intraseasonal variability of the saharan heat low and its link with midlatitudes abstract the saharan heat low shl is thought to be a key feature of the west african monsoon and its variations during the summer season have not yet been systematically assessed to characterize the intraseasonal variations of the shl real and complex empirical orthogonal function analyses were applied to the 850 hpa potential temperature field over northern africa and the mediterranean using ncep department of energy doe atmospheric model intercomparison project amip ii reanalysis results a robust quasi propagative mode was highlighted over north africa and the mediterranean this mode consists of two phases the west phase corresponds to a maximum temperature over the coast of morocco mauritania propagating southwestward and a minimum between libya and sicily propagating southeastward the east phase corresponds to the opposite temperature structure which propagates as in the west phase a lag composite analysis revealed that this shl mode was preceded by large scale midlatitude intrase

regional circulation characteristics associated with a cold surge event over east asia during winter monex abstract a case study is presented of the cold surge over east asia during 9 13 december 1978 using nine vertical levels of winter monex data the surge event is manifested by a rapid meridional mass flow in the lower troposphere from the midlatitudes 30 n to the equator within three days during the period studied the lower tropospheric circulation dramatically changed in extent and intensity with the onset of the surge event the main divergent maximum began shifting from the south china sea to southeast china at the same time the upper tropospheric circulation correspondingly changed in a reversed order from that of the lower troposphere and a direct vertical coupling between flows in the low and high troposphere was observed the time averaged meridional mass circulation between 100 and 126 e reveals a two cell structure the southern cell is located between the northern south china sea and the equator and the northern cell between midlatitudes and the northern south china sea analysis of s

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

the seasonal cycle in the lower troposphere over west africa from sounding observations in situ sounding observations from selected sites in west and central africa were used to document the seasonal cycle in the lower troposphere associated with the annual migration of precipitation the main purpose of this exercise is to provide a description of the seasonal cycle free of the errors and biases of numerical models which can be substantial in this region many features in the circulation typically associated with the west african monsoon wam exist through almost the entire seasonal cycle in the region the low level easterly jet known as the african easterly jet during the monsoon season remains prominent and closely related to the rain band in other phases of the seasonal cycle even when the rain band is located near the equator the surface southerly monsoon onshore flow penetrates through the rain band in the entire season cycle the depth of the southerly surface monsoon flow undergoes a seasonal variation and is greatest during the peak season of the monsoon a unique feature is a low level 700 hpa northerly flow penetrating through the rain band in various phases of the seasonal cycle risking possible biases of land bound sounding data these observations are synthesized into a schematic describing the salient features in the low level circulation associated with the rain band in different phases of the seasonal cycle this synthesized circulation pattern in west africa is in contrast to the traditional conceptual circulation pattern associated with the marine itcz and other monsoon systems the results from this study call for more research attention to the low level meridional circulation for a full understanding of the dynamics of the wam this study also advocates the study of the wam to be conducted in a broad context of the seasonal cycle in the region and in comparison to the marine itcz copyright 2006 royal meteorological society

the role of the indian monsoon onset in the west african monsoon onset observations and agcm nudged simulations in spring the inland penetration of the west african monsoon wam is weak and the associated rainband is located over the guinean coast then within a few days deep convection weakens considerably and the rainband reappears about 20 days after over the sahel where it remains until late september signalling the summer rainy season over the period 1989 2008 a teleconnection induced by the indian monsoon onset is shown to have a significant impact on the wam onset by performing composite analyses on both observational data sets and atmospheric general circulation model simulations ensembles where the model is nudged to observations over the indian monsoon sector the initiation of convective activity over the indian subcontinent north of 15 n at the time of the indian monsoon onset results in a westward propagating rossby wave establishing over north africa 7 15 days after a back trajectory analysis shows that during this period dry air originating from the westerly subtropical jet entrance is driven to subside and move southward over west africa inhibiting convection there at the same time the low level pressure field over west africa reinforces the moisture transport inland after the passage of the wave the dry air intrusions weaken drastically hence 20 days after the indian monsoon onset convection is released over the sahel where thermodynamic conditions are more favourable this scenario is very similar in the observations and in the nudged simulations meaning that the indian monsoon onset is instrumental in the wam onset and its predictability at intraseasonal scale

the structure and evolution of cold surges east of the rocky mountains abstract northerly surges of cold air often move southward along the eastern side of the rockies from southern canada into mexico the strongest surges which generally develop in midwinter are associated with temperature decreases and pressure rises of 20 30 c and 15 30 mb respectively within 24 h surges are usually accompanied by a meridionally elongated pressure ridge and strong low level ageostrophic winds that parallel the terrain the width of the pressure ridging is approximately 1000 km over the southern plains but decreases to only a few hundred kilometers when the surge enters mexico this paper provides a detailed description of a northerly surge to the east of the rocky mountains that occurred on 12 14 november 1986 using both observational and model data the structural evolution of the surge is analyzed in addition the dynamics of the event is explored by diagnosing the momentum thermodynamic energy and vorticity equations to determine the typical synoptic scale evolution of these

role of inertial instability in the west african monsoon jump the west african monsoon jump is a sudden shift in the latitude of the west african precipitation maximum from the guinean coast near 4 n into sahel near 12 n in late june or early july an examination of reanalyses and observations indicates that the sahel rainy season develops smoothly and the monsoon jump occurs because of an abrupt decrease in guinean coast rainfall we show that this abrupt end of the coastal rainy season occurs when inertial instability develops over the region 1 month later than it develops in the vicinity of the marine atlantic intertropical convergence zone the reason for this delay is the presence of the african easterly jet which places strong negative meridional zonal wind gradients over the coast to preserve the inertially stable environment when the african easterly jet moves farther north due to the seasonal solar forcing these gradients weaken and then reverse to satisfy the threshold condition for inertial instability the rapid end of the guinean coast rainy season follows the northward movement and intensity of the african easterly jet are controlled by the seasonal development of strong meridional land surface temperature gradients and are independent of the formation of the atlantic cold tongue this explanation for the west african monsoon jump relates the phenomenon to the shape and location of the african continent including the low latitude position of the guinean coast and the large expanse of the continent to the north

the salinity temperature and δ18o of the glacial deep ocean we use pore fluid measurements of the chloride concentration and the oxygen isotopic composition from ocean drilling program cores to reconstruct salinity and temperature of the deep ocean during the last glacial maximum lgm our data show that the temperatures of the deep pacific southern and atlantic oceans during the lgm were relatively homogeneous and within error of the freezing point of seawater at the ocean s surface our chloride data show that the glacial stratification was dominated by salinity variations in contrast with the modern ocean for which temperature plays a primary role during the lgm the southern ocean contained the saltiest water in the deep ocean this reversal of the modern salinity contrast between the north and south atlantic implies that the freshwater budget at the poles must have been quite different a strict conversion of mean salinity at the lgm to equivalent sea level change yields a value in excess of 140 meters however the storage of fresh water in ice shelves and or groundwater reserves implies that glacial salinity is a poor predictor of mean sea level

west african convection regimes and their predictability from submonthly forecasts recurrent convection regimes are identified during the extended west african monsoon wam season may nov using a k means clustering of 1980 2013 noaa daily outgoing longwave radiation olr and are well reproduced in 1996 2015 ecmwf week 1 reforecasts despite systematic biases one regime of broad drying across the sahel in the early may jun and late oct wam is of particular interest regarding the prediction of onset date this regime is associated with an anticyclonic cell along the atlantic coast of west africa leading to a weakened monsoon flow and subsiding anomalies across the sahel teleconnections of this regime with the indian monsoon sector are identified through modulations of the walker circulation alongside relationships to mjo phase 3 more than 10 days in advance when convection is enhanced over the indian ocean other regimes are associated with westward propagating anomalous convective cells along two distinct wave trains at 15 circ hbox n and 24 circ hbox n during the core jul sep and late oct nov wam respectively and translate into wet anomalies transiting across the sahel a regime of broad sahel wetting in the core wam more frequent since the 1990s is related to global sst warming agreeing with the observed recovery of sahel rainfall ecmwf skill in forecasting regime sequences decreases from week 1 to 4 leads except in the case of the above mentioned regime associated with early season dry spells translating into the potential for skillful wam onset date predictions our analysis suggests that sources of predictability include relationships to the mjo and the indian monsoon sector which need to be further examined to benefit subseasonal forecasting efforts in west africa and ultimately agricultural planning and food security across the sahel

chronostratigraphy of odp 181 site 1121 sediment core southwest pacific ocean using 10be 9be dating of entrapped ferromanganese nodules a 10be 9be based chronostratigraphy has been determined for odp 181 site 1121 sediment core recovered from the foot of the campbell plateau southwest pacific ocean this core was drilled through the campbell skin drift in ca 4500 m water depth on the mid western margin of the extensive campbell nodule field beneath the flow of the major cold water deep western boundary current dwbc in the absence of detailed biostratigraphy beryllium isotopes have provided essential time information to allow palaeo environmental interpretation to be undertaken on the upper 7 m of the core measured 10be 9be ratios of sediment and of ferromanganese nodules entrapped in the sediment decrease systematically with depth in the core in accordance with radioactive decay however the 10be 9be data diverge from ca 3 m below the seafloor mbsf to the top of the core giving rise to several possible geochronological models the preferred model assumes that the measured 10be 9be ratios of the nodule rims reflect initial 10be 9be ratios equivalent to contemporary seawater and that these can be used to derive the true age of the sediment where the nodules occur the nodule rim ages can be then used to interpret the sediment 10be 9be data which indicate an overall age to ca 7 mbsf of ca 17 5 ma the derived chronology is consistent with diatom biostratigraphy which indicates an age of 2 2 3 6 ma at 1 mbsf calculated sedimentation rates range from 8 to 95 cm m y 1 with an overall rate to 7 mbsf of ca 39 cm m y 1 the lowest rates generally coincide with the occurrence of entrapped nodules and reflect periods of increased bottom current flow causing net sediment loss growth rates of individual nodules decrease towards the top of the sediment core similar to the observed decrease in growth rate from core to rim of seafloor nodules from the campbell nodule field this may be related to an overall increase in the vigour of the dwbc from ca 10 ma to the present

the dynamics of the west african monsoon part v the detection and role of the dominant modes of convectively coupled equatorial rossby waves abstract this study is the last in a series of papers addressing the dynamics of the west african summer monsoon at intraseasonal time scales between 10 and 90 days the signals of convectively coupled equatorial rossby er waves within the summer african monsoon have been investigated after filtering noaa outgoing longwave radiation olr data within a box delineated by the dispersion curves of the theoretical er waves two families of waves have been detected in the 10 100 day periodicity band by performing a singular spectrum analysis on a regional index of er filtered olr for each family the first eof mode has been retained to focus on the main convective variability signal within the periodicity band of 30 100 days an er wave pattern with an approximate wavelength of 13 500 km has been depicted this er wave links the mjo mode in the indian monsoon sector with the main mode of convective variability over west and central africa this confirms the investigations carried out in previous studies wi

cold surges in tropical and extratropical south america the strong event in june 1994 the authors investigate one case of surges of polar air that occasionally propagate into southeastern brazil during wintertime and are harmful to coffee production because of the freezing conditions associated with them the cooling is also observed in southern and with less intensity in western amazonia the event of 26 june 1994 is studied the frost event caused a sharp drop in coffee production and similarly dramatic increases in coffee prices the event was characterized by the presence of an anticyclonic perturbation off the coast of chile that enters south america and crosses the andes mountains south of 458s hours later the anticyclone center tracks equatorward until it reaches 208 308s and then shifts eastward toward the atlantic based on a diagnostic analysis using the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis a two stage process can be distinguished in the starting period midtropospheric troughing is established east of the andes over central argentina due to stretching and or vorticity advection this troughing is responsible for cold advection along the eastern flank of the andes which produces substantial temperature drops and deepens the upper trough locally the increase in the magnitude of the zonal temperature gradients east of the cold advection zone at levels near 700 hpa is another result of the cooling process this feature would ensure that the cold advection be extended eastward producing an upper level trough local deepening there this intensification would produce an increase in the cyclonic vorticity advection which would tend to produce sea level pressure drops underneath this near surface low pressure area would eventually contribute to the southeastern brazil sb cooling due to the associated southerly winds another feature is the near surface local anticyclonic generation due to midtropospheric cold advection associated with descending motions low level divergence and anticyclonic growth eventually this high pressure would also contribute to produce southwesterlies to transport cold air to sb the paper is divided in two sections the first part is a study of synoptic and climatic aspects of the cold surge episodes by using daily surface climatic observations the purpose of the second part is to analyze the dynamic aspects of this cold episode and to study the cooling mechanisms by using the four times daily surface and upper air ncep ncar reanalysis as well as to look for possible predictors

intensified deep pacific inflow and ventilation in pleistocene glacial times the production of cold deep waters in the southern ocean is an important factor in the earth s heat budget1 the supply of deep water to the pacific ocean is presently dominated by a single source the deep western boundary current east of new zealand here we use sediment records deposited under the influence of this deep western boundary current to reconstruct deep water properties and speed changes during the pleistocene epoch in physical and isotope proxies we find evidence for intensified deep pacific ocean inflow and ventilation during the glacial periods of the past 1 2 million years the changes in throughflow may be directly related to an increased production of antarctic bottom water during glacial times possible causes for such an increased bottom water production include increasing wind strengths in the southern ocean or an increase in annual sea ice formation leaving dense water after brine rejection and thereby enhancing deep convection we infer also that the global thermohaline circulation was perturbed significantly during the mid pleistocene climate transition between 0 86 and 0 45 million years ago

major depositional events under the deep pacific inflow the main inflow of deep water to the pacific is via a deep western boundary current as the current passes along the continental margin off eastern new zealand it receives much terrigenous sediment which together with the biogenic pelagic supply has been reworked into a suite of large drifts ocean drilling program site 1123 on the north chatham drift and site 1124 on the rekohu drift have yielded high resolution and well dated records of carbonate and terrigenous fluxes for the past 3 m y deposition at both drifts was affected by a deep western boundary current background sediment flux and by glacial interglacial cycles in addition drifts received site specific flux pulses caused by 1 sediment remobilization under a vigorous antarctic circumpolar current 2 increased oceanic productivity 3 overspill of sediment from the newly formed hikurangi channel and 4 generally increased discharge from the hikurangi channel under intensified quaternary uplift and paleoclimatic oscillations

intermittent african easterly wave activity in a dry atmospheric model influence of the extratropics abstracta dynamical model is constructed of the northern summertime global circulation maintained by empirically derived forcing based on the same dynamical code that has recently been used to study african easterly waves aews as convectively triggered perturbations thorncroft et al leroux and hall in the configuration used here the model faithfully simulates the observed mean distributions of jets and transient disturbances and explicitly represents the interactions between them this simple gcm is used to investigate the origin and intraseasonal intermittency of aews in an artificially dry no convection context a long integration of the model produces a summertime climatology that includes a realistic african easterly jet and westward propagating 3 5 day disturbances over west africa these simulated waves display intraseasonal intermittency as the observed aews also do further experiments designed to discern the source of this intermittency in the model show that the simulated waves are

planetary and synoptic scale signatures associated with central american cold surges abstract motivated by outstanding issues from a previous case study of a midlatitude cold surge that affected mexico and central america the climatology of central american cold surges is examined in this paper an independently derived listing of 177 cold surge events is employed for which the following properties are tabulated onset date duration time between cold surge events latitude of maximum equatorward penetration min and 48 h maximum surface temperature change at merida mexico t these data show that 75 of the cold surges have durations of 2 6 days the same timescale as mobile disturbances in the westerlies also there does not appear to be any relationship between t and the duration of the event although cold surges that penetrate to low latitudes min 7 10 n have a weak tendency to persist longer than those that do not penetrate to low latitudes min 15 20 n in addition the reding data indicate that the cold surges tend to reach their most equatorward extent where

easterly wave regimes and associated convection over west africa and tropical atlantic results from the ncep ncar and ecmwf reanalyses ncep ncar and ecmwf daily reanalyses are used to investigate the synoptic variability of easterly waves over west africa and tropical atlantic at 700 hpa in northern summer between 1979 1995 1979 1993 for ecmwf spectral analysis of the meridional wind component at 700 hpa highlighted two main periodicity bands between 3 and 5 days and 6 and 9 days the 3 5 day easterly wave regime has already been widely investigated but only on shorter datasets these waves grow both north and south of the african easterly jet aej the two main tracks noted over west africa at 5 n and 15 n converge over the atlantic on latitude 17 5 n these waves are more active in august september than in june july their average wavelength phase speed varies from about 3000 km 8 m s 1 north of the jet to 5000 km 12 m s 1 south of the jet rainfall convection and monsoon flux are significantly modulated by these waves convection in the inter tropical convergence zone itcz being enhanced in the trough and ahead of it with a wide meridional extension compared to the 3 5 day waves the 6 9 day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics the only main track is located north of the aej along 17 5 n both over west africa and the atlantic the mean wavelength is higher about 5000 km long and the average phase speed is about 7 m s 1 then the wind field perturbation is mostly evident at the aej latitude and north of it the perturbation structure is similar to that of 3 5 days in the north except that the more developed circulation centers moving more to the north lead to a large modulation of the jet zonal wind component south of the aej the wind field perturbation is weaker and quite different the zonal wind core of the jet appears to be an almost symmetric axis in the 6 9 day wind field pattern a clockwise circulation north of the aej being associated with a counter clockwise circulation south of the jet and vice versa these 6 9 day easterly waves also affect significantly rainfall convection and monsoon flux but in a different way inducing large zonal convective bands in the itcz mostly in the trough and behind it as opposed to the 3 5 day wave regime these rainfall anomalies are associated with anomalies of opposite sign over the guinea coast and the sahelian regions over the continent these waves are more active in june july and in august september over the ocean gate phase i gave an example of such an active 6 9 day wave pattern considered as a sequence of weak easterly wave activity this phase was also a sequence of high 6 9 day easterly wave activity we suggest that the 6 9 day regime results from an interaction between the 3 5 day easterly wave regime maintained by the barotropic baroclinic instability of the aej and the development of strong anticyclonic circulations north of the jet over west africa and both north and south of the jet over the atlantic significantly affecting the jet zonal wind component the permanent subtropical anticyclones azores libya st helena could help initiation and maintenance of such regime over west africa and tropical atlantic based on an a priori period band criterion our synoptic classification has enabled us to point out two statistical and meteorological easterly wave regimes over west africa and tropical atlantic ncep ncar and ecmwf reanalyses are in good agreement the main difference being a more developed easterly wave activity in the ncep ncar reanalyses especially for the 3 5 day regime over the atlantic

orbitally influenced vegetation record of the mid pleistocene climate transition offshore eastern new zealand odp leg 181 site 1123 a census of plio pleistocene 5 0 ma terrestrial palynomorph assemblages from odp site 1123 located 1100 km offshore eastern new zealand and in a water depth of 3290 m reveals marked variations in warm cyathea tall tree podocarpus prumnopitys dacrydium cupressinum and cold halocarpus phyllocladus nothofagus fusca type coprosma climate indicator species at milankovitch scale periodicities time series analysis indicates that the vegetation record is covariant with marine climate proxies carbonate content and is strongly coherent at the 40 ka and 100 ka orbital frequency a pronounced increase in amplitude and a coeval decrease in frequency of climate cycles from 40 to 100 ka occurs between 0 92 and 0 62 ma and provides a rare vegetation record of a fundamental reorganisation of earth s climate system the mid pleistocene climate transition despite the long distance from land and great water depth terrestrial palynomorphs are relatively abundant in this high resolution marine sediment record where they reflect a southern north island source the predominance of robust spores and buoyant pollen cyathea podocarpus prumnopitys indicates considerable current sorting and degradation of more delicate morphotypes during transport across the eastern north island shelf little evidence is found for sorting and transport processes being significantly modulated by climate

case study of an intense african easterly wave abstract the life cycle of an intense african easterly wave aew over the african continent is examined using european centre for medium range weather forecasts ecmwf operational analyses meteosat satellite images and synoptic observations this system the strongest aew of 2000 can be tracked from central north africa into the eastern atlantic ocean where it is associated with the genesis of hurricane alberto synoptic analysis of the kinematic and thermodynamic fields is supplemented by analysis of potential vorticity pv allowing exploration at the role of multiple scales in the evolution of this aew the authors analysis promotes the division of the aew life cycle into three distinctive phases i initiation the aew development is preceded by a large convective event composed of several mesoscale convective systems over elevated terrain in sudan this convection provides a forcing on the baroclinically and barotropically unstable state that exists over tropical north africa ii baroclini

factors affecting cold air outbreaks east of the rocky mountains abstract a two layer isentropic model is used to examine the parameters controlling the severity of cold air outbreaks east of the rocky mountains and in particular the channeling effect of the mountain range the case that is scrutinized is the cold air outbreak of early february 1989 the parameters investigated include a the baro clinicity expressed as the ratio of upper to lower layer zonal scale velocities b the rossby number c the height of the topography d the initial longitudinal position of the cold air mass relative to the mountain ridge and e the presence of a wave in the upper level flow it is found that the rossby number determines the intensity of a cold air outbreak whereas the baroclinicity as well as the height of the orography and the initial position of the cold air mass control the extent to which the outbreak is channeled by the mountain range an upper level trough over the central united states favors the evolution of an outbreak

planktonic foraminiferal evidence on late pliocene quaternary near surface water masses at odp site 1123b northern chatham rise east of new zealand planktonic foraminifera are used to identify late pliocene quaternary near surface water masses on the northeastern flank of chatham rise by comparison with faunas in core tops east of new zealand in an overview study distance measures ordinations and discriminant analysis are applied to 32 faunas from site 1123b to identify similar faunas among 35 core tops between 35 and 61 s east of new zealand many site 1123b faunas in the 2 72 myr interval sampled compare with those in core tops on the northern side of chatham rise from a similar latitude and are identified as transitional zone assemblages now associated with the subtropical gyre this result is consistent with studies of late quaternary planktonic foraminifera from this region and suggests that typically the subtropical front was locked to chatham rise through glacial and interglacial periods at least back to the late pliocene however a fauna at ca 1 17 ma compares with subpolar assemblages in core tops between 44 and 48 s and identifies cooler surface water expectedly closer sampling may reveal additional periods when southern water moved over the northeastern flank of chatham rise although the dominance of globorotalia inflata a species typical of the southern margin of subtropical gyres is a principal feature of site 1123b faunas in a minority it is replaced as the most abundant species by dextral populations of neogloboquadrina pachyderma particularly about the time of the middle pleistocene transition close analogues of these variant transitional assemblages are not present in core tops about chatham rise but sediment trap and coretop data from other regions suggest that they identify high fertility in the mixed layer associated with upwelling or mixing of water masses the proportion of sinistrally coiled neogloboquadrina pachyderma rises to ca 0 6 between ca 2 45 and 2 57 ma soon after the intensification of northern hemisphere glaciation although the coiling data indicate subantarctic near surface water the species remains rare as the faunas retain their transitional zone character only minor entrainment of subantarctic water may have occurred

biostratigraphy and paleoceanographic significance of paleocene radiolarians from offshore eastern new zealand abstract a 100 m thick paleocene sequence of mainly pelagic sediments at odp site 1121 on the eastern flanks of the campbell plateau contains few to common radiolarians of relatively low diversity in the lower 40 m early to early late paleocene and abundant diverse radiolarian assemblages in the upper 60 m mid late paleocene the 150 taxa recorded from the entire paleocene interval are thought to under represent the actual species diversity by at least one half as many morphotypes have not been differentiated below the level of genus assemblages in the lower 40 m are similar to those described from onland new zealand and dsdp site 208 northern lord howe rise they are correlated with south pacific radiolarian zones rp4 and rp5 assemblages in the upper 60 m differ from other known late paleocene assemblages in the great abundance of plagiacanthids and cycladophorids similarities are noted with later cenozoic cool water assemblages this upper interval is correlated with south pacific zone rp6 as revised herein based on comparison with faunas from site 208 and marlborough new zealand the interval is also correlated with the upper part of north atlantic zone rp6 rp6b c based on the presence of aspis velutochlamydosaurus plectodiscus circularis and pterocodon poculum other species such as buryella tetradica and buryella pentadica are valuable for local correlation but exhibit considerable diachroneity between the pacific indian and atlantic oceans an age model for the paleocene interval at site 1121 based on well constrained nannofossil and radiolarian datums indicates that the rate of compacted sediment accumulation doubles from 15 to 30 mm ka at the rp5 rp6 zonal boundary in large part this is due to a sudden and pronounced increase in accumulation rates for all siliceous fossils radiolarians and larger diatoms increase from 10 000 specimens cm2 ka this apparent increase in biosiliceous productivity is age equivalent to a mid paleocene cooling event 57 59 ma identified from global stable isotope records that is associated with the heaviest 13c values for the entire cenozoic

monsoons and the dynamics of deserts the existence of subtropical deserts such as the sahara has often been attributed to the annual mean zonal mean hadley circulation which shows strong descent in the subtropics however the zonal mean hadley circulation shows considerable evolution over the course of the year with very strong subtropical descent during winter but practically no zonal mean subtropical descent during summer when rainfall over the eastern sahara and the mediterranean is least charney 1975 proposed a biosphere albedo feedback mechanism whereby local anthropogenic effects related to over grazing could affect the radiative balance enhancing summertime diabatic descent and leading to desertification of the subtropics in general the present study which uses an idealized model suggests a monsoon desert mechanism for desertification whereby remote diabatic heating in the asian monsoon region can induce a rossby wave pattern to the west integral with the rossby wave solution is a warm thermal structure that interacts with air on the southern flank of the mid latitude westerlies causing it to descend this adiabatic descent is localized over the eastern sahara and mediterranean and over the kyzylkum desert to the south east of the aral sea by the mountains of north africa and south west asia trajectories indicate that the monsoon desert mechanism does not represent a simple walker type overturning cell instead the descending air is seen to be mainly of mid latitude origin it is speculated that the monsoon forced adiabatic descent may result in clear air and therefore a local diabatic enhancement which effectively doubles the strength of descent with this mechanism desertification can be forced by remote changes in monsoon strength rather than by local effects this conclusion is supported by the observed dramatic strengthening of descent over the mediterranean and east sahara during the onset of the asian monsoon and on the longer timescale by relating prehistoric lake levels to milankovitch monsoon forcing the latter may help to explain the perceived discrepancies between the palaeoclimate of the eastern sahara and the strength of a tropic wide monsoon the monsoon desert mechanism may not be confined to the asian monsoon alone and the existence of other monsoon climate regions over the globe may in a similar way explain the observed summertime strengthening of the oceanic sub tropical anticyclones and the existence of western continental deserts and mediterranean type climate regions

cold air incursions over subtropical south america mean structure and dynamics abstract synoptic scale incursions of midlatitude air moving into subtropical south america to the east of the andes cordillera are observed to occur year round with a periodicity of about 1 2 weeks during wintertime they have a profound impact upon the low level temperature field and extreme episodes produce freezing conditions from central argentina to southern brazil and bolivia warm season episodes produce less dramatic variations of temperature but they organize deep convection in the form of synoptic scale bands of convective cloudiness along the leading edge of the cool air on the basis of 17 yr of ncep ncar reanalysis and outgoing longwave radiation fields the mean synoptic scale structure and evolution of these incursions is documented using a simple compositing technique the underlying physical mechanisms responsible for the occurrence of these incursions are also investigated by diagnosing the leading dynamic and thermodynamic forcing of their development

evidence of two independent modes of convection at intraseasonal timescale in the west african summer monsoon 1 two main independent modes of convection at intraseasonal timescale in the west african summer monsoon are highlighted one depicts a meridional dipole of convection over west africa linked to a modulation of the latitude of the inter tropical convergence zone itcz it is associated with a westward propagative signal of the convection over the sahel and to a modulation of cyclonic vorticity at these latitudes the other one is characterized by a stationary uniform modulation of convection in the itcz without any significant modulation of its latitudinal location it is associated with a modulation of the zonal wind component over the eastern equatorial atlantic the dominant periodicity of these two modes is around 15 days it is necessary to take into account the relative parts of these two modes in order to accurately analyze the mechanisms involved in intraseasonal variability of the african monsoon

the west african monsoon dynamics part i documentation of intraseasonal variability intraseasonal variability in the west african monsoon is documented by using daily gridded datasets of rainfall and convection and reanalyzed atmospheric fields over the period 1968 90 rainfall and convection over west africa are significantly modulated at two intraseasonal timescales 10 25 and 25 60 day leading to variations of more than 30 of the seasonal signal a composite analysis based on the dates of the maximum minimum of a regional rainfall index in wet dry sequences shows that these sequences last on average 9 days and belong to a main quasiperiodic signal of about 15 days a secondary periodicity of 38 days is present but leads to a weaker modulation during a wet dry sequence convection in the itcz is enhanced weakened and its northern boundary moves to the north south while the speed of the african easterly jet decreases increases the speed of the tropical easterly jet increases decreases and the monsoon flow becomes stronger weaker all these features being similar to the ones associated with interannual variability characterizing wet and dry years this modulation of convection at intraseasonal timescales is not limited to west africa but corresponds to a westward propagating signal from eastern africa to the western tropical atlantic an enhanced monsoon phase is associated with stronger cyclonic activity in the low levels over the sahel associated with stronger moisture advection over west africa five days before the full development of the wet phase a stronger cyclonic circulation at 20 e induces enhanced southerly winds along 25 e where convection enhances while another westwardpropagating cyclonic circulation is located at 0 this atmospheric pattern is linked to the enhancement of the subsiding branch of the northern hadley cell at 35 n northerly advection of drier air over west africa and to increased dry convection in the heat low at 20 n it propagates westward leading to a zonally extended area of enhanced monsoon winds over west africa consistent with the occurrence of the wet phase

cold air incursions over subtropical and tropical south america a numerical case study abstract synoptic scale incursions of cold midlatitude air that penetrate deep into the tropics are frequently observed to the east of the andes cordillera these incursions are a distinctive year round feature of the synoptic climatology of this part of south america and exhibit similar characteristics to cold surges observed in the lee of the rocky mountains and the himalayan plateau while their large scale structure has received some attention details of their mesoscale structural evolution and underlying dynamics are largely unknown this paper advances our understanding in these matters on the basis of a mesoscale numerical simulation and analysis of the available data during a typical case that occurred in may of 1993 the large scale environment in which the cold air incursion occurred was characterized by a developing midlatitude wave in the middle and upper troposphere with a ridge immediately to the west of the andes and a downstream trough over eastern south america at the surface a migra

evolution of the sedimentary system beneath the deep pacific inflow off eastern new zealand results from ocean drilling program sites 1121 1124 show the eastern new zealand oceanic sedimentary system enzoss evolved in response to 1 the inception of the circum antarctic circulation 2 orbital and non orbital regulation of the global thermohaline flow and 3 development of the new zealand plate boundary enzoss began in the early oligocene following opening of the tasmanian gateway and inception of the ancestral antarctic circumpolar current acc and sw pacific deep western boundary current dwbc widespread erosion marked by the marshall paraconformity was followed by extensive drift formation in the late oligocene early miocene alternating nannofossil chalk and nannofossil rich mud deposited in response to 41 kyr orbital regulation of the abyssal circulation with the mudstones representing times of increased inflow of corrosive southern source waters drift deposition at the deepest sites was interrupted by bouts of erosion coincident with mi1 5 isotopic events signifying expansions of the east antarctic ice sheet and enhanced bottom water formation by late miocene times the basic enzoss was established south of bounty trough the energetic acc instigated an erosional low depositional regime to the north where the dwbc prevailed orbitally regulated drift deposition continued increased convergence at the new zealand plate boundary enhanced the terrigenous supply but little of this sediment reached the deep enzoss as the three main sediment conduits solander bounty and hikurangi channels had not fully developed the plio pleistocene heralded a change from a carbonate to terrigenous dominant supply caused by interception of the dwbc by the three channels 1 6 ma for bounty and hikurangi time of solander interception unknown the solander and bounty fans and hikurangi fan drift systems formed and drifts downstream of those systems received terrigenous detritus supply increased with accelerating uplift along the plate boundary but delivery to the dwbc was regulated by eustatic fluctuations of sea level times of maximum supply to all three channels was during glacial lowstands whereas the supply either ceased bounty solander or reduced hikurangi in highstands in glacial times sediment was entrained by a dwbc invigorated by an increased input of antarctic bottom water the acc also accelerated under strengthened glacial winds thus glacials were times of optimum sediment supply to enzoss depocentres where depositional rates were 2 3 times more than interglacial rates

factors influencing the distribution patterns of recent deep sea benthic foraminifera east of new zealand southwest pacific ocean abstract this study investigates which combination of environmental factors most strongly influences the distribution patterns of benthic foraminferal tests 0 63 m in a region bisected by the subtropical front east of new zealand seafloor sample sites extend from outer shelf 90 m to abyssal 4700 m depths across substrates ranging from biogenic terrigenous gravelly sand to hemipelagic mud and occur under the influence of antarctic intermediate water aaiw and circumpolar deep waters as well as receiving detritus from both subtropical and subantarctic surface water masses elevated values of the planktic foraminiferal fragmentation index and reworked small paleogene planktic foraminifera at outer shelf and bathyal depths coincide with areas of strong bottom currents q mode cluster analysis on the census counts of 398 benthic species clusters the 66 samples into three large groups shallow bathyal abyssal and at a lower level 10 mappable associations are recognised a combination of canonical correspondence analysis and a correlation coefficient matrix was used to relate the faunal data to a set of measured environmental proxies these analyses show that factors that have a relationship with depth are the most significant in determining foraminiferal distribution the principal environmental factors which appear to most strongly influence this benthic foraminiferal distribution are dissolved oxygen content in bottom waters sustainability of organic carbon flux rates seasonality of food supply lateral advection of water masses bottom water carbonate corrosiveness energetic state at the benthic boundary layer grain size composition of substrate salinity and temperature of the bottom waters shallow water associations 90 1250 m dominated by cassidulina carinata and trifarina angulosa occur within coarse substrates under well oxygenated high energy regimes and sustained food input the occurrence of the bathyal associations 230 2840 m dominated by c carinata alabaminella weddellensis and abditodentrix pseudothalmanni closely mirrors the distribution of aaiw within a region of variable food supply the sustainability of food supply combined with bottom water type and associated ventilation and dissolution strongly influence the composition of abyssal associations 1200 4700 m mostly dominated by epistominella exigua and a weddellensis

cenozoic oceanographic evolution of the sw pacific gateway introduction the region from the southern tip of the campbell plateau to the northern hikurangi plateau spans 21 of latitude equivalent to the distance between southern florida and newfoundland in the north atlantic from new zealand eastwards along the chatham rise to the louisville seamounts is 22 of longitude about 2000 km this region equivalent in area to a major part of the north american basin of the north atlantic now contains 11 stratigraphically useful drill holes seven more than the four of the dsdp sites 277 278 279 and 594 the latest holes ocean drilling program sites 1119 1125 were drilled on odp leg 181 the rst visit of the drilling ship joides resolution to the region in august october 1998 under arduous southern winter conditions fig 1

benthic foraminiferal proxy evidence for the neogene palaeoceanographic history of the southwest pacific east of new zealand abstract canonical correspondence analysis indicates that the distribution of neogene benthic foraminiferal faunas 63 m in seven dsdp and odp sites 500 4500 m water depth east of new zealand 38 51 s 170 e 170 w is most strongly influenced by depth water mass stratification and secondly by age palaeoceanographic changes influencing faunal composition and biotic evolution stratigraphic faunal changes are interpretted in terms of the pulsed sequential development of southern and later northern polar glaciation and consequent cooling of bottom waters increased vertical and lateral stratification of ocean water masses and increased overall and seasonal surface water productivity oligocene initiation of the antarctic circumpolar current and deep western boundary current dwbc flowing northwards past new zealand resulted in extensive hiatuses throughout the southwest pacific some extending through into the miocene planktic foraminiferal fragmentation index values indicate that carbonate dissolution was significant at abyssal depths throughout most of the neogene peaking at upper abyssal depths in the late miocene 11 7 ma with the lysocline progressively deepened thereafter miocene abyssal faunas are dominated by globocassidulina subglobosa and oridorsalis umbonatus with increasing epistominella exigua after 16 ma at upper abyssal depths peak abundances of epistominella umbonifera indicate increased input of cold southern component water to the dwbc at 7 6 ma faunal association changes imply establishment of the modern oxygen minimum zone upper circumpolar deep water in the latest miocene significant latitudinal differences between the benthic foraminiferal faunas at lower bathyal depths indicate the existence of an oceanic front along the chatham rise location of present subtropical front since the early late miocene at least with more pulsed productivity higher e exigua along the south side modern antarctic intermediate water faunal associations were established north of the chatham rise at 10 9 ma and south of it at 3 1 5 ma middle upper bathyal faunas on the campbell plateau are dominated by reticulate bolivinids during the early and middle miocene indicative of sustained productivity above relatively sluggish suboxic bottom waters faunal changes and hiatuses indicate increased current vigour over the campbell plateau from the latest miocene on surface water productivity food supply appears to have increased in three steps at times of enhanced global cooling marked by substantially increased relative abundance of 1 abditodentrix pseudothalmanni alabaminella weddellensis cassidulina norvangi 16 15 ma increased pulsed productivity 2 bulimina marginata f aculeata nonionella auris trifarina angulosa uvigerina peregrina 3 1 5 ma increased overall productivity and 3 cassidulina carinata 1 0 5 ma increased overall productivity three intervals of deep sea benthic foraminiferal taxonomic turnover are recognised 16 15 11 5 10 2 0 5 ma corresponding to intervals of enhanced global cooling and possible productivity changes the late pliocene middle pleistocene extinction associated with increasing northern hemisphere glaciation culminating in the middle pleistocene climatic transition was more significant in the study area than the earlier neogene turnovers

shear wave velocity as function of cone penetration test measurements in sand and silt mixtures abstract a database of research quality piezocone test cptu results and shear wave velocity v s measurements assembled in the context of an ambitious research project carried out at the treporti test site venice italy has been interpreted in order to study the seismic response of the predominantly silty sediments forming the venetian lagoon basin as well as to explore potential correlations for such intermediate soils the experience gained so far with the treporti database and other field data collected in the lagoon area has shown that the mechanical behavior of venetian soils cannot be easily interpreted using the existing and well established cptu based approaches hence the interest in examining whether or not the seismic response of these sediments expressed in terms of shear wave velocity follows the framework published for other soils the applicability to the available data of a number of existing correlations originally developed either for sands or fine sediments has been first investigated and updated relationships have been then suggested furthermore the small strain stiffness calculated from v s measurements has been compared with the field stiffness derived from the observed performance of a full scale test embankment built in the treporti site so as to verify common design assumptions the verification and calibration of existing approaches are likely to constitute a useful contribution to the practice of geotechnics not only in the venetian lagoon area but also in the silty deposits usually present in the surrounding alluvial plain and other similar depositional environments allowing rapid estimates of seismic soil classification as well as small strain stiffness properties for preliminary design or at sites where only cone penetration data are available

simple solutions for the summer shallow atmospheric circulation over north africa the spring to summer shallow atmospheric circulation over north africa is driven by two desert heat lows dhls one over sahara shl and one over arabia ahl with cyclones embedded in the convective layer capped by mid tropospheric anticyclones the aim of this work is to produce a minimal theoretical framework for these dynamical features in terms of kelvin mixed and planetary mode solutions to a two layer matsuno gill model with a lower rayleigh frictional layer dynamically coupled to an almost frictionless upper layer results show that a dhl drives a walker like cell with rising air over the low and subsiding air to its west with the lower level cyclone to the east of the mid tropospheric anticyclone the latter being bounded to the south west by an unstable easterly jet these features related to frictional differences between the two layers are enhanced by ambient easterly winds in spring the stronger shl drives a cell with rising air over west africa and subsiding over the atlantic the weaker ahl drives a cell with rising air over arabia and subsiding over east africa in summer the stronger ahl drives a cell with rising air over arabia and subsiding over libya and the weaker shl drives a cell with rising air over west sahara and subsiding over the atlantic the concurrent contributions of easterly winds monsoonal low level convergence and seasonal airborne dust warming strengthen the shl making its winds stronger than those of the ahl in addition a third order expansion of the coriolis parameter strengthens the zonal winds and weakens the meridional winds these contributors strengthen anticyclones over the east mediterranean and to the west of gibraltar the ability of the model in representing the main features of the north african atmospheric circulations suggests a potential for better understanding climate dynamics in connected regions mediterranean tropical atlantic

sea surface temperature estimates from the southwest pacific based on planktonic foraminifera and oxygen isotopes planktonic foraminiferal census data were collected on 34 surficial sediment samples from east of new zealand sea surface temperature estimates were derived from the data using the modern analogue technique mat and compared to data from climap s fa 20 equation they were also computed from oxygen isotope analyses of globigerina bulloides the temperature estimates compared well between the three methods and were found to correlate closely with the appropriate seasonal sea surface temperature as measured by satellite the foraminiferal assemblages and derivative temperatures clearly identify the major watermasses and oceanic fronts in the region and thus can be applied to palaeoceanographic analysis of southwest pacific sediments

a systematic comparison of tropical waves over northern africa part ii dynamics and thermodynamics abstractthis study presents the first systematic comparison of the dynamics and thermodynamics associated with all major tropical wave types causing rainfall modulation over northern tropical afric

pleistocene biogeochemical record in the south west pacific ocean images site md97 2114 chatham rise through a multidisciplinary approach based on novel micropaleontological and geochemical analyses the main paleoceanographic and paleoclimate changes that have influenced the surface and deep water circulation in the sw pacific ocean chatham rise eastern new zealand during the last million years are reconstructed this region represents a key area for investigating the climate evolution during the pleistocene because here the largely wind driven antarctic circumpolar current interacts with the west pacific ocean circulation via the deep western boundary current the major source of deep water for the whole pacific ocean to understand coupling or decoupling events between sea surface and bottom waters a continuous marine sedimentary succession since 1 1 ma recovered by the images international marine past global change study cruise in the sw pacific ocean core md97 2114 has been investigated based on calcareous planktonic and benthic microfossil content and c and o isotope record performed on planktonic and benthic foraminiferal tests results show the occurrence of long and short term patterns of climate and ocean circulation in the last million years as the result of the interplay of ice sheet dynamics surface tropical versus polar water inflow and trophic status of the surface water copyright 2012 john wiley sons ltd

horizontal structure and seasonality of large scale circulations associated with submonthly tropical convection the relationship between deep tropical convection and large scale atmospheric circulation in the 6 30 day period range is examined regression relationships between filtered outgoing longwave radiation at various locations in the tropics and 200 and 850 mb circulation are mapped for the standard seasons and the spatial structure and seasonal dependence of the results are interpreted in view of the basic state circulation in regions where the convection is embedded in upper level easterlies anomalous equatorial easterly flow is typically present at 200 mb within and to the west of the convective signal along with patterns of meridional outflow into subtropical anticyclonic perturbations lagged relationships suggest that the convection is forcing the circulation in many of these cases the outflow and subtropical circulations are strongest into the winter hemisphere during the solstitial seasons with more symmetric signals about the equator seen in the equinoctial seasons the longitudinal positioning of the subtropical features with respect to the convection varies but is generally located due poleward or just to the east of the convection there tends to be a first baroclinic mode vertical structure to these circulations such that equatorial westerlies are present at 850 mb within the convection with closed circulations on either side of the equator resembling equatorial rossby modes especially common over the atlantic and pacific sectors as a contrast in regions located within upper level westerlies or along the margin of influence of upper westerly disturbances convection appears to be forced by upper level wave energy propagating into the deep tropics with the heating located in the upward motion region ahead of upper level troughs this occurs over the atlantic and eastern pacific sectors during northern winter and spring and over australia the south pacific and south america during southern summer when upper westerlies are at relatively low latitudes where they can interact with deep tropical convection the results confirm theoretical and modeling ideas that suggest that rossby wave energy is able to propagate into the deep tropics in regions where upper level westerlies exist in the tropics

african easterly jet barotropic instability waves and cyclogenesis abstract this study investigates the structure of the african easterly jet focusing on instability processes on a seasonal and subseasonal scale with the goal of identifying features that could provide increased predictability of atlantic tropical cyclogenesis the modern era retrospective analysis for research and applications merra is used as the main investigating tool merra is compared with other reanalyses datasets from major operational centers around the world and was found to describe very effectively the circulation over the african monsoon region in particular a comparison with precipitation datasets from the global precipitation climatology project shows that merra realistically reproduces seasonal precipitation over that region the verification of the generalized kuo barotropic instability condition computed from seasonal means is found to have the interesting property of defining well the location where observed tropical storms are detected this property does not appear to be an arti

prediction of long term settlement on soft clay using shear wave velocity and damping characteristics this paper presents a method of calculating long term settlement of a loaded pad on soft clay at klang selangor malaysia where the soil model is treated as an anelastic material of viscoelastic property initially an elastic shear modulus g value from shear wave velocity profiles of the seismic tests from spectral analysis of surface wave sasw and continuous surface wave csw tests was obtained a value of damping d at an equivalent elastic strain is then calculated from the hysteresis of the plate load tests corrected to equivalent strain using the damping strain formula the calculated elastic settlement and its equivalent damping are then used to calculate the long term settlement by applying the generalised viscoelastic formula comparisons to traditional methods of settlement predictions were made and the viscoelastic formula has shown better agreement to the observed settlement further modification of the settlement formula is introduced to improve the settlement accuracy to 10

global deep sea extinctions during the pleistocene ice ages the dark near freezing environment of the deep oceans is regarded as one of the most stable habitats on earth and this stability is generally reflected in the slow turnover rates extinctions and appearances of the organisms that live there by far the best fossil record of deep sea organisms is provided by the shells of benthic foraminifera protista a little known global extinction of deep sea benthic foraminifera occurred during the pleistocene ice ages in the southwest pacific it caused the disappearance of at least two families 15 genera and 48 species 15 25 of the fauna of dominantly uniserial elongate foraminifera with distinctive apertural modifications these forms progressively died back and became extinct during glacial periods in the late pliocene to middle pleistocene ca 2 5 0 6 ma most extinctions occurred between 1 0 and 0 6 ma at the time of the middle pleistocene climatic revolution this first high resolution study of this extinction event indicates that it was far more significant for deep sea diversity loss than previously reported 10 species the middle pleistocene extinction was the most dramatic last phase of a worldwide decline in the abundance of these elongate forms a phase that began during cooling near the eocene oligocene boundary and continued during the middle miocene clearly these taxa declined when the world cooled but the reason is yet to be resolved

microbial sulfate reduction in deep sediments of the southwest pacific odp leg 181 sites 1119 1125 evidence from stable sulfur isotope fractionation and pore water modeling interstitial water samples from seven odp sites leg 181 sites 1119 1125 of the southwestern pacific ocean have been analyzed for the stable sulfur isotopic composition of dissolved sulfate along with major and minor ions sulfate from the interstitial fluids 34s values between 20 7 and 60 vs the so2 based vienna canyon diablo troilite standard was enriched in 34s with respect to modern sea water 34s 20 6 indicating that microbial sulfate reduction takes place to different extents at all investigated sites microbial sulfate reduction msr was found at all sites the intensity depending on the availability of organic matter which is controlled by paleo sedimentation conditions sedimentation rate presence of turbidites and productivity microbial net sulfate reduction was additionally confirmed by modeling interstitial water sulfate profiles areal net sulfate reduction rates up to 14 mmol m 2 yr 1 have been calculated which were positively related to sedimentation rates total reduced inorganic sulfur tris essentially pyrite as a product of microbial sulfate reduction was isotopically characterized in squeeze cake samples and gave 34s values between 51 and 9 indicating pyrite formation both close to the sediment water interface and later diagenetic contributions

west african monsoon demise climatology interannual variations and relationship to seasonal rainfall the climatology and interannual variations of the west african monsoon demise are investigated using the tropical rainfall measuring mission and the global precipitation climatology project precipitation data sets along with the era interim reanalysis during monsoon demise the rainfall maximum progresses southward from the sahel to the gulf of guinea the west african monsoon demise is smooth with no evidence of a meridional jump of the rainfall maximum as occurs during monsoon onset the climatological monsoon demise date defined by using a 2 mm d 1 rainfall threshold for the sahel is 20 october the demise date varies up to 25 days during the period 1979 2012 between 5 and 30 october an early late demise is associated with an anomalously strong weak north atlantic subtropical high which extends over the mediterranean and the sahara throughout the demise period the monsoon season total rainfall is found to be significantly correlated with the demise date in a composite of early late demise cases northerly southerly moisture flux anomalies which enhance reduce moisture divergence are associated with an anomalously strong weak north atlantic subtropical high in addition the early late demise of the west african monsoon is accompanied by cold warm sea surface temperature anomalies in the subtropical north atlantic

the west african monsoon dynamics part iii the quasi biweekly zonal dipole abstract this paper presents an investigation of the mechanisms giving rise to the main intraseasonal mode of convection in the african monsoon during northern summer here identified as the quasi biweekly zonal dipole qbzd the qbzd is primarily characterized by a quasi stationary zonal dipole of convection whose dimension is larger than the west african monsoon domain with its two poles centered along the guinean coast and between 30 and 60 w in the equatorial atlantic the qbzd dynamical processes within the atlantic africa domain are examined in some detail the qbzd has a dipole pattern associated with a walker type circulation in the near equatorial zonal plane it is controlled both by equatorial atmospheric dynamics through a kelvin wave like disturbance propagating eastward between its two poles and by land surface processes over africa inducing combined fluctuations in surface temperatures surface pressure and low level zonal winds off the coast of west africa when convection is at a min

development of sediment drifts approaching an active plate margin under the sw pacific deep western boundary current the abyssal pacific ocean is fed by a 1000 km wide deep western boundary current dwbc that flows northward along the continental margin east of new zealand between the passive margin of chatham rise and the subduction zone of kermadec trench a distance of 1200 km the dwbc has formed a suite of sediment drifts over a depth range of 2200 5700 m airgun and 3 5 khz profiles record a variety of drift types that reflect regional variations in bathymetry sediment supply and the tectonic volcanic framework on chatham rise the dwbc has deposited a sinuous linear body along the south flank 3000 m an extensive apronlike drift on the north flank 2200 4500 m and a ridgelike drift about the rise base 4500 5200 m the flow has also deposited a body of sediment over 400 km long within a moat at the base of the nearby louisville seamount chain further downcurrent the 250 km long rekohu drift 3600 4190 m has developed northward to 39 s south of this latitude drifts comprise mainly reworked pelagic hemipelagic material and sediment transported from distant southerly sources in contrast drifts north of 39 s have received a major injection of terrigenous sediment from hikurangi channel which runs 1400 km from new zealand eastward across the hikurangi plateau to disgorge on to the abyssal floor at the plateau edge en route turbidity current overspill from the channel has moved north under the influence of the shallow dwbc to contribute to a series of small ridge and patch drifts among the numerous seamounts on the plateau at 3500 4200 m off hikurangi channel mouth a large fan has accumulated the dwbc has extended the fan into a drift running over 250 km along the base of hikurangi plateau 5150 5770 m toward kermadec trench here drift sediment becomes increasingly disrupted by mass wasting associated with the active subduction in this area the seismic stratigraphy reveals the drifts to rest mainly on a widespread erosional surface that is interpreted to mark the inception of the dwbc in the region with the late oligocene opening of the australian antarctic seaway drift construction commenced during the miocene but was punctuated in the late miocene by another period of erosion that coincided with increased bottom water production in antarctica deposition resumed in plio pleistocene times when large quantities of sediment from the rapidly rising landmass of new zealand were injected into the boundary current the modern flow continues to affect drift deposition as manifest by an active boundary channel along the foot of hikurangi plateau and widespread scour zones and sediment wave fields

response of surface water masses and circulation to late quaternary climate change east of new zealand a series of cores from east of new zealand have been examined to determine the paleoceanographic history of the late quaternary in the sw pacific using planktonic foraminiferal data distinct shifts of species can be seen between glacial and interglacial times especially south of chatham rise east of south island foraminiferal fragmentation ratios and benthic planktonic foraminiferal ratios both show increased dissolution during glacials especially isotope stage 2 to the south of chatham rise the present day subtropical convergence appears to be tied to the chatham rise at 44 s but during glacial times this rise separated cold water to the south from much warmer water to the north with an associated strong thermal gradient across the rise we estimate that this gradient could have presented as much as an 8 c temperature change across 4 of latitude during the maximum of the last ice age there is only weak evidence of the younger dryas cool event but there is a clear climatic optimum between 8 and 6 4 ka with temperatures 1 2 c higher than the present day the marine changes compare well with vegetational changes on both south and north island

late pleistocene to holocene composite speleothem 18o and 13c chronologies from south island new zealand did a global younger dryas really exist oxygen and carbon data from eight stalagmites from northwest south island are combined to produce composite records of delta 18 o and delta 13 c from 23 4 ka to the present the chronology is anchored by 43 thermal ionization mass spectrometry tims uranium series ages delta o 18 values are interpreted as having a first order positive relationship to temperature but also to be influenced by precipitation in a complex manner delta c 13 is interpreted as responding negatively to increases in atmospheric co concentration biological activity and precipitation amount six climatic phases are recognized after adjustment of 1 2parts per thousand for the ice volume effect the delta 18 o record between 23 and 18 ka varies around 3 72parts per thousand compared to the holocene average of 3 17parts per thousand late glacial warming commenced between 18 2 and 17 8 ka and accelerated after 16 7 ka culminating in a positive excursion between 14 70 and 13 53 ka this was followed by a significant negative excursion between 13 53 and 11 14 ka of up to 0 55parts per thousand depth that overlapped the antarctic cold reversal acr and spanned the younger dryas yd positive delta 18 o excursions at 11 14 ka and 6 91 6 47 ka represent the warmest parts of the holocene the mid holocene from 6 to 2 ka was marked by negative excursions that coincide with increased glacial activity in the south island a short positive excursion from 0 71 to 0 57 ka was slightly later than the medieval warm period of europe delta c 13 values were high until 17 79 ka after which there was an abrupt decrease to 17 19 ka followed by a steady decline to a minimum at 10 97 ka then followed a general increase suggesting a drying trend to 3 23 ka followed by a further general decline the abrupt decrease in delta values after 17 79 ka probably corresponds to an increase in atmospheric co2 concentration biological activity and wetness at the end of the last glaciation but the reversal identified in the delta 18 o record from 13 53 to 11 14 ka was not reflected in delta 13 c changes the lowest delta 13 c values coincided with the early holocene climatic suboptimum when conditions were relatively wet as well as mild major trends in the delta 18 o c record are similar to the northern hemisphere but second order detail is often distinctly different consequently at the millennial scale a more convincing case can be made for asymmetric climatic response between the two hemispheres rather than synchronicity c 2004 elsevier b v all rights reserved

pliocene sea surface temperature changes in odp site 1125 chatham rise east of new zealand abstract planktonic foraminiferal census counts were converted to sea surface temperature sst estimates using the modern analogue technique mat for the middle late pliocene 4 0 2 37 ma in odp site 1125 north side of chatham rise sw pacific ocean mat sstwarm records range between 8 c and 20 5 c and mat sstcold records parallel that pattern but with a temperature range of 5 15 c the modern position of site 1125 is just north of the subtropical front and has an annual temperature range of 14 18 c pliocene warmest temperatures are 1 2 warmer than modern summers whereas cold season sst records are up to 6 10 c cooler than modern winters overall average temperatures at the site are 2 3 c cooler than modern temperatures during a time of sustained global warmth three major cold excursions centred on 3 35 3 0 and 2 8 ma showed warm season temperatures over 5 c colder than the last glacial maximum experiencing temperatures typical of modern subantarctic waters two minor cold excursions at 2 7 ma and 2 4 ma experienced temperatures cooler than modern winters but not as cold as last glacial conditions cold season ssts show a shift to warmer climate upward through the study interval whereas warm season estimates remain essentially unchanged we interpret the strong regional cooling of subtropical southwest pacific water through the middle late pliocene as having been caused by increased upwelling it is also possible that the subtropical frontal zone moved north over the site in the pliocene however this is considered the least likely interpretation our record of cool conditions in the southwest pacific corroborate evidence of cooler than modern conditions in other regions of the western pacific through the mid pliocene despite overall global warming

paleocurrent reconstruction of the deep pacific inflow during the middle miocene reflections of east antarctic ice sheet growth today the deep western boundary current dwbc east of new zealand is the most important route for deep water entering the pacific ocean large scale changes in deep water circulation patterns are thought to have been associated with the development of the east antarctic ice sheet eais close to the main source of bottom water for the dwbc here we reconstruct the changing speed of the southwest pacific dwbc during the middle miocene from 15 5 12 5 ma a period of significant global ice accumulation associated with eais growth sortable silt mean grain sizes from ocean drilling program site 1123 reveal variability in the speed of the pacific inflow on the timescale of the 41 kyr orbital obliquity cycle similar orbital period flow changes have recently been demonstrated for the pleistocene epoch collectively these observations suggest that a strong coupling between changes in the speed of the deep pacific inflow and high latitude climate forcing may have been a persistent feature of the global thermohaline circulation system for at least the past 15 myr furthermore long term changes in flow speed suggest an intensification of the dwbc under an inferred increase in southern component water production this occurred at the same time as decreasing tethyan outflow and major eais growth between 15 5 and 13 5 ma these results provide evidence that a major component of the deep thermohaline circulation was associated with the middle miocene growth of the eais and support the view that this time interval represents an important step in the development of the neogene icehouse climate

soudano saharan depressions and their importance for precipitation and dust a new perspective on a classical synoptic concept according to a classical synoptic concept soudano saharan depressions ssds are surface lows that track westward over tropical west africa curve anticyclonically across the sahara and may then transform into eastward moving mediterranean cyclones occurrence frequency and track location undergo a marked seasonal cycle interactions between troughs in the upper level westerlies and mid level african easterly waves have been suggested as a mechanism for their formation ssds have been reported to be associated with dust storms and precipitation over northern africa this paper presents the first ever systematic investigation of ssds using re analysis and satellite data depressions are identified and tracked objectively based on closed contours in 0000 utc fields of 925 hpa geopotential height from the european centre for medium range weather forecasts era interim re analysis 1989 2008 to classify as potential ssds tracks must i start to the south of 20 n ii intersect 15 30 n 10 w 30 e iii cover a meridional distance of at least 10 latitude and iv have a minimum lifetime of 24 hours even with a relatively low threshold of 4 gpm only 50 potential ssds are found annual average 2 5 monthly range 0 6 lagrangian and eulerian composite analyses reveal that the identified systems i are mostly shallow lee troughs of the central saharan and atlas mountains during the warm season without a well defined cyclonic wind field ii do not show the seasonal track variation described in the literature iii mostly occur in association with high pressure anomalies over the mediterranean sea and iv are not associated with significant increases in dustiness and precipitation these results strongly suggest that the disturbances described as ssds do not manifest themselves as traceable low level depressions calling for a fundamental revision of the classical concept in the literature copyright 2011 royal meteorological society

late quaternary oceanic fronts along chatham rise indicated by phytoplankton assemblages and refined calcareous nannofossil stratigraphy for the mid latitude sw pacific abstract a quantitative analysis was carried out of planktonic diatoms biogenic opal and calcareous nannofossils biogenic calcite in late quaternary sediments mis 1 6 from four cores along a n s transect east of new zealand from 39 50 s to 50 04 s across the e w trending submarine ridge the chatham rise this was done to trace movements of oceanic fronts and to improve calcareous nannofossil stratigraphy for the last 130 000 yr in the sw pacific sites odp 1123 and q 858 are below present day subtropical surface waters north of chatham rise site dsdp 594 is below present day mixed temperate subantarctic surface water south of the rise and site odp 1120 is below subantarctic surface water the more diverse and opportunistic planktonic diatoms provided marker species for subtropical surface waters alveus marina fragilariopsis doliolus rhizosolenia bergonii and azpeitia nodulifer and others for subantarctic surface waters nitzschia kerguelensis thalassiosira lentiginosa application of these tracers permits the following conclusions 1 subtropical conditions persisted north of chatham rise throughout the past 130 000 yr in spite of the cooling of surface waters during colder periods 2 during warm times mis 5 and mis 3 and in mis 1 the sporadic occurrence of subtropical species south of chatham rise indicates occasional admixture of subtropical surface waters that far south 3 subantarctic waters extended to the southern slopes of the chatham rise during mis 5b late mis 5a to early mis 4 during the warmer time intervals in early mis 3 and during latest mis 3 to early mis 2 4 subantarctic frontal conditions existed over southern chatham rise during early mis 4 and late mis 3 to early mis 2 and 5 it is probable that during cooler times mis 6 mis 5b and in mis 2 intensified particle transport from the bounty trough to the northern flank of chatham rise occurred by intensified boundary currents the larger abundance fluctuations in both microfossil groups at the sites south of chatham rise than north of chatham rise reflect northward shifts of the circumpolar subantarctic water csw and a contemporaneous disappearance of australasian subantarctic water asw implying an elevated temperature gradient between the surface water masses north and south of the chatham rise at the times of such northward shifts of csw calcareous nannofossils are less diverse than diatoms and are less specialised some calcareous nannofossil species show abundance shifts at the same time at different latitudes two of these abundance shifts can be used for correlation between subtropical and subantarctic sediments in the sw pacific 1 reversal in the relative abundance of calcidiscus leptoporus and coccolithus pelagicus associated with the mis 2 1 boundary and 2 drop in abundance of gephyrocapsa muellerae or medium sized gephyrocapsa at the mis 4 3 boundary an additional abundance shift seems to be restricted to subtropical to mixed temperate subtropical subantarctic surface waters 3 increase in abundance of g muellerae or medium sized gephyrocapsa at the beginning of mis 2 below the okareka tephra

west african monsoon influence on the summer euro atlantic circulation 1 the west african monsoon wam influence on the interannual variability of the summer atmospheric circulation over north atlantic and europe is investigated over the period 1971 2000 a set of sensitivity experiments performed through the arpege climat atmospheric general circulation model is analyzed using the so called grid point nudging technique where the simulated atmospheric fields in the wam region are relaxed towards the era40 reanalysis observations confirm that a sizable part of the euro atlantic circulation variability is related to the wam with anomalies of reinforced convection in the sudan sahel region associated with positive north atlantic oscillation nao phases and subsidence over eastern mediterranean the nudged simulations highlights the role of the wam in driving the mid latitude circulation a strong monsoon is related to high pressure anomalies over the azores and positive nao phases

comparison of the synoptic conditions in midlatitudes accompanying cold surges over eastern asia for the months of december 1974 and 1978 part i monthly mean fields and individual events abstract east asian cold surges during two periods of contrasting intensity december 1974 strong and december 1978 weak are studied it is shown that the midlatitude mechanisms initiating cold surges during both months are quite similar in nature synoptic scale short waves passing through the long wave trough position near the east asian coast act to release the cold air southward from the main reservoir over eastern siberia dynamic descent is initiated by these synoptic scale waves entering the northwesterly flow on the western side of the long wave trough anchored on the coast this is the descent center in the local direct circulation in the entrance region of the east asian jet maximum at 200 mb the frequency and intensity of the cold air surges for a given month depend on the large scale circulation pattern the average flow over asia for december 1974 and 1978 presents a contrast with respect to ridging over the midcontinent in december 1978 the flow was characterized by a low zonal inde

glacial to interglacial changes in non carbonate and carbonate accumulation in the sw pacific ocean new zealand abstract fluxes of carbonate and non carbonate terrigenous biogenic siliceous sediment back to a maximum of marine isotope stage mis 6 were determined for 54 cores from off eastern new zealand mass accumulation rates mars were derived using dry bulk densities measured directly from fresh cores or in the case of dry archived samples on estimates from a density carbonate relationship mars exhibit wide spatial and temporal variability in response to i the location of cores relative to the terrigenous source of new zealand current pathways and local oceanographic features and ii palaeoclimatic change for mis 1 2 non carbonate accumulated at a high rate between 27 and 18 ka calendar years due to an influx of waterborne and aeolian terrigenous detritus together with localised production of biogenic silica under a northward intrusion of subantarctic waters and an increase in wind induced upwelling by 18 12 ka non carbonate mars reduced as winds ameliorated and sea level rose to confine some of the terrigenous load to the continental shelf the holocene transgression was accompanied by warmer sea surface temperatures the formation of coastal and lacustrine sediment traps and more widespread terrestrial vegetation these conditions further reduced the deep oceanic non carbonate flux to levels about half those of glacial periods in contrast carbonate mars were lower in cold periods reflecting lower productivity increased dissolution and in the case of the generally productive waters of chatham rise to redistribution by bottom currents however off central new zealand carbonate production was higher on the continental margin possibly because of enhanced upwelling under the strong wind regime of mis 2 such localised effects produced glacial carbonate mars higher than interglacial mis 1 values for the rest of the region carbonate fluxes increased to an early to middle mis 1 maximum before generally declining in recent times

the sedimentary regime beneath the deep western boundary current inflow to the southwest pacific ocean abstract a review of hydrographic sections single channel seismic profiles bottom photographs and nephelometer profiles collected by the u s n s eltanin plus new seismic and sedimentary data from the r v rapuhia provide an insight into the sedimentary regime beneath the pacific deep western boundary current dwbc where it enters the sw pacific off southernmost new zealand at macquarie ridge and campbell plateau there the dwbc is reinforced by the antarctic circumpolar current acc largely through the generation of deep reaching eddies formed by interaction of the acc with the pronounced bathymetry it is therefore a region of high eddy kinetic energy which induces widespread abyssal erosion it is also where the dwbc receives its first major injection of terrigenous se iment which is transferred from southern new zealand along the 450 km long solander channel from macquarie ridge the acc dwbc travels northeast onto the subantarctic slope of campbell plateau this high steep western boundary intensifies the flow causing erosion along the slope base east of the scour the 800 km long campbell drift has been deposited this drift extends to 51 s where the acc swings east to continue its circum antarctic journey while the dwbc continues northeast to bounty trough where the combination of abundant sediment supplied by bounty channel a subdued western boundary and a less energetic dwbc have allowed the bounty fan to form directly across the current path apart from localized winnowing near zones of high relief evidence for widespread dwbc in luence on the fan is inconclusive however current effects become more apparent north of bounty fan where the steep western boundary resumes in the form of chatham rise eroded fan sediments have contributed to a series of small drifts at 4400 5000 m depth whereas at 2400 3400 m depth on the flank of chatham rise the dwbc has molded a sinuous terrace like drift viewed regionally sedimentation beneath the boundary current south of chatham rise contrasts strongly with the regime to the north the southern current is energetic and terrigenous supply is intermittent being restricted to glacial lowstands consequently erosion is common and drifts are few and are themselves subject to erosion by comparison the current north of the rise is generally slower and has a more consistent terrigenous supply particularly near hikurangi channel which has been active irrespective of sea level thus widespread drift deposition is favored

a mechanism for african monsoon breaks mediterranean cold air surges 1 surges of cold air from the mediterranean into northern africa during the boreal summer are documented and their influence on monsoon breaks is analyzed using tropical rainfall measuring mission rainfall estimates and reanalysis products between 1998 and 2006 6 10 cold air surges occurred each summer with low level temperature anomalies ranging from less than 1 k to over 6 k composite analysis indicates that cold air surges over northern africa persist for 2 10 days and travel equatorward at approximately 5 5 m s 1 which is 0 5 1 5 m s 1 faster than the observed climatological low level meridional flow northern african cold surges have characteristics similar to surges observed elsewhere in the world including a hydrostatically induced ridge of surface pressure and an amplified upper tropospheric ridge trough pattern the african cold surge is preceded by the passage of a shortwave trough and an intensification of the upper tropospheric subtropical westerly jet streak over the mediterranean sea these events are associated with increased confluence in the jet entrance region over the central mediterranean an enhanced direct secondary circulation subsidence and low level ageostrophic northerly flow over northeastern africa composite analysis shows that the passage of a cold surge is associated with an enhancement in convective activity over southern algeria western niger northern mali and mauritania 2 to 5 days before the surge reaches the eastern sahel 17 5 n when northeasterly flow channeled between the atlas and ahaggar mountains strengthens and transports relatively moist air from the western mediterranean and eastern north atlantic over the region and increases moisture convergence over western africa north of 20 n over the eastern sahel of sudan and eastern chad the composite results reveal a break in convective activity and decrease in low level convergence when the surge arrives that persists for about 6 days these results offer great promise for improving the short range prediction of rainfall over northern africa

regional sediment recycling in the abyssal southwest pacific ocean an active plate boundary with a high sediment output three major submarine channels and the world9s largest deep western boundary current dwbc make up an extensive recycling system along the 4500 km continental margin east of new zealand seismic reflection sedimentary and oceanographic data demonstrate that detritus from the rising mountains of the new zealand plate boundary is transferred to the southwest pacific abyssal floor by turbidity currents flowing along solander 450 km long bounty 950 km and hikurangi 1400 km channels these conduits discharge directly into the dwbc which transports material north to form a series of sediment drifts the northernmost drift containing sediment from hikurangi channel and eroded drifts to the south is now subducting into kermadec trench geochemical data suggest that sediment is recycled through the mantle to re emerge in the arc volcanic rocks thus one cycle is completed and a new one begins

neogene history of the deep western boundary current at rekohu sediment drift southwest pacific odp site 1124 abstract odp site 1124 located 600 km east of the north island of new zealand records post middle oligocene variations in the pacific deep western boundary current dwbc and new zealand s climatic and tectonic evolution sediment parameters such as terrigenous grain size flux magnetic fabric and non depositional episodes are used to interpret dwbc intensity and antarctic climate interpretations of dwbc velocities indicate that the antarctic circumpolar current reached modern intensities at 23 ma as the tectonic seaways expanded completing the thermal isolation of antarctica periods of more intense bottom water formation are suggested by the presence of hiatuses formed under the dwbc at 22 5 17 6 16 5 15 and 14 11 ma the oldest interval of high current intensity occurs within a climatically warm period during which the intensity of thermohaline circulation around antarctica increased as a result of recent opening of circum antarctic gateways the younger hiatuses represent glacial periods on antarctica and major fluctuations in the east antarctic ice sheet whereas intervals around the hiatuses represent times of relative warmth but with continued current activity the period between 11 to 9 ma is characterized by conditions surrounding a high velocity dwbc around the time of the formation and stabilization of the west antarctic ice sheet the increased terrigenous input may result from either changing antarctic conditions or more direct sediment transport from new zealand the pacific dwbc did not exert a major influence on sedimentation at site 1124 from 9 ma to the present the late miocene to pleistocene sequence is more influenced by the climatic and tectonic history of new zealand despite the apparent potential for increased sediment supply to this site from changes in sediment channeling increasing rates of mountain uplift and volcanic activity terrigenous fluxes remain low and constant throughout this younger period

a characterization of african easterly waves on 2 5 6 day and 6 9 day time scales abstractthis study shows that the african easterly wave aew activity over the african monsoon region and the northern tropical atlantic can be divided in two distinct temporal bands with time scales of 2 5 6 and 6 9 days the results are based on a two dimensional ensemble empirical mode decomposition 2d eemd of the modern era retrospective analysis for research and applications merra the novel result of this investigation is that the 6 9 day waves appear to be located predominantly to the north of the african easterly jet aej originate at the jet level and are different in scale and structure from the well known low level 2 5 6 day waves that develop baroclinically on the poleward flank of the aej moreover they appear to interact with midlatitude eastward propagating disturbances with the strongest interaction taking place at the latitudes where the core of the atlantic high pressure system is located composite analyses applied to the mode decomposition indicate that the interaction of the

the formation of a large summertime saharan dust plume convective and synoptic scale analysis haboobs are dust storms produced by the spreading of evaporatively cooled air from thunderstorms over dusty surfaces and are a major dust uplift process in the sahara in this study observations reanalysis and a high resolution simulation using the weather research and forecasting model are used to analyze the multiscale dynamics which produced a long lived over 2 days saharan mesoscale convective system mcs and an unusually large haboob in june 2010 an upper level trough and wave on the subtropical jet 5 days prior to mcs initiation produce a precipitating tropical cloud plume associated with a disruption of the saharan heat low and moistening of the central sahara the restrengthening saharan heat low and a mediterranean cold surge produce a convergent region over the hoggar and air mountains where small convective systems help further increase boundary layer moisture emerging from this region the mcs has intermittent triggering of new cells but later favorable deep layer shear produces a mesoscale convective complex the unusually large size of the resulting dust plume over 1000 km long is linked to the longevity and vigor of the mcs an enhanced pressure gradient due to lee cyclogenesis near the atlas mountains and shallow precipitating clouds along the northern edge of the cold pool dust uplift processes identified are 1 strong winds near the cold pool front 2 enhanced nocturnal low level jet within the aged cold pool and 3 a bore formed by the cold pool front on the nocturnal boundary layer

the 1993 superstorm cold surge frontal structure gap flow and tropical impact in the wake of the eastern united states cyclone of 12 14 march 1993 a cold surge originating over alaska and western canada brought northerlies exceeding 20 m s21 and temperature decreases up to 158c over 24 h into mexico and central america this paper addresses the multiscale aspects of the surge from the planetary scale to the mesoscale focusing on 1 the structure and evolution of the leading edge of the cold surge 2 the reasons for its extraordinary intensity and equatorward extent and 3 the impact of the surge on the tropics specifically on the strength of the trade winds and on the sea surface temperature in the eastern pacific the cold surge was initiated as a developing cyclone over the gulf of mexico and an upstream anticyclone east of the rockies caused an along barrier pressure gradient to form forcing topographically channeled northerlies along the rocky and sierra madre mountains to transport cold air equatorward on the mesoscale the leading edge of the cold surge possessed nonclassical frontal structure for example as the cold surge entered mexico the coldest air and the strongest wind arrived at about 900 hpa before affecting the surface suggestive of a tipped forward leading edge to the surge also satellite imagery and surface observations indicate that the leading edge appeared to be successively regenerated in the warm presurge air the cold surge had characteristics reminiscent of a kelvin wave a tipped forward cold front a pressure jump line a bore and a gravity current but none of these conceptual dynamical models was fully applicable associated with the cold surge gap winds up to 25 m s21 were observed in the gulfs of tehuantepec a tehuantepecer fonseca papagayo and panama owing to the strong cross mountain pressure gradient in the case of the tehuantepecer a rope cloud emanated from the isthmus of tehuantepec and turned anticyclonically consistent with an inertial oscillation on the synoptic and planetary scales the extraordinary equatorward extent of the cold surge was aided by topographic channeling similar to cold air damming by a low latitude upper tropospheric trough and by the lower branch of the secondary circulation associated with a confluent jet entrance region aloft the cold surge also impacted the tropical atmosphere and ocean by contributing to the strengthening of the northeast trade winds over the eastern pacific ocean and by inducing local cooling of the sea surface temperature in the gulfs of tehuantepec and papagayo by about 48 88c

a review of the structure and dynamics of upper level frontal zones abstract this article presents a review of upper level fronts with the intent of synthesizing observational and modeling studies into a conceptual and dynamical description of these fronts and their evolution relative to the life cycle of midlatitude baroclinic waves the discussion begins by tracing present day concepts concerning the structure of upper level frontal systems which are based on composite analyses of radiosonde and aircraft data from their origins in the pioneering analyses of upper air data in the 1930s perspectives from scales both smaller and larger than upper level frontal systems are provided respectively by considering the effects of turbulent processes on frontal structure and dynamics and by relating variations in frontal structure to the evolution of the baroclinic waves that provide the dynamical environment for upper level frontogenesis the dynamics of upper level fronts are shown to comprise the interactions between the primary geostrophic and secondary ageostrophic cir