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Anthony Sharwood, 14 May 2024, 3:54 AM UTC

Why such low tides on our southern coastline?

Why such low tides on our southern coastline?

When Victorians have looked up this week, they've seen the Aurora Australis in all its sky-blazing glory. But when Victorians who live near the coast look down, they might have noticed something unusual on the shoreline: extremely low tides.

Tides have been lower than usual most days for at least a month now across the coastline of Victoria, South Australia and further afield. For that, we can thank unusually strong and persistent high pressure systems.

Last Friday, Melbourne-based climatologist Andrew Watkins posted an image of the shoreline of Port Phillip Bay looking very exposed at low tide.

As Watkins explained, the sea level drops when surface-level air pressure is high. That's because high air pressure at the surface exerts force on the water, pushing it elsewhere.

High-pressure systems also tend to bring calm or light winds to coastal regions, leading to reduced wave action and less mixing of ocean waters near the coast. This can result in a slight decrease in tide heights due to less wind-driven water being pushed toward the shore.

Why is this happening now?

Over the last month or so, unrelenting high pressure systems centred south of the mainland have dominated Australia's weather.

The chart below shows the sea level pressure anomaly (difference from the average) in just one recent week from April 28 to May 5. The areas where the air pressure was much higher than usual are shown in red (the biggest difference), orange, yellow and green.

Source: NOAA / PSL

  • High pressure = low tides, that's the simple equation.
  • You're generally talking about 1 cm of sea level for every 1 hectopascal of pressure.
  • So when air pressure is 10-20 hectopascals above normal, that's a 10-20 cm reduction in sea level, which is particularly noticeable at low tide.

The effect is not easily noticeable everywhere. But when you have a gently sloping shoreline with sandy or muddy areas, anyone familiar with the coast will definitely notice the difference.

You can also see the effect illustrated with data, as on the chart below for Port Welshpool on Victoria's South Gippsland coast.

Source: Gippslandports.

See how the red line (actual tide level) is dipping below the blue line (predicted tide level)? That's our old mate high pressure at work.

Port Welshpool is around three hours SE of Melbourne, but that's close enough to compare air pressure, and if you check out the air pressure in Melbourne this Tuesday, you'll see it peaked at 1029 hectopascals – a relatively high reading – just before midday.

  • To date in May 2024, Melbourne's mean sea level pressure has averaged 1030 at 9 am and 1027.5 at 3 pm.
  • In May 2023, it was much lower at 1022.3 at 9 am and 1020.4 at 3 pm.
  • May 2023 was also much rainier than May 2024 to date in Melbourne, which again fits the pattern, as rain tends to accompany low pressure systems.

Image: The long jetty at Port Welshpool with Wilsons Promontory in the background. Source: iStock.

Long story short, if you live along Australia's southern coastline and if (like this reporter) you lost your wedding ring in the shore breakers a while back, now might be a good time to break out the metal detector.

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