4-9 September 2022, Bonn, Germany
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UP3.2

Mid-latitude atmospheric teleconnection dynamics

Covariability between remote regions – often named teleconnections – are at the basis of our current knowledge of a large part of Earth’s climate variations and represent an important source of weather and climate predictability. Tropospheric and stratospheric pathways have been suggested to play a role in connecting internally-generated and radiatively-forced anomalies at mid-latitudes, as well as in settling tropical-extratropical and polar-nonpolar interactions. However, the underlying processes behind these linkages are still not properly understood, misled by different metrics and diagnostics, and/or generally poorly simulated by global climate models (GCMs). A continuous assessment of these atmospheric teleconnections is thus necessary, since advances in process understanding could translate into improving climate models and predictions.

This session aims at gathering studies on both empirical and modelling approaches, dealing with a dynamical characterization of mid-latitude atmospheric teleconnections. It invites contributions using observational datasets; coupled and uncoupled (atmosphere-only) GCM simulations; pre-industrial, present, and future climate conditions; idealised sensitivity experiments; or theoretical models.

Keynote talk:

TBD - ""

Convener: Javier Garcia-Serrano | Co-conveners: Paolo Davini, Yannick Peings
Orals
| Tue, 06 Sep, 11:00–13:00 (CEST)|Room HS 3-4
Posters
| Attendance Tue, 06 Sep, 14:00–15:30 (CEST) | Display Tue, 06 Sep, 08:00–18:00|b-IT poster area

Tue, 6 Sep, 14:00–15:30

Chairpersons: Yannick Peings, Bianca Mezzina

EMS2022-301
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Onsite presentation
Ewa Łupikasza and Łukasz Małarzewski

Precipitation phase depends on air temperature; therefore, the variabilities in snowfall and rainfall are counted among the indicators of current climate change. The shifts in the structure of precipitation phases have significant environmental implications and influence public perception of climate change. Based on data from 42 synoptic stations for the period 1966-2020, this study discusses trends in the indices of liquid (Lq), mixed (Mx) and solid (Sd) precipitation describing their totals (T), frequencies (Nd) and contribution to overall precipitation totals (T%) and to a number of days with overall precipitation (Nd%) from a seasonal perspective. Moreover, the influence of seasonal air temperature on long-term variability in the indices and changes in the probability of precipitation phases in 1 °C  wide ranges of daily air temperatures between  1966-1985 and 2001-2020 were also analysed. Precipitation phases were discriminated based on the observations of weather phenomena noted as current and past weather in SYNOP messages, considering days with precipitation ≥0.1mm.

In Poland, precipitation phases significantly reacted to current warming. Trend directions corresponded to warming; Thus, trends in liquid precipitation indices were positive, while trends in mixed and solid precipitation indices were negative, both resulting from redistribution of particular phases. The strongest and widespread were increasing trends in winter rains (+1.74 for LqNd, +5.35 mm for LqT, +3.51% for LqNd%, +4.08% for LqT% per decade) and decreasing trends in winter snowfalls (–1.92 days for SdNd, –3.12 mm for SdT, –3.65% for SdNd%, –3.26% for SdT% per decade). In winter, air temperature explained 60–80% variance in the frequency indices. In spring and autumn, increasing trends were found in LqNd% due to the transformation of solid to liquid phase, and in autumn due to melting of mixed and solid phases. In spring, air temperature explained at most 30–50% of the variance in the indices of snowfall frequency. Negative trends in mixed precipitation were significant in transitional seasons, being more prominent in MAM than in SON. A diminished percentage of winter mixed precipitation and summer rains on days with air temperature below 0 °C and 15 °C, respectively, was compensated by an increase in their frequency above these thresholds, thus producing no long-term trends. In most cases, trends in air temperature and indices of precipitation phases diminished the correlations between these variables, particularly in transitional seasons in the case of mixed and solid phases. Solid and mixed phases are becoming sporicidal phases in Polish transitional seasons.

How to cite: Łupikasza, E. and Małarzewski, Ł.: Trends in the indices of precipitation phases under the background of current warming in Poland, 1966-2020, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-301, https://doi.org/10.5194/ems2022-301, 2022.

Orals

11:00–11:15
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EMS2022-4
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Online presentation
Investigating links between the Arctic and East Asian cold extremes with self-organizing maps
(withdrawn)
Chueh-Hsin (Shing) Chang et al.
11:15–11:30
|
EMS2022-668
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Onsite presentation
Steffen Tietsche et al.

Arctic-midlatitude linkages are state-dependent and multi-facetted. Their faithful quantification on sub-seasonal time scales therefore requires methods that account for differences in the background state and reject spurious links arising from common drivers and strong autocorrelation. Here, we use causal network discovery methods that have these properties to establish causal linkages of sub-seasonal time series of selected atmospheric circulation and surface condition indices in the Arctic and Northern mid-latitudes. In order to obtain state-dependent and uncertainty-quantified linkages, we use a large set of sub-seasonal reforecasts from the ECMWF model cycle 47R1 with 11-member ensembles initialised on each day of the winters 1999/2000 to 2018/2019. Pooling forecasts started within a 10-day window allows enough samples to robustly estimate a causal-effect network valid for the window. State-dependence and uncertainty quantification of Arctic-midlatitude linkages is then possible by analysing he seasonal and year-to-year changes of the networks. As an application of the general methods, we discuss the causal links diagnosed for Northern Europe surface air temperature in winter. We chose this index because of its societal relevance and previously reported strong links between extreme cold winters in this region and Arctic sea ice anomalies. Somewhat surprisingly, on sub-seasonal time scales we do not detect significant links between Northern Europe surface air temperatures and Arctic sea ice indices, nor with the polar vortex strength. However, the North Atlantic Oscillation index and geopotential height at 500 hPa over the polar cap do have causal links to Northern Europe surface air temperatures as expected. Link strengths have a seasonal dependence, as well as substantial year-to-year changes. Our results demonstrate the value of large forecast ensembles for quantifying explicitly the state-dependence and uncertainty of Arctic-midlatitude linkages, and might provide guidance for reconciling some previously reported conflicting results.

How to cite: Tietsche, S., Vitart, F., and Balmaseda, M.: Sub-seasonal Arctic-midlatitude linkages as causal networks, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-668, https://doi.org/10.5194/ems2022-668, 2022.

11:30–11:45
|
EMS2022-303
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Onsite presentation
Jet Steam Waviness Response to Polar Amplification on an Aquaplanet
(withdrawn)
Thomas J. Batelaan et al.
11:45–12:00
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EMS2022-208
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Online presentation
Sara Ivasić and Ivana Herceg Bulić

Atmospheric variability and predictable components over the North Atlantic-European region (NAE) were analysed in the late-winter season using a general circulation model of intermediate complexity (ICTP AGCM). The method of empirical orthogonal functions (EOF) analysis was applied to 200-hPa geopotential heights (GH200) to extract individual modes of variability occurring in the ensemble of numerical simulations. The same variable was selected for the signal-to-noise optimal patterns method, which identifies the patterns maximising the signal-to-noise ratio, following Straus et al. (2003).

Six experiments based on a 35-member ensemble of 156-year long simulations were conducted to detect the potential impact of tropical sea surface temperatures. Each experiment was forced with observed sea surface temperature anomalies prescribed in different ocean areas: globally, in the tropics, in the tropical Atlantic region, in the tropical Pacific, and the tropical Indian Ocean. In late winter, the leading EOF pattern calculated for all individual ensemble members projects onto the North Atlantic Oscillation, while the second EOF pattern projects onto the East Atlantic pattern. However, EOF modes based on the ensemble mean, which should reflect the forced component of the signal, have different spatial characteristics. 

Alongside the classical analysis of signal and noise, results of the signal-to-noise optimal patterns method suggest that the optimal patterns and signal-to-noise ratio are affected by the boundary forcing of the oceans. Relatively high correlations between the first principal component of NOAA SST anomalies in different ocean basins and the principal components of GH200, as well as with the time series of the first optimal pattern, further confirm the connection between remote SSTs and potential predictability of the GH200 over the NAE region in late winter. Furthermore, the resemblance between the first optimal pattern and the EOF1 pattern based on the ensemble mean points toward the vital role of the lower-boundary-forced signal in establishing potential predictability in the NAE region. 

How to cite: Ivasić, S. and Herceg Bulić, I.: Impact of tropical SSTs on the variability and predictable components of late-winter atmospheric circulation in the North Atlantic-European region, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-208, https://doi.org/10.5194/ems2022-208, 2022.

12:00–12:15
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EMS2022-608
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Presentation form not yet defined
Bianca Mezzina et al.

The late-winter signal associated with the El Niño-Southern Oscillation (ENSO) over the European continent is unsettled. Two main anomalous patterns of sea-level pressure (SLP) can be identified: a “wave-like” pattern with two opposite-signed anomalies over Europe, and a pattern showing a single anomaly (“semi-isolated”). In this work, potential paths of the tropospheric ENSO teleconnection to Europe and their role in favoring a more wave-like or semi-isolated pattern are explored. Outputs from historical runs (1850-2005, 3 members) of two versions of the MPI-ESM coupled model, which simulate these two types of patterns, are examined. The two versions of MPI-ESM have same horizontal resolution in the atmosphere (T63/1.9º), same top (0.01 hPa), but different vertical resolution: 47 levels in the low-resolution (LR) version and 95 in the mixed-resolution (MR) one. Results from the models are compared with observational ones using the NOAA-20CR reanalysis. A novel ray-tracing approach that accounts for zonal asymmetries in the background flow is used to test potential propagation paths in these simulations and in observations; three source regions are considered: the tropical Pacific, the North America/North Atlantic, and the tropical Atlantic. The semi-isolated pattern is suggested to be related to the well-known Rossby wave train emanating from the tropical Pacific, either via a split over northern North America or via reflection due to inhomogeneities in the background flow. The wave-like pattern, in turn, appears to be related to a secondary wave train emerging from the tropical Atlantic. The competition between these two pathways contributes to determining the actual surface response.

How to cite: Mezzina, B., Garcia-Serrano, J., Ambrizzi, T., Matei, D., Manzini, E., and Bladé, I.: Tropospheric pathways of the late-winter ENSO teleconnection to Europe, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-608, https://doi.org/10.5194/ems2022-608, 2022.

12:15–12:30
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EMS2022-399
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Onsite presentation
Priyanka Yadav et al.

The Madden-Julian Oscillation (MJO) influences the North Atlantic region on timescales of days to weeks. The extratropical response has been found to depend on the propagation speed of the MJO in the region of the Indian to the Pacific ocean: Here, we define slow (fast) MJO events as events that take more (less) than 16 (10) days to propagate from the Indian Ocean (phase 3) to the Pacific Ocean (phase 6), and the MJO amplitude has to be greater than 1 for at least 3 consecutive days in phase 3 and phase 6. Slowly propagating MJO events lead to a stronger North Atlantic Oscillation (NAO) response than fast MJO events, and the positive (negative) NAO response for slow events occurs after a lag of 10 days following phase 4 (phases 7-8). The increased persistence of the tropical thermal forcing in slow events may contribute to a stronger extratropical response.

Furthermore, the MJO can influence the strength of the stratospheric polar vortex, which in turn can impact the NAO via downward coupling. While the tropospheric pathway for teleconnections from MJO events with varying phase speed is well understood, the stratospheric pathways for MJO events of different propagation speeds has not yet been explored.

In this study, we discuss the stratospheric pathways during fast and slow MJO episodes using reanalysis data with respect to the strength of the Northern Hemisphere stratospheric polar vortex and subsequent downward coupling to the troposphere. The results suggest that the stratosphere plays an important role in generating a strong NAO response during slowly propagating MJO episodes.

How to cite: Yadav, P., Domeisen, D., and Garfinkel, C.: The role of the stratosphere in teleconnections arising from slow and fast MJO episodes, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-399, https://doi.org/10.5194/ems2022-399, 2022.

12:30–12:45
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EMS2022-680
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Onsite presentation
Esteban Rodríguez-Guisado and Ernesto Rodríguez-Camino

Although most operational seasonal forecasting systems are based on dynamical
models, empirical forecasting systems, built on statistical relationships between
present and future at seasonal time horizons conditions of the climate system, provide
a feasible and realistic alternative and a source of supplementary information. Here, a
new empirical model based on partial least squares regression is presented. Originally
designed as a flexible tool, the system is able to automatically select predictors from an initial pool and explore spatial fields looking for additional predictors.
the model can be run with many configurations including different predictands,
resolutions, leads and aggregation times. The model benefits from specific predictors
for the Mediterranean region unveiled in the frame of the MEDSCOPE project.  We
present here 2 sets of results: the first one from a configuration producing probabilistic forecasts of seasonal
(3 month averages) temperature and precipitation over the Mediterranean area, their verification and comparison
against a selection of state-of-the-art seasonal forecast systems based on dynamical
models in a hindcast period (1994-2015). The model is able to produce spatially
coherent anomaly patterns, and reach levels of skill comparable to those based on
dynamical models. To explore the potential of the model for producing skilful forecasts over reduced areas, a second set of results are calculated using higher resolution predictands over Iberia, again comparing its skill with that of a set of state of the art models. Examples of the model usage for evaluating the impact on skill of
certain predictor helping in the search and understanding of new sources of
predictability are also shown. 

How to cite: Rodríguez-Guisado, E. and Rodríguez-Camino, E.: Sources of predictability over the Mediterranean at seasonal time-scale: building up anempirical forecasting model, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-680, https://doi.org/10.5194/ems2022-680, 2022.

12:45–13:00
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EMS2022-650
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Online presentation
Daniela I.V. Domeisen et al.

Extreme weather events have devastating impacts on human health, economic activities, ecosystems, and infrastructure. It is therefore crucial to anticipate extremes and their impacts to allow for preparedness and emergency measures. There is indeed potential for probabilistic subseasonal prediction on timescales of several weeks for selected cases of extreme events that are linked to remote drivers and large-scale teleconnections. We here present a range of case studies, including heatwaves, cold spells, and tropical cyclones, where precursors and global linkages may have improved sub-seasonal predictability. These linkages include teleconnections from the tropics as well as the stratosphere, in addition to circumglobal teleconnections. For example, cold extremes can be induced by stratospheric or tropical teleconnections, while the predictability of tropical cyclones tends to be increased when an active Madden-Julian Oscillation signal is present. Prediction can further be improved through increased physical process understanding of the drivers of extremes, an improved representation of these processes in prediction systems, as well as the development of post-processing techniques that improve model output. While the case studies presented only represent some of the major types of extreme events, they provide an informative overview of the current ability of prediction models to benefit from teleconnections. These case studies clearly illustrate the potential for event – dependent advance warnings for a wide range of extreme events globally.The subseasonal predictability of extreme events allows for an extension of warning horizons, can provide advance information to impact modelers, and informs communities and stakeholders affected by the impacts of extreme weather events.

How to cite: Domeisen, D. I. V., White, C. J., Afargan-Gerstman, H., Antoine, S., Ardilouze, C., Batté, L., Camargo, S. J., Collins, D., Ferranti, L., Infanti, J. M., Janiga, M. A., Kolstad, E. W., LaJoie, E., Magnusson, L., Strazzo, S., Vitart, F., and Wulff, C. O.: Teleconnection-driven extreme events: Relevant case studies, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-650, https://doi.org/10.5194/ems2022-650, 2022.

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