Hi
Phil Watson.
Some comments about sea level changes ...
"As you are aware, there are numerous meteorological and oceanographic
processes which affect ocean water levels around the New Zealand mainland
over differing time and spatial scales. Several influences are on timescales
less than a year such as seasonal factors, weather systems and variability
of ocean water properties (temperature, salinity, density, pH, etc). Other
longer term influences such as El Niño Southern Oscillation (up to 10
years), nodal tide (‰18.6 years) and Pacific Decadal Oscillation (20-30
years) will potentially influence ocean water levels over much longer
timeframes. These influencing processes, are generally considered to be
cyclical in nature, that is they oscillate back and forth influencing ocean
water levels but, do not result in a permanent imprint on the position of
mean sea level.
The aforementioned influences on ocean water levels can be quite dynamic and
vary significantly from location to location. The difficulty arises in
attempting to isolate from tide gauge records the comparatively smaller,
omnipresent signal of climate change induced sea level rise from these
larger dynamic influences which are operating over considerably different
time and spatial scales. One technique is to consider ocean water level
records which are sufficiently long that the more dynamic and cyclical
influences can be averaged out. The above-mentioned timescales of relevant
processes provide some guidance on the length of record required to mitigate
their influence on measured ocean water levels.
The National Tidal Centre (Australia) advises that large inter-annual and
inter-decadal sea level fluctuations associated with climate variability
(such as El Niño) can obscure underlying trends of climate change induced
sea level rise where the station record is less than 25 years (NTC, 2008).
Record lengths of at least 60 years are recommended to distinguish between
trends and long-period relative sea level fluctuations in individual records
(Douglas, Kearney and Leatherman, 2001). Similarly, analysis of the long
Fort Denison tide gauge record suggests that the record length may have to
exceed 50-60 years in order to isolate with reasonable confidence, the low
amplitude sea level rise signal from the large dynamic influences on ocean
water level records (De lange, Lord and Watson, 2009).
With tidal data records of sufficient length, there are numerous analytical
filtering and averaging procedures that can then be applied to sufficiently
smooth the dynamic ocean water level influences to reveal the low amplitude
sea level rise (Watson, 2011). Notwithstanding, tide gauge data provides a
measure of the water level ³relative² to a fixed, land-based reference mark.
What is often not known, however, is whether or not the land upon which the
tide gauge is positioned is actually moving. There are in fact many
processes that can contribute to the land moving including solid earth
tides, plate tectonics, glacial isostatic adjustment and localised factors
that contribute predominantly to subsidence (e.g. aquifer extraction,
reclamation and development loadings). In order to determine a rigorous
estimation of mean sea level change, these processes need to be taken into
account (Watson, et al, 2012)."
References
Douglas, B.C.; Kearney, M., and Leatherman, S., (2001). Sea Level Rise.
History and Consequences. Academic Press, San Diego.
National Tidal Centre (NTC, 2008). Australian Mean Sea Level Survey 2008.
Bureau of Meteorology, Australia.
Watson, P.J. (2011). Is There Evidence Yet of Acceleration in Mean Sea Level
Rise Around Mainland Australia. Journal of Coastal Research, Vol 27 (2), pp.
368-377, DOI: 10.2112/JCOASTRES-D-10-00141.1, February.
Watson, P., Commins, R., Janssen, V., McElroy, S., Batman, G., and Connors,
D., (2012). Augmenting the Utility of NSW Longest Tide Gauge Records with
Continuous GNSS Technology. Proceedings of the 21st NSW Coastal Conference,
Kiama, November.
You, J-Z.; Lord, D.B., and Watson, P.J., (2009). Estimation of Relative Mean
Sea Level Rise from Fort Denison Tide Gauge Data. Proceedings of the 19th
Australasian Coastal and Ocean Engineering Conference, Wellington, New
Zealand.