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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]

Previous studies have shown that models overestimate the strength of ENSO teleconnections to the North Pacific during springtime, but the underlying reasons for this bias remain unknown. In this work, the relative contributions from basic-state and thermodynamic/dynamic forcing factors are disentangled through idealized experiments with the Community Earth System Model and a range of stationary wave modeling experiments. It is revealed that in CESM1 the diabatic heating biases over the tropical Indian Ocean and tropical central-western Pacific jointly favor a cyclonic (anticyclonic) circulation bias to occur in the North Pacific during the springtime of El Niño (La Niña) events. On one hand, the difference in the modeled and observed climatological basic state does not lead to the bias formation directly, as the diabatic heating biases are the primary cause. On the other hand, the springtime basic state is conducive to a more vigorous stationary wave response to the biased diabatic heating than the wintertime state, and this explains why the teleconnection bias occurs during springtime but not in winter. An iterative bias-correction approach is then implemented in the atmospheric model component of CESM1 to verify the linkage between the tropical diabatic heating bias and the teleconnection bias. Moreover, this explanation is shown to be relevant in other models of phase 5 of the Coupled Model Intercomparison Project (CMIP5) as a strong relationship is found between biases in ENSO-related tropical central-western Pacific/Indian Ocean precipitation and North Pacific circulation across models in spring. Significance Statement: The purpose of this study is to explain why climate models tend to overestimate the springtime ENSO teleconnection to the North Pacific. Through both simplified and comprehensive model experiments, we found that the diabatic heating biases over the tropical Indian Ocean and central-western Pacific basins are the main cause behind the circulation bias. Although similar heating biases also occur in winter, the spring mean climate state is more sensitive to the biased heating than the winter mean state. These findings are useful for developing future climate models that would better simulate the springtime climate response during the ENSO events, as the same problem can be found in many other models. [ABSTRACT FROM AUTHOR]