modelling glacial climatic millennial scale variability related to changes in the atlantic meridional overturning circulation a review abstract since the discovery of glacial abrupt variability in greenland and the north atlantic about two decades ago its modelling remains a conceptual and technical challenge while the global climatic signature of heinrich events can reasonably be ascribed to the fresh water discharge related to the massive iceberg armadas released by the laurentide ice sheet in the north atlantic the existence of such a triggering mechanism is elusive as far as dansgaard oeschger type variability is concerned even if other mechanisms have been proposed the majority of modelling attempts to better understand glacial climate variability has assumed it to be linked to abrupt variations in the atlantic meridional overturning circulation amoc in this review we introduce the rationale behind this approach i e the possible existence of several deep ocean circulation equilibria associated with distinct climate signatures we then review the published fresh water hosing experiments and the mechanisms at work to explain the climate differences related to the different amoc states most of these works have been performed under pre industrial boundary conditions the few hosing experiments run under glacial conditions show that the sensitivity of the climate system to fresh water hosing is very dependent on the background climate state in order to rigourously compare several types of experiments equilibrium runs vs transient runs with pre industrial vs glacial boundary conditions we present experiments especially run for this review with two climate models of intermediate complexity uvic and loveclim although there are similarities between the responses simulated by these models there are also many differences which would benefit being compared to reconstructions such as those presented in this special issue however a strict comparison will require to consider boundary conditions as realistic as possible for marine isotopic stage 3 modelling glacial variability to compare model results with paleoclimatic data remains a big challenge

towards a quantitative understanding of millennial scale antarctic warming events abstract the interhemispheric temperature response to a meltwater induced weakening of the atlantic meridional overturning circulation amoc is characterized by a northern hemispheric cooling and a southern hemispheric warming this so called bipolar seesaw pattern explains many millennial scale features identified in paleo proxy records from both hemispheres here we present modeling evidence that suggests that the southern hemispheric response to a weakening of the amoc includes elements that have previously been overlooked under present day conditions an amoc weakening leads to an intensification of the negative phase of the pacific south america psa pattern with its southernmost pole north of the ross sea an intensified psa pattern may lead to a regional cooling of marie byrd land thereby favoring an in phase temperature relationship between the northern hemisphere and the western side of the west antarctic ice sheet on millennial timescales however under glacial conditions due to reduced tropical diabatic forcing and reduced tropical extratropical teleconnections the weakening of the amoc induces a different austral winter time atmospheric response pattern that is characterized by a wavenumber 2 structure and different air temperature anomalies we furthermore demonstrate that the observed co 2 changes that accompanied major disruptions of the amoc may have helped to amplify antarctic warming events a events due to radiative forcing and the polar amplification mechanism our modeling results provide a new framework to explain a considerable fraction of the observed millennial scale variance in southern hemispheric climate records during marine isotope stage 3 mis3

marine derived 14c calibration and activity record for the past 50 000 years updated from the cariaco basin abstract an expanded cariaco basin 14c chronology is tied to 230th dated hulu cave speleothem records in order to provide detailed marine based 14c calibration for the past 50 000 years the revised high resolution cariaco 14c calibration record agrees well with data from 230th dated fossil corals back to 33 ka with continued agreement despite increased scatter back to 50 ka suggesting that the record provides accurate calibration back to the limits of radiocarbon dating the calibration data document highly elevated 14c during the glacial period carbon cycle box model simulations show that the majority of observed 14c change can be explained by increased 14c production however from 45 to 15 ka 14c remains anomalously high indicating that the distribution of radiocarbon between surface and deep ocean reservoirs was different than it is today additional observations of the magnitude spatial extent and timing of deep ocean 14c shifts are critical for a complete understanding of observed glacial 14c variability

a review of the bipolar see saw from synchronized and high resolution ice core water stable isotope records from greenland and east antarctica numerous ice core records are now available that cover the last glacial cycle both in greenland and in antarctica recent developments in coherent ice core chronologies now enable us to depict with a precision of a few centuries the relationship between climate records in greenland and antarctica over the millennial scale variability of the last glacial period stacks of greenland and antarctic water isotopic records nicely illustrate a seesaw pattern with the abrupt warming in greenland being concomitant with the beginning of the cooling in antarctica at the antarctic isotopic maximum aim in addition from the precise estimate of chronological error bars and additional high resolution measurements performed on the edc and taldice ice cores we show that the seesaw pattern does not explain the regional variability in antarctic records with clear two step structures occurring during the warming phase of aim 8 and 12 our antarctic high resolution data also suggest possible teleconnections between changes in low latitude atmospheric circulation and antarctic without any greenland temperature fingerprint

effects of glacial meltwater in the giss coupled atmosphere ocean model 2 a bipolar seesaw in atlantic deep water production the relationship between the northern hemisphere and southern hemisphere deep water circulation systems is explored in experiments with gradual and impulsive freshwater input through the st lawrence with sufficient freshwater volume input 50 sv years north atlantic deep water nadw cessation occurs as does substantial cooling in the northern hemisphere the colder temperatures are accompanied by increased mass and sea level pressure in the northern hemisphere with corresponding lower pressure in the southern hemisphere the low pressure response occurs at high southern latitudes consistent with the antarctic annular mode the leading mode of variability in the current climate stronger winds associated with this increased cyclonicity intensify the antarctic circumpolar current acc with heat flux divergences in the south atlantic and convergences and warming in the indian ocean weddell sea bottom water production increases in response to the south atlantic high latitude cooling and sea ice growth hence acting as a seesaw with the decreasing nadw and even global antarctic bottom water increases although not as strongly the initial seesaw response occurs within a few years although it takes some 100 years to maximize due to the response time of the acc the south atlantic cooling occurs approximately in phase with the north atlantic so the seesaw is not in temperature within that ocean basin however warming in portions of the southern indian ocean occurs out of phase with the northern hemisphere cooling nadw does not resume of its own accord once complete cessation occurs even when freshwater input is stopped but when increased evaporation is used to force nadw formation temporarily weddell sea bottom water decreases accordingly

coupling of the hemispheres in observations and simulations of glacial climate change abstract we combine reconstructions climate model simulations and a conceptual model of glacial climate change on millennial time scales to examine the relation between the high latitudes of both hemispheres a lead lag analysis of synchronised proxy records indicates that temperature changes in greenland preceded changes of the opposite sign in antarctica by 400 500 yr a composite record of the dansgaard oeschger events shows that rapid warming cooling in greenland was followed by a slow cooling warming phase in antarctica the amplitudes rates of change and time lag of the interhemispheric temperature changes found in the reconstructions are in excellent agreement with climate model simulations in which the formation of north atlantic deep water is perturbed the simulated time lag between high northern and southern latitudes is mainly determined by the slow meridional propagation of the signal in the southern ocean our climate model simulations also show that increased deep water formation in the north atlantic leads to a reduction of the antarctic circumpolar current through diminishing meridional density gradients in the southern ocean we construct a simple conceptual model of interhemispheric dansgaard oeschger oscillations this model explains major features of the recorded temperature changes in antarctica as well as the general shape of the north south phase relation found in the observations including a broad peak of positive correlations for a lead of antarctica over greenland by 1000 2000 yr the existence of this peak is due to the regularity of the oscillations and does not imply a southern hemisphere trigger mechanism contrary to previous suggestions our findings thus further emphasise the role of the thermohaline circulation in millennial scale climate variability

simulation of atmospheric radiocarbon during abrupt oceanic circulation changes trying to reconcile models and reconstructions abstract an abrupt increase in the atmospheric 14 c content at the beginning of the younger dryas about 13 000 years ago has been related to a slow down of the global oceanic circulation this has been well simulated by box models but the amplitude and timing of such a 14 c change could not be represented with dynamical models we have forced a climate model of intermediate complexity with a meltwater discharge to test three model parameters that have the potential to strongly influence the simulated 14 c change and may help us to reconcile the model results with reconstructions we find that a decrease of the tropical wind speed accompanying a slow down of the oceanic circulation largely amplifies the circulation induced 14 c change and brings it closer to the reconstructions second we find that vertical mixing in a dynamical ocean model leads to a substantial damping in the 14 c response of simple models third we demonstrate that neglecting radiocarbon reservoir age variations during abrupt climate change may lead to a substantial overestimate of atmospheric 14 c fluctuations reconstructed from paleoceanographic archives

time frequency variability of enso and stochastic simulations abstract the time frequency spectral structure of el nino southern oscillation enso time series holds much information about the physical dynamics of the enso system the authors have analyzed changes of the spectrum with time of three enso indices the conventional southern oscillation index soi nino3 sea surface temperatures and a tropical pacific rain index over the period 1871 1995 three methods of time frequency analysis windowed fourier transform wavelet analysis and windowed prony s method were used and the results are in good agreement the time frequency spectra of all the series show strong multidecadal variations over the past century in particular there was reduced activity of enso in the 2 3 yr periodicity range during the period 1920 60 compared with both the earlier and later periods the dominant frequencies in the spectra do not appear to be constrained to certain frequency bands and there is no evidence that the enso system has fixed modes of oscillation the qualitative b

a numerical investigation of the organization and interaction of the convective and stratiform regions of tropical squall lines abstract a set of 13 two dimensional numerical simulations based on the 22 and 23 june soundings from the convectionprofonde tropicale in 1981 copt81 experiment in west africa is used to study the organization and interactionof the convective and stratiform regions of squall line type convective systems the initial wind profiles arecharacterized by the african easterly jet aej and the tropical easterly jet tej located at about 3 5 km and14 km respectively the physical processes that generate and maintain the mesoscale inflow at the rear of squall line type mesoscaleconvective systems are thereby examined horizontal potential temperature gradients generated by a combinationof latent heat release in the convective region and unsaturated mesoscale descent both modulated by evaporation cause a horizontal pressure gradient and generate horizontal line parallel vorticity the rear inflow is a consequenceof these processes the convective activity induces a significant upscale influence ahead of the

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

abrupt ice age shifts in southern westerly winds and antarctic climate forced from the north the mid latitude westerly winds of the southern hemisphere play a central role in the global climate system via southern ocean upwelling carbon exchange with the deep ocean agulhas leakage transport of indian ocean waters into the atlantic and possibly antarctic ice sheet stability meridional shifts of the southern hemisphere westerly winds have been hypothesized to occur in parallel with the well documented shifts of the intertropical convergence zone in response to dansgaard oeschger do events abrupt north atlantic climate change events of the last ice age shifting moisture pathways to west antarctica are consistent with this view but may represent a pacific teleconnection pattern forced from the tropics the full response of the southern hemisphere atmospheric circulation to the do cycle and its impact on antarctic temperature remain unclear here we use five ice cores synchronized via volcanic markers to show that the antarctic temperature response to the do cycle can be understood as the superposition of two modes a spatially homogeneous oceanic bipolar seesaw mode that lags behind northern hemisphere climate by about 200 years and a spatially heterogeneous atmospheric mode that is synchronous with abrupt events in the northern hemisphere temperature anomalies of the atmospheric mode are similar to those associated with present day southern annular mode variability rather than the pacific south american pattern moreover deuterium excess records suggest a zonally coherent migration of the southern hemisphere westerly winds over all ocean basins in phase with northern hemisphere climate our work provides a simple conceptual framework for understanding circum antarctic temperature variations forced by abrupt northern hemisphere climate change we provide observational evidence of abrupt shifts in the southern hemisphere westerly winds which have previously documented ramifications for global ocean circulation and atmospheric carbon dioxide these coupled changes highlight the necessity of a global rather than a purely north atlantic perspective on the do cycle

retracted southward migrations of the atlantic equatorial currents during the younger dryas previous studies suggested that a weakened gulf stream may have contributed to the younger dryas yd cold event however the physical mechanism of the gulf stream weakening is unclear here we utilize paired mg ca and 18o measurements of planktonic foraminifera from the northerly and southerly marine sediments in the middle atlantic ocean to reconstruct past changes in tropical surface ocean temperature and salinity over the past 14 ka we demonstrate that during the yd the sea surface temperature sst at the northerly core 0 16 s was up to 2 1 2 7 c cooler whereas the sst at the southerly core 14 53 s was up to 0 8 1 6 c warmer than modern temperatures the 18o salinity values accumulation rates and other hydrological elements at the northerly core all exhibit opposite patterns with those at the southerly core the antiphased variations of the paleohydrological elements at the northerly and southerly cores suggest that the atlantic equatorial currents shifted southward significantly from its present day position during the yd possibly resulting in a decrease of the gulf stream in turn a cool northern hemisphere

rapid changes in ocean circulation and atmospheric radiocarbon a latitude depth coupled global ocean ice atmosphere model is extended to include a simple biosphere component a physically reasonable adjustment of runoff into the north atlantic is invoked to achieve a transient response to glacial meltwater perturbations which closely resembles the younger dryas climate event we then investigate the evolution of the isotopic ratio of atmospheric radiocarbon 14c due to the rapid changes of deep ocean circulation when the north atlantic branch of the conveyor belt circulation is interrupted the oceanic uptake of radiocarbon is reduced resulting in an increase of atmospheric 14c by about 35 the reduction of ventilation in the north atlantic is partly compensated by an increase of the 14c ratios of the biosphere the southern ocean and the upper ocean above 1000 m depth a plateau of the 14c year calendar year relation can be generated at the time of the rapid reinitiation of deep ocean ventilation which begins coincident with the major temperature increase and lasts for about 60 years it is hence significantly shorter than that found by analyzing tree rings during the termination of younger dryas longer than 400 years a sensitivity study reveals that the duration of the plateau depends strongly on the transient evolution of the gas exchange rate and can increase to 150 300 years if changes of pco2 or sea ice coverage are taken into account

timing of the descent into the last ice age determined by the bipolar seesaw we present planktonic foraminiferal fauna and isotope records from the se atlantic that highlight the nature of millennial scale variability over the last 100 kyr we derive an hypothesis driven age model for our records based on the empirical link between variations in greenland temperature ocean circulation and carbonate preservation in the deep se atlantic our results extend earlier findings of an anti phase seesaw relationship between north and south for the largest abrupt events of marine isotope stage mis 3 2 and the last deglaciation in particular we find that heinrich stadials were paralleled by inferred southward shifts of the thermal subtropical front these were followed by pronounced rebounds of the front with the return to interstadial conditions in the north our results also shed light on the mechanism of glaciation in contrast to the last deglaciation which was a globally symmetric change superposed by interhemispheric asynchronicity we find that the descent into full glacial conditions at the onset of mis 4 70 ka displayed interhemispheric synchrony we suggest that this globally synchronous descent into glacial mis 4 was preconditioned by orbital changes but that the timing was ultimately determined by abrupt changes in ocean atmosphere circulation patterns i e the bipolar seesaw

deglacial changes in ocean circulation from an extended radiocarbon calibration temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon based age estimates of biogenic material that do not agree with true calendar age this problem is particularly severe beyond the limit of the high resolution radiocarbon calibration based on tree ring data which stretches back only to1 2 about 11 8 kyr before present bp near the termination of the younger dryas cold period if a wide range of palaeoclimate records are to be exploited for better understanding the rates and patterns of environmental change during the last deglaciation extending the well calibrated radiocarbon timescale back further in time is crucial several studies attempting such an extension using uranium thorium dated corals3 4 5 and laminae counts in varved sediments6 7 8 9 show conflicting results here we use radiocarbon data from varved sediments in the cariaco basin in the southern caribbean sea to construct an accurate and continuous radiocarbon calibration for the period 9 to 14 5 kyr bp nearly 3 000 years beyond the tree ring based calibration a simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that north atlantic deep water formation shut down during the younger dryas period but was gradually replaced by an alternative mode of convection possibly via the formation of north atlantic intermediate water

large scale characteristics of the distribution of blowing snow sublimation to consider the large scale characteristics of blowing snow sublimation and its importance in the hydrological cycle in the cryosphere we investigated the sublimation of blowing snow particles on a global scale using the global datasets of the european centre for medium range weather forecasts ecmwf re analysis data and the international satellite land surface climatology project islscp initiative i data for 1987 the sublimation fluxes of blowing snow particles were estimated globally with 2 58 resolution at 6 hour intervals we found that the sublimation of blowing snow particles occurs more widely in the northern hemisphere than in the southern hemisphere does not increase monotonously with latitude and becomes more active in the polar coast regions and highlands although the annual mean sublimation fluxes of the northern and southern hemispheres are almost equal in addition we confirmed the characteristic seasonal changes in the area of sublimation in the northern hemisphere although we need to incorporate continuous parameters from systematic ground based studies of the structure of blowing snow in specific fields to reduce uncertainty regarding the characteristics of blowing snow our results point to a need to review the current understanding of the hydrological cycle

interhemispheric atlantic seesaw response during the last deglaciation the asynchronous relationship between millennial scale temperature changes over greenland and antarctica during the last glacial period has led to the notion of a bipolar seesaw which acts to redistribute heat depending on the state of meridional overturning circulation within the atlantic ocean here we present new records from the south atlantic that show rapid changes during the last deglaciation that were instantaneous within dating uncertainty and of opposite sign to those observed in the north atlantic our results demonstrate a direct link between the abrupt changes associated with variations in the atlantic meridional overturning circulation and the more gradual adjustments characteristic of the southern ocean these results emphasize the importance of the southern ocean for the development and transmission of millennial scale climate variability and highlight its role in deglacial climate change and the associated rise in atmospheric carbon dioxide

the two step shape and timing of the last deglaciation in antarctica the two step character of the last deglaciation is well recognized in western europe in greenland and in the north atlantic for example in greenland a gradual temperature decrease started at the bolling b around 14 5 ky bp spanned through the allerod a and was followed by the cold younger dryas yd event which terminated abruptly around 11 5 ky bp recent results suggest that this ba yd sequence may have extended throughout all the northern hemisphere but the evidence of a late transition cooling is still poor for the southern hemisphere here we present a detailed isotopic record analyzed in a new ice core drilled at dome b in east antarctica that fully demonstrates the existence of an antarctic cold reversal acr these results suggest that the two step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences thus the coldest part of the acr which shows a temperature drop about three times weaker than that recorded during the yd in greenland may have preceded the yd antarctica did not experienced abrupt changes and the two warming periods started there before they started in greenland the links between southern and northern hemisphere climates throughout this period are discussed in the light of additional information derived from the antarctic dust record

wavelet analysis of simulated tropical convective cloud systems part ii decomposition of convective scale and mesoscale structure abstract numerically simulated three dimensional tropical convective systems were analyzed using a wavelet approach in which the vertical shear of the ambient wind is used as a decomposition criterion the method objectively decomposes the simulated systems into their two primary components namely organized deep convection and a mesoscale stratiform region this reproduces the standard model of highly organized mesoscale convective systems as well as giving an objective characterization of less organized regimes it demonstrates the vertical component of wind shear as a statistically useful criterion in addition to its better known role as a quasi deterministic mechanism controlling the organization of convection and its scale selection

simulated resumption of the north atlantic meridional overturning circulation slow basin wide advection and abrupt local convection abstract the resumption process of the north atlantic meridional overturning circulation moc is investigated in a series of freshwater hosing experiments using a comprehensive coupled climate model four different freshwater perturbations are applied to the north atlantic between 50 n and 70 n leading to a substantial weakening of moc and an expansion of winter sea ice cover over the greenland iceland norwegian gin seas deactivating the freshwater forcing leads to a two phase recovery of the moc the first phase is a slow basin wide advection process whereas the second phase is an abrupt decadal scale transition located in the north atlantic the slow basin wide advection process to an intermediate state of the moc is characterized by a restart of the deep water production in the area of the subpolar gyre and the irminger basin however the ice cover over the gin seas remains whereas the surface density gradually increases in the gin seas on a century time scale the length of this intermediate state depends on the strength of freshwater perturbation which determines the magnitude of the meridional density gradient in the north atlantic the second transition phase is triggered by a model specific density threshold reached in the gin seas when passed the moc abruptly intensifies by 60 within 60 80 years the analysis shows that this abrupt intensification only depends on surface salinity in the gin seas the model results reveal that the simulated sea ice retreat in the gin seas and a rapid increase of the greenland air temperature of the second phase of the resumption of the moc are similar in amplitude to temperature proxies observed during dansgaard oeschger events of the last ice age

cloud resolving modeling of cloud systems during phase iii of gate part ii effects of resolution and the third spatial dimension abstract two and three dimensional simulations of cloud systems for the period of 1 7 september 1974 in phase iii of the global atmospheric research programme garp atlantic tropical experiment gate are performed using the approach discussed in part i of this paper the aim is to reproduce cloud systems over the gate b scale sounding array comparison is presented between three experiments driven by the same large scale conditions i a fully three dimensional experiment ii a two dimensional experiment that is an east west section of the three dimensional case and iii a high resolution version of the two dimensional experiment differences between two and three dimensional frameworks and those related to spatial resolution are analyzed the three dimensional experiment produced a qualitatively realistic organization of convection nonsquall clusters a squall line and scattered convection and transitions between regimes were simulated the two dimensional experiments produced convective organ

the 8k event cause and consequences of a major holocene abrupt climate change abstract a prominent abrupt climate event about 8200 years ago brought generally cold and dry conditions to broad northern hemisphere regions especially in wintertime in response to a very large outburst flood that freshened the north atlantic changes were much larger than typical climate variability before and after the event with anomalies up to many degrees contributing to major displacement of vegetative patterns this 8k event provides a clear case of cause and effect in the paleoclimatic realm and so offers an excellent opportunity for model testing the response to north atlantic freshening has the same general anomaly pattern as observed for older events associated with abrupt climate changes following north atlantic freshening and so greatly strengthens the case that those older events also reflect north atlantic changes the north atlantic involvement in the 8k event helps in estimating limits on climate anomalies that might result in the future if warming caused ice melt and hydrologic cycle intensification at high latitudes lead to major changes in north atlantic circulation few model experiments have directly addressed the 8k event and most studies of proxy records across this event lack the time resolution to fully characterize the anomalies so much work remains to be done

grain size dependence of eolian saltation lengths during snow drifting eolian saltation a primary process in the transport of fine granular material by wind produces a variety of geophysical effects on earth and other planetary surfaces wind tunnel experiments were carried out to investigate the dependence of saltation on grain size the saltation length of snow particles was estimated at size intervals of 0 05 mm in diameter by measuring local vertical mass fluxes in 17 snow collectors arrayed at the lee end of the snow surface the measured mean saltation length of snow particles of 0 01 1 mm in diameter ranged from 0 1 to 1 0 m at wind velocities of 5 10 m s mean saltation length decreased with increasing diameter and decreasing wind speed we suggest that the probability of the saltation length of a particle at each diameter is described by a monotonically decreasing distribution function that is the shorter the saltation length the higher the frequency of its occurrence one ramification of this distribution is that the mean saltation length does not imply the dominance of saltating particles of this length

paleocean circulation during the last deglaciation a bipolar seesaw hughen et al 1998 have documented that during the first 200 years of younger dryas time the 14c content of atmospheric co2 increased by 50 and that during the remainder of this 1200 year duration cold event it steadily declined the initial increase in 14c c was likely the result of a reduction in the atlantic s conveyor circulation however were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump then it is difficult to understand why the climate conditions in the northern atlantic basin remained cold throughout the younger dryas modeling exercises by stocker and wright 1996 mikolajewicz 1998 and schiller et al 1998 show that if the conveyor is terminated the transfer of radiocarbon into the deep sea shifts to the southern ocean thereby stabilizing the atmospheric 14c c ratio paleoclimatic evidence from the antarctic continent suggests that this model based scenario might have been played out in the real world while the younger dryas cooling has been documented in many places around the world including new zealand denton and hendy 1994 sowers and bender 1995 using their 18o in o2 based correlation between the ice core 18o in ice records for antarctica and greenland have demonstrated that in antarctica the younger dryas was a time of maximum warming the point of this paper is that the steep rise in 18o rise in antarctic ice which commenced close to the onset of the younger dryas might have been caused by heat released to the atmosphere in response to an increase in deep sea ventilation in the southern ocean

synchronous climate changes in antarctica and the north atlantic central greenland ice cores provide evidence of abrupt changes in climate over the past 100 000 years many of these changes have also been identified in sedimentary and geochemical signatures in deep sea sediment cores from the north atlantic confirming the link between millennial scale climate variability and ocean thermohaline circulation it is shown here that two of the most prominent north atlantic events the rapid warming that marks the end of the last glacial period and the bolling allerod younger dryas oscillation are also recorded in an ice core from taylor dome in the western ross sea sector of antarctica this result contrasts with evidence from ice cores in other regions of antarctica which show an asynchronous response between the northern and southern hemispheres

deep sea corals evidence periodic reduced ventilation in the north atlantic during the lgm holocene transition abstract nine new intermediate water ages estimated with coupled uranium series and radiocarbon measurements on deep sea corals from the northeastern atlantic are presented together with six intermediate and deep water ages from the western north atlantic adkins et al science 280 1998 725 728 and one from the equatorial atlantic mangini et al nature 392 1998 347 348 they span the time period from 15 400 to 560 yr ago and show abrupt variations of intermediate and deep water ages from 270 up to 2320 yr both major pulses of meltwater discharge mwp 1a following heinrich event 1 and mwp 1b after the younger dryas cold event are followed by enhanced supply of southern source water at intermediate depths in the north atlantic together with reduced formation of well ventilated glacial north atlantic intermediate water 1000 2000 m this result gives strong support to numerical models predicting fast and sharp increase of bottom water age in the atlantic triggered by pulses of freshwater discharge into the north atlantic rahmstorf nature 372 1994 82 85 stocker and wright radiocarbon 40 1998 359 366

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

modeling study of a tropical squall type convective line abstract a multidimensional and time dependent cloud scale model is used to investigate the dynamic and micro physical processes associated with convective and stratiform regions within a tropical squall type convective line the evolution of the total convective and stratiform portions of rainfall is also estimated by using model output a three dimensional version of the model covers a horizontal domain about 96 96 km2 frequently the horizontal extent of an observed stratiform region is over a few hundred kilometers therefore a two dimensional version of the model with a 512 km horizontal length is also used to incorporate a complete stratiform region two dimensional model result recapture many interesting features as observed in particular the fractional portion of stratiform rain as well as its fractional area coverage are in good agreement with observations a significant amount of ice particles melted to rain near the freezing level in the trailing part of the modeled squall system during i

1 f noise and pulse like events in the tropical atmospheric surface variabilities this study discusses the 1 f noise behaviour and its connection with pulse like events in tropical surface meteorological data the 1 f noise withf being frequency refers to the unique spectrum shape of signals that represents the same degree of variability for a whole spectrum range with very long lag correlations the pulse like events are characterized by their high localizations in time and a lack of a distinct spectrum peak approximate 1 f noise behaviour is found in tropical surface temperature moisture and wind speed for periods of 1 hour to beyond 10 days and in oceanic wind stress up to 2 year periods the origin of the 1 f noise can be closely tied to pulse like events with a highly intermittent nature in the time series examples for such pulse like events include the cumulus convection events leading to convective downdraughts and intraseasonal westerly wind events associated with the madden julian oscillations mjo a method based on the wavelet is introduced to extract individual pulse like events these events are characterized by a 1 f spectrum extending up to 1 2 decades of frequency centred around their characteristic durations their successive occurrences are mutually uncorrelated and the individual pulse like events themselves constitute a 1 f spectrum for a frequency range around their durations the superposition of these events of various durations yields the 1 f noise spectrum for much wider ranges physical implications of these findings to convective quasi equilibrium and mjo el nino southern oscillation relationship are discussed copyright 2004 royal meteorological society

optical properties of blowing snow calc ulation procedures are developed and results shown for the exact calculation of extinction and meteorological visual range using the blowin g snow mass in the atmosp here and particle radius results of the calculations show i for monochromatic radiation geometrical optics approx imations of the extin ction efficiency are found to provide results of only moderate accuracy in calculating the extinction of radiation by a single particle 2 for broad band radiation the geomet rical optics appro ximation is sufficiently accurate for many single particle measurement instru ments and appli cations except in the infra red band where mie theory should be used 3 for typical blowing s now particle size dist ributions the shape parameter of the distribution of particle radii and the mean particle radius are very important in broa d band extinction and visua l range modelling estimates of blowing snow quantities from broad band extinction measurements or visual range from blowin g snow quantities should address the shape and mean value of the snow particle radius distribution

the potential added value of regional climate models in south america using a multiresolution approach this paper aims to identify those regions within the south american continent where the regional climate models rcms have the potential to add value pav compared to their coarser resolution global forcing for this we used a spatial scale filtering method based on the wavelet theory to distinguish the regional climatic signal present in atmospheric surface fields from observed data cpc and trmm and 6 rcm simulations belonging to the cordex project the wavelet used for filtering was haar wavelet but a comparative analysis with daubechies 4 wavelet indicated that meteorological fields or regional indices were not very sensitive to the wavelet selected once the longer wavelengths were filtered we focused on analyzing the spatial variability of extreme rainfall and the spatiotemporal variability of maximum and minimum surface air temperature on a daily basis the results obtained suggest essential differences in the spatial distribution of the small scale signal of extreme precipitation between trmm and regional models together with a large dispersion between models while trmm and cpc register a large signal throughout the continent the rcms place it over the andes cordillera and some over tropical south america pav signal for surface air temperature was found over the andes cordillera and the brazilian highlands which are regions characterized by complex topography and also on the coasts of the continent the signal came specially from the small scale stationary component the transient part is much smaller than the stationary one except over la plata basin where they are of the same order of magnitude also the rcms and cpc showed a large spread between them in representing this transient variability the results confirm that rcms have the potential to add value in the representation of extreme precipitation and the mean surface temperature in south america however this condition is not applicable throughout the whole continent but is particularly relevant in those terrestrial regions where the surface forcing is strong such as the andes cordillera or the coasts of the continent

cloud resolving modeling of tropical cloud systems during phase iii of gate part i two dimensional experiments abstract a formal framework is established for the way in which cloud resolving numerical models are used to investigate the role of precipitating cloud systems in climate and weather forecasting models emphasis is on models with periodic lateral boundary conditions that eliminate unrealistic numerically generated circulations caused by open boundary conditions in long term simulations defined in this formalism is the concept of large scale forcing and the cloud environment interactions that are consistent with the periodic boundary conditions two dimensional numerical simulations of the evolution of cloud systems during 1 7 september 1974 in phase iii of the global atmospheric research program atlantic tropical experiment gate are conducted based on the above formalism a simple technique is used to force an anelastic cloud resolving model with evolving large scale horizontal wind field and large scale forcing for the temperature and moisture obtained from the gate data the 7 day period selected i

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

rapid reorganizations of the marine environments in the equatorial atlantic during the younger dryas abstract the younger dryas event is one of the most abrupt climate changes observed in northern hemisphere paleoclimatic records the possible driver despite being proposed by a number of authors is still under vigorous debate with no scientific consensus most studies investigated the amoc with its shutdown being responsible for a reduction of heat and salt in the north atlantic however by reanalyzing and comparing the paleoclimatic records from the northern and southern sites of the brazilian cape we consider a significant decrease of the north brazil current to be the main cause for the heat and salt transport to high latitudes through coupled effects of the brazilian promontory and a southward shift of the equatorial currents in the equatorial atlantic we obtained the 18o 13c toc caco3 records at the northern and southern sites from the pangaea database the results demonstrate a pronounced antiphased variation of the paleoclimatic records between the northern and southern sites during the yd thus suggesting a significant southward migration of the equatorial currents

the last glacial cycle transient simulations with an aogcm a number of transient climate runs simulating the last 120 kyr have been carried out using famous a fast atmosphere ocean general circulation model aogcm this is the first time such experiments have been done with a full aogcm providing a three dimensional simulation of both atmosphere and ocean over this period our simulation thus includes internally generated temporal variability over periods from days to millennia and physical detailed representations of important processes such as clouds and precipitation although the model is fast computational restrictions mean that the rate of change of the forcings has been increased by a factor of 10 making each experiment 12 kyr long atmospheric greenhouse gases ghgs northern hemisphere ice sheets and variations in solar radiation arising from changes in the earth s orbit are treated as forcing factors and are applied either separately or combined in different experiments the long term temperature changes on antarctica match well with reconstructions derived from ice core data as does variability on timescales longer than 10 kyr last glacial maximum lgm cooling on greenland is reasonably well simulated although our simulations which lack ice sheet meltwater forcing do not reproduce the abrupt millennial scale climate shifts seen in northern hemisphere climate proxies or their slower southern hemisphere counterparts the spatial pattern of sea surface cooling at the lgm matches proxy reconstructions reasonably well there is significant anti correlated variability in the strengths of the atlantic meridional overturning circulation amoc and the antarctic circumpolar current acc on timescales greater than 10 kyr in our experiments we find that ghg forcing weakens the amoc and strengthens the acc whilst the presence of northern hemisphere ice sheets strengthens the amoc and weakens the acc the structure of the amoc at the lgm is found to be sensitive to the details of the ice sheet reconstruction used the precessional component of the orbital forcing induces 20 kyr oscillations in the amoc and acc whose amplitude is mediated by changes in the eccentricity of the earth s orbit these forcing influences combine to first order in a linear fashion to produce the mean climate and ocean variability seen in the run with all forcings

effects of glacial meltwater in the giss coupled atmosphereocean model 1 north atlantic deep water response varying magnitudes of freshwater input through the st lawrence are added to different versions of the giss coupled atmosphere ocean model the results show a generally linear response in percentage north atlantic deep water nadw reduction to the volume of water added without any obvious threshold effects when the estimated best guess freshwater input for the allerod younger dryas interval is added only small reductions in nadw production occur in less than a century with complete cessation requiring more than 300 years if the ocean circulation was weaker in the allerod preceding the younger dryas somewhat larger percentage reductions occur and it would take 150 200 years for a complete shutdown the freshening is aided by a positive feedback associated with the reduction in evaporation relative to precipitation in the north atlantic amounting to some 20 30 of the added freshwater runoff when the freshwater input is stopped after complete nadw shutdown nadw production does not resume with complete nadw cessation cooling eventually occurs throughout the northern hemisphere including regions where it is not thought to have arisen during the younger dryas and the globe cools by 1 c due primarily to the sea ice albedo feedback while in many regions the observed cooling is reproduced the cooling in greenland is much less than is estimated with only 50 nadw cessation cooling is primarily around the north atlantic no rapid warming is seen in response to freshwater cessation the sensitivity displayed here for nadw reduction due to freshwater input from the st lawrence is roughly similar to that associated with increased precipitation accompanying global warming in the next century so a similar sensitivity may arise with younger dryas freshwater inputs from other suspected locations greenland europe

snow particle characteristics in the saltation layer shadowgraphy was employed to study snow saltation in boundary layer wind tunnel experiments with fresh naturally deposited snow the shadowgraphy method allowed for a temporally and spatially high resolution investigation of snow particle characteristics within a measurement area of up to 50 mm x 50 mm snow particle size and number characteristics and their variation with height in the saltation layer were analysed the following observations and findings were made for the saltation layer 1 the particle number decreases exponentially with height 2 the mean particle diameter is fairly constant with a very slight tendency to decrease with height 3 the maximum particle diameter decreases linearly with height and 4 the snow particle size distribution can be adequately described by gamma probability density functions the shape and scale parameters of the gamma distribution were found to vary systematically though only slightly with height over ground and between experiments with different snowpack characteristics

back analysis of drifting snow measurements over an instrumented mountainous site the nemo numerical model of drifting snow whose general outlines are presented in this paper is based on a physical model for saltation and turbulent diffusion the model needs a set of input parameters including fall velocity threshold shear velocity shear velocity mass concentration and roughness which are obtained from empirical formulae and wind speed measured at a given height to better determine the required field data in an alpine context our experimental site col du lac blanc 2700 m a s l french alps was first equipped with one anemometer and blowing snow acoustic sensors which proved not to be accurate enough for research purposes in the current state of development even though a new calibration curve was used we therefore set up a snow particle counter and we returned to the traditional robust mechanical traps and a 10 m mast with six anemometers two temperature sensors and a depth sensor to better determine friction velocity and aerodynamic roughness based on the studied drifting snow events we conclude 1 the proportionality of the aerodynamic roughness to the square of the friction velocity was confirmed but with a varying proportionality ratio depending on the snowdrift event 2 values of s u f were relatively well approximated by empirical formulae from data originating from antarctica and 3 snowdrift concentration profiles obtained by pomeroy s semi empirical formulae for the saltation layer coupled with a theoretical approach for the diffusion layer overestimated the concentration profiles for the studied blowing snow event

linkages between rapid climate variability and deep sea benthic foraminifera in the deep subantarctic south atlantic during the last 95 kyr we present a high resolution record of benthic foraminifera fauna from a sediment core retrieved from the south cape basin subantarctic south atlantic spanning the last glacial cycle 95 kyr information provided by benthic foraminiferal assemblages together with paleoclimate proxies from the same core allow us to interpret changes in the style of primary production episodic versus sustained in relation to abrupt climate oscillations our results indicate that fluctuations in the abundance of the phytodetritus related species epistominella exigua are concomitant with millennial scale high latitude climate perturbations episodic phytoplankton blooms increased during a negative mode of the bipolar seesaw irrespective of the magnitude of the perturbation i e heinrich stadial versus non heinrich stadial events we provide a hypothesis linking the frequency and intensity of these events to atmospheric perturbations interhemispheric climate variability and millennial scale changes in atmospheric co2 a notable exception to the overall pattern is the generally high abundance of e exigua across the globally synchronous onset of glacial marine oxygen isotope stage mis 4 a period generally characterized by increased dustiness and low quality organic carbon as inferred by the percentage of the nonphytodetritus species this highlights the special characteristics governing the onset of mis 4 in the subantarctic

wavelet analysis of simulated tropical convective cloud systems part i basic analysis abstract a wavelet analysis of a three dimensional 7 day explicit simulation of the tropical cloud systems in the global atmosphere research programme garp atlantic tropical experiment phase iii is performed three physically distinct regimes squall line nonsquall cloud cluster and scattered convection in a doubly periodic domain are analyzed using discrete meyer wavelets these wavelets are complete and facilitate the inversion of the decomposed modes the full wavelet spectra well characterize the spatial localization of each physical variable in particular the vertical velocity and the condensate fields the probability distribution of the wavelet coefficients is non gaussian despite the horizontal winds the temperature and humidity being closely gaussian in physical space this demonstrates the effectiveness of the wavelet basis for analyzing cloud system organization the full wavelet spectrum also selects a preferred spatial orientation of the convective organization in physical space a p

deglacial temperature history of west antarctica the most recent glacial to interglacial transition constitutes a remarkable natural experiment for learning how earth s climate responds to various forcings including a rise in atmospheric co2 this transition has left a direct thermal remnant in the polar ice sheets where the exceptional purity and continual accumulation of ice permit analyses not possible in other settings for antarctica the deglacial warming has previously been constrained only by the water isotopic composition in ice cores without an absolute thermometric assessment of the isotopes sensitivity to temperature to overcome this limitation we measured temperatures in a deep borehole and analyzed them together with ice core data to reconstruct the surface temperature history of west antarctica the deglacial warming was formula see text c approximately two to three times the global average in agreement with theoretical expectations for antarctic amplification of planetary temperature changes consistent with evidence from glacier retreat in southern hemisphere mountain ranges the antarctic warming was mostly completed by 15 kybp several millennia earlier than in the northern hemisphere these results constrain the role of variable oceanic heat transport between hemispheres during deglaciation and quantitatively bound the direct influence of global climate forcings on antarctic temperature although climate models perform well on average in this context some recent syntheses of deglacial climate history have underestimated antarctic warming and the models with lowest sensitivity can be discounted

intermediate depth warming in the tropical atlantic related to weakened thermohaline circulation combining paleoclimate data and modeling results for the last deglaciation 1 benthic foraminiferal oxygen isotope ratios from two sediment cores recovered at 426 and 1299 m water depth in the eastern and western tropical atlantic show that a slowdown of the thermohaline circulation thc during heinrich event h1 and the younger dryas was accompanied by rapid and intense warming of intermediate depth waters millennial scale covariations of low paleosalinities in the subpolar north atlantic with decreased benthic oxygen isotope ratios in the eastern tropical atlantic throughout the past 10 000 years suggest that thc weakening might be related to middepth warming during the holocene period as well climate model experiments simulating a strong reduction of the thc in the atlantic ocean under present day and glacial conditions reveal that the increase of temperature in the middepth tropical and south atlantic is a common feature for both climatic states caused by a reduced ventilation of cold intermediate and deep waters in conjunction with downward mixing of heat from the thermocline from the similarity of the paleoclimatic records with the model simulations we infer that the characteristic pattern of temperature change in the atlantic ocean related to weakened thermohaline circulation can serve as an indicator of present day and future thc slowdown

precise interpolar phasing of abrupt climate change during the last ice age a new ice core from west antarctica shows that during the last ice age abrupt northern hemisphere climate variations were followed two centuries later by a response in antarctica suggesting an oceanic propagation of the climate signal to the southern hemisphere high latitudes