Friday, December 5, 2014

Hurricanes self-feed, sucking up El Nino heat from the deep ocean

It has long been asserted that El Nino events, warming the tropical Pacific as they do, promote hurricanes—but it may not occur as we’ve previously assumed.

(Image: Hurricane tracks shown in black in the eastern and central Pacific. Credit: Jin/SOEST.)

University of Hawai`i researchers have identified a two or three-season delay that explains a lot about hurricane behavior. The El Nino hot water sinks, moves in the ocean, and then surges back to the surface to fuel tropical cyclones.

And that’s important information in terms of predicting hurricane frequency and ferocity.

Fei-Fei Jin and Julien Boucharel, both of the UH Mānoa School of Ocean and Earth Science and Technology (SOEST), published their study in the journal Nature. 

The researchers looked into the phenomenon in which El Nino is strongest in winter, but hurricane frequency picks up in the following summer and fall. At that time, most of the heated water from the El Nino is stored deep in the ocean of the eastern North Pacific.

But it may be that the activity of a young hurricane can suck that warm water to the surface, in essence finding the heat needed to fuel itself and supercharge the hurricane’s strength.

“We did not connect the discharged heat of El Niño to the fueling of hurricanes until recently, when we noticed another line of active research in the tropical cyclone community that clearly demonstrated that a strong hurricane is able to get its energy not only from the warm surface water, but also by causing warm, deep water – up to 100 meters deep – to upwell to the surface,” Jin said. He was quoted in a University of Hawai`i press release

Boucharel said that extra heat provides a lot of destructive energy.

“The Northeastern Pacific is a region normally without abundant subsurface heat. El Niño’s heat discharged into this region provides conditions to generate abnormal amount of intense hurricanes that may threaten Mexico, the southwest of the U.S. and the Hawaiian islands,” he said.

The authors wrote in the Nature paper: “we show that El Niño—the warm phase of an ENSO cycle—effectively discharges heat into the eastern North Pacific basin two to three seasons after its wintertime peak, leading to intensified TCs.”

They continue: “As a result of the time involved in ocean transport, El Niño’s equatorial subsurface ‘heat reservoir’, built up in boreal winter, appears in the eastern North Pacific several months later during peak (tropical cylone) season (boreal summer and autumn). 

"By means of this delayed ocean transport mechanism, ENSO provides an additional heat supply favourable for the formation of strong hurricanes.”

© Jan TenBruggencate 2014

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