We present an unprecedented set of high-resolution climate simulations, consisting of a 500-year pre-industrial control simulation and a 250-year historical and future climate simulation from 1850 to 2100. A high-resolution configuration of the Community Earth System Model version 1.3 (CESM1.3) is used for the simulations with a nominal horizontal resolution of 0.25°for the atmosphere and land models and 0.1°for the ocean and sea-ice models. At these resolutions, the model permits tropical cyclones and ocean mesoscale eddies, allowing interactions between these synoptic and mesoscale phenomena with large-scale circulations. An overview of the results from these simulations is provided with a focus on model drift, mean climate, internal modes of variability, representation of the historical and future climates, and extreme events. Comparisons are made to solutions from an identical set of simulations using the standard resolution (nominal 1°) CESM1.3 and to available observations for the historical period to address some key scientific questions concerning the impact and benefit of increasing model horizontal resolution in climate simulations. An emerging prominent feature of the high-resolution pre-industrial simulation is the intermittent occurrence of polynyas in the Weddell Sea and its interaction with an Interdecadal Pacific Oscillation. Overall, high-resolution simulations show significant improvements in representing global mean temperature changes, seasonal cycle of sea-surface temperature and mixed layer depth, extreme events and in relationships between extreme events and climate modes. Plain Language Summary Although the current generation of climate models has demonstrated high fidelity in simulating and projecting global temperature change, these models show large uncertainties when it comes to questions concerning how rising global temperatures will impact local weather conditions. This is because the resolution (~100 km) at which the majority of climate models simulate the climate is not fine enough to resolve these small-scale regional features. Conducting long-term (multi-centuries) high-resolution (~10 km) climate simulations has been a great challenge for the research community due to the extremely high computational demands. Through international

Sea surface temperature (SST) is a critical field to get right in simulations of the climate system because it is the key variable linking changes in the ocean to the atmosphere. SST anomalies associated with the El Niño-Southern Oscillation (ENSO), the Pacific Decadal Variability (PDV) and the Atlantic Multidecadal Variability (AMV) are all capable of modulating extreme weather globally (Di Lorenzo & Mantua, 2016;McCabe et al., 2004;McPhaden et al., 2006). The mesoscale SST anomalies in the northwestern Pacific can also influence heavy rainfall in western North America (Liu et al., 2021). To date, SST is also among the best measured climate fields, with long-term, global coverage obtained by ships (Smith et al., 2019;Woodruff et al., 2005), mooring buoys (TAO/ TRITON, PIRATA, RAMA), Argo floats (Roemmich et al., 2009), and satellites (Reynolds &Smith, 1994).Despite the fact that SST data sets are extremely useful for understanding global climate, it is still challenging to decipher the physical processes that govern SST, such as the relative roles of ocean mixing and advection, which

Upwelling along ocean eastern boundaries is expected to intensify due to coastal wind strengthening driven by increasing land-sea contrast according to the Bakun hypothesis. Here, the latest high-resolution climate simulations that exhibit drastic improvements of upwelling processes reveal far more complex future upwelling changes. The Southern Hemisphere upwelling systems show a future strengthening in coastal winds with a rapid coastal warming, whereas the Northern Hemisphere coastal winds show a decrease with a comparable warming trend. The Bakun mechanism cannot explain these changes. Heat budget analysis indicates that temperature change in the upwelling region is not simply controlled by vertical Ekman upwelling, but also influenced by horizontal heat advection driven by strong near-coast wind stress curl that is neglected in the Bakun hypothesis and poorly represented by the low-resolution models in the Coupled Model Intercomparison Project. The high-resolution climate simulations also reveal a strong spatial variation in future upwelling changes, which is missing in the low-resolution simulations.

No abstract