The Inverse Barometer Effect
Most surfers know of the daily tidal cycles and the effects it has on the surf.
A fuller tide will generally result in slower, slopey and more forgiving waves, while a lower tide, sharper steeper more abrupt waves. Along with this, some reefs/points and banks only work on high tides, and others on low.
Besides the natural daily variation in tide due to the gravitational effects of the moon, sun and other planets, there's one more lesser known but major contributing factor to the current sea level.
That being the atmospheric air pressure.
The atmosphere is constantly exerting pressure down on the Earth and ocean, with one cubic meter of air at sea level weighing about one kilogram.
The constantly changing weather along with high and low pressure cells moving across us has a greater effect on the tidal range than you'd first think.
For every 10hPa change in surface air pressure, a change of 10cm is seen at sea level.
So when low pressure systems move over a region, the sea level rises by a relative amount, while high pressure systems push down on the ocean, creating a drop in sea level. This is called the inverse barometer effect, as the higher the pressure, the lower the sea level, and vice versa.
It should be noted that the sea level doesn't change instantaneously, but more so responds to the average change in pressure over a larger area.
In general changes in surface pressure are in the vicinity of 20-30hPa, and in turn the sea level doesn't change more than 20-30cm, but if you combine the inverse barometer effect with wind setup (discussed further below) created by strong onshore winds, you can get coastal inundation.
Now, global weather models assume an average Mean Sea Level Pressure of 1013hPa, so when the pressure varies from this we see the inverse barometer effect modifying the actual tidal heights and this modification is usually noted as a residual.
All around the country, tidal observation sites show the forecast tide heights and the residual, a useful tool for boaties and savvy surfers trying to navigate a shallow bar crossing or get your favourite reef on.
This data is invaluable for those fickle spots that need a certain tidal range of height to break or be surfable.
Say for example you need at least 2m of tide to surf a particular reef. If the broader synoptic pressure is around 1030hPa, then the inverse barometer effect can lower the sea level down to around 1.83m (17hpa difference in pressure = 17cm drop in water level). This would potentially affect the surfability of the reef.
For Queensland the tidal observations and residuals can be accessed here.
The current Gold Coast observations are shown below.
We can see there is a slight negative tide residual, and this is because the current air pressure over the Gold Coast is 1020hPa (7 above the standard 1013hPa) resulting in the actual sea level being 7cm lower than the predicted tide.
There is one more factor that effects the sea level and that's the wind. If the coast is under a persistent strong onshore wind, increased sea levels are observed and this is called wind setup. The opposite is seen with strong offshore winds, while cross-shore breezes can set in motion a Coastally-Trapped wave, causing the sea level to vary as it moves along the coast.