Reconstructions of past climate behavior often describe prominent anomalous periods that are not necessarily captured in climate simulations. Here, we illustrate the contrast between an interdecadal ...strong positive phase of the winter Pacific/North American pattern (PNA) in the early 19th century that is described by a PNA reconstruction based on tree rings from northwestern North America, and a slight tendency towards negative winter PNA anomalies during the same period in an ensemble of state-of-the-art coupled climate simulations. Additionally, a pseudo-proxy investigation with the same simulation ensemble allows for assessing the robustness of PNA reconstructions using solely geophysical predictors from northwestern North America for the last millennium. The reconstructed early 19th-century positive PNA anomaly emerges as a potentially reliable feature, although the pseudo-reconstructions are subject to a number of sources of uncertainty and deficiencies highlighted especially at multidecadal and centennial timescales. The pseudo-reconstructions demonstrate that the early 19th-century discrepancy between reconstructed and simulated PNA does not stem from the reconstruction process. Instead, reconstructed and simulated features of the early 19th-century PNA can be reconciled by interpreting the reconstructed evolution during this time as an expression of internal climate variability, which is unlikely to be reproduced in its exact temporal occurrence by a small ensemble of climate simulations. However, firm attribution of the reconstructed PNA anomaly is hampered by known limitations and deficiencies of coupled climate models and uncertainties in the early 19th-century external forcing and background climate state.
The response of the global climate-carbon cycle system to an extremely large Northern Hemisphere mid-latitude volcanic eruption is investigated using ensemble integrations with the comprehensive ...Earth System Model MPI-ESM. The model includes dynamical compartments of the atmosphere and ocean and interactive modules of the terrestrial biosphere as well as ocean biogeochemistry. The MPI-ESM was forced with anomalies of aerosol optical depth and effective radius of aerosol particles corresponding to a super eruption of the Yellowstone volcanic system. The model experiment consists of an ensemble of fifteen model integrations that are started at different pre-ENSO states of a control experiment and run for 200 years after the volcanic eruption. The climate response to the volcanic eruption is a maximum global monthly mean surface air temperature cooling of 3.8 K for the ensemble mean and from 3.3 K to 4.3 K for individual ensemble members. Atmospheric pCO2 decreases by a maximum of 5 ppm for the ensemble mean and by 3 ppm to 7 ppm for individual ensemble members approximately 6 years after the eruption. The atmospheric carbon content only very slowly returns to near pre-eruption level at year 200 after the eruption. The ocean takes up carbon shortly after the eruption in response to the cooling, changed wind fields and ice cover. This physics-driven uptake is weakly counteracted by a reduction of the biological export production mainly in the tropical Pacific. The land vegetation pool shows a decrease by 4 GtC due to reduced short-wave radiation that has not been present in a smaller scale eruption. The gain of the soil carbon pool determines the amplitude of the CO2 perturbation and the long-term behaviour of the overall system: an initial gain caused by reduced soil respiration is followed by a rather slow return towards pre-eruption levels. During this phase, the ocean compensates partly for the reduced atmospheric carbon content in response to the land's gain. In summary, we find that the volcanic eruption has long-lasting effects on the carbon cycle: After 200 years, the ocean and the land carbon pools are still different from the pre-eruption state by 3 GtC and 4 GtC, respectively, and the land carbon pools (vegetation and soil) show some long-lasting local anomalies that are only partly visible in the global signal.
In the present investigation, for the first time, fundamental characteristics of autumn and winter average sequences of sea level heights (SLH) that were recorded in the Lagoon of Venice (northern ...Adriatic, in the Mediterranean Sea) during the period 1872–2004 are investigated. Interannual‐to‐decadal variability of Venetian SLH is found to reflect the variability of the most prominent Euro‐Atlantic teleconnections (EATs). In particular, the North Atlantic Oscillation (NAO), the East Atlantic/Western Russian, and the Scandinavian patterns are found to contribute to generate the ∼5‐year, ∼8‐year and ∼22‐year peaks that dominate the spectra of seasonal Venetian SLH. Among the possible oceanic and atmospheric phenomena downscaling interannual‐to‐decadal large‐scale atmospheric signals into the observed variability in the Venetian SLH, we explore inverse barometer effect, wind‐driven setup, and the thermohaline circulation of both the Adriatic and the Mediterranean seas. All these phenomena are assessed to display some of the typical features of the shared interannual‐to‐decadal variability of both Venetian SLH and EATs. Our analysis shows also that the decadal variability of winter Venetian SLH is closely linked with variations in solar activity: in particular, the winter SLH multidecadal pattern is found to be correlated, with very high statistical confidence, to the Hale Cycles pattern (∼22 years), which describes the series of sunspot cycles with alternating opposite polarity. The marked signature of Hale Cycles on the leading mode of multidecadal sea level pressure winter variability (which is practically indistinguishable from the inverse wintertime NAO) is also detailed to further support the hypothesis of a Sun‐Venetian SLH association.
We investigate the various methods currently available for distinguishing between the Central Pacific (CP) El Niño (or “El Niño Modoki”) and the canonical El Niño by considering nine different ...methods and five sea surface temperature (SST) datasets from 1880 to 2010. This is aimed to demonstrate the variety which exists between different classification methods as well as to help identify years which can be more confidently classified as CP events. Classifying CP El Niños based on the greatest convergence between methods and between SST datasets provides a more robust identification of these events. Analysis of the SST patterns of the CP years identified demonstrates several misclassifications, stressing the importance of not relying solely on indices. After removal, 14 years which are classified the most consistently as CP events include the following: 1885/1886, 1914/1915, 1940/1941, 1958/1959, 1963/1964, 1968/1969, 1977/1978, 1986/1987, 1990/1991, 1991/1992, 1994/1995, 2002/2003, 2003/2004, and 2004/2005. Our findings also indicate the intermittent appearance of CP events throughout the time period investigated, inciting the role of multidecadal natural climate variability in generating CP El Niños.
The decadal evolution of Arctic and Antarctic sea ice following strong volcanic eruptions is investigated in four climate simulation ensembles performed with the COSMOS-Mill version of the Max Planck ...Institute Earth System Model. The ensembles differ in the magnitude of the imposed volcanic perturbations, with sizes representative of historical tropical eruptions (1991 Pinatubo and 1815 Tambora) and of tropical and extra-tropical "supervolcano" eruptions. A post-eruption Arctic sea-ice expansion is robustly detected in all ensembles, while Antarctic sea ice responds only to supervolcano eruptions, undergoing an initial short-lived expansion and a subsequent prolonged contraction phase. Strong volcanic forcing therefore emerges as a potential source of inter-hemispheric interannual-to-decadal climate variability, although the inter-hemispheric signature is weak in the case of eruptions comparable to historical eruptions. The post-eruption inter-hemispheric decadal asymmetry in sea ice is interpreted as a consequence mainly of the different exposure of Arctic and Antarctic regional climates to induced meridional heat transport changes and of dominating local feedbacks that set in within the Antarctic region. Supervolcano experiments help to clarify differences in simulated hemispheric internal dynamics related to imposed negative net radiative imbalances, including the relative importance of the thermal and dynamical components of the sea-ice response. Supervolcano experiments could therefore serve the assessment of climate models' behavior under strong external forcing conditions and, consequently, favor advancements in our understanding of simulated sea-ice dynamics.
Dominant Euro‐Atlantic modes of large‐scale atmospheric variability significantly affect interannual‐to‐decadal Euro‐Mediterranean climate fluctuations, especially in winter. Here, we investigate the ...robustness of historical and projected state and variability of such modes in a CMIP6 multi‐model ensemble of historical and ssp585 future scenario simulations, focusing on the winter season. Results show overall good skills of the historical ensemble to reproduce the observed temporal, spectral and distributional properties of all considered modes. At the end of the 21st Century the ssp585 ensemble yields non‐significant distributional changes for NAO, EAWR, and SCA indices and a transition to a baroclinic structure for EA, with persistent positive anomalies in the mid‐troposphere enhancing globally‐driven warming over the Euro‐Mediterranean region. The hemispheric spatial correlation patterns with temperature and precipitation significantly change for all modes, that is, we observe a significant modulation of the teleconnections associated with each index.
Plain Language Summary
Modes of variability in the large‐scale atmospheric circulation over the Euro‐Atlantic region are evaluated in a multi‐model ensemble of simulations contributing to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) under historical conditions and under a projected future global warming scenario. The modes' key features in the winter season are analyzed in terms of temporal, spectral and distributional properties. Results show that overall CMIP6 models reproduce well the observed properties. The mode known as Eastern Atlantic pattern (EA) is projected to change substantially in the course of the 21st Century above the boundary layer, where the EA index evolves toward persistent positive values. As a positive EA is associated with anomalous warming over Europe, this contributes to enhancing the projected globally‐driven warming.
Key Points
Euro‐Atlantic climate modes are evaluated in CMIP6 historical and ssp585 ensembles using a box‐based method for climate index definition
Models robustly simulate observed spatial patterns of Euro‐Atlantic modes, including their impacts on Mediterranean temperature and precipitation
The ssp585 ensemble indicates non‐significant trends in NAO, EAWR, and SCA, and a progression toward a persistent positive phase of EA
Are simulations and reconstructions of past climate and its variability consistent with each other? We assess the consistency of simulations and reconstructions for the climate of the last millennium ...under the paradigm of a statistically indistinguishable ensemble. In this type of analysis, the null hypothesis is that reconstructions and simulations are statistically indistinguishable and, therefore, are exchangeable with each other. Ensemble consistency is assessed for Northern Hemisphere mean temperature, Central European mean temperature and for global temperature fields. Reconstructions available for these regions serve as verification data for a set of simulations of the climate of the last millennium performed at the Max Planck Institute for Meteorology. Consistency is generally limited to some sub-domains and some sub-periods. Only the ensemble simulated and reconstructed annual Central European mean temperatures for the second half of the last millennium demonstrates unambiguous consistency. Furthermore, we cannot exclude consistency of an ensemble of reconstructions of Northern Hemisphere temperature with the simulation ensemble mean. If we treat simulations and reconstructions as equitable hypotheses about past climate variability, the found general lack of their consistency weakens our confidence in inferences about past climate evolutions on the considered spatial and temporal scales. That is, our available estimates of past climate evolutions are on an equal footing but, as shown here, inconsistent with each other.
Abyssal temperature and velocity observations performed within the framework of the Neutrino Mediterranean Observatory, a project devoted to constructing a km(3)-scale underwater telescope for the ...detection of high-energy cosmic neutrinos, demonstrate cross-fertilization between subnuclear physics and experimental oceanography. Here we use data collected south of Sicily in the Ionian abyssal plain of the Eastern Mediterranean (EM) basin to show for the first time that abyssal vortices exist in the EM, at depths exceeding 2,500 m. The eddies consist of chains of near-inertially pulsating mesoscale cyclones/anticyclones. They are embedded in an abyssal current flowing towards North-Northwest. The paucity of existing data does not allow for an unambiguous determination of the vortex origin. A local generation mechanism seems probable, but a remote genesis cannot be excluded a priori. The presence of such eddies adds further complexity to the discussion of structure and evolution of water masses in the EM.
Venice is an iconic place and a paradigm of huge historical and cultural values at risk. The frequency of the flooding of the city centre has dramatically increased in recent decades, and this threat ...is expected to continue to grow – and even accelerate – through this century. This special issue is a collection of three review articles addressing different and complementary aspects of the hazards causing the floods of Venice, namely (1) the relative sea level rise, (2) the occurrence of extreme water heights, and (3) the prediction of extreme water heights and floods. It emerges that the effect of compound events poses critical challenges to the forecast of floods, particularly from the perspective of effectively operating the new mobile barriers (Modulo Sperimentale Elettromeccanico – MoSE) in Venice and that the relative sea level rise is the key factor determining the future growth of the flood hazard, so that the present defence strategy is likely to become inadequate within this century under a high-emission scenario. Two strands of research are needed in the future. First, there is a need to better understand and reduce the uncertainty of the future evolution of the relative sea level and its extremes at Venice. However, this uncertainty might not be substantially reduced in the near future, reflecting the uncertain anthropogenic emissions and structural model features. Hence, complementary adaptive planning strategies appropriate for conditions of uncertainty should be explored and developed in the future.
Floods in the Venice city centre result from the superposition of several factors: astronomical tides; seiches; and atmospherically forced fluctuations, which include storm surges, meteotsunamis, and ...surges caused by atmospheric planetary waves. All these factors can contribute to positive water height anomalies individually and can increase the probability of extreme events when they act constructively. The largest extreme water heights are mostly caused by the storm surges produced by the sirocco winds, leading to a characteristic seasonal cycle, with the largest and most frequent events occurring from November to March. Storm surges can be produced by cyclones whose centres are located either north or south of the Alps. Historically, the most intense events have been produced by cyclogenesis in the western Mediterranean, to the west of the main cyclogenetic area of the Mediterranean region in the Gulf of Genoa. Only a small fraction of the inter-annual variability in extreme water heights is described by fluctuations in the dominant patterns of atmospheric circulation variability over the Euro-Atlantic sector. Therefore, decadal fluctuations in water height extremes remain largely unexplained. In particular, the effect of the 11-year solar cycle does not appear to be steadily present if more than 100 years of observations are considered. The historic increase in the frequency of floods since the mid-19th century is explained by relative mean sea level rise. Analogously, future regional relative mean sea level rise will be the most important driver of increasing duration and intensity of Venice floods through this century, overcompensating for the small projected decrease in marine storminess. The future increase in extreme water heights covers a wide range, largely reflecting the highly uncertain mass contributions to future mean sea level rise from the melting of Antarctica and Greenland ice sheets, especially towards the end of the century. For a high-emission scenario (RCP8.5), the magnitude of 1-in-100-year water height values at the northern Adriatic coast is projected to increase by 26–35 cm by 2050 and by 53–171 cm by 2100 with respect to the present value and is subject to continued increase thereafter. For a moderate-emission scenario (RCP4.5), these values are 12–17 cm by 2050 and 24–56 cm by 2100. Local subsidence (which is not included in these estimates) will further contribute to the future increase in extreme water heights. This analysis shows the need for adaptive long-term planning of coastal defences using flexible solutions that are appropriate across the large range of plausible future water height extremes.
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