First Alert Weather 101: What causes Hawaii’s high north shore winter surf?

HONOLULU (HawaiiNewsNow) - Although it may not feel like it, the seasons are starting to change here in Hawaii.

We’re already starting to see a glimpse of winter surf.

North shores across the island chain were placed under a high surf advisory Monday morning. The National Weather Service forecasted for wave heights up to 14 to 18 feet through Tuesday evening.

As we creep closer to winter, south shore swells are on the way out, and north shore residents are preparing for monstrous winter swells.

What causes this annual switch up between south shore surf and the large mountains of water that buffet our north shores annually?

Like most things that change with the seasons, it comes down to temperature differences.

“When we get into our northern hemisphere winter, the sun angle is mainly the strongest over the southern hemisphere,” said National Weather Service forecaster Genki Kino, “so then we can get these large temperature differences in the northern hemisphere that create these large mid-latitude cyclones.”

The difference in temperature between the northern Pacific and equatorial Pacific is one of the main reasons behind strong northern Pacific storms. In meteorology speak, this is called a mid-latitude cyclone.

Kino says the wintertime also sees what’s called the Aleutian Low.

“It’s kind of a semipermanent, low-pressure system that sets up near the Aleutian Islands, and that’s what really sends us the northwest swells, the continuous northwest swell here in the winter,” Kino said.

The air around a low-pressure system rotates counterclockwise, pulling up warm air from the south to the east of the low‘s center. Cold air is also pulled down from the north to the west of the low’s center.

The movement of this cold air is what is referred to as a cold front, and the air behind it moves in from the north and northwest, churning and adding energy to the ocean below.

This continuous movement of air is what pushes these large north shore swells toward the Hawaiian Islands.

As we transition to the winter surf season, make sure to pay attention to all posted signage and ask the lifeguard if it is safe to head out.

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