Bob Blog 14 Aug

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

Compiled Sunday 14 August 2022

Tides of New Zealand

Ahh…when we start moving out of the depths of winter my first reminder that summer and brighter times are coming is the annual arrival of the new edition of the NZ Nautical Almanac (thanks to Boat Books). It is online now (and the astronomical data is at http://www.linz.govt.nz/sea/nautical-information/astronomical-information) but serious boats have a place on board for their own hard copy.

When the almanac arrives, it reminds me also of the time back in the early 1970s when I was doing a course in oceanography as part of my meteorological degree at Victoria University in Wellington and we were given as homework the task of studying the tidal movement in Wellington harbour as an example of the application of Coriolis forcing (things that move around the southern hemisphere feel a tug to the left).

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J. W. Brodie’s analysis of Wellington harbour tidal flow in 1958

See //doi.org/10.1080/00288306.1958.10423176

What impressed me at the time was that the “tug to the left” from Coriolis should cause the water in an oceanic basin in the Southern Hemisphere to spin clockwise, but the tides around the NZ Coast rotate counter-clockwise.

The tide at Onehunga is around three and a half hours after that at Westhaven. If ever a canal is built between these two harbours, it could be a perpetual power station but would need locks for vessels.

The combined attraction of the Moon and the Sun generates ocean tides and Earth. Calculating tides is not easy. Distance and angle of separation of the Sun and Moon keep changing, and the shape and size of ocean basins complicate the calculation, and these keep changing too. Each component has a cycle with its own amplitude and period (a sine wave) and these are each calculated then added back together. The main component in our area is M2 (the moon-twice-a-day component) due to the attraction of a ‘virtual’ Moon placed on a perfectly circular orbit in the Earth’s equatorial plane. It has two high and two low tides per day (semi-diurnal wave). The K1 wave, with a diurnal period, reflects declination variations of the Moon and Sun. In certain areas, a hundred components have to be added together to obtain a precise forecast.

Here is what M2 looks like (note that it repeats in 12hr and 25mins)

/en.wikipedia.org/wiki/File:Global_surface_elevation_of_M2_ocean_tide.webm

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There is a Tidal nodal point or amphidromic point (Greek for around running) over the mainland near central New Zealand

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The nearest bay to this point as been Oteranga Bay, but now that the centre of NZ has shifted since the Kaikoura earthquake in 2016, I wonder if this too has changed?

Maupiha’a (also known as Mopelia) and Maupiti in the Society Islands west of Borabora in western French Polynesia, are very close to a Tidal null point or amphidromic point.

Although the lunar component of their tide is nil, there is still a solar component, so slack water is around noon each day. There are also ever-changing weather components. Whenever the lagoon water drains out to sea it does so in a rush. These are NARROW PASSAGES and If a STRONG CURRENT encounters an opposing wind or swell then a vigorous short chop results.

Maupiti’s lagoon passage is on the south side of the island and may be unsafe for several days whenever there a southerly swell.

Maupiha’a has a pass on its northwestern side. The pass can have up to a 9 knot continual outflowing current with a wall of breakers across the entrance.  In 1917 the German raider Count Felix von Luckner Seeadler’s three masted schooner Seeadler was wrecked in these breakers.

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Here is a link to a video of SV KOLOHE exiting from the passage on NW side of Maupiha’a n a day when the surrounding swells where 3 to 3.5 significant metres. The current was an estimated 5kt and encountered waves up to 2m.

See drive.google.com/open?id=1a8TWsADTKPYChc9o7YYneyEVjLUcScJG

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TROPICS

The latest cyclone activity report is at tropic.ssec.wisc.edu and Tropical Cyclone Potential is from www.ssd.noaa.gov/PS/TROP/TCFP/index.html

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Cyclone Ten is off the Mexican west coast, going NW. MEARI is fading near Korea.

WEATHER ZONES

Weather Zones Mid-week GFS model showing isobars, winds, waves (purple), rain (red), STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) and CAPE (pink)

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CAPE mid-week as seen by ECMWF and GFS from Predictwind.com

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Different models seem to produce different values.

SPCZ=South Pacific ConvL1ergence zone and STR (Sub tropical ridge).

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Rain Accumulation next five days from windy.com

The SPCZ stretches from PNG to New Caledonia and from Vanuatu to Samoa. A convergence zone/Trough from Samoa to Southern Cooks is expected to travel northeast to FP by mid-week and then go westwards and form a low over the Niue area by weekend. Avoid.

HIGHS and LOWS

High H1 east of Northland expected to travel southeast along 35 to 40S to around 160W and then intensify and become slow-moving. Squash zone expected to north of H1 from mid-week.

L1 near Tasmania expected to travel southeast across the south Tasman Sea and fade away by mid-week.

L2 is expected to form over New Caledonia on Monday and travel southeast crossing NZ on Tuesday to Thursday followed by another low L3 on Friday

H2 is expected to standby over eastern Australia until after L3 and then cross the Tasman Sea this weekend.

Not a good week for departing NZ

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

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Contact is bob@metbob.com or txt 64277762212

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