How often does severe clear air turbulence occur over tropical oceans?

The motivation for writing this paper came from a report in Weather of prolonged and severe clear air turbulence (CAT) over the tropical Pacific by Blanchard (1999), followed shortly by a reply from Bysouth (2000) showing that this incident was forecast using the Met Office operational numerical forecast model- or Unified Model. This seemed anomalous, since CAT is normally associated with temperate (polar front) and subtropical jet streams usually found poleward of 30 degree latitude.