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Sea Levels: South-West Pacific

There is concern that sea level rise might threaten the existence of some small island communities. Since the start of the 1990s the Australian Meteorological Bureau has run the South Pacific Sea Level & Climate Monitoring Project. Every month the tidal centre publishes an update on their work. Figures 1-12 show the 12 island groups involved in the study (see below):

In these graphs we present values of monthly average sea level relative to a local datum for each of the sites. The numbers were obtained by digitising graphs of the sea levels as the reports do not give data values. We estimate that the error in any individual month is of the order 5 to 10 mm, not significant relative to level changes of the order of 100s of millimetres. We have also plotted values of the Southern Oscillation Index (SOI). The index used by the Bureau of Meteorology is based on ten times the monthly anomaly of the difference in mean sea level pressure between Tahiti and Darwin, divided by the long-term standard deviation of that difference for the relevant month. For both sea levels and the SOI we also plot 12 month moving averages.

What all of these graphs show is that there are significant variations in sea level and that these are very much influenced by the atmospheric pressure, as indicated the SOI. For many of the measuring sites the lowest levels occurred in 1998 and the large pressure anomaly in 1997. This is also a feature, seen most clearly in the moving average plots; the effect the SOI on sea levels is only seen after a period of 6 to 9 months. The same anomaly was also related to the El Nino of 1997 which was followed by a world-wide temperature increase. The graphs also show that for the period of record sea levels are tending to increase, though the SOI has also tended to increase over this period and may be responsible for at least part of the level increase.

The Bureau of Meteorology estimates the rate of rise to be in the range of 3.3 to 9.4 mm/year for the islands except for the Federal States of Micronesia which has a short record. They advise caution in interpreting these figures which are based (in terms appropriate for sea level change) on short records.

One of the stations is for Papua New Guinea where there is record for Rabual in Papua New Guinea in the file prints.rlr from the Proudman Oceanographic Laboratory (POL). Figure 13 shows data for the 1975 to 2009 from two records, the above and the one from POL. For a few months there were missing values but the period of missing data was never more than one month and this case the average of the previous and following month was used. The values of the early record were adjusted to have the same average as the later record for the 21 month period of overlap. This graph confirms the importance of variations in atmospheric pressure on sea level. It also confirms the general tendency of sea levels to rise.

Data Sources

Data for the 12 South-West Pacific were extracted from:

http://www.bom.gov.au/ntc/IDO60101/IDO60101.201110.pdf. The file was downloaded in November 2011 and has data up to October 2011.

The data for the Rabual were extracted from the file “prints.rlr” downloaded from www.pol.ac.uk/psmsl/pub/prints.rlr .

Update: Dec 2011