Article in "Hamburger Abendblatt": “Deep Waves” in Climate Models

01.10.2014

News from Climate Science: Once a month, climate researchers report on their latest findings in the newspaper "Hamburger Abendblatt." Prof. Detlef Stammer is an oceanographer, Co-Speaker of CliSAP and Director of the Center for Earth System Research and Sustainability (CEN) at Universität Hamburg.

Even if it looks like a quiet sea - oceans are actually never at rest.
Prof. Detlef Stammer is an oceanographer, Co-Speaker of CliSAP and Director of the Center for Earth System Research and Sustainability (CEN) at Universität Hamburg

If you look at a quiet sea on a warm summer evening, it’s hard to imagine that our oceans are actually never at rest. Three factors keep them in constant motion: the wind that the water produces on its surface; differing temperatures and levels of salinity in the ocean, as a result of which the colder, heavier water masses sink deeper and warmer water takes their place; and lastly the tides, which are created because the Earth’s rotation and the gravitational forces produced by the sun and moon keep the seas in motion.

All three movement patterns help to shape our climate, as ocean currents are important “conveyor belts” for heat and energy. For example, the Gulf Steam moves heat from the Gulf of Mexico to Europe’s North Sea, ensuring we enjoy mild temperatures. Unlike the wind, which merely mixes up the upper layers of the seawater, or sinking masses of colder water, which can only be found in certain regions, tidal movements affect the ocean as a whole – and they can be felt all the way down, to ocean depths of several thousand meters.

The nature of the ocean floor is what makes it so difficult to predict these movements for the purposes of climate models – our oceans not only display different depths, but are also riddled with countless trenches, ridges and slopes. When a tidal wave encounters one of these obstacles or simply sweeps along the ocean floor, it is slowed, divided, redirected or in some cases even accelerated – e.g., when the water has to pass through a narrow chasm.

The range and energy of these “deep waves” and their interaction with the ocean floor represent important parameters for climate research, as they have a considerable influence on currents and heat transfer in the ocean – and the climate in the process. At the same time, they are also connected to rising sea levels. The very first climate model ever developed in Hamburg (in the 1950s) was a tide model for the North Sea. Nevertheless it wasn’t until the 21st century that international research groups were able to develop models to simulate these processes on a global scale.

At the Center for Earth System Research and Sustainability we have now for the first time systematically compared and assessed these global tide models. The result: global maps that show where and how different tidal patterns overlap, and where the energy input is especially high or low. In a second step, we used our HAMTIDE model to calculate the associated energy dissipation. The results tell us how tidal energy is distributed across the ocean, which routes it follows, and whether climate-relevant ocean currents are tending to grow stronger or weaker.

We’ve come a long way since the 1970s: with very few exceptions, we now have comprehensive data on the topography of the ocean floor, coupled with detailed tidal observations on the surface. At the same time, today’s models are sufficiently powerful to combine these two types of data, allowing them to represent the complex patterns of energy distribution and movement in the ocean better and better. If we can successfully integrate these findings into global climate calculations, it will mean a major step forward.
 

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