Effects of strongly eddying oceans on multidecadal climate variability in the Community Earth System Model

Abstract

Abstract. Climate variability on multidecadal timescales appears to be organized in pronounced patterns with clear expressions in sea surface temperature, such as the Atlantic Multidecadal Variability and the Pacific Decadal Oscillation. These patterns are now well studied both in observations and global climate models and are important in the attribution of climate change. Results from CMIP5 models have indicated large biases in these patterns with consequences for ocean heat storage variability and the global mean surface temperature. In this paper, we use two multi-century Community Earth System Model simulations at coarse (1∘) and fine (0.1∘) ocean model horizontal grid spacing to study the effects of the representation of mesoscale ocean flows on major patterns of multidecadal variability. We find that resolving mesoscale ocean flows both improves the characteristics of the modes of variability with respect to observations and increases the amplitude of the heat content variability in the individual ocean basins. In the strongly eddying model, multidecadal variability increases compared to sub-decadal variability. This shift of spectral power is seen in sea surface temperature indices, basin-scale surface heat fluxes, and the global mean surface temperature. This implies that the current CMIP6 model generation, which predominantly does not resolve the ocean mesoscale, may systematically underestimate multidecadal variability.

Increased wintertime European atmospheric blocking frequencies in General Circulation Models with a coupled eddy-permitting ocean

“…Another major outlook from this research is the potential benefit of employing GCMs with an eddy-rich ocean resolution, which can be obtained for nominal resolutions as low as 0.1°x0.1°2 7,28 . Unlike EP ocean grids, which only include mesoscale eddies with broader horizontal extents (100 km), eddy-rich ocean grids incorporate smaller-scale eddies (20 to 100 km), resulting in a significantly higher number of these mesoscale eddies and a better represented mean climate 17,27,28,29,37 . The use of an eddy-rich grid was notably shown to further improve most benefits and bias reductions seen in EP GCMs, notably in terms of SST mean biases 27 .…”

“…At the surface, the horizontal distribution of heat driven by mesoscale eddies importantly contribute to sharp SST gradients, thus acting on the atmospheric boundary layer and higher atmosphere through variations in surface stability on momentum transfer, cloud cover and pressure gradients 26 . They have notably been shown to contribute significantly to the realism of air-sea interactions in GCMs, as well as large-scale oceanic features 27,28,29 .…”

Climate Variability and Response in High-Resolution Earth System Models

“…5.2), must be acknowledged (Trenberth et al, 2011). A multidecadal variability signal, significant with respect to a red noise null hypothesis, also exists in the HR-CESM simulation (Jüling et al, 2021a). This could potentially influence the results, but the magnitude of the response to the strong RCP forcing is very large compared to this internal variability.…”

“…This Chapter is published as: A. Jüling, A. S. von der Heydt, and H. A. Dijkstra (2021a). "Effects of strongly eddying oceans on multidecadal climate variability in the Community Earth System Model".…”

“…In the HR-CESM Southern Ocean multidecadal variability occurs which modulates both the RCP and 4⇥CO 2 sea ice response (Fig. 6.13c/d; Jüling et al (2021a)). While the RCP Arctic maximum sea ice extent evolution is very similar between the models, HR-CESM experiences a sudden drop in the last decade of the century (Fig.…”