A critical evaluation of decadal solar cycle imprints in the MiKlip historical ensemble simulations
Authors: Spiegl Tobias C. , Langematz Ulrike , Pohlmann Holger , Kröger Jürgen 
Published: September 2023

Abstract. Studies concerning solar–terrestrial connections over the last decades claim to have found evidence that the quasi-decadal solar cycle can have an
influence on the dynamics in the middle atmosphere in the Northern
Hemisphere (NH) during the winter season. It has been argued that feedbacks between the intensity of the UV part of the solar spectrum and low-latitude
stratospheric ozone may produce anomalies in meridional temperature
gradients which have the potential to alter the zonal-mean flow in middle to high latitudes. Interactions between the zonal wind and planetary waves can lead to a downward propagation of the anomalies, produced in the middle
atmosphere, down to the troposphere. More recently, it has been proposed
that top-down-initiated decadal solar signals might modulate surface climate and synchronize the North Atlantic Oscillation. A realistic representation of the solar cycle in climate models was suggested to significantly enhance decadal prediction skill. These conclusions have been debated controversial since then due to the lack of realistic decadal prediction model setups and more extensive analysis. In this paper we aim for an objective and improved evaluation of possible
solar imprints from the middle atmosphere to the surface and with that from
head to toe. Thus, we analyze model output from historical ensemble
simulations conducted with the state-of-the-art Max Planck Institute for Meteorology Earth System Model in high-resolution configuration (MPI-ESM-HR). The target of these simulations was to isolate the most crucial model physics to foster basic research on decadal climate prediction and to develop an operational ensemble decadal prediction system within the “Mittelfristige Klimaprognose” (MiKlip) framework. Based on correlations and multiple linear regression analysis we show that
the MPI-ESM-HR simulates a realistic, statistically significant and robust
shortwave heating rate and temperature response at the tropical stratopause, in good agreement with existing studies. However, the dynamical response to this initial radiative signal in the NH during the boreal winter season is weak. We find a slight strengthening of the polar vortex in midwinter during solar maximum conditions in the ensemble mean, which is consistent with the so-called “top-down” mechanism. The individual ensemble members, however, show a large spread in the dynamical response with opposite signs in response to the solar cycle, which might be a result of the large overall internal variability compensating for rather small solar imprints. We also analyze the possible surface responses to the 11-year solar cycle
and review the proposed synchronization between the solar forcing and the
North Atlantic Oscillation. We find that the simulated westerly wind
anomalies in the lower troposphere, as well as the anomalies in the mean sea level pressure, are most likely independent from the timing of the solar signal in the middle atmosphere and the alleged top-down influences. The pattern rather reflects the decadal internal variability in the troposphere, mimicking positive and negative phases of the Arctic and North Atlantic oscillations throughout the year sporadically, which is then assigned to the solar predictor time series without any plausible physical connection and sound solar contribution. Finally, by applying lead–lag correlations, we find that the proposed
synchronization between the solar cycle and the decadal component of the
North Atlantic Oscillation might rather be a statistical artifact, affected
for example by the internal decadal variability in the ocean, than a
plausible physical connection between the UV solar forcing and quasi-decadal variations in the troposphere.