timing and climate forcing of volcanic eruptions for the past 2 500 years volcanic eruptions contribute to climate variability but quantifying these contributions has been limited by inconsistencies in the timing of atmospheric volcanic aerosol loading determined from ice cores and subsequent cooling from climate proxies such as tree rings here we resolve these inconsistencies and show that large eruptions in the tropics and high latitudes were primary drivers of interannual to decadal temperature variability in the northern hemisphere during the past 2 500 years our results are based on new records of atmospheric aerosol loading developed from high resolution multi parameter measurements from an array of greenland and antarctic ice cores as well as distinctive age markers to constrain chronologies overall cooling was proportional to the magnitude of volcanic forcing and persisted for up to ten years after some of the largest eruptive episodes our revised timescale more firmly implicates volcanic eruptions as catalysts in the major sixth century pandemics famines and socioeconomic disruptions in eurasia and mesoamerica while allowing multi millennium quantification of climate response to volcanic forcing

a modified method for detecting incipient bifurcations in a dynamical system 1 we assess the proximity of a system to a bifurcation point using a degenerate fingerprinting method that estimates the declining decay rate of fluctuations in a time series as an indicator of approaching a critical state the method is modified by employing detrended fluctuation analysis dfa which improves the estimation of short term decay especially in climate records which generally possess power law correlations when the modified method is applied to genie 1 model output that simulates collapse of the atlantic thermohaline circulation the bifurcation point is correctly anticipated in greenland ice core paleotemperature data for which the conventional degenerate fingerprinting is not applicable due to the short length of the series the modified method detects the transition from glacial to interglacial conditions the technique could in principle be used to anticipate future bifurcations in the climate system but this will require high resolution time series of the relevant data

simulated decadal oscillations of the atlantic meridional overturning circulation in a cold climate state the significance of the atlantic meridional overturning circulation moc for regional and hemispheric climate change requires a complete understanding using fully coupled climate models here we present a persistent decadal oscillation in a coupled atmosphere ocean general circulation model while the present study is limited by the lack of comparisons with paleo proxy records the purpose is to reveal a new theoretically interesting solution found in the fully coupled climate model the model exhibits two multi century long stable states with one dominated by decadal moc oscillations the oscillations involve an interaction between anomalous advective transport of salt and surface density in the north atlantic subpolar gyre their time scale is fundamentally determined by the advection in addition there is a link between the moc oscillations and north atlantic oscillation nao like sea level pressure anomalies the analysis suggests an interaction between the nao and an anomalous subpolar gyre circulation in which sea ice near and south of the labrador sea plays an important role in generating a large local thermal anomaly and a meridional temperature gradient the latter induces a positive feedback via synoptic eddy activity in the atmosphere in addition the oscillation only appears when the nordic sea is completely covered by sea ice in winter and deep convection is active only near the irminger sea such conditions are provided by a substantially colder north atlantic climate than today

sea ice cover sea surface salinity and halo thermocline structure of the northwest north atlantic modern versus full glacial conditions abstract seasonal sea ice develops along the eastern continental margins in the northern north atlantic where freshwater and or meltwater outflow are responsible for relatively low salinity in surface waters and very pronounced water mass stratification sea ice constitutes a major parameter in the marine ecosystem since the duration and extent of its seasonal spreading constrain the plankton distribution and the related microfossil assemblages on the sea floor organic walled dinoflagellate cysts that are highly resistant to dissolution were recovered from surface sediments of the northern north atlantic and used to develop transfer functions best analogue method for the reconstruction of the seasonal spreading and duration of sea ice cover in addition to salinity and temperature of the warmest month of the year application of the best analogue approach to cores from the labrador sea reveals large variations in sea ice cover and sea surface conditions throughout the last glacial stage and during the early holocene isotopic analyses in epipelagic and mesopelagic planktonic foraminifers also suggest important changes in salinity and temperature gradients between the surface and sub surface water masses specific study of the last glacial maximum lgm time slice 16 20 ka on a 14 c time scale in the northwestern north atlantic shows much more extensive sea ice than at present with perennial sea ice lying along the continental margins of eastern canada seasonal spreading of the cover of sea ice offshore was accompanied by large seasonal contrasts in temperature with very cold winters but relatively warm summers a pattern linked to strong stratification between a buoyant low saline surface layer having a low thermal inertia and the underlying intermediate oceanic waters

influence of a thin veneer of low hydraulic conductivity sediment on modelled exchange between river water and groundwater in response to induced infiltration a thin layer of fine grained sediment commonly is deposited at the sediment water interface of streams and rivers during low flow conditions and may hinder exchange at the sediment water interface similar to that observed at many riverbank filtration rbf sites results from a numerical groundwater flow model indicate that a low permeability veneer reduces the contribution of river water to a pumping well in a riparian aquifer to various degrees depending on simulated hydraulic gradients hydrogeological properties and pumping conditions seepage of river water is reduced by 5 10 when a 2 cm thick low permeability veneer is present on the bed surface increasing thickness of the low permeability layer to 0 1 m has little effect on distribution of seepage or percentage contribution from the river to the pumping well a three orders of magnitude reduction in hydraulic conductivity of the veneer is required to reduce seepage from the river to the extent typically associated with clogging at rbf sites this degree of reduction is much larger than field measured values that were on the order of a factor of 20 25 over 90 of seepage occurs within 12 m of the shoreline closest to the pumping well for most simulations virtually no seepage occurs through the thalweg near the shoreline opposite the pumping well although no low permeability sediment was simulated for the thalweg these results are relevant to natural settings that favour formation of a substantial low permeability sediment veneer as well as central pivot irrigation systems and municipal water supplies where river seepage is induced via pumping wells published in 2011 by john wiley sons ltd

spatial analysis of early warning signals for a north atlantic climate transition in a coupled gcm the climate system can potentially switch from one stable state to another the closer a system is to a bifurcation point i e tipping point the more likely it is that even small perturbations can force the system to experience a state shift e g a collapsing atlantic meridional overturning circulation amoc and associated cooling in parts of the north atlantic here we present an abrupt state transition from a warm to a cold north atlantic climate state with expanded sea ice during an orbitally forced transient holocene simulation performed with the community climate system model version 3 the state transition is associated with a weakening of the amoc by about 33 in this simulation the changing background climate induced by slow external orbital forcing plays an important role for the abrupt climate shift the model allows the identification of regions and variables that play a key role for a potential climate transition and show early warning signals increase in autocorrelation and standard deviation as well as trends in skewness especially for sea surface salinity in the northern north atlantic are identified as robust early warning signals whereas no early warning signals are found in the time series of the amoc stream function

effect of surficial disturbance on exchange between groundwater and surface water in nearshore margins 1 low permeability sediments situated at or near the sediment water interface can influence seepage in nearshore margins particularly where wave energy or currents are minimal seepage meters were used to quantify flow across the sediment water interface at two lakes where flow was from surface water to groundwater disturbance of the sediment bed substantially increased seepage through the sandy sediments of both lakes seepage increased by factors of 2 6 to 7 7 following bed disturbance at seven of eight measurement locations at mirror lake new hampshire where the sediment representing the greatest restriction to flow was situated at the sediment water interface although the veneer of low permeability sediment was very thin and easily disturbed accumulation on the bed surface was aided by a physical setting that minimized wind generated waves and current at lake belle taine minnesota where pre disturbance downward seepage was smaller than at mirror lake but hydraulic gradients were very large disturbance of a 20 to 30 cm thick medium sand layer resulted in increases in seepage of 2 to 3 orders of magnitude exceptionally large seepage rates some exceeding 25 000 cm d were recorded following bed disturbance since it is common practice to walk on the bed while installing or making seepage measurements disruption of natural seepage rates may be a common occurrence in nearshore seepage studies disturbance of the bed should be avoided or minimized when utilizing seepage meters in shallow nearshore settings particularly where waves or currents are infrequent or minimal

pseudotachylites in the eastern peninsular ranges of california abstract a continuous zone containing pseudotachylites in the eastern peninsular ranges of california extends over more than 15 km from deep canyon in the north to at least toro canyon in the south pseudotachylites are found in rocks of tonalitic to dioritic composition while the overall compositions of host rock and pseudotachylite veins are similar veins are characteristically enriched in fe and ti and depleted in si many veins are cataclastic and all contain fragments but a large number have a groundmass with skeletal and spherulitic microlites of calcic plagioclase an 40 50 biotite and ilmenite indicative of partial melting the pseudotachylites formed mainly during a late brittle event 56 62 ma and postdate the high strain ductile mylonitic deformation in the santa rosa mylonite zone 65 87 ma as ascertained by 40ar 39ar ages the size of veins ranges from less than 1 mm to more than 10 cm in thickness and aspect ratios display a broad log normal distribution three quarters of the veins are more or less parallel to the foliation the orientation of the veins is consistent with a top to the west displacement similar to that for the ductile mylonites from the sizes energies required for melting are estimated to range between 105 j for smaller veins and 1010 j for larger veins interestingly the energy distribution and geometry of these pseudotachylites correspond closely with energy distributions of current microseismic events along the san andreas fault at parkfield suggesting that basic mechanisms may be similar and due to intrinsic mechanical properties of rocks in particular these indicate that local stress concentrations rather than average crustal stresses are relevant for earthquakes

mechanisms of abrupt climate change of the last glacial period 1 more than a decade ago ice core records from greenland revealed that the last glacial period was characterized by abrupt climate changes that recurred on millennial time scales since their discovery there has been a large effort to determine whether these climate events were a global phenomenon or were just confined to the north atlantic region and also to reveal the mechanisms that were responsible for them in this paper we review the available paleoclimate observations of abrupt change during the last glacial period in order to place constraints on possible mechanisms three different mechanisms are then reviewed ocean thermohaline circulation sea ice feedbacks and tropical processes each mechanism is tested for its ability to explain the key features of the observations particularly with regard to the abruptness millennial recurrence and geographical extent of the observed changes it is found that each of these mechanisms has explanatory strengths and weaknesses and key areas in which progress could be made in improving the understanding of their long term behavior both from observational and modeling approaches are suggested finally it is proposed that a complete understanding of the mechanisms of abrupt change requires inclusion of processes at both low and high latitudes as well as the potential for feedbacks between them some suggestions for experimental approaches to test for such feedbacks with coupled climate models are given

unsaturated zone wedge beneath a large natural lake lake belle taine 480 ha in north central minnesota receives on average 76 000 m3 d 1 of water from little sand creek but has no outlet water seeps out of the lake flows through glacial outwash and discharges into three nearby lakes with stages 13 14 m lower than belle taine seepage meter data indicate water is seeping out of belle taine at velocities up to 263 cm d 1 hydraulic head measurements made in the lake bed indicate the sediments are unsaturated beneath a portion of the lake edge and a wedge of unsaturated sediments extends beneath the lake bed as much as 20 m from the shoreline at the shoreline the water table is as much as 6 7 m below the lake surface modeling results of a similar hypothetical setting indicate that the horizontal extent of an unsaturated zone beneath a lake depends on 1 the permeability contrast between the outwash and the lake bed 2 anisotropy 3 lake bed slope and 4 thickness of the lower permeability lake bed sediments although rarely documented unsaturated sediments beneath a lake may not be extremely uncommon similar much smaller unsaturated zone areas also have been observed beneath two other lakes in minnesota

analysis of pumping induced unsaturated regions beneath a perennial river 1 the presence of an unsaturated region beneath a streambed during groundwater pumping near streams can reduce the pumping capacity change flow paths and alter the types of biological transformations in the streambed sediments a three dimensional multiphase flow model of two horizontal collector wells along the russian river near forestville california was developed to investigate the impact of varying the ratio of the aquifer to streambed permeability on 1 the formation of an unsaturated region beneath the stream 2 the pumping capacity 3 stream water fluxes through the streambed and 4 stream water traveltimes to the collector wells the aquifer to streambed permeability ratio at which the unsaturated region was initially observed ranged from 10 to 100 the size of the unsaturated region beneath the streambed increased as the aquifer to streambed permeability ratio increased the simulations also indicated that for a particular aquifer permeability decreasing the streambed permeability by only a factor of 2 3 from the permeability where desaturation initially occurred resulted in reducing the pumping capacity in some cases the stream water fluxes increased as the streambed permeability decreased however the stream water residence times increased and the fraction of stream water that reached that the wells decreased as the streambed permeability decreased indicating that a higher streambed flux does not necessarily correlate to greater recharge of stream water around the wells

hydrologic response of the crow wing watershed minnesota to mid holocene climate change in this study we have integrated a suite of holocene paleoclimatic proxies with mathematical modeling in an attempt to obtain a comprehensive picture of how watersheds respond to past climate change a three dimensional surface water groundwater model was developed to assess the effects of mid holocene climate change on water resources within the crow wing watershed upper mississippi basin in north central minnesota the model was first calibrated to a 50 yr historical record of average annual surface water discharge monthly ground water levels and lake level fluctuations the model was able to reproduce reasonably well long term historical records 1949 1999 of water table and lake level fluctuations across the watershed as well as stream discharge near the watershed outlet the calibrated model was then used to reproduce paleo groundwater and lake levels using climate reconstructions based on pollen transfer functions from williams lake just outside the watershed computed declines in mid holocene lake levels for two lakes at opposite ends of the watershed were between 6 and 18 m simulated streamflow near the outlet of the watershed decreased to 70 of modern average annual discharge after 200 yr the area covered by wetlands for the entire watershed was reduced by 16 the mid holocene hydrologic changes indicated by these model results and corroborated by several lake core records across the crow wing watershed may serve as a useful proxy of the hydrologic response to future warm dry climatic forecasts ca 2050 made by some atmospheric general circulation models for the glaciated midwestern united states

can north atlantic sea ice anomalies account for dansgaard oeschger climate signals abstract north atlantic sea ice anomalies are thought to play an important role in the abrupt dansgaard oeschger d o cycles of the last glacial period this model study investigates the impacts of changes in north atlantic sea ice extent in glacial climates to help provide geographical constraints on their involvement in d o cycles based on a coupled climate model simulation of the last glacial maximum 21 ka the nordic seas and western north atlantic broadly south of greenland are identified as two plausible regions for large and persistent displacements of the sea ice edge in the glacial north atlantic sea ice retreat scenarios targeting these regions are designed to represent ice cover changes associated with the cold to warm stadial to interstadial transitions of d o cycles the atmospheric responses to sea ice retreat in the nordic seas and in the western north atlantic are tested individually and together using an atmospheric general circulation model the nordic seas ice retreat causes 1

role of a groundwater lake interface in controlling seepage of water and nitrate summary the effects of the hydraulic characteristics of a groundwater lake interface on discharge and nitrate loading to a lake were investigated the interface is defined as the zone separating the adjacent aquifer 10 s of m and the lake bed 10 s of cm itself the study combines field data using several tracers water oxygen isotopes and nitrate and numerical modeling the hydraulic head distribution a nitrate plume and seepage rates were observed over a two year period along a 100 m long transect reaching from an agricultural field into the lake the groundwater lake interface system was simulated with a 2d steady state flow and nitrate transport model feflow the observations showed that discharge to the lake was doubled peaked with a peak discharge near the shore line followed by an almost classical exponential decrease and a second peak further off shore the nitrate plume also extended 60 80 m off shore by calibrating the model to measured discharge and the outline of the nitrate plume it was demonstrated that 1 the ratio of horizontal to vertical hydraulic conductivity anisotropy was very important and on the order of 50 and 2 the lake bed acted as a hydraulic barrier by having a much lower hydraulic conductivity than that of the relatively homogeneous aquifer we suggest that the barrier is formed by an extensive plant cover that can trap finer materials and produce a surface colmation layer the simulation results show that when a barrier is present the total groundwater discharge to the lake can be up to a factor of two lower and that approximately 50 of the nitrate bypasses the barrier this proportion of the nitrate loading will therefore also bypass the plant cover and discharge directly to the lake off shore potentially leading to algal blooms under n limited conditions in the lake water column

millennial variability in an idealized ocean model predicting the amoc regime shifts abstracta salient feature of paleorecords of the last glacial interval in the north atlantic is pronounced millennial variability commonly known as dansgaard oeschger events it is believed that these events are related to variations in the atlantic meridional overturning circulation and heat transport here the authors formulate a new low order model based on the howard malkus loop representation of ocean circulation capable of reproducing millennial variability and its chaotic dynamics realistically it is shown that even in this chaotic model changes in the state of the meridional overturning circulation are predictable accordingly the authors define two predictive indices which give accurate predictions for the time the circulation should remain in the on phase and then stay in the subsequent off phase these indices depend mainly on ocean stratification and describe the linear growth of small perturbations in the system thus monitoring particular indices of the ocean state could help predict

stochastic atmospheric forcing as a cause of greenland climate transitions abstractan unforced simulation of the community climate system model version 4 ccsm4 is found to have greenland warming and cooling events that resemble dansgaard oeschger cycles in pattern and magnitude with the caveat that only three transitions were available to be analyzed it is found that the transitions are triggered by stochastic atmospheric forcing the atmospheric anomalies change the strength of the subpolar gyre leading to a change in labrador sea sea ice concentration and meridional heat transport the changed climate state is maintained over centuries through the feedback between sea ice and sea level pressure in the north atlantic indications that the initial atmospheric pressure anomalies are preceded by precipitation anomalies in the western pacific warm pool are discussed the full evolution of the anomalous climate state depends crucially on the climatic background state

tipping points early warning and wishful thinking 1 the causes for and possible predictions of rapid climate changes are poorly understood the most pronounced changes observed beside the glacial terminations are the dansgaard oeschger events present day general circulation climate models simulating glacial conditions are not capable of reproducing these rapid shifts it is thus not known if they are due to bifurcations in the structural stability of the climate or if they are induced by stochastic fluctuations by analyzing a high resolution ice core record we exclude the bifurcation scenario which strongly suggests that they are noise induced and thus have very limited predictability

variability of sea ice conditions in the fram strait over the past 30 000 years sea ice is a critical component of the climate and oceanographic system in the north atlantic ocean a biomarker record reveals millennial scale and glacial interglacial fluctuations in sea ice coverage in the northernmost atlantic ocean over the past 30 000 years

amplification of holocene multicentennial climate forcing by mode transitions in north atlantic overturning circulation 1 using a three dimensional global climate model we show that mode transitions in north atlantic deep water production can provide an amplifying mechanism of relatively weak climate perturbations during the holocene under pre industrial boundary conditions a freshwater forcing in the labrador sea pushes the north atlantic overturning circulation into a deterministically bistable regime characterized by stochastic on and off switches in labrador sea convection on a multicentennial time scale these stochastic mode transitions can be phaselocked by a small subthreshold periodic freshwater forcing the local small periodic forcing is effectively amplified with the assistance of noise to have a large scale impact on north atlantic overturning circulation and climate these results suggest a stochastic resonance mechanism that can operate under holocene boundary conditions and indicate that changes in the threedimensional configuration of north atlantic deep water formation can be an important component of multicentennial climate variability during interglacials citation jongma j i m prange h renssen and m schulz 2007 amplification of holocene multicentennial climate forcing by mode transitions in north atlantic overturning circulation geophys res lett 34 l15706 doi 10 1029 2007gl030642

instability of the atlantic overturning circulation during marine isotope stage 3 variations in the strength of the atlantic meridional overturning circulation amoc were involved in the occurrences of dansgaard oeschger d o events during marine isotope stage 3 mis3 the stability of the amoc to north atlantic freshwater perturbations is studied using a comprehensive climate model under mis3 boundary conditions an amoc stability diagram constructed from a series of equilibrium freshwater perturbation experiments reveals a highly nonlinear dependence of amoc strength on freshwater forcing the mis3 baseline state is remarkably unstable with respect to minor perturbations the global climate signal associated with a change in amoc strength is consistent with a transition from an interstadial to a stadial state including an annual mean surface air temperature drop of 8 k in central greenland we suggest that minor freshwater perturbations in the hydrologic cycle e g related to ice sheet processes had the potential to trigger d o type climate shifts associated with a threshold in the atmosphere ocean system

mid to late holocene climate change an overview the last 6000 years are of particular interest to the understanding of the earth system because the boundary conditions of the climate system did not change dramatically in comparison to larger glacial interglacial changes and because abundant detailed regional palaeoclimatic proxy records cover this period we use selected proxy based reconstructions of different climate variables together with state of the art time series of natural forcings orbital variations solar activity variations large tropical volcanic eruptions land cover and greenhouse gases underpinned by results from general circulation models gcms and earth system models of intermediate complexity emics to establish a comprehensive explanatory framework for climate changes from the mid holocene mh to pre industrial time the redistribution of solar energy due to orbital forcing on a millennial timescale was the cause of a progressive southward shift of the northern hemisphere nh summer position of the intertropical convergence zone itcz this was accompanied by a pronounced weakening of the monsoon systems in africa and asia and increasing dryness and desertification on both continents the associated summertime cooling of the nh combined with changing temperature gradients in the world oceans likely led to an increasing amplitude of the el nin o southern oscillation enso and possibly increasingly negative north atlantic oscillation nao indices up to the beginning of the last millennium on decadal to multi century timescales a worldwide coincidence between solar irradiance minima tropical volcanic eruptions and decadal to multi century scale cooling events was not found however reconstructions show that widespread decadal to multi century scale cooling events accompanied by advances of mountain glaciers occurred in the nh e g in scandinavia and the european alps this occurred namely during the little ice age lia between ad w1350 and 1850 when the lower summer insolation in the nh due to orbital forcing coincided with solar activity minima and several strong tropical volcanic eruptions the role of orbital forcing in the nh cooling the southward itcz shift and the desertifi cation of the sahara are supported by numerous model simulations other simulations have suggested that the fingerprint of solar activity variations should be strongest in the tropics but there is also evidence that changes in the ocean heat transport took place during the lia at high northern latitudes with possible additional implications for climates of the southern hemisphere sh

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