A High Sea in Auckland

This year I gave students a lecture about sea level and storm surge plannning in New Zealand. This was partly informed by the Parliamentary Commissioner for Environment's report Rising Seas: Certainty and Uncertainty. It was also informed by the recent South Dunedin flooding incident, and a lesser known one that affected Auckland on a fine Sunday, 23 January 2011.

At the time I took a few pictures in Devonport, and did a post about it.

The urban policy course provided an opportunity to talk with students about the various factors in play that day that caused the kind of coastal inundation recorded in the photos. Students explored various datasets, most of which are readily available.

First off all they checked out weatheronline for rainfall. It's available there for free for Auckland Airport. You can see that it did rain on the 23rd of January - but not a lot - about 15mm. It rained much more heavily around the 28th January, but there was no coastal flooding.

You can find this comprehensive weather information from timeanddate.com. At the bottom are the wind speeds and directions. Unusually the winds are strong South Easterlies until mid-day, around 33 kph. They would have been blowing straight up Waitemata Harbour (but they were also doing that the day before).

This one also come from weatheronline. It shows the atmospheric air pressure over Auckland. You can see that on the Sunday in question it dropped to 995 hectopascals. That's quite low, and has the effect of lifting sea levels, because higher pressures elsewhere push the sea down there, pushing it up where air pressure is lower.

And this is the really interesting graph students wre able to produce from Ports of Auckland tidal charts provided by POAL for the purpose of this course. This graph shows the sea levels at POAL wharves in Waitemata Harbour for the month of January, 2011. There are two high tides (and two low tides) each 24 hours. The sea level recorded at 23rd January is 4.129 metres - and you can see that the "normal" tidal level around that time was about 3.6 metres - a typical peak high tide level in a month.

Thus the sea level in Auckland's Waitemata Harbour was half a metre higher than normal on the 23rd of January 2011. What was the main cause?

Was it the rainfall? If you look at the rainfall graph you will note that heavy rain fell on the 28th January, but the sea level on the 28th January (counting 10 peaks after 23rd January peak) was only 3.1 metres. So, almost certainly not.

Was it the wind? There were strong winds the day before - but not as strong. So the wind would certainly have contributed, by pushing the sea up Waitemata Harbour.

Was it low air pressure? Check out the PCE's report for the physics of this. That report confirms the direct relationship that exists between low air pressure, and elevated sea levels. Often called a "storm surge". This is the name given to elevated sea levels in a storm where winds circulate around an area of low air pressure.

Big storms are associated with very low air pressure, and very elevated sea levels. Hurricane Katrina elevated the sea level at New Orleans by 7 metres. Hurricane Sandy elevated the sea level at New York by 2 metres. The storm that sunk the Wahine in 1968 was recorded as a low pressure weather system of 970 hectopascals (the lowest recorded around NZ) which elevated the sea level at Ports of Tauranga by 88 centimetres and caused widespread coastal flooding - quite apart from sinking the Wahine.

Tropical storm systems are moving further and further south with climate change, and the frequency of their arrival offshore of New Zealand is increasing. It is only a matter of time before air pressure and winds combine to increase the sea level in Auckland's Waitemata Harbour a metre above normal high tide.

Are we ready, what's the plan, and how much warning would we have?