Water management in a semi-arid region: an analogue algorithm approach for rainfall seasonal forecasting

In a semi-arid region, like those of Sub-Saharian Africa, water and crop management is a basilar tool to avoid serious health problem (famine, epidemic,…). But to obtain the best results from these type of policy information on precipitation and temperature anomalies must be available a few months before. Thus, seasonal Forecasting tends to assume an increasing role, despite of the large uncertainties still present. At the same time, methods and results of this recent branch of atmospheric sciences must the most simple and accessible as possible. For this reason, the Institute of Biometeorology has been developed a simple, physically-based, statistical approach to obtain seasonal forecasts, regarding rainfall precipitation, over Sahel region. The method is based on the “similarity” conditions of the sea surface temperature (SST) in three areas of the world defined as: Niño-3 (5S-5N;150W-90W), Guinea Gulf (10S-5N;20W-10E), Indian Ocean (5S-15N;60E-90E) which, in literature, are indicated as the most important areas to drive the precipitation patterns (Indian Ocean and Southern Atlantic with regards of the trend of precipitation and Niño-3 with regards of the interannual time scale). Each month of the year is defined by six variables: three are the sea surface temperatures standardized anomaly (SSTAs) while the other three take into account their respective tendencies (namely “Change Rate” CRs) and are defined as the difference between the current SSTAs and those of the previous month. The standardization is obtained with the subtraction of the 1979-2003 climatological mean and the division by the 1979-2003 climatological standard deviation. The “similarity” to the current SST conditions is evaluated by means of the minimization of the Euclidean distance to find the most similar year (namely analogue) and assign the values observed in that year to the forecast rainfall field. Precipitation anomaly and percentage anomaly are then computed, compared with the 1979-2003 climatological mean. Due to the specific dynamical behaviour of the West African Monsoon this simple analogue characterization is able to catch main features of rainfall precipitation patterns during the JJA period and a validation of this approach, through analysis of forecast skills, shows encouraging results.