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Whenever I'm watching the evening news and I hear the words, "blustery tradewinds, 15-30 miles per hour," I perk up. This is my cue to head over to the Windward side to see what treasures the wind may be blowing in.

During such a windy spell last week, I drove to Kailua Beach. It looked promising. Windsurfers and kite sailors flitted around the bay like butterflies, and salt crystals seasoned the air. I pulled on my sweats and headed down the beach.

But I was soon disappointed. Although the wind had been blowing onshore for days, the ocean offered up little of its usual fare. The beach was bare.

Why are our Windward beaches sometimes loaded with flora and fauna from the open ocean but other times, under similar weather conditions, are virtually empty?

The answer has to do with ocean currents. These circulation patterns involve air and water temperatures, salinity gradients, winds, upwelling, down-welling, gyres, eddies and more. This branch of oceanography gets complicated in a hurry. But a study of some drifting Nike brand sneakers a few years ago clarifies the picture. By following them we can see what happens at the surface in one part of the Pacific.

In 1990 a storm in the North Pacific caused a Korean container ship to lose five sneaker-laden containers overboard. Four of these containers broke open, releasing nearly 62,000 athletic shoes into the open ocean. Since the two sneakers of each pair were not tied together, each shoe set off on its journey alone.

Six months later, the footwear began appearing on the West Coast of North America. Since the shoes were wearable after washing, and worth about $100 a pair, coastal residents began holding swap meets to match pairs. This sparked the interest of Seattle oceanographer Curtis Ebbesmeyer, who began mapping the times and locations of the recovered sneakers.

After studying computer models of Pacific Ocean currents, Ebbesmeyer found that the sneaker drift rates agreed with the model's predicted currents.

The model also predicted there would be little scattering of the shoes as the current carried them the 1,500 miles to shore. People found the shoes, however, from California to northern British Columbia.

Researchers explained this as the effect of coastal currents, which run north and south as the seasons change.

But as good as the computer models are, Mother Nature still has some tricks up her sleeve. In 1992 three Nike sneakers washed up on the north end of the Big Island at Pololu. Also, since it takes four or five years for an object to drift completely around the North Pacific, oceanographers predicted the sneakers would show up on Japanese beaches in 1994 and 1995. None have been reported there to this day.

Ebbesmeyer also was curious about where the shoes would have gone if they had been lost on the same date in other years. The model showed that in 1951 the sneakers would have been stuck swirling in a current called the Alaska Loop; in 1973 they would have come ashore near the Columbia River.

The story of the great sneaker spill helps explain the variation of stuff we find on Hawaii's beaches. Sometimes the fickle flow of water brings plants, animals and objects close enough to be blown ashore. Other times, the currents trap their treasures far out at sea.

Over the eons the tricky combinations of winds and currents determined which plants and animals made it to the islands. Today, they make prowling Hawaii's beaches unpredictable and fun.