ABSTRACT

Numerical weather prediction only became practically possible in the 1960s with the availability of sufficiently powerful computers. Prior to then, scientifically-based weather forecasting was based on statistical relationships informed by understanding of the physical processes of weather. A seasonal climate forecast is an indication of some aspect of the expected weather conditions aggregated over a period of between one and about six months, and typically starting a few weeks to a few months in the future. A typical seasonal climate forecast differs from a typical weather forecast in important ways, although exceptions can be found. Variability in tropical Pacific sea-surface temperatures is dominated by the El Nino – Southern Oscillation (ENSO). The ENSO is the main reason why seasonal forecasts are possible, because it has a stronger influence on temporal variability in climate at seasonal scales than anything other than the cycle of summer and winter.