Resolution, physics and atmosphere–ocean interaction – How do they influence climate model representation of Euro-Atlantic atmospheric blocking?

Abstract

Atmospheric blocking events are known to locally explain a large part of climate variability. However, despite their relevance, many current climate models still struggle to represent the observed blocking statistics. In this study, simulations of the global climate model EC-Earth are analysed with respect to atmospheric blocking. Seventeen simulations map the uncertainty space defined by the three-model characteristics: atmospheric resolution, physical parameterization and complexity of atmosphere-ocean interaction, namely an atmosphere coupled to an ocean model or forced by surface data. Representation of the real-world statistics is obtained from reanalyses ERA-20C, JRA-55 and ERA-Interim which agree on Northern Hemisphere blocking characteristics. Blocking events are detected on a central blocking latitude which is individually determined for each simulation. The frequency of blocking events tends to be underestimated relative to ERA-Interim over the Atlantic and western Eurasia in winter and overestimated during spring months. However, only few model setups show statistically significant differences compared to ERA-Interim which can be explained by the large inter-annual variability of blocking. Results indicate slightly larger biases relative to ERA-Interim in coupled than in atmosphere-only models but differences between the two are not statistically significant. Although some resolution dependence is present in spring, the signal is weak and only statistically significant if the physical parameterizations of the model are improved simultaneously. Winter blocking is relatively more sensitive to physical parameterizations, and this signal is robust in both atmosphere-only and coupled simulations, although stronger in the latter. Overall, the model can capture blocking frequency well despite biases in representing the mean state of geopotential height over this area. Blocking signatures of geopotential height are represented more similar to ERA-Interim and only weak sensitivities to model characteristics remain.

Multi-model evaluation of the sensitivity of the global energy budget and hydrological cycle to resolution

“…It has been used by e.g. Hartung et al (2017) and Koenigk and Brodeau (2017). EC-EARTH3.0.1 is based on ECM-WF's atmospheric circulation model IFS, cycle 36r4.…”

The extremely warm summer of 2018 in Sweden – set in a historical context

“…We use three sources of observational data (Table 1): (i) the gridded daily and monthly climatology E-OBS version 19.0e (Haylock et al, 2008) that covers Europe at a 12.5 km horizontal resolution for the 1951-2018 period, (ii) monthly averages for Sweden derived from SMHI observational stations starting in 1860, and (iii) a long-term record of homogenized daily temperatures derived from thermometer readings at the Stockholm Observatory starting in 1756. Here, we describe the Swedish data sets.…”

Diagnosing topographic forcing in an atmospheric dataset: The case of the North American Cordillera

“…The index is one-dimensional and picks up blocking at the latitude of the climatological storm track. For more details on the blocking index and data, see Hartung et al (2017). We composite topographic forcing based on the PNA index, which is representative of the upstream variability, and European blocking is used as an indicator for downstream regimes.…”