marine ice sheet stability we examine the stability of two dimensional marine ice sheets in steady state the dynamics of marine ice sheets is described by a viscous thin film model with two stefan type boundary conditions at the moving boundary or grounding line that marks the transition from grounded to floating ice one of these boundary conditions constrains ice thickness to be at a local critical value for flotation which depends on depth to bedrock at the grounding line the other condition sets ice flux as a function of ice thickness at the grounding line depending on the shape of the bedrock multiple equilibria may be possible using a linear stability analysis we confirm a long standing heuristic argument that asserts that the stability of these equilibria is determined by a simple mass balance consideration if an advance in the grounding line away from its steady state position leads to a net mass gain the steady state is unstable and stable otherwise this also confirms that grounding lines can only be stable in positions where bedrock slopes downwards sufficiently steeply

precessional forcing of coccolithophore assemblages in the northern arabian sea implications for monsoonal dynamics during the last 200 000 years abstract the paleoceanographic potential of coccolithophores was used to decipher the monsoon induced paleoproductivity changes in the northern arabian sea during the past 200 000 years core so 90 94 kl was taken in the north eastern part of the study area representing the primarily ne monsoon influenced domain for comparison core odp117 723 a from the western side of the arabian sea off oman under the dominating influence of the sw monsoon was investigated samples were prepared using a quantitative dilution filtering technique and investigated in the sem we focus here only on the most important contributors to the assemblages gephyrocapsa oceanica gephyrocapsa ericsonii emiliania huxleyi florisphaera profunda contributing at least 63 up to 92 5 of the coccolith assemblages abundances of the two species e huxleyi and g ericsonii were summed up and depicted as a combined record ehuxgeric the 200 000 year record from the north eastern arabian sea yielded a significant precession derived signal in the abundances of ehuxgeric which could be related by cross spectral analysis to maximum low latitudinal insolation in contrast g oceanica seemed to correspond to insolation minima relative abundances of f profunda are generally rather low compared to other oceanographic regions and exhibit no distinct variations except for the thermal maxima of isotope stages ii i and v a schematic model is presented to show the changes in surface water stratification in the arabian sea driven by changing monsoon intensities over periodicities of the precessional band of milankovitch cycles this model is based on the variations of the two taxa groups g oceanica and ehuxgeric together with changes in the abundance of cocco total and f profunda probably the arabian sea remained eutrophic at least during one season of the year despite drastic changes in the summer monsoon strength lower productivities during the summer monsoon were probably compensated by higher phytoplankton growth due to stronger winter monsoons

stability of ice rises and uncoupled marine ice sheets an analysis of the linear stability of marine ice sheets uncoupled from associated ice shelves is presented the principal feature is a zero eigenvalue associated with infinitesimal shifts along the line of neutral equilibrium in phase space termed the equilibrium manifold a finite difference scheme is constructed which respects this stability property the zero eigenvalue appears to allow modelling errors to accumulate rather than dissipate as occurs in land based ice sheets the practical significance of this is that even rather fine spatial grids may allow substantial numerical error to accumulate

linear stability analysis of an ice sheet interacting with the ocean a linear stability analysis of a two dimensional flow of an isothermal ice sheet interacting with the ocean is considered the set of boundary conditions determining motion of the grounding line is adopted to describe hydrostatic equilibrium of ice in water and a cubic dependence of the mass flow rate on ice thickness the numerical analysis shows that the zero growth zero eigenvalue mode found for linear bed slopes and constant accumulation rates indeed determines neutral equilibrium and separates stable and unstable solutions it is also argued that provided some conditions of regularity of the solutions are satisfied finding only one stable and one unstable solution would be enough to ascertain that the condition determining a zero eigenvalue also determines neutral equilibrium this supports the intuitive understanding of ice sheet stability ice sheets are stable on bed slopes that ensure that the mass flow rate at the grounding line increases faster than the cumulative ice accumulation rate at the surface when the grounding line is perturbed and ice sheets are unstable otherwise

coccolithophore response to oceanographic changes in the equatorial atlantic during the last 200 000 years a deep sea core from the eastern equatorial atlantic meteor core 16772 1 21 s 11 58 w revealed strong primary productivity variations 70 230 gc m2 year based on the relative abundance of the deep living coccolithophore florisphaera profunda with productivity maxima following temperature minima during the last 200 000 years the response of the coccolithophore flora was investigated by relative and absolute abundance records of coccoliths the wind strength and the resulting equatorial upwelling intensity are thought to comprise the most important control of coccolith abundance and species composition which reflects the different abundance of coccolithophores living in the upper or lower photic zone relative abundances of the different species varies widely throughout the core although a wide range of species were present in the different samples three different taxa dominated the assemblage the lower photic zone species f profunda was the most abundant with a mean relative abundance of 49 4 throughout the core the upper photic zone flora were divided into three groups emiliania huxleyi together with small gephyrocapsa 3 m was the second most abundant species group mean 20 8 gephyrocapsa oceanica was the third most important species in the core and had a mean relative abundance of 16 8 all other species together represented a mean of 13 highest absolute abundance of coccoliths number gram sediment occurs in sediments deposited during periods of high sst mainly in warm isotope stages 1 and 5 the accumulation rates of coccoliths number cm2 ky show peaks in isotope stages 1 and 5 but also in some parts of cool isotope stages 4 and 6 high relative and absolute abundance of f profunda were found in warm isotope stages which suggests that a deep nutricline existed and that primary productivity was low absolute and relative abundance records of e huxleyi and small gephyrocapsa 3 m mainly showed higher values in sediments from warm periods the other upper photic zone coccolith species were generally more abundant during periods of low sst particularly in isotope stages 2 4 and 6 which were times of high primary productivity and a shallow nutricline this suggest that e huxleyi and small gephyrocapsa probably were able to live deeper in the water column than most other species during periods of low nutrient supply to the upper photic zone in summary the composition of the coccolith flora can be explained by the nutricline depth level with three different assemblages being recognized a deep photic zone assemblage consisting of f profunda a middle upper photic zone assemblage of e huxleyi and small gephyrocapsa and an upper photic zone assemblage consisting of all other species

the last 1 million years of the extinct genus discoaster plio pleistocene environment and productivity at site u1476 mozambique channel abstract a detailed paleoenvironment reconstruction from the mozambique channel western indian ocean based on the calcareous nannoplankton assemblages was conducted for the interval between 2 85 and 1 85 myr this study covers the period during which the successive extinction of the last five species of discoasters occurred new productivity data obtained from the abundances of the discoaster species discoaster brouweri d triradiatus d pentaradiatus d surculus and d tamalis and other indicative calcareous nannoplankton taxa showed abundance variations which were at paced with the 100 41 and 23 kyr astronomical periodicities a shift in the productivity and water column stratification proxies occurred at 2 4 ma after the onset of the northern hemisphere glaciation here we propose that the variability recorded at international ocean discovery program site u1476 reflects the interplay between forcing associated with warm tropical pacific and cold southern ocean influences the former is shown by consistent occurrence of warm water taxa calcidiscus leptoporus oolithotus spp rhabdosphaera clavigera syracosphaera spp umbellosphaera spp typical of indonesian throughflow surface waters on the other hand the occurrence of coccolithus pelagicus indicates the influence of cold nutrient rich sub antarctic surface waters a more mixed water column initiated at 2 4 ma and a consequent productivity increase led to the gradual reduction of the discoaster species until their extinction at 1 91 ma this period was characterized by the low values of the florisphaera profunda index and high abundances of upper photic zone flora indicative of nutrient rich surface water conditions high productivity at the location during this period could have also been amplified by localized upwelling events driven by the mozambique channel eddies

timing and style of late pleistocene glaciation in the queer shan northern hengduan mountains in the eastern tibetan plateau glacial landforms and sediments provide evidence for the existence of two late pleistocene major glacial advances in the queer shan northern hengduan mountains in the eastern tibetan plateau in the current study optically stimulated luminescence and electron spin resonance dating results reveal that the two glacial advances occurred during marine isotope stage mis 3 and the last glacial maximum lgm in mis 2 respectively geomorphic evidence shows that the glacial advance during mis 3 was more extensive than that in mis 2 this glacial advance is synchronous with other glaciated areas in the himalaya and tibet but contrasts with global ice volumes that reached their maximum extent during the lgm glaciers in the queer shan are of the summer accumulation type and are mainly fed by precipitation from the south asian monsoon palaeoclimate proxies show that during mis 3 the south asian monsoon strengthened and extended further north into the tibetan plateau to supply more precipitation as snow at high altitudes this in turn led to positive glacier mass balances and caused glaciers to advance however during the lgm despite cooler temperature than in mis 3 the weakened south asian monsoon and the associated reduced precipitation were not as favourable for glacier expansion as in mis 3 copyright c 2010 john wiley sons ltd glacial landforms and sediments provide evidence for the existence of two late pleistocene major glacial advances in the queer shan northern hengduan mountains in the eastern tibetan plateau in the current study optically stimulated luminescence and electron spin resonance dating results reveal that the two glacial advances occurred during marine isotope stage mis 3 and the last glacial maximum lgm in mis 2 respectively geomorphic evidence shows that the glacial advance during mis 3 was more extensive than that in mis 2 this glacial advance is synchronous with other glaciated areas in the himalaya and tibet but contrasts with global ice volumes that reached their maximum extent during the lgm glaciers in the queer shan are of the summer accumulation type and are mainly fed by precipitation from the south asian monsoon palaeoclimate proxies show that during mis 3 the south asian monsoon strengthened and extended further north into the tibetan plateau to supply more precipitation as snow at high altitudes this in turn led to positive glacier mass balances and caused glaciers to advance however during the lgm despite cooler temperature than in mis 3 the weakened south asian monsoon and the associated reduced precipitation were not as favourable for glacier expansion as in mis 3 copyright c 2010 john wiley sons ltd

the structure and annual variation of antisymmetric fluctuations of tropical convection and their association with rossby gravity waves abstract the signature of 4 5 day period rossby gravity waves is searched for in the tropical convection field across the indian pacific oceans the convergence divergence field of these waves in the lower troposphere is anticipated to produce an antisymmetric fluctuation in tropical convection antisymmetric fluctuations of tropical convection are shown to exhibit a pronounced spectral peak at a 4 5 day period only during boreal fall and only within about 30 longitude of the date line the peak amplitude occurs around 7 5 latitude these fluctuations propagate westward at 15 20 m s 1 with zonal wavelength of about 7000 9000 km the fluctuations of convection are coherent and out of phase with the equatorial meridional wind which also exhibits a pronounced spectral peak at a 4 5 day period in the lower troposphere near the date line the antisymmetric zonal wind also is strongly coherent with the antisymmetric convective fluctuations in this region the horizontal distributions of the 4 5 day power an

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

insolation forcing of coccolithophore productivity in the western tropical indian ocean over the last two glacial interglacial cycles we present a new coccolithophore productivity reconstruction spanning the last 300 ka in core geob12613 1 retrieved from the western tropical indian ocean io an area that mainly derives its warm and oligotrophic surface waters from the eastern io application of a calibrated assemblage based productivity index indicates a reduction in estimated primary productivity epp from 300 ka to the present with reconstructed epp values ranging from 91 to 246 g c m2 yr coccolithophore assemblages and coccolith fraction sr ca indicate three main phases of productivity change with major changes at 160 and 46 ka the productivity and water column stratification records show both dominant precession and obliquity periodicities which appear to control the paleoproductivity in the study area over the last two glacial interglacial cycles shallowing of the thermocline due to strengthening of the trade winds in response to insolation maxima resulted to peaks in epp comparison with the eastern io productivity and stratification coccolithophore data reveals good correspondence with our records indicating a strong tropical pacific influence in our study area both of these records show high productivity from 300 ka to 160 ka interpreted to be due to stronger walker circulation while the declining productivity from 160 ka to the present day is a consequence of its weakening intensity

identification of density stratified waters in the late pleistocene north atlantic a faunal derivation abstract an expanded study of the radiolarian cycladophora davisiana in late pleistocene north atlantic marine sediments shows that over the last several hundred thousand years this species exhibits large variations in relative abundance the c davisiana curves in the north atlantic cores are quite similar with easily recognizable features common to all records minor deviations from the general pattern of this species abundance apparently reflect the response of c davisiana to specific oceanographic conditions characteristic of a particular area within the north atlantic c davisiana occurs today in high abundance 20 only in the sea of okhotsk extensive winter and early spring sea ice cover coupled with low surface water salinities during summer and fall is responsible for maintaining near freezing subsurface temperatures in this northwest pacific marginal sea as well as relatively stable temperatures and salinities at depths below a shallow subsurface temperature minimum during periods in the late pleistocene high c davisiana abundances 20 in the north atlantic were probably associated with oceanographic properties similar to those that exist in the sea of okhotsk today because of the relationship between relatively stable subsurface temperatures and salinities and high abundance levels of c davisiana analysis of this species abundance pattern at several locations throughout the high latitude north atlantic should assist in identifying source areas of deep water formation and determining the duration of deep convective processes at these sites

interannual variations in mixed rossby gravity waves and their impacts on tropical cyclogenesis over the western north pacific abstract the present study investigates the transition from mixed rossby gravity mrg waves to tropical depression td type disturbances and its interannual variations over the western north pacific wnp using ecmwf high resolution data for the years of 1980 2001 as the equatorially trapped mrg waves propagate westward into the wnp the mrg waves transit to td type disturbances because of background flow change interannual variations in the transition of mrg waves are related to monsoon circulation change in response to tropical convective heating over the warm pool wp region when the wp is in a warm state convective heating is enhanced in the western part of the wnp and the monsoon trough retreats westward which induces a westward shift of the wave transition zone in contrast when the wp is in a relatively cold state the eastward penetration of convection and monsoon trough shifts the wave transition to the eastern part of the wnp the zonal wind convergence and shear in the monsoon trough

synoptic scale waves in sheared background flow over the western north pacific abstracttropical depression td type waves are the dominant mode of synoptic scale fluctuations over the western north pacific by applying spatiotemporal filters to the observed olr data and the ncep doe amip ii reanalysis data for 1979 2013 this study reveals the characteristics and energetics of convectively coupled td type waves under the effects of different circulation patterns in association with vertical wind shear results exhibit that different ambient sheared flows significantly affect the vertical structure of westward propagating td type waves with a lower tropospheric mode in an easterly sheared background and an upper tropospheric mode in a westerly sheared background energetic diagnoses demonstrate that when the disturbance is trapped in the lower upper level by easterly westerly shear the horizontal mean flow in the lower upper level favors wave growth by converting energy from the shear of the zonal mean flow from the convergence of the meridional mean flow during the penet

observation of 3 6 day meridional wind oscillations over the tropical pacific 1973 1992 vertical structure and interannual variability abstract rawinsonde data from tropical pacific stations were examined for westward propagating 3 6 day meridional wind oscillations in the troposphere and lower stratosphere 1973 1992 four types were identified from cross spectrum and principal component analysis 1 the dominant oscillation near 250 mb had a period slightly greater than 5 days zonal wavenumber 4 6 and modified rossby gravity structure near the date line 2 in the western pacific lower troposphere there was broadband activity with short zonal scale coupled to upper tropospheric waves in nh summer 3 in the central pacific during nh autumn there was a well defined 4 day oscillation with maximum amplitude in the lower troposphere and baroclinic phase tilt above the vertical structure suggested coupling to deep tropical convection this interpretation was supported by correlation of meridional wind with antisymmetric outgoing longwave radiation 4 in the stratosphere rossby gravity waves had periods 4 days and zonal wavenu

dust impact on marine biota and atmospheric co2 during glacial periods 1 we assess the impact of high dust deposition rates on marine biota and atmospheric co2 using a state of the art ocean biogeochemistry model and observations our model includes an explicit representation of two groups of phytoplankton and colimitation by iron silicate and phosphate when high dust deposition rates from the last glacial maximum lgm are used as input our model shows an increase in the relative abundance of diatoms in today s iron limited regions causing a global increase in export production by 6 and an atmospheric co2 drawdown of 15 ppm when the combined effects of changes in dust temperature ice cover and circulation are included the model reproduces roughly our reconstruction of regional changes in export production during the lgm based on several paleoceanographic indicators in particular the model reproduces the latitudinal dipole in the southern ocean driven in our simulations by the conjunction of dust sea ice and circulation changes in the north pacific the limited open ocean data suggest that we correctly simulate the east west gradient in the open ocean but more data are needed to confirm this result from our model data comparison and from the timing of the dust record at vostok we argue that our model estimate of the role of dust is realistic and that the maximum impact of high dust deposition on atmospheric co2 must be 30 ppm

on the interpretation of eof analysis of enso atmospheric kelvin waves and the mjo abstractempirical orthogonal function eof analysis is frequently applied to derive patterns and indexes used to identify and track weather and climate modes as expressed in state variables or proxies of convection individual eofs or pairs of eofs are often taken to be a complete description of the phenomenon they are intended to index at the same time in the absence of projection of the phenomenon onto multiple eofs yielding multiple similar eigenvalues each eof is often assumed to represent a physically independent phenomenon this project analyzed the leading eofs of the earth s skin temperature on the equator and outgoing longwave radiation olr anomalies filtered for atmospheric equatorial kelvin waves results show that the leading two eofs of the skin temperature data including east pacific el nino and el nino modoki frequently evolve as a quadrature pair during el nino events even though the first eof explains roughly 6 times as much variance as the second they together diagnose the longit

late pleistocene variability of arabian sea summer monsoon winds and continental aridity eolian records from the lithogenic component of deep sea sediments the modern indian ocean summer monsoon is driven by differential heating between the asian continent and the indian ocean to the south this differential heating produces a strong pressure gradient which drives southwest monsoon winds during june july and august satellite and meteorological observations aerosol measurements sediment trap studies and mineralogical studies indicate an atmospheric mode of transport for modern lithogenic sediments in the northwest arabian sea analyses of lithogenic grain size and mass accumulation rate mar records from the owen ridge indicate that eolian transport has been the primary mode of transport for the past 370 kyr visual inspection shows that the mar record is positively correlated with global ice volume as indicated by the marine 18o record in contrast the grain size record varies at a much higher frequency showing little correlation to either the mar or the 18o records spectral analyses confirm these relationships indicating that the lithogenic grain size and mar records are coherent only over the precession band whereby the grain size leads the mar by 124 8 kyr we conclude that an eolian transport mechanism is the only mechanism that allows for this phase difference and at the same time is supported by comparison of the grain size and mar with independent eolian records we use lithogenic grain size as a paleoclimatic indicator of summer monsoon wind strength and lithogenic mar as a paleoclimatic indicator of source area aridity these interpretations are supported by comparison of the lithogenic records to independent indicators of wind strength globigerina bulloides upwelling record and aridity a loess record from central china such comparisons indicate high coherence and zero phase relationships our work supports the findings of previous studies which have documented the link between monsoon strength and the earth s axial precession cycles both the lithogenic mar and the grain size records have high coherency with precessional insolation maximum lithogenic mar source area aridity is in phase with 18o global ice volume and leads maximum precessional insolation by 88 6 kyr we attribute this lead to the influence of glacial conditions on the aridity and therefore the deflation potential of the source areas maximum lithogenic grain size summer monsoon wind strength lags maximum precession by 148 9 kyr we attribute this lag both to the influence of global and or local ice volume and to the availability of latent heat from the southern hemisphere indian ocean the two of which combine to determine the strength of the indian ocean monsoon

reconstructing sea surface temperature and salinity using δ 18 o and alkenone records the oxygen isotope 18o composition of foraminiferal tests from deep sea sediments is widely used as a palaeoclimate proxy but it includes contributions from sea surface temperature global ice volume and local salinity which are difficult to separate recently a new technique for deriving palaeotemperatures has been developed which is based on the abundance ratios of unsaturated alkenones in phytoplankton algae1 2 here we use a combination of oxygen isotope and alkenone records in a deep sea core from the juncture of the arabian sea and the bay of bengal to extract the salinity signal from the former record variations in salinity are related to the balance between evaporation and precipitation3 and are thus a sensitive indicator of climate change our 170 kyr salinity record enables us to reconstruct changes in the indian monsoon over this period considerably extending earlier studies which reached back to 18 kyr ago 4 8 like these previous studies we find that large variations in the monsoon occurred during the transition from the last glacial period to the present interglacial but our results also provide a view of the monsoon throughout the last glacial and demonstrate the potential of this approach for reconstructing palaeosalinity

a 400 kyr record of combustion oxygen demand in the western equatorial pacific evidence for a precessionally forced climate response we have developed a combustion analysis technique for sediments which measures the amount of o2 consumed by the reduced species we have measured this quantity which we call combustion oxygen demand cod on a carbonate rich sediment core from the ontong java plateau in the western equatorial pacific back to marine oxygen isotope stage 11 the precision of the cod technique is 6 3 mol o2 g 1 which corresponds to 0 0076 wt corg assuming oxidation of organic carbon dominates the signal the cod time series is characterized by values which are about twice as high during glacials as during interglacials the largest shift occurring from 401 mol o2 g 1 in midstage 6 to 144 mol o2 g 1 at 5e and is coherent with the oxygen isotope curve of globigerinoides sacculifer in the same core at the milankovitch frequencies of 100 and 41 kyr pronounced variations in the 19 23 kyr band suggest that the climate of the western equatorial pacific is sensitive to precessional forcing a response not apparent from other records obtained in this region

the effect of bottom boundary conditions in the ice sheet to ice shelf transition zone problem uniqueness of the transition zone solution is discussed it is argued that when it is assumed that the stresses are continuous at the grounding line the ice shelf solution at the grounding line should possess a zero slope in order to avoid issues caused by the mathematical singularity of the solution the same technique as that used to study the transition zone is applied to a similar problem of lifting an elastic sheet from a rigid substrate which allows a better physical understanding this exemplifies the effect of the bottom boundary condition of no vertical motion imposed up to a fixed grounding line position while the forcing parameters such as the flow rate vary its effect is to produce multiple solutions due to suppression of the separation mechanism ice sheet motion is determined by the transition zone dynamics that couple the grounded part of an ice sheet to its floating part the ice shelf as the shallow ice approxima tion makes equations governing the grounding part of the ice sheet simpler it is convenient to decouple the grounded ice sheet problem from the ice shelf problem this leads to the necessity of imposing boundary conditions at the grounding line although it has been argued that due to its low stress the presence of the ice shelf does not constrain the ice sheet dynamics in any way apart from the condition of hydrostatic equilibrium at the grounding line hindmarsh 1993 here we discuss the other possibility namely that the ice sheet motion is uniquely determined by its external and initial conditions the latter implies two boundary conditions at the grounding line that together with the condition of zero mass flux at the ice divide close the isothermal ice sheet problem described by a non linear parabolic second order differential equation solved between the ice divide and the unknown grounding line position to date no work has been able to show uniqueness of the transition zone solution that apart from the hydrostatic equilibrium condition would allow us to impose an additional unique boundary condition at the grounding line to relate the ice thickness to the mass flow rate the progress in doing so has been hindered by the complexity of the problem especially due to a singularity of the solution at the grounding line caused by a jump in the bottom boundary

the effect of buttressing on grounding line dynamics determining the position and stability of the grounding line of a marine ice sheet is a major challenge for ice sheet models here we investigate the role of lateral shear and ice shelf buttressing in grounding line dynamics by extending an existing boundary layer theory to laterally confined marine ice sheets we derive an analytic expression for the ice flux at the grounding line of confined marine ice sheets that depends on both local bed properties and non local ice shelf properties application of these results to a laterally confined version of the mismip 1a experiment shows that the boundary condition at the ice shelf front i e the calving law is a major control on the location and stability of the grounding line in the presence of buttressing allowing for both stable and unstable grounding line positions on downwards sloping beds these results corroborate the findings of existing numerical studies that the stability of confined marine ice sheets is influenced by ice shelf properties in contrast to unconfined configurations where grounding line stability is solely determined by the local slope of the bed consequently the marine ice sheet instability hypothesis may not apply to buttressed marine ice sheets

dynamics of the monsoon in the equatorial indian ocean over the last 260 000 years abstract the coccolithophorids are excellent indicators of the variations in ocean productivity as an example the relative abundance of florisphaera profunda a species which lives exclusively in the deepest part of the photic zone constitutes a reliable monitor of the depth of the nutricline as a function of climatic variability productivity in the indian ocean is closely linked with the monsoon its intensity affects the depth of the mixing zone which deepens as the wind stress increases during the monsoon season the nutricline is shallow resulting in a higher productivity the variations in composition of coccolith assemblages in well dated oceanic sediments allows to describe the dynamics of productivity and therefore the fluctuation of the monsoon intensity core md900963 was retrieved in the equatorial indian ocean east of the maldives at a water depth of 2400 m a precise chronology was established by fine tuning the high resolution 18o record with the specmap stack coccoliths were counted from samples taken at 10 cm intervals in the core providing a resolution of 2000 years for the last 260 000 years the productivity estimates made from coccolith counts show that productivity varied greatly in the maldives area during this time the variations of the coccolith productivity index match the variations of the organic carbon content in the sediment spectral analysis reveals strong precessional cycles and weak obliquity and eccentricity cycles this implies that the solar radiation has a dominant effect on the monsoon variability south of india the productivity maxima occur during even numbered 18o stages this may indicate that the productivity events are related to increase of the winter monsoon however the analysis of the phase between the palaeoproductivity and seasonal solar radiation curves calculated for the past 260 000 years suggest an alternative hypothesis the westerlies which blow in april in the maldives area there the onset of the monsoon is in advance on the arabian sea would have a major effect on productivity since the insolation curve of the end of march at 5 n is in perfect phase with the paleoproductivity record for the last 260 000 years

coarse fraction fluctuations in pelagic carbonate sediments from the tropical indian ocean a 1500 kyr record of carbonate dissolution appendix table al is available with entire article onmicrofiche order from the american geophysical union 2000florida avenue n w washington d c 20009 documentp94 001 2 50 payment must accompany order we examined coarse fraction contents of pelagic carbonates deposited between 2000 and 3700 m water depth in the tropical indian ocean using ocean drilling program odp sites 722 owen ridge arabian sea and 758 ninetyeast ridge eastern equatorial indian ocean and four giant piston cores collected by the french r v marion dufresne during the seymama expedition over the last 1500 kyr coarse fraction records display high amplitude oscillations with an irregular wavelength on the order of 500 kyr these oscillations can be correlated throughout the entire equatorial indian ocean from the seychelles area eastward to the ninetyeast ridge and into the arabian sea changes in grain size mainly result from changes in carbonate dissolution as evidenced by the positive relationship between coarse fraction content and a foraminiferal preservation index based on test fragmentation the well known mid bruhes dissolution cycle represents the last part of this irregular long term dissolution oscillation the origin of this long term oscillation is still poorly understood our observations suggest that it is not a true cycle it has an irregular wavelength and we propose that it may result from long term changes in ca flux to the ocean sites 722 and 758 18o records provide a high resolution stratigraphy that allows a detailed intersite comparison of the two coarse fraction records over the last 1500 kyr site 722 2030 m lies above the present and late pleistocene lysocline the lysocline shoaled to the position of site 758 2925 m only during the interglacial intervals that occurred between about 300 and 500 ka peterson and prell 1985a despite these supralysoclinal positions of the two sites short term changes in coarse fraction contents are correctable from one site to another and probably result from regional or global dissolution pulses by stacking the normalized coarse fraction records from sites 722 and 758 we constructed a composite coarse fraction index ccfi curve in which most of the local signals cancelled out the last 800 kyr of this curve appear to compare extremely well with the composite dissolution index curve from core v34 53 ninetyeast ridge which unambiguously records past variations of carbonate dissolution in the equatorial indian ocean peterson and prell 1985a in the late pleistocene the ccfi variations are mainly associated with glacial interglacial changes they show strong 100 and 41 kyr periodicities but no clear precession related periodicities as proposed earlier by peterson and prell 1985a the lack of precession frequencies may suggest that the regional carbonate dissolution signal is driven by changes in deepwater circulation we cannot totally reject the possibility however that low temporal resolution and or bioturbation degrade somehow the precessional signal at odp sites 722 and 758 in contrast spectral density of dissolution cycles in the giant 53 m long piston core md900963 maldives area displays clear maxima centered on the precession frequencies 23 and 19 kyr 1 as well as on the kyr 1 frequency but shows little power at the 100 kyr 1 frequency these high frequency changes most probably result from changes in surface productivity associated with monsoon variability dissolution at this site may be ultimately controlled by the oxidation of organic matter which appears to be incorporated into the sediments in greater quantity during periods of weak sw monsoon and or increased dry ne monsoon

precessional forcing of nutricline dynamics in the equatorial atlantic climate control of nutricline depth in the equatorial atlantic can be monitored by variations in the abundance of the phytoplankton species florisphaera profunda a conceptual model based on in situ evidence associates high abundances of f profunda with a deep nutricline and low abundances with a shallow nutricline a 200 000 year record of f profunda relative abundances obtained from a deep sea core sited beneath the region of maximum equatorial divergence at 10 w has 52 percent of its variance centered on the 23 000 year precessional band cross spectral analysis between the signals of f profunda and sea surface temperature independently derived from zooplankton species shows their 23 000 year cycles to be coherent and nearly in phase abundance minima of f profunda coincide with times of december perihelion whereas abundance maxima coincide with june perihelion these relations indicate that nutricline dynamics in the divergence region of the equatorial atlantic are controlled by variations in the tropical easterlies forced by the precessional component of orbital insolation on time scales greater than 10 000 years

a coupled marine ice stream ice shelf model theorie des variations climatiques du pleistocene a aujourd hui liees a la dynamique des couvertures marines de glace couplee aux glaces libres flottantes on neglige les effets thermodynamiques et on etudie les processus de flottaison en relation avec les rapports de densite entre la place et l eau de mer

oxygen and carbon isotopic composition and biogeographic distribution of planktonic foraminifera in the indian ocean five planktonic foraminiferal assemblages were encountered along a north south transect in the indian ocean with the following dominant species globorotalia truncatulinoides in the southern subtropical assemblage globigerinoides ruber globigerinella aequilateralis and globorotalia menardii in the northern subtropical assemblage globigerinoides sacculifer and pulleniatina obliquiloculata in the tropical assemblage neogloboquadrina dutertrei and globigerina bulloides in the tropical upwelling assemblage and globigernoides conglobatus in the tropical arabian sea assemblage oxygen and carbon isotopic analyses were performed on three surface dwelling species of planktonic foraminifera in tropical and subtropical waters 22 30 c the 18o of globigerinoides ruber and g sacculifer varies with surface water temperature and the slope of the isotope temperature regression line is similar to that of epstein et al 1953 for marine molluscs however foraminiferal 18o values are significantly lighter than those of molluscs indicating isotopic disequilibrium comparison between plankton tows taken within and below the mixed layer at the same stations shows that g ruber has similar low 18o values in both sets suggesting that it secretes its shell within the mixed layer by contrast g sacculifer has slightly higher 18o values below than within the mixed layer because it continues to secrete calcite below the thermocline no significant 18o variations with shell size were found on the other hand 13c values were strongly size dependent i e the smaller the size the lighter the 13c comparison of living g ruber and g sacculifer with recent specimens from the surface sediment shows that the living foraminifera are generally lighter in 18o than the fossils supporting independent observations that the latter have secreted additional calcite during gametogenesis when the organisms settle freely from the euphotic zone for about 12 h and pass through waters lower in temperature than the epipelagic waters where the foraminifera form the bulk of their shells

variations in coccolithophore productivity off south africa over the last 500 kyr abstract we present a 500 kyr productivity reconstruction of the southwest indian ocean md96 2077 natal valley and the eastern south atlantic sites 1266 walvis ridge from coccolithophore assemblages and coccolith geochemistry the study sites are situated in two different hydrographic regimes one located in the upper boundary of the agulhas current ac and the other in the open ocean branch of the benguela current bc we aim to decipher how marine phytoplankton communities responded to variations in water column characteristics over multiple glacial interglacial cycles gephyrocapsa species g caribbeanica g ericsonii dominates the assemblage in the walvis ridge whereas florisphaera profunda is more prominent and co dominates with gephyrocapsa species g oceanica g ericsonii in the natal valley higher abundance of f profunda in md96 2077 than at site 1266 suggests that natal valley has a deeper nutricline thermocline and lower productivity than in the walvis ridge over the last 500 kyrs asynchrony in the productivity changes is suggested to be related to different driving mechanisms between the eastern and the western sides off south africa the equatorward stf migration during glacial periods has potentially reduced the intensity of the ac and promoted surface water productivity in the natal valley by contrast productivity in the walvis ridge shows opposite patterns showing enhanced productivity during interglacial periods which we attribute to a localized mixing at the confluence zone between the eastward flowing south atlantic current and the benguela oceanic current and or an increased agulhas leakage in combination with stronger westerlies thereby promoting upwelling of nutrients to the surface

glacial to interglacial contrasts in the northern indian ocean differences between the 18o 16o ratios of planktonic foraminifera deposited during the last glacial maximum about 18 000 yr ago and those deposited during the holocene show that in the indian ocean the south west monsoon was weaker than today but that the north east monsoon was stronger the upwelling observed in modern conditions along the southern coast of arabia had disappeared because of the low speed of the southwestern winds during glacial summers the reduction in rainfall and runoff over the continent caused a reduction of the salinity gradient in the bay of bengal and along the western coast of india increased precipitation fell on the sea south of 10 n while strong evaporation over the northernmost arabian sea produced an enhanced salinity gradient in that area

evolution of a coupled marine ice sheet sea level model we investigate the stability of marine ice sheets by coupling a gravitationally self consistent sea level model valid for a self gravitating viscoelastically deforming earth to a 1 d marine ice sheet shelf model the evolution of the coupled model is explored for a suite of simulations in which we vary the bed slope and the forcing that initiates retreat we find that the sea level fall at the grounding line associated with a retreating ice sheet acts to slow the retreat in simulations with shallow reversed bed slopes and or small external forcing the drop in sea level can be sufficient to halt the retreat the rate of sea level change at the grounding line has an elastic component due to ongoing changes in ice sheet geometry and a viscous component due to past ice and ocean load changes when the ice sheet model is forced from steady state on short timescales 500 years viscous effects may be ignored and grounding line migration at a given time will depend on the local bedrock topography and on contemporaneous sea level changes driven by ongoing ice sheet mass flux on longer timescales an accurate assessment of the present stability of a marine ice sheet requires knowledge of its past evolution

revisiting reconstructed indian monsoon rainfall variations during the last 25 ka from planktonic foraminiferal δ18o from the eastern arabian sea abstract a careful reanalysis of recently published large body of surface seawater salinity and 18 o data indicates that the variations in the surface seawater 18 o of the eastern arabian sea appear to be governed mainly by the influx of low salinity water through the winter monsoon current from the relatively fresher bay of bengal and transported northwards by the west india coastal current armed with this calibration for the present day we revisit the south asian summer monsoon variability during the past 25 ka by compiling high resolution fossil planktonic foraminiferal 18 o data from a large number of well dated sediment cores from the eastern arabian sea we clearly demonstrate that there is no evidence to believe that the south asian summer monsoon steadily declined during the holocene as reported in a few earlier papers from the western arabian sea and speleothems from oman on the contrary a majority of sediment core data corroborate the inference of a much earlier paper sarkar et al 2000 of a steadily increasing monsoon rain during the holocene that the south asian monsoons were driven solely by past insolation change appears invalid

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

low frequency equatorial waves in vertically sheared zonal flow part i stable waves abstract the mechanism by which a vertically sheared zonal flow affects large scale low frequency equatorial waves is investigated with two level equatorial plane and spherical coordinates models vertical shears couple barolinic and barotropic components of equatorial wave motion affecting significantly the rossby wave and westward propagating yanai wave but not the kelvin wave this difference results from the fact that the barotropic component is a modified rossby mode and can be resonantly excited only by westward propagating internal waves the barotropic components emanate poleward into the extratropics with a pronounced amplitude while the baroclinic components remain equatorially trapped a westerly vertical shear favors the trapping of rossby and yanai waves in the upper troposphere whereas an easterly shear tends to confine them in the lower troposphere as such their westward propagation is slowed down by both westerly and easterly shears when the strength of the vertical shear varies w

sea level as a stabilizing factor for marine ice sheet grounding lines climate change could potentially destabilize marine ice sheets which would affect projections of future sea level rise1 2 3 4 specifically an instability mechanism5 6 7 8 has been predicted for marine ice sheets such as the west antarctic ice sheet that rest on reversed bed slopes whereby ice sheet thinning or rising sea level leads to irreversible retreat of the grounding line however existing analyses of this instability mechanism have not accounted for deformational and gravitational effects that lead to a sea level fall at the margin of a rapidly shrinking ice sheet9 10 11 here we present a suite of predictions of gravitationally self consistent sea level change following grounding line migration our predictions vary the initial ice sheet size and also consider the contribution to sea level change from various subregions of the simulated ice sheet using these results we revisit a canonical analysis of marine ice sheet stability5 and demonstrate that gravity and deformation induced sea level changes local to the grounding line contribute a stabilizing influence on ice sheets grounded on reversed bed slopes we conclude that accurate treatments of sea level change should be incorporated into analyses of past and future marine ice sheet dynamics

mixed rossby gravity waves and western pacific tropical cyclogenesis part i synoptic evolution abstract a large amplitude mixed rossby gravity wave packet is identified in the western pacific using 6 10 day bandpass filtered winds individual disturbances of 2300 3000 km wavelength propagated westward as the packet moved slowly eastward the packet first appeared and subsequently amplified within a region of active convection associated with the madden julian oscillation mjo which was isolated by low pass filtered outgoing longwave radiation the packet lasted about 5 weeks then rapidly dispersed as the active mjo moved away from it to the east west of 150 e individual disturbances within the packet turned northwestward away from the equator indicating an apparent transition from mixed rossby gravity waves to off equatorial tropical depression td type disturbances cyclones filled with cloud and anticyclones cleared during the transition nevertheless convective structure consistent with mixed rossby gravity waves remained outside the circulation centers and three tropical cyclones for

a climatology of waves in the equatorial region abstract propagating anomalies of moisture and moist deep convection in the tropics are organized into a variety of large scale modes these include but are not limited to the so called intraseasonal oscillations convectively coupled waves similar to those predicted by shallow water theory on the equatorial beta plane and tropical depression type disturbances along with the annual and diurnal cycles these modes act and interact to control much of the variance of tropical convection analyses of 10 yr of outgoing longwave radiation olr and precipitable water pw data are carried out to develop comparative climatologies of these wavelike modes the analysis relaxes the commonly used cross equatorial symmetry constraints which allows study of the portions of the wavelike processes that are asymmetric across the equator mean background states are found for olr and for pw as functions of day of the year examination of anomalies together with the background reveals much about how the waves are affec

oxygen isotope evidence for a stronger winter monsoon current during the last glaciation a variety of palaeoclimate data from the northern indian ocean has shown that the asian summer monsoon was weaker during the last glacial maximum 18 000 years ago than it is today1 5 it has also been inferred from these data that the winter monsoon was stronger at that time5 here we present evidence that the transport of low salinity water from the bay of bengal to the arabian sea caused by the winter monsoon6 was higher during the last glacial maximum this inference is based on a negative excursion up to about 1 in magnitude in the oxygen isotope ratios of three planktonic foraminiferal species detected in a sediment core from the arabian sea by high resolution 1 2 cm sampling our results place an upper limit of about 4 000 years on the length of time during which the winter monsoon was stronger than today

radiolarian assemblage distributions and paleoceanography of the eastern equatorial pacific ocean during the last 127 000 years abstract mathematical treatment q mode factor analysis of radiolarian species abundance data in equatorial pacific core tops yields four oceanographically meaningful assemblages the distribution of these assemblages in surface sediments can be related to the major surface and subsurface characteristics of the equatorial circulation system western tropical mixed layer flow coastal upwelling and the oxygen minimum layer eastern equatorial divergence upwelling and the equatorial undercurrent and the subtropical water masses of both hemispheres changes in the distribution of radiolarian assemblages during the glacial interglacial cycle spanning the last 127 000 yr are observed in two equatorial pacific piston cores these changes reflect fluctuations in the boundaries of equatorial water masses in response to variations in the position and strength of the tradewind system and tend to precede major glacial interglacial transitions by several thousand years a history of atmospheric circulation in the equatorial pacific has been developed for the last 127 000 yr fluctuations in tradewind intensity north south shifts in the latitudinal position of the tradewind maxima and perhaps a change in the zonality of the tradewinds during the last 127 000 yr have been inferred from downcore quartz abundance and radiolarian assemblage distribution patterns oxygen isotopic fluctuations during the last 127 000 yr indicate that continental ice volume was much greater during glacial stage 2 29 000 b p to 11 000 b p than during glacial stage 4 73 000 b p to 61 000 b p large changes in atmospheric and oceanic circulation patterns also are indicated by the distribution of radiolarian assemblages during the two glacial periods during stage 2 oceanic circulation was more intense the southeast tradewind maximum was closer to the equator and wind stress was more meridional than during stage 4

bubbly ice densification in ice sheets i theory dry snow on the surface of polar ice ice sheets is first densified and metamorphosed to produce firn bubbly ice is the next stage of the transformation process which takes place below the depth of pore closure this stage extends to the transition zone where due to high pressures and low temperatures air trapped in bubbles and ice begins to form the mixed air clathrate hydrates while the gas phase progressively disappears here we develop a model of bubbly ice rheology and ice sheet dynamics taking into account glacier ice compressibility the interaction between hydrostatic compression of air bubbles deviatoric uniaxial compressive deformation of the ice matrix and global deformations of the glacier body is considered the ice matrix pressure and the absolute load pressure are distinguished similarity theory and scale analysis are used to examine the resultant mathematical model of bubbly ice densification the initial rate of bubble compression in ice sheets appears to be relatively high so that the pressure density relaxation process takes place only 150 200 m in depth below pore close off to reach its asymptotic phase wherein the minimal drop between bubble and ice pressures is governed by the rate of loading ice accumulation this makes it possible to consider densification under stationary present day conditions of ice formation as a special case of primary interest the computational tests performed with the model indicate that both ice porosity and bubble pressure profiles in ice sheets are sensitive to variations of the rheological parameters of pure ice however only the bubble pressure profile distinguishes between the rheological properties at low and high stresses the porosity profile at the asymptotic phase is mostly determined by the air content in the ice in the companion paper lipenkov and others 1997 we apply the model to experimental data from polar ice cores and deduce through an inverse procedure the rheological properties of pure ice as well as the mean air content in holocene and glacial ice sediments at vostok station antarctica

mercury content as a new indicator of ocean stratification and primary productivity in quaternary sediments off bahama bank in the caribbean sea abstract we report the first evidence of hg content in marine sediments changing in connection with the climate driven changes in ocean stratification during the quaternary period based on core samples from odp hole 1006a off great bahama bank in the caribbean sea the hg content ranged from 5 9 to 60 7 ng g with an average value of 33 1 ng g during 350 and 1330 ka and changed inversely with 18oplanktonic values the change in hg content was positively correlated with total organic carbon toc content indicating connections between the 15norg and 13corg values of organic matter and the absolute abundance of a deep dwelling calcareous nannoplankton florisphaera profunda the marine hg is thought to have been incorporated into the organic matter produced by deep dwelling phytoplankton based on these results we propose a mechanism by which marine hg can collect in a thermocline formed in the stratified lower photic zone mercury content and nannoplankton assemblage in marine sediment provide information about the extent of stratification of the oceanic photic zone and the role of surface and deep dwelling phytoplankton in producing marine organic matter and changing its 15norg and 13corg values

insolation cycles as a major control of equatorial indian ocean primary production analysis of a continuous sedimentary record taken in the maldives indicates that strong primary production fluctuations 70 to 390 grams of carbon per square meter per year have occurred in the equatorial indian ocean during the past 910 000 years the record of primary production is coherent and in phase with the february equatorial insolation whereas it shows diverse phase behavior with 18o depending on the orbital frequency eccentricity obliquity or precession examined these observations imply a direct control of productivity in the equatorial oceanic system by insolation in the equatorial indian ocean productivity is driven by the wind intensity of westerlies which is related to the southern oscillation therefore it is suggested that a precession forcing on the southern oscillation is responsible for the observed paleoproductivity dynamics

suppression of marine ice sheet instability a long standing open question in glaciology concerns the propensity for ice sheets that lie predominantly submerged in the ocean marine ice sheets to destabilise under buoyancy this paper addresses the processes by which a buoyancy driven mechanism for the retreat and ultimate collapse of such ice sheets the marine ice sheet instability is suppressed by lateral stresses acting on its floating component the ice shelf the key results are to demonstrate the transition between a mode of stable easily reversible retreat along a stable steady state branch created by ice shelf buttressing to tipped almost irreversible retreat across a critical parametric threshold the conditions for triggering tipped retreat can be controlled by the calving position and other properties of the ice shelf profile and can be largely independent of basal stress in contrast to principles established from studies of unbuttressed grounding line dynamics the stability and recovery conditions introduced by lateral stresses are analysed by developing a method of constructing grounding line stability bifurcation diagrams which provide a rapid assessment of the steady state positions their natures and the conditions for secondary grounding giving clear visualisations of global stabilisation conditions a further result is to reveal the possibility of a third structural component of a marine ice sheet that lies intermediate to the fully grounded and floating components the region forms an extended grounding area in which the ice sheet lies very close to flotation and there is no clearly distinguished grounding line the formation of this region generates an upsurge in buttressing that provides the most feasible mechanism for reversal of a tipped grounding line the results of this paper provide conceptual insight into the phenomena controlling the stability of the west antarctic ice sheet the collapse of which has the potential to dominate future contributions to global sea level rise

variations of the monsoon regime during the upper quaternary evidence from carbon isotopic record of organic matter in north indian ocean sediment cores abstract the 13 c 12 c ratios and contents of organic carbon in deep sea sediment cores of the north indian ocean have been compared for the different climatic stages as determined by oxygen isotope stratigraphy using the differences in the 13 c values between marine and terrestrial organic matter the terrestrial contribution to the eastern gulf of bengal and the andaman sea has been determined for modern and glacial times the geographical distribution of the terrestrial organic carbon linked to sea surface circulation shows that during the last glacial period the ne monsoon circulation was the dominant feature the organic carbon abundance within the sediment which is related to local productivity was greater during glacial times in the areas where production depends on the ne monsoon strength coast of north india in the arabian sea and andaman sea suggesting its reinforcement

paleoproductivity changes in the upwelling system of socotra somali basin nw indian ocean during the last 72 000years evidence from biological signatures abstract high resolution analyses of foraminiferal and radiolarian assemblages were performed on core md 85682 10 53 5n 52 23 e 3092 m depth 7 23 m length located close to the socotra island beneath the 10 n gyre of the upwelling area of the somali basin nw indian ocean in order to reconstruct the paleoproductivity changes during the last 72 kyr correspondence and cluster analyses of combined planktonic foraminiferal and radiolarian quantitative data show that the distribution pattern of the microfauna is partly controlled by temperature and hydrographic structures and their interrelations on water mass fertility the continuous occurrence of high abundances of foraminiferal species well known in fertile areas such as globigerinita glutinata globigerina bulloides and globigerina falconensis as well as radiolarian species characteristic of upwelling systems testifies to a sustained activity of the upwelling during the last 72 kyr downcore evolution of these biological proxies suggests an increase in paleoproductivity during isotope stages 1 and 3 which reflects a stronger upwelling activity during these periods these observations are in agreement with the results previously obtained by geochemical markers such as biogenic barium phosphorus organic matter and also trace elements related to the organic matter and indicators of redox conditions vanadium samples from isotope stage 2 record a more complex biological and geochemical signature resulting from increased terrigenous input comparison between the biological signatures obtained both under the socotra 10 n and somali 5 n upwelling areas indicates higher productivity throughout the last 72 kyr record of the socotra upwelling system in both areas a similar evolution of biological proxies indicative of high fertility is observed during isotope stages 1 and 3 however minor differences in the biological and geochemical signatures are related to local conditions prevailing beneath the two gyres

high resolution palaeoenvironmental reconstruction of odp hole 963d sicily channel during the last deglaciation based on calcareous nannofossils abstract a palaeoenvironmental reconstruction of ocean drilling program odp hole 963d sicily channel has been obtained on the basis of quantitative abundance fluctuations in the calcareous nannofossil assemblages the studied hole is characterized by a very high sedimentary resolution and covers the last ca 20 kyr a new palaeoclimatic curve based on the gephyrocapsa muellerae upper photic zone upz group ratio permitted the detection of the rapid climatic fluctuations that characterized the last deglaciation cold water masses occupied the sicily channel during the glacial period and the younger dryas whereas they were generally warm during the bolling allerod and the holocene an approach based on a direct relationship between actualistic data water samples and palaeontological records in the coccolithophores has been attempted in order to obtain the palaeoecological characterization with the proposition of two floristic groups pertaining to the upper photic zone and the lower photic zone lpz further information has been provided by several additional taxa during the investigated time interval stratification in the water column and oligotrophic conditions are always linked to the warm periods while high productivity caused by continuous convection of nutrient fluxes existed during the cold periods finally a simple model with two end members is proposed to explain the relative abundance of the deep dwelling species florisphaera profunda relative to other coccolithophores and to the physical conditions of the photic zone

mixed rossby gravity waves triggered by lateral forcing abstract the mechanisms associated with the excitation of mixed rossby gravity waves mrgws in the upper troposphere are studied using wind and outgoing longwave radiation olr data from 1979 to 1991 the largest anomalies in meridional wind associated with mrgws at 200 mb generally appear in the northern hemisphere summer fall periods and they are pronounced in the central eastern pacific where equatorial westerlies form the olr field in the intertropical convergence zone shows a spectral peak with time and space scales similar to those of mrgws at 200 mb however tropical convective activity does not show a clear contrast between years of strong and weak 200 mb mrgw activity during the northern hemisphere summer weak easterlies or westerlies often form over the equatorial central eastern pacific allowing disturbances to propagate from the southern hemisphere midlatitudes into the deep tropics some of these disturbances that possess spatial and temporal scales similar to those of the observed mrgw

bubbly ice densification in ice sheets ii applications a mathematical model for simulating the densification of bubbly glacier ice is used to interpret the following experimental data from the vostok central antarctica ice core two ice porosity profiles obtained by independent methods and a bubble pressure profile obtained by direct measurements of air pressure within individual bubbles the rheological properties of pure polycrystalline ice are deduced from the solution of the inverse problem the model and the inferred ice flow law are then validated using porosity profiles from seven other ice cores drilled in antarctica and greenland in the temperature range from 55 to 20 c the following expression is adopted for the constitutive law 2e 1 2 exp q 1 t s 1 t r s where e and are the effective strain rate and stress respectively is the creep exponent taken as 3 5 r s is the gas constant and t t s is the temperature standard temperature the numerical values obtained for the linear and non linear viscosities are 1 2 9 1 3 mpayear and 2 0 051 0 019 mpa year and the apparent activation energy q is confirmed to be 60 kj mole 1 the corresponding flow law is in good agreement with results of both mechanical tests and independent estimations based on the analysis of different natural phenomena associated with glacier ice deformation when the model is constrained by the porosity and bubble pressure profiles from vostok the mean air content in holocene ice is inferred to be about 0 088 cm 3 g 1 the corresponding mean air pressure in bubbles at the end of pore closure is about 0 083 mpa whereas the atmospheric pressure at this depth level would be 0 063 mpa the influence of the climatic change on the ice porosity profile is discussed it resulted in an increased air content in ice at vostok during the last glacial maximum 0 096 cm 3 g 1

extratropical forcing of tropical wave disturbances along the indian ocean itcz the role of extratropical waves in the excitation of tropical waves on submonthly timescales is explored along the indian ocean intertropical convergence zone itcz during the austral summer using japanese reanalysis products and noaa outgoing longwave radiation data the analysis period is december february of 1979 1980 to 2008 2009 the submonthly tropical waves are regarded as a type of rossby wave propagating along the mean monsoon westerly flow they play an important role in modulating the indian ocean itcz convection the linkage between the tropical and extratropical waves which is responsible for the formation and strengthening of tropical waves is examined composite analysis results linked to the tropical wave train development show that the midlatitude rossby wave train progresses eastward and northeastward from the south atlantic into the subtropical indian ocean as a trough and ridge that form part of the midlatitude wave train approach the southern africa southwest indian ocean swio region a southwest northeast oriented wave train is subsequently established originating from this feature and is strengthened across the tropical indian ocean the midlatitude wave propagation toward the subtropics induces the growth of the trough and ridge over the swio wave activity flux diagnostics indicate that the amplified trough and ridge over the swio act as an energy source for northeastward amplification of the tropical waves through the wave energy dispersion process the results suggest that the propagation of the midlatitude wave toward the swio is the fundamental mechanism behind the development of the tropical waves along the indian ocean itcz

convectively coupled equatorial waves 1 convectively coupled equatorial waves ccews control a substantial fraction of tropical rainfall variability their horizontal structures and dispersion characteristics correspond to matsuno s 1966 solutions of the shallow water equations on an equatorial beta plane namely kelvin equatorial rossby mixed rossby gravity and inertio gravity waves because of moist processes the tilted vertical structures of ccews are complex and their scales do not correspond to that expected from the linear theory of dry waves the dynamical structures and cloud morphology of ccews display a large degree of self similarity over a surprisingly wide range of scales with shallow convection at their leading edge followed by deep convection and then stratiform precipitation mirroring that of individual mesoscale convective complexes ccews have broad impacts within the tropics and their simulation in general circulation models is still problematic although progress has been made using simpler models a complete understanding of ccews remains a challenge in tropical meteorology

palynological record of tropical rain forest vegetation and sea level fluctuations since 140 ka from sediment core south eastern arabian sea palyno chronological study of a 552 cm deep sediment core from the south eastern arabian sea covers a time span of 140 ka the age estimate is based on oxygen isotopic data of planktic foraminifera six zones were identified on the basis of palynology covering partially mis 6 and mis 5 to mis 1 marine isotopic stages the interglacial period mis 5 shows high rate of sedimentation corresponding to high percentage of terrestrial rain forest pollen the palynological study reveals highstand and intense rain forest vegetation during mis 5a which corresponds to the similar earlier records of estuarine ecosystem on land in kerala the relative sea level rise and fall since 140 ka have been inferred through quantitative estimate of marine versus terrestrial palynomorphs the terrigenous non pollen forms comprised abundant neogene extinct nanoplanktons discoaster spp until mis 3 which was transported from the varkala formation highlands of the kerala coast the pollen assemblage is comparable to the present day flora in kerala except ongokea gore which is now present in south africa and therefore the provenance of pollen recorded in the studied core is western ghats of india high percentage of planktic foraminifera and dinoflagellate cysts throughout the core indicates high productivity that suggests upwelling induced by seasonal reversals in the monsoon winds even during the glacial periods botryococcus remains and poaceous leaf cuticles dominated by the pooideae group in the marine realm also point towards active winter monsoon considerable fragmentation of rain forest took place post 74 ka and during the lgm as the sea retreated to its maximum which resulted into the vast exposure of land favoring the expanse of grassland with pockets of rain forest plant refugia on land neogene rain forest flora recorded earlier from the varkala formation and the present record of its existence since mis 6 in the region provide an understanding that the monsoon circulation over southern india was similar since the middle miocene climate optimum and the flora in the south western ghats is a fossil ecosystem and a relic of the tertiary period

dispersion characteristics and circulation associated with boreal summer westward traveling mixed rossby gravity wave like disturbances abstractan algorithm has been developed to track synoptic scale westward traveling mixed rossby gravity mrg wave like disturbances with a cross equatorial component applied to space time filtered meridional wind data this algorithm finds locations with gaussian shaped wind structures stated in the solutions of shallow water equations swes based on 850 hpa meridional wind from the global national centers for environmental prediction ncep climate forecast system reanalysis cfsr dataset local and instantaneous wave properties including the occurrence time wavenumber intrinsic frequency and magnitude were examined it was found that these low level mrg wave like disturbances can be classified into a longer wavelength group and a shorter wavelength group while most waves identified in the eastern pacific give longer wavelengths disturbances in the western pacific tend to have a wider range of wavenumbers composite analysis revealed that east of 140 e low level disturbances are characterize

enso phase dependent td and mrg wave activity in the western north pacific the three dimensional structure and evolution characteristics of tropical depression td and mixed rossby gravity wave mrg type disturbances in the tropical western north pacific during el nino and la nina summers are investigated based on observational and reanalysis data a clear mrg to td transition was observed during el nino summers while such a transition is unclear during la nina summers the vertical structure of the td mrg waves appears equivalent barotropic during el nino but becomes tilted eastward with height during la nina the diagnosis of barotropic energy conversion shows that both the rotational and divergent components of the background flow change associated with e1 nino southern oscillation enso are responsible for energy conversion from the mean flow to the td mrg perturbations a further examination of the pure mrg mode shows that its intensity does not vary between el nino and la nina while its phase speed does a faster slower westward propagation speed during la nina el nina is attributed to enhanced reduced mean easterlies in the western equatorial pacific the heating associated with the mrg wave appears more anti symmetric during la nina than during el nino in contrast to the mrg waves the amplitude of the td waves depends greatly on the enso phase the enhanced suppressed td disturbances during el nino la nina is attributed to greater less barotropic energy conversion associated with the background flow change the vertical structure of the td waves appears quasi barotropic in the geopotential height field but baroclinic in the divergence field

phytoplankton dynamics in the eastern mediterranean sea during marine isotopic stage 5e abstract calcareous nannoplankton diatoms and organic walled dinoflagellate cysts from seven eastern mediterranean sediment cores were investigated to clarify the climatic and paleoceanographic conditions that influenced the deposition of sapropel s5 during marine isotopic stage 5e warming of surface waters during s5 deposition is indicated by the high abundance of the calcareous nannofossil small gephyrocapsa group the presence of the dinoflagellate species spiniferites mirabilis spiniferites pachydermus lingulodinium machaerophorum and polysphaeridium zoharyi and the tropical subtropical diatom pseudosolenia calcar avis chaetoceros resting spores rhizosolenids and thalassionema group increased productivity accompanied s5 deposition as evidenced by a general decrease in abundance of calcareous nannofossil superficial species and a coeval increase of dinocysts and diatoms the productivity increase is further supported by an increase of the deep dwelling calcareous nannofossil species florisphaera profunda stratification of near surface waters is recorded by the presence of the dinocyst species l machaerophorum and p zoharyi together with the co occurrence of the diatom chaetoceros resting spore group thalassionema frauenfeldii and thalassionema nitzschioides var parva which reflect transport of neritic waters off shore and by the presence of some freshwater and transport related diatom taxa cyclotella diatomella and diploneis gradual shoaling of the nutricline and a gradual weakening of water stratification at the end of s5 deposition is indicated by an increase in the deep dwelling calcareous nannoplankton species gladiolithus flabellatus which suggests a less deep niche compared to the deeper dwelling f profunda time transgressive variations in the distribution of the calcareous nannofossil emiliana huxleyi and changes in the abundances of gephyrocapsa oceanica small gephyrocapsa g flabellatus and of the dinocysts p zoharyi and l machaerophorum lead to paleocirculation considerations as they always reflect the main current path moreover the occurrence of upwelling related diatom and dinocyst taxa such as chaetoceros resting spores and s pachydermus suggests the presence of a former gyre structure south of crete that was active during s5 time

sediment sources and east asian monsoon intensity over the last 450 ky mineralogical and geochemical investigations on south china sea sediments a coupled approach based on clay mineral assemblages and isotopic data sr and nd of sediment from ocean drilling program odp site 1145 has been used to trace the sources of sediment feeding the northern part of the south china sea and to investigate the evolution of east asian monsoon intensity over the last 450 ky clay mineral assemblages are dominated by illite and smectite with lesser abundance of chlorite and kaolinite 87 sr 86 sr and end 0 isotopic data combined with the smectite illite chlorite ratio indicate that the pearl river is the main contributor for detrital material to the northern margin of the scs with variable continental input of volcanic material derived from the erosion of the luzon arc these inputs follow the low latitude solar insolation with a 23 ky periodicity as recorded by a periodic change of the clay mineralogy for the first time this study presents a scs deep sea sediment high resolution record of climatic changes in the last 450 ky showing that smectite illite chlorite variations are mainly related to changes in the east asian summer monsoon intensity because the mineralogical record is independent of other external forcing phenomena such as global glacial interglacial oscillations the smectite illite chlorite ratio has been used here as a proxy to reconstruct past changes of the south east asian summer monsoon intensity d 2005 elsevier b v all rights reserved

evolution of weathering patterns in the indo burman ranges over the last 280 kyr effects of sediment provenance on 87sr 86sr ratios tracer a high resolution study of mineralogy and major element geochemistry combined with sr and nd isotopes has been conducted on high sedimentation rate cores collected off the irrawaddy river mouth in the andaman sea and the bay of bengal to reconstruct the erosional and weathering history of the irrawaddy river basin in both cores nd 0 values imply that both glacial and interglacial sediments share a common crustal source the irrawaddy river strong glacial interglacial cycles are recorded by 87sr 86sr interglacial periods yield values between 0 713 and 0 717 whereas glacial periods show higher values between 0 717 and 0 719 variations of the pedogenic clays smectite and kaolinite to primary mineral feldspar quartz illite and chlorite ratios show strong precessional cycles suggesting a control by past changes in the summer monsoon intensity each increase in pedogenic clays content is also associated with a net loss of labile elements na k and ca from the detrital minerals under chemical weathering wet periods of summer monsoon reinforcement correspond to an increase in weathering of the irrawaddy plain soils and a decrease of 87sr 86sr ratio plotting 87sr 86sr versus 87rb 86sr gives a pseudo isochrons interpreted as a mixing line representing the strength of chemical weathering during glacial stages enhanced physical erosion induced by glacier scour and frost action in the highland of the irrawaddy river basins produced high volumes of unaltered rb rich minerals the low sea level of glacial times constricted the river to the main channel in the lower reaches and permitted an efficient transport of unaltered rb rich minerals with high radiogenic sr composition from the high relief of the indo burman ranges and the tibetan plateau to the indian ocean

multi scale interactions of equatorial waves associated with tropical cyclogenesis over the western north pacific the tropical cyclone tc centric approach developed in previous studies tends to overemphasize the direct impact of tropical waves and underestimates their large scale modulation of western north pacific wnp tropical cyclogenesis tcg to overcome these limitations this study proposes a new approach based on empirical orthogonal function analyses to re examine associations of multiple waves including the madden julian oscillation mjo quasi biweekly oscillation qbwo convective equatorial rossby er waves and synoptic scale waves ssws with wnp tcg events there is a close association between wnp tcg events total of 273 tcs in this study and ssws 64 mjo 68 qbwo 64 and er 65 waves these suggest that ssw is critical for many tcg events and all of these intra seasonal waves significantly modulate tcg events a majority of tcs 79 were found to be related to more than one wave type indicating an important role for a combination of dynamic and thermodynamic conditions associated with multiple waves in tcg events further analyses show that ssw activity is strongly modulated by the mjo and er waves while the qbwo has no significant impact on ssws during convectively active mjo and er phases stronger ssw trains with more distinct southeast northwestward aligned structures can be found compared to convectively inactive phases similar intra seasonal modulation of the transition from the mixed rossby gravity waves from the equatorial central pacific to the ssw trains over the wnp by the three waves is noted with such transitions largely favored during convectively active mjo and er phases

the early development of the 2015 16 quasi biennial oscillation disruption abstractan unprecedented disruption of the quasi biennial oscillation qbo started to develop from late 2015 the early development of this event is analyzed using the space time spectra of eddies

a 3 d coupled ice sheet sea level model applied to antarctica through the last 40 ky abstract we present results from a three dimensional ice sheet shelf model of antarctica coupled to a gravitationally self consistent global sea level model that incorporates maxwell viscoelastic deformation of the solid earth the coupled model captures complex post glacial changes in sea level associated with the gravitational deformational and rotational effects of the evolving surface mass ice plus ocean load over the global ocean including at the grounding lines of marine based ice the simulations are initiated at 40 ka and we focus on ice distributions and sea levels from the last glacial maximum to present our results extend and confirm the key conclusions of earlier work using a simplified 1 d ice sheet model by demonstrating that the sea level coupling has a significant stabilizing influence on marine ice sheet grounding lines the feedback of sea level changes into the ice sheet model acts to slow down the retreat and advance of the grounding line relative to simulations in which the full coupling is not incorporated differences in ice thickness between these simulations can reach 1 km close to the grounding line finally we perform preliminary comparisons of our results to relative sea level rsl histories and gps derived present day uplift rates at sites near the margins of antarctica we find that the coupling improves fits to uplift rates in several regions and that the rsl predictions of the coupled model yield a fit to the observations that is comparable to recent uncoupled simulations in which the underlying earth model was varied to obtain a best fit to the rsl histories

observation of 3 6 day meridional wind oscillations over the tropical pacific 1973 1992 horizontal structure and propagation abstract twenty years of rawinsonde data 1973 1992 were examined in conjunction with european centre for medium range weather forecasts ecmwf analyses and outgoing longwave radiation olr in 1980 1989 to determine the horizontal structure propagation and convective coupling of 3 6 day meridional wind oscillations over the tropical pacific wave properties from ecmwf data determined by lag correlation with respect to rawinsonde or ecmwf principal components were consistent with what could be determined from the sparse rawinsonde network alone gridded analyses allowed a clearer distinction between equatorially trapped rossby gravity waves rgw and off equatorial tropical depression td disturbances so that the contrasting properties of these waves including their seasonal and interannual variation could be studied in better detail significant correlations with olr were found increasing in magnitude from eastern to western pacific the apparent group propagation of disturbances was equator

spatial heterogeneity of land cover response to climatic change in the nilgiri highlands southern india since the last glacial maximum fourteen hillslope soil profiles were sampled under natural vegetation i e grassland or forest and plantations in the nilgiri highlands southern india delta 13c ratios were measured at different depths and 14c ages determined for six profiles in these highland soils where the turnover rate of organic matter is extremely low the 13c ratios of entire soil profiles have recorded signatures of past land cover by correlating the data with results previously obtained from peat bogs and with knowledge concerning the history of human settlement we distinguish three contrasting trajectories of palaeoenvironmental history and landscape change since the last glacial maximum in the central nilgiris between 18 and 10 ka bp forest expansion occurred due to the conjunction of a wetter climate the maximum of southwest monsoon related humidity occurring at ca 11 ka bp and higher temperatures since 10 ka bp the reversal towards grassland vegetation is attributed to drier conditions in the western nilgiris where strong southwest monsoon winds permanently restrict forest patches to sheltered valley sites steady but limited expansion of forest from 18 ka bp to the present is recorded and attributed to rising temperatures the southern and eastern nilgiris where the northeast monsoon contributes 20 of the annual rainfall are the less sensitive to fluctuations in the southwest monsoon in these areas rapid and extensive expansion of forest occurred mainly as a consequence of higher temperatures from 18 ka bp to the present massive deforestation by badaga cultivators and europeans planters followed after the 16th century ad as a result and in contrast with the western nilgiris where the land cover mosaic has remained remarkably stable in the last 18 ka bp the current landscape differs sharply from the land cover pattern detected by the soil record

equatorial indian ocean productivity during the last 33kyr and possible linkage to westerly jet variability the top 1 m radiocarbon dated section of a 5 6 m long sediment core retrieved from the equatorial indian ocean is studied for productivity changes in response to climate variability that have taken place during the last 33 kyr the robust indicators of marine productivity such as phytol and brassicasterol exhibit distinctly higher contents av 5 8 ng g and 4 8 ng g respectively during the lgm 25 17 ky bp as compared to the succeeding period 16 5 ky bp av 1 9 ng g each and preceding period 33 25 ky bp av 1 8 and 0 8 ng g respectively suggest increased productivity during the cold and dry climate of the lgm the corg content is also relatively higher during this period than the warmer holocene however marginally higher c n ratios 7 8 and enrichment of 13 corg 19 8 during the lgm than the holocene c n 6 4 and 13 corg 21 5 and also the higher carbon preference index cpi of n alkanes and n alkanols together suggest the presence of terrestrial organic matter in the sediment the increased equatorial productivity and terrestrial organic matter input together during the lgm may indicate intensified inter monsoon equatorial westerly jets resulting in elevated productivity

marine ice sheet dynamics part 1 the case of rapid sliding marine ice sheets are continental ice masses resting on bedrock below sea level their dynamics are similar to those of land based ice sheets except that they must couple with the surrounding floating ice shelves at the grounding line where the ice reaches a critical flotation thickness in order to predict the evolution of the grounding line as a free boundary two boundary conditions are required for the diffusion equation describing the evolution of the grounded ice thickness by analogy with stefan problems one of these conditions imposes a prescribed ice thickness at the grounding line and arises from the fact that the ice becomes afloat the other condition must be determined by coupling the ice sheet to the surrounding ice shelves here we employ matched asymptotic expansions to study the transition from ice sheet to ice shelf flow for the case of rapidly sliding ice sheets our principal results are that the ice flux at the grounding line in a two dimensional ice sheet is an increasing function of the depth of the sea floor there and that ice thicknesses at the grounding line must be small compared with ice thicknesses inland these results indicate that marine ice sheets have a discrete set of steady surface profiles if they have any at all and that the stability of these steady profiles depends on the slope of the sea floor at the grounding line

enso related variation of equatorial mrg and rossby waves and forcing from higher latitudes the contrasting behaviour of westward moving mixed rossby gravity wmrg and the first rossby r1 waves in el nino en and la nina ln seasons is documented with a focus on the northern hemisphere winter the eastward moving variance in the upper troposphere is dominated by wmrg and r1 structures that appear to be doppler shifted by the flow and are referred to as wmrg e and r1 e in the east pacific and atlantic the years with stronger equatorial westerly winds have the stronger wmrg and wmrg e in the east pacific r1 is also a maximum in ln however r1 e exhibits an eastward shift between ln and en the changes with enso phase provide a test bed for the understanding of these waves in the east pacific and atlantic the stronger wmrg e and wmrg with stronger westerlies are in accord with the dispersion relation with simple doppler shifting by the zonal flow the possible existence of free waves can also explain stronger r1 in en in the eastern hemisphere 1 d free wave propagation theory based on wave activity conservation is also important for r1 however this theory is unable to explain the amplitude maxima for other waves observed in the strong equatorial westerly regions in the western hemisphere and certainly not their enso related variation the forcing of equatorial waves by higher latitude wave activity and its variation with enso phase is therefore examined propagation of extratropical eastward moving rossby wave activity through the westerly ducts into the equatorial region where it triggers wmrg e is favoured in the stronger westerlies in ln in the east pacific and en in the atlantic it is also found that wmrg is forced by southern hemisphere westward moving wavetrains arching into the equatorial region where they are reflected the most significant mechanism for both r1 and r1 e appear to be lateral forcing by subtropical wavetrains

development of synoptic scale disturbances over the summertime tropical northwest pacific abstract this study addresses the origin of the synoptic scale disturbances that occur in the tropical western north pacific ocean wp region in northern hemisphere summer these have been called easterly waves and tropical depression type td disturbances this analysis uses the national center for environmental prediction national center for atmospheric research reanalysis dataset by performing a regression analysis on several terms in the vorticity equation at 850 hpa it is shown that the td disturbances propagate approximately as barotropic rossby waves at 850 hpa given this ray tracing calculations and the wave activity diagnostic introduced by plumb are used to show that wave accumulation is a promising candidate for the initial development mechanism of the td disturbances the expected local growth rate from this mechanism is simply the convergence of the group velocity which reaches values corresponding to a growth timescale of 3 days this convergence is dominated by but somewhat lar

palaeoproductivity of the eastern arabian sea during the past 200 ka a multi proxy investigation abstract two sediment cores from the eastern arabian sea were investigated for reconstructing the late quaternary palaeoproductivity using a variety of proxies the relative abundance of florisphaera profunda in the total coccolithophore flora fp organic carbon c org and calcium carbonate contents the low abundance of fp during glacial periods suggests a significantly enhanced primary productivity during those periods the high fp abundance during terminations ii and i implies that the productivity was low during deglaciation the reduced productivity was probably a result of the intensification of the sw monsoon during the two terminations the high c org content documented for glacial periods marine isotopic stages 2 4 and 6 also suggests that productivity was high during glacial periods the consistent results obtained from two independent proxies fp and c org for palaeoproductivity confirm that productivity was indeed high during glacial periods and hence the high c org content is not just an artifact of better preservation of organic matter due to high sedimentation rate down core variations in the caco 3 content and 18 o show coherent fluctuations and typical glacial interglacial cyclicity glacial periods were characterised by low caco 3 and interglacials by high caco 3 content interestingly the low caco 3 content during glacials is in contrast to the high productivity suggested by the low relative abundance of fp and the high c org content therefore the low carbonate content during glacial periods was probably due to dilution by terrigenous material and or increased biogenic opal production spectral analysis of data on palaeoproductivity proxies shows a 23 ka periodicity indicating that the earth s precessional cycle exerts a significant control on the palaeoproductivity of the eastern arabian sea

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

evolution and variability of the asian monsoon system state of the art and outstanding issues the asian monsoon is comprised of the indian and east asian subsystems these two components are linked to one another in varying degrees by regions of strong sensible heating indo asian landmass and strong latent heat export the western pacific warm pool and the southern subtropical indian ocean variability within the indian and east asian subsystems interactions among them and the extent to which they interact with other climate phenomena e g enso are current topics of modern and paleoclimate research this work provides an overview of past and current paleomonsoon research on tectonic to interannual time scales with a primary focus on marine sediment records while past and current work has contributed greatly to our understanding of paleomonsoon variability at all time scales additional efforts are required to make further progress in two critical areas 1 additional efforts are needed in terms of proxy development and evaluation requiring concerted efforts at long term sediment trap deployments in key monsoon influenced regions as well as development of adequate and widely available core top databases these are necessary to assess the impact of modern oceanographic and seafloor processes on potential monsoon proxies 2 additional effort is also needed in acquiring a sufficient geographic distribution of downcore records to assess linkages among the subsystems and their role in the context of extratropical climate change these records should have high sedimentation rates including varved sections this requires substantial survey support to identify the most appropriate coring and drilling targets c 2004 elsevier ltd all rights reserved

microfossil evidence for changing salinity patterns in the bay of bengal over the last 20 000 years the monsson climate characterizing the northern indian ocean produces salinity gradients that are large compared with those in other open ocean areas these gradients are substantially independent of thermal and other ecologically important gradients this situation makes it possible not only to isolate the effect of salinity on sea bed distribution patterns of planktonic foraminifera but also to investigate past salinity changes along a north south transect of piston cores in the bay of bengal analysis of 152 surface sediment samples reveals that at least three factors other than salinity are important in controlling foraminiferal distribution sea bed dissolution surface productivity and surface temperature of these dissolution is the most important therefore in order to reconstruct past salinity patterns in the bay of bengal faunal indices need to be developed that isolate the effect of salinity we have developed two independent approaches the first uses ternary diagrams of the three dominant dissolution resistant species globoquadrina dutertrei globorotalia menardii pulleniatina obliquiloculata high values of g dutertrei are associated with low surface salinities the second approach uses dissolution buffered salinity transfer functions based on the total foraminiferal fauna in the 152 surface samples from the northern indian ocean applied to downcore samples these methods show the same basic historical pattern of salinity variation which is in turn consistent with independent inferences based on preliminary oxygen isotope data duplessy 1980 historical changes in the bay of bengal salinity are reconstructed along a north south transect of seven cores at 90 e the stratigraphy of the transect is controlled by oxygen isotope profiles that are similar to the generalized patterns observed in other areas over the past 20 000 yr salinity gradients are reconstructed for three time intervals 1 the last glacial maximum lgm 2 the mid termination mt and 3 the holocene during the lgm interval salinity was highger in the bay of bengal particularly in the north thus reducing the north south salinity gradient during the mt the middle of the transition from glacial to interglacial global climatic regimes northern bay of bengal surface salinity was considerably reduced while salinity to the south was generally similar to today thus the salinity gradient between 10 n and 15 n was considerably steeper than today in contrast the holocene gradient was approximately the same as the modern gradient with absolute values along the transect being slightly but uniformly higher these changes in the pattern of salinity are explained in terms of changes in the local patterns of precipitation and river run off during the lgm the salinity pattern probably results from decreased precipitation and decreased run off by the ganges brahmaputra river system during the mt decreased salinity in the northern bay of bengal is caused by increased precipitation and run off rather than by the influx of glacial meltwater the steeper mt salinity gradient in the central bay of bengal suggests increased eastward advection of equatorial surface waters by a southwest monsoon current somewhat more vigorous than today these patterns can be correlated with faunal evidence from the western arabian sea that suggest decreased upwelling during the lgm and increased upwelling during the mt this coupling of events may be associated with changes in the pattern or intensity of circulation during the southwest monsoon decreased intensity at the lgm 18 000 b p and increased intensity at the mt 10 500 to 12 500 b p

conditions for a steady ice sheet ice shelf junction abstract this paper investigates the conditions under which a marine ice sheet may adopt a steady profile the ice is treated as a linear viscous fluid caused to flow from a rigid base to and over water treated as a denser but inviscid fluid the solutions in the region around the point of flotation or transition zone are calculated numerically in flow and out flow conditions appropriate to ice sheet and ice shelf flow are applied at the ends of the transition zone and the rigid base is specified the flow and steady free surfaces are determined as part of the solutions the basal stress upstream and the basal deflection downstream of the flotation point are examined to determine which of these steady solutions satisfy contact conditions that would prevent i the steady downstream basal deflection contacting the downstream base and ii the upstream ice commencing to float in the event it was melted at the base in the case that the upstream bed is allowed to slide we find only one mass flux that satisfies the contact conditions when no sliding is allowed at the bed however we find a range of mass fluxes satisfy the contact conditions the effect of backpressure on the solutions is investigated and is found to have no affect on the qualitative behaviour of the junctions to the extent that the numerical linearly viscous treatment may be applied to the case of ice flowing out over the ocean we conclude that when sliding is present weertman s instability hypothesis holds