Extreme Tides
The below charts summarise annual extreme tides recorded at Auckland and Taranaki for the years I've been able to obtain the relevant observed tide gauge data, i.e. 1999-2020 for Auckland and 1955-2019 for Taranaki. Both the highest observed and predicted tides are shown for each year. Note these don't always agree for the reasons given below. Also shown is the frequency (%) of observed high tides that exceeded Highest Astronomical Tide (HAT) each year.
Predicted tides never exceed HAT which is as per the definition. Predictions are made for normal meteorological conditions. Although predicted and their corresponding observed tide times generally agree well, the observed heights can differ from their corresponding predicted heights as a result of a deviation from normal meteorological conditions, such as strong or prolonged winds, and/or unusually high or low barometric pressure.
In the charts below, the prevalence of observed high tides exceeding HAT generally occurs around the years when the highest annual predicted tide is close to or equals HAT, which last occurred in 2015 and will next do so again around 2033. This is due to the regular wobble in the Moon’s orbit that takes 18.6 years to complete. In half of the Moon’s 18.6-year cycle, Earth’s regular daily tides are suppressed: High tides are lower than normal, and low tides are higher than normal. In the other half of the cycle, tides are amplified: High tides get higher, and low tides get lower.
Global sea level rise pushes high tides in only one direction – higher. So half of the 18.6-year lunar cycle counteracts the effect of sea level rise on high tides, and the other half increases the effect. We can therefore expect an increased probability of High-tide floods – also called nuisance floods or sunny day floods to next occur in approximately the 9.3 years centred around 2033.
High tide times in the below charts are in New Zealand Standard Time (NZST) and are therefore not corrected for Daylight Saving.
I'm currently working on similar charts for Wellington, Lyttelton and Dunedin and will post these below when complete.
The charts on this page are best viewed in Landscape Mode if viewing from a mobile device.
Documentation
I produced the above Auckland predictions from 140 harmonic constituents, resulting from a continuous analysis in uTide for MATLAB of observed Auckland tide gauge data between 2001-2020. While the Taranaki predictions are from 114 harmonic constituents, resulting from a continuous analysis in uTide for MATLAB of observed Port Taranaki tide gauge data between 1955-2019.
Given most of the Auckland observed tide gauge data kindly supplied by Ports of Auckland Ltd was only available as hourly readings, I used Python to upsample each tide gauge record by interpolating the 59 minutes between each hourly reading. With the tide gauge record in one minute readings I then used Python to extract the highest tide of each year and also the frequency of high tides that exceeded Highest Astronomical Tide (HAT). The same process was used to derive the observed Taranaki high tide information.
For 2019 and 2020 Ports of Auckland Ltd kindly supplied the observed tide gauge data in one minute readings. I used these to compare against the one minute readings interpolated by Python using the above process and found the residuals (observed-interpolated) to have both a mean and standard deviation of approximately 7 mm. Unfortunately I have not been able to source any Taranaki observed tide gauge data at one minute intervals to conduct the same process.
Port Taranaki hourly data for 1955-2019 is from Caldwell, P. C., M. A. Merrifield, P. R. Thompson (2015), Sea level measured by tide gauges from global oceans - the Joint Archive for Sea Level holdings (NCEI Accession 0019568), Version 5.5, NOAA National Centers for Environmental Information, Dataset, doi:10.7289/V5V40S7W and downloaded November 2020 from the University of Hawaii Sea Level Center.