Tropical sea surface temperatures (SSTs) play a crucial role as a source of boundary-forced predictability for the atmosphere in the extratropics, influencing atmospheric dynamics characterised by significant internal variability and limited predictability. One of the most influential events determining global climate variability is the El Niño-Southern Oscillation (ENSO). Over the North-Atlantic European region (NAE), many processes obscure the manifestation of ENSO. In this study, we analyse the ENSO-induced signal over the NAE region and investigate the contributions of the individual tropical basins and the mid-latitude North Atlantic SSTs. Using an intermediate-complexity atmospheric general circulation model (ICTP AGCM), we performed a set of targeted experiments using 35-member ensembles of long integrations with SST anomalies in different regions that served as a lower boundary forcing for the model. This experimental framework facilitated the separation of the influences from individual basins and allowed the estimation of their respective contributions to the overall signal. Our analysis shows a recognisable atmospheric response occurring primarily in the late winter months, with the strongest signal associated with ENSO events. The competing influences emanating from individual tropical basins are highlighted. At the same time, the superposition effect of the extratropical North Atlantic SSTs is demonstrated through the modulation of the storm tracks. Although the atmospheric response to the midlatitude SST forcing is weak, extratropical air-sea interaction still can be an important modifier of local atmospheric variability.  Both the model results and the NOAA-CIRES-DOE 20th Century Reanalysis variance of geopotential heights (GH200) show the presence of an ENSO signature, which manifests itself as a distinct pattern in the North Atlantic and projects onto the East Atlantic pattern.

How to cite: Herceg-Bulić, I., Ivasić, S., and Popović, M.: The impact of the extratropical North Atlantic SSTs on the ENSO signal in the North Atlantic-European region, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-133, https://doi.org/10.5194/ems2024-133, 2024.

El Niño-Southern Oscillation (ENSO) is the main mode of climate variability at interannual timescales, impacting the climate of many regions worldwide through atmospheric teleconnections. Characterising these teleconnections is relevant to improve seasonal prediction skill over the regions affected by ENSO. However, the early-winter (November-December; ND) North Atlantic and European (NAE) atmospheric circulation response to ENSO and its associated impacts on surface air temperature in Europe remain unclear. Hence, the ENSO early-winter teleconnection to temperature in Europe is analysed in this study. Unlike most of the previous studies addressing this teleconnection, our analysis is carried out in different time periods, taking into account the climate shifts that occurred in the Pacific in the 1970’s and in the 2000’s. This fact allows for the possibility of a non-stationary teleconnection due to changes in the mean circulation. The study also aims at analysing the ability of the European Centre for Medium Range Weather Forecasts’ (ECMWF) seasonal forecast system System 5 (SEAS5) to reproduce the ENSO early-winter teleconnection. Results show that ENSO has a significant impact on the early-winter temperature in Europe, with a clearly non-stationary behaviour of the teleconnection. Between the 1950s and mid-1970s, impacts on temperature were restricted to the far north of Europe, and since the late 1970s, they have extended to much of the rest of Europe, with the largest impact since the 2000s being located in the southwest. In this region, ENSO shows significant correlation with the leading mode of seasonal early-winter temperature variability. These changes are related to interdecadal changes in the ENSO wave train pattern that reaches the NAE region. ECMWF’s SEAS5 captures reasonably well the spatial pattern of the teleconnection, although with a notably weaker signal compared to observations. Overall, these results may contribute to enhance early-winter seasonal predictability over Europe.

How to cite: Fernández-Castillo, P., Losada, T., Rodríguez-Fonseca, B., García-Maroto, D., Durán, L., and Mohino, E.: Multidecadal variability of the ENSO teleconnection to early-winter temperature in Europe, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-789, https://doi.org/10.5194/ems2024-789, 2024.

We investigate the impact of Tropical Atlantic seasonal forecast biases on the North Atlantic. The analysis focuses on the hindcasts of the Met Office Global Seasonal forecast system (GloSea).  A tripolar bias occurs in Tropical Atlantic rainfall, where the seasonal forecast system is too dry on the equator and too wet to the north and south of the equator, showing a “double ITCZ” pattern. Our analysis uses a novel ensemble-based method to estimate the impact of this tropical rainfall bias on forecasts of the Extratropical North Atlantic. Prior to completing the analysis, ENSO is regressed out of the fields to ensure this common teleconnection is not the source of this bias. The inter-ensemble spread of the forecast model is used to estimate the impact of the bias in Tropical Atlantic rainfall on the North Atlantic by selecting model members that happen to produce forecast anomalies that most closely resemble the tropical rainfall bias and using these as a proxy for the model error. The Tropical Atlantic rainfall bias impacts Rossby wave sources over the Subtropical Atlantic and there is a clear Rossby wave pattern originating from this area which is comparable to the mean bias in hindcasts. The bias impacts the PMSL over the North Atlantic and projects onto the NAO pattern. We show that the bias is not limited to the GloSea System and is also seen in other seasonal forecast systems. Our results suggest that Tropical Atlantic rainfall errors explain a significant amount of the mean bias in seasonal forecasts over the Extratropical North Atlantic.

How to cite: Collier, T., Kettleborough, J., Scaife, A., Hermanson, L., and Davis, P.: Tropical Atlantic rainfall drives bias in extratropicalseasonal forecasts, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-119, https://doi.org/10.5194/ems2024-119, 2024.

Tropical Atlantic Variability (TAV) exerts a significant influence on the climate of different regions through distinct mechanisms that can be identified. Understanding these mechanisms and their impacts can improve the predictability of the North Atlantic-European winter climate. The Atlantic Niño (ATLN) or Equatorial Mode is known for being the dominant pattern of TAV, whose dynamics are similar to El Niño-Southern Oscillation in the tropical Pacific. This study aims at exploring the atmospheric response to positive/negative winter ATLN, as it has been much less documented than the summer ATLN. To this end, a set of sensitivity experiments has been performed with two global atmosphere models of different complexity (EC-EARTH and SPEEDY) to ensure that results are model-independent. The experiments have been designed as atmosphere-only simulations that comprise 150-year long integrations keeping the radiative forcing fixed at present-day climate conditions, covering the extended winter season from November to March: a control run with observed climatological SSTs over 1980-2010 (CTRL), and two sensitivity runs prescribing positive (ATLNiño) and negative (ATLNiña) SST anomalies with climatology elsewhere. The atmospheric response is evaluated by comparing the ensemble mean of each sensitivity run with CTRL. The results show a local Gill-type structure, symmetrically straddling the equator, whose amplitude is slightly larger in response to the heating than to the cooling, and increases from November-December to January-February. In the extratropics, the upper-tropospheric circulation displays a dipolar structure with cyclonic (anticyclonic) anomalies at mid-latitudes and anticyclonic (cyclonic) anomalies at subpolar latitudes, associated with ATLNiño (ATLNiña), which is different from the North Atlantic Oscillation (NAO). In addition to the impact on tropical convection, depicting a southward (northward) shift during ATLNiño (ATLNiña), the precipitation anomalies show a robust and approximately-linear signal on the European continent.

How to cite: Gil-Reyes, L., García-Serrano, J., and Kucharski, F.: Atmospheric response to the winter Atlantic Niño/Niña, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-568, https://doi.org/10.5194/ems2024-568, 2024.

Monsoons are composed by a lower cyclone coupled to an upper anticyclone by an intense mid-tropospheric updraft, surrounded by a larger region of weaker downdraft. In this work, the dynamics of the South Asia monsoon (SAM) and of the West African monsoon (WAM) are analyzed using Gill's tropospheric model (GTM). 

Since their dynamics are fueled by the latent heat released by the marine air masses, which cyclonically spiral inland from nearby tropical oceans in the warm season, the focus of this work is on the impact of the presence and in the absence of an Ekman frictional layer (EFL), of the Somali mountains and of the desert heal lows (DHLs) on these trajectories.

Results show that the addition of a lower Ekman frictional layer (EFL) to the GTM brakes the antisymmetry between the upper and the lower layer, making the lower cyclone deeper and more compact than the upper anticyclone; hence, the marine air particles reach the monsoons with tighter spirals. In addition, the presence of Ekman pumping weakens the low level subsidence about the monsoons, widening the monsoonal updraft region. 

In the absence of the East Africa mountains, particles from the Atlantic ocean can reach SAM, and particles from the Indian ocean can reach WAM. The Somali mountains separate WAM's catch basin of marine air from SAM's catch basin in the EFL; thus, the Atlantic and the Mediterranean air particles reach WAM, whilst the Indian ocean air particles reach SAM; while, the two monsoons are still coupled in the upper levels.

The Atlantic air particles sink over the Azores, the Mediterranean air particle sink over the Eastern Atlantic. The particles originating from the Indian ocean sink over the Arabian and Persian deserts. 

The Saharan-Arabian desert heat lows (DHL) deform the air particle trajectories and the intertropical front (ITF) over Africa; the Western Sahara heat low displaces the ITF northwards; whilst, the eastern Saharan-Arabian heat low displaces it southwards.

How to cite: Dalu, G. and Baldi, M.: Marine air particle trajectories into the Asia-Africa monsoonal system, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-788, https://doi.org/10.5194/ems2024-788, 2024.

The majority of Asian mountain region is made up of the Tibetan Plateau (TP), often referred to as the “Third Pole”. The TP, characterized by its high elevation, is a critical and sensitive region for climate change, experiencing a warming trend at a rate twice that of the global average. Its significance extends beyond local climate impacts, exerting a profound influence in shaping global climate dynamics. While its influence on the Asian is well-documented, its impact on the remote Arctic region has remained unclear. By conducting orographically sensitive experiments using the Whole Atmosphere Community Climate Model Version 6, we shed light on the intricate relationship between the TP and Arctic climate variability. The results reveal that the Asian topography significantly influences Arctic pressure anomalies by modulating the Aleutian Low–Icelandic Low seesaw teleconnection. Furthermore, the topography facilitates the stratosphere-troposphere coupling, leading to the change of the stratospheric polar vortex in its strength and position. In addition to the impact on atmospheric circulation, the response of the Arctic ozone levels to the removal of the Asian mountain is also investigated. It is demonstrated that the topography significantly enhances the Arctic stratospheric ozone during winter, with a notable increase of 15% in the lower Arctic stratosphere. This increase is attributed to the intensified residual mean circulation transport, which is primarily influenced by the robust strengthening of planetary waves induced by Asian topography. The enhancement of planetary waves results in a weakened polar vortex, further promoting ozone accumulation in the Arctic region, ultimately influencing the dynamics and chemistry of the Arctic stratosphere. Overall, this research underlines the significant role of the Asian topography in driving Arctic climate variability and stratospheric ozone dynamics, providing valuable insights into the mechanisms underlying paleoclimatic evolution and atmospheric circulation.

How to cite: Pan, Z., Duan, A., and Wang, Q.: Influence of Asian Mountains on Arctic Pressure System and the Stratospheric Ozone, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-94, https://doi.org/10.5194/ems2024-94, 2024.

Most studies on atmospheric variability in the Northern Hemisphere (NH) have traditionally focused on the North Atlantic and North Pacific regions, due to the prominence of two main teleconnection patterns: the North Atlantic Oscillation (NAO) and the Pacific-North America (PNA). Despite being relatively overlooked compared to those two regions, Eurasia has gained an emerging interest due to its role in Arctic-mid latitude linkages, particularly in the so-called Warm Arctic-Cold Eurasia (WACE) pattern. The dynamics underlying this latter covariance seem to include an anticyclonic circulation over the Siberian coast whose identification has been ambiguous in literature. Here, we have diagnosed and characterized two distinct atmospheric variability modes, namely the Ural-Siberian (USib) pattern and the Siberian High (SH). The Ural-Siberian pattern, sometimes referred to as the Scandinavian (SCAND) pattern at upper levels, is identified as a regional mode of circulation variability maintained by transient-eddy activity and with a barotropic wave-like structure associated with Rossby wave propagation. Whereas Siberian High variability is mainly driven by radiative processes, i.e. the cooling effect of snow cover, and displays a baroclinic structure. While the signature on surface temperature of both atmospheric modes exhibits a dipole that can be identified as the WACE pattern, our results show clear spatial differences among them, being actually in quadrature (out-of-phase). The USib-related temperature signal shows positive anomalies over the Barents-Kara Seas associated with sea ice reduction and negative anomalies over the whole Eurasian continent related to northerly cold advection from the Arctic. In contrast, the SH signal depicts positive anomalies over northern Eurasia linked to warm advection from the south and negative anomalies around the core of the Siberian High due to an increase in snow cover. Further, we find that the vertical extension into the stratosphere of both patterns is distinct, with only the USib signal being consistent with an impact on the polar vortex.

How to cite: Santolaria-Otín, M., García-Serrano, J., Mori, M., Okajima, S., Nakamura, H., Martineau, P., and Wegmann, M.: Observational evidence differentiating Siberian High variability from the Ural-Siberian pattern, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-865, https://doi.org/10.5194/ems2024-865, 2024.

How to cite: Gargiulo, F., Ruggieri, P., Famooss Paolini, L., and Di Sabatino, S.: A hybrid statistical-dynamical approach for seasonal prediction of the boreal winter stratosphere, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-558, https://doi.org/10.5194/ems2024-558, 2024.

In the last decades, there has been an ongoing discussion about whether the winter NAO has a deep barotropic and annular signature, being the stratosphere a key player of this variability, or, on the contrary, the NAO presents a wave-like behavior with its perturbations originating in the troposphere and propagating upward and downstream.

In our work we tackle this discussion through zonal wavenumber decomposition of the NAO-related anomalies in both reanalysis (ERA5 and NCEP-NCAR) and climate models of different complexity and resolution (SPEEDO/L8, CAM5.3/L46 and EC-EARTH3.3/L91). We find that the NAO exhibits a non-annular, wave-like structure in all levels, particularly in the upper troposphere (300 hPa) but also in the middle stratosphere (30 hPa). Indeed, we identify that the troposphere-stratosphere coupling associated with the NAO includes a wavenumber 2 structure tilting westward and increasing in amplitude with height, indicating upward propagation and forcing of the polar vortex. This recurrent NAO-related wavenumber 2 anomaly could contribute to the characteristic wavenumber 2 pattern of the climatological stratospheric circulation. Wavenumber 1, on the other hand, shows no statistical significance in the stratosphere; while in the troposphere it appears to be an artifact of the method, reflecting the prominence of regional dynamics in the North Atlantic. At upper-tropospheric levels, wavenumber 3 seems to dominate the wave-like structure of the total field at mid/high latitudes, whereas wavenumber 5 is more prominent at lower latitudes. Finally, wavenumber 4 does not significantly contribute to the NAO-related anomalies. These results are robust in reanalysis and in the climate models, although some differences are noticeable, likely linked to model biases in the mean flow.

To further interpret the vertical propagation of long waves (wavenumbers 1 and 2) and the horizontal propagation of small waves (wavenumber 3 and 5), additional diagnostics such as eddy heat flux and wave activity flux will be analyzed.

How to cite: Brotons, M., García-Serrano, J., and Haarsma, R.: Zonal wavenumber characterization of the North Atlantic Oscillation-associated anomalies, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-545, https://doi.org/10.5194/ems2024-545, 2024.

The projected changes of the wintertime North Atlantic eddy-driven jet (EDJ) under climate change conditions have been extensively studied in recent years. Although most previous studies agree on a squeezing and elongation of the EDJ over Europe, there are still large uncertainties related to the intensity and latitude. In particular, some studies detect an intensification of the jet core region. However, there is no consensus as some studies find it insignificant or do not detect it. Similarly, the EDJ latitude in winter present a strong inter-model spread, spanning from poleward to equatorward responses. Here, we use a novel multiparametric description of the EDJ to scrutinize the EDJ projections of a CMIP6 multi-model ensemble. Further, we analyze separately these projections in early and late winter. Our results show a non-stationary response of the EDJ latitude through the winter, presenting a poleward migration in early winter and equator shift in late winter. This intra-seasonal shift is related to the climate change response of the different drivers. In particular, thermodynamic processes, involving the change in the 200hPa meridional temperature gradient and the North Atlantic sea surface temperature warming hole are found to influence the early winter change of the EDJ latitude. As for late winter, dynamical process related to the stratospheric vortex response to climate change plays a major role on the EDJ projections. Model biases also have an effect in the EDJ latitudinal projections. Relatedly, the well-established future squeezing of the winter EDJ is no longer found, suggesting it is an artefact from mixing intra-seasonal responses.

How to cite: Garcia-Burgos, M., Ayarzagüena, B., Barriopedro, D., Woollings, T., and Garcia-Herrera, R.: Intraseasonal shift in the wintertime North Atlantic jet structure projected by CMIP6 models, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-595, https://doi.org/10.5194/ems2024-595, 2024.

Key characteristics of anthropogenic climate change are polar amplification and upper tropospheric tropical warming. These large-scale spatial warming patterns alter the equator-to-pole temperature gradient in the lower and upper troposphere. The modified meridional temperature gradients affect the tropospheric jet streams. How the future jet streams will be affected is not fully understood. We perform four aquaplanet simulations with different sea surface temperature (SST) distributions to mimic large scale spatial warming patterns. Compared to the control run the SSTs of the SST4 simulations are increased with 4 K. In the Reduced Temperature Gradient (RTG) simulation the SSTs are gradually warmed from the equator with the maximum temperature increase of 5 K occurring poleward of 60° latitude. The Polar Amplification (PA) simulation uses the SST distribution of control between 45°S and 45°N, with SSTs set to 5 K poleward of these latitudes. We quantify the impact of these sea surface temperature distributions on jet stream strength, wave amplitudes and jet stream  waviness, quantified by a modified Sinuosity Index.

Large-scale spatial warming strengthens the jet stream by a uniform warming scenario SST4 and weakens the jet stream in the two scenarios RTG and PA where the meridional temperature gradient is reduced. However, all scenarios indicate substantial decreases in the magnitude of large wave amplitudes, extreme jet stream waviness and reduced variability of these diagnostics. Our results contradict the earlier proposed mechanism that low-level  polar warming weakens the jet stream and increases wave amplitudes and jet stream waviness. We conclude that a weaker jet stream does not become necessarily wavier.

How to cite: Batelaan, T. J., Weijenborg, C., Steeneveld, G.-J., van Heerwaarden, C., and Sinclair, V.: The Response of Extreme Jet Stream Waviness on Large-Scale Spatial Warming on an Aquaplanet, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-808, https://doi.org/10.5194/ems2024-808, 2024.

The Mediterranean region is highly sensitive to climate change, having experienced an intense warming and drying trend in recent decades. In the context of decision-making processes, there is a growing interest in understanding the near-term climate evolution of this region. Climate change projections consistently indicate that southern Europe and the Mediterranean region will undergo significantly drier conditions in the latter half of the 21st century. However, if our focus shifts to the next decade or so, as is the case for many stakeholders and decision-makers, do climate change projections remain the best tool for understanding climate evolution during this timeframe? 

In this study, using retrospective forecasts from eight decadal prediction systems contributing to the CMIP6 Decadal Climate Prediction Project (CMIP6 DCPP) and the corresponding ensemble of non-initialized simulations (historical and projections), we compare the capabilities of the state-of-the-art climate models in predicting the near-term climate anomalies of the wintertime Mediterranean region for some key quantities so as to assess the added value of initialization. Our findings indicate that decadal predictions exhibit higher reliability than projections, particularly over the subpolar gyre for surface temperature and over southern Europe for precipitation. Therefore, their use should be preferred over projections for a decadal time horizon.

We also inspect the role of the North Atlantic Oscillation (NAO) index and subpolar sea surface temperature (SP-SST) in influencing the predictability of the Mediterranean climate. Therefore, we develop a statistical model to explore the potential impact of the NAO and SP-SST indices on Mediterranean precipitation predictability. The hybrid approach offers higher predictive skill, suggesting that both oceanic and atmospheric factors contribute to the increased skill in predicting European rainfall and can be potential predictors of Euro-Mediterranean rainfall variability on decadal timescale.

How to cite: Nicolì, D., Gualdi, S., and Athanasiadis, P.: Decadal predictions outperform projections in forecasting winter precipitation over the Mediterranean region, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-526, https://doi.org/10.5194/ems2024-526, 2024.

The frequency, duration and intensity of summer extreme temperatures over Europe are increased since the middle of the twentieth century. These trends are expected to further increase in the future due to global warming, which will influence both the thermodynamics and dynamics of these events. In this context, the role played by the thermodynamic changes on trends of extreme temperature events has been largely studied. Differently, the role of dynamic changes in explaining their historical trends as well as their future projections is still under debate.

In the present study, we assess the role played by dynamic changes on trends of summer extreme temperatures by assessing the trends in the occurrence of atmospheric circulation patterns (analogs) associated with the most severe heat waves during 1940—2022. Heat wave events are identified adopting the Heat Wave Magnitude Index daily (HWMId) as suggested by Russo et al. (2015). Furthermore, the trends in the number of summer extreme temperatures in the regions of the identified heat waves are decomposed as suggested by Horton et al. (2015), in order to evaluate the relative contribution of dynamic, thermodynamic and interactive terms on the total extreme temperature trends.

The analyses are performed using a multi-model initial-condition large ensemble, composed by CMIP5 and CMIP6 models forced with the Representative Concentration Pathway 8.5 (RCP8.5) and Shared Socioeconomic Pathways 585 (ssp585), respectively. This allows us to take into account both the global warming influence on extreme temperature trends and the uncertainties arising from model differences and internal climate variability. The analyses are performed for the time periods 1950—2010 and 2011—2100, in order to assess the role played by the different contributions on the historical and future trends of summer extreme temperatures, respectively.

Bibliography

Horton, D. E., Johnson, N. C., Singh, D., Swain, D. L., Rajaratnam, B., & Diffenbaugh, N. S. (2015). Contribution of changes in atmospheric circulation patterns to extreme temperature trends. Nature, 522(7557), 465-469.

Russo, S., Sillmann, J., & Fischer, E. M. (2015). Top ten European heatwaves since 1950 and their occurrence in the coming decades. Environmental Research Letters, 10(12), 124003.

How to cite: Famooss Paolini, L., Pascale, S., Ruggieri, P., Brattich, E., and Di Sabatino, S.: The role of atmospheric circulation in explaining the summer extreme temperature trends over Europe, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-1069, https://doi.org/10.5194/ems2024-1069, 2024.