simulations of two last glacial maximum ocean states two stable thermohaline circulation states were produced in simulations of the last glacial maximum lgm ocean both forced by the same prescribed atmosphere direct application of this atmosphere led to an equilibrium state with strong north atlantic deep water formation northern sinking state nss upon applying a weak freshwater flux anomaly to the north atlantic for a short time a state was entered with little north atlantic but significant southern ocean deep water formation southern sinking state sss a fully dynamic and thermodynamic iceberg trajectory model was developed and icebergs were seeded into both states distinctly different pathways were found particularly in the eastern atlantic where the sss icebergs tend to move south along the european coast while they move north in the nss while neither ocean state is claimed to be a definitive model for the lgm ocean paleoclimate data are more consistent with the characteristics of the nss

interannual enso and interdecadal enso like variability in the southern hemisphere tropospheric circulation abstract recent work has identified variability in the pacific ocean sst with a structure qualitatively similar to enso but at lower frequencies than enso zhang et al have documented the atmospheric circulation anomalies in the tropics and northern hemisphere that are associated with decadal enso like variability and compared these anomalies to those associated with the interannual enso cycle here the authors extend the study of zhang et al to the southern hemisphere using the national centers for environmental prediction national center for atmospheric research reanalysis data for 1958 96 consistent with previous studies the southern hemisphere circulation anomalies associated with enso display a teleconnection pattern from the central tropical pacific into the far southeastern pacific ocean comparatively larger circulation anomalies are found in the southern hemisphere associated with the decadal enso like variability though aloft the structure of the anomalies emphasizes the polar vortex wit

the mutual interaction between continental scale ice sheets and atmospheric stationary waves the great continental ice sheets of the pleistocene represented significant obstacles to the northern hemisphere midlatitude westerlies they must therefore have forced large changes in the atmospheric circulation and consequently also in the patterns of accumulation and melting over the ice sheets themselves a simplified threedimensional coupled ice sheet stationary wave model is developed in order to understand the ice sheet s response to the circulation changes that it induces consistent with ice age climate simulations the ice sheet topography induces an anticyclonic circulation over the ice sheet causing a slight warming over the western slopes and a stronger cooling over the remainder the modeled feedbacks significantly affect the ice sheet configuration with the most important influences being the patterns of summer temperature and the topographically induced precipitation field the time evolution of the ice sheet is also changed by the atmospheric feedbacks and the results suggest the possibility of multiple equilibrium solutions

stationary rossby wave propagation in a baroclinic atmosphere observational studies of teleconnections in both soisticial seasons have suggested various patterns of behaviour linked mainly to westerly jets and equatorial westerlies these have been interpreted using barotropic rossby wave theory and simulated using stationary forcing in barotropic models linearized about 300 hpa flows in this paper the relevant propagation behaviour is investigated using a baroclinic model with three dimensional climatological basic states time integrations are performed using localized thermal forcing as a wavemaker the propagation results are qualitatively very similar to those obtained with the barotropic model though there are quantitative differences the westerly jets still act as strong waveguides the shorter wavelengths and smaller wave activity speeds found with the baroclinic model are generally in better agreement with observation propagation into and from the upper tropospheric equatorial westerlies in the east pacific propagation across north america and propagation from europe across the arabian gulf are all found the extent to which upper tropospheric westerlies reach towards the equator influences the propagation into those regions barotropic models can only represent such behaviour if the basic state reflects the near equatorial upper tropospheric zonal wind structure

late glacial stage and holocene tropical ice core records from huascaran peru two ice cores from the col of huascaran in the north central andes of peru contain a paleoclimatic history extending well into the wisconsinan wurm glacial stage and include evidence of the younger dryas cool phase glacial stage conditions at high elevations in the tropics appear to have been as much as 8 to 12 c cooler than today the atmosphere contained about 200 times as much dust and the amazon basin forest cover may have been much less extensive differences in both the oxygen isotope ratio 18o 8 per mil and the deuterium excess 4 5 per mil from the late glacial stage to the holocene are comparable with polar ice core records these data imply that the tropical atlantic was possibly 5 to 6 c cooler during the late glacial stage that the climate was warmest from 8400 to 5200 years before present and that it cooled gradually culminating with the little ice age 200 to 500 years before present a strong warming has dominated the last two centuries

a tropical mechanism for northern hemisphere deglaciation 1 we investigate the role of the tropics in the melting and reforming of the laurentide ice sheet on glacial timescales using an atmospheric general circulation model it is found that warming of tropical sea surface temperatures ssts from glacial boundary conditions as observed at the end of glacial periods causes a large increase in summer temperatures centered over the ice sheet forming regions of canada this highlatitude response to tropical change is due to relatively small changes in the circulation of the extratropical atmosphere which lead to changes in the vertical profiles of temperature and moisture in the extratropical atmosphere the maximum perturbation in the summer radiative balance over the laurentide ice sheet 25 w m 2 due to the changes in the local atmospheric water vapor inventory is much larger than that induced by glacial to interglacial changes in atmospheric co2 these changes via an atmospheric bridge between the tropics and extratropics represent a mechanism for deglaciations which is consistent with timing constraints in contrast a cold perturbation to tropical sst for interglacial boundary conditions results in only very small changes in the delivery of water vapor to the laurentide region and therefore almost no cooling over the canadian region this implies that tropical ssts could play a more important role in melting ice sheets in the northern hemisphere than in reforming them possibly providing a mechanism which could help to explain the rapidity of deglaciation relative to glacial inception

enso variability and teleconnections during glacial climates abstract understanding the origin of sub milankovitch and millennial scale abrupt climate change still poses a major challenge an important role of communicating the orbital scale forcing into abrupt climate change on millennial and centennial timescales has been hypothesized for the tropics in earlier studies based on the assumption of stationary teleconnections this paper presents a modelling study for the last glacial maximum and marine isotope stage 3 using the comprehensive climate model ccsm3 glacial boundary conditions and freshwater hosing at high latitudes of the north atlantic are imposed to mimic a heinrich stadial and dansgaard oeschger stadials and interstadials it is shown that glacial enso variability is strongly enhanced by a substantial slowdown of the atlantic meridional overturning circulation the simulations also suggest that glacial boundary conditions induce major modifications to enso teleconnections and that the blueprint of modern enso teleconnections should only be applied with caution to glacial climate periods

the influence of continental ice atmospheric co2 and land albedo on the climate of the last glacial maximum the contributions of expanded continental ice reduced atm9spheric co2 and changes in land albedo to the maintenance of the climate of the last glacial maximum lgm are examined a series of experiments is perform ed using an atmosphere mixed layer ocean model in which these changes in boundary conditions are incorporated either singly or in combination the model used has been shown to produce a reasonably reajistic simulation of the reduced temperature of the lgm manabe and broccoli 1985 by comparing the results from pairs of ex periments the effects of each of these envirornnental changes can be determined expanded continental ice and reduced atmospheric co2 are found to have a substantial impact on global mean temperature the ice sheet effect is confined almost ex clusively to the northern hemisphere while lowered co2 cools both hemispheres changes in land albedo over ice free areas have only a minor thermal effect on a global basis the reduction of co2 content in the atmosphere is the primary contributor to the cooling of the southern hemisphere the model sensitivity to both the ice sheet and co2 effects is characterized by a high latitude amplification and a late autumn and early winter maximum

synchronous high frequency oscillations in tropical sea surface temperatures and north atlantic deep water production during the last glacial cycle stable isotopic measurements of g sacculifer and c wuellerstorfi in a core from the western equatorial atlantic imply that there are parallel suborbital oscillations in surface water hydrography and deep water circulation occurring during oxygen isotope stages 2 and 3 low values of g sacculifer 18o accompany high values of c wuellerstorfi 13c linking warmer sea surface temperatures ssts in the tropics with increased production of lower north atlantic deep water nadw the amplitude of the 18o oscillations is 0 6 or 2 3 c which is superimposed on a glacial interglacial amplitude of about 2 1 using the g sacculifer 18o data we calculate that surface waters were colder during stage 2 than calculated by climap 1976 1981 the longer period 2 kyr oscillations in air temperature recorded in the greenland and antarctic ice cores appear to correlate with oscillations in sea surface temperature in the equatorial atlantic the magnitude of these oscillations in tropical sst is too large to have resulted from changes in meridional heat transport caused by the global conveyor alone the apparent synchroneity of equatorial sst and polar air temperature changes as well as the amplitude of the sst changes at the equator are consistent with the climate effects expected from changes in the atmosphere s greenhouse gas content h2ovapor co2 and ch4

the importance of tropical sea surface temperature patterns in simulations of last glacial maximum climate abstract prescribed sst experiments are performed using the national center for atmospheric research s community climate model version 3 general circulation model to isolate the contribution of the tropical ssts reconstructed by the climate long range investigation mapping and prediction study climap to the modeled global atmospheric circulation anomalies at the last glacial maximum lgm the changes in tropical sst patterns cause changes in tropical convection that force large 300 m in 500 mb geopotential height changes in northern hemisphere wintertime circulation these midlatitude circulation changes occur despite the small 1 c change in the mean tropical sst between the present and the climap reconstruction in fact the midlatitude circulation changes due to the difference in the tropical sst pattern between the present and the climap reconstruction are greater than the circulation changes due to a uniform tropical sst cooling of 3 c or those due to the presence of the lgm ice sheets the

thermomechanical modelling of northern hemisphere ice sheets with a two level mass balance parameterization a 3 d time dependent thermomechanical ice sheet model was used together with a two level snow accumulation runoff mass balance model to investigatethe quaternary ice sheets of the northern hemisphere the model freely generates the ice sheet geometry in response to specified changes of surfacetemperature and mass balance and includes bedrock adjustment basal sliding and a full temperature calculation within the ice the mass balanceparameterisation makes a distinction between snowfall and melting yearly snow fall rates depend on the present precipitation distribution and are variedproportional to changes of surface temperature and the moisture content of the air the ablation model is based on the positive degree day method anddistinguishes between ice and snow melting this paper discusses steady state characteristics conditions for growth and retreat and response time scales ofice sheets as a function of a prescribed lowering of summer temperature most notably the modelled extents of the eurasian ice sheet for a summertemperature lowering of 6 7 k and of the laurentide ice sheet for a cooling of 9 10 k are in reasonable agreement with most reconstructions based ongeological evidence except for the presence of a large ice sheet stretching from alaska across the bering strait to most of eastern siberia in addition wetbasal conditions turned out to be always confined to the margin whereas central areas in these reconstructions remained always cold based this is ofrelevance for processes involving reduced basal traction

cross equatorial response to middle latitude forcing in a zonally varying basic state abstract a nonlinear model based on the shallow water equations is used to study cross equatorial propagation of forced waves in the presence of a longitudinally varying time mean basic state zonal wind field it is found that global scale planetary waves are unable to propagate past a critical latitude where the mean zonal wind speed vanishes however if the longitudinally asymmetric basic state includes a duet in which the zonal winds are westerly waves of zonal scale less than the zonal scale of the westerly duct may propagate from one hemisphere to the other even though the zonally symmetric mean zonal wind remains easterly in the equatorial region the amplitude of the response in one hemisphere to forcing in the opposite hemisphere increases strongly with the magnitude of the westerlies in the equatorial duct the existence and annual variations of a westerly duct region in the upper troposphere in the eastern pacific appear to account for some features of the low frequency variability in the no

ice age paleotopography a gravitationally self consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the last glacial maximum lgm the results show that lgm ice volume was approximately 35 percent lower than suggested by the climap reconstruction and the maximum heights of the main laurentian and fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level use of these ice age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the climap data set

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

glacial interglacial dynamics of the eastern equatorial pacific cold tongue intertropical convergence zone system reconstructed from oxygen isotope records record mean late holocene lh last glacial maximum lgm d 18 o amplitudes ranging between 1 0 and 1 3 we estimate that mean sea surface temperatures sst in this region during the lgm were on average 1 5 0 5 c lower than the lh larger d 18 o amplitudes are observed in sites north of the equator indicating a spatial pattern of reduced meridional sst gradient across the equator during the lgm this result is supported by comparison of mg ca sst reconstructions from two sites straddling the equator we interpret the reduction of this gradient during the lgm as evidence for a less intense cold tongue intertropical convergence zone itcz frontal system a more southerly position of the itcz and weaker southeast equatorial trades in the eep indexterms 1620 global change climate dynamics 3309 3339 meteorology and atmospheric dynamics ocean atmosphere interactions 0312 4504 3344 meteorology and atmospheric dynamics paleoclimatology 4231 oceanography general equatorial oceanography 4267 oceanography general paleoceanography keywords cold tongue itcz oxygen isotopes tropical pacific

sea ice control of plio pleistocene tropical pacific climate evolution marine proxies of sea surface temperature sst indicate that the tropical pacific thermal gradient intensified through the plio pleistocene and peaked during pleistocene glaciations the cause of this variability which has been linked to the initiation of the walker circulation is uncertain here we hypothesize that plio pleistocene tropical climate variability was coupled to high latitude southern hemisphere climate change specifically sea ice extent we use a coupled ocean atmosphere general circulation model to investigate the influences of sea ice extent and atmospheric co2 on the tropical pacific thermal structure in the model co2 radiative forcing in the absence of any sea ice feedbacks has little influence on the tropical sst gradient a 180 ppm reduction in co2 causes the sst gradient to decrease by 0 4 c in comparison an expansion of southern hemisphere high latitude sea ice reduces tropical ssts and enhances the sst gradient in the tropical pacific by as much as 3 5 c tropical cooling is primarily due to the advection and upwelling of waters into the eastern equatorial pacific that were cooled by sensible heat loss at the sea ice margin in the southern pacific an energy balance analysis indicates that the ocean heat flux into the eastern equatorial pacific decreases by 44 this mechanism provides an intimate coupling between the tropical pacific and the high latitude southern hemisphere through the thermocline circulation

environmental processes of the ice age land oceans glaciers epilog knowledge of the state of the earth at the last glacial maximum lgm an interval around 21 000 years ago is an important benchmark for understanding the sensitivity of global environmental systems to change much progress in understanding climates of the lgm has occurred in the 20 years since the end of the climap project of the 1970s climate long range investigation mapping and prediction here we review this progress based on presentations and discussion at a rst open science meeting of the epilog project environmental processes of the ice age land oceans glaciers held in delmenhorst germany may 1999 we outline key controversies and document protocols for epilog contributions so that a new synthesis of the lgm earth can emerge as an open project of the world s community of scientists 2001 elsevier science ltd all rights reserved

barotropic wave propagation and instability and atmospheric teleconnection patterns abstract a global barotropic model linearized about the 300 mb climatological mean january flow is perturbed by applying a series of localized forcings distributed throughout the tropics and subtropics structures which resemble the observed pacific north american and east atlantic teleconnection patterns noted by wallace and gutzler 1981 tend to recur in the responses similar patterns are found to result from the dispersion of isolated initial perturbations placed at a variety of locations in the tropics and midlatitudes it is shown that these structures are related to the most rapidly growing mode associated with barotropic instability of the zonally varying climatological basic state in the absence of damping this mode has an e folding time of about a week and a period close to 50 days in localized regions the instantaneous growth rates can be competitive with those of baroclinic instability these episodes of rapid local barotropic growth are interspersed with intervals in which the local

terminating the 100 kyr ice age cycle we report a simulation of the most recent 100 000 year glaciationdeglaciation cycle of the late pleistocene ice age a simulation that delivers an ice sheet chronology that is in close accord with that inferred from the geological record our analyses are performed with a reduced model of the climate system that incorporates significant improvements to the representation of both climate forcing and mass balance response in a previously described theory based upon a coupled one level energy balance model ebm and vertically integrated ice sheet model ism the theory fully incorporates the influences of orbital insolation forcing glacial isostatic adjustment and variations in the atmospheric concentrations of greenhouse gases it correctly predicts the main geographical regions of the northern hemisphere that were glaciated at last glacial maximum 21 000 years ago as well as the abrupt termination of the glacial epoch that occurred subsequently the latter feature of the ice age cycle is obtained without the need to incorporate unconstrained and therefore controversial physical processes into the model a limitation of all previous attempts to understand this global scale climate cycle our analyses suggest that the radiative impact on surface glaciation due to the changing atmospheric concentration of co2 is critical to the ability of the model to deliver a synthetic history of glaciation and deglaciation that is in accord with inferences based upon surface geological and geomorphological evidence with the incorporation of this influence model predicted ice thickness distributions at last glacial maximum lgm are very similar to those of the recently described ice 4g reconstruction that was based upon the inversion of postglacial relative sea level histories

glacial to holocene surface hydrography of the tropical eastern indian ocean quantifying the spatial and temporal sea surface temperature sst and salinity changes of the indo pacific warm pool is essential to understand the role of this region in connection with abrupt climate changes particularly during the last deglaciation in this study we reconstruct sst and seawater delta o 18 of the tropical eastern indian ocean for the past 40 000 years from two sediment cores geob 10029 4 1 degrees 30 s 100 degrees 08 e and geob 10038 4 5 degrees 56 s 103 degrees 15 e retrieved offshore sumatra our results show that annual mean ssts increased about 2 3 degrees c at 19 000 years ago and exhibited southern hemisphere like timing and pattern during the last deglaciation our sst records together with other mg ca based sst reconstructions around indonesia do not track the monsoon variation since the last glacial period as recorded by terrestrial monsoon archives however the spatial sst heterogeneity might be a result of changing monsoon intensity that shifts either the annual mean ssts or the seasonality of g ruber towards the warmer or the cooler season at different locations seawater delta o 18 reconstructions north of the equator suggest fresher surface conditions during the last glacial and track the northern high latitude climate change during the last deglaciation in contrast seawater delta o 18 records south of the equator do not show a significant difference between the last glacial period and the holocene and lack bolling allerod and younger dryas periods suggestive of additional controls on annual mean surface hydrology in this part of the indo pacific warm pool c 2010 elsevier b v all rights reserved

enso like interdecadal variability 1900 93 a number of recent studies have reported an enso like eof mode in the global sea surface temperature sst field whose time variability is marked by an abrupt change toward a warmer tropical eastern pacific and a colder extratropical central north pacific in 1976 77 the present study compares this pattern with the structure of the interannual variability associated with the enso cycle and documents its time history back to 1900 the analysis is primarily based on the leading eofs of the sst anomaly and anomaly deviation fields in various domains and the associated expansion coefficient or principal component time series which are used to construct global regression maps of sst sea level pressure slp and a number of related variables the use of anomaly deviations i e departures of local sst anomalies from the concurrent global mean sst anomaly reduces the influence of global mean sst trends upon the structure of the eofs and their expansion coefficient time series an important auxiliary time series used in this study is a southern oscillation index based on marine surface observations by means of several different analysis techniques the time variability of the leading eof of the global sst field is separated into two components one identified with the enso cycle related variability on the interannual timescale and the other a linearly independent residual comprising all the interdecadal variability in the record the two components exhibit rather similar spatial signatures in the global sst slp and wind stress fields the sst signature in the residual variability is less equatorially confined in the eastern pacific and it is relatively more prominent over the extratropical north pacific the corresponding slp signature is also stronger over the extratropical north pacific and its counterpart in the cold season 500 mb height field more closely resembles the pna pattern the amplitude time series of the enso like pattern in the residual variability reflects the above mentioned shift in 1976 77 as well as a number of other prominent features including a shift of opposite polarity during the 1940s

terrestrial conditions at the last glacial maximum and climap sea surface temperature estimates are they consistent climap 1981 seasonal reconstruction of the earth s surface at the last glacial maximum geological society of america map and chart series mc 36 boundary conditions were used as inputs to the giss general circulation model and the last glacial maximum lgm climate was simulated for six model years the simulation was compared with snow line depression and pollen inferred temperature data at low latitudes specifically for hawaii colombia east africa and new guinea the model does not produced as much cooling at low latitudes as is implied by the terrestrial evidence an alternative experiment in which the climap sea surface temperatures were uniformly lowered by 2 c produces a better fit to the land data although in hawaii model temperatures are still too warm the relatively warm climap tropical sea surface temperatures also provide for only a slight decrease in the hydrologic cycle in the model in contrast to both evidence of lgm tropical aridity and the results of the experiment with colder ocean temperatures with the climap sea surface temperatures the lgm global annual mean surface air temperature is 3 6 c colder than at present if the ocean temperatures were allowed to cool in conformity with the model s radiation balance the lgm simulation would be 5 6 c colder than today and in better agreement with the tropical land evidence

evolution of east asian monsoon clay mineral evidence in the western philippine sea over the past 700 kyr clay mineral assemblage of the core md06 3047 in the western philippine sea has been used to trace sediment provenance and transport process and thus to constrain the east asian monsoon evolution as well as its forcing mechanism over the past 700 kyr clay minerals mainly consist of smectite 61 and illite 23 with minor chlorite 10 and kaolinite 6 provenance analyses suggest that the smectite originates predominantly from the alteration of nearby luzon volcanic rocks illite is primarily derived from the asian continent with eolian origin illite contents show strong glacial interglacial cyclicity as constrained by oxygen isotope stratigraphy variations of illite contents are ultimately controlled by the east asian monsoon evolution characterized by higher illite values during glacials with relatively stronger east asian winter monsoon but weaker east asian summer monsoon during interglacials enhanced east asian summer monsoon and rainfall could produce more smectite with poor crystallinity from luzon to the study core spectral analyses of illite content and smectite crystallinity display a strong periodicity at 100 kyr for the east asian winter monsoon and 100 kyr 30 kyr and 23 kyr periodicities for the east asian summer monsoon respectively suggesting the unique forcing processes of the east asian winter monsoon from high latitudes i e ice volume and the east asian summer monsoon from both high latitudes and low latitudes i e tropical pacific c 2012 elsevier ltd all rights reserved

el nino like pattern in ice age tropical pacific sea surface temperature sea surface temperatures ssts in the cold tongue of the eastern equatorial pacific exert powerful controls on global atmospheric circulation patterns we examined climate variability in this region from the last glacial maximum lgm to the present using a sst record reconstructed from magnesium calcium ratios in foraminifera from sea floor sediments near the galapagos islands cold tongue sst varied coherently with precession induced changes in seasonality during the past 30 000 years observed lgm cooling of just 1 2 c implies a relaxation of tropical temperature gradients weakened hadley and walker circulation southward shift of the intertropical convergence zone and a persistent el nino like pattern in the tropical pacific this is contrasted with mid holocene cooling suggestive of a la nina like pattern with enhanced sst gradients and strengthened trade winds our results support a potent role for altered tropical pacific sst gradients in global climate variations

a high resolution absolute dated holocene and deglacial asian monsoon record from dongge cave china abstract we present a continuous record of the asian monsoon over the last 16 ka from 18o measurements of stalagmite calcite over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230th dates 18o and therefore asian monsoon intensity generally follows changes in insolation although changes in 18o are generally accommodated in abrupt shifts in contrast to smoothly varying insolation indicating that threshold effects may be important 18o decreased dramatically 3 at the start of the holocene 11 5 ka and remained low for 6 ka four positive 18o events centered at 11225 97 yr bp 1 05 10880 117 yr bp 1 15 9165 75 yr bp 1 4 and a double event centered at 8260 64 yr bp 1 1 and 8080 74 yr bp 1 0 punctuated this period of high monsoon intensity all four events correlate within error with climate changes in greenland ice cores thus the relationship between the asian monsoon and the north atlantic observed during the glacial period appears to continue into the early holocene in addition three of the four events correlate within error with outburst events from lake agassiz the decline of monsoon intensity in the mid late holocene is characterized by an abrupt positive shift in 18o which occurs at 3550 59 yr bp 1 1 in 100 yr in addition the holocene is punctuated by numerous centennial and multi decadal scale events amplitudes 0 5 to 1 up to half the amplitude of the glacial interstadial events seen in the last glacial period thus holocene centennial and multi decadal scale monsoon variability is significant although not as large as glacial millennial scale variability the monsoon shows a strong connection with northern south american hydrological changes related by changes in itcz position spectral analysis of the 18o record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing however there are numerous other significant peaks including peaks at el nino frequencies observed for high resolution portions of the record between 8110 and 8250 yr which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling holocene monsoon climate in addition for this high resolution portion we observe a distinctive biennial oscillation of the asian monsoon which has been associated with the tropospheric biennial oscillation tbo

response in atmospheric circulation and sources of greenland precipitation to glacial boundary conditions the response in northern hemisphere atmospheric circulation and the resulting changes in moisture sources for greenland precipitation to glacial boundary conditions are studied in ncar s ccm3 atmospheric general circulation model fitted with a moisture tracking functionality we employ both the climap sst reconstruction and a modification thereto with reconstructions of glacial ice sheets and land masks the individual components of the boundary conditions are added first one at a time and finally together these steps show the atmospheric circulation to respond approximately linearly to the boundary condition changes and the full glacial change may thus be decomposed into contributions from sst and topography changes respectively we find that using the climap sst reconstruction leads to a shift from atlantic toward pacific source regions not found with the modified reconstruction having cooler tropics and less sea ice the occurrence of such a shift depends chiefly on the sst reconstruction and not on the existence of the large northern hemisphere glacial ice sheets the influence of these circulation changes on important factors for ice core interpretation such as precipitation seasonality condensation temperatures and source temperatures are assessed

lithium isotope geochemistry of sediments and hydrothermal fluids of the guaymas basin gulf of california abstract lithium isotopic compositions of hydrothermally altered sediments of deep sea drilling project dsdp site 477 477a as well as high temperature vent fluids of the guaymas basin have been determined to gain an understanding of lithium exchange during fluid sediment interaction at this sediment covered spreading center unaltered turbidite of the basin has a 6 li value of 10 5 7 heavier than fresh oceanic basalts contact metamorphism induced by a shallow sill intrusion results in a decrease of the lithium content of the adjacent sediments and a lighter isotopic value 8 below the sill sediments altered by a deep seated hydrothermal system show strong depletions in lithium while lithium isotopic compositions vary greatly ranging from 11 to 1 the shift to lighter composition is the result of preferential retention of the lighter isotope in recrystallized phases after destruction of the primary minerals the complexity of the isotope profile is attributed to inhomogeneity in mineral composition the tortuous pathway of fluids and the temperature effect on isotopic fractionation the range of lithium concentration and 6 li values for the vent fluids sampled in 1982 and 1985 overlaps with that of the sediment free mid ocean ridge systems the lack of a distinct expression of sediment input is explained in terms of a flow through system with continuous water recharge the observations on the natural system agree well with the results of laboratory hydrothermal experiments the experimental study demonstrates the importance of temperature pressure water rock ratio substrate composition and reaction time on the lithium isotopic composition of the reacted fluid high temperature authigenic phases do not seem to constitute an important sink for lithium and sediments of a hydrothermal system such as guaymas are a source of lithium to the ocean the ready mobility of lithium in the sediment under elevated temperature and pressure conditions also has important implications for lithium cycling in subduction zones

evolution of the zonal gradients across the equatorial pacific during the miocene pleistocene combining u k 37 0 and tex 86 derived temperatures and oxygen isotopic values of mixed layer and thermocline species from the iodp site u1338 east equatorial pacific and odp site 806 west equatorial pacific we assess the evolution of the zonal sea surface temperature gradients and thermocline depth across the equatorial pacific from the late miocene through the pleistocene data suggest a long term shoaling of the thermocline along the equator throughout the miocene pliocene that accelerated around 5 3 ma we identify a critical transition at about 3 8 ma from a el nino like dominated mean state during the late miocene and early pliocene to a la nina like dominated state during the late pliocene pleistocene this transition coincides with the restriction of the indonesian seaway and the onset of ice growth in the northern hemisphere and in antarctica that led to the long term strengthening of the walker circulation and affected low latitude zonal gradient

orbital and suborbital climate variability in the sulu sea western tropical pacific 1 a detailed record of planktic 18o from a sediment core in the sulu sea located between the south china sea and the western pacific warm pool reveals that for the past 400 kyr 1 kyr 1000 years 18o variability on orbital timescales is similar to that caused by changes in ice volume alone this result indicates that in the sulu sea temperature driven changes in planktic 18o on orbital times scales were generally compensated for by the effects of sea level and changes in seasonal monsoon intensity on the local freshwater budget as well as by other changes in the tropical hydrologic cycle and their attendant effects on surface water 18o increased freshening of the western tropical pacific warm pool is reminiscent of la nina conditions however we argue that the mean tropical climate state was not more la nina like than today on broader spatial scales suborbital variability occurred in the sulu sea throughout the past 400 kyr suggesting little sensitivity to ice volume or to glacial interglacial changes in tropical hydrology variations on 4 10 kyr timescales appear to be linked to those in the north atlantic region suggesting a common forcing of that variability we suggest that sulu sea salinity variations were a response to suborbital climate variability in the north atlantic region transmitted via changes in the intensity of the east asian summer monsoon we suggest that a north atlantic origin of that tropical suborbital variability can be reconciled with weak glacial amplification in the tropics if the tropical response is nonlinear

simulated influence of carbon dioxide orbital forcing and ice sheets on the climate of the last glacial maximum a coupled atmosphere ocean sea ice model is used to investigate the climate of the last glacial maximum 21 000 years ago and the relative climate forcing effects of atmosphere co2 the earth s orbital parameters and ice sheet albedo tropical temperatures are found to be 2 2 c less than today s slightly colder than indicated by the climap palaeoclimate reconstruction this result is consistent with a low to medium climate sensitivity to radiative perturbations temperatures are colder still in the northern north atlantic region owing to a weakening and shallowing of the thermohaline circulation a sensitivity analysis suggests that changes in ocean circulation since the last glacial maximum have not contributed directly to the global mean temperature change since that time

more humid interglacials in ecuador during the past 500 kyr linked to latitudinal shifts of the equatorial front and the intertropical convergence zone in the eastern tropical pacific 1 studying past changes in the eastern equatorial pacific ocean dynamics and their impact on precipitation on land gives us insight into how the intertropical convergence zone itcz movements and the el nino southern oscillation modulate regional and global climate in this study we present a multiproxy record of terrigenous input from marine sediments collected off the ecuadorian coast spanning the last 500 kyr in parallel we estimate sea surface temperatures sst derived from alkenone paleothermometry for the sediments off the ecuadorian coast and complement them with alkenone based sst data from the panama basin to the north in order to investigate sst gradients across the equatorial front near the equator today s river runoff is tightly linked to sst reaching its maximum either during the austral summer when the itcz migrates southward or during el nino events our multiproxy reconstruction of riverine runoff indicates that interglacial periods experienced more humid conditions than the glacial periods the north south sst gradient is systematically steeper during glacial times suggesting a mean background climatic state with a vigorous oceanic cold tongue resembling modern la nina conditions this enhanced north south sst gradient would also imply a glacial northward shift of the intertropical convergence zone at least in vicinity of the cold tongue a pattern that has not yet been reproduced in climate models

foraminiferal faunal estimates of paleotemperature circumventing the no analog problem yields cool ice age tropics the sensitivity of the tropics to climate change particularly the amplitude of glacial to interglacial changes in sea surface temperature sst is one of the great controversies in paleoclimatology here we reassess faunal estimates of ice age ssts focusing on the problem of no analog planktonic foraminiferal assemblages in the equatorial oceans that confounds both classical transfer function and modern analog methods a new calibration strategy developed here which uses past variability of species to define robust faunal assemblages solves the no analog problem and reveals ice age cooling of 5 o to 6oc in the equatorial current systems of the atlantic and eastern pacific oceans classical transfer functions underestimated temperature changes in some areas of the tropical oceans because core top assemblages misrepresented the ice age faunal assemblages our finding is consistent with some geochemical estimates and model predictions of greater ice age cooling in the tropics than was inferred by climate long range investigation mapping and prediction climap 1981 and thus may help to resolve a long standing controversy our new foraminiferal transfer function suggests that such cooling was limited to the equatorial current systems however and supports climap s inference of stability of the subtropical gyre centers

paleoclimatic tracers an investigation using an atmospheric general circulation model under ice age conditions 2 water isotopes the linear relationship observed between the water isotopic contents of precipitation and surface air temperatures leads to the use of the water isotopes h218o and hdo in paleoclimatology applied to the measurements of the isotopic content of paleowaters like groundwaters and deep ice cores this relationship is used to infer paleotemperatures however this interpretation of paleo isotopic contents is only valid if the isotope temperature relationship is not affected by climate change to address this problem we have developed a water isotope modeling inside an atmospheric general circulation model agcm and performed simulations of both the present day and last glacial maximum lgm climatic conditions agcms are indeed the only appropriate tools able to account the whole complexity of the atmospheric circulation for the present day climate preliminary results for january were presented by joussaume et al 1984 and are complemented by new simulations performed for both february and august climatic conditions with a higher resolution version of the model model results are well corroborated by observations they also exhibit some effects of the atmospheric circulation on the isotopic fields for the simulated lgm climate the model results compare well with paleoclimatic data of water isotopic contents except for a higher than observed spatial variability the overall patterns of the simulated 18o temperature relationship for the lgm climate are practically unchanged which tends to comfort the use of water isotopes in paleoclimatology however concerning the deuterium excess i e the relationship between oxygen 18 and deuterium the model results are not sufficiently valid to allow a discussion of the use of deuterium excess in paleoclimatology