A major component of the monthly variation in sea level is the annual cycle.

The annual cycle is calculated by taking monthly means of MLOS and generating a table like that shown below (for Newton King Wharf at Port Taranaki). The table has months across and years down. At the bottom there are three rows showing the minimum, mean and maximum for each month. To the right there is a column showing the mean for each year. N/A for any month means there are more than 5 days of missing data in the month.

Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Mean
2004 N/A N/A 1.920 1.931 2.019 1.950 1.934 1.969 1.884 1.887 1.889 1.948 1.933
2005 1.875 1.994 2.010 1.927 2.033 1.906 1.940 1.910 1.970 1.888 N/A 1.957 1.946
2006 1.868 1.926 1.948 2.003 1.954 1.908 1.888 1.926 1.859 1.862 N/A N/A 1.914
2007 1.907 1.910 1.967 1.950 1.956 1.974 1.928 N/A N/A N/A N/A N/A 1.942
2008 N/A 2.045 2.006 2.014 1.995 1.959 1.932 1.841 1.736 1.796 1.874 1.975 1.925
2009 1.918 1.897 1.895 1.972 2.010 1.919 1.974 1.891 1.907 1.901 1.835 1.838 1.913
2010 1.874 1.857 1.858 1.902 2.003 1.959 1.914 1.991 1.903 1.792 1.819 1.908 1.898
2011 1.930 1.923 1.941 N/A N/A N/A N/A N/A N/A N/A N/A N/A 1.931
Min 1.868 1.857 1.858 1.902 1.954 1.906 1.888 1.841 1.736 1.792 1.819 1.838 1.855
Mean 1.895 1.936 1.943 1.957 1.996 1.939 1.930 1.921 1.876 1.855 1.854 1.925 1.919
Max 1.930 2.045 2.010 2.014 2.033 1.974 1.974 1.991 1.970 1.901 1.889 1.975 1.976

In the plots that follow, the annual cycle is plotted for each month as a bar between the minimum and maximum, with the mean marked with a cross. The means for each month are joined by a line to indicate the annual cycle.
The extent of each bar indicates the scatter in the data. In some cases, the extent of the bars exceeds the range of the annual cycle. This indicates seasonality has little effect on sea levels at the site.

The plots are updated every month to accommodate both new data and any corrections that have been made to the historical data as a result of data processing.

An alternative way to calculate the annual cycle is to fit the annual and semiannual tides, as a cosine series. A comparison of that method with the method used here has shown there is no significant difference between the results.