kinematics of the nicaraguan forearc from gps geodesy 1 campaign gps data from a network in the nicaraguan forearc show a strong component of arc parallel motion indicating northwest translation of a nearly rigid forearc sliver our measured mean velocity for forearc sites of 15 1 mm yr 1 agrees well with the arc parallel sliver motion predicted previously by demets 2001 derived from closure constraints on oblique convergence between the cocos and caribbean plates the lack of a northeasterly oriented arc normal component of motion in forearc velocities indicates that there are complexities involved beyond a simple interpretation of sliver motion being driven by oblique convergence the forearc is reasonably well fit by rigid rotation about an euler pole with a rms misfit of residual velocities of 4 9 2 6 mm yr 1 current motion of the forearc sliver relative to the stable caribbean plate yields predominantly boundary parallel nw motion with boundary normal extension in the northwestern region averaging 5m m yr 1 citation turner h l iii p lafemina a saballos g s mattioli p e jansma and t dixon 2007 kinematics of the nicaraguan forearc from gps geodesy geophys res lett 34 l02302 doi 10 1029 2006gl027586

active strike slip faulting in el salvador central america several major earthquakes have affected el salvador central america during the past 100 yr as a consequence of oblique subduction of the cocos plate under the caribbean plate which is partitioned between trench orthogonal compression and strike slip deformation parallel to the volcanic arc focal mechanisms and the distribution of the most destructive earthquakes together with geomorphologic evidence suggest that this transcurrent component of motion may be accommodated by a major strike slip fault el salvador fault zone we present field geological structural and geomorphological data collected in central el salvador that allow the constraint of the kinematics and the quaternary activity of this major seismogenic strike slip fault system data suggest that the el salvador fault zone consists of at least two main e w fault segments san vicente and berlin segments with associated secondary synthetic wnw ese and antithetic nnw sse riedel shears and nw se tensional structures the two main fault segments overlap in a dextral en echelon style with the formation of an intervening pull apart basin our original geological and geomorphologic data suggest a late pleistocene holocene slip rate of 11 mm yr along the berlin segment in contrast with low historical seismicity the kinematics and rates of deformation suggested by our new data are consistent with models involving slip partitioning during oblique subduction and support the notion that a trench parallel component of motion between the caribbean and cocos plates is concentrated along e w dextral strike slip faults parallel to the volcanic arc

intraseasonal variability over tropical africa during northern summer abstract the intraseasonal variability over africa during northern summer was analyzed using 25 years of ncep ncar reanalysis and satellite data the dominant pattern of variability was one of enhanced deep convection over the whole african monsoon region it appeared to arise at least partly as a remote response to the intraseasonal madden julian oscillation over the warm pool sector twenty days prior to the maximum in convection over africa there was no signal over africa but convection was reduced over the equatorial warm pool an equatorial kelvin wave response to this change in warm pool convection propagated eastward and an equatorial rossby wave response propagated westward and between them they completed a circuit of the equator and met up 20 days later over africa where the negative midtropospheric temperature anomalies in the kelvin and rossby waves favored deep convection over west africa the kelvin wave component contained lower tropospheric westerly anomalies that acted to increase t

asymptotic models for tropical intraseasonal oscillations and geostrophic balance abstractin the tropics rainfall is coupled with waves in the form of for example convectively coupled equatorial waves ccews and the madden julian oscillation mjo in perhaps the simplest vi

a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

large scale forcing of summertime monsoon surges over the gulf of california and the southwestern united states synoptic forcing of the gulf of california summertime low level wind field is described using a nested regional modeling system under appropriate synoptic conditions strong surge events develop that are characterized by persistent vertically extensive 1 2 km southerly flow extending along the entire gulf of california and into arizona california and even southern nevada these surge periods are initiated either by tropical cyclone activity to the south of baja california or by the westward propagation of lower tropospheric troughs from over the sierra madre occidental to the eastern pacific the flow over the gulf is primarily geostrophic and is associated with the presence of these low pressure centers to the south and west of baja california

the madden julian oscillation barotropic dynamics and north pacific tropical cyclone formation part i observations low level barotropic dynamics may help to explain the modulation of eastern and western north pacific tropical cyclones by the madden julian oscillation mjo during northern hemisphere summer the mjo is characterized by alternating periods of westerly and easterly 850 mb zonal wind anomalies across the tropical pacific ocean when mjo 850 mb wind anomalies are westerly small scale slow moving eddies grow through barotropic eddy kinetic energy eke conversion from the mean flow these growing eddies together with strong surface convergence 850 mb cyclonic shear and high mean sea surface temperatures create a favorable environment for tropical cyclone formation periods of strong mjo easterlies over the pacific are characterized by lesser eke and negligible eddy growth by barotropic conversion the term 2 x is a leading contributor to low level barotropic eke conversion during mjo westerly 2 u9 u periods across the pacific indicating the importance of zonal variations in the westerly jet for producing concentrations of eddy energy this mechanism can be described as wave accumulation associated with variations of the low level zonal flow the conversion term 2 y contributes a smaller portion of the total conversion u9y9 u over the eastern pacific but is of comparable importance to 2 x during westerly mjo events in the 2 u9 u western pacific

active seismic deformation in the grabens of northern central america and its relationship to the relative motion of the north america caribbean plate boundary abstract the active seismic deformation along the grabens of northern central america is calculated using a method which relates relative plate velocity with the seismic strain rate tensor the latter is in turn obtained from seismic moment tensors we calculate the typical or average moment tensor from available centroid moment tensors and fault plane solutions this average tensor is then used to obtain the sum of moment tensors of historical and modern earthquakes reported in the literature both historical and modern earthquakes yield an average extension rate of 8 mm yr along the grabens of northern central america while the relative motion between the north america and caribbean plates expressed along the motagua polochic fault system is 20 mm yr this result suggests that part of the seismic deformation related to the plate boundary is taken up as extension along the grabens which play the role of fault termination structures for the motagua polochic system

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

clustering of synoptic activity by indian summer monsoon intraseasonal oscillations 1 active and break phases of the indian summer monsoon are characterized by enhancement and decrease of precipitation over the monsoon trough region using genesis data of monsoon low pressure systems lps and circulation data for the period 1954 to 1993 it is shown that the frequency of occurrence of lps is nearly 3 5 times higher in the active phase of monsoon as compared to the break phase in addition the tracks of these synoptic systems are also strongly spatially clustered along the monsoon trough during the active phase of the monsoon the enhanced decreased frequency of occurrence of lps during active break phases is due to modulation of meridional shear of zonal winds and cyclonic vorticity along the monsoon trough by the intraseasonal oscillations iso

cloud components of the intertropical convergence zone 1 this study explores the roles of different synoptic scale cloud systems in the making of the intertropical convergence zone itcz outgoing longwave radiation olr data are used to represent both the mean itcz and cloud systems an olr threshold of 205 w m 2 is used to distinguish daily mean deep convective clouds from nondeep clouds zonally moving synoptic scale cloud systems are identified through a cloud tracking method mean olr of the itcz is decomposed into five components westward and eastward moving synoptic scale deep convective clouds random nonpropagating deep convective clouds nondeep clouds and clear sky backgrounds hypothetical itczs each composed of one of the cloud components and the clear sky background are constructed and compared the largest synoptic scale cloud constituents of the itcz are nondeep clouds and deep clouds with no persistent tendency of zonal propagation each of the two comprises about 40 45 of total cloud signals in the itcz a weak itcz could exist solely because of westward propagating synoptic scale deep convective clouds in reality these westward propagating clouds contribute 25 40 of deep convective clouds and 10 20 of total clouds in the itcz contributions from eastward propagating synoptic scale clouds are much smaller 8 of the deep convective clouds and less than 5 of total clouds in the itcz an itcz without any zonally propagating synoptic scale deep convective clouds would be diffused in latitude the cloud compositions in the itcz and other tropical convective zones are compared their discrepancies indicate fundamental characteristics in large scale environments for the cloud components in different regions

atlantic seasonal hurricane frequency part i el nino and 30 mb quasi biennial oscillation influences abstract this is the first of two papers on atlantic seasonal hurricane frequency in this paper seasonal hurricane frequency as related to e1 nino events during 1900 82 and to the equatorial quasi biennial oscillation qbo of stratospheric zonal wind from 1950 to 1982 is discussed it is shown that a substantial negative correlation is typically present between the seasonal number of hurricanes hurricane days and tropical storms and moderate or strong 15 cases el nin off the south american west coast a similar negative anomaly in hurricane activity occurs when equatorial winds at 30 mb are from an easterly direction and or are becoming more easterly with time during the hurricane season this association of atlantic hurricane activity with el nino can also be made with the southern oscillation index by contrast seasonal hurricane frequency is slightly above normal in non el nino years and substantially above normal when equatorial stratospheric winds blow from a westerly direction and or are bec

cenozoic tectonics of the nicaraguan depression nicaragua and median trough el salvador based on seismic reflection profiling and remote sensing data lakes nicaragua and managua are the two largest lakes in central america and they cover a combined area of 9000 km 2 of the presently active nicaraguan depression and central america volcanic front as part of the subduction factory focus area of the u s national science margins program 1925 km of shallow geophysical data were acquired over lakes nicaragua and managua in may 2006 to establish their late quaternary structural and stratigraphic history and to better constrain regional models for active tectonics in western nicaragua the gulf of fonseca and the median trough in el salvador in order to investigate regional upper crustal deformation resulting from forearc sliver transport and or slab rollback of the cocos plate these new data were integrated with relocated earthquake epicenters earthquake focal mechanisms high resolution digital topography from the national aeronautics and space administration nasa shuttle radar topography mission srtm published global positioning system gps vectors onland geologic maps previous maps of lake bathymetry and bottom sediment types a previously unpublished regional aeromagnetic data set and multichannel seismic reflection profiles from the gulf of fonseca and pacific ocean these new data sets have improved bathymetric bottom sediment and recent fault maps for both nicaraguan lakes and can be used as new constraints on the regional geology and tectonics three regional structural cross sections across the nicaraguan depression indicate that the basin is a highly asymmetrical half graben bounded to the southwest by northeast dipping oblique slip normal faults late oligocene to holocene extension resulted in footwall uplift along the elevated and folded area of the nicaraguan isthmus and the area of greatest subsidence occurs on the footwall block underlying lake nicaragua in the southeast a similar but younger pattern of footwall uplift adjacent to the down thrown footwall block is present to the northeast beneath lake managua and the gulf of fonseca we interpret this structural pattern as a time transgressive rift opening where the oldest extension late oligocene early miocene began in the southeast and migrated to the northwest gps data indicate that this earlier phase of intra arc normal rifting is presently being superimposed by arc parallel right lateral shear related to the northwestward transport of the central america forearc sliver

simulation of the intraseasonal variability over the eastern pacific itcz in climate models during boreal summer convective activity over the eastern pacific epac inter tropical convergence zone itcz exhibits vigorous intraseasonal variability isv previous observational studies identified two dominant isv modes over the epac i e a 40 day mode and a quasi biweekly mode qbm the 40 day isv mode is generally considered a local expression of the madden julian oscillation however in addition to the eastward propagation northward propagation of the 40 day mode is also evident the qbm mode bears a smaller spatial scale than the 40 day mode and is largely characterized by northward propagation while the isv over the epac exerts significant influences on regional climate weather systems investigation of contemporary model capabilities in representing these isv modes over the epac is limited in this study the model fidelity in representing these two dominant isv modes over the epac is assessed by analyzing six atmospheric and three coupled general circulation models gcms including one super parameterized gcm spcam and one recently developed high resolution gcm gfdl hiram with horizontal resolution of about 50 km while it remains challenging for gcms to faithfully represent these two isv modes including their amplitude evolution patterns and periodicities encouraging simulations are also noted in general spcam and hiram exhibit relatively superior skill in representing the two isv modes over the epac while the advantage of spcam is achieved through explicit representation of the cumulus process by the embedded 2 d cloud resolving models the improved representation in hiram could be ascribed to the employment of a strongly entraining plume cumulus scheme which inhibits the deep convection and thus effectively enhances the stratiform rainfall the sensitivity tests based on hiram also suggest that fine horizontal resolution could also be conducive to realistically capture the isv over the epac particularly for the qbm mode further analysis illustrates that the observed 40 day isv mode over the epac is closely linked to the eastward propagating isv signals from the indian ocean western pacific which is in agreement with the general impression that the 40 day isv mode over the epac could be a local expression of the global madden julian oscillation mjo in contrast the convective signals associated with the 40 day mode over the epac in most of the gcm simulations tend to originate between 150 e and 150 w suggesting the 40 day isv mode over the epac might be sustained without the forcing by the eastward propagating mjo further investigation is warranted towards improved understanding of the origin of the isv over the epac

impact of resolution and downscaling technique in simulating recent atlantic tropical cylone activity using the global environmental multiscale gem model we investigate the impact of increasing model resolution from 2 to 0 3 on atlantic tropical cyclone activity there is a clear improvement in the realism of atlantic storms with increased resolution in part linked to a better representation of african easterly waves the geographical distribution of a genesis potential index composed of large scales fields known to impact cyclone formation coincides closely in the model with areas of high cyclogenesis the geographical distribution of this index also improves with resolution we then compare two techniques for achieving local high resolution over the tropical atlantic a limited area model driven at the boundaries by the global 2 gem simulation and a global variable resolution model gvar the limited area domain and high resolution part of the gvar model coincide geographically allowing a direct comparison between these two downscaling options these integrations are further compared with a set of limited area simulations employing the same domain and resolution but driven at the boundaries by reanalysis the limited area model driven by reanalysis produces the most realistic atlantic tropical cyclone variability the gvar simulation is clearly more accurate than the limited area version driven by gem global degradation in the simulated interannual variability is partly linked to the models failure to accurately reproduce the impact of atmospheric teleconnections from the equatorial pacific and sahel on atlantic cyclogenesis through the use of a smaller limited area grid driven by gem global 2 we show that an accurate representation of african easterly waves is crucial for simulating atlantic tropical cyclone variability

effect of dry large scale vertical motions on initial mjo convective onset anomalies of eastward propagating large scale vertical motion with 30 day variability at addu city maldives move into the indian ocean from the west and are implicated in madden julian oscillation mjo convective onset using ground based radar and large scale forcing data derived from a sounding array typical profiles of environmental heating moisture sink vertical motion moisture advection and eulerian moisture tendency are computed for periods prior to those during which deep convection is prevalent and those during which moderately deep cumulonimbi do not form into deep clouds convection with 3 7 km tops is ubiquitous but present in greater numbers when tropospheric moistening occurs below 600 hpa vertical eddy convergence of moisture in shallow to moderately deep clouds is likely responsible for moistening during a 3 7 day long transition period between suppressed and active mjo conditions although moistening via evaporation of cloud condensate detrained into the environment of such clouds may also be important reduction in large scale subsidence associated with a vertical velocity structure that travels with a dry eastward propagating zonal wavenumbers 1 1 5 structure in zonal wind drives a steepening of the lapse rate below 700 hpa which supports an increase in moderately deep moist convection as the moderately deep cumulonimbi moisten the lower troposphere more deep convection develops which itself moistens the upper troposphere reduction in large scale subsidence associated with the eastward propagating feature reinforces the upper tropospheric moistening helping to then rapidly make the environment conducive to formation of large stratiform precipitation regions whose heating is critical for mjo maintenance

crustal structure across the pacific margin of nicaragua evidence for ophiolitic basement and a shallow mantle sliver seismic wide angle measurements across the pacific margin of nicaragua were carried out using ocean bottom hydrophones and land stations recording marine airgun shots the structure and the p wave velocity of the subducting cocos and overriding caribbean plates were determined by modelling wide angle data and further constrained by coincident seismic reflection borehole and gravity data the oceanic crust of the cocos plate is 5 5 km thick with a thin pelagic sediment cover the plate boundary can be traced to 40 km depth and is generally similar to configurations derived earlier from the local seismic network a major feature of the upper plate is an 80 km wide high velocity high density rock unit reaching from the front of the margin to about the middle of the shelf this wedge shaped unit is 15 km thick beneath the shelf edge and is composed of a 5 km thick upper part with velocities increasing from 3 5 km s 1 near the trench to 5 2 km s 1 at the shelf break overlying a 10 km thick lower part with velocities of 6 0 km s 1 analysis of the gravity field requires densities of 2 6 2 7 g cm 3 for the upper part and 2 9 g cm 3 for the lower part of this unit these velocities and densities suggest that the margin wedge is composed of ophiolitic rock similar to those sampled offshore guatemala and exposed on the nicoya peninsula costa rica the velocity structure of this ophiolitic unit ends about 50 km offshore landward the basement underneath the sandino basin is formed by older igneous rock drilled beneath upper cretaceous sedimentary rock onshore nicaragua beneath the ophiolitic basement we find a sliver with velocities typical of mantle material that begins at depths of 12 18 km and coincides with the down dip limit of the seismogenic zone mantle densities are required for the sliver to match the gravity data in a tectonic reconstruction the suture of an oceanic plateau on the farallon plate against the chortis block in upper cretaceous time is suggested suturing left the former trench and margin in deep water consistent with the late cretaceous to palaeocene deep water sediment of the rivas formation at the base of the sandino basin suture of the farallon plate and chortis block might have initiated the strike slip movement along the motagua polochic fault system the development of a new subduction zone in eocene oligocene times left the ophiolitic basement and a sliver of oceanic mantle attached to the chortis block and shifted the volcanic arc about 70 km southwestwards close to its present position

centered composite analysis of variations associated with the madden julian oscillation abstract centered composite analysis is described and applied to gain a better understanding of the initial phases of the madden julian oscillation mjo centered composite analysis identifies the dates and central locations of key events the elements of the composite means are centered on these central locations before averages are calculated in this way much of the spatial fuzziness which is inherent in traditional composite analysis is removed the results for the mjo based on mjo filtered outgoing longwave radiation for the reference data and 40 yr ecmwf re analysis era 40 and ncep ncar reanalysis products for the composites show highly significant composites of unfiltered data for not only zero lag but also lags back to 20 days before the target events these composites identify propagating patterns of surface pressure upper and lower troposphere zonal winds surface temperature and 850 hpa specific humidity associated with mjo convective events in the indian ocean the propagation chara

influence of the subducting plate velocity on the geometry of the slab and migration of the subduction hinge abstract geological observations indicate that along two active continental margins east asia and mediterranean major phases of overriding plate extension resulting from subduction hinge retreat occurred synchronously with a reduction in subducting plate velocity in this paper results of fluid dynamical experiments are presented to test the influence of the velocity of the subducting plate on the hinge migration velocity and on the geometry of the slab results show that hinge retreat decreases with increasing subducting plate velocity in addition phases of hinge retreat alternate with phases of hinge advance for relatively high subducting plate velocities due to interaction of the slab with the bottom of the box simulating the upper lower mantle discontinuity such slab kinematics could explain the episodic behaviour of back arc opening observed in convergent settings the geometry of the slab and the kinematics of subduction are significantly affected by the velocity of the subducting plate three subduction modes with accompanying slab geometry can be recognized a relatively low subducting plate velocity is accompanied by relatively fast hinge retreat with backward sinking of the slab and a backward draping slab geometry with increasing subducting plate velocity hinge migration is relatively small resulting in subvertical sinking of the slab and a folded slab piling geometry for a very high velocity the hinge migrates forward resulting in forward oriented subduction vectors and a forward draping slab geometry

effects of monsoon trough intraseasonal oscillation on tropical cyclogenesis over the western north pacific abstractthe effects of intraseasonal oscillation iso of the western north pacific wnp monsoon trough on tropical cyclone tc formation were investigated using the advanced research weather research and forecasting arw model a weak vortex was specified initially and inserted into the background fields containing climatological mean anomalies associated with active and inactive phases of monsoon trough isos the diagnosis of simulations showed that monsoon trough iso can modulate tc development through both dynamic and thermodynamic processes the dynamic impact is attributed to the lower midtropospheric large scale vorticity associated with monsoon trough iso interactions between cyclonic vorticity in the lower middle troposphere during the active iso phase and a vortex lead to the generation of vortex scale outflow at the midlevel which promotes the upward penetration of friction induced ascending motion and thus upward moisture transport in addition the low level convergence associated with ac

the influence of the mjo on upstream precursors to african easterly waves abstractthe madden julian oscillation mjo produces alternating periods of increased and reduced precipitation and african easterly wave aew activity in west africa this study documents the influence of the mjo on the west african monsoon system during boreal summer using reanalysis and brightness temperature fields mjo related west african convective anomalies are likely induced by equatorial kelvin and rossby waves generated in the indian ocean and west pacific by the mjo which is consistent with previous studies the initial modulation of tropical african convection occurs upstream of west africa near the entrance of the african easterly jet aej previous studies have hypothesized that an area to the east of lake chad is an initiation region for aews called the trigger region in this study this area exhibits significant intraseasonal convection and wave activity anomalies prior to the wet and dry mjo phases in the west african monsoon region in the trigger region cold tropospheric tempera

a climatology of central american gyres abstractcentral american gyres cags are large closed cyclonic circulations that occur during the rainy season may november which can yield exceptional rainfall leading to catastrophic flooding and large societal impacts a reanalysis based climatology of cags is developed from an algorithm that distinguishes cag cases from other systems this algorithm identified cag cases based on circulation intensity a broad radius of maximum winds and the existence of closed earth relative cyclonic flow based on these criteria 47 cag cases were identified from 1980 to 2010 featuring a bimodal distribution of cases with maxima in may june and september november cag cases are composited into two categories based on their upper tropospheric pv structure nonbaroclinic cags are more common n 42 and characterized by an upper tropospheric anticyclone while baroclinic cags are less common n 5 and characterized by an upper tropospheric trough whereas a nonbaroclinic cag has anomalous moisture and preci

diagnosis of the mjo modulation of tropical cyclogenesis using an empirical index the modulation of tropical cyclone activity by the madden julian oscillation mjo is explored using an empirical genesis potential gp index composite anomalies of the genesis index associated with the different mjo phases are consistent with the composite anomalies in tc genesis frequency that occur in the same phases indicating that the index captures the changes in the environment that are at least in part responsible for the genesis frequency changes of the four environmental variables that enter the genesis potential index the midlevel relative humidity makes the largest contribution to the mjo composite gp anomalies the second largest contribution comes from the low level absolute vorticity and only very minor contributions come from the vertical wind shear and potential intensity when basin integrated mjo composite anomalies of the gp index are regressed against basin integrated composite anomalies of tc genesis frequency the results differ quantitatively from those obtained from the analogous calculation performed on the annual climatologies in the two quantities the gp index captures the mjo modulation of tc genesis to a lesser degree than the climatological annual cycle of genesis to which it was originally tuned this may be due to weaknesses of the reanalysis or indicative of the importance of precursor disturbances not well captured in the gp index computed from weekly data to the intraseasonal tc genesis frequency fluctuations

global association of the madden julian oscillation with monsoon lows and depressions previous research has revealed that monsoon lows and depressions are modulated on intraseasonal time scales in a few regions including india australia and the east pacific this study examines whether such modulation occurs on a global scale and in particular how the madden julian oscillation mjo is associated with changes in synoptic scale vortices across all monsoon regions the spatial climatology of monsoon disturbances is largely insensitive to mjo amplitude however monsoon disturbance frequency mdf varies substantially with mjo phase with regional perturbations of 25 to 90 percent of the seasonal mean value across the tropics in off equatorial locations mdf maxima occur in locations where the mjo enhances low level cyclonic vorticity typically near the western edge of mid level moisture perturbations in contrast equatorial mdf perturbations are in phase with mjo moisture and rainfall anomalies with maxima in regions with strong low level zonal wind convergence

mjo related sst variations over the tropical eastern pacific during northern hemisphere summer abstract the madden julian oscillation mjo significantly modulates eastern tropical pacific sea surface temperatures ssts during northern hemisphere summer typical sst variations during an mjo life cycle are 0 4 0 5 c considerably higher variations occur with the strongest events 1 2 c the magnitudes of these variations are comparable to those associated with west pacific mjo variability during nh winter sst anomalies to the north of 4 n where the strongest modulation of east pacific convection by the mjo occurs are 180 out of phase with equatorial anomalies these sst variations are responsible for variations in surface saturation equivalent potential temperatures of greater than 3 c the highest ssts lead maximum convection by about 10 days and ssts begin to cool with the onset of enhanced convection off equatorial sst anomalies are most likely due to variations in surface latent heat flux although shortwave flux anomalies are important in the heart of the mjo convective region decrea

roles of the synoptic scale wave train the intraseasonal oscillation and high frequency eddies in the genesis of typhoon manyi 2001 abstractexperiments using the weather research and forecasting wrf model were conducted to investigate the effects of multiscale motions on the genesis of typhoon manyi 2001 in the western north pacific the precursor signal associated with this typhoon genesis was identified as a northwest southeast oriented synoptic scale wave train swt the model successfully simulated the genesis of the typhoon in the wake of the swt further experiments were conducted to isolate the effects of the swt the intraseasonal oscillation iso and high frequency shorter than 3 days eddies in the typhoon formation removing the swt in the initial and boundary conditions eliminates the typhoon genesis this points out the importance of the swt in the typhoon genesis it was noted that the swt strengthened the wake cyclone through southeastward energy dispersion the strengthening wake cyclone triggered multiple episodes of strong sustained convective updrafts leading to aggregation of vertical vorticity and formatio

mixed layer modeling in the east pacific warm pool during 2002 two vertical mixing models the modified dynamic instability model of price et al pwp and k profile parameterizaton kpp are used to analyze intraseasonal sea surface temperature sst variability in the northeast tropical pacific near the costa rica dome during boreal summer of 2002 anomalies in surface latent heat flux and shortwave radiation are the root cause of the three intraseasonal sst oscillations of order 1 c amplitude that occur during this time although surface stress variations have a significant impact on the third event a slab ocean model that uses observed monthly varying mixed layer depths and accounts for penetrating shortwave radiation appears to well simulate the first two sst oscillations but not the third the third oscillation is associated with small mixed layer depths 5 m forced by and acting with weak surface stresses and a stabilizing heat flux that cause a transient spike in sst of 2 c intraseasonal variations in freshwater flux due to precipitation and diurnal flux variability do not significantly impact these intraseasonal oscillations these results suggest that a slab ocean coupled to an atmospheric general circulation model as used in previous studies of east pacific intraseasonal variability may not be entirely adequate to realistically simulate sst variations further while most of the results from the pwp and kpp models are similar some important differences that emerge are discussed

systematic variation of summertime tropical cyclone activity in the western north pacific in relation to the madden julian oscillation abstract the variability of observed tropical cyclone tc activity i e genesis track and landfall in the western north pacific wnp is examined in relation to the various categories of the madden julian oscillation mjo during summer june september for the period 1979 2004 the mjo categories are defined based on the empirical orthogonal function analysis of outgoing longwave radiation data the number of tcs increases when the mjo related convection center is located in the wnp the axis of a preferable genesis region systematically shifts like a seesaw in response to changes in the large scale environments associated with both the eastward and northward propagation of the mjo and the intraseasonal variability of the wnp subtropical high furthermore the authors show that the density of tc tracks in each mjo category depends on the systematic shift in the main genesis regions at first order also the shift is affected by the prevailing large scale steering flows in each mjo category when th

propagation of the madden julian oscillation through the maritime continent and scale interaction with the diurnal cycle of precipitation the convectively active part of the madden julian oscillation mjo propagates eastward through the warm pool from the indian ocean through the maritime continent the indonesian archipelago to the western pacific the maritime continent s complex topography means the exact nature of the mjo propagation through this region is unclear model simulations of the mjo are often poor over the region leading to local errors in latent heat release and global errors in medium range weather prediction and climate simulation using 14 northern winters of trmm satellite data it is shown that where the mean diurnal cycle of precipitation is strong 80 of the mjo precipitation signal in the maritime continent is accounted for by changes in the amplitude of the diurnal cycle additionally the relationship between outgoing long wave radiation olr and precipitation is weakened here such that olr is no longer a reliable proxy for precipitation the canonical view of the mjo as the smooth eastward propagation of a large scale precipitation envelope also breaks down over the islands of the maritime continent instead a vanguard of precipitation anomalies of 2 5 mm day 1 over 10 6 km 2 jumps ahead of the main body by approximately 6 days or 2000 km hence there can be enhanced precipitation over sumatra borneo or new guinea when the large scale mjo envelope over the surrounding ocean is one of suppressed precipitation this behaviour can be accommodated into existing mjo theories frictional and topographic moisture convergence and relatively clear skies ahead of the main convective envelope combine with the low thermal inertia of the islands to allow a rapid response in the diurnal cycle which rectifies onto the lower frequency mjo hence accurate representations of the diurnal cycle and its scale interaction appear to be necessary for models to simulate the mjo successfully

neotectonic development of western nicaragua this study presents a reconstruction of the neotectonic evolution of western nicaragua by means of structural analysis of outcrop scale joints faults and bedding planes integrated with macroscale structures the results indicate three clearly separable deformation phases which are closely related to specific volcanic events and account for the major tectonic features of nicaragua the first deformation phase was a late miocene to early pliocene shortening that produced the pacific plain anticlines and controlled the contemporaneous extrusion of the el coyol group volcanic rocks the second phase active between the pliocene and the pleistocene resulted in extensional structures such as the 500 km long and 50 km wide nw se trending nicaragua depression this phase was accompanied by a gradual trenchward migration of the volcanic activity responsible for the formation of the las sierras group the third deformation phase is still active and is responsible for the present shallow focus seismic activity associated with n s trending volcanic vents and the n s managua pull apart graben the neotectonic development of western nicaragua as interpreted here is closely related to the contemporaneous evolution of the neighboring areas of guatemala south of the jocotan fault honduras el salvador and costa rica north of the valle central the valle central of costa rica represents the southern boundary of this structurally homogeneous domain and may indicate the southern boundary of the neogene chortis block changes in deformation style and the trenchward migration of volcanism which occurred during the pliocene may indicate an increase in the subduction angle of the cocos plate and may also be correlated to pliocene shortening of jamaica and hispaniola

mjo and tropical cyclogenesis in the gulf of mexico and eastern pacific case study and idealized numerical modeling the role of the madden julian oscillation mjo in modulating the frequency and location of tropical cyclogenesis over the eastern pacific and the gulf of mexico during august september 1998 is examined during the nonconvective phase of the mjo convection and low level cyclonic vorticity occurred primarily in conjunction with the intertropical convergence zone itcz during the convective phase convection low level cyclonic vorticity and convergence expanded into the northeastern pacific and the gulf of mexico this was accompanied by enhanced eddy kinetic energy and barotropic energy conversions as compared to the nonconvective phase consistent with previous research during the nonconvective phase of the mjo vertical shear was relatively weaker but tropical cyclones tended to form mainly within the itcz on the contrary during the convective phase vertical wind shear exceeded 10 m s 1 over much of this region and tropical cyclone development occurred north of the itcz near the mexican pacific coast and the gulf of mexico idealized numerical experiments are conducted using a barotropic model with time invariant basic states representative of the nonconvective and convective phases of the mjo the simulations indicate that the propagation paths as well as the amplification of the eddies differ substantially between the two phases during the nonconvective phase the waves tend to propagate westward into the eastern pacific during the convective phase stronger southerlies steer the waves into the gulf of mexico the mjo related modulation of tropical cyclogenesis in the eastern pacific and gulf of mexico thus appears to involve anomalous convergence cyclonic vorticity vertical wind shear and differing tracks of easterly waves

genesis of typhoon chanchu 2006 from a westerly wind burst associated with the mjo part i evolution of a vertically tilted precursor vortex abstract although previous studies have shown the relationship between the madden julian oscillation mjo and tropical cyclogenesis tcg many scale interactive processes leading to tcg still remain mysterious in this study the larger scale flow structures and evolution during the pregenesis genesis and intensification of typhoon chanchu 2006 near the equator are analyzed using ncep s final analysis satellite observations and 11 day nested numerical simulations with the advanced research weather research and forecast model arw wrf results show that the model could reproduce the structures and evolution of a synoptic westerly wind burst wwb associated with the mjo during the genesis of chanchu including the eastward progression of a wwb from the indian ocean into the pacific ocean the modulation of the associated quasi symmetric vortices the initial slow spinup of a northern pre chanchu disturbance at the northeastern periphery of the wwb and its general track and intensification it i

the leading edge of the greater himalayan crystalline complex revealed in the nw indian himalaya implications for the evolution of the himalayan orogen the three himalayan lithologic units the lesser himalayan sequence the greater himalayan crystalline complex and the tethyan himalayan sequence have a specific structural correlation with the main central thrust and south tibet detachment in the central himalaya there the main central thrust places the greater himalayan crystalline complex over the lesser himalayan sequence and the south tibet detachment places the tethyan himalayan sequence over the greater himalayan crystallines although this division has formed the basis for all himalayan tectonic models it fails to explain aspects of the geology of the western himalaya where the main central thrust places the tethyan himalayan sequence directly above the lesser himalayan sequence our mapping in nw india shows that this relationship results from southward merging of the main central thrust and south tibet detachment this finding in conjunction with observed alternating shear senses on the south tibet detachment is inconsistent with the wedge extrusion and erosion induced channel flow models both require only top to the n motion on the south tibet detachment but is consistent with a tectonic wedging model

propagating versus nonpropagating madden julian oscillation events abstractbasinwide convective anomalies over the indian ocean io associated with the madden julian oscillation mjo sometimes propagate eastward and reach the west pacific wp but sometimes do not long term observations and reanalysis products are used to investigate the difference between the propagating and nonpropagating mjo events io convection onset events associated with the mjo are grouped into three categories based on the strengths of the simultaneous dry anomalies over the eastern maritime continent and wp the io convection anomaly preferentially makes eastward propagation and reaches the wp when the dry anomaly is stronger analysis of the column integrated moist static energy mse budget shows that horizontal advection moistens the atmosphere to the east of the positive mse anomaly associated with the active convection over the io and is of sufficient magnitude to explain the eastward propagation of the positive mse anomaly interpretation is complicated however by lack of closure in

climate modulation of north atlantic hurricane tracks the variability of north atlantic tropical storm and hurricane tracks and its relationship to climate variability is explored tracks from the north atlantic hurricane database for the period 1950 2007 are objectively separated into four groups using a cluster technique that has been previously applied to tropical cyclones in other ocean basins the four clusters form zonal and meridional separations of the tracks the meridional separation largely captures the separation between tropical and more baroclinic systems while the zonal separation segregates gulf of mexico and cape verde storms general climatologies of the seasonality intensity landfall probability and historical destructiveness of each cluster are documented and relationships between cluster membership and climate variability across a broad spectrum of time scales are identified composites with respect to cluster membership of sea surface temperature and other environmental fields show that regional and remote modes of climate variability modulate the cluster members in substantially

the managua graben and las sierras masaya volcanic complex nicaragua pull apart localization by an intrusive complex results from analogue modeling abstract there is a well documented association between pull apart basins in strike slip zones and large volcanic caldera complexes las sierras masaya volcanic complex nicaragua is a large basaltic lava and ignimbrite shield with nested calderas the widest caldera is partly underlain by a large and dense intrusive complex evidenced by a positive gravity anomaly the las sierras caldera relating to this cumulate complex is less than 30 000 years old and is probably still hot and ductile the inner masaya caldera hosts an active magmatic system the volcanic complex is in a dextral transtensional tectonic context of the nicaraguan depression the highly active managua graben is on the northern part of the volcano we speculate that the graben and volcano are linked tectonically with the graben initiating in response of a regional stress field modified around the dense ductile intrusive complex previous field work and reappraisal of structures with digital elevation model morphological analysis show that the volcano is surrounded by a rhombic fault pattern that may form a nascent pull apart basin we have done scaled analogue models to test the effect of intrusion density and rheology anomalies on strike slip fault geometries the models show that intrusion density variations alone do not significantly change fault patterns in contrast ductile rocks silicone in the models as analogue for hot mafic intrusive rocks markedly alter strike slip fault patterns in transtension the presence of a ductile intrusion causes the formation of a pull apart while in pure strike slip or transpression uplift and thrusting is generated pull apart and uplift structures are rhomb shaped even when the ductile inclusion is circular we conclude that a pull apart is developing at las sierras masaya volcanic complex in response to the transtensive regional deformation regime and the dense ductile intrusive complex we suggest that the volcano and graben are one dynamic system and should be monitored as one entity

tectonic interpretation of the morphology of honduras the morphotectonics of honduras have been studied using existing geologic and topographic maps and imaging done from satellites and the following interpretations have been made during the late oligocene to early miocene a wide strike slip boundary existed in western honduras and eastern guatemala and produced east west trending basins containing the coarse clastic rocks of the subinal formation marine geophysical studies of the continental shelf of honduras show that during the rest of the miocene and the early pliocene the boundary was inactive but motion resumed in the late pliocene to pleistocene and produced horsts on the north coast of honduras in northwestern honduras the east northeast trending alignment of the jocotan copan and charnelecon river valleys may result from crustal flexuring rather than from faulting as previously proposed because the postulated fault the jocotan chamelecon fault cannot be reconciled with available geologic maps the northwest and northeast flowing rivers of central and eastern honduras define domains of strike slip faulting that have probably rotated 30 from an original set of conjugate fractures widespread deposition of conglomerates suggests that strike slip movements began in the mid cretaceous and reoccurred at intervals culminating in the development of pull apart basins in western honduras during the late pliocene at the present time no further strike slip movement is possible as the faults defining each domain lie parallel and perpendicular to the middle america trench new conjugate fractures however cut honduras along north south and east northeast trends and have also separated into domains in the domain of north south fractures easterly directed ductile extension of the lithosphere beneath western honduras and guatemala is opening up rifts but the domain of east northeast fractures is characterized by strike slip motion that has produced left lateral pull aparts in central honduras early workers identified no rift valley extending from the gulf of fonseca to the caribbean and there is no fresh evidence upon which to base the existence of one today the notion of the honduras depression which has persisted over the last half century stems from misinterpretation of the maps and writings of karl sapper

the itcz in the central and eastern pacific on synoptic time scales the itcz in the central and eastern pacific on synoptic time scales is highly dynamic the active season extends roughly from may through october during the active season the itcz continuously breaks down and re forms and produces a series of tropical disturbances the life span of the itcz varies from several days to 3 weeks sixty five cases of itcz breakdown have been visually identified over five active seasons 1999 2003 in three independent datasets itcz breakdown can be triggered by two mechanisms 1 interaction with westward propagating disturbances wpds and 2 the vortex rollup vr mechanism results show that the frequency of occurrence of itcz breakdown from these two mechanisms is the same the vr mechanism may have been neglected because the produced disturbances are rather weak and they may dissipate quickly the itcz shows a strong tendency to re form within 1 2 days in the same location the itcz may break down via the vr mechanism without any other support and thus it may continuously generate numerous tropical disturbances throughout the season there are two main differences between the two mechanisms 1 the wpds induced itcz breakdown tends to create one or two vortices that may be of tropical depression strength the vr induced itcz breakdown generates several nearly equal sized weak disturbances 2 the wpds tend to disturb the itcz in the eastern pacific only disturbances generally move along the mexican coast after shedding off from the itcz and do not further disturb the itcz in the central pacific therefore the vr mechanism is observed more clearly and is the dominating mechanism for itcz breakdown in the central pacific

the influence of the madden julian oscillation on tropical cyclone activity in the fiji region abstract this study examines the modulation of tropical cyclone tc activity by the madden julian oscillation mjo in the fiji samoa and tonga regions fst region using joint typhoon warning center best track cyclone data and the mjo index developed by wheeler and hendon results suggest strong mjo tc relationships in the fst region the tc genesis patterns are significantly altered over the fst region with approximately 5 times more cyclones forming in the active phase than in the inactive phase of the mjo this modulation is further strengthened during el nino periods the large scale environmental conditions i e low level relative vorticity upper level divergence and vertical wind shear associated with tc genesis show a distinct patterns of variability for the active and inactive mjo phases the mjo also has a significant effect on hurricane category and combined gale and storm category cyclones in the fst region the occurrences of both these cyclone categories are increased in the active

thermochemical structure and dynamics of the african superplume 1 we present a new three dimensional 3 d model of the thermochemical structure of the african superplume region obtained from simultaneous inversion of global seismic and convection related data convection related observations include the global free air gravity field tectonic plate motions dynamic surface topography and the excess ellipticity of the core mantle boundary a 3 d image of the chemically induced density perturbations provides direct evidence that intrinsically dense material is entrained within the superplume and concentrated into a rounded structure 1000 km above the core mantle boundary the thermally induced density perturbations are greater in magnitude than the chemically induced implying overall positive buoyancy throughout the superplume the observed morphology and density signatures are consistent with a thermochemical plume that has risen from a compositionally distinct pile at the base of the mantle and may be currently deforming under the influence of its intrinsic negative chemical buoyancy citation simmons n a a m forte and s p grand 2007 thermochemical structure and dynamics of the african superplume geophys res lett 34 l02301 doi 10 1029 2006gl028009

the role of interactions between multiscale circulations on the observed zonally averaged zonal wind variability associated with the madden julian oscillation abstractthe mechanisms driving the upper tropospheric zonal mean intraseasonal zonal wind associated with the madden julian oscillation are examined through budget analysis during boreal winter to diagnose the role of nonlinear and cross scale interactions the wind fields are decomposed into three temporal bands including the intraseasonal time scale 30 100 days and periods shorter and longer than the intraseasonal time scales the intraseasonal zonal mean circulation and its driving mechanisms are first examined based on the leading eofs of the intraseasonal zonal wind consistent with previous studies of intraseasonal atmospheric angular momentum the upper troposphere zonal mean intraseasonal zonal wind anomaly begins in the tropics and propagates poleward results show that interaction between the background state and intraseasonal time scale zonally symmetric and asymmetric circulation helps drive changes in the tropical intraseasonal zonal wind and its poleward propagation the intraseasonal a

triggering of destructive earthquakes in el salvador we investigate the existence of a mechanism of static stress triggering driven by the interaction of normal faults in the middle american subduction zone and strike slip faults in the el salvador volcanic arc the local geology points to a large strike slip fault zone the el salvador fault zone as the source of several destructive earthquakes in el salvador along the volcanic arc we modeled the coulomb failure stress cfs change produced by the june 1982 and january 2001 subduction events on planes parallel to the el salvador fault zone the results have broad implications for future risk management in the region as they suggest a causative relationship between the position of the normal slip events in the subduction zone and the strike slip events in the volcanic arc after the february 2001 event an important area of the el salvador fault zone was loaded with a positive change in coulomb failure stress 0 15 mpa this scenario must be considered in the seismic hazard assessment studies that will be carried out in this area

the madden julian oscillation s influence on african easterly waves and downstream tropical cyclogenesis abstractthe influence of the madden julian oscillation mjo over tropical africa and atlantic is explored during the northern hemisphere summer months the mjo is assessed by using real time multivariate mjo rmm indices these indices divide the active convective signal of the mjo into 8 phases convection associated with the mjo is enhanced over tropical africa during rmm phases 8 1 and 2 convection becomes suppressed over tropical africa during the subsequent rmm phases phases 3 7 african convective signals are associated with westward propagating equatorial rossby waves the mjo modulates african easterly wave aew activity aew activity is locally enhanced during rmm phases 1 3 and suppressed during rmm phases 6 8 enhanced aew activity occurs during periods of enhanced convection over tropical africa consistent with stronger or more frequent triggering of aews as well as more growth associated with latent heat release enhanced aew activity occurs during the low level westerly wind phase of

gps measurement of relative motion of the cocos and caribbean plates and strain accumulation across the middle america trench global positioning system gps measurements in 1988 and 1991 on cocos island cocos plate san andres island caribbean plate and liberia caribbean plate mainland costa rica provide an estimate of relative motion between the cocos and caribbean plates the data for cocos and san andres islands both located more than 400 km from the middle america trench define a velocity that is equivalent within two standard errors 7 mm yr rate 5 degrees azimuth to the nuvel 1 plate motion model the data for liberia 120 km from the trench define a velocity that is similar in azimuth but substantially different in rate from nuvel 1 the discrepancy can be explained with a simple model of elastic strain accumulation with a subduction zone that is locked to a relatively shallow 20 z 5 km depth

genesis of super cyclone pam 2015 modulation of low frequency large scale circulations and the madden julian oscillation by sea surface temperature anomalies abstractsuper cyclone pam 2015 formed in the central tropical pacific under conditions that included el nino modoki and the passage of a convectively enhanced phase of the madden julian oscillation mjo in the western pacific this study examines the influence that sea surface temperature anomalies sstas have on the mjo and low frequency large scale circulation and establishes how they modulated the genesis of pam two series of numerical experiments were conducted by using a nonhydrostatic global atmospheric model with observed obssst and climatological clmsst ssts the results suggested that low frequency westerly winds at 850 hpa u850 were intensified in the central tropical pacific due to the observed ssta the amplitude of the mjo simulated in obssst was larger than in clmsst in addition the experiments initialized 26 february 2 march exhibited that the phase of the mjo in obssst was ahead of that in clmsst and that the genesis location in obssst was 10 to the east of that in clmsst

links between tropical cyclone activity and madden julian oscillation phase in the north atlantic and northeast pacific basins abstract the leading intraseasonal mode of atmospheric and oceanic variability the madden julian oscillation mjo influences tropical and extratropical sea level pressure temperature divergent and rotational wind components moisture and deep convection as a 40 to 50 day oscillation the mjo is also known to influence tropical phenomena including tropical cyclone tc activity in various tc basins the links between the mjo and multiple measures of tc activity including genesis landfall and an integrative accumulated cyclone energy ace index were quantified for multiple tc formation basins across the western hemisphere including the north atlantic and northeast pacific ocean and subbasins for the period 1978 2006 using this relatively long 29 yr tc dataset and employing an upper tropospheric mjo diagnostic that is physically meaningful over the entire western hemisphere this study extends existing research on the relationships between the mjo and tcs the noaa climate prediction cente

eastward propagating intraseasonal oscillation represented by chikira sugiyama cumulus parameterization part i comparison with observation and reanalysis abstractthe eastward propagating intraseasonal oscillation represented by the chikira sugiyama cumulus parameterization in an atmospheric general circulation model is compared with observations and reanalyses the scheme is characterized by state dependent entrainment rates that vertically vary affected by the environment the zonal wavenumber frequency power spectrum shows a strong signal corresponding to the madden julian oscillation the eastward propagating feature of the convective region and accompanying anomalous zonal wind structure is well extracted by the first and second modes of the combined empirical orthogonal function ceof with a reasonable explained variance for the first mode though the second mode is not sufficiently reproduced the basic features of the composited anomalous fields including moisture temperature and vertical and zonal winds resemble those of the reanalysis in both the free troposphere and surface air the anomalous free tropospheric moisture exhibits its westward ti

variation of tropical cyclone activity in the south indian ocean el nino southern oscillation and madden julian oscillation effects 1 the present study examines variation of tropical cyclone tc activity in the south indian ocean sio during tc seasons december march for the period 1979 2004 the impact of the el nino southern oscillation and the madden julian oscillation mjo on the variation is revealed through a composite analysis during el nino periods tc genesis was shifted westward enhancing the formation west of 75 e and reducing it east of 75 e these changes in the genesis correspond to a westward shift of convection it may be explained by a remote effect on the sio that is the increase in sea surface temperature in the central eastern pacific alters the walker circulation and forms an anomalous anticyclonic circulation in the east sio during el nino the spatial difference in tc passages between el nino and la nina shows a significant decrease to the southeast of madagascar but a moderate increase in the central midlatitude sio indicating that tcs move farther east during el nino this change is possibly due to the anomalous southwesterlies east of madagascar variation of tc activity also depends on various mjo phases frequent tc passages for phases 2 4 strong convective activity straddles along the equatorial indian ocean versus infrequent tc passages for other phases tc tracks tend to be more south oriented in phase 3 compared with those in phases 2 and 4 this is possibly caused by the increased steering northerlies which are a part of the anticyclonic rossby wave of gill type in response to the suppressed mjo related convection in the maritime continent

seismic cycle and plate margin deformation in costa rica gps observations from 1994 to 1997 global positioning system gps observations in costa rica from 1994 to 1997 reveal a complex pattern of motion consistent with the superposition of seismic cycle and secular plate margin deformation in the south velocity vectors are consistent with motion of the panama block plus postseismic deformation following the 1991 limon earthquake and interseismic strain due to partial locking of the middle america trench mat thrust in the northwest sites west of the volcanic arc are moving to the nw as a forearc sliver superimposed on this sliver motion are vertical and horizontal interseismic deformations from the adjacent nicoya segment of the mat we apply two different inverse methods to understand the source of the seismic strain in nw costa rica we compare fault locking models derived using a singular value decomposition inversion with that of a simulated annealing global optimization approach both methods yield similar models for partial locking of the thrust interface beneath the nicoya peninsula our results define an area of nearly fully locked fault beneath the outer coast of the southern portion of the peninsula with somewhat lower coupling beneath the northern half and with low coupling elsewhere these initial results show the promise for detailed imaging of the locked portion of a thrust interface responsible for future large subduction zone earthquakes

clustering of eastern north pacific tropical cyclone tracks enso and mjo effects 1 a probabilistic clustering technique is used to describe tropical cyclone tracks in the eastern north pacific on the basis of their shape and location the best track data set is decomposed in terms of three clusters these clusters are analyzed in terms of genesis location trajectory landfall intensity seasonality and their relationships with the el nino southern oscillation enso and madden julian oscillation mjo longitudinal track location plays a strong discriminating role in the regression mixture model s solution with the average track orientation becoming more zonal toward the west this progression encapsulates well the relationship between tropical cyclones over the eastern tropical pacific and the mjo or enso two of the clusters describe tropical cyclones tcs with tracks that lie near the coast of mexico and central america the most frequent cluster contains tracks that trend west northwestward while the second most frequent one has genesis locations that lie slightly to the southeast of those in the most frequent cluster and tracks that run typically parallel to the central american coast this second cluster is shown to be significantly associated with the westerly phase of the mjo the third least frequent cluster contains tcs with westward trajectories over the central and eastern equatorial pacific some of these tcs have an impact on hawaii and other islands as far as the central and western north pacific regions the least frequent cluster is strongly related to enso while the others are not it occurs significantly more frequently during el nino conditions examination of the large scale patterns of atmospheric circulation and sea surface temperature associated with each of our three clusters are consistent with previous studies anomalous low level westerly zonal winds from the monsoon trough and mjo meet anomalous easterlies near the region of genesis in each cluster

on the westward turning of hurricane sandy 2012 effect of atmospheric intraseasonal oscillations abstracthurricane sandy 2012 experienced an unusual westward turning and made landfall in new jersey after its northward movement over the atlantic ocean the landfall caused severe casualties an

modulation of hurricane activity in the gulf of mexico by the madden julian oscillation the madden julian oscillation mjo is a large scale episodic modulation of tropical winds and precipitation that travels eastward from asia to america with a characteristic repeat time of 30 to 60 days here it is shown that when mjo wind anomalies in the lower troposphere of the eastern pacific are westerly gulf of mexico and western caribbean hurricane genesis is four times more likely than when the mjo winds are easterly accurate predictions of the mjo may lead to improved long range forecasts of tropical cyclone activity

on wind convection and sst variations in the northeastern tropical pacific associated with the madden julian oscillation abstract lagged maximum covariance analysis lmca is used to examine the intraseasonal variability of zonal wind sea surface temperature sst and outgoing longwave radiation olr in the northeastern tropical pacific ocean during northern hemisphere summertime the analysis shows a strong temporal asymmetry in that wind and convection anomalies lead to sst anomalies but sst anomalies are not followed by comparably strong wind and convection anomalies this suggests that sst anomalies associated with the mjo in the northeastern tropical pacific are largely subject to atmospheric variability

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

madden julian oscillation in a climate model with a well resolved stratosphere 1 this work evaluates how well a coupled ocean atmospheric climate model with a well resolved stratosphere captures the observed madden julian oscillation mjo signal in the upper troposphere and lower stratosphere the model is the centro euro mediterraneo per i cambiamenti climatici cmcc coupled modeling system cmcc cms with 95 atmospheric levels cmcc cms produces mjo composites of precipitation that are very similar to those of the observations and the era 40 reanalysis product furthermore model precipitation is found to propagate eastward near the observed speed there is also strong agreement in the tropics and subtropics in the temperature and flows at 100 and 200 hpa upon comparing the composites based on the full 340 years of model output and those for the past 20 years of the model output with those of the era 40 reanalysis at the 100 hpa level the temperature anomalies in the model output and reanalysis propagate eastward at about 7 m s 1 at the equator 25 n and 65 n longitude and height cross sections at 25 n suggest vertically propagating rossby waves up to near the 10 hpa level in both the model and observations latitude and height cross sections of temperature and zonal and meridional winds indicate strong vertical propagation in the tropics in both the model and reanalysis overall the cmcc cms model shows mjo characteristics in the tropical and subtropical troposphere and lower stratosphere that are in very good agreement with observed analyses

modulation of tropical cyclogenesis location and frequency over the indo western north pacific by the intraseasonal indo western pacific convection oscillation during the boreal extended summer abstractthe influence of the intraseasonal indo western pacific convection oscillation ipco on tropical cyclone tc genesis location and frequency over the indo western north pacific wnp during the boreal extended summer may october is explored observational analysis shows that the impacts of the intraseasonal ipco on tcs over the indo wnp include an evident phase lock of tc genesis location and distinct differences in tc frequency in the wnp in the positive intraseasonal ipco phase the atmosphere gains heat through the release of latent heat in cumulus convective condensation and the anomalous cyclonic circulation weakens the western pacific subtropical high wpsh and enhances tc genesis thereby tending to produce many more tcs moreover the diminished wpsh and the westward shift of the centers of anomalous cyclonic circulations lock tc genesis locations to the west wnp and lower latitudes around 5 20 n especially in the south china sea the almost opposite situation occurs in a nega

extension induced by volcanic loading in regional strike slip zones volcanoes and strike slip pull apart structures are commonly found together we use theoretical predictions and analogue modeling to evaluate the effect of volcanic loading in strike slip zones and the converse effect of regional strike slip faults on volcano structure theoretical predictions indicate that regional stress is reoriented by volcano loading to cause extension analogue models indicate that volcanoes in strike slip zones develop extensional pull apart structures a graben typically forms normal to the minimum regional stress axis 3 at a cone cones emplaced onto preexisting faults increase that fault9s tensional component if the substratum is weak enough to permit volcano spreading thrusts form normal to the maximum stress axis 1 a feedback situation can arise i e loading related extension allows more magma influx more eruption and thus increased loading this process can create large volcano tectonic depressions volcano structures may also be used as indicators of regional strike slip movements

frictional moisture convergence in a composite life cycle of the madden julian oscillation abstract a composite life cycle of the madden julian oscillation mjo is constructed using an index based on the first two eofs of the bandpass filtered 20 80 days 850 mb zonal wind averaged from 5 n to 5 s every 2 5 around the equator precipitation 1000 mb convergence 850 mb wind and 200 mb wind are composited for the period 1979 95 water vapor integrated from the surface to 300 mb is composited for the period 1988 92 frictional moisture convergence at the equator is shown to play an important role in the life cycle of the madden julian oscillation mjo regions of boundary layer convergence foster growth of positive water vapor anomalies to the east of convection this convergence coincides with 850 mb easterly wind anomalies as is consistent with kelvin wave dynamics drying of the atmosphere occurs rapidly after the passage of convection with the onset of 850 mb westerly perturbations possible mechanisms for this drying include boundary layer divergence and subsidence or horizontal advect

el nino southern oscillation the madden julian oscillation and atlantic basin tropical cyclone rapid intensification 1 both el nino southern oscillation enso and the madden julian oscillation mjo have previously been documented to impact atlantic basin tropical cyclone tc activity through alterations in large scale fields such as vertical wind shear mid level moisture sea level pressure and sea surface temperature atlantic tc activity has been shown to be enhanced when la nina conditions are present in the tropical pacific while activity is reduced when el nino conditions occur atlantic tc activity is enhanced when the convectively active phase of the mjo is over africa and the western indian ocean phases 1 2 while tc activity is suppressed when the convectively active phase of the mjo is over the tropical pacific phases 6 7 these relationships are shown to extend to atlantic basin rapid intensification ri events typically defined as intensification of 30 knots or greater in 24 h with nearly three times as many ri events in la nina years when compared with el nino years in addition approximately four times more ri episodes occur when the mjo exceeds one standard deviation in phases 1 2 than when the mjo exceeds one standard deviation in phases 6 7 storms forming in phases 1 2 are twice as likely to undergo at least one ri episode during their lifetime as storms forming in phases 6 7 even stronger relationships are seen when the mjo and enso are considered in combination

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

westerly wind events and precipitation in the eastern indian ocean as predictors for el nino climatology and case study for the 2002 2003 el nino 1 this study expands on recent work linking intraseasonal to seasonal variability in observed precipitation and wind from september to march in the eastern indian ocean with the initiation of el nino events during the last 25 years first westerly wind burst wwb events are defined as days when westerly wind speeds averaged over 5 15 s and 70 100 e were greater than 1 5 standard deviations from the mean the number of wwb days from september to march was high before the onset of the 1982 1983 1991 1992 1997 1998 and 2002 2003 el nino events but not the 1986 1987 el nino this study suggests that for the 1979 2002 period variations in precipitation in the eastern indian ocean is a more robust predictor of el nino onset than analyzed winds on the basis of the work of curtis et al 2002 a real time precipitation based el nino onset index is presented which during the austral summer of 2001 2002 successfully predicted the 2002 2003 el nino the index focuses on the magnitude of 30 60 day oscillations and mean conditions in the precipitation field case studies of high resolution satellite based data sets of precipitation wind and sea surface temperature sst for the 2001 2002 season are examined to better understand how events in the indian ocean are linked to pacific ocean wind disturbances and sst changes twice during this season maxima in precipitation and zonal winds propagated eastward the first near the equator and the second to the south for the southern case warm waters preceded heavy precipitation in the eastern indian ocean which preceded strong westerly winds a cooling of the sea surface followed the wind rain system this sequence of events moved through the ocean passage between indonesia and australia suggesting a coupling of convection wind and sea surface temperatures on the timescale of days these case studies provide a basis for how the east indian ocean variations are linked to subsequent events in the pacific ocean including the initiation of el nino events

on the madden julian oscillation atlantic hurricane relationship abstract the large scale equatorial circulation known as the madden julian oscillation mjo has been shown to impact tropical cyclone activity in several basins around the globe in this paper the author utilizes an mjo index created by wheeler and hendon to examine its impacts on tropical genesis and intensification in the atlantic large differences in frequency and intensity of tropical cyclone activity are seen both in the tropical atlantic as well as in the northwest caribbean and gulf of mexico depending on the mjo phase coherent changes in upper and lower level winds and relative humidity are likely responsible for these differences since the mjo shows potential predictability out to about two weeks the relationships discussed in this paper may be useful for short term predictions of the probability of tropical cyclone activity in the atlantic as a complement to the already available longer term seasonal predictions

contributions of tropical waves to tropical cyclone genesis over the western north pacific the present study investigates the relationship between the tropical waves and the tropical cyclone tc genesis over the western north pacific wnp for the period 1979 2011 five wave types are considered in this study it is shown that the tc genesis is strongly related to enhanced low level vorticity and convection of tropical waves and significant difference are detected in the tc modulation by dynamic and thermodynamic components of the waves more tcs tend to form in regions of waves with overlapping cyclonic vorticity and active convection about 83 2 of tcs form within active phase of tropical waves mainly in a single wave and two coexisting waves each wave type related genesis accounts for about 30 of all tc geneses except for the kelvin waves that account for only 25 2 of tc geneses the number of each wave type related tc genesis consistently varies seasonally with peak in the tc season july november which is attributed to a combined effect of active wave probability and intensity change the interannual variation in the tc genesis is well reproduced by the tropical wave related tc genesis especially in the region east of 150 e an eastward extension of the enhanced monsoon trough coincides with increased tropical wave activity by accelerated wave mean flow interaction

potential vorticity easterly waves and eastern pacific tropical cyclogenesis abstract a significant sign reversal in the meridional potential vorticity gradient was found during the summer of 1991 on the 310 k isentropic surface near 700 mb over the caribbean sea the charney stern necessary condition for instability of the mean flow is met in this region it is speculated that the sign reversal permits either invigoration of african waves or actual generation of easterly waves in the caribbean during the same season a correlation existed between the strength of the negative potential vorticity gradient in the caribbean and subsequent cyclogenesis in the eastern pacific the meridional pv gradient convective heating measured by outgoing longwave radiation data and eastern pacific cyclogenesis all varied on the timescale of the madden julian oscillation mjo it is hypothesized that upstream wave growth in the dynamically unstable region provides the connection between the mjo or any other convective forcing and the associated enhanced downstream tropical cyclogenesis

an anomalous genesis potential index for mjo modulation of tropical cyclones abstractmodulation of tropical cyclone tc genesis by the madden julian oscillation mjo has been quantitatively diagnosed by using a climatological genesis potential index gpi analysis of tc genesis during november april of 1979 2014 indicates the most effective factors controlling intraseasonal tc genesis are 850 hpa relative vorticity weighted by the coriolis parameter f r850 and 500 hpa vertical motion 500 the total vertical wind shear and maximum potential intensity are unimportant and the role of 600 hpa relative humidity is greatly represented by 500 the mjo modulates tc genesis primarily through changing low level vorticity induced by its rossby wave gyres and meridional shears of equatorial zonal winds a new intraseasonal gpi isgpi is proposed to quantify the mjo s modulation of tc genesis the isgpi significantly improves representation of intraseasonal variation of tc genesis in the tropics and in each subregion of the southern indian ocean australian monsoon and south pacific i

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

a new stratigraphy for the tertiary volcanic rocks of the nicaraguan highland three major volcanic events have occurred in nicaragua since middle tertiary time first the oligocene was dominated by rhyolite shield volcanism producing the highland ignimbrite second creation of northeast and later northwest striking dilatational fracture zones allowed the extrusion of basaltic to andesitic magmas along the pacific coast constructing the northwest oriented coyol volcanic arc this miocene arc can be followed all along the pacific coast of the chortis block third a pliocene pleistocene southwestward shift of arc volcanism created the modern volcanic arc this volcanic history is supported by a new stratigraphy based on volcanic centers and related facies the old cyclic layer cake stratigraphy of the coyol group has been abandoned

the geometry of the wadati benioff zone under southern central america and its tectonic significance results from a high resolution local seismographic network abstract we present here a detailed geometry of the wadati benioff zone under costa rica obtained from seismicity recorded by a dense local seismographic network jointly operated by the costa rica volcanological and seismological observatory national university and the charles f richter seismological laboratory university of california santa cruz underneath the nicaragua costa rica border the wadati benioff zone smoothly contorts from steep to shallow dip angles nw to se but does not show evidence of a brittle tear as postulated by others however further to the se ne of puerto caldera costa rica the wadati benioff zone does show a segmentation the quesada sharp contortion at intermediate depths h 70km nw of this sharp contortion the deepest portion of the seismically active slab dips at about 80 and reaches maximum depths ranging from 200 km near the nicaragua costa rica border to 135 km under ciudad quesada to the se the deeper portion of the wadati benioff zone dips at about 60 and the seismicity does not extend below depths ranging from 125 km behind the volcanic arc to 50 km east of quepos in southern costa rica east of 83 55 w we find no evidence of the wadati benioff zone deeper than 50 km the obtained geometry and other known tectonic features related to the subduction of the cocos plate under the caribbean plate along the southern terminus of the middle america trench nicaragua and costa rica correlate well with along trench variations in age of the subducted cocos plate some of these tectonic features are 1 the shallowing of middle america trench bathymetry from nw to se 2 variations in the energy release within the subducted slab 3 differences in coupling between cocos and caribbean plates 4 the termination of the central america volcanic chain in central costa rica 5 distinct stress field variations on the overriding caribbean plate the subduction of the cocos ridge under southern costa rica is partially responsible for some of these features shallowing of the middle america trench in southern costa rica and for the high uplift rates of the outer arc however as the presence of the panama fracture zone limits the subducted extension of the cocos ridge to less than 100 km from the trench we propose that the overall geometry of the wadati benioff zone is controlled by abrupt along trench changes in the age of the subducted cocos plate

on the mechanism of tectonic transport in zones of oblique subduction abstract many cases of oblique subduction are known throughout the world today and many more must have existed in the past in at least one modern case sumatra the over riding plate has responded to oblique subduction by forming a strike slip fault that traverses the magmatic arc and displaces its leading edge in the direction of the transverse component of oblique convergence conditions favoring this sort of tectonic response are shown to be 1 1 a high angle of oblique convergence 2 2 a low angle of subduction and 3 3 relative thermal softening of the magmatic arc this mechanism may have been responsible in part for northward transport of western cordilleran magmatic arc and subduction zone rocks in the late cretaceous and earliest tertiary it also may help to explain evidence of southward transport and left lateral shear in northern mexico and the caribbean region in pre jurassic time

bookshelf faulting in nicaragua oblique subduction at a high rate of convergence along much of the middle america trench results in northwest directed trench parallel block motion accommodation of this motion along northwest striking dextral strike slip faults has been postulated however in nicaragua such faults are not well developed we suggest instead that this motion is accommodated by bookshelf faulting that includes northeast striking left lateral faults we present earthquake epicenter and focal mechanism data and mapped fracture and fault data consistent with this model trenchward migration of the volcanic arc since the miocene and reactivation of northeast striking miocene structures may have led to the development of this arc and trench normal fault system

environmental patterns associated with active and inactive caribbean hurricane seasons this study of hurricanes passing through the caribbean in the 1950 2005 period reveals that seasons with more intense hurricanes occur with the onset of pacific la nina events and when atlantic ssts west of africa are above normal composites of ncep reanalysis fields with regard to caribbean hurricanes reveal de velopment of an anomalous equatorial atlantic zonal overturning circulation upper easterly lower westerly that shifts toward the caribbean coincident with a westward spread of the cold tongue in the east pacific ocean atmosphere coupling is promoted through interaction of the southern hadley cell and the atlantic itcz a heat budget analysis suggests that evaporation governs ssts in the major development region mdr and near venezuela but the signal is weak prior to may using the knowledge gained statistical algorithms are developed to predict caribbean hurricanes at seasonal lead times these make use of equa torial pacific sst subtropical atlantic sst and the zonal walker cell over the atlantic

barotropic aspects of itcz breakdown abstract in satellite images the itcz intertropical convergence zone is sometimes observed to undulate and break down into a series of tropical disturbances tropical cyclones may later develop within these disturbances and move into higher latitudes allowing the itcz to reform it has been proposed that itcz breakdown results from a convectively modified form of combined barotropic and baroclinic instability of the mean flow an unstable mean flow can be produced by itcz convection in just a couple of days in this sense the itcz produces favorable conditions for its own instability and breakdown in this study a nonlinear shallow water model on the sphere is used to simulate barotropic aspects of itcz breakdown in the shallow water model the itcz is simulated by a prescribed zonally elongated mass sink near the equator the mass sink produces a cyclonic potential vorticity pv anomaly that has a reversed meridional pv gradient on its poleward side according to the ripa theorem a flow that has a