impact of tropical heat sources on the south american tropospheric upper circulation and subsidence the nonlinear adjustment of the wind and mass fields to idealized tropical heat sources is studied with a simple nonlinear primitive equation model with emphasis on the upper level circulation over south america and neighboring oceanic regions during the austral summer numerical experiments are performed with 1 an idealized symmetrical heat source in the amazon region 2 the asymmetry induced in source 1 by the sacz 3 the effect of the atlantic itcz off the amazon mouth 4 the african heat source 5 the west pacific source and 6 the central pacific source during the warm phase of enso the linear response is obtained through the reduction of the heat source by a factor of 10 and subsequent multiplication of the results by the same factor two basic questions are discussed 1 are localized heat sources important for the development of the observed cyclonic flow in the midequatorial atlantic and 2 where is the compensating subsidence associated with the amazon heat source located the nonlinearity helps organizing a weak cyclonic curvature in the midequatorial atlantic with the inclusion of source 2 the basic state generated by the west pacific source and primarily by the central pacific source has a large impact on the cyclonic curvature on the equatorial atlantic the compensating subsidence associated with the amazon source is concentrated on the southwest side of the source the sacz extension helps to enhance the subsidence over the northern argentina and the atlantic itcz enhances the subsidence over northeast brazil and central equatorial atlantic nonlinearity weakens the subsidence at the 500 hpa level inducing a more barotropic structure in the dynamical response to the heating

stationary rossby wave propagation in a baroclinic atmosphere observational studies of teleconnections in both soisticial seasons have suggested various patterns of behaviour linked mainly to westerly jets and equatorial westerlies these have been interpreted using barotropic rossby wave theory and simulated using stationary forcing in barotropic models linearized about 300 hpa flows in this paper the relevant propagation behaviour is investigated using a baroclinic model with three dimensional climatological basic states time integrations are performed using localized thermal forcing as a wavemaker the propagation results are qualitatively very similar to those obtained with the barotropic model though there are quantitative differences the westerly jets still act as strong waveguides the shorter wavelengths and smaller wave activity speeds found with the baroclinic model are generally in better agreement with observation propagation into and from the upper tropospheric equatorial westerlies in the east pacific propagation across north america and propagation from europe across the arabian gulf are all found the extent to which upper tropospheric westerlies reach towards the equator influences the propagation into those regions barotropic models can only represent such behaviour if the basic state reflects the near equatorial upper tropospheric zonal wind structure

tropospheric waveguide teleconnections and their seasonality abstractone point correlation maps of the subseasonal variability of 200 hpa meridional wind in nature and an atmospheric general circulation model are systematically analyzed to quantify the impact of the climatological mean jets on tropospheric covariability as a result of the jets acting as waveguides for the propagation of rossby waves as anticipated by linear theory signatures of jet influence are detected in terms of i the geographical position of the strongest teleconnections ii the zonal orientation and extent of prominent patterns of variability and iii the scale of the features that make up those patterns further evidence of jet waveguide influence comes from examining the seasonality of these teleconnection attributes during winter covariability can be essentially circumglobal while during summer it tends to be confined within two separate sectors of the globe where the jets are especially strong experiments with a multilevel linear planetary wave model confirm that the analyzed

multiple weather regimes and baroclinically forced spherical resonance abstract systematically recurrent geographically fixed weather regimes forced by a single isolated mountain in a two layer high resolution quasigeostrophic model modified for the sphere are found to be robust phenomena while the climatological stationary wave is often confined to or has maximum amplitude in the region just downstream of the orography giving the appearance of a wave train propagating into the tropics the regional maximum centers of low frequency variance appear around the hemisphere giving the appearance of zonal resonance or some type of zonally confined propagation this result is not anticipated in light of rossby wave dispersion theory on the sphere on the other hand baroclinic disturbances developing on a meridional temperature gradient of finite extent force subtropical and polar easterlies as well as a sharpened midlatitude westerly jet which provides a zonal waveguide by refraction and or reflection for the rossby waves these conditions are favorable for the establis

the morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae experimental studies of subcritical unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves flow depth was 0 11 m mean flow velocities were 0 86 1 0 m s 1 shear velocities were 0 058 0 71 m s 1 and dimensionless shear stresses were 0 56 0 87 bedwaves are asymmetrical in profile and range from 0 75 to 11 mm in height mainly 2 6 mm from 0 7 to 1 3 m in wavelength and have mean celerities of 10 mm s 1 flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs high resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave

tropical plumes in a barotropic model a product of rossby wave generation in the tropical upper troposphere abstract a divergent barotropic model produces tropical cyclonic rossby wave sources rwss and tropical plumes tps when forced by upper tropospheric ut convergence and subsidence over the tropical central and eastern pacific tcep experiments show that tps form when equatorward amplification and zonal contraction of the tcep trough cause rotationally balanced midlatitude flow to transition to buoyancy driven ageostrophic flow equatorward of 15 n this transition results from the sharp reduction of the flow s horizontal length scale below the rossby radius of deformation in local near equatorial regions sudden adjustment of mass and momentum fields in the absence of rotational balance produces concentrated convergence divergence couplets and jet exit entrance regions which define actual tcep tps simulated tps display remarkable feature agreement with observed tps even sharing a common length scale evolution extensive ut cold advection convergence and subsidence exist west of the tcep trough

a transition phase teleconnection of the pacific quasi decadal oscillation the atmospheric circulation patterns associated with the pacific quasi decadal oscillation qdo are investigated using available observational data from 1948 to 2007 previous studies indicate that the pacific qdo is characterized by a distinct lifecycle in the form of sea surface temperature sst patterns in the warm and cool phases of the pacific qdo the sst patterns resemble those associated with the el nino southern oscillation enso during the warm cool and cool warm transitions of the pacific qdo recurrent sst patterns are also clearly visible the rotated empirical orthogonal function analysis on the 10 15 year filtered data shows that the evolutions of sst and atmospheric circulation are well coupled while the warm cool phase pacific qdo generates an enso like circulation pattern the transition phases form a distinct short wave train emanating from southeast asia towards north america this short wave train is particularly robust in the streamfunction of water vapor flux diagnostic analyses of the heat budget the stationary wave flux and a barotropic model indicate that this short wave train is thermodynamically maintained and is likely forced by diabatic heating near southeast asia additional modulations of forcing sources in the western and eastern tropical pacific on this short wave teleconnection are also discussed

large scale motions of the tropics in observations and theory recent observations of the tropical and subtropical atmosphere are interpreted in terms of scaling arguments and wave propagation theory advanced bycharney 1963 1969 charney s idealizations describe the tropical atmosphere in terms of large regions of quasi nondivergent flow containing small subdomains of heavy convection and divergence and place emphasis upon the quasi rotational regions fgge first garp global experiment observations suggest that strongly divergent local tropical circulations are forced by latent heating and produce important direct modifications of the total wind field we describe the extent to which the resulting field consists of divergent and rotational components in different analyses of the fgge data and present independent supporting documentation of the results in terms of heating estimates and rainfall observations local tropical heating rates on the order of 10 c day are apparently due to latent heat release associated with precipitation rates as large as 6 cm day during extended periods

the british baikal corridor a teleconnection pattern along the summertime polar front jet over eurasia abstractthe british baikal corridor bbc pattern a new teleconnection along the summertime upper tropospheric polar front jet pfj is investigated based on observational and reanalysis datasets

circumglobal teleconnections and wave packets associated with israeli winter precipitation this investigation performs diagnostic analyses with ncep ncar reanalysis data and israeli precipitation data to examine the dynamical processes that drive the teleconnection pattern associated with israeli winter precipitation anomalies a dipole teleconnection pattern with one centre of action over western europe and another centre of action over the eastern mediterranean em is found to be associated with the winter precipitation anomalies in israel this pattern is referred to as the southern levant sl pattern the positive negative phase of the sl pattern is found to be closely associated with enhanced reduced precipitation in israel composites of the 300 hpa stream function field show that both phases of the sl pattern are associated with eastward propagating wave packets that traverse about three quarters of the distance across the globe these wave packets which have a phase speed that is close to zero originate over the northeast pacific the east coast of north america for the positive negative sl phase and propagate eastward across the north atlantic western europe the em and southern asia until they eventually decay over the northwest pacific as these wave packets pass over western europe and the em the sl pattern is triggered and anomalies in israeli winter precipitation occur the sl pattern is found to have an e folding time scale of 4 days which is determined by the group velocity of the wave packet the time averaged spatial structure of the circumglobal wave packet closely resembles the first empirical orthogonal function of the hemispheric 300 hpa meridional wind field this suggests that the circumglobal wave packet may be a fundamental pattern of variability which is relevant for the entire northern hemisphere copyright 2008 royal meteorological society

circumglobal teleconnections the jet stream waveguide and the north atlantic oscillation abstract monthly and seasonally averaged upper tropospheric northern hemisphere winter fields are examined to determine whether the waveguiding effect of the time averaged tropospheric jets on low frequency disturbances that is predicted by theory does affect the behavior of these disturbances it is found that indeed disturbances in the vicinity of the mean jets particularly the jet that stretches across south asia are fundamentally different from those that reside in regions where the mean winds have weaker meridional gradients like the mid pacific patterns of variability in the jets tend to be smaller scale and to consist of zonally oriented chains of anomalies while variability in the mid pacific is composed of patterns with distinct meridional orientation because they are meridionally trapped and zonally elongated patterns associated with the jet stream waveguide connect activity at points that are much farther apart than do patterns in other regions of the globe within the south asian waveg

tsunami deposits beneath tidal marshes on northwestern vancouver island british columbia two sand sheets underlying tidal marshes at fair harbour neroutsos inlet and koprino harbour on the northwestern coast of vancouver island british columbia were probably deposited by tsunamis the sand sheets become thinner and finer grained landward drape former land surfaces contain marine microfossils are locally graded or internally stratified and can be correlated with earthquakes that generated tsunamis in the region 137cs dating and historical accounts indicate that the upper sand sheet was deposited by the tsunami from the great alaska earthquake in 1964 radiocarbon ages on plant fossils within and on top of the lower sand sheet show that it was deposited sometime after about a d 1660 we attribute the lower sand sheet to a tsunami from the most recent plate boundary earthquake on the cascadia subduction zone about 300 yr ago extending the documented effects of this earthquake north of the nootka fault zone the 1964 tsunami deposits differ little in thickness and continuity among the three marshes in contrast the lower sand sheet becomes thinner and less continuous to the north implying a tsunami source south of the study area

summertime enso north african asian jet teleconnection and implications for the indian monsoons 1 recent findings indicate that enso may modulate the strength of the indian monsoons by altering the meridional tropospheric temperature gradient between asia and the equatorial indian ocean here we show that during northern hemisphere summertime el nino events both colder upper tropospheric temperatures and increased vorticity anomalies are observed within the north african asian naa jet the temperature response within the naa jet is the dominant enso related change to the meridional tropospheric temperature gradient between asia and the equatorial indian ocean forced solutions of the linearized barotropic vorticity equation indicate that these anomalies within the naa jet are a response to northern hemisphere summertime el nino related convective activity over the equatorial pacific ocean additional experiments indicate that westward propagating anomalies in both the tropics and middle latitudes produce the anomaly response within the naa jet these findings suggest that the teleconnection linking enso and the monsoons is mediated by the response of the naa jet to westward propagating rossby waves citation shaman j and e tziperman 2007 summertime enso north african asian jet teleconnection and implications for the indian monsoons geophys res lett 34 l11702 doi 10 1029 2006gl029143

interhemispheric influence of the atlantic warm pool on the southeastern pacific abstract the atlantic warm pool awp is a large body of warm water comprising the gulf of mexico caribbean sea and western tropical north atlantic the awp can vary on seasonal interannual and multidecadal time scales the maximum awp size is in the boreal late summer and early fall with the largest extent in the year being about 3 times the smallest one the awp alternates with the amazon basin in south america as the seasonal heating source for circulations of the hadley and walker type in the western hemisphere during the boreal summer fall a strong hadley type circulation is established with ascending motion over the awp and subsidence over the southeastern tropical pacific this is accompanied by equatorward flow in the lower troposphere over the southeastern tropical pacific as dynamically required by the sverdrup vorticity balance it is shown by analyses of observational data and ncar community atmospheric model simulations that an anomalously large small awp during the boreal summer f

the effect of model bias on the equatorward propagation of extratropical waves the effect of model bias on the equatorward propagation of extratropical waves in a gcm simulation is assessed within the context of simple wave guiding principles the modes representing these waves are identified through the use of empirical orthogonal function analysis performed on the 200 mb zonal wind filtered to retain variations between 6 and 30 days the temporal evolution and vertical structure of these modes are examined through the use of time lagged composite analysis the differences between observed and simulated wave propagation is examined in relationship to the theoretical wave guiding properties associated with the observed and simulated time mean flow the utility of simple wave guiding theory for describing the observed and modeled wave propagation is assessed the model bias in the time mean flow is closely associated with the differences in the propagation of the transient waves the excessively strong wave guiding properties associated with the simulated pacific jet appear to inhibit the proper meridional propagation of wave energy into the tropical central and eastern pacific in the simulation waves that do propagate into the tropical pacific either dissipate or are reflected near the equator while in the observations wave energy propagates into the southern hemisphere on the other hand the wave guiding by the subtropical and midlatitude jets over the atlantic is weaker in the simulation than in the observations in this region wave energy propagates primarily into the tropics in the simulation while some of the observed wave energy is reflected toward the east and northeast over the atlantic and northern africa the locations of the theoretical critical lines and wave guides of the time mean flow although based on many simplifying assumptions are remarkably consistent with the propagation characteristics of these waves

sensitivity of stationary wave amplitude to regional changes in laurentide ice sheet topography in single layer models of the atmosphere climate variability on millennial timescales has been observed in many geologic records covering the last glacial cycle a potential source of this variability is the laurentide ice sheet lis in its periodic discharge of large quantities of icebergs to the north atlantic the present analysis considers whether regional variations in lis topography could exert a significant influence on the atmosphere s stationary wave circulation the maximum effect that regional changes in lis topography have on the atmosphere s stationary wave circulation is determined using single layer models of the atmosphere model experiments measure the individual contribution of 4 5 7 5 sections of the lis and greenland topography to global mean stationary wave amplitude results show the possibility for a limited region of topography to control a disproportionate amount of the atmosphere s total response to topography moreover the possibility exists for a reduction in topographic forcing to increase stationary wave amplitude these results can be understood by considering how the mean flow controls the horizontal propagation of wave energy and superposition of wave amplitude the location of regions with enhanced stationary wave sensitivity to topographic alteration is found to be sensitive to mean topographic height but not mean wind strength the latter is found to primarily affect the overall amplitude of sensitivity rather than the pattern the impact of two hypothetical changes in lis topography is considered and they are found to have widely different effects on the global stationary wave field stationary wave sensitivity to topography within the single layer models suggests that variations in the size or shape of the lis can be one factor important to climate variability on millennial timescales

simulation of the east asian summer monsoon using a variable resolution atmospheric gcm the east asia summer monsoon easm is simulated with a variable resolution global atmospheric general circulation model gcm developed at the laboratoire de meteorologie dynamique france the version used has a local zoom centered on china this study validates the model s capability in reproducing the fundamental features of the easm the monsoon behaviors over east asia revealed by the ecmwf reanalysis data are also addressed systematically providing as observational evidence the mean state of the easm is generally portrayed well in the model including the large scale monsoon airflows the monsoonal meridional circulation the cross equatorial low level jets the monsoon trough in the south china sea the surface cold high in australia and the upper level northeasterly return flow while the performance of simulating large scale monsoonal climate is encouraging the model s main deficiency lies in the rainfall the marked rainbelt observed along the yangtze river valley is missed in the simulation this is due to the weakly reproduced monsoonal components in essence and is directly related to the weak western pacific subtropical high which leads to a fragile subtropical southwest monsoon on its western flank and results in a weaker convergence of the southwest monsoon flow with the midlatitude westerlies the excessively westward extension of the high together with the distorted indian low also makes the contribution of the tropical southwest monsoon to the moisture convergence over the yangtze river valley too weak in the model the insufficient plateau heating and the resulting weak land sea thermal contrast are responsible for the weakly reproduced monsoon it is the deficiency of the model in handling the low level cloud cover over the plateau rather than the horizontal resolution and the associated depiction of plateau topography that results in the insufficient plateau heating comparison with the simulation employing regular coarser mesh model reveals that the local zoom technique improves in a general manner the easm simulation

freshwater megaflood sedimentation what can we learn about generic processes abstract there is growing recognition that sedimentary deposits related to high energy large scale freshwater floods are widespread across the continents and in the main can be related to quaternary outbreak flows from ice dammed lakes such deposits may also occur in the neogene and earlier recognition of these megaflood deposits is hindered by the lack of well conditioned stratigraphic models of depositional successions however descriptions of successions from many locations often exhibit commonalities when compared carefully thus the primary purpose of this paper is to examine and condense the published stratigraphic and sedimentological evidence so as to identify the key signatures of megaflood successions the deposits often are the only record of a flood and so the secondary purpose is to interpret the sedimentary sequences in order to recreate the behaviour of floods through single and multiple events finally some pointers are provided as to those areas of sedimentological research that might be profitably explored in more depth to improve understanding of megaflood dynamics a key characteristic of megaflood deposits is the apparent massive appearance of single units which on closer inspection usually are amalgamated layers of similar grain size distribution individual beds and sets of beds are commonly near horizontal planar and laterally extensive successions evidently are not channelized and extensivity is indicative of simultaneous and coherent deposition over large areas localised large scale clinoforms or smaller scale cross beds are common associated with bar front or dune progradation respectively but otherwise extensivity implies that cross cutting erosional contacts notably steeply inclined examples and small scale scours are not common except immediately around large isolated clasts such as dropstones and ice blocks a common vertical stack of sequences in any one succession consists of 1 basal thick coarse parallel bedded units 2 large scale clinoforms 3 horizontally bedded thin laminated units 4 ripple and dune cross beds 5 silt beds and 6 succession capping debris flow deposits in some cases such a succession usually is indicative of a single cycle of waxing and waning flood flow generally dominated by high concentration suspension deposition onto plane beds the high power allows the transport of fine gravel and coarse sand in suspension leading upon deposition to relatively coarse beds from suspension fall out clay and silt is generally sparse or absent being transported further down the system at the scale of the hydrograph the successions indicate that flows initially accelerate and then decelerate with significant shorter period flow pulses also evident in the sedimentary signatures in parallel bedded sequences primarily during waxing flows bedload transport often was ineffective in sorting sediment due to the high rates of deposition from the highly sediment charged waters resulting in diffuse accretion surfaces at each location similar sequence stacks are common leading to the impression of rhythmic deposition from pulsing flow including from repetitive series of floods although often composed of coarse grains many sequence stacks excluding any debris flows are not dissimilar to turbidity flow sequences the bouma sequence in particular although there is limited experimental evidence for turbidity currents associated with high energy freshwater flows theory does not preclude the deposition of similar sedimentary motifs from high concentration flows such that an analogy between some megaflood sedimentary beds and turbidites is not unreasonable despite considerable variation in megaflood sedimentary successions commonalities occur which are indicative of similar processes of deposition from high energy floods at geographically different locations the changes in style of deposition throughout a succession provide clues as to the nature of the flood flows and this information could be used to improve estimates of flood hydrograph behaviour however hypotheses linking deposit characteristics to flow dynamics are currently ill conditioned and often poorly tested the majority of published studies describe the coarser scale stratigraphy with only passing reference to the fine scale structure of individual beds closer attention to lamination styles particle orientation and syn depositional deformation structures for example should provide information on the flow mechanics at the time of bed deposition thus better consideration of the sedimentology of megaflood deposit beds will lead to better understanding of the controlling hydraulics and should result in better models of flood deposition

mechanisms of global teleconnections associated with the asian summer monsoon an intermediate model analysis the indian summer monsoon ism and western north pacific summer monsoon wnpsm are two subsystems of the asian summer monsoon and they exhibit different global teleconnection patterns the enhanced ism strengthens the south asian high and mascarene high and the wnpsm excites a meridional tripolar wave train in the northern hemisphere and affects the australian high in the southern hemisphere to understand the dynamics behind these global teleconnections especially the processes responsible for the cross equatorial teleconnection an intermediate model describing a two level troposphere and a steady planetary boundary layer pbl is linearized from the background horizontal wind field the model results indicate that the ism heating located under the strong easterly vertical shear vs and close to the westerly jet in the northern hemisphere can excite a barotropic rossby wave that emanates northwestward and then propagates downstream along the westerly jet since the wnpsm heating is far away from the westerly jet over the north pacific it only excites a weak rossby wave train which cannot explain the meridional tripolar teleconnection associated with the wnpsm it is found that both the ism and wnpsm heating excite strong teleconnections in the southernhemisphere via an advectionmechanism that is the background upper level northerly winds can transport energy across the equator from the northern hemisphere summer monsoon to the southern hemisphere westerly jet in addition the pbl enhances monsoon teleconnections by trapping more energy in the upper troposphere the elevated maximum monsoon heating also reinforces upper level perturbations and enhances the teleconnection pattern

tropical extratropical teleconnections in boreal summer observed interannual variability abstractmaximum covariance analysis is performed on the fields of boreal summer tropical rainfall and northern hemisphere nh 200 hpa height for the 62 yr period of record of 1948 2009 the leading mode which appears preferentially in summers preceding the peak phases of the el nino southern oscillation enso cycle involves a circumglobal teleconnection cgt pattern in the nh extratropical 200 hpa height field observed in association with indian monsoon rainfall anomalies the second mode which tends to occur in summers following enso peak phases involves a western pacific north america wpna teleconnection pattern in the height field observed in association with western north pacific summer monsoon rainfall anomalies the cgt pattern is primarily a zonally oriented wave train along the westerly waveguide while the wpna pattern is a wave train emanating from the western pacific monsoon trough and following a great circle the cgt is accompanied by a pronounced tropical extratropical seesaw in t

local variation of inundation sedimentary characteristics and mineral assemblages of the 2011 tohoku oki tsunami on the misawa coast aomori japan abstract the 2011 tohoku oki tsunami caused severe damage to the coastal regions of eastern japan and left a sediment veneer over affected areas we discuss differences in depositional characteristics of the 2011 tohoku oki tsunami from the viewpoint of the sediment source coastal topography and flow height the study area on the misawa coast northern tohoku includes a 20 km long coastline with sandy beaches coastal dunes and a gently sloping lowland this landscape assemblage provides an opportunity to examine the effects of topography on the characteristics of the tsunami deposit during field surveys conducted from april 10 to may 2 2011 we described the thickness facies and structure of the tsunami deposit we also collected sand samples at approximately 20 m intervals along 13 shore perpendicular transects extending up to 550 m inland for grain size and mineral assemblage analysis the tsunami flow height was estimated by measuring the elevation of debris found in trees broken tree limbs or water marks on buildings the nature of the coastal lowland affected the flow height and inundation distance in the southern part of the study area where there is a narrow 100 m wide low lying coastal strip the run up height reached 10 m on the landward terrace slopes to the north the maximum inundation reached 550 m with a run up height of 3 2 m on the wider low lying coastal topography the average flow height was 4 5 m the tsunami eroded coastal dunes and formed small scarps along the coast immediately landward of the coastal dunes the tsunami deposit was more than 20 cm thick but thinned markedly inland from this point close to the dunes the deposit was composed largely of medium sand 1 2 with planar and parallel bedding but with no apparent upward fining or coarsening the grain size was similar to that of the coastal dune and we infer that the dunes were the local source material for the tsunami deposit at this point the mineral assemblage of the tsunami deposit was dominated by orthopyroxene and clinopyroxene and was also similar to the dune and beach sand at sites more than half the inundation distance inland the thinner tsunami deposit consisted mainly of fine sand 2 375 with some upward fining the difference in grain size and sedimentary characteristics was probably caused by differences in sediment transportation and depositional processes we infer that the well sorted finer sediments were deposited out of suspension whereas the relatively coarse sands were laid down from traction flows the depositional characteristics of the 2011 tohoku oki tsunami deposit appeared to have been affected mainly by the coastal topography and the extent of erosion at any one point as opposed to flow height

a southern hemisphere wave response to enso with implications for southern africa precipitation ensemble gcm simulations with an imposed idealized warming of the eastern pacific ocean reveal two wave anomalies in the southern hemisphere one in the eastern and one in the western hemisphere both are statistically significant at the 99 confidence level application of a steady state linear model and a rossby wave source analysis is used to diagnose the causes of the waves the western hemisphere wave is forced by the advection and stretching of planetary vorticity by the divergent flow from the southern hemisphere component of the central pacific twin anticyclones that straddle the equator during warm events the eastern hemisphere wave is a result of the northeastward shift of the south indian convergence zone sicz that in turn is forced from the upper troposphere by convergence to the north an upper level convergence maximum over the equatorial indian ocean induces divergence to the south encouraging vertical motion and precipitation to the northeast of the sicz s normal position the resulting anomalous upper level convergence in the climatological position of the sicz as well as the anomalous vorticity flux convergence by the transients associated with an equatorward shift of the storm track behind the sicz force the eastern hemisphere rossby wave since a shift of the sicz is a fairly robust observed consequence of enso events these results suggest the mechanism by which drought conditions develop over southern africa at the height of many warm events seasonal prediction capabilities in this region can be improved by monitoring and understanding the details and consequences of the adjustment of the walker circulation near the equator outside of the pacific ocean basin

swept away by a turbidity current in mendocino submarine canyon california we present unique observations and measurements of a dilute turbidity current made with a remotely operated vehicle in 400 m water depth near the head of mendocino canyon california the flow had a two layer structure with a thin 0 5 to 30 m relatively dense 0 04 vol and fast up to 1 7 m s wedge shaped lower layer overlain by a thicker up to 89 m more dilute and slower current the fast moving lower layer lagged the slow moving dilute flow front by 14 min which we infer resulted from the interaction of two initial pulses the two layers were strongly coupled and the sharp interface between the layers was characterized by a wave like instability this is the first field scale data from a turbidity current to show i the complex dynamics of the head of a turbidity current and ii the presence of multiple layers within the same event

downstream development of the summertime tropical cyclone submonthly wave pattern in the extratropical north pacific abstract the impact of tropical perturbation on the extratropical wave activity in the north pacific in the submonthly time scale is demonstrated here previous studies identified a tropical cyclone tc submonthly wave pattern which propagated north northwestward in the philippine sea and recurved in the oceanic region between japan and taiwan this study found that after the arrival of the tc submonthly wave pattern at the recurving region the eastward propagating wave activity in the extratropical north pacific was significantly enhanced it is suggested that the tc submonthly wave pattern which is originated in the tropical western north pacific enhances the eastward energy propagation of rossby wave like perturbation in the extratropical north pacific and may have an impact on the long range weather predictability in the eastern north pacific and western north america

on the effects of the hadley circulation and westerly equatorial flow on planetary wave reflection the nonlinear behaviour of quasi stationary planetary waves that are excited by mid latitude orographic forcing is considered in a global shallow water model taken to represent the upper troposphere the waves propagate toward low latitudes where the background flow is normally weak and the waves are therefore likely to break nonlinear pseudomomentum conservation relations are used to quantify the absorption reflection behaviour of the wave breaking region two different flow scenarios are represented i initial states without a representation of the hadley circulation but where the axisymmetric equatorial background flow changes from being weak and easterly to moderate westerly ii initial states that include a representation of the hadley circulation and that have weak equatorial easterlies based on linear arguments both i and ii are expected to influence the progression of the wave train the nonlinear behaviour in the presence of low latitude westerly background flows is different from linear predictions for large amplitude forcing wave breaking takes place even though there is no zero wind line in the initial state and the cross equatorial wave propagation that took place for small amplitude forcing is stopped before it can reach the equator nonlinear reflection is found to take place back into the hemisphere of origin but not across to the other hemisphere in the presence of a hadley circulation representative of winter conditions the nonlinear reflection takes longer to get established i e it requires more forcing but a reflected wave train is still present in the numerical simulations both for a longitudinally symmetric forcing and for the more realistic case of an isolated forcing a summer hadley circulation allows wave activity to get to the winter hemisphere as the forcing is increased wave breaking occurs and eventually nonlinear reflection

quasi stationary waves in the southern hemisphere part i observational data abstract this part i presents selected major features of the quasi stationary monthly mean wave field in the troposphere and stratosphere of the southern hemisphere it is confirmed that the quasi stationary wave with zonal wavenumber 1 qs wave 1 is by far the dominant component of the geopotential height field at tropospheric and stratospheric levels the amplitude of this wave is largest at about 60 s all year round and reaches a maximum during september and october in the upper troposphere and stratosphere analysis of the elliasen palm flux vector suggests that at high latitudes the quasi stationary wave field is primarily forced from lower latitudes most prominently from the indian ocean region during june and october orographic and thermal forcing from antarctic regions seem to also be important sources of wave activity in polar and high latitudes particularly over southern south america and the atlantic ocean the contribution to the quasi stationary flow by the transient component of the fl

maintenance of austral summertime upper tropospheric circulation over tropical south america the bolivian high nordeste low system using the nasa geos reanalysis data for 1980 95 the austral summer stationary eddies in the tropical subtropical southern hemisphere are examined in two wave regimes long and short wave wave 1 and waves 2 6 respectively the basic structure of the bolivian high nordeste low bh nl system is formed by a shortwave train across south america but modulated by the long wave regime the short wave train exhibits a monsoonlike vertical phase reversal in the midtroposphere and a quarter wave phase shift relative to the divergent circulation as inferred from a the spatial relationship between the streamfunction and velocity potential and b the structure of the divergent circulation the short wave train forming the bh nl system is maintained by south american local heating and remote african heating while the long wave regime is maintained by western tropical pacific heating the maintenance of the stationary waves in the two wave regimes is further illustrated by a simple diagnostic scheme that includes the velocity potential maintenance equation which links velocity potential and diabatic heating and the streamfunction budget which is the inverse laplacian transform of the vorticity equation some simple relationships between streamfunction and velocity potential for both wave regimes are established to substantiate the links between diabatic heating and streamfunction of particular interest is a sverdrup balance in the short wave regime this simplified vorticity equation explains the vertical structure of the short wave train associated with the bh nl system and its spatial relationship with the divergent circulation based upon the diagnostic analysis of its maintenance a simple forced barotropic model is adopted to simulate the bh nl system with idealized forcings which imitates the real 200 mb divergence centers over south america africa and the tropical pacific numerical simulations demonstrate that the formation of the bh nl system is affected not only by the african remote forcing but also by the tropical pacific forcing

the effect of the hadley circulation on the meridional propagation of stationary waves the effect on the meridional propagation of stationary planetary waves of the inclusion of the zonal mean hadley circulation in linearized models of the atmosphere is examined in a forced barotropic vorticity equation model a meridional wind in the basic state enables wave propagation through tropical easterlies in the direction of the wind this is shown to be consistent with the wkb solution in a multi level linearized steady primitive equation model with an idealized meridional circulation tropical forcing can still produce response through easterlies although propagation from one mid latitude to the other is much less than in the barotropic result this is explained through wkb and wave tracing methods for a simplified three dimensional vorticity equation the near non divergence of the unforced tropical motion permits a wave to travel largely within a layer of favourable winds in a twelve level model using observed january and july zonal mean conditions the hadley circulation changes the response to earth topographic forcing mainly in the subtropics however tropical heat forcing can now produce a large mid latitude response in the winter hemisphere especially the resulting subtropical jets suggest a mechanism for the maintenance of those observed over australia during the northern monsoon season and adjacent to the anomalous forcing of the 1983 el nino

rossby wave packets on the midlatitude waveguide a review abstractrossby wave packets rwps are rossby waves for which the amplitude has a local maximum and decays to smaller values at larger distances this review focuses on upper tropospheric transient rwps along the midlatitude jet stream their central characteristic is the propagation in the zonal direction as well as the transfer of wave energy from one individual trough or ridge to its downstream neighbor a process called downstream development these rwps sometimes act as long range precursors to extreme weather and presumably have an influence on the predictability of midlatitude weather systems the paper reviews research progress in this area with an emphasis on developments during the last 15 years the current state of knowledge is summarized including a discussion of the rwp life cycle as well as rossby waveguides recent progress in the dynamical understanding of rwps has been based in part on the development of diagnostic methods these methods include algorithms to identify and track rwps

analysis of bed load motion at high shear stress recent pipeline experiments performed in saskatchewan show the existence of a shear layer in which the solids concentration decreases linearly with height this finding has already been used to calculate the velocity distribution within a constant stress shear layer the analysis has now been extended to obtain the solids discharge per unit breadth of bed the result agrees with the modified meyer peter and muller equation and with the writer s sand data for larger nylon particles the solids discharge per unit breadth fell below that expected on the basis of the sand data and the new analysis shows the reason for this apparent anomaly it also shows that a shear layer several grain diameters thick will not be uncommon in open channel flow contrary to the conventional wisdom the new analysis predicts that the threshold of motion curve on the shields diagram must approach a constant value at sufficiently small abscissae a finding which appears to be in accord with recent experimental evidence

a simple atmospheric model of the local and teleconnection responses to tropical heating anomalies abstract a minimal complexity model of both the local and remote stationary responses of the atmosphere to tropical heating anomalies is described and demonstrated two levels are recast as baroclinic and barotropic components with thermal advection in the tropics neglected the model is linearized about some idealized and realistic background wind fields and forced with a localized heating for illustration in the tropics the baroclinic responses are familiar from the matsuno gill model these excite barotropic responses by advective interactions with vertical background wind shear the barotropic signals are in turn transmitted to high latitudes only in the presence of barotropic background westerly winds for an el nino like equatorial heating the barotropic response has anticyclones to the north and south of the heating reinforcing opposing the anticyclonic cyclonic baroclinic gyres in the upper lower troposphere with realistic background flows the model reproduces the hemispheric asymmetry

teleconnection between the south atlantic convergence zone and the southern indian ocean implications for tropical cyclone activity a link between anomalous austral summer convection over the south atlantic convergence zone sacz and atmospheric conditions over the south indian ocean is investigated interannual north south shifts in the sacz produce a dipole of anomalous convection and precipitation over south america the south atlantic convergence zone index saczi capturing this variability is presented and associated with a midlatitude rossby wave train that propagates from south america eastward and south of africa before curving north into the tropical indian ocean this wave train is reproduced using rossby wave ray tracing and simulations with a linearized barotropic vorticity equation model forced with divergence and convergence consistent with the observed dipole of anomalous convection the wave train acts to excite anomalies in wind shear and relative humidity over the tropical south indian ocean which in turn impact tropical cyclone tc genesis and distribution over this region we find that changes in the wind shear and relative humidity associated with the saczi effect changes in tc genesis which result in a change in observed tc days over portions of the south indian ocean

a stepwise screening system to select storm sensitive stalagmites taking a targeted approach to speleothem sampling methodology a stepwise stalagmite sample screening process was applied in order to select a stalagmite as a sensitive recorder of recent tropical cyclone rainfall events near belize field and laboratory criteria were combined in series to screen out candidate stalagmites whose characteristics indicated lower sensitivity to stormwater infiltration the approach described here was designed to increase the likelihood that selected stalagmites would increase the signal noise ratio of the target phenomenon in the resulting proxy records this four stage method for sample selection may be an important methodological consideration for researchers attempting to use speleothems for stable isotope paleotempestology hallmarks of this approach include 1 establishing a priori scientific targets 2 applying sample criteria in the form of contra indicators and 3 organizing the sample screening protocol into a series of practical stages the overall approach to stalagmite selection presented here supports cave conservation and can be adapted readily by others in support of different scientific goals it is hoped that other investigators will also publish field and laboratory sample selection schemes as increased transparency will provide new information invaluable to replication efforts and to advancing the field of speleothem paleo environmental studies

origins of massive type sandstones in braided river systems abstract this study details largely ignored massive type predominantly structureless sandstones preserved within braided fluvial successions of carboniferous to triassic age architectural element analysis reveals that these sediments were deposited within sand dominated perennial systems of low braiding index cross stratified braid bar deposits are interbedded with and laterally equivalent to geometrically distinct largely structureless massive type sandbodies identified as two separate architectural elements channel like smc and sheet like sms sub divisions within these broad categories define six geometric units which are texturally distinct from each other and from the structured sediments of the same lithological unit since massive type sandstone elements have many features in common with the deposits of highly concentrated laminar sediment water flows they are interpreted in terms of similar depositional processes smc elements form elongate channel like features which trend both at high angles to and parallel with the palaeoflow of host fluvial channels the lower bounding surfaces of smc elements may be either erosive or non erosive and describe symmetrical cross sections with margins dipping 250 m parallel and transverse to the fluvial palaeoflow direction established from cross stratified sandstones of adjacent architectural elements the basal surface of sms elements may either be undulose where the sandbodies are termed smsu or erosional where they are termed smse internally smsu elements preserve parallel laminae marginal to basal scours diffuse sweeping laminae isolated cross sets and water escape structures the smsu sediments are compositionally identical to the structured sandstones with which they are associated and are interpreted as the result of deposition from highly concentrated stream flows developed through flooding and entrainment of sandy floodplain material along the basal surface of smse elements scours in excess of 4 5 m are preserved which are elongate both parallel and oblique to fluvial flow these are commonly lined with a mudclast breccia and or massive sandstone with crude laminae developed parallel with the margin in the upper portion of smse elements diffuse dune structures and sweeping laminae are preserved smse units are texturally and compositionally immature with respect to the cross stratified sandstones with which they are associated thus they are believed to be of first cycle origin and are interpreted as the deposits of highly concentrated flows possibly generated by glacial outburst floods or jokulhaups

rotational atmospheric circulation during north atlantic european winter the influence of enso the dominant variability modes of the north atlantic european rotational flow are examined by applying a principal component analysis pca eof to the 200 hpa streamfunction mid winter anomalies jan feb monthly means the results reveal that when this norm is used the leading mode eof1 does not correspond to the traditional north atlantic oscillation nao which appears in our analysis as the second leading mode eof2 but is the local manifestation of the leading hemispheric streamfunction eof the regression of this regional mode onto the global sst field exhibits a clear el nino signature with no signal over the atlantic while the associated upper height anomalies resemble the tropical northern hemisphere tnh pattern east of north america this tnh like wavetrain produces a meridional dipole like pattern at lower levels although in some ways this pattern resembles the nao eof2 the dynamics of these two modes are very different in that only eof2 is associated with a latitudinal shift of the north atlantic stormtrack thus the choice of the streamfunction norm in the eof analysis allows the separation of two different phenomena that can produce similar dipolar surface pressure anomalies over the north atlantic but that have different impact on european climate these two modes also differ on their contribution to variability at lower levels while nao eof2 is mostly confined to the north atlantic tnh eof1 has a more annular global character at upper levels nao eof2 also produces a global pattern but with no annular structure reminiscent of the circumglobal teleconnection

the ocean atmosphere response to wind induced thermocline changes in the tropical south western indian ocean in the indian ocean basin the sea surface temperatures ssts are most sensitive to changes in the oceanic depth of the thermocline in the region of the seychelles dome observational studies have suggested that the strong sst variations in this region influence the atmospheric evolution around the basin while its impact could extend far into the pacific and the extra tropics here we study the adjustments of the coupled atmosphere ocean system to a winter shallow doming event using dedicated ensemble simulations with the state of the art ec earth climate model the doming creates an equatorial kelvin wave and a pair of westward moving rossby waves leading to higher sst 1 2 months later in the western equatorial indian ocean atmospheric convection is strengthened and the walker circulation responds with reduced convection over indonesia and cooling of the sst in that region the pacific warm pool convection shifts eastward and an oceanic kelvin wave is triggered at thermocline depth the wave leads to an sst warming in the east equatorial pacific 5 6 months after the initiation of the seychelles dome event the atmosphere responds to this warming with weak anomalous atmospheric convection the changes in the upper tropospheric divergence in this sequence of events create large scale rossby waves that propagate away from the tropics along the atmospheric waveguides we suggest to repeat these types of experiments with other models to test the robustness of the results we also suggest to create the doming event in june so that the east pacific warming occurs in november when the atmosphere is most sensitive to sst anomalies and el nino could possibly be triggered by the doming event under suitable conditions

in situ measurements of velocity structure within turbidity currents 1 turbidity currents are thought to be the main mechanism to move 500 000 m3 of sediments annually from the head of the monterey submarine canyon to the deep sea fan indirect evidence has shown frequent occurrences of such turbidity currents in the canyon but the dynamic properties of the turbidity currents such as maximum speed duration and dimensions are still unknown here we present the first ever in situ measurements of velocity profiles of four turbidity currents whose maximum along canyon velocity reached 190 cm s two turbidity currents coincided with storms that produced the highest swells and the biggest stream flows during the year long deployment

the role of transient disturbances in the dynamics of the pacific north american pattern abstract a 25 yr dataset is used to investigate the role of transient eddies in the dynamics of the pacific north american pna pattern monthly mean vorticity and sensible heat flux divergences associated with submonthly transients are computed over the northern hemisphere for each winter month these fields are composited over months with strong pna patterns and the average over all winter months is subtracted to obtain anomaly fields the vorticity flux divergence anomaly is found to be well correlated with the pna height field particularly in the upper troposphere where an eddy vorticity flux convergence divergence is found in the low high height regions of the pna anomaly the sensible heat flux divergence on the other hand is negatively correlated with the pna temperature anomaly so that the transient eddies produce a sensible heat flux out of the warm regions of the pna and into the cold regions thus tending to destroy the temperature anomaly a linear quasi nondivergent global steady s

the sedimentology of middle holocene tsunami facies in northern sutherland scotland uk abstract lagoonal sediments attributed to the main holocene marine transgression in strath halladale northern sutherland contain a complex coarser layer believed to have been deposited during the tsunami associated with the second storegga slide off south west norway the coarser sequence is dated at between 7590 50 and 7290 50 radiocarbon years bp 6507 6260 cal bc and 6228 6029 cal bc detailed stratigraphical analysis has determined a distinctive suite of sedimentary sub units within the coarser layer in marked contrast to the sediments which occur above and below a pronounced erosional unconformity with the underlying sediments is recorded with the base of the tsunami layer characterised by eroded material from the underlying peat the presence of a mixed diatom assemblage although fragmentary indicates a chaotic accumulation of the deposit with all habitats represented variations in particle size within the sequence disclose striking similarities with those from contemporary tsunami deposits the run up of the tsunami sediments is calculated at a minimum of 4 6 m this is the first occasion on which a deposit of the second storegga slide tsunami has been found outside the north sea basin and indicates that the area affected by the tsunami may have been larger than has been previously described

the global atmospheric circulation response to tropical diabatic heating associated with the madden julian oscillation during northern winter abstractthe detailed dynamical mechanisms of the upper tropospheric circulation response to the madden julian oscillation mjo convection are examined by integrating a primitive equation model a series of initial value calculations with the climatological boreal winter background flow forced by the mjo like thermal forcing successfully capture the key aspects of the observed circulation response to the mjo convection this suggests that a large fraction of mjo related circulation anomalies are direct responses to tropical heating in both the tropics and extratropics and can be largely explained by linear dynamics it is found that mjo like dipole heatings not only intensify tropical upper tropospheric anomalies but also weaken them at certain regions because of the interaction between equatorial kelvin and rossby waves the rossby wave train primarily excited by horizontal divergence of upper level perturbation flow propagates northeastward and then heads back to the equator in this way rossby wave act

rossby wave propagation into the tropics in two gfdl general circulation models during the northern winter the eastern pacific is characterized by upper level westerly flow extending from the equator into the midlatitudes of both hemispheres theoretical and simple modeling studies suggest that such a region should favor the penetration of rossby waves into the tropics from higher latitudes observational results by kiladis and weickmann using 200 mb data indicate that rossby waves do indeed propagate freely into the tropical eastern pacific during the northern winter from the asian jet exit region they also confirmed that cross equatorial dispersion of energy from the northern into the southern hemisphere occurs frequently the present study examines these interactions in climatological runs of two gfdl gcms the northern wintertime mean states of these models are characterized by a rather realistically simulated upper level westerly regime in the tropical pacific despite the relative weakness of the asian jet and wave activity with respect to observations propagation of rossby waves into the tropics is present in both models and these waves are strongly positively tilted as seen in the observations a momentum budget of the zonal wind and e vector diagnostics over the tropical pacific indicate that these transients are an important component of the momentum balance of the equatorial westerlies in both the observations and in the models

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

horizontal energy propagation in a barotropic atmosphere with meridional and zonal structure abstract to investigate how the propagation of energy away from steady sources may be influenced by the horizontal structure of the time averaged flow in the troposphere solutions to a barotropic model are displayed and interpreted the model is steady and linearized about a basic state which varies in latitude and longitude emphasis is placed on cases where the longitudinal variations are gradual many of the results can be analyzed by locally applying tools developed for zonally symmetric basic states e g ray tracing and refractive indices simple background flows are constructed which have longitudinally confined critical lines reflective surfaces wave guides and regions conducive to propagation an example is given in which longitudinal derivatives of the basic state are significant the theory of ray tracing in a two dimensional wind must be employed as an interpretive tool in this case it is concluded that zonal variations tend to enhance poleward equatorward propagation in large scale tr

is midlatitude convection an active or a passive player in producing global circulation patterns abstract the ability of persistent midlatitude convective regions to influence hemispheric circulation patterns during the northern hemisphere summer is investigated global rainfall data over a 15 yr period indicate anomalously large july total rainfalls occurred over mesoscale sized midlatitude regions of north america and or southeast asia during the years of 1987 1991 1992 and 1993 the anomalous 200 hpa vorticity patterns for these same years are suggestive of rossby wave trains emanating from the regions of anomalous rainfall in the midlatitudes results from an analysis of an 11 yr mean monthly 200 hpa july wind field indicate that in the climatological mean rossby waveguides are present that could assist in developing a large scale response from mesoscale sized regions of persistent convection in the midlatitudes this hypothesis is tested using a barotropic model linearized about the 200 hpa july time mean flow and forced by the observed divergence anomalies the model results are in qualit

upper regime plane bed the analysis of a laboratory and field data set for sand bed channels defines the characteristics of upper regime plane bed with sediment transport the occurrence of upper regime plane bed relates to the laminar sublayer thickness and depends on shear velocity u median grain size d50 and grain shear reynolds number r two different boundary conditions are recognized 1 transition to hydraulically smooth when r 11 6 in the first case upper regime plane bed is obtained when u 2 g where g is the gravitational acceleration in the second case upper regime plane bed is observed when d50 2 temperature effects are possible when r approaches 11 6 which explains the observations on the missouri river resistance to flow for plane bed with sediment transport increases with the shields parameter

traction carpet stratification in turbidites fact or fiction abstract the spaced stratification that characterizes some thick deep water sandstones consists of a stack of layers 5 10 cm thick that are inversely graded at their base and structureless at their top these layers have been attributed to deposition from a succession of collapsed inertia layers traction carpets prior to collapse each layer was believed to have been maintained by shear from the overriding turbidity current this depositional model is based on the mistaken assumption that the distribution of shear stress in such a traction carpet is indistinguishable from the distribution of shear stress in a grain flow on a steep slope instead shear stress in an externally sheared traction carpet should be approximately constant from the top to the base of the layer and deposition sho ld be a gradual process leaving no record of sequentially active inertia layers inertia layers that do form under large turbidity currents are probably never as thick as 5 10 cm based on available data from shear cell experiments instead inertia layers are probably about 1 cm thick similar to the thickness of the inversely graded basal part of each layer in field examples of spaced stratification the rest of each inversely graded stratum probably forms due to rapid fallout from suspension the basal inversely graded 1 cm is believed to record strong shear that may accompany impingement on the bed of large turbulent eddies hence spaced stratification may not be a structure produced beneath steady flows but may record strong y fluctuating hydrodynamic conditions and vigorous burst sweep cycles in large turbidity currents

spatial pattern and zonal shift of the north atlantic oscillation part i a dynamical interpretation abstract this paper presents a possible dynamical explanation for why the north atlantic oscillation nao pattern exhibits an eastward shift from the period 1958 77 p1 to the period 1978 97 p2 or 1998 2007 p3 first the empirical orthogonal function analysis of winter mean geopotential heights during p1 p2 and p3 reveals that the nao dipole anomaly exhibits a northwest southeast nw se tilting during p1 but a northeast southwest ne sw tilting during p2 and p3 the nao pattern especially its northern center undergoes a more pronounced eastward shift from p1 to p2 the composite calculation of nao events during p1 and p2 also indicates that the negative positive nao phase dipole anomaly can indeed exhibit such a nw se ne sw tilting second a linear rossby wave formula derived in a slowly varying basic flow with a meridional shear is used to qualitatively show that the zonal phase speed of the nao dipole anomaly is larger smaller in higher latitudes and smaller larger in lower latitud

teleconnections from tropics to northern extratropics through a southerly conveyor rossby wave propagation theory predicts that rossby waves in a tropical easterly flow cannot escape from the tropics to the extratropics here the authors show that a southerly flow component in the basic state a southerly conveyor may transfer a rossby wave source northward thus a forcing embedded in the deep tropical easterlies may excite a rossby wave response in the extratropical westerlies it is shown that the southerly conveyor determines the location of the effective rossby wave source and that the extratropical response is relatively insensitive to the location of the tropical forcing provided that the tropical response can reach the southerly conveyor a stronger southerly flow favors a stronger extratropical response and the spatial structure of the extratropical response is determined by the extratropical westerly basic flows

on the dynamics of persistent states and their secular trends in the waveguides of the southern hemisphere troposphere we identify the dynamical drivers of systematic changes in persistent quasi stationary states regimes of the southern hemisphere troposphere and their secular trends we apply a purely data driven approach whereby a multiscale approximation to nonstationary dynamical processes is achieved through optimal sequences of locally stationary fast vector autoregressive factor processes to examine a high resolution atmospheric reanalysis over the period encompassing 1958 2013 this approach identifies regimes and their secular trends in terms of the predictability of the flow and is granger causal a comprehensive set of diagnostics on both isentropic and isobaric surfaces is employed to examine teleconnections over the full hemisphere and for a set of regional domains composite states for the hemisphere obtained from nonstationary nonparametric cluster analysis reveal patterns consistent with a circumglobal wave 3 polar wave 5 subtropical pattern while regional composites reveal the pacific south american pattern and blocking modes the respective roles of potential vorticity sources stationary rossby waves and baroclinic instability on the dynamics of these circulation modes are shown to be reflected by the seasonal variations of the waveguides where rossby wave sources and baroclinic disturbances are largely contained within the waveguides and with little direct evidence of sustained remote tropical influences on persistent synoptic features warm surface temperature anomalies are strongly connected with regions of upper level divergence and anticyclonic rossby wave sources the persistent states identified reveal significant variability on interannual to decadal time scales with large secular trends identified in all sectors apart from a region close to south america

the seasonal effects of enso on atmospheric conditions associated with european precipitation model simulations of seasonal teleconnections abstractthe seasonal upper tropospheric teleconnection between enso and the north atlantic european sector is explored through a series of model experiments a barotropic vorticity equation model is linearized about climatological conditions for each season of the year and divergence forcing is applied over the equatorial pacific to mimic el nino related convective activity during boreal fall winter and spring this forcing similarly excites a northeastward propagating stationary barotropic rossby wave train that extends across the north atlantic to the european coast strong anomalies develop over the british isles in the vicinity of the north atlantic jet exit solutions during boreal summer produce no clear wave train however evidence exists for a north atlantic response because of both eastward and westward propagating signals these direct responses over the atlantic and europe are qualitatively similar to observed enso associated anomalies during boreal spring and fall but differ structurall

the response of an idealized atmosphere to orographic forcing zonal versus meridional propagation abstracta dry atmospheric general circulation model is forced with large scale gaussian orography in an attempt to isolate a regime in which the model responds linearly to orographic forcing and then to study the departures from linearity as the orography is increased in amplitude in contrast to previous results which emphasized the meridional propagation of orographically forced stationary waves using the standard held suarez h s control climate it is found that the linear regime is characterized by a meridionally trapped zonally propagating wave meridionally trapped waves of this kind have been seen in other contexts where they have been termed circumglobal waves as the height of the orography is increased the circumglobal wave coexists with a meridionally propagating wave and for large enough heights the meridionally propagating wave dominates the response a barotropic model on a sphere reproduces this trapped wave in the linear regime and also reproduces the transition to meridional pro

the effect of enso on tibetan plateau snow depth a stationary wave teleconnection mechanism and implications for the south asian monsoons abstract an atmospheric stationary wave teleconnection mechanism is proposed to explain how enso may affect the tibetan plateau snow depth and thereby the south asian monsoons using statistical analysis the short available record of satellite estimates of snow depth and ray tracing it is shown that wintertime enso conditions in the central pacific may produce stationary barotropic rossby waves in the troposphere with a northeastward group velocity these waves reflect off the north american jet turning equatorward and enter the north african asian jet over the eastern atlantic ocean once there the waves move with the jet across north africa south asia the himalayas and china anomalous increases in upper tropospheric potential vorticity and increased wintertime snowfall over the tibetan plateau are speculated to be associated with these rossby waves the increased snowfall produces a larger tibetan plateau snowpack which persists through the spring and summer and weakens the intensity of the

baroclinic to barotropic pathway in el nino southern oscillation teleconnections from the viewpoint of a barotropic rossby wave source abstractthe baroclinic to barotropic pathway in enso teleconnections is examined from the viewpoint of a barotropic rossby wave source that results from decomposition into barotropic and baroclinic components diagnoses using the ncep ncar reanalysis are supplemented by analysis of the response of a tropical atmospheric model of intermediate complexity to the ncep ncar barotropic rossby wave source among the three barotropic rossby wave source contributions shear advection vertical advection and surface drag the leading contribution is from shear advection and more specifically the mean baroclinic zonal wind advecting the anomalous baroclinic zonal wind vertical advection is the smallest term while surface drag tends to cancel and reinforce the shear advection in different regions through damping on the baroclinic mode which spins up a barotropic response there are also nontrivial impacts of transients in the barotropic wind response to enso both tropical and subtropical baroclinic vorticity

structureless coarse tail graded bouma ta formed by internal hydraulic jump of the turbidity current abstract turbidity currents are commonly super critical if they travel on slopes steeper than 0 001 hence upon deceleration such flows must pass a hydraulic jump it is therefore surprising that sedimentary features related to the hydraulic jump are so poorly known and not clearly related to the bouma sequence to study this missing link a stability diagram of bed forms generated by decelerating initially super critical turbidity currents is presented here to sketch the position of hydraulic jump deposits relative to super and sub critical bed forms observed in turbidites the axes of the diagram are formed by the densiometric froude number the grain size and the particle fall out rate experimental evidence shows that the hydraulic jump is hardly manifested in the deposit if turbidity currents pass the jump as a low concentration single phase suspension flow only those turbidity currents that develop two phase suspension flow with a super critical flowing traction carpet cause vigorous erosion and liquefaction of the substrate at the jump locality to produce rip up of the substrate rip up clasts and flame structures which are trapped in structureless coarse tail graded deposits characteristic of the classic bouma ta unit the location of the jump is sensitive to density velocity variations of the traction carpet and can shift rapidly up and down the slope this would explain why structureless coarse tail graded deposits such as bouma ta develop sheet like

bed load motion at high shear stress dune washout and plane bed flow sediment motion at high shear stress is investigated in a pressurized conduit system the bed materials include three sands and bakelite as the dimensionless shear stress was increased toward unity the steepness of the bedforms disappeared abruptly this behavior is indicative of a sudden shift from the bedform regime to the upper plane bed regime not a gradual transition the results for the upper plane bed regime reveal that this type of flow has a larger frictional resistance than that of a conventional rough boundary also determined is that frictional characteristics of the bedform regime cannot be expressed solely in terms of dimensionless shear stress the observed behavior also suggests that as a solid transport mechanism sand waves may be more efficient than upper plane bed flow

an atmospheric teleconnection linking enso and southwestern european precipitation abstract numerous studies have demonstrated statistical associations between the el nino southern oscillation enso and precipitation in the mediterranean basin the dynamical bases for these teleconnections have yet to be fully identified here observational analyses and model simulations are used to show how enso variability affects rainfall over southwestern europe iberia southern france and italy a precipitation index for the region named southwestern european precipitation swep is used the observational analyses show that enso modulates swep during the september december wet season these precipitation anomalies are associated with changes in large scale atmospheric fields to the west of iberia that alter low level westerly winds and onshore moisture advection from the atlantic the vorticity anomalies associated with swep variability are linked to enso through a stationary barotropic rossby wave train that emanates from the eastern equatorial pacific and propagates eastward to the atlant

multidecadal and interannual changes of stationary rossby waves changes in stationary rossby waves srw are connected to regional climate change and the propagation of these waves has a large influence on the evolution and development of the general circulation since the late 1970s and early 1980s many studies have noted a major shift in the structure of large scale circulation occurring in both hemispheres this paper explores the changes in the srw pattern and their propagation associated with these climate shifts first multidecadal changes in srw activity are examined using ncep ncar reanalysis data for two periods 1948 1978 and 1979 2000 for comparison of these two periods the zonal wind srw kinetic energy pattern along the waveguide and stationary wavenumber ks are analysed the results showed that in the northern hemisphere nh the srw propagation is favoured in the asian jet stream ajs waveguide ks 7 in the period 1979 2000 mainly in the east asia region there are indications that this could be associated with the pacific decadal oscillation warm phase a favoured srw propagation also occurs in the north atlantic subtropical jet nasj waveguide ks 5 region in the latter period and could be linked with positive north atlantic oscillation in the southern hemisphere sh a favoured srw propagation in the polar jet waveguide ks 4 region is observed in the latter period and this could be associated indirectly with tropical sea surface temperature fluctuations the interannual changes in the srw propagation during stronger el nino and la nina events which occurred from 1948 to 2000 are then examined the results showed that during stronger el nino events in nh the srw propagation in the ajs mainly in the east asia region and in the nasj waveguides ks 7 5 respectively is reduced on the other hand in sh the srw propagation in australian and polar waveguides ks 7 4 respectively is favoured copyright 2011 royal meteorological society

causes of extreme ridges that induce california droughts abstractcalifornia droughts are often caused by high amplitude and persistent ridges near and off the west coast of north america without apparent connections with enso here with a hierarchy of climate models it is demonstrated that extreme ridges in this region are associated with a continuum of zonal wavenumber 5 circumglobal teleconnection patterns that originate from midlatitude atmospheric internal dynamics although tropical diabatic heating anomalies are not essential to the formation and maintenance of these wave patterns certain persistent heating anomalies may double the probability of ridges with amplitudes in the 90th percentile occurring on interannual time scales those heating anomalies can be caused by either natural variability or possibly by climate change and they do not necessarily depend on enso the extreme ridges that occurred during the 2013 14 and 2014 15 winters could be examples of ridges produced by heating anomalies that are not associated with enso this mechanism could p

influence of the background state on rossby wave propagation into the great lakes region based on observations and model simulations abstractthe authors investigate the relationship between hydrology in the great lakes basin namely overlake precipitation and transient rossby waves using the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis data and historical output from phase 3 of the coupled model intercomparison project cmip3 the preferred path of observed rossby wave trains associated with overlake precipitation on lake superior depends strongly on season and appears to be related to the time mean upper level flow during summer and fall the northern hemisphere extratropical jet is relatively narrow and acts as a waveguide such that rossby wave trains traversing the great lakes region travel along the extratropical pacific and atlantic jets during other months the pacific jet is relatively broad which allows more wave activity originating in the tropics to penetrate into the midlatitudes and influence lake superior precipitation analysis is extended to cmip3 mod

a dynamic link between high intensity precipitation events in southwestern north america and europe at the last glacial maximum abstract both hydroclimate proxies and modeling studies alike present compelling evidence for abundant precipitation in southwestern north america and europe at the last glacial maximum lgm however model simulations exhibit an apparent disagreement between changes in precipitation and the orientation of the midlatitude jet streams relative to modern this is surprising as it is well known from modern observations that the jet streams and storm tracks closely follow one another in the time mean the present study shows that a substantial fraction between 35 and 50 of the precipitation along the western seaboards of the northern hemisphere continents at the lgm is due to short lived high intensity events rather than a permanent rearrangement of the circulation moreover these precipitation events are shown to be dynamically linked by a wavenumber 5 stationary wave packet that is trapped in a midlatitude waveguide when the wave energy is concentrated in the eastern north pacific an atmospheric river is excited that makes landfall along the north american west coast days later when the wave packet has reached europe the north atlantic jet stream assumes a zonally oriented state that brings abundant precipitation to the western mediterranean sector understanding the dynamics of this teleconnection pattern is not only key for interpreting proxy data signals in past climates but may also be important for improving predictions of storm track dynamics and water resources availability in the face of climate variability and change

the dynamics of the enso atlantic hurricane teleconnection enso related changes to the north african asian jet affect atlantic basin tropical cyclogenesis abstract the nature of the teleconnection linking enso variability with atlantic basin tropical storm formation is investigated solutions of the linearized barotropic vorticity equation forced with august october el nino event divergence produce upper tropospheric vorticity anomalies over the sahel and at the mouth of the north african asian naa jet over the tropical atlantic these responses are similar in magnitude and orientation to observed enso vorticity variability for this region further investigation reveals that the vorticity anomalies over the subtropical atlantic develop primarily in response to very low wavenumber westward propagating stationary rossby waves excited by el nino related convective activity over the equatorial pacific ocean however the dynamics of this teleconnection change as the atlantic basin hurricane season progresses in august and september the response is dominated by the westward propagating stationary rossby waves that alter vorticity within the naa jet and to it

modulation of tropical cyclone tracks over the western north pacific by intra seasonal indo western pacific convection oscillation during the boreal extended summer this study investigates the effect of the intra seasonal indo western pacific convection oscillation ipco on tropical cyclone tc tracks over the western north pacific wnp during the boreal extended summer may october the number of west and northwest moving tc tracks is found to sharply increase over the wnp in the positive intra seasonal ipco phase recurving tracks have greater weight in the negative intra seasonal ipco phase possible physical mechanisms are further examined in terms of steering flow energy conversion and energy propagation when the intra seasonal ipco phase is positive the first order moment term of perturbation potential energy ppe1 converts into perturbation kinetic energy pke at lower latitudes the pressure trough spreads farther to the east meanwhile rossby waves emanating from the convective centers of the intra seasonal ipco over the wnp wavewnp and eeio waveeeio travel into the trough region thereby deepening the trough these features enhance the westward and northwestward steering flow between 20 n and about 30 n sharply increasing the number of straight west and northwest moving tc tracks over the wnp when the intra seasonal ipco is in a negative phase conversion from ppe1 to pke at lower latitudes is suppressed and the trough weakens more ppe1 converts to pke in the climatological western pacific subtropical high wpsh region and the wpsh is intensified moreover waveeeio intensifies the north south ridge of the wpsh over the southern indian peninsula meanwhile part wavewnp propagate northeastward these features favor northeastward motion of tcs over the wnp

sediment transport in oscillatory boundary layers in cases of rippled beds and sheet flow an experimental study was focused on the process of sediment transport in unsteady flow conditions due to wave action wave induced oscillatory flow conditions near the seabed were simulated at full scale 1 1 in a new large oscillating water tunnel two sets of experiments series a and b were carried out during series a bed forms and wave cycle averaged suspended sediment concentrations were measured under sinusoidal waves series b focused on measurement of the wave cycle averaged sediment transport rates under regular and random asymmetric oscillatory flows upper shoreface conditions and was aimed at the verification of quasi steady formulas for the description of cross shore sediment transport bedform dimensions appeared to decrease considerably under the influence of wave asymmetry and wave randomness only the measured ripple dimensions under asymmetric waves series b showed good agreement with nielsen s 1979 relations for most of the series b experiments the bed was plane sheet flow the net sediment transport was directed onshore and the measured transport rates showed a strong correlation with the velocity moment an empirical quasi steady transport model is proposed which is based on the new tunnel data and other existing data sets the limitations of the quasi steady model approach became clear in the rippled bed regime and through the presence of a consistent influence of the wave period in plane bed conditions in rippled bed conditions the suspended concentration profiles followed a negative exponential distribution for most of the experiments the measured concentration decay length showed a linear relation with the ripple height in plane bed sheet flow conditions the measured suspended concentration profiles followed a negative power function the power or concentration decay parameter was constant 2 1 for a wide range of velocity conditions it is suggested that the mobile bed sheet flow layer has a strong damping effect on the mixing of suspended sediments turbulence damping

origins of the 1988 north american drought the 1988 summer drought in the united states was the most extensive in many years because the drought developed in different places at different times not all regional effects can be traced to the same cause along the west coast and in the northwestern united states drought conditions developed during 1987 in association with the 1986 to 1987 el nino in the tropical pacific ocean record low rainfalls from april to june 1988 led to rapid development of drought in the north central united states strong anticyclonic conditions and a northward displaced jet stream in the upper atmosphere over north america throughout this period were only part of pronounced and distinctive wavetrain of anomalies in the atmospheric circulation that appeared to emanate from the tropical pacific below average sea surface temperatures along the equator in the pacific in the northern spring of 1988 combined with warmer than normal water from 10 degrees to 20 degrees n led to a northward displaced but still active intertropical convergence zone itcz southeast of hawaii results from a steady state planetarywave atmospheric model indicate that the atmospheric heating anomalies associated with the displaced itcz can force an anomalous wavetrain across north america similar to that observed land surface processes probably contributed to the severity and persistence of the drought however the large scale atmospheric circulation perturbations associated with natural variations in the coupled atmosphere ocean system in the tropical pacific were most likely the primary cause

classifying el nino modoki i and ii by different impacts on rainfall in southern china and typhoon tracks based on their opposite influences on rainfall in southern china during boreal fall this paper classifies el nino modoki into two groups el nino modoki i and ii which show different origins and patterns of sst anomalies the warm sst anomalies originate in the equatorial central pacific and subtropical northeastern pacific for el nino modoki i and ii respectively thus el nino modoki i shows a symmetric sst anomaly distribution about the equator with the maximum warming in the equatorial central pacific whereas el nino modokiii displays an asymmetric distribution with the warmsstanomaliesextending fromthe northeastern pacifictotheequatorialcentralpacific additionally thewarmsstanomaliesintheequatorialcentralpacific extend farther westward for el nino modoki ii than for el nino modoki i similar to the canonical el nino el nino modoki i is associated with an anomalous anticyclone in the philippine sea that induces south westerly wind anomalies along the south coast of china and carries the moisture for increasing rainfall in southern china for el nino modoki ii an anomalous cyclone resides east of the philippines associated with northerly wind anomalies and a decrease in rainfall in southern china the canonical el nino and el nino modoki i are associated with a westward extension of the western north pacific subtropical high wnpsh whereas el nino modoki ii shifts the wnpsh eastward differing from canonical el nino and el nino modoki i el nino modoki ii corresponds to northwesterly anomalies of the typhoon steeringflow which are unfavorable for typhoons to make landfall in china

the large scale modulation of subtropical cyclogenesis in the central and eastern pacific ocean abstract an analysis of the composite large scale circulation associated with periods of enhanced active or diminished inactive cyclogenesis in the subtropical central and eastern pacific ocean is presented composites were constructed using surface and tropospheric analyses from the ecmwf tropical ocean global atmosphere toga dataset for 10 northern hemisphere cool seasons 1986 96 active periods of subtropical cyclogenesis were defined to be periods in which two or more cyclones developed in close succession to each other while inactive periods were defined to be periods of at least 10 days duration during which no cyclones with a subtropical origin were present in the pacific basin the analysis revealed that the occurrence of subtropical cyclones in the central and eastern pacific ocean is strongly linked to the strength and location of the asian jet with active periods characterized by a weaker zonally retracted asian jet while inactive periods are characterized by a stronger zonally elon

summertime planetary wave resonance in the northern and southern hemispheres abstractslow moving planetary waves of high amplitudes are often associated with persistent surface weather conditions this persistence can lead to extreme weather events with potentially serious implications for society and nature quasi resonant amplification qra of planetary waves has been proposed as a mechanism to generate high amplitude hemisphere wide patterns of wavenumbers 6 8 in the northern hemisphere nh summer here this mechanism is studied in both hemispheres analyzing 1979 2015 reanalysis data evidence for qra in the southern hemisphere sh is found for wavenumbers 4 and 5 it is shown that the difference in resonating wavenumbers between hemispheres stems from the different magnitude and latitudinal shape of the respective zonal mean zonal winds during resonance events a strong and narrow jet is observed in both hemispheres whereas the emergence of a second zonal mean jet at high latitudes i e a double jet is seen in the nh only strong and narrow jets can form stable zona

extratropical atmospheric response to the atlantic nino decaying phase abstract the atlantic nino or atlantic equatorial mode em is the dominant coupled variability phenomenon in the tropical atlantic basin during boreal summer from the 1970s the mode has changed evolving in time from east to west and without persisting until the following winter in a previous observational work the authors have studied the atmospheric response to the em during the 1979 2005 period proposing three main issues along the decaying phase of this mode 1 the continuous confinement of the anomalous deep convection over northeastern brazil following the thermal forcing decay 2 an increasing dipole like precipitation anomaly with dry conditions in the florida gulf of mexico region and 3 the excitation of rossby waves forced by the remaining upper tropospheric divergence that are trapped into the subtropical jet but do not show a robust impact on the european sector in this work a 10 member ensemble simulation for the recent em with the university of california los angeles agcm model

interhemispheric propagation of stationary rossby waves in a horizontally nonuniform background flow abstractsignificant interhemispheric teleconnections are identified that span the tropical easterlies in the boreal summer 300 hpa streamfunction such as the north africa antarctic naa and the north pacific south america npsa patterns these patterns are not supported by traditional wave theory since stationary waves in a basic state without meridional wind would be trapped in the easterlies to describe the interhemispheric responses more realistically two dimensional spherical rossby wave theory in a horizontally nonuniform basic state is considered conditions sufficient for the existence of one propagating wave are obtained and the meridional group velocity of the wave is shown to have the same direction as the meridional basic wind at the traditional critical latitude it is concluded that stationary waves with a specific wavelength can propagate across the easterlies from south north to north south via southerly northerly flows hence energy transport by stationary waves on a horizont

el nino southern oscillation sea level pressure anomalies in the western pacific why are they there abstractalthough sea level pressure slp anomalies in the western pacific have long been recognized as an integral part of the classic southern oscillation pattern associated with el nino southern oscillation enso there is an unresolved question regarding the dynamics that maintain these anomalies traditional studies of the enso response in the tropics assume a single deep baroclinic mode associated with the tropospheric temperature anomalies however the slp anomalies in the western pacific are spatially separated from the baroclinic signal in the ncep ncar reanalysis cmip5 models and an intermediate complexity model a quasi equilibrium tropical circulation model qtcm separation of enso slp anomalies in the tropical pacific into baroclinic and barotropic components indicates that the barotropic component contributes throughout the tropics and constitutes the primary contribution in the western pacific to demonstrate the roles of baroclinic and barotropic modes in enso teleconnections within

equatorial windows and barriers for stationary rossby wave propagation abstractrossby waves can cross the equator and connect the northern hemisphere nh and southern hemisphere sh or be blocked in the vicinity of the equator this work explores the windows and ba

intraseasonal teleconnection between the summer eurasian wave train and the indian monsoon this study investigated the most recurrent coupled pattern of intraseasonal variability between midlatitude circulation and the indian summer monsoon ism the leading singular vector decomposition svd pattern reveals a significant coupled intraseasonal variation between a rossby wave train across the eurasian continent and the summer monsoon convection in northwestern india and pakistan hereafter referred to as nism the wave train associated with an active phase of nism rainfall displays two high pressure anomalies one located over central asia and the other over northeastern asia they are accompanied by increased rainfall over the western siberia plain and northern china and decreased rainfall over the eastern mediterranean sea and southern japan the circulation of the wave train shows a barotropic structure everywhere except the anomalous central asian high located to the northwest of india where a heat induced baroclinic circulation structure dominates the time lagged svd analysis shows that the midlatitude wave train originates from the northeastern atlantic and traverses europe to central asia the wave train enhances the upper level high pressure and reinforces the convection over the nism region meanwhile it propagates farther toward east asia along the waveguide provided by the westerly jet after an outbreak of nism convection the anomalous central asian high retreats westward composite analysis suggests a coupling between the central asian high and the convective fluctuation in the nism the significance of the midlatitude ism interaction is also revealed by the close resemblance between the individual empirical orthogonal functions and the coupled svd modes of the midlatitude circulation and the ism it is hypothesized that the eastward and southward propagation of the wave train originating from the northeastern atlantic contributes to the intraseasonal variability in the nism by changing the intensity of the monsoonal easterly vertical shear and its associated moist dynamic instability on the other hand the rainfall variations over the nism reinforce the variations of the central asian high through the monsoon desert mechanism thus reenergizing the downstream propagation of the wave train the coupling between the eurasian wave train and nism may be instrumental for understanding their interaction and can provide a way to predict the intraseasonal variations of the indian summer monsoon and east asian summer monsoon

dynamics of an interhemispheric teleconnection across the critical latitude through a southerly duct during boreal winter abstractin this study an interhemispheric teleconnection pattern across the critical latitude from southern africa through south asia to the north pacific was revealed in boreal winter monthly averaged 250 hpa streamfunction fields obtained from both the 40 yr ecmwf re analysis era 40 and the ncep ncar reanalysis data from 1957 58 to 2001 02 classical rossby wave theory for zonally varying flow in which the effects of the basic state meridional wind are ignored predicts that stationary rossby waves cannot propagate across easterlies to elucidate the underlying mechanisms responsible for this interhemispheric teleconnection the theoretical basis for stationary wave propagation across the critical latitude is considered taking into account meridional ambient flow the theoretical results suggest that the southerly flow over east africa the western indian ocean and south asia creates a path for the northward propagation of stationary waves across the critical latitude stationary wavenumber and grou