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).