A new paper on identifying and investigating seasonal extremes accepted in J. Climate

Seasonal extremes are difficult to study in the observational record because, by the definition of the word extreme, only very few such seasons occurred at any location on the globe. In a new INTEXseas study led by Matthias Röthlisberger and in collaboration with colleagues from the group of Reto Knutti at ETH as well as with Christoph Frei from MeteoSwiss we propose to tackle this problem by spatially pooling extreme season events. We use a statistical modeling approach to quantify the local return period of any summer or winter seasonal mean two meter temperature value in the ERA-Interim data set from 1979 to 2018 and identify contiguous spatial extreme season objects from these return periods. Applying the same methodology to 1200 years of data from the Community Earth System Model Large Ensemble (CESM-LENS) then yields large samples of extreme season events with comparable characteristics to even the most extreme observed ones. These large samples of events allows studying extreme seasons in hitherto unexplored ways. We reveal a striking co-occurrence of El Niño to La Niña transitions and the largest mid-latitude extreme summer objects and evaluate CESM with regard to  extreme season characteristics. Finally, we quantify regional return periods of extreme season, which inform about the occurrence of an extreme season with particular size and intensity characteristics within a predefined region, e.g., how often does a winter with comparable area and intensity characteristics as the cold North American 2013/14 winter occur in Europe. The approach is currently being extended to other variables such as seasonal mean precipitation and wind, and will thus serve as a basis for several subsequent INTEXseas studies.
The Figure above shows the six largest ERA-Interim hot summer and cold winter objects in the mid-latitudes. Shading depicts the local return period of the respective seasonal mean T2m value and stippling shows the identified extreme season objects.


INTEXseas at the virtual AMS 101th annual meeting

We are very much looking forward to attend the first virtual AMS annual meeting on 10-15 January 2021 where we have the opportunity to present work from the INTEXseas project. The presentation by Heini Wernli on “Characteristics and dynamics of extreme seasons – a research topic at the interface of weather and climate” provides an overview of the project activities and achievements in the first two years. Mauro Hermann presents recent results from Matthias Röthlisberger et al. about “A new framework for identifying and investigating seasonal climate extremes” in data from ERA-Interim and the CESM large ensemble. And Luise Fischer uses data from the same large ensemble to investigate “How do North Atlantic – European weather regimes change with climate change?”. We are curious to receive feedback about our work from the AMS community!

Philipp Zschenderlein joins INTEXseas!

Philipp joins us from Karlsruhe Institute of Technology (KIT), where he recently obtained his PhD. Philipp is an expert on heat wave dynamics and intensively studied heat waves across Europe from a Lagrangian point of view.

Philipp now joins the INTEXseas project as a PostDoc. In INTEXseas, Philipp will investigate, among other things, under which temperature regimes large amounts of precipitation occur and he will study the meteorological reasons for that. Moreover, Philipp plans to identify extreme summer objects in numerous large ensemble simulations from various modelling centers using an identifiaction scheme that has previousely been developped in INTEXseas.

A new INTEXseas paper on extreme wet seasons

In a study led by Emmanouil Flaounas we identified and studied the occurrence and synoptic dynamics of extreme wet seasons unsing ERA-Interim and an extensive collection of weather feature identification climatologies. In this study we employed a temporally flexible definition of “wet seasons” and their extremes by identifying at each grid point the 90-day periods with the most accumulated precipitation. Connecting these extreme wet seasons to spatial objects (see figure below) then allows to study the dynamical drivers of these individual events. These analyses reveal a wide palette of synoptic storylines for extreme wet seasons that strongly vary in space. For example, an extreme wet season over the tropical Atlantic resulted from a single particularly strong tropical cyclone, while over Iberia and the US West Coast extreme wet seasons occurred due to anomalousely frequent tropical moisture exports and extratropical cyclones and an extreme wet season over Northern Australia resulted form a complex interplay between extratropical Rossby wave breaking, a landfalling tropical cyclone and anomalousely frequent surface cyclones. This paper is the first to comprehensively chracterize extreme wet seasons around to globe with regard to their weather feature characteirstics and is now in the discussion phase in Weather and Climate Dynamics.

The Figure above shows the 100 largest extreme wet season objects in the ERA-Interim period 1979-2018.

New paper on concurrent and sequential Scandinavian and Central European heatwaves with INTEXseas co-authors

Together with colleagues from Karlsruhe Institute of Technology (Germany) and University of Bergen (Norway) we have analyzed the dynamical mechanisms that lead to concurrent and sequential heat waves in Scandinavia and Central Europe. Such events are socio-economically relevant, because the affected area of concurrent and sequential heat waves can far exceed the area affected by individual heat waves. The study led by Clemens Spensberger (University of Bergen) has now been accepted in the Quarterly Journal of the Royal Meteorological Society (QJRMS). Several INTEXseas members (Maxi Böttcher, Lukas Papritz, Michael Sprenger and Matthias Röthlisberger) contributed to this study which highlights the relevance of weak pressure gradient situations over Central Europe for co-occurring Central European and Scandianvian heat waves. Such weak pressure gradient situations are conducive to heat waves over central Europe and typically form at the southern fringe of the a blocking anticyclone, which fosters heat over Scandinavia. Moreover, concurrent heat waves are observed more frequently than one would expect under the assumption that Scandinavian heat waves are statistically independend from Central European heat waves. Overall, the study thus highlights several key dynamical aspects of heat waves affecting unusually large areas.