Tides, Storm Surges and Long Waves at Timaru
The plot below shows two graphs. In the top one, the raw data as retrieved from the sea-level
recorder are plotted (blue), along with the forecast tide (red). The plot also
shows an empty yellow circle for New Moon.
Lunar perigee is indicated by a yellow diamond. In the lower plot, we compare storm surge
(SS) with inverted barometer (IB).
Storm surge is calculated by subtracting the forecast tide from the raw
data to form a residual. The residual
is then filtered to remove residual tide, high frequency oscillations (i.e.,
waves), and long-period fluctuations (annual cycle, ENSO effects, long-term
sea-level rise). Storm surge around New
Zealand is mainly the result of changes in atmospheric pressure. In the deep ocean, far from land, sea level
changes in response to changing pressure by the "inverted barometer" relationship. This says that for every rise in pressure of
1 hPa, there is a 1 cm drop in sea level, and for every fall in pressure by 1
hPa there is a 1 cm rise in sea level.
When storm surge and inverted barometer diverge, it usually means that
shelf waves have propagated in to the region, or there are large winds
offshore. In the case below, we see that the shapes of the two curves are similar,
but the SS curve is displaced upwards. This means that there has been a long-term
(a week or two) rise in sea level, but it is responding to local short-term (a
few days) pressure changes.
The plot below shows 3 graphs. The top one is a plot of the
long waves in the record. The signal has been calculated from raw data from the
recorder sampled at 1-minute intervals. The tide and storm surge have been
stripped away, along with instrument errors and the top plot is the remaining
signal. The middle plot shows the height of the waves shown in the top plot and
the bottom plot is their period. The height is called Hs, the
significant wave height, being the average height (i.e., distance from trough
to crest) of the highest one-third of the waves. Much of the time, the long
waves are just a random signal, characterised by periods from 10 to 30 minutes
and significant heights of 300 mm or less. When a long wave event occurs, the
period will drop below 10 minutes, and the height will rise above 300 mm.
Long waves go unnoticed at the beach because they are completely masked by the breakers. But in a port they can cause serious problems for berthing and loading or unloading because they cause “surging” in big ships. Surging is the forwards and backwards movement of a ship against her moorings. Long wave events occur from time to time (there were 6 in the winter of 2003). They are caused partly by swell waves that have come from hundreds of km away and formed into groups of large waves as they propagate into shallow water, and partly by moving low-pressure systems (depressions). We call such events “rissaga”, a Catalan word used in the Balearic Islands of the Western Mediterranean to describe these waves.
I gratefully acknowledge PrimePort Timaru for allowing me to
use their data.
Last Updated: 18 November 2003
For more information contact: d.goring@mulgor.co.nz