern ocean and relationships to geophysical surface features the spatial and seasonal distributions of phytoplankton pigment concentration over the entire southern ocean have been studied for the first time using the coastal zone color scanner historical data set from october 1978 through june 1986 enhanced pigment concentrations are observed between 35 s and 55 s throughout the year with such enhanced regions being more confined to the south in the austral summer and extending further north in the winter north and south of the polar front phytoplankton blooms 1 mg m3 are not uniformly distributed around the circumpolar region instead blooms appear to be located in regions of ice retreat or high melt areas such as the scotia sea and the ross sea in relatively shallow areas e g the patagonian and the new zealand shelves in some regions of ekman upwelling like the tasman sea and near areas of high eddy kinetic energy such as the agulhas retroflection among all features examined by regression analysis bathymetry appears to be the one most consistently correlated with pigments correlation coefficient being about 0 3 for the entire region the cause of negative correlation with bathymetry is unknown but is consistent with the observed abundance of iron in shallow areas in the antarctic region it is also consistent with resuspension of phytoplankton cells by wind induced mixing especially in shallow waters on the other hand in the deep ocean especially at latitudes 30 in average pigment concentration over the entire region during different seasons indicates possible influence of time dependent parameters

observations of open ocean deep convection in the labrador sea from subsurface floats abstract the occurrence and extent of deep convection in the labrador sea in winters 1996 97 and 1997 98 is investigated from measurements of over 200 neutrally buoyant subsurface profiling autonomous lagrangian circulation explorer palace and sounding oceanographic lagrangian observer solo floats in addition to providing drift velocity data and vertical profiles of temperature and salinity 55 floats are equipped with vertical current meters vcms time series of vertical velocity derived from measured pressure and vertical flow past the float and temperature are obtained from the vcm floats mixed layer depths estimated from profile measurements indicate that convection reached depths greater than 1300 m in 1997 but no deeper than 1000 m in 1998 deep mixed layers were concentrated in the western basin although a number of deep mixed layers were observed southwest of cape farewell and also north of 60 n the highest variance in vertical velocity and the lowest mean temperatures were found in t

baby it s cold outside climate model simulations of the effects of the asteroid impact at the end of the cretaceous sixty six million years ago the end cretaceous mass extinction ended the reign of the dinosaurs flood basalt eruptions and an asteroid impact are widely discussed causes yet their contributions remain debated modeling the environmental changes after the chicxulub impact can shed light on this question existing studies however focused on the effect of dust or used one dimensional noncoupled atmosphere models here we explore the longer lasting cooling due to sulfate aerosols using a coupled climate model depending on aerosol stratospheric residence time global annual mean surface air temperature decreased by at least 26 c with 3 to 16 years subfreezing temperatures and a recovery time larger than 30 years the surface cooling triggered vigorous ocean mixing which could have resulted in a plankton bloom due to upwelling of nutrients these dramatic environmental changes suggest a pivotal role of the impact in the end cretaceous extinction

physical analysis of an antarctic ice core towards an integration of micro and macrodynamics of polar ice microstructures from deep ice cores reflect the dynamic conditions of the drill location as well as the thermodynamic history of the drill site and catchment area in great detail ice core parameters crystal lattice preferred orientation lpo grain size grain shape mesostructures visual stratigraphy as well as borehole deformation were measured in a deep ice core drilled at kohnen station dronning maud land dml antarctica these observations are used to characterize the local dynamic setting and its rheological as well as microstructural effects at the edml ice core drilling site european project for ice coring in antarctica in dml the results suggest a division of the core into five distinct sections interpreted as the effects of changing deformation boundary conditions from triaxial deformation with horizontal extension to bedrock parallel shear region 1 uppermost approx 450 m depth with still small macroscopic strain is dominated by compression of bubbles and strong strain and recrystallization localization region 2 approx 450 textendash 1700 m depth shows a girdle type lpo with the girdle plane being perpendicular to grain elongations which indicates triaxial deformation with dominating horizontal extension in this region approx 1000 m depth the first subtle traces of shear deformation are observed in the shape preferred orientation spo by inclination of the grain elongation region 3 approx 1700 textendash 2030 m depth represents a transitional regime between triaxial deformation and dominance of shear which becomes apparent in the progression of the girdle to a single maximum lpo and increasing obliqueness of grain elongations the fully developed single maximum lpo in region 4 approx 2030 textendash 2385 m depth is an indicator of shear dominance region 5 below approx 2385 m depth is marked by signs of strong shear such as strong spo values of grain elongation and strong kink folding of visual layers the details of structural observations are compared with results from a numerical ice sheet model pism isotropic for comparison of strain rate trends predicted from the large scale geometry of the ice sheet and borehole logging data this comparison confirms the segmentation into these depth regions and in turn provides a wider view of the ice sheet this article is part of the themed issue textquoteleft microdynamics of ice textquoteright

hydrography of the labrador sea during active convection the hydrographic structure of the labrador sea during wintertime convection is described the cruise part of the deep convection experiment took place in february march 1997 amidst an extended period of strong forcing in an otherwise moderate winter because the water column was preconditioned by previous strong winters the limited forcing was enough to cause convection to approximately 1500 m the change in heat storage along a transbasin section relative to an occupation done the previous october gives an average heat loss that is consistent with calibrated national centers for environmental prediction surface heat fluxes over that time period 200 wm 22 deep overturning was observed both seaward of the western continental slope which was expected as well as within the western boundary current itself something that had not been directly observed previously these two geographical regions separated by roughly the 3000 m isobath produce separate water mass products the offshore water mass is the familiar cold fresh dense classical labrador sea water lsw the boundary current water mass is a somewhat warmer saltier lighter vintage of classical lsw though in the far field it would be difficult to distinguish these products the offshore product was formed within the cyclonic recirculating gyre measured by lavender et al in a region that is limited to the north most likely by an eddy flux of buoyant water from the eastern boundary current the velocity measurements taken during the cruise provide a transport estimate of the boundary current throughput and offshore recirculation finally the overall trends in stratification of the observed mixed layers are described

a new fully three dimensional numerical model for ice dynamics the problem of describing ice dynamics has been faced by many researchers in this paper a fully three dimensional model for ice dynamics is presented and tested using an approach followed by other researchers ice is considered a non linear incompressible viscous fluid so that a fluid dynamic approach can be used the model is based on the full three dimensional stokes equations for the description of pressure and velocity fields on the saint venant equation for the description of the free surface time evolution and on a constitutive law derived from glen s law for the description of ice viscosity the model computes the complete pressure field by considering both the hydrostatic and hydrodynamic pressure components it is time evolutive and uses high order numerical approximation for equations and boundary conditions moreover it can deal with both constant and variable viscosity three theoretical tests and two applications to priestley glacier antarctica are presented in order to evaluate the performance of the model and to investigate important phenomena of ice dynamics such as the influence of viscosity on pressure and velocity fields basal sliding and flow over perturbed bedrocks all these applications demonstrate the importance of treating the complete pressure and stress fields

rates and mechanisms of water mass transformation in the labrador sea as inferred from tracer observations time series of hydrographic and transient tracer 3h and 3he observations from the central labrador sea collected between 1991 and 1996 are presented to document the complex changes in the tracer fields as a result of variations in convective activity during the 1990s between 1991 and 1993 as atmospheric forcing intensified convection penetrated to progressively increasing depths reaching 2300 m in the winter of 1993 over that period the potential temperature salinity s properties of labrador sea water stayed nearly constant as surface cooling and downward mixing of freshwater was balanced by excavating and upward mixing of the warmer and saltier northeast atlantic deep water it is shown that the net change in heat content of the water column 150 2500 m between 1991 and 1993 was negligible compared to the estimated mean heat loss over that period 110 w m 2 implying that the lateral convergence of heat into the central labrador sea nearly balances the atmospheric cooling on a surprisingly short timescale interestingly the 3h 3he age of labrador sea water increased during this period of intensifying convection starting in 1995 winters were milder and convection was restricted to the upper 800 m between 1994 and 1996 the evolution of 3h 3he age is similar to that of a stagnant water body in contrast the increase in and s over that period implies exchange of tracers with the boundaries via both an eddy induced overturning circulation and along isopycnal stirring by eddies with an exchange coefficient of o 500 m2 s 1 the authors construct a freshwater budget for the labrador sea and quantitatively demonstrate that sea ice meltwater is the dominant cause of the large annual cycle of salinity in the labrador sea both on the shelf and the interior it is shown that the transport of freshwater by eddies into the central labrador sea 140 cm between march and september can readily account for the observed seasonal freshening finally the authors discuss the role of the eddy induced overturning circulation with regard to transport and dispersal of the newly ventilated labrador sea water to the boundary current system and compare its strength 2 3 sv to the diagnosed buoyancy forced formation rate of labrador sea water

refining the southern extent of the 1872 owens valley earthquake rupture through paleoseismic investigations in the haiwee area southeastern california recent upward revision of the 1872 owens valley earthquake from mw 7 4 7 5 to 7 7 7 9 implies either additional unrecognized rupture length or anoma louslystronggroundmotionsassociatedwiththisevent weinvestigatethefirstpossibility through paleoseismic trenching south of the mapped surface rupture in the haiwee area where historical accounts suggest significant surface deformation following the earth quake trenching focused on a prominent north striking scarp herein termed the sage flat fault expressed in pleistocene alluvial fans east of haiwee reservoir surficial map ping and ground based light detection and ranging lidar surveying suggest that this faultaccommodateseast downnormalmotion andpossiblyacomparableamountofdex tralslip trenchingandluminescencedatingbracketsthetimingofthemostrecentsurface rupturing earthquake between 25 7 and 30 1 ka and provides evidence for an earlier event predating this time in combination with scarp profiling these dates also suggest am aximum rate of normal dip slip fault motion up to 0 1 mm yr over this period although we discovered no evidence for recent surface rupture on the sage flat fault a series of subvertical fractures and fissures cut across young trench stratigraphy consis tentwithsecondarydeformationassociatedwithseismicshaking assuch wesuggestthat possible ground disturbance in the haiwee area during the 1872 event primarily reflected ground shaking or liquefaction related deformation rather than triggered slip in addition we infer a structural and kinematic connection between the owens valley fault and oblique dextral faults north of lower cactus flat in the northwestern coso range rather thanawest stepintonorthernorwesternrosevalley considerationofthesestructuresin the total extent of the owens valley fault suggests a length of 140 km of which at least 113 km ruptured during the 1872 event online material procedural details and expanded results from the osl sample analyses as well as high resolution paleoseismic trench logs

a possible 20th century slowdown of southern ocean deep water formation chlorofluorocarbon 11 inventories for the deep southern ocean appear to confirm physical oceanographic and geochemical studies in the southern ocean which suggest that no more than 5 x 10 6 cubic meters per second of ventilated deep water is currently being produced this result conflicts with conclusions based on the distributions of the carbon 14 carbon ratio and a quasi conservative property po 4 in the deep sea which seem to require an average of about 15 x 10 6 cubic meters per second of southern ocean deep ventilation over about the past 800 years a major reduction in southern ocean deep water production during the 20th century from high rates during the little ice age may explain this apparent discordance if this is true a seesawing of deep water production between the northern atlantic and southern oceans may lie at the heart of the 1500 year ice rafting cycle

rapid climate transitions in a coupled ocean atmosphere model recent geochemical data1 2 have challenged the view that rapid climate fluctuations in the north atlantic at the end of the last glacial were caused by the thermohaline circulation of the ocean being switched on or off instead these data suggest that the circulation pattern must have switched between a warm deep mode and a cold shallow mode probably associated with different sites of deep convection3 here i present simulations with a three dimensional ocean model coupled to an idealized atmosphere which show this kind of transition the mechanism for the transition is a rearrangement of convection in the north atlantic triggered by a brief freshwater pulse this results in a drop in sea surface temperature in the north atlantic by up to 5 c within less than 10 years the rate of north atlantic deep water nadw forma tion is the same in the cold climate as in the warm climate but nadw sinks to intermediate depths only while antarctic bottom water pushes north to fill the entire abyssal atlantic

atlantic subsurface temperatures response to a shutdown of the overturning circulation and consequences for its recovery abstract using the coupled climate model of intermediate complexity climber 3 changes in the vertical thermal structure associated with a shutdown of the atlantic meridional overturning circulation amoc are investigated when north atlantic deep water formation is inhibited by anomalous freshwater forcing intermediate depth ventilation can remain active and cool the subsurface water masses i e the cold case however if intermediate ventilation is completely suppressed relatively warm water coming from the south penetrates to a high northern latitude beneath the halocline and induces a strong vertical temperature inversion between the surface and intermediate depth i e the warm case both types of temperature anomalies emerge within the first decade after the beginning of the freshwater perturbation the sign of subsurface temperature anomaly has a strong implication for the recovery of the amoc once the anomalous freshwater forcing is removed while the amoc recovers from the cold case

simulation of the antarctic ice sheet with a three dimensional polythermal ice sheet model in support of the epica project ii nested high resolution treatment of dronning maud land antarctica

lava fingerprinting using paleomagnetism and innovative x ray fluorescence spectroscopy a case study from the coso volcanic field california 1 we demonstrate an efficient method of rigorously separating difficult to distinguish lavas into eruptive units based on paleomagnetic remanence direction and rapid x ray fluorescence spectroscopy xrf for rb sr y zr and nb of polished paleomagnetic core samples called pc xrf combined use of paleomagnetic remanence and pc xrf for lava fingerprinting allows correlation of individual eruptive units from one locality to another permitting compilation of composite stratigraphy and paleomagentic measurement of relative vertical axis rotation of fault bounded blocks we test this lava fingerprinting method on rocks from the coso volcanic field california against similar fingerprinting using xrf analysis by established methods resulting unit definitions and correlations are the same by both xrf techniques when coupled with paleomagnetic data but at great time and cost savings with pc xrf pc xrf analysis is possible because 1 matrix and grain size effects are minimal for the element set analyzed 2 moderately phyric to aphyric polished paleomagnetic cores are already homogenous on spatial scales of 2 cm and 3 use of element ratios cancels out some analytical uncertainties as well as minimizes effects of concentration variations due to fractional crystallization paleomagnetic remanence direction is an indispensable part of fingerprinting because it distinguishes flows of similar chemistry and can also place constraints on the duration of emplacement of each eruptive unit

fault block kinematics at a releasing stepover of the eastern california shear zone partitioning of rotation style in and around the coso geothermal area and nascent metamorphic core complex abstract pliocene lavas and sediments of wild horse mesa in the coso range ca exhibit clockwise vertical axis rotation of fault bounded blocks this indicates localization of one strand of the eastern california shear zone walker lane belt within a large scale transtensional dextral releasing stepover we measured rotations paleomagnetically relative to two different reference frames at two localities we averaged secular variation through sedimentary sections to reveal rotation or its absence relative to paleogeographic north where sediments are lacking we used areally extensive lava flows from individual cooling units or short eruptive episodes to measure the relative rotation of localities by comparing their paleomagnetic remanence directions to one another at the western edge of wild horse mesa the fanglomerate member of the coso formation c a 3 ma exhibits between 8 4 7 8 and 26 2 9 0 two endmember models of a continuum absolute clockwise rotation within wild horse mesa 3 3 5 ma lavas at 5 different localities exhibit about 12 0 4 6 weighted mean clockwise rotation relative to the margins of the area a result statistically indistinguishable from the absolute rotation hence the segment of the eastern california shear zone passing through wild horse mesa has caused vertical axis rotation of fault bounded blocks as part of the overall dextral shear strain the magnitude of block rotation at wild horse mesa suggests that rotation has accommodated 1 1 5 km of dextral shear along an azimuth of about north 30 west since ca 3 ma between the area s bounding faults and 2 2 km of extension perpendicular to the coso wash normal fault during this same period this corresponds to 13 25 extension across the mesa in contrast to wild horse mesa the opposite western side of the trace of the coso wash normal fault hosts the coso geothermal area and what monastero et al f c monastero a m katzenstein j s miller j r unruh m c adams k richards dinger the coso geothermal field a nascent metamorphic core complex geol soc amer bull 117 2005 1534 1553 characterize as a nascent metamorphic core complex consistent with upper plate disruption above a detachment surface rocks i e the upper plate of the detachment system at the coso geothermal area are tilted westward however they appear to exhibit no detectable rotation thus the style of block rotation may be partitioned with clockwise vertical axis rotation dominating in the wild horse mesa and horizontal axis rotation tilting in the geothermal area

the response of southern ocean eddies to increased midlatitude westerlies a non eddy resolving model study 1 the midlatitude westerlies of the southern hemisphere have intensified since the 1970s non eddy resolving general circulation models respond to such wind intensification with steeper isopycnals a faster antarctic circumpolar current acc and a stronger atlantic meridional overturning circulation amoc however hydrographic observations show little change in the slope of the southern ocean isopycnals over the past 40 years this insensitivity seems to result from a compensating mechanism whereby an initial increase in the slope of the isopycnals causes eddy activity to intensify and forces the isopycnal slopes down climate models do not yet resolve ocean eddies and the eddy parameterizations included in them do not capture well the compensation mechanism mentioned above we present simulations with a non eddy resolving model incorporating an eddy parameterization in which eddy compensation is greatly enhanced by the use of a non constant spatially varying thickness diffusivity the sensitivity of the simulated acc and amoc to increased southern hemisphere westerlies is greatly reduced compared to simulations using constant and uniform diffusivities

climatic interpretation of the recently extended vostok ice records a new ice core drilled at the russian station of vostok in antarctica reached 2755 m depth in september 1993 at this depth the glaciological time scale provides an age of 260 ky bp 25 we refine this estimate using records of dust and deuterium in the ice and of 18o of o2 in the entrapped air 18o of o2 is highly correlated with insolation over the last two climatic cycles if one assumes that the egt chronology overestimates the increase of age with depth by 12 for ages older than 112 ky bp this modified age depth scale gives an age of 244 ky bp at 2755 m depth and agrees well with the age depth scale of walbroeck et al in press derived by orbital tuning of the vostok d record we discuss the temperature interpretation of this latter record accounting for the influence of the origin of the ice and using information derived from deuterium excess data we conclude that the warmest period of stage 7 was likely as warm as today in antarctica a remarkable feature of the vostok record is the high level of similarity of proxy temperature records for the last two climatic cycles stages 6 and 7 versus stages 1 5 this similarity has no equivalent in other paleorecords

on the freshwater forcing of the thermohaline circulation in the limit of low diapycnal mixing 1 a conjecture is offered on the stability characteristics of the thermohaline circulation in the limit of very low diapycnal mixing in this limit the action of the winds on the antarctic circumpolar current acc can sustain a deep overturning pattern known as the reconfigured conveyor consisting of upwelling around antarctica and sinking in the north atlantic as shown by the work of toggweiler and others it is conjectured that in this limit northern sinking should be stabilized in an on state because of the penetration of freshwater into the ocean interior via isopycnal layers that outcrop to the surface within and south of the acc this conjecture is supported by qualitative arguments and by a hydraulic model for the reconfigured conveyor the hydraulic model takes into account the freshwater budgets of the atlantic basin antarctic surface waters and the remaining oceans it also takes into account in simple terms wind driven antarctic upwelling eddy transports and mixing within the acc changes in pycnocline depth the role of temperature forcing and advective feedbacks on salinity the hydraulic model suggests that multiple on off states of the reconfigured conveyor are possible but only if the deep waters that form in the northern hemisphere are fresher than the intermediate waters that form in the vicinity of the acc in the southern hemisphere a condition that is not satisfied in the modern ocean

regional tectonics of the coso geothermal area along the intracontinental plate boundary in central eastern california three dimensional vp and vp vs models spatial temporal seismicity patterns and seismogenic deformation we synthesize the tectonics of the southern walker lane belt and coso range in central eastern california using regional earthquake data first we invert for three dimensional models of the v p and v p v s structure of the upper and middle crust using these models we also determine three dimensional vs and poisson s ratio models the changes in seismic velocities across the region are small except for low velocities in sedimentary basins and a 2 km positive elevation of the basement velocities v p 6 km s beneath the southern sierra nevada localized low v p and low v s zones beneath the central coso range image a geothermal reservoir at 0 to 3 km depth as well as distinct low velocity anomalies in the depth range of 8 to 12 km because the v p v s has average crustal values within this broader zone we interpret the anomaly to indicate a zone of few percent geothermal brines extending from 8 to 12 km depth in addition an embedded highly localized poorly resolved zone possibly as small as 1 km 3 of slightly above average v p v s and higher poisson s ratio is a tentative suggestion of a small volume percent of magma present at depth of 10 km second we relocated the seismicity in the region using absolute traveltimes and differential traveltimes determined from waveform cross correlation the relocated seismicity forms several spatially clustered lineaments along the southeast side of the sierra nevada and in the indian wells valley and vicinity of the coso geothermal field which coincide with mapped late quaternary faults in the region the base of seismicity shallows from a regional depth of about 11 to 5 km beneath the central coso range which we interpret as evidence for shallowing of the brittle ductile transition zone beneath the geothermal field in addition to abundant background seismicity two large earthquake swarms located 5 to 8 km to the west of coso occurred in april to may 1992 and may to june 2001 two dual main shock aftershock sequences also occurred as follows the 1994 sequence near ridgecrest and the later coso earthquake sequence from late 1996 to early 1998 with the pairs of main shocks spaced 47 days and 16 months apart respectively kinematic analysis of the focal mechanisms indicates that the crustal stress loading process varies across the region the low vp anomaly abundant seismicity and crustal thinning provide quantitative evidence for the coso region being an extensional releasing step over between two northwest striking dextral faults the little lake and airport lake fault zones to the south and the owens valley fault to the north

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

mixing of a tracer in the pycnocline a patch of sulfur hexafluoride was released in may 1992 in the eastern north atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces the diapycnal eddy diffusivity k estimated for the first 6 months was 0 12 0 02 cm2 s while for the subsequent 24 months it was 0 17 0 02 cm2 s the vertical tracer distribution remained very close to gaussian for the full 30 months as the root mean square rms dispersion grew from 5 to 50 m lateral dispersion was measured on several scales the growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0 07 m2 s at scales of 0 1 to 1 km larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing teasing of the overall patch initially about 25 km across into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3 0 5 10 7 s 1 the rms width of these streaks estimated as 3 km and maintained in the face of the streak growth indicates an isopycnal diffusivity of 2 m2 s at scales of 1 to 10 km much greater than expected from internal wave shear dispersion the patch was painted in albeit streakily by 12 months confirming expectations from analytical and numerical models homogenization of the patch continued during the subsequent 18 months while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2 s acting at scales of 30 to 300 km

a zonally averaged ocean model for the thermohaline circulation part ii interocean circulation in the pacific atlantic basin system abstract the zonally averaged latitude depth ocean model developed in part i is extended to a two basin system representing the atlantic and pacific steady states are calculated under two different surface boundary conditions to study a possible global thermohaline circulation linking the pacific and the atlantic if the surface temperature and salinity profiles are identical functions of latitude in both the pacific and the atlantic the steady state under restoring boundary conditions consists in spite of the different basin extensions of a two cell structure in each basin upon switching to mixed boundary conditions the state is unstable and a one cell circulation with downwelling at high northern latitudes develops in both basins for a realistic surface salinity profile that is fresher in the pacific the steady state under restoring boundary conditions is completely different it exhibits a global thermohaline circulation with strong interocean man exchange deep water is formed primarily in

the role of stratification dependent mixing for the stability of the atlantic overturning in a global climate model using the climber 3 coupled climate model of intermediate complexity the effect of a stratification dependent vertical diffusivity on the sensitivity of the atlantic ocean meridional overturning circulation to perturbations in freshwater forcing is investigated the vertical diffusivity is calculated as n where n is the local buoyancy frequency and the parameter is a measure of the sensitivity of the vertical diffusivity to changes in stratification independent of the stratification of the deep ocean is weakly increased as a response to an anomalous freshwater flux in the north atlantic in these experiments in the region of freshwater forcing and north of it this is caused by the freshwater anomaly itself but farther south it is associated with anomalously warm surface waters caused by a reduction of the northward oceanic heat transport subsequently and in opposition to results from previous studies the overturning is reduced by the anomalous freshwater flux independent of the choice of however the amount of reduction in overturning following a freshwater perturbation is found to depend critically on the choice of the mixing sensitivity i f cr the response is similar to the model s response using constant vertical diffusivity 0 for cr a sharp increase of the sensitivity is found the value of cr is found to be between 0 5 and 1 a general feedback is proposed explaining this threshold behavior if is large both positive and negative perturbations of stratification are amplified by associated changes in diffusivity in the experiments presented here this enhances the initial positive stratification anomaly in northern high latitudes which is created by the anomalous freshwater flux as a result convection is strongly reduced and the overturning is significantly weakened

stochastic resonance in the north atlantic large abrupt changes between warm and cold modes of north atlantic climate exhibit spectral power at 1500 years yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic we hypothesize that a weak periodic forcing has combined with noise from ice sheet related events to cause the observed mode switches this stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models the ice isotopic data from central greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested we thus support arguments for the existence of a periodicity of 1500 years in the north atlantic climate system and for the importance of ice sheet events in forcing north atlantic changes

the last glacial maximum climate over europe and western siberia a pmip comparison between models and data under the framework of the palaeoclimate modelling intercomparison project pmip 17 climate models 16 of which are atmospheric general circulation models have been run to simulate the climate of the last glacial maximum 21 000 years ago using the same set of boundary conditions parallel to these numerical experiments new consistent data bases have been developed on a continental scale the present work compares the range of the model responses to the large perturbation corresponding to the conditions of the last glacial maximum with consistently derived climate reconstructions from pollen records over europe and western siberia it accounts for the differences in the model results due to the models themselves and directly compares this error bar due to the models to the uncertainties in the climate reconstructions from the pollen records overall the last glacial maximum climate simulated by the models over western europe is warmer especially in winter and wetter than the one depicted by the reconstructions this is the region where the reconstructed increase in temperature precipitation and moisture index from the last glacial maximum to the present conditions is largest the same disagreement but of smaller amplitude is found over central europe and the eastern mediterranean basin while models and data are in broad agreement over western siberia the numerous modelling results allow a study of the link between the changes in atmospheric circulation and those in temperature and an interpretation of the discrepancies in precipitation in terms of those in temperature

simulation of an abrupt change in saharan vegetation in the mid holocene climate variability during the present inter glacial the holocene has been rather smooth in compar ison with the last glacial nevertheless there were some rather abrupt climate changes one of these changes the desertication of the saharan and arabian region some 4 6 thousand years ago was presumably quite important for human society it could have been the stimulus leading to the foundation of civilizations along the nile euphrat and tigris rivers here we argue that saharan and arabian de sertication was triggered by subtle variations in the earth s orbit which were strongly amplied by atmosphere vegeta tion feedbacks in the subtropics the timing of this tran sition however was mainly governed by a global interplay between atmosphere ocean sea ice and vegetation

the role of the deep ocean in north atlantic climate change between 70 and 130 kyr ago the suggestion1 that changes in north atlantic deep water nadw production are linked through surface heat flux to the atmospheric temperature over greenland is supported by earlier indications2 3 that nadw production decreased during glacial times and by the subsequent finding4 6 that it declined during the younger dryas cool period at the end of the last glaciation changes in north atlantic surface temperatures have been found7 to mirror high frequency temperature changes recorded in greenland ice cores over the past 80 kyr but the connection to abyssal circulation has yet to be established except for one or two isolated oscillations8 9 here we present carbon and oxygen isotope analyses of benthic foraminifera in a high resolution north atlantic deep sea sediment core for the period 70 130 kyr ago these data allow us to reconstruct the history of nadw production which shows a close correlation with greenland climate variability for much of this time interval suggesting that the climate influence of nadw variability was widespread we see no evidence however for changes in nadw production during substage 5e the eemian interglacial period in contrast with recent ice core data10 which suggest severe climate instability in greenland during this time period our results may support suggestions based on data from a second ice core that this apparent instability is an artefact caused by ice flow11 alternatively the eemian climate instability may have had a different origin from the subsequent climate events

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

effects of mesoscale eddies on subduction and distribution of subtropical mode water in an eddy resolving ogcm of the western north pacific abstract the effects of mesoscale eddies on the subduction and distribution of the north pacific subtropical mode water stmw are investigated using an eddy resolving ocean general circulation model ogcm first the subduction rate is calculated and the contribution of eddies to the subduction of stmw is estimated it is found that eddy subduction significantly contributes to the total subduction of stmw second eddy thickness transport and diapycnal flux are directly diagnosed to investigate the large scale eddy induced transport process of stmw the large southward eddy thickness transport in the stmw core density is consistent with eddy subduction the eddy transport on the isopycnal surface of stmw is directed down the thickness gradient and traverses the mean flow the meridional eddy transport streamfunction indicates two eddy circulation cells south of 30 n associated with the circulation of stmw these cells flatten density surfaces similar to the effect of the gent and mcwilliams gm schem

quantifying the heterogeneity of the tectonic stress field using borehole data the heterogeneity of the tectonic stress field is a fundamental property which influences earthquake dynamics and subsurface engineering self similar scaling of stress heterogeneities is frequently assumed to explain characteristics of earthquakes such as the magnitude frequency relation however observational evidence for such scaling of the stress field heterogeneity is scarce we analyze the local stress orientations using image logs of two closely spaced boreholes in the coso geothermal field with sub vertical and deviated trajectories respectively each spanning about 2 km in depth both the mean and the standard deviation of stress orientation indicators borehole breakouts drilling induced fractures and petal centerline fractures determined from each borehole agree to the limit of the resolution of our method although measurements at specific depths may not we find that the standard deviation in these boreholes strongly depends on the interval length analyzed generally increasing up to a wellbore log length of about 600 m and constant for longer intervals we find the same behavior in global data from the world stress map this suggests that the standard deviation of stress indicators characterizes the heterogeneity of the tectonic stress field rather than the quality of the stress measurement a large standard deviation of a stress measurement might be an expression of strong crustal heterogeneity rather than of an unreliable stress determination robust characterization of stress heterogeneity requires logs that sample stress indicators along a representative sample volume of at least 1 km

chronology of tectonic geomorphic and volcanic interactions and the tempo of fault slip near little lake california new geochronologic and geomorphic constraints on the little lake fault in the eastern california shear zone reveal steady modest rates of dextral slip during and since the mid to late pleistocene we focus on a suite of offset fluvial landforms in the pleistocene owens river channel that formed in response to periodic interaction with nearby basalt flows thereby recording displacement over multiple time intervals overlap between 40 ar 39 ar ages for the youngest intracanyon basalt flow and 10 be surface exposure dating of downstream terrace surfaces suggests widespread channel incision during a prominent outburst flood through the little lake channel at ca 64 ka older basalt flows flanking the upper and lower canyon margins indicate localization of the owens river in its current position between 212 14 and 197 11 ka coupled with terrestrial light detection and ranging lidar and digital topographic measurements of dextral offset the revised little lake chronology indicates average dextral slip rates of at least 0 6 0 7 mm yr and 4 to 10 5 yr despite previous geodetic observations of relatively rapid interseismic strain along the little lake fault we find no evidence for sustained temporal fluctuations in slip rates over multiple earthquake cycles instead our results indicate that accelerated fault loading may be transient over much shorter periods 10 1 yr and perhaps indicative of time dependent seismic hazard associated with eastern california shear zone faults

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

changes in east atlantic deepwater circulation over the last 30 000 years eight time slice reconstructions using 95 epibenthic 13c records eight time slices were reconstructed to trace the distribution of east atlantic deepwater and intermediate water masses over the last 30 000 years our results show that there have been three distinct modes of deepwater circulation near the stage 3 2 boundary the origin of north atlantic deep water nadw was similar to today mode 1 however after late stage 3 the source region of the nadw end member shifted from the norwegian greenland sea to areas south of iceland mode 2 a reduced nadw flow persisted during the last glacial maximum with constant preformed 13c values the nutrient content of nadw increased markedly near the azores fracture zone from north to south probably because of the mixing of upwelled antarctic bottom water aabw from below which then advected with much higher flux rates into the northeast atlantic later the spread of glacial meltwater over the north atlantic led to a marked short term ventilation minimum below 1800 m about 13 500 14c years ago mode 3 the formation of nadw recommenced abruptly north of iceland 12 800 12 500 years ago and reached a volume approaching that of the holocene in the younger dryas 10 800 10 350 years b p another short term shutdown of deepwater formation followed between 10 200 and 9 600 years b p linked to a further major meltwater pulse into the atlantic each renewal of deepwater formation led to a marked release of fossil co2 from the ocean the likely cause of the contemporaneous 14c plateaus over the last 9000 years deepwater circulation varied little from today apart from a slight increase in aabw about 7000 14c years ago it is also shown that the oxygenated mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30 000 years

hydrological impact of heinrich events in the subtropical northeast atlantic reconstructing the impact of heinrich events outside the main belt of ice rafting is crucial to understanding the underlying causes of these abrupt climatic events a high resolution study of a marine sediment core from the iberian margin demonstrates that this midlatitude area was strongly affected both by cooling and advection of low salinity arctic water masses during the last three heinrich events these paleoclimatic time series reveal the internal complexity of each of the last three heinrich events and illustrate the value of parallel studies of the organic and inorganic fractions of the sediments

performance of a second order moments advection scheme in an ocean general circulation model 1 the reliability of ocean general circulation models ogcms strongly depends on the quality of their tracer advection schemes for the sake of simplicity and computing time tracer advection schemes most commonly used in large scale ogcms tend to be low order schemes which suffer from spurious numerical diffusion and dispersion that result in distorted solutions the application of high order schemes would reduce numerical errors but at a considerable cost in terms of computing time an alternative to the use of high order methods is the implementation of algorithms that take into account the sub grid distribution of tracers one such method is the second order moments som scheme of prather 1986 which is more accurate than a fourth order scheme but at the time consumption of a second order algorithm this article presents results from coarse resolution global ocean simulations with very low explicit diapycnal mixing in which active and passive tracers were advected with the som method we compare the performance of the method with that of more traditional schemes namely the fct flux corrected transport and quicker quadratic upstream interpolation for convective kinematics schemes in general the use of the som method significantly improves tracer distributions and transports compared to fct and quicker thus leading to a better representation of ocean currents notably boundary currents and frontal systems while model simulations employing the fct and quicker schemes recreate a global overturning circulation with strong upwelling occurring in low latitudes the som simulations admit a circulation pattern closer to that known as the reconfigured conveyor belt toggweiler and samuels 1993 in which the bulk of the global ocean upwelling occurs in the southern ocean

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

a decomposition of the atlantic meridional overturning circulation into physical components using its sensitivity to vertical diffusivity abstract the sensitivity of the atlantic ocean meridional overturning circulation to the vertical diffusion coefficient in the global coupled atmosphere ocean sea ice model climber 3 is investigated an important feature of the three dimensional ocean model is its low diffusive tracer advection scheme the strength mmax of the atlantic overturning is decomposed into three components 1 the flow ms exported southward at 30 s 2 the large scale upward flow that balances vertical diffusion in the atlantic and 3 a wind dependent upwelling flux wbound along the atlantic boundaries that is not due to vertical diffusion the export of water at 30 s varies only weakly with but is strongly correlated with the strength of the overflow over the greenland scotland ridge the location of deep convection is found to be mixing dependent such that a shift from the nordic seas to the irminger sea is detected for high values of the ratio r ms mmax gives a measure of the interhemispheric overturning efficienc

a century of accumulation and temperature changes in dronning maud land antarctica a mass balance program was initiated in the vestfjella heimefrontfjella area of western dronning maud land during the austral summer of 1988 1989 as a part of this program spatial and temporal variations in snow accumulation and temperature stable isotopes are measured using shallow firn cores in this paper we present surface accumulation data and discuss the climatic implications of the stable isotope records from two shallow firn cores one 30 m deep core obtained about 200 km from the coast at 700 m asl 73 36 s 12 26 w covers the period 1932 1991 the other core was drilled at about 500 km from the coast at 2900 m asl 75 00 s 2 00 e and covers the period 1865 1991 the recent accumulation increase that has been reported from several areas of the antarctic continent is not present in either of these records instead our coastal record suggests a significant decrease in accumulation with the strongest trend from about 1975 there is a positive trend in the oxygen isotope signal in both cores which in the coastal core corresponds to a temperature increase of about 1 8 c since the early 1930s and in the high altitude core to about 0 8 c since 1865 however it is likely that part of this increase in 18o is due to a change of moisture source

the relation of meridional pressure gradients to north atlantic deep water volume transport in an ocean general circulation model we use a coarse resolution ocean general circulation model to study the relation between meridional pressure and density gradients in the southern ocean and north atlantic and the atlantic meridional overturning circulation in several experiments we artificially modify the meridional density gradients by applying different magnitudes of the gent mcwilliams isopycnal eddy diffusion coefficients in the southern ocean and in the north atlantic and investigate the response of the simulated atlantic meridional overturning to such changes the simulations are carried out close to the limit of no diapycnal mixing with a very small explicit vertical diffusivity and a tracer advection scheme with very low implicit diffusivities our results reveal that changes in eddy diffusivities in the north atlantic affect the maximum of the atlantic meridional overturning but not the outflow of north atlantic deep water into the southern ocean in contrast changes in eddy diffusivities in the southern ocean affect both the south atlantic outflow of north atlantic deep water and the maximum of the atlantic meridional overturning results from these experiments are used to investigate the relation between meridional pressure gradients and the components of the atlantic meridional overturning pressure gradients and overturning are found to be linearly related we show that in our simulations zonally averaged deep pressure gradients are very weak between 20 s and about 30 n and that between 30 n and 60 n the zonally averaged pressure grows approximately linearly with latitude this pressure difference balances a westward geostrophic flow at 30 40 n that feeds the southbound deep atlantic western boundary current we extend our analysis to a large variety of experiments in which surface freshwater forcing vertical mixing and winds are modified in all experiments the pycnocline depth assumed to be the relevant vertical scale for the northward volume transport in the atlantic is found to be approximately constant at least within the coarse vertical resolution of the model the model behaviour hence cannot directly be related to conceptual models in which changes in the pycnocline depth determine the strength of atlantic meridional flow and seems conceptually closer to stommel s box model in all our simulations the atlantic overturning seems to be mainly driven by southern ocean westerlies however the actual strength of the atlantic meridional overturning is not determined solely by the southern ocean wind stress but as well by the density pressure gradients created between the deep water formation regions in the north atlantic and the inflow outflow region in the south atlantic