A critical evaluation of decadal solar cycle imprints in the MiKlip historical ensemble simulations

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

Extreme Solar Events: Setting up a Paradigm

The Sun is magnetically active and often produces eruptive events on different energetic and temporal scales. Until recently, the upper limit of such events was unknown and believed to be roughly represented by direct instrumental observations. However, two types of extreme events were discovered recently: extreme solar energetic particle events on the multi-millennial time scale and super-flares on sun-like stars. Both discoveries imply that the Sun might rarely produce events, called extreme solar events (ESE), whose energy could be orders of magnitude greater than anything we have observed during recent decades. During the years following these discoveries, great progress has been achieved in collecting observational evidence, uncovering new events, making statistical analyses, and developing theoretical modelling. The ESE paradigm lives and is being developed. On the other hand, many outstanding questions still remain open and new ones emerge. Here we present an overview of the current state of the art and the forming paradigm of ESE from different points of view: solar physics, stellar–solar projections, cosmogenic-isotope data, modelling, historical data, as well as terrestrial, technological and societal effects of ESEs. Special focus is paid to open questions and further developments. This review is based on the joint work of the International Space Science Institute (ISSI) team #510 (2020–2022)

Extreme Solar Events: Setting up a Paradigm

The Sun is magnetically active and often produces eruptive events on different energetic and temporal scales. Until recently, the upper limit of such events was unknown and believed to be roughly represented by direct instrumental observations. However, two types of extreme events were discovered recently: extreme solar energetic particle events on the multi-millennial time scale and super-flares on sun-like stars. Both discoveries imply that the Sun might rarely produce events, called extreme solar events (ESE), whose energy could be orders of magnitude greater than anything we have observed during recent decades. During the years following these discoveries, great progress has been achieved in collecting observational evidence, uncovering new events, making statistical analyses, and developing theoretical modelling. The ESE paradigm lives and is being developed. On the other hand, many outstanding questions still remain open and new ones emerge. Here we present an overview of the current state of the art and the forming paradigm of ESE from different points of view: solar physics, stellar–solar projections, cosmogenic-isotope data, modelling, historical data, as well as terrestrial, technological and societal effects of ESEs. Special focus is paid to open questions and further developments. This review is based on the joint work of the International Space Science Institute (ISSI) team #510 (2020–2022)

Effects of a potential Grand Solar Minimum on the climate system against the background of anthropogenic climate change

Das langanhaltende Minimum des solaren Zyklus 23 sowie das besonders schwache Maximum des Zyklus 24, deuten auf ein mögliches Grand Solar Minimum innerhalb der kommenden drei bis fünf Jahrzehnte hin. Für die vergangenen 1.000 Jahre können zumindest 5 solcher Grand Solar Minima (mit einer Dauer zwischen 60 und 100 Jahren) festgestellt werden, welche auf einer Abschwächung des Magnetfelds der Sonne beruhen. Das letzte Grand Solar Minimum, das Maunder Minimum (1645-1715), fällt mit der ’kleinen Eiszeit’ in Europa zusammen. Einer Epoche mit überdurchschnittlich tiefen Temperaturen insbesondere im Winter. Eine derartige Reduktion des Strahlungsantriebs vor dem Hintergrund stetig ansteigender Treibhausgasemissionen führt häufig zu einer Missinterpretation in der medialen Berichterstattung und einer Infragestellung des anthropogenen Einflusses auf das Klimasystem. Da die Abschätzung der Strahlungsreduktion mit hohen Unsicherheiten verbunden ist, wurden zur Simulation eines zukünftigen Grand Solar Minimum unter den Bedingungen eines RCP6.0-Szenarios, 3 unterschiedliche Rekonstruktionen des Strahlungsantriebs während der Maunder Minimum Periode verwendet. Dabei unterscheiden sich die Datensätze, sowohl hinsichtlich der TSI (total solar irradiance) als auch der SSI (solar spectral irradiance). Alle transienten Simulationen wurden mit dem gekoppelten, Klima- Chemiemodell EMAC-O durchgeführt. Das Modell verfügt neben einer interaktiven Ozonchemie und einem hohen Modelloberrand (0.01 hPa), über ein hochauflösendes Strahlungsschema im kurzwelligen Wellenbereich. Die Ergebnisse der vorliegenden Dissertation deuten auf eine gewisse Abschwächung des globalen anthropogenen Klimawandels während der Kernperiode eines möglichen Grand Solar Minimum hin. Es konnten jedoch auch Regionen identifiziert werden, welche überdurchschnittlich stark von den Auswirkungen eines Grand Solar Minimum betroffen wären. Zu diesen Regionen gehören die hohen Breiten der Nordhemisphäre aber auch weite Teile der inneren Tropen. Weiterhin kann eine Modulation von ENSO unter den Bedingungen eines Grand Solar Minimum festgestellt werden. Diese ist geprägt von einer schwächeren Amplifizierung der ENSO-Amplitude mit Fortschreiten des anthropogenen Klimawandels im Vergleich zur Referenzsimulation. Die durch den Anstieg anthropogener Treibhausgase bedingte Abkühlung in der mittleren Atmosphäre wird unter den Bedingungen eines Grand Solar Minimum weiter verstärkt. Dies ist auf eine Reduzierung des kurzwelligen Strahlungsantriebs, eine abgeschwächte Ozonproduktion und daraus resultierende negative Anomalien der kurzwelligen solaren Heizraten zurückzuführen. Darüber hinaus zeigt sich eine deutliche Verzögerung der Totalozonerholung in den Simulationen unter Grand Solar Minimum Bedingungen. Abschließend kann eine deutliche Beeinflussung der winterlichen Dynamik innerhalb der mittleren Atmosphäre der Nordhemisphäre unter reduzierter solarer Einstrahlung aufgezeigt werden. Diese Beeinflussung äußert sich in Form einer Abschwächung des Polarwirbels, häufigeren Stratosphärenerwärmungen im Frühwinter und einer Modulation der troposphärischen Variabilität, welche mit dem Top-down Mechanismus in Einklang steht.The long-lasting minimum of Solar Cycle 23 as well as the overall weak maximum of Cycle 24, reveal the possibility for a return to Grand Solar Minimum conditions within the next three to five decades. The past millennium featured at least 5 excursions (lasting 60–100 years) of exceptionally low solar activity, induced by a weak magnetic field of the Sun. The last Grand Solar Minimum, the so called Maunder Minimum (1645–1715), coincides with the ’Little Ice Age’ in Europe, a time of severe cold, especially during winter season. The quantification of the implications of such a projected decrease in solar forcing is of ultimate importance, given the on-going public discussion of the role of carbon dioxide emissions for global warming, and the possible role a cooling due to decreasing solar activity could be ascribed to. Since the magnitude of a future reduction in solar forcing, due to a possible Grand Solar Minimum, is highly uncertain, 3 different solar reconstruction datasets for the Maunder Minimum period that show significant differences in both, TSI (total solar irradiance) and SSI (solar spectral irradiance), were used to simulate a future Grand Solar Minimum under RCP6.0 conditions. All transient simulations were carried out using the ocean-coupled chemistry-climate model EMAC-O. Besides interactive ozone chemistry and a high model top (0.01 hPa) the model includes a high-resolution shortwave radiation scheme. The results obtained, suggest a certain slowdown of global-scale temperature increase mainly during the core period of a future Grand Solar Minimum. However, some regions show a rather strong response to Grand Solar Minimum conditions. These include the high latitudes of the Northern Hemisphere, but also vast areas of the inner tropics. Furthermore it shows, that a Grand Solar Minimum might be associated with a modulation of the ENSO amplitude which faces less amplification during the 21st century under reduced solar forcing compared to the reference simulation. In the middle atmospere a Grand Solar Minimum might reinforce the expected cooling due to rising greenhouse gas emissions. This additional cooling can be attributed to reduced short wave radiation during a Grand Solar Minimum, less ozone production and an associated decrease in short wave heating rates. It also can be shown that a Grand Solar Minimum interferes with a recovery of total column ozone, which is markedly delayed under the influence of a Grand Solar Minimum. Conclusively, a distinct influence on middle atmosphere dynamics in Northern Hemisphere winter was found under Grand Solar Minimum conditions. These solar induced changes are characterized by a weakening of the polar vortex, more frequent sudden stratospheric warmings in early winter and a top-down modulation of tropospheric winter variability

Modeling the Transport and Deposition of 10Be Produced by the Strongest Solar Proton Event During the Holocene

Prominent excursions in the number of cosmogenic nuclides (e.g., 10Be) around 774 CE/775 document the most severe solar proton event (SPE) throughout the Holocene. Its manifestation in ice cores is valuable for geochronology, but also for solar-terrestrial physics and climate modeling. Using the ECHAM/MESSy Atmospheric Chemistry (EMAC) climate model in combination with the Warning System for Aviation Exposure to SEP (WASAVIES), we investigate the transport, mixing, and deposition of the cosmogenic nuclide 10Be produced by the 774 CE/775 SPE. By comparing the model results to the reconstructed 10Be time series from four ice core records, we study the atmospheric pathways of 10Be from its stratospheric source to its sink at Earth's surface. The reconstructed post-SPE evolution of the 10Be surface fluxes at the ice core sites is well captured by the model. The downward transport of the 10Be atoms is controlled by the Brewer-Dobson circulation in the stratosphere and cross-tropopause transport via tropopause folds or large-scale sinking. Clear hemispheric differences in the transport and deposition processes are identified. In both polar regions the 10Be surface fluxes peak in summertime, with a larger influence of wet deposition on the seasonal 10Be surface flux in Greenland than in Antarctica. Differences in the peak 10Be surface flux following the 774 CE/775 SPE at the drilling sites are explained by specific meteorological conditions depending on the geographic locations of the sites

Assessment of the 11-year solar cycle signals in the middle atmosphere in multiple-model ensemble simulations

To better understand possible reasons for the diverse modeling results and large discrepancies of the detected solar fingerprints, we took one step back and assessed the "initial" solar signals in the middle atmosphere based on large ensemble simulations with multiple climate models — FOCI, EMAC, and MPI-ESM-HR. Consistent with previous work, we find that the 11-year solar cycle signals in the short wave heating rate (SWHR) and ozone anomalies are robust and statistically significant in all three models. These "initial" solar cycle signals in SWHR, ozone, and temperature anomalies are sensitive to the strength of the solar forcing. Correlation coefficients of the solar cycle with the SWHR, ozone, and temperature anomalies linearly increase along with the enhancement of the solar cycle amplitude, and this reliance becomes more complex when the solar cycle amplitude exceeds a certain threshold. In addition, the cold bias in the tropical stratopause of EMAC dampens the subsequent results of the "initial" solar signal. The warm pole bias in MPI-ESM-HR leads to a weak polar night jet (PNJ), which may limit the top-down propagation of the initial solar signal. Although FOCI simulated a so-called top-down response as revealed in previous studies in a period with large solar cycle amplitudes, its warm bias in the tropical upper stratosphere results in a positive bias in PNJ and can lead to a "reversed" response in some extreme cases. We suggest a careful interpretation of the single model result and further re-examination of the solar signal based on more climate models

Isolated alkaline basalt occurrences in the northern Spessart, Germany: Outposts of the Early Miocene Vogelsberg shield volcano?

Four isolated occurrences of Tertiary volcanic rocks in the northern Spessart at Beilstein, Hoher Berg, Madstein and Kasselgrund are relics of volcanic vents or dikes. They display alkaline basalts (s. l.) with mainly trachybasaltic composition, which, from normative mineral contents, may be designated as nepheline-bearing alkali-olivine basalts and basanites. In part, centimetre-sized xenoliths of spinel lherzolite occur. According to Ar-Ar dating, the alkaline basalts (s. l.) from Kasselgrund have erupted at 18.1 ± 0.3 or 19.3 ± 0.4 Ma, those of Hoher Berg between c. 18 and c. 21 Ma. These ages correspond to the Vogelsberg eruption stage I. A slightly younger Ar-Ar age of 16.8 ± 0.3 Ma was recorded for the Beilstein basalt, which is in chronological accordance to the turn of Vogelsberg eruption stages II and III. Samples of all four occurrences reveal major and trace element compositions, which are different from those of the Vogelsberg basalts. Compositions of basalts of the stage III from Vogelsberg coincide most with the Spessart basalts. This signals a special position of the northern Spessart volcanic rocks either as a discrete spatial part of the Vogelsberg volcanic suite or as smaller, independent eruption centres

DNA-binding and protein structure of nuclear factors likely acting in genetic information processing in the Paulinella chromatophore

The chromatophores in Paulinella are evolutionary-early-stage photosynthetic organelles. Biological processes in chromatophores depend on a combination of chromatophore and nucleus-encoded proteins. Interestingly, besides proteins carrying chromatophore-targeting signals, a large arsenal of short chromatophore-targeted proteins (sCTPs; <90 amino acids) without recognizable targeting signals were found in chromatophores. This situation resembles endosymbionts in plants and insects that are manipulated by host-derived antimicrobial peptides. Previously, we identified an expanded family of sCTPs of unknown function, named here "DNA-binding (DB)-sCTPs". DB-sCTPs contain a ~45 amino acid motif that is conserved in some bacterial proteins with predicted functions in DNA processing. Here, we explored antimicrobial activity, DNA-binding capacity, and structures of three purified recombinant DB-sCTPs. All three proteins exhibited antimicrobial activity against bacteria involving membrane permeabilization, and bound to bacterial lipids in vitro. A combination of in vitro assays demonstrated binding of recombinant DB-sCTPs to chromatophore-derived genomic DNA sequences with an affinity in the low nM range. Additionally, we report the 1.2 Å crystal structure of one DB-sCTP. In silico docking studies suggest that helix α2 inserts into the DNA major grove and the exposed residues, that are highly variable between different DB-sCTPs, confer interaction with the DNA bases. Identification of photosystem II subunit CP43 as a potential interaction partner of one DB-sCTP, suggests DB-sCTPs to be involved in more complex regulatory mechanisms. We hypothesize that membrane binding of DB-sCTPs is related to their import into chromatophores. Once inside, they interact with the chromatophore genome potentially providing nuclear control over genetic information processing