By Warren Cornwall
Even before Hurricane Patricia lashed into Mexico this weekend, the central Pacific Ocean had already been flogged by a record number of tropical storms. Give some credit to El Niño.
As this year’s strong El Niño raises ocean temperatures in the central and eastern Pacific, it appears to be following a classic pattern of acting both as a match that helps ignite tropical storms and as gasoline that makes them grow stronger.
El Niño is one phase in a years-long back-and-forth in Pacific Ocean temperatures and winds near the equator. Low-altitude trade winds that typically blow to the west weaken or reverse course during an El Niño.
And warm water that would usually shift toward Asia instead builds up in the eastern and central Pacific. When a tropical thunderstorm hits, the unusually warm water can set off a cascading series of events that generates and sustains the swirling vortex of a full-blown hurricane.
Changes in Pacific wind patterns tied to El Niño—particularly the weakening of winds blowing from the west high in the atmosphere—also set the stage for bigger storms by reducing wind shear, says Phil Klotzbach, a Colorado State University atmospheric scientist who studies tropical storms. Wind shear occurs when air currents at a lower altitude blow in a different direction from winds higher in the atmosphere, which can destabilize and weaken a hurricane.
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