The northeast trade winds play a huge role in providing Hawai‘i with some of the best weather on the planet, but that might be changing.
The air in the Koʻolau mountains is still this morning. With no wind to rustle the trees, the chatter of birds, the whirl of a helicopter, and the beeping of a garbage truck fill the silence. The air feels thick with humidity, making me not want to move, except I do dance around to fend off mosquitoes. In the distance, the ocean shimmers like glass, reflecting the cloudless sky. On days like this, everyone knows there are no northeast trade winds.
When the islands’ famous northeast trade winds are present, they arrive on the windward side of Hawaiʻi, where they wrap around the shoreline, creating small waves, cool breezes, clouds, and rain, which feeds the watershed.
A sublime breeze persists and the air is clean, as any volcanic haze or industrial pollution is blown off shore. Everything feels right.
It’s easy to take for granted something we don’t necessarily “see,” but the northeast trade winds affect the everyday reality of locals, inviting keiki to ride waves at Makapuʻu Beach, encouraging us to eat lunch outside, and providing much of the water that comes out of the tap.
Commonly known as Hawaiʻi’s “natural air conditioner,” the northeast trade winds have a powerful cooling effect as they blow across your skin. Trade winds also figure largely into the idealized version of Hawaiʻi: Palm trees sway, hula skirts flutter, and a surfer is seen through the perforated mist dashing off a wave.
The trade winds are fundamental to the climate of Hawaiʻi, influencing the different environments from makai to mauka and windward to leeward.
Northeast trade winds blow more frequently in the summer, cooling the islands down when the temperature starts to heat up. Native Hawaiians recognize two seasons: kau, from May to October, when the sun shines directly overhead and the trade winds are more reliable, and hoʻoilo, from November to April, when the weather is cooler and the trade winds are less reliable. This aligns closely with the two seasons determined by modern climatologists, although the hotter season is shorter by a month.
But there has been a significant decline of northeast trade winds in the past four decades. Climatologist and University of Hawai‘i at Mānoa professor Pao-Shin Chu, along with other researchers, published a study in 2012 that found the frequency and intensity of northeast trade winds had decreased between 1973 and 2009. Data collected at Honolulu’s airport showed that in 1973, northeast trade winds occurred 291 days per year, but by 2009, the number had dropped to only 210 days. Chu called the findings “really alarming” and said it was not good for Hawaiʻi’s future.
These discoveries are consistent with what would be expected as climate change causes the planet to heat up. The study indicates fewer days were cooled off by northeast trade winds. It also found slightly increased frequency of easterly trade winds, which don’t bring the same amount of cool air to Hawaiʻi.
This phenomenon seemed obvious over the long, hot summer of 2019. It was evident in the headlines on the local news—“Hawaii has broken or tied more than 120 heat records since April” (Hawaii News Now) and “NE Trade wind days nearly cut in half” (KITV). It was also clear in the extra-tense conversations people had about things that happen every summer, like mango thieves. My auntie, who lives in Mānoa, decided to buy an air-conditioning unit. There was a shave-ice shortage, as the machines that make the large ice blocks overheated.
European sailors, who followed the winds’ consistent paths from Europe to the Americas and then across the Pacific Ocean to Hawaiʻi and Asia, named them the trade winds. These early sailors figured out that the best route was not necessarily in a straight line, but instead in a sort of circular pattern that follows the winds. Polynesians also used the winds to explore the seas, and Hawaiians have hundreds of names for the winds on each island that flow across, away, or toward the islands.
“The trade winds are created by an atmospheric circulation system that basically takes heat away from the tropics and distributes it to the north and south of the tropics,” explained Chip Fletcher, a professor at the UH at Mānoa’s School of Ocean and Earth Science and Technology.
Hawaiʻi’s proximity to the equator means that the sun is often shining overhead, heating up the land and in turn warming the air. This warm air rises into the atmosphere and is pulled into cooler parts of the Earth in the form of high-altitude jet streams. To replace the hot air, cool air then flows from the northeast along the surface of the Earth, collecting moisture and creating trade winds. This cycle operates as Hawai‘i’s natural ventilation system.
Without the blissful northeast trade winds in Hawaiʻi, we will feel the warmth and we will notice that trade wind rainfall has decreased, according to Chu. “We depend very much on trade-wind rainfall for agriculture, tourism, water resources, and all kind of things,” Chu said.
The trade winds are fundamental to the climate of Hawaiʻi, influencing the different environments from makai to mauka and windward to leeward. By shaping the dense cloud banks that surround the ridge tops in Hawaiʻi, the winds create what is known as a cloud forest.
Water droplets are deposited on leaves and branches, then drip onto the ground and travel down into the islands’ aquifers. The flora and fauna that live in the upper watersheds along Hawai‘i’s windward slopes have evolved in these moist conditions.
The extraction of moisture as the air is lifted over or around the windward slopes leads to a “rain shadow” on the leeward slopes, creating dry mountains. This is why a drive from cloudy Kailua on Oʻahu through the Nuʻuanu Pali tunnels to Honolulu is often like sliding into a parallel land where it is very sunny.
The phenomena of global warming is expanding the tropics and it is changing atmospheric circulation.
Each Hawaiian island interacts with the trade winds in different ways. On Oʻahu and Kaua‘i, the trade winds flow over the mountains. The taller peaks on Hawaiʻi Island and Maui force the clouds to go around them, creating dry, desert conditions at the highest elevations. But an increase in eastern tradewinds means the mountains are getting different windy results that produce fewer trade showers and fewer clouds, according to Fletcher.
“The phenomena of global warming is expanding the tropics and it is changing atmospheric circulation,” Fletcher said. Hawaiʻi does see more extremes, he said, and it will continue to see changes. “I’m saying ‘changing’ because there’s so much about how climate change influences the atmosphere and ocean that we just don’t understand.”
I have lived close to the top of O‘ahu’s Mount Tantalus, also known as Puʻu ʻŌhiʻa, for most of my life, so I’ve grown up embedded in the upper watershed ecosystem. Misty days that blocked the sunlight used to be fairly common, but now they feel infrequent enough that I am delighted when they arrive.
Many of my neighbors still have working fireplaces, and the smell of burning embers fills the air on these days. Cloudy trade wind weather is an excuse to stay home and to feel cozy, like how the Danish do hygge, but tropical style with slippers and socks.