Fig. 6: Power spectral densities (PSDs) of sea surface temperature anomalies in the Niño 3.4 region for the period 1870–2014. The black line gives the observed (OBS) spectrum after Rayner et al. (2003). The five historical ensemble members with AWI-CM are given in blue. Grey shading denotes the 5–95 % confidence interval of an AR(1)-process fitted to OBS, based on a Monte Carlo approach with 10,000 realizations, as detailed by Rackow et al. (2018). The total (integrated) observed Niño 3.4 variance [K2] is 0.57, for AWI-CM-MR the range is (0.75–1.01).
To assess the temporal behaviour further, we apply a diagnostic that quantifies the seasonal phase locking of Niño 3.4 SST anomalies to the seasonal cycle (Fig. 7). Observed SST variability associated with ENSO, as diagnosed from monthly standard deviation, tends to peak in boreal winter, with a minimum in spring. Especially in boreal winter, the 5 ensemble members capture the corresponding U-shape and its magnitude relatively well; however, there is a positive bias in spring. A bias of similar magnitude had already been identified in a previous configuration of AWI-CM, using a globally relatively low resolution mesh but with tropical ocean grid refinement at 0.25° (Rackow and Juricke, 2020). The bias appears to be rather sensitive to the applied tropical ocean resolution since the secondary peak in spring is much stronger at a coarser resolution of 1°, using the same atmospheric resolution (see Fig. 6 in Rackow et al., 2014).