When it comes to fundamental drivers of climate and weather across the Earth, it is hard to think of a region more important than the Indo-Pacific Warm Pool, an enormous area stretching across the Pacific and Indian oceans on both sides of the equator.
This is, basically, the biggest body of warm water there is. Indeed, the warm pool, which is fueled by the intense sunlight striking the equator and tropics, is defined as the area where the average surface ocean temperature is greater than about 82 degrees Fahrenheit all year round (a temperature, incidentally, that is well above the threshold level needed for tropical cyclone or hurricane formation).
The warm pool drives monsoons, tropical cyclones and much more. Its warm ocean surface is the home to deep atmospheric “convection,” or the rising of warm, moist air, which leads to atmospheric circulation and rainfall patterns that influence the entire planet.
And the warm pool is growing.
“It is about four or five times larger than Australia,” said Seung-Ki Min, a researcher at Pohang University of Science and Technology in South Korea and an author of a new study in Science Advances on the warm pool’s expansion. “It has been increasing about 32 percent over the last 60 years in size.”
The new study, which Min co-authored with Evan Weller of Pohang University as well as colleagues in China, Canada and Australia, proves for what is apparently the first time that this spatial expansion — which has implications for hurricane landfalls, rising seas (warm water expands and takes up more area) and much more — is caused by human-induced climate change.
It is not, of course, news that the global ocean itself is warming. That’s been documented many times. Indeed, the ocean is sucking up 90 percent of the total additional heat that climate change is retaining within the Earth’s system.
But because of the warm pool’s enormous heat and role in the global atmospheric circulation, its changes reverberate very widely indeed.
“We have more energy available from the hotter ocean,” said Min. “That means the atmosphere will be enhanced to transport more energy from the tropical ocean to the high latitude zone.”
The new study used a series of model runs, with and without human greenhouse gas emissions included, to study how well the different simulations reproduced the actual, observed expansion of the warm pool. And here was the result:
The chart on the top left shows the actual expansion of the warm pool (dotted lines show its extent from 1953 to 1959, and solid lines, its extent in the 2000 to 2012 time period). Below that is what climate models produce when they only include the role of greenhouse gases. In the top right is what they produce when both greenhouse gases and natural factors, like the Pacific Decadal Oscillation, are also included. And finally, to the bottom right is what models produce with only natural factors included.
It’s clear that the models that include both natural factors and greenhouse gases best capture the expansion. “About 12 to 18 percent has been due to natural variability in the ocean, but the remaining part is overall due to the greenhouse gas increase,” said Min.
The maps above also give a sense of the kinds of consequences that this expansion will have and indeed is already having. For instance, the warm pool, which is quite conducive to generating hurricanes, is now much closer to many hurricane-vulnerable regions — including Bangladesh, which has seen the deadliest tropical cyclone strikes in world history.
More warm water means that there are more areas conducive to the formation of tropical cyclones, and it also affects how far they can travel, in any particular direction. A storm “can survive longer, because we now have a larger area of hot water, giving more energy to the tropical cyclone,” Min said.
When it comes to climate change, much of the most vivid imagery is far away from the warm pool — in the rapidly warming Arctic, for instance, where sea ice is vanishing and glaciers are breaking off city-sized pieces.
That’s all very dramatic, but the slow, creeping expansion of the warm pool is hard to match when it comes to ultimate consequences.