on the stability of the atlantic meridional overturning circulation during the last deglaciation using a generalized stability indicator l we explore the stability of the atlantic meridional overturning circulation amoc during the last deglaciation based on a paleoclimate simulation from the last glacial maximum as forced by various external climate forcings notably the meltwater forcing the amoc experiences a collapse and a subsequent rapid recovery in the early stage of deglaciation this change of the amoc induces an anomalous freshwater divergence and later convergence across the atlantic and therefore leads to a positive l suggesting a negative basin scale salinity advection feedback and in turn a mono stable deglacial amoc further analyses show that most anomalous freshwater is induced by the amoc via the southern boundary of the atlantic at 34 s where the freshwater transport m ovs is about equally controlled by the upper branch of the amoc and the upper ocean salinity along 34 s from 19 to 17 ka as a result of multiple climate feedbacks associated with the amoc change the upper ocean at 34 s is largely salinified which helps to induce a switch in m ovs from import to export our study has important implications to the deglacial simulations by climate models a decomposition of l shows that the amoc stability is mostly determined by two terms the salinity stratification at 34 s and the change of stratification with the amoc both terms appear positive in model however the former is likely to be distorted towards positive as associated with a common bias existing over the south atlantic in climate models therefore the amoc is potentially biased towards mono stability in most paleoclimate simulations

frequency and magnitude biases in the fryberger model with implications for characterizing geomorphically effective winds abstract the fryberger model uses standardized wind data to estimate regional aeolian sediment drift potential dp which allows for interpretation and classification of aeolian dune landscapes this paper identifies systematic frequency and magnitude biases in the model that result from i variations in wind direction sector range which affects frequency bin size for determining percent occurrence of transporting winds and ii use of wind speed class mid point values over more statistically representative values e g mean median or minimally classified whole knot wind speeds in the magnitude weighting factor for dp calculations the significance of these biases is tested using unclassified data from stanhope prince edward island canada frequency bias resulting from wind data aggregation into direction sectors of varying size produces statistically significant discrepancies in dp and rdp estimates for this dataset when 36 point 10 s of degree data are reduced to 16 sectors cardinal directions receive one extra direction class and the magnitude of dp and rdp values decreases by 1 4 and 2 8 respectively compared to 16 equal 22 5 sectors derived from original data this results from under representation of intermediate direction classes for this dataset the effects on rdd less than 1 and directional variability ratio rdp dp are insignificant magnitude biases result from the use of the wind speed class mid point values instead of other statistically representative measures of wind speed in dp calculations in that wind speed distributions are often positively skewed mid point values yield an over estimate of total dp by as much as 34 and rdp by up to 22 for this dataset over those derived using mean median and whole knot minimally classified values this difference is statistically significantly at both the individual wind speed direction category level and at the aggregate directionally summed level and causes rdp vectors to shift 3 5 to the south though the impacts of these frequency magnitude biases are site specific and modest the effects on rdp estimation may be more significant in complex i e multi modal wind regimes and or in environments with more frequent high magnitude winds from the cardinal directions given the increased availability of more precise unclassified wind data most of these systematic frequency magnitude biases can be avoided recommendations on reducing inaccuracies imposed by these biases include i using to the degree wind data where available and categorizing into 16 equal 22 5 direction sectors ii using either wind speed class statistical mean values or minimally classified whole knot values in dp calculations and iii recognizing that converting 36 point 10 s of degrees data to 16 direction classes may introduce a frequency bias toward the cardinal directions and will cause inaccuracies in dp and rdp estimates of an amount that depends on the wind regime further research is needed to assess the implications of such biases in different wind regimes and into the influence of localized supply and transport limiting factors on regional scale assessments of dune morphodynamics and mobility these findings are also relevant for other applications that use categorized wind speed direction data such as dust or contaminant plume dispersion modelling

aeolian abrasion and modes of fine particle production from natural red dune sands an experimental study a series of experiments was conducted to determine the potential for aeolian abrasion of natural dune sands to produce fine particles 125 m by 1 the release of resident fines 2 spalling chipping and breakage of particles and 3 the removal of grain surface coatings parent samples were obtained from the surfaces of four active continental dunes and abraded using a glass test tube chamber for up to 120h the fine particles produced by this abrasion process were trapped at varying time intervals and subject to detailed particle size analyses using a coulter multisizer the abrasion of untreated parent samples produced fine particles in one of two main size classes 50 m but when the parent sample was sieved to exclude particles 250 m relatively more material in the range 10 50 m was produced for unsieved parent samples the size range associated with the dominant mode varied according to the length of the abrasion time the coarsest mode 63 m was dominant during the first 16 h of abrasion then became less significant and is thought to be associated with the release of resident fines into suspension the finest mode 10 m was absent or very weak during the first 16h of abrasion then became more significant and in some instances dominated the distribution as abrasion continued removal of grain surface coatings is the main source of fine material 10 m and this may be a significant source of fine material in areas where sands are dominated by subrounded and rounded particles by comparison with previous studies of aeolian particle abrasion these natural dune sands produced very low quantities of fine material by weight but their spatial extent makes them potentially a significant source of dust sized particles at the global scale

sensitivity of the atlantic meridional overturning circulation to south atlantic freshwater anomalies the sensitivity of the atlantic meridional overturning circulation amoc to changes in basin integrated net evaporation is highly dependent on the zonal salinity contrast at the southern border of the atlantic biases in the freshwater budget strongly affect the stability of the amoc in numerical models the impact of these biases is investigated by adding local anomaly patterns in the south atlantic to the freshwater fluxes at the surface these anomalies impact the freshwater and salt transport by the different components of the ocean circulation in particular the basin scale salt advection feedback completely changing the response of the amoc to arbitrary perturbations it is found that an appropriate dipole anomaly pattern at the southern border of the atlantic ocean can collapse the amoc entirely even without a further hosing the results suggest a new view on the stability of the amoc controlled by processes in the south atlantic

deep ocean early warning signals of an atlantic moc collapse a future collapse of the atlantic meridional overturning circulation moc has been identified as one of the most dangerous tipping points in the climate system it is therefore crucial to develop early warning indicators for such a potential collapse based on relatively short time series so far attempts to use indicators based on critical slowdown have been marginally successful based on complex climate network reconstruction we here present a promising new indicator for the moc collapse that efficiently monitors spatial changes in deep ocean circulation through our analysis of the performance of this indicator we formulate optimal locations of measurement of the moc to provide early warning signals of a collapse our results imply that an increase in spatial resolution of the atlantic moc observations i e at more sections can improve early detection because the spatial coherence in the deep ocean arising near the transition is better captured

seasonal variability in the origin of recent atmospheric mineral dust at northgrip greenland abstract isotopic sr and nd and mineralogical analyses of dust extracted from snow deposits from the last decade at the northgrip greenland ice camp 75 1 n 042 3 w confirm the eastern asian source for greenland dust and extend it to the present day the 1998 and 1999 samples which represent a 2 4 month resolution of dust deposition show that there are at least two east asian sources for the greenland dust that vary seasonally the provenance of dust during the major spring summer deposition is the takla makan desert of northwest china

an indicator of the multiple equilibria regime of the atlantic meridional overturning circulation recent model results have suggested that there may be a scalar indicator s monitoring whether the atlantic meridional overturning circulation moc is in a multiple equilibrium regime the quantity s is based on the net freshwater transport by the moc into the atlantic basin it changes sign as soon as the steady atlantic moc enters the multiple equilibrium regime because of an increased freshwater input in the northern north atlantic this paper addresses the issue of why the sign of s is such a good indicator for the multiple equilibrium regime changes in the atlantic freshwater budget over a complete bifurcation diagram and in finite amplitude perturbation experiments are analyzed in a global ocean circulation model the authors show that the net anomalous freshwater transport into or out of the atlantic resulting from the interactions of the velocity perturbations and salinity background field is coupled to the background steady state state freshwater budget and hence to s the sign of s precisely shows whether this net anomalous freshwater transport is stabilizing or destabilizing the moc therefore it can indicate whether the moc is in a single or multiple equilibrium regime

the stability of the moc as diagnosed from model projections for pre industrial present and future climates the stability of the atlantic meridional overturning circulation moc is investigated for various climate scenario runs using data from the cmip3 archive of coupled atmosphere ocean models apart from atmospheric feedbacks the sign of the salt flux into the atlantic basin that is carried by the moc determines whether the moc is in the single or multiple equilibria regime this salt advection feedback is analyzed by diagnosing the freshwater and salt budgets for the combined atlantic and arctic basins consistent with the finding that almost all coupled climate models recover from hosing experiments it is found that most models feature a negative salt advection feedback in their pre industrial climate freshwater perturbations are damped by this feedback excluding the existence of a stable off state for the moc all models feature enhanced evaporation over the atlantic basin in future climates but for a moderate increase in radiative forcing b1 and 2 co2 scenarios there is a decrease of the fresh water flux carried by the moc into the atlantic the deficit is made up by increased fresh water transport by the gyre circulation in this forcing regime the salt advection feedback becomes less negative for three models from an ensemble of eight it is positive in a 2 co2 climate while two models feature a positive feedback in the pre industrial climate for even warmer climates a1b equilibrium and 4 co2 the salt feedback becomes more negative damping again it is shown that the decrease in northward fresh water transport at 34 s by the moc in b1 equilibrium and 2 co2 is due to a reduction of the inflow of intermediate waters relative to thermocline waters associated with a robust shoaling of the moc in future warmer climates in a1b and 4 co2 climates northward freshwater transport increases again the moc keeps shoaling but both intermediate and thermocline water masses freshen

control of dust emissions by geomorphic conditions wind environments and land use in northern china an examination based on dust storm frequency from 1960 to 2003 the degree to which dust emissions are controlled by geomorphic conditions wind environments and land use was investigated using the dust storm frequency dsf and data from more than 300 meteorological stations throughout northern china our analysis showed that most dust emissions originated in gobi deserts that developed in piedmont alluvial fans of the kunlun qilian and helan mountains dust emissions are low from other gobi desert regions such as the northern gurbantunggut and eastern taklimakan where high vegetation coverage restrained dust emissions or where dust size particles are not abundant after a long period of strong wind erosion sandy deserts with relatively high vegetation coverage or an extensive cover by mobile sands are not a major dust source although the highest dust emissions did not appear in regions with the highest wind energy dsf trends in each region from 1960 to 2003 were closely related to local wind activity dsf was low in regions with high levels of human activity where the mean dsf from 1960 to 2003 did not exceed 4 days year even from the 1960s to the early 1970s the period with the greatest dsf frequency did not exceed 8 days year which indicates that extensive land use did not contribute to dsf the low dsf in these areas might result from the fact that although land use could produce abundant fine soil fractions vegetation coverage and soil moisture remained higher than in the gobi deserts of arid china thereby decreasing dust storm occurrence

the thermohaline circulation and the control of ice ages abstract it is found that the global salinity variations associated with the thermohaline circulation may have a tendency to make the circulation increasingly asymmetric with respect to the equator as a consequence the salinity difference between the pacific and the atlantic ocean may be slowly increasing such a process could have a time scale long enough to be comparable with the time span between major glaciations a speculative glaciation cycle is proposed which involves the above mentioned property of the thermohaline circulation in this cycle the role of a northern hemisphere glaciation is to bring excess freshwater from the pacific to the atlantic

the stability of an evolving atlantic meridional overturning circulation 1 in this study we propose a generalized stability indicator l for a slowly evolving and quasi steady atlantic meridional overturning circulation amoc which represents a feedback related to the amoc and its associated freshwater transport within the atlantic basin as an improvement from previous indicators for the amoc in equilibrium this generalized indicator does not require a divergence free freshwater transport in the atlantic for a collapsed amoc which enables it to correctly monitor the amoc stability through the amoc hysteresis loop in the coupled atmosphere ocean general circulation models from the simulation the indicator l suggests that the amoc is in a stable regime with single equilibrium under the present day and the last glacial maximum lgm climates however under the present day climate a bering strait bs closure will diminish the freshwater outflow from the north atlantic into the arctic as the amoc collapses resulting in a freshwater convergence in the atlantic basin and making the amoc reside in a stable collapsed state i e the amoc exhibits characteristics of multiple equilibria further analysis shows that the bs effect is much reduced under the lgm climate this generalized indicator l has great implications for paleoclimate studies in understanding the abrupt climate change due to the instability of the amoc

thermohaline circulation hysteresis a model intercomparison we present results from an intercomparison of 11 different climate models of intermediate complexity in which the north atlantic ocean was subjected to slowly varying changes in freshwater input all models show a characteristic hysteresis response of the thermohaline circulation to the freshwater forcing which can be explained by stommel s salt advection feedback the width of the hysteresis curves varies between 0 2 and 0 5 sv in the models major differences are found in the location of present day climate on the hysteresis diagram in seven of the models present day climate for standard parameter choices is found in the bi stable regime in four models this climate is in the mono stable regime the proximity of the present day climate to the stommel bifurcation point beyond which north atlantic deep water formation cannot be sustained varies from less than 0 1 sv to over 0 5 sv

what is the thermohaline circulation the term thermohaline circulation is widely used in the context of past and future climate change in his perspective wunsch argues that there are many different and inconsistent definitions for this term he defines ocean circulation in terms of mass fluxes which are largely driven by surface forces such as wind and tides and argues that thermohaline circulation should only refer to the separate transport of heat and salinity in the ocean

a diagnostic indicator of the stability of the atlantic meridional overturning circulation in ccsm3 a diagnostic indicator dmov is proposed in this paper to monitor the stability of the atlantic meridional overturning circulation amoc the dmov is a diagnostic for a basinwide salt advection feedback and defined as the difference between the freshwater transport induced by the amoc across the southern border of the atlantic ocean and the overturning liquid freshwater transport from the arctic ocean to the north atlantic as validated in the community climate system model version 3 ccsm3 for an amoc in the conveyor state a positive dmov freshwater convergence in the atlantic basin indicates a monostable amoc and a negative dmov freshwater divergence indicates a bistable amoc based on dmov the authors investigate the amoc stability in the last glacial maximum lgm and analyze the modulation of the amoc stability by an open closed bering strait moreover the authors estimate that the real amoc is likely to be bistable in the present day since some observations suggest a negative dmov freshwater divergence is currently in the atlantic basin however this estimation is very sensitive to the choice of the observational data

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

fingerprints for early detection of changes in the amoc abstractdifferent strategies have been proposed in previous studies for monitoring the atlantic meridional overturning circulation amoc as well as arrays to directly monitor the amoc strength v

multimodel analysis on the response of the amoc under an increase of radiative forcing and its symmetrical reversal the response of the atlantic meridional overturning circulation amoc to an increase of radiative forcing ramp up and a subsequent reversal of radiative forcing ramp down is analyzed for four different global climate models due to changes in ocean temperature and hydrological cycle all models show a weakening of the amoc during the ramp up phase once the external forcing is reversed the results become model dependent for ipsl cm5a lr the amoc continues its weakening trend for most of the ramp down experiment for hadgem2 es the amoc trend reverses once the external forcing also reverses without recovering its initial value for ec earth and mpi esm lr the recovery is anomalously strong yielding an amoc overshoot a robust linear dependency can be established between amoc and density difference between north atlantic na deep water formation region and south atlantic sa in particular amoc evolution is primarily controlled by a meridional salinity contrast between these regions during the warming scenario the subtropical atlantic becomes saltier while the na experiences a net freshening which favours an amoc weakening the different behaviour in the models during the ramp down is dependent on the response of the ocean at the boundaries of na and sa the way in which the positive salinity anomaly stored in the subtropical atlantic during the ramp up is subsequently released elsewhere characterizes the recovery an out of phase response of the salinity transport at 48 circ hbox n and 34 circ hbox s boundaries is able to control the meridional density contrast between na and sa during the transient experiments such a non synchronized response is mainly controlled by changes in gyre salinity transport rather than by changes in overturning transport thus suggesting a small role of the salt advection feedback in the evolution of the amoc

response of the south atlantic circulation to an abrupt collapse of the atlantic meridional overturning circulation the south atlantic response to a collapse of the north atlantic meridional overturning circulation amoc is investigated in the echam5 mpi om climate model a reduced agulhas leakage about 3 1 sv 1 sv 106 m3 s 1 is found to be associated with a weaker southern hemisphere sh supergyre and indonesian throughflow these changes are due to reduced wind stress curl over the sh supergyre associated with a weaker hadley circulation and a weaker sh subtropical jet the northward cross equatorial transport of thermocline and intermediate waters is much more strongly reduced than agulhas leakage in relation with an amoc collapse a cross equatorial gyre develops due to an anomalous wind stress curl over the tropics that results from the anomalous sea surface temperature gradient associated with reduced ocean heat transport this cross equatorial gyre completely blocks the transport of thermocline waters from the south to the north atlantic the waters originating from agulhas leakage flow somewhat deeper and most of it recirculates in the south atlantic subtropical gyre leading to a gyre intensification this intensification is consistent with the anomalous surface cooling over the south atlantic most changes in south atlantic circulation due to global warming featuring a reduced amoc are qualitatively similar to the response to an amoc collapse but smaller in amplitude however the increased northward cross equatorial transport of intermediate water relative to thermocline water is a strong fingerprint of an amoc collapse

does net e p set a preference for north atlantic sinking abstractthe present day global meridional overturning circulation moc with formation of north atlantic deep water nadw and the absence of a deep water formation in the north pacific is often considered to be caused by the fact that the north pacific basin is a net precipitative while the north atlantic is a net evaporative basin in this paper the authors study the effect of asymmetries in continent geometry and freshwater fluxes on the moc both in an idealized two dimensional model and in a global ocean model this study approaches the problem from a multiple equilibria perspective where asymmetries in external factors constrain the existence of steady moc patterns both this multiple equilibria perspective and the fact that a realistic global geometry is used add new aspects to the problem in the global model it is shown that the atlantic forced by net precipitation can have a meridional overturning circulation with northern sinking and a sea surface salinity that resembles the present day sali

meridional overturning circulation transport variability at 34 5 s during 2009 2017 baroclinic and barotropic flows and the dueling influence of the boundaries

holocene shorelines and lake evolution in juyanze basin southern mongolian plateau revealed by luminescence dating holocene environment change in the gobi desert of mongolia and northwestern china is of global significance as it is one of the key dust source areas of the world the juyanze basin located in the central gobi desert of southern mongolian plateau is one of the three terminal basins of the heihe river that flows north from the qilian mountain a series of geomorphic shorelines at different elevations around the basin indicate that large lake levels fluctuated during the past in this study we measured the exact elevation of shorelines in the juyanze basin using digital global positioning system and found seven shorelines at 37 36 34 30 28 24 and 22 m above modern basin floor a m b f optically stimulated luminescence dating was employed to date the deposition of beach sand and gravel sequences at these shorelines results indicate a paleolake developed in the basin at 5 ka when lake levels reached 26 m a m b f the lake level then gradually increased to 29 m a m b f at 3 3 ka and r

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

correction to bistability of the atlantic overturning circulation in a global climate model and links to ocean freshwater transport 1 in the paper bistability of the atlantic overturning circulation in a global climate model and links to ocean freshwater transport by e hawkins et al geophysical research letters 38 l10605 doi 10 1029 2011gl047208 2011 an error was made in figure 3 the corrected figure 3 is shown here and the conclusions are unchanged 2 the error was in the calculation of fov for depresysff pink line the corrected estimate is now more consistent with the results for the other ocean reanalyses references

implications for the history of cenozoic opal deposition from a quantitative model in addition to being a time of enhanced crustal activity and a warmer climate the eocene also saw prolonged periods of silica accumulation on the ocean floor the peak of this accumulation occurred at approximately 50 ma creating horizon ac a layer of silica rich sediments spanning approximately 4 my horizon ac is comparatively isolated in time from known silicic acid addition processes leading mcgowran 1989 mcgowran b 1989 silica burp in the eocene ocean geology 17 pp 857 860 to suggest a novel mechanism the silica burp hypothesis that centres on the temporal decoupling of silicic acid supply and burial by climatic variation here we examine this hypothesis using a quantitative biogeochemical model of the silicon cycle our results unequivocally show that mcgowran s hypothesis is unable to account for horizon ac the model s residence time and ocean capacity for silicic acid are insufficient to permit the degree of temporal decoupling proposed by mcgowran inverting the model s results suggests that instead the sediment record may be a useful proxy for silicic acid additions to the ocean

eddy contributions to the meridional transport of salt in the north atlantic 1 the meridional transport of salt in the atlantic ocean is an important process for climate controlling the stability of the meridional overturning circulation the contribution of transient eddies to this transport is quantified in an eddy resolving north atlantic model at 1 12 resolution natl12 and compared with lower resolution north atlantic and global 1 4 models in natl12 between 10 n and 40 n there is a volume loss by evaporation of 0 6 sverdrups sv the divergence of the eddy flux of salt normalized by a reference salinity of 34 8 is 0 2 sv over the region a significant fraction of the total air seawater exchange but it is compensated by an opposite convergent transport of salt by the mean flow so that the total transport of salt is small the compensation between eddy and mean salt transport is almost complete in a multicentury long global model experiment but less effective in natl12 because the short integration time does not allow the salt content to equilibrate and the model drift is large eddies arising from baroclinic instability contribute to the meridional salt transports at the northern and southern boundary of the subtropical gyre where they appear consistent with a lateral diffusion acting on the mean salinity gradient however the eddy transport of salt is the sum of two terms an advective contribution arising from the correlations of velocity and isopycnal thicknesses and a diffusion along isopycnals both components have the same amplitude at the southern boundary of the subtropical gyre while diffusion is dominant at the northern boundary

meridional overturning circulation stability and ocean feedbacks in a box model a box model of the inter hemispheric atlantic meridional overturning circulation is developed including a variable pycnocline depth for the tropical and subtropical regions the circulation is forced by winds over a periodic channel in the south and by freshwater forcing at the surface the model is aimed at investigating the ocean feedbacks related to perturbations in freshwater forcing from the atmosphere and to changes in freshwater transport in the ocean these feedbacks are closely connected with the stability properties of the meridional overturning circulation in particular in response to freshwater perturbations a separate box is used for representing the region north of the antarctic circumpolar current in the atlantic sector the density difference between this region and the north of the basin is then used for scaling the downwelling in the north these choices are essential for reproducing the sensitivity of the meridional overturning circulation observed in general circulation models and therefore suggest that the southernmost part of the atlantic ocean north of the drake passage is of fundamental importance for the stability of the meridional overturning circulation with this configuration the magnitude of the freshwater transport by the southern subtropical gyre strongly affects the response of the meridional overturning circulation to external forcing the role of the freshwater transport by the overturning circulation m ov as a stability indicator is discussed it is investigated under which conditions its sign at the latitude of the southern tip of africa can provide information on the existence of a second permanently shut down state of the overturning circulation in the box model m ov will be an adequate indicator of the existence of multiple equilibria only if salt advection feedback dominates over other processes in determining the response of the circulation to freshwater anomalies m ov is a perfect indicator if feedbacks other than salt advection are negligible

the atlantic freshwater budget as a diagnostic for the existence of a stable shut down of the meridional overturning circulation 1 which physical processes effectively determine the stability regime of the atlantic meridional overturning circulation moc is not yet fully understood we investigate the role of the oceanic freshwater transport into the basin employing a coupled model of intermediate complexity by modifying the longitudinal variation of surface salinities near the southern border at 33 s the amount of salt flowing out of the atlantic via the brazil current can be regulated in turn this will influence whether the moc exports or imports salt water the latter is associated with a basin scale salinity overturning feedback which can be either positive or negative pulse experiments strongly suggest that its sign determines the existence of a monostable or bistable regime in our model

optimal initial excitations of decadal modification of the atlantic meridional overturning circulation under the prescribed heat and freshwater flux boundary conditions within a three dimensional ocean circulation model the nonlinear optimal initial perturbations noip of sea surface salinity sss and sea surface temperature sst to excite variability in the atlantic meridional overturning circulation amoc were obtained under prescribed heat and freshwater flux boundary conditions using the conditional nonlinear optimal perturbation cnop method after 10 years the optimal sss and sst perturbations lead to reductions of the amoc by 3 6 and 2 5 sv 1 sv 10 6 m 3 s 1 respectively followed by multidecadal oscillations with a period of about 50 years during the first 30 years nonlinear processes have an important influence on the amoc strength convection strengthens the amoc during years 0 2 zonal density advection promotes the slowdown of the amoc during years 7 20 and meridional density advection inhibits the slowdown of meridional velocities in the upper ocean during years 5 18 the linear optimal initial perturbation loip was also computed using the first singular vector fsv method for sss perturbations with an amplitude of 0 5 psu the loip will cause an underestimation of the amplitude of the multidecadal amoc variability by about 1 sv compared to that induced by the noip this underestimation will become more significant as the amplitudes of sss perturbations increase

the history and nature of wind erosion in deserts recently the importance of wind erosion has been reevaluated many low angle surfaces developed on susceptible materials possess closed depressions pans and these often have a distinctive morphology and lunette dunes on their lee sides it has also become apparent that in drylands there are extensive areas of stone pavements some of which have been molded by deflation however it is now recognized that other horizontal sorting processes have played a role especially movement of fines by overland flow vertical sorting associated with wetting and drying salt heave bioturbation frost action and dust accretion have also played a role in hyperarid areas especially with unidirectional winds bedrock outcrops and old lake beds have been molded to give aerodynamic forms called yardangs the study of dust storms by analysis of climatological data and remote sensing has revealed the importance of deflation especially in hyperarid areas with centripetal drainage deflation hot spots such as the bo

response of a strongly eddying global ocean to north atlantic freshwater perturbations abstractthe strongly eddying version of the parallel ocean program pop is used in two 45 yr simulations to investigate the response of the atlantic meridional overturning circulation amoc to strongly enhanced freshwater input due to greenland melting with an integrated flux of 0 5 sverdrups sv 1 sv 106 m3 s 1 for comparison a similar set of experiments is performed using a noneddying version of pop the aim is to identify the signature of the salt advection feedback in the two configurations for this reason surface salinity is not restored in these experiments the freshwater input leads to a quantitatively comparable reduction of the overturning strength in the two models to examine the importance of transient effects in the relation between amoc strength and density distribution the results of the eddy resolving model are related to water mass transformation theory the freshwater forcing leads to a reduction of the rate of light to dense water conversion in the north atlantic but there

geochemical and magnetic characteristics of fine grained surface sediments in potential dust source areas implications for tracing the provenance of aeolian deposits and associated palaeoclimatic change in east asia abstract we investigated the geochemical and magnetic characteristics of fine grained surface sediments fractions

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

the energetics of a collapsing meridional overturning circulation a well studied example of natural climate variability is the impact of large freshwater input to the polar oceans simulating glacial melt release or an amplification of the hydrological cycle such forcing can reduce or entirely eliminate the formation of deep water in the polar latitudes and thereby weaken the atlantic meridional overturning circulation moc this study uses a series of idealized eddy permitting numerical simulations to analyze the energetic constraints on the atlantic ocean s response to anomalous freshwater forcing inthismodel thechangesinmocarenotcorrelatedwiththeglobalinputofmechanicalenergy both kinetic energy and available potential energy ape increase with northern freshwater forcing while the moc decreases however a regional analysis of ape density supports the notion that local maxima in ape density control the response of the moc to freshwater forcing perturbations a coupling between ape input and changes in local density anomalies accounts for the difference in time scales between the recovery and collapse of the moc

north atlantic observations sharpen meridional overturning projections atlantic meridional overturning circulation amoc projections are uncertain due to both model errors as well as internal climate variability an amoc slowdown projected by many climate models is likely to have considerable effects on many aspects of global and north atlantic climate previous studies to make probabilistic amoc projections have broken new ground however they do not drift correct or cross validate the projections and do not fully account for internal variability furthermore they consider a limited subset of models and ignore the skill of models at representing the temporal north atlantic dynamics we improve on previous work by applying bayesian model averaging to weight 13 coupled model intercomparison project phase 5 models by their skill at modeling the amoc strength and its temporal dynamics as approximated by the northern north atlantic temperature based amoc index we make drift corrected projections accounting for structural model errors and for the internal variability cross validation experiments give approximately correct empirical coverage probabilities which validates our method our results present more evidence that amoc likely already started slowing down while weighting considerably moderates and sharpens our projections our results are at low end of previously published estimates we project mean amoc changes between periods 1960 1999 and 2060 2099 of 4 0 sv and 6 8 sv for rcp4 5 and rcp8 5 emissions scenarios respectively the corresponding average 90 credible intervals for our weighted experiments are 7 2 1 2 and 10 5 3 7 sv respectively for the two scenarios

the role of african dust in atlantic climate during heinrich events increased ice discharge in the north atlantic is thought to cause a weakening or collapse of the atlantic meridional overturning circulation amoc during heinrich events paleoclimate records indicate that these periods were marked by severe tropical aridity and dustiness although the driver of these events is still under debate large freshwater input is necessary for climate models to simulate the magnitude geographical extent and abruptness of these events indicating that they may be missing feedbacks we hypothesize that the dust climate feedback is one such feedback that has not been previously considered here we analyze the role of dust climate feedbacks on the amoc by parameterizing the dust radiative effects in an intermediate complexity model and consider uncertainties due to wind stress forcing and the magnitude of both atmospheric dust loading and freshwater hosing we simulate both stable and unstable amoc regimes by changing the prescribed wind stress forcing in the unstable regime additional dust loading during heinrich events cools and freshens the north atlantic and abruptly reduces the amoc by 20 relative to a control simulation in the stable regime however additional dust forcing alone does not alter the amoc strength including both freshwater and dust forcing results in a cooling of the subtropical north atlantic more comparable to proxy records than with freshwater forcing alone we conclude that dust climate feedbacks may provide amplification to heinrich cooling by further weakening amoc and increasing north atlantic sea ice coverage

quantifying dust emissions from desert landforms eastern mojave desert usa abstract the measurement of natural dust emissions from desert landforms is crucial in environmental hazard assessment and field checking the accuracy of global dust models more than 500 individual dust measurements from eight common desert landforms in southern california were collected using the pi swerl portable in situ wind erosion lab the largest emitters of dust are dry washes 13 787 to 0 007 mg m 2 s 1 dunes playa margins distal alluvial fans and lacustrine beaches low emitters include salt crusted playas 0 692 to 0 002 mg m 2 s 1 silt clay crusted playas and desert pavements high emissions are a function of saltating sand that bombards the surface liberating dust sized particles for entrainment low dust emissions are primarily a function of surface crusting gravel armoring and vegetation density pi swerl measurements reveal that emission rates can vary by at least three orders of magnitude reflecting local variability in soil texture and continuity of surface crusts shear stress partitioning models can be applied to dust data measured by the pi swerl to account for large surface roughness features such as vegetation the results presented here give an approximation of the contributions to atmospheric dust loading by landforms in the mojave desert and can potentially be used to improve atmospheric dust models

processes governing the recovery of a perturbed thermohaline circulation in hadcm3 abstract in an experiment with the latest version of the hadley centre climate model the model response has been analyzed after the thermohaline circulation thc in the atlantic ocean has been suppressed the suppression is induced by a strong initial perturbation to the salinity distribution in the upper layer of the northern north atlantic the model is then allowed to adjust freely salinity gradually increases and deep water formation in the greenland and norwegian seas restarts later also in the labrador sea the meridional overturning recovers after about 120 yr in the first few decades when the overturning is very weak surface air temperature is dominated by cooling of much of the northern hemisphere and weak warming of the southern hemisphere leading to maximum global cooling of 0 9 c the disruption to the atmosphere s radiation balance results in a downward flux anomaly at the top of the atmosphere maximally 0 55 w m 2 in the first decade then decreasing with the thc recovery the processes