It is widely accepted that the monsoon trough of the northwest Pacific Ocean plays a critical role in the meteorology of the region. In particular, a number of authors have stated that 75-80% of western Pacific tropical cyclones form “within the monsoon trough”. None of these authors, however, has provided evidence to support such an assertion.

We begin with a deceptively simple question: is this percentage correct? The answer depends upon the definition of the monsoon trough. It will be shown that current definitions create some dilemmas. For instance, the Glossary of Meteorology defines a monsoon trough as “a line on a weather map showing the locations of relative minimum pressure in a monsoon region”. Not many tropical cyclones can form within a line! We will calculate the percentage of tropical cyclones that form in the monsoon trough using vorticity, pressure, and rainfall-based definitions, for various time means.

The definition of the monsoon trough is part of a larger discussion regarding how we label the components of the tropical circulation. Commonly used terminology confuses the long-term mean flow with day-to-day or week-to-week variations. This issue has been with us since the debates between the climatological and perturbation schools of tropical meteorology described by Palmer (1952). We will address the divide between current operational practice and the equatorial wave/MJO/ENSO paradigms that dominate the research literature.