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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
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