strain gradients in zones of ductile thrusting insights from the external hellenides abstract new finite strain data from a thrust sense shear zone which extends for 600 km along strike in the high pressure belt of the external hellenides greece were used to describe the variation of ductile strain with structural distance d from the basal thrust sampling was carried out along four traverses across the central and southeastern parts of the zone the strain ratio in xz sections r xz of 120 samples was obtained using the theta curve the mean object ellipse and the mean radial length methods all methods give very consistent results based on these results four strain profiles were constructed depicting a non linear increase of r xz values with proximity to the basal thrust these profiles in combination with published strain data from the northwestern parts of the zone enabled us to obtain an empirical logarithmic function describing the relationship between r xz and d this function includes only one varying parameter which represents the strain gradient in linear r xz ln d space the value of strain gradient is directly correlated with the strain level in the thrust zone and provides information for strain localization at its base the proposed function may also be used for quantifying the strain profiles of ductile to semi ductile thrust zones in other orogenic belts such as the alps the appalachians and himalaya

ionospheric signatures of dayside magnetopause transients a case study using satellite and ground measurements a case study is presented of coordinated ground and space measurements featuring a set of transient auroral fragments located on the poleward side of a stable cusp cleft arc optical ground data from ny alesund svalbard and heiss island franz josef land were combined with dmsp f9 satellite measurements to examine the characteristics of these auroral features a stable red arc stretched across most of the dayside auroral zone in a region dominated by westward convection in accordance with the orientation of the imf poleward of the red arc were several westward moving auroral jets having characteristics similar to midday auroral breakup events such events may be ground signatures of transitory magnetic merging at the dayside magnetopause if so the driven convective motion of these structures should contribute to the polar cap potential within this limited data set we find that although the transitory structures have an inherent potential associated with the motion of the optical signatures the structures on the whole contribute a small fraction of the total polar cap potential

interannual and decadal variability of the subsurface thermal structure in the pacific ocean 1961 90 heat content anomalies are analyzed to understand subsurface variability on both aparticular focus on the evolving basinwide patterns and oceanic connections between the extratropics and tropics various analyses indicate two distinct modes one interannual and the other decadal that involve the tropics and the north pacific subtropical gyre respectively interannual variability is associated with el nino in the tropics with a prominent see saw pattern alternately on and off the equator and in the east and west respectively the interannual cycle features a coherent propagation of subsurface signals around the tropical pacific eastward along the equator but westward off the equator at 10 15 n decadal signals are dominant in the subtropics and midlatitudes but also have a tropical component that appears to be independent of interannual variations an oceanic connection can be seen between subsurface anomalies in the midlatitudes in the subtropics and tropics on decadal time scales subsurface thermal anomalies associated with midlatitude decadal variability can propagate through the subtropics into the tropics which may modulate the intensity of interannual variability in the tropics for example in the middle and late 1970s a significant warm temperature anomaly appeared to penetrate into the western and central tropics at depth warming the tropical upper ocean and depressing the thermocline during the development of el nino therefore an extratropically preconditioned subsurface state e g an enhanced positive heat content anomaly in the western and central tropical pacific would favor a warmer sea surface temperature anomaly in the eastern equatorial pacific potentially increasing the intensity of ocean atmosphere coupling these changes in the thermocline structure and possibly in the coupling strength can further alter the very character of tropical air sea interactions this may help to explain decadal variability of el nino evolution in the tropical pacific as observed in the 1980s our subsurface variability analysis presents observational evidence for the detailed space time structure of decadal oceanic links between the extratropics and the tropics

how different wind stress patterns affect the tropical subtropical circulations of the upper ocean abstract an oceanic gcm is used to investigate the response of the tropical and subtropical thermocline circulation and structure to different wind stress patterns although the subtropical winds do not affect the transport or the speed of the equatorial undercurrent significantly they do change the equatorial temperature field in the lower part of the equatorial thermocline significantly a weaker subtropical wind curl causes a cooling of the subsurface equatorial region and hence an intensification of the equatorial thermocline a weakening of the subtropical wind curl by a factor of 2 cools the equatorial lower thermocline water by 2 c

interdecadal sea surface temperature variability in the equatorial pacific ocean part ii the role of equatorial off equatorial wind stresses in a hybrid coupled model abstract many modeling studies have been carried out to investigate the role of oceanic rossby waves linking the off equatorial and equatorial pacific ocean although the equatorial ocean response to off equatorial wind stress forcing alone tends to be relatively small it is clear that off equatorial oceanic rossby waves affect equatorial pacific ocean variability on interannual through to interdecadal time scales in the present study a hybrid coupled model hcm of the equatorial pacific between 12 5 s and 12 5 n was developed and is used to estimate the magnitude of equatorial region variability arising from off equatorial poleward of 12 5 latitude wind stress forcing the hcm utilizes a reduced gravity ocean shallow water model and a statistical atmosphere derived from monthly output from a 100 yr australian bureau of meteorology research centre now the centre for australian weather and climate research coupled general circulation model integration the equatorial region wind stress forcing i

on the entry and transit of high density plasma across the polar cap 1 observations are presented from superdarn and the eiscat svalbard radar of two intense polar cap patch events on 6 february 2001 the interplanetary magnetic field imf was dominated by a large positive by component and for both events the electron density exceeded 1012 m 3 in the f region with superdarn we tracked the events all the way across the polar cap from the dayside svalbard sector to the nightside alaska sector the flow speed was highly dynamic and pulsed and both patches underwent substantial rotation in the polar cap on the nightside the leading edge had become the trailing edge it suggests that the first patch to enter the polar cap on the dayside may not always be the first patch to reach the nightside plasma might be stagnant in the polar cap or even overtaken the study also provides evidence that momentum transfer in the dayside polar cap can last significantly longer than 10 min after reconnection especially for extremely long field lines where imf by is dominating i e on old open field lines knowledge of the solar wind driver and the coupling processes is therefore extremely important for predicting the motion of a polar cap patch event across the polar cap gradients in the plasma flow associated with the rotation of the extreme density may in itself lead to a stronger growth of ionospheric irregularities these irregularities may continue to grow all the way across the polar cap the result is more efficient creation of ionospheric irregularities

application of the rfφ technique to ellipitical markers deformed by pressure solution abstract the applicability of the r f technique of finite strain analysis to elliptical markers whose shape is modified by pressure solution is investigated variably oriented elliptical markers shortened by removing material from one or both sides in open and closed systems are compared to markers shortened homogeneously by the same amount for the models examined it was found that the r f technique correctly predicts the principal directions the initial marker axial ratio and the axial ratio of a circle deformed in a like manner interpretation of strain ratios in terms of percent shortening requires knowing how much volume was lost equations relating the strain ratio to percent shortening are given for both constant volume and volume loss deformation

finite strain analysis of the zhangbaling metamorphic belt se china crustal thinning in transpression abstract finite strain analysis of pebbles in the metatillite and phyllite shows that the deformation of the metatillite phyllite zone is flattening to plane strain dominated results of spo analysis of the rigid plagioclase porphyroclasts in three perpendicular planes indicate monoclinic strain symmetry of the zhangbaling schist and oblate geometry of the clast forward strain modeling of rigid clasts is performed based on the shape fabric of the clast population integrated results of finite strain modeling and finite strain analysis indicate that this midcrustal section experienced at least 18 36 thinning in the oblique convergence between north and south china blocks

origin of upper ocean warming and el nino change on decadal scales in the tropical pacific ocean the cause of decadal scale variability in the tropical pacific ocean such as that marked by the 1976 77 shift in the el nino southern oscillation1 2 3 4 5 6 7 is poorly understood unravelling the mechanism of the recent decade long warming in the tropical upper ocean is a particularly important challenge given the link to el nino variability but establishing the hypothesized interannual decadal oceanic connections between middle latitudes and tropics has proved elusive8 here we present observational evidence that pacific upper ocean warming and decadal changes in the el nino southern oscillation after 1976 may originate from decadal mid latitude variability in the middle 1970s the north pacific ocean is observed to have undergone a clear phase transition a see saw subsurface temperature anomaly pattern that rotates clockwise around the subtropical gyre at middle latitudes a subsurface warm anomaly formed in the early 1970s from subducted surface waters and penetrated through the subtropics and into the tropics thus perturbing the tropical thermocline and driving the formation of a warm surface water anomaly that may have influenced el nino in the 1980s the identification of this teleconnection of extratropical thermal anomalies to the tropics through a subsurface ocean bridge may enable improved prediction of decadal scale climate variability

tectonic significance of ductile deformation in low grade sandstones in the mesozoic otago subduction wedge new zealand this study demonstrates how ductile strain measurements can determine the tectonic evolution of a large and long lived subduction wedge we provide a synthesis of the geology of the south island of new zealand with an emphasis on a wedge tectonics perspective that contrasts with a traditional view that interprets new zealand geology in the context of terrane collision and accretion we argue that the otago subduction wedge evolved in a steady fashion throughout its 290 to 105 ma history in response to accretion of trench fill and abyssal plain sediments and slow erosion of a subaerially exposed forearc high maximum temperatures for rocks in the flanks of the forearc high were no greater than 150 to 300 c with solution mass transfer active as the dominant ductile mechanism the 54 studied samples provide information about the absolute ductile strains acquired all along their flow path from the site of accretion to exhumation in the forearc high we use tensor averages to estimate strain at a regional scale these show plane strain uniaxial flattening given that the tensor averages for sy and sx are close to one on average sz is approximately 0 77 and this is balanced by a mass loss of about 23 percent the average z direction is sub horizontal in the prowedge and moderately plunging in the retrowedge a difference attributed to spatial variations in the mode of accretion we infer that rocks presently in the pro side of the otago high were sourced by frontal accretion and those in the retro side were underplated this result highlights the important role of accretion in determining the style of within wedge deformation and also demonstrates the benefit of using a tensor averaging approach to examine regional strain

calculation of shortening due to outcrop scale deformation and its relation to regional deformation patterns abstract the objectives of this study are to assess the regional contribution of outcrop scale structures in well exposed rocks of a tectonic province and evaluate how failure to measure this scale of structure affects the assessment of regional deformation the contribution of outcrop scale structures to regional deformation has not been quantified previously because of the need for long continuous exposures to regionally characterize the structures this study used 18 6 km of semi continuous roadcut and streambed exposures in well exposed devonian clastic rocks to construct 1 200 scale profiles to address the objectives key results are 1 outcrop scale structures contribute about 1 6 to 1 4 of total regional shortening 2 submapscale shortening components from microscale and outcrop scale structures equal mapscale shortening that would be interpreted from a regional cross section 3 analysis of submapscale structures in the target blind thrust belt is needed to identify that the roof sequence deformation preceded and continued through duplex formation 4 the kinematic response of the roof sequence would be incorrectly identified as backthrusting if the shortening contribution of outcrop scale structures is ignored 5 although measured outcrop scale shortening does not match that predicted by the fractal analysis of other workers the fractal approach to predicting deformation as a function of scale may still be valid

flux transfer events on the magnetopause spatial distribution and controlling factors magnetic field profiles across the dayside magnetopause obtained during the first five years 1977 1981 of the isee mission have been examined in boundary normal coordinates to determine the spatial distribution and controlling factors of flux transfer events fte s owing to the natural evolution of the orbital plane of the spacecraft the dayside magnetopause region is eventually sampled both above and below the magnetic equator nevertheless at the end of the period surveyed the satellites still had not yet reached latitudes as far south of the equator as they were initially north of the equator two types of magnetic signature in the component normal to the magnetopause are regularly observed when an fte passes the spacecraft in the northern dawn quadrant the perturbation along the normal direction is usually first outward and then inward whereas in the southern dusk quadrant the reverse behavior occurs the dividing line between the two signatures is tilted somewhat with respect to the equator along a south dawn north dusk direction however deviations from this simple pattern are observed in both hemispheres examination of simultaneous imp 8 and isee 3 interplanetary data reveals that fte s occur almost exclusively during southward interplanetary magnetic field imf conditions fte s occur about 45 of the time when the imf has a southward component with the highest rates of occurrence for due southward imf the sign of the east west component of the imf does not exert control on the rate of fte occurrence

strain patterns within part of the willard thrust sheet idaho utah wyoming thrust belt abstract the willard thrust sheet located in northern utah usa contains a 10 15 km thick sequence of strata that were internally deformed and translated about 50 km ese upper levels of the sheet are relatively little deformed lower levels have minor folds and cleavage that increase in intensity downward and rotate into parallelism with the thrust and the basal fault zone contains mylonite and ultracataclasite finite strain displays systematic changes with structural position and lithology basal levels have strain ratios r r xz of 3 5 and angles between stretch directions and the thrust of r of 1 5 3 and of 10 35 with overall correlations between increasing r decreasing and greater mica content overlying micaceous quartzite has relatively low strain with r of 1 3 2 and 50 principal stretching x directions trend perpendicular to oblique to minor fold axes which are locally oblique to the overall transport direction reflecting partial decoupling of internal deformation from thrust slip principal axial ratios have r yz r xy but microfabrics indicate only limited extension in the intermediate stretching direction y corresponding to volume losses of about 10 40 strain is factored into components of thrust perpendicular thinning and thrust parallel shear that increase downward and in mica rich layers minor thrust parallel shortening to extension minor longitudinal extension and minor longitudinal wrench shear these patterns reflect litho mechanical layering of the thrust sheet which includes a very weak basal fault zone that formed at elevated fluid pressures a weak mica rich lower level with widespread crystal plastic and mass transfer deformation and a stronger upper part with limited internal deformation

temporal and spatial aspects of the cusp inferred from local and global ground and space based observations in a case study 1 global and local observations from both ground and space facilities are brought to bear to understand the origin of a stepped cusp ion dispersion observed 3 hours prenoon under an interplanetary magnetic field characterized by strongly positive by conditions under which the cusp is believed to be shifted postnoon our main aim is to distinguish the temporal from the spatial cusp features the local observations from svalbard are from approximately the 0900 1200 mlt sector while the in situ observations are from polar at 0900 mlt the global auroral observations were acquired by the earth camera on polar the aurora from the ground perspective appeared as a periodic sequence of brightening events located on either side of a midday gap aurora centered at about 1030 mlt at noon the aurora appeared as a semiperiodic sequence of brightening events these were concurrent with enhancements of the overhead ionospheric hall current polar observed two steps of a staircase ion energy latitude dispersion these two steps were in turn concurrent with two consecutive auroral brightening events we argue from this complement of observations that the stepped cusp was temporal in nature and resulted from a time varying reconnection process global uv imagery was employed to complement the ground observations for the purpose of separating the temporal from the spatial aspects the imager captured namely two bands of strong uv emission in the prenoon and postnoon sectors between which there appeared a persistent weak uv aurora we interpret this to be an underlying spatial substratum patchy and bursty uv enhancements coincided with the transients seen in the local ground and space measurements corroborating further our distinction between space and time in the cusp comparing these inferences with recent work on the response of the dayside aurora to reconnection at the magnetopause we find a close correspondence in addition we document the field aligned currents associated with the stepped cusp which in turn compare favorably with recent theoretical ideas this work is a contribution to the current debate on the nature of the stepped cusp and suggests that multiple data sets are essential to study this problem

structural and strain analyses of the middle part of the talassian alatau ridge middle asia kirgizstan abstract the middle part of the talassian alatau ridge is composed of upper precambrian lower paleozoic greenschists the usunakhmatian sheet which were overthrust on unmetamorphosed rocks of nearly the same age the talassian and kumishtagian sheets three phases of deformation can be recognized in the usunakhmatian sheet the most intensive of which was the late silurian early devonian d 2 deformation in the usunakhmatian sheet the axis of maximum extension is subhorizontal and parallel to f 2 fold axes r xz varies from 2 4 to 7 5 and k from 0 03 to 1 75 flattening is widespread but the strain of many samples is nearly plane strain in the talassian sheet r xz varies from less than 1 8 to 4 4 and k from 0 to 0 35 r xz and k values increase near large faults r xz values determined by quartz and chert grains were found to be linearly related a three stage strain history is related to d 2 deformation the earliest was simple shear parallel to the axial surfaces of folds following this the fold and thrust system was transformed by pure shear with an orogen parallel axis of extension some minor increments of strain are locally represented by pressure shadows around pyrite crystals

a simple model of the mass exchange between the subtropical and tropical ocean abstract a simple ventilated thermocline model is used to study the subtropical tropical mass exchange it is found that the water subducted in the western subtropical gyre recirculating window tends to recirculate within the subtropical gyre while the water subducted in the eastern part exchange window tends to penetrate equatorward the exchange window expands with an increased easterly wind or basin width on the southern boundary of the subtropical gyre but shrinks with an increased wind curl within the subtropical gyre furthermore the total exchange transport increases with the easterly wind or the width of the basin on the southern boundary of the subtropical gyre but it is independent of subtropical wind the ventilation mechanism is important in supporting the exchange transport for wind with realistic strength at the southern boundary the reduction of the exchange transport is about 15 30 of the ekman transport finally relative to the exchange transport in the interior of the ocean

response of the dayside auroral and electrodynamic processes to variations in the interplanetary magnetic field the response of the dayside auroral and electrodynamic processes to variations in the interplanetary magnetic field imf is examined with multi instrument data from the viking and dmsp f7 satellites as well as ground magnetometer data the event selected which occurred on october 5 1986 was previously identified as a high latitude dayside form by murphree and elphinstone 1988 imf by was positive during most of the viking auroral uv observation whereas imf bz made a transient southward excursion followed by a sharp increase to 8 nt the results are summarized as follows 1 there were two auroral belts extending westward from the early afternoon sector the equatorward belt was persistent throughout the event and was embedded in the cps bps precipitation region whereas the poleward belt was in the open field line region and was associated with northward imf bz 2 a new auroral spot emerged in the afternoon sector just poleward of the previously active region delayed 10 min from the sudden increase in imf bz this time lag is ascribed to the travel time of the new imf orientation from the subsolar point to a solar wind magnetosphere interaction site tailward of the dayside cusp as well as to the response time of the dayside auroral acceleration process 3 the new auroral activity expanded both eastward and westward during the first few minutes and then expanded primarily westward across the noon meridian forming the poleward belt the speed of the westward extension in the second phase was comparable to the speed of the ionospheric convection 4 the dayside auroral process has a finite decay constant which is inferred to be at least 10 min 5 the response time of the midday ionospheric convection to imf variations is estimated to be a few to several minutes

decadal variability of temperature at a depth of 400 meters in the north pacific ocean the gyre scale structure and evolution of decadal ocean thermocline variability in the north pacific is described by analyzing historical temperature data at a depth of 400 meters decadal thermocline anomalies are characterized by a distinct see saw pattern in the midlatitudes and subtropics that appears to circulate coherently in the north pacific basin with a period on the order of 20 years there are coherent spatial and temporal relationships between temperature anomalies around the subtropical gyre with phase differences at depth and at the sea surface suggesting that thermocline variability evolves independently of local sst anomalies in the subtropics subsurface anomalies can be seen to propagate all the way across the basin and thereby lead sst anomalies providing a possible phase transition mechanism for sst the basinwide anomaly pathways suggest a clockwise advection of subsurface temperature anomalies around the subtropical gyre an active role played by the mean geostrophic flow in decadal variability

upper ocean thermal variations in the north pacific during 1970 1991 abstract a newly available extensive compilation of upper ocean temperature profiles was used to study the vertical structure of thermal anomalies between the surface and 400 m depth in the north pacific during 1970 1991 a prominent decade long perturbation in climate occurred during this time period surface waters cooled by 1 c in the central and western north pacific and warmed by about the same amount along the west coast of north america from late 1976 to 1988 comparison with data from coads suggests that the relatively sparse sampling of the subsurface data is adequate for describing the climate anomaly the vertical structure of seasonal thermal anomalies in the central north pacific shows a series of cold pulses beginning in the fall of 1976 and continuing until late 1988 that appear to originate at the surface and descend with time into the main thermocline to at least 400 m depth individual cold events descend rapidly 100 m yr 1 superimposed upon a slower cooling 15 m yr 1 the inter

characteristics of ionospheric convection and field aligned current in the dayside cusp region the assimilative mapping of ionospheric electrodynamics amie technique has been used to estimate global distributions of high latitude ionospheric convection and field aligned current by combining data obtained nearly simultaneously both from ground and from space therefore unlike the statistical patterns the snapshot distributions derived by amie allow us to examine in more detail the distinctions between field aligned current systems associated with separate magnetospheric processes especially in the dayside cusp region by comparing the field aligned current and ionospheric convection patterns with the corresponding spectrograms of precipitating particles the following signatures have been identified 1 for the three cases studied which all had an imf with negative y and z components the cusp precipitation was encountered by the dmsp satellites in the postnoon sector in the northern hemisphere and in the prenoon sector in the southern hemisphere the equatorward part of the cusp in both hemispheres is in the sunward flow region and marks the beginning of the flow rotation from sunward to antisunward 2 the pair of field aligned currents near local noon i e the cusp mantle currents are coincident with the cusp or mantle particle precipitation in distinction the field aligned currents on the dawnside and duskside i e the normal region 1 currents are usually associated with the plasma sheet particle precipitation thus the cusp mantle currents are generated on open field lines and the region 1 currents mainly on closed field lines 3 topologically the cusp mantle currents appear as an expansion of the region 1 currents from the dawnside and duskside and they overlap near local noon when by is negative in the northern hemisphere the downward field aligned current is located poleward of the upward current whereas in the southern hemisphere the upward current is located poleward of the downward current 4 under the assumption of quasi steady state reconnection the location of the separatrix in the ionosphere is estimated and the reconnection velocity is calculated to be between 400 and 550 m s the dayside separatrix lies equatorward of the dayside convection throat in the two cases examined

structure and cycle of decadal variability of upper ocean temperature in the north pacific abstract yearly upper ocean in situ temperature anomaly data for the period 1961 90 are analyzed to reveal spatial structure and evolution of decadal variability in the north pacific ocean an eof analysis has been performed on individual temperature anomaly fields at upper ocean standard levels as well as simultaneously on the entire upper ocean data to depict the combined three dimensional structure in a coherent manner time evolution of anomaly fields is depicted by using a regression analysis the analyses detect the principal basin scale structure of decadal warm period dwp and decadal cold period dcp there is a well defined subsurface thermal anomaly pattern characterized by a prominent seesaw structure with opposite anomaly polarity between the midlatitude north pacific and the subtropical regions during a dwp a positive temperature anomaly is found in the central midlatitude upper ocean with the maximum at about 100 m depth this is accompanied by a corresponding negative anomaly in the

structure and evolution of interannual variability of the tropical pacific upper ocean temperature n n yearly in situ temperature anomaly data for the period 1961 1990 are analyzed to reveal the dominant structure and evolution of interannual variability of the tropical pacific upper ocean we use multivariate empirical orthogonal function eof analyses to detect the principal three dimensional structure related to el nino there are well defined subsurface thermal patterns characterized by a prominent sea saw structure with opposite anomaly polarity in the equatorial and off equatorial tropical north pacific regions during an el nino year a positive temperature anomaly is found in the e astern and central tropical upper ocean this is accompanied by a corresponding negative anomaly at subsurface depths in the west with a maximum at 100 150 m off the equator unlike sea surface temperature whose variations are confined largely to the east with one dominant polarity temperature variations at 50 200 m depths have a dipole pattern with out of phase oscillations in the central equatorial pacific and in the western tropical north pacific a reverse pattern of these anomalies is observed during a la nina year evolution between el nino and la nina involves a significant zonal transfer of anomaly phase across the equatorial basin as well as across the off equatorial tropical north pacific showing consistent and coherent variations from west to east from subsurface to sea surface and from on the equator to off the equator this phase propagation is more evident at subsurface depths than that at the sea surface suggesting a continual movement of anomaly pattern in succession eastward along the equator and westward off the equator of the tropical north pacific with spatial inhomogeneities of zonal propagation in longitude our analyses present evidence of the manner in which temperature anomalies evolve at subsurface ocean depths in the tropical pacific thus providing an observational basis for evaluating theoretical studies and model simulations the dynamical implication of these results is also discussed

ability of the fry method to characterize pressure solution deformation abstract the applicability of the fry method to marker distributions deformed by pressure solution is examined four types of pressure solution deformation were modeled they differ by the spacing of the dissolution surfaces relative to the distance between marker centers the amount of volume loss and whether or not markers in contact with dissolution surfaces are dissolved completely the results show that the fry method correctly characterizes each type of deformation but without additional information such as the amount of volume loss it can be used only where the deformation is constant volume and where dissolution surfaces are more closely spaced than marker centers

interdecadal variability of the pacific ocean model response to observed heat flux and wind stress anomalies variability of the pacific ocean is examined in numerical simulations with an ocean general circulation model forced by observed anomalies of surface heat flux wind stress and turbulent kinetic energy tke over the period 1970 88 the model captures the 1976 77 winter time climate shift in sea surface temperature as well as its monthly seasonal and longer term variability as evidenced in regional time series and empirical orthogonal function analyses examination of the surface mixed layer heat budget reveals that the 1976 77 shift was caused by a unique concurrance of sustained heat flux input anomalies and very strong horizontal advection anomalies during a multi month period preceding the shift in both the central pacific region where cooling occurred and the california coastal region where warming occurred in the central pacific the warm conditions preceding and the cold conditions following the shift tend to be maintained by anomalous vertical mixing due to increases in the atmospheric momentum flux tke input into the mixed layer which deepens in the model after the shift from the early 1970s to the late 1970s and 1980s since the ocean model does not contain feedback to the atmosphere and it succeeds in capturing the major features of the 1976 77 shift it appears that the midlatitude part of the shift was driven by the atmosphere although effects of midlatitude ocean atmosphere feedback are still possible the surface mixed layer heat budget also reveals that in the central pacific the effects of heat flux input and vertical mixing anomalies are comparable in amplitude while horizontal advection anomalies are roughly half that size in the california coastal region in contrast where wind variability is much weaker than in the central pacific horizontal advection and vertical mixing effects on the mixed layer heat budget are only one quarter the size of typical heat flux input anomalies

scintillation and loss of signal lock from poleward moving auroral forms in the cusp ionosphere we present two examples from the cusp ionosphere over svalbard where poleward moving auroral forms pmafs are causing significant phase scintillation in signals from navigation satellites the data were obtained using a combination of ground based optical instruments and a newly installed multiconstellation navigation signal receiver at longyearbyen both events affected signals from gps and global navigation satellite system glonass when one intense pmaf appeared the signal from one gps spacecraft also experienced a temporary loss of signal lock although several polar cap patches were also observed in the area as enhancements in total electron content the most severe scintillation and loss of signal lock appear to be attributed to very intense pmaf activity this shows that pmafs are locations of strong ionospheric irregularities which at times may cause more severe disturbances in the cusp ionosphere for navigation signals than polar cap patches

roof sequence response to emplacement of the wills mountain duplex the roles of forethrusting and scales of deformation abstract this paper focuses on the behavior of a roof sequence in the appalachian plateau of west virginia u s a and emplacement of the wills mountain duplex with 17 5 km of displacement unlike the plateau along strike in pennsylvania and new york where forethrusting was previously documented this roof sequence lacks an underlying salt dominated roof decollement kinematic analyses reveal that the roof sequence in the west virginian plateau accommodated about two thirds of the 17 5 km of shortening by the adjacent wills mountain duplex as a forethrusting kinematic response the remaining shortening imbalance of about 5 km between the duplexes and younger roof sequence rocks is accommodated by additional forethrusting further into the foreland and local compensation this kinematic response matches that along strike in the central appalachians despite the loss of the salt decollement we interpret that an ordovician shale dominated formation was sufficiently weak to substitute for the salt horizon thus a weak mechanical unit rather than specifically a salt decollement is a necessary prerequisite for forethrusting a contributing factor to forethrusting may be the subvertical front of the wills mountain duplex which inhibited other responses by the roof sequence mesoscale and smaller processes including grain to grain pressure solution twinning and cleavage formation account for over 75 of the shortening in the roof sequence and if ignored would result in an erroneous interpretation of backthrusting or local compensation this result suggests that failure to consider all deformation scales could lead to incorrect kinematic conclusions in other tectonic systems

late winter generation of spiciness on subducted isopycnals the origins of density compensating anomalies of temperature and salinity spice are investigated using a model forced with the most realistic surface products available over the 40 years 1958 97 in this hindcast the largest interannual spiciness anomalies are found in the pacific ocean near the isopycnal s 0 5 25 5 where deviations as great as 1 28c and 0 6 psu are generated equatorward of winter outcropping in the eastern subtropics in both hemispheres these source regions are characterized by very unstable salinity gradients and low mean density stratification in winter two related signatures of winter mixing in the southeast pacific sep are density that is well mixed deeper than either temperature or salinity and subsurface density ratios that approach 1 all ocean basins in the model are shown to have regions with these characteristics and signatures however the resultant spiciness signals are focused on different isopycnals ranging from s 0 5 25 0 in the northeast pacific to s 0 5 26 5 in the south indian ocean a detailed examination of the sep finds that large positive anomalies are generated by diapycnal mixing across subducted isopycnals e g s 0 5 25 5 whereas negative anomalies are the result of a steady isopycnal advection moderated by vertical advection and heave there is considerable interannual variability in the strength of anomalies and in the density on which they occur historical observations are consistent with the model results but are insufficient to verify all aspects of the hindcast including the processes of anomaly generation in the sep it was not possible to relate isopycnal anomaly genesis to local surface forcing of any kind a complex scenario involving basinwide circulation of both the ocean and atmosphere especially of surface water through the subtropical evaporation zones is put forward to explain the decadal time scale evident in sep salinity anomalies on s 0 5 25 5 pacific anomalies generated on s 0 5 25 5 can be traced along mean geostrophic streamlines to the western boundary where decadal salinity variations at 78s are about 2 times as large order 60 1 psu as at 128n although there may be more variance on shallower isopycnals in the north at least portions of the s 0 5 25 5 signals appear to continue along the boundary to a convergence at the equator suggesting that the most robust sources of pacific spiciness variance coincide with equatorial exchange pathways

evidence for stronger el nino southern oscillation enso events in a mid holocene massive coral we present a 47 year long record of sea surface temperature sst derived from sr ca and u ca analysis of a massive porites coral which grew at 4150 calendar years before present b p in vanuatu southwest tropical pacific ocean mean sst is similar in both the modern instrumental record and paleorecord and both exhibit el nino southern oscillation enso frequency sst oscillations however several strong decadal frequency cooling events and a marked modulation of the seasonal sst cycle with power at both enso and decadal frequencies are observed in the paleorecord which are unprecedented in the modern record

observation of a correspondence between poleward moving auroral forms and stepped cusp ion precipitation in a case study we document for the first time a one to one association between three auroral forms which are separated in latitude and moving poleward and three intervals of almost constant but different low energy cutoffs in the ion precipitation in the cusp region stepped cusp precipitation the ground observations are from ny alesund at 75 magnetic latitude mlat and the precipitation pattern is observed by the dmsp f13 satellite as it crosses the noon meridian into the morning sector the energy cutoffs increase with decreasing latitude the auroral forms brighten up at different times but at practically the same mlat 73 and start fading 5 min later at 76 mlat interplanetary conditions monitored by wind show a stable slow and cold solar wind with an interplanetary magnetic field which points south and west by 0 staircase ion precipitation signatures such as observed in this example have been successfully interpreted in terms of a sequence of reconnection bursts at the low latitude dayside magnetopause the time history of the auroral activity observed from the ground confirms this interpretation we exclude the possibility that the auroral forms are triggered by changes in the interplanetary medium because the latter is very steady in particular the low dynamic pressure varies little about a mean value of 1 6 npa

empirical high latitude electric field models electric field measurements from the dynamics explorer 2 satellite have been analyzed to extend the empirical models previously developed from dawn dusk ogo 6 measurements j p heppner 1977 the analysis embraces large quantities of data from polar crossings entering and exiting the high latitudes in all magnetic local time zones paralleling the previous analysis the modeling is based on the distinctly different polar cap and dayside convective patterns that occur as a function of the sign of the y component of the interplanetary magnetic field the objective which is to represent the typical distributions of convective electric fields with a minimum number of characteristic patterns is met by deriving one pattern model bc for the northern hemisphere with a y interplanetary magnetic field imf and southern hemisphere with a y imf and two patterns models a and de for the northern hemisphere with a y imf and southern hemisphere with a y imf the most significant large scale revisions of the ogo 6 models are 1 on the dayside where the latitudinal overlap of morning and evening convection cells reverses with the sign of the imf y component 2 on the nightside where a westward flow region poleward from the harang discontinuity appears under model bc conditions and 3 magnetic local time shifts in the positions of the convection cell foci the modeling above was followed by a detailed examination of cases where the imf z component was clearly positive northward neglecting the seasonally dependent cases where irregularities obscure pattern recognition the observations range from reasonable agreement with the new bc and de models to cases where different characteristics appeared primarily at dayside high latitudes the analysis of these differences for strong bz conditions using passes through different mlt zones led to finding that the deviations from both the bc and de models could be modeled by extending the evening cell foci toward and beyond noon with a deformation that roughly resembles a rotational twist of the extended evening cell the morning cell accommodation to the model bc deformation produces a narrow tongue of eastward convection near 75 invariant latitude on the dayside the model de deformation compresses the morning cell toward the night hours such that the sunward convection in the morning cell is diverted to polar latitudes in the 0400 to 0700 mlt sector in both cases the deformations of the two cell patterns lead to sunward convection in dayside polar regions while maintaining the integrity of the nightside convection pattern thus the nightside dilemmas that plague three and four cell models designed to explain sunward convection in polar regions under bz conditions do not appear

events of enhanced convection and related dayside auroral activity in this paper we study the high latitude plasma flow variations associated with a periodic 8 min sequence of auroral forms moving along the polar cap boundary which appear to be the most regularly occuring dayside auroral phenomenon under conditions of southward directed interplanetary magnetic field satellite data on auroral particle precipitation and ionospheric plasma drifts from dmsp f10 and f11 are combined with ground based optical and ion flow measurements for january 7 1992 ionospheric flow measurements of 10 s resolution over the range of invariant latitudes from 71 to 76 were obtained by operating both the european incoherent scatter eiscat uhf and vhf radars simultaneously the optical site ny alesund svalbard and the eiscat radar field of view were located in the postnoon sector during the actual observations the west greenland magnetometers provided information about temporal variations of high latitude convection in the prenoon sector satellite observations of polar cap convection in the northern and southern hemispheres show a standard two cell pattern consistent with a prevailing negative by component of the interplanetary magnetic field the 630 0 nm auroral forms located poleward of the persistent cleft aurora and the flow reversal boundary in the 1440 1540 mlt sector were observed to coincide with magnetosheath like particle precipitation and a secondary population of higher energy ions and they propagated eastward tailward at speeds comparable with the convection velocity it is shown that these optical events were accompanied by bursts of sunward return flow at lower latitudes in both the morning and the afternoon sectors consistent with a modulation of dungey cell convection the background level of convection was low in this case kp 2 the variability of the high latitude convection may be explained as resulting from time varying reconnection at the magnetopause in that case this study indicates that time variations of the reconnection rate effectively modulates ionospheric convection