synoptic climatology and intrahemispheric associations of cold air mesocyclones in the australasian sector satellite ir images for seven months in 1992 are interpreted for cold air mesoscale cyclones mesocyclones occurring in the australasian sector 70 e 150 w of the southern ocean time averaged monthly seasonal distributions of mesocyclogenesis mesocyclolysis and tracks of movement along with statistical summaries of mesocyclone attributes e g cloud vortex size speeds of movement are presented and discussed in the context of the larger scale atmospheric circulation maximum frequencies of mesocyclones occurred in the transitional months of april and october 1992 with a secondary peak in july statistically significant differences in mesocyclone track length between months appear related dominantly to changes in speed of the background flow associated with the semiannual oscillation sao of tropospheric pressure height the associations of mesocyclone outbreaks with composite anomaly fields of pressure and height are identified for three subareas of the australasian sector suggested by the analysis of mesocyclone spatial patterns outbreaks occur in the strong southerly geostrophic airflow located between pressure and height anomalies that are negative positive to the eastward westward when outbreaks occurred in the new zealand subarea in 1992 a similarly strong couplet of pressure height anomalies developed in the southern south america antarctic peninsula sector but not when outbreaks occurred south of australia the mesocyclone remote association that is suggested is evaluated by using polar orbiter ir imagery for the southeastern pacific region frequencies of mesocyclones increase decrease west of chile but decrease increase through drake passage when mesocyclone outbreaks occur near new zealand south of australia these long distance associations of mesocyclone outbreaks are consistent with the connectivity of the baroclinic waves and might prove useful in the development of techniques to forecast mesocyclones over the southern ocean

lateral variation of the mantle transition zone beneath the tibetan plateau insight into thermal processes during indian asian collision abstract the indian plate began to subduct under the eurasian plate in the late mesozoic since the eocene continuous continental collision has resulted in the uplift of the tibetan plateau with extensive volcanic activities geophysical and geochemical studies have suggested that the indian plate subducted and that volcanism originated in the deep upper mantle however the depth range of the subduction and volcanism and the relationship between them are still unclear here to image the mantle transition zone mtz structures we deployed new seismic stations and collected as much seismic data as possible from the tibetan plateau we then calculated the receiver functions and analyzed them using the common conversion point stacking method we found that the mtz is thickened by 20 km under the lhasa block where high velocity anomalies are obvious in the mtz apparent thinning of the mtz by 20 km was revealed under the qiangtang and songpan ganze blocks confirming a component of depleted mid oceanic ridge basalt in the magmatic rocks our results suggest that the indian plate has at least subducted to the mtz which has further induced mantle upwelling and volcanism in the tibetan plateau

reconciling subduction dynamics during tethys closure with large scale asian tectonics insights from numerical modeling we use three dimensional numerical models to investigate the relation between subduction dynamics and large scale tectonics of continent interiors the models show how the balance between forces at the plate margins such as subduction ridge push and far field forces controls the coupled plate margins and interiors evolution removal of part of the slab by lithospheric break off during subduction destabilizes the convergent margin forcing migration of the subduction zone whereas in the upper plate large scale lateral extrusion rotations and back arc stretching ensue when external forces are modeled such as ridge push and far field forces indentation increases with large collisional margin advance and thickening in the upper plate the balance between margin and external forces leads to similar convergent margin evolutions whereas major differences occur in the upper plate interiors here three strain regimes are found large scale extrusion extrusion and thickening along the collisional margin and thickening only when negligible far field forces ridge push and larger far field forces respectively add to the subduction dynamics the extrusion tectonics develops a strong asymmetry toward the oceanic margin driven by large scale subduction with no need of preexisting heterogeneities in the upper plate because the slab break off perturbation is transient the ensuing plate tectonics is time dependent the modeled deformation and its evolution are remarkably similar to the cenozoic asian tectonics explaining large scale lithospheric faulting and thickening and coupling of indentation extrusion and extension along the asian convergent margin as a result of large scale subduction process

polar low le cygne satellite observations and numerical simulations a polar low pl which occurred in october 1993 over the norwegian sea is investigated from an observational and a numerical point of view this pl has several unique features it developed early in the season it lasted for about 3 days and its trajectory was such that it passed over weather stations so that conventional observations of the low are available the conditions of the formation development and decay of the pl are investigated using a double approach satellite data from several instruments are used together to document the mesoscale structure of the low and two versions of a limited area model are run to investigate the dynamics of the low numerical model fields are compared to quantities derived from tiros n operational vertical sounder the special sensor microwave imager and satellite radar altimeter data in spite of a better spatial resolution of the models humidity and surface wind speeds are less organized in the simulations than in satellite retrievals the number of vertical levels especially for the lowest layers of the atmosphere appears to be an essential component for a good simulation of the trajectory of the low there is however good overall agreement between modelled and satellite derived fields and the good quality of the simulations allows inferences to be made regarding the essential physical and dynamical processes taking place during the formation and development of the pl we find that the pl was the result of favourable flow conditions at the surface in the form of a shallow arctic front established south of the ice edge together with an upper level potential vorticity anomaly setting the stage for a positive interaction later on the strong surface sensible and latent heat fluxes contributed to the extensive vertical development this study demonstrates the usefulness of the approach adopted here which relies not only on simulations but also on observations to get a very complete description of such disturbances copyright 2004 royal meteorological society

mesoscale vortices in the weddell sea region antarctica abstract preliminary results of investigations of antarctic mesocyclones in the weddell sea region are presented for the antarctic summer periods 1983 88 based on noaa and meteor satellite images a total of 195 mesoscale vortices scale less than 1000 km were observed and classified using a simple classification scheme depending on lifetime and cortex diameter the highest frequency of vortices was found over the ice free part of the eastern weddell sea mean synoptic conditions for vortices with scales less than 400 km near halley station indicate an offshore flow being most favorable for their development a case study with satellite images surface measurements synoptic analyses derived from tovs data shows the evolution of several mesoscale vortices in a pool of cold air in the eastern weddell sea

effects of baroclinicity on the cloud pattern and structure of polar lows a high resolution numerical experiment 1 high resolution numerical experiments using a three dimensional non hydrostatic model with horizontal grid size of 5 or 2 km are performed to clarify what determines the structure and cloud pattern of polar lows an axisymmetric initial vortex is imposed in a baroclinic basic state for which thermal wind balance holds it is found that when the baroclinicity is absent a hurricane like vortex with spiral cloud bands a cloud free eye and a warm core develops when the baroclinicity is strong on the other hand a vortex with a comma shaped cloud and slightly larger horizontal scale develops the former appears to develop due to cisk wishe mechanism while the latter due to baroclinic instability modified by latent heating

mantle kinematics driving collisional subduction insights from analogue modeling abstract since several decades the processes allowing for the subduction of the continental lithosphere less dense than the mantle in a collision context have been widely explored but models that are based upon the premise that slab pull is the prominent driver of plate tectonics fail the india asia collision where several episodes of continental subduction have been documented constitute a case study for alternative views one of these episodes occurred in the early collision time within the asian plate where continental lithosphere not attached to any oceanic lithosphere subducted southward in front of the indian lithosphere during its northward subduction that followed the oceanic subduction of the tethys ocean this process known as collisional subduction has a counter intuitive behavior since the subduction is not driven by slab pull it has been speculated that the mantle circulation can play an important role in triggering collisional subduction but a detailed qualitative analysis of it is not available yet in this work we explore the southward subduction dynamics of the asian lithosphere below tibet by means of analogue experiments with the aim to highlight how the mantle circulation induces or responds to collisional subduction we found that during the northward oceanic subduction analogue of tethys subduction attached to the indenter indian analogue the main component of slab motion is driven vertically by its negative buoyancy while the trench rolls back in the mantle the convective pattern consists in a pair of wide convective cells on both sides of the slab but when the indenter starts to bend and plunge in the mantle trench motion reverses its advance transmits the far field forces to two upper plates asian analogues the more viscous frontal plate thickens and the less viscous hinterland plate which is attached to the back wall of the box subducts during this transition a pair of sub lithospheric convective cells is observed on both sides of the asian analogue slab driven by the shortening of the frontal plate it favors the initiation of the backwall plate subduction such subduction is maintained during the entire collision by a wide cell with a mostly horizontal mantle flow below tibet passively advecting the asian analogue slab experimental results suggest that once the tectonic far field force related to the forward horizontal motion becomes dominant upon the buoyancy forces trench advancing and the transmission of the tectonic force to the upper and backwall plates are promoted this peculiar condition triggers the subduction of the backwall plate despite it is light and buoyant

surface wind fields of antarctic mesocyclones derived from ers 1 scatterometer data the effectiveness of ers 1 scatterometer data as a tool for studying the wind fields of antarctic mesocyclones is analyzed by using 1 year s observations of these systems derived from satellite imagery encompassing the antarctic peninsula and bellingshausen and weddell seas it is shown that the scatterometer processing scheme utilized can affect the resultant wind fields particularly the accuracy of the external forecast winds used for meteorological ambiguity removal very few of the mesocyclones 9 were captured by the scatterometer data because 1 60 of the vortices did not form over the open ocean 25 of those that did were included in the scatterometer data and 2 the narrow scatterometer swath results in a poor temporal resolution that is less than ideal for observing small sized short lived antarctic mesocyclones over the southern ocean close agreement between the derived wind fields and those predicted by a simple model suggests that a mesocyclone may be considered as a vorticity anomaly superimposed on a uniform flow most systems had a surface circulation varying from a slight deviation from the geostrophic flow to a closed circulation however 25 of mesocyclones did not exhibit a surface circulation four case studies are used to illustrate the different types of observed circulation derived maximum cyclonic relative vorticity values those most negative in the southern hemisphere varied between 100 and 400 10 6 s 1 an order of magnitude lower than that calculated for an arctic mesocyclone the scatterometer wind field data represent an important new tool for studying antarctic mesocyclones but need to be used in conjunction with sensors having a higher frequency of coverage to allow multitemporal analyses of such systems throughout their development

miocene subsidence and surface uplift of southernmost tibet induced by indian subduction dynamics the indus yarlung suture of southernmost tibet marks the initial collisional zone the ongoing india asia collision and yet more than 30 million years after the onset of collision a thick detrital sedimentary unit was deposited just north of the suture the kailas formation the mechanism permitting subsidence of the deep intracontinental kailas basin in a compressional tectonic regime remains uncertain we present new apatite 16 11 ma and zircon 24 19 ma fission track aft and zft ages from the gangdese batholith just north of the kailas basin zft analysis of modern river sand from the northern gangdese magmatic arc indicates an exhumation at 27 3 1 3 ma thermal modeling indicates that the batholith experienced reheating between 28 and 20 ma coeval with deposition in the kailas basin between 26 and 21 ma followed by overall rapid cooling between 20 and 17 ma we interpret this thermal history as a phase of regional oligocene miocene sedimentary burial followed by exhumation by modeling mantle dynamics in the geodynamic framework of the india asia collision we show that transient dynamic topography over the relative southward folding of the indian slab is consistent with burial and exhumation of the gangdese magmatic arc during oligocene miocene time the northward migration of the indian continent relative to its own stati onary slab created a wave of dynamic topography that caused subsidence in the overriding plate north of the himalaya followed by a phase of surface uplift since 27 ma of the northern gangdese magmatic arc during latest oligocene early miocene time the dynamic deflection center was in the kailas area and it progressively relocated southward to its present position at the ganges basin

reconstructing greater india paleogeographic kinematic and geodynamic perspectives abstract key in understanding the geodynamics governing subduction and orogeny is reconstructing the paleogeography of greater india the indian plate lithosphere that subducted since tethyan himalayan continental collision with asia here we discuss this reconstruction from paleogeographic kinematic and geodynamic perspectives and isolate the evolution scenario that is consistent with all three we follow recent constraints advocating a 58 ma initial collision and update a previous kinematic restoration of intra asian shortening with a recently proposed model that reconciles long debated large and small estimates of indochina extrusion our new reconstruction is tested against paleomagnetic data and against seismic tomographic constraints on paleo subduction zone locations the resulting restoration shows 1000 1200 km of post collisional intra asian shortening leaving a 2600 3400 km wide greater india from a paleogeographic sediment provenance perspective eocene sediments in the lesser himalaya and on undeformed india may be derived from tibet suggesting that all greater indian lithosphere was continental but may alternatively be sourced from the contemporaneous western indian orogen unrelated to india asia collision a quantitative kinematic paleomagnetic perspective prefers major cretaceous extension and a greater india basin opening within greater india but data uncertainty may speculatively allow for minimal extension finally from a geodynamic perspective assuming a fully continental greater india would require that subduction rates close to 20 cm yr was driven by a down going lithosphere crust assemblage more buoyant than the mantle which seems physically improbable we conclude that the greater india basin scenario is the only sustainable one from all three perspectives we infer that old pre collisional lithosphere rapidly entered the lower mantle sustaining high subduction rates whilst post collisional continental and young greater india basin lithosphere did not inciting the rapid india asia convergence deceleration 8 myr after collision subsequent absolute northward slab migration and overturning caused flat slab subduction tibetan shortening arc migration and arc volume decrease

modification of polar low development by orography and sea ice abstractthe life cycles of intense high latitude mesoscale cyclones and polar lows are strongly shaped by their ambient environments this study focuses on the influence of the orography of svalbar

hemispheric atmospheric variations and oceanographic impacts associated with katabatic surges across the ross ice shelf antarctica numerical simulations and surface based observations show that katabatic winds persistently converge toward and blow across the siple coast part of west antarctica onto the ross ice shelf about 14 of the time during winter april to august 1988 thermal infrared satellite images reveal the horizontal propagation of this negatively buoyant katabatic airstream for about 1000 km across the ice shelf to its northwestern edge a trajectory that nearly parallels the transantarctic mountains this takes place when the pressure field supports such airflow and is caused by synoptic scale cyclones that decay near and or over marie byrd land the northwestward propagation of the katabatic winds is accompanied by other changes in the hemispheric long wave pattern an upper level ridge develops over wilkes land resulting in an enhancement of the split jet in the pacific ocean then more frequent and or intensified synoptic scale cyclones are steered toward marie byrd land where they become nearly stationary to the northeast of the climatological location the resulting isobaric configuration accelerates the katabatic winds crossing siple coast and supports their horizontal propagation across the ross ice shelf an immediate impact of this katabatic airflow that crosses from the ice shelf to the ross sea is expansion of the persistent polynya that is present just to the east of ross island this polynya is a conspicuous feature on passive microwave images of antarctic sea ice and plays a central role in the salt budget of water masses over the ross sea continental shelf the impact of this katabatic airflow upon mesoscale cyclogenesis over the south pacific ocean is also discussed

southern ocean mesocyclones and polar lows from manually tracked satellite mosaics a new reference dataset of mesocyclone activity over the southern ocean has been developed from the manual analysis of high resolution infrared satellite mosaics for winter 2004 of the total 1735 mesocyclones which were identified and analyzed about three quarters were classified as being polar lows i e intense systems see rasmussen and turner 2003 the dataset includes mesocyclone track size associated cloud vortex type and background synoptic conditions maxima in track density were observed over the bellingshausen sea and around east antarctica and are highly correlated with cyclogenesis regions a comparison against quikscat and reanalyses wind characteristics shows that the reanalyses while capturing mesocyclone events tend to considerably underestimate their wind speed by up to 10 ms 1 this mesocyclone dataset is available as a reference for further analysis of mesocyclones and for the evaluation and development of cyclone tracking algorithms

deep burial of asian continental crust beneath the pamir imaged with local earthquake tomography an inclined zone of intermediate depth seismicity beneath the pamir orogen in central asia has been interpreted as southward subduction of a slab of asian lithosphere however it is not known whether asian lithosphere subducts intact or only partially we used arrival times of shallow and intermediate depth earthquakes recorded with a temporary 2008 2010 seismic network in this region to invert for 3d models of seismic velocities in an attempt to answer this question with local seismicity reaching depths of up to 240 km the deep structure of the pamir could be illuminated with high resolution the resulting velocity models show a north south contrast in crustal seismic velocities in the pamir with very low p velocities 5 7 5 9 km s at 15 30 km depth coupled with relatively low vp vsvp vs 1 70 at mid crustal levels in the southern part of the orogen at sub moho depths we image an arcuate high velocity 8 2 8 6 km s slab dipping south in the eastern pamir and east in the pamir s southwest underlying the intermediate depth earthquakes on top of the high velocity slab and just above the onset of deep seismicity between a depth of 60 to 100 km very low compressional wavespeeds around 7 1 km s and high vp vsvp vs ratios 1 80 attest to subducted crustal rocks additionally we carried out 2d numerical thermomechanical modeling of the continental collision in the pamir focusing on the fate of the crust and mantle lithosphere of the asian and indian plates seismic velocities were computed from the modeling results and the resulting images were compared with the velocity distributions obtained from seismic traveltimes combining tomography and modeling results we infer that a substantial amount of crustal material is pulled down beneath the pamir by cold mantle lithosphere to depths of at least 80 100 km from there on only lower crust and mantle lithosphere continue their subduction and earthquakes occur inside the lower crustal layer probably due to metamorphic reactions the buoyant asian upper and middle crust does not penetrate deeper into the mantle but pools at this depth level from where it might eventually exhume or relaminate

the alternative density structures of cold saltwater pools on a sloping bottom the role of friction observed density sections through dense water pools or lenses on sloping topography typically have an asymmetric structure one side of the dense lens usually is bounded by isopycnals that slope steeply down to the seabed while on the other side the slope of isopycnals is more gentle a common situation is for the steepest sloping isopycnals to be on the upslope side of a lens which is termed the head up state but the reverse is occasionally true which is called the head down state here a 1 layer reduced gravity model which resolves the bottom boundary layer to provide physical insight into the three dimensional evolution of these alternative forms is used it is found that the head up state arises when the thickness of the central core of a lens exceeds about two ekman depth scales and the head down state arises when the converse is true the speed of along and cross slope motion of the central thick core of a dense lens is also investigated and the results from an ensemble of runs with the three dimensional reduced gravity model are found to accord suprisingly well with some approximations derived from bulk dynamics from a practical point of view the results concerning the shape of isopycnals bounding dense lenses on slopes can provide valuable information from which to infer important aspects of the underlying dynamics

seafloor evidence for the interaction between cascading and along slope bottom water masses 1 the steep slope of the sw adriatic margin central mediterranean is impacted today by two water masses having markedly different origin but a similar southward flow component the levantine intermediate water liw and the north adriatic deep water naddw the liw forms through evaporation in the eastern mediterranean and enters the adriatic along a counterclockwise path the naddw forms through winter cooling on the north adriatic shelf and cascades across the western slope of the south adriatic basin bathymetric and side scan sonar tobi images of the sw adriatic margin accompanied by chirp sonar profiles define an area of extreme seafloor complexity characterized by a variety of sediment waves erosional scours longitudinal furrows and giant comet marks these distinctive bottom current features are not randomly distributed but appear genetically linked and have a consistent down current arrangement we ascribe this seafloor pattern to the constructive interference of the highly saline liw flowing along the slope between 200 and 600 m water depth and the cold naddw plunging off shelf obliquely to the slope stratigraphic data indicate that this oceanographic regime has been active since the last glacial interglacial transition the cascading naddw is density driven but is not short lived as a turbidity current would be consequently the interaction between downslope and along slope currents is here more dynamical as two water masses act simultaneously over a prolonged interval somewhat diverting their paths and leading to locally enhanced energetic conditions at the seafloor

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

climatology of polar mesocyclones over the sea of japan using a new objective tracking method abstractpolar mesocyclones pmcs are mesoscale cyclonic vortices that develop poleward of the main polar front this article reports on a new algorithm for the objective tracking of pmcs including meso scale vortices which will facilitate the study of their climatology the algorithm is based mainly on the vorticity field and consists of three parts the identification of vortices the connection of vortices at consecutive time steps and discrimination between pmcs and synoptic scale disturbances the objective tracking method was applied to mesoscale analysis ma data provided by the japan meteorological agency which has a horizontal resolution of 5 km the detected tracks of pmcs were confirmed by subjective analysis of the ma data and satellite images the method used here to discriminate between pmcs and synoptic scale disturbances differs from that used in previous studies which is based on the difference between the sea surface temperature and the temperature at 500 hpa but gives a consist

mesoscale cyclone activity over antarctica during 1991 2 near the antarctic peninsula a survey of mesoscale cyclogenesis near the antarctic peninsula has been carried out for 1991 based on digital satellite imagery collected at palmer station slightly higher cyclonic activity was found over the bellingshausen sea sector bss than in the weddell sea sector wss both sides were characterized by a pronounced formation maximum in summer and a minimum in winter comma clouds were the dominant vortex type and their average diameter was around 370 km trajectories show that many vortices in the bss move northeastward toward the drake passage with some of the most well developed moving north northeastward to the southern tip of south america bringing severe weather conditions to that area little systematic motion was noted over the wss the fraction of mesoscale cyclones with middle and or high cloud on the west side of the antarctic peninsula was much higher than on the east side these numerous and deep mesoscale cyclones are a consequence of the frequent outbreaks of cold air associated with synoptic scale cyclones that move cold air northward over the relatively warm bellingshausen sea with convective instability resulting this happens much less often in the wss where low level baroclinic instability and vortex stretching are the dominant formation mechanisms and air sea interaction is usually much less vigorous due to the stabilizing influence of the extended sea ice cover

evolution and mesoscale structure of a polar low outbreak an observational study of an unusual polar low outbreak is described in which four lows formed in succession near south east iceland because all the lows tracked south east close to scotland an unusually dense network of surface observations was available for analysis including radar this complemented the geostationary and polar orbiting satellite data that are more commonly used in polar low studies each of the four polar lows displayed a different structure and evolution the first was a circular convective low the second a relatively weak surface signature of an upper level feature while the final two cyclones appeared as a wave train with one cyclone decaying and the other developing into an intense disturbance because of the high resolution of the surface data it was possible to identify unambiguously mesoscale frontal features within the polar lows these are described in detail a close examination was made of the final intense low which showed that its evolution and frontal structure closely matched those found in standard conceptual models of rapidly developing maritime cyclones this polar low also showed evidence of a warm core and symmetric instability within an extensive cloud head

observations and modeling of dense water cascading from the northwestern laptev sea shelf 1 we examine dense water cascading from the northwestern laptev sea shelf in the arctic ocean this region is believed to be a potential site of dense water formation through ice freezing and brine ejection climate data indicate the presence of saltier denser water on the shelf all year round tongues of dense water spreading from the shelf to the continental slope and reaching a depth of 300 400 m are reported in some historical hydrographic surveys in this study we recreate dense water cascading from the laptev sea shelf by filling a gap between two sequential hydrographic surveys with numerical model output from october 1984 to april 1985 ice formation over the shelf provided salt input almost 5 times greater than the amount that was accumulated in the shelf waters this excessive salt was transported off the shelf mainly by baroclinic eddies and also by mean outflow through the bottom boundary layer shelf origin dense water would have reached at least 1000 m depth but strong stratification prevented it from penetrating deeper than 400 m downslope contribution of mean downslope flow to heat and salt exchange between shelf and slope is on the average 4 times smaller than that caused by eddy fluxes hence although the volume transport associated with cascading is relatively small 0 02 sv heat and salt fluxes are large enough to provide cooling and freshening of atlantic water which propagates along the siberian continental slope

baroclinic instability in an environment of small stability to slantwise moist convection part i two dimensional models abstract in the semigeostrophic system the growth rate of baroclinic waves varies with the inverse square root of the potential vorticity which acts as the effective static stability recent observations in the ascent regions of middle latitude cyclones show that the effective potential vorticity for saturated air is very near zero in this paper we examine the structure and rate of growth of baroclinic cyclones when the effective potential vorticity is small for upward saturated displacements but large in regions of descent analytic solutions for two dimensional disturbances in a two layer semigeostrophic model and numerical simulations using a multilevel semigeostrophic model show that when the effective potential vorticity is small in regions of upward motion growth rates are modestly increased and the region of ascent intensifies and collapses onto a thin ascending sheet in the limit of zero moist potential vorticity the fastest growing wave has a finite growth rate which is about 2 5 times the

modeling the evolution of continental subduction processes in the pamir hindu kush region several geological and geophysical studies suggest the presence of two converging subduction zones in the western syntaxis of the india eurasia collision zone with steep northward subduction of indian lithosphere beneath the hindu kush and southward subduction of asian lithosphere under the pamir we investigate the geometry and the timing of these subduction processes seismic tomography images are used to constrain the geometry of the two slabs under the pamir and the hindu kush by measuring the tomographically inferred length of the indian slab under the hindu kush region and by comparison with paleomagnetic reconstructions we estimate that the process of indian slab break off most likely occurred at the early stage of the collision at ca 44 48 ma we infer that after slab break off western india continued its northward motion until its northern boundary reached the present hindu kush region where it began to subduct we estimate an age for the initiation of subduction in this region of about 8 ma we apply 2d thermo kinematic and rheological numerical modeling to compute the temperature distribution and brittle field along two vertical sections across the hindu kush and pamir regions for the hindu kush we introduce parameters reproducing fast and near vertical subduction whereas in the pamir region we use the results from published studies to reproduce slower subduction modeling results indicate that faster subduction under the hindu kush results in a deeper brittle region which is consistent with observed deeper seismicity than in the pamir region

topography and grounding in a simple bottom layer model a reduced gravity model for a dense bottom layer under a motionless deep layer is used to investigate bottom layer flow with topography the two competing effects in the model are topography which introduces a topographic beta effect and strong layer thickness variation which allows for the possibility of zero layer thickness or grounding the goal is to arrive at a simple observationally relevant characterization of the model which can help to distinguish different dynamical mechanisms operating in the ocean several regimes are identified including situations for which the layer depth goes to zero with and without topography ways of distinguishing observationally between competing effects are suggested the study is motivated by the observed shift of antarctic bottom water in the western north atlantic ocean from the west to the east this phenomenon is used to discriminate key elements of the model

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

near bottom current speed maxima in north atlantic contourite environments inferred from current induced bedforms and other seabed evidence abstract seabed acoustic data e g deep tow side scan sonar together with information from seabed photography is presented for estimating near bottom current speed maxima in north atlantic contourite environments these include the southern iceland faroe ridge flank the southeast and southwest greenland margin and the greater antilles outer ridge in the southwestern part of the north atlantic geomorphological response to north atlantic deep water circulation is expressed in a variety of dynamic bedforms and other current controlled seabed features various types of dynamic bedforms reflect different hydraulic energy levels maximum near bottom flow speed of almost 1 0 m s or more is illustrated by infinite sand ribbons found on lag sediment while a much lower maximum of near bottom water flow speed

thermohaline circulation of shallow tidal seas the mechanisms controlling the temperature and salinity structure of shallow continental shelf seas have been understood for over thirty years yet knowledge of what drives their large scale circulation has remained relatively unknown here we describe a decade long programme of measurements using satellite tracked drifting buoys on the northwest european shelf to draw attention to a striking picture of highly organised thermohaline circulation consisting of narrow near surface fast flowing jets these are ubiquitous above sharp horizontal gradients in bottom temperatures and or salinities the circulation phenomena we describe are likely to be prevalent on all similar wide tidally energetic continental shelves including those off north eastern china argentina and parts of the arctic the robust repeatable observation of the key role of jets above bottom fronts results in a fundamental reassessment of how we view the dynamics of shelf seas

a polar low over the japan sea on 21 january 1997 part ii a numerical study abstract a remarkable isolated polar low observed over the japan sea on 21 january 1997 is studied by numerical simulations using a 20 km mesh regional primitive equation model regional spectral model rsm of the japan meteorological agency and a 5 km mesh nonhydrostatic mesoscale model meteorological research institute nonhydrostatic model mri nhm and by sensitivity experiments using the mri nhm the 24 h integration of rsm starting from 0000 utc 21 january reproduces quite well the polar low s movement and development including the initial wrapping of the west part of an east west oriented vorticity zone the vorticity budget analysis indicates that the stretching term plays a dominant role in the polar low development the simulation by mri nhm reproduces surprisingly well the wrapping of the western part of the east west oriented vortcity zone associated with a cloud band the spiral shaped cloud pattern and the eye structure near the vortex center the wind speed and resulting sea surface

mantle conveyor beneath the tethyan collisional belt abstract collisional belts are generated by the arrival of continental lithosphere into a subduction zone the tethyan suture from the bitlis to the himalayas is a prime example where the arabian and indian plates collided with eurasia during the cenozoic while the kinematics of this process are well established its dynamics are more uncertain india and arabia intriguingly keep advancing in spite of large collisional resisting forces and in the absence of a substantial upper mantle slab driving force at present day we perform global mantle circulation computations to test the role of deep mantle flow as a driving force for the kinematics of the tethyan collisional belt evaluating different boundary conditions and mantle density distributions as inferred from seismic tomography or slab models our results show that mantle drag exerted on the base of the lithosphere by a large scale convective conveyor belt with an active upwelling component is likely the main cause for the ongoing indentation of the indian and arabian plates into eurasia

polar lows over the gulf of alaska in conditions of reverse shear abstract the formation of two polar lows over the gulf of alaska are studied using observations taken during the ocean storms field experiment synoptic scale and mesoscale analyses were constructed using noaa p 3 aircraft flight level and radar data and drop windsonde profiles in addition to conventional data sources the synoptic scale analyses show that polar low development occurred in a low level mesoscale baroclinic zone near the center of a mature occluded synoptic scale low pressure system the disturbances propagated along the zone in the reverse shear sense i e in the direction opposite to the thermal wind within the zone the mesoscale analysis reveals a boundary layer jet of 38 m s 1 on the cold side of the baroclinic zone accompanying the leading polar low indirect evidence suggests that a sawyer eliassen secondary circulation was present polar low development occurred in the region of a frontogenetical geostrophic deformation convective activity was not prominent during the growth p

simulated interannual variability of the greenland sea deep water formation and its connection to surface forcing a fully prognostic arctic ice ocean model is used to study the interannual variability of deepwater formation in the greenland sea gyre based on the simulations for the arctic ice ocean system for the period 1955 and 1960 1985 the model uses monthly climatology for thermodynamic forcing components such as air temperature and cloudiness together with constant annual net precipitation and river runoff the daily wind forcing is derived from analyzed sea level air pressures from the national center for atmospheric research in summary the model shows that the occurrence of deep convection in the greenland sea gyre is controlled by the extensive fram strait ice export and or local wind conditions in the greenland sea in the latter case the weakening of the local wind curl allows the polar front to move eastward the movement of the polar front causes adverse ice conditions often together with much larger than normal ice export from the arctic such as in 1968 which can block convection in the gyre the density difference between upper and lower layers is investigated as an indication of water mass formation through convection occurring as strong diffusion in the model the model simulated density difference between the average top 100 m and deep levels reveals that the period 1960 1985 had only a few distinct years with weak stratification and especially the model predicts no deep convection since the mid 1970s the common factor for the years of the weakest stratification is extreme wintertime wind events deep convection is also evident as a decrease of the model predicted heat content of the upper 2000 m which can to a high degree be explained by local heat loss the salt content changes in the greenland gyre are independent of the heat content changes and are determined by the advection of fresh or salty anomalies either from the arctic or from the frontal movement the model indicates that the surface salinity flux favors freshening even in wintertime so that brine rejection is less important in buoyancy loss than heat loss in leading to deep convection the model shows that the system has a memory deep convection in one year will precondition the following year

climatology of polar lows over the sea of japan using the jra 55 reanalysis abstractpolar lows are intense meso scale cyclones that develop over the oceans poleward of the main baroclinic zone a number of previous studies have reported polar low formation over the sea of japan within the east asian winter monsoon to understand the climatology of polar lows over the sea of japan a tracking algorithm for polar lows is applied to the recent jra 55 reanalysis the polar low tracking is applied to 36 cold seasons october march from october 1979 to march 2015 the polar lows over the sea of japan reach their maximum intensity on the southeastern side of the midline between the japanese islands and the asian continent consistent with previous case studies composite analysis demonstrates that the polar low development is associated with the enhanced northerly flow on the western side of a synoptic scale extratropical cyclone with the cold trough in the midtroposphere and with increased heat fluxes from the sea surface furthermore the present climatological study has revealed

cascading of water down the sloping sides of a deep lake in winter during winter the temperature of the water overlying the shallow typically 2 5 m deep shelf region around the edge of the 310 m deep lake geneva falls more rapidly than that over deeper areas this causes the spilling or cascading of relatively dense water from the shallows down the sloping sides of the lake in the form of gravity currents 2 15 m thick and typically 0 1 c cooler than the ambient the flow is intermittent with slugs of cold water lasting on average for 8 hrs with mean downslope speeds of 5 2 cms 1 the temperature and thickness of the slugs is however variable with pulses of colder water lasting for 1 3 hrs each preceded by a front in which thickness increases and temperature falls by about 0 01 c per min the net volume flux carried by the slugs is 18 5 times the mean winter flow into the lake from rivers

high frequency environmental changes and deposition processes in a 2 kyr long sedimentological record from the cap breton canyon bay of biscay the core md03 2693 43 39 258 n 01 39 805 w 431 m water depth was collected on an abandoned meander of the capbreton canyon se bay of biscay filled over the last millennia by very high sedimentation rates mean sedimentation rate of 1 2 cm yr linked to its specific environmental location and fine grained clayed sediment decantation from the proximal canyon axis this archive thus permits to undertake the study of late holocene regional climatic patterns at a decadal temporal resolution in the present work we use data derived from planktonic foraminifera assemblages coupled to a multiproxy approach that associates grain size measurements x ray fluorescence xrf elemental analysis and stable oxygen isotope on globigerina bulloides shells to infer sea surface temperature sst and sea surface salinity sss changes over the last two millennia signals reconstructed in the bay of biscay show significant oscillations that are consistent with well known temperature anomalies such as the little ice a

the 1987 aegean dense water formation a streamtube investigation by comparing theoretical model results satellite field and numerical data with contourite distribution abstract we here discuss a detailed investigation of the dense water formation evolution and spreading in the aegean sea during the year 1987 immediately prior to the onset of the eastern mediterranean transient emt we use hydrologic data collected during the lia cruise satellite images for sst sea surface temperature and protheus data a coupled ocean atmosphere numeric model along with theoretical streamtube models these hydrological analyses are related to late quaternary sedimentary drifts in the cyclades plateau and in the myrtoon basin our analysis shows that streamtube dynamics provide a novel model of dense water evolution and spreading in the aegean basin applying this model to dense water masses observed in winter and spring 1987 near samothrace and over the limnos lesbos plateau results in a geostrophic flow of this dense cold water towards the limnos sporades channel in the north aegean sea there it mixes with dense water from the limnos lesbos plateau and finally both move geostrophically towards the cyclades plateau these results indicate that most of the dense water observed near the cyclades formed initially about 3 months earlier at samothrace and limnos shelves during its long pathway it partially mixed with adjacent water masses although our analysis concerns only one year of dense water analyses these results are thought to reflect a more general and recurrent phenomenon in the aegean basin indeed high resolution airgun 10 in seismic reflection data from the cyclades plateau reveal the presence of late quaternary sediment drifts these observations are concordant with results from our theoretical model this suggests a direct link between such a dense water cascading and contourite dynamics the continuation of sediment drifts into the deep basin floor 900 m deep of the myrtoon basin moreover indicates a cascading character of such bottom currents at the flanks of the basin a feature that set further investigations

a polar low over the labrador sea interactions with topography and an upper level potential vorticity anomaly and an observation by radarsat 1 sar 1 polar lows high latitude marine mesoscale cyclones are an enigmatic atmospheric phenomenon with their small spatial scales their short life times and their tendency to develop in remote data sparse regions they remain difficult to observe and to model in this paper we use a variety of data sets to discuss the development of a polar low over the labrador sea that is of interest for several reasons first it developed out of the remnants of a synoptic scale cyclone that underwent a bifurcation due to an interaction with the high topography of southern greenland second the transformation of the resulting circulation into a polar low was aided by an interaction with an upper tropospheric potential vorticity anomaly finally the event was observed by the synthetic aperture radar on the radarsat 1 satellite the resulting image of the ocean s surface provides a new and provocative view of a most interesting weather system

mesoscale cyclogenesis over the southwestern ross sea linked to strong katabatic winds abstract two years of automatic weather station aws observations and satellite images have been used to study mesoscale cyclogenesis along the transantarctic mountains twice daily regional sea level pressure analyses revealed the frequent formation of mesoscale cyclones adjacent to two regions where the discharge of cold boundary layer air from east antarctica is concentrated near terra nova bay franklin island and byrd glacier between one and two new vortices on average formed each week in the former location with weak frequency maxima in december march and august september there was a large difference between the cyclogenetic activity in the two years the aws array expanded in 1985 and resolved another cyclogenetic area near byrd glacier this feature was half as active as the franklin island area and exhibited many of the same characteristics about half of the byrd glacier cyclones developed simultaneously with vortices near franklin island these developments are the result of a dry baroclinic

orocline and syntaxes formation during subduction and collision 1 the present work investigates the formation of curved ranges and syntaxes with scaled laboratory experiments we simulated subduction and collision processes comparable to india asia configuration involving a continental upper plate and a subducting plate composed of an oceanic lithosphere and a continental indenter the experiments reveal that the shape of the mountain range concave straight or convex and the development of syntaxes are controlled by the subduction interface the buoyancy number fb of the upper plate i e thickness and viscosity and the boundary conditions four end members regimes of indentation can be defined depending on the range shape and dynamics of the upper plate the curvature of the range is convex toward the subducting plate with syntaxes for a weak subduction fault and concave without syntaxes for a strong subduction fault convex curvature and syntaxes form by overthrusting of upper plate material on the subducting plate which is faster at the center than at the extremities they are associated with a rather flat slab underthrusting during continental collision low fb experiments show less pronounced curvatures associated to thickening comparable to the early stages of the india asia collision in contrast a thick and weak upper plate high fb leads to gravity collapse that increases the amplitude of the curvature and lateral escape similar to the late evolution of the himalaya tibet system important lateral decoupling on the sides of the indenter enhances the indentation and produces sharper syntaxes

3d geodynamic models for the development of opposing continental subduction zones the hindu kush pamir example abstract the development of opposing continental subduction zones remains scantly explored in three dimensions the hindu kush pamir orogenic system at the western end of the himalayan orogen provides a rare example of continental collision linked to two opposing intra continental subduction zones the subducted plates feature a peculiar 3d geometry consisting of two distinct lithospheric fragments with different polarities subduction angles and slab curvatures beneath the hindu kush and pamir respectively using 3d geodynamic modeling we simulate possible development of two opposing continental subduction zones to understand the dynamic evolution of the hindu kush pamir orogenic system our geodynamic model reproduces the major tectonic elements observed 1 the deeper subduction depth the steeper dip angle and the southward offset of the hindu kush subduction zone relative to the pamir naturally occur if convergence direction of the subducting indian plate and dip direction of the hindu kush subduction zone match 2 the formation of the highly asymmetrically curved pamir region and the south dipping subduction is promoted by the initial geometry of the indenting indian lithosphere together with the existence of a major strike slip fault on the eastern margin of the pamir region 3 subduction of only the lower continental crust during continental collision can occur if the coupling between upper and lower crusts is weak enough to allow a separation of these two components and that 4 the subduction of mainly lower crust then facilitates that conditions for intermediate depth seismicity can be reached 5 the secondary tectonic features modeled here such as strike slip fault growth north northwest striking extension zone and lateral flow of the thickened ductile upper crust are comparable to the current tectonics of the region 6 model results are further compared to the potentially similar orogenic system i e the alpine orogen in terms of the curved western alpine arc and the two opposing subducted slabs beneath the alps and the dinarides

cascades of dense water around the world ocean dense water overflow off continental shelves cascading is one of the contributing processes of shelf deep ocean exchange and of topical interest to climate studies and nutrient fluxes dense water originating from cooling evaporation freezing and salinization on a shallow shelf spills over the shelf edge and may develop as near bottom gravity current or an intermediate depth intrusion it is difficult to observe in nature due to its intermittent character this paper provides an extensive inventory of observed cases of water cascades around the world ocean summarises their locations and individual properties and provides statistics of the identified cases the search for cascading was carried out using oceanographic databases and a literature review this study identified 61 confirmed cases world wide including 25 cases in the arctic seas 12 at mid latitudes seven in sub tropical and tropical regions and 17 off the antarctic shelves eighteen cascades had not been reported before we analyze a set of numerical parameters of dense water cascades allowing us to quantify compare and contrast the properties of water cascades the overall average density contrast between the confirmed cascades and ambient water is 0 37 kg m3 it can be as much as 2 kg m3 on some arctic shelves frequently initiated by strong cooling at the surface cascades often remain colder through the descent thus supplying the deep ocean with colder and fresher water in non dimensional variables the data from all climate zones fit well to a unique curve which represents a relationship between a cascade s internal structure and the parameters describing its forcing on average the down slope volumetric flux provided by dense water cascades is estimated as 0 05 to 0 08 sv per 100 km of shelf edge regional terms arctic antarctic north atlantic ocean barents sea mediterranean sea skagerrak tasman sea sea of okhotsk

response of sea ice to the arctic oscillation abstract data collected by the international arctic buoy programme from 1979 to 1998 are analyzed to obtain statistics of sea level pressure slp and sea ice motion sim the annual and seasonal mean fields agree with those obtained in previous studies of arctic climatology the data show a 3 hpa decrease in decadal mean slp over the central arctic ocean between 1979 88 and 1989 98 this decrease in slp drives a cyclonic trend in sim which resembles the structure of the arctic oscillation ao regression maps of sim during the wintertime january march ao index show 1 an increase in ice advection away from the coast of the east siberian and laptev seas which should have the effect of producing more new thin ice in the coastal flaw leads 2 a decrease in ice advection from the western arctic into the eastern arctic and 3 a slight increase in ice advection out of the arctic through fram strait taken together these changes suggest that at least part of the thinning of sea ice recently observed ov

application of a transient reduced gravity plume model to the denmark strait overflow a hydrostatic reduced gravity two dimensional primitive equation numerical model for the simulation of transient bottom arrested gravity plumes was developed by adopting an algorithm from a wadden sea model a model that simulates the wetting and drying of mud flats sand banks etc with a movable lateral boundary the vertically integrated nonlinear rotational model accounts for the dissipation of momentum due to bottom friction and for the entrainment of water mass properties from a spatially structured but stagnant ambient water body it predicts the temporal and spatial evolution of the flow field and the water mass modification within a gravity plume that descends on an arbitrary topography previous model investigations considered a laterally integrated streamtube whereas our model resolves the plume horizontally in the streamtube approach it was assumed that the plume descends in a stationary almost geostrophic balance modified only by bottom drag and entrainment the results of this model demonstrate that this balance may be disturbed whenever the plume encounters topographic disturbances which for the initial intruding phase in particular accounts for its highly transient character the model results are in good agreement with theoretical and laboratory investigations and with observations as is demonstrated by some basic test experiments an application to the overflow of dense water through the denmark strait is presented

mediterranean outflow mixing and dynamics the mediterranean sea produces a salty dense outflow that is strongly modified by entrainment as it first begins to descend the continental slope in the eastern gulf of cadiz the current accelerates to 1 3 meters per second which raises the internal froude number above 1 and is intensely turbulent through its full thickness the outflow loses about half of its density anomaly and roughly doubles its volume transport as it entrains less saline north atlantic central water within 100 kilometers downstream the current is turned by the coriolis force until it flows nearly parallel to topography in a damped geostrophic balance the mixed mediterranean outflow continues westward slowly descending the continental slope until it becomes neutrally buoyant in the thermocline where it becomes an important water mass

cascading off the west greenland shelf a numerical perspective cascading of dense water from the shelf to deeper layers of the adjacent ocean basin has been observed in several locations around the world the west greenland shelf wgs however is a region where this process has never been documented in this study we use a numerical model with a 1 4 resolution to determine i if cascading could happen from the wgs ii where and when it could take place iii the forcings that induce or halt this process and iv the path of the dense plume results show cascading happening off the wgs at davis strait dense waters form there due to brine rejection and slide down the slope during spring once the dense plume leaves the shelf it gradually mixes with waters of similar density and moves northward into baffin bay our simulation showed events happening between 2003 2006 and during 2014 but no plume was observed in the simulation between 2007 2013 we suggest that the reason why cascading was halted in this period is related to the increased freshwater transport from the arctic ocean through fram strait the additional sea ice melting in the region and the reduced presence of irminger water at davis strait during fall early winter although observations at davis strait show that our simulation usually overestimates the seasonal range of temperature and salinity they agree with the overall variability captured by the model this suggests that cascades have the potential to develop on the wgs albeit less dense than the ones estimated by the simulation

atmospheric teleconnections involving the southern ocean 1 i review the characteristic patterns of low frequency interannual and subdecadal variation in atmospheric circulation over the southern hemisphere sh extratropics and their climatic associations e g temperature precipitation winds sea surface temperatures ssts and sea ice conditions for the southern ocean the considerable spatial and temporal limitations of the observational database have strongly conditioned the pace of our understanding of sh teleconnections particularly for the pacific sector where the tropical el nino southern oscillation enso exerts a dominant influence an enso signal appears in the intensity interannual variations of the amundsen sea mean low pressure asl jet streams and long waves these variations comprise the so called pacific south america psa teleconnection patterns of low frequency variability in sh circulation that are mostly extratropical in origin but which can interact with enso are dominated by the zonally symmetric high latitude mode or antarctic oscillation aao the aao involves an alternation of atmospheric mass between middle and high southern latitudes a zonally asymmetric mode of wave number 1 is represented by an oscillation in pressure height between australian and south american sectors depicted by a trans polar circulation index a significant wave number 2 oscillation in the coupled atmosphere ocean ice system or antarctic circumpolar wave acw has a similar periodicity to enso and is strongest in the pacific sw atlantic sectors atmospheric teleconnections involving the southern ocean including those to enso show evidence of recent changes and the climatic implications of these are discussed

a long term climatology of north atlantic polar lows 1 a dynamical downscaling of the 6 hourly 1948 2006 ncep ncar re analyses for the subarctic region of the north atlantic has been used to derive a climatology of polar lows by means of a tracking algorithm based on the simulated bandpass filtered mslp fields this climatology is consistent with the limited observational evidence in terms of frequency and spatial distribution the climatology exhibits strong year to year variability but weak decadal variability and a negligible trend a canonical correlation analysis describes the conditioning of the formation of polar lows by characteristic meridional seasonal mean flow regimes which favor cold air outbreaks and upper air troughs

forward and reverse shear environments during polar low genesis over the northeast atlantic abstractthe synoptic and subsynoptic environments associated with polar low genesis are examined ambient pre polar low environments are classified as forward or reverse shear conditions based on the angle between the thermal and mean wind forward shear environments are associated with a synoptic scale ridge over scandinavia featuring a zonally oriented baroclinic zone extending throughout the troposphere with a wind speed maximum at the tropopause similar to typical midlatitude cyclogenesis concurrent wavelike development occurs both in the lower and upper troposphere along the baroclinic zone and the mean propagation direction is eastward parallel to isolines of sea surface temperature reverse shear environments exhibit a distinctly different structure and are characterized by a trough over scandinavia associated with a synoptic scale occluded cyclone the genesis area exhibits strong cold air advection on its right hand side and polar low development occurs on the warm side of an intense low le

a comparison of arctic and antarctic mesoscale vortices the mesoscale less than 100km vortices occurring in the two polar regions are considered in terms of their geographical and seasonal distribution satellite cloud signatures and forcing mechanisms environmental conditions important in the development of the vortices are considered including sensible heat flux stability throughout the troposphere and synoptic factors such as baroclinicity and upper air cold pools a scheme to classify the observed vortices in the two polar regions in terms of the physical mechanisms behind their formation and development is proposed the processes considered important are baroclinic instability convection and vorticity generation through cyclonic vorticity advection and topographic forcing the major difference between the systems observed in the two polar regions is the lack of deep convection in the southern hemisphere which precludes the development of many of the vigorous types of system found in the north and the major role that topography plays in the antarctic coastal region the most common type of vortex found in the antarctic occurs over the ice free ocean to the west of synoptic scale disturbances and is similar to the type of northern system know as a comma cloud

a climatology of polar low cyclogenetic regions over the north pacific ocean abstract a climatology of polar low cyclogenesis in the north pacific ocean for seven winter seasons november march is presented the data are collected by the subjective interpretation of defense meteorological satellite program dmsp infrared imagery the results demonstrate that on average the most active polar low cyclogenesis takes place in the western extratropical north pacific with the greatest rates of formation found just to the east of northern japan few polar lows form east of the date line agreeing with theory and earlier observations comma clouds are by far the most common form of polar low in the north pacific and spiraliforms are relatively rare intra annual variability is explained by the seasonal cycle of the large scale ocean atmosphere system with early winter polar lows forming off the land or ice edge in the more northerly parts of the basin and midseason cyclones forming southward and increasingly away from land as the circumpolar vortex expands late in the season the

accelerating dense water flow down a slope abstract where water is denser on a shallow shelf than in the adjacent deep ocean it tends to flow down the slope from shelf to ocean the flow can be in a steady bottom boundary layer for moderate combinations of upslope density gradient x and bottom slope angle to horizontal here g is acceleration due to gravity 0 is a mean density and f is twice the component of the earth s rotation normal to the sloping bottom for stronger combinations of the horizontal density gradient and bottom slope the flow accelerates analysis of an idealized initial value problem shows that when b 1 there is a bottom boundary layer with downslope flow intensifying exponentially at a rate fb2 1 b 1 2 2 and slower growing flow higher up for stronger stratification b 21 2 that is a relatively weak coriolis constraint the idealized problem posed here may not be the most apposite but suggests that the whole water column accelerates at a rate 0 1 x g sin 1 2 if f is negligible

large scale atmospheric circulation patterns during polar low events over the nordic seas 1 the large scale atmospheric circulation during polar low events over the nordic seas the north atlantic between greenland and novaya zemlya is investigated on the basis of a recently compiled data set that was derived purely from satellite observations a classification system is applied that divides the polar lows into four types western polar lows eastern polar lows greenland lee polar lows and storm track polar lows type specific large scale atmospheric circulation patterns are identified from national centers for environmental prediction reanalysis data these are distinct in sea level pressure upper level geopotential height and the difference between the skin temperature of the ocean and upper level temperature eastern polar lows are found to be associated with a strong blocking situation caused by anomalously high pressure over iceland and a synoptic scale low pressure anomaly over the barents sea a weaker blocking situation with an anomalous ridge over greenland reaching into the irminger sea and a low pressure anomaly over the norwegian sea favors the development of western polar lows typical values for polar low genesis are a geopotential height of 5030 geopotential meters at 500 hpa and a difference of 48 k between the skin temperature of the ocean and the temperature at 500 hpa the locations of upper level low pressure anomalies relative to the locations of the related anomalies at the sea level show that western and eastern polar lows form on average in a much less baroclinic large scale environment than greenland lee and storm track polar lows

an arctic hurricane over the bering sea abstract a mesoscale arctic hurricane developed over the western bering sea on 7 march 1977 and traveled eastward parallel to the ice edge along a zone of large sea surface temperature gradient satellite imagery reveals spiral cloud bands of unusual symmetry and mesoscale dimension associated with the mature stage of the low the track of the low pressure center passed over the rawinsonde station at st paul island where time series of surface data show a pronounced maximum in equivalent potential temperature at the core of the low the storm made landfall with surface winds 30 m s 1 at cape newenham alaska on 9 march and rapidly dissipated thereafter synoptic analyses show that the arctic hurricane formed at the leading edge of an outflow of arctic air that originated over the ice and passed over the open water of the western bering 5u in the mid and upper troposphere a large cold core low dominated the bering sea region quasi geostrophic analysis at 0000 utc 7 march 1977 reveals conditions co

a numerical modeling study of mesoscale cyclogenesis to the east of the korean peninsula numerical simulations and the analysis of observational data are employed to understand the mesoscale cyclogenesis in a polar airstream that occurred over the sea to the east of the korean peninsula on 28 29 january 1995 the observational analysis shows that a mesoscale low develops over the southeastern east sea japan sea on 29 january 1995 satellite imagery also indicates that a meso b scale vortex forms on the lee side of the northern korean mountain complex kmc which is located in the northern korean peninsula and that a meso a scale cyclone develops over the southeastern east sea at a later time the mesoscale cyclone forms in the lower troposphere with strong baroclinicity and cyclonic circulation under the influence of an upper level synoptic scale cold vortex numerical simulation has captured major features of the observed cyclogenesis very well the cyclogenesis occurs in a progressive manner basically four distinctive stages of the cyclogenesis are identified 1 first a surface pressure trough forms on the lee side of the kmc under a northwesterly synoptic scale flow that is deflected anticyclonically over the kmc 2 second the lee trough deepens further into a strong convergence zone and a meso b scale vortex 3 next the meso b scale vortex develops into a meso a scale vortex as the vortex and the trough begin to move southeastward from the lee of the kmc 4 finally the surface trough deepens into a closed low and the meso a scale vortex becomes collocated with this deepening surface low to form a meso a scale cyclone over the southeastern east sea several sensitivity experiments are performed to isolate the effects of a topography warmer sea surface diurnal thermal forcing and latent heat release during stages 1 and 2 it is found that the kmc and low level baroclinicity are responsible for generating the strong lee trough and vortex during stage 3 the development of the meso a scale vortex is brought on by the tilting of horizontal vorticity and vertical stretching in a synopticscale cyclonic circulation in the final stage the condensational heating plays the key role for the development of the meso a scale cyclone under the influence of an upper level synoptic scale cold vortex the presence of the warm sea surface is found to be a necessary condition for the development of a polar air convergence zone and the mesoscale cyclone it is also found that the low level baroclinicity is essential for the present case of mesoscale cyclogenesis

high resolution numerical simulation of a polar low 1 the first high resolution simulation of an observed polar low using a non hydrostatic mesoscale numerical model with 2 km horizontal resolution is reported the case studied is a remarkable polar low that developed over the japan sea on 21 january 1997 the simulation successfully reproduced the observed features of the polar low such as its horizontal scale movement spiral bands a cloud free eye and cloud streets associated with the cold air outbreak detailed three dimensional structures of the simulated polar low which are difficult to observe by in situ measurement have been clarified and its development mechanism is discussed

magmatic expression of tectonic transition from oceanic subduction to continental collision insights from the middle triassic rhyolites of the north qiangtang block abstract the tectonic transition from subduction to collision is a fundamental process during orogenesis yet the magmatic expression of this transition and related deep geodynamic processes remain unclear this study focuses on a newly identified volcanic belt within the moyun zaduo sulu area of the north qiangtang block and presents new zircon u pb data that indicate that this belt formed during the middle triassic 247 241 ma a time characterized by a regional transition from subduction to collisional tectonism the volcanic belt is located to the south of a permian to early triassic arc and is dominated by high k calc alkaline and peraluminous rhyolites these rhyolites have low mg nb ta and eu values contain low contents of sr have high rb sr and whole rock end t values and show positive zircon ehf t values all of which suggest that they formed from magmas generated by the dehydration melting of juvenile crustal material the migration of middle triassic volcanism in this region was most likely caused by rollback of the subducting longmucuo shuanghu tethyan oceanic slab combining our new data with previously published results of numerical modeling of subduction collisional processes and regional data from north central tibet yields insights into the magmatic expressions and related deep geodynamics of the transition from oceanic subduction to continental collision this combination of data also suggests that variations in oxygen fugacity can be used as a proxy for the discrimination of magmatism related to subduction the transition from subduction to collision and collisional tectonism

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

large sediment waves over the gulf of roses upper continental slope nw mediterranean recently acquired swath bathymetry and seismic datasets on the gulf of roses continental slope revealed the presence of large sediment waves that have developed between 200 and 400 m water depth geometric parameters were computed from the multibeam dataset and the obtained results showed mean sediment wave lengths of 2000 m and maximum heights of 60 m the analysis of seismic profiles showed the general architecture of the late quaternary deposits over the gulf of roses outer shelf and slope the outer continental shelf is characterized by morphological erosional features affecting the upper part of the most recent seismic unit over the continental slope five main unconformity bounded seismic units are differentiated with sediment waves developed in all of them the sediment waves observed over the continental slope are presumably generated by bottom currents intensified during major dense shelf water cascading events flowing downslope at an oblique angle with respect to the main bathymetry

improved extended reconstruction of sst 1854 1997 abstract an improved sst reconstruction for the 1854 1997 period is developed compared to the version 1 analysis in the western tropical pacific the tropical atlantic and indian oceans more variance is resolved in the new analysis this improved analysis also uses sea ice concentrations to improve the high latitude sst analysis and a modified historical bias correction for the 1939 41 period in addition the new analysis includes an improved error estimate analysis uncertainty is largest in the nineteenth century and during the two world wars due to sparse sampling the near global average sst in the new analysis is consistent with the version 1 reconstruction the 95 confidence uncertainty for the near global average is 0 4 c or more in the nineteenth century near 0 2 c for the first half of the twentieth century and 0 1 c or less after 1950

deep planetary circulation and topography simple models of midocean flows abstract the presence of broad flowing water masses in the deep ocean requires dynamical explanation vertical density diffusion and effect are invoked in classical theories here we show how topographic potential vorticity can control broad baroclinic flows with diffusion playing a secondary role the theory and numerical experiments involve a zonal flow over topography such as one finds in the southern ocean although similar effects occur in source sink flows at middle latitudes a 1 layer model that is one active deep layer beneath a thick upper layer of the deep circulation shows intense interactions with both small scale o 500 km wide and large scale o 2500 km wide seafloor slopes broad gentle slopes can block an initially uniform flow expelling the circulation to an outer rim defined by hyperbolic characteristics where it forms into jets with smaller scale topography planetary scale flow encountering a midocean ridge develops transient distortions of the flow and density fields whi

a case study of explosive cyclogenesis in the eastern pacific abstract an explosive cyclogenesis that occurred in the eastern pacific on 13 november 1981 is described and discussed on the basis of surface and upper air analyses and satellite imagery during one 12 h period the storm deepened by nearly 40 mb and subsequently attained a central pressure of 950 mb or less peak surface winds were in the 45 50 m s 1 range the limited area fine mesh lfm model 24 h forecast initialized at 0000 gmt on the thirteenth completely failed to predict the storm development the development took place within a strong baroclinic zone as a shallow frontal wave traveled from the relatively stable environment of a long wave ridge to the unstable environment of a long wave trough winds at jet stream level exceeded 90 m s 1 prior to the explosive deepening the lifted index in and ahead of the storm diminished and became negative prior to the rapid intensification measurements revealed that sensible and latent heat fluxes from the ocean contributed significantly to the reduced sta

acceleration and deceleration of india asia convergence since the cretaceous roles of mantle plumes and continental collision 1 a strong 50 35 ma decrease in india asia convergence is generally ascribed to continent continent collision however a convergence rate increase of similar magnitude occurred between 65 50 ma an earlier increase occurred at 90 ma both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province lip on the indian plate we here first confirm these convergence rate trends reassessing the indo atlantic plate circuits then using two different numerical models we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent plume distance may explain deceleration even without continental collision the results show that plume head arrival can indeed lead to absolute indian plate motion accelerations on the order of several cm yr followed by decelerations on timescales similar to the reconstructed fluctuations the 90 ma increase could potentially be explained as response to the morondova mantle plume alone the 65 50 ma convergence rate increase however is larger than can be explained by plume head spreading alone we concur with previous hypotheses that plume induced weakening of the indian continental lithosphere asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase the post 50 ma decrease is best explained by orogeny related increased trench resistivity decreased slab pull due to continental subduction and possibly restrengthening of lithosphere asthenosphere coupling upon plume demise

control of near surface winds over an antarctic ice shelf simple models are used to assess the factors controlling near surface winds over an antarctic ice shelf observations from halley station and an automatic weather station situated close to the coastal slopes adjoining the brunt ice shelf are presented these suggest that katabatic flows originating over the continental slopes adjust more quickly to the regional easterly flow over the ice shelf than is predicted by these simple models it is suggested that nonlinear mechanisms such as hydraulic jumps or internal gravity wave radiation could lead to flow adjustment on the short space and time scales observed in contrast with the present observations katabatic flows have been observed to propagate across the ross ice shelf for great distances with little modification however in this latter region the topography favors the formation of intense channeled katabatic flow while the katabatic flow onto the brunt ice shelf is unconfined and consequently much weaker

asian collisional subduction a key process driving formation of the tibetan plateau using silicone slabs as a model analogue for lithospheric plates subducting into a box of glucose syrup as an analogue of the mantle we explore the subduction of continental lithosphere in a context of intercontinental collision the continental indenter pushed by a piston reproducing the collision attached to a dense oceanic plate subducts to two thirds of the depth of the mantle box we show that surprisingly the continental plate attached to the back wall of the box subducts even if not attached to a dense oceanic slab the engine of this subduction is not the weight of the slab because the slab is lighter than the mantle but the motion of the piston which generates horizontal tectonic forces these are transmitted to the back wall plate through the indenter and the upper plate at the surface and by the advancing indenter slab through the mantle at shallow depth we define this process as collisional subduction occurring in a compressional context the collisional subduction absorbs between 14 and 20 of the convergence and represents an unexplored component of collisional mass balance the transmission of tectonic forces far from the collision front favors the formation of a wide plateau our experiments reproduce adequately the amount and geometry of the asian lithosphere subduction episodes inferred during the collision leading us to conclude that it reproduces adequately the physics of such process

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

associations between large scale atmospheric circulation and polar low developments over the north atlantic during winter 1 polar lows are intense mesoscale weather systems which develop in response to combinations of upper tropospheric vorticity advection low level baroclinicity near the sea ice ocean margin and strong air sea thermodynamical fluxes in this paper an evaluation of the associations between dominant patterns of low frequency variability in the atmospheric circulation and polar low developments over the north atlantic during winter is presented because polar lows are rather poorly represented in reanalyses the temperature t500 and geopotential height at 500 hpa the wind at 925 hpa the sst and the difference sst t500 which are critical for their formation are considered the north atlantic oscillation nao has a strong impact on all these variables but it is contrasted spatially and temporally polar low formation is stimulated in the labrador sea for a positive nao phase while over the norwegian sea there is an increase of polar lows forming on the flank of synoptic storms still over the norwegian sea higher sst from january on favor the formation of polar lows of predominantly convective nature while to the east of greenland the reduction of sea ice plus the northerly low level flow lead to a larger number of polar lows other teleconnection patterns also exhibit associations with polar low developments the scandinavia pattern shows a strong negative association with the studied variables over the norwegian and barents seas but only marginally affects the labrador sea the formation of polar lows is encouraged over the norwegian and greenland seas when the polar eurasia pattern is in its negative phase

amount of asian lithospheric mantle subducted during the india asia collision abstract body wave seismic tomography is a successful technique for mapping lithospheric material sinking into the mantle focusing on the india asia collision zone we postulate the existence of several asian continental slabs based on seismic global tomography we observe a lower mantle positive anomaly between 1100 and 900 km depths that we interpret as the signature of a past subduction process of asian lithosphere based on the anomaly position relative to positive anomalies related to indian continental slab we propose that this anomaly provides evidence for south dipping subduction of north tibet lithospheric mantle occurring along 3000 km parallel to the southern asian margin and beginning soon after the 45 ma break off that detached the tethys oceanic slab from the indian continent we estimate the maximum length of the slab related to the anomaly to be 400 km adding 200 km of presently asian subducting slab beneath central tibet the amount of asian lithospheric mantle absorbed by continental subduction during the collision is at most 600 km using global seismic tomography to resolve the geometry of asian continent at the onset of collision we estimate that the convergence absorbed by asia during the indentation process is 1300 km we conclude that asian continental subduction could accommodate at most 45 of the asian convergence the rest of the convergence could have been accommodated by a combination of extrusion and shallow subduction underthrusting processes continental subduction is therefore a major lithospheric process involved in intraplate tectonics of a supercontinent like eurasia

structure and environment of polar mesocyclones over the northeastern part of the sea of japan abstractpolar mesocyclones pmcs are mesoscale cyclonic vortices that develop poleward of the main polar front the sea of japan is one region where pmcs frequently occur during winter in this paper the general characteristics of the structure and environment of pmcs that form over the northeastern part of the sea of japan and move southward are examined using composite analysis and numerical simulation the composite analyses show that the synoptic scale environment of the pmcs is characterized by a zonal temperature gradient at lower levels and a trough accompanied by cold air at upper levels these elements of the environmental field form a reverse shear the mesoscale structure of the pmcs exhibits characteristics of baroclinic development while it is also accompanied by condensational heating the numerical simulation in which the composite fields are used for the initial and boundary conditions successfully reproduces a realistic pmc in this numerical simulation the mesoscale structures are al

mountain building and mantle dynamics 1 mountain building at convergent margins requires tectonic forces that can overcome frictional resistance along large scale thrust faults and support the gravitational potential energy stored within the thickened crust of the orogen a general dynamic model for this process is still lacking here we propose that mountain belts can be classified between two end members first those of slab pull type where subduction is mainly confined to the upper mantle and rollback trench motion lead to moderately thick crustal stacks such as in the mediterranean second those of slab suction type where whole mantle convection cells conveyor belts lead to the more extreme expressions of orogeny such as the largely thickened crust and high plateaus of present day tibet and the altiplano for the slab suction type deep mantle convection produces the unique conditions to drag plates toward each other irrespective of their nature and other boundary conditions we support this hypothesis by analyzing the orogenic volcanic and convective history associated with the tertiary formation of the andes after 40 ma and himalayas after collision at 55 ma based on mantle circulation modeling and tectonic reconstructions we surmise that the forces necessary to sustain slab suction mountain building in those orogens derive after transient slab ponding from the mantle drag induced upon slab penetration into the lower mantle and from an associated surge of mantle upwelling beneath africa this process started at 65 55 ma for tibet himalaya when the tethyan slab penetrated into the lower mantle and 10 myr later in the andes when the nazca slab did this surge of mantle convection drags plates against each other generating the necessary compressional forces to create and sustain these two orogenic belts if our model is correct the available geological records of orogeny can be used to decipher time dependent mantle convection with implications for the supercontinental cycle

dynamics of dense water cascades at the shelf edge abstract plumes of dense shelf water cascade down continental slopes in many parts of the world s oceans and provide a mechanism for shelf ocean exchange in this paper a nonlinear process orientated theory is developed and used to examine the dynamics of cascading the theory is formulated in terms of a 1 layer model and incorporates bottom topography earth rotation internal and bottom friction and entrainment as well as externally imposed pressure gradients the theory occupies a niche between the stream tube class of model which considers only bulk properties of a plume and the full three dimensional primitive equation approach the model provides useful insights into the complex interplay between the controlling forces and it allows one to recover the shape and trajectory of dense plumes as well as the three dimensional flow field inside the bottom layer asymptotic limits are investigated and lead to several basic results a typical thickness of a fully developed plume is found to be twice t

cyclone development in polar air streams over the wintertime continent abstract a polar air stream cyclogenesis over the wintertime north american continent is documented numerous similarities are found between this land case and oceanic polar lows studied harrold and browning 1969 reed 1979 and mullen 1979 the main difference between the land case and oceanic polar lows is that the land cyclone develops in an air mass with negligible water vapor content whereas oceanic ones clearly do not since they are characterized by enhanced cumulus activity during their early stage of development hence it is concluded that cumulus convection and its associated latent heat release are not essential for cyclone development in a polar air stream it is also shown that the surface occlusion which forms from the merger of the polar trough with the main polar front does not follow the typical surface occlusion process as described by the norwegian cyclone model

deep india meets deep asia lithospheric indentation delamination and break off under pamir and hindu kush central asia abstract subduction of buoyant continental lithosphere is one of the least understood plate tectonic processes yet under the pamir hindu kush at the northwestern margin of the india asia collision zone unusual deep earthquakes and seismic velocity anomalies suggest subduction of asian and indian lithosphere here we report new precise earthquake hypocenters detailed tomographic images and earthquake source mechanisms which allow distinguishing a narrow sliver of indian lithosphere beneath the deepest hindu kush earthquakes and a broad arcuate slab of asian lithosphere beneath the pamir we suggest that this double subduction zone arises by contrasting modes of convergence under the pamir and hindu kush imposed by the different mechanical properties of the three types of lithosphere involved while the buoyant northwestern salient of cratonic india bulldozes into cratonic asia forcing delamination and rollback of its lithosphere india s thinned western continental margin separates from cratonic india and subducts beneath asia this torn off narrow plate sliver forms a prominent high velocity anomaly down to the mantle transition zone our images show that its uppermost section is thinned or already severed and that intermediate depth earthquakes cluster at the neck connecting it to the deeper slab providing a rare glimpse at the ephemeral process of slab break off

meridional transport of eddy sensible heat in winters marked by extremes of the north atlantic oscillation 1948 49 1979 80 abstract composite patterns of eddy sensible heat transport at 700 mb on the northern hemisphere are derived for winters december january february djf marked by extremes of the north atlantic oscillation nao occurring in the 1948 49 1979 80 period statistically significant heat flux differences between the two nao modes ga greenland above gb greenland below normal temperatures relative to northern europe occur dominantly in the north atlantic sector these are connected to marked reversals in sea level pressure slp and an out of phase behavior of the zonal component of the geostrophic wind related to changes in the amplitude of wavenumber 2 teleconnections of the nao in terms of both pressure and eddy heat transport to the north pacific and east asian regions are found to be weak for those extreme events occurring since 1948 49 partitioning the total meridional heat flux into its transient and quasi stationary components confirms the dominance of the latter for the hemisphere as a who

polar mesoscale cyclone climatology for the nordic seas based on era interim abstractpolar mesoscale cyclones pmcs are automatically detected and tracked over the nordic seas using the melbourne university algorithm applied to the era interim reanalysis the novelty of this study lies in the length of the dataset 1979 2014 the usage of pmc tracks to infer relationships to large scale flow patterns and elucidating the sensitivity to different selection criteria when defining pmcs and polar lows and their genesis environments the angle between the ambient mean and thermal wind is used to distinguish two different pmcs genesis environments the forward shear environment thermal and mean wind in same direction features typical baroclinic conditions with a temperature gradient at the surface and a strong jet stream at the tropopause the reverse shear environment thermal and mean wind in opposite direction features an occluded cyclone with a barotropic structure throughout the entire troposphere and a low level jet in contrast to previous studies pmc occurrence does neither