A Cautionary Note on Decadal Sea Level Pressure Predictions from GCMs

Date of Submission: 
February 6, 2018
Report Number: 
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Decadal prediction of sea level pressure (SLP) plays an important role in regional climate prediction, because it shows changes in atmospheric behavior on time scales that are relevant for policy makers. These changes consist of a combination of externally forced and internally driven climate system characteristics.
A comparison of SLP trends in a subset of seven Coupled Model Intercomparison Project (CMIP) phase 5 general circulation models (GCM) during the satellite-era 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 a two-decade initialization period for each ensemble member and then ranked based on their performance relative to observations in all five zones over the same time period.  The same method is used to rank the ensemble members during the following decade. In CMIP3, 17 out of 38 ensemble members retain their rank in the 2001-2011 hindcast period and 3 retain the neighboring rank. However, these numbers are much lower in more recent CMIP5 decadal predictions over a similar period with the same number of ensembles. The conclusion to consider the forecast skill in CMIP3 predictions during the 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.