Davide Zanchettin

Bi-decadal variability excited in the coupled ocean–atmosphere system by strong tropical volcanic eruptions

Decadal and bi-decadal climate responses to tropical strong volcanic eruptions (SVEs) are inspected in an ensemble simulation covering the last millennium based on the Max Planck Institute—Earth system model. An unprecedentedly large collection of pre-industrial SVEs (up to 45) producing a peak annual-average top-of-atmosphere radiative perturbation larger than −1.5xa0Wm−2 is investigated by composite analysis. Post-eruption oceanic and atmospheric anomalies coherently describe a fluctuation in the coupled ocean–atmosphere system with an average length of 20–25xa0years. The study provides a new physically consistent theoretical framework to interpret decadal Northern Hemisphere (NH) regional winter climates variability during the last millennium. The fluctuation particularly involves interactions between the Atlantic meridional overturning circulation and the North Atlantic gyre circulation closely linked to the state of the winter North Atlantic Oscillation. It is characterized by major distinctive details. Among them, the most prominent are: (a) a strong signal amplification in the Arctic region which allows for a sustained strengthened teleconnection between the North Pacific and the North Atlantic during the first post-eruption decade and which entails important implications from oceanic heat transport and from post-eruption sea ice dynamics, and (b) an anomalous surface winter warming emerging over the Scandinavian/Western Russian region around 10–12xa0years after a major eruption. The simulated long-term climate response to SVEs depends, to some extent, on background conditions. Consequently, ensemble simulations spanning different phases of background multidecadal and longer climate variability are necessary to constrain the range of possible post-eruption decadal evolution of NH regional winter climates.

Aerosol size confines climate response to volcanic super-eruptions

Extremely large volcanic eruptions have been linked to global climate change, biotic turnover, and, for the Younger Toba Tuff (YTT) eruption 74,000 years ago, near-extinction of modern humans. One of the largest uncertainties of the climate effects involves evolution and growth of aerosol particles. A huge atmospheric concentration of sulfate causes higher collision rates, larger particle sizes, and rapid fall out, which in turn greatly affects radiative feedbacks. We address this key process by incorporating the effects of aerosol microphysical processes into an Earth System Model. The temperature response is shorter (9–10 years) and three times weaker (−3.5 K at maximum globally) than estimated before, although cooling could still have reached −12 K in some midlatitude continental regions after one year. The smaller response, plus its geographic patchiness, suggests that most biota may have escaped threshold extinction pressures from the eruption.

Central Pacific El Niño, the “subtropical bridge,” and Eurasian climate

[1]xa0This study contributes to the discussion on possible effects of El Nino on North Atlantic/European regional climates. We use NCEP/NCAR reanalysis data to show how the two different types of El Ninos (the central Pacific, or CP, and the east Pacific, or EP) result in remarkably different European winter temperature anomalies, specifically weak warming during EP and significant cooling during CP El Ninos, the latter being associated with a negative phase of the winter North Atlantic Oscillation (NAO). Our results diverge from former suggestions addressing the weakened stratospheric polar vortex as the dominant factor contributing to the El Nino/NAO teleconnection. We propose a tropospheric bridge as the mechanism primarily responsible for the establishment of a negative NAO phase and of associated cold European winters. This mechanism includes the subtropical jet (STJ) waveguide being activated only during CP El Ninos, when anomalous convective heating occurs near the edge of the Pacific warm pool. Under these conditions the STJ is enhanced by planetary wave flux divergence in the subtropical upper troposphere, providing favorable conditions for the propagation of a wave number 5 disturbance around the subtropical Northern Hemisphere. This wave contributes to weakening of the Azores High and, hence, to the negative NAO phase. As global warming scenarios project an increase in the frequency of CP El Nino events, the distinctive nature of this mechanism implies that the probability of cold European winters may increase as well in future decades.

European summer temperatures since Roman times

The spatial context is criticalwhen assessing present-day climate anomalies, attributing them to potential forcings and making statements regarding their frequency and severity in a long-term perspective. Recent international initiatives have expanded the number of high-quality proxy-records and developed new statistical reconstruction methods. These advances allow more rigorous regional past temperature reconstructions and, in turn, the possibility of evaluating climate models on policy-relevant, spatiotemporal scales. Here we provide a new proxy-based, annually-resolved, spatial reconstruction of the European summer (June-August) temperature fields back to 755 CE based on Bayesian hierarchical modelling (BHM), together with estimates of the European mean temperature variation since 138 BCE based on BHM and composite-plus-scaling (CPS). Our reconstructions compare well with independent instrumental and proxy-based temperature estimates, but suggest a larger amplitude in summer temperature variability than previously reported. Both CPS and BHM reconstructions indicate that the mean 20th century European summer temperature was not significantly different from some earlier centuries, including the 1st, 2nd, 8th and 10th centuries CE. The 1st century (in BHM also the 10th century) may even have been slightly warmer than the 20th century, but the difference is not statistically significant. Comparing each 50 yr period with the 1951-2000 period reveals a similar pattern. Recent summers, however, have been unusually warm in the context of the last two millennia and there are no 30 yr periods in either reconstruction that exceed the mean average European summer temperature of the last 3 decades (1986-2015 CE). A comparison with an ensemble of climate model simulations suggests that the reconstructed European summer temperature variability over the period 850-2000 CE reflects changes in both internal variability and external forcing on multi-decadal time-scales. For pan-European temperatures we find slightly better agreement between the reconstruction and the model simulations with high-end estimates for total solar irradiance. Temperature differences between the medieval period, the recent period and the Little Ice Age are larger in the reconstructions than the simulations. This may indicate inflated variability of the reconstructions, a lack of sensitivity and processes to changes in external forcing on the simulated European climate and/or an underestimation of internal variability on centennial and longer time scales.

Multidecadal-to-centennial SST variability in the MPI-ESM simulation ensemble for the last millennium

We assess the responses of North Atlantic, North Pacific, and tropical Indian Ocean Sea Surface Temperatures (SSTs) to natural forcing and their linkage to simulated global surface temperature (GST) variability in the MPI-Earth System Model simulation ensemble for the last millennium. In the simulations, North Atlantic and tropical Indian Ocean SSTs show a strong sensitivity to external forcing and a strong connection to GST. The leading mode of extra-tropical North Pacific SSTs is, on the other hand, rather resilient to natural external perturbations. Strong tropical volcanic eruptions and, to a lesser extent, variability in solar activity emerge as potentially relevant sources for multidecadal SST modes’ phase modulations, possibly through induced changes in the atmospheric teleconnection between North Atlantic and North Pacific that can persist over decadal and multidecadal timescales. Linkages among low-frequency regional modes of SST variability, and among them and GST, can remarkably vary over the integration time. No coherent or constant phasing is found between North Pacific and North Atlantic SST modes over time and among the ensemble members. Based on our assessments of how multidecadal transitions in simulated North Atlantic SSTs compare to reconstructions and of how they contribute characterizing simulated multidecadal regional climate anomalies, past regional climate multidecadal fluctuations seem to be reproducible as simulated ensemble-mean responses only for temporal intervals dominated by major external forcings.

Delayed winter warming: A robust decadal response to strong tropical volcanic eruptions ?

[1]xa0Climate simulations suggest that strong tropical volcanic eruptions (SVEs) induce decadal dynamical responses in the coupled ocean-atmosphere system, which protract the climate recovery beyond the short-lived radiative forcing. Here, for the first time, we diagnose the signature of such responses in European seasonal climate reconstructions over the past 500 years. The signature consists of a decadal-scale positive phase of the winter North Atlantic Oscillation accompanied by winter warming over Europe peaking approximately one decade after a major eruption. The reconstructed delayed winter warming is compatible with formerly suggested mechanisms behind simulated SVE-driven climate responses, thus corroborating the existence of SVE-driven decadal climate variability. Historical climate-state uncertainty may, however, hamper unambiguous statistical and dynamical assessments both for multiple and for individual SVEs.

Impact of variations in solar activity on hydrological decadal patterns in northern Italy

[1]xa0Using spectral and statistical analyses of discharges and basin average precipitation rates acquired over the Po River since the early 1800s, we investigate the impact of variations in solar activity on hydrological decadal patterns over northern Italy. Wet and dry periods appear to alternate in accordance with polarized sunspot cycles. Intriguingly, a solar signature on Po River discharges is detected to be highly significant since the late 1800s, before the onset of sunspots hyperactivity established by the middle 1900s. In particular, observed hydrological patterns over northern Italy are significantly correlated, under periods of quiet sunspot activity, with parameters characterizing the Suns orbital motion, specifically with the time derivative of the solar angular momentum (τ) which is thought to modulate the strength of the solar wind and sunspot dynamics under weak sunspot activity. The North Atlantic Oscillation (NAO) is detected as potential link between the Sun and Po River discharges, since it is significantly correlated with both solar activity and the decadal variability in the north Italian climate. In particular, positive (negative) NAO anomalies, which are associated with comparatively lower (higher) Po River discharges, are assessed to alternatively correlate at decadal timescales either with τ or with the Earths geomagnetic activity (GA), which closely follows sunspot activity. This changing correlation seems to be regulated by the strength of sunspot activity: under periods of quiet sunspot activity, a weakening of the GA-NAO connection and a reinforcement of the τ-NAO connection is observed. In this sense, the strength of solar activity apparently modulates the connection between the NAO and Po River discharges.

Intermittent multidecadal-to-centennial fluctuations dominate global temperature evolution over the last millennium,.

[1]xa0Observed climate time series covering several centuries are often characterized by fluctuations on multidecadal-to-centennial timescales. These are not homogeneously distributed in time: Instead, they appear within irregularly intermittent temporal intervals, whose irregular duration varies, in general, with the signal fluctuation frequency. A similar irregularly intermittent, frequency-dependent appearance of energetic fluctuations is found in long-term Earth system model integrations, consisting of a multi-millennia control experiment (i.e., an unforced simulation) and forced simulations covering the last millennium. Here, for the first time, we investigate the long-term relative importance of internal and externally-driven variability and their possible interferences on Global Surface Temperature (GST). Multidecadal GST fluctuations are mostly associated to internal variability. Externally-forced perturbations acting predominantly on centennial timescales tend to overwhelm such variability and to enhance O(∼200 years) GST fluctuations. Externally-forced perturbations tend also to correspond to major changes in the coherency among internal climate processes, and among them and GST.

Influence of the circumglobal wave-train on European summer precipitation

We investigate European summer (July–August) precipitation variability and its global teleconnections using the NCEP/NCAR reanalysis data (1950–2010) and a historical Coupled Model Intercomparison Project climate simulation (1901–2005) carried out using the ECHAM6/MPIOM climate model. A wavelike pattern is found in the upper tropospheric levels (200xa0hPa) similar to the summer circumglobal wave train (CGT) extending from the North Pacific to the Eurasian region. The positive phase of the CGT is associated with upper level anomalous low (high) pressure over western (eastern) Europe. It is further associated with a dipole-like precipitation pattern over Europe entailing significantly enhanced (reduced) precipitation over the western (eastern) region. The anomalous circulation features and associated summer precipitation pattern over Europe inverts for the negative CGT phase. Accordingly, the global teleconnection pattern of a precipitation index summarizing summer precipitation over Western Europe entails an upper level signature which consists of a CGT-like wave pattern extending from the North Pacific to Eurasia. The imprint of the CGT on European summer precipitation is distinct from that of the summer North Atlantic Oscillation, despite the two modes of variability bear strong similarities in their upper level atmospheric pattern over Western Europe. The analysis of simulated CGT features and of its climatic implications for the European region substantiates the existence of the CGT-European summer precipitation connection. The summer CGT in the mid-latitude therefore adds to the list of the modes of large-scale atmospheric variability significantly influencing European summer precipitation variability.

Different flavors of the Atlantic Multidecadal Variability

We investigate how differently-constructed indices for North Atlantic sea-surface temperatures (NASSTs) describe the “Atlantic Multidecadal Variability” (AMV) in a suite of unperturbed as well as externally-forced millennial (pre-industrial period) climate simulations. The simulations stem from an ensemble of Earth system models differing in both resolution and complexity. Different criteria exist to construct AMV indices capturing different aspects of the phenomenon. Although all representations of the AMV maintain strong multidecadal variability, they depict different characteristics of simulated low-frequency NASST variability, evolve differently in time and relate to different hemispheric teleconnections. Due to such multifaceted signatures in the ocean-surface as well as in the atmosphere, reconstructions of past AMV may not univocally reproduce multidecadal NASST variability. AMV features under simulated externally-forced pre-industrial climate conditions are not unambiguously distinguishable, within a linear framework, from AMV features in corresponding unperturbed simulations. This prevents a robust diagnosis of the simulated pre-industrial AMV as a predominantly internal rather than externally-forced phenomenon. We conclude that a multi-perspective assessment of multidecadal NASSTs variability is necessary for understanding the origin of the AMV, its physics and its climatic implications.