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to predict quantities of interest when other methods are not available. Statistical methods depend on correlations between predictors (the quantities used to make the prediction) and the quantities of interest (predictands). For example, the rainfall in the Nordeste region of Brazil (the predictand) correlates with sea surface temperature in both the tropical Pacific and subtropical Atlantic (the predictors), and statistical forecast schemes using both of these these predictors have proven useful in predicting rainfall in the Brazilian northeast (e.g., Hastenrath, 1990; Uvo et al., 1998). When the predictors are correctly chosen (including, perhaps, internal ocean data) and the relationship between the predictors and predictands is simple and direct, there is no reason that statistical methods would not have as high a skill as numerical methods. In general, numerical models contain most of the processes in the atmosphere and the ocean and keep track of them in a consistent way. Thus, they have the potential to provide more accurate and complete information. However, there is no reason that statistical methods that keep track of all the predictors should not have a comparable skill to numerical methods. Which method is preferred when both are available is judged by the skill of prediction.

Which Quantities Are Forecast?

Scientists forecast sea surface temperature (SST) by numerical methods, but, in general, it is temperature and precipitation over land that people most want to predict. At the moment, only SST in the tropical Pacific Ocean characteristic of ENSO is forecast; however, because ENSO has such a global influence, forecasting tropical Pacific SST has predictive value for temperature and precipitation in many specific regions around the world (Figure 2-1). We emphasize that forecasts of ENSO predict a physical quantity, the SST. When the SST in the tropical Pacific is predicted to be anomalously high, it may be said that forecasters have predicted El Niño, but since this term has no agreed-on definition in terms of value of SST, this is an interpretation. The key is that the value of SST is predicted and the value of the forecast resides in the consequences of the predicted value of SST. The statement that El Niño has been forecast is a journalistic rather than a scientific statement.

In the tropics, atmospheric circulations are driven directly by the latent heat released in regions of persistent precipitation. Thus in the far western Pacific, the normal persistent rainfall is accompanied by rising motion and lowered surface pressure. In the eastern Pacific, the circuit is completed with downward motion, lack of precipitation, and higher surface pressure. These regions of persistent precipitation can emit planetary waves, which propagate to higher latitudes and affect local circulations