2009 High-Water Event:
The intra-seasonal high-water event of summer 2009 is evident in this
water-level graph from the Chesapeake Bay Bridge Tunnel. The red line shows
that the 30-day average water level was elevated from June through mid-July. Data
from NOAA's National Ocean Service.
By David Malmquist
“Intra-seasonal”
variability impacts forecasting and ecosystems
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.
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 30 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.
JJ 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 light declining exponentially with depth it could mean
added stress to plants at the deeper edges of the grass beds.”
Periods of unusually low water
could also affect seagrasses and other marine life, says Brubaker, but the
impacts are likely to be less significant because the long-term rise in sea
level tempers their effects.
VIMS professor John Brubaker
checks the VIMS weather station.
The Summer 2009 Event
Brubaker’s interest in
intra-seasonal variability was piqued during summer 2009—when a prolonged period
of high water affected the U.S. East Coast—and again during the shorter period
of unusually high water during May 2011, which he experienced first-hand while
teaching a course at VIMS’ Eastern Shore Lab in the seaside village
of Wachapreague.
“The 2009 event got a lot of
attention—eventually,” says Brubaker. “It wasn’t dramatic and it took quite a
while to gain much attention, but at some point NOAA posted a notice about it
in response to questions and concerns from the public, who had noticed week
after week of abnormally high tides.”
If intra-seasonal changes in sea
level aren’t generated by regular tides, storm surge, seasonal heating or
cooling, or long-term sea-level rise, what is their cause? Brubaker says the northeasterly
winds contributed to high water along the coast due to “Ekman transport,” a
phenomenon in which surface waters begin to move to the right of the prevailing
wind because of the Coriolis force. “The Ekman transport associated with these
winds would push the water towards the shore,” says Brubaker. Persistent
offshore winds from the northeast were also measured during May 2011.
The slow-down in the Gulf Stream
contributed to 2009’s persistently high water levels through the re-positioning
of what oceanographers call a “geostrophic slope.” “The Gulf Stream creates a
geostrophic slope that’s related to the speed of the current,” says Brubaker.
“If the current speeds up, the slope gets 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 15 years
along the U.S. East Coast to better understand the frequency, magnitude, and
duration of high-water events in the region. He’s 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 6 weeks from June into July, but
at the Bridge Tunnel they only exceeded HAT for 24 hours, a cumulative total of
2 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."
Excerpted from http://www.vims.edu/newsandevents/topstories/brubaker_intraseasonal_variability.php
