L5: Future Climate Applications of Mixing Parameterisations in Earth-System Models

In phase 2, subproject L5 has focused on applying the consistent IDEMIX closure for wave-driven mixing in two different climate states of the past, the Last Glacial Maximum and the Mid-Cretaceous. Fig. 1 of the „Objectives“ section illustrates how the deep circulation and the sea level changed in these periods compared to today’s climate.

The Last Glacial Maximum (LGM; 19,000-21,000 years before present) is characterized by a large ice sheet cover of northern Europe, America and Asia, a colder climate and approximately 130 m lower sea levels. The latter implies that the continental shelves, where in today’s climate a notable fraction of tidal energy dissipates, were exposed and the tidally driven mixing was shifted toward the open ocean.

The section along 30°W (fig. 1) illustrates ho w IDEMIX increases the vertical diffusivity throughout the water column as already seen in earlier applications for today’s climate. The effect on the Atlantic meridional overturning circulation (AMOC) is to weaken its upper cell while strengthening the lower cell relative to today’s climate (fig. 2). Observational references are strongly limited for past climates. Instead, we need to rely on proxy data from paleo-archives like marine sediments or glacial ice cores. These indicate that the upper cell was indeed weaker during the LGM (e.g., Hesse et al., 2011, Menviel et al., 2012), indicating that IDEMIX improves the simulations. There is ambiguity in the proxy data about the strength of the lower cell, but some (e.g., Negre et al., 2010, Hesse et al., 2011) support the behavior predicted by IDEMIX.

The second time period in Earth’s history to be investigated with the state-of-the-art mixing parameterization IDEMIX is the Cenomanian-Turonian boundary (approximately 94 million years ago), characterized by the warmer climates of the  Mid-Cretaceous. Sea levels were at least 100 m higher than today, which implies that the continental shelves were flooded and the mixing hot spots might have shifted from the open ocean toward the continents, which had a vastly different configuration than today (figs 3).

The Community Earth System Model uses as default parameterization of tidally driven mixing the scheme of Simmons et al, 2004, building on the scaling laws of Jayne and St. Laurent (2001). The recommendation for deep time simulations is to switch it off entirely because several present-day features are hard-coded. Fig. 4 illustrates how these two options compare with the vertical diffusivity modeled by IDEMIX, which similar to present-day (e.g., Olbers & Eden, 2013; Brüggemann et al., 2024) and LGM-conditions increases the vertical diffusivity throughout the water column over rough bottom topography.