Intraseasonal variability of global land monsoon precipitation and its recent trend

Abstract

Accurate prediction of global land monsoon rainfall on a sub-seasonal (2-8 weeks) time scale has become a worldwide demand. Current forecasts of weekly-mean rainfall in most monsoon regions, however, have limited skills beyond two weeks, calling for a more profound understanding of monsoon intraseasonal variability (ISV). 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, generally dominating the sub-seasonal predictability of various land monsoons, while the low-frequency (LF; 20-70 days)’s contribution is comparable to HF only over Australia (AU; 47%), South Asia (SA; 43%), and South America (SAM; 40%). The leading modes of HFISVs in Northern Hemisphere (NH) monsoons primarily originate from different convectively coupled equatorial waves, while from mid-latitude wave trains for Southern Hemisphere (SH) monsoons and East Asian (EA) monsoon. The Madden-Julian Oscillation (MJO) directly regulates LFISVs in Asian-Australian monsoon and affects 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 Asian (Asian-Australian) monsoon but weakened over American (SAM) monsoon. Sub-seasonal to seasonal (S2S) prediction models exhibit higher sub-seasonal prediction skills over AU, SA, and SAM monsoons that have larger LFISV contributions than other monsoons. These 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, especially under the global warming scenarios.

Meridional circulation dominates the record-breaking “Dragon Boat Water” rainfall over south China in 2022

“…The second flood season is generally called the "second rainy season" or "late rainy season" of China, which occurs from July to September. In the first rainy season, the rainfall in SC is mainly influenced by monsoon circulations, including the subtropical jet, western Pacific subtropical high, South Asia high, and lowlevel southwest jet in the subtropics and the East Asian trough and northeastern cold vortex in the mid-high latitudes (Yuan et al, 2012;Li et al, 2018;Miao et al, 2019;Liu et al, 2022). In the second rainy season, the rainfall in SC is mainly influenced by typhoons or tropical depressions (Lee et al, 2010;Yuan et al, 2019).…”