POLEX

Our consortium includes the following scientists:

Alfred Wegener Institute, section  Atmospheric Physics: Dr. Annette Rinke, Dr. Dörthe Handorf, Dr. Ralf Jaiser, Dr. Wolfgang Dorn, Dr. Heidrun Matthes, Dr, Xiayong Yu, Sara Khoshravi, Raphael Köhler, Johannes Riebold

Alfred Wegener Institute, section Polar Meteorology: Dr. Christof Lüpkes, Dr. Vladimir Gryanik, Dr. Dmitry Chechin

Alfred Wegener Institute, Climate office for polar regions and sea level rise: Dr. Renate Treffeisen

Obukhov Institute for Atmospheric Physics, Russian Academy of Sciences, Moscow: Prof. Dr. Michael Kurgansky, Dr. Irina Repina, Prof. Igor Mokhov, Dr. Vladimir Semenov, Dr. Mirseid Akperov, Dr. Alexey Eliseev, Dr. Alexander Chernokulsky, Andrey Debolskiy, Anna Shestakova, Olga Kibanova

During the past decade, a large number of extreme weather events in the Northern Hemisphere mid-latitudes occurred with hazardous impacts on traffic, economy, and ecosystems. Recent studies emphasize that accelerated and amplified Arctic climate change can increase the probability of preferred mid-latitude atmospheric circulation patterns which favor extreme events in mid- and high latitudes.
To date, climate models have still large deficits in reproducing the observed Arctic circulation and sea ice development, which is partly due to shortcomings in the representation of physical processes governing air-sea ice-ocean interaction. The improved representation of Arctic atmospheric processes in climate models is therefore a fundamental contribution to advance the understanding of the mechanisms underlying the linkages between Arctic climate changes, changes in the mid-latitude atmospheric circulation and subsequent changes in extreme events.
Air-sea ice-ocean interaction is connected in particular with boundary-layer turbulence in the lowest layers above the surface. Most climate models are currently using parametrizations developed for mid-latitude conditions not appropriate for Polar regions. Our project aims to overcome this drawback by introducing a new suite of parametrizations specifically designed for polar conditions.
These new parametrizations will be implemented in a regional and a global climate model. We will investigate and quantify the impact of the improved physical representation of polar air-sea ice-ocean interaction on the variability and changes of Arctic atmospheric circulation, Arctic sea ice and mid-latitude atmospheric circulation. Therefore we investigate the full chain of dynamical linkages between Arctic climate changes and weather and climate extremes. On the other hand, we will assess the subsequent recent and future changes in extreme events in the key regions of the Arctic, Middle Europe and Russia, which are important for reliable assessments of potential environmental and socio-economic impacts. Accordingly, the objectives of our project comprise:

1. An improvement of the representation of atmosphere-sea ice-ocean interaction in climate models by developing and implementing a suite of advanced turbulence parametrizations for polar conditions
2. Performance of present day and future climate change simulations for the 21st century using the new suite of parametrizations
3. Understanding the dynamical linkages between Arctic climate change, mid-latitude atmospheric circulation changes and subsequent changes in extreme events
4. Determination of recent and future changes in extreme events over the Arctic, Middle Europe and Russia and assessment of their impact on Northern Sea route, wildfire and vegetation over the key regions Arctic and Russia.
   Our project will strongly enhance the climate models capability to represent Arctic processes that are known to be critical for simulating linkages between the Arctic and mid-latitudes.

1. Improvement of the representation of atmosphere-sea ice interaction in climate models.
   Computationally fast parametrizations will be developed and implemented into regional and global climate models available at the German and Russian partners. For the evaluation of parametrizations and models, observational data from previous and upcoming campaigns by AWI (e.g., STABLE 2013, AFLUX 2019, MOSAIC 2019-2020) and by Russian partners (e.g., Tiksi observatory, NABOS, ?North-Pole? drifting stations) will be used. We will quantify uncertainties of the simulated climate in polar regions related to the surface flux parametrization. Regions with large response to modified parametrizations will be identified.
2. Performance of climate simulations using the new suite of parametrizations
   Multi-decadal-long simulations of the present-day climate will be carried out with the coupled regio-nal climate model HIRHAM-NAOSIM for the Arctic and the coupled global climate model AWI-CM with an improved representation of atmosphere-sea ice interaction. Additionally, climate change projections for the 21st century will be performed with the improved global model AWI-CM. The comparison between the reference runs and those with improved turbulence schemes will be used to estimate the effect of parametrizations on the simulated present and future climate.
3. Understanding of the linkage between Arctic climate change, atmospheric circulation changes and subsequent changes in extreme weather and climate events
   We are aiming at an improved understanding of the full chain of dynamical linkages between Arctic climate change, changes in NH atmospheric circulation and subsequent changes in extreme weather and climate events. In particular, the effect of the newly improved near-surface turbulence parametrizations on these linkages will be analyzed. Regarding the linkage mechanisms we will focus our work on the tropospheric and stratospheric pathways underlying Arctic-mid-latitude linkages.
4. Determination of recent and future changes in the characteristics of extreme weather events over the Arctic, Russia and Middle Europe and impact case studies
   We are aiming to study present and future changes in extreme weather/climate events over the key regions Arctic, Russia and Middle Europe and of their impact on environment. We address the recent and future changes in temperature and wind extremes and in particular severe convective events over the key regions in different seasons. Case studies will be performed with regard to the impact of extreme events on navigability of Northern sea route over the Arctic Ocean, wildfires and changes in vegetation characteristics specifically over Russia.

Models are typically applying parametrizations of near-surface turbulent fluxes in Polar regions, developed originally for mid-latitude conditions. Our goal is to replace these parametrizations in the Polar regions by a refined scheme. By this approach, for the first time the sensitivity of climate model results on the use of a large number of different formulations of stability functions and of transfer coefficients (for momentum, heat and humidity) can be tested systematically. This will help to identify the most reasonable parametrization. E.g., results might hint to the usage of one specific scheme rather than to a matching of different schemes or vice versa. For this decision also a comparison with observations and re-analyses will be helpful. The quantification of effects includes the Arctic region but it will be highly important to consider effects on Arctic-mid-latitude linkages as well.
Milestone M1.1 Development of turbulence parametrization
Milestone M1.2 Implementation of turbulence parametrization
Milestone M1.3 Sensitivity studies

Multi-decadal-long simulations of the present-day climate will be carried out with the coupled regional climate model HIRHAM-NAOSIM for the Arctic and the coupled global climate model AWI-CM. Both coupled models share the same atmospheric parametrization module from ECHAM. The con-templated simulations will be performed with the model versions with an improved representation of atmosphere-sea ice-ocean interaction (from WP1). The reference runs using the standard ECHAM turbulence scheme are available from other projects: The reference simulations of AWI-CM are currently running as part of the AWI contribution to CMIP6, those of HIRHAM-NAOSIM are performed in the framework of the project (AC)3:Arctic Amplification: Climate relevant Atmospheric and Surface Processes (Transregional Collaborative Research Centre TR 172). Additionally, climate change projections for the 21st century will be performed with the improved global model AWI-CM and compared with CMIP6 simulations carried out at AWI. Comparison between the reference runs and those with improved turbulence schemes will be used to estimate the effect of parametrizations on the simulated present and future climate.
We apply here concertedly this hierarchical approach from regional to global models. With the help of the regional model we can determine the regional-scale impacts of the turbulence parametrization on the turbulent fluxes, vertical atmospheric structure, and atmosphere-ice feedbacks in the Arctic. These regional-scale modifications of the energy fluxes and atmospheric structure will affect the regional circulation, which in turn impacts and interacts with the large-scale hemispheric-wide circulation to be studied in the global model simulations.
Milestone M2.1 Set-up of regional and global climate simulations with improved parametrizations
Milestone M2.2 Evaluation of new model simulations

This WP is aiming at an improved understanding of the dynamical linkage between Arctic climate change, changes in NH atmospheric circulation and subsequent changes in extreme events. The effect of the improved turbulence parametrizations on these linkages will be analyzed. We concentrate our work on the tropospheric and stratospheric pathways underlying Arctic-mid-latitude linkages and put special emphasis on the role of synoptic and planetary wave interactions.
We will provide a characterization of the recent changes in the characteristics of NH atmospheric circulation, an analysis of the relationships between Arctic climate changes (mainly sea-ice and surface temperature) and atmospheric circulation changes and subsequent investigations of the linkage mechanisms. To close the chain of linkages, we will determine the contribution of atmospheric circulation changes to changes in extreme events.
The research conducted in this WP has close links to WP2 through exploiting the new climate model simulations, and by providing tools for analyses of the sensitivity runs in WP2. The link to the extreme event studies in WP4 is established through the study of changes in extreme events.
Milestone M3.1 Metrics of NH atmospheric circulation
Milestone M3.2 Detection of Arctic-mid-latitude linkages
Milestone M3.3 Detection of linkages between atmospheric circulation changes and extremes

This WP is aiming at the study of present and future changes in extreme weather/climate events over the key regions Arctic, Russia and Middle Europe and of their impact on environment. First three tasks address the recent and future changes in temperature and wind extremes and in particular severe convective events over the key regions. Extreme events will be analyzed on the basis of extended observational data sets for the Arctic, Russia and Middle Europe, of new available reanalysis data and of data from global and regional climate model simulations (simulations performed under WP2, and from CMIP and CORDEX projects). We will particularly focus on changes in different seasons over the key regions. The concluding tasks deal with impacts of extreme events on navigability of Northern sea route over the Arctic Ocean, wildfires and changes in vegetation characteristics over Russia.
Milestone M4.1 Implementation of improved ocean wave-ice interaction model
Milestone M4.2 Implementation of improved version of IAP natural fire module
Milestone M4.3 Implementation of improved version of IAP carbon cycle module
Milestone M4.4 Estimation of changes in extreme events over key regions

This WP comprises the coordination of the research activities, the monitoring of the research progress and the communication and exchange of scientific results within the project and outside the project by ensuring international collaboration. Special emphasis will be put on the education of involved young scientists and on the knowledge transfer to the society. One aim of our project is to strengthen the German ? Russian collaboration.
Milestone M5.1 1st plenary meeting and 1st Science days in Potsdam
Milestone M5.2 2nd plenary meeting and 2nd Science days in Moskau
Milestone M5.3 3rd plenary meeting and 3rd Science days in Bremerhaven