Impact of model resolution on Holocene climate simulations of the Northern Hemisphere

Abstract. This study demonstrates the dependence of simulated surface air temperatures on variations in grid resolution and resolution-dependent orography in simulations of the Mid-Holocene. A set of Mid-Holocene sensitivity experiments is carried out with the atmospheric general circulation model ECHAM5 forced with sea surface temperature and sea ice fields from coupled simulations. Each experiment was performed in two resolution modes: low (~ 3.75°, 19 vertical levels) and high (~ 1.1°, 31 vertical levels). Results are compared to respective preindustrial runs. It is found that the large-scale temperature anomalies for the Mid-Holocene (compared to the preindustrial) are significantly different in the low- and high-resolution versions. For boreal winter, differences are related to circulation changes caused by the response to thermal forcing in conjunction with orographic resolution. For summer, shortwave cloud radiative forcing emerges as the predominant factor. In summary, the simulated Mid-Holocene temperature differences (low versus high resolution) reveal a response that regionally exceeds the Mid-Holocene to preindustrial modelled temperature anomalies, and show partly reversed signs across the same geographical regions. Our results imply that climate change simulations sensitively depend on the chosen grid resolutions.