Why does sea level change?
Short term variability
Some of the processes that drive short term (hours to days) changes in sea level are:
Gravitational forces from the Moon and the Sun produce changes in sea level mostly on daily
or half-daily time scales - tides. In many places the tidal signal is a mix of daily and
half-daily tides. In the deep ocean the tidal range is typically a few tens of centimeters
but in coastal regions it can be up to several metres. There are also longer term
changes (particularly the modulation of the daily tide range of the Spring/Neap cycle),
and smaller amplitude longer-period tides (including modulation of the tidal amplitude over
the 18.6 year lunar cycle).
Waves steepen and break as they encounter shallow coastal waters. The wave breaking leads to
loss of energy and loss of wave height. Therefore, the further offshore these conditions
are encountered the smaller the waves will be that finally reach the shore. Conversely, deeper
waters lying adjacent to coastlines enable waves to travel closer into shore before finally
breaking. These two situations are illustrated in Figure 1. Paradoxically, the conditions that
ensure that waves lose energy before reaching the shore are the same conditions that favour
larger storm surges.
Figure 1: Shallow coastal waters (left) cause wave breaking and reduction of
wave energy to occur further offshore while deeper coastal waters (right) allow
higher energy waves to reach the shore.
As the waves move progressively into shallower water, they break and lose energy. Some of
this energy is transferred into a shoreward momentum flux which acts to raise the mean sea
level slightly close into shore. This sea level increase is called wave setup. Typically wave
setup values range from about 10 to 15% of deep water significant wave heights where the
significant wave height is, by definition, the average height of the top one third of waves
occurring at some deep water location (Guide to wave analysis and forecasting. World
Meteorological Organisation, Report no. 702, 1988).
Climate variability and change has a significant impact on wind and wave conditions globally.
For example, studies in the North Atlantic Ocean have shown that wave heights have increased
over the last few decades, displaying a strong relationship with the North Atlantic Oscillation,
and inter-annual variability as great as 20 % (Woolf et al., 2002). Sterl and Caires (2005), in
their analysis of the ERA-40 global waves re-analysis, identified significant trends in wave
height, particularly in the Southern Ocean, the North Atlantic and the North Pacific. These
trends are more pronounced in the high quantiles, indicating that the large wave events are
increasing at a greater rate than the mean. In the Southern Ocean, Sterl and Caires (2005)
identified an increased number of Southern Ocean storms as the reason for the observed trend
in the model record.
Interannual variability of wave direction in the Australian region has been recorded via
beach rotation processes at a number of short embayed beaches along the East Australian
coast (Ranasinghe et al. 2004). This rotation is linked to wave climate variability associated
with the Southern Oscillation Index. During El-Nino phases, the beach rotates clockwise in
response to changes in the wave climate, with the northern end of the beach accreting, and
the southern end of the beach eroding. During the La-Nina phase, the opposite occurs with a
net anti-clockwise rotation of the beach observed. Hemer et al. (2007) indicated a
positive correlation between wave heights and the southern annular mode index along the
southern and western margins of Australia, suggesting that a similar relationship may hold
for these regions.
Sea level rise will be felt most severely in response to storm driven wave and surge events,
which lead to wave induced erosion of coastal landforms. Wave induced erosion is dependent on
the water elevation relative to the height of the fronting beach face junction. The water
level depends on the mean sea level, a tidal component, any storm surge component, and an
increase in water level produced by waves, including the set-up and run-up of individual waves.
Larger waves are therefore more easily able to erode the shoreline. In addition to the impacts
of wave height, the rate at which beach material is redistributed along the shore is also
dependent on the angle at which waves arrive in the coastal zone. Therefore, coastal
morphology depends strongly on the wave climate the coast is exposed to. Changes to the
wave climate, such as a shift in wave direction, or increase/decrease in wave heights, will
therefore lead to erosion of the coast. Slott et al (2006) found shoreline change in response to
changing wave climate due to changing storm patterns could be an order of magnitude greater
than that caused by rising sea levels.
The effects of wave erosion
A commonly applied rule to explain erosion of sandy shores in response to sea level rise is
the Bruun Rule (Bruun, 1962). It describes the cross-shore response of a beach to sea level
rise. Bruun states that within the closure zone of the beach (typically the limit of
significant wave-driven sediment transport), the beach will adjust to maintain its equilibrium
profile relative to the still water level. This is achieved by translating the profile landwards
and upwards, with eroded sediments at the landward end of the profile being deposited in the
lower portion of the profile, and raising the bed, maintaining a net sediment balance. The
landward translation is given by geometry, and is expressed as R = SL/(hd+f), Where S is the
amount of sea level rise, L is the active length of the profile, hd is the closure depth, and
f is the freeboard. R is typically of the order 50-100 times the magnitude of S.
Figure 2: Effect of wave action on a shore.
The Bruun rule has several limitations. Firstly, the rule does not account for
longshore interactions, and secondly, the rule assumes the wave climate is steady and hence
the equilibrium profile remains the same - simply translated landwards and upwards with the
rise in mean sea level. Such limitations should be considered when the Bruun rule is applied.
Onshore winds will pile up water onto a coastline, and offshore winds will do the opposite.
Changes in atmospheric pressure also produce changes in sea level (lower atmospheric pressure
leading to higher sea levels), so the effect of a severe storm (very low atmospheric pressure)
with strong onshore winds can lead to very high coastal sea levels (Storm Surges) with, at
times, severe coastal damage, especially when the large waves produced by the strong winds
are added! Less extreme weather also produces less extreme changes in sea level. These storm
surges can have devastating impacts - over 500,000 people were killed in the 1970 storm surge
in Bangladesh. Since 1700 there have been 23 storm surges in Bangladesh that have (each) killed at
least 10,000 people. The
storm surge and surface waves generated by Hurricane Katrina had
a devastating effect on New Orleans in August 2005, with over 1000 people killed and over
$US100Billion in damages.
Storm surges are long period waves with a wavelength of several tens of kilometres that
are generated by the winds and falling pressure associated with severe weather events. There
are two ways in which the wind can act to elevate coastal sea levels. The first, called 'wind
setup' (Figure 3a), is effectively the pushing of the water against the coast, leading to
cyclone as it approaches the coast. Wind setup can also occur within semi-enclosed bays when
the wind has been blowing steadily from a particular direction over a number of hours. The
second way in which winds can generate elevated sea levels at the coast is via 'current setup'
(Figure 3b). When the wind blows in a largely coast parallel direction, it induces coast
parallel flow. If these currents are sustained over periods of a day or more, the rotation
of the earth (the Coriolis effect) causes the currents to become deflected to the left (right)
of the direction of flow in the southern (northern) hemisphere. If a coastal barrier blocks
the deflected flow, elevated coastal sea levels will result. This process however, is more
important in the mid-latitudes where the Coriolis effect is stronger and storm depressions
are of larger size, are generally less intense and are of longer duration than tropical
Figure 3: The main dynamic processes leading to increased sea
levels at the coast due to the
The falling atmospheric pressure at the centre of a tropical cyclone can also increase
sea levels by acting like a suction on the ocean surface. This process is known as the
inverted or inverse barometer effect. The rate of sea level rise is approximately 1 cm per
hPa fall in pressure.
Coastal geometry also influences the severity of storm surges experienced at the coast.
Storm surges are amplified by wide continental shelves. This is because the currents caused
by the wind are slowed down by the friction created by the ocean floor over the shallower shelf
region and this in turn causes the water depth to increase. Other topographic features such
as headlands can also either amplify or protect a coastal region from storm surge depending
on the prevailing wind direction in relation to the headland.
The storm conditions that create storm surges will also create large wind-generated waves.
In some situations and locations, such as coastlines and islands with little or no continental
shelf, waves may produce a greater impact than the storm surge in a severe storm. Unlike storm
surges which are generated by storm systems in the immediate vicinity, the sources of waves
include not only local weather conditions but also distant storms. This is because waves can
travel long distances in deep water from their point of origin with little loss of energy. Such
waves are called swell.
Extreme sea levels - causes
Extreme sea levels can be caused by high tides, particularly spring high tides, storm surges
and wave breaking processes at the coast. The interaction of these various processes is illustrated
in Figure 4.
Figure 4: Contributions to coastal sea level from tides, storm surge and wave
Some earthquakes cause a very rapid vertical movement of the ocean floor and in these
cases generate tsunamis (tidal waves), such as the 2004 Boxing Day Tsunami in the Indian Ocean.
Tsunamis travel at speeds of about 200 metres per second in the deep ocean (about 700 kilometres
per hour), taking several hours to cross an ocean basin allowing time for tsunamis warnings to
be issued. Tsunami warning systems are
being strengthened since the 2004 Boxing Day Tsunami.
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Hemer, M.A., J.A. Church and J.R. Hunter (2007), Waves and climate change on the Australian
coast. Journal of Coastal Research. SI, 50, 432-437.
Ranasinghe, R., R. McLoughlin, A. Short and G. Symonds (2004), The Southern Oscillation Index,
wave climate, and beach rotation. Marine Geology, 204, 273-287.
Slott, J.M., A.B. Murray, A.D. Ashton and T.J. Crowley (2006), Coastline responses to
changing storm patterns. Geophysical Research Letters, 33,
Sterl, A. and S. Caires (2005), Climatology, Variability and Extrema of Ocean Waves - The
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Woolf. D.K., P.G. Challenor and P.D. Cotton (2002), Variability and predictibility of the
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