mechanisms by which gulf of guinea and eastern north atlantic sea surface temperature anomalies can influence african rainfall abstract the sensitivity of precipitation over west africa to sea surface temperature anomalies sstas in the gulf of guinea and the eastern north atlantic is studied using a gcm results from nine perpetual july simulations with various imposed sstas are presented and analyzed to reveal associations between the precipitation and sst fields via large scale circulation and atmospheric moisture anomalies rainfall increases over the guinean coast and decreases over the congo basin when warm sstas are present in the gulf of guinea these precipitation perturbations are related to the forcing of a kelvin and a rossby wave the former is associated with a weakening of the walker circulation while the latter strengthens the west african monsoon rainfall over west africa is less sensitive to cold sstas than to warm anomalies three contributing factors are identified as follows 1 latitude of the sst forcing 2 background flow and 3 nonlinearity of the clausius clapeyron equation no more than a 20 effec

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

a revised picture of the structure of the monsoon and land itcz over west africa this article presents an overview of the land itcz intertropical convergence zone over west africa based on analysis of ncar ncep reanalysis data the picture that emerges is much different than the classic one the most important feature is that the itcz is effectively independent of the system that produces most of the rainfall rainfall linked directly to this zone of surface convergence generally affects only the southern sahara and the northern most sahel and only in abnormally wet years in the region a second feature is that the rainbelt normally assumed to represent the itcz is instead produced by a large core of ascent lying between the african easterly jet and the tropical easterly jet this region corresponds to the southern track of african easterly waves which distribute the rainfall this finding underscores the need to distinguish between the itcz and the feature better termed the tropical rainbelt the latter is conventionally but improperly used in remote sensing studies to denote the surface itcz over west africa the new picture also suggests that the moisture available for convection is strongly coupled to the strength of the uplift which in turn is controlled by the characteristics of the african easterly jet and tropical easterly jet rather than by moisture convergence this new picture also includes a circulation feature not generally considered in most analyses of the region this feature a low level westerly jet termed the african westerly jet plays a significant role in interannual and multidecadal variability in the sahel region of west africa included are discussions of the how this new view relates to other aspects of west africa meteorology such as moisture sources rainfall production and forecasting desertification climate monitoring hurricanes and interannual variability the west african monsoon is also related to a new paradigm for examining the interannual variability of rainfall over west africa one that relates changes in annual rainfall to changes in either the intensity of the rainbelt or north south displacements of this feature the new view presented here is consistent with a plethora of research on the synoptic and dynamic aspects of the african easterly waves the disturbances that are linked to rainfall over west africa and spawn hurricanes over the atlantic and with our knowledge of the prevailing synoptic and dynamic features this article demonstrate a new aspect of the west africa monsoon a bimodal state with one mode linked to dry conditions in the sahel and the other linked to wet conditions the switch between modes appears to be linked to an inertial instability mechanism with the cross equatorial pressure gradient being a critical factor the biomodal state has been shown for the month of august only but this month contributes most of the interannual variability this new picture of the monsoon and interannual variability shown here appears to be relevant not only to interannual variability but also to the multidecadal variability evidenced in the region between the 1950s and 1980s

chemical parameters characterizing the sedimentary environment in a nw adriatic coastal area italy abstract organic matter constituents c n p have been measured in surficial sediment samples from a coastal area often subjected to eutrophication phenomena chemical and physico chemical parameters are substantially homogeneous throughout the area investigated indicating a mainly autochthonous origin of the sedimented organic matter and allowing some considerations on the diagenetic processes possibly occurring at the sediment water interface in relation to nutrient release

lunar basalt chronology mantle differentiation and implications for determining the age of the moon despite more than 40 years of studying apollo samples the age and early evolution of the moon remain contentious following the formation of the moon in the aftermath of a giant impact the result

shallow meridional circulations in the tropical atmosphere abstract a shallow meridional circulation smc in the tropical atmosphere features a low level e g 700 hpa flow that is in the opposite direction to the boundary layer monsoon or trade wind flow and is distinct from the meridional flow above representations of the smc in three global reanalyses show both similarities and astonishing discrepancies while the smc over west africa appears to be the strongest it also exists over the eastern atlantic and eastern pacific oceans and over the indian subcontinent with different strength and structure all smcs undergo marked seasonal cycles the smcs are summarized into two types one associated with the marine itcz and the other with the summer monsoon the large scale conditions for these two types of smcs are similar a strong meridional gradient in surface pressure linked to surface temperature distributions and an absence of deep moist convection the processes responsible for these conditions are different for the two types of smcs as are their str

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

a physical basis for the interannual variability of rainfall in the sahel a major factor in rainfall variability over sahelian west africa is the latitudinal location of the tropical rainbelt when it is displaced abnormally far northward the sahel experiences a wet year an anomalous southward displacement results in drought in this paper we examine the question of what controls the location during the boreal summer hypothesizing that inertial instability plays a role an analysis of surface pressure and temperature fields wind fields divergence and vertical motion show that the criteria for inertial instability are satisfied in wet augusts but not in dry ones the key determinant appears to be the surface pressure gradient between the continent and the equatorial atlantic when this is large inertial instability results in the development of a low level westerly jet the presence of this jet enhances the horizontal and vertical shear and displaces the african easterly jet northwestward associated with this situation is strong vertical motion over the sahel and subsidence over the guinea coast producing dry conditions over the latter the result is a rainfall dipole one of two major modes of variability over west africa important factors include sea surface temperatures ssts in the equatorial atlantic and pressure in the south atlantic the first of these factors suggests a link with the atlantic nino mode of tropical atlantic variability while the second suggests a possible link with the pacific and the extratropical south atlantic overall our study relates the well known sst influence on sahel rainfall to atmospheric dynamics over the continent copyright 2007 royal meteorological society

recent deposits and suspended sediments off the po della pila po river main mouth italy abstract present sedimentary conditions in the prodeltaic marine area immediately in front of the po della pila po river main mouth are characterized by a maximum thickness of holocene muds of 10 12 m at a depth of 20 25 m and by two different sedimentary cycles with the more recent lens shaped unit 2 5 4 0 m thick probably deposited in the last 300 yrs the nearshore prodelta area is characterized by rapidly accumulating deposits 2 4 cm yr enriched in smectite gas organic material 1 2 and trace metals the water system presented well marked vertical stratification during the summer of 1985 revealing three distinct water bodies 1 riverine and diluted waters average depth of 3 m 2 middle depth waters 10 20 m and 3 bottom waters temperature oxygen and particulate organic carbon decrease with depth whilst total suspended matter yellow substances and total particulate carbon show maxima at the surface and in the bottom layers and minima in the intermediate waters where the organic fraction comprises 42 of the total

variability of the intertropical front itf and rainfall over the west african sudan sahel zone abstract the intertropical front itf is a fundamental feature of the atmospheric circulation over west africa it separates the wedge of warm moist southwesterly monsoon flow off the tropical atlantic from much hotter and very dry northeasterly wind from the sahara desert here the daily temperature humidity and rainfall data for 1974 2003 are analyzed to document the northward advance and southward retreat of the itf between boreal spring and autumn and assess its role in sudan sahel 10 20 n rainfall patterns using largely dekadal 10 day and monthly resolutions analyses are performed for the 30 yr average seasonal time scale and sets of extreme years with a major focus on concurrent monthly itf rainfall relations the seasonal rainfall predictive potential of the early season itf latitude is also investigated as is the secular variation of itf latitude weather system rainfall associations during 1974 2003 the northward advance of the itf across the sudan sahel from april to early august i

mesoscale convective systems in relation to african and tropical easterly jets abstractthis paper documents the interaction processes between mesoscale convective systems mcs the tropical easterly jet tej and the african easterly jet aej over west africa during the monsoon peak of 2006 observed during the african monsoon multidisciplinary analyses amma project the results highlight the importance of the cloud system localization relative to the jets in order to explain their duration and life cycle a systematical study reveals that intense and long lived mcss correspond to a particular pattern where clouds associated with deep convection are located in entrance regions of tej and in exit regions of aej a case study on a particularly well documented convective event characterizes this link and infers the importance of jet streaks in promoting areas of divergence favoring the persistence of mcss

modern and last glacial maximum eolian sedimentation patterns in the atlantic ocean interpreted from sediment iron oxide content eolian dust derived from the desert regions of north africa is blown far into the tropical atlantic ocean by persistent easterly and northeasterly winds in this paper we demonstrate that the iron oxides hematite and goethite are a worthwhile addition to proxy monitors of eolian sedimentation in the tropical atlantic iron oxides are identified by diffuse reflectance spectrophotometry a technique capable of identifying these minerals in concentrations as low as 0 01 by weight we analyze samples from both the modern and last glacial maximum lgm synoptic levels from 178 sample locations yielding a total of 356 samples distributed throughout the atlantic ocean to determine the relative contribution of the iron oxides we factor analyzed the modern and lgm levels as a single data set the iron oxide factor explains about 25 of the variance in the combined core top and lgm data set mapped factor scores for the lgm and modern ocean indicate high iron oxide values are present in just two regions one off eastern north america and the other off northwest africa in the region off eastern north america iron oxide occurs primarily during the lgm as the previously noted brick red lutite a unique sediment type derived from the erosion of permo carboniferous red beds in atlantic canada a larger lobe shaped area of iron oxide rich sediment is present off northwest africa in both the modern and lgm levels the modern iron oxide lobe is coincident with the distribution of eolian dust as determined by observations from ships satellites and analysis of air samples during the lgm iron oxides exhibit a similar distribution except the southern margin of the region shifts equatorward and iron oxide concentration increases

identification of processes driving low level westerlies in west equatorial africa abstractthis paper investigates and characterizes the control mechanisms of the low level circulation over west equatorial africa wea using four reanalysis datasets emphasis is placed on the contribution of the divergent and rotational circulation to the total flow additional focus is made on analyzing the zonal wind component in order to gain insight into the processes that control the variability of the low level westerlies llw in the region the results suggest that the control mechanisms differ north and south of 6 n in the north the llw are primarily a rotational flow forming part of the cyclonic circulation driven primarily by the heat low of the west african monsoon system this northern branch of the llw is well developed from june to august and disappears in december february south of 6 n the seasonal variability of the llw is controlled by the heating contrast between cooling associated with subsidence over the ocean and heating over land regions largely south of the equator where as

150 years of eutrophication in the northern adriatic sea evidence from a benthic foraminiferal record the vertical distribution of benthic foraminifera in a sediment core in front of the po delta has been studied in detail according to our age model based on 210pb and 137cs analyses of another core from exactly the same locality the studied core spans the past 160 years the radio isotope profiles further show that sediment mixing is largely restricted to the top centimeter suggesting that the core should provide an extremely detailed record of the youngest history of the northern adriatic sea benthic foraminiferal patterns and grain size analyses indicate a number of substantial changes in sedimentation rate and food oxygen availability in the benthic ecosystem changes occurring at about 1840 and 1880 can be attributed to man induced changes in the main outflow canals of the po river the first one led to an important reduction of the marine vegetation cover which probably was present up to that date the second change resulted in the present day situation in which the po outflow is passing the studied core locality close by the local benthic foraminiferal associations indicate a steadily increasing nutrient load from 1900 ad onwards this trend is interpreted as the effect of anthropogenic eutrophication due to agriculture and waste water disposal although the faunal record as discussed here only gives a limited impression of the basin wide development a marked faunal transition around 1930 indicates intensification of the eutrophication around 1960 the first signs of an increasing importance of anoxic events can be recognized the faunal changes in the last decade which are ascribed to changes in preservation potential indicate that more intense or more prolonged anoxia started about 10 years ago and that the ecological health of this part of the northern adriatic probably is still in decline

west african monsoon dynamics and eastern equatorial atlantic and pacific sst anomalies 1970 88 abstract the laboratoire de meteorologie dynamique atmospheric gcm is used to investigate relationships between west african monsoon dynamics and sst anomalies in the eastern equatorial atlantic and pacific for the period 1970 88 positive sst anomalies in the eastern equatorial pacific mainly associated with a larger east west divergent circulation over the tropical atlantic are found to coincide with negative rainfall anomalies over west africa this is the case for the composite enso warm episodes of 1972 1976 1982 and 1983 by contrast positive sst anomalies in the eastern equatorial atlantic are accompanied by a southward shift of the intertropical convergence zone along with negative rainfall anomalies in the sahel and positive rainfall anomalies in the guinean region this was the case in 1987 the enso warm event during this year had apparently no significant impact on west african monsoon dynamics a zonal atmospheric coupling associated with differences of sst anomalies between the eastern

global equatorial variability of 850 and 200 hpa zonal winds from rawinsondes between 1963 and 1989 the longitude height time variability of 3 month averaged zonal wind anomalies at 850 and 200 hpa over the equatorial area 5 degrees n 5 degrees s is analyzed using a three dimensional dataset constructed from rawinsonde data 1963 1989 the first mode closely related to the southern oscillation index suggests a strong vertical coupling associated with a horizontal out of phase pattern between the central western pacific and the remainder of the equatorial belt the vertical coupling appears to be phase locked to the annual cycle with strongest intensities found over south america and near the maritime continent early in the calendar year and over the pacific basin and africa during the second half of the year this mode of variability can be viewed as a standing pattern superimposed with an eastward migrating component coherent with the annual cycle this westerly moving mode originates near the maritime continent during the northern autumn and tends to precede el nino southern oscillation events in the central and eastern pacific ocean variance is mainly concentrated in the 3 8 year low frequency time scale throughout the tropics and in the 2 3 year quasi biennal band from 110 degrees 120 degrees e to 180 degrees 24 refs 4 figs

wind field coherence and its variations over west africa abstract the wind field coherence over west africa is investigated during two contrasted periods 1953 63 abnormally wet and 1963 75 abnormally dry in the sahel zonal and meridional components of the monthly mean wind are computed with radiosonde data relative to the 950 100 hpa layer a descriptive analysis reveals the main features of the west african atmospheric circulation and a correlation analysis points out their dynamical coherence the exiwnce of hadley type cells of northern and southern hemispheres are confirmed and a mutual interaction in the lower layers is depicted the functioning of a walker type cell defined by a coupling between the zonal components of the monsoonal flow and of the tropical easterly jet tej is shown during northern summer over the whole of west africa there is no correlation between the tej velocity and the african easterly jet aej velocity over the sahelian belt during the dry period the coherence of the cell circulations vanishes in concomitance with a

interdecadal variation of the west african summer monsoon during 1979 2010 and associated variability this paper addresses the interdecadal variation of the west african summer monsoon wasm along with its background of atmospheric circulation and possible physical mechanism over the past 32 years 1979 2010 it is indicated that the wasm starts to strengthen from 1998 as the rainfall begins to increase over western west africa on the whole which shows a new interdecadal variation in this interdecadal variation the strengthened ascending motion corresponding to enhanced divergence convergence movement on the upper lower troposphere is prone to develop the local circulation of the monsoon moreover the strengthened southwestern eastern wind on the lower upper level leads to more moisture from the atlantic and the gulf of guinea transported to the west african continent in addition the summer subtropical high over the north atlantic and western west africa is strong and northward and the tropical east wind is also strong statistically the weaker stronger the spring north atlantic oscillation nao is the stronger weaker the tropical easterly is and then the wasm is also stronger but the effect of the nao on the decadal variation of the wasm is not so significant from the north atlantic anomaly sensitivity simulation with a single model this is also an indication that the relationship between the wasm and nao is complicated in an interdecadal time scale and is needed further study in terms of sea surface temperature sst variation the tendency is toward warming in the subtropical north pacific the south pacific and north atlantic numerical simulation experiments and data analysis show that the sst variation in the north pacific plays an important role in the latest interdecadal strengthening of the wasm during the past 32 years while the influences of the south pacific and the north atlantic sst anomalies are not so significant to the associated atmospheric circulation changes

composition and longshore dispersal of sands from the po and adige rivers since the pre etruscan age abstract the sands presently carried by the po river differ from those of the adige river mainly in the content of dolomite and of fragments of acidic volcanic rocks and to a lesser degree in the content of some heavy mineral species such differences individuate two petrographic provinces compositional data of samples from paleoriver beds and bars reveal that the same differences were present during past times complex admixtures of sands from the two petrographic provinces are often observed in sands from beach ridges whose shape also depends on eolian activity and on the consequent formation of dunes the beach ridges are the product of an evolution from protruding deltaic bodies to a marked regularity of the coastline such modifications are mainly due to marine erosional processes which are of greatest relative importance during periods of decreased fluvial discharge under these conditions the river supplied sands are redistributed with resultant admixture of sands from the two petrographic provinces the sands from deltaic channel mouth bars have compositions that become increasingly uniprovincial from early to late depositional stages these bars form at times of increased fluvial deposition when the rivers build prominent deltas that modify a previously straight coastline under such conditions longshore transport of sands gradually become less important so that mixing of sediments derived from the different provenances gradually becomes negligible since pre etruscan times the sands carried by the adige river were transported longshore towards the south and were mixed with sands from the po river moving north the convergence zone between the two provinces was usually controlled by whichever distributary of the po had the higher discharge among northern po branches that distributary would produce a marked deltaic body that modified the coastline until the southward drift of the adige sands ceased by comparing compositional data with the historical geomorphic evolution of the system the most active mouths of the po in terms of discharge have been identified from the etruscan age to the present

generation of the african easterly jet and its role in determining west african precipitation abstract an examination of analyses and model simulations is used to show that the african easterly jet forms over west africa in summer as a result of strong meridional soil moisture gradients in a series of gcm experiments the imposition of realistic surface wetness contrasts between the sahara and equatorial africa leads to strong positive meridional temperature gradients at the surface and in the lower troposphere the associated easterly shear in the atmosphere is strong enough to establish easterly flow the african easterly jet above the monsoon westerlies at the surface positive temperature gradients associated with the summertime distributions of solar radiation ssts or clouds are not large enough to produce the easterly jet in the absence of soil moisture gradients a thermally direct ageostrophic circulation is identified that can accelerate the largely geostrophic zonal flow and maintain the jet while moisture converges throughout the lower troposphere over east africa moisture divergenc