Low Frequency Winter Rainfall Variability in the North West of Morocco

2016 ◽  
pp. 133-152
Author(s):  
Ali Bellichi
Keyword(s):  
Rainfall Variability ◽  
Low Frequency ◽  
Winter Rainfall ◽  
North West ◽  
The North

Mechanisms for nitrogen supply to the Australian North West Shelf

1985 ◽  
Vol 36 (6) ◽  
pp. 753 ◽  
Author(s):  
PE Holloway ◽  
SE Humphries ◽  
M Atkinson ◽  
J Imberger
Keyword(s):  
River Flow ◽  
Tidal Flow ◽  
Vertical Mixing ◽  
Low Frequency ◽  
Transport Processes ◽  
Physical Processes ◽  
Mixing Processes ◽  
North West ◽  
Tidal Motion ◽  
The North

An upper bound for the rate of supply of new nitrate required to maintain the observed primary production on the North West Shelf is estimated to be 0.1 g N m-2 day -1. Nitrate concentrations over the shelf and slope regions are high ( > 100 mg N m-3, in water deeper than - 100 m and usually low (~10 mg N m-3), on the shelf. River flow is weak and carries little nutrient into the shelf waters and so it remains for ocean physical processes to advect and mix the nutrient-rich deep waters onto the shallower shelf regions to meet the nutrient demand. Several mechanisms are reviewed to determine their potential in carrying out the required transport processes. Estimates of the advection of nitrate onto the shelf show that both semi-diurnal tidal flow and low-frequency (periods > 35 h) upwelling events can each contribute approximately half the required demand, providing there is rapid use of nutrients. The upwelling events occur in summer and are associated with reversals of the south-west-flowing Leeuwin Current. Tropical cyclones are also shown to be capable of meeting a small, but significant, portion of the demand through enrichment of the surface layers in the offshelf waters by upwelling and vertical mixing. The enriched water can then be advected onto the shelf. Both tidal and internal tidal motion have the potential to transport nitrate onto the shelf from deeper water through vertical and horizontal mixing processes. However, these processes are difficult to quantify accurately. It is concluded that nitrogen is supplied to this shelf ecosystem by physical processes that are regular throughout the year, as opposed to large sporadic events that occur only once or twice a year.

Accuracy of wavelets, seismic inversion, and thin-bed resolution

2017 ◽  
Vol 5 (4) ◽  
pp. T523-T530
Author(s):  
Ehsan Zabihi Naeini ◽  
Mark Sams
Keyword(s):  
Seismic Data ◽  
Reservoir Characterization ◽  
Low Frequency ◽  
Seismic Inversion ◽  
Inversion Method ◽  
Frequency Content ◽  
Frequency Model ◽  
North West ◽  
The North ◽  
Well Data

Broadband reprocessed seismic data from the North West Shelf of Australia were inverted using wavelets estimated with a conventional approach. The inversion method applied was a facies-based inversion, in which the low-frequency model is a product of the inversion process itself, constrained by facies-dependent input trends, the resultant facies distribution, and the match to the seismic. The results identified the presence of a gas reservoir that had recently been confirmed through drilling. The reservoir is thin, with up to 15 ms of maximum thickness. The bandwidth of the seismic data is approximately 5–70 Hz, and the well data used to extract the wavelet used in the inversion are only 400 ms long. As such, there was little control on the lowest frequencies of the wavelet. Different wavelets were subsequently estimated using a variety of new techniques that attempt to address the limitations of short well-log segments and low-frequency seismic. The revised inversion showed greater gas-sand continuity and an extension of the reservoir at one flank. Noise-free synthetic examples indicate that thin-bed delineation can depend on the accuracy of the low-frequency content of the wavelets used for inversion. Underestimation of the low-frequency contents can result in missing thin beds, whereas underestimation of high frequencies can introduce false thin beds. Therefore, it is very important to correctly capture the full frequency content of the seismic data in terms of the amplitude and phase spectra of the estimated wavelets, which subsequently leads to a more accurate thin-bed reservoir characterization through inversion.

scholarly journals First look at the giant radio galaxy 3C 236 with LOFAR

2019 ◽  
Vol 628 ◽  
pp. A69 ◽  
Author(s):  
A. Shulevski ◽  
P. D. Barthel ◽  
R. Morganti ◽  
J. J. Harwood ◽  
M. Brienza ◽  
...  
Keyword(s):  
Spectral Index ◽  
Radio Galaxy ◽  
Ambient Medium ◽  
Low Frequency ◽  
Low Frequencies ◽  
North West ◽  
Link Type ◽  
The North ◽  
Morphological Differences ◽  
Inner Region

We have examined the giant radio galaxy 3C 236 using LOFAR at 143 MHz down to an angular resolution of 7″, in combination with observations at higher frequencies. We used the low frequency data to derive spectral index maps with the highest resolution yet at these low frequencies. We confirm a previous detection of an inner hotspot in the north-west lobe and for the first time observe that the south-east lobe hotspot is in fact a triple hotspot, which may point to an intermittent source activity. Also, the spectral index map of 3C 236 shows that the spectral steepening at the inner region of the northern lobe is prominent at low frequencies. The outer regions of both lobes show spectral flattening, in contrast with previous high frequency studies. We derive spectral age estimates for the lobes, as well as particle densities of the IGM at various locations. We propose that the morphological differences between the lobes are driven by variations in the ambient medium density as well as the source activity history.

scholarly journals Large-scale changes of the semidiurnal tide along North Atlantic coasts from 1846 to 2018

Ocean Science ◽  
2021 ◽  
Vol 17 (1) ◽  
pp. 17-34
Author(s):  
Lucia Pineau-Guillou ◽  
Pascal Lazure ◽  
Guy Wöppelmann
Keyword(s):  
Sea Level ◽  
North Atlantic ◽  
Large Scale ◽  
Low Frequency ◽  
Underlying Mechanism ◽  
East Atlantic ◽  
North West ◽  
North East ◽  
The North ◽  
The North Atlantic

Abstract. We investigated the long-term changes of the principal tidal component M2 along North Atlantic coasts, from 1846 to 2018. We analysed 18 tide gauges with time series starting no later than 1940. The longest is Brest with 165 years of observations. We carefully processed the data, particularly to remove the 18.6-year nodal modulation. We found that M2 variations are consistent at all the stations in the North-East Atlantic (Cuxhaven, Delfzijl, Hoek van Holland, Newlyn, Brest), whereas some discrepancies appear in the North-West Atlantic. The changes started long before the 20th century and are not linear. The secular trends in M2 amplitude vary from one station to another; most of them are positive, up to 2.5 mm/yr at Wilmington since 1910. Since 1990, the trends switch from positive to negative values in the North-East Atlantic. Concerning the possible causes of the observed changes, the similarity between the North Atlantic Oscillation and M2 variations in the North-East Atlantic suggests a possible influence of the large-scale atmospheric circulation on the tide. Our statistical analysis confirms large correlations at all the stations in the North-East Atlantic. We discuss a possible underlying mechanism. A different spatial distribution of mean sea level (corresponding to water depth) from one year to another, depending on the low-frequency sea-level pressure patterns, could impact the propagation of the tide in the North Atlantic basin. However, the hypothesis is at present unproven.

scholarly journals Relationship between winter orographic precipitation with synoptic and large-scale atmospheric circulation: The case of mount Olympus, Greece

2018 ◽  
Vol 52 (1) ◽  
pp. 45 ◽  
Author(s):  
Michael Nikolaos Styllas ◽  
Dimitrios Kaskaoutis
Keyword(s):  
Atmospheric Circulation ◽  
Large Scale ◽  
Rainfall Variability ◽  
Circulation Pattern ◽  
Western Mediterranean ◽  
The Arctic ◽  
Central Mediterranean ◽  
Winter Rainfall ◽  
Atmospheric Circulation Patterns ◽  
The North

The relationship between the winter (DJFM) precipitation and the atmospheric circulation patterns is examined around Mount Olympus, Greece in order to assess the effects of orography and atmospheric dynamics over a small (less than 100 x 100 km) spatial domain. Winter accumulated rainfall datasets from 8 stations spread along the eastern (marine) and western (continental) sides of the Mount Olympus at elevations between 30 m and 1150 m are used during the period 1981 to 2000. Synoptic scale conditions of mean sea-level pressure and geopotential heights at 850 hPa and 500 hPa, were used to explain the multiyear rainfall variability. High pressure systems dominated over the central Mediterranean and most parts of central Europe during the late 1980’s and early 1990’s, are associated with minimum winter rainfall along both sides of Mount Olympus. The winter of 1996 was associated with peak in rainfall along the marine side of the mountain and was characterized by enhancement of upper level trough over the western Mediterranean and increased low tropospheric depressions over the southern Adriatic and the Ionian Seas. This atmospheric circulation pattern facilitated a southeasterly air flow that affected more (less) the marine (continental) sides of the mountain. In contrast, dominance of low pressure systems with cores over the Gulf of Genoa and the Central Mediterranean affect the study area mostly from west/southwest revealing higher correlations with the precipitation in the continental side of the mountain (r= -0.80; Elassona station) and considerably lower correlations with the marine side (r = -0.67; Katerini station). This highlights the orographic barrier of the Mount Olympus revealing large differences between the upward and leeward sides. Large scale atmospheric patterns like the North Atlantic Oscillation and the Arctic Oscillation seem to influence the winter rainfall in the lowlands along the continental side of the mountain.

scholarly journals Using Twitter Data for the Study of Language Change in Low-Resource Languages. A Panel Study of Relative Pronouns in Frisian

2021 ◽  
Vol 4 ◽  
Author(s):  
Jelske Dijkstra ◽  
Wilbert Heeringa ◽  
Lysbeth Jongbloed-Faber ◽  
Hans Van de Velde
Keyword(s):  
Language Change ◽  
Panel Study ◽  
Low Frequency ◽  
Background Information ◽  
Birth Cohorts ◽  
North West ◽  
Low Resource ◽  
The North ◽  
Twitter Data ◽  
Relative Pronouns

This paper investigates the usability of Twitter as a resource for the study of language change in progress in low-resource languages. It is a panel study of a vigorous change in progress, the loss of final t in four relative pronouns (dy't, dêr't, wêr't, wa't) in Frisian, a language spoken by ± 450,000 speakers in the north-west of the Netherlands. This paper deals with the issues encountered in retrieving and analyzing tweets in low-resource languages, in the analysis of low-frequency variables, and in gathering background information on Twitterers. In this panel study we were able to identify and track 159 individual Twitterers, whose Frisian (and Dutch) tweets posted in the era 2010–2019 were collected. Nevertheless, a solid analysis of the sociolinguistic factors in this language change in progress was hampered by unequal age distributions among the Twitterers, the fact that the youngest birth cohorts have given up Twitter almost completely after 2014 and that the variables have a low frequency and are unequally spread over Twitterers.

scholarly journals Climate-scale changes of the semidiurnal tide over the North Atlantic coasts from 1846 to 2018

2020 ◽  
Author(s):  
Lucia Pineau-Guillou ◽  
Pascal Lazure ◽  
Guy Wöppelmann
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Low Frequency ◽  
Underlying Mechanism ◽  
East Atlantic ◽  
North West ◽  
North East ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract. We investigated the long-term changes of the principal tidal component M2 over the North Atlantic coasts, from 1846 to 2018. We analysed 9 tide gauges with time series starting no later than 1920. The longest is Brest with 165 years of observations. We carefully processed the data, particularly to remove the 18.6-year nodal modulation. We found that M2 variations are consistent at all the stations in the North East Atlantic (Newlyn, Brest, Cuxhaven), whereas some discrepancies appear in the North West Atlantic. The changes started long before the XXth century, and are not linear. The trends vary from a station to another; they are overall positive, up to 0.7 mm/yr. Since 1990, the trends switch from positive to negative values. Concerning the possible causes of the observed changes, the similarity between the North Atlantic Oscillation and M2 variations in the North East Atlantic suggests a possible influence of the large-scale atmospheric circulation on the tide. We discuss a possible underlying mechanism. A different spatial distribution of water heights from one year to another, depending on the low-frequency sea-level pressure patterns, could impact the propagation of the tide in the North Atlantic basin. However, the hypothesis is at present unproven.

SEISMIC STRATIGRAPHIC EVALUATION OF A NEOCOMIAN SUBMARINE FAN SYSTEM, BROWSE BASIN, NORTH WEST SHELF

1992 ◽  
Vol 32 (1) ◽  
pp. 171
Author(s):  
L.R. Miller ◽  
W.J. Stuart
Keyword(s):  
High Frequency ◽  
Seismic Reflection ◽  
Low Frequency ◽  
High Amplitude ◽  
Seismic Facies ◽  
Submarine Fan ◽  
North West ◽  
The North ◽  
Braided Channel ◽  
Low Amplitude

A possible submarine fan system of Valanginian age occurs in the south of the western half of Permit WA-212-P in the Browse Basin. Seismic mapping and interpretation have allowed the recognition of five seismic facies which are considered representative of this fan system.The five seismic facies are the upper-middle fan braided channel facies, the upper-middle fan braided interchannel facies, the lower fan channel facies, the lower fan sheet facies, and the lower fan lobe fringe facies. The reflections of the upper-middle braided channel fan facies are discontinuous, disrupted, convex up, low amplitude and high frequency. The interchannel facies has reflections that are concave up, continuous, low frequency and moderate to high amplitude. The lower fan channel facies are recognised by convex up, discontinuous, high frequency and low amplitude reflections. The lower fan sheet facies is noted by mounded configurations with continuous, moderate to high amplitude, moderate frequency reflections. The lower fan lobe fringe facies reflections are flat, often shingled reflections with moderate discontinuity, moderate to high amplitude and low to moderate frequency.Since no wells penetrate the submarine fan, the interpretation is based on seismic reflection configurations which are considered typical of submarine fan segments. The interpreted ancient submarine fan occurs on the basin floor adjacent to a probable ramp type margin, and manifests shape and setting consistent with known submarine fans, such as the Eocene Frigg Fan of the North Sea, and the Lower Cretaceous Barrow Group turbidites in the Carnarvon Basin.Seismic facies mapping, in conjunction with sequence stratigraphy concepts, is particularly useful in areas such as the Browse Basin where considerable marine shale sections exist with little structure, and sequences with reservoir potential continue to be a problem to locate. For instance, in Caswell-1, a well drilled in 1977 immediately north of the permit area, 200 barrels of oil flowed from thin sands within a shale sequence of Albian age. Results of this study indicate that local seismic reflection signatures may be indicative of potential sandstone reservoirs in the vicinity of the study area.

scholarly journals Rainfall Response in Northeast Brazil from Ocean Climate Variability during the Second Half of the Twentieth Century

2011 ◽  
Vol 24 (23) ◽  
pp. 6174-6184 ◽  
Author(s):  
Daisy Beserra Lucena ◽  
Jacques Servain ◽  
Manoel Francisco Gomes Filho
Keyword(s):  
Twentieth Century ◽  
Rainfall Variability ◽  
Low Frequency ◽  
Northeast Brazil ◽  
Drought Impact ◽  
The North ◽  
Tropical Oceans ◽  
The Pacific ◽  
The North Atlantic Oscillation

Abstract The authors investigated the rainfall variability response in northern Northeast Brazil (NNEB) from El Niño/La Niña (EN/LN) events and from the meridional sea surface temperature gradient (MGRAD) over the tropical Atlantic during the period 1948–97. The diagnostic analysis was stratified according to four climatic series of scenarios associated with EN, LN, and positive and negative MGRAD. During ENs, which were more numerous and more intense after the 1970s, the MGRAD was generally not noticeable, and the drought impact in NNEB was mainly due to the warm Pacific influence. Conversely, during LNs, the MGRAD signal was important, but there was an inverse relationship between the third and the fourth quarters of the twentieth century. Thus, before the 1970s the LNs were associated with positive MGRAD, which led to an inverse influence inducing minor changes in seasonal rainfall in NNEB. After the 1970s the LNs were linked to negative MGRAD, which induced a cumulative wet influence in NNEB. The positive MGRADs were generally associated with ENs, which reinforced the drought impact in NNEB. The well-marked negative MGRADs, which all occurred after the beginning of the 1970s, were generally linked with large LNs that induced very consistent wet episodes in NNEB. Interestingly, the two low-frequency variations in the tropical oceans observed during the second half of the twentieth century (i.e., from a few to several strong ENs and from none to numerous strong negative MGRADs) occurred concomitantly with symmetric long-term changes in the Pacific decadal oscillation (PDO) and the North Atlantic Oscillation (NAO). This symmetrical long-term climate behavior during the second half of the twentieth century could have lead to an inverse influence on the climate over the north Northeast Brazil, in agreement with a quasi-null long-term trend of the rainfall observed in that region all along this period. Such symmetrical behavior seems to have been unique during the last 150 years.

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