Monday, July 31, 2006

Hurricanes in Southern California?

The temperature of the Pacific Ocean off the coast of California is ominously warm - San Clemente pier had an ocean temperature of 79 degrees yesterday - and this may make possible an unprecedented event: a full-strength hurricane strike on southern California in the next two months.

To review the science: hurricanes require a warm ocean surface (about 82 degrees Fahrenheit or above) in order to grow and maintain themselves. Colder water will quickly kill a hurricane, which is why California has never had a hurricane. There are ominous signs that this is changing.

Note: there is a usful article that discusses the subject of the diary at
Compared to a similar latitude on the east coast of the united States, California's coastal water temperatures are usually about 20 degrees cooler during summer, due to phenomenon called "coastal upwelling". Winds in summer come out of the northwest, which moves water down the coast. The Coriolis effect, due to the Earth's rotation, then makes this south-moving current curve out to sea, and cold, deep water wells up to the surface to replace it. Hence, the water temperature around California is often in the upper forties to low fifties, and usually no higher than an occasional spurt to the upper sixties - far too cold to feed a hurricane.
This whole system has collapsed this summer, and we have extremely warm (for California) waters at the coast. Southern California is experiencing conditions characteristic of Hawaii or South Carolina - humid, warm nights (due to the proximity of a bathwater-warm ocean), and thunderstorms during the afternoons and evenings.

If the coastal water temperature rises another 3-6 degrees, there would be very little to keep an east pacific hurricane from maintaining its strength and plowing into southern California late this summer or fall. The only controlling factor would be the steering winds, which tend to push Pacific hurricanes out to sea, rather than up the coast - but there have been near-misses (most recently in 1997) where the only thing that saved California was the now-vanished cold water at the coast.

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