interannual modulation of east african early short rains by the winter arctic oscillation in the present study we analyzed the interannual linkage between the boreal winter arctic oscillation ao and east african early short rains when the indian ocean dipole and el nino southern oscillation variance are excluded by linear regression the boreal winter ao index is significantly correlated with the october east african precipitation over the domain of 5 n 5 s and 35 45 e for the period 1979 2014 r 0 46 the upper ocean heat content likely acts as a medium that links the ao and east african precipitation significant subsurface warming and positive upper ocean heat content anomalies occur over the western indian ocean during the autumn following positive ao winters which enriches the atmospheric moisture intensifies convection and enhances precipitation oceanic dynamics play a key role in causing this subsurface warming winter ao related atmospheric circulation creates anomalous wind stress which forces a downwelling oceanic rossby wave between 60 75 e and 5 10 s where the thermocline significantly deepens this rossby wave propagates westward and accompanies significant subsurface warming along the thermocline the rossby wave arrives at the western indian ocean in the late summer significantly warming the region to the west of 55 e at a depth of 60 100 m this warming remains significant through october correspondingly the upper ocean heat content significantly increases by approximately 2 3 108 j m 2 in the region west of 60 e between 5 and 10 s the role of these oceanic dynamics in linking the winter ao and anomalous subsurface warming was tested by numerical experiments with an oceanic general circulation model the experiments were performed with the forcing of ao related wind stress anomalies over the indian ocean in the winter the oceanic rossby wave generated in the central indian ocean during boreal winter the consequent subsurface warming and the anomalous upper ocean heat content in october over the western indian ocean were all adequately reproduced the winter ao can serve as a precursor of east african short rain anomalies

experiments on the generation of internal waves over continental shelf topography experiments were performed to examine the generation of internal waves by a barotropic tide forcing a continuously stratified fluid over idealized continental shelf slope topography a range of responses was observed including the generation of both internal wave beams and boundary layer boluses primarily dependent on the values of both the reynolds number and the topographic steepness parameter the formation of beams required a critical bottom slope whilst for bolus formation a large vertical fluid excursion was necessary a bolus formed when the non dimensional vertical excursion parameter hn w 0 3 2 here h is the vertical excursion n is the buoyancy frequency and w 0 is the near bottom vertical velocity associated with the local depth averaged velocity we simplified the classification of the observed flow regimes using a generation parameter g defined as the ratio of a reynolds number to the topographic steepness parameter the estimated flow regime boundaries were for g g g g 400 there was no bolus observed we estimated that approximately 4 of the barotropic energy was converted to baroclinic energy when beams were generated

global glacial isostatic adjustment palaeogeodetic and space geodetic tests of the ice 4g vm2 model analyses of the global process of glacial isostatic adjustment and post glacial relative sea level change continue to deliver important insights into earth system form and process one successful model of the related phenomenology is based upon a spherically symmetric internal viscoelastic structure for the solid earth which has been denoted vm2 and a model of the most recent deglaciation event of the current ice age denoted ice 4g the primary purpose of this paper is to describe several new a posteriori tests that have recently been performed to further investigate the quality of this global solution to the inverse problem for both mantle viscosity and deglaciation history that is posed by the observables associated with this large scale geodynamic phenomenon i focus especially upon the misfits of observations to the theoretical predictions of this model which i am currently using to further refine its properties and upon predictions made using it of geophysical signals that should soon become visible in the context of the gravity recovery and climate experiment grace satellite mission among the required refinements to ice 4g one that is necessary to eliminate a recently revealed misfit to space geodetic constraints on the present day rate of radial motion at the yellowknife location well to the west of hudson bay and a similar misfit to absolute gravity measurements to the southwest of the bay is the insertion of a keewatin dome of thick ice centred over yellowknife with a ridge of ice extending to the south east in the geomorphological literature the existence of such a keewatin dome previously has been hypothesised but chronological control was lacking on the surface features that suggested its former existence an important additional constraint that requires the late glacial existence of this important feature consists of new inferences of the last glacial maximum lowstand of the sea from sites in the far field of the main concentrations of land ice copyright 2002 john wiley sons ltd

atmospheric hydrospheric mechanisms of climate anomalies in the western equatorial indian ocean atmosphere ocean mechanisms of rainfall anomalies at the coast of eastern africa are studied using long term ship observations in the indian ocean surface current measurements subsurface casts upper air analyses by the european centre for medium range weather forecasts rain gauge series in eastern africa and india and an index of the southern oscillation so the high so phase being defined by anomalously high low pressure at tahiti darwin the causalities of precipitation anomalies at the coast of eastern africa differ for the two rainy seasons centered on april may and october november and only the latter is strongly related to the so in the high so phase april may pressure is low over the entire indian ocean domain whereas in october november pressure is high in the west and low in the east concomitantly surface waters are anomalously cold in the west and strong westerlies sweep the equatorial zone of the indian ocean eastern african rainfall anomalies are related to the so through a combination of cooperative mechanisms that function most effectively in the boreal autumn rainy season of eastern africa 1 equatorial westerly winds are conducive to lower tropospheric divergence over equatorial east africa and in the high so phase these are accelerated especially in october november owing to the anomalous eastward pressure gradient 2 the equatorial westerly winds drive the eastward equatorial jet in the upper hydrosphere which entails cold water upwelling in the western extremity of the basin where sea surface temperature further hydrostatically affects the zonal pressure gradient and thus feeds back into the equatorial westerly winds 3 in addition cold water anomalies in the western indian ocean most pronounced in october november during the high so phase also suppress convection 4 in the high so phase the indian summer monsoon tends to be strong leaving behind an anomalously cold western indian ocean which in turn feeds into mechanisms 1 to 3 the eastward equatorial jet thus has a role to play in feedback mechanisms contributing to the anomalies of the boreal autumn rains at the coast of eastern africa

dynamical mechanisms for asymmetric ssta patterns associated with some indian ocean dipoles indian ocean dipole iod can be divided into two types according to their sst anomaly ssta patterns the symmetric iods and the asymmetric iods dynamical mechanisms for the two types of iods are investigated using an ocean general circulation model and the numerical experiments indicate that the setup time of the equatorial easterly wind anomalies eewa associated with iods is crucially important for the symmetrical characteristics of the ssta patterns early setup of the eewa in spring can induce intense cooling in the east pole but weak warming in the west pole making the two poles asymmetric the intense cooling in the east pole is linked to the seasonal variations of thermocline depth horizontal sst gradient and unusually long duration of the eewa and the weak warming in the west pole is due to unusually cold zonal advection resulted from zonal sst gradient change the numerical experiments demonstrate that the strength of the eewa also plays an important role in the symmetrical characteristics of iods the ssta in the two poles both intensify as the eewa strength increases only when the eewa speed is below a critical value above the critical value the warming in the west pole starts to decrease while the cooling in the east pole still keeps increasing leading to an asymmetric ssta pattern the mechanism behind this phenomenon is similar to the one in the situation when eewa sets up earlier

effects on post glacial rebound from the hard rheology in the transition zone summary we analyse the influences of a viscosity increase in the transition zone between 420 and 670 km on the geophysical signatures induced by post glacial rebound ranging from the perturbations in the earth s rotation to the short wavelength features associated with the migration of the peripheral bulge a self gravitating model is adopted consisting of an elastic lithosphere a three layer viscoelastic mantle and an inviscid core the horizontal displacements and velocities and the stress pattern are extremely sensitive to the viscosity increase and to the chemical stratification of the transition zone the hardening of the upper and the chemical density jumps in mantle below the 420 discontinuity induces a channel effect which contaminates the horizontal deformation both in the near field and in the far field from the ice sheets these findings indicate that intraplate geodetic data can be used to put bounds on the viscosity increase in the transition zone and on the amount of chemical stratification in the mantle the stress field induced in the lithosphere by the pleistocenic ice sheet disintegration is a very sensitive function of mantle viscosity stratification the existence of seismic activity along passive continental margins of previously glaciated areas requires a substantial viscosity increase in the mantle with the viscosity of the transition zone acting as a controlling parameter a viscously stratified mantle is responsible for a delayed upward migration of stress in the lithosphere which can account for the seismicity today

diagnosing the 2005 drought in equatorial east africa abstract equatorial east africa suffered severe drought during its 2005 short rains centered on october november the circulation mechanisms of such precipitation anomalies are examined using long term upper air and surface datasets and based on diagnostic findings from earlier empirical investigations the steep eastward pressure gradient is conducive to fast westerlies over the central equatorial indian ocean surface manifestation of a powerful zonal circulation cell with subsidence over east africa and ascending motion over indonesia with fast westerlies rainfall in east africa is deficient and they tend to be accompanied by anomalously cold waters in the northwestern and warm anomalies in the southeastern extremity of the equatorial indian ocean basin without any seesaw between these domains in october november 2005 pressure in the west was anomalously high entailing a steep eastward pressure gradient along the equator conducive to fast westerlies and further symptomatic of the zonal ci

significance of the fundamental mantle rotational relaxation mode in polar wander simulations summary in some studies on glacially induced true polar wander tpw the tidal effective relaxation of the fundamental mantle mode mo is lacking we show that this is caused by the deletion of the chandler wobble in an early stage of the theory development to facilitate the retrieval of the rotational relaxation modes we derive an analytical approximation formula for the mo rotational relaxation mode including the chandler wobble which can be of practical value for tpw simulations with realistically stratified earth models however we point out that the contribution of the mo rotational relaxation mode has to a high approximation the same effect on secular tpw as the contribution from an elastic term in models that do not have the mo rotational relaxation mode the two model approaches lead to the same polar wander results whenever the chandler wobble is filtered from models in which the mo rotational relaxation mode is retained

tidal conversion by supercritical topography calculations are presented of the rate of energy conversion of the barotropic tide into internal gravity waves above topography on the ocean floor the ocean is treated as infinitely deep and the topography consists of periodic obstructions a green function method is used to construct the scattered wavefield the calculations extend the previous results of balmforth et al for subcritical topography wherein waves propagate along rays whose slopes exceed that of the topography everywhere by allowing the obstacles to be arbitrarily steep or supercritical so waves propagate at shallower angles than the topographic slopes and are scattered both up and down a complicated pattern is found for the dependence of energy conversion on e the ratio of maximum topographic slope to wave slope and the ratio of obstacle amplitude and separation this results from a sequence of constructive and destructive interferences between scattered waves that has implications for computing tidal conversion rates for the global ocean

an investigation of interannual rainfall variability in africa abstract interannual variations in african rainfall are examined using rotated principal component analysis pca applied to anomalies from the annual mean as well as seasonal anomalies the rotated pca loading patterns suggest several preferred continental scale rainfall anomaly patterns the dominant features of year to year variations in african rainfall appear to be the high spatial coherence of rainfall anomalies over large portions of the continent in addition several dipole regions that is adjacent regions which tend to experience rainfall anomalies of opposite sign are found and discussed one dipole region the sub saharan region appears to have a relationship with a characteristic atlantic sea surface temperature anomaly pattern during boreal summer during austral summer the tendency for a large scale dipole pattern in southeast africa is apparent as is an association of this pattern with the warm and cold phases of the el nin southern oscillation enso phenomenon normalized rainfal

carbon and oxygen isotopic composition of a guam coral and their relationships to environmental variables in the western pacific we examine the high resolution 32 samples year carbon and oxygen isotopic composition 13ccoral and 18ocoral in a coral core porites lobata from double reef guam over the years 1980 2000 the 13ccoral shows clear seasonal variations with mean seasonal amplitude of 1 89 which roughly corresponds with seasonal variations in solar radiation the seasonal amplitude of 18ocoral variations are small 0 23 0 57 but they are significantly correlated with sea surface temperature sst and salinity sss the 18ocoral and sst are more strongly correlated during el nino southern oscillation enso warm phases r 0 81 p 28 c and lower values of 18osw at guam these differences in oceanic parameters result from movements of the western pacific warm pool wpwp during the different phases of enso anomalies in 18osw inferred from the 18ocoral and instrumental sst are consistent with sss anomalies for the years 1980 2000 these 18osw anomalies may reflect changes in sss and evaporation precipitation due to movements of the wpwp this detailed analysis of a coral from guam suggests that it may contain an excellent archive of past enso events

glacial isostatic adjustment and earth rotation refined constraints on the viscosity of the deepest mantle we explore the extent to which two properties of earth s present day rotational state may be applied to constrain the viscosity of the lower regions of the lower mantle the analysis builds upon recent advances in understanding the radial resolving power of the relative sea level data of postglacial rebound which have demonstrated anew that no significant increase of viscosity across the 660 km seismic discontinuity appears to be allowed by these data such observations do not constrain the viscosity of the mantle below about 1400 km depth however and here we estimate the average viscosity in this deepest region by invoking observations of the present day rate and direction of polar wander and the present day magnitude of the nontidal acceleration of the rate of planetary rotation since the rotational response to the glaciation cycle depends only upon the degree 2 spherical harmonic components of the induced deformation this response is expected to provide the most specific sampling possible of deep mantle properties our analysis is based upon the use of glacial excitation functions calculated from complete gravitationally self consistent solutions of the sea level equation for the most recent refinement of the history of ice sheet thickness variations across the last glacial interglacial transition namely the ice 4g model this analysis shows that when earth rotation observations are combined with relative sea level constraints the viscosity of the deepest mantle is required to be 1 order of magnitude higher than the viscosity of the upper part of the lower mantle such significant elevation of viscosity in the lowermost region of the lower mantle is also in accord with recent inferences based upon analysis of the nonhydrostatic geoid the analyses presented herein therefore suffice to effect a reconciliation between the requirements of these distinct geophysical observations

late quaternary productivity changes in the equatorial indian ocean odp hole 716a abstract the equatorial indian ocean is swept by the indian ocean equatorial westerlies iew which intensify in monsoon transitions during april may and october november driving eastward equatorial current eec to the upper ocean the eec brings significant changes in the surface productivity in the equatorial region influencing the life style of the surface and deep sea fauna this study is aimed at understanding 300 kyr record 450 150 kyrs ago of productivity changes linked to variations in iew eec in the equatorial indian ocean using high resolution microfaunal record from integrated ocean drilling program iodp hole 716a maldives ridge in total 451 core samples of 10 cm 3 volume from the 300 kyr long sequence ranging 450 150 kyrs ago were analyzed at every 1 cm interval to generate census data of the benthic foraminiferal fauna factor and cluster analyses using percentages of 53 highest ranked benthic foraminiferal species were run that enabled us to identify five biofacies indicating the varied nature of deep sea environments during the late quaternary the results show a major shift roughly 300 kyr ago across marine isotope stages 9 and 8 mis 9 8 recorded in this study as mid brunhes transition at hole 716a the present study also combines published planktic foraminiferal fauna and pteropod records from hole 716a to understand if the surface to shallow and intermediate water paleoenvironments were coupled with deep water changes during the late quaternary in the maldivian archipelago biofacies robulus nicobarensis trifarina reussi rn tr uvigerina porrecta reussella simplex upo rs and cymbaloporetta squammosa bolivinita sp cs bsp dominate before the mid brunhes transition and document high organic flux with relatively low oxygen paleoenvironment similar to the planktic foraminifer globigerina bulloides population while benthic foraminiferal biofacies hoeglundina elegans miliolinella subrotunda he ms and uvigerina peregrina quinqueloculina seminulum upe qs record high seasonality in food supply with well oxygenated deep water following the mid brunhes climatic transition 300 kyr ago at hole 716a these changes are also recorded in the population of planktic foraminiferal species globigerinoides ruber and neogloboquadrina dutertrei as well as pteropods organic carbon flux linked to changes in the iew eec strength appears to be the most plausible factor controlling faunal assemblages and hence paleoproductivity dynamics in the maldivian region our results suggest that the strength of the iew is inversely related with the indian ocean dipole iod the iod strengthened across mis 9 8 during which time the iew weakened leading to major changes at hole 716a

internal wave breaking at concave and convex continental slopes abstract internal wave reflection from a sloping topographic boundary may lead to enhanced shear if the topographic angle to the horizontal is close to that of the internal wave group velocity vector previous analytic studies have suggested that shear enhancement is reduced at concave slopes as compared with convex and planar slopes near the critical angle here the internal wave reflection from concave and convex slopes that pass through the critical angle is investigated numerically using the nonhydrostatic massachusetts institute of technology general circulation model mitgcm overturning shear instability and resultant mixing are examined results are compared with simulations of wave reflection from planar slopes with angles greater than less than and equal to the critical angle in contrast to the analytic predictions no reduction in mixing is found for the concave slope as compared with the other slopes in all cases stratification is eroded in a band above the slope bounded at its outer

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

isostatic deformations and polar wander induced by redistribution of mass within the earth the effects at the surface of the earth of time dependent mantle mass anomalies are analyzed within the framework of a viscoelastic mantle with maxwell rheology the implications for the earth s rotation are developed using the linearized liouville equations valid for small polar displacements our approach is appropriate for a simplified modeling of subduction the displacement of the earth s axis of rotation called true polar wander is very sensitive to the viscosity profile of the mantle and to the nature of the 670 km seismic discontinuity phase change models generally yield a huge amount of polar wander except for large viscosity increases for chemically stratified models true polar wander is drastically reduced as a consequence of dynamic compensation of the mass anomalies at the upper lower mantle interface when the source is embedded in the upper mantle in the proximity of the chemical density jump the polar wander stops after a finite time controlled by the slowest isostatic mode

the seasonal development of an sst anomaly in the indian ocean and its relationship to enso abstract the seasonal development of the sea surface temperature sst anomaly in the indian ocean is investigated in relation to el nino southern oscillation enso using noaa optimally interpolated sst and ncep reanalysis data the result shows that the onset season of el nino affects the seasonal development of surface wind anomalies over the equatorial eastern indian ocean eeio these surface wind anomalies in turn determine whether the sst anomaly in the eeio evolves into the eastern pole of the dipole pattern in years when the dipole pattern develops surface zonal wind anomalies over the eeio switch from westerly to easterly in spring as la nina switches to el nino the seasonal zonal wind over the eeio also switches from westerly to easterly in spring and the anomalous wind strengthens seasonal wind from winter to summer stronger winds and resultant thermal forcings produce the negative sst anomaly in the eeio in winter and its amplitude increases in summer the sst anomaly becomes the ea

stable isotopes in east african precipitation record indian ocean zonal mode observational stable isotope data and model simulations show that the 18 o composition of precipitation in east africa is closely related to the coupled ocean atmosphere system over the indian ocean during the rainy seasons 18 o is a very sensitive recorder of the indian ocean zonal mode iozm the link between 18 o and the iozm is established through adjustments in the large scale overturning circulation over the indian ocean and associated changes in convective activity and precipitation over east africa the iozm is recorded as significant departures in 18 o not only over east africa but as a dipole mode with opposite sign to the east and west of the indian ocean

ice 3g a new global model of late pleistocene deglaciation based upon geophysical predictions of post glacial relative sea level change a new high resolution global model of late pleistocene deglaciation is inferred on the basis of geophysical predictions of postglacial relative sea level variations in which the ice ocean solid earth interaction is treated in a gravitationally self consistent fashion for the purpose of these analyses the radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem only radiocarbon controlled relative sea level histories from sites that were actually ice covered with one or two additions are employed to constrain the model leaving relative sea level rsl data from sites that were not ice covered to be employed to confirm its consistency results for these confirmatory analyses are reported elsewhere here the new deglaciation model referred to as ice 3g is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories including geologically controlled retreat isochrones oxygen isotope data from deep sea sedimentary cores and coral terrace elevations the latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial interglacial transition

mayotte coral reveals hydrological changes in the western indian ocean between 1881 and 1994 we reconstruct the hydrologic history of the tropical western indian ocean by calculating the 18oseawater from coupled coral sr ca and 18o measurements in a massive porites coral from mayotte comoros between 1881 and 1994 we found that the precipitation evaporation balance varies naturally on time scales of 5 6 years and 18 25 years high low ssts are associated with positive negative 18oseawater implying that atmospheric variability is linked with remote climate modes in the indian ocean and the tropical extratropical pacific ocean warm el nino southern oscillation events are associated with a negative freshwater balance at mayotte this case study demonstrates that a much denser network of 18oseawater reconstructions is crucial for understanding the spatial patterns of hydrological conditions

potential predictability of the sea surface temperature forced equatorial east african short rains interannual variability in the 20th century in this article the predictability of the 20th century sea surface temperature sst forced east african short rains variability is analyzed using observational data and ensembles of long atmospheric general circulation model agcm simulations to our knowledge such an analysis for the whole 20th century using a series of agcm ensemble simulations is carried out here for the first time the physical mechanisms that govern the influence of sst on east african short rains in the model are also investigated it is found that there is substantial skill in reproducing the east african short rains variability given that the ssts are known consistent with previous recent studies it is found that the indian ocean and in particular the western pole of the indian ocean dipole iod play a dominant role for the prediction skill whereas ssts outside the indian ocean play a minor role the physical mechanism for the influence of the western indian ocean on east african rainfall in the model is consistent with previous findings and consists of a gill type response to a warm cold anomaly that induces a westerly easterly low level flow anomaly over equatorial africa and leads to moisture flux convergence divergence over east africa on the other hand a positive el nino southern oscillation enso anomaly leads to a spatially non coherent reducing effect over parts of east africa but the relationship is not strong enough to provide any predictive skill in our model the east african short rains prediction skill is also analyzed within a model derived potential predictability framework and it is shown that the actual prediction skill is broadly consistent with the model potential prediction skill low frequency variations of the prediction skill are mostly related to ssts outside the indian ocean region and are likely due to an increased interference of enso with the indian ocean influence on east african short rains after the mid 1970s climate shift

pleistocene deglaciation and the earth s rotation a new analysis summary this paper is concerned with an analysis of the effect of the pleistocene glacial cycle upon the earth s rotation we demonstrate that two important geophysical observables may be explained as aspects of the rotational response to surface mass loading by ice sheets these are the astronomically observed non tidal component of the acceleration of planetary rotation and the secular drift of the rotation pole relative to the surface geography which is evident in the ils pole path the former observation is shown to provide an unambiguous constraint upon the viscosity of the planetary mantle and requires a depth dependence of this parameter which is the same as that which has previously been inferred in studies of the relative sea level variations and free air gravity anomalies associated with post glacial rebound the observed secular drift of the pole cannot provide an independent estimate of the viscosity of the mantle because it depends jointly upon mantle viscosity and lithospheric thickness with the viscosity profile fixed by the rebound data the observed speed of polar wander suggests a continental lithospheric thickness which is in excess of that appropriate for old ocean basins but nevertheless in accord with independent constraints

idealized solutions for the energy balance of the finescale internal wave field abstract recent fine and microstructure observations indicate enhanced finescale shear and strain in conjunction with bottom intensified turbulent dissipation above rough bathymetry in the brazil basin such observations implicate the bottom boundary as an energy source for the finescale internal wave field simple analytical and numerical solutions to an equation governing the spatial and temporal evolution of the finescale wave field are described here the governing equation implicitly treats the effects of wave breaking on the vertical propagation of internal waves through a flux representation of nonlinear transports associated with internal wave wave interactions these solutions identify the rate of dissipation of turbulent kinetic energy with downscale energy transports at high vertical wavenumber resulting in an estimate of dissipation versus depth the sensitivity of the turbulent dissipation depth profile to various environmental parameters is examined observed dissipation profiles and shear

circulation mechanisms of kenya rainfall anomalies abstract expanding earlier studies on the boreal spring and autumn rainy seasons in equatorial east africa pending challenges on the mechanisms of rainfall variability are investigated eastward pressure gradient and slack south indian ocean trade winds allow surface equatorial westerlies in spring and autumn complementing that upper tropospheric easterlies are required for the development of a zonal vertical circulation cell along the indian ocean equator because of the summer warming and high stand of upper tropospheric topography over south asia strong upper tropospheric easterlies over the tropical northern and equatorial indian ocean persist from summer into autumn thus allowing the development of a zonal vertical circulation cell by contrast the winter cooling entails low stand of upper tropospheric topography in the north thus hindering easterlies over the equator consequently an equatorial zonal circulation cell does not develop in boreal spring the equatorial zonal circulation cell

internal tide generation by seamounts ridges and islands a numerical investigation is made into the generation of semidiurnal internal tides by tidal flow over steep topographic features in the deep ocean a fully three dimensional free surface nonlinear hydrostatic model the princeton ocean model is used with real stratification representative tidal forcing and gaussian shaped seamounts ridges and islands for the topography the efficiencies of the different topographies in extracting energy from the barotropic tide and in generating an internal tide are considered each topography produces an internal tide characterized by signals propagating away from the feature as beams that follow internal wave characteristic paths the strength of the signals however varies markedly for different topographies this is largely attributed to the way in which the barotropic tide interacts with the three dimensional topography internal wave generation requires a significant vertical displacement of stratified water by the barotropic tide as it flows over topography it is found that for symmetric seamounts and islands the barotropic flow tends to go around the feature producing only a weak internal tide when elongated into a ridge barotropic flow is forced across isobaths generating an energetic internal tide changing the horizontal aspect ratio of the topography from 1 1 a seamount to 3 1 a ridge increases the resulting baroclinic energy flux by nearly an order of magnitude the slope of the topography relative to that of the internal wave characteristics is also shown to be important with strongest generation occurring at regions of critical or supercritical topographic slope

polar wandering and the forced responses of a rotating multilayered viscoelastic planet changes in the earth s rotation produced by the waxing and waning of large mobile ice sheets can be used to infer the viscosity of the mantle from a comparison with the recent secular polar drift as revealed in the international latitude service data and also in the estimates of the nontidal acceleration of the length of the day we have developed by using an analytical approach a physical model of a rotating earth consisting of an elastic lithosphere a viscoelastic mantle and an inviscid core forcings from the two major ice sheets laurentide and fennoscandia have been considered sensitivity analysis of the parameters governing the external forcings such as the length and the time of termination of the deglaciation phase show in general that there exist two families of mean mantle viscosities 0 1022 p and 0 1023 p which can match the data satisfactorily however we find it an improvement to incorporate some amount of shrinkage from the antarctic ice sheet obtaining simultaneously a better fit with the two rotational data sets to the lower viscosity solutions hence forcings from the southern hemisphere may be able to remove the ambiguity associated with the extraction of mantle viscosity from rotational data we have also solved the initial value problem of the components of instantaneous angular velocity by means of the laplace transform for applied loads characterizing the continental ice sheets which since the mid pliocene have periodically expanded and contracted over the northern hemisphere in consequence of this continual forcing polar wander of around 5 10 degrees can take place for a mean mantle viscosity 0 1022 p recent reanalysis of paleomagnetic data in conjunction with hot spot tracks have revealed that several degrees of true polar wander could have occurred in the last 5 m y these calculations thus provide a physical explanation for the observed movement of the rotation axis membrane stresses of 0 10 bars can develop from this secular polar motion

an estimate of tidal energy lost to turbulence at the hawaiian ridge abstract an integrated analysis of turbulence observations from four unique instrument platforms obtained over the hawaiian ridge leads to an assessment of the vertical cross ridge and along ridge structure of turbulence dissipation rate and diffusivity the diffusivity near the seafloor was on average 15 times that in the midwater column at 1000 m depth the diffusivity atop the ridge was 30 times that 10 km off the ridge decreasing to background oceanic values by 60 km a weak factor of 2 spring neap variation in dissipation was observed the observations also suggest a kinematic relationship between the energy in the semidiurnal internal tide e and the depth integrated dissipation d such that d e1 0 5 at sites along the ridge this kinematic relationship is supported by combining a simple knife edge model to estimate internal tide generation with wave wave interaction time scales to estimate dissipation the along ridge kinematic relationship and the observed vertical and cross ridge st

tidally generated turbulence over the knight inlet sill very high turbulent dissipation rates above 5 1024 wk g 21 were observed in the nonlinear internal lee waves that form each tide over a sill in knight inlet british columbia this turbulence was due to both shear instabilities and the jumplike adjustment of the wave to background flow conditions away from the sill turbulent dissipation was significantly lower 5 1027 to 5 1028 wk g 21 energy removed from the barotropic tide was estimated using a pair of tide gauges a peak of 20 mw occurred during spring tide approximately twothirds of the barotropic energy loss radiated away as internal waves while the remaining one third was lost to processes near the sill observed dissipation in the water column does not account for the near sill losses but energy lost to vortex shedding and near bottom turbulence though not measured could be large enough to close the energy budget

topographic amplitude dependence of internal wave generation by tidal forcing over idealized three dimensional topography 1 this numerical study using the princeton ocean model investigates the conversion of energy from an eastward propagating surface tide to the internal wave field as an oscillating tidal current flows over and around idealized three dimensional gaussian seamounts in particular the relationship between the energy flux in the internal wave field and the height of the topography is explored using a range of horizontal aspect ratios in agreement with theory the energy flux is found to increase as the square of the topographic amplitude ho for subcritical topography for supercritical topographies the functional relationship is more complex aspect ratio dependent it increases more rapidly than ho2 for near critical topography for larger circular seamounts the energy flux increases much more slowly with the rate of increase dropping with amplitude as the seamounts are elongated in the cross tidal wave north south direction the energy flux increases more rapidly with height and for a two dimensional ridge increases as approximately ho2 2 the directional dependence of the energy flux also depends on the aspect ratio and amplitude of the seamount for circular seamounts energy flux has its maximum in directions between 20 and 45 counterclockwise from the east west axis northern hemisphere for elongated seamounts the energy flux is directed in the east west direction

the rotational stability of an ice age earth summary predictions of glaciation induced changes in the earth s rotation vector exhibit sensitivities to earth structure that are unique within the suite of long wavelength observables associated with glacial isostatic adjustment henceforth gia and despite nearly a quarter of a century of research these sensitivities remain enigmatic previous predictions of present day true polar wander tpw speed driven by gia have indicated for example a strong sensitivity to variations in the thickness of the elastic lithosphere and the treatment phase or chemical of the density discontinuity at 670 km depth nakada recently presented results that suggest that the predictions are also sensitive to the adopted rheology of the lithosphere however his results have introduced an intriguing paradox in particular predictions generated using a model with an extremely high viscosity lithospheric lid do not converge to results for a purely elastic lithosphere of the same thickness mitrovica as cited by nakada has suggested that the paradox originates from an inaccuracy in the traditional rotation theory e g wu peltier associated with the treatment of the background equilibrium rotating form upon which any load and rotation induced perturbations are superimposed we revisit these issues using a new treatment of the linearized euler equations governing load induced rotation perturbations on viscoelastic earth models we demonstrate that our revised theory in which the background form of the planet combines a hydrostatic component and an observationally inferred excess ellipticity resolves the apparent paradox calculations using the revised theory indicate that earlier predictions based on earth models with purely elastic lithospheric lids are subject to large errors indeed previously noted sensitivities of tpw speed predictions to the thickness and rheology elastic versus viscous of the lithosphere largely disappear in the application of the new theory significant errors are also incurred by neglecting the stabilizing influence of the earth s excess ellipticity finally we demonstrate that the contribution from rotational feedback on predictions of present day rates of change of the geoid sea surface and crustal velocities are overestimated by the traditional rotation theory and this has implications for analyses of ongoing satellite e g grace missions and geodetic gps surveys

african hydroclimatic variability during the last 2000 years abstract the african continent is characterised by a wide range of hydroclimate regimes ranging from humid equatorial west africa to the arid deserts in the northern and southern subtropics the livelihoods of much of its population are also vulnerable to future climate change mainly through variability in rainfall affecting water resource availability a growing number of data sources indicate that such hydroclimatic variability is an intrinsic component of africa s natural environment this paper co authored by members of the pages africa 2k working group presents an extensive assessment and discussion of proxy historical and instrumental evidence for hydroclimatic variability across the african continent spanning the last two millennia while the african palaeoenvironmental record is characterised by spatially disjunctive datasets with often less than optimal temporal resolution and chronological control the available evidence allows the assessment of prominent spatial patterns of palaeomoisture variability through time in this study we focus sequentially on data for six major time windows the first millennium ce the medieval climate anomaly 900 1250 ce the little ice age 1250 1750 ce the end of the lia 1750 1850 ce the early modern period 1850 1950 and the period of recent warming 1950 onwards this results in a continent wide synthesis of regional moisture balance trends through history allowing consideration of possible driving mechanisms and suggestions for future research

zonal circulations over the equatorial indian ocean abstract the ncep ncar 1958 97 upper air dataset and surface observations have been analyzed for evidence of zonal vertical circulations along the indian ocean equator and their role in climatic variability the long term mean upper tropospheric circulation is dominated in boreal winter by divergent outflow from the southern indian ocean northwestward into southern asia and in summer from southern asia southwestward into the southern hemisphere in boreal autumn only divergent easterlies blow from indonesia along the equator into an upper tropospheric convergence band over east africa and only then a closed zonal vertical circulation cell materializes along the indian ocean equator between the centers of ascending motion over indonesia and of subsidence over equatorial east africa and featuring westerlies in the lower layers the boreal autumn zonal vertical circulation varies interannually a regime of intense circulation features accelerated equatorial surface westerlies enhanced subsidence and d

sea surface temperature productivity and hydrological changes in the northern indian ocean maldives during the interval 575 175 ka mis 14 to 7 abstract the south asian monsoon sam drives seasonal changes in the atmospheric and ocean circulation of the tropical indian ocean affecting precipitation on land and oceanic primary productivity this work examined sediments from the international ocean discover program iodp site u1467 iodp expedition 359 located in the maldives inner sea northern indian ocean at a water depth of 487 m the maldives inner sea is a perfect location to study past changes in tropical climate and ocean circulation related to monsoon dynamics in the equatorial indian ocean this study focuses on the 575 175 ka interval from marine isotope stage mis 14 to 7 an important period for human evolution and dispersal to eurasia we reconstructed sea surface temperature sst based on alkenone unsaturation index u 37 k hydrological changes based on terrestrial input of n alkanes and past surface ocean productivity based on total c37 alkenones concentration the u 37 k sst record shows a difference of about 1 5 c between glacial and interglacial periods clearly showing all stages and substages from mis 14 to 7 and revealing a connection between ice sheets extension and sst at the equatorial region the n alkanes concentration and average chain length index indicate vegetation changes at the indian peninsula with drier conditions generally associated with glacial periods precipitation increases abruptly at the end of terminations lasting for a variable time interval in each interglacial period except for mis 13 however other mechanisms superimposed to the glacial interglacial forcing such as the indian ocean dipole iod mode have been invoked to explain shorter scale variability in precipitation over india the total alkenone concentration record indicates that primary productivity at site u1467 is strongly associated with orbital changes probably related to the summer inter tropical insolation gradient sitig 23n 23s on june 21st high primary productivity occurred during intervals of low sitig which resulted in strong inter monsoon april may and october november indian ocean equatorial westerlies iew and reduced precipitation in the equatorial region this mechanism may also be related to the iod which affects the strength of the iew

polar wander caused by the quaternary glacial cycles and fluid love number abstract perturbations of the earth s rotation caused by the quaternary glacial cycles provide an important constraint on the viscosity of the deep mantle because they represent a long wavelength response of the earth to surface load redistribution the predicted present day polar wander speed pws is however sensitive to both the lower mantle viscosity lm the density jump at 670 km depth and the lithospheric thickness and viscosity e g sabadini and peltier geophys j r astron soc 66 1981 553 578 yuen et al j geophys res 87 1982 10745 10762 peltier and wu geophys res lett 10 1983 181 184 wu and peltier geophys j r astron soc 76 1984 753 791 peltier j geophys res 89 1984 11303 11316 vermeersen et al j geophys res 102 1997 27689 27702 mitrovica and milne j geophys res 103 1998 985 1005 johnston and lambeck geophys j int 136 1999 537 558 nakada geophys j int 143 2000 230 238 for earth models with lm 21 pa s and an elastic lithosphere the present day pws is very sensitive to the m1 mode buoyancy mode related to the density jump at 670 km depth mitrovica and milne j geophys res 103 1998 985 1005 the contribution of the m1 mode however is less significant for earth models with a viscoelastic lithosphere nakada geophys j int 143 2000 230 238 this is due to the fact that this contribution depends on the relative strength of the m1 mode k 2 t m1 k f t where k 2 t m1 is the magnitude of tidal love number k 2 t of the m1 mode and k f t is the value of k 2 t in the fluid limit fluid love number the magnitude of k f t for earth models with a viscoelastic lithosphere is larger than that for an elastic lithosphere and it is smaller for a thicker elastic lithosphere than for a thinner one thus for earth models with a viscoelastic lithosphere the pws is mainly sensitive to the lower mantle viscosity regardless of the behavior of the 670 km density discontinuity this relation also explains why the predicted pws increases with increasing thickness of an elastic lithosphere that is since the value of k 2 t m1 k f t with a thicker elastic lithosphere is larger than that with a thinner elastic lithosphere the m1 mode will have a higher contribution in the case of a thicker elastic lithosphere

empirical predictability study of october december east african rainfall relationships between september atmospheric dynamics over the global tropics and the east africa short rains over the domain 4 s 4 n 30 39 e were investigated to define a seasonal rainfall forecasting scheme from total and partial correlation analyses over the period 1968 97 three major and useful signals appear in september precursor components of the october december east west circulation cell appear over the equatorial indian ocean eio they were discovered through various indices of sea surface temperature sst zonal wind vertical velocity and moist static energy computed over the western central and eastern parts of the eio principal component analyses were applied to these indices to form two synthetic indices which discriminate ocean atmosphere variability in the eastern and western parts of the basin the indian monsoon dynamics control the ssts off the somalia coast which are known to play an important role in the variability of the short rains ridge trough systems develop off south africa which when persistent enough drive sst anomalies in the eastern tropical and equatorial atlantic these in turn are associated with anomalies in the middle levels over an equatorial band ranging from the guinea gulf to the western indian ocean the three september signals used as predictors in a multiple linear regression model explain 64 of the rainfall variance a comparable efficiency is also achieved with a linear discriminant analysis model copyright 2002 royal meteorological society

preliminary simulations of internal waves and mixing generated by finite amplitude tidal flow over isolated topography much recent observational evidence suggests that energy from the barotropic tides can be used for mixing in the deep ocean here the process of internal tide generation and dissipation by tidal flow over an isolated gaussian topography is examined using two dimensional numerical simulations employing the mitgcm four different topographies are considered for five different amplitudes of barotropic forcing thereby allowing a variety of combinations of key nondimensional parameters while much recent attention has focused on the role of relative topographic steepness and height in modifying the rate of conversion of energy from barotropic to baroclinic modes here attention is focused on parameters dependent on the flow amplitude for narrow topography large amplitude forcing gives rise to baroclinic responses at higher harmonics of the forcing frequency tall narrow topographies are found to be the most conducive to mixing dissipation rates in these calculations are most efficient for the narrowest topography

long term true polar wander of the earth including the effects of convective processes in the mantle and continental drift summary long term true polar wander tpw of the earth was examined by taking into account the effects of simplified convective processes in the earth s mantle and continental drift the tpw for a given viscoelastic earth model is wholly determined by both the magnitude of non forcing elements of moment of inertia i 11 i 22 and i 33 and i 12 element of product of inertia and the rates for forcing elements di 13 dt and di 23 dt the forcing rates are largely related to time dependent convective processes in the mantle and also continental drift in this study i examined the tpw on a convecting mantle with oscillating moments of inertia inferred from a convective process of alternating degree one and degree two structure changes of mantle convection by zhongetal 2007 in the phase for a relatively hydrostatic earth corresponding to largely degree one planform the predicted tpw is sensitive to the viscosity structure of the mantle particularly to the lithospheric viscosity structure and its magnitude may be larger than 30 even for the forcing rates with 10 30 kg m 2 myr 1 related to continental drift dickman 1979 in the phase for a non hydrostatic earth characterized by largely degree two planform such as for the present day however the tpw is less sensitive to the viscosity structure and its magnitude may be 10 at most these results may provide quantitative constraints on examining relationship between rheological structure and convective processes in the mantle continental drift and tpw

subtropical sst dipole events in the southern indian ocean sea surface temperature sst anomalies in the subtropical southern indian ocean show interannual dipole events that are seasonally phase locked to the austral summer a positive phase of the event is characterized by cold sst anomalies in the eastern part i e off australia and warm sst anomalies in the southwestern part south of madagascar such an event is found to produce above normal rainfall over many regions in south central africa the cooling of sst in the eastern part is mainly caused by the enhanced evaporation this is associated with stronger winds along the eastern edge of the subtropical high which is strengthened and shifted slightly to the south during the event on the other hand relative decrease in the seasonal latent heat loss due to reduced evaporation dominates the warming in the southwestern part evolution of such subtropical dipole events shows quite a contrast to that of the tropical dipole events discovered recently in the indian ocean

indian ocean response to anomalous conditions in 2006 1 the equatorial indian ocean eio exhibited anomalous conditions characteristic of an indian ocean dipole iod during 2006 the eastern eio had cold sea surface temperature anomalies ssta lower sea level shallow thermocline and higher chlorophyll than normal the anomalies in the east restricted to the south of the equator were highest off sumatra the western pole of the iod was marked by warm ssta and deeper thermocline with maxima on either side of the equator an ocean general circulation model of the indian ocean forced by quikscat winds reproduces the iod of 2006 remarkably well the switch over to cooling in the east and warming in the west happened during may and july respectively in the east airsea heat flux initiated cold ssta in the model which were sustained later by oceanic processes in the west surface heat fluxes and horizontal advection caused warm ssta and contribution by the latter decreased after august citation vinayachandran p n j kurian and c p neema 2007 indian ocean response to anomalous conditions in 2006 geophys res lett 34 l15602 doi 10 1029 2007gl030194

abrupt rainfall transitions over the greater horn of africa observations and regional model simulations 1 we examine the yearly occurrence of a monsoon jump of approximately 20 latitude during the boreal spring and summer rainy seasons over the greater horn of africa gha this jump is in contrast with a simple model of a smoothly varying itcz over the region the rainfall jump is observed annually during april and may in three precipitation data sets and in regional climate model simulations using the psu ncar mesoscale model mm5 the mm5 simulations show the rainfall jump to be roughly coincident with abrupt circulation changes that occur as the somali jet develops during april may and june in particular the cross equatorial meridional branch of the somali jet forms along the east african coast in april bringing moisture and rainfall northward to the southern slopes of the ethiopian plateau this meridional branch forms well before the northern zonal branch of the jet which diverts moisture eastward from southern ethiopia and feeds the indian monsoon these results establish a framework for understanding the precipitation cycle over the gha and provide a foundation for improving subseasonal forecasts over drought prone regions of eastern africa

the rotational stability of a triaxial ice age earth 1 mitrovica et al 2005 following calculations by nakada 2002 demonstrated that the traditional approach for computing rotation perturbations driven by glacial isostatic adjustment significantly overestimates present day true polar wander tpw speeds by underestimating the background oblateness on which the ice age loading is superimposed the underestimation has two contributions the first originates from the treatment of the hydrostatic form and the second from the neglect of the earth s excess ellipticity supported by mantle convection in mitrovica et al 2005 the second of these two contributions was computed assuming a biaxial nonhydrostatic form i e the principal equatorial moments of inertia were assumed to be equal to their mean value in this article we outline an extended approach that accounts for a triaxial planetary form we show that differences in the tpw speed predicted using the mitrovica et al 2005 approach and our triaxial theory are relatively minor 0 1 myr and are limited to earth models with lower mantle viscosity less than 5 1021 pa s however for this same class of earth models the angle of tpw predicted for a triaxial earth is rotated westward toward the axis of maximum equatorial inertia by as much as 20 relative to the biaxial case we demonstrate that these effects are a consequence of the geometry of the ice age forcing which has a dominant equatorial direction that is intermediate to the axes defining the principal equatorial moments of inertia of the planet we complete the study by computing updated frechet kernels for the tpw speed datum which provide a measure of the detailed depth dependent sensitivity of the predictions to variations in mantle viscosity we show in contrast to earlier efforts to explore this sensitivity based on the traditional rotation theory that the datum does not generally have a sensitivity to viscosity that peaks near the base of the mantle

mechanisms for anomalous warming in the western indian ocean during dipole mode events 1 during a typical indian ocean dipole mode iod event the weakening and reversal of winds in the central equatorial indian ocean lead to the development of unusually warm sea surface temperatures ssts in the western indian ocean here we report an analysis of model results and observations for the 1990 1999 period showing the anomalous warming that occurred during 1997 1998 and 1994 1995 iod events in the western indian ocean it was primarily the result of the internal oceanic processes within the arabian sea rather than a direct response to external forcing from the eastern indian ocean as previously suggested we compare the evolution of subsurface temperatures in the arabian sea just prior to 1996 1997 representative of a normal year and during the iod event 1997 1998 to infer the physical processes responsible for the anomalous warming during the iod events in a normal year such as 1996 the existence of a unique open ocean upwelling feature the arabian sea dome asd raises the thermocline in the southern arabian sea a result of the positive wind stress curl of the winter monsoon during an iod year 1997 with the collapse of the monsoon wind system in the arabian sea the cold tongue the strip of cool water indicative of asd that normally occupies the southern arabian sea fails to develop as a result the thermocline remains deep which in turn enhances the heat content of the upper ocean leading to warm ssts volume integrated heat budget terms in the asd region illustrate the role of horizontal and vertical advection in the upper ocean warming an implication is that ssts remain higher than 28 c in the southern arabian sea thereby supporting large scale deep convection in the atmosphere during the winter monsoon a band of deep convective cloud forms directly over the warm asd region during the iod events which bring greater than average rainfall over the southern arabian sea and east africa whereas convection is suppressed over the cold asd region during the normal years resulting in less than average rainfall

generation of internal waves in the deep ocean by tides 1 a direct computation of the tidal generation of internal waves over the global ocean is presented it is based on linear wave theory and high resolution data for the bottom topography the geographical distribution of the energy flux from tides to internal waves is determined with a spatial resolution of a few kilometers the total flux over the area with a depth greater than 500 m is found to be 1 2 tw the greatest uncertainties of the computation are due to unresolved topography and to nonlinear effects caused by supercritical bottom slope

a benchmark study for glacial isostatic adjustment codes the study of glacial isostatic adjustment gia is gaining an increasingly im portant role within the geophysical community understanding the response of the earth to loading is crucial in various contexts ranging from the the interpretation of modern satellite geodetic measurements e g grace and goce to the projections of future sea level trends in response to climate change modern modeling approaches to gia are based on various techniques that range from purely analytical formulations to fully numerical methods despite various teams independently investigating gia we do not have a suitably large set of explicitly validated numerical results through which they may be validated a community benchmark data set would clearly be valuable following the example of the mantle convection community here we present for the first time the results of a benchmark study of codes designed to model gia the approaches benchmarked are based on significantly different codes and different techniques this effort is performed within the working group 4 wg4 of the esf cost action es0701 improved constraints on models of glacial isostatic adjustment our aims are i testing the codes currently in use by the various teams ii establish a sufficiently large set of agreed results iii correct possible systematic errors embedded in the various physical formulations or computer implementations and iv facilitate the dissemination of numerical tools for surface loading studies to the community and to young scientists the test computations are mainly based on models with spherical symmetry and maxwell rheology and include inputs from various different methods and solution techniques viscoelastic normal modes spectral finite elements and finite elements the tests involve the loading and tidal love numbers and their relaxation spectra the deformation and gravity variations driven by surface loads characterized by simple geometry and time history and the rotational fluctuations in response to glacial unloading in spite of the significant differences in the numerical methods employed the test computations show a satisfactory agreement between the results provided by the participants most of the existing misfits have been addressed during the preparation of the manuscript some others are currently the subject of analysis within the wg4 community

changes in rotation induced by pleistocene ice masses with stratified analytical earth models the rotational response of the earth to pleistocene deglaciation is studied by means of a multilayered viscoelastic earth model based on the preliminary reference earth model prem incompressible viscoelastic deformation is evaluated from a self compressed initial state the novelty of our approach stands on the application of a fully analytical normal mode theory to the response of the earth to surface loads and variations in the centrifugal potential for large numbers of viscoelastic layers as requested by prem assuming that both present day true polar wander tpw and changes in the second degree component of the geopotential are solely due to pleistocene postglacial rebound we obtain for a two layer viscosity model that the upper mantle viscosity must be lowered to about 1 5 1020 pa s with respect to the classical value of 1021 pa s this upper mantle viscosity is accompanied by an increase of the lower mantle viscosity by a factor of 25 in agreement with some recent relative sealevel rsl data analyses and convectively supported long wavelength geoid anomalies when the viscosity contrast is located at 1470 km depth tpw and require a viscosity of 1021 pa s in the upper part of the mantle above 1470 km depth with a moderate viscosity increase in the lowermost portion of the mantle this result indicates that a viscosity of 1021 pa s is appropriate for a wider portion of the mantle than the upper mantle in agreement with haskell s 1935 estimate that was not limited to the seismically inferred 670 km boundary

interannual displacements of convection and surface circulation over the equatorial indian ocean the interannual variability of surface observations in the equatorial indian ocean is investigated from 23 years of ship reports 1954 1976 in october and november during certain years the monthly analyses show strong wind anomalies in the eastern and central equatorial indian ocean simultaneously cloud cover anomalies of opposite signs are observed in the eastern indian ocean and in the central indian ocean a linear analysis of heat induced circulation suggests that the surface wind anomalies are forced by anomalies of rainfall this circulation is probably dissipative with time scales in the range two to five days the reasons for the maintenance of anomalous circulations are not well known it is noted that during most el nino events in the pacific ocean anomalies are observed in the indian ocean however there are other years with the same anomalous pattern 1961 for example during the years with anomalously little cloudiness in the eastern indian ocean sea surface temperatures lower than normal are found in the eastern indian ocean this suggests that coupled air sea dynamics must be taken into account to understand the interannual variability

on transient rheology and glacial isostasy theoretical predictions from analytical self gravitating layered earth models with maxwell burgers body and standard linear solid rheologies are employed to explore the extent to which transient creep may have influenced the inference of long term mantle viscosity from analysis of the various signatures derived from the process of glacial isostatic adjustment the compatibility of the various models is checked by their ability to fit simultaneously the relative sea level free air gravity and the rotational data we conclude that transient creep cannot operate throughout the entire mantle on account of the extreme sensitivity of the uplift and gravity data to any significant departures of the steady upper mantle viscosity from 1021 pa s transient rheology would most likely be unimportant in the upper mantle restricting transient rheology to the lower mantle we find that models with an adiabatic density jump at 670 km depth are not satisfactory because of their inability to produce sufficiently large gravity anomalies and polar wander speeds our calculations show that the preferred models with transient creep in the lower mantle are chemically stratified with a nonadiabatic density change of around 10 the long term viscosities in the lower mantle associated with compatible transient rheological models lie between 5 1022 and 1023 pa s these values are consistent with viscosity estimates derived from correlation of long wavelength geoid anomalies and the density perturbations in the deep mantle

perturbations of the earth s rotation and their implications for the present day mass balance of both polar ice caps summary the present day perturbations of the earth s rotation are sensitive to the glacial isostatic adjustment gia arising from the late pleistocene glacial cycles and also to the recent mass balance of polar ice caps in this study we evaluate the polar wander and the change of degreetwo harmonic of the earth s geopotential j 2 proportional to the rotation rate for four late pleistocene ice models we examine these perturbations as a function of lower and uppermantle viscosities and lithospheric thickness and rheology elastic or viscoelastic in which a compressible earth model with elasticity and density given by the seismological model prem is used by considering the observations and predictions including the gia process arising from the late pleistocene ice and recent mass balance of polar ice caps we discuss the recent mass balance of the antarctic and greenland ice sheets we also examine the effects of internal processes and the melting of mountain glaciers although this work is only preliminary the results shown below seem to be supported even if these effects are included two solutions are obtained for source areas of the recent antarctic melting we denote an equivalent sea level esl rise mm yr 1 from the greenland and antarctic ice sheets as gl and

planktonic foraminifera stable isotopes and water column structure disentangling ecological signals differential carbon and oxygen stable isotope 13c and 18o fractionation between planktonic foraminifera test calcite and sea water related to ecology and life stage confound the potential for reconstructing palaeo water column temperature and carbon gradients multi species analysis and strict selection of test sizes are useful methods for identifying these fractionation processes also known as vital effects in fossil taxa however there are a limited number of species with adequate size controlled data sets needed for ground truthing the approach in the modern here we report 13c and 18o measurements made on twelve species of modern planktonic foraminifera across a range of fourteen tightly constrained size windows from a tropical indian ocean core top sample this data set includes more test size windows per species especially from the smallest identifiable test size classes and a wider range of species than previously attempted we use the size controlled 18o calcite trajectories to infer depth habitats and calculate species specific calcification temperatures the temperatures are then used to constrain species specific calcification depths along the modern vertical temperature profile in the western tropical indian ocean by overlaying the per species 13c calcite trajectories on local water column 13cdic profiles we estimate if and when i e at which test sizes the planktonic foraminifera species investigated approach ambient 13cdic values the profiling shows significant size controlled 13c deviation from seawater values in all species at some life growth stage which we attribute to i metabolic fractionation in tests 300 m of mixed layer photosymbiotic taxa for most species there is a size window where these effects appear to be at a minimum and or in balance exceptions are globigerinita glutinata a small 200 m surface living species globigerina bulloides which is highly opportunistic and deep living globorotalia tumida and globorotaloides hexagonus the latter two species being affected by various unexplained 13c vital effects use of our refined guidelines for test size selection should improve the potential for making realistic reconstructions of water column 13cdic in a modern tropical stratified setting and potentially in the distant geological past when there are no living analogues present

mountain building and earth rotation summary whereas the present day true polar wander and the secular non tidal acceleration of the earth have usually been attributed to postglacial rebound it has recently been suggested that non glacially induced vertical tectonic movements taking place under non isostatic conditions can also be effective in changing the earth s rotation we present a case study in which we analyse the effects of some simple uplift histories of the himalayas and the tibetan plateau on the rotational axis and on the second degree zonal harmonic of the geoid for time scales of up to a few million years we first assume a permanent amount of overcompensation which is consistent with observed geoid anomalies over the himalayas and then we model by means of the normal mode techniques the viscous relaxation in the mantle with the elastic lithosphere supporting elastically 2 per cent of isostatic disequilibrium in our normal mode analysis the earth is divided into five layers an effectively elastic lithosphere a viscoelastic shallow upper mantle transition zone and lower mantle characterized by the maxwell rheology and an inviscid core the readjustment of the equatorial bulge due to viscous flow in the mantle is taken into account in our studies by solving the linearized liouville equations for the conservation of angular momentum via the love numbers formalism polar wander is sensitive to the rate of relaxation of the modes m1 and m2 due to the discontinuities between the three mantle layers positioned at 420 and 670 kilometres depth the rate of readjustment is sensitive to the viscosity of the transition zone whenever the lower mantle shallow upper mantle viscosity ratio is small the highest present day velocity of polar wander due to himalayan and tibetan plateau uplift is estimated to be 1 myr 1 for an isoviscous mantle that has the same magnitude of the observed value reduced to 0 1 myr 1 for a factor 50 viscosity increase in the lower mantle these numbers are about the same as those found from postglacial rebound that occurs on the short time scale of a thousand years instead of the million years of our analysis but represent upper bounds for mountain building obtained only in the case in which a permanent deviation from isostasy of at least 2 per cent is assumed in general the proposed mechanism is less efficient in driving long term rotation instabilities than deep seated processes characterized by the same time scale of a million years such as subduction polar wander velocity is extremely sensitive to the depth of the uncompensated anomalous root of the topography for the models in which full mantle relaxation is allowed

internal tides generated on a corrugated continental slope part ii along slope barotropic forcing recent measurements in a region of continental slope characterized by ridges and valleys running up and down the slope reveal interesting high mode structure in the tidal band velocity signals with enhanced mixing above the corrugations in order to understand these observations numerical simulations of the internal tide generation in this region of topography were performed here the focus is on the response of the flow to along slope barotropic tidal forcing for small amplitude barotropic forcing internal waves are generated over the continental slope that propagate toward the ocean surface and toward shallower water when higher amplitude forcing is combined with large amplitude corrugations the flow is locally supercritical downstream of ridges and transient internal hydraulic jumps result as the flow relaxes each half tidal period these jumps are released as internal wave packets which propagate up into the thermocline the internal hydraulic jumps are a source of mixing in the valleys while the small scale shears associated with the internal waves could lead to mixing higher up the water column over the forcing range considered the response is dominated by slightly higher harmonics of the tidal forcing frequency than predicted by existing analytic theories

reconstructing east african rainfall and indian ocean sea surface temperatures over the last centuries using data assimilation the relationship between the east african rainfall and indian ocean sea surface temperatures ssts is well established the potential interest of this covariance to improve reconstructions of both variables over the last centuries is examined here this is achieved through an off line method of data assimilation based on a particle filter using hydroclimate related records at four east african sites lake naivasha lake challa lake malawi and lake masoko and ssts related records at six oceanic sites spread over the indian ocean to constrain the last millennium ensemble of simulations performed by cesm1 skillful reconstructions of the indian ssts and east african rainfall can be obtained based on the assimilation of only one of these variables when assimilating pseudo proxy data deduced from the model cesm1 the skill of these reconstructions increases with the number of particles selected in the particle filter although the improvement becomes modest beyond 99 particles when considering a more realistic framework the skill of the reconstructions is strongly deteriorated because of the model biases and the uncertainties of the real proxy based reconstructions however it is still possible to obtain a skillful reconstruction of ssts over most of the indian ocean only based on the assimilation of the six sst related proxy records selected as far as a local calibration is applied at all individual sites this underlines once more the critical role of an adequate integration of the signal inferred from proxy records into the climate models for reconstructions based on data assimilation

glaciation induced perturbations in the earth s rotation a new appraisal we revisit the theory and predictions of glaciation induced perturbations in both the rotation rate of the earth or j 2 and the wander of the rotation pole with respect to the surface geography the two most common theoretical formulations associated with polar wander driven by the surface loading of a viscoelastic earth e g sabadini et al 1982 wu and peltier 1984 appear to disagree by one on the number of rotational normal modes associated with a surface gravitational response having n normal modes we show using a generalized theory that the two formalisms yield essentially equivalent predictions we next consider a benchmark comparison of recent predictions of present day polar wander speed due to glacial isostatic adjustment henceforth gia these comparisons confirm the validity of certain previous analyses e g spada et al 1992a they also reveal that predictions of polar wander speed can be sensitive to an assumption of incompressibility previous studies have noted that predictions of j 2 rotation rate as a function of lower mantle viscosity differ significantly in form from analogous predictions of polar wander speed we investigate these differences by considering the contribution to the predicted anomalies from the individual normal modes of relaxation we find that the normal mode structure of the j 2 rotation rate predictions is fundamentally different from that which characterizes the polar wander response and this difference reflects the distinct physical mechanisms which govern these anomalies finally using a large suite of forward computations as well as the calculation of a set of frechet kernels we investigate the sensitivity of our predictions to variations in the viscoelastic earth model we find for example that a previously noted sensitivity of polar wander speed predictions to variations in lithospheric thickness is only valid for earth models with relatively weak lower mantle viscosities 5 x 10 21 pa s

application of the ncar regional climate model to eastern africa 2 simulation of interannual variability of short rains we have applied the ncar regcm2 to the simulation of the interannual variability of precipitation over eastern africa for the short rains season by performing a set of experiments for the years 1982 to 1993 the model reproduced the observed interannual variability of precipitation in most of the years the results show that remote factors play a dominant role in determining the precipitation anomalies interannual variability of precipitation over tanzania is closely related to el nino events in their mature phase and sea surface temperature sst anomalies over the indian and atlantic oceans the southward shift of the arabian high results in a southward shift of the zonal component of the intertropical convergence zone itcz which is responsible for early onset of the rainy season e g 1982 and 1986 the enhanced st helena high and weaker mascarene high lead to the eastward shift of the meridional branch of the itcz for the wet years model simulations confirmed a strong positive correlation between precipitation anomalies over lake victoria and the warm el nino southern oscillation events by which enhanced moist westerly flow from the atlantic ocean and the mainly easterly flow from the indian ocean converge over lake victoria during wet years the interannual variability of precipitation over lake victoria and the western kenya highlands wkh are strongly coupled positive precipitation anomalies over the wkh region are usually associated with weaker arabian high and mascarene high which weaken the large scale divergence over the wkh region and favor the development of convection the interannual variability of precipitation over eastern kenya highlands ekh is not directly related to the el nino events but the association with a warm sst anomaly pattern over the western indian ocean is evident during wet years an el nino signal is however evident for wet years over the turkana channel warm sst anomalies over the northern indian ocean contribute enhanced water vapor transport over the region

total meltwater volume since the last glacial maximum and viscosity structure of earth s mantle inferred from relative sea level changes at barbados and bonaparte gulf and gia induced j2

effect of the viscoelastic lithosphere on polar wander speed caused by the late pleistocene glacial cycles previous studies of the wander of the rotation pole associated with the late pleistocene glacial cycles indicate that the predicted polar wander speed is sensitive to the density jump at the 670 km discontinuity the thickness of the elastic lithosphere and the lower mantle viscosity in particular the m1 mode related to the density jump at 670 km depth has been shown to contribute a dominant portion of predicted polar wander speed for sufficiently small lower mantle viscosities in this study we examine the sensitivity of polar wander to variations in the viscosity of the viscoelastic lithosphere using simplified compressible maxwell viscoelastic earth models model calculations for earth models with a viscoelastic lithosphere of finite viscosity indicate that the contribution of the m1 mode is similar to those associated with the density discontinuity at the core mantle boundary c0 mode and the lithosphere l0 mode we speculate that this is due to the interaction between the m1 mode and the transient mode associated with the viscoelastic lithosphere which reduces the magnitude of polar wander rates therefore the m1 mode does not contribute a dominant portion of the predicted polar wander speed for earth models with a viscoelastic lithosphere of finite viscosity in this case predictions of polar wander speed as a function of lower mantle viscosity exhibit the qualitative form of an inverted parabola as predicted for the j 2 curve we caution however that these results are obtained for simplified earth models and the results for seismological earth models such as prem may be complicated by the interaction between the m1 mode and the large set of transient modes

mid brunhes strengthening of the indian ocean dipole caused increased equatorial east african and decreased australasian rainfall 1 the tropical indian ocean is an important component of the largest warm pool marked by changes in sea surface temperatures and depths of thermocline and mixed layer in its western and eastern extremities leading to the development of a dipole mode the indian ocean dipole iod a narrow band of westerlies 7 n to 7 s sweep the equatorial indian ocean during the april may and october november transitions between the summer and winter monsoon seasons these indian ocean equatorial westerlies iew are closely related to the iod intensifying the upper ocean eastward equatorial current also known as wyrtki jets the strength of the iod iew determines the moisture content in east africa a major decrease in the strength of the iew strengthening or positive mode of the iod during the mid brunhes epoch 300 250 kyr bp coincides with a wetter equatorial east africa a drier australasia and a stronger indian summer monsoon indicating that the iod iew play a significant role in driving climate change in east africa australasia and south asia

a recipe for simulating the interannual variability of the asian summer monsoon and its relation with enso this study investigates how accurately the interannual variability over the indian ocean basin and the relationship between the indian summer monsoon and the el nino southern oscillation enso can be simulated by different modelling strategies with a hierarchy of models from an atmospherical general circulation model agcm forced by observed sst to a coupled model with the ocean component limited to the tropical pacific and indian oceans the role of heat fluxes and of interactive coupling is analyzed whenever sea surface temperature anomalies in the indian basin are created by the coupled model the inverse relationship between the enso index and the indian summer monsoon rainfall is recovered and it is preserved if the atmospherical model is forced by the ssts created by the coupled model if the ocean model domain is limited to the indian ocean changes in the walker circulation over the pacific during el nino years induce a decrease of rainfall over the indian subcontinent however the observed correlation between enso and the indian ocean zonal mode iozm is not properly modelled and the two indices are not significantly correlated independently on season whenever the ocean domain extends to the pacific and enso can impact both the atmospheric circulation and the ocean subsurface in the equatorial eastern indian ocean modelled precipitation patterns associated both to enso and to the iozm closely resemble the observations

mechanisms of climate anomalies in the equatorial indian ocean 1 building on earlier work on the interannual variability of the boreal autumn equatorial westerlies ueq over the indian ocean and concomitant rainfall anomalies at the coast of east africa and in indonesia the inherent circulation mechanisms are here explored further from long term surface and upper air data fast ueq and deficient east african rainfall come with positive sea level pressure p and negative sea surface temperature t departures in a domain w at the northwestern extremity and opposite departures in a domain e at the southeastern extremity of the equatorial indian ocean however there is no seesaw between w and e in either p or t and no indication of local forcing of t on p the large scale pressure field in particular the zonal pressure gradient along the equator and the south indian ocean pressure and southern tradewinds control the evolution of ueq fast ueq steepens the zonal temperature gradient thus tightening the inverse relationships between the zonal gradients of pressure and temperature the rainfall anomalies associated with the interannual variability of ueq surface manifestation of a zonal circulation cell along the indian ocean equator are favored by the kinematic and thermodynamic conditions in w and e thus with fast ueq the domain w features departure lower tropospheric divergence and subsidence and favored by the cold t and subsidence reduced precipitable water all conducive to deficient precipitation by contrast e has departure lower tropospheric convergence and ascending motion and favored by the warm t and ascending motion enhanced precipitable water in conjunction conducive to abundant rainfall the interannual variability of the boreal autumn equatorial westerlies dominated as it is by the large scale pressure field is crucial in the climate dynamics of the equatorial indian ocean region this leads to the question what controls the pressure pattern over the indian ocean basin

assessing the impact of diagenesis on foraminiferal geochemistry from a low latitude shallow water drift deposit abstract due to their large heat and moisture storage capabilities the tropics are fundamental in modulating both regional and global climate furthermore their thermal response during past extreme warming periods such as super interglacials is not fully resolved in this regard we present high resolution analytical foraminiferal geochemical 18 o and mg ca records for the last 1800 kyr from the shallow 487 m inner sea drift deposits of the maldives archipelago in the equatorial indian ocean considering the diagenetic susceptibility of these proxies in carbonate rich environments we assess the integrity of a suite of commonly used planktonic and benthic foraminifera geochemical datasets globigerinoides ruber white globigerinita glutinata with bulla pulleniatina obliquiloculata with cortex and cibicides mabahethi and their use for future paleoceanographic reconstructions using a combination of spot secondary ion mass spectrometer electron probe micro analyzer and scanning electron microscope image data it is evident that authigenic overgrowths are present on both the external and internal test shell surfaces yet the degree down core as well as the associated bias is shown to be variable across the investigated species and proxies given the elevated authigenic overgrowth mg ca 12 22 mmol mol and 18 o values closer to the benthic isotopic compositions the whole test planktonic g ruber w geochemical records are notably impacted beyond 627 4 ka 24 7 mcd yet considering the setting i e bottom water location for overgrowth formation the benthic foraminifera 18 o record is markedly less impacted with only minor diagenetic bias beyond 790 0 ka 28 7 mcd even though only the top of the g ruber w and c mabahethi records whole test data would be suitable for paleo reconstructions of absolute values i e sea surface temperature salinity seawater 18 o the long term cycles while dampened appear to be preserved furthermore planktonic species with thicker tests i e p obliquiloculata w c might be better suited in comparison to thinner test counter parts i e g glutinata w b g ruber w for traditional whole test geochemical studies in shallow carbonate rich environments a thicker test equates to a smaller overall bias from the authigenic overgrowth overall if the diagenetic impact is constrained as done in this study these types of diagenetically altered geochemical records can still significantly contribute to studies relating to past tropical seawater temperatures latitudinal scale ocean current shifts and south asian monsoon dynamics

tidally driven processes leading to near field turbulence in a channel at the crest of the mendocino escarpment abstractin situ observations of tidally driven turbulence were obtained in a small channel that transects the crest of the mendocino ridge a site of mixed diurnal and semidiurnal tides diurnal tides are subinertial at this latitude and once per day a trapped tide leads to large flows through the channel giving rise to tidal excursion lengths comparable to the width of the ridge crest during these times energetic turbulence is observed in the channel with overturns spanning almost half of the full water depth a high resolution nonhydrostatic 2 5 dimensional simulation is used to interpret the observations in terms of the advection of a breaking tidal lee wave that extends from the ridge crest to the surface and the subsequent development of a hydraulic jump on the flanks of the ridge modeled dissipation rates show that turbulence is strongest on the flanks of the ridge and that local dissipation accounts for 28 of the energy converted from the barotropic tide into baroclinic motion

postglacial sea level change on a rotating earth first results from a gravitationally self consistent sea level equation we present and solve a gravitationally self consistent sea level equation which governs postglacial sea level variations on a spherically symmetric self gravitating viscoelastic and rotating earth we find that the inclusion of a glacio isostatically induced rotational excitation can significantly affect previous predictions of both present day sea level rates and postglacial sea level histories which were based on a theory that assumed a non rotating earth model to illustrate we consider present day sea level rates and tide gauge corrections along the us east coast and relative sea level curves in the far field of the late pleistocene ice sheets

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

effects of mode coupling and location of rotational axis on glacial induced rotational deformation in a laterally heterogeneous viscoelastic earth summary lateral viscosity variations introduce two problems in the study of glacial induced rotational deformation one of them is mode coupling the other arises because centrifugal deformations are dependent on the location of the rotational axis relative to the lateral heterogeneities in the earth the effects of mode coupling and displacement of the rotational axis on centrifugal induced potential perturbations are studied on a layered earth with lateral viscosity variations inferred from seismic tomography the modified coupled laplace finite element method is used to calculate the centrifugal deformations it is found that mode coupling is weak because the lateral viscosity variation in the deep mantle is relatively small the effect of displacing the rotational axis is to shift the spectral amplitude from degree 2 and order 0 harmonic to other orders within degree 2 as is required for changing the orientation of the applied centrifugal potential but again the leakage of spectral amplitude to other harmonics are small this implies that rotational motion in an earth with lateral viscosity variations can be approximated by the calculation of the corresponding laterally homogeneous earth

importance of the indian ocean for simulating rainfall anomalies over eastern and southern africa the relative contributions of the indian ocean and pacific ocean sea surface temperatures ssts to the rainfall variability over eastern central and southern africa during the austral spring summer are examined the variability of african rainfall is statistically related to both oceans but the variability in the two oceans is also related to separate the effects of the indian and pacific oceans a suite of numerical model simulations is presented goga the atmosphere is forced by observed ssts globally ioga the atmosphere is forced by observed ssts only in the indian ocean basin and poga the atmosphere is forced by observed ssts only in the tropical pacific basin while the sst variability of the tropical pacific exerts some influence over the african region it is the atmospheric response to the indian ocean variability that is essential for simulating the correct rainfall response over eastern central and southern africa analyses of the dynamical response s seen in the numerical experiments and in the observations indicate that the pacific and indian oceans have a competing influence over the indian ocean african region this competition is related to the influence of the two oceans on the walker circulation and the consequences of that variability on low level fluxes of moisture over central and southern africa finally given the high correlation found between sst variability in the indian and pacific oceans with the pacific leading by 3 months we speculate on an approach to long lead dynamical climate prediction over central east and southern africa

interaction between the enso and the asian monsoon in a coral record of tropical climate the oxygen isotopic composition of a banded coral from the western equatorial indian ocean provides a 150 year long history of the relation between the el nino southern oscillation enso phenomenon and the asian monsoon interannual cycles in the coral time series were found to correlate with pacific coral and instrumental climate records suggesting a consistent linkage across ocean basins despite the changing frequency and amplitude of the enso however decadal variability that is characteristic of the monsoon system also dominates the coral record which implies important interactions between tropical and midlatitude climate variability one prominent manifestation of this interaction is the strong amplitude modulation of the quasi biennial cycle

dynamics of the surface wind field over the equatorial indian ocean the annual cycle and interannual variability of the equatorial westerlies over the indian ocean are examined in the context of the equations of motion using long term surface ship observations eastward pressure gradient along the equator may accelerate equatorial westerlies but the process is more intricate the south indian ocean trade winds can recurve in the southern hemisphere only with an eastward pressure gradient this is counteracted by the coriolis acceleration directed at right angles to the left of the motion with weak trades and strong eastward pressure gradient the flow can recurve at relatively high southern latitudes thus allowing a broad equatorial zone in which westerlies can develop in the annual cycle the southern indian ocean high and trade winds are closest to the equator and strongest in austral winter when however the eastward pressure gradient in the equatorial zone is also steepest accordingly the latitude of flow recurvature in the southern trade winds stays farthest away from the equator in boreal spring and autumn consequently the equatorial westerlies peak in these short transition seasons the boreal autumn equatorial westerlies are the surface manifestation of a powerful zonal circulation cell along the indian ocean equator the interannual variability of boreal autumn is characterized by weak westerlies accompanying a slack zonal pressure gradient and strong southern trade winds with recurvature near the equator such an ensemble of circulation departures is characteristic of abundant rainfall in east africa and deficient precipitation in indonesia copyright 2004 royal meteorological society

glacial isostatic adjustment on a rotating earth summary we extend and complete previous work to compute the influence of perturbations to the rotation vector on a suite of observables associated with glacial isostatic adjustment gia we emphasize observables relevant to present and future geodetic missions for example present day 3 d crustal motions relative sea level change and geoid or absolute sea level variations our calculations adopt spherically symmetric self gravitating maxwell viscoelastic earth models while incorporating realistic mass ice plus ocean load and rotation variations the predicted rotation induced signals are dominated by the influence of true polar wander tpw the spatial geometry of the tpw induced relative sea level geoid and radial velocity fields is primarily that of a degree two order one surface spherical harmonic the spatial variation of the horizontal velocity vectors is given by the gradient of this harmonic the peak radial and horizontal velocities are of the order of 0 5 mm yr 1 however we show that this value is sensitive to the adopted profile of mantle viscosity we also demonstrate that an accurate prediction of tpw induced sea level and 3 d crustal deformation rates requires that a realistic number of glacial cycles be incorporated into the ice load history we conclude that geodetic observations of the gia process should be analysed using a gia theory valid for a rotating planet finally we also consider variations in rotation driven by simple present day polar melting scenarios and predict the influence of these variations on a suite of geophysical observables we find that the rotational feedback associated with greenland melting is capable of significantly perturbing both relative and absolute sea level variations

relative role of dynamic and thermodynamic processes in the development of the indian ocean dipole an ogcm diagnosis 1 the relative role of oceanic dynamics and surface heat fluxes in the initiation and development of the indian ocean dipole was investigated by analyzing results from an oceanic general circulation model the model was forced by observed surface wind stress and heat flux fields for 1958 1997 the results show that it was capable of reproducing observed dipole events over the tropical indian ocean the diagnosis of the mixed layer heat budget indicates that the sst anomaly ssta in the east pole is primarily induced by anomalous surface latent heat flux and vertical temperature advection whereas in the west pole it is mainly caused by meridional and vertical temperature advection anomalies in both regions shortwave radiation anomalies tend to damp the ssta the ocean rossby waves are essential in linking the anomalous wind and sst off sumatra and subsurface temperature variations in southwest indian ocean

ice age earth rotation modern predictions of the rotational stability of an ice age earth reflect a convergence of two classic problems in geophysical analysis the modeling of the glacial isostatic adjustment gia process and the rotational stability of terrestrial planets recent theoretical advances in this area have been motivated not by conventional applications such as the inference of earth s deep mantle viscosity but rather by efforts to address vexing problems in global climate change research these advances have demonstrated that traditional calculations of the ongoing motion of the rotation pole relative to the surface geography or true polar wander tpw driven by ice age loading have systematically overestimated this motion by up to a factor of 4 by underestimating by 1 the background flattening of earth s oblate form the physics of this sensitivity is related to concepts that appear in canonical mid twentieth century discussions of earth rotation and avoiding the associated inaccuracy resolves numerous p

tidal influence on particulate organic carbon export fluxes around a tall seamount as tall seamounts may be stepping stones for dispersion and migration of deep open ocean fauna an improved understanding of the productivity at and food supply to such systems needs to be formed here the 234th 238u approach for tracing settling particulate matter was applied to senghor seamount a tall sub marine mountain near the tropical cape verde archipelago in order to elucidate the effects of topographically influenced physical flow regimes on the export flux of particulate organic carbon poc from the near surface topmost 100 m into deeper waters the comparison of a suitable reference site and the seamount sites revealed that poc export at the seamount sites was 2 4 times higher than at the reference site for three out of five seamount sites the calculated poc export fluxes are likely to be underestimates if this is taken into account it can be concluded that poc export fluxes increase while the passing waters are advected around and over the seamount with the highest export fluxes occurring on the downstream side of the seamount this supports the view that biogeochemical and biological effects of tall seamounts in surface ocean waters might be strongest at some downstream distance from rather than centred around the seamount summit based on measured vessel mounted adcp and modelled regional flow field aviso internal tides at senghor mitgcm flow dynamics it is proposed that tidally generated internal waves result in a screen of increased rates of energy dissipation that runs across the seamount and leads to a combination of two factors that caused the increased poc export above the seamount 1 sudden increased upward transport of nutrients into the euphotic zone driving brief pulses of primary production of new particulate matter followed by the particles export into deeper waters and 2 pulses of increased shear driven aggregation of smaller slower settling into larger faster settling particles this study shows that under certain conditions there can be an effect of a tall seamount on aspects of surface ocean biogeochemistry with tidal dynamics playing a prominent role it is speculated that these effects can control the spatiotemporal distribution of magnitude and nutritional quality of the flux of food particles to the benthic and benthic pelagic communities at and near tall seamounts

enhanced multidecadal climate variability in the seventeenth century from coral isotope records in the western indian ocean 1 the slowly growing coral diploastreaheliopora affords a novel opportunity to obtain multicentury records of paleoclimate including details of the interannual rhythms associated with the el nino southern oscillation enso system we examine climate variability in an enso teleconnected region using new oxygen isotope records from d heliopora spanning most of the twentieth 1896 1998 and seventeenth 1622 1722 centuries from the mafia archipelago tanzania the modern record demonstrates coherency with relevant instrumental and proxy time series documents twentieth century warming and displays significant power at enso periodicities the seventeenth century record lacks any trend exhibits interannual variance comparable to the modern record and displays a pronounced interdecadal signal not evident in the twentieth century that correlates with other tropical and hemispheric climate records we find no clear evidence of solar irradiance influence on interannual variability in the western indian ocean during these intervals although the mean sea surface temperature appears to vary inversely with insolation

orbital scale variation of primary productivity in the central equatorial indian ocean maldives during the early pliocene abstract we investigate the response of the central equatorial indian ocean odp site 716 maldives upwelling system to orbital forcing between 3 64 and 4 19 ma this time interval in the early pliocene was characterized by global average temperatures considerably warmer than present and reduced zonal and meridional sea surface temperature gradients across the major oceans chlorin a degradation product of chlorophyll and a potential indicator of primary productivity was previously shown to record orbital scale variations at this site to confirm that chlorin records primary productivity and to understand the mechanism controlling the orbital variations we compare changes in the planktic foraminiferal assemblages with the chlorin content of the sediment cross wavelet analysis demonstrates the in phase relationship between the abundance of thermocline species and chlorin content indicating that upwelling and a shallower thermocline drive the productivity changes the aragonite content in periplatform carbonates is known to reflect variations in sea level and or climate with high values generally indicating sea level highstands the aragonite content in the studied interval shows precession and eccentricity astronomical cycles whereas the primary productivity proxies show only precessional cycles the negative correlation between aragonite content and the relative abundance of thermocline species on the precessional band implies that the primary productivity increased during sea level lowstands this phase coupling is consistent with quaternary productivity records from the central indian ocean that were proposed to be controlled by orbital induced variations of the el nino southern oscillation enso we therefore propose that a similar mechanism was active in the early pliocene despite major changes in the global boundary conditions

potential relation between equatorial sea surface temperatures and historic water level variability for lake turkana kenya summary water level in lake turkana kenya in the past ca 150 years is controlled primarily from the biannual passage of the east and west african monsoon with rainfall volume related partially to sea surface temperatures ssts in the western indian and east atlantic oceans empirical orthogonal function analyses show significant correlation between eastern atlantic or western indian ssts and lake level anomalies with the first mode accounting for 66 and 55 of the variability the primary geographic loadings are consistent with a gulf of guinea moisture source and positive indian ocean dipole iod state the second mode explains 10 of variability and reflects the westward extension of an indian ocean cool pool potentially indicative of a normal to a negative iod state there is significant spatial correlation between basin rainfall anomalies associated with eastern atlantic ssts and a low in the continental divide between the kenyan and the ethiopian highlands which is a passage for moisture from the congo basin linear regression analysis with bootstrap sampling and monte carlo simulations define numeric relations between western indian and eastern atlantic ssts and lake level change for ad 1992 2013 the monthly and yearly lake level reconstructions based on this numeric analysis capture the decadal scale variability and the 15 m drop in water level in the early 20th century meter scale variability in lake level since ca ad 1930 is associated with precipitation sourced from the western indian ocean with iod variability whereas the 15 m drop in water level in the early 20th century may reflect a profound decrease in moisture from atlantic congo basin source these numerical solutions are poised to reconstruct water level variations in the past ca 300 years for lake turkana with new proxy records of ssts from the western indian ocean and the gulf of guinea

internal tide reflection and turbulent mixing on the continental slope abstract observations of turbulence internal waves and subinertial flow were made over a steep corrugated continental slope off virginia during may june 1998 at semidiurnal frequencies a convergence of low mode onshore energy flux is approximately balanced by a divergence of high wavenumber offshore energy flux this conversion occurs in a region where the continental slope is nearly critical with respect to the semidiurnal tide it is suggested that elevated near bottom mixing k 10 3 m2 s 1 observed offshore of the supercritical continental slope arises from the reflection of a remotely generated low mode m2 internal tide based on the observed turbulent kinetic energy dissipation rate the high wavenumber internal tide decays on time scales o 1 day no evidence for internal lee wave generation by flow over the slope s corrugations or internal tide generation at the shelf break was found at this site

postglacial rebound and sea level contributions to changes in the geoid and the earth s rotation axis summary the rate of change of the spherical harmonic degree 2 component of the earth s gravitational potential coa 20 and polar wander velocity are two signals which are sensitive to late pleistocene deglaciation current changes in sea level and deep mantle viscosity diierent load and earth models have been used in earlier papers to predict the component of these geophysical signals caused by the collapse of the last great ice sheets and recent melting of polar ice caps in this paper we present a systematic analysis of the dependence of the predictions on parameters of the ice and earth model we show that the key parameters of the ice model which govern the predictions are the mass the location of the centre of mass and the midpoint of the deglaciation phase of secondary importance is the length of the deglaciation phase and the mean ice load prior to the last glacial maximum these conclusions enable us to make a more robust inference of mantle viscosity than has been made before allowing for the uncertainties in the model of late pleistocene and present deglaciation as previous authors have shown the lower mantle viscosity is the most important rheological parameter and therefore thecoa 20 observation complements sea level observations which are primarily sensitive to lithospheric thickness and the viscosity of the upper part of the mantle using realistic constraints on the sizes locations and timing of deglaciation of the late pleistocene ice sheets and current changes in polar ice caps the observation of coa 20 is used to infer lower mantle viscosity as a function of the present rate of sea level change if the present rate of non steric sea level change is 1 mm yr 1 and that change has been occurring for less than 1000 years then the lower mantle viscosity satis es log10 glm 21 82 0 15 which is consistent with inferences drawn from recent sea level analyses and con rms other analyses of thecoa 20 observation if the polar wander signal is produced entirely by postglacial rebound and current sea level change no more than 20 per cent of the present contribution to global sea level change comes from greenland the above conclusions also hold if the density discontinuities at 420 and 670 km are modelled as phase boundaries rather than material chemical boundaries

present day secular variations in the low degree harmonics of the geopotential sensitivity analysis on spherically symmetric earth models 1 we predict present day rates of change of the long wavelength components of the earth s geopotential due to the late pleistocene glacial cycles hereinafter referred to as glacial isostatic adjustment or gia these predictions are generated using spherically symmetric self gravitating maxwell viscoelastic earth models previous studies have generally considered only zonal i e azimuthally independent harmonics or the so called coefficients we extend these efforts to focus on the nonzonal harmonics and explore the sensitivity of our predictions to changes in a variety of model inputs as an example we examine the influence on the gia predictions of ignoring rotational perturbations and assuming an elastically incompressible model and find that these assumptions can have a significant impact on the degree 2 order 1 coefficients while the other harmonics are generally affected by 10 though the gia predictions are generally insensitive to variations in lithospheric thickness and upper mantle viscosity reasonable variations in either lower mantle viscosity or the late pleistocene ice history will have a more significant effect mindful of upcoming dedicated gravity missions we show that predictions of the gia induced changes of the geoid can vary by at least 0 5 mm yr 1 over a plausible range of earth and ice models