role of topography in forcing low level jets in the central united states during the 1993 flood altered terrain simulations regional model sensitivity simulations in which the height of elevated terrain was reduced to explore simulated changes in features of the low level jet llj are presented such an approach has not been reported and it provides complementary insight to the previous llj studies the simulations were carried out for a 45 day period during the 1993 summer flood in the central united states when strong lljs were frequent the simulations illustrate directly the significance of topographical blocking leeside cyclogenesis and terrain thermal effects exerted by the rocky mountains in support of llj formation in particular it is shown that in the absence of topography the ridging from the bermuda high extended considerably westward with weaker southerly flow over the high plains thus diminishing the potential for llj development the slope induced nocturnal horizontal thermal gradient was indicated to have a significant role in the formation of the llj

el nino evolution during the holocene revealed by a biomarker rain gauge in the galapagos islands abstract the el nino southern oscillation enso represents the largest perturbation to the climate system on an inter annual time scale but its evolution since the end of the last ice age remains debated due to the lack of unambiguous enso records lasting longer than a few centuries changes in the concentration and hydrogen isotope ratio of lipids produced by the green alga botryococcus braunii which blooms during el nino rains in the galapagos islands indicate that the early holocene 9200 5600 yr bp was characterized by alternating extremes in the intensity and or frequency of el nino events that lasted a century or more our data from the core of the enso region thus calls into question earlier studies that reported a lack of el nino activity in the early holocene in agreement with other proxy evidence from the tropical pacific the mid holocene 5600 3500 yr bp was a time of consistently weak el nino activity as were the early middle ages 1000 1500 yr bp el nino activity was moderate to high during the remainder of the last 3500 years periods of strong or frequent el nino tended to occur during peaks in solar activity and during extended droughts in the united states great plains linked to la nina these changing modes of enso activity at millennial and multi centennial timescales may have been caused by variations in the seasonal receipts of solar radiation associated with the precession of the equinoxes and or changes in solar activity respectively

rapid flooding of the sunda shelf a late glacial sea level record the increase in sea level from the last glacial maximum has been derived from a siliciclastic system on the tectonically stable sunda shelf in southeast asia the time from 21 to 14 thousand calendar years before the present has been poorly covered in other records the record generally confirms sea level reconstructions from coral reefs the rise of sea level during meltwater pulse 1a was as much as 16 meters within 300 years 14 6 to 14 3 thousand years ago

impacts of el nino and la nina on the u s climate during northern summer the impacts of el nino and la nina on the u s climate during northern summer are analyzed separately composite analyses reveal that a continental scale anomalous high dominates over most of north america during la nina events and leads to hot and dry summers over the central united states however the impacts of el nino over north america are weaker and more variable a linear barotropic model is used to explore the maintenance of the anomalous patterns various forcing terms derived from observations via a single level vorticity budget analysis are used to drive the model when the barotropic model is driven by the total forcing rossby wave source plus transient eddy forcing plus nonlinear interactions the model simulations resemble the observed patterns and a strong and extensive anticyclone is reproduced in the la nina simulation the model responses to the individual forcing terms suggested that the vorticity stretching term fd and the transient eddy forcing contribute most to the responses over north america the stretching term fd excites a low in the el nino simulation and a high in the la nina simulation over north america however the transient eddy forcing favors an anomalous high over north america in both el nino and la nina simulations such that it weakens the el nino pattern and strengthens the la nina pattern

n alkane and pollen reconstruction of terrestrial climate and vegetation for n w africa over the last 160 kyr abstract proxy environmental records have been obtained for the last 160 kyr from the well studied odp site 658 ca 200 km west of cap blanc n w africa 20 5 n 18 35 w 2263 m water depth this collective assessment of several terrigenous proxies lithogenic fraction n alkane content and 13 c values and pollen counts provides a better understanding of the climate and vegetation history of the n w african hinterland the results indicate that for this site beneath the main dust pathway of the saharan air layer sal the dust flux increased during dry glacial periods but the amounts of pollen and n alkanes delivered were lower in accord with the decreased density of vegetation cover of their source areas the c 4 pollen was also very low but the heavy 13 c values of the n alkanes accorded with a mainly c 4 plant origin evidently the load of sal pollen was much reduced but not the sal wind strength

paleo sea levels reconsidered from direct observation of paleoshoreline position during glacial maxima for the last 500 000 yr the drastic climatic changes which characterise the cooling trend of the last few million years of earth history led to variations in eustatic sea level that had tremendous impact on the geology and ecology of continental margins reconstructing a sea level curve back in time is not an easy task observations of shoreline positions are always a local measurement of relative sea level that needs to be corrected from the effect of tectonic and thermal subsidence sediment loading compaction and glacio hydro isostasy extensive studies have been done for the last deglaciation and for the last 100 000 yr cycle but very few studies deal with position of sea level during earlier cycles simply because conditions are very rarely favourable for the preservation of such witnesses the shelf of the golfe du lion western mediterranean reveals a unique record of shoreline paleopositions during glacial maxima of at least the last five circa 100 kyr glacial interglacial cycles in fact it is the entire glacial deltaic lobe of up to 50 m thick from delta front or shoreface to prodelta that has been preserved in place and which provides direct and independent constraints for relative sea level minima we measure a relative sea level of 112m 128 134 246 and 262 m for mis 2 6 8 10 and 12 respectively after corrections taking into account postdepositional movement of strata subsidence we find that sea level dropped to a depth of 102 6 m during the last three glaciations mis2 mis6 mis8 but reached exceptionally low values of more than 150 10 m during the preceding glaciations mis10 and mis 12 at about 340 and 434 kyr bp this general time framework and sedimentological interpretation has been confirmed by preliminary results from two deep drillings during the promess cruise july 2004 which validate our methodology however no detailed and absolute datings of such witnesses are available so far so that we cannot prove that these levels are the lowest ever reached during each glacials but they correspond undoubtedly to the last preserved shoreface before rapid sea level rise we also suggest that the abrupt change in sea level maxima might be the overprint of 400 kyr orbital periodicity cycles last but not least these results prove that the golfe du lion is indeed a unique laboratory to study paleoclimates and sea level variations on a larger time scale further work is needed for a complete glacio hydro sedimento isostatic modelling of each sequence and each glacial to further constraint local sea level versus global sea level and quantify in particular the relative effect of glacio hydro isostatic effect which differ according to ice sheet extend but also of erosion sedimentation isostatic effect erosion on land and deposition on the outer shelf and slope

tracking the last sea level cycle seafloor morphology and shallow stratigraphy of the latest quaternary new jersey middle continental shelf seafloor geomorphology and surficial stratigraphy of the new jersey middle continental shelf provide a detailed record of sea level change during the last advance and retreat of the laurentide ice sheet 120 kyr b p to present a nw se oriented corridor on the middle shelf between water depths of 40 m the mid shelf paleo shore and 100 m the franklin paleo shore encompasses 500 line km of 2d huntec boomer profiles 500 3500 hz an embedded 4 6 km2 3d volume and a 490 km2 swath bathymetry map we use these data to develop a relative stratigraphy core samples from published studies also provide some chronological and sedimentological constraints on the upper 5 m of the stratigraphic succession the following stratigraphic units and surfaces occur from bottom to top 1 r a high amplitude reflection that separates sediment 46 5 kyr old by ams 14c dating from overlying sediment wedges 2 the outer shelf wedge a marine unit up to 50 m thick that onlaps r 3 channels a reflection sub parallel to the seafloor that incises r and appears as a dendritic system of channels in map view 4 channels fill the upper portion of which is sampled and known to represent deepening upward marine sediments 12 3 kyr in age 5 the t horizon a seismically discontinuous surface that caps channels fill 6 oblique ridge deposits coarse grained shelly units comprised of km scale shallow shelf bedforms and 7 ribbon floored swales bathymetric depressions parallel to modern shelf currents that truncate the oblique ridges and cut into surficial deposits we interpret this succession of features in light of a global eustatic sea level curve and the consequent migration of the coastline across the middle shelf during the last 120 kyr the morphology of the new jersey middle shelf shows a discrete sequence of stratigraphic elements and reflects the pulsed episodicity of the last sea level cycle r is a complicated marine non marine erosional surface formed during the last regression while the outer shelf wedge represents a shelf wedge emplaced during a minor glacial retreat before maximum wisconsin lowstand i e marine oxygen isotope stage 3 1 channels is a widespread fluvial subarial erosion surface formed at the late wisconsin glacial maximum 22 kyr b p the shoreline migrated back across the mid shelf corridor non uniformly during the period represented by channels fill oblique ridges are relict features on the new jersey middle shelf while the ribbon floored swales represent modern shelf erosion there is no systematic relationship between modern seafloor morphology and the very shallowly buried stratigraphic succession

sea level changes in the tectonically stable malay thai peninsula abstract the malay thai peninsula is part of sundaland a geological region of relative crustal stability therefore the positions of old shorelines of the past 20 000 years or so can be considered to indicate positions of actual past sea levels over 200 shoreline indicators at positions below as well as above present sea level have been radiocarbon dated these dated positions indicate that prior to 6 ka the sea rose from at least 90 m below present sea level at rates that initially amounted to 15 mm year and later became 6 mm year at ca 6 ka the rising sea surpassed its current position and at ca 5 ka it reached 5 m during this maximum marine inundation the peninsula was narrower since the low coastal plains did not yet exist in the period following this maximum transgression the available shoreline data indicate that regional sea level subsided through a series of fluctuations that were characterized by amplitudes of 2 m and periods of about 2000 years the peaks and troughs of the fluctuations became progressively lower the latest peak was at about 1 7 m or 0 7 m above present high tide a few hundred years ago the cause of these fluctuations is not yet known but the trend suggests that in the near future regional sea level may subside at annual rates between 1 5 mm and 2 mm this rate of descent is 25 to 30 of the estimated rate of sea level rise that may result from global warming in other words for the malay thai peninsula the effect of sea level rise on account of manmade greenhouse gases will be partly compensated by actual subsidence of regional sea level

late quaternary sedimentation within a submarine channel levee system offshore cap timiris mauritania aeolian and fluvial sediment transport to the atlantic ocean offshore mauritania were reconstructed based on grain size distributions of the carbonate free silt fraction of three marine sediment records of cap timiris canyon to monitor the climatic evolution of present day arid north western africa during the late pleistocene predominantly coarse grained particles which are interpreted as windborne dust characterise glacial dry climate conditions with a low sea level and extended sand seas that reach onto the exposed continental shelf off mauritania subsequent particle fining and the abrupt decrease in terrigenous supply are attributed to humid climate conditions and dune stabilisation on the adjacent african continent with the onset of the holocene humid period indications for an ancient drainage system which was discharging fluvial mud offshore via cap timiris canyon are provided by the finest end member for early to mid holocene times however in comparison to the senegal and niger river further south the river system connecting cap timiris canyon with the mauritanian hinterland was starved during the late holocene and is non discharging under present day arid climate conditions

multiscale diagnosis of the north american monsoon system using a variable resolution gcm abstract the onset and evolution of the north american monsoon system during the summer of 1993 were examined from regional to large scales using the national aeronautics and space administration nasa goddard earth observing system geos stretched grid gcm the model s grid spacing for the dynamical core ranges from 0 4 0 5 in latitude longitude over the united states to about 2 5 3 5 at the antipode and the physical package is solved on an intermediate 1 1 uniform grid a diagnostic analysis of the monsoon s onset reveals the development of a positive potential temperature anomaly at the surface that favors a lower level cyclonic circulation while a negative potential vorticity pv anomaly below the tropopause induces an upper level anticyclonic circulation ignoring diabatic effects this pattern is consistent with the superimposition of idealized pv and anomalies as previously discussed in the literature the inclusion of the smaller scale features of the core monsoon in the mod

the senegal river mud belt a high resolution archive of paleoclimatic change and coastal evolution abstract fine grained sediment depocenters on continental shelves are of increased scientific interest since they record environmental changes sensitively a north south elongated mud depocenter extends along the senegalese coast in mid shelf position shallow acoustic profiling was carried out to determine extent geometry and internal structures of this sedimentary body in addition four sediment cores were retrieved with the main aim to identify how paleoclimatic signals and coastal changes have controlled the formation of this mud depocenter a general paleoclimatic pattern in terms of fluvial input appears to be recorded in this depositional archive intervals characterized by high terrigenous input high sedimentation rates and fine grain sizes occur roughly contemporaneously in all cores and are interpreted as corresponding to intensified river discharge related to more humid conditions in the hinterland from 2750 to 1900 and from 1000 to 700 cal a bp wetter conditions are recorded off senegal an observation which is in accordance with other records from nw africa nevertheless the three employed proxies sedimentation rate grain size and elemental distribution do not always display consistent inter core patterns major differences between the individual core records are attributed to sediment remobilization which was linked to local hydrographic variations as well as reorganizations of the coastal system the senegal mud belt is a layered inhomogeneous sedimentary body deposited on an irregular erosive surface early holocene deceleration in the rate of the sea level rise could have enabled initial mud deposition on the shelf these favorable conditions for mud deposition occur coevally with a humid period over nw africa thus high river discharge sedimentation started preferentially in the northern areas of the mud belt during mid holocene a marine incursion led to the formation of an embayment afterwards sedimentation in the north was interrupted in association with a remarkable southward shift in the location of the active depocenter as it is reflected by the sedimentary architecture and confirmed by radiocarbon dates these sub recent shifts in depocenters location are caused by migrations of the senegal river mouth during late holocene times the weakening of river discharge allowed the longshore currents to build up a chain of beach barriers which have forced the river mouth to shift southwards

rapid rise of sea level 19 000 years ago and its global implications evidence from the irish sea basin supports the existence of an abrupt rise in sea level meltwater pulse at 19 000 years before the present b p climate records indicate a large reduction in the strength of north atlantic deep water formation and attendant cooling of the north atlantic at this time indicating a source of the meltwater pulse from one or more northern hemisphere ice sheets warming of the tropical atlantic and pacific oceans and the southern hemisphere also began at 19 000 years b p these responses identify mechanisms responsible for the propagation of deglacial climate signals to the southern hemisphere and tropics while maintaining a cold climate in the northern hemisphere

abrupt post glacial climate events in west asia and north africa monsoon domains regions beyond the present or past penetration of the indian and african monsoons have experienced several large and abrupt climatic fluctuations over the past 13 14c kyr pollen and lake records from west asia western tibet and rajasthan east africa ethiopia and west africa western sahara sahel and subequatorial africa were selected on the basis of chronological control sensitivity of both site and environmental indicators to climate change the continuity of the record and interdisciplinary control of the palaeoclimatic interpretation conditions wetter than those of today prevailed during the early mid holocene period but major dry spells are recorded at all sites during the intervals 11 0 9 5 kyr bp 8 7 kyr bp and 3 4 kyr bp several records also suggest dry events of minor amplitude around 6 kyr bp potential boundary forcings of insolation and sea surface and tropical land surface conditions are discussed the solar radiation accounts for the general envelop of the post glacial monsoon fluctuations but explains neither the timing nor the amplitude of the short term changes in spite of apparent covariation between fluctuations in sea surface conditions in the north atlantic and the monsoon record no direct mechanism could be found relating the intensity of the oceanic thermohaline conveyor belt to the monsoon strength changes in tropical land surface conditions soil moisture negative feedback and changes in ch4 production from wetlands provide a more satisfactory hypothesis for explaining abrupt reversal events

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

on the fundamental difference between coal rank and coal type abstract this article addresses the fundamental difference between coal rank and coal type while theoretically settled long ago as being different aspects of coal systems science the two concepts are still often confounded in recent years this has resulted in the publication of several works stating that coal type changes with coal rank coal type refers solely to coals depositional origin and the maceral mineral admixture resulting from that origin coal types typically fall in to two categories humic coals developed from peat and sapropelic coals developed from organic mud either type may be allocthonous or autochthonous and within types further refinement of depositional environment can be made coal rank refers to the changes in geochemistry and resultant changes in reflectance caused by increasing thermal maturity of the coal thus it provides an overprint of maturity on existing coal types with proper techniques such as use of crossed polars and etching maceral forms can be differentiated even at high ranks and the original coal type determined

physical processes involved in the 1988 drought and 1993 floods in north america abstract an analysis of the spring summer 1988 drought and 1993 floods over north america reveals a reversal in the sign of anomalies in several fields large sea surface temperature anomalies of opposite signs existed in the tropical pacific with strong la nina conditions in 1988 and a mature el nino in 1993 the distribution of tropical convection in the convergence zones and associated latent heating of the atmosphere were correspondingly altered implying a large scale switch in the anomalous tropical heating and forcing of extratropical quasi stationary waves in the atmosphere influencing the subtropical jet stream over the north pacific and across north america in 1988 the jet stream and the closely related storm track of high frequency disturbances in the upper troposphere were displaced into canada well north of the normal location the farthest north of any year from 1979 to 1993 in 1993 a broader jet stream and the storm track were displaced well south of normal to a more springlike location a

the changing relationship between enso variability and moisture balance in the continental united states the el nino southern oscillation enso system orchestrates a well documented suite of climate anomalies worldwide the details of enso s extratropical influence vary among events but this variability has not been described or diagnosed beyond the past few decades and previous descriptions have looked at inter event differences rather than decadal patterns we use a new tree ring based drought reconstruction for the continental u s and instrumental enso indices to document systematic decadal changes in the u s drought enso relationship since the late 19th century significant enso drought correlation occurs consistently in the southwest u s but the strength of penetration of moisture anomalies into the continent varies substantially the most striking change over the past 130 years is the initiation of a bipolar enso drought signature around 1920 producing opposite sign moisture anomalies in the southwest and mid atlantic states shifts in teleconnection patterns coincide with variations in the strength of enso and in a north pacific mode

climate and sea level induced turbidite activity in a canyon system offshore the hyperarid western sahara mauritania the timiris canyon a major outcome from studying the sedimentary archives of the timiris canyon off mauritania is that climate shifts in the saharan hinterland not only influenced offshore pelagic sedimentation processes but also controlled turbidite activity in the canyon system sediment supply to the slope predominantly by eolian dust and remobilised eolian sands was greatest during hyperarid glacial and deglacial periods and due to overall increased trade wind strengths upwelling induced pelagic productivity attained highest magnitudes turbidite activity in the timiris canyon responded rapidly to these shifts in the surrounding sedimentary regime on the continental slope during high glacial and early deglacial times the tributary gullies on the uppermost slope were rapidly and repetitively filled subsequently turbidity currents were released and large turbulent flows passed through the lower timiris canyon leading to overspill sequences on its levees in addition to this scenario partial flooding of the exposed outer shelf during successive sea level rises in mis 3 may have caused re mobilization of huge aeolian dune fields that had expanded close to the shelf edge during glacial exposure during the holocene no turbidite activity is recorded in the lower timiris canyon whereas a core from a widened area in the upper timiris canyon reveals an interesting climate controlled 900 year cyclicity of carbonate and mud turbidite events throughout the holocene

a 10 000 year record of dune activity dust storms and severe drought in the central great plains dune fi elds and loess deposits of the great plains of north america contain stratigraphic records of eolian activity that can be used to extend the short observational record of drought we present a 10 000 yr reconstruction of dune activity and dust production in the central great plains region based on 95 optically stimulated luminescence ages the integration of data from both eolian sand and loess is an important new aspect of this record clusters of ages defi ne episodes of extensive eolian activity which we interpret as a response to frequent severe drought at 1 0 0 7 ka and 2 3 4 5 ka with peaks centered on 2 5 and 3 8 ka sustained eolian activity occurred from 9 6 to 6 5 ka parts of this record may be consistent with hypotheses linking holocene drought to sea surface temperature anomalies in the pacifi c or atlantic oceans or to the el nino southern oscillation phenomenon but the record as a whole is diffi cult to reconcile with any of these hypotheses

post glacial filling of a semi enclosed basin the arguin basin mauritania semi enclosed basins are not very common features in the world and are most frequently the result of tectonic movements studies of their filling are usually based on the micropaleontological analyses of sediment cores torgersen et al 1988 reeves et al 2007 or seismic analyses lykousis et al 2007 cagatay et al 2009 van daele et al 2011 the morphology of semi enclosed basins is generally simple and bowl shaped and their edges are marked by one or more sills their depths range from a few dozen to several thousand meters semi enclosed basins are however present in some regions in the world the semi enclosed basin of the golfe d arguin northwest africa is present on a wide shallow shelf bordering the sahara desert in a stable tectonic context its sedimentary filling took place during the end of the post glacial transgression the current knowledge on sedimentary filling of semi enclosed basins is rather limited and inadequate to fully understand the processes at play three campaigns have allowed the creation of the first morpho bathymetric map of the golfe d arguin shedding new light on its large scale morphology a series of rocky shoals banc d arguin interrupted by two sills in the north and south divides the shelf into two parts the inner and outer the inner part of the golfe d arguin called the arguin basin forms a shallow semi enclosed depression the basin is situated on the stable west african margin and the lack of any vertical tectonic movement provides an ideal situation for studying the progradation regression variations and the sediment depositional conditions caused by the post glacial sea level changes based on the analysis of very high resolution seismic data seven units were identified the sedimentary sequence of deposition of the arguin basin was interpreted in relation to the bedrock morphology the sea level rise and the climatic changes their chronology was established in comparison to the regional sea level curve basin physiography and unit distribution the arguin basin is interpreted as a land locked freshwater lake during the post glacial sea level rise corresponding to wet climatic conditions the inner part was flooded ca 8 7 ka bp when the sea level reached the sills the filling then corresponded to a marine estuarine environment the climatic aridification and the sea level stabilization from 6 5 ka bp onwards allowed the deposition of the last units which are composed of aeolian sand combined with a significant marine biogenic carbonate fraction bedrock morphology plays a major role in determining the depositional sequence architecture it controls the available accommodation space and in conjunction with climate changes influences the environmental processes that shape the deposit geometry

tropical extratropical teleconnections in boreal summer observed interannual variability abstractmaximum covariance analysis is performed on the fields of boreal summer tropical rainfall and northern hemisphere nh 200 hpa height for the 62 yr period of record of 1948 2009 the leading mode which appears preferentially in summers preceding the peak phases of the el nino southern oscillation enso cycle involves a circumglobal teleconnection cgt pattern in the nh extratropical 200 hpa height field observed in association with indian monsoon rainfall anomalies the second mode which tends to occur in summers following enso peak phases involves a western pacific north america wpna teleconnection pattern in the height field observed in association with western north pacific summer monsoon rainfall anomalies the cgt pattern is primarily a zonally oriented wave train along the westerly waveguide while the wpna pattern is a wave train emanating from the western pacific monsoon trough and following a great circle the cgt is accompanied by a pronounced tropical extratropical seesaw in t

centennial eolian cyclicity in the great plains usa a dominant climate pattern of wind transport over the past 4000 years proxy evidence at decadal resolution from late holocene sediments from pickerel lake northeastern south dakota shows distinct centennial cycles 400e700 years in magnetic susceptibility contents of carbonate organic carbon and major elements abundance in ostracodes and d 18 o and d 13 c values in calcite proxies indicate cyclic changes in eolian input productivity and temperature maxima in magnetic susceptibility are accompanied by maxima in aluminum and iron mass accumulation rates mars and in abundances of the ostracode fabaeformiscandona rawsoni this indicates variable windy and dry conditions with westerly wind dominance including during the medieval climate anomaly maxima in carbonates organic carbon phosphorous and high d 13 c values of endogenic calcite indicate moister and less windy periods with increased lake productivity including during the little ice age and alternate with maxima of eolian transport times of the maunder sporer and wolf sunspot minima are characterized by maxima in d 18 o values and aluminum mars and minima in d 13 c values and organic carbon content we interpret these lake conditions during sunspot minima to indicate decreases in lake surface water temperatures of up to 4e5 c associated with decreases in epilimnetic productivity during

an extended pacific north american index from upper air historical data back to 1922 abstract this paper presents a reconstruction of a pacific north america pna index from historical upper level data for the period 1922 47 the data have been compiled from a number of sources and cover the pacific north american sector relatively well over this time period temperature and geopotential height profiles from aircraft kite and radiosonde ascents back to 1922 have been digitized and validated wind speed and direction from pilot balloon data back to the early 1920s provided by ncar have also been used a statistical regression approach is used for the reconstruction and calibrated in the post 1948 period using ncep ncar reanalysis data split sample validation experiments were performed within the ncep ncar period and sensitivity experiments with different subsets of predictors were performed similar reconstructions and validation experiments were carried out using a 540 yr control run from the community climate system model version 3 ccsm3 the reconstructed index series together

coastal evolution in senegal and mauritania at 103 102 and 101 year scales natural and human records abstract previous works have shown that the range of marine level eustatic changes on the coast of the senegalo mauritanian basin after 5500 bp must have been relatively small on the basis of regional geological mapping this position is confirmed in two littoral regions in mauritania baie de saint jean cape timiris and in senegal saloum delta the evolution of the shoreline is linked mainly to the balance between deposition and reworking by the sea in mauritania the reworking capacity is high before 4000 bp under a more humid climate than the present one it becomes insufficient around 3000 bp as areas of aeolo marine progradation develop in the sebkhas at the inner part and the shorelines of the baie de saint jean the deposits are subjected to a constant sea level and are therefore formed by the rhythm of recurrent aridity crises on the millenary or the centenary scale in the saloum delta the evolution of the shoreline results from several processes involving more varied timescales an essential factor is the positioning of littoral sand barriers shaped like long sandy spits which divert the flow of the river towards the south and are arranged according to a basal surface also adjusted to a constant sea level the development of sand barriers such as the present sangomar spit and also the langue de barbarie occurs at a centennial rhythm higher probability events at the decade scale are likely to perturb this development by inducing gaps in the sand barrier and as the river goes beyond the sand barrier the latter becomes incorporated in the deltaic plain forcing the littoral drift to build a new sand barrier further forward these events are linked to a conjunction of factors involving different timescales aridity crises at the 10 3 or 10 2 year scales sedimentation waves at the 10 year scale seasonal exceptional swells river floods variations of the tidal range determined by modifications in the estuarian morphology

deciphering signals of late holocene fluvial and aeolian supply from a shelf sediment depocentre off senegal north west africa high resolution climatic records of the late holocene along the north west african continental margin are scarce here we combine sediment grain size elemental distribution and mineral assemblage data to trace dust and riverine sources at a shallow marine sediment depocentre in the vicinity of the senegal river mouth the aim is to understand how these terrigenous components reflect climate variability during the late holocene major element contents were measured and mineral identification was performed on three sub fractions of our sediment core i fluvial material 2 m ii aeolian material of 18 63 m and iii a sub fraction of dual origin material of 2 18 m results show that more than 80 of the total al and fe terrigenous bulk content is present in the fluviogenic fraction in contrast ti k and si cannot be considered as proxies for one specific source off senegal the al ca ratio recording the continental river runoff reveals two dry periods from 3010 to 2750 cal a bp and from 1900 to 1000 cal a bp and two main humid periods from 2750 to 1900 cal a bp and from 1000 to 700 cal a bp the match between i intervals of low river runoff inferred by low al ca values ii reduced river discharge inferred by integrated palynological data from offshore senegal and iii periods of enhanced dune reactivation in mali confirms this interpretation copyright 2011 john wiley sons ltd

the upwelling area off northwest africa a description of phenomena related to coastal upwelling abstract this paper presents a brief review of the oceanography along the upwelling coast of northwest africa the description contains information on seasonal variations of large scale winds ocean circulation and some gross of features of the hydrography characteristic meso scale patterns of currents during the upwelling season between 16 n and 26 n air and water temperature differences during upwelling season diurnal and semi diurnal variations of temperature atmospheric pressure wind and current regional peculiarities canyons and the banc d arguin

north american droughts of the mid to late nineteenth century a history simulation and implication for mediaeval drought unlike the major droughts of the twentieth century that are readily identified in the instrumental record similar events in the nineteenth century have to be identified using a combination of proxy data historical accounts and a sparse collection of early instrumental records in the usa three distinct periods of widespread and persistent drought stand out in these records for the latter half of the nineteenth century 1856 1865 1870 1877 and 1890 1896 each of these events is shown to coincide with the existence of an anomalously cool la nina like tropical pacific to examine the physical mechanisms behind these droughts two ensembles of simulations with an atmosphere general circulation model agcm were generated the first forces an agcm with the observed history of sea surface temperatures ssts everywhere from 1856 to 2001 the goga experiment the second forces the agcm only with tropical pacific ssts being coupled to a two layer entraining mixed layer ml ocean elsewhere the poga ml experiment owing to a sparsity of instrumental precipitation data at this time proxy evidence from tree rings is used as verification a comparison of modelled soil moisture with tree ring reconstructions of the palmer drought severity index pdsi a proxy for soil moisture from the north american drought atlas is made both the poga ml and goga ensemble means capture the three multi year droughts of the mid to late nineteenth century indicating that the droughts were sst forced the similarity of the poga ml and goga simulations implies that the component of each drought signal that is forced by the sst is driven ultimately by the la nina like tropical pacific the global atmosphere ocean context of each of the mid to late nineteenth century droughts reveals a zonally and hemispherically symmetric pattern consistent with forcing from the tropics in addition rossby wave propagation from the cooler equatorial pacific amplifies dry conditions over the usa finally using published coral data for the last millennium to reconstruct a nino 3 4 history the modern day relationship between nino 3 4 and north american drought is applied to recreate two of the severest mediaeval drought epochs in the western usa the large scale spatial similarity to the drought atlas data demonstrates the potential link between a colder eastern equatorial pacific and the persistent north american droughts of the mediaeval period

late pleistocene and holocene dune activity and wind regimes in the western sahara desert of mauritania the western sahara desert in mauritania is dominated by extensive sand seas consisting largely of linear dunes analyses of landsat images geomorphic and stratigraphic studies and optically stimulated luminescence dating of dunes in the azefal agneitir and akchar sand seas provide evidence that three main generations of dunes were formed during the periods 25 15 ka centered around the last glacial maximum 10 13 ka spanning the younger dryas event and after 5 ka the wind regimes that occurred during each of these periods were significantly different leading to the formation of dunes on three distinct superimposed trends northeast north northeast and north and the development of the sand seas as composite geomorphic features

a late quaternary sedimentary shelf system under hyperarid conditions unravelling climatic oceanographic and sea level controls golfe d arguin mauritania nw africa abstract the sedimentation on the mauritanian shelf off the western saharan desert is strongly influenced by dust supply and is investigated in this study by combining surface mapping shallow acoustic profiling and the analysis of sediment cores coarse grained carbonates dominate wide areas of the open shelf at places where the shelf is exposed to the atlantic ocean the sheltered golfe d arguin in contrast is characterized by muddy shoals in its inner area and locally restricted and wedge shaped muddy depocenters which extend contour parallel in mid shelf settings in both cases cross shelf currents control sediment distribution and accumulation not location or amount of primary eolian input a major discontinuity on the outer shelf is located in 85 110 m water plus sediment depth and is refilled by a sedimentary facies succession of shoreface tidal and open shelf deposits this succession has formed during temporary lower sea levels along the last overall regression mis 4 and mis 3 the last glacial maximum and during the past deglacial sea level rise significant sediment deposition and preservation is limited to very restricted areas concerning the overall shelf architecture sea level was the main factor enabling or suppressing deposition in terms of 1 creating local erosional swales which have favored subsequent trapping and preservation of sediments 2 determining the type of sedimentary facies refilling these depressions under a variety of nearshore conditions 3 interacting with the seafloor morphology and by this controlling timing of initiation and growth geometry of mud wedges these mud wedges have formed since ca 9 cal ka under a continuous sedimentation regime these bodies must have formed in hydraulic equilibrium but they are detached from a defined sediment point source which is in contrast to common mudbelt formation on shelves in general sediment storage on shelves under hyperarid conditions seems to be very limited due to comparably low sediment input strong ravinement activity during changing sea levels significant bypassing and winnowing processes and a low preservation potential of the non cohesive materials these conditions lead to the development of almost sediment starved systems with low subsidence rates over longer geological timescales

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

variations in north american summer precipitation driven by the atlantic multidecadal oscillation understanding the development and variation of the atmospheric circulation regimes driven by the atlantic multidecadaloscillation amo isessentialbecausethesecirculationsinteractwithotherforcingsondecadaland interannual time scales collectively they determine the summer june july and august precipitation variations for north america in this study a general circulation model gcm is used to obtain such understanding with a focus on physical processes connecting the amo and the summertime precipitation regime change in northamerica twoexperimentalrunsareconductedwithseasurfacetemperature sst anomaliesimposedin the north atlantic ocean that represent the warm and cold phases of the amo climatological ssts are used elsewhere in the oceans model results yield summertime precipitation anomalies in north america closely matching the observed anomaly patterns in north america suggesting that the amo provides a fundamental control on summertime precipitation in north america at decadal time scales the impacts of the amo are achieved by a chain of events arising from different circulation anomalies during warm and cold phases of the amo during the warm phase the north atlantic subtropical high pressure system nash weakens and the north american continent is much less influenced by it a massive body of warm air develops over the heated land in north america from june august associated with high temperature and low pressure anomalies in the lowertroposphereandhighpressureanomaliesintheuppertroposphere incontrast duringthecoldphaseofthe amo the north american continent particularly to the west is much more influenced by an enhanced nash cooler temperatures and high pressure anomalies prevail in the lower troposphere and a frontal zone forms in the upper troposphere these different circulation anomalies further induce a three cell circulation anomaly pattern over north america in the warm and cold phases of the amo in particular during the cold phase the three cell circulation anomaly pattern features a broad region of anomalous low level southerly flow from the gulf of mexico into the u s great plains superimposed with an upper troposphere front more frequent summertime storms develop and excess precipitation occurs over most of north america a nearly reversed condition occurs during the warm phase of the amo yielding drier conditions in north america this new understandingprovides a foundation for further studyand better prediction of the variations of north american summer precipitation especially when modulated by other multidecadal variations for example the pacific decadal oscillation and interannual variations associated with the enso and the arctic oscillation

the regional forcing of northern hemisphere drought during recent warm tropical west pacific ocean la nina events northern hemisphere circulations differ considerably between individual el nino southern oscillation events due to internal atmospheric variability and variation in the zonal location of sea surface temperature forcing over the tropical pacific ocean this study examines the similarities between recent northern hemisphere droughts associated with la nina events and anomalously warm tropical west pacific sea surface temperatures during 1988 1989 1998 2000 2007 2008 and 2010 2011 in terms of the hemispheric scale circulations and the regional forcing of precipitation over north america and asia during the cold season of november through april the continental precipitation reductions associated with recent central pacific la nina events were most severe over north america eastern africa the middle east and southwest asia high pressure dominated the entire northern hemisphere mid latitudes and weakened and displaced storm tracks northward over north america into central canada regionally over north america and asia the position of anomalous circulations within the zonal band of mid latitude high pressure varied between each la nina event over the northwestern and southeastern united states and southern asia the interactions of anomalous circulations resulted in consistent regional temperature advection which was subsequently balanced by similar precipitation modifying vertical motions over the central and northeastern united states the spatial variation of anomalous circulations resulted in modest inter seasonal temperature advection variations which were balanced by varying vertical motion and precipitation patterns over the middle east and eastern africa the divergence of moisture and the advection of dry air due to anomalous circulations enhanced each of the droughts

late holocene morphodynamics in the littoral zone of the iwik peninsula area banc d arguin mauritania abstract in the littoral zone of the banc d arguin both littoral sedimentary units and man made deposits shell middens abound they were formed during the late holocene after the final onset of the post glacial transgression here a geo archaeological approach is used to study them the geomorphological and sedimentological characteristics of the coastal features define two distinct coastal sedimentary units in the iwik aouatil zone between an ancient shoreline which formed on the tafaritian substratum ca 6 7 5 7 cal ka bp and the present one i very extensive sand flats and ii linear relief features both covered by a thin veneer of anthropogenic anadara senilis shell middens they developed under a stable sea level regime the sand flats expanded at variable rates throughout the period the linear relief features represent beach barriers and coastal dunes which can be dated archaeologically thanks to the superimposed shell middens supplemented by 14 c dating two series of linear sedimentary units are identified first a complex north south system of dunes and hook ended sand barriers developed from 4 8 to 4 1 cal ka bp and secondly two regular and linear beach barriers developed between 3 7 and 3 3 cal ka bp their morphological characteristics suggest that they were generated through littoral drift processes directional changes observed between the first and the second groups demonstrate that the wave refraction pattern was dramatically modified after the closure of the strait between the former iwik island and the land a comparison with the nearby jerf el oustani coastal area in the late holocene shows that both sites shared similarities in depositional patterns but also differences in the chronology of the sedimentary units processes involved in the development of the linear units are indicative of the enhancement of both sand influx and longshore drift due to climatic change

warm season subseasonal variability and climate extremes in the northern hemisphere the role of stationary rossby waves abstractthis study examines the nature of boreal summer subseasonal atmospheric variability based on the new nasa modern era retrospective analysis for research and applications merra for the period 1979 2010 an analysis of the june july and august subseasonal 250 hpa meridional wind anomalies shows distinct rossby wave like structures that appear to be guided by the mean jets on monthly subseasonal time scales the leading waves the first 10 rotated empirical orthogonal functions reofs of the 250 hpa wind explain about 50 of the northern hemisphere wind variability and account for more than 30 60 of the precipitation surface temperature variability over a number of regions of the northern middle and high latitudes including the u s northern great plains parts of canada europe and russia the first reof in particular consists of a rossby wave that extends across northern eurasia where it is a dominant contributor to monthly surface temperature and precipitation variability and

mid to late quaternary environments and stratigraphy of the southern sierra leone shelf west africa the middle to upper quaternary succession of the southern sierra leone shelf appears to have been deposited in response to three glacial sea level cycles the last of which has left the most complete sedimentary record the unconformable base of the succession is deeply incised off larger rivers inshore but has little relief toward the shelf break this difference results from the steep gradient of the inner shelf and the gentle gradient of the valley9s long profile incision was caused by high fluvial discharge during sea level fall decline of discharge and sediment supply during and after glacial maximum resulted in cessation of incision and in the formation of cemented beach ridges at the shelf break the lowstand wedge of the last glacial cycle is well developed beyond the shelf break and where the pre existing shelf was deep where sampled it is deposited below wave base its preservation is apparently the result of protection from wave and shelf current action during regression the subsequent transgressive wedge entirely dominates shelf deposition infilling incised valleys and contains vertically stacked fluvial estuarine littoral and sub littoral elements it is thin or absent beyond the shelf as a result of active oceanic currents hindering sedimentation active oceanic currents also result in only weak development of the highstand wedge energy regime as represented by moderate shelf currents and variable fluvial discharge has been the fundamental control on sediment architecture sediment supply has been of only secondary importance

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

is midlatitude convection an active or a passive player in producing global circulation patterns abstract the ability of persistent midlatitude convective regions to influence hemispheric circulation patterns during the northern hemisphere summer is investigated global rainfall data over a 15 yr period indicate anomalously large july total rainfalls occurred over mesoscale sized midlatitude regions of north america and or southeast asia during the years of 1987 1991 1992 and 1993 the anomalous 200 hpa vorticity patterns for these same years are suggestive of rossby wave trains emanating from the regions of anomalous rainfall in the midlatitudes results from an analysis of an 11 yr mean monthly 200 hpa july wind field indicate that in the climatological mean rossby waveguides are present that could assist in developing a large scale response from mesoscale sized regions of persistent convection in the midlatitudes this hypothesis is tested using a barotropic model linearized about the 200 hpa july time mean flow and forced by the observed divergence anomalies the model results are in qualit

maintenance of circulation anomalies during the 1988 drought and 1993 floods over the united states the large scale circulation anomalies associated with the 1988 drought and the 1993 floods are investigated with the national centers for environmental prediction reanalysis data and a linear stationary wave model the transient vorticity and thermal forcings are explicitly calculated and the diabatic heating is derived as a residual in the thermodynamic energy equation using the april june amj data for 1988 and june august jja data for 1993 the linear stationary wave model is able to reproduce the main features of the geopotential height anomaly for the two seasons when all forcings are included this provides a basis for further investigation of stationary wave response to different forcing mechanisms using the linear model within the linear model framework the linear model responses to different forcings are examined separately the results indicate that the 1988 anomaly over the united states is a result of both the diabatic heating and the transient vorticity and thermal forcings the large anticyclonic anomalies over the north pacific and canada are forced mainly by the diabatic heating the 1993 anomaly however is dominated by the response to transient vorticity forcing by further separating the linear model responses to regional diabatic heating anomalies in 1988 the results indicate that the western north pacific heating is entirely responsible for the anticyclonic center over the north pacific which causes the northward shift and intensification of the pacific jet stream the eastern north pacific heating cooling couplet is the most important for maintaining the north american circulation

delta development and channel incision during marine isotope stages 3 and 2 in the western south yellow sea abstract approximately 4100 km of high resolution seismic profiles recently obtained from the western south yellow sea were subdivided into seven seismic units by six major seismic surfaces three sediment cores between 60 and 71 m in length obtained from localities on the seismic profile lines were analyzed for sedimentary characteristics clay mineral components and benthic foraminiferal and ostracod assemblages and were also dated by ams 14 c and optically stimulated luminescence to document the environmental history of the region stratigraphic units in the cores were confidently correlated with seismic units in the profiles stratigraphic correlation among these three cores and the previous well studied core qc 2 was facilitated by the cold water deposits in marine isotope stage mis 5 the three cores document the sedimentary sequence during the period from mis 6 up to mis 1 strata formation and sedimentary environment changes since mis 5 are strongly controlled by sea level fluctuations most of the successions were formed in mis 5 mis 3 and mis 1 with major erosions in mis 4 and mis 2 particularly our data revealed the presence of a large delta active during mis 3 likely from the early stage of mis 3 to about 40 cal kyr bp that was subsequently affected by channel incision the delta succession and incised channel fills have a high concentration of smectite in their clay mineral components suggesting that the paleo yellow river was the main contributor to the delta development and channel incision the delta sequence has a coarsening upward trend and consists of two seismic facies the upper facies shows lateral alternations of chaotic and oblique or very low angle reflectors which correspond to sandy proximal delta front and delta plain facies and the lower facies is characterized by seaward dipping clinoforms which correspond to muddy distal delta front and prodelta facies the top surface of the delta inclines very gently seaward consistent with a deltaic development associated with the sea level fall in mis 3 the incised channel system consists of two main channels with multiple tributaries in a dendritic pattern the incised channel fills begin with fluvial and then estuarine sediments which are truncated by a transgressive ravinement surface and capped by transgressive deposits the main incision channels have a terraced cross sectional morphology which is interpreted to reflect multiple episodes of sea level fall during incision presumably from the late mis 3 to the last glacial maximum

amplified contiguous united states summer rainfall variability induced by east asian monsoon interdecadal change the strength of the dominant variability of contiguous united states conus summer rainfall during 1960 2013 experiences an interdecadal change in the early 1990s before the early 1990s the variation in conus summer rainfall is relatively small in amplitude standard deviation 0 64 mm day 1 whereas after it amplifies remarkably with its standard deviation 1 31 mm day 1 roughly doubling observational diagnoses and simulation results show that enhanced east asian subtropical monsoon variability plays a direct role in strengthening the conus summer rainfall dipole variability besides a northward shift of the east asian summer monsoon is also responsible for the amplification of the conus summer rainfall variability this northward shift of the east asian rain belt pushes the rainfall perturbation farther to the north much closer to the subtropical east asian upper level westerly jet stream as a result the east asian subtropical monsoon heating induces the upper level asia north america teleconnection pattern more effectively leading to the larger amplitude of conus summer rainfall variability

a method for simulating effects of quasi stationary wave anomalies on regional climate abstract a combined observational and numerical modeling methodology was employed to examine the interaction of large scale quasi stationary waves with regional forcing in the drought of 1988 and flood of 1993 in the north central united states the study implements and tests a new lateral boundary condition closure approach enabling regional climate modeling sensitivity studies to account for anomalies external to the simulated domain thirty day regional simulations of the continental united states using the mm5 regional model successfully reproduced the anomalous drought and flood conditions in an additional set of simulations the observed large scale quasi stationary wave anomalies in the dynamic and thermodynamic fields were filtered out through adjustment of the observationally based lateral boundary conditions in these filtered simulations the simulated meteorological fields over the drought flood areas for both years tended toward those for normal years suggesting that the proposed filtering te

la nina diversity and northwest indian ocean rim teleconnections the differences in tropical pacific sea surface temperature sst expressions of el nino southern oscillation enso events of the same phase have been linked with different global atmospheric circulation patterns this study examines the dynamical forcing of precipitation during october december ond and march may mam over east africa and during december march djfm over central southwest asia for 1950 2010 associated with four tropical pacific sst patterns characteristic of la nina events the cold phase of enso the self organizing map method along with a statistical distinguishability test was used to isolate la nina events and seasonal precipitation forcing was investigated in terms of the tropical overturning circulation and thermodynamic and moisture budgets recent la nina events with strong opposing sst anomalies between the central and western pacific ocean phases 3 and 4 force the strongest global circulation modifications and drought over the northwest indian ocean rim over east africa during mam and ond subsidence is forced by an enhanced tropical overturning circulation and precipitation reductions are exacerbated by increases in moisture flux divergence over central southwest asia during djfm the thermodynamic forcing of subsidence is primarily responsible for precipitation reductions with moisture flux divergence acting as a secondary mechanism to reduce precipitation eastern pacific la nina events in the absence of west pacific sst anomalies phases 1 and 2 are associated with weaker global teleconnections particularly over the indian ocean rim the weak regional teleconnections result in statistically insignificant precipitation modifications over east africa and central southwest asia

effects of quasi stationary large scale anomalies on some mesoscale features associated with the 1993 flood a regional model simulation we investigate the effect of quasi stationary large scale anomalies on mesoscale factors contributing to the 1993 u s midwest flood by performing 30 day simulations with a mesoscale model the contributions of anomalous mesoscale features to the flood were quantified by contrasting model integrations forced by initial and lateral boundary conditions with and without large scale anomalous stationary waves that are believed to have been responsible for the flood supplementing previous investigations the present study finds the following 1 the large scale anomalies produced environmental conditions that favored a generation of mcc like systems rather than individual convective cells or squall lines that would have produced less precipitation over large areas ambient conditions favoring mesoscale convective complex mcc include an intensified east west orientation of quasistationary fronts weaker upper tropospheric inertial stability and a strong low level jet llj 2 the anomalous ageostrophic wind component to the far south of the flood region texas area and geostrophic wind component over the flood region bridged together to form an extended moisture conveyer that sustained the long lasting 1993 flood

connection between sea surface anomalies and atmospheric quasi stationary waves abstractlarge scale quasi stationary atmospheric waves qsws are known to be strongly connected with extreme events and general weather conditions yet despite their importance there is still a

high resolution proxy record of holocene climate from a loess section in southwestern nebraska usa abstract multi proxy analysis was used to produce a high resolution paleoclimatic record from an exceptionally thick section of the holocene bignell loess near wauneta southwestern nebraska in the central great plains the wauneta section has excellent age control based on optically stimulated luminescence osl and radiocarbon dating and records multiple episodes of rapid loess deposition alternating with slower deposition and soil formation the lowermost and uppermost osl ages obtained from the bignell loess are 10 250 610 years 5 9 m depth and 100 10 years 0 1 m depth respectively as a result the holocene has been temporally confined stratigraphically the bignell loess overlies the late pleistocene peoria loess deposited 21 14 ka and the two units are separated by the brady soil which is distinguished by its color and other pedogenic features l a b color parameters and organic carbon content of bignell loess are sensitive proxies to differentiate drought induced aeolian sediment layers from the intercalated soil horizons soil organic carbon derived 13 c data suggest that the c 3 dominated floral environment during peoria loess deposition shifted dramatically to a c 4 dominated environment during brady soil formation in response to a warming trend even greater c 4 abundance characterized the late holocene high resolution 13 c data support the contention that c 3 vs c 4 vegetation change in the holocene reflects ecosystem response to frequent vegetation disturbance under arid conditions time series analysis reveals that 13 c and color parameters display high frequency variation with periodicities of 103 118 years and 103 years respectively similar periodicities were also reported in studies of north dakota lakes though the physical mechanism responsible is uncertain comparison of bignell loess color and tropical pacific sea surface temperatures ssts allows evaluation of a proposed teleconnection between drought in the great plains and la nina like conditions in the tropical pacific the loess color index and eastern tropical pacific sst display broad similarities through the late pleistocene and holocene that are consistent with this teleconnection on the other hand drought centered at 3800 years ago is not consistent with the teleconnection and the end of early holocene aridity at the wauneta section around 6500 years ago is much earlier than the corresponding rise in sst and increase in el nino frequency in the eastern tropical pacific

u s summer precipitation and temperature patterns following the peak phase of el nino abstractevidence for spatially coherent but different u s summer precipitation and surface air temperature anomalies during the evolving phase and during the summers following the peak phase of the winter el nino is presented the spatial patterns during the decaying phase of el nino are distinctive from patterns in the preceding summer when el nino is in its evolving phase that is the traditional simultaneous composite patterns associated with el nino the analysis of a multimodel ensemble of global atmospheric models forced by observed sea surface temperature further confirms that the differences in the u s summer precipitation and surface temperature anomalies between the developing and decaying phases of el nino are a result of the atmospheric response to tropical warm sst anomalies that are shifted eastward and are confined east of 120 w during the decaying phase of el nino given the distinctive pattern and relatively large amplitude of these anomalies during the decaying phase of el nino

regional atmospheric circulation and rainfall variability in south equatorial africa abstractthis study examines daily precipitation data during december march over south equatorial africa sea and proposes a new zonal asymmetric pattern zap that explains the leading mode of weather scale precipitation variability in the region the eastern and western components of the zap separated at about 30 e appear to be a consequence of an anomalous zonal atmospheric cell triggered by enhanced low level westerly winds the enhanced westerlies are generated by a diagonal interhemispheric pressure gradient between the southwestern indian and north tropical atlantic oceans in eastern sea these winds hit the east african plateau producing low level convergence and convection that further intensifies the westerlies in western sea a subsiding branch develops in response closing the circulation cell the system gradually dissipates as the pressure gradient weakens through this mechanism simultaneous changes in two hemispheres generate a regional zonally oriented circulation that relies on clima

provenance of present day eolian dust collected off nw africa 1 atmospheric dust samples collected along a transect off the west african coast have been investigated for their physical grain size distribution mineralogical and chemical major elements composition on the basis of these data the samples were grouped into sets of samples that most likely originated from the same source area in addition shipboard collected atmospheric meteorological data modeled 4 day back trajectories for each sampling day and location and total ozone mapping spectrometer aerosol index data for the time period of dust collection february march 1998 were combined and used to reconstruct the sources of the groups of dust samples on the basis of these data we were able to determine the provenance of the various dust samples it appears that the bulk of the wind blown sediments that are deposited in the proximal equatorial atlantic ocean are transported in the lower level 900 hpa ne trade wind layer which is a very dominant feature north of the intertropical convergence zone itcz however south of the surface expression of the itcz down to 5 s where surface winds are southwesterly we still collected sediments that originated from the north and east carried there by the ne trade wind layer as well as by easterly winds from higher altitudes the fact that the size of the wind blown dust depends not only on the wind strength of the transporting agent but also on the distance to the source hampers a direct comparison of the dust s size distributions and measured wind strengths however a comparison between eolian dust and terrigenous sediments collected in three submarine sediment traps off the west coast of nw africa shows that knowledge of the composition of eolian dust is a prerequisite for the interpretation of paleorecords obtained from sediment cores in the equatorial atlantic

geometry lateral variation and preservation of downlapping regressive shelf deposits eastern tyrrhenian sea margin italy abstract the shelf of the eastern tyrrhenian sea margin changes substantially in width shelf break depth and sea floor steepness over relatively short distances largely owing to pronounced lateral changes in subsurface geologic structure remnants of late pleistocene prograded coastal deposits are locally preserved on the middle and outer parts of the shelf we recognize two major controls on the distribution lateral extent thickness and preservation potential of these prograded deposits first prograded downlapping deposits formed only where the physiographic shelf break was deeper than the lowstand shoreline thus providing accommodation space for the lowstand deposits second the proximity and relative size of sediment sources and the local coastal dispersal system apparently infl enced the geometry of the deposit midshelf and shelf margin bodies composed of seaward steepening downlapping reflectors were deposited as thin to thick continuous prograding sheets over an irregular eroded shelf surface and onto the shelf edge during the latest fall and lowstand of sea level decreased sedimentation at the end of this lowstand resulted in net erosion during sea level rise shoreface erosion created a major marine erosional ravinement surface landward of the 120 m isobath and much of the downlapping deposits were removed preservation of these downlapping deposits is largely a function of their thickness thick continuous deposits are common on the shelf edge whereas on the midshelf only thin remnants are preserved locally where depressions or morphologic steps were present in the shelf surface