Tuesday, November 27, 2012

“Intra-seasonal” variability of sea level affects forecasting and ecosystems

Virginia Institute of Marine Science: 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 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.”

Intra-seasonal variability—which Brubaker says takes place on time-scales of 10 to 90 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....

Intra-seasonal variability can raise water levels a moderate amount over long periods (shown in the graph agove for 3 weeks). Hurricanes and nor'easters (e.g., Irene) generate higher, shorter peaks. Sandy was unusual in generating a storm surge that persisted through 5 tidal cycles. All data are from the Chesapeake Bay Bridge Tunnel. MLLW = Mean Lower Low Water. Graph compiled from NOAA tide-gauge data by Dr. John Brubaker.

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