dynamical mechanisms for african monsoon changes during the mid holocene 1 during the mid holocene about 6 ka bp the african monsoon was stronger and extended farther north than in present day climate associated with differences in orbital parameters affecting insolation earlier research suggests that land surface vegetation interaction and atmosphere ocean coupling may be important to the northward extension of the holocene monsoon rainfall but different climate models produce different results in terms of rainfall distribution this study examines the dynamical mechanisms that modulate the orbital forcing effect on the african monsoon convection a series of simulations with 6 ka bp orbital parameters modified land surface albedo that mimics grassland extending over north africa and alterations of the model dynamics are used it is found that the ventilation mechanism inflow of low moist static energy air from oceanic or poleward regions is a key process in setting the northern rainfall boundary for the african monsoon in particular advection and diffusion of less moist air oppose the increase of moisture that would be required to meet the increased convective instability criteria set by the warmer tropospheric air above the continent while changes in heat fluxes into the atmospheric column favor convection in principle the ventilation mechanism limits the poleward extent of the region that can actually convect

climate evolution during the holocene a study with an earth system model of intermediate complexity an earth system model of intermediate complexity mobidic has been used to simulate the transient variations in continental temperature sea surface temperature sst thermohaline circulation thc and sea ice cover over the last 9000 years 9 kyr experiments were designed to determine a the deviation of the climatic system with respect to equilibrium over the last 9 kyr b the individual contributions of oceans and vegetation to climatic changes as well as the potential synergies between these components and c the relative importance of precession obliquity and co2 concentration changes during this period results show a monotonous cooling trend in the northern high latitudes between 9 kyr bp and the present day both over the oceans and the continents north of 60 n this cooling is noticed throughout the year but the largest variations appear in spring and summer up to 6 c over continents along with this cooling the model exhibits a southward shift of the northern treeline by about 600 km most of this shift takes place between 4 and 1 kyr bp during this period reorganisations of the boreal forest introduce a lag of about 200 years in the system with respect to a state in equilibrium with the external forcing sensitivity experiments illustrate the strong impact of this vegetation shift both on the oceans and the continents especially in spring and early summer however the model exhibits a weak synergy between vegetation and ocean throughout the holocene finally a sensitivity study to the forcing components shows the dominant role of the astronomical forcing with respect to co2 as well as the non linear behaviour of climate in response to obliquity and precession

a multi model analysis of the role of the ocean on the african and indian monsoon during the mid holocene we investigate the role of the ocean feedback on the climate in response to insolation forcing during the mid holocene 6 000 year bp using results from seven coupled ocean atmosphere general circulation models we examine how the dipole in late summer sea surface temperature sst anomalies in the tropical atlantic increases the length of the african monsoon how this dipole structure is created and maintained and how the late summer sst warming in the northwest indian ocean affects the monsoon retreat in this sector similar mechanisms are found in all of the models including a strong wind evaporation feedback and changes in the mixed layer depth that enhance the insolation forcing as well as increased ekman transport in the atlantic that sharpens the atlantic dipole pattern we also consider changes in interannual variability over west africa and the indian ocean the teleconnection between variations in sst and sahelian precipitation favor a larger impact of the atlantic dipole mode in this region in the indian ocean the strengthening of the indian dipole structure in autumn has a damping effect on the indian dipole mode at the interannual time scale

el nino la nina and sahel precipitation during the middle holocene simulations with a synchronously coupled atmo sphere land ocean sea ice model indicate el nino la nina like events in the eastern tropical pacific ocean during the middle holocene 6 ka with similar intensities and frequencies as at present 0 ka july august september sahel precipitation shifts northward as expected from proxy data for present day interannual decadal variability of western sahel precipi tation is correlated with both pacific el nino la nina sea sur face temperature sst anomalies and the atlantic dipole teleconnections at 6 ka between sahel precipitation and el nino la nina sst anomalies are absent with tropical atlantic ssts asserting a dominant influence these results illustrate potential problems with using present day teleconnection patterns in interpreting past climate variability from proxy data

mechanisms limiting the northward extent of the northern summer monsoons over north america asia and africa abstract mechanisms determining the poleward extent of summer monsoon convergence zones for north america asia and africa are examined in an intermediate atmospheric model coupled with a simple land model and a mixed layer ocean observations show that thermodynamical factors associated with the net heat flux into the atmospheric column provide favorable conditions for the monsoon convergence zone to extend farther poleward than actually occurs to understand the discrepancy a series of experiments are designed to test the importance of mechanisms previously examined in the south american case by the authors namely soil moisture ventilation and the interactive rodwell hoskins mechanism the latter refers to the interaction between baroclinic rossby wave dynamics and convective heating in north america experiments suggest that ventilation by both temperature and moisture advection is a leading effect the interactive rodwell hoskins mechanism tends to favor east coast rainfall and west coast dryne

variations of indian and african monsoons induced by insolation changes at 6 and 9 5 kyr bp this study investigates the role of insolation in controlling the indian and african monsoon evolutions during the holocene using coupled ocean atmosphere simulations of 0 6 9 5 kyr bp climates for which only the variations of earth s orbital configuration are considered the two monsoon systems are enhanced at 6 and 9 5 kyr bp compared to 0 kyr bp as a result of the intensified seasonal cycle of insolation in the northern hemisphere the analysis of daily climatologies indicates that even though the length of the celestial summer season is shorter at 9 5 kyr bp the rainy season is longer than at present emphasis is put on the impact of the precession on the seasonality which partly explains why the relative amplification of the indian and african monsoon varies between 9 5 and 6 kyr bp moreover the changes in snow cover over the tibetan plateau play a critical role in reinforcing the 9 5 kyr bp monsoon in india during spring the results suggest that the teleconnection between convection over india and subsidence over the mediterranean regions through the rodwell and hoskins mechanism has an impact on the development of the african monsoon at 9 5 kyr bp

relationship between the indian monsoon rainfall and the tropospheric temperature over the eurasian continent based on climatological data such as the all india rainfall air for 1949 98 air temperatures in the troposphere and at the land surface for 1949 98 as well as precipitation and outgoing long wave radiation olr for 1979 98 the statistical relationship between the indian monsoon rainfall and tropospheric temperature tt over the eurasian continent is examined by using the correlation analysis and singular value decomposition svd analysis methods the main results are summarized as follows 1 the june september jjas air has significant positive correlations with the 200 700 hpa tt averaged for eurasia in jjas the area with the highest simultaneous correlation of upper tt with air for jjas is located in western eurasia there is also a region with positive simultaneous correlations in the southern subtropics covering southern africa the jjas air also shows significant positive correlations with tt over the eurasian continent in march may mam and with the tt averaged over the tibetan plateau in september and october although some precursory springtime temperature signals for the anomalous summer monsoon are found in the upper tt over eurasia the variation in the upper tt appears mostly independent from that of the land surface temperature 2 the anomalies of upper tt in jjas are closely coupled with the variations in olr related to the monsoon rainfall over tropical asia and africa the spatial structure of the first svd mode for olr anomalies shows the coherent variations in convective activity from northern tropical africa to india while the pattern of the first svd mode for 300 hpa temperature anomalies shows a distribution which is nearly symmetric with respect to the equator with variations of the same sign over subtropical areas in both hemispheres and variations of the opposite sign around equatorial africa this pair of coupled patterns suggests that when the monsoon rains from tropical africa to india become more intense the upper troposphere from northern africa to western asia and over southern africa becomes warmer 3 the observed temperature variation in the upper troposphere over the eurasian continent especially western eurasia during the summer monsoon season is considered to be a result of the anomalous subsidence and thermal advection dynamically induced by anomalous tropical heating associated with the enhanced monsoon rainfall this study suggests that the tropical convective activity linked to the indian monsoon rainfall has an impact on the atmospheric variability on a global scale

holocene paleoclimates of india we present a comprehensive summary of the available palaeoclimate records from india and compare the results from different proxies the results indicate i fluctuating lake levels during the early holocene the period of relatively higher lake levels from increased precipitation efficiency was reached only 7 2 6 0 cal kyr bp possibly due to increased contribution from winter rainfall ii the onset of aridity in nw india could have begun as early as 5 3 cal kyr bp subsequently there were multiple wet events but of shorter duration and smaller magnitude than during the mid holocene iii there is evidence of several short term climate events in the proxy record however in the absence of a rigorous chronological framework a detailed regional correlation is not possible at this stage finally a comparison between marine and terrestrial records indicates that episodes of strongest and weakest monsoon winds were not always associated with wettest and driest episodes respectively in the nw indian lakes

mid holocene land surface conditions in northern africa and the arabian peninsula a data set for the analysis of biogeophysical feedbacks in the climate system large changes in the extent of northern subtropical arid regions during the holocene are attributed to orbitally forced variations in monsoon strength and have been implicated in the regulation of atmospheric trace gas concentrations on millenial timescales models that omit biogeophysical feedback however are unable to account for the full magnitude of african monsoon amplification and extension during the early to middle holocene 9500 5000 years b p a data set describing land surface conditions 6000 years b p on a 1 1 grid across northern africa and the arabian peninsula has been prepared from published maps and other sources of palaeoenvironmental data with the primary aim of providing a realistic lower boundary condition for atmospheric general circulation model experiments similar to those performed in the palaeoclimate modelling intercomparison project the data set includes information on the percentage of each grid cell occupied by specific vegetation types steppe savanna xerophytic woods scrub tropical deciduous forest and tropical montane evergreen forest open water lakes and wetlands plus information on the flow direction of major drainage channels for use in large scale palaeohydrological modeling the data set is available in digital form by anonymous ftp

impact of precession on monsoon characteristics from coupled ocean atmosphere experiments changes in indian monsoon and indian ocean climatology abstract using a fully coupled ocean atmosphere model we have investigated the response of the indian and southeast monsoons to changes in precession with special emphasis on the indian ocean we designed our experiments in order to explore the differences between periods where the monsoon activity is strong compared to the period of reference 115 ka bp and the seasonal timing of the insolation forcing is different the two extreme cases are such that the maximum insolation occurs between the vernal equinox and the summer solstice in one case and between the summer solstice and the autumnal equinox in the other our results show that even though the changes in the land sea contrast that drive the monsoon flow follow quite well the differences in the insolation forcing the regional distribution of monsoon characteristics monsoon flow precipitation etc from africa to southeast asia and over the ocean substantially varies from one simulation to the other large differences are found in the simulated surface temperature and salinity in the indian ocean they are mainly due to changes in the hydrological cycle over the basin precipitation evaporation and river runoff our results strongly emphasise that the timing of the seasonal cycle must be taken into account in analyses of past monsoons and that it may explain some of the leads and lags reported between insolation and various monsoon proxies reported in the literature

abrupt onset and termination of the african humid period rapid climate responses to gradual insolation forcing a detailed ca 100 yr resolution and well dated 18 ams 14 c dates to 23 cal ka bp record of latest pleistocene holocene variations in terrigenous eolian sediment deposition at odp site 658c off cap blanc mauritania documents very abrupt large scale changes in subtropical north african climate the terrigenous record exhibits a well defined period of low influx between 14 8 and 5 5 cal ka bp associated with the african humid period when the sahara was nearly completely vegetated and supported numerous perennial lakes an arid interval corresponding to the younger dryas chronozone punctuates this humid period the african humid period has been attributed to a strengthening of the african monsoon due to gradual orbital increases in summer season insolation however the onset and termination of this humid period were very abrupt occurring within decades to centuries both transitions occurred when summer season insolation crossed a nearly identical threshold value which was 4 2 greater than present these abrupt climate responses to gradual insolation forcing require strongly non linear feedback processes and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non linear climate sensitivity the african monsoon climate system is thus a low latitude corollary to the bi stable behavior of high latitude deep ocean thermohaline circulation which is similarly capable of rapid and large amplitude climate transitions

sensitivity of paleoclimate simulation results to season definitions according to the milankovitch theory slow variations of the earth s orbital parameters change the amplitude of the seasonal cycle of insolation and are considered to be the main forcing mechanism of glacial interglacial cycles because of the precession and changes in eccentricity the length of seasons also varies no absolute phasing is then possible between the insolation curves of two different periods various solutions to compare different periods have been given either for astronomical computations e g berger and loutre 1991 laskar et al 1993 or for model simulations e g kutzbach and otto bliesner 1982 mitchell et al 1988 but the sensitivity of model results to the different possible solutions has never been quantified our results based on simulations of the last interglacial climate 126 kyr b p where changes in the length of the seasons are large clearly show that phase leads or lags between the various solutions used introduce biases in the analysis of insolation and climate change of the same order of magnitude as the milankovitch forcing our main conclusions are that 1 when comparing various model simulations the date of the vernal equinox i e the phasing of the seasonal cycle of insolation as well as the definition of seasons must be the same for all models in order to avoid artificial differences 2 seasons based on astronomical positions are preferred to seasons defined with the same lengths as today since they better account for the phasing of insolation curves however insolation is not the only forcing in most atmospheric general circulation model simulations we also discuss the impact of the calendar hidden behind the definition of the seasonal cycle of the other boundary conditions such as sea ice or sea surface temperatures

a reassessment of lake and wetland feedbacks on the north african holocene climate 1 large parts of the sahara were vegetated during the early to mid holocene several positive feedbacks most notably related to vegetation have been shown to have favored the northward migration of the desert boundary during this period numerous lakes and wetlands existed in the sahara region and might have acted as a local moisture source however earlier model studies of the effects of open water surfaces on the mid holocene north african climate suggested that these were weak and did not contribute significantly to this northward migration of the north african climate zones using a state of the art climate model we suggest that the effect of open water surfaces on the mid holocene north african climate might have been much stronger than previously estimated regionally more than doubling the simulated precipitation rates it is thus possible that this effect combined to other known positive feedbacks favored the appearance of the green sahara

mechanism of heating and the boundary layer over the tibetan plateau abstract the structure of the boundary layer and the mechanism of heating over the tibetan plateau are examined using the data obtained from the first garp global atmospheric research program global experiment and the chinese qinghai xizang tibet plateau meteorological experiment from may to august 1979 the meteorological elements near the plateau surface exhibit pronounced diurnal variations there is a large ground air temperature difference during the daytime generating a thin layer of superadiabatic lapse rates near the surface the horizontal wind speed attains the minimum in the morning and the maximum in the evening a deep and well mixed layer of potential temperature is observed over the western and central plateau in the evening 1200 utc however moisture is not well mixed vertically and water vapor mixing ratio is larger in the morning 0000 utc than in the evening the mixed layer becomes shallower from the western to the central plateau and it disappears over the eastern plateau the

monsoons and the dynamics of deserts the existence of subtropical deserts such as the sahara has often been attributed to the annual mean zonal mean hadley circulation which shows strong descent in the subtropics however the zonal mean hadley circulation shows considerable evolution over the course of the year with very strong subtropical descent during winter but practically no zonal mean subtropical descent during summer when rainfall over the eastern sahara and the mediterranean is least charney 1975 proposed a biosphere albedo feedback mechanism whereby local anthropogenic effects related to over grazing could affect the radiative balance enhancing summertime diabatic descent and leading to desertification of the subtropics in general the present study which uses an idealized model suggests a monsoon desert mechanism for desertification whereby remote diabatic heating in the asian monsoon region can induce a rossby wave pattern to the west integral with the rossby wave solution is a warm thermal structure that interacts with air on the southern flank of the mid latitude westerlies causing it to descend this adiabatic descent is localized over the eastern sahara and mediterranean and over the kyzylkum desert to the south east of the aral sea by the mountains of north africa and south west asia trajectories indicate that the monsoon desert mechanism does not represent a simple walker type overturning cell instead the descending air is seen to be mainly of mid latitude origin it is speculated that the monsoon forced adiabatic descent may result in clear air and therefore a local diabatic enhancement which effectively doubles the strength of descent with this mechanism desertification can be forced by remote changes in monsoon strength rather than by local effects this conclusion is supported by the observed dramatic strengthening of descent over the mediterranean and east sahara during the onset of the asian monsoon and on the longer timescale by relating prehistoric lake levels to milankovitch monsoon forcing the latter may help to explain the perceived discrepancies between the palaeoclimate of the eastern sahara and the strength of a tropic wide monsoon the monsoon desert mechanism may not be confined to the asian monsoon alone and the existence of other monsoon climate regions over the globe may in a similar way explain the observed summertime strengthening of the oceanic sub tropical anticyclones and the existence of western continental deserts and mediterranean type climate regions

global monsoons in the mid holocene and oceanic feedback the response of the six major summer monsoon systems the north american monsoon the northern africa monsoon the asia monsoon the northern australasian monsoon the south america monsoon and the southern africa monsoon to mid holocene orbital forcing has been investigated using a coupled ocean atmosphere general circulation model foam with the focus on the distinct roles of the direct insolation forcing and oceanic feedback the simulation result is also found to compare well with the ncar csm the direct effects of the change in insolation produce an enhancement of the northern hemisphere monsoons and a reduction of the southern hemisphere monsoons ocean feedbacks produce a further enhancement of the northern africa monsoon and the north american monsoon however ocean feedbacks appear to weaken the asia monsoon although the overall effect direct insolation forcing plus ocean feedback remains a strengthened monsoon the impact of ocean feedbacks on the south american and southern african monsoons is relatively small and therefore these regions especially the south america experienced a reduced monsoon regime compared to present however there is a strong ocean feedback on the northern australian monsoon that negates the direct effects of orbital changes and results in a strengthening of austral summer monsoon precipitation in this region a new synthesis is made for mid holocene paleoenvironmental records and is compared with the model simulations overall model simulations produce changes in regional climates that are generally consistent with paleoenvironmental observations

a snapshot of climate variability at tahiti at 9 5 ka using a fossil coral from iodp expedition 310 1 the integrated ocean drilling program iodp expedition 310 recovered drill cores from the drowned reefs around the island of tahiti 17 40 s 149 30 w many of which contained samples of massive corals from the genus porites herein we report on one well preserved fossil coral sample a 13 6 cm long porites sp dated by uranium series techniques at 9523 33 years monthly 18o and sr ca determinations reveal nine clear and robust annual cycles coral 18o and sr ca determinations estimate a mean temperature of 24 3 c 3 2 c colder than modern for tahiti at 9 5 ka however this estimate is viewed with caution since potential sources of cold bias in coral geochemistry remain to be resolved the interannual variability in coral 18o is similar between the 9 5 ka coral record and a modern record from nearby moorea the seasonal cycle in coral sr ca is approximately the same or greater in the 9 5 ka coral record than in modern coral records from tahiti paired analysis of coral 18o and sr ca indicates cold wet warm dry interannual anomalies opposite from those observed in the modern instrumental record

modeling climate shift of el nino variability in the holocene a coupled ocean atmosphere general circulation model is used to investigate climatic shift of el nino in the holocene the model simulates a reduced enso intensity in the early and mid holocene in agreement with paleoclimate record the enso reduction is proposed to be caused by both an intensified asian summer monsoon and a warm water subduction from the south pacific into the equatorial thermocline

detecting decadal changes in enso using neural networks the present manuscript analyzes monthly equatorial pacific indices by using a specific neural algorithm the so called self organizing maps soms the main result is a change found in the nature of the transitions between cold to warm and warm to cold extreme events from 1950 to present around the late 1970s som is an unsupervised clustering technique which allows one to reduce high dimensional data space in this case three indices over 636 months in terms of a smaller set of three dimensional reference vectors 100 characterizing pertinent situations these reference vectors which are displayed on a two dimension map are closely related by a topological relationship leading us to discriminate la nina conditions from the opposite el nino conditions in a second step a hierarchical agglomerative clustering hac method is used to further group the reference vectors into a small number of clusters 12 whose spatial and temporal characteristics can be analyzed and interpreted in terms of physical parameters schematically these 12 clusters can be divided into two warm clusters six neutral or transition clusters and four cold clusters in each particular group warm neutral cold the clusters mainly differ from each other by the amplitude of the anomalies their spatial patterns and their temporal variability some clusters are found to be strongly linked to the boreal spring period while others have barely any records during that season other clusters are associated with records mainly observed either prior to or after 1980 this suggests that the method is able to identify changes in the variability of the tropical pacific basin observed on decadal time scales 1976 climate shift in our case each monthly record can be summarized by the cluster to which it belongs the temporal evolution of this value during extreme enso events shows similar patterns persistence in specific clusters and transition between groups of clusters associated with comparable el nino or la nina events the methodology described in the present study som plus hac is suggested to be useful both for seasonal enso predictability and for the detection of decadal changes in enso behavior

observed relationship of spring and summer east asian rainfall with winter and spring eurasian snow abstract this study investigates the relationship between spring and summer rainfall in east asia and the preceding winter and spring snow cover depth over eurasia using station rainfall observations satellite observed snow cover satellite derived snow water equivalent and station observations of the number of days of snow cover and snow depth correlation analysis shows that snow depth anomalies can persist from winter to spring whereas snow cover anomalies cannot in most regions of eurasia locally snow cover and snow depth anomalies in february are not related in most regions to the north of 50 n but those anomalies in april display consistent year to year variations the results suggest that the winter snow cover cannot properly represent all the effects of snow and it is necessary to separate the winter and spring snow cover in addressing the snow monsoon relationship spring snow cover in western siberia is positively correlated with spring rainfall in southern china the circulation anomalies

holocene itcz and indian monsoon dynamics recorded in stalagmites from oman and yemen socotra abstract high resolution oxygen isotope 18o profiles of holocene stalagmites from four caves in northern and southern oman and yemen socotra provide detailed information on fluctuations in precipitation along a latitudinal transect from 12 n to 23 n 18o values reflect the amount of precipitation which is primarily controlled by the mean latitudinal position of the itcz and dynamics of the indian summer monsoon ism during the early holocene rapidly decreasing 18o values indicate a rapid northward displacement in the mean latitudinal position of the summer itcz and the associated ism rainfall belt with decadal to centennial scale changes in monsoon precipitation correlating well with high latitude temperature variations recorded in greenland ice cores during the middle to late holocene the summer itcz continuously migrated southward and monsoon precipitation decreased gradually in response to decreasing solar insolation a trend which is also recorded in other monsoon records from the indian and east asian monsoon domains importantly there is no evidence for an abrupt middle holocene weakening in monsoon precipitation although abrupt monsoon events are apparent in all monsoon records they are short lived and clearly superimposed on the long term trend of decreasing monsoon precipitation for the late holocene there is an anti correlation between ism precipitation in oman and inter monsoon spring autumn precipitation on socotra revealing a possible long term change in the duration of the summer monsoon season since at least 4 5 ka bp together with the progressive shortening of the ism season gradual southward retreat of the mean summer itcz and weakening of the ism the total amount of precipitation decreased in those areas located at the northern fringe of the indian and asian monsoon domains but increased in areas closer to the equator

response of the african monsoon to orbital forcing and ocean feedbacks in the middle holocene simulations with a climate model that asynchronously couples the atmosphere and the ocean showed that the increased amplitude of the seasonal cycle of insolation in the northern hemisphere 6000 years ago could have increased tropical atlantic sea surface temperatures in late summer the simulated increase in sea surface temperature and associated changes in atmospheric circulation enhanced the summer monsoon precipitation of northern africa by more than 25 percent compared with the middle holocene simulation with prescribed modern sea surface temperatures and provided better agreement with paleorecords of enhanced monsoons

an idealized two dimensional framework to study the west african monsoon part i validation and key controlling factors abstract an idealized vertical meridional zonally symmetric model is developed in order to recover a july typical monsoon regime over west africa in response to surface conditions the model includes a parameterization to account for heat and momentum fluxes associated with eddies the sensitivity of the simulated west african monsoon equilibrium regime to some major processes is explored it allows confirmation of the important role played by the sun s latitudinal position the aerosols the albedo and the sst s magnitude in the gulf of guinea and in the mediterranean sea the important role of aerosols in warming the saharan lower layers and their effect on the whole monsoon is underlined model results also stress the importance of the mediterranean sea which is needed to obtain the extreme dryness of the sahara the use of this idealized model is finally discussed for studying the scale interactions and coupling involved in the west african monsoon as explored in a companion paper

an andean ice core record of a middle holocene mega drought in north africa and asia an ice core from the nevado huascaran col in the cordillera blanca of northern peru contains high resolution time series of dust concentrations and size distributions since the end of the last glacial stage a large dust peak dated 4500 years ago is contemporaneous with a widespread and prolonged drought that apparently extended from north africa to eastern china evidence of which occurs in historical archeological and paleoclimatic records this event may have been associated with several centuries of weak asian indian african monsoons possibly linked with a protracted cooling in the north atlantic during the second half of the 20th century high austral summer dust concentrations in the huascaran record are significantly correlated with atmospheric conditions such as sea level pressure and zonal wind velocities that are consistent with el nino southern oscillation enso and positive north atlantic oscillation nao indices and with aridity in north africa southwest asia and the middle east therefore the dominant submicron fraction of the dust may have been transported by more intense northeasterly trade winds from the african dry regions across the tropical atlantic during a period of frequent and or intense enso activity the proposed enso conditions that may have been linked with drought in the monsoon region may also have contributed to aridity in tropical south america including the cordillera blanca

quantifying the role of biosphere atmosphere feedbacks in climate change coupled model simulations for 6000 years bp and comparison with palaeodata for northern eurasia and northern africa the lmd agcm was iteratively coupled to the global biome1 model in order to explore the role of vegetation climate interactions in response to mid holocene 6000 y bp orbital forcing the sea surface temperature and sea ice distribution used were present day and co2 concentration was pre industrial the land surface was initially prescribed with present day vegetation initial climate anomalies differences between agcm results for 6000 y bp and control were used to drive biome1 the simulated vegetation was provided to a further agcm run and so on results after five iterations were compared to the initial results in order to identify vegetation feedbacks these were centred on regions showing strong initial responses the orbitally induced high latitude summer warming and the intensification and extension of northern hemisphere tropical monsoons were both amplified by vegetation feedbacks vegetation feedbacks were smaller than the initial orbital effects for most regions and seasons but in west africa the summer precipitation increase more than doubled in response to changes in vegetation in the last iteration global tundra area was reduced by 25 and the southern limit of the sahara desert was shifted 2 5 n north to 18 n relative to today these results were compared with 6000 y bp observational data recording forest tundra boundary changes in northern eurasia and savana desert boundary changes in northern africa although the inclusion of vegetation feedbacks improved the qualitative agreement between the model results and the data the simulated changes were still insufficient perhaps due to the lack of ocean surface feedbacks

tropical pacific climate at the mid holocene and the last glacial maximum simulated by a coupled ocean atmosphere general circulation model 1 simulations for the mid holocene 6000 years before present 6 ka and the last glacial maximum lgm 21 ka have been performed by a global ocean atmosphere coupled general circulation model gcm after the initial spin up periods both runs were integrated for about 200 years for 6 ka the model shows an enhanced seasonal variation in surface temperature and a northward shift of the african and the indian summer monsoon rain area overall circulation features in the tropics correspond to a strong walker circulation state with negative sea surface temperature sst and precipitation anomalies in the central pacific and positive precipitation anomalies over the indian and australian monsoon regions it is noted that there is about a 0 35 c cooling of the global mean sst in contrast to the 6 ka result the simulated tropical climate anomaly at 21 ka corresponds to a weak walker circulation state the simulated lgm sst decrease is about 2 c over the tropical western pacific it is larger about 5 c over the caribbean sea this sst anomaly is in broad agreement with the observed proxy data interestingly the model simulates a warmer than present sst over the subtropical pacific this may be related to a weaker subtropical anticyclone due to weakened monsoon circulation at the lgm

timing of the southward retreat of the itcz at the end of the holocene humid period in southern arabia data model comparison new pollen data from omani mangroves kwar al jaramah 22 49 n 59 76 e and filim 20 61 n 58 17 e in addition to previously published paleohydrological records from southern arabia improve our understanding of the timing and amplitude of the southward retreat of the indian monsoon influence in southern arabia along a north south transect comparison with simulations performed with the ipsl climate model considering both snapshot experiments and transient simulations from 6000 cal yr bp to the present confirm the latitudinally time transgressive nature of the humid arid transition at the end of the holocene humid period this occurred in two steps respectively dated at around 5000 and 2700 cal yr bp at around 5000 cal yr bp the southward itcz shift was orbitally driven and led to the abrupt aridification at kwar al jaramah and the progressive increase of dryness at filim as the mean position of the itcz was centered ca 22 30 n at 2700 cal yr bp aridity was fully in place over all of southern arabia due to increased climate variability more intense rainy events during the last millennium however may have contributed to the discrete hydrological improvement without any impact on the regional vegetation which has remained desert to the present day

mid holocene climate in new caledonia southwest pacific coral and pmip models monthly resolved results abstract the mid holocene climate is characterized by an insolation seasonality decrease in the southern hemisphere but measurements of its actual impact on monthly resolved sea surface temperature sst in the southwest pacific region are still insufficient a new caledonian 5 5 ka cal bp coral provides a 20 year long seasonally resolved record of ocean surface conditions as inferred from coral sr ca and ba ca results were compared to monthly series of paleoclimate modeling intercomparison project phase 2 pmip2 model simulations anomalous stable isotope and u ca values are observed in a restricted area of the skeleton related to dissolution features the mid holocene sst seasonal amplitude mean and variability are higher than presently in new caledonia the increased seasonal amplitude in the mid holocene being most probably due to the occurrence of colder winters other southwest pacific mid holocene coral data showed also such an increased seasonal amplitude this could mean that the south pacific convergence zone spcz was weaker or reached locations more northerly than at present which could fit with northward shifts of the inter tropical convergence zone itcz during south hemisphere winter inversely strong rainfalls during the summer deduced from the ba ca signal and reconstructed sea surface salinity sss were interpreted as reflecting pronounced southwestward shifts of the spcz in summer as those occurring today during la nina events none of the six pmip2 models used reproduce the proxy based mid holocene increase of sst seasonal amplitude model maps show a less intense spcz in winter that would be consistent with higher sst seasonal amplitude finally we stress the need for more seasonally resolved data to validate this enhanced mid holocene sst seasonal amplitude in the southwest pacific region and to better understand the underlying mechanisms

indian monsoon variations during three contrasting climatic periods the holocene heinrich stadial 2 and the last interglacial glacial transition in contrast to the east asian and african monsoons the indian monsoon is still poorly documented throughout the last climatic cycle last 135 000 years pollen analysis from two marine sediment cores nghp 01 16a and nghp 01 19b collected from the offshore godavari and mahanadi basins both located in the core monsoon zone cmz reveals changes in indian summer monsoon variability and intensity during three contrasting climatic periods the holocene the heinrich stadial hs 2 and the marine isotopic stage mis 5 4 during the ice sheet growth transition during the first part of the holocene between 11 300 and 4200 cal years bp characterized by high insolation minimum precession maximum obliquity the maximum extension of the coastal forest and mangrove reflects high monsoon rainfall this climatic regime contrasts with that of the second phase of the holocene from 4200 cal years bp to the present marked by the development of drier vegetation in a context of low insolation maximum precession minimum obliquity the historical period in india is characterized by an alternation of strong and weak monsoon centennial phases that may reflect the medieval climate anomaly and the little ice age respectively during the hs 2 a period of low insolation and extensive iceberg discharge in the north atlantic ocean vegetation was dominated by grassland and dry flora indicating pronounced aridity as the result of a weak indian summer monsoon the mis 5 4 glaciation also associated with low insolation but moderate freshwater fluxes was characterized by a weaker reduction of the indian summer monsoon and a decrease of seasonal contrast as recorded by the expansion of dry vegetation and the development of artemisia respectively our results support model predictions suggesting that insolation changes control the long term trend of the indian monsoon precipitation but its millennial scale variability and intensity are instead modulated by atmospheric teleconnections to remote phenomena in the north atlantic eurasia or the indian ocean

centennial to millennial scale variability of the indian monsoon during the early holocene from a sediment pollen and isotope record from the desert of yemen lacustrine deposits of al hawa 15 52 n 46 53 e 710 m above sea level document the climatic and environmental history of the inland desert of yemen during the early to mid holocene a freshwater lake expanded in one of the most arid areas of the world in response to increased indian monsoon fluxes from 12 000 to 7500 cal b p three dry intervals punctuated the lacustrine phase recording episodes of weaker summer monsoon activity over arabia dry intervals were coeval with cold periods recorded in the north atlantic confirming the links between the indian monsoon and the north atlantic systems during the holocene we demonstrate that the regional vegetation remained of semi arid character throughout the lacustrine period

reinterpretation of tractus fossae region as an asymmetric rift system on mars the tractus fossae region of mars is a wide area dominated by grabens normal faults and pit crater chains in this work based on previous studies on the area and the new interpretation of topographic data and morphological units based on images as a geologic framework we present a new insight on the origin of the graben structures as well as on the processes associated with the formation of the volcanic features here we propose a new model for this region on the basis of new measurements of graben extension and geological interpretations together with a reinterpretation of the stratigraphy and the geologic history of the area finally it is postulated that this region underwent a tectonic regime analog to an asymmetric rift system on earth

evolution and variability of the asian monsoon system state of the art and outstanding issues the asian monsoon is comprised of the indian and east asian subsystems these two components are linked to one another in varying degrees by regions of strong sensible heating indo asian landmass and strong latent heat export the western pacific warm pool and the southern subtropical indian ocean variability within the indian and east asian subsystems interactions among them and the extent to which they interact with other climate phenomena e g enso are current topics of modern and paleoclimate research this work provides an overview of past and current paleomonsoon research on tectonic to interannual time scales with a primary focus on marine sediment records while past and current work has contributed greatly to our understanding of paleomonsoon variability at all time scales additional efforts are required to make further progress in two critical areas 1 additional efforts are needed in terms of proxy development and evaluation requiring concerted efforts at long term sediment trap deployments in key monsoon influenced regions as well as development of adequate and widely available core top databases these are necessary to assess the impact of modern oceanographic and seafloor processes on potential monsoon proxies 2 additional effort is also needed in acquiring a sufficient geographic distribution of downcore records to assess linkages among the subsystems and their role in the context of extratropical climate change these records should have high sedimentation rates including varved sections this requires substantial survey support to identify the most appropriate coring and drilling targets c 2004 elsevier ltd all rights reserved