The heat is on as mountains combine with ocean warming

Few places in Hawaii escaped the summer heat wave, and no place was hotter than Maui, which consistently recorded some of the state’s highest temperatures — including a scorching 97 degrees on Sept. 16.

Remarkably, Maui was the only island that saw a daily maximum temperature of at least 90 degrees throughout September, according to preliminary data collected at Kahului Airport by the National Weather Service, and on 13 days the mercury reached record-breaking highs.

In fact, the average high temperature for August was 91.1 degrees, and for September, nearly 92.5 degrees.

Residents aren’t the only ones feeling the heat. Maui Electric Co. says that over the four-month summer period, residential power use increased 5% from the previous year. In July alone, usage was up 13% on Maui.

MECO attributed most of the increase to air conditioning. The percentage of homes in Maui County with air conditioning has grown to 53% from 44% just five years ago, the utility reported.

Meanwhile, the county Department of Water Supply issued a plea Sept. 24 for its customers in West Maui to conserve water use. The agency reported that dry weather and the sweltering heat have resulted in “low raw water flows” in the ditch intakes for both the Mahinahina and Lahaina water treatment facilities, limiting production.

So why is it so darn hot on Maui?

Aside from the warmer ocean temperatures and diminished tradewinds that have been affecting weather across the state, Maui’s topography plays a role in the island’s consistently hotter temperatures, according to experts.

It’s not called the Valley Isle for nothing: Central Maui, where nearly 40% of the island’s 154,000 residents live, lies on a broad isthmus connecting the West Maui Mountains and the 10,000-foot Haleakala volcano. Nasir Gazdar, a geology professor at Kapiolani Community College, said the two mountain formations trap the heat rising from the flat, arid central valley.

“The colder air of the two mountains makes a blanket over the hot air,” he said.

Leeward sections of Maui and the western part of Hawaii island also trend 1 to 2 degrees hotter than other areas on their respective islands because Mauna Loa and Mauna Kea on Hawaii island further block the flow of tradewinds, according to Jamison Gove, a research oceanographer with the National Ocean and Atmospheric Administration’s Pacific Islands Fisheries Science Center.

“(Tradewinds) are really important for mixing the water and cooling ocean temperatures, and so one of the contributing factors is just you’re in the lee of some really giant mountains that are blocking the wind and are exacerbating this larger-scale warming pattern,” he said.

Gove and other researchers say ocean temperatures strongly correlate with the temperature and humidity on land, and warmer ocean water has been approaching from southwest of the state.

A “borderline to weak” El Nino, which lessens tradewinds and results in warmer ocean-surface temperatures near the equator, ended in July, but the warm waters around and to the northeast of Hawaii have remained — an unusual phenomenon.

“The ocean-surface temperature near Hawaii has been very warm, about a 1 to 2 degrees Centigrade above the mean value,” said Pao-Shin Chu, the state climatologist. “We have this subtropical higher-latitude warming that’s kind of unusual. … This keeps temperatures very hot here.”

The heat is being exacerbated by human-driven climate change, the research- ers say.

“There’s no question climate change is contributing to the warm temperatures we’re experiencing,” Gove said, adding climate change “is not the sole factor; it’s a contributing factor.”

Although cooler conditions can be expected as the winter months arrive, Maui residents should prepare for continued hot and sticky weather. The National Weather Service Forecast Office in Honolulu predict a 65% chance temperatures in Kahului will exceed the average temperature for the three-month period from October through December.