Annual climate variability in much of the tropics is affected by the El Niņo Southern Oscillation (ENSO) phenomenon. The southern oscillation index (SOI), defined as the standardized difference in atmospheric pressure between Tahiti and Darwin, is a key indicator of ENSO. The SOI correlates significantly with rainfall and cloud cover in areas affected by ENSO. Rainfall forecasting is improved when month-to-month change in the SOI is considered. SOI phases, which are defined as consistently negative, consistently positive, rapidly falling, rapidly rising, or remaining near zero, have been used to generate skillful seasonal climate forecasts. In many tropical Asian countries, irrigated rice can be grown anytime of the year, but rainfed rice is possible only from June to October. In temperate areas like northern Japan, rice is grown in May and harvested in September. This study examined the impact of seasonal climate forecasts based on the ENSO phenomenon on rice production in Asia. Historic time series of rice production data were collected from several rice-growing areas in Asia. For tropical countries, the reported production data of irrigated rice during the wet and dry seasons and those of rainfed rice during the wet season were analyzed separately. Monthly SOI values and the month-to-month SOI phases were available for each month of the corresponding years. Monthly SOI correlated significantly with the amount of rice produced in countries bordering the Pacific Ocean. The best correlations were obtained for the months of March to June. For years when the SOI falls rapidly from April to May, yields of both irrigated and rainfed rice grown during the wet season in the tropics may decrease by 40% compared with those obtained during an 'average' year. In the Philippines, decreasing SOI throughout the year is associated with drought. As early as May, the Philippine government may prepare for a possible rice shortage by improving the irrigation system for the wet season cropping, or consider importing rice. In Japan, decreasing SOI from April to May is associated with warm temperatures in the immediately following crop season, but cooler temperatures for the next year's crop season. When temperatures are low during the reproductive stage of the crop, usually during the months of July and August, the potential production of rice in Japan decreases.
Key words: climate variability, El Niņo, probability distribution, Southern Oscillation Index