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.
Highlights
The ocean plays a key role in the climate system’s heat budget, absorbing some 93 % of the additional heat retained due to anthropogenic greenhouse gases (Stocker et al, 2013)
The results are divided into four subsections: the first describes the variability in sea surface temperature (SST) by means of the chosen indices, the second focuses on the surface heat fluxes (SHF), the third explores the spatial structure of multidecadal variability of the ocean heat content (OHC), and the fourth shows the consequences for the global mean surface temperature (GMST)
We investigated the effect of ocean model resolution on multidecadal variability by contrasting two multi-century simulations with the Community Earth System Model: one with a non-eddying ocean typical of the CMIP5 models (LR-CESM) and one with a strongly eddying ocean (HRCESM)
Summary
The ocean plays a key role in the climate system’s heat budget, absorbing some 93 % of the additional heat retained due to anthropogenic greenhouse gases (Stocker et al, 2013). The instrumental record of sea surface temperature (SST) is only about ∼ 1.5 centuries, the observations indicate the existence of spatially correlated patterns of variability on multidecadal timescales, referred to as (statistical) modes of variability (Deser et al, 2010) These modes are thought to be part of the internal variability of the climate system, and they affect the oceanic heat content by altering heat fluxes and the global energy budget (Trenberth and Shea, 2006; Dijkstra, 2013; Zhang and Wang, 2013; Frajka-Williams et al, 2017). Societies are impacted significantly by these SST patterns through associated changes in temperature extremes (e.g., Ruprich-Robert et al, 2018), droughts (e.g., McCabe and Palecki, 2006; Delworth et al, 2015), hurricanes (e.g., Zhang and Delworth, 2006), precipitation patterns (Sutton and Hodson, 2005), and ecosystem productivity (Mantua et al, 1997). Zhang et al (2019) reviews the climate impacts of the Atlantic Multidecadal Variability
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