recent changes in the pacific subtropical cells inferred from an eddy resolving ocean circulation model abstract in this study the subtropical cells stc in the pacific ocean are analyzed using an eddy resolving ocean general circulation model driven by atmospheric forcing for the years 1992 2003 in particular the authors seek to identify decadal changes in the stcs in the model and to compare them with observations in order to understand the consequences of such changes for the equatorial ocean heat and mass budgets the simulation shows a trend toward increasing pycnocline volume transport at 9 n and 9 s across the basin from 1992 to 2003 this increase 4 9 1 0 sv sv 106 m3 s 1 is in qualitative agreement with observations and is attributed primarily to changes in the interior ocean transport which are partially compensated by opposing western boundary transports the subtropical meridional volume transport convergence anomalies in the model pycnocline are found to be consistent with anomalous volume transports in both the observed and modeled equatorial undercurrent as well as with the magni

origins and dynamics of the 90 day and 30 60 day variations in the equatorial indian ocean abstract sea level observations in the equatorial indian ocean show a dominant spectral peak at 90 days and secondary peaks at 30 60 days over an intraseasonal period 20 90 days a detailed investigation of the origins and dynamics of these variations is carried out using an ocean general circulation model namely the hybrid coordinate ocean model hycom two parallel experiments are performed in the tropical indian ocean basin for the period 1988 2001 one is forced by ncep 3 day mean forcing fields together with the climate prediction center cpc merged analysis of precipitation cmap pentad precipitation and the other is forced by monthly mean fields to help to understand the role played by the wind driven equatorial wave dynamics a linear continuously stratified ocean model is also used both the observed and modeled 90 day sea level anomaly fields and hycom surface current clearly show equatorial kelvin and first meridional mode rossby wave structures that are forced by the 90 day winds the

3d diffraction imaging of linear features and its application to seismic monitoring many subsurface features such as faults fractures cracks or fluid content terminations are defined by geological discontinuities the seismic response from such features is encoded in diffractions we develop an algorithm for imaging such discontinuities by detecting edge diffractions the algorithm exploits phase reversal phenomena of edge diffractions and uses them as a criterion to separate these diffractions from specular reflections and diffractions produced by a leaner object the performance of the method is demonstrated on both synthetic and real 3d seismic data the output image focuses the diffracted energy back to its origin and shows high semblance values at the edge of the object the method is applied on conventionally stacked data producing an image that contains only diffraction events called the d volume we also reveal the potential of diffractions to image and track the changes of a co2 plume using time lapse analysis and detect any possible co2 seepage from its primary containment

anisotropy parameter inversion in vertical axis of symmetry media using diffractions diffractions play a vital role in seismic processing as they can be utilized for high resolution imaging applications and analysis of subsurface medium properties like velocity they are particularly valuable for anisotropic media as they inherently possess a wide range of dips necessary to resolve the angular dependence of velocity however until recently the focus of diffraction imaging or inversion algorithms have been only on the isotropic approximation of the subsurface using diffracted waves we develop a framework to invert for the effective model this effective model is obtained through scanning over possible effective values and selecting the one that best fits the observed moveout curve for each diffractor location the obtained effective model is then converted to an interval model using a dix type inversion formula the inversion methodology holds the potential to reconstruct the true model with sufficiently high accuracy and resolution properties however it relies on an accurate estimation of diffractor locations which in turn requires good knowledge of the background velocity model we test the effectiveness and applicability of our method on the vertical transverse isotropic marmousi model the inversion results yield a reasonable match even for the complex marmousi model

holocene paleoclimates of india we present a comprehensive summary of the available palaeoclimate records from india and compare the results from different proxies the results indicate i fluctuating lake levels during the early holocene the period of relatively higher lake levels from increased precipitation efficiency was reached only 7 2 6 0 cal kyr bp possibly due to increased contribution from winter rainfall ii the onset of aridity in nw india could have begun as early as 5 3 cal kyr bp subsequently there were multiple wet events but of shorter duration and smaller magnitude than during the mid holocene iii there is evidence of several short term climate events in the proxy record however in the absence of a rigorous chronological framework a detailed regional correlation is not possible at this stage finally a comparison between marine and terrestrial records indicates that episodes of strongest and weakest monsoon winds were not always associated with wettest and driest episodes respectively in the nw indian lakes

subsurface equatorial zonal current in the eastern indian ocean 1 variations of subsurface zonal current in the eastern equatorial indian ocean are investigated by examining 6 year data december 2000 november 2006 from acoustic doppler current profiler adcp mooring at 0 s 90 e the analysis indicates the presence of an eastward equatorial subsurface current between 90 and 170 m depths during both boreal winter and summer during boreal winter the generation of eastward pressure gradient which drives an eastward flow in the thermocline is caused primarily by upwelling equatorial kelvin waves excited by prevailing easterly winds on the other hand the downwelling rossby waves generated by the reflection of the spring downwelling kelvin waves in the eastern boundary as well as the upwelling equatorial kelvin waves triggered by easterlies create an oceanic state that favors the generation of the eastward pressure gradient during boreal summer the subsurface current reveals a distinct seasonal asymmetry the maximum eastward speed of 63 cm s 1 is observed in april and secondary maximum of 49 cm s 1 is seen in october the zonal transport per unit width within depth of the subsurface current exhibits similar variations reaching maximum eastward transport of 35 m2 s 1 in april and secondary maximum of 29 m2 s 1 in october moreover the subsurface current during boreal summer undergoes significant interannual variations it was absent in 2003 but it was anomalously strong during 2006

observed interannual variability of zonal currents in the equatorial indian ocean thermocline and their relation to indian ocean dipole interannual variability of zonal currents in the eastern equatorial indian ocean thermocline is significantly correlated with sea surface temperature sst in the eastern pole of the indian ocean dipole iod this relationship is more significant than for zonal currents and the dipole mode index the latter of which measures zonal sst anomaly gradient the variability of zonal currents in the thermocline is consistent with the variations of eastward pressure gradient force in the thermocline and the equatorward geostrophic thermocline flows associated with the shallow meridional overturning circulations in the eastern tropical indian ocean our analysis suggests a positive feedback between zonal currents in the thermocline and sst in the eastern pole of iod the combination of equatorial wind and off equatorial wind stress curls associated with sst anomalies in the eastern pole of the iod drives interannual variability in equatorial zonal thermocline flows and its covariability with the shallow meridional overturning cells

impacts of the iod associated temperature and salinity anomalies on the intermittent equatorial undercurrent anomalies the study of equatorial undercurrent euc has attracted a broad attention in recent years due to its strong response and feedback to the indian ocean dipole in this paper we first produce a high quality simulation of three dimensional temperature salinity and zonal current simulation from 1982 to 2014 using a high resolution ocean general circulation model on this basis with two sensitivity experiments we investigate the role of temperature and salinity anomalies in driving and enhancing the euc during the positive iod events by examining the variation of the euc seasonal cycle and diagnosing the zonal momentum budget along the equatorial indian ocean our results show that during january march the euc can appear along the entire equatorial indian ocean in all years but during august november the euc can appear and reach the eastern indian ocean only during the positive iod events the zonal momentum budget analysis indicates that the pressure gradient force contributes most to the variation of the eastward acceleration of zonal currents in the subsurface during the positive iod events strong negative subsurface temperature anomalies exist in the eastern indian ocean with negative surface salinity anomalies in the central and eastern indian ocean resulting in a large pressure gradient force to drive euc during the august november further the results of two sensitivity experiments indicate that the temperature anomalies significantly impact the pressure gradient force playing a leading role in driving the euc while the surface salinity anomalies can secondarily help to intensify the eastward euc through increasing the zonal density gradient in the eastern indian ocean and impacting the vertical momentum advection in the subsurface

strong coherence between solar variability and the monsoon in oman between 9 and 6 kyr ago variations in the amount of solar radiation reaching the earth are thought to influence climate but the extent of this influence on timescales of millennia to decades is unclear a number of climate records show correlations between solar cycles and climate1 but the absolute changes in solar intensity over the range of decades to millennia are small2 and the influence of solar flux on climate is not well established the formation of stalagmites in northern oman has recorded past northward shifts of the intertropical convergence zone3 whose northward migration stops near the southern shoreline of arabia in the present climate4 here we present a high resolution record of oxygen isotope variations for the period from 9 6 to 6 1 kyr before present in a th u dated stalagmite from oman the 18o record from the stalagmite which serves as a proxy for variations in the tropical circulation and monsoon rainfall allows us to make a direct comparison of the 18o record with the 14c record from tree rings5 which largely reflects changes in solar activity6 7 the excellent correlation between the two records suggests that one of the primary controls on centennial to decadal scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation

the effect of vertical shear on tropical cyclone intensity change abstract the effect of vertical shear on tropical cyclone intensity change is usually explained in terms of ventilation where heat and moisture at upper levels are advected away from the low level circulation which inhibits development a simple two layer diagnostic balance model is used to provide an alternate explanation of the effect of shear when the upper layer wind in the vortex environment differs from that in the lower layer the potential vorticity pv pattern associated with the vortex circulation becomes tilted in the vertical the balanced mass field associated with the tilted pv pattern requires an increased midlevel temperature perturbation near the vortex center it is hypothesized that this midlevel warming reduces the convective activity and inhibits the storm development previous studies have shown that diabatic heating near the storm center acts to reduce the vertical tilt of the vortex circulation these studies have also shown that there is an adiabatic process that acts to redu

seismic monitoring of diffraction images for detection of local heterogeneities diffracted waves contain valuable information regarding both the structure and composition of the media they are in in seismic data processing however these waves are usually regarded as noise in this paper we present an attempt to use scattered diffracted waves for the detection of local heterogeneities the method is based on the detection of diffracted waves by concentrating the signal amplitudes from diffracting points on the seismic section this is done using a correlation procedure that enhances the amplitude of the seismic signal at the location of the diffractors on the common diffraction point section d section the new local time correction for diffraction traveltime curve parameterization is based on the radius of curvature of the diffracted wavefront and near surface velocity we use the idea of seismic monitoring for detection and delineating local objects which may occur within the subsurface resulting from human activity or fast geological processes the method consists of continuou

imaging discontinuities on seismic sections in routine seismic processing normal moveout nmo corrections are performed to enhance the reflected signals on common depth point or common midpoint stacked sections however when faults are present reflection interference from the two blocks and the diffractions from their edges hinder fault location determination destruction of diffraction patterns by poststack migration further inhibits proper imaging of diffracting centers this paper presents a new technique which helps in the interpretation of diffracting edges by concentrating the signal amplitudes from discontinuous diffracting points on seismic sections it involves application to the data of moveout and amplitude corrections appropriate to an assumed diffractor location the maximum diffraction amplitude occurs at the location of the receiver for which the diffracting discontinuity is beneath the source receiver midpoint since the amplitudes of these diffracted signals drop very rapidly on either side of the midpoint an appropriate amplitude correction must be applied also because the diffracted signals are present on all traces one can use all of them to obtain a stacked trace for one possible diffractor location repetition of this procedure for diffractors assumed to be located beneath each surface point results in the common fault point cfp stacked section which shows diffractor locations by high amplitudes the method was tested for synthetic data with and without noise it proves to be quite effective but is sensitive to the velocity model used for moveout corrections therefore the velocity model obtained from nmo stacking is generally used for enhancing diffractor locations by stacking finally the technique was applied to a field reflection data set from an area south of princess well in alberta

poststack velocity analysis by separation and imaging of seismic diffractions small geologic features manifest themselves in seismic data in the form of diffracted waves which are fundamentally different from seismic reflections using two field data examples and one synthetic example we demonstrate the possibility of separating seismic diffractions in the data and imaging them with optimally chosen migration velocities our criteria for separating reflection and diffraction events are the smoothness and continuity of local event slopes that correspond to reflection events for optimal focusing we develop the local varimax measure the objectives of this work are velocity analysis implemented in the poststack domain and high resolution imaging of small scale heterogeneities our examples demonstrate the effectiveness of the proposed method for high resolution imaging of such geologic features as faults channels and salt boundaries

paleoclimatic record of speleothems in a tropical region study of laminated sequences from a holocene stalagmite in central west brazil abstract this paper presents a detailed mineralogical and geochemical study of a 44 cm long stalagmite from mato grosso do sul state brazil the 230 th 234 u dating of this speleothem indicates a constant growth rate since 3800 years ago and allows a mean growth rate estimation of 116 m yr the stalagmite is entirely constituted of aragonite more than 1500 visible laminae were observed most of which correspond to the alternation of thin brown and thicker clear aragonite layers associated in couplets averaging 100 m thickness the quite fair matching between the mean yearly growth rate determined by the u th dating and the observed thickness of the couplets support the idea that these couplets are annual at the base of the stalagmite between 3800 and 2500 yr bp the variations of the aragonite fabric reveal a repetitive sequential arrangement which is related to recurrent decrease of the speleothem growth rate and water disponibility a comparison of the paleorecord provided by the stalagmite with respect to other holocene paleoclimatic data in south america reveals that its growth history is consistent with the known regional pattern of climate change this confirms the potential of such stalagmite as climatic paleorecord

the role of moist baroclinic instability in the growth and structure of monsoon depressions abstract the effect of latent heat of condensation on baroclinic instability process is studied using the model developed by bonatti and rao this model includes the effect of latent heat of condensation as parameterized by mak a zonal wind profile that varies linearly with pressure and a constant lapse rate of 6 c km 1 the effect of dissipative terms and vertical variation of heating on baroclinic instability with positive and negative zonal wind shears is studied dissipative terms and the type and intensity of heating change the selection of the most unstable wave viscosity and newtonian cooling stabilize short waves the characteristics of the most unstable wave obtained using a double maximum in the vertical distribution of heating and observed vertical profiles of zonal wind and temperature representative of the summer monsoon of india resemble those of observed monsoon depressions the inclusion of dissipative effects makes the green mode the most unstable wave

precipitation variability in the indian central himalaya during last ca 4 000 years inferred from a speleothem record impact of indian summer monsoon ism and westerlies abstract we report the first high resolution stalagmite record from indian central himalaya by using a combined oxygen and carbon isotopic study spanning the last 4000 years in addition to the multi decadal events the stalagmite data also provide information on the variability in the intensity of precipitation indian summer monsoon ism and westerlies in north indian hills during the late holocene the 18o values show strong variability between 2 1 and 8 9 the extremely high variability points to the monsoon from two different sources compared to other similar aged caves which are primarily controlled by one monsoon such as the east asian summer monsoon easm although ism has penetrated into some parts in the areas of easm in the past or the westerlies therefore we suggest that the westerlies played an important role in the late holocene climate of the indian himalaya as the source of the westerlies is mediterranean atlantic the north atlantic oscillation nao which was responsible for comparatively stronger westerlies in southern europe may have brought higher precipitation to the north indian hills the stronger westerlies may also have resulted in the highest precipitation between 0 5 and 0 25 ka bp 1450 1700 ad a part of the lia in contrast to the weakening of the indian summer monsoon ism in peninsular india the stalagmite has also recorded the driest period peaking at 3 2 ka bp which may be nearly synchronous with the final collapse of the harappan culture in northwest india

continuously stratified models of the steady state equatorial ocean abstract two linearized vertically diffusive steady state models are formulated on an equatorial plane the purpose is a to investigate the vertical boundary layer structure in a continuously stratified ocean spanning the equator and b to test the sensitivity of the results to different turbulence parameterizations both models are analytically tractable in a horizontally unbounded basin one is characterized by newtonian cooling the other has biharmonic friction for either model the equations are analogous to the well known equations governing equatorial wave motion this analogy is exploited in both obtaining and interpreting the solutions in both models zonal wind forcing leads to features such as the equatorial undercurrent south equatorial current and equatorial intermediate current structures resembling the recently discovered subsurface countercurrents are also generated the depth velocity and other scales are model dependent but the basic dynamics are not specifically near the eq

palaeoclimatic conditions in the upper pleistocene and holocene bhimtal naukuchiatal lake basin in south central kumaun north india abstract a 52 m thick upper pleistocene and holocene terrestrial succession in the bhimtal naukuchiatal basin south central kumaun himalaya india was studied using chronological palaeontological palynological and 13c measurements the section recorded evidence for climatic changes at least two phases of arid climate and one phase of humid climate were recognised preliminary palaeomagnetic studies revealed a reversal of polarity presumably correlatable with the mono lake excursion prior to this no reversal event in the upper pleistocene holocene terrestrial sediments of indian subcontinent is known a fossiliferous horizon discovered in the lower part of the section consisted of sorex and mus this is the only report of a late pleistocene micromammalian assemblage in the kumaun himalaya

decadal variations in the subtropical cells and equatorial pacific sst 1 a mechanism for generating decadal sea surface temperature sst variability in the equatorial pacific is investigated using an ocean general circulation model forced by observed wind stress equatorial sst variability is governed by different ocean dynamics on interannual and decadal time scales at interannual time scales equatorial sst anomalies are mainly driven by equatorial winds at decadal time scales on the other hand they are forced both by equatorial winds and by off equatorial winds in the tropics while the contribution from midlatitude winds poleward of 25 is negligible trade wind variations in the off equatorial tropics force equatorial sst variability by spinning up and down the subtropical cells that transport cold water into the equatorial upwelling zone equatorial sst anomalies induced by this mechanism lag behind the response to local equatorial winds by about two years

mid late holocene monsoonal variations from mainland gujarat india a multi proxy study for evaluating climate culture relationship a multi proxy study involving palynology phytoliths sedimentology clay mineralogy carbon isotopes and magnetic mineralogy was carried out on wadhwana lake sediments from sub humid zone of mainland gujarat to determine the mid holocene climatic fluctuations and its possible impact on the harappan culture the 1 03 m sediment profile of wadhwana lake shows five paleoclimatic phases the study reveals high lake stand during phase i 7500 5560 cal yr bp a considerable cool and moist climatic condition during phase i is inferred due to the presence of pollen belonging to wet evergreen taxa and high phytolith climatic index ic later part of phase i show gradual replacement of evergreen to deciduous pollen taxa decrease in ic value and dominance of smectite over kaolinite indicating a reduction in wet climatic condition due to decline in the precipitation and prevalence of seasonally dry climate however a large variety of madhuca meliaceae asteraceae pollen taxa abundant fresh water algae chrozophora and ceralia pollen in this period provides evidence of intensified arboriculture and agricultural activity low lake level and dry climate have been documented during phase ii 5560 to 4255 cal yr bp and are synchronous with the lake records of western indian region high values of phytolith aridity index iph high primary minerals increase in the i 13c values provide evidence for excessive dry climatic conditions at 4255 cal yr bp phase iii shows a gradual strengthening of sw monsoon after 3500 cal yr bp phase iv shows a short pulse of dry climatic conditions 3238 to 2709 cal yr bp followed by somewhat similar to present day climate for phase v the study concludes that onset of dry climate after 5500 cal yr bp is a regionally spread synchronous event that has been documented in several lake records of western india it is surmised that the urbanization in harappan civilization in north west india was coincident with the initial phase of declining rainfall of mid holocene the emergence of cultural complexity of harappan civilization should be seen as an initial adaptation to earliest phase of environmental deterioration and its subsequent decline is probably linked with the changing seasonality pattern and excessive dry climate of later phase 4200 4255 cal yr bp of mid holocene

persistent multidecadal power of the indian summer monsoon article i nfo the instrumental record of indian summer monsoon ism precipitation displays two complete manifesta tions of an inferred multidecadal cycle hitherto no precipitation sensitive proxy record from the indian subcontinent has had the necessary resolution and length to adequately assess whether this observed feature is an inherent aspect of the ism system on longer timescales here we present compelling evidence for persistence of this cycle using a millennial length ad 600 1550 and sub annually resolved speleothem oxygen isotope record 18 o from dandak cave in east central india the record displays a high degree of correlation with a speleothem based asian monsoon reconstruction from wanxiang cave in north central china on annual to decadal timescales showing theregional significanceof these findings theobserved period inourmonsoonreconstructionissimilartothatassociatedwiththegleissbergsolarcycleandmultidecadalsea surface temperature variability in the north atlantic amo both of which are often cited as the prominent pacemakers of ism variability on multidecadal timescales we document transient coherence between ism precipitation amount and solar variability that persists exclusively in the century prior to and during the medieval climate anomaly nominally ad 950 1300 the non stationary nature of the sfv monsoon relationship presented here may be evidence of the time varying influence of tropical ocean atmosphere dynamics on the solar monsoon link however it is not possible to show with significance that this period of coherenceisanythingmorethananartifactoftwotimeserieswithsimilarspectra wetherefore areinclinedto interpret our record as evidence of a minimal role of solar variability in driving persistent multidecadal variability of the ism multidecadal sst variability in the north atlantic remains as the likely alternative driver for persistence of this cycle regardless of the causative mechanism s the amplitude and regional signature of the observed cycle in ism precipitation highlight its societal importance with respect to forecasting ism precipitation on multidecadal timescales

a linear stratified ocean model of the equatorial undercurrent a linear stratified ocean model is used to study the wind driven response of the equatorial ocean the model is an extension of the lighthill 1969 model that allows the diffusion of heat and momentum into the deeper ocean and so can develop non trivial steady solutions to retain the ability to expand solutions into sums of vertical normal modes mixing coefficients must be inversely proportional to the square of the back ground vaisala frequency the model is also similar to the earlier homogeneous ocean model of stommel i960 he extended ekman dynamics to the equator by allowing his model to generate a barotropic pressure field the present model differs in that the presence of stratification allows the generation of a baroclinic pressure field as well the most important result of this paper is that linear theory can produce a realistic equatorial current structure the model undercurrent has a reasonable width and depth scale there is westward flow both above and below the undercurrent the meridional circulation conforms to the classical picture suggested by cromwell 1953 unlike the stommel solution the response here is less sensitive to variations of parameters ocean boundaries are not necessary for the existence of the undercurrent but are necessary for the existence of the deeper equatorial intermediate current the radiation of equatorially trapped rossby and kelvin waves is essential to the development of a realistic undercurrent because the system supports the existence of these waves low order vertical modes can very nearly adjust to sverdrup balance defined below which in a bounded ocean and for winds without curl is a state of rest as a result higher order vertical modes are much more visible in the total solution this property accounts for the surface trapping and narrow width scale of the equatorial currents the high order modes tend to be in yoshida balance defined below and generate the characteristic meridional circulation pattern associated with equatorial ekman pumping

time lapse seismic reservoir monitoring time lapse seismic reservoir monitoring has advanced rapidly over the past decade there are currently about 75 active projects worldwide and more than 100 cumulative projects in the past decade or so the present total annual expenditures on 4 d seismic projects are on the order of 50 100 million us this currently represents a much smaller market than 3 d seismic but the use of 4 d seismic has grown exponentially over the past decade and is expected to continue to do so

impact of indian ocean dipole and el nino southern oscillation wind forcing on the wyrtki jets 1 interannual variability of the wyrtki jets is studied in the context of indian ocean dipole iod and el nino and southern oscillation enso wind forcing using a three dimensional numerical ocean model and observations the boreal fall october november wyrtki jet is more significantly affected than the boreal spring april may wyrtki jet since both the iod and enso tend to peak toward the end of the calendar year various statistical methods are used in an attempt to separate the impacts of the iod and enso on these jets with emphasis on the fall jet the first two modes of an empirical orthogonal function eof decomposition account for about 90 and 85 of variability in zonal currents and wind stress respectively along the equator in the indian ocean but eof analysis does not cleanly separate out iod and enso forcing and response partial correlation analysis reveals that iod wind forcing and zonal equatorial current response are stronger on average than for enso and extend further west across the basin composite analysis of iod only enso only and combined iod and enso years provides a complementary definition of the relative contributions of these two phenomena on wyrtki jet variability and in general is consistent with the results of the partial correlation analysis

repeatability analysis of land time lapse seismic data co2crc otway pilot project case study time lapse seismics is the methodology of choice for remotely monitoring changes in oil gas reservoir depletion reservoir stimulation or co 2 sequestration due to good sensitivity and resolving power at depths up to several kilometres this method is now routinely applied offshore however the use of time lapse methodology onshore is relatively rare the main reason for this is the relatively high cost of commercial seismic acquisition on land a widespread belief of a relatively poor repeatability of land seismic data prevents rapid growth in the number of land time lapse surveys considering that co 2 sequestration on land is becoming a necessity there is a great need to evaluate the feasibility of time lapse seismics for monitoring therefore an understanding of the factors influencing repeatability of land seismics and evaluating limitations of the method is crucially important for its application in many co 2 sequestration projects we analyse several repeated 2d and 3d surveys acquired within the otway co2 sequestration pilot project operated by the cooperative research centre for greenhouse technologies co2crc in australia in order to determine the principal limitations of land time lapse seismic repeatability and investigate the influence of the main factors affecting it our findings are that the intrinsic signal to noise ratio s n signal to coherent and background noise levels and the normalized root meansquare nrms difference are controlled by the source strength and source type however the post stack s n ratio and corresponding nrms residuals are controlled mainly by the data fold for very high fold data the source strength and source type are less critical

diffraction imaging by focusing defocusing an outlook on seismic superresolution diffractions always need more advertising it is true that conventional seismic processing and migration are usually successful in using specular reflections to estimate subsurface velocities and reconstruct the geometry and strength of continuous and pronounced reflectors however correct identification of geological discontinuities such as faults pinch outs and small size scattering objects is one of the main objectives of seismic interpretation the seismic response from these structural elements is encoded in diffractions and diffractions are essentially lost during the conventional processing migration sequence hence we advocate a diffraction based data oriented approach to enhance image resolution as opposed to the traditional image oriented techniques which operate on the image after processing and migration even more it can be shown that at least in principle processing of diffractions can lead to superresolution and the recovery of details smaller than the seismic wavelength the so called reflection stack is capable of effectively separating diffracted and reflected energy on a prestack shot gather by focusing the reflection to a point while the diffraction remains unfocused over a large area muting the reflection focus and defocusing the residual wavefield result in a shot gather that contains mostly diffractions diffraction imaging applies the classical isotropic diffraction stack to these diffraction shot gathers this focusingmuting defocusing approach can successfully image faults small size scattering objects and diffracting edges it can be implemented both in model independent and modeldependent contexts the resulting diffraction images can greatly assist the interpreter when used as a standard supplement to full wave images

steered migration in hard rock environments hard rock seismic exploration normally has to deal with rather complex geological environments these types of environments are usually characterized by a large number of local heterogeneity e g faults fracture zones and steeply dipping interfaces the seismic data from such environments often have a poor signal to noise ratio because of the complexity of hard rock geology to be able to obtain reliable images of subsurface structures in such geological conditions processing algorithms that are capable of handling seismic data with a low signal to noise ratio are required for a reflection seismic exploration in this paper we describe a modification of the 3d kirchhoff post stack migration algorithm that utilizes coherency attributes obtained by the diffraction imaging algorithm in 3d to steer the main kirchhoff summation the application to a 3d synthetic model shows the stability of the presented steered migration to the presence of high level of the random noise a test on the 3d seismic volume acquired on a mine site located in western australia reveals the capability of the approach to image steep and sharp objects such as fracture and fault zones and lateral heterogeneity

on the role of sea surface temperature gradients in forcing low level winds and convergence in the tropics abstract we examine the importance of pressure gradients due to surface temperature gradients to low level p 700 mb flow and convergence in the tropics over time scales 1 month the latter plays a crucial role in determining the distribution of cumulonimbus convection and rainfall our approach is to consider a simple one layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically consistent with ecmwf analyzed data the top of the layer is taken at 700 mb the influence from higher levels is intentionally suppressed by setting horizontal pressure gradients and frictional stresses to zero at the top of the layer horizontal convergence within the layer is taken up by cumulonimbus mass flux however the development of the cumulonimbus mass flux is associated with a short relaxation time o hr roughly the development time for such convection during this short time horizontal convergence acts to redistribute mass so as to reduce horizontal pressur

direct measurements of upper ocean currents and water properties across the tropical pacific during the 1990s abstract meridional sections of upper ocean zonal currents potential temperature and salinity are estimated at ten longitudes from 143 e to 95 w using conductivity temperature depth and acoustic doppler current profiler data from 172 synoptic sections taken in the tropical pacific between 138 e and 86 w mostly in the 1990s data reduction is carried out in a potential isopycnal and mixed layer framework to preserve a sharp pycnocline a mixed layer water property extrema and velocity extrema mean zonal currents potential temperatures and salinities are produced at each longitude the seasonal cycles of these fields are also estimated as well as a simple el nino southern oscillation enso cycle zonal sections along the equator are also presented properties of the near equatorial zonal currents including transports temperatures and salinities are estimated separately from the synoptic sections the seasonal cycles of these quantities and their correlations with the southern oscillation index are investigated the work is distinguished from most existing literature in that direct estimates of zonal velocity are combined with contemporaneous temperature and salinity data allowing trans pacific estimates of near equatorial current transports and properties including those of the northern branch of the south equatorial current the new guinea coastal undercurrent and the equatorial undercurrent

variability of upper ocean characteristics and tropical cyclones in the south west indian ocean track and intensity are key aspects of tropical cyclone behavior intensity may be impacted by the upper ocean heat content relevant for tc intensification known as tdy and barrier layer thickness blt here the variability of tdy and blt in the south west indian ocean and their relationships with tropical cyclones are investigated it is shown that rapid cyclone intensification is influenced by large tdy values thick barrier layers and the presence of anticyclonic eddies for tc generation in the south west indian ocean the parameter tdy was found to be important large blt values overlay with large tdy values during summer both fields are modulated by the westward propagation of rossby waves which are often associated with enso for example the 1997 1998 el nino shows a strong signal in tdy sst and blt over the south west indian ocean after this event an increasing trend in tdy occurred over most of the basin which may be associated with changes in atmospheric circulation increasing sst power dissipation index and frequency of category 5 tropical cyclones also occurred from 1980 to 2010 to further examine the links between tropical cyclones tdy and blt the ocean response to category 5 tropical cyclone bansi that developed near madagascar during january 2015 was analyzed its unusual track was found to be linked with the strengthening of the monsoonal north westerlies while its rapid intensification from category 2 to category 4 was linked to a high tdy region associated with a warm core eddy and large blt

differences in draft core statistics from the wet to dry spell over gadanki india 13 5 n 79 2 e abstract the indian mesosphere stratosphere troposphere mst radar observations during the passage of 37 convective systems are utilized to investigate the characteristics of vertical air velocity w in different convection categories shallow deep and decaying and also the differences in draft core statistics from the wet to dry spell the radar and optical rain gauge measurements show pronounced differences in core statistics in terms of their vertical structure draft strength size number and the elevation angle and surface rainfall characteristics from the dry to wet spell the shallow convective cores are preponderant in the dry spell composite w profiles retrieved from all deep cases and also from individual convection cases depict an upper tropospheric peak in the wet spell and a bimodal distribution peaks at 5 and 11 13 km in the dry spell illustrating that they are characteristic features of wet and dry spells the average vertical extents of the cores are nearly equal about 8 km

changes in surface productivity and subsurface denitrification during the holocene a multiproxy study from the eastern arabian sea spatial and temporal variations in the intensity of subsurface denitrification in the water column of the eastem arabian sea have been investigated using chemical and isotopic proxies the data show a steady increase in the denitrification intensity from 10 ka to 2 ka bp probably caused by changes in surface productivity fuelled by strengthening of upwelling monsoonal intensity in addition changes in redox conditions and detrital material input are assessed using abundances of a suite of major and trace elements the temporal variations in some of the monsoonal proxies are characterized by periodicities of 1400 and 700 years similar to those reported in the south asian chinese monsoon and high latitude climate north atlantic region

diffraction imaging in depth high resolution imaging is of great value to an interpreter for instance to enable identification of small scale faults and to locate formation pinch out positions standard approaches to obtain high resolution information such as coherency analysis and structure oriented filters derive attributes from stacked migrated images since they are image driven these techniques are sensitive to artifacts due to an inadequate migration velocity in fact the attribute derivation is not based on the physics of wave propagation diffracted waves on the other hand have been recognized as physically reliable carriers of high or even super resolution structural information however high resolution information encoded in diffractions is generally lost during the conventional processing sequence indeed migration kernels in current migration algorithms are biased against diffractions we propose here methods for a diffraction based data oriented approach to image resolution we also demonstrate the different behaviour of diffractions compared to specular reflections and how this can be leveraged to assess characteristics of subsurface features in this way a rough surface such as a fault plane or unconformity may be distinguishable on a diffraction image and not on a traditional reflection image we outline some characteristic properties of diffractions and diffraction imaging and present two novel approaches to diffraction imaging in the depth domain the first technique is based on reflection focusing in the depth domain and subsequent filtering of reflections from prestack data the second technique modifies the migration kernel and consists of a reverse application of stationary phase migration to suppress contributions from specular reflections to the diffraction image both techniques are proposed as a complement to conventional full wave pre stack depth migration and both assume the existence of an accurate migration velocity

did the indo asian summer monsoon decrease during the holocene following insolation a few studies from the western arabian sea indicate that the indian summer or southwest monsoon ism after attaining its maximum intensity at ca 9 ka declined during the holocene as did insolation in contrast earlier and later observations from both the eastern and the western arabian sea do not support this inference analysis of multiple proxies of productivity in a new sediment core from the western arabian sea fails to confirm the earlier single proxy e g abundance of globigerina bulloides based inference of the holocene weakening of ism following insolation the reason for the observed decreasing trend in foraminiferal abundance the basis for the earlier inference could be the favouring of silicate rather than carbonate productivity by the increased ism wind strength although ism exhibits several multi millennial scale fluctuations there is no evidence from several multi proxy data to conclude that it declined during the holocene this is consistent with the phase lag analysis of longer time series of monsoon proxies thus on sub milankovitch timescales ism did not follow insolation highlighting the importance of internal feedbacks a comparison with east asian summer monsoon easm records suggests that both ism and easm varied in unison implying common forcing factors on such longer timescales copyright 2010 john wiley sons ltd

formal subdivision of the holocene series epoch a discussion paper by a working group of intimate integration of ice core marine and terrestrial records and the subcommission on quaternary stratigraphy international commission on stratigraphy this discussion paper by a working group of intimate integration of ice core marine and terrestrial records and the subcommision on quaternary stratigraphy sqs of the international commission on stratigraphy ics considers the prospects for a formal subdivision of the holocene series epoch although previous attempts to subdivide the holocene have proved inconclusive recent developments in quaternary stratigraphy notably the definition of the pleistocene holocene boundary and the emergence of formal subdivisions of the pleistocene series epoch mean that it may be timely to revisit this matter the quaternary literature reveals a widespread but variable informal usage of a tripartite division of the holocene early middle or mid and late and we argue that this de facto subdivision should now be formalized to ensure consistency in stratigraphic terminology we propose an early middle holocene boundary at 8200 a bp and a middle late holocene boundary at 4200 a bp each of which is linked to a global stratotype section and point gssp should the proposal find a broad measure of support from the quaternary community a submission will be made to the international union of geological sciences iugs via the sqs and the ics for formal ratification of this subdivision of the holocene series epoch copyright 2012 john wiley sons ltd

the dynamics of zonal current variations in the central equatorial indian ocean 1 this paper examines the depth integrated zonal momentum balance in the central equatorial indian ocean 0 80 5 e for two periods in 1993 1994 and 2004 2006 using a combination of in situ and satellite observations one to two year record length mean balances are approximately linear and steady state monthly to seasonal time scale variations are also governed by linear dynamics with imbalances between the pressure gradient force and surface wind stress leading to zonal mass transport variations along the equator interannual variations in zonal transports along the equator associated with indian ocean dipole events in 1994 and 2006 are likewise consistent with linear dynamics these results provide a valuable benchmark for evaluating models of equatorial indian ocean circulation on seasonal time scales

climatic fluctuations during the lia and post lia in the kumaun lesser himalaya india evidence from a 400 y old stalagmite record this paper presents the first stable isotope delta o 18 and delta c 13 data of a similar to 400 years 1590 2006 ad long annual to decadal resolution speleothem record collected from the indian lesser himalaya the data show a variation from 2 7 to 5 9 parts per thousand in delta o 18 and 5 3 to 8 8 parts per thousand in delta c 13 the isotopic analyses indicate that the climate during this period can be divided into two stages a wet phase during the little ice age lia 1590 1850 ad and comparatively dry phase during the post lia after 1850 ad however the record also documents the minor dry events during the lia and a wet episode after the lia within the age uncertainty the dry spells during the lia are linked with the historical drought events in the indian subcontinent and similar latitudes the isotopic record is consistent with a number of previous studies in the areas influenced by the westerlies but appears to be conflicting to the regions dominated by the indian summer monsoon ism this may be due to the possible changes in the strength of westerlies in the study area and added by negative anomaly of north atlantic oscillation nao during the lia c 2012 elsevier ltd and inqua all rights reserved

characteristics of vertical velocity cores in different convective systems observed over gadanki india abstract the indian mesosphere stratosphere troposphere mst radar measurements during the passage of 60 convective systems are used to study the vertical air velocity w characteristics of tropical convection the up and downdraft cores and various stages types of convection shallow deep and decaying are discerned from radar time intensity maps of w the characteristics of cores speed size orientation vertical extent gravity wave activity etc at different stages of convection are discussed with the help of three case studies the cores stratified based on the type of convection are mostly erect in nature in all types of convective systems except for deep updraft cores a considerable percentage 35 of deep updraft cores show inclined structure with elevation angles as low as 0 20 the variation of the horizontal wind field with height and the internal dynamics of mesoscale convective systems mcss are thought to be responsible for this geometry further the vertical extent of draft

holocene moisture evolution in arid central asia and its out of phase relationship with asian monsoon history we synthesize palaeoclimate records from the mid latitude arid asian region dominated today by the westerlies arid central asia aca to evaluate spatial and temporal patterns of moisture changes during the holocene sediment records from i i takes with reliable chronologies and robust proxies were selected to reconstruct moisture histories based on a five class ordinal wetness index with assigned scores from the driest to wettest periods at individual sites for 200 year time slices the proxies used in these records include pollen and diatom assemblages sediment lithology lake levels and geochemistry mainly isotope data the results of our synthesis show that aca as a whole experienced synchronous and coherent moisture changes during the holocene namely a dry early holocene a wetter less dry early to mid holocene and a moderately wet late holocene during the early holocene most of the lakes experienced very low water levels and even dried out before ca 8 ka 1 ka 1000cal a bp hence the effective moisture history in aca is out of phase with that in monsoonal asia as documented by numerous palaeoclimate records in monsoonal asia a strong summer monsoon and humid climate characterized the early holocene and a weakened summer monsoon and drier climate prevailed during the late holocene which were mainly controlled by changes in low latitude summer insolation in contrast we propose that the pattern of holocene effective moisture evolution in the westerly dominated aca was mainly determined by north atlantic sea surface temperatures ssts and high latitude air temperatures that affect the availability amount and transport of water vapor also topography of the tibetan plateau and adjacent asian highlands could have contributed to the intensification of dry climate in aca during the early holocene as a result of strengthening the subsidence of dry air masses associated with stronger uplift motion on the plateau by intense heating under a stronger summer insolation summer insolation might have played a key role in directly controlling moisture conditions in aca but only after the northern hemisphere ice sheets had disappeared in the mid and late holocene c 2007 elsevier ltd all rights reserved

a 3000 yr annually laminated stalagmite record of the last glacial maximum from hulu cave china abstract a high resolution annual layer counted and 230 th dated multi proxy record is constructed from a stalagmite in hulu cave china these proxies including 18 o annual layer thickness alt gray level gl and sr ca cover a time span of 3000 yr from 21 to 24 ka the physical proxies alt and gl and the geochemical index sr ca all primarily reflecting karst hydrological processes vary in concert and their coherence is supported by wavelet analyses variations in the 18 o data agree with fluctuations in the alt and sr ca records on multi decadal to centennial scales suggesting that the hulu 18 o signal is strongly associated with varying local rainfall amounts on short timescales a monsoon failure event at 22 2 ka correlates with a decrease in tropical rainfall a reduction in global ch 4 and an ice rafted event in the north atlantic this correlation highlights roles of the asian monsoon and tropical hydrological cycle in modulating global ch 4 because the high latitude emission was inhibited during the last glacial maximum lgm spectral analysis of the 18 o record displays peaks at periodicities of 139 59 53 43 30 23 and 19 15 yr the absence of typical centennial solar cycles may be related to muted changes in ocean circulation during the lgm

a 900 year 600 to 1500 a d record of the indian summer monsoon precipitation from the core monsoon zone of india 1 we present a near annually resolved record of the indian summer monsoon ism rainfall variations for the core monsoon region of india that spans from 600 to 1500 a d from a 230th dated stalagmite oxygen isotope record from dandak cave our rainfall reconstruction which spans the medieval warm period mwp and the earliest portion of the little ice age lia indicates that the short instrumental record of ism underestimates the magnitude of monsoon rainfall variability periods of severe drought lasting decades occurred during the 14th and mid 15th centuries and coincided with several of india s most devastating famines

characteristics of the tropical easterly jet long term trends and their features during active and break monsoon phases 1 using high vertical resolution indian mesosphere stratosphere troposphere mst radar and gps sonde observations from gadanki 13 5 n 79 2 e an attempt has been made for the first time to delineate the characteristics of indian summer monsoon winds with special emphasis on the tropical easterly jet stream tej its day to day monthly and interannual variability using observations available for more than a decade 1996 2008 for comparison data available during 1996 2008 from routine radiosonde observations from nearby india meteorological department imd observatory located at chennai 13 0 n 80 1 e and national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis data sets have been considered relatively poor vertical resolution data from imd lead to underestimation of the jet characteristics like jet width vertical shear above and below the jet and the wind reversal height although reanalysis data sets represented well the spatial variability they underestimate the peak zonal winds by about 5 m s when compared to indian mst radar and gps sonde observations large day to day variability of tej characteristics peak jet speed jet height jet width zonal wind reversal height and vertical shear of zonal wind over gadanki is noticed no significant trend is observed in peak jet speed using reanalysis data sets either over gadanki grid or over jet core region in indian summer monsoon months however mst radar and gps sonde observations show increasing trend 1 m s yr in peak jet speed particularly from the year 2000 characteristics of the tej during active and break phases showed quite different features thus statistics generated using the long term database from indian mst radar and gps sonde provide valuable information for studying the tej characteristics and will be useful to incorporate in the future reanalysis data sets

argo profiles a rare occurrence of three consecutive positive indian ocean dipole events 2006 2008 1 during 2006 2008 the indian ocean io experienced a rare realization of three consecutive positive io dipoles piods including an unusual occurrence with a la nina in 2007 common to all three piods is an early excitation of equatorial easterly anomalies argo profiles reveal that for the 2008 and 2006 piods the wind anomalies are generated by the following process upwelling rossby waves propagating into the western io and their subsequent reflection as equatorial upwelling kelvin waves enhance the seasonal upwelling changing sea surface temperature sst gradients for the 2007 piod coastal upwelling kelvin waves off the sumatra java coast associated with the 2006 piod el nino radiate into the io as upwelling rossby waves they curve sharply equatorward to arrive at the central equatorial io inducing easterly anomalies upwelling kelvin waves and the unusual piod our results suggest that real time argo observations when assimilated into predictive systems will enhance iod forecasting skills

climate change at the 4 2 ka bp termination of the indus valley civilization and holocene south asian monsoon variability 1 planktonic oxygen isotope ratios off the indus delta reveal climate changes with a multi centennial pacing during the last 6 ka with the most prominent change recorded at 4 2 ka bp opposing isotopic trends across the northern arabian sea surface at that time indicate a reduction in indus river discharge and suggest that later cycles also reflect variations in total annual rainfall over south asia the 4 2 ka event is coherent with the termination of urban harappan civilization in the indus valley thus drought may have initiated southeastward habitat tracking within the harappan cultural domain the late holocene drought cycles following the 4 2 ka bp event vary between 200 and 800 years and are coherent with the evolution of cosmogenic 14c production rates this suggests that solar variability is one fundamental cause behind holocene rainfall changes over south asia

equatorial indian ocean subsurface current variability in an ocean general circulation model the variability of subsurface currents in the equatorial indian ocean is studied using high resolution ocean general circulation model ogcm simulations during 1958 2009 february march eastward equatorial subsurface current esc shows weak variability whereas strong variability is observed in northern summer and fall esc an eastward subsurface current with maximum amplitude in the pycnocline is prominent right from summer to winter during strong indian ocean dipole iod years when air sea coupling is significant on the other hand during weak iod years both the air sea coupling and the esc are weak this strongly suggests the role of esc on the strength of iod the extension of the esc to the summer months during the strong iod years strengthens the oceanic response and supports intensification and maintenance of iods through modulation of air sea coupling although the esc is triggered by equatorial winds the coupled air sea interaction associated with iods strengthens the esc to persist for several seasons thereby establishing a positive feedback cycle with the surface this suggests that the esc plays a significant role in the coupled processes associated with the evolution and intensification of iod events by cooling the eastern basin and strengthening thermocline sst sea surface temperature interaction as the impact of iod events on indian summer monsoon is significant only during strong iod years understanding and monitoring the evolution of esc during these years is important for summer monsoon forecasting purposes there is a westward phase propagation of anomalous subsurface currents which persists for a year during strong iod years whereas such persistence or phase propagation is not seen during weak iod years supporting the close association between esc and strength of air sea coupling during strong iod years in this study we report the processes which strengthen the iod events and the air sea coupling associated with iod it also unravels the connection between equatorial indian ocean circulation and evolution and strengthening of iod

contribution of diverse monsoon precipitation over central and northern india during mid to late holocene abstracts the indian summer monsoon ism precipitation pattern over indian subcontinent is looked through various high resolution speleothem oxygen 18o isotope records from mid to late holocene the six cave locations distributed from central to northern india describing regional climatic conditions due to stalagmite deposition are considered the stalagmite deposition reflects precipitation patterns by temporal variations of 18o values oxygen isotope values reflect cumulative precipitation that is basically interpreted by averaged latitudinal site of inter tropical convergence zone itcz under the paleoclimate modelling intercomparison project phase iii pmip3 simulation commonwealth scientific and industrial research organisation mk3l version 1 0 part 2 csiro mk3l 1 2 model output shows itcz movement from bay of bengal bob to arabian sea as during mid to late holocene which resulted due to solar insolation changes it reflects precipitation shift from bob to as as well as to core monsoon zone cmz to the western himalayas the past millennium epoch which consists little ice age lia and middle warm period mwp shows large variations in 850 hpa geopotential height leading to high anomalous precipitation variability all over india during ism the high fluctuation in 18o values during mid holocene over northern india led to drought conditions due to which indus valley civilization vanished the present study shows that the himalayas received higher precipitation during late holocene that is basically governed by western disturbances wd as well as ism present study summarizes that the mean annual precipitation cycle for entire epoch was roughly similar but spatial distribution of precipitation during various epoch was quite divergent from mid to late holocene the onset of ism was also similar but withdrawal was comparatively unalike wherever peaks of seasonal precipitation were alike over indian subcontinent region spatial distribution of precipitation were unlike

in a changing climate weakening tropical easterly jet induces more violent tropical storms over the north indian ocean 1 tropical easterly jet tej of summer monsoon over the north indian ocean is weakening in recent years the absolute easterly shear shows a strong negative correlation significant at 99 9 level by students two sided t test with the number of severe storms suggesting that a decrease in easterly shear is favorable for the formation of more severe tropical storms for the first time in recorded history a category 5 hurricane formed in june 2007 together with two more severe tropical storms over the north indian ocean thus if the present decreasing trend of tej intensity continues there is a strong likelihood of the formation of tropical cyclones of hurricane intensity even during the summer monsoon presently these intense systems are known to form only in the pre and post monsoon seasons when the vertical wind shear is small

seasonal to interannual time scale dynamics of the equatorial undercurrent in the indian ocean abstractthis paper investigates the structure and dynamics of the equatorial undercurrent euc of the indian ocean by analyzing in situ observations and reanalysis data and performing ocean model experiments using an ocean general circulation model and a linear continuously stratified ocean model the results show that the euc regularly occurs in each boreal winter and spring particularly during february and april consistent with existing studies the euc generally has a core depth near the 20 c isotherm and can be present across the equatorial basin the euc reappears during summer fall of most years with core depth located at different longitudes and depths in the western basin the euc results primarily from equatorial kelvin and rossby waves directly forced by equatorial easterly winds in the central and eastern basin however reflected rossby waves from the eastern boundary play a crucial role while the first two baroclinic modes make the largest contribution intermediate modes 3 8 are also

holocene forcing of the indian monsoon recorded in a stalagmite from southern oman a high resolution oxygen isotope record from a thorium uranium dated stalagmite from southern oman reflects variations in the amount of monsoon precipitation for the periods from 10 3 to 2 7 and 1 4 to 0 4 thousand years before the present ky b p between 10 3 and 8 ky b p decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in greenland ice cores indicating that early holocene monsoon intensity is largely controlled by glacial boundary conditions after approximately 8 ky b p monsoon precipitation decreases gradually in response to changing northern hemisphere summer solar insolation with decadal to multidecadal variations in monsoon precipitation being linked to solar activity

physical and numerical model study of diffraction effects on seismic profiles over simple structures summary 2 d scale models and synthetic seismograms have been used to study diffraction artifacts and interpretation pitfalls on seismic profiles over two classes of simple shaped structures a vertical fault model and a rectangular mound model the fault throw was varied from one eighth of a wavelength 8 to two wavelengths 2 for throws greater than 2 the structure is resolved in the x t domain but diffractions which are present convey misleading structure amplitude and wavelet anomalies are observed for small fault displacement 2 which serve to detect but not to define the exact fault position and extent migration improves but does not necessarily overcome the problem mound structures having heights of 8 to 2 and length of one half to twice the fresnel zone radius were investigated lateral resolution is improved as the length of the structure increases but only for mounds which are vertically resolved height greater than 2 for low relief mounds resolution deteriorates with increasing mound length the resulting edge diffractions which interfere with the reflection events serve to detect and identify the target amplitude anomalies are observed across the structures but it is not possible without calibration to interpret the edges of the model even for thick mounds the diffraction patterns complicate the seismic picture and can lead to false interpretation the migration aperture needs to be about 10 times the fresnel zone radius to produce satisfactory results however for vertically unresolved structures migration may not significantly help complicating factors with post stack migration are lack of knowledge of the true velocity function and inadequacy of the cmp zero offset equivalence assumption

variability of zonal currents in the eastern equatorial indian ocean on seasonal to interannual time scales this study examines equatorial zonal current variations in the upper layers of eastern indian ocean in relation to variations in the indian ocean dipole iod the analysis utilizes data from the research moored array for african asian australian monsoon analysis and prediction rama and the european centre for medium range weather forecasts ocean reanalysis system 4 ecmwf oras4 surface currents are characterized by semiannual eastward flowing wyrtki jets along the equator in boreal spring and fall forced by westerly monsoon transition winds the fall jet intensifies during negative iod niod events when westerlies are anomalously strong but significantly weakens during positive iod piod events when westerlies are anomalously weak as zonal wind stress weakens during piod events sea surface height becomes unusually low in the eastern basin and high in the west setting up an anomalous pressure force that drives increased eastward transport in the thermocline opposite tendencies are evident during niod events in response to intensified equatorial westerlies current transport adjustments to anomalous zonal wind forcing during iod events extend into the following year consistent with the cycling of equatorial wave energy around the basin a surface layer mass budget calculation for the eastern sea surface temperature sst pole of the iod indicates upwelling of 2 9 0 7 sv during normal periods increasing by 40 50 during piod events and reducing effectively to zero during niod events iod related variations in wyrtki jet and thermocline transports are major influences on these upwelling rates and associated water mass transformations which vary consistently with sst changes

the railroad switch effect of seasonally reversing currents on the bay of bengal high salinity core the southwest monsoon current smc flows eastward from the arabian sea into the bay of bengal bob during summer advecting a core of high salinity water this high salinity core has been linked with arabian sea high salinity water that is presumed to enter the bob directly from the arabian sea via the smc here we show that the high salinity core originates primarily from the western equatorial indian ocean reaching the bob via the somali current the equatorial undercurrent and the smc years with anomalously saline high salinity cores are linked with the east africa coastal current and the somali current winter convergence and an anomalously strong equatorial undercurrent seasonal reversals that occur at the somali current and smc junctions act as railroad switches diverting water masses to different basins in the northern indian ocean interannual fluctuations of the equatorial undercurrent are linked to wind stress and el nino