a laboratory study of surface boundary currents application to the algerian current the problem of a gravity current along a coast is considered in the laboratory the flow originates from a continuous pointsource and when facing downstream has the shore to its right the shore is the vertical wall of a 13 m diameter rotating basin the density of the current r1 is slightly less than that of the surrounding fluid this buoyant flow is shown to depend upon three parameters namely the burger and ekman numbers and the aspect ratio of the current at its source disregarding friction and taking the flow as hydrostatic so that the current aspect ratio does not need to be considered the stability characteristics of the current are shown to depend mostly upon the burger number to study its role in the flow we performed several experiments by varying the burger number bu between 0 15 and 0 82 for large burger numbers the current is stable but as it decreases increasing numbers of anticyclonic disturbances develop and grow along the axis of the current measurements of the normalized distance from the source and times for these disturbances to develop are given as functions of the burger number the dependence of the normalized separation distance of disturbances upon the burger number in cases of multiple instabilities is also discussed finally these laboratory results are compared with the algerian current where both anticyclonic and cyclonic eddies have been observed the experimental current simulates the algerian current when the burger number is 0 15

timing and structure of mega sacz events during heinrich stadial 1 a substantial strengthening of the south american monsoon system sams during heinrich stadials hs points toward decreased cross equatorial heat transport as the main driver of monsoonal hydroclimate variability at millennial time scales in order to better constrain the exact timing and internal structure of hs1 over tropical south america we assessed two precisely dated speleothem records from central eastern and northeastern brazil in combination with two marine records of terrestrial organic and inorganic matter input into the western equatorial atlantic during hs1 we recognize at least two events of widespread intensification of the sams across the entire region influenced by the south atlantic convergence zone sacz at 16 11 14 69 kyr b p and 18 1 16 66 kyr b p labeled as hs1a and hs1c respectively separated by a dry excursion from 16 66 to 16 11 kyr b p hs1b in view of the spatial structure of precipitation anomalies the widespread increase of monsoon precipitation over the sacz domain was termed mega sacz

the surface circulation of the balearic sea until recently the surface circulation of the balearic sea has been viewed as largely cyclonic with a fairly quiescent central dome however recent studies involving ship data tracked drifters current meter moorings and satellite imagery indicate that this sea has strong mesoscale variability and a more complex general circulation these studies together with an examination of registered satellite imagery collected during the western mediterranean circulation experiment wmce indicate that the surface circulation is strong year round and characterized by two permanent density fronts located on the continental shelf slope the catalan front and the balearic islands shelf slope the balearic front the catalan front is the more active of the two fronts in the northern area a plume of cold water is frequently observed moving southward along the continental slope region shedding dipole eddies along the leading edge of the plume in addition the catalan front continuously spawns energetic filaments that appear to be associated with the plume of cool water the likely mechanism of formation of these filaments is the deflection of the cold water by the regional submarine canyons

sea surface water variability during the mid brunhes inferred from calcareous plankton in the western mediterranean odp site 975 abstract planktonic foraminifera and calcareous nannoplankton were analyzed in core ocean drilling program site 975 located in the western mediterranean in order to study their response to environmental changes across the mid brunhes interval and specifically through marine isotope stage mis 15 and 8 ca 650 230 ka multivariate analysis of assemblage composition revealed that the calcareous plankton dynamics were mainly driven by surface water temperatures both on glacial interglacial and stadial interstadial time scales throughout the studied interval foraminiferal and nannoplankton assemblages responded to environmental forcing in concert except during mis 15 when multiple environmental factors appear to have been superimposed on the otherwise dominating effect of temperature variability sea surface temperature sst has been estimated from planktonic foraminiferal assemblages using the artificial neural networks method pointing out relevant seasonal variations through the record our reconstructions reveal that calcareous plankton assemblages also responded to other environmental factors than temperature including modification in surface water stratification nutrient availability and detrital input specifically a more stable stratified water column seems to have accompanied the interval through mis 15 13 on the other hand across the mid brunhes interval from mis 12 onwards the enhanced glacial strength affected both planktonic foraminifera and calcareous nannofossils planktonic foraminifera indicate that mixed and productive surface waters developed during glacials and stadials accompanied by episodes of increased detrital input as suggested by calcareous nannoplankton assemblages enhanced food availability was likely driven by wind induced upwelling and or by continental coastal input during dry conditions persisting in the mediterranean region and over north africa finally spectral and wavelet analyses of calcareous plankton time series allowed to identify the pattern of periodic response of the plankton at milankovitch cycles and in some cases at millennial variability the dominant 100 kyr periodicity recorded in surface water temperature proxies clearly reflects a major response of the planktonic community to the global glacial interglacial climate pattern high frequency variation 5 8 11 13 ky of surface water temperature proxies and of selected taxa abundance highlights the occurrence of millennial scale instability superimposed on the paleoenvironmental modifications occurring through the mid brunhes interval climate transition this framework accounts for the occurrence of different surface water dynamics across the mid brunhes interval and is in favor of the hypothesis that the millennial scale surface water temperature variations are modulated by the harmonics of the precession forcing

statistical analyses of eddies in the western mediterranean sea based on synthetic aperture radar imagery abstract we analysed 7188 sentinel 1a synthetic aperture radar sar images acquired in a 3 year period between october 2014 and september 2017 in the western mediterranean sea and found imprints of about 14 300 oceanic eddies with diameters ranging from 0 4 km to 160 1 km those eddies manifest on the sar imagery either through the accumulation of surfactants or through wave current interaction and considering the favourable wind speed ranges for both mechanisms we performed statistical analyses to gain insight into the eddies spatio temporal distribution their typical sizes and shapes and into the predominant mechanisms that are responsible for their formation the vast majority of the detected eddies is elliptical with a tendency for only large mesoscale eddies towards a circular shape we found no general influence of the wind on the eddies orientation although mesoscale eddies tend to orient into the main current direction in each subregion submesoscale eddies do not show such tendency submesoscale eddies in the western mediterranean sea are predominantly cyclonic 90 and with increasing eddy diameter there is a transition from cyclonic to anticyclonic sense of rotation in general we found highest densities of submesoscale eddies in autumn when the biological activity is still high and when mean regional winds are already strong but variable in addition instabilities of thermal fronts during summer and autumn and instabilities of the main and secondary currents influence the formation of submesoscale eddies some of our results confirm those previously obtained for other regions

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

temporal variability of currents in the eastern ligurian sea surface and intermediate waters enter the ligurian sea from the tyrrhenian and algero provencal basins via northward flows along the east and west coasts of corsica the temporal variability of these flows is examined by means of a 1 year time series of current data october 1986 to september 1987 and hydrographic data from december march and may cruises the flow through the corsica channel increases suddenly during december and then gradually decreases after march reaching its lowest values in late summer the seasonal variability in the channel seems associated mainly with the steric sea level differences between the tyrrhenian sea and ligurian sea the west corsican current appears more stable throughout the one year of data the two flows converge just north of corsica into the ligurian current because of the water masses differences between the two currents in the area of convergence considerable smaller scaled structure is observed during winter the less dense tyrrhenian water provides available potential energy to the ligurian shelf area and thereby helps sustain the large scale cyclonic circulation in the ligurian basin during the other seasons the spatial structures over the ligurian shelf are less organized and the spatial scale of the flow is a few tens of kilometers

high latitude forcing of the south american summer monsoon during the last glacial the climate of the last glacial period 10 000 to 110 000 years ago was characterized by rapid millennial scale climate fluctuations termed dansgaard oeschger d o and heinrich events we present results from a speleothem derived proxy of the south american summer monsoon sasm from 16 000 to 50 000 years ago that demonstrate the occurrence of d o cycles and heinrich events this tropical southern hemisphere monsoon reconstruction illustrates an antiphase relationship to northern hemisphere monsoon intensity at the millennial scale our results also show an influence of antarctic millennial scale climate fluctuations on the sasm this high resolution precisely dated tropical precipitation record can be used to establish the timing of climate events in the high latitudes of the northern and southern hemispheres

climate variability in the western mediterranean between 121 and 67 ka derived from a mallorcan speleothem record abstract the western mediterranean region is exceptionally vulnerable to predicted climate changes of increasing temperature and aridity characterizing past climate oscillations since the last interglacial lig is critical to understand climate patterns during the present warm period here we present an accurately dated speleothem record cam 1 of paleoclimate between 121 and 67 ka from the island of mallorca the growth history combined with the isotopic record and mineral changes provides evidence of dramatic climatic shifts in the western mediterranean isotopic equilibrium deposition was assessed by hendy tests and by comparing the 18o in modern drip water with newly precipitated calcite we argue that variability of 18o in cam 1 is mainly related to changes in source of precipitation whereas high 13c values reflect a dry climate calcite rather than aragonite growth is related to cooler drier climate the mis 5e 5d transition c24 c23 and c21 cold events concomitantly recorded in cam 1 odp 984 and ngrip highlights the teleconnection between high and mid latitudes in the northern hemisphere our record reveals a prolonged aridity during mis 5c and a sudden climate shift from drier to wetter conditions beginning with mis 5b a growth hiatus at 67 to 53 ka probably marks the driest period after which slow calcite growth indicates markedly drier climate during the last glacial and holocene than during mis 5 many of the significant changes in growth rates and stable isotopic composition are synchronous with events that are likely driven by regional climate and large circulation patterns linked to the north atlantic

correlated millennial scale changes in surface hydrography and terrigenous sediment yield inferred from last glacial marine deposits off northeastern brazil abstract the stable isotope composition of planktonic foraminifera correlates with evidence for pulses of terrigenous sediment in a sediment core from the upper continental slope off northeastern brazil stable oxygen isotope records of the planktonic foraminiferal speciesglobigerinoides sacculiferandglobigerinoides ruber pink reveal sub milankovitch changes in sea surface hydrography during the last 85 000 yr warming of the surface water coincided with terrigenous sedimentation pulses that are inferred from high xrf intensities of ti and fe and which suggest humid conditions in northeast brazil these tropical signals correlate with climatic oscillations recorded in greenland ice cores dansgaard oeschger cycles and in sediment cores from the north atlantic heinrich events trade winds may have caused changes in the north brazil current that altered heat and salt flux into the north atlantic thus affecting the growth and decay of the large glacial ice sheets

north atlantic forcing of amazonian precipitation during the last ice age the last glacial period was marked by rapid reorganizations of oceanic and atmospheric circulation speleothem records from the amazon basin suggest that precipitation variability was linked to these events the last glacial period was marked by multiple abrupt reorganizations of ocean and atmosphere circulation1 on thousand year timescales slowing of the atlantic meridional overturning circulation was associated with cooling in the high northern latitudes whereas strengthened circulation was linked to northern warming1 2 in the tropics these millennial scale events were primarily reflected in altered patterns of precipitation3 these hydrologic fluctuations induced ecological changes in the atlantic seaboard and the high andes2 but less is known about the amazon basin here we reconstruct precipitation over amazonian ecuador over the past 94 000 years using a 18o record from speleothems collected in santiago cave in western amazonia we interpret the variability of the 18o record as changes in the source and amount of precipitation with the exception of the period between 40 000 and 17 000 years ago abrupt high frequency changes coincide with shifts in north atlantic circulation indicating a high latitude influence on amazonian precipitation over millennial timescales on longer timescales the record shows a relationship to precessional changes in the earth s orbit in light of the lack of extreme aridity in our records we conclude that ecosystems in western amazonia have not experienced prolonged drying over the past 94 000 years

coupling of millennial scale changes in sea surface temperature and precipitation off northeastern brazil with high latitude climate shifts during the last glacial period 1 high resolution records of alkenone derived sea surface temperatures and elemental ti ca ratios from a sediment core retrieved off northeastern brazil 4 s reveal short term climate variability throughout the past 63 000 a large pulses of terrigenous sediment discharge caused by increased precipitation in the brazilian hinterland coincide with heinrich events and the younger dryas period terrigenous input maxima related to heinrich events h6 h2 are characterized by rapid cooling of surface water ranging between 0 5 and 2 c this signature is consistent with a climate model experiment where a reduction of the atlantic meridional overturning circulation amoc and related north atlantic cooling causes intensification of ne trade winds and a southward movement of the intertropical convergence zone resulting in enhanced precipitation off northeastern brazil during deglaciation the surface temperature evolution at the core site predominantly followed the antarctic warming trend including a cooling prior to the younger dryas period an abrupt temperature rise preceding the onset of the bolling allerod transition agrees with model experiments suggesting a southern hemisphere origin for the abrupt resumption of the amoc during deglaciation caused by southern ocean warming and associated with northward flow anomalies of the south atlantic western boundary current

the role of north brazil current transport in the paleoclimate of the brazilian nordeste margin and paleoceanography of the western tropical atlantic during the late quaternary abstract reconstructions of surface paleoceanographic conditions of the western equatorial atlantic and past climates of the adjacent northeast brazilian the nordeste continental margin were undertaken by analyzing sediments from a piston core and associated gravity and box cores recovered from 3107 meter water depth at 0 20 n on the equatorial brazilian continental slope the record is dated by radiocarbon analysis and oxygen isotopic stratigraphy of planktonic foraminifers and spans from near modern to approximately 110 ka high resolution xrf analysis provides insight into the paleoclimate history of the nordeste during the last glacial interval several large amplitude and abrupt peaks are observed in the time series of ti ca and are usually accompanied by peaks of fe k together these record periods of increased precipitation and intense weathering on the adjacent continent and increased terrestrial sediment discharge from nordeste rivers into the atlantic within the limits of dating accuracy most ti ca peaks correlate with heinrich events in the north atlantic this record thus corroborates and extends back in time the previous record of arz et al 1998 determined on sediment cores from farther southeast along the nordeste margin stable oxygen isotopic analysis and mg ca paleothermometry on the near surface dwelling planktonic foraminiferal species globierinoides ruber find that mean sea surface temperature sst during glacial time 20 55 ka n 97 was 23 89 0 79 c and the mean sst during the late holocene 0 5 ka n 14 was 26 89 0 33 c ssts were 0 5 2 c higher and inferred sea surface salinities were lower during most of the periods of elevated ti ca thus as observed in previous studies the western equatorial atlantic was warm at least locally and the adjacent southern tropical continent was wet at the same time that the high latitude north atlantic was cold using the syntrace ccsm3 fully coupled climate model with transient forcing for the period 22 ka to present we find that decreased transport of the north brazil current co occurs with reduced atlantic meridional overturning circulation and colder than normal ssts in the north atlantic region these simulated conditions are invariably associated with significantly increased precipitation in the nordeste region

abrupt glacial climate shifts controlled by ice sheet changes the volume of the northern hemisphere ice sheet controlled abrupt millennial scale climate changes during the last glacial dansgaard oeschger events are millennial scale warm periods interrupting cold glacial climates although they are perhaps the most prominent example of abrupt change observed in the ice core record of the past 800 000 years their underlying physical controls remain debated xu zhang and colleagues now use a fully coupled climate model to show that dansgaard oeschger events tend to occur when ice sheets are at an intermediate height a condition in which small perturbations to the coupled atmosphere ocean sea ice system can lead to rapid nonlinear feedbacks and abrupt changes in climate during glacial periods of the late pleistocene an abundance of proxy data demonstrates the existence of large and repeated millennial scale warming episodes known as dansgaard oeschger do events1 this ubiquitous feature of rapid glacial climate change can be extended back as far as 800 000 years before present bp in the ice core record2 and has drawn broad attention within the science and policy making communities alike3 many studies have been dedicated to investigating the underlying causes of these changes but no coherent mechanism has yet been identified3 4 5 6 7 8 9 10 11 12 13 14 15 here we show by using a comprehensive fully coupled model16 that gradual changes in the height of the northern hemisphere ice sheets nhiss can alter the coupled atmosphere ocean system and cause rapid glacial climate shifts closely resembling do events the simulated global climate responses including abrupt warming in the north atlantic a northward shift of the tropical rainbelts and southern hemisphere cooling related to the bipolar seesaw are generally consistent with empirical evidence1 3 17 as a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets minor changes in the height of the nhiss and the amount of atmospheric co2 can trigger the rapid climate transitions via a local positive atmosphere ocean sea ice feedback in the north atlantic our results although based on a single model thus provide a coherent concept for understanding the recorded millennial scale variability and abrupt climate changes in the coupled atmosphere ocean system as well as their linkages to the volume of the intermediate ice sheets during glacials

floodplains of large rivers weathering reactors or simple silos abstract large river sediments are mostly derived from tectonically active mountain belts but then undergo a series of sedimentation temporary storage and reworking episodes on their journey to the ocean the long transfer time of these sediments through active floodplains might result in a significant chemical maturation via weathering reactions which is of critical importance for biogeochemical cycles at the earth surface this study reports the chemical composition of river sediments from different locations throughout the courses of the main tributaries of the amazon basin sampling along river depth profiles yields access to the whole grain size and chemical composition range of river sediment here weathering intensities i e losses of na k mg and ca associated with chemical weathering in floodplains are 1 examined as a function of grain size and 2 integrated over the whole grain size range for three selected long river reaches flowing through the foreland and the lowland of the amazon basin the upper maranon the beni and the lower madeira rivers a relatively small na loss through plagioclase dissolution is observed in the madeira reach while an important ca loss due to carbonate dissolution occurs in the two foreland reaches maranon and beni no significant loss of k and mg is observed in any of the reaches showing the low alterability of primary k and mg bearing minerals and suggesting retention of k and mg in the particulate phase by secondary minerals the combination of these findings with previously reported data on the downstream change of dissolved na k mg and ca fluxes suggests that chemical weathering of stable alluvial deposits could also significantly contribute to theweathering flux generated in foreland and lowland areas the comparison between the three amazon reaches and the gangetic plain tends to show that the features observed in the amazon are valid on a global scale finally we show that although resulting in a relatively small change in the chemical composition of the river sediment silicate weathering in the lower madeira floodplain can lead to a co2 drawdown equivalent to ca 10 of the total co2 consumption flux of the whole madeira basin

interhemispheric anti phasing of rainfall during the last glacial period abstract we have obtained a high resolution oxygen isotopic record of cave calcite from caverna botuvera 27 13 s 49 09 w southern brazil which covers most of the last 36 thousand years ka with an average resolution of a few to several decades the chronology was determined with 46 u th ages from two stalagmites tests for equilibrium conditions show that oxygen isotopic variations are primarily caused by climate change we interpret our record in terms of meteoric precipitation changes hence the variability of south american monsoon sam intensity the oxygen isotopic profile broadly follows local insolation changes and shows clear millennial scale variations during the last glacial period with amplitudes as large as 3 but with smaller centennial scale shifts 1 during the holocene the overall record is strikingly similar to but strongly anti correlated with a number of records from the northern hemisphere we compared our record to other precisely dated contemporaneous records from hulu cave eastern china minima in 18o wet periods intense sam at our site are synchronous with maxima in 18o dry periods weak east asian monsoon eam in eastern china within precise dating errors and vice versa this anti phased precipitation relationship between two low latitude locations may be interhemispheric in extent based on comparison with records from other sites precipitation anti phasing may be related to north south shifts in the mean position of the intertropical convergence zone itcz and asymmetry in hadley circulation in two hemispheres associated not with seasonal changes as observed today but with millennial scale climate shifts the millennial scale atmospheric see saw patterns that we observe could have important controls and feedbacks on climate within hemispheres because of water vapor s greenhouse properties

late quaternary productivity changes in the western equatorial atlantic evidence from 230th normalized carbonate and organic carbon accumulation rates paleoproductivity changes in the western equatorial atlantic have been estimated from carbonate and marine organic carbon accumulation for the last 180 000 yr from a ceara rise sediment core geob 1523 1 accumulation rates were calculated by normalizing to excess 230th activity paleoproductivity from carbonate accumulation was estimated by a correlation between carbonate and organic carbon fluxes derived from sediment traps deployed in oligotrophic waters results indicate minor productivity changes varying around 30 gc m 2 yr 1 for organic carbon derived estimations and around 45 gc m 2 yr 1 for carbonate derived estimations suggesting that the study area remained a low productivity region throughout the time period examined maxima in western atlantic paleoproductivity occurred during the warm substages of glacials and interglacials in contrast to the eastern tropical atlantic where maxima are recorded in the cold substages this contrast might be caused by a deepening of the thermocline and nutricline in the west connected with a synchronous shallowing in the east the carbonate preservation record was examined for the last 380 000 yr based on variations in percent planktonic foraminiferal fragments and percent coarse fraction only minor carbonate dissolution above 3300 m water depth is observed except for three main dissolution events at isotope stages 4 8 4 and 10 considerable carbonate loss during these intervals is attributed to a decreased production of north atlantic deep water which is associated with vertical expansion of southern source deep water

dynamic boundary monsoon intensity hypothesis evidence from the deglacial amazon river discharge record glacioeustatic and temperature corrected planktonic foraminiferal oxygen isotope d 18 o records from odp site 942 on the amazon fan provide a means of monitoring past changes in the outflow of the amazon river this study focuses on the last deglaciation and reveals that during this period there were significant variations in the outflow which implies large changes in moisture availability in the amazon basin aridity in the amazon basin seems to occur between 20 5 ka calendar to 17 0 ka and 13 6 ka to 11 ka the second arid period correlates with the start of the antarctic cold reversal and aridity continues throughout the younger dryas period we find that the large scale trends in amazon river outflow are dissimilar to high latitude variability in either hemisphere instead high resolution variations correlate with the d 18 o difference between greenland and antarctica ice core temperature records this suggests a link between hemispheric temperature gradients and moisture availability over the amazon based on our results and previously published work we present a new testable dynamic boundary monsoon intensity hypothesis which suggests that tropical moisture is not a simple belt that moves north or south rather the northern and southern boundaries of the south american summer monsoon sasm are independently dynamic and driven by temperature gradients within their individual hemispheres the intensity of rainfall within the sasm however is driven by precessionally modulated insolation and the resultant convection strength combining these two influences produces the dynamic heterogenic changes in the moisture availability observed over tropical south america since the last glacial maximum

nature and causes of quaternary climate variation of tropical south america abstract this selective review of the quaternary paleoclimate of the south american summer monsoon sasm domain presents viewpoints regarding a range of key issues in the field many of which are unresolved and some of which are controversial 1 el nino southern oscillation variability while the most important global scale mode of interannual climate variation is insufficient to explain most of the variation of tropical south american climate observed in both the instrumental and the paleoclimate records 2 significant climate variation in tropical south america occurs on seasonal to orbital i e multi millennial time scales as a result of sea surface temperature sst variation and ocean atmosphere interactions of the tropical atlantic 3 decadal scale climate variability linked with this tropical atlantic variability has been a persistent characteristic of climate in tropical south america for at least the past half millennium and likely far beyond 4 centennial to millennial climate events in tropical south america were of longer duration and perhaps larger amplitude than any observed in the instrumental period which is little more than a century long in tropical south america these were superimposed upon both precession paced insolation changes that caused significant variation in sasm precipitation and eccentricity paced global glacial boundary conditions that caused significant changes in the tropical south american moisture balance as a result river sediment and water discharge increased and decreased across tropical south america lake levels rose and fell paleolakes arose and disappeared on the altiplano glaciers waxed and waned in the tropical andes and the tropical rainforest underwent significant changes in composition and extent to further evaluate climate forcing over the last glacial cycle 125 ka we developed a climate forcing model that combines summer insolation forcing and a proxy for north atlantic sst forcing to reconstruct long term precipitation variation in the sasm domain the success of this model reinforces our confidence in assigning causation to observed reconstructions of precipitation in addition we propose a critical correction for speleothem stable oxygen isotopic ratios which are among the most significant of paleoclimate proxies in tropical south america for reconstruction of variation of paleo precipitation or sasm intensity however it is already well known that any particular 18o value observed in speleothem carbonate is affected by two processes that have nothing to do with changes in precipitation amount the influence of temperature on carbonate water isotopic fractionation in the cave and the influence of changing 18o of seawater quantitatively accounting for both artifacts can significantly alter the interpretations of speleothem records in tropical south america both adjustments act in the same direction and have the tendency to increase the true amplitude of the paleo hydrologic signal but by different amounts in glacial and inter glacial stages these corrections have even graver implications for the interpretation of tropical northern hemisphere speleothem records e g chinese speleothems where the combined adjustments tend to decrease or even eliminate the true signal amplitude

significance of the sedimentary al ti ratio as an indicator for variations in the circulation patterns of the equatorial north atlantic this paper presents results of geochemical investigations conducted on three gravity cores recovered along an e w transect 3 5 n in the equatorial atlantic considering the high terrigenous content of the sediments approximately 12 74 wt al and ti concentrations are predominantly associated with the input of lithogenous phases the terrigenous fraction of western equatorial atlantic sediments is more influenced by material derived from the amazon river than by deposition of african dust particles as a possible route of transport the north equatorial countercurrent is considered fluctuations in al ti records generally reflect changes in the low latitude insolation cycle 19 23 kyr and thus indicate a close coupling to variations of the trade wind systems and continental climate changes however characteristics of the terrigenous signals in the eastern and western equatorial atlantic differ significantly while the precessional signal in the al ti record and low latitude portion in the 18o record are in phase in the eastern equatorial atlantic variations in the terrigenous fraction westward of the mid atlantic ridge precede the marine signal by 2 3 kyr which we ascribe to a shift between trade wind systems of the northern and southern hemispheres

the last 100 000 years in the western mediterranean sea surface water and frontal dynamics as revealed by coccolithophores a quantitative analysis was carried out on coccolith assemblages from two pleistocene cores k1 and k10 from the western mediterranean the distribution of selected coccolithophore species provides new paleoclimatic and paleoceano graphic data a continuous sequence from the top of isotope stage 5 to the holocene was recorded the reversal in dominance between gephyrocapsa muellerae and emiliania huxleyi was dated in both cores at ca 73 ka at about 47 ka e huxleyi shows a regular increase whereas g muellerae progressively decreases in abundance during interglacial periods high concentrations of coccoliths are observed whereas in glacial times coccoliths are more diluted and the percentage of reworked forms increases as a consequence of the higher terrigenous input after taking careful account of the dilution factor we conclude that the production of coccolithophores was higher during warm periods maxima in coccolith concentrations coincide with highstand episodes probably as a result of the intensification of the atlantic flux into the mediterranean across the gibraltar strait this intensification could have produced an increase in nutrient content in the surface mediterranean waters during cold periods the western mediterranean front underwent a reduction in activity probably due to an increase in the saline and or thermal gradients between the superficial waters and intermediate waters in the liguro provencal basin

origin of amazon mudbanks along the northeastern coast of south america abstract seismic profiles sediment cores and water column measurements were collected along the northeastern coast of brazil to examine the origin of mudbanks in the amazon coastal mud belt these 10 60 km long shore attached features previously had been observed to migrate along the 1200 km coast of the guianas in response to wave forcing chirp 3 5 khz seismic profiles of the shoreface and inner shelf located two mudbanks updrift of the previous eastern limit in french guiana 210 pb geochronology shows that these two banks are migrating to the northwest over a relict mud surface in 5 20 m water depth the mudbanks are 3 4 m thick and are translating over a modern shoreface mud wedge deposited by previous mudbank passage in

some features of the algerian current an analysis of infrared images leads to a new conception of the dynamics of the atlantic water in the algerian basin in situ current measurements are not yet available but coherent hypotheses are presented with the aim to initiate further experimental and theoretical works the algerian current flows steadily along the coast near 0 and becomes unstable near 1 2 e eddies of both signs are generated that are advected by the mean current but only the anticyclonic eddies increase in size diameter about 100 km as they extract more energy from the mean current they are advected more slowly and they may detach from the coast and drift for several weeks in the algerian basin they grow as deep as 1000m at least and they are able to pull lenses of intermediate water seaward from the sardinian continental slope coastal upwelling cells are generated near 1 2 e between the cyclonic and the anticyclonic eddies they are also advected by the mean current and are definitively not wind induced these turbulent structures create intense mixing between the atlantic and mediterranean waters which account for the large eastward gradient of salinity encountered in the surface layer therefore the algerian basin is characterized by a large mesoscale variability mainly due to the instability of the algerian current it appears to be a reservoir in which the water of atlantic origin is amassed and it forms a buffer zone which disconnects the flow of atlantic water coming in at gibraltar from the flow of hygly modified atlantic water exiting the basin through the strait of sardinia and into the ligurian sea

eddy properties in the western mediterranean sea from satellite altimetry and a numerical simulation three different eddy detection and tracking methods are applied to the outputs of a high resolution simulation in the western mediterranean sea in order to extract mesoscale eddy characteristics the results are compared with the same eddy statistics derived from satellite altimetry maps over the same period eddy radii are around 30 km in altimetry maps whereas in the model they are around 20 km this is probably due to the inability of altimetry maps to resolve the smaller mesoscale in the region about 30 eddies are detected per day in the basin with a very heterogeneous spatial distribution and relatively short lifespans median life around 13 days unlike other areas of the open ocean they do not have a preferred direction of propagation but appear to be advected by mean currents the number of detected eddies seems to present an annual cycle when separated according to their lifespan with the numerical simulation we show that anticyclones extend deeper in the water column and have a more conic shape than cyclones

links between ocean temperature and iceberg discharge during heinrich events the last glacial period was punctuated by several periods of massive iceberg discharge from the northern hemisphere ice sheets numerical simulations indicate that these discharge events are linked to an interplay between ice sheet accumulation marine ice shelf stability and periodically oscillating surface ocean temperature

multiscale variability in the balearic sea an altimetric perspective 1 the present day availability of an 18 year record of 1 8 merged mediterranean sea sea level anomaly sla data enables a contemporary description of long term mesoscale activity in the balearic sea sla data from satellite altimetry are used to study the variability of sea level and surface geostrophic circulation at different spatial and temporal scales within this complex and relatively understudied region in the western mediterranean wmed we find that the mean northern current along the iberian slope is strongest in autumn although higher variability in winter leads to stronger peaks in kinetic energy the balearic current which flows along the northern slopes of the balearic islands also has its maximum expression in autumn across the two balearic channels ibiza and mallorca key locations that partly regulate meridional exchange in the wmed observed seasonal variability in geostrophic velocity anomalies conforms rather well to prior descriptions suggesting cautious confidence in the use of the mediterranean merged altimeter product in nearshore regions circulation through the channels is maximum in winter the channel data support the hypothesis that the channel circulation may be hindered by the intermittent presence of the western intermediate water mass which sometimes forms in winter in the gulf of lions this is the first time that an analysis of variability in the balearic channels has been performed using altimetric data

the greenland ice core chronology 2005 15 42 ka part 2 comparison to other records a new greenland ice core chronology gicc05 based on multi parameter counting of annual layers has been obtained for the last 42 ka here we compare the glacial part of the new time scale which is based entirely on records from the northgrip ice core to existing time scales and reference horizons covering the same period these include the grip and northgrip modelled time scales the meese sowers gisp2 counted time scale the shackleton fairbanks grip time scale sfcp04 based on 14c calibration of a marine core the hulu cave record three volcanic reference horizons and the laschamp geomagnetic excursion event occurring around greenland interstadial 10 gicc05 is generally in good long term agreement with the existing greenland ice core chronologies and with the hulu cave record but on shorter time scales there are significant discrepancies around the last glacial maximum there is a more than 1 ka age difference between gicc05 and sfcp04 and a more than 0 5 ka discrepancy in the same direction between gicc05 and the age of a recently identified tephra layer in the northgrip ice core both sfcp04 and the tephra age are based on 14c dated marine cores and fixed marine reservoir ages for the laschamp event gicc05 agrees with a recent independent dating within the uncertainties

calibration of xrf core scanners for quantitative geochemical logging of sediment cores theory and application abstract on line analysis of split sediment cores by xrf core scanners has become increasingly popular in the past decade because it allows nondestructive extraction of near continuous records of element intensities from sediment cores with a minimum of analytical effort a disadvantage of xrf core scanning relative to conventional geochemical analysis is the problematic conversion of core scanner output to element concentrations the main reason for this long standing problem is the poorly constrained measurement geometry attributable to inhomogeneity of the specimens e g variable water content and grain size distribution irregularities of the split core surface and in some setups spatial variations in thickness of an adhesive pore water film which forms directly below a protective foil covering the core surface we propose a log ratio calibration model for xrf core scanners derived from a combination of xrf spectrometry theory principles of compositional data analysis and empirical evidence the log ratio calibration model provides accurate and precise predictions of sediment composition element concentrations from xrf core scanner output with a limited number of parameters namely 2 d 1 where d equals the number of chemical elements whose concentrations are to be estimated the model can accommodate the inherent non linearity of the relation between relative intensities and concentrations which is apparent from the fact that it provides unbiased estimates an immediate corollary of our results is that log ratios of element intensities which are related to log ratios of element abundances by a simple linear transformation provide the most easily interpretable signals of relative changes in chemical composition consistent use of log ratios of element intensities or concentrations should minimise the risk of drawing erroneous conclusions from geochemical proxies the relative standard deviation precision of predicted element concentrations in core geob7920 is less than 2 stochastic simulations indicate that this level of precision can be attained with 40 randomly selected calibration specimens improved control over input errors and development of robust goodness of fit statistics allows xrf core scanning to be developed into a rigorous quantitative measurement technology the log ratio calibration equation derived in this study may be adapted to inter laboratory and inter instrument calibration as well

a 43 ka record of paleoenvironmental change in the central american lowlands inferred from stable isotopes of lacustrine ostracods we present a continuous ostracod isotope 18o and 13c record from lake peten itza peten guatemala in the northern lowland neotropics that spans the last 43 cal ka bp variations in oxygen and carbon isotopes closely follow lithologic variations which consist of alternating gypsum and clay deposits that were deposited under relatively dry and wet climate respectively during the last glacial period the greatest 18o and 13c values coincide with gypsum deposited during lake lowstands under arid climate conditions that were correlated previously with north atlantic heinrich events in contrast interstadials and the entirety of the last glacial maximum 24 19 cal ka bp are marked by clay deposition and lower 18o and 13c values reflecting higher lake levels and relatively moister climate isotope results and pollen data along with independently inferred past water levels show the early deglacial period 19 15 cal ka bp was the time of greatest aridity and lowest lake stage of the past 43 ka this period occurred during heinrich stadial 1 hs 1 when an extensive tropical megadrought has been postulated stager et al 2011 heinrich stadial 1 is represented by two episodes of gypsum precipitation and high 18o and 13c values in peten itza interrupted by an intervening period of lower 18o and 13c and clay deposition centered on 17 cal ka bp the two periods of inferred maximum cold and or arid conditions at 17 5 and 16 1 cal ka bp coincide approximately with two pulses of ice rafted debris ird recorded off southern portugal bard et al 2000 at 15 cal ka bp coinciding with the start of the bolling allerod period 18o and 13c decrease and gypsum precipitation ceases indicating a transition to warmer and or wetter conditions gypsum precipitation resumed while 18o and 13c increased at the start of the younger dryas at 13 1 cal ka bp and continued until 10 4 cal ka bp near the onset of the holocene precipitation changes during the last glacial period in the northern hemisphere neotropics were closely linked with freshwater forcing to the high latitude north atlantic and sensitive to changes in the location of meltwater input climate was coldest driest when meltwater directly entered the high latitude north atlantic permitting sea ice expansion and weakening of atlantic meridional overturning circulation amoc which resulted in a more southerly position of the intertropical convergence zone itcz upon deglaciation when meltwater was directed to the gulf of mexico at 17 ka and during the bolling allerod period 15 13 ka precipitation increased in the northern hemisphere neotropics as north atlantic sea ice retreated and the itcz shifted northward results from lake peten itza offer some support for the meltwater routing hypothesis of clark et al 2001

recent advances in observing the physical oceanography of the western mediterranean sea the mediterranean sea has been investigated intensively since the early nineties using modern techniques and collaborative approaches this overview summarizes some of the resulting advances that were made concerning the physical oceanography of the western mediterranean the water mass formation processes are now much better understood and have been quantified to a large extent the boundary conditions of the system in terms of surface fluxes and strait transports can be determined with improved accuracy thus enabling future investigation of interannual variability the dynamics of the surface and intermediate layers have revealed a variety of eddy and mesoscale processes that are important for the circulation and spreading of water masses the deep circulation is being investigated with lagrangian techniques tracers and floats first results show a large component of the deep water originating from the tyrrhenian sea and intense cyclonic and anticyclonic eddy flows

multivariate analysis of the surface circulation in the balearic sea this study gives a comprehensive view of the regional surface circulation of the balearic sea in early summer its spatial variability and dynamics are described from a multivariate analysis of axbt ctd and adcp data collected during the mesoscale experiment fe 91 may 22 june 2 1991 the results provide evidence of two energetic frontal jets that drive the circulation over the slopes the southward continental current 0 9sv and the northward balearic current 0 5sv in the southern balearic basin to the north of the ibiza sill the analyses show that the slope circulation is drastically modified by a wide pool 70km of winter intermediate waters wiw in this region both the continental and balearic currents leave the slopes and merge offshore as a result the continental current veers cyclonically to the north and flows along the insular slope numerous mesoscale structures eddies meanders and filaments are found in the centre of the basin directly related to instabilities of the frontal jets often induced by sharp topographic irregularities and subsurface eddies and involving significant offshore transport of slope waters assuming stationarity of the velocity field a lagrangian integration is performed to establish the trajectories and time scales associated with the analysed velocity field the calculations indicate that a drifter released in the core of the continental current at 22m depth drifts cyclonically around the balearic basin and reaches the north of mallorca in less than 40 days this is consistent with data from an in situ surface drifter deployed in june 1993 and suggests that the velocity field obtained from fe 91 data is representative of the summer circulation of the balearic sea all these results are discussed in the perspective of the general circulation of the western mediterranean

a real time oceanographic nowcast forecast system for the mediterranean sea we describe here a nowcast forecast system for the entire mediterranean sea designed for real time forecasts and closely resembling operational numerical weather prediction systems the core of the system is a high resolution 10 km three dimensional primitive equation based sigma coordinate numerical circulation model assimilating remotely sensed multi channel sea surface temperature and in situ expendable bathythermograph conductivity temperature depth observational data using an optimal interpolational scheme we present results for 1993 and 1994 from this data assimilation model focusing principally on the mesoscale features prevalent in the western and eastern mediterranean sea we show that the model exhibits considerable skill in simulating both the permanent and transient mesoscale features in the mediterranean in particular hitherto less well known and less well studied circulation features in the adriatic and aegean seas are carefully examined and presented particular care is given to discussing the circulation in the entire mediterranean with particular attention to flows through various straits and their variability

vertical structure and temporal evolution of an anticyclonic eddy in the balearic sea western mediterranean 1 an anticyclonic eddy in the balearic sea western mediterranean was described using data from a mooring line deployed at the northern slope of mallorca island at about 900 m deep its surface signature was investigated using sea surface height and sea surface temperature images the eddy which lasted around 1 month modified the thermohaline characteristics and the currents of the entire water column levantine intermediate waters usually resident in the region were displaced by colder and fresher western mediterranean intermediate waters associated with the eddy along slope main currents toward ne were completely reversed at 500 m and significantly deviated at 900 m interestingly near bottom velocities were found to be systematically larger than those at intermediate depths furthermore during the eddy velocities reached values up to 26 cm s at the bottom 5 times larger than the bottom average speed the recurrence of the phenomenon was explored with an eddy detection tool applied to satellite observations results indicated that anticyclonic eddies are common structures in the balearic current

a comparison of seasonal winds with currents on the continental slope of the catalan sea northwestern mediterranean the waters of the northwestern region of the mediterranean sea follow the continental shelf break in a flow that appears to be linked to a permanent shelf slope density front several studies are underway to evaluate the role of the possible forces that drive this circulation the continental margin located off the ebro river is an area of persistent and high intensity winds long term representative seasonal wind and current meter data are presented here in a comparative analysis of the temporal variability of both fields the data seem to indicate that the local wind field is not responsible for the characteristics of the marine circulation the wind is only able to develop significant inertial oscillations in spring and may contribute to the intensification of the currents only during the cold season in addition the variability of the current field shows no relationship to wind variability important mesoscale activity is probably associated with variations of the density structure and not with the wind in essence the wind provides no appreciable direct contribution to modifying the general circulation

distribution of major elements in atlantic surface sediments 36 n 49 s imprint of terrigenous input and continental weathering 1 numerous studies use major element concentrations measured on continental margin sediments to reconstruct terrestrial climate variations the choice and interpretation of climate proxies however differ from site to site here we map the concentrations of major elements ca fe al si ti k in atlantic surface sediments 36 n 49 s to assess the factors influencing the geochemistry of atlantic hemipelagic sediments and the potential of elemental ratios to reconstruct different terrestrial climate regimes high concentrations of terrigenous elements and low ca concentrations along the african and south american margins reflect the dominance of terrigenous input in these regions single element concentrations and elemental ratios including ca e g fe ca are too sensitive to dilution effects enhanced biological productivity carbonate dissolution to allow reliable reconstructions of terrestrial climate other elemental ratios reflect the composition of terrigenous material and mirror the climatic conditions within the continental catchment areas the atlantic distribution of ti al supports its use as a proxy for eolian versus fluvial input in regions of dust deposition that are not affected by the input of mafic rock material the spatial distributions of al si and fe k reflect the relative input of intensively weathered material from humid regions versus slightly weathered particles from drier areas high biogenic opal input however influences the al si ratio fe k is sensitive to the input of mafic material and the topography of andean river drainage basins both ratios are suitable to reconstruct african and south american climatic zones characterized by different intensities of chemical weathering in well understood environmental settings

seasonal variations in the horizontal structure of the amazon plume inferred from historical hydrographic data near surface salinity observations from the national oceanographic data center hydrographic data archive for the region 4 s 10 n 60 40 w show that the amazon river discharge forms a plume of low salinity water that extends northwestward into the western tropical north atlantic the average near surface salinity within the plume increases away from the amazon river mouth consistent with an average entrainment of oceanic water of 2 105 m3 s every 1 of latitude there are two notable seasonal variations in the vicinity of the amazon river mouth 0 2 n the plume extends about 200 300 km eastward from june through december however from march to may the plume was often much wider 400 500 km in the vicinity of the river mouth possibly due to the combination of near maximum river discharge and winds with a southeastward component north of 5 n the amazon plume fans out over a broad region of the western tropical north atlantic during january through june low salinity amazon plume water north of 5 n is primarily found west of 52 w spreading northwestward toward the caribbean beginning typically in may some of the amazon plume water is deflected eastward in the north brazil current nbc retroflection from august through october surface salinities suggest 70 of the amazon plume water is carried eastward in the nbc retroflection while the remaining 30 flows northwestward toward the caribbean

general patterns of circulation sediment fluxes and ecology of the palamos la fonera submarine canyon northwestern mediterranean abstract currents particle fluxes and ecology were studied in the palamos submarine canyon also known as the fonera canyon located in the northwestern mediterranean seven mooring arrays equipped with current meters and sediment traps were deployed along the main canyon axis on the canyon walls and on the adjacent slope additionally local and regional hydrographic cruises were carried out current data showed that mean near surface and mid depth currents were oriented along the mean flow direction ne sw although at 400 and 1200 m depth within the canyon current reversals were significant indicating a more closed circulation inside the canyon mean near bottom currents were constrained by the local bathymetry especially at the canyon head the most significant frequency at all levels was the inertial frequency a second frequency of about three days attributed to a topographic wave was observed at all depths suggesting that this wave was probably not trapped near the bottom the current field observed during the most complete survey revealed a meandering pattern with cyclonic vorticity just upstream from and within the canyon the associated vertical velocity ranged between 10 and 20 m day and was constrained to the upper 300 m this latter feature together with other computations suggests that during this survey the meander was not induced by the canyon but by some kind of instability of the mean flow in the canyon suspended sediment concentration downward particle fluxes chlorophyll and particulate c and n were significantly higher up canyon from about 1200 m depth than offshore defining along with the different hydrodynamics two canyon domains one from the canyon head to about 1200 m depth more affected by the canyon confinement and the other deeper than 1200 m depth more controlled by the mean flow and the shelf slope front the higher near bottom downward total mass fluxes were recorded in the canyon axis at 1200 m depth along with sharp turbidity increases and are related to sediment gravity flows during the deployment period the increase in downward particle fluxes occurred by mid november when a severe storm took place on the canyon walls at 1200 m depth suspended sediment concentrations downward particle fluxes chlorophyll and particulate c and n were higher on the southern wall than on the northern wall inversely to the current s energy this could be caused by an upward water supply on the southern canyon wall and or the mean flow interacting with the canyon bathymetry in the swimmers collected by the sediment traps the dominant species was an elasipod holothurian which has not been recorded in other canyons or elsewhere in the mediterranean indicating particular speciation

variable winter moisture in the southwestern united states linked to rapid glacial climate shifts the last glacial period was characterized by large rapid climate fluctuations an analysis of a speleothem from new mexico shows that the coldest conditions over greenland coincide with increased winter precipitation in the southwestern united states which can be attributed to a southward displacement of the polar jet stream and the north american storm track

instability of the atlantic overturning circulation during marine isotope stage 3 variations in the strength of the atlantic meridional overturning circulation amoc were involved in the occurrences of dansgaard oeschger d o events during marine isotope stage 3 mis3 the stability of the amoc to north atlantic freshwater perturbations is studied using a comprehensive climate model under mis3 boundary conditions an amoc stability diagram constructed from a series of equilibrium freshwater perturbation experiments reveals a highly nonlinear dependence of amoc strength on freshwater forcing the mis3 baseline state is remarkably unstable with respect to minor perturbations the global climate signal associated with a change in amoc strength is consistent with a transition from an interstadial to a stadial state including an annual mean surface air temperature drop of 8 k in central greenland we suggest that minor freshwater perturbations in the hydrologic cycle e g related to ice sheet processes had the potential to trigger d o type climate shifts associated with a threshold in the atmosphere ocean system

strong and deep atlantic meridional overturning circulation during the last glacial cycle extreme abrupt northern hemisphere climate oscillations during the last glacial cycle 140 000 years ago to present were modulated by changes in ocean circulation and atmospheric forcing1 however the variability of the atlantic meridional overturning circulation amoc which has a role in controlling heat transport from low to high latitudes and in ocean co2 storage is still poorly constrained beyond the last glacial maximum2 3 4 here we show that a deep and vigorous overturning circulation mode has persisted for most of the last glacial cycle dominating ocean circulation in the atlantic whereas a shallower glacial mode with southern sourced waters filling the deep western north atlantic prevailed during glacial maxima3 5 our results are based on a reconstruction of both the strength and the direction of the amoc during the last glacial cycle from a highly resolved marine sedimentary record in the deep western north atlantic parallel measurements of two independent chemical water tracers the isotope ratios of 231pa 230th and 143nd 144nd 6 7 8 which are not directly affected by changes in the global cycle reveal consistent responses of the amoc during the last two glacial terminations any significant deviations from this configuration resulting in slowdowns of the amoc were restricted to centennial scale excursions during catastrophic iceberg discharges of the heinrich stadials severe and multicentennial weakening of north atlantic deep water formation occurred only during heinrich stadials close to glacial maxima with increased ice coverage probably as a result of increased fresh water input in contrast the amoc was relatively insensitive to submillennial meltwater pulses during warmer climate states and an active amoc prevailed during dansgaard oeschger interstadials greenland warm periods

moisture variability in the southwestern united states linked to abrupt glacial climate change the atmospheric response to millennial scale circulation changes in the north atlantic ocean during the last glacial period has been difficult to constrain cave deposits from southwestern north america reveal that atmospheric moisture in this region increased in response to slowdowns of the atlantic meridional overturning circulation

surface circulation variability in the balearic basin the balearic basin is defined as a wide region of the western mediterranean located in between the liguro provencal and algerian basins the dynamics of the liguro provencal basin in the north is dominated by the dense water formation process associated with a permanent circulation along the continental slope the northern current the algerian basin in the south is dominated by intense mesoscale eddies and their interactions with the unstable algerian current the balearic basin is a transition region between these two different dynamic regimes it includes the catalan sea located between the balearic islands and the iberian peninsula and the wide gulf of valencia in the south of this sea some new ideas about the surface circulation variability have been inferred from a series of noaa satellite advanced very high resolution radiometer images taken mainly from the autumn winter period during 1981 1988 and also from various in situ data the northern current which flows southward along the iberian peninsula slope is covered in summer by a warm surface layer spreading over the whole catalan sea this local warming creates one of the most intense thermal fronts in the western mediterranean the pyrenees front near the balearic islands the flow through the sills of recent atlantic water partially deflected by anticyclonic eddies from the algerian current creates the balearic front and contributes to its mesoscale variability this variability appears to be much more intense than previously described the balearic front is clearly recognized as the westward continuation of the north balearic front already defined in the open sea as the northern limit of the recent atlantic water reservoir finally the gulf of valencia is frequently influenced by water entering from the algerian basin which appears to be important in the disruption of the northern current and the formation of the balearic current the geostrophic flow associated with the balearic front

mesoscale variability in the alboran sea synthetic aperture radar imaging of frontal eddies in autumn 1992 the entire alboran sea western mediterranean was sampled by the spanish r v garcia del cid the incoming jet of atlantic water its associated meandering front and the two big anticyclonic gyres were described from conductivity temperature depth ctd and acoustic doppler current profiler adcp data smaller scale eddies were also observed additionally 36 ers 1 synthetic aperture radar sar scenes were obtained from mid september to mid october the sar images capture these features when wind conditions are suitable the current shear is mainly depicted as narrow lines of low backscatter because of the damping of waves by natural surface films these lines delineate the northern border of both gyres and the beginning of the alongslope algerian current at the eastern limit of the alboran sea adcp observations confirm that lines on sar imagery follow the direction of the surface currents the two gyres present high backscatter values in their center while their frontal boundaries appear modulated by the variation of the marine atmospheric boundary layer stratification due to the surface thermal front sar has observed small spiral eddies that were not evidenced by the almost contemporaneous but too coarse ctd in situ sampling good spatial correspondence between radar detected and in situ measured structures occurs when comparing sar images to the surface dynamic topography rather than strictly surface water characteristics

a stratigraphic framework for abrupt climatic changes during the last glacial period based on three synchronized greenland ice core records refining and extending the intimate event stratigraphy due to their outstanding resolution and well constrained chronologies greenland ice core records provide a master record of past climatic changes throughout the last interglacial glacial cycle in the north atlantic region as part of the intimate integration of ice core marine and terrestrial records project protocols have been proposed to ensure consistent and robust correlation between different records of past climate a key element of these protocols has been the formal definition and ordinal numbering of the sequence of greenland stadials gs and greenland interstadials gi within the most recent glacial period the gs and gi periods are the greenland expressions of the characteristic dansgaard oeschger events that represent cold and warm phases of the north atlantic region respectively we present here a more detailed and extended gs gi template for the whole of the last glacial period it is based on a synchronization of the ngrip grip and gisp2 ice core records that allows the parallel analysis of all three records on a common time scale the boundaries of the gs and gi periods are defined based on a combination of stable oxygen isotope ratios of the ice 18o reflecting mainly local temperature and calcium ion concentrations reflecting mainly atmospheric dust loading measured in the ice the data not only resolve the well known sequence of dansgaard oeschger events that were first defined and numbered in the ice core records more than two decades ago but also better resolve a number of short lived climatic oscillations some defined here for the first time using this revised scheme we propose a consistent approach for discriminating and naming all the significant abrupt climatic events of the last glacial period that are represented in the greenland ice records the final product constitutes an extended and better resolved greenland stratotype sequence against which other proxy records can be compared and correlated it also provides a more secure basis for investigating the dynamics and fundamental causes of these climatic perturbations

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

different precipitation patterns across tropical south america during heinrich and dansgaard oeschger stadials abstract detailed knowledge about tropical south american precipitation patterns during heinrich h and dansgaard oeschger do stadials provides relevant insights into the possible evolution of amazonian hydroclimate under future climate change sediment core geob16224 1 ca 7 n raised from a site in the continental slope off french guiana in western equatorial atlantic under the influence of the amazon river discharge documents the impacts of h and do stadials on both inorganic i e fe ca record and organic i e alkenone c37 concentration and c37 c38 ratio geochemistry between 41 and 13 ka bp our results show millennial scale covariations of increased fe ca values with decreased c37 concentration and c37 c38 ratios during h and do stadials comparing our high temporal resolution data with previously published records from ca 17 n to 4 s we are able to differentiate the influence of h and do stadials upon tropical south american precipitation we find that records under the influence of the south american summer monsoon e g western amazon and the northern edge of the intertropical convergence zone itcz e g northernmost south america exhibit strong climate shifts during both h and do stadials but regions under the influence of the southern edge of the itcz e g northeastern brazil experience a weaker reaction during do stadials than during h stadials

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

variations in the structure of the anticyclonic gyres found in the alboran sea historical satellite aircraft and in situ data have shown that two anticyclonic gyres the western and eastern alboran gyres are major ocean features of the alboran sea an examination of several years of satellite imagery indicates that large variations in the surface expression of these two gyres occur and that on occasion one or the other gyre disappears the disappearance of both gyres at the same time was not seen the initial disappearance of either gyre occurs on a time scale of a week to 2 weeks whereas the return may take from 3 weeks to 2 months various forcing mechanisms i e winds mass flux inflow through the straits of gibraltar and sicily and or density have been used in numerical ocean circulation models to study the dynamics of the western mediterranean sea various model results show relationships similar to those shown by the satellite imagery however no single forcing mechanism has been positively identified as the source of the disappearances and the events may be a result of a combination of forcing mechanisms

sahel megadroughts triggered by glacial slowdowns of atlantic meridional overturning 1 the influence of the large scale ocean circulation on sahel rainfall is elusive because of the shortness of the observational record we reconstructed the history of eolian and fluvial sedimentation on the continental slope off senegal during the past 57 000 years our data show that abrupt onsets of arid conditions in the west african sahel were linked to cold north atlantic sea surface temperatures during times of reduced meridional overturning circulation associated with heinrich stadials climate modeling suggests that this drying is induced by a southward shift of the west african monsoon trough in conjunction with an intensification and southward expansion of the midtropospheric african easterly jet

a study of an intense density front in the eastern alboran sea the almeria oran front abstract studies of satellite imagery and space shuttle photographs of the western mediterranean have indicated that the main path of inflowing atlantic water is around two large anticyclonic gyres in the alboran sea and along the algerian coast these studies have also shown that a strong ocean front is present between almeria spain and oran algeria which is part of the easternmost segment of the eastern alboran gyre based on these satellite studies the first in situ investigation of the front called here the almeria oran front was conducted in march 1986 as part of the winter campaign of the western mediterranean circulation experiment wmce analyses of the resulting data show that the almeria oran front is a large scale density front formed by the convergence of two distinct water masses and controlled by the geographic position and strength of the eastern alboran gyre physical and biochemical data indicate that the front is limited to the upper 300 m with a strong southward baroclinic jet

flow modification and shelf slope exchange induced by a submarine canyon off the northeast spanish coast we present results from a study of the modifications induced by the short timescale response of a shelf break baroclinic jet to the presence of a downstream canyon the field site was the canyon of palamos located in the northeastern edge of spain to determine the short time scale variability of the area three intensive surveys were carried out at approximately 10 day intervals data indicate that the jet s location usually over the slope is subjected to significant onshore offshore displacements analysis of the data has shown the importance of a small scale temporal variability induced by river runoff and the climatology of the area this variability generates on off shore displacements of the jet which interacts with the head mouth of the canyon the different interactions with the canyon induced by these displacements produce different regimes of shelf slope exchange in the canyon domain that strongly modify the biomass patterns in the area