Updated Aug. 27 with preliminary data showing near-record rainfall in Hawaii.
As Hurricane Lane's rain bands deluged the Hawaiian Islands, scientists looked to the ocean temperature for evidence of connections to climate change and clues to what may be ahead for this region where hurricane landfalls have been rare.
Climate scientists have been warning that warmer oceans and atmosphere will supercharge tropical weather systems. Globally, they generally expect fewer tropical storms overall but an increase in the most intense storms. But they also say it's important to understand that there will be regional nuances.
In some areas—including the waters near Hawaii—hurricanes will probably become more common by the end of the century, said Hiroyuki Murakami, a climate researcher with the Geophysical Fluid Dynamics Laboratory at Princeton University who focuses on extreme weather.
In a 2013 study, Murakami and colleagues projected that tropical storm frequency for the area would double by 2100 under a global temperature increase of 2 degrees Celsius.
Global climate models are consistent in projecting a significant increase in sea surface temperatures in the vast Central Pacific, which would drive an increase in tropical storms that could affect Hawaii, Guam and other islands in the region, he said.
"It's very rare that we see multiple hurricanes approaching Hawaii in a single season, but just a few weeks ago, we had Hector passing south of the islands," he said. "In our dynamic climate model, we found out that hurricanes increase if we add more global warming."
"Extreme seasons like this one are in line with what we project," he said. "I think this is a signature of global warming."
Waters 1 Degree Celsius Warmer than Normal
Natural sea surface temperature cycles in the Pacific are important, Murakami said.
Currently, the subtropical Pacific south of Hawaii is about 1 degree Celsius warmer than average, and El Niño—which typically tamps down hurricane activity in the Atlantic Basin but can fuel it the Pacific—is likely developing over the coming months. Other cyclical sea surface temperature changes also affect the frequency of storms in the region, Murakami said.
"Both natural variability and global warming contribute, but it's difficult to separate what percentage is coming from global warming," he said.
Overall, the research is mixed when it comes to trends in the number of tropical storms in the various ocean basins, said Penn State climate scientist Michael Mann. "But there does seem to be an emerging consensus that we will see more intense storms," he said.
He noted some of the strongest storms on record—such as Hurricane Patricia, which exploded from a tropical storm to a powerful Category 5 hurricane within 24 hours off the coast of Central America in 2015—have occurred within the past four years while ocean temperatures were at record levels.
"In short, warmer ocean surface temperatures mean more energy available to strengthen these storms," Mann said.
Global Warming and an Uptick in Pacific Storms
Some research has suggested an uptick in tropical storm activity, especially in the western North Pacific. Without effective measures to control greenhouse gas emissions, global warming could significantly increase the average number of annual tropical storms worldwide by 2075, one study found.
In a separate study, Murakami found that global warming contributed to the record 2015 Eastern Pacific hurricane season. Another study he authored projects tropical storm intensity increasing in Japan, a trend that could be related to an increase in the strength of key ocean currents.
What About Record Warm Water Off California?
The Southern California coast could also face an increasing risk in years like this one, with record-warm ocean temperatures off San Diego and northern Baja California.
Usually, when the Eastern Pacific storms that form off the coast of Mexico veer north, they weaken when they run into cooler water along the Pacific Coast of North America. But as that part of the ocean warms, it could sustain their strength closer to land, Murakami said.
Slow Moving Storms, Too Much Rain
Hurricane Lane was moving slowly as it passed by Hawaii, subjecting the islands to days of extreme rainfall and flooding.
One National Weather Service rain gauge on the Big Island recorded 52.02 inches over four days, which, if validated, would be a record for Hawaii and the second-highest U.S. storm total from a tropical cyclone after Hurricane Harvey, which struck Texas one year earlier. The effects of the slow-moving storm on Hawaii were reminiscent of what Houston experienced as Harvey sat over the city for days.
When it comes to the slow movement of some recent destructive storms, like Harvey and Lane, the jury is still out on a warming connection. Kevin Trenberth, an atmospheric scientist with the National Center for Atmospheric Research, said the slow movement is reflective of the local weather situation: "There could be a link to global warming, but it's not identifiable."