The relationship between African easterly waves and daily rainfall over West Africa: observations and regional climate simulations

2014 ◽  
Vol 44 (1-2) ◽  
pp. 385-404 ◽  
Author(s):  
Julien Crétat ◽  
Edward K. Vizy ◽  
Kerry H. Cook
Keyword(s):  
West Africa ◽  
Regional Climate ◽  
Daily Rainfall ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
Climate Simulations ◽  
The Relationship

scholarly journals Late 21st Century Projected Changes in the Relationship between Precipitation, African Easterly Jet, and African Easterly Waves

Atmosphere ◽  
2020 ◽  
Vol 11 (4) ◽  
pp. 353
Author(s):  
Ibourahima Kebe ◽  
Ismaila Diallo ◽  
Mouhamadou Bamba Sylla ◽  
Fernando De Sales ◽  
Arona Diedhiou
Keyword(s):  
High Resolution ◽  
West Africa ◽  
21St Century ◽  
Regional Climate ◽  
Historical Period ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
African Easterly Jet ◽  
Late 21St Century ◽  
Sahel Region

The present study utilizes three high-resolution simulations from the Regional Climate Model version 4 (RegCM4) to examine the late 21st century changes (2080–2099) in the West African Monsoon (WAM) features. A set of three Earth System Models are utilized to provide initial and lateral boundary conditions to the RegCM4 experiments. Our analysis focuses on seasonal mean changes in WAM large-scale dynamical features, along with their connections with the summer monsoon precipitation. In the historical period, the simulation ensemble means mimic reasonably well the intensity and spatial distribution of the WAM rainfall as well as the WAM circulation patterns at different scales. The future projection of the WAM climate exhibits warming over the whole West Africa leading to precipitation reduction over the Sahel region, and a slight increase over some areas of the Guinea Coast. The position of the African Easterly Jet (AEJ) is shifted southward and the African Easterly Waves (AEWs) activities are reduced, which affect in turn the WAM rainbelt characteristics in terms of position and strength. Overall the changes in simulated AEJ and AEWs contribute substantially to reduce the seasonal summer mean precipitation in West Africa by the late 21st century, with prevailing negative changes in the Savanna-Sahel region. To further explore the robustness of the relationships revealed in this paper, future studies using different high-resolution regional climate models with large ensemble are recommended.

scholarly journals Trains of African Easterly Waves and Their Relationship to Tropical Cyclone Genesis in the Eastern Atlantic

2017 ◽  
Vol 145 (2) ◽  
pp. 599-616 ◽  
Author(s):  
Abdou L. Dieng ◽  
Saidou M. Sall ◽  
Laurence Eymard ◽  
Marion Leduc-Leballeur ◽  
Alban Lazar
Keyword(s):  
West African ◽  
Dynamical Structure ◽  
African Easterly Waves ◽  
The South ◽  
The West ◽  
Easterly Waves ◽  
West African Coast ◽  
African Coast ◽  
Tropical Depressions ◽  
The Relationship

In this study, the relationship between trains of African easterly waves (AEWs) and downstream tropical cyclogenesis is studied. Based on 19 summer seasons (July–September from 1990 to 2008) of ERA-Interim reanalysis fields and brightness temperature from the Cloud User Archive, the signature of AEW troughs and embedded convection are tracked from the West African coast to the central Atlantic. The tracked systems are separated into four groups: (i) systems originating from the north zone of the midtropospheric African easterly jet (AEJ), (ii) those coming from the south part of AEJ, (iii) systems that are associated with a downstream trough located around 2000 km westward (termed DUO systems), and (iv) those that are not associated with such a close downstream trough (termed SOLO systems). By monitoring the embedded 700-hPa-filtered relative vorticity and 850-hPa wind convergence anomaly associated with these families along their trajectories, it is shown that the DUO generally have stronger dynamical structure and statistically have a longer lifetime than the SOLO ones. It is suggested that the differences between them may be due to the presence of the previous intense downstream trough in DUO cases, enhancing the low-level convergence behind them. Moreover, a study of the relationship between system trajectories and tropical depressions occurring between the West African coast and 40°W showed that 90% of tropical depressions are identifiable from the West African coast in tracked systems, mostly in the DUO cases originating from the south zone of the AEJ.

scholarly journals Three-Dimensional Structure and Dynamics of African Easterly Waves. Part I: Observations

2006 ◽  
Vol 63 (9) ◽  
pp. 2212-2230 ◽  
Author(s):  
George N. Kiladis ◽  
Chris D. Thorncroft ◽  
Nicholas M. J. Hall
Keyword(s):  
West Africa ◽  
Three Dimensional ◽  
Companion Paper ◽  
Radiosonde Data ◽  
Dimensional Structure ◽  
African Easterly Waves ◽  
Three Dimensional Structure ◽  
Easterly Waves ◽  
The Mean ◽  
The Waves

Abstract The mean structure of African easterly waves (AEWs) over West Africa and the adjacent Atlantic is isolated by projecting dynamical fields from reanalysis and radiosonde data onto space–time-filtered satellite-derived outgoing longwave radiation. These results are compared with previous studies and an idealized modeling study in a companion paper, which provides evidence that the waves bear a close structural resemblance to the fastest-growing linear normal mode of the summertime basic-state flow over Africa. There is a significant evolution in the three-dimensional structure of AEWs as they propagate along 10°N across West Africa. At this latitude, convection occurs in northerly flow to the east of the Greenwich meridian, then shifts into the wave trough, and finally into southerly flow as the waves propagate offshore into the Atlantic ITCZ. In contrast, to the north of the African easterly jet along 15°N convection remains in southerly flow throughout the waves’ trajectory. Along 10°N over West Africa, the location of convection is consistent with the adiabatic dynamical forcing implied by the advection of perturbation vorticity by the mean thermal wind in the zonal direction, as in the companion paper. Offshore, and along 15°N, the relationship between the convection and dynamics is more complex, and not as easily explained in terms of dynamical forcing alone.

scholarly journals Atlantic Hurricane response to Sahara greening and reduced dust emissions during the mid-Holocene

2020 ◽  
Author(s):  
Samuel Dandoy ◽  
Francesco S.R. Pausata ◽  
Suzana J. Camargo ◽  
René Laprise ◽  
Katja Winger ◽  
...  
Keyword(s):  
Climate Model ◽  
Regional Climate ◽  
Boreal Summer ◽  
Orbital Forcing ◽  
African Easterly Waves ◽  
Warm Climate ◽  
Solar Insolation ◽  
Airborne Dust ◽  
Easterly Waves ◽  
Hurricane Season

Abstract. We use a high-resolution regional climate model to investigate the changes in Atlantic tropical cyclone (TC) activity during a warm climate state, the mid-Holocene (MH: 6,000 yrs BP). This period was characterized by increased boreal summer insolation, a vegetated Sahara, and reduced airborne dust concentrations. A set of sensitivity experiments were conducted in which solar insolation, vegetation and dust concentrations were changed in turn to disentangle their impacts on TC activity. Results show that the greening of the Sahara and reduced dust loadings (MHGS+RD) lead to a larger increase in the number of Atlantic TCs (27 %) relative to the pre-industrial climate (PI) than the orbital forcing alone (MHPMIP; 9 %). The TC seasonality is also highly modified in the MH climate, showing a decrease in TC activity during the beginning of the hurricane season (June to August), with a shift of its maximum towards October and November in the MHGS+RD experiment relative to PI. MH experiments simulate stronger hurricanes compared to PI, similar to future projections. Moreover, they suggest longer lasting cyclones relative to PI. Our results also show that changes in the African Easterly Waves are not relevant in altering the frequency and intensity of TCs, but they may shift the location of their genesis. This work highlights the importance of considering vegetation and dust changes over the Sahara region when investigating TC activity under a different climate state.

scholarly journals Classification of Intense Rainfall Days in Southern West Africa and Associated Atmospheric Circulation

Atmosphere ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 188
Author(s):  
Pierre Camberlin ◽  
Marc Kpanou ◽  
Pascal Roucou
Keyword(s):  
West Africa ◽  
Atmospheric Circulation ◽  
Daily Rainfall ◽  
Tropical Rainfall Measuring Mission ◽  
Heavy Precipitation ◽  
Reanalysis Data ◽  
Intense Rainfall ◽  
Rainfall Events ◽  
Easterly Waves ◽  
Atmospheric Circulation Anomalies

Daily rainfall in southern West Africa (4–8° N, 7° W–3° E) is analyzed with the aim of documenting the intense rainfall events which occur in coastal Ivory Coast, Ghana, Togo, and Benin. The daily 99th percentile (P99) shows that the coastline experiences higher intensity rainfall than inland areas. Using Tropical Rainfall Measuring Mission (TRMM) rainfall data for 1998–2014, a novel way of classifying the intense events is proposed. We consider their space-time structure over a window of 8° latitude-longitude and five days centered on the event. A total 39,680 events (62 at each location) are classified into three major types, mainly found over the oceanic regions south of 5° N, the Bight of Benin, and the inland regions respectively. These types display quite distinct rainfall patterns, propagation features, and seasonal occurrence. Three inland subtypes are also defined. The atmospheric circulation anomalies associated with each type are examined from ERA-interim reanalysis data. Intense rainfall events over the continent are mainly a result of westward propagating disturbances. Over the Gulf of Guinea, many intense events occur as a combination of atmospheric disturbances propagating westward (mid-tropospheric easterly waves or cyclonic vortices) and eastward (lower tropospheric zonal wind and moisture anomalies hypothesized to reflect Kelvin waves). Along the coast, there is a mixture of different types of rainfall events, often associated with interacting eastward- and westward-moving disturbances, which complicates the monitoring of heavy precipitation.

scholarly journals Observations of Seven African Easterly Waves in the East Atlantic during 2006

2010 ◽  
Vol 67 (1) ◽  
pp. 26-43 ◽  
Author(s):  
Jonathan Zawislak ◽  
Edward J. Zipser
Keyword(s):  
Large Scale ◽  
High Amplitude ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
Wave Trough ◽  
Wind Structure ◽  
Easterly Wave ◽  
Global Data Assimilation System ◽  
Low Amplitude ◽  
The Relationship

Abstract The African Monsoon Multidisciplinary Analyses (AMMA) experiment and its downstream NASA extension, NAMMA, provide an unprecedented detailed look at the vertical structure of consecutive African easterly waves. During August and September 2006, seven easterly waves passed through the NAMMA domain: two waves developed into Tropical Cyclones Debby and Helene, two waves did not develop, and three waves were questionable in their role in the development of Ernesto, Florence, and Gordon. NCEP Global Data Assimilation System (GDAS) analyses are used to describe the track of both the vorticity maxima and midlevel wave trough associated with each of the seven easterly waves. Dropsonde data from NAMMA research flights are used to describe the observed wind structure and as a tool to evaluate the accuracy of the GDAS to resolve the structure of the wave. Finally, satellite data are used to identify the relationship between convection and the organization of the wind structure. Results support a necessary distinction between the large-scale easterly wave trough and smaller-scale vorticity centers within the wave. An important wave-to-wave variability is observed: for NAMMA waves, those waves that have a characteristically high-amplitude wave trough and well-defined low-level circulations (well organized) may contain less rainfall, do not necessarily develop, and are well resolved in the analysis, whereas low-amplitude (weakly organized) NAMMA waves may have stronger vorticity centers and large persistent raining areas and may be more likely to develop, but are not well resolved in the analysis.

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