Twofold expansion of the Indo-Pacific warm pool warps the MJO life cycle

The Madden-Julian Oscillation (MJO) is the most dominant mode of subseasonal variability in the tropics, characterized by an eastward-moving band of rain clouds. The MJO modulates the El Niño Southern Oscillation¹, tropical cyclones^2,3 and the monsoons^4-10, and contributes to severe weather events over Asia, Australia, Africa, Europe and the Americas. MJO events travel a distance of 12,000-20,000 km across the tropical oceans, covering a region that has been warming during the twentieth and early twenty-first centuries in response to increased anthropogenic emissions of greenhouse gases¹¹, and is projected to warm further. However, the impact of this warming on the MJO life cycle is largely unknown. Here we show that rapid warming over the tropical oceans during 1981-2018 has warped the MJO life cycle, with its residence time decreasing over the Indian Ocean by 3-4 days, and increasing over the Indo-Pacific Maritime Continent by 5-6 days. We find that these changes in the MJO life cycle are associated with a twofold expansion of the Indo-Pacific warm pool, the largest expanse of the warmest ocean temperatures on Earth. The warm pool has been expanding on average by 2.3 × 10⁵ km² (the size of Washington State) per year during 1900-2018 and at an accelerated average rate of 4 × 10⁵ km² (the size of California) per year during 1981-2018. The changes in the Indo-Pacific warm pool and the MJO are related to increased rainfall over southeast Asia, northern Australia, Southwest Africa and the Amazon, and drying over the west coast of the United States and Ecuador.