Seas Are Rising In South Florida. So Why Don't Tide Forecasts Show That?

Oct 18, 2019

Some of the most dramatic sea rise around South Florida has occurred in the last two decades: at least five inches near Virginia Key since 1992.

Yet most of that increase is not reflected in the National Oceanic and Atmospheric Administration’s tide predictions that draw so much attention during king tides. That will change in the next year when NOAA updates the baseline it uses to predict low and high tide heights for the first time in nearly 20 years as part of a recalculation it has been performing for decades.

At least two more inches - the average of the last two decades of tides - will likely be added to tides around South Florida, said University of Miami Rosenstiel School of Marine and Atmospheric Science researcher Brian McNoldy.

But with climate change upending the planet’s reliable patterns, it may now also be time to rethink how often NOAA recalculates the baseline, said oceanographer William Sweet.

“Every time we make these adjustments, they tend to continue to increase the amount we need to adjust,” he said. “Sea level rise is going to make that challenging - how we go about providing baseline information - because that baseline is rapidly changing.”

To come up with tide predictions, NOAA looks at many factors: new moons, moon cycles and where the earth is in relation to the sun or moon, McNoldy said.

“Because none of those things are unknowns” he said.

Sweet said it’s like listening for individual drummers in a drumline.

“There's some very cyclical-like drum beats, so we actually monitor dozens of beats that occur due to the sun, moon and earth system. And we determine how loud each beat is,” he said. “We find patterns that explain the typical rhythm of the highs and lows. And once we have that pattern, we can then project that into the future so we can tell you when high tide will be two months from now.”

NOAA uses the 19-year timeline because it covers an orbital path followed by the earth and moon that amplifies tides called the Metonic cycle, after the Greek astronomer who discovered it. 

The cycle creates a push in the gravitational forces that propel tides. It also provides a framework for long-term observations that allow oceanographers to block out noise from periodic events like hurricanes. The current baseline uses the average tide height for the cycle that ran from 1983 to 2001.

While tide heights are part of the predictions, they do not incorporate winds, storms and other weather conditions so are not intended to be used as flood forecasts.

“The way we’re programmed to think of predictions is they’re tied to what’s observed,” McNoldy said. “But [tide predictions] aren’t actually forecasts attuned to the current conditions. These are just climatological and astronomical types of things.”

For timing, the tide predictions are remarkably reliable - something ship captains, fishermen and others have come to rely on.

“They’re timed miraculously close to perfect,” McNoldy said. “They’re timed to within five minutes. And that part is spot on.”

Even though the predictions aren’t meant to be flood forecasts, Sweet said they should still more closely reflect existing conditions.

“The fact of the matter is sea levels are rising and now it's starting to spill in the streets and causing flooding much more often than it ever did in the past,” he said. “The struggle is you really want to characterize what your tides are doing - your earth moon and sun cycles and how they changed - but you also need to stay relevant to the current conditions that folks are experiencing.”