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Title: Modelling the intraseasonal variability of the West African monsoon : influence of the Madden-Julian oscillation and regional soil moisture anomalies
Author: Lavender, Sally L.
ISNI:       0000 0003 5219 6095
Awarding Body: University of East Anglia
Current Institution: University of East Anglia
Date of Award: 2008
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The mechanisms controlling the intraseasonal variability of the West African monsoon (WAM) are investigated. Knowledge of these mechanisms will help to improve forecasting capabilities over the West African region. There are two dominant modes of variability, one within the 25-60-day band and one with an average period of 15 days. Observations show that the 25-60-day variability in rainfall over West Africa is influenced by the Madden-Julian oscillation (MJO). A number of mechanisms have been suggested. However, previous observational studies are not able to easily distinguish between cause and effect. Intraseasonal convective anomalies over West Africa are simulated in an atmospheric general circulation model (AGCM) as a response to imposed sea surface temperature (SST) anomalies associated with the MJO over the equatorial warm pool region. Negative midtropospheric temperature anomalies, associated with negative SSTs and reduced convection, over the warm pool propagate eastwards as a Kelvin wave and westwards as a Rossby wave, reaching Africa approximately 10 days later. The negative midtropospheric temperature anomalies act to destabilise the troposphere resulting in .enhanced convection over West Africa. Variability in soil moisture has the potential to feedback on the atmosphere and hence rainfall, on a regional scale. The I5-day westward-propagating mode found previously in rainfall is detected in soil moisture observations. A set of AGCM experiments is performed to analyse the influence of soil moisture on the WAM. The I5-day westwardpropagating signal in precipitation is found to exist in the model independent of soil moisture. However, soil moisture is found to feedback into the atmosphere via changes in evaporation which ultimately result in anomalies in the low level circulation. The anomalies in low-level circulation can then change the supply of moist, unstable air to the region, influencing the convective anomalies.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available