role of seasonal transitions and the westerlies in the interannual variability of the east asian summer monsoon precipitation a recent hypothesis holds that changes to the east asian summer rainfall are characterized by changes in the timing and duration of its intraseasonal stages controlled by the meridional position of the westerlies relative to the tibetan plateau this hypothesis is examined in the context of the leading mode of east asian summer july aug rainfall one phase of this tripole mode characterized by less rainfall over central eastern china and increased rainfall over northeastern and southeastern china is tied to an earlier termination of meiyu that results in a significantly shorter meiyu and longer midsummer stage this phase also exhibits an earlier northward transition of the westerlies to the north of the plateau essentially mirroring the changes to precipitation seasonality the reverse does not hold true for the opposite phase our results show direct observational evidence for the meridional position of the westerlies to control east asian summer monsoon seasonality

ocean crust formation processes at very slow spreading centers a model for the mohns ridge near 72 n based on magnetic gravity and seismic data the mohns ridge in the norwegian greenland sea is one of the slowest spreading centers of the mid ocean ridge system 8 mm yr half rate sea beam data acquired with r v jean charcot near 72 n show that its rift valley floor is characterized by en echelon volcanic ridges oriented obliquely relatively to the average strike of the ridge axis these ridges are regularly spaced along the axis about every 40 km and are separated by nontransform discontinuities sharp positive magnetic anomalies centered over the topographic highs suggest that they are eruptive centers considered as the surficial expression of active spreading cells over the rift valley bouguer anomalies obtained by subtracting the predicted effects due to seafloor topography from the measured free air gravity field are consistent with a low density body within the lower crust having its upper surface lying at about 2 km below the sea surface this body if it exists probably corresponds to the zone of low viscosity that can be inferred from the model of chen and morgan 1990b which predicts the existence of a decoupling region between the upper crust and the asthenophere below its width varies rapidly along strike from less than about 5 km to more than 15 km in plan view it has a pinch and swell form which defines a series of spreading cells the center of one cell being where the bouguer anomaly is widest short wavelength less than 10 to 20 km along strike variations such as bouguer anomaly lows centered on the topographic highs reflect local effects associated with the presence of the eruptive centers seismic tomography data from a 20 10 km active oblique volcanic ridge near 72 22 n tend to indicate that the links between the main low velocity body at depth and the magma injections centers which lie within the rift valley inner floor are probably complex

orographic effects of the tibetan plateau on the east asian summer monsoon an energetic perspective this paper investigates the dynamical processes through which the tibetan plateau tp influences the east asian summer monsoon easm within the framework of the moist static energy mse budget using both observations and atmospheric general circulation model agcm simulations the focus is on the most prominent feature of the easm the so called meiyu baiu mb which is characterized by a well defined southwest northeast elongated quasi stationary rainfall band spanning from eastern china to japan and into the northwestern pacific ocean between mid june and mid july observational analyses of the mse budget of the mb front indicate that horizontal advection of moist enthalpy and primarily of dry enthalpy sustains the front in a region of otherwise negative net energy input into the atmospheric column a decomposition of the horizontal dry enthalpy advection into mean transient and stationary eddy fluxes identifies the longitudinal thermal gradient due to zonal asymmetries and the meridional stationary eddy velocity as the most influential factors determining the pattern of horizontal moist enthalpy advection numerical simulations in which the tp is either retained or removed show that the tp influences the stationary enthalpy flux and hence the mb front primarily by changing the meridional stationary eddy velocity with reinforced southerly wind over the mb region and northerly wind to its north changes in the longitudinal thermal gradient are mainly confined to the near downstream of the tp with the resulting changes in zonal warm air advection having a lesser impact on the rainfall in the extended mb region

a different pattern of ridge segmentation and mantle bouguer gravity anomalies along the ultra slow spreading southwest indian ridge 15 30 e to 25 e abstract the results of a recent bathymetric and geophysical investigation of a 650 km long portion of the very slowly opening 16 mm yr full rate southwest indian ridge swir between 15 30 e and 25 e are presented bathymetry and mantle bouguer gravity anomalies mba caused by variations in crustal thickness and or crustal and upper mantle densities show different characteristics from those observed at faster spreading centers like the mid atlantic ridge mar 20 30 mm yr full rate with the exception of the du toit transform none of the ridge axis discontinuities have offsets greater than 10 km and few of the discontinuities have clearly defined off axis traces the mba patterns associated with individual segments are much more complex than the simple circular bull s eyes lows reported along the mar while the short wavelength ridge segment length is comparable to that of the mar there is little correlation with mba amplitude and segment length and axial relief furthermore an eastward propagating magma source and an 84 km long zone of oblique spreading appears to define a fundamental boundary along the swir between two 250 300 km long sections characterized by distinctly different axial morphology and gravity signatures we interpret these results to indicate a long wavelength segmentation pattern of the underlying upwelling mantle melt separates from the upwelling mantle at the base of the lithosphere and is channeled to the surface along dikes fissure eruptions within the rift valley build linear ridges defining a short wavelength spatial pattern of ridge segmentation that is not directly related to the segmentation pattern of the upwelling mantle our results and interpretation are quite different than that predicted by extending current models of the faster spreading mar to these ultra slow spreading rates

origins of east asian summer monsoon seasonality author s chiang jch kong w wu ch battisti ds abstract 2020 american meteorological society the east asian summer monsoon is unique among summer monsoon systems in its complex seasonality exhibiting distinct intraseasonal stages previous studies have alluded to the downstream influence of the westerlies flowing around the tibetan plateau as key to its existence we explore this hypothesis using an atmospheric general circulation model that simulates the intraseasonal stages with fidelity without a tibetan plateau east asia exhibits only one primary convective stage typical of other monsoons as the plateau is introduced the distinct rainfall stages spring pre mei yu mei yu and midsummer emerge and rainfall becomes more intense overall this emergence coincides with a pronounced modulation of the westerlies around the plateau and extratropical northerlies penetrating northeastern china the northerlies meridionally constrain the moist southerly flow originating from the tropics leading to a band of lower tropospheric convergence and humidity front that produces the rainband the northward migration of the westerlies away from the northern edge of the plateau leads to a weakening of the extratropical northerlies which coupled with stronger monsoonal southerlies leads to the northward migration of the rainband when the peak westerlies migrate north of the plateau during the midsummer stage the extratropical northerlies disappear leaving only the monsoon low level circulation that penetrates northeastern china the rainband disappears leaving isolated convective rainfall over northeastern china in short east asian rainfall seasonality results from the interaction of two seasonally evolving circulations the monsoonal southerlies that strengthen and extend northward and the midlatitude northerlies that weaken and eventually disappear as summer progresses

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

westerly jet east asian summer monsoon connection during the holocene 1 the causes of spatial and temporal changes in east asian summer monsoon easm precipitation on millennial to multimillennial time scales during the holocene have not yet been resolved here we examine the relationship between spatial variations in easm precipitation and the westerly jet wj path over east asia during the holocene using the provenance of eolian dust in japan sea sediments which we interpret to reflect changes of the wj path over east asia and or surface conditions of desert areas the contribution of dust from the mongolian gobi desert relative to that from the taklimakan desert shows millennial scale to multimillennial scale minima at 11 5 10 7 5 and 3 5 1 5 kyr b p which we attribute to earlier seasonal northward progression of the wj these dates correlate with precipitation maxima along the present northwestern easm margin and minima in eastern northeast china and the yangtze river basin suggesting that the wj shifted northward earlier in the year allowing earlier northward migration of the easm rainband and generating abundant precipitation in the northwestern easm margin therefore during the holocene changes of the wj path probably contributed to the millennial scale to multimillennial scale easm precipitation changes and its northwest southeast contrast within china our findings suggest that wj path changes are critical to understand the dynamics of easm in the past present and future

interaction of the westerlies with the tibetan plateau in determining the mei yu termination abstractthis study explores how the termination of the mei yu is dynamically linked to the westerlies impinging on the tibetan plateau it is found that the mei yu stage terminates when the maximum

oceanic crustal thickness versus spreading rate a compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness away from plateaus is 6 km no systematic increase of crustal thickness with spreading rate is observed instead the data show large variations in crustal thickness at slow spreading ridges 3 8 km for half rates 30 mm yr the large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid ocean ridges both data sets support the speculation of a transition from a 3 d structure of crustal accretion at slow ridges to a 2 d accretion pattern at fast ridges

nonlinear intraseasonal phases of the east asian summer monsoon extraction and analysis using self organizing maps abstractthe hypothesis that regional characteristics of the east asian summer monsoon easm result from the presence of nonlinear coupled features that modulate the seasonal circulation and rainfall at the intraseasonal time scale is advanced in this study to examine this hypothesis the authors undertake the analysis of daily easm variability using a nonlinear multivariate data classifying algorithm known as self organizing mapping som on the basis of various som node analyses four major intraseasonal phases of the easm are identified the first node describes a circulation state corresponding to weak tropical and subtropical pressure systems strong upper level jets weakened monsoonal winds and cyclonic upper level vorticity this mode related to large rainfall anomalies in southeast china and southern japan is identified as the mei yu baiu phase the second node represents a distinct circulation state corresponding to a strengthened subtropical high monsoonal winds and anticyclonic upper lev

a systematic analysis of the mid atlantic ridge morphology and gravity between 15 n and 40 n constraints of the thermal structure multibeam bathymetry data obtained along a 2400 km long section of the mid atlantic ridge mar from 15 n to the azores platform 40 n and satellite derived gravity data were used to calculate the mantle bouguer anomaly mba along this portion of the mar both data sets were used to determine the relations between gravity anomalies and topographic variations and discuss these in terms of thermal difference a long wavelength influence of the azores hot spot is characterized by a gentle continuous slope of the average ridge axial depth and a general gradient in the along axis mba profile this thermal influence of the azores hot spot controls a systematic southward propagation of the spreading segments at least to 26 30 n south of 26 30 n the direction of the segment propagation is controlled by the local difference in thermal state between adjacent segments except on the azores platform the systematic along axis 11 90 km long wavelength segmentation is independent of the long wavelength influence of the azores at the segment center the axial morphology is linked to the thermal state of the segments between 1 hotter segments characterized by a smooth axial morphology a well defined shallow inner valley high mba and a long length 2 colder segments which present a rough axial morphology with a deep wide and well defined rift valley a low mba and a small length for hotter segments the formation of the abyssal hills is mainly due to a magmato tectonic cycle over periods of 0 3 to 1 myr whereas on colder segments the axial morphology is mainly controlled by a tectonic rift valley formation we propose that these different segment types correspond to a temporal evolution of the rift valley morphology over periods of several million years

role of seasonal transitions and westerly jets in east asian paleoclimate abstract the summer rainfall climate of east asia underwent large and abrupt changes during past climates in response to precessional forcing glacial interglacial cycles as well as abrupt changes to the north atlantic during the last glacial however current interpretations of said changes are typically formulated in terms of modulation of summer monsoon intensity and do not account for the known complexity in the seasonal evolution of east asian rainfall which exhibits sharp transition from the spring regime to the meiyu and then again from the meiyu to the summer regime we explore the interpretation that east asian rainfall climate undergoes a modulation of its seasonality during said paleoclimate changes following previous suggestions we focus on role of the westerly jet over asia namely that its latitude relative to tibet is critical in determining the stepwise transitions in east asian rainfall seasons in support of this linkage we show from observational data that the interannual co variation of june july august rainfall and upper tropospheric zonal winds show properties consistent with an altered timing of the transition to the meiyu summer and with more northward shifted westerlies for earlier transitions we similarly suggest that east asian paleoclimate changes resulted from an altered timing in the northward evolution of the jet and hence the seasonal transitions in particular the transition of the jet from south of the plateau to the north that determines the seasonal transition from spring rains to the meiyu in an extreme scenario which we speculate the climate system tended towards during stadial cold phases of d o stadials and periods of low northern hemisphere summer insolation the jet does not jump north of the plateau essentially keeping east asia in prolonged spring conditions we argue that this hypothesis provides a viable explanation for a key paleoproxy signature of d o stadials over east asia namely the heavier mean 18o of precipitation as recorded in speleothem records the southward jet position prevents the low level monsoonal flow which is isotopically light from penetrating into the interior of east asia as such precipitation there will be heavier consistent with speleothem records this hypothesis can also explain other key evidences of east asian paleoclimate changes in particular the occurrence of dusty conditions during north atlantic stadials and the southward migration of the holocene optimal rainfall

seasonal transitions and the westerly jet in the holocene east asian summer monsoon abstractthe holocene east asian summer monsoon easm was previously characterized as a trend toward weaker monsoon intensity paced by orbital insolation it is demonstrated here that this evolution is more accurately characterized as changes in the transition timing and duration of the easm seasonal stages spring pre mei yu mei yu midsummer and tied to the north south displacement of the westerlies relative to tibet to this end time slice simulations across the holocene are employed using an atmospheric general circulation model self organizing maps are used to objectively identify the transition timing and duration of the easm seasonal stages compared to the late holocene an earlier onset of mei yu and an earlier transition from mei yu to midsummer in the early to mid holocene are found resulting in a shortened mei yu and prolonged midsummer stage these changes are accompanied by an earlier northward positioning of the westerlies relative to tibet invoking changes to seasonal transitions al

holocene moisture changes in western china central asia inferred from stalagmites abstract central asia lies at the convergence between the mediterranean and asian monsoon climates and there is a complex interaction between the westerlies with the monsoon to form the climate of that region and its variability the region is highly vulnerable to changes in rainfall highlighting the need to understand the underlying controls we present a stalagmite based 18 o record from kesang cave in western china using mc icp ms u series dating and stable isotope analysis stalagmite calcite 18 o largely documents changes in the 18 o of precipitation 18 o in stalagmites was low during the early and middle holocene 10 0 3 0 ka bp and shifted to higher values between 3 0 and 2 0 ka bp after 2 0 ka bp 18 o fluctuates with distinct centennial scale variations drawing from results of state of the art atmospheric general circulation model simulations for the preindustrial period and 9 ka bp we propose that changes in moisture source regions and the wetter climate both contributed to the isotopic depletion of precipitation during the early and middle holocene multiple records from surrounding regions indicate a generally wetter climate during the early and mid holocene supporting our interpretation on the speleothem 18 o changes in precipitation seasonality do not appear to be a viable explanation for the observed changes nor increased penetration of monsoonal moisture to the study site we speculate that the climatic regime shifted around 3 0 2 0 ka bp towards a drier climate resulting in temperature having dominant control on precipitation 18 o the demise of three settlements around 500ad at the margin of tarim basin coincided with a period of decreased precipitation and increased temperature that likely affected local water resources underscoring the potential impact of climate on human habitation in this region

the southwest indian ridge between 49 15 e and 57 e focused accretion and magma redistribution abstract bathymetric gravity magnetic and backscattering strength data have been used to characterise the segmentation of an 800 km long portion of the ultraslow spreading southwest indian ridge swir full rate 14 mm yr between 49 15 e and 57 e this analysis reveals that the segmentation defined by along axis variations of depth and by occurrence of axial offsets does not systematically correspond to the segmentation determined by the along axis variations of backscattering strength mantle bouguer anomaly mba and amplitude of the central magnetic anomalies cma at axial discontinuities with offsets larger than 15 km thin crust and reduced volcanic production are suggested by the occurrence of mba highs almost non existent cma and 50 lower backscattering strength relative to the segment centres by contrast smaller non transform discontinuities with offsets smaller than 15 km correspond to very weak variations or to no variation of the mba the cma or the reflectivity of the seafloor suggesting that there is little or no variation of volcanic production and crustal thickness associated with those small discontinuities these small axial discontinuities bound low relief bathymetric segments 500 700 m corresponding to weak or no mba lows amplitude 1000 m corresponding to large mba lows amplitude 30 mgal we suggest that the magma supply to these low relief segments is controlled by near surface processes such as melt migration and or crustal plumbing from adjacent high relief segments pronounced mba lows at high relief segments are thought to correspond to spreading cells where magma supply is focused in the mantle these spreading cells are spaced by about 100 km along the swir axis we suggest that the spacing of spreading cells on slow spreading ridges is primarily controlled by the spreading rate with larger spacing between spreading cells on ultraslow spreading ridges than on slow spreading ridges

seismic gap beneath logachev seamount indicator for melt focusing at an ultraslow mid ocean ridge ultraslow spreadingmid ocean ridges have a low magma budget and melt is distributed unevenly along the ridge axis there is little or no basaltic crust between isolated magmatic centers the processes that focus melts to segments of robust magmatism are not yet understood during a seismic survey of the ultraslow spreading knipovich ridge in the norwegian greenland sea with ocean bottom seismometers we discovered a seismic gap in the upper mantle beneath logachev seamount where micro earthquakes clearly delineate a shallowing of the maximum depth of faulting a topography of the lithosphere that allows melts to travel laterally along its base and rise in areas of thin lithosphere has been proposed as a possible mechanism to explain the focusing of melts at volcanic centers but has never been confirmed observationally our results are the first geophysical evidence for an along axis variation of the lithospheric thickness at an ultraslow spreading ridge

precession and glacial cycle controls of monsoon precipitation isotope changes over east asia during the pleistocene abstract precipitation isotope reconstructions derived from speleothems and plant waxes are important archives for understanding hydroclimate dynamics their climatic significance in east asia however remains controversial here we present terrestrial plant wax stable hydrogen isotope dwax records over periods covering the last four interglacials and glacial terminations from sediment cores recovered from the northern south china sea scs as an archive of regionally integrated precipitation isotope changes in southeast china combined with previous precipitation isotope reconstructions from china we find that the scs dwax and southwest central china stalagmite 18 o records show relatively enriched and depleted isotopic values respectively during interglacial peaks but relatively similar isotopic variations during most sub interglacials and glacial periods over the past 430 thousand years during interglacial peaks strong summer insolation should have intensified the convection intensity the isotopic fractionation along moisture trajectories and the seasonality which are all in favor of causing isotopically depleted rainfall over the east asian monsoon regime these effects in combination with a relatively high proportion of indian ocean versus pacific sourced moisture influx should have resulted in strongly depleted precipitation isotopes stalagmite 18 o over most parts of china however southeast china should have been affected by a relatively low ratio of indian ocean versus pacific sourced moisture influx which dominated over effects yielding depleted precipitation isotopes and led to enriched precipitation isotopes dwax it is thus concluded that glacial boundary conditions and insolation forcing are the two most important factors for causing regional differences in precipitation isotope compositions over subtropical east asia on orbital timescales

two long and pronounced cold periods 3 000 5 000 and 6 600 8 400 years b p in east asia and the southward migration of the westerly jet abstract to reconstruct the variations in sea surface temperature sst during the holocene the alkenone unsaturation index of marine sediments from two deep drilled cores and 81 different core top surface sediments recovered from the heuksan mud belt located off the southwestern coast of the korean peninsula was measured first comparison of the alkenone temperature estimates of 81 core top sediments with in situ temperatures indicated that the alkenone temperatures corresponded to the average sst in april to october the spatial distribution pattern of the core top alkenone sst showed a north south temperature gradient which represented the in situ temperatures well this indicated that the effects of the resuspension and lateral transport of sediments on the past temperature estimation might be insignificant based on the two deep drilled cores variations in the alkenone sst during the holocene were reconstructed in general the alkenone sst decreased by less than 1 c from the early to late holocene an interesting feature was the presence of two long and pronounced cold approximately 2 c periods which occurred at 3 5 kyr and 6 6 8 4 kyr b p these cold periods were also observed in other mid latitude regions in east asia indicating that the cooling pattern was regional the cold periods appeared to be associated with the southward migration of the westerly jet at that time investigations into the effects of local sea level changes and resultant changes in the tidal regime on changes in the sst during the holocene period suggested that they were insignificant in the study area

asymmetric geophysical signatures in the greenland norwegian and southern labrador seas and the eurasia basin a thorough examination of geophysical data from the greenland norwegian sea eurasia basin and southern labrador sea shows significant asymmetry of several parameters basement topography adjusted for sediment loading free air gravity anomaly spreading half rate and seismicity with respect to crustal age 1 1 average zero age depth 0 57 m y b p depth of highest rift mountain summits and depth to magnetic basement 10 30 km from axis of mohns and knipovich ridges is less on the north american plate flanks the zero age depth asymmetry is 400 500 m for the eurasia basin 0 57 m y b p and for mohns ridge 57 22 m y b p and 150 200 m for younger mohns ridge crust 22 0 m y b p and for the extinct aegir ridge 57 27 m y b p there is little or no asymmetry in the labrador sea except near the extinct rift valley where the east flank is 150 300 m shallower magnetic depth to source computations provide an independent confirmation of basement asymmetry the belts 10 30 km from the axis of mohns and knipovich ridges are 100 150 m shallower on the west flank of these ridges the shallower ridge flank is topographically rougher so that average rift mountain summits are 300 m shallower on the west flanks of the mohns knipovich ridges a larger asymmetry than for average zero age depth the amount of topographic asymmetry is greatest near the mohns knipovich bend asymmetry appears to be greatest for ridges oriented normal to the spreading direction and less for oblique spreading 2 2 free air gravity anomaly asymmetries of 5 to 20 mgal sign indicates west flank is more positive are associated with topographic asymmetry at least within 10 15 m y of the axis of mohns and knipovich ridges gravity is reduced on the older flanks west of the extinct mid labrador ridge and east of mohns ridge asymmetric crustal layer thicknesses or densities provide one possible explanation although deep seated sources e g mantle convection unrelated to the crust cannot be excluded 3 3 spreading half rate was about 5 15 lower on the north american plate flanks of mohns ridge 57 35 m y and in the eurasia basin 0 57 m y thus the fast spreading flank tends to produce deeper smoother crust however topographic asymmetry cannot relate only to spreading rate asymmetry since for the young mohns ridge crust 9 m y b p faster spreading and higher topography are both associated with the west flank 4 4 mid plate seismicity is higher on the eurasia eastern flank of mohns and knipovich ridge but this effect may be unrelated to the other three the fluid dynamical model of stein et al correctly explains the sense of spreading rate asymmetry the north american plate moving faster over mantle is growing more slowly however the other asymmetries and their causal relationships remain theoretically unexplained

response of heterogeneous rainfall variability in east asia to hadley circulation reorganization during the late quaternary abstract the quaternary east asian summer rainfall evolution reconstructed with the chinese speleothem 18o records shows the consistency throughout the region of east asia and has long been considered equivalent to the monsoon intensity its variation and teleconnection with global climate were usually interpreted by the meridional shift of the intertropical convergence zone itcz however many other proxy records climate simulations and meteorological observations suggest the inconsistent rainfall patterns in the different regions of east asia on multi timescales such spatial heterogeneity in quaternary east asian summer rainfall indicates that the hydroclimate in this region is not fully paced by the migration of the itcz here we report a sediment record of rainfall evolution during the last 400 ka in the northern east china sea and this record in combination with a transient climate model simulation indicates an out of phase relationship between rainfall over middle southeastern east asia and northern and southwestern east asia on the precession band with high boreal summer insolation corresponding to the increased rainfall intensity in the northern and southwestern east asia however decreased rainfall intensity in the middle east asia we attribute this regional heterogeneity in east asian rainfall to the reorganization of the hadley circulation including shifts in the ascending branch itcz and descending branch subtropical westerly jet in response to changes in the hemispheric meridional temperature gradient our results highlight the crucial role of the hadley circulation in the east asian hydroclimate and have important implications for future climate projections

prediction of sediment thickness in the norwegian greenland sea from gravity inversion 1 we explore the feasibility of inverting freely available altimetry derived gravity grids for sediment thickness on oceanic crust a four layer earth model is adopted assuming that the gravity signal from the base of the sedimentary layer can be fully isolated from the observed free air gravity the following additional data are required 1 shipboard bathymetry for reliable bouguer correction 2 oceanic crustal ages to correct for gravity anomalies related to the thermal history of lithospheric breakup and seafloor spreading and 3 some wide angle seismic observations of sediment thickness a fixed relationship between the base of the sedimentary layer and the moho is also required because of limited gravity resolution at large depths the final inversion setup searches for three control parameter values density of uppermost sediments density of oceanic crystalline crust and thermal diffusivity of mantle rock that minimize misfit between the earth model and observations applied to the norwegian greenland sea the inversion successfully predicts locations of 20 25 km wide depocenters and crystalline crustal highs yet 1 km thickness errors are common even in well constrained areas the largest errors are associated with underplated crust active mid ocean ridges and the continent ocean boundary indicating locally incorrect assumptions of mantle thermal structure and crystalline crustal thickness the thicknesses of 4 km thick submarine fans are underpredicted probably because of too low sedimentary densities in the earth model if these limitations are acceptable or do not apply because of simple crustal structure gravity inversion may be used for mapping regional sedimentary patterns e g in the arctic ocean

evidence of recent volcanic activity on the ultraslow spreading gakkel ridge seafloor spreading is accommodated by volcanic and tectonic processes along the global mid ocean ridge system as spreading rate decreases the influence of volcanism also decreases and it is unknown whether significant volcanism occurs at all at ultraslow spreading rates 1 5 cm yr 1 here we present three dimensional sonar maps of the gakkel ridge earth s slowest spreading mid ocean ridge located in the arctic basin under the arctic ocean ice canopy we acquired this data using hull mounted sonars attached to a nuclear powered submarine the uss hawkbill sidescan data for the ultraslow spreading approximately 1 0 cm yr 1 eastern gakkel ridge depict two young volcanoes covering approximately 720 km2 of an otherwise heavily sedimented axial valley the western volcano coincides with the average location of epicentres for more than 250 teleseismic events detected in 1999 suggesting that an axial eruption was imaged shortly after its occurrence these findings demonstrate that eruptions along the ultraslow spreading gakkel ridge are focused at discrete locations and appear to be more voluminous and occur more frequently than was previously thought

monsoons itczs and the concept of the global monsoon earth s tropical and subtropical rainbands such as intertropical convergence zones itczs and monsoons are complex systems governed by both large scale constraints on the atmospheric general circulation and regional interactions with continents and orography and coupled to the ocean monsoons have historically been considered as regional large scale sea breeze circulations driven by land sea contrast more recently a perspective has emerged of a global monsoon a global scale solstitial mode that dominates the annual variation of tropical and subtropical precipitation this results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone regional subsystems are embedded in this global monsoon localized by surface boundary conditions parallel with this much theoretical progress has been made on the fundamental dynamics of the seasonal hadley cells and convergence zones via the use of hierarchical modeling approaches including aquaplanets here we review the theoretical progress made and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings after summarizing the dynamical and energetic balances that distinguish an itcz from a monsoon we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets limitations of current theories are discussed including the need for a better understanding of the influence of zonal asymmetries and transients on the large scale tropical circulation

impacts of the spatial extent of pollen climate calibration set on the absolute values range and trends of reconstructed holocene precipitation abstract pollen based quantitative reconstructions of past climate variables is a standard palaeoclimatic approach despite knowing that the spatial extent of the calibration set affects the reconstruction result guidance is lacking as to how to determine a suitable spatial extent of the pollen climate calibration set in this study past mean annual precipitation pann during the holocene since 11 5 cal ka bp is reconstructed repeatedly for pollen records from qinghai lake 36 7 n 100 5 e north east tibetan plateau gonghai lake 38 9 n 112 2 e north china and sihailongwan lake 42 3 n 126 6 e north east china using calibration sets of varying spatial extents extracted from the modern pollen dataset of china and mongolia 2559 sampling sites and 168 pollen taxa in total results indicate that the spatial extent of the calibration set has a strong impact on model performance analogue quality and reconstruction diagnostics absolute value range trend optimum generally these effects are stronger with the modern analogue technique mat than with weighted averaging partial least squares wa pls with respect to fossil spectra from northern china the spatial extent of calibration sets should be restricted to radii between ca 1000 and 1500 km because small scale calibration sets 1500 km radius will include taxa with very different pollen climate relationships

evidence from gravity anomalies for interactions of the marion and bouvet hotspots with the southwest indian ridge effects of transform offsets abstract the ultra slow spreading southwest indian ridge swir presents a unique environment to study the interactions between hotspots and ridges with highly segmented geometry using recently available satellite free air gravity and shipboard bathymetry data we obtain mantle bouguer mba and residual mantle bouguer anomalies rmba by removing from free air gravity the attractions of seafloor topography sediment thickness variations a reference crust and theoretically predicted effects of lithospheric cooling the bouvet hotspot previously observed to be associated with anomalous bathymetry and geochemistry near the bouvet triple junction has an mba axial gravity low of 100 mgal implying pronounced localized crustal thickening off axis the rmba lows along previously calculated bouvet hotspot tracks are variable in amplitude suggesting the possibilities that bouvet flux varies in time or that hotspot magmatism is enhanced by proximity to a spreading center along axis geophysical anomalies suggest that the marion hotspot has a significant effect on accretionary processes in the central portion of the swir importantly the marion axial anomaly appears to be compartmentalized between the andrew bain and discovery ii fracture zones implying that transform offsets play a significant role in governing the distribution of plume material in a highly segmented ultra slow spreading system

gravity evidence of very thin crust at the gakkel ridge arctic ocean abstract gakkel ridge the active spreading center in the arctic ocean is the slowest spreading portion of the global mid ocean ridge system total spreading rates range from 0 6 cm yr in the east where the ridge disappears beneath the laptev shelf to 1 3 cm yr in the west near greenland bathymetry and gravity surveys of four sections of the gakkel ridge were carried out in 1996 by the u s navy nuclear submarine uss pogy as part of scicex 96 in order to sample variations in seafloor morphology and gravity anomalies as a function of spreading rate the ridge axis throughout the survey area is characterized by a continuous axial rift valley similar to that observed at other slow spreading ridges the continuous rift axis suggests that well organized seafloor spreading is occurring at total spreading rates of less than 1 cm yr in three faster spreading 1 13 1 24 cm yr western survey areas located between 7 e and 54 e the gakkel ridge is deep compared with other ridge axes axial depths range between 4600 and 5100 m and ridge flanks at about 3200 m the ridge flank morphology is very blocky and is characterized by large scarps and deep fault bounded troughs very large amplitude free water anomalies with peak to trough amplitudes of 85 150 mgal are observed centered on the axis of the gakkel ridge modeling of the free water anomalies by varying the crustal thickness and average crustal density including the gravity effect of the cooling of the mantle away from the axis implies that if the average crustal density is less than 2900 kg m3 the crustal thickness must be less than 4 km the axial rift valley at the fourth survey area near 98 e where the total spreading rate is 0 99 cm yr is buried by sediments the axis in this region is associated with a continuous 70 mgal gravity minimum implying the presence of a large buried rift valley the rift flanks at 95 e are at a depth of greater than 3800 m 600 m deeper than the average depth at the gakkel ridge axis west of 60 e simple isostatic calculations suggest that the crust in this region may be vanishingly thin beneath the sediment cover these observations indicate a relationship between melt production and seafloor spreading rate at very slow spreading rates suggesting that ultra slow spreading may suppress melt production or delivery at the gakkel ridge

rifting in the northern norwegian greenland sea thermal tests of asymmetric spreading the analysis of heat flow seismic and topographic data collected in the northern norwegian greenland sea reveals an asymmetric evolution of the eurasian and north american plates these data are compared to predictions from three kinematic models of extension which produce asymmetry about the knipovich ridge 1 uniform asymmetric pure shear 2 lithospheric simple shear and 3 rift ridge jumping the data are consistent with a range of deformation scenarios from one ridge jump occurring at about 25 m y after the initiation of spreading to continuous asymmetric extension the simple shear model can match the data only when a detachment fault dips more steeply than 45 under svalbard tectonic and heat flow evidence suggests that asymmetry may have evolved from a combination of all three models when the mohns ridge propagating to the east encountered the preexisting northward trending spitsbergen shear zone the direction of ridge propagation shifted to the north being influenced by the abrupt change in the regional stress field at the shear zone ridge axis intersection as a result the nascent knipovich ridge entered into and propagated along the shear zone consequently formerly active shear faults became the new detachment surfaces along which new crust is minted and asymmetrically extended the high level of deviatoric stress about the mohns and knipovich ridge intersection may cause a gradual eastward migration of the knipovich ridge resulting in multiple zones of magma intrusion off axial bands of high heat flow and volcanism located along the barents sea and svalbard margin along the yermak plateau and on continental svalbard just southeast of the yermak plateau may be evidence for this migration propagation from the nansen ridge may have entered the same shear zone from the north explaining the creation of the small molloy ridge and off axis volcanism on the yermak plateau

the spatial temporal patterns of asian summer monsoon precipitation in response to holocene insolation change a model data synthesis highlights slice and transient simulations of holocene climate change were performed spatial temporal patterns of holocene asian summer precipitation are investigated a tripole pattern of summer precipitation can be seen over monsoonal asia insolation change is a key factor for holocene asian summer monsoon change internal feedbacks are important to holocene asian summer precipitation changes abstract paleoclimate proxy records of precipitation effective moisture show spatial temporal inhomogeneous over asian monsoon and monsoon marginal regions during the holocene to investigate the spatial differences and diverging temporal evolution over monsoonal asia and monsoon marginal regions we conduct a series of numerical experiments with an atmosphere ocean sea ice coupled climate model the kiel climate model kcm for the period of holocene from 9 5 ka bp to present 0 ka bp the simulations include two time slice equilibrium experiments for early holocene 9 5 ka bp and present day 0 ka bp respectively and one transient simulation ht using a scheme for model acceleration regarding to the earth s orbitally driven insolation forcing for the whole period of holocene from 9 5 to 0 ka bp the simulated summer precipitation in the equilibrium experiments shows a tripole pattern over monsoonal asia as depicted by the first modes of empirical orthogonal function eof1 of h0k and h9k the transient simulation ht exhibits a wave train pattern in the summer precipitation across the asian monsoon region associated with a gradually decreased trend in the strength of asian summer monsoon as a result of the response of asian summer monsoon system to the holocene orbitally forced insolation change both the synthesis of multi proxy records and model experiments confirm the regional dissimilarity of the holocene optimum precipitation effective moisture over the east asian summer monsoon region monsoon marginal region and the westerly dominated areas suggesting the complex response of the regional climate systems to holocene insolation change in association with the internal feedbacks within climate system such as the air sea interactions associated with the el nino southern oscillation enso and shift of the intertropical convergence zone itcz in the evolution of asian summer monsoon during the holocene

new gravity data in the arctic ocean comparison of airborne and ers gravity new gravity fields from airborne gravimetry and from ers 1 and 2 satellite altimetry cover extensive portions of the arctic ocean these two data sets may constitute as much as 60 of the data contributions to the arctic gravity project compilation here we evaluate the accuracy and resolution of these data and quantify their impact on the compilation both gravity determinations compare favorably with geological survey of canada surface measurements in the beaufort sea airborne 1 86 2 09 mgal rms ers 2 64 3 11 mgal rms comparisons between the airborne and ers data over the chukchi borderlands reveal a 4 38 mgal rms difference over the smoother region of the field and 7 36 mgal rms over the rugose field generated by the shallow ridges and deep troughs coherency between the two data sets in the chukchi region implies a resolution of 19 km comparison with science ice expedition submarine measurements over chukchi plateau suggests that the ers field resolves even shorter wavelength signal than the airborne data whereas in the beaufort sea the airborne data showed better coherence to ground truth data long wavelength differences exist between the two data sets expressed as a 2 3 mgal offset over the chukchi region this study highlights the respective strengths of the two data sets the ers gravity field has the advantage of ubiquitous coverage of the ocean south of 81 5 degreesn a denser sampling of the gravity field and a recovery of signal down to similar to 15 km the airborne data cover a significant portion of the polar hole in the satellite coverage have lower measurement noise and recover somewhat higher anomaly amplitudes in the 25 100 km wavelength range

lucky strike seamount implications for the emplacement and rifting of segment centered volcanoes at slow spreading mid ocean ridges the history of emplacement tectonic evolution and dismemberment of a central volcano within the rift valley of the slow spreading mid atlantic ridge at the lucky strike segment is deduced using near bottom sidescan sonar imagery and visual observations volcano emplacement is rapid 1 myr associated with focused eruptions and with effusion rates feeding lava flows that bury tectonic features developed prior to and during volcano construction this volcanic phase likely requires efficient melt pooling and a long lived crustal magma chamber as a melt source a reduction in melt supply triggers formation of an axial graben rifting the central volcano and the onset of seafloor spreading may eventually split it at lucky strike this results in two modes of crustal construction eruptions and tectonic activity focus at a narrow graben that bisects the central volcano and contains the youngest lava flows accumulating a thick layer of extrusives away from the volcano summit deformation and volcanic emplacement is distributed throughout the rift valley floor lacking a clear locus of accretion and deformation volcanic emplacement on the rift floor is characterized by axial volcanic ridges fed by dikes that propagate from the central axial magma chamber the mode of rapid volcano construction and subsequent rifting observed at the lucky strike seamount is common at other central volcanoes along the global mid ocean ridge system

mechanisms for spatially inhomogeneous changes in east asian summer monsoon precipitation during the mid holocene abstractthe east asian summer monsoon easm intensified during the early to mid holocene relative to the present primarily due to orbital forcing however on the regional scale changes in the mo

hydrothermal activity on the ultra slow spreading southern knipovich ridge we report first evidence for hydrothermal activity from the southern knipovich ridge an ultra slow spreading ridge segment in the norwegian greenland sea evidence comes from optical backscatter anomalies collected during a systematic side scan sonar survey of the ridge axis augmented by the identification of biogeochemical tracers in the overlying water column that are diagnostic of hydrothermal plume discharge mn ch4 atp analysis of coregistered geologic and oceanographic data reveals that the signals we have identified are consistent with a single high temperature hydrothermal source located distant from any of the axial volcanic centers that define second order segmentation along this oblique ridge system rather our data indicate a hydrothermal source associated with highly tectonized seafloor that may be indicative of serpentinizing ultramafic outcrops consistent with this hypothesis the hydrothermal plume signals we have detected exhibit a high methane to manganese ratio of 2 3 1 this is higher than that typical of volcanically hosted vent sites and provides further evidence that the source of the plume signals reported here is most probably a high temperature hydrothermal field that experiences some ultramafic influence compare to rainbow and logachev sites mid atlantic ridge while such sites have previously been invoked to be common on the sw indian ridge this may be the first such site to be located along the arctic ultra slow spreading ridge system

inter relationship and environmental significance of stalagmite δ13c and δ18o records from zhenzhu cave north china over the last 130 ka abstract there is a vigorous and ongoing debate about the interpretation of the stalagmite 18 o record from east china as an indicator of east asian summer monsoon easm intensity this is mainly because of the inconsistent spatial variation of precipitation amount in east china but the generally consistent variation of precipitation 18 o on various timescales however a direct comparison of a proxy record of precipitation amount with the 18 o record from the same stalagmite has rarely been reported especially in north china here we present precisely dated high resolution stalagmite 13 c and 18 o records for the past 130 ka 1 ka 1000 years from zhenzhu zz cave in north china on multi millennial to orbital timescales the 13 c and 18 o records exhibit a high degree of in phase co variation however there are differences in the trend and amplitude of change between various marine isotopic stages mis specifically mis5e mis5c mis5a mis3 and mis1 modern monitoring results of two years from zz cave together with a comparison of the 13 c record from the cave with a pedogenic carbonate 13 c record from the western chinese loess plateau clp demonstrate that the 13 c record is a sensitive and reliable indicator of easm rainfall amount considering that variations in both the amount and 18 o of precipitation can be modulated by the atmospheric circulation system we conclude that the amplitude and trend of the stalagmite 18 o variations from north china are not an absolute indicator of easm intensity but rather an indicator of changes in the phase of the easm we suggest that stalagmite 18 o records combined with other unambiguous proxy indicators from stalagmites can provide a more comprehensive history of changes of the easm

mantle flow and melting beneath oceanic ridge ridge ridge triple junctions abstract plate boundary geometry likely has an important influence on crustal production at mid ocean ridges many studies have explored the effects of geometrical features such as transform offsets and oblique ridge segments on mantle flow and melting this study investigates how triple junction tj geometry may influence mantle dynamics an earlier study georgen j e lin j 2002 three dimensional passive flow and temperature structure beneath oceanic ridge ridge ridge triple junctions earth planet sci lett 204 115 132 suggested that the effects of a ridge ridge ridge configuration are most pronounced under the branch with the slowest spreading rate thus we create a three dimensional finite element variable viscosity model that focuses on the slowest diverging ridge of a triple junction with geometry similar to the rodrigues tj this spreading axis may be considered to be analogous to the southwest indian ridge within 100 km of the tj temperatures at depths within the partial melting zone and crustal thickness are predicted to increase by 40 c and 1 km respectively we also investigate the effects of differential motion of the tj with respect to the underlying mantle by imposing bottom model boundary conditions replicating a absolute plate motion and b a three dimensional solution for plate driven and density driven asthenospheric flow in the african region neither of these basal boundary conditions significantly affects the model solutions suggesting that the system is dominated by the divergence of the surface places finally we explore how varying spreading rate magnitudes affects tj geodynamics when ridge divergence rates are all relatively slow i e with plate kinematics similar to the azores tj significant along axis increases in mantle temperature and crustal thickness are calculated at depths within the partial melting zone temperatures are predicted to increase by 150 c similar to the excess temperatures associated with mantle plumes likewise crustal thickness is calculated to increase by approximately 6 km over the 200 km of ridge closest to the tj these results could imply that some component of the excess volcanism observed in geologic settings such as the terceira rift may be attributed to the effects of tj geometry although the important influence of features like nearby hotspots e g the azores hotspot cannot be evaluated without additional numerical modeling

deformation partitioning in a slow spreading ridge undergoing oblique extension mohns ridge norwegian sea although oceanic spreading is often perpendicular to the ridge trends in some cases the angle between these two directions can be significantly less than 90 40 50 this occurs because of either a bend of the ridge trend or a change of the spreading direction we here describe oblique spreading in the mohns ridge resulting in deformation partitioning between the valley walls which are dominantly affected by strike slip displacements and the axial valley which is subject to nearly pure extension the axial valley walls are characterized by en echelon normal faults affecting the walls while the axial valley is affected by parallel faults grouped into oblique sets these fault sets define different structures horst or tilted blocks that are regularly spaced inside the axial valley moreover some ridge segments mainly undergo pure extension whereas others are affected by oblique extension we explain this faulting pattern including the along strike and transverse variations as a consequence of depth variations of the brittle ductile transition

asynchronous holocene optimum of the east asian monsoon abstract the spatial and temporal distribution of summer monsoon precipitation or effective moisture during the holocene has been reconstructed on the basis of geological data including lake levels pollen profiles and loess paleosol records in addition the summer july precipitation increment effective precipitation and monsoon strength index have been obtained from numerical modeling experiments both geological data and numerical modeling indicate that the holocene optimum as defined by peak east asian summer monsoon precipitation was asynchronous in central and eastern china reaching a maximum at different times in different regions e g ca 10 000 8000 yr ago in northeastern china 10 000 7000 yr ago in north central and northern east central china ca 7000 5000 yr ago in the middle and lower reaches of the yangtze river and ca 3000 yr ago in southern china in southwestern china the maximum appeared ca 11 000 yr ago but probably was related to the maximum landward extension of the indian summer monsoon the regional shift in the maximum precipitation belt from northwest to southeast over the past 10 000 yr is interpreted as a response to changing seasonality related to orbital forcing of the climate the southeastward shift of the east asian summer monsoon maximum is consistent with the progressive weakening of the summer monsoon as the summer solar radiation anomaly decreased progressively through the holocene and the east asian monsoon index declined while the early maximum in southwestern china matches the maximum of the indian monsoon index

preliminary analysis of the knipovich ridge segmentation influence of focused magmatism and ridge obliquity on an ultraslow spreading system abstract bathymetry gravity and deep tow sonar image data are used to define the segmentation of a 400 km long portion of the ultraslow spreading knipovich ridge in the norwegian greenland sea northeast atlantic ocean discrete volcanic centers marked by large volcanic constructions and accompanying short wavelength mantle bouguer anomaly mba lows generally resemble those of the gakkel ridge and the easternmost southwest indian ridge these magmatically robust segment centers are regularly spaced about 85 100 km apart along the ridge and are characterized by accumulated hummocky terrain high relief off axis seamount chains and significant mba lows we suggest that these eruptive centers correspond to areas of enhanced magma flux and that their spacing reflects the geometry of underlying mantle upwelling cells the large scale thermal structure of the mantle primarily controls discrete and focused magmatism and the relatively wide spacing of these segments may reflect cool mantle beneath the ridge segment centers along the southern knipovich ridge are characterized by lower relief and smaller mba anomalies than along the northern section of the ridge this suggests that ridge obliquity is a secondary control on ridge construction on the knipovich ridge as the obliquity changes from 35 to 49 from north to south respectively while spreading rate and axial depth remain approximately constant the increased obliquity may contribute to decreased effective spreading rates lower upwelling magma velocity and melt formation and limited horizontal dike propagation near the surface we also identify small magmatically weaker segments with low relief little or no mba anomaly and no off axis expression we suggest that these segments are either fed by lateral melt migration from adjacent magmatically stronger segments or represent smaller discrete mantle upwelling centers with short lived melt supply

focused magmatism versus amagmatic spreading along the ultra slow spreading southwest indian ridge evidence from tobi side scan sonar imagery 1 the analysis of the towed ocean bottom instrument tobi side scan sonar images along the southwest indian ridge between 63 40 e and 65 40 e reveals strong focusing of magmatic activity and long amagmatic accretionary ridge segments fresh looking volcanic terrains are observed at distinct locations along the axis separated by highly tectonized and sedimented terrains of an along axis extent as much as 82 km the largest tectonized section corresponds to a dramatically thin crust area with moderate magnetization anomalies we suggest that seafloor spreading is mainly amagmatic in this tectonized section of the southwest indian ridge with upper mantle rocks exposed at the seafloor amagmatic accretionary ridge segments of such dimensions are quite distinct from what is observed at the mid atlantic ridge but are also recognized at the gakkel ridge and may thus be characteristic of ultra slow spreading ridges the correlation between the distribution of fresh looking volcanic terrains the occurrence of shallow areas crowned by axial volcanic ridges and high magnetization values suggests a shallow segmentation of the ridge mainly related to variation in the thickness and or the intrinsic magnetization of the basaltic source layer by contrast strong along axis variations of the gravity derived crustal thickness are shrunken in length relative to this shallow segmentation of the ridge and occur only beneath the elevated segments adjacent to these elevated segments small bathymetric swells with fresh looking volcanic constructions do not correspond to thicker crust areas this suggests a highly focused melt supply beneath the elevated segments which may feed volcanic constructions up to 60 km from the center of these segments by shallow lateral melt migration in the crust probably through large dikes neither the ultra slow spreading rate nor the ridge obliquity explains the variation of the magmatic vigor along the ridge mantle source heterogeneities together with lower mantle temperatures beneath the easternmost southwest indian ridge could partly control its segmentation

the continuum of northern hemisphere teleconnection patterns and a description of the nao shift with the use of self organizing maps abstract in this study the method of self organizing maps soms is used with ncep ncar reanalysis data to advance the continuum perspective of northern hemisphere teleconnection patterns and to shed light on the secular eastward shift of the north atlantic oscillation nao that began in the late 1970s a 20 pattern som analysis of daily wintertime northern hemisphere sea level pressure reveals a continuum of patterns that correspond closely with well known teleconnection patterns this analysis also reveals that interdecadal variability of the hemispheric sea level pressure field may be understood in terms of changes in the frequency distribution within the continuum of sea level pressure patterns described by the som based on the continuum perspective illustrated with the som the above secular shift of the nao may be understood as a change in dominance from westward displaced negative nao like patterns to eastward displaced positive nao like patterns though westward and eastward displaced nao

formation of the axial relief at the very slow spreading southwest indian ridge 49 to 69 e the comparison of segment lengths relief and gravity signature along the very slow spreading southwest indian ridge swir between 49oe and 69oe suggests that the marked change in segmentation style that occurs across the melville transform 60o45 e reflects a change in the modes of formation of the axial topography we propose that the axial relief east of melville is largely due to volcanic constructions that load the axial lithosphere from above by contrast the axial relief in segments west of the melville fracture zone appears to be primarily due as proposed for segments of the faster spreading mid atlantic ridge to along axis changes in the depth of the axial valley and to partial compensation of negative loads thicker lower crust and or lighter upper mantle acting within the plate or at the bottom of the plate in terms of geology this means that the contribution of the uppermost effusive part of the crust to along axis crustal thickness variations may be greater east of melville than in other regions of the study area regional axial depths suggest that the ridge east of melville is also characterized by a low melt supply and is underlain by cold mantle a simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region and the shallower region west of the gallieni transform 52o20 e are of the order of 80oc and 4 km respectively

westerly jet stream controlled climate change mode since the last glacial maximum in the northern qinghai tibet plateau abstract climate change from the northern qinghai tibet plateau qtp is affected by the combined effects of the asian summer monsoon and the westerlies however changes in monsoon vapor transport and the westerly jet stream as well as their effect to climate change mode there are not clear at different time scales here we present composite research using paleoclimate reconstructions and ensemble simulations since the last glacial maximum lgm on the basis of an eolian sedimentary sequence in the northern qtp results from 317 surface sediment samples show that pedogenic carbonate 18 o is affected both by precipitation and temperature and organic carbon isotope 13 corg reflects the vegetation growth status in areas where precipitation is greater than 290 mm proxies from the sedimentary sequence and the paleoclimate modeling intercomparison project 3 pmip3 simulations show a close relationship between climate change mode in the northern qtp and the westerly jet stream whose movement is triggered by warm or cold conditions in the high latitude regions that is when the climate is in cold glacial period the westerly jet stream strengthens and moves southward and the mountains become wetter this climate change mode can be validated by records from mountains of north america and africa according to the cmip5 simulations the westerly jet stream will strengthen in the future and climate in the northern qtp and the high altitude region of the mid latitudes will become wetter with global warming

oblique rifting at oceanic ridges relationship between spreading and stretching directions from earthquake focal mechanisms the relationship between spreading and stretching directions is investigated at oblique spreading oceanic ridges using earthquake focal mechanisms the stretching direction at ridge axes corresponds to the direction of the greatest principal strain 31 taken as the mean trend of the seismic t axes of extensional earthquake focal mechanisms it is compared with the spreading direction provided by global plate motion models we find that the stretching direction trends approximately halfway between the spreading direction and the normal to the ridge trend a result in line with analogue experiments of oblique rifting this result is satisfactorily accounted for with an analytical model of oblique rifting for which the direction of 31 is calculated with respect to rifting obliquity for different amounts of stretching using continuum mechanics for low stretching factors typical of incremental seismic deformations 31 obliquity is two times lower than rifting obliquity for higher stretching factors the stretching and spreading directions become parallel 2006 elsevier ltd all rights reserved

seasonality and interannual variability of the westerly jet in the tibetan plateau region abstract in this study 40 yr ecmwf re analysis era 40 data are used for the description of the seasonal cycle and the interannual variability of the westerly jet in the tibetan plateau region to complement results based on the analysis of monthly mean horizontal wind speeds an occurrence based jet climatology is constructed by identifying the locations of the jet axes at 6 hourly intervals throughout 1958 2001 thus a dataset describing the highly transient and localized features of jet variability is obtained during winter and summer the westerly jet is located respectively to the south and north of the tibetan plateau during the spring and autumn seasons there are jet transitions from south to north and vice versa the median dates for these transitions are 28 april and 12 october the spring transition is associated with large interannual variations while the fall transition occurs more reliably within a 3 week period the strength of the jet exhibits a peculiar seasonal cycle during northw

east asian hydroclimate modulated by the position of the westerlies during termination i speleothem oxygen isotope records have revolutionized our understanding of the paleo east asian monsoon yet there is fundamental disagreement on what they represent in terms of the hydroclimate changes we report a multiproxy speleothem record of monsoon evolution during the last deglaciation from the middle yangtze region which indicates a wetter central eastern china during north atlantic cooling episodes despite the oxygen isotopic record suggesting a weaker monsoon we show that this apparent contradiction can be resolved if the changes are interpreted as a lengthening of the meiyu rains and shortened post meiyu stage in accordance with a recent hypothesis model simulations support this interpretation and further reveal the role of the westerlies in communicating the north atlantic influence to the east asian climate

the meridional shift of the midlatitude westerlies over arid central asia during the past 21 000 years based on the trace 21ka simulations

mid ocean ridge eruptive vent morphology and substructure evidence for dike widths eruption rates and evolution of eruptions and axial volcanic ridges high resolution side scan sonar data of mid atlantic ridge mar inner valley floor axial volcanic ridges reveal details of their architectural elements we develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features inhibition of gas exsolution lack of magma disruption and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling leading to more rapid centralization of eruptions along the widest places in the dike dikes are predicted to initially feed eruptions from fissure vents producing lines of hummocky ridges centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts the widest dikes are predicted to produce smooth flows up to several kilometers in length which should pond in adjacent lows edifice sizes predicted on the basis of volume fluxes and flow lengths implied by the widths of dikes observed on the seafloor in iceland and in ophiolite complexes are in quantitative agreement with the dimensions of observed features each ridge is made up of the products of a variety of these individual dike emplacement and extrusive events involving various dike widths cooling times and eruption durations on the basis of typical mar spreading rates about one dike emplacement event would be expected every 40 years since not all dikes reach the surface the actual mean interval between eruptions will exceed this mar dikes should solidify between average dike emplacement events dike emplacement events are more frequent on the east pacific rise epr and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes differences between the morphology of the mar axial volcanic ridge and the sheet flow dominated epr are attributed to on average wider dikes erupting with greater frequency along the epr

gas hydrates free gas distribution and fault pattern on the west svalbard continental margin summary a 3 d tomographic analysis of seismic velocity and attenuation fields is presented with the purpose to analyse the intimate relation of gas hydrates and free gas distribution with the fault pattern the 3 d four component seismic data have been acquired offshore western svalbard the analysis of the subbottom topography of the base of the stability field of gas hydrates indicated by the bottom simulating reflector and the thickness of the underlying free gas bearing zone suggest a fault induced compartmentalization of the gas reservoir the proposed fluid and gas circulation scheme assumes deep sourced warm fluids moving upwards mainly along high permeability faults and fractures below the gas hydrate stability zone faults change from being a pathway of fluid flow to being barriers to fluid flow in the hydrate stability zone consequently below the hydrate stability zone free gas is transferred from the permeable faults to non faulted but fault bounded sediments where it accumulates below the base of the hydrate stability zone producing a significantly thicker free gas layer the tomographic approach is therefore proposed as an effective procedure to provide detailed information on 3 d p and s wave velocity and attenuation distribution in marine sediments and specifically to examine the details of the distribution of the gas hydrates and of the free gas for which the dependence on the vp vs and qp is now fairly well known

the relationship between buoyant mantle flow melt migration and gravity bull s eyes at the mid atlantic ridge between 33 n and 35 n abstract crustal thickness and mantle bouguer anomaly mba patterns along the mid atlantic ridge mar between the oceanographer and hayes transforms 33 35 n are predicted from a three dimensional mantle flow model driven by plate spreading and mantle density variations assuming a two or three dimensional model of melt extraction our two dimensional melt extraction model focuses melt to the nearest ridge segment by assuming that melt moves only vertically and parallel to the spreading direction in contrast the three dimensional model assumes melt migrates vertically to the top of the melting region and then up the local slope of this surface the shape of this surface which is controlled by enhanced conductive cooling at ridge offsets directs melt toward the center of segments our results suggest that buoyant mantle flow driven by temperature variations and a small retained melt fraction

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

millennial scale oscillations of the westerly jet path during the last glacial period abstract it has been established that east asian summer monsoon intensity varies with dansgaard oeschger cycles suggesting a connection between the climates of east asia and the north atlantic on a millennial timescale however the dynamics of such a connection are still unsolved here we demonstrate that temporal changes in the provenance of aeolian dust in japan sea sediments which we interpret to reflect changes in the westerly jet path over east asia exhibit variations in harmony with dansgaard oeschger cycles the dominance of dust with a mongolian gobi provenance during stadials suggests a westerly jet axis located to the south of the himalaya tibetan plateau throughout most of the year whereas the co dominance of dust from both the taklimakan desert and the mongolian gobi during interstadials suggests that the westerly jet axis jumped to the north of the tibetan plateau at latest in summer as the shift of the westerly jet axis to the north of the tibetan plateau is closely related to the onset of the east asian summer monsoon changes of the westerly jet path apparently critically affect the teleconnection between the climates of asia and north atlantic on a millennial timescale

on the interpretation of chinese loess as a paleoclimate indicator the records of wind blown dust i e loess in china are some of the most important terrestrial records of past climate changes stretching back over the last 10 ma in the paleoclimate literature intervals of increased dust generation have been almost always interpreted as being associated with more intense or prolonged wintertime conditions here it is shown that in accordance with modern observations dust outbreaks in asia are predominantly springtime phenomena during spring frequent cyclogenesis in the lee of the mongolian altai and the passage of strong cold fronts produce the intense windstorms that loft and entrain dust into the air the meteorology governing such outbreaks is likely robust in past climates contrary to the common paleoclimate presumption it is actually the breakdown of the siberian high that permits the dust producing windstorms to occur the importance of cold fronts in generating such windstorms suggests that cooling of high latitude climate during the miocene or during glacial intervals might play a significant role in the signal recorded in the loess deposits the unique springtime factors that generate dust storms are an example of why the common partitioning of asian climate into a winter and summer monsoon is oversimplified and can be misleading

analysis of geometry of volcanoes and faults in terceira island azores evidence for reactivation tectonics at the eur afr plate boundary in the azores triple junction abstract the late pliocene to quaternary 5 ma volcanism in the central and eastern azores archipelago is related to the eurasia africa divergence but a clear deformation pattern has not yet been established at this location this work focuses on the contribution of synthetic aperture radar sar scenes and digital elevation models dem complemented with geophysical geodetic and morpho structural data to establish the geometric relationships between volcanic edifices and tectonic structures in the central and eastern azores archipelago bathymetric data were also used to extend field observations to the significant submarine area of the azores plateau strikes of extension fractures directly observed or inferred from elongated volcanic vents or linear volcanic clusters in terceira island indicate that volcanism is mainly controlled by regional extension as given by nuvel 1a plate motion model additionally other directions were also detected for extension fractures around the santa barbara volcano defining a radial pattern and in the central part of the island exhibiting an s shape pattern although most of the volcanic vents are controlled by extension fractures some seem to be controlled by faulting such as the case of the ones rooted in releasing bends along strike slip or oblique slip faults in the central part of the island concerning the azores plateau most of the structures have directions that do not directly fit with present day direction of relative motion n70 between eurasia and africa directions ranging from n110 to n125 found mainly along the terceira rift are interpreted as ancient transform directions reactivating as transtensional fault zones due to the present day plate motion n s directions are also visible in the plateau being interpreted as former middle oceanic rift faults reactivated as left lateral fault zones these results contrast with the volcanic expression in other hotspot dominated oceanic islands such as the canaries or the hawaii islands chain probably due to the tectonic complexity promoted by the azores triple junction instability through time

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

large scale dynamics of the meiyu baiu rainband environmental forcing by the westerly jet abstract meiyu baiu is the major rainy season from central china to japan brought by a zonally elongated rainband from june to mid july large scale characteristics and environmental forcing of this important phenomenon are investigated based on a reanalysis dataset the meiyu baiu rainband is accompanied by a trough of sea level pressure horizontal shears and sharp moisture gradients near the surface a westerly jet tilted northward with height and large northeastward moisture transport from the south the analysis here reveals the westerly jet as an important culprit for meiyu baiu along the rainband mean ascending motion corresponds well with a band of warm horizontal temperature advection in the midtroposphere throughout summer this adiabatic induction of upward motion originates from the advection of warm air by the westerlies from the eastern flank of the tibetan plateau the ascending motion both induces convection and is enhanced by the resultant condensational heating the westerly jet anch

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

volcano tectonic events and sedimentation since late miocene times at the mohns ridge near 72 n in the norwegian greenland sea abstract the combined analysis of structural and magnetic data from the mohns ridge near 72 n in the norwegian greenland sea shows the existence of different phases of seafloor spreading from anomaly 5a 12 ma which represents the limit of the magnetic survey onwards for the period comprised between anomaly 5 9 ma and anomaly 3a 6 5 ma most particularly a dramatic drecrease of spreading rate is observed by as much as 50 between anomaly 3a and 2a 3 2 ma an acceleration of spreading is observed together with a global uplift of the acoustic basement this latter period may correspond to a local enhancement of tectono volcanic activity the existence of ridges located at 25 and 50 km from the ridge axis and parallel to it suggests that both the beginning 5 4 5 ma and the end 3 2 5 ma of this phase were marked by intense volcanism in the sedimentary basins of the survey area two at least intra sedimentary horizons appear at respectively 140 160 ms and 340 360 ms two way travel time beneath the sediments surface using an inferred value of sedimentation rate of 50 70 m myr the age of the first horizon between 3 and 2 ma corresponds to the beginning of mid pliocene glaciations in the northern hemisphere the age of the second one between 6 5 and 4 5 ma to the pliocene miocene boundary our data and other data from other areas indicate that the three major sources of sediments western spitzbergen margin bear island trough and storfjord trough which supply the mohns ridge area near 72 n have been dramatically more active after 5 5 ma in addition local unconformities recorded in the single channel seismic sections indicate that some tectonic event occurred off axis little after the date corresponding to the deposition of the second horizon s2 if one accepts that the knipovich ridge migrated eastward 6 5 ma ago it can be hypothesized that the tectonic release of the lena trough knipovich ridge mohns ridge system forced the sediment production to increase at each one of the three sediment sources after the miocene pliocene transition

magmatic segmentation of the northern knipovich ridge evidence for high pressure fractionation at an ultraslow spreading ridge 1 the ultraslow spreading knipovich ridge shows axial topographic highs that are associated with off axis linear arrays of seamounts that parallel the spreading direction these linear arrays suggest that the segmentation has been stable for at least 7 8 m y axial topographic highs are marked with distinct gravity anomalies suggesting that they represent thicker crust and hence volcanic segment centers dredging on the northern knipovich ridge between 76 30 and 77 50 n shows that the axial and off axial seamounts and the deeper axial rift valley between the segment centers are composed of volcanic rocks the basalts are subdivided into high intermediate and low feo groups low feo basalts with generally more enriched radiogenic isotope ratios and ree patterns were only dredged midway between segment centers whereas dredges close to the segment centers comprised high and intermediate feo basalts the variations in isotopes and ree patterns suggest that the mantle below the northern knipovich ridge is heterogeneous and the along axis variations can be explained by higher degrees of melting of this heterogeneous source close to segment centers the high and intermediate feo basalts define different mgo feo liquid lines of descents that may reflect differences in the crystallization pressures or the water contents of the magmas similar water contents of the high and intermediate feo basalts suggest that the differences in feo primarily are a pressure effect and that the intermediate feo group fractionated at higher pressure than the high feo group the high feo basalts show both low and high pressure phenocryst assemblages suggesting crystallization at high pressures 6 7 kbar followed by shallow low pressure fractionation on the basis of the new data we propose that magmas at this ultraslow spreading ridge fractionate at mantle depth except at segment centers where magma productivity occasionally is high enough to sustain shallow level magma reservoirs

tobi sidescan sonar imagery of the very slow spreading southwest indian ridge evidence for along axis magma distribution new deep tow sidescan sonar data from the southwest indian ridge reveal complex volcanic tectonic interrelationships in the axial zone of this ultra slow spreading ridge while some constructional volcanic features resemble examples documented at the slow spreading mid atlantic ridge such as axial volcanic ridges hummocky and smooth lava flows their distribution and dimensions differ markedly the largest axial volcanic ridges occur at segment centres but fresh looking volcanic constructions also occur at segment ends and in the deep basins marking the non transform discontinuities the orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes this observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes with associated fractures opening in response to magma pressure may control local emplacements of axial volcanic ridges at obliquely spreading ridges this non systematic pattern of ridge characteristics suggests an along axis variation between focused and distributed magmatic supply a model which is supported by our interpretation of low amplitude mantle bouguer anomalies calculated for the area we propose that a change of the axial segmentation pattern from two segments to the present day three segments may have introduced additional instability into the crustal accretion process

crustal thickness and earthquake distribution south of the logachev seamount knipovich ridge 1 during rv polarstern cruise ark xxiv 3 2009 a geophysical study along the ultraslow spreading knipovich ridge was conducted the survey located in the rift valley south of the logachev seamount 76 36 n provides a crustal thickness of 4 5 km in the amagmatic parts and 5 7 km underneath the seamount itself the velocity depth function indicates the presence of a thick oceanic layer 2 4 5 km but no indication for a thick oceanic layer 3 the only exception is the area underneath the logachev seamount where velocities higher than 6 km s are detected this indicates a stronger and focussed melt supply underneath the seamount local seismicity was analysed for two days in total 191 quakes were identified in the vicinity of the rift valley with magnitudes up to ml 2 6 at least 48 of them are located in the upper mantle up to 18 km below sea level supporting models predicting a cold mantle or conductive cooling underneath ultraslow ridges to explain the reduced melt supply

diurnal variations of precipitation deep convection and lightning over and east of the eastern tibetan plateau abstract diurnal cycles of total rainfall precipitation features mesoscale convective systems mcss deep convection precipitation vertical structure and lightning over the eastern tibetan plateau tp and eastward through china are investigated using 11 yr of tropical rainfall measuring mission trmm measurements diurnal cycles of rainfall and precipitation features present apparent phase propagation eastward from the eastern tp for about 1000 km the phase propagation is most evident during the pre mei yu and mei yu seasons however it weakens with the northward progress of the east asian monsoon and ceases in midsummer during the pre mei yu season diurnal cycles of storm population total rainfall deep convection and lightning over the central and eastern tp foothills are in phase peaking during the early morning another striking feature of the pre mei yu season is that nocturnal rainfall and mcss prevail over the southeastern tp foothills following the deep convection and lightning maxim

oblique rifting in a slow spreading ridge in oceanic rifts the spreading direction is usually nearly perpendicular to the ridge axis but at the reykjanes1 and mohns2 4 ridges this direction is highly oblique 30 to 40 to the axis these ridges possess axial valleys containing oblique raised features highs corresponding to second or third order ridge axis discontinuities5 at the reykjanes ridge searle and laughton1 observed small highs which they interpreted as volcanic domes here we present a fault map of the mohns ridge obtained from image processing of seabeam data we argue that the oblique highs in this ridge are of tectonic origin the axial valley is interpreted as the surface trace of a deformable band of oceanic lithosphere inherited from an earlier stage of perpendicular spreading we describe laboratory experiments on brittle ductile models which support this interpretation in which stretching at 30 to a deformable band reproduces the en echelon horst and graben pattern of the mohns ridge

cluster analysis of northern hemisphere wintertime 500 hpa flow regimes during 1920 2014 abstractclusters in the northern hemisphere wintertime 10 day low pass filtered 500 hpa height field are identified using the method of self organizing maps soms results are based on 1 a 57 winter record of era and 2 a 93 winter record of the noaa twentieth century reanalysis 20cr the clusters derived from soms appear to be more robust and more linearly independent than their counterparts derived from ward s method and clusters with comparable numbers of member days are more distinctive in terms of the standardized euclidean distances of their centroids from the centroid of the dataset the reproducible som clusters in the hemispheric domain are 1 the negative polarity of the north atlantic oscillation nao 2 a pattern suggestive of alaska blocking with a downstream wave train extending over north america and the north atlantic 3 an enhancement of the climatological mean stationary wave pattern in the western hemisphere that projects positively upon the pacific north america pna pattern

field test of a permanent in well fiber optic seismic system a deviated borehole was drilled and completed to test systems for deployment of permanent in well three component fiber optic seismic sensors two systems were developed and tested for deployment of these sensors the first system involved using a passive mandrel device to enable coupling of the sensors simply using the weight of the tubing the second system involved use of an active mandrel that incorporates a spring loaded pad device to isolate the sensor from the tubing vibration in addition reference wireline vertical seismic profile data were acquired both in the empty cased borehole and through the tubing at the same location as the fiber optic sensor analysis of the data showed that best results were obtained using the active design tubing related resonances were noted on data acquired using the passive design whereas the active design did a good job of discriminating among these undesirable signals