Changes in synoptic weather patterns and Greenland precipitation in the 20th and 21st centuries: 1. Evaluation of late 20th century simulations from IPCC models

Using the self-organizing map (SOM) technique, the sea level pressure synoptic climatology and precipitation of 15 Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) models are compared to that of the ERA-40 reanalysis for the North Atlantic region for the period 1961 to 1999. Three of the models, the CCCMA-CGCM3.1(T63), the MIROC3.2(hires), and the MPI-ECHAM5, best reproduce the ERA-40 synoptic climatology and are chosen for further analysis of precipitation over Greenland. The MIROC3.2(hires) is the best single performing model, in that it best matches ERA-40. Although the three-model ensemble simulates the same mean annual precipitation over Greenland as ERA-40, the ensemble simulates the mean annual precipitation differently than ERA-40. A dry bias in the CCCMA-CGCM3.1(T63) and a wet bias from the MPI-ECHAM5 cancel in the ensemble average. The mean annual precipitation difference between the model ensemble, as well as each individual model, and ERA-40 is then attributed to differences in intrapattern variability and pattern frequency components in the models that make up the ensemble. Pattern frequency differences between the model and ERA-40 indicate a difference in the occurrence of synoptic weather patterns, while intrapattern variability differences denote differences in the amount of precipitation produced when a given synoptic weather pattern occurs. Intrapattern variability differences between the models and ERA-40 are predominantly responsible for Greenland precipitation differences, but pattern frequency (circulation) differences in the models also play a small role. Part 2 of this paper uses this three-model ensemble to analyze and attribute predicted increases in precipitation over the Greenland ice sheet for the 21st century.