Different criteria have been used to define the monsoon onset and retreat over different monsoon regions and even over different parts of the same monsoon region, which make it very difficult to intercompare and understand the short-term and intraseasonal variability of the global monsoon system.

Here we propose an objective criterion to define, for the first time, globally unified summer monsoon onset (or retreat) dates using the NVAP global daily 1deg by 1 deg precipitable water (i.e, the total column water vapor) data with the threshold value being the Golden Ratio (0.618).

Over different monsoon regions, our onset and retreat dates are consistent with those determined using local criteria. The use of a single criterion globally also enables us to suggest the existence of two branches of the North American monsoon system, just like its counterpart in Asia. Our monsoon onset and retreat (and hence monsoon duration) dates also help quantify the geographical extent of global monsoon systems.

References:

Zeng, X., and E. Lu, 2002: Globally Unified Monsoon Onset and Retreat Indexes, J. Climate, submitted.