“Intra-seasonal” variability in sea-level change

steeper, and if the current slows down, the slope levels off. We on the East Coast are on the low side of the geostrophic slope, so as the Gulf Stream slowed down during the summer of 2009, the slope flattened out and water levels rose.”

Preliminary results
Brubaker’s interest in intra-seasonal variability focuses on using tidal records from the last fifteen years along the U.S. East Coast to better understand the frequency, magnitude, and duration of high-water events in the region. He is also interested in how these events propagate spatially through coastal water bodies like Chesapeake Bay.

“Our results are very preliminary at this point,” says Brubaker, “but there are a few things that stand out. One is that July and August are typically relatively quiet in terms of intra-seasonal high-water events. Another is that there is a lot of year-to-year variability. There seem to be more active years in terms of these events and then multi-year periods of relative quiet.”

He says the data also suggest an intriguing correlation between the high-water events and the occurrence of El Niño in the Pacific, as measured by the “Oceanic Niño Index,” a commonly used measure of El Niño-La Niña activity.

“You can’t help but notice,” he says, “that the spikes in the duration of high-water events seem to correspond to the very strong El Niño event in 1997-98, and again in 2009-10, which is the next biggest El Niño peak. There’s obviously not a direct correlation through the years, but El Niño is known for its teleconnections and effects that happen at great distances. So it’s not unreasonable to think that there might be some connection there. It’s something we continue to keep track of.”

Intra-seasonal variability in Chesapeake Bay
Brubaker’s study of how high-water pulses propagate through Chesapeake Bay has also produced some interesting preliminary results. “The magnitude of the peaks in non-tidal water level are pretty consistent across the lower, middle, and upper parts of the Bay,” says Brubaker. “But their impacts can be quite different because of the different tidal ranges in those areas.”

Brubaker notes that tides in Chesapeake Bay are driven by the ebb and flow of water at its mouth.

“The tidal range is higher at the mouth of the Bay, drops to lower levels in the mid-Bay, and then picks up again in the upper Bay,” says Brubaker. “Because of that, a coherent peak in water level during an intra-seasonal event will have the greatest impacts in the mid-Bay, where the tidal range is lowest.”

Explaining further, Brubaker introduces the term “highest astronomical tide,” or HAT, which is the highest high tide predicted for any particular tidal station. “Near the Bay mouth, at the Chesapeake Bay Bridge Tunnel, the HAT is 2.5 feet above mean sea level,” says Brubaker, “but at Solomons Island in the mid-Bay, it’s only 1.1 feet — so there’s a big difference. In 2009, water levels were elevated at all the tidal stations in Chesapeake Bay for about six weeks from June into July, but at the Bridge Tunnel they only exceeded HAT for twenty-four hours, a cumulative total of two days. At Solomons, by contrast, the water level exceeded HAT during almost every high tide. In fact, there was a period when the whole tidal cycle, from high through low tide, remained above HAT. All told, water levels at Solomons exceeded the HAT for a total of 15 days from June into July.

“The bottom line,” says Brubaker, “is that the same rise in water level will have different impacts at different locations in the Bay. Even though the water rises uniformly, the relative impact will differ by location and the level of the highest astronomic tide. HAT is an important datum for ecosystems, and it should be an important datum for people too. City planners, waterfront property owners, and land-use decision makers shouldn’t build too close to the HAT where they live. When they do, they’re likely to get in trouble.”