If you’re planning to spend any time out on the water this summer, make sure you check the LINZ tide tables first.
Whether you’re launching a boat from a ramp or accessing the rocky foreshore for fishing, it will be important to know the times for high and low water on the day.
In addition, keep an eye on the weather forecast. The height of the tide can be significantly impacted by meteorological effects such as barometric pressure, wind and storm surges.
Spring tides over the holiday period
Spring tides generally occur a day or two after full and new moon, a delay which is caused by friction between the ocean and seabed in shallow waters. Following the Full Moon on 27 December, spring tides will be experienced at most coastal locations around New Zealand through to 30 December. However, the first spring tides of the new year, over the 14 -16 January period, will result in an even greater tidal range. This is because the New Moon on 11 January occurs close to lunar perigee (the Moon’s closest point to Earth) on 13 January, amplifying its gravitational pull. Expect neap tides from 5 to 7 January.
Tidal bulge around New Zealand - it’s always high tide somewhere
At any one given time, there’s always a high tide (and a low tide) occurring somewhere around the New Zealand coast. The tidal bulge takes approximately 13 hours to travel in an anti-clockwise direction around the North and South Islands. This pattern of flow is the reason why the state of the tides in Auckland’s Waitemata and Manukau Harbours are never the same despite their close proximity. The tide has to travel from Auckland around Northland to reach the Manukau Harbour – a journey that takes about 3 1⁄2 hours on average.
Beware of fast moving tidal streams and tide rips
The effect of the tidal bulge also means that one entrance to Cook Strait can be experiencing high tide, while it’s close to low tide at the other. This difference in water level drives very fast tidal currents. The streams often run in one direction for 8 to 10 hours, but cases have been reported of them going for 18 hours or more.
In the vicinity of Karori Rock and Cape Terawhiti, rates of up to 7 knots are frequently experienced, but as a rule do not last for more than an hour. Small vessels are warned to keep well clear of tide rips as they may lose steerage way and, in extreme cases, capsize. Tidal streams can be very strong through the east entrance to Tory Channel / Kura te Au, reaching rates of 5 knots.
Meanwhile in Te Aumiti / French Pass, the flood and ebb streams attain rates of 5 to 7 knots. Slack water lasts for only 20 minutes or so. Mariners are cautioned against taking a vessel through Te Aumiti / French Pass against the stream. The extraordinary irregularity of the sea floor, together with the narrowness of the channel, creates many eddies in this area. Care must be taken to prevent a vessel being swung round by the eddies.
Winds and storm surges
The effect of the wind on sea level, and therefore on tidal heights and times, is variable and depends largely on the topography of the area. A strong wind blowing onshore will pile up the water and cause the sea level to be higher than predicted, while winds blowing off the land will have the reverse effect.
The combination of wind setup and the inverted barometer effect associated with storms can create a pronounced increase in sea level, often called a storm surge. A long surface wave travelling with the storm depression can further exaggerate this sea level increase.
Barometric pressure affects sea levels
This one is for our hard core weather techies, tide predictions are computed for a standard barometric pressure of 1013 hectopascals (hPa) or millibars. A difference of 1hPa from the average can cause a difference in height of 1cm. Low barometric pressure will allow the sea level to rise, while high pressure tends to depress it.
However, the water level does not adjust itself immediately to a change of pressure, but responds to the average change over a considerable area. While changes in sea level due to barometric pressure alone seldom exceed 30 cm, LINZ has recorded discrepancies of up to 60cm with the passing of very low pressure storm systems.