climate at high obliquity abstract the question of climate at high obliquity is raised in the context of both exoplanet studies e g habitability and paleoclimates studies evidence for low latitude glaciation during the neoproterozoic and the snowball earth hypothesis states of high obliquity are distinctive in that for 54 the poles receive more solar radiation in the annual mean than the equator opposite to the present day situation in addition the seasonal cycle of insolation is extreme with the poles alternatively facing the sun and sheltering in the dark for months the novelty of our approach is to consider the role of a dynamical ocean in controlling the surface climate at high obliquity which in turn requires understanding of the surface winds patterns when temperature gradients are reversed to address these questions a coupled ocean atmosphere sea ice gcm configured on an aquaplanet is employed except for the absence of topography and modified obliquity the set up is earth like two large obliquities 54 and 90 are compared to today s earth value 23 5 three key results emerge at high obliquity 1 despite reversed temperature gradients mid latitudes surface winds are westerly and trade winds exist at the equator as for 23 5 although the westerlies are confined to the summer hemisphere 2 a habitable planet is possible with mid latitude temperatures in the range 300 280 k and 3 a stable climate state with an ice cap limited to the equatorial region is unlikely we clarify the dynamics behind these features notably by an analysis of the potential vorticity structure and conditions for baroclinic instability of the atmosphere interestingly we find that the absence of a stable partially glaciated state is critically linked to the absence of ocean heat transport during winter a feature ultimately traced back to the high seasonality of baroclinic instability conditions in the atmosphere

processes of 30 90 days sea surface temperature variability in the northern indian ocean during boreal summer during summer the northern indian ocean exhibits significant atmospheric intraseasonal variability associated with active and break phases of the monsoon in the 30 90 days band in this paper we investigate mechanisms of the sea surface temperature sst signature of this atmospheric variability using a combination of observational datasets and ocean general circulation model sensitivity experiments in addition to the previously reported intraseasonal sst signature in the bay of bengal observations show clear sst signals in the arabian sea related to the active break cycle of the monsoon as the atmospheric intraseasonal oscillation moves northward sst variations appear first at the southern tip of india day 0 then in the somali upwelling region day 10 northern bay of bengal day 19 and finally in the oman upwelling region day 23 the bay of bengal and oman signals are most clearly associated with the monsoon active break index whereas the relationship with signals near somali upwelling and the southern tip of india is weaker in agreement with previous studies we find that heat flux variations drive most of the intraseasonal sst variability in the bay of bengal both in our model regression coefficient 0 9 against 0 25 for wind stress and in observations 0 8 regression coefficient 60 of the heat flux variation is due do shortwave radiation and 40 due to latent heat flux on the other hand both observations and model results indicate a prominent role of dynamical oceanic processes in the arabian sea wind stress variations force about 70 100 of sst intraseasonal variations in the arabian sea through modulation of oceanic processes entrainment mixing ekman pumping lateral advection our 100 km resolution model suggests that internal oceanic variability i e eddies contributes substantially to intraseasonal variability at small scale in the somali upwelling region but does not contribute to large scale intraseasonal sst variability due to its small spatial scale and random phase relation to the active break monsoon cycle the effect of oceanic eddies however remains to be explored at a higher spatial resolution

intraseasonal variability of sst and precipitation in the arabian sea during the indian summer monsoon impact of ocean mixed layer depth abstractthis study investigates sea surface temperature sst and precipitation variations in the eastern arabian sea eas induced by the northward propagating indian summer monsoon ism intraseasonal oscillations misos through analyzing satellite observations and the climate forecast system reanalysis cfsr and performing ocean general circulation model ogcm experiments misos in the eas achieve the largest intensity in the developing stage may june of the ism the misos induce intraseasonal sst variability primarily through surface heat flux forcing contributed by both shortwave radiation and turbulent heat flux and secondarily through mixed layer entrainment the shallow mixed layer depth mld 29 c in the developing stage leads to enhanced response of miso convection to sst anom

size matters another reason why the atlantic is saltier than the pacific abstractthe surface salinity in the north atlantic controls the position of the sinking branch of the meridional overturning circulation moc the north atlantic has higher salinity so deep water formation occurs there rather than in the north pacific here it is shown that in a 3d primitive equation model of two basins of different widths connected by a reentrant channel there is a preference for sinking in the narrow basin even under zonally uniform surface forcing this preference is linked to the details of the velocity and salinity fields in the sinking basin the southward western boundary current associated with the wind driven subpolar gyre has higher velocity in the wide basin than in the narrow basin it overwhelms the northward western boundary current associated with the moc for wide basin sinking so freshwater is brought from the far north of the domain southward and forms a pool on the western boundary in the wide basin the fresh pool suppresses local convection and spreads eastward

past climate and the role of ocean heat transport model simulations for the cretaceous a series of general circulation model experiments using global environmental and ecological simulation of interactive systems genesis were executed to evaluate the sensitivity of simulated mid cretaceous climate to small perturbations in ocean heat transport three experiments were performed 1 mixed layer ocean with no ocean heat transport zeroq 2 ocean heat transport specified as required for genesis to best match modern observations oneq and 3 doubled ocean heat transport twoq the oneq experiment represents an ocean heat transport which is actually about 15 of the values given by carissimo et al 1985 from modern observations as a sensitivity experiment these model simulations represent a doubling of the role of the ocean however relative to the observations they represent small perturbations to the total poleward heat transport in the model with the exception of the tropics no major changes in the structure of the general circulation of the atmosphere resulted from the modification of the ocean heat transport however relatively small increases in ocean heat transport resulted in a number of significant differences between simulations including tropical cooling polar warming weakened equator to pole surface temperature gradients weakened midlatitude jets decreased land sea pressure contrast and decreased midlatitude storminess the experiments indicate that changes in ocean heat transport which are well within the realm of possibility for earth history can have significant climatic impact although ocean heat transport may be a significant factor in explaining cretaceous polar warmth the changes specified in these experiments alone are not sufficient to explain the polar warmth of the cretaceous

coupling between northward propagating intraseasonal oscillations and sea surface temperature in the indian ocean using a hybrid atmosphere ocean coupled model it is shown that during the boreal summer northwardpropagating intraseasonal oscillations npisos are strongly coupled to the underlying sea surface temperature sst in the indian ocean sector on the one hand the intraseasonal atmospheric convection changes the sst through solar radiation latent heat flux and mixed layer entrainment on the other the induced sst fluctuations feed back to affect the intraseasonal convection the preferential northward rather than southward propagation of boreal summer isos in the indian ocean is partially explained by an interaction among the summer mean climate state the atmospheric disturbances and the ocean surface temperature a solution to an atmosphere only model forced with daily sst produces much stronger npisos than a similar solution forced with monthly mean sst amip type run the atmosphere only model however even when it is forced by daily sst from the coupled model with a small amount of noise in the initial and or boundary conditions is unable to reproduce the npisos in the coupled case in the coupled system intraseasonal sst anomalies are forced by intraseasonal atmospheric convection and hence are in quadrature with the convection in the stand alone atmospheric model however sst acts only as a boundary forcing and the resultant atmospheric convection has almost the same phase with the underlying sst one consequence is that the intensity of the sst forced intraseasonal convection in the stand alone atmospheric model is considerably weaker than in the coupled model finally solutions indicate that the northward movement of the off equatorial convection in the northern indian ocean is more closely related to local intraseasonal sst anomalies than to the equatorial eastward moving madden julian oscillation positive negative sst anomalies in the northern indian ocean lead the active break phases of the intraseasonal convection by about 2 pentads 10 days therefore intraseasonal sst anomalies in the northern indian ocean are potentially a useful index to forecast active break spells of the south asian summer monsoon

oceanic control of multidecadal variability in an idealized coupled gcm idealized ocean models are known to develop intrinsic multidecadal oscillations of the meridional overturning circulation moc here we explore the role of ocean atmosphere interactions on this low frequency variability we use a coupled ocean atmosphere model set up in a flat bottom aquaplanet geometry with two meridional boundaries the model is run at three different horizontal resolutions 4 2 and 1 in both the ocean and atmosphere at all resolutions the moc exhibits spontaneous variability on multidecadal timescales in the range 30 40 years associated with the propagation of large scale baroclinic rossby waves across the atlantic like basin the unstable region of growth of these waves through the long wave limit of baroclinic instability shifts from the eastern boundary at coarse resolution to the western boundary at higher resolution increasing the horizontal resolution enhances both intrinsic atmospheric variability and ocean atmosphere interactions in particular the simulated atmospheric annular mode becomes significantly correlated to the moc variability at 1 resolution an ocean only simulation conducted for this specific case underscores the disruptive but not essential influence of air sea interactions on the low frequency variability this study demonstrates that an atmospheric annular mode leading moc changes by about 2 years as found at 1 resolution does not imply that the low frequency variability originates from air sea interactions

observations inferences and mechanisms of the atlantic meridional overturning circulation a review this is a review about the atlantic meridional overturning circulation amoc its mean structure temporal variability controlling mechanisms and role in the coupled climate system the amoc plays a central role in climate through its heat and freshwater transports northward ocean heat transport achieved by the amoc is responsible for the relative warmth of the northern hemisphere compared to the southern hemisphere and is thought to play a role in setting the mean position of the intertropical convergence zone north of the equator the amoc is a key means by which heat anomalies are sequestered into the ocean s interior and thus modulates the trajectory of climate change fluctuations in the amoc have been linked to low frequency variability of atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses on intra annual timescales variability in amoc is large and primarily reflects the response to local wind forcing meridional coherence of anomalies is limited to that of the wind field on interannual to decadal timescales amoc changes are primarily geostrophic and related to buoyancy anomalies on the western boundary a pacemaker region for decadal amoc changes is located in a western transition zone along the boundary between the subtropical and subpolar gyres decadal amoc anomalies are communicated meridionally from this region amoc observations as well as the expanded ocean observational network provided by the argo array and satellite altimetry are inspiring efforts to develop decadal predictability systems using coupled atmosphere ocean models initialized by ocean data

the climatic significance of late ordovician early silurian black shales the ordovician silurian transition approximate to 455 430ma is characterized by repeated climatic perturbations concomitant with major changes in the global oceanic redox state best exemplified by the periodic deposition of black shales the relationship between the climatic evolution and the oceanic redox cycles however remains largely debated here using an ocean atmosphere general circulation model accounting for ocean biogeochemistry mitgcm we investigate the mechanisms responsible for the burial of organic carbon immediately before during and right after the latest ordovician hirnantian 445 444ma glacial peak our results are compared with recent sedimentological and geochemical data we show that the late katian time slice approximate to 445ma typified by the deposition of black shales at tropical latitudes represents an unperturbed oceanic state with regional organic carbon burial driven by the surface primary productivity during the hirnantian our experiments predict a global oxygenation event in agreement with the disappearance of the black shales in the sedimentary record this suggests that deep water burial of organic matter may not be a tenable triggering factor for the positive carbon excursion reported at that time our simulations indicate that the perturbation of the ocean circulation induced by the release of freshwater in the context of the post hirnantian deglaciation does not sustain over sufficiently long geological periods to cause the rhuddanian approximate to 444ma oceanic anoxic event input of nutrients to the ocean through increased continental weathering and the leaching of newly exposed glaciogenic sediments may instead constitute the dominant control on the spread of anoxia in the early silurian

investigating the role of the tibetan plateau in the formation of atlantic meridional overturning circulation abstractthe tibetan plateau tp over the eurasian continent has significant effects on both regional and global climate it can even affect the remote atlantic meridional overturning circulation

bay of bengal salinity stratification and indian summer monsoon intraseasonal oscillation 2 impact on sst and convection the indian summer monsoon intraseasonal oscillations misos induce pronounced intraseasonal sea surface temperature sst variability in the bay of bengal bob which has important feedbacks to atmospheric convection an ocean general circulation model ogcm is employed to investigate the upper ocean processes affecting intraseasonal sst variability and its feedback to the miso convection in the bob the miso induces intraseasonal sst variability predominantly through surface heat flux forcing with comparable contributions from shortwave radiation and turbulent heat flux and to a much smaller extent through wind driven ocean mixed layer entrainment the ocean salinity stratification represented by mixed layer depth mld and barrier layer thickness blt has a strong control on sst but weak impact on convection of the miso the mld is critical for the amplitude of sst response to various forcing processes while the blt mainly affects entrainment by determining the temperature difference between the mixed layer and the water below from may to mid june the shallow mld and thin barrier layer greatly enhance intraseasonal sst anomalies which can amplify convection fluctuations of the miso through air sea interaction and leads to intense but short duration postconvection break spells when either the mld or the blt is large intraseasonal ssts tend to be weak further investigation reveals that freshwater flux of the monsoon gives rise to the shallow mld and thick barrier layer and its overall effect on intraseasonal ssts is a 20 enhancement these results provide implications for improving the simulation and forecast of the miso in climate models

is the gulf stream responsible for europe s mild winters is the transport of heat northward by the gulf stream and north atlantic drift and its subsequent release into the midlatitude westerlies the reason why europe s winters are so much milder than those of eastern north america and other places at the same latitude here it is shown that the principal cause of this temperature difference is advection by the mean winds south westerlies bring warm maritime air into europe and north westerlies bring frigid continental air into north eastern north america further analysis of the ocean surface heat budget shows that the majority of the heat released during winter from the ocean to the atmosphere is accounted for by the seasonal release of heat previously absorbed and not by ocean heat flux convergence therefore the existence of the winter temperature contrast between western europe and eastern north america does not require a dynamical ocean two experiments with an atmospheric general circulation model coupled to an ocean mixed layer confirm this conclusion the difference in winter temperatures across the north atlantic and the difference between western europe and western north america is essentially the same in these models whether or not the movement of heat by the ocean is accounted for in an additional experiment with no mountains the flow across the ocean is more zonal western europe is cooled the trough east of the rockies is weakened and the cold of north eastern north america is ameliorated in all experiments the west coast of europe is warmer than the west coast of north america at the same latitude whether or not ocean heat transport is accounted for in summary the deviations from zonal symmetry of winter temperatures in the northern hemisphere are fundamentally caused by the atmospheric circulation interacting with the oceanic mixed layer copyright 2002 royal meteorological society

representation of convective plumes by vertical adjustment open ocean deep water formation involves the interplay of two dynamical processes plumes 1 km wide driven by upright convection and geostrophic eddies 5 km wide driven by baroclinic instability numerical twin experiments are used to address two questions about the plumes can they be represented by a simple mixing process in large scale models if so is it important that the mixing occurs over a finite time tmix or would instantaneous mixing produce the same effect on large scale properties in numerical simulations which resolve the geostrophic eddies we represent the plumes with a slow convective adjustment algorithm which is broadly equivalent to an enhanced vertical diffusivity of density in statically unstable regions the diffusivity depends on tmix the mixing timescale the fidelity of the plume parameterization is then evaluated by comparison with plume resolving simulations of open ocean deep convection integral properties of the plumes such as the temperature census of the convected water and the strength of the rim current that encircles the convecting region are all accurately reproduced by the slow adjustment scheme the importance of choosing an appropriate finite value for tmix is explored by setting tmix 12 hours in some experiments in accordance with scaling considerations and tmix 0 in others corresponding to instantaneous adjustment the conventional assumption in the case of convection into a moderately or strongly stratified ocean the behavior does not significantly depend on tmix however in neutral conditions the slow adjustment does improve the parametric representation our experiments confirm the picture of plumes homogenizing the water column over a time tmix

oceanic isopycnal mixing by coordinate rotation abstract current numerical models of oceanic circulation differentiate between the eddy diffusion and viscosity transport along the geopotential horizontal and vertical directions only in order to model the effect of anisotropic turbulence as diffusive transport along and across density surfaces the isopycnal mixing tensor has been transformed from a diagonal second rank tensor in the isopycnal coordinate system to a tensor containing off diagonal elements in the geopotential coordinate system

does net e p set a preference for north atlantic sinking abstractthe present day global meridional overturning circulation moc with formation of north atlantic deep water nadw and the absence of a deep water formation in the north pacific is often considered to be caused by the fact that the north pacific basin is a net precipitative while the north atlantic is a net evaporative basin in this paper the authors study the effect of asymmetries in continent geometry and freshwater fluxes on the moc both in an idealized two dimensional model and in a global ocean model this study approaches the problem from a multiple equilibria perspective where asymmetries in external factors constrain the existence of steady moc patterns both this multiple equilibria perspective and the fact that a realistic global geometry is used add new aspects to the problem in the global model it is shown that the atlantic forced by net precipitation can have a meridional overturning circulation with northern sinking and a sea surface salinity that resembles the present day sali

multi stage onset of the summer monsoon over the western north pacific the climatological summer monsoon onset displays a distinct step wise northeastward movement over the south china sea and the western north pacific wnp 110 160 e 10 20 n monsoon rain commences over the south china sea philippines region in mid may extends abruptly to the southwestern philippine sea in early to mid june and finally penetrates to the northeastern part of the domain around mid july in association three abrupt changes are identified in the atmospheric circulation specifically the wnp subtropical high displays a sudden eastward retreat or quick northward displacement and the monsoon trough pushes abruptly eastward or northeastward at the onset of the three stages the step wise movement of the onset results from the slow northeastward seasonal evolution of large scale circulation and the phase locked intraseasonal oscillation iso the seasonal evolution establishes a large scale background for the development of convection and the iso triggers deep convection the iso over the wnp has a dominant period of about 20 30 days this determines up the time interval between the consecutive stages of the monsoon onset from the atmospheric perspective the seasonal sea surface temperature sst change in the wnp plays a critical role in the northeastward advance of the onset the seasonal northeastward march of the warmest sst tongue sst exceeding 29 5 c favors the northeastward movement of the monsoon trough and the high convective instability region the seasonal sst change in turn is affected by the monsoon through cloud radiation and wind evaporation feedbacks

active break cycles diagnosis of the intraseasonal variability of the asian summer monsoon in this study various diagnostics have been applied to daily observed outgoing longwave radiation olr and ecmwf reanalysis era products to provide a comprehensive description of the active break cycles associated with the asian summer monsoon and to address the differing behaviour of the two dominant time scales of intraseasonal variability 10 20 days and 30 60 days composite analysis of olr based on filtered daily all india rainfall air for the 40 day 30 60 days intraseasonal mode indicates that during active phases convection is significantly enhanced over the indian continent extending over the bay of bengal maritime continent and equatorial west pacific while convection is suppressed over the equatorial indian ocean and northwest tropical pacific resulting in a quadrapole structure over the asian monsoon domain in response to this heating pattern the large scale hadley lateral and the two east west transverse tropical circulations are enhanced there is also a significant impact on the extra tropical circulation through excitation and propagation of rossby waves in contrast the 15 day mode is more regional to the monsoon domain and has a prominent east west orientation in convection only the local hadley circulation over the monsoon region is modulated by this mode the evolution of these two modes as revealed by pop principal oscillation pattern analysis shows that the 40 day mode originates over the equatorial indian ocean once formed it has poleward propagation on either side of the equator and eastward propagation into the equatorial west pacific from the equatorial west pacific northward propagation over the west pacific and westward propagation into the indian longitudes are prominent the propagative features are complex and interactive and are responsible for the quadrapole structure in convection seen from the composites the interannual variability assessed from the pop coefficient time series indicates that the 40 day mode is strong during the onset phase of the monsoon in all the years but systematic propagation over the entire season depends crucially on the activity of the oceanic tcz tropical convergence zone the pop analysis of the 15 day mode indicates that this event originates over the equatorial west pacific associated with westward propagating rossby waves amplifies over the northwest tropical pacific and modulates both the continental and oceanic tczs over indian longitudes simultaneously this mode is pronounced during the established phase of the monsoon due to the complexity in the propagational features of both the intraseasonal modes it is concluded that understanding the subseasonal variability of one regional component of the asian summer monsoon asm requires understanding the entire asm system

the role of intraseasonal variability in the nature of asian monsoon precipitation abstract the structure of the mean precipitation of the south asian monsoon is spatially complex embedded in a broad precipitation maximum extending eastward from 70 e to the northwest tropical pacific ocean are strong local maxima to the west of the western ghats mountain range of india in cambodia extending into the eastern china sea and over the eastern tropical indian ocean and the bay of bengal bob where the strongest large scale global maximum in precipitation is located in general the maximum precipitation occurs over the oceans and not over the land regions distinct temporal variability also exists with time scales ranging from days to decades neither the spatial nor temporal variability of the monsoon can be explained simply as the response to the cross equatorial pressure gradient force between the continental regions of asia and the oceans of the southern hemisphere as suggested in classical descriptions of the monsoon monthly 1979 2005 and daily 1997 present rainfall estimates

antarctic ocean and sea ice response to ozone depletion a two time scale problem theresponseofthesouthernoceantoarepeatingseasonalcycleofozonelossisstudiedintwocoupledclimate models and is found to comprise both fast and slow processes the fast response is similar to the interannual signature of the southern annular mode sam on sea surface temperature sst onto which the ozone hole forcing projects in the summer it comprises enhanced northward ekman drift inducing negative summertime sst anomalies around antarctica earlier sea ice freeze up the following winter and northward expansion of the sea ice edge year round the enhanced northward ekman drift however results in upwelling of warm waters from below the mixed layer in the region of seasonal sea ice with sustained bursts of westerly winds induced by ozone hole depletion this warming from below eventually dominates over the cooling from anomalous ekman drift the resulting slow time scale response years to decades leads to warming of ssts around antarctica and ultimately a reduction in sea ice cover year round this two time scale behavior rapid cooling followed by slow but persistent warming is found in the two coupled models analyzed one with an idealized geometry and the other with a complex global climate model with realistic geometry processes that control the time scale of the transition from cooling to warming and their uncertainties are described finally the implications of these results are discussed for rationalizing previous studies of the effect of the ozone hole on sst and sea ice extent

localization of deep water formation role of atmospheric moisture transport and geometrical constraints on ocean circulation a series of coupled atmosphere ocean ice aquaplanet experiments is described in which topological constraints on ocean circulation are introduced to study the role of ocean circulation on the mean climate of the coupled system it is imagined that the earth is completely covered by an ocean of uniform depth except for the presence or absence of narrow barriers that extend from the bottom of the ocean to the sea surface the following four configurations are described aqua no land ridge one barrier extends from pole to pole drake one barrier extends from the north pole to 358s and ddrake two such barriers are set 908 apart and join at the north pole separating the ocean into a large basin and a small basin connected to the south on moving from aqua to ridge to drake to ddrake the energy transports in the equilibrium solutions become increasingly realistic culminating in ddrake which has an uncanny resemblance to the present climate remarkably the zonal average climates of drake and ddrake are strikingly similar exhibiting almost identical heat and freshwater transports and meridional overturning circulations however drake and ddrake differ dramatically in their regional climates the small and large basins of ddrake exhibit distinctive atlantic like and pacific like characteristics respectively the small basin is warmer saltier and denser at the surface than the large basin and is the main site of deep water formation with a deep overturning circulation and strong northward ocean heat transport a sensitivity experiment with ddrake demonstrates that the salinity contrast between the two basins and hence the localization of deep convection results from a deficit of precipitation rather than an excess of evaporation over the small basin it is argued that the width of the small basin relative to the zonal fetch of atmospheric precipitation is the key to understanding this salinity contrast finally it is argued that many gross features of the present climate are consequences of two topological asymmetries that have profound effects on ocean circulation a meridional asymmetry circumpolar flow in the southern hemisphere blocked flow in the northern hemisphere and a zonal asymmetry a small basin and a large basin

in situ granulation by thermal stress during subaqueous volcanic eruptions some of the most complex volcanic thermodynamic processes occur when erupting magma interacts with water in shallow water surtseyan eruptions are spectacular and they efficiently fragment magma into fine ash particles the aviation hazard from these eruptions depends the amount of transportable fine ash that is generated and whether it is aggregated into particle coatings or accretions to investigate both mechanisms we analyzed ash encased lapilli from the surtseyan eruptions of capelinhos azores 1957 1958 and hunga tonga hunga ha apai tonga 2014 2015 using x ray computed microtomography and electron microscopy we discovered pyroclasts that were not coated sensu stricto but had enveloping ash produced by in situ granulation of the particle surface this was caused by thermal stress as pyroclasts briefly traveled through water and were quenched during eruption in situ granulation is thus an important secondary disruption process in shallow subaqueous eruptions our results imply that ash encasement is not always evidence of particle aggregation and accretion but it may also result from new ash formation shallow water conditions produce the most efficient ash generation conditions leading to the greatest hazard to downwind populations and air traffic

influence of sea surface temperature on the intraseasonal variability of the south china sea summer monsoon the objective of this study is to examine based on recently available high resolution satellite and observational data the evolution and role of sea surface temperature sst in influencing the intraseasonal variability of the south china sea scs summer monsoon sm the study focuses on the 30 60 day timescale when the northward propagating anomalies are dominant over the scs composite analysis of the sst maximum events during scs sm shows that increased sst anomalies over the scs are significantly influenced by the downward shortwave radiation flux anomalies with the suppressed surface latent heat flux anomalies supplementing to it a thermal damping of the positive sst anomalies induces positive upward heat fluxes which then destabilize the lower atmosphere between 1 000 and 700 hpa the positive sst anomalies lead the positive precipitation anomalies over the scs by 10 days with a significant correlation r 0 44 between the sst precipitation anomalies the new findings here indicate an ocean to atmosphere effect over the scs where underlying sst anomalies tend to form a favorable condition for convective activity and sustain enhanced precipitation during the scs sm it is also argued based on our observations that the negative sea level pressure anomalies induced by the positive sst anomalies play a role in enhancing the northward propagation of the intraseasonal anomalies over the scs

horizontal structure and seasonality of large scale circulations associated with submonthly tropical convection the relationship between deep tropical convection and large scale atmospheric circulation in the 6 30 day period range is examined regression relationships between filtered outgoing longwave radiation at various locations in the tropics and 200 and 850 mb circulation are mapped for the standard seasons and the spatial structure and seasonal dependence of the results are interpreted in view of the basic state circulation in regions where the convection is embedded in upper level easterlies anomalous equatorial easterly flow is typically present at 200 mb within and to the west of the convective signal along with patterns of meridional outflow into subtropical anticyclonic perturbations lagged relationships suggest that the convection is forcing the circulation in many of these cases the outflow and subtropical circulations are strongest into the winter hemisphere during the solstitial seasons with more symmetric signals about the equator seen in the equinoctial seasons the longitudinal positioning of the subtropical features with respect to the convection varies but is generally located due poleward or just to the east of the convection there tends to be a first baroclinic mode vertical structure to these circulations such that equatorial westerlies are present at 850 mb within the convection with closed circulations on either side of the equator resembling equatorial rossby modes especially common over the atlantic and pacific sectors as a contrast in regions located within upper level westerlies or along the margin of influence of upper westerly disturbances convection appears to be forced by upper level wave energy propagating into the deep tropics with the heating located in the upward motion region ahead of upper level troughs this occurs over the atlantic and eastern pacific sectors during northern winter and spring and over australia the south pacific and south america during southern summer when upper westerlies are at relatively low latitudes where they can interact with deep tropical convection the results confirm theoretical and modeling ideas that suggest that rossby wave energy is able to propagate into the deep tropics in regions where upper level westerlies exist in the tropics

the three dimensional overturning circulation of the southern ocean during the woce era a box inverse model of the southern ocean during the world ocean circulation experiment is constructed to investigate the three dimensional structure of the regional overturning circulation in that era the model has many features in common with various preceding inverse studies but also contains several novel elements that make it well suited for addressing many of the significant uncertainties that surround the circulation at present the net overturning circulation of the southern ocean is found to consist of two well defined cells of similar strength the upper cell consists of a northward transport of 18 8 5 5 sv of surface mode and intermediate waters lighter than the 27 5 kg m 3 isoneutral and an equivalent southward flow in the approximate 27 5 27 9 kg m 3 neutral density range encompassing the bulk of the upper circumpolar deep water the lower cell involves the northward export of 18 6 0 9 sv of antarctic bottom water and lower circumpolar deep water denser than 28 08 kg m 3 and an opposing transport in the lighter classes of that water mass substantial structural differences between the overturning circulations of the atlantic indian and pacific basins are indicated by the model s solution overall the diagnosed southern ocean circulation shares many qualitative and some quantitative features with previous inverse estimates particularly as regards the large scale depth integrated lateral circulation and associated energy fluxes in the subtropics and in the antarctic circumpolar current and the strength of the upper overturning cell however it also suggests several significant adjustments to current views of the regional circulation most notable amongst these are the subpolar circulation of the southern ocean is more vigorous and zonally interconnected than generally thought the associated lower overturning cell is more intense than indicated by most preceding estimates contrary to common perception sub surface mixing processes play a role of comparable importance to air sea ice exchanges of buoyancy in underpinning the dianeutral closure of the southern ocean overturning even at shallow mode and intermediate water levels and the connection between north atlantic deep water formation and southern ocean upwelling is fundamentally three dimensional such that deep waters from the north atlantic must upwell dianeutrally before being returned to the permanent pycnocline of the northern oceans

structures and mechanisms of the northward propagating boreal summer intraseasonal oscillation abstract the spatial and temporal structures of the northward propagating boreal summer intraseasonal oscillation bsiso are revealed based on the analysis of both the echam4 model simulation and the ncep ncar reanalysis the bsiso structure and evolution characteristics simulated by the model bear many similarities to those derived from the ncep ncar reanalysis the most notable features are the remarkable meridional asymmetries relative to the bsiso convection in the vorticity and specific humidity fields a positive vorticity perturbation with an equivalent barotropic structure appears a few latitude degrees north of the convection center the maximum specific humidity also shows a clear northward shift in the lower troposphere two internal atmospheric dynamics mechanisms are proposed to understand the cause of the northward propagation of the bsiso the first is the vertical shear mechanism the key process associated with this mechanism is the generation of barotropic vorticity due to the couplin

structures and mechanisms of the first branch northward propagating intraseasonal oscillation over the tropical indian ocean the first branch northward propagating intra seasonal oscillation fniso over the tropical indian ocean io often triggers the onset of the asian summer monsoon in this study we investigate the structures and mechanisms associated with fniso through the diagnosis of era interim reanalysis data for the period of 1990 2009 a composite analysis is conducted to reveal the structure and evolution characteristics of the fniso and associated background circulation changes it is found that the fniso convection originates from the southwest ern io and propagates eastward after reaching the eastern io the major convective branch moves northward toward the northern bay of bengal bob two possible mecha nisms may contribute to the northward propagation of the fniso one is the meridional asymmetry of the back ground convective instability a greater background con vective instability over the northern bob may destabilize rossby waves and cause convection to shift northward the other is the meridional phase leading of perturbation humidity in the planetary boundary layer pbl maximum pbl moisture appears to the north of the convection center which promotes a convectively unstable stratification ahead of the convection and leads to the northward prop agation of the fniso a pbl moisture budget analysis reveals that anomalous zonal advection is a dominant process in contributing to the moisture asymmetry

coupling of trade winds with ocean circulation damps itcz shifts abstractthe position of the intertropical convergence zone itcz is sensitive to the atmosphere s hemispheric energy balance lying in the hemisphere most strongly heated by radiative and turbulent surface energy fluxes this study examines how the ocean circulation through its cross equatorial energy transport and associated surface energy fluxes affects the itcz s response to an imposed interhemispheric heating contrast in a coupled atmosphere ocean general circulation model shifts of the itcz are strongly damped owing to a robust coupling between the atmosphere s hadley cells and the ocean s subtropical cells by the trade winds and their associated surface stresses an anomalous oceanic wind driven cross equatorial cell transports energy across the equator strongly offsetting the imposed heating contrast the circulation of this cell can be described by the combination of trade wind anomalies and the meridional gradient of sea surface temperature which sets the temperature contrast between its up

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

seasonal variation abrupt transition and intraseasonal variability associated with the asian summer monsoon in the gla gcm abstract in this paper a description of the global and regional features of the asian summer monsoon in the goddard laboratory atmospheres gla general circulation model gcm is presented based on a 10 yr 1979 1988 integration of the model under the atmospheric model intercomparison project it is found that the earliest signal of onset of the asian monsoon is represented by the emergence of convection over the indo china land bridge in early may followed by the sudden jump of the equatorial itcz intertropical convergence zone to 10 n over the south china sea and indian ocean in mid may this sudden jump is linked to an abrupt northward shift of the ascending branch of the local hadley circulation possibly stemming from symmetric instability of the basic flow in may in agreement with observations the model shows a pronounced intraseasonal oscillation along the equator and enhanced synoptic scale activities identified as easterly waves in the tropical and subtropical western pacific these int

the boreal summer intraseasonal oscillation relationship between northward and eastward movement of convection the summertime intraseasonal oscillation iso is an important component of the south asian monsoon lagged regressions of intraseasonally filtered 25 80 days outgoing longwave radiation olr reveal that centers of convection move both northward and eastward from the central equatorial indian ocean subsequent to the initiation of an iso eastward movement of convection is also seen at indian subcontinent latitudes 10 8 208n based on the regression results the summertime iso convection signal appears as a band tilting northwestward with latitude and stretching from the equator to about 208n viewed along any meridian convection appears to propagate northward while equatorial convection propagates to the east to examine the robustness of the connection between eastward and northward movement individual isos are categorized and composited relative to the strength of the large scale eastward component of convection in the central equatorial indian ocean it is found that the majority of isos that exhibit northward movement onto the indian subcontinent 42 out of 54 isos or 78 also exhibit eastward movement into the western pacific ocean it is also found that when convection in the central indian ocean is not followed within 10 20 days by convection in the western pacific ocean 12 out of 54 isos or 22 the independent northward movement of convection in the indian ocean region is somewhat stunted the link between the eastward and northward movement of convection is consistent with an interpretation of the summertime iso in terms of propagating equatorial modes the northward moving portion of convection is forced by surface frictional convergence into the low pressure center of the rossby cell that is excited by equatorial iso convection a similar convergence pattern is seen for the northern winter iso but it does not generate poleward movement due to relatively cool ssts underlying the surface convergence

origins of eddy kinetic energy in the bay of bengal by analyzing satellite observational data and ocean general circulation model experiments this study investigates the key processes that determine the spatial distribution and seasonality of intraseasonal eddy kinetic energy eke within the bay of bengal bob it is revealed that a complicated mechanism involving both local and remote wind forcing and ocean internal instability is responsible for the generation and modulation of eke in this region high level eke mainly resides in four regions east of sri lanka region 1 the western bob region 2 northwest of sumatra region 3 and the coastal rim of the bob region 4 the high eke levels in regions 1 and 2 are predominantly produced by ocean internal instability which contributes 90 and 79 respectively prominent seasonality is also observed in these two regions with higher eke levels in boreal spring and fall due to enhanced instability of the east indian coast current and the southwest monsoon current respectively in contrast ocean internal instability contributes 49 and 52 of the total eke in regions 3 and 4 respectively whereas the atmospheric forcing of intraseasonal oscillations isos also plays an important role isos produce eke mainly through wind stress involving both the remote effect of equatorial winds and the local effect of monsoonal winds equatorial origin wave signals significantly enhance the eke levels in regions 3 and 4 in the form of reflected rossby waves and coastal kelvin waves respectively the local wind forcing effect through ekman pumping also has a significant contribution in regions 3 and 4 24 and 22 respectively

interannual variability of the onset of the indian summer monsoon and its association with atmospheric features el nino and sea surface temperature anomalies abstract the long term mean date of the monsoon onset over kerala mok varies between 30 may and 2 june according to different estimates with a standard deviation of 8 9 days the earliest date of mok and the most delayed one during the last 100 years differ by 46 days 7 may and 22 june respectively mok switches on a spatially large and intense convective heat source over south asia lasting from june to september whose moisture supply is made available through the cross equatorial low level jet stream superposed epoch analysis of 10 years of outgoing longwave radiation olr data shows that mok is a significant stage in the evolution of the olr field in the tropics of the eastern hemisphere at the time of mok there is increased convection in a band about 5 10 degrees wide meridionally extending from the south arabian sea to south china and convection is suppressed all around particularly in the western pacific ocean in 1983 when mok was delayed by about 3 pentads olr data showed that the

coupled climate impacts of the drake passage and the panama seaway tectonically active gateways between ocean basins have modified ocean circulation over earth history today the atlantic and pacific are directly connected via the drake passage which forms a barrier to the time mean geostrophic transport between the subtropics and antarctica in contrast during the warm early cenozoic era when antarctica was ice free the drake passage was closed instead at that time the separation of north and south america provided a tropical seaway between the atlantic and pacific that remained open until the isthmus of panama formed in the relatively recent geological past ocean circulation models have previously been used to explore the individual impacts of the drake passage and the panama seaway but rarely have the two gateways been considered together and most explorations have used very simple atmospheric models here we use a coupled ocean ice atmosphere model gfdl s cm2mc to simulate the impacts of a closed drake passage both with and without a panama seaway we find that the climate response to a closed drake passage is relatively small when the panama seaway is absent similar to prior studies although the coupling to a dynamical atmosphere does increase the temperature change however with a panama seaway closing drake passage has a much larger effect due to the cessation of deep water formation in the northern hemisphere both gateways alter the transport of salt by ocean circulation with the panama seaway allowing fresh pacific water to be imported to the north atlantic and the drake passage preventing the flow of saline subtropical water to the circum antarctic a flow that is particularly strong when the panama seaway is open thus with a panama seaway and a closed drake passage the southern ocean tends to be relatively salty while the north atlantic tends to be relatively fresh such that the deep ocean is ventilated from the circum antarctic ensuing changes in the ocean heat transport drive a bi polar shift of surface ocean temperatures and the intertropical convergence zone migrates toward the warmer southern hemisphere the response of clouds to changes in surface ocean temperatures amplifies the climate response resulting in temperature changes of up to 9 c over antarctica even in the absence of land ice feedbacks these results emphasize the importance of tectonic gateways to the climate history of the cenozoic and support a role for ocean circulation changes in the glaciation of antarctica

on the relationship between decadal buoyancy anomalies and variability of the atlantic meridional overturning circulation abstractowing to the role of the atlantic meridional overturning circulation amoc in ocean heat transport amoc variability is thought to play a role in climate variability on a wide range of time scales this paper focuses on the potential role of the amoc in climate variability on decadal time scales coupled and ocean only general circulation models run in idealized geometries are utilized to study the relationships between decadal amoc and buoyancy variability and determine whether the amoc plays an active role in setting sea surface temperature on decadal time scales decadal amoc variability is related to changes in the buoyancy field along the western boundary according to the thermal wind relation buoyancy anomalies originate in the upper ocean of the subpolar gyre and travel westward as baroclinic rossby waves when the buoyancy anomalies strike the western boundary they are advected southward by the deep western boundary current leading to latitudinally coherent amoc variability the amoc i

mechanism of the northward propagating intraseasonal oscillation insights from a zonally symmetric model abstract the propagation and initiation mechanisms of the boreal summer intraseasonal oscillation bsiso in the south asian summer monsoon are examined with a zonally symmetric atmospheric model in the axially symmetric model the effects of zonally propagating atmospheric waves are intentionally excluded the model specifies mean flows and depicts the lowest baroclinic mode and a barotropic mode in the free troposphere the two vertical modes are coupled by the time mean vertical wind shear the model atmosphere produces a 15 20 day oscillation which is characterized by northward propagation of convection from south of the equator to the indian monsoon trough region and a reinitiation of convection in the region between 10 s and the equator the northward propagation in the model is produced by the free troposphere barotropic divergence which leads convection by about a quarter of a cycle the vertical advection of summer mean easterly vertical wind shear by perturbation vertical motion inside the con

an ocean atmosphere interaction mechanism for the active break cycle of the asian summer monsoon a typical active break cycle of the asian summer monsoon is taken as beginning with maximum sst pentad 0 over the north bay of bengal when the oceans to its west and east from longitude 40 160 e and between latitudes 10 and 25 n area a also has maximum sst during this pentad the recently found cold pool of the bay of bengal between latitudes 3 n and 10 n has its minimum sst an area of convection takes genesis over the bay of bengal immediately after pentad 0 in the zone of large sst gradient north of the cold pool and it pulls the monsoon low level jetstream llj through peninsular india convection and the llj westerlies then spread to the western pacific ocean during pentads 1 4 taken as the active phase of the monsoon during which convection and llj have grown in a positive feed back process the cyclonic vorticity to the north of the llj axis is hypothesized to act as a flywheel maintaining the convection during the long active phase against the dissipating effect of atmospheric stabilization by each short spell of deep convection by the end of pentad 4 the sst over area a has cooled and the convection weakens there when the llj turns clockwise over the arabian sea and flows close to the equator in the indian ocean a band of convection develops at pentad 5 between the equator and latitude 10 s over the indian ocean and it is nourished by the cyclonic vorticity of the llj now near the equator and the moisture supply through it this is taken as the break monsoon phase lasting for about three to four pentads beginning from pentad 5 of a composite active break cycle of 40 day duration with reduced wind and convection over the area a during the break phase solar radiation and light winds make the sst there warm rapidly and a new active break cycle begins sst convection llj and the net heat flux at the ocean surface have important roles in this new way of looking at the active break cycle as a coupled ocean atmosphere phenomenon

role of north indian ocean air sea interaction in summer monsoon intraseasonal oscillation abstractair sea coupling processes over the north indian ocean associated with the indian summer monsoon intraseasonal oscillation miso are investigated observations show that miso convection an

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

structure genesis and scale selection of the tropical quasi biweekly mode the quasi biweekly mode qbm and the 30 60 day mode are two major intraseasonal oscillations isos in the tropics the qbm is known to have a major influence in determining the active and break conditions of the indian monsoon during the northern summer a westward propagating equatorial rossby wave with quasi biweekly period influences the australian monsoon during the northern winter universality between the summer and winter qbm is established through analysis of daily circulation and convection data for 10 years it is shown that the mean spatial structure of the qbm in circulation and convection resembles that of a gravest meridional mode equatorial rossby wave with wavelength of about 6000 km and westward phase speed of approximately 4 5 m s 1 however the maximum zonal wind occurs at around 5 n 5 s during the northern summer winter the wave structure appears to be translated northward southward by about 5 during the northern summer winter the relationship between outgoing long wave radiation and circulation data indicates that the mode is driven unstable by coupling with moist convection similarity in temporal and spatial characteristics of the mode during the two seasons leads us to propose that the same mechanism governs the genesis and scale selection of the mode in both the seasons an acceptable mechanism for genesis and scale selection of the qbm has been lacking in the present study a mechanism for genesis and scale selection of the observed qbm is proposed a simple 2 layer model that includes a steady ekman boundary layer bl formulation incorporating effect of entrainment mixing is constructed for the convectively coupled equatorial waves without influence of the background mean flow moist feedback in the presence of frictional bl convergence drives the gravest meridional mode equatorial rossby wave unstable with observed wavelength and period but with zonal winds symmetric about the equator potential temperature perturbation associated with the rossby wave is in phase with relative vorticity perturbation at low level the bl drives moisture convergence in phase with the relative vorticity at the top of the bl release of latent heat associated with the bl convergence enhances the potential temperature leading to a positive feedback the mean flow over the indian ocean and western pacific at low levels is such that the zero ambient absolute vorticity or the dynamic equator shifts to around 5 n 5 s during summer winter and results in a shift of the unstable rossby waves towards the north south by about 5 the resulting structure of the unstable rossby mode resembles the observed structure of the biweekly mode it is shown that neither evaporation wind feedback nor vertical shear of the mean flow is crucial for the existence of the mode however these processes marginally modify the growth rate and make the structure of the unstable wave more realistic