Thanks Nick, I will look that up. Pressure does effect sea-level, via the *inverted barometer effect* decribed [here](http://www.bom.gov.au/oceanography/tides/met_effects.shtml).
"In addition, oceanographic effects such as Southern Ocean Oscillation (El Niño) can produce large scale variations in mean sea level of up to 0.5m with corresponding changes in rate and direction of tidal streams."
I will post an interesting fit to the tidal variation shortly, which involves an exact two-year sinusoidal forcing function. This is starting to make some sense.