causes of enhanced sst variability over the equatorial atlantic and its relationship to the atlantic zonal mode in cmip5 abstracta spurious band of enhanced sea surface temperature sst variance sbev is identified over the northern equatorial atlantic in the geophysical fluid dynamics laboratory gfdl climate model version 2 1 the sbev is especially pronounced in boreal spring owing to the combined effect of both anomalous atmospheric thermal forcing and oceanic vertical upwelling the sbev is a common bias in phase 5 of the coupled model intercomparison project cmip5 found in 14 out of 23 models the sbev in cmip5 is associated with the atmospheric thermal forcing and the oceanic vertical upwelling similar to gfdl cm2 1 while the tropical north atlantic variability is only weakly correlated with the atlantic zonal mode azm in observations the sbev in cmip5 produces conditions that drive and intensify the azm variability via triggering the bjerknes feedback this partially explains why azm is strong in some cmip5 models even though the equatorial cold tongue and easterly trades are biased low

ageostrophic geopotential fluxes in downstream and upstream development of baroclinic waves abstract with the use of a simple primitive equation model it is demonstrated that the convergence divergence of ageostrophic geopotential fluxes can be a major source sink of kinetic energy for both downstream and upstream development of baroclinic waves and can play a dominant role during the early stages of wave development it is also shown that both surface friction and effects lead to an asymmetry in the upstream versus downstream development with downstream development much stronger a total group velocity is defined based on ageostrophic fluxes and its relationship to the rate of wave packet spreading and to convective and absolute instability is discussed

on the connection between benguela and equatorial atlantic ninos and the role of the south atlantic anticyclone for the eastern tropical atlantic two recurring el nino like phenomena with high interannual sst anomalies have been described one centered in the equatorial region as part of the atlantic zonal mode and one off angola referred to as benguela nino both events are supposed to be generated not locally but by a relaxation of the trade winds in the western equatorial atlantic here the connection between sst variations in the two regions is investigated with observational data sets as well as ocean model simulations they are correlated to such an extent that joint events should rather be viewed as one atlantic nino an intriguing feature counterintuitive in view of the remote forcing mechanism is that sst anomalies off angola precede those in the equatorial atlantic we show this behavior to be related to the difference in thermocline depths and a different seasonality of interannual sst variability in the two regions while benguela ninos peak in austral fall due to the angola benguela front being located furthest to the south and high interannual variability in coastal kelvin wave activity warm events at the equator are phase locked to austral winter when the thermocline is shallow perturbation experiments confirm the importance of remote forcing from the equator for sst variability off angola and demonstrate the leading role of wind stress in the generation of sst anomalies in the eastern tropical atlantic these wind stress variations are shown to be linked to fluctuations in the strength of the south atlantic anticyclone a connection that might be important with respect to the predictability of atlantic ninos

the source of benguela ninos in the south atlantic ocean 1 the intermittent occurrence of anomalous warm events in the upwelling regions of the pacific el ninos has been intensively studied pacific ninos have striking effects on the local ecosystem hence on the fisheries and on rainfall similar dramatic events have been observed in the south atlantic off the coasts of angola and namibia and named benguela ninos they tend not to occur in unison with their pacific counterpart and may thus have unrelated forcing mechanisms using an ocean general circulation model forced by real winds and verified with satellite data it is shown that benguela ninos are generated by specific wind stress events in the west central equatorial atlantic and progress from there as subsurface temperature anomalies that eventually outcropped only at the south west african coast these results suggest that it now may be possible to predict the occurrence of these disruptive events with a lead time of 2 months

role of tropical sst variability on the formation of subtropical dipoles abstractinterannual variations of sea surface temperature sst in the midlatitudes of the southern hemisphere play an important role in the rainfall variability over the surrounding countries by modulating synoptic scale atmospheric disturbances these are frequently associated with a northeast southwest oriented dipole of positive and negative sst anomalies in each oceanic basin referred to as a subtropical dipole this study investigates the role of tropical sst variability on the generation of subtropical dipoles by conducting sst nudging experiments using a coupled general circulation model in the experiments where the simulated sst in each tropical basin is nudged to the climatology of the observed sst the subtropical dipoles tend to occur as frequently as the case in which the simulated sst is allowed to freely interact with the atmosphere it is found that without the tropical sst variability the zonally elongated atmospheric mode in the mid to high latitudes called the antarctic oscillation

relationship between the equatorial and meridional modes of climatic variability in the tropical atlantic the tropical atlantic ocean exhibits two primary modes of interannual climate variability an equatorial mode analogous to but weaker than the pacific el nino phenomenon and a meridional mode that does not have a pacific counterpart the equatorial mode is responsible for warm and cold sea surface temperature sst events mainly in the gulf of guinea and is identifiable by abnormal changes in the equatorial thermocline slope resulting from zonal wind anomalies in the western tropical atlantic the meridional mode is characterized by a north south interhemispheric gradient of sst anomalies here it is shown using observed surface and subsurface oceanic temperatures that the meridional mode is linked to the equatorial mode at both decadal and short interannual 1 2 years time scales both modes involve north south displacements of the itcz as in the annual response

is the boreal spring tropical atlantic variability a precursor of the equatorial mode the equatorial mode em governs the tropical atlantic inter annual variability during boreal summer it has profound impacts on the climate of adjacent and remote areas however predicting the em is one of the most challenging and intriguing issues for the scientific community recent studies have suggested a possible connection between the boreal spring meridional mode mm and the em through ocean wave propagation here we use a set of sensitivity experiments with a medium resolution ocean model to determine the precursor role of a mm to create equatorial sst variability our results demonstrate that boreal summer equatorial ssts following a mm are subject to two counteracting effects the local wind forcing and remotely excited oceanic waves for a positive mm the anomalous easterly winds blowing along the equator shallow the thermocline cooling the sea surface via vertical diffusion and meridional advection anomalous wind curl excites a downwelling rossby wave north of equator which is reflected at the western boundary becoming an equatorial kelvin wave kw this downwelling kw propagates eastward deepening the thermocline and activating the thermocline feedbacks responsible for the equatorial warming moreover the local wind forcing and rw reflected mechanism have a significant and comparable impact on the equatorial sst variability changes in the intensity and persistence of these distinct forcings will determine the equatorial sst response during boreal summer our results give a step forward to the improvement of the em predictability

fluid flow triggered migration of events in the 1989 dobi earthquake sequence of central afar during the august 1989 sequence of earthquakes beneath the dobi graben in central afar principal events including 10 shocks with ml 5 5 propagated 50 km north westwards in 50 h the sequence is analyzed in terms of triggering by propagation of a critical pore pressure wave in a compressible fluid through a fractured medium the macroscopic fluid diffusion tensor d in the crust beneath the graben is estimated its principal eigenvector strikes n115 e nearly parallel to the graben axis with a value it is about 10 times larger than the other two consistent with fissure anisotropy in the graben the order of magnitude of the permeability 10 8 m is compatible with characteristic fissure widths ranging from 1 mm to 3 cm estimating such parameters in other extensional regions might help to understand better the temporal evolution of seismic swarms

quasigeostrophic transient wave activity flux updated climatology and its role in polar vortex anomalies abstract the climatology of transient wave activity flux defined by plumb has been calculated for each calendar month for high frequency hf and low frequency lf waves using the ncar ncep reanalyses for both hemispheres wave activity flux of both hf and lf waves shows upward propagation of waves from the lower troposphere into the upper troposphere then into the lower stratosphere during the summer and at least up to the midstratosphere during other seasons while the upward flux emanating from the lower troposphere is particularly large in the two storm tracks in the northern hemisphere nh it is large in most of the extratropics in the southern hemisphere sh the hf waves radiate equatorward most noticeably in the upper troposphere whereas the lf waves do not show visible signs of equatorward radiation the total horizontal flux is generally dominated by the advective flux that represents the eddy enstrophy advection by the mean flow and appears predominantly pseudoeastward divergence of th

seismic amplitude ratio analysis of the 2014 2015 bar arbunga holuhraun dike propagation and eruption magma is transported in brittle rock through dikes and sills this movement may be accompanied by the release of seismic energy that can be tracked from the earth s surface locating dikes and deciphering their dynamics is therefore of prime importance in understanding and potentially forecasting volcanic eruptions the seismic amplitude ratio analysis sara method aims to track melt propagation using the amplitudes recorded across a seismic network without picking the arrival times of individual earthquake phases this study validates this methodology by comparing sara locations filtered between 2 and 16hz with the earthquake locations same frequency band recorded during the 2014 2015 bar arbunga holuhraun dike intrusion and eruption in iceland integrating both approaches also provides the opportunity to investigate the spatiotemporal characteristics of magma migration during the dike intrusion and ensuing eruption during the intrusion sara locations correspond remarkably well to the locations of earthquakes several exceptions are however observed 1 a low frequency signal was possibly associated with a subglacial eruption on 23 august 2 a systematic retreat of the seismicity was also observed to the back of each active segment during stalled phases and was associated with a larger spatial extent of the seismic energy source this behavior may be controlled by the dike s shape and or by dike inflation 3 during the eruption sara locations consistently focused at the eruptive site 4 tremor rich signal close to ice cauldrons occurred on 3 september this study demonstrates the power of the sara methodology provided robust site amplification quality factors and seismic velocities are available plain language summary locating earthquakes usually implies picking phase arrivals p and s waves another technique called seismic amplitude ratio analysis sara was recently introduced to locate them only by using the amplitude recorded at different pairs of seismic stations however this technique was never proven to be true this study shows that the earthquake locations derived by sara compares remarkably well with the locations of 30 000 seismic events triggered when magma migrated in the icelandic crust prior to the 2014 2015 holuhraun eruption but the results also provide new insight into the magma dynamics that led to the largest eruption of the last two centuries in europe we show that ground vibration was continuously triggered during the 2week period preceding the eruption when magma forced its way toward the eruption site but also during the eruption itself several intriguing features were observed including low frequency vibrations possibly associated with eruption below the ice or large patches of seismic activity when the magma stopped propagating toward the eruption site this methodology performs very well provided some parameters are available and allows to gain insights into the complex dynamics associated with magma movements

twentieth century tropical sea surface temperature trends revisited 1 this study compares the global distribution of 20th century sst and marine air temperature trends from a wide variety of data sets including un interpolated archives as well as globally complete reconstructions apart from the eastern equatorial pacific all datasets show consistency in their statistically significant trends with warming everywhere except the far northwestern atlantic the largest warming trends are found in the middle latitudes of both hemispheres two of the sst reconstructions exhibit statistically significant cooling trends over the eastern equatorial pacific in disagreement with the un interpolated sst and marine air temperature datasets which show statistically significant warming in this region twentieth century trends in tropical marine cloudiness precipitation and slp from independent data sets provide physically consistent evidence for a reduction in the strength of the atmospheric walker circulation accompanied by an eastward shift of deep convection from the western to the central equatorial pacific

an equatorial extratropical dipole structure of the atlantic nino abstractequatorial atlantic variability is dominated by the atlantic nino peaking during the boreal summer studies have shown robust links of the atlantic nino to fluctuations of the st helena subtropical anticyclone and benguela nino events furthermore the occurrence of opposite sea surface temperature sst anomalies in the eastern equatorial and southwestern extratropical south atlantic ocean sao also peaking in boreal summer has recently been identified and termed the sao dipole saod however the extent to which and how the atlantic nino and saod are related remain unclear here an analysis of historical observations reveals the atlantic nino as a possible intrinsic equatorial arm of the saod specifically the observed sporadic equatorial warming characteristic of the atlantic nino 0 4 k is consistently linked to southwestern cooling 0 4 k of the atlantic ocean during the boreal summer heat budget calculations show that the saod is largely driven by the surface net heat flux anoma

characteristics of wave packets in the upper troposphere part i northern hemisphere winter abstract gridded data produced by the ecmwf reanalysis project have been analyzed to document the properties of wave packets in the northern hemisphere winter midlatitude upper troposphere based on results from earlier investigations 300 hpa meridional wind variations were chosen for analysis wave packet envelopes were also defined by performing complex demodulation on the wind data the properties of the wave packets are mainly illustrated using time lagged one point correlation maps performed both on and wave packet envelopes the results show that over most regions in the northern hemisphere winter with the exception of the deep tropics and near the aleutian low medium scale waves dominant wavenumber 5 8 exhibit the characteristics of downstream development and occur within wave trains that propagate with eastward group velocities much faster than the phase speeds of individual phases of the waves their group velocity is highly correlated with the local time mean 200 400 hpa wind while the

evolution of interannual warm and cold events in the southeast atlantic ocean extreme warm episodes in the southeast atlantic ocean known as benguela ninos have devastating envi ronmental impacts and have been shown to be remotely forced to place these extreme events into perspective the investigation is here extended to minor warm events as well as to cold episodes to this end different sets of observations have been combined with outputs from a numerical simulation of the tropical atlantic for the period 1982 99 it is shown that both warm and cold surface events develop regularly in the same specific region along the coast of angola and namibia some cold events compete in magnitude with major warm episodes local sea air heat flux exchanges do not seem to precondition the sea surface in the angola benguela region prior to the arrival of an event most warm and cold episodes are large scale events despite their limited surface signature they appear to be generated by wind anomalies in the western and central equatorial atlantic in the same way as benguela ninos seasonal fluctuations of the depth and shape of the tropical thermocline seem partly to control the way subsurface anomalies eventually impact the surface during austral summer surface anomalies create an identifiable pool centered near 158s whereas in winter they show an elongated pattern along the coast stretching toward the equator local upwelling or downwelling favorable wind regimes as well as local net heat fluxes may modulate the surface expression of events

two paradigms of baroclinic wave life cycle behaviour two idealized baroclinic wave 6 life cycles examined here suggest a framework of opposite extremes a in which to view the behaviour of real synoptic scale disturbances in middle latitudes and b in which to interrelate the synoptic and wave theoretic viewpoints using the saturation propagation saturation sps picture of wave 6 life cycle behaviour the two life cycles dented by lc1 and lc2 are higher resolution versions of the simmons hoskins basic and anomalous cases showing strong and weak late decay of eddy kinetic energy eke they illustrate in varying degrees two extreme types of behaviour here disignated anticyclonic and cyclonic and epitomized by strongly contrasting upper air trough behaviour anticyclonic behaviour dominates the late stages of lc1 and is characterized by backward tilted thinning troughs being advected anticyclonically and equatorward as in the commoner cases of planetary scale mid stratospheric rossby wave breaking cyclonic behaviour dominates lc2 and is characterized by forward tilted broadening troughs wrapping themselves up cyclonically and poleward producing major cut off cyclones in high latitudes these morphologies are visualized by upper air maps of potential temperature on the nominal tropopause defined as a constant potential vorticity surface some atmospheric mid latitude disturbances examined here using the same visualization applied to operational analyses show the same two extreme types of trough behaviour together with intermediate cases the sps picture is re examined using eliassen palm and refractive index cross sections it is shown in particular by reference to a wave activity theorem of haynes that the late stages of lc2 can be looked upon as a remarkably clear and morphologically novel large amplitude counterpart of the nonlinear reflection scenario of rossby wave critical layer theory the late stages of lc1 by constrast look more akin to an nonlinear critical layer absorption scenario lc2 exhibits region of largely undular pv contours adjacent to a nonlinear of irreversiby deformen pv contous in the latter region pv rearrangement and hence absorption of rossby wave activity has largely ceased this accounts for the more persistent ekee in the lc2 case

interannual and interdecadal variability of the south atlantic convergence zone interannual variations of the summertime january march atmospheric circulation over subtropical south america are examined during the period 1958 97 using the national centers for environmental prediction national center for atmospheric research reanalysis data it is found from an empirical orthogonal function analysis that an anomalous upper tropospheric large scale stationary eddy in the lee of the andes tends to accompany a dipole in anomalous vertical motion an anomalous cyclonic anticyclonic eddy accompanies an intensified diffuse south atlantic convergence zone sacz with anomalous descent ascent to the southwest the cold core equivalent barotropic vertical structure of the anomalous cyclonic eddy and the 200 hpa vorticity balance are both characteristic of a stationary rossby wave the tendency for the eddy to be advected downstream by the mean westerlies is compensated by meridional advection of planetary vorticity and stretching associated with vertical motion the anomalous cyclonic flow at low levels reinforces the thermally direct circulation associated with the sacz a weak funneling of submonthly rossby wave activity into this descent region is also identified the interannual time series of the eddy is significantly correlated with north south dipolar sea surface temperature sst anomalies over the southwest atlantic one standard deviation 200 hpa wind speed anomalies of up to 5 m s21 are accompanied by sst anomalies of up to 0 38c a near cyclic 15 yr component is identified which the authors corroborate from independent analyses of southwest atlantic ssts and river flows both are

an energetic view of the relation between the mediterranean storm track and the north atlantic oscillation the time tendency equation of eddy kinetic energy eke includes some important forcing terms such as baroclinic conversion bcc barotropic conversion btc and energy flux this article compares the ensemble mean distribution of these forcing terms as well as the baroclinic generation bcg term in critical positive and critical negative months of the north atlantic oscillation nao focusing on the mediterranean and middle east regions results show that all the computed terms attain larger values and all the extrema are stronger in the positive phase the energy flux vectors point to energy transfer from the west of the atlantic and north of europe to the mediterranean region with the transfer being weaker in the positive phase there is energy flux divergence over the central region of the mediterranean sea which is more extensive in the positive phase relative to the negative phase the energy flux divergence leads to a radiation source in this region the direction of the energy flux vectors then indicates that the energy radiated from the central mediterranean region is transferred southeastward leading to an energy flux convergence over the red sea and the northeast of africa this convergence is stronger and more extensive in the positive phase which may be responsible for the observed larger value of the upper level eddy kinetic energy in the positive phase the patterns of the bcg and bcc together with the energy flux convergence suggest that while the mediterranean storm track is affected by the north atlantic storm track more strongly in the negative phase it acts as an independent centre of action by radiating energy southeastward in the positive phase copyright 2011 royal meteorological society

the impact of enso on the south atlantic subtropical dipole mode abstractthe impact of el nino southern oscillation enso on the south atlantic subtropical dipole mode sasd is investigated using both observations and model simulations the sasd is the dominant mode of coupled ocean atmosphere variability in the south atlantic this study focuses on austral summer when both enso and sasd peak it is shown that negative sasd events are associated with central pacific el nino events by triggering the pacific south american wave train psa the latter resembles the third leading mode of atmospheric variability in the southern hemisphere psa2 and causes a weakening and meridional shift of the south atlantic subtropical high which then generates the negative sasd events on the other hand a strengthening of the south atlantic subtropical high related to central la nina teleconnections causes positive sasd events the results herein show that the psa2 triggered by central pacific enso events connects the tropical pacific to the atlantic this connection is absent f

evidence for interannual and interdecadal climate variability in the south atlantic a singular value decomposition svd analysis is used to determine the coupled modes of variability of monthly sea surface temperature sst and sea level pressure slp data from the south atlantic region for the period 1953 1992 we find that the three leading svd modes respectively account for 63 20 and 6 of the total square covariance the first mode represents an approximately 15 year period oscillation in the strength of the subtropical anticyclone accompanied by fluctuations of a north south dipole structure in the sst it appears to be linked to the global scale interdecadal 15 year joint mode in sst and slp recently studied by mann and park the second mode is characterized by east west displacements of the anticyclone center in association with strong 6 to 7 year period fluctuations of sst off the coast of africa the third mode is characterized by north south displacements of the anticyclone and 4 year period fluctuations in the sst in a broad band across the central south atlantic this mode is strongly correlated with enso

variability in the south atlantic anticyclone and the atlantic nino mode sea surface temperature sst anomalies in the eastern equatorial atlantic are connected to modulations in the strength of the south atlantic subtropical high pressure system referred to as the south atlantic anticyclone saa using ocean and atmosphere reanalysis products we show here that the strength of the saa from february to may impacts the timing of the cold tongue onset and the intensity of its development in the eastern equatorial atlantic eea via anomalous tropical wind power this modulation of the timing and amplitude of the seasonal cold tongue development manifests as anomalous sst events peaking between june and august the timing and impact of this connection is not completely symmetric for warm and cold events for cold events an anomalously strong saa in february and march leads to positive wind power anomalies from february to june resulting in an early cold tongue onset and subsequent cold sst anomalies in june and july for warm events the anomalously weak saa persists until may generating negative wind power anomalies that lead to a late cold tongue onset as well as a suppression of the cold tongue development and associated warm sst anomalies mechanisms by which saa induced wind power variations south of the equator influence eea sst are discussed including ocean adjustment via rossby and kelvin wave propagation meridional advection and local intraseasonal wind variations

decadal and interannual sst variability in the tropical atlantic ocean abstract the mechanisms regulating interannual and decadal variations of sea surface temperature sst in the tropical atlantic are examined observed variations of sea surface temperature are typically in the range of 0 3 0 5 c and are linked to fluctuations in rainfall on both the african and south american continents the authors use a numerical model to simulate the observed time series of sea surface temperature for the period 1960 1989 based on the results experiments are conducted to determine the relative importance of heat flux and momentum forcing two dominant timescales for variability of sst are identified a decadal timescale that is controlled by latent heat flux anomalies and is primarily responsible for sst anomalies off the equator and an equatorial mode with a timescale of 2 5 years that is dominated by dynamical processes the interhemispheric gradient of anomalous sst the sst dipole is primarily linked to the former process and thus results from the gradual strengthening and weak

are the winters 2010 and 2012 archetypes exhibiting extreme opposite behavior of the north atlantic jet stream abstractthe atmospheric circulation over the north atlantic european sector experienced exceptional but highly contrasting conditions in the recent 2010 and 2012 winters november march with the year dated by the relevant january evidence is given for the remarkably different locations of the eddy driven westerly jet over the north atlantic in the 2010 winter the maximum of the jet stream was systematically between 30 and 40 n south jet regime whereas in the 2012 winter it was predominantly located around 55 n north jet regime these jet features underline the occurrence of either weak flow 2010 or strong and persistent ridges throughout the troposphere 2012 this is confirmed by the very different occurrence of blocking systems over the north atlantic associated with episodes of strong cyclonic anticyclonic rossby wave breaking in 2010 2012 winter these dynamical features underlie strong precipitation and temperature anomalies over parts of europe with detrimental impacts on many soc

earthquakes at loihi submarine volcano and the hawaiian hot spot loihi is an active submarine volcano located 35 km south of the island of hawaii and may eventually grow to be the next and southernmost island in the hawaiian chain the hawaiian volcano observatory recorded two major earthquake swarms located there in 1971 1972 and 1975 which were probably associated with submarine eruptions or intrusions the swarms were located very close to loihi s bathymetric summit except for earthquakes during the second stage of the 1971 1972 swarm which occurred well onto loihi s southwest flank the flank earthquakes appear to have been triggered by the preceding activity and possible rifting along loihi s long axis similar to the rift flank relationship at kilauea volcano other changes accompanied the shift in locations from loihi s summit to its flank including a shift from burst to continuous seismicity a rise in maximum magnitude a change from small earthquake clusters to a larger elongated zone a drop in b value and a presumed shift from concentrated volcanic stresses to a more diffuse tectonic stress on loihi s flank the 1971 1972 swarm began at depths of 20 50 km about 1 month before the shallow swarm started and suggests an upward migration of seismicity as the first stage of the 13 month swarm the seismic root of kilauea volcano is well defined by earthquakes which plunge to the south and southwest to a depth of 50 60 km terminating in a region in which earthquakes are associated with deep harmonic tremor and magma the zone of these deeper earthquakes and tremor lies between kilauea loihi and mauna loa volcanoes may feed magma to all three volcanoes and probably locates the hawaiian hot spot

multiple causes of interannual sea surface temperature variability in the equatorial atlantic ocean the tropical atlantic ocean shows sea surface temperature variability on interannual timescales observational and model data suggest that some of this variability can be attributed to the advection of anomalously warm northern subtropical waters toward the equator

influence of the south atlantic ocean dipole on west african summer precipitation abstract this paper demonstrates a causal link between the air sea phenomenon referred to as the south atlantic ocean sao dipole saod and the interannual variability of precipitation over west africa during the boreal summer monsoon rainy season in which most of the annual rainfall is recorded using an array of observational datasets analyses show that positive precipitation anomalies exceeding 40 mm month 1 over most locations at the guinea coast are associated with the positive phase of the saod which is characterized by warm sea surface temperature anomalies sstas in the northeastern part of the sao or the northeast pole nep that is the atlantic nino sector and cool ssta in the southwestern part southwest pole swp off the argentina uruguay brazil coast on the other hand interannual variability in the sahel is closely connected to the west african summer monsoon and the atlantic nino the results of this study reveal that the well known influence of the atlantic nino on guinea coast pre

real time assessment of potential seismic migration within a monitoring network using red flag sara abstract magma opening new fluid pathways through the crust can generate migrating seismic sources following the trail of the magma by using seismic amplitude ratio analysis sara it is possible to detect this seismic migration simply from the amplitudes of continuous data recorded at different stations in a network without having to do any picking of seismic phases in this study we present a modified method red flag sara which adapts sara for real time monitoring red flag sara provides a quantitative tool to analyse amplitude ratios between stations in a network and detect temporal changes in these ratios since such changes imply seismic source location variations red flag sara is a handy tool during seismic crises to quickly answer the question of whether seismic activity and therefore magma is migrating or not we tested red flag sara on synthetic data and validated it using real data from two volcanoes piton de la fournaise reunion island and gede indonesia for three scenarios 1 magma migration ending as intrusion 2 migration leading to eruption and 3 a burst of seismicity with no magma migration

the bjerknes feedback in the tropical atlantic in cmip5 models coupled state of the art general circulation models still perform relatively poorly in simulating tropical atlantic ta climate to investigate whether lack of air sea interaction might be responsible for their biases we investigate the bjerknes feedback bf in the ta the driver of the dominant interannual variability in that region first we analyse this mechanism from reanalysis data then we compare our findings to model output from the coupled model intercomparison project phase 5 the feedback is subdivided into three components the first one consists of the influence of eastern equatorial sea surface temperature anomalies sst on zonal wind stress anomalies tau u in the western basin the second component is the influence of wind stress anomalies in the western ta on eastern equatorial oceanic heat content anomalies hc the third component is the local response of overlying sst to hc in the eastern ta all three components are shown to be present in era interim and oras4 reanalysis by correlating the two variables of each component with each other the obtained patterns are compared to the ones from model output via pattern correlation per component while the models display errors in the annual cycles of sst tau u and hc as well as in the seasonality of the feedback the impact of sst on wind stress and the impact of wind stress on hc are simulated relatively well by most of the models this is especially the case when correcting for the error in seasonality the third component of the bf the impact of hc on sst in the eastern part of the basin deviates from what we find in reanalysis we find an influence of hc anomalies on overlying ssts in the eastern equatorial ta but it is weaker than in the reanalysis and it is not strongly confined to the equator longitude depth cross sections of equatorial temperature variance and correlation between subsurface temperature anomalies and sst in the cold tongue region show that flawed simulation and slow adjustment of the subsurface ocean are responsible for this

simulated impacts of the south atlantic ocean dipole on summer precipitation at the guinea coast an intermediate complexity atmospheric general circulation model has been used to investigate the influence of the south atlantic ocean sao dipole saod on summer precipitation over the guinea coast of west africa two ensemble integrations in which idealized but realistic saod type sea surface temperature sst anomaly is prescribed only in the sao and then globally are performed and inter compared consistently above below the average precipitation is simulated over the guinea coast during the positive negative phase of the saod comparison of the two set of experiments reveal that in its active years the saod is a dominant mechanism that shapes the spatial character of summer precipitation at the guinea coast the global sst variability merely slightly moderate its effects during the saod cool sst anomaly in the extra tropical sao off the brazil uruguay argentina coast gives rise to suppressed convection and mass divergence in turn the subsidence tends to amplify the sub tropical arm of anomalous hadley type circulation and consequently large scale convection and mass flux convergence in the equatorial atlantic ocean gulf of guinea region bordering on the coastal fringes of west africa precipitation is therefore increased at the guinea coast

atmosphere ocean coupled variability in the south atlantic abstract the climate variability of the south atlantic region is determined from 40 yr 1953 92 of comprehensive ocean atmosphere data set monthly sea surface temperature sst and sea level pressure slp data using the empirical orthogonal function eof and the singular value decomposition svd analysis methods the eof method is applied to each field separately whereas the svd method is applied to both fields simultaneously the significance of the atmosphere ocean interaction is revealed by a strong resemblance between individual eof and coupled svd modes of sst and slp the three leading modes of coupled variability on interannual and interdecadal timescales are discussed in some detail the first coupled mode which accounts for 63 of the total square covariance represents a 14 16 yr period oscillation in the strength of the subtropical anticyclone accompanied by fluctuations of a north south dipole structure in the sst the atmosphere ocean coupling is strongest during the southern summe

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

interannual atmospheric variability forced by the deep equatorial atlantic ocean climate variability in the tropical atlantic ocean is determined by large scale ocean atmosphere interactions which particularly affect deep atmospheric convection over the ocean and surrounding continents1 apart from influences from the pacific el nino southern oscillation2 and the north atlantic oscillation3 the tropical atlantic variability is thought to be dominated by two distinct ocean atmosphere coupled modes of variability that are characterized by meridional4 5 and zonal6 7 sea surface temperature gradients and are mainly active on decadal and interannual timescales respectively8 9 here we report evidence that the intrinsic ocean dynamics of the deep equatorial atlantic can also affect sea surface temperature wind and rainfall in the tropical atlantic region and constitutes a 4 5 yr climate cycle specifically vertically alternating deep zonal jets of short vertical wavelength with a period of about 4 5 yr and amplitudes of more than 10 cm s 1 are observed in the deep atlantic to propagate their energy upwards towards the surface10 11 they are linked at the sea surface to equatorial zonal current anomalies and eastern atlantic temperature anomalies that have amplitudes of about 6 cm s 1 and 0 4 c respectively and are associated with distinct wind and rainfall patterns although deep jets are also observed in the pacific12 and indian13 oceans only the atlantic deep jets seem to oscillate on interannual timescales our knowledge of the persistence and regularity of these jets is limited by the availability of high quality data despite this caveat the oscillatory behaviour can still be used to improve predictions of sea surface temperature in the tropical atlantic deep jet generation and upward energy transmission through the equatorial undercurrent warrant further theoretical study

an image of a magma intrusion process inferred from precise hypocentral migrations of the earthquake swarm east of the izu peninsula summary earthquake swarms in the area east of the izu peninsula central japan have been active and have been repeated intermittently since 1978 after 40 years of quiescence the activities were always accompanied by crustal deformations which were well modelled by dyke intrusions to study the process of a dyke intrusion precise hypocentres of the latest activity occurring in 1998 are obtained in this paper using waveform similarity by which new images of volcanic processes have been successfully acquired at several volcanoes the relocated hypocentres are mainly aligned on a thin vertical plane with a circular shape at a depth of 3 7 km and there is an aseismic area at the centre the normal direction to the plane coincides well with the direction of tectonic extensional stress around the hypocentral area and matches theoretical models well at the beginning of the activity a small fraction of the events occurred at greater depth where hypocentres align on a vertical line and migrate upward at a rate of 1 km h 1 the migration rate in this stage agrees well with a theoretical model of buoyancy driven dyke propagation after 1 day from the beginning earthquakes began to occur around the aseismic area and hypocentres spread on the thin circular plane described above during this activity the hypocentres seemed to migrate downwards and upwards from the centre the earthquakes are caused by shear fracture at the tip of the dyke and the migration of hypocentres reflects that the dyke expands downward and upward from the neutrally buoyant layer that is located at the centre of the swarm in this paper we also propose a process of magma intrusion based on the precise migration of hypocentres and show that the 1998 activity is composed of a few simple volcanic processes magma rising by buoyancy it staying at a neutral buoyancy point and spreading outward from the neutral point with elastic fracture by an inside excess magma pressure

imaging the dynamics of dyke propagation prior to the 2000 2003 flank eruptions at piton de la fournaise reunion island 1 the relationship between shallow magma storage and flank injections in volcanic edifices is rarely documented we analyse the time series of geodetic and seismic data collected at piton de la fournaise volcano la reunion during 9 flank eruptions from 2000 to 2003 the data depicts how magma injections supplied flank eruptions at piton de la fournaise volcano and allows to precisely determine the direction and duration of dike intrusions indicating a two phases pattern consistent over all the eruptions 1 fast vertical migration of the dyke 2 m s from the magma chamber to the surface in 10 to 50 minutes and 2 slower lateral migration 0 2 0 8 m s in tens of minutes to hours controlled by the rift zone geometry

the atlantic itcz bias in cmip5 models precipitation over the tropical atlantic in 24 atmospheric models is analyzed using an object based approach which clusters rainy areas in the models as precipitation objects and calculates their properties such as size amplitude and location based on the distribution of precipitation objects over land and over ocean two classes of models emerge the first class of models has a reasonable representation of objects over land but misplaces the ocean object westward near the coast of brazil instead of the central atlantic as observed the second class of models show small sized objects over land with intense precipitation values for these models the ocean object is located eastward near the coast of guinea the atlantic intertropical convergence zone structure in the models exhibits either the west or the east atlantic bias no model matches the observed precipitation distribution the two distinct model behaviors in the mean state are traced to the coastal precipitation bias of the models in boreal spring in this season the two model groups place the main precipitation object on opposite coasts one group puts it at the south coast of brazil and the other group places it at the gulf of guinea this west east partitioning of precipitation is sustained in boreal summer resulting in the west and east atlantic bias in the annual mean it is found that models with the east atlantic bias tend to be high resolution models which rain excessively over the gulf of guinea starting from boreal spring

weakening of the equatorial atlantic cold tongue over the past six decades seasonal and interannual variations of the equatorial cold tongue are defining features of the tropical atlantic ocean an analysis of bias corrected observations suggests that cold tongue variability has weakened over the past six decades

volcano tectonic earthquakes a new tool for estimating intrusive volumes and forecasting eruptions abstract we present data on 136 high frequency earthquakes and swarms termed volcano tectonic vt seismicity which preceded 111 eruptions at 83 volcanoes plus data on vt swarms that preceded intrusions at 21 other volcanoes we find that vt seismicity is usually the earliest reported seismic precursor for eruptions at volcanoes that have been dormant for decades or more and precedes eruptions of all magma types from basaltic to rhyolitic and all explosivities from vei 0 to ultraplinian vei 6 at such previously long dormant volcanoes because large eruptions occur most commonly during resumption of activity at long dormant volcanoes vt seismicity is an important precursor for the earth s most dangerous eruptions vt seismicity precedes all explosive eruptions of vei 5 and most if not all vei 4 eruptions in our data set surprisingly we find that the vt seismicity originates at distal locations on tectonic fault structures at distances of one or two to tens of kilometers laterally from the site of the eventual eruption and rarely if ever starts beneath the eruption site itself the distal vt swarms generally occur at depths almost equal to the horizontal distance of the swarm from the summit out to about 15 km distance beyond which hypocenter depths level out we summarize several important characteristics of this distal vt seismicity including swarm like nature onset days to years prior to the beginning of magmatic eruptions peaking of activity at the time of the initial eruption whether phreatic or magmatic and large non double couple component to focal mechanisms most importantly we show that the intruded magma volume can be simply estimated from the cumulative seismic moment of the vt seismicity from log10 v 0 77 log moment 5 32 with volume v in cubic meters and seismic moment in newton meters because the cumulative seismic moment can be approximated from the size of just the few largest events and is quite insensitive to precise locations the intruded magma volume can be quickly and easily estimated with few short period seismic stations notable cases in which distal vt events preceded eruptions at long dormant volcanoes include nevado del ruiz 1984 1985 pinatubo 1991 unzen 1989 1995 soufriere hills 1995 shishaldin 1989 1999 tacana 1985 1986 pacaya 1980 1984 rabaul 1994 and cotopaxi 2001 additional cases are recognized at frequently active volcanoes including popocateptl 2001 2003 and mauna loa 1984 we present four case studies pinatubo soufriere hills unzen and tacana in which we demonstrate the above mentioned vt characteristics prior to eruptions using regional data recorded by neic we recognized in near real time that a huge distal vt swarm was occurring deduced that a proportionately huge magmatic intrusion was taking place beneath the long dormant sulu range new britain island papua new guinea that it was likely to lead to eruptive activity and warned rabaul volcano observatory days before a phreatic eruption occurred this confirms the value of this technique for eruption forecasting we also present a counter example where we deduced that a vt swarm at volcan cosiguina nicaragua indicated a small intrusion insufficient to reach the surface and erupt finally we discuss limitations of the method and propose a mechanism by which this distal vt seismicity is triggered by magmatic intrusion

locating pyroclastic flows on soufriere hills volcano montserrat west indies using amplitude signals from high dynamic range instruments pyroclastic flows are located using amplitude signals from a seven station high dynamic range seismograph array located 1 9 6 1 km from soufriere hills volcano in montserrat west indies locations are determined by measuring the seismograph signal amplitude for an event recorded at several stations in a moving time window analysis for a given window the measured amplitudes are corrected to a trial source location by removing the effect of the surface wave geometric spreading instrument gain and the attenuation at calculated travel times the trial source location is then compared to other trial locations via an iterative localised grid search where the root mean squared amplitude residual a is minimised the process is repeated for subsequent time steps resulting in a best fit event location and size through time the method has been tested on four small events occurring on april 8 1999 august 12 1999 february 25 2001 and july 4 2001 when visual observations of pyroclastic flows coincided with good seismograph station coverage number of stations 5 azimuthal gap 160 based on the location results the four events propagated 0 5 1 4 1 3 and 1 0 km from the dome and had maximum attenuation corrected reduced displacements drq of 9 0 2 8 6 9 and 2 3 cm2 and maximum pyroclastic flow velocities of 7 30 20 and 8 ms 1 respectively a time lapse video of the event of august 12 1999 shows that amplitude based location through time closely matches the observed run out distance and velocity in contrast amplitude based locations for the events of april 8 1999 and july 4 2001 underestimated the actual flow run out by 1 5 km underestimation of the true run out distance is probably due to both the increased distribution of sources as coherent dome material disaggregates into many blocks and signal contamination from other sources results indicate that pyroclastic flows and rockfalls can be located using amplitude signals from high dynamic range seismograph stations yielding estimates of size and trajectory regardless of visibility conditions on the volcano this new method is being tested as a hazard mitigation and research tool on montserrat

on the mechanisms governing dike arrest insight from the 2000 miyakejima dike injection abstract magma stored beneath volcanoes is sometimes transported out of the magma chambers by means of laterally propagating dikes which can lead to fissure eruptions if they intersect the earth s surface the driving force for lateral dike propagation can be a lateral tectonic stress gradient the stress gradient due to the topographic loads the overpressure of the magma chamber or a combination of those forces the 2000 dike intrusion at miyakejima volcano izu arc japan propagated laterally for about 30 km and stopped in correspondence of a strike slip system sub perpendicular to the dike plane then the dike continued to inflate without further propagation abundant seismicity was produced including five m 6 earthquakes one of which occurred on the pre existing fault system close to the tip of the dike at approximately the time of arrest it has been proposed that the main cause for the dike arrest was the fault induced stress here we use a boundary element numerical approach to study the interplay between a propagating dike and a pre stressed strike slip fault and check the relative role played by dike fault interaction and topographic loading in arresting the miyakejima dike we calibrate the model parameters according to previous estimates of dike opening and fault displacement based on crustal deformation observations by computing the energy released during the propagation our model indicates whether the dike will stop at a given location we find that the stress gradient induced by the topography is needed for an opening distribution along the dike consistent with the observed seismicity but it cannot explain its arrest at the prescribed location on the other hand the interaction of dike with the fault explains the arrest but not the opening distribution the joint effect of the topographic load and the stress interaction with strike slip fault is consistent with the observations provided the pre existing fault system is pre loaded with a significant stress released gradually during the dike fault interplay our results reveal how the mechanical interaction between dikes and faults may affect the propagation of magmatic intrusions in general this has implications for our understanding of the geometrical arrangement of rift segments and transform faults in mid ocean ridges and for the interplay between dikes and dike induced graben systems

dynamical feedbacks and the persistence of the nao the persistence of the north atlantic oscillation nao is studied using observations of the threedimensional vorticity budget in the atlantic sector analysis of the relative vorticity tendency equation shows that convergence of eddy vorticity flux in the upper troposphere counteracts the effect of anomalous largescale divergence at the upper level at low levels the convergence associated with this large scale vertical circulation cell maintains the relative vorticity anomaly against frictional drag the eddy vorticity flux convergence thus acts to sustain the vorticity anomaly associated with the nao against drag and increases the persistence of the nao vorticity anomaly the adiabatic cooling associated with the rising motion in the vorticity maximum sustains the thermal structure of the nao anomaly enhancing the baroclinicity and thus eddy generation this constitutes a positive eddy feedback that helps maintain the nao the positive eddy feedback occurs only in the midlatitude region and is strongest during the negative phase of the nao when the atlantic jet is displaced toward the equator with a high pressure anomaly to the north and a low pressure anomaly to the south the stronger feedback demonstrated during the negative phase is consistent with the greater persistence observed for this phase of the nao the positive feedback appears to be associated with anomalous northward eddy propagation away from the jet

on the triggering of benguela ninos remote equatorial versus local influences 1 the relative importance of remote and local influences in the development of benguela ninos is examined using observations and a coupled general circulation model capable of simulating interannual variability in the tropical atlantic while previous studies have emphasized the role of equatorially excited kelvin waves the present study finds that meridional wind anomalies along the southwest african coast contribute substantially both observations and model output indicate that sea surface temperatures along the southwest african coast respond rapidly to changes in meridional wind stress these wind anomalies form part of a basin scale weakening of the subtropical anticyclone that extends to the equator as the weakening begins three months before the peak of the event it might have some predictive potential results also indicate that the close correlation between benguela and atlantic ninos in observations might result from the large spatially coherent wind stress anomalies associated with the weakened anticyclone

theoretical basis of the amplitude source location method for volcano seismic signals the source location method using high frequency seismic amplitudes based on the assumption of isotropic radiation of s waves has been used successfully to locate seismic events and tremor at volcanoes this amplitude source location asl method overcomes the limitations of traditional hypocenter determination methods that use onset arrival times and has great potential to improve volcano seismic monitoring and the investigation of source processes of volcano seismic signals however theoretical justification of the basic assumption used in the asl method has not been provided in previous studies in this study we tested the asl method by using seismic waveforms simulated with a finite difference method in realistic heterogeneous volcanic structures with intrinsic attenuation and topography our results showed that asl determinations were successful when using waveforms characterized by multiple scattering that can be approximated by the diffusion model we found that the energy solution of the diffusion model provided an amplitude distance relation that is similar to that used in the asl method our results suggest that the asl method is applicable to high frequency seismograms in highly heterogeneous media with transport mean free paths of 103 m or smaller and strong intrinsic attenuation these medium parameters are consistent with those estimated at various volcanoes our study validates the basic assumption of the asl method and justifies its use to locate the sources of high frequency seismic signals observed at volcanoes

the dynamics of nao teleconnection pattern growth and decay this investigation performs both diagnostic analyses with ncep ncar re analysis data and forced barotropic model calculations to examine the dynamical mechanisms associated with the growth and decay of the north atlantic oscillation nao teleconnection pattern the diagnostic calculations include projection and composite analyses of each term in the stream function tendency equation the results of the analyses reveal a complete life cycle of growth and decay within approximately two weeks the positive nao phase is found to develop after anomalous wavetrain propagation across the north pacific to the east coast of north america this contrasts with the negative nao phase which appeared to develop in situ both high frequency period 10 days transient eddy fluxes drive the nao growth after the nao anomaly attains its maximum amplitude the high frequency transient eddy fluxes continue to drive the nao anomaly in a manner that is consistent with a positive feedback process the decay of the nao occurs through both the divergence term and the low frequency transient eddy fluxes the temporal and spatial properties of the divergence term are found to be consistent with ekman pumping these results illustrate many important differences between the nao and pacific north american pna teleconnection patterns perhaps most striking being that the nao life cycle is dominated by nonlinear processes whereas the pna evolution is primarily linear in addition the relation between the nao and the zonal index is discussed copyright 2003 royal meteorological society

imaging the dynamics of magma propagation using radiated seismic intensity 1 at shallow depth beneath the earth s surface magma propagates through strongly heterogeneous volcanic material inversion of buoyancy and or solidification have strong impacts on the dynamics of propagation without any change of magma supply in this paper we study the spatial and time evolution of magma intrusions using induced seismicity we propose a new method based on ratio analysis of estimates of radiated seismic intensities recorded at different stations during seismic swarms by applying this method to the january 2010 piton de la fournaise volcano eruption we image complex dike propagation dynamics which strongly differ from a model of constant velocity dike propagation we provide a new method to image in real time the dynamics of dike propagation and to infer the position of eruptive fissures

magma migration at the onset of the 2012 13 tolbachik eruption revealed by seismic amplitude ratio analysis abstract in contrast of the 1975 76 tolbachik eruption the 2012 13 tolbachik eruption was not preceded by any striking change in seismic activity by processing the klyuchevskoy volcano group seismic data with the seismic amplitude ratio analysis sara method we gain insights into the dynamics of magma movement prior to this important eruption a clear seismic migration within the seismic swarm started 20 hours before the reported eruption onset 05 15 utc 26 november 2012 this migration proceeded in different phases and ended when eruptive tremor corresponding to lava flows was recorded at 11 00 utc 27 november 2012 in order to get a first order approximation of the magma location we compare the calculated seismic intensity ratios with the theoretical ones as expected the observations suggest that the seismicity migrated toward the eruption location however we explain the pre eruptive observed ratios by a vertical migration under the northern slope of plosky tolbachik volcano followed by a lateral migration toward the eruptive vents another migration is also captured by this technique and coincides with a seismic swarm that started 16 20 km to the south of plosky tolbachik at 20 31 utc on november 28 and lasted for more than 2 days this seismic swarm is very similar to the seismicity preceding the 1975 76 tolbachik eruption and can be considered as a possible aborted eruption

decadal change of the south atlantic ocean angola benguela frontal zone since 1980 high resolution simulations with a regional atmospheric model coupled to an intermediate level mixed layer ocean model along with multiple atmospheric and oceanic reanalyses are analyzed to understand how and why the angola benguela frontal zone abfz has changed since 1980 a southward shift of 0 05 0 55 latitude decade 1 in the annual mean abfz position accompanied by an intensification of 0 05 to 0 13 k 100 km decade 1 has occurred as ocean mixed layer temperatures have warmed cooled equatorward poleward of the front over the 1980 2014 period these changes are captured in a 35 year model integration the oceanic warming north of the abfz is associated with a weakening of vertical entrainment reduced cooling associated with vertical diffusion and a deepening of the mixed layer along the angola coast these changes coincide with a steady weakening of the onshore atmospheric flow as the zonal pressure gradient between the eastern equatorial atlantic and the congo basin weakens oceanic cooling poleward of the abfz is primarily due to enhanced advection of cooler water from the south and east increased cooling by vertical diffusion and shoaling of the mixed layer depth in the atmosphere these changes are related to an intensification and poleward shift of the south atlantic sub tropical anticyclone as surface winds hence the westward mixed layer ocean currents intensify in the benguela upwelling region along the namibian coast with a few caveats these findings demonstrate that air sea interactions play a prominent role in influencing the observed decadal variability of the abfz over the southeastern atlantic since 1980

air sea interaction and the seasonal cycle of the subtropical anticyclones abstract the causes of the seasonal cycles of the subtropical anticyclones and the associated zonal asymmetries of sea surface temperature sst across the subtropical oceans are examined in all basins the cool waters in the east and warm waters in the west are sustained by a mix of atmosphere and ocean processes when the anticyclones are best developed during local summer subsidence and equatorward advection on the eastern flanks of the anticyclones cool ssts while poleward flow on the western flanks warms ssts during local winter the sst asymmetry across the subtropical north atlantic and north pacific is maintained by warm water advection in the western boundary currents that offsets the large extraction of heat by advection of cold dry air of the continents and by transient eddies in the southern hemisphere ocean processes are equally important in cooling the eastern oceans by upwelling and advection during local winter ocean dynamics are important in amplifying the sst asymmetry as experi

barotropic effects on atmospheric storm tracks the effect of barotropic dynamics on atmospheric midlatitude storm tracks is investigated for the northern hemisphere winter season using both observed data analyses and linear barotropic model experiments it is shown that when the model flow is initialized with a realistic wave packet barotropic processes alone can produce key features of the observed storm track structure the author attributes this result both to the barotropic waveguide effect and to the fact that the geographical locations of barotropic growth coincide reasonably well with the baroclinic growth of the atmospheric storm track eddies this study also shows that barotropic dynamics are more relevant for storm tracks in lower latitudes than for those in higher latitudes consistent with the fact that the lower latitude storm tracks are more closely associated with the subtropical jet rather than with the polar front jet that is driven by baroclinic eddies

a comparative stability analysis of atlantic and pacific nino modes abstractel nino southern oscillation enso in the pacific and the analogous atlantic nino mode are generated by processes involving coupled ocean atmosphere interactions known as the bjerknes feedback it has been argued that the atlantic nino mode is more strongly damped than enso which is presumed to be closer to neutrally stable in this study the stability of enso and the atlantic nino mode is compared via an analysis of the bjerknes stability index this index is based on recharge oscillator theory and can be interpreted as the growth rate for coupled modes of ocean atmosphere variability using observational data an ocean reanalysis product and output from an ocean general circulation model the individual terms of the bjerknes index are calculated for the first time for the atlantic and then compared to results for the pacific positive thermocline feedbacks in response to wind stress forcing favor anomaly growth in both basins but they are twice as large in the pacific compared to the atlanti

influence of tropical south atlantic sea surface temperatures on the indian summer monsoon in cmip5 models in this study the teleconnection from the tropical south atlantic to the indian monsoon has been assessed in observations and in 32 models from the world climate research programme wcrp coupled model intercomparison project phase 5 cmip5 all models show that the regression pattern of tropics wide atlantic sea surface temperature sst anomalies onto the tropical south atlantic index correlates well with that in observations even though with varying spatial standard deviations however only about half of the 32 models considered show the correct sign of rainfall response over india to a warm anomaly in the south tropical atlantic which is a reduction of rainfall on the other hand models generally do show large scale responses broadly consistent with the observations and the signal over india depends on relatively subtle changes in the response this response to a tropical south atlantic warm cold anomaly is a low level quadrupole in stream function with an anticyclonic cyclonic anomaly over the arabian sea and india this anticyclonic cyclonic anomaly leads to a weakening strengthening of the somali jet and low level divergence convergence over india both inducing a reduction an increase of indian rainfall the models which do not show the correct rainfall response over india also show a response similar to the one indicated above but with maximum of the anticyclonic cyclonic response shifted to the western pacific the large scale walker circulation adjustment to the tropical south atlantic sst anomalies is identified as one of the factors which account for the differences in the low level stream function response models and the observations with the correct sign of the rainfall signal over india show the dominant upper level convergence divergence as response to a warm cold tropical south atlantic in the western pacific region whereas models with the wrong sign of the rainfall signal show it predominantly in the central eastern pacific the fact that such relatively subtle changes in the response lead to a reversal of the rainfall signal over india raises the question about the robustness of the tropical south atlantic indian monsoon teleconnection

dominant modes of variability in the south atlantic a study with a hierarchy of ocean atmosphere models using an atmosphere model of intermediate complexity and a hierarchy of ocean models the dominant modes of interannual and decadal variability in the south atlantic ocean are studied the atmosphere simplified parameterizations primitive equation dynamics speedy model has t30l7 resolution the physical package consists of a set of simplified physical parameterization schemes based on the same principles adopted in the schemes of state of the art agcms it is at least an order of magnitude faster whereas the quality of the simulated climate compares well with those models the hierarchy of ocean models consists of simple mixed layer models with an increasing number of physical processes involved such as ekman transport wind induced mixing and wind driven barotropic transport finally the atmosphere model is coupled to a regional version of the miami isopycnal coordinate ocean model micom covering the south atlantic with a horizontal resolution of 1 and 16 vertical layers the coupled modes of mean sea level pressure and sea surface temperature simulated by speedy micom strongly resemble the modes as analyzed from the ncep ncar reanalysis indicating that this model configuration possesses the required physical mechanisms for generating these modes of variability using the ocean model hierarchy the authors were able to show that turbulent heat fluxes ekman transport and wind induced mixing contribute to the generation of the dominant modes of coupled sst variability the different roles of these terms in generating these modes are analyzed variations in the wind driven barotropic transport mainly seem to affect the sst variability in the brazil malvinas confluence zone the spectra of the mixed layer models appeared to be too red in comparison with the fully coupled speedy micom model due to the too strong coupling between sst and surface air temperatures sats resulting from the inability to advect and subduct sst anomalies by the mixed layer models in speedy micom anomalies in the southeastern corner of the south atlantic are subducted and advected toward the north brazilian coast on a time scale of about 6 yr

physical processes associated with the tropical atlantic sst gradient during the anomalous evolution in the southeastern ocean abstract this work is focused on the evolution of the dominant air sea coupled mode in the equatorial and southeastern tropical atlantic and the associated physical processes it is shown that in june august jja the evolution of the dominant mode is mainly dynamically driven and displays a coherent warming or cooling pattern extending from the angola coast toward the equator in the gulf of guinea for anomalies peaking in jja the warming cooling is initiated near the angola coast in march may it is suggested that sst anomalies along the coast and near the equator are physically connected the air sea interaction along the coast may be a major factor in triggering the development of sst anomalies near the equator which is intensified by local positive feedbacks that may include bjerknes and ekman processes in return the warming or cooling near the equator weaken the sst anomalies along the coast by changing the direction of anomalous wind slow westward rossby wave propagation may also play a role

breaking waves at the tropopause in the wintertime northern hemisphere climatological analyses of the orientation and the theoretical lc1 2 classification abstract breaking waves on the tropopause are viewed as potential vorticity pv streamers on middle world isentropic levels a northern hemisphere winter climatology of the streamers spatial distribution and meridional orientation is derived from the 40 yr ecmwf re analysis era 40 dataset and used to assess the nature and frequency of occurrence of breaking synoptic scale waves the streamers are grouped into two classes related to the so called cyclonic lc2 and anticyclonic lc1 patterns and the ambient wind strength and wind shear is also noted it is shown that the occurrence of cyclonic and anticyclonic pv streamers exhibits a distinct spatial variability in the horizontal and the vertical the majority of cyclonic pv streamers are found on lower isentropic levels that intersect the tropopause at more poleward latitudes whereas anticyclonic streamers predominate at higher elevations in the subtropics an analysis of the streamer patterns for the two phases of the north atlantic oscillation

dynamics of upwelling annual cycle in the equatorial atlantic ocean the annual upwelling is an important component of the equatorial atlantic annual cycle a simple theory is proposed using the framework of zebiak cane zc ocean model for insights into the dynamics of the upwelling annual cycle it is demonstrated that in the atlantic equatorial region this upwelling is dominated by ekman processing in the west whereas in the east it is primarily owing to shoaling and deepening of the thermocline resulting from equatorial mass meridional recharge discharge and zonal redistribution processes associated with wind driven equatorial ocean waves this wind driven wave upwelling plays an important role in the development of the annual cycle in the sea surface temperature sst of the cold tongue in the eastern equatorial atlantic

cloud sst feedback in southeastern tropical atlantic anomalous events 1 using satellite based cloud measurements for 1984 2004 the interannual variability of the low level cloud cover over the tropical atlantic ocean in austral winter june july august jja is examined it is found that the leading pattern of the low cloud anomalies in this season is a modulation of the climatological center of the cloud cover over the southeastern tropical atlantic ocean off the angola and namibia coasts the fluctuation of cloud amount there occurs on both interannual and longer timescales the relationship between this low cloud anomalous pattern and basinwide ocean atmosphere anomalies is studied through a composite analysis based on the objectively selected major low cloud deficit and excess years for the composites we intentionally use data sets mainly based on satellite measurements for the past one to two decades to minimize the potential influences of the bias in the model based ocean atmosphere analyses the composites show that the jja anomalous cloud pattern is strongly influenced by the sea surface temperature sst anomalies of the equatorial and southeastern tropical atlantic ocean in the previous summer the anomalous surface warm events off the southwestern coast of africa near 15 s in january and february are usually initiated dynamically by remote forcing from the westerly wind anomalies over the western equatorial atlantic preceding the cloud anomalies during the next few months the warm water is spread into the southeastern tropical atlantic ocean and is conducive to deficit low cloud cover in subsequent jja the reduced low cloud cover in turn forces positive sst tendency in a larger area of the southeastern atlantic ocean by changing the amount of the local solar radiation reaching the sea surface in june and july this process moves the major center of the sst anomalies away from the coast and closer to the equator when the coastal process weakens the low cloud radiation sst feedback also plays a role in the slow westward expansion of the sst anomalies in late austral winter and spring overall the influence of the cloud fluctuation is an important component in the evolution of the southeastern tropical atlantic anomalous events

is there evidence of changes in tropical atlantic variability modes under amo phases in the observational record abstractthe atlantic multidecadal oscillation amo is the leading mode of atlantic sea surface temperature sst variability at multidecadal time scales previous studies have shown that the amo could modulate el nino southern oscillation enso variance however the role played by the amo in the tropical atlantic variability tav is still uncertain here it is demonstrated that during negative amo phases associated with a shallower thermocline the eastern equatorial atlantic sst variability is enhanced by more than 150 in boreal summer consequently the interannual tav modes are modified during negative amo the atlantic nino displays larger amplitude and a westward extension and it is preceded by a simultaneous weakening of both subtropical highs in winter and spring in contrast a meridional seesaw slp pattern evolving into a zonal gradient leads the atlantic nino during positive amo the north tropical atlantic nta mode is related to a scandinavian blocking pattern during winter and spring

south atlantic variability arising from air sea coupling local mechanisms and tropical subtropical interactions abstract interannual variability in the southern and equatorial atlantic is investigated using an atmospheric general circulation model agcm coupled to a slab ocean model som in the atlantic in order to isolate features of air sea interactions particular to this basin simulated covariability between sea surface temperatures ssts and atmosphere is very similar to the observed non enso related covariations in both spatial structures and time scales the leading simulated empirical coupled mode resembles the zonal mode in the tropical atlantic despite the lack of ocean dynamics and is associated with baroclinic atmospheric anomalies in the tropics and a rossby wave train extending to the extratropics suggesting an atmospheric response to tropical sst forcing the second non enso mode is the subtropical dipole in the sst with a mainly equivalent barotropic atmospheric anomaly centered on the subtropical high and associated with a midlatitude wave train consistent with atmospheric forcing of the sub

dynamics of simulated atlantic upwelling annual cycle in cmip5 models as one of the prominent climatic component of tropical climate system the annual cycle of the equatorial atlantic cold tongue region has been a subject of significant research in this work we examine the fidelity of the key part of the annual cycles namely the upwelling annual cycles from the simulations of the coupled model intercomparison project phase 5 cmip5 utilizing the simple framework we have recently developed based on the zebiak cane zc ocean model we first tested the validity of the simple framework of upwelling as a combination of the local wind driven ekman upwelling and nonlocal wind driven wave upwelling it is demonstrated that the theoretically reconstructed upwelling annual cycles to large extent are in good agreement with the simulations in these coupled climatic models comparing with observations we show that the semi annual component of the upwelling is systematically too strong this significant bias is attributable to the dynamic amplification to the wind driven wave upwelling originated from a less significant bias in the semi annual harmonic component in the central equatorial wind field the latter is dynamically amplified in the wind driven wave upwelling owing to its relatively fast timescale thus the realistic simulations of equatorial annual cycles may sensitively depend on good simulations of the semi annual harmonic components in equatorial wind fields

energetics of the tropical atlantic zonal mode sea surface temperature in the central eastern equatorial atlantic has a seasonal cycle far bigger than that of the pacific but interannual anomalies smaller than those of the pacific given the amplitude of seasonal sst variability one wonders whether the seasonal cycle in the atlantic is so dominant that it is able to strongly influencetheevolutionofitsinterannualvariability inthisstudy interannualupper oceanvariabilitywithinthe tropical atlanticis viewed from an energetics perspective and the role ofocean dynamics in particularthe role ofoceanmemory withinzonalmodeeventsisinvestigated unlikeinthepacificwhereseasonalandinterannual variability involve distinctly different processes the results suggest that the latter is a modulation of the former

what determines the position and intensity of the south atlantic anticyclone in austral winter an agcm study abstract the south atlantic anticyclone is a major feature of the austral winter climatology an atmospheric general circulation model agcm is used to study the dynamics of the south atlantic anticyclone by means of control simulations and experiments to investigate sensitivity to prescribed orography sea surface temperatures and soil wetness the south atlantic anticyclone in the first control simulation is unrealistically zonally elongated and centered too far west errors typical of coupled ocean atmosphere gcms results of the sensitivity experiments suggest that these deficiencies are associated with another family of systematic model errors the overprediction of convection over the tropical land surfaces particularly over eastern tropical africa and india and the concurrent large scale westward shift in the divergence center at upper levels and the convergence center at lower levels the results also confirm the important role of south american and african orography in localizing the south atl

on the atlantic pacific ninos connection a multidecadal modulated mode atlantic and pacific el nino are the leading tropical oceanic variability phenomena at interannual timescales recent studies have demonstrated how the atlantic nino is able to influence on the dynamical processes triggering the development of the pacific la nina and vice versa however the stationarity of this interbasin connection is still controversial here we show for the first time that the atlantic pacific ninos connection takes place at particular decades coinciding with negative phases of the atlantic multidecadal oscillation amo during these decades the atlantic pacific connection appears as the leading coupled covariability mode between tropical atlantic and pacific interannual variability the mode is defined by a predictor field the summer atlantic sea surface temperature sst and a set of predictand fields which represent a chain of atmospheric and oceanic mechanisms to generate the pacific el nino phenomenon alteration of the walker circulation surface winds in western pacific oceanic kelvin wave propagating eastward and impacting on the eastern thermocline and changes in the pacific sst by internal bjerknes feedback we suggest that the multidecadal component of the atlantic acts as a switch for el nino prediction during certain decades putting forward the amo as the modulator acting through changes in the equatorial atlantic convection and the equatorial pacific sst variability these results could have a major relevance for the decadal prediction systems

baroclinic wave breaking and the internal variability of the tropospheric circulation abstract a simple model of the tropospheric circulation based on a 10 level primitive equation model is forced by linearly relaxing the potential temperature toward an idealized zonally symmetric equilibrium field the model equations are integrated in time until a statistically steady state is obtained the local relationship between the state of the background flow the direction of wave propagation and subsequent wave breaking at the tropopause level is then investigated maps of potential vorticity pv on isentropic surfaces are analyzed and all four different types of wave breaking described recently by peters and waugh are shown to occur it is found that cyclonic wave breaking events are usually initiated by poleward fluxes of wave activity and anticyclonic events by equatorward fluxes composites are then used to show that equatorward fluxes are associated with a jet that is locally broad and weak with relatively strong isentropic pv gradients to its equatorward flank by contrast poleward