A cautionary note on decadal sea level pressure predictions from GCMs

摘要

A comparison of sea level pressure (SLP) trends in a subset of seven Coupled Model Intercomparison Project (CMIP) phase 5 general circulation models (GCM),namely decadal simulations with CCSM4,CanCM4,MPI-ESM-LR,FGOALS-g2,MIROC4h,MIROC5,and MRI-CGCM3,to their CMIP3 counterparts reveals an unrealistically strong forecast skill in CMIP3 models for trend predictions for 2001-2011 when using the 1979-2000 period to train the forecast.Boreal-winter SLP trends over five high-,mid-,and low-latitude zones were calculated over the 1979-2000 initialization period for each ensemble member and then ranked based on their performance relative to HadSLP2 observations.The same method is used to rank the ensemble members during the 2001-2011 period.In CMIP3,17 out of 38 ensemble members retain their rank in the 2001-2011 hindcast period and 3 retain the neighboring rank.However,numbers are much lower in more recent CMIP5 decadal predictions over the similar 2001-2010 period when using the 1981-2000 period as initialization with the same number of ensembles.Different periods were used for CMIP3 and CMIP5 because although the 1979-2000 initialization is widely used for CMIP3,CMIP5 decadal predictions are only available for 30-year periods.The conclusion to consider the forecast skill in CMIP3 predictions during 2001-2011 as unrealistic is corroborated by comparisons to earlier periods from the 1960s to the 1980s in both CMIP3 and CMIP5 simulations.Thus,although the 2001-2011 CMIP3 predictions show statistically significant forecast skill,this skill should be treated as a spurious result that is unlikely to be reproduced by newer more accurate GCMs.