Trends in global monsoon area and precipitation over the past 30 years

The analysis of the GPCP and CMAP datasets during the past 30 years (1979-2008) indicates that there are consistent increasing trends in both the global monsoon area (GMA) and the global monsoon total precipitation (GMP). This positive monsoon rainfall trend differs from previous studies that assumed a fixed global monsoon domain. Due to the increasing trends in both the GMA and GMP, a global monsoon intensity (GMI) index, which measures the global monsoon precipitation amount per unit area, is introduced. The GMI measures the strength of the global monsoon. Our calculations with both the GPCP and CMAP datasets show a consistent downward trend in the GMI over the past 30 years. This decreasing trend is primarily attributed to a greater percentage increase in the GMA than in the GMP. A further diagnosis reveals that the decrease of the GMI is primarily attributed to the land monsoon in the GPCP, but to the oceanic monsoon in the CMAP.