1. The tide is the forecast tide. It differs slightly from the
tide in the Nautical Almanac because the tidal analysis and forecast
algorithms used here are superior to those used by LINZ.
2. The tidal residual is the difference between the forecast tide and the
tidal band of the measured signal. It contains both the error in forecasting
the tide and any basin-wide seiche with periods between 2 and 36 hours.
3. Port Taranaki has no significant basin-wide seiche, so the tidal residual is
primarily error in tide forecasts. Notice that the tidal residual is usually zero
at high and low tide, and has a maximum on the falling (ebbing) tide and a minimum on the
rising (flooding) tide. This indicates a slight misshape in the tidal signal, with the
rising limb being slightly steeper than the falling limb. Usually,
this is caused by shallow water tides that have not been correctly identified in the
tidal analysis. Shallow water tides are generated by nonlinear interaction of the major
tides as they propagate into shallow water. Nonlinear interaction of physical phenomena
generates what is known as "chaos". Chaos differs from noise, which is random fluctuations.
If we were able to correctly model the ocean, we could forecast the chaotic signal,
but we are not clever enough to do that (yet).
On the other hand, we have no hope of ever forecasting noise.
4. The tidal residual is roughly Gaussian distributed, which means we can use the
standard deviation to estimate the confidence intervals in the forecast tide as follows:
For 95% of the time, the tide is forecast within ±1.96*SD m.
Thus, for example, if SD = 0.050 m, then for 95% of the time shown in the figure,
the forecast tide is within ±0.098 m of the measured tide.
5. The time of Moon's phase and perigee are also shown on the figure
(if they occur in the period). These are important because they determine
when the tidal range will be large and when it will be small.
When the Moon is Full or New, the Sun is lined up with the Moon and both
exert gravitational forces on Earth's waters in the same direction. Thus,
the tide has a larger range just after Full Moon and New Moon. These are called
spring tides. At First and Third Quarter, the Moon and Sun are at right angles
and the tidal range is small. These are called neap tides.
The Moon has an elliptical orbit, meaning that every month it is closer to the Earth
(called lunar perigee) and two weeks later it is futher away (called lunar apogee).
In lunar perigee, the tides have a higher ranger than in lunar apogee.
Every 7 months, lunar perigee occurs when there is a Full or New Moon.
This produces large tides, sometimes known as king tides, but more properly perigean spring tides.
Conversely, when lunar apogee coincides with First or Third Quarter, we get small
tides called apogean neap tides.
CD is chart datum. Everything is expressed in m above it.
It is at approximately LAT (see below).
HAT is highest astronomical tide. It is the highest high tide that will occur between
2005 and 2024.
MHWPS is mean high water, perigean springs. It occurs every 7 months when lunar perigee
coincides with a Full or New Moon.
MHWS is mean high water springs. It occurs every two weeks just after Full or New Moon.
MHWN is mean high water neaps. It occurs every two weeks just after First or Third Quarter.
MHWAN is mean high water, apogean neaps. It occurs every 7 months when lunar apogee
coincides with First or Third Quarter.
MLWAN is mean low water, apogean neaps. It occurs every 7 months when lunar apogee
coincides with First or Third Quarter.
MLWN is mean low water neaps. It occurs every two weeks just after First or Third Quarter.
MLWS is mean low water springs. It occurs every two weeks just after Full or New Moon.
MLWPS is mean low water, perigean springs. It occurs every 7 months when lunar perigee
coincides with a Full or New Moon.
LAT is lowest astronomical tide. It is the lowest low tide that will occur between
2005 and 2024.
MSL is mean sea level. It is the mean level of the sea for the year 2000.
SD is standard deviation. It can be used to estimate the effect of tidal residual on
tide forecasts (see below).