The North Atlantic-European (NAE) region is influenced by many climate factors that could be cooccurring and/or coinciding. These various influences, combined with the pronounced internal atmospheric variability, make separating the distinct impacts on the NAE area difficult. Besides the North Atlantic Oscillation (NAO), which is the strongest pattern of climate variability in the mid and high latitudes of the Northern hemisphere, the NAE region is also affected by the El Niño-Southern Oscillation (ENSO). ENSO is an atmosphere-ocean phenomenon involving SST changes in the tropical Pacific. Its remote influence can be manifested directly through the atmosphere, but also indirectly involving different parts of the ocean.
In this study, we use a set of sensitivity experiments to investigate the potential impact of tropical sea surface temperatures (SSTs) on the signal of geopotential heights at 200 hPa (GH200) over the NAE region. Using an intermediately complex atmospheric general circulation model (ICTP AGCM), we have designed five experiments based on 35-member ensembles of long integrations with SST anomalies prescribed in various regions acting as lower boundary forcing for the model. In the AGCM experiments, the SST forcing was prescribed globally, in the tropical zone of all oceans, only inside the tropical Atlantic area, in the tropical Indian Ocean and limited to the tropical Pacific. Additionally, an experiment containing only climatological SSTs was analysed.
The monthly GH200 signal was calculated based on the difference between the ensemble mean of each experiment and the climatological mean for the considered period. In addition, to separate the impact of the ENSO from other influences, the signal was calculated for ENSO and non-ENSO years, respectively. The influence of ENSO was analyzed in the context of the direct tropical Pacific SST forcing, but also from the aspect of the indirect contribution of other parts of the tropical oceans. The linearity of the response was analyzed with regard to the amplitude of the forcing, but also with regard to the contribution of individual parts of the tropical oceans.
Results have shown that the GH200 signal is the strongest in the late-winter months (January-March) in all experiments. The AGCM experiment with SST boundary forcing prescribed only in the tropical Atlantic consistently yielded the least amount of signal. The strongest signal linked to ENSO events was found in the experiment with the SST forcing prescribed only in the tropical Pacific. It is also demonstrated that the potentially predictable ENSO-related signal over the NAE region is projected onto the East Atlantic pattern.

How to cite: Herceg-Bulić, I., Ivasić, S., and Popović, M.: Impact of tropical SSTs on the late-winter signal over the North Atlantic-European region, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-445, https://doi.org/10.5194/ems2023-445, 2023.

The El-Niño Southern Oscillation (ENSO) is the main known predictor of interannual natural climate variability. It presents several known teleconnections to weather in different regions of the world. When it comes to the influence of ENSO on European climate, the existing literature is mainly focused on the variability of precipitation and, more recently, temperature. In most of these studies, the variability modes of the target variables are directly recovered with techniques such as empirical orthogonal functions, and the role of atmospheric dynamics is assessed a posteriori by considering composites of dynamical variables. Here we take on a different approach, and we directly assess the influence of ENSO on atmospheric dynamics over the Europe and North Atlantic (ENA) region. This is particularly relevant when considering summer, given the vulnerability of parts of this region to increasing risk of drought and summer heatwaves. To conduct this assessment, we use Latent Dirichlet Allocation (LDA), a generative statistical topic model traditionally used in natural language processing, which can also be utilized as a weather pattern recognition technique. In particular, we apply LDA to the ERA5 daily 500 hPa geopotential height anomaly field (Z500). As a result, we obtain a prescribed number of positive and negative Z500 anomaly patterns, each associated with a daily weight. The analysis of the weights time series for each LDA pattern shows a significant association between the ENSO phase and the importance of some Z500 patterns during summer over ENA. By clustering the pattern weights after conditioning on the ENSO phase, we show how this identified teleconnection affects precipitation and temperature variability over the ENA region. 

How to cite: Pons, F. and Yiou, P.: The dynamic fingerprint of ENSO on European summer weather regimes, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-503, https://doi.org/10.5194/ems2023-503, 2023.

During the last decades, CMIP5 models simulate a warming trend in the tropical eastern Pacific that has not been present in observations (Seager et al., 2019). Associated with this, the Walker circulation has experienced a westward migration while CMIP5 models simulate an eastward migration. This mismatch is still present in CMIP6 models and might affect climate projections worldwide through different teleconnections.

In the Caribbean region, CMIP6 models project a strong drying at the end of the 21st century. El Niño-like changes in the Walker circulation are the dominant processes driving the Caribbean drying. The models that project a strong Caribbean drying also simulate generally a strong equatorial eastern Pacific warming trend over the recent decades. Thus, the mismatch between observed and simulated warming trends over the equatorial eastern Pacific questions the reliability of the Caribbean precipitation projections. The warming bias might also have implications for tropical cyclones’ projections in the Atlantic and Pacific through the effect of vertical wind shear, which is related to shifts in the Walker circulation. In addition, the double Intertropical Convergence Zone (ITCZ) bias might be influenced by the mismatching trends. Based on our understanding of the physical processes affecting Caribbean drying, we carry out an emergent constraint analysis to reduce the projected Caribbean precipitation spread. We find that the future drying in the Caribbean might be weaker than the one projected by CMIP6 models.

The strong influence of El Niño-Southern Oscillation (ENSO) dynamics on the world’s climate through different teleconnections demands more in-depth studies addressing the drivers of the Walker circulation and the equatorial Pacific warming bias.

How to cite: Brotons, M., Haarsma, R., Bloemendaal, N., de Vries, H., and Allen, T.: Impact of tropical eastern Pacific warming bias on Caribbean climate., EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-533, https://doi.org/10.5194/ems2023-533, 2023.

The temporal and spatial variability of atmospheric compounds and pollutants is largely driven by a combination of local, mesoscale and synoptic meteorological conditions influencing atmospheric processes, such as horizontal and vertical dispersion, deposition, chemical reactions rates and velocity. At the same time, different anthropogenic and natural radionuclides have long been used as tracers to understand and describe surface and atmospheric processes and their influence on transport and deposition. Among these substances, ⁷Be, a cosmogenic radionuclide produced by  spallation reactions in the stratosphere and upper troposphere, is frequently used as tracer of different atmospheric processes. Many studies have reported how the variability of ⁷Be surface concentration is driven by the movement of air masses, atmospheric deposition processes, tropospheric vertical mixing, and the vertical exchange between the stratosphere and the troposphere. 

With an aim to elucidate the influence of teleconnection and tropospheric circulation dynamics, several studies have investigated the influence of the main modes of large scale and regional climate variability in Europe, such as North Atlantic Oscillation (NAO), Arctic Oscillation (AO), East Atlantic (EA), East Atlantic/Western Russia (EA/WR), Scandinavian pattern (SCAND), and Western Mediterranean Oscillation (WeMO) on the surface ⁷Be activity concentrations. This paper, in particular, presents a summary of recent results achieved by our team working on this topic, focusing firstly at the European scale and then to northern Europe. 

Analysing ⁷Be surface concentrations measured at 15 European sampling stations over 2005–2014 and combining the calculation of air mass trajectories and cluster analysis with time series of teleconnection indices, we found that extremely high values of the ⁷Be surface concentrations are largely connected with the negative phase of NAO and We-MO, and with the positive EA phase. These results showed a latitudinal division between the northern and southern sites, with a similar influence of teleconnection patterns, while the sites located in the central part of Europe present a larger variability in the impact of teleconnection patterns. A detailed analysis in northern Europe revealed that extreme ⁷Be surface concentrations observed during winter were associated with variability of the Arctic polar vortex and linked with high values of the SCAND index.

The findings provide a classification of meteorological conditions associated with high surface ⁷Be concentrations and thus offer a basis for forecasting these events. In a general view, the findings also help in understanding the seasonal and inter-annual atmospheric variability at different scales and its impact on airborne concentrations of radionuclides and pollutants.

How to cite: Hernández-Ceballos, M. Á., Ajtić, J., and Brattich, E.: Understanding the variability of 7Be surface concentrations in Europe: the role of teleconnection patterns, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-204, https://doi.org/10.5194/ems2023-204, 2023.

Characterizing hydrometeorological extreme events can be hampered by the limited availability of observations of past meteorological conditions. Seasonal forecasts, which are commonly used to represent the temporal evolution of the predictable components of the Earth system over periods of a few weeks or months, have been recently applied to obtain information about extreme events and their dynamical drivers: indeed, they have a spatial resolution which is generally higher than that of climate projections and they can be used in order to get a better description of the complex and non-linear interactions taking place between the individual components of the Earth system. In this work, an application of seasonal forecast for the study of meteorological drought over Europe is presented. The paper is organized in three steps. First of all, the realism of simulations provided by C3S multi-system seasonal forecasts is assessed through a bias correction method and a statistical characterization. Then, it is shown how the applied method allows an estimate of the probability of new precipitation deficit which is consistent with that obtained from observations alone over the period 1993-2022 and that presents a markedly reduced uncertainty. At last, the investigation of the 500 hPa geopotential anomaly associated with the most intense meteorological drought events simulated by the model suggests that seasonal forecasts can be a powerful tool to better understand the dynamical drivers of climate extremes at the regional scale and the role played by the involved teleconnections, as highlighted by the similarity with the circulation pattern of the 2022 spring-summer drought event.

How to cite: Buccellato, M., Ruggieri, P., and Porcù, F.: An application of seasonal forecasts to the study of extreme meteorological events: the case of drought., EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-290, https://doi.org/10.5194/ems2023-290, 2023.

Several studies show that the occurrence of summer extreme temperatures over Europe is increased since the middle of the twentieth century and is expected to further increase in the future due to global warming. Thus, due to the impacts that extreme temperatures have on socio-economic and environmental systems, predicting heat extremes and their statistics several months ahead is crucial.

State-of-the-art dynamical prediction systems show low skills in predicting European heat extremes on seasonal timescale. This is deemed to be due to the combination of the internal chaotic nature of the atmosphere and the underestimation of predictable component of the climate variability in the model ensemble. Recent studies have shown that our skills in predicting extratropical climate can be largely improved by refining the ensemble-based dynamical prediction systems with statistical post-processing techniques. Such techniques are based on sub-sampling the model ensemble by selecting only members that verify specific conditions.

The present study assesses the prediction skill of summer extreme temperatures over Europe on seasonal timescale in the Copernicus Climate Change Service (C3S) multi-systems seasonal forecasts for the period 1993—2016. Then, a hybrid statistical-dynamical prediction system is presented, where the model ensemble is sub-sampled by retaining only a subset of members which predict the summer extreme temperature statistics over Europe in agreement with a teleconnection-based statistical prediction (following Dobrynin et al. 2022). This approach relies on predictors of European heat extremes during spring season (e.g. sea surface temperature in the North Atlantic and tropical Pacific, soil moisture and sea-ice concentration) and thus allows us to retain only those members with a reasonable representation of summer heat extreme teleconnections.

Results on the skill of the hybrid statistical-dynamical prediction system and its potential applications for the health-sector are discussed.

How to cite: Famooss Paolini, L., Ruggieri, P., Pascale, S., Brattich, E., and Di Sabatino, S.: Hybrid statistical-dynamical seasonal prediction of summer extreme temperatures over Europe, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-440, https://doi.org/10.5194/ems2023-440, 2023.

Previous studies have shown that the centers of action of the North Atlantic Oscillation (NAO), leading mode of regional atmospheric variability, shifted eastward in the mid-1970s compared to the preceding decades of the mid-twentieth century. They pointed at increasing greenhouse gas forcing and/or low-frequency oceanic variability as potential players. Here, the possible driving mechanisms of this shift are further investigated using long-term reanalysis data (NOAA 20CR, ERA 20CR) and two 500-year climate simulations performed with EC-EARTH 3.3 (CMIP6 version) keeping fixed the radiative forcing at present-day conditions [year 2000], in atmosphere-only and coupled ocean-atmosphere configurations. 30-year running empirical orthogonal functions (EOFs) of sea level pressure anomalies in the North Atlantic-European region are computed to track the longitudinal displacements of the NAO pattern, first showing that a similar eastward shift happened at the beginning of the observational record. Recurrent eastern and western locations of the NAO pattern are properly simulated by the model, with marginal differences between the atmosphere-only and coupled configurations. Moreover, the fraction of variance explained by the running EOFs varies with time displaying a comparable range (~30% to 60%) in observations and model simulations. Composites of eastern and western NAO patterns, based on upper and lower terciles, are built to show that the longitudinal shifts appear linked to changes in the variability of the regional eddy-driven zonal wind. The similarity in the NAO displacements between observations, atmosphere-only and coupled simulations suggests that the observed eastward shift could have occurred previously and might not be due to anthropogenic or oceanic forcing but to pure internal variability. 

How to cite: Santolaria-Otín, M. and García-Serrano, J.: On the origin of North Atlantic Oscillation longitudinal displacements, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-492, https://doi.org/10.5194/ems2023-492, 2023.

The interaction between the North Atlantic Oscillation (NAO) and the latitudinal shifts of Gulf Stream sea surface temperature front (GSF) has been the subject of extensive investigations. There are indications of non-stationarity in this interaction, but differences in the methodologies used in previous studies make it difficult to draw consistent conclusions. Furthermore, there is a lack of consensus on the key mechanisms underlying the response of the GSF to the NAO forcing. This study assesses the possible non-stationarity in the NAO—GSF interaction and the mechanisms underlying this interaction over the last few decades, using reanalysis data.

Results show that the NAO and GSF indices covary on the decadal timescales but only during 1972—2018. A secondary peak in the NAO-GSF covariability emerges on multiannual timescales but only during 2005—2015. The non-stationarity in the decadal NAO-GSF covariability is also manifested through the dependency of their lead—lag relationship on the analyzed time period. Indeed, the NAO leads the GSF shifts by 3 years during 1972—1990 and by 2 years during 1990—2018.

The lag between GSF shifts and NAO can be interpreted as the joint effect of the fast response of wind-driven oceanic circulation, the lagged response of deep oceanic circulation, and the propagation of Rossby waves. However, there is evidence of Rossby wave propagation only before 1990. Here it is suggested that the non-stationarity of Rossby wave propagation causes the time lag between the NAO and the GSF latitudinal position on decadal timescales to differ before and after 1990.

Considering the impact that the GSF variability has on the North Atlantic variability, the non-stationarity in the NAO—GSF covariability has important implications for the predictability of the North Atlantic sector.

How to cite: Famooss Paolini, L., Omrani, N., Bellucci, A., Athanasiadis, P. J., Ruggieri, P., Patrizio, C. R., and Keenlyside, N.: Non-stationary NAO—Gulf Stream SST front interaction, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-11, https://doi.org/10.5194/ems2023-11, 2023.

Vegetation is a relevant and highly dynamic component of the Earth System controlling, amongst others, surface roughness, albedo and evapotranspiration; its variability shows changes in seasons, interannual, decadal and longer timescales. In this study, we investigate the effects of improved representation of vegetation dynamics on climate predictions at different timescales: seasonal and decadal. To this aim, the latest generation satellite datasets of vegetation characteristics have been exploited, and a novel and improved parameterization of the effective vegetation cover has been developed. The new parameterization is implemented in the land surface scheme HTESSEL shared by two state-of-the-art Earth system models: ECMWF SEAS5 and EC-Earth3. The former model is used for sensitivity at seasonal timescale, while the latter is for sensitivity at decadal timescale.

Both seasonal and decadal experiments show considerable sensitivity of models' surface climate bias with large effects on December-January-February (DJF) T2M, mean sea level pressure and zonal wind over middle-to-high latitudes. Consistently, a significant improvement in the skill for DJF T2M is found, especially over Euro-Asian Boreal forests. For seasonal experiments, this improvement displays a strong interannual coupling with the local surface albedo. 

From the region with the most considerable T2M improvement, over Siberia, originates a large-scale effect on circulation encompassing Northern Hemisphere middle-to-high latitudes from Siberia to the North Atlantic. As a result, in seasonal experiments, the correlation between the model NAO index against the ERA5 NAO index improves significantly.

These results show a non-negligible effect of the vegetation cover on the general circulation, especially for the northern hemisphere and on the prediction skill.

How to cite: Di Carlo, E., Alessandri, A., van Oorschot, F., Cherchi, A., Corti, S., Balsamo, G., Boussetta, S., and Stockdale, T.: Effects of the realistic vegetation cover on the large-scale circulation at seasonal and decadal time scales., EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-559, https://doi.org/10.5194/ems2023-559, 2023.

How to cite: D'Andrea, F., Portal, A., Davini, P., Hamouda, M. E., and Pasquero, C.: Atmospheric conditions associated with cold East-Asian orography in climate models, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-332, https://doi.org/10.5194/ems2023-332, 2023.

Recent studies propose that the Asian-Bering-North American (ABNA) teleconnection is a distinct atmospheric pattern that is related to Eurasian and North American winter climate besides the Pacific-North America (PNA) pattern, while its origin remains elusive. This study investigates the interannual variability of the ABNA during the past 42 winters (1979-2020) and the associated prior surface boundary forcings. The ABNA explains coherent surface air temperature changes in northern Asia, Eastern Siberia-Alaska, and eastern North America, even after removing the impacts of the PNA, the Arctic Oscillation, the North Atlantic Oscillation, and the North Pacific Oscillation. Surface boundary conditions linked to the ABNA could be traced back to a Eurasian snow cover dipole pattern (ESCDP) and a Maritime Continent sea surface temperature anomaly (MCSST) in November. The ESCDP leads to a displacement of the Arctic stratospheric polar vortex via troposphere-stratosphere coupling. The anomalous polar vortex propagates downwards in the following winter and generates the tropospheric ABNA pattern. The MCSST induces a diabatic heating anomaly, which is associated with a tropical western Pacific precipitation anomaly (TWPP) in winter. The TWPP excites a poleward Rossby wave train that propagates across the North Pacific and directly strengthens the ABNA. The above physical processes can be well reproduced by a linear baroclinic model (LBM). Based on the ESCDP and MCSST predictors, an empirical model is established and shows a promising prediction skill of the ABNA during the hindcast period. This can provide a useful strategy for seasonal prediction of winter climate in the Northern Hemisphere extratropics. 

How to cite: Zhong, W. and Wu, Z.: Interannual variability of the wintertime Asian-Bering-North American teleconnection linked to Eurasian snow cover and Maritime Continent sea surface temperature, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-582, https://doi.org/10.5194/ems2023-582, 2023.