Graduate Student Reinvents Tides

Rising sea levels already altering tides in Chesapeake Bay

Researchers have found evidence that sea-level rise is already affecting high and low tides in both the Chesapeake and Delaware bays, two large estuaries of the eastern United States.

The team combined a computer model with 100 years of observations to tease out the fact that global sea-level rise is increasing the tidal range, or the distance between the high and low tides, in many areas throughout each bay.

Tides, or the rising and falling of the ocean’s surface, occur on regular intervals and as a result of numerous factors, the biggest of which is the gravitational pull from the sun and moon.
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In 2015, Andrew Ross, a meteorology doctoral student at Penn State, noticed an odd pattern emerging while testing a numerical computer model for tidal research. Adding one meter of sea-level rise to the model resulted in a distinct pattern of changes to the high and low tides throughout the Chesapeake Bay.

“We weren’t sure why it was there, but it was unique enough that we thought it should show up in observations, too, if it was actually real,” says Ross, now a postdoctoral research associate at Princeton University. “So we started looking at the observations, doing more comparisons.”
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The team analyzed tidal gauge records from 15 locations in the Delaware and Chesapeake bays, the oldest of which dates back to 1901. They also studied nearby cities outside of each bay to control for larger changes affecting the ocean more broadly.

Once they isolated sea-level rise’s influence on the tides using the tide gauge records, they compared this information with a computer model in which they could adjust the overall sea level, simulating the effects of the past or future.

As they report in the Journal of Geophysical Research: Oceans, both the tide gauge data and the model generally agreed on the main effects of sea-level rise on tides. The analysis shows that one meter of sea-level rise increased the tidal range by up to 20 percent in some areas. The exact amount of change from sea-level rise varied based on physical characteristics such as the shape of the bay.

Tides can be pretty complicated in a place like Chesapeake Bay with many tributaries, and regions with different depths. We understand the astronomical factors which drive tides, but the actual motion is also a function of water depth and shape of the basin. But we know from first principals that increasing the depth will change the tides. This has happened before, as sea level fell and rose with the passing of ice ages.

Did they also consider the counter-effect of sedimentation having the effect of countering the effects of sea level rise bu making the water shallower? Sedimentation rates in the Bay are on the same order of magnitude as sea level rise, but are much more variable geographically. Probably not, because they weren't looking for contrary arguments.