scholarly journals Tropical Atlantic moisture availability and precipitation over West Africa: Application to DEMETER hindcasts

2006 ◽  
Vol 33 (21) ◽  
Author(s):  
Philippe Rogel ◽  
Yves M. Tourre ◽  
Vincent Benoit ◽  
Lionel Jarlan
Keyword(s):  
West Africa ◽  
Tropical Atlantic ◽  
Moisture Availability

scholarly journals Understanding the variability of the rainfall dipole in West Africa using the EC-Earth last millennium simulation

2021 ◽  
Author(s):  
Qiong Zhang ◽  
Ellen Berntell ◽  
Qiang Li ◽  
Fredrik Charpentier Ljungqvist
Keyword(s):  
West Africa ◽  
Model Simulation ◽  
Rainfall Variability ◽  
Low Pressure ◽  
Last Millennium ◽  
Tropical Atlantic ◽  
Gulf Of Guinea ◽  
Dipole Pattern ◽  
Sahel Region ◽  
Heat Low

AbstractThere is a well-known mode of rainfall variability associating opposite hydrological conditions over the Sahel region and the Gulf of Guinea, forming a dipole pattern. Previous meteorological observations show that the dipole pattern varies at interannual timescales. Using an EC-Earth climate model simulation for last millennium (850–1850 CE), we investigate the rainfall variability in West Africa over longer timescales. The 1000-year-long simulation data show that this rainfall dipole presents at decadal to multidecadal and centennial variability and long-term trend. Using the singular value decomposition (SVD) analysis, we identified that the rainfall dipole present in the first SVD mode with 60% explained variance and associated with the variabilities in tropical Atlantic sea surface temperature (SST). The second SVD mode shows a monopole rainfall variability pattern centred over the Sahel, associated with the extra-tropical Atlantic SST variability. We conclude that the rainfall dipole-like pattern is a natural variability mode originated from the local ocean–atmosphere-land coupling in the tropical Atlantic basin. The warm SST anomalies in the equatorial Atlantic Ocean favour an anomalous low pressure at the tropics. This low pressure weakens the meridional pressure gradient between the Saharan Heat Low and the tropical Atlantic. It leads to anomalous northeasterly, reduces the southwesterly moisture flux into the Sahel and confines the Gulf of Guinea's moisture convergence. The influence from extra-tropical climate variability, such as Atlantic multidecadal oscillation, tends to modify the rainfall dipole pattern to a monopole pattern from the Gulf of Guinea to Sahara through influencing the Sahara heat low. External forcing—such as orbital forcing, solar radiation, volcanic and land-use—can amplify/dampen the dipole mode through thermal forcing and atmosphere dynamical feedback.

Evidence of forest extension in west Africa since 22,000 BP: A pollen record from the eastern tropical Atlantic

1993 ◽  
Vol 12 (3) ◽  
pp. 203-210 ◽  
Author(s):  
Anne-Marie Lézine ◽  
Colette Vergnaud-Grazzini
Keyword(s):  
West Africa ◽  
Pollen Record ◽  
Tropical Atlantic

scholarly journals Entrainment and its dependency on environmental conditions and convective organization in convection-permitting simulations

2021 ◽  
Author(s):  
Tobias Becker ◽  
Cathy Hohenegger
Keyword(s):  
South America ◽  
West Africa ◽  
Deep Convection ◽  
Static Stability ◽  
Tropical Atlantic ◽  
Shallow Convection ◽  
Squall Lines ◽  
Environmental Humidity ◽  
Turbulence Generation ◽  
Convective Organization

AbstractIn this study, we estimate bulk entrainment rates for deep convection in convection-permitting simulations, conducted over the tropical Atlantic Ocean, encompassing parts of Africa and South America. We find that, even though entrainment rates decrease with height in all regions, they are, when averaging between 600 and 800 hPa, generally higher over land than over ocean. This is so because, over Amazonia, shallow convection causes an increase of bulk entrainment rates at lower levels and because, over West Africa, where entrainment rates are highest, convection is organized in squall lines. These squall lines are associated with strong mesoscale convergence, causing more intense updrafts and stronger turbulence generation in the vicinity of updrafts, increasing the entrainment rates. With the exception of West Africa, entrainment rates differ less across regions than across different environments within the regions. In contrast to what is usually assumed in convective parameterizations, entrainment rates increase with environmental humidity. Moreover, over ocean, they increase with static stability, while over land, they decrease. In addition, confirming the results of a recent idealized study, entrainment rates increase with convective aggregation, except in regions dominated by squall lines, like over West Africa.

scholarly journals Linking horizontal and vertical transports of biomass fire emissionsto the tropical Atlantic ozone paradox during the Northern Hemisphere winter season: climatology

2004 ◽  
Vol 4 (2) ◽  
pp. 449-469 ◽  
Author(s):  
G. S. Jenkins ◽  
J.-H. Ryu
Keyword(s):  
South America ◽  
West Africa ◽  
Northern Hemisphere ◽  
Outgoing Longwave Radiation ◽  
Winter Season ◽  
Longwave Radiation ◽  
Tropical Atlantic ◽  
Hemisphere Winter ◽  
Column Ozone ◽  
Northern Hemisphere Winter

Abstract. During the Northern hemisphere winter season, biomass burning is widespread in West Africa, yet the total tropospheric column ozone values (<30DU) over much of the Tropical Atlantic Ocean (15°N-5°S) are relatively low. At the same time, the tropospheric column ozone values in the Southern Tropical Atlantic are higher than those in the Northern Hemisphere (ozone paradox). We examine the causes for low tropospheric column ozone values by considering the horizontal and vertical transport of biomass fire emissions in West Africa during November through March, using observed data which characterizes fires, aerosols, horizontal winds, precipitation, lightning and outgoing longwave radiation. We have found that easterly winds prevail in the lower troposphere but transition to westerly winds at pressure levels lower than 500hPa. A persistent anticyclone over West Africa at 700hPa is responsible for strong easterly winds, which causes a net outflow of ozone/ozone precursors from biomass burning in West Africa across the Atlantic Ocean towards South America. The lowest outgoing longwave radiation (OLR) and highest precipitation rates are generally found over the central Atlantic, some distance downstream of fires in West Africa making the vertical transport of ozone and ozone precursors less likely and ozone destruction more likely. However, lightning over land areas in Central Africa and South America can lead to enhanced ozone levels in the upper troposphere especially over the Southern tropical Atlantic during the Northern Hemisphere winter season.

scholarly journals Space-borne observations link the tropical atlantic ozone maximum and paradox to lightning

2004 ◽  
Vol 4 (2) ◽  
pp. 361-375 ◽  
Author(s):  
G. S. Jenkins ◽  
J.-H. Ryu
Keyword(s):  
South America ◽  
West Africa ◽  
Biomass Burning ◽  
Central Africa ◽  
Tropical Atlantic ◽  
Upper Troposphere ◽  
The North ◽  
Seasonal Basis ◽  
June July ◽  
Column Ozone

Abstract. The potential enhancement of tropospheric column ozone values over the Tropical Atlantic Ocean on a seasonal basis by lightning is investigated using satellite derived ozone data, TRMM lightning data, ozonesonde data and NCEP reanalysis during 1998-2001. Our results show that the number of lightning flashes in Africa and South America reach a maximum during September, October and November (SON). The spatial patterns of winds in combination with lightning from West Africa, Central Africa and South America is likely responsible for enriching middle/upper troposphere ozone over the Tropical South Atlantic during SON. Moreover, lightning flashes are high in the hemisphere opposite to biomass burning during December, January, and February (DJF) and June, July and August (JJA). This pattern leads to an enrichment of ozone in the middle/upper troposphere in the Southern Hemisphere Tropics during DJF and the Northern Hemisphere Tropics during JJA. During JJA the largest numbers of lightning flashes are observed in West Africa, enriching tropospheric column ozone to the north of 5S in the absence of biomass burning. During DJF, lightning is concentrated in South America and Central Africa enriching tropospheric column ozone south of the Equator in the absence of biomass burning.

scholarly journals Tropical Atlantic Hurricanes, Easterly Waves, and West African Mesoscale Convective Systems

2010 ◽  
Vol 2010 ◽  
pp. 1-13 ◽  
Author(s):  
Yves K. Kouadio ◽  
Luiz A. T. Machado ◽  
Jacques Servain
Keyword(s):  
West Africa ◽  
West African ◽  
Mesoscale Convective Systems ◽  
Tropical Atlantic ◽  
Atmospheric Conditions ◽  
Easterly Waves ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Sst Anomaly ◽  
Atlantic Hurricanes

The relationship between tropical Atlantic hurricanes (Hs), atmospheric easterly waves (AEWs), and West African mesoscale convective systems (MCSs) is investigated. It points out atmospheric conditions over West Africa before hurricane formation. The analysis was performed for two periods, June–November in 2004 and 2005, during which 12 hurricanes (seven in 2004, five in 2005) were selected. Using the AEW signature in the 700 hPa vorticity, a backward trajectory was performed to the African coast, starting from the date and position of each hurricane, when and where it was catalogued as a tropical depression. At this step, using the Meteosat-7 satellite dataset, we selected all the MCSs around this time and region, and tracked them from their initiation until their dissipation. This procedure allowed us to relate each of the selected Hs with AEWs and a succession of MCSs that occurred a few times over West Africa before initiation of the hurricane. Finally, a dipole in sea surface temperature (SST) was observed with a positive SST anomaly within the region of H generation and a negative SST anomaly within the Gulf of Guinea. This SST anomaly dipole could contribute to enhance the continental convergence associated with the monsoon that impacts on the West African MCSs formation.

scholarly journals Seasonal Predictions of Holopelagic Sargassum Across the Tropical Atlantic Accounting for Uncertainty in Drivers and Processes: The SARTRAC Ensemble Forecast System

2021 ◽  
Vol 8 ◽  
Author(s):  
Robert Marsh ◽  
Kwasi Appeaning Addo ◽  
Philip-Neri Jayson-Quashigah ◽  
Hazel A. Oxenford ◽  
Ava Maxam ◽  
...  
Keyword(s):  
West Africa ◽  
Early Warning ◽  
Ocean Model ◽  
Ensemble Forecast ◽  
Boreal Summer ◽  
Tropical Atlantic ◽  
Seasonal Forecasts ◽  
Forecast System ◽  
Trade Winds ◽  
The Caribbean

The holopelagic macroalgae sargassum has proliferated across the tropical Atlantic since 2011, of consequence for coastal populations from West Africa to the Caribbean with limited early warning of major beaching events. As part of an interdisciplinary project, ‘Teleconnected SARgassum risks across the Atlantic: building capacity for TRansformational Adaptation in the Caribbean and West Africa’ (SARTRAC), an ensemble forecast system, SARTRAC-EFS, is providing seasonal predictions of sargassum drift. An eddy-resolving ocean model hindcast provides the winds and currents necessary to generate ensemble members. Ensemble forecasts are then obtained for different combinations of ‘windage’, the fractional influence of winds on sargassum mats, and in situ rates of growth, mortality, and sinking. Forecasts for north and south of Jamaica are evaluated with satellite-observed distributions, associated with beaching events in specific years of heavy inundation, 2015 and 2018-20. These seasonal forecasts are evaluated, on lead times of up to 180 days. Forecasts are subject to leading modes of tropical climate variability, in particular the Atlantic Meridional Mode (AMM). More accurate forecasts for a given year are obtained with ensemble members from hindcast years with a similar spring AMM-index. This is most clearly evident during negative AMM phases in spring of 2015 and 2018, when positive sea surface temperature anomalies and anomalously weak trade winds were established across the northern tropics. On this evidence, SARTRAC-EFS is potentially useful in providing early warning of high sargassum prevalence. Extended to sargassum drift off West Africa, extensive cloud cover limits availability of the satellite data needed for full application and evaluation of SARTRAC-EFS in this region, although experimental forecasts off the coast of Ghana are found highly sensitive to the windage that is associated with strong onshore winds during boreal summer. Alongside other forecast systems, SARTRAC-EFS is providing useful early warnings of sargassum inundation at seasonal timescale.

Evidence of two regimes of easterly waves over West Africa and the tropical Atlantic

1998 ◽  
Vol 25 (15) ◽  
pp. 2805-2808 ◽  
Author(s):  
Arona Diedhiou ◽  
Serge Janicot ◽  
Alain Viltard ◽  
Pierre de Felice
Keyword(s):  
West Africa ◽  
Tropical Atlantic ◽  
Easterly Waves
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