scholarly journals Maintenance of the Midtropospheric North African Summer Circulation: Saharan High and African Easterly Jet

2005 ◽  
Vol 18 (15) ◽  
pp. 2943-2962 ◽  
Author(s):  
Tsing-Chang Chen
Keyword(s):  
Lower Troposphere ◽  
Monsoon Circulation ◽  
North African ◽  
Summer Circulation ◽  
African Easterly Jet ◽  
Differential Heating ◽  
The North ◽  
Budget Analysis ◽  
Eastern North Atlantic ◽  
East West

Abstract The conspicuous feature of the midtropospheric North African summer circulation is the Saharan high surrounded on its southern rim by the African easterly jet (AEJ). Like a major monsoon circulation, the Saharan high is juxtaposed with the North African divergent center to the east and the eastern Atlantic convergent center to the west. Different from a major monsoon circulation, these pronounced midtropospheric circulation components are overlaid by the western part of the Tibetan high. Because of the unique roles played by the Saharan high and the African easterly jet in the North African summer circulation, an effort is made to explore maintenance mechanisms of these two midtropospheric circulation elements. Major findings of this effort are summarized as follows:In terms of the velocity potential maintenance equation, it is shown that the North African divergent center over the Chad–Sudan region is maintained by the vertical differential heating established by the Saharan thermal-low heating in the lower troposphere and the Saharan radiative cooling in the upper troposphere.The Saharan high is spatially in quadrature with the North African divergent center. It is inferred from the streamfunction budget analysis that the Saharan high is maintained by the east–west circulation, which is formed by the east–west differential heating between the Saharan thermal-low heating and the eastern North Atlantic cooling. This inference is further substantiated by forced barotropic model simulations.The AEJ around the tropical periphery of the Saharan high is almost perpendicular to the equatorward divergent northerlies spilling out of the North African divergent center. The energetic interaction between divergent and rotational flows reveals that this jet is maintained by the Coriolis acceleration associated with these divergent winds. These findings not only reveal the maintenance mechanism of the Saharan high and the associated AEJ, but also facilitate the search for answers to some problems of the North African summer weather/climate system.

scholarly journals Pivotal role of the North African Dipole Intensity (NAFDI) on alternate Saharan dust export over the North Atlantic and the Mediterranean, and relationship with the Saharan Heat Low and mid-latitude Rossby waves

2016 ◽  
Author(s):  
E. Cuevas ◽  
Á. J. Gómez-Peláez ◽  
S. Rodríguez ◽  
E. Terradellas ◽  
S. Basart ◽  
...  
Keyword(s):  
Rossby Wave ◽  
North Atlantic ◽  
Rossby Waves ◽  
Lower Troposphere ◽  
North African ◽  
Zonal Wind Anomaly ◽  
Western Sahara ◽  
The North ◽  
The Impact ◽  
Heat Low

Abstract. In this study, we revise the index that quantifies the North African Dipole Intensity (NAFDI), and explain its relationship with the Saharan Heat Low (SHL) and mid-latitude Rossby waves. We find outstanding similarities of meteorological patterns associated with the positive NAFDI and the SHL West-phase on the one hand, and with the negative NAFDI and the SHL East-Phase, on the other hand. We introduce the daily NAFDI index and the daily SHL West-East Displacement Index (SHLWEDI). The Pearson correlation coefficient between the daily SHLWEDI 1-day lagged and the daily NAFDI for the period 1980–2013 20 June–17 September is fairly high (r = 0.77). The correlation reduces to 0.69 if the SHLWEDI is not lagged. We observe that the SHL West-phase is significantly more frequent than the SHL East-phase, and that the SHL is more intense during its East-phase. We find positive aerosol optical depth (AOD) anomalies in the Western Sahara during positive NAFDI/SHL West-phase, and negative AOD anomalies in the central and eastern Sahara during negative NAFDI/SHL East-phase. A significant positive (negative) NE-SW axis AOD anomaly over the Subtropical North Atlantic for positive (negative) NAFDI is found. Remarkable patterns of positive (negative) AOD anomalies over the tropical Atlantic and the Central-Western Mediterranean during negative (positive) NAFDI are observed. The impact of mid-latitude Rossby waves on NAFDI variations depends on both the amplitude and phase of the Rossby wave at 200–300 hPa, which is quantified in this study by the daily Zonal Wind Anomaly at 300 hPa over South Morocco (ZWA300), and the penetration of the Rossby wave into the lower troposphere, quantified by the daily Omega at 500 hPa over Northwest Algeria (O500). The correlation of both ZWA300 and O500 with NAFDI is significant: 0.48 and 0.53, respectively, when we apply 5-day running means to the time series before calculating the correlation coefficients, and increases to 0.66 when a multi-linear regression is performed. The results suggest that ZWA300 drives almost one day in advance the NAFDI, whereas O500 might be ahead respect to NAFDI less than 12 hours. The power spectra of the NAFDI, SHL, ZWA300 and O500 times series in the intermediate time scale range (between 10 and 30 days) show 10 especially intense NAFDI spectral peaks, most of them also present in the SHLWEDI spectrum, finding that for many of the NAFDI/SHLWEDI peaks there is associated an O500 and/or ZWA300 peak. Our results indicate that the modes of oscillation of both the NAFDI and the SHL are driven by those mid-latitudes Rossby waves that go deep enough into the lower troposphere imposing their perturbation to the background meteorological fields. A comprehensive top-down conceptual model is introduced to explain the relationships between the NAFDI, the SHL and the mid-latitude Rossby waves and their impact in dust mobilization and transport in Northern Africa.

Faunal specificity, endemism and paleobiogeography: the post-glacial (Hirnantian-early Rhuddanian) graptolite fauna of the North-African border of Gondwana: a case study

2009 ◽  
Vol 180 (4) ◽  
pp. 353-367 ◽  
Author(s):  
Philippe Legrand
Keyword(s):  
Water Density ◽  
North African ◽  
Specific Level ◽  
Selective Filter ◽  
The North ◽  
Starting Point ◽  
Glacial Rebound ◽  
Marine Clays ◽  
East West

Abstract Several types of stratigraphic successions are found at the top of the glacial to periglacial “Complexe terminal” forming the uppermost Ordovician of the North-African border of Gondwana. Locally, there may be a progressive transition from microconglomeratic clays (diamictites) with dropstones to more normal marine clays accompanied by the almost immediate reappearance of graptolites. Study of the graptolites has revealed a distinctive composition at the specific level, differing from contemporaneous faunal associations in adjacent regions, which consist of such classical species as “Glyptograptus” persculptus, Akidograptus ascensus, Parakidograptus acuminatus, etc. The faunal composition has long posed a chronostratigraphic problem, which has now been largely resolved, the fauna being interpreted as Hirnantian to early Rhuddanian in age. On the other hand, the clear evidence of faunal specificity poses a number of problems in the post-glacial context, namely: the disappearance of graptolites in the mid-Caradoc from these regions poses a problem as to the origin of the new specific fauna. Three hypotheses are considered, none of which is satisfactory. Most available data point to an eastern communication, although it is possible that the rejuvenation of the Mauritanides to the west “erased” all prior evidence, giving a wrong idea as to the communications which existed in the area under consideration; what mechanisms caused and maintained this isolation? The hypothesis of a wide, east-west trending depression produced by the overloading of the frontal ice sheet and its progressive disappearance concomitant with the glacial rebound is being considered; what were the effects of isolation on the morphology of graptolites and their population? The virgella and the virgula in several species are remarkably long and this could be attributed to a reduction in water density. The size of monospecific populations also attests to an adaptation to a restricted regime with sandy deposition; how can one explain why some species like N. pseudovenustus, N. inazzaouae, N. normalis brenansi and Ps. kiliani, occur also in other parts of the world? If these species occur where the classical species are absent, the opposite is even more difficult to explain, leading one to postulate the presence of a selective “filter”; how did this faunal specificity disappear progressively? The extent of the sea with Nd. africanus and Nd. fezzanensis put an end to isolation, although it respected the east-west trend. However, there was an opening to the adjacent regions corresponding to present-day Libya. To conclude, if pelagic faunas are considered to be poor paleogeographic tools, faunal specificity rather than endemism should be regarded as the starting point for further reflection. However, all faunal specificity must be fully documented and the results integrated in a framework that includes all aspects of sedimentology, tectonics and climatology. Seven new species are briefly described: Normalograptus nseirati sp. nov., Normalograptus gelidus sp. nov., Normalograptus arrikini sp. nov., Normalograptus pretilokensis sp. nov., Neodiplograptus inezzani sp. nov., Neodiplograptus incommodus sp. nov. and “Glyptograptus” saharensis sp. nov.

scholarly journals Attribution of Global Monsoon Response to the Last Glacial Maximum Forcings

2019 ◽  
Vol 32 (19) ◽  
pp. 6589-6605 ◽  
Author(s):  
Jian Cao ◽  
Bin Wang ◽  
Libin Ma
Keyword(s):  
Last Glacial Maximum ◽  
Climate Model ◽  
Ice Sheet ◽  
Early Summer ◽  
Glacial Maximum ◽  
Monsoon Circulation ◽  
Global Monsoon ◽  
Last Glacial ◽  
The North ◽  
Budget Analysis

Abstract Investigation of global monsoon (GM) responses to external forcings is instrumental for understanding its formation mechanism and projected future changes. Coupled climate model experiments are performed to assess how the individual and full Last Glacial Maximum (LGM) forcings change GM precipitation. Under the full LGM forcing, the annual and local summer-mean GM precipitation are reduced by 8.5% and 10.8%, respectively, compared to the results in the preindustrial control run; and the reduction of Northern Hemisphere (NH) summer monsoon (NHSM) precipitation is twice as large as its Southern Hemisphere (SH) counterpart (SHSM). The NH–SH asymmetric response is mainly caused by the monsoon circulation change–induced moisture convergence rather than the reduction of moisture content, but the root cause is the continental ice sheet forcing. The NHSM precipitation changes dramatically differ among various single-forcing experiments, while this is not the case for their SH counterparts. The moisture budget analysis indicates the NHSM is dynamically oriented, but SHSM is thermodynamically oriented. The markedly different NHSM circulation changes are caused by different forcing-induced sea surface temperature (SST) patterns, including the North Atlantic cooling pattern forced by the continental ice sheet, the mega–La Niña–like pattern resulting from the greenhouse gas forcing, and the Indian Ocean dipole–like SST pattern caused by the land–sea configuration forcing. Moreover, the distinctive change of “monsoonality” in the Australian–Indonesian monsoon is predominantly forced by the exposure of the land shelf, which enhances precipitation during early summer (November–December) but weakens it in the rest of the year.

scholarly journals Harmattan, Saharan heat low and West African Monsoon circulation: Modulations on the Saharan dust outflow towards the north Atlantic

2017 ◽  
Author(s):  
Kerstin Schepanski ◽  
Bernd Heinold ◽  
Ina Tegen
Keyword(s):  
North Atlantic ◽  
Dust Emission ◽  
Temporal Distribution ◽  
Saharan Dust ◽  
Monsoon Circulation ◽  
North African ◽  
Dust Source ◽  
The North ◽  
The North Atlantic ◽  
Spatio Temporal

Abstract. The outflow of dust from the North African continent towards the north Atlantic is stimulated by the atmospheric circulation over North Africa, which modulates the spatio-temporal distribution of dust source activation and consequently the entrainment of mineral dust into the boundary layer, as well as the transport of dust out of the source regions. The atmospheric circulation over the North African dust source regions, predominantly the Sahara and the Sahel, is characterised by three major circulation regimes: (1) the Harmattan (trade winds), (2) the Saharan heat low (SHL), and (3) the West African Monsoon circulation. The strength of the individual regimes controls the Saharan dust outflow by affecting the spatio-temporal distribution of dust emission, transport pathways, and deposition fluxes. This study aims at investigating the atmospheric circulation pattern over North Africa with regard to its role favouring dust emission and dust export towards the tropical North Atlantic. The focus of the study is on summer 2013 (June to August), during which also the SALTRACE (Saharan Aerosol Long-range TRansport and Aerosol-Cloud interaction Experiment) field campaign took place. It involves satellite observations by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) flying on-board the geostationary Meteosat Second Generation (MSG) satellite, which are analysed and used to infer a data set of active dust sources. The spatio-temporal distribution of dust source activation frequencies (DSAF) allows for linking the diurnal cycle of dust source activations to dominant meteorological controls on dust emission. In summer, Saharan dust source activations clearly differ from dust source activations over the Sahel regarding the time-of-day when dust emission begins. The Sahara is dominated by morning dust source activations predominantly driven by the break-down of the nocturnal low-level jet. In contrast, dust source activations in the Sahel are predominantly activated during the second half of the day when down-drafts associated with deep moist convection are the major atmospheric driver. Complementary to the satellite-based analysis on dust source activations and implications from their diurnal cycle, simulations on atmosphere and dust life-cycle were performed using the meso-scale atmosphere-dust model system COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model). Fields from this simulation were analysed regarding the variability of the Harmattan, the Saharan heat low, and the Monsoon circulation as well as their impact on the variability of the Saharan dust outflow towards the north Atlantic. This study illustrates the complexity of the interaction among the three major circulation regimes and their modulation of the North African dust outflow. Enhanced westward dust fluxes frequently appear following a phase characterised by a deep SHL. Ultimately, findings from this study contribute to the quantification of the interannual variability of the atmospheric dust burden.

Skeletochronology and mark–recapture assessments of growth in the North African agamid lizard (Agama impalearis)

1999 ◽  
Vol 249 (4) ◽  
pp. 455-461
Author(s):  
El Hassan El Mouden ◽  
Mohammed Znari ◽  
Richard P. Brown
Keyword(s):  
North African ◽  
Mark Recapture ◽  
Agamid Lizard ◽  
The North

scholarly journals A Low frequency Southern Sky Survey Using the Mauritius Radio Telescope

2002 ◽  
Vol 199 ◽  
pp. 25-31
Author(s):  
N. Udaya Shankar
Keyword(s):  
Radio Telescope ◽  
Low Frequency ◽  
Fourier Synthesis ◽  
North East ◽  
The North ◽  
Sky Survey ◽  
Correlation Receiver ◽  
Complex Correlation ◽  
Right Ascension ◽  
East West

The Mauritius Radio Telescope (MRT) is a Fourier synthesis instrument which has been built to fill the gap in the availability of deep sky surveys at low radio frequencies in the southern hemisphere. It is situated in the north-east of Mauritius at a southern latitude of 20°.14 and an eastern longitude of 57°.73. The aim of the survey with the MRT is to contribute to the database of southern sky sources in the declination range −70° ≤ δ ≤ −10°, covering the entire 24 hours of right ascension, with a resolution of 4' × 4'.6sec(δ + 20.14°) and a point source sensitivity of 200 mJy (3σ level) at 151.5 MHz.MRT is a T-shaped non-coplanar array consisting of a 2048 m long East-West arm and a 880 m long South arm. In the East-West arm 1024 fixed helices are arranged in 32 groups and in the South arm 16 trolleys, with four helices on each, which move on a rail are used. A 512 channel, 2-bit 3-level complex correlation receiver is used to measure the visibility function. At least 60 days of observing are required for obtaining the visibilities up to the 880 m spacing. The calibrated visibilities are transformed taking care of the non-coplanarity of the array to produce an image of the area of the sky under observation.This paper will describe the telescope, the observations carried out so far, a few interesting aspects of imaging with this non-coplanar array and present results of a low resolution survey (13' × 18') covering roughly 12 hours of right ascension, and also present an image with a resolution of 4' × 4'.6sec(δ + 20.14°) made using the telescope.

scholarly journals Defatted Hydroethanolic Extract of Ammodaucus leucotrichus Cosson and Durieu Seeds: Antidiabetic and Anti-Inflammatory Activities

2020 ◽  
Vol 10 (24) ◽  
pp. 9147
Author(s):  
Imane Es-Safi ◽  
Hamza Mechchate ◽  
Amal Amaghnouje ◽  
Anna Calarco ◽  
Smahane Boukhira ◽  
...  
Keyword(s):  
Wistar Rats ◽  
Fasting Blood Glucose ◽  
Diabetic Mice ◽  
North African ◽  
Glucose Levels ◽  
The North ◽  
Blood Glucose Levels ◽  
Anti Inflammatory ◽  
Hydroethanolic Extract ◽  
Inflammatory Effect

The seeds of Ammodaucus leucotrichus Cosson and Durieu have been used in the North African Sahara as a traditional medicine to treat diabetes. The present study investigates the antidiabetic, antihyperglycemic, and anti-inflammatory properties of the defatted hydroethanolic extract of Ammodaucus leucotrichus (DHEAM). The antidiabetic and the antihyperglycemic studies were assessed on alloxan-induced diabetic with orally administered doses of DHEAM (100 and 200 mg/kg). At the same time, its anti-inflammatory propriety was evaluated by measuring edema development in the Wistar rats paw induced with carrageenan. Treatment of diabetic mice with DHEAM for four weeks managed their high fasting blood glucose levels, improved their overall health, and also revealed an excellent antihyperglycemic activity. Following the anti-inflammatory results, DHEAM exhibited a perfect activity. HPLC results revealed the presence of seven molecules (chlorogenic acid, 3-p-coumaroylquinic acid, gallic acid, ferulic acid, myricetin, quercetin, luteolin). This work indicates that the DHEAM has an important antidiabetic, antihyperglycemic, and anti-inflammatory effect that can be well established as a phytomedicine to treat diabetes.

scholarly journals Identification and Characterization of the Most Common Genetic Variant Responsible for Acephalic Spermatozoa Syndrome in Men Originating from North Africa

2021 ◽  
Vol 22 (4) ◽  
pp. 2187
Author(s):  
Caroline Cazin ◽  
Yasmine Boumerdassi ◽  
Guillaume Martinez ◽  
Selima Fourati Ben Mustapha ◽  
Marjorie Whitfield ◽  
...  
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
North Africa ◽  
Posterior Part ◽  
North African ◽  
The North ◽  
Transmission Electron ◽  
Variant Filtering ◽  
North African Population ◽  
Sperm Nuclei

Acephalic spermatozoa syndrome (ASS) is a rare but extremely severe type of teratozoospermia, defined by the presence of a majority of headless flagella and a minority of tail-less sperm heads in the ejaculate. Like the other severe monomorphic teratozoospermias, ASS has a strong genetic basis and is most often caused by bi-allelic variants in SUN5 (Sad1 and UNC84 domain-containing 5). Using whole exome sequencing (WES), we investigated a cohort of nine infertile subjects displaying ASS. These subjects were recruited in three centers located in France and Tunisia, but all originated from North Africa. Sperm from subjects carrying candidate genetic variants were subjected to immunofluorescence analysis and transmission electron microscopy. Moreover, fluorescent in situ hybridization (FISH) was performed on sperm nuclei to assess their chromosomal content. Variant filtering permitted us to identify the same SUN5 homozygous frameshift variant (c.211+1_211+2dup) in 7/9 individuals (78%). SUN5 encodes a protein localized on the posterior part of the nuclear envelope that is necessary for the attachment of the tail to the sperm head. Immunofluorescence assays performed on sperm cells from three mutated subjects revealed a total absence of SUN5, thus demonstrating the deleterious impact of the identified variant on protein expression. Transmission electron microscopy showed a conserved flagellar structure and a slightly decondensed chromatin. FISH did not highlight a higher rate of chromosome aneuploidy in spermatozoa from SUN5 patients compared to controls, indicating that intra-cytoplasmic sperm injection (ICSI) can be proposed for patients carrying the c.211+1_211+2dup variant. These results suggest that the identified SUN5 variant is the main cause of ASS in the North African population. Consequently, a simple and inexpensive genotyping of the 211+1_211+2dup variant could be beneficial for affected men of North African origin before resorting to more exhaustive genetic analyses.

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