Waves
There are several classes of waves:
- Sea Waves (also known as "chop") have periods of a few seconds. They are generated by local winds.
- Swell Waves have periods from 8 up to 20 seconds. They are generated by weather hundreds or perhaps thousands of km away.
- Long Waves have periods greater than 20 seconds. There are several categories of long waves, each with a different generation mechanism:
- Infra Gravity Waves - generated by breaking swell waves.
- Far Infra Gravity Waves - associated with groups of swell waves.
- Rissaga Waves - generated by moving low-pressure systems.
Sea Levels
Sea level has a number of components, each with a different timescale:
- Tides and Tidal Residual - from 4 to 24 hour timescales:
- Tides - astronomical forcing of Earth's waters by Sun and Moon.
- Tidal Residual - difference between forecast and actual tide.
- Storm Surge - from 36 to 384 hours - response to changing atmospheric pressure and wind
- Mean Level of the Sea - 16 days to centuries - response to large scale meteorological forcing
- Available Depth - depth of water available for navigation, taking all of these factors into account
Weather Maps
Weather forecasts in terms of maps of atmospheric pressure at sea level can provide warning of future long wave events. We are looking for low pressure systems coming down out of the tropics and propagating southwards past New Zealand to the east. Such systems generate rissaga waves that can affect Marsden Point even though the local weather is calm.