Simulating the influence of Madden Julian oscillation on the MAMJ intra-seasonal variations over West Africa

2021 ◽  
Vol 134 (1) ◽  
Author(s):  
M. O. Adeniyi ◽  
Z. Lin
Keyword(s):  
West Africa ◽  
Seasonal Variations ◽  
Madden Julian Oscillation

scholarly journals Response of the West African Monsoon to the Madden–Julian Oscillation

2009 ◽  
Vol 22 (15) ◽  
pp. 4097-4116 ◽  
Author(s):  
Sally L. Lavender ◽  
Adrian J. Matthews
Keyword(s):  
West Africa ◽  
Monsoon Season ◽  
West African Monsoon ◽  
Circulation Model ◽  
Negative Temperature ◽  
Equatorial Waves ◽  
Madden Julian Oscillation ◽  
Global Circulation Model ◽  
Kelvin Wave ◽  
Sst Anomalies

Abstract Observations show that rainfall over West Africa is influenced by the Madden–Julian oscillation (MJO). A number of mechanisms have been suggested: 1) forcing by equatorial waves; 2) enhanced monsoon moisture supply; and 3) increased African easterly wave (AEW) activity. However, previous observational studies are not able to unambiguously distinguish between cause and effect. Carefully designed model experiments are used to assess these mechanisms. Intraseasonal convective anomalies over West Africa during the summer monsoon season are simulated in an atmosphere-only global circulation model as a response to imposed sea surface temperature (SST) anomalies associated with the MJO over the equatorial warm pool region. 1) Negative SST anomalies stabilize the atmosphere leading to locally reduced convection. The reduced convection leads to negative midtropospheric latent heating anomalies that force dry equatorial waves. These waves propagate eastward (Kelvin wave) and westward (Rossby wave), reaching Africa approximately 10 days later. The associated negative temperature anomalies act to destabilize the atmosphere, resulting in enhanced monsoon convection over West and central Africa. The Rossby waves are found to be the most important component, with associated westward-propagating convective anomalies over West Africa. The eastward-propagating equatorial Kelvin wave also efficiently triggers convection over the eastern Pacific and Central America, consistent with observations. 2) An increase in boundary layer moisture is found to occur as a result of the forced convective anomalies over West Africa rather than a cause. 3) Increased shear on the African easterly jet, leading to increased AEW activity, is also found to occur as a result of the forced convective anomalies in the model.

Seasonal variations and distribution of diatom flora of Lake Ahémé (Benin, West Africa)

2019 ◽  
Vol 167 (1) ◽  
pp. 160-173 ◽  
Author(s):  
Isabella Yasmine Olodo ◽  
Christine Cocquyt ◽  
Youssouf Abou ◽  
Kouami Kokou
Keyword(s):  
West Africa ◽  
Seasonal Variations ◽  
Diatom Flora

scholarly journals Statistical Connection between the Madden–Julian Oscillation and Large Daily Precipitation Events in West Africa

2017 ◽  
Vol 30 (6) ◽  
pp. 1999-2010 ◽  
Author(s):  
Awolou Sossa ◽  
Brant Liebmann ◽  
Ileana Bladé ◽  
Dave Allured ◽  
Harry H. Hendon ◽  
...  
Keyword(s):  
West Africa ◽  
Daily Precipitation ◽  
Statistical Tests ◽  
West African ◽  
Seasonal Precipitation ◽  
Madden Julian Oscillation ◽  
Wet Season ◽  
Precipitation Regime ◽  
The Impact ◽  
Precipitation Events

Abstract This study focuses on the impact of the Madden–Julian oscillation (MJO)—as monitored by a well-known multivariate index—on large daily precipitation events in West Africa for the period 1981–2014. Two seasons are considered: the near-equatorial wet season in March–May (MAM) and the peak of the West African monsoon during July–September (JAS), when the intertropical convergence zone (ITCZ) is at its most northerly position. Although the MJO-related interannual variation of seasonal mean rainfall is large, the focus here is on the impacts of the MJO on daily time scales because variations in the frequency of intense, short-term, flood-causing, rainfall events are more important for West African agriculture than variations in seasonal precipitation, particularly near the Guinean coast, where precipitation is abundant. Using composites based on thresholds of daily precipitation amounts, changes in mean precipitation and frequency of the heaviest daily events associated with the phase of the MJO are investigated. The expected modulation of mean rainfall by the MJO is much stronger during MAM than during JAS; yet the modulation of the largest events (i.e., daily rainfall rates above the 90th percentile) is comparable in both seasons. Conservative statistical tests of local and field significance indicate unambiguous impacts of the MJO of the expected sign during certain phases, but the nature of the impact depends on the local seasonal precipitation regime. For instance, in JAS, the early stages of the MJO increase the risk of flooding in the Sahel monsoon region while providing relief to the dry southern coast.

A numerical study of the seasonal variations for tracer redistribution by clouds over West Africa

1995 ◽  
Vol 20 (3) ◽  
pp. 237-258 ◽  
Author(s):  
M. Renard ◽  
N. Chaumerliac ◽  
S. Cautenet ◽  
N. Audiffren ◽  
E. C. Nickerson
Keyword(s):  
West Africa ◽  
Seasonal Variations ◽  
Numerical Study

Observations on Nematode Infections of Goats and Sheep in West Africa

1958 ◽  
Vol 32 (4) ◽  
pp. 195-210 ◽  
Author(s):  
E. E. Edwards ◽  
A. S. B. Wilson
Keyword(s):  
West Africa ◽  
Seasonal Variations ◽  
Worm Burden ◽  
Egg Production ◽  
High Peak ◽  
Forest Belt ◽  
Nematode Eggs ◽  
Moniezia Expansa ◽  
High Level

An account is given of the results of preliminary studies of the worm-burden of goats and sheep in Ghana, West Africa, based on fortnightly counts of nematode eggs in their faeces over a period of three years, 1954 to 1956, inclusive. Only eggs of nematodes and Moniezia expansa were encountered apart from an occasional egg of Schistosoma sp., probably S. bovis, in two sheep. Four centres were involved in these investigations differing appreciably in regard to the amount and distribution of the rainfall. It would seem from the results obtained that:—There are in the drier regions two distinct periods in the year when worm-egg production in goats and sheep reaches a high level compared with that at other times of the year, a high peak of production in June, sometimes a month either earlier or later, and a slight peak in November or December.There are seasonal variations in the forest belt regions but here the peaks of egg-production are less evident and more irregular in respect to the time of their occurrences in different years.

Sign in / Sign up

Export Citation Format

Share Document