For all the Atlantic Ocean’s composure this hurricane season, the eastern Pacific is turbulent after already producing its 15th storm.
Tropical Storm Odile formed southwest of Lazaro Cardenas, Mexico, yesterday, almost a full month before its 15th storm formed last year. As that was happening, forecasters were watching a 16th potential storm farther in the Pacific. In the Atlantic, the day came and went with nothing more than a little rain far out to sea.
“The Atlantic and eastern Pacific are usually out of phase with their hurricane seasons — when one is active, the other is inactive,” said Jeff Masters, co-founder of Weather Underground in Ann Arbor, Michigan. “This occurs because when the large- scale atmospheric circulation favors rising air and low pressure over one ocean basin, there must be high pressure and dry, sinking air elsewhere to compensate.”
The eastern Pacific has spawned 15 named storms, 10 hurricanes and seven major systems with winds in excess 111 miles per hour [178 km/h]. On the other side of the continent, the Atlantic has produced four storms, lagging behind its 30-year pace.
The eastern Pacific has been dominated by moist, rising air and low pressure, which leads to “above-average vertical instability” that helps create hurricanes, Masters said in an e-mail interview.
Across the Atlantic, the air has been falling, said Gerry Bell, lead hurricane forecaster for the U.S. Climate Prediction Center in College Park, Maryland. That increases atmospheric stability and keeps thunderstorms from forming.
There are other differences as well. The eastern Pacific has been 1 degree Fahrenheit (0.6 Celsius) above normal, Masters said. The Atlantic has been cooler than in recent years.
Wind shear is another factor. A greater level of shear, which occurs when winds blow at different speeds or directions at varying altitudes, can weaken storms or prevent them from forming. In the eastern Pacific wind shear is 20 percent below normal, Masters said.
The Atlantic has been more active than normal since 1995. In 2005, it produced a record 28 storms, including Hurricane Katrina, which touched off the destruction of New Orleans, and Hurricane Wilma, the last major storm to hit the U.S.
In the same time period, the eastern Pacific has been less active, according to the U.S. National Hurricane Center in Miami. From 1995 to 2013, 42 percent of the eastern Pacific seasons were below normal, 37 percent normal and 21 percent above normal.
With the hurricane center watching two more systems in the eastern Pacific, this season may be in the above-average category by the end of today.
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