Past and projected future changes of North Atlantic polar low frequency 

Matthias Zahn and Hans von Storch 

I would like to present the results of our recent work on frequency changes of North Atlantic polar lows, over the past six decades and 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 for the past on the one hand side and by global data of IPCC future projections on the other.  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.