Intraseasonal variability of global land monsoon precipitation and recent change

Accurate prediction of global land monsoon rainfall on a subseasonal (2–8 weeks) time scale has become a worldwide demand. However, the current forecasts of weekly-mean rainfall in most monsoon regions have limited skills beyond two weeks. Given that two-thirds of the world’s population lives in the monsoon regions, this challenge calls for a more profound understanding of monsoon intraseasonal variability (ISVs). We show that the high-frequency (HF; 8–20 days) ISV, crucial for the Week 2 and Week 3 predictions, accounts for about 53–70% of the total (8–70 days) ISV in various land monsoon regions. The low-frequency (LF; 20–70 days) ISV has a relatively high contribution over Australia (47%), South Asia (43%), and South America (40%). The leading modes of HFISVs in Northern Hemisphere monsoons primarily originate from convectively coupled equatorial Rossby waves (Asia), mixed Rossby-gravity waves (North America), and Kelvin waves (West Africa), while in Southern Hemisphere monsoons, from mid-latitude wave trains. The Madden-Julian Oscillation (MJO) directly regulates LFISVs in the Asian-Australian monsoon while affecting the American and African monsoons by exciting Kelvin waves and mid-latitude teleconnections. During the past four decades, the HF (LF) ISVs have considerably intensified over the Asian (Asian-Australian) monsoon but weakened over the American (South American) monsoon. The presented characteristics of the ISV in each monsoon region are helpful for empirical subseasonal prediction and for identifying models’ strengths and weaknesses in reproducing regional monsoon ISVs. The results suggest an urgent need to improve the simulation of convectively coupled equatorial waves and two-way interactions between regional monsoon ISVs and mid-latitude processes and between MJO and regional monsoons.