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Our 10-year forecast: Summers will be wet

By BILL COATS
Published September 13, 2005

The Tampa Bay area can expect rainier than normal summers for a decade or more, thanks to changing temperature cycles in the Atlantic Ocean, the same phenomenon that has revved up hurricane seasons. Afterward, we can expect 20- to 30-year periods of relative dryness when the ocean cycle changes.

Even scientists such as Marty Kelly admit this is a fundamentally new way of looking at weather. "Some years, you get a lot of rain; some years you don't get a lot of rain," Kelly said. "Most people think it kind of evens out over just a matter of years." But Kelly, a top manager at the Southwest Florida Water Management District, knows better today.

To study rainfall, he compared river flows when the Atlantic's surface waters were warm to those when they were cool. Rivers in peninsular Florida shrank during the ocean's cool phases and bulged during warm ones. Hillsborough County's Alafia River ran 27 percent higher during the warm phases.

Scientists call this ocean cycle the Atlantic Multidecadal Oscillation, or AMO. It was discovered in connection with global warming. The AMO's warm phases were contributing to global warming, but its cool phases were offsetting it.

Researchers linked the cycles to tropical storms after the AMO switched to a warm phase in 1995 and promptly spawned a wicked hurricane season.

In 1999, Miami oceanologist David Enfield checked the connection to rain. Enfield, of the National Oceanic and Atmospheric Administration, compared the ocean temperature cycles to rainfall flowing into Lake Okeechobee. The inflows had been changing as much as 40 percent. "In hydrological terms, that's huge," Enfield said.

The Tampa Bay area has become rainier and stormier in the AMO's warm phase. Last year, bay area counties averaged the most rain since 1983.

The finding that the AMO drives these cycles carries major implications for Florida's embattled water supplies.

Over the last decade, Tampa Bay authorities invested more than $600-million in water projects aimed at reducing pumping from well fields. Rain, triggered by El Nino and the AMO, became an unexpected ally.

Regional water officials say it's nice to know rain may be plentiful for years, but they can't rely on that.

NOAA's Enfield thinks water officials should be planning for the next dry phase, which could last a quarter century or more. "Is it time for them to start pumping more water into underground aquifers because they're coming up on a dry period of several decades?"

[Last modified September 13, 2005, 08:02:02]