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Atmospheric Rivers (ARs), corridors of intensive water vapor transport, have received much attention recently due to their potentially large economic and societal impacts. However, a comprehensive understanding of the interannual variability of ARs over East Asia is still lacking. This study examines the frequency of AR occurrence over East Asia based on the EAR5 reanalysis. It is shown that the frequency of AR occurrence over East Asia displays obvious seasonality, with its peak in early boreal summer. ARs contribute to up to 50% of total precipitation and largely modulate the interannual variability of precipitation in East Asia during early boreal summer. The interannual variability of ARs over East Asia during early boreal summer is largely controlled by the Indo-western Pacific Ocean Capacitor (IPOC) effect, which is mainly triggered by El Niño-Southern Oscillation and sometimes by Indian Ocean dipole. During early boreal summer, a higher frequency of AR occurrence over East Asia is associated with anomalous anticyclonic circulation (AAC) over the Indo-western Pacific region. The AAC, the atmospheric signal of the IPOC mode, indicates an enhanced East Asian summer monsoon and a northwestward shift of the western North Pacific subtropical high, favoring the AR occurrence over East Asia. This study illustrates that a strong positive Indian Ocean dipole event could impact AR activity over East Asia in the following summer by triggering the IPOC effect. The IPOC-related signals 2–4 months ahead could be useful to predict the AR occurrence over East Asia during early boreal summer. [ABSTRACT FROM AUTHOR]

This study examines the climate response to a sea surface temperature (SST) warming imposed over the southwest tropical Indian Ocean (TIO) in a coupled ocean–atmosphere model. The results indicate that the southwest TIO SST warming can remotely modulate the atmospheric circulation over the western North Pacific Ocean (WNP) via interbasin air–sea interaction during early boreal summer. The southwest TIO SST warming induces a "C shaped" wind response with northeasterly and northwesterly anomalies over the north and south TIO, respectively. The northeasterly wind anomalies contribute to the north TIO SST warming via a positive wind–evaporation–SST (WES) feedback after the Asian summer monsoon onset. In June, the easterly wind response extends into the WNP, inducing an SST cooling by WES feedback on the background trade winds. Both the north TIO SST warming and the WNP SST cooling contribute to an anomalous anticyclonic circulation (AAC) over the WNP. The north TIO SST warming, WNP SST cooling, and AAC constitute an interbasin coupled mode called the Indo-western Pacific Ocean capacitor (IPOC), and the southwest TIO SST warming could be a trigger for IPOC. While the summertime southwest TIO SST warming is often associated with antecedent El Niño, the warming in 2020 seems to be related to extreme Indian Ocean dipole during the autumn of 2019. The strong southwest TIO SST warming seems to partly explain the strong summer AAC of 2020 over the WNP even without a strong antecedent El Niño. [ABSTRACT FROM AUTHOR]

The 2019 positive Indian Ocean Dipole (IOD) was the strongest event since the 1960s which developed independently without coinciding El Niño. The dynamics is not fully understood. Here we show that in March–May, westward propagating oceanic Rossby waves, a remnant consequence of the weak 2018 Pacific warm condition, led to anomalous sea surface temperature warming in the southwest tropical Indian Ocean (TIO), inducing deep convection and anomalous easterly winds along the equator, which triggered the initial cooling in the east. In June–August, the easterly wind anomalies continued to evolve through ocean‐atmosphere coupling involving Bjerknes feedback and equatorial nonlinear ocean advection, until its maturity in September–November. This study clarifies the contribution of oceanic Rossby waves in the south TIO in different dynamic settings and reveals a new triggering mechanism for extreme IOD events that will help to understand IOD diversity. Plain Language Summary: The Indian Ocean Dipole (IOD) is an ocean‐atmosphere coupled climatic phenomenon which can cause severe social and economic losses in the surrounding regions such as drought in the Maritime Continent/Australia and flooding in East Africa. The IOD features a see‐saw structure accompanied by an anomalous sea surface temperature gradient, winds, and oceanic adjustments. The El Niño–Southern Oscillation in the Pacific is an important trigger to a strong IOD event. However, an extreme positive IOD event occurred in 2019 without a concurrent or ensuing El Niño. We show that the thermocline warming associated with anomalous ocean downwelling in the southwest tropical Indian Ocean triggered atmospheric convection, inducing anomalous easterly winds along the equator and hence, positive feedbacks associated with an IOD event. This study may help to understand the evolution of extreme IOD and improve IOD predictability. Key Points: The oceanic downwelling Rossby waves in the south tropical Indian Ocean is key to the 2019 extreme positive Indian Ocean Dipole (IOD)The Rossby waves induced thermocline warming, triggering wind‐evaporation‐SST feedback thus anomalous easterly winds along the equatorThe easterly wind anomalies further triggered the Bjerknes feedback and other positive feedbacks and established an extreme IOD [ABSTRACT FROM AUTHOR]

This study identifies a new triggering mechanism of the Indian Ocean Dipole (IOD) from the Southern Hemisphere. This mechanism is independent from the El Niño‐Southern Oscillation (ENSO) and tends to induce the IOD before its canonical peak season. The joint effects of this mechanism and ENSO may explain different lifetimes and strengths of the IOD. During its positive phase, development of sea surface temperature cold anomalies commences in the southern Indian Ocean, accompanied by an anomalous subtropical high system and anomalous southeasterly winds. The eastward movement of these anomalies enhances the monsoon off Sumatra‐Java during May–August, leading to an early positive IOD onset. The pressure variability in the subtropical area is related with the Southern Annular Mode, suggesting a teleconnection between high‐latitude and midlatitude climate that can further affect the tropics. To include the subtropical signals may help model prediction of the IOD event. Plain Language Summary: An Indian Ocean Dipole (IOD) at its positive phase featuring anomalously high and low sea surface temperature (SST) in the west and east equatorial Indian Ocean, respectively, shifts atmosphere convection westward, causing severe floods and droughts in surrounding west and east Indian Ocean‐rim regions. Known triggering mechanisms, such as the external El Niño‐Southern Oscillation (ENSO), cannot explain development or intensity of many IOD events. Here we find a novel triggering mechanism of the IOD from the Southern Hemisphere, in which the subtropical high pressure and wind anomalies forcing cool SST anomalies evolve to trigger onset of an IOD event via enhancing the monsoon off Sumatra‐Java. This Southern Hemisphere Mechanism can operate independently from the ENSO and commences earlier than the ENSO forcing, thus providing explanation of different IOD characteristics and a longer prediction lead time. Key Points: A triggering mechanism of the IOD originated from the Southern Hemisphere is identifiedThe Southern Hemisphere Mechanism is independent from ENSO and associated with subtropical and high‐latitude climatic variabilityThe joint effects of such mechanism and ENSO produce different characteristics of the IOD [ABSTRACT FROM AUTHOR]

The Indo-western Pacific Ocean capacitor (IPOC) effect induces coherent climate variations over the Indo-western Pacific Ocean with significant sea-surface temperature (SST) anomalies over the South China Sea (SCS). Most IPOC events are related to El Ni n ∼ o-Southern Oscillation (ENSO), occurring in the post-ENSO years. The coral δ18O and Sr/Ca precisely recorded climate variations as proxies for SST. This study investigated the IPOC via a reduced major axis (RMA) regression analysis based on coral records in the SCS. The coral records of Yongxing Island show a double SST peak during El Niño events after the 1960s, one in the mature phase and the other in the post-El Ni n ∼ o summer. Tropical Indian Ocean warming persists into summer and anchors an anomalous anticyclonic circulation over the western North Pacific, extending the ENSO effect on the SCS SST through the IPOC effect, despite the decay of El Ni n ∼ o. The corals from Yongle Atoll, Dongsha Atoll, and the region off central Vietnam well recorded the IPOC impacts associated with two strong El Ni n ∼ o events (2009–2010 and 1997–1998) on SST anomalies in summer. According to coral records off central Vietnam, the 1959 IPOC effect occurred without an antecedent El Ni n ∼ o event. The results show that coral proxies could capture the IPOC events in the SCS and prove that IPOC could act as an intrinsic climate mode in the Indo-western Pacific Ocean. • Coral proxies from Yongxing Island record the IPOC events since 1960. • Most IPOC events recorded by corals are associated with the El Ni n ∼ o events. • Some IPOC events, e.g., recorded off central Vietnam in 1959, occur without antecedent El Ni n ∼ o events. [ABSTRACT FROM AUTHOR]