|
Since 1850, only seven tropical cyclones have brought gale-force winds to the southwestern United States. They are: The 1858 San Diego hurricane that was reconstructed as just missing landfall in 1858, the 1939 Long Beach tropical storm that made landfall near San Pedro in 1939, the remnants of Tropical Storm Jennifer-Katherine in 1963, the remnants of Hurricane Emily in 1965, the remnants of Hurricane Joanne in 1972, the remnants of Hurricane Kathleen in 1976, and Hurricane Nora after being downgraded to a tropical storm in 1997. In most cases, rainfall is the only effect that these cyclones have on California. Sometimes, the rainfall is severe enough to cause flooding and damage. For example, floods from Hurricane Kathleen devastated Ocotillo, California and killed several people. Precipitation other than rainfall is unusual in tropical cyclones, because of their weak updrafts and lack of supercooled water. ==An improbable event== There are two reasons why tropical cyclones rarely strike California: sea surface temperatures, and the usual upper level steering winds in the eastern Pacific, with sea surface temperatures being more important. Tropical cyclones usually require very warm water to depth, generally above 26.5 °C (80 °F) extending to a depth of 50 meters (160 ft). The waters off California are cold even in summer. They rarely rise above 24 °C (75 °F) in near-shore southern California, and usually don't get above 17 °C (63 °F) along most of the rest of the coast and outer coastal waters, although El Niño events may warm the waters somewhat. This is due primarily to the extensive upwelling of colder sub-surface waters caused by the prevailing northwesterly winds acting through the Ekman Effect. The winds drive surface water to the right of the wind flow, that is offshore, which draws water up from below to replace it. The upwelling further cools the already cool California Current which runs north to south along coastal California and even much of coastal Baja California. This is the same mechanism which produces coastal California's characteristic fog. The second reason is the general path of tropical cyclones in the eastern Pacific. They generally move north-westward or westward due to steering by the prevailing upper level winds, which takes them far out to sea and away from land.〔 These factors make eastern Pacific landfalls improbable north of about central Baja California. In those instances when upper level steering winds do allow a more northerly path, much cooler sea surface temperatures quickly weaken tropical cyclones that approach California, although torrential rainfall can still occur. For example, the September 24-hour rainfall record for Los Angeles is held by the 1939 Long Beach Tropical Storm, as of January 2007. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「List of California hurricanes」の詳細全文を読む スポンサード リンク
|