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19 Factsheet 8: Long-term sea-level fluctuations

Long-term fluctuations
Long-term fluctuations could alter the mean
level of the sea by up to ±0.25 m when all
longer-period sea-level cycles of at least six
months are included.  However, such a
combination would occur infrequently and
last for only a short period of time (ie, mean
annual sea level would not fluctuate by as
much).

Longer-term fluctuations (lasting at least a month) in the mean level of the sea are important components when assessing inundation and erosion hazards.  These fluctuations are typically related to:

  • the annual heating and cooling cycle caused by the influence of the sun on the ocean.  Mean sea levels tend to be higher in late summer and autumn and, over a year, can fluctuate around ± 0.04 m on average, but up to ± 0.08 m in some years
  • interannual 2–4 year El Niño-Southern Oscillation (ENSO) cycles.  Mean level of the sea is depressed during El Niño phases, and is higher during La Niña phases, with fluctuations of up to ± 0.12 m on both east and west coasts of the upper North Island.  An analysis of the magnitude of fluctuations further south is currently underway
  • interdecadal 20–30 year Interdecadal Pacific Oscillation (IPO) cycles.  The rate of sea-level rise tends to be higher during negative phases of IPO and tends to flatten out during the positive phases of IPO.  The IPO facilitates sea-level fluctuations of up to ± 0.05 m.  The IPO has been in a negative phase since about 1999.

 

Variations in mean annual sea level (black line) from the Port of Auckland sea-
level gauge.  The line suggests that long-term fluctuations since 1899 have
resulted in a variation in mean annual sea level of about 0.17 m relative to the
linear trend in sea-level rise (straight red line).

The influence of IPO can also be seen in the Port of Auckland data, with higher
sea levels generally occurring during negative phases (smoothed red line
showing the 20-year moving average).
Variations in mean annual sea level

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