During normal ENSO conditions, the trade winds blow from east to west along the equator in the Pacific Ocean, causing warm water to accumulate in the western Pacific. This warm water is then cooled as it rises to the surface, resulting in a stable and relatively cool ocean temperature in the eastern Pacific.

If you cannot locate the original document, do not despair. Several open-access resources replicate its teachings:

The search for "El Niño Normal Illingworth" suggests a mix-up between two unrelated subjects: (a climate phenomenon) and

El Niño-Southern Oscillation (ENSO) is a complex climate pattern that affects the Pacific Ocean and involves fluctuations in the atmosphere and ocean temperatures. ENSO has three phases: El Niño, La Niña, and Neutral (or normal).

Some of the best ENSO literature is published by the American Meteorological Society (AMS) or Springer. The PDF may be a chapter from El Niño: Historical and Paleoclimatic Aspects or a similar book. If Illingworth contributed a chapter titled "The Normal Baseline Problem," the PDF might be behind a $40 paywall.

Once the "Normal" conditions have stacked enough warm water in the Western Pacific, the system becomes top-heavy. Illingworth described how a small trigger—perhaps a westerly wind burst—can send a massive, slow-moving wave (a Kelvin wave) traveling eastward across the Pacific.