Infrastructure protection“Intra-seasonal” variability in sea-level change

The effects of storm surge and sea-level rise have become topics of everyday conversation in the days and weeks following Hurricane Sandy’s catastrophic landfall along the mid-Atlantic coast.

Ongoing research by professor John Brubaker of the Virginia Institute of Marine Science (VIMS) is throwing light on another, less-familiar component of sea-level variability — the “intra-seasonal” changes that occupy the middle ground between rapid, storm-related surges in sea level and the long-term increase in sea level due to global climate change.

“These are cases when the water is just ‘running high,’” says Brubaker, ”but not from an obvious direct cause of a storm. It isn’t necessarily windy, it’s just an elevated water level without a clear cause.”

A VIMS release reports that intra-seasonal variability — which Brubaker says takes place on time-scales of ten to ninety days and can add or detract a foot or more from the predicted tide — is likely due to shifts in oceanic currents and large-scale movements of water masses along the coast. It often goes unacknowledged in discussions of sea-level trends, but can play an important role in water-level forecasts, coastal activities, and ecosystem health.

“Intra-seasonal variability has significant impacts,” says Brubaker. “For instance, being aware of these non-tidal, non-storm anomalies is very important for forecasting. If you’re experiencing a relative high during the approach of a storm, with water levels already elevated by a foot or more above predicted tides, that could make a big difference in terms of storm surge and coastal flooding.” Indeed, graduate student Carissa Wilkerson, whom Brubaker co-advises, is studying how intra-seasonal anomalies combine with storm surge as part of her Master’s research at VIMS.

Brubaker, who teams with researchers John Boon and David Forrest on the Tidewatch Forecast system at VIMS, says the Tidewatch forecasts account for at least some part of intra-seasonal variability by using as their starting point a moving average of the most recent thirty days of sea-level measurements. Other forecasts use mean sea level, a tidal datum that NOAA defines as the average measured over the years 1983-2001.

Brubaker notes that intra-seasonal variability can also impact marine life, most notably underwater grasses. Dr. J. J. Orth, head of the Seagrass Monitoring and Restoration Program at VIMS, raised concern during a period of unusually high water in May 2011, noting that “with water levels this high above predicted, it means less light for seagrasses, and with