New 3D models show how the warming climate affects underwater tides

This article was reviewed according to Science


Changes in the size of the twice-daily M2 tides caused by recent ocean warming. Large simulations with a global three-dimensional ocean model were performed on JUWELS to determine both the change of the baroclinic tide (panel a) and the barotropic tide (panel b). Values ​​are trends of the respective tidal surface amplitude over a 28-year period. Credit: Michael Schindelegger and Lana Opel.

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Changes in the size of the twice-daily M2 tides caused by recent ocean warming. Large simulations with a global three-dimensional ocean model were performed on JUWELS to determine both the change of the baroclinic tide (panel a) and the barotropic tide (panel b). Values ​​are trends of the respective tidal surface amplitude over a 28-year period. Credit: Michael Schindelegger and Lana Opel.

Few things in nature are as predictable as the ocean’s tides. Powered by the gravitational forces of the moon and the sun, these persistent, short-lived and large-scale phenomena are clearly visible in almost all types of oceanographic and satellite observations. They also directly influence the pace of life of millions of people and countless ecosystems.

But recently, researchers have noticed subtle changes in surface tide measurements that don’t coincide with changes in the moon’s and sun’s gravity. Instead, the collected data and theory indicate that a warming ocean surface could be behind these observations.

To investigate these phenomena, Dr. Michael Schindelegger from the University of Bonn used supercomputing resources from the Jülich Supercomputing Center (JSC) to better understand observational data collected between 1993 and 2020, improving the accuracy of three-dimensional (3D) ocean circulation. models in the process.

The research has been published in the journal Communication Earth & Environment.

“Tides often mask other potentially interesting and less predictable signals related to, for example, ocean general circulation or the effects of climate change,” Schindelegger explains. “Retrieving climate signals from oceanographic observations also depends on the accuracy with which we can model tides, including their potential changes over time.”

Internal flows add complexity

Scientists estimate that the upper 700 meters of the ocean absorbs about 90% of the excess heat trapped in the warming climate system. As this zone of the ocean warms, it also expands and becomes less dense, leading to a greater contrast in water density compared to lower levels of the ocean that remain cooler and denser.

Specifically, Schindelegger and his colleagues investigate the interactive relationship between a warming climate, ocean stratification as a measure of density contrast, and two types of tidal currents: barotropic tides, which refer to the periodic movement of ocean currents related to gravitational forces; and baroclinic or internal tides, which occur when barotropic tides flow against underwater topography, such as a ridge, causing waves of denser water from the depths to push upward into less dense surface water.

“Ocean warming improves the energy transfer from barotropic to baroclinic tides, so that open ocean tides now lose a few percent more tidal energy to internal waves than they did 30 years ago,” Schindelegger explains. To assess the severity of these changes and predict their impact on coastal areas, simulations have become an essential tool.

Observational data and modeling must work together

Observing and modeling ocean tides is nothing new, and new data becomes available to work with every hour of the day. However, data collected near the coast can suffer from “noise” and errors, while computer models are always simplified representations of real-world processes. This is why, according to Schindelegger, it is imperative to consider both observational data and models when testing for tidal changes.

Furthermore, considering tides in a more realistic, layered ocean – including these baroclinic tides – means that established 2D ocean models would need to be extended to include depth as a third dimension and have higher horizontal resolution to achieve useful accuracy.

“Initial modeling attempts were limited to a single-layer, constant-density ocean model, which I could even run on one CPU,” says Schindelegger. “But when I started investigating the causes of ocean tide changes, especially the effects of stratification, general 3D circulation models became essential.”

Schindelegger says he spent about five years gradually adding complexity to the model, but it became clear that more computing power would be needed to achieve the necessary resolution for accurate 3D models. For this reason, Schindelegger and his colleagues turned to JSC’s supercomputer, JUWELS.

“Since the calculation grid also extends in the vertical direction, we have approximately 300 million grid points to diagnose the relevant variables of pressure, temperature and salinity based on the model’s equations,” says Schindelegger.

“We needed a million core hours to successfully execute the project. Distributing the task across a large number of computing nodes was the key to achieving feasible runtimes and avoiding memory issues. The resources available on JUWELS provided the necessary basis for these types of applications.”

Predicting future tides

Schindelegger says that while these surface tidal changes have been subtle so far — a drop of about an inch over decades on the coast, and even less in the deep ocean — it’s still worth continuing to improve of the 3D model until it can predict using predictions. reasonable accuracy as to how these changes in ocean stratification will affect coastal areas in the future. Especially for places like the Gulf of Maine or northern Australia, where tides are pronounced and involve complex underwater topography, even these small changes can have significant impacts.

With continued access to supercomputing resources, Schindelegger and his collaborators will deploy a powerful tool to complement studying observational data. Taken together, these two research methods will help researchers in the geosciences better understand the role a warming ocean plays on the tides and their role in the climate system.

More information:
Lana Opel et al., A likely role for stratification in long-term changes of global ocean tides, Communication Earth & Environment (2024). DOI: 10.1038/s43247-024-01432-5

Brought to you by Gauss Center for Supercomputing

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