the equatorial atlantic oscillation and its response to enso an internal equatorial atlantic oscillation has been identified by analyzing sea surface temperature sst observations the equatorial atlantic oscillation can be viewed as the atlantic analogue of the el nino southern oscillation enso phenomenon in the equatorial pacific but it is much less vigorous the equatorial atlantic oscillation is strongly influenced by the pacific enso with the equatorial atlantic sea surface temperature lagging by about six months this lag can be explained by the dynamical adjustment time of the equatorial atlantic to low frequency wind stress variations and the seasonally varying background state which favours strongest growth of perturbations in summer results of an extended range simulation with a coupled ocean atmosphere gcm support this picture

eight centuries of north atlantic ocean atmosphere variability climate in the tropical north atlantic is controlled largely by variations in the strength of the trade winds the position of the intertropical convergence zone and sea surface temperatures a high resolution study of caribbean sediments provides a subdecadally resolved record of tropical upwelling and trade wind variability spanning the past 825 years these results confirm the importance of a decadal 12 to 13 year mode of atlantic variability believed to be driven by coupled tropical ocean atmosphere dynamics although a well defined interdecadal mode of variability does not appear to be characteristic of the tropical atlantic there is evidence that century scale variability is substantial the tropical atlantic may also have been involved in a major shift in northern hemisphere climate variability that took place about 700 years ago

analysis of a nontornadic storm during vortex 95 abstract analysis of a supercell storm that did not produce a tornado near hays kansas is presented a well defined midlevel mesocyclone was apparent throughout most of the storm s life cycle numerous shallow circulations were observed along the rear flank gust front during the data collection period six of these circulations strengthened into intense low level mesocyclones each of these mesocyclones failed to produce a tornado the strongest low level mesocyclone referred to as vortex 4 underwent a life cycle that was consistent with other tornadic mesocyclones documented in the literature these results illustrate that the presence of a long lived mesocyclone at low levels is not sufficient for tornadogenesis to occur the kinematic structure of the low level mesocyclone that did not produce a tornado is compared with a tornadic mesocyclone from another storm in order to understand the characteristic differences between these circulations the results lead to the conclusion that the presence of

the life cycle of thunderstorm gust fronts as viewed with doppler radar and rawinsonde data abstract this paper presents the time dependent analysis of the thunderstorm gust front with the use of project nimrod data rhi cross sections of reflectivity and doppler velocity are constructed to determine the entire vertical structure the life cycle of the gust front is divided into four stages 1 the formative stage 2 the early mature stage 3 the late mature stage and 4 the dissipation stage a new finding is a horizontal roll detected in the reflectivity pattern resulting from airflow that is deflected upward by the ground while carrying some of the smaller precipitation ahead of the main echo core of the squall line this feature is called a precipitation roll as determined from rawinsonde data the cold air behind the gust front accounts for the observed surface pressure rise calculations confirm that the collision of two fluids produce a nonhydrostatic pressure at the leading edge of the outflow the equation governing the propagation speed of a density current accurately predicts t

the numerical simulation of non supercell tornadogenesis part i initiation and evolution of pretornadic misocyclone circulations along a dry outflow boundary abstract high resolution three dimensional simulations are used to study misocyclone initiation and development along the leading edge of an outflow boundary model conditions were designed such that this development could be simulated independent of moist processes the outflow boundary is allowed to propagate into a region of southerly low level flow which results in a vertical vortex sheet along the outflow s leading edge lobe and cleft instabilities follow and provide perturbations for the subsequent development of horizontal shearing instabilities these growing instabilities are the inaugural circulations of leading edge misocyclones with wavelengths ranging from 1 6 to 3 2 km the structure of these modeled misocyclones compares favorably to observed pretornadic misocyclones along outflow boundaries in northeast colorado vortex sheet dynamics are observed to exert substantial control over the structure of the evolving outflow leading edge the vertical vortex sheet passes through discrete evoluti

el nino during the last interglacial period recorded by a fossil coral from indonesia measurements of oxygen isotopes and elemental ratios in a fossil coral that grew 124 000 years ago in north sulawesi indonesia reflect interannual variability in precipitation and sea surface temperature sst due to the el nino southern oscillation enso this indicates that enso was robust during the last interglacial period a time when global climate was slightly warmer than the present the pattern of enso frequency behavior in the past is similar to variability in modern instrumental records but distinct from the most recent period since the mid 1970s supporting the hypothesis that enso behavior in recent decades is anomalous with respect to natural variability

a study of the tornadic region within a supercell thunderstorm abstract the transition of a supercell thunderstorm into its tornadic phase is investigated through high resolution numerical cloud model simulations initiated within the interior portion of a previously simulated mature supercell storm with the enhanced grid resolution the low level cyclonic vorticity increases dramatically and the gust front rapidly occludes as small scale downdrafts develop in the vicinity of the low level center of circulation as the occlusion progresses a ring of high vorticity air surrounds the circulation center and could be conducive to multiple vortex tornado formation numerous features of the simulated transition bear resemblance to those observed in tornadic storms in the model simulation the large low level vorticity is generated through the tilting and intense stretching of air from the inflow side of the storm this vertical vorticity is derived from the horizontal vorticity of the environmental shear and also from horizontal vorticity generated solenoidally as low l

a numerical simulation of cyclic mesocyclogenesis abstract a three dimensional nonhydrostatic numerical model the advanced regional prediction system is used to study the process of cyclic mesocyclogenesis in a classic supercell thunderstorm during the 4 h simulation the storm s mesocyclone undergoes two distinct occlusions with the beginning of a third indicated at the end of the simulation the occlusion process exhibits a period of approximately 60 min and is qualitatively similar in each case initial midlevel 3 7 km mesocyclogenesis proceeds according to the classic picture that is via tilting of streamwise environmental vorticity the development of an evaporatively driven rear flank downdraft rfd signals the beginning of the occlusion process the developing rfd wraps cyclonically around the mesocyclone causing the gust front to surge outward simultaneously the occluding mesocyclone rapidly intensifies near the surface trajectory analyses demonstrate that this intensification follows from the tilting and stretching of near ground

simulation and analysis of tornado development and decay within a three dimensional supercell thunderstorm abstract a three dimensional numerical simulation using a two way interactive nested grid is to study tornado genesis within a supercell during a 40 minute period two tornadoes grow and decay within the storm s mesocyclone the tornadoes have life spans of approximately 10 minutes maximum ground relative surface wind speeds exceed 60 m s 1 during both tornadoes and horizontal pressure gradients reach 18 hpa km 1 during the second tornado comparison of the simulated storm evolution with doppler and field observations of supercells and tornadoes shows many similar features vertical vorticity in the mesocyclone and the tornado vortex at low levels is initially created by the tilting of the environmental vorticity and baroclinically generated vorticity along the forward gland gust front of the storm tornadogenesis is initiated when mesocyclone rotation increase above cloud base the increased rotation generates lower pressure in the mesocyclone increasing the upward pressure gradient forces the upwar

glacial interglacial differences in circulation and carbon cycling within the upper western north atlantic we investigated glacial interglacial changes in the circulation and carbon cycling in the western north atlantic subtropical gyre using hydrographic data and downcore records of the stable isotopic compositions of individual shells of bahamian benthic foraminifera potential temperature salinity depth relations show that modern thermocline 200 1000 m and deep 1000 2000 m waters in the providence channels bahamas originate in the sargasso sea and are typical of the subtropical gyre gradients in the stable isotopic compositions of late holocene planulina and cibicidoides species from the bank margins 400 to 1500 m depth reflect temperature nutrient and isotopic gradients of modern subtropical gyre waters the difference between the 18o of glacial maximum and late holocene foraminifera is 2 1 for the upper 900 m of the water column and 1 6 for deeper waters indicating that these waters were 4 c and 2 c cooler respectively during glacial time the glacial temperature gradient dt dz was similar to today while the base of the thermocline was 100 m shallower these results differ significantly from our earlier results from multiple shell 18o analyses which implied upper thermocline waters were only 1 c cooler and dt dz was greater during the glacial maximum the difference occurs because bioturbation adversely affects multiple shell analyses of glacial aged samples from shallow water depths at all depths above 1500 m foraminiferal 13c are greater during the glacial maximum than the late holocene by at least 0 1 to 0 2 as much as 0 6 in the lower thermocline indicating that nutrient concentrations throughout the thermocline were reduced and there was no oxygen minimum zone during the glacial maximum this suggests greater more uniform ventilation of the thermocline results of single and multiple shell 13c analyses of glacial age foraminifera compare favorably because samples most affected by mixing correspond to water depths where the glacial interglacial change of 13c was small cooler upper ocean waters during the glacial maximum reflect cooler temperatures at the ocean surface where isopycnal surfaces outcrop including large areas of the subtropical ocean a shallower thermocline base is consistent with southward migration of the northern edge of the subtropical gyre or increased mode water production enhanced thermocline ventilation is consistent with more vigorous winds and all isopycnal surfaces outcropping in the area of ekman downwelling

linear and nonlinear propagation of supercell storms abstract a nonlinear formula for updraft motion in supercell storms is derived from petterssen s formula for the motion of systems and the vertical equation of motion and tested on form preserving disturbances at each level continuous propagation of an updraft maximum is determined largely by the horizontal gradient of the nonhydrostatic vertical pressure gradient force nhvpgf at the updraft center the nhvpgf is deduced from the formal solution of the poisson equation for nonhydrostatic pressure in anelastic flow subject to homogeneous neumann boundary conditions at the ground and top boundary recourse also is made to published fields of partitioned vertical pressure gradient force updraft motion is partitioned into parts forced by horizontal gradients of hydrostatic pressure linear interaction between the environmental shear and updraft and nonlinear dynamical effects the dynamics of supercell storms for nearly straight and highly curved hodographs are found to be different nonlinear rotation

local and regional geochemical signatures of surface sediments from the cariaco basin and orinoco delta venezuela we have analyzed the chemical compositions of 87 samples from the cariaco basin and 20 samples from the orinoco delta venezuela in order to better characterize the fluvial sources of material to the cariaco basin we observe distinctive regional patterns in composition shelf sediments found near the tuy river manzanares river the araya margarita region and in the northern portion of the unare platform record reproducible compositional differences however linear mixing models using only the composition from these rivers do not satisfy the variability observed in modern sediment trap samples collected from the basin average trap values are best approximated by mixing upper crust 30 50 with local river sources 50 70 of either the tuy unare or neveri rivers and eolian dust

the dependence of numerically simulated convective storms on vertical wind shear and buoyancy abstract the effects of vertical wind shear and buoyancy on convective storm structure and evolution are investigated with the use of a three dimensional numerical cloud model by varying the magnitude of buoyant energy and one directional vertical shear over a wide range of environmental conditions associated with severe storms the model is able to produce a spectrum of storm types qualitatively similar to those observed in nature these include short lived single cells certain types of multicells and rotating supercells the relationship between wind shear and buoyancy is expressed in terms of a nondimensional convective parameter which delineates various regimes of storm structure and in particular suggests optimal conditions for the development of supercell type storms applications of this parameter to well documented severe storm cases agree favorably with the model results suggesting both the value of the model in studying these modes of convection as well as the value of this representation i

changes in foraminiferal assemblages and environmental conditions during the t oae early jurassic in the northern lusitanian basin portugal abstract the foraminiferal assemblages and other microfossils from the rabacal area maria pares section in the lusitanian basin portugal provide new insights regarding the toarcian oceanic anoxic event t oae complementary analysis of the 13c total organic carbon toc and geochemical redox and palaeoproductivity proxies reflect environmental changes that affected the microfossil assemblages detailed analyses of foraminiferal assemblages their diversity abundance morphogroups and lifestyle and geochemistry led us to discern phases of the biotic crisis and subsequent stages of recovery environmental instability which began in the lattermost part of the polymorphum zone is marked by a lesser abundance of foraminifera and ostracods increased opportunist foraminifera fluctuations in toc values and the enrichment factor of phosphorous pef as well as the onset of a negative carbon isotopic excursion cie the peak phase of the biotic crisis corresponds with the lowest values for foraminiferal diversity and abundance including a barren sample for benthic microfossils the abundance and diversity of echinoderms brachiopods dinoflagellate cysts and calcareous nannofossils dropped sharply during the peak phase this biotic crisis also entails extinction of the order metacopina and a local disappearance of echinoderms and brachiopods anoxic conditions at sea bottom did not develop given the almost continuous record of foraminifera and trace fossils yet oxygen depleted conditions occurred the lower part of the levisoni zone contains thin nodular limestones corresponding to a tempestitic turbiditic facies in this context the re oxygenation of bottom waters was favoured as indicated by the presence of thalassinoides and the low values of toc and redox sensitive elements mo ni and u the survival phase in the wake of the biotic crisis is characterized by greater diversity and abundance of foraminifera and echinoderms holothuroids and ophiuroids the lowest 13c values are recorded during this phase with a dominance of opportunist eoguttulina and facultative opportunist paralingulina tenera foraminifera finally recovery after the biotic crisis is evidenced by high values for specialists and potentially deep infaunal foraminifera indicating oxic conditions in the sediment

brief interstadial events in the santa barbara basin ne pacific during the past 60 kyr the instability of the northern hemisphere glacial climate over the past 100 kyr has been revealed by at least 20 brief warm interstadial episodes called dansgaard oeschger events recorded in greenland ice cores1 3 and in north atlantic sedimentary records4 5 a few of these events have been recognized elsewhere6 8 here we describe a record of ocean oxygenation and circulation from the santa barbara basin in the northeast pacific ocean which correlates well with the greenland ice core records we see 19 of the 20 dansgaard oeschger events in the form of laminated sediments deposited under anoxic conditions and we can correlate at least 16 of these with the 17 ice core interstadials of the past 60 kyr thus these short term events were not restricted to the north atlantic region the events had substantial ecological and oceanographic effects in the santa barbara basin including changes in benthic faunal populations and in the age and composition of bottom waters similar ventilation changes have been seen in the gulf of california9 10 suggesting that these changes may have been widespread and synchronous along the northeast pacific margin these results suggest sensitivity of broad areas of the ocean atmosphere cryosphere system to short term climate change

vertical distribution and isotopic fractionation of living planktonic foraminifera from the panama basin a model1 2 which explains the vertical and seasonal distribution of planktonic foraminifera was developed using data from the western north atlantic and is comprised of three tenets 1 the conditions in the photic zone upper 80 100 m exclusively dictate planktonic foraminifera species composition and abundance throughout the water column 2 species are vertically stratified within the photic zone according to their temperature preference depth distribution according to temperature preference has been proposed by several authors3 based on isotope analyses of sediments due to seasonal temperature variations seasonal variations in the 18o composition of seawater and species specific isotope fractionation rigorous proof of this hypothesis cannot be provided from sediment analyses alone 3 the chlorophyll maximum is a major food source zone which is preferentially exploited by foraminifera our data from the panama basin which are presented here are confirmation of this model the panama basin in the eastern equatorial pacific which has a shallow and steep thermocline was sampled for living planktonic foraminifera a multiple opening closing net and environmental sensing system mocness 4 was used to collect 13 sets of eight vertically stratified oblique samples we report here the quantitative distribution and stable isotopic composition of planktonic foraminifera from four mocness tows taken over 5 days during august 1979 rv knorr cruise 73 at 5 20 n 8 50 w each tow consisted of eight samples which integrated 12 5 m intervals from 0 to 100 m for moc 126 25 m intervals from 0 to 200 m for moc 127 100 150 m intervals from 0 to 1 000 m on moc 128 and 250 m intervals from 0 to 2 000 m on moc 131

mechanisms of abrupt climate change of the last glacial period 1 more than a decade ago ice core records from greenland revealed that the last glacial period was characterized by abrupt climate changes that recurred on millennial time scales since their discovery there has been a large effort to determine whether these climate events were a global phenomenon or were just confined to the north atlantic region and also to reveal the mechanisms that were responsible for them in this paper we review the available paleoclimate observations of abrupt change during the last glacial period in order to place constraints on possible mechanisms three different mechanisms are then reviewed ocean thermohaline circulation sea ice feedbacks and tropical processes each mechanism is tested for its ability to explain the key features of the observations particularly with regard to the abruptness millennial recurrence and geographical extent of the observed changes it is found that each of these mechanisms has explanatory strengths and weaknesses and key areas in which progress could be made in improving the understanding of their long term behavior both from observational and modeling approaches are suggested finally it is proposed that a complete understanding of the mechanisms of abrupt change requires inclusion of processes at both low and high latitudes as well as the potential for feedbacks between them some suggestions for experimental approaches to test for such feedbacks with coupled climate models are given

meridional shifts in the marine itcz and the tropical hydrologic cycle over the last three glacial cycles 1 paleoproxy studies show a strong correlation between tropical climate and high latitude temperature variability recorded in the greenland ice cores over the last glacial cycle in particular abrupt cooling events in the greenland ice sheet project ii 18o ice record appear synchronous with a southward migration of the intertropical convergence zone itcz in the atlantic a weakening of the indian and east asian monsoon systems and a strengthening of the south american monsoon system because this high to low latitude climate teleconnection significantly alters the tropical hydrologic cycle around the globe it plays a critical role in regulating global climate on glacial interglacial time scales we compare 18oseawater reconstructions a salinity proxy generated from previously published mg ca and oxygen isotope data on globigerinoides ruber white var obtained from western caribbean core odp 999a and western equatorial pacific core 806b across the last three glacial cycles to show that systematic variations in surface salinity at these sites suggest the tropical hadley cell hydrologic system undergoes systematic reorganizations that differ dramatically between warm interglacial and cold glacial periods furthermore cross spectral and phase angle analyses of the ice volume corrected caribbean and western pacific 18osw records reveal a 100 kyr frequency in both records that is almost 180 out of phase and a 23 kyr frequency that is nearly in phase this results in the development of a very large 18osw gradient between the caribbean and the western equatorial pacific on glacial interglacial time scales that is best explained by a southward shift in both the atlantic and pacific itcz during north atlantic cold phases

a three dimensional numerical simulation of splitting severe storms on 3 april 1964 abstract a three dimensional numerical storm model is used to investigate the observed splitting of several reflectivity echoes on 3 april 1964 in oklahoma representative soundings from this day exhibit a nearly one directional environmental wind shear vector and the presence of strong low level wind shear in the numerical simulation an initial cloud splits into two long lived rotating storms one that moves to the left of the mean winds and the other to the right the left moving storm develops more slowly than the right moving one due to the deviation of the environmental wind hodograph from a straight line below 1 km further the left mover eventually splits convergence induced by the cold low level storm outflow plays a major role in the development of both the first and second splits however the second split appears to be dynamically different than the first as the left moving updraft remains essentially unchanged while a new updraft forms immediately adjacent to it because of the different p

reconstructing marine productivity of the cariaco basin during marine isotope stages 3 and 4 using organic walled dinoflagellate cysts 1 an organic walled dinoflagellate cyst analysis was carried out on sediment core md03 2622 retrieved from the cariaco basin the core comprises the 73 30 ka interval down core changes in cyst abundance accumulation rate and composition of cyst assemblages were used to identify climatic and oceanographic changes at orbital and millennial time scales in this near equatorial seasonal upwelling area throughout the sequence dinoflagellate cyst assemblages were dominated by heterotrophic dinocysts mainly brigantedinium spp with the exception of four autotrophic dominated mainly spiniferites ramosus intervals around 58 53 46 and 37 ka at orbital time scales changes in the dinoflagellate productivity seem to follow low latitude insolation with the highest productivities coinciding with maximum february insolation 47 38 ka at millennial scales cyst accumulation rates appeared to coincide with dansgaard oeschger d o variability with significant increments occurring during warm interstadials the opposite was true during stadials short periods of high nutrient availability and stratified conditions followed heinrich events h4 h5 h5a and h6 and concurred with enhanced river runoff spectral analyses confirm the existence of these and other higher frequency periodicities and support the idea of a tightly coupled terrestrial marine and tropical high latitude climate system during the last glacial period

the association of significant tornadoes with a baroclinic boundary on 2 june 1995 abstract on 2 june 1995 the large scale environment of eastern new mexico and western texas was generally favorable for the occurrence of supercells because of the presence of strong deep shear and storm relative helicity as well as sufficient convective available potential energy cape indeed many supercells occurred but the only storms to produce tornadoes were those supercells that crossed or developed and persisted on the immediate cool side of a particular outflow boundary generated by earlier convection surface conditions vertical vorticity and horizontal vorticity near this boundary are documented using conventional and special observations from the vortex field program it is shown that the boundary was locally rich in horizontal vorticity had somewhat enhanced vertical vorticity and enhanced cape theoretical arguments indicate that the observed horizontal vorticity around 1 10 2 s 1 largely parallel to the boundary can be readily produced with the type of buoyancy contrast obse

new constraints on the timing of sea level fluctuations during early to middle marine isotope stage 3 1 to settle debate on the timing of sea level fluctuations during marine isotope stage mis 3 we present records of 18oruber sea level proxy and magnetic susceptibility from the same samples within the single sediment archive i e coregistered of central red sea core geotu kl11 core scanning x ray fluorescence and environmental magnetic data establish the suitability of magnetic susceptibility as a proxy for eolian dust content in red sea sediments the eolian dust data record similar variability as greenland 18oice during early to middle mis 3 in agreement with previous observations that regional arabian sea climate fluctuated with a timing similar to that of greenland climate variations in contrast the sea level record fluctuates with a timing similar to that of antarctic style climate variations the coregistered nature of the two records in core kl11 unambiguously reveals a distinct offset in the phase relationship between sea level and eolian dust fluctuations the results confirm that sea level rises indicated by shifts in red sea 18oruber to lighter values occurred during cold episodes in greenland during early to middle mis 3 this indicates that the amplitudes of the reconstructed mis 3 sea level fluctuations would not be reduced by inclusion of regional climate fluctuations in the red sea sea level method our analysis comprehensively supports our earlier conclusions of large amplitude sea level variations during mis 3 with a timing that is strongly similar to antarctic style climate variations

glacial interglacial surface water variations in the caribbean sea during the last 300 ky based on calcareous nannofossil analysis abstract quantitative and qualitative analyses of nannofossil assemblages from odp site 999 in the caribbean sea were performed to reveal changes in upper seawater structures and implications for climatic changes during the last 300 000 years overall there is an inverse relationship in stratigraphic fluctuations between the absolute abundance aa of a lower photic taxon florisphaera profunda and 18 o this fluctuation pattern is considered to represent climatic changes in the subtropical under water suw in the caribbean cross spectral analysis demonstrates that the suw was characterized by nutrient rich water during interglacials changes in orbitally driven gyre circulation system strongly affected fluctuations in the suw in the caribbean conversely the surface water was enriched in nutrients during glacials as the result of increased terrigenous materials and shallowing of the thermocline based on changes of the n ratio abundance of reticulofenestrids relative to f profunda and results of cross spectral analysis between them and 18 o the relationship between the thermocline variation and the ice volume fluctuation suggests that changes in the subtropical gyre circulation system in the north atlantic have had stronger influences on the caribbean thermocline variations and there may be a little contribution of climatic fluctuations that originated in the southern ocean to the caribbean sea a large floral turnover in nannofossil assemblages between oxygen isotope stages 6 and 5 was observed in hole 999a which corresponds to termination ii on the basis of floral compositions photosynthetic primary productions had been more active than today before termination ii

variability in the mean latitude of the atlantic intertropical convergence zone as recorded by riverine input of sediments to the cariaco basin venezuela abstract changes in the strength and position of the intertropical convergence zone itcz are an important component of climate variability in the tropical atlantic the cariaco basin located on the northern margin of venezuela is sensitive to tropical atlantic climate change and its sediments provide a record of past itcz behavior today the cariaco basin experiences two distinct seasons that reflect the annual migration of the atlantic itcz between january and march when the itcz lies south of the equator northeasterly trade winds sit directly over cariaco basin and strong coastal upwelling and dry conditions dominate beginning in june july as the itcz moves north local rainfall reaches a maximum and the upwelling diminishes or disappears here we summarize new and previously published data on the river derived terrigenous fraction of cariaco basin sediments as well as comparisons to other paleoclimate records which together suggest a coherent climatologic response in the tropical atlantic triggered by a pattern of itcz migration that mimics the seasonal cycle during periods of cooler north atlantic ssts on time scales ranging from the little ice age to the younger dryas to the cold stadials of the last glacial decreased detrital delivery to cariaco basin from local rivers suggests a southward shift in the mean latitudinal position of the itcz during warm interstadials and periods of holocene and deglacial warmth northward shifts in itcz position and its belt of convective rainfall are inferred from increased detrital delivery to the basin whether the rapid shifts in itcz position and precipitation recorded by cariaco basin sediments and other regional records reflect a response to forcing originating in the high latitude atlantic or to forcing potentially sourced in the tropics is a key question yet to be fully answered

the garden city kansas storm during vortex 95 part ii the wall cloud and tornado abstract on 16 may 1995 a supercell storm produced an f1 tornado near garden city kansas during vortex verification of the origins of rotation in tornadoes experiment this event provided the first opportunity to synthesize data collected by a new airborne radar platform called eldora electra doppler radar developed by the national center for atmospheric research the evolution of the low and midlevel mesocyclone was presented in a companion paper in this paper the sequence of events that triggered tornadogenesis within the mesocyclone circulation is shown the occlusion downdraft documented in part i appears to promote vortex breakdown resulting in the generation of multiple vorticity centers it is one of these centers that intensifies into the garden city tornado the structural relationship between the observed wall cloud and the radar reflectivity associated with the developing hook echo and the kinematic wind field is also presented although the wall cloud appears to be uniform there

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

low level mesocyclonic concentration by nonaxisymmetric transport part i supercell and mesocyclone evolution an idealized simulation of a supercell using the regional atmospheric modeling system rams was able to produce a low level mesocyclone near the intersection of the forward and rear flank downdrafts the creation of the low level mesocyclone is similar to previous studies after 3600 s the low level mesocyclone underwent a period of rapid intensification during which its form changed from an elongated patch to a compact center this transition was also accompanied by a sudden decrease in pressure to 12 mb below that of the neighboring flow and was found to occur even in the absence of nested grids it is shown that the stage of strong intensification does not begin aloft as in the dynamic pipe effect and then descend to the surface rather the vortex is initiated near the surface and then builds upward the process is completed in 5 min and the final vortex can be clearly distinguished from the larger scale mesocyclone at the cloud base the reduction of pressure can be explained as a consequence of the evacuation of mass in the horizontal convergence equation this is in contrast to axisymmetric models of vortex intensification which generally rely on the evacuation of mass in the vertical divergence equation in the latter cases a positive horizontal convergence tendency is what initiates the concentrated vortex however nondivergent models prove that vorticity concentration can occur in the absence of any horizontal convergence here the concentration is associated with a negative horizontal convergence tendency

decadal to centennial scale tropical atlantic climate variability across a dansgaard oeschger cycle 1 the patterns and forcing mechanisms of climate variability on decadal to centennial time scales represent a major void in our current understanding of earth s climate system furthermore the response of the low latitudes to abrupt climate change is also not well understood as most high resolution paleoclimate studies are from midlatitudes and high latitudes this study explores the tropical atlantic response to a dansgaard oeschger cycle interstadial 12 using ultra high resolution 2 3 years foraminiferal census data from cariaco basin sediments the interpretation of the abundance records for the onset of interstadial 12 is complicated by the competing effects of rising sea level on ekman induced upwelling within the cariaco basin and migrating intertropical convergence zone associated variations in trade wind location and fluvial nutrient delivery to the basin the foraminiferal abundance records for the latter part of the interstadial suggest a southerly shift in the average annual position of the intertropical convergence zone that acted to enhance upwelling and productivity within the cariaco basin sea level eventually reached a critical point in the transition back to stadial conditions that led to upwelling of nutrient depleted waters and a decline in productivity within the basin spectral analyses of theglobigerina bulloidesabsolute abundance records reveal significant variability ranging from subdecadal to centennial scale atlantic multidecadal scale climate variability is only evident in the warmest interval of interstadial 12 suggesting that variability on this scale may only operate during warm climate periods something that has significant implications for modern and near future climate variability

observations of nontornadic low level mesocyclones and attendant tornadogenesis failure during vortex abstract three storms intercepted during the verification of the origins of rotation in tornadoes experiment generated a moderate to strong mesocyclone within the lowest several hundred meters above the ground and qualitatively appeared capable of tornadogenesis yet did not produce a tornado such novel observations of what is considered tornadogenesis failure are documented and used to show the insufficiency of a low level mesocyclone for tornadogenesis possible modes of failure are discussed

an 8 century tropical atlantic sst record from the cariaco basin baseline variability twentieth century warming and atlantic hurricane frequency 1 we present the first direct comparison and calibration of a downcore foraminiferal mg ca record to historical instrumental sea surface temperature sst mg ca measured on the planktic foraminifer globigerina bulloides from a cariaco basin sediment core strongly correlate with spring march may instrumental ssts between a d 1870 and 1990 a mg ca sst equation is derived and a paleo sst record is presented spanning the last 8 centuries an interval that includes the end of the medieval warm period and the little ice age the long term record displays a surprising amount of variability the temperature swings are not necessarily related to local upwelling variability but instead represent wider conditions in the caribbean and western tropical atlantic the mg ca sst record also captures the decadal and multidecadal variability observed in records of global land and sea surface temperature anomalies and atlantic tropical storm and hurricane frequency over the late nineteenth and twentieth centuries a divergence between the sst proxy record and atlantic storm frequency around 1970 appears to reflect a fundamental change in atlantic hurricane behavior noted in historical data on average twentieth century temperatures are not the warmest in the entire record but they do show the largest increase in magnitude and fastest rate of sst change over the last 800 a

the relationship between overshooting tops in a tornadic supercell and its radar observed evolution abstractthis is a study of a tornadic supercell in kansas on 14 may 2018 in which data of relatively high spatiotemporal resolution from a mobile polarimetric x band doppler radar were integrate

origin of magnetic mineral concentration variation in the southern ocean 1 in the southern ocean magnetic mineral concentration increases in glacial periods the variation pattern closely resembles eolian dust flux records from antarctic ice cores but the cause of the linkage remains unclear as the dust flux is too small for the source of terrigenous materials in the southern ocean we have conducted an environmental magnetic study of late pleistocene sediments from the south indian ocean to investigate the origin of the magnetic concentration changes biogenic magnetites can be detected using the characteristics of almost no magnetostatic interactions and narrow coercivity distribution reflecting occurrence of single domain magnetites in a chain we interpret that a non interacting component on first order reversal curve diagrams and low coercivity components with small dispersion from isothermal remanent magnetization irm component analyses represent biogenic magnetites and that the interacting and middle coercivity components represent terrigenous maghemites the ratio of anhysteretic remanent magnetization susceptibility to saturation irm reflects relative abundance of the biogenic and terrigenous components it was revealed that biogenic magnetites are a dominant constituent of the magnetic minerals in glacials the abundance of both biogenic and terrigenous components increased with increased proportions of the latter increased ocean productivity in glacials is suggested from increased proportions of biogenic magnetites with elongated morphologies indicative of less oxic conditions and increased sedimentation rates these observations suggest that the increased magnetic concentration in glacials in the southern ocean may be explained by iron fertilization the production of biogenic magnetites was enhanced associated with increased ocean productivity which was fueled by increased eolian dust flux

the numerical simulation of nonsupercell tornadogenesis part ii evolution of a family of tornadoes along a weak outflow boundary abstract nonsupercell tornadogenesis along a weak outflow boundary has been simulated using a three dimensional moist convective cloud model thermodynamic conditions similar to those observed for nonsupercell tornado nst events of the high plains were utilized in the model initialization as the ensemble system of storm outflow boundary and leading edge vortices evolve six distinct life cycle stages for the development and decay of nsts are documented that span a period of about 35 min consistent with the results of part i of this numerical study vortex sheet dynamics exert considerable control over the outflow leading edge the progression of pretornadic life cycle stages serves to concentrate vertical vorticity effectively along the outflow boundary in discrete misocyclone circulations aligned in a 3 km wavelength pattern the organization of larger scale misocyclones and ultimate intensification to initial tornadic intensity occurs coincident with the rapid development of deep convection overhe

climate change in the circum north atlantic region during the last deglaciation a survey of new and published palaeoclimate data indicates that both the high and low latitude north atlantic regions were characterized by at least three synchronous periods of abrupt climate change during the last glacial to interglacial transi tion climate model results suggest that changes in the melting history of the laurentide ice sheet may explain much of this nonlinear response of the climate system to astronomical milankovitch forcing

influences of the local environment on supercell cloud to ground lightning radar characteristics and severe weather on 2 june 1995 abstract radar cloud to ground cg lightning characteristics and storm reports were documented for 20 long lived supercell thunderstorms that occurred during a 6 h period in the west texas panhandle on 2 3 june 1995 these thunderstorms occurred in proximity to a preexisting mesoscale outflow boundary storms that remained on the warm side of the mesoscale outflow boundary and storms that formed directly on the boundary tended to produce weaker low level rotation lower maximum heights for the 40 dbz echo top and had the largest negative cg flash rates the largest negative flash rate was produced as each storm was gradually weakening in contrast out of 11 boundary crossing storms several important radar based measurands increased unambiguously after storms crossed the boundary 40 dbz echo top height in 5 cases radar reflectivity above the environmental freezing level in 6 cases and low level mesocyclone strength in 9 cases trends of the first two measurands were ambiguous for 4 of 11 cases aff

the influence of the shear induced pressure gradient on thunderstorm motion abstract in the present investigation we propose a simple theory to explain how a veering environmental wind shear vector can cause an initially symmetric updraft to grow preferentially to the right of the shear vector and acquire cyclonic rotation the explanation offered is based on linear theory which predicts that interaction of the mean shear with the updraft produces favorable vertical pressure gradients along its right flank to asses the validity of linear theory for large amplitude updrafts the three dimensional shallow anelastic equations are numerically integrated using a simple parameterization for latent heating within a cloud and the linear and nonlinear forcing terms are separately analyzed these results suggest that although the nonlinear effects strongly promote splitting of the updraft the linear forcing remains the dominant factor in preferentially enhancing updraft growth on the right flank we believe this differential forcing is a major contributor to the observed predominance o

hook echoes and rear flank downdrafts a review abstract nearly 50 years of observations of hook echoes and their associated rear flank downdrafts rfds are reviewed relevant theoretical and numerical simulation results also are discussed for over 20 years the hook echo and rfd have been hypothesized to be critical in the tornadogenesis process yet direct observations within hook echoes and rfds have been relatively scarce furthermore the role of the hook echo and rfd in tornadogenesis remains poorly understood despite many strong similarities between simulated and observed storms some possibly important observations within hook echoes and rfds have not been reproduced in three dimensional numerical models

on the interaction of tropical cyclone scale vortices ii discrete vortex patches the interaction of vortex patches in relation to the observed scales and features reported in part i of this paper is investigated it is found that the initial approach of compound vortices such as tropical cyclones arises from distortion of their weaker outer vorticity fields care needs to be taken in applying simple concepts such as the propagation of each vortex on the gradient of its neighbour for similar vortices the initial interaction consists of mutual orbit and approach with only minor fluctuations in shape once the vortices approach to within a critical separation distance rapid merger occurs a simplified expression for this critical distance is derived and it indicates that binary tropical cyclones must approach to within 150 300 km before merger of the cores can occur when vortices have substantial differences in size or intensity the smaller weaker system shears into the outer circulation of the other and no core merger occurs it is suggested that this is typical of the interaction between tropical cyclones and both the monsoon trough and embedded mesoscale convective complexes the motion of interacting vortices consists largely of a mutual orbit which scales according to their relative sizes and intensities an important finding however is that higher frequency meanders may develop this indicates that a tropical cyclone interacting with a monsoonal shear zone may develop internal asymmetries and a trochoidal oscillations as a direct result of such interaction further weak mesoscale vortices within the cyclone circulation may continue to cause a track meander long after they have been sheared beyond the resolving capacity of atmospheric observations

the influence of preexisting boundaries on supercell evolution abstract a three dimensional nonhydrostatic cloud model is used to study the evolution of supercell thunderstorms with emphasis on the low level mesocyclone interacting with preexisting boundaries the impacts of low level environmental shear storm motion relative to boundary orientation and boundary strength are assessed in the low level shear experiments significant low level rotation is consistently observed earlier tends to be stronger and is longer lived in storms interacting with a boundary than in storms initiated in a homogeneous environment low level rotation is weaker in storms crossing the boundary and moving into the colder air in contrast all storms moving along or into the warm air ahead of the boundary develop significant low level rotation increasing the temperature gradient and shear across the boundary has little impact on the low level mesocyclone evolution storms interacting with a boundary characterized by only horizontal shear produce weaker mesocyclones than those creat

direct surface thermodynamic observations within the rear flank downdrafts of nontornadic and tornadic supercells abstract despite the long surmised importance of the hook echo and rear flank downdraft rfd in tornadogenesis only a paucity of direct observations have been obtained at the surface within hook echoes and rfds in this paper in situ surface observations within hook echoes and rfds are analyzed these mobile mesonet data have unprecedented horizontal spatial resolution and were obtained from the verifications of the origins of rotation in tornadoes experiment vortex and additional field experiments conducted since the conclusion of vortex the surface thermodynamic characteristics of hook echoes and rfds associated with tornadic and nontornadic supercells are investigated to address whether certain types of hook echoes and rfds are favorable or unfavorable for tornadogenesis tornadogenesis is more likely and tornado intensity and longevity increase as the surface buoyancy potential buoyancy as measured by the convective available potential energy and equivalent potential temperature in the r

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

the dissipation of a left moving cell in a severe storm environment abstract observations have shown that thunderstorms sometimes undergo updraft splitting where one updraft moves to the right of the mean tropospheric wind and the other to the left observations also show that the left moving updraft tends to dissipate approximately 15 min after the splitting process the right moving cell however may exist for up to a few hours idealized modeling studies suggest that this behavior is related to the clockwise turning of the environmental shear vectors with height the interaction between the environmental shear and the storms updraft produces a high low pressure couplet oriented downshear this pressure pattern produces favorable vertical accelerations for the right mover this same process inhibits upward motion for the left mover in this paper an idealized simulation is presented that suggests an additional process that contributed to the decay of the left moving updraft analysis of low level storm relative winds for the left moving cell indicated that the inflow

the garden city kansas storm during vortex 95 part i overview of the storm s life cycle and mesocyclogenesis abstract analysis of a supercell storm that produced an f1 tornado near garden city kansas is presented this event provided one of the first opportunities to synthesize data collected by a new airborne radar platform called eldora electra doppler radar developed by the national center for atmospheric research the early stages of development of the midlevel mesocyclone and the entire evolution of the low level mesocyclone are captured over a 70 min period the low level mesocyclone began as an incipient shallow circulation along a synoptic scale trough the circulation intensified and grew in depth via vortex stretching under the influence of a strong updraft as this rotation built up from the boundary layer it initially remained separate and distinct from the midlevel mesocyclone subsequently the two mesocyclones merge to produce a single column of rotation 4 5 km in diameter an occlusion downdraft develops within the mesocyclone circulation during the last passes by the storm signaling the beg

severe downslope windstorm calculations in two and three spatial dimensions using anelastic interactive grid nesting a possible mechanism for gustiness abstract the clark nonhydrostatic anelastic code is extended to allow for interactive grid nesting in both two and three spatial dimensions tests are presented which investigate the accuracy of three different quadratic interpolation formulae which are used to derive boundary conditions for the fine mesh model application of the conservation condition of kurihara and others is shown to result in significant improvements in the treatment of interactive nesting a significant improvement in the solutions for interactive versus parasitic nesting is also shown in the context of forced gravity wave flow this result for the anelastic system is in agreement with the earlier results of phillips and shukla who considered the hydrostatic shallow water system of equations the interactive nesting model is applied to the simulation of the severe downslope windstorm of 11 january 1972 in boulder using both two and three spatial dimensions the three dimensional simulation results in a gustiness signature in the

dominant northern hemisphere climate control over millennial scale glacial sea level variability abstract based on a radiocarbon and paleomagnetically dated sediment record from the northern red sea and the exceptional sensitivity of the regional changes in the oxygen isotope composition of sea water to the sea level dependent water exchange with the indian ocean we provide a new global sea level reconstruction spanning the last glacial period the sea level record has been extracted from the temperature corrected benthic stable oxygen isotopes using coral based sea level data as constraints for the sea level oxygen isotope relationship although the general features of this millennial scale sea level records have strong similarities to the rather symmetric and gradual southern hemisphere climate patterns we observe in constrast to previous findings pronounced sea level rises of up to 25 m to generally correspond with northern hemisphere warmings as recorded in greenland ice core interstadial intervals whereas sea level lowstands mostly occur during cold phases corroborated by climber 2 model results the close connection of millennial scale sea level changes to northern hemisphere temperature variations indicates a primary climatic control on the mass balance of the major northern hemisphere ice sheets and does not require a considerable antarctic contribution

a high resolution late quaternary upwelling record from the anoxic cariaco basin venezuela results are presented of a high resolution study of the planktonic foraminiferal faunas from two piston cores recovered from the cariaco basin in the southern caribbean sea the cariaco basin is a small anoxic marine basin on the northern continental margin of venezuela in an area today characterized by both seasonal trade wind induced upwelling and pronounced dry and wet seasons our data indicate that large changes in the intensity of upwelling and hence trade wind strength occurred in this region during the last glacial interglacial transition and throughout the holocene during the last glacial lowstand of sea level the cariaco basin was effectively isolated from the open caribbean along its northern margin by the then largely emergent tortuga bank oxic conditions existed in the deep cariaco basin at this time and surface productivity was low about 12 600 years ago the abrupt initiation of strong upwelling over the basin and the onset of permanent anoxia in the deep waters are coincident with the rapid rise of sea level that accompanied the peak interval of meltwater discharge from the laurentide ice sheet into the gulf of mexico strong upwelling between 12 600 and about 10 000 years ago may be related to intensified trade winds resulting in part from cooler sea surface temperatures in the caribbean and gulf of mexico after about 10 000 years ago upwelling intensity was reduced though highly variable a preliminary frequency domain analysis of the holocene portion of the cariaco basin time series suggests that solar forcing may explain a significant component of the century scale variability observed in the record of upwelling and trade wind strength

hurricane spiral bands abstract the spiral bands that occur in tropical cyclones can be conveniently divided into two classes outer bands and inner bands evidence is presented here that the outer bands form as the result of nonlinear effects during the breakdown of the intertropical convergence zone itcz through barotropic instability in this process a zonal strip of high potential vorticity the itcz shear zone or monsoon trough begins to distort in a varicose fashion with the potential vorticity pv becoming pooled in local regions that are connected by filaments of high pv as the pooled regions become more axisymmetric the filaments become thinner and begin to wrap around the pv centers it is argued that inner bands form in a different manner as a tropical cyclone intensifies due to latent heat release the pv field becomes nearly circular with the highest values of pv in the cyclone center the radial gradient of pv provides a state on which pv waves the generalization of rossby waves can propagate the nonline