Use the Wayback Machine (archive.org) on defunct university course websites. Many professors hosted PDFs on personal pages that no longer exist but are archived. Forums like Weatherzone or American Weather occasionally have links in old threads discussing "Illingworth’s normalization."
While Julian Illingworth is a prominent figure in physical oceanography and fluid dynamics, he is arguably most famous for a specific conceptual breakthrough regarding how El Niño events terminate. This is often discussed in physical oceanography textbooks and papers (such as J. Phys. Oceanogr. or Geophys. Astrophys. Fluid Dyn.), though a single PDF by that exact title might be elusive. el nino normal illingworth pdf
Here is the "interesting piece" regarding his contribution to the topic: Use the Wayback Machine (archive