Seasonal Cycles of Precipitation and Precipitable Water and Their Use in Monsoon Onset and Retreat

Abstract

Precipitation (P) and precipitable water (W) are important components of the hydrological cycles in the earth system, and their seasonal cycles are closely related to monsoon circulations over monsoon regions. Through theoretical analyses and extensive analysis of data from in-situ measurements, satellite remote sensing, and regional reanalysis, significant progress has been made (via four peer-reviewed publications) in four areas related to P, W, and monsoon onset and retreat. First, based on the normalized W index, a novel unified method is proposed to determine global monsoon onset and retreat dates. The results are consistent with those obtained from different local criteria. Second, theoretical and data analyses demonstrate that, because of the large annual range of temperature, W can increase from winter to summer anywhere except in the tropics, including both monsoon and nonmonsoon regions. Third, while the seasonal variation of P is, in general, caused by complex processes (e.g., atmospheric circulations), thermodynamic derivations and data analysis demonstrate that the variation of P from winter to summer can be easily understood from the comparative strength between the change of water vapor and the change of temperature. In monsoon regions, the change of water vapor from winter to summer is much greater than the change of temperature, so P has an in-phase relation with W. While in some of the nonmonsoon regions, where winter is the rainy season, the change of temperature is much greater than the change of water vapor, leading to an out-of-phase relation between P and W, and, relative to summer, the coldness of the winter air is much more significant than its dryness. Finally, the satisfactory performance of the globally unified monsoon index can be understood by comparing the seasonal cycles of P and W. The significant positive correlations between P and W at seasonal and synoptic scales imply that W has the ability to indicate both the means and the interannual variations of the monsoon onset and retreat. Since large increase of W from winter to summer can occur in both monsoon and nonmonsoon regions, the global monsoon regions cannot be obtained from the seasonal change of W.