Using multiple observational and reanalysis data, this paper investigates the impact of the interdecadal shift in summer Arctic Oscillation (AO) on precipitation in East Asia, by removing ENSO influences. The results indicate that the lower-layer activity center of summer AO in Atlantic shifted eastward after the mid-1980s. This regime shift of summer AO has a significant impact on precipitation in East Asia. Before the mid-1980s, the key regions in which precipitation was affected by AO in East Asia were northern East Asia and Northeastern China and adjacent regions. After the mid-1980s, the key regions in which precipitation was affected by AO in East Asia were central Inner Mongolia and Southern China. The mechanism of precipitation changes can be attributed to changes in atmospheric circulation and water vapor transport related to AO changes. After the mid-1980s, the influence of AO on geopotential height over northern East Asia weakened; meanwhile, the impact of AO on geopotential height over China increased. Consistent with the changes in atmospheric circulation, water vapor transport in East Asia also underwent interdecadal changes before and after the mid-1980s. The differences in atmospheric circulation and water vapor transport in East Asia can be traced back to the North Atlantic. Before the mid-1980s, wave activity flux related to summer AO tended to propagate in high latitudes and subtropics; after the mid-1980s, the wave activity flux changed in its subtropical path and propagated eastward from the North Atlantic through the Middle East to China, significantly affecting the summer precipitation in China.

In 1998 and 2016, boreal summer intraseasonal oscillation (BSISO) could reach the middle-lower reaches of the Yangtze River basin (YRB), leading to extreme precipitation. Based on multiple daily data, this study reveals the differences in BSISO events and mechanisms between 1998 and 2016. In June–July of 1998 (2016), YRB precipitation was impacted by 30–60-day oscillation, i.e., BSISO1 (10–30-day oscillation, i.e., BSISO2), with two strong (three) precipitation events occurring. In 1998, when BSISO1 was in phases 1–4 (phases 5–8), the YRB experienced a wet (dry) episode. In 2016, when BSISO2 was in phases 1–2 and 7–8 (phases 3–6), the YRB experienced a wet (dry) episode. In 1998, in event 1, the active convection of the YRB first originated in the South China Sea–western Pacific (SCS–WP) and then in the tropical Indian Ocean (IO). In 1998, in event 2, the active convection of the YRB originated in the SCS–WP. In 2016, in events 1 and 3, the active convection of the YRB originated from the SCS–WP. In 2016, in event 2, the active convection of the YRB originated from the tropical IO and the extratropical WP. Different SST and atmospheric circulations explain different BSISO modes that dominate in the YRB. In 1998 (2016), in summer, (no) strong easterly wind anomalies occurred in the SCS–WP, which are favorable (unfavorable) for the enhancement of BSISO1. Accompanying the suppressed BSISO1, BSISO2 was enhanced in 2016.

A double intertropical convergence zone (ITCZ) pattern emerges over the eastern Pacific Ocean in March-April. Their inter-annual variabilities may exert major climatic and ecological effects. El Niño-Southern Oscillation (ENSO) can significantly regulate the inter-annual variability of double ITCZs. ENSO diversity has increased in recent years. A mixed-type El Niño occurred in 2009/2010. Our results reveal that the meridional SSTA pattern in mixed-type El Niño is a combination of “SST warming with a maximum at the equator” (i.e., EP-type delta SST) and “north-warm and south-cold” (i.e., CP-type delta SST) patterns. In a mixed-type El Niño, both meridional EP-type and CP-type delta SST played a role and had corresponding climate impacts. Under the coordinated modulation of EP-type and CP-type delta SST in a mixed-type El Niño, the climate effect of EP-type delta SST (increasing equatorial precipitation) is weakened; while the climate effect of CP-type delta SST (enhancing the northern ITCZ and weakening the southern ITCZ) is amplified. The climate effect of mixed-type El Niño finally resembles that of typical CP-type El Niños, intensifying the north-south asymmetry of the eastern Pacific double ITCZs. This study will contribute to a better understanding of the impacts of ENSO diversity on double ITCZs in the eastern Pacific region.

The El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) coupling exhibits important globe climate impacts. The relationship of ENSO with the IOD’s eastern pole (EP) and western pole (WP) and its interdecadal variation have not been determined. Based on observations and reanalysis datasets, this study reveals that the ENSO and IOD-EP relationship significantly weakened, while the ENSO and IOD-WP relationship strengthened after 2000. Changes in ENSO-IOD-EP connection outweigh that in ENSO-IOD-WP connection. The El Niño-positive IOD (pIOD) coupling weakened and the La Niña-negative IOD (nIOD) coupling strengthened. In addition, the changes in El Niño-pIOD coupling dominate. From the atmospheric view, an anomalous ascending branch of Walker circulation in the Maritime Continent intensified after 2000. The new status of Walker circulation favored weakened wind easterlies and SST warming in IOD-EP, which hindered the development of pIOD events but promoted the development of nIOD events. From the oceanic wave process view, the equatorial and extra-equatorial downwelling Rossby waves both favored SST warming in IOD-WP after 2000, which promoted the generation of pIOD events but hindered the generation of nIOD events. The equatorial Rossby waves reflected by Kelvin waves may be even more dominant in regulating the evolution of IOD events. The presence of the Seychelles thermocline dome region amplifies the impact of oceanic wave processes. Using information flow, a causal analysis verifies that the influence of atmospheric processes on IOD-EP decreased and the influence of oceanic wave processes on IOD-WP increased. Such weakened ENSO-IOD-EP connection can be attributed to the weakened wind easterlies in pIOD events associated with the La Niña-like condition in Pacific and the declined Bjerknes feedback induced by the deepened thermocline condition in IOD-EP. While the strengthened ENSO-IOD-WP connection can be attributed to the strengthened downwelling Rossby waves in pIOD events, which were related to IO Basin (IOB) warming induced by El Niño in the previous year. Increased consecutive El Niños associated with La Niña-like condition are favorable for an enhanced ENSO-pIOD-WP connection. Under the background of ENSO properties transition occurring around 2000, our research will contribute to a better understanding on the ENSO-IOD coupling and its implications.

In July 2021, a rare and extremely severe rainstorm and flood disaster occurred in Henan Province, China. Using multiple daily observational and reanalysis data, this study investigates the modulation mechanisms of the boreal summer intraseasonal oscillation (BSISO) on the Henan extreme rainfall event. The analysis is conducted within the framework of the dual-mode BSISO indices (BSISO1: 30–60-day oscillation; BSISO2: 10–30-day oscillation), employing wavelet analysis, dry and wet phase analysis, and refined identification of active convection source regions. The results show that the extreme rainfall event was jointly modulated by BSISO1 and BSISO2, with BSISO2 being stronger than BSISO1. The deep convection of BSISO1 mainly originated from the western Indian Ocean, and its wet phase evolved through three stages. The deep convection of BSISO2 mainly originated from the South China Sea and the western North Pacific; convection from the South China Sea persisted throughout the rainstorm period, while convection from the western North Pacific was even stronger. This study not only provides direct evidence from BSISO phase composite evolution confirming that BSISO2 is stronger than BSISO1, but also refines the source region distinctions within the Indian Ocean and between the South China Sea and the western North Pacific. Against the background of increasing extreme events, this study offers a new perspective for understanding the multi-scale mechanisms of extreme precipitation under La Niña conditions, and provides a valuable reference for improving subseasonal-to-seasonal prediction of summer precipitation in Central China.

The warm water volume (WWV), representing oceanic heat content, is the most widely used oceanic predictor of El Niño–Southern Oscillation (ENSO). However, the decadal variation in the relationship between WWV and ENSO remain unclear. Here we investigate the decadal changes in WWV’s predictive skill on ENSO from two aspects: the positive-negative ENSO event asymmetry and the Western-Eastern Pacific asymmetry. In this study, it’s found that an asymmetric decadal shift occurs in the predictive ability of WWV on El Niño and La Niña. Before 2000, WWV can predict ENSO beyond one-year lead, with the predictability reaching maximum at 8 months lead. This lead relation between WWV and ENSO decreased after 2000, mainly for the dramatically weakened La Niña predictability with the lead time of maximum predictability changing from 8 months to 3 months, while the El Niño predictability remained robust beyond 8 months lead. Such decreased predictive ability of WWV on La Niña is linked to the increased multi-year La Niña after 2000. We also identified the distinct roles and changes of WWV in the Western and Eastern Pacific Oceans. Revealed by information flow, after 2000 the stabilizing influences of Western Pacific WWV on ENSO increased and the key region expanded southward (from 4°S–3°N to 10°S–3°N), while the destabilizing influences of Eastern Pacific WWV on ENSO weakened and its scope narrowed (from 4°S–3°N to 2°S–2°N). Specifically in the Western Pacific after 2000, not only is the equatorial WWV important, but the off-equator (6°S–10°S) WWV in southern hemisphere is also significant. These results further extend our understanding on the prediction of ENSO by WWV.