Supercomputer shows how climate change affects tides

Six hours straight, six hours on and off…and again, there are few things as predictable as the tides. Under the influence of the gravity of the Moon and to a lesser extent the Sun, sea water rises and falls again. This has been the case since time immemorial. So far: Scientists have discovered that tides are changing, perhaps due to warming ocean waters.

Researchers from University of Bonn Used supercomputer To better study tidal observations between 1993 and 2020. “Tides often mask other, less predictable signals related to, for example, the general ocean circulation or the effects of climate change,” explains researcher Michael Schindleger. “But whether we can extract signs of global warming from ocean observations also depends on the accuracy with which we can model tides, including their potential change over time.” It turns out that this requires 3D models of the supercomputer.

Ocean layers
Scientists estimate that the upper 700 meters of the ocean absorbs about 90 percent of the heat generated by the warming. As this layer of water warms, it expands and becomes less dense, leading to greater variation in water density than in the deeper ocean layers where it remains colder and the water density is greater.

The researchers compared this density variation with two types of tidal currents: barotropic tides, which are essentially the natural ebb and flow of tides, and baroclinic or internal tides, which occur underwater when normal tides move against the current. Think of a ridge, pushing waves of denser water from the depths into less dense surface water.

Ocean stratification
By ocean stratification we mean the natural separation of seawater into different layers, which arises due to the difference in water density. Hot water floats on cold water. The water is heated by the sun, making these different layers more stable.

The transfer of energy
“Ocean warming improves the transfer of energy from barotropic to baroclinic tides, so open ocean tides are now losing a few percent more energy to underwater waves than they were thirty years ago,” explains Schindliger. The question, of course, is how bad this will be and what impact it will have on coastal areas. 3D simulations are essential for this purpose, so that you can also perceive depth. Only then will you get a realistic picture of what is happening in the different layers of the ocean.

Schindliger explains how he first used a two-dimensional ocean model with constant density. “But when I started investigating the causes of ocean tidal changes, and especially the effects of stratification, 3D circulation models became essential.”

Supercomputer gems
It took the researcher at least five years to achieve the required complexity of the model, but it turned out that much more computing power was needed to create truly accurate 3D models. Then came the JUWELS supercomputer from Germany Jülich Supercomputing Center in the picture.

“Since the calculation grid also extends in the vertical direction, we have approximately 300 million grid points to define the relevant variables of pressure, temperature and salinity based on the model equations,” says Schindliger.
“We needed a million computer hours to successfully implement the project. The key was to distribute the task across a large number of computing nodes to make it possible.” Operating times Prevent memory problems.

Small changes, big consequences
Tidal changes at the surface are small so far. This relates to the decline of the coast by about one centimeter over decades. In the deep ocean the difference is smaller. However, it is worth continuing research to see how large the future impact will be on coastal areas, such as in the Gulf of Maine or in northern Australia where tides are more extreme, and even small changes can have serious consequences. to have.

The supercomputer is an important addition to the notes. In this way, researchers can better understand the role of ocean warming in changes in high and low tides.