Neues vom stratosphärischen Solarverstärker

Eines des großen ungelösten Rätsel der Klimawissenschaften ist die Frage, auf welchem Wege solare Aktivitätsschwankungen zu klimatischen Veränderungen führen. Eine Vielzahl von geologisch-paläoklimatologischen Untersuchungen belegt einwandfrei, dass es einen solaren Einfluss auf das Klima gibt. Allerdings wird hierzu ein solarer Verstärkermechanismus benötigt, da die Sonne-Schwankungen im sichtbaren Strahlungsbereich des Lichtes wohl zu gering sind, um den beobachteten Effekt zu erzeugen. In unserem Buch „Die kalte Sonne“ haben wir die beiden wahrscheinlichsten Verstärker-Kandidaten vorgestellt. Zum einen wäre hier der Svensmark-Wolkeneffekt zu nennen, wobei von der Sonne modulierte galaktische Strahlung Keime für Wolken bilden könnte. Zum anderen geht es um die UV-Strahlung, die viel stärker schwankt als der sichtbare Lichtanteil der Sonne. Das UV erzeugt in der Stratosphäre, Ozon. Mittlerweile gibt es sich verdichtenede Hinweise darauf, dass sich das Geschehen in der Stratosphäre auch in die tieferen Atmosphärenstockwerke durchpaust, wo das Wettergeschehen stattfindet. Im Folgenden wollen einen Streifzug durch die neuere Literatur unternehmen. Was gibt es Neues vom stratosphärischen Solarverstärker?

Im November 2012 wartete eine Gruppe um David Thompson in Nature mit einer großen Überraschung auf: Die Forscher hatten einen neuen Datensatz zur Temperaturentwicklung der mittleren und oberen Stratosphäre ermittelt, der sich signifikant von früheren Temperaturkurven unterschied. Hierdurch wurde nun alles durcheinandergewirbelt. Offenbar stimmten die früheren Modelle zur Entwicklung hinten und vorne nicht. Auch Modelle mussten nun plötzlich auf den Prüfstand, die nur die veraltete Temperaturkurve nachvollziehen konnten, nicht jedoch die neue. Dies ist insbesondere bedenklich, da hier Emissionen von CO2 und ozonzerstörenden Gasen eine Rolle spielen. Hier der Abstract der Arbeit:

The mystery of recent stratospheric temperature trends
A new data set of middle- and upper-stratospheric temperatures based on reprocessing of satellite radiances provides a view of stratospheric climate change during the period 1979–2005 that is strikingly different from that provided by earlier data sets. The new data call into question our understanding of observed stratospheric temperature trends and our ability to test simulations of the stratospheric response to emissions of greenhouse gases and ozone-depleting substances. Here we highlight the important issues raised by the new data and suggest how the climate science community can resolve them.

Auch Katja Matthes vom Kieler Geomar ist zusammen mit Kollegen an der Erforschung des stratosphärischen Solarverstärkers beteiligt. Hier gelangten in den letzten Jahren gleich drei Arbeiten zur Publikation, die wir hier vorstellen möchten. So erschien im September 2012 im Journal of Geophysical Research ein Paper, das von Christof Petrick angeführt wurde. In der Studie geht es um solare UV-Schwankungen, die stratosphärische Veränderungen hervorrufen, die sich über Ozeanzyklen in die Ozeane fortpflanzen. Die Matthes-Gruppe nennt dies „Top-Down-Mechanismus“. Hier der Abstract:

Impact of the solar cycle and the QBO on the atmosphere and the ocean
The Solar Cycle and the Quasi-Biennial Oscillation are two major components of natural climate variability. Their direct and indirect influences in the stratosphere and troposphere are subject of a number of studies. The so-called “top-down’ mechanism describes how solar UV changes can lead to a significant enhancement of the small initial signal and corresponding changes in stratospheric dynamics. How the signal then propagates to the surface is still under investigation. We continue the “top-down’ analysis further down to the ocean and show the dynamical ocean response with respect to the solar cycle and the QBO. For this we use two 110-year chemistry climate model experiments from NCAR’s Whole Atmosphere Community Climate Model (WACCM), one with a time varying solar cycle only and one with an additionally nudged QBO, to force an ocean general circulation model, GFZ’s Ocean Model for Circulation and Tides (OMCT). We find a significant ocean response to the solar cycle only in combination with a prescribed QBO. Especially in the Southern Hemisphere we find the tendency to positive Southern Annular Mode (SAM) like pattern in the surface pressure and associated wind anomalies during solar maximum conditions. These atmospheric anomalies propagate into the ocean and induce deviations in ocean currents down into deeper layers, inducing an integrated sea surface height signal. Finally, limitations of this study are discussed and it is concluded that comprehensive climate model studies require a middle atmosphere as well as a coupled ocean to investigate and understand natural climate variability.

Im April 2013 publizierte die Matthes-Gruppe mit Ermolli et al 2013 im Fachblatt Atmospheric Chemistry and Physics eine weitere Studie zum Thema. Die Hauptnachricht: Klimamodelle unterschätzten bislang die UV-Schwankungen um einen Faktor von 4-6. Das ist enorm. In Wirklichkeit waren die UV-Schwankungen also im Mittel 5 mal so hoch wie angenommen. Ermolli und Kollegen prognostizieren, dass auch die atmosphärischen Effekte entsprechend viel größer sind als zuvor modelliert. Hier der Abstract:

Recent variability of the solar spectral irradiance and its impact on climate modelling
The lack of long and reliable time series of solar spectral irradiance (SSI) measurements makes an accurate quantification of solar contributions to recent climate change difficult.
Whereas earlier SSI observations and models provided a qualitatively consistent picture of the SSI variability, recent measurements by the SORCE (SOlar Radiation and Climate Experiment) satellite suggest a significantly stronger variability in the ultraviolet (UV) spectral range and changes in the visible and near-infrared (NIR) bands in anti-phase with the solar cycle. A number of recent chemistry-climate model (CCM) simulations have shown that this might have significant implications on the Earth’s atmosphere. Motivated by these results, we summarize here our current knowledge of SSI variability and its impact on Earth’s climate.
We present a detailed overview of existing SSI measurements and provide thorough comparison of models available to date. SSI changes influence the Earth’s atmosphere, both directly, through changes in shortwave (SW) heating and therefore, temperature and ozone distributions in the stratosphere, and indirectly, through dynamical feedbacks. We investigate these direct and indirect effects using several state-of-the art CCM simulations forced with measured and modelled SSI changes. A unique asset of this study is the use of a common comprehensive approach for an issue that is usually addressed separately by different communities.
We show that the SORCE measurements are difficult to reconcile with earlier observations and with SSI models. Of the five SSI models discussed here, specifically NRLSSI (Naval Research Laboratory Solar Spectral Irradiance), SATIRE-S (Spectral And Total Irradiance REconstructions for the Satellite era), COSI (COde for Solar Irradiance), SRPM (Solar Radiation Physical Modelling), and OAR (Osservatorio Astronomico di Roma), only one shows a behaviour of the UV and visible irradiance qualitatively resembling that of the recent SORCE measurements. However, the integral of the SSI computed with this model over the entire spectral range does not reproduce the measured cyclical changes of the total solar irradiance, which is an essential requisite for realistic evaluations of solar effects on the Earth’s climate in CCMs.
We show that within the range provided by the recent SSI observations and semi-empirical models discussed here, the NRLSSI model and SORCE observations represent the lower and upper limits in the magnitude of the SSI solar cycle variation.
The results of the CCM simulations, forced with the SSI solar cycle variations estimated from the NRLSSI model and from SORCE measurements, show that the direct solar response in the stratosphere is larger for the SORCE than for the NRLSSI data. Correspondingly, larger UV forcing also leads to a larger surface response.
Finally, we discuss the reliability of the available data and we propose additional coordinated work, first to build composite SSI data sets out of scattered observations and to refine current SSI models, and second, to run coordinated CCM experiments.

In unserer Monatskolumne “Die Sonne im August 2015“ hatten wir bereits eine weitere aktuelle Matthes-Arbeit besprochen, Thiéblemont et al., die im September 2015 in Nature Communications erschien. Laut der Arbeit ist die Nordatlantische Oszillation mit einer Verzögerung von 1-2 Jahren an die Sonnenaktivität gekoppelt. Die Forscher benutzten ein Klimamodell, das die Atmosphäre bis zu einer Höhe von 140 km modelliert  und somit die Wirkung der UV-Strahlung auf die Chemie der Stratosphäre, etwa der Ozonbildung, besser berücksichtigen kann. Hier der Abstract:

Solar forcing synchronizes decadal North Atlantic climate variability
Quasi-decadal variability in solar irradiance has been suggested to exert a substantial effect on Earth’s regional climate. In the North Atlantic sector, the 11-year solar signal has been proposed to project onto a pattern resembling the North Atlantic Oscillation (NAO), with a lag of a few years due to ocean-atmosphere interactions. The solar/NAO relationship is, however, highly misrepresented in climate model simulations with realistic observed forcings. In addition, its detection is particularly complicated since NAO quasi-decadal fluctuations can be intrinsically generated by the coupled ocean-atmosphere system. Here we compare two multi-decadal ocean-atmosphere chemistry-climate simulations with and without solar forcing variability. While the experiment including solar variability simulates a 1–2-year lagged solar/NAO relationship, comparison of both experiments suggests that the 11-year solar cycle synchronizes quasi-decadal NAO variability intrinsic to the model. The synchronization is consistent with the downward propagation of the solar signal from the stratosphere to the surface.

Mithilfe eines Klimamodells, das auch die Stratosphäre berücksichtigt, konnte ein Forscherteam um Lon Hood einen Einfluss des 11-Jahres-Sonnenzyklus in der winterlichen Temperaturentwicklung des Pazifiks nachweisen. Das macht Hoffnung. Die Arbeit erschien im Oktober 2013 im Journal of Climate. Hier der Abstract:

The Surface Climate Response to 11-Yr Solar Forcing during Northern Winter: Observational Analyses and Comparisons with GCM Simulations
The surface climate response to 11-yr solar forcing during northern winter is first reestimated by applying a multiple linear regression (MLR) statistical model to Hadley Centre sea level pressure (SLP) and sea surface temperature (SST) data over the 1880–2009 period. In addition to a significant positive SLP response in the North Pacific found in previous studies, a positive SST response is obtained across the midlatitude North Pacific. Negative but insignificant SLP responses are obtained in the Arctic. The derived SLP response at zero lag therefore resembles a positive phase of the Arctic Oscillation (AO). Evaluation of the SLP and SST responses as a function of phase lag indicates that the response evolves from a negative AO-like mode a few years before solar maximum to a positive AO-like mode at and following solar maximum. For comparison, a similar MLR analysis is applied to model SLP and SST data from a series of simulations using an atmosphere–ocean general circulation model with a well-resolved stratosphere. The simulations differed only in the assumed solar cycle variation of stratospheric ozone. It is found that the simulation that assumed an ozone variation estimated from satellite data produces solar SLP and SST responses that are most consistent with the observational results, especially during a selected centennial period. In particular, a positive SLP response anomaly is obtained in the northeastern Pacific and a corresponding positive SST response anomaly extends across the midlatitude North Pacific. The model response versus phase lag also evolves from a mainly negative AO-like response before solar maximum to a mainly positive AO response at and following solar maximum.

In eine ähnliche Richtung ging bereits eine Arbeit von Chiodo et al., die im März 2012 im Journal of Geophysical Research erschienen war. Auch hier fanden die Autoren einen klimatischen Effekt auf die winterliche Atmosphäre. Im Folgenden die Kurzfassung:

The 11 year solar cycle signal in transient simulations from the Whole Atmosphere Community Climate Model
The atmospheric response to the 11 year solar cycle (SC) and its combination with the quasi-biennal oscillation (QBO) are analyzed in four simulations of the Whole Atmosphere Community Climate Model version 3.5 (WACCM3.5), which were performed with observed sea surface temperatures, volcanic eruptions, greenhouse gases, and a nudged QBO. The analysis focuses on the annual mean response of the model to the SC and on the evolution of the solar signal during the Northern Hemispheric winter. WACCM3.5 simulates a significantly warmer stratosphere under solar maximum conditions compared to solar minimum. The vertical structure of the signal in temperature and ozone at low latitudes agrees with observations better than previous versions of the model. The temperature and wind response in the extratropics is more uncertain because of its seasonal dependence and the large dynamical variability of the polar vortex. However, all four simulations reproduce the observed downward propagation of zonal wind anomalies from the upper stratosphere to the lower stratosphere during boreal winter resulting from solar-induced modulation of the polar night jet and the Brewer-Dobson circulation. Combined QBO-SC effects in the extratropics are consistent with observations, but they are not robust across the ensemble members. During boreal winter, solar signals are also found in tropospheric circulation and surface temperature. Overall, these results confirm the plausibility of proposed dynamical mechanisms driving the atmospheric response to the SC. The improvement of the model climatology and variability in the polar stratosphere is the basis for the success in simulating the evolution and magnitude of the solar signal.

Eine Gruppe um Michael Beckmann ließ im April 2014 mit einer Arbeit aufhorchen, die in Methods in Ecology and Evolution publiziert wurde. Beckmann und Kollegen stellten einen UV-B Datensatz zusammen und fanden eine gute Korrelation zwischen der globalen Durchschnittstemperatur und der UV-B-Intensität. Hier die Kurzfassung:

glUV: a global UV-B radiation data set for macroecological studies

  1. Macroecology has prospered in recent years due in part to the wide array of climatic data, such as those provided by the WorldClim and CliMond data sets, which has become available for research. However, important environmental variables have still been missing, including spatial data sets on UV-B radiation, an increasingly recognized driver of ecological processes.
  2. We developed a set of global UV-B surfaces (glUV) suitable to match common spatial scales in macroecology. Our data set is based on remotely sensed records from NASA’s Ozone Monitoring Instrument (Aura-OMI). Following a similar approach as for the WorldClim and CliMond data sets, we processed daily UV-B measurements acquired over a period of eight years into monthly mean UV-B data and six ecologically meaningful UV-B variables with a 15-arc minute resolution. These bioclimatic variables represent Annual Mean UV-B, UV-B Seasonality, Mean UV-B of Highest Month, Mean UV-B of Lowest Month, Sum of Monthly Mean UV-B during Highest Quarter and Sum of Monthly Mean UV-B during Lowest Quarter. We correlated our data sets with selected variables of existing bioclimatic surfaces for land and with Terra–MODIS Sea Surface Temperature for ocean regions to test for relations to known gradients and patterns.
  3. UV-B surfaces showed a distinct seasonal variance at a global scale, while the intensity of UV-B radiation decreased towards higher latitudes and was modified by topographic and climatic heterogeneity. UV-B surfaces were correlated with global mean temperature and annual mean radiation data, but exhibited variable spatial associations across the globe. UV-B surfaces were otherwise widely independent of existing bioclimatic surfaces.
  4. Our data set provides new climatological information relevant for macroecological analyses. As UV-B is a known driver of numerous biological patterns and processes, our data set offers the potential to generate a better understanding of these dynamics in macroecology, biogeography, global change research and beyond. The glUV data set containing monthly mean UV-B data and six derived UV-B surfaces is freely available for download at: http://www.ufz.de/gluv.

Überraschend auch das Ergebnis einer Studie einer Gruppe um Tao Li, die im Juni 2013 in den Geophysical Research Letters zur Veröffentlichung kam. Die Wissenschaftler fanden heraus, dass sich das El Nino-Phänomen bis in höchste Atmosphärenschichten durchpaust. Li und Kollegen wiesen El Nino-Effekte bis in die Mesosphäre nach, die sich in einem Höhenstockwerk von 50 bis 85 km erstreckt und oberhalb der Stratosphäre liegt. Ein weiterer Hinweis auf eine signifikante klimatische Verknüpfung der verschiedenen Atmosphärenstockwerke, die in den bisherigen IPCC-Klimamodellen kaum berücksichtigt wurde. Hier der Abstract:

Influence of El Niño-Southern Oscillation in the mesosphere
Using the middle atmosphere temperature data set observed by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite experiment between 2002 and 2012, and temperatures simulated by the Whole Atmospheric Community Climate Model version 3.5 (WACCM3.5) between 1953 and 2005, we studied the influence of El Niño-Southern Oscillation (ENSO) on middle atmosphere temperature during the Northern Hemisphere (NH) wintertime. For the first time, a significant winter temperature response to ENSO in the middle mesosphere has been observed, with an anomalous warming of ~1.0 K/MEI (Multivariate ENSO Index) in the tropics and an anomalous cooling of ~ −2.0 K/MEI in the NH middle latitudes. The observed temperature responses to ENSO in the mesosphere are opposite to those in the stratosphere, in agreement with previous modeling studies. Temperature responses to ENSO observed by SABER show similar patterns to those simulated by the WACCM3.5 model. Analysis of the WACCM3.5 residual mean meridional circulation response to ENSO reveals a significant downwelling in the tropical mesosphere and upwelling in the NH middle and high latitudes during warm ENSO events, which is mostly driven by anomalous eastward gravity wave forcing in the NH mesosphere.

Eine weitere interessante Studie zum Thema stammt von Maycock et al., die im Juni 2015 im Journal of Geophysical Research herauskam. Die Autoren modellierten ein großes solares Minimum, das die Mehrheit der Solarphysiker für die kommenden Jahrzehnte vorhersagt. Dabei kamen sie auf eine Abkühlung der oberen Stratosphären-Grenzschicht von 1,2°C. Mit den noch immer begrenzten Modellen konnten die Forscher kaum einen Temperatureffekt am Erdboden ermitteln. Allerdings erkennten sie bereits, dass in gewissen Regionen der Erde wohl auch mit den bestehenden Modellen bereits Klimaveränderungen im Zusammenhang mit der großen solaren Schwächephase zu erwarten sind. Hier der Abstract:

Possible impacts of a future grand solar minimum on climate: Stratospheric and global circulation changes
It has been suggested that the Sun may evolve into a period of lower activity over the 21st century.
This study examines the potential climate impacts of the onset of an extreme “Maunder Minimum-like” grand solar minimum using a comprehensive global climate model. Over the second half of the 21st century, the scenario assumes a decrease in total solar irradiance of 0.12% compared to a reference Representative Concentration Pathway 8.5 experiment. The decrease in solar irradiance cools the stratopause (1 hPa) in the annual and global mean by 1.2 K. The impact on global mean near-surface temperature is small (−0.1 K), but larger changes in regional climate occur during the stratospheric dynamically active seasons. In Northern Hemisphere wintertime, there is a weakening of the stratospheric westerly jet by up to 3–4 m s−1, with the largest changes occurring in January–February. This is accompanied by a deepening of the Aleutian Low at the surface and an increase in blocking over Northern Europe and the North Pacific. There is also an equatorward shift in the Southern Hemisphere midlatitude eddy-driven jet in austral spring. The occurrence of an amplified regional response during winter and spring suggests a contribution from a top-down pathway for solar-climate coupling; this is tested using an experiment in which ultraviolet (200–320 nm) radiation is decreased in isolation of other changes. The results show that a large decline in solar activity over the 21st century could have important impacts on the stratosphere and regional surface climate.

Eine weitere Arbeit zur engen Verknüpfung von Stratosphäre und Troposphäre stammt von einer Gruppe um Mengchu Tao, die im Juni 2015 in den Geophysical Research Letters erschien. In den „Key Points“ zur Studie erklären die Autoren, dass sie eine Beeinflussung des tropischen Wasserdampfgehaltes als Folge von stratosphärischen Erwärmungsereignissen gefunden hätten („Tropical water vapor response to stratospheric major warming“). Hier der Abstract:

Impact of stratospheric major warmings and the quasi-biennial oscillation on the variability of stratospheric water vapor
Based on simulations with the Chemical Lagrangian Model of the Stratosphere for the 1979–2013 period, driven by the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalysis, we analyze the impact of the quasi-biennial oscillation (QBO) and of Major Stratospheric Warmings (MWs) on the amount of water vapor entering the stratosphere during boreal winter. The amplitude of H2O variation related to the QBO amounts to 0.5 ppmv. The additional effect of MWs reaches its maximum about 2–4 weeks after the central date of the MW and strongly depends on the QBO phase. Whereas during the easterly QBO phase there is a pronounced drying of about 0.3 ppmv about 3 weeks after the MW, the impact of the MW during the westerly QBO phase is smaller (about 0.2 ppmv) and more diffusely spread over time. We suggest that the MW-associated enhanced dehydration combined with a higher frequency of MWs after the year 2000 may have contributed to the lower stratospheric water vapor after 2000.

Interessant auch eine Publikation in Nature Communications von Andersson et al. aus dem Oktober 2014. Schon der Titel macht neugierig. Es geht um den intensiv gesuchten Klimazusammenhang zwischen Sonne und Erde, über einen Ozon-Verstärker on der Mesosphäre. Die im Artikel aufgeführte „Energetic electron precipitation” (EEP) wird von der Sonnenaktivität beinflusst. Die Autoren beschreiben einen Prozess, der suggeriert, dass EEP eine bedeutende Rolle in der Frage einer solaren Beeinflussung des Erdklimas spielt. Hier der Abstract:

Missing driver in the Sun–Earth connection from energetic electron precipitation impacts mesospheric ozone
Energetic electron precipitation (EEP) from the Earth’s outer radiation belt continuously affects the chemical composition of the
polar mesosphere. EEP can contribute to catalytic ozone loss in the mesosphere through ionization and enhanced production of odd hydrogen. However, the long-term mesospheric ozone variability caused by EEP has not been quantified or confirmed to date. Here we show, using observations from three different satellite instruments, that EEP events strongly affect ozone at 60–80 km, leading to extremely large (up to 90%) short-term ozone depletion. This impact is comparable to that of large, but much less frequent, solar proton events. On solar cycle timescales, we find that EEP causes ozone variations of up to 34% at 70–80 km. With such a magnitude, it is reasonable to suspect that EEP could be an important part of solar influence on the atmosphere and climate system.

Und schließlich: Solare Temperatureffekte im höchsten Teil der Atmosphäre, der Thermosphäre, wurden im April 2014 von einer Gruppe um Martin Mlynczak in den Geophysical Research Letters beschrieben. Temperaturen schwankten hier im Takte des 11-Jahres-Sonnenzyklus, im Zusammenhang mit NO and CO2-Änderungen. Hier der Abstract:

Influence of solar variability on the infrared radiative cooling of the thermosphere from 2002 to 2014
Infrared radiative cooling of the thermosphere
by carbon dioxide (CO2, 15 µm) and by nitric oxide (NO, 5.3 µm) has been observed for 12 years by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics satellite. For the first time we present a record of the two most important thermospheric infrared cooling agents over a complete solar cycle. SABER has documented dramatic variability in the radiative cooling on time scales ranging from days to the 11 year solar cycle. Deep minima in global mean vertical profiles of radiative cooling are observed in 2008–2009. Current solar maximum conditions, evidenced in the rates of radiative cooling, are substantially weaker than prior maximum conditions in 2002–2003. The observed changes in thermospheric cooling correlate well with changes in solar ultraviolet irradiance and geomagnetic activity during the prior maximum conditions. NO and CO2 combine to emit 7 × 1018 more Joules annually at solar maximum than at solar minimum.