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

the tectonic evolution of the south atlantic from late jurassic to present an improved tectonic database for the south atlantic has been compiled by combining magnetic anomaly geosat altimetry and onshore geologic data we used this database to obtain a revised plate kinematic model starting with a new fit reconstruction for the continents around the south atlantic we present a high resolution isochron map from chron m4 to present fit reconstructions of south america and africa that require rigid continental plates result in substantial misfits either in the southern south atlantic or in the equatorial atlantic to achieve a fit without gaps we assume a combination of complex rift and strike slip movements 1 along the south american parana chacos basin deformation zone 2 within marginal basins in south america salado colorado basin and 3 along the benue trough niger rift system in africa these faults are presumed to have been active before or during the breakup of the continents our model describes a successive unzipping of rift zones starting in the southern south atlantic between 150 ma tithonian and approximately 130 ma hauterivian rifting propagated to 38 s causing tectonic movements within the colorado and salado basins subsequently between 130 ma and chron m4 126 5 ma the tip of the south atlantic rift moved to 28 s resulting in intracontinental deformation along the parana chacos basin deformation zone between chron m4 and chron mo 118 7 ma rifting propagated into the benue trough and niger rift inducing rift and strike slip motion after chron mo the equatorial atlantic began to open while rifting and strike slip motion still occurred in the benue trough and niger rift since chron 34 84 ma the opening of the south atlantic is characterized by simple divergence of two rigid continental plates

isentropic analysis of polar cold airmass streams in the northern hemispheric winter abstractan analysis method is proposed for polar cold airmass streams from generation to disappearance it designates a threshold potential temperature t at around the turning point of the extratropical direct etd meridional circulation from downward to equatorward in the mass weighted isentropic zonal mean mim and clarifies the geographical distributions of the cold air mass the negative heat content nhc their horizontal fluxes and their diabatic change rates on the basis of conservation relations of the air mass and thermodynamic energy in the northern hemispheric winter the polar cold air mass below t 280 k has two main streams the east asian stream and the north american stream the former grows over the northern part of the eurasian continent flows eastward turns down southeastward toward east asia via siberia and disappears over the western north pacific ocean the latter grows over the arctic ocean flows toward the eastern coast of north america via hudson bay and disappears ov

importance of latent heating in mesocyclones for the decay of cold air outbreaks a numerical process study from the pacific sector of the southern ocean abstractin this study the dynamical mechanisms shaping the evolution of a marine cold air outbreak cao that occurred over the ross amundsen and bellingshausen seas in june 2010 are investigated in an isentropic framework the drainage of cold air from west antarctica into the interior ross sea its subsequent export and the formation of a dome of cold air off the sea ice edge are shown to be intimately linked to a lower tropospheric cyclone as well as the cyclonic breaking of an upper level potential vorticity trough the dome formation is accompanied by an extreme deepening of the boundary layer whose top reaches to the height of the low lying tropopause within the trough potentially allowing for deep stratosphere troposphere exchange a crucial finding of this study is that the decay of the cao is essentially driven by the circulation associated with a train of mesocyclones and the release of latent heat in their warm sectors sensitivity experiments with switched off fluxes of sensible and late

the rio grande fracture zone in the western south atlantic and its tectonic implications abstract a broad zone of linear mappable basement structures is observed north and northeast of the rio grande rise in the south atlantic ocean these structures lie along the same flow line as the sa o paulo ridge the florianopolis high and onshore lineaments suggesting that they all comprise the same tectonic trend the rio grande fracture zone the morphology developed along this fracture zone during the early opening of the south atlantic ocean formed a barrier to open ocean circulation during the aptian and allowed the formation of extensive evaporite deposits to the north of it

identification of surface wind patterns over the ross ice shelf antarctica using self organizing maps abstractthe interaction of synoptic and mesoscale circulations with the steep topography surrounding the ross ice shelf antarctica greatly influences the wind patterns in the region of the ross ice shelf the topography provides forcing for features such as katabatic winds barrier winds and barrier wind corner jets the combination of topographic forcing and synoptic and mesoscale forcing from cyclones that traverse the ross ice shelf sector create a region of strong but varying winds this paper identifies the dominant surface wind patterns over the ross ice shelf using output from the weather research and forecasting model run within the antarctic mesoscale prediction system and the method of self organizing maps som the dataset has 15 km grid spacing and is the first study to identify the dominant surface wind patterns using data at this resolution the analysis shows that the ross ice shelf airstream a dominant stream of air flowing northward from the interior of the continent over the western

on the impact of forced roll convection on vertical turbulent transport in cold air outbreaks we investigated the impact of roll convection on the convective boundary layer and vertical transports in different cold air outbreak cao scenarios using large eddy simulations les the organization of convection into rolls was triggered by upstream heterogeneities in the surface temperature representing ice and water by changing the sea ice distribution in our les we were able to simulate a roll and a nonroll case for each scenario furthermore the roll wavelength was varied by changing the scale of the heterogeneity the characteristics of the simulated rolls and cloud streets such as aspect ratios orientation of the roll axes and downstream extensions of single rolls agreed closely with observations in cao situations the vertical turbulent fluxes calculated for each simulation were decomposed into contributions from rolls and from unorganized turbulence even though our results confirmed that rolls triggered by upstream heterogeneities can substantially contribute to vertical turbulent fluxes the total fluxes were not affected by the rolls

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

reply to comment by cinthia labails and walter r roest on breakup of pangaea and plate kinematics of the central atlantic and atlas regions summary in their comment labails and roest have raised several points against the kinematic model of schettino and turco for the central atlantic region which requires the existence of an independent moroccan plate during the oligocene and the early miocene we thank these authors for giving us the opportunity to clarify some controversial points but disagree with most of their objections and show that they are based either on a different interpretation of existing data or a distortion of the facts presented by schettino and turco

cyclonically forced barrier winds along the transantarctic mountains near ross island abstract the effect of the transantarctic mountains on cyclonically forced boundary layer winds in the vicinity of ross island antarctica 77 5 s 167 e is discussed when cyclones are present over the western ross ice shelf and ross sea the low level easterly airflow is toward the mountains a barrier wind regime is set up as the flow is turned northward and becomes parallel to the mountain range it is found that cyclonically forced barrier winds occurred around 5 and 8 of the time during 1984 and 1985 respectively the case histories of two well defined barrier wind events lasting for 24 h are discussed in detail with regional analyses based on satellite photographs and automatic weather station data one case is for a katabatic wind forced mesoscale cyclone forming to the north of ross island and the other is for a synoptic scale cyclone moving through the western ross ice shelf ross sea region a numerical model for the vertically integrated boundary layer flow that calculates two horizontal

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

roll and cell convection in wintertime arctic cold air outbreaks abstract cold air outbreaks from the polar ice caps or winterly continents over the open ocean lead to organized convection that typically starts as longitudinal roll patterns and changes to cellular patterns in downstream direction during the field experiments arktis 1991 and arktis 1993 aircraft missions were conducted in 13 cold air outbreak events over the greenland and barents seas to determine the characteristic parameters of both the mean primary flow and the superimposed organized convection secondary flow the measurements are classified into four categories with respect to the convective pattern form longitudinal rolls with small and wider horizontal wavelengths transitional forms between rolls and cells and cells rolls were observed for boundary layer depths h 1 4 km with 8 km and h between 4 and 12 the amplitudes of the secondary flow scale varia

temporal and spatial variability of seafloor spreading processes in the northern south atlantic the paper reports the results of an aeromagnetic study of the mid atlantic ridge in the northern south atlantic the study consists of 38 000 km of low altitude magnetics tracks centered on the ridge covering nearly 700 000 km2 south of ascension island most tracks extend out to crust of anomaly 5b age large areal coverage coupled with high resolution allows comparison of crustal generation processes over space along ridge strike and time across strike temporal heterogeneities revealed in the data include a pattern of change in spreading rate and direction of plate motion the total spreading rate increased sharply at 10 ma and then decreased between 6 ma and the present the time variation of spreading rate is similar to that seen in other areas such as the north atlantic and the southern and eastern pacific the plate boundary has evolved over time in response to the shift in plate motion direction through fragmentation and rotation of ridge segments between fracture zones in several cases the location of the spreading axis has shifted through jumps or propagation across fracture zones the periods of plate boundary modification correlate with the increase and decrease in spreading rate spatial heterogeneities are caused by the influence of the ascension hotspot on the mid atlantic ridge in the northern portion of the study area and include two apparent seamount chains formed by the hotspot ascension island and several of the seamounts appear to have been produced some distance away from the hotspot location presently about 225 km southeast of ascension island by channeled subaxial magma flow beneath the spreading center the seamount chains therefore do not mark hotspot tracks large local variations in spreading rate and degree of asymmetry along strike are also found

high resolution inversion for south atlantic plate kinematics using joint altimeter and magnetic anomaly data we present an inversion for plate kinematics that solves for finite rotation parameters using fracture zone fz and magnetic anomaly location data jointly we define misfit functions that incorporate properties unique to each data type in particular the fz misfit function does not depend upon alignment of conjugate fz traces in the same way as magnetic lineations under the finite rotations this property is useful for fz locations in which the signals on conjugate sides of the ridge may include systematic differences or where data from one side of the ridge are sparse or missing formal error bounds estimated for the pole parameters show that the magnetic and fz data are complementary in their information content error bounds computed for the joint inversion are substantially smaller than for either the fz or magnetics data used separately indicating that simultaneous use of the data in an inversion is crucial we apply this method to seasat altimeter data and magnetic anomaly picks in the south atlantic we solve for finite poles corresponding to magnetic anomalies 5 6 8 13 21 22 25 30 33 33r and 34 these define a smooth path over the past 84 m y indicating that the plate motions have not been as erratic as found previously

morphology and tectonics of the mid atlantic ridge 25 27 30 s from sea beam and magnetic data high resolution sea beam bathymetry and sea surface magnetic data have been collected between 25 s and 27 30 s on the mid atlantic ridge analysis of the morphology and structure of the ridge axis and flanking terrain as well as the results of a three dimensional inversion of sea surface magnetic data are presented this 227 km long portion of the mid atlantic ridge is offset 42 km by the right lateral rio grande transform at 25 40 s and 9 5 km by the left lateral nontransform moore discontinuity at 26 30 s a third nontransform discontinuity the midway discontinuity exists near 26 12 s and is defined by a small displacement of the axial magnetization distribution and the disruption in the along strike continuity of ridge parallel terrain elements although the detailed survey presented in this paper extends out to only 5 to 7 m y old crust a regional compilation of magnetic data from this area by cande et al 1988 indicates that the relative positions and dimensions of the spreading cells have characterized this part of the mid atlantic ridge for at least the past 55 m y the offsets dimensions and geometries however have changed markedly because of prolonged differential asymmetric spreading between segments between and within the accretionary units that are spreading at uniform rates 35 mm yr total opening rate the morphology of the ridge axis varies dramatically from a well defined rift valley 20 25 km wide 1 1 5 km deep to an axial swell that rises 1500 m above the adjacent valley floor the axial magnetic structure is also highly variable along strike the inversion solution shows a series of generally linear short wavelength 10 km magnetization highs within the central brunhes anomaly over each of the four segments in the survey area these anomalies are interpreted to be the central anomaly magnetization high that is observed over the neovolcanic zones of many of the world s mid ocean spreading centers the distinct axial magnetization highs can extend out to 2 m y old crust at ridge discontinuity intersections suggesting that in some instances they may also be the product temporal variations in the geochemistry of the extrusive rocks e g highly fractionated and strongly magnetized basalts enriched in iron and titanium the magnetic inversion shows that the central anomaly is locally attenuated at the minimum depth points of three of the four spreading segments in the survey area both inversion and forward modeling of magnetic data show that topography alone cannot produce the anomalously low values observed in the case of an axial swell centered at 26 s attenuation of the central anomaly is attributed to the effective thinning of the source layer by demagnetization effects of a crustal thermal anomaly and or a zone of substantial alteration taken together the bathymetric data and magnetic modeling document strong along axis gradients in crustal and upper mantle properties

thermomechanical evolution of oceanic fracture zones a fracture zone fz model is constructed from existing models of the thermal and mechanical evolution of the oceanic lithosphere as the lithosphere cools by conduction thermal and mechanical boundary layers develop and increase in thickness as age 1 2 surface expressions of this development such as topography and deflection of the vertical i e gravity field are most apparent along major fz s because of the sharp age contrast a simple model including the effects of lateral heat transport but with no elastic layer predicts that variations in seafloor depth and deflection of the vertical will become increasingly smooth and ultimately disappear as the fz ages observations however show that both the fz topography and the deflection of the vertical remain sharp as the fz evolves these two observations as well as the observed asymmetry in deflection of the vertical profiles across the udintsev romanche and mendocino fz s are explained by including a continuous elastic layer in the model the asymmetry in deflection of the vertical is a consequence of elastic thickness variations across the fz modeling also shows that the evolution of the fz topography is extremely sensitive to the initial thermal structure near the ridge transform intersection model geoid steps and their development with age are used to access techniques for measuring geoid offsets across fz s reasonable step estimation techniques will underestimate the overall step amplitude by up to 50 this implies that abnormally thin thermal boundary layers derived from studies of geoid height versus age are not required by the data

cretaceous palaeopositions of the falkland plateau relative to southern africa using mesozoic seafloor spreading anomalies summary mesozoic seafloor spreading anomalies mo m10 have been identified in the natal valley between the south eastern african margin and the mozambique ridge these identifications confirm that seafloor spreading north and south of the falkland agulhas fracture zone fafz began simultaneously the natal valley anomalies are offset 1300km by the fafz from their equivalents in the southern cape basin their positions confirm that the tugela ridge marks the continent ocean boundary cob in the natal valley we have computed successive falkland plateau palaeopositions using natal valley and georgia basin anomalies these show that the offset in spreading ridges at the fafz remained 1300km long from m10 to mo time by anomaly 34 time the offset was 1270 km therefore no major ridge jumps had occurred by then dating mo as 108 myr bp and anomaly 34 as 80 myr bp the average halfspreading rate immediately south of the fafz for the cretaceous normal polarity epoch is 4 2 cm yr using this we date a the change in early pole of rotation at 105 myr b a reconstruction which juxtaposes salt boundaries in the brazil and angolan basins at 103 7 myr c final separation of the falkland plateau from southern africa at 98 3 myr d the formation of the oceanic northern part of the agulhas plateau at 97 3 90 7 myr comparison of magnetic data with implied cob positions in the southernmost cape and argentine basins and the georgia basin suggests continental separation began 122 1 27 myr bp with undeformed magnetic anomalies dating from 122 myr m10 these dates are consistent with micropalaeontological and sedimentological data around southern africa

an extreme cold air outbreak over the labrador sea roll vortices and air sea interaction abstract observational data from two research aircraft flights are presented the flights were planned to investigate the air sea interaction during an extreme cold air outbreak associated with the passage of a synoptic scale low pressure system over the labrador sea during 8 february 1997 this is the first such aircraft based investigation in this remote region both high level dropsonde and low level flight level data were collected the objectives were twofold to map out the structure of the roll vortices that cause the ubiquitous cloud streets seen in satellite imagery and to estimate the sensible and latent heat fluxes between the ocean and atmosphere during the event the latter was achieved by a lagrangian analysis of the flight level data the flights were part of the labrador sea deep convection experiment investigating deep oceanic convection and were planned to overpass a research vessel in the area the aircraft observed roll vortices had a characteristic wavelength of 4 5 km particular

thermal stresses in the oceanic lithosphere evidence from geoid anomalies at fracture zones models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate however there has so far been little direct evidence for the magnitude and distribution of thermal stresses we present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones specifically our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones including the effect of varying thermal bending moments thin plate theory predicts vertical nonisostatic displacements of the lithosphere by plate flexure the predicted amplitude of the resulting geoid anomaly is large enough to be observed in seasat altimeter profiles furthermore the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable the anomaly due to thermal stresses has been clearly identified in geoid profiles across the clarion and the udintsev fracture zones the amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700 c consistent with the maximum depth of seismicity in the oceanic lithosphere the distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes

an investigation of small offset fracture zone geoid waveforms approximately 1 500 geoid cross sections across 15 small offset south atlantic fracture zones fz s are compiled from seasat altimeter data and organized according to crustal age these profiles provide a basis for the comparison of the different fz s and the evolution of each over geological time i use an empirical orthogonal function decompostion to investigate the dependence of profile shape and amplitude upon crustal age the geoid cross sections are found to be coherent in form down the length of each fz and possess amplitudes that are inversely related to the relative spreading rate at the time of formation this observation is consistent with a simple model in which the active portion of a fracture zone the transform fault remains a fixed spatial length yielding a variable age offset across the fz

geophysical applications of satellite altimetry publications related to geophysical applications of seasat and geosat altimetry are reviewed for the period 1987 1990 problems discussed include geoid and gravity errors regional geoid heights and gravity anomalies local gravity field flexure plate tectonics and gridded geoid heights gravity anomalies 99 refs

investigations of relative plate motions in the south atlantic using seasat altimeter data fracture zones in the world s oceans appear clearly in the seasat altimeter geoid data at wavelengths of about 100 km and less in the north and south atlantic these signatures are stable in form and amplitude from the ridge crest to crust tens of millions of years old and can be mapped across nearly the width of the basin the signatures diminish in amplitude markedly on crust older than 70 to 80 m y b p apparently when thickly covered by sediments because fracture zone trajectories closely approximate plate motion flow lines they may be used to infer the plate kinematic history of the ocean basin in this paper i describe a new method for estimating the relative poles of opening describing the history of motion between two plates by using fracture zone trajectories the method is an iterative inversion whereby the trajectories of synthetic flow lines are linearized with respect to the pole parameters when applied to the seasat altimeter track data a waveform inversion based upon this method is possible in which the altimeter data are modeled using a synthetic geoid fracture zone signal this waveform inversion is applied to 17 prominent fracture zones in the central and south atlantic to estimate the poles of opening between the south american and african plates altogether 1022 seasat fracture zone crossings are used for each of these a waveform comparison is made the new method is found to be robust and converges to a stable set of pole solutions even if the initial flow lines are small circles about a single pole that poorly approximates the observed trajectories the solution poles document a cenozoic change in plate motion near anomaly 13 time 36 m y b p similar to that in the north atlantic at anomaly 1 3 21 time 36 50 m y b p

seasat altimetry and the south atlantic geoid 2 short wavelength undulations this paper is a discussion of the short wavelength 25 100 km undulations of the geoid recovered from seasat altimeter data in the south atlantic these undulations are known to be highly correlated to seafloor topography and they considerably improve our knowledge about global bathymetry of the south atlantic which remains poorly surveyed the orientation of fracture zones fz with respect to the seasat profiles ensures an efficient recovery of the fz pattern in the younger 65 ma parts of the south atlantic between 20 n and 50 s we used 22 well marked fzs regularly distributed between 12 n and 47 s to compute eulerian poles describing the finite relative movement between africa and south america from the present back to 35 ma fz trends south of the equator are well accounted for with a single rotation and standard statistical tests do not indicate a need for distinct and successive rotations however these tests indicate an improvement of the fit when distinct poles are adjusted to separate subsets of our 22 fzs either fzs north and south of the equator or fzs north and south of tristan da cunha this points out the possibility of the existence of either corridors in which slight relative movements occur or diffuse elastic deformation over the entire south atlantic although this is really a second order effect we also present and discuss a map of the roughness of the geoid for the 25 to 110 km wave band strong disagreements between the roughness map and the gebco maps exist in regions of sparse ship track coverage for example the rio grande rise has an associated roughness feature which appears elongated and more symmetric with respect to the walvis ridge than what is displayed on the gebco maps the roughness map also clearly documents a long wavelength 400 km segmentation of the mid atlantic ridge between the equator and 35 s

the role of katabatic winds on the antarctic surface wind regime abstract antarctica is known for its strong and persistent surface winds that are directed along topographic pathways surface winds are especially strong during the winter period the high directional constancy of the wind and the close relationship of the wind direction to the underlying terrain can be interpreted as evidence of katabatic wind activity observations show that the directional constancy of the antarctic surface wind displays little seasonal variation summertime winds cannot be expected to contain a significant katabatic component owing to enhanced solar heating of the ice slopes observations also show that the coastal environs are subjected to wide variation in atmospheric pressure associated with frequent cyclone activity the robust unidirectional nature of the antarctic surface wind throughout the year implies that significant topographic influences other than those from katabatic forcing must be acting idealized numerical simulations have been performed to illustrate the potential

the impact of polar mesoscale storms on northeast atlantic ocean circulation every year thousands of mesoscale storms termed polar lows cross the climatically sensitive subpolar north atlantic ocean high resolution numerical simulations of the ocean circulation taking into account the effect of these storms on deep water formation suggest that polar lows significantly affect the global ocean circulation

the fifteen twenty fracture zone and the north american south american plate boundary the fifteen twenty fracture zone offsets the mid altantic ridge over 195 km in a sinistral sense it was surveyed in 1982 between 30 w and 60 i e from anomaly 34 on the african plate to anomaly 34 on the american plate where it disappears as the barracuda ridge into the caribbean subduction zone the fossil limbs of the fifteen twenty fracture zone show a remarkable symmetry to both sides of the mid atlantic ridge barracuda ridge appears to have undergone a later deformation the sections of the fifteen twenty fz between 49 w and 54 w to the west of the mid atlantic ridge and between 37 5 w and 42 5 w to the east do not exhibit the normal characteristics of a fracture zone but have elongate and extremely narrow transverse ridges dykes along the fracture zone axis the present transform domain shows signs of leaking this is interpreted as a result of a fairly recent about 7 ma bp change in spreading direction of the area this change in spreading direction seems to have affected only the area south of 17 50 n it may have resulted from a northward migration of the plate boundary between the north american and south american plates and the coming into existence of a triple point near 16 n from this reconstruction the location of the differential pole between the north american and south american plates is 16 n 53 5 w this location leads to increasing compression from 53 5 to 60 w affecting the barracuda ridge and trough with a n s shortening of about 16 km in 7 ma from 53 5 to 45 w n s extension occurred about 19 km near the mid atlantic ridge which led to the origin of the volcano tectonic complex of researcher ridge researcher trough and royal trough to the west of the mid atlantic ridge

centroid depths of mid ocean ridge earthquakes dependence on spreading rate from inversions of teleseismic p and sh waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962 1983 on slowly spreading mid ocean ridges all events are characterized by predominantly normal faulting on planes that dip at approximately 45 and strike parallel to the local trend of the ridge axis centroid depths range from 1 to 6 km beneath the seafloor the p waves from these earthquakes show strong water column reverberations suggesting that fault rupture extended to the seafloor under the assumption that the centroid depth marks the mean depth of fault slip earthquake faulting extended to a depth of 2 10 km for these earthquakes the water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley the maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate and the maximum seismic moment may also decrease with increasing spreading rate these results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region the concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley

marine gravity of the southern ocean and antarctic margin from geosat in november of 1986 the u s navy satellite geosat began collecting unclassified gravity altimeter data as part of its exact repeat mission erm for national security reasons the geosat orbit was arranged so that it closely follows the seasat satellite altimeter ground track however there are two advantages of the geosat data over the seasat data first because of improvements in altimeter design geosat profiles are about 3 times more precise than seasat profiles this corresponds to an accuracy of 2 3 rad i e 2 3 mgal for wavelengths greater than 20 km second the geosat altimeter data were collected when the antarctic ice coverage was minimal february 1987 to march 1987 while seasat was only active during an antarctic winter june 1978 to september 1978 these new data reveal many previously uncharted seamounts and fracture zones in the extreme southern ocean areas adjacent to antarctica seven large age offset fracture zones apparent in the geosat data record the early breakup of gondwana finally the new data reveal the detailed gravity signatures of the passive and active continental margins of antarctica these data are an important reconnaissance tool for future studies of these remote ocean areas

seafloor spreading in the weddell sea and southwest atlantic since the late cretaceous geosat radar altimetry defines a pattern of closely spaced curvilinear gravity anomalies in the northern weddell sea and adjacent parts of the southwest atlantic these anomalies are caused by a fracture zone pattern resulting from seafloor spreading mainly on the southern flank of a ridge system which now exists only to the east of the south sandwich trench flowlines derived from these anomalies are combined with isochrons resulting from an interpretation of all available magnetic anomaly profiles in the region to produce a new tectonic summary chart this chart reveals major changes in spreading direction during i the cretaceous magnetic superchron ii the latest cretaceous paleocene 65 ma and iii the mid eocene 50 ma the latter two changes enclose a period of very slow spreading and define a kink in the flowlines which is discernable throughout most of the weddell sea preliminary modelling indicates that trends from the western weddell sea to near the south atlantic triple junction can be explained by the separation of just two plates there is some disagreement between the observed flowlines and those predicted by rotations published for the south atlantic and southwest indian ridge but abrupt changes in spreading direction and a slowing of rates are observed on all three branches of the south america africa antarctica plate system between the times of anomalies c31 and c21 this suggests that the lack of closure results from errors in the calculated rotations and that the seafloor of the weddell sea and southwest atlantic was created as a direct consequence of south america antarctica plate motion the data do not support the existence of a late cretaceous malvinas plate within the weddell sea although microplates in the northernmost weddell sea do appear to have formed briefly as a result of interactions between the former spreading ridge and a subduction zone located at the south scotia ridge

geoid roughness and long wavelength segmentation of the south atlantic spreading ridge the configuration of accreting plate boundaries tends to reflect the geometry of the original continental break up yet there are also many indications that the detailed pattern of spreading ridge and transform fault segments may be an inherent part of the spreading process itself1 2 the mid atlantic ridge for example is segmented with a typical spacing of 40 km between transform faults3 4 here we use the short wavelength 25 110 km part of the seasat altimetry data set to derive a map of the geoid rough ness in the south atlantic between 1 n and 39 s this map reflects changes in the local amplitude envelope of the geoid signal primarily associated with topography zones of high roughness correlate with major fracture zones aseismic ridges and the mid atlantic ridge the most remarkable feature is that the roughness is not uniform along the ridge but displays quasi periodic variations with a wavelength of 400 km a segmentation an order of magnitude larger than described so far which has persisted for at least 60 myr this segmentation may reflect either fabric generated at the ridge by normal plate tectonic processes1 5 or small scale convective instabilities rising from 80 km deep region of the upper mantle3 6 7

breakup of pangaea and plate kinematics of the central atlantic and atlas regions summary a new central pangaea fit type a is proposed for the late ladinian 230 ma together with a plate motions model for the subsequent phases of rifting continental breakup and initial spreading in the central atlantic this model is based on 1 a reinterpretation of the process of formation of the east coast magnetic anomaly along the eastern margin of north america and the corresponding magnetic anomalies at the conjugate margins of northwest africa and the moroccan meseta 2 an analysis of major rifting events in the central atlantic atlas and central mediterranean and 3 a crustal balancing of the stretched margins of north america moroccan meseta and northwest africa the process of fragmentation of central pangaea can be described by three major phases spanning the time interval from the late ladinian 230 ma to the tithonian 147 7 ma during the first phase from the late ladinian 230 ma to the latest rhaetian 200 ma rifting proceeded along the eastern margin of north america the northwest african margin and the high saharan and tunisian atlas determining the formation of a separate moroccan microplate at the interface between gondwana and laurasia during the second phase from the latest rhaetian 200 ma to the late pliensbachian 185 ma oceanic crust started forming between the east coast and blake spur magnetic anomalies whereas the morrocan meseta simply continued to rift away from north america during this time interval the atlas rift reached its maximum extent finally the third phase encompassing the time interval from the late pliensbachian 185 ma to chron m21 147 7 ma was triggered by the northward jump of the main plate boundary connecting the central atlantic with the tethys area therefore as soon as rifting in the atlas zone ceased plate motion started along complex fault systems between morocco and iberia whereas a rift drift transition occurred in the northern segment of the central atlantic between morocco and the conjugate margin of nova scotia the inversion of the atlas rift and the subsequent formation of the atlas mountain belt occurred during the oligocene early miocene time interval in the central atlantic this event was associated with higher spreading rates of the ridge segments north of the atlantis fz an estimate of 170 km of dextral offset of morocco relative to northwest africa in the central atlantic is required by an analysis of marine magnetic anomalies five plate tectonic reconstructions and a computer animation are proposed to illustrate the late triassic and jurassic process of breakup of pangaea in the central atlantic and atlas regions

characteristics of the ross ice shelf air stream as depicted in antarctic mesoscale prediction system simulations 1 streamlines of the mean annual near surface winds over the antarctic continent suggest a confluent channeling of the drainage flows off the ice sheets and onto the ross ice shelf a persistent cyclonic circulation to the north of the ice shelf supports a large scale pressure field that reinforces the continental drainage flows owing to these two processes an enhanced low level airflow is present along the southern and western sections of the ross ice shelf the resulting air stream known as the ross ice shelf air stream ras is one of the persistent and prominent low level wind features seen in the antarctic real time mesoscale simulations of the antarctic atmosphere and high southern latitudes using a modified version of the pennsylvania state university national center for atmospheric research mesoscale modeling system have been ongoing since the 1999 2000 austral field season model results from the 1 year period november 2001 to october 2002 have been analyzed to investigate the mean structure and modulation of the ross ice shelf air stream analyses of model results show the low level air stream over the western ross ice shelf has a wind speed maxima that is linked to the steep topography to the west individual cases of strong wind events appear to contain a significant barrier wind component that arises from cold air damming against the transantarctic mountains cyclones that frequently form in the ross sea are shown to establish conditions that promote barrier wind dynamics and thus significantly modulate the intensity of the ras

diagnosis of the katabatic wind influence on the wintertime antarctic surface wind field from numerical simulations abstract katabatic winds have long been recognized as one of the key climatic variables of the low level antarctic environment antarctic surface winds display a high degree of persistence with mean directions related to the local topographic configuration of the ice sheet consistent with katabatic forcing continental orography also constrains the atmospheric boundary layer motions through blocking and cold air damming finally the coastal rim about the antarctic continent is among the most active baroclinic zones on earth the establishment of the low level wind field over antarctica is thus potentially the result of a number of interacting processes to quantify the forcing of the wintertime surface wind field over the antarctic continent two numerical strategies are presented first idealized numerical simulations are conducted to illustrate the strong orographic control of the low level wind field second a series of daily numerical simulations using the fifth generation pennsylvania state unive

observations of the ocean response to cold air outbreaks and polar lows over the nordic seas 1 the impact of intense atmospheric mesocyclones polar lows on the ocean circulation at high latitudes as well as the role of ocean feedbacks on the evolution of these atmospheric systems themselves is under debate here the upper ocean response to atmospheric forcing before during and after polar lows events over the nordic seas is studied a set of 96 unique polar low tracks from 2002 to 2010 are collocated with satellite based sea surface temperature and altimeter observations and with surface drifter observations the satellite data show systematic temperature and sea level drops and enhanced geostrophic kinetic energies over the days leading up to polar low events these data however reveal little information about the ocean response to the polar lows themselves the drifter observations largely agree with the satellite data on the response to synoptic conditions but they also give indication of enhanced upper ocean kinetic energies immediately following the passage of polar lows

fracture zones in the north atlantic from combined geosat and seasat data we present a new tectonic fabric map of the northern and central north atlantic ocean floor from 10 s to 66 n based on combined geosat and seasat altimetry data short wavelength vertical gravity along track was derived by hilbert transforming and high pass filtering geosat and seasat deflection of the vertical profiles the along track gravity data were used in conjunction with magnetic anomaly data to locate a large number of fracture zones as well as extinct spreading ridges and v shaped structures we traced many fracture zones more continuously and closer to adjacent continental margins than was previously possible we also mapped a number of unnamed fracture zones and fracture zone segments many of which were previously uncharted we have evaluated the agreement between the mapped fracture zones and synthetic flow lines as predicted from recent plate models for the opening of the north central north and south atlantic most fracture zones in this area correlate quite well with synthetic flow lines supporting the proposition that geosat seasat altimetry data serve as a useful tool to map fracture zones where insufficient bathymetric data are available many fracture zones including some small offset fracture zones in the central north atlantic have been stable and follow tectonic flow lines for tens of millions of years however many other fracture zones do not reflect a railroad track pattern of plate motions the geometry of many small and medium offset fracture zones indicates that the ridge axis discontinuities that produce them can migrate along the ridge causing off axis traces that deviate from tectonic flow lines velocity space diagrams constructed using a recent relative and absolute present day plate motion model indicate that some low angle v shaped troughs and ridges are likely formed by migration of the plate boundary relative to a mesospheric reference frame other low angle v shaped structures reflect slow migrations oscillations of small offset ridge axis discontinuities that are distinct from both tectonic flow lines and v shapes reflecting absolute plate motions high angle v shaped structures are the result of fast migrating ridge axis discontinuities propagating rifts

a numerical study of an extreme cold air outbreak over the labrador sea sea ice air sea interaction and development of polar lows in this paper the ability of the mm5 mesoscale forecast model to simulate the air sea interaction boundary layer development and mesoscale structure associated with a cold air outbreak over the labrador sea is investigated the case chosen was one for which research aircraft data and satellite imagery are available for validation the default surface layer parameterization included in the model is shown to grossly overestimate the magnitude of the air sea interaction resulting in forecasts of boundary layer growth and mesoscale development that differ substantially from observations it is also shown that a representation of the inhomogeneities in sea ice cover results in a significant improvement in simulations of the air sea interaction boundary layer development and mesoscale structure both within the marginal ice zone and downstream over the open ocean finally the mesoscale cyclones or polar lows observed in the wake of the cold air outbreak are shown to be coupled to the evolution of an upper tropospheric potential vorticity anomaly that was advected over the region the model simulations suggest that mm5 most probably due to inaccuracies in and the limited resolution of the analyzed fields that supply the initial and boundary conditions to the model was unable to correctly simulate the development and track of this anomaly and this ultimately led to an incorrect forecast of the polar lows finite amplitude behavior

a stochastic model for the creation of abyssal hill topography at a slow spreading center the present study is an attempt to model abyssal hill topography i e the randomly varying topography that is superimposed on the subsidence of the ocean floor due to thermal contraction we assume that abyssal hill topography is created by faulting and that it can be represented as the convolution of a series of stochastic fault offsets with a fault impulse response as a constraint the power spectrum of the model generated topography will have to resemble the power spectrum of a profile of abyssal hill topography across a typical slow spreading ridge such as found in the south atlantic using a series of fault offsets positively correlated in space and fault impulse responses limited to a finite width w profiles that statistically resemble abyssal hill topography have been generated if the fault offsets are constrained to be positive i e to cause uplift the synthetic profiles contain a steady state depression at the axis the fit of the synthetically derived topography to the observed topography is very sensitive to the width w of the fault impulse response our results suggest that the effects of a faulting event must be limited to distances of about 3 5 km and that tectonic activity must take place in the immediate vicinity of the inner wall of the axial valley if faulting is indeed the process that creates abyssal hill topography the best fit flexural impulse response to faulting implies a flexural rigidity of 5 1017 n m if instead constructional volcanism contributes significantly to the topography our study suggests that the emplacement of volcanic constructions must also take place within a narrow width w about the axis

satellite observations of katabatic wind propagation for great distances across the ross ice shelf abstract five winter months april august 1988 of thermal infrared satellite images were examined to investigate the occurrence of dark warm signatures across the ross ice shelf in the antarctic continent these features are inferred to be generated by katabatic winds that descend from southern marie byrd land and then blow horizontally across the ice shelf significant mass is added to this airstream by katabatic winds blowing from the major glaciers that flow through the transantarctic mountains from east antarctica these negatively buoyant katabatic winds can reach the northwestern edge of the shelf a horizontal propagation distance of up to 1000 km 14 of the time where the airstream crosses from the ice shelf to the ice covered ross sea a prominent coastal polynya is formed because the downslope buoyancy force is near zero over the ross ice shelf the northwestward propagation of this katabatic air mass requires pressure gradient support the study shows that the extended horizontal propagat

a comprehensive mass flux scheme for cumulus parameterization in large scale models abstract observational studies indicate that a mass flux approach may provide a realistic framework for cumulus parameterization in large scale models but this approach through the introduction of a spectral cloud ensemble leads normally to rather complex schemes in this paper the question is addressed whether much simpler schemes can already provide realistic values of the thermal forcing by convection under various synoptic conditions this is done through verifying such a scheme first on data from field experiments for periods of tropical penetrative convection gate marshall islands tradewind cumuli atex bomex and extratropical organized convection sesame 79 and then in a nwp model the scheme considers a population of clouds where the cloud ensemble is described by a one dimensional bulk model as earlier applied by yanai et al in a diagnostic study of tropical convection cumulus scale downdrafts are included various types of convection are represented i e penetrative convection in c

tectonic evolution of the antarctic phoenix plate system since 15 ma abstract joint inversion of magnetic isochron and fracture zone data from the extinct antarctic phoenix spreading system in sw drake passage yields seven new finite reconstruction poles the inversion results are very well constrained for such a short length of plate boundary although this is partly because the finite poles are located close to the reconstructed region the optimum use of fracture zone identifications from satellite derived free air gravity data is also important as the stability of stage poles throughout the short intervals in the model affirms the model results describe a well organised spreading system since magnetic anomaly chron c5ad 15 ma in which the phoenix plate rotated about stage poles nearby to the southwest stage pole locations are broadly consistent with a hypothesis of pivoting subduction as the driving force of phoenix plate movement and there is some evidence in the progression of stage poles for late stage movement of the subduction pivot in response to the changing azimuth of the subduction zone at which the phoenix plate was being consumed the model kinematics alone provide no unequivocal support for previous interpretations of disruption of the subducted part of the phoenix plate the very latest stages of spreading saw falling spreading rates between magnetic anomaly chrons c4 8 1 ma and c2a 3 3 ma when the antarctic phoenix ridge became extinct this is consistent with an increase in shear stress across the plate bounding shackleton fracture zone due to a plate reorganisation in the neighbouring scotia sea following the cessation of spreading on the west scotia ridge

pole of rotation analysis of present day juan de fuca plate motion convergence rates between the juan de fuca and north america plates are calculated by means of their relative present day pole of rotation subduction rates as measured normal to the strike of the washington oregon trench are predicted to range from about 40 mm yr under washington to 20 mm yr under oregon an integral part of this analysis is the formulation of a numerical inversion scheme which calculates both the pole of rotation between two plates and the associated errors this method is used to determine the euler pole for pacific juan de fuca this pole can then be vectorially added to previously published poles for north america pacific and hot spot pacific to obtain north america juan de fuca and hot spot juan de fuca respectively the errors associated with these resultant poles are also determined by propagating the errors of the two summed angular velocity vectors with the assumption that hot spots are fixed with respect to a mantle reference frame if is computed that the average absolute velocity of the juan de fuca plate is approximately 15 mm yr thereby making it the slowest moving of the oceanic plates

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