In July-August 2022, Yangtze River Valley (YRV) experienced unprecedented hot summer, with the number of heatwave days exceeding climatology by four standard deviations. The heatwaves and associated severe droughts affected about 38 million people and caused devastating economic losses of about five billion US dollars. Here we present convergent empirical and modelling evidence to show that the record-breaking Pakistan rainfall, along with the 2022 tripe-dip La Niña, produces anomalous high pressure over YRV, causing intense heatwaves. The La Niña-induced second-highest sea surface temperature gradient in the equatorial western Pacific suppresses western Pacific convection and extends the subtropical high westward. More importantly, the tremendous diabatic heating associated with the unprecedented Pakistan rainfall reinforces the downstream Rossby wave train, extending the upper-level South Asia High eastward and controlling the entire YRV. The overlay of the two high-pressure systems sustains sinking motion and increases solar radiation reaching the ground, causing recurrent heat waves.

Climate variation of global monsoon (GM) precipitation involves both internal feedback and external forcing. Here, we focus on strong volcanic forcing since large eruptions are known to be a dominant mechanism in natural climate change. It is not known whether large volcanoes erupted at different latitudes have distinctive effects on the monsoon in the Northern Hemisphere (NH) and the Southern Hemisphere (SH). We address this issue using a 1500-year volcanic sensitivity simulation by the Community Earth System Model version 1.0 (CESM1). Volcanoes are classified into three types based on their meridional aerosol distributions: NH volcanoes, SH volcanoes and equatorial volcanoes. Using the model simulation, we discover that the GM precipitation in one hemisphere is enhanced significantly by the remote volcanic forcing occurring in the other hemisphere. This remote volcanic forcing-induced intensification is mainly through circulation change rather than moisture content change. In addition, the NH volcanic eruptions are more efficient in reducing the NH monsoon precipitation than the equatorial ones, and so do the SH eruptions in weakening the SH monsoon, because the equatorial eruptions, despite reducing moisture content, have weaker effects in weakening the off-equatorial monsoon circulation than the subtropical-extratropical volcanoes do.

Five out of six La Niña events since 1998 have lasted two to three years. Why so many long-lasting multiyear La Niña events have emerged recently and whether they will become more common remains unknown. Here we show that ten multiyear La Niña events over the past century had an accelerated trend, with eight of these occurring after 1970. The two types of multiyear La Niña events over this time period followed either a super El Niño or a central Pacific El Niño. We find that multiyear La Niña events differ from single-year La Niñas by a prominent onset rate, which is rooted in the western Pacific warming-enhanced zonal advective feedback for the central Pacific multiyear La Niña events type and thermocline feedback for the super El Niño multiyear La Niña events type. The results from large ensemble climate simulations support the observed multiyear La Niña events–western Pacific warming link. More multiyear La Niña events will exacerbate adverse socioeconomic impacts if the western Pacific continues to warm relative to the central Pacific.

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.

The authors investigate the effects of El Niño and La Niña on the intraseasonal oscillation (ISO) in the boreal summer (May–October) over the western North Pacific (WNP). It is found that during El Niño summers, the ISO is dominated by a higher-frequency oscillation with a period of around 20–40 days, whereas during La Niña summers the ISO is dominated by a lower-frequency period of around 40–70 days. The former is characterized by northwestward-propagating convection anomalies in the WNP, and the latter is characterized by northward- and eastward-propagating convective signals over the tropical Indian Ocean/Maritime Continent. The possible mechanisms through which El Niño–Southern Oscillation (ENSO)-induced background mean state changes influence the ISO behavior are examined through idealized numerical experiments. It is found that enhanced (weakened) mean moisture and easterly (westerly) vertical wind shear in the WNP during El Niño (La Niña) are the main causes of the strengthened (weakened) 20–40-day northwestward-propagating ISO mode, whereas the 40–70-day ISO initiated from the Indian Ocean can only affect the WNP during La Niña years because the dry (moist) background moisture near the Maritime Continent during El Niño (La Niña) suppresses (enhances) the ISO over the Maritime Continent, and the ISO propagates less over the Maritime Continent during El Niño years than in La Niña years.