14.5 Forecasting El Niño
The importance of El Niño to global weather patterns has led to many schemes for forecasting events in the equatorial Pacific. Several generations of models have been produced, but the skill of the forecasts has not always increased. Models worked well for a few years, then failed. Failure was followed by improved models, and the cycle continued. Thus, the best models in 1991 failed to predict weak El Niños in 1993 and 1994 (Ji, Leetma, and Kousky, 1996). The best model of the mid 1990s failed to predict the onset of the strong El Niño of 1997-1998 although a new model developed by the National Centers for Environmental Prediction made the best forecast of the development of the event. In general, the more sophisticated the model, the better the forecasts (Kerr, 1998).
The following recounts some of the more recent work to improve the forecasts. For simplicity, I describe the technique used by the National Centers for Environmental Prediction (Ji, Behringer, and Leetma, 1998). But Chen et al., (1995), Latif et al., (1993), and Barnett et al., (1993), among others, have all developed useful prediction models.
The first results indicate that none of the models were able to duplicate all important interseasonal variability of the tropical atmosphere on timescales of 2 to 80 days. Models with weak intraseasonal activity tended to have a weak annual cycle. Most models seemed to simulate some important aspects of the interannual variability including El Niño. The length of the time series was, however, too short to provide conclusive results on interannual variability.
The results of the substudy imply that numerical models of the atmospheric general circulation need to be improved if they are to be used to study tropical variability and the response of the atmosphere to changes in the tropical ocean. Some of the improvement is coming from new knowledge gained from COARE.
Ji, Behringer, and Leetma (1998) at the National Centers for Environmental Prediction have modified the Geophysical Fluid Dynamics Laboratory’s Modular Ocean Model for use in the tropical Pacific (see §15.4 for more information about this model). It’s domain is the Pacific between 45°S and 55°N and between 120°E and 707°W. The zonal resolution is 1.5.° The meridional resolution is 1/3° within 10° of the equator, increasing smoothly to 1° poleward of 20° latitude. It has 28 vertical levels, with 18 in the upper 400m to resolve the mixed layer and thermocline. The model is driven by mean winds from Hellerman and Rosenstein (1983), anomalies in the wind field from Florida State University, and mean heat fluxes from Oberhuber (1988). It assimilates subsurface temperature from the TAO array and XBTS, and surface temperatures from the monthly optimal-interpolation data set (Reynolds and Smith, 1994).
The output of the model is an ocean analysis, the density and current field that best fits the data used in the analysis (Figures 14.3 and 14.4). This is used to drive a coupled ocean-atmosphere model to produce forecasts.
As computer power decreases in cost, the models are becoming ever more complex. The trend is to global coupled models able to include other coupled ocean-atmosphere systems in addition to ENSO. We return to the problem in §15.6 where I describe global coupled models.
The forecasts of the very strong 1997 El Niño have been carefully studied. Jan van Oldenborg et al (2005) and Barnston et al (1999) found no models successfully forecast the earliest onset of the El Niño in late 1996 and early 1997. The first formal announcements of the El Niño were made in May 1997. Nor did any model forecast the large temperature anomalies observed in the eastern equatorial Pacific until the area had already warmed. There was no clear distinction between the accuracy of the dynamical or statistical forecasts.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, email@example.com
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Updated on November 7, 2007