Monsoon is one of the key elements of the global climate system and strongly affects agricultural and other human activities in the monsoon regions (including North America). Here an objective criterion is proposed to define, for the first time, the globally unified summer monsoon onset (or retreat) dates using the global daily 1 deg by 1 deg precipitable water (i.e, the total column water vapor) data with the only threshold value being the Golden Ratio (0.618).

Over different monsoon regions, our onset and retreat dates are found to be consistent with those determined using local criteria. The use of a single criterion globally also enables us to identify the existence of two branches of the North American monsoon system, just as its counterpart in Asia. Because an area with monsoon climate should have identifiable monsoon onset and retreat dates as well as long enough monsoon duration, our data also help quantify the geographical extent of monsoon systems.

Reference:

X. Zeng and E. Lu, 2002: Globally Unified Monsoon Onset and Retreat Indexes, Nature, to be submitted.