Warmest part of Indo-Pacific ocean expanding yearly by an area the size of California, impacting rains in US
Warming of the Indo-Pacific Ocean is changing rainfall patterns from the tropics to the US and causing declines in rainfall on the west and east coasts of the US.
This Indo-Pacific warm pool—in what is already the warmest part of the global ocean—is expanding each year by an area the size of California, according to researchers from the National Oceanic and Atmospheric Administration (NOAA). The researchers report a doubling in the size of the warm pool of water spanning the western Pacific and the eastern Indian Ocean in recent years.
The expansion, says the team, is changing a key weather and climate feature called the Madden-Julian Oscillation (MJO), which is characterized by a band of rain clouds that move over the tropical ocean from the Seychelles off Africa toward India and into the Pacific Ocean.
The alterations in Madden-Julian Oscillation is causing changes in rainfall patterns globally, say experts. For example, this is influencing heat waves and flooding in the US.
Each year, weather variability costs the global economy over $2 trillion, with $700 billion alone in the US. The MJO contributes to more than 55% of this weather variability over the tropics and modulates the Asian, Australian, African, and American monsoons, tropical cyclones, and the El Niño Southern Oscillation (ENSO).
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“The MJO modulates the El Niño Southern Oscillation, tropical cyclones, and the monsoons, 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 gases, and is projected to warm further,” says the team in their analysis published in Nature.
However, the impact of this warming on the Madden-Julian Oscillation life cycle has been largely unknown, until now.
The researchers found that changes in the life cycle of Madden-Julian Oscillation are "associated with a two-fold 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,” says the study.
Such changes, say experts, have direct implications on the global weather and climate. The researchers explain that changes in the behavior of Madden-Julian Oscillation have brought a decline in rainfall to the central Pacific, the west and east coasts of the US, north India, east Africa and the Yangtze basin in China. The same changes, they say, are causing increased rainfall over northern Australia, Amazon basin, southwest Africa and Southeast Asia.
Though the entire Indo-Pacific Ocean has warmed, the warmest waters are over the west Pacific, creating a temperature contrast that drives moisture from the Indian Ocean to the west, enhancing cloud formation. This has changed the life cycle of the MJO. The length of time these clouds linger over the Indian Ocean has shrunk by four days from an average of 19 to 15 days. Over the west Pacific, the clouds now reside five more days.
"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. It is this change in the residence of MJO-driven clouds that is altering weather patterns around the globe," the findings state.
Climate model projections suggest further warming of the warm pool region, which may intensify the observed changes in global rainfall patterns, says the team. According to them, to meet these challenges presented by a warming world, climate models must be updated to accurately simulate, monitor, and predict the changes.