Spatio-temporal Interplay of RWTs and Cyclones in the North Atlantic

Tuesday, 16 December 2014
Mareike Schuster and Uwe Ulbrich, Free University of Berlin, Berlin, Germany
We examine the relation of Rossby-Wave-Trains (RWTs) and cyclones in the North Atlantic. Extra-tropical cyclones are known to have high socioeconomic impacts (high windspeed and large amounts of precipitation). Long lived RWTs have been shown to be precursors for extreme events. Therefore they may impact the predictability of mid-latitude (extreme) weather systems. Current results point at the following relationship: The weakening or dissolving of a long lived RWT that was prevalent over the U.S. and the eastern Pacific for up to 5 days facilitates the genesis of a cyclone downstream - in the whole North Atlantic. In turn, the temporal and spacial coincidence of a newly developing or rather reinforcing RWT off the western coast of Europe and these pre-existing cyclone leads to a strengthening of the cyclone and triggers an explosive development just off the European coast. We apply automated schemes for the identification and tracking of RWTs and cyclones, respectively and relate their characteristics, with a focus on the impact for European climate. By evaluating reanalysis and model data of historical/uninitialized hindcast simulations, we aim to identify spatio-temporal connections between these objectively identified RWTs and cyclones. We then evaluate the interplay of RWT and cyclones in initialized hindcasts and a two way nested (TWN) model simulation. All named simulations are part of the MiKlip project (decadal climate prediction; funded by the German Ministry of Education and Research - BMBF). We also evaluate the decadal variability of cyclones and RWTs in the MiKlip simulations. For our TWN model setup, the regional climate model COSMO-CLM (CCLM) is nested into the atmosphere- ocean general circulation model ECHAM6/MPI-OM (MPI-ESM) in order to investigate the feedback of the meso-scales on the large scales and vice versa. Focus is laid on the development and propagation of synoptic systems (e.g. Rossby Wave Trains and cyclones) that are affecting Europe. The Two-Way-Nested region, thus the CCLM domain, covers Central America and the North Atlantic (CANA) and therefore includes the Gulf stream region, whose prevalent strong meridional SST gradients favor the development of perturbations which then propagate downstream, commonly develop into extra-tropical cyclones and strike Europe.