Tide tables have a reputation for being rock solid. And to be honest, they deserve it.
The rise and fall of the sea is driven mainly by the Moon and the Sun, whose movements can be calculated centuries ahead.
In truth and theory, tidal predictions for the year 3,000 could be printed today and still be right. How useful would they be for surfers and sailors of today? Maybe not that much.
Yet anyone who has watched waves flood a seaside car park earlier than expected, or seen a "small" high tide spill over a seawall, knows something else is going on.
The ocean is always in tune with the sky, the wind, and the shape of the coast.
The thing is, those influences are harder to pin down, and they can quietly rewrite what the tide was supposed to do.
Could they influence a planned weekend surf session? Definitely. Here's how.
Wind that pushes the ocean sideways
Wind does more than ruffle or caress the surface of the oceans and water in general. When it blows hard and long enough, it physically moves water.
So, a strong onshore wind pushes seawater toward the land and raises the local water level in a process known as wind setup.
It's an effect that is especially noticeable along open coastlines and shallow continental shelves.
In extreme cases, such as during gale-force winds combined with large swell, the water level at high tide can be several meters higher than predicted by tide tables alone.
Episodes like this could change the expected behavior of a surf break, for instance, like Nazaré's Praia do Norte.
Coastal engineers and oceanographers have documented cases where wind and swell together added multiple meters to the expected height of the sea.
Can you imagine a wind so powerful that it is capable of competing with the Moon's gravitational pull?
The pragmatic consequence of that on surfing is that waves could break farther inland than usual, sandbars could be dramatically moved or see their shape altered, and currents could feel stronger than forecast.
If you live by the sea, it wouldn't be surprising to witness flooded streets on what should have been an ordinary high tide.
Air pressure that lets the sea rise
But there is another meteorological phenomenon that can affect the precision of tides.
The atmosphere constantly presses down on the ocean and, when that pressure changes, the sea reacts accordingly.
Low atmospheric pressure allows the ocean surface to rise, while high pressure pushes it down. It's like a balloon effect when you press it.
The relationship is simple and well established in hydrostatics. Let's try to simplify it with a simple rule of thumb.
A drop of about 100 millibars in air pressure raises the average sea level by roughly one meter. It's actually considerable.
In other words, it means that a deep low-pressure system at around 950 millibars can "lift" the sea significantly compared to a strong high-pressure system near 1,050 millibars.
The effect is subtle on calm days, but during storms, it becomes relevant.
Also, it's important to note that the ocean does not rise evenly everywhere. Local geography plays a decisive role in how much of that extra water shows up along the coast.
Storm surge and the shape of the coast
So, when low pressure, strong onshore wind, and large swell arrive together, the result is a storm surge, one of the most dangerous and unpredictable tidal disturbances.
We need to stress one thing, though - a storm surge is not a wave. Instead, it is a sustained rise in sea level that can persist for hours.
Low-lying regions are especially vulnerable, and history has made that very clear.
The southern North Sea is often cited as a textbook example. A large storm forming in the north of the basin can send swell southward at the same time the tide is rising.
As the surge moves into the narrowing southern end of the sea, the coastline acts like a funnel, forcing water higher and higher.
The mechanism has threatened parts of the Netherlands and eastern England for centuries, and that's one of the reasons modern defenses exist.
It's a similar phenomenon to Nazaré's underwater canyon.
The UK's Storm Tide Warning Service tracks these surges along the coast, while the Thames Barrier was built to block storm-driven water from flooding central London.
As for the surf, a storm surge can dramatically change a break's behavior, sometimes annihilating shallow reefs or lighting up normally dry sandbars.
Long waves, big swell, and water piling up
A swell does not change the tide itself, but it changes how high the water rises at the shore.
Large waves carry momentum.
And when they reach shallow water, that energy pushes water landward and raises the average water level along the coast, a process known as wave setup.
The event becomes stronger with long-period swell (groundswell), the kind generated by distant storms taking place thousands of miles away from the shore.
During high tide, wave setup itself can be the difference between waves staying on the beach and waves running through beachfront structures.
And now the nearly surreal part.
Coastal flooding often happens on clear days after a storm has passed far offshore. The tide is doing its usual thing. The swell is not.
Climate cycles that tilt the ocean
Some tidal changes unfold over weeks or months, not hours. One of the most important examples is the El Niño Southern Oscillation (ENSO).
During certain El Niño events, a large pool of warm water shifts eastward across the Pacific Ocean in a movement that changes sea level across vast regions.
In some areas, temporary sea-level changes of up to half a meter have been observed and documented by satellite altimeters and tide gauges.
For Pacific coastlines, this can alter tidal ranges, change how waves break, and increase the frequency of coastal flooding even without major storms.
The phenomenon also explains why the same tide chart can behave differently from one winter to the next.
Rivers, rain, and the water that comes from land
Now, this one is pretty straightforward.
Heavy rainfall and river flooding can also raise coastal water levels, particularly near estuaries and deltas. Makes sense, right?
When large volumes of freshwater flow into the sea, they slow the outgoing tide and add to the incoming one.
It's like they are "canceling" the incoming tide.
The effect is obviously localized but real, and it becomes more noticeable during prolonged wet periods.
Surfers will witness murkier water, altered currents, and sandbars that shift faster than usual.
The ground beneath the tide gauge
Last but not least, sometimes the tide looks higher because the land has moved.
Yes, you read it right. Earthquakes can lift or drop entire sections of coastline in seconds.
Gradual land subsidence from groundwater extraction or sediment compaction can have the same effect over decades.
Tide gauges measure sea level relative to land, so when the land sinks, the tide appears to rise.
All these changes are not part of daily tide prediction, but they shape long-term coastal risk and redefine what "normal" high tide really means.
Words by Luís MP | Founder of SurferToday.com
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