Tropical waves are "faster," allowing for smaller, thinner boards and more fun. Up north it's like surfing with the emergency brake stuck on. It's the salt. The spray from northern waves often smells fresh, like mist from a garden sprinkler. How much more salt is there (on average) in equatorial ocean water than, say, the Pacific off of San Francisco?
-- Lance McDougal, Ocean Beach
Worked like a moondoggie on this one. Scripps scientists, people who surf and then write about it for Surfing and Surfer, ordinary people who surf and don't write about it... Almost all confirm your observations about the tropical "fun" factor. But the salt explanation? I couldn't find anyone to support it. There is a difference in salinity in various parts of the world's oceans, but the spread is minuscule and wouldn't have anything to do with speed on a wave.
I don't expect you to buy this at all, Lance. In the 60, 70 years I've been pushing this rock up the Reader hill, I've noticed something about surfers. Each has his/her particular world view and doesn't take kindly to being challenged by a nonsurfer. When we identified the famous Hawaii Five-0 wave as having been filmed at Rockpile, on the north shore of Oahu, we were pelted with messages from outraged surfers who were sure they had the correct answer, and it wasn't Rockpile. They'd studied the wave on their tiny TV screens for years and arrived at their own conclusions, supported with excruciating descriptive detail, and they didn't want their particular apple carts upset. Never mind that we'd talked to the guy who'd actually filmed it.
So when I say it's the tropics themselves, not the salt, I don't expect you to buy it. One informant at Surfing magazine has noticed the same phenomenon you did and asked Scripps scientists about it and was told the same thing I was. Worldwide, dissolved salts in parts per thousand are between 32 and 37 -- lowest at the poles, highest south of the equator. Hawaii, about 35; San Francisco, about 33; between 36 and 37 in the Caribbean, the Gulf of Mexico, and off the coast of Brazil. None of it is enough to affect drag or flotation or the whoopee factor. Water in general is not much changed by the addition of salt. Can you believe, as my source concluded, that surfing in the tropics is more fun, because it's more fun in the tropics? He attributes it to sunny skies, balmy air, warm water, mai tais, and maybe the heavy wetsuit you wear in cold water. I really tried to find some physics, but all I could find was psychology.
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From Thomas Busch-Sorensen of O.B., re: mushy surfing up north vs. the glide in the tropics.
I never argue with guys with two names. "What's the main difference between the water off San Francisco and, say, Waimea Bay? The temperature. What's the physical parameter that determines how easy water glides over a surface? The viscosity, calculated as follows: u=u20 exp (-0.0284 x [T-20]). If the viscosity of water at 20 dec.C (u20) is 1 centipoise, then off the coast of San Francisco (8 deg.C) it is 1.41 centipoise. At Waimea Bay (28 deg.C) it is 0.80 centipoise. Your surfboard glides 1.41/0.8= 76% easier in tropical water."