precipitation over eastern south america and the south atlantic sea surface temperature during neutral enso periods the dominant mode of coupled variability over the south atlantic ocean is known as south atlantic dipole sad and is characterized by a dipole in sea surface temperature sst anomalies with centers over the tropical and the extratropical south atlantic previous studies have shown that variations in sst related to sad modulate large scale patterns of precipitation over the atlantic ocean here we show that variations in the south atlantic sst are associated with changes in daily precipitation over eastern south america rain gauge precipitation satellite derived sea surface temperature and reanalysis data are used to investigate the variability of the subtropical and tropical south atlantic and impacts on precipitation sad phases are assessed by performing singular value decomposition analysis of sea level pressure and sst anomalies we show that during neutral el nino southern oscillation events sad plays an important role in modulating cyclogenesis and the characteristics of the south atlantic convergence zone positive sst anomalies over the extratropical south atlantic sad negative phase are related to increased cyclogenesis near southeast brazil as well as the migration of extratropical cyclones further north as a consequence these systems organize convection and increase precipitation over eastern south america

the relationship of sea surface temperature to thermocline depth at annual and interannual time scales in the tropical atlantic ocean expendable bathythermograph data derived from sequal focal and toga sponsored volunteer observing ship programs in the tropical atlantic ocean are used to study the relationship between the sea surface temperature sst and the underlying thermocline time series up to 8 years long allow investigation of both the annual and interannual variability between 2n and 2s along selected shipping lanes for the annual harmonic a warm sst is correlated with a shallow thermocline everywhere except in the eastern equatorial region near the intertropical convergence zone itcz the 180 phase shift in the annual harmonic of both the sst and the thermocline depth occurs at slightly different latitudes over most of the tropical atlantic the annual cycle of sst is linked to the phase of the net surface heat flux and only in the eastern equatorial and coastal upwelling regions does heat flux across the thermocline become a significant factor for the annual cycle of the sst canonical correlation analysis of the interannual variability reveal two distinct modes with structures similar to the annual cycle the first in which a warm sst is correlated with a shallow thermocline is a dipole with centers of maximum amplitude off the equator at approximately 8s at 12n and a node at the itcz the second mode for which a warm sst is correlated with a deep thermocline peaks at the equator the predominance of the first mode suggests that for most of the tropical atlantic ocean the dynamical state beneath the surface mixed layer has only a limited influence on sst fluctuations

iron in plagioclase in the bushveld and skaergaard intrusions implications for iron contents in evolving basic magmas the evolved iron rich rocks of the tholeiitic bushveld and skaergaard intrusions are similar in containing cumulus magnetite ilmenite plagioclase clinopyroxene apatite and olivine and also orthopyroxenes pigeonite in bushveld here we evaluate their liquid evolution trends using the total iron content in plagioclase determined by electron microprobe analyses to aid this analysis a revised mass balance model for the liquid evolution of skaergaard is presented for plagioclase in the upper zone of skaergaard it was previously demonstrated that total feo increases from 0 25 to 0 45 wt with differentiation and correlates inversely with an 100 ca na ca the reverse trend is observed in two recently published datasets for bushveld showing that total feo in plagioclase decreases upward through the magnetite bearing upper zone from 0 30 to 0 15 and from 0 40 to 0 25 in the western and northern limbs respectively and correlates positively with an the partition coefficient of total iron between plagioclase and magma increases with oxidation and polymerisation in the liquid although bushveld formed under slightly more oxidizing conditions than skaergaard differences in the partition coefficients cannot explain the two observed trends we therefore conclude that the differentiation trends of the liquids subsequent to magnetite saturation were fundamentally different the inferred liquid composition for bushveld contained about 15 wt total feo at the level of magnetite in which is slightly less than the total feo content of the subsequent cumulates in contrast the skaergaard liquid contained more total feo than the ensuing cumulates as a result in bushveld residual liquids total feo decreased after magnetite saturation whereas in skaergaard the residual liquids continued to become enriched in iron this conclusion is corroborated by simple mass balance calculations between modelled residual liquids and extracted cumulate rocks despite the mineralogical similarities of evolved iron rich rocks of skaergaard and bushveld their liquid evolution trends were very different and generalizations about the extent of iron enrichment in tholeiitic magmas should be avoided

on the skaergaard intrusion and forward modeling of its liquid line of descent a reply to principles of applied experimental igneous petrology by morse 2008 lithos 105 pp 395 399 abstract forward modeling based on an experimental investigation successfully duplicated main features of the gabbros in the skaergaard layered series thy p lesher c e nielsen t f d and brooks c k 2006 experimental constraints on the skaergaard liquid line of descent lithos 92 154 180 the foundation for the modeling was equilibrium melting experiments that were controlled by temperature and oxygen fugacity at low pressure conditions the experimental techniques and methods were chosen to represent a reasonable approximation to the inferred emplacement and crystallization conditions of the skaergaard intrusion the dike rocks used as starting materials define a strong differentiation trend that represents the skaergaard liquid line of descent this suite of dikes allowed liquidus conditions to be defined for a range of composition and temperature the initial redox conditions were chosen based on measured and calculated estimates for the gabbros of interest the melting experiments defined liquidus and subliquidus conditions that were used to understand crystallization of the lower and middle zones of the skaergaard layered series and can be extrapolated to the upper zone gabbros assuming perfect fractional crystallization the forward modeling reproduces the cryptic variation seen in the main gabbro minerals olivine augite plagioclase well into the upper zone and provides reasonable liquidus temperatures and compositions it can be shown that based on the assumption of fe ti oxide modes in the middle and upper zones a range of oxygen fugacity trends can be obtained we repeat our previous conclusion that iron depletion and strong reduction in oxygen fugacity in the upper zone are only feasible for very high fe ti oxide modes that exceed the experimental evidence as well as the observations from the gabbros a strong drop in oxygen fugacity in the upper zone requires a significant sink for fe ti oxides that so far has not been identified we thus reject morse s morse s a 2008 principles of applied experimental igneous petrology a comment on experimental constraints on the skaergaard liquid line of descent by thy lesher nielsen and brooks 2006 lithos 92 154 180 lithos 105 pp 395 399 contention that we violated in our original study established principles of applied experimental igneous petrology such principles dictate that experimental and forward models are carefully tested against field observations before petrologic processes can be verified

circulation and teleconnection mechanisms of northeast brazil droughts abstract the northern nordeste of brazil has its short rainy season narrowly concentrated around march april when the interhemispheric southward gradient of sea surface temperature sst is weakest and the intertropical convergence zone itcz which is the main rainbearing system for the nordeste reaches its southernmost position in the course of the year the recurrent secas droughts have a severe socio economic impact in this semi arid region in drought years the pre season october january rainfall is scarce the interhemispheric sst gradient weakened and the basin wide southerly northerly wind component enhanced reduced all manifestations of an anomalously far northward itcz position apart from this ensemble of atlantic indicators the secas also tend to be preceded by anomalously warm equatorial pacific waters in january during el nino years an upper tropospheric wave train extends from the equatorial eastern pacific to the northern tropical atlantic affecting the patterns of upper tropospheric topography and divergence and hence of vertical motion over the atlantic the altered vertical motion leads to a weaker meridional pressure gradient on the equatorward flank of the north atlantic subtropical high and thus weaker north atlantic tradewinds the concomitant reduction of evaporation and wind stirring allows for warmer surface waters in the tropical north atlantic and thus steeper interhemispheric meridional thermal gradient consequently the itcz stays anomalously far north and the nordeste rainy season becomes deficient

eight centuries of north atlantic ocean atmosphere variability climate in the tropical north atlantic is controlled largely by variations in the strength of the trade winds the position of the intertropical convergence zone and sea surface temperatures a high resolution study of caribbean sediments provides a subdecadally resolved record of tropical upwelling and trade wind variability spanning the past 825 years these results confirm the importance of a decadal 12 to 13 year mode of atlantic variability believed to be driven by coupled tropical ocean atmosphere dynamics although a well defined interdecadal mode of variability does not appear to be characteristic of the tropical atlantic there is evidence that century scale variability is substantial the tropical atlantic may also have been involved in a major shift in northern hemisphere climate variability that took place about 700 years ago

the multidecadal variability of the asymmetric mode of the boreal autumn hadley circulation and its link to the atlantic multidecadal oscillation abstractprevious studies show that the first principal mode of the variability of the seasonal mean hadley circulation hc is an equatorial asymmetric mode am with long term trend this study demonstrates that the variability of the boreal autumn september november son hc is also dominated by an am but with multidecadal variability the son am has ascending and descending branches located at approximately 20 n and 20 s respectively and explains about 40 of the total variance further analysis reveals that the am is closely linked to the atlantic multidecadal oscillation amo which is associated with a large cross equatorial sea surface temperature sst gradient and sea level pressure slp gradient the cross equatorial thermal contrast further induces an equatorial asymmetric hc anomaly numerical simulations conducted on an atmospheric general circulation model also suggest that amo associated sst anomalies can also induce a cross equatorial slp gradient and anomalous vertical shear of the

tropical atlantic surface current variability from 10 years of topex poseidon altimetry 1 10 years of surface geostrophic currents from topex poseidon altimetric data are used to describe the low frequency variability of the tropical atlantic circulation through empirical orthogonal function analysis the seasonal variability clearly agrees with previous studies based on climatological data it shows the tropical atlantic response to seasonal fluctuations of the overlying wind system more interesting is the capability using altimetry to reach for the first time on a basin scale the year to year variability from measurements abnormal events occur in 1996 1997 and in 2001 with different spatial scales regarding both large scale zonal distribution and regional variability located in the north western basin a first attempt to link these events to climatic indexes el nino southern oscillation north atlantic oscillation is also evocated

the skaergaard liquid line of descent revisited there is a fundamental conflict between the suggestion that the iron content of skaergaard liquids increases during fe ti oxide fractionation and the observation that at the same time oxygen fugacity f text o text 2 drops by two log units below the fayalite magnetite quartz oxygen buffer fmq a new petrographic study of average skaergaard gabbros shows that the total modal content of fe ti oxides is about 22 in the early lzc and markedly decreases to below 5 in the uzc forward modeling based on these modal constraints as well as experimental results on skaergaard related dikes predicts that fractionation of troctolitic lza gabbros drives the derivative liquid towards a high iron content strong iron enrichment continues together with a small decline in silica during lzb crystallization due to the appearance of augite as a fractionating phase the fractionation of fe ti oxides in the lzc initially suppresses iron enrichment and reverses the silica trend to one of slight enrichment however continued evolution into the uz produces liquids with maximum uzc feo content of 23 25 wt and sio2 content of 53 wt feo is total iron as feo the maximum in feo is dependent on several factors of which the fe ti oxide mode has the strongest effect the f text o text 2 during crystallization of the lzc is widely thought to have been at or slightly below the fayalite magnetite quartz oxygen buffer fmq under closed system evolution incorporation of ferric iron into augite during formation of the lzb restricts the increase in f text o text 2 to about 0 1 log units above fmq 0 1 fmq likewise crystallization of the lzc through the uza involving fe ti oxide minerals leads to a decline in f text o text 2 of less than 0 1 fmq crystallization of the uzb c gabbros results in oxidation to a maximum of 0 5 fmq this behavior can account for the iron rich character of the uz gabbros as well as the low modal content of fe ti oxides thus evolved skaergaard liquids are high in iron and contain a modest amount of sio2 our modeling result do not account for a strong drop in f text o text 2 through the layered series such a drop would require an unacceptably high proportion of fe ti oxides and high magnetite content in the fractionating assemblage

relationship between the equatorial and meridional modes of climatic variability in the tropical atlantic the tropical atlantic ocean exhibits two primary modes of interannual climate variability an equatorial mode analogous to but weaker than the pacific el nino phenomenon and a meridional mode that does not have a pacific counterpart the equatorial mode is responsible for warm and cold sea surface temperature sst events mainly in the gulf of guinea and is identifiable by abnormal changes in the equatorial thermocline slope resulting from zonal wind anomalies in the western tropical atlantic the meridional mode is characterized by a north south interhemispheric gradient of sst anomalies here it is shown using observed surface and subsurface oceanic temperatures that the meridional mode is linked to the equatorial mode at both decadal and short interannual 1 2 years time scales both modes involve north south displacements of the itcz as in the annual response

climatic aspects and vertical structure circulation associated with the severe drought in northeast brazil 2012 2016 this study provides an updated dynamical analysis of the observed climate conditions during the longest most severe and continuous drought 2012 2016 ever recorded in northeast brazil neb this investigation was carried out based on temperature precipitation sea surface temperature outgoing long wave radiation and era interim reanalysis data from 1981 to 2016 this was done by first looking for the area of the neb where the drought was most severe and then calculating the main rainy season anomalies and atmospheric parameters from 2012 to 2016 in relation to the climatological conditions 1981 2010 results show this drought event was influenced mainly by two distinct oceanic conditions that induced anomalous atmospheric circulation patterns the observed negative precipitation anomalies were firstly related to the northward displacement of the intertropical convergence zone itcz followed by an anomalous upward motion over the western amazon as well as the anomalous precipitation deficit along the south atlantic convergence zone that was slightly displaced southward of the climatological position from 2015 to 2016 the subsidence over neb brazil was linked to the descending branch of the walker circulation as well as to the strong anomalous flow induced by convection in the itcz over africa and gulf of mexico

north northeast brazil rainfall and its decadal scale relationship to wind stress and sea surface temperature in this study low frequency variability of rainfall data for the north part of northeast nne brazil is examined the time scale dependence of nne brazil rainfall and the associated sea surface temperature sst field is also described area averaged monthly rainfall data for the nne brazil from 1911 to 1989 exhibits not only a distinct interannual behavior but also significant decadal variability examination of a 30 year data set of tropical atlantic sst and wind stress show similar behavior it was found that the relationships between sst nne brazil rainfall and meridional pseudo wind stress on the interdecadal time scale are very similar to those on the interannual and seasonal time scale in both time scales sst in the tropical atlantic are in phase with the meridional displacement of the intertropical convergence zone and associated meridional pseudo wind stress and out of phase with rainfall

a further study of the tropical western hemisphere warm pool abstract variability of the tropical western hemisphere warm pool whwp of water warmer than 28 5 c which extends seasonally over parts of the eastern north pacific the gulf of mexico the caribbean and the western tropical north atlantic tna was previously studied by wang and enfield using the da silva data from 1945 93 using additional datasets of the ncep ncar reanalysis field and the ncep sst from 1950 99 and the levitus climatological subsurface temperature the present paper confirms and extends the previous study of wang and enfield the whwp alternates with northern south america as the seasonal heating source for the walker and hadley circulations in the western hemisphere during the boreal winter a strong hadley cell emanates northward from the amazon heat source with subsidence over the subtropical north atlantic north of 20 n sustaining a strong north atlantic anticyclone and associated northeast ne trade winds over its southern limb in the tna this circulation including the ne

relationship between zonal and meridional modes in the tropical atlantic the tropical atlantic displays two main modes of variability the meridional or dipole mode and the equatorial zonal mode it was proposed recently that these two modes are correlated and are both forced within the tropics by latitudinal displacements of the itcz this modeling study shows that while the two modes are correlated for limited record lengths prior to and after 1976 the correlation falls apart when longer time series from 1949 to 2000 are considered the 1976 climate shift also occurred in the tropical atlantic seen as a thermocline shift similar to the pacific forced dynamically within the tropics the first eof of the simulated thermocline depth captures the inter decadal mode with the 1976 shift the first eof of sst anomalies prior to after 1976 represents the meridional zonal mode consistent with the previous finding that the relation between the eastern pacific and atlantic itcz is stronger in the 1980 90 s

how ubiquitous is the dipole relationship in tropical atlantic sea surface temperatures several kinds of analysis are applied to sea surface temperature anomalies ssta 1856 1991 to determine the degree to which ssta of opposite sign in the tropical north and south atlantic occur antisymmetric dipole configurations of ssta on basin scales are not ubiquitous in the tropical atlantic unless the data are stratified by both season and frequency inherent dipole behavior cannot be demonstrated upon removing the global el nino southern oscillation signal in ssta which is symmetric between the north and south atlantic from the data the regions north or south of the intertropical convergence zone have qualitatively different temporal variabilities and are poorly correlated dipole configurations do occur infrequently 12 15 of the time but no more so than expected by chance for stochastically independent variables nondipole configurations that imply significant meridional ssta gradients occur much more frequently nearly half of the time cross spectral analysis of seasonally averaged ssta indices for the north and south atlantic show marginally significant coherence with antisymmetric phase in two period bands 8 12 years for the boreal winter spring and 2 3 years for the boreal summer fall antisymmetric coherence is optimal for a small subregion west of angola in the south atlantic with respect to ssta of basin scale in the tropical north atlantic dipole variability even where optimal explains only a small fraction of the total variance in tropical atlantic ssta 7

flow induced transport and enrichment of erosional sediment from a well aggregated and uniformly textured oxisol sediment enrichment and transport from a highly weathered well aggregated and uniformly textured soil were determined using field rainfall simulation overland flow on an agriculturally managed ridge and furrow system was generated by applying four successive rainfall events a dry run on bare soil with a rainfall intensity of 35 mm h 1 a wet run on bare soil with the same rainfall intensity a very wet run on bare soil with a rainfall intensity of 62 mm h 1 and a final run on partially plastic mulched soil to simulate pineapple culture with a rainfall intensity of 50 mm h 1 stream power and shear stress of overland flow during the dry and wet runs did not exceed the critical level for rill flow detachment and resulted in runoff sediment concentrations between 1 7 3 5 g l 1 with 70 90 of sediment consisting of fine aggregates wet run 1 1 for oc and 1 2 for ext p very wet run 1 03 for oc and 1 1 for ext p plastic mulch run close to 1 for both oc and ext p this enrichment was associated with selective transport of fine aggregates which were chemically richer than the coarser ones

prediction of tropical atlantic sea surface temperatures using linear inverse modeling the predictability of tropical atlantic sea surface temperature on seasonal to interannual timescales by linear inverse modeling is quantified the authors find that predictability of caribbean sea and north tropical atlantic sea surface temperature anomalies sstas is enhanced when one uses global tropical sstas as predictors compared with using only tropical atlantic predictors this predictability advantage does not carry over into the equatorial and south tropical atlantic indeed persistence is a competitive predictor in those regions to help resolve the issue of whether or not the dipole structure found by applying empirical orthogonal function analysis to tropical atlantic ssts is an artifact of the technique or a physically real structure the authors combine empirically derived normal modes and their adjoints to form influence functions maps highlighting the geographical areas to which the north tropical atlantic and the south tropical atlantic ssts are most sensitive at specified lead times when the analysis is confined to the atlantic basin the 6 month influence functions in the north and south tropical atlantic tend to be of the opposite sign and evolve into clear dipoles within 6 months when the analysis is performed on global tropical ssts the 6 month influence functions are connected

moist teleconnection mechanisms for the tropical south american and atlantic sector abstract teleconnections have traditionally been studied for the case of dry dynamical response to a given diabatic heat source important anomalies often occur within convective zones for instance in the observed remote response to el nino the reduction of rainfall and teleconnection propagation in deep convective regions poses theoretical challenges because feedbacks involving convective heating and cloud radiative effects come into play land surface feedbacks including variations of land surface temperature and ocean surface layer temperature response must be taken into account during el nino descent and negative precipitation anomalies often extend across equatorial south america and the atlantic intertropical convergence zone analysis of simulated mechanisms in a case study of the 1997 98 el nino is used to illustrate the general principals of teleconnections occurring in deep convective zones contrasting land and ocean regions comparison to other simulated events shows similar behavior t

a central tropical pacific coral demonstrates pacific indian and atlantic decadal climate connections while instrumental and proxy based climate records describe significant decadal scale climate variabil ity throughout the tropical pacific indian and atlantic oceans the processes responsible for these variations and their interactionsare not readily apparent from the obs er vations a new 112 yr coral based sea surface temperature sst reconstruction from palmyra island in the central tropical pacific ctp exhibitss trong decadal variability with an amplitude of roughly 0 3 c a 12 13yr period sig nal in thiscoral record ishighly coherent with long equa torial atlantic and indian ocean climate records implying a unified phenomenon the atlantic pattern suggests that it may fall under direct influence of anomaloussst in the ctp as it does over interannual timescales while the in dian ocean pattern exhibits maximum response during the switch between warm cold states in the tropical pacific the results demonstrate that the ctp has played a significant role in determining the expression of global decadal climate variability over the twentieth century

a review of late quaternary inland dune systems of south america east of the andes abstract inland dune systems of south america occur in a variety of environmental settings under different climatic conditions from humid subhumid to semiarid arid this contribution provides the state of the art on dune research of inland systems east of the andes tropical northern 10 n 10 s including the dune fields of llanos del orinoco and the amazonian basin are mainly formed from parabolic blowout or deflation basins and linear dunes developed on river floodplains aeolian activity was reported to occur near the late pleistocene holocene boundary in llanos del orinoco over the last 17 ka in the branco river basin and 32 6 ka in the negro river basin both in the amazonian region in general the attributed aeolian origin is controversial the morphology difficult to determine and the chronology still extremely limited and rather debatable in some cases the southern tropical region 10 s 22 s includes aeolian systems situated across the extensive lowlands of the gran chaco and those located in fluvial basins related to the cratonic brazilian shield sao francisco river pantanal and mato grosso the oldest barchanoid barchan ridges at the gran chaco were dated 36 33 ka and subsequent aeolian sand accumulation occurred at 18 ka 14 12 ka 10 9 ka and mid to late holocene times parabolic dunes at the sao francisco river floodplain show an oldest episode generated between 28 and 11 ka followed by eolian activity during the early to mid holocene no chronological data is available for the linear and lunette dunes described at the pantanal linear ridges were distinguished in the upper parana river mato grosso with associated sand accumulation at different intervals of the holocene the last reviewed region is the andean piedmont and western pampas at subtropical eastern south america comprising a large variety of dune morphologies barchan barchanoid linear parabolic transverse and lunette dunes deflation basins and sand sheets luminescence ages suggest the occurrence of aeolian activity intervals during late glacial times and the holocene the general understanding on the dynamic of south american dune systems is hampered by the paucity of chronological information and the representativeness of some dune records whether they reflect local or regional environmental conditions is still a matter of discussion in several areas that need further studies

influence of surface processes over africa on the atlantic marine itcz and south american precipitation abstract previous studies show that the climatological precipitation over south america particularly the nordeste region is influenced by the presence of the african continent here the influence of african topography and surface wetness on the atlantic marine itcz ami and south american precipitation are investigated cross equatorial flow over the atlantic ocean introduced by north south asymmetry in surface conditions over africa shifts the ami in the direction of the flow african topography for example introduces an anomalous high over the southern atlantic ocean and a low to the north this results in a northward migration of the ami and dry conditions over the nordeste region the implications of this process on variability are then studied by analyzing the response of the ami to soil moisture anomalies over tropical africa northerly flow induced by equatorially asymmetric perturbations in soil moisture over northern tropical africa shifts the ami southward increasing the climatological pre

a mid twentieth century reduction in tropical upwelling inferred from coralline trace element proxies abstract the cariaco basin is an important archive of past climate variability given its response to inter and extratropical climate forcing and the accumulation of annually laminated sediments within an anoxic water column this study presents high resolution surface coral trace element records montastrea annularis and siderastrea siderea from isla tortuga venezuela located within the upwelling center of this region a two fold reduction in cd ca ratios 3 5 1 7 nmol mol is observed from 1946 to 1952 with no concurrent shift in ba ca ratios this reduction agrees with the hydrographic distribution of dissolved cadmium and barium and their expected response to upwelling significant anthropogenic variability is also observed from pb ca analysis observing three lead maxima since 1920 kinetic control of trace element ratios is inferred from an interspecies comparison of cd ca and ba ca ratios consistent with the sr ca kinetic artifact but these artifacts are smaller than the environmental signal and do not explain the cd ca transition the trace element records agree with historical climate data and differ from sedimentary faunal abundance records suggesting a linear response to north atlantic extratropical forcing cannot account for the observed historical variability in this region

variability of the atlantic meridional mode during the atlantic hurricane season abstract an observational and modeling study is conducted to investigate the structure of the atlantic meridional mode amm during the atlantic hurricane season and the relationship between amm related sst anomalies and environmental conditions that influence seasonal tropical cyclone activity the observational analysis shows that during the atlantic hurricane season the amm exhibits a similar sst and low level wind structure as during boreal spring when the amm is most active observed amm sst variations are accompanied by air temperature and moisture anomalies that are limited to the boundary layer and an anomalous baroclinic circulation structure in the northern subtropical atlantic with an anomalous lower level cyclonic circulation residing under an anomalous upper level anticyclone during a warm phase this baroclinic structure contributes to a reduction in vertical wind shear over the tropical atlantic that is dominated by changes in the upper level flow two sets of model experiments were cond

adjustment of the coupled ocean atmosphere system to a sudden change in the thermohaline circulation 1 the adjustment of the coupled ocean atmosphere system to a fresh water anomaly in the north atlantic ocean is investigated using a coupled gcm in response to the anomaly the atlantic thermohaline circulation thc collapses and the associated reduction in northward heat transport causes sea surface temperatures sst to fall over the north atlantic and to increase over the tropical south atlantic atmospheric feedbacks are excited as soon as the sst anomalies reach sufficient magnitude a key stage in the adjustment process occurs 4 6 years after the perturbation is introduced when a significant sst dipole develops in the tropical atlantic this dipole causes a southward shift of the itcz and leads in year 7 to the triggering of an el nino event it is concluded that atmospheric feedbacks could spread globally the influence of a sudden change in the thc much more quickly and efficiently than could ocean processes alone

rapid 20th century warming in the caribbean and impact of remote forcing on climate in the northern tropical atlantic as recorded in a guadeloupe coral we have generated 104 year long 1895 1999 monthly 18o and sr ca time series from a fast growing diploria strigosa coral core drilled off guadeloupe island lesser antilles coral sr ca reliably records interannual to decadal surface air temperature sat variations in the region and shows a pronounced warming of approximately 1 5 c since 1950 with the strongest warming 1 2 c occurring since 1975 this warming is also evident in sat measured at guadeloupe which ends in 1951 thus our sr ca series extends the air temperature record by 56 years we find that the past few decades are the warmest years over the entire period of record the accelerated warming since 1950 is accompanied by a pronounced decrease in regional precipitation this dampens the warming signal indicated by coral 18o which is too low only 0 7 0 8 c since 1951 consistently 18osw estimated from the coral proxies also shows a strong decrease since 1950 our data suggest an inverse relationship between sat and precipitation i e warmer and drier for the latter half of the 20th century with the strongest trends since the mid 1970s this is consistent with recent observational and model data which report that while over the tropical oceans rainfall has increased due to an increase in sea surface temperatures precipitation over land regions is reduced a continuation of this warming and drying trend over caribbean land regions would have severe societal consequences especially in the context of anthropogenic warming the el nino southern oscillation enso and the north atlantic oscillation nao are the two major climate modes affecting large scale sst variability in the northern tropical atlantic both sr ca and 18o show a close relationship to enso and the nao a quantitative comparison between extremes in mean march may coral 18o and the nino3 and nao indices imply that climate variability in the northern tropical atlantic is mainly forced by tropical pacific and north atlantic variability spectral analysis suggests that the relative importance of enso and the nao is frequency dependent with enso dominating at interannual and the nao dominating at interdecadal time scales

regionalization and spatial properties of ceara state rainfall in northeast brazil the overall daily rainfall pattern of ceara for 19 years 1974 1992 was analyzed in terms of physiography and atmospheric circulation this information was used to suggest a regionalization of the state s rainfall into homogeneous areas the three large scale precipitation mechanisms intertropical convergence zone cold fronts and upper air vortices interact and create a certain rainfall pattern over the state during different months an example of this interaction is the opposite behavior that the northern and southern parts of the state tend to have when considering the amount of precipitation during the rainy season that is in general large amounts of rain in the southern part of the state tend to be associated with small amounts of rain in the northern part and vice versa as influenced by the opposite influence of the cold fronts and the upper air vortices at the same time the orographical influence enhances the precipitation in the northern part of the state and decreases precipitation in the central part of the state the state could be divided into six homogeneous precipitation regions 1 the northwestern part of the state locally influenced by the mountain massif of serra da ibiapaba 2 3 the northern and northeastern ceara influenced locally by sea breeze and the slopes of the serra de baturite 4 5 the central state influenced by the chapada do apodi and the serra de uruburetama and 6 the southern part of the state influenced mostly by cold fronts and by the slopes of the serra do araripe less

the impacts of inter el nino variability on the tropical atlantic and northeast brazil climate abstractin this study observations and numerical simulations are used to investigate how different el nino events affect the development of sst anomalies in the atlantic and how this relates to the brazilian northeast ne precipitation the results show that different types of el nino have different impacts on the sst anomalies of the equatorial and tropical south atlantic but a similar sst response in the tropical north atlantic strong and long weak and short el ninos with the main heating source located in the eastern central pacific generate cold warm anomalies in the cold tongue and benguela upwelling regions during boreal winter and spring when the sst anomalies in the eastern equatorial and tropical south atlantic are cold warm the meridional sst gradient across the equator is positive negative and the itcz is not allowed allowed to move southward during the boreal spring as a consequence the precipitation is below above the average over the ne thus strong and long weak and s

physical processes associated with the tropical atlantic sst meridional gradient abstract the major modes of seasonal sea surface temperature sst meridional gradient and their connection with some regional mean sst indices in the atlantic ocean are examined using reanalysis data the focus of the work is on the evolution of the dominant mode of the meridional sst gradient in boreal spring and the associated physical processes the spatial distribution of the dominant mode in boreal spring is a seesaw pattern reflecting the opposite variation of the meridional sst gradient between the subtropical and tropical north atlantic which resulted from a coherent warming or cooling with maxima along 10 15 n it is confirmed that this mode is dominated by the wind evaporation sst feedback the feedback persists a longer time in the western atlantic than in the eastern the contribution to the sst variation is mainly from latent heat flux the surface longwave and shortwave cloud radiative forcings are mainly determined by low cloud cover variations the authors also found that the thermodyn

two leading modes of the interannual variability in south american surface air temperature during austral winter the first two empirical orthogonal function eof modes of the surface air temperature sat interannual variability in the south american sa continent have been revealed in several previous studies this presentation focuses on winter season and furtherly investigates the detailed advection and cloud radiation processes and teleconnections from tropical sea surface temperature anomalies ssta combining statistical analysis with rossby wave dynamics and modelling experiments the eof1 featured with the anomalous center in the central part is related to the tropical eastern pacific ssta which may impact on the sa sat variability through the walker circulation and a regional hadley cell the anomalous center is largely attributed to low level advection transported by the hadley cell the eof2 as a fluctuation between anomalies in the southeast brazil and the southern tip is related to the ssta surrounding the maritime continent which may generate a barotropic wave train propagating to the sa continent this wave train can strengthen high latitude westerly flow transporting warm advection to the southern tip and generate southeast anomalous flow transporting cold advection to the southeast brazil meanwhile the cloud radiation processes are also involved to enhance the advection induced sat anomalies in both areas

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

wintertime variability in the euro atlantic region in observations and in ecmwf seasonal ensemble experiments an empirical orthogonal function eof decomposition of monthly mean analyses of 500 hpa height 1949 94 is used to describe the interannual variability of the large scale flow in the euro atlantic region during winter the first four eofs resemble low frequency variability patterns identified in previous studies such as the north atlantic oscillation and the eastern atlantic and eurasian teleconnection patterns the second eof and fourth eof eof4 are associated with the occurrence of el nino like sea surface temperature sst anomalies according to an observational analysis for eof4 correlations with sst anomalies in the west pacific are also important indications of an influence of tropical sst anomalies on the blocking like third eof emerging from the european centre for medium range weather forecasts ecmwf re analysis era data are not confirmed by a longer sst record a negligible correlation exists between the first eof and el nino indices during the era period although a link may be detected in earlier decades in the second part of the paper a 14 winter set of seasonal ensemble simulations performed with the ecmwf atmospheric model forced by observed sst is validated by comparing the projection of era data onto the four leading euro atlantic eofs with those of ensemble experiments and computing composite anomalies of analyses and model fields the performance of the model is uneven and depends on the large scale pattern considered model biases and flow dependent errors affect the simulations of some of the eofs in particular the model has strong problems reproducing the occurrence of european blocks the best correlation 69 between the time series of analysis and ensemble mean principal components is obtained for eof4 which seems to be forced by sst anomalies in the west pacific associated with el nino southern oscillation enso events the effects of this forcing are felt on the zonal wind structure over eurasia and are reasonably reproduced by the ecmwf model conversely the propagation into the atlantic region of planetary waves originating in the east pacific is poorly simulated thus degrading the model performance in predicting euro atlantic anomalies during some strong enso events

sensitivity of tropical atlantic climate to mixing in a coupled ocean atmosphere model the sensitivity of tropical atlantic climate to upper ocean mixing is investigated using an ocean only model and a coupled ocean atmosphere model the upper ocean thermal structure and associated atmospheric circulation prove to be strongly related to the strength of upper ocean mixing using the heat balance in the mixed layer it is shown that an excessively cold equatorial cold tongue can be attributed to entrainment flux at the base of the oceanic mixed layer that is too large enhanced entrainment efficiency acts to deepen the mixed layer and causes strong reduction in the upper ocean divergence in the central equatorial atlantic as a result the simulated sea surface temperature thermocline structure and upwelling velocities are close to the observed estimates in the coupled model the seasonal migration of the intertropical convergence zone itcz reduces when the entrainment efficiency in the oceanic mixed layer is enhanced the precipitation rates decrease in the equatorial region and increase along 10 n resulting in a more realistic atlantic marine itcz the reduced meridional surface temperature gradient in the eastern tropical atlantic prohibits the development of convective precipitation in the southeastern part of the tropical atlantic also the simulation of tropical atlantic variability as expressed in the meridional gradient mode and the eastern cold tongue mode improves when the entrainment efficiency is enhanced

structure of interannual to decadal climate variability in the tropical atlantic sector abstract a search for coupled modes of atmosphere ocean interaction in the tropical atlantic sector is presented previous studies have provided conflicting indications of the existence of coupled modes in this region the subject is revisited through a rotated principal component analysis performed on datasets spanning the 36 yr period 1958 93 the analysis includes four variables sea surface temperature oceanic heat content wind stress and atmospheric diabatic heating the authors find that the first rotated principal component is associated with fluctuations in the subtropical wind system and correlates with the north atlantic oscillation nao while the second and third modes which are the focus of interest are related to tropical variability the second mode is the atlantic nino mode with anomalous sea surface temperature and anomalous heat content in the eastern equatorial basin wind stress weakens to the west of anomalously warm water while convection is shifted south and eastward surface

the role of the mean state in meridional mode structure and growth abstractthis study uses a simple linear coupled model to investigate the role of the wes feedback and itcz mean states in meridional mode variability optimal structures that maximize transient growth are calculated for mean states characteristic of boreal spring and boreal fall in the tropical atlantic during boreal spring the leading optimal structure is a zonal mode that propagates westward and does not resemble the observed meridional mode in contrast the leading optimal structure during fall is a sea surface temperature sst monopole over the northern hemisphere nh that propagates equatorward and westward and that closely matches meridional mode variability during this season it is found that the boreal fall optimal growth greatly exceeds growth of the corresponding optimal during boreal spring despite the observed boreal spring peak in atlantic meridional mode variance sensitivity studies are used to explore the role of northern or southern hemisphere initial conditions itcz width and itcz

influence of el nino on the upper ocean circulation in the tropical atlantic ocean abstract this study investigates the influence of el nino on the upper ocean circulation in the tropical atlantic ocean via changes in the atlantic trade winds by analyzing observed sea surface temperature sst together with an ocean general circulation model integration forced by the ncep ncar reanalysis during periods with anomalously warm cold eastern equatorial pacific sst the southern atlantic tropical cell is strengthened weakened the difference of the cell strength between el nino and la nina years is about 20 of the mean cell strength however the variability of the cell is not dominated by the remote forcing from the eastern equatorial pacific but seems to be caused by intrinsic tropical atlantic variability a strengthening weakening for periods with anomalously warm cold eastern equatorial pacific sst is also found for the zonal surface and subsurface currents topex poseidon altimetry data are used to validate the results based on the ogcm integration

recording of el nino in ice core δ18o records from nevado huascaran peru the 68 year monthly resolved time series of 18 o from ice cores retrieved from the glaciated col of nevado huascaran peru 9 s 77 w 6050 m reflects climate variability over amazonia and the western tropical atlantic over the 25 year period 1968 1993 of midtroposphere observations the interannual variations in huascaran 18 o relate closely with the zonal wind variations over tropical south america at the 500 hpa level additionally there is some evidence that the spatial distribution of temperature anomalies in the western tropical atlantic has influence on the 500 hpa circulation and hence the isotopic fractionation of moisture advected across amazonia during el nino warming the moisture convergence axis over the atlantic ocean is commonly diverted northward leading to unusual warm and dry conditions over northeast brazil and 18 o enriched snowfall at huascaran this enrichment phase is enhanced when the peak pacific warming occurs during the first half of the calendar year coincident with the wet season over amazonia approximately 12 months later the el nino demise is affiliated with a reprisal of strong trade wind circulation and the resultant cool pluvial environment over amazonia triggers a reversal to strongly depleted isotope anomalies

evolution of the lorenz energy cycle in the intertropical convergence zone in the south american sector of the atlantic ocean abstractthe intertropical convergence zone itcz in the south american sector of the atlantic ocean is identified using outgoing longwave radiation olr data in order to investigate the evolution of the lorenz energy cycle in the region dominated by this large scale feature the ncep reanalysis data are utilized to calculate the zonal and eddy components denoted by z and e respectively of kinetic energy k and available potential energy a i e kz ke az and ae and their conversions on a daily basis a wavelet decomposition of the time series is performed to detect long term cycles trends in the atlantic itcz region this work also investigates trends in sea surface temperature sst and sea level pressure slp in the itcz region and connections between the itcz and the southern oscillation index soi a strong annual cycle in all the energy components with high peaks in austral summer is observed approximately 91 of the zonal component of energy is contained on decadal or longer time scales

coupled ocean atmosphere surface variability and its climate impacts in the tropical atlantic region this study examines time evolution and statistical relationships involving the two leading ocean atmosphere coupled modes of variability in the tropical atlantic and some climate anomalies over the tropical 120 w 60 w region using selected historical files 75 y near global ssts and precipitation over land more recent observed data 30 y sst and pseudo wind stress in the tropical atlantic and reanalyses from the us national centers for environmental prediction ncep ncar reanalysis system on the period 1968 1997 surface air temperature sea level pressure moist static energy content at 850 hpa precipitable water and precipitation the first coupled mode detected through singular value decomposition of the sst and pseudo wind stress data over the tropical atlantic 30 n 20 s expresses a modulation in the thermal transequatorial gradient of sst anomalies conducted by one month leading wind stress anomalies mainly in the tropical north atlantic during northern winter and fall it features a slight dipole structure in the meridional plane its time variability is dominated by a quasi decadal signal well observed in the last 20 30 ys and when projected over longer term sst data in the 1920s and 1930s but with shorter periods the second coupled mode is more confined to the south equatorial tropical atlantic in the northern summer and explains considerably less wind stress sst cross covariance its time series features an interannual variability dominated by shorter frequencies with increased variance in the 1960s and 1970s before 1977 correlations between these modes and the enso like nino3 index lead to decreasing amplitude of thermal anomalies in the tropical atlantic during warm episodes in the pacific this could explain the nonstationarity of meridional anomaly gradients on seasonal and interannual time scales overall the relationships between the oceanic component of the coupled modes and the climate anomaly patterns denote thermodynamical processes at the ocean atmosphere interface that create anomaly gradients in the meridional plane in a way which tends to alter the north south movement of the seasonal cycle this appears to be consistent with the intrinsic non dipole character of the tropical atlantic surface variability at the interannual time step and over the recent period but produces abnormal amplitude and or delayed excursions of the intertropical convergence zone itcz connections with continental rainfall are approached through three ncep ncar and observed rainfall indexes over the nordeste region in brazil and the guinea and sahel zones in west africa these indices appear to be significantly linked to the sst component of the coupled modes only when the two atlantic modes the enso like nino3 index are taken into account in the regressions this suggests that thermal forcing of continental rainfall is particularly sensitive to the linear combinations of some basic sst patterns in particular to those that create meridional thermal gradients the first mode in the atlantic is associated with transequatorial pressure moist static energy and precipitable water anomaly patterns which can explain abnormal location of the itcz particularly in northern winter and hence rainfall variations in nordeste the second mode is more associated with in phase variations of the same variables near the southern edge of the itcz particularly in the gulf of guinea during the northern spring and winter it is primarily linked to the amplitude and annual phase of the itcz excursions and thus to rainfall variations in guinea connections with sahel rainfall are less clear due to the difficulty for the model to correctly capture interannual variability over that region but the second atlantic mode and the enso like pacific variability are clearly involved in the sahel climate interannual fluctuations anomalous dry wet situations tend to occur when warmer cooler waters are present in the eastern pacific and the gulf of guinea in northern summer which contribute to create a northward southward transequatorial anomaly gradient in sea level pressure over west africa

a review of north atlantic modes of natural variability and their driving mechanisms 1 this paper reviews three modes of natural variability that have been identified in the north atlantic ocean namely the north atlantic oscillation nao the atlantic multidecadal oscillation amo and the atlantic meridional mode amm this manuscript focuses on the multidecadal fluctuations of these three modes a range of different mechanisms to initiate phase reversals in these modes on multidecadal timescales has been suggested previously we propose a systematic grouping of these mechanisms into three types that involve respectively 1 the dependency of the atlantic thermohaline circulation thc on salinity 2 the sensitivity of the thc to changes in ocean heat transport and 3 the dependency of the nao to changes in the atlantic meridional temperature gradient some new density data is also provided demonstrating physical links between the thc and the amo

on the dynamics of droughts in northeast brazil observations theory and numerical experiments with a general circulation model abstract it is proposed that a possible mechanism for the occurrence of severe droughts over northeast brazil is the establishment of a thermally direct local circulation which has its ascending branch at about 10 n and its descending branch over northeast brazil and the adjoining oceanic region the driving for this anomalous circulation is provided by warming due to enhanced moist convection associated with warmer sea surface temperature anomalies over the northern tropical atlantic and cooling associated with colder sea surface temperature anomalies in the southern tropical atlantic the combined effects of thermally forced subsidence and reduced evaporation and moisture flux convergence produces severe drought conditions over northeast brazil we have examined the monthly mean sea surface temperature anomalies over the tropical atlantic and rainfall anomalies over two selected stations fortaleza 3 46 s 38 31 w and quixeramobim 5 12 s 39 18 w for 25 years 1948 72 it is found that the most sever

extratropical forcing of tropical atlantic variability during boreal summer and fall abstractthe connection between midlatitude atlantic sea surface temperature sst anomalies and tropical sst variations during boreal summer and fall are investigated using a coupled general circulation model gcm this research follows on an observational study that finds that using linear inverse modeling lim predictions of boreal summer tropical atlantic meridional mode amm variations can be made with skill exceeding persistence with lead times of about one year the lim framework identified extratropical atlantic sst anomalies as important precursors to the amm variations the authors have corroborated this finding using a general circulation model coupled to a slab ocean which represents a completely different physical basis from the lim initializing the gcm with the lim derived optimal sst anomaly in november results in a steady equatorward propagation of sst anomalies into the subtropics during the following boreal spring thereafter the gcm suggests that two possible feedbacks propagate

dominant modes of variability in the south atlantic a study with a hierarchy of ocean atmosphere models using an atmosphere model of intermediate complexity and a hierarchy of ocean models the dominant modes of interannual and decadal variability in the south atlantic ocean are studied the atmosphere simplified parameterizations primitive equation dynamics speedy model has t30l7 resolution the physical package consists of a set of simplified physical parameterization schemes based on the same principles adopted in the schemes of state of the art agcms it is at least an order of magnitude faster whereas the quality of the simulated climate compares well with those models the hierarchy of ocean models consists of simple mixed layer models with an increasing number of physical processes involved such as ekman transport wind induced mixing and wind driven barotropic transport finally the atmosphere model is coupled to a regional version of the miami isopycnal coordinate ocean model micom covering the south atlantic with a horizontal resolution of 1 and 16 vertical layers the coupled modes of mean sea level pressure and sea surface temperature simulated by speedy micom strongly resemble the modes as analyzed from the ncep ncar reanalysis indicating that this model configuration possesses the required physical mechanisms for generating these modes of variability using the ocean model hierarchy the authors were able to show that turbulent heat fluxes ekman transport and wind induced mixing contribute to the generation of the dominant modes of coupled sst variability the different roles of these terms in generating these modes are analyzed variations in the wind driven barotropic transport mainly seem to affect the sst variability in the brazil malvinas confluence zone the spectra of the mixed layer models appeared to be too red in comparison with the fully coupled speedy micom model due to the too strong coupling between sst and surface air temperatures sats resulting from the inability to advect and subduct sst anomalies by the mixed layer models in speedy micom anomalies in the southeastern corner of the south atlantic are subducted and advected toward the north brazilian coast on a time scale of about 6 yr

thermodynamic analysis of fe and mg partitioning between plagioclase and silicate liquid thermodynamic analysis of fe and mg bearing plagioclase and silicate liquid was carried out based on reported element partitioning data between plagioclase and silicate liquid in reduced conditions solution properties of ternary feldspar standard state properties of plagioclase endmembers and solution properties of multicomponent silicate liquid derived mixing properties of fe and mg bearing plagioclase are in harmony with estimated results from synthetic experiments in the systems caal2si2o8 cafesi3o8 and caal2si2o8 camgsi3o8 based on the determined solution properties of the plagioclase a computer program to calculate the element partition relationships between fe and mg bearing plagioclase and multicomponent silicate liquid was developed the feo mgo and mgo mgo feo in plagioclase predicted from known liquid compositions and pressure are in agreement with measurements within 0 2 wt 0 1 wt and 0 1 mol ratio respectively the fe3 content in plagioclase crystallized at high oxygen fugacity can be estimated with this program the fe3 total fe ratio in plagioclase crystallized near the quartz fayalite magnetite buffer ranges from 0 to 0 5 which is consistent with previous study on natural plagioclase in submarine basalt derived solution properties of the fe and mg bearing plagioclase are also used to calculate equilibrium composition relationship between olivine and plagioclase change of xfo in olivine coexisting with plagioclase affects mgo and feo contents in plagioclase greatly the present model predicts xfo of coexisting olivine from the chemical composition of plagioclase to 0 1 accuracy at given pressure and temperature

atlantic climate variability and its associated atmospheric circulation cells abstract phenomena important for atlantic climate variability include the atlantic zonal equatorial mode the tropical atlantic meridional gradient mode and the north atlantic oscillation nao these climate phenomena and their associated atmospheric circulation cells are described and discussed using the ncep ncar reanalysis field and the ncep sea surface temperature sst from january 1950 to december 1999 atmospheric divergent wind and vertical motion are used for the identification of atmospheric circulation cells during the peak phase of the atlantic equatorial mode the atlantic walker circulation weakens and extends eastward which results in surface westerly wind anomalies in the equatorial western atlantic these westerly wind anomalies are partly responsible for warming in the equatorial eastern atlantic that occurs in the second half of the year the atlantic equatorial mode involves a positive ocean atmosphere feedback associated with the atlantic walker circulation similar to the pacific

multimodel study of tropical atlantic variability and change 1 interannual variability associated with the zonal and the meridional mode in the tropical atlantic is studied in nine coupled ocean atmosphere models for twentieth century climate conditions tc and the sres a1b scenario for future greenhouse gas concentrations for tc the subtropical part of the meridional mode is reasonably well simulated in contrast to the deep tropical part of the meridional mode and the zonal mode a common model bias is that the onset of the meridional mode is preceded by the presence of a zonal mode in boreal fall that extends towards the western boundary of the atlantic basin and which initiates a wind evaporation sst feedback as a result of this there is a spuriously strong interaction between the zonal and the meridional mode the models that seem to best represent the meridional mode show a weakening for future climate conditions biases in the zonal mode are too strong to assess changes citation breugem w p w hazeleger and r j haarsma 2006 multimodel study of tropical atlantic variability and change geophys res lett 33 l23706 doi 10 1029 2006gl027831

the differentiation trend of the skaergaard intrusion and the timing of magnetite crystallization iron enrichment revisited abstract initial studies of the skaergaard intrusion l r wager j petrol 1 1960 364 398 and much of the subsequent work r j williams am j sci 271 1971 132 146 s a morse et al am j sci 280a 1980 159 170 a r mcbirney h r naslund contrib mineral petrol 104 1990 235 247 c tegner contrib mineral petrol 128 1997 45 51 a r mcbirney contrib mineral petrol 132 1998 103 105 concluded that the skaergaard magma followed an iron enrichment trend with little or no silica enrichment until the final stages of crystallization several recent reports r h hunter r s j sparks contrib mineral petrol 95 1987 451 461 r h hunter r s j sparks contrib mineral petrol 104 1990 248 254 however have suggested that the skaergaard magma began to follow a silica enrichment trend in lower zone c lzc of the layered series where magnetite first became an abundant mineral magnetite in lzc however generally occurs in aggregates of magnetite ulvospinel and ilmenite hematite that have undergone extensive subsolidus reequilibration and exsolution e a vincent neues jahrb mineral abh 94 1960 993 1016 e a vincent geochim cosmochim acta 6 1954 1 26 a f buddington d h lindsley j petrol 5 1964 310c357 h r naslund j petrol 25 1984 185 212 a r mcbirney j petrol 30 1989 363 397 y d jang petrological geochemical and mineralogical variations in the skaergaard intrusion east greenland ph d dissertation state university of new york binghamton ny 1999 219 pp as a result it is not clear if magnetite in these samples was an equilibrium liquidus mineral fractionated from the main magma reservoir or if magnetite crystallized as a later interstitial mineral and did not directly affect the differentiation trend of the main skaergaard magma the timing of the initial crystallization of abundant magnetite and ilmenite is a key factor in understanding the trend of skaergaard differentiation because v is a strongly included element in oxides and is not strongly included in silicate minerals the v content of an evolving magma is generally controlled by the fractionation of oxide minerals in particular magnetite the initial crystallization of magnetite should therefore be accompanied by a sudden decrease in the v content of the evolving magma and in all of the coexisting mafic phases in equilibrium with that magma as well the v content in skaergaard pyroxene does not decrease significantly until the upper part of the middle zone mz suggesting that the onset of extensive magnetite fractionation is much later than has previously been thought and that the magnetite in lzc and the lower part of the mz might not have been a liquidus phase at that level the observed v trend in skaergaard pyroxene can be modeled almost perfectly using published partition coefficients for the coexisting minerals in the skaergaard intrusion assuming that no magnetite fractionation occurred until the upper part of the mz independently calculated trends for f o 2 in the skaergaard magma r j williams am j sci 271 1971 132 146 s a morse et al am j sci 280a 1980 159 170 a r mcbirney h r naslund contrib mineral petrol 104 1990 235 247 change in the upper part of the mz to more reducing conditions the onset of magnetite fractionation would remove fe 2 o 3 from the magma and could initiate such a change the timing of magnetite fractionation will have a strong effect on whether magma evolves towards iron enrichment or silica enrichment

some overlooked features of tropical atlantic climate leading to a new nino like phenomenon abstract the atlantic nino an equatorial zonal mode akin to the pacific el nino southern oscillation enso is phase locked to boreal summer when the equatorial easterly winds intensify and the thermocline shoals in the gulf of guinea a suite of satellite and in situ observations reveals a new mode of tropical atlantic variability that displays many characteristics of the zonal mode but instead peaks in november december nd this new mode is found to be statistically independent from both the atlantic nino in the preceding summer and the pacific enso the origin of this nd zonal mode lies in an overlooked aspect of the seasonal cycle in the equatorial atlantic in november the equatorial easterly winds intensify for the second time increasing upwelling and lifting the thermocline in the gulf of guinea an analysis of high resolution climatological data shows that these dynamical changes induce a noticeable sst cooling in the central equatorial atlantic the shoaling thermocline and increased upwelli

tropical atlantic moisture flux convection over northeastern brazil and pertinence of the pirata network abstract this study aims to examine the relationship between the tropical atlantic latent heat flux and convective cloud coverage over northeast brazil neb during the four months of the main rainy season february may the correlation with anomalies of these data is investigated both without lag and with a 1 month lag the heat flux in advance in both cases a significant positive correlation appears in the northwestern tropical atlantic and a significant negative correlation is obtained for a limited area off eastern neb these two correlation patterns are linked to anomalies in the trade wind intensity and in the meridional position of the intertropical convergence zone itcz which relate to the latent heat flux anomalies and neb convective coverage anomalies respectively the positive correlation pattern is spread over a large part of the northern tropical atlantic whereas the negative correlation pattern is confined off neb this indicates the existence of different regional mechanisms in t

influence of the atlantic zonal mode on monsoon depressions in the bay of bengal during boreal summer the influence of the atlantic zonal mode azm or the atlantic nia o on monsoon depressions in the bay of bengal during the boreal summer june august is studied our analysis shows that there is a statistically significant difference in the number of monsoon depressions in the bay of bengal between the warm and cold phases of the azm more fewer monsoon depressions form during the cold warm phase of azm it also shows that there are differences in spatial pattern of trajectories of monsoon depressions during the cold phase of azm the tracks are relatively long and seem to cluster along the axis of core monsoon region compared to the warm phase of azm the analysis indicates an increase a reduction in low level cyclonic vorticity and midtropospheric humidity but a reduction an increase in vertical wind shear due to anomalous circulation pattern all of these changes are favorable for the enhancement suppression of monsoon depressions during the cold warm phase of the azm our analysis further shows a teleconnection pathway by which the azm can influence the remote indian ocean this could have implications for enhancing monsoon prediction skill especially during non el nia o southern oscillation years key points influence of the azm on monsoon depressions during boreal summer cold warm azm increases decreases number of monsoon depressions the difference in large scale atmospheric conditions in the bob during azm

the influence of amazon rainfall on the atlantic itcz through convectively coupled kelvin waves abstract using outgoing longwave radiation olr and tropical rainfall measuring mission trmm daily rain rate data systematic changes in intensity and location of the atlantic intertropical convergence zone itcz were detected along the equator during boreal spring it is found that the changes in convection over the tropical atlantic may be induced by deep convection in equatorial south america lagged regression analyses demonstrate that the anomalies of convection developed over the land propagate eastward across the atlantic and then into africa the eastward propagating disturbances appear to be convectively coupled kelvin waves with a period of 6 7 5 days and a phase speed of around 15 m s 1 these waves modulate the intensity and location of the convection in the tropical atlantic and result in a zonal variation of the atlantic itcz on synoptic time scales the convectively coupled kelvin wave has substantial signals in both the lower and upper troposphere both a reanalysis dataset and the qui

iron in plagioclase as a monitor of the differentiation of the skaergaard intrusion a discussion of christian tegner contrib mineral petrol 128 45 51

the dependence of caribbean rainfall on the interaction of the tropical atlantic and pacific oceans abstract seasonally stratified analyses of rainfall anomalies over the intra americas sea and surrounding land areas and of onset and end dates of the central american rainy season show that the variability of the tropical atlantic sea surface temperature anomaly ssta is more strongly associated with rainfall over the caribbean and central america than is tropical eastern pacific ssta seasonal differences include the importance of antisymmetric configurations of tropical atlantic ssta in the dry season but not in the rainy season both oceans are related to rainfall but the strength of the rainfall response appears to depend on how ssta in the tropical atlantic and eastern pacific combine the strongest response occurs when the tropical atlantic is in the configuration of a meridional dipole antisymmetric across the itcz and the eastern tropical pacific is of opposite sign to the tropical north atlantic when the tropical north atlantic and tropical pacific are of the same sign the rainfall respons

impact of atlantic sea surface temperature anomalies on the summer climate in the western north pacific during 1997 1998 1 the summers of 1997 and 1998 were characterized by large anomalous atmospheric circulations in the tropical western pacific which in turn were associated with anomalous atmospheric circulation and precipitation in the adjacent areas we use an atmospheric general circulation model to investigate the role of sea surface temperatures ssts on the anomalous atmospheric circulations and associated precipitation in the tropics in these two summers it is found that the circulation and rainfall anomalies are well reproduced when the model is forced with the observed global ssts a further experiment is performed to investigate the role of the sst anomalies in the atlantic in summer 1997 the atlantic sst anomalies compared with the sst anomalies outside the atlantic play a secondary role in influencing the circulation and rainfall anomalies in the tropical indian ocean and pacific by contrast it shows that the sst anomalies in the atlantic play a comparable role to those outside the atlantic in influencing the circulation and rainfall anomalies in the remote tropics in summer 1998 in this summer equatorial stationary waves similar to a kelvin wave and a rossby wave are clearly illustrated by the results implied by the atlantic sst anomalies the eastward equatorial stationary wave extends into the tropical western pacific and suppresses the atmospheric convection in the intertropical convergence zone in the indian ocean and pacific the evidence presented here for the role of atlantic ocean conditions on summer climates suggests that attention should be paid to accurate simulation of the atlantic ocean state as well as the states of the pacific and indian oceans in developing a seasonal forecasting system

enhancement of enso variability by a weakened atlantic thermohaline circulation in a coupled gcm abstract a coupled ocean atmosphere general circulation model is used to investigate the modulation of el nino southern oscillation enso variability due to a weakened atlantic thermohaline circulation thc the thc weakening is induced by freshwater perturbations in the north atlantic and leads to a well known sea surface temperature dipole and a southward shift of the intertropical convergence zone itcz in the tropical atlantic through atmospheric teleconnections and local coupled air sea feedbacks a meridionally asymmetric mean state change is generated in the eastern equatorial pacific corresponding to a weakened annual cycle and westerly anomalies develop over the central pacific the westerly anomalies are associated with anomalous warming of sst causing an eastward extension of the west pacific warm pool particularly in august february and enhanced precipitation these and other changes in the mean state lead in turn to an eastward shift of the zonal wind anomalies associated with el nin

circulation mechanisms related to northeast brazil rainfall anomalies this study explores the role of the upper ocean for circulation anomalies in the overlying atmosphere with focus on the late boreal winter and extreme climatic events in northeast brazil data sources comprise rainfall records in the nordeste surface ship observations of the sea surface temperature sst pressure and wind fields coads period 1948 1990 satellite observations of tropical convection hrc 1971 1988 and upper air analyses of the european centre for medium range weather forecasts ecmwf 1980 1991 all for the tropical atlantic sector interhemispheric sst gradients are most strongly associated with north south contrasts in pressure and the meridional wind component whereas the inverse sst pressure relations in situ are less close enhanced northward temperature increase in the tropical atlantic is accompanied by steeper meridional pressure gradient and accelerated southerly wind component which is representative of a northward displaced intertropical convergence zone itcz with the latter in turn leading to drought in northeast brazil interhemispheric contrasts in the 850 1000 mbar layer mean temperature are similar to those of sst and account for most of the meridional surface pressure gradient during wet as compared to dry years in northeast brazil the tropical north atlantic is cooler and the south atlantic warmer and accordingly the lower tropospheric thickness is reduced to the north but inflated to the south of the equator resulting in increased reduced surface pressure over the north south atlantic as well as reduced southerly surface wind stronger subsidence over the outer tropics of the northern hemisphere and intensified ascending motion and convective activity over the nordeste related to a southward displaced near equatorial convection belt the surface and upper air evidence thus indicates the ways in which the interhemispheric sst gradients exert a hydrostatic control on the lower tropospheric thickness pattern and thus force the south north surface pressure gradients and the surface meridional wind component and hence modulate the latitude position of the itcz and nordeste rainfall

variability of the atlantic off equatorial eastward currents during 1993 2010 using a synthetic method 1 we have developed validated and applied a synthetic method to monitor the off equatorial eastward currents in the central tropical atlantic this method combines high density expendable bathythermograph xbt temperature data along the ax08 transect with altimetric sea level anomalies slas to estimate dynamic height fields from which the mean properties of the north equatorial countercurrent necc the north equatorial undercurrent neuc and the south equatorial undercurrent seuc and their variability can be estimated on seasonal to interannual timescales on seasonal to interannual timescales the synthetic method is well suited for reconstructions of the necc variability reproduces the variability of the neuc with considerable skill and less efficiently describes variations of the seuc which is located in a region of low sla variability a positive correlation is found between interannual variations of the necc transport and two indices based on an interhemispheric sea surface temperature sst gradient and southeasterly wind stress in the central tropical atlantic the neuc is correlated on interannual timescales with ssts and meridional wind stress in the gulf of guinea and zonal equatorial wind stress this study shows that both altimetry and xbt data can be effectively combined for near real time inference of the dynamic and thermodynamic properties of the tropical atlantic current system

the role of ocean dynamics in the interaction between the atlantic meridional and equatorial modes this study examines a mechanism of the interaction between the tropical atlantic meridional and equatorial modes to derive robust heat content hc variability the ensemble mean hc anomalies hca of six state of the art global ocean reanalyses for 1979 2007 are analyzed compared with previous studies characteristic oceanic processes are distinguished through their dominant time scales using the ensemble empirical mode decomposition eemd method the hc fields are first decomposed into components with different time scales the authors analysis shows that these components are associated with distinctive ocean dynamics thehigh frequency firstthree componentscanbecharacterizedastheequatorialmodes whereas the low frequency the fifth and sixth components are featured as the meridional modes in between the fourth component on the time scale of 3 4 yr demonstrates mixed characteristics of the meridional and equatorial modes because of an active transition from the predominant meridional to zonal structures on this time scale physically this transition process is initiated by the discharge of the off equatorial hca which is first accumulated as a part of the meridional mode into the equatorial waveguide which is triggered by the breakdown of the equilibrium between the cross equatorial hc contrast and the overlying wind forcing and resultsinamajorheattransportthroughtheequatorialwaveguideintothesoutheasterntropicalatlantic itis

the low frequency variability of the tropical atlantic ocean abstract upper ocean temperature variability in the tropical atlantic is examined from the comprehensive ocean atmosphere data set coads as well as from an ocean model simulation forced by coads anomalies appended to a monthly climatology the findings are as follows only the sea surface temperatures sst in the northern tropics are driven by heat fluxes while the southern tropical variability arises from wind driven ocean circulation changes the subsurface temperatures in the northern and southern tropics are found to have a strong linkage to buoyancy forcing changes in the northern north atlantic evidence for kelvin like boundary wave propagation from the high latitudes is presented from the model simulation this extratropical influence is associated with wintertime north atlantic oscillation nao forcing and manifests itself in the northern and southern tropical temperature anomalies of the same sign at depths of 100 200 m as result of a rossby wave propagation away from the eastern boundary i

unusual conditions in the tropical atlantic ocean in 1984 au cours de la 1ere moitie de 1984 les conditions oceaniques et atmospheriques dans l atlantique tropical ont ete a bien des egards similaires a celles dans le pacifique au cours de l el nino l ocean superieur a ete inhabituellement chaud dans sa partie orientale les pluies ont ete abondantes sur des regions normalement arides au sud de l equateur et l upwelling cotier a ete contrarie dans une region sud ouest africain ou il constitue un phenomene saisonnier le changement de tsm sst qui apparait lorsque d inhabituels courants portant a l est au sud de l equateur transportent de l eau chaude de surface vers l afrique contribue au changement des conditions atmospheriques ces dernieres en retour contribuent au changement de la circulation oceanique et de la tsm