39
devastating flooding to many parts of North Carolina and neighboring states. At
Oregon Inlet however, the wind field from Florence pushed enough ocean water
away from the coast to cause sea level to be 30 cm (~1 foot) lower than expected
before the storm track and wind field moved and sea level rose quickly to be 30 cm
higher than expected. This 60 cm swing in sea level took place over approximately
20 h. The highest elevation reached by sea level in 2018 at Oregon Inlet was during
Tropical Storm Michael in October when a storm surge of 1.3 m was measured. This
is a nice example of how the path and speed of a particular storm, rather than its raw
strength, controls the height of a storm surge. It just goes to show that a large storm
surges are not limited to the biggest and most powerful hurricanes. One way to predict when a hurricane will hit North Carolina is to ask one of us (Horton) when we
are going on summer vacation to the Outer Banks because, over the years, several
family vacations (often with some fieldwork thrown in for good measure) were cut
short by the need to evacuate a coastline in the path of an incoming hurricane!
The strength and/or position of ocean currents also affects relative sea level. The
Gulf Stream is a warm and fast-moving current that hugs the U.S. southeastern
Atlantic coast as it moves northward until it reaches Cape Hatteras, North Carolina
(Fig. 3.1), from where it hangs a sharp right turn into the Atlantic and heads toward
northern Europe. The Gulf Stream (in combination with other currents in the North
Atlantic Ocean) sustains a gradient in the height of the sea-surface, such that it is
lower on the U.S. Atlantic coast and higher near the center of the ocean (near
Bermuda for example). The more powerful the Gulf Stream becomes, the more
pronounced this gradient becomes. In contrast, if the Gulf Stream weakens, the
gradient is diminished and relative sea-level rise occurs on the U.S. Atlantic coast.
Instruments in the ocean and along our coasts that record this effect on timescales
of months to a few years. Computer simulations of how the climate of the Atlantic
Ocean will likely change during the twenty-first century predict that the Gulf Stream
will get weaker and ocean water will slosh on to the U.S. Atlantic coast causing
relative sea-level rise.
Long-Term Processes Controlling Relative Sea Level
When we thought about why relative sea level varies over time periods of hours to
a couple of decades, we could reasonably assume that the land-surface in North
Carolina was stable and that the changes in the height of the sea surface were caused
by processes (such as tides and winds) which moved water that was already in the
ocean from one place to another. Now we need to think longer term and we will see
that if our observer stays on the dock for many decades to millennia, these assumptions must be reconsidered. To illustrate this point, let’s return to the tide gauge at
Oregon Inlet. During the course of a day (Fig. 3.3a), a month (Fig. 3.3b), or a single
year (Fig. 3.3c), the changes in relative sea level follow the expected pattern of rise
and fall by tides with the occasional evidence of a storm impacting the coast. If
instead we remove this short-term noise (with the caveat that one scientist’s noise is
3 Time and Tide Wait for No Man
devastating flooding to many parts of North Carolina and neighboring states. At
Oregon Inlet however, the wind field from Florence pushed enough ocean water
away from the coast to cause sea level to be 30 cm (~1 foot) lower than expected
before the storm track and wind field moved and sea level rose quickly to be 30 cm
higher than expected. This 60 cm swing in sea level took place over approximately
20 h. The highest elevation reached by sea level in 2018 at Oregon Inlet was during
Tropical Storm Michael in October when a storm surge of 1.3 m was measured. This
is a nice example of how the path and speed of a particular storm, rather than its raw
strength, controls the height of a storm surge. It just goes to show that a large storm
surges are not limited to the biggest and most powerful hurricanes. One way to predict when a hurricane will hit North Carolina is to ask one of us (Horton) when we
are going on summer vacation to the Outer Banks because, over the years, several
family vacations (often with some fieldwork thrown in for good measure) were cut
short by the need to evacuate a coastline in the path of an incoming hurricane!
The strength and/or position of ocean currents also affects relative sea level. The
Gulf Stream is a warm and fast-moving current that hugs the U.S. southeastern
Atlantic coast as it moves northward until it reaches Cape Hatteras, North Carolina
(Fig. 3.1), from where it hangs a sharp right turn into the Atlantic and heads toward
northern Europe. The Gulf Stream (in combination with other currents in the North
Atlantic Ocean) sustains a gradient in the height of the sea-surface, such that it is
lower on the U.S. Atlantic coast and higher near the center of the ocean (near
Bermuda for example). The more powerful the Gulf Stream becomes, the more
pronounced this gradient becomes. In contrast, if the Gulf Stream weakens, the
gradient is diminished and relative sea-level rise occurs on the U.S. Atlantic coast.
Instruments in the ocean and along our coasts that record this effect on timescales
of months to a few years. Computer simulations of how the climate of the Atlantic
Ocean will likely change during the twenty-first century predict that the Gulf Stream
will get weaker and ocean water will slosh on to the U.S. Atlantic coast causing
relative sea-level rise.
Long-Term Processes Controlling Relative Sea Level
When we thought about why relative sea level varies over time periods of hours to
a couple of decades, we could reasonably assume that the land-surface in North
Carolina was stable and that the changes in the height of the sea surface were caused
by processes (such as tides and winds) which moved water that was already in the
ocean from one place to another. Now we need to think longer term and we will see
that if our observer stays on the dock for many decades to millennia, these assumptions must be reconsidered. To illustrate this point, let’s return to the tide gauge at
Oregon Inlet. During the course of a day (Fig. 3.3a), a month (Fig. 3.3b), or a single
year (Fig. 3.3c), the changes in relative sea level follow the expected pattern of rise
and fall by tides with the occasional evidence of a storm impacting the coast. If
instead we remove this short-term noise (with the caveat that one scientist’s noise is
3 Time and Tide Wait for No Man