late eocene to early miocene ice sheet dynamics and the global carbon cycle paired benthic foraminiferal trace metal and stable isotope records have been constructed from equatorial pacific ocean drilling program site 1218 the records include the two largest abrupt 1 myr increases in the cenozoic benthic oxygen isotope record oi 1 in the earliest oligocene 34 ma and mi 1 in the earliest miocene 23 ma the paired mg ca and oxygen isotope records are used to calculate seawater 18o w calculated w suggests that a large antarctic ice sheet formed during oi 1 and subsequently fluctuated throughout the oligocene on both short 0 5 myr and long 2 3 myr timescales between about 50 and 100 of its maximum earliest oligocene size the magnitudes of these fluctuations are consistent with estimates of sea level derived from sequence stratigraphy the transient expansion of the antarctic ice sheet at mi 1 is marked in the benthic 18o record by two positive excursions between 23 7 and 22 9 ma each with a duration of 200 300 kyr bottom water temperatures decreased by 2 c over the 150 kyr immediately prior to both rapid 18o excursions however the onset of each of these phases of ice growth is synchronous within the resolution of the records with the onset of a 2 c warming over 150 kyr we suggest that the warming during these glacial expansions reflect increased greenhouse forcing prompted by a sudden decrease in global chemical weathering rates as antarctic basement silicate rocks became blanketed by an ice sheet this represents a negative feedback process that might have operated during major abrupt growth phases of the antarctic ice sheet

marked decline in atmospheric carbon dioxide concentrations during the paleogene the relation between the partial pressure of atmospheric carbon dioxide pco2 and paleogene climate is poorly resolved we used stable carbon isotopic values of di unsaturated alkenones extracted from deep sea cores to reconstruct pco2 from the middle eocene to the late oligocene approximately 45 to 25 million years ago our results demonstrate that pco2 ranged between 1000 to 1500 parts per million by volume in the middle to late eocene then decreased in several steps during the oligocene and reached modern levels by the latest oligocene the fall in pco2 likely allowed for a critical expansion of ice sheets on antarctica and promoted conditions that forced the onset of terrestrial c4 photosynthesis

atmospheric and oceanic impacts of antarctic glaciation across the eocene oligocene transition the glaciation of antarctica at the eocene oligocene transition approx 34 million years ago was a major shift in the earth s climate system but the mechanisms that caused the glaciation and its effects remain highly debated a number of recent studies have used coupled atmosphere ocean climate models to assess the climatic effects of antarctic glacial inception with often contrasting results here using the hadcm3l model we show that the global atmosphere and ocean response to growth of the antarctic ice sheet is sensitive to subtle variations in palaeogeography using two reconstructions representing eocene and oligocene geological stages the earlier stage eocene priabonian which has a relatively constricted tasman seaway shows a major increase in sea surface temperature over the pacific sector of the southern ocean in response to the ice sheet this response does not occur for the later stage oligocene rupelian which has a more open tasman seaway this difference in temperature response is attributed to reorganization of ocean currents between the stages following ice sheet expansion in the earlier stage the large ross sea gyre circulation decreases in size stronger zonal flow through the tasman seaway allows salinities to increase in the ross sea deep water formation initiates and multiple feedbacks then occur amplifying the temperature response this is potentially a model dependent result but it highlights the sensitive nature of model simulations to subtle variations in palaeogeography and highlights the need for coupled ice sheet climate simulations to properly represent and investigate feedback processes acting on these time scales

the impact of climate vegetation interactions on the onset of the antarctic ice sheet a global coupled atmosphere vegetation model and a dynamic ice sheet model were employed to study the impact of climate vegetation interactions on the onset of the antarctic ice sheet during the eocene oligocene transition we found that the co2 threshold for antarctic glaciation is highly sensitive to the prevailing vegetation in our experiments the co2 threshold is less than 280 ppm if the antarctic vegetation is dominated by forests and between 560 and 1120 ppm for tundra and bare ground conditions the large impact of vegetation on inception is attributed to the ability of canopies to shade the snow covered ground which leads to a weaker snow albedo feedback and higher summer temperatures however the overall effect of canopy shading on the antarctic climate also depends on features like local cloudiness and atmospheric meridional heat transport our results suggest that vegetation feedbacks on climate are crucial for the timing of the antarctic glaciation

oligocene plant ecological strategies in low latitude asia unraveled by leaf economics abstract the oligocene represents an important epoch in the history of the stepwise global climate evolution from a warm climate to a cooler climate the change probably introduced selective pressures influencing ecological strategies and caused the past ecosystems restructured however little is known about the paleoenvironment and adaptive plant ecological strategies in the terrestrial realm of the low latitudes here based on the plant fossils from the baigang formation of the baise basin low latitude china the paleoclimate paleovegetational type and paleoecological strategy are reconstructed using leaf physiognomic analyses integrated plant record and leaf economics spectrum respectively the results show that under a warm and wet sub tropical climate with low seasonality the oligocene baise flora was dominated by slow return maximized resource retention species but mixed with a considerable proportion of fast return rapid resource acquisition species these ecological strategies might promote the community to maximize the use of limited living resources and at the same time to better tolerate potential extreme situations these results allow us to examine which ecological strategies that fossil plants selected under a given paleoclimate condition and might potentially provide a clue to predict the direction of ecological strategy changes in a future climate change scenario

observations of warm water volume changes in the equatorial pacific and their relationship to el nino and la nina abstract this paper describes observed changes in surface winds sea surface temperature sst and the volume of water warmer than 20 c wwv in the equatorial pacific ocean for the period 1980 99 the purpose is to test recent hypotheses about the relationship between variations in wwv and the el nino southern oscillation enso cycle the results confirm inferences based on theory models and previous empirical analyses using proxy data namely sea level that enso involves a recharge and discharge of wwv along the equator and that the cyclic nature of enso results from a disequilibrium between zonal winds and zonal mean thermocline depth the authors also find that the magnitude of enso sst anomalies is directly related to the magnitude of zonal mean wwv anomalies furthermore for a given change in equatorial wwv the corresponding warm el nino sst anomalies are larger than the corresponding cold la nina anomalies this asymmetry between the warm and cold phases of the enso cycle implies differences

contrasting eastern pacific and central pacific types of enso abstract surface observations and subsurface ocean assimilation datasets are examined to contrast two distinct types of el nino southern oscillation enso in the tropical pacific an eastern pacific ep type and a central pacific cp type an analysis method combining empirical orthogonal function eof analysis and linear regression is used to separate these two types correlation and composite analyses based on the principal components of the eof were performed to examine the structure evolution and teleconnection of these two enso types the ep type of enso is found to have its sst anomaly center located in the eastern equatorial pacific attached to the coast of south america this type of enso is associated with basinwide thermocline and surface wind variations and shows a strong teleconnection with the tropical indian ocean in contrast the cp type of enso has most of its surface wind sst and subsurface anomalies confined in the central pacific and tends to onset develop and decay in situ

cenozoic paleotemperatures and leaf physiognomy a european perspective for 21 european leaf floras with a focus on central europe which span a stratigraphic range from the late eocene to the pliocene paleoclimate estimates have been calculated using five different quantitative techniques a leaf margin analysis lma1 using a regression model based on data from east asia b the multivariate climate leaf analysis multivariate programm clamp technique based on data from east asia and n america c a recently developed multivariate technique based on modern european vegetation elpa d a provisional lma regression model based on the vegetation of several wet localities from n america lma2 and e the coexistence approach ca a technique based on comparisons with the nearest living relatives nlrs of fossil taxa according to our results there seems to be certain discrepancies where the different techniques are compared for paleotemperatures estimates depending mainly on the stratigraphic age of the floras for paleogene floras both multivariate leaf physiognomic techniques are in rather good agreement with ca although both techniques may differ considerably from each other in contrast for the neogene clamp shows a tendency to produce estimates that are considerably colder than ca whereas elpa provides generally warmer estimates and is in better agreement with ca and other independent evidence our results and interpretations add some caveats to temperature reconstructions based on leaf physiognomy especially when applied to european floras from older periods i e paleogene cretaceous possible changes of the relationship between climate and leaf physiognomy over time should be taken into account as a possible source of error whenever such techniques are used there is the possibility that the actual correlation between climate and leaf form may be modified by long time evolutionary responses or floral changes leading to erroneous paleoclimate estimates if a calibration data set is used which is not suited for the region and time interval in question however further research will be needed to test whether such changes in the relationship between climate and leaf physiognomy over time can also be detected on other continents or whether this is a problem restricted to europe

palaeobotanical evidence for a warm cretaceous arctic ocean the cretaceous period was a time of global warmth1 4 mid cretaceous equatorial temperatures were similar to today s5 but the equator to pole temperature gradient is the subject of some controversy5 7 although it is unlikely that the poles were ice free8 10 fossil evidence3 5 11 12 indicates that near polar temperatures were much higher than they are today little is known moreover about oceanic poleward heat transport and this makes it hard to model the cretaceous climate or to evaluate the extent to which it provides an analogue for a greenhouse world warmed by increased atmospheric co2 alone here we use relationships between leaf physiognomy such as shape and size and modern climate to determine cretaceous climate conditions in the arctic region from fossil leaves we find that the arctic ocean was relatively warm remaining above 0 c even during the winter months this implies that there was significant poleward heat transport during all seasons

strong and moderate nonlinear el nino regimes it has been previously proposed that two el nino en regimes strong and moderate exist but the historical observational record is too short to establish this conclusively here 1200 years of simulations with the gfdl cm2 1 model allowed us to demonstrate their existence in this model and by showing that the relevant dynamics are also evident in observations we present a stronger case for their existence in nature in cm2 1 the robust bimodal probability distribution of equatorial pacific sea surface temperature sst indices during en peaks provides evidence for the existence of the regimes which is also supported by a cluster analysis of these same indices the observations agree with this distribution with the en of 1982 1983 and 1997 1998 corresponding to the strong en regime and all the other observed en to the moderate regime the temporal evolution of various indices during the observed strong en agrees very well with the events in cm2 1 providing further validation of this model as a proxy for nature the two regimes differ strongly in the magnitude of the eastern pacific warming but not much in the central pacific observations and model agree in the existence of a finite positive threshold in the sst anomaly above which the zonal wind response to warming is strongly enhanced such nonlinearity in the bjerknes feedback which increases the growth rate of en events if they reach sufficiently large amplitude is very likely the essential mechanism that gives rise to the existence of the two en regimes oceanic nonlinear advection does not appear essential for the onset of strong en the threshold nonlinearity could make the en regimes very sensitive to stochastic forcing observations and model agree that the westerly wind stress anomaly in the central equatorial pacific in late boreal summer has a substantial role determining the en regime in the following winter and it is suggested that a stochastic component at this time was key for the development of the strong en towards the end of 1982

interannual to decadal predictability of tropical and north pacific sea surface temperatures abstract a multivariate empirical model is used to show that predictability of the dominant patterns of tropical and north pacific oceanic variability el nino southern oscillation enso and the pacific decadal oscillation pdo is mostly limited to little more than a year despite the presence of spectral peaks on decadal time scales the model used is a linear inverse model lim derived from the observed simultaneous and 1 yr lag correlation statistics of july june averaged sst from the hadley centre global sea ice and sea surface temperature hadisst dataset for the years 1900 2002 the model accurately reproduces the power spectra of the data including interannual and interdecadal spectral peaks that are significant relative to univariate red noise eigenanalysis of the linear dynamical operator yields propagating eigenmodes that correspond to these peaks but have very short decay times and thus limited predictability longer term predictability does exist however due to two stationary eigen

application of three dimensional lagrangian residual transport three dimensional lagrangian residual circulation computed from an intratidal hydrodynamic model is used to drive an intertidal water quality transport model the sum of the eulerian residual and stokes drift is used as a first order approximation for the lagrangian residual current the stokes drift approximates residual currents induced by the nonlinear interactions of the tidal currents computed residual velocities show close agreement with a two dimensional vertical longitudinal analytical result the methodology is applied to chesapeake bay for the entire year of 1985 salinity computed with the intertidal transport model i e with lagrangian residuals shows good agreement with observed salinity data and that computed with the intratidal hydrodynamic model the basic effect of the stokes drift is to transport water mass e g salinity up estuary the procedures provide a practical means of interfacing intratidal hydrodynamic model information to water quality models for making long term in

factors affecting enso s period accurately capturing the observed mean period of enso in general circulation models gcms is often challenging and it is therefore useful to understand which parameters and processes affect this period a computationally efficient simulation based approach is used to extract both the dominant eigenvalues and corresponding eigenvectors of the linearized model from the zebiak cane intermediate complexity model of enso without having to directly construct the linearization the sensitivity of the period to a variety of parameters is examined including atmosphere ocean coupling atmospheric heating parameterization thermocline depth zonal profile western boundary reflection coefficient atmospheric and ocean wave speeds or rossby radii of deformation ocean decay time and the strength of the annual cycle in addition to the sensitivity information the spatial structures of the main fields sst thermocline thickness and more that are involved in period changes are obtained to aid in the physical interpretation of the sensitivities there are three main time lags that together compose one half of a model enso period the rossby plus kelvin wave propagation time for a wind caused central pacific disturbance to propagate to the western ocean and back sst dynamics that determine the lag between eastern ocean thermocline anomalies and eastern ocean sst anomalies and the accumulation lag of integrating a sufficient delayed wave signal arriving from the western ocean to cancel the eastern ocean anomalies for any of the parameter changes considered the eigenvector changes show that the largest contributor to the period change is from changes to the last of these three mechanisms physical mechanisms that affect this accumulation delay are discussed and the case is made that any significant change to enso s period is in turn likely to involve changes to this delay

modelling late oligocene c4 grasses and climate evidence suggests that c4 grasses adapted to conditions of low co2 high temperatures and water stressed environments expanded relatively quickly and in several geographically distinct regions during the late miocene approximately 9 7 ma using a variety of modelling tools we attempt to explain this expansion in terms of changes in global climate we use a fully coupled atmosphere ocean general circulation model aogcm hadcm3l to simulate global climate before and after the expansion the late oligocene 27 ma and pre industrial we then use the simulated climates to drive a process based global vegetation model lpj contrary to indications from palaeo data the vegetation model predicts a greater c4 distribution during the late oligocene than the pre industrial the principal reason for this is that the relatively high tropical temperatures which we predict for the late oligocene confer a competitive advantage on c4 grasses over c3 grasses which outweighs the disadvantage imposed by the higher co2 concentrations we find that this result is robust to prescription of different co2 levels in the late oligocene aogcm simulation and to the vegetation model used of the several possible explanations proposed for the model data discrepancy three seem most likely first that the vegetation models do not adequately represent certain processes under extreme climate conditions such as fire disturbance or key parameters related to the competition between c3 and c4 grasses second that there existed an evolutionary niche for c4 grasses prior to their expansion in the late miocene which for some reason was not filled third that we have not adequately modelled the late oligocene climate with the aogcm future research should concentrate on the collection and analysis of additional data for the assessment of palaeo tropical land temperatures and on further testing of the sensitivity of the aogcm results

optimal growth of central and east pacific enso events central pacific cp type and eastern pacific ep type el ni no and the southern oscillation enso events are investigated using linear inverse modeling optimal initial conditions and growth rates for cp or ep enso events are identified explicitly using a cp or ep enso norm the dominant difference in initial conditions that lead to cp and ep enso events is the role of the second empirical orthogonal function of tropical sea surface temperature which represents the pacific meridional mode pmm optimal initial conditions for cp type enso events include warm sst anomalies in the central subtropical pacific a characteristic of the pmm while optimal initial conditions for ep type enso events are focused in the eastern equatorial pacific and southern hemisphere subtropics along about 25 s thermocline anomalies differ in initial structures and in their influence on sst for cp and ep events results point to different roles of the pmm and thermocline variations in the evolution of cp and ep enso events

timing and nature of the deepening of the tasmanian gateway 1 tectonic changes that produced a deep tasmanian gateway between australia and antarctica are widely invoked as the major mechanism for antarctic cryosphere growth and antarctic circumpolar current acc development during the eocene oligocene e o transition 34 33 ma ocean drilling program odp leg 189 recovered near continuous marine sedimentary records across the e o transition interval at four sites around tasmania these records are largely barren of calcareous microfossils but contain a rich record of siliceous and organic walled marine microfossils in this study we integrate micropaleontological sedimentological geochemical and paleomagnetic data from site 1172 east tasman plateau to identify four distinct phases a d in the e o tasmanian gateway deepening that are correlative among odp leg 189 sites phase a prior to 35 5 ma minor initial deepening characterized by a shallow marine prodeltaic setting with initial condensation episodes phase b 35 5 33 5 ma increased deepening marked by the onset of major glauconitic deposition and inception of energetic bottom water currents phase c 33 5 30 2 ma further deepening to bathyal depths with episodic erosion by increasingly energetic bottom water currents phase d 30 2 ma establishment of stable open ocean warm temperate oligotrophic settings characterized by siliceous carbonate ooze deposition our combined evidence indicates that this early oligocene tasmanian gateway deepening initially produced an eastward flow of relatively warm surface waters from the australo antarctic gulf into the southwestern pacific ocean this proto leeuwin current fundamentally differs from previous regional reconstructions of eastward flowing cool water e g a proto acc during the early oligocene and thereby represents an important new constraint for reconstructing regional to global scale dynamics for this major global change event

a stability analysis of tropical ocean atmosphere interactions bridging measurements and theory for el nino abstract interactions between the tropical oceans and atmosphere permit a spectrum of natural modes of oscillation whose properties period intensity spatial structure and direction of propagation depend on the background climatic state i e the mean state this mean state can be described by parameters that include the following the time averaged intensity of the pacific trade winds the mean depth h of the thermocline and the temperature difference across the thermocline t a stability analysis by means of a simple coupled ocean atmosphere model indicates two distinct families of unstable modes one has long periods of several years involves sea surface temperature variations determined by vertical movements of the thermocline that are part of the adjustment of the ocean basin to the fluctuating winds requires a relatively deep thermocline and corresponds to the delayed oscillator the other family requires a shallow thermocline has short periods of a year or two has sea surface tempe

miocene pollen assemblages from the zeku basin northeastern tibetan plateau and their palaeoecological and palaeoaltimetric implications abstract a comprehensive understanding of the tectonic uplift history of the tibetan plateau tp a 2000 km wide region with a mean altitude of 4000 5000 m above sea level asl is crucial for interpreting global cenozoic climate change collisional tectonics and the evolution of the asian climatic system however the timing degree and extent of the uplift of the tp remain controversial here we report new palynological data from early to mid miocene lacustrine swamp sediments in the zeku basin northeastern tp the palynoflora includes 68 palynotaxa at a family or genus level comprising gymnosperms 16 2 angiosperms 77 9 pteridophytes 2 9 and others 2 9 and is suggestive of a mixed warm temperate forests of coniferous e g picea abies and tsuga and deciduous broadleaved trees e g ulmus zelkova betula and quercus with some subtropical elements e g carya and liquidambar three periods of vegetation succession can be recognized from the caergen section indicating a stepwise warming and drying trend from bottom to top the local palaeoclimatic parameters obtained by applying the coexistence approach coa to palynologic assemblages rendered mean annual temperature mat values of 14 2 16 1 c and mean annual precipitation map values of 797 1113 mm this study departs from the traditional application of the coa relying on published determination of plant climate relationships rather than on data derived from the paleoflora database which is currently not fully open access based on the difference in temperatures between the early to mid miocene and the present day and the temperature lapse rate corrected to take account of miocene global temperature differences the estimated palaeoaltitude of the zeku basin during the early to mid miocene was 1200 1400 m asl this palaeoaltitude estimate suggests that this basin has experienced about 2200 2500 m of uplift since the early to mid miocene findings therefore do not support previous hypotheses that the northeastern tp attained its present altitude before the miocene

improving el nino prediction using a space time integration of indo pacific winds and equatorial pacific upper ocean heat content 1 equatorial westerly easterly wind anomalies phase locked to the seasonal cycle typically propagate from the eastern equatorial indian ocean and into the western pacific immediately before an el nino la nina a space time integration of this indo pacific signal yields an index that for 11 out of 12 months of the calendar year leads the el nino index nino3 4 with a correlation of 0 5 or greater for at least some lead in the range 10 15 months cross validated hindcasts suggest that a linear combination of this atmospheric index and the ocean indices nino3 4 and t the anomalous equatorial pacific upper ocean heat content is an excellent predictor of el nino it can predict across the nearest spring persistence barrier but not the one after that the present el nino should die over the spring leaving near neutral conditions for the rest of 2003

harmonic analysis of tides and tidal currents in south san francisco bay california abstract water level observations from tide stations and current observations from current meter moorings in south san francisco bay south bay california have been harmonically analysed at each tide station 13 harmonic constituents have been computed by a least squares regression without inference tides in south bay are typically mixed there is a phase lag of approximately 1 h and an amplification of 1 5 from north to south for a mean semi diurnal tide because most of the current meter records are between 14 and 29 days only the five most important harmonics have been solved for east west and north south velocity components the eccentricity of tidal current ellipse is generally very small which indicates that the tidal current in south bay is strongly bidirectional the analyses further show that the principal direction and the magnitude of tidal current are well correlated with the basin bathymetry patterns of eulerian residual circulation deduced from the current meter data show an anticlockwise gyre to the west and a clockwise gyre to the east of the main channel in the summer months due to the prevailing westerly wind opposite trends have been observed during winter when the wind was variable

collective role of thermocline and zonal advective feedbacks in the enso mode the vertical advection of anomalous subsurface temperature by the mean upwelling and the zonal advection of mean sea surface temperature sst by anomalous current are known to be essential for the equatorial sst anomaly associated with the el nino southern oscillation enso in the coupled model these two processes are referred to as the thermocline feedback and the zonal advective feedback respectively using a version of a recharge oscillator model for enso obtained from the stripped down approximation of the cane zebiak type model it is demonstrated that these two feedbacks which are linked dynamically through the geostrophic approximation tend constructively to contribute to the growth and phase transition of enso however these two feedbacks control the leading coupled mode in different ways the thermocline feedback leads to a coupled mode through the merging of the damped sst mode and ocean adjustment mode whereas the zonal advective feedback tends to destabilize the gravest ocean basin mode with both of these feedbacks the leading modes of the coupled model still can be traced back to these different origins under moderate changes in the model setup the main consequence of these sensitivities is that the growth rate and frequency of the enso mode may be sensitive to slight changes in basic state parameters which control the strength of these feedbacks

characterizing decadal to centennial variability in the equatorial pacific during the last millennium 1 the magnitude of sea surface temperature variability in the nino3 4 region of the equatorial pacific on decadal and longer timescales is assessed in observational data state of the art coupled model intercomparison project 5 climate model simulations and a new ensemble of paleoclimate reconstructions on decadal to multidecadal timescales variability in these records is consistent with the null hypothesis that it arises from multivariate red noise a multivariate ornstein uhlenbeck process generated from a linear inverse model of tropical ocean atmosphere dynamics on centennial and longer timescales both a last millennium simulation performed using the community climate system model 4 ccsm4 and the paleoclimate reconstructions have variability that is significantly stronger than the null hypothesis however the time series of the model and the reconstruction do not agree with each other in the model variability primarily reflects a thermodynamic response to reconstructed solar and volcanic activity whereas in the reconstruction variability arises from either internal climate processes forced responses that differ from those in ccsm4 or nonclimatic proxy processes that are not yet understood these findings imply that the response of the tropical pacific to future forcings may be even more uncertain than portrayed by state of the art models because there are potentially important sources of century scale variability that these models do not simulate

rossby waves in the tropical north pacific and their role in decadal thermocline variability abstract a band of enhanced thermocline variability at 10 15 n in the pacific found in nature also occurs in an ocean general circulation model forced with observed fluxes of momentum heat and freshwater over the period 1958 97 the variability in the model is primarily associated with long baroclinic rossby waves characterized by periods in the decadal range 7 10 yr the waves are forced by westward propagating ekman pumping anomalies east of the date line and propagate at a speed of 13 cm s 1 which is slower than the phase speed of the first mode unforced baroclinic waves 15 16 cm s 1 west of the date line the correlations between thermocline displacements and local ekman pumping are relatively small and the ocean signals have a phase speed of 20 cm s 1 very similar to the phase speed of the first baroclinic mode in the western half of the basin 18 20 cm s 1 the phase speeds of the ocean model signals have been estimated using cospectral analysis while the wkb approximation has been us

warm and wet conditions in the arctic region during eocene thermal maximum 2 several episodes of abrupt and transient warming each lasting between 50 000 and 200 000 years punctuated the long term warming during the late palaeocene and early eocene 58 to 51 myr ago epochs1 2 these hyperthermal events such as the eocene thermal maximum 2 emt2 that took place about 53 5 myr ago2 are associated with rapid increases in atmospheric co2 content however the impacts of most events are documented only locally86 here we show on the basis of estimates from the tex 86 proxy that sea surface temperatures rose by 3 5 c in the arctic ocean during the emt2 dinoflagellate fossils demonstrate a concomitant freshening and eutrophication of surface waters which resulted in euxinia in the photic zone the presence of palm pollen implies5 that coldest month mean temperatures over the arctic land masses were no less than 8 c in contradiction of model simulations that suggest hyperthermal winter temperatures were below freezing6 in light of our reconstructed temperature and hydrologic trends we conclude that the temperature and hydrographic responses to abruptly increased atmospheric co2 concentrations were similar for the etm2 and the better described palaeocene eocene thermal maximum7 8 55 5 myr ago 2009 macmillan publishers limited all rights reserved

paleogene monsoons across india and south china drivers of biotic change monsoonal climates at low latitudes 32 n are an inevitable consequence of seasonal migrations of the inter tropical convergence zone itcz but the character of these monsoons depends on continental configuration orographic expression and the strength of hadley circulation to explore the evolution of monsoon systems across southern asia we compare climate signatures archived in ten paleogene floras from northern india tibet and southern china occupying low palaeolatitudes at a time of extreme global warmth and elevated co2 fossil leaf form reveals that under such hothouse conditions megathermal early eocene to earliest miocene forests were exposed to strong monsoonal climates typical of those experienced today arising from annual migrations of the itcz possibly enhanced by a lower equator to pole temperature gradient throughout the paleogene an elevated tibetan highland produced no discernable modification of this itcz monsoon although rainfall seasonality similar to that of the modern south asia monsoon sam is observed in northern india as early as the beginning of the eocene despite its near equatorial palaeoposition in south china rainfall seasonality increased progressively achieving modern monsoon like wet season dry season precipitation ratios by the early oligocene despite evidencing weak rainfall seasonality overall fossil leaves from south china have exhibited monsoon adapted morphologies comparable to those seen in today s indonesia australia monsoon for at least 45 million years together the indian and south china fossil leaf assemblages show that the evolution of megathermal ecosystems across southern asia has been influenced profoundly by monsoonal climates for at least the last 56 million years the paleogene itcz driven monsoon system strongly impacted india as it transited the equator likely eliminating gondwanan taxa not able to adapt to seasonal precipitation extremes furthermore powerful seasonally reversing winds and associated surface ocean currents are likely to have facilitated two way biotic transfer between india and eurasia long before closure of the tethys ocean

the heartbeat of the oligocene climate system a 13 million year continuous record of oligocene climate from the equatorial pacific reveals a pronounced heartbeat in the global carbon cycle and periodicity of glaciations this heartbeat consists of 405 000 127 000 and 96 000 year eccentricity cycles and 1 2 million year obliquity cycles in periodically recurring glacial and carbon cycle events that climate system response to intricate orbital variations suggests a fundamental interaction of the carbon cycle solar forcing and glacial events box modeling shows that the interaction of the carbon cycle and solar forcing modulates deep ocean acidity as well as the production and burial of global biomass the pronounced 405 000 year eccentricity cycle is amplified by the long residence time of carbon in the oceans

is enso a cycle or a series of events 1 after early ideas that saw el ninos as isolated events the advent of coupled models brought the conception of enso as a cycle in which each phase led to the next in a self sustained oscillation twenty two years of observations that represent the el nino and la nina peaks east pacific sst and the memory of the system zonal mean warm water volume suggest a distinct break in the cycle in which the coupled system is able to remain in a weak la nina state for up to two years so that memory of previous influences would be lost similarly while the amplitude of anomalies persists from the onset of a warm event through its termination there is no such persistence across the la nina break these observations suggest that el ninos are in fact event like disturbances to a stable basic state requiring an initiating impulse not contained in the dynamics of the cycle itself

simultaneous evolution of gyre and atlantic meridional overturning circulation anomalies as an eigenmode of the north atlantic system abstractthe authors identify an interdecadal oscillatory mode of the north atlantic atmosphere ocean system in a general circulation model gfdl cm2 1 via a linear inverse model lim the oscillation mechanism is mostly embedded in the subpolar gyre anomalous advection generates density anomalies in the eastern subpolar gyre which propagate along the mean gyre circulation and reach the subpolar gyre center around 10 years later when associated anomalous advection of the opposite sign starts the other half cycle as density anomalies reach the labrador sea deep convection region atlantic meridional overturning circulation amoc anomalies are also induced both the gyre and amoc anomalies then propagate equatorward slowly following the advection of density anomalies the oscillation is further demonstrated to be more likely an ocean only mode while excited by the atmospheric forcing in particular it can be approximated as a linearly driven damped oscillator that is partly excited by the north atla

eocene circulation of the southern ocean was antarctica kept warm by subtropical waters 1 near the eocene s close 34 million years ago the climate system underwent one of the largest shifts in earth s history antarctic terrestrial ice sheets suddenly grew and ocean productivity patterns changed previous studies conjectured that poleward penetration of warm subtropical currents the east australian current eac in particular caused eocene antarctic warmth late eocene opening of an ocean gateway between australia and antarctica was conjectured to have disrupted the eac cooled antarctica and allowed ice sheets to develop here we reconstruct eocene paleoceanographic circulation in the tasmanian region using 1 biogeographical distributions of phytoplankton including data from recently drilled ocean drilling program leg 189 sites and 2 fully coupled climate model simulations we find that the eac did not penetrate to high latitudes and ocean heat transport in the region was not greater than modern our results do not support changes in thermal isolation as the primary driver of the eocene oligocene climatic transition

tidal and residual circulations in coupled restricted and leaky lagoons abstract finite element numerical modelling based on field data is used to study the tidal and tidally induced residual circulation dynamics of a coupled restricted and leaky coastal lagoon system located in the magdalen islands gulf of saint lawrence havre aux maisons lagoon hml is of a restricted nature with a neutral inlet in terms of tidal asymmetry grande entree lagoon gel is of a leaky nature with a marked ebb dominance at the inlet due to direct interactions between the main astronomical tidal constituents the imbalance caused by the different tidal filtering characteristics of both inlets combines with the internal morphological asymmetries of the system to produce a residual throughflow from hml to gel the residual circulation is also characterized by strongest values at both inlets very weak residual currents in hml deep basin and a dipole of residual eddies over the deeper areas of gel further investigations including numerical tracer experiments will be necessary to achieve a full understanding of the long term circulation of this lagoonal system

the respective role of atmospheric carbon dioxide and orbital parameters on ice sheet evolution at the eocene oligocene transition the continental scale initiation of the antarctic ice sheet at the eocene oligocene boundary eocene oligocene transition eot 34 ma is associated with a global reorganization of the climate if data studies have assessed the precise timing and magnitudes of the ice steps modeling studies have been unable to reproduce a transient ice evolution during the eocene oligocene transition in agreement with the data here we simulate this transition using general circulation models coupled to an ice sheet model our simulations reveal a threshold for continental scale glaciation of 900 ppm 100 to 150 ppm higher than previous studies this result supports the existence of ephemeral ice sheets during the middle eocene as similar co 2 levels 900 1000 ppm have been reached episodically during this period transient runs show that the ice growth is accurately timed with eot 1 and oi 1 the two 18 o excursions occurring during the transition we show that co 2 and orbital variations are crucial in initiating these steps with eot 1 corresponding to the occurrence of low summer insolation whereas oi 1 is controlled by a major co 2 drop the two 18 o steps record both ice growth and temperature representing some 10 30 m eustatic sea level fall and 2 4 c cooling at eot 1 and 70 20 m and 0 2 c for oi 1 the simulated magnitude of the ice steps 10 m for eot 1 and 63 m for oi 1 and the overall cooling at various locations show a good agreement with the data which supports our results concerning this critical transition

observed horizontal temperature advection by tropical instability waves 1 velocity data from moored current meters is combined with satellite sea surface temperature sst to compute oceanic mixed layer temperature advection by tropical instability waves tiws for the years 2002 to 2005 it is found that this process heats the equatorial mixed layer at an annual mean rate of 0 8 c month at 0 n 140 w and 2 8 c month at 0 n 110 w at 0 n 110 w approximately 25 of the heating is contributed by zonal temperature advection a process that has often been assumed to be negligible from a nine month segment of data may 2004 february 2005 the zonal temperature advection at 2 n 140 w has been estimated to be approximately 0 7 c month much larger than the equatorial value for the same time period thus the data supports a recent hypothesis that tropical instability waves contribute a significant mean zonal temperature advection with off equatorial maxima to the equatorial mixed layer heat budget comparisons with numerical model results suggest that current ocean general circulation models can realistically simulate important aspects of tropical eddy mixed layer interactions

upper level features associated with winter monsoon surges over south china abstract the upper level features associated with the two kinds of winter monsoon surges over south china are studied the easterly surge es and the northerly surge ns the study is similar to that used by wu and chan except that a broader region 0 60 n 70 160 e is considered the outbreak of an ns is associated with a breakdown of the siberia mongolia high the upper level features suggest that the evolution of the siberia mongolia high in an ns can be related to an eastward passage of a short wave trough and the polar jet based on the quasigeostrophic theory the intensification of the siberia mongolia high appears likely to be caused mainly by the adiabatic cooling due to rising motion associated with the trough and the polar jet passages after the passage of the trough and the jet the siberia mongolia high breaks down in response to the sinking motion upstream of the trough causing a surge of the northerly winds over the south china coast for the es the passage of an upper ridge is obse

a new view of near shore dynamics based on observations from hf radar abstract since 1984 the oscr hf radar system has been used in over 50 deployments to measure near shore surface currents for both scientific and engineering applications the enhanced scope resolution and accuracy of these measurements have yielded new insights into the tidal wind and density driven dynamics of the near shore zone tidal current ellipses obtained from these radar measurements have been shown to be in good aggrement with values calculated by numerical models both for the predominant constituents and also for higher harmonics coherent patterns of wind forced currents ahve been determined with strong evidence of a slab like surface response in one deployment with offshore winds blowing over relatively deep water this slab rotated clockwise at near inertial frequency strong up to 20cm s 1 persistent surface residual currents are commonly observed these are almost certainly generated by small horizontal density gradients these observed surface residuals provide ideal data for rigorous testing of 3 d numerical models with a threatened rise in sea level hf radar is well suited for observing the expected changes in the dynamics of near shore regions continuing development of these radar systems offers exciting prospects of remote sensing of both surface waves and currents future applications may extend beyond the near shore region to measurements along the shelf edge in oceanic gyres and for beach processes

estuarine salt conservation a lagrangian approach abstract understanding tidal and mean flow transport mechanisms that maintain the estuarine salt balance against seaward transport by river flow is one of the fundamental questions of estuarine oceanography previous salt transport studies have failed to resolve this problem for two principal methodological reasons in addition to the inherent variability of estuaries first salt transport expansions used to represent the salt balance have included a large number of terms that could not be related to any theory of estuarine circulation and whose physical meaning was thus unclear second it has proven difficult to measure small mean velocities in the presence of much larger tidal variations a new analysis method that expresses lagrangian scalar property transport in terms of time and space variations of eulerian variables is much simpler than expansions previously used and yields new insights into estuarine transport processes this approach like previous expansions is valid for narrow systems in which cross channel transport processes are weak and the ratio of tidal amplitude to mean depth is small a surface boundary condition is used to eliminate one class of troublesome terms the equivalence of the tidal cycle average transport of tidal variations in salinity and the stokes transport of mean salinity is then employed to show that the vertically integrated tidal flow plays no role in the overall salt balance that is seaward transport of salt caused by the river flow can only be balanced by landward transport resulting from correlations between shear and stratification in the mean tidal and overtide flows in a hypothetical inviscid estuary tide and overtide flows would be vertically uniform and outward fluvial salt transport could only be balanced by shear and stratification in the mean gravitational circulation in a more realistic example with strong friction the gravitational circulation would be severely damped and inward transport could only be accomplished by correlations of shear and stratification in the tide and overtide flows

the coexistence approach theoretical background and practical considerations of using plant fossils for climate quantification the coexistence approach was established by mosbrugger and utescher 1997 as a plant based method to reconstruct palaeoclimate by considering recent climatic distribution ranges of the nearest living relatives of each fossil taxon during its existence for over more than 15 years its basics have been tested and reviewed in comparison with other terrestrial and marine climate reconstruction techniques and climate modelling data however some controversies remain about its underlying data or its applicability in general in view of these controversies this paper discusses the power and limitations of the coexistence approach by summarising past results and new developments we give insights into the details and problems of each step of the application from the assignment of the fossil plant to the most suitable nearest living relative the crucial consideration of the usefulness of specific taxa towards their climatic values and the correct interpretation of the software based suggested palaeoclimatic intervals furthermore we reflect on the fundamental data integrated in the coexistence approach by explaining different concepts and usages of plant distribution information and the advantages and disadvantages of modern climatic maps additionally we elaborate on the importance of continually updating the information incorporated in the database due to new findings in e g palaeo botany meteorology and computer technology finally for a transparent and appropriate use we give certain guidelines for future applications and emphasize to users how to carefully consider and discuss their results we show the coexistence approach to be an adaptive method capable of yielding palaeoclimatic and palaeoenvironmental information through rime and space c 2014 elsevier b v all rights reserved

deep water temperature carbonate ion and ice volume changes across the eocene oligocene climate transition 1 paired benthic foraminiferal stable isotope and mg ca data are used to estimate bottom water temperature bwt and ice volume changes associated with the eocene oligocene transition eot the largest global climate event of the past 50 myr we utilized odp sites 1090 and 1265 in the south atlantic to assess seawater 18o w antarctic ice volume and sea level changes across the eot 33 8 33 54 ma we also use benthic 13c data to reconstruct the sources of the deep water masses in this region during the eot our data together with previously published records indicate that a pulse of northern component water influenced the south atlantic immediately prior to and following the eot benthic 18o records show a 0 5 increase at 33 8 ma eot 1 that represents a 2 c cooling and a small 10 m eustatic fall that is followed by a 1 0 increase associated with oi 1 the expected cooling of deep waters at oi 1 33 54 ma is not apparent in our mg ca records we suggest the cooling is masked by coeval changes in the carbonate saturation state co32 which affect the mg ca data to account for this the bwt ice volume and w estimates are corrected for a change in the co32 of deep waters on the basis of recently published work corrected bwt at sites 1090 and 1265 show a 1 5 c cooling coincident with oi 1 and an average w increase of 0 75 the increase in ice volume during oi 1 resulted in a 70 m drop in global sea level and the development of an antarctic ice sheet that was near modern size or slightly larger

on tide induced lagrangian residual current and residual transport 2 residual transport with application in south san francisco bay california the transports of solutes and other tracers are fundamental to estuarine processes the apparent transport mechanisms are convection by tidal current and current induced shear effect dispersion for processes which take place in a time period of the order of a tidal cycle however as emphasis is shifted toward the effects of intertidal processes the net transport is mainly determined by tide induced residual circulation and by residual circulation due to other processes the commonly used intertidal conservation equation takes the form of a convection dispersion equation in which the convective velocity is the eulerian residual current and the dispersion terms are often referred to as the phase effect dispersion or sometimes as the tidal dispersion the presence of these dispersion terms is merely the result of a fickian type hypothesis since the actual processes are not fickian thus a fickian hypothesis obscures the physical significance of this equation recent research results on residual circulation have suggested that long term transport phenomena are closely related to the lagrangian residual current or the lagrangian residual transport in this paper a new formulation of an intertidal conservation equation is presented and examined in detail in a weakly nonlinear tidal estuary the resultant intertidal transport equation also takes the form of a convection dispersion equation without the ad hoc introduction of phase effect dispersion in a form of dispersion tensor the convective velocity in the resultant equation is the first order lagrangian residual current the sum of the eulerian residual current and the stokes drift the remaining dispersion terms are important only in higher order solutions they are due to shear effect dispersion and turbulent mixing there exists a dispersion boundary layer adjacent to shoreline boundaries an order of magnitude estimate of the properties in the dispersion boundary layer is given the present treatment of intertidal transport processes is illustrated by an analytical solution for an amphidromic system and by a numerical application in south san francisco bay california the present formulation reveals that the mechanism for long term transport of solutes is mainly convection due to the lagrangian residual current in the interior of a tidal estuary this result also points out the weakness in the tidal dispersion formulation and explains the large variability of the observed values for tidal dispersion coefficients further research on properties of the dispersion boundary layer is needed

analysis of the atlantic meridional mode using linear inverse modeling seasonality and regional influences abstractpredictability and variability of the tropical atlantic meridional mode amm is investigated using linear inverse modeling lim analysis of the lim using an energy norm identifies two optimal structures that experience some transient growth one related to el nino southern oscillation enso and the other to the atlantic multidecadal oscillation amo amm patterns analysis of the lim using an amm norm identifies an amm optimal with similar structure to the second energy optima opt2 both the amm optimal and opt2 exhibit two bands of sst anomalies in the mid to high latitude atlantic the amm optimal also contains some elements of the first energy optimal enso indicating that the lim captures the well known relationship between enso and the amm seasonal correlations of lim predictions with the observed amm show enhanced amm predictability during boreal spring and for long lead around 11 15 months forecasts initialized around september regional lims were constructed to determine the

asian eocene monsoons as revealed by leaf architectural signatures abstract the onset and development of the asian monsoon systems is a topic that has attracted considerable research effort but proxy data limitations coupled with a diversity of definitions and metrics characterizing monsoon phenomena have generated much debate failure of geological proxies to yield metrics capable of distinguishing between rainfall seasonality induced by migrations of the inter tropical convergence zone itcz from that attributable to topographically modified seasonal pressure reversals has frustrated attempts to understand mechanisms underpinning monsoon development and dynamics here we circumvent the use of such single climate parameter metrics in favor of detecting directly the distinctive attributes of different monsoon regimes encoded in leaf fossils leaf form adapts to the prevailing climate particularly under the extreme seasonal stresses imposed by monsoons so it is likely that fossil leaves carry a unique signature of past monsoon regimes leaf form trait spectra obtained from fossils from eocene basins in southern china were compared with those seen in modern leaves growing under known climate regimes the fossil leaf trait spectra including those derived from previously published fossil floras from northwestern india were most similar to those found in vegetation exposed to the modern indonesia australia monsoon i am which is largely a product of seasonal migrations of the itcz the presence of this distinctive leaf physiognomic signature suggests that although a monsoon climate existed in eocene time across southern asia the characteristics of the modern topographically enhanced south asia monsoon had yet to develop by the eocene leaves in south asia had become well adapted to an i am type regime across many taxa and points to the existence of a pervasive monsoon climate prior to the eocene no fossil trait spectra typical of exposure to the modern east asia monsoon were seen suggesting the effects of this system in southern china were a much later development

resolving apparent conflicts between oceanographic and antarctic climate records and evidence for a decrease in pco2 during the oligocene through early miocene 34 16 ma abstract an apparent mismatch between published oxygen isotopic data and other paleoclimate proxies for the span from 26 16 ma is resolved by calibration against global sea level estimates obtained from backstripping continental margin stratigraphy ice volume estimates from calibrated oxygen isotope data compare favorably with stratigraphic and palynological data from antarctica and with estimates of atmospheric pco2 throughout the oligocene to early miocene 34 16 ma isotopic evidence for an east antarctic ice sheet eais as much as 30 larger than its present day volume at glacial maxima during that span is consistent with seismic reflection and stratigraphic evidence for an ice sheet covering much of the antarctic continental shelf at the same glacial maxima palynological data suggest long term cooling during the oligocene with cold near tundra environments developing along the coast at glacial minima no later than the late oligocene a possible mechanism for this long term cooling is a decrease in atmospheric pco2 from the middle eocene to oligocene reaching near pre industrial levels by the latest oligocene and remaining at those depressed levels throughout the miocene

tibetan plateau aridification linked to global cooling at the eocene oligocene transition the eocene oligocene transition about 33 5 million years ago was a major global climate event the end of the eocene was unusually warm with no significant ice on antarctica but the oligocene saw the arrival of a permanent antarctic ice sheet two papers this week relate to the continental effects of this global change dupont nivet et al examined sedimentary records from the tibetan plateau and find a drop in atmospheric water which caused cooling and aridification coincident with antarctic cooling previous studies attributed this phenomenon to the rapid uplift of the tibetan plateau but this new work suggests that regional tibetan climate was influenced by global events in an unrelated paper on the same climate transition zanazzi et al explore the cooling in north america at the time using stable isotope measurements from fossil teeth and bones to create a proxy temperature record they find a large drop in mean annual temperature of 8 2 c a greater fall than seen in the oceans this continental transition may explain why many cold blooded reptiles and amphibians became extinct whereas mammals able to regulate their body temperature escaped relatively unscathed precisely dated evidence details that aridification on the tibetan plateau occurred at the time of the eocene oligocene transition in the marine records this suggests a direct link between the palaeontological and palaeoenvironmental changes recorded in continental asia at the time and global climate cooling continental aridification and the intensification of the monsoons in asia are generally attributed to uplift of the tibetan plateau and to the land sea redistributions associated with the continental collision of india and asia1 2 3 whereas some studies suggest that past changes in asian environments are mainly governed by global climate4 5 6 the most dramatic climate event since the onset of the collision of india and asia is the eocene oligocene transition an abrupt cooling step associated with the onset of glaciation in antarctica 34 million years ago7 8 9 however the influence of this global event on asian environments is poorly understood here we use magnetostratigraphy and cyclostratigraphy to show that aridification which is indicated by the disappearance of playa lake deposits in the northeastern tibetan plateau occurred precisely at the time of the eocene oligocene transition our findings suggest that this global transition is linked to significant aridification and cooling in continental asia recorded by palaeontological and palaeoenvironmental changes10 11 12 and thus support the idea that global cooling is associated with the eocene oligocene transition13 14 15 we show that with sufficient age control on the sedimentary records global climate can be distinguished from tectonism and recognized as a major contributor to continental asian environments

relationship between optimal precursory disturbances and optimally growing initial errors associated with enso events implications to target observations for enso prediction by superimposing initial sea temperature disturbances in neutral years we determine the precursory disturbances that are most likely to evolve into el nino and la nina events using an earth system model these precursory disturbances for el nino and la nina events are deemed optimal precursory disturbances because they are more likely to trigger strong enso events specifically the optimal precursory disturbance for el nino exhibits negative sea surface temperature anomalies sstas in the central eastern equatorial pacific additionally the subsurface temperature component exhibits negative anomalies in the upper layers of the eastern equatorial pacific and positive anomalies in the lower layers of the western equatorial pacific the optimal precursory disturbance for la nina is almost opposite to that of el nino the optimal precursory disturbances show that both el nino and la nina originate from precursory signals in the subsurface layers of the western equatorial pacific and in the surface layers of the eastern equatorial pacific we find that the optimal precursory disturbances for el nino and la nina are particularly similar to the optimally growing initial errors associated with el nino prediction that have been presented in previous studies the optimally growing initial errors show that the optimal precursor source areas represent the sensitive areas for target observations associated with enso prediction combining the optimal precursory disturbances and the optimally growing initial errors for enso we infer that additional observations in these sensitive areas can reduce initial errors and be used to detect precursory signals thereby improving enso predictions

late oligocene miocene mid latitude aridification and wind patterns in the asian interior the asian interior has the largest mid latitude arid zone in the northern hemisphere and so has become increasingly attractive for studying the initiation and the past extent of aridification in this zone given the enormousness of the asian interior it remains unclear how old and extensive the eolian deposits might have been and what wind regimes have been responsible for the formation of the mid latitude arid zone here we report new eolian records of widespread tertiary eolian deposits in a region far from the chinese loess plateau the giant junggar inland basin of northwestern china our results demonstrate that the earliest eolian deposition initiated ca 24 ma we interpret that the tertiary eolian dust in the junggar basin was transported by westerly winds possibly from areas in kazakhstan the dust differs from the airborne dust transported by winter monsoon winds from the deserts of mongolia and northern china that accumulated on the loess plateau these results further reveal that the climate pattern similar to that of the present has prevailed at least since the latest oligocene in central asia

interannual variability in a tropical atmosphere ocean model influence of the basic state ocean geometry and nonlinearity abstract the behavior of a tropical coupled atmosphere ocean model is analyzed for a range of different background states and ocean geometries the model is essentially that of cane and zebiak for the tropical pacific except only temporally constant background states are considered here for realistic background states and ocean geometry the model solutions feature oscillations of period of 3 5 yr by comparing the full model solution with a linearized version of the model it is shown that the basic mechanism of the oscillation is contained within linear theory a simple linear analog model is derived that describes the nature of the interannual variability in the coupled tropical atmosphere ocean system the analog model highlights the properties that produce coupled atmosphere ocean instability in the eastern ocean basin and the equatorial wave dynamics in the western ocean basin that are responsible for a delayed negative feedback into this instability growth the growth rate of the local instabil

the trident pacific model part 2 role of long equatorial wave reflection on sea surface temperature anomalies during the 1993 1998 topex poseidon period the present study aims to give a quantitative description of the role played by long equatorial waves on sea surface temperature anomalies during the 1993 1998 topex poseidon period and more specifically during the 1997 1998 el nino using a new pacific ocean model called trident in a companion paper the trident dynamical component was described and validated with observations briefly the model exhibits skill in simulating not only sea level but also surface zonal current variability in the equatorial wave guide here the model thermodynamics are described and validated the thermodynamics of trident consist of one single equation for interannual sea surface temperature anomalies compared to other similar models the introduction in the temperature equation of a term equivalent to a vertical mixing term improves significantly the temperature simulations in the eastern pacific thus the model comparison to interannual sea level zonal current and sea surface temperature anomalies is fairly good over the entire equatorial pacific ocean the model is found to be sensitive to both subsurface variability central and eastern pacific and to zonal current advection especially near the dateline where the data suggest that advection is a first order process for determining sea surface temperature changes the role of long equatorial wave reflection observed in topex poseidon data is studied by either cancelling the eastern boundary or western boundary reflection first at the eastern boundary although the reflected rossby waves were found to act against the warming during the onset phase of the 1997 1998 el nino through zonal advection the major impact of these reflected rossby waves was to reinforce the deepening of the thermocline in the eastern pacific initiated by the impinging kelvin waves therefore all things considered the rossby waves strongly contributed to the warming in 1997 1998 east of 120 w secondly at the western boundary rossby wave reflection acted as suggested by the delayed action oscillator theory the reflected kelvin waves caused shoalling of the thermocline in the central and eastern pacific where they weakened the warm or cold conditions observed in the pacific ocean during the 1997 1998 event specifically they contributed to about a third of the upwelling kelvin wave amplitude propagating in the central pacific the other two thirds of the amplitude were found to be actually wind forced strongly suggesting that during that event easterly wind anomalies in the western pacific played a significant role in the termination of the recent 1997 1998 el nino event and its switch to the la nina period

the coexistence approach a method for quantitative reconstructions of tertiary terrestrial palaeoclimate data using plant fossils abstract the coexistence approach is introduced as a method for quantitative terrestrial palaeoclimate reconstructions in the tertiary it is based on the assumption that tertiary plant taxa have similar climatic requirements to their nearest living relatives the aim of the coexistence approach is to find for a given fossil flora and a given climate parameter the climatic interval in which all nearest living relatives of the fossil flora can coexist for this purpose we have developed a data base climbot containing for over 800 tertiary plant taxa the nearest living relatives and their climatic requirements and an algorithm for analysis climst for all types of fossil floras the coexistence approach thus allows the rapid quantitative reconstruction of 10 different climate parameters including the mean annual temperature temperature of the warmest month temperature of the coldest month mean annual precipitation maximum monthly precipitation minimum monthly precipitation precipitation of the warmest month relative humidity potential evaporation as well as the ratio of mean annual precipitation over potential evaporation the reliability and resolution of the coexistence approach are tested with various techniques and proved to be reasonably good for instance the resolution with respect to the mean annual temperature can be up to 1 c applications to various modern and fossil floras are illustrated and the advantages and disadvantages of the coexistence approach are discussed

on the impact of local feedbacks in the central pacific on the enso cycle abstract while sea surface temperature sst anomalies in the eastern equatorial pacific are dominated by the thermocline feedback in the central equatorial pacific local wind effects such as zonal advection are important as well el nino southern oscillation enso simulations with a linear model improve markedly if these effects are included as a local wind stress feedback on sst an atmosphere model that reacts both to eastern and central pacific sst anomalies is needed for producing a realistic enso cycle first simulations are studied of a linear 1 5 layer reduced gravity ocean model and a linear sst anomaly equation forced by observed monthly wind stress if only the thermocline feedback is present in the sst equation sst can be simulated well in the eastern pacific but contrary to observations central pacific sst is out of phase with the eastern pacific if a wind stress feedback is added in the sst equation as a term proportional to the zonal wind stress correlations between observed an

correlation of himalayan exhumation rates and asian monsoon intensity although most data suggest that the india eurasia continental collision began 45 55 myr ago the architecture of the himalayan tibetan orogen is dominated by deformational structures developed in the neogene period 23 myr ago the stratigraphic record and thermochronometric data indicate that erosion of the himalaya intensified as this constructional phase began and reached a peak around 15 myr ago it remained high until 10 5 myr ago and subsequently slowed gradually to 3 5 myr ago but then began to increase once again in the late pliocene and pleistocene epochs here we present weathering records from the south china sea bay of bengal and arabian sea that permit asian monsoon climate to be reconstructed back to the earliest neogene these indicate a correlation between the rate of himalayan exhumation as inferred from published thermochronometric data and monsoon intensity over the past 23 myr we interpret this correlation as indicating dynamic coupling between neogene climate and both erosion and deformation in the himalaya although the india eurasia collision initiated 50 myr ago major deformation and exhumation of the himalaya did not begin until the early neogene 23 myr ago this coincides with the increased intensity of the asian monsoons as indicated by weathering records from the south china sea bay of bengal and arabian sea and hints at a dynamic coupling between climate and both erosion and deformation in the himalaya

a delayed action oscillator shared by biennial interannual and decadal signals in the pacific basin 1 biennial interannual and decadal signals in the pacific basin are observed to share patterns and evolution in covarying sea surface temperature sst 18 c isotherm depth z18 zonal surface wind zsw and wind stress curl wsc anomalies from 1955 to 1999 each signal has warm sst anomalies propagating slowly eastward along the equator generating westerly zsw anomalies in their wake these westerly zsw anomalies produce cyclonic wsc anomalies off the equator which pump baroclinic rossby waves in the western central tropical north pacific ocean these rossby waves propagate westward taking 6 12 and 36 months to reach the western boundary near 7 n 12 n and 18 n on biennial interannual and decadal period scales respectively there they reflect as equatorial coupled waves propagating slowly eastward in covarying sst z18 and zsw anomalies taking 6 12 and 24 months to reach the central eastern equatorial ocean these equatorial coupled waves produce a delayed negative feedback to the warm sst anomalies there the decrease in rossby wave phase speed with latitude the increase in meridional scale of equatorial sst anomalies with period scale and the associated increase in latitude of rossby wave forcing are consistent with the delayed action oscillator dao model used to explain el nino however this is not true of the western boundary reflection of rossby waves into slow equatorial coupled waves this requires modification of the extant dao model we construct a modified dao model demonstrating how the various mechanisms and the size and sources of their delays yield the resulting frequency of each signal

the role of ocean gateways in the dynamics and sensitivity to wind stress of the early antarctic circumpolar current the date of inception of the antarctic circumpolar current is debated due to uncertainty in the relative opening times of drake passage and the tasman seaway using an idealized eddy resolving numerical ocean model we investigate whether both ocean gateways have to be open to allow for a substantial circumpolar current we find that overlapping continental barriers do not impede a circumpolar transport in excess of 50sv as long as a circumpolar path can be traced around the barriers however the presence of overlapping barriers does lead to an increased sensitivity of the current s volume transport to changes in wind stress this change in sensitivity is interpreted in terms of the role of pressure drops across continental barriers and submerged bathymetry in balancing the momentum input by the surface wind stress specifically when the pressure drop across continents is the main balancing sink of momentum the zonal volume transport is sensitive to changes in wind stress changes in zonal volume transport take place via altering the depth independent part of the circumpolar transport rather than that arising from thermal wind shear in such a scenario isopycnals continue to slope steeply across the model southern ocean implying a strong connection between the deep and surface oceans this may have consequences for the meridional overturning circulation and its sensitivity to wind stress

increased seasonality through the eocene to oligocene transition in northern high latitudes a profound global climate shift took place at the eocene oligocene transition approximately 33 5 million years ago when cretaceous early palaeogene greenhouse conditions gave way to icehouse conditions during this interval changes in the earth s orbit and a long term drop in atmospheric carbon dioxide concentrations resulted in both the growth of antarctic ice sheets to approximately their modern size and the appearance of northern hemisphere glacial ice however palaeoclimatic studies of this interval are contradictory although some analyses indicate no major climatic changes others imply cooler temperatures increased seasonality and or aridity climatic conditions in high northern latitudes over this interval are particularly poorly known here we present northern high latitude terrestrial climate estimates for the eocene to oligocene interval based on bioclimatic analysis of terrestrially derived spore and pollen assemblages preserved in marine sediments from the norwegian greenland sea our data indicate a cooling of approximately 5 degrees c in cold month winter mean temperatures to 0 2 degrees c and a concomitant increased seasonality before the oi 1 glaciation event these data indicate that a cooling component is indeed incorporated in the delta 18 o isotope shift across the eocene oligocene transition however the relatively warm summer temperatures at that time mean that continental ice on east greenland was probably restricted to alpine outlet glaciers

south pacific decadal climate variability and potential predictability the south pacific decadal oscillation spdo characterizes the southern hemisphere contribution to the pacific wide interdecadal pacific oscillation ipo and is analogous to the pacific decadal oscillation pdo centered in the north pacific in this study upper ocean variability and potential predictability of the spdo is examined in hadisst data and an atmosphere forced ocean general circulation model the potential predictability of the ipo related variability is investigated in terms of both the fractional contribution made by the decadal component in the south tropical and north pacific oceans and in terms of a doubly integrated first order autoregressive ar1 model despite explaining a smaller fraction of the total variance we find larger potential predictability of the spdo relative to the pdo we identify distinct local drivers in the western subtropical south pacific where nonlinear baroclinic rossby wave topographic interactions act to low pass filter decadal variability in particular we show that the kermadec ridge in the southwest pacific enhances the decadal signature more prominently than anywhere else in the pacific basin applying the doubly integrated ar1 model we demonstrate that variability associated with the pacific south american pattern is a critically important atmospheric driver of the spdo via a reddening process analogous to the relationship between the aleutian low and pdo in the north pacific albeit that the relationship in the south pacific appears to be even stronger our results point to the largely unrecognized importance of south pacific processes as a key source of decadal variability and predictability

paleogene temperature gradient seasonal variation and climate evolution of northeast china abstract continental paleogene climates have been well studied in europe and north america but very little is known from asia because paleoclimatic results have only been reported from particular geological intervals here based on 29 plant assemblages from 8 well age controlled fossiliferous sites we quantitatively reconstruct the climates through most of the paleogene of northeast china and discuss related seasonal variations our results demonstrate that the mean annual temperature mat gradient was fairly shallow 0 27 c 1 latitude during the paleocene throughout this region in the eocene seasonality was high in the region indicated by marked differences in both temperature and precipitation between winters and summers of the sites the paleo east asian monsoon must have had intensified at least in the late mid eocene shown by apparent differences in annual precipitation distribution at all the sites regarding the paleogene climatic evolution of northeast china our quantitative results suggest that mat overall declined from warm in the paleocene and eocene to moderate in the oligocene generally consistent with the trend of marine records but with some distinctions two significant cooling events are recognized in the early and mid eocene with mat 3 4 c and 3 8 c lower and winter temperature 5 8 c and 4 7 c lower respectively in similar magnitudes to corresponding variations in europe and north america furthermore the present results show that mat rebounded in the late mid eocene and then decreased until the oligocene a similar pattern demonstrated in europe during the mid eocene to oligocene interval

large temperature drop across the eocene oligocene transition in central north america the eocene oligocene transition about 33 5 million years ago was a major global climate event the end of the eocene was unusually warm with no significant ice on antarctica but the oligocene saw the arrival of a permanent antarctic ice sheet two papers this week relate to the continental effects of this global change dupont nivet et al examined sedimentary records from the tibetan plateau and find a drop in atmospheric water which caused cooling and aridification coincident with antarctic cooling previous studies attributed this phenomenon to the rapid uplift of the tibetan plateau but this new work suggests that regional tibetan climate was influenced by global events in an unrelated paper on the same climate transition zanazzi et al explore the cooling in north america at the time using stable isotope measurements from fossil teeth and bones to create a proxy temperature record they find a large drop in mean annual temperature of 8 2 c a greater fall than seen in the oceans this continental transition may explain why many cold blooded reptiles and amphibians became extinct whereas mammals able to regulate their body temperature escaped relatively unscathed an analysis of fossil tooth enamel and fossil bones is used to derive a continental temperature record for the eocene oligocene transition and reports a large drop in mean annual temperature over roughly 400 000 years this cooling explains the decline in cold blooded terrestrial animals whereas most mammals in the region were unaffected the eocene oligocene transition towards a cool climate 33 5 million years ago was one of the most pronounced climate events during the cenozoic era1 the marine record of this transition has been extensively studied however significantly less research has focused on continental climate change at the time yielding partly inconsistent results on the magnitude and timing of the changes2 3 4 5 6 7 8 here we use a combination of in vivo stable isotope compositions of fossil tooth enamel with diagenetic stable isotope compositions of fossil bone to derive a high resolution about 40 000 years continental temperature record for the eocene oligocene transition we find a large drop in mean annual temperature of 8 2 3 1 c over about 400 000 years the possibility of a small increase in temperature seasonality and no resolvable change in aridity across the transition the large change in mean annual temperature exceeding changes in sea surface temperatures at comparable latitudes9 10 and possibly delayed in time with respect to marine changes by up to 400 000 years explains the faunal turnover for gastropods amphibians and reptiles whereas most mammals in the region were unaffected our results are in agreement with modelling studies that attribute the climate cooling at the eocene oligocene transition to a significant drop in atmospheric carbon dioxide concentrations

changing atmospheric co2 concentration was the primary driver of early cenozoic climate the early eocene climate optimum eeco which occurred about 51 to 53 million years ago was the warmest interval of the past 65 million years with mean annual surface air temperature over ten degrees celsius warmer than during the pre industrial period subsequent global cooling in the middle and late eocene epoch especially at high latitudes eventually led to continental ice sheet development in antarctica in the early oligocene epoch about 33 6 million years ago however existing estimates place atmospheric carbon dioxide co2 levels during the eocene at 500 3 000 parts per million and in the absence of tighter constraints carbon climate interactions over this interval remain uncertain here we use recent analytical and methodological developments to generate a new high fidelity record of co2 concentrations using the boron isotope 11 b composition of well preserved planktonic foraminifera from the tanzania drilling project revising previous estimates although species level uncertainties make absolute values difficult to constrain co2 concentrations during the eeco were around 1 400 parts per million the relative decline in co2 concentration through the eocene is more robustly constrained at about fifty per cent with a further decline into the oligocene provided the latitudinal dependency of sea surface temperature change for a given climate forcing in the eocene was similar to that of the late quaternary period this co2 decline was sufficient to drive the well documented high and low latitude cooling that occurred through the eocene once the change in global temperature between the pre industrial period and the eocene caused by the action of all known slow feedbacks apart from those associated with the carbon cycle is removed both the eeco and the late eocene exhibit an equilibrium climate sensitivity relative to the pre industrial period of 2 1 to 4 6 degrees celsius per co2 doubling 66 per cent confidence which is similar to the canonical range 1 5 to 4 5 degrees celsius indicating that a large fraction of the warmth of the early eocene greenhouse was driven by increased co2 concentrations and that climate sensitivity was relatively constant throughout this period

el nino modoki and its possible teleconnection 1 using observed data sets mainly for the period 1979 2005 we find that anomalous warming events different from conventional el nino events occur in the central equatorial pacific this unique warming in the central equatorial pacific associated with a horseshoe pattern is flanked by a colder sea surface temperature anomaly ssta on both sides along the equator empirical orthogonal function eof analysis of monthly tropical pacific ssta shows that these events are represented by the second mode that explains 12 of the variance since a majority of such events are not part of el nino evolution the phenomenon is named as el nino modoki pseudo el nino modoki is a classical japanese word which means a similar but different thing the el nino modoki involves ocean atmosphere coupled processes which include a unique tripolar sea level pressure pattern during the evolution analogous to the southern oscillation in the case of el nino hence the total entity is named as el nino southern oscillation enso modoki the enso modoki events significantly influence the temperature and precipitation over many parts of the globe depending on the season the impacts over regions such as the far east including japan new zealand western coast of united states etc are opposite to those of the conventional enso the difference maps between the two periods of 1979 2004 and 1958 1978 for various oceanic atmospheric variables suggest that the recent weakening of equatorial easterlies related to weakened zonal sea surface temperature gradient led to more flattening of the thermocline this appears to be a cause of more frequent and persistent occurrence of the enso modoki event during recent decades

effect of the deepening of the tasman gateway on the global ocean 1 we examine the effect of the deepening of the tasman seaway at the end of the eocene in a climate model with realistic late eocene bathymetry and winds for this we have constructed an eocene numerical model based on the university of victoria climate model with wind forcing derived from a fully coupled eocene simulation the model climate state is characterized by an oceanic meridional overturning circulation moc involving southern hemisphere sinking and a northward atmospheric moisture transport across the equator the deepening of the tasman seaway in the presence of an open drake passage and the associated establishment of the antarctic circumpolar current acc have a limited climatic impact on antarctica nonetheless the antarctic deep sinking regions cool sufficiently to lead to a global deep ocean cooling of 3 c no initiation of northern component water is found indicating that this may require the development of a more mature acc previous studies suggest that the ross sea gyre cools the east coast of australia and expected the deepening of the tasman seaway to lead to a warming east of australia due to the introduction of warmer water from the australo antarctic gulf we here find that this warming is limited to close to the australian coast and that widespread cooling prevails further off shore

the linear response of enso to the madden julian oscillation the possibility that the tropical pacific coupled system linearly amplifies perturbations produced by the madden julian oscillation mjo is explored this requires an estimate of the low frequency tail of the mjo using 23 yr of ncep ncar reanalyses of surface wind and reynolds sst we show that the spatial structure that dominates the intraseasonal band i e the mjo also dominates the low frequency band once the anomalies directly related to enso have been removed this low frequency contribution of the intraseasonal variability is not included in most enso coupled models used to date its effect in a coupled model of intermediate complexity has therefore been studied it is found that this mjo forcing mjo can explain a large fraction of the interannual variability in an asymptotically stable version of the model this interaction is achieved via linear dynamics that is it is the cumulative effect of individual events that maintains ensos in this model the largest coupled wind anomalies are initiated after a sequence of several downwelling kelvin waves of the same sign have been forced by mjo the cumulative effect of the forced kelvin waves is to persist the small sst anomalies in the eastern pacific just enough for the coupled ocean atmosphere dynamics to amplify the anomalies into a mature enso event even though mjo explains just a small fraction of the energy contained in the stress not associated with enso a large fraction of the modeled enso variability is excited by this forcing the characteristics that make mjo an optimal stochastic forcing for the model are discussed the large zonal extent is an important factor that differentiates the mjo from other sources of stochastic forcing

convergent cenozoic co2 history reconstructions of atmospheric carbon dioxide concentrations over the past 65 million years are heading towards consensus it is time for systematic testing of the proxies against measurements and against each other

pacific meridional mode and el nino southern oscillation 1 we present intriguing evidence that the majority of el nino events over the past four decades are preceded by a distinctive sea surface warming and southwesterly wind anomaly in the vicinity of the intertropical convergence zone itcz during the boreal spring this phenomenon known as the meridional mode mm is shown to be intrinsic to the thermodynamic coupling between the atmosphere and ocean the mm effectively acts as a conduit through which the extratropical atmosphere influences enso modeling results further suggest that the mm plays a vital role in the seasonal phase locking behavior of enso the findings provide a new perspective for understanding the important role of thermodynamic ocean atmosphere feedback in enso and may have profound implications for enso prediction particularly the unresolved issue of the spring predictability barrier citation chang p l zhang r saravanan d j vimont j c h chiang l ji h seidel and m k tippett 2007 pacific meridional mode and el nino southern oscillation geophys res lett 34 l16608 doi 10 1029 2007gl030302

effect of orogeny plate motion and land sea distribution on eurasian climate change over the past 30 million years the eurasian climates of today 10 million and 3o million years ago are simulated using an atmospheric general circulation model that incorporates realistic continental geography and epicontinental sea distributions the resulting climates compare well with various palaeoclimate records the retreat of the paratethys an epicontinental sea shifts the central asian climate from temperate to continental conditions and plays as important a role as uplift of the himalayan tibetan plateau in driving the asian monsoon changes

an empirical model of tropical ocean dynamics to extend the linear stochastically forced paradigm of tropical sea surface temperature sst variability to the subsurface ocean a linear inverse model lim is constructed from the simultaneous and 3 month lag covariances of observed 3 month running mean anomalies of sst thermocline depth and zonal wind stress this lim is then used to identify the empirically determined linear dynamics with physical processes to gauge their relative importance to enso evolution optimal growth of sst anomalies over several months is triggered by both an initial sst anomaly and a central equatorial pacific thermocline anomaly that propagates slowly eastward while leading the amplifying sst anomaly the initial sst and thermocline anomalies each produce roughly half the sst amplification if interactions between the sea surface and the thermocline are removed in the linear dynamical operator the sst anomaly undergoes less optimal growth but is also more persistent and its location shifts from the eastern to central pacific optimal growth is also found to be essentially the result of two stable eigenmodes with similar structure but differing 2 and 4 year periods evolving from initial destructive to constructive interference variations among enso events could then be a consequence not of changing stability characteristics but of random excitation of these two eigenmodes which represent different balances between surface and subsurface coupled dynamics as found in previous studies the impact of the additional variables on lim sst forecasts is relatively small for short time scales over time intervals greater than about 9 months however the additional variables both significantly enhance forecast skill and predict lag covariances and associated power spectra whose closer agreement with observations enhances the validation of the linear model moreover a secondary type of optimal growth exists that is not present in a lim constructed from sst alone in which initial sst anomalies in the southwest tropical pacific and indian ocean play a larger role than on shorter time scales apparently driving sustained off equatorial wind stress anomalies in the eastern pacific that result in a more persistent equatorial thermocline anomaly and a more protracted and predictable enso event

the tidally averaged momentum balance in a partially and periodically stratified estuary abstract observations of turbulent stresses and mean velocities over an entire spring neap cycle are used to evaluate the dynamics of tidally averaged flows in a partially stratified estuarine channel in a depth averaged sense the net flow in this channel is up estuary due to interaction of tidal forcing with the geometry of the larger basin the depth variable tidally averaged flow has the form of an estuarine exchange flow downstream at the surface upstream at depth and varies in response to the neap spring transition the weakening of the tidally averaged exchange during the spring tides appears to be a result of decreased stratification on the tidal time scale rather than changes in bed stress the dynamics of the estuarine exchange flow are defined by a balance between the vertical divergence of the tidally averaged turbulent stress and the tidally averaged pressure gradient in the lower water column in the upper water column tidal stresses are important contributors particularly during the n

global cooling during the eocene oligocene climate transition about 34 million years ago earth s climate shifted from a relatively ice free world to one with glacial conditions on antarctica characterized by substantial ice sheets how earth s temperature changed during this climate transition remains poorly understood and evidence for northern hemisphere polar ice is controversial here we report proxy records of sea surface temperatures from multiple ocean localities and show that the high latitude temperature decrease was substantial and heterogeneous high latitude 45 degrees to 70 degrees in both hemispheres temperatures before the climate transition were 20 c and cooled an average of 5 c our results combined with ocean and ice sheet model simulations and benthic oxygen isotope records indicate that northern hemisphere glaciation was not required to accommodate the magnitude of continental ice growth during this time

studies of el nino and interdecadal variability in tropical sea surface temperatures using a nonnormal filter abstract a dynamically based filter is used to separate tropical sea surface temperatures ssts into three components the evolving el nino signal the global tropical trend and the background the components thus isolated are not independent on the contrary this procedure allows us to see the importance of the interdecadal signal to the predictability of el nino the data filtered in this way reveal el nino signals in the equatorial indian ocean and in the north tropical atlantic ocean that are remarkably similar a signature of el nino in the south tropical atlantic leads nino 3 4 sst anomalies by about 9 months the time series of a global tropical trend is found to have a very smooth parabolic structure in unfiltered data this trend conspires with el nino to obscure a meridional tropical atlantic dipole which is significant in the filtered background sst data

atmospheric carbon dioxide through the eocene oligocene climate transition geological and geochemical evidence1 2 3 indicates that the antarctic ice sheet formed during the eocene oligocene transition4 33 5 34 0 million years ago modelling studies5 6 suggest that such ice sheet formation might have been triggered when atmospheric carbon dioxide levels fell below a critical threshold of 750 p p m v but the timing and magnitude of relative to the evolution of the ice sheet has remained unclear here we use the boron isotope ph proxy7 8 on exceptionally well preserved carbonate microfossils from a recently discovered geological section in tanzania9 10 to estimate before during and after the climate transition our data suggest that a reduction in occurred before the main phase of ice growth followed by a sharp recovery to pre transition values and then a more gradual decline during maximum ice sheet growth was between 450 and 1 500 p p m v with a central estimate of 760 p p m v the ice cap survived the period of recovery although possibly with some reduction in its volume implying as models predict11 a nonlinear response to climate forcing during melting overall our results confirm the central role of declining in the development of the antarctic ice sheet in broad agreement with carbon cycle modelling12 and help to constrain mechanisms and feedbacks associated with the earth s biggest climate switch of the past 65 myr

quasi quadrennial and quasi biennial variability in the equatorial pacific evaluation of competing el nino southern oscillation enso theories requires one to identify separate spectral peaks in equatorial wind and sea surface temperature sst time series to sharpen this identification we examine the seasonal to interannual variability of these fields by the data adaptive method of multi channel singular spectrum analysis m ssa m ssa is applied to the equatorial band 4 n 4 s using 1950 1990 data from the comprehensive ocean and atmosphere data set two major interannual oscillations are found in the equatorial sst and surface zonal wind fields u the main peak is centered at about 52 months we refer to it as the quasi quadrennial qq mode quasi biennial qb variability is split between two modes with periods near 28 months and 24 months a faster 15 month oscillation has smaller amplitude the qq mode dominates the variance and has the most distinct spectral peak in time longitude reconstructions of this mode the sst has the form of a standing oscillation in the eastern equatorial pacific while the u field is dominated by a standing oscillation pattern in the western pacific and exhibits also slight eastward propagation in the central and western pacific the locations of maximum anomalies in both qb modes are similar to those of the qq mode slight westward migration in sst across the eastern and central and eastward propagation of u across the western and central pacific are found the significant wind anomaly covers a smaller region than for the qq the qq and qb modes together represent the enso variability well and interfere constructively during major events the sharper definition of the qq spectral peak and its dominance are consistent with the devil s staircase interaction mechanism between the annual cycle and enso

on lagrangian residual currents with applications in south san francisco bay california the lagrangian residual circulation has often been introduced as the sum of the eulerian residual circulation and the stokes drift unfortunately this definition of the lagrangian residual circulation is conceptually incorrect because both the eulerian residual circulation and the stokes drift are eulerian variables in this paper a classification of various residual variables are reviewed and properly defined the lagrangian residual circulation is then studied by means of a two stage formulation of a computer model the tidal circulation is first computed in a conventional eulerian way and then the lagrangian residual circulation is determined by a method patterned after the method of markers and cells to demonstrate properties of the lagrangian residual circulation application of this approach in south san francisco bay california is considered with the aid of the model results properties of the eulerian and lagrangian residual circulation are examined it can be concluded that estimation of the lagrangian residual circulation from eulerian data may lead to unacceptable error particularly in a tidal estuary where the tidal excursion is of the same order of magnitude as the length scale of the basin a direction calculation of the lagrangian residual circulation must be made and has been shown to be feasible

the simplest enso recharge oscillator 1 eastern pacific sea surface temperature sst and mean equatorial pacific thermocline depth are key variables in el nino southern oscillation enso a linear fit to observations leads to a remarkably simple picture enso can be represented by a classical damped oscillator with sst and thermocline depth playing the roles of momentum and position respectively an independent fit of observed relationships between western and eastern thermocline depth central wind stress and eastern pacific sst yields the same picture and supports a recharge oscillator interpretation the oscillation arises from the interaction between the recharge time of the warm pool and the time delay between east and west pacific both finite kelvin wave speed and sst dynamics contribute to the time delay including seasonality in the description we find two periods of relative instability boreal spring with a large phase progression and autumn with nearly stationary phase

enso like interdecadal variability 1900 93 a number of recent studies have reported an enso like eof mode in the global sea surface temperature sst field whose time variability is marked by an abrupt change toward a warmer tropical eastern pacific and a colder extratropical central north pacific in 1976 77 the present study compares this pattern with the structure of the interannual variability associated with the enso cycle and documents its time history back to 1900 the analysis is primarily based on the leading eofs of the sst anomaly and anomaly deviation fields in various domains and the associated expansion coefficient or principal component time series which are used to construct global regression maps of sst sea level pressure slp and a number of related variables the use of anomaly deviations i e departures of local sst anomalies from the concurrent global mean sst anomaly reduces the influence of global mean sst trends upon the structure of the eofs and their expansion coefficient time series an important auxiliary time series used in this study is a southern oscillation index based on marine surface observations by means of several different analysis techniques the time variability of the leading eof of the global sst field is separated into two components one identified with the enso cycle related variability on the interannual timescale and the other a linearly independent residual comprising all the interdecadal variability in the record the two components exhibit rather similar spatial signatures in the global sst slp and wind stress fields the sst signature in the residual variability is less equatorially confined in the eastern pacific and it is relatively more prominent over the extratropical north pacific the corresponding slp signature is also stronger over the extratropical north pacific and its counterpart in the cold season 500 mb height field more closely resembles the pna pattern the amplitude time series of the enso like pattern in the residual variability reflects the above mentioned shift in 1976 77 as well as a number of other prominent features including a shift of opposite polarity during the 1940s

cenozoic continental climatic evolution of central europe continental climate evolution of central europe has been reconstructed quantitatively for the last 45 million years providing inferred data on mean annual temperature and precipitation and winter and summer temperatures although some regional effects occur the european cenozoic continental climate record correlates well with the global oxygen isotope record from marine environments during the last 45 million years continental cooling is especially pronounced for inferred winter temperatures but hardly observable from summer temperatures correspondingly cenozoic cooling in central europe is directly associated with an increase of seasonality in contrast inferred cenozoic mean annual precipitation remained relatively stable indicating the importance of latent heat transport throughout the cenozoic moreover our data support the concept that changes in atmospheric co2 concentrations although linked to climate changes were not the major driving force of cenozoic cooling

what triggers the transition of palaeoenvironmental patterns in china the tibetan plateau uplift or the paratethys sea retreat abstract geological research has illustrated the transition of palaeoenvironmental patterns by the earliest miocene from a planetary wind dominant type to a monsoon dominant type indicating that the east asian monsoon became markedly intensified and played a leading role in the east asian climate from a modeling point of view the pioneering research using the reduced number of scenarios had demonstrated that both the tibetan plateau uplift and the paratethys sea retreat were important for understanding the asian monsoon evolution however the sensitivity of the paratethys retreat to the east asian climate still needs further studies based on the more detailed scenarios thirty numerical experiments under the six paratethys sea and the five tibetan plateau conditions illustrate the shifts from zonal climate to the monsoon climate in east asia the results confirm again that both the paratethys retreat and the tibetan plateau uplift play important roles in the formation of the monsoon dominant environmental pattern and show that the paratethys retreat can strengthen the east asian monsoon and greatly increase humidity and aridity respectively in the monsoon areas and northwest china which is similar to the impact of the tibetan plateau uplift on the east asian climate furthermore the fact that the paratethys sea retreats to the turan plate is found to be the key criterion for the palaeoenvironmental patterns transition in china the shrinkage of paratethys sea leads to the reconstructions of the pressure system and the atmospheric circulations which result in the variations of precipitation and the transition of palaeoenvironmental patterns

the response of enso flavors to mid holocene climate implications for proxy interpretation the response of el nino southern oscillation enso to mid holocene boundary conditions remains an open question paleoclimate proxies and climate model simulations do not agree in the magnitude of the reduction of enso variability while recent proxy evidence from fossil corals from the central pacific show that the reduction in mid holocene enso variability compared to the end of the twentieth century is not different from the reduction during other holocene time intervals this is inconsistent with the interpretation of lake and ocean sediment records from the eastern pacific which show a significant reduction compared to all other holocene periods in order to reconcile the seemingly conflicting proxy evidence from the eastern and central pacific we hypothesize that enso remained active during the mid holocene however there was a change in the spatial pattern of the sea surface temperature anomalies also known as enso flavors using national center for atmospheric research s community climate system model version 4 forced with mid holocene orbital conditions we find that the frequency of occurrence of the strongest eastern pacific ep events decreases in the mid holocene and their variance is reduced by 30 while the frequency of central pacific cp events slightly increases and their variance does not change we also find a shift in the seasonality of ep events but not in that of cp events lastly mid holocene ep events develop more slowly and decay faster the differential response of enso flavors to mid holocene forcing is remotely forced by the west pacific where a weakening of the trade winds in early boreal spring in the mid holocene initiates an anomalous downwelling annual kelvin wave which reaches the eastern pacific during the enso development season weakens the upper ocean stratification and results in reduced enso upwelling feedback the simulated reduction in the ep flavor versus the cp flavor in the mid holocene is consistent with proxy evidence the teleconnection patterns of the two flavors with temperature precipitation and salinity are distinct and proxies from different regions of the pacific might be recording variability associated with only one of the flavors or some combination of their relative effects

nonmodal growth of atmospheric disturbances relevant to the east asian pressure surge in boreal winter significant events of east asian pressure surge are identified based on sea level pressure for boreal winters of the period 1979 2016 these events typically grow from a high pressure anomaly over northern europe and the northwestern russia and attain their peak over siberia around ten days after the initiation the structure of the pressure surge at its peak intensity mimics that of the semi permanent siberian high with a two level quasi geostrophic qg model a nonmodal instability analysis of the winter climatological flow reveals that one of the most rapidly amplifying optimal disturbances with an optimization time of 2 days resembles the structure of the east asian pressure surge this optimal disturbance originates from small scale precursor disturbances over northern europe and the barents sea such initial disturbances grow in magnitude via siberia towards east asia forming a low level ridge extending to around 50 n the optimal disturbance successfully captures the temporal and spatial scales and the overall structure of the circulation anomaly associated with the pressure surge albeit with varying degrees of accuracy of the location of individual trough ridge of the surge anomaly additional experiments suggest that 1 adding damping effect to the two level qg model extends the low level ridge further south to around 40 n and 2 the upper level wave train feature associated with pressure surge can be captured even in a barotropic model by optimal disturbances with an optimization time of 4 days

oligocene climate signals and forcings in eurasia revealed by plant macrofossil and modelling results abstract the oligocene represents a transitional time period from a warm climate to a cooler climate that is more representative of the modern day yet a general view of continental climate pattern and forcings are still lacking different proxies and models show striking disparities especially in mid high latitudes requiring validation of oligocene climate reconstruction in order to understand the large scale processes that drive the observed climate changes here we compiled 149 macrofossil floras in the mid high latitudes of eurasia then quantitatively reconstructed the oligocene climate using coexistence approach ca and combined previously published paleoclimate data during the oligocene eurasian mid high latitudes were mainly dominated by a humid subtropical climate mean annual temperature mat ranged between 5 4 c and 25 5 c with mean annual precipitation map ranging from 338 to 2453 mm three regions europe central eurasia and eastern asia indicate different climatic regimes with a generally warmer and wetter climate in europe and a colder and drier climate in central eurasia when compared to eastern asia no significant reorganization of climate was observed between the early and late oligocene the climate anomalies between the oligocene and present indicate that geographic changes e g retreat of the paratethys sea played an important role in shaping the climate pattern of eurasia by comparing the fossil data to a range of different hadcm3l model simulations of the oligocene with differing boundary conditions e g co2 and topography we demonstrate similar large scale climate spatial patterns between models and fossil data however models simulated much higher temperature seasonality lower simulated winter temperatures and higher simulated summer temperatures in eurasia mean annual temperature analysis indicates that simulations with 560 and 1120 ppmv co2 matched better with fossil data when compared to other simulations depending on the topography these results provide some constraints that should be considered for future paleoclimate modelling

deciphering the role of southern gateways and carbon dioxide on the onset of the antarctic circumpolar current 1 growth of antarctic ice sheet during the cenozoic 34 million years ago appears as a potential tipping point in the long term cooling trend that began 50 ma ago for decades the onset of the antarctic circumpolar current acc following the opening of the drake passage and of the tasman seaway has been suggested as the main driver of the continental scale antarctic glaciation however recent modeling works emphasized that the eocene oligocene atmospheric carbon dioxide co2 lowering could be the primary forcing of the antarctic glaciation questioning the acc theory here we investigate the response of the acc to changes in co2concentrations occurring from the late eocene to the late oligocene we used a fully coupled atmosphere ocean model foam with a mid oligocene geography we find that the opening of southern oceanic gateways does not trigger the onset of the acc for co2typical of the late eocene 840 ppm a cooler background climatic state such as the one prevalent at the end of the oligocene is required to simulate a well developed acc in this cold configuration the intensified sea ice development around antarctica and the resulting brine formation lead to a strong latitudinal density gradient in the southern ocean favoring the compensation of the ekman transport and consequently the acc our results imply that the acc has acted as a feedback rather than as a driver of the global cooling

the nonnormal nature of el nino and intraseasonal variability abstract the idea that intraseasonal variability in the tropical west pacific can act as an effective means of stochastically forcing enso episodes is explored using the ideas of generalized linear stability theory as they apply to nonnormal dynamical systems the physical attributes of the coupled ocean atmosphere system in the tropics that allow perturbations with structures that are dissimilar to enso to act as precursors for enso episodes are examined using a coupled ocean atmosphere model two particularly important factors are identified that contribute to the nonnormality of the coupled system nonsolar atmospheric heating directly related to sst changes and the dissimilarity between the equatorial ocean wave reflection process at eastern and western boundaries the latter is intrinsic to the dynamics of the ocean while the former is related to the presence of the west pacific warm pool and its relationship with the walker circulation