Ocean tides are not simple.
If our planet had no continents,
tides would be hemispheric-sized bulges of water moving westward with the moon and sun.
This animation shows the tides as a complex system of rotating and trapped waves with a mixture of frequencies.
In many oceans we see waves rotating clockwise or anticlockwise,
with small amplitudes in the middle of the ocean and high amplitudes around the boundaries,
especially along the coasts of northwest Europe and Britain.
Waves are trapped and rotating around New Zealand,
causing a high tide on one side of the islands with a simultaneous low tide on the other side.
The Topex/Poseidon and Jason satellite altimeter missions were designed to observe and record this complexity.
Altimeters, on these missions,
acted as flying tide gauges.
After several years collecting data,
researchers could analyze the signals at each ocean location to determine the tidal characteristics.
With that knowledge,
plus near-perfect knowledge of the motion of the sun and moon,
the tide can be predicted at any location and at any time in the future.
Even now,
after three decades of measurements,
we continue to add new details to the picture.
The large, short-scale waves on continental shelves and marginal seas,
as well as polar regions outside the orbits of Topex and Jason,
are still inadequately known.
Scientists expect that SWOT,
the wide-swath altimeter launching soon,
will help improve tidal knowledge in these regions.