interfacial turbulent mixing in stratified magma reservoirs abstract we analyse the interfacial turbulent mixing of molten magmas in a two layer stratified convecting magma chamber we examine situations where convection is driven by the heat and or compositional flux associated with the melting of a solid boundary the model applies to the case in which the rayleigh number is sufficiently large that the effect of a viscosity difference on turbulent entrainment is negligible and so the model predictions represent an upper bound for interfacial mixing our results suggest that the extent of mixing induced by the thermal buoyancy flux is relatively small however the compositional buoyancy fluxes associated with phase change may be much larger and the mixing they induce may significantly affect the evolution of the reservoir our analysis of the melting of the floor of a two layer stratified magma chamber shows that turbulent mixing at the interface between the two magmas may suppress the melting and decrease the final amount of melt produced when the system reaches thermal equilibrium it is also shown that although floor melting tends to decrease the density of the lower layer of magma somewhat counterintuitively turbulent interfacial mixing with the overlying layer of smaller density may inhibit large scale overturn and preserve the stratified state of the chamber in the case in which both the floor and roof of a chamber melt turbulent mixing at the interface between the newly formed light melt at the roof and the lower layer of dense magma induces a shift of the melting process from the floor to the roof for typical parameter values in a basaltic magma chamber we estimate that under conditions of very large rayleigh number the amount of crustal melt produced at the roof may be as high as 50 of that produced at the floor

a sea ice climate switch mechanism for the 100 kyr glacial cycles a box model of the ocean atmosphere sea ice land ice climate system is used to study a novel mechanism for self sustained oscillations of the climate system on a time scale of 100 000 years without external forcing the oscillation in land ice volume has the familiar sawtooth shape of climate proxy records the most novel aspect of the climate oscillations analyzed here is the crucial role played by the sea ice the sea ice acts as a switch of the climate system switching it from a growing land glaciers mode to a retreating land glaciers mode and shaping the oscillation s sawtooth structure a simple explanation of the 100 kyr timescale is formulated on the basis of the mechanism seen in the model finally rapid sea ice changes such as those seen in our model and their drastic effects on the climate system may provide an explanation to some of the rapid climate changes observed to be a part of the variability at all timescales in the paleorecord

diapycnal mixing in a conceptual model of the atlantic meridional overturning circulation abstract a three box model of the atlantic ocean is used to examine the influence different parameterisations of the diapycnal mixing may have on the large scale dynamics of the ocean circulation special emphasis is given to the northward volume and heat transports the vertical diffusivity is taken in the general form n where n is the buoyancy frequency and the parameter space 0 3 is explored an imposed freshwater forcing of the northern high latitude box is used as a test case to investigate the behaviour of the model compared to similar types of sensitivity experiments carried out with for instance general circulation models four different solution states are identified separated by three critical values of for small values of both heat and volume fluxes decrease with increasing freshwater flux increasing leads first into a parameter domain where the transport of heat increases with increasing freshwater forcing then at even higher also the volume transport increases finally a fourth state is found where strong advection of warm water leads to an increase of the northern box temperature as opposed to cooling for small the behaviour of the critical values of is discussed with respect to model dynamics and parameters a subtle interplay between changes in the thermocline depth and the advective fluxes from the thermocline box into northern box which both are modulated by diapycnal mixing is found to be the key process in determining the behaviour of the model finally the implications for modelling and understanding of basic features of the ocean circulation are discussed

thermohaline circulation stability a box model study part ii coupled atmosphere ocean model abstract a thorough analysis of the stability of a coupled version of an interhemispheric three box model of thermohaline circulation thc is presented this study follows a similarly structured analysis of an uncoupled version of the same model presented in part i of this paper the model consists of a northern high latitude box a tropical box and a southern high latitude box which can be thought of as corresponding to the northern tropical and southern atlantic ocean respectively this paper examines how the strength of thc changes when the system undergoes forcings representing global warming conditions since a coupled model is used a direct representation of the radiative forcing is possible because the main atmospheric physical processes responsible for freshwater and heat fluxes are formulated separately each perturbation to the initial equilibrium is characterized by the total radiative forcing realized by the rate of increase and by the north south asymmetry although only weakly asymm

the stability of an evolving atlantic meridional overturning circulation 1 in this study we propose a generalized stability indicator l for a slowly evolving and quasi steady atlantic meridional overturning circulation amoc which represents a feedback related to the amoc and its associated freshwater transport within the atlantic basin as an improvement from previous indicators for the amoc in equilibrium this generalized indicator does not require a divergence free freshwater transport in the atlantic for a collapsed amoc which enables it to correctly monitor the amoc stability through the amoc hysteresis loop in the coupled atmosphere ocean general circulation models from the simulation the indicator l suggests that the amoc is in a stable regime with single equilibrium under the present day and the last glacial maximum lgm climates however under the present day climate a bering strait bs closure will diminish the freshwater outflow from the north atlantic into the arctic as the amoc collapses resulting in a freshwater convergence in the atlantic basin and making the amoc reside in a stable collapsed state i e the amoc exhibits characteristics of multiple equilibria further analysis shows that the bs effect is much reduced under the lgm climate this generalized indicator l has great implications for paleoclimate studies in understanding the abrupt climate change due to the instability of the amoc

instability of the thermohaline circulation with respect to mixed boundary conditions is it really a problem for realistic models abstract this comment discusses two issues raised by a recent study of the stability of the thermohaline circulation tziperman et al a numerical problem is pointed out that may have affected the results of the reported calculations in addition it is argued that the state transition of the model circulation found in the above study is triggered by a convective instability mechanism not the advective mechanism discussed by its authors should this be the case then the conclusion of tziperman et al that the ocean is close to a stability transition point must be questioned

evidence for planetary wave effects on midlatitude backscatter and sporadic e layer occurrence in this paper a large database of midlatitude e region coherent backscatter obtained with a 50 mhz doppler system is used to investigate the long term variability in echo occurrence the backscatter is found to be dominated by pronounced quasi periodic variations with periods in the range from about 2 to 9 days that persist for time intervals from about 10 to maybe more than 20 days and have no relation to geomagnetic activity the most commonly observed periods appear in two preferential bands that is the 2 to 3 day and the 4 to 6 day band using concurrent ionosonde data we find the variations in backscatter to be exactly in phase with similar periodicities in the occurrence of relatively strong sporadic e layers the present findings support the possibility that planetary waves are responsible for the observed long term periodicities which indicates also a close relation between planetary waves and the well known but not well understood seasonal es dependence we suggest the planetary wave option constitutes a new component into the research of midlatitude sporadic e layer formation and occurrence that needs to be considered and closely investigated

a wind induced thermohaline circulation hysteresis and millennial variability regimes abstract the multiple equilibria of the thermohaline circulation thc used here in the sense of the meridional overturning circulation as function of the surface freshwater flux has been studied intensively following a stommel paper from 1961 it is shown here that multistability and hysteresis of the thc also exist when the wind stress amplitude is varied as a control parameter both the massachusetts institute of technology ocean general circulation model mitgcm and a simple three box model are used to study and explain different dynamical regimes of the thc and thc variability as a function of the wind stress amplitude starting with active winds and a thermally dominant thermohaline circulation state the wind stress amplitude is slowly reduced to zero over a time period of 40 000 yr 40 kyr and then increased again to its initial value over another 40 kyr it is found that during the decreasing wind stress phase the thc remains thermally dominant until very low wind stress amplitude at which

evidence of a role for modulated atmospheric tides in the dependence of sporadic e layers on planetary waves 1 a large amplitude 7 day period westward propagating s 1 planetary wave pw of global response has been reported from ground radar and satellite wind measurements in the mesosphere lower thermosphere mlt during the second half of august and well into september 1993 following recent suggestions that pw might play a role in the formation of midlatitude sporadic e layers es haldoupis and pancheva 2002 found a strong 7 day periodicity present in all stations concurrently with the 7 day planetary wave reported elsewhere by analyzing sporadic e critical frequency foes time series from eight midlatitude ionosonde stations covering a longitudinal zone from about 58 e to 157 w this study provided the first direct proof in favor of a pw role on es formation in the present paper we further investigate this role by considering the same pw event and correlating the 7 day periodicity in foes directly with concurrent variations in the mesospheric neutral wind measured with atmospheric radars in saskatoon canada and in sheffield united kingdom although our analysis cannot exclude a direct pw role on es formation it shows clearly that es is affected indirectly by the pw through the action of the diurnal and semidiurnal tides which are strongly modulated by the same pw apparently through a nonlinear interaction process at altitudes below 100 km this 7 day pw modulation was found to be clearly present simultaneously in the amplitude of the zonal 12 hour tidal wind the meridional 24 hour tidal wind and in both the 12 hour and 24 hour periodicities which existed in the foes time series the results here provide a new physical explanation for the observed relation between sporadic e layers and planetary waves

effects of a wind driven gyre on thermohaline circulation variability a simplified model is used to study the possible effects of the horizontal upper ocean wind driven circulation wdc on the variability of the overturning meridional circulation driven by buoyancy fluxes it is found that the added interaction with the wdc adds interesting new classes of variability first self sustained variability of the thermohaline circulation thc becomes possible on time scales of interdecades to a few centuries furthermore these oscillations may be either small amplitude or large amplitude and either periodic or chaotic depending on the amplitude of the freshwater forcing and on the strength of the wdc even a relatively weak wdc changes the well known stability properties of the thc that are seen in numerous models of the thc alone the variability modes found here may account for similar modes of variability observed in gcm studies

vertical coupling from the lower atmosphere to the ionosphere observations inferred from indian mst radar gps radiosonde ionosonde magnetometer olr noaa and saber timed instrument over gadanki

a diagnostic indicator of the stability of the atlantic meridional overturning circulation in ccsm3 a diagnostic indicator dmov is proposed in this paper to monitor the stability of the atlantic meridional overturning circulation amoc the dmov is a diagnostic for a basinwide salt advection feedback and defined as the difference between the freshwater transport induced by the amoc across the southern border of the atlantic ocean and the overturning liquid freshwater transport from the arctic ocean to the north atlantic as validated in the community climate system model version 3 ccsm3 for an amoc in the conveyor state a positive dmov freshwater convergence in the atlantic basin indicates a monostable amoc and a negative dmov freshwater divergence indicates a bistable amoc based on dmov the authors investigate the amoc stability in the last glacial maximum lgm and analyze the modulation of the amoc stability by an open closed bering strait moreover the authors estimate that the real amoc is likely to be bistable in the present day since some observations suggest a negative dmov freshwater divergence is currently in the atlantic basin however this estimation is very sensitive to the choice of the observational data

quasi periodic radar echoes from midlatitude sporadic e and role of the 5 day planetary wave using measurements of magnetic aspect sensitive radar echoes from midlatitude sporadic e collected over a two month period from tanegashima japan we show that while their occurrence duration from night to night did not exhibit any systematic variation that of the so called quasi periodic qp echoes varied sinusoidally with a period of 5 days we have interpreted this behavior in terms of effects produced by a planetary wave and identified its presence through neutral wind measurements made with a partial reflection drift radar located nearby at yamagawa we propose that the occurrence of qp echoes is affected both by a contribution of the wind to the dynamo electric field and by the direction of the neutral wind we argue that because the wind vector of the planetary wave is elliptically polarized at midlatitudes a preferred wind direction conducive to the generation of qp echoes occurs once every 5 days on the other hand this wave is linearly polarized and directed zonally over the geographic equator the fact that qp echoes are most fully developed at midlatitudes and less so at lower latitudes suggests that zonal flow is not particularly favorable for qp echo production

simulated decadal oscillations of the atlantic meridional overturning circulation in a cold climate state the significance of the atlantic meridional overturning circulation moc for regional and hemispheric climate change requires a complete understanding using fully coupled climate models here we present a persistent decadal oscillation in a coupled atmosphere ocean general circulation model while the present study is limited by the lack of comparisons with paleo proxy records the purpose is to reveal a new theoretically interesting solution found in the fully coupled climate model the model exhibits two multi century long stable states with one dominated by decadal moc oscillations the oscillations involve an interaction between anomalous advective transport of salt and surface density in the north atlantic subpolar gyre their time scale is fundamentally determined by the advection in addition there is a link between the moc oscillations and north atlantic oscillation nao like sea level pressure anomalies the analysis suggests an interaction between the nao and an anomalous subpolar gyre circulation in which sea ice near and south of the labrador sea plays an important role in generating a large local thermal anomaly and a meridional temperature gradient the latter induces a positive feedback via synoptic eddy activity in the atmosphere in addition the oscillation only appears when the nordic sea is completely covered by sea ice in winter and deep convection is active only near the irminger sea such conditions are provided by a substantially colder north atlantic climate than today

oceanic responses to gradual transitions of equator to pole temperature gradients summary responses of an ocean model to gradual transitions of equator to pole gradients show that several quasiequilibrium states within the thermally driven regime are generated under identical forcing conditions these different states are associated with different patterns the location and the penetration depth of oceanic convection small changes in the thermal forcing conditions can push the system into an oscillatory regime once in this regime oscillations appear to proceed as internal processes one important implication of these results is that the presentday ocean climate may not recover fully if the atmospheric coz level after doubling is lowered to the present day level

on the role of topography and wind stress on the stability of the thermohaline circulation a frictional geostrophic model is used to examine how the stability of the thermohaline circulation is affected by idealized topographic variations and the presence or absence of wind stress if the flow exhibits collapses the authors consider how topography and wind stress affect the ensuing oscillations large scale slope up toward the north or the west can significantly destabilize the circulation by modifying the barotropic flow and reducing the depth of convection wind stress stabilizes the circulation by deepening the thermocline in the subtropical gyre wind driving can also radically reduce the period of oscillations by destabilizing the northern halocline in the collapsed phase the overall period of the oscillation is usually governed by the time taken for diffusive warming to destabilize the deep ocean

measurements and modelling of intermediate descending and sporadic layers in the lower ionosphere results and implications for global scale ionospheric thermospheric studies we demonstrate that conventional ionosondes can provide long term observations of intermediate descending and transitional layers in the 100 200 km altitude region of the ionosphere using 15 consecutive days of observations at townsville australia during the sundial campaign of september 1989 we tracked the birth of the layers at altitudes above 150 km and their systematic downward motion to the 110 km region at rates between 4 and 5 km hr the observations are compared with ncar tigcm simulations and the results show i that the layering process is identifiable with meridional wind shear node convergence of ions ii that zonal wind controls of the layers are insignificant under the prevailing conditions and iii that electric fields play an important role in the effectiveness of the ion convergence and downward transport processes at altitudes above 125 km the measurement and modelling comparisons are the first of a kind providing insight into the relative roles of winds and electric fields and opening possibilities for determining the global characteristics of the layers and their cause effect roles in the dynamics of the lower ionospheric thermospheric domain

a 6 5 day westward propagating planetary wave origin and characteristics a large 10 30 m s 1 6 7 day period westward propagating planetary wave was seen in both ground radar and satellite data within the 80 100 km height regime during august and september 1993 previous analyses of these data argue that it was most likely a manifestation of the 1 2 rossby planetary wave commonly referred to as the 5 day wave perhaps shifted to longer periods by interaction with the mean zonal winds in this study the global scale wave model gswm hagan et al 1993 is used to analyze the characteristics of this westward propagating wave we propose that the observed wave is not a doppler shifted 5 day wave but rather an unstable mode drawing energy from unstable regions in the upper mesosphere whose realization is global in scale the concept of atmospheric instability is used to determine a likely source region and the model is used determine the characteristic period growth rate and perturbation fields of the instability driven wave

multiple oscillatory modes of the global ocean circulation for the first time the linear stability of the global ocean circulation has been determined explicitly in a low resolution general circulation model a steady state is computed directly by solving the elliptic boundary value problem the stability of this solution is determined by solving the generalized eigenvalue problem although the steady global circulation is linearly stable there are two interesting oscillatory modes among the least stable ones with periods of about 3800 and 2300 yr these modes are characterized by buoyancy anomalies that propagate through the ocean basins as they are advected by the global overturning circulation the millennial timescale is set by the time it takes for anomalies to travel at depth from the north atlantic to the north pacific further analyses confirm that the advective feedback between the steady flow and buoyancy anomalies is an essential process in the propagation mechanism the growth rate of the millennial modes is controlled by vertical mixing it is argued that these internal ocean modes may be a relevant mechanism for global climate variability on millennial timescales

signatures of ultra fast kelvin waves in the equatorial middle atmosphere and ionosphere 1 in the equatorial atmosphere oscillations with periods of 3 to 4 days have been observed in the meteor radar zonal wind at cariri 7 4 s 36 5 w in the ionospheric minimum virtual height h f and the maximum critical frequency fof2 at fortaleza 3 9 s 38 4 w and in the timed saber satellite temperature data in the stratosphere mesosphere wavelet analyses of these time series reveal that the 3 4 day oscillation was observed for all of these data during the period from march 1 to 11 2005 from the characteristics of the downward phase propagation wavelength of 40 km longitudinal and latitudinal extension we conclude that this oscillation must be a 3 5 day ultra fast kelvin ufk wave this is the first report of clear evidence of propagation of a ufk wave from the stratosphere to the ionosphere the ufk wave could have an important role in the day to day variability of the equatorial ionosphere evening uplift

deep water renewal by turbidity currents in the sulu sea the ventilation of the intermediate and deep layers of the ocean is governed by convection dense water formed at the sea surface through air sea interaction processes mainly at high latitudes sinks to great depths convection determines the heat salt and dissolved gas budgets of the ocean s interior and therefore exerts a significant influence on the earth s climate convective processes can also be initiated by the flow of dense water over submarine sills separating ocean basins with water masses of different buoyancy here we report the observation of relatively light deep water in the overflow region near the sill in the northern part of the 5 000 m deep sulu sea because of its buoyancy this water could not have reached the sea floor of its own accord turbidity currents induced by changes in water density that result from sediment influx occur in this region at intervals of several decades as indicated by the frequent deposition of graded sequences of silty mud on the abyssal plain we suggest that such turbidity currents combined with the effects of classical hydrological plume convection are capable of bringing light water to large depths in tropical regions where normal oceanic convection is relatively weak this mechanism may contribute significantly to the ventilation of deep basins

quantifying uncertainty in model predictions for the pliocene plio qump initial results examination of the mid pliocene warm period mpwp 3 3 to 3 0 ma bp provides an excellent opportunity to test the ability of climate models to reproduce warm climate states thereby assessing our confidence in model predictions to do this it is necessary to relate the uncertainty in model simulations of mpwp climate to uncertainties in projections of future climate change the uncertainties introduced by the model can be estimated through the use of a perturbed physics ensemble ppe developing on the uk met office quantifying uncertainty in model predictions qump project this paper presents the results from an initial investigation using the end members of a ppe in a fully coupled atmosphere ocean model hadcm3 running with appropriate mpwp boundary conditions prior work has shown that the unperturbed version of hadcm3 may underestimate mpwp sea surface temperatures at higher latitudes initial results indicate that neither the low sensitivity nor the high sensitivity simulations produce unequivocally improved mpwp climatology relative to the standard whilst the high sensitivity simulation was able to reconcile up to 6 c of the data model mismatch in sea surface temperatures in the high latitudes of the northern hemisphere relative to the standard simulation it did not produce a better prediction of global vegetation than the standard simulation overall the low sensitivity simulation was degraded compared to the standard and high sensitivity simulations in all aspects of the data model comparison the results have shown that a ppe has the potential to explore weaknesses in mpwp modelling simulations which have been identified by geological proxies but that a best fit simulation will more likely come from a full ensemble in which simulations that contain the strengths of the two end member simulations shown here are combined

terdiurnal tidelike variability in sporadic e layers 1 time series analysis was performed on a large database of ionosonde recordings taken over 12 years for the 6 month period from the beginning of may to the end of october when sporadic e layer occurrence is frequent the results show that in addition to the well known 24 and 12 hour tidal variations there is also a weaker but regular 8 hour periodicity in midlatitude sporadic e layers es statistically this terdiurnal periodicity is significant and occurs in both the sporadic e layer critical frequency foes and the layer virtual height h es becoming strongest around summer solstice and later in october after a minimum in september at times a weak 6 hour periodicity can also be present in es but its effects are much less significant than those of the 8 hour oscillation the 8 hour periodicity in es is attributed to the terdiurnal tide presumably acting through its vertical wind shear forcing of the metallic ions in the lower thermosphere the relation to the terdiurnal tide was inferred from simultaneous ionosonde recordings from three widely spaced stations in the same midlatitude zone showing the 8 hour oscillation to be present simultaneously in all ionosonde stations and have phases which are consistent with a westward propagating terdiurnal wave of zonal wave number 3 the present study establishes the regular occurrence of a terdiurnal tidelike oscillation in es which needs to be incorporated into the physics and modeling of sporadic e layer formation and dynamics

simple theoretical model may explain apparent climate instability abstract the authors propose a simple theory to explain a climatic drift previously found in a series of coupled general circulation model gcm experiments their theory places the gcm results on a simple stability diagram for the thermohaline circulation in the atlantic it is argued that the drift is due to stommel s salt advection feedback and arises because the particular spinup procedure of the coupled model has in some cases resulted in an initial state on an unstable solution branch such drift may result from model initialization with unsuitable relaxation boundary conditions rather than being a climatic transition that could occur in the real world

erosion of a stable density gradient by sedimentation driven convection stable density gradients are common in the world s oceans and lakes and play a crucial part in their physical chemical and biological evolution it has recently been suggested1 that turbidity currents provide an indirect mechanism for eroding this stratification in these currents light ambient fluid that has been mixed by cyclones with dense suspended sediment is transported to greater depths where it convectively erodes the gradient as the sediment settles here i present experimental results which show that in these circumstances the density gradient is eroded in a number of cycles of decreasing intensity and increasing duration in each cycle some of the sediment settles to the base of a stagnant region of unimpeded sedimentation while the rest is mixed into an overlying region that is vigorously convecting my experimental observations agree with a simple physical model that predicts the rate and extent of vertical mixing as well as the variation of particle sizes in the resulting turbidite

coherent resonant millennial scale climate oscillations triggered by massive meltwater pulses the role of mean and stochastic freshwater forcing on the generation of millennial scale climate variability in the north atlantic is studied using a low order coupled atmosphere ocean sea ice model it is shown that millennial scale oscillations can be excited stochastically when the north atlantic ocean is fresh enough this finding is used in order to interpret the aftermath of massive iceberg surges heinrich events in the glacial north atlantic which are characterized by an excitation of dansgaard oeschger events based on model results it is hypothesized that heinrich events trigger dansgaard oeschger cycles and that furthermore the occurrence of heinrich events is dependent on the accumulated climatic effect of a series of dansgaard oeschger events this scenario leads to a coupled ocean ice sheet oscillation that shares many similarities with the bond cycle further sensitivity experiments reveal that the timescale of the oscillations can be decomposed into stochastic linear and nonlinear deterministic components a schematic bifurcation diagram is used to compare theoretical results with paleoclimatic data

reorganization of miocene deep water circulation in response to the shoaling of the central american seaway 1 the response of ocean circulation to the shoaling of the central american seaway cas is investigated using the massachusetts institute of technology mit ocean general circulation model ogcm in contrast to earlier model studies it is found that significant amounts of deep water are formed in the north atlantic prior to the closure of the cas however the circulation pattern is fundamentally different from the modern ocean in the upper layers of the cas there is a relatively strong geostrophic flow from the pacific to the atlantic controlled by the pressure difference across the seaway however when the cas is deeper than the level of north atlantic deep water nadw outflow a significant amount of nadw passes through the cas to the pacific ocean in the pacific the deep water traverses the basin from east to west in a relatively narrow zonal jet and becomes a southward flowing western boundary current before it joins with the antarctic circumpolar current acc to the south this implies that deep sea sediment records from the miocene pacific ocean could have been influenced by relatively young nadw and provides a new framework for the interpretation of geochemical tracer data diversification of benthic foraminifer fauna suggests that the cas shoaled to a depth of about 1000 m toward the end of the middle miocene this would have prevented the passage of nadw to the pacific and established the modern deep water circulation pattern at that time

frontogenesis in the north pacific oceanic frontal zones a numerical simulation abstract a primitive equation model geophysical fluid dynamics laboratory s gfdl s mom 2 with one degree horizontal resolution is used to simulate the seasonal cycle of frontogenesis in the subarctic frontal zone safz and the subtropical frontal zone stfz of the north pacific ocean the safz in the model contains deep greater than 500 m in some places regions with seasonally varying high gradients in temperature and salinity the gradients generally weaken toward the east the stfz consists of a relatively shallow less than 200 m in most places region of high gradient in temperature that disappears in the summer fall the high gradient in salinity in the stfz maintains its strength year round and extends across almost the entire basin the model simulates the location and intensity of the frontal zones in good agreement with climatological observations generally to within two degrees of latitude and usually at the same or slightly stronger intensity the seasonal cycle of the frontal zones al

glacial abrupt climate changes and dansgaard oeschger oscillations in a coupled climate model 1 there are three fundamental features which characterize large glacial millennial dansgaard oeschger oscillations 1 the climatic transitions were abrupt and large 2 the lengths of both interstadials and stadials and the period of dansgaard oeschger oscillations were not uniform and 3 there were no large millennial oscillations during an early stage of a glacial period and a peak glacial period in this modeling study we offer a consistent explanation for these three features by employing an earth system model of intermediate complexity we demonstrate that a moderate global cooling forces the atlantic meridional overturning circulation moc into an unstable state and hence causes the flip flop of the atlantic moc between a strong mode and a weak mode the durations of both interstadials and stadials associated with these millennial oscillations are modulated by the changing background climate in qualitative agreement with the observations in a warm climate the atlantic moc is strong and stable with the deep water formed mainly by intense heat loss to the atmosphere in a cold climate the atlantic moc is weak and stable and this mode is largely maintained by the process of sea ice brine rejection since the dansgaard oeschger oscillations result from an alternation between these two moc states during an intermediate phase climate we conclude that brine rejection plays a necessary role in the oscillations confirming a hypothesis suggested in some proxy data studies

proximity of the present day thermohaline circulation to an instability threshold the relation between the mean state of the thermohaline circulation thc and its stability is examined using a realistic geometry primitive equation coupled ocean atmosphere ice global general circulation model the main finding is that a thermohaline circulation that is 25 weaker and less dominated by thermal forcing than that of today s ocean is unstable within this coupled gcm unstable initial ocean climates lead in the coupled model to an increase of the thc to strong oscillations or to a thc collapse the existence of an unstable range of weak states of the thc provides a natural explanation for largeamplitude thc variability seen in the paleo record prior to the past 10 000 years a weakening of the thc due to an external forcing e g ice melting and freshening of the north atlantic may push it into the unstable regime once in this regime the thc strongly oscillates due to the inherent instability of a weak thc hence the strong thc variability in this scenario does not result from switches between two or more quasi stable steady states

planetary waves and midlatitude sporadic e layers strong experimental evidence for a close relationship 1 a large amplitude 7 day period westward propagating s 1 planetary wave has been reported from ground radar and satellite wind measurements in the mesosphere lower thermosphere mlt during the second half of august and well into september 1993 following recent suggestions that planetary waves might play a role in the formation of midlatitude sporadic e layers es we have obtained and analyzed for the period from august 1 to september 30 1993 the sporadic e critical frequency foes time series from eight midlatitude ionosonde stations covering a large longitudinal zone from 58 e to 157 w the analysis revealed that all eight station foes data showed a strong 7 day periodicity occurring concurrently with the 7 day planetary wave reported elsewhere using independent methods for the analysis of the foes time series we computed identical estimates for the propagation direction zonal wave number and phase velocity of the 7 day wave which are in agreement with those reported from radar and satellite neutral wind mlt measurements the present findings provide the first direct evidence proving that planetary waves play an important role in the physics of midlatitude sporadic e region layers in addition our results include an important implication that the es parameters measured routinely and rather reliably with a dense global network of digital ionosondes as well as the enormous ionogram databases existing in world data centers may be used as an alternative means of studying large scale neutral atmospheric dynamics in the mlt region

the spatial and temporal structure of twin peaks and midday bite out in fof2 with associated height changes in the australian and south pacific low midlatitude ionosphere morning and afternoon peaks in daytime critical frequency fof2 defining a midday bite out were found to occur regularly across northern australia during april 2008 this behavior was sufficiently repetitive to appear in the monthly median values of fof2 the twin peak and bite out phenomenon was observable across the entire longitudinal range of ionosondes which were accessible for this study from niue in mid pacific to learmonth on the western side of australia the high geographic density of ionosondes operating in australia enabled the limited latitudinal range of occurrence to be established the phenomenon ceasing to be present in the southern half of australia or at equatorial latitudes while strongest in 2008 the fof2 bite out and associated variation in hmf2 the peak height of the ionosphere continued to be seen in monthly april medians from sunspot minimum 2007 2008 to the current low sunspot maximum 2011 2013 while diminishing in magnitude this phenomenon was also present around the september equinox though not of such magnitude or consistency as during the april equinox the morning and afternoon peaks in fof2 occurred during periods of falling virtual and true height and were associated with the maximum compression of the ionosphere at this time as measured by the subpeak equivalent parabolic layer thickness ym meridional winds in the f2 layer are suggested as a driver of the twin peak enhancements in fof2 which may possibly be related to a simultaneous occurrence of strong tidal influences as seen to be present in descending sporadic e

rapid switch like sea ice growth and land ice sea ice hysteresis 1 rapid and extensive growth of sea ice cover was suggested to play a major role in the sea ice switch mechanism for the glacial cycles as well as on shorter millennial scales gildor and tziperman 2000 this mechanism also predicts a hysteresis between sea ice and land ice such that land ice grows when sea ice cover is small and withdraws when sea ice cover is more extensive the switch like sea ice growth and the hysteresis were previously demonstrated using a simple highly idealized box model in this work we demonstrate a switch like sea ice behavior as well as the sea ice land ice hysteresis using a coupled climate model that is continuous in the latitudinal dimension it is shown that the switch like sea ice growth occurs when the initial meridional atmospheric temperature gradient is not too strong it is also shown that the meridional extent to which sea ice grows in a switch like manner is not affected by the intensity of the thermohaline circulation which does however influence the climate cooling that is needed to trigger such rapid sea ice growth

the stability of thermohaline circulation in a two box model abstract in stommel s simple two box model which has provided an insight on the thermohaline circulation and climate instability mechanisms a linear mass transport was used however a scaling law based on geostrophy and advective diffusive heat balance suggests a nonlinear mass transport relation for the oceans by including this nonlinear mass transport relation to stommel s box model it is possible to study the effects of the thermocline which was not considered before on the stability of the thermohaline circulation while keeping the simplicity of stommel s box model the results were compared with those obtained with the traditional model using a linear mass transport relation the thermal mode circulation of the nonlinear model is significantly more stable than that of the linear model suggesting the thermohaline catastrophe is less likely to occur in the present north atlantic if the thermocline is considered in the nonlinear model the circulation removes density anomalies rapidly so that s

experiments on gas ash separation processes in volcanic umbrella plumes abstract we present a series of analogue laboratory experiments which simulate the separation of ash and gas and the formation of secondary intrusions from finite volcanic umbrella plumes we examined the lateral spreading of mixtures of freshwater and particles released into a laboratory tank containing a uniformly stratified aqueous solution for times smaller than the sedimentation time of particles through the intrusion the current remains coherent and intrudes laterally as some of the particles settle into the underlying ambient fluid a layer of particle depleted fluid develops below the upper surface of the current and the density of the residual fluid is reduced over longer times the intrusion ceases to be coherent with small fingers of relatively buoyant particle depleted fluid rising from the upper part of the intrusion into the overlying fluid meanwhile the lateral motion of the injected solution induces a return flow in the ambient fluid which sweeps some of the particles sedimenting from the lower surface of the intrusion inwards as a result relatively dense particle laden fluid collects below the intrusion and then sinks into the underlying fluid eventually this fluid reaches a new neutral buoyancy height where it intrudes to form a second laterally spreading current below the original intrusion the process then repeats to form further weaker intrusions below these results of the separation of the ash and volcanic gas in an umbrella plume are consistent with field observations at sakurajima volcano where positively charged plumes thought to consist of volcanic gas have been observed above negatively charged plumes of ash this work also suggests that volcanic aerosols may form up to a kilometer above the original injection height of the ash in a strong wind shear this could result in very different trajectories of the ash and gas and so be important for evaluating the impact of ash plumes on both aviation safety and volcanic aerosol formation the separation of the ash from the umbrella plume after a time of order 10 3 10 4 s corroborates recent models of the may 18 1980 eruption at mount st helens that after a few hours the motion of the ash was simply controlled by wind dispersal and gravitational settling the complex motion of the sedimenting ash may also explain some of the observations of multiple intrusions in short lived umbrella plumes such as observed at redoubt alaska in april 1990

the intrusion of fluid mechanics into geology during the last few years a number of fluid dynamicists often in collaboration with geologists have been investigating problems suggested by geology and creating the new field of geological fluid mechanics the research has uncovered many new fluid mechanical phenomena and the studies have ranged widely over theoretical contributions laboratory experiments and confirmatory field observations this paper presents an overview of part of the field with a strong emphasis on the novel fluid mechanical concepts the presentation includes discussion of some of the effects of cooling and crystallization and the resulting convective motions the flow of gravity currents at both high and low reynolds number and the erosion of solid retaining walls by hot turbulent flows

imperfections of the thermohaline circulation latitudinal asymmetry and preferred northern sinking abstract the present atlantic thermohaline circulation is dominated by deep water formation in the north despite the fact that surface buoyancy forcing has relatively modest latitudinal asymmetry many studies have shown that even with buoyancy forcing that is symmetric about the equator spontaneous symmetry breaking can produce a single overturning cell with intense sinking in the north this occurs by salt advection at sufficiently large freshwater forcing in this symmetry breaking case a southern sinking solution and a symmetric solution are also possible a simple coupled ocean atmosphere model of the zonally averaged thermohaline circulation is used to examine the effect of latitudinal asymmetries in the boundary conditions the greater continental area in the northern hemisphere combined with the slight asymmetry in the observed freshwater flux induce a strong preference for the northern sinking solution examining the relation to the solution under symmetric conditions the salt advection mech

a predictability study of simulated north atlantic multidecadal variability the north atlantic is one of the few places on the globe where the atmosphere is linked to the deep ocean through air sea interaction while the internal variability of the atmosphere by itself is only predictable over a period of one to two weeks climate variations are potentially predictable for much longer periods of months or even years because of coupling with the ocean this work presents details from the first study to quantify the predictability for simulated multidecadal climate variability over the north atlantic the model used for this purpose is the gfdl coupled ocean atmosphere climate model used extensively for studies of global warming and natural climate variability this model contains fluctuations of the north atlantic and high latitude oceanic circulation with variability concentrated in the 40 60 year range oceanic predictability is quantified through analysis of the time dependent behavior of large scale empirical orthogonal function eof patterns for the meridional stream function dynamic topography 170 m temperature surface temperature and surface salinity the results indicate that predictability in the north atlantic depends on three main physical mechanisms the first involves the oceanic deep convection in the subpolar region which acts to integrate atmospheric fluctuations thus providing for a red noise oceanic response as elaborated by hasselmann the second involves the large scale dynamics of the thermohaline circulation which can cause the oceanic variations to have an oscillatory character on the multidecadal time scale the third involves nonlocal effects on the north atlantic arising from periodic anomalous fresh water transport advecting southward from the polar regions in the east greenland current when the multidecadal oscillatory variations of the thermohaline circulation are active the first and second eof patterns for the north atlantic dynamic topography have predictability time scales on the order of 10 20 y whereas eof 1 of sst has predictability time scales of 5 7 y when the thermohaline variability has weak multidecadal power the hasselmann mechanism is dominant and the predictability is reduced by at least a factor of two when the third mechanism is in an extreme phase the north atlantic dynamic topography patterns realize a 10 20 year predictability time scale additional analysis of sst in the greenland sea in a region associated with the southward propagating fresh water anomalies indicates the potential for decadal scale predictability for this high latitude region as well the model calculations also allow insight into regional variations of predictability which might be useful information for the design of a monitoring system for the north atlantic predictability appears to break down most rapidly in regions of active convection in the high latitude regions of the north atlantic

the dynamic ionosphere over arecibo a theoretical investigation numerical simulations of the f region ionosphere over arecibo puerto rico are presented we deduce from these simulations that the dynamic behavior of the arecibo ionosphere particularly the so called midnight collapse phenomenon is strongly influenced by the upward propagating semidiurnal tidal component of the neutral meridional wind excited below thermospheric levels presence of a terdiurnal component in the wind field improves the agreement with experiment and could result from nonlinear interactive coupling between the neutral wind and ion drag the steep underside density gradients occurring in conjunction with the collapse are due to vertical shear in the meridional wind field associated with the propagating semidiurnal tide these gradients are capable of triggering the gradient drift instability thus accounting for plasma irregularity formation and observations of vhf scintillations associated with the collapse phenomenon

observations of the 5 day wave in the mesosphere and lower thermosphere the 5 day planetary wave has been detected in the winds measured by the high resolution doppler imager hrdi on the upper atmosphere research satellite uars in the mesosphere and lower thermosphere 50 110 kin the appearances of the 5 day wave are transient with a lifetime of 10 20 days in the two year data set the structures of selected 5 day wave events are in generally good agreement with the 1 1 rossby normal mode for both zonal and meridional components a climatology of the 5 day wave is presented for an altitude of 95 km and latitudes mainly between 40os and 40on

the different nature of the interdecadal variability of the thermohaline circulation under mixed and flux boundary conditions the differences between the interdecadal variability under mixed and constant flux boundary conditions are investigated using a coarse resolution ocean model in an idealized flat bottom single hemisphere basin objective features are determined that allow one type of oscillation to be distinguished versus the other first by performing a linear stability analysis of the steady state obtained under restoring boundary conditions it is shown that the interdecadal variability under constant flux and mixed boundary conditions arises respectively from the instability of a linear mode around the mean stratification and circulation and from departure from the initial state based on the budgets of density variance it is shown next that the two types of oscillations have different energy sources under the constant flux boundary condition the thermal mode the downgradient meridional eddy heat flux in the western boundary current regions sustains interdecadal variability whereas under mixed boundary conditions the salinity mode a positive feedback between convective adjustment and restoring surface heat flux is at the heart of the existence of the decadal oscillation furthermore the positive correlations between temperature and salinity anomalies in the forcing layer are shown to dominate the forcing of density variance in addition the vertical structure of perturbations reveals vertical phase lags at different depths in all tracer fields under constant flux while under mixed boundary conditions only the temperature anomalies show a strong dipolar structure the authors propose that these differences will allow one to identify which type of oscillation if any is at play in the more exhaustive climate models

on convection and mixing driven by sedimentation the sedimentation of small particles from a suspension and the concomitant release of light interstitial fluid may constitute a buoyancy source for the development of convective motions when the dense suspension is emplaced beneath a stratified fluid an intermediate convecting layer between the sedimenting front and the density gradient above gradually grows in depth by erosion of the overlying stratified fluid novel laboratory experiments involving sedimentation below a two layer stratified region show that turbulent mixing and entrainment across the top density interface is significant for a broad range of the richardson number a simple theoretical model predicting the rate of erosion of the stratification above the convecting layer agrees well with these experiments the model is then extended to include the case of an overlying continuous density gradient and compared successfully with both new experimental data and the original data of kerr 1991 owing to the effects of dispersion of grain sizes small particles in the convecting fluid may lower the efficiency of the interfacial mixing by the turbulent eddies our model calculations suggest that turbulent mixing and entrainment driven by sedimentation may be significant in the atmospheric and oceanic contexts in both of which stratification is weak such mixing may also occur in molten magma chambers following the sedimentation of suspended crystals and in this case it may suppress large scale overturning events

a triple sea ice state mechanism for the abrupt warming and synchronous ice sheet collapses during heinrich events 1 abrupt switch like changes in sea ice cover are proposed as a mechanism for the large amplitude abrupt warming that seemed to have occurred after each heinrich event sea ice changes are also used to explain the colder than ambient glacial conditions around the time of the glacier discharge the abrupt warming events occur in this mechanism owing to rapid sea ice melting which warmed the atmosphere via the strong sea ice albedo and insulating feedbacks such abrupt sea ice changes can also account for the warming observed during dansgaard oeschger events the sea ice changes are caused by a weak order of 5 sv response of the thermohaline circulation thc to glacier discharges the main point of this work is therefore that sea ice may be thought of as a very effective amplifier of a weak thc variability explaining the abrupt temperature changes over greenland synchronous ice sheet collapses from different ice sheets around the north atlantic indicated by some proxy records are shown to be possible via the weak coupling between the different ice sheets by the atmospheric temperature changes caused by the sea ice changes this weak coupling can lead to a nonlinear phase locking of the different ice sheets which therefore discharge synchronously it is shown that the phase locking may also lead to precursor glacier discharge events from smaller ice sheets before the laurentide ice sheet discharges the precursor events in this mechanism are the result rather than the cause of the major glacier discharges from the laurentide ice sheet

sea ice switches and abrupt climate change we propose that past abrupt climate changes were probably a result of rapid and extensive variations in sea ice cover we explain why this seems a perhaps more likely explanation than a purely thermohaline circulation mechanism we emphasize that because of the significant influence of sea ice on the climate system it seems that high priority should be given to developing ways for reconstructing high resolution in space and time sea ice extent for past climate change events if proxy data can confirm that sea ice was indeed the major player in past abrupt climate change events it seems less likely that such dramatic abrupt changes will occur due to global warming when extensive sea ice cover will not be present

effects of glacial meltwater in the giss coupled atmosphereocean model 1 north atlantic deep water response varying magnitudes of freshwater input through the st lawrence are added to different versions of the giss coupled atmosphere ocean model the results show a generally linear response in percentage north atlantic deep water nadw reduction to the volume of water added without any obvious threshold effects when the estimated best guess freshwater input for the allerod younger dryas interval is added only small reductions in nadw production occur in less than a century with complete cessation requiring more than 300 years if the ocean circulation was weaker in the allerod preceding the younger dryas somewhat larger percentage reductions occur and it would take 150 200 years for a complete shutdown the freshening is aided by a positive feedback associated with the reduction in evaporation relative to precipitation in the north atlantic amounting to some 20 30 of the added freshwater runoff when the freshwater input is stopped after complete nadw shutdown nadw production does not resume with complete nadw cessation cooling eventually occurs throughout the northern hemisphere including regions where it is not thought to have arisen during the younger dryas and the globe cools by 1 c due primarily to the sea ice albedo feedback while in many regions the observed cooling is reproduced the cooling in greenland is much less than is estimated with only 50 nadw cessation cooling is primarily around the north atlantic no rapid warming is seen in response to freshwater cessation the sensitivity displayed here for nadw reduction due to freshwater input from the st lawrence is roughly similar to that associated with increased precipitation accompanying global warming in the next century so a similar sensitivity may arise with younger dryas freshwater inputs from other suspected locations greenland europe

an upper e region quarterdiurnal tide at arecibo studies of the time height trajectories of ionosphere e region layers at arecibo have revealed not only the usual diurnal sporadic e and semidiurnal intermediate layer structures but also that on occasion extra layers form and descend through the e region in particular during january 1981 four layers per day were observed to descend through the upper e region this four per day layer system was remarkably coherent over the 3 days of observation and suggests the existence of a 6 hour period or quarterdiurnal tidal wind system in the upper e region numerical simulations of ion trajectories in a realistic model atmosphere with diurnal semidiurnal and quarterdiurnal tidal wind systems of appropriate amplitude and phase structures confirm that the presence of a quarterdiurnal tide is sufficient to explain the observations

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

a study of tidal and planetary wave periodicities present in midlatitude sporadic e layers the diurnal and semidiurnal atmospheric tides are known to be of fundamental importance in the formation of midlatitude sporadic e layers acting through their vertical windshear forcing of the long living metallic ions in the lower thermosphere also recent studies suggested that planetary waves play a role on sporadic e generation as well a fact that went unnoticed in the long going research of sporadic layers in this paper a methodology is employed to investigate the tidal and planetary wave periodicities imprinted onto sporadic e critical frequencies foes in this approach standard analysis techniques used in neutral atmospheric dynamics are applied on foes time series obtained during summertime when sporadic e occurrence is nearly continuous it is shown that besides the dominant and known 24 hour and 12 hour tidal periodicities in foes there is often a weaker terdiurnal 8 hour oscillation present as well in addition there are planetary wave periodicites in foes with periods near the normal rossby modes that is 2 5 10 and 16 days it is also found that the tidal oscillations in foes undergo a strong amplitude modulation with periods comparable to the dominant planetary wave periodicities present in the data our results are in line with recent findings based on a single event study which suggested that sporadic e layers are affected indirectly by planetary waves through their nonlinear interaction and modulation of the atmospheric tides at lower altitudes the close relationship between neutral wave dynamics and midlatitude sporadic e periodicities suggests that the ionosonde data can be used as an alternative means of studying tidal and planetary wave characteristics and their climatology in the lower thermosphere

the impact of surface flux parameterizations on the modeling of the north atlantic ocean abstract the response of an ocean general circulation model to several distinct parameterizations of the surface heat and freshwater fluxes which differ primarily by their representation of the ocean atmosphere feedbacks is investigated in a realistic configuration for the north atlantic ocean the impact of explicitly introducing oceanic information climatological sea surface temperature into the computation of the heat flux through a haney type restoring boundary condition as opposed to the case in which the flux is based on atmosphere only climatologies and is computed with the full bulk formulation is considered the strong similarity between these two approaches is demonstrated and the sources of possible differences are discussed when restoring boundary conditions are applied to the surface salinity however an unphysical feedback mechanism is being introduced the model s response to this restoring is contrasted to the response to a flux boundary condition that prescribes the freshwater fl

multiple equilibria and transitions in a coupled ocean atmosphere box model abstract a six box model is employed as a prototype of the coupled atlantic ocean atmosphere system ice dynamics are excluded numerical integration of this system shows that different thermohaline circulation patterns are possible under the same forcing conditions they consist of a global thermal mode with oceanic poleward surface flow a global saline mode with equatorward surface flow and two intermediate modes that are combinations of the two global modes the stability of the modern day like intermediate mode to finite amplitude freshwater flux perturbations in the high latitude north atlantic meant as a model of glacial melting is explored it is found that freshwater fluxes of the proper inferred magnitude are close to critical and can induce a transition of the coupled system to a saline mode however paleoclimatic data argues the last deglaciation was subcritical further working in a realistic subcritical parameter regime the box model yields an unrealistic temperature record this argue

convection and particle entrainment driven by differential sedimentation when a suspension of small particles is overlain by a clear fluid whose density is greater than that of the interstitial fluid but less than that of the bulk suspension the settling of the dense suspended particles can lead to vigorous convection in the overlying fluid this novel situation is investigated experimentally and theoretically a sharp interface is observed between the convecting upper region and a stagnant lower region in which there is unimpeded sedimentation at low reynolds number there is no transport of fluid from the upper region into the lower though there is mixing of both buoyant fluid and entrained particles from the lower region into the upper the interface between the two regions is found to descend at a constant velocity systematic laboratory measurements have determined how this velocity depends on the densities of the layers and the distributions of settling velocities of the particles a theoretical description is developed which calculates the evolution of the density of the lower region due to differential sedimentation of polydisperse particles buoyancy arguments based on the calculated density profile are used to placc upper and lower bounds on the amount of particle entrainment into the upper layer and on the rate of fall of the interface between the convecting and sedimenting regions the theoretical predictions are in good agreement with the experimental observations the analysis of the interaction between convection and sedimentation in the system considered here may be particularly relevant to the description of evolving crystal rich layers in magma chambers and of silt laden outflow from rivers and has a wide range of other industrial environmental and geological applications