Bob Blog 14 August

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
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).

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.
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.

An animation of M2 looks like (note that it repeats in 12hr and 25mins) is
at
/en.wikipedia.org/wiki/File:Global_surface_elevation_of_M2_ocean_tide.webm

There is a Tidal nodal point or amphidromic point (Greek for around running)
over the mainland near central New Zealand
The nearest bay to this point has been Oteranga Bay near Cook strait 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 almost 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's three masted
schooner Seeadler was wrecked in these breakers.

Here is a link to a video of SV KOLOHE exiting from the passage on NW side
of Maupiha'a on 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


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

WEATHER ZONES
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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