Past and projected future changes of North Atlantic polar low frequency

Matthias Zahn, Institute of Coastal Research, Helmholtz Centre Geesthacht

I will present methodology and results of my work on frequency changes of North Atlantic polar lows. Changes of annual polar low numbers were investigated in the past six decades and in an anthropogenically warmed atmosphere in a projected IPCC future.  I will show that by means of dynamical downscaling it is possible to simulate these storms with a Regional Climate Model (RCM).  The results are used to develop an automated detection procedure for polar lows, which then is applied to the output fields of long-term RCM simulations.  

The RCM was driven by NCEP reanalysis data of the past and by global data of IPCC future projections.  While there could not be found any systematic changes of polar low frequency for the past, a significant decrease was discovered associated with projected atmospheric warming.  This decrease is linked to an increase in atmospheric stability with warming, which is a result of faster rising air temperatures compared to sea surface temperatures. 

I will finalise my presentation showing preliminary results of our current project, in which we dynamically downscale NCEP data using a global climate model (GCM) in order to compile a global high resolution long-term hindcast.  These data are used to detect tropical cyclones and investigate their climatological properties.