During the first phase of MiKlip, a significant positive skill has been established for probabilistic forecasts of cyclone and windstorm frequencies in the North Atlantic. This positive prediction skill exists towards the climatological forecasts, as well as towards uninitialised historical simulations (Kruschke et al., 2015). The STOC (STorm OCean interaction) project aims at a better understanding of mechanisms leading to this skill. The multi-annual ocean variability is the basis for decadal prediction and it is known that an increase in the meridional overturning circulation induces baroclinicity in the North Atlantic region which favors European windstorm development (Nissen et al., 2014). This project investigates the relationship between the ocean state, in particular the North Atlantik, and European windstorms in the MiKlip decadal prediction system.
Relevant oceanic variables and the associated time scales for windstorm development are identified and serve as a basis for the model validation of oceanic processes leading to European winter windstorm development (In cooperation with project VALOCEAN). STOC analyses how far the representation of the identified oceanic variables influences the forecast skill for winter windstorms and extratropical cyclones. This helps understanding and communicating forecast skill found in the operational forecast system in regards to winter storms, and to explore possibilities to identify oceanic states leading to improved skill.
The aim of STOC is the analysis of interactions between the oceanic state and european winter windstorm activity on decadal time scales. Therefore, STOC contributes to process-oriented studies investigating decadal storm forecast skill. It is aimed to define relevant oceanic parameters and states in the North Atlantic, which influence the winter windstorm development on a decadal scale. These processes will be identified for observations and for the MiKlip model system. Analysis of modes of co-variability will be implemented as plug-ins for the central evaluation system. Results guide a stratified verification of winter windstorm forecast and thus STOC contributes to a better understanding and improvement of the MiKlip operational system. Three project goals can be formulated:
1. Atmosphere-ocean interaction in reanalysis data
The reanalysis dataset "20th century reanalysis (20CR)" will be evaluated in regards to ocean-storm-interaction in the North Atlantic region. Due to the limitation in availability of oceanic parameters within this reanalysis, the interactions here are based on sea surface temperatures only. Ocean-storm interactions exist for different time scales (Nissen et al., 2014) and are potentially model dependent. We identify time scales of co-variability and associated patterns in seas surface temperature. In cooperation with project VALOCEAN, the MiKlip model system will be validated for oceanic mechanisms leading to these storm relevant patterns in the sea surface temperatures.
2. Atmosphere-ocean interaction in the Earth system model MPI-ESM
We investigate ocean-storm-interaction in the MPI-ESM climate model using the pre-industrial control run. The relationship between the increase in meridional overturning circulation in the Atlantic and the European storm frequency is explored with respect to decadal time scales. Modes of co-variability of oceanic parameters and the appearance of winter windstorms are analysed for different time scales, and compared to the observed relationships.
3. Forecast skill of windstorms analysing decadal hindcasts
Previous findings are synthesised to see how far the found oceanic processes lead to a decadal predictability for European windstorms; together with the project VALOCEAN, the presence of the relevant oceanic processes in the initialised decadal hindcasts is investigated. Based on a framework developed in the project ECO stratified verification for European winter windstorms along relevant oceanic parameters will be carried out.
The variability of European wind storms has been investigated for different frequency bands. For Eastern Europe an analysis was done for both bands, 3-5 years and 6-15 years. Storm frequencies have been correlated with North Atlantic (NA) sea surface temperatures (SST). It was found that storm occurrence for both frequency bands is connected with enhanced meridional temperature gradients in the NA.
Statistically significant co-variability of marine indices and storm frequency are found for several annual marine indices describing the position and the state of the North Atlantic subpolar gyre, as for example the position of the marine sub-polar front and the windstorm frequency in Central Asia are statistically linked.
Kruschke, T., Rust, H., Kadow, C., Müller, W., Pohlmann, H., Leckebusch, G., and Ulbrich, U., 2015: Probabilistic evaluation of northern hemisphere winter storm frequencies in the miklip decadal prediction system. Meteorol. Z., under revision.
Nissen, K. M, U. Ulbrich, G. C. Leckebusch, and I. Kuhnel, 2014: Decadal windstorm activity in the North Atlantic-European sector and its relationship to meridional overturning circulation in an ensemble of simulations with a coupled climate model, Clim. Dyn, 43, 1545-1555, doi: 10.1007/s00283-013-1975-6
Freie Universität Berlin, Institute for Meteorology
Prof. Dr. U. Ulbrich
Prof. Dr. H. Rust
Dipl.-Math. I. Höschel