36
Short-Term Processes Controlling Relative Sea Level
Tides
During a single day we already recognized that our observer sitting on the dock
witnessed a regularly-paced rise and fall of the sea surface as the tide came in and
retreated. On this very short timescale, the land surface is essentially stable and the
cause of relative sea-level change is the pull of gravity in the Earth-Moon-Sun system. Over the course of a month (a particularly generous and relaxing vacation
perhaps), the careful observer would note that the height of the high and low tides is
not the same every day. When the Sun and the Moon are working together the high
tides get a little higher while the low tides get a little lower and we say that we are
experiencing spring tides. During these few days tidal range (the difference between
high and low tides) is large. In contrast, at those times of the month when the Sun
and the Moon are working against one another, high tides are a little lower and low
tides are a little higher. These neap tides cause a period where tidal range is small.
Since the procession of the Earth, the Moon, and the Sun is well known, tides can
usually be predicted well in advance with a high degree of confidence. Tides must
be accurately predicted to ensure (for example) that ships can enter and leave ports
and harbors. In North Carolina, the sea level changes caused by astronomical tides
are actually very small. At Oregon Inlet (Fig. 3.1), there is a tide gauge (Fig. 3.2)
that automatically measures relative sea level every 6 min (and has done so since
1974). It shows that the difference between and high and low tide is, on average, just
0.36 m (14 inches). In contrast, the Bay of Fundy in Nova Scotia has the biggest
tidal range in the world, the inflow and outflow of one billion tonnes of water twice
each day results in an astonishingly large tidal range of 16 m (53 feet). This is our
first example that relative sea level varies from place to place and through time.
Even the most diehard holiday maker is unlikely to want to spend an entire year,
or a couple of decades, sitting on the dock, but relative sea level continues to change
on these timescales. Just as the tidal ranges were larger or smaller at particular times
within a month, so they also vary among different months. For North Carolina, the
largest tidal range occurs in January (when Earth is a little closer to the Sun, a position called perihelion) and the smallest in July (when Earth is a little further from
the Sun, a position called aphelion). On top of all this, the tides vary in a predictable
cycle that lasts 18.6 years.
Winds and Ocean Currents
Astronomical tides, however, are not the only cause of relative sea level changes to
influence the dock on timescales of hours to decades. Prevailing winds and individual weather systems can push water onto, or away from, the coast causing relative sea-level rise and fall respectively by tens of centimeters. The Wright brothers
A. C. Kemp and B. P. Horton
Short-Term Processes Controlling Relative Sea Level
Tides
During a single day we already recognized that our observer sitting on the dock
witnessed a regularly-paced rise and fall of the sea surface as the tide came in and
retreated. On this very short timescale, the land surface is essentially stable and the
cause of relative sea-level change is the pull of gravity in the Earth-Moon-Sun system. Over the course of a month (a particularly generous and relaxing vacation
perhaps), the careful observer would note that the height of the high and low tides is
not the same every day. When the Sun and the Moon are working together the high
tides get a little higher while the low tides get a little lower and we say that we are
experiencing spring tides. During these few days tidal range (the difference between
high and low tides) is large. In contrast, at those times of the month when the Sun
and the Moon are working against one another, high tides are a little lower and low
tides are a little higher. These neap tides cause a period where tidal range is small.
Since the procession of the Earth, the Moon, and the Sun is well known, tides can
usually be predicted well in advance with a high degree of confidence. Tides must
be accurately predicted to ensure (for example) that ships can enter and leave ports
and harbors. In North Carolina, the sea level changes caused by astronomical tides
are actually very small. At Oregon Inlet (Fig. 3.1), there is a tide gauge (Fig. 3.2)
that automatically measures relative sea level every 6 min (and has done so since
1974). It shows that the difference between and high and low tide is, on average, just
0.36 m (14 inches). In contrast, the Bay of Fundy in Nova Scotia has the biggest
tidal range in the world, the inflow and outflow of one billion tonnes of water twice
each day results in an astonishingly large tidal range of 16 m (53 feet). This is our
first example that relative sea level varies from place to place and through time.
Even the most diehard holiday maker is unlikely to want to spend an entire year,
or a couple of decades, sitting on the dock, but relative sea level continues to change
on these timescales. Just as the tidal ranges were larger or smaller at particular times
within a month, so they also vary among different months. For North Carolina, the
largest tidal range occurs in January (when Earth is a little closer to the Sun, a position called perihelion) and the smallest in July (when Earth is a little further from
the Sun, a position called aphelion). On top of all this, the tides vary in a predictable
cycle that lasts 18.6 years.
Winds and Ocean Currents
Astronomical tides, however, are not the only cause of relative sea level changes to
influence the dock on timescales of hours to decades. Prevailing winds and individual weather systems can push water onto, or away from, the coast causing relative sea-level rise and fall respectively by tens of centimeters. The Wright brothers
A. C. Kemp and B. P. Horton