Prediction of the Start of the Rainy Season in West Africa

Abstract: Since most of the inhabitants in West Africa is working with, or are dependent on agricultural work, accurate weather forecasts are important in their daily work. Knowledge of when to start to sow is one of the most important features from a farmer’s point of view. It can be devastating for the farmers if the soil is not moist enough when planting since the crops risks to dry out, but also planting too late needs to be avoided since it will affect the growing time and therefore might reduce the production. In this thesis, investigations whether the start of the rainy season in Ghana and parts of Burkina Faso is predictable, only with the use of patterns in rainfall and changes in sea surface temperature in the Gulf of Guinea. The region of interest has been divided into four equally sized areas with a latitudinal width of 2  from south to north. The models are first of all predicting the start of the rainy season in the southernmost area by use of four different methods, three that are based on precipitation patterns and one based on changes in sea surface temperature. Thereafter, the three northerly areas are predicted with a linear function based on when the rainy season started in the southernmost area. The results shows that the model is acceptable in its predictability but is very good in indicating if the rainy season will start earlier or later than the year before. This is of major benefits for the farmers in the region. On a long‐range average, the rainy season starts in the southernmost area first and then it starts further north, but this is not always the case in individual years, which makes the models complicated to use in some years. In order to give reliable forecasts to the farmers, the rainy season needs to be defined so it fulfils the conditions that are needed for plants to grow. Therefore, the start of the rainy season is defined as when 40 mm of precipitation is received during a five‐day period with at least 16 mm in one of these five days. Thereafter, the next 30 days cannot contain more than 18 days without precipitation.