causes of enhanced sst variability over the equatorial atlantic and its relationship to the atlantic zonal mode in cmip5 abstracta spurious band of enhanced sea surface temperature sst variance sbev is identified over the northern equatorial atlantic in the geophysical fluid dynamics laboratory gfdl climate model version 2 1 the sbev is especially pronounced in boreal spring owing to the combined effect of both anomalous atmospheric thermal forcing and oceanic vertical upwelling the sbev is a common bias in phase 5 of the coupled model intercomparison project cmip5 found in 14 out of 23 models the sbev in cmip5 is associated with the atmospheric thermal forcing and the oceanic vertical upwelling similar to gfdl cm2 1 while the tropical north atlantic variability is only weakly correlated with the atlantic zonal mode azm in observations the sbev in cmip5 produces conditions that drive and intensify the azm variability via triggering the bjerknes feedback this partially explains why azm is strong in some cmip5 models even though the equatorial cold tongue and easterly trades are biased low

the equatorial atlantic oscillation and its response to enso an internal equatorial atlantic oscillation has been identified by analyzing sea surface temperature sst observations the equatorial atlantic oscillation can be viewed as the atlantic analogue of the el nino southern oscillation enso phenomenon in the equatorial pacific but it is much less vigorous the equatorial atlantic oscillation is strongly influenced by the pacific enso with the equatorial atlantic sea surface temperature lagging by about six months this lag can be explained by the dynamical adjustment time of the equatorial atlantic to low frequency wind stress variations and the seasonally varying background state which favours strongest growth of perturbations in summer results of an extended range simulation with a coupled ocean atmosphere gcm support this picture

predicting the state of the southern oscillation using principal oscillation pattern analysis abstract principal oscillation pattern pop analysis is a diagnostic technique for deriving the space time characteristics of a dataset objectively a multiyear dataset of monthly mean sea level pressure slp in the area 15 s to 40 s is examined with the pop technique in the low frequency band one physically significant pair of patterns is identified which is clearly associated with the southern oscillation so according to the pop analysis the 50 may be described as a damped oscillatory sequence of patterns p1 p2 p1 p2 p1 having a time scale of two to three years the first pattern p1 representative of the peak phase of enso exhibits a dipole with anomalies of opposite sign over the central and eastern pacific and over the indian ocean australian sector the second p2 pattern is dominated by an anomaly in the spcz region and describes an intermediate or onset phase the time coefficients of the two patterns p1 and p2 may be interpreted as a bivariate index of the so generalizing

on the connection between benguela and equatorial atlantic ninos and the role of the south atlantic anticyclone for the eastern tropical atlantic two recurring el nino like phenomena with high interannual sst anomalies have been described one centered in the equatorial region as part of the atlantic zonal mode and one off angola referred to as benguela nino both events are supposed to be generated not locally but by a relaxation of the trade winds in the western equatorial atlantic here the connection between sst variations in the two regions is investigated with observational data sets as well as ocean model simulations they are correlated to such an extent that joint events should rather be viewed as one atlantic nino an intriguing feature counterintuitive in view of the remote forcing mechanism is that sst anomalies off angola precede those in the equatorial atlantic we show this behavior to be related to the difference in thermocline depths and a different seasonality of interannual sst variability in the two regions while benguela ninos peak in austral fall due to the angola benguela front being located furthest to the south and high interannual variability in coastal kelvin wave activity warm events at the equator are phase locked to austral winter when the thermocline is shallow perturbation experiments confirm the importance of remote forcing from the equator for sst variability off angola and demonstrate the leading role of wind stress in the generation of sst anomalies in the eastern tropical atlantic these wind stress variations are shown to be linked to fluctuations in the strength of the south atlantic anticyclone a connection that might be important with respect to the predictability of atlantic ninos

impact of the equatorial atlantic on the el nino southern oscillation observations indicate that the atlantic zonal mode influences el nino southern oscillation enso in the pacific as already suggested in previous studies here we demonstrate for the first time using partial coupled experiments that the atlantic zonal mode indeed influences enso the partial coupling experiments are performed by forcing the coupled general circulation model echam5 mpi om with observed sea surface temperature sst in the tropical atlantic but with full air sea coupling allowed in the pacific and indian ocean the ensemble mean of a five member simulation reproduces the observational results well analysis of observations reanalysis and coupled model simulations all indicate the following mechanism sst anomalies associated with the atlantic zonal mode affect the walker circulation driving westward wind anomalies over the equatorial pacific during boreal summer the wind stress anomalies increase the east west thermocline slope and enhance the sst gradient across the pacific the bjerknes positive feedback acts to amplify these anomalies favouring the development of a la nina like anomalies the same mechanisms act for the cold phase of atlantic zonal mode but with opposite sign in contrast to previous studies the model shows that the influence on enso exists before 1970 furthermore no significant influence of the tropical atlantic on the indian monsoon precipitation is found in observation or model

on the origin of equatorial atlantic biases in coupled general circulation models many coupled ocean atmosphere general circulation models gcms suffer serious biases in the tropical atlantic including a southward shift of the intertropical convergence zone itcz in the annual mean a westerly bias in equatorial surface winds and a failure to reproduce the eastern equatorial cold tongue in boreal summer the present study examines an ensemble of coupled gcms and their uncoupled atmospheric component to identify common sources of error it is found that the westerly wind bias also exists in the atmospheric gcms forced with observed sea surface temperature but only in boreal spring during this time sea level pressure is anomalously high low in the western eastern equatorial atlantic which appears to be related to deficient excessive precipitation over tropical south america africa in coupled simulations this westerly bias leads to a deepening of the thermocline in the east which prevents the equatorial cold tongue from developing in boreal summer thus reducing atmospheric model errors during boreal spring may lead to improved coupled simulations of tropical atlantic climate

equatorial atlantic variability and its relation to mean state biases in cmip5 coupled general circulation model gcm simulations participating in the coupled model intercomparison project phase 5 cmip5 are analyzed with respect to their performance in the equatorial atlantic in terms of the mean state 29 out of 33 models examined continue to suffer from serious biases including an annual mean zonal equatorial sst gradient whose sign is opposite to observations westerly surface wind biases in boreal spring play an important role in the reversed sst gradient by deepening the thermocline in the eastern equatorial atlantic and thus reducing upwelling efficiency and sst cooling in the following months both magnitude and seasonal evolution of the biases are very similar to what was found previously for cmip3 models indicating that improvements have only been modest the weaker than observed equatorial easterlies are also simulated by atmospheric gcms forced with observed sst they are related to both continental convection and the latitudinal position of the intertropical convergence zone itcz particularly the latter has a strong influence on equatorial zonal winds in both the seasonal cycle and interannual variability the dependence of equatorial easterlies on itcz latitude shows a marked asymmetry from the equator to 15 n the equatorial easterlies intensify approximately linearly with itcz latitude when the itcz is south of the equator on the other hand the equatorial easterlies are uniformly weak despite serious mean state biases several models are able to capture some aspects of the equatorial mode of interannual sst variability including amplitude pattern phase locking to boreal summer and duration of events the latitudinal position of the boreal spring itcz through its influence on equatorial surface winds appears to play an important role in initiating warm events

principal oscillation pattern analysis of the 30 to 60 day oscillation in general circulation model equatorial troposphere a new technique is described for identifying time dependent patterns i e principal oscillation patterns or pops in a set of geophysical time series pops are defined as the normal modes of a linear dynamical representation of the data in terms of a first order autoregressive vector process with residual noise forcing pops associated with real eigenvalues represent nonpropagating nonoscillatory patterns which decay exponentially pops associated with complex eigenvalues occur in complex conjugate pairs and can represent standing wave structures if one pattern is much stronger than the other propagating waves if both patterns are periodic and have the same structure except for a quarter wavelength shift or in general an arbitrary amphidromal oscillation after the pops have been defined for a selected set of primary variables associated correlation or composite patterns may be derived for additional secondary fields to gain further insight into the structure of the interaction mechanisms the method is illustrated by analyzing the tropical variability structure of a 10 year numerical simulation with the t21 general circulation model gcm of the european centre for medium range weather forecasts reading england the pop analysis is applied to the 200 hpa horizontal divergence field along the equator for time scales shorter than 10 weeks the associated patterns are determined for a number of additional fields in the tropical belt between 30 s and 30 n one dominant pop pair is found its spatial scale corresponds to zonal wave number 1 the patterns travel eastward with an average period of 24 days and an e folding decay time of 10 days the maximum variance occurs in the area with maximum tropical precipitation between 100 e and the dateline these features correspond rather closely to the characteristics of the observed tropical 30 to 60 day wave except for the smaller period a frequency wave number analysis confirms that this 30 to 60 day wave is the most dominant regular oscillation in the tropical gcm troposphere the associated patterns of sea level pressure precipitation and other quantities exhibit a number of intriguing aspects namely 1 the phase velocity of the 30 to 60 day wave varies with longitude from 6 m s 1 in the indonesian area to more than 30 m s 1 over the eastern pacific small phase speeds being associated with large amplitudes and high phase speeds with small amplitudes 2 in the high amplitude regions rainfall and upper air velocity potential are in phase while in the low amplitude regions rainfall and velocity potential appear uncorrelated 3 at the surface a pattern strongly resembling gill s theoretical response to an equatorial heating source is found with a trough to the east and two off equatorial cyclones to the west of the heating the lack of meridional winds is probably related to the disregard of the seasonal asymmetries

thallium isotope evidence for a permanent increase in marine organic carbon export in the early eocene article the first high resolution thallium tl isotope records in two ferromanganese crusts fe mn crusts cd29 and d11 from the pacific ocean are presented the crusts record pronounced but systematic changes in 205 tl 203 tl that are unlikely to reflect diagenetic overprinting or changes in isotope fractionation between seawater and fe mn crusts it appears more likely that the fe mn crusts track the tl isotope composition of seawater over time the present day oceanic residence time of tl is estimated to be about 20 000 yr such that the isotopic composition should reflect ocean wide events new and published os isotope data are used to construct age models for these crusts that are consistent with each other and significantly different from previous age models application of these age models reveals that the tl isotope composition of seawater changed systematically between 55 ma and 45 ma using a simple box model it is shown that the present day tl isotope composition of seawater depends almost exclusively on the ratio between the two principal output fluxes of marine tl these fluxes are the rate of removal of tl from seawater via scavenging by authigenic fe mn oxyhydroxide precipitation and the uptake rate of tl during low temperature alteration of oceanic crust it is highly unlikely that the latter has changed greatly therefore assuming that the marine tl budget has also not changed significantly during the cenozoic the low 205 tl

on sub enso variability multichannel singular spectrum analysis mssa of surface zonal wind sea surface temperature sst 20 isotherm depth and surface zonal current observations between 1990 and 2004 identifies three coupled ocean atmosphere modes of variability in the tropical pacific the el nino southern oscillation enso the annual cycle and a mode with a 14 18 month period which is referred to as sub enso in this study the sub enso mode accounts for the near 18 month near annual variability prior to following the 1997 98 el nino event it was strongest during this el nino event with sst anomalies exceeding 1 c sub enso peak sst anomalies are enso like in structure and are associated with eastward propagating heat content variations however the sst anomalies are preceded by and in near quadrature with relatively strong remotely forced westward propagating zonal current variations suggesting the sub enso mode arises from the zonal advective feedback the sub enso mode is found to exist also in an intermediate complexity model icm of the tropical pacific a heat budget analysis of the model s sub enso mode shows it indeed arises from the zonal advective feedback in the model both enso and sub enso modes coexist but there is a weak nonlinear interaction between them experiments also show that the observed changes in sub enso s characteristics may be explained by changes in the relative importance of zonal and vertical advection sst tendencies

observations of warm water volume changes in the equatorial pacific and their relationship to el nino and la nina abstract this paper describes observed changes in surface winds sea surface temperature sst and the volume of water warmer than 20 c wwv in the equatorial pacific ocean for the period 1980 99 the purpose is to test recent hypotheses about the relationship between variations in wwv and the el nino southern oscillation enso cycle the results confirm inferences based on theory models and previous empirical analyses using proxy data namely sea level that enso involves a recharge and discharge of wwv along the equator and that the cyclic nature of enso results from a disequilibrium between zonal winds and zonal mean thermocline depth the authors also find that the magnitude of enso sst anomalies is directly related to the magnitude of zonal mean wwv anomalies furthermore for a given change in equatorial wwv the corresponding warm el nino sst anomalies are larger than the corresponding cold la nina anomalies this asymmetry between the warm and cold phases of the enso cycle implies differences

tropical response to the atlantic equatorial mode agcm multimodel approach on the frame of the amma eu project sensitivity experiments for an atlantic equatorial mode aem which origin development and damping resembles the observed one during the last decades of the 20th century has been analysed in order to investigate the influence on the anomalous summer west african rainfall recent studies raise the matter of the aem influence on the next pacific enso episodes and also on the indian monsoon this paper evaluates the response of four different atmospheric global circulation models using the above mentioned aem sensitivity experiments to study the tropical forcing associated with the atlantic nino mode the results show a remote signal in both the pacific and indian basins for a warm phase of the aem the associated southward location of the itcz with rising motions over the equatorial atlantic leads to a global subsidence over the rest of the tropics weakening the asian monsoon and favouring the la nina conditions in the central pacific although ocean atmosphere coupled experiments are required to test the latter hypothesis the present studies shows how the aem is able to influence the rest of the tropics a result with important implications on enso seasonal predictability

an advective reflective conceptual model for the oscillatory nature of the enso recent findings about zonal displacements of the pacific warm pool required a notable modification of the delayed action oscillator theory the current leading theory for the el nino southern oscillation enso simulations with a linearized coupled ocean atmosphere model resulted in 3 to 6 year enso like oscillations with many of the variable model parameters found to be very close to their observed values this simple model suggests that ocean processes that are ignored or underestimated in the delayed action oscillator theory such as zonal current convergence zonal advection of sea surface temperature and equatorial wave reflection from the eastern ocean boundary are fundamental to the development of the enso in particular to its manifestations in the central equatorial pacific

behavior of li and its isotopes during serpentinization of oceanic peridotites analyses of li and li isotopes in serpentinized peridotites have been performed using thermo ionisation mass spectrometry tims and secondary ion mass spectrometry sims techniques on samples collected from the southwest indian ridge swir in the bulk samples li concentrations range from 0 6 to 8 2 ppm while whole rock 6li values range from 2 9 to 14 in situ analyses display a greater range in both li concentration 0 1 19 5 ppm and li isotopic composition 27 to 19 with the serpentinized portions having higher li concentrations than the associated relict phases these variations may reflect changes in li partitioning and isotopic fractionation between serpentine and fluid with temperature and water rock ratio they may also be explained by changes in the composition of the serpentinizing fluid over the course of serpentinization as the serpentine forms by interaction with a circulating fluid it preferentially removes 6li causing the li in the fluid to become isotopically heavier the isotopic composition of the initial hydrothermal fluid is dominated by basalt derived li which easily overwhelms the very low li content originally present in seawater as this fluid circulates through ultramafic rocks it induces the formation of serpentine that incorporates this mantle derived li hence li in serpentine is mainly derived from oceanic crust rather than from seawater and serpentinization involves li recycling within this crust consequently li isotopes are good tracers of the hydrothermal contribution in serpentinizing fluid these results imply that serpentinized peridotites are probably only a minor sink of oceanic li

lithium isotope geochemistry of the hawaiian plume results from the hawaii scientific drilling project and koolau volcano 1 we determined lithium isotopic compositions of mauna loa and mauna kea basalts from the 3 1 km drill hole of the hawaiian scientific drilling project hsdp for comparison li isotopic ratios were also determined for basalts from koolau volcano these two suites of samples define geochemical extremes in the range of hawaiian shield lavas the 400 ka record of mauna kea in the hsdp core shows temporal fluctuations between low 7li 4 relative to the l svec standard and high 7li 5 6 suggesting that the source components in the hawaiian plume are heterogeneous in li isotopic composition based on sio2 content and isotopic ratios of he li nd hf and pb three geochemical groups are identified in mauna kea lavas mauna kea basalts between 1900 and 2500 mbsl have relatively low 7li of about 4 they are low sio2 lavas distinguished by the highest 3he 4he and 208pb 204pb and low 176hf 177hf and 143nd 144nd like basalt from loihi seamount this mauna kea group is considered to originate from the core of the plume above 1900 mbsl high 7li lavas with high sio2 contents appear in both the submarine and subaerial sections they are marked by low 3he 4he and high 176hf 177hf the 7li rich signature of some samples 7li up to 5 7 is indicative of recycled oceanic crust in the plume this magma group defines the kea component the low sio2 lavas in the subaerial section have low 7li 4 3he 4he and 208pb 204pb their 7li values overlap the range of 7li in unaltered mid ocean ridge basalt morb and are consistent with upper mantle material entrained by the plume or contamination of plume derived magmas by the pacific lithosphere the 7li of koolau lavas mostly fall within the range of 4 5 0 3 exceptions are two samples that have 7li of 2 3 the lightest isotopic values may indicate subducted li that was isotopically fractionated during slab dehydration in contrast to other isotopic systems most koolau samples however resemble mauna kea samples in li isotopic composition mauna loa samples have 7li values of 3 5 to 4 9 within the range of the koolau and mauna kea lavas based on these data the loa trend volcanoes and kea trend volcanoes have largely overlapping li isotopic compositions in summary the hawaiian plume is not highly variable in li isotopic composition 7li is typically 4 with perturbations by subducted components to lower and higher ratios 2 5 to 5 7 the overlap of most hawaiian basalt and morb in their range of li isotopic ratios suggests minor influence of recycled oceanic crust in the plume and perhaps similar li isotopic ratios in the upper and lower mantle

lithium isotopic composition of alaskan type intrusion and its implication abstract to determine whether fractionation of li isotopes could occur at high temperatures we examined major element and li isotopic compositions of olivine from the xiadong intrusion an alaskan type complex in the central asian orogenic belt olivine in thirteen dunites displaying characteristic cumulus textures yielded large variations in li concentration 0 10 to 11 18 ppm and isotopic composition 7 li 7 18 to 34 41 these variations are too large to be attributed entirely to diffusive processes the correlations between li elemental or isotopic composition and differentiation indices such as fo and mno contents of olivine and nio content of chromite suggest probable li isotope fractionation during early stage of differentiation we speculate that while li behaves mildly incompatible during differentiation 7 li is preferentially incorporated into olivine relative to 6 li during early stage crystallization from arc magmas relatively high li concentrations and low 7 li in arc magmas might be the result of substantial olivine fractionation prior to eruption

seasonal cycle in the upper equatorial atlantic ocean the dynamics of the seasonal cycle in the upper equatorial atlantic ocean are studied using observations and a hierarchy of ocean models distinctive features of the seasonal cycle are strong annual and semiannual components eastward westward propagating sea surface height ssh and thermocline depth at the equator off the equator and westward propagating surface zonal currents at the equator modelling results show that linear theory can explain the seasonal cycle in thermocline depth and ssh while first order linear theory can also explain the structure of the seasonal cycle of surface zonal currents at the equator nonlinear terms are required they weaken the variability and improve its phase and zonal extent the important terms are meridional and vertical advection and vertical diffusion of zonal momentum the linear solution is essentially determined by the four gravest baroclinic modes and kelvin and first meridional mode rossby waves the eastward propagation in thermocline depth at the equator results from the kelvin wave contribution while the westward propagation in thermocline depth off the equator and surface zonal currents at the equator result from the first meridional mode rossby wave the contribution of kelvin and rossby waves generated by boundary reflections equals that of the directly forced waves the semiannual cycle in zonal winds although much weaker than the annual component forces a strong semiannual component in ssh and surface zonal currents because it excites the basin mode of the second baroclinc mode this explains the observed feature in the seasonal cycle from march to august

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

combined sr nd pb and li isotope geochemistry of alkaline lavas from northern james ross island antarctic peninsula and implications for back arc magma formation abstract we present a comprehensive geochemical data set for a suite of back arc alkaline volcanic rocks from james ross island volcanic group jrivg antarctic peninsula the elemental and isotopic sr nd pb and li composition of these cenozoic basalts emplaced east of the antarctic peninsula is different from the compositions of the fore arc alkaline volcanic rocks in southern shetlands and nearby bransfield strait the variability in elemental and isotopic composition is not consistent with the jrivg derivation from a single mantle source but rather it suggests that the magma was mainly derived from a depleted mantle with subordinate oib like enriched mantle component em ii the isotopic data are consistent with mantle melting during extension and possible roll back of the subducted lithosphere of the antarctic plate magma contamination by triassic early tertiary clastic sediments deposited in the back arc basin was only localized and affected li isotopic composition in two of the samples while most of the basalts show very little variation in 7 li values as anticipated for mantle driven li isotopic composition these variations are difficult to resolve with radiogenic isotope systematics but li isotopes may prove sensitive in tracking complex geochemical processes acting through the oceanic crust pile including hydrothermal leaching and seawater equilibration

main processes of the atlantic cold tongue interannual variability the interannual variability of the atlantic cold tongue act is studied by means of a mixed layer heat budget analysis a method to classify extreme cold and warm act events is proposed and applied to ten various analysis and reanalysis products this classification allows 5 cold and 5 warm act events to be selected over the period 1982 2007 cold warm act events are defined by the presence of negative positive sea surface temperature sst anomalies at the center of the equatorial atlantic in late boreal spring preceded by negative positive zonal wind stress anomalies in the western equatorial atlantic an ocean general circulation model capable of reconstructing the interannual variability of the act correctly is used to demonstrate that cold act events develop rapidly from may to june mainly due to intense cooling by vertical mixing and horizontal advection the simulated cooling at the center of the basin is the result of the combined effects of non local and local processes the non local process is an upwelling associated with an eastward propagating kelvin wave which makes the mixed layer more shallow and preconditions the upper layers to be cooled by an intense heat loss at the base of the mixed layer which is amplified by a stronger local injection of energy from the atmosphere the early cooling by vertical mixing in march is also shown to be a good predictor of june cooling in july horizontal advection starts to warm the mixed layer abnormally and damps sst anomalies the advection anomalies which result from changes in the horizontal temperature gradient are associated in some cases with the propagation of rossby waves along the equator during warm act events processes are reversed generating positive sst anomalies a downwelling kelvin wave triggers stratification anomalies and mixed layer depth anomalies amplified by a weaker injection of energy from the atmosphere in may june in july warm act events are abnormally cooled due to negative horizontal advection anomalies resulting from processes similar to those that occur during cold act events this additional cooling process extends the period of cooling of the act reducing sst anomalies

the bjerknes feedback in the tropical atlantic in cmip5 models coupled state of the art general circulation models still perform relatively poorly in simulating tropical atlantic ta climate to investigate whether lack of air sea interaction might be responsible for their biases we investigate the bjerknes feedback bf in the ta the driver of the dominant interannual variability in that region first we analyse this mechanism from reanalysis data then we compare our findings to model output from the coupled model intercomparison project phase 5 the feedback is subdivided into three components the first one consists of the influence of eastern equatorial sea surface temperature anomalies sst on zonal wind stress anomalies tau u in the western basin the second component is the influence of wind stress anomalies in the western ta on eastern equatorial oceanic heat content anomalies hc the third component is the local response of overlying sst to hc in the eastern ta all three components are shown to be present in era interim and oras4 reanalysis by correlating the two variables of each component with each other the obtained patterns are compared to the ones from model output via pattern correlation per component while the models display errors in the annual cycles of sst tau u and hc as well as in the seasonality of the feedback the impact of sst on wind stress and the impact of wind stress on hc are simulated relatively well by most of the models this is especially the case when correcting for the error in seasonality the third component of the bf the impact of hc on sst in the eastern part of the basin deviates from what we find in reanalysis we find an influence of hc anomalies on overlying ssts in the eastern equatorial ta but it is weaker than in the reanalysis and it is not strongly confined to the equator longitude depth cross sections of equatorial temperature variance and correlation between subsurface temperature anomalies and sst in the cold tongue region show that flawed simulation and slow adjustment of the subsurface ocean are responsible for this

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

interannual atmospheric variability forced by the deep equatorial atlantic ocean climate variability in the tropical atlantic ocean is determined by large scale ocean atmosphere interactions which particularly affect deep atmospheric convection over the ocean and surrounding continents1 apart from influences from the pacific el nino southern oscillation2 and the north atlantic oscillation3 the tropical atlantic variability is thought to be dominated by two distinct ocean atmosphere coupled modes of variability that are characterized by meridional4 5 and zonal6 7 sea surface temperature gradients and are mainly active on decadal and interannual timescales respectively8 9 here we report evidence that the intrinsic ocean dynamics of the deep equatorial atlantic can also affect sea surface temperature wind and rainfall in the tropical atlantic region and constitutes a 4 5 yr climate cycle specifically vertically alternating deep zonal jets of short vertical wavelength with a period of about 4 5 yr and amplitudes of more than 10 cm s 1 are observed in the deep atlantic to propagate their energy upwards towards the surface10 11 they are linked at the sea surface to equatorial zonal current anomalies and eastern atlantic temperature anomalies that have amplitudes of about 6 cm s 1 and 0 4 c respectively and are associated with distinct wind and rainfall patterns although deep jets are also observed in the pacific12 and indian13 oceans only the atlantic deep jets seem to oscillate on interannual timescales our knowledge of the persistence and regularity of these jets is limited by the availability of high quality data despite this caveat the oscillatory behaviour can still be used to improve predictions of sea surface temperature in the tropical atlantic deep jet generation and upward energy transmission through the equatorial undercurrent warrant further theoretical study

local coupled equatorial variability versus remote enso forcing in an intermediate coupled model of the tropical atlantic the relative roles played by the remote el nino southern oscillation enso forcing and the local air sea interactions in the tropical atlantic are investigated using an intermediate coupled model icm of the tropical atlantic the oceanic component of the icm consists of a six baroclinic mode ocean model and a simple mixed layer model that has been validated from observations the atmospheric component is a global atmospheric general circulation model developed at the university of california los angeles ucla in a forced context the icm realistically simulates both the sea surface temperature anomaly ssta variability in the equatorial band and the relaxation of the atlantic northeast trade winds and the intensification of the equatorial westerlies in boreal spring that usually follows an el nino event the results of coupled experiments with or without pacific enso forcing and with or without explicit air sea interactions in the equatorial atlantic indicate that the background energy in the equatorial atlantic is provided by enso however the time scale of the variability and the magnitude of some peculiar events cannot be explained solely by enso remote forcing it is demonstrated that the peak of ssta variability in the 1 3 yr band as observed in the equatorial atlantic is due to the local air sea interactions and is not a linear response to enso seasonal phase locking in boreal summer is also the result of the local coupling the analysis of the intrinsic sustainable modes indicates that the atlantic el nino is qualitatively a noisedriven stable system such a system can produce coherent interdecadal variability that is not forced by the pacific or extraequatorial variability it is shown that when a simple slab mixed layer model is embedded into the system to simulate the northern tropical atlantic nta sst variability the warming over nta following el nino events have characteristics location and peak phase that depend on air sea interaction in the equatorial atlantic in the model the interaction between the equatorial mode and nta can produce a dipolelike structure of the ssta variability that evolves at a decadal time scale the results herein illustrate the complexity of the tropical atlantic ocean atmosphere system whose predictability jointly depends on enso and the connections between the atlantic modes of variability

a unified oscillator model for the el nino southern oscillation the delayed oscillator the western pacific oscillator the recharge discharge oscillator and the advective reflective oscillator have been proposed to interpret the oscillatory nature of the el nino southern oscillation enso all of these oscillator models assume a positive ocean atmosphere feedback in the equatorial eastern and central pacific the delayed oscillator assumes that the western pacific is an inactive region and wave reflection at the western boundary provides a negative feedback for the coupled system to oscillate the western pacific oscillator emphasizes an active role of the western pacific in enso the recharge discharge oscillator argues that discharge and recharge of equatorial heat content cause the coupled system to oscillate the advective reflective oscillator emphasizes the importance of zonal advection associated with wave reflection at both the western and eastern boundaries motivated by the existence of these different oscillator models a unified oscillator model is formulated and derived from the dynamics and thermodynamics of the coupled ocean atmosphere system consistent with enso anomaly patterns observed in the tropical pacific this oscillator model considers sea surface temperature anomalies in the equatorial eastern pacific zonal wind stress anomalies in both the equatorial central pacific and the equatorial western pacific and thermocline depth anomalies in the off equatorial western pacific if the western pacific wind forced response is neglected thermocline and zonal wind stress anomalies in the western pacific are decoupled from the coupled system and the unified oscillator reduces to the delayed oscillator if wave reflection at the western boundary is neglected the unified oscillator reduces to the western pacific oscillator the mathematical form of the recharge discharge oscillator can also be derived from this unified oscillator most of the physics of the advective reflective oscillator are implicitly included in the unified oscillator and the negative feedback of wave reflection at the eastern boundary is added to the unified oscillator with appropriate model parameters chosen to be consistent with those of previous oscillator models the unified oscillator model oscillates on interannual timescales

phase locking of equatorial atlantic variability through the seasonal migration of the itcz the equatorial atlantic is marked by significant interannual variability in sea surface temperature sst that is phase locked to late boreal spring and early summer the role of the atmosphere in this phase locking is examined using observations reanalysis data and model output the results show that equatorial zonal surface wind anomalies which are a main driver of warm and cold events typically start decreasing in june despite sst and sea level pressure gradient anomalies being at their peak during this month this behavior is explained by the seasonal northward migration of the intertropical convergence zone itcz in early summer the north equatorial position of the atlantic itcz contributes to the decay of wind anomalies in three ways 1 horizontal advection associated with the cross equatorial winds transports air masses of comparatively low zonal momentum anomalies from the southeast toward the equator 2 the absence of deep convection leads to changes in vertical momentum transport that reduce the equatorial wind anomalies at the surface while anomalies aloft remain relatively strong 3 the cross equatorial flow is associated with increased total wind speed which increases surface drag and deposit of momentum into the ocean previous studies have shown that convection enhances the surface wind response to sst anomalies the present study indicates that convection also amplifies the surface zonal wind response to sea level pressure gradients in the western equatorial atlantic where sst anomalies are small this introduces a new element into coupled air sea interaction of the tropical atlantic

an overlooked feature of tropical climate inter pacific atlantic variability 1 both the tropical pacific and atlantic host an equatorial mode of interannual variability called the pacific el nino and the atlantic nino respectively although the pacific el nino does not correlate with the atlantic nino anomalous warming or cooling of the two equatorial oceans can form an inter pacific atlantic sea surface temperature sst gradient variability that induces surface zonal wind anomalies over equatorial south america and over some regions of both ocean basins the zonal wind anomalies act to bridge the interaction of the two ocean basins reinforcing the inter pacific atlantic sst gradient through atmospheric walker circulations and oceanic dynamics thus a positive feedback seems to exist for climate variability of the tropical pacific atlantic oceans and atmosphere system in which the inter basin sst gradient is coupled to the overlying atmospheric wind rainfall responds to the inter pacific atlantic sst gradient by showing an anti symmetric configuration between the two equatorial oceans suggesting that rainfall is sensitive to the equatorial inter basin sst gradient regardless of which ocean is anomalously warm or cold

a comparative stability analysis of atlantic and pacific nino modes abstractel nino southern oscillation enso in the pacific and the analogous atlantic nino mode are generated by processes involving coupled ocean atmosphere interactions known as the bjerknes feedback it has been argued that the atlantic nino mode is more strongly damped than enso which is presumed to be closer to neutrally stable in this study the stability of enso and the atlantic nino mode is compared via an analysis of the bjerknes stability index this index is based on recharge oscillator theory and can be interpreted as the growth rate for coupled modes of ocean atmosphere variability using observational data an ocean reanalysis product and output from an ocean general circulation model the individual terms of the bjerknes index are calculated for the first time for the atlantic and then compared to results for the pacific positive thermocline feedbacks in response to wind stress forcing favor anomaly growth in both basins but they are twice as large in the pacific compared to the atlanti

a western pacific oscillator paradigm for the el nino southern oscillation a data based hypothesis is presented on the mechanism of the el nino southern oscillation enso a major determinant of interannual global climate variability the hypothesis emphasizes the importance of off equator sea surface temperature and sea level pressure variations west of the dateline for initiating equatorial easterly winds over the far western pacific these winds compete with westerly winds over the equatorial central pacific enabling the coupled ocean atmosphere system to oscillate consistent with this hypothesis an analogical oscillator model is constructed that produces enso like oscillations the proposed mechanism differs from the delayed oscillator paradigm in that wave reflection at the western boundary is not a necessary condition for the coupled ocean atmosphere system to oscillate

physical processes associated with the tropical atlantic sst gradient during the anomalous evolution in the southeastern ocean abstract this work is focused on the evolution of the dominant air sea coupled mode in the equatorial and southeastern tropical atlantic and the associated physical processes it is shown that in june august jja the evolution of the dominant mode is mainly dynamically driven and displays a coherent warming or cooling pattern extending from the angola coast toward the equator in the gulf of guinea for anomalies peaking in jja the warming cooling is initiated near the angola coast in march may it is suggested that sst anomalies along the coast and near the equator are physically connected the air sea interaction along the coast may be a major factor in triggering the development of sst anomalies near the equator which is intensified by local positive feedbacks that may include bjerknes and ekman processes in return the warming or cooling near the equator weaken the sst anomalies along the coast by changing the direction of anomalous wind slow westward rossby wave propagation may also play a role

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

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

equatorial atlantic interannual variability role of heat content 1 the dynamics of the equatorial atlantic zonal mode are studied using observed sea surface height ssh sea surface temperature sst and heat flux and reanalysis wind stress and upper ocean temperature principal oscillation pattern pop analysis shows that the zonal mode is an oscillatory normal mode of the observed coupled system obeying the delayed action recharge oscillator paradigm for el nino southern oscillation variations in equatorial averaged ssh a proxy for upper ocean heat content precede sst anomalies in the cold tongue by 4 5 months about a quarter of the pop period positive subsurface temperature anomalies appear in the west as a delayed response to the preceding cold event these propagate eastward where due to the shallow thermocline they can influence sst leading to the next warm event although sst variations exhibit weak westward propagation during some zonal mode events pop analysis indicates that to first order there is no zonal propagation in sst net surface heat flux anomalies generally act to damp sst anomalies the zonal mode explains a large amount 70 of sst variability in the east and a significant fraction 19 of equatorial variability thus the predictability potential in the equatorial atlantic on seasonal time scales may be considerably higher than currently thought

energetics of the tropical atlantic zonal mode sea surface temperature in the central eastern equatorial atlantic has a seasonal cycle far bigger than that of the pacific but interannual anomalies smaller than those of the pacific given the amplitude of seasonal sst variability one wonders whether the seasonal cycle in the atlantic is so dominant that it is able to strongly influencetheevolutionofitsinterannualvariability inthisstudy interannualupper oceanvariabilitywithinthe tropical atlanticis viewed from an energetics perspective and the role ofocean dynamics in particularthe role ofoceanmemory withinzonalmodeeventsisinvestigated unlikeinthepacificwhereseasonalandinterannual variability involve distinctly different processes the results suggest that the latter is a modulation of the former

warm events in the tropical atlantic abstract sea surface temperature in the eastern equatorial atlantic ocean undergoes anomalous warming events of 1 2 c every few years the warm anomalies reach their maximum strength in northern hemisphere summer when equatorial upwelling normally brings cold thermocline water to the surface by compositing surface observations from a 28 year record we are able to identify consistent features in anomalies of sst and winds the composites show that the sst anomalies in northern summer are confined to the eastern equatorial region with reduced zonal winds to the west and reduced northward trade winds to the east accompanying these changes in winds are enhanced convection near the equator caused by a southward shift and intensification of the intertropical convergence zone later in the year sst south of the equator becomes elevated as a result by spring of the year following the equatorial anomaly convection in the western side of the basin is much higher than normal to understand the ocean dynami