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Climate Change

The Need for Speed: The velocities of tides in the Mid-Atlantic

References: Brunner, K.; Lwiza, K. M. M. (2020). Tidal velocities on the Mid-Atlantic Bight continental shelf using high-frequency radar. J. Oceanogr. 76, 289-306.

DOI: https://doi.org/10.1007/s10872-020-00545-7

Reading Time: 5 minutes

“Folks, start your engines” you hear from a distant, indistinguishable voice. You look out in front of you and see almost the entirety of the eastern seaboard in America. You’re a singular water molecule ready to drive right into the coastline, and you are not the only one. Every 12 hours and 25 minutes, a rush of water move into the coast to create a high tide. These tides happen because of the moon. The moon can pull water toward itself using gravity, and as it rotates around Earth the water moves too! So the question is this, how fast does the water move? Currently, global sea levels are rising and it is becoming significantly important to understand how these coastal tides move as millions of Americans live along the eastern seaboard. How will local tides economically affect a small fishing town in North Carolina? Will the tides force naval bases to move? These are questions asked by two scientists at Stony Brook University, and what they found was quite interesting!

What did they find? How fast does tidal water move?

How fast do the tides move along the eastern seaboard? Image Credit: NASA Goddard Photo and Video

The pair studied the tides between Cape Hatteras, North Carolina and Cape Cod, Massachusetts (the Mid-Atlantic Bight) and found that the fastest speed of tidal water was 50 centimeters per second. This speed was measured at Nantucket Shoals and is much faster in comparison to the rest of the coast. For example, the tidal water moves fastest along the mid-shelf of the continental shelf at only 10 centimeters per second. The scientists hypothesize that the tide is so much faster at Nantucket Shoals because of internal waves (waves underwater with different temperatures) and sand waves. Sand waves are special ways in which sand stacks up on the ocean floor: The moving water can feel the floor beneath it and respond to these changes in sand. Last, large differences in the temperature of water between estuaries and the open ocean (otherwise known as stratification) can also affect tidal speeds. Internal waves, sand waves, and stratification are stated as important factors which determine the velocity of the tides.

In addition, the researchers found that Nantucket Shoals experiences a large amount of variability in the speed of its tide between seasons. While the majority of areas along the coastline see some changes in their tidal speed depending on the season, they are statistically small (about 1 centimeter per second change). Near Nantucket Shoals, the tidal speed can change close to 5 centimeters per second either faster or slower depending on the season!

For some additional findings, the pair found that tides can account for 32% of all of the energy of water moving on the Atlantic continental shelf. During the spring and fall, this number bumps up close to 40%. This means that the currents on the Atlantic continental shelf could slow down during the spring and fall.

How did they do it?

Cape Cod and Nantucket Shoals. Image Credit: NASA Goddard Photo and Video

The researchers used high-frequency radar (HFR) data from the SeaSonde HFR (from Rutgers University) and the MARACOOS project. There are a total of 15 SeaSonde stations that shoot 5,000,000 Hertz (Hz) sound waves into the ocean (the human ear can only hear between 20 Hz to 20,000 Hz) to a depth of 2.4 meters. By studying how the sound waves bounce back to the station, the researchers can determine the speed of the water at that point. To make sure their data was sound (pun intended), the pair compared their data to a group from 1984. The scientists at Stony Brook University found that their tidal velocities matched what previous research has told us so far.

Why does it matter?

This study highlights the value in long-term monitoring of ocean dynamics, as their data can provide new insights into how the ocean flows around the rest of the world. By studying the Mid-Atlantic Bight for a longer period of time than previous research has, the duo could analyze how tidal velocities changed with the seasons. These observations are extremely important to understand how the ocean could impact the communities along the eastern seaboard.

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