Expanding Indo-Pacific warm pool changing global rainfall patterns: Study

In the longstanding effort to understand how global climatic weather patterns have been changing in a warming environment, there is perhaps no bigger mystery than the Madden-Julian Oscillation (MJO). The MJO, a coupled phenomenon between oceanic and atmospheric circulations between the Indian and the Pacific Oceans, has long been known to be the most prominent influencer of sub-seasonal meteorological variability in the tropics travelling some 12,000-20,000 kms along the equator. But how the MJO, characterised by an eastward propagation of clouds, wind, rainfall and pressure along the equator, has been responding to increased warming due to GHG emissions has thus far been poorly understood. A new study now has shed light on how the life cycle of the MJO has changed over recent decades and what this means for weather and rainfall patterns in the tropical belt.

In a study published in the journal Nature, researchers report that the Indo-Pacific warm pool has nearly doubled in the last four decades from an area of 2.2 × 10⁷ km² during 1900–1980, to an area of 4 × 10⁷ km² during 1981–2018. This expanse- the largest such pool hosting the warmest ocean temperatures on Earth, is currently noted to be expanding at 4 × 10⁵ km² annually. This is equivalent to the area of California. The expansion of the warm pool and increases in the sea surface temperatures (SSTs) has in effect changed the MJO to bring increased rainfall during the months between November and April to northern Australia, west Pacific, Amazon basin, southwest Africa and southeast Asia while declining rainfall trends have been observed over rainfall over central Pacific, along the west and east coast of United States (e.g., California), north India, east Africa, and the Yangtze basin in China.

Researchers have also noted that while the total time period of the MJO has remained largely unchanged, there have been significant changes in the lifecycle of the MJO and its movement. The paper explains that although the entire Indo-Pacific belt has warmed, the warmest waters are over the west Pacific, creating a temperature contrast that drives moisture from the Indian Ocean to the west Pacific Maritime Continent. The result has been enhanced cloud formation over the western Pacific where MJO clouds are now active for an additional 5-6 days, increasing the average phase duration from 17.5 days to 23 days. Meanwhile, the time spent by MJO clouds over the Indian Ocean has shortened by 4 days bringing the average down from 19 days to 15 days during the November-April period.

The 2013-14 California drought, 2011 floods in South East Asia and 2011 east Africa droughts have all been linked in the paper to the increase in duration of the MJO over the Maritime Pacific Continent and western Pacific. The study also points out that extreme flooding events in South and Latin America, such as those seen in 2011 in Brazil, resulted from interactions  between a strong MJO and the South Atlantic Convergence Zone.

According to the latest GHG bulletin published by the World Meteorological Organisation, global CO² atmospheric concentrations rose to above 407 ppm last year, the highest it has been in over 3 million years. With no slowdown in sight, it seems inevitable that warming trends will continue to alter global atmpspheric and oceanic circulation, and by extension their coupled phenomena. “Climate model simulations indicate that continued warming of the Indo-Pacific Ocean is highly likely, which may further intensify these changes in global rainfall patterns in the future,” said lead author Roxy Matthew Koll, a scientist at the Indian Institute of Tropical Meteorology (IITM) in Pune. “This means that we need to enhance our ocean observational arrays to monitor these changes accurately, and update our climate models to skillfully predict the challenges presented by a warming world” he said.

The findings of the study are expected to be crucial in our understanding of climatic patterns and weather anomalies are likely to develop over the century as GHG emissions continue to rise. “Our results provide a critical benchmark for determining which computer models to trust for extended range weather forecasting, based on their ability to simulate the observed behaviour of the MJO in a changing climate,” said Michael McPhaden, senior scientist at the U.S. National Oceanic and Atmospheric Administration (NOAA), and a co-author of the study.

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