Dr Scott Osprey FRMetS

Abstract:

Blocking of the tropospheric jet stream during Northern Hemisphere winter (December-January-February) is examined in a multi-model ensemble of coupled atmosphere-ocean general circulation models (GCMs) obtained from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The CMIP5 models exhibit large biases in blocking frequency and related biases in tropospheric jet latitude, similar to earlier generations of GCMs. Underestimated blocking at high latitudes, especially over Europe, is common. In general, model biases decrease as model resolution increases. Increased blocking frequency at high latitudes in both the Atlantic and Pacific basins, as well as more realistic variability of Atlantic jet latitude, are associated with increased vertical resolution in the mid-troposphere to lowermost stratosphere. Finer horizontal resolution is associated with higher blocking frequency at all latitudes in the Atlantic basin but appears to have no systematic impact on blocking near Greenland or in the Pacific basin. Results from the CMIP5 analysis are corroborated by additional controlled experiments using selected GCMs. Key PointsCMIP5 models have large blocking biases and associated jet biasesIncreased spatial resolution is associated with reduced blocking and jet biasesVertical and horizontal resolution give blocking changes in different regions ©2013. American Geophysical Union. All Rights Reserved.

Abstract:

There was a numerical error in the abstract of Mitchell et al. (2012). In the fourth sentence of the abstract the number should be 7 events per decade, not 0.7. The full sentence should read, Analysis of the standard stratospheric zonal mean wind diagnostic shows no significan increase over the twenty-first century in the number of major sudden stratospheric warmings (SSWs) from its historical value of 7 events per decade, although the monthly distribution of SSWs does vary, with events becoming more evenly dispersed through thwinter.