direct evidence of active deformation in the eastern indian oceanic plate conventional plate tectonics theory postulates that plates only deform on their boundaries to the contrary there is ample evidence of intraplate deformation in the equatorial indian ocean west of the ninetyeast aseismic ridge prior to this study no direct evidence of deforma tion east of the ninetyeast ridge was available we present the results of a multipurpose geo physical cruise showing that intraplate deformation also occurs in this area long at least 1000 km left lateral north south strike slip faults are active and reactivate fossil fracture zones this style of deformation is strikingly different from the east west folds and reverse faults that affect the region west of the ninetyeast ridge contrasting processes of convergence at the northern plate boundaries can account for the two styles of deformation west of the ninetyeast ridge there is a continent continent collision and east of the ridge oceanic lithosphere subducts along the sumatra trench the ninetyeast aseismic ridge therefore appears to be a mechanical border separating two distinct deformed areas

structure and isostatic compensation of the comorin ridge north central indian ocean summary bathymetry gravity and magnetic data about 9200 lkm of the comorin ridge north central indian ocean were investigated using the transfer function and forward model techniques to understand the mode of isostatic compensation and origin of the ridge the ridge extends for about 500 km in nnw sse direction and associates with low amplitude gravity anomalies ranging from 25 to 30 mgal compared to the ridge relief suggesting that the anomalies are compensated at deeper depths from admittance analysis an airy model or local compensation with an elastic plate thickness t e of about 3 km and crust thickness t of 15 20 km are suggested for the southern part of the comorin ridge south of 5 n whereas for the northern part a flexural plate model with an elastic thickness of about 15 km is obtained admittance analysis together with the results from gravity forward modelling reveal that the south part was emplaced on relatively weak oceanic crust with both surface and subsurface loading while the north part was emplaced on the continental crust based on present studies and published plate kinematic models we interpret that the comorin ridge was evolved at about 90 ma during the rift stage of madagascar from the southwest of india we have also demarcated the continent ocean boundary cob west of sri lanka and southern tip of india which runs across the strike of the ridge placing the northern part of the ridge on continent and southern part on oceanic crust on the southern part of the ridge eastern flank is steep faulted up to 0 6 km and is controlled by the 79 e fz and then by cob

the isostatic compensation of the azores plateau a 3d admittance and coherence analysis the compensation of the azores plateau is re examined by means of a 3d admittance study involving computations of the admittance between the bathymetry and 1 the geoid 2 the free air gravity and 3 the mantle bouguer anomalies the geoid to bathymetry relationship rather than being analysed in the space domain is studied in the spectral domain in this way the information related to the flexural response of the lithosphere is retained in the geoid to bathymetry signal and can be compared to that of the gravity to bathymetry we find that the anomalously shallow depths of the azores plateau are not due to dynamic forces sustained by mantle upwelling the plateau is supported by a thickened crust which mainly results from large volumes of accreted extrusives and consequent deflection of the underlying elastic plate both the free air gravity and the mantle bouguer admittance point to a flexural isostatic model with a moho depth of 12 km and an elastic thickness in the range of 3 6 km however the analysis of the coherency between bathymetry and the mantle bouguer anomaly indicates that buoyant material at the base of the crust partially accounts for the uplift of the plateau in our best fitting model the average elastic thickness is 4 km and the amplitude of the buoyant load at the moho is 1 3 of that of the surface volcanic load buoyancy forces at the moho may arise from underplated material resulting from subcrustal plutonism

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

three dimensional estimation of elastic thickness under the louisville ridge a three dimensional approach to estimating elastic thickness is presented which uses dense satellite altimetry and sparse ship bathymetry this technique is applied to the louisville ridge system to study the tectonic history of the region the inversion is performed as both a first order approximation and a nonlinear relationship between gravity and topography based on parker s 1973 equation while the higher order effect on the gravity anomaly is nearly zero for most of the region the magnitude is significant over the summits of the ridge nevertheless the inclusion of the nonlinear terms has only a minor influence on the elastic thickness estimate within each region lowering the value by 1 2 km compared with the linear result the incorrect assumption of two dimensionality for circular features exhibits a marked effect on the gravitational anomaly resulting in false sidelobe structure of nearly 20 mgal for large seamounts our elastic thickness estimates are compared with the contradictory values obtained in previous studies by cazenave and dominh 1984 and watts et al 1988 we find an increasing elastic thickness along the chain from southeast to northwest with a discontinuity along the wishbone scarp the jump in elastic thickness values northwest of the scarp appears to be an indication of an age discontinuity caused by an extinct spreading center north of the ridge

the evolution of el nino past and future abstract we review forecasts of the future of el nino and the southern oscillation enso a coupled instability of the ocean atmosphere system in the tropical pacific with global impacts enso in the modern world is briefly described and the physics of the enso cycle is discussed particular attention is given to the bjerknes feedback the instability mechanism which figures prominently in enso past and future our knowledge of enso in the paleoclimate record has expanded rapidly within the last 5 yr the enso cycle is present in all relevant records going back 130 kyr it was systematically weaker during the early and middle holocene and model studies indicate that this results from reduced amplification in the late summer and early fall a consequence of the altered mean climate in response to boreal summer perihelion data from corals shows substantial decadal and longer variations in the strength of the enso cycle within the past 1000 yr it is suggested that this may be due to solar and volcanic variations in solar insolation amplified by the bjerknes feedback there is some evidence that this feedback has operated in the 20th century and some model results indicate that it will hold sway in the greenhouse future but it is very far from conclusive the comprehensive general circulation models used for future climate projections leave us with an indeterminate picture of enso s future some predict more enso activity some less with the highly uncertain consensus forecast indicating little change

crustal structure and tectonics of the ninetyeast ridge from seismic and gravity studies seismic reflection and refraction gravity and bathymetric data across and along the central part of the ninetyeast ridge were analyzed to determine the crustal structure of the ridge and to understand its tectonics the ridge in the study area rises to 3 5 km above the seafloor of adjacent ocean basins basement is covered by sediments up to 0 8 km thick but exposed at some locations seismic refraction and gravity models of the ridge show that the crustal thickness is 22 km greater than the average thickness 7 km of oceanic crust estimated in the central indian and wharton basins the ridge is compensated by flexure of layers 2a 2b and 3a to the extent of 2 km and by a 12 km thick deep crustal body layer 3b of underplated material upper crustal velocities layer 2a beneath the ridge are lower up to 3 4 km s 1 than in adjacent basins because of weathering process and hydrothermal circulation of the waters in the tectonized blocks while velocities in deep crustal body layer 3b are higher up to 7 8 km s 1 and seem to be resulted from magmatic rocks accreted within the lithosphere steep down ward faulting of 2 km along the eastern flank of the ninetyeast ridge possibly originated because of compressional and extensional stresses applied along the 89 e fracture zone fz the stresses are interpreted as resulting from multiple ridge jumps of wharton spreading centers during the late cretaceous and middle eocene the velocity depth model suggests that the rocks of layers 2a and 2b and layer 3a are exposed on the eastern flank of the ninetyeast ridge and 90 e fz respectively they are consistent with basalt dolerite and gabbro rocks collected from the fault scarps the central part of the ridge between 1 n and 11 s has uniformly deepened by 0 7 km and is bounded by major sublatitudinal deep seated fractures we interpret that the deepened structural block and associated fractures resulted from the activity of the intense deformation of the central indian ocean

abandoned paleocene spreading center in the northeastern indian ocean evidence from magnetic and seismic reflection data abstract magnetic data from the northeastern indian ocean reveal east west trending magnetic lineations 28 through 34 an abandoned spreading center asc and part of the cretaceous magnetic quiet zone cmqz which are used to reconstruct past tectonic processes involved in the evolution of the ocean offsets in the magnetic lineations outline the existence of four n s trending fracture zones 80 e indira 84 5 e and 86 e the 84 5 e fracture zone fz acts as a boundary separating oceanic crust with different magnetic isochron patterns the pairs of magnetic lineations 30 through 32n 2 between the 86 e fz and the ninetyeast ridge reveal an asc of about 65 ma age parallel to 0 5 s latitude the structure of the asc is well identified in seismic reflection data as an undulating basement topographic rise covered by up to 2 km of bengal fan sediments the spreading center might have initiated its activity along with other spreading centers of the wharton ridge and india antarctica ridge after the first major plate reorganization of the indian ocean about 95 5 ma but ceased shortly after formation of anomaly 30 about 65 ma then it jumped southward between anomalies 32n 2 and 33 the jump captured the oceanic crust of anomalies 30 through 32n 2 that initially formed on the antarctica plate but were later transferred to the indian plate the spreading centers in the vicinity of the ninetyeast ridge jumped several times southward to maintain a close proximity to the kerguelen hotspot

the indian monsoon and its variability for over 300 years the monsoon has been viewed as a gigantic land sea breeze it is shown in this paper that satellite and conventional observations support an alternative hypothesis which considers the monsoon as a manifestation of seasonal migration of the intertropical convergence zone itcz with the focus on the indian monsoon the mean seasonal pattern is described and why it is difficult to simulate it is discussed some facets of the intraseasonal variation such as active weak cycles break monsoon and a special feature of intraseasonal variation over the region namely poleward propagations of the itcz at intervals of 2 6 weeks are considered vertical moist stability is shown to be a key parameter in the variation of monthly convection over ocean and land as well as poleward propagations special features of the bay of bengal and the monsoon brought out by observations during a national observational experiment in 1999 are briefly described

early pre 8 ma fault activity and temporal strain accumulation in the central indian ocean the diffuse deformation zone in the central indian ocean is the classical example of distributed deformation of the oceanic lithosphere where shortening between the indian and capricorn plates is manifest as reverse faulting 5 10 km spaced faults and long wavelength 100 300 km folding the onset of this deformation is commonly regarded as a key far field indicator for the start of major uplift of the himalayas and tibet some 4000 km further to the north due to increased deviatoric stresses within the wider india asia area there has been disagreement concerning the likely timing for the onset of deformation between plate motion inversions and seismic reflection based studies in the present study fault displacement data from seismic reflection profiles within the central indian ocean demonstrate that compressional activity started much earlier than previously thought at around 15 4 13 9 ma from reconstructions of fault activity histories we show that 12 of the total reverse fault population had been activated and 14 of the total strain accumulated prior to a sharp increase in the deformation rate at 8 0 7 5 ma there is no evidence for any regional unconformity before 8 0 7 5 ma early shortening was accommodated by activity on single isolated fault blocks total strain estimates derived are more variable and complex than those predicted from plate inversion and they do not show simple west to east increase

depth and age in the north pacific the concept of the oceanic lithosphere as a cooling thermal boundary layer on top of a viscous upper mantle accounts for the increase in depth with age of the ocean floor analyses of the exact nature of this relation provide constraints on the rheology and structure of this layer in past studies problems with uncertainties of age and in sediment thickness have limited the value of this relation on older ocean floor in this study we evaluate different tectonic histories for the quiet zones and construct a revised isochron chart for the north pacific we check and modify the current sediment thickness charts using new deep sea drilling data and revised ideas concerning reflectors in the southwestern north pacific averaging the synbaps digital data base into half by half degree elements and correcting for sediment load we plot basement depth versus age for the entire north pacific because of the skewness in the data associated with seamounts we present the data as contours about the mode and find that two thirds of the observations lie within 300 m of this value on crust between 0 and 80 ma the ocean floor subsides with the square root of age on ocean floor older than 100 ma the depths in general flatten with age with areally correlateable swells and valleys the hawaiian and line island swells correlate positively and the mariana basin valley negatively with geoid height the flattening in the depths indicates that the thermal boundary layer has reached equilibrium conditions under older ocean floor further the correlation of the swells and valleys with geoid height presents evidence for organized convection in the upper mantle beneath older lithosphere in the north pacific

resolution of the scripps noaa marine gravity field from satellite altimetry the july 1995 declassification of the entire geosat gm satellite altimeter data set enabled a joint scripps noaa effort to compute a new version 7 2 marine gravity field on a 2 minute grid this gravity field covers the world oceans between 72 n and 72 s and is derived from a combination of ers 1 and geosat gm and erm data an earlier noaa geosat only gravity field solution was confined to the southern latitudes because the 1992 declassification was limited to gm data south of 30 s a simple coherence analysis between accurately navigated ship gravity profiles and comparable gravity profiles obtained from the gravity grids reveals that the scripps noaa gravity field is coherent with ship gravity down to 23 30 km this slight increase in resolution over the previous noaa geosat only gravity field short wavelength resolution of 26 30 km implies that the increased spatial coverage provided by the ers i altimeter when combined with geosat improves the solution coherence analyses between satellite gravity and ship topography and ship gravity and ship topography show that even shorter wavelength gravity anomalies 13 km are present in sea surface measurements made by ship even so the scripps noaa marine gravity field does an excellent job of resolving most of the short wavelength gravity anomalies covering the world oceans

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

crustal evolution and sedimentation history of the bay of bengal since the cretaceous multichannel seismic reflection gravity magnetic and bathymetric investigations were made along five latitudinal profiles from the eastern shelf of india to the andaman shelf and four nw se profiles in the western basin off madras seismics on three latitudinal profiles in the bay of bengal the trend of the fracture zones the locations of the magnetic chron 34 and the cretaceous magnetic quiet zone suggest that greater india separated from antarctica after a period of transform motion in the early cretaceous that is about polarity chron m0 120 ma and drifted northwestward negative gravity anomalies are associated with basement rises including the 85 e ridge between the continental margin of india and the ninetyeast ridge magnetic reversals and northward trend of the 85 e ridge support a hotspot origin of the ridge and its emplacement most likely after the cretaceous quiet period juxtaposition of high amplitude hyperbolic reflections down faulted continental blocks buried under thick sediments and associated gravity and magnetic anomalies mark the boundary between continental and oceanic rocks at the foot of continental slope about 80 km seaward of the present continental shelf edge eight seismic sequences as thick as 8 5 km overlie the early cretaceous oceanic basement and include four unconformities lower eocene upper oligocene upper miocene and upper pleistocene which have been correlated to the major geologic tectonic events we also interpret the late cretaceous early tertiary features on the eastern flank of the buried 85 e ridge as carbonate reefs the observation of steep subduction of older cold indian plate beneath the burmese plate near the andaman islands suggests the sunda arc in this region as low to intermediate stress subduction zone

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

the tectonic origin of the bay of bengal and bangladesh we are able to decipher the tectonic evolution of the bay of bengal a puzzle which has not been satisfactorily solved in the past and we are also able to shed new light on origin of the buried 85 e ridge we do so by incorporating a number of disparate items into a unified solution these items are the marine magnetic anomalies in the western basin of the bay of bengal the rajmahal and sylhet traps and deep seismic sounding lines in india a prominent magnetic anomaly doublet and seismic seaward dipping reflectors in bangladesh and a new precise gravity map of the bay of bengal we identify seafloor spreading magnetic anomalies ranging in age from 132 ma m12n to 120 ma m0 in the western basin these anomalies are one sided the conjugate anomalies lie in the western enderby basin off east antarctica the direction of spreading was approximately nw se and the half spreading rates varied from 2 5 to 4 0 cm yr with the arrival of the kerguelen plume around m0 time seafloor spreading was reorganized and a new spreading axis opened at or close to the line joining the rajmahal and sylhet traps the prominent magnetic anomaly doublet connecting the rajmahal and sylhet traps indicates that these traps are not individual eruptions at about 118 ma but rather together define the new line of opening spreading started at this line and subsequently india changed direction from west to north the new oceanic crust thus generated underlies bangladesh and the eastern basin of the bay of bengal and is younger than 118 ma the western boundary of the new ocean floor is a transform fault which was generated by the spreading axis jump this transform fault appears as the 85 e ridge and further north on land as a negative free air gravity anomaly strip a unique feature of the northern boundary of the new oceanic crust is that due to the later deposition of enormous sediments derived from the himalayan orogeny it lies onshore bangladesh in contrast to most continent ocean boundaries in the world which lie offshore

tectonics of the ninetyeast ridge derived from spreading records in adjacent oceanic basins and age constraints of the ridge 1 analysis of new and existing geophysical data for the central indian and wharton basins of the indian ocean were used to understand the formation and evolution of the ninetyeast ridge ner especially its relationship to the kerguelen hot spot and the wharton spreading ridge satellite gravity data and magnetic anomalies 34 through 19 define crustal isochrons and show fracture zones striking n5 e one of these at 89 e crosses the n10 e trending ner impacting the ner morphology from 77 to 43 ma the ner lengthened at a rate of 118 km myr twice that of the 48 58 km myr accretion rate of adjacent oceanic crust this difference can be explained by southward jumps of the wharton spreading ridge toward the hot spot which transferred portions of crust from the antarctic plate to the indian plate lengthening the ner magnetic anomalies document a small number of large spreading ridge jumps in the ocean crust immediately to the west of the ner especially two leaving observable 65 and 42 ma fossil spreading ridges in contrast complex magnetic anomaly progressions and morphology imply that smaller spreading ridge jumps occurred at more frequent intervals beneath the ner comparison of the ner dates and magnetic anomaly ages implies that the hot spot first emplaced ner volcanoes on the indian plate at a distance from the wharton ridge but as the northward drifting spreading ridge approached the hot spot the two interacted keeping later ner volcanism near the spreading ridge crest by spreading center jumps

volcanism and vertical tectonics in the pacific basin related to global cretaceous transgressions abstract the dominance of volcanic processes and the importance of vertical tectonics in the geological evolution of the pacific basin has been recognised since the time of charles darwin data gathered on several legs of the deep sea drilling project dsdp and numerous marine expeditions in the past decade have confirmed menard s postulate that the pacific basin was the scene of volcanism on an enormous scale in mesozoic time widespread mid plate volcanism between 110 and 70 m y b p characterised the area bounded by the line islands the mid pacific mountains and the nauru basin marshall islands heating of the pacific lithospheric plate during this period of volcanism resulted in regional uplift and the bathymetric evolution of the area diverged significantly from a normal parsons sclater subsidence curve the farallon plate now almost entirely subducted was also the scene of mid plate volcanism that produced such features as the nicoya plateau now found as an allochthonous ophiolitic terrain landward of the middle america trench large benthonic reef associated foraminifera comprising a pseudorbitoid fauna hitherto considered to be largely restricted to central america have now been additionally recorded from dsdp sites 165 315 and 316 in the line islands site 462 in the nauru basin and in new guinea the distribution of this fauna of campanian maastrichtian age is interpreted as indicating stepping stone connections aseismic ridges plateaus and seamounts between the caribbean farallon and pacific plates 70 80 m y b p similarities between the geology of the nauru basin and the caribbean ocean crust reinforce the interpretation of the latter as a former part of the farallon plate estimates of the sea level and continental freeboard change caused by the thermally induced uplift of the pacific and farallon plates as well as substantial areas in the atlantic and indian ocean basins indicate that such shallowing of the sea floor could have been the major factor in causing global cretaceous transgressions

crustal architecture and deep structure of the ninetyeast ridge hotspot trail from active source ocean bottom seismology a 550 km long transect across the ninetyeast ridge a major indian ocean hotspot trail provided seismic refraction and wide angle reflection data recorded on 60 ocean bottom instruments about 24 000 crustal and 15 000 upper mantle arrivals have been picked and used to derive an image of the hotspot track two approaches have been chosen i a first arrival tomographic inversion yielding crustal properties and ii forward modelling of mantle phases revealing the structure at the crust mantle boundary region and of the uppermost mantle away from the volcanic edifice seismic recordings show the typical phases from oceanic crust that is two crustal refraction branches pg a wide angle reflection from the crust mantle boundary pmp and a wave group turning within the upper mantle pn approaching the edifice three additional phases have been detected we interpret these arrivals as a wide angle reflection from the base of material trapped under the pre hotspot crust pm2p and as a wide angle reflection pnp and its associated refraction branch pn from a layered upper mantle the resulting models indicate normal oceanic crust to the west and east of the edifice crustal thickness averages 6 5 7 km wide angle reflections from both the pre hotspot and the post hotspot crust mantle boundary suggest that the crust under the ridge has been bent downwards by loading the lithosphere and hotspot volcanism has underplated the pre existing crust with material characterized by seismic velocities intermediate between those of mafic lower crustal and ultramafic upper mantle rocks 7 5 7 6 km s 1 in total the crust is up to 24 km thick the ratio between the volume of subcrustal plutonism forming the underplate and extrusive and intrusive volcanism forming the edifice is about 0 7 an important observation is that underplating continued to the east under the wharton basin during the shield building phase however ninetyeast ridge was located adjacent to the broken ridge and was subsequently pulled apart along a transform fault boundary therefore underplating eastwards of the fracture zone separating the edifice from the wharton basin suggests that prolonged crustal growth by subcrustal plutonism occurred over millions of years after the major shield building stage this fact however requires mantle flow along the fossil hotspot trail the occurrence of pnp and pn arrivals is probably associated with a layered and anisotropic upper mantle due to the preferential alignment of olivine crystals and may have formed by rising plume material which spread away under the base of the lithosphere

extratropical influences on the inter annual variability of south asian monsoon the effects of extratropical dynamics on the interannual variations in south asian monsoon sam are examined based on ncep ncar reanalysis and cru precipitation data a conditional maximum covariance analysis is performed on sea level pressure 200 hpa geopotential heights and the sam rainfall by removing the linear effects of el nino southern oscillation from the fields it is found that two modes provide a strong connection between the upper level circulation in the atlantic european region and sam rainfall the circumglobal teleconnection cgt and the summer north atlantic oscillation snao the structures in the 200 hpa heights of both modes in the atlantic region are similar in the atlantic region and their southeastward extension to south asia sa also corresponds to upper level ridges in their positive phases in slightly different positions nevertheless the influence of both modes on sam rainfall is distinct whereas a positive cgt is related to a widespread increase of rainfall in sam a positive snao is related to a precipitation dipole with its positive phase over pakistan and the negative phase over northern india the physical mechanisms for the influence of cgt and snao on sam are related to the upper level geopotential anomaly which affects the amplitude and position of the low level convergence the small displacements of the centers of these responses and the low level cold advection from the north east of sa in case of snao explain the differences in the corresponding sam rainfall distributions these findings are confirmed with the relatively high resolution coupled climate model ec earth which gives confidence in the physical basis and robustness of these extratropical variability modes and their influence on the south asian monsoon rainfall

analysis of satellite gravity and bathymetry data over ninety east ridge variation in the compensation mechanism and implication for emplacement process 1 we investigate the mode of compensation emplacement history and deep density structure of the ninety east ridge indian ocean using spectral analyses and forward modeling of satellite gravity and bathymetry data we find that the northern 0 10 n and the southern 20 30 s parts of the ridge are flexurally compensated with an effective elastic thickness 15 km whereas the central part 0 20 s is locally compensated furthermore we find that for a part of central block 10 20 s over osborn knoll the compensation depth is unreasonably very high 30 40 km therefore we favor a model with subsurface loading and interpret this to be due to underplating of mafic material at the base of the crust a hypothesis that is supported by seismic results and direct modeling of gravity data along some profiles these results suggest that the northern and southern parts of ninety east ridge were emplaced off to a ridge axis compared to the central one which might have been emplaced on or near a spreading center locally compensated large topography thick underplated crust in the central part near osborn knoll might result from an interaction of a hot spot with the extinct wharton spreading ridge

tectonic model for the evolution of oceanic crust in the northeastern indian ocean from the late cretaceous to the early tertiary bathymetry and magnetic studies part of the trans indian ocean geotraverse investigations in the northeastern indian ocean revealed seafloor topographic features magnetic lineations 19 through 32b and abandoned spreading centers the seafloor topography of the ninetyeast ridge is relatively wider and shallower south of 15 s the magnetic anomalies indicate nine fracture zones two of them are newly identified some of the fracture zones are reflected in the bathymetry abandoned spreading centers between 86 e fracture zone fz and 92 e fz are interpreted as the western extensions of the wharton ridge they ceased spreading along with other spreading centers in the wharton basin soon after the formation of magnetic anomaly 19 around 42 ma and merged the indian and australian plates as single indo australian plate the pattern of magnetic lineations between 86 e fz and 90 e fz indicate a series of southerly ridge jumps at anomalies 30 26 royer et al 1991 and other workers and 19 these ridge jumps transferred portions of the antarctic plate to the indian plate the captured portions and offset along 86 e fz between india antartica ridge and wharton ridge resulted in an anomalous extra oceanic crust between 86 e fz and ninetyeast ridge spanning 11 in latitude

variability of the intertropical convergence zone recorded in coral isotopic records from the central indian ocean chagos archipelago we have analyzed the stable oxygen isotopic composition of two porites corals from the chagos archipelago which is situated in the geographical center of the indian ocean coral 18o at this site reliably records temporal variations in precipitation associated with the intertropical convergence zone itcz precipitation maxima occur in boreal winter when the itcz forms a narrow band across the indian ocean the chagos then lies within the center of the itcz and rainfall is strongly depleted in 18o a 120 yr coral isotopic record indicates an alternation of wet and dry intervals lasting 15 to 20 yr the most recent 2 decades are dominated by interannual variability which is tightly coupled to the el nino southern oscillation enso this is unprecedented in the 120 yr of coral record as the itcz is governed by atmospheric dynamics this provides evidence of a major change in the coupled enso monsoon system

high resolution residual geoid and gravity anomaly data of the northern indian ocean an input to geological understanding geoid data are more sensitive to density distributions deep within the earth thus the data are useful for studying the internal processes of the earth leading to formation of geological structures and activities in this paper we present much improved version of high resolution 1 1 geoid anomaly map of the northern indian ocean generated from the altimeter data obtained from geodetic missions of geosat and ers 1 along with ers 2 topex posideon and jason satellites the geoid map of the indian ocean is dominated by a significant low of 106 m south of sri lanka named as the indian ocean geoid low iogl whose origin is not clearly known yet the residual geoid data are retrieved from the geoid data by removing the long wavelength core mantle density effects using recent spherical harmonic coefficients of earth gravity model 2008 egm 2008 up to degree and order 50 from the observed geoid data the coefficients are smoothly rolled off between degrees 30 70 in order to avoid artifacts related to the sharp truncation at degree 50 with this process we observed significant improvement in the residual geoid data when compared to the previous low spatial resolution maps majumdar and bhattacharyya 2004 majumdar et al 2006 the previous version was superposed by systematic broad regional highs and lows like checker board with amplitude up to 12 m though the trends of geoid in general match in both versions these methodical artifacts in the previous version may have arisen due to the use of old rapp s geo potential model coefficients as well as sharp truncation of reference model at degree and order 50 geoid anomalies are converted to free air gravity anomalies and validated with cross over corrected ship borne gravity data of the arabian sea and bay of bengal the present satellite derived gravity data matches well with the ship borne data with root mean square error rmse of 5 17 8 mgal spectral analysis of ship borne and satellite data suggested that the satellite gravity data have a resolution down to 16 18 km further the geoid residual geoid and gravity anomalies are integrated with seismic data along two profiles in the bay of bengal and arabian sea and inferences have been made in terms of density distributions at different depths the new residual geoid anomaly map shows excellent correlation with regional tectonic features such as sunda subduction zone volcanic traces chagoslaccadive ninetyeast and 85 e ridges etc and mid ocean ridge systems central indian and carlsberg ridges

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

variation of the apparent compensation depth of hotspot swells with age of plate abstract most oceanic hotspot swells are associated with anomalously shallow seafloor and broad geoid anomalies these depth and geoid anomalies are generally positively correlated and linearly related the linear relationship can be used to infer an apparent depth of isostatic compensation of the swell we have analysed geoid and depth anomalies over 23 oceanic hotspot swells almost the majority of oceanic hotspots and deduced the apparent depth of compensations of these features the data indicate a linear increase of the compensation depth with the square root of seafloor age t on the order of 7 km m y 1 2 this trend is comparable to that predicted by conductive cooling models for the deepening of the isotherms inside the oceanic lithosphere

thin oceanic crust below bay of bengal inferred from 3 d gravity interpretation abstract the bay of bengal bob lithosphere evolved due to rifting between india and antarctica and subsequent seafloor spreading process since the early cretaceous is known for the presence of thick bengal fan sediments covering the entire bob with maximum sediment accumulation of 22 km in the offshore bangladesh three dimensional crustal structure of the bob is investigated by integrating the sediment isopach map seismic reflection refraction and wide angle reflection data with the 3 d gravity modelling large amount of sediment seismic velocities have been used to construct depth varying density contrast with respect to the underlying crustal layer due to deeper burial effect by a quadratic density function 3 d gravity effect of the sedimentary layer computed for the derived density distribution shows large variations ranging from 5 mgal over the ninetyeast ridge near the equator to as low as 210 mgal at the apex of bengal fan in the bangladesh shelf region the residual crustal bouguer anomaly map obtained after subtracting the effect of water and sediment layers and normalised for 6 km uniform crust display crustal thickness variations with depth to moho in the bob region varying from 11 km in the south to as much as 35 km in the bangladesh shelf with moho depressed locally below both 85 e and ninetyeast ridges the total crust map obtained after subtracting the water and sediment layers from the moho depth map shows an unusually thin oceanic crust of 2 4 km in the deep basinal areas of bob and 10 12 km of thick crust below 85 e and ninetyeast ridges due to the widespread nature of observed thin crust as well as the presence of plume traces that have emplaced 85 e and ninetyeast ridges we infer its formation during plume spreading interactions in the eastern indian ocean further higher flexural rigidity of lithosphere and cold mantle in the bob region indicates that huge amount of bengal fan sediments were emplaced over the unusually thin oceanic crust and a strong mantle similar to that observed in the amazon fan

seismic reflection images of intraplate deformation central indian ocean and their tectonic significance multichannel seismic reflection profiles collected from the intraplate deformation area in the central indian ocean basin are used to describe brittle structures produced under compressive stress reflectors within oceanic basement are divided into four types north dipping and south dipping reverse faults that are interpreted as reactivations of structures formed at the spreading centre as outward and inward dipping faults respectively lower angle north dipping reflectors that probably represent new faults or faults just initiating and sub horizontal reflectors within the uppermost crust that are interpreted as hydrothermal alteration fronts within the sedimentary cover upwards fault propagation shows the faults steepening from c 40 just above basement to near vertical and is preceded by sediment folding a fractal analysis of faulting suggests two fault populations possibly reflecting different criteria for brittle failure measurements of north south crustal shortening indicate a shortening rate of 2 5 0 9 mm a 1 which is at the lower end of predictions from plate motions but significant enough to recognize this area as a diffuse plate boundary the formation of long wavelength basement undulations and the reactivation of fracture zones and ridge parallel fault fabrics are linked in a unified tectonic model driven by the high level of intraplate compressive stress in the area there is little evidence from the seismic profiles for intraplate deformation starting before the widespread unconformity dated as 7 ma

temporal evolution of the kerguelen plume geochemical evidence from 38 to 82 ma lavas forming the ninetyeast ridge abstract basaltic basement has been recovered by deep sea drilling at seven sites on the linear ninetyeast ridge in the eastern indian ocean studies of the recovered lavas show that this ridge formed from 82 to 38 ma as a series of subaerial volcanoes that were created by the northward migration of the indian plate over a fixed magma source in the mantle the sr nd and pb isotopic ratios of lavas from the ninetyeast ridge range widely but they largely overlap with those of lavas from the kerguelen archipelago thereby confirming previous inferences that the kerguelen plume was an important magma source for the ninetyeast ridge particularly important are the 81 ma ninetyeast ridge lavas from dsdp site 216 which has an anomalous subsidence history coffin 1992 these lavas are feti rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic upper miocene lavas in the kerguelen archipelago the isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the kerguelen plume weis et al 1993a b despite the overlap in isotopic ratios there are important compositional differences between lavas erupted on the ninetyeast ridge and in the kerguelen archipelago the ninetyeast ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios such as la yb and zr nb which are intermediate between those of indian ocean morb mid ocean ridge basalt and the transitional to alkalic basalts erupted in the kerguelen archipelago these compositional differences reflect a much larger extent of melting for the ninetyeast ridge lavas and the proximity of the plume to a spreading ridge axis this tectonic setting contrasts with that of the recent alkalic lavas in the kerguelen archipelago which formed beneath the thick lithosphere of the kerguelen plateau from 82 to 38 ma there was no simple systematic temporal variation of sr nd and pb isotopic ratios in ninetyeast ridge lavas therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume although class et al 1993 propose that some of the isotopic variations reflect such aging we infer that most of the isotopic heterogeneity in lavas from the ninetyeast ridge and kerguelen archipelago can be explained by mixing of the kerguelen plume with a depleted morb like mantle component however with this interpretation some of the youngest 42 44 ma lavas from the southern ninetyeast ridge which have206pb 204pb ratios exceeding those in indian ocean morb and kerguelen archipelago lavas require a component with higher206pb 204pb such as that expressed in lavas from st paul island

crustal architecture and moho topography beneath the eastern indian and bangladesh margins new insights on rift evolution and the continent ocean boundary the thick sedimentary cover 18 km along the eastern continental margin of india ecmi and over the bay of bengal has hindered understanding of the mechanisms of rifting that operated on the eastern indian and bangladesh margins analysis of multichannel seismic reflection data together with 3d gravity inversion and 2d gravity forward modelling illuminates basement configuration crustal thickness and moho topography thereby revealing the modes of rift evolution and the location of the continent ocean boundary cob along these margins the basement particularly in the shelf and slope regions of eastern peninsular india is traversed by nearly coast perpendicular graben and their seaward continuity is delineated up to 125 km from the coastline three different types of continental margin are present along the ecmi 1 sheared rift on the southern segment of the ecmi up to 14 n latitude offshore the southern granulite terrain 2 hyper extended rift in the central segment between 14 n and 17 n offshore the dharwar craton and 3 hypo extended rift on the northern segment between 17 n and 20 n offshore the eastern ghats mobile belt the graben terminations crustal thickness and moho topography of both the ecmi and offshore bengal basin clearly suggest that the cob runs nearly parallel to the coastline of peninsular india but takes an orthogonal turn offshore in the bengal basin and connects the palaeo continental shelf and the rajmahal sylhet line onshore in the bengal basin crustal thickness 8 10 km and depth to the moho discontinuity 28 km beneath the bangladesh margin are unusual because the crustal thickness increased through interaction of the kerguelen plume with the existing oceanic lithosphere the moho deepened largely due to the load of the bengal fan sediments the absence of rifted crustal blocks on the bangladesh margin and the continuity of the cob into the onshore bengal basin together imply that the present bangladesh region was under marine conditions at least until the beginning of bengal fan sedimentation i e no less than 23 ma

structure mechanical properties and evolution of the lithosphere below the northwest continental margin of india the continental breakup history at the northwest continental margin of india remained conjectural due to lack of clearly discernable magnetic anomaly identifications and the presence of several enigmatic structural basement features whose structure was partly obscured by the late cretaceous deccan magmatic event in this study a detailed analysis of the existing seismic and seismological data covering both onshore and offshore areas of the northwest indian margin along with 3 d 2 d constrained potential field gravity magnetic and geoid modeling has been carried out the crustal structure and lithosphere asthenosphere boundary lab delineated across the margin provided valuable insights on the mechanism of continental extension an analysis of the residual geoid anomaly degree 10 map and the modeled lab below deccan volcanic province dvp revealed significant variation in upper mantle characteristics between the northwest nw and south central sc parts of dvp having thinner lithosphere in the nw part the depth to lab ranges 80 130 km at the margin with gradual thinning towards the western offshore having sharp gradient in the south sc part of dvp and gentle gradient in the north nw part of dvp the moho configuration obtained from seismically constrained 3 d gravity inversion reveals that moho depths vary 34 42 km below dvp and gradually thins to 16 20 km in the western offshore the effective elastic thickness te map computed through 3 d flexural modeling indicates that the te values are in general lower in the region and range 12 25 km such lower te values could be ascribed to the combined effect of the lithosphere stretching during gondwana fragmentation in the mesozoic and subsequent thermal influence of the reunion plume based on the crustal stretching factors te estimates and the modeled lithosphere geometry at the margin in this study we propose that the lithosphere below laxmi gop basin region 3 0 had undergone continuous stretching since india madagascar rifting 88 ma much prior to this event however this continuous stretching did not lead to breakup due to syn rift cooling the developed necking zone brittle ductile deformation got ceased and led to the development of a new necking zone between seychelles and laxmi ridge subsequent stretching between seychelles and the laxmi ridge contemporaneous with the deccan flood basalts eruption led to the seafloor spreading in the western basin anomaly c28n thus the laxmi ridge became a continental sliver

on the weakening relationship between the indian monsoon and enso analysis of the 140 year historical record suggests that the inverse relationship between the el nino southern oscillation enso and the indian summer monsoon weak monsoon arising from warm enso event has broken down in recent decades two possible reasons emerge from the analyses a southeastward shift in the walker circulation anomalies associated with enso events may lead to a reduced subsidence over the indian region thus favoring normal monsoon conditions additionally increased surface temperatures over eurasia in winter and spring which are a part of the midlatitude continental warming trend may favor the enhanced land ocean thermal gradient conducive to a strong monsoon these observations raise the possibility that the eurasian warming in recent decades helps to sustain the monsoon rainfall at a normal level despite strong enso events

crustal structure flexure and subsidence history of the hawaiian islands seismic reflection profile data have been used to estimate the long term 106 years mechanical properties of the oceanic lithosphere underlying the hawaiian islands the data show prominent reflectors associated with the top of the oceanic crust reflector 2 the m discontinuity and the base of the crust reflector 3 the two way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three dimensional elastic plate models a good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere te of 40 km the main discrepancies with the refraction data occur beneath oahu and molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of hawaii where it is too deep we attribute the thickened crust beneath oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of hawaii to crustal melting beneath the youngest part of the chain the effective te of 40 km is higher than expected for 80 m y oceanic lithosphere possibly due to the effects of subsurface loads that act at or near the base of the flexed crust according to free air gravity anomaly and bathymetry data te does not change significantly along the island chain retaining a similar value for hawaii as it does for oahu we have used this result to estimate the pattern of subsidence and uplift that would be expected along the hawaiian islands because of flexural loading of hawaii the observed depth of the haleakala wave cut terrace northeast of maui can be generally explained by the flexure model the main discrepancies between observed and the predicted depths occur northwest of hawaii where they suggest a somewhat higher value of te possibly due to incomplete adjustment of the lithosphere we suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle hotspot is accommodated by progressive flexing downstream causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge the progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the hawaiian islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat a progressive flexure model may also apply to the case of a continental plate moving over a mantle hotspot and could explain the observations of 1 a clinoform pattern of stratigraphy in some flood basalt provinces and 2 a seaward dipping reflector sequence at some passive continental margins

decreasing monsoon precipitation in southwest china during the last 240 years associated with the warming of tropical ocean based on an absolutely dated stalagmite 18o record from yunnan province china we reconstructed monsoon precipitation variations in southwest china since 1760 ad with a resolution of about 2 years combining the speleothem 18o and observed rainfall records we find an overall decreasing trend in monsoon precipitation in this region and suggest that the recent drought in 2009 2012 ad has been the driest since 1760 ad our speleothem record is consistent with the monsoon precipitation records reconstructed from tree rings in the nepal himalaya and southeastern tibetan plateau however it is anti correlated with a speleothem record from central india which confirms the observed anti phase variations of indian monsoon precipitation with moistures from the bay of bengal and arabian sea on multi decadal to centennial timescales during historical time the long term warming of tropical ocean may have caused the decrease of the land sea thermal gradient and the amount of moisture transported from the bay of bengal which may reduce precipitations in southwest china during the last 240 years on decadal scale el nino like conditions of tropical pacific sea surface temperature may cause drought in this region climate model simulations suggest el nino like conditions exist in tropical pacific under global warming scenarios as a result it is crucial to have adaptive strategies to overcome future declines in precipitation and or drought events in southwest china

cretaceous volcanism and jurassic magnetic anomalies in the nauru basin western pacific ocean the nauru basin contains a well documented pattern of late jurassic magnetic anomalies and a cretaceous volcanic complex that has not disturbed the older magnetic structure the following hypotheses are offered to explain this paradox the cretaceous basalts are normally and uniformly magnetized making them invisible to a surface towed magnetometer tension cracks and fractures associated with thermal uplift and or magma wedging provided pathways for the cretaceous magma through the jurassic basement without disrupting the jurassic magnetic structure the same tension cracks admitted sea water that convectively removed heat from the magnetic layer and thus maintained the jurassic basement below its magnetic blocking temperature for extended periods of time during intrusive thermal episodes finally the maximum uplift associated with cretaceous volcanism in the nauru basin could have been equal to but was probably something less than that associated with nearby seamount and atoll chains

magma production rate along the ninetyeast ridge and its relationship to indian plate motion and kerguelen hot spot activity the ninetyeast ridge a linear trace of the kerguelen hot spot in the indian ocean was emplaced on a rapidly drifting indian plate magma production rates along the ridge track are computed using gravity derived excess crustal thickness data the production rates change between 2 and 15 m3 s over timescales of 3 16 myr major variations in magma production rates are primarily associated with significant changes in the indian plate velocity with low production phases linked to high plate velocity periods the lowest magma production rate 2 m3 s at 62 ma is associated with the rapid northward drift of indian plate under the influence of the reunion mantle plume the contemporaneous slowing of the african plate coincides with increase in magma production rate along the walvis ridge in the atlantic ocean the present study suggests that variations in the indian plate motion and frequent ridge jumps have a major role in controlling the magma production particularly on long period cycles 16 myr short period variations 5 myr in magma productions may be associated with intrinsic changes in the plume possibly due to the presence of solitary waves in the plume conduit

global gravity bathymetry and the distribution of submarine volcanism through space and time 1 the seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin relatively few of these features 0 1 however have been dated and so little is known about their tectonic setting one parameter that is sensitive to whether a seamount formed on near or far from a mid ocean ridge is the elastic thickness te which is a proxy for the long term strength of the lithosphere most previous studies are based on using the bathymetry to calculate the gravity anomaly for different values of te and then comparing the calculated and observed gravity anomaly the problem with such an approach is that bathymetry data are usually limited to single beam echo sounder data acquired along a ship track and these data are too sparse to define seamount shape we therefore use the satellite derived gravity anomaly to predict the bathymetry for different values of te by comparing the predicted bathymetry to actual shipboard soundings in the vicinity of each locality in the wessel global seamount database we have obtained 9758 te estimates from a wide range of submarine volcanic features in the pacific indian and atlantic oceans comparisons where there are previous estimates show that bathymetric prediction is a robust way to estimate te and its upper and lower bounds te at sites where there is both a sample and crustal age show considerable scatter however and there is no simple relationship between te and age nevertheless we are able to tentatively assign a tectonic setting to each te estimate the most striking results are in the pacific ocean where a broad swath of on ridge volcanism extends from the foundation seamounts and ducie island easter island ridge in the southeast across the equator to the shatsky and hess rises in the northwest interspersed among the on ridge volcanism are flank ridge and off ridge features the indian and atlantic oceans also show a mix of tectonic settings off ridge volcanism dominates in the eastern north atlantic and northeast indian oceans while flank ridge volcanism dominates the northeastern indian and western south atlantic oceans we have been unable to assign the flank ridge and off ridge estimates an age but the on ridge estimates generally reflect we believe the age of the underlying oceanic crust we estimate the volume of on ridge volcanism to be 1 1 106 km3 which implies a mean seamount addition rate of 0 007 km3 yr 1 rates appear to have varied through geological time reaching their peak during the late early cretaceous and then declining to the present day

large fault fabric of the ninetyeast ridge implies near spreading ridge formation 1 ninetyeast ridge ner is a linear volcanic ridge in the indian ocean thought to have formed by hotspot volcanism on the northward drifting indian plate geological data from the ridge are sparse so its tectonic evolution is poorly known we studied satellite derived gravity data seismic reflection profiles and multibeam bathymetry to examine ner structure gravity data show that the ridge displays a series of nearly e w trending lineations with average spacing 0 4 45 km in seismic and bathymetry data these lineations correlate with horsts and grabens that probably formed near the time of ridge emplacement from their extensional nature and trends we infer that these faulted structures formed near the spreading ridge that separated the indian and antarctic plates and their ubiquity implies the hotspot was never far from this spreading ridge

crustal underplating and its implications for subsidence and state of isostasy along the ninetyeast ridge hotspot trail recent seismic field work has revealed high lower crustal velocities under ninetyeast ridge indian ocean indicating the presence of crustal underplating grevemeyer et al 2000 we used results from ocean drilling program odp drill cores and cross spectral analysis of gravity and bathymetric data to study the impact of the underplating body on the subsidence history and the mode of isostatic compensation along ninetyeast ridge compared with the adjacent indian basin the subsidence of ninetyeast ridge is profoundly anomalous within the first few millions of years after crustal emplacement the ridge subsided rapidly thereafter however subsidence slowed down significantly the most reliable model of isostasy suggests loading of a thin elastic plate on and beneath the seafloor isostatic compensation of subsurface loading occurs at a depth of about 25km which is in reasonably good agreement with seismic constraints subsurface loading is inherently associated with buoyant forces acting on the lithosphere the low subsidence may therefore be the superposition of cooling of the lithosphere and uplift due to buoyant material added at the base of the crust a model including prolonged crustal growth in the form of subcrustal plutonism may account for all observations

extremely thin crust in the indian ocean possibly resulting from plume ridge interaction the thickness of the crust created at ocean spreading centres depends on the spreading rate and melt production in the mantle it is 5 8 km for a crust formed at slow and fast spreading centres and 2 4 km at ultra slow spreading centres away from hotspots and mantle anomalies the crust is generally thin at the fracture zones and thick beneath hotspots and large igneous provinces here we present results for the crust generated at the fast wharton spreading centre 55 58 ma ago using seismic reflection and refraction data we find that the crust over a 200 km segment of the wharton basin is only 3 5 4 5 km thick the thinnest crust ever observed in a fast spreading environment a thin crust could be produced by the presence of depleted and or cold mantle numerical simulations and recent laboratory experiments studying the impact of a hot plume under a lithosphere show that a curtain of weak cold downwellings of depleted mantle material is likely to develop around the edges of the hot plume pond because of a strongly temperature dependent viscosity of lithospheric material the hotter therefore less viscous bottom of the lithosphere can be mobilized by an impinging plume if sampled by a spreading centre the locally cold and depleted mantle should result in low production of melt we suggest that the observed thin crust in the wharton basin is likely to have been formed by the interaction between the kerguelen mantle plume and the wharton spreading centre 55 ma ago

evolution of the late cretaceous crust in the equatorial region of the northern indian ocean and its implication in understanding the plate kinematics summary analysis of 3100 km of newly acquired marine magnetic data constrained by satellite and shipborne free air gravity anomalies in the corridor between the 86 e fracture zone and ninetyeast ridge north of the equator reveals the evolutionary history of the late cretaceous crust characterised by anomaly 34 through 31 83 5 68 7 ma under complex tectonic settings seafloor spreading model studies suggest that the crust particularly between the chrons 33r and 33 79 0 73 6 ma was formed with variable and slightly higher half spreading rates 4 8 7 1 cm yr than the crust of similar age either in the regions west of 86 e fracture zone east of the ninetyeast ridge or the southern crozet basin further the interpretation of magnetic anomalies suggests the presence of fossil spreading ridge segments and extra oceanic crust on the indian plate that has been transferred from the antarctica plate by discrete southward ridge jumps these ridge jumps are caused by thermal instability of the spreading center as the indian plate moved northward over the kerguelen mantle plume the present study indicates that the spreading ridge plume interaction is the prime mechanism for these ridge jumps which have occurred since 75 8 ma the newly identified magnetic anomalies 34 through 31 and the inferred n3 e trending fracture zones refined the plate reconstruction models for that period

geoid height versus topography for oceanic plateaus and swells oceanic plateaus and swells are a major component of the seafloor topography yet they remain among the most poorly understood features this is especially true of the oceanic plateaus which show large variations in crustal thickness to determine the depth and mode of compensation for 53 of the largest plateaus and swells we analyzed the relationship between geoid height and topography in polygonal areas containing each feature both geoid height and topography were first band pass filtered 400 km i 4000 km to isolate the signal associated with local compensation from flexural and deep mantle signals the ratio of geoid height to topography was then determined by fitting a straight line to the data except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with airy and thermal compensation models eighteen features have high geoid topography ratios that cannot be explained by the airy compensation model of crustal thickening these features thermal swells are partially supported by thermal buoyancy forces in the lower half of the lithosphere the ratios are highest for active hot spot swells and decay with the thermal age of the swell to values consistent with airy compensation of the enduring volcanic edifice the remaining features plateaus have lower geoid topography ratios in agreement with the airy compensation model those plateaus with average height greater than 4 km are thought to be continental fragments the shorter plateaus tend to be volcanic features modified continental plateaus presumably small fragments of extended and intruded continental margin crust cluster around heights of n3 km overlapping the range associated with oceanic plateaus since the origin of many plateaus is poorly understood this global geoid topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells

sediment volume and mass beneath the bay of bengal rates of sediment accumulation and the amount of sedimentary fill in depocenters lying downstream of erosion in the himalayas and tibet can provide some insight into tectonics and geological history the objective of this paper is to put on record the best estimates which are possible with existing data of the volume and mass of sediments sedimentary rock and metasedimentary rock beneath the sea floor of the bay of bengal the sedimentary section in the bay of bengal is divided into two parts 1 eocene through holocene sediments and sedimentary rocks which post date the initial india asia collision volume 12 5 106 km3 mass 2 88 1016 t this is most of the bengal fan including its eastern lobe the nicobar fan plus some of the outer bengal delta 2 early cretaceous through paleocene pre collision sedimentary and metasedimentary rocks volume 4 36 106 km3 mass 1 13 to 1 18 1016 t these are interpreted as continental rise and pelagic deposits

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

on the interpretation of frequency response functions for oceanic gravity and bathymetry summary oceanic bathymetry and gravity comprise a geophysical system which is often characterized by a linear frequency response function or admittance the actual behaviour of this system differs in important respects however from that of an ideal linear model three sources of such differences are considered in this study the presence of noise in the data non linearity due to finite amplitude and intrinsic two dimensionality of the data fields the high positive correlation between admittance amplitude and coherence estimates obtained in a number of studies indicates that the noise is not normally distributed with zero mean as generally assumed non linear modification of the admittance results from the finite amplitude of the bathymetry and the non linear elasticity of the lithosphere the former effect increases with bathymetric roughness and is most pronounced for shortwavelength uncompensated bathymetry the latter is significant only if the deflection of the lithosphere is comparable to its elastic thickness as for certain large seamounts in the pacific ocean which were emplaced on lithosphere near ridge crests the most severe problem arises for features which are markedly two dimensional features typical of observed oceanic seamounts are characterized by admittances along tracks which differ substantially from those appropriate to one dimensional bathymetry this difference occurs primarily in the long wavelength portion of the admittance rendering problematic the comparison of observed admittances or gravity anomalies with theoretical expressions

active faulting on the ninetyeast ridge and its relation to deformation of the indo australian plate 1 the 4500 km long ninetyeast ridge ner in the northeastern indian ocean crosses a broad zone of deformation where the indo australian plate is fracturing into three smaller plates india capricorn australia separated by diffuse boundaries whose extents are poorly defined new multichannel seismic reflection profiles image active faults along the entire length of the ner and show spatial changes in the style of deformation along the ridge the northern ner 0 n 5 n displays transpressional motion along wnw ese faults observed fault patterns confirm strike slip motion at the western extent of the april 2012 wharton basin earthquake swarm in the central ner 5 s 8 s deformation on wnw ese trending thrust faults implies nearly n s compression an abrupt change in fault style occurs between 8 and 11 s with modest extension characterizing the southern ner 11 s 27 s although extension is dominant narrow zones of faults with strike slip or compressional character also occur in the southern ner suggesting a complex combination of fault motions at all sites active faulting is controlled by the reactivation of original spreading center formed normal faults implying that deformation is opportunistic and focused along existing zones of weakness even when original fault trend is oblique to the direction of relative plate motion observed faulting can be interpreted as india australia deformation in the northern ner and capricorn australia deformation in the southern ner the india capricorn boundary is directly adjacent to the northern ner and this juxtaposition combined with a different style of faulting to the east of the ner imply that the ridge is a tectonic boundary

increased eurasian tropical temperature amplitude difference in recent centuries implications for the asian monsoon 1 a warmer eurasian continent and stronger land sea temperature gradient between eurasia and the tropical oceans contribute to an intensified summer monsoon we evaluate changes in the temperature difference between eurasia and the tropics over 250 years using proxies of eurasian surface air temperatures tree rings and tropical sea surface temperatures indo pacific corals these records show low frequency correspondence with each other over this interval and with other temperature and precipitation sensitive proxies from the asian monsoon regime greater warming is estimated for eurasia amplitude 1 70 0 28 c 1801 20 vs 1976 95 relative to the tropics 0 61 0 29 c 1806 25 vs 1937 56 the amplitude change from the 18th to 20th centuries is thus estimated to be about three times 1 5 6 greater over eurasia than the tropics this change may have contributed to an intensified asian monsoon system over recent centuries and to a decoupling of the monsoon and el nino southern oscillation in recent decades

lacustrine record from the eastern tibetan plateau associated with asian summer monsoon changes over the past 6 ka and its links with solar and enso activity a multi parameter absolute dated lacustrine record from the eastern tibetan plateau provides a continuous history of the asian summer monsoon asm over the past 6000 years the record broadly follows the trend in northern hemisphere summer insolation however it diverges during a period of increased moisture and thermal conditions during the past 1300 years broadly corresponding to the asm 2 kyr shift this shift as referred by cheng et al nature 534 640 646 10 1038 nature18591 2016 may be related to atlantic meridional overturning circulation variations and potentially to increased forcing by greenhouse gases coincident with reduced enso activity the record is punctuated by several weak monsoon intervals cross correlation of the sub millennial scale monsoon record with solar activity and enso records shows that most of the monsoon variability potentially results from changes in solar irradiation and has a strong teleconnection with enso activity

seismic stratigraphy and sedimentation history of the east mariana basin western pacific abstract an interpretation of the seismic stratigraphy and sedimentation history of the east mariana basin has been made using recently collected seismic reflection and refraction data this mesozoic age basin between the marshall islands and the mariana trench is subdivided into three regions the central region with about 1000 m of sediment probably records jurassic to late cretaceous sedimentation of a pelagic biogenic and clay rich section overlain by a thick section of mainly cenozoic carbonates shed from nearby volcanic platforms a western region is characterized by a thinner sediment cover and a shallower acoustic basement with a similar sedimentation history except that the upper section is thinner as a consequence of fewer nearby volcanic highs extensive late cretaceous mid plate volcanics apparently masks the lower section and forms acoustic basement the shallower eastern region east of 157 5 e contains wnw trending ridges which may be either fracture zones or high amplitude abyssal hills the sedimentation appears controlled by the same factors as in the other regions but the area was bypassed by most cenozoic basin filling turbidites because of its elevation the isostatistically corrected basement depths between the three regions suggest that the crust in the east may be substantially younger than in the rest of the east mariana basin perhaps cretaceous in age this requires the existence of a tectonic boundary within the basin

late cretaceous and eocene volcanism in the southern line islands and implications for hotspot theory rocks dredged from a seamount 100 km northwest of caroline island at the southern end of the line islands chain contain late cretaceous fossils associated with volcanic debris this association is evidence for the existence of a reef bearing volcanic edifice with a minimum age of late cretaceous 70 to 75 m y near caroline island with the discovery of this seamount the known occurrences of late cretaceous reef capped volcanic edifices now extend a distance of 2 500 km from deep sea drilling project site 165 to 100 km northwest of caroline island the apparent synchroneity of late cretaceous volcanism over this distance argues against the proposition that a single hotspot of the hawaiian emperor type produced the line islands chain biochronologic data from the line islands indicate that the chain is not the temporal equivalent of the emperor chain volcanic edifices of cretaceous age are now known to extend from the line islands through the mid pacific mountains to the marshall islands and the western margin of the pacific plate from japan to the marianas a volcanic event occurred in the southern line islands during middle eocene time eocene sediments were engulfed and altered by a volcanic eruption the occurrence of both cretaceous and eocene volcanism in the southern line islands indicates that the history of the line islands is similar to that of the marshall islands

spatial variations in isostatic compensation mechanisms of the ninetyeast ridge and their tectonic significance the ninetyeast ridge ner one of the longest linear volcanic features on the earth offers an excellent opportunity of understanding the isostatic response to the interactions of mantle plume with the migrating mid ocean ridge bathymetry geoid and gravity ship borne and satellite data along 72 closely spaced transects and 17 overlapping grids on the ner are analyzed and modeled to determine the effective elastic thickness te beneath the entire ridge the results of 2 d and 3 d flexural modeling of the ner show large spatial variations in te values ranging from 4 to 35 km suggesting that the ridge was compensated along its length by different isostatic mechanisms the southern south of 22 s latitude and northern north of 2 n latitude parts of the ner have te values of 10 and 23 km respectively revealing that the southern part was emplaced on a lithosphere of intermediate strength possibly on flank of the indian plate whereas the northern part was emplaced in an intraplate setting in contrast in the central part of the ner between latitudes 22 s and 2 n highly variable te values 4 22 km are estimated the scattered te values in the central ner suggest that this part may have evolved due to the occurrence of more frequent ridge jumps caused by the interaction of kerguelen hot spot with rapid northward migration of the wharton spreading ridge residual mantle bouguer anomaly rmba map of the ner and adjacent basins reveals that the entire length of the ner is associated with a significant negative anomaly up to 200 mgal indicating the presence of thickened crust or less dense mantle beneath the ridge 3 d crustal thickness map of the ner generated by inversion of the rmba data shows a thick crust ranging from 15 to 19 km the present study clearly shows that ner possesses a highly segmented isostatic pattern with the occurrence of sub crustal underplating or sub surface loading

lithosphere structure and upper mantle characteristics below the bay of bengal the oceanic lithosphere in the bay of bengal bob formed 80 120 ma following the breakup of eastern gondwanaland since its formation it has been affected by the emplacement of two long n s trending linear aseismic ridges 85oe and ninetyeast and by the loading of ca 20 km of sediments of the bengal fan here we present the results of a combined spatial and spectral domain analysis of residual geoid bathymetry and gravity data constrained by seismic reflection and refraction data self consistent geoid and gravity modeling defined by temperature dependent mantle densities along a n s transect in the bob region revealed that the depth to the lithosphere asthenosphere boundary lab deepens steeply from 77 km in the south to 127 km in north with the greater thickness being anomalously thick compared to the lithosphere of similar age beneath the pacific ocean the geoid topography ratio gtr analysis of the 85 e and ninetyeast ridges indicate that they are compensated at shallow depths effective elastic thickness te estimates obtained through admittance coherence analysis as well as the flexural modeling along these ridges led to the conclusions i 85 e ridge was emplaced in off ridge environment te 10 15 km ii the higher te values of 25 km over the afanasy nikitin seamount ans reflect the secondary emplacement of the seamount peaks in off ridge environment iii that the emplacement of the ninetyeast ridge north of 2 n occurred in an off ridge environment as indicated by higher te values 25 30 km furthermore the admittance analysis of geoid and bathymetry revealed that the admittance signatures at wavelengths 800 km are compensated by processes related to upper mantle convection

lithospheric structure and compensation mechanisms of the galapagos archipelago volcanic islands of the galapagos archipelago are the most recent subaerial expression of the galapagos hotspot these islands and numerous seamounts are constructed mainly upon a broad volcanic platform that overlies very young 10 m y oceanic lithosphere just south of the active galapagos spreading center the 91 w fracture zone crosses the platform and creates an estimated 5 m y age discontinuity in the lithosphere major tectonic features of the galapagos include an unusually broad distribution of volcanic centers pronounced structural trends such as the nw se wolf darwin lineament wdl and a steep escarpment along the western and southern margins of the archipelago we use shipboard gravity and bathymetry data along with geosat geoid data to explain the tectonic and structural evolution of the galapagos region we model the gravity anomalies using a variety of compensation models including airy isostasy continuous elastic flexure of the lithosphere and an elastic plate with embedded weaknesses and we infer significant lithospheric strength variations across the archipelago the outboard parts of the southern and western escarpment are flexurally supported with an effective elastic thickness of 12 km this area includes the large shield volcanoes of fenandina and isabela islands where the lithosphere regionally supports these volcanic loads the central platform is weaker with an elastic thickness of 6 km or less and close to airy isostasy the greatest depths to the moho are located beneath eastern isabela island and the central platform thinner lithosphere in this region may account for the broad distribution of volcanoes the extended period of eruption of the central volcanoes and their reduced size the transition from strong to weak lithosphere along the southern escarpment appears to be abrupt within the resolution of our models and can be best represented by a free end or faultlike discontinuity also modeling the wdl as a lithospheric fault increases the match to the observed gravity anomalies the primary feature is the weak central platform whose existence is a logical convergence of several factors the 91 w transform age offset the possible increase in age of the lithosphere along the southern part of the platform due to ridge jumping and the formation of the central platform at a spreading ridge

evidence for multiphase folding of the central indian ocean lithosphere long wavelength 100 300 km folding in the central indian ocean associated with the diffuse plate boundary separating the indian australian and capricorn plates is earth s most convincing example of organized large scale lithospheric deformation to test the timing and mechanics of this deformation as implied by plate kinematic and deformation models we present a new analysis of the seismic stratigraphy of the bengal fan sediments this analysis shows that the folding of the oceanic lithosphere was multiphase with major events occurring in the miocene 8 0 7 5 ma pliocene 5 0 4 0 ma and pleistocene 0 8 ma the miocene phase was the most intense and involved deformation of an area south of 1 s whereas in the pliocene the activity shifted northward in the final phase pleistocene the activity was focused in the equatorial region no evidence was found for deformation prior to 8 0 7 5 ma the spatial extent of the pleistocene folding event overlaps the pliocene and or miocene folding events and coincides with both the area of most active faulting and the zone of greatest historical seismicity the seismic data show that the timing of reverse faulting and thus more significant shortening of the lithosphere generally coincided with the phases of folding but there are examples of folding of the oceanic lithosphere without associated reverse faulting

rough versus smooth topography along oceanic hotspot tracks observations and scaling analysis some hotspot tracks are topographically smooth and broad nazca carnegie cocos galapagos walvis iceland while others are rough and discontinuous easter sala y gomez tristan gough louisville st helena hawaiian emperor smooth topography occurs when the lithospheric age at emplacement is young favoring intrusive magmatism whereas rough topography is due to isolated volcanic edifices constructed on older thicker lithosphere the main controls on the balance of intrusive versus extrusive magmatism are expected to be the hotspot swell volume flux qs plate hotspot relative speed v and lithospheric elastic thickness te which can be combined as a dimensionless parameter r qs v 1 2 te which represents the ratio of plume heat to the lithospheric heat capacity observational constraints show that except for the ninetyeast ridge r is a good predictor of topographic character for r 3 they are smooth and dominated by intrusion

crustal underplating and effective elastic plate thickness of the laxmi ridge northern arabian sea abstract laxmi ridge a prominent basement high feature with negative gravity anomaly in the northern arabian sea is generally considered as stretched continental crust however recently it is proposed that the ridge could be an extinct spreading ridge ship borne and satellite altimetry derived bathymetry gravity and geoid data over the laxmi ridge are analysed and modelled to determine effective elastic plate thickness te and crustal structure beneath the ridge the estimation of te and compensation mechanism of the ridge are examined based on 3d coherence and admittance analysis considering surface and subsurface loads which involve computation of coherence and admittance between mantle bouguer anomaly mba and sediment corrected bathymetry the best fit model reveals that te is about 3 4 km and amplitude of subsurface load at moho is approximately equal to the surface load low value of te implies elastically weak lithospheric plate which can be attributed to either weakening of lithosphere by re heating and underplating caused by the then nearby reunion hotspot or volcanic emplacement on the young oceanic crust further all along the length of the laxmi ridge the mba is negative and its magnitude reaches up to 115 mgal which indicates either presence of thickened crust or less dense mantle beneath the ridge 2d crustal models suggest that underplating material is prevalent all along the laxmi ridge high amplitude 400 nt magnetic anomaly over the southern part of the ridge reveals several lineations in wnw ese direction suggesting magma outpouring along week zones geoid to topographic ratio over the ridge is negative 1 98 m km possibly indicating shallow subsurface load

the asian australian monsoon and el nino southern oscillation in the ncar climate system model features associated with the asian australian monsoon system and el nino southern oscillation enso are described in the national center for atmospheric research ncar global coupled climate system model csm simulation characteristics are compared with a version of the atmospheric component of the csm the ncar ccm3 run with time evolving ssts from 1950 to 1994 and with observations the csm is shown to represent most major features of the monsoon system in terms of mean climatology interannual variability and connections to the tropical pacific this includes a representation of the southern oscillation links between strong asian australian monsoons and associated negative sst anomalies in the eastern equatorial pacific the equatorial sst gradient across the pacific in the csm is shown to be similar to the observed with somewhat cooler mean ssts across the entire pacific by about 18 28c the seasonal cycle of ssts in the eastern equatorial pacific has the characteristic signature seen in the observations of relatively warmer ssts propagating westward in the first half of the year followed by the reestablishment of the cold tongue with relatively colder ssts propagating westward in the second half of the year like other global coupled models the propagation is similar to the observed but with the establishment of the relatively warmer water in the first half of the year occurring about 1 2 months later than observed the seasonal cycle of precipitation in the tropical eastern pacific is also similar to other global coupled models in that there is a tendency for a stronger than observed double itcz year round particularly in northern spring but with a well reproduced annual maximum of itcz strength north of the equator in the second half of the year time series of area averaged ssts for the nino3 region in the eastern equatorial pacific show that the csm is producing about 60 of the amplitude of the observed variability in that region consistent with most other global coupled models global correlations between nino3 time series global surface temperatures and sea level pressure slp show that the csm qualitatively reproduces the major spatial patterns associated with the southern oscillation lower slp in the central and eastern tropical pacific when nino3 ssts are relatively warmer and higher slp over the far western pacific and indian oceans with colder water in the northwest and southwest pacific indices of asian australian monsoon strength are negatively correlated with nino3 ssts as in the observations spectra of time series of indian monsoon australian monsoon and nino3 sst indices from the csm show amplitude peaks in the southern oscillation and tropospheric biennial oscillation frequencies 3 6 yr and about 2 3 yr respectively as observed lag correlations between the nino3 sst index and upper ocean heat content along the equator show eastward propagation of heat content anomalies with a phase speed of about 0 3 m s21 compared to observed values of roughly 0 2 m s21 composites of el nino la nina events in the csm show similar seasonal evolution to composites of observed events with warming cooling of greater than several tenths of a degree beginning early in northern spring of year 0 and diminishing around northern spring of year 11 but with a secondary resurgence in the csm events later in northern spring of year 11 the csm also shows the largest amplitude enso sst and low level wind anomalies in the western tropical pacific with enhanced interannual variability of ssts extending northeastward and southeastward toward the subtropics compared to largest interannual sst variability in the central and eastern tropical pacific in the observations